Automobile Rear Module

Abstract

This automotive rear module is an automotive rear module for reinforcing the side outer of an automobile, which is formed by hot stamping a plurality of integrated steel plates. When viewed from the vertical direction of the reference plane, the projected area is S (m 2 ), among the components of the automotive rear module, the total weight of the components is W (kg), and the total weight of the steel components with a plate thickness of 1.5 mm or less and a minimum Vickers hardness of HV230 or more is W A . When, W / S is 24 or less, and W A / W is 0.30 or more.

Description

Technical Field

[0001] The present invention relates to an automotive rear module for reinforcing the side outer of an automobile. This application claims priority based on Japanese Patent Application No. 2024-045131 filed in Japan on March 21, 2024, and incorporates its content herein by reference.

Background Art

[0002] Recently, from the perspective of preventing global warming, it has become more important to suppress the emission of greenhouse gases (hereinafter referred to as GHG) such as carbon dioxide (CO 2 2). Under such circumstances, with the emergence of electric vehicles, hybrid vehicles, etc., whose GHG emissions are suppressed compared to conventional internal combustion engine-powered vehicles, it is expected to reduce GHG emissions from vehicles during driving. In addition, by adopting materials with excellent weight reduction properties such as aluminum and carbon as materials for constructing automobiles, it is expected to reduce GHG emissions from vehicles during driving.

[0003] Regarding the automobile body, for example, Patent Document 1 below discloses a vehicle body structure with excellent productivity. Also, related to the vehicle body structure, Patent Document 2 below discloses a vehicle body rear structure that provides improved impact resistance in the case of a rear impact on a vehicle, and a manufacturing method including a step of forming a tailored welded blank into a desired shape. Furthermore, Patent Document 3 below discloses an automobile body capable of reducing the total amount of GHG generated during a series of life cycles from the manufacture, use, and disposal of an automobile.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

[0005] Considering the life cycle of automobiles, in order to reduce the total amount of GHG emitted in the global environment, it is insufficient to focus only on the reduction of GHG during vehicle use (driving). In addition, no consideration has been given to reducing LC-GHG (hereinafter referred to as life cycle GHG or LC-GHG) generated during a series of life cycles from the manufacturing, use, to disposal of automotive rear modules.

[0006] Therefore, an object of the present invention is to provide an automotive rear module capable of reducing LC-GHG per unit area. MEANS FOR SOLVING THE PROBLEMS

[0007] The gist of the present disclosure is as follows.

[0008] (1) A first aspect of the present invention is an automotive rear module having an integrated component formed by hot stamping a plurality of integrated steel plates, wherein the projected area when viewed from the vertical direction of the reference plane is S (m 2 ), among the components of the automotive rear module, the total weight of the components is W (kg), and the total weight of the steel components with a plate thickness of 1.5 mm or less and a minimum Vickers hardness of HV230 or more is W A When, W / S is 24 or less, W A / W is 0.30 or more. (2) The automotive rear module according to (1) above may include at least one of element technology A1, and element technologies B1, C1, C2, D1, and D2. The element technology A1 is a skeletal member formed by hot stamping a steel sheet. The skeletal member has a closed cross-sectional portion whose cross-section perpendicular to the longitudinal direction is a closed cross-section. The closed cross-sectional portion has at least two flat portions where the radius of curvature is larger than the maximum outer dimension in the cross-section, and a concave bead portion formed between the two flat portions. The concave bead portion is a pair of wall portions with a radius of curvature of 50 mm or more, and has a pair of wall portions that protrude toward the inside of the closed cross-sectional portion through a pair of bent portions that bend inward from the opposite ends of the two flat portions toward the inside of the closed cross-section. The Vickers hardness at the center of the plate thickness of the wall portion is 520 Hv or more, and the width of the wall portion is 0.5 times or more and 2.5 times or less of the effective width W obtained from the effective width formula of Karman. The standard deviation ratio obtained by dividing the standard deviation of the hardness frequency distribution in the surface layer portion of the wall portion by the standard deviation of the hardness frequency distribution at the center of the plate thickness of the wall portion is less than 1.0. e It is a skeletal member. The element technology B1 is a structural member, including a first steel sheet having the smallest plate thickness and a second steel sheet having a plate thickness larger than that of the first steel sheet, and is formed by a plurality of steel sheets joined to each other, and includes a member body having an annular shape in a plan view. The value of the coefficient A calculated by the following formula (1) using the chemical composition of the first steel sheet is larger than the value of the coefficient A calculated by the following formula (1) using the chemical composition of the second steel sheet. A = 1.48×(2.7×C + 0.4×Si + Mn + 0.45×Ni + 0.8×Cr + 2×Mo) 3.42 (1) However, the content (mass%) of the corresponding element is substituted into the element symbol in the above formula (1). The element technology C1 is a structural member for a vehicle body. (C1a) It includes a pair of side frames and a cross member connecting the side frames. The side frames and the cross member include a first steel sheet having the smallest plate thickness and a second steel sheet having a plate thickness larger than that of the first steel sheet, and are formed by a plurality of steel sheets joined to each other. On the first steel sheet, 0.001 g / m of one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide2 is provided with a film containing the following (C1b) a pair of side frames and a cross member connecting the side frames, comprising, wherein the side frames and the cross member are formed of a plurality of steel plates joined to each other, including a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate, and on the first steel plate, carbon black is 0.500 g / m 2 is provided with a film containing the following (C1c) a pair of side frames and a cross member connecting the side frames, comprising, wherein the side frames and the cross member are formed of a plurality of steel plates joined to each other, including a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate, and the first steel plate and the second steel plate are plated steel plates each having an aluminum-based plating layer on both surfaces of the base steel plate, and the thickness of the aluminum-based plating layer in the first steel plate is smaller than the thickness of the aluminum-based plating layer in the second steel plate, is a structural member satisfying at least one of the following The element technology C2 is a structural member for a vehicle body, (C2a) a pair of side frames and a cross member connecting the side frames, wherein the side frames and the cross member are formed of a plurality of steel plates joined to each other, including a first steel plate and a second steel plate having an end portion that overlaps and is joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate, and on the surfaces of the first steel plate and the second steel plate located outside the overlap portion in each of them, a film containing 0.001 g / m or more of one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide is provided, 2 is provided with a film containing the following (C2b) It includes a pair of side frames and a cross member connecting the side frames. The structural member includes a first steel plate and a second steel plate having an end portion that is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate. It is formed by a plurality of steel plates joined to each other. On the surface of each of the first steel plate and the second steel plate located outside the overlap portion, a film containing 0.500 g / m 2 is provided. (C2c) It includes a pair of side frames and a cross member connecting the side frames. The side frames and the cross member include a first steel plate, a second steel plate having an end portion that is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate, and a third steel plate. It is formed by a plurality of steel plates joined to each other. At least one of the first steel plate and the second steel plate, and the third steel plate are each a plated steel plate having aluminum-based plating layers on both surfaces of a base steel plate. The thickness of the aluminum-based plating layer in at least one of the first steel plate and the second steel plate is smaller than the thickness of the aluminum-based plating layer in the third steel plate. It is a structural member that satisfies at least one of the above. In the element technology D1, a plurality of partial blanks made of steel plates are joined. (D1a) At least two of the partial blanks are joined at a plurality of joining portions in an overlapping portion formed by partially overlapping, and in a cross-section perpendicular to the surface of the partial blank including the center of the joining portion of the outermost partial blank among the partial blanks, at a position 1 / 4 of the plate thickness from the surface in contact with the other partial blanks of the partial blanks, when the Vickers hardness at a position more than 15 mm away from the center of the joining portion and where joining is not performed is Hvm, a part of the plurality of joining portions has a difference ΔHv between the maximum hardness and the minimum hardness in terms of the Vickers hardness in a range within 5 mm from the end of the joining portion toward the base material side (or within 12 mm from the center of the joining portion) that is less than 0.2Hvm, preferably 0.1Hvm or less, and the other joining portions (joining portions other than the part) of the plurality of joining portions have the ΔHv of 0.2Hvm or more, preferably 0.3Hvm, 0.4Hvm, or 0.5Hvm or more. (D1b) At least two of the partial blanks are joined by a plurality of spot welds in an overlapping portion formed by partially overlapping, and in a cross-section including the center of the spot weld of the outermost partial blank among the partial blanks, at a position 1 / 4 of the plate thickness from the surface of the partial blank, when the hardness at a position more than 15 mm away from the center of the spot weld and where the spot weld is not performed is Hvm, a part of the plurality of spot welds has a difference ΔHv between the maximum hardness and the minimum hardness in a range within a radius of 12 mm from its center that is less than 0.2Hvm, and the spot welds other than the part have the ΔHv of 0.2Hvm or more. It is a press-formed part that satisfies at least one of the above. The element technology D2 is (D2a) A press-formed part having a bent portion, wherein a patchwork made of a steel plate is superposed on the surface of a base blank made of a steel plate and joined at a joint, and the difference between the maximum hardness within 2 mm from the outer edge of the joint on the surface of the base blank and the hardness of the base material of the base blank is 7% or more of the hardness of the base material of the base blank. When there is one bent portion, the joint exists only in one of the regions on either side of the surface of the base blank with the bent portion as the boundary. When there are two or more bent portions, the joint exists only in the region between two adjacent bent portions on the surface of the base blank. (D2b) A press-formed part having a bent portion, wherein a patchwork made of a steel plate is superposed on the surface of a base blank made of a steel plate and joined by spot welding at a joint, and the difference between the maximum hardness at a position 5 mm away from the center of the joint on the surface of the base blank and the hardness of the base material of the base blank is 7% or more of the hardness of the base material of the base blank. When there is one bent portion, the joint exists only in one of the regions on either side of the surface of the base blank with the bent portion as the boundary. When there are two or more bent portions, the joint exists only in the region between two adjacent bent portions on the surface of the base blank. A press-formed part that satisfies at least one of the above. (3) The automotive rear module according to (2) above may include the element technology A1 and the element technology B1. (4) The automotive rear module according to (2) above may include the element technology A1 and at least one of the element technology C1 and the element technology C2. (5) The automotive rear module according to (2) above may include the element technology A1 and at least one of the element technology D1 and the element technology D2. (6) The automotive rear module according to (2) above may include the element technology A1, the element technology B1, at least one of the element technology C1 and the element technology C2, and at least one of the element technology D1 and the element technology D2.

Advantages of the Invention

[0009] According to the present disclosure, an automotive rear module capable of reducing LC-GHG per unit area can be provided.

Brief Description of the Drawings

[0010]

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Embodiments for Carrying Out the Invention

[0011] As described above, considering the life cycle of automobiles, in order to reduce the total amount of GHG emitted in the global environment, it is insufficient to focus only on GHG reduction during vehicle use (driving). In addition, since the automotive rear module accounts for about 3 to 10% of the total vehicle weight, the contribution to reduction by reducing GHG related to the automotive rear module is significant. Currently, the main focus is on making automobiles multi-material using materials such as aluminum and carbon to reduce the weight of automobiles. However, the present inventors 1.GHG generated in the material manufacturing of automobile rear modules (hereinafter referred to as "material manufacturing GHG"); 2.GHG generated in the manufacturing process of automobile rear modules (hereinafter referred to as "process GHG"); 3. The contribution of GHGs generated by the rear module of a vehicle while the vehicle is running (hereinafter referred to as "driving GHGs"); and 4.GHG generated when the automobile rear module is disposed of (hereinafter referred to as "GHG at the time of disposal"), The inventors focused on these four types of GHGs and investigated ways to reduce their total amounts.

[0012] In this specification, the rear module refers to a group of structural members including the rear floor of the vehicle body and the frame parts joined thereto. The rear module is mainly composed of an upper frame and an underframe. Additionally, the rear module includes reinforcing or stiffening components that are joined to these sections or members. In this specification, the CO generated during the life cycle 2 GHGs including CO are called LC-GHGs. 2 The amount is CO 2 It is stipulated that GHGs other than those mentioned above are also calculated and added together in equivalent mass. CO 2 Other GHGs include methane, nitrous oxide, and ozone-depleting substances such as fluorocarbons. 2 The equivalent mass is calculated using the conversion factors listed in Table 1, which are set for each material, process, use, and recycling category. "CO 2 "CO equivalent" 2 Also referred to as "CO equivalent mass" in this specification. 2 "equivalent", "CO 2 Equivalent mass” “CO 2 "Equivalent amount" is defined as having the same meaning. "CO 2 The equivalent mass is CO 2 (global warming potential: 1) and CO 2 Gases other than methane, e.g., CH4 (The greenhouse effect per unit mass is 25 times that of CO 2 : global warming potential 25), nitrous oxide N 2 O (the greenhouse effect per unit mass is 298 times that of CO 2 : global warming potential 298), weighted by the global warming potential, and the mass converted to CO 2 is calculated.

[0013]

Table 1

[0014] (GHG in material manufacturing) As a material, steel has the lowest GHG emissions per unit weight compared to other materials. Figure 1 is a characteristic diagram showing the environmental load (GHG emissions) during the manufacturing of each material for the automotive rear module. The vertical axis shows normal steel sheets, high-strength steel sheets, aluminum, and carbon fiber reinforced plastics (CFRP) as materials for the automotive rear module. The horizontal axis shows the GHG emissions per equivalent function [kg-CO 2 equivalent / kg-equivalent parts].[[]END]] As shown in Figure 1, steel materials (normal steel sheets, high-strength steel sheets) have significantly lower GHG emissions compared to other materials (aluminum, carbon fiber reinforced plastics). It can be seen that mainly using steel materials as the main material for the automotive rear module can greatly contribute to the reduction of LC-GHG.

[0015] (GHG in processes) In the manufacturing of the automotive rear module, GHG is mainly generated in welding processes, heating processes, painting processes, etc. Therefore, by performing appropriate process design while ensuring the required performance of the automotive rear module, it is possible to contribute to the reduction of LC-GHG.

[0016] (GHG during driving) By reducing the weight of the automotive rear module, the load on the power source such as the internal combustion engine can be reduced. Therefore, by reducing the weight of the automotive rear module, it is possible to contribute to the reduction of LC-GHG.

[0017] (GHG at the time of disposal) Steel can contribute to the reduction of -1.60 kg-CO per kg by scrap recycling. 2 eq of GHG. Compared with aluminum, the reduction effect of emissions per kg of steel is small, but when using high-strength steel, the weight required to obtain the necessary strength is small. Therefore, it can be said that using high-strength steel can contribute to the reduction of LC-GHG. That is, similar to the GHG during material production, the GHG at the time of disposal can be reduced by mainly using steel materials as the materials constituting the automotive rear module, and it can contribute to the reduction of LC-GHG.

[0018] As described above, in order to reduce LC-GHG, the total GHG emissions should be reduced from four perspectives. For example, "GHG during material production and GHG at the time of disposal" and "GHG during driving" are considered to be in a trade-off relationship. Furthermore, generally, in order to manufacture lightweight and high-functional parts, "process GHG" tends to increase, and there is a trade-off relationship between "process GHG" and "GHG during driving". Conventionally, GHG reduction methods in each life cycle have been studied and discussed, but the optimal examples of material selection and process design for reducing LC-GHG have not been disclosed in the past.

[0019] The inventors of the present invention focused on reducing LC-GHG in consideration of the GHG in the above four categories including the trade-off relationship, and by controlling the weight of the components per predetermined area and the weight of the high-strength components with a plate thickness of 1.5 mm or less and a minimum Vickers hardness of HV230 or more within an appropriate range, it was found that the LC-GHG of the automotive rear module can be reduced while satisfying the required strength (crashworthiness).

[0020] Hereinafter, the automotive rear module according to an embodiment of the present invention based on the above findings will be described with reference to the drawings. However, these descriptions are merely illustrative of preferred embodiments of the present invention and are not intended to limit the present invention to such specific embodiments.

[0021] The automotive rear module 100 according to this embodiment is applied to an automotive body configured by a frame having a monocoque structure including a shock-absorbing skeletal member. FIG. 2 shows an exploded perspective view of the automotive rear module 100 according to this embodiment. As shown in FIG. 2, the automotive rear module 100 includes an integrated upper frame 10 and an integrated underframe 20.

[0022] The integrated upper frame 10 is composed of a rear member front portion 11, a rear member rear portion 12, two cross members 13, a portion equivalent to a cross member extension joined to the cross member 13, a rear floor portion 15, a rear floor side panel portion 16, and a rear wheel house portion 17.

[0023] The integrated upper frame 10 is manufactured by hot stamping a single tailored blank into an integrated component. A single tailored blank can be obtained by joining a plurality of hot stamping steel sheets corresponding to each part to be an integrated component and laminating and joining (patchwork joining) hot stamping steel sheets to be formed into reinforcement members as necessary.

[0024] The integrated underframe 20 is composed of a rear member front portion 21, a rear member rear portion 22, three cross members 23, and a portion equivalent to a cross member extension joined to the cross member 23.

[0025] The integrated underframe 20 is manufactured by hot stamping a single tailored blank into an integrated component. One tailored blank can be obtained by joining a plurality of hot stamping steel sheets corresponding to each part to be integrated, and by stacking and joining (patchwork joining) hot stamping steel sheets that are formed into reinforcement members as needed.

[0026] Thus, the automotive rear module 100 according to the present embodiment has parts (integrated underframe 110, integrated upper frame 120) that are made into integrated parts by hot stamping one tailored blank each. And by hot stamping the tailored blank, integrated parts with different characteristics (weight, hardness, strength, plate thickness) for each part can be obtained. In addition, additional reinforcing parts and brackets may be attached to the automotive rear module 100 after hot stamping.

[0027] In addition, the parts to be integrated are not limited to the above examples. For example, in the present embodiment, the rear floor part 15, the rear floor side panel part 16, and the rear wheel house part 17 are integrated with the upper frame 10, but after forming an integrated part composed of the front part 11 of the rear member, the rear part 12 of the rear member, three cross members 13, and a part corresponding to the cross member extension joined to the cross member 13, it may be retrofitted. Also, the rear wheel house part 17 may be integrated with the integrated underframe 20.

[0028] Thus, in the automotive rear module 100 according to this embodiment, since an integrated part formed by joining a plurality of hot stamping steel sheets and then performing hot stamping is used, compared with the case where press forming is performed for each part, unnecessary parts are appropriately trimmed, and then joined by welding, the GHG in material production can be reduced. Further, when hot stamping is performed for each part and then welding is performed, the number and time of the heating process of hot stamping are required. However, in the automotive rear module 100 according to this embodiment, by performing hot stamping after welding a plurality of steel sheets, the number and time of the heating process can be reduced. Therefore, according to the automotive rear module 100 according to this embodiment, the process GHG can be reduced while achieving the performance required for the automotive rear module.

[0029] Furthermore, in the automotive rear module 100 according to this embodiment, by controlling the weight of the component per projected area and the weight of the high-strength component having a thickness of 1.5 mm or less and a minimum Vickers hardness of HV230 or more within an appropriate range, it is possible to reduce the LC-GHG of the automotive rear module.

[0030] Specifically, when the projected area viewed from the vertical direction of the reference plane is S (m 2 ), among the components of the automotive rear module, the total weight of the components is W (kg), and the total weight of the steel components having a thickness of 1.5 mm or less and a minimum Vickers hardness of HV230 or more is W A , when W / S is 24 or less and W A / W is 0.30 or more, it is possible to reduce the LC-GHG of the automotive rear module. There has been no automotive rear module that satisfies the above conditions, and these are conditions that could not be easily conceived without the above-mentioned findings of the present inventors. Note that the reference plane is a plane perpendicular to the vehicle height direction in the automotive rear module in a state where it is attached to the vehicle body.

[0031] W AThe value of 2 tends to increase as the size of the large vehicle body or side door module increases. Therefore, in this application, the value of W corresponding to the projected area S of the automotive rear module, i.e., W / S (m ) is used as an index. When the value of W / S is 24 or less, weight reduction according to the size of the side door module can be achieved, so it is possible to reduce GHG during driving. For the purpose of reducing GHG during driving, the smaller the value of W / S, the better. Preferably, it is 23 or less, more preferably 22 or less.

[0032] W A When the value of t / W is 0.30 or more, among the members used in the side door module, the proportion of high-strength members with a plate thickness of 1.5 mm or less and a minimum Vickers hardness of HV230 or more is high, so it is possible to reduce GHG in material manufacturing. For the purpose of reducing GHG in material manufacturing, the higher the value of t / W, the better. Preferably, it exceeds 0.40, more preferably exceeds 0.45. A Furthermore, from the perspective of reducing GHG in material manufacturing, when the weight of the component with a plate thickness of 1.3 mm or less and a minimum Vickers hardness of HV230 or more is defined as t W, it is preferably that the value of t / W is 0.18 or more, more preferably 0.20 or more. B W B The method for measuring the Vickers hardness is as follows.

[0033] The method for measuring the Vickers hardness is as follows. A sample having a cross-section perpendicular to the plate surface is taken from the flat portions of each part, the cross-section is prepared as a measurement surface, and the measurement surface is subjected to a hardness test. The method for preparing the measurement surface is carried out in accordance with JIS Z 2244:2009. After polishing the measurement surface using silicon carbide paper from #600 to #1500, the measurement surface is finished to a mirror surface using a liquid in which diamond powder with a particle size of 1 μm to 6 μm is dispersed in a diluent such as alcohol or pure water. The hardness test is carried out by the method described in JIS Z 2244:2009. Using a micro-Vickers hardness tester, at the 3 / 8 position of the plate thickness of the sample, 30 points are measured at an interval of more than 3 times the indentation with a load of 1000 gf, and the average value thereof is taken as the hardness at the center of the plate thickness.

[0034] In the present application, (1) Element technology A1, and (2) at least one of element technology B1, element technology C1, element technology C2, element technology D1, and element technology D2, and By applying to the components of the automotive rear module 100, the weight ratio of the steel material in the automotive body is increased to reduce the above-mentioned material manufacturing GHG, and the GHG during driving is reduced by the weight reduction of the automotive body. As a result, a significant reduction in LC-GHG is achieved compared to the conventional automotive body.

[0035] Said element technology A1, and Said element technology B1, and It is more preferable to apply. Said element technology A1, and At least one of said element technology C1 and said element technology C2, and It is more preferable to apply. Said element technology A1, and At least one of said element technology D1 and said element technology D2, and It is more preferable to apply. Said element technology A1, and Said element technology B1, and At least one of said element technology C1 and said element technology C2, and At least one of said element technology D1 and said element technology D2, and It is more preferable to apply

[0036] In this specification, an automobile means an automobile with excellent rear - end collision safety. For example, if it obtains the highest evaluation in the rear - end neck injury protection test by IIHS (Insurance Institute for Highway Safety), it can be said that it is fully capable of driving on public roads.

[0037] Moreover, the vehicle body of the motor vehicle for public road driving to which the motor vehicle rear module according to this embodiment is applied is not limited to the vehicle body of an engine vehicle or an electric vehicle, and may be a vehicle body of a hybrid vehicle, a fuel cell vehicle, a hydrogen engine vehicle, etc., which uses an internal combustion engine and an electric motor as drive sources. Furthermore, the vehicle body to which the motor vehicle rear module is applied is not limited to a vehicle body having a monocoque - structured frame, and may be a vehicle body having a ladder - frame structure. Moreover, as vehicle types of motor vehicles for public road driving, it includes passenger cars or commercial vehicles such as sedans, hatchbacks, station wagons, one - box vehicles, pickup trucks, etc. Furthermore, the motor vehicle for public road driving includes loading vehicles such as trucks.

[0038] (Example) Hereinafter, the present invention will be specifically described by way of examples. The conditions of the examples are an example adopted to confirm the feasibility and effects of the present disclosure, and the present disclosure is not limited to the conditions of the examples. The present disclosure can adopt various conditions without departing from its gist and as long as its purpose is achieved.

[0039] As various characteristic values of the motor vehicle rear modules according to the invention example and the comparative example in Table 2, · The number of components, · The weight ratio of steel plates, · The projected area S (m 2 ) · The total weight W (kg) of the components, · W / S, · The total weight W of components with a thickness of 1.5 mm or less and a minimum Vickers hardness of HV230 or more A · W A / W (kg / m 2 )、 · The total weight W of components with a thickness of 1.3 mm or less and a minimum Vickers hardness of HV230 or more B 、 · W B / W、 shows the calculated values of.

[0040]

Table 2

[0041] The automotive rear module of Invention Example 1 includes 56 components. Among them, 4 are integrated components. The weight of each component is as shown in the graph of Fig. 3A. In the graph of Fig. 3A, the vertical axis represents the component weight of that part. The hatched area is the part that meets the condition of a thickness of 1.5 mm or less and a minimum Vickers hardness of HV230 or more. The dotted area is the part that has a minimum Vickers hardness of HV230 or more but a thickness greater than 1.5 mm. From left to right on the horizontal axis, the components are arranged in ascending order of thickness. To the left of the dashed line are components with a thickness of 1.5 mm or less. Components No. 2, 3, 4, and 5 are integrated components obtained by hot stamping from a single tailored blank.

[0042] Also, the same weights, thicknesses, and hardnesses were determined for Invention Examples 2 - 8.

[0043] The automotive rear module of Comparative Example 1 includes 129 components. There are no integrated components, only individually formed components. The weight of each component is as shown in the graph of Fig. 3B. In the graph of FIG. 3B, the vertical axis represents the component weight of that part. The hatched filling indicates the part that meets the condition of having a plate thickness of 1.5 mm or less and a minimum Vickers hardness of HV230 or more. The dot filling indicates the part that has a minimum Vickers hardness of HV230 or more but does not have a plate thickness of 1.5 mm or less. From left to right on the horizontal axis, they are arranged in ascending order of plate thickness. To the left of the dashed line are the components with a plate thickness of 1.5 mm or less.

[0044] Also, for Comparative Examples 2 to 8, the same weights, plate thicknesses, and hardnesses were compared.

[0045] The weight (kg) of the parts shown in Table 2 is the weight when the automotive rear module is cut along the welding line trace of the tailored blank. Each weight in the comparative examples was obtained by disassembling the automotive body of a generally circulating public road vehicle, measuring and analyzing the data of the shape and weight measurement. Each weight in some of the comparative examples and the inventive examples was obtained by measuring and analyzing the design and development data by CAD (Computer - Aided Design). The hardness HV was determined as follows. Samples having a cross-section perpendicular to the plate surface were taken from the flat part of each part, and the cross-section was prepared as a measurement surface and subjected to a hardness test. The method for preparing the measurement surface was carried out in accordance with JIS Z 2244:2009. After polishing the measurement surface using silicon carbide paper from #600 to #1500, the measurement surface was finished to a mirror surface using a liquid in which diamond powder with a particle size of 1 μm to 6 μm was dispersed in a diluent such as alcohol or pure water. The hardness test was carried out by the method described in JIS Z 2244:2009. Using a micro-Vickers hardness tester, at the 3 / 8 position of the plate thickness of the sample, 30 points were measured at intervals of more than 3 times the indentation with a load of 1000 gf, and their average value was taken as the hardness at the center of the plate thickness.

[0046] In Table 3, for each of the inventive examples and comparative examples, · Total GHG emissions LC - GHG (kg, CO 2 -eq), and, · LC - GHG / S (kg, CO 2 -eq / m2 ) shows the calculated value.

[0047]

Table 3

[0048] The total GHG emissions are calculated from the above material production GHG, process GHG, driving GHG, and end-of-life GHG by calculating the CO 2 equivalent mass and summing them, which corresponds to the emissions of LC-GHG. The total GHG emissions are the values calculated by the method described below.

[0049] Each characteristic value of Invention Examples 1-8 was obtained by the inventors measuring and analyzing an automobile body configured by the inventors using each of the above-described elemental technologies. In addition, each characteristic value of Comparative Examples 1-8 was obtained by the inventors measuring and analyzing an automobile body of a general-purpose passenger car on public roads. For some of the comparative examples, values described as default values on the website of World Auto Steel (WAS) were used. WAS is an automotive division of the World Steel Association and is composed of 17 steelmakers from around the world. Note that the analysis of LC-GHG emissions was 「Roland Geyer, Parametric Assessment of Climate Change Impacts of Automotive Material Substitution, Environmental Science & Technology 2008 42 (18), 6973-6979, DOI: 10.1021 / es800314w」 performed based on.

[0050] (Calculation of Material Production GHG) The default settings of the GHG analysis software were used as the basic conditions. In these default settings, the charging rate of scrap into the blast furnace is 11.9%, and the usage rates of recycled materials using scrap are set based on statistical data as 5% for sheet materials, 85% for bar and wire materials, and 100% for cast iron. Assuming these as the base conditions, values were input and calculated to obtain the various material compositions shown in Table 1.

[0051] (Calculation of Process GHG) The default settings of the GHG analysis software were used as the basic conditions. The material yields in the production of automotive parts were assumed to be 55% for steel sheets, 52% for aluminum alloy sheets, 75% for bar and wire materials, and 80% for cast iron, aluminum extruded materials, and aluminum casting materials. Values were input and calculated to obtain the various material compositions shown in Table 1.

[0052] (Calculation of GHG during Driving) An electric vehicle was selected as the power train type of the target vehicle model. Based on the size and weight of each analyzed vehicle, a mid-sized electric vehicle was set. The driving pattern of the vehicle was set as the following WLTP (Class 3b) mode. WLTP Mode · Average speed ··· 36.57 km / h · Maximum speed ··· 97.4 km / h · Driving time ··· 1477 seconds · Driving distance ··· 15.01 km · Idling ratio ··· 15.4% · Cold start ratio ··· 100% Assuming a driving distance of 110,000 km, the size of the power train was considered with the vehicle body weight reduction factored in. Also, for the power consumption during the driving of the electric vehicle, electricity generated in Japan was used, and the values of the power consumption of the rear module contribution obtained by the inventors in the analysis were input and calculated.

[0053] (Calculation of GHG at End-of-Life) Based on the default settings of the software for GHG analysis at the time of disposal, assuming a recycling rate of 90.3% for steel materials and 78.6% for aluminum alloy materials, and also considering the energy recovery from recycling to other than automobiles as the CO 2 absorption amount for the setting.

[0054] CO 2 The calculation of the equivalent mass uses the coefficient for calculating the equivalent mass of GHG in the manufacturing process of the materials shown in Table 1, the manufacturing process of the vehicle, the manufacturing and use process of the fuel, and the recycling process of the materials and the vehicle, and calculates the CO 2 equivalent mass using the weight or energy amount. These numerical values are the default setting values of the software for GHG analysis and are set based on the statistical data of GHG emissions in each substance and each process. 2 In the above procedure, the CO equivalent mass is calculated from the material manufacturing GHG, process GHG, driving GHG, and disposal GHG, and the LC-GHG described in Table 3 is calculated by summing them up. 2 In Invention Examples 1-8, as a result of increasing the usage ratio of steel materials in the automotive rear module and applying and integrating the above-described respective element technologies in combination, it was possible to make W / S 24 or less and W

[0055] / W 0.30 or more. Thereby, the LC-GHG of the automotive rear module could be reduced. A On the other hand, in Comparative Examples 1-8, W

[0056] / W was 0.27 or less, and it was necessary to increase W / S in order to ensure the collision performance, and the reduction effects of material manufacturing GHG and driving GHG could not be sufficiently obtained. A In Comparative Examples 1-8 that do not adopt an integrated structure, press forming is performed for each part, unnecessary parts are trimmed and removed, and then joined by welding. Therefore, the reduction effects of material manufacturing GHG and driving GHG could not be sufficiently obtained.

[0057] In addition, in Comparative Examples 1-8 that do not adopt an integrated structure, press forming is performed for each part, unnecessary parts are trimmed and removed, and then joined by welding. Therefore, the reduction effects of material manufacturing GHG and driving GHG could not be sufficiently obtained.

[0058] Figure 4A is a graph showing, for an example, W / W on the horizontal axis and W / S (kg / m A ) plotted on the vertical axis. 2 ) Figure 4B is a graph showing, for an example, W / W on the horizontal axis and LC-GHG / S (kg,CO A -eq / m 2 ) plotted on the vertical axis. 2 ) From these graphs, according to the example of the present invention, it can be confirmed that LC-GHG is significantly reduced compared to the conventional structure. Thus, according to the present invention, when the projected area when viewed from the vertical direction of the reference plane is S (m 2 ), among the components of the automobile rear module, the total weight of the components is W (kg), and the total weight of the steel components with a plate thickness of 1.5 mm or less and a minimum Vickers hardness of HV230 or more is W A , by satisfying W / S ≤ 24 and W A / W ≥ 0.30, it is possible to reduce the LC-GHG of the automobile rear module.

[0059] (Example 2) Table 4 shows the results of collision tests for the inventive examples and comparative examples. In Table 4, the test results for the collisions are shown by evaluation values A and B. In this evaluation, first, the test results in the IIHS rear collision test are disclosed, and numerical analysis of the rear collision test in the IIHS rear collision analysis model is performed on the vehicle body of Comparative Example 6 with the highest evaluation of collision safety performance, and the obtained intrusion amount is used as a reference (Evaluation B). Note that the vehicle bodies of Comparative Examples 1-8 are also vehicle bodies that have received type approval in various countries' regulations. The safety performance evaluation results of some vehicle bodies are described in comparison with Comparative Example 6. For Inventive Examples 1-8, numerical analysis of the rear collision test with only the rear module replaced was performed, and the safety performance was evaluated based on the relative intrusion amount into the rear module and the absorbed energy during the collision. Then, those with test results better than the vehicle with the highest evaluation (Comparative Example 6) in the IIHS rear-end collision simulation test were rated as Evaluation A. Also, those with results equivalent to the safety test results of Comparative Example 6 and without component separation were rated as Evaluation B.

[0060]

Table 4

[0061] As shown in Table 4, in Invention Examples 1-8, for rear-end collisions, results (Evaluation A or Evaluation B) equivalent to or better than the safety test results of vehicles with a good evaluation in the IIHS rear-end test were obtained.

[0062] Therefore, according to Invention Examples 1-8, it is possible to reduce LC-GHG while satisfying the safety test results of vehicles with a good evaluation in the IIHS rear-end test.

[0063] The outline of the element technologies applied to the automotive rear module 100 according to this embodiment is as follows. Note that the reference numerals for components, formulas, embodiments, examples, etc. in the description of each element technology are assigned for each element technology to simplify the description. Therefore, the same reference numeral may be assigned in the descriptions of different element technologies. Also, the term "invention" in the description of the element technology should be read as "element technology".

[0064] Elemental technology A1 is a skeletal member formed by hot stamping a steel plate. The skeletal member has a closed cross-sectional portion where the cross-section perpendicular to the longitudinal direction is a closed cross-section. The closed cross-sectional portion has at least two flat portions where the radius of curvature is larger than the maximum outer dimension in the cross-section, and a concave bead portion formed between the two flat portions. The concave bead portion is a pair of wall portions with a radius of curvature of 50 mm or more, and has a pair of wall portions that project toward the inside of the closed cross-sectional portion through a pair of bent portions that bend inward from the opposing ends in the two flat portions toward the inside of the closed cross-section. The Vickers hardness at the center of the plate thickness of the wall portion is 520 Hv or more, and the width of the wall portion is 0.5 times or more and 2.5 times or less of the effective width W e obtained from the Karman effective width formula, and the standard deviation ratio obtained by dividing the standard deviation of the hardness frequency distribution in the surface layer portion of the wall portion by the standard deviation of the hardness frequency distribution at the center of the plate thickness of the wall portion is less than 1.0.

[0065] Elemental technology A1 is the technology disclosed in International Publication No. 2022 / 018963. According to this elemental technology A1, it is possible to provide a skeletal member with excellent energy absorption efficiency.

[0066] (Elemental technology B1) The elemental technology B1 is a structural member including a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness larger than that of the first steel plate, and is formed of a plurality of steel plates joined to each other, and includes a member body having an annular shape in plan view. The value of coefficient A calculated by the following formula (1) using the chemical composition of the first steel plate is larger than the value of coefficient A calculated by the following formula (1) using the chemical composition of the second steel plate. A = 1.48×(2.7×C + 0.4×Si + Mn + 0.45×Ni + 0.8×Cr + 2×Mo) 3.42 (1) However, the content (mass%) of the corresponding element is substituted for the element symbol in the above formula (1).

[0067] According to the elemental technology B1, a structural member excellent in impact absorption performance can be provided.

[0068] For example, as described in "Masakatsu Ueno, Kintaro Ito, 'New Prediction Formula for Hardenability of Steel Substituting for GROSSMANN's Formula', Iron and Steel, Japan Iron and Steel Federation, 74th year (1988), No. 6, p. 1073 - 1080", conventionally, the critical cooling rate V c90 has been used as an index of the hardenability of steel. The critical cooling rate V c90 is the critical cooling rate (°C / s) at which a martensite structure with a volume fraction of 90% or more can be obtained, and is expressed by the formula logV c90 = 2.94 - 0.75β. β is calculated by 2.7×C + 0.4×Si + Mn + 0.45×Ni + 0.8×Cr + 2×Mo. β represents the influence degree of each element on the hardenability based on the Mn content. The larger β is, the smaller the critical cooling rate V c90 becomes, and the better the hardenability of the steel.

[0069] There is a correlation between β indicating the influence degree of each element on the hardenability and the time (transformation start time) from the completion of heating of the steel to the start of diffusion transformation. The inventors heated hot stamping steel plates A, B, C, and D (plate thickness 1.2 mm) similar to the examples (Table 5) described later in a heating furnace at 900°C for 1 minute, then took them out of the heating furnace and air-cooled them, and measured the time (transformation start time) until the phase transformation started. Then, regression analysis was performed using the measured transformation start time data, and the formula A = 1.48×β 3.42 for converting β into the transformation start time was constructed. A obtained by this formula is a coefficient (index value) different for each steel depending on the chemical composition. The coefficient A corresponds to the transformation start time when only the influence of the elements is considered, and means that the larger the coefficient A, the better the hardenability of the steel. The inventors further examined the appropriate arrangement of the steel in the blank using the coefficient A. As a result, the inventors completed the blank according to the embodiment.

[0070] The blank for hot stamping according to the embodiment includes a plurality of steel plates. The plurality of steel plates are arranged and joined so as to have an annular shape in a plan view of the blank. The plurality of steel plates include a first steel plate and a second steel plate. The first steel plate has the smallest plate thickness among the plurality of steel plates. The second steel plate has a plate thickness larger than the plate thickness of the first steel plate. The value of coefficient A calculated by the following formula (1) using the chemical composition of the first steel plate is larger than the value of coefficient A calculated by the following formula (1) using the chemical composition of the second steel plate (first configuration). A = 1.48×(2.7×C + 0.4×Si + Mn + 0.45×Ni + 0.8×Cr + 2×Mo) 3.42 (1) However, the content (mass%) of the corresponding element is substituted into the element symbol in the above formula (1).

[0071] For example, when the hardenability of the materials is the same between the first steel plate having the smallest plate thickness and the second steel plate having a plate thickness larger than that of the first steel plate, the thin first steel plate starts the phase transformation (diffusion transformation) from austenite to ferrite earlier than the thick second steel plate after the heating of the blank for hot stamping is completed. However, in the blank according to the first configuration, the coefficient A calculated based on the chemical composition of the first steel plate is larger than the coefficient A calculated based on the chemical composition of the second steel plate. That is, the first steel plate is composed of a material with higher hardenability compared to the second steel plate, in other words, a material with a later start of diffusion transformation during cooling. Therefore, after the heating of the blank is completed, the start of diffusion transformation in the first steel plate is delayed, and the difference in the transformation start time between the first steel plate and the second steel plate can be reduced. As a result, when the blank is hot stamped, not only the relatively thick second steel plate but also the first steel plate with the minimum plate thickness can be properly hardened, and the hardness of the structural member formed from the blank is likely to be uniformized. In addition, since the stress is canceled out by transformation plasticity due to good hardening of the first steel plate and the residual stress becomes small, the occurrence of torsion caused by the concentration of residual stress in the annular structural member can be suppressed, and as a result, the deterioration of dimensional accuracy can be reduced. Here, the application of the invention to an annular automobile door ring component will be described. However, it can also be used for the rear module of an automobile or the like, and it is possible to suppress the occurrence of torsion, warping, etc. caused by the concentration of residual stress.

[0072] Thus, in the first configuration, although the annular blank includes the first steel plate that is thinner than the second steel plate, it is possible to equalize the hardness of the structural member formed by hot stamping from the blank and suppress the deterioration of dimensional accuracy. Therefore, it is possible to improve the impact absorption performance (crashworthiness) of the annular structural member, particularly a large-sized and annular structural member or a structural member such as a rear module.

[0073] In the blank according to the first configuration, when the plate thickness of the first steel plate is t min and the maximum plate thickness among the plate thicknesses of the plurality of steel plates is t max , t max -t min may be ≧ 0.2 (mm) (second configuration).

[0074] The manufacturing method of the structural member according to the embodiment includes a step of preparing a blank according to the first or second configuration, a step of heating the plurality of steel plates included in the blank to a temperature equal to or higher than the austenite transformation completion temperature, and a step of using a mold to form the heated blank into an annular structural member in plan view and performing quenching (third configuration).

[0075] The structural member according to the embodiment includes a member body. The member body has an annular shape in plan view. The member body is formed by a plurality of steel plates joined to each other. The plurality of steel plates include a first steel plate having the minimum plate thickness and a second steel plate having a plate thickness larger than the plate thickness of the first steel plate. The value of the coefficient A calculated by the following formula (1) using the chemical composition of the first steel plate is larger than the value of the coefficient A calculated by the following formula (1) using the chemical composition of the second steel plate (fourth configuration). A = 1.48×(2.7×C + 0.4×Si + Mn + 0.45×Ni + 0.8×Cr + 2×Mo) 3.42 (1) However, the element symbols in the above formula (1) are substituted with the contents (mass %) of the corresponding elements.

[0076] The structural member according to the fourth configuration may be, for example, an automobile door ring part, a rear module, or the like. In the case of an automobile door ring part, the member body may include a front pillar, a center pillar, and a rocker connecting the front pillar and the center pillar (fifth configuration).

[0077] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The same or corresponding components in each figure are denoted by the same reference numerals, and the same description will not be repeated.

[0078] <First Embodiment> [Structural Member] FIG. 5 is a view (plan view) seen from above in a state where the structural member B1-10 according to the present embodiment is placed on a horizontal plane. The structural member B1-10 is used, for example, in an automobile body. The structural member B1-10 is typically an automobile door ring part. In the present embodiment, an example in which the structural member B1-10 is a door ring part will be described.

[0079] The structural member B1-10 is a hot stamping member. That is, the structural member B1-10 is formed by hot stamping (hot press working) a blank composed of a plurality of steel plates. The structural member B1-10 includes a member body B1-11. The member body B1-11 has an annular shape in a plan view of the structural member B1-10. The member body B1-11 includes a front pillar B1-111, a B1-center pillar B1-112, and a rocker B1-113. When the structural member B1-10 is assembled to an automobile body, the center pillar B1-112 is disposed behind the front pillar B1-111. The center pillar B1-112 extends substantially in the vertical direction of the vehicle body. The front pillar B1-111 extends toward the center pillar B1-112. When the structural member B1-10 is assembled to an automobile body, the rocker B1-113 is disposed below the front pillar B1-111 and the center pillar B1-112. The rocker B1-113 connects the front pillar B1-111 and the center pillar B1-112.

[0080] In the present embodiment, the member body B1-11 is formed by a plurality of steel plates B1-21, B1-22, B1-23 joined to each other. In the example of FIG. 5, the front pillar B1-111 is mainly composed of the steel plates B1-21, B1-22. The center pillar B1-112 is mainly composed of the steel plate B1-23. The rocker B1-113 is composed of the steel plates B1-21, B1-23.

[0081] FIG. 6 is a sectional view taken along line II-II of FIG. 5. FIG. 6 shows a cross-section obtained by cutting the structural member B1-10 along its plate thickness direction at the position of the steel plate B1-21. As shown in FIG. 6, the steel plate B1-21 has an open cross-section. The steel plate B1-21 has, for example, a generally hat-shaped cross-section when viewed in the cross-section of the structural member B1-10. More specifically, the steel plate B1-21 includes a top plate B1-211, vertical walls B1-212 and B1-213, and flanges B1-214 and B1-215. The vertical wall B1-212 is disposed on the opposite side of the vertical wall B1-213 with respect to the top plate B1-211. In the cross-sectional view of the structural member B1-10, one end portions of the vertical walls B1-212 and B1-213 are connected by the top plate B1-211. In the cross-sectional view of the structural member B1-10, flanges B1-214 and B1-215 are connected to the other end portions of the vertical walls B1-212 and B1-213, respectively. The flanges B1-214 and B1-215 respectively protrude from the vertical walls B1-212 and B1-213 to the outside of the structural member B1-10.

[0082] In the structural member B1-10, the width W of the steel plate B1-21 at the position shown in FIG. 6, that is, the width W at the lower part of the front pillar B1-111 (FIG. 5), may be 30 mm or more and 750 mm or less. The height H at the lower part of the front pillar B1-111 may be 25 mm or more and 150 mm or less. The width W is the distance from the R stop on the vertical wall B1-212 side of the corner portion between the top plate B1-211 and the vertical wall B1-212 to the R stop on the vertical wall B1-213 side of the corner portion between the top plate B1-211 and the vertical wall B1-213 in the cross-section of the structural member B1-10. The height H is the distance along the plate thickness direction of the top plate B1-211 from the top plate B1-211 to the flanges B1-214 and B1-215. The width W of the portion of the steel plate B1-21 corresponding to the rocker B1-113 (FIG. 5) is, for example, 30 mm or more and 300 mm or less. The height of the portion of the steel plate B1-21 corresponding to the rocker B1-113 may be 25 mm or more and 150 mm or less.

[0083] Although illustrations are omitted, other steel plates B1-22 and B1-23 (Fig. 5) also have an open cross-section similar to that of the steel plate B1-21. The steel plates B1-22 and B1-23 can also have a generally hat-shaped cross-section, for example, when viewed in cross-section of the structural member B1-10. The width of the portion of the steel plate B1-22 corresponding to the upper part of the front pillar B1-111 may be 15 mm or more and 300 mm or less. The height of the portion of the steel plate B1-22 corresponding to the upper part of the front pillar B1-111 may be 10 mm or more and 150 mm or less. The width of the portion of the steel plate B1-23 corresponding to the center pillar B1-112 may be 15 mm or more and 300 mm or less. The height of the portion of the steel plate B1-23 corresponding to the center pillar B1-112 may be 10 mm or more and 150 mm or less.

[0084] In plan view, the size of the annular structural member B1-10 is, for example, 1.0 m or more. The size of the structural member B1-10 may be, for example, 4.0 m or less. The size of the structural member B1-10 refers to the length of the line segment connecting the two points with the farthest distance among any two points on the outer periphery of the structural member B1-10 when the structural member B1-10 is placed on a horizontal plane and viewed along the vertical direction.

[0085] [Manufacturing method of structural member] Hereinafter, the manufacturing method of the structural member B1-10 will be described with reference to Figs. 7A to 7G. The manufacturing method of the structural member B1-10 according to the present embodiment includes a step of preparing a blank B1-20, a step of heating the blank B1-20, and a step of forming the heated blank B1-20 into the structural member B1-10.

[0086] (Preparation step) As shown in Fig. 7A, in the preparation step, a blank B1-20 having a shape obtained by developing the structural member B1-10 is prepared. The blank B1-20 includes a plurality of steel plates B1-21, B1-22, and B1-23. The steel plates B1-21, B1-22, and B1-23 are arranged and joined so as to have an annular shape in plan view of the blank B1-20.

[0087] Figures 7B, 7C, and 7D are cross-sectional views of blank B1-20 showing the joints of steel plates B1-21, B1-22, and B1-23. Figures 7B, 7C, and 7D are the IIIB-IIIB cross-sectional view, the IIIC-IIIC cross-sectional view, and the IIID-IIID cross-sectional view of Figure 7A, respectively. Referring to Figures 7B and 7C, steel plate B1-21 is butt-jointed to each of steel plates B1-22 and B1-23. That is, these end faces are joined in a state where the end face of steel plate B1-21 abuts against the end face of steel plate B1-22, and these end faces are joined in a state where the other end face of steel plate B1-21 abuts against the end face of steel plate B1-23. Referring to Figure 7D, steel plate B1-22 is butt-jointed not only to steel plate B1-21 but also to steel plate B1-23. The end face of steel plate B1-22 is joined to the end face in a state where it abuts against the end face of steel plate B1-23. Steel plates B1-21, B1-22, and B1-23 are joined by, for example, laser welding. In the present embodiment, blank B1-20 is a so-called tailor-welded blank.

[0088] Referring to FIGS. 7B to 7D, the steel plate B1-21 has a plate thickness t 1 Steel plate B1-22 has a plate thickness t 2 Steel plate B1-23 has a plate thickness t 3 The plate thicknesses t 2 , t 3 of steel plates B1-22 and B1-23 are larger than the plate thickness t 1 of steel plate B1-21. That is, the plate thickness t 1 of steel plate B1-21 is the minimum plate thickness t min among steel plates B1-21, B1-22, and B1-23. In the example of the present embodiment, the plate thickness t 2 of steel plate B1-22 is larger than the plate thickness t 3 of steel plate B1-23. Therefore, the plate thickness t 2 of steel plate B1-22 is the maximum plate thickness t max among steel plates B1-21, B1-22, and B1-23. However, steel plate B1-22 does not necessarily have to have the maximum plate thickness t max among steel plates B1-21, B1-22, and B1-23. The plate thickness t 2 of steel plate B1-22 is the plate thickness t of the other steel plate B1-233 It may be as follows.

[0089] In steel plates B1-21, B1-22, and B1-23, the minimum plate thickness t min and the maximum plate thickness t max are, for example, t max -t min satisfy ≧0.2 (mm). The plate thickness t min , t max is, t max -t min may satisfy ≦3.2 (mm). The plate thickness t of steel plate B1-21 min is, for example, less than 1.4 mm. The plate thickness t min may be 0.8 mm or more. The maximum plate thickness t max is, for example, less than 4.0 mm. The plate thickness t max may be 1.4 mm or more.

[0090] Steel plates B1-21, B1-22, and B1-23 may have a chemical composition known as a steel plate for hot stamping. For example, the chemical compositions of steel plates B1-21, B1-22, and B1-23 are, respectively, in mass %, C: 0.05 to 0.50%, Si: 0.020 to 1.000%, Mn: 0.20 to 2.50%, Ni: 0 to 0.50%, Cr: 0 to 0.50%, Mo: 0 to 0.5%, and B: 0.0005 to 0.0050%. Each chemical composition of steel plates B1-21, B1-22, and B1-23 may further contain one or more selected from the group consisting of Cu: 0.005 to 3.000%, Co: 0.005 to 0.500%, Sn: 0.005 to 0.500%, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, REM: 0.0005 to 0.0050%, and Sb: 0.0005 to 0.0200% in mass %.

[0091] In steel plates B1-21, B1-22, and B1-23, the minimum plate thickness t minThe chemical composition of the steel plate B1-21 having [specific property] is different from the chemical compositions of the thicker steel plates B1-22 and B1-23. The value of the coefficient A calculated by the following formula (1) using the chemical composition of the steel plate B1-21 is different from the coefficient A calculated by formula (1) using the chemical composition of each of the steel plates B1-22 and B1-23. A = 1.48×(2.7×C + 0.4×Si + Mn + 0.45×Ni + 0.8×Cr + 2×Mo) 3.42 (1)

[0092] In the element symbols in formula (1), the content (mass%) of the corresponding element is substituted. That is, the coefficient A of the steel plate B1-21 is calculated by substituting the content (mass%) of each element in the chemical composition of the steel plate B1-21 into the corresponding element symbol in formula (1). Similarly, the coefficient A of the steel plate B1-22 is calculated by substituting the content (mass%) of each element in the chemical composition of the steel plate B1-22 into the corresponding element symbol in formula (1). The value of the coefficient A calculated by formula (1) using the chemical composition of the steel plate B1-21 is larger than the coefficient A calculated by formula (1) using the chemical composition of the steel plate B1-22. The coefficient A of the steel plate B1-22 is the smallest among the coefficients A calculated by formula (1) for the steel plates B1-22 and B1-23 other than the steel plate B1-21. Let the coefficient A of the steel plate B1-21 be A 1 and the coefficient A of the steel plate B1-22 be A 2 . When 1 - A 2 is considered, it is preferably 0.10 or more, more preferably 0.20 or more. A 1 - A 2 may be, for example, 11.50 or less.

[0093] The coefficient A of the steel plate B1-23 is calculated by substituting the content (mass%) of each element in the chemical composition of the steel plate B1-23 into the corresponding element symbol in formula (1). The value of the coefficient A calculated by formula (1) using the chemical composition of the steel plate B1-23 is equal to or greater than the value of the coefficient A calculated by formula (1) using the chemical composition of the steel plate B1-22. The value of the coefficient A of the steel plate B1-23 is preferably the minimum plate thickness t minis smaller than the value of coefficient A of steel sheet B1-21 having the same. That is, among the plurality of steel sheets B1-21, B1-22, and B1-23 included in blank B1-20, the minimum plate thickness t min It is preferable that the coefficient A of the steel sheet B1-21 having is the largest. Let the coefficient A of the steel sheet B1-21 be A 1 , and the coefficient A of the steel sheet B1-23 be A 3 When it is set as, A 1 - A 3 is preferably 0.10 or more, more preferably 0.20 or more. Although not particularly limited, A 1 - A 3 may be 11.50 or less. However, the coefficient A 3 of the steel sheet B1-23 is the coefficient A 1 of the steel sheet B1-21 or more (A 1 - A 3 ≦ 0) may be sufficient.

[0094] The chemical compositions of the steel sheets B1-21, B1-22, and B1-23 included in the blank B1-20 may be measured by a general analysis method. For example, test pieces for analysis are taken from each of the steel sheets B1-21, B1-22, and B1-23, and the chemical compositions of the steel sheets B1-21, B1-22, and B1-23 can be obtained by measuring the test pieces using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). In each test piece for analysis, C may be measured using the combustion-infrared absorption method.

[0095] (Heating process) The prepared blank B1-20 is formed into a structural member B1-10 (Figs. 5 and 6) by hot stamping (hot press working). During hot stamping, the blank B1-20 is subjected to a heating process. Referring to Fig. 7E, in the heating process, for example, the blank B1-20 is heated by a heating furnace B1-30. The plurality of steel plates B1-21, B1-22, B1-23 included in the blank B1-20 are heated to a temperature equal to or higher than the austenite transformation completion temperature (Ac3 point). The steel plates B1-21, B1-22, B1-23 are heated to, for example, 900°C or higher. As a result, the microstructure of the steel plates B1-21, B1-22, B1-23 transforms into, for example, all or almost the austenite phase.

[0096] (Forming process) Referring to Fig. 7F, in the forming process, using a mold B1-40, the heated blank B1-20 is formed into an annular structural member B1-10 (Figs. 5 and 6) in plan view and quenched. The blank B1-20 heated by the heating process is taken out from the heating furnace B1-30 (Fig. 7E) and conveyed to the mold B1-40. The mold B1-40 is attached to a known press device. The mold B1-40 includes, for example, a punch B1-41 and a die B1-42. The blank B1-20 is disposed between the punch B1-41 and the die B1-42.

[0097] Referring to Fig. 7G, after the blank B1-20 is disposed between the punch B1-41 and the die B1-42, the die B1-42 approaches relatively to the punch B1-41. The blank B1-20 is clamped (pressed) by the punch B1-41 and the die B1-42 and formed into a shape along the forming surfaces of the punch B1-41 and the die B1-42. The blank B1-20 is held while being clamped by the punch B1-41 and the die B1-42. The blank B1-20 is heat-extracted (quenched) by the mold B1-40, and its microstructure transforms into martensite. Thereby, the structural member B1-10 can be manufactured from the blank B1-20.

[0098] Referring again to Fig. 5, the chemical compositions of the steel plates B1-21, B1-22, and B1-23 do not change before and after hot stamping. Therefore, in the structural member B1-10, the value of the coefficient A calculated by the above formula (1) using the chemical composition of the steel plate B1-21 having the minimum plate thickness tmin is larger than the coefficient A calculated by formula (1) using the chemical composition of the steel plate B1-22. The value of the coefficient A calculated by formula (1) using the chemical composition of the steel plate B1-21 is preferably larger than the coefficient A calculated by formula (1) using the chemical composition of the steel plate B1-23. However, the value of the coefficient A calculated by formula (1) using the chemical composition of the steel plate B1-21 may be less than or equal to the coefficient A calculated by formula (1) using the chemical composition of the steel plate B1-23.

[0099] The chemical compositions of the steel plates B1-21, B1-22, and B1-23 in the structural member B1-10 after hot stamping can be obtained by the same analysis method as the chemical compositions of the steel plates B1-21, B1-22, and B1-23 at the blank B1-20 stage.

[0100] Referring to Fig. 6, at the position of the steel plate B1-21 having the minimum plate thickness t min When the value obtained by subtracting the minimum martensite fraction (%) from the maximum martensite fraction (%) in the cross-section of the structural member B1-10 at the position of the steel plate B1-21 having is taken as the variation in the martensite fraction, the variation in the martensite fraction is, for example, 20% or less. The variation in the martensite fraction is preferably 15% or less, and more preferably 10% or less. The variation in the martensite fraction can be measured as follows. That is, in the cross-section of the structural member B1-10 at the position of the steel plate B1-21 having the minimum plate thickness t min After cutting out 10 or more analysis samples (for example, having a size of about 10 mm on the long side) from positions more than 20 mm away from the end and each more than 10 mm away from each other in the cross-section of the structural member B1-10 at the position of the steel plate B1-21 having the minimum plate thickness t, each is mirror-polished and etched with a Lepera reagent so that the plate thickness direction becomes the observation surface. Then, for the region from 1 / 8 of the plate thickness from the steel plate surface to 3 / 8 of the plate thickness from the steel plate surface (the region from 1 / 4 of the plate thickness from the steel plate surface), using an optical microscope at a magnification of 1000 times, one field of view is 2,400 μm 2Take 30 visual fields of the above tissue photographs and perform image analysis on the obtained tissue photographs.

[0101] As the image analysis method, the maximum brightness value Lmax and the minimum brightness value Lmin of the image are obtained from the image, and the portion having pixels with brightness from Lmax - 0.3(Lmax - Lmin) to Lmax is defined as the white region. By calculating the ratio of the number of pixels in the white region to the total number of pixels in the image, the martensite fraction is measured. For a total of 30 observation visual fields of each analysis sample, such image analysis is performed to obtain the martensite fraction, and the average value thereof is taken as the martensite fraction of each analysis sample. Further, the difference between the maximum value and the minimum value of the martensite fraction in 10 or more analysis samples is defined as the variation in the martensite fraction in the cross section of the structural member B1-10 at the position of the steel plate B1-21 having the minimum plate thickness t min and is defined as the variation in the martensite fraction in the cross section of the structural member B1-10 at the position of the steel plate B1-21 having the minimum plate thickness t min If there are a plurality of steel plates having the minimum plate thickness t in the structural member B1-10, such analysis is performed for each steel plate to obtain the martensite fraction, and the maximum variation in the martensite fraction among these steel plates is taken as the variation in the martensite fraction in the structural member B1-10.

[0102] Note that depending on the steel plate, the area ratio of martensite obtained by image analysis, that is, the area ratio of the white region, may contain a few percent of the area ratio of retained austenite. However, since the variation in the martensite fraction is calculated as a difference, its influence is minor.

[0103] After the forming process (hot stamping), the steel sheet B1-21 can have a tensile strength of, for example, 0.5 GPa or more, preferably 1.0 GPa or more. Similarly, after the forming process (hot stamping), the steel sheets B1-22 and B1-23 (Fig. 5) can have a tensile strength of, for example, 0.5 GPa or more, preferably 1.0 GPa or more. At least one of the steel sheets B1-21, B1-22, and B1-23 may have a tensile strength of 1.5 GPa or more after the forming process. The tensile strength of each of the steel sheets B1-21, B1-22, and B1-23 may be the same as or different from the tensile strength of other steel sheets.

[0104] [Effect] In the blank B1-20 according to the present embodiment, the coefficient A of the steel sheet B1-21 having the minimum plate thickness t min is larger than the coefficient A of the thicker steel sheet B1-22 1 . The coefficient A 2 , A 1 is a value obtained by calculating the formula (1): A = 1.48 × (2.7 × C + 0.4 × Si + Mn + 0.45 × Ni + 0.8 × Cr + 2 × Mo) based on the chemical composition of each of the steel sheets B1-21 and B1-22 2 . The coefficient A 3.42 , A 1 corresponds to the time (transformation start time) until the steel sheets B1-21 and B1-22 each start diffusion transformation after the heating of the blank B1-20 for hot stamping is completed from the heating furnace B1-30. However, the coefficient A 2 , A 1 is a value corresponding to the transformation start time when considering only the influence of elements without considering the influence of the plate thickness for each of the steel sheets B1-21 and B1-22. When the coefficient A 2 is larger than the coefficient A 1 , it means that the steel sheet B1-21 is composed of a material with higher hardenability compared to the steel sheet B1-22, in other words, a material with a slower start of diffusion transformation during cooling. By setting the coefficient A 2 > coefficient A 1 as in the present embodiment, the minimum plate thickness t 2 minIt is possible to delay the start of diffusional transformation in the steel sheet B1-21 having it, and the coefficient A 1 is the coefficient A 2 When it is below, the difference in the transformation start time between the steel sheets B1-21 and B1-22 can be reduced. Therefore, when hot stamping the blank B1-20, not only the relatively thick steel sheet B1-22 but also the steel sheet B1-21 with the minimum plate thickness t min can be quenched well, and the hardness of the structural member formed from the blank B1-20 is likely to be made uniform. Further, since the stress is canceled by transformation plasticity due to good hardening of the steel sheet B1-21 and the residual stress becomes small, in the annular structural member B1-10, the generation of torsion due to the concentration of the residual stress can be suppressed. As a result, the deterioration of the dimensional accuracy of the structural member B1-10 can be reduced.

[0105] Thus, in the present embodiment, although the annular blank B1-20 includes the steel sheet B1-21 thinner than the steel sheet B1-22, the hardness of the structural member B1-10 formed by hot stamping from the blank B1-20 can be made uniform, and the deterioration of the dimensional accuracy can be suppressed. Therefore, the impact absorption performance (crashworthiness) of the annular structural member B1-10, particularly the large-sized and annular structural member B1-10, can be improved.

[0106] In the present embodiment, since the hardenability of the relatively thin steel sheet B1-21 is improved, the hardness of the structural member B1-10 formed from the blank B1-20 can be made uniform. More specifically, the minimum plate thickness t minSince the steel sheet B1-21 having [it] can be well hardened, the variation in the martensite fraction in the steel sheet B1-21 can be made 20% or less. As a result, for example, when a collision load is input to the structural member B1-10, deformation concentration is less likely to occur, and the structural member B1-10 is more likely to exhibit high impact absorption performance. Therefore, even when forming the annular structural member B1-10 including the steel sheet B1-21 with a small thickness, particularly the large-sized and annular structural member B1-10 from the blank B1-20, the strength defect of the structural member B1-10 can be reduced, and the impact absorption performance of the structural member B1-10 can be improved.

[0107] The smaller the variation in the martensite fraction, the less the non-uniformity of the mechanical properties within the structural member B1-10, which is preferable from the viewpoint of the function of the structural member B1-10. On the other hand, when the variation in the martensite fraction is large, it indicates that the quenching insufficient parts, that is, the low hardness parts, are unevenly distributed within the structural member B1-10. When the structural member B1-10 undergoes a collision deformation, deformation is likely to concentrate on the low hardness parts, so the function of the structural member B1-10 deteriorates.

[0108] <Second Embodiment> FIG. 8 is a cross-sectional view of the blank B1-20A according to the second embodiment. In FIG. 8, the steel sheet B1-21 having the minimum plate thickness t min and the joint of the steel sheet B1-22 having a larger plate thickness t 2 are shown. The blank B1-20A according to the present embodiment has substantially the same configuration as the blank B1-20 according to the first embodiment, but is different from the blank B1-20 according to the first embodiment in that a film B1-24 is provided on the steel sheet B1-21. In the example of FIG. 8, one surface of the steel sheet B1-21 having the minimum plate thickness t min is covered with the film B1-24.

[0109] The film B1-24 is a substantially black film. For example, when the lightness L* value (CIE 1976 lightness index L* defined in JIS Z8781-4 (2013)) from the surface of the film B1-24 is 60 or less, it can be determined that the film B1-24 is black. The film B1-24 may be a carbon-based surface treatment film (a film containing carbon (C)).

[0110] The film B1-24 can contain, for example, carbon black. The film B1-24 can further contain a metal oxide. The metal oxide is, for example, one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide. The film B1-24 may contain silica.

[0111] As the film B1-24, for example, the surface treatment film described in International Publication No. 2022 / 215229 can be used. That is, the film B1-24 can contain graphite or soot instead of or in addition to carbon black. Alternatively, the film B1-24 can contain, for example, an acicular compound having a hexagonal crystal structure with an aspect ratio of 4 or more and 50 or less. The compound having a hexagonal crystal structure is typically graphite (C), but may also be lanthanum silicate, magnesium diboride, beryllium oxide (beryllia), zinc oxide, β-quartz, acicular nickel ore (NiS), wurtzite (ZnS), etc.

[0112] The blank B1-20A according to this embodiment is formed into an annular structural member B1-10 (Figs. 5 and 6) by the same manufacturing method as in the first embodiment. In the blank B1-20A, the emissivity of the surface of the steel sheet B1-21 is increased by the substantially black film B1-24 provided on the surface of the steel sheet B1-21. The surface of the steel sheet B1-21 coated with the film B1-24 has an emissivity of 60% or more, for example, at a measurement temperature of 25°C and a wavelength of 8.0 μm. Thus, the minimum plate thickness t minBy increasing the emissivity of the steel sheet B1-21, when heating the blank B1-20A during hot stamping, the temperature rise of the steel sheet B1-21 can be accelerated. Therefore, the steel sheet B1-21 can be quickly heated to the temperature in the austenite region, and the high-temperature holding time of the steel sheet B1-21 can be ensured to be long. As a result, the austenite crystal grains in the microstructure of the steel sheet B1-21 become coarser, and the diffusion transformation of the steel sheet B1-21 after the heating process is completed can be delayed more. Therefore, quenching can be better performed on the steel sheet B1-21. At least a part of the coating B1-24 can remain on the surface of the steel sheet B1-21 after hot stamping.

[0113] In this embodiment, one surface of the steel sheet B1-21 is covered by the coating B1-24. However, both surfaces of the steel sheet B1-21 may be substantially covered by the black coating B1-24. The coating B1-24 may or may not be provided on one or both surfaces of the other steel sheets B1-22 and B1-23. However, from the viewpoint of ensuring a longer high-temperature holding time of the steel sheet B1-21 from the start to the completion of heating of the blank B1-20A, it is preferable that the coating B1-24 is not provided on at least one of the other steel sheets B1-22 and B1-23.

[0114] <Third Embodiment> FIG. 9 is a cross-sectional view of the blank B1-20B according to the third embodiment. In FIG. 9, the minimum plate thickness t min and the joint between the steel sheet B1-21 having the steel sheet B1-22 having a larger plate thickness t 2 are shown. The blank B1-20B according to this embodiment has substantially the same configuration as the blank B1-20 according to the first embodiment, but is different from the blank B1-20 according to the first embodiment in that the steel sheets B1-21 and B1-22 are plated steel sheets.

[0115] In the example of FIG. 9, the steel plate B1-21 has a base metal steel plate B1-21a and an aluminum-based plating layer B1-21b. The aluminum-based plating layer B1-21b covers both surfaces of the base metal steel plate B1-21a. The aluminum-based plating layer B1-21b is provided over the entire or substantially the entire surface of both surfaces of the base metal steel plate B1-21a. Similarly, the steel plate B1-22 has a base metal steel plate B1-22a and an aluminum-based plating layer B1-22b. The aluminum-based plating layer B1-22b covers both surfaces of the base metal steel plate B1-22a. The aluminum-based plating layer B1-22b is provided over the entire or substantially the entire surface of both surfaces of the base metal steel plate B1-22a. In the present embodiment, the plate thickness t of the steel plate B1-21 min is the plate thickness including the base metal steel plate B1-21a and the aluminum-based plating layer B1-21b. Also, the plate thickness t of the steel plate B1-22 2 is the plate thickness including the base metal steel plate B1-22a and the aluminum-based plating layer B1-22b.

[0116] The chemical compositions of the aluminum-based plating layers B1-21b and B1-22b are not particularly limited. As the aluminum-based plating layers B1-21b and B1-22b, known aluminum-based plating layers (plating layers mainly composed of aluminum) can be adopted. Although not particularly limited, the aluminum-based plating layers B1-21b and B1-22b are, for example, Al-Si-based plating layers. The chemical compositions of the aluminum-based plating layers B1-21b and B1-22b may be the same or different.

[0117] When the deposition amount of the aluminum-based plating layer B121b in the steel plate B1-21 is W1 (g / m 2 ) and the deposition amount of the aluminum-based plating layer B1-22b in the steel plate B1-22 is W2 (g / m 2 ), the deposition amounts W1 and W2 are each 20 g / m 2 or more and 120 g / m 2The following may be applicable. The deposition amount W1 of the aluminum-based plating layer B1-21b is the average deposition amount on both surfaces of the base steel plate B1-21a. The deposition amount W2 of the aluminum-based plating layer B1-22b is the average deposition amount on both surfaces of the base steel plate B1-22a.

[0118] The deposition amounts W1 and W2 are each preferably 30 g / m 2 or more, more preferably 35 g / m 2 or more. The deposition amounts W1 and W2 are each preferably 115 g / m 2 or less, more preferably 100 g / m 2 or less. However, the deposition amount W1 of the aluminum-based plating layer B1-21b in the steel plate B1-21 having the minimum plate thickness t min is less than the deposition amount W2 of the aluminum-based plating layer B1-22b in the steel plate B1-22 having a larger plate thickness t 2 . The difference between the deposition amounts W1 and W2: W2 - W1 is, for example, 10 (g / m 2 ) or more. W2 - W1 is preferably 20 (g / m 2 ) or more, more preferably 30 (g / m 2 ) or more. W2 - W1 may be 80 (g / m 2 ) or less. W2 - W1 is preferably 70 (g / m 2 ) or less, more preferably 60 (g / m 2 ) or less. The deposition amounts W1 and W2 satisfy the relationship W2 / W1 > 1.0. The deposition amounts W1 and W2 can preferably satisfy the relationship W2 / W1 ≥ 1.2, more preferably W2 / W1 ≥ 1.5.

[0119] When the steel plates B1-21 and B1-22 are plated steel plates, the coefficients A 1 , A 2 are calculated using the chemical compositions of the base steel plates B1-21a and B1-22a. That is, the coefficient A 1 of the steel plate B1-21 is calculated by substituting the content (mass%) of each element in the chemical composition of the base steel plate B1-21a into the above formula (1). Similarly, the coefficient A 2is calculated by substituting the content (mass%) of each element in the chemical composition of the base steel plate B1-22a into Equation (1). Also in this embodiment, similar to the first embodiment, the coefficient A of the steel plate B1-21 1 is the coefficient A of the steel plate B1-22 2 is larger. The coefficient A of the steel plate B1-22 2 is the minimum coefficient A among the coefficients A calculated by Equation (1) for each of the plurality of steel plates included in the blank B1-20B.

[0120] The blank B1-20B according to this embodiment is formed into an annular structural member B1-10 (FIGS. 5 and 6) by the same manufacturing method as in the first embodiment. In the blank B1-20B according to this embodiment, the minimum plate thickness t min The deposition amount W1 of the aluminum-based plating layer B1-21b of the steel plate B1-21 having is less than the deposition amount W2 of the aluminum-based plating layer B1-22b of the steel plate B1-22 having a larger plate thickness t 2 . As a result, when the blank B1-20B is heated during hot stamping, the heating rate of the steel plate B1-21 becomes significantly higher compared to the steel plate B1-22. Specifically, since the aluminum-based plating layer B1-21b on the surface layer of the steel plate B1-21 is relatively thin, when the blank B1-20B is heated, the alloying of the aluminum-based plating layer B1-21b and the iron contained in the base steel plate B1-21a rapidly proceeds to the surface of the steel plate B1-21, and both surfaces of the steel plate B1-21 change to black or a color close thereto. That is, the emissivity of both surfaces of the steel plate B1-21 increases during the heating process. Therefore, the steel plate B1-21 can be rapidly heated to the temperature in the austenite region, and a long holding time at a high temperature of the steel plate B1-21 can be ensured. As a result, the austenite crystal grains in the microstructure of the steel plate B1-21 become coarser, and the diffusion transformation of the steel plate B1-21 after completion of the heating process can be delayed more. Therefore, quenching can be performed better with the steel plate B1-21.

[0121] In the structural member B1-10 (Figs. 5 and 6) formed from the blank B1-20B, when the average thickness (plating thickness) of the aluminum-based plating layer B1-21b on both surfaces of the steel plate B1-21 is K1 (μm) and the average thickness (plating thickness) of the aluminum-based plating layer B1-21b on both surfaces of the steel plate B1-22 is K2 (μm), the plating thickness K1 of the steel plate B1-21 is smaller than the plating thickness K2 of the steel plate B1-22. The difference in plating thicknesses K1 and K2: K2 - K1 is, for example, 7 (μm) or more. K2 - K1 may be 33 (μm) or less. Also, the plating thicknesses K1 and K2 can satisfy the relationship K2 / K1 > 1.0. K2 / K1 is preferably 1.2 or more, more preferably 1.5 or more.

[0122] In the present embodiment, the steel plate B1-23 (Figs. 7C and 7D) may be a plated steel plate having a base steel plate and a plating layer, similar to the steel plates B1-21 and B1-22, or may be a steel plate (bare material) having no plating layer on its surface. When the steel plate B1-23 is a plated steel plate, the plating layer may be an aluminum-based plating layer or a metal plating layer other than aluminum. When the steel plate B1-23 is a plated steel plate, the adhesion amount and thickness of the plating layer to the base steel plate are not particularly limited. When the steel plate B1-23 is a plated steel plate, the coefficient A of the steel plate B1-23 3 is calculated by substituting the content (mass%) of each element in the chemical composition of the base steel plate into the above formula (1). Even when the steel plates B1-21, B1-22, and B1-23 are plated steel plates, the chemical compositions of the steel plates B1-21, B1-22, and B1-23 can be measured by the general analysis method described in the first embodiment. The analysis of the chemical compositions of the steel plates B1-21, B1-22, and B1-23 may be performed after removing the plating layer on their surfaces by mechanical grinding.

[0123] The configuration of the blank B1-20B according to this embodiment can also be combined with the blanks B1-20 and B1-20A according to the first and second embodiments respectively. That is, in each of the blanks B1-20 and B1-20A, the steel plate B1-21 is a plated steel plate having a base steel plate B1-21a and an aluminum-based plating layer B1-21b, and the steel plate B1-22 is a plated steel plate having a base steel plate B1-22a and an aluminum-based plating layer B1-22b. The adhesion amount W1 of the aluminum-based plating layer B1-21b in the steel plate B1-21 may be less than the adhesion amount W2 of the aluminum-based plating layer B1-22b in the steel plate B1-22.

[0124] However, in the first and second embodiments, the adhesion amount W1 of the aluminum-based plating layer B1-21b in the steel plate B1-21 may be equal to or more than the adhesion amount W2 of the aluminum-based plating layer B1-22b in the steel plate B1-22. Also, in the first and second embodiments, the plating layers of the steel plates B1-21 and B1-22 may be metal plating layers other than aluminum, or the steel plates B1-21 and B1-22 may be steel plates (bare materials) having no plating layer on their surfaces.

[0125] As described above, the embodiments according to the present disclosure have been described. However, the present disclosure is not limited to the above embodiments, and various modifications are possible without departing from the spirit thereof.

[0126] In the above embodiments, the blanks B1-20, B1-20A, and B1-20B each include three steel plates B1-21, B1-22, and B1-23. However, the number of steel plates included in the blanks B1-20, B1-20A, and B1-20B is not limited to this. The blanks B1-20, B1-20A, and B1-20B may be composed of two steel plates B1-21 and B1-22, or may include four or more steel plates. The blanks B1-20, B1-20A, and B1-20B should at least include a steel plate B1-21 having a minimum plate thickness t min and a steel plate B1-22 having a plate thickness t min greater than the plate thickness t 2 .

[0127] In plan view, the annular blanks B1-20, B1-20A, and B1-20B can typically include three or more steel plates. The steel plate B1-21 is joined directly or indirectly to the steel plate B1-22. In the blanks B1-20, B1-20A, and B1-20B, the arrangement of the plurality of steel plates including the steel plates B1-21 and B1-22 is not particularly limited. In the blanks B1-20, B1-20A, and B1-20B, the coefficient A of the steel plate B1-21 1 is the coefficient A calculated by the formula (1) for the other steel plates B1-2i i (i = 2, 3, ···) and is larger than the coefficient A of the steel plate B1-22 which is the smallest among them. When there are a plurality of steel plates B1-21 having the minimum plate thickness t 2 in any of the blanks B1-20, B1-20A, and B1-20B, it is preferable that the coefficient A of all the steel plates B1-21 min is larger than the coefficient A of the steel plate B1-22. When the blanks B1-20, B1-20A, and B1-20B include three or more steel plates, the coefficient A of the steel plates other than the steel plates B1-21 and B1-22 is equal to or greater than the coefficient A of the steel plate B1-22 1 2 . 2

[0128] ​​In the above-described embodiment, the steel plate B1-21 is butt-joined to each of the steel plates B1-22 and B1-23. The steel plate B1-22 is butt-joined to the steel plate B1-23. However, the steel plate B1-21 may be lap-joined to at least one of the steel plates B1-22 and B1-23. That is, as shown in FIG. 10A, the steel plates B1-21 and B1-22 may form an overlap portion B1-25 by joining the end portion of the steel plate B1-21 to the end portion of the steel plate B1-22 in a state where the end portion of the steel plate B1-21 is overlapped with the end portion of the steel plate B1-22 by, for example, spot welding or laser welding. Similarly, as shown in FIG. 10B, the steel plates B1-21 and B1-23 may form an overlap portion B1-25 by joining the end portion of the steel plate B1-21 to the end portion of the steel plate B1-23 in a state where the end portion of the steel plate B1-21 is overlapped with the end portion of the steel plate B1-23 by, for example, spot welding or laser welding. Further, as shown in FIG. 10C, the steel plates B1-22 and B1-23 may form an overlap portion B1-25 by joining the end portion of the steel plate B1-23 to the end portion of the steel plate B1-22 in a state where the end portion of the steel plate B1-22 is overlapped with the end portion of the steel plate B1-23 by, for example, spot welding or laser welding. In the blanks B1-20, B1-20A, and B1-20B, the joining method of adjacent steel plates may be butt-joining or lap-joining.

[0129] In each of the blanks B1-20, B1-20A, and B1-20B according to the above-described embodiment, the plurality of steel plates arranged so as to have an annular shape in plan view may each be a single layer or a multi-layer. That is, each of the plurality of steel plates may be a single steel plate or a plate material formed by overlapping a plurality of steel plates.

[0130] In the above-described embodiment, the mold B1-40 used for hot stamping the blanks B1-20, B1-20A, and B1-20B includes a punch B1-41 and a die B1-42. However, the configuration of the mold B1-40 is not limited to the example described in the above-described embodiment. The mold B1-40 may further include, for example, a pad or a blank holder.

[0131] In the above embodiment, the main body B1-11 of the structural member B1-10 includes a front pillar B1-111, a center pillar B1-112, and a rocker B1-113. However, the member main body B1-11 can further include other components. For example, as shown in FIG. 11, the member main body B1-11 can further include a rear pillar B1-114. The structural member B1-10 according to the above embodiment is a door ring part (single door ring part) having a single ring shape. On the other hand, the structural member shown in FIG. 11 is a door ring part (double door ring part) having a double ring shape. When manufacturing a double door ring part, the blank used as the material also has a double ring shape.

[0132] Hereinafter, the present disclosure will be described in more detail by way of examples. However, the present disclosure is not limited to the following examples.

[0133] [First Embodiment] To confirm the effects of the present disclosure, for the press forming (hot stamping) of a structural member that is a single door ring part, while changing the steel plate included in the structural member and the splitting pattern of the structural member, CAE analysis was performed using commercially available software (AUTOFORM R.10, manufactured by AUTOFORM).

[0134] [Table 5]

[0135] Table 5 shows the types of steel plates (materials) used in this analysis. In Table 5, for each material, the content (mass%) of each element in the base material, the type of plating, and the coefficient A calculated by the above-described formula (1) are shown.

[0136] The division patterns of the structural members are shown in FIGS. 12A to 12G. FIGS. 12A to 12G show the number of steel plates (materials) included in the structural members, which are single-door ring parts, and the positions of the joints between the steel plates in the structural members. In FIGS. 12A to 12G, numbers are given to each steel plate in parentheses.

[0137]

Table 6

[0138] Referring to Table 6, in Example 1, for the material (3) having the minimum plate thickness t min : 1.2 mm among the materials (1) to (3), the value of the coefficient A is the largest. In Example 1, the coefficient A of the thinnest material (3) 1 is significantly larger than the minimum coefficient A 2 in the other materials (1) and (2). Also in Example 2, for the material (1) having the minimum plate thickness t min : 1.2 mm among the materials (1) to (3), the coefficient A 1 of the material (1) is significantly larger than the minimum coefficient A 2 in the other materials (2) and (3). In Example 3, for the material (2) having the minimum plate thickness t min : 1.2 mm among the materials (1) to (3), the value of the coefficient A is the largest. Also in Example 3, the coefficient A of the thinnest material (2) 1 is significantly larger than the minimum coefficient A 2 in the other materials (1) and (3). In contrast, in Comparative Examples 1 and 2, the coefficient A 1 of the thinnest material is equal to or less than the coefficient A of the other materials.

[0139] The coefficient A corresponds to the transformation start time for each material when only considering the influence of the elements as described above. However, the actual transformation start time of each material is also affected by the plate thickness, and it becomes shorter as the plate thickness decreases. The "phase transformation start time" in Table 6 is the shortest time (the time until the thinnest material starts the phase transformation) from when the blank is heated at a furnace temperature of 920°C for 5 minutes and 30 seconds until the phase transformation to ferrite starts after being taken out of the heating furnace. As shown in Table 6, in Examples 1 to 3, the phase transformation start time was longer compared to Comparative Examples 1 and 2. For example, comparing Example 3 and Comparative Example 2 where the conditions are the same except for the material of the thinnest material (2), it can be seen that in Example 3, the phase transformation start time is slower than that in Comparative Example 2. In Example 3, the coefficient A of the thinnest material (2) 1 is larger than the smallest coefficient A among the other materials (1) and (3) 2 , and the hardenability of the thinnest material is higher than that of the other materials. On the other hand, in Comparative Example 2, the coefficient A of the thinnest material (2) 1 is less than or equal to the smallest coefficient A among the other materials (1) and (3) 2 , and the hardenability of the thinnest material is equal to or lower than that of the other materials. In Example 3, by making the hardenability of the thinnest material higher than that of the other materials, the phase transformation start time of the thinnest material is extended compared to Comparative Example 2, and the phase transformation start time of the thinnest material and the phase transformation start time of the other relatively thick materials are made uniform. Therefore, in Example 3, after the heating of the blank is completed, it becomes easier to start forming before the phase transformation to ferrite starts in the thinnest material, and it becomes easier to uniformly harden the structural member.

[0140] Regarding the division patterns 3 and 4 shown in FIGS. 12C and 12D, the analysis conditions and results are shown in Table 7. In FIGS. 12C and 12D, the structural member is formed by four materials (1) to (4).

[0141]

Table 7

[0142] Referring to Table 7, in Examples 4 to 11, the coefficient A of the material with the smallest plate thickness t among the materials (1) to (4) min is the smallest coefficient A among the other relatively thick materials 1 and is larger than that. In contrast, in Comparative Examples 3 to 7, the coefficient A of the material with the smallest plate thickness t among the materials (1) to (4) 2 is the smallest coefficient A among the other relatively thick materials min and is equal to or less than that. In each Example and each Comparative Example, when there are multiple materials having the smallest plate thickness t 1 the minimum value among the coefficients A of the materials having the smallest plate thickness t is taken as the coefficient A 2 and compared with the coefficients A of the other materials min min 1 2 1 2 1 2

[0143] Comparing each Example with the corresponding Comparative Example, it can be seen that the start time of the phase transformation is extended. For example, comparing Example 6 and Comparative Example 5 where the conditions are the same except for the material of the thinnest material (4), in Example 6, the start time of the phase transformation is later than that in Comparative Example 5. In Example 6, the coefficient A of the thinnest material (4) 1 is larger than the smallest coefficient A among the other materials (1) to (3) 2 and the hardenability of the thinnest material is higher than that of one or more of the other materials. On the other hand, in Comparative Example 5, the coefficient A of the thinnest material (4) 1 is equal to or less than the smallest coefficient A among the other materials (1) to (3) 2 and the hardenability of the thinnest material is equal to or less than that of the other materials. In Example 6, by making the hardenability of the thinnest material higher than that of the other materials, the start time of the phase transformation of the thinnest material is extended compared to Comparative Example 5, and the start time of the phase transformation of the thinnest material and the start time of the phase transformation of the other relatively thick materials are made uniform. Therefore, in Example 6, after the heating of the blank is completed, it becomes easier to start forming before the phase transformation to ferrite starts in the thinnest material, and it becomes easier for the structure member to be uniformly hardened.

[0144] For the division patterns 5 to 7 shown in FIGS. 12E to 12G, Table 8 shows the analysis conditions and results. In FIGS. 12E to 12G, the structural member is formed by five materials (1) to (5).

[0145]

Table 8

[0146] Referring to Table 8, in Examples 12 to 17, among the materials (1) to (5), the coefficient A of the material with the minimum plate thickness t min is larger than the coefficient A of the other relatively thick materials 1 and is the smallest coefficient A among the other relatively thick materials 2 On the other hand, in Comparative Examples 8 to 11, among the materials (1) to (5), the coefficient A of the material with the minimum plate thickness t min is less than or equal to the smallest coefficient A among the other relatively thick materials 1 and is the smallest coefficient A among the other relatively thick materials 2 In each example and each comparative example, when there are multiple materials having the minimum plate thickness t min , the minimum value among the coefficients A of the materials having the minimum plate thickness t min is taken as the coefficient A 1 , and compared with the coefficient A of the other materials 2 .

[0147] Comparing each example with the corresponding comparative example, it can be seen that the start time of the phase transformation is prolonged. For example, comparing Example 12 and Comparative Example 8, where the conditions are the same except for the materials of the thinnest materials (1) and (4), in Example 12, the start time of the phase transformation is later than that in Comparative Example 8. In Example 12, the coefficient A 1 of the thinnest materials (1) and (4) is larger than the smallest coefficient A 2 among the other materials (2), (3), and (5), and the hardenability of the thinnest materials is higher than that of one or more of the other materials. In Example 12, the coefficient A 1 of the thinnest materials (1) and (4) is larger than all the coefficients A of the other materials (2), (3), and (5). On the other hand, in Comparative Example 8, the coefficient A 1is the smallest coefficient A among the other materials (2), (3), and (5) 2 is as follows, and the hardenability of the thinnest material is equal to or lower than that of the other materials. In Example 12, by making the hardenability of the thinnest material higher than that of the other materials, the phase transformation start time of the thinnest material was extended compared to Comparative Example 8, and the phase transformation start time of the thinnest material and the phase transformation start time of the other relatively thick materials were made uniform. Therefore, in Example 12, after the heating of the blank was completed, it became easier to start forming before the phase transformation to ferrite started in the thinnest material, and it became easier for the structural member to be uniformly hardened.

[0148] Also, for example, when comparing Example 13 and Comparative Example 9 where the conditions are the same except for the materials of the thinnest materials (1), (3), and (5), in Example 13, the phase transformation start time is slower than that of Comparative Example 9. In Example 13, the smallest coefficient A 1 is the smallest coefficient A among the other materials (2) and (4) 2 is larger than, and the hardenability of the thinnest material is higher than that of one or more of the other materials. In Example 13, all the coefficient As of the thinnest materials (1), (3), and (5) are larger than the coefficient As of the other materials (2) and (4). On the other hand, in Comparative Example 9, the smallest coefficient A 1 is the smallest coefficient A among the other materials (2) and (4) 2 is as follows, and the hardenability of the thinnest material is equal to or lower than that of the other materials. In Example 13, by making the hardenability of the thinnest material higher than that of the other materials, the phase transformation start time of the thinnest material was extended compared to Comparative Example 9, and the phase transformation start time of the thinnest material and the phase transformation start time of the other relatively thick materials were made uniform. Therefore, in Example 13, after the heating of the blank was completed, it became easier to start forming before the phase transformation to ferrite started in the thinnest material, and it became easier for the structural member to be uniformly hardened.

[0149] In each of the examples and comparative examples shown in Tables 6 to 8, the materials are joined by laser joining (butt joint) after butting against each other. On the other hand, in each of the examples and comparative examples shown in Table 9 below, some of the materials are joined by, for example, spot welding to form an overlap portion.

[0150]

Table 9

[0151] Referring to Table 9 and FIG. 12F, in Example 18 and Comparative Example 12, the materials (2) and (5), the materials (3) and (4), and the materials (4) and (5) form an overlap portion at their respective joints. Referring to Table 9 and FIG. 12E, in Example 19 and Comparative Example 13, the materials (2) and (5), and the materials (3) and (4) form an overlap portion at their respective joints.

[0152] As shown in Table 9, in Examples 18 and 19, the coefficient A min of the material with the smallest plate thickness t 1 among the materials (1) to (5) is larger than the smallest coefficient A 2 among the other relatively thick materials. In contrast, in Comparative Examples 12 and 13, the coefficient A min of the material with the smallest plate thickness t 1 among the materials (1) to (5) is less than or equal to the smallest coefficient A 2 among the other relatively thick materials.

[0153] From Table 9, in Examples 18 and 19, the phase transformation start time is slower than that in Comparative Examples 12 and 13. Therefore, even when there is an overlap portion in the blank, by making the coefficient A 1 of the thinnest material larger than the smallest coefficient A 2 among the other materials, it was confirmed that the phase transformation start times of the respective materials in the blank can be made uniform.

[0154] [Second Embodiment] Regarding the press forming (hot stamping) of the structural member that is a double door ring part, an analysis similar to that in the first embodiment was carried out while changing the material and plate thickness of the material contained in the structural member, as well as the division pattern of the structural member.

[0155] The steel plates used as materials were selected from those shown in Table 5, as in the first embodiment. The division pattern of the structural members is as shown in Figs. 13A to 13D. Figs. 13A to 13D show the number of steel plates (materials) included in the structural members that are double door ring parts, and the positions of the joints between the steel plates in the structural members. In Figs. 13A to 13D, each steel plate used as material is given a number in parentheses.

[0156] Regarding division patterns 8 and 9 shown in Figures 13A and 13B, the analysis conditions and results are shown in Table 10. In Figures 13A and 13B, the structural member is formed from six pieces of material (1) to (6).

[0157] [Table 10]

[0158] Referring to Table 10, in Examples 20 and 21, the minimum plate thickness t min Coefficient A of 1 The coefficient A is the smallest among other materials with relatively thick walls. 2 On the other hand, in Comparative Examples 14 and 15, the minimum plate thickness t min Coefficient A of 1 The coefficient A is the smallest among other materials with relatively thick walls. 2 The details are as follows.

[0159] From Table 10, A 1 -A 2 In Examples 20 and 21, A 1 -A 2It can be seen that the phase transformation start time is extended compared to Comparative Examples 14 and 15 where it is ≤0. Therefore, by making the hardenability of the thinnest material higher than that of one or more other materials, it becomes easier to start blank forming before the phase transformation to ferrite starts in the thinnest material, and it can be said that it is possible to uniformly harden the structural member.

[0160] Regarding the division patterns 10 and 11 shown in FIGS. 13C and 13D, Table 11 shows the analysis conditions and results. In FIGS. 13C and 13D, the structural member is formed by seven materials (1) to (7).

[0161] [Table 11]

[0162] Referring to Table 11, in Examples 22 to 24, among the materials (1) to (7), the minimum coefficient A min in the material having the minimum plate thickness t 1 is larger than the minimum coefficient A 2 in the other relatively thick materials. In contrast, in Comparative Examples 16 and 17, among the materials (1) to (7), the minimum coefficient A min in the material having the minimum plate thickness t 1 is less than or equal to the minimum coefficient A 2 in the other relatively thick materials.

[0163] From Table 11, in Examples 22 to 24 where A 1 -A 2 >0, it can be seen that the phase transformation start time is extended compared to Comparative Examples 16 and 17 where A 1 -A 2 ≤0. Therefore, by making the hardenability of the thinnest material higher than that of one or more other materials, it becomes easier to start blank forming before the phase transformation to ferrite starts in the thinnest material, and it can be said that it is possible to uniformly harden the structural member.

[0164] [Third Embodiment] The blank was heated until the entire blank reached 910°C. After heating at a furnace temperature of 920°C, it was transferred to a press device in 17 seconds, and hot stamping was performed at a forming speed of 40 mm / s. It was held at the bottom dead center for 20 seconds while being pressurized at 3000 kN to obtain a hot stamp structural member. From the thinnest part of these structural members, analysis samples were collected by the method described in the above embodiment, and the variation in the martensite fraction was measured. Separately, shape accuracy measurement and impact absorption performance measurement were performed on these structural members. The results of each evaluation are shown in Table 12.

[0165]

Table 12

[0166] Examples 14 and 16 in Table 12 are examples under the same conditions as Examples 14 and 16 shown in Table 8. Example 15A has the same combination of split pattern and material as Example 15 shown in Table 8, but some materials form an overlap portion at the joint. Comparative Example 10A has the same combination of split pattern and material as Comparative Example 10 shown in Table 8, but some materials form an overlap portion at the joint. In Example 15A and Comparative Example 10A, materials (1) and (2), materials (1) and (3), materials (2) and (5), and materials (4) and (5) form overlap portions at their respective joints. Comparative Example 11 is a comparative example under the same conditions as Comparative Example 11 shown in Table 8.

[0167] In Table 12, the variation in the martensite fraction is, as described in the above embodiment, the value obtained by subtracting the minimum martensite fraction (%) from the maximum martensite fraction (%) in the cross-section of the structural member at the position of the material having the minimum plate thickness t min .

[0168] Regarding the shape accuracy, when a substantially hat-shaped structural member in cross-section was attached to another member, it was evaluated by how much the structural member was separated from the mating member at the overlapping portion between the structural member and the mating member. In Table 12, when the distance from the surface of the mating member was within ±2.0 mm, it was indicated as ○, when it was more than ±2.0 mm and within ±3.0 mm, it was indicated as △, and when it was more than ±3.0 mm, it was indicated as ×.

[0169] Regarding the shock absorption performance, for each structural member (door ring outer part) of the examples and comparative examples shown in Table 12, the door ring inner part was joined to the door ring outer part by spot welding to produce an assembled part of the door ring, and this was used as a test body for partial structure evaluation. Then, the periphery of each test body was fixed with a restraining jig for reproducing the deformation during vehicle body collision, and a barrier was collided from the side of the door ring outer part (the side of the vehicle body), and the maximum intrusion amount at this time was evaluated as the shock absorption performance. The shock absorption performance was evaluated by comparing it with the shock absorption performance of the door ring outer part formed by joining each material after hot stamping. In Table 12, the shock absorption performance equivalent to the base shock absorption performance was indicated as good, the shock absorption performance superior to the base shock absorption performance was indicated as better, the shock absorption performance slightly lower than the base shock absorption performance was indicated as marginal, and the shock absorption performance even lower was indicated as poor. The comprehensive evaluation of the shape accuracy and the shock absorption performance is also shown in four levels of poor, marginal, good, and better.

[0170] Each example shown in Table 12 is A 1 -A 2 >0 is satisfied, while in each comparative example, A 1 -A 2 ≦0. In Comparative Examples 10A and 11, the variation in the martensite fraction exceeded 20%, while in Examples 14, 15A, and 16, the variation in the martensite fraction was 20% or less. In Examples 14, 15A, and 16, the variation in the martensite fraction was reduced to 15% or less. In Examples 14, 15A, and 16, the shape accuracy was also good compared to Comparative Examples 10A and 11.

[0171] In Examples 14, 15A, and 16 where the variation in the martensite fraction was small, the impact absorption performance was also improved as compared with Comparative Examples 10A and 11. In particular, in Examples 14 and 15A where the variation in the martensite fraction was 10% or less, impact absorption performance equal to or better than that of the base could be ensured. That is, even though a plurality of materials were integrated at the blank stage to form an annular structural member, impact absorption performance equal to or better than that of a structural member joined after individually press-forming the materials could be ensured. Although the present invention has been described so far with respect to an annular automobile door ring, the present invention can be applied to a rear module or the like, and in that case, dimensional accuracy can be improved and impact absorption performance can be ensured.

[0172] (Element technology C1) The element technology C1 is a structural member for a vehicle body, (C1a) a pair of side frames, a cross member connecting the side frames, and is provided with the side frames and the cross member include a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness larger than that of the first steel plate, and are formed of a plurality of steel plates joined to each other, on the first steel plate, a film containing 0.001 g / m or more of at least one oxide selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide is provided 2 (C1b) a pair of side frames, a cross member connecting the side frames, and is provided with the side frames and the cross member include a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness larger than that of the first steel plate, and are formed of a plurality of steel plates joined to each other, on the first steel plate, a film containing 0.500 g / m or less of carbon black is provided, 2 ​​(C1c) A pair of side frames, a cross member connecting the side frames, and are provided with, the side frames and the cross member include a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than the plate thickness of the first steel plate, and are formed by a plurality of steel plates joined to each other, the first steel plate and the second steel plate are each a plated steel plate having an aluminum-based plating layer on both surfaces of the base steel plate, the thickness of the aluminum-based plating layer in the first steel plate is smaller than the thickness of the aluminum-based plating layer in the second steel plate, is a structural member that satisfies at least one of.

[0173] According to the element technology C1, a structural member excellent in impact absorption performance can be provided.

[0174] The blank for hot stamping according to the embodiment includes a plurality of steel plates. The plurality of steel plates are arranged and joined so as to form two long portions and a connecting portion. The long portions are arranged side by side in the lateral direction in a plan view of the blank. The connecting portion connects the long portions. The plurality of steel plates include a first steel plate and a second steel plate. The first steel plate has the minimum plate thickness among the plurality of steel plates. The second steel plate has a plate thickness greater than the plate thickness of the first steel plate. At least one of the both surfaces of the first steel plate is subjected to a treatment for increasing the emissivity as compared with the both surfaces of the second steel plate (first configuration).

[0175] The blank according to the first configuration includes a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness larger than that of the first steel plate. At least one surface of the first steel plate is treated to increase the emissivity as compared with both surfaces of the second steel plate. Thereby, when the blank is heated during hot stamping, the heating rate of the first steel plate, which is the thin-walled part, can be increased. Therefore, the first steel plate can be heated to the temperature in the austenite region more quickly, and the holding time of the first steel plate at that temperature can be ensured to be long. Thus, the austenite crystal grains in the first steel plate can be coarsened. As a result, since the ferrite transformation region (ferrite nose) in the CCT diagram shifts to the long-time side, after the heating of the blank is completed, the start of the transformation of the first steel plate into ferrite can be delayed, and the forming of the blank can be started while maintaining the microstructure of the first steel plate in the austenite phase. That is, the hardenability of the first steel plate with a small plate thickness can be improved. Note that the phrase "is treated to increase the emissivity" includes not only the case where the emissivity of at least one surface of the first steel plate is higher than the emissivities of both surfaces of the second steel plate before the heating of the blank, but also the case where the emissivity of at least one surface of the first steel plate becomes higher than the emissivities of both surfaces of the second steel plate during the heating of the blank.

[0176] In the blank according to the first configuration, since the hardenability of the first steel plate with a small plate thickness is improved, the first steel plate can be satisfactorily quenched when forming a structural member from the blank by hot stamping. Therefore, the hardness of the structural member is likely to be uniformized, and a partial decrease in the strength of the structural member can be suppressed. Further, since non-uniform stress is less likely to occur in the structural member, even if the structural member is large, torsion, warpage, etc. are less likely to occur, and good dimensional accuracy can be ensured in the structural member. Therefore, when forming a structural member, particularly a large-sized structural member, including the first steel plate having a smaller plate thickness than the second steel plate from a blank, poor strength and poor dimensional accuracy of the structural member can be reduced, and the impact absorption performance (crashworthiness) of the structural member can be improved.

[0177] In the blank according to the first configuration, a treatment for increasing the emissivity is applied to the first steel plate with the minimum plate thickness as compared to the second steel plate with a relatively large plate thickness. In this case, when the blank is heated during hot stamping, the first steel plate heats up faster than the second steel plate. Compared with the case where the first steel plate has the same emissivity as the second steel plate, the high-temperature holding time of the first steel plate, that is, the time from when the first steel plate reaches the temperature in the austenite region until the second steel plate and the entire blank reach the temperature in the austenite region becomes longer. As a result, after the heating of the blank is completed, the non-uniformity of the phase transformation due to the difference in the cooling rate between the steel plates is reduced. Specifically, the start of the phase transformation from austenite to ferrite of the first steel plate with the minimum plate thickness can be delayed, and the difference in the phase transformation start time between the first steel plate with the minimum plate thickness and the other steel plates becomes smaller. As a result, the hardenability can be made uniform between the first steel plate with the minimum plate thickness and the other steel plates.

[0178] In the blank according to the first configuration, the first steel plate may have a plate thickness of less than 1.4 mm (second configuration).

[0179] When the plate thickness of the first steel plate is less than 1.4 mm as in the second configuration, the first steel plate is particularly likely to be heat-extracted after the heating of the blank is completed, and the hardenability of the first steel plate is more likely to deteriorate. However, even when the plate thickness of the first steel plate is less than 1.4 mm, by applying a treatment for increasing the emissivity to at least one surface of the first steel plate as compared to the relatively thick second steel plate, when the blank is heated during hot stamping, the temperature rise of the first steel plate can be promoted and the high-temperature holding time of the first steel plate can be ensured to be long. Therefore, the hardenability of the first steel plate can be improved.

[0180] In the blank according to the first or second configuration, the first steel plate may be a plated steel plate. The plated steel plate can have a base steel plate and an aluminum-based plating layer provided on the base steel plate (third configuration).

[0181] When the first steel plate is a plated steel plate having an aluminum-based plating layer as in the third configuration, when the blank is heated during hot stamping, the heating rate of the first steel plate tends to be low. Since the aluminum-based plating layer is close to white, it easily reflects thermal energy and inhibits the temperature rise of the first steel plate. However, even when the first steel plate is a plated steel plate having an aluminum-based plating layer, by performing a treatment for increasing the emissivity on at least one surface of the first steel plate as compared with the relatively thick second steel plate, when the blank is heated during hot stamping, the temperature rise of the first steel plate can be promoted. Therefore, a long high-temperature holding time of the first steel plate can be ensured, and the hardenability of the first steel plate can be improved.

[0182] In the blank according to any one of the first to third configurations, a film may be formed on at least one surface of the first steel plate as a treatment for increasing the emissivity. This film may have an emissivity of 60% or more at a wavelength of 8.0 μm at 25 °C (fourth configuration).

[0183] In the blank according to any one of the first to third configurations, a film may be formed on at least one surface of the first steel plate as a treatment for increasing the emissivity. The film can contain carbon black, one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide, and 0 to 0.30 g / m 2 of silica. In this case, when the content of carbon black in the film is X CB (g / m 2 ) and the content of the oxide is X Oxide (g / m 2 ), X CB and X Oxide may satisfy the following formula (1) (see Patent Document 1) (fifth configuration). 118.9 ≦ 24280 / {6700 / (100 + 76×X CB ) + 18000 / (130 + 65×X Oxide )} ≦ 332.0 (1)

[0184] In the blank according to any one of the first to fifth configurations, the first steel plate and the second steel plate may each be a plated steel plate. The plated steel plate can have a base steel plate and an aluminum-based plating layer covering both surfaces of the base steel plate. As a treatment for increasing the emissivity, the adhesion amount (g / m 2 ) of the aluminum-based plating layer to both surfaces of the base steel plate in the first steel plate may be less than the adhesion amount (g / m 2 ) of the aluminum-based plating layer to both surfaces of the base steel plate in the second steel plate (sixth configuration).

[0185] In the sixth configuration, both the first steel plate and the second steel plate are plated steel plates having an aluminum-based plating layer. However, the adhesion amount of the aluminum-based plating layer to both surfaces of the base steel plate is less in the first steel plate than in the second steel plate. As a result, when the blank is heated during hot stamping, alloying of the aluminum-based plating layer and iron progresses to the surface with the thinner first steel plate prior to the relatively thicker second steel plate, and both surfaces of the first steel plate change from silver-white to black or a color close thereto. Therefore, during heating of the blank, the emissivity of both surfaces of the first steel plate becomes higher than the emissivity of both surfaces of the second steel plate. Accordingly, the temperature of the first steel plate can be raised to the temperature in the austenite region more quickly, and the holding time of the first steel plate at that temperature can be ensured to be longer. Thus, the austenite crystal grains in the first steel plate can be coarsened. As a result, since the ferrite transformation region (ferrite nose) in the CCT diagram shifts to the longer time side, the start of transformation to ferrite in the first steel plate can be delayed after the heating of the blank is completed. Therefore, the hardenability of the first steel plate can be improved.

[0186] In the blank according to any one of the first to sixth configurations, the first steel plate may be a plated steel plate having a base steel plate and a plating layer provided on the base steel plate. In this case, when the plate thickness of the first steel plate is t min and the plate thickness of the steel plate having the maximum plate thickness among the plurality of steel plates is t max , 1.0 < t max / t minIt is preferably ≤3.2 (seventh configuration).

[0187] Among the plurality of steel plates included in the blank, the minimum plate thickness t min of the first steel plate and the maximum plate thickness t max of the other steel plate are large, it becomes difficult to secure the process window in the manufacture of the structural member. For example, when the minimum plate thickness t min and the maximum plate thickness t max are large, when the blank is heated during hot stamping, while waiting for the steel plate having the maximum plate thickness t max to reach the temperature in the austenite region, the alloying of the plating layer of the first steel plate that has been previously heated to the temperature in the austenite region progresses, and the diffusion layer becomes thick, and the corrosion resistance or weldability of the first steel plate by the plating layer may not be ensured. Therefore, in the seventh configuration, the ratio of the maximum plate thickness t min to the minimum plate thickness t max is set to 3.2 or less. Thereby, since the heating rate of the steel plate having the maximum plate thickness t max and the heating rate of the first steel plate having the minimum plate thickness t min do not deviate too much, the heating of the other steel plate can be completed before the alloying of the plating layer of the first steel plate progresses excessively. Therefore, the structural member can be manufactured while maintaining the corrosion resistance or weldability of the first steel plate, and the process window in the manufacture of the structural member can be secured.

[0188] In the blank according to any one of the first to seventh configurations, the first steel plate may be a plated steel plate having a base steel plate and a plating layer provided on the base steel plate. The blank can further include an overlap portion. The overlap portion is formed by overlapping the ends of two adjacent steel plates among the plurality of steel plates, which are steel plates other than the second steel plate. The overlap portion can have a total plate thickness of 4.0 mm or less. On the surfaces located outside the overlap portion in each of the two steel plates, a treatment for increasing the emissivity may be performed as compared with both surfaces of the second steel plate (eighth configuration).

[0189] When there is an overlap portion formed by overlapping the ends of two steel plates in a blank, it may not be possible to secure a process window in the manufacture of a structural member. Specifically, when heating the blank during hot stamping, while waiting for the overlap portion to reach the temperature in the austenite region, the alloying of the plating layer of the first steel plate having the minimum plate thickness progresses, and the diffusion layer becomes thick, and the corrosion resistance or weldability of the first steel plate by the plating layer may not be ensured. Therefore, in the eighth configuration, in each of the two steel plates forming the overlap portion, a treatment for increasing the emissivity is performed on the outer surface of the overlap portion. As a result, the temperature rise of the overlap portion can be promoted, so that the heating of the overlap portion can be completed before the alloying of the plating layer of the first steel plate progresses excessively, and the structural member can be manufactured while maintaining the corrosion resistance or weldability of the first steel plate. That is, it becomes easier to secure a process window in the manufacture of the structural member. However, even when the emissivity of the overlap portion is increased, if the total plate thickness of the overlap portion becomes excessive, it becomes difficult to secure a process window. Therefore, the total plate thickness of the overlap portion is preferably 4.0 mm or less.

[0190] In the blank according to the eighth configuration, the second steel plate and the two steel plates may each be a plated steel plate having a base steel plate and an aluminum-based plating layer covering both surfaces of the base steel plate. In this case, the adhesion amount (g / m 2 ) of the aluminum-based plating layer to both surfaces of the base steel plate in each of the two steel plates may be less than the adhesion amount (g / m 2 ) of the aluminum-based plating layer to both surfaces of the base steel plate in the second steel plate (ninth configuration).

[0191] In the ninth configuration, two steel plates forming an overlap portion and the second steel plate are aluminum-plated steel plates. The amount of aluminum-based plating layer adhered to both surfaces of the base steel plate of each steel plate forming the overlap portion is less than that of the second steel plate. As a result, when the blank is heated during hot stamping, alloying of the aluminum-based plating layer and iron progresses relatively quickly to the surface in the overlap portion, and both surfaces of the overlap portion change from silver-white to black or a color close thereto. That is, during heating of the blank, the emissivity of both surfaces of the overlap portion increases. As a result, the temperature rise of the overlap portion can be promoted, so that the process window in the manufacture of the structural member is easily secured.

[0192] The method for manufacturing a structural member according to the embodiment includes a step of preparing a blank according to any one of the first to ninth configurations, a step of heating a plurality of steel plates included in the blank to a temperature equal to or higher than the austenite transformation completion temperature, and a step of forming and quenching the heated blank using a mold (tenth configuration).

[0193] The structural member for a vehicle body according to the embodiment includes a pair of side frames and a cross member. The cross member connects the side frames. The side frames and the cross member are formed of a plurality of steel plates joined to each other. The plurality of steel plates includes a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness larger than the plate thickness of the first steel plate. A film is provided on the first steel plate. The film contains 0.001 g / m or more of one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide. (Eleventh configuration). 2 or more (eleventh configuration).

[0194] The structural member for a vehicle body according to another embodiment includes a pair of side frames and a cross member. The cross member connects the side frames. The side frames and the cross member are formed of a plurality of steel plates joined to each other. The plurality of steel plates includes a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness larger than that of the first steel plate. A film is provided on the first steel plate. The film contains carbon black at 0.500 g / m 2 (12th configuration).

[0195] The structural member for a vehicle body according to still another embodiment includes a pair of side frames and a cross member. The cross member connects the side frames. The side frames and the cross member are formed of a plurality of steel plates joined to each other. The plurality of steel plates includes a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness larger than that of the first steel plate. The first steel plate and the second steel plate are plated steel plates each having an aluminum-based plating layer on both surfaces of a base steel plate. The thickness of the aluminum-based plating layer in the first steel plate is smaller than the thickness of the aluminum-based plating layer in the second steel plate (13th configuration).

[0196] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The same or corresponding components in each figure are denoted by the same reference numerals, and the same description will not be repeated.

[0197] <First Embodiment> [Structural Member] FIG. 14 is an exploded perspective view of structural members C1-10 and C1-20 according to this embodiment. The structural members C1-10 and C1-20 are used for a vehicle body such as an automobile. In the example shown in FIG. 14, the structural members C1-10 and C1-20 constitute a front under module of the vehicle body.

[0198] The structural member C1-10 is an upper module. That is, in a state where the structural member C1-10 is assembled to the vehicle body, it is disposed above the structural member C1-20. The structural member C1-10 includes a pair of side frames C1-11L and C1-11R, and at least one cross member C1-12. The side frames C1-11L and C1-11R and the cross member C1-12 each have an elongated shape.

[0199] The side frames C1-11L and C1-11R are arranged side by side in the left-right direction of the vehicle body in a state where the structural member C1-10 is assembled to the vehicle body. The side frames C1-11L and C1-11R each extend in the front-rear direction of the vehicle body. Each of the side frames C1-11L and C1-11R includes a front portion C1-111 and a rear portion C1-112. The rear portion C1-112 is disposed behind the front portion C1-111 in a state where the structural member C1-10 is assembled to the vehicle body.

[0200] The cross member C1-12 extends in the left-right direction of the vehicle body in a state where the structural member C1-10 is assembled to the vehicle body. The cross member C1-12 extends from the side frame C1-11L to the side frame C1-11R. The cross member C1-12 connects the side frames C1-11L and C1-11R. In the example shown in FIG. 14, the cross member C1-12 connects the side frames C1-11L and C1-11R at one end side in the longitudinal direction of the side frames C1-11L and C1-11R. The cross member C1-12 is disposed, for example, at the rear end portion of the structural member C1-10 in a state where the structural member C1-10 is assembled to the vehicle body. However, the cross member C1-12 may connect the intermediate portions of the side frames C1-11L and C1-11R.

[0201] The structural member C1-20 is a lower module. That is, the structural member C1-20 is disposed below the structural member C1-10 in a state where it is assembled to the vehicle body. The structural member C1-20 includes a pair of side frames C1-21L and C1-21R, and at least one cross member C1-22. The side frames C1-21L and C1-21R and the cross member C1-22 each have an elongated shape.

[0202] The side frames C1-21L and C1-21R are arranged side by side in the left-right direction of the vehicle body in a state where the structural member C1-20 is assembled to the vehicle body. The side frames C1-21L and C1-21R each extend in the front-rear direction of the vehicle body. Each of the side frames C1-21L and C1-21R includes a front portion C1-211 and a rear portion C1-212. The rear portion C1-212 is disposed behind the front portion C1-211 in a state where the structural member C1-10 is assembled to the vehicle body.

[0203] The lower side frames C1-21L and C1-21R are each joined to the upper side frames C1-11L and C1-11R. The side frames C1-21L and C1-21R together with the side frames C1-11L and C1-11R form a closed cross-section. Fig. 15 shows the closed cross-section formed by the side frames C1-21L and C1-21R together with the side frames C1-11L and C1-11R respectively. Hereinafter, when there is no need to particularly distinguish between the side frames C1-11L and C1-11R, the side frames C1-11L and C1-11R are collectively referred to as the side frame C1-11. Similarly, when there is no need to particularly distinguish between the side frames C1-21L and C1-21R, the side frames C1-21L and C1-21R are collectively referred to as the side frame C1-21.

[0204] Fig. 15 is a cross-sectional view (transverse cross-sectional view) when the side frames C1-11 and C1-21 are cut by a plane perpendicular to the longitudinal direction. In the example of Fig. 15, the side frames C1-11 and C1-21 each have a substantially hat-shaped cross-section.

[0205] Referring to FIG. 15, the side frame C1-11 includes a top plate C1-113, vertical walls C1-114, C1-115, and flanges C1-116, C1-117. In a cross-sectional view of the side frame C1-11, one end of the vertical walls C1-114, C1-115 is connected by the top plate C1-113. In a cross-sectional view of the side frame C1-11, flanges C1-116, C1-117 are respectively connected to the other ends of the vertical walls C1-114, C1-115. The flanges C1-116, C1-117 respectively protrude outward from the vertical walls C1-114, C1-115.

[0206] The side frame C1-21 includes a top plate C1-213, vertical walls C1-214, C1-215, and flanges C1-216, C1-217. In a cross-sectional view of the side frame C1-21, one end of the vertical walls C1-214, C1-215 is connected by the top plate C1-213. In a cross-sectional view of the side frame C1-21, flanges C1-216, C1-217 are respectively connected to the other ends of the vertical walls C1-214, C1-215. The flanges C1-216, C1-217 respectively protrude outward from the vertical walls C1-214, C1-215.

[0207] The top plate C1-213 of the lower side frame C1-21 is arranged to face the top plate C1-113 of the upper side frame C1-11. In a cross-sectional view of the side frames C1-11 and C1-21, the vertical walls C1-214 and C1-215 of the side frame C1-21 extend from the top plate C1-213 toward the side frame C1-11 side. The flanges C1-216 and C1-217 of the side frame C1-21 are joined to the flanges C1-116 and C1-117 of the side frame C1-11, respectively. The flanges C1-216 and C1-217 are joined to the flanges C1-116 and C1-117 by, for example, spot welding or the like. In the example of FIG. 15, the flanges C1-116 and C1-117 of the upper side frame C1-11 are directly joined to the flanges C1-216 and C1-217 of the lower side frame C1-21. However, other members such as a floor panel may be provided between the side frame C1-11 and the side frame C1-21.

[0208] Returning to FIG. 14, the cross member C1-22 extends in the left-right direction of the vehicle body in a state where the structural member C1-20 is assembled to the vehicle body. The cross member C1-22 extends from the side frame C1-21L to the side frame C1-21R. The cross member C1-22 connects the side frames C1-21L and C1-21R. In the example shown in FIG. 14, the cross member C1-22 connects the side frames C1-21L and C1-21R at one end side in the longitudinal direction of the side frames C1-21L and C1-21R. The cross member C1-22 is arranged, for example, at the rear end portion of the structural member C1-20 in a state where the structural member C1-20 is assembled to the vehicle body, similar to the upper cross member C1-12. However, the cross member C1-22 may connect the intermediate portions of the side frames C1-21L and C1-21R. The cross member C1-22 may be joined to the upper cross member C1-12 by, for example, spot welding or the like.

[0209] The structural members C1-10 and C1-20 are hot stamping members. That is, the structural member C1-10 is formed by hot stamping (hot press working) a blank formed by a plurality of steel plates (sub blanks). Similarly, the structural member C1-20 is formed by hot stamping a blank formed by a plurality of steel plates.

[0210] In the upper structural member C1-10, for example, the side frames C1-11L and C1-11R may each be formed of a plurality of steel plates C1-31 and C1-32. In each of the side frames C1-11L and C1-11R, for example, the front portion C1-111 may be formed of the steel plate C1-31, and the rear portion C1-112 may be formed of the steel plate C1-32. The plate thickness of the steel plate C1-32 forming the rear portion C1-112 may be larger than the plate thickness of the steel plate C1-31 forming the front portion C1-111. Also, the tensile strength of the steel plate C1-32 may be larger than the tensile strength of the steel plate C1-31. The cross member C1-12 may mainly be formed of a steel plate C1-33 different from the steel plates C1-31 and C1-32 forming the side frames C1-11L and C1-11R. Among the steel plates C1-31, C1-32, and C1-33, adjacent steel plates are joined by welding.

[0211] Similarly, in the lower structural member C1-20, for example, the side frames C1-21L and C1-21R may each be formed of a plurality of steel plates C1-41 and C1-42. In each of the side frames C1-21L and C1-21R, for example, the front portion C1-211 may be formed of the steel plate C1-41, and the rear portion C1-212 may be formed of the steel plate C1-42. The thickness of the steel plate C1-42 forming the rear portion C1-212 may be greater than the thickness of the steel plate C1-41 forming the front portion C1-211. Also, the tensile strength of the steel plate C1-42 may be greater than the tensile strength of the steel plate C1-41. The cross member C1-22 may mainly be formed of a steel plate C1-43 different from the steel plates C1-41 and C1-42 forming the side frames C1-21L and C1-21R. Among the steel plates C1-41, C1-42, and C1-43, adjacent steel plates are joined by welding.

[0212] [Manufacturing method of structural member] Hereinafter, a method for manufacturing the structural members C1-10 and C1-20 according to the present embodiment will be described with reference to FIGS. 16A to 16G. The manufacturing method of the structural member C1-10 includes a step of preparing a blank C1-30, a step of heating the blank C1-30, and a step of forming the heated blank C1-30 into the structural member C1-10. Similarly, the manufacturing method of the structural member C1-20 includes a step of preparing a blank C1-40, a step of heating the blank C1-40, and a step of forming the heated blank C1-40 into the structural member C1-20.

[0213] (Preparation step) As shown in FIG. 16A, when manufacturing the upper structural member C1-10 (FIG. 14), in the preparation step, a blank C1-30 is prepared. The blank C1-30 has a shape obtained by developing the structural member C1-10. The blank C1-30 includes a plurality of steel plates (sub-blanks) C1-31, C1-32, and C1-33. The steel plates C1-31, C1-32, and C1-33 are arranged and joined so as to form two long portions C1-34L and C1-34R and at least one connecting portion C1-35.

[0214] The long portions C1-34L and C1-34R are arranged side by side in a lateral direction in a plan view of the blank C1-30. The long portion C1-34L is a portion of the blank C1-30 that corresponds to the side frame C1-11L (FIG. 14). The long portion C1-34R is a portion of the blank C1-30 that corresponds to the side frame C1-11R (FIG. 14). In the example of FIG. 16A, the long portions C1-34L and C1-34R are formed by steel plates C1-31 and C1-32, respectively.

[0215] The connecting portion C1-35 connects the long portions C1-34L and C1-34R to each other. The connecting portion C1-35 is a portion of the blank C1-30 that corresponds to the cross member C1-12 (FIG. 14). In the example of FIG. 16A, the connecting portion C1-35 includes the steel plate C1-33. The connecting portion C1-35 can further include a part of the steel plate C1-32.

[0216] FIGS. 16B and 16C are cross-sectional views of the blank C1-30 showing the joints of the steel plates C1-31, C1-32, and C1-33. FIGS. 16B and 16C are the IIIB-IIIB cross-sectional view and the IIIC-IIIC cross-sectional view of FIG. 16A, respectively. Referring to FIG. 16B, the steel plate C1-31 is butt-jointed to the steel plate C1-32. That is, these end faces are joined in a state where the end face of the steel plate C1-31 abuts against the end face of the steel plate C1-32. Referring to FIG. 16C, the steel plate C1-32 is butt-jointed to the steel plate C1-33. That is, these end faces are joined in a state where the other end face of the steel plate C1-32 abuts against the end face of the steel plate C1-33. The steel plates C1-31, C1-32, and C1-33 are joined by, for example, laser welding. In the present embodiment, the blank C1-30 is a so-called tailor welded blank. However, the steel plates C1-31, C1-32, and C1-33 may be joined in a state where the ends of adjacent steel plates overlap each other (overlap joint). In this case, the steel plates C1-31, C1-32, and C1-33 may be joined by spot welding. In particular, the intersection of the cross member C1-12 (FIG. 14) extending in the left-right direction of the vehicle body and the side frames C1-11L and C1-11R (FIG. 14) extending in the front-rear direction of the vehicle body may have an overlap structure as necessary.

[0217] Referring to FIGS. 16B and 16C, the steel plate C1-31 has a plate thickness t 1 The steel plate C1-32 has a plate thickness t 2 The steel plate C1-33 has a plate thickness t 3 In this embodiment, the t 1 of the steel plate C1-31 is the minimum plate thickness t min among the steel plates C1-31, C1-32, and C1-33. The plate thickness t 2 of the steel plate C1-32 is greater than the plate thickness t 1 of the steel plate C1-31. The plate thickness t 3 of the steel plate C1-33 is equal to or greater than the plate thickness t 1 of the steel plate C1-31. In this embodiment, the plate thickness t 2 of the steel plate C1-32 is the maximum plate thickness t max among the steel plates C1-31, C1-32, and C1-33. However, the plate thickness t 3 of the steel plate C1-33 may be the maximum plate thickness t max among the steel plates C1-31, C1-32, and C1-33. That is, the plate thickness t 3 of the steel plate C1-33 may be equal to or greater than the plate thickness t 2 of the steel plate C1-32.

[0218] The plate thickness t min of the steel plate C1-31 is typically less than 1.4 mm. The plate thickness t min may be, for example, 0.8 mm or more. The plate thickness t min and the plate thickness t max preferably satisfy 1.0 < t max / t min ≦ 3.2, and 1.3 ≦ t max / t min More preferably, it satisfies ≦3.2.

[0219] The minimum plate thickness t min On at least one of the two surfaces of the steel plate C1-31 having the minimum plate thickness t, a treatment for increasing the emissivity is applied as compared with the two surfaces of the thicker steel plate C1-32. In the present embodiment, the emissivity of at least one surface of the steel plate C1-31 is higher than the emissivities of the two surfaces of the steel plate C1-32 before the heating step of the blank C1-30.

[0220] For example, the emissivity at a wavelength of 8.0 μm at 25°C is 60% or more on one or both surfaces of the steel plate C1-31 and less than 60% on both surfaces of the steel plate C1-32. The emissivity at a wavelength of 8.0 μm at 25°C on one or both surfaces of the steel plate C1-31 is more preferably 70% or more, and even more preferably 80% or more. The minimum plate thickness t min The difference in emissivity at a wavelength of 8.0 μm at 25°C between the steel plate C1-31 having the minimum plate thickness t and another steel plate C1-32 is preferably greater than 5%, more preferably greater than 10%, and even more preferably greater than 20%. The emissivity can be measured in accordance with JIS R 1801:2002. In this case, a sample collected from the steel plate to be measured is set in a Fourier transform infrared spectrophotometer, and the emissivity is calculated by measuring the radiation intensity at a wavelength of 8.0 μm at 25°C. Alternatively, it is also possible to calculate the emissivity from the ratio of the radiation intensity of the site of interest at 25°C to the radiation intensity of a black body by using a radiation thermometer with the measurement wavelength set to 8.0 μm.

[0221] In this embodiment, a film C1-50 is formed on one surface of the steel sheet C1-31 as a treatment for increasing the emissivity. One surface of the steel sheet C1-31 is, for example, entirely covered by the film C1-50. On the other hand, the film C1-50 is not provided on both surfaces of the steel sheet C1-32. As a result, the emissivity of one surface of the steel sheet C1-31 is higher than that of both surfaces of the steel sheet C1-32. However, the film C1-50 may be provided on both surfaces of the steel sheet C1-31.

[0222] The film C1-50 is, for example, a substantially black film. For example, the lightness L from the surface of the film C1-50 * value (CIE 1976 lightness index L defined in JIS Z8781-4:2013 * ) is 60 or less, the film C1-50 can be determined to be substantially black. The film C1-50 may be a carbon-based surface treatment film (a film containing carbon (C)). The emissivity of the film C1-50 at a wavelength of 8.0 μm at 25°C is 60% or more, preferably 70% or more, more preferably 80% or more. That is, the emissivity of the surface of the steel sheet C1-31 to which the film C1-50 is applied at a wavelength of 8.0 μm at 25°C is 60% or more, preferably 70% or more, more preferably 80% or more. The emissivity of the film C1-50 at a wavelength of 8.0 μm at 700°C may be 60% or more. As the film C1-50, for example, the surface treatment film described in Patent Document 1 can be used. Specifically, the film C1-50 can contain carbon black and one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide. The film C1-50 may or may not contain silica. That is, the silica content of the film C1-50 is 0 g / m 2 or more. The silica content of the film C1-50 may be 0.30 g / m 2 or less. The silica content is more preferably 0.10 g / m 2 or less, and even more preferably 0.05 g / m 2 or less.

[0223] Carbon black and oxides can be dispersed throughout the surface perpendicular to the thickness direction of steel sheet C1-31 in film C1-50. Let the content of carbon black be X CB (g / m 2 ), and let the content of one or more oxides (metal oxides) selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide be X Oxide (g / m 2 ). When this is the case, it is preferable that X CB and X Oxide satisfy the following formula (1). 118.9 ≦ 24280 / {6700 / (100 + 76×X CB ) + 18000 / (130 + 65×X Oxide )} ≦ 332.0 (1)

[0224] In formula (1), the value calculated by the central formula: 24280 / {6700 / (100 + 76×X CB ) + 18000 / (130 + 65×X Oxide )} is preferably 119.0 or more, more preferably 170.0 or more, and even more preferably 220.0 or more. The value calculated by the central formula is preferably 330.0 or less, more preferably 310.0 or less, and even more preferably 300.0 or less.

[0225] The dispersion state of carbon black and metal oxides in film C1-50 can be confirmed by performing surface analysis of film C1-50 with respect to elements derived from carbon black (for example, C) and elements derived from oxides (Zr, Zn, and Ti) using an electron probe micro analyzer (EPMA). The content X of carbon black CBcan be measured by cross-sectional analysis of the film C1-50 using a transmission electron microscope (TEM). That is, cross-sectional analysis of the film C1-50 is performed by TEM-EDS analysis on a region of a predetermined size (film thickness of the film C1-50 × 5 μm), and the film thickness of the film C1-50 and the area ratio occupied by particles having a carbon content of 70% by mass or more in the region are measured. When the density of carbon black is ρ (ton / m 3 ), the film thickness is d (μm), and the area ratio is a (%), the value represented by ρ × d × a is the carbon black content X CB (g / m 2 ). The oxide content X Oxide can be determined by performing elemental analysis from the surface of the film C1-50 using a fluorescent X-ray analyzer (ZSX Primus manufactured by Rigaku Corporation) and quantifying metallic Zr, metallic Zn, and metallic Ti.

[0226] The carbon black content X CB in the film C1-50 is preferably 0.030 g / m 2 or more, and more preferably 0.100 g / m 2 or more. The content X CB may be set within the range that satisfies formula (1), but is preferably 0.800 g / m 2 or less, and more preferably 0.600 g / m 2 or less.

[0227] The film C1-50 can contain 5.0 or more carbon black by volume%, preferably 8.0 or more carbon black by volume%. Further, the film C1-50 can contain 40.0 or less carbon black by volume%, preferably 30.0 or less carbon black by volume%.

[0228] The metal oxide content X Oxide in the film C1-50 is preferably 0.030 g / m 2 or more, and preferably 0.060 g / m 2It is more preferable that it is as described above. Content X Oxide may be set within a range that satisfies formula (1), preferably 0.500 g / m 2 or less, more preferably 0.300 g / m 2 or less.

[0229] Coating C1-50 can contain a metal oxide in a volume percentage of 1.0 or more. Further, coating C1-50 can contain a metal oxide in a volume percentage of 30.0 or less, preferably a metal oxide in a volume percentage of 25.0 or less.

[0230] Content X of carbon black CB (g / m 2 ) and content X of metal oxide Oxide (g / m 2 ) and the ratio thereof: X Oxide / X CB is preferably 0.20 or more and 200.00 or less. X Oxide / X CB is more preferably 0.40 or more and 10.00 or less, and even more preferably 0.60 or more and 5.00 or less.

[0231] In addition to the above carbon black and metal oxide, coating C1-50 can contain various binder components and additives.

[0232] The binder component is preferably a water-dispersible or water-soluble resin. The content of the binder component is preferably 40% by volume or more based on the total volume of the film C1-50. As the binder component selected from water-dispersible or water-soluble resins, various known resins showing water-dispersibility or water-solubility can be used. Examples of such resins showing water-dispersibility or water-solubility include polyurethane resins, polyester resins, acrylic resins, epoxy resins, fluororesins, polyamide resins, polyolefin resins, polymer compounds obtained by hydrolysis and polycondensation of silane coupling agents, and the like. The binder component is more preferably one or more selected from the group consisting of polyester resins, polyurethane resins, polyolefin resins, acrylic resins, epoxy resins, fluororesins, and polyamide resins. When using a polyurethane resin as the binder component, the polyurethane resin is preferably a polyether-based polyurethane resin.

[0233] Additives include, for example, leveling agents, water-soluble solvents, metal stabilizers, etching inhibitors, and the like. The leveling agent is, for example, a nonionic or cationic surfactant. Examples of nonionic or cationic surfactants include polyethylene oxide or polypropylene oxide adducts, acetylene glycol compounds, and the like. Examples of water-soluble solvents include alcohols such as ethanol, isopropyl alcohol, t-butyl alcohol, and propylene glycol; cellosolves such as ethylene glycol monobutyl ether and ethylene glycol monoethyl ether; esters such as ethyl acetate and butyl acetate; and ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone. Examples of metal stabilizers include chelate compounds such as EDTA (ethylenediaminetetraacetic acid) and DTPA (diethylenetriaminepentaacetic acid). Examples of etching inhibitors include amine compounds such as ethylenediamine, triethylenetetramine, guanidine, and pyrimidine.

[0234] The film C1-50 can be formed, for example, by applying an organic or inorganic treatment liquid containing carbon black and metal oxide to the entire surface of the steel sheet C1-31 and then drying the volatile components in the treatment liquid. The treatment liquid can be applied to the surface of the steel sheet C1-31 by, for example, a roll coater, a curtain coater, or an inkjet. In the case of an inkjet, the film thickness of the film C1-50 can also be continuously changed. The film thickness of the film C1-50 is, for example, 0.5 μm or more and 5.0 μm or less. The film thickness of the film C1-50 is preferably 1.0 μm or more and 3.0 μm or less. The film thickness of the film C1-50 is negligibly small compared to the plate thickness t of the steel sheet C1-31. Therefore, the plate thickness measured including the film C1-50 in the steel sheet C1-31 can be treated as the plate thickness t of the steel sheet C1-31. min The steel sheet C1-31 may be a plated steel sheet. In this case, the steel sheet C1-31 has a base steel sheet C1-31a and a plating layer C1-31b. The type of the base steel sheet C1-31a is not particularly limited. The plating layer C1-31b is provided on the base steel sheet C1-31a. The plating layer C1-31b covers the entire or substantially the entire surface of both sides of the base steel sheet C1-31a. The plating layer C1-31b is a metal plating layer. The plating layer C1-31b may be, for example, a molten aluminum plating, a molten zinc plating, an alloyed molten zinc plating, or an electro-galvanized plating. As the steel sheet C1-31, known aluminum-plated steel sheets, zinc-plated steel sheets, etc. can be used. min The plating layer C1-31b is typically a plating layer mainly composed of aluminum (aluminum-based plating layer). The composition of the aluminum-based plating layer is not particularly limited. As the plating layer C1-31b, a known aluminum-based plating layer can be adopted. When the steel sheet C1-31 is a plated steel sheet, the plate thickness t of the steel sheet C1-31 is the combined plate thickness of the base steel sheet C1-31a and the plating layer C1-31b.

[0235] The plating layer C1-31b is typically a plating layer mainly composed of aluminum (aluminum-based plating layer). The composition of the aluminum-based plating layer is not particularly limited. As the plating layer C1-31b, a known aluminum-based plating layer can be adopted. When the steel sheet C1-31 is a plated steel sheet, the plate thickness t of the steel sheet C1-31 is the combined plate thickness of the base steel sheet C1-31a and the plating layer C1-31b.

[0236] The plating layer C1-31b is typically a plating layer mainly composed of aluminum (aluminum-based plating layer). The composition of the aluminum-based plating layer is not particularly limited. As the plating layer C1-31b, a known aluminum-based plating layer can be adopted. When the steel sheet C1-31 is a plated steel sheet, the plate thickness t of the steel sheet C1-31 is the combined plate thickness of the base steel sheet C1-31a and the plating layer C1-31b. min is the combined plate thickness of the base steel sheet C1-31a and the plating layer C1-31b.

[0237] The steel plates C1-32 and C1-33 may be known plated steel plates, similar to the steel plate C1-31. The steel plates C1-32 and C1-33 may be aluminum-plated steel plates or zinc-plated steel plates. The steel plates C1-32 and C1-33 may be plated steel plates of the same type as the steel plate C1-31, or may be plated steel plates of a different type from the steel plate C1-31. Also, the steel plate C1-32 may be a plated steel plate of the same type as the steel plate C1-33, or may be a plated steel plate of a different type from the steel plate C1-33. When the steel plate C1-32 is a plated steel plate, the plate thickness t 2 of the steel plate C1-32 is the combined plate thickness of the base metal steel plate and the plating layer. Similarly, when the steel plate C1-33 is a plated steel plate, the plate thickness t 3 of the steel plate C1-33 is the combined plate thickness of the base metal steel plate and the plating layer. When two or more of the steel plates C1-31, C1-32, and C1-33 are plated steel plates, the plating weight per unit area of each steel plate may be the same as or different from that of the other steel plates. However, in this embodiment, the steel plates C1-31, C1-32, and C1-33 may be steel plates (bare materials) having no plating layer on the surface.

[0238] As shown in FIG. 16D, when manufacturing the lower structural member C1-20 (FIG. 14), in the preparation process, a blank C1-40 is prepared. The blank C1-40 has a shape obtained by developing the structural member C1-20. The blank C1-40 includes a plurality of steel plates (sub-blanks) C1-41, C1-42, and C1-43. The steel plates C1-41, C1-42, and C1-43 are arranged and joined so as to form two long portions C1-44L and C1-44R and at least one connecting portion C1-45.

[0239] The long portions C1-44L and C1-44R are arranged side by side in the lateral direction in a plan view of the blank C1-40. The long portion C1-44L is a portion of the blank C1-40 corresponding to the side frame C1-21L (FIG. 14). The long portion C1-44R is a portion of the blank C1-40 corresponding to the side frame C1-21R (FIG. 14). In the example of FIG. 16D, the long portions C1-44L and C1-44R are formed by the steel plates C1-41 and C1-42, respectively.

[0240] The connecting part C1-45 connects the long parts C1-44L and C1-44R. The connecting part C1-45 is the part corresponding to the cross member C1-22 (Fig. 14) in the blank C1-40. In the example of Fig. 16D, the connecting part C1-45 includes the steel plate C1-43. The connecting part C1-45 can further include a part of the steel plate C1-42.

[0241] The blank C1-40 is configured in the same manner as the blank C1-30 (Figs. 16B and 16C) with respect to the steel plates C1-41, C1-42, and C1-43. That is, the configuration of the steel plates C1-31, C1-32, and C1-33 of the blank C1-30 (Figs. 16B and 16C) can be directly applied to the steel plates C1-41, C1-42, and C1-43. Therefore, a detailed description of the configuration of the steel plates C1-41, C1-42, and C1-43 is omitted.

[0242] (Heating process) The prepared blanks C1-30 and C1-40 are formed into the structural members C1-10 and C1-20 (Fig. 14) respectively by hot stamping. During hot stamping, the blanks C1-30 and C1-40 are subjected to a heating process. Referring to Fig. 16E, in the heating process, for example, the blank C1-30 is heated by a heating furnace. The plurality of steel plates C1-31, C1-32, and C1-33 included in the blank C1-30 are heated to a temperature equal to or higher than the austenite transformation completion temperature (A c3 point). The steel plates C1-31, C1-32, and C1-33 are heated to, for example, 900 °C or higher. As a result, the microstructure of the steel plates C1-31, C1-32, and C1-33 transforms into the austenite phase. Although not shown, the plurality of steel plates C1-41, C1-42, and C1-43 (Fig. 16D) included in the blank C1-40 are also heated to a temperature equal to or higher than the austenite transformation completion temperature (A c3 point) in the heating process.

[0243] (Forming process) Referring to FIG. 16F, in the forming process, using die C1-60, the heated blank C1-30 is formed into the structural member C1-10 (FIG. 14) and quenched. The blank C1-30 heated by the heating process is taken out from the heating furnace and conveyed to the die C1-60. The die C1-60 may be attached to a known press device. The die C1-60 includes, for example, a punch C1-61 and a die C1-62. The blank C1-30 is disposed between the punch C1-61 and the die C1-62.

[0244] Referring to FIG. 16G, after the blank C1-30 is disposed between the punch C1-61 and the die C1-62, the die C1-62 approaches relatively to the punch C1-61. The blank C1-30 is clamped (pressed) by the punch C1-61 and the die C1-62 and formed into a shape along the forming surfaces of the punch C1-61 and the die C1-62. The blank C1-30 is held while being clamped by the punch C1-61 and the die C1-62. The blank C1-30 is heat-extracted (quenched) by the die C1-60, and its microstructure transforms into martensite. Thereby, the structural member C1-10 can be manufactured from the blank C1-30.

[0245] Although not shown, the blank C1-40 shown in FIG. 16D is also subjected to a forming process similar to that of the blank C1-30. That is, using a die, the heated blank C1-40 is formed into the structural member C1-20 (FIG. 14) and quenched. The structural member C1-20 is joined to the structural member C1-10 (FIG. 14) by, for example, welding.

[0246] Figure 17 is a cross-sectional view of the structural member C1-10 after hot stamping. In Figure 17, a cross-section of the structural member C1-10 is shown at the position of the steel plate C1-31 (Figure 16B) where the black film C1-50 was applied at the stage of the blank C1-30. In the example of Figure 17, the structural member C1-10 includes the film C1-13. The film C1-13 is provided on the steel plate C1-31. The black film C1-50 (Figure 16B) applied to the steel plate C1-31 in the blank C1-30 becomes the film C1-13 through hot stamping. The film C1-13 is provided on at least one surface of the steel plate C1-31. When the film C1-50 before hot stamping contains carbon black, this carbon black almost disappears due to high-temperature heating during hot stamping, but it may also remain. When the film C1-50 before hot stamping satisfies the above formula (1), the film C1-13 after hot stamping may not contain carbon black, or 0.500 g / m 2 It may also contain the following carbon black. However, when the film C1-50 before hot stamping satisfies the above formula (1), the film C1-13 after hot stamping is 0.500 g / m 2 It is preferably contained with the following carbon black. When the film C1-13 after hot stamping contains carbon black, the content of carbon black in the film C1-13 is 0 g / m 2 Ultra, and more desirably 0.001 g / m 2 Or more. When the film C1-50 (Figure 16B) was provided on the steel plate C1-31 before hot stamping so that the film C1-13 contains carbon black after hot stamping, carbon black will exist on the steel plate C1-31 even in the later stage of the heating process, and the emissivity of the steel plate C1-31 is ensured. Therefore, even in the later stage of the heating process, the minimum plate thickness t minThe steel sheet C1-31 having [something] is prone to heating. Also, when the steel sheet C1-31 is a plated steel sheet, the film C1-13 after the heating process contains carbon black, so that in the forming process (hot stamping), the adhesion of the plating layer C1-31b (Fig. 16B) to the mold C1-60 is suppressed, and the friction coefficient between the steel sheet C1-31 and the mold C1-60 (Figs. 16F and 16G) can be reduced. When the film C1-50 before hot stamping satisfies the above formula (1), in the film C1-13 after hot stamping, the value calculated by the central formula: 24280 / {6700 / (100 + 76×X CB ) + 18000 / (130 + 65×X Oxide )} is, for example, 120.0 or more and 150.0 or less.

[0247] When the film C1-50 (Fig. 16B) before hot stamping satisfies the above formula (1), the film C1-13 after hot stamping contains, for example, 0 g / m 2 super, more desirably 0.001 g / m 2 or more of one or more oxides (metal oxides) selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide. The content of the metal oxide in the film C1-13 is, for example, 0.500 g / m 2 or less. When the metal oxide remains in the structural member C1-10 in this way, that is, when the film C1-13 contains more than 0 g / m 2 of the metal oxide, it is more desirable because the corrosion resistance of the structural member C1-10 is improved. When the film C1-50 before hot stamping satisfies the above formula (1), the film C1-13 after hot stamping contains 0 to 0.30 g / m 2 of silica.

[0248] The carbon black content, metal oxide content, and silica content in the film C1-13 can be measured in the same manner as the film C1-50 (Figure 16B) at the stage of the blank C1-30. Specifically, the vehicle body parts are disassembled to obtain the structural member C1-10, and an analysis sample is obtained from this structural member C1-10 by, for example, laser cutting or the like. For example, analysis samples are obtained from each of the plurality of steel plates included in the structural member C1-10. The acquisition position of the analysis sample is, for example, the center or the vicinity of the top plate of each steel plate having an open cross-section. The obtained analysis sample is adjusted by polishing the cross-section to outside the heat-affected zone during laser cutting or the like to prepare a film analysis sample. For this sample, surface analysis of the film C1-13 is performed by EPMA for elements derived from carbon black (for example, C) and elements derived from oxides (Zr, Zn, and Ti), whereby the dispersion states of carbon black and metal oxides in the film C1-13 can be confirmed. Since the film C1-13 exists on the front side and / or the back side of the structural member C1-10 depending on the part of the structural member C1-10, the front side and the back side of the analysis sample are analyzed.

[0249] In many cases, an electrodeposition coating film or the like exists on the outermost layer of the structural member C1-10. In that case, the film layer existing under the electrodeposition coating film layer and above the alloyed metal plating layer is analyzed. The carbon black content X in the film C1-13 CB can be measured by cross-sectional analysis of the film C1-50 using TEM. That is, cross-sectional analysis of the film C1-13 is performed by TEM-EDS analysis for a region of a predetermined size (film thickness of the film C1-13 × 5 μm), and the film thickness of the film C1-13 and the area ratio occupied by particles having a carbon content of 70 mass% or more in the region are measured. When the density of carbon black is ρ (ton / m 3 ), the film thickness is d (μm), and the area ratio is a (%), the value represented by ρ × d × a is the carbon black content X CB (g / m 2 ). The oxide content X OxideIt can be determined by performing elemental analysis on the coating layer existing below the electrodeposited coating layer and above the alloyed metal plating layer using the fluorescent X-ray analyzer described above and quantifying metallic Zr, metallic Zn, and metallic Ti.

[0250] Minimum plate thickness t min In the cross-section of the structural member C1-10 at the position of the steel plate C1-31 having the minimum plate thickness t, when the value obtained by subtracting the minimum martensite fraction (%) from the maximum martensite fraction (%) is defined as the variation in the martensite fraction, the variation in the martensite fraction is, for example, 15% or less. The variation in the martensite fraction is preferably 10% or less. The variation in the martensite fraction can be measured as follows. That is, the minimum plate thickness t min In the cross-section of the structural member C1-10 at the position of the steel plate C1-31 having the minimum plate thickness t, after cutting out 10 or more analysis samples (for example, having a size of about 10 mm on the long side) from positions more than 20 mm away from the end and more than 10 mm apart from each other, each is mirror-polished and etched with a repeller reagent so that the plate thickness direction becomes the observation surface. Then, for the region from 1 / 4 of the plate thickness from the steel plate surface (the region from 1 / 8 of the plate thickness from the steel plate surface to 3 / 8 of the plate thickness from the steel plate surface), 30 fields of view with a magnification of 1000 times and a field of view of 2,400 μm 2 or more are photographed with an optical microscope, and image analysis is performed on the obtained tissue photographs.

[0251] As the image analysis method, the maximum brightness value Lmax and the minimum brightness value Lmin of the image are obtained from the image, the portion having pixels with brightness from Lmax - 0.3(Lmax - Lmin) to Lmax is defined as the white region, and the ratio of the number of pixels in the white region to the total number of pixels is calculated to measure the martensite fraction. For a total of 30 observation fields of each analysis sample, such image analysis is performed to obtain the martensite fraction, and the average value is taken as the martensite fraction of each analysis sample. Further, the difference between the maximum value and the minimum value of the martensite fraction in 10 or more analysis samples is the minimum plate thickness t minIt is defined as the variation in the martensite fraction in the cross-section of the structural member C1-10 at the position of the steel plate C1-31 having it. In the structural member C1-10, the minimum plate thickness t min When there are a plurality of steel plates having it, such an analysis is performed for each steel plate to obtain the martensite fraction, and the maximum variation in the martensite fraction among these steel plates is taken as the variation in the martensite fraction in the structural member C1-10.

[0252] Note that depending on the steel plate, the area ratio of martensite obtained by image analysis, that is, the area ratio of the white region, may contain a few percent of the area ratio of retained austenite. However, since the variation in the martensite fraction is calculated by difference, the influence is minor.

[0253] After the forming process (hot stamping), the steel plate C1-31 can have a tensile strength of, for example, 0.5 GPa or more, preferably 1.0 GPa or more. Similarly, after the forming process (hot stamping), the steel plates C1-32, C1-33 (Fig. 14) can have a tensile strength of, for example, 0.5 GPa or more, preferably 1.0 GPa or more. At least one of the steel plates C1-31, C1-32, C1-23 may have a tensile strength of 1.5 GPa or more after the forming process. The tensile strength of each of the steel plates C1-31, C1-32, C1-33 may be the same as or different from the tensile strength of other steel plates.

[0254] Although not shown, the lower structural member C1-20 (Fig. 14) can also include a film C1-13 similar to the upper structural member C1-10 after hot stamping. The film C1-13 is disposed on at least one surface of the steel plate having, for example, the minimum plate thickness t min For the structural member C1-20 as well, the variation in the martensite fraction at the position of the steel plate having the minimum plate thickness t min is, similar to the structural member C1-10, for example, 15% or less, more preferably 10% or less.

[0255] [Effect] In the blank C1-30 according to the present embodiment, the emissivity of one surface of the steel plate C1-31 having the minimum plate thickness t min is greater than the emissivity of both surfaces of the steel plate C1-32 having a larger plate thickness t 2 . That is, the surface of the steel plate C1-31 is treated to increase the emissivity as compared with both surfaces of the steel plate C1-32. As a result, when the blank C1-30 is heated during hot stamping, the temperature increase rate of the steel plate C1-31 becomes significantly higher than that of the steel plate C1-32. Therefore, in the heating process, the temperature of the steel plate C1-31 can be quickly raised to the temperature in the austenite region, and a long holding time at a high temperature of the steel plate C1-31 can be ensured. As a result, the austenite crystal grains in the microstructure of the steel plate C1-31 become coarser, and the ferrite transformation region (ferrite nose) in the CCT diagram shifts to the long time side. Therefore, after the blank C1-30 is taken out from, for example, a heating furnace, it is possible to prevent austenite from transforming into ferrite in the steel plate C1-31 until the forming by the mold C1-60 is started. Therefore, it is possible to start the forming of the blank C1-30 by the mold C1-60 while maintaining the microstructure of the steel plate C1-31 in the austenite phase, and improve the hardenability of the steel plate C1-31 having the minimum plate thickness t min .

[0256] In the present embodiment, since the hardenability of the relatively thin steel plate C1-31 is improved, the hardness of the structural member C1-10 formed from the blank C1-30 can be made uniform. More specifically, the minimum plate thickness t minSince the steel sheet C1-31 having [it] can be quenched well, the variation in the martensite fraction in the steel sheet C1-31 can be made 15% or less. As a result, for example, when a collision load is input to the structural member C1-10, deformation concentration is less likely to occur, and the structural member C1-10 is more likely to exhibit high impact absorption performance. Therefore, even when forming the structural member C1-10 including the steel sheet C1-31 with a small thickness, particularly the relatively large structural member C1-10 used for a vehicle body or the like, from the blank C1-30, the strength defect of the structural member C1-10 can be reduced, and the impact absorption performance of the structural member C1-10 can be improved.

[0257] The smaller the variation in the martensite fraction, the less the non-uniformity of the mechanical properties within the structural member C1-10, which is preferable from the viewpoint of the function of the structural member C1-10. On the other hand, when the variation in the martensite fraction is large, it indicates that the quenching insufficient parts, that is, the insufficient hardness parts, are unevenly distributed within the structural member C1-10. Therefore, during the collision deformation of the structural member C1-10, deformation is likely to concentrate on the insufficient hardness parts, and the function of the structural member C1-10 deteriorates.

[0258] In this embodiment, by improving the hardenability of the relatively thin steel sheet C1-31, non-uniformity of stress is also less likely to occur in the structural member C1-10. Therefore, even when forming a large structural member C1-10 from the blank C1-30, for example, torsion, warping, etc. are less likely to occur in the structural member C1-10. Therefore, even when forming a large structural member C1-10 including the steel sheet C1-31 with a small thickness from the blank C1-30, the dimensional accuracy defect of the structural member C1-10 can be reduced, and the impact absorption performance of the structural member C1-10 can be improved.

[0259] In the blank C1-30 according to this embodiment, the surface of the steel sheet C1-31 with the minimum plate thickness t min is substantially covered with the black film C1-50, while the plate thickness t 2The steel sheet C1-32 is not provided with the film C1-50. As a result, the emissivity of the surface of the steel sheet C1-31 is larger in advance than the emissivities of both surfaces of the steel sheet C1-32. In this case, when the blank C1-30 is heated during hot stamping, since the steel sheet C1-31 heats up faster than the steel sheet C1-32, the high-temperature holding time of the steel sheet C1-31 becomes longer compared to the case where the steel sheet C1-31 has the same emissivity as the steel sheet C1-32. Therefore, after the heating of the blank C1-30 is completed, it is possible to reduce the non-uniformity of the phase transformation due to the difference in the cooling rate among the steel sheets C1-31, C1-32, and C1-33. Specifically, the minimum plate thickness t min For the steel sheet C1-31, the start of the phase transformation from austenite to ferrite can be delayed, so the difference in the start time of the phase transformation between the steel sheet C1-31 and the other steel sheets C1-32 and C1-33 becomes smaller. As a result, it is possible to equalize the hardenability of the steel sheets C1-31, C1-32, and C1-33 included in the blank C1-30.

[0260] For example, when the plating layer C1-31b of the steel sheet C1-31 is an aluminum-based plating layer, the heating rate of the steel sheet C1-31 tends to be slow in the heating process. Since the aluminum-based plating layer is white, it easily reflects thermal energy and inhibits the heating of the steel sheet C1-31. However, in the blank C1-30 according to the present embodiment, a process for increasing the emissivity is performed on the surface of the steel sheet C1-31. Therefore, even if the steel sheet C1-31 is a plated steel sheet having an aluminum-based plating layer, it is possible to promote the heating of the steel sheet C1-31 in the heating process and ensure a long high-temperature holding time of the steel sheet C1-31. Therefore, it is possible to ensure the hardenability in the thin steel sheet C1-31.

[0261] In the present embodiment, in the heating process, first, the steel sheet C1-31 having the minimum plate thickness t min reaches the temperature in the austenite region, and then the steel sheets C1-32 and C1-33 reach the temperature in the austenite region in ascending order of the plate thickness. Here, in the steel sheets C1-31, C1-32, and C1-33, the ratio of the minimum plate thickness t min to the maximum plate thickness t max :tmax / t min is preferably 3.2 or less. Thereby, before the alloying of the plating layer C1-31b of the steel sheet C1-31 having the plate thickness t proceeds excessively and the diffusion layer grows to lose corrosion resistance or weldability, the steel sheet C1-33 having the plate thickness t min can be sufficiently heated until the phase transformation to austenite of the steel sheet C1-33 having the plate thickness t is completed. Therefore, a process window can be secured in the manufacture of the structural member C1-10. max

[0262] In the present embodiment, in order to increase the emissivity of the steel sheet C1-31, the coating film C1-50 can be applied to the steel sheet C1-31. The emissivity (temperature 25°C and wavelength 8.0 μm) of the coating film C1-50 is, for example, 60% or more. Thereby, the steel sheet C1-31 can be efficiently radiatively heated, and the temperature increase rate of the steel sheet C1-31 in the heating process is more likely to increase.

[0263] In the present embodiment, the coating film C1-50 can contain carbon black, one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide, and 0 to 0.30 g / m 2 or less of the following silica. The content X CB (g / m 2 ) of carbon black and the content X Oxide (g / m 2 ) of the oxide preferably satisfy the above formula (1). As described in Patent Document 1, the formula (1) defines the relationship between the magnification (%) of the increase in the temperature increase rate (°C / s) and the content X CB of carbon black and the content X Oxide of the oxide. The formula (1) represents that carbon black mainly functions as a heat absorber in the range up to 700°C, and the oxide mainly functions as a heat absorber in the range of 700°C or higher. When the coating film C1-50 satisfies the formula (1), the emissivity at a wavelength of 8.0 μm at 25°C on the surface of the steel sheet C1-31 to which the coating film C1-50 is applied is likely to be 60% or more.

[0264] Carbon black and oxides can be dispersed throughout the surface of the steel sheet C1-31 perpendicular to the thickness direction of the coating C1-50. This makes it easier to uniformize the emissivity of the surface of the steel sheet C1-31. Therefore, in the heating process, the steel sheet C1-31 having the minimum plate thickness t min can be heated quickly and uniformly.

[0265] However, the structure of the coating C1-50 is not limited to this. The coating C1-50 may be a substantially black coating as long as it increases the emissivity of the steel sheet C1-31 compared to the untreated case. For example, the coating C1-50 may contain graphite or soot instead of or in addition to carbon black. Alternatively, the coating C1-50 may contain, for example, an acicular compound having a hexagonal crystal structure with an aspect ratio of 4 or more and 50 or less in order to increase the emissivity of the steel sheet C1-31. Compounds having a hexagonal crystal structure are typically graphite (C), but may also be lanthanum silicate, magnesium diboride, beryllium oxide (beryllia), zinc oxide, β-quartz, acicular nickel ore (NiS), wurtzite (ZnS), etc.

[0266] The lower structural member C1-20 and the blank C1-40 have the same configuration as the upper structural member C1-10 and the blank C1-30. Therefore, the lower structural member C1-20 and the blank C1-40 can achieve the same effects as described above.

[0267] <Second Embodiment> [Blank] Figures 18A and 18B are cross-sectional views of the blank C1-30A according to the second embodiment. Figures 18A and 18B are cross-sectional views of the blank C1-30 according to the first embodiment: corresponding to Figures 16B and 16C. In the first embodiment, in the blank C1-30 for the structural member C1-10, the minimum plate thickness t minThe surface of the steel sheet C1-31 having [a certain property] is treated on at least one surface so that its emissivity becomes higher than that of the surface of another steel sheet C1-32 before the heating process. In the first embodiment, for example, a film C1-50 for increasing the emissivity is formed on one or both surfaces of the steel sheet C1-31 (Fig. 16B). On the other hand, in this embodiment, the surface of the steel sheet C1-31 is treated in the blank C1-30A so that its emissivity becomes higher than that of the surface of the steel sheet C1-32 during the heating process.

[0268] Referring to Figs. 18A and 18B, in this embodiment, the steel sheets C1-31, C1-32, and C1-33 are each a plated steel sheet. More specifically, the steel sheets C1-31, C1-32, and C1-33 are all aluminum-plated steel sheets. The steel sheet C1-31 has a base steel sheet C1-31a and an aluminum-based plating layer C1-31b. The steel sheet C1-32 has a base steel sheet C1-32a and an aluminum-based plating layer C1-32b. The steel sheet C1-33 has a base steel sheet C1-33a and an aluminum-based plating layer C1-33b.

[0269] In the steel sheet C1-31, the aluminum-based plating layer C1-31b covers both surfaces of the base steel sheet C1-31a. The aluminum-based plating layer C1-31b is provided over the entire or substantially the entire surface of both surfaces of the base steel sheet C1-31a. Similarly, in the steel sheet C1-32, the aluminum-based plating layer C1-32b covers both surfaces of the base steel sheet C1-32a. The aluminum-based plating layer C1-32b is provided over the entire or substantially the entire surface of both surfaces of the base steel sheet C1-32a. Also, in the steel sheet C1-33, the aluminum-based plating layer C1-33b covers both surfaces of the base steel sheet C1-33a. The aluminum-based plating layer C1-33b is provided over the entire or substantially the entire surface of both surfaces of the base steel sheet C1-33a.

[0270] The chemical compositions of the aluminum-based plating layers C1-31b, C1-32b, and C1-33b are not particularly limited. As the aluminum-based plating layers C1-31b, C1-32b, and C1-33b, known aluminum-based plating layers (plating layers mainly composed of aluminum) can be adopted. Although not particularly limited, the aluminum-based plating layers C1-31b, C1-32b, and C1-33b are, for example, Al-Si-based plating layers. The aluminum-based plating layers C1-31b, C1-32b, and C1-33b may be the same as or different from the aluminum-based plating layers of the other steel plates C1-31, C1-32, and C1-33, respectively.

[0271] The types of the base steel plates C1-31a, C1-32a, and C1-33a are not particularly limited. The base steel plates C1-31a, C1-32a, and C1-33a may be the same as or different from the other base steel plates, respectively.

[0272] Referring to FIGS. 18A and 18B, similar to the first embodiment, the steel plate C1-31 has the smallest plate thickness t among the steel plates C1-31, C1-32, and C1-33. min The steel plate C1-32 has a plate thickness t greater than the plate thickness t of the steel plate C1-31. min The steel plate C1-33 has a plate thickness t equal to or greater than the plate thickness t of the steel plate C1-31. 2 The plate thickness t of the steel plate C1-31 is the combined plate thickness of the base steel plate C1-31a and the aluminum-based plating layer C1-31b, and is the average plate thickness of the steel plate C1-31. The plate thickness t of the steel plate C1-32 is the combined plate thickness of the base steel plate C1-32a and the aluminum-based plating layer C1-32b, and is the average plate thickness of the steel plate C1-32. The plate thickness t of the steel plate C1-33 is the combined plate thickness of the base steel plate C1-33a and the aluminum-based plating layer C1-33b, and is the average plate thickness of the steel plate C1-33. min The plate thickness t of the steel plate C1-31 is the combined plate thickness of the base steel plate C1-31a and the aluminum-based plating layer C1-31b, and is the average plate thickness of the steel plate C1-31. The plate thickness t of the steel plate C1-32 is the combined plate thickness of the base steel plate C1-32a and the aluminum-based plating layer C1-32b, and is the average plate thickness of the steel plate C1-32. The plate thickness t of the steel plate C1-33 is the combined plate thickness of the base steel plate C1-33a and the aluminum-based plating layer C1-33b, and is the average plate thickness of the steel plate C1-33. 3 The plate thickness t of the steel plate C1-31 is the combined plate thickness of the base steel plate C1-31a and the aluminum-based plating layer C1-31b, and is the average plate thickness of the steel plate C1-31. The plate thickness t of the steel plate C1-32 is the combined plate thickness of the base steel plate C1-32a and the aluminum-based plating layer C1-32b, and is the average plate thickness of the steel plate C1-32. The plate thickness t of the steel plate C1-33 is the combined plate thickness of the base steel plate C1-33a and the aluminum-based plating layer C1-33b, and is the average plate thickness of the steel plate C1-33. min The plate thickness t of the steel plate C1-31 is the combined plate thickness of the base steel plate C1-31a and the aluminum-based plating layer C1-31b, and is the average plate thickness of the steel plate C1-31. The plate thickness t of the steel plate C1-32 is the combined plate thickness of the base steel plate C1-32a and the aluminum-based plating layer C1-32b, and is the average plate thickness of the steel plate C1-32. The plate thickness t of the steel plate C1-33 is the combined plate thickness of the base steel plate C1-33a and the aluminum-based plating layer C1-33b, and is the average plate thickness of the steel plate C1-33. 2 The plate thickness t of the steel plate C1-31 is the combined plate thickness of the base steel plate C1-31a and the aluminum-based plating layer C1-31b, and is the average plate thickness of the steel plate C1-31. The plate thickness t of the steel plate C1-32 is the combined plate thickness of the base steel plate C1-32a and the aluminum-based plating layer C1-32b, and is the average plate thickness of the steel plate C1-32. The plate thickness t of the steel plate C1-33 is the combined plate thickness of the base steel plate C1-33a and the aluminum-based plating layer C1-33b, and is the average plate thickness of the steel plate C1-33. 3 The plate thickness t of the steel plate C1-31 is the combined plate thickness of the base steel plate C1-31a and the aluminum-based plating layer C1-31b, and is the average plate thickness of the steel plate C1-31. The plate thickness t of the steel plate C1-32 is the combined plate thickness of the base steel plate C1-32a and the aluminum-based plating layer C1-32b, and is the average plate thickness of the steel plate C1-32. The plate thickness t of the steel plate C1-33 is the combined plate thickness of the base steel plate C1-33a and the aluminum-based plating layer C1-33b, and is the average plate thickness of the steel plate C1-33.

[0273] Referring to FIG. 18A, the smallest plate thickness t minFor the steel sheet C1-31 having [specific property], the adhesion amount W1 (g / m 2 ) of the aluminum-based plating layer C1-31b on both surfaces of the base metal steel sheet C1-31a, and for the steel sheet C1-32 having a larger plate thickness t 2 , when the adhesion amount W2 (g / m 2 ) of the aluminum-based plating layer C1-32b on both surfaces of the base metal steel sheet C1-32a is considered, the adhesion amount W1 of the aluminum-based plating layer C1-31b in the steel sheet C1-31 is less than the adhesion amount W2 of the aluminum-based plating layer C1-32b in the steel sheet C1-32. The adhesion amount W1 of the aluminum-based plating layer C1-31b in the steel sheet C1-31 is the average adhesion amount on both surfaces of the base metal steel sheet C1-31a. Usually, the adhesion amount (g / m 2 ) of the aluminum-based plating layer C1-31b on one surface of the base metal steel sheet C1-31a is substantially equal to the adhesion amount (g / m 2 ) of the aluminum-based plating layer C1-31b on the other surface of the base metal steel sheet C1-31a. However, for example, due to various conditions during manufacturing, etc., the adhesion amount of the aluminum-based plating layer C1-31b may vary between the front and back surfaces of the base metal steel sheet C1-31a. The adhesion amount of the aluminum-based plating layer C1-31b may be different between one surface and the other surface. Similarly, the adhesion amount W2 of the aluminum-based plating layer C1-32b in the steel sheet C1-32 is the average adhesion amount on both surfaces of the base metal steel sheet C1-32a. The adhesion amount (g / m 2 ) of the aluminum-based plating layer C1-32b on one surface of the base metal steel sheet C1-32a is usually substantially equal to the adhesion amount (g / m 2 ) of the aluminum-based plating layer C1-32b on the other surface of the base metal steel sheet C1-32a. However, for example, due to various conditions during manufacturing, etc., the adhesion amount of the aluminum-based plating layer C1-32b may vary between the front and back surfaces of the base metal steel sheet C1-32a. The adhesion amount of the aluminum-based plating layer C1-32b may be different between one surface and the other surface.

[0274] The deposition amount W1 of the aluminum-based plating layer C1-31b on the steel sheet C1-31 and the deposition amount W2 of the aluminum-based plating layer C1-32b on the steel sheet C1-32 may each be 20 g / m 2 or more and 120 g / m 2 or less. The deposition amounts W1 and W2 are each preferably 30 g / m 2 or more, and more preferably 35 g / m 2 or more. The deposition amounts W1 and W2 are each preferably 115 g / m 2 or less, and more preferably 100 g / m 2 or less. The difference between the deposition amounts W1 and W2: W2 - W1 is, for example, 10 (g / m 2 ) or more. W2 - W1 is preferably 20 (g / m 2 ) or more, and more preferably 30 (g / m 2 ) or more. W2 - W1 may be 80 (g / m 2 ) or less. W2 - W1 is preferably 70 (g / m 2 ) or less, and more preferably 60 (g / m 2 ) or less. Also, the deposition amounts W1 and W2 satisfy the relationship W2 / W1 > 1.0. The deposition amounts W1 and W2 can preferably satisfy the relationship W2 / W1 ≥ 1.2, and more preferably W2 / W1 ≥ 1.5.

[0275] Referring to FIG. 18B, when the deposition amount of the aluminum-based plating layer C1-33b on both surfaces of the base steel sheet C1-33a in the steel sheet C1-33 having a plate thickness t 3 is W3 (g / m 2 ), the deposition amount W3 of the aluminum-based plating layer C1-33b in the steel sheet C1-33 may be less than the deposition amount W2 of the aluminum-based plating layer C1-32b in the steel sheet C1-32. The deposition amount W3 of the aluminum-based plating layer C1-33b is the average deposition amount on both surfaces of the base steel sheet C1-33a. Usually, the deposition amount (g / m 2 ) of the aluminum-based plating layer C1-33b on one surface of the base steel sheet C1-33a is the deposition amount (g / m 2) is substantially equal. However, for example, due to various conditions during manufacturing, etc., the adhesion amount of the aluminum-based plating layer C1-33b may vary between the front and back surfaces of the base steel plate C1-33a. The adhesion amount of the aluminum-based plating layer C1-33b may be different between one surface and the other surface of the base steel plate C1-33a.

[0276] The adhesion amount W3 of the aluminum-based plating layer C1-33b in the steel plate C1-33 is also, similar to the steel plates C1-31 and C1-32, for example, 20 g / m 2 or more and 120 g / m 2 or less may also be acceptable. The adhesion amount W3 is preferably 30 g / m 2 or more, and more preferably 35 g / m 2 or more. The adhesion amount W3 is preferably 115 g / m 2 or less, and more preferably 100 g / m 2 or less. When W2 > W3, the difference between the adhesion amounts W2 and W3: W2 - W3 is, for example, 10 (g / m 2 ) or more. W2 - W3 is preferably 20 (g / m 2 ) or more, and more preferably 30 (g / m 2 ) or more. W2 - W3 may be 80 (g / m 2 ) or less. W2 - W3 is preferably 70 (g / m 2 ) or less, and more preferably 60 (g / m 2 ) or less. The adhesion amount W3 of the aluminum-based plating layer C1-33b in the steel plate C1-33 may be equal to the adhesion amount W1 of the aluminum-based plating layer C1-31b of the steel plate C1-31 having the minimum plate thickness t min , or may be more than the adhesion amount W1.

[0277] The method of forming aluminum-based plating layers C1-31b, C1-32b, and C1-33b on base steel plates C1-31a, C1-32a, and C1-33a respectively is not particularly limited, but for example, it is a common molten plating method. That is, by immersing the base steel plate C1-31a in a molten aluminum plating bath and performing gas wiping with nitrogen, air, etc., an aluminum-plated steel plate C1-31 with the adhesion amount W1 of the aluminum-based plating layer C1-31b adjusted can be obtained. Similarly, by immersing the base steel plate C1-32a in a molten aluminum plating bath and performing gas wiping with nitrogen, air, etc., an aluminum-plated steel plate C1-32 with the adhesion amount W2 of the aluminum-based plating layer C1-32b adjusted can be obtained. Also, by immersing the base steel plate C1-33a in a molten aluminum plating bath and performing gas wiping with nitrogen, air, etc., an aluminum-plated steel plate C1-33 with the adhesion amount W3 of the aluminum-based plating layer C1-33b adjusted can be obtained. When forming an aluminum-based plating layer by the molten plating method, an Al-Fe alloy layer is formed at the interface between the base steel plate and the aluminum-based plating layer due to the elution of Fe during the molten plating process.

[0278] As a method for measuring the adhesion amounts W1, W2, and W3 of the aluminum-based plating layers C1-31b, C1-32b, and C1-33b, for example, the sodium hydroxide-hexamethylenetetramine·hydrochloric acid stripping gravimetric method described in JIS G 3314:2019 can be mentioned. Specifically, in accordance with JIS G 3314:2019, from each of the steel plates C1-31, C1-32, and C1-32 of the blank C1-30A, a predetermined surface area S (mm 2Collect five or more test pieces (e.g., 50 mm × 50 mm), and measure the weight w1 (g) of each test piece. Then, immerse each test piece in an aqueous sodium hydroxide solution, and after confirming that the foaming caused by the dissolution of the plating has subsided, take out each test piece from the aqueous sodium hydroxide solution and wash it with water. Subsequently, immerse each test piece, which remains wet after washing, in an aqueous hydrochloric acid solution added with hexamethylenetetramine until the foaming caused by the dissolution of the plating subsides. Immediately wash and dry the test piece taken out from the hexamethylenetetramine-hydrochloric acid aqueous solution, and measure the weight w2 (g) of the test piece again. The adhesion amount W (g / m 2 ) of the aluminum-based plating layer on each test piece can be obtained by {(w1 - w2) / S} × 10 6 . Take the average value of the adhesion amount W of five or more test pieces collected for each steel plate as the adhesion amount of the aluminum-based plating layer on the steel plate.

[0279] However, when the size of the test pieces collected from each of the steel plates C1-31, C1-32, and C1-32 is small, observe the cross-section of each of the aluminum-based plating layers C1-31b, C1-32b, and C1-33b with an optical microscope (area: 100 μm × 100 μm), and measure the thickness (μm) of the plating layer in the same manner for three fields of view. The adhesion amount can be converted by multiplying the average value of the thickness measured in the three fields of view by 3. At this time, for each of the steel plates C1-31, C1-32, and C1-33, calculate the adhesion amount for each side of the base steel plate, and take the average value (average value of both sides) of the obtained adhesion amounts as the adhesion amount of the aluminum-based plating layer. When an Al-Fe alloy layer exists at the interface between the base steel plate and the aluminum-based plating layer, the thickness of the aluminum-based plating layer is the thickness including the Al-Fe alloy layer. The thickness of the aluminum-based plating layer C1-31b of the thinnest steel plate C1-31 is smaller than the thickness of the aluminum-based plating layer C1-32b of the other steel plate C1-32. In the present embodiment, the thickness of the aluminum-based plating layer C1-33b of the steel plate C1-33 is also smaller than the thickness of the aluminum-based plating layer C1-32b of the steel plate C1-32. The thicknesses of the aluminum-based plating layers C1-31b and C1-33b may be different from each other or the same.

[0280] [Structural member] The blank C1-30A is subjected to the same heating process and forming process as in the first embodiment. As a result, as shown in FIGS. 19A to 19C, a structural member C1-10A similar to that of the first embodiment is manufactured from the blank C1-30A. FIGS. 19A to 19C are cross-sectional views of the structural member C1-10A after the forming process (hot stamping). FIG. 19A shows the minimum plate thickness t min of the cross-section of the structural member C1-10A at the position of the steel plate C1-31. FIG. 19B shows the cross-section of the structural member C1-10A at the position of the steel plate C1-32 having a plate thickness t min larger than the plate thickness t 2 of the steel plate C1-31. FIG. 19C shows the cross-section of the structural member C1-10A at the position of the steel plate C1-33 having a plate thickness t min equal to or greater than the plate thickness t 3 of the steel plate C1-31.

[0281] Referring to FIG. 19A, also in the structural member C1-10A after hot stamping, the steel plate C1-31 is a plated steel plate having aluminum-based plating layers C1-31b on both surfaces of the base steel plate C1-31a. Referring to FIG. 19B, the steel plate C1-32 is a plated steel plate having aluminum-based plating layers C1-32b on both surfaces of the base steel plate C1-32a. Referring to FIG. 19C, the steel plate C1-33 is a plated steel plate having aluminum-based plating layers C1-33b on both surfaces of the base steel plate C1-33a. However, the aluminum-based plating layers C1-31b, C1-32b, C1-33b in the structural member C1-10A are undergoing alloying with iron by the heating process as compared with the state of the blank C1-30A (FIGS. 18A and 18B).

[0282] Referring to FIGS. 19A and 19B, when the average thickness (plating thickness) of the aluminum-based plating layer C1-31b on both surfaces of the steel sheet C1-31 is K1 (μm) and the average thickness (plating thickness) of the aluminum-based plating layer C1-32b on both surfaces of the steel sheet C1-32 is K2 (μm), the plating thickness K1 of the steel sheet C1-31 is smaller than the plating thickness K2 of the steel sheet C1-32. The difference between the plating thicknesses K1 and K2: K2 - K1 is, for example, 7 (μm) or more. K2 - K1 may be 33 (μm) or less. Also, the plating thicknesses K1 and K2 can satisfy the relationship K2 / K1 > 1.0. K2 / K1 is preferably 1.2 or more, more preferably 1.5 or more.

[0283] Referring to FIGS. 19B and 19C, when the average thickness (plating thickness) of the aluminum-based plating layer C1-33b on both surfaces of the steel sheet C1-33 is K3 (μm), in the example of this embodiment, the plating thickness K3 of the steel sheet C1-33 is smaller than the plating thickness K2 of the steel sheet C1-32. The difference between the plating thicknesses K2 and K3: K2 - K3 is, for example, 7 (μm) or more. K2 - K3 may be 33 (μm) or less.

[0284] The thicknesses K1, K2, and K3 of the aluminum-based plating layers C1-31b, C1-32b, and C1-33b in the structural member C1-10A can be measured as follows. That is, the vehicle body parts are disassembled to obtain the structural member C1-10A, and an analysis sample is obtained from this structural member C1-10A by, for example, laser cutting or the like. For example, analysis samples are obtained from each of the plurality of steel sheets included in the structural member C1-10A. The acquisition position of the analysis sample is the center or the vicinity of the top plate of each steel sheet having an open cross section. For the test pieces obtained from each of the plurality of steel sheets, after the cross section of the aluminum-based plating layer is nitrided and etched, it is observed with an optical microscope (area: 100 μm × 100 μm), and the thickness of the plating layer is measured in three fields of view. The average value of the plating layer thicknesses measured in the three fields of view can be taken as the plating thickness. In many cases, an electrodeposition coating film or the like exists on the outermost layer of the structural member C1-10A. In that case, the plating layer existing below the electrodeposition coating film layer and above the base metal steel sheet is observed.

[0285] Also in the structural member C1-10A according to the present embodiment, similar to the first embodiment, the minimum plate thickness t min The variation in the martensite fraction in the cross section at the position of the steel plate C1-31 having is, for example, 15% or less, more preferably 10% or less. The variation in the martensite fraction can be measured by the method described in the first embodiment.

[0286] Although illustration is omitted, the blank C1-40 (FIGS. 14 and 16D) for the structural member C1-20 can adopt the same configuration as the blank C1-30A. That is, the configurations of the steel plates C1-31, C1-32, and C1-33 in the blank C1-30A can be directly applied to the steel plates C1-41, C1-42, and C1-43 (FIG. 16D) of the blank C1-40. In this case, in the structural member C1-20 manufactured from the blank C1-40 through the heating process and the forming process, the steel plates C1-41, C1-42, and C1-43 have the same configuration as the steel plates C1-31, C1-32, and C1-33 of the structural member C1-10A shown in FIGS. 19A to 19C.

[0287] [Effect] In the blank C1-30A according to the present embodiment, as a treatment for increasing the emissivity of the surface of the steel plate C1-31 compared to both surfaces of the steel plate C1-32, the minimum plate thickness t min The adhesion amount W1 of the aluminum-based plating layer C1-31b of the steel plate C1-31 having is larger than the plate thickness t 2It is less than the adhesion amount W2 of the aluminum-based plating layer C1-32b of the steel sheet C1-32. As a result, when the blank C1-30A is heated during hot stamping, the heating rate of the steel sheet C1-31 becomes significantly higher compared to the steel sheet C1-32. Specifically, since the aluminum-based plating layer C1-31b on the surface layer of the steel sheet C1-31 is relatively thin, when the blank C1-30A is heated, the alloying of the aluminum-based plating layer C1-31b and the iron contained in the base steel sheet C1-31a proceeds rapidly to the surface of the steel sheet C1-31, and both surfaces of the steel sheet C1-31 change to black or a color close to it. That is, the emissivity of both surfaces of the steel sheet C1-31 increases during the heating process. Therefore, the steel sheet C1-31 can be heated to the temperature in the austenite region more quickly, and the high-temperature holding time of the steel sheet C1-31 can be ensured for a longer time. As a result, the austenite crystal grains in the microstructure of the steel sheet C1-31 become coarser, and the ferrite transformation region (ferrite nose) in the CCT diagram shifts to the long-time side. Therefore, after being taken out from the heating furnace, until the forming by the mold C1-60 is started, it is possible to prevent the austenite in the steel sheet C1-31 from transforming into ferrite. Therefore, it is possible to start the forming of the blank C1-30A by the mold C1-60 while maintaining the microstructure of the steel sheet C1-31 in the austenite phase, and the minimum plate thickness t min The hardenability of the steel sheet C1-31 having min can be improved.

[0288] In the present embodiment, since the hardenability of the relatively thin steel sheet C1-31 is improved, the hardness of the structural member C1-10A formed from the blank C1-30A can be made uniform. More specifically, also in the present embodiment, the minimum plate thickness t min Since the steel sheet C1-31 having min can be well hardened, the variation in the martensite fraction in the steel sheet C1-31 can be made 15% or less. Therefore, as described in the first embodiment, the strength defect of the structural member C1-10A can be reduced, and the impact absorption performance of the structural member C1-10A can be improved.

[0289] Similar to the first embodiment, by improving the hardenability of the relatively thin steel sheet C1-31, stress non-uniformity is less likely to occur in the structural member C1-10A. As a result, even when forming a large structural member C1-10A from a blank C1-30A including the steel sheet C1-31, dimensional accuracy defects of the structural member C1-10A can be reduced, and the impact absorption performance of the structural member C1-10A can be improved.

[0290] In the blank C1-30A according to the present embodiment, the adhesion amount W1 of the aluminum-based plating layer C1-31b in the steel sheet C1-31 with the minimum plate thickness t min is less than the adhesion amount W2 of the aluminum-based plating layer C1-32b in the steel sheet C1-32 thicker than the steel sheet C1-31. As a result, the temperature of the steel sheet C1-31 can be raised faster than that of the steel sheet C1-32, so the high-temperature holding time of the steel sheet C1-31 is longer than that when the adhesion amount W1 of the aluminum-based plating layer C1-31b in the steel sheet C1-31 is the same as or greater than the adhesion amount W2 of the aluminum-based plating layer C1-32b in the steel sheet C1-32. Therefore, after the heating of the blank C1-30A is completed, non-uniformity of the phase transformation due to the difference in the cooling rate among the steel sheets C1-31, C1-32, and C1-33 can be reduced. Specifically, since the start of the phase transformation from austenite to ferrite of the steel sheet C1-31 with the minimum plate thickness t min can be delayed, the difference in the phase transformation start time between the steel sheet C1-31 and the other steel sheets C1-32 and C1-33 becomes smaller. As a result, the hardenability of the steel sheets C1-31, C1-32, and C1-33 included in the blank C1-30 can be made uniform.

[0291] As shown in FIG. 20, in the blank C1-30A according to the present embodiment, the minimum plate thickness t minThe steel sheet C1-31 having [description of what it has] may be provided with a film C1-50 similar to that of the first embodiment. The film C1-50 can be provided on at least one surface of the steel sheet C1-31. That is, the film C1-50 may cover only one surface of the steel sheet C1-31, or may cover both surfaces of the steel sheet C1-31. Thereby, since the emissivity of the surface of the steel sheet C1-31 can be increased in advance, when the blank C1-30A is heated during hot stamping, the temperature rise of the steel sheet C1-31 becomes faster. Therefore, the high-temperature holding time of the steel sheet C1-31 can be ensured to be longer. Therefore, the hardenability of the thinnest steel sheet C1-31 can be further improved.

[0292] When the film C1-50 is provided on the thinnest steel sheet C1-31 in the blank C1-30A, in the formed structural member C1-10A, a film C1-13 (FIG. 17) similar to that of the first embodiment exists on the steel sheet C1-31.

[0293] The lower structural member C1-20 (FIG. 14) and the blank C1-40 (FIG. 16D) can have the same configuration as the upper structural member C1-10A and the blank C1-30A in the present embodiment. Therefore, the lower structural member C1-20 and the blank C1-40 can also exhibit the same effects as described above.

[0294] <Third Embodiment> FIG. 21 is a plan view of a blank C1-30B according to the third embodiment. The blanks C1-30, C1-30A according to the first and second embodiments are tailored blanks in which the steel sheets C1-31, C1-32, C1-33 are butt-jointed to each other. The blank C1-30B according to the present embodiment mainly differs from the first and second embodiments in the arrangement of the steel sheets and the form of the joint portions.

[0295] Referring to FIG. 21, the blank C1-30B includes a plurality of steel plates C1-31, C1-32, C1-33, C1-36. In the example of FIG. 21, the steel plates C1-31, C1-32, C1-33, C1-36 are arranged and joined so as to form long portions C1-34L, C1-34R and a plurality of connecting portions C1-35. The minimum plate thickness t min On at least one surface of the steel plate C1-31 having [the minimum plate thickness t], a treatment for increasing the emissivity as compared with both surfaces of the steel plate C1-32 is performed. That is, at least one surface of the steel plate C1-31 is subjected to the same treatment as in the first or second embodiment so that the emissivity becomes higher than that of the steel plate C1-32 before or during heating. Therefore, the blank C1-30B according to the present embodiment can also achieve the same effects as in the first embodiment.

[0296] The blank C1-30B includes an overlap portion C1-37. FIG. 22 is a cross-sectional view taken along line IX-IX of FIG. 21 and shows a cross-section of the overlap portion C1-37. In the present embodiment, the overlap portion C1-37 is formed by overlapping the ends of two adjacent steel plates C1-33, C1-36. The end of the steel plate C1-33 is joined in a state of being overlapped with the end of the steel plate C1-36. The steel plates C1-33, C1-36 are joined to each other by, for example, spot welding or laser welding.

[0297] The overlap portion C1-37 has a total plate thickness t. The total plate thickness t is the plate thickness obtained by adding the plate thickness t 3 of the steel plate C1-33 and the plate thickness t 6 of the steel plate C1-36. When at least one of the steel plates C1-33, C1-36 is a plated steel plate, the total plate thickness t also includes the thickness of the plating layer. The total plate thickness t of the overlap portion C1-37 is, for example, 4.0 mm or less. The total plate thickness t of the overlap portion C1-37 may be greater than 2.5 mm.

[0298] On the surface of each of the steel plates C1-33 and C1-36, which is located outside the overlap portion C1-37, a treatment for increasing the emissivity is performed as compared with both surfaces of another steel plate C1-32 (FIG. 16B). In each of the steel plates C1-33 and C1-36, on the surface located outside the overlap portion C1-37, for example, a treatment for increasing the emissivity is performed over the entire surface. For example, the emissivity of the surface of the steel plate C1-33 that is located outside the overlap portion C1-37, that is, the surface located on the opposite side of the mating steel plate C1-36, is larger than the emissivity of both surfaces of the steel plate C1-32 in advance. Similarly, the emissivity of the surface of the steel plate C1-36 that is located outside the overlap portion C1-37, that is, the surface located on the opposite side of the mating steel plate C1-33, is larger than the emissivity of both surfaces of the steel plate C1-32 in advance. For example, the emissivity at a wavelength of 8.0 μm at 25°C is 60% or more, preferably 70% or more, more preferably 80% or more on the surface of the steel plates C1-33 and C1-36 that is located outside the overlap portion C1-37. The difference in emissivity at a wavelength of 8.0 μm at 25°C between the surface of the steel plates C1-33 and C1-36 that is located outside the overlap portion C1-37 and both surfaces of another steel plate C1-32 is preferably greater than 5%, more preferably greater than 10%, and even more preferably greater than 20%. In the steel plates C1-33 and C1-36, the emissivity of the surface located inside the overlap portion C1-37 may be larger than the emissivity of both surfaces of the steel plate C1-32, or may be equal to or less than the emissivity of both surfaces of the steel plate C1-32.

[0299] For example, due to the film C1-50 described in the first embodiment, the surfaces of the steel plates C1-33 and C1-36 located outside the overlap portion C1-37 may have a higher emissivity than the steel plate C1-32 (FIG. 16B). The film C1-50 is provided on at least the surface of both surfaces of the steel plate C1-33 that is located outside (front side) of the overlap portion C1-37, and covers the entire surface. Also, the film C1-50 is provided on at least the surface of both surfaces of the steel plate C1-36 that is located outside (front side) of the overlap portion C1-37, and covers the entire surface. Since the film thickness of the film C1-50 is very small as described in the first embodiment, the plate thickness measured including the film C1-50 for the overlap portion C1-37 can be treated as the total plate thickness t.

[0300] Alternatively, the surfaces of the steel plates C1-33 and C1-36 located outside the overlap portion C1-37 may be processed so as to have a higher emissivity than both surfaces of the steel plate C1-32 (FIG. 21) by heating the blank C1-30. For example, as shown in FIG. 23, the steel plates C1-33 and C1-36 may be plated steel plates with thinner plating compared to the steel plate C1-32 (FIG. 18A). In this case, the steel plate C1-33 is a plated steel plate having a base steel plate C1-33a and an aluminum-based plating layer C1-33b, similar to the second embodiment. The steel plate C1-36 is also a plated steel plate having a base steel plate C1-36a and an aluminum-based plating layer C1-36b. The base steel plates C1-33a and C1-36a may be the same type of steel plate or different types of steel plates. Similarly, the aluminum-based plating layers C1-33b and C1-36b may be the same type of plating layer or different types of plating layers. The other steel plates C1-31 and C1-32 (FIGS. 18A and 18B) are also plated steel plates having base steel plates C1-31a and C1-32a and aluminum-based plating layers C1-33b, respectively, and have the same configuration as in the second embodiment.

[0301] When the steel plates C1-33 and C1-36 are plated steel plates, the total plate thickness t of the overlap portion C1-37 is the plate thickness including the thicknesses of the plating layers C1-33b and C1-36b. In the example of FIG. 23, the adhesion amount W3 (g / m 2 ) of the aluminum-based plating layer C1-33b to both surfaces of the base steel plate C1-33a of the steel plate C1-33, and the adhesion amount W6 (g / m 2 ) of the aluminum-based plating layer C1-36b to both surfaces of the base steel plate C1-36a of the steel plate C1-36, the adhesion amounts W3 and W6 are less than the adhesion amount W2 of the aluminum-based plating layer C1-32b (FIG. 18A) to both surfaces of the base steel plate C1-32a of the steel plate C1-32.

[0302] As described in the second embodiment, the adhesion amount W3 of the aluminum-based plating layer C1-33b in the steel plate C1-33 is the average adhesion amount on both surfaces of the base steel plate C1-33a. Similarly, the adhesion amount W6 of the aluminum-based plating layer C1-36b in the steel plate C1-36 is the average adhesion amount on both surfaces of the base steel plate C1-36a. Usually, the adhesion amount (g / m 2 ) of the aluminum-based plating layer C1-36b to one surface of the base steel plate C1-36a is substantially equal to the adhesion amount (g / m 2 ) of the aluminum-based plating layer C1-36b to the other surface of the base steel plate C1-36a. However, for example, due to various conditions during manufacturing, etc., the adhesion amount of the aluminum-based plating layer C1-36b may vary between the front and back surfaces of the base steel plate C1-36a. The adhesion amount of the aluminum-based plating layer C1-36b may be different between one surface and the other surface of the base steel plate C1-36a. The adhesion amounts W3 and W6 may each be 20 g / m 2 or more and 120 g / m 2 or less. The adhesion amounts W3 and W6 are each preferably 30 g / m 2 or more, and more preferably 35 g / m 2 or more. The adhesion amounts W3 and W6 are each preferably 115 g / m 2 or less, and more preferably 100 g / m 2 or less. The difference in the adhesion amounts W2 and W3 between the steel plates C1-32 and C1-33: W2 - W3 is, for example, 20 (g / m2 ) is as above. W2 - W3 may be 80 (g / m 2 ) or less. Similarly, the difference in the deposition amounts W2 and W6 between the steel plates C1-32 and C1-36: W2 - W6 is, for example, 20 (g / m 2 ) or more. W2 - W6 may be 80 (g / m 2 ) or less. The deposition amount W3 of the aluminum-based plating layer C1-33b on the steel plate C1-33 and the deposition amount W6 of the aluminum-based plating layer C1-36b on the steel plate C1-36 may be the same or different. Also, the plate thickness t 3 of the steel plate C1-33 and the plate thickness t 6 of the steel plate C1-36 may be the same or different. The plate thickness t 3 of the steel plate C1-33 and the plate thickness t 6 of the steel plate C1-36 are respectively the average plate thicknesses of the steel plates C1-33 and C1-36, and are the plate thicknesses including the aluminum-based plating layers C1-33b and C1-36b.

[0303] The deposition amount W3 of the aluminum-based plating layer C1-33b on the steel plate C1-33 and the deposition amount W6 of the aluminum-based plating layer C1-36b on the steel plate C1-36 can be measured by the method described in the second embodiment.

[0304] When the total plate thickness t of the overlap portion C1-37 exceeds, for example, 2.5 mm, since it is difficult for the overlap portion C1-37 to heat up, when the blank C1-30B is heated during hot stamping, until the overlap portion C1-37 reaches the temperature in the austenite region, the minimum plate thickness t minThe plating layer C1-31b of the steel sheet C1-31 (Figs. 16B and 18A) having [the relevant property] may be alloyed, the diffusion layer may become thick, and the corrosion resistance or weldability of the steel sheet C1-31 may not be ensured. However, in the example of Fig. 22, for example, the emissivity of the overlap portion C1-37 is increased in advance by the film C1-50 so that the temperature rise of the overlap portion C1-37 can be promoted in the heating process. In the example of Fig. 23, by making the adhesion amounts W3 and W6 of the plating layers C1-33b and C1-36b in the overlap portion C1-37 less than the adhesion amount W2 of the plating layer C1-32b of the other steel sheet C1-32, the emissivity of the overlap portion C1-37 becomes higher than that of the steel sheet C1-32 during heating, thereby promoting the temperature rise of the overlap portion C1-37. Therefore, even when the total plate thickness t of the overlap portion C1-37 is greater than 2.5 mm, before the alloying of the plating layer C1-31b of the steel sheet C1-31 progresses and the diffusion layer becomes thick and the corrosion resistance or weldability is lost, the overlap portion C1-37 can be sufficiently heated until the phase transformation to austenite is completed. Therefore, a process window in the manufacture of the structural member can be ensured.

[0305] As shown in Fig. 24, even when the steel sheets C1-33 and C1-36 are plated steel sheets having aluminum-based plating layers C1-33b and C1-36b, a film C1-50 may be provided on the surface located outside the overlap portion C1-37 in at least one of the steel sheets C1-33 and C1-36. The film C1-50 may be provided on the surfaces located outside the overlap portion C1-37 in both of the steel sheets C1-33 and C1-36. The surfaces located inside the overlap portion C1-37 in each of the steel sheets C1-33 and C1-36 may be covered with the film C1-50, but from the viewpoint of uniformizing the heating of the blank C1-30B, it is preferably not covered with the film C1-50.

[0306] As a result, the emissivity of the overlapping portion C1-37 can be increased in advance. Therefore, when the blank C1-30B is heated during hot stamping, the temperature rise of the overlapping portion C1-37 can be accelerated. Thus, the plate thickness t min Before the alloying of the plating layer C1-31b of the steel plate C1-31 (Figs. 16B and 18A) having reaches an excessive level, the overlapping portion C1-37 is likely to be heated to the temperature in the austenite region, and a structural member can be manufactured while maintaining the corrosion resistance or weldability of the steel plate C1-31. Therefore, it becomes easier to secure a process window in the manufacture of the structural member.

[0307] In the blank C1-30B according to the present embodiment, the steel plate C1-31 may be butt-joined to the steel plate C1-32, or may form an overlapping portion with the steel plate C1-32 like the steel plates C1-33 and C1-36. The steel plate C1-32 may be butt-joined to the steel plate C1-36, or may form an overlapping portion with the steel plate C1-36 like the steel plates C1-33 and C1-36.

[0308] The lower blank C1-40 (Fig. 16D) can have the same configuration as the upper blank C1-30B in the present embodiment. In this case, the lower blank C1-40 can also exhibit the same effects as described above.

[0309] <Fourth Embodiment> Fig. 25 is an exploded perspective view of the structural members C1-10C and C1-20C according to the present embodiment. The structural members C1-10, C1-10A, and C1-20 according to the above embodiment constitute the front under module of the vehicle body. On the other hand, the structural members C1-10C and C1-20C according to the present embodiment constitute the rear under module of the vehicle body.

[0310] Referring to FIG. 25, the structural member C1-10C includes a pair of side frames C1-11L and C1-11R and at least one cross member C1-12, similar to the above-described embodiment. The structural member C1-20C also includes a pair of side frames C1-21L and C1-21R and at least one cross member C1-22, similar to the above-described embodiment. In the example shown in FIG. 25, the cross member C1-12 connects the middle portions of the side frames C1-11L and C1-11R. Similarly, the cross member C1-22 connects the middle portions of the side frames C1-21L and C1-21R. The configurations of the structural members C1-10, C1-10A and the structural member C1-20 described in other embodiments can be applied to the structural member C1-10C and the structural member C1-20C of the present embodiment, respectively.

[0311] The structural member C1-10C can be manufactured from the blank C1-30C shown in FIG. 26 by a manufacturing method similar to the manufacturing method described in the first embodiment. The blank C1-30C includes steel plates C1-31, C1-32, and C1-33. The steel plates C1-31, C1-32, and C1-33 are arranged and joined so as to form two long portions C1-34L and C1-34R and at least one connecting portion C1-35. The blank C1-30C can have a configuration similar to any of the blanks C1-30, C1-30A, and C1-30B described in other embodiments.

[0312] In the present embodiment, the steel plate C1-31 forming each rear portion C1-112 (FIG. 25) of the side frames C1-11L and C1-11R has a smaller plate thickness than the steel plate C1-32 forming the front portion C1-111 (FIG. 25). The tensile strength of the steel plate C1-31 may be smaller than the tensile strength of the steel plate C1-32. In the structural member C1-10C according to the present embodiment and the structural members C1-10, C1-10A (FIGS. 14 and 19A to 19C) according to other embodiments, it is preferable that the plate thickness and / or the tensile strength of the steel plate located more outward in the longitudinal direction of the vehicle body is smaller than that of the steel plate located more inward. Thereby, when a collision load in the longitudinal direction is input to the vehicle body, in the structural members C1-10, C1-10A, and C1-10C, the portion located more outward in the vehicle body deforms to absorb the collision energy, while the portion located more inward in the vehicle body is less likely to deform, and peripheral components can be protected.

[0313] The structural member C1-20C can be manufactured from the blank C1-40C shown in FIG. 27 by the same manufacturing method as the manufacturing method described in the first embodiment. The blank C1-40C includes steel plates C1-41, C1-42, and C1-43. The steel plates C1-41, C1-42, and C1-43 are arranged and joined so as to form two long portions C1-44L and C1-44R and at least one connecting portion C1-45. The blank C1-40C can have the same configuration as any of the blanks C1-30, C1-30A, and C1-30B described in other embodiments.

[0314] In the present embodiment, the steel plate C1-41 forming each rear portion C1-212 (FIG. 25) of the side frames C1-21L and C1-21R has a smaller plate thickness than the steel plate C1-42 forming the front portion C1-211 (FIG. 25). The tensile strength of the steel plate C1-41 may be smaller than the tensile strength of the steel plate C1-42. Also in the lower structural members C1-20 and C1-20C (FIGS. 14 and 25), it is preferable that the plate thickness and / or the tensile strength of the steel plate located more outward in the longitudinal direction of the vehicle body is smaller than that of the steel plate located more inward, similar to the upper side. Thereby, when a collision load in the longitudinal direction is input to the vehicle body, in the structural members C1-20 and C1-20C, the portion located more outward in the vehicle body deforms to absorb the collision energy, while the portion located more inward in the vehicle body is less likely to deform, and peripheral components can be protected.

[0315] As described above, the embodiments according to the present disclosure have been described. However, the present disclosure is not limited to the above embodiments, and various modifications are possible without departing from the spirit thereof.

[0316] In the blank C1-30 according to the first embodiment, the minimum plate thickness t minAs a treatment for increasing the emissivity of the surface of the steel sheet C1-31 having [a certain property] compared to that of another steel sheet C1-32, a film C1-50 is formed on at least one surface of the steel sheet C1-31. In the blank C1-30A according to the second embodiment, as a treatment for increasing the emissivity of the surface of the steel sheet C1-31 compared to that of another steel sheet C1-32, the adhesion amount W1 of the aluminum-based plating layer C1-31b on both surfaces of the steel sheet C1-31 is less than the adhesion amount W2 of the aluminum-based plating layer C1-32b of another steel sheet C1-32. However, the treatment for increasing the emissivity of the steel sheet C1-31 is not limited to these. For example, by making the surface roughness of at least one surface of the steel sheet C1-31 larger than the surface roughness of both surfaces of the steel sheet C1-32, the emissivity of at least one surface of the steel sheet C1-31 can also be increased compared to that of the steel sheet C1-32. Similarly, in the blank C1-30B according to the third embodiment, when performing a treatment for increasing the emissivity on the outer surface of the overlap portion C1-37 of the steel sheets C1-33 and C1-36, it is not limited to the method described in that embodiment.

[0317] In the second embodiment above, the minimum plate thickness t min Not only the steel sheet C1-31 having [a certain property], but also the steel sheet C1-33 having a plate thickness t 3 An example where the adhesion amount W3 of the aluminum-based plating layer C1-33b in the steel sheet C1-33 is less than the adhesion amount W2 of the aluminum-based plating layer C1-32b in another steel sheet C1-32 has been described. However, among the plurality of steel sheets C1-31, C1-32, C1-33 constituting the blank C1-30, at least the minimum plate thickness t minIn the steel sheet C1-31 having [specific condition], it is sufficient that the adhesion amount W1 of the aluminum-based plating layer C1-31b on the steel sheet C1-31 is less than the adhesion amount W2 of the aluminum-based plating layer C1-32b on the steel sheet C1-32. The adhesion amount W3 of the aluminum-based plating layer C1-33b on the steel sheet C1-33 other than the steel sheet C1-32 can also be set to be equal to or greater than the adhesion amount W2 of the aluminum-based plating layer C1-32b on the steel sheet C1-32. In this case, in the structural member C1-10A after hot stamping, the plating thickness K3 of the steel sheet C1-33 is also equal to or greater than the plating thickness K2 of the steel sheet C1-32. In the above second embodiment, the steel sheets other than the steel sheets C1-31 and C1-32 do not necessarily have to be aluminum-plated steel sheets.

[0318] In the first embodiment described above, the blank C1-30 includes steel sheets C1-31 and C1-32 corresponding to the long portions C1-34L and C1-34R (side frames C1-11L and C1-11R), respectively, and a steel sheet C1-33 corresponding to the connecting portion C1-35 (cross member C1-12). However, the number and arrangement of the steel sheets included in the blank C1-30 and the structural member C1-10 manufactured from the blank C1-30 are not limited to this. As shown in FIGS. 28 and 29, the number and arrangement of the steel sheets can be appropriately changed.

[0319] As shown in FIGS. 28 and 29, in the blank C1-30 corresponding to the structural member C1-10 (FIG. 14) of the front under module, the long portions C1-34L and C1-34R may each be composed of a single steel sheet C1-31. In the blank C1-30, the connecting portion C1-35 may connect one end portion in the longitudinal direction of the long portions C1-34L and C1-34R to each other, similar to the first embodiment. That is, on the end side arranged in the front or rear when the structural member C1-10 is assembled to the vehicle body, the long portions C1-34L and C1-34R may be connected by the connecting portion C1-35. As shown in FIG. 29, the blank C1-30 may include a plurality of connecting portions C1-35. These connecting portions C1-35 are formed of separate steel sheets C1-32 and C1-33.

[0320] Although illustrations are omitted, the number and arrangement of steel plates for the structural member C1-10A and blanks C1-30A, C1-30B according to other embodiments, and the lower structural member C1-20 and blank C1-40 are not particularly limited. The structural members C1-10, C1-10A, C1-20 and blanks C1-30, C1-30A, C1-30B, C1-40 only need to include two or more joined steel plates respectively. The structural members C1-10, C1-10A, C1-20 and blanks C1-30, C1-30A, C1-30B, C1-40 preferably include three or more steel plates respectively. Each of the blanks C1-30, C1-30A, C1-30B, C1-40 has at least a first steel plate having a minimum plate thickness t min and a second steel plate having a plate thickness greater than t min It is sufficient to include. The first steel plate is joined directly to the second steel plate or indirectly via another steel plate. On one or both sides of the first steel plate, a treatment for increasing the emissivity is performed as compared with both sides of the second steel plate. For other steel plates, a treatment for increasing the emissivity may or may not be performed. When there are a plurality of steel plates having the minimum plate thickness t min in each of the blanks C1-30, C1-30A, C1-30B, C1-40, it is preferable that a treatment for increasing the emissivity is performed on one or both sides of all these steel plates as compared with both sides of the steel plate having a larger plate thickness.

[0321] In the fourth embodiment above, the blank C1-30C corresponds to the structural member C1-10C of the rear under module. In this blank C1-30C, it includes steel plates C1-31, C1-32 corresponding to the respective long portions C1-34L, C1-34R (side frames C1-11L, C1-11R), and a steel plate C1-33 corresponding to the connecting portion C1-35 (cross member C1-12). However, the number and arrangement of the steel plates included in the blank C1-30C and the structural member C1-10C manufactured from the blank C1-30C are not limited to this. As shown in FIGS. 30 to 40, the number and arrangement of the steel plates can be changed as appropriate.

[0322] For example, as shown in FIGS. 30 and 31, in the blank C1-30C, the long portions C1-34L and C1-34R may each be composed of a single steel plate C1-31. In this case, the connecting portion C1-35 may connect the long portions C1-34L and C1-34R at one end side in the longitudinal direction. For example, the long portions C1-34L and C1-34R may be connected by the connecting portion C1-35 at the end side that is arranged forward when the structural member C1-10C (FIG. 25) is assembled to the vehicle body. The long portions C1-34L and C1-34R may be connected by the connecting portion C1-35 at their intermediate portions.

[0323] As shown in FIG. 32, in the blank C1-30C, even when the long portions C1-34L and C1-34R are formed of a plurality of steel plates C1-31 and C1-32, the long portions C1-34L and C1-34R may be connected by the connecting portion C1-35 at the end side that is arranged forward when the structural member C1-10C (FIG. 25) is assembled to the vehicle body. For example, when a portion corresponding to the front portion C1-111 (FIG. 25) of the side frames C1-11L and C1-11R is formed of the steel plate C1-31 and a portion corresponding to the rear portion C1-112 (FIG. 25) is formed of the steel plate C1-32, the connecting portion C1-35 may be joined to the steel plate C1-31 as shown in FIGS. 32 and 33, or may be joined to the steel plate C1-32 as shown in FIG. 34.

[0324] In the above-described fourth embodiment, since a single cross member C1-12 is provided in the structural member C1-10C (FIG. 25), the blank C1-30C for the structural member C1-10C also includes a single connecting portion C1-35. However, the structural member C1-10C can include a plurality of cross members C1-12. In this case, as shown in FIGS. 35 to 40, the blank C1-30C also includes a plurality of connecting portions C1-35. The connecting portion C1-35 is formed of separate steel plates C1-32, C1-33, steel plates C1-33, C1-36, or steel plates C1-32, C1-33, C1-36. In this case, as shown in FIGS. 35, 36, and 40, the long portions C1-34L, C1-34R may each be formed of a single steel plate C1-31, or as shown in FIGS. 37 to 39, may each be formed of a plurality of steel plates C1-31, C1-32.

[0325] Although illustration is omitted, in the fourth embodiment, the number and arrangement of the steel plates for the lower structural member C1-20C and the blank C1-40C are not particularly limited. The structural members C1-10C, C1-20C and the blanks C1-30C, C1-40C may each include two or more joined steel plates. The structural members C1-10C, C1-20C and the blanks C1-30C, C1-40C preferably each include three or more steel plates. Each of the blanks C1-30C, C1-40C includes at least a first steel plate having a minimum plate thickness t min and a second steel plate having a plate thickness greater than the plate thickness t min That's all right. The first steel plate is joined directly to the second steel plate or indirectly via another steel plate. On one or both sides of the first steel plate, a treatment for increasing the emissivity is performed as compared with both sides of the second steel plate. For the other steel plates, a treatment for increasing the emissivity may or may not be performed. When there are a plurality of steel plates having the minimum plate thickness t min in each of the blanks C1-30C, C1-40C, it is preferable that a treatment for increasing the emissivity is performed on one or both sides of all of these steel plates as compared with both sides of the steel plate having a larger plate thickness.

[0326] In the above-described embodiment, the plurality of steel plates (sub-blanks) included in each blank and each structural member may be single-layered or multi-layered. That is, each sub-blank may be a single steel plate or a plate material formed by laminating a plurality of steel plates.

[0327] In the above-described embodiment, the mold C1-60 used in the forming process includes a punch C1-61 and a die C1-62. However, the configuration of the mold C1-60 is not limited to the example described in the above embodiment. The mold C1-60 can further include, for example, pads and blank holders. The mold C1-60 may be configured according to the target structural member.

[0328] In the structural members C1-10, C1-10A, C1-10C, C1-20, C1-20C according to the above-described embodiment, the side frames C1-11, C1-21 have a substantially hat-shaped cross section. However, the shape of the cross section of the side frames C1-11, C1-21 is not necessarily limited to this. For example, as shown in FIG. 41, the side frames C1-11, C1-21 may have a shape in which one side in the width direction is open in a cross-sectional view. In this case, another member (not shown) may be joined to the open portion of the side frames C1-11, C1-21, and a closed cross section may be formed by the side frames C1-11, C1-21 and the other member. Similarly, the cross members C1-12, C1-22 may each have a substantially hat-shaped cross section or may have a cross section of another shape.

[0329] Hereinafter, the present disclosure will be described in more detail by way of examples. However, the present disclosure is not limited to the following examples.

[0330] To confirm the effects of the present disclosure, while changing the type (material type) and thickness of the steel sheet included in the structural member, and the division pattern of the structural member, CAE analysis was performed using commercially available software (AUTOFORM R.10, manufactured by AUTOFORM) for the press forming (hot stamping) of the structural member which is a front or rear under module.

[0331] Table 13 shows the steel sheets (material types) used in this analysis.

[0332]

Table 13

[0333] In Table 13, the material types are listed in the order of plating type, tensile strength, and application (hot stamping). Regarding the coating specifications, the black coating is a black coating containing carbon black and metal oxides. "Black coating - one side" means that the entire one side of the steel sheet is coated with the black coating. "Black coating - both sides" means that the entire both sides of the steel sheet are coated with the black coating. In this analysis, the material types were selected from Table 13 to constitute the target structural member.

[0334] The division patterns of the structural members are shown in FIGS. 42A to 42F. The structural member shown in FIG. 42A is the upper or lower structural member of the front under module. The structural members shown in FIGS. 42B to 42F are the upper or lower structural members of the rear under module. FIGS. 42A to 42F show the number of steel sheets (materials) included in the structural member and the positions of the joints between the steel sheets in the structural member. In FIGS. 42A to 42F, each steel sheet is assigned a number in parentheses.

[0335] Regarding the structural members shown in FIGS. 42A and 42B, the analysis conditions and results are shown in Table 14. In FIGS. 42A and 42B, the side frames of the structural members are each formed by two materials (1) and (2). The cross member is formed by material (3). Materials (1) to (3) are butt - joined to adjacent materials respectively.

[0336]

Table 14

[0337] Referring to Table 14 and FIGS. 42A and 42B, Examples 1 and 2, and Comparative Examples 1 and 2 are structural members on the upper side of the rear under module, and Examples 3 and Comparative Example 3 are structural members on the upper side of the front under module. In Example 1, a black film is applied to both sides of Material (3) having the minimum plate thickness t min : 1.0 mm. In Example 2, a black film is applied to one side of Material (3) having the minimum plate thickness t min : 1.0 mm. In Example 3, a black film is applied to one side of Material (2) having the minimum plate thickness t min : 1.0 mm. On the other hand, in Comparative Examples 1 to 3, no black film is applied to any of Materials (1) to (3). In Comparative Examples 1 to 3, no black film is applied to the material having the minimum plate thickness t min : 1.0 mm.

[0338] In Table 14, the "910°C arrival time" is the time required from the start of heating of the blank until 910°C (A c3 point or higher) is reached for the material that has been heated to 910°C earliest among the materials included in the blank. The "phase transformation start time" is the shortest time until the phase transformation to ferrite starts after the blank is heated at a furnace temperature of 920°C for 5 minutes and 30 seconds and then taken out of the heating furnace. From Table 14, it can be seen that in Examples 1 to 3 where the emissivity of the thinnest material was increased by the black film, the 910°C arrival time was shortened by about 20 seconds compared to Comparative Examples 1 to 3, and the heating rate of the thinnest material in the heating process was increased. Furthermore, in Examples 1 to 3, the phase transformation start time was later than that in Comparative Examples 1 to 3, so it becomes easier to start forming the blank before the start of ferrite transformation, and it becomes possible to uniformly heat the blank in the forming process.

[0339] Regarding the structural members shown in FIGS. 42C to 42F, the analysis conditions and results are shown in Tables 15 and 16. Examples 4 to 16 and Comparative Examples 4 to 16 in Tables 15 and 16 are all rear under modules, but Examples 4 to 8 and 14 to 16, and Comparative Examples 4 to 8 and 14 to 16 are upper side structural members, and Examples 9 to 13 and Comparative Examples 9 to 13 are lower side structural members. In FIGS. 42C to 42E, the side frames of the structural members are each formed by two materials (1) and (2). The structural members in FIGS. 42C to 42E include a plurality of cross members formed of material (3) or material (4). In FIG. 42F, the side frame of the structural member is formed by one material (1). The structural member in FIG. 42F includes a plurality of cross members formed of any of materials (2) to (4). Materials (1) to (4) are butt-joined to adjacent materials, respectively.

[0340]

Table 15

[0341]

Table 16

[0342] Referring to Table 15, in Examples 4 to 13, a black film is applied to the material having the smallest plate thickness t min : 1.0 mm among materials (1) to (4). In Examples 4 to 13, one side or both sides of the material with the smallest plate thickness t min are covered with a black film. When there are multiple materials with the smallest plate thickness t min , black films are applied to all of these materials. On the other hand, in Comparative Examples 4 to 13, no black film is applied to any of materials (1) to (4). That is, in Comparative Examples 4 to 13, no black film is applied to the material having the smallest plate thickness t min : 1.0 mm.

[0343] From Table 15, it can be seen that in Examples 4 to 13, the time to reach 910°C was shortened by about 20 seconds compared with Comparative Examples 4 to 13, and the heating rate of the thinnest material in the heating process increased. Further, in Examples 4 to 13, the phase transformation start time was later than that in Comparative Examples 4 to 13. Therefore, it becomes easier to start forming the blank before the start of the ferrite transformation, and it becomes possible to uniformly heat the blank in the forming process.

[0344] Referring to Table 16, in Examples 14 to 16, a black film is applied to the material having the minimum plate thickness t min : 1.2 mm among Materials (1) to (4). In Examples 14 to 16, one side or both sides of the material with the minimum plate thickness t min are covered with a black film. On the other hand, in Comparative Examples 14 to 16, no black film is applied to any of Materials (1) to (4). In Comparative Examples 14 to 16, no black film is applied to the material having the minimum plate thickness t min : 1.2 mm.

[0345] From Table 16, it can be seen that in Examples 14 to 16, the time to reach 910°C was shortened by 45 seconds or more compared with Comparative Examples 14 to 16, and the heating rate of the thinnest material in the heating process increased. Further, in Examples 14 to 16, the phase transformation start time exceeded 20 seconds and was later than that in Comparative Examples 14 to 16. Therefore, it becomes easier to start forming the blank before the start of the ferrite transformation, and it becomes possible to uniformly heat the blank in the forming process.

[0346] Regarding Examples 6, 11, and 16, and Comparative Examples 6, 11, and 16, analysis samples were collected from the thinnest part of the structural member by the method described in the above embodiment, and the variation in the martensite fraction was measured. Separately, shape accuracy measurement and impact absorption performance measurement were performed on these structural members. The evaluation results are shown in Table 17.

[0347]

Table 17

[0348] In Table 17, the variation in the martensite fraction, as described in the above embodiment, is the value obtained by subtracting the minimum martensite fraction (%) from the maximum martensite fraction (%) in the cross-section of the structural member at the position of the material having the minimum plate thickness t min In the cross-section of the structural member at the position of the material having the minimum plate thickness t, it is the value obtained by subtracting the minimum martensite fraction (%) from the maximum martensite fraction (%).

[0349] Regarding the shape accuracy, when the structural member was attached to another member, it was evaluated by how much the structural member was separated from the mating member at the overlapping portion of the structural member and the mating member. In Table 17, when the distance from the surface of the mating member was within ±2.0 mm, it was indicated as ○, when it was more than ±2.0 mm and within ±3.0 mm, it was indicated as △, and when it was more than ±3.0 mm, it was indicated as ×.

[0350] Regarding the impact absorption performance, assuming a rear collision and a side collision in a state where the structural member was assembled to a vehicle, an impactor simulating the vehicle was made to collide with the structural member, and the maximum intrusion amount at the time of the rear collision and the maximum intrusion amount at the time of the side collision were evaluated respectively. The impact absorption performance was evaluated by comparison with the impact absorption performance of a rear under module formed by joining each material after forming by hot stamping as a base. In Table 17, an impact absorption performance equivalent to the base impact absorption performance was indicated as good, an impact absorption performance superior to the base impact absorption performance was indicated as better, an impact absorption performance slightly lower than the base impact absorption performance was indicated as marginal, and an even lower impact absorption performance was indicated as poor.

[0351] As described above, in Examples 6, 11, and 16, a black film was applied to the surface of the thinnest material. On the other hand, in Comparative Examples 6, 11, and 16, no black film was applied to each material including the thinnest material. As shown in Table 17, in any of Examples 6, 11, and 16, the variation in the martensite fraction was 15% or less, and the variation in the martensite fraction was significantly reduced compared to Comparative Examples 6, 11, and 16. In Examples 6, 11, and 16, the shape accuracy was also better than that in Comparative Examples 6, 11, and 16.

[0352] In Examples 6, 11, and 16 where the variation in the martensite fraction was small, the impact absorption performance was also improved as compared with Comparative Examples 6, 11, and 16. In Examples 6, 11, and 16, despite forming the structural member by integrating a plurality of materials at the blank stage, it was possible to secure impact absorption performance equal to or better than that of a structural member joined after individually press-forming the materials.

[0353] (Elemental technology C2) The elemental technology C2 is a structural member for a vehicle body, (C2a) a pair of side frames, and a cross member connecting the side frames, and includes the side frame and the cross member include a first steel plate and a second steel plate having an end portion that is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate, and are formed by a plurality of steel plates joined to each other, on the surface located outside the overlap portion in each of the first steel plate and the second steel plate, a film containing 0.001 g / m or more of at least one oxide selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide is provided 2 (C2b) a pair of side frames, and a cross member connecting the side frames, and includes the side frame and the cross member include a first steel plate and a second steel plate having an end portion that is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate, and are formed by a plurality of steel plates joined to each other, on the surface located outside the overlap portion in each of the first steel plate and the second steel plate, a film containing 0.500 g / m or less of carbon black is provided, 2 (C2c) a pair of side frames, and ​​A cross member that connects the side frames, comprises, the side frame and the cross member include a first steel plate, a second steel plate having an end portion that is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate, and a third steel plate, and are formed by a plurality of steel plates joined to each other, at least one of the first steel plate and the second steel plate, and the third steel plate are each a plated steel plate having an aluminum-based plating layer on both surfaces of a base steel plate, the thickness of the aluminum-based plating layer in at least one of the first steel plate and the second steel plate is smaller than the thickness of the aluminum-based plating layer in the third steel plate, is a structural member that satisfies at least one of the above.

[0354] According to Element Technology C2, a structural member can be provided that combines the strength of the overlap portion having the maximum plate thickness and the rust prevention function.

[0355] The blank for hot stamping according to the embodiment includes a plurality of steel plates. The plurality of steel plates are arranged and joined so as to form two long portions and a connecting portion. The long portions are arranged side by side in the lateral direction in a plan view of the blank. The connecting portion connects the long portions. The plurality of steel plates include a first steel plate and a second steel plate. The second steel plate has an end portion that is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate. The overlap portion has the maximum plate thickness in the blank. At least one of the plurality of steel plates is a plated steel plate having a base steel plate and a plating layer provided on the base steel plate. A surface located outside the overlap portion in at least one of the first steel plate and the second steel plate is subjected to a treatment for increasing the emissivity compared to at least one other surface of the plurality of steel plates (first configuration).

[0356] In the blank according to the first configuration, the end of the first steel plate and the end of the second steel plate form an overlapping portion having the maximum plate thickness in the blank. On at least one of the surfaces of the first steel plate and the second steel plate located outside the overlapping portion, a treatment for increasing the emissivity is performed as compared with at least one other surface of the plurality of steel plates included in the blank. Thereby, when the blank is heated during hot stamping, the temperature rising rate of the overlapping portion can be increased, so that the heating time of the overlapping portion can be shortened. Therefore, before the alloying of the plating layer proceeds excessively and the diffusion layer becomes thick in the plated steel plate included in the blank, the heating of the blank required for hot stamping can be completed. As a result, in the structural member formed from the blank, the strength of the overlapping portion can be ensured by hot stamping, and the corrosion resistance (rust prevention property) can be ensured. Note that the phrase "a treatment for increasing the emissivity is performed" includes not only the case where the surface located outside the overlapping portion of at least one of the first steel plate and the second steel plate has a higher emissivity than the other surfaces before heating the blank, but also the case where the surface located outside the overlapping portion of at least one of the first steel plate and the second steel plate has a higher emissivity than the other surfaces during heating the blank.

[0357] Thus, according to the blank according to the first configuration, it becomes possible to manufacture a structural member having both the strength of the overlapping portion having the maximum plate thickness and the rust prevention function of the plated steel plate portion. That is, according to the blank, the temperature rising rate of the overlapping portion having the maximum plate thickness can be increased, and as a result, it becomes easier to secure the process window of the heating conditions in the manufacture of the structural member. Further, by increasing the temperature rising rate of the overlapping portion, the heating time of the blank for hot stamping can be shortened, so that the productivity of the structural member can be improved. Furthermore, since the heating time of the blank is shortened, the energy consumption in the manufacture of the structural member is suppressed, and the amount of greenhouse gas generated when heating the blank during hot stamping can be reduced.

[0358] In the blank according to the first configuration, among the plurality of steel sheets, the steel sheet constituting the portion having the minimum plate thickness in the blank may be a plated steel sheet (second configuration).

[0359] When the blank is heated during hot stamping, the steel sheet with the minimum plate thickness heats up relatively quickly. Therefore, when the steel sheet with the minimum plate thickness in the blank is a plated steel sheet, while raising the temperature of the overlap portion having the maximum plate thickness to ensure its strength by hot stamping, alloying of the plating layer easily proceeds in the plated steel sheet with the minimum plate thickness that has been heated earlier, and the corrosion resistance due to the plating layer may decrease or disappear. However, in the blank according to the embodiment, in at least one of the first steel sheet and the second steel sheet, a treatment for increasing the emissivity is applied to the surface located outside the overlap portion. Therefore, the heating rate of the overlap portion can be increased and its heating time can be shortened, and before the alloying of the plating layer proceeds excessively in the steel sheet with the minimum plate thickness that has been heated earlier, the heating of the blank required for hot stamping can be completed. Therefore, even if the steel sheet with the minimum plate thickness is a plated steel sheet as in the second configuration, it is possible to ensure its corrosion resistance.

[0360] In the blank according to the first or second configuration, a treatment for increasing the emissivity may be applied to the surface located outside the overlap portion in each of the first steel sheet and the second steel sheet, as compared with at least one other surface among the plurality of steel sheets (third configuration).

[0361] In the blank according to the third configuration, in each of the first steel plate and the second steel plate, a treatment for increasing the emissivity is applied to the surface located outside the overlap portion. That is, the emissivities of the surfaces on both outer sides of the overlap portion are higher than those of the other surfaces before heating the blank, or are higher than those of the other surfaces during heating of the blank. In this case, the temperature increase rate of the overlap portion can be further increased during heating of the blank, and the heating time of the overlap portion can be further shortened. Therefore, it becomes easier to secure the process window of the heating conditions in the manufacture of the structural member.

[0362] In the blank according to any one of the first to third configurations, the plurality of steel plates may each be a plated steel plate (fourth configuration).

[0363] In the fourth configuration, each steel plate included in the blank is a plated steel plate. In this case, when forming the blank into a structural member by hot stamping, generation of oxide scale can be suppressed. Therefore, after hot stamping, it is not necessary to apply a treatment for removing oxide scale, such as shot blasting treatment, to the structural member. Thus, the productivity of the structural member can be increased. Further, since each steel plate is a plated steel plate, the corrosion resistance of the structural member is also easily ensured.

[0364] In the blank according to the fourth configuration, the plating layer of each steel plate is an aluminum-based plating layer (fifth configuration).

[0365] When the plated steel sheet has an aluminum-based plating layer for each steel sheet as in the fifth configuration, when heating the blank during hot stamping, in particular, a difference in the heating rate is likely to occur between the overlapping portion having the maximum plate thickness and the non-overlapping portion. Since the aluminum-based plating layer is close to white, it easily reflects thermal energy and inhibits the temperature rise of the overlapping portion. However, even when the plated steel sheet has an aluminum-based plating layer for each steel sheet, by performing a treatment for increasing the emissivity on at least one of the outer surfaces of both sides of the overlapping portion, when heating the blank during hot stamping, the temperature rise of the overlapping portion can be promoted, and the heating time of the overlapping portion can be shortened. Therefore, the corrosion resistance in the plated steel sheet portion can be ensured, and the productivity of the structural member manufactured from the blank can be improved.

[0366] In the blank according to any one of the third to fifth configurations, a film may be formed as a treatment for increasing the emissivity on the surface located outside the overlapping portion in each of the first steel sheet and the second steel sheet. This film may have an emissivity of 60% or more at a wavelength of 8.0 μm at 25 °C (sixth configuration).

[0367] In the blank according to any one of the third to fifth configurations, a film may be formed as a treatment for increasing the emissivity on the surface located outside the overlapping portion in each of the first steel sheet and the second steel sheet. This film may contain carbon black, one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide, and 0 to 0.30 g / m 2 of silica. In this case, when the content of carbon black in the film is X CB (g / m 2 ), and the content of the oxide is X Oxide (g / m 2 ), X CB and X Oxide may satisfy the following formula (1) (see Patent Document 1) (seventh configuration). 118.9 ≦ 24280 / {6700 / (100 + 76 × XCB ) + 18000 / (130 + 65×X Oxide )} ≤ 332.0 (1)

[0368] In the blank according to any one of the third to seventh configurations, the plurality of steel plates may include two or more steel plates having different plate thicknesses. In this case, if the plate thickness of the overlap portion is t max , and the plate thickness of the steel plate constituting the portion having the minimum plate thickness in the blank is t min , when max / t min ≤ 3.2, it may be acceptable (eighth configuration).

[0369] When two or more steel plates having different plate thicknesses are included in the blank, compared with the case where the plate thicknesses of all the steel plates included in the blank are the same, the plate thickness difference between the overlap portion and the steel plate constituting the portion having the minimum plate thickness in the blank becomes larger. When the plate thickness difference is excessive, when manufacturing a structural member from the blank by hot stamping, the temperature rise of the overlap portion becomes slower compared with the steel plate constituting the portion having the minimum plate thickness in the blank. Therefore, for example, when the thinnest steel plate is a plated steel plate, there is a possibility that alloying of the plating layer proceeds excessively in the thinnest steel plate, and it becomes more difficult to secure the process window of the heating conditions. However, in the eighth configuration, the ratio of the plate thickness t max of the overlap portion to the minimum plate thickness t min : t max / t min is set to 3.2 or less. Thereby, even when two or more steel plates having different plate thicknesses are included in the blank, it becomes easier to complete the heating of the thickest overlap portion before the alloying of the plating layer proceeds excessively, and it becomes easier to secure the process window of the heating conditions.

[0370] In the blank according to the first configuration, the plurality of steel plates may further include a third steel plate. At least one of the first steel plate and the second steel plate, and the third steel plate may each be a plated steel plate having an aluminum-based plating layer as a plating layer on both surfaces of the base steel plate. In this case, as a process for increasing the emissivity of the surface located outside the overlap portion as compared with the emissivity of the surface of the third steel plate, for example, the adhesion amount (g / m 2 ) of the aluminum-based plating layer on both surfaces of the base steel plate in at least one of the first steel plate and the second steel plate is 60 or less, and may be less than the adhesion amount (g / m 2 ) of the aluminum-based plating layer on both surfaces of the base steel plate in the third steel plate (ninth configuration).

[0371] The blank according to the ninth configuration includes a third steel plate in addition to the first steel plate and the second steel plate. At least one of the first steel plate and the second steel plate, and the third steel plate are aluminum-based plated steel plates. The adhesion amount of the aluminum-based plating layer in at least one of the first steel plate and the second steel plate is less than the adhesion amount of the aluminum-based plating layer in the third steel plate and is 60 g / m 2 or less. By making the first steel plate and / or the second steel plate a relatively thinly plated steel plate in this way, when the blank is heated during hot stamping, the alloying of the aluminum-based plating layer and iron in the first steel plate and / or the second steel plate proceeds quickly, and both surfaces of the first steel plate and / or the second steel plate quickly change from silver-white to black or a color close thereto. Therefore, during the heating of the blank, the emissivity of the overlap portion formed by t...

Claims

1. An automobile rear module having an integrated part formed by hot stamping a plurality of integrated steel plates, The projected area when viewed from the perpendicular direction of the reference plane is S (m ２ ), Among the components of the automobile rear module, The total weight of the components is W (kg), The total weight of steel components having a plate thickness of 1.5 mm or less and a minimum Vickers hardness of HV230 or more is W Ａ When W / S is 24 or less, W Ａ / W is 0.30 or more An automobile rear module comprising:

2. Elemental technology A1; At least one of element technology B1, element technology C1, element technology C2, element technology D1, and element technology D2; Equipped with The elemental technology A1 is a skeletal member formed by hot stamping a steel plate, the skeletal member having a closed cross-section portion in which a cross-section perpendicular to a longitudinal direction is a closed cross-section, the closed cross-section portion having at least two flat portions having a radius of curvature larger than a maximum outer dimension of the cross-section, and a concave bead portion formed between the two flat portions, the concave bead portion having a pair of wall portions having a radius of curvature of 50 mm or more, the pair of wall portions protruding toward the inside of the closed cross-section portion from opposing ends of the two flat portions via a pair of bent portions that bend toward the inside of the closed cross-section, the Vickers hardness of the plate thickness center portion of the wall portions being 520 Hv or more, and the width of the wall portions being an effective width W calculated from Kármán's effective width formula. ｅ a standard deviation ratio calculated by dividing the standard deviation of the hardness frequency distribution in the surface layer portion of the wall portion by the standard deviation of the hardness frequency distribution in the thickness center portion of the wall portion is smaller than 1.0, The elemental technology B1 is a structural member, The member body is formed by a plurality of steel plates joined together, including a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than the plate thickness of the first steel plate, and has an annular shape in a plan view; A structural member, wherein a value of coefficient A calculated by the following formula (1) using a chemical composition of the first steel plate is greater than a value of coefficient A calculated by the following formula (1) using a chemical composition of the second steel plate, A=1.48×(2.7×C+0.4×Si+Mn+0.45×Ni+0.8×Cr+2×Mo) ３．４２ (1) In the formula (1), the element symbols are substituted with the contents (mass%) of the corresponding elements. The elemental technology C1 is a structural member for a vehicle body, (C1a) A vehicle body comprising a pair of side frames and a cross member connecting the side frames, the side frames and the cross member including a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate, the side frames and the cross member being formed by a plurality of steel plates joined together, the first steel plate being provided with at least one oxide selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide in an amount of 0.001 g / m ２ A coating containing the above is provided. (C1b) A vehicle body comprising a pair of side frames and a cross member connecting the side frames, the side frames and the cross member including a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate, the side frames and the cross member being formed by a plurality of steel plates joined together, and carbon black is applied to the first steel plate at a concentration of 0.500 g / m ２ A coating is provided which contains: (C1c) A vehicle body comprising a pair of side frames and a cross member connecting the side frames, wherein the side frames and the cross member are formed by a plurality of steel plates joined together, including a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate, the first steel plate and the second steel plate are each a plated steel plate having an aluminum-based plating layer on both surfaces of a base steel plate, and a thickness of the aluminum-based plating layer in the first steel plate is smaller than a thickness of the aluminum-based plating layer in the second steel plate, A structural member that satisfies at least one of the following: The elemental technology C2 is a structural member for a vehicle body, (C2a) A vehicle body comprising a pair of side frames and a cross member connecting the side frames, wherein the side frames and the cross member include a first steel plate and a second steel plate having an end portion that is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate, and are formed by a plurality of steel plates joined to each other, and each of the first steel plate and the second steel plate has a surface located outside the overlap portion, and the surface of the first steel plate and the second steel plate is coated with at least one oxide selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide in an amount of 0.001 g / m ２ A coating containing the above is provided. (C2b) A structural member comprising a pair of side frames and a cross member connecting the side frames, the structural member including a first steel plate and a second steel plate having an end portion that is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate, the structural member being formed by a plurality of steel plates joined to each other, and carbon black is added at 0.500 g / m on a surface of each of the first steel plate and the second steel plate that is positioned outside the overlap portion. ２ A coating is provided which contains: (C2c) A vehicle comprising a pair of side frames and a cross member connecting the side frames, wherein the side frames and the cross member include a first steel plate, a second steel plate having an end portion that is overlapped and joined to an end portion of the first steel plate to form an overlap portion together with the end portion of the first steel plate, and a third steel plate, and are formed by a plurality of steel plates joined to each other, wherein at least one of the first steel plate and the second steel plate and the third steel plate are plated steel plates having an aluminum-based plating layer on both surfaces of a base steel plate, and a thickness of the aluminum-based plating layer on at least one of the first steel plate and the second steel plate is smaller than a thickness of the aluminum-based plating layer on the third steel plate, A structural member that satisfies at least one of the following: The elemental technology D1 is a method in which a plurality of partial blanks made of steel plates are joined together, (D1a) At least two of the partial blanks are joined at a plurality of joints at an overlapping portion formed by partially overlapping each other, and in a cross section perpendicular to the surface of the partial blank including the center of the joint of the outermost partial blank among the partial blanks, at a position 1 / 4 of the plate thickness from the surface of the partial blank in contact with the other partial blanks, the Vickers hardness at a position 15 mm or more away from the center of the joint and not joined is Hvm. In the case of a portion of the plurality of joints, the difference ΔHv between the maximum hardness and the minimum hardness in Vickers hardness within a range of 5 mm from the end of the joint to the base material side (or within 12 mm from the center of the joint) is less than 0.2 Hvm, preferably 0.1 Hvm or less, and the other joints of the plurality of joints (joints other than the portion) have the ΔHv of 0.2 Hvm or more, preferably 0.3 Hvm, 0.4 Hvm, or 0.5 Hvm or more. (D1b) At least two of the partial blanks are joined by a plurality of spot welds at overlapping portions where the partial blanks are partially overlapped, and in a cross section including the center of the spot weld of the outermost partial blank among the partial blanks, at a position 1 / 4 of the plate thickness from the surface of the partial blank, when the hardness at a position 15 mm or more away from the center of the spot weld and at a position where the spot weld is not applied is Hvm, a portion of the plurality of spot welds has a ΔHv, which is the difference between the maximum hardness and the minimum hardness within a range of a radius of 12 mm from the center, of less than 0.2 Hvm, and the ΔHv of the spot welds other than the portion is 0.2 Hvm or more; A press-molded part that satisfies at least one of the following: The elemental technology D2 is (D2a) A press-molded part having a bent portion, in which a patchwork made of steel plate is superimposed on the surface of a base blank made of steel plate and joined at a joint, wherein the difference between the maximum hardness within 2 mm outward from the outer edge of the joint on the surface of the base blank and the hardness of the base material of the base blank is 7% or more of the hardness of the base material of the base blank, and the joint exists only in one of the regions on the surface of the base blank separated by the bent portion when there is one bent portion, and only in the region between two adjacent bent portions on the surface of the base blank when there are two or more bent portions. (D2b) A press-molded part having a bent portion, in which a patchwork made of steel plate is overlapped on the surface of a base blank made of steel plate and joined by spot welding at the joining points, wherein the difference between the maximum hardness at a position 5 mm away from the center of the joining point on the surface of the base blank and the hardness of the base material of the base blank is 7% or more of the hardness of the base material of the base blank, and the joining is present only in one of the regions on either side of the bent portion on the surface of the base blank when there is one bent portion, and only in the region between two adjacent bent portions on the surface of the base blank when there are two or more bent portions. It is a press molded part that satisfies at least one of the following:

2. The vehicle rear module according to claim 1.

3. The elemental technology A1, The elemental technology B1, 3. The vehicle rear module according to claim 2, further comprising:

4. The elemental technology A1, At least one of the elemental technology C1 and the elemental technology C2; 3. The vehicle rear module according to claim 2, further comprising:

5. The elemental technology A1, At least one of the elemental technology D1 and the elemental technology D2; 3. The vehicle rear module according to claim 2, further comprising:

6. The elemental technology A1, The elemental technology B1, At least one of the elemental technology C1 and the elemental technology C2; At least one of the elemental technology D1 and the elemental technology D2; 3. The vehicle rear module according to claim 2, further comprising:

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