Blank, method for producing structural member, and structural member

Abstract

A blank (30, 30A, 30B, 30C) comprises a plurality of steel sheets. The plurality of steel sheets are arranged and joined so as to form two long sections (34L, 34R) and a connection section (35). The plurality of steel sheets include a first steel sheet (31) and a second steel sheet (32). The first steel sheet (31) has a minimum sheet thickness (tmin). The second steel sheet (32) has a sheet thickness (t2) greater than the sheet thickness (tmin) of the first steel sheet (31). The value of a coefficient A calculated by formula (1) using the chemical composition of the first steel sheet (31) is greater than the value of the coefficient A calculated by formula (1) using the chemical composition of the second steel sheet (32).　Formula (1): A = 1.48 × (2.7 × C + 0.4 × Si + Mn + 0.45 × Ni + 0.8 × Cr + 2 × Mo)3.42

Description

Blank, manufacturing method of structural member, and structural member

[0001] The present disclosure relates to a blank, a method for manufacturing a structural member, and a structural member.

[0002] Structures such as automobile bodies are formed from multiple structural members. Structural members are manufactured, for example, by press-forming a blank. To ensure high strength and good dimensional accuracy, 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 press-formed using a die. The blank is then 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 a plurality of blanks. In Patent Document 1, the plurality of 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 body structure. The method for manufacturing a vehicle rear body 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 portion of the aft rail is greater than the yield strength of the middle portion, which in turn is greater than the yield strength of the aft portion. Patent Document 2 describes that when two adjacent portions of an aft rail have the same composition but are intended to ultimately have different yield strengths, the different yield strengths can be obtained by one or a combination of the following methods: during hot forming, the portion intended to have a lower yield strength is heated to a lower temperature than the portion intended to have a higher yield strength; after hot forming, the portion intended to have a lower yield strength is cooled at a slower rate than the portion intended to have a higher yield strength; and / or these portions are subjected to the same hot forming and cooling after the hot forming process, but the portion intended to have a lower yield strength is subsequently subjected to an additional heat treatment to reduce its yield strength.

[0006] Special table No. 2021-528248 Publication No. 2019-503920

[0007] In recent years, in order to simplify the manufacturing process of structures, integration of two or more components from the blank stage has been considered. For example, as described in Patent Documents 1 and 2, hot stamping of a composite blank (tailor-welded blank) including multiple steel plates (sub-blanks) has been considered to form structural components that were previously formed separately into a single component. However, if the composite blank contains multiple steel plates with different thicknesses, the performance of the structural component may be reduced. Specifically, during hot stamping, the blank is heated, for example, in a heating furnace until its microstructure is austenitized, and then formed into a structural component using a die. However, because thinner steel plates are more easily cooled than thicker steel plates, the portions of the blank where the thinner steel plates are located may undergo further cooling and undergo diffusion transformation before the blank is removed from the heating furnace and forming begins, potentially making the structural component less resistant to hardening. This can easily result in non-uniform hardness in the structural component. Furthermore, thinner steel plates are less resistant to hardening and are more likely to retain residual stress, which can lead to twisting, warping, and other problems in the structural component, potentially reducing the dimensional accuracy of the structural component. 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 structural components, particularly large structural components, when the structural components include a steel plate having a thickness smaller than that of other steel plates.

[0009] A hot stamping blank 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 the horizontal direction when viewed from above 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 the smallest thickness among the plurality of steel plates. The second steel plate has a 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 x (2.7 x C + 0.4 x Si + Mn + 0.45 x Ni + 0.8 x Cr + 2 x Mo) ３．４２ (1) However, the element symbols in the above formula (1) are substituted with the contents (mass%) of the corresponding elements.

[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 than other steel plates is formed, the performance of the component can be improved.

[0011] FIG. 1 is an exploded perspective view of a structural member according to a first embodiment. FIG. 2 is a cross-sectional view of a side frame included in each of the structural members shown in FIG. 1. FIG. 3A is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment, showing a blank corresponding to one of the structural members shown in FIG. 1. FIG. 3B is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment, showing a cross-sectional view of the blank shown in FIG. 3A. FIG. 3C is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment, showing another cross-sectional view of the blank shown in FIG. 3A. FIG. 3D is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment, showing a blank corresponding to the other of the structural members shown in FIG. 1. FIG. 3E is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment. FIG. 3F is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment. FIG. 3G is a schematic view for explaining a method for manufacturing a structural member according to the first embodiment. FIG. 4 is a cross-sectional view of a blank according to a second embodiment. FIG. 5 is a cross-sectional view of a blank according to a third embodiment. FIG. 6 is an exploded perspective view of a structural member according to a fourth embodiment. FIG. 7 is a plan view of a blank according to the fourth embodiment. FIG. 8 is a plan view of another blank according to the fourth embodiment. FIG. 9A is a cross-sectional view of a blank according to a modification of each embodiment. FIG. 9B is another cross-sectional view of a blank according to a modification of each embodiment. FIG. 10 is a plan view of a blank according to a modification of the first embodiment. FIG. 11 is a plan view of a blank according to another modification of the first embodiment. FIG. 12 is a plan view of a blank according to yet another modification of the first embodiment. FIG. 13 is a plan view of a blank according to a modification of the fourth embodiment. FIG. 14 is a plan view of a blank according to another modification of the fourth embodiment. FIG. 15 is a plan view of a blank according to yet another modification of the fourth embodiment. FIG. 16 is a plan view of a blank according to yet another modification of the fourth embodiment. FIG. 17 is a plan view of a blank according to yet another modification of the fourth embodiment. FIG. 18 is a plan view of a blank according to yet another modification of the fourth embodiment. FIG. 19 is a plan view of a blank according to yet another modification of the fourth embodiment. FIG. 20 is a plan view of a blank according to yet another modification of the fourth embodiment.

[0047] Fig. 21 is a plan view of a blank according to yet another modified example of the fourth embodiment. Fig. 22 is a plan view of a blank according to yet another modified example of the fourth embodiment. Fig. 23 is a plan view of a blank according to yet another modified example of the fourth embodiment. Fig. 24 is a cross-sectional view of a side frame included in a structural member according to the modified examples of each embodiment. Fig. 25A is a diagram showing a division pattern of a structural member in an example. Fig. 25B is a diagram showing another division pattern of a structural member in an example. Fig. 25C is a diagram showing yet another division pattern of a structural member in an example. Fig. 25D is a diagram showing yet another division pattern of a structural member in an example. Fig. 25E is a diagram showing yet another division pattern of a structural member in an example. Fig. 25F is a diagram showing yet another division pattern of a structural member in an example. Fig. 25G is a diagram showing yet another division pattern of a structural member in an example.

[0012] For example, as described in "Ueno Masakatsu and Ito Kametaro, 'A new prediction formula for the hardenability of steel to replace the GROSSMANN formula,' Iron and Steel, The Iron and Steel Institute of Japan, 74th year (1988) No. 6, pp. 1073-1080," the critical cooling rate V has conventionally been used as an index of the hardenability of steel. ｃ９０ The critical cooling rate V ｃ９０ is the critical cooling rate (°C / s) at which a martensite structure of 90% or more in volume fraction is obtained, 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 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 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 (plate thickness: 1.2 mm) similar to those in the examples (Table 1) described below in a heating furnace at 900°C for 1 minute, then removed from the heating furnace and air-cooled, and measured the time until the phase transformation starts (transformation start time). Then, regression analysis was performed using the measured transformation start time data, and the formula for converting β to the transformation start time was used: A = 1.48 × β ３．４２ The inventors further utilized coefficient A to study the appropriate arrangement of steel materials in a blank. As a result, the inventors completed a blank according to an embodiment.

[0014] A hot stamping blank 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 the horizontal direction when viewed from above 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 the smallest thickness among the plurality of steel plates. The second steel plate has a 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 x (2.7 x C + 0.4 x Si + Mn + 0.45 x Ni + 0.8 x Cr + 2 x 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, if a first steel plate having a minimum thickness and a second steel plate having a greater thickness have the same hardenability, the thin-walled first steel plate will begin its austenite-to-ferrite phase transformation (diffusion transformation) earlier than the thick-walled second steel plate after the blank is heated for hot stamping. However, in a blank according to the first configuration, the coefficient A calculated based on the chemical composition of the first steel plate is greater 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 with higher hardenability than the second steel plate, i.e., a material that initiates diffusion transformation later during cooling. Therefore, after the blank is heated, the initiation of diffusion transformation in the first steel plate is delayed, thereby reducing the difference in transformation initiation time between the first steel plate and the second steel plate. As a result, when the blank is hot stamped, not only the relatively thick second steel plate but also the minimum-thick first steel plate can be well hardened, which facilitates uniform hardness of the structural component formed from the blank. Furthermore, since the first steel plate is well hardened, 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 the concentration of residual stress, and as a result, reduces the deterioration of dimensional accuracy.

[0016] In this way, in the first configuration, even though the blank including the two long portions and the connecting portion 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, thereby improving the impact absorption performance (crash resistance) of structural members, particularly large structural members.

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

[0018] A method for manufacturing 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 included in the blank to a temperature equal to or higher than the austenite transformation completion temperature, and forming and quenching the heated blank using a mold (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 together. The plurality of steel plates include 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 value of coefficient A calculated using the chemical composition of the first steel plate by the following formula (1) is greater than the value of coefficient A calculated using the chemical composition of the second steel plate by the following formula (1) (fourth configuration). A = 1.48 x (2.7 x C + 0.4 x Si + Mn + 0.45 x Ni + 0.8 x Cr + 2 x 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 equivalent components are designated by the same reference numerals, and the same description will not be repeated.

[0021] <First embodiment> [Structural member] 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 vehicle body.

[0022] The structural member 10 is an upper module. That is, when assembled to the vehicle body, the structural member 10 is disposed above the structural member 20. 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-to-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] When the structural member 10 is assembled to the vehicle body, the cross member 12 extends in the left-right direction of 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 of the side frames 11L, 11R in the longitudinal direction. 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 also connect the middle portions of the side frames 11L, 11R.

[0025] The structural member 20 is a lower module. That is, when assembled to the vehicle body, the structural member 20 is disposed below the structural member 10. 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 behind 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. Figure 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 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 side frames 21.

[0028] 2 is a cross-sectional view (horizontal cross-section) of the side frames 11, 21 taken 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 and 115, and flanges 116 and 117. In a cross-sectional view of the side frame 11, one ends of the vertical walls 114 and 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 and 115 are connected to flanges 116 and 117, respectively. The flanges 116 and 117 protrude outward from the vertical walls 114 and 115, respectively.

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

[0031] The top plate 213 of the lower side frame 21 is disposed 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 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 longitudinal end of the side frames 21L and 21R. Like the upper cross member 12, 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. However, the cross member 22 may also connect the middle portions of the side frames 21L and 21R. The cross member 22 may also 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 working) a blank formed from a plurality of steel plates (sub-blanks). Similarly, the structural member 20 is formed by hot stamping a blank formed from a plurality of steel plates.

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

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

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

[0037] (Preparation Process) As shown in Figure 3A, in the preparation process for manufacturing the upper structural member 10 (Figure 1), a blank 30 is prepared. The blank 30 has the shape of the structural member 10 when unfolded. 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 to form two long portions 34L and 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 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 from 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 between the steel plates 31, 32, and 33. FIGS. 3B and 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 face of the steel plate 31 is joined to the end face of the steel plate 32 with the end face abutting against the end face of the steel plate 32. Referring to FIG. 3C, the steel plate 32 is butt-joined to the steel plate 33 with the other end face of the steel plate 32 abutting against the end face of the steel plate 33 with the other end face being joined. 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-welded blank. However, the steel plates 31, 32, and 33 may also be joined to adjacent steel plates with their ends overlapping (overlap joint). In this case, the steel plates 31, 32, 33 may be joined by spot welding. In particular, the intersections 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 t １ The steel plate 32 has a plate thickness t ２ The steel plate 33 has a plate thickness t ３ In this embodiment, the t １ is the smallest thickness t of the steel plates 31, 32, and 33 ｍｉｎ The thickness t of the steel plate 32 ２ is the thickness t of the steel plate 31 １ The thickness t of the steel plate 33 is larger than ３ is the thickness t of the steel plate 31 １ In this embodiment, the plate thickness t ２ is the maximum thickness t of the steel plates 31, 32, and 33 ｍａｘ However, the thickness t of the steel plate 33 ３ is the maximum thickness t among the steel plates 31, 32, and 33 ｍａｘ That is, the thickness t ３ is the thickness t of the steel plate 32 ２ It may be more than that.

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

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

[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 1.5 μm is different from the chemical compositions of thicker steel plates 32 and 33. The value of coefficient A calculated using the chemical composition of steel plate 31 with the following formula (1) is different from the coefficient A calculated using the chemical composition of each of steel plates 32 and 33 with the formula (1): A=1.48×(2.7×C+0.4×Si+Mn+0.45×Ni+0.8×Cr+2×Mo) ３．４２ (1)

[0045] The element symbols in formula (1) are substituted with the content (mass%) of the corresponding element. That is, the coefficient A of steel plate 31 is calculated by substituting the content (mass%) of each element in the chemical composition of steel plate 31 into the corresponding element symbol in formula (1). Similarly, the coefficient A of steel plate 32 is calculated by substituting the content (mass%) of each element in the chemical composition of steel plate 32 into the corresponding element symbol in formula (1). The value of coefficient A calculated by formula (1) using the chemical composition of steel plate 31 is larger than the coefficient A calculated by formula (1) using the chemical composition of steel plate 32. The coefficient A of steel plate 32 is the smallest of the coefficients A calculated by formula (1) for steel plates 32 and 33 other than steel plate 31. The coefficient A of steel plate 31 is calculated as A １ , the coefficient A of the steel plate 32 is A ２ When this is done, A １ -A ２ is preferably 0.10 or more, and more preferably 0.20 or more. １ -A ２ may be, for example, 11.50 or less.

[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 formula (1) using the minimum plate thickness t ｍｉｎ That is, among the plurality of steel plates 31, 32, and 33 included in the blank 30, 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 coefficient A is the largest. １ , the coefficient A of the steel plate 33 is A ３ When this is done, A １ -A ３ is preferably 0.10 or more, more preferably 0.20 or more. １ -A ３ However, the coefficient A of the steel plate 33 may be 11.50 or less. ３ is the coefficient A of the steel plate 31 １ The above (A １ -A ３≦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 may be taken from each of the steel plates 31, 32, and 33, and the test pieces may be 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 Figure 3D, in manufacturing the lower structural member 20 (Figure 1), a blank 40 is prepared in the preparation step. The blank 40 has the shape of the structural member 20 when unfolded. The blank 40 includes a plurality of steel plates (sub-blanks) 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.

[0049] The long portions 44L, 44R are arranged side by side in the horizontal direction when viewed from above 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 from 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 has the same configuration as 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 applied directly 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 Step) The prepared blanks 30, 40 are formed into the structural members 10, 20 (FIG. 1) 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 a temperature above the austenite transformation completion temperature (A ). 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 plurality of steel plates 41, 42, and 43 ( FIG. 3D ) included in the blank 40 are also heated to a temperature above the austenite transformation completion temperature (A ). ｃ３ It is heated above this temperature (point).

[0053] (Forming Process) Referring to Fig. 3F, in the forming process, the heated blank 30 is formed into the structural member 10 (Fig. 1) using a die 60 and quenched. The blank 30 heated in the heating process 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 placed 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 moves relatively close to the punch 61. The blank 30 is clamped (pressed) between the punch 61 and the die 62 and formed into a shape that conforms to the forming surfaces of the punch 61 and the die 62. The blank 30 remains clamped between 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 the same forming process as 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 plates 31, 32, and 33 do not change before and after hot stamping. Therefore, in the structural member 10, the minimum plate thickness t ｍｉｎ The value of coefficient A calculated by the above formula (1) using the chemical composition of steel plate 31 having the formula (I) is larger than the coefficient A calculated by the formula (I) using the chemical composition of steel plate 32. The value of coefficient A calculated by the formula (I) using the chemical composition of steel plate 31 is preferably larger than the coefficient A calculated by the formula (I) using the chemical composition of steel plate 33. However, the value of coefficient A calculated by the formula (I) using the chemical composition of steel plate 31 may be equal to or smaller than the coefficient A calculated by the formula (I) using the chemical composition of steel plate 33.

[0057] The chemical composition of the steel plates 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 plates 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 martensite fraction (%), the martensite fraction variation is, for example, 20% or less, when the difference is the maximum martensite fraction (%) minus the minimum martensite fraction (%). The martensite fraction variation is preferably 15% or less, and more preferably 10% or less. The martensite fraction variation can be measured as follows. That is, ｍｉｎ Ten or more analysis samples (for example, a length of about 10 mm) are cut out from positions at least 20 mm away from the end of the cross section of the structural member 10 at the position of the steel plate 31 having the above structure, and each sample is at least 10 mm away from the end. Then, each sample is mirror-polished and etched with LePeller's reagent so that the observation surface is in the thickness direction. Then, an optical microscope is used to observe the area from the surface of the steel plate to a depth of 1 / 4 of the plate thickness (the area from the surface of the steel plate to a depth of 1 / 8 of the plate thickness to a depth of 3 / 8 of the plate thickness) with a magnification of 1000x and a field of view of 2,400 μm. ２ The above structural photographs are taken from 30 fields of view, and the obtained structural photographs are subjected to image analysis.

[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 of each analysis sample to determine the martensite fraction, and the average value is used 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 If there are multiple steel plates having the above structure, such 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] In some steel sheets, 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 plates 31, 32, and 33 can have a tensile strength of, for example, 0.5 GPa or more. The steel plates 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 plates 31, 32, and 33 may have a tensile strength of 1.5 GPa or more. The tensile strength of each of the steel plates 31, 32, and 33 may be the same as or different from the tensile strength of the other steel plates.

[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 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 A of the steel plate 31 having １ is the coefficient A of the thicker steel plate 32 ２ Coefficient A is greater than １ , A ２ is calculated based on the chemical compositions of the steel plates 31 and 32, using the formula (1): A = 1.48 × (2.7 × C + 0.4 × Si + Mn + 0.45 × Ni + 0.8 × Cr + 2 × Mo) ３．４２ The coefficient A is the value obtained by calculating １ , A ２ corresponds to the time (transformation start time) from the completion of heating of the blank 30 for hot stamping until the steel sheets 31 and 32 start to undergo diffusion transformation. １ , A ２ is a value corresponding to the transformation start time when only the influence of elements is considered for each of the steel plates 31 and 32, without considering the influence of the plate thickness. １ is coefficient A ２ If the coefficient A is larger than 0.01, it means that the steel plate 31 is made of a material with higher hardenability than the steel plate 32, in other words, a material that starts diffusion transformation slowly during cooling. １ > Coefficient A ２ By doing so, the minimum plate thickness t ｍｉｎ The start of the diffusion transformation in the steel sheet 31 having the coefficient A １ is coefficient A ２Therefore, when the blank 30 is hot stamped, not only the relatively thick steel sheet 32 ​​but also the steel sheet 31 having the minimum thickness t ｍｉｎ This allows the steel plate 31 to be well quenched, which makes it easier to uniformize the hardness of the structural member formed from the blank 30. 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 the concentration of residual stress. As a result, deterioration in the dimensional accuracy of the structural member 10 can be reduced.

[0064] As described above, 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 a thickness of 1000 .0 mm or less can be quenched well, the variation in the martensite fraction in the steel plate 31 can be kept to 20% or less. This makes it less likely for deformation concentration to occur when a collision load is input to the structural member 10, making it easier for the structural member 10 to exhibit high impact absorption performance. Therefore, even when a structural member 10 including a thin steel plate 31, particularly a relatively large structural member 10 used in 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 non-uniformity there is 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 parts with insufficient hardenability, i.e., parts with insufficient hardness, are unevenly distributed within the structural member 10, and when the structural member 10 is deformed by a collision, deformation tends to concentrate in the parts with insufficient hardness, thereby reducing the functionality of the structural member 10.

[0067] The lower-side structural member 20 and blank 40 have the same configuration as the upper-side structural member 10 and blank 30. Therefore, the lower-side structural member 20 and 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 a second embodiment. ｍｉｎ and a steel plate 31 having a larger plate thickness t ２ The blank 30A 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 a coating 50 is provided on the steel plate 31. In the example of FIG. 4, ｍｉｎ The steel plate 31 having the above structure has one surface covered with a coating 50.

[0069] The coating 50 is a substantially black coating. For example, the lightness L ＊ Value (CIE 1976 lightness index L defined in JIS Z8781-4:2013) ＊ ) is 60 or less, 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 also contain silica. For example, the surface treatment coating described in WO 2022 / 215229 may be used as the coating 50.

[0071] The coating 50 can contain graphite, soot, or the like instead of or in addition to carbon black. Alternatively, the coating 50 can 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 coated 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, the temperature rise of the steel sheet 31 can be accelerated when the blank 30A is heated during hot stamping. Therefore, the steel sheet 31 can be quickly heated to a temperature in the austenite range, ensuring a long high-temperature holding time for the steel sheet 31. As a result, the austenite crystal grains in the microstructure of the steel sheet 31 become coarse, which can further delay the diffusion transformation of the steel sheet 31 after the heating process is completed. Therefore, the steel sheet 31 can be more satisfactorily quenched. At least a portion 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 plate 31 is coated with the coating 50. However, both surfaces of the steel plate 31 may be coated with a substantially black coating 50. The coating 50 may or may not be provided on one or both surfaces of the other steel plates 32, 33 (FIG. 3C). However, from the viewpoint of ensuring a longer high-temperature holding time for the steel plate 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 plates 32, 33.

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

[0075] Third Embodiment Fig. 5 is a cross-sectional view of a blank 30B according to a third embodiment. ｍｉｎ and a steel plate 31 having a larger plate thickness t ２ 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 plate 31 has a base steel plate 31a and an aluminum-based plating layer 31b. The aluminum-based plating layer 31b covers both surfaces of the base steel plate 31a. The aluminum-based plating layer 31b is provided over both surfaces or over almost the entire surface of the base steel plate 31a. Similarly, the steel plate 32 has a base steel plate 32a and an aluminum-based plating layer 32b. The aluminum-based plating layer 32b covers both surfaces of the base steel plate 32a. The aluminum-based plating layer 32b is provided over the entire surface or over almost the entire surface of both surfaces of the base steel plate 32a. In this embodiment, the plate thickness t ｍｉｎ is the plate thickness including the base steel plate 31a and the aluminum-based plating layer 31b. ２is the plate thickness including the base steel plate 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 containing aluminum as a main component) 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 plate 32 is W2 (g / m ２ ), the deposition amounts W1 and W2 are 20 g / m ２ 120g / m or more ２ 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 deposition amounts W1 and W2 are preferably 30 g / m ２ More preferably, 35 g / m ２ The deposition amounts 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 larger plate thickness t ２ The difference between the coating weights W1 and W2 (W2-W1) is, for example, 10 (g / m ２ ) or more. W2-W1 is preferably 20 (g / m ２ ) or more, more preferably 30 (g / m ２ ) or more. W2-W1 is 80 (g / m ２) or less. W2-W1 is preferably 70 (g / m ２ ) or less, more preferably 60 (g / m ２ The adhesion amounts W1 and W2 satisfy the relationship W2 / W1>1.0. The adhesion amounts W1 and W2 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, coefficient A １ , A ２ is calculated using the chemical compositions of the base steel plates 31a and 32a. １ is calculated by substituting the content (mass%) of each element in the chemical composition of the base steel plate 31a into the above-mentioned formula (1). ２ is calculated by substituting the content (mass%) of each element in the chemical composition of the base steel plate 32a into formula (1). １ is the coefficient A of the steel plate 32 ２ The coefficient A of the steel plate 32 is larger than ２ is the smallest coefficient A among the coefficients A calculated by equation (1) for each of the multiple steel plates included in the blank 30B.

[0081] The blank 30B according to this embodiment is formed into the structural member 10 (FIG. 1) by the same manufacturing method as in the first embodiment. ｍｉｎ The coating weight W1 of the aluminum-based plating layer 31b of the steel sheet 31 having a larger plate thickness t ２The thickness W2 of the aluminum-based plating layer 32b of the steel sheet 32 ​​having the thickness W1 is smaller than the thickness W2 of the aluminum-based plating layer 32b of the steel sheet 32 ​​having the thickness W1. 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, because 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 turn black or a color similar 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, ensuring a long high-temperature holding time for the steel sheet 31. As a result, austenite grains in the microstructure of the steel sheet 31 coarsen, further delaying the diffusion transformation of the steel sheet 31 after the heating process is completed. Therefore, the steel sheet 31 can be more effectively quenched.

[0082] In a 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. Furthermore, the plating thicknesses K1 and K2 may 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, similar to 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 A of the steel sheet 33 ３is 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 analytical method described in the first embodiment. Analysis of the chemical compositions 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. 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 smaller 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. Furthermore, 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 the same configuration as the upper blank 30B of 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] 6 is an exploded perspective view of 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] Referring to Figure 6, the structural member 10C, like the above-described embodiment, includes a pair of side frames 11L, 11R and at least one cross member 12. Similarly, the structural member 20C, like the above-described embodiment, includes a pair of side frames 21L, 21R and at least one cross member 22. In the example shown in Figure 6, the cross member 12 connects the middle portions of the side frames 11L, 11R. Similarly, the cross member 22 connects the middle portions 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 Figure 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 plates 31 forming the rear portions 112 ( FIG. 6 ) of the side frames 11L, 11R have a thickness smaller than that of the steel plates 32 forming the front portions 111 ( FIG. 6 ). The tensile strength of the steel plates 31 may be smaller than that of the steel plates 32. In the structural member 10C according to this embodiment and the structural member 10 ( FIG. 1 ) according to other embodiments, it is preferable that the thickness and / or tensile strength of the steel plates located further outward in the fore-aft direction of the vehicle body be smaller than that of the steel plates located further inward. As a result, when a collision load is input to the vehicle body in the fore-aft direction, in the structural members 10, 10C, the portions located further outward in the vehicle body deform to absorb the collision energy, while the portions located further inward in the vehicle body are less likely to deform, thereby protecting surrounding components.

[0091] The structural member 20C can be manufactured from a blank 40C shown in Figure 8 using a manufacturing method similar to that 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 plates 41 forming the rear portions 212 ( FIG. 6 ) of the side frames 21L, 21R have a smaller thickness than the steel plates 42 forming the front portions 211 ( FIG. 6 ). The tensile strength of the steel plates 41 may be smaller than the tensile strength of the steel plates 42. In the lower structural members 20, 20C ( FIGS. 1 and 6 ), similar to the upper structural members, it is preferable that the thickness and / or tensile strength of the steel plates located further outward in the fore-aft direction of the vehicle body be smaller than those of the steel plates located further inward. As a result, when a longitudinal collision load is applied to the vehicle body, the outer portions of the structural members 20, 20C deform to absorb the collision energy, while the inner portions are less likely to deform, thereby protecting 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 embodiments, the steel plate 31 is butt-joined to the steel plate 32. 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 lap-joined to another 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, thereby forming an overlap portion 36 between the steel plates 31 and 32. 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, thereby forming the overlap portion 36 between the steel plates 32 and 33. In the blanks 30, 30A, 30B, and 30C, the joining method for adjacent steel plates may be either butt joining or lap joining. Similarly, in the blanks 40 and 40C, the joining method for adjacent steel plates may be either 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 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 Figures 10 to 12, the number and arrangement of steel plates can be changed as appropriate.

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

[0097] In the first embodiment described above, 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 multiple cross members 12. In this case, as shown in Figures 11 and 12, the blank 30 also includes multiple connecting portions 35. These connecting portions 35 are formed from separate steel plates 32, 33 or steel plates 33, 37. The long portions 34L, 34R may each be formed from a single steel plate 31 as shown in Figure 11, or may each be formed from multiple steel plates 31, 32 as shown in Figure 12.

[0098] Although not shown, the number and arrangement of steel plates in the blanks 30A, 30B and the structural members 10 formed therefrom according to other embodiments, as well as the lower structural member 20 and blank 40, are not particularly limited. The structural member 10 and blanks 30, 30A, 30B, 40 each need only include two or more joined steel plates. Preferably, the structural members 10, 20 and blanks 30, 30A, 30B, 40 each 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 first steel plate is joined to the second steel plate directly or indirectly via another steel plate. ｍｉｎ The coefficient A of the steel plate 31 having １ is the coefficient A calculated by the formula (1) for the other steel plate 3i ｉ The coefficient A of the steel plate 32 that is the smallest among (i = 2, 3, ...) ２ The minimum thickness t of any of the blanks 30, 30A, and 30B is greater than ｍｉｎ When there are a plurality of steel plates 31 having the coefficients A of all the steel plates 31, １ is the coefficient A of the steel plate 32 ２ When the blanks 30, 30A, and 30B include three or more steel plates, the coefficients A of the steel plates other than the steel plates 31 and 32 are preferably greater than the coefficient A of the steel plate 32. ２ Similarly, in the blank 40, the coefficient A of the steel plate having the smallest plate thickness is１ is larger than the smallest value of the coefficient A calculated by the formula (1) for the steel plate having a larger plate thickness.

[0099] In the fourth embodiment, the blank 30C corresponds to the structural member 10C of the rear under module. This 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 Figures 13 to 23, the number and arrangement of the steel plates can be changed as appropriate.

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

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

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

[0103] Although not shown, 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. Preferably, the structural members 10C, 20C and blanks 30C, 40C each 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. ｍｉｎ The coefficient A of the steel plate 31 having １ is the coefficient A calculated by the formula (1) for the other steel plate 3i ｉ The coefficient A of the steel plate 32 that is the smallest among (i = 2, 3, ...) ２ The minimum thickness t of the blank 30C is larger than ｍｉｎ When there are a plurality of steel plates 31 having the coefficients A of all the steel plates 31, １ is the coefficient A of the steel plate 32 ２ When the blank 30C includes three or more steel plates, the coefficient A of the steel plates other than the steel plates 31 and 32 is preferably greater than the coefficient A of the steel plate 32. ２ Similarly, in blank 40C, the coefficient A of the steel plate having the smallest plate thickness is １is larger than the smallest value of the coefficient A calculated by the formula (1) for the steel plate having a larger plate thickness.

[0104] In the above embodiment, each of the blanks and the multiple steel plates (sub-blanks) included in each structural member may be single-layered or multi-layered, i.e., each of the sub-blanks may be a single steel plate or a plate material formed by stacking 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 may further include, for example, a pad and a blank holder. The mold 60 may be configured according to the desired 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 cross-sectional shape 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 widthwise side is open in cross section. In this case, another member (not shown) may be joined to the open portion of the side frame 11 and 21, so that the side frame 11 and the other member form a closed cross section. Similarly, the cross members 12 and 22 may each have a substantially hat-shaped cross section or a cross-sectional shape of another shape.

[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, CAE analysis was performed on the press-formed (hot stamped) structural members 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 included in the structural member, as well as the division pattern of the structural member.

[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]

[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 member and the positions of the joints between the steel plates in the structural member. 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 Figures 25A to 25C. In Figures 25A to 25C, the side frames of the structural members are each made of two pieces of material (1) and (2). The cross member is mainly made of material (3). In each structural member, materials (1) to (3) are each butt-jointed with the adjacent material.

[0113]

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

[0115] As described above, coefficient A corresponds to the transformation start time for each material when only the influence of elements is considered. However, the actual transformation start time for each material is also affected by plate thickness, and becomes shorter as the plate thickness decreases. The "phase transformation start time" in Table 2 refers to the shortest time until the phase transformation to ferrite begins 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 begins phase transformation). As shown in Table 2, Examples 1 to 3 had longer phase transformation start times than Comparative Examples 1 and 2. For example, comparing Example 3 and Comparative Example 2, which are identical except for the material of the thinnest material (3), it can be seen that Example 3 has a slower phase transformation start time than Comparative Example 2. In Example 3, coefficient A for the thinnest material (3) １ The coefficient A is the smallest among the other materials (1) and (2). ２ On the other hand, in Comparative Example 2, the coefficient A of the thinnest material (3) is larger than that of the thinnest material (4), and the hardenability of the thinnest material is higher than that of the other materials. １ 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 the phase transformation start time of the thinnest material is extended compared to Comparative Example 2, and the phase transformation start time of the thinnest material and the phase transformation start time of the other relatively thick materials are made uniform. Therefore, in Example 3, after heating of the blank is completed, it becomes easier to start forming before the phase transformation to ferrite begins 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 each formed from two pieces of material (3) and (4). 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 each formed from two pieces of material (1) and (3). 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]

[0118] Referring to Table 3, in Examples 4 to 11, the minimum plate thickness t ｍｉｎ Material coefficient A １ The coefficient A is the smallest among other relatively thick materials. ２ On the other hand, in Comparative Examples 3 to 7, the minimum thickness t ｍｉｎ Coefficient A of １ The smallest coefficient A among other relatively thick materials ２ The minimum plate thickness t ｍｉｎ If there are multiple pieces of material with the minimum thickness t ｍｉｎ The smallest value of the coefficient A of the material having the coefficient A １ As the coefficient A of other materials ２ We decided to compare it with the following.

[0119] When comparing each Example with the corresponding Comparative Example, it can be seen that the phase transformation start time is extended. For example, when comparing Example 6 and Comparative Example 5, which are identical except for the material of the thinnest material (4), the phase transformation start time in Example 6 is delayed compared to Comparative Example 5. In Example 6, the coefficient A of the thinnest material (3) １ is the smallest coefficient A among the other materials (1), (2), and (4). ２ On the other hand, in Comparative Example 5, the coefficient A of the thinnest material (3) is larger than that of the thinnest material (3), and the hardenability of the thinnest material is higher than that of the other one or more materials. １is the smallest coefficient A among the other materials (1), (2), and (4). ２ or less, and the hardenability of the thinnest material is equal to or less than that of the other materials. In Example 6, the hardenability of the thinnest material is made higher than that of the other materials, so 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 made uniform. Therefore, in Example 6, after heating of the blank is completed, it becomes easier to start forming before the phase transformation to ferrite begins in the thinnest material, and it becomes easier to harden the structural member 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 each formed from two pieces of material (1) and (2). The structural member in Figure 25F includes a cross member formed from material (3) and a cross member formed from materials (4) and (5). The structural member in Figure 25G includes a cross member formed from materials (3) and (4) and a cross member formed from material (5). In each structural member, materials (1) to (5) are each butt-jointed with the adjacent material.

[0121]

[0122] Referring to Table 4, in Examples 12 to 17, the minimum plate thickness t ｍｉｎ Material coefficient A １ The coefficient A is the smallest among other relatively thick materials. ２ On the other hand, in Comparative Examples 8 to 11, the minimum thickness t ｍｉｎ Material coefficient A １ The coefficient A is the smallest among other relatively thick materials. ２ The minimum plate thickness t ｍｉｎ If there are multiple pieces of material with the same thickness, the smallest thickness t ｍｉｎ The smallest value of the coefficient A of the material having the coefficient A １ As the coefficient A of other materials ２ We decided to compare it with the following.

[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 are compared, which are identical except for the materials of the thinnest materials (2) and (4), the phase transformation start time of Example 12 is delayed compared to Comparative Example 8. In Example 12, the coefficient A of the thinnest materials (2) and (4) １ is the smallest coefficient A among the other materials (1), (3), and (5). ２ In Example 12, the coefficient A of the thinnest materials (2) and (4) is greater than that of the thinnest material, and the hardenability of the thinnest material is higher than that of the other one or more materials. １ is larger than the coefficient A of all the other materials (1), (3), and (5). On the other hand, in Comparative Example 8, the coefficient A of the thinnest materials (2) and (4) is １ is less than the coefficient A of the other materials (1), (3), and (5), and the hardenability of the thinnest material is equal to or less than that of the other materials. In Example 12, the hardenability of the thinnest material is made higher than that of the other materials, so the phase transformation start time of the thinnest material is extended compared to Comparative Example 8, and the phase transformation start time of the thinnest material and the phase transformation start time of the relatively thick other materials are made uniform. Therefore, in Example 12, after heating of the blank is completed, it becomes easier to start forming before the phase transformation to ferrite begins in the thinnest material, and it becomes easier to harden the structural member uniformly.

[0124] In addition, for example, when Example 13 and Comparative Example 9 are compared, which are identical in conditions except for the materials of the thinnest materials (1), (2), and (5), the phase transformation start time is slower in Example 13 than in Comparative Example 9. In Example 13, the smallest coefficient A １ The coefficient A is the smallest among the other materials (3) and (4). ２ 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 coefficient A is the smallest among the thinnest materials (1), (2), and (5). １ The coefficient A is the smallest among the other materials (3) and (4).２ or less, and the hardenability of the thinnest material was equal to or less than that of the other materials. In Example 13, the hardenability of the thinnest material was made higher than that of the other materials, so 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 relatively thick other 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 began in the thinnest material, and it became easier to harden the structural member uniformly.

[0125] In the examples and comparative examples shown in Table 4, the materials are butted together and then laser-joined (butt welding), 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]

[0127] In the examples and comparative examples shown in Table 5, similar to the examples and comparative examples shown in Table 4, the materials are arranged in the division patterns shown in Figures 25F and 25G. Among the examples and comparative examples shown in Table 5, in 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. Among the examples and comparative examples shown in Table 5, in 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 Material coefficient A １ The coefficient A is the smallest among other relatively thick materials. ２ On the other hand, in Comparative Examples 12 and 13, the minimum thickness t ｍｉｎ Coefficient A of １ The coefficient A is the smallest among other relatively thick materials. ２ The details are as follows.

[0129] As can be seen from Table 5, in Examples 18 and 19, the phase transformation start time is delayed compared to Comparative Examples 12 and 13. Therefore, even when an overlap portion exists in the blank, the coefficient A of the thinnest material is １ The smallest coefficient A among other materials ２ It was confirmed that by making the thickness larger than 1 / 2 mm, the phase transformation start time of each material in the blank can be made uniform.

[0130] The blank was heated at a furnace temperature of 920°C until the entire blank reached 910°C, then transferred to a press in 17 seconds. Hot stamping was performed at a forming speed of 40 mm / s, and the blank was held at bottom dead center for 20 seconds while applying a pressure of 3000 kN to obtain hot-stamped structural components. Analysis samples were taken from the thinnest portions of these structural components using the method described in the above embodiment, and the variation in martensite fraction was measured. Furthermore, shape accuracy and impact absorption performance were measured separately for these structural components. The evaluation results are shown in Table 6.

[0131]

[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 plate thickness t ｍｉｎ The martensite fraction (%) is the maximum martensite fraction (%) minus the minimum martensite fraction (%) in the cross section of the structural member at the location of the material having the martensite fraction (%).

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

[0134] Regarding impact absorption performance, when the structural member was a rear module (FIG. 25F), a rear-end collision and a side collision were assumed with the structural member assembled to a vehicle, and an impactor simulating a vehicle was collided with the structural member, and the maximum intrusion amount during a rear-end collision and the maximum intrusion amount during a side collision were evaluated. On the other hand, when the structural member was a front module (FIG. 25E), a front-end collision and a side collision were assumed with the structural member assembled to a vehicle, and an impactor simulating a vehicle was collided with the structural member, and the maximum intrusion amount during a front-end collision and the maximum intrusion amount during a side collision were evaluated. Impact absorption performance was evaluated based on the impact absorption performance of an under-module formed by hot stamping each material and then joining them, and compared to the impact absorption performance of the base. In Table 6, impact absorption performance equivalent to the impact absorption performance of the base is represented by "good," impact absorption performance superior to the impact absorption performance of the base is represented by "better," impact absorption performance slightly lower than the impact absorption performance of the base is represented by "marginal," and impact absorption performance even lower than the impact absorption performance is represented by "poor."

[0135] Each example shown in Table 6 is A １ -A ２ >0, whereas in each comparative example, A １ -A ２ ≦0. In Comparative Examples 5 and 12, the variation in martensite fraction exceeded 20%, whereas in Examples 5, 6, and 18, the variation in martensite fraction was 20% or less. In Examples 5, 6, and 18, the variation in martensite fraction was reduced to 15% or less. In Examples 5, 6, and 18, the shape accuracy was also better 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 also improved compared to Comparative Examples 5 and 12. In particular, in Examples 5 and 18, impact absorption performance equal to or greater 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 greater than that of a structural member formed by joining the materials after press-forming them individually was ensured.

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

Claims

1. A blank for hot stamping, comprising a plurality of steel plates arranged and joined to form two long portions arranged side by side in a horizontal direction in a plan view of the blank and a connecting portion connecting the long portions, the plurality of steel plates including: a first steel plate having the smallest thickness among the plurality of steel plates; and a second steel plate having a thickness greater than that of the first steel plate, 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 x (2.7 x C + 0.4 x Si + Mn + 0.45 x Ni + 0.8 x Cr + 2 x Mo). ３．４２ (1) However, the element symbols in the above formula (1) are substituted with the contents (mass%) of the corresponding elements.

2. The blank according to claim 1, wherein 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 When this is the case, t ｍａｘ -t ｍｉｎ ≧0.2 (mm), blank.

3. A method for manufacturing a structural component, comprising the steps of: preparing a blank as described in claim 1 or 2; heating the 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 mold and quenching it.

4. A structural member for a vehicle body, comprising a pair of side frames and a cross member connecting the side frames, wherein the side frames and the cross member are formed by a plurality of steel plates joined together, including a first steel plate having a minimum thickness and a second steel plate having a thickness greater than that of the first steel plate, and 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 x (2.7 x C + 0.4 x Si + Mn + 0.45 x Ni + 0.8 x Cr + 2 x Mo) ３．４２ (1) However, the element symbols in the above formula (1) are substituted with the contents (mass%) of the corresponding elements.

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