Blank, manufacturing method of structural member, and structural member

Abstract

The blank (30, 30A, 30B, 30C) includes a plurality of steel plates. The plurality of steel plates are arranged and joined to form two long portions (34L, 34R) and a connecting portion (35). The plurality of steel plates include a first steel plate (31) and a second steel plate (32). The first steel plate (31) has a minimum plate thickness (t min The second steel plate (32) has a plate thickness (t min ) greater than the plate thickness (t 2 The value of coefficient A calculated by formula (1) using the chemical composition of the first steel plate (31) is larger than the value of coefficient A calculated by formula (1) using the chemical composition of the second steel plate (32). A=1.48×(2.7×C+0.4×Si+Mn+0.45×Ni+0.8×Cr+2×Mo) 3.42 (1)

Description

[Technical field]

[0001] The present disclosure relates to a blank, a method for manufacturing a structural member, and a 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 method for manufacturing an automobile body side structural frame from multiple blanks. In Patent Document 1, multiple blanks are joined to form a composite blank, and the composite blank is press-formed to manufacture the body side structural frame. Patent Document 1 also describes hot forming (hot stamping) the composite blank.

[0004] Patent Document 2 discloses a method for manufacturing a vehicle rear structure. The method for manufacturing a vehicle rear structure includes a step of manufacturing a rear rail. The step includes a step of providing a tailored welded blank and a step of forming the tailored welded blank into a desired shape. The forming step is, for example, a step of hot forming the tailored welded blank. In this case, the hot forming is followed by a step of cooling the hot-formed tailored welded blank at a controlled cooling rate.

[0005] In Patent Document 2, for example, the yield strength of the front part of the rear rail is greater than that of the middle part, which in turn is greater than that of the rear part. Patent Document 2 describes that when two adjacent parts of the rear rail have the same composition but are intended to have different final yield strengths, the different yield strengths can be obtained by one or a combination of the following methods: during hot forming, the part intended to have a lower yield strength is heated to a lower temperature than the part intended to have a higher yield strength; after hot forming, the part intended to have a lower yield strength is cooled at a slower rate than the part intended to have a higher yield strength; and / or, these parts undergo the same hot forming and cooling after the hot forming process, but the part intended to have the lower yield strength is subsequently subjected to an additional heat treatment to reduce its yield strength. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Special Publication No. 2021-528248 [Patent Document 2] Special Publication No. 2019-503920 Summary of the Invention [Problem to be solved by the invention]

[0007] 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. For example, as described in Patent Documents 1 and 2, it has been considered to provide a composite blank (tailor welded blank) containing multiple steel plates (sub-blanks) to hot stamping and form structural members that have been formed separately in the past as one member. However, if a composite blank contains multiple steel plates with different plate thicknesses, the performance of the 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 into a structural member 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, cooling proceeds by the time the blank is taken out of the heating furnace and forming is started, causing diffusion transformation, which may make it difficult to quench. This makes it easy for the hardness of the structural member to become uneven. In addition, since a steel plate with a small plate thickness is difficult to quench and stress is likely to remain in the steel plate with a small plate thickness, twisting, warping, etc. may occur in the structural member, which may deteriorate the dimensional accuracy of the structural member. Non-uniform hardness and poor dimensional accuracy can lead to a decrease in the impact absorption performance (crash resistance) of structural members. The decrease in impact absorption performance becomes more pronounced as the structural members become larger.

[0008] An object of the present disclosure is to provide a hot stamping blank that can improve the performance of a structural component, particularly a large structural component, when the structural component includes a steel plate having a smaller plate thickness compared to other steel plates. [Means for solving the problem]

[0009] 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 form two long portions and a connecting portion. The long portions are arranged side by side in a horizontal direction in a plan view of the blank. The connecting portion connects the long portions to each other. The plurality of steel plates include a first steel plate and a second steel plate. The first steel plate has a smallest plate thickness among the plurality of steel plates. The second steel plate has a plate thickness greater than that of the first steel plate. The value of coefficient A calculated by the following formula (1) using the chemical composition of the first steel plate is greater than the value of coefficient A calculated by the following formula (1) using the chemical composition of the second steel plate. A=1.48×(2.7×C+0.4×Si+Mn+0.45×Ni+0.8×Cr+2×Mo) 3.42 (1) However, the element symbols in the above formula (1) are substituted with the contents (mass%) of the corresponding elements. Effect of the Invention

[0010] According to the hot stamping blank of the present disclosure, when a structural component, particularly a large structural component, including a steel plate having a smaller plate thickness compared to other steel plates is formed, the performance of the component can be improved. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is an exploded perspective view of a structural member according to a first embodiment. [Diagram 2] FIG. 2 is a cross-sectional view of a side frame included in each of the structural members shown in FIG. [Figure 3A] FIG. 3A is a schematic diagram for explaining the method for manufacturing a structural member according to the first embodiment, showing a blank corresponding to one of the structural members shown in FIG. [Figure 3B] FIG. 3B is a schematic diagram for illustrating the method for manufacturing a structural member according to the first embodiment, and is a cross-sectional view of the blank shown in FIG. 3A. [Figure 3C] 3C is a schematic view for explaining the method for manufacturing a structural member according to the first embodiment, and is another cross-sectional view of the blank shown in FIG. 3A. FIG. [Figure 3D] 1. FIG. 3D is a schematic diagram for explaining the method for manufacturing a structural member according to the first embodiment, showing a blank corresponding to the other structural member shown in FIG. [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 blank according to the second embodiment. [Diagram 5] FIG. 5 is a cross-sectional view of a blank according to the third embodiment. [Figure 6] FIG. 6 is an exploded perspective view of a structural member according to a fourth embodiment. [Figure 7] FIG. 7 is a plan view of a blank according to the fourth embodiment. [Figure 8] FIG. 8 is a plan view of another blank according to the fourth embodiment. [Figure 9A] FIG. 9A is a cross-sectional view of a blank according to a modification of each embodiment. [Figure 9B] FIG. 9B is another cross-sectional view of a blank according to a modification of each embodiment. [Figure 10] FIG. 10 is a plan view of a blank according to a modified example of the first embodiment. [Figure 11] FIG. 11 is a plan view of a blank according to another modified example of the first embodiment. [Figure 12] FIG. 12 is a plan view of a blank according to still another modified example of the first embodiment. [Figure 13] FIG. 13 is a plan view of a blank according to a modified example of the fourth embodiment. [Figure 14] FIG. 14 is a plan view of a blank according to another modified example of the fourth embodiment. [Figure 15] FIG. 15 is a plan view of a blank according to still another modified example of the fourth embodiment. [Figure 16] FIG. 16 is a plan view of a blank according to still another modified example of the fourth embodiment. [Figure 17] FIG. 17 is a plan view of a blank according to still another modified example of the fourth embodiment. [Figure 18] FIG. 18 is a plan view of a blank according to still another modified example of the fourth embodiment. [Figure 19] FIG. 19 is a plan view of a blank according to still another modified example of the fourth embodiment. [Figure 20] FIG. 20 is a plan view of a blank according to still another modified example of the fourth embodiment. [Figure 21] FIG. 21 is a plan view of a blank according to still another modified example of the fourth embodiment. [Figure 22] FIG. 22 is a plan view of a blank according to still another modified example of the fourth embodiment. [Figure 23] FIG. 23 is a plan view of a blank according to still another modified example of the fourth embodiment. [Figure 24] FIG. 24 is a cross-sectional view of a side frame included in a structural member according to a modified example of each embodiment. [Figure 25A] FIG. 25A is a diagram showing a division pattern of a structural member in an embodiment. [Figure 25B] FIG. 25B is a diagram showing another division pattern of the structural member in the embodiment. [Figure 25C] FIG. 25C is a diagram showing yet another division pattern of a structural member in an embodiment. [Figure 25D] FIG. 25D is a diagram showing yet another division pattern of a structural member in an embodiment. [Figure 25E] FIG. 25E is a diagram showing yet another division pattern of a structural member in an embodiment. [Figure 25F] FIG. 25F is a diagram showing yet another division pattern of a structural member in an embodiment. [Figure 25G] FIG. 25G is a diagram showing yet another division pattern of a structural member in an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] For example, as described in "Masakatsu Ueno and Kametaro Ito, "A new prediction formula for steel hardenability replacing the GROSSMANN formula", Iron and Steel, The Iron and Steel Institute of Japan, Vol. 6, 1988, p. 1073-1080, the critical cooling rate V was used as an index of the hardenability of steel. c90 The critical cooling rate V c90 is the critical cooling rate (℃ / s) at which 90% or more martensite structure is obtained by volume fraction, and logV c90 =2.94-0.75β. β is calculated as 2.7×C+0.4×Si+Mn+0.45×Ni+0.8×Cr+2×Mo. β represents the degree of influence of each element on hardenability based on the amount of Mn. The larger β is, the higher the critical cooling rate V c90 becomes smaller, and the hardenability of the steel material becomes good.

[0013] There is a correlation between β, which indicates the degree of influence of each element on hardenability, and the time until the diffusion transformation starts after the heating of the steel material is completed (transformation start time). The inventors held hot stamping steel sheets A, B, C, and D (sheet thickness 1.2 mm) similar to those in the examples (Table 1) described below at 900°C for 1 minute in a heating furnace, then removed from the heating furnace and air-cooled, and measured the time until the phase transformation starts (transformation start time). Then, a regression analysis was performed using the measured transformation start time data, and the formula for converting β to the transformation start time was: A = 1.48 x β 3.42 The formula A is a coefficient (index value) that differs for each steel material depending on the chemical composition. The coefficient A corresponds to the transformation start time when only the effects of elements are considered, and a larger coefficient A means that the steel material has better hardenability. The inventors further used the coefficient A to consider an appropriate arrangement of the steel material in the blank. As a result, the inventors completed a blank according to the embodiment.

[0014] 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 form two long portions and a connecting portion. The long portions are arranged side by side in a horizontal direction in a plan view of the blank. The connecting portion connects the long portions to each other. The plurality of steel plates include a first steel plate and a second steel plate. The first steel plate has a smallest plate thickness among the plurality of steel plates. The second steel plate has a plate thickness greater than that of the first steel plate. The value of coefficient A calculated by the following formula (1) using the chemical composition of the first steel plate is greater than the value of coefficient A calculated by the following formula (1) using the chemical composition of the second steel plate (first configuration). A=1.48×(2.7×C+0.4×Si+Mn+0.45×Ni+0.8×Cr+2×Mo) 3.42 (1) However, the element symbols in the above formula (1) are substituted with the contents (mass%) of the corresponding elements.

[0015] For example, when the hardenability of the material is the same between 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 thin first steel plate starts a phase transformation (diffusion transformation) from austenite to ferrite earlier than the thick second steel plate after the blank heating for hot stamping is completed. However, in the blank according to the first configuration, the coefficient A calculated based on the chemical composition of the first steel plate is larger than the coefficient A calculated based on the chemical composition of the second steel plate. That is, the first steel plate is made of a material having a higher hardenability than the second steel plate, in other words, a material that starts diffusion transformation during cooling late. Therefore, after the blank heating is completed, the start of diffusion transformation in the first steel plate is delayed, and the difference in transformation start time between the first steel plate and the second steel plate can be reduced. As a result, when the blank is hot stamped, not only the relatively thick second steel plate but also the first steel plate having a minimum thickness can be well hardened, and the hardness of the structural member formed from the blank is easily uniformed. In addition, since the first steel plate is well quenched, stress is offset by transformation plasticity and residual stress is reduced, which makes it possible to suppress the occurrence of twisting, warping, etc. in structural components caused by concentration of residual stress, thereby reducing deterioration in dimensional accuracy.

[0016] In this way, in the first configuration, even though the blank including the two long parts and the connecting part includes the first steel plate that is thinner than the second steel plate, the hardness of the structural member formed by hot stamping from the blank can be made uniform and deterioration of dimensional accuracy can be suppressed. Therefore, the impact absorption performance (crash resistance performance) of the structural member, especially a large structural member, can be improved.

[0017] In the blank according to the first configuration, the plate thickness of the first steel plate is t min The maximum thickness among multiple steel plates is t max Then, t max -t min ≧0.2 (mm) (second configuration).

[0018] A manufacturing method for a structural component according to an embodiment includes the steps of preparing a blank according to the first or second configuration, heating a plurality of steel plates contained in the blank to a temperature equal to or higher than the austenite transformation completion temperature, and forming the heated blank using a die and quenching it (third configuration).

[0019] A vehicle body structural member according to an embodiment includes a pair of side frames and a cross member. The cross member connects the side frames. The side frames and the cross member are formed by a plurality of steel plates joined to each other. 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 value of coefficient A calculated by the following formula (1) using the chemical composition of the first steel plate is greater than the value of coefficient A calculated by the following formula (1) using the chemical composition of the second steel plate (fourth configuration). A=1.48×(2.7×C+0.4×Si+Mn+0.45×Ni+0.8×Cr+2×Mo) 3.42 (1) However, the element symbols in the above formula (1) are substituted with the contents (mass%) of the corresponding elements.

[0020] Hereinafter, embodiments 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 characters, and the same description will not be repeated.

[0021] First Embodiment [Structural Members] Fig. 1 is an exploded perspective view of structural members 10, 20 according to this embodiment. The structural members 10, 20 are used in the body of an automobile or the like. In the example shown in Fig. 1, the structural members 10, 20 constitute a front under module of the body.

[0022] The structural member 10 is an upper module. That is, the structural member 10 is disposed above the structural member 20 when assembled to the vehicle body. The structural member 10 includes a pair of side frames 11L, 11R and at least one cross member 12. The side frames 11L, 11R and the cross member 12 each have an elongated shape.

[0023] The side frames 11L, 11R are arranged side by side in the left-right direction of the vehicle body when the structural member 10 is assembled to the vehicle body. The side frames 11L, 11R each extend in the front-rear direction of the vehicle body. Each of the side frames 11L, 11R includes a front portion 111 and a rear portion 112. The rear portion 112 is arranged behind the front portion 111 when the structural member 10 is assembled to the vehicle body.

[0024] The cross member 12 extends in the left-right direction of the vehicle body when the structural member 10 is assembled to the vehicle body. The cross member 12 extends from the side frame 11L to the side frame 11R. The cross member 12 connects the side frames 11L, 11R. In the example shown in FIG. 1, the cross member 12 connects the side frames 11L, 11R to each other at one end side of the longitudinal direction of the side frames 11L, 11R. When the structural member 10 is assembled to the vehicle body, the cross member 12 is disposed, for example, at the rear end of the structural member 10. However, the cross member 12 may connect the middle portions of the side frames 11L, 11R.

[0025] The structural member 20 is a lower module. That is, the structural member 20 is disposed below the structural member 10 when assembled to the vehicle body. The structural member 20 includes a pair of side frames 21L, 21R and at least one cross member 22. The side frames 21L, 21R and the cross member 22 each have an elongated shape.

[0026] The side frames 21L, 21R are arranged side by side in the left-right direction of the vehicle body when the structural member 20 is assembled to the vehicle body. The side frames 21L, 21R each extend in the front-rear direction of the vehicle body. Each of the side frames 21L, 21R includes a front portion 211 and a rear portion 212. The rear portion 212 is arranged rearward of the front portion 211 when the structural member 20 is assembled to the vehicle body.

[0027] The lower side frames 21L, 21R are joined to the upper side frames 11L, 11R, respectively. The side frames 21L, 21R form a closed cross section together with the side frames 11L, 11R. FIG. 2 shows the closed cross section formed by the side frames 21L, 21R together with the side frames 11L, 11R, respectively. Hereinafter, when there is no need to particularly distinguish between the side frames 11L, 11R, the side frames 11L, 11R will be collectively referred to as the side frames 11. Similarly, when there is no need to particularly distinguish between the side frames 21L, 21R, the side frames 21L, 21R will be collectively referred to as the side frames 21.

[0028] Fig. 2 is a cross-sectional view (transverse cross-sectional view) of the side frames 11, 21 cut along a plane perpendicular to the longitudinal direction. In the example of Fig. 2, each of the side frames 11, 21 has a substantially hat-shaped transverse cross-section.

[0029] 2, the side frame 11 includes a top plate 113, vertical walls 114, 115, and flanges 116, 117. In a cross-sectional view of the side frame 11, one ends of the vertical walls 114, 115 are connected by the top plate 113. In a cross-sectional view of the side frame 11, the other ends of the vertical walls 114, 115 are connected to flanges 116, 117, respectively. The flanges 116, 117 protrude outward from the vertical walls 114, 115, respectively.

[0030] The side frame 21 includes a top plate 213, vertical walls 214, 215, and flanges 216, 217. In a cross-sectional view of the side frame 21, one ends of the vertical walls 214, 215 are connected by the top plate 213. In a cross-sectional view of the side frame 21, the other ends of the vertical walls 214, 215 are connected to flanges 216, 217, respectively. The flanges 216, 217 protrude outward from the vertical walls 214, 215, respectively.

[0031] The top plate 213 of the lower side frame 21 is disposed so as to face the top plate 113 of the upper side frame 11. In a cross-sectional view of the side frames 11, 21, the vertical walls 214, 215 of the side frame 21 extend from the top plate 213 toward the side frame 11. The flanges 216, 217 of the side frame 21 are joined to the flanges 116, 117 of the side frame 11, respectively. The flanges 216, 217 are joined to the flanges 116, 117 by, for example, spot welding. In the example of FIG. 2, the flanges 116, 117 of the upper side frame 11 are directly joined to the flanges 216, 217 of the lower side frame 21. However, other members such as, for example, a floor panel may be provided between the side frame 11 and the side frame 21.

[0032] Returning to FIG. 1, the cross member 22 extends in the left-right direction of the vehicle body when the structural member 20 is assembled to the vehicle body. The cross member 22 extends from the side frame 21L to the side frame 21R. The cross member 22 connects the side frames 21L and 21R. In the example shown in FIG. 1, the cross member 22 connects the side frames 21L and 21R to each other at one end side of the longitudinal direction of the side frames 21L and 21R. The cross member 22 is disposed, for example, at the rear end of the structural member 20 when the structural member 20 is assembled to the vehicle body, similar to the upper cross member 12. However, the cross member 22 may connect the middle parts of the side frames 21L and 21R. The cross member 22 may be joined to the upper cross member 12 by, for example, spot welding.

[0033] The structural members 10 and 20 are hot stamped members. That is, the structural member 10 is formed by hot stamping (hot press processing) a blank formed of a plurality of steel plates (sub-blanks). Similarly, the structural member 20 is formed by hot stamping a blank formed of a plurality of steel plates.

[0034] In the upper structural member 10, for example, the side frames 11L, 11R may each be formed of a plurality of steel plates 31, 32. In each of the side frames 11L, 11R, for example, the front portion 111 may be formed of the steel plate 31, and the rear portion 112 may be formed of the steel plate 32. The plate thickness of the steel plate 32 forming the rear portion 112 may be greater than the plate thickness of the steel plate 31 forming the front portion 111. In addition, the tensile strength of the steel plate 32 may be greater than the tensile strength of the steel plate 31. The cross member 12 may be mainly formed of a steel plate 33 different from the steel plates 31, 32 forming the side frames 11L, 11R. Adjacent steel plates of the steel plates 31, 32, 33 are joined to each other by welding.

[0035] Similarly, in the lower structural member 20, for example, the side frames 21L, 21R may each be formed of a plurality of steel plates 41, 42. In each of the side frames 21L, 21R, for example, the front portion 211 may be formed of the steel plate 41, and the rear portion 212 may be formed of the steel plate 42. The plate thickness of the steel plate 42 forming the rear portion 212 may be greater than the plate thickness of the steel plate 41 forming the front portion 211. In addition, the tensile strength of the steel plate 42 may be greater than the tensile strength of the steel plate 41. The cross member 22 may be formed mainly of a steel plate 43 different from the steel plates 41, 42 forming the side frames 21L, 21R. Adjacent steel plates of the steel plates 41, 42, 43 are joined to each other by welding.

[0036] [Manufacturing methods for structural components] Hereinafter, a method for manufacturing the structural members 10 and 20 according to this embodiment will be described with reference to Figures 3A to 3G. The method for manufacturing the structural member 10 includes a step of preparing a blank 30, a step of heating the blank 30, and a step of forming the heated blank 30 into the structural member 10. Similarly, the method for manufacturing the structural member 20 includes a step of preparing a blank 40, a step of heating the blank 40, and a step of forming the heated blank 40 into the structural member 20.

[0037] (preparation process) As shown in Fig. 3A, in manufacturing the upper structural member 10 (Fig. 1), a preparation step involves preparing a blank 30. The blank 30 has a shape obtained by developing the structural member 10. The blank 30 includes a plurality of steel plates (sub-blanks) 31, 32, and 33. The steel plates 31, 32, and 33 are arranged and joined so as to form two long portions 34L, 34R and at least one connecting portion 35.

[0038] The long portions 34L, 34R are arranged side by side in the horizontal direction when viewed from above of the blank 30. The long portion 34L is a portion of the blank 30 that corresponds to the side frame 11L (FIG. 1). The long portion 34R is a portion of the blank 30 that corresponds to the side frame 11R (FIG. 1). In the example of FIG. 3A, the long portions 34L, 34R are formed of steel plates 31, 32, respectively.

[0039] The connecting portion 35 connects the long portions 34L, 34R to each other. The connecting portion 35 is a portion of the blank 30 that corresponds to the cross member 12 (FIG. 1). In the example of FIG. 3A, the connecting portion 35 includes a steel plate 33. The connecting portion 35 may further include a portion of the steel plate 32.

[0040] 3B and 3C are cross-sectional views of the blank 30 showing the joints of the steel plates 31, 32, and 33. FIG. 3B and FIG. 3C are cross-sectional views taken along lines IIIB-IIIB and IIIC-IIIC in FIG. 3A, respectively. Referring to FIG. 3B, the steel plate 31 is butt-joined to the steel plate 32. That is, the end faces of the steel plate 31 are joined in a state where the end face of the steel plate 31 is in contact with the end face of the steel plate 32. Referring to FIG. 3C, the steel plate 32 is butt-joined to the steel plate 33. That is, the other end face of the steel plate 32 is in contact with the end face of the steel plate 33. The steel plates 31, 32, and 33 are joined by, for example, laser welding. In this embodiment, the blank 30 is a so-called tailor weld blank. However, the steel plates 31, 32, and 33 may be joined in a state where the ends of the adjacent steel plates are overlapped (overlap joint). In this case, the steel plates 31, 32, and 33 may be joined by spot welding. In particular, an intersection between the cross member 12 (FIG. 1) extending in the left-right direction of the vehicle body and the side frames 11L, 11R (FIG. 1) extending in the front-rear direction of the vehicle body may have an overlap structure as necessary.

[0041] 3B and 3C, the steel plate 31 has a plate thickness t1. The steel plate 32 has a plate thickness t2. The steel plate 33 has a plate thickness t3. In this embodiment, the t1 of the steel plate 31 is the smallest plate thickness t min The plate thickness t2 of the steel plate 32 is greater than the plate thickness t1 of the steel plate 31. The plate thickness t3 of the steel plate 33 is greater than or equal to the plate thickness t1 of the steel plate 31. In this embodiment, the plate thickness t2 of the steel plate 32 is the maximum plate thickness t max However, the plate thickness t3 of the steel plate 33 is the largest plate thickness t max That is, the plate thickness t3 of the steel plate 33 may be equal to or greater than the plate thickness t2 of the steel plate 32.

[0042] The minimum plate thickness t of steel plates 31, 32, and 33 min and maximum plate thickness t max For example, max -t min ≧0.2(mm) Plate thickness t min ,t max t max -t min The thickness t of the steel plate 31 may satisfy ≦3.2 (mm). min For example, the thickness t is less than 1.4 mm. min may be 0.8 mm or more. max For example, the thickness t is less than 4.0 mm. max may be 1.4 mm or more.

[0043] The steel sheets 31, 32, and 33 may have a known chemical composition as a steel sheet for hot stamping. For example, the chemical composition of the steel sheets 31, 32, and 33 contains, in mass%, C: 0.05 to 0.50%, Si: 0.020 to 1.000%, Mn: 0.20 to 2.50%, Ni: 0 to 0.50%, Cr: 0 to 0.50%, Mo: 0 to 0.5%, and B: 0.0005 to 0.0050%, respectively. The chemical composition of each of the steel plates 31, 32, and 33 may further contain, in mass%, one or more elements selected from the group consisting of Cu: 0.005-3.000%, Co: 0.005-0.500%, Sn: 0.005-0.500%, Ca: 0.0005-0.0050%, Mg: 0.0005-0.0050%, REM: 0.0005-0.0050%, and Sb: 0.0005-0.0200%.

[0044] In the steel plates 31, 32, and 33, the minimum plate thickness t min The chemical composition of steel plate 31 having a thickness of 100 mm differs from the chemical compositions of thicker steel plates 32 and 33. The value of coefficient A calculated by the following formula (1) using the chemical composition of steel plate 31 differs from the coefficient A calculated by formula (1) for each of steel plates 32 and 33 using their respective chemical compositions. A=1.48×(2.7×C+0.4×Si+Mn+0.45×Ni+0.8×Cr+2×Mo) 3.42 (1)

[0045] The content (mass%) of the corresponding element is substituted for the element symbol in formula (1). That is, the coefficient A of the steel plate 31 is calculated by substituting the content (mass%) of each element in the chemical composition of the steel plate 31 for the corresponding element symbol in formula (1). Similarly, the coefficient A of the steel plate 32 is calculated by substituting the content (mass%) of each element in the chemical composition of the steel plate 32 for the corresponding element symbol in formula (1). The value of the coefficient A calculated by formula (1) using the chemical composition of the steel plate 31 is larger than the coefficient A calculated by formula (1) using the chemical composition of the steel plate 32. The coefficient A of the steel plate 32 is the smallest among the coefficients A calculated by formula (1) for the steel plates 32 and 33 other than the steel plate 31. When the coefficient A of the steel plate 31 is A1 and the coefficient A of the steel plate 32 is A2, A1-A2 is preferably 0.10 or more, more preferably 0.20 or more. A1-A2 may be, for example, less than or equal to 11.50.

[0046] The coefficient A of the steel plate 33 is calculated by substituting the content (mass%) of each element in the chemical composition of the steel plate 33 into the corresponding element symbol in formula (1). The value of the coefficient A calculated by formula (1) using the chemical composition of the steel plate 33 is equal to or greater than the value of the coefficient A calculated by formula (1) using the chemical composition of the steel plate 32. The value of the coefficient A of the steel plate 33 is preferably calculated by the minimum plate thickness t min That is, the value of the coefficient A of the steel plate 31 having the smallest plate thickness t min It is preferable that the coefficient A of the steel plate 31 having the above coefficient A is the largest. When the coefficient A of the steel plate 31 is A1 and the coefficient A of the steel plate 33 is A3, A1-A3 is preferably 0.10 or more, and more preferably 0.20 or more. Although not particularly limited, A1-A3 may be 11.50 or less. However, the coefficient A3 of the steel plate 33 may be equal to or greater than the coefficient A1 of the steel plate 31 (A1-A3≦0).

[0047] The chemical compositions of the steel plates 31, 32, and 33 included in the blank 30 may be measured by a general analytical method. For example, analytical test pieces are taken from each of the steel plates 31, 32, and 33, and the test pieces are measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry), thereby obtaining the chemical compositions of the steel plates 31, 32, and 33. In each analytical test piece, C may be measured using a combustion-infrared absorption method.

[0048] As shown in Fig. 3D, in manufacturing the lower structural member 20 (Fig. 1), a preparation step involves preparing a blank 40. The blank 40 has a shape obtained by developing the structural member 20. The blank 40 includes a plurality of steel plates (sub-blanks) 41, 42, 43. The steel plates 41, 42, 43 are arranged and joined so as to form two long portions 44L, 44R and at least one connecting portion 45.

[0049] The long portions 44L, 44R are arranged side by side in the horizontal direction when viewed from above of the blank 40. The long portion 44L is a portion of the blank 40 that corresponds to the side frame 21L (FIG. 1). The long portion 44R is a portion of the blank 40 that corresponds to the side frame 21R (FIG. 1). In the example of FIG. 3D, the long portions 44L, 44R are formed of steel plates 41, 42, respectively.

[0050] The connecting portion 45 connects the long portions 44L, 44R to each other. The connecting portion 45 is a portion of the blank 40 that corresponds to the cross member 22 (FIG. 1). In the example of FIG. 3D, the connecting portion 45 includes a steel plate 43. The connecting portion 45 may further include a portion of the steel plate 42.

[0051] The blank 40 is configured similarly to the blank 30 (FIGS. 3B and 3C) with respect to the steel plates 41, 42, and 43. That is, the configurations of the steel plates 31, 32, and 33 of the blank 30 (FIGS. 3B and 3C) can be directly applied to the steel plates 41, 42, and 43. Therefore, detailed description of the configurations of the steel plates 41, 42, and 43 will be omitted.

[0052] (Heating process) The prepared blanks 30, 40 are formed into the structural members 10, 20 (FIG. 1), respectively, by hot stamping. During the hot stamping, the blanks 30, 40 are subjected to a heating step. Referring to FIG. 3E, in the heating step, the blank 30 is heated, for example, by a heating furnace. The plurality of steel plates 31, 32, 33 included in the blank 30 are heated to an austenite transformation completion temperature (A c3 The steel plates 31, 32, and 33 are heated to, for example, 900° C. or higher. This causes the microstructures of the steel plates 31, 32, and 33 to transform into an austenite phase. Although not shown, the multiple steel plates 41, 42, and 43 (FIG. 3D) included in the blank 40 are also heated to an austenite transformation completion temperature (A c3 It is heated to above this temperature.

[0053] (molding process) Referring to Fig. 3F, in the forming step, the heated blank 30 is formed into the structural member 10 (Fig. 1) and quenched using a die 60. The blank 30 heated in the heating step is removed from the heating furnace and transported to the die 60. The die 60 may be attached to a known press device. The die 60 includes, for example, a punch 61 and a die 62. The blank 30 is disposed between the punch 61 and the die 62.

[0054] 3G, after the blank 30 is placed between the punch 61 and the die 62, the die 62 approaches relatively to the punch 61. The blank 30 is clamped (pressed) by the punch 61 and the die 62, and formed into a shape along the forming surfaces of the punch 61 and the die 62. The blank 30 is held while being clamped by the punch 61 and the die 62. The blank 30 is cooled (quenched) by the die 60, and its microstructure is transformed into martensite. In this way, the structural member 10 can be manufactured from the blank 30.

[0055] Although not shown, the blank 40 shown in Fig. 3D is also subjected to a forming process similar to that of the blank 30. That is, using a die, the heated blank 40 is formed into the structural member 20 (Fig. 1) and quenched. The structural member 20 is joined to the structural member 10 (Fig. 1) by, for example, welding.

[0056] Referring again to FIG. 1, the chemical compositions of the steel sheets 31, 32, and 33 do not change before and after hot stamping. Therefore, in the structural member 10, the minimum sheet thickness t min The value of coefficient A calculated by the above formula (1) using the chemical composition of steel plate 31 having the above formula (1) is larger than the coefficient A calculated by the formula (1) using the chemical composition of steel plate 32. The value of coefficient A calculated by the formula (1) using the chemical composition of steel plate 31 is preferably larger than the coefficient A calculated by the formula (1) using the chemical composition of steel plate 33. However, the value of coefficient A calculated by the formula (1) using the chemical composition of steel plate 31 may be equal to or smaller than the coefficient A calculated by the formula (1) using the chemical composition of steel plate 33.

[0057] The chemical composition of the steel sheets 31, 32, 33 in the structural member 10 after hot stamping can be obtained by the same analytical method as that for the chemical composition of the steel sheets 31, 32, 33 at the blank 30 stage.

[0058] Minimum plate thickness t min In the cross section of the structural member 10 at the position of the steel plate 31 having the minimum plate thickness t, when the variation in martensite fraction is calculated by subtracting the minimum martensite fraction (%) from the maximum martensite fraction (%), the variation in martensite fraction is, for example, 20% or less. The variation in martensite fraction is preferably 15% or less, and more preferably 10% or less. The variation in martensite fraction can be measured as follows. That is, when the minimum plate thickness t minTen 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 31 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.

[0059] 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 31 having the minimum plate thickness t min When 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.

[0060] 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.

[0061] After the forming process (hot stamping), the steel sheets 31, 32, and 33 can have a tensile strength of, for example, 0.5 GPa or more. The steel sheets 31, 32, and 33 preferably have a tensile strength of 1.0 GPa or more. Similarly, after the forming process (hot stamping), at least one of the steel sheets 31, 32, and 33 may have a tensile strength of 1.5 GPa or more. The tensile strength of each of the steel sheets 31, 32, and 33 may be the same as or different from the tensile strength of the other steel sheets.

[0062] The lower structural member 20 can have the same configuration as the upper structural member 10. That is, in the structural member 20, the value of the coefficient A calculated by the above formula (1) using the chemical composition of the steel plate having the smallest plate thickness is also larger than the coefficient A calculated by the formula (1) using the chemical composition of another steel plate having a larger plate thickness. min The variation in the martensite fraction at the position of the steel plate having the above structure is, for example, 20% or less, preferably 15% or less, and more preferably 10% or less, similar to the structural member 10.

[0063] [effect] In the blank 30 according to this embodiment, the minimum plate thickness t min The coefficient A1 of the steel plate 31 having the above thickness is larger than the coefficient A2 of the thicker steel plate 32. The coefficients A1 and A2 are determined based on the chemical compositions of the steel plates 31 and 32, and are expressed by the formula (1): A=1.48×(2.7×C+0.4×Si+Mn+0.45×Ni+0.8×Cr+2×Mo). 3.42 The coefficients A1 and A2 correspond to the time (transformation start time) until the steel sheets 31 and 32 start diffusion transformation, respectively, after the heating of the blank 30 for hot stamping is completed. However, the coefficients A1 and A2 correspond to the transformation start time when only the influence of the elements is considered for each of the steel sheets 31 and 32, without considering the influence of the sheet thickness. When the coefficient A1 is larger than the coefficient A2, it means that the steel sheet 31 is made of a material having a higher hardenability than the steel sheet 32, in other words, a material that starts diffusion transformation during cooling slowly. By making the coefficient A1 larger than the coefficient A2 as in this embodiment, the minimum sheet thickness tmin The start of the diffusion transformation in the steel sheet 31 having the minimum sheet thickness t can be delayed, and the difference in the transformation start time between the steel sheets 31 and 32 can be made smaller than when the coefficient A1 is equal to or smaller than the coefficient A2. Therefore, when the blank 30 is hot stamped, not only the relatively thick steel sheet 32 ​​but also the steel sheet having the minimum sheet thickness t min This allows the steel plate 31 to be well quenched, and the hardness of the structural member formed from the blank 30 is easily made uniform. Furthermore, by well quenching the steel plate 31, stress is offset by transformation plasticity and residual stress is reduced, making it possible to suppress the occurrence of twisting, warping, and the like in the structural member 10 due to concentration of residual stress. As a result, deterioration of the dimensional accuracy of the structural member 10 can be reduced.

[0064] Thus, in this embodiment, even though the blank 30 includes the steel plate 31 that is thinner than the steel plate 32, the hardness of the structural member 10 formed by hot stamping from the blank 30 can be made uniform, and deterioration of dimensional accuracy can be suppressed. Therefore, the impact absorption performance (crash resistance performance) of the structural member 10, particularly of a large structural member 10, can be improved.

[0065] In this embodiment, the hardenability of the relatively thin steel plate 31 is improved, so that the hardness of the structural member 10 formed from the blank 30 can be made uniform. More specifically, the minimum plate thickness t min Since the steel plate 31 having the above properties is well quenched, the variation in martensite fraction in the steel plate 31 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 a structural member 10 including a steel plate 31 having a small plate thickness, particularly a relatively large structural member 10 used for a vehicle body or the like, is formed from the blank 30, it is possible to reduce strength defects in the structural member 10 and improve the impact absorption performance of the structural member 10.

[0066] 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.

[0067] The lower-side structural member 20 and the blank 40 have the same configuration as the upper-side structural member 10 and the blank 30. Therefore, the lower-side structural member 20 and the blank 40 can achieve the same effects as those described above.

[0068] <Second embodiment> FIG. 4 is a cross-sectional view of a blank 30A according to the second embodiment. In FIG. min 4 shows a joint between a steel plate 31 having a minimum plate thickness t2 and a steel plate 32 having a greater plate thickness t3. The blank 30A according to this embodiment has a configuration similar to that of the blank 30 according to the first embodiment, but differs from the blank 30 according to the first embodiment in that a coating 50 is provided on the steel plate 31. In the example of FIG. min The steel plate 31 having the above structure is covered on one surface with a coating 50.

[0069] The coating 50 is a substantially black coating. For example, the lightness L * Value (CIE 1976 Lightness Index L as specified in JIS Z8781-4:2013 * ) is equal to or less than 60, the coating 50 can be determined to be substantially black. The coating 50 may be a carbon-based surface treatment coating (a coating containing carbon (C)).

[0070] The coating 50 may contain, for example, carbon black. The coating 50 may further contain a metal oxide. The metal oxide may be, for example, one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide. The coating 50 may contain silica. As the coating 50, for example, the surface treatment coating described in International Publication No. 2022 / 215229 may be used.

[0071] The coating 50 may contain graphite or soot instead of or in addition to carbon black. Alternatively, the coating 50 may contain, for example, an acicular compound having a hexagonal crystal structure with an aspect ratio of 4 to 50. 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.

[0072] The blank 30A according to this embodiment is formed into a structural member 10 (FIG. 1) by the same manufacturing method as in the first embodiment. In the blank 30A, the emissivity of the surface of the steel plate 31 is increased by a substantially black coating 50 applied to the surface of the steel plate 31. The surface of the steel plate 31 covered with the coating 50 has an emissivity of 60% or more at a measurement temperature of 25° C. and a wavelength of 8.0 μm, for example. In this way, the minimum plate thickness t min By increasing the emissivity of the steel sheet 31 having the above-mentioned emissivity, when the blank 30A is heated during hot stamping, the temperature of the steel sheet 31 can be increased. Therefore, the temperature of the steel sheet 31 can be increased quickly to the austenite region temperature, and the high-temperature holding time of the steel sheet 31 can be secured for a long time. As a result, the austenite crystal grains in the microstructure of the steel sheet 31 become coarse, and the diffusion transformation of the steel sheet 31 after the heating process is completed can be further delayed. Therefore, the steel sheet 31 can be more satisfactorily quenched. At least a part of the coating 50 can remain on the surface of the steel sheet 31 after hot stamping.

[0073] In this embodiment, one surface of the steel sheet 31 is coated with the coating 50. However, both surfaces of the steel sheet 31 may be coated with the coating 50, which is substantially black. The coating 50 may or may not be provided on one or both surfaces of the other steel sheets 32, 33 (FIG. 3C). However, from the viewpoint of ensuring a longer high-temperature retention time of the steel sheet 31 from the start to the completion of heating of the blank 30A, it is preferable that the coating 50 is not provided on at least one of the other steel sheets 32, 33.

[0074] The lower blank 40 (FIG. 3D) and the structural member 20 formed therefrom can have a similar configuration to the upper blank 30A in this embodiment and the structural member 10 formed therefrom. Therefore, the lower blank 40 and the structural member 20 can also achieve the same effects as those described above.

[0075] <Third embodiment> 5 is a cross-sectional view of a blank 30B according to a third embodiment. min 1 shows a joint between a steel sheet 31 having a thickness t1 and a steel sheet 32 ​​having a larger thickness t2. The blank 30B according to this embodiment has a configuration generally similar to that of the blank 30 according to the first embodiment, but differs from the blank 30 according to the first embodiment in that the steel sheets 31 and 32 are plated steel sheets.

[0076] In the example of FIG. 5, the steel sheet 31 has a base steel sheet 31a and an aluminum-based plating layer 31b. The aluminum-based plating layer 31b covers both surfaces of the base steel sheet 31a. The aluminum-based plating layer 31b is provided over both surfaces or over almost the entirety of the base steel sheet 31a. Similarly, the steel sheet 32 ​​has a base steel sheet 32a and an aluminum-based plating layer 32b. The aluminum-based plating layer 32b covers both surfaces of the base steel sheet 32a. The aluminum-based plating layer 32b is provided over both surfaces or over almost the entirety of the base steel sheet 32a. In this embodiment, the sheet thickness t of the steel sheet 31 is 100 μm. minis the plate thickness including the base steel sheet 31a and the aluminum-based plating layer 31b. Moreover, the plate thickness t2 of the steel sheet 32 ​​is the plate thickness including the base steel sheet 32a and the aluminum-based plating layer 32b.

[0077] The chemical composition of the aluminum-based plating layers 31b, 32b is not particularly limited. A known aluminum-based plating layer (a plating layer mainly composed of aluminum) can be used as the aluminum-based plating layers 31b, 32b. Although not particularly limited, the aluminum-based plating layers 31b, 32b are, for example, Al-Si-based plating layers. The chemical compositions of the aluminum-based plating layers 31b, 32b may be the same or different.

[0078] The coating weight of the aluminum-based plating layer 31b on the steel sheet 31 is W1 (g / m 2 ), the coating weight of the aluminum-based plating layer 32b on the steel sheet 32 ​​is W2 (g / m 2 ), the deposition weights W1 and W2 are 20 g / m 2 More than 120g / m 2 or less. The coating weight W1 of the aluminum-based plating layer 31b is the average coating weight on both surfaces of the base steel sheet 31a. The coating weight W2 of the aluminum-based plating layer 32b is the average coating weight on both surfaces of the base steel sheet 32a.

[0079] The coating weights W1 and W2 are preferably 30 g / m 2 More preferably, it is 35 g / m 2 The deposition weights W1 and W2 are preferably 115 g / m 2 More preferably, it is 100 g / m or less. 2 However, the minimum plate thickness t min The coating weight W1 of the aluminum-based plating layer 31b in the steel sheet 31 having a thickness t2 is smaller than the coating weight W2 of the aluminum-based plating layer 32b in the steel sheet 32 ​​having a thickness t2 larger than that of the steel sheet 32. The difference between the coating weights W1 and W2, i.e., W2-W1, is, for example, 10 (g / m 2 ) or more. W2-W1 is preferably 20 (g / m 2 ) or more, more preferably 30 (g / m2 ) or more. W2-W1 is 80 (g / m 2 ) or less. W2-W1 is preferably 70 (g / m 2 ) or less, more preferably 60 (g / m 2 ) or less. The adhesion amounts W1 and W2 satisfy the relationship W2 / W1>1.0. The adhesion amounts W1 and W2 can preferably satisfy the relationship W2 / W1≧1.2, and more preferably W2 / W1≧1.5.

[0080] When the steel sheets 31 and 32 are plated steel sheets, the coefficients A1 and A2 are calculated using the chemical compositions of the base steel sheets 31a and 32a. That is, the coefficient A1 of the steel sheet 31 is calculated by substituting the content (mass%) of each element in the chemical composition of the base steel sheet 31a into the above-mentioned formula (1). Similarly, the coefficient A2 of the steel sheet 32 ​​is calculated by substituting the content (mass%) of each element in the chemical composition of the base steel sheet 32a into the formula (1). In this embodiment, as in the first embodiment, the coefficient A1 of the steel sheet 31 is larger than the coefficient A2 of the steel sheet 32. The coefficient A2 of the steel sheet 32 ​​is the smallest coefficient A among the coefficients A calculated by the formula (1) for each of the multiple steel sheets included in the blank 30B.

[0081] The blank 30B according to this embodiment is formed into a structural member 10 (FIG. 1) by the same manufacturing method as that of the first embodiment. minThe coating weight W1 of the aluminum-based plating layer 31b of the steel sheet 31 having the thickness t2 is smaller than the coating weight W2 of the aluminum-based plating layer 32b of the steel sheet 32 ​​having the thickness t2 larger than that of the steel sheet 32. As a result, when the blank 30B is heated during hot stamping, the temperature rise rate of the steel sheet 31 is significantly higher than that of the steel sheet 32. Specifically, since the aluminum-based plating layer 31b on the surface of the steel sheet 31 is relatively thin, when the blank 30B is heated, alloying of the aluminum-based plating layer 31b with the iron contained in the base steel sheet 31a progresses rapidly to the surface of the steel sheet 31, and both surfaces of the steel sheet 31 change to black or a color close to black. That is, the emissivity of both surfaces of the steel sheet 31 increases during the heating process. Therefore, the steel sheet 31 can be quickly heated to a temperature in the austenite range to ensure a long high-temperature holding time of the steel sheet 31. As a result, the austenite crystal grains in the microstructure of the steel sheet 31 become coarse, and the diffusion transformation of the steel sheet 31 after the heating process is completed can be further delayed. Therefore, the steel plate 31 can be more satisfactorily hardened.

[0082] In the structural member 10 (FIG. 1) formed from the blank 30B, when the average thickness (plating thickness) of the aluminum-based plating layer 31b on both surfaces of the steel sheet 31 is K1 (μm) and the average thickness (plating thickness) of the aluminum-based plating layer 32b on both surfaces of the steel sheet 32 ​​is K2 (μm), the plating thickness K1 of the steel sheet 31 is smaller than the plating thickness K2 of the steel sheet 32. The difference between the plating thicknesses K1 and K2: K2-K1, is, for example, 7 (μm) or more. K2-K1 may be 33 (μm) or less. In addition, the plating thicknesses K1 and K2 can satisfy the relationship K2 / K1>1.0. K2 / K1 is preferably 1.2 or more, more preferably 1.5 or more.

[0083] In this embodiment, the steel sheet 33 (FIG. 3C) may be a plated steel sheet having a base steel sheet and a plating layer like the steel sheets 31 and 32, or may be a steel sheet (bare material) without a plating layer on the surface. When the steel sheet 33 is a plated steel sheet, the plating layer may be an aluminum-based plating layer or a metal plating layer other than aluminum. When the steel sheet 33 is a plated steel sheet, the adhesion amount and thickness of the plating layer on the base steel sheet are not particularly limited. When the steel sheet 33 is a plated steel sheet, the coefficient A3 of the steel sheet 33 is calculated by substituting the content (mass%) of each element in the chemical composition of the base steel sheet into the above-mentioned formula (1). Even when the steel sheets 31, 32, and 33 are plated steel sheets, the chemical compositions of the steel sheets 31, 32, and 33 can be measured by the general analysis method described in the first embodiment. The analysis of the chemical composition of the steel sheets 31, 32, and 33 may be performed after removing the plating layer on the surface by mechanical grinding.

[0084] The configuration of the blank 30B according to this embodiment can also be combined with the blanks 30, 30A according to the first and second embodiments, respectively. That is, in each of the blanks 30, 30A, the steel sheet 31 is a plated steel sheet having a base steel sheet 31a and an aluminum-based plating layer 31b, and the steel sheet 32 ​​is a plated steel sheet having a base steel sheet 32a and an aluminum-based plating layer 32b, and the coating weight W1 of the aluminum-based plating layer 31b on the steel sheet 31 may be less than the coating weight W2 of the aluminum-based plating layer 32b on the steel sheet 32.

[0085] However, in the first and second embodiments, the coating weight W1 of the aluminum-based plating layer 31b on the steel sheet 31 may be equal to or greater than the coating weight W2 of the aluminum-based plating layer 32b on the steel sheet 32. In addition, in the first and second embodiments, the plating layers of the steel sheets 31, 32 may be plating layers of a metal other than aluminum, or the steel sheets 31, 32 may be steel sheets (bare materials) that do not have a plating layer on their surfaces.

[0086] The lower blank 40 (FIG. 3D) and the structural member 20 formed therefrom can have a configuration similar to that of the upper blank 30B in this embodiment and the structural member 10 formed therefrom. Therefore, the lower blank 40 and the structural member 20 can also achieve the same effects as those described above.

[0087] <Fourth embodiment> 6 is an exploded perspective view of the structural members 10C, 20C according to this embodiment. The structural members 10, 20 according to the above embodiments constitute a front under module of the vehicle body. On the other hand, the structural members 10C, 20C according to this embodiment constitute a rear under module of the vehicle body.

[0088] 6, the structural member 10C includes a pair of side frames 11L, 11R and at least one cross member 12, similar to the above embodiment. The structural member 20C includes a pair of side frames 21L, 21R and at least one cross member 22, similar to the above embodiment. In the example shown in FIG. 6, the cross member 12 connects the middle parts of the side frames 11L, 11R. Similarly, the cross member 22 connects the middle parts of the side frames 21L, 21R. The configurations of the structural member 10 and the structural member 20 described in the other embodiments can be applied to the structural member 10C and the structural member 20C of this embodiment, respectively.

[0089] The structural member 10C can be manufactured from a blank 30C shown in Fig. 7 by a manufacturing method similar to the manufacturing method described in the first embodiment. The blank 30C includes steel plates 31, 32, and 33. The steel plates 31, 32, and 33 are arranged and joined to form two long portions 34L and 34R and at least one connecting portion 35. The blank 30C can have a configuration similar to any of the blanks 30, 30A, and 30B described in the other embodiments.

[0090] In this embodiment, the steel plate 31 forming each rear portion 112 (FIG. 6) of the side frames 11L, 11R has a smaller plate thickness than the steel plate 32 forming the front portion 111 (FIG. 6). The tensile strength of the steel plate 31 may be smaller than the tensile strength of the steel plate 32. In the structural member 10C according to this embodiment and the structural member 10 according to the other embodiments (FIG. 1), it is preferable that the plate thickness and / or tensile strength of the steel plate located further outboard in the longitudinal direction of the vehicle body is smaller than that of the steel plate located further inboard. As a result, when a collision load in the longitudinal direction is input to the vehicle body, in the structural members 10, 10C, the portion located further outboard in the vehicle body deforms to absorb the collision energy, while the portion located further inboard in the vehicle body is less likely to deform, thereby protecting the surrounding components.

[0091] The structural member 20C can be manufactured from a blank 40C shown in Fig. 8 by a manufacturing method similar to the manufacturing method described in the first embodiment. The blank 40C includes steel plates 41, 42, and 43. The steel plates 41, 42, and 43 are arranged and joined to form two long portions 44L and 44R and at least one connecting portion 45. The blank 40C can have a configuration similar to any of the blanks 30, 30A, and 30B described in the other embodiments.

[0092] In this embodiment, the steel plate 41 forming each rear portion 212 (FIG. 6) of the side frames 21L, 21R has a smaller plate thickness than the steel plate 42 forming the front portion 211 (FIG. 6). The tensile strength of the steel plate 41 may be smaller than the tensile strength of the steel plate 42. In the lower structural members 20, 20C (FIGS. 1 and 6), similarly to the upper side, it is preferable that the plate thickness and / or tensile strength of the steel plate located more outboard in the longitudinal direction of the vehicle body is smaller than that of the steel plate located more inboard. As a result, when a collision load in the longitudinal direction is input to the vehicle body, in the structural members 20, 20C, the portion located more outboard in the vehicle body deforms to absorb the collision energy, while the portion located more inboard in the vehicle body is less likely to deform, thereby protecting the surrounding components.

[0093] 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.

[0094] In the blanks 30, 30A, 30B, and 30C according to the above-mentioned embodiments, the steel plate 31 is butt-joined to the steel plate 32. Also, the steel plate 32 is butt-joined to the steel plate 33. However, in the blanks 30, 30A, 30B, and 30C, each steel plate may be overlapped and joined to the other steel plate. For example, as shown in FIG. 9A, the end of the steel plate 31 may be overlapped with the end of the steel plate 32 and joined to the end of the steel plate 32 by, for example, spot welding or laser welding, so that the steel plates 31 and 32 form an overlap portion 36. Similarly, as shown in FIG. 9B, the end of the steel plate 32 may be overlapped with the end of the steel plate 33 and joined to the end of the steel plate 33 by, for example, spot welding or laser welding, so that the steel plates 32 and 33 form an overlap portion 36. In the blanks 30, 30A, 30B, and 30C, the joining method for adjacent steel plates may be butt joining or lap joining. Similarly, in the blanks 40 and 40C, the joining method for adjacent steel plates may be butt joining or lap joining.

[0095] In the first embodiment, the blank 30 includes steel plates 31, 32 corresponding to the long portions 34L, 34R (side frames 11L, 11R), and a steel plate 33 corresponding to the connecting portion 35 (cross member 12). However, the number and arrangement of the steel plates included in the blank 30 and the structural member 10 manufactured from the blank 30 are not limited to this. As shown in Figs. 10 to 12, the number and arrangement of the steel plates can be changed as appropriate.

[0096] For example, as shown in Fig. 10, in a blank 30 corresponding to a structural member 10 (Fig. 1) for a front under module, the long portions 34L, 34R may each be formed of a single steel plate 31. In the blank 30, the connecting portion 35 may connect one end of the long portions 34L, 34R in the longitudinal direction to each other, as in the first embodiment. That is, the long portions 34L, 34R may be connected by the connecting portion 35 at the end side that is disposed in the front or rear when the structural member 10 is assembled to the vehicle body.

[0097] In the first embodiment, the structural member 10 is provided with a single cross member 12, and therefore the blank 30 for the structural member 10 also includes a single connecting portion 35. However, the structural member 10 may include a plurality of cross members 12. In this case, as shown in Figs. 11 and 12, the blank 30 also includes a plurality of connecting portions 35. These connecting portions 35 are formed of separate steel plates 32, 33 or steel plates 33, 37. The long portions 34L, 34R may each be formed of a single steel plate 31 as shown in Fig. 11, or may each be formed of a plurality of steel plates 31, 32 as shown in Fig. 12.

[0098] Although not shown, the number and arrangement of steel plates are not particularly limited for the blanks 30A, 30B according to other embodiments and the structural member 10 formed therefrom, and for the lower structural member 20 and blank 40. The structural member 10 and the blanks 30, 30A, 30B, 40 each need only include two or more joined steel plates. The structural members 10, 20 and the blanks 30, 30A, 30B, 40 each preferably include three or more steel plates. Each of the blanks 30, 30A, 30B, 40 has at least a minimum plate thickness t min A first steel plate having a plate thickness t min The blank 30, 30A, and 30B may include a second steel plate having a thickness greater than t. The first steel plate is joined to the second steel plate directly or indirectly via another steel plate. min The coefficient A1 of the steel plate 31 having the above-mentioned i(i=2, 3, . . . ) is larger than the coefficient A2 of the steel plate 32, which is the smallest among the blanks 30, 30A, and 30B. min When there are multiple steel plates 31 having the above thickness, it is preferable that the coefficients A1 of all the steel plates 31 are greater than the coefficient A2 of the steel plate 32. When the blanks 30, 30A, 30B each contain three or more steel plates, the coefficients A of the steel plates other than the steel plates 31, 32 are greater than or equal to the coefficient A2 of the steel plate 32. Similarly, in the blank 40, the coefficient A1 of the steel plate having the smallest thickness is greater than the smallest value of the coefficients A calculated by formula (1) for the steel plates having larger thicknesses.

[0099] In the fourth embodiment, the blank 30C corresponds to the structural member 10C of the rear under module. The blank 30C includes steel plates 31 and 32 corresponding to the long portions 34L and 34R (side frames 11L and 11R), respectively, and a steel plate 33 corresponding to the connecting portion 35 (cross member 12). However, the number and arrangement of the steel plates included in the blank 30C and the structural member 10C manufactured from the blank 30C are not limited to this. As shown in Figs. 13 to 23, the number and arrangement of the steel plates can be changed as appropriate.

[0100] For example, as shown in Figures 13 and 14, in a blank 30C, the long portions 34L, 34R may each be formed of a single steel plate 31. In this case, the connecting portion 35 may connect the long portions 34L, 34R at one end side in the longitudinal direction. For example, the long portions 34L, 34R may be connected by the connecting portion 35 at the end side that is disposed forward when the structural member 10C (Figure 11) is assembled to the vehicle body. The long portions 34L, 34R may be connected by the connecting portion 35 at their intermediate portions.

[0101] As shown in Fig. 15, in the blank 30C, even when the long portions 34L, 34R are formed of a plurality of steel plates 31, 32, the long portions 34L, 34R may be connected by a connecting portion 35 at the end side disposed forward when the structural member 10C (Fig. 6) is assembled to the vehicle body. For example, when the portion corresponding to the front portion 111 (Fig. 6) of the side frames 11L, 11R is formed of the steel plate 31 and the portion corresponding to the rear portion 112 (Fig. 6) is formed of the steel plate 32, the connecting portion 35 may be joined to the steel plate 31 as shown in Figs. 15 and 16, or may be joined to the steel plate 32 as shown in Fig. 17.

[0102] In the fourth embodiment, since the structural member 10C (FIG. 6) is provided with a single cross member 12, the blank 30C for the structural member 10C also includes a single connecting portion 35. However, the structural member 10C may include a plurality of cross members 12. In this case, as shown in FIGS. 18 to 23, the blank 30C also includes a plurality of connecting portions 35. The connecting portion 35 is formed of separate steel plates 32, 33, steel plates 33, 37, or steel plates 32, 33, 37. In this case, the long portions 34L, 34R may each be formed of a single steel plate 31 as shown in FIGS. 18, 19, and 23, or may each be formed of a plurality of steel plates 31, 32 as shown in FIGS. 20 to 22.

[0103] Although not shown in the drawings, in the fourth embodiment, the number and arrangement of steel plates in the lower structural member 20C and blank 40C are not particularly limited. The structural members 10C, 20C and blanks 30C, 40C each need only include two or more joined steel plates. The structural members 10C, 20C and blanks 30C, 40C each preferably include three or more steel plates. Each of the blanks 30C, 40C has at least a minimum plate thickness t min A first steel plate having a plate thickness t min The blank 30C may include a second steel plate having a thickness greater than t. The first steel plate is joined to the second steel plate directly or indirectly via another steel plate. minThe coefficient A1 of the steel plate 31 having the above-mentioned i (i=2, 3, . . . ) is larger than the coefficient A2 of the steel plate 32, which is the smallest among the steel plates 32. min When there are multiple steel plates 31 having the above thickness, it is preferable that the coefficients A1 of all the steel plates 31 are greater than the coefficient A2 of the steel plate 32. When the blank 30C includes three or more steel plates, the coefficients A of the steel plates other than the steel plates 31 and 32 are greater than or equal to the coefficient A2 of the steel plate 32. Similarly, in the blank 40C, the coefficient A1 of the steel plate having the smallest thickness is greater than the smallest value of the coefficients A calculated by formula (1) for the steel plates having larger thicknesses.

[0104] In the above embodiment, each of the blanks and the multiple steel plates (sub-blanks) included in each structural member may be a single layer or multiple layers. That is, each of the sub-blanks may be a single steel plate or a plate material formed by overlapping multiple steel plates.

[0105] In the above embodiment, the mold 60 used in the forming process includes a punch 61 and a die 62. However, the configuration of the mold 60 is not limited to the example described in the above embodiment. The mold 60 can further include, for example, a pad and a blank holder. The mold 60 may be configured according to the target structural member.

[0106] In the structural members 10, 10C, 20, and 20C according to the above embodiments, the side frames 11 and 21 have a substantially hat-shaped cross section. However, the shape of the cross section of the side frames 11 and 21 is not necessarily limited to this. For example, as shown in FIG. 24, the side frames 11 and 21 may have a shape in which one side in the width direction is open in cross section. In this case, other members (not shown) may be joined to the open portion of the side frames 11 and 21, and the side frames 11 and 21 and the other members may form a closed cross section. Similarly, the cross members 12 and 22 may also have a substantially hat-shaped cross section, or may have a cross section of another shape. EXAMPLES

[0107] 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.

[0108] In order to confirm the effects of the present disclosure, a CAE analysis was performed on the press forming (hot stamping) of the structural components of the front or rear under module 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 components, as well as the division pattern of the structural components.

[0109] The types of steel sheets (materials) used in this analysis are shown in Table 1. For each material, Table 1 shows the content (mass%) of each element in the base material, the type of plating, and the coefficient A calculated by the above-mentioned formula (1). In this analysis, materials were selected from Table 1 to construct the target structural members.

[0110] [Table 1]

[0111] The division patterns of the structural members are shown in Figures 25A to 25G. The structural members shown in Figures 25A, 25B, 25D, and 25E are structural members on the upper or lower side of the front under-module. The structural members shown in Figures 25C, 25F, and 25G are structural members on the upper or lower side of the rear under-module. Figures 25A to 25G show the number of steel plates (materials) included in the structural members and the positions of the joints between the steel plates in the structural members. In Figures 25A to 25G, each steel plate is given a number in parentheses.

[0112] Table 2 shows the analysis conditions and results for the structural members shown in Figs. 25A to 25C. In Figs. 25A to 25C, the side frames of the structural members are each formed from two pieces of material (1) and (2). The cross member is mainly formed from material (3). In each structural member, materials (1) to (3) are each butt-jointed with the adjacent material.

[0113] [Table 2]

[0114] Referring to Table 2, in Example 1, the minimum plate thickness t min The coefficient A is the largest for the material (2) having a thickness of 1.2 mm. In Example 1, the coefficient A1 of the thinnest material (2) is significantly larger than the smallest coefficient A2 of the other materials (1) and (3). In Example 2, the coefficient A1 of the thinnest material (2) having a thickness of 1.2 mm is the largest for the material (2) having a thickness of 1.2 mm. min The coefficient A1 of the material (1) having a thickness of 1.2 mm is significantly larger than the minimum coefficient A2 of the other materials (2) and (3). In Example 3, the coefficient A1 of the material (1) having the minimum thickness t min The coefficient A value is the largest for material (3) having a thickness of 1.2 mm. In Example 3, the coefficient A1 of the thinnest material (3) is also significantly larger than the smallest coefficient A2 of the other materials (1) and (2). In contrast, in Comparative Examples 1 and 2, the coefficient A1 of the thinnest material is equal to or smaller than the coefficient A of the other materials.

[0115] As described above, the coefficient A corresponds to the transformation start time for each material when only the influence of the elements is considered, but the actual transformation start time for each material is also affected by the plate thickness, and becomes shorter as the plate thickness becomes smaller. The "phase transformation start time" in Table 2 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 (the time until the thinnest material starts phase transformation). As shown in Table 2, the phase transformation start time was longer in Examples 1 to 3 than in Comparative Examples 1 and 2. For example, when comparing Example 3 and Comparative Example 2, which are the same conditions except for the material of the thinnest material (3), it can be seen that the phase transformation start time in Example 3 is slower than that in Comparative Example 2. In Example 3, the coefficient A1 of the thinnest material (3) is larger than the smallest coefficient A2 among the other materials (1) and (2), and the hardenability of the thinnest material is higher than the hardenability of the other materials. On the other hand, in Comparative Example 2, the coefficient A1 of the thinnest material (3) is smaller than the coefficient A of the other materials (1) and (2), and the hardenability of the thinnest material is lower than that of the other materials. In Example 3, the hardenability of the thinnest material is made higher than that of the other materials, so that the phase transformation start time of the thinnest material is extended compared to Comparative Example 2, and the phase transformation start time of the thinnest material and the phase transformation start time of the relatively thick other materials are uniformed. Therefore, in Example 3, after the heating of the blank is completed, it becomes easier to start forming before the phase transformation to ferrite starts in the thinnest material, and it becomes easier to harden the structural member uniformly.

[0116] Table 3 shows the analysis conditions and results for the structural members shown in Figures 25D and 25E. In Figure 25D, the side frames of the structural member are formed from two pieces of material (3) and (4), respectively. The structural member in Figure 25D includes multiple cross members formed from material (1) or (2). In Figure 25E, the side frames of the structural member are formed from two pieces of material (1) and (3), respectively. The structural member in Figure 25E includes multiple cross members formed from material (2) or (4). In each structural member, materials (1) to (4) are each butt-jointed with the adjacent material.

[0117] [Table 3]

[0118] Referring to Table 3, in Examples 4 to 11, the minimum plate thickness t min The coefficient A1 of the material is larger than the minimum coefficient A2 among the other relatively thick materials. In contrast, in Comparative Examples 3 to 7, the coefficient A1 of the material (1) to (4) is the minimum thickness t min The coefficient A1 of the thickness t is smaller than the minimum coefficient A2 among other materials with relatively thick walls. min If there are multiple pieces of material with the same thickness, the minimum thickness t min The smallest value among the coefficients A of the materials having this property was set as coefficient A1, and this was compared with the coefficients A2 of the other materials.

[0119] When each Example is compared with the corresponding Comparative Example, it can be seen that the phase transformation start time is extended. For example, when Example 6 and Comparative Example 5, which are the same except for the material of the thinnest material (4), are compared, the phase transformation start time of Example 6 is delayed compared to Comparative Example 5. In Example 6, the coefficient A1 of the thinnest material (3) is larger than the smallest coefficient A2 among the other materials (1), (2), and (4), and the hardenability of the thinnest material is higher than the hardenability of one or more other materials. On the other hand, in Comparative Example 5, the coefficient A1 of the thinnest material (3) is equal to or smaller than the smallest coefficient A2 among the other materials (1), (2), and (4), and the hardenability of the thinnest material is equal to or smaller than the hardenability of the other materials. In Example 6, the hardenability of the thinnest material is made higher than the hardenability of the other materials, so that the phase transformation start time of the thinnest material is extended compared to Comparative Example 5, and the phase transformation start time of the thinnest material and the phase transformation start time of the relatively thick other materials are uniformed. Therefore, in Example 6, after the heating of the blank was completed, forming was likely to begin before the phase transformation to ferrite began in the thinnest material, and the structural member was likely to be quenched uniformly.

[0120] Table 4 shows the analysis conditions and results for the structural members shown in Figures 25F and 25G. In Figures 25F and 25G, the side frames of the structural members are formed of two pieces of material (1) and (2), respectively. The structural member in Figure 25F includes a cross member formed of material (3) and a cross member formed of materials (4) and (5). The structural member in Figure 25G includes a cross member formed of materials (3) and (4) and a cross member formed of material (5). In each structural member, materials (1) to (5) are butt-jointed to the adjacent materials.

[0121] [Table 4]

[0122] Referring to Table 4, in Examples 12 to 17, the minimum plate thickness t min The coefficient A1 of the material is larger than the minimum coefficient A2 among the other relatively thick materials. In contrast, in Comparative Examples 8 to 11, the coefficient A1 of the material with the smallest plate thickness t min The coefficient A1 of the material is smaller than the minimum coefficient A2 among other relatively thick materials. min If there are multiple pieces of material with the same thickness, the smallest thickness t min The smallest value among the coefficients A of the materials having this property was set as coefficient A1, and this was compared with the coefficients A2 of the other materials.

[0123] When each Example is compared with the corresponding Comparative Example, it can be seen that the phase transformation start time is extended. For example, when Example 12 and Comparative Example 8, which are the same except for the materials of the thinnest materials (2) and (4), are compared, the phase transformation start time of Example 12 is delayed compared to Comparative Example 8. In Example 12, the coefficient A1 of the thinnest materials (2) and (4) is larger than the smallest coefficient A2 among the other materials (1), (3), and (5), and the hardenability of the thinnest material is higher than the hardenability of one or more other materials. In Example 12, the coefficient A1 of the thinnest materials (2) and (4) is larger than all the coefficients A of the other materials (1), (3), and (5). On the other hand, in Comparative Example 8, the coefficient A1 of the thinnest materials (2) and (4) is equal to or smaller than the coefficients A of the other materials (1), (3), and (5), and the hardenability of the thinnest material is equal to or smaller than the hardenability of the other materials. In Example 12, the hardenability of the thinnest material was made higher than that of the other materials, so that the phase transformation start time of the thinnest material was extended compared to Comparative Example 8, and the phase transformation start time of the thinnest material and the phase transformation start time of the other relatively thick materials were made uniform. Therefore, in Example 12, after heating of the blank was completed, it became easier to start forming before the phase transformation to ferrite started in the thinnest material, and it became easier to quench the structural member uniformly.

[0124] Also, for example, comparing Example 13 and Comparative Example 9, which are the same except for the materials of the thinnest materials (1), (2), and (5), the phase transformation start time is delayed in Example 13 compared to Comparative Example 9. In Example 13, the smallest coefficient A1 among the thinnest materials (1), (2), and (5) is larger than the smallest coefficient A2 among the other materials (3) and (4), and the hardenability of the thinnest material is higher than the hardenability of the other materials. In Example 13, all of the coefficients A of the thinnest materials (1), (2), and (5) are larger than the coefficients A of the other materials (3) and (4). On the other hand, in Comparative Example 9, the smallest coefficient A1 among the thinnest materials (1), (2), and (5) is equal to or smaller than the smallest coefficient A2 among the other materials (3) and (4), and the hardenability of the thinnest material is equal to or smaller than the hardenability of the other materials. In Example 13, the hardenability of the thinnest material was made higher than that of the other materials, so that the phase transformation start time of the thinnest material was extended compared to Comparative Example 9, and the phase transformation start time of the thinnest material and the phase transformation start time of the other relatively thick materials were made uniform. Therefore, in Example 13, after heating of the blank was completed, it became easier to start forming before the phase transformation to ferrite started in the thinnest material, and it became easier to quench the structural member uniformly.

[0125] In the examples and comparative examples shown in Table 4, the materials are butted together and then laser-jointed (butt joint), whereas in the examples and comparative examples shown in Table 5 below, some of the materials form overlapping portions and are joined by, for example, spot welding.

[0126] [Table 5]

[0127] In the examples and comparative examples shown in Table 5, similarly to the examples and comparative examples shown in Table 4, materials are arranged in the division patterns shown in Figures 25F and 25G. In the examples and comparative examples shown in Table 5 with the division pattern of Figure 25F, materials (3) and (4) are each overlap-joined to material (1). Meanwhile, materials (1) and (2), and materials (4) and (5) are butt-joined. In the examples and comparative examples shown in Table 5 with the division pattern of Figure 25G, materials (4) and (5) are each overlap-joined to material (1). Meanwhile, materials (1) and (2), and materials (3) and (4) are butt-joined.

[0128] As shown in Table 5, in Examples 18 and 19, the minimum plate thickness t min The coefficient A1 of the material is larger than the minimum coefficient A2 among the other relatively thick materials. On the other hand, in Comparative Examples 12 and 13, the coefficient A1 of the material having the smallest plate thickness t min The coefficient A1 is smaller than the smallest coefficient A2 among other relatively thick materials.

[0129] As can be seen from Table 5, the phase transformation start times of Examples 18 and 19 are slower than those of Comparative Examples 12 and 13. Therefore, it was confirmed that even when an overlapping portion exists in a blank, the phase transformation start times of each material in the blank can be made uniform by making the coefficient A1 of the thinnest material larger than the smallest coefficient A2 among the other materials.

[0130] The blank was heated at a furnace temperature of 920°C until the entire blank reached 910°C, and then transferred to a press machine in 17 seconds, where hot stamping was performed at a forming speed of 40 mm / s, and the blank was held at the bottom dead center for 20 seconds while being pressed at 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 6.

[0131] [Table 6]

[0132] Examples 5, 6, and 18 in Table 6 are examples formed under the same conditions as Examples 5, 6, and 18 shown in Tables 3 and 5. Comparative Examples 5 and 12 in Table 6 are comparative examples formed under the same conditions as Comparative Examples 5 and 12 shown in Tables 3 and 5. In Table 6, the variation in martensite fraction refers to the variation in the minimum sheet thickness t min The 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.

[0133] Shape accuracy was evaluated based on how far apart a structural component is from the mating component at the overlapping portion when the structural component is attached to another component. In this analysis, a structural component was given an A rating if 90% or more of the surface of the mating component was within ±0.5 mm of the structural component, a B rating if 70% or more but less than 90% of the surface was within ±0.5 mm of the mating component, and a C rating if less than 70% of the surface was within ±0.5 mm of the mating component.

[0134] Regarding the impact absorption performance, when the structural member is a rear module (FIG. 25F), a rear impact and a side impact with the structural member assembled to the vehicle were assumed, and an impactor simulating a vehicle was collided with the structural member, and the maximum intrusion amount during the rear impact and the maximum intrusion amount during the side impact were evaluated. On the other hand, when the structural member is a front module (FIG. 25E), a front impact and a side impact with the structural member assembled to the vehicle were assumed, and an impactor simulating a vehicle was collided with the structural member, and the maximum intrusion amount during the front impact and the maximum intrusion amount during the side impact were evaluated. The impact absorption performance was evaluated based on the impact absorption performance of the under module formed by hot stamping each material and then joining them, and compared with the impact absorption performance of the base. In Table 6, impact absorption performance equivalent to the impact absorption performance of the base is indicated 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 the impact absorption performance as poor.

[0135] While each example shown in Table 6 satisfies A1-A2>0, each comparative example satisfies A1-A2≦0. While the martensite fraction variation exceeded 20% in comparative examples 5 and 12, the martensite fraction variation was 20% or less in examples 5, 6, and 18. The martensite fraction variation was reduced to 15% or less in examples 5, 6, and 18. The shape accuracy was also better in examples 5, 6, and 18 than in comparative examples 5 and 12.

[0136] In Examples 5, 6, and 18, which had small variations in the martensite fraction, the impact absorption performance was improved compared to Comparative Examples 5 and 12. In particular, in Examples 5 and 18, impact absorption performance equal to or higher than that of the base was ensured. In other words, even though a structural member was formed by integrating multiple materials at the blank stage, impact absorption performance equal to or higher than that of a structural member formed by joining the materials after press molding them individually was ensured. [Explanation of symbols]

[0137] 10, 10C: Structural members 11, 11L, 11R: Side frame 12: Cross member 20, 20C: Structural members 21, 21L, 21R: Side frame 22: Cross member 30, 30A, 30B, 30C: Blank 31,32,33,37: Steel plate 34L, 34R: Long section 35:Connection part 40,40C:Blank 41,42,43: Steel plate 44L, 44R: Long section 45:Connection part 60: Mold

Claims

1. A blank for hot stamping, The blank has a plurality of steel plates that are arranged and joined to form two long portions arranged side by side in a plan view of the blank and a connecting portion that connects the long portions to each other, 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, in which the value of coefficient A calculated by the following formula (1) using the chemical composition of the first steel plate is greater than the value of coefficient A calculated by the following formula (1) using the chemical composition of the second steel plate. A=1.48×(2.7×C+0.4×Si+Mn+0.45×Ni+0.8×Cr+2×Mo) ３．４２ (1) In the above formula (1), the element symbols are substituted with the contents (mass%) of the corresponding elements.

2. 2. The blank of claim 1, The thickness of the first steel plate is t ｍｉｎ The maximum thickness among the thicknesses of the plurality of steel plates is t ｍａｘ Then, t ｍａｘ -t ｍｉｎ ≧0.2 (mm).

3. A method for manufacturing a structural member, comprising the steps of: Providing a blank according to claim 1 or 2; Heating the plurality of steel plates included in the blank to an austenite transformation completion temperature or higher; forming the heated blank with a die and quenching it; A manufacturing method comprising:

4. A structural member for a vehicle body, comprising: A pair of side frames; A cross member connecting the side frames; Equipped with The side frame and the cross member are formed by a plurality of steel plates joined together, including a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than the plate thickness of the first steel plate, A structural member, wherein the value of coefficient A calculated by the following formula (1) using the chemical composition of the first steel plate is greater than the value of coefficient A calculated by the following formula (1) using the chemical composition of the second steel plate. A=1.48×(2.7×C+0.4×Si+Mn+0.45×Ni+0.8×Cr+2×Mo) ３．４２ (1) In the above formula (1), the element symbols are substituted with the contents (mass%) of the corresponding elements.

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