Framework member

Abstract

Provided is a framework member that has a tensile strength between 980 MPa and 1.8 GPa (both inclusive) and is excellent in axial collapse characteristics.　This framework member uses a steel sheet having a base steel sheet. A cross-sectional shape thereof includes a top plate portion extending in the width direction and two vertical wall portions extending from both ends in the width direction of the top plate portion in a direction different from the extension direction of the top plate portion. The base steel sheet of the top plate portion includes, in the surface layer, a surface layer soft layer of which the Vickers hardness is 84% or less of that at a 1 / 4 sheet thickness position. The thickness of the surface layer soft layer satisfies the following formula (1). The tensile strength of the framework member is between 980 MPa and 1.8 GPa (both inclusive). (1): 20 ≤ X ≤ 0.1 × t - 3800 × [Sb] - 1900 × [Sn] In the formula, X is the thickness (μm) of the surface layer soft layer, t is the thickness (μm) of the base steel sheet, and [Sb] and [Sn] are the contents (mass %) of Sb and Sn in the base steel sheet, respectively.

Description

Frame members

[0001] The present invention relates to a frame member having excellent axial crushing properties, and particularly to a frame member having excellent collision properties when used as a frame member for an automobile.

[0002] The background art will be described using the case of a frame member for an automobile as an example.

[0003] In recent years, the use of high-strength steel plates in automobile body materials has been increasing in order to reduce the weight of automobiles while maintaining crash performance. However, the increased strength of the materials reduces their ductility, increasing the risk of fracture during a collision.

[0004] In particular, energy absorption components located at the front and rear of a vehicle are required to absorb collision energy by plastically deforming during a collision. However, axial collapse of energy absorption components often results in fracture due to the generation of large localized strain. If such fractures develop into large cracks and the component collapses, the collision load may be significantly reduced, potentially destabilizing the energy absorption performance. This destabilization of energy absorption performance is a major issue when applying high-strength steel plates to vehicle bodies. Therefore, it is important to improve the collision performance of vehicle bodies by applying materials to energy absorption components that are high in strength, can withstand high collision loads, and have stable energy absorption performance by suppressing collision fracture. Non-Patent Document 1 discloses that 980 MPa-class materials with different material structures were applied to test specimens simulating energy absorption members, and an axial crushing test was conducted. The results showed that, compared to DP (Dual Phase) steel consisting of ferrite and martensite and TRIP (Transformation-Induced Plasticity) steel containing retained austenite, dual-phase steel containing tempered martensite and bainite in addition to retained austenite, the generation of voids during bending deformation is suppressed, and therefore the bending fracture resistance during a collision is superior.

[0005] Komine et al., Transactions of the Society of Automotive Engineers of Japan, Vol. 52, No. 1 (2021), 94-99.

[0006] Non-Patent Document 1 only examines the basic characteristics of axial crushing for materials with a material strength of 905 to 925 MPa. Furthermore, when impact deformation is even more severe, it is conceivable that further improvements in bending fracture resistance may be necessary. For example, when considering the application of even higher strength steels of 1180 MPa or higher, it is unclear whether impact fracture can be suppressed simply by improving the material structure, and it may be necessary to apply a material with even better bending properties.

[0007] In view of the above, the present invention aims to provide a skeleton member having excellent axial crushing properties by forming a base steel sheet for the top plate portion into a soft layer in the surface layer portion and having a tensile strength of 980 MPa to 1.8 GPa. In particular, the present invention aims to provide a skeleton member for automobiles that achieves both load-bearing properties and fracture-resistance performance during a collision.

[0008] Here, "excellent axial crushing properties" means that when an axial crushing test is carried out, the material has a load-bearing property and fracture-resisting property (crack resistance) at or above a predetermined level, and is excellent in impact absorption properties.

[0009] The present inventors conducted extensive research to achieve the above-mentioned object, and as a result, have found the following. That is, they have found that the effects intended by the present invention can be achieved by satisfying the following [1] to [5], and have arrived at the present invention. [1] A skeleton member using a steel plate having a base steel plate, the cross-sectional shape of which has: a top plate portion extending in the width direction; and two vertical wall portions extending from both ends of the top plate portion in the width direction in a direction different from the extending direction of the top plate portion, the base steel plate of the top plate portion having a surface soft layer at its surface, the Vickers hardness of which is 84% ​​or less of the Vickers hardness at a position 1 / 4 of the plate thickness, the thickness of the soft surface layer satisfying the following formula (1), and the tensile strength of the soft surface layer being 980 MPa or more and 1.8 GPa or less. 20≦X≦0.1×t−3800×[Sb]−1900×[Sn] (1) Here, X is the thickness (μm) of the surface soft layer, t is the thickness (μm) of the base steel sheet, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the base steel sheet, respectively. [2] The skeletal member according to [1], wherein the base steel plate of the top plate portion has a structure in the soft surface layer, in which the area ratio of ferrite is 60.0% or more, and among the structures other than ferrite, the value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less, and the area ratio of retained austenite is 3.0% or less, and the structure at a 1 / 4 thickness position of the base steel plate has an area ratio of ferrite of 55.0% or less (including 0.0%), and the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is more than 40.0%, and the area ratio of fresh martensite is 10.0% or less (including 0.0%).[3] The skeletal member according to [1] or [2], wherein the base steel plate of the top plate portion contains, in mass%, C: 0.050% or more and 0.400% or less, Si: 0.02% or more and 3.00% or less, Mn: 1.50% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.005% or more and 2.000% or less, N: 0.0100% or less, Sb: 0.200% or less (including 0%), and Sn: 0.200% or less (including 0%), with the remainder being Fe and unavoidable impurities. [4] The base steel sheet of the top plate portion further contains, in mass%, Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, The skeletal member according to [3] above, containing at least one element selected from As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less. [5] The skeletal member according to any of [1] to [4] above, wherein the base steel sheet of the top plate portion has an R / t value of 2.0 or less as measured in a 90-degree V-bend test in accordance with JIS Z 2248. Here, R is the limiting bending radius (μm), and t is the thickness of the base steel sheet (μm).

[0010] According to the present invention, it is possible to provide a member having excellent axial crushing properties, i.e., high load-bearing properties and fracture-inhibiting properties (crack resistance), and excellent impact absorbing properties, which is typified by its application to automotive frame members.

[0011] Fig. 1 is a diagram illustrating a specific example of a cross-sectional shape pattern of a skeletal member according to an embodiment of the present invention. Fig. 2 is a diagram illustrating an example of a process for manufacturing a skeletal member according to an embodiment of the present invention. Fig. 3 is a diagram illustrating an axial crushing test method. Fig. 4(a) shows a photograph of an example of a fractured portion after an axial crushing test in a comparative example, and Fig. 4(b) shows a photograph of an example of a fractured portion after an axial crushing test in an example of the present invention.

[0012] [Skeleton Member] The skeletal member of the present invention has a cross-sectional shape including a top plate portion extending in the width direction and two vertical wall portions extending from both ends of the top plate portion in a direction different from the extending direction of the top plate portion. The skeletal member can have various configurations, such as a processed part only, a combination of a processed part and a flat-plate-shaped reinforcing part that reinforces the processed part, or a combination of multiple processed parts.

[0013] Examples of cross-sectional shape patterns are shown in FIG. 1. In FIG. 1, each pattern shown in (a) to (f) shows a cross-sectional shape and a perspective view. As shown in FIGS. 1(a) to (c), in the case of a configuration consisting of only a processed part 10, the cross-sectional shape may not have a closed cross section (see FIG. 1(a)), or may have a closed cross section (see FIGS. 1(b) and 1(c)). Furthermore, as shown in FIGS. 1(d) to 1(f), in the case of a reinforcing part 11, holes or irregularities may be present at any location on the processed part 10 or the reinforcing part 11 (FIG. 1(d) is an example without holes or irregularities, while FIGS. 1(e) and 1(f) are examples with holes or irregularities). Furthermore, a specific cross section may not only be a shape extending in the normal direction of the cross section, but may also be a shape extending along a curve or a shape extending in a certain direction while being magnified.

[0014] The base steel sheet of the top plate portion has a soft surface layer in a region from the surface to 20 μm or more in the plate thickness direction and within 0.1 times the plate thickness, where the Vickers hardness is 84% ​​or less of the Vickers hardness at 1 / 4 of the plate thickness. The soft surface layer refers to a decarburized layer. The soft surface layer contributes to suppressing the propagation of bending cracks during collision deformation, thereby improving performance as a skeletal member. Furthermore, when such a base steel sheet is press-formed into a skeletal member, bendability during press-forming is improved. It is preferable that the vertical wall portion and flange portion also have a soft surface layer.

[0015] The Vickers hardness is measured based on JIS Z 2244-1 (2020) with a load of 9.8 × 10 －２ It can be measured as N.

[0016] Here, the term "base steel sheet" refers to a steel sheet that serves as the base material of the framework member of the present invention. The surface of the base steel sheet may be subjected to plating, painting, film treatment, etc., but the plating layer, painting layer, film layer, etc. are not included in the base steel sheet.

[0017] Here, the soft surface layer has a thickness that satisfies the following formula (1): 20≦X≦0.1×t−3800×[Sb]−1900×[Sn] (1) In formula (1), X is the thickness (μm) of the soft surface layer, t is the thickness (μm) of the base steel sheet, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the base steel sheet, respectively.

[0018] If the soft surface layer thickness (X) is less than 20 μm, the desired bendability intended by the present invention cannot be obtained. On the other hand, if the soft surface layer thickness (X) exceeds (0.1 × t - 3800 × [Sb] - 1900 × [Sn]) μm, the high strength and fracture suppression performance intended by the present invention cannot be simultaneously achieved. Therefore, the soft surface layer thickness (X) is specified to be 20 μm or more and (0.1 × t - 3800 × [Sb] - 1900 × [Sn]) μm or less. In the present invention, Sb and Sn are added as needed to improve plating ability and chemical conversion treatability. However, when Sb and Sn are added, the surface segregation of these elements lowers the allowable upper limit of the soft surface layer thickness (X) that affects bend cracking. For this reason, the upper limit of the soft surface layer that provides good bendability is (0.1 × t - 3800 × [Sb] - 1900 × [Sn]) μm. The thickness of the surface soft layer is preferably 30 μm or more, more preferably 40 μm or more. The thickness of the surface soft layer is preferably 0.1×t μm or less, more preferably 0.08×t μm or less.

[0019] The base steel sheet in the top plate portion has a tensile strength of 980 MPa or more and 1.8 GPa or less. The tensile strength (TS) and yield stress (YS) can be measured by a tensile test in accordance with JIS Z 2241 (2011).

[0020] Next, the steel structure of the skeletal member according to one embodiment of the present invention will be described. <Structure in the surface soft layer of the base steel plate in the top plate portion> Preferred conditions for the structure of the surface soft layer of the base steel plate according to one embodiment of the present invention will be described.

[0021] Ferrite area ratio: 60.0% or more When subjected to bending, the surface layer is deformed more than the interior. Therefore, voids are more likely to form in the surface layer. In the present invention, by controlling the ferrite area ratio in the soft surface layer to 60.0% or more, voids that serve as crack initiation points are less likely to form in the surface layer, and crack propagation is suppressed. Therefore, the ferrite area ratio is preferably 60.0% or more. The ferrite area ratio is more preferably 80.0% or more, and even more preferably 90.0% or more. The ferrite area ratio may be 100.0%. The ferrite area ratio may be less than 100.0%. The ferrite area ratio may be 98.0% or less, or 96.0% or less.

[0022] The value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite): 0.5 or less. If the area ratio of fresh martensite in the soft surface layer is excessively increased, the fresh martensite becomes a void generation origin, making it impossible to achieve the desired bendability inside the base steel sheet. From the viewpoint of ensuring good bendability inside the base steel sheet, when the area ratio of ferrite in the soft surface layer is less than 100.0%, it is preferable that the value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) be 0.5 or less. This value may be 0.4 or less, or may be 0.35 or less. Note that the lower limit of the value obtained by dividing the area ratio of fresh martensite in the soft surface layer by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is not particularly limited and may be 0.0. This value may be 0.1 or greater, or may be 0.15 or greater.

[0023] Area Fraction of Retained Austenite: 3.0% or Less If the area fraction of retained austenite exceeds 3.0%, hard martensite is generated by the strain-induced transformation of the retained austenite during bending deformation during a collision, and voids are generated and cracks propagate at the heterophase boundary with the hard martensite, making it difficult to ensure bendability at the edge of the base steel sheet. Therefore, the area fraction of retained austenite is preferably 3.0% or less. The area fraction of retained austenite is more preferably 2.0% or less, and even more preferably 1.0% or less. There is no particular lower limit for the area fraction of retained austenite, but the area fraction of retained austenite is preferably 0.1% or more, and more preferably 0.3% or more.

[0024] <Structure at 1 / 4 position of plate thickness of base steel sheet> Next, preferred conditions for the structure at 1 / 4 position of plate thickness of a steel sheet according to one embodiment of the present invention will be described.

[0025] Ferrite area fraction: 55.0% or less (including 0.0%) Soft ferrite is a phase that improves ductility. However, if the ferrite area fraction increases excessively, it becomes difficult to achieve a TS of 980 MPa or more. It also leads to a decrease in YS. Therefore, the ferrite area fraction is preferably 55.0% or less. The ferrite area fraction is more preferably 45.0% or less, and more preferably 30.0% or less. The lower limit of the ferrite area fraction is not particularly limited and may be 0.0%. The ferrite area fraction may be 1.0% or more, or 2.0% or more.

[0026] Total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite): greater than 40.0%. Bainitic ferrite and tempered martensite have hardness intermediate between soft ferrite and hard fresh martensite, and are important phases for ensuring good bendability, shear deformation end face bendability, and axial crush resistance of the base steel sheet. Bainitic ferrite is also useful for obtaining an appropriate amount of retained austenite by utilizing the diffusion of C from bainitic ferrite to untransformed austenite. Tempered martensite is effective for improving TS. Therefore, the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite): preferably greater than 40.0%. The total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is more preferably 65.0% or more, and even more preferably 80.0% or more. On the other hand, the upper limit of the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) may be 100.0%. The total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) may be 96.0% or less, or 92.0% or less. Bainitic ferrite is upper bainite with little carbide that is formed in a relatively high temperature range.

[0027] Area fraction of fresh martensite: 10.0% or less (including 0.0%) In the present invention, if the area fraction of fresh martensite is excessively increased, the fresh martensite becomes the starting point for void generation, making it impossible to obtain the desired bending performance. Therefore, the area fraction of fresh martensite is preferably 10.0% or less. Furthermore, the area fraction of fresh martensite is more preferably 5.0% or less. The lower limit of the area fraction of fresh martensite is not particularly limited and may be 0.0%. The area fraction of fresh martensite may be 1.0% or more, or may be 2.0% or more. The fresh martensite referred to here is as-quenched (untempered) martensite. Furthermore, the fresh martensite referred to here also includes (isolated) island-like fresh martensite within ferrite grains.

[0028] Area fraction of retained austenite: less than 3.5% In the present invention, in order to ensure bending performance, the area fraction of retained austenite is preferably less than 3.5%. The area fraction of retained austenite is more preferably 3.0% or less. The area fraction of retained austenite is further preferably 2.5% or less, and even more preferably 2.0% or less. The lower limit of the area fraction of retained austenite is not particularly limited, and may be 0%. The area fraction of retained austenite is preferably 0.1% or more, and more preferably 0.2% or more. The retained austenite referred to here also includes (isolated) island-like retained austenite within ferrite grains, which will be described later.

[0029] The area ratio of the remaining structure other than the above is preferably 10.0% or less. The area ratio of the remaining structure is more preferably 7.0% or less, and even more preferably 5.0% or less. The area ratio of the remaining structure may also be 0.0%.

[0030] The remaining structure is not particularly limited, and examples thereof include carbides such as pearlite and cementite. The type of the remaining structure can be confirmed by observation using, for example, a scanning electron microscope (SEM).

[0031] Here, the area ratios of ferrite, bainitic ferrite, tempered martensite, and hard second phase (fresh martensite + retained austenite) at a quarter-thickness position of the base steel sheet are measured as follows. Specifically, a sample is cut out so that the observation surface is a thickness cross section (L cross section) parallel to the longitudinal direction of the base steel sheet. The observation surface of the sample is then polished with diamond paste, and then finish-polished using alumina. The observation surface of the sample is then etched with 1 vol. % nital to reveal the structure. Next, five fields of view are observed at a magnification of 3000x using an SEM, with the observation position being set at a quarter-thickness position of the base steel sheet ±100 μm. The observation field is selected within a range of ¼ of the steel sheet thickness, with each field of view being 38 μm × 30 μm. From the obtained structural images, Adobe Photoshop from Adobe Systems Inc. was used to calculate the area ratio of each constituent structure (ferrite, bainitic ferrite, tempered martensite, and hard second phase (fresh martensite + retained austenite)) divided by the measured area for five fields of view, and these values ​​were averaged to determine the area ratio of each structure. At the outermost surface, the galvanized layer was excluded, and the image was taken so as to include the internal oxide layer.

[0032] Ferrite: A region that is black and has a blocky shape. It contains almost no carbides. Isolated islands of fresh martensite and isolated islands of retained austenite within ferrite grains are not included in the area ratio of ferrite. Bainitic ferrite: A region that is black to dark gray and has a blocky or amorphous shape. It contains a relatively small number of carbides. Tempered martensite: A region that is gray and has an amorphous shape. It contains a relatively large number of carbides. Hard second phase (retained austenite + fresh martensite): A region that is white to light gray and has an amorphous shape. It does not contain carbides. Carbides: A region that is white and has a dotted or linear shape. It is contained in bainite, tempered bainite, and tempered martensite. Remaining structure: Examples include the above-mentioned pearlite and cementite, and the morphology of these is well known.

[0033] The area fraction of retained austenite is measured as follows. The base steel sheet is mechanically ground in the thickness direction (depth direction) to a position one-quarter of the sheet thickness, and then chemically polished with oxalic acid to obtain an observation surface. The observation surface is then observed by X-ray diffraction. MoKα rays are used as the incident X-rays, and the ratios of the diffraction intensities of the (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensities of the (200), (211), and (220) planes of bcc iron are calculated, and the volume fraction of retained austenite is calculated from the ratio of the diffraction intensities of each plane. The retained austenite is then considered to be three-dimensionally homogeneous, and the volume fraction of retained austenite is taken as the area fraction of retained austenite.

[0034] The area fraction of fresh martensite is determined by subtracting the area fraction of retained austenite from the area fraction of the hard second phase determined as described above: [Area fraction of fresh martensite (%)] = [Area fraction of hard second phase (%)] - [Area fraction of retained austenite (%)]

[0035] The area ratio of the remaining structure is calculated by subtracting the area ratio of ferrite, the area ratio of bainitic ferrite, the area ratio of tempered martensite, and the area ratio of the hard second phase calculated as described above from 100.0%: [Area ratio of remaining structure (%)] = 100.0 - [Area ratio of ferrite (%)] - [Area ratio of bainitic ferrite (%)] - [Area ratio of tempered martensite (%)] - [Area ratio of hard second phase (%)]

[0036] Furthermore, the structure of the soft surface layer can be identified at a position halfway through the thickness of the soft surface layer in the same manner as in identifying the structure at a position halfway through the thickness of the base steel sheet.

[0037] <Composition of Base Steel Sheet> A preferred composition of the base steel sheet used in the base material of the present invention will be described. The base steel sheet used in the base material of the present invention preferably contains, by mass%, C: 0.050% to 0.400%, Si: 0.02% to 3.00%, Mn: 1.50% to less than 3.50%, P: 0.001% to 0.100%, S: 0.0001% to 0.0200%, Al: 0.005% to 2.000%, N: 0.0100% or less, Sb: 0.200% or less (including 0%), and Sn: 0.200% or less (including 0%), with the balance being Fe and unavoidable impurities. Note that although the unit of composition is always "mass %," hereinafter, unless otherwise specified, it will be simply referred to as "%".

[0038] C: 0.050% or more and 0.400% or less. C is an effective element for generating appropriate amounts of tempered martensite, bainite, and tempered bainite, thereby ensuring a TS of 980 MPa or more, as well as high YS and YR. Here, if the C content is less than 0.050%, the area fraction of ferrite increases, making it difficult to achieve a TS of 980 MPa or more. This may also result in a decrease in YS and YR. On the other hand, if the C content exceeds 0.400%, the area fraction of fresh martensite increases excessively, resulting in an excessively high TS and a decrease in El. Furthermore, fresh martensite serves as a starting point for void formation during bending deformation of the base steel sheet, which may prevent the desired bendability and shear edge bendability of the base steel sheet from being achieved. Furthermore, the area fraction of retained austenite increases excessively, significantly increasing the fraction of fresh martensite generated by the stress-induced transformation of retained austenite during shear deformation, promoting subsequent void generation and crack propagation, and making it more difficult to achieve the desired bendability of the shear-deformed end face. Therefore, the C content is preferably 0.050% or more and 0.400% or less. The C content is more preferably 0.070% or more. The C content is even more preferably 0.080% or more, and even more preferably 0.090% or more. The C content is more preferably 0.300% or less. The C content is even more preferably 0.280% or less, and even more preferably 0.250% or less.

[0039] Si: 0.02% or more and 3.00% or less Si is an element that suppresses excessive softening of tempered martensite. Here, if the Si content is less than 0.02%, the tempered martensite may be excessively softened, making it difficult to ensure a TS of 980 MPa or more. On the other hand, if the Si content exceeds 3.00%, the C concentration in austenite during annealing may increase excessively due to an excessive increase in the ferrite area ratio, and the desired bendability of the shear deformation end face may not be achieved. Therefore, the Si content is preferably 0.02% or more and 3.00% or less. The Si content is more preferably 0.10% or more. The Si content is further preferably 0.20% or more, and even more preferably 0.30% or more. The Si content is more preferably 1.80% or less. The Si content is further preferably 1.70% or less, and even more preferably 1.60% or less.

[0040] Mn: 1.50% or more and less than 3.50% Mn is an element that adjusts the area ratio of bainitic ferrite and tempered martensite. Here, if the Mn content is less than 1.50%, the area ratio of ferrite increases, which may make it difficult to achieve a TS of 980 MPa or more. This may also result in a decrease in YS and YR. On the other hand, if the Mn content is 3.50% or more, the martensitic transformation start temperature Ms (hereinafter simply referred to as the Ms point or Ms) decreases, which may reduce the amount of martensite generated during the cooling process. As a result, the area ratio of fresh martensite ultimately increases, and the desired bending performance may not be ensured. Therefore, the Mn content is preferably 1.50% or more and less than 3.50%. The Mn content is more preferably 2.00% or more. The Mn content is even more preferably 2.20% or more. The Mn content is more preferably 3.20% or less. The Mn content is more preferably 3.10% or less, and even more preferably 3.00% or less.

[0041] P: 0.001% or more and 0.100% or less. P has a solid solution strengthening effect and increases the TS and YS of steel sheets. To achieve this effect, the P content is preferably 0.001% or more. On the other hand, if the P content exceeds 0.100%, P may segregate at prior austenite grain boundaries and embrittle the grain boundaries. As a result, during a V-bend test, voids may form and cracks may propagate along the prior austenite grain boundaries, making it difficult to achieve the desired R / t. Therefore, the P content is preferably 0.001% or more and 0.100% or less. The P content is more preferably 0.002% or more, and even more preferably 0.004% or more. The P content is more preferably 0.030% or less. The P content is more preferably 0.025% or less, and even more preferably 0.020% or less.

[0042] S: 0.0001% or more and 0.0200% or less S exists as sulfides in steel. In particular, if the S content exceeds 0.0200%, voids may be generated and cracks may propagate from the sulfides during a V-bend test, making it difficult to obtain the desired R / t. Therefore, the S content is preferably 0.0200% or less. The S content is more preferably 0.0080% or less. The S content is further preferably 0.0050% or less, and even more preferably 0.0030% or less. On the other hand, due to constraints in production technology, the S content is preferably 0.0001% or more. The S content is further preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0043] Al: 0.005% or more and 2.000% or less. Al promotes ferrite transformation during annealing and the cooling process after annealing. That is, Al is an element that affects the area ratio of ferrite. Here, if the Al content is less than 0.005%, the area ratio of ferrite may decrease, and ductility may deteriorate. On the other hand, if the Al content exceeds 2.000%, the area ratio of ferrite may increase excessively, making it difficult to achieve a TS of 980 MPa or more. This may also result in a decrease in YS and YR. Therefore, the Al content is preferably 0.005% or more and 2.000% or less. The Al content is more preferably 0.010% or more. The Al content is more preferably 0.015% or more. The Al content is further preferably 0.020% or more, and even more preferably 0.030% or more. The Al content is more preferably 1.000% or less. The Al content is more preferably 0.800% or less, and even more preferably 0.500% or less.

[0044] N: 0.0100% or less N exists as nitrides in steel. In particular, if the N content exceeds 0.0100%, voids may form and cracks may propagate from the nitrides during a V-bend test, making it difficult to obtain the desired R / t. Therefore, the N content is preferably 0.0100% or less. The N content is more preferably 0.0050% or less. The N content is even more preferably 0.0045% or less, and even more preferably 0.0040% or less. Although there is no particular lower limit for the N content, due to production technology constraints, the N content is preferably 0.0005% or more. The N content is more preferably 0.0010% or more, and even more preferably 0.0015% or more.

[0045] Sb: 0.200% or less (including 0%) Sb is a useful element that segregates on the surface of the base steel sheet during annealing to improve galvanizability and chemical conversion treatability. Therefore, the Sb content may be 0%, but is preferably 0.002% or more. The Sb content is more preferably 0.005% or more. The Sb content is even more preferably 0.007% or more, and even more preferably 0.008% or more. On the other hand, if the Sb content exceeds 0.200%, the effect of improving galvanizability and chemical conversion treatability saturates, and there is a risk of deterioration in the internal bendability and crack propagation resistance of the steel sheet. Therefore, when Sb is contained, the Sb content is preferably 0.200% or less. The Sb content is more preferably 0.020% or less. The Sb content is even more preferably 0.015% or less. The Sb content is even more preferably 0.012% or less. Furthermore, the Sb content is even more preferably 0.011% or less.

[0046] Sn: 0.200% or less (including 0%) Like Sb, Sn is a useful element that segregates to the surface of the base steel sheet during annealing to improve galvanizability and chemical conversion treatability. Therefore, the Sn content may be 0%, but is preferably 0.002% or more. The Sn content is more preferably 0.003% or more. On the other hand, if the Sn content exceeds 0.200%, the effect of improving galvanizability and chemical conversion treatability saturates, which may result in a decrease in the internal bendability and crack propagation resistance of the steel sheet. Therefore, when Sn is contained, the Sn content is preferably 0.200% or less. The Sn content is more preferably 0.020% or less. The Sn content is even more preferably 0.012% or less. The Sn content is even more preferably 0.008% or less, and even more preferably 0.004% or less.

[0047] While a suitable basic chemical composition of a base steel sheet according to one embodiment of the present invention has been described above, the base steel sheet according to one embodiment of the present invention has a chemical composition that contains the above basic chemical components with the balance including Fe (iron) and unavoidable impurities. Here, it is preferable that the base steel sheet according to one embodiment of the present invention has a chemical composition that contains the above basic chemical components with the balance consisting of Fe and unavoidable impurities.

[0048] In addition to the basic components described above, a base steel sheet according to one embodiment of the present invention may contain at least one selected from the optional components shown below. Note that the effects of the present invention can be obtained as long as the optional components shown below are contained in amounts up to the upper limit amounts shown below, so no lower limit is particularly set. Note that when the optional elements listed below are contained in amounts less than the preferred lower limit values ​​described below, the elements are considered to be included as inevitable impurities.

[0049] Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less , Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.100 At least one selected from the group consisting of 0% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less

[0050] Nb: 0.200% or less Nb forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. To achieve this effect, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more. On the other hand, if the Nb content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. Since the coarse precipitates and inclusions can become the starting points for voids and cracks during bending deformation, it may be difficult to ensure the bendability required for a skeletal member. Therefore, when Nb is contained, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.

[0051] Ti: 0.200% or less Like Nb, Ti increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. To achieve this effect, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more. On the other hand, if the Ti content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. Since the coarse precipitates and inclusions can become the starting points for voids and cracks during bending deformation, it may be difficult to ensure the bendability required for a skeletal component. Therefore, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.

[0052] V: 0.200% or less Like Nb and Ti, V forms fine carbides, nitrides, or carbonitrides during hot rolling and annealing, thereby increasing TS and YS. To achieve this effect, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. The V content is further preferably 0.010% or more, and even more preferably 0.030% or more. On the other hand, if the V content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. These coarse precipitates and inclusions may become the starting points for voids and cracks during bending deformation, making it difficult to ensure the bendability required for a skeletal component. Therefore, when V is contained, the V content is preferably 0.200% or less. The V content is more preferably 0.060% or less.

[0053] B: 0.0100% or less B is an element that segregates at austenite grain boundaries to improve hardenability. Furthermore, B controls the formation and grain growth of ferrite during cooling after annealing. To achieve this effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more. The B content is even more preferably 0.0005% or more, and even more preferably 0.0007% or more. On the other hand, if the B content exceeds 0.0100%, cracks may occur inside the base steel sheet during hot rolling, which is a manufacturing process for the base steel sheet. Furthermore, when subjected to bending deformation (as a skeletal member), these internal cracks may become the starting point for cracks, making it difficult to ensure the bendability of the end portion of the base steel sheet. Therefore, when B is contained, the B content is preferably 0.0100% or less. The B content is more preferably 0.0050% or less.

[0054] Cr: 1.000% or less. Cr is an element that improves hardenability. Adding Cr produces an appropriate amount of tempered martensite, thereby increasing TS and YS. To achieve this effect, the Cr content is preferably 0.0005% or more. The Cr content is more preferably 0.010% or more. The Cr content is even more preferably 0.030% or more, and even more preferably 0.050% or more. The Cr content is more preferably 0.100% or more, and even more preferably 0.150% or more. On the other hand, if the Cr content exceeds 1.000%, the area ratio of fresh martensite increases, reducing bendability (as a skeletal member), and it may be difficult to ensure bendability, particularly at the ends of the base steel sheet. Therefore, when Cr is contained, the Cr content is preferably 1.000% or less. The Cr content is more preferably 0.800% or less, and even more preferably 0.700% or less.

[0055] Ni: 1.000% or less. Ni is an element that improves hardenability, and the addition of Ni results in the formation of a large amount of tempered martensite, thereby increasing TS and YS. To achieve this effect, the Ni content is preferably 0.005% or more. The Ni content is more preferably 0.020% or more. The Ni content is even more preferably 0.040% or more, and even more preferably 0.060% or more. On the other hand, if the Ni content exceeds 1.000%, the area ratio of fresh martensite increases, reducing bendability (as a skeletal member), and it may be difficult to ensure bendability, particularly at the ends of the base steel sheet. Therefore, when Ni is contained, the Ni content is preferably 1.000% or less. The Ni content is more preferably 0.800% or less. The Ni content is even more preferably 0.600% or less, and even more preferably 0.400% or less.

[0056] Mo: 1.000% or less Mo is an element that improves hardenability, and the addition of Mo results in the formation of a large amount of tempered martensite, thereby increasing TS and YS. To achieve this effect, the Mo content is preferably 0.010% or more. The Mo content is more preferably 0.030% or more. The Mo content is even more preferably 0.100% or more, and even more preferably 0.150% or more. On the other hand, if the Mo content exceeds 1.000%, the area ratio of fresh martensite increases, reducing bendability (as a skeletal member), and it may be difficult to ensure bendability, particularly at the ends of the base steel sheet. Therefore, when Mo is contained, the Mo content is preferably 1.000% or less. The Mo content is more preferably 0.500% or less, even more preferably 0.450% or less, and even more preferably 0.400% or less. The Mo content is more preferably 0.350% or less, and even more preferably 0.300% or less.

[0057] Cu: 1.000% or less Cu is an element that improves hardenability, and therefore, the addition of Cu results in the formation of a large amount of tempered martensite, thereby increasing TS and YS. To achieve this effect, the Cu content is preferably 0.005% or more. The Cu content is more preferably 0.008% or more, and even more preferably 0.010% or more. The Cu content is more preferably 0.020% or more. The Cu content is further preferably 0.100% or more, and even more preferably 0.150% or more. On the other hand, if the Cu content exceeds 1.000%, the area ratio of fresh martensite may increase excessively. Furthermore, large amounts of coarse precipitates and inclusions may be formed. In such cases, the excessively formed fresh martensite and coarse precipitates and inclusions may become the starting points for voids and cracks during bending deformation, making it difficult to ensure the bendability required for a skeletal member. Therefore, when Cu is contained, the Cu content is preferably 1.000% or less, and more preferably 0.200% or less.

[0058] Ta: 0.100% or less Like Ti, Nb, and V, Ta increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. Additionally, Ta partially dissolves in Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb, Ta)(C, N). This suppresses coarsening of precipitates and stabilizes precipitation strengthening. This further improves TS and YS. To achieve this effect, the Ta content is preferably 0.001% or more. The Ta content is more preferably 0.002% or more, and even more preferably 0.004% or more. On the other hand, if the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions may be formed. In such cases, excessively coarse precipitates and inclusions may become the starting points for voids and cracks during bending deformation, making it difficult to ensure the bendability required for a skeletal member. Therefore, when Ta is contained, the Ta content is preferably 0.100% or less. The Ta content is more preferably 0.090% or less, and even more preferably 0.080% or less. The Ta content is more preferably 0.030% or less, and even more preferably 0.010% or less.

[0059] W: 0.500% or less. W is an element that improves hardenability, and the addition of W results in the formation of a large amount of tempered martensite, thereby increasing TS and YS. To achieve this effect, the W content is preferably 0.001% or more. The W content is more preferably 0.010% or more, and even more preferably 0.030% or more. On the other hand, if the W content exceeds 0.500%, the area ratio of fresh martensite increases, reducing bendability (as a skeletal member), and it may be difficult to ensure bendability, particularly at the ends of the base steel sheet. Therefore, when W is contained, the W content is preferably 0.500% or less. The W content is more preferably 0.450% or less, and even more preferably 0.400% or less. The W content is even more preferably 0.300% or less.

[0060] Mg: 0.0200% or less. Mg is an element that effectively spheroidizes inclusions such as sulfides and oxides and improves the hole expandability and bendability of the base steel sheet. To achieve this effect, the Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0005% or more, and even more preferably 0.0010% or more. The Mg content is even more preferably 0.0030% or more. On the other hand, if the Mg content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, excessively coarse precipitates and inclusions may become the starting points for voids and cracks during bending deformation, making it difficult to ensure the bendability required for a skeletal component. Therefore, when Mg is contained, the Mg content is preferably 0.0200% or less. The Mg content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Mg content is even more preferably 0.0100% or less.

[0061] Zn: 0.0200% or less Zn is an element effective in spheroidizing the shape of inclusions and improving the hole expandability and bendability of the base steel sheet. To achieve this effect, the Zn content is preferably 0.0010% or more. The Zn content is more preferably 0.0020% or more, and even more preferably 0.0030% or more. On the other hand, if the Zn content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be formed. In such cases, excessively coarse precipitates and inclusions may become the starting point for voids and cracks during bending deformation, making it difficult to ensure the bendability required for a skeletal component. Therefore, when Zn is contained, the Zn content is preferably 0.0200% or less. The Zn content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.

[0062] Co: 0.0200% or less Like Zn, Co is an effective element for spheroidizing inclusions and improving the hole expandability and bendability of the base steel sheet. To achieve this effect, the Co content is preferably 0.0010% or more. The Co content is more preferably 0.0020% or more, and even more preferably 0.0030% or more. On the other hand, if the Co content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be formed. In such cases, excessively coarse precipitates and inclusions may become the starting point for voids and cracks during bending deformation, making it difficult to ensure the bendability required for a skeletal component. Therefore, when Co is contained, the Co content is preferably 0.0200% or less. The Co content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.

[0063] Zr: 0.1000% or less Like Zn and Co, Zr is an element that effectively spheroidizes the shape of inclusions and improves the hole expandability and bendability of the base steel sheet. To achieve this effect, the Zr content is preferably 0.0010% or more. On the other hand, if the Zr content exceeds 0.1000%, excessively coarse precipitates and inclusions may become the starting point for voids and cracks during bending deformation, making it difficult to ensure the bendability required for a skeletal component. Therefore, when Zr is contained, the Zr content is preferably 0.1000% or less. The Zr content is more preferably 0.0300% or less, even more preferably 0.0150% or less, and even more preferably 0.0100% or less.

[0064] Ca: 0.0200% or less Ca exists as inclusions in steel. If the Ca content exceeds 0.0200%, a large amount of coarse inclusions may be formed. In such cases, excessively coarse precipitates and inclusions may become the starting point for voids and cracks during bending deformation, making it difficult to ensure the bendability required for a skeletal component. Therefore, when Ca is contained, the Ca content is preferably 0.0200% or less. The Ca content is preferably 0.0020% or less. The Ca content is more preferably 0.0019% or less, and even more preferably 0.0018% or less. While the lower limit of the Ca content is not particularly limited, the Ca content is preferably 0.0005% or more. Due to production technology constraints, the Ca content is more preferably 0.0010% or more.

[0065] Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less. Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are all effective elements for improving the hole expandability and bendability of the base steel sheet. To achieve this effect, the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are each preferably 0.0001% or more. On the other hand, if the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM each exceed 0.0200% and / or the content of As each exceeds 0.0500%, large amounts of coarse precipitates and inclusions may be generated. In such cases, the excessively coarse precipitates and inclusions may become the starting points for voids and cracks during bending deformation, making it difficult to ensure the bendability required for a skeletal member. Therefore, when at least one of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM is contained, it is preferable that the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM each be 0.0200% or less, and the content of As be 0.0500% or less.

[0066] The Se content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Se content is even more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Se content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Te content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Te content is even more preferably 0.0010% or more, and even even more preferably 0.0050% or more. The Te content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Ge content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Ge content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Ge content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The As content is more preferably 0.0010% or more, and even more preferably 0.0015% or more. The As content is even more preferably 0.0050% or more. The As content is more preferably 0.0400% or less, and even more preferably 0.0300% or less. The Sr content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Sr content is even more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Sr content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Cs content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Cs content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Cs content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Hf content is more preferably 0.0005% or more, and even more preferably 0.0008% or more.The Hf content is even more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Hf content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Pb content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Pb content is even more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Pb content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Bi content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Bi content is even more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Bi content is even more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Bi content is even more preferably 0.0100% or less. The REM content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The REM content is more preferably 0.0010% or more, and even more preferably 0.0020% or more. The REM content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The REM content is even more preferably 0.0100% or less. In the present invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM concentration in the present invention refers to the total content of one or more elements selected from the above-mentioned REM elements. REM is not particularly limited, but is preferably Sc, Y, Ce, or La.

[0067] The steel sheet used as the base material of the skeletal member of the present invention may be a cold-rolled steel sheet, which is a base steel sheet, or may be a hot-dip galvanized steel sheet (GI) in which a plating layer is formed on a base steel sheet, or a galvannealed steel sheet (GA).

[0068] Plated Layer In the present invention, a plated layer is formed on a base steel sheet (on the surface of the base steel sheet), and this plated layer may be formed on only one surface of the base steel sheet, or on both surfaces. That is, in the present invention, a base steel sheet may be provided, and a plated layer (such as a zinc plated layer or an aluminum plated layer) may be formed on the base steel sheet.

[0069] The plating layer (galvanized layer) referred to here refers to a plating layer containing Zn as the main component (Zn content of 50.0% or more), and examples thereof include a hot-dip galvanized layer and a hot-dip galvannealed layer.

[0070] Here, the hot-dip galvanized layer is preferably composed of, for example, Zn, 20.0 mass% or less Fe, and 0.001 mass% to 1.0 mass% Al. The hot-dip galvanized layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0 mass% to 3.5 mass%. The Fe content of the hot-dip galvanized layer is more preferably less than 7.0 mass%. The remainder other than the above elements is unavoidable impurities.

[0071] The galvannealed layer is preferably composed of, for example, 20.0 mass% or less Fe and 0.001 mass% to 1.0 mass% or less Al. The galvannealed layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0 mass% to 3.5 mass%. The Fe content of the galvannealed layer is more preferably 7.0 mass% or more, and even more preferably 8.0 mass% or more. The Fe content of the galvannealed layer is more preferably 15.0 mass% or less, and even more preferably 12.0 mass% or less. The remainder other than the above elements is unavoidable impurities.

[0072] In addition, the coating weight of the plating layer (zinc plating layer) per side is not particularly limited, but is preferably 20 g / m ２80g / m or more ２ It is preferable to do the following:

[0073] The coating weight of the plating layer (zinc plating layer) is measured as follows. That is, a treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe ("Ivit 700BK" (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10 mass % aqueous hydrochloric acid solution. Next, a steel sheet (zinc-plated steel sheet) to be used as a test material is immersed in the treatment solution to dissolve the plating layer (zinc plating layer). The mass loss of the test material before and after dissolution is measured, and this value is divided by the surface area of ​​the base steel sheet (the surface area of ​​the part that was covered with the plating) to determine the coating weight (g / m ２ ) is calculated.

[0074] The thickness of the steel plate according to one embodiment of the present invention is not particularly limited, but is preferably 0.5 mm or more, more preferably 0.6 mm or more, and even more preferably 0.8 mm or more. The thickness of the steel plate is preferably 2.3 mm or less, and more preferably 1.6 mm or less.

[0075] Next, the bendability of the base steel sheet in the top plate portion according to one embodiment of the present invention will be described. Regarding bendability, it is preferable that R (limit bending radius) / t (sheet thickness) be 2.0 or less as measured in a 90-degree V-bend test in accordance with JIS Z 2248 (2022). Excellent bendability means excellent fracture resistance during collision deformation.

[0076] [Manufacturing Method (Manufacturing Method of Skeleton Member and Manufacturing Method of Steel Sheet)] First, according to one embodiment of the present invention, a manufacturing method of a steel sheet having a base steel sheet used as a base material for a skeleton member will be described. The manufacturing method of the steel sheet used as a base material of the present invention includes a hot rolling step of hot rolling a steel slab to form a hot-rolled steel sheet, a pickling step of pickling the hot-rolled steel sheet, a cold rolling step of cold rolling, an annealing step of annealing the steel sheet after the cold rolling step, a cooling step of cooling the steel sheet, and, if necessary, a plating step of plating the steel sheet with zinc. Examples of the zinc plating process include hot-dip galvanizing and galvannealing.

[0077] Hereinafter, a preferred example of a method for manufacturing a steel sheet to be used as a base material for a skeletal member according to one embodiment of the present invention will be described. The method for manufacturing a base steel sheet of the present invention includes a hot rolling step of hot rolling a steel slab having the above-described chemical composition at a finish rolling temperature of 820°C or higher, an annealing step of heating the steel sheet after the hot rolling step and annealing it under conditions of an annealing temperature of 750°C to 900°C, a soaking time of 20 seconds or longer, and a dew point of -10°C or higher in an atmosphere, and further satisfying the conditions of formulas (2) and (3), a cooling step of cooling the steel sheet after the annealing step to a cooling stop temperature of less than 100°C, a first holding step of reheating the steel sheet after the cooling step to a reheat holding temperature range of the cooling stop temperature to 440°C and holding for 10 seconds or longer, and a first holding step of holding the steel sheet after the first holding step to a tensile strength of 19.6 N / mm ２ and a second holding step of holding the steel sheet after the surface strain introduction step in the reheating holding temperature range for 10 seconds or more, or further includes a cold rolling step of cold rolling the steel sheet after the hot rolling step and before the annealing step at a rolling reduction of 20% or more and 80% or less to obtain a cold-rolled steel sheet. 2400≦Y≦20000 (2) Y=[{(T−Ac1)×t1} / 2}]+{(T−Ac1)×t2} (3) Here, in formula (3), T: annealing temperature (°C), t1: time (s) from 650°C to the annealing temperature T during temperature rise in the annealing step, t2: soaking time (s), Ac1 (°C): 727.0−32.7×[%C]+14.9×[%Si]+2.0×[%Mn], [%C]: C content in the steel plate, [%Si]: Si content in the steel plate, [%Mn]: Mn content in the steel plate.

[0078] In the present invention, the method for melting the steel material (steel slab) is not particularly limited, and any known melting method such as a converter or electric furnace is suitable. Furthermore, the steel slab is preferably produced by a continuous casting method to prevent macrosegregation, but it can also be produced by an ingot casting method, a thin slab casting method, or the like. Furthermore, in addition to the conventional method in which the steel slab is cooled to room temperature after production and then reheated, energy-saving processes such as direct rolling, in which the slab is charged into a heating furnace as a hot slab without being cooled to room temperature, or is immediately rolled after a short heat retention period, can also be applied without any problems.

[0079] (Hot rolling process) When heating a slab, the slab heating temperature is preferably 1100°C or higher from the viewpoint of dissolving carbides and reducing the rolling load. Furthermore, in order to prevent an increase in scale loss, the slab heating temperature is preferably 1300°C or lower. The slab heating temperature is the temperature of the slab surface. Furthermore, the slab is made into a sheet bar by rough rolling under normal conditions. However, when the heating temperature is low, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling from the viewpoint of preventing problems during hot rolling.

[0080] Finish rolling temperature: 820°C or higher A low finish rolling temperature leads to an increase in the rolling load. Furthermore, a low finish rolling temperature increases the reduction ratio in the unrecrystallized state of austenite, causing the development of an abnormal structure elongated in the rolling direction, which reduces the ductility, hole expandability, and bendability of the final material. For this reason, the finish rolling temperature is set to 820°C or higher. The finish rolling temperature is preferably 830°C or higher, more preferably 850°C or higher. Furthermore, the finish rolling temperature is preferably 1080°C or lower, more preferably 1050°C or lower.

[0081] The coiling temperature after hot rolling is not particularly limited, but it is necessary to consider the possibility of reducing the ductility, hole expandability, and bendability of the final material. Therefore, the coiling temperature after hot rolling is preferably 300°C or higher. Furthermore, the coiling temperature after hot rolling is preferably 700°C or lower.

[0082] During hot rolling, the rough-rolled sheets may be joined together and continuously finished. The rough-rolled sheets may also be temporarily wound. To reduce the rolling load during hot rolling, some or all of the finish rolling may be performed as lubricated rolling. Performing lubricated rolling is also effective from the viewpoint of uniforming the shape and material properties of the steel sheet. The coefficient of friction during lubricated rolling is preferably in the range of 0.10 to 0.25.

[0083] (Pickling step) The hot-rolled steel sheet produced as described above may be subjected to pickling. Pickling can remove oxides from the steel sheet surface, and therefore can be performed to ensure good chemical conversion treatability and plating quality in the final high-strength steel sheet product. Furthermore, pickling may be performed once or multiple times.

[0084] (Cold Rolling Step) The hot-rolled pickled sheet or hot-rolled steel sheet obtained as described above is subjected to cold rolling as necessary. When cold rolling is performed, the pickled sheet may be subjected to cold rolling as it is after hot rolling, or may be subjected to heat treatment and then cold rolling. Optionally, the cold-rolled steel sheet obtained after cold rolling may be subjected to pickling. Cold rolling is performed by multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling.

[0085] If necessary, cold rolling reduction ratio: 20% or more and 80% or less When cold rolling is performed, the cold rolling reduction ratio (cumulative reduction ratio) is not particularly limited, but is preferably 20% or more and 80% or less. If the cold rolling reduction ratio is less than 20%, the steel structure is likely to become coarse and non-uniform in the annealing process, which may result in a decrease in TS and bendability in the final product. On the other hand, if the cold rolling reduction ratio exceeds 80%, the steel sheet is likely to have a defective shape, and when zinc plating is performed in the subsequent process, the zinc plating deposition amount may become non-uniform.

[0086] (Annealing Step) In one embodiment of the present invention, the steel sheet after the hot rolling step (when cold rolling is not performed) or after the cold rolling step (when cold rolling is performed) is heated and annealed in an atmosphere with an annealing temperature of 750°C or higher and 900°C or lower, a soaking time of 20 seconds or longer, and a dew point of -10°C or higher, preferably under conditions that satisfy formulas (2) and (3). 2400≦Y≦20000 ... formula (2) Y = [{(T - Ac1) × t1} / 2}] + {(T - Ac1) × t2} ... formula (3) Where, in formula (3), T is the annealing temperature (°C), t1 is the time (s) from 650°C to the annealing temperature T during the temperature rise in the annealing step, and t2 is the soaking time (s).

[0087] Annealing temperature: 750°C or higher and 900°C or lower If the annealing temperature is lower than 750°C, the austenite generation rate during heating in the two-phase region of ferrite and austenite becomes insufficient. As a result, the area ratio of ferrite increases excessively after annealing, and the desired TS and YS cannot be obtained. On the other hand, if the annealing temperature exceeds 900°C, the ductility decreases. Therefore, the annealing temperature is preferably set to 750°C or higher and 900°C or lower. The annealing temperature is more preferably 880°C or lower. The annealing temperature is even more preferably 870°C or lower. The annealing temperature is more preferably 780°C or higher, and even more preferably 800°C or higher. The annealing temperature is the maximum temperature reached in the annealing process (soaking temperature).

[0088] Soaking time: 20 seconds or more. If the soaking time is less than 20 seconds, the austenite generation rate during heating in the two-phase region of ferrite and austenite becomes insufficient. As a result, the area ratio of ferrite increases excessively after annealing, preventing TS and YS from being obtained. Therefore, the soaking time is preferably 20 seconds or more. The soaking time is more preferably 30 seconds or more, and more preferably 50 seconds or more. Although the upper limit of the soaking time is not particularly limited, the soaking time is preferably 900 seconds or less, and more preferably 800 seconds or less. The annealing time is even more preferably 600 seconds or less, and even more preferably 250 seconds or less. The soaking time is the holding time in a temperature range of (annealing temperature -40°C) or more and the annealing temperature or less. In other words, the soaking time includes not only the holding time at the annealing temperature but also the residence time in a temperature range of (annealing temperature -40°C) or more and the annealing temperature or less during heating and cooling before and after reaching the annealing temperature. The number of annealing steps may be two or more, but one step is preferred from the viewpoint of energy efficiency.

[0089] Dew point of the atmosphere in the annealing step (annealing atmosphere): -10°C or higher In one embodiment of the present invention, the dew point of the atmosphere in the annealing step (annealing atmosphere) is preferably -10°C or higher. By performing annealing in the annealing step with a dew point of the annealing atmosphere of -10°C or higher, the decarburization reaction is promoted and a deeper soft surface layer can be formed. The dew point of the annealing atmosphere in the annealing step is more preferably -5°C or higher, even more preferably 0°C or higher, and most preferably +5°C or higher. There is no particular upper limit for the dew point of the annealing atmosphere in the annealing step, but in order to suitably prevent oxidation of the surface of the Fe-based electroplating layer and improve plating adhesion when a zinc plating layer is formed, the dew point of the annealing atmosphere in the annealing step is preferably 30°C or lower. The dew point of the annealing atmosphere in the annealing step is more preferably 25°C or lower, and even more preferably 20°C or lower.

[0090] 2400≦Y≦20000 (2) Y=[{(T−Ac1)×t1} / 2}]+{(T−Ac1)×t2} (3) In formula (3), T is the annealing temperature (°C), t1 is the time (s) from 650°C to the annealing temperature T during the temperature rise in the annealing process, and t2 is the soaking time (s). t1 is the time (s) from 650°C to the annealing temperature (soaking temperature) T during the temperature rise in the annealing process, t2 is the soaking time (holding time) (s) in the annealing process, and Ac1 is the Ac1 point (°C). In the present invention, it is preferable to perform annealing under annealing conditions that satisfy formulas (2) and (3). If Y in formula (3) is less than 2400, the soft surface layer defined in the present invention may be less than 20 μm. On the other hand, if Y exceeds 20,000, the soft surface layer defined in the present invention may have a thickness exceeding (0.1 × t - 3,800 × [Sb] - 1,900 × [Sn]) μm. Therefore, Y in formula (3) is set to 2,400 or more and 20,000 or less. Y is more preferably 9,000 or more, and even more preferably 12,000 or more. Furthermore, Y is more preferably 19,000 or less, and even more preferably 18,000 or less. t1 is preferably 30 s or more. Furthermore, t1 is preferably 80 s or less.

[0091] The Ac1 point (°C) is calculated by the following formula: Ac1 point (°C) = 727.0 - 32.7 × [%C] + 14.9 × [%Si] + 2.0 × [%Mn], where [%C] is the C content of the steel plate, [%Si] is the Si content of the steel plate, and [%Mn] is the Mn content of the steel plate.

[0092] (Cooling Step) Cooling to a Cooling Stop Temperature of Less than 100°C Average cooling rate: 10°C / s or more and 50°C / s or less, atmospheric dew point: -20°C or less (Preferred Conditions) In the cooling step, it is preferable to cool the steel sheet after the annealing step to a cooling stop temperature of less than 100°C. In this case, the cooling start temperature can be 750°C or more and 900°C or less. The cooling stop temperature is more preferably 80°C or less, and even more preferably 60°C or less. The cooling stop temperature is more preferably 5°C or more, and even more preferably 15°C or more. The average cooling rate during this cooling step is preferably 10°C / s or more and 50°C / s or less. The steel structure specified in the present invention can be obtained in this cooling step. Furthermore, the atmospheric dew point during this cooling step is preferably -20°C or less. If the atmospheric dew point exceeds -20°C, the thickness of the soft surface layer of the steel sheet varies greatly in the in-plane direction, and the tensile strength specified in the present invention may not be obtained. Therefore, it is preferable to set the dew point of the atmosphere in this cooling step to −20° C. or lower. The average cooling rate (° C. / s) is obtained by dividing the difference between the cooling start temperature (° C.) and the cooling stop temperature (° C.) in the cooling step by the cooling time (s).

[0093] (First Holding Step (Reheating Holding Step)) In the first holding step (reheating holding step), it is preferable to reheat the steel sheet to a reheating holding temperature range of not less than the cooling stop temperature but not more than 440°C and hold the temperature for 10 seconds or more. By reheating the steel sheet to a reheating holding temperature range of not less than the cooling stop temperature but not more than 440°C and holding the temperature for 10 seconds or more, it becomes easy to form bainitic ferrite and tempered martensite as defined in the present invention. The reheating holding temperature range is more preferably not more than 420°C, and even more preferably not more than 400°C. The holding time is more preferably not less than 20 seconds, and even more preferably not less than 30 seconds. The holding time is more preferably not more than 100 seconds, and even more preferably not more than 80 seconds.

[0094] (Surface Strain Inducing Step) In the surface strain introducing step, the steel sheet after the first holding step is subjected to a strain of 19.6 N / mm ２The tension is calculated by multiplying the total value of the loads (N) of the load cells on the left and right sides of the rolls through which the steel sheet passes while in contact with the roll by the cross-sectional area of ​​the steel sheet (= sheet thickness (mm) × sheet width (mm)) (mm ２ ) and the tension is obtained by dividing the load cell by the tension direction. The load cell must be placed parallel to the tension direction. The load cell is preferably placed 200 mm from both ends of the roll. The length of the roll used is preferably 1500 to 2500 mm. The tension is preferably 21.5 N / mm ２ More preferably, it is 23.5 N / mm ２ The tension is preferably 147 N / mm ２ More preferably, 98 N / mm ２ This tension is even more preferably 39.2 N / mm ２ The following is the result.

[0095] (Second Holding Step) In the second holding step, the steel sheet after the surface strain introduction step is preferably held in the reheating holding temperature range for 10 seconds or more. The holding time in the second holding step is more preferably 15 seconds or more, and even more preferably 20 seconds or more. The holding time is preferably 60 seconds or less, and more preferably 50 seconds or less.

[0096] (Plating Step (Hot-Dip Galvanizing Step, Galvannealed Hot-Dip Galvanizing Step)) In the present invention, after the annealing step, the steel sheet is subjected to a galvanizing treatment in the plating step to obtain a galvanized steel sheet. Examples of the galvanizing treatment include a hot-dip galvanizing treatment and a galvannealed hot-dip galvanizing treatment.

[0097] In the case of hot-dip galvanizing treatment, it is preferable to immerse the steel sheet in a galvanizing bath at a temperature of 440° C. or higher and 500° C. or lower, and then adjust the coating weight by gas wiping, etc. The hot-dip galvanizing bath is not particularly limited as long as it provides the above-mentioned composition of the galvanized layer, but it is preferable to use, for example, a plating bath having an Al content of 0.10 mass % or higher and 0.23 mass % or lower, with the balance consisting of Zn and unavoidable impurities.

[0098] Furthermore, in the case of alloying hot-dip galvanizing treatment, it is preferable to perform alloying treatment by heating the hot-dip galvanized steel sheet to an alloying temperature of 450°C or higher and 600°C or lower after performing the hot-dip galvanizing treatment as described above. If the alloying temperature is lower than 450°C, the Zn-Fe alloying rate will be slow, and alloying may become difficult. On the other hand, if the alloying temperature exceeds 600°C, untransformed austenite will transform to pearlite, making it difficult to achieve a TS of 980 MPa or higher. The alloying temperature is more preferably 500°C or higher, and even more preferably 510°C or higher. The alloying temperature is more preferably 570°C or lower.

[0099] The coating weight of both the hot-dip galvanized steel sheet (GI) and the galvannealed steel sheet (GA) is 20 to 80 g / m per side. ２ The plating weight can be adjusted by gas wiping or the like.

[0100] The steel sheet obtained as described above may further be subjected to temper rolling. If the temper rolling reduction exceeds 2.00%, the yield stress increases, which may result in a decrease in dimensional accuracy when the steel sheet is formed into a component. Therefore, the temper rolling reduction is preferably 2.00% or less. The lower limit of the temper rolling reduction is not particularly limited, but it is preferably 0.05% or more from the viewpoint of productivity. Furthermore, temper rolling may be performed on an apparatus continuous with the annealing apparatuses used for the above-mentioned processes (online), or may be performed on an apparatus discontinuous with the annealing apparatuses used for the above-mentioned processes (offline). Furthermore, the number of times temper rolling may be one, two, or more. Furthermore, rolling using a leveler or the like may be used as long as it can impart an elongation rate equivalent to that of temper rolling.

[0101] Other conditions of the manufacturing method are not particularly limited, but from the viewpoint of productivity, it is preferable that the series of processes such as the annealing, hot-dip galvanizing, and alloying treatment of the galvanizing be carried out in a continuous galvanizing line (CGL). After hot-dip galvanizing, wiping can be performed to adjust the coating weight of the coating. Note that plating conditions other than those described above can be based on conventional hot-dip galvanizing methods.

[0102] Next, a method for manufacturing a skeletal member according to one embodiment of the present invention will be described. FIG. 2 shows an example of a process for manufacturing a skeletal member using a steel plate as a base material. The skeletal member 1 may include a hat-shaped component (processed component 10) consisting of a top plate portion, vertical wall portions, and flange portions, and a back plate component (reinforcement component 11). The hat-shaped component shown in FIG. 2( a) is processed using a typical thin plate material forming method such as press forming, bending, roll forming, or extrusion. As shown in FIG. 2( b), the hat-shaped component (processed component 10) and the back plate component (reinforcement component 11) are assembled into an integrated component (skeletal member 1) by joining methods such as spot welding (see symbol A in the figure), laser welding, bolting, or adhesive bonding, or by caulking. Next, as shown in FIG. 2( c), an end fixing jig 12 can be further attached to the integrated component (skeletal member 1) including the hat-shaped component and the back plate component to obtain a hollow cross-section component 13. Note that 40 mm, 40 mm, and 20 mm shown in Figure 2(a) are examples of the lengths of the top plate portion, vertical wall portion, and flange portion in the cross-sectional shape, respectively, but the lengths of the top plate portion, vertical wall portion, and flange portion are not limited to these lengths.

[0103] The back plate component is joined to the hat-shaped component to form a closed cross section, thereby suppressing the opening of the cross section of the hat-shaped component and serving as a reinforcing member for stably and significantly deforming the frame member. For example, a cold-rolled steel sheet or a galvanized steel sheet can be used for this back plate component. While there are no particular restrictions on the tensile strength or ductility of the back plate component, a steel sheet with a tensile strength of 590 MPa or more is preferred.

[0104] The above describes an example in which the base steel sheet is treated in the annealing step, cooling step, first holding step, surface strain introduction step, and second holding step while it is still in the steel sheet state. However, in the present invention, the heat treatment steps from the annealing step to the second holding step do not necessarily have to be performed while the base steel sheet is in the steel sheet state. The heat treatment steps from the annealing step to the second holding step may also be performed after the base steel sheet is press-formed into the shape of a skeleton member.

[0105] The skeletal member of the present invention has excellent crashworthiness, particularly when used as a skeletal member for an automobile, but its use is not limited to this. The skeletal member of the present invention can be used as a skeletal member for an architectural structure or a skeletal member for a mechanical structure. For example, when used as a strength member such as a column or beam in an architectural structure, it can be a strong skeletal member with excellent impact resistance in the event of an earthquake. Furthermore, when used as a skeletal member for a mechanical structure, it can be a strong skeletal member with excellent impact resistance when some kind of impact load is applied to the mechanical structure.

[0106] <Steel Sheet Manufacturing Method> Slabs having the chemical compositions shown in Table 1 were produced by melting in a converter and continuous casting. The steel slabs were heated to 1200°C, and after heating, the steel slabs were subjected to hot rolling, consisting of rough rolling and finish rolling at a finish rolling temperature of 900°C, to produce hot-rolled steel sheets. The resulting hot-rolled steel sheets were then pickled and cold-rolled at a cold reduction of 30 to 60% to produce cold-rolled steel sheets with a thickness of 1.6 mm. The resulting cold-rolled steel sheets were then subjected to annealing and cooling. Some of the steel sheets were also subjected to hot-dip galvanizing or galvannealing. Table 2 shows the conditions for the annealing process, cooling process, plating process during the cooling process, reheating and holding process (first holding process), surface strain introduction process, and second holding process. In Table 2, the type of plating process GA is a condition in which alloyed hot-dip galvanizing treatment was performed, GI is a condition in which hot-dip galvanizing treatment was performed, and CR is a condition in which the base steel sheet was not subjected to plating treatment.

[0107]

[0108]

[0109] <Method for Manufacturing Components> Blanks for hat-shaped skeletal components were cut from the above steel plate. Specifically, material was prepared so that the hat-shaped components would have a constant cross-section with an axial length of 200 mm after molding, and the hat-shaped components were fabricated by press molding. The cross-sectional shape of the hat-shaped components is shown in Figure 2. The top plate and vertical wall portions were 40 mm long, and the flange portions were 20 mm long. All bending ridges connecting the top plate and vertical wall portions and the vertical wall and flange portions were formed to have an internal bending radius of 5 mm. The hat-shaped components and back plate members were then spot-welded to join the flanges of the hat-shaped components to the back plate member, producing a skeletal component with a hollow cross-section. The welding pitch was 45 mm. This back plate member was a cold-rolled steel plate with a thickness of 1.6 mm and a tensile strength of 1180 MPa. By joining the back plate member to the hat-shaped components to form a closed cross-section, it serves as a reinforcing component for effectively and stably suppressing cross-sectional opening of the hat-shaped components in the axial crushing test described below, thereby enabling large deformation. In order to stabilize deformation when conducting axial crushing tests to evaluate collision performance, a jig for fixing the end cross section was arc-welded to one end of the frame member in the axial direction of the test specimen.

[0110] <Method for Measuring the Soft Surface Layer> The method for measuring the soft surface layer is as follows: A steel plate test piece was cut out from the top plate portion of the hat-shaped frame member, and the plate thickness cross section (L cross section) perpendicular to the longitudinal direction of the hat-shaped frame member was smoothed by wet polishing. Then, the hardness of the test piece was measured using a Vickers hardness tester under a load of 9.8 × 10 －２ Measurements were made at 1 μm intervals from a position 1 μm from the surface of the steel plate test piece in the thickness direction to a position 120 μm in the thickness direction using a 1000 kJ / N gauge. Measurements were then made at 20 μm intervals up to the center of the plate thickness. The region where the hardness was reduced to 84% or less compared to the hardness at the 1 / 4 position in the plate thickness was defined as the soft layer (surface soft layer), and the thickness of this region in the thickness direction was defined as the thickness of the soft layer.

[0111] <Method of Measuring Structure> The structure at the position halfway through the thickness of the surface soft layer and the structure at the position one-quarter through the thickness of the base steel sheet were measured by the method described above.

[0112] <Tensile Test> The tensile test was performed in accordance with JIS Z 2241 (2011). That is, a JIS No. 5 test piece was taken from the top plate portion of the hat-shaped frame member. Using the taken test piece, a tensile test was performed at a crosshead speed of 10 mm / min, and TS and YS were measured.

[0113] <90° V-Bend Test> The V (90°) bend test was conducted in accordance with JIS Z 2248 (2022). 100 mm x 35 mm test specimens were obtained from the top plate of the hat-shaped frame member by shearing and edge grinding. Bending radius R: Varying in 0.1 to 0.5 mm increments. Test method: Die support, punch indentation. Forming load: 10 ton. Test speed: 30 mm / min. Hold time: 5 s. Bending direction: Transverse to rolling direction (C). Three tests were performed, and the minimum bending radius (critical bending radius) R at which cracks did not appear was divided by the thickness t of the base steel sheet to calculate R / t. Additionally, cracks of 200 μm or longer were considered to be cracks using a Leica stereo microscope at 25x magnification.

[0114] <Axial Crush Test> FIG. 3 illustrates an axial crush test method used to evaluate the crashworthiness of a frame component (hollow cross-section component) 13 manufactured according to this embodiment. The axial crush test was performed using an autograph bending tester. The end fixing jig 12 was placed on the flat floor of the non-moving section 20 of the bending tester, with the axial direction of the test component aligned with the direction of movement of the bending tester. A punch 22 with a flat surface was attached to the moving section 21 of the bending tester so that the flat surface of the punch 22 was perpendicular to the direction of movement of the tester (see symbol X in FIG. 3 ). The moving section 21 of the bending tester was then moved to crush the test component (hollow cross-section component 13). The axial crush test was performed at a punch moving speed of 10 mm / min and a punch stroke of 100 mm. Three components using each material were tested. The load applied to the punch during the crush test was measured, and the maximum load value at initial buckling was obtained. The average of the three values ​​was calculated and designated as the maximum load N3 average. Regarding the pass / fail of the average maximum load N3, those of 220 kN or more were judged as good, and those of less than 220 kN were judged as bad.

[0115] To evaluate the impact fracture characteristics of the steel plate itself during the axial crush test, the fracture state was confirmed. Figure 4 shows examples of fractures after the axial crush test of frame parts using the materials of Experiment No. 10 (Comparative Example) and Experiment No. 3 (Inventive Example). The fractures shown in Figure 4 are bending fractures of the base material (the steel plate itself). As shown in Figure 4(a) of the Comparative Example, the fracture propagated through the base material, resulting in a large crack. As shown in Figure 4(b) of the Inventive Example, the fracture state was a minor crack, where the base material fractured but remained a partial crack in the bending deformation area. In the fracture state shown in Figure 4(a), the arrow in Figure 4(a) indicates the location of a large bending crack in the base material, and the arrow in Figure 4(b) indicates the location of a minor bending crack in the base material. In this example, for bending fractures of the base material that could be visually confirmed by appearance, fracture locations with a fracture length of 20 mm or more were judged to be major cracks, and fracture locations with a fracture length of less than 20 mm were judged to be minor cracks, and the number of fracture locations was measured. The above fracture judgment and quantity measurement were performed for all three bodies, and the total value was calculated, and this total value was taken as the total number of fractures. Those with one or more major cracks or five or more minor cracks were judged to be poor, and all others were judged to be good.

[0116] Table 3 shows the results of this example.

[0117]

[0118] Nos. 5 to 9, 11, and 13 are comparative examples in which the tensile strength of the base steel sheet of the top plate portion is less than 980 MPa. Furthermore, Comparative Examples 9, 10, and 52 have thin soft surface layers, resulting in large R / t values ​​in the bending test and large cracks occurring in two or more locations in the axial crushing test. On the other hand, the total number of fractures in the inventive examples is lower than in Comparative Examples 9, 10, and 52, demonstrating excellent bending fracture resistance properties of the base material during a collision. Furthermore, it can be seen that the inventive examples have higher YS values ​​and therefore higher maximum loads than Comparative Examples 5 to 9, 11, and 13. Furthermore, Inventive Example No. In Examples 1 to 4, 12, 16, 19 to 29, 31 to 34, 36 to 38, 42, 43, 45 to 51, and 53 to 61, the soft surface layer was thick and R / t was 1.5 or less, which provided particularly excellent bendability, and therefore the total number of fractures was kept to two or less minor cracks, demonstrating even better bending fracture resistance. As described above, according to this embodiment, it is possible to manufacture automotive frame parts with excellent crash properties that combine a high crash load, which can be achieved by increasing the yield strength even at the same tensile strength level through the engineering of the material structure of the base steel sheet, with stable crash deformability, which can be achieved by suppressing crash fractures through the engineering of the surface layer structure.

[0119] REFERENCE SIGNS LIST 1 skeletal member 10 processed part 11 reinforcing part 12 end fixing jig 13 hollow cross-section part 20 non-moving part of bending tester 21 moving part of bending tester 22 punch

Claims

1. A skeletal member using a steel plate having a base steel plate, the cross-sectional shape of which comprises a top plate portion extending in the width direction and two vertical wall portions extending from both widthwise ends of the top plate portion in a direction different from the extending direction of the top plate portion, the base steel plate of the top plate portion having a surface soft layer whose Vickers hardness is 84% ​​or less of the Vickers hardness at 1 / 4 of the plate thickness, the thickness of the soft surface layer satisfying the following formula (1), and having a tensile strength of 980 MPa or more and 1.8 GPa or less: 20≦X≦0.1×t−3800×[Sb]−1900×[Sn] (1) where X is the thickness of the soft surface layer (μm), t is the thickness of the base steel plate (μm), and [Sb] and [Sn] are the Sb and Sn contents (mass%) in the base steel plate, respectively.

2. A skeletal member according to claim 1, wherein the base steel plate of the top plate portion has a structure in the soft surface layer, in which the area ratio of ferrite is 60.0% or more, and among the structures other than ferrite, the value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less, and the area ratio of retained austenite is 3.0% or less, and the structure at a 1 / 4 thickness position of the base steel plate has an area ratio of ferrite of 55.0% or less (including 0.0%), and the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) exceeds 40.0%, and the area ratio of fresh martensite is 10.0% or less (including 0.0%).

3. A skeletal member according to claim 1 or 2, wherein the base steel plate of the top plate portion has a component composition containing, in mass%, C: 0.050% or more and 0.400% or less, Si: 0.02% or more and 3.00% or less, Mn: 1.50% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.005% or more and 2.000% or less, N: 0.0100% or less, Sb: 0.200% or less (inclusive), and Sn: 0.200% or less (inclusive), with the balance being Fe and unavoidable impurities.

4. The base steel sheet of the top plate portion further includes, in mass %, the following composition of elements: Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, 4. The skeletal member according to claim 3, containing at least one element selected from As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less.

5. A frame member according to any one of claims 1 to 4, wherein the base steel sheet of the top plate has an R / t value of 2.0 or less as measured in a 90-degree V-bend test in accordance with JIS Z 2248, where R is the limiting bending radius (µm) and t is the thickness of the base steel sheet (µm).

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