Blank, structural member manufacturing method, and structural member

Abstract

To provide a structural member including a steel plate smaller in plate thickness than other steel plates, and a hot stamp blank capable of improving performance of the structural member especially when the large structural member is molded.SOLUTION: A blank (30,30A,30B,30C) comprises a plurality of steel plates. The steel plates are arranged to form two long parts (34L and 34R) and a connection part (35), and joined. The steel plates include a first steel plate (31) and a second steel plate (32). The first steel plate (31) has a smallest plate thickness (tmin) among the steel plates. The second steel plate (32) has a plate thickness (t2) larger than that (tmin) of the first steel plate (31). On at least one of both the surfaces of the first steel plate (31), a treatment for increasing emissivity more than those of both the surfaces of the second steel plate (32) has been executed.SELECTED DRAWING: Figure 3B

Description

[Technical Field]

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

[0002] Structures such as automobile bodies are made up of 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 using 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 steel sheet (blank) for hot stamping. The steel sheet of Patent Document 1 has a surface treatment film on the entire surface of at least one surface, the surface treatment film having an emissivity of 60% or more at 25°C at a wavelength of 8.0 μm. According to Patent Document 1, the surface of the steel sheet to which the surface treatment film is applied has an increased emissivity, and the heat transfer effect by radiation is large. Therefore, when the steel sheet is heated during hot stamping, the steel sheet undergoes a transformation from the A phase to the A phase, where the metal structure is transformed into the austenite phase. c3 The temperature is quickly raised to a temperature equal to or higher than the temperature point. Patent Document 1 describes that this makes it possible to shorten the heating time and improve the productivity of hot stamped members.

[0004] Patent Document 2 discloses an overlapping blank for hot stamping. The overlapping blank in Patent Document 2 includes a first steel sheet and a second steel sheet having an area smaller than that of the first steel sheet. The second steel sheet is overlapped on the surface of the first steel sheet and welded to the first steel sheet. Both the first steel sheet and the second steel sheet are aluminum-plated steel sheets, and the coating weight of the plating layer on both surfaces is 20 g / m 2 More than 120g / m 2When the average coating weight of the aluminum-based plating layer on both surfaces of the first steel sheet is W1, the coating weight of the aluminum-based plating layer on the surface of the second steel sheet not in contact with the first steel sheet is W2, the sheet thickness of the first steel sheet is t1, and the sheet thickness of the second steel sheet is t2, the overlapping blank of Patent Document 2 satisfies the following conditions: 30≦(W1−W2)≦100, and (W1 / W2) 2 × (t1 / t2) ≥ 1.5 is satisfied. According to Patent Document 2, by satisfying these relationships, the alloying reaction of the coating layer, which increases the emissivity during heating in hot stamping, can be rapidly advanced to the surface in the overlapping portion between the first steel sheet and the second steel sheet. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2022 / 215229 [Patent Document 2] Patent No. 6642777 Summary of the Invention [Problem to be solved by the invention]

[0006] In recent years, efforts have been made to simplify the manufacturing process of structures by integrating two or more components from the blank stage. Specifically, hot stamping a composite blank containing multiple steel plates (sub-blanks) has been considered, allowing structural components that were previously formed separately to be formed into a single component. However, if the composite blank contains steel plates of different thicknesses, the performance of the resulting structural component may be degraded. Specifically, during hot stamping, the blank is heated, for example, in a heating furnace until its microstructure is austenitized, and then formed using a die. However, because thinner steel plates cool more easily than thicker steel plates, the portions of the blank containing the thinner steel plates may begin to transform to ferrite before the blank is removed from the heating furnace and forming begins, potentially resulting in poor hardenability. Partially degraded hardenability of the blank can result in non-uniform hardness in the structural component formed from the blank, potentially reducing its impact absorption performance. Furthermore, partial degraded hardenability can also result in non-uniform stress in the structural component. This can cause the structural members to twist or warp, which can lead to a deterioration in dimensional accuracy. The deterioration in impact absorption performance (crash resistance) due to deterioration in hardenability or dimensional accuracy becomes more pronounced as the structural members become larger.

[0007] 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 they are formed using a steel plate having a thickness smaller than that of other steel plates. [Means for solving the problem]

[0008] A hot stamping blank according to the present disclosure comprises a plurality of steel plates. The plurality of steel plates are arranged and joined to form two long portions and a connecting portion. The long portions are arranged side by side in a horizontal direction in a plan view of the blank. The connecting portion connects the long portions to each other. The plurality of steel plates include a first steel plate and a second steel plate. The first steel plate has the smallest plate thickness among the plurality of steel plates. The second steel plate has a plate thickness greater than that of the first steel plate. At least one of both surfaces of the first steel plate is treated to increase the emissivity compared to both surfaces of the second steel plate. [Effects of the Invention]

[0009] 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. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is an exploded perspective view of a structural member according to a first embodiment. [Figure 2] FIG. 2 is a cross-sectional view of a side frame included in each of the structural members shown in FIG. [Figure 3A] FIG. 3A is a schematic view for explaining the method for manufacturing a structural member according to the first embodiment, showing a blank corresponding to one of the structural members shown in FIG. [Figure 3B] FIG. 3B is a schematic view for explaining the method for manufacturing a structural member according to the first embodiment, and is a cross-sectional view of the blank shown in FIG. 3A. [Figure 3C] FIG. 3C is a schematic view for explaining the method for manufacturing a structural member according to the first embodiment, and is another cross-sectional view of the blank shown in FIG. 3A. [Figure 3D] FIG. 3D is a schematic view for explaining the method for manufacturing a structural member according to the first embodiment, showing a blank corresponding to the other structural member shown in FIG. [Figure 3E] FIG. 3E is a schematic view for explaining the method for manufacturing a structural member according to the first embodiment. [Figure 3F] FIG. 3F is a schematic view for explaining the method for manufacturing a structural member according to the first embodiment. [Figure 3G] FIG. 3G is a schematic view for explaining the method for manufacturing a structural member according to the first embodiment. [Figure 4] FIG. 4 is a cross-sectional view of a structural member manufactured by the manufacturing method according to the first embodiment. [Figure 5A] FIG. 5A is a cross-sectional view of a blank according to a second embodiment. [Figure 5B] FIG. 5B is another cross-sectional view of the blank according to the second embodiment. [Figure 6A] FIG. 6A is a cross-sectional view of a structural member according to a second embodiment. [Figure 6B] FIG. 6B is another cross-sectional view of the structural member according to the second embodiment. [Figure 6C] FIG. 6C is yet another cross-sectional view of a structural member according to the second embodiment. [Figure 7] FIG. 7 is a cross-sectional view of a blank according to a modification of the second embodiment. [Figure 8] FIG. 8 is a plan view of a blank according to the third embodiment. [Figure 9] FIG. 9 is a cross-sectional view of a blank according to the third embodiment. [Figure 10] FIG. 10 is a cross-sectional view of a blank according to a modification of the third embodiment. [Figure 11] FIG. 11 is a cross-sectional view of a blank according to another modification of the third embodiment. [Figure 12] FIG. 12 is an exploded perspective view of a structural member according to a fourth embodiment. [Figure 13] FIG. 13 is a plan view of a blank according to the fourth embodiment. [Figure 14] FIG. 14 is a plan view of another blank according to the fourth embodiment. [Figure 15] FIG. 15 is a plan view of a blank according to a modified example of the first embodiment. [Figure 16] FIG. 16 is a plan view of a blank according to another modified example of the first embodiment. [Figure 17] FIG. 17 is a plan view of a blank according to a modification of the fourth embodiment. [Figure 18] FIG. 18 is a plan view of a blank according to another modified example of the fourth embodiment. [Figure 19] FIG. 19 is a plan view of a blank according to yet another modified example of the fourth embodiment. [Figure 20] FIG. 20 is a plan view of a blank according to yet another modified example of the fourth embodiment. [Figure 21] FIG. 21 is a plan view of a blank according to yet another modified example of the fourth embodiment. [Figure 22] FIG. 22 is a plan view of a blank according to yet another modified example of the fourth embodiment. [Figure 23] FIG. 23 is a plan view of a blank according to yet another modified example of the fourth embodiment. [Figure 24] FIG. 24 is a plan view of a blank according to yet another modified example of the fourth embodiment. [Figure 25] FIG. 25 is a plan view of a blank according to yet another modified example of the fourth embodiment. [Figure 26] FIG. 26 is a plan view of a blank according to yet another modified example of the fourth embodiment. [Figure 27] FIG. 27 is a plan view of a blank according to yet another modified example of the fourth embodiment. [Figure 28] FIG. 28 is a cross-sectional view of a side frame included in a structural member according to a modification of each embodiment. [Figure 29A] FIG. 29A is a diagram showing a division pattern of a structural member in an embodiment. [Figure 29B] FIG. 29B is a diagram showing another division pattern of the structural member in the embodiment. [Figure 29C] FIG. 29C is a diagram showing yet another division pattern of a structural member in an embodiment. [Figure 29D] FIG. 29D is a diagram showing yet another division pattern of a structural member in an embodiment. [Figure 29E]FIG. 29E is a diagram showing yet another division pattern of a structural member in an embodiment. [Figure 29F] FIG. 29F is a diagram showing yet another division pattern of a structural member in an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] 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 together. The plurality of steel plates include a first steel plate and a second steel plate. The first steel plate has the smallest thickness of the plurality of steel plates. The second steel plate has a thickness greater than that of the first steel plate. At least one of both surfaces of the first steel plate is treated to increase the emissivity compared to both surfaces of the second steel plate (first configuration).

[0012] The blank according to the first configuration includes a first steel plate having a minimum thickness and a second steel plate having a thickness greater than that of the first steel plate. At least one surface of the first steel plate is treated to increase the emissivity compared to both surfaces of the second steel plate. This allows the heating rate of the thin-walled portion of the first steel plate to be increased when the blank is heated during hot stamping. This allows the first steel plate to be heated to the austenite temperature more quickly and the first steel plate to be held at that temperature for a longer period. This allows the austenite grains in the first steel plate to be coarsened. As a result, the ferrite transformation region (ferrite nose) in the CCT diagram is shifted to the long-time side, delaying the onset of ferrite transformation in the first steel plate after heating of the blank. This allows the blank to be formed while maintaining the microstructure of the first steel plate in the austenite phase. In other words, the hardenability of the thin first steel plate can be improved. The phrase "treated to increase emissivity" not only refers to a case where the emissivity of at least one surface of the first steel plate is higher than the emissivity of both surfaces of the second steel plate before the blank is heated, but also refers to a case where the emissivity of at least one surface of the first steel plate becomes higher than the emissivity of both surfaces of the second steel plate during the heating of the blank.

[0013] In the blank according to the first configuration, the hardenability of the first steel plate, which has a smaller thickness, is improved, so that the first steel plate can also be well hardened when a structural component is formed from the blank by hot stamping. This makes it easier to uniformize the hardness of the structural component, thereby preventing partial reductions in the structural component's strength. Furthermore, because uneven stress is less likely to occur in the structural component, twisting, warping, and other problems are less likely to occur even when the structural component is large, ensuring good dimensional accuracy in the structural component. Therefore, when a structural component, particularly a large structural component, including a first steel plate having a smaller thickness than the second steel plate is formed from the blank, poor strength and dimensional accuracy of the structural component can be reduced, and the impact absorption performance (crash resistance) of the structural component can be improved.

[0014] In the blank according to the first configuration, the first steel sheet with the minimum thickness is treated to have a higher emissivity than the second steel sheet with a relatively large thickness. In this case, when the blank is heated during hot stamping, the first steel sheet heats up faster than the second steel sheet. This increases the high-temperature holding time of the first steel sheet, i.e., the time from when the first steel sheet reaches the austenite temperature range until the second steel sheet and the entire blank reach the austenite temperature range, compared to when the first steel sheet and the second steel sheet have the same emissivity. This reduces non-uniformity in phase transformation due to differences in cooling rates between the steel sheets after the blank is fully heated. Specifically, the initiation of the austenite-to-ferrite phase transformation in the first steel sheet with the minimum thickness can be delayed, thereby reducing the difference in the phase transformation initiation time between the first steel sheet with the minimum thickness and the other steel sheets. As a result, the hardenability can be made uniform between the first steel sheet with the minimum thickness and the other steel sheets.

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

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

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

[0018] When the first steel sheet is a plated steel sheet having an aluminum-based plating layer, as in the third configuration, the temperature rise rate of the first steel sheet tends to be slow when the blank is heated during hot stamping. Because the aluminum-based plating layer is nearly white, it tends to reflect heat energy and inhibit the temperature rise of the first steel sheet. However, even when the first steel sheet is a plated steel sheet having an aluminum-based plating layer, by subjecting at least one surface of the first steel sheet to a treatment that increases the emissivity compared to the relatively thick second steel sheet, the temperature rise of the first steel sheet can be accelerated when the blank is heated during hot stamping. This allows the first steel sheet to be held at a high temperature for a long time, thereby improving the hardenability of the first steel sheet.

[0019] In the blank according to any one of the first to third configurations, a coating may be formed on at least one surface of the first steel plate as a treatment for increasing emissivity, and this coating may have an emissivity of 60% or more at a wavelength of 8.0 μm at 25° C. (fourth configuration).

[0020] In the blank according to any one of the first to third configurations, a coating may be formed on at least one surface of the first steel plate as a treatment for increasing emissivity. The coating may comprise carbon black, one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide, and a 0 to 0.30 g / m 2 In this case, the content of carbon black in the coating can be X CB (g / m 2 ), oxide content is X Oxide (g / m 2 ), then X CB and X Oxide may satisfy the following formula (1) (see Patent Document 1) (fifth configuration): 118.9≦24280 / {6700 / (100+76×X CB )+18000 / (130+65×X Oxide )}≦332.0 (1)

[0021] In the blank according to any one of the first to fifth configurations, the first steel sheet and the second steel sheet may each be a plated steel sheet. The plated steel sheet may have a base steel sheet and an aluminum-based plating layer covering both surfaces of the base steel sheet. As a treatment for increasing the emissivity, the coating weight (g / m) of the aluminum-based plating layer on both surfaces of the base steel sheet in the first steel sheet may be increased. 2 ) is the coating weight (g / m) of the aluminum-based coating layer on both surfaces of the base steel sheet of the second steel sheet. 2 ) (sixth configuration).

[0022] In the sixth configuration, both the first steel sheet and the second steel sheet are plated steel sheets having an aluminum-based plating layer. However, the coating weight of the aluminum-based plating layer on both surfaces of the base steel sheet is smaller on the first steel sheet than on the second steel sheet. As a result, when the blank is heated during hot stamping, alloying of the aluminum-based plating layer and iron progresses to the surface of the thin first steel sheet before that of the relatively thick second steel sheet, causing both surfaces of the first steel sheet to change from silvery white to black or a color close to black. As a result, the emissivity of both surfaces of the first steel sheet is higher than that of both surfaces of the second steel sheet during heating of the blank. This allows the first steel sheet to be heated to the austenite temperature range more quickly and the first steel sheet to be held at that temperature for a longer period of time. This allows the austenite grains in the first steel sheet to be coarsened. As a result, the ferrite transformation region (ferrite nose) in the CCT diagram shifts to the longer time side, delaying the onset of ferrite transformation in the first steel sheet after heating of the blank. Therefore, the hardenability of the first steel plate can be improved.

[0023] In the blank according to any one of the first to sixth configurations, the first steel sheet may be a plated steel sheet having a base steel sheet and a plated layer provided on the base steel sheet. In this case, the thickness of the first steel sheet may be t min , the thickness of the steel plate with the largest thickness among the multiple steel plates is t max When this is done, it becomes 1.0 <t max / t minIt is preferable that the ratio is ≦3.2 (seventh configuration).

[0024] Among the multiple steel plates included in the blank, the smallest plate thickness t min The first steel plate has a maximum plate thickness t max If the difference in thickness between the steel plate and other steel plates is large, it becomes difficult to ensure the process window in the manufacture of structural components. For example, min and maximum plate thickness t max If the difference is large, the maximum thickness t max While waiting for the steel sheet having the minimum thickness t to reach the austenite temperature, alloying of the coating layer of the first steel sheet, which has been heated to the austenite temperature in advance, progresses, and the diffusion layer becomes thick, which can make it impossible to ensure the corrosion resistance or weldability of the first steel sheet due to the coating layer. min Maximum plate thickness t max The ratio is set to 3.2 or less. This allows the maximum plate thickness t max Heating rate and minimum thickness t of steel plate min Since the heating rate of the first steel sheet does not deviate too much from that of the other steel sheets, heating of the other steel sheets can be completed before excessive alloying of the coating layer of the first steel sheet progresses. Therefore, a structural component can be manufactured while maintaining the corrosion resistance or weldability of the first steel sheet, and a process window for manufacturing the structural component can be secured.

[0025] In the blank according to any one of the first to seventh configurations, the first steel sheet may be a plated steel sheet having a base steel sheet and a plating layer provided on the base steel sheet. The blank may further include an overlap portion. The overlap portion is formed by overlapping the ends of two adjacent steel sheets among the plurality of steel sheets, the steel sheets being other than the second steel sheet. The overlap portion may have a total thickness of 4.0 mm or less. The surface of each of the two steel sheets located outside the overlap portion may be treated to increase the emissivity compared to both surfaces of the second steel sheet (eighth configuration).

[0026] When a blank has an overlap portion formed by overlapping the ends of two steel sheets, it may be impossible to ensure the process window for manufacturing structural components. Specifically, when the blank is heated during hot stamping, alloying of the coating layer of the first steel sheet with the smallest sheet thickness progresses while waiting for the overlap portion to reach the austenite temperature range, resulting in a thick diffusion layer, making it impossible to ensure the corrosion resistance or weldability of the first steel sheet. Therefore, in the eighth configuration, the outer surface of each of the two steel sheets forming the overlap portion is treated to increase the emissivity. This accelerates the temperature rise of the overlap portion, allowing heating of the overlap portion to be completed before excessive alloying of the coating layer of the first steel sheet progresses, thereby enabling the manufacturing of structural components while maintaining the corrosion resistance or weldability of the first steel sheet. In other words, the process window for manufacturing structural components is easily ensured. However, even if the emissivity of the overlap portion is increased, it becomes difficult to ensure the process window if the total sheet thickness of the overlap portion becomes excessive. Therefore, it is preferable that the total sheet thickness of the overlap portion be 4.0 mm or less.

[0027] In the blank according to the eighth configuration, the second steel sheet and the two steel sheets may each be a plated steel sheet having a base steel sheet and an aluminum-based plating layer covering both surfaces of the base steel sheet. In this case, the coating weight (g / m) of the aluminum-based plating layer on both surfaces of the base steel sheet in each of the two steel sheets may be 2 ) is the coating weight (g / m) of the aluminum-based coating layer on both surfaces of the base steel sheet of the second steel sheet. 2 ) (ninth configuration).

[0028] In a ninth configuration, the two steel sheets forming the overlap portion and the second steel sheet are aluminum-plated steel sheets. The aluminum-based plating layer on both surfaces of the base steel sheet of each steel sheet forming the overlap portion is less than that of the second steel sheet. As a result, when the blank is heated during hot stamping, alloying of the aluminum-based plating layer and iron in the overlap portion progresses relatively quickly to the surface, and both surfaces of the overlap portion change from silver-white to black or a color close to silver-white. In other words, the emissivity of both surfaces of the overlap portion increases during heating of the blank. As a result, the temperature rise in the overlap portion can be accelerated, making it easier to ensure a process window in the manufacture of structural components.

[0029] A manufacturing method for a structural component according to an embodiment includes the steps of preparing a blank according to any one of the first to ninth configurations, heating a plurality of steel plates contained in the blank to a temperature equal to or higher than the austenite transformation completion temperature, and forming and quenching the heated blank using a mold (tenth configuration).

[0030] 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 includes a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate. A coating is provided on the first steel plate. The coating contains at least one oxide selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide at a concentration of 0.001 g / m 2 The above is contained (eleventh configuration).

[0031] A vehicle body structural member according to another embodiment includes a pair of side frames and a cross member. The cross member connects the side frames. The side frames and the cross member are formed by a plurality of steel plates joined together. The plurality of steel plates includes a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate. A coating is provided on the first steel plate. The coating contains carbon black at 0.500 g / m 2 The following is contained (12th configuration).

[0032] A vehicle body structural member according to yet another embodiment includes a pair of side frames and a cross member. The cross member connects the side frames. The side frames and the cross member are formed by a plurality of steel plates joined together. The plurality of steel plates include a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than that of the first steel plate. The first steel plate and the second steel plate are each plated steel plates having an aluminum-based plating layer on both surfaces of a base steel plate. The thickness of the aluminum-based plating layer on the first steel plate is smaller than the thickness of the aluminum-based plating layer on the second steel plate (thirteenth configuration).

[0033] 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.

[0034] First Embodiment [Structural Members] Fig. 1 is an exploded perspective view of structural members 10, 20 according to this embodiment. The structural members 10, 20 are used in the body of an automobile or the like. In the example shown in Fig. 1, the structural members 10, 20 constitute a front under module of the body.

[0035] 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.

[0036] The side frames 11L, 11R are arranged side by side in the left-right direction of the vehicle body when the structural member 10 is assembled to the vehicle body. The side frames 11L, 11R each extend in the front-rear direction of the vehicle body. Each of the side frames 11L, 11R includes a front portion 111 and a rear portion 112. The rear portion 112 is arranged behind the front portion 111 when the structural member 10 is assembled to the vehicle body.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] Fig. 2 is a cross-sectional view (transverse cross-section) of the side frames 11, 21 cut along a plane perpendicular to the longitudinal direction. In the example of Fig. 2, the side frames 11, 21 each have a substantially hat-shaped transverse cross-section.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] Returning to FIG. 1 , the cross member 22 extends in the left-right direction of the vehicle body when the structural member 20 is assembled to the vehicle body. The cross member 22 extends from the side frame 21L to the side frame 21R. The cross member 22 connects the side frames 21L and 21R. In the example shown in FIG. 1 , the cross member 22 connects the side frames 21L and 21R to each other at one end of the longitudinal direction 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.

[0046] 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.

[0047] 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 of the steel plates 31, 32, 33 are joined together by welding.

[0048] 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 among the steel plates 41, 42, 43 are joined together by welding.

[0049] [Manufacturing methods for structural components] 3A to 3G, a method for manufacturing the structural members 10 and 20 according to this embodiment will be described below. 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.

[0050] (preparation process) As shown in FIG. 3A, in manufacturing the upper structural member 10 (FIG. 1), a blank 30 is prepared in a preparation step. 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.

[0051] The long portions 34L, 34R are arranged side by side in the horizontal direction when the blank 30 is viewed from above. 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.

[0052] 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.

[0053] 3B and 3C are cross-sectional views of the blank 30 showing the joints of 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, and these end faces are joined. 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, and these end faces are 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 joining). In this case, the steel plates 31, 32, and 33 may be joined by spot welding. In particular, the intersection of the cross member 12 (FIG. 1) extending in the left-right direction of the vehicle body and the side frames 11L and 11R (FIG. 1) extending in the front-rear direction of the vehicle body may have an overlap structure as necessary.

[0054] 3B and 3C, the steel plate 31 has a plate thickness t1. The steel plate 32 has a plate thickness t2. The steel plate 33 has a plate thickness t3. In this embodiment, the t1 of the steel plate 31 is the smallest plate thickness t1 among the steel plates 31, 32, and 33. min The plate thickness t2 of the steel plate 32 is larger than the plate thickness t1 of the steel plate 31. The plate thickness t3 of the steel plate 33 is equal to or larger than the plate thickness t1 of the steel plate 31. In this embodiment, the plate thickness t2 of the steel plate 32 is the largest plate thickness t max However, the thickness t3 of the steel plate 33 is the largest among the steel plates 31, 32, and 33. max That is, the thickness t3 of the steel plate 33 may be equal to or greater than the thickness t2 of the steel plate 32.

[0055] Thickness t of steel plate 31 min is typically less than 1.4 mm. min The thickness t may be, for example, 0.8 mm or more. min and plate thickness tmax is 1.0 <t max / t min ≦3.2, and 1.3≦t max / t min It is more preferable that the value be ≦3.2.

[0056] Minimum plate thickness t min At least one of the two surfaces of the steel plate 31 having a thickness of 1000 nm is treated to increase the emissivity compared to the two surfaces of the thicker steel plate 32. In this embodiment, the emissivity of at least one surface of the steel plate 31 is higher than the emissivity of the two surfaces of the steel plate 32 even before the heating process of the blank 30.

[0057] For example, the emissivity at a wavelength of 8.0 μm at 25° C. is 60% or more on one or both surfaces of the steel plate 31, and less than 60% on both surfaces of the steel plate 32. The emissivity at a wavelength of 8.0 μm at 25° C. on one or both surfaces of the steel plate 31 is more preferably 70% or more, and even more preferably 80% or more. min The difference in emissivity at a wavelength of 8.0 μm at 25°C between the steel sheet 31 having the above-mentioned characteristic and the other steel sheet 32 ​​is preferably greater than 5%, more preferably greater than 10%, and even more preferably greater than 20%. The emissivity can be measured in accordance with JIS R 1801:2002. In this case, a sample taken from the steel sheet to be measured is placed in a Fourier transform infrared spectrophotometer, and the radiation intensity at a wavelength of 8.0 μm at 25°C is measured to calculate the emissivity. Alternatively, it is also possible to measure the radiation intensity of a target area at 25°C using a radiation thermometer set to a measurement wavelength of 8.0 μm, and calculate the emissivity from the ratio to the radiation intensity of a blackbody.

[0058] In this embodiment, a coating 50 is formed on one surface of the steel plate 31 as a treatment for increasing emissivity. For example, one surface of the steel plate 31 is entirely covered with the coating 50. On the other hand, the coating 50 is not provided on either surface of the steel plate 32. As a result, the emissivity of one surface of the steel plate 31 is higher than the emissivity of both surfaces of the steel plate 32. However, the coating 50 may be provided on both surfaces of the steel plate 31.

[0059] The coating 50 is, for example, a substantially black coating. * Value (CIE 1976 lightness index L specified 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)). The emissivity of the coating 50 at a wavelength of 8.0 μm at 25°C is 60% or more, preferably 70% or more, and more preferably 80% or more. That is, the emissivity of the surface of the steel sheet 31 to which the coating 50 is applied at a wavelength of 8.0 μm at 25°C is 60% or more, preferably 70% or more, and more preferably 80% or more. The coating 50 may have an emissivity of 60% or more at a wavelength of 8.0 μm at 700°C. For example, the surface treatment coating described in Patent Document 1 can be used as the coating 50. Specifically, the coating 50 can contain carbon black and one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide. The coating 50 may or may not contain silica. That is, the silica content of the coating 50 is 0 g / m 2 The silica content of the coating 50 is 0.30 g / m 2 The silica content may be 0.10 g / m or less. 2 or less, and more preferably 0.05 g / m 2 The following is the result.

[0060] The carbon black and oxides can be dispersed throughout the entire surface of the coating 50 that is perpendicular to the thickness direction of the steel sheet 31. CB (g / m 2 ), Zr oxide, Zn oxide, and Ti oxide, and the content of one or more oxides (metal oxides) selected from the group consisting of X Oxide (g / m 2 ), then X CB and X Oxide It is preferable that the following formula (1) is satisfied. 118.9≦24280 / {6700 / (100+76×X CB )+18000 / (130+65×X Oxide )}≦332.0 (1)

[0061] In equation (1), the middle equation is: 24280 / {6700 / (100+76×X CB )+18000 / (130+65×X Oxide )} is preferably 119.0 or more, more preferably 170.0 or more, and even more preferably 220.0 or more. The value calculated by the middle formula is preferably 330.0 or less, more preferably 310.0 or less, and even more preferably 300.0 or less.

[0062] The dispersion state of the carbon black and metal oxide in the coating 50 can be confirmed by performing an area analysis of the coating 50 using an electron probe micro analyzer (EPMA) to determine the elements derived from the carbon black (e.g., C) and the elements derived from the oxide (Zr, Zn, and Ti). CBcan be measured by cross-sectional analysis of the coating 50 using a transmission electron microscope (TEM). That is, a cross-sectional analysis of the coating 50 is performed by TEM-EDS analysis in an area of ​​a predetermined size (thickness of the coating 50 × 5 μm), and the thickness of the coating 50 and the area ratio of particles with a carbon content of 70 mass % or more in that area are measured. The density of carbon black is expressed as ρ (ton / m 3 ), the film thickness is d (μm), the area ratio is a (%), and the value expressed as ρ×d×a is the carbon black content X CB (g / m 2 ) The oxide content X Oxide can be determined by performing elemental analysis on the surface of the coating 50 using an X-ray fluorescence analyzer (ZSX Primus, manufactured by RIGAKU Corporation) and quantifying the amounts of metal Zr, metal Zn, and metal Ti.

[0063] Carbon black content in film 50 X CB is 0.030g / m 2 It is preferable that the content is 0.100 g / m or more. 2 It is more preferable that the content X is equal to or greater than the above. CB is set within a range that satisfies formula (1), but is preferably 0.800 g / m 2 or less, more preferably 0.600 g / m 2 The following is the result.

[0064] Coating 50 can contain 5.0% or more carbon black by volume, and preferably 8.0% or more carbon black by volume. Coating 50 can contain 40.0% or less carbon black by volume, and preferably 30.0% or less carbon black by volume.

[0065] Metal oxide content in film 50 X Oxide is 0.030g / m 2 It is preferable that the content is 0.060 g / m or more. 2 It is more preferable that the content X is equal to or greater than the above. Oxideis set within a range that satisfies formula (1), but is preferably 0.500 g / m 2 or less, more preferably 0.300 g / m 2 The following is the result.

[0066] Coating 50 can contain 1.0% or more metal oxide by volume. Coating 50 can also contain 30.0% or less metal oxide by volume, and preferably contains 25.0% or less metal oxide by volume.

[0067] Carbon black content X CB (g / m 2 ) and metal oxide content X Oxide (g / m 2 ) ratio: X Oxide / X CB is preferably 0.20 or more and 200.00 or less. Oxide / X CB is more preferably 0.40 or more and 10.00 or less, and further preferably 0.60 or more and 5.00 or less.

[0068] In addition to the carbon black and metal oxides described above, the coating 50 may contain various binder components and additives.

[0069] The binder component is preferably a water-dispersible or water-soluble resin. The content of the binder component is preferably 40% by volume or more relative to the total volume of the coating 50. Various known water-dispersible or water-soluble resins can be used as the binder component selected from water-dispersible or water-soluble resins. Examples of such water-dispersible or water-soluble resins include polyurethane resins, polyester resins, acrylic resins, epoxy resins, fluororesins, polyamide resins, polyolefin resins, and polymer compounds obtained by hydrolysis and condensation polymerization of silane coupling agents. The binder component is more preferably one or more selected from the group consisting of polyester resins, polyurethane resins, polyolefin resins, acrylic resins, epoxy resins, fluororesins, and polyamide resins. When a polyurethane resin is used as the binder component, the polyurethane resin is preferably a polyether-based polyurethane resin.

[0070] Examples of additives include leveling agents, water-soluble solvents, metal stabilizers, and etching inhibitors. Leveling agents include nonionic or cationic surfactants. Examples of nonionic or cationic surfactants include polyethylene oxide or polypropylene oxide adducts and acetylene glycol compounds. Examples of water-soluble solvents include alcohols such as ethanol, isopropyl alcohol, t-butyl alcohol, and propylene glycol; cellosolves such as ethylene glycol monobutyl ether and ethylene glycol monoethyl ether; esters such as ethyl acetate and butyl acetate; and ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone. Examples of metal stabilizers include chelating compounds such as EDTA (ethylenediaminetetraacetic acid) and DTPA (diethylenetriaminepentaacetic acid). Examples of etching inhibitors include amine compounds such as ethylenediamine, triethylenepentamine, guanidine, and pyrimidine.

[0071] The coating 50 can be formed by applying an organic or inorganic treatment liquid containing, for example, carbon black and a metal oxide to the entire surface of the steel sheet 31, and then drying the volatile components in the treatment liquid. The treatment liquid can be applied to the surface of the steel sheet 31 by, for example, a roll coater, a curtain coater, or an inkjet. In the case of inkjet coating, the thickness of the coating 50 can be changed continuously. The thickness of the coating 50 is, for example, 0.5 μm or more and 5.0 μm or less. The thickness of the coating 50 is preferably 1.0 μm or more and 3.0 μm or less. The thickness of the coating 50 is determined based on the thickness t of the steel sheet 31. min Therefore, the thickness of the steel plate 31 measured including the coating 50 is min can be treated as

[0072] The steel sheet 31 may be a plated steel sheet. In this case, the steel sheet 31 has a base steel sheet 31a and a plating layer 31b. The type of base steel sheet 31a is not particularly limited. The plating layer 31b is provided on the base steel sheet 31a. The plating layer 31b covers the entire or almost the entire surface of both sides of the base steel sheet 31a. The plating layer 31b is a metal plating layer. The plating layer 31b may be, for example, hot-dip aluminum plating, hot-dip galvanneal plating, or electrogalvanization. As the steel sheet 31, a known aluminum-plated steel sheet, zinc-plated steel sheet, etc. can be used.

[0073] The plating layer 31b is typically a plating layer containing aluminum as a main component (aluminum-based plating layer). The configuration of the aluminum-based plating layer is not particularly limited. A known aluminum-based plating layer can be used as the plating layer 31b. When the steel sheet 31 is a plated steel sheet, the thickness t min is the combined thickness of the base steel sheet 31a and the plating layer 31b.

[0074] Like the steel sheet 31, the steel sheets 32 and 33 may be known plated steel sheets. The steel sheets 32 and 33 may be aluminum-plated steel sheets or zinc-plated steel sheets. The steel sheets 32 and 33 may be the same type of plated steel sheet as the steel sheet 31, or a different type of plated steel sheet from the steel sheet 31. Furthermore, the steel sheet 32 ​​may be the same type of plated steel sheet as the steel sheet 33, or a different type of plated steel sheet from the steel sheet 33. When the steel sheet 32 ​​is a plated steel sheet, the thickness t2 of the steel sheet 32 ​​is the combined thickness of the base steel sheet and the plated layer. Similarly, when the steel sheet 33 is a plated steel sheet, the thickness t3 of the steel sheet 33 is the combined thickness of the base steel sheet and the plated layer. When two or more of the steel sheets 31, 32, and 33 are plated steel sheets, the coating weight of each steel sheet may be the same as or different from that of the other steel sheets. However, in this embodiment, the steel sheets 31, 32, and 33 may be steel sheets (bare materials) that do not have a plating layer on the surface.

[0075] As shown in FIG. 3D, in manufacturing the lower structural member 20 (FIG. 1), a blank 40 is prepared in a 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.

[0076] The long portions 44L, 44R are arranged side by side in the horizontal direction when the blank 40 is viewed from above. 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.

[0077] 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.

[0078] 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.

[0079] (Heating process) The prepared blanks 30, 40 are formed into structural members 10, 20 (FIG. 1), respectively, by hot stamping. During the hot stamping, the blanks 30, 40 are subjected to a heating step. Referring to FIG. 3E, in the heating step, the blank 30 is heated, for example, by a heating furnace. The plurality of steel plates 31, 32, 33 included in the blank 30 are heated to an austenite transformation completion temperature (A c3 The steel plates 31, 32, and 33 are heated to a temperature of, 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 an austenite transformation completion temperature (A c3 It is heated above this temperature.

[0080] (molding 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 transferred 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.

[0081] 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 (rapidly cooled) 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.

[0082] 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.

[0083] FIG. 4 is a cross-sectional view of the structural member 10 after hot stamping. FIG. 4 shows a cross-section of the structural member 10 at the position of the steel sheet 31 (FIG. 3B) to which the black coating 50 was applied at the blank 30 stage. In the example of FIG. 4, the structural member 10 includes a coating 13. The coating 13 is provided on the steel sheet 31. The black coating 50 (FIG. 3B) that was applied to the steel sheet 31 in the blank 30 becomes the coating 13 after hot stamping. The coating 13 is provided on at least one surface of the steel sheet 31. If the coating 50 before hot stamping contains carbon black, this carbon black will almost completely disappear due to the high temperature heating during hot stamping, but it may remain. If the coating 50 before hot stamping satisfies the above formula (1), the coating 13 after hot stamping may not contain carbon black, or may contain 0.500 g / m 2 However, if the coating 50 before hot stamping satisfies the above formula (1), the coating 13 after hot stamping may contain 0.500 g / m 2 It is preferable that the coating 13 contains the following carbon black: When the coating 13 after hot stamping contains carbon black, the carbon black content in the coating 13 is 0 g / m2 more preferably 0.001 g / m 2 That is all. If the coating 50 (FIG. 3B) is provided on the steel sheet 31 before hot stamping so that the coating 13 contains carbon black after hot stamping, carbon black will be present on the steel sheet 31 even in the later stage of the heating process, and the emissivity of the steel sheet 31 will be ensured. Therefore, even in the later stage of the heating process, the minimum sheet thickness t min The steel sheet 31 having the above formula (1) is easily heated. Furthermore, when the steel sheet 31 is a plated steel sheet, the coating 13 after the heating step contains carbon black, which suppresses adhesion of the plating layer 31b (FIG. 3B) to the die 60 in the forming step (hot stamping), and can reduce the coefficient of friction between the steel sheet 31 and the die 60 (FIGS. 3F and 3G). When the coating 50 before hot stamping satisfies the above formula (1), the coating 13 after hot stamping satisfies the formula in the center: 24280 / {6700 / (100+76×X CB )+18000 / (130+65×X Oxide )} is, for example, between 120.0 and 150.0.

[0084] When the coating 50 (FIG. 3B) before hot stamping satisfies the above formula (1), the coating 13 after hot stamping contains one or more oxides (metal oxides) selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide at a concentration of, for example, 0 g / m 2 More preferably, 0.001 g / m 2 The metal oxide content of the coating 13 is, for example, 0.500 g / m 2 In this way, when the metal oxide remains on the structural member 10, that is, when the coating 13 is 0 g / m 2 It is more desirable for the coating 50 before hot stamping to contain more than 0.30 g / m of metal oxide, since this improves the corrosion resistance of the structural member 10. When the coating 50 before hot stamping satisfies the above formula (1), the coating 13 after hot stamping has a metal oxide content of 0 to 0.30 g / m 2 Contains silica.

[0085] The carbon black content, metal oxide content, and silica content in the coating 13 can be measured in the same manner as for the coating 50 ( FIG. 3B ) at the blank 30 stage. Specifically, a vehicle body part is disassembled to obtain a structural member 10, and an analytical sample is obtained from the structural member 10, for example, by laser cutting. For example, an analytical sample is obtained from each of the multiple steel plates included in the structural member 10. The analytical sample is obtained, for example, from the center or its vicinity of the top plate of each steel plate having an open cross section. The obtained analytical sample is adjusted by polishing the cross section to outside the heat-affected zone during laser cutting, to prepare a sample for coating analysis. This sample is subjected to surface analysis of the coating 13 for elements derived from carbon black (e.g., C) and elements derived from oxides (e.g., Zr, Zn, and Ti) using an EPMA, allowing the dispersion state of the carbon black and metal oxide in the coating 13 to be confirmed. Because the coating 13 is present on the front and / or back side of the structural member 10 depending on the location, both the front and back sides of the analytical sample are analyzed.

[0086] In many cases, the outermost surface layer of the structural member 10 is, for example, an electrodeposition coating film. In this case, the coating layer below the electrodeposition coating film layer and above the alloyed metal plating layer is analyzed. The carbon black content X in the coating 13 is CB can be measured by cross-sectional analysis of the coating 50 using a TEM. That is, a cross-sectional analysis of the coating 13 is performed by TEM-EDS analysis in an area of ​​a predetermined size (thickness of the coating 13 × 5 μm), and the thickness of the coating 13 and the area ratio of particles with a carbon content of 70 mass % or more in that area are measured. The density of carbon black is defined as ρ (ton / m 3 ), the film thickness is d (μm), the area ratio is a (%), and the value expressed as ρ×d×a is the carbon black content X CB (g / m 2 ) The oxide content X Oxide can be determined by performing elemental analysis of the coating layer that is located below the electrodeposition coating layer and above the alloyed metal plating layer using the above-mentioned X-ray fluorescence analyzer, and quantifying the amounts of metal Zr, metal Zn, and metal Ti.

[0087] 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, 15% or less. The martensite fraction variation is preferably 10% or less. The martensite fraction variation can be measured as follows. That is, min Ten or more analysis samples (for example, about 10 mm on the long side) are cut out from the cross section of the structural member 10 at the position of the steel plate 31 having the above structure, at positions 20 mm or more away from the end and 10 mm or more away from each other, and then each is mirror-polished so that the plate thickness direction becomes the observation surface, and etched with LePeller's reagent. Then, an optical microscope is used to observe the area from the steel plate surface to a depth of 1 / 4 of the plate thickness (the area from the steel plate surface to a depth of 1 / 8 of the plate thickness to a depth of 3 / 8 of the plate thickness) at a magnification of 1000x, with one field of view of 2,400 μm. 2 Photographs of the above structures are taken from 30 fields of view, and image analysis is performed on the obtained structure photographs.

[0088] 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 percentage of pixels in the white area to the total number of pixels to measure the martensite fraction. This type of image analysis is performed on a total of 30 observation fields for each analysis sample to determine the martensite fraction, and the average value is used as the martensite fraction for each analysis sample. Furthermore, the difference between the maximum and minimum martensite fraction values ​​for 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 minIf 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.

[0089] Depending on the steel sheet, the area fraction of martensite obtained by image analysis, i.e., the area fraction of the white area, 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.

[0090] After the forming process (hot stamping), steel sheet 31 may have a tensile strength of, for example, 0.5 GPa or more, preferably 1.0 GPa or more. Similarly, after the forming process (hot stamping), steel sheets 32 and 33 (FIG. 1) may have a tensile strength of, for example, 0.5 GPa or more, preferably 1.0 GPa or more. At least one of steel sheets 31, 32, and 23 may have a tensile strength of 1.5 GPa or more after the forming process. The tensile strength of each of steel sheets 31, 32, and 33 may be the same as or different from the tensile strength of the other steel sheets.

[0091] Although not shown, the lower structural member 20 (FIG. 1) may also include the coating 13 similar to that of the upper structural member 10 after hot stamping. The coating 13 is formed on the structural member 20 at a thickness of, for example, the minimum thickness t min The structural member 20 is also arranged on at least one surface of a steel plate having a minimum plate thickness t min The variation in martensite fraction at the position of the steel plate having the above structure is, for example, 15% or less, more preferably 10% or less, similar to the structural member 10.

[0092] [effect] In the blank 30 according to this embodiment, the minimum plate thickness t minThe emissivity of one surface of the steel sheet 31 having a thickness t2 is higher than the emissivity of both surfaces of the steel sheet 32 ​​having a larger thickness t2. That is, the surface of the steel sheet 31 is treated to increase the emissivity compared to both surfaces of the steel sheet 32. As a result, when the blank 30 is heated during hot stamping, the temperature rise rate of the steel sheet 31 is significantly higher than that of the steel sheet 32. Therefore, in the heating process, 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 grains in the microstructure of the steel sheet 31 coarsen, and the ferrite transformation region (ferrite nose) in the CCT diagram shifts to the long-time side. Therefore, after the blank 30 is removed from, for example, a heating furnace, the transformation of austenite to ferrite in the steel sheet 31 can be prevented before the forming of the blank 30 using the die 60 begins. Therefore, the microstructure of the steel sheet 31 can be maintained in the austenite phase while the forming of the blank 30 using the die 60 begins, and the minimum sheet thickness t min The hardenability of the steel plate 31 having this structure can be improved.

[0093] In this embodiment, the hardenability of the relatively thin steel plate 31 is improved, so that the hardness of the structural member 10 formed from the blank 30 can be made uniform. More specifically, the minimum plate thickness t min Since the steel plate 31 having the above structure is well quenched, the variation in the martensite fraction in the steel plate 31 can be kept to 15% or less. As a result, for example, when a collision load is input to the structural member 10, deformation concentration is less likely to occur, and the structural member 10 is more likely to exhibit high impact absorption performance. Therefore, even when a structural member 10 including a 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.

[0094] 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 standpoint of the functionality of the structural member 10. On the other hand, a large variation in the martensite fraction indicates that there are unevenly distributed parts of the structural member 10 that lack hardenability, i.e., parts that lack hardness, and when the structural member 10 is deformed by a collision, deformation tends to concentrate in the parts that lack hardness, thereby reducing the functionality of the structural member 10.

[0095] In this embodiment, the hardenability of the relatively thin steel plate 31 is improved, which makes it less likely that uneven stress will occur in the structural member 10. Therefore, even when a large structural member 10 is formed from the blank 30, twisting, warping, etc., are less likely to occur in the structural member 10. Therefore, even when a large structural member 10 including a thin steel plate 31 is formed from the blank 30, it is possible to reduce poor dimensional accuracy of the structural member 10 and improve the impact absorption performance of the structural member 10.

[0096] In the blank 30 according to this embodiment, the minimum thickness t min The surface of the steel sheet 31 having a thickness t2 greater than that of the steel sheet 31 is substantially covered with a black coating 50, while the coating 50 is not provided on the steel sheet 32 ​​having a thickness t2 greater than that of the steel sheet 31. As a result, the emissivity of the surface of the steel sheet 31 is already greater than the emissivity of both surfaces of the steel sheet 32. In this case, when the blank 30 is heated during hot stamping, the steel sheet 31 heats up faster than the steel sheet 32, so the high-temperature holding time of the steel sheet 31 is longer than when the steel sheet 31 has the same emissivity as the steel sheet 32. Therefore, after heating of the blank 30 is completed, non-uniformity in phase transformation due to differences in cooling rates among the steel sheets 31, 32, and 33 can be reduced. Specifically, min Since the start of the phase transformation from austenite to ferrite can be delayed for the steel plate 31, the difference in the start time of the phase transformation between the steel plate 31 and the other steel plates 32 and 33 is reduced. As a result, the hardenability of the steel plates 31, 32, and 33 included in the blank 30 can be made uniform.

[0097] For example, if the plating layer 31b of the steel sheet 31 is an aluminum-based plating layer, the rate of temperature rise of the steel sheet 31 is likely to be slow during the heating process. Because the aluminum-based plating layer is white, it tends to reflect heat energy, inhibiting the temperature rise of the steel sheet 31. However, in the blank 30 according to this embodiment, the surface of the steel sheet 31 is treated to increase the emissivity. Therefore, even if the steel sheet 31 is a plated steel sheet having an aluminum-based plating layer, the temperature rise of the steel sheet 31 during the heating process can be accelerated and the high-temperature holding time of the steel sheet 31 can be ensured to be long. Therefore, the hardenability of the thin-walled steel sheet 31 can be ensured.

[0098] In this embodiment, in the heating process, first, the minimum plate thickness t min The steel plate 31 having the smallest thickness t reaches the austenite temperature range, and then the steel plates 32 and 33 reach the austenite temperature range in the order of thickness. min and maximum plate thickness t max Ratio to:t max / t min It is preferable that the thickness t min The thickness t max Therefore, the steel plate 33 having the above-mentioned structure can be heated sufficiently until the phase transformation to austenite is completed, thereby ensuring a process window in the manufacture of the structural member 10.

[0099] In this embodiment, a coating 50 can be applied to the steel sheet 31 in order to increase the emissivity of the steel sheet 31. The emissivity of the coating 50 (at a temperature of 25°C and a wavelength of 8.0 μm) is, for example, 60% or more. This allows the steel sheet 31 to be efficiently radiantly heated, and the rate at which the temperature of the steel sheet 31 rises in the heating process can be more easily increased.

[0100] In this embodiment, the coating 50 is made of carbon black, one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide, and a carbon black powder having a concentration of 0 to 0.30 g / m2 The following silica and carbon black content X may be contained. CB (g / m 2 ), and oxide content X Oxide (g / m 2 ) preferably satisfies the above formula (1). As described in Patent Document 1, formula (1) is a function of the ratio (%) of the increase in the temperature rise rate (°C / s) and the carbon black content X CB and oxide content X Oxide Equation (1) defines the relationship between the carbon black and oxides. Equation (1) indicates that carbon black primarily functions as a heat absorbing material in the temperature range up to 700°C, and oxides primarily function as heat absorbing materials in the temperature range of 700°C or higher. When coating 50 satisfies equation (1), the surface of steel sheet 31 to which coating 50 is applied tends to have an emissivity of 60% or higher at a wavelength of 8.0 μm at 25°C.

[0101] The carbon black and oxides can be dispersed throughout the entire surface of the coating 50 that is perpendicular to the thickness direction of the steel sheet 31. This makes it easier to make the emissivity of the surface of the steel sheet 31 uniform. Therefore, in the heating process, min The steel plate 31 having the above structure can be heated quickly and uniformly.

[0102] However, the configuration of the coating 50 is not limited to this. The coating 50 may be a substantially black coating to increase the emissivity of the steel sheet 31 compared to an untreated steel sheet. For example, the coating 50 may contain graphite or soot instead of or in addition to carbon black. Alternatively, the coating 50 may contain, for example, an acicular compound having a hexagonal crystal structure with an aspect ratio of 4 to 50 in order to increase the emissivity of the steel sheet 31. 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.

[0103] The lower structural member 20 and blank 40 have the same configuration as the upper structural member 10 and blank 30. Therefore, the lower structural member 20 and blank 40 can achieve the same effects as those described above.

[0104] Second Embodiment [blank] 5A and 5B are cross-sectional views of a blank 30A according to a second embodiment. FIGS. 5A and 5B are cross-sectional views of the blank 30 according to the first embodiment, corresponding to FIGS. 3B and 3C. In the first embodiment, the blank 30 for the structural member 10 has a minimum plate thickness t min At least one surface of the steel plate 31 is treated so that the emissivity of the surface of the steel plate 31 having the above-mentioned structure is higher than that of the surface of the other steel plate 32 before the heating step. In the first embodiment, for example, a coating 50 for increasing the emissivity is formed on one or both surfaces of the steel plate 31 (FIG. 3B). On the other hand, in the present embodiment, the surface of the steel plate 31 in the blank 30A is treated so that the emissivity of the surface of the steel plate 31 is higher than that of the surface of the steel plate 32 during the heating step.

[0105] 5A and 5B, in this embodiment, steel sheets 31, 32, and 33 are each plated steel sheets. More specifically, the steel sheets 31, 32, and 33 are all aluminum-plated steel sheets. The steel sheet 31 has a base steel sheet 31a and an aluminum-based plating layer 31b. The steel sheet 32 ​​has a base steel sheet 32a and an aluminum-based plating layer 32b. The steel sheet 33 has a base steel sheet 33a and an aluminum-based plating layer 33b.

[0106] In the steel sheet 31, an aluminum-based plating layer 31b covers both surfaces of the base steel sheet 31a. The aluminum-based plating layer 31b is provided over the entire or almost the entire surfaces of both surfaces of the base steel sheet 31a. Similarly, in the steel sheet 32, an aluminum-based plating layer 32b covers both surfaces of the base steel sheet 32a. The aluminum-based plating layer 32b is provided over the entire or almost the entire surfaces of both surfaces of the base steel sheet 32a. Furthermore, in the steel sheet 33, an aluminum-based plating layer 33b covers both surfaces of the base steel sheet 33a. The aluminum-based plating layer 33b is provided over the entire or almost the entire surfaces of both surfaces of the base steel sheet 33a.

[0107] The chemical composition of the aluminum-based plating layers 31b, 32b, and 33b is not particularly limited. Known aluminum-based plating layers (plating layers containing aluminum as a main component) can be used as the aluminum-based plating layers 31b, 32b, and 33b. While not particularly limited, the aluminum-based plating layers 31b, 32b, and 33b are, for example, Al-Si-based plating layers. The aluminum-based plating layers 31b, 32b, and 33b may be the same as or different from the aluminum-based plating layers of the other steel sheets 31, 32, and 33, respectively.

[0108] There is no particular limitation on the type of the base steel plates 31a, 32a, 33a, and each of the base steel plates 31a, 32a, 33a may be the same as or different from the other base steel plates.

[0109] 5A and 5B, similarly to the first embodiment, the steel plate 31 has the smallest plate thickness t min The steel plate 32 has a thickness t min The steel plate 33 has a plate thickness t2 that is larger than the plate thickness t min The steel plate 31 has a plate thickness t3 of at least 100 mm. minis the combined thickness of the base steel plate 31a and the aluminum-based plating layer 31b, and is the average thickness of the steel plate 31. The thickness t2 of the steel plate 32 is the combined thickness of the base steel plate 32a and the aluminum-based plating layer 32b, and is the average thickness of the steel plate 32. The thickness t3 of the steel plate 33 is the combined thickness of the base steel plate 33a and the aluminum-based plating layer 33b, and is the average thickness of the steel plate 33.

[0110] Referring to Figure 5A, the minimum plate thickness t min The coating weight of the aluminum-based plating layer 31b on both surfaces of the base steel sheet 31a of the steel sheet 31 having the above structure is W1 (g / m 2 ), and the coating weight of the aluminum-based plating layer 32b on both surfaces of the base steel sheet 32a of the steel sheet 32 ​​having a larger sheet thickness t2 is W2 (g / m 2 ), the coating weight W1 of the aluminum-based plating layer 31b on the steel sheet 31 is smaller than the coating weight W2 of the aluminum-based plating layer 32b on the steel sheet 32. The coating weight W1 of the aluminum-based plating layer 31b on the steel sheet 31 is the average coating weight on both surfaces of the base steel sheet 31a. Usually, the coating weight (g / m) of the aluminum-based plating layer 31b on one surface of the base steel sheet 31a is 2 ) is the coating weight (g / m) of the aluminum-based plating layer 31b on the other surface of the base steel sheet 31a. 2 ) is substantially equal to the coating weight (g / m) of the aluminum-based plating layer 31b on one surface of the base steel sheet 31a. However, depending on various conditions during manufacturing, for example, the coating weight of the aluminum-based plating layer 31b may vary between the front and back surfaces of the base steel sheet 31a. The coating weight of the aluminum-based plating layer 31b on one surface of the base steel sheet 31a may be different from the coating weight on the other surface of the base steel sheet 31a. Similarly, the coating weight W2 of the aluminum-based plating layer 32b on the steel sheet 32 ​​is the average coating weight on both surfaces of the base steel sheet 32a. The coating weight (g / m) of the aluminum-based plating layer 32b on one surface of the base steel sheet 32a is 2 ) is usually the coating weight (g / m) of the aluminum-based plating layer 32b on the other surface of the base steel sheet 32a. 2) However, depending on various conditions during manufacturing, for example, the coating weight of the aluminum-based plating layer 32b may vary between the front and back surfaces of the base steel sheet 32a. The coating weight of the aluminum-based plating layer 32b may be different between one surface and the other surface.

[0111] The coating weight W1 of the aluminum-based plating layer 31b on the steel sheet 31 and the coating weight W2 of the aluminum-based plating layer 32b on the steel sheet 32 ​​are each 20 g / m 2 More than 120g / m 2 The deposition amounts W1 and W2 are preferably 30 g / m or less. 2 More preferably, it is 35 g / m or more. 2 The deposition weights W1 and W2 are preferably 115 g / m 2 or less, more preferably 100 g / m 2 The difference between the adhesion amounts W1 and W2 (W2-W1) is, for example, 10 (g / m 2 ) or more. W2-W1 is preferably 20 (g / m 2 ) or more, more preferably 30 (g / m 2 ) or more. W2-W1 is 80 (g / m 2 ) or less. W2-W1 is preferably 70 (g / m 2 ) or less, more preferably 60 (g / m 2 ) or less. The adhesion amounts W1 and W2 satisfy the relationship W2 / W1>1.0. The adhesion amounts W1 and W2 preferably satisfy the relationship W2 / W1≧1.2, and more preferably W2 / W1≧1.5.

[0112] Referring to FIG. 5B, the coating weight of the aluminum-based plating layer 33b on both surfaces of the base steel sheet 33a of the steel sheet 33 having a thickness t3 is W3 (g / m 2), the coating weight W3 of the aluminum-based plating layer 33b on the steel sheet 33 may be smaller than the coating weight W2 of the aluminum-based plating layer 32b on the steel sheet 32. The coating weight W3 of the aluminum-based plating layer 33b is the average coating weight on both surfaces of the base steel sheet 33a. Usually, the coating weight (g / m) of the aluminum-based plating layer 33b on one surface of the base steel sheet 33a is 2 ) is the coating weight (g / m) of the aluminum-based plating layer 33b on the other surface of the base steel sheet 33a. 2 ) is substantially equal to the thickness of the aluminum-based plating layer 33b. However, depending on various conditions during manufacturing, for example, the adhesion weight of the aluminum-based plating layer 33b may vary between the front and back surfaces of the base steel sheet 33a. The adhesion weight of the aluminum-based plating layer 33b may be different between one surface and the other surface of the base steel sheet 33a.

[0113] The coating weight W3 of the aluminum-based plating layer 33b on the steel sheet 33 is also, like the steel sheets 31 and 32, 20 g / m 2 More than 120g / m 2 The coating weight W3 is preferably 30 g / m 2 More preferably, it is 35 g / m or more. 2 The adhesion amount W3 is 115 g / m 2 It is preferable that the density is 100 g / m or less. 2 When W2>W3, the difference between the deposition amounts W2 and W3 (W2-W3) is, for example, 10 (g / m 2 ) or more. W2-W3 is preferably 20 (g / m 2 ) or more, more preferably 30 (g / m 2 ) or more. W2-W3 is 80 (g / m 2 ) or less. W2-W3 is preferably 70 (g / m 2 ) or less, more preferably 60 (g / m 2 The coating weight W3 of the aluminum-based plating layer 33b on the steel sheet 33 is equal to or less than the minimum sheet thickness t min The thickness may be equal to or greater than the coating weight W1 of the aluminum-based plating layer 31b of the steel sheet 31 having the thickness W1.

[0114] The method for forming the aluminum-based plating layers 31b, 32b, and 33b on the base steel sheets 31a, 32a, and 33a, respectively, is not particularly limited, but may be, for example, a general hot-dip plating method. That is, by immersing the base steel sheet 31a in a molten aluminum plating bath and gas wiping with nitrogen, air, or the like, an aluminum-plated steel sheet 31 having an adjusted coating weight W1 of the aluminum-based plating layer 31b can be obtained. Similarly, by immersing the base steel sheet 32a in a molten aluminum plating bath and gas wiping with nitrogen, air, or the like, an aluminum-plated steel sheet 32 ​​having an adjusted coating weight W2 of the aluminum-based plating layer 32b can be obtained. Furthermore, by immersing the base steel sheet 33a in a molten aluminum plating bath and gas wiping with nitrogen, air, or the like, an aluminum-plated steel sheet 33 having an adjusted coating weight W3 of the aluminum-based plating layer 33b can be obtained. When an aluminum-based plating layer is formed by hot-dip galvanization, an Al-Fe-based alloy layer is formed at the interface between the base steel sheet and the aluminum-based plating layer due to the elution of Fe during the hot-dip galvanization process.

[0115] The coating weights W1, W2, and W3 of the aluminum-based plating layers 31b, 32b, and 33b can be measured, for example, by the sodium hydroxide-hexamethylenetetramine-hydrochloric acid stripping weight method described in JIS G 3314:2019. Specifically, in accordance with JIS G 3314:2019, a predetermined surface area S (mm 2 ) (for example, 50mm x 50mm) and measure the weight w1 (g) of each test piece. Then, immerse each test piece in a sodium hydroxide solution, and after confirming that the foaming caused by the dissolution of the plating has subsided, remove each test piece from the sodium hydroxide solution and rinse with water. Next, immerse each test piece, still wet after rinsing, in a hydrochloric acid solution containing added hexamethylenetetramine until the foaming caused by the dissolution of the plating has subsided. Immediately rinse and dry the test piece after removing it from the hexamethylenetetramine-hydrochloric acid solution, and measure the weight w2 (g) of the test piece again. The adhesion weight W (g / m) of the aluminum-based plating layer of each test piece is 2 ) is {(w1-w2) / S}×10 6The average value of the coating weight W of five or more test pieces taken from each steel sheet is taken as the coating weight of the aluminum-based coating layer on that steel sheet.

[0116] However, if the size of the test specimens taken from each of the steel sheets 31, 32, and 33 is small, the cross section of each of the aluminum-based plating layers 31b, 32b, and 33b can be observed using an optical microscope (area: 100 μm × 100 μm) and the thickness (μm) of the plating layer can be measured in three fields of view. The average of the thicknesses measured in the three fields of view can be tripled to convert it into the coating weight. In this case, the coating weight is calculated for each side of the base steel sheet for each of the steel sheets 31, 32, and 33, and the average of the obtained coating weights (average of both sides) is taken as the coating weight of the aluminum-based plating layer. If an Al-Fe-based alloy layer is present at the interface between the base steel sheet and the aluminum-based plating layer, the thickness of the aluminum-based plating layer includes the thickness of the Al-Fe-based alloy layer. The thickness of the aluminum-based plating layer 31b of the thinnest steel sheet 31 is smaller than the thickness of the aluminum-based plating layer 32b of the other steel sheet 32. In this embodiment, the thickness of the aluminum-based plating layer 33b of the steel plate 33 is also smaller than the thickness of the aluminum-based plating layer 32b of the steel plate 32. The thicknesses of the aluminum-based plating layers 31b, 33b may be different from each other or may be the same.

[0117] [Structural Members] The blank 30A is subjected to the heating step and forming step similar to those in the first embodiment. As a result, as shown in Figures 6A to 6C, a structural member 10A similar to that in the first embodiment is manufactured from the blank 30A. Figures 6A to 6C are cross-sectional views of the structural member 10A after the forming step (hot stamping). Figure 6A shows the structural member 10A with a minimum plate thickness t min 6B shows a cross section of the structural member 10A at the position of the steel plate 31 having a thickness t min 6C shows a cross section of the structural member 10A at the position of the steel plate 32 having a plate thickness t2 greater than the thickness t2 of the steel plate 31. min The cross section of the structural member 10A at the position of the steel plate 33 having the above plate thickness t3 is shown.

[0118] Referring to Fig. 6A, even in the structural member 10A after hot stamping, the steel sheet 31 is a plated steel sheet having aluminum-based plating layers 31b on both surfaces of a base steel sheet 31a. Referring to Fig. 6B, the steel sheet 32 ​​is a plated steel sheet having aluminum-based plating layers 32b on both surfaces of a base steel sheet 32a. Referring to Fig. 6C, the steel sheet 33 is a plated steel sheet having aluminum-based plating layers 33b on both surfaces of a base steel sheet 33a. However, compared to the state of the blank 30A (Figs. 5A and 5B), the aluminum-based plating layers 31b, 32b, 33b in the structural member 10A have been alloyed with iron more effectively by the heating process.

[0119] 6A and 6B, 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.

[0120] 6B and 6C, when the average thickness (plating thickness) of the aluminum-based plating layer 33b on both surfaces of the steel sheet 33 is K3 (μm), in this embodiment, the plating thickness K3 of the steel sheet 33 is smaller than the plating thickness K2 of the steel sheet 32. The difference between the plating thicknesses K2 and K3, K2 - K3, is, for example, 7 (μm) or more. K2 - K3 may be 33 (μm) or less.

[0121] The thicknesses K1, K2, and K3 of the aluminum-based plating layers 31b, 32b, and 33b in the structural member 10A can be measured as follows. Specifically, the structural member 10A is obtained by disassembling a vehicle body part, and an analysis sample is obtained from the structural member 10A, for example, by laser cutting. For example, an analysis sample is obtained from each of the multiple steel plates included in the structural member 10A. The analysis sample is obtained from the center or its vicinity of the top plate of each steel plate having an open cross section. For test pieces obtained from each of the multiple steel plates, the cross section of the aluminum-based plating layer is nital-etched and then observed with an optical microscope (area: 100 μm × 100 μm), and the thickness of the plating layer is measured in three fields of view. The average value of the thicknesses of the plating layer measured in the three fields of view can be used as the plating thickness. In many cases, the outermost layer of the structural member 10A contains, for example, an electrodeposition coating film. In such cases, the plating layer that exists below the electrodeposition coating film and above the base steel sheet is observed.

[0122] In the structural member 10A according to this embodiment, as in the first embodiment, the minimum plate thickness t min The variation in martensite fraction in the cross section at the position of the steel plate 31 having this structure is, for example, 15% or less, and more preferably 10% or less. The variation in martensite fraction can be measured by the method described in the first embodiment.

[0123] Although not shown, the blank 40 (FIGS. 1 and 3D) for the structural member 20 can have the same configuration as the blank 30A. That is, the configuration of the steel plates 31, 32, and 33 in the blank 30A can be applied as is to the steel plates 41, 42, and 43 (FIG. 3D) of the blank 40. In this case, in the structural member 20 manufactured from the blank 40 through the heating process and the forming process, the steel plates 41, 42, and 43 have the same configuration as the steel plates 31, 32, and 33 of the structural member 10A shown in FIGS. 6A to 6C.

[0124] [effect] In the blank 30A according to this embodiment, as a treatment for increasing the emissivity of the surface of the steel plate 31 compared to both surfaces of the steel plate 32, minThe coating weight W1 of the aluminum-based plating layer 31b of the steel sheet 31 having the thickness t2 is smaller than the coating weight W2 of the aluminum-based plating layer 32b of the steel sheet 32 ​​having the greater thickness t2. As a result, when the blank 30A 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 30A 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. In other words, the emissivity of both surfaces of the steel sheet 31 increases during the heating process. Therefore, the steel sheet 31 can be heated to a temperature in the austenite range more quickly, and the high-temperature holding time of the steel sheet 31 can be extended. As a result, the austenite grains in the microstructure of the steel sheet 31 become coarse, and the ferrite transformation region (ferrite nose) in the CCT diagram shifts to the long-time side. Therefore, it is possible to prevent austenite from transforming into ferrite in the steel sheet 31 after it is removed from the heating furnace and before forming with the die 60 begins. Therefore, it is possible to start forming the blank 30A with the die 60 while maintaining the microstructure of the steel sheet 31 in the austenite phase, and it is possible to reduce the minimum sheet thickness t min The hardenability of the steel plate 31 having this structure can be improved.

[0125] In this embodiment, the hardenability of the relatively thin steel plate 31 is improved, so that the hardness of the structural member 10A formed from the blank 30A can be made uniform. More specifically, in this embodiment, the minimum plate thickness t min Since the steel plate 31 having the above structure is well quenched, the variation in martensite fraction in the steel plate 31 can be reduced to 15% or less. Therefore, as described in the first embodiment, it is possible to reduce the strength defect of the structural member 10A and improve the impact absorption performance of the structural member 10A.

[0126] As in the first embodiment, the improved hardenability of the relatively thin steel plate 31 makes it less likely that non-uniform stress will occur in the structural member 10A. This makes it possible to reduce dimensional accuracy defects in the structural member 10A, even when forming, for example, a large structural member 10A from a blank 30A including the steel plate 31, and improve the impact absorption performance of the structural member 10A.

[0127] In the blank 30A according to this embodiment, the minimum plate thickness t min The coating weight W1 of the aluminum-based plating layer 31b on the steel sheet 31 is smaller than the coating weight W2 of the aluminum-based plating layer 32b on the steel sheet 32 ​​that is thicker than the steel sheet 31. This allows the steel sheet 31 to be heated more quickly than the steel sheet 32, and the high-temperature holding time of the steel sheet 31 is longer than when the coating weight W1 of the aluminum-based plating layer 31b on the steel sheet 31 is the same as or greater than the coating weight W2 of the aluminum-based plating layer 32b on the steel sheet 32. Therefore, after heating of the blank 30A is completed, non-uniformity in phase transformation caused by differences in cooling rates among the steel sheets 31, 32, and 33 can be reduced. Specifically, min Since the start of the phase transformation from austenite to ferrite can be delayed for the steel plate 31, the difference in the start time of the phase transformation between the steel plate 31 and the other steel plates 32 and 33 is reduced. As a result, the hardenability of the steel plates 31, 32, and 33 included in the blank 30 can be made uniform.

[0128] As shown in FIG. 7, in the blank 30A according to this embodiment, the minimum plate thickness t minThe steel sheet 31 having the above structure may be provided with a coating 50 similar to that of the first embodiment. The coating 50 can be provided on at least one surface of the steel sheet 31. That is, the coating 50 may cover only one surface of the steel sheet 31, or may cover both surfaces of the steel sheet 31. This allows the emissivity of the surface of the steel sheet 31 to be increased in advance, so that when the blank 30A is heated during hot stamping, the temperature of the steel sheet 31 rises more quickly. Therefore, the high-temperature holding time of the steel sheet 31 can be secured longer. Therefore, the hardenability of the thinnest steel sheet 31 can be further improved.

[0129] When the coating 50 is provided on the thinnest steel plate 31 in the blank 30A, the coating 13 (FIG. 4) similar to that in the first embodiment is present on the steel plate 31 in the formed structural member 10A.

[0130] The lower structural member 20 (FIG. 1) and blank 40 (FIG. 3D) can have the same configuration as the upper structural member 10A and blank 30A in this embodiment. Therefore, the lower structural member 20 and blank 40 can also achieve the same effects as those described above.

[0131] <Third embodiment> 8 is a plan view of a blank 30B according to the third embodiment. The blanks 30 and 30A according to the first and second embodiments are tailored blanks in which steel plates 31, 32, and 33 are butt-joined to one another. The blank 30B according to this embodiment differs from the first and second embodiments mainly in the arrangement of the steel plates and the mode of the joint.

[0132] 8, the blank 30B includes a plurality of steel plates 31, 32, 33, and 36. In the example of FIG. 8, the steel plates 31, 32, 33, and 36 are arranged and joined to form long portions 34L and 34R and a plurality of connecting portions 35. The minimum plate thickness t minAt least one surface of the steel plate 31 having the above structure is subjected to a treatment to increase the emissivity compared to both surfaces of the steel plate 32. That is, at least one surface of the steel plate 31 is subjected to the same treatment as in the first or second embodiment so that the emissivity is higher than that of the steel plate 32 before or during heating. Therefore, the blank 30B according to this embodiment can also achieve the same effects as in the first embodiment.

[0133] The blank 30B has an overlap portion 37. Fig. 9 is a cross-sectional view taken along line IX-IX in Fig. 8, showing a cross section of the overlap portion 37. In this embodiment, the overlap portion 37 is formed by overlapping the ends of two adjacent steel plates 33, 36. The end of the steel plate 33 is joined to the end of the steel plate 36 in an overlapping state. The steel plates 33, 36 are joined to each other by, for example, spot welding or laser welding.

[0134] The overlap portion 37 has a total thickness t. The total thickness t is the sum of the thickness t3 of the steel plate 33 and the thickness t6 of the steel plate 36. If at least one of the steel plates 33 and 36 is a plated steel plate, the total thickness t also includes the thickness of the plated layer. The total thickness t of the overlap portion 37 is, for example, 4.0 mm or less. The total thickness t of the overlap portion 37 may be greater than 2.5 mm.

[0135] The surface of each of the steel sheets 33, 36 located outside the overlap portion 37 is treated to increase its emissivity compared to both surfaces of the other steel sheet 32 ​​(FIG. 3B). The surface of each of the steel sheets 33, 36 located outside the overlap portion 37 is treated, for example, over its entire surface, to increase its emissivity. For example, the emissivity of the surface of the steel sheet 33 located outside the overlap portion 37, i.e., the surface located opposite the mating steel sheet 36, is set to be higher than the emissivity of both surfaces of the steel sheet 32. Similarly, the emissivity of the surface of the steel sheet 36 located outside the overlap portion 37, i.e., the surface located opposite the mating steel sheet 33, is set to be higher than the emissivity of both surfaces of the steel sheet 32. For example, the emissivity at a wavelength of 8.0 μm at 25°C of the surfaces of the steel sheets 33, 36 located outside the overlap portion 37 is 60% or more, preferably 70% or more, and more preferably 80% or more. The difference in emissivity at a wavelength of 8.0 μm at 25° C. between the surface of the steel plates 33, 36 located outside the overlap portion 37 and both surfaces of the other steel plate 32 is preferably greater than 5%, more preferably greater than 10%, and even more preferably greater than 20%. The emissivity of the surface of the steel plates 33, 36 located inside the overlap portion 37 may be greater than the emissivity of both surfaces of the steel plate 32, or may be equal to or less than the emissivity of both surfaces of the steel plate 32.

[0136] For example, the coating 50 described in the first embodiment may cause the surface of each of the steel sheets 33, 36 located outside the overlap portion 37 to have a higher emissivity than the steel sheet 32 ​​( FIG. 3B ). The coating 50 is provided on at least the surface located outside (the front side) of the overlap portion 37, of both surfaces of the steel sheet 33, and covers the entire surface. The coating 50 is also provided on at least the surface located outside (the front side) of the overlap portion 37, of both surfaces of the steel sheet 36, and covers the entire surface. Because the film thickness of the coating 50 is very small as described in the first embodiment, the plate thickness of the overlap portion 37, measured including the coating 50, can also be treated as the total plate thickness t.

[0137] Alternatively, the surfaces of the steel sheets 33, 36 located outside the overlap portion 37 may be treated so that the surfaces have a higher emissivity than both surfaces of the steel sheet 32 ​​( FIG. 8 ) when the blank 30 is heated. For example, as shown in FIG. 10 , the steel sheets 33, 36 may be plated steel sheets having a thinner plating than the steel sheet 32 ​​( FIG. 5A ). In this case, the steel sheet 33 is a plated steel sheet having a base steel sheet 33a and an aluminum-based plating layer 33b, as in the second embodiment. The steel sheet 36 is also a plated steel sheet having a base steel sheet 36a and an aluminum-based plating layer 36b. The base steel sheets 33a, 36a may be steel sheets of the same type or different types. Similarly, the aluminum-based plating layers 33b, 36b may be plated layers of the same type or different types. The other steel sheets 31, 32 (FIGS. 5A and 5B) are also plated steel sheets each having a base steel sheet 31a, 32a and an aluminum-based plating layer 33b, and have the same configuration as in the second embodiment.

[0138] When the steel sheets 33 and 36 are plated steel sheets, the total thickness t of the overlap portion 37 is a thickness including the thickness of the plated layers 33b and 36b. In the example of FIG. 10, the coating weight of the aluminum-based plated layer 33b on both surfaces of the base steel sheet 33a of the steel sheet 33 is W3 (g / m 2 ), and the coating weight of the aluminum-based plating layer 36b on both surfaces of the base steel sheet 36a of the steel sheet 36 is W6 (g / m 2 ), the coating weights W3 and W6 are smaller than the coating weight W2 of the aluminum-based plating layer 32b (FIG. 5A) on both surfaces of the base steel sheet 32a of the steel sheet 32.

[0139] As explained in the second embodiment, the coating weight W3 of the aluminum-based plating layer 33b on the steel sheet 33 is the average coating weight on both surfaces of the base steel sheet 33a. Similarly, the coating weight W6 of the aluminum-based plating layer 36b on the steel sheet 36 is the average coating weight on both surfaces of the base steel sheet 36a. Usually, the coating weight (g / m) of the aluminum-based plating layer 36b on one surface of the base steel sheet 36a is 2 ) is the adhesion weight (g / m) of the aluminum-based plating layer 36b on the other surface of the base steel sheet 36a.2 ) are substantially equal to the thickness W3 and W6. However, depending on various conditions during manufacturing, for example, the coating weight of the aluminum-based plating layer 36b may vary between the front and back surfaces of the base steel sheet 36a. The coating weights W3 and W6 may be different between one surface and the other surface of the base steel sheet 36a. 2 More than 120g / m 2 The deposition weights W3 and W6 are preferably 30 g / m or less. 2 More preferably, it is 35 g / m or more. 2 The deposition weights W3 and W6 are preferably 115 g / m 2 or less, more preferably 100 g / m 2 The difference between the adhesion weights W2 and W3 between the steel plates 32 and 33, W2-W3, is, for example, 20 (g / m 2 ) or more. W2-W3 is 80 (g / m 2 Similarly, the difference between the adhesion weights W2 and W6 between the steel plates 32 and 36, i.e., W2-W6, may be, for example, 20 (g / m 2 ) or more. W2-W6 is 80 (g / m 2 ) or less. The coating weight W3 of the aluminum-based plating layer 33b of the steel sheet 33 and the coating weight W6 of the aluminum-based plating layer 36b of the steel sheet 36 may be the same or different. Furthermore, the sheet thickness t3 of the steel sheet 33 and the sheet thickness t6 of the steel sheet 36 may be the same or different. The sheet thickness t3 of the steel sheet 33 and the sheet thickness t6 of the steel sheet 36 are the average thicknesses of the steel sheets 33 and 36, respectively, and are sheet thicknesses including the aluminum-based plating layers 33b and 36b.

[0140] The coating weight W3 of the aluminum-based plating layer 33b on the steel sheet 33 and the coating weight W6 of the aluminum-based plating layer 36b on the steel sheet 36 can be measured by the method described in the second embodiment.

[0141] If the total thickness t of the overlap portion 37 exceeds, for example, 2.5 mm, the overlap portion 37 is difficult to heat up. Therefore, when the blank 30B is heated during hot stamping, the overlap portion 37 may reach the minimum thickness t before it reaches the temperature in the austenite region. min In some cases, the coating layer 31b (FIGS. 3B and 5A) of the steel sheet 31 having the coating layer 33b (FIG. 3B) may be alloyed, resulting in a thick diffusion layer and making it impossible to ensure the corrosion resistance or weldability of the steel sheet 31. However, in the example of FIG. 9, the emissivity of the overlap portion 37 is increased in advance by, for example, using a coating 50, thereby facilitating the heating of the overlap portion 37 during the heating process. In the example of FIG. 10, the coating weights W3 and W6 of the coating layers 33b and 36b in the overlap portion 37 are made smaller than the coating weight W2 of the coating layer 32b of the other steel sheet 32, thereby increasing the emissivity of the overlap portion 37 compared to the steel sheet 32 ​​during heating and facilitating the heating of the overlap portion 37. Therefore, even if the total sheet thickness t of the overlap portion 37 is greater than 2.5 mm, the overlap portion 37 can be heated sufficiently until the phase transformation to austenite is completed before the alloying of the coating layer 31b of the steel sheet 31 progresses and the diffusion layer thickens, resulting in a loss of corrosion resistance or weldability. Therefore, a process window can be secured in the manufacture of structural members.

[0142] 11 , even when the steel sheets 33, 36 are plated steel sheets having aluminum-based plating layers 33b, 36b, the coating 50 may be provided on the surface of at least one of the steel sheets 33, 36 located outside the overlap portion 37. The coating 50 may be provided on the surface of both of the steel sheets 33, 36 located outside the overlap portion 37. The surface of each of the steel sheets 33, 36 located inside the overlap portion 37 may be coated with the coating 50, but it is preferable that it is not coated with the coating 50 from the viewpoint of uniform heating of the blank 30B.

[0143] This allows the emissivity of the overlap portion 37 to be increased in advance, so that when the blank 30B is heated during hot stamping, the temperature of the overlap portion 37 can be increased more quickly. min This makes it easier for the overlap portion 37 to be heated to a temperature in the austenite range before alloying of the plating layer 31b of the steel sheet 31 (FIGS. 3B and 5A) having this structure progresses excessively, allowing the structural member to be manufactured while maintaining the corrosion resistance or weldability of the steel sheet 31. This makes it easier to ensure a process window in the manufacture of structural members.

[0144] In the blank 30B according to this embodiment, the steel plate 31 may be butt-joined to the steel plate 32, or may form an overlapping portion with the steel plate 32, as in the steel plates 33 and 36. The steel plate 32 may be butt-joined to the steel plate 36, or may form an overlapping portion with the steel plate 36, as in the steel plates 33 and 36.

[0145] The lower blank 40 (FIG. 3D) may have the same configuration as the upper blank 30B in this embodiment, in which case the lower blank 40 can also achieve the same effects as those described above.

[0146] <Fourth embodiment> 12 is an exploded perspective view of structural members 10C and 20C according to this embodiment. The structural members 10, 10A, and 20 according to the above embodiments constitute a front under module of the vehicle body. On the other hand, the structural members 10C and 20C according to this embodiment constitute a rear under module of the vehicle body.

[0147] Referring to FIG. 12, structural member 10C, like the above-described embodiment, includes a pair of side frames 11L, 11R and at least one cross member 12. Similarly, 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 FIG. 12, cross member 12 connects the middle portions of side frames 11L, 11R. Similarly, cross member 22 connects the middle portions of side frames 21L, 21R. The configurations of structural members 10, 10A, and structural member 20 described in other embodiments can be applied to structural member 10C and structural member 20C of this embodiment, respectively.

[0148] The structural member 10C can be manufactured from a blank 30C shown in Fig. 13 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.

[0149] In this embodiment, the steel plates 31 forming the rear portions 112 (FIG. 12) of the side frames 11L, 11R have a smaller thickness than the steel plates 32 forming the front portions 111 (FIG. 12). The tensile strength of the steel plates 31 may be smaller than the tensile strength of the steel plates 32. In the structural member 10C according to this embodiment and the structural members 10, 10A according to the other embodiments (FIG. 1 and FIGS. 6A to 6C), 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 collision load is input to the vehicle body in the fore-aft direction, in the structural members 10, 10A, 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.

[0150] The structural member 20C can be manufactured from a blank 40C shown in Fig. 14 by a manufacturing method similar to the manufacturing method described in the first embodiment. The blank 40C includes steel plates 41, 42, and 43. The steel plates 41, 42, and 43 are arranged and joined to form two long portions 44L and 44R and at least one connecting portion 45. The blank 40C can have a configuration similar to any of the blanks 30, 30A, and 30B described in the other embodiments.

[0151] In this embodiment, the steel plate 41 forming the rear portion 212 (FIG. 12) of each of the side frames 21L, 21R has a smaller thickness than the steel plate 42 forming the front portion 211 (FIG. 12). The tensile strength of the steel plate 41 may be smaller than the tensile strength of the steel plate 42. As with the upper side, in the lower structural members 20, 20C (FIGS. 1 and 12), it is preferable that the thickness and / or tensile strength of the steel plate located further outward in the fore-and-aft direction of the vehicle body is smaller than that of the steel plate located further inward. As a result, when a collision load is input to the vehicle body in the fore-and-aft direction, the structural members 20, 20C in 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.

[0152] 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.

[0153] In the blank 30 according to the first embodiment, the minimum thickness t minAs a treatment for increasing the emissivity of the surface of the steel sheet 31 compared to the other steel sheets 32, a coating 50 is formed on at least one surface of the steel sheet 31. In the blank 30A according to the second embodiment, as a treatment for increasing the emissivity of the surface of the steel sheet 31 compared to the other steel sheets 32, the coating weight W1 of the aluminum-based plating layer 31b on both surfaces of the steel sheet 31 is made smaller than the coating weight W2 of the aluminum-based plating layer 32b on the other steel sheets 32. However, the treatment for increasing the emissivity of the steel sheet 31 is not limited to this. For example, the emissivity of at least one surface of the steel sheet 31 can be increased compared to the steel sheet 32 ​​by making the surface roughness of at least one surface of the steel sheet 31 greater than the surface roughness of both surfaces of the steel sheet 32. Similarly, in the blank 30B according to the third embodiment, the treatment for increasing the emissivity of the outer surface of the overlap portion 37 of the steel sheets 33 and 36 is not limited to the method described in the embodiment.

[0154] In the second embodiment, the minimum plate thickness t min The above description concerns not only the steel sheet 31 having the minimum thickness t but also the steel sheet 33 having the minimum thickness t3, in which the coating weight W3 of the aluminum-based plating layer 33b is smaller than the coating weight W2 of the aluminum-based plating layer 32b of the other steel sheets 32. However, among the plurality of steel sheets 31, 32, 33 constituting the blank 30, at least the steel sheets 31, 32, 33 having the minimum thickness t min It is sufficient that the coating weight W1 of the aluminum-based plating layer 31b in the steel sheet 31 having the above-mentioned thickness is less than the coating weight W2 of the aluminum-based plating layer 32b in the steel sheet 32. The coating weight W3 of the aluminum-based plating layer 33b in the steel sheet 33 other than the steel sheet 32 ​​may be equal to or greater than the coating weight W2 of the aluminum-based plating layer 32b in the steel sheet 32. In this case, in the structural member 10A after hot stamping, the plating thickness K3 of the steel sheet 33 is also equal to or greater than the plating thickness K2 of the steel sheet 32. In the second embodiment, the steel sheets other than the steel sheets 31, 32 do not necessarily have to be aluminum-plated steel sheets.

[0155] In the first embodiment described above, the blank 30 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 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 15 and 16, the number and arrangement of steel plates can be changed as appropriate.

[0156] As shown in FIGS. 15 and 16 , in a blank 30 corresponding to the structural member 10 ( FIG. 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. That is, the long portions 34L, 34R may be connected by the connecting portion 35 at the end side that is disposed in the front or rear when the structural member 10 is assembled to the vehicle body. As shown in FIG. 16 , the blank 30 may also include multiple connecting portions 35. These connecting portions 35 are formed from separate steel plates 32, 33.

[0157] Although not shown, the number and arrangement of steel plates in the structural member 10A and blanks 30A, 30B according to other embodiments, and in the lower structural member 20 and blank 40, are not particularly limited. The structural members 10, 10A, 20 and blanks 30, 30A, 30B, 40 each need only include two or more joined steel plates. Preferably, the structural members 10, 10A, 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 minThe blanks 30, 30A, 30B, and 40 may each include a first steel plate and 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. One or both sides of the first steel plate are treated to increase the emissivity compared to both sides of the second steel plate. The other steel plates may or may not be treated to increase the emissivity. min When there are multiple steel plates having the above structure, it is preferable that one or both surfaces of all of these steel plates are treated to increase the emissivity more than both surfaces of a steel plate with a greater plate thickness.

[0158] 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 17 to 27, the number and arrangement of the steel plates can be changed as appropriate.

[0159] 17 and 18, 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. 12) is assembled to the vehicle body. The long portions 34L, 34R may also be connected by the connecting portion 35 at their intermediate portions.

[0160] 19, even when the long portions 34L, 34R of the blank 30C are formed of a plurality of steel plates 31, 32, the long portions 34L, 34R may be connected by the connecting portion 35 at the end side that is disposed forward when the structural member 10C (FIG. 12) is assembled to the vehicle body. For example, when the portion corresponding to the front portion 111 (FIG. 12) of the side frames 11L, 11R is formed of the steel plate 31 and the portion corresponding to the rear portion 112 (FIG. 12) is formed of the steel plate 32, the connecting portion 35 may be joined to the steel plate 31 as shown in FIGS. 19 and 20, or may be joined to the steel plate 32 as shown in FIG. 21.

[0161] In the fourth embodiment, the structural member 10C (FIG. 12) 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. 22 to 27, the blank 30C also includes multiple connecting portions 35. The connecting portion 35 is formed from separate steel plates 32, 33, steel plates 33, 36, or steel plates 32, 33, 36. In this case, the long portions 34L, 34R may each be formed from a single steel plate 31, as shown in FIGS. 22, 23, and 27, or may each be formed from multiple steel plates 31, 32, as shown in FIGS. 24 to 26.

[0162] Although not shown in the drawings, in the fourth embodiment, the number and arrangement of steel plates in the lower structural member 20C and blank 40C are not particularly limited. The structural members 10C, 20C and blanks 30C, 40C each need only include two or more joined steel plates. Preferably, the structural members 10C, 20C and blanks 30C, 40C each include three or more steel plates. Each of the blanks 30C, 40C should have at least a minimum plate thickness t min a first steel plate having a plate thickness t minThe blanks 30C and 40C may each include a first steel plate and 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. One or both sides of the first steel plate are treated to increase the emissivity compared to both sides of the second steel plate. The other steel plates may or may not be treated to increase the emissivity. min When there are multiple steel plates having the above structure, it is preferable that one or both surfaces of all of these steel plates are treated to increase the emissivity more than both surfaces of a steel plate with a greater plate thickness.

[0163] 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. That is, each of the sub-blanks may be a single steel plate or a plate material formed by overlapping multiple steel plates.

[0164] 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.

[0165] In the structural members 10, 10A, 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. 28 , the side frames 11 and 21 may have a shape in which one side in the width direction is open in cross section. In this case, 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 may have a cross-sectional shape of another shape. [Example]

[0166] 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.

[0167] In order to confirm the effects of this disclosure, CAE analysis was performed using commercially available software (AUTOFORM R.10, manufactured by AUTOFORM) on the press forming (hot stamping) of structural members that are front or rear under modules, while changing the type (material type) and thickness of the steel plate contained in the structural member, as well as the division pattern of the structural member.

[0168] The steel plates (material types) used in this analysis are shown in Table 1.

[0169] [Table 1]

[0170] In Table 1, the material type is listed in the order of plating type, tensile strength, and application (hot stamping). Regarding the coating specifications, the black coating is a black coating containing carbon black and metal oxides. "Black coating - one side" means that one entire side of the steel sheet is coated with a black coating. "Black coating - both sides" means that both entire sides of the steel sheet are coated with a black coating. In this analysis, material types were selected from Table 1 to construct the target structural members.

[0171] The division patterns of the structural members are shown in Figures 29A to 29F. The structural member shown in Figure 29A is a structural member on the upper or lower side of the front under module. The structural member shown in Figures 29B to 29F is a structural member on the upper or lower side of the rear under module. Figures 29A to 29F 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 29A to 29F, each steel plate is given a number in parentheses.

[0172] Table 2 shows the analysis conditions and results for the structural member shown in Figures 29A and 29B. In Figures 29A and 29B, the side frames of the structural member are each made of two pieces of material (1) and (2). The cross member is made of material (3). Each of materials (1) to (3) is butt-joined to the adjacent material.

[0173] [Table 2]

[0174] Referring to Table 2 and Figures 29A and 29B, Examples 1 and 2 and Comparative Examples 1 and 2 are structural members on the upper side of a rear under-module, and Example 3 and Comparative Example 3 are structural members on the upper side of a front under-module. In Example 1, the material with the smallest thickness t min In Example 2, the material (3) having the smallest thickness t of the materials (1) to (3) is provided with a black coating on both sides. min In Example 3, a black coating is applied to one side of the material (3) having the smallest thickness t min A black coating is applied to one side of the material (2) having a thickness of 1.0 mm. On the other hand, in Comparative Examples 1 to 3, a black coating is not applied to any of the materials (1) to (3). In Comparative Examples 1 to 3, min : No black coating is applied to the material with a thickness of 1.0 mm.

[0175] In Table 2, the "time to reach 910°C" is the time that the material contained in the blank reaches 910°C (A c3The "phase transformation start time" is the time required for a blank heated to a furnace temperature of 920°C for 5 minutes and 30 seconds to reach 910°C after removal from the furnace. The "phase transformation start time" is the shortest time required for the phase transformation to begin after the blank is heated to a furnace temperature of 920°C for 5 minutes and 30 seconds and removed from the furnace. Table 2 shows that in Examples 1 to 3, in which the emissivity of the thinnest blank is increased by a black coating, the time to reach 910°C is approximately 20 seconds shorter than in Comparative Examples 1 to 3, and the temperature rise rate of the thinnest blank during the heating process is higher. Furthermore, in Examples 1 to 3, the phase transformation start time is slower than in Comparative Examples 1 to 3, making it easier to start forming the blank before the onset of ferrite transformation, and enabling the blank to be uniformly quenched during the forming process.

[0176] Tables 3 and 4 show the analysis conditions and results for the structural members shown in Figures 29C to 29F. Examples 4 to 16 and Comparative Examples 4 to 16 in Tables 3 and 4 are all rear under modules, but Examples 4 to 8 and 14 to 16 and Comparative Examples 4 to 8 and 14 to 16 are upper-side structural members, while Examples 9 to 13 and Comparative Examples 9 to 13 are lower-side structural members. In Figures 29C to 29E, the side frames of the structural members are each formed from two pieces of material (1) and (2). The structural members in Figures 29C to 29E include multiple cross members formed from material (3) or material (4). In Figure 29F, the side frames of the structural member are formed from a single piece of material (1). The structural member in Figure 29F includes multiple cross members formed from any of materials (2) to (4). Materials (1) to (4) are each butt-jointed to adjacent materials.

[0177] [Table 3]

[0178] [Table 4]

[0179] Referring to Table 3, in Examples 4 to 13, the minimum thickness tmin In Examples 4 to 13, a black coating was applied to a material having a minimum plate thickness t min The material is covered with a black coating on one or both sides. min When there are multiple blanks having the same thickness, all of these blanks are provided with a black coating. On the other hand, in Comparative Examples 4 to 13, none of the blanks (1) to (4) are provided with a black coating. That is, in Comparative Examples 4 to 13, min : No black coating is applied to the material with a thickness of 1.0 mm.

[0180] Table 3 shows that in Examples 4 to 13, the time to reach 910°C was about 20 seconds shorter than in Comparative Examples 4 to 13, and the temperature rise rate of the thinnest material in the heating process was higher. Furthermore, in Examples 4 to 13, the phase transformation start time was slower than in Comparative Examples 4 to 13, making it easier to start forming the blank before the ferrite transformation begins, and making it possible to uniformly harden the blank in the forming process.

[0181] Referring to Table 4, in Examples 14 to 16, the minimum thickness t min In Examples 14 to 16, a black coating is applied to a material having a minimum plate thickness t min On the other hand, in Comparative Examples 14 to 16, no black coating was applied to any of the materials (1) to (4). In Comparative Examples 14 to 16, the minimum plate thickness t min : No black coating is applied to the material with a thickness of 1.2 mm.

[0182] Table 4 shows that in Examples 14 to 16, the time to reach 910°C was 45 seconds or more shorter than in Comparative Examples 14 to 16, and the temperature rise rate of the thinnest material in the heating process was higher. Furthermore, in Examples 14 to 16, the phase transformation start time exceeded 20 seconds, which was later than in Comparative Examples 14 to 16. This made it easier to start forming the blank before the onset of ferrite transformation, and enabled the blank to be quenched uniformly in the forming process.

[0183] For Examples 6, 11, and 16 and Comparative Examples 6, 11, and 16, analysis samples were taken from the thinnest parts of the structural members using the method described in the above embodiment, and the variation in martensite fraction was measured. Furthermore, these structural members were separately measured for shape accuracy and impact absorption performance. The evaluation results are shown in Table 5.

[0184] [Table 5]

[0185] In Table 5, the variation in martensite fraction is 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 a structural member at the location of the material having the martensite fraction (%).

[0186] Shape accuracy was evaluated by the distance between the structural member and the mating member at the overlapping portion when the structural member was attached to another member. In Table 5, a value of ○ indicates that the distance from the surface of the mating member was within ±2.0 mm, a value of △ indicates that the distance was between ±2.0 mm and ±3.0 mm, and an x ​​indicates that the distance was greater than ±3.0 mm.

[0187] For impact absorption performance, we simulated rear and side collisions with the structural members installed on a vehicle, and collided an impactor simulating a vehicle with the structural members to evaluate the maximum intrusion amount during a rear collision and the maximum intrusion amount during a side collision. Impact absorption performance was evaluated based on the impact absorption performance of a rear under module made by hot stamping each material and then joining them together, and compared it to the impact absorption performance of the base. In Table 5, impact absorption performance equivalent to the base impact absorption performance is indicated as good, impact absorption performance superior to the base impact absorption performance is indicated as better, impact absorption performance slightly lower than the base impact absorption performance is indicated as marginal, and impact absorption performance even lower than that is indicated as poor.

[0188] As described above, in Examples 6, 11, and 16, a black coating was applied to the surface of the thinnest material. On the other hand, in Comparative Examples 6, 11, and 16, no black coating was applied to any material, including the thinnest material. As shown in Table 5, in all of Examples 6, 11, and 16, the variation in martensite fraction was 15% or less, which was significantly lower than that in Comparative Examples 6, 11, and 16. In Examples 6, 11, and 16, the shape accuracy was also better than that of Comparative Examples 6, 11, and 16.

[0189] In Examples 6, 11, and 16, where the variation in martensite fraction was small, the impact absorption performance was also improved compared to Comparative Examples 6, 11, and 16. In Examples 6, 11, and 16, even though the structural members were formed by integrating multiple materials at the blank stage, it was possible to ensure impact absorption performance equal to or greater than that of structural members formed by joining the materials after press-molding them individually. [Explanation of symbols]

[0190] 10, 10A, 10C: Structural members 11, 11L, 11R: Side frame 12: Cross member 13: Film 20, 20C: Structural members 21, 21L, 21R: Side frame 22: Cross member 30, 30A, 30B, 30C: Blank 31,32,33,36: Steel plate 31a, 32a, 33a, 36a: Base steel plate 31b, 32b, 33b, 36b: plating layer 34L, 34R: Long section 35:Connection part 37: Overlap section 40,40C:Blank 41,42,43: Steel plate 44L, 44R: Long section 45:Connection part 50: Film 60: Mold

Claims

1. A blank for hot stamping, The blank includes a plurality of steel plates that are arranged and joined to form two elongated portions that are arranged side by side in the horizontal direction in a plan view of the blank and a connecting portion that connects the elongated portions to each other, The plurality of steel plates are a first steel plate having a smallest plate thickness among the plurality of steel plates; a second steel plate having a thickness greater than that of the first steel plate; Including, A blank, wherein at least one of both surfaces of the first steel plate is treated to increase the emissivity compared to both surfaces of the second steel plate.

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

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

4. 10. The blank of claim 1, A blank, wherein a coating having an emissivity of 60% or more at a wavelength of 8.0 μm at 25°C is formed on at least one surface of the first steel plate as the treatment for increasing emissivity.

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

6. 10. The blank of claim 1, the first steel sheet and the second steel sheet are each plated steel sheets having a base steel sheet and an aluminum-based plating layer covering both surfaces of the base steel sheet, As the treatment for increasing the emissivity, the coating weight (g / m) of the aluminum-based plating layer on both surfaces of the base steel sheet of the first steel sheet is ２ ) is the coating weight (g / m) of the aluminum-based plating layer on both surfaces of the base steel sheet of the second steel sheet. ２ ) are less than blank.

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

8. 10. The blank of claim 1, the first steel sheet is a plated steel sheet having a base steel sheet and a plating layer provided on the base steel sheet, The blank further includes an overlap portion formed by overlapping ends of two adjacent steel plates among the plurality of steel plates, the steel plates being other than the second steel plate, and having a total plate thickness of 4.0 mm or less, A blank, wherein the surface of each of the two steel plates located outside the overlap portion is treated to increase the emissivity compared to both surfaces of the second steel plate.

9. 9. The blank of claim 8, the second steel plate and the two steel plates are plated steel plates each having a base steel plate and an aluminum-based plating layer covering both surfaces of the base steel plate, The coating weight (g / m) of the aluminum-based plating layer on both surfaces of the base steel sheet for each of the two steel sheets ２ ) is the coating weight (g / m) of the aluminum-based plating layer on both surfaces of the base steel sheet of the second steel sheet. ２ ) less than blank.

10. A method for manufacturing a structural member, comprising: Providing a blank according to any one of claims 1 to 9; heating the plurality of steel plates included in the blank to an austenite transformation completion temperature or higher; forming the heated blank using a die and quenching it; A manufacturing method comprising:

11. A structural member for a vehicle body, A pair of side frames; a cross member connecting the side frames; Equipped with The side frames and the cross members are formed by a plurality of steel plates joined together, including a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than the plate thickness of the first steel plate, On the first steel plate, one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide are applied in an amount of 0.001 g / m ２ A structural member provided with a coating containing the above.

12. A structural member for a vehicle body, A pair of side frames; a cross member connecting the side frames; Equipped with The side frames and the cross members are formed by a plurality of steel plates joined together, including a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than the plate thickness of the first steel plate, On the first steel plate, carbon black was applied at 0.500 g / m ２ A structural member provided with a coating comprising:

13. A structural member for a vehicle body, A pair of side frames; a cross member connecting the side frames; Equipped with The side frames and the cross members are formed by a plurality of steel plates joined together, including a first steel plate having a minimum plate thickness and a second steel plate having a plate thickness greater than the plate thickness of the first steel plate, the first steel sheet and the second steel sheet are plated steel sheets each having an aluminum-based plating layer on both surfaces of a base steel sheet, a thickness of the aluminum-based plating layer on the first steel plate being smaller than a thickness of the aluminum-based plating layer on the second steel plate.

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