Blank, manufacturing method of structural member, and structural member

Abstract

A blank (20, 20A) for hot stamping is provided with a plurality of steel sheets. The plurality of steel sheets are arranged and joined with one another so as to form a ring shape when seen in a plan view of the blank (20, 20A). The plurality of steel sheets include a first steel sheet (21) and a second steel sheet (22). The first steel sheet (21) has the lowest sheet thickness (tmin) among the plurality of steel sheets. The second steel sheet (22) has a sheet thickness which is greater than the sheet thickness (tmin) of the first steel sheet (21). The first steel sheet (21) is configured in a manner such that the emissivity of at least one surface of the two surfaces thereof is greater than the emissivity of the two surfaces of the second steel sheet (22).

Description

[Technical field]

[0001] The present disclosure relates to a blank for hot stamping. The present disclosure also relates to a method for manufacturing a structural member using the blank, and to the structural member. [Background technology]

[0002] Structures such as the body of an automobile are formed by a plurality of structural members. The structural members are manufactured, for example, by press forming a blank. In order to ensure high strength and good dimensional accuracy, the structural members are sometimes manufactured by a press forming method called hot stamping. Hot stamping is a technique in which a blank, which is a steel plate, is heated to a temperature in the austenite range, and then the blank is press formed using a die, and the blank is held in the die and quenched by removing heat (rapid cooling).

[0003] Patent Document 1 discloses a steel sheet (blank) for hot stamping. The steel sheet of Patent Document 1 has a surface treatment film having an emissivity of 60% or more at a wavelength of 8.0 μm at 25° C., covering the entire surface of at least one surface. According to Patent Document 1, the surface of the steel sheet to which the surface treatment film is applied has an increased emissivity, and the heat transfer effect by radiation is large. Therefore, when the steel sheet is heated during hot stamping, the steel sheet is heated to the A temperature at which the metal structure transforms into the austenite phase. c3 The temperature is quickly raised to a temperature equal to or higher than the heating point. Patent Document 1 describes that this makes it possible to shorten the heating time and improve the productivity of hot stamped members.

[0004] Patent Document 2 discloses a method for manufacturing an automobile body side structural frame from a plurality of blanks. In Patent Document 2, a composite blank is formed by joining a plurality of blanks, and the composite blank is press-formed to manufacture the body side structural frame. Patent Document 2 also describes hot forming (hot stamping) the composite blank. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2022 / 215229 [Patent Document 2] Special Publication No. 2021-528248 Summary of the Invention [Problem to be solved by the invention]

[0006] In recent years, in order to simplify the manufacturing process of structures, it has been considered to integrate two or more members from the blank stage. Patent Document 2, for example, discloses that a composite blank having an annular shape in a plan view is subjected to hot stamping to form an annular body side structural frame in which pillars, rockers, etc. are integrated. However, if the composite blank contains steel plates with different plate thicknesses, the performance of the formed structural member may be reduced. Specifically, in hot stamping, the blank is heated, for example, in a heating furnace until its microstructure is austenitized, and then formed by a die. However, since a steel plate with a small plate thickness is more easily cooled than a steel plate with a large plate thickness, in the portion of the blank where the steel plate with a small plate thickness is arranged, transformation to ferrite may begin before the blank is removed from the heating furnace and forming is started, and the hardenability may be deteriorated. If the hardenability of the blank is partially deteriorated, the structural member formed from the blank may have non-uniform hardness, which may reduce its impact absorption performance. In addition, if the hardenability is partially deteriorated, non-uniform stress may be caused in the structural member. This can lead to twisting, particularly in the case of annular structural members, and can result in poor dimensional accuracy. The deterioration of impact absorption performance (crash resistance) due to poor hardenability or dimensional accuracy becomes more pronounced the larger the annular structural member becomes.

[0007] An object of the present disclosure is to provide a hot stamping blank that can improve the performance of an annular structural component, particularly a large and annular structural component, when the component is formed, the blank including a steel plate having a smaller plate thickness compared to other steel plates. [Means for solving the problem]

[0008] A blank for hot stamping according to the present disclosure includes a plurality of steel plates. The plurality of steel plates are arranged and joined 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 a 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. The first steel plate is configured such that the emissivity of at least one of both surfaces of the first steel plate is greater than the emissivity of both surfaces of the second steel plate. Effect of the Invention

[0009] According to the hot stamping blank of the present disclosure, when annular structural components, particularly large and annular structural components, containing a steel plate having a smaller plate thickness compared to other steel plates are formed, the performance of the components can be improved. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a plan view of a structural member according to a first embodiment. [Diagram 2] FIG. 2 is a cross-sectional view taken along line II-II of FIG. [Figure 3A] FIG. 3A is a schematic diagram for explaining the method for manufacturing a structural member according to the first embodiment, and is a diagram showing a blank according to the first embodiment. [Figure 3B] FIG. 3B is a schematic diagram for explaining the method for manufacturing a structural member according to the first embodiment, and is a diagram showing a blank according to the first embodiment. [Figure 3C] FIG. 3C is a schematic diagram for explaining the method for manufacturing a structural member according to the first embodiment, and is a diagram showing a blank according to the first embodiment. [Figure 3D] FIG. 3D is a schematic diagram for explaining the method for manufacturing a structural member according to the first embodiment, and is a diagram showing a blank according to the first embodiment. [Figure 3E]FIG. 3E is a schematic view for explaining the method for manufacturing a structural member according to the first embodiment. [Figure 3F] FIG. 3F is a schematic view for explaining the method for manufacturing a structural member according to the first embodiment. [Figure 3G] FIG. 3G is a schematic view for explaining the method for manufacturing a structural member according to the first embodiment. [Figure 4] FIG. 4 is a cross-sectional view of a structural member manufactured by the manufacturing method according to the first embodiment. [Diagram 5] FIG. 5 is a plan view of a blank according to the second embodiment. [Figure 6] FIG. 6 is a cross-sectional view taken along line VI-VI of FIG. [Figure 7] FIG. 7 is a plan view of a structural member according to the second embodiment. [Figure 8] FIG. 8 is a plan view of a structural member according to a modified example of each embodiment. [Figure 9A] FIG. 9A is a diagram showing a division pattern of a structural member in the first embodiment. [Figure 9B] FIG. 9B is a diagram showing another division pattern of the structural member in the first embodiment. [Figure 9C] FIG. 9C is a diagram showing still another division pattern of the structural member in the first embodiment. [Figure 9D] FIG. 9D is a diagram showing still another division pattern of the structural member in the first embodiment. [Figure 9E] FIG. 9E is a diagram showing still another division pattern of the structural member in the first embodiment. [Figure 9F] FIG. 9F is a diagram showing still another division pattern of the structural member in the first embodiment. [Figure 9G] FIG. 9G is a diagram showing still another division pattern of the structural member in the first embodiment. [Figure 10A] FIG. 10A is a diagram showing a division pattern of a structural member in the second embodiment. [Figure 10B] FIG. 10B is a diagram showing another division pattern of the structural member in the second embodiment. [Figure 10C] FIG. 10C is a diagram showing still another division pattern of the structural member in the second embodiment. [Figure 10D] FIG. 10D is a diagram showing still another division pattern of the structural member in the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] A blank for hot stamping according to an embodiment includes a plurality of steel plates. The plurality of steel plates are arranged and joined 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 a 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. The first steel plate is configured such that the emissivity of at least one of both surfaces of the first steel plate is greater than the emissivity of both surfaces of the second steel plate (first configuration).

[0012] The blank according to the first configuration includes a first steel plate having a minimum thickness and a second steel plate having a thickness greater than that of the first steel plate. The emissivity of at least one surface of the first steel plate is greater than the emissivity of both surfaces of the second steel plate. This allows the heating rate of the first steel plate, which is the thin-walled portion, to be increased when the blank is heated during hot stamping. Therefore, the first steel plate can be heated to the austenite region temperature more quickly, and the holding time of the first steel plate at that temperature can be secured longer. Thus, the austenite grains in the first steel plate can be coarsened. As a result, the ferrite transformation region (ferrite nose) in the CCT diagram shifts to the long-time side, so that after the heating of the blank is completed, the start of the transformation to ferrite in the first steel plate can be delayed, and the blank can be formed while maintaining the microstructure of the first steel plate in the austenite phase. That is, the hardenability of the first steel plate having a small thickness can be improved.

[0013] In the blank according to the first configuration, the hardenability of the first steel plate having a small plate thickness is improved, so that the first steel plate can be well hardened when the blank is formed into a structural member by hot stamping. Therefore, the hardness of the structural member is easily uniformed, and partial reduction in strength of the structural member can be suppressed. In addition, since unevenness in stress is unlikely to occur in the structural member, even if the formed structural member is annular, twisting is unlikely to occur, and good dimensional accuracy can be ensured in the structural member. Therefore, when an annular structural member, particularly a large and annular structural member, including the first steel plate having a plate thickness smaller than that of the second steel plate is formed from the blank, it is possible to reduce strength defects and dimensional accuracy defects of the structural member, and improve the impact absorption performance (crash resistance performance) of the structural member.

[0014] In the blank according to the first configuration, the emissivity of the first steel plate with the minimum thickness is greater than that of the second steel plate with a relatively large thickness. In this case, the first steel plate heats up faster than the second steel plate, and 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, is longer than when the first steel plate has the same emissivity as the second steel plate. This reduces the non-uniformity of phase transformation caused by the difference in cooling rate between the steel plates after the heating of the blank is completed. Specifically, the start of the phase transformation from austenite to ferrite for the first steel plate with the minimum thickness can be delayed, and the difference in the start time of phase transformation between the first steel plate with the minimum thickness and the other steel plates is reduced. As a result, the hardenability can be made uniform between the first steel plate with the minimum thickness and the other steel plates.

[0015] In the first configuration, the first steel plate may have a plate thickness of less than 1.4 mm (second configuration).

[0016] When the thickness of the first steel plate is less than 1.4 mm as in the second configuration, the first steel plate is particularly susceptible to heat dissipation after the heating of the blank is completed, and the hardenability of the first steel plate is more likely to deteriorate. However, even if the thickness of the first steel plate is less than 1.4 mm, by making the emissivity of at least one surface of the first steel plate higher than that of the relatively thick second steel plate, it is possible to promote the temperature rise of the first steel plate when the blank is heated during hot stamping, and to ensure a long high-temperature holding time for the first steel plate. Therefore, the hardenability of the first steel plate can be improved.

[0017] In the first or second configuration, the first steel sheet may be a plated steel sheet. In this case, the first steel sheet may have a base steel sheet and an aluminum-based plating layer provided on the base steel sheet (third configuration).

[0018] In the case where the first steel sheet is a plated steel sheet having an aluminum-based plating layer as in the third configuration, the temperature rise rate of the first steel sheet tends to be slow when the blank is heated during hot stamping. The aluminum-based plating layer is close to white, so it tends to reflect heat energy and inhibits the temperature rise of the first steel sheet. However, even if the first steel sheet is a plated steel sheet having an aluminum-based plating layer, by making the emissivity of at least one surface of the first steel sheet higher than that of the second steel sheet having a relatively large thickness, the temperature rise of the first steel sheet can be promoted when the blank is heated during hot stamping. Therefore, the high temperature retention time of the first steel sheet can be secured for a long time, and the hardenability of the first steel sheet can be improved.

[0019] In any one of the first to third configurations, at least one surface of the first steel plate may be covered with a coating having an emissivity of 60% or more at a wavelength of 8.0 μm at 25° C. (fourth configuration).

[0020] In any one of the first to third configurations, at least one surface of the first steel plate may be coated with a coating, which is a coating containing carbon black, one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide, and a 0 to 0.30 g / m 2 The content of carbon black in the coating can be X. CB (g / m 2 ), the oxide content is X Oxide (g / m 2 ), then X CB and X Oxide It is preferable that the following formula (1) (see Patent Document 1) is satisfied (fifth configuration). 118.9≦24280 / {6700 / (100+76×X CB )+18000 / (130+65×X Oxide )}≦332.0 (1)

[0021] In any one of the first to fifth configurations, the first steel sheet may be a plated steel sheet having a base steel sheet and a plating layer provided on the base steel sheet. In this case, the sheet thickness of the first steel sheet is t min The thickness of the steel plate with the largest thickness among the multiple steel plates is t max When you set it to 1.0, <t max / t min It is preferable that the ratio is ≦3.2 (sixth configuration).

[0022] Among the multiple steel plates included in the blank, the smallest plate thickness t min The first steel plate having a maximum plate thickness t max If the difference in thickness between the steel plate and other steel plates having the minimum thickness t is large, it becomes difficult to ensure the process window in the manufacture of structural components. min and maximum plate thickness t max If the difference between the maximum thickness and the maximum thickness t is large, the blank is heated during hot stamping. maxWhile waiting for the steel sheet having the minimum sheet thickness t to reach the austenite temperature, alloying of the coating layer of the first steel sheet, which has been heated to the austenite temperature in advance, may progress, causing the diffusion layer to become thicker and making it impossible to ensure the corrosion resistance or weldability of the coating layer of the first steel sheet. min Maximum plate thickness t max The ratio of the maximum plate thickness t max Heating rate and minimum plate thickness t of steel plate min Since the heating rate of the first steel sheet having the above-mentioned characteristic does not deviate too much from that of the first steel sheet having the above-mentioned characteristic, the heating of the other steel sheet can be completed before the alloying of the coating layer of the first steel sheet progresses excessively. Therefore, a structural component can be manufactured while maintaining the corrosion resistance or weldability of the first steel sheet, and a process window in the manufacture of the structural component can be secured.

[0023] In any one of the first to sixth configurations, the first steel sheet may be a plated steel sheet having a base steel sheet and a plating layer provided on the base steel sheet. The blank may further include an overlapping portion. The overlapping portion is formed by overlapping ends of two adjacent steel sheets among the plurality of steel sheets, other than the second steel sheet. The overlapping portion may have a total sheet thickness of more than 2.5 mm and not more than 4.0 mm. In this case, it is preferable that the two steel sheets are each configured so that the emissivity of the surface located outside the overlapping portion is greater than the emissivity of both surfaces of the second steel sheet (seventh configuration).

[0024] When there is an overlapping portion formed by overlapping the ends of two steel sheets in a blank, it may not be possible to ensure the process window in the manufacture of a structural member. Specifically, when the blank contains a plurality of steel sheets with different thicknesses and the total thickness of the overlapping portion exceeds 2.5 mm, when the blank is heated during hot stamping, while waiting for the overlapping portion to reach the temperature in the austenite region, alloying of the plating layer of the first steel sheet having the smallest thickness progresses, and the diffusion layer becomes thick, so that the corrosion resistance or weldability of the first steel sheet due to the plating layer may not be ensured. Therefore, in the seventh configuration, the emissivity of the outer surface of the overlapping portion is increased in each of the two steel sheets forming the overlapping portion. This can promote the temperature rise of the overlapping portion, so that the heating of the overlapping portion can be completed before the alloying of the plating layer of the first steel sheet progresses excessively, and a structural member can be manufactured while maintaining the corrosion resistance or weldability of the first steel sheet. That is, the process window in the manufacture of a structural member is easily ensured. However, even if the emissivity of the overlapping portion is increased, if the total plate thickness of the overlapping portion becomes excessively large, it becomes difficult to ensure the process window, so the total plate thickness of the overlapping portion is preferably 4.0 mm or less.

[0025] A manufacturing method for a structural component according to an embodiment includes the steps of preparing a blank according to any one of the first to seventh configurations, heating a plurality of steel plates contained in the blank to a temperature equal to or higher than the austenite transformation completion temperature, and using a die to form the heated blank into a structural component that is annular in plan view and quenching it (eighth configuration).

[0026] A structural member according to an embodiment includes a member body and a coating. The member body is formed by a plurality of steel plates joined together and has an annular shape in a plan view. The plurality of steel plates include 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 coating is provided on the first steel plate. The coating contains at least one oxide selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide at a concentration of 0.001 g / m 2Contains the above (ninth aspect).

[0027] A structural member according to an embodiment includes a member body and a coating. The member body is formed by a plurality of steel plates joined together and has an annular shape in a plan view. The plurality of steel plates include 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 coating is provided on the first steel plate. The coating contains carbon black at 0.500 g / m 2 Contains the following (tenth configuration).

[0028] In the ninth or tenth configuration, the structural member may be a door ring part of an automobile. In this case, the member body may include a front pillar, a center pillar, and a rocker connecting the front pillar and the center pillar (eleventh configuration).

[0029] Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. In each drawing, the same or corresponding components are denoted by the same reference numerals, and the same description will not be repeated.

[0030] First Embodiment [Structural Members] 1 is a diagram (plan view) of a structural member 10 according to this embodiment, viewed from above in a state in which the structural member 10 is placed on a horizontal surface. The structural member 10 is used, for example, in the body of an automobile. The structural member 10 is typically a door ring part of an automobile. In this embodiment, an example in which the structural member 10 is a door ring part will be described.

[0031] The structural member 10 is a hot stamped member. That is, the structural member 10 is formed by hot stamping (hot press processing) a blank made of a plurality of steel plates. The structural member 10 includes a member body 11. The member body 11 has an annular shape in a plan view of the structural member 10. The member body 11 includes a front pillar 111, a center pillar 112, and a rocker 113. When the structural member 10 is assembled to the body of an automobile, the center pillar 112 is disposed behind the front pillar 111. The center pillar 112 extends generally in the up-down direction of the vehicle body. The front pillar 111 extends toward the center pillar 112. When the structural member 10 is assembled to the body of an automobile, the rocker 113 is disposed below the front pillar 111 and the center pillar 112. The rocker 113 connects the front pillar 111 and the center pillar 112.

[0032] In this embodiment, the member body 11 is formed of a plurality of steel plates 21, 22, and 23 joined together. In the example of FIG. 1, the front pillar 111 is mainly formed of the steel plates 21 and 22. The center pillar 112 is mainly formed of the steel plate 23. The rocker 113 is formed of the steel plates 21 and 23.

[0033] FIG. 2 is a cross-sectional view taken along the line II-II in FIG. 1. FIG. 2 shows a cross section of the structural member 10 cut along the thickness direction at the position of the steel plate 21. As shown in FIG. 2, the steel plate 21 has an open cross section. The steel plate 21 has, for example, a roughly hat-shaped cross section in the cross section of the structural member 10. More specifically, the steel plate 21 includes a top plate 211, vertical walls 212 and 213, and flanges 214 and 215. The vertical wall 212 is disposed on the opposite side of the vertical wall 213 with respect to the top plate 211. In the cross section of the structural member 10, one ends of the vertical walls 212 and 213 are connected by the top plate 211. In the cross section of the structural member 10, the other ends of the vertical walls 212 and 213 are connected to flanges 214 and 215, respectively. The flanges 214 and 215 protrude from the vertical walls 212 and 213 to the outside of the structural member 10.

[0034] In the structural member 10, the width W of the steel plate 21 may be 30 mm or more and 750 mm or less. The height H of the steel plate 21 may be 25 mm or more and 150 mm or less. The width W is the distance from the end of the R on the vertical wall 212 side of the corner portion between the top plate 211 and the vertical wall 212 to the end of the R on the vertical wall 213 side of the corner portion between the top plate 211 and the vertical wall 213 in the cross section of the structural member 10. The height H is the distance from the top plate 211 to the flanges 214, 215 in the plate thickness direction of the top plate 211.

[0035] Although not shown in the figures, the other steel plates 22, 23 also have an open cross section like the steel plate 21. The steel plates 22, 23 can also have, for example, a roughly hat shape in a cross-sectional view of the structural member 10. The width of the steel plates 22, 23 may be 15 mm or more and 300 mm or less. The height of the steel plates 22, 23 may be 10 mm or more and 150 mm or less.

[0036] The size of the structural member 10, which is annular in plan view, is, for example, 1.0 m or more. The size of the structural member 10 may be, for example, 4.0 m or less. The size of the structural member 10 is the length of a line segment connecting any two points on the outer periphery of the structural member 10 that are farthest apart when the structural member 10 is placed on a horizontal surface and viewed in the vertical direction.

[0037] [Manufacturing methods for structural components] 3A to 3G, a method for manufacturing the structural member 10 will be described below. The method for manufacturing the structural member 10 according to this embodiment includes the steps of preparing a blank 20, heating the blank 20, and forming the heated blank 20 into the structural member 10.

[0038] (preparation process) 3A, in the preparation step, a blank 20 having a shape obtained by developing the structural member 10 is prepared. The blank 20 includes a plurality of steel plates 21, 22, and 23. The steel plates 21, 22, and 23 are arranged and joined so as to have an annular shape in a plan view of the blank 20.

[0039] 3B, 3C, and 3D are cross-sectional views of the blank 20 showing the joints of the steel plates 21, 22, and 23. FIG. 3B, FIG. 3C, and FIG. 3D are cross-sectional views taken along lines IIIB-IIIB, IIIC-IIIC, and IIID-IIID in FIG. 3A, respectively. Referring to FIG. 3B and FIG. 3C, the steel plate 21 is butt-joined to each of the steel plates 22 and 23. That is, the end faces of the steel plate 21 are joined in a state where the end face of the steel plate 21 is in contact with the end face of the steel plate 22, and the other end face of the steel plate 21 is joined in a state where the end faces of the steel plate 23 are joined. Referring to FIG. 3D, the steel plate 22 is butt-joined to the steel plate 23 in addition to the steel plate 21. The end face of the steel plate 22 is joined to the end face of the steel plate 23 in a state where the end face of the steel plate 22 is in contact with the end face of the steel plate 23. The steel plates 21, 22, and 23 are joined by, for example, laser welding. In this embodiment, the blank 20 is a so-called tailor welded blank.

[0040] 3B and 3C, among the steel plates 21, 22, and 23, the steel plate 21 has a minimum plate thickness t min The steel plate 22 has a thickness t min The steel plate 23 has a plate thickness t min In this embodiment, the thickness of each of the steel plates 22 and 23 is equal to or greater than the thickness t min In this embodiment, the steel plate 23 has the largest plate thickness t max The steel plate 22 has a thickness t min is larger than the thickness t of steel plate 23. max Plate thickness t smaller than mid However, the steel plate 22 may have a thickness equal to or greater than that of the steel plate 23. That is, among the steel plates 21, 22, and 23, the steel plate 22 has the maximum plate thickness t max It is also possible to have

[0041] Plate thickness t of steel plate 21 min is typically less than 1.4 mm. min The thickness t of the steel plate 21 may be, for example, 0.8 mm or more. min and the thickness t of the steel plate 23 maxis 1.0 <t max / t min ≦3.2, and 1.3≦t max / t min It is more preferable that the ratio satisfies ≦3.2.

[0042] Minimum thickness t in blank 20 min The steel plate 21 having the above structure is configured so that the emissivity of at least one of its two surfaces is greater than the emissivity of both surfaces of the steel plate 22. For example, the emissivity at a wavelength of 8.0 μm at 25° C. is 60% or more at one or both surfaces of the steel plate 21, and less than 60% at both surfaces of the steel plate 22. The emissivity at a wavelength of 8.0 μm at 25° C. of one or both surfaces of the steel plate 21 is more preferably 70% or more, and even more preferably 80% or more. min The difference in emissivity at a wavelength of 8.0 μm at 25° C. between the steel plate 21 having the above-mentioned characteristic and the other steel plate 22 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 taken from the steel plate to be measured is set in a Fourier transform infrared spectrophotometer, and the radiation intensity at a wavelength of 8.0 μm is measured at 25° C. to calculate the emissivity. Alternatively, it is also possible to measure the radiation intensity of a target portion at 25° C. using a radiation thermometer set to a measurement wavelength of 8.0 μm, and calculate the emissivity from the ratio to the radiation intensity of a black body.

[0043] In this embodiment, one surface of the steel plate 21 is covered with a coating 26. On the other hand, the steel plate 22 is not provided with a coating 26. As a result, the emissivity of the one surface of the steel plate 21 is higher than the emissivity of both surfaces of the steel plate 22.

[0044] The coating 26 is, for example, a black coating. For example, the lightness L * Value (CIE 1976 Lightness Index L as specified in JIS Z8781-4 (2013) *) is 60 or less, the coating 26 can be determined to be black. The coating 26 may be a carbon-based surface treatment coating (a coating containing carbon (C)). The emissivity of the coating 26 at a wavelength of 8.0 μm at 25° C. is 60% or more, preferably 70% or more, and more preferably 80% or more. That is, the emissivity of the surface of the steel sheet 21 to which the coating 26 is applied at a wavelength of 8.0 μm at 25° C. is 60% or more, preferably 70% or more, and more preferably 80% or more. The coating 26 may have an emissivity of 60% or more at a wavelength of 8.0 μm at 700° C. As the coating 26, for example, the surface treatment coating described in Patent Document 1 can be used. Specifically, the coating 26 can contain carbon black and one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide. The coating 26 may or may not contain silica. That is, the silica content of the coating 26 is 0 g / m 2 The silica content of the coating 26 is 0.30 g / m 2 The silica content may be 0.10 g / m or less. 2 or less, more preferably 0.05 g / m 2 The following is the result.

[0045] The carbon black and the oxide can be present dispersedly throughout the surface of the coating 26 perpendicular to the thickness direction of the steel sheet 21. CB (g / m 2 ), Zr oxide, Zn oxide, and Ti oxide, and the content of one or more oxides (metal oxides) selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide is X Oxide (g / m 2 ), then X CB and X Oxide It is preferable that the following formula (1) is satisfied. 118.9≦24280 / {6700 / (100+76×X CB )+18000 / (130+65×X Oxide )}≦332.0 (1)

[0046] In formula (1), the middle formula is 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 middle formula is preferably 330.0 or less, more preferably 310.0 or less, and even more preferably 300.0 or less.

[0047] The dispersion state of the carbon black and the metal oxide in the coating 26 can be confirmed by performing a surface analysis of the coating 26 for elements derived from the carbon black (e.g., C) and elements derived from the oxide (Zr, Zn, and Ti) using an electron probe micro analyzer (EPMA). CB can be measured by a cross-sectional analysis of the coating 26 using a transmission electron microscope (TEM). That is, a cross-sectional analysis of the coating 26 is performed by TEM-EDS analysis in an area of ​​a predetermined size (thickness of the coating 26 × 5 μm), and the thickness of the coating 26 and the area ratio of particles with a carbon content of 70 mass % or more in that area are measured. The density of carbon black is expressed as ρ (ton / m 3 ), the film thickness is d (μm), and the area ratio is a (%). The value expressed by ρ×d×a is the carbon black content X CB (g / m 2 ) The oxide content X Oxide can be obtained by performing elemental analysis on the surface of the coating 26 using an X-ray fluorescence analyzer (ZSX Primus, manufactured by RIGAKU Corporation) and quantifying the amounts of metallic Zr, metallic Zn, and metallic Ti.

[0048] Carbon black content X in coating 26 CB is 0.030g / m 2 It is preferable that the content is 0.100 g / m or more. 2 More preferably, the content X is equal to or greater than the above. CBis set within a range that satisfies formula (1), but is preferably 0.800 g / m 2 or less, more preferably 0.600 g / m 2 The following is the result.

[0049] Coating 26 may contain 5.0% or more of carbon black by volume, and preferably 8.0% or more of carbon black by volume. Coating 26 may contain 40.0% or less of carbon black by volume, and preferably 30.0% or less of carbon black by volume.

[0050] Metal oxide content X in coating 26 Oxide is 0.030g / m 2 It is preferable that the content is 0.060 g / m or more. 2 It is more preferable that the content is equal to or more than X. Oxide is set within a range that satisfies formula (1), but is preferably 0.500 g / m 2 More preferably, it is 0.300 g / m or less. 2 The following is the result.

[0051] Coating 26 may contain 1.0 or more volume percent of metal oxide. Coating 26 may also contain 30.0 or less volume percent of metal oxide, and preferably contains 25.0 or less volume percent of metal oxide.

[0052] Carbon black content X CB (g / m 2 ) and metal oxide content X Oxide (g / m 2 ) ratio: X Oxide / X CB is preferably 0.20 or more and 200.00 or less. Oxide / X CB is more preferably 0.40 or more and 10.00 or less, and further preferably 0.60 or more and 5.00 or less.

[0053] In addition to the carbon black and metal oxides described above, the coating 26 may contain various binder components and additives.

[0054] 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 with respect to the total volume of the film 26. As the binder component selected from the water-dispersible or water-soluble resins, various known resins exhibiting water dispersibility or water solubility can be used. Examples of such resins exhibiting water dispersibility or water solubility include polyurethane resins, polyester resins, acrylic resins, epoxy resins, fluororesins, polyamide resins, polyolefin resins, and polymer compounds obtained by hydrolysis and condensation polymerization of silane coupling agents. It is more preferable that the binder component is one or more selected from the group consisting of polyester resins, polyurethane resins, polyolefin resins, acrylic resins, epoxy resins, fluororesins, and polyamide resins. When a polyurethane resin is used as the binder component, the polyurethane resin is preferably a polyether-based polyurethane resin.

[0055] The additives include, for example, a leveling agent, a water-soluble solvent, a metal stabilizer, and an etching inhibitor. The leveling agent is, for example, a nonionic or cationic surfactant. Examples of the nonionic or cationic surfactant include polyethylene oxide or polypropylene oxide adducts and acetylene glycol compounds. Examples of the water-soluble solvent 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 the metal stabilizer include chelate compounds such as EDTA (ethylenediaminetetraacetic acid) and DTPA (diethylenetriaminepentaacetic acid). Examples of the etching inhibitor include amine compounds such as ethylenediamine, triethylenepentamine, guanidine, and pyrimidine.

[0056] The coating 26 can be formed by applying an organic or inorganic treatment liquid containing, for example, carbon black and a metal oxide to the entire surface of the steel sheet 21, and then drying the volatile components in the treatment liquid. The treatment liquid can be applied to the surface of the steel sheet 21 by, for example, a roll coater, a curtain coater, or an inkjet. In the case of an inkjet, the thickness of the coating 26 can also be changed continuously. The thickness of the coating 26 is, for example, 0.5 μm or more and 5.0 μm or less. The thickness of the coating 26 is preferably 1.0 μm or more and 3.0 μm or less. The thickness of the coating 26 is determined based on the sheet thickness t of the steel sheet 21. min Therefore, the thickness of the steel plate 21 measured including the coating 26 is set to the thickness t min It can be treated as.

[0057] The steel sheet 21 may be a plated steel sheet. In this case, the steel sheet 21 has a base steel sheet 216 and a plated layer 217. The type of the base steel sheet 216 is not particularly limited. The plated layer 217 is provided on the base steel sheet 216. The plated layer 217 covers the entire or almost the entire both sides of the base steel sheet 216. The plated layer 217 is a metal plated layer. The plated layer 217 may be, for example, hot-dip aluminum plating, hot-dip galvanizing, alloyed hot-dip galvanizing, or electrolytic galvanizing. As the steel sheet 21, a known aluminum-plated steel sheet, galvanized steel sheet, or the like can be used.

[0058] The plating layer 217 is typically a plating layer containing aluminum as a main component (aluminum-based plating layer). The configuration of the aluminum-based plating layer is not particularly limited. A known aluminum-based plating layer can be adopted as the plating layer 217. When the steel sheet 21 is a plated steel sheet, the sheet thickness t min is the combined thickness of the base steel sheet 216 and the plating layer 217.

[0059] The steel sheets 22 and 23 may be known plated steel sheets, similar to the steel sheet 21. The steel sheets 22 and 23 may be aluminum plated steel sheets or zinc plated steel sheets. The steel sheets 22 and 23 may be the same type of plated steel sheet as the steel sheet 21, or may be a plated steel sheet of a different type from the steel sheet 21. Furthermore, the steel sheet 22 may be the same type of plated steel sheet as the steel sheet 23, or may be a plated steel sheet of a different type from the steel sheet 23. When the steel sheet 22 is a plated steel sheet, the sheet thickness t of the steel sheet 22 may be 0.01 to 0.01 mm. mid Similarly, when the steel sheet 23 is a plated steel sheet, the sheet thickness t max is the combined thickness of the base steel sheet and the plating layer. When two or more of the steel sheets 21, 22, and 23 are plated steel sheets, the coating weight of each steel sheet may be the same as or different from the other steel sheets.

[0060] (Heating process) The prepared blank 20 is formed into a structural member 10 (FIGS. 1 and 2) by hot stamping. During the hot stamping, the blank 20 is subjected to a heating step. Referring to FIG. 3E, in the heating step, the blank 20 is heated by, for example, a heating furnace 30. The plurality of steel plates 21, 22, and 23 included in the blank 20 are heated to an austenite transformation completion temperature (A c3 The steel plates 21, 22, 23 are heated to, for example, 900° C. or higher. This causes the microstructures of the steel plates 21, 22, 23 to be transformed, for example, entirely or almost entirely into an austenite phase.

[0061] (molding process) Referring to Fig. 3F, in the forming process, the heated blank 20 is formed into a structural member 10 (Figs. 1 and 2) having an annular shape in plan view using a die 40 and quenched. The blank 20 heated in the heating process is removed from the heating furnace 30 (Fig. 3E) and transported to the die 40. The die 40 is attached to a known pressing device. The die 40 includes, for example, a punch 41 and a die 42. The blank 20 is disposed between the punch 41 and the die 42.

[0062] 3G, after the blank 20 is placed between the punch 41 and the die 42, the die 42 approaches relatively to the punch 41. The blank 20 is clamped (pressed) by the punch 41 and the die 42, and formed into a shape along the forming surfaces of the punch 41 and the die 42. The blank 20 is held while being clamped by the punch 41 and the die 42. The blank 20 is cooled (quenched) by the die 40, and its microstructure is transformed into martensite. In this way, the structural member 10 can be manufactured from the blank 20.

[0063] FIG. 4 is a cross-sectional view of the structural member 10 after hot stamping. FIG. 4 shows a cross-section of the structural member 10 at the position of the steel plate 21 (FIGS. 3B and 3C) to which the black coating 26 was applied at the blank 20 stage. This structural member 10 includes a member body 11 and a coating 12. The coating 12 is provided on the steel plate 21. The black coating 26 (FIGS. 3B and 3C) applied to the steel plate 21 in the blank 20 becomes the coating 12 through hot stamping. If the coating 26 contains carbon black, this carbon black is almost completely lost due to high-temperature heating during hot stamping, but may remain on the member body 11. If the coating 26 before hot stamping satisfies the above formula (1), the coating 12 after hot stamping may not contain carbon black, or may have a carbon black content of 0.500 g / m 2 In some cases, the coating 12 after hot stamping contains carbon black. In the case where the coating 12 contains carbon black, the carbon black content in the coating 12 is 0 g / m 2 In addition, when the coating 26 before hot stamping satisfies the above formula (1), the coating 12 after hot stamping satisfies the central formula: 24280 / {6700 / (100+76×X CB )+18000 / (130+65×X Oxide )} is, for example, between 120.0 and 150.0.

[0064] When the coating 26 (FIGS. 3B and 3C) before hot stamping satisfies the above formula (1), the coating 12 after hot stamping contains one or more oxides (metal oxides) selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide at a concentration of, for example, 0 g / m 2 More preferably, 0.001 g / m 2 The metal oxide content of the coating 12 is, for example, 0.500 g / m 2 In this way, when the metal oxide remains on the structural member 10, that is, when the coating 12 is 0 g / m 2 In the case where the coating 26 before hot stamping satisfies the above formula (1), the coating 12 after hot stamping has a metal oxide content of 0 to 0.30 g / m or more, which is more preferable because it improves the corrosion resistance of the structural member 10.2 Contains silica.

[0065] The carbon black content, metal oxide content, and silica content in the coating 12 can be measured in the same manner as the coating 26 at the blank 20 stage. Specifically, the vehicle body part is disassembled to obtain a ring-shaped structural member 10, and an analysis sample is obtained from this structural member 10, for example, by laser cutting or the like. For example, an analysis sample is obtained from each of the multiple steel plates included in the structural member 10. The analysis sample is obtained from the center or its 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 the outside of the heat-affected zone during laser cutting, etc., to prepare a sample for coating analysis. The surface analysis of the coating 12 for elements derived from carbon black (e.g., C) and elements derived from oxides (Zr, Zn, and Ti) is performed on this sample using an EPMA, and the dispersion state of the carbon black and metal oxide in the coating 12 can be confirmed.

[0066] In many cases, the outermost layer of the structural member 10 is, for example, an electrodeposition coating film. In that case, the coating layer that is below the electrodeposition coating film layer and above the alloyed metal plating layer is analyzed. The carbon black content X of the coating 12 is CB can be measured by a cross-sectional analysis of the coating 26 using a TEM. That is, a cross-sectional analysis of the coating 12 is performed by TEM-EDS analysis in an area of ​​a predetermined size (thickness of the coating 12 × 5 μm), and the thickness of the coating 12 and the area ratio of particles with a carbon content of 70 mass % or more in that area are measured. The density of carbon black is expressed as ρ (ton / m 3 ), the film thickness is d (μm), and the area ratio is a (%). The value expressed 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 of the coating layer that is located below the electrodeposition coating layer and above the alloyed metal plating layer using the above-mentioned X-ray fluorescence analyzer, and quantifying the amounts of metallic Zr, metallic Zn, and metallic Ti.

[0067] Minimum plate thickness t min In the cross section of the structural member 10 at the position of the steel plate 21 having the minimum martensite fraction (%), the martensite fraction variation is, for example, 20% or less. The martensite fraction variation is preferably 15% or less, and more preferably 10% or less. The martensite fraction variation can be measured as follows. That is, when the minimum plate thickness t min Ten or more analysis samples (for example, a size of about 10 mm on the long side) are cut out from positions 20 mm or more away from the end and 10 mm or more away from each other in the cross section of the structural member 10 at the position of the steel plate 21 having the above structure, and then each is mirror-polished and etched with LePeller's reagent so that the plate thickness direction becomes the observation surface. Then, for a depth of 1 / 4 of the plate thickness from the steel plate surface (a 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), a magnification of 1000 times is used to observe the area of ​​2,400 μm in one field of view. 2 Photographs of the above structures are taken from 30 fields of view, and image analysis is performed on the obtained structure photographs.

[0068] The image analysis method involves obtaining the maximum brightness value Lmax and minimum brightness value Lmin from the image, designating the area with pixels whose brightness ranges from Lmax-0.3 (Lmax-Lmin) to Lmax as a white area, and calculating the ratio of the number of pixels in the white area to the total number of pixels to measure the martensite fraction. This type of image analysis is performed on a total of 30 observation fields for each analysis sample to determine the martensite fraction, and the average value is taken as the martensite fraction for each analysis sample. Furthermore, the difference between the maximum and minimum martensite fractions in 10 or more analysis samples is calculated based on the minimum plate thickness t min The variation in martensite fraction in the cross section of the structural member 10 at the position of the steel plate 21 having the minimum plate thickness t minWhen there are multiple steel plates having the above structure, such an analysis is performed on each steel plate to determine the martensite fraction, and the maximum variation in martensite fraction among these steel plates is taken as the variation in martensite fraction in the structural member 10.

[0069] Depending on the steel sheet, the area fraction of martensite obtained by image analysis, i.e., the area fraction of the white region, may contain a few percent of the area fraction of retained austenite. However, since the variation in the martensite fraction is calculated as a difference, the impact is minor.

[0070] After the forming process (hot stamping), the steel plate 21 may have a tensile strength of, for example, 0.5 GPa or more, and preferably has a tensile strength of 1.0 GPa or more. Similarly, after the forming process (hot stamping), the steel plates 22 and 23 (FIG. 1) may have a tensile strength of, for example, 0.5 GPa or more, and preferably has a tensile strength of 1.0 GPa or more. At least one of the steel plates 21, 22, and 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 21, 22, and 23 may be the same as or different from the tensile strength of the other steel plates.

[0071] [effect] In the blank 20 according to this embodiment, the minimum plate thickness t minThe emissivity of one surface of the steel sheet 21 having a thickness of 0.1 mm is greater than the emissivity of both surfaces of the steel sheet 22 having a thickness greater than 0.1 mm. That is, the surface of the steel sheet 21 is treated to increase the emissivity. As a result, when the blank 20 is heated during hot stamping, the temperature rise rate of the steel sheet 21 is significantly higher than that of the steel sheet 22. Therefore, in the heating process, the steel sheet 21 can be quickly heated to a temperature in the austenite range, and the high-temperature holding time of the steel sheet 21 can be secured for a long time. As a result, the austenite crystal grains in the microstructure of the steel sheet 21 become coarse, and the ferrite transformation region (ferrite nose) in the CCT diagram shifts to the long-time side. Therefore, it is possible to prevent the austenite from transforming into ferrite in the steel sheet 21 after it is removed from the heating furnace 30 and before the forming by the die 40 is started. Therefore, the forming of the blank 20 by the die 40 can be started while the microstructure of the steel sheet 21 is maintained in the austenite phase, and the minimum sheet thickness t min The hardenability of the steel plate 21 having this structure can be improved.

[0072] In this embodiment, the hardenability of the relatively thin steel plate 21 is improved, so that the hardness of the structural member 10 formed from the blank 20 can be made uniform. More specifically, the minimum plate thickness t min Since the steel plate 21 having the above properties is well quenched, the variation in martensite fraction in the steel plate 21 can be made 20% or less. As a result, for example, when a collision load is input to the structural member 10, deformation concentration is less likely to occur, and the structural member 10 is more likely to exhibit high impact absorption performance. Therefore, even when an annular structural member 10 including a steel plate 21 having a thin plate thickness, particularly a large and annular structural member 10, is formed from the blank 20, it is possible to reduce strength defects in the structural member 10 and improve the impact absorption performance of the structural member 10.

[0073] The smaller the variation in the martensite fraction, the less the non-uniformity in the mechanical properties within the structural member 10, which is preferable from the viewpoint of the functionality of the structural member 10. On the other hand, a large variation in the martensite fraction indicates that hardenability-insufficient parts, i.e., hardness-insufficient parts, are unevenly distributed within the structural member 10, and when the structural member 10 is subjected to deformation due to a collision, deformation tends to concentrate in the hardness-insufficient parts, thereby reducing the functionality of the structural member 10.

[0074] In this embodiment, the hardenability of the relatively thin steel plate 21 is improved, so that non-uniform stress is less likely to occur in the structural member 10. Therefore, even when the annular structural member 10 is formed from an annular blank 20, twisting is less likely to occur in the structural member 10. Therefore, even when an annular structural member 10 including a steel plate 21 with a thin plate thickness, particularly a large and annular structural member 10, is formed from a blank 20, it is possible to reduce poor dimensional accuracy of the structural member 10 and improve the impact absorption performance of the structural member 10.

[0075] In the blank 20 according to this embodiment, the minimum plate thickness t min The surface of the steel plate 21 is substantially covered with a black coating 26, while the steel plate 22, which has a thickness greater than that of the steel plate 21, is not provided with the coating 26. Therefore, the emissivity of the surface of the steel plate 21 is greater than the emissivity of both surfaces of the steel plate 22. In this case, the steel plate 21 heats up faster than the steel plate 22, and the high-temperature retention time of the steel plate 21 is longer than when the steel plate 21 has the same emissivity as the steel plate 22. This makes it possible to reduce unevenness in phase transformation caused by differences in cooling rates between the steel plates 21, 22, and 23 after heating of the blank 20 is completed. Specifically, min Since the start of the phase transformation from austenite to ferrite can be delayed for the steel plate 21, the difference in the phase transformation start time between the steel plate 21 and the other steel plates 22 and 23 is reduced. As a result, the hardenability of the steel plates 21, 22, and 23 included in the blank 20 can be made uniform.

[0076] For example, when the plating layer 217 of the steel sheet 21 is an aluminum-based plating layer, the temperature rise rate of the steel sheet 21 is likely to be slow in the heating step. Since the aluminum-based plating layer is white, it is likely to reflect thermal energy and inhibit the temperature rise of the steel sheet 21. However, in the blank 20 according to the present embodiment, the surface of the steel sheet 21 is treated to increase the emissivity. Therefore, even if the steel sheet 21 is a plated steel sheet having an aluminum-based plating layer, the temperature rise of the steel sheet 21 in the heating step can be promoted and the high-temperature retention time of the steel sheet 21 can be ensured to be long. Therefore, the hardenability of the thin steel sheet 21 can be ensured.

[0077] In this embodiment, in the heating process, the minimum plate thickness t min The steel plate 21 having a thickness of t mid Steel plate 22 having a maximum plate thickness t max The steel plate 23 having the minimum plate thickness t min and maximum plate thickness t max Ratio to:t max / t min It is preferable that the thickness t is 3.2 or less. min The alloying of the plating layer 217 of the steel sheet 21 having the thickness t max Therefore, the steel plate 23 having the above-mentioned structure can be heated sufficiently until the phase transformation to austenite is completed. Therefore, the process window can be secured in the manufacturing of the structural member 10.

[0078] In this embodiment, a coating 26 can be applied to the steel sheet 21 in order to increase the emissivity of the steel sheet 21. The emissivity of the coating 26 (at a temperature of 25°C and a wavelength of 8.0 μm) is, for example, 60% or more. This allows the steel sheet 21 to be efficiently radiatively heated, and the temperature rise rate of the steel sheet 21 in the heating process is more likely to be increased.

[0079] In this embodiment, the coating 26 is made of carbon black, one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide, and a carbon black powder having a density of 0 to 0.30 g / m 2 The following silica and carbon black content X may be contained. CB (g / m 2 ), and oxide content X Oxide (g / m 2 ) preferably satisfies the above formula (1). As described in Patent Document 1, formula (1) is a function of the ratio (%) of the increase in the temperature rise rate (°C / s) and the carbon black content X CB and oxide content X Oxide This is an equation that defines the relationship between the above. Equation (1) expresses that carbon black mainly functions as a heat absorbing material in the range up to 700°C, and oxides mainly function as heat absorbing materials in the range of 700°C or higher. When coating 26 satisfies equation (1), the surface of steel sheet 21 to which coating 26 is applied tends to have an emissivity of 60% or more at a wavelength of 8.0 μm at 25°C.

[0080] The carbon black and oxides can be dispersed throughout the entire surface of the coating 26 that is perpendicular to the thickness direction of the steel sheet 21. This makes it easier to make the emissivity of the surface of the steel sheet 21 uniform. Therefore, in the heating process, min The steel plate 21 having the above structure can be heated quickly and uniformly.

[0081] However, the configuration of the coating 26 is not limited to this. The coating 26 may be a substantially black coating in order to increase the emissivity of the steel sheet 21 compared to an untreated case. For example, the coating 26 may contain graphite or soot instead of or in addition to carbon black. Alternatively, the coating 26 may contain, for example, an acicular compound having an aspect ratio of 4 to 50 and a hexagonal crystal structure in order to increase the emissivity of the steel sheet 21. 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, goethite (NiS), wurtzite (ZnS), or the like.

[0082] <Second embodiment> 5 is a plan view of a blank 20A according to the second embodiment. The blank 20 according to the first embodiment is a tailored blank in which steel plates 21, 22, and 23 are butt-joined to one another. The blank 20A according to this embodiment differs from the first embodiment mainly in the state of the joint portion of the steel plates.

[0083] 5, the blank 20A includes a plurality of steel plates 21, 22, 23, 24, and 25. The steel plates 21, 22, 23, 24, and 25 are arranged and joined to have an annular shape in a plan view of the blank 20. As in the first embodiment, the steel plate 21 has a minimum plate thickness t min and the emissivity of at least one of the surfaces is configured to be greater than the emissivity of both surfaces of the steel plate 22. Therefore, the blank 20A according to this embodiment can also achieve the same effects as the first embodiment.

[0084] The blank 20A has an overlap portion 27. Fig. 6 is a cross-sectional view taken along line VI-VI in Fig. 5, showing a cross section of the overlap portion 27. In this embodiment, the overlap portion 27 is formed by overlapping ends of two adjacent steel plates 23, 24. The end of the steel plate 23 is joined to the end of the steel plate 24 in an overlapping state. The steel plates 23, 24 are joined to each other by, for example, spot welding or laser welding.

[0085] The overlap portion 27 has a total thickness t. The total thickness t is the sum of the thickness of the steel plate 23 and the thickness of the steel plate 24. When at least one of the steel plates 23 and 24 is a plated steel plate, the total thickness t includes the thickness of the plated layer. The total thickness t of the overlap portion 27 is, for example, greater than 2.5 mm and equal to or less than 4.0 mm. In this case, the steel plate 23 is configured so that the emissivity of the surface located outside the overlap portion 27, i.e., the surface opposite the steel plate 24, is greater than the emissivity of both surfaces of the steel plate 22 (FIG. 5). The steel plate 24 is configured so that the emissivity of the surface located outside the overlap portion 27, i.e., the surface opposite the steel plate 23, is greater than the emissivity of both surfaces of the steel plate 22. That is, in each of the steel plates 23 and 24, the surface located outside the overlap portion 27 is treated to increase the emissivity throughout its entirety. For example, the emissivity at a wavelength of 8.0 μm at 25° C. is 60% or more, preferably 70% or more, and more preferably 80% or more, on the surfaces of the steel plates 23, 24 located outside the overlap portion 27. The difference in emissivity at a wavelength of 8.0 μm at 25° C. between the surfaces of the steel plates 23, 24 located outside the overlap portion 27 and both surfaces of the other steel plate 22 is preferably greater than 5%, more preferably greater than 10%, and even more preferably greater than 20%. The emissivity of the surfaces of the steel plates 23, 24 located inside the overlap portion 27 may be greater than the emissivity of both surfaces of the steel plate 22, or may be less than or equal to the emissivity of both surfaces of the steel plate 22.

[0086] In the steel plates 23, 24, the surfaces positioned outside the overlap portion 27 may be covered with the coating 26, so that the emissivity of the surfaces may be greater than that of the steel plate 22 (FIG. 5). The coating 26 used in the steel plates 23, 24 may have a similar configuration to the coating 26 used in the steel plate 21. As described above, the thickness of the coating 26 is very small, so the total plate thickness t of the overlap portion 27 may be the plate thickness measured including the coating 26.

[0087] When the total thickness t of the overlap portion 27 exceeds, for example, 2.5 mm, the overlap portion 27 is difficult to heat up. Therefore, when the blank 20A is heated during hot stamping, the overlap portion 27 is heated to the minimum thickness t before it reaches the austenite region. min In some cases, the plating layer 217 (FIGS. 3B and 3C) of the steel sheet 21 having the above structure is alloyed, and a diffusion layer grows, making it impossible to ensure the corrosion resistance or weldability of the steel sheet 21. However, in this embodiment, the emissivity of the overlap portion 27 is increased to promote heating, so that even if the total sheet thickness t of the overlap portion 27 is greater than 2.5 mm, the overlap portion 27 can be sufficiently heated until the phase transformation to austenite is completed before the alloying of the plating layer 217 of the steel sheet 21 progresses and the diffusion layer becomes thicker, causing the corrosion resistance or weldability to be lost. Therefore, a process window can be ensured in the manufacture of structural members.

[0088] In the blank 20A according to this embodiment, the steel plate 21 may be butt-joined to the steel plates 22 and 23, or may form an overlap portion 27 with one or both of the steel plates 22 and 23. The steel plate 25 may be butt-joined to the steel plates 22 and 24, or may form an overlap portion 27 with one or both of the steel plates 22 and 24. When the steel plates 21 and 23 form the overlap portion 27 and the total plate thickness t of the overlap portion 27 exceeds 2.5 mm, the steel plates 21 and 23 may be configured so that the emissivity of the surface located outside the overlap portion 27 is greater than the emissivity of both surfaces of the steel plate 22. Similarly, when the steel plates 24 and 25 form the overlap portion 27 and the total plate thickness t of the overlap portion 27 exceeds 2.5 mm, the steel plates 24 and 25 may be configured so that the emissivity of the surface located outside the overlap portion 27 is greater than the emissivity of both surfaces of the steel plate 22.

[0089] Fig. 7 is a plan view of a structural member 10A manufactured from a blank 20A. The structural member 10A has generally the same configuration as the structural member 10 of the first embodiment (Figs. 1 and 2). However, in the structural member 10A, the member body 11 is formed from five steel plates 21, 22, 23, 24, and 25. The method of manufacturing the structural member 10A from the blank 20A is the same as in the first embodiment.

[0090] The structural member 10A can have a size similar to that of the structural member 10 according to the first embodiment. That is, the size of the structural member 10A, which is annular in plan view, is, for example, 1.0 m or more and 4.0 mm or less. In the structural member 10A, each of the steel plates 21, 22, 23, 24, and 25 has an open cross section similar to that of the first embodiment. Each of the steel plates 21, 22, 23, 24, and 25 can have, for example, a roughly hat shape in a cross-sectional view of the structural member 10A.

[0091] Although not shown in the drawings, in the structural member 10A, the width of the steel plate 21 arranged on the upper part of the front pillar 111 is, for example, 15 mm or more and 300 mm or less. The height of the steel plate 21 may be, for example, 10 mm or more and 150 mm or less. In the structural member 10A, the width of the steel plate 22 arranged on the lower part of the front pillar 111 is, for example, 30 mm or more and 750 mm or less. The height of the steel plate 22 may be, for example, 25 mm or more and 150 mm or less. In the structural member 10A, the width of the steel plates 23, 24 arranged at the position of the center pillar 112 is, for example, 15 mm or more and 300 mm or less. The height of the steel plates 23, 24 may be, for example, 10 mm or more and 150 mm or less. In the structural member 10A, the width of the steel plate 25 arranged at the position of the locker 113 is, for example, 30 mm or more and 300 mm or less. The height of the steel plate 25 may be, for example, 25 mm or more and 150 mm or less.

[0092] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present disclosure.

[0093] In each of the above embodiments, the minimum plate thickness t minIn order to increase the emissivity of the surface of steel plate 21 having the above-mentioned roughness more than that of steel plate 22, the surface of steel plate 21 is covered with coating 26. However, the method of increasing the emissivity of the surface of steel plate 21 is not limited to coating 26. For example, the emissivity of the surface of steel plate 21 can be increased more than that of steel plate 22 by making the surface roughness of steel plate 21 larger than that of steel plate 22. Similarly, the method of increasing the emissivity of the outer surface of overlap portion 27 is not limited to coating 26.

[0094] In each of the above embodiments, the minimum plate thickness t min In the above example, the emissivity of only one of the two surfaces of the steel plate 21 having the above-mentioned structure is made higher than the emissivity of the steel plate 22. However, the emissivity of both surfaces of the steel plate 21 may be made higher than the emissivity of the steel plate 22. For example, both surfaces of the steel plate 21 may be covered with the coating 26. Alternatively, the surface roughness of both surfaces of the steel plate 21 may be made higher than the surface roughness of the steel plate 22.

[0095] In the first embodiment, the steel plates 21, 22, and 23 included in the blank 20 may each be a single layer or multiple layers. That is, the steel plates 21, 22, and 23 may each be a single steel plate or a plate material formed by overlapping a plurality of steel plates. Similarly, in the second embodiment, the steel plates 21, 22, 23, 24, and 25 may each be a single layer or multiple layers.

[0096] In the first embodiment, the blank 20 includes three steel plates 21, 22, and 23. In the second embodiment, the blank 20A includes five steel plates 21, 22, 23, 24, and 25. However, the number of steel plates included in the blanks 20 and 20A is not limited to this. The blanks 20 and 20A have at least a minimum plate thickness t min and a steel plate 21 having a plate thickness t minand a steel plate 22 having a thickness greater than t. The emissivity of one or both sides of the steel plate 21 is greater than the emissivity of both sides of the steel plate 22. The steel plate 21 is directly or indirectly joined to the steel plate 22. The blank 20, 20A, which is annular in plan view, can typically include three or more steel plates. In the blank 20, 20A, the steel plates other than the steel plates 21, 22 may or may not be treated to increase the emissivity. In either of the blanks 20, 20A, the minimum plate thickness t min When there are multiple steel plates 21 having the above structure, it is preferable that the emissivity of one side or both sides of all the steel plates 21 is higher than the emissivity of both sides of the other steel plates 22. In the annular blanks 20, 20A, the arrangement of the multiple steel plates including the steel plates 21, 22 is not particularly limited.

[0097] In each of the above embodiments, the minimum plate thickness t min In the above example, the steel sheet 21 having the above-mentioned characteristics is a plated steel sheet. However, the steel sheet 21 does not necessarily have to be a plated steel sheet. The steel sheet 21 may be a steel sheet (bare material) that does not have a plating layer on its surface. Similarly, steel sheets other than the steel sheet 21 may be plated steel sheets or bare materials.

[0098] In the first embodiment, the die 40 used for hot stamping the blank 20 includes a punch 41 and a die 42. However, the configuration of the die 40 is not limited to the example described in the first embodiment. The die 40 may further include, for example, a pad and a blank holder.

[0099] In the above embodiment, the main body 11 of the structural member 10, 10A includes a front pillar 111, a center pillar 112, and a rocker 113. However, the member main body 11 can further include other components. For example, as shown in FIG. 8, the member main body 11 can further include a rear pillar 114. The structural members 10, 10A according to the above embodiment are door ring parts (single door ring parts) having a single ring shape. On the other hand, the structural member shown in FIG. 8 is a door ring part (double door ring part) having a double ring shape. When manufacturing a double door ring part, the blank that is the material for the part also has a double ring shape. EXAMPLES

[0100] The present disclosure will be described in more detail below with reference to examples, although the present disclosure is not limited to the following examples.

[0101] [First Example] In order to confirm the effects of this disclosure, a CAE analysis was performed on the press forming (hot stamping) of a structural member, which is a single door ring part, using commercially available software (AUTOFORM R.10, manufactured by AUTOFORM) while changing the type (material type) and thickness of the steel plate contained in the structural member, as well as the division pattern of the structural member.

[0102] The steel plates used in this analysis are shown in Table 1.

[0103] [Table 1]

[0104] In Table 1, the material type is listed in the following order: plating type, tensile strength, and application (hot stamping). Regarding the coating specifications, the black coating is a black coating containing carbon black and metal oxide. "Black coating - one side" means that one entire side of the steel sheet is covered with a black coating. "Black coating - both sides" means that both entire sides of the steel sheet are covered with a black coating.

[0105] The division patterns of the structural members are shown in Figures 9A to 9G. Figures 9A to 9G show the number of steel plates (materials) included in the structural members that are single door ring parts, and the positions of the joints between the steel plates in the structural members. In Figures 9A to 9G, each steel plate is given a number in parentheses.

[0106] Regarding division patterns 1 and 2 shown in Figures 9A and 9B, the analysis conditions and results are shown in Table 2. In Figures 9A and 9B, the structural member is formed from three pieces of material (1) to (3).

[0107] [Table 2]

[0108] Referring to Table 2, in Example 1, the minimum plate thickness t min In Example 2, the material (3) having the smallest plate thickness t of the materials (1) to (3) is provided with a black coating on both sides. min In Example 3, a black coating is applied to one side of the material (1) having a thickness of 1.2 mm. min A black coating is applied to both sides of the material (2) having a thickness of 1.2 mm. On the other hand, in Comparative Examples 1 and 2, a black coating is not applied to any of the materials (1) to (3). In Comparative Examples 1 and 2, min : No black coating is applied to the material with a thickness of 1.2 mm.

[0109] In Table 2, the "910℃ reaching time" is the time when the material contained in the blank reaches 910℃ (A c3The "phase transformation start time" is the time required for the blank to reach 910 ° C. from the start of heating for the material heated to 920 ° C. (point or more). 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 removed from the heating furnace. From Table 2, it can be seen that in Examples 1 to 3 in which the emissivity of the thinnest material is increased by the black coating, the time to reach 910 ° C. is shorter by 20 seconds or more than in Comparative Examples 1 and 2, and the temperature rise rate of the thinnest material in the heating process is higher. Furthermore, in Examples 1 to 3, the phase transformation start time exceeds 20 seconds, which is later than the phase transformation start time in Comparative Examples 1 and 2. Therefore, it becomes easier to start forming the blank before the start of ferrite transformation, and it becomes possible to uniformly quench the blank in the forming process.

[0110] Regarding division patterns 3 and 4 shown in Figures 9C and 9D, the analysis conditions and results are shown in Table 3. In Figures 9C and 9D, the structural member is formed from four pieces of material (1) to (4).

[0111] [Table 3]

[0112] Referring to Table 3, in Examples 4 to 10, the minimum plate thickness t min In Examples 4 to 10, the black coating is applied to the material having the minimum plate thickness t min The material is covered on one or both sides with a black coating. min In the case where there are multiple blanks having the same thickness, all of these blanks are provided with a black coating. On the other hand, in Comparative Examples 3 to 7, none of the blanks (1) to (4) are provided with a black coating. In Comparative Examples 3 to 7, min No black coating is applied to the material having the above structure.

[0113] From Table 3, it can be seen that in Examples 4 to 10, the time to reach 910°C was 20 seconds or more shorter than in Comparative Examples 3 to 7, and the temperature rise rate of the thinnest material in the heating process was higher. Furthermore, in Examples 4 to 10, the phase transformation start time exceeded 20 seconds, which is later than the phase transformation start time in Comparative Examples 3 to 7. This makes it easier to start forming the blank before the start of ferrite transformation, and makes it possible to uniformly quench the blank in the forming process.

[0114] Regarding division patterns 5 to 7 shown in Figures 9E to 9G, the analysis conditions and results are shown in Table 4. In Figures 9E to 9G, the structural member is formed from five pieces of material (1) to (5).

[0115] [Table 4]

[0116] Referring to Table 4, in Examples 11 to 15, the minimum plate thickness t min In Examples 11 to 15, the black coating is applied to the material having the minimum plate thickness t min The material is covered on one or both sides with a black coating. min In the case where there are multiple blanks having the same thickness, all of these blanks are provided with a black coating. On the other hand, in Comparative Examples 8 to 11, none of the blanks (1) to (5) are provided with a black coating. In Comparative Examples 8 to 11, min No black coating is applied to the material having the above structure.

[0117] From Table 4, it can be seen that in Examples 11 to 15, the time to reach 910°C was 20 seconds or more shorter than in Comparative Examples 8 to 11, and the temperature rise rate of the thinnest material in the heating process was higher. Furthermore, in Examples 11 to 15, the phase transformation start time exceeded 20 seconds, which was later than in Comparative Examples 8 to 11. Therefore, it became easier to start forming the blank before the start of ferrite transformation, and it became possible to uniformly quench the blank in the forming process.

[0118] In each of the examples and comparative examples shown in Tables 2 to 4, the materials are butted together and then laser-joined (butt joint). On the other hand, in each of the examples and comparative examples shown in Table 5 below, some of the materials are joined by, for example, spot welding to form overlapping portions.

[0119] [Table 5]

[0120] Referring to Table 5 and Fig. 9F, in Example 16 and Comparative Example 12, material (2) and material (5), material (3) and material (4), and material (4) and material (5) form overlapping portions at their joints. Referring to Table 5 and Fig. 9E, in Example 17 and Comparative Example 13, material (2) and material (5), and material (3) and material (4) form overlapping portions at their joints.

[0121] As shown in Table 5, in Examples 16 and 17, the minimum plate thickness t min On the other hand, in Comparative Examples 12 and 13, a black coating is not applied to any of the materials (1) to (5). In Comparative Examples 12 and 13, min No black coating is applied to the material having the above structure.

[0122] As can be seen from Table 5, in Examples 16 and 17, the time to reach 910°C is shorter by 20 seconds or more, and the time to start phase transformation is also delayed, compared to Comparative Examples 12 and 13. Therefore, it was confirmed that even when an overlapping portion exists in a blank, by increasing the emissivity of the thinnest material, the same effect as in the case of butt welding can be obtained.

[0123] [Second Example] Regarding the press forming (hot stamping) of the structural members that are double door ring parts, an analysis similar to that in the first embodiment was carried out while changing the material type and plate thickness contained in the structural members, as well as the division pattern of the structural members.

[0124] As in the first embodiment, the steel plates used as materials were selected from those shown in Table 1. The division pattern of the structural members is as shown in Figs. 10A to 10D. Figs. 10A to 10D show the number of steel plates (materials) included in the structural members that are double door ring components, and the positions of the joints between the steel plates in the structural members. In Figs. 10A to 10D, each steel plate used as material is given a number in parentheses.

[0125] Regarding division patterns 8 and 9 shown in Figures 10A and 10B, the analysis conditions and results are shown in Table 6. In Figures 10A and 10B, the structural member is formed from six pieces of material (1) to (6).

[0126] [Table 6]

[0127] Referring to Table 6, in Examples 18 to 20, the minimum plate thickness t min In Examples 18 to 20, the black coating is applied to the material having the minimum plate thickness t min On the other hand, in Comparative Examples 14 and 15, no black coating was applied to any of the materials (1) to (6). In Comparative Examples 14 and 15, the minimum plate thickness t min No black coating is applied to the material having the above structure.

[0128] From Table 6, it can be seen that in Examples 18 to 20, the time to reach 910°C was about 20 seconds shorter than in Comparative Examples 14 and 15, and the temperature rise rate of the thinnest material in the heating process was higher. Furthermore, in Examples 18 to 20, the phase transformation start time was slower than in Comparative Examples 14 and 15. Therefore, it can be said that by increasing the emissivity of the thinnest material, it becomes easier to start forming the blank before the start of ferrite transformation, and it becomes possible to uniformly harden the blank in the forming process.

[0129] Regarding the division patterns 10 and 11 shown in Figures 10C and 10D, the analysis conditions and results are shown in Table 7. In Figures 10C and 10D, the structural member is formed from seven pieces of material (1) to (7).

[0130] [Table 7]

[0131] Referring to Table 7, in Examples 21 to 23, the minimum plate thickness t min In Examples 21 to 23, a black coating is applied to a material having a minimum plate thickness t min On the other hand, in Comparative Examples 16 and 17, no black coating was applied to any of the materials (1) to (7). In Comparative Examples 16 and 17, the minimum plate thickness t min No black coating is applied to the material having the above structure.

[0132] From Table 7, it can be seen that in Examples 21 to 23, the time to reach 910°C was shorter and the temperature rise rate of the thinnest material in the heating process was higher than in Comparative Examples 16 and 17. Furthermore, in Examples 21 to 23, the phase transformation start time was slower than in Comparative Examples 16 and 17. Therefore, it becomes easier to start forming the blank before the start of ferrite transformation, and it becomes possible to uniformly quench the blank in the forming process.

[0133] [Third Example] In order to confirm the influence of plate thickness on the press forming (hot stamping) of the structural member of the single door ring part, the same analysis as in Example 1 was carried out while changing the combination of materials and the joining method. The analysis conditions and results are shown in Table 8.

[0134] [Table 8]

[0135] The division pattern of test examples 1 and 5 is division pattern 7 shown in FIG. 9G. The division pattern of test example 4 is division pattern 5 shown in FIG. 9E. In test examples 1, 4, and 5, some of the materials form overlapping portions at the joints. More specifically, in test examples 1 and 5, material (1) and material (2), material (1) and material (3), material (2) and material (5), and material (4) and material (5) each form overlapping portions at their joints. In test example 4, material (1) and material (2), material (1) and material (3), material (3) and material (4), and material (2) and material (5) each form overlapping portions at their joints. In all of these test examples, the minimum plate thickness t min In each of these test examples, a black coating is applied to both outer sides of the overlap portion having the maximum total plate thickness t.

[0136] The division pattern of Test Examples 2, 3, and 6 is the division pattern 7 shown in FIG. 9G. In Test Examples 2, 3, and 6, the materials are joined by butt joints. In all of these Test Examples, the minimum plate thickness t min A black coating is applied to the material having the above structure.

[0137] The "process window (PW)" in Table 8 is the allowable heating time for the material contained in the blank that reaches 910°C the earliest, minus the time it takes for the entire blank to reach 910°C after the earliest temperature reaches 910°C. If the process window is 5 seconds or less, alloying of the plating layer of the material that reaches 910°C the earliest in the heating process will progress, meaning that the corrosion resistance of the plating layer cannot be ensured and it will be difficult to manufacture structural components satisfactorily.

[0138] As shown in Table 8, the minimum plate thickness t min Maximum plate thickness t max Ratio of: t max / t min In the test examples 5 and 6 in which t exceeded 3.2, the process window was negative, and the process window required for good manufacturing of the structural member could not be secured.max / t min In Test Examples 1 to 4, in which the difference was 3.2 or less, the process window could be ensured.

[0139] As shown in Table 8, in Test Example 5, where the maximum total plate thickness t of the overlap portion exceeded 4.0 mm, the process window was negative and the process window necessary for good manufacturing of structural components could not be secured. On the other hand, in Test Examples 1 and 4, where the maximum total plate thickness t of the overlap portion was greater than 2.5 mm and equal to or less than 4.0 mm, a process window of 5 seconds or more could be secured.

[0140] Reference Example 1 is an example of division pattern 5 similar to Test Example 4, but no black coating is applied to each material, including the overlapping portion. However, in Reference Example 1, there is no difference in sheet thickness between the materials, and no problem occurs with the progression of alloying of the plating layer in the thinnest material, so a process window of 5 seconds or more can be secured.

[0141] For Test Examples 1 to 4 and Comparative Examples 10 and 13A, the blank was heated at a furnace temperature of 920°C until the entire blank reached 910°C, and then conveyed to a press machine in 17 seconds, hot stamped at a forming speed of 40 mm / s, and held at bottom dead center for 20 seconds while applying pressure of 3000 kN to obtain hot stamped structural members. Analysis samples were taken from the thinnest parts of these structural members by the method described in the above embodiment, and the variation in martensite fraction was measured. In addition, shape accuracy and impact absorption performance were measured separately for these structural members. The evaluation results are shown in Table 9.

[0142] [Table 9]

[0143] In Table 9, the variation in martensite fraction is, as explained in the above embodiment, the variation in the minimum plate thickness t minThe martensite fraction (%) is calculated by subtracting the minimum martensite fraction (%) from the maximum martensite fraction (%) in the cross section of a structural component at the location of the material having the above structure.

[0144] The shape accuracy was evaluated based on how far the structural member was separated from the mating member at the overlapping portion of the structural member and mating member when the structural member, which is roughly hat-shaped in cross section, was attached to another member. In Table 9, cases where the distance from the surface of the mating member was within ±2.0 mm are marked with ○, cases where it was between ±2.0 mm and ±3.0 mm are marked with △, and cases where it was over ±3.0 mm are marked with ×.

[0145] Regarding the impact absorption performance, assuming a small overlap (SOL) collision and a side collision with the structural members installed in the vehicle, an impactor simulating a vehicle was collided with the structural members to evaluate the maximum intrusion amount during an SOL collision and the maximum intrusion amount during a side collision. The impact absorption performance was evaluated based on the impact absorption performance of door ring parts made by bonding each material after forming them by hot stamping, and compared with the impact absorption performance of the base. In Table 9, impact absorption performance equivalent to the impact absorption performance of the base is displayed as "good," impact absorption performance superior to the impact absorption performance of the base as "better," impact absorption performance slightly lower than the impact absorption performance of the base as "marginal," and impact absorption performance even lower than that as "poor."

[0146] Comparative Example 10 is a comparative example of the first embodiment, and is a butt-jointed laser joint of materials similar to Test Example 2. Comparative Example 13A is the same plate assembly as Comparative Example 13 of the first embodiment, but similar to Test Example 4, the materials are partially overlapped and joined. In Comparative Example 10 and Comparative Example 13A, unlike each test example, no black coating is applied to each material, including the thinnest material. As shown in Table 9, the variation in martensite fraction in all of Test Examples 1 to 4 was 20% or less, and the variation in martensite fraction was significantly reduced compared to Comparative Examples 10 and 13A. In Test Examples 1 to 4, the shape accuracy was also better than Comparative Examples 10 and 13A.

[0147] In Test Examples 1 to 4, in which the variation in the martensite fraction was small, the impact absorption performance was improved compared to Comparative Examples 10 and 13A. In particular, in Test Examples 1 to 3, in which the variation in the martensite fraction was 15% or less, impact absorption performance equal to or higher than that of the base could be ensured. That is, even though a ring-shaped structural member was formed by integrating a plurality of materials at the blank stage, impact absorption performance equal to or higher than that of a structural member in which the materials were individually press-formed and then joined could be ensured. In Test Examples 2 and 3, the materials were butt-joined to each other, which improved the strength against the axial force, and therefore the impact absorption performance was better than that of Test Example 1. Test Examples 2 and 3 demonstrated high impact absorption performance, especially for SOL collisions. [Explanation of symbols]

[0148] 10, 10A: Structural members 11: Main body of material 111: Front pillar 112: Center pillar 113: Locker 12: Film 20,20A:Blank 21: Steel plate (first steel plate) 216: Base material steel plate 217: Plating layer 22: Steel plate (second steel plate) 23,24,25: Steel plate 26: Membrane 27: Overlap section 40: Mold

Claims

1. A blank for hot stamping, The blank includes a plurality of steel plates that are arranged and joined to have an annular shape in a plan view, The plurality of steel plates include A first steel plate having a smallest plate thickness among the plurality of steel plates; A second steel plate having a plate thickness greater than a plate thickness of the first steel plate; Including, A blank, wherein the first steel plate is configured such that the emissivity of at least one of both surfaces of the first steel plate is greater than the emissivity of both surfaces of the second steel plate.

2. 2. The blank of claim 1, The first steel plate has a plate thickness of less than 1.4 mm.

3. 2. The blank of claim 1, The first steel sheet is a plated steel sheet having a base steel sheet and an aluminum-based plating layer provided on the base steel sheet.

4. 2. The blank of claim 1, A blank, wherein the at least one surface of the first steel plate is covered with a coating having an emissivity of 60% or more at a wavelength of 8.0 μm at 25° C.

5. 2. The blank of claim 1, At least one surface of the first steel plate is covered with a coating; The coating comprises carbon black, one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide, and a weight ratio of 0 to 0.30 g / m ２ and silica, The content of the carbon black in the coating is X ＣＢ (g / m ２ ), the content of the oxide is X Ｏｘｉｄｅ (g / m ２ ) and then X ＣＢ and X Ｏｘｉｄｅ A blank satisfies the following formula (1): 118.9≦24280 / {6700 / (100+76×X ＣＢ )+18000 / (130+65×X Ｏｘｉｄｅ )}≦332.0 (1)

6. 2. The blank of claim 1, The first steel sheet is a plated steel sheet having a base steel sheet and a plating layer provided on the base steel sheet, The thickness of the first steel plate is t ｍｉｎ The thickness of the steel plate having the largest thickness among the plurality of steel plates is t ｍａｘ When 1.0<t ｍａｘ / t ｍｉｎ ≦3.2, blank.

7. 2. The blank of claim 1, The first steel sheet is a plated steel sheet having a base steel sheet and a plating layer provided on the base steel sheet, The blank further includes an overlap portion formed by overlapping ends of two adjacent steel plates among the plurality of steel plates, the ends being made of steel plates other than the second steel plate, and having a total plate thickness of greater than 2.5 mm and less than or equal to 4.0 mm; A blank, wherein the two steel plates are each configured so that the emissivity of a surface located outside the overlap portion is greater than the emissivity of both surfaces of the second steel plate.

8. A method for manufacturing a structural member, comprising the steps of: Providing a blank according to any one of claims 1 to 7; Heating the plurality of steel plates included in the blank to an austenite transformation completion temperature or higher; forming the heated blank into an annular structural member in a plan view using a die and quenching the blank; A manufacturing method comprising:

9. A structural member, A member body 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 having an annular shape in a plan view; A coating layer is provided on the first steel plate, and contains 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, A structural member comprising:

10. A structural member, A member body 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 having an annular shape in a plan view; A carbon black layer having a thickness of 0.500 g / m2 is provided on the first steel plate. ２ A coating containing the following: A structural member comprising:

11. A structural member according to claim 9 or 10, The structural member is a door ring part of an automobile, The member body is a structural member including a front pillar, a center pillar, and a rocker connecting the front pillar and the center pillar.

Images