Framework member

Abstract

To provide a structural member with a tensile strength of 980 MPa or more and 1.8 GPa or less and good axial crushing characteristics. A structural member including a steel sheet having a base steel sheet, wherein the structural member has a cross-sectional shape that includes a top sheet portion extending in a width direction, and two vertical wall portions extending from both ends in a width direction of the top sheet portion in a direction different from a direction of extension of the top sheet portion, the base steel sheet of the top sheet portion has, in a surface layer, a surface soft layer with a Vickers hardness that is 84% or less of a Vickers hardness at a quarter thickness, the surface soft layer has a thickness satisfying the following formula (1), and the structural member has a tensile strength of 980 MPa or more and 1.8 GPa or less, 20≤X≤0.1×t−3800×Sb−1900×Sn wherein X denotes the thickness of the surface soft layer (µm), t denotes a thickness of the base steel sheet (µm), and [Sb] and [Sn] denote an Sb content and a Sn content (% by mass) of the base steel sheet, respectively.

Description

Technical Field

[0001] The present invention relates to a structural member with good axial crushing characteristics. In particular, the present invention relates to a structural member with good collision characteristics when used as an automotive structural member.Background Art

[0002] The background art will be described using a case of an automotive structural member as an example.

[0003] In recent years, to reduce the weight of automobiles while maintaining collision performance, high-strength steel sheets have been increasingly used as materials for vehicle bodies, but due to a decrease in the ductility of the materials caused by reinforcement of the materials, the risk of fracture in case of a collision increases.

[0004] In particular, although an energy-absorbing member disposed at the front and rear of a vehicle body is required to absorb collision energy by plastic deformation of the member in case of a collision, in the axial crushing of the energy-absorbing member, a large strain is locally generated, so that fracture often occurs. When such fracture progresses to large cracking and the member collapses, there is a possibility that the collision load greatly decreases and the energy absorption performance becomes unstable, and the instability of the energy absorption performance is a major issue in the application of the high-strength steel sheet to vehicle bodies. It is therefore important to improve the collision performance of a vehicle body by applying to an energy-absorbing member a material that has high strength and high collision load and that provides stable energy absorption performance by suppressing collision fracture.

[0005] Non-patent Literature 1 discloses that, as a result of applying 980 MPa grade materials with different material microstructures to specimens simulating energy-absorbing members and performing an axial crushing test, a multi-phase steel containing tempered martensite and bainite in addition to retained austenite has higher bending fracture resistance in case of a collision by suppressing the occurrence of a void during bending deformation than a dual phase (DP) steel composed of ferrite and martensite and a transformation-induced plasticity (TRIP) steel containing retained austenite.Citation ListNon Patent Literature

[0006] NPL 1: Komine et. al., Transactions of the Society of Automotive Engineers of Japan, Vol. 52, No. 1 (2021), 94-99.Summary of InventionTechnical Problem

[0007] In Non-patent Literature 1, only the basic characteristics of axial crushing are studied for materials with a material strength in the range of 905 to 925 MPa. Furthermore, in a case where collision deformation is severer, there may be a case where it is necessary to further improve bending fracture resistance. For example, in the case of considering the application of further high-strength steel of 1180 MPa grade or higher, it is unclear whether collision fracture can be suppressed only by making a similar material microstructure, and there is a possibility that the application of a material with further higher bendability is required.

[0008] The present invention focuses on the above points and aims to provide a structural member with good axial crushing characteristics by forming a base steel sheet of a top sheet portion as a structural member having a soft layer in a surface layer portion and having a tensile strength of 980 MPa or more and 1.8 GPa or less. In particular, the present invention aims to provide an automotive structural member that achieves both load bearing characteristics and fracture suppression performance in case of a collision.

[0009] The term "good axial crushing characteristics", as used herein, means that when an axial crushing test is performed, it has load bearing characteristics and fracture suppression performance (crack resistance) of a predetermined level or more and has high impact absorption performance.Solution to Problem

[0010] As a result of extensive studies to achieve the above objects, the present inventors have obtained the following findings. That is, the present invention has been completed by finding that the advantages intended in the present invention can be achieved by satisfying the following [1] to [5]. [1] A structural member including a steel sheet having a base steel sheet, wherein the structural member has a cross-sectional shape that includes a top sheet portion extending in a width direction and two vertical wall portions extending from both ends in a width direction of the top sheet portion in a direction different from a direction of extension of the top sheet portion, the base steel sheet of the top sheet portion has, in a surface layer, a surface soft layer with a Vickers hardness that is 84% or less of a Vickers hardness at a quarter thickness, the surface soft layer has a thickness satisfying the following formula (1), and the structural member has a tensile strength of 980 MPa or more and 1.8 GPa or less, 20 ≤ X ≤ 0.1 × t − 3800 × Sb − 1900 × Sn wherein X denotes the thickness of the surface soft layer (µm), t denotes a thickness of the base steel sheet (µm), and [Sb] and [Sn] denote an Sb content and a Sn content (% by mass) of the base steel sheet, respectively. [2] The structural member according to [1], wherein in the base steel sheet of the top sheet portion, as microstructures in the surface soft layer, ferrite has an area fraction of 60.0% or more, among microstructures other than ferrite, an area fraction of fresh martensite divided by a total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less, and retained austenite has an area fraction of 3.0% or less, as microstructures at the quarter thickness of the base steel sheet, ferrite has an area fraction of 55.0% or less (including 0.0%), bainitic ferrite and tempered martensite (excluding retained austenite) have a total area fraction of more than 40.0%, and fresh martensite has an area fraction of 10.0% or less (including 0.0%). [3] The structural member according to [1] or [2], wherein the base steel sheet of the top sheet portion has a chemical composition containing, on a mass percent basis, 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 composed of Fe and incidental impurities. [4] The structural member according to [3], wherein the base steel sheet of the top sheet portion further contains, as the chemical composition, on a mass percent basis, at least one element selected from 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, 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 structural member according to any one of [1] to [4], wherein the base steel sheet of the top sheet portion has an R / t value of 2.0 or less as measured by a 90-degree V-bending test according to JIS Z 2248, wherein R denotes a critical bending radius (µm), and t denotes a thickness (µm) of the base steel sheet. Advantageous Effects of Invention

[0011] The present invention can provide a member with good axial crushing characteristics, that is, with high load bearing characteristics and fracture suppression performance (crack resistance), and with high impact absorption performance, which is typically applied to an automotive structural member.Brief Description of Drawings

[0012] [Fig. 1] Fig. 1 is an explanatory view of a specific example of a cross-sectional shape pattern of a structural member according to an embodiment of the present invention. [Fig. 2] Fig. 2 is an explanatory view of an example of a process of producing a structural member according to an embodiment of the present invention. [Fig. 3] Fig. 3 is an explanatory view of an axial crushing test method. [Fig. 4] Fig. 4(a) is a photograph of an example of a fracture portion after an axial crushing test in a comparative example, and Fig. 4(b) is a photograph of an example of a fracture portion after an axial crushing test in an example of the present invention. Description of Embodiments[Structural Member]

[0013] A structural member according to the present invention has a cross-sectional shape including a top sheet portion extending in the width direction and two vertical wall portions extending from both ends in the width direction of the top sheet portion in a direction different from a direction of extension of the top sheet portion.

[0014] The structural member can have various configurations, such as only a fabricated part, a combination of a fabricated part and a flat sheet-shaped reinforcing part that reinforces the fabricated part, or a combination of a plurality of fabricated parts.

[0015] Fig. 1 illustrates examples of the cross-sectional shape pattern. In Fig. 1, each pattern illustrated in (a) to (f) has a cross-sectional shape and a perspective view.

[0016] As illustrated in Figs. 1(a) to 1(c), in a configuration of only a fabricated part 10, it may be a cross-sectional shape with no closed cross section (see Fig. 1(a)) or a cross-sectional shape with a closed cross section (see Figs. 1(b) and 1(c)). As illustrated in Figs. 1(d) to (f), in the case of having a reinforcing part 11, the fabricated part 10 or the reinforcing part 11 may have a hole shape or an uneven shape at any position (Fig. 1(d) illustrates an example of not having a hole shape or an uneven shape, and Figs. 1(e) and 1(f) illustrate examples of having a hole shape or an uneven shape). It may also be not only a shape in which a specific cross section is extended in the direction normal to the cross section but also a shape in which the specific cross section is extended along a certain curve or a shape in which the specific cross section is extended in a certain direction while being magnified.

[0017] The base steel sheet of the top sheet portion has a surface soft layer with a Vickers hardness of 84% or less relative to the Vickers hardness at the quarter thickness in a region from the surface to 20 µm or more in the thickness direction and within 0.1 times the sheet thickness. The surface soft layer refers to a decarburized layer.

[0018] The surface soft layer contributes to suppression of bending crack propagation during collision deformation and improves performance as a structural member. Furthermore, such a base steel sheet press-formed into a structural member improves bendability in press forming. A vertical wall portion and a flange portion also preferably have the surface soft layer.

[0019] The Vickers hardness can be measured in accordance with JIS Z 2244-1 (2020) at a load of 9.8 x 10 -2< N.

[0020] The term "base steel sheet", as used herein, refers to a steel sheet that serves as a base material portion in a structural member according to the present invention. Although coating treatment, painting treatment, covering treatment, and the like can be applied to the surface of the base steel sheet, a coated layer, a paint layer, a cover layer, and the like are not included in the base steel sheet.

[0021] Here, the surface soft layer has a thickness satisfying the following formula (1). 20 ≤ X ≤ 0.1 × t − 3800 × Sb − 1900 × Sn

[0022] In the formula (1), X denotes the thickness of the surface soft layer (µm), t denotes the thickness of the base steel sheet (µm), and [Sb] and [Sn] denote the Sb content and the Sn content (% by mass) of the base steel sheet, respectively.

[0023] When the surface soft layer thickness (X) is less than 20 µm, the desired bendability intended in the present invention cannot be achieved.

[0024] On the other hand, when the surface soft layer thickness (X) exceeds (0.1 x t - 3800 x [Sb] - 1900 x [Sn]) µm, it is not possible to achieve both the high strength and the fracture suppression performance in case of a collision intended in the present invention. Thus, the surface soft layer thickness (X) is set to 20 µm or more and (0.1 x t - 3800 x [Sb] - 1900 x [Sn]) µm or less.

[0025] In the present invention, Sb and Sn are added as required to improve the coatability and chemical convertibility, but due to the surface segregation of these elements, the addition of Sb and Sn reduces the allowable upper limit of the surface soft layer thickness (X) affecting bending cracks. For this reason, the upper limit of the surface soft layer for high bendability is (0.1 x t - 3800 x [Sb] - 1900 x [Sn]) µm.

[0026] The surface soft layer preferably has a thickness of 30 µm or more, more preferably 40 µm or more.

[0027] The surface soft layer preferably has a thickness of 0.1 x t µm or less, more preferably 0.08 x t µm or less.

[0028] The base steel sheet in the top sheet portion has a tensile strength of 980 MPa or more and 1.8 GPa or less.

[0029] The tensile strength (TS) and the yield stress (YS) can be measured by a tensile test according to JIS Z 2241 (2011).

[0030] Next, the steel microstructure of a structural member according to an embodiment of the present invention will be described.<Microstructure in Surface Soft Layer of Base Steel Sheet in Top Sheet Portion>

[0031] Suitable conditions for the microstructure of a surface soft layer of a base steel sheet according to an embodiment of the present invention will be described below.Area fraction of ferrite: 60.0% or more

[0032] When subjected to bending, the surface layer is deformed more greatly than the inner portion. Thus, a void is likely to be formed in the surface layer. In the present invention, by controlling the area fraction of ferrite in the surface soft layer to 60.0% or more, a void that acts as a starting point of cracking is less likely to be formed in the surface layer, and the propagation of cracking is suppressed. Thus, the area fraction of ferrite is preferably 60.0% or more. The area fraction of ferrite is more preferably 80.0% or more, even more preferably 90.0% or more.

[0033] The area fraction of ferrite may be 100.0%. The area fraction of ferrite may be less than 100.0%. The area fraction of ferrite may be 98.0% or less or 96.0% or less.

[0034] Area fraction of fresh martensite divided by total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite): 0.5 or less

[0035] When the area fraction of fresh martensite in the surface soft layer is excessively increased, the fresh martensite becomes a starting point of void formation, and the desired bendability in the inner portion of the base steel sheet cannot be achieved. From the perspective of ensuring high bendability in the inner portion of the base steel sheet, in a case where ferrite has an area fraction of less than 100.0% in the surface soft layer, the area fraction of fresh martensite divided by the total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is preferably 0.5 or less. This value may be 0.4 or less or 0.35 or less.

[0036] The lower limit of the area fraction of fresh martensite divided by the total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) in the surface soft layer may be, but is not limited to, 0.0. This value may be 0.1 or more or 0.15 or more.Area fraction of retained austenite: 3.0% or less

[0037] In a case where retained austenite has an area fraction of more than 3.0%, when bending deformation occurs in case of a collision, hard martensite is formed as a result of deformation-induced transformation of retained austenite, and void formation and crack growth occur at a heterophase boundary with the hard martensite, which makes it difficult to ensure bendability at an end portion of the base steel sheet. Thus, retained austenite preferably has an area fraction of 3.0% or less. The retained austenite more preferably has an area fraction of 2.0% or less, even more preferably 1.0% or less.

[0038] Although the lower limit of the area fraction of retained austenite is not particularly limited, the area fraction of retained austenite is preferably 0.1% or more, more preferably 0.3% or more.<Microstructure at Quarter Thickness of Base Steel Sheet>

[0039] Next, suitable conditions of the microstructure at the quarter thickness of a steel sheet according to an embodiment of the present invention will be described.Area fraction of ferrite: 55.0% or less (including 0.0%)

[0040] Soft ferrite is a phase that improves ductility. However, when the area fraction of ferrite is excessively increased, it is difficult to achieve a TS of 980 MPa or more. This also results in a decrease in YS. Thus, ferrite preferably has an area fraction of 55.0% or less. Ferrite more preferably has an area fraction of 45.0% or less, more preferably 30.0% or less.

[0041] The lower limit of the area fraction of ferrite may be, but is not limited to, 0.0%. Ferrite may have an area fraction of 1.0% or more or 2.0% or more.Total area fraction of bainitic ferrite and tempered martensite (excluding retained austenite): more than 40.0%

[0042] Bainitic ferrite and tempered martensite have an intermediate hardness between soft ferrite and hard fresh martensite and are important phases to ensure high bendability of the base steel sheet, bendability of a shear deformation end face, and axial crushing characteristics. Bainitic ferrite is also a phase useful for obtaining an appropriate amount of retained austenite by utilizing the diffusion of C from the bainitic ferrite to non-transformed austenite. Tempered martensite is effective in improving TS. Thus, bainitic ferrite and tempered martensite (excluding retained austenite) preferably have a total area fraction of more than 40.0%. Bainitic ferrite and tempered martensite (excluding retained austenite) more preferably have a total area fraction of 65.0% or more, even more preferably 80.0% or more.

[0043] On the other hand, the upper limit of the total area fraction of bainitic ferrite and tempered martensite (excluding retained austenite) may be 100.0%. Bainitic ferrite and tempered martensite (excluding retained austenite) may have a total area fraction of 96.0% or less or 92.0% or less.

[0044] The term "bainitic ferrite" refers to upper bainite that is formed in a relatively high temperature region and contains a small amount of carbide.Area fraction of fresh martensite: 10.0% or less (including 0.0%)

[0045] In the present invention, when the area fraction of fresh martensite is excessively increased, the fresh martensite becomes a starting point of void formation, so that the desired bendability cannot be achieved. Thus, fresh martensite preferably has an area fraction of 10.0% or less. Fresh martensite more preferably has an area fraction of 5.0% or less.

[0046] The lower limit of the area fraction of fresh martensite may be, but is not limited to, 0.0%. Fresh martensite may have an area fraction of 1.0% or more or 2.0% or more.

[0047] The term "fresh martensite", as used herein, refers to as-quenched (untempered) martensite. The term "fresh martensite", as used herein, also includes (isolated) fresh Martensite-Austenite islands in ferrite grains.Area fraction of retained austenite: less than 3.5%

[0048] In the present invention, to ensure bendability, retained austenite preferably has an area fraction of less than 3.5%.

[0049] Retained austenite more preferably has an area fraction of 3.0% or less. Retained austenite even more preferably has an area fraction of 2.5% or less, even further more preferably 2.0% or less. The lower limit of the area fraction of retained austenite may be, but is not limited to, 0%. Retained austenite preferably has an area fraction of 0.1% or more, more preferably 0.2% or more.

[0050] The "retained austenite", as used herein, also includes island-like retained austenite (isolated) in ferrite grains described later.

[0051] The area fraction of the remaining microstructure other than the above is preferably 10.0% or less. The area fraction of the remaining microstructure is more preferably 7.0% or less, even more preferably 5.0% or less. The area fraction of the remaining microstructure may be 0.0%.

[0052] The remaining microstructure is, for example, but not limited to, carbide, such as pearlite or cementite. The type of the remaining microstructure can be determined, for example, by scanning electron microscope (SEM) observation.

[0053] The area fractions of ferrite, bainitic ferrite, tempered martensite, and a hard second phase (fresh martensite + retained austenite) at the quarter thickness of the base steel sheet are measured as described below.

[0054] That is, a sample is cut out to form a cross section in the thickness direction (an L cross section) parallel to the longitudinal direction of the base steel sheet as an observation surface. The observation surface of the sample is then polished with a diamond paste and is then subjected to final polishing using alumina. The observation surface of the sample is then etched with 1% by volume nital to expose the microstructure.

[0055] Five visual fields are then observed at the quarter thickness of the base steel sheet as an observation position using SEM at a magnification of 3000 times. The visual fields to be observed are selected in the range of 1 / 4 of the thickness of the steel sheet ± 100 µm, and one visual field is 38 µm x 30 µm. From the microstructure image thus obtained, the area fraction obtained by dividing the area of each constituent microstructure (ferrite, bainitic ferrite, tempered martensite, and hard second phase (fresh martensite + retained austenite)) by the measurement area is calculated for five visual fields using Adobe Photoshop of Adobe Systems, and the values are averaged to obtain the area fraction of each microstructure. At the outermost surface layer position, a photograph is taken so as to exclude the galvanized layer and include the internal oxidation layer.

[0056] Ferrite: a region with a black color and with a massive form. Almost no carbides are included. Fresh Martensite-Austenite islands and island-like retained austenite isolated in ferrite grains are not included in the area fraction of ferrite.

[0057] Bainitic ferrite: a region with a black to dark gray color and with a massive form, an indefinite form, or the like. It also includes a relatively small number of carbides.

[0058] Tempered martensite: a region with a gray color and with an indefinite form. A relatively large number of carbides are included.

[0059] Hard second phase (retained austenite + fresh martensite): a region with a white to light gray color and with an indefinite form. No carbides are included.

[0060] Carbide: a region with a white color and with a dot-like or linear form. It is included in bainite, tempered bainite, and tempered martensite.

[0061] Remaining microstructure: pearlite, cementite, and the like, as described above, and the forms and the like thereof are known.

[0062] The area fraction of retained austenite is measured as described below.

[0063] The base steel sheet is mechanically ground to a quarter thickness position in the sheet thickness direction (depth direction) and is then chemically polished with oxalic acid to form an observation surface. The observation surface is then observed by X-ray diffractometry. A MoKα ray is used as an incident X-ray, the ratio of the diffraction intensity of each of the (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensity of each of the (200), (211), and (220) planes of bcc iron is determined to calculate the volume fraction of retained austenite from the ratio of the diffraction intensity of each plane. Assuming that the retained austenite is three-dimensionally homogeneous, the volume fraction of the retained austenite is defined as the area fraction of the retained austenite.

[0064] 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.

[0065] The area fraction of the remaining microstructure is calculated by subtracting the area fraction of ferrite, the area fraction of bainitic ferrite, the area fraction of tempered martensite, and the area fraction of the hard second phase determined as described above from 100.0%.

[0066] The microstructure of the surface soft layer can be identified at the half thickness of the surface soft layer in the same manner as in the identification of the microstructure at the quarter thickness of the base steel sheet described above.<Chemical Composition of Base Steel Sheet>

[0067] A suitable chemical composition of a base steel sheet used for a base material according to the present invention will be described below.

[0068] A base steel sheet used for a base material according to the present invention preferably has a chemical composition containing, on a mass percent basis, 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 composed of Fe and incidental impurities.

[0069] Although the unit in the chemical composition is "% by mass" in all cases, the unit is hereinafter simply indicated by "%" unless otherwise specified.C: 0.050% or more and 0.400% or less

[0070] C is an element that is effective in achieving a TS of 980 MPa or more, a high YS, and a high YR by forming appropriate amounts of tempered martensite, bainite, tempered bainite, and the like. A C content of less than 0.050% may result in an increase in the area fraction of ferrite and make it difficult to achieve a TS of 980 MPa or more. Furthermore, YS and YR may decrease.

[0071] On the other hand, a C content of more than 0.400% results in an excessive increase in the area fraction of fresh martensite, an excessive increase in TS, and a decrease in El. Furthermore, fresh martensite acts as a starting point of void formation when bending deformation occurs in a base steel sheet, and the desired bendability of the base steel sheet and desired bendability of a sheared end face may not be achieved. Furthermore, the area fraction of retained austenite may increase excessively, the fraction of fresh martensite formed by deformation-induced transformation of the retained austenite when receiving shear deformation may greatly increase, the subsequent void formation and crack growth may be promoted, and it may become more difficult to achieve desired bendability of a shear deformation end face.

[0072] Thus, 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, even further 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, even further more preferably 0.250% or less.Si: 0.02% or more and 3.00% or less

[0073] Si is an element that suppresses excessive softening of tempered martensite. A Si content of less than 0.02% may result in excessive softening of tempered martensite and make it difficult to achieve a TS of 980 MPa or more.

[0074] On the other hand, a Si content of more than 3.00% may result in, due to an excessive increase in the area fraction of ferrite, an excessive increase in the C concentration of austenite during annealing and make it impossible to achieve desired bendability of a shear deformation end face.

[0075] Thus, 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 even more preferably 0.20% or more, even further more preferably 0.30% or more. The Si content is more preferably 1.80% or less. The Si content is even more preferably 1.70% or less, even further more preferably 1.60% or less.Mn: 1.50% or more and less than 3.50%

[0076] Mn is an element that adjusts the area fractions of bainitic ferrite and tempered martensite. A Mn content of less than 1.50% may result in an increase in the area fraction of ferrite and make it difficult to achieve a TS of 980 MPa or more. Furthermore, YS and YR may be decreased.

[0077] On the other hand, a Mn content of 3.50% or more may result in a decrease in martensite start temperature Ms (hereinafter also referred to simply as an Ms temperature or Ms) and a decrease in martensite formed in a cooling step. In the end, this may result in an increase in the area fraction of fresh martensite and may not ensure the desired bendability. Thus, 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 even more preferably 3.10% or less, even further more preferably 3.00% or less.P: 0.001% or more and 0.100% or less

[0078] P is an element that has a solid-solution strengthening effect and increases TS and YS of a steel sheet. To produce such an effect, the P content is preferably 0.001% or more.

[0079] On the other hand, when the P content is more than 0.100%, P may segregate at a prior-austenite grain boundary and embrittle the grain boundary. Thus, in a V-bending test, void formation and crack growth may occur along the prior-austenite grain boundary, and desired R / t may not be achieved. Thus, the P content is preferably 0.001% or more and 0.100% or less. The P content is more preferably 0.002% or more, 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, even more preferably 0.020% or less.S: 0.0001% or more and 0.0200% or less

[0080] S is present as a sulfide in steel. In particular, when the S content is more than 0.0200%, void formation and crack growth may occur from the sulfide in a V-bending test, and desired R / t may not be achieved. Thus, the S content is preferably 0.0200% or less. The S content is more preferably 0.0080% or less. The S content is even more preferably 0.0050% or less, even further more preferably 0.0030% or less.

[0081] On the other hand, due to constraints on production technology, the S content is preferably 0.0001% or more. The S content is even more preferably 0.0003% or more, even further more preferably 0.0005% or more.Al: 0.005% or more and 2.000% or less

[0082] Al promotes ferrite transformation during annealing and in a cooling process after annealing. That is, Al is an element that affects the area fraction of ferrite. An Al content of less than 0.005% may result in a decrease in the area fraction of ferrite and a decrease in ductility.

[0083] On the other hand, an Al content of more than 2.000% may result in an excessive increase in the area fraction of ferrite and make it difficult to achieve a TS of 980 MPa or more. Furthermore, YS and YR may be decreased. Thus, 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 even more preferably 0.020% or more, even further more preferably 0.030% or more.

[0084] The Al content is more preferably 1.000% or less. The Al content is even more preferably 0.800% or less, even further more preferably 0.500% or less.N: 0.0100% or less

[0085] N is present as a nitride in steel. In particular, when the N content is more than 0.0100%, void formation and crack growth may occur from the nitride in a V-bending test, and desired R / t may not be achieved. Thus, 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, even further more preferably 0.0040% or less.

[0086] The N content may have any lower limit but is preferably 0.0005% or more due to constraints on production technology. The N content is more preferably 0.0010% or more, even more preferably 0.0015% or more.Sb: 0.200% or less (including 0%)

[0087] Sb is a useful element that can segregate on the surface of a base steel sheet during annealing and improve coatability and chemical convertibility. Thus, 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, even further more preferably 0.008% or more.

[0088] On the other hand, an Sb content of more than 0.200% may result in saturation of the effects of improving coatability and chemical convertibility and a decrease in bendability and crack propagation resistance in a steel sheet. Thus, 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 further more preferably 0.012% or less. The Sb content is even further more preferably 0.011% or less.Sn: 0.200% or less (including 0%)

[0089] Like Sb, Sn is a useful element that can segregate on the surface of a base steel sheet during annealing and improve coatability and chemical convertibility. Thus, the Sn content may be 0% but is preferably 0.002% or more. The Sn content is more preferably 0.003% or more.

[0090] On the other hand, a Sn content of more than 0.200% may result in saturation of the effects of improving coatability and chemical convertibility and a decrease in bendability and crack propagation resistance in a steel sheet. Thus, 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, even further more preferably 0.004% or less.

[0091] Although a suitable base chemical composition of a base steel sheet according to an embodiment of the present invention has been described above, a base steel sheet according to an embodiment of the present invention has a chemical composition containing the base components with the remainder other than the base components containing iron (Fe) and incidental impurities. A base steel sheet according to an embodiment of the present invention preferably has a chemical composition that contains the base components and the remainder composed of Fe and incidental impurities.

[0092] A base steel sheet according to an embodiment of the present invention may contain at least one selected from the following optional components in addition to the base components. Provided that the following optional components are contained in an amount equal to or lower than the upper limit amount described below, the advantages of the present invention can be achieved, and the lower limits are not particularly defined. The following optional elements, when contained below the appropriate lower limits described later, are contained as incidental impurities.

[0093] At least one selected from 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, 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 lessNb: 0.200% or less

[0094] Nb increases TS and YS by forming fine carbide, nitride, or carbonitride during hot rolling or annealing. To produce such an effect, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more.

[0095] On the other hand, a Nb content of more than 0.200% may result in a large number of coarse precipitates or inclusions. A coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, when Nb is contained, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.Ti: 0.200% or less

[0096] Like Nb, Ti increases TS and YS by forming fine carbide, nitride, or carbonitride during hot rolling or annealing. To produce such an effect, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more.

[0097] On the other hand, a Ti content of more than 0.200% may result in a large number of coarse precipitates or inclusions. A coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.V: 0.200% or less

[0098] Like Nb or Ti, V increases TS and YS by forming fine carbide, nitride, or carbonitride during hot rolling or annealing. To produce such an effect, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. The V content is even more preferably 0.010% or more, even further more preferably 0.030% or more.

[0099] On the other hand, a V content of more than 0.200% may result in a large number of coarse precipitates or inclusions. A coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, when V is contained, the V content is preferably 0.200% or less. The V content is more preferably 0.060% or less.B: 0.0100% or less

[0100] B is an element that segregates at an austenite grain boundary and thereby enhances hardenability. B is also an element that controls the formation and grain growth of ferrite during cooling after annealing. To produce such an 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, even further more preferably 0.0007% or more.

[0101] On the other hand, a B content of more than 0.0100% may result in cracking in the inner portion of a base steel sheet during hot rolling in the process of producing the base steel sheet. Furthermore, during bending deformation (as a structural member), the internal crack may act as a starting point of cracking and make it difficult to ensure the bendability at an end portion of a base steel sheet. Thus, when B is contained, the B content is preferably 0.0100% or less. The B content is more preferably 0.0050% or less.Cr: 1.000% or less

[0102] Cr is an element that increases hardenability, and the addition of Cr forms an appropriate amount of tempered martensite and increases TS and YS. To produce such an effect, the Cr content is preferably 0.0005% or more. The Cr content is more preferably 0.010% or more. Cr is even more preferably 0.030% or more, even further more preferably 0.050% or more. The Cr content is more preferably 0.100% or more, even more preferably 0.150% or more.

[0103] On the other hand, a Cr content of more than 1.000% may result in an increase in the area fraction of fresh martensite and a decrease in bendability (as a structural member) and particularly make it difficult to ensure the bendability at an end portion of a base steel sheet. Thus, when Cr is contained, the Cr content is preferably 1.000% or less. The Cr content is more preferably 0.800% or less, even more preferably 0.700% or less.Ni: 1.000% or less

[0104] Ni is an element that increases hardenability, and the addition of Ni forms a large amount of tempered martensite and increases TS and YS. To produce such an 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, even further more preferably 0.060% or more.

[0105] On the other hand, a Ni content of more than 1.000% may result in an increase in the area fraction of fresh martensite and a decrease in bendability (as a structural member) and particularly make it difficult to ensure the bendability at an end portion of a base steel sheet. Thus, 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, even further more preferably 0.400% or less.Mo: 1.000% or less

[0106] Mo is an element that increases hardenability, and the addition of Mo forms a large amount of tempered martensite and increases TS and YS. To produce such an 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, even further more preferably 0.150% or more.

[0107] On the other hand, a Mo content of more than 1.000% may result in an increase in the area fraction of fresh martensite and a decrease in bendability (as a structural member) and particularly make it difficult to ensure the bendability at an end portion of a base steel sheet. Thus, 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, even further more preferably 0.400% or less. The Mo content is more preferably 0.350% or less, even more preferably 0.300% or less.Cu: 1.000% or less

[0108] Cu is an element that increases hardenability, and the addition of Cu forms a large amount of tempered martensite and increases TS and YS. To produce such an effect, the Cu content is preferably 0.005% or more. The Cu content is more preferably 0.008% or more, even more preferably 0.010% or more. The Cu content is more preferably 0.020% or more. The Cu content is even more preferably 0.100% or more, even further more preferably 0.150% or more.

[0109] On the other hand, a Cu content of more than 1.000% may result in an excessive increase in the area fraction of fresh martensite. Furthermore, a large number of coarse precipitates or inclusions may be formed. In such a case, excessively formed fresh martensite and a coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, when Cu is contained, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.200% or less.Ta: 0.100% or less

[0110] Like Ti, Nb, and V, Ta increases TS and YS by forming fine carbide, nitride, or carbonitride during hot rolling or annealing. Furthermore, Ta partially dissolves in Nb carbide or Nb carbonitride and forms a complex precipitate, such as (Nb, Ta) (C, N). This suppresses coarsening of the precipitate and stabilizes precipitation strengthening. This further improves TS and YS. To produce such an effect, the Ta content is preferably 0.001% or more. The Ta content is more preferably 0.002% or more, even more preferably 0.004% or more.

[0111] On the other hand, a Ta content of more than 0.100% may result in a large number of coarse precipitates or inclusions. In such a case, an excessively coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, when Ta is contained, the Ta content is preferably 0.100% or less. The Ta content is more preferably 0.090% or less, even more preferably 0.080% or less. The Ta content is more preferably 0.030% or less, even more preferably 0.010% or less.W: 0.500% or less

[0112] W is an element that increases hardenability, and the addition of W forms a large amount of tempered martensite and increases TS and YS. To produce such an effect, the W content is preferably 0.001% or more. The W content is more preferably 0.010% or more, even more preferably 0.030% or more.

[0113] On the other hand, a W content of more than 0.500% may result in an increase in the area fraction of fresh martensite and a decrease in bendability (as a structural member) and particularly make it difficult to ensure the bendability at an end portion of a base steel sheet. Thus, when W is contained, the W content is preferably 0.500% or less. The W content is more preferably 0.450% or less, even more preferably 0.400% or less. The W content is even further more preferably 0.300% or less.Mg: 0.0200% or less

[0114] Mg is an element that is effective in spheroidizing the shape of an inclusion of sulfide, oxide, or the like and improving the flangeability and bendability of a base steel sheet. To produce such an effect, the Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0005% or more, even more preferably 0.0010% or more. The Mg content is even further more preferably 0.0030% or more.

[0115] On the other hand, a Mg content of more than 0.0200% may result in a large number of coarse precipitates or inclusions. In such a case, an excessively coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, when Mg is contained, the Mg content is preferably 0.0200% or less. The Mg content is more preferably 0.0180% or less, even more preferably 0.0150% or less. The Mg content is even further more preferably 0.0100% or less.Zn: 0.0200% or less

[0116] Zn is an element that is effective in spheroidizing the shape of an inclusion and improving the flangeability and bendability of a base steel sheet. To produce such an effect, the Zn content is preferably 0.0010% or more. The Zn content is more preferably 0.0020% or more, even more preferably 0.0030% or more.

[0117] On the other hand, a Zn content of more than 0.0200% may result in a large number of coarse precipitates or inclusions. In such a case, an excessively coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, when Zn is contained, the Zn content is preferably 0.0200% or less. The Zn content is more preferably 0.0180% or less, even more preferably 0.0150% or less.Co: 0.0200% or less

[0118] Like Zn, Co is an element that is effective in spheroidizing the shape of an inclusion and improving the flangeability and bendability of a base steel sheet. To produce such an effect, the Co content is preferably 0.0010% or more. The Co content is more preferably 0.0020% or more, even more preferably 0.0030% or more.

[0119] On the other hand, a Co content of more than 0.0200% may result in a large number of coarse precipitates or inclusions. In such a case, an excessively coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, when Co is contained, the Co content is preferably 0.0200% or less. The Co content is more preferably 0.0180% or less, even more preferably 0.0150% or less.Zr: 0.1000% or less

[0120] Like Zn and Co, Zr is an element that is effective in spheroidizing the shape of an inclusion and improving the flangeability and bendability of a base steel sheet. To produce such an effect, the Zr content is preferably 0.0010% or more.

[0121] On the other hand, when the Zr content exceeds 0.1000%, in such a case, an excessively coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, 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, even further more preferably 0.0100% or less.Ca: 0.0200% or less

[0122] Ca is present as an inclusion in steel. A Ca content of more than 0.0200% results in a large number of coarse inclusions. In such a case, an excessively coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, 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, even more preferably 0.0018% or less. The Ca content may have any lower limit and is preferably 0.0005% or more. Due to constraints on production technology, the Ca content is more preferably 0.0010% or more.

[0123] 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

[0124] Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are elements effective in improving the flangeability and bendability of a base steel sheet. To produce such an effect, each of the Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM contents is preferably 0.0001% or more.

[0125] On the other hand, Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM contents each exceeding 0.0200% and / or an As content of more than 0.0500% may result in a large number of coarse precipitates or inclusions. In such a case, an excessively coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, when at least one of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM is contained, each of the Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM contents is preferably 0.0200% or less, and the As content is preferably 0.0500% or less.

[0126] The Se content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Se content is even further more preferably 0.0010% or more, even further more preferably 0.0050% or more. The Se content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0127] The Te content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Te content is even further more preferably 0.0010% or more, even further more preferably 0.0050% or more. The Te content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0128] The Ge content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Ge content is more preferably 0.0010% or more, even more preferably 0.0050% or more. The Ge content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0129] The As content is more preferably 0.0010% or more, even more preferably 0.0015% or more. The As content is even further more preferably 0.0050% or more. The As content is more preferably 0.0400% or less, even more preferably 0.0300% or less.

[0130] The Sr content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Sr content is even further more preferably 0.0010% or more, even further more preferably 0.0050% or more. The Sr content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0131] The Cs content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Cs content is even further more preferably 0.0010% or more, even further more preferably 0.0050% or more. The Cs content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0132] The Hf content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Hf content is even further more preferably 0.0010% or more, even further more preferably 0.0050% or more. The Hf content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0133] The Pb content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Pb content is even further more preferably 0.0010% or more, even further more preferably 0.0050% or more. The Pb content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0134] The Bi content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Bi content is even further more preferably 0.0010% or more, even further more preferably 0.0050% or more. Bi is more preferably 0.0180% or less, even more preferably 0.0150% or less. The Bi content is even further more preferably 0.0100% or less.

[0135] REM is more preferably 0.0005% or more, even more preferably 0.0008% or more. The REM content is more preferably 0.0010% or more, even more preferably 0.0020% or more. REM is more preferably 0.0180% or less, even more preferably 0.0150% or less. The REM content is even further more preferably 0.0100% or less.

[0136] The term "REM", as used herein, refers to scandium (Sc) with an atomic number of 21, yttrium (Y) with an atomic number of 39, and lanthanoids from lanthanum (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71.

[0137] The term "REM concentration", as used herein, refers to the total content of one or two or more elements selected from the REM.

[0138] The REM is preferably, but not limited to, Sc, Y, Ce, or La.

[0139] A steel sheet to be used as a base material of a structural member according to the present invention may be a cold-rolled steel sheet that is a base steel sheet, or may be a hot-dip galvanized steel sheet (GI) or a hot-dip galvannealed steel sheet (GA) in which a coated layer is formed on a base steel sheet.Coated Layer

[0140] In the present invention, a coated layer is formed on a base steel sheet (on the surface of the base steel sheet), and the coated layer may be provided on only one surface of the base steel sheet or may be provided on both surfaces thereof.

[0141] That is, in the present invention, a base steel sheet may be provided, and a coated layer (such as a galvanized layer or an aluminum coated layer) may be formed on the base steel sheet.

[0142] The term "coated layer (galvanized layer)", as used herein, refers to a coated layer containing Zn as a main component (Zn content: 50.0% or more), for example, a hot-dip galvanized layer or a hot-dip galvannealed layer.

[0143] The hot-dip galvanized layer is preferably composed of, for example, Zn, 20.0% or less by mass of Fe, and 0.001% or more by mass and 1.0% or less by mass of Al. The hot-dip galvanized layer may optionally contain one or two 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% or more by mass and 3.5% or less by mass. The Fe content of the hot-dip galvanized layer is more preferably less than 7.0% by mass. The remainder other than the above elements is incidental impurities.

[0144] The hot-dip galvannealed layer is preferably composed of, for example, 20.0% or less by mass of Fe and 0.001% or more by mass and 1.0% or less by mass of Al. The hot-dip galvannealed layer may optionally contain one or two 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% or more by mass and 3.5% or less by mass. The Fe content of the hot-dip galvannealed layer is more preferably 7.0% or more by mass, even more preferably 8.0% or more by mass. The Fe content of the hot-dip galvannealed layer is more preferably 15.0% or less by mass, even more preferably 12.0% or less by mass. The remainder other than the above elements is incidental impurities.

[0145] Furthermore, the coating weight per side of a coated layer (galvanized layer) is preferably, but not limited to, 20 g / m 2< or more and 80 g / m 2< or less.

[0146] The coating weight of a coated layer (galvanized layer) is measured as described below.

[0147] That is, a treatment liquid is prepared by adding 0.6 g of a corrosion inhibitor for Fe ("Ibit 700BK" (registered trademark) manufactured by Asahi Chemical Co., Ltd.) to 1 L of 10% by mass aqueous hydrochloric acid. A steel sheet (galvanized steel sheet) as a specimen is then immersed in the treatment liquid to dissolve a coated layer (galvanized layer). The mass loss of the specimen due to the dissolution is then measured and is divided by the surface area of a base steel sheet (the surface area of a coated portion) to calculate the coating weight (g / m 2< ).

[0148] The thickness of a steel sheet according to an embodiment of the present invention is preferably, but not limited to, 0.5 mm or more, more preferably 0.6 mm or more, even more preferably 0.8 mm or more. The steel sheet preferably has a thickness of 2.3 mm or less, more preferably 1.6 mm or less.

[0149] Next, the bendability of a base steel sheet in a top sheet portion according to an embodiment of the present invention will be described.

[0150] Regarding the bendability, R (critical bending radius) / t (sheet thickness) measured by a 90-degree V-bending test according to JIS Z 2248 (2022) is preferably 2.0 or less. High bendability means good fracture resistance characteristics during collision deformation.[Production Method (Method for Producing Structural Member and Method for Producing Steel Sheet)]

[0151] First, according to an embodiment of the present invention, a method for producing a steel sheet having a base steel sheet used as a base material of a structural member will be described.

[0152] The method for producing a steel sheet used as a base material according to the present invention includes a hot rolling step of hot-rolling a steel slab into 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, when necessary, a coating step of subjecting the steel sheet to a galvanizing treatment. The galvanizing treatment is, for example, a hot-dip galvanizing treatment or a galvannealing treatment.

[0153] A suitable example of a method for producing a steel sheet used for a base material of a structural member according to an embodiment of the present invention will be described.

[0154] The method for producing a base steel sheet according to the present invention includes a hot rolling step of hot-rolling a steel slab with the chemical composition described above at a finish rolling temperature of 820°C or more, an annealing step of heating the steel sheet after the hot rolling step and annealing the steel sheet at an annealing temperature of 750°C or more and 900°C or less for a soaking time of 20 seconds or more in an atmosphere with a dew-point temperature of -10°C or more under the conditions satisfying the formulae (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 reheating and holding temperature range of the cooling stop temperature or more and 440°C or less and holding the steel sheet for 10 seconds or more, a surface layer strain introduction step of applying a tension of 19.6 N / mm 2< or more to the steel sheet after the first holding step in the reheating and holding temperature range, and a second holding step of holding the steel sheet after the surface layer strain introduction step in the reheating and holding temperature range for 10 seconds or more, or further a cold rolling step of cold-rolling the steel sheet after the hot rolling step and before the annealing step at a rolling reduction ratio of 20% or more and 80% or less to produce a cold-rolled steel sheet. 2400 ≤ Y ≤ 20000 Y = T − Ac 1 × t 1 / 2 + T − Ac 1 × t 2

[0155] In the formula (3), T denotes the annealing temperature (°C), t1 denotes the time (s) from 650°C to the annealing temperature T during heating in the annealing step, t2 denotes the soaking time (s), Ac1 (°C) denotes 727.0 - 32.7 x [%C] + 14.9 x [%Si] + 2.0 x [%Mn], [%C] denotes the C content of the steel sheet, [%Si] denotes the Si content of the steel sheet, and [%Mn] denotes the Mn content of the steel sheet.

[0156] In the present invention, a steel material (steel slab) may be produced by any melting method and may be produced by any known melting method using a converter, an electric arc furnace, or the like. A steel slab (slab) is preferably produced by a continuous casting method to prevent macrosegregation but may also be produced by an ingot casting method, a thin slab casting method, or the like. Furthermore, in addition to a known method of producing a steel slab, then temporarily cooling the steel slab to room temperature, and then heating the steel slab again, it is also possible without problems to apply an energy-saving process, such as hot direct rolling or direct hot charge rolling, in which a hot piece is charged into a furnace as it is without being cooled to room temperature or is subjected to slight heat retention and is then immediately rolled.(Hot Rolling Step)

[0157] When a slab is heated, from the perspective of dissolution of carbide and reduction of rolling force, the slab heating temperature is preferably 1100°C or more. Furthermore, to prevent an increase in scale loss, the slab heating temperature is preferably 1300°C or less. The slab heating temperature is the temperature of the slab surface. Furthermore, although a slab is formed into a sheet bar by rough rolling under normal conditions, when the heating temperature is lowered, from the perspective of preventing troubles during hot rolling, the sheet bar is preferably heated with a bar heater or the like before finish rolling.Finish rolling temperature: 820°C or more

[0158] A low finish rolling temperature results in an increase in rolling load. A low finish rolling temperature also results in an increase in the rolling reduction ratio of austenite in an unrecrystallized state, growth of an abnormal microstructure elongated in the rolling direction, and a decrease in the ductility, flangeability, and bendability of a final material. Thus, the finish rolling temperature is 820°C or more. The finish rolling temperature is preferably 830°C or more, more preferably 850°C or more. The finish rolling temperature is preferably 1080°C or less, more preferably 1050°C or less.

[0159] Furthermore, although the coiling temperature after hot rolling is not particularly limited, it is necessary to consider a case where the ductility, flangeability, and bendability of the final material are degraded. Thus, the coiling temperature after hot rolling is preferably 300°C or more. The coiling temperature after hot rolling is preferably 700°C or less.

[0160] During hot rolling, sheets after rough rolling may be joined and continuously subjected to finish rolling. A sheet after rough rolling may be once coiled. Furthermore, to reduce the rolling force during hot rolling, part or all of the finish rolling may be subjected to lubrication rolling. The lubrication rolling is also effective in making the shape and the material quality of a steel sheet uniform. The friction coefficient during the lubrication rolling is preferably 0.10 or more and 0.25 or less.(Pickling Step)

[0161] A hot-rolled steel sheet thus produced may be pickled. Pickling can remove oxides on the surface of a steel sheet and can therefore be performed to ensure high chemical convertibility and coating quality of a high-strength steel sheet as the final product. Furthermore, pickling may be performed once or may be divided into a plurality of times.(Cold Rolling Step)

[0162] A pickled sheet after hot rolling or a hot-rolled steel sheet produced as described above is cold-rolled as required. In cold rolling, after hot rolling, a pickled sheet may be cold-rolled as it is or may be cold-rolled after heat treatment. Optionally, a cold-rolled steel sheet after cold rolling may be pickled.

[0163] The cold rolling is performed, for example, by multipass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling.Rolling reduction ratio of cold rolling as required: 20% or more and 80% or less

[0164] In cold rolling, the rolling reduction ratio (cumulative rolling reduction ratio) of the cold rolling is preferably, but not limited to, 20% or more and 80% or less. When the rolling reduction ratio of the cold rolling is less than 20%, coarsening and non-uniformity of the steel microstructure are likely to occur in the annealing step, and the final product may have a decrease in TS and bendability. On the other hand, a rolling reduction ratio of more than 80% in the cold rolling tends to result in a steel sheet with a poor shape and, in a case where galvanizing is performed in the subsequent step, may result in an uneven galvanized coating weight.(Annealing Step)

[0165] In an embodiment of the present invention, a steel sheet after the hot rolling step (in a case where cold rolling is not performed) or after the cold rolling step (in a case where cold rolling is performed) is preferably heated and annealed at an annealing temperature of 750°C or more and 900°C or less for a soaking time of 20 seconds or more in an atmosphere with a dew-point temperature of -10°C or more under the conditions satisfying the formulae (2) and (3). 2400 ≤ Y ≤ 20000 Y = T − Ac 1 × t 1 / 2 + T − Ac 1 × t 2

[0166] In the formula (3), T denotes the annealing temperature (°C), t1 denotes the time (s) from 650°C to the annealing temperature T during heating in the annealing step, and t2 denotes the soaking time (s).Annealing temperature: 750°C or more and 900°C or less

[0167] An annealing temperature of less than 750°C results in an insufficient formation ratio of austenite during heating in a two-phase region of ferrite and austenite. Thus, the area fraction of ferrite increases excessively after annealing, and desired TS and YS cannot be achieved.

[0168] On the other hand, an annealing temperature of more than 900°C results in a decrease in ductility.

[0169] Thus, the annealing temperature is preferably 750°C or more and 900°C or less.

[0170] The annealing temperature is more preferably 880°C or less. The annealing temperature is even more preferably 870°C or less. The annealing temperature is more preferably 780°C or more, even more preferably 800°C or more.

[0171] The annealing temperature is the highest temperature (soaking temperature) reached in the annealing step.Soaking time: 20 seconds or more

[0172] A soaking time of less than 20 seconds results in an insufficient proportion of austenite formed during heating in a two-phase region of ferrite and austenite. Thus, the area fraction of ferrite increases excessively after annealing, and TS and YS cannot be achieved. Thus, the soaking time is preferably 20 seconds or more. The soaking time is more preferably 30 seconds or more, even 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, more preferably 800 seconds or less. The annealing time is even more preferably 600 seconds or less, even further more preferably 250 seconds or less.

[0173] The term "soaking time" refers to the holding time in the temperature range of (annealing temperature - 40°C) or more and the annealing temperature or less. That is, the soaking time includes, in addition to the holding time at the annealing temperature, the residence time in the temperature range of (annealing temperature - 40°C) or more and the annealing temperature or less in the heating and cooling before and / or after reaching the annealing temperature.

[0174] The annealing may be performed twice or more but is preferably performed once from the perspective of energy efficiency.Dew-point temperature of atmosphere of annealing step (annealing atmosphere): -10°C or more

[0175] In an embodiment of the present invention, the dew-point temperature of the atmosphere in the annealing step (annealing atmosphere) is preferably -10°C or more. Annealing at a dew-point temperature of -10°C or more in the annealing atmosphere in the annealing step can promote a decarburization reaction and more deeply form a surface soft layer. The dew-point temperature of the annealing atmosphere in the annealing step is more preferably -5°C or more, even more preferably 0°C or more, most preferably +5°C or more. Although the dew-point temperature of the annealing atmosphere in the annealing step may have any upper limit, to suitably prevent the oxidation of the surface of an Fe-based electroplated layer and to improve the adhesiveness of coating when a galvanized layer is provided, the dew-point temperature of the annealing atmosphere in the annealing step is preferably 30°C or less. The dew-point temperature of the annealing atmosphere in the annealing step is more preferably 25°C or less, even more preferably 20°C or less. 2400 ≤ Y ≤ 20000 Y = T − Ac 1 × t 1 / 2 + T − Ac 1 × t 2

[0176] In the formula (3), T denotes the annealing temperature (°C), t1 denotes the time (s) from 650°C to the annealing temperature T during heating in the annealing step, and t2 denotes the soaking time (s).

[0177] t1 denotes the time (s) from 650°C to the annealing temperature (soaking temperature) T during heating in the annealing step, t2 denotes the soaking time (holding time) (s) in the annealing step, and Ac1 denotes the Ac1 temperature (°C).

[0178] In the present invention, annealing is preferably performed under conditions that satisfy the formulae (2) and (3). Y of less than 2400 in the formula (3) may result in a surface soft layer defined in the present invention below 20 µm. On the other hand, Y of more than 20000 may result in a surface soft layer defined in the present invention exceeding (0.1 x t - 3800 x [Sb] - 1900 x [Sn]) µm. Thus, Y in the formula (3) is 2400 or more and 20000 or less.

[0179] Y is preferably 9000 or more, more preferably 12000 or more. Y is preferably 19000 or less, more preferably 18000 or less.

[0180] t1 is preferably 30 s or more. t1 is preferably 80 s or less.

[0181] The Ac1 temperature (°C) is calculated by the following formula: wherein [%C] denotes the C content of the steel sheet, [%Si] denotes the Si content of the steel sheet, and [%Mn] denotes the Mn content of the steel sheet.(Cooling Step)Cooling to cooling stop temperature of less than 100°C

[0182] The average cooling rate is 10°C / s or more and 50°C / s or less, and the dew-point temperature of atmosphere is - 20°C or less (preferred requirement)

[0183] In the cooling step, a steel sheet after the annealing step is preferably cooled to a cooling stop temperature of less than 100°C.

[0184] At this time, the cooling start temperature can be 750°C or more and 900°C or less.

[0185] The cooling stop temperature is more preferably 80°C or less, even more preferably 60°C or less.

[0186] The cooling stop temperature is more preferably 5°C or more, even more preferably 15°C or more.

[0187] The average cooling rate in the cooling step is preferably 10°C / s or more and 50°C / s or less. In this cooling step, a steel microstructure defined in the present invention can be formed.

[0188] The dew-point temperature of the atmosphere in the cooling step is preferably -20°C or less. An atmosphere with a dew-point temperature of more than -20°C may result in a large variation in the thickness of a surface soft layer in the in-plane direction of the steel sheet, and the tensile strength defined in the present invention may not be achieved. Thus, the dew-point temperature of the atmosphere in the cooling step is preferably -20°C or less.

[0189] The average cooling rate (°C / s) is calculated 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).(First Holding Step (Reheating and Holding Step))

[0190] In the first holding step (reheating and holding step), preferably, a steel sheet is reheated to a reheating and holding temperature range of the cooling stop temperature or more and 440°C or less and is held for 10 seconds or more.

[0191] Reheating the steel sheet to the reheating and holding temperature range of the cooling stop temperature or more and 440°C or less and holding the steel sheet for 10 seconds or more facilitate the formation of bainitic ferrite and tempered martensite defined in the present invention. The reheating and holding temperature range is more preferably 420°C or less, even more preferably 400°C or less.

[0192] The holding time is more preferably 20 seconds or more, even more preferably 30 seconds or more.

[0193] The holding time is more preferably 100 seconds or less, even more preferably 80 seconds or less.(Surface Layer Strain Introduction Step)

[0194] In the surface layer strain introduction step, a tension of 19.6 N / mm 2< or more is preferably applied to a steel sheet after the first holding step in the reheating and holding temperature range.

[0195] The tension is calculated by dividing the total value of the loads (N) of load cells at the left and right of a roll through which the steel sheet passes while contacting the roll by the cross-sectional area of the steel sheet (= sheet thickness (mm) x sheet width (mm)) (mm 2< ). The load cells should be arranged parallel to the direction of the tension.

[0196] The load cells are preferably disposed at a position of 200 mm from both ends of the roll. The length of the roll to be used preferably ranges from 1500 to 2500 mm.

[0197] The tension is preferably 21.5 N / mm 2< or more, more preferably 23.5 N / mm 2< or more. The tension is preferably 147 N / mm 2< or less, more preferably 98 N / mm 2< or less. The tension is even more preferably 39.2 N / mm 2< or less.(Second Holding Step)

[0198] In the second holding step, a steel sheet after the surface layer strain introduction step is preferably held in the reheating and holding temperature range for 10 seconds or more. The holding time in the second holding step is more preferably 15 seconds or more, even more preferably 20 seconds or more. The holding time is preferably 60 seconds or less, more preferably 50 seconds or less.(Coating Step (Hot-Dip Galvanizing Step, Hot-Dip Galvannealing Step))

[0199] In the present invention, in a coating step after the annealing step, a steel sheet can be subjected to a galvanizing treatment to produce a galvanized steel sheet.

[0200] The galvanizing treatment is, for example, a hot-dip galvanizing treatment or a hot-dip galvannealing treatment.

[0201] In the hot-dip galvanizing treatment, preferably, a steel sheet is immersed in a galvanizing bath at 440°C or more and 500°C or less, and the coating weight is then adjusted by gas wiping or the like. The hot-dip galvanizing bath is not particularly limited, provided that the galvanized layer has the composition described above, and is preferably, for example, a coating bath having a composition with an Al content of 0.10% or more by mass and 0.23% or less by mass and with the remainder being composed of Zn and incidental impurities.

[0202] In the hot-dip galvannealing treatment, after the hot-dip galvanizing treatment performed in the manner described above, the hot-dip galvanized steel sheet is preferably heated to an alloying temperature of 450°C or more and 600°C or less to perform an alloying treatment.

[0203] An alloying temperature of less than 450°C may result in a low Zn-Fe alloying speed and make alloying difficult. On the other hand, an alloying temperature of more than 600°C results in transformation of non-transformed austenite into pearlite and makes it difficult to achieve a TS of 980 MPa or more. The alloying temperature is more preferably 500°C or more, even more preferably 510°C or more. The alloying temperature is more preferably 570°C or less.

[0204] The coating weight of each of the hot-dip galvanized steel sheet (GI) and the hot-dip galvannealed steel sheet (GA) preferably ranges from 20 to 80 g / m 2< per side. The coating weight can be adjusted by gas wiping or the like.

[0205] Furthermore, the steel sheet thus produced may be further subjected to temper rolling.

[0206] A rolling reduction ratio of more than 2.00% in the temper rolling may result in an increase in yield stress and a decrease in dimensional accuracy when the steel sheet is formed into a member. Thus, the rolling reduction ratio of the temper rolling is preferably 2.00% or less. The lower limit of the rolling reduction ratio in the temper rolling is preferably, but not limited to, 0.05% or more from the perspective of productivity. The temper rolling may be performed with an apparatus coupled to an annealing apparatus for each step (on-line) or with an apparatus separated from the annealing apparatus for each step (offline). The number of times of temper rolling may be one or two or more. Provided that the elongation percentage can be similar to that of temper rolling, rolling with a leveler or the like may be used.

[0207] Although the other conditions of the production method are not particularly limited, from the perspective of productivity, a series of treatments, such as the annealing, hot-dip galvanizing, and alloying treatment of galvanization, are preferably performed in a continuous galvanizing line (CGL), which is a hot-dip galvanizing line. After the hot-dip galvanizing, wiping can be performed to adjust the coating weight. The conditions of coating and the like other than the above-described conditions can be the same as in a usual hot-dip galvanizing method.

[0208] Next, a method for producing a structural member according to an embodiment of the present invention will be described.

[0209] Fig. 2 illustrates an example of a process of producing a structural member using a steel sheet having a base steel sheet as a base material. A structural member 1 may have a hat-shaped part (fabricated part 10) composed of a top sheet portion, a vertical wall portion, and a flange portion, and a back sheet part (reinforcing part 11). The hat-shaped part illustrated in Fig. 2(a) is fabricated by press forming, bending, roll forming, extrusion, or other ordinary methods for forming a thin sheet material. As illustrated in Fig. 2(b), the hat-shaped part (fabricated part 10) and the back sheet part (reinforcing part 11) are fabricated into an integral part (structural member 1) by a joining method, such as spot welding (see reference sign A in the drawing), laser welding, bolt fastening, or adhesive joining, or assembly by caulking or the like.

[0210] Next, as illustrated in Fig. 2(c), an integral part (structural member 1) having a hat-shaped part and a back sheet part is further assembled with an end fixing jig 12 to fabricate a hollow cross-section part 13. It should be noted that 40 mm, 40 mm, and 20 mm in Fig. 2(a) are examples of the lengths of the top sheet portion, the vertical wall portion, and the flange portion in the cross-sectional shape, respectively, but the lengths of the top sheet portion, the vertical wall portion, and the flange portion are not limited to these lengths.

[0211] The back sheet part is joined to the hat-shaped part to form a closed cross section, thereby suppressing opening of the cross section of the hat-shaped part, and is a reinforcing member for stably and largely deforming the structural member. The back sheet part can be, for example, a cold-rolled steel sheet or a galvanized steel sheet. The back sheet part may have any tensile strength and ductility and is preferably a steel sheet with a tensile strength of 590 MPa or more.

[0212] In the above examples, treatments in the annealing step, the cooling step, the first holding step, the surface layer strain introduction step, and the second holding step are performed in a state in which the base steel sheet is a steel sheet. However, in the present invention, it is not necessary to perform the treatments in the heat treatment steps from the annealing step to the second holding step in the state of the steel sheet. After a base steel sheet is press-formed into the shape of a structural member, the treatments in the heat treatment steps from the annealing step to the second holding step may be performed.

[0213] Although a structural member according to the present invention is a structural member with good collision characteristics particularly when used as an automotive structural member, the application is not limited thereto. A structural member according to the present invention can be used as a structural member of a building structure or a structural member of a machine structure. For example, by using it as a strength member, such as a pillar or a beam for a building structure, it can be a strong structural member with high impact resistance at the time of occurrence of an earthquake. Furthermore, when used as a structural member of a machine structure, it is possible to obtain a strong structural member with high impact resistance when any impact load is applied to the machine structure.EXAMPLES<Method for Producing Steel Sheet>

[0214] Slabs containing the chemical components shown in Table 1 were produced by melting in a converter and a continuous casting method. The steel slabs were heated to 1200°C and, after the heating, were subjected to hot rolling composed of rough rolling and finish rolling at a finish rolling temperature of 900°C to form hot-rolled steel sheets. The hot-rolled steel sheets were then pickled and cold-rolled at a cold-rolling reduction ratio in the range of 30% to 60% to produce cold-rolled steel sheets with a thickness of 1.6 mm. The cold-rolled steel sheets were then subjected to an annealing treatment and a cooling treatment. Some of the steel sheets were subjected to a hot-dip galvanizing treatment or a hot-dip galvannealing treatment. Table 2 shows the conditions in the annealing step, the cooling step, the coating step during the cooling step, the reheating and holding step (first holding step), the surface layer strain introduction step, and the second holding step. In Table 2, the type of coating step GA is a condition for the hot-dip galvannealing treatment, GI is a condition for the hot-dip galvanizing treatment, and CR is a condition under which a base steel sheet was produced without coating treatment. [Table 1]Steel gradeChemical composition (% by mass)OthersAc1 (°C)CSiMnPSAlNSbSnA0.1250.562.880.0960.00120.0300.00320.0100--737B0.1150.532.730.0180.01950.0370.00650.01100.0030-737C0.1200.352.960.0080.00100.0320.0027---734D0.1410.663.210.0090.00090.0380.00380.0090--739E0.1330.602.890.0090.00110.5000.00320.0080--737F0.0200.502.820.0100.00150.0280.00490.0090--739G0.1283.202.750.0120.00180.0350.00320.0060--776H0.1250.541.200.0140.00140.0270.00420.0060--733I0.1180.572.720.0100.00080.0320.00450.0010-Ti: 0.040737J0.1150.632.650.0090.00090.0350.00300.0100-Ti: 0.028, B: 0.0022738K0.1240.592.620.0100.00120.0460.00330.0050-Nb: 0.018, Ti: 0.022, B: 0.0014737L0.1210.452.510.0110.00120.0290.00320.00300.0010Nb: 0.035, Ti: 0.015, B: 0.0012, Cr: 0.580735M0.1370.693.120.0090.00080.0320.00350.0010-Nb: 0.012, Ti: 0.023, B: 0.0015739N0.1650.572.520.0110.00090.0390.00220.0080-Nb: 0.025, Ti: 0.025, B: 0.0010, Cr: 0.52073500.0750.612.550.0140.00150.0340.00510.0010-Nb: 0.035, Ti: 0.020, B: 0.0015, Cr: 0.440739P0.1150.732.530.0120.00250.0350.00330.00400.0020Nb: 0.040, Ti: 0.010, B: 0.0014, Cr: 0.680739Q0.1190.072.500.0150.00380.0540.00380.0070-Nb: 0.015, Ti: 0.015, B: 0.0020, Cr: 0.350729R0.1210.433.350.0180.00200.0420.00290.0010-Nb: 0.025, Ti: 0.015, B: 0.0012, Cr: 0.550736S0.1160.582.150.0120.00140.0310.00260.0090-Nb: 0.020, Ti: 0.020, B: 0.0015, Cr: 0.600736T0.1250.602.630.0100.00120.0320.00350.01100.0010V: 0.055737U0.1300.552.780.0090.00120.0450.00320.0060-Cu: 0.180737v0.1130.502.720.0100.00100.0380.00190.0010-Cr: 0.590736W0.1090.182.950.0080.00090.0520.00260.00900.0030Ni: 0.150732X0.1210.452.760.0090.00230.0410.0037--Mo: 0.200735Y0.1180.322.720.0110.00120.0350.00220.0030-Nb: 0.032, Ta: 0.007733Z0.0980.512.980.0100.00050.0520.00250.0005-Ta: 0.008737AA0.1550.482.480.0150.00290.0490.00440.0110-W: 0.090734AB0.1220.562.810.0040.00180.0300.00530.0005-Mo: 0.0050737AC0.1170.622.720.0090.00200.0320.00340.0030-Zn: 0.0060738AD0.1390.472.320.0110.00150.0380.00320.0005-Co: 0.0080734AE0.1100.532.730.0160.00090.0250.00270.0030-Zr: 0.0030737AF0.1000.582.820.0120.00160.0420.00380.0010-Ca: 0.0018738AG0.1170.492.780.0450.00090.0280.00320.0005-Se: 0.0075736AH0.1120.532.730.0180.00780.0370.00650.01100.0030Te: 0.0140737AI0.1270.282.680.0270.00070.0320.00270.0020-Ge: 0.0060732AJ0.1180.452.710.0160.00320.0390.0071--As: 0.0210735AK0.0910.603.180.0090.00170.0340.00350.0005-Sr: 0.0070739AL0.1220.522.690.0070.00210.0190.00390.00800.0010Cs: 0.0100736AM0.0990.413.290.0100.00360.0300.00310.01000.0020Hf: 0.0070736AN0.1150.472.750.0210.00190.0310.0033--Pb: 0.0100736AO0.1170.502.720.0130.00110.0420.00350.0005-Bi: 0.0050736AP0.1160.482.770.0110.00150.0350.00420.0010-REM: 0.0040736AQ0.1000.342.190.0130.00120.0340.00310.01000.0010Nb: 0.180, Ti: 0.180, V: 0.180, B: 0.0075, Cr: 0.950, Ni: 0.960, Mo: 0.950, Cu: 0.900, Ta: 0.095, W: 0.450, Mg: 0.0170, Zn: 0.0180, Co: 0.0180, Zr: 0.0930, Ca: 0.0180, Se: 0.0190, Te: 0.0185, Ge: 0.0190, As: 0.0400, Sr: 0.0180, Cs: 0.0185, Hf: 0.0185, Pb: 0.0190, Bi: 0.0190, REM: 0.0190733BA0.0520.753.020.0200.00060.0110.00360.0130--743BB0.3940.891.500.0160.00170.0210.00410.0110--730BC0.2200.022.590.0050.00150.0120.00380.0140--725BD0.0893.032.740.0180.00120.0340.00450.0110--775BE0.2930.651.460.0170.00050.0100.00260.0130--730BF0.1180.452.700.0160.00320.0050.0071---735BG0.1310.602.890.0090.00111.9800.00320.0080--737BH0.1190.452.710.0160.00320.0390.0100---735BI0.1250.552.880.0110.00120.0300.00320.0360--737BJ0.1130.532.730.0180.00780.0370.0065-0.0730-737- The remainder other than those described above is composed of Fe and incidental impurities. [Table 2] No.Steel gradeAnnealing stepCooling stepFirst holding stepSurface layer strain introduction stepSecond holding stepCoating stepAnnealing temperature T (°C)Heating t1 (s)Annealing (soaking) t2 (s)Annealing dew-point temperature (°C)Formula (3) Y (-)Cooling stop temperature (°C)Holding temperature (°C)Holding time (s)Average tension (N / mm 2< )Holding temperature (°C)Holding time (s)TypeAlloying temperature (°C)1A850481001014010403503028.434025GA5002C8404590-511901353402527.432020CR-3D850341001013029162702031.426040GA5204E83077701010050313106027.431060GA5105F88076801016587323806026.536010GA4906G8903590012256343407037.236010GI-7H8804570513564364105030.439060GA5108A7006516015-7126363706031.436010GI-9A860511004366353303026.533025GA51010A8504396-2013276303403025.533020GA54011A820515201452433505021.635040GA49012A81580170-1016377483307019.632050GA51013A84080801012358173605030.445010GI-14A86037801512114173405028.420010GA52015A8505490-513219303609032.336010GA52016B82053100510551343303032.335040GA49017I85077801513382363904035.339010GA51018J8507170-511824533403039.234010GA52019K8005670106176603706027.435060GI-20L8505170511005373304038.233010GA51021M8105680207664363503034.337010GI-22N8307680011194173307038.233010GI-2308406390512304183405033.333010GA51024P84064702010284563604030.436020GI-25Q86069901016291323309036.332010GA52026R8304990510746333806033.340040GI-27S83041120-5131865533010031.433010GA51028T8507870012305473304024.532040GA52029U8305280209911593706024.537050GI-30v8404390011574153305037.231050GA51031W85063110-1016694243607031.435010GA51032X8505590-513481593507025.535010GA49033Y83064100-512758302905032.328030GA51034Z8306570-59496433605039.238010GA48035AA840411151514357533502019.635020GA51036AB8506370011472283002024.530020GI-37AC8407290512871583403031.433030GA52038AD8204980208977113306037.233010GI-39AE85057901013419483606036.334010GA52040AF85070110516238404206037.242020CR-41AG8406470010604354004028.438010GA50042AH82053100510538343303032.335040GA49043AI83076802011519423804032.838010GA50044AJ85073901014514263504038.234010GA51045AK81065120010778283603037.238020CR-46AL82054100010650263106020.629010GA51047AM8406490-512633573305021.632010GA50048AN8503880011311233509028.433010GA51049AO83077702010192243307035.331010GA50050AP85061110516031433304036.335040GI-51AQ83059902011570473203027.431040GA50052A8606880-1114020173802026.538010GA53053A84055120-515190383805024.537010GA51054A84052120515036163206030.431020GA50055BA8006312008709323603034.338020GA52056BB7907310008139353804031.437020GA53057BC85062110517585293305031.432015GA51058BD8007280102933313508033.333020GA51059BE8804570513873364105030.439060GA51060BF82054100010764263106020.629010GA51061BG83077701010043313106027.431060GA51062BH85073901014518263504038.234010GA51063BI850481001014028403503028.434025GA50064BJ82053100510542343303032.335040GA490 <Method for Producing Member>

[0215] A blank for a hat-shaped structural member was cut out from these steel sheets.

[0216] That is, a material was prepared so that a hat-shaped member after forming has a constant cross section with an axial length of 200 mm and was press-formed into the hat-shaped member. Fig. 2 illustrates the cross-sectional shape of the hat-shaped member. It was formed so that the top sheet portion and the vertical wall portions had a length of 40 mm, the flange portions had a length of 20 mm, and all the bending ridge line portions between the top sheet portion and the vertical wall portions and between the vertical wall portions and the flange portions had an inner bending R of 5 mm. Subsequently, the hat-shaped member and a back sheet member were joined to each other by spot welding so that the flange portions of the hat-shaped member and the back sheet part were joined to each other, thereby producing a structural member having a hollow cross section. The weld pitch was 45 mm.

[0217] The back sheet member is a cold-rolled steel sheet with a thickness of 1.6 mm and a tensile strength of 1180 MPa and is a reinforcing member that is joined to the hat-shaped part to form a closed cross section, thereby suppressing opening of the cross section of the hat-shaped part and effectively and stably causing large deformation in an axial crushing test described later.

[0218] To stabilize deformation in an axial crushing test for evaluating collision performance, one end portion of the structural member in the axial direction of the specimen was joined by arc welding to a jig for fixing an end cross section.<Surface Soft Layer Measurement Method>

[0219] Measurement is performed on a surface soft layer as described below. A steel sheet test specimen was cut out from the top sheet portion of the hat-shaped structural member, a cross section in the thickness direction (L cross section) perpendicular to the longitudinal direction of the hat-shaped structural member was smoothed by wet grinding, and measurement was then performed with a Vickers hardness tester at a load of 9.8 x 10 -2< N at intervals of 1 µm from a position of 1 µm in the sheet thickness direction from the steel sheet test specimen surface to a position of 120 µm in the sheet thickness direction. The measurement was then performed at intervals of 20 µm up to the center of the sheet thickness. A region where the hardness decreased to 84% or less relative to the hardness at the quarter thickness was defined as a soft layer (surface soft layer), and the thickness of the region in the sheet thickness direction was defined as the thickness of the soft layer.<Microstructure Measurement Method>

[0220] The microstructure at the half thickness of a surface soft layer and the microstructure at the quarter thickness of a base steel sheet were measured by the method described above.<Tensile Test>

[0221] The tensile test was performed in accordance with JIS Z 2241 (2011). That is, a JIS No. 5 specimen was taken from the top sheet portion of the hat-shaped structural member. The test specimen was subjected to a tensile test at a crosshead speed of 10 mm / min to measure TS and YS.<90-Degree V-Bending Test>

[0222] The V (90-degree) bending test was performed in accordance with JIS Z 2248 (2022).

[0223] A 100 mm x 35 mm test specimen was taken from the top sheet portion of the hat-shaped structural member by shearing and end grinding. Bending radius R: change in 0.1 to 0.5 mm pitch Test method: die support, punch indentation Forming load: 10 ton Test speed: 30 mm / min Holding time: 5 s Bending direction: direction (C) perpendicular to rolling direction

[0224] Evaluation was performed three times to calculate R / t by dividing the minimum bending radius (critical bending radius) R at which no cracking occurred in any case by the thickness t of the base steel sheet. A crack with a length of 200 µm or more was determined to be a crack using a stereomicroscope manufactured by Leica at a magnification of 25 times.<Axial Crushing Test>

[0225] Fig. 3 is an explanatory view of an axial crushing test method performed to evaluate the collision performance of a structural part (hollow cross-section part) 13 fabricated based on the present embodiment. The axial crushing test was performed with an Autograph bending tester. The end fixing jig 12 was installed on a flat floor surface of a non-movable portion 20 of the bending tester in such a direction that the axial direction of the specimen was the movable direction of the bending tester. A punch 22 with a flat surface was attached to a movable portion 21 of the bending tester such that the flat surface of the punch 22 was perpendicular to the movable direction of the tester (see the reference sign X in Fig. 3), and the movable portion 21 of the bending tester was moved to crush the specimen (the hollow cross-section part 13).

[0226] In the axial crushing test, the movable speed of the punch was 10 mm / min, and the movable stroke of the punch was 100 mm. In the test, three members to which each material was applied were tested. The load applied to the punch during the crushing test was measured, the maximum load at the time of initial buckling was collected, and the average value of the three members was calculated as the maximum load N3 average. The maximum load N3 average of 220 kN or more was determined to be good, and the maximum load N3 average of less than 220 kN was determined to be poor.

[0227] To evaluate the collision fracture characteristics of a steel sheet itself in the axial crushing test, the fracture state was checked. Fig. 4 shows examples of fracture portions after the axial crushing test of the structural parts to which materials of Experiment No. 10 (Comparative Example) and Experiment No. 3 (Example) are applied.

[0228] The fractures shown in Fig. 4 are bending fractures of the base material (steel sheet itself), the comparative example of Fig. 4(a) shows a fracture form in which the fracture progresses in the base material and becomes large cracking, and the example of Fig. 4(b) shows a fracture form of minor cracking in which the base material is bent and fractured but the fracture is limited to partial cracking of the bent and deformed portion.

[0229] In the fracture states of Fig. 4, an arrow in Fig. 4(a) indicates the position of large cracking in the base material bending, and an arrow illustrated in Fig. 4(b) indicates the position of minor cracking in the base material bending. In the present example, regarding the bending fracture of the base material that could be confirmed by visual observation of the appearance, a fracture portion with a fracture portion length of 20 mm or more was determined to be large cracking, a fracture portion with a fracture portion length of less than 20 mm was determined to be minor cracking, and the number of fracture portions was counted. The determination of the fracture and the number count were performed for all the three members, and the total value was calculated as the total number of fractures. One or more large cracks or five or more minor cracks were determined to be poor, and the other was determined to be good.

[0230] Table 3 shows the results of the present example.

[0231] Nos. 5 to 9, 11, and 13 are comparative examples in which the tensile strength of the base steel sheet of the top sheet portion is less than 980 MPa.

[0232] Comparative Example Nos. 9, 10, and 52 have a thin surface soft layer, a high R / t in the bending test, and therefore have large cracking at two or more locations in the axial crushing test.

[0233] On the other hand, Examples have smaller total numbers of fractures than Comparative Example Nos. 9, 10, and 52 and have better bending fracture resistance characteristics of the base material in case of a collision. Furthermore, it can be seen that Examples have higher YS and maximum loads than Comparative Example Nos. 5 to 9, 11, and 13.

[0234] It can also be seen that Example Nos. 1 to 4, 12, 16, 19 to 29, 31 to 34, 36 to 38, 42, 43, 45 to 51, and 53 to 61 have a thick surface soft layer and an R / t of 1.5 or less, exhibiting particularly high bendability, and the total number of fractures is therefore suppressed to two or less minor cracks, indicating that these examples have much better bending fracture resistance characteristics.

[0235] As described above, according to the present embodiment, an automotive structural part with good collision characteristics can be fabricated, which combines a high collision load achieved by increasing the yield strength even at the same tensile strength level through the formation of a controlled material microstructure in the base steel sheet, and stable collision deformability achieved by suppressing collision fracture through the formation of a controlled microstructure in the surface layer.Reference Signs List

[0236] 1structural member 10fabricated part 11reinforcing part 12end fixing jig 13hollow cross-section part 20non-movable portion of bending tester 21movable portion of bending tester 22punch

Examples

examples

[0214]Slabs containing the chemical components shown in Table 1 were produced by melting in a converter and a continuous casting method. The steel slabs were heated to 1200°C and, after the heating, were subjected to hot rolling composed of rough rolling and finish rolling at a finish rolling temperature of 900°C to form hot-rolled steel sheets. The hot-rolled steel sheets were then pickled and cold-rolled at a cold-rolling reduction ratio in the range of 30% to 60% to produce cold-rolled steel sheets with a thickness of 1.6 mm. The cold-rolled steel sheets were then subjected to an annealing treatment and a cooling treatment. Some of the steel sheets were subjected to a hot-dip galvanizing treatment or a hot-dip galvannealing treatment. Table 2 shows the conditions in the annealing step, the cooling step, the coating step during the cooling step, the reheating and holding step (first holding step), the surface layer strain introduction step, and the second holding step. In Table 2,...

Technical Field

[0001] The present invention relates to a structural member with good axial crushing characteristics. In particular, the present invention relates to a structural member with good collision characteristics when used as an automotive structural member.Background Art

[0002] The background art will be described using a case of an automotive structural member as an example.

[0003] In recent years, to reduce the weight of automobiles while maintaining collision performance, high-strength steel sheets have been increasingly used as materials for vehicle bodies, but due to a decrease in the ductility of the materials caused by reinforcement of the materials, the risk of fracture in case of a collision increases.

[0004] In particular, although an energy-absorbing member disposed at the front and rear of a vehicle body is required to absorb collision energy by plastic deformation of the member in case of a collision, in the axial crushing of the energy-absorbing member, a large strain is locally generated, so that fracture often occurs. When such fracture progresses to large cracking and the member collapses, there is a possibility that the collision load greatly decreases and the energy absorption performance becomes unstable, and the instability of the energy absorption performance is a major issue in the application of the high-strength steel sheet to vehicle bodies. It is therefore important to improve the collision performance of a vehicle body by applying to an energy-absorbing member a material that has high strength and high collision load and that provides stable energy absorption performance by suppressing collision fracture.

[0005] Non-patent Literature 1 discloses that, as a result of applying 980 MPa grade materials with different material microstructures to specimens simulating energy-absorbing members and performing an axial crushing test, a multi-phase steel containing tempered martensite and bainite in addition to retained austenite has higher bending fracture resistance in case of a collision by suppressing the occurrence of a void during bending deformation than a dual phase (DP) steel composed of ferrite and martensite and a transformation-induced plasticity (TRIP) steel containing retained austenite.Citation ListNon Patent Literature

[0006] NPL 1: Komine et. al., Transactions of the Society of Automotive Engineers of Japan, Vol. 52, No. 1 (2021), 94-99.Summary of InventionTechnical Problem

[0007] In Non-patent Literature 1, only the basic characteristics of axial crushing are studied for materials with a material strength in the range of 905 to 925 MPa. Furthermore, in a case where collision deformation is severer, there may be a case where it is necessary to further improve bending fracture resistance. For example, in the case of considering the application of further high-strength steel of 1180 MPa grade or higher, it is unclear whether collision fracture can be suppressed only by making a similar material microstructure, and there is a possibility that the application of a material with further higher bendability is required.

[0008] The present invention focuses on the above points and aims to provide a structural member with good axial crushing characteristics by forming a base steel sheet of a top sheet portion as a structural member having a soft layer in a surface layer portion and having a tensile strength of 980 MPa or more and 1.8 GPa or less. In particular, the present invention aims to provide an automotive structural member that achieves both load bearing characteristics and fracture suppression performance in case of a collision.

[0009] The term "good axial crushing characteristics", as used herein, means that when an axial crushing test is performed, it has load bearing characteristics and fracture suppression performance (crack resistance) of a predetermined level or more and has high impact absorption performance.Solution to Problem

[0010] As a result of extensive studies to achieve the above objects, the present inventors have obtained the following findings. That is, the present invention has been completed by finding that the advantages intended in the present invention can be achieved by satisfying the following [1] to [5]. [1] A structural member including a steel sheet having a base steel sheet, wherein the structural member has a cross-sectional shape that includes a top sheet portion extending in a width direction and two vertical wall portions extending from both ends in a width direction of the top sheet portion in a direction different from a direction of extension of the top sheet portion, the base steel sheet of the top sheet portion has, in a surface layer, a surface soft layer with a Vickers hardness that is 84% or less of a Vickers hardness at a quarter thickness, the surface soft layer has a thickness satisfying the following formula (1), and the structural member has a tensile strength of 980 MPa or more and 1.8 GPa or less, 20 ≤ X ≤ 0.1 × t − 3800 × Sb − 1900 × Sn wherein X denotes the thickness of the surface soft layer (µm), t denotes a thickness of the base steel sheet (µm), and [Sb] and [Sn] denote an Sb content and a Sn content (% by mass) of the base steel sheet, respectively. [2] The structural member according to [1], wherein in the base steel sheet of the top sheet portion, as microstructures in the surface soft layer, ferrite has an area fraction of 60.0% or more, among microstructures other than ferrite, an area fraction of fresh martensite divided by a total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less, and retained austenite has an area fraction of 3.0% or less, as microstructures at the quarter thickness of the base steel sheet, ferrite has an area fraction of 55.0% or less (including 0.0%), bainitic ferrite and tempered martensite (excluding retained austenite) have a total area fraction of more than 40.0%, and fresh martensite has an area fraction of 10.0% or less (including 0.0%). [3] The structural member according to [1] or [2], wherein the base steel sheet of the top sheet portion has a chemical composition containing, on a mass percent basis, 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 composed of Fe and incidental impurities. [4] The structural member according to [3], wherein the base steel sheet of the top sheet portion further contains, as the chemical composition, on a mass percent basis, at least one element selected from 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, 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 structural member according to any one of [1] to [4], wherein the base steel sheet of the top sheet portion has an R / t value of 2.0 or less as measured by a 90-degree V-bending test according to JIS Z 2248, wherein R denotes a critical bending radius (µm), and t denotes a thickness (µm) of the base steel sheet. Advantageous Effects of Invention

[0011] The present invention can provide a member with good axial crushing characteristics, that is, with high load bearing characteristics and fracture suppression performance (crack resistance), and with high impact absorption performance, which is typically applied to an automotive structural member.Brief Description of Drawings

[0012] [Fig. 1] Fig. 1 is an explanatory view of a specific example of a cross-sectional shape pattern of a structural member according to an embodiment of the present invention. [Fig. 2] Fig. 2 is an explanatory view of an example of a process of producing a structural member according to an embodiment of the present invention. [Fig. 3] Fig. 3 is an explanatory view of an axial crushing test method. [Fig. 4] Fig. 4(a) is a photograph of an example of a fracture portion after an axial crushing test in a comparative example, and Fig. 4(b) is a photograph of an example of a fracture portion after an axial crushing test in an example of the present invention. Description of Embodiments[Structural Member]

[0013] A structural member according to the present invention has a cross-sectional shape including a top sheet portion extending in the width direction and two vertical wall portions extending from both ends in the width direction of the top sheet portion in a direction different from a direction of extension of the top sheet portion.

[0014] The structural member can have various configurations, such as only a fabricated part, a combination of a fabricated part and a flat sheet-shaped reinforcing part that reinforces the fabricated part, or a combination of a plurality of fabricated parts.

[0015] Fig. 1 illustrates examples of the cross-sectional shape pattern. In Fig. 1, each pattern illustrated in (a) to (f) has a cross-sectional shape and a perspective view.

[0016] As illustrated in Figs. 1(a) to 1(c), in a configuration of only a fabricated part 10, it may be a cross-sectional shape with no closed cross section (see Fig. 1(a)) or a cross-sectional shape with a closed cross section (see Figs. 1(b) and 1(c)). As illustrated in Figs. 1(d) to (f), in the case of having a reinforcing part 11, the fabricated part 10 or the reinforcing part 11 may have a hole shape or an uneven shape at any position (Fig. 1(d) illustrates an example of not having a hole shape or an uneven shape, and Figs. 1(e) and 1(f) illustrate examples of having a hole shape or an uneven shape). It may also be not only a shape in which a specific cross section is extended in the direction normal to the cross section but also a shape in which the specific cross section is extended along a certain curve or a shape in which the specific cross section is extended in a certain direction while being magnified.

[0017] The base steel sheet of the top sheet portion has a surface soft layer with a Vickers hardness of 84% or less relative to the Vickers hardness at the quarter thickness in a region from the surface to 20 µm or more in the thickness direction and within 0.1 times the sheet thickness. The surface soft layer refers to a decarburized layer.

[0018] The surface soft layer contributes to suppression of bending crack propagation during collision deformation and improves performance as a structural member. Furthermore, such a base steel sheet press-formed into a structural member improves bendability in press forming. A vertical wall portion and a flange portion also preferably have the surface soft layer.

[0019] The Vickers hardness can be measured in accordance with JIS Z 2244-1 (2020) at a load of 9.8 x 10 -2< N.

[0020] The term "base steel sheet", as used herein, refers to a steel sheet that serves as a base material portion in a structural member according to the present invention. Although coating treatment, painting treatment, covering treatment, and the like can be applied to the surface of the base steel sheet, a coated layer, a paint layer, a cover layer, and the like are not included in the base steel sheet.

[0021] Here, the surface soft layer has a thickness satisfying the following formula (1). 20 ≤ X ≤ 0.1 × t − 3800 × Sb − 1900 × Sn

[0022] In the formula (1), X denotes the thickness of the surface soft layer (µm), t denotes the thickness of the base steel sheet (µm), and [Sb] and [Sn] denote the Sb content and the Sn content (% by mass) of the base steel sheet, respectively.

[0023] When the surface soft layer thickness (X) is less than 20 µm, the desired bendability intended in the present invention cannot be achieved.

[0024] On the other hand, when the surface soft layer thickness (X) exceeds (0.1 x t - 3800 x [Sb] - 1900 x [Sn]) µm, it is not possible to achieve both the high strength and the fracture suppression performance in case of a collision intended in the present invention. Thus, the surface soft layer thickness (X) is set to 20 µm or more and (0.1 x t - 3800 x [Sb] - 1900 x [Sn]) µm or less.

[0025] In the present invention, Sb and Sn are added as required to improve the coatability and chemical convertibility, but due to the surface segregation of these elements, the addition of Sb and Sn reduces the allowable upper limit of the surface soft layer thickness (X) affecting bending cracks. For this reason, the upper limit of the surface soft layer for high bendability is (0.1 x t - 3800 x [Sb] - 1900 x [Sn]) µm.

[0026] The surface soft layer preferably has a thickness of 30 µm or more, more preferably 40 µm or more.

[0027] The surface soft layer preferably has a thickness of 0.1 x t µm or less, more preferably 0.08 x t µm or less.

[0028] The base steel sheet in the top sheet portion has a tensile strength of 980 MPa or more and 1.8 GPa or less.

[0029] The tensile strength (TS) and the yield stress (YS) can be measured by a tensile test according to JIS Z 2241 (2011).

[0030] Next, the steel microstructure of a structural member according to an embodiment of the present invention will be described.<Microstructure in Surface Soft Layer of Base Steel Sheet in Top Sheet Portion>

[0031] Suitable conditions for the microstructure of a surface soft layer of a base steel sheet according to an embodiment of the present invention will be described below.Area fraction of ferrite: 60.0% or more

[0032] When subjected to bending, the surface layer is deformed more greatly than the inner portion. Thus, a void is likely to be formed in the surface layer. In the present invention, by controlling the area fraction of ferrite in the surface soft layer to 60.0% or more, a void that acts as a starting point of cracking is less likely to be formed in the surface layer, and the propagation of cracking is suppressed. Thus, the area fraction of ferrite is preferably 60.0% or more. The area fraction of ferrite is more preferably 80.0% or more, even more preferably 90.0% or more.

[0033] The area fraction of ferrite may be 100.0%. The area fraction of ferrite may be less than 100.0%. The area fraction of ferrite may be 98.0% or less or 96.0% or less.

[0034] Area fraction of fresh martensite divided by total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite): 0.5 or less

[0035] When the area fraction of fresh martensite in the surface soft layer is excessively increased, the fresh martensite becomes a starting point of void formation, and the desired bendability in the inner portion of the base steel sheet cannot be achieved. From the perspective of ensuring high bendability in the inner portion of the base steel sheet, in a case where ferrite has an area fraction of less than 100.0% in the surface soft layer, the area fraction of fresh martensite divided by the total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is preferably 0.5 or less. This value may be 0.4 or less or 0.35 or less.

[0036] The lower limit of the area fraction of fresh martensite divided by the total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) in the surface soft layer may be, but is not limited to, 0.0. This value may be 0.1 or more or 0.15 or more.Area fraction of retained austenite: 3.0% or less

[0037] In a case where retained austenite has an area fraction of more than 3.0%, when bending deformation occurs in case of a collision, hard martensite is formed as a result of deformation-induced transformation of retained austenite, and void formation and crack growth occur at a heterophase boundary with the hard martensite, which makes it difficult to ensure bendability at an end portion of the base steel sheet. Thus, retained austenite preferably has an area fraction of 3.0% or less. The retained austenite more preferably has an area fraction of 2.0% or less, even more preferably 1.0% or less.

[0038] Although the lower limit of the area fraction of retained austenite is not particularly limited, the area fraction of retained austenite is preferably 0.1% or more, more preferably 0.3% or more.<Microstructure at Quarter Thickness of Base Steel Sheet>

[0039] Next, suitable conditions of the microstructure at the quarter thickness of a steel sheet according to an embodiment of the present invention will be described.Area fraction of ferrite: 55.0% or less (including 0.0%)

[0040] Soft ferrite is a phase that improves ductility. However, when the area fraction of ferrite is excessively increased, it is difficult to achieve a TS of 980 MPa or more. This also results in a decrease in YS. Thus, ferrite preferably has an area fraction of 55.0% or less. Ferrite more preferably has an area fraction of 45.0% or less, more preferably 30.0% or less.

[0041] The lower limit of the area fraction of ferrite may be, but is not limited to, 0.0%. Ferrite may have an area fraction of 1.0% or more or 2.0% or more.Total area fraction of bainitic ferrite and tempered martensite (excluding retained austenite): more than 40.0%

[0042] Bainitic ferrite and tempered martensite have an intermediate hardness between soft ferrite and hard fresh martensite and are important phases to ensure high bendability of the base steel sheet, bendability of a shear deformation end face, and axial crushing characteristics. Bainitic ferrite is also a phase useful for obtaining an appropriate amount of retained austenite by utilizing the diffusion of C from the bainitic ferrite to non-transformed austenite. Tempered martensite is effective in improving TS. Thus, bainitic ferrite and tempered martensite (excluding retained austenite) preferably have a total area fraction of more than 40.0%. Bainitic ferrite and tempered martensite (excluding retained austenite) more preferably have a total area fraction of 65.0% or more, even more preferably 80.0% or more.

[0043] On the other hand, the upper limit of the total area fraction of bainitic ferrite and tempered martensite (excluding retained austenite) may be 100.0%. Bainitic ferrite and tempered martensite (excluding retained austenite) may have a total area fraction of 96.0% or less or 92.0% or less.

[0044] The term "bainitic ferrite" refers to upper bainite that is formed in a relatively high temperature region and contains a small amount of carbide.Area fraction of fresh martensite: 10.0% or less (including 0.0%)

[0045] In the present invention, when the area fraction of fresh martensite is excessively increased, the fresh martensite becomes a starting point of void formation, so that the desired bendability cannot be achieved. Thus, fresh martensite preferably has an area fraction of 10.0% or less. Fresh martensite more preferably has an area fraction of 5.0% or less.

[0046] The lower limit of the area fraction of fresh martensite may be, but is not limited to, 0.0%. Fresh martensite may have an area fraction of 1.0% or more or 2.0% or more.

[0047] The term "fresh martensite", as used herein, refers to as-quenched (untempered) martensite. The term "fresh martensite", as used herein, also includes (isolated) fresh Martensite-Austenite islands in ferrite grains.Area fraction of retained austenite: less than 3.5%

[0048] In the present invention, to ensure bendability, retained austenite preferably has an area fraction of less than 3.5%.

[0049] Retained austenite more preferably has an area fraction of 3.0% or less. Retained austenite even more preferably has an area fraction of 2.5% or less, even further more preferably 2.0% or less. The lower limit of the area fraction of retained austenite may be, but is not limited to, 0%. Retained austenite preferably has an area fraction of 0.1% or more, more preferably 0.2% or more.

[0050] The "retained austenite", as used herein, also includes island-like retained austenite (isolated) in ferrite grains described later.

[0051] The area fraction of the remaining microstructure other than the above is preferably 10.0% or less. The area fraction of the remaining microstructure is more preferably 7.0% or less, even more preferably 5.0% or less. The area fraction of the remaining microstructure may be 0.0%.

[0052] The remaining microstructure is, for example, but not limited to, carbide, such as pearlite or cementite. The type of the remaining microstructure can be determined, for example, by scanning electron microscope (SEM) observation.

[0053] The area fractions of ferrite, bainitic ferrite, tempered martensite, and a hard second phase (fresh martensite + retained austenite) at the quarter thickness of the base steel sheet are measured as described below.

[0054] That is, a sample is cut out to form a cross section in the thickness direction (an L cross section) parallel to the longitudinal direction of the base steel sheet as an observation surface. The observation surface of the sample is then polished with a diamond paste and is then subjected to final polishing using alumina. The observation surface of the sample is then etched with 1% by volume nital to expose the microstructure.

[0055] Five visual fields are then observed at the quarter thickness of the base steel sheet as an observation position using SEM at a magnification of 3000 times. The visual fields to be observed are selected in the range of 1 / 4 of the thickness of the steel sheet ± 100 µm, and one visual field is 38 µm x 30 µm. From the microstructure image thus obtained, the area fraction obtained by dividing the area of each constituent microstructure (ferrite, bainitic ferrite, tempered martensite, and hard second phase (fresh martensite + retained austenite)) by the measurement area is calculated for five visual fields using Adobe Photoshop of Adobe Systems, and the values are averaged to obtain the area fraction of each microstructure. At the outermost surface layer position, a photograph is taken so as to exclude the galvanized layer and include the internal oxidation layer.

[0056] Ferrite: a region with a black color and with a massive form. Almost no carbides are included. Fresh Martensite-Austenite islands and island-like retained austenite isolated in ferrite grains are not included in the area fraction of ferrite.

[0057] Bainitic ferrite: a region with a black to dark gray color and with a massive form, an indefinite form, or the like. It also includes a relatively small number of carbides.

[0058] Tempered martensite: a region with a gray color and with an indefinite form. A relatively large number of carbides are included.

[0059] Hard second phase (retained austenite + fresh martensite): a region with a white to light gray color and with an indefinite form. No carbides are included.

[0060] Carbide: a region with a white color and with a dot-like or linear form. It is included in bainite, tempered bainite, and tempered martensite.

[0061] Remaining microstructure: pearlite, cementite, and the like, as described above, and the forms and the like thereof are known.

[0062] The area fraction of retained austenite is measured as described below.

[0063] The base steel sheet is mechanically ground to a quarter thickness position in the sheet thickness direction (depth direction) and is then chemically polished with oxalic acid to form an observation surface. The observation surface is then observed by X-ray diffractometry. A MoKα ray is used as an incident X-ray, the ratio of the diffraction intensity of each of the (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensity of each of the (200), (211), and (220) planes of bcc iron is determined to calculate the volume fraction of retained austenite from the ratio of the diffraction intensity of each plane. Assuming that the retained austenite is three-dimensionally homogeneous, the volume fraction of the retained austenite is defined as the area fraction of the retained austenite.

[0064] 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.

[0065] The area fraction of the remaining microstructure is calculated by subtracting the area fraction of ferrite, the area fraction of bainitic ferrite, the area fraction of tempered martensite, and the area fraction of the hard second phase determined as described above from 100.0%.

[0066] The microstructure of the surface soft layer can be identified at the half thickness of the surface soft layer in the same manner as in the identification of the microstructure at the quarter thickness of the base steel sheet described above.<Chemical Composition of Base Steel Sheet>

[0067] A suitable chemical composition of a base steel sheet used for a base material according to the present invention will be described below.

[0068] A base steel sheet used for a base material according to the present invention preferably has a chemical composition containing, on a mass percent basis, 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 composed of Fe and incidental impurities.

[0069] Although the unit in the chemical composition is "% by mass" in all cases, the unit is hereinafter simply indicated by "%" unless otherwise specified.C: 0.050% or more and 0.400% or less

[0070] C is an element that is effective in achieving a TS of 980 MPa or more, a high YS, and a high YR by forming appropriate amounts of tempered martensite, bainite, tempered bainite, and the like. A C content of less than 0.050% may result in an increase in the area fraction of ferrite and make it difficult to achieve a TS of 980 MPa or more. Furthermore, YS and YR may decrease.

[0071] On the other hand, a C content of more than 0.400% results in an excessive increase in the area fraction of fresh martensite, an excessive increase in TS, and a decrease in El. Furthermore, fresh martensite acts as a starting point of void formation when bending deformation occurs in a base steel sheet, and the desired bendability of the base steel sheet and desired bendability of a sheared end face may not be achieved. Furthermore, the area fraction of retained austenite may increase excessively, the fraction of fresh martensite formed by deformation-induced transformation of the retained austenite when receiving shear deformation may greatly increase, the subsequent void formation and crack growth may be promoted, and it may become more difficult to achieve desired bendability of a shear deformation end face.

[0072] Thus, 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, even further 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, even further more preferably 0.250% or less.Si: 0.02% or more and 3.00% or less

[0073] Si is an element that suppresses excessive softening of tempered martensite. A Si content of less than 0.02% may result in excessive softening of tempered martensite and make it difficult to achieve a TS of 980 MPa or more.

[0074] On the other hand, a Si content of more than 3.00% may result in, due to an excessive increase in the area fraction of ferrite, an excessive increase in the C concentration of austenite during annealing and make it impossible to achieve desired bendability of a shear deformation end face.

[0075] Thus, 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 even more preferably 0.20% or more, even further more preferably 0.30% or more. The Si content is more preferably 1.80% or less. The Si content is even more preferably 1.70% or less, even further more preferably 1.60% or less.Mn: 1.50% or more and less than 3.50%

[0076] Mn is an element that adjusts the area fractions of bainitic ferrite and tempered martensite. A Mn content of less than 1.50% may result in an increase in the area fraction of ferrite and make it difficult to achieve a TS of 980 MPa or more. Furthermore, YS and YR may be decreased.

[0077] On the other hand, a Mn content of 3.50% or more may result in a decrease in martensite start temperature Ms (hereinafter also referred to simply as an Ms temperature or Ms) and a decrease in martensite formed in a cooling step. In the end, this may result in an increase in the area fraction of fresh martensite and may not ensure the desired bendability. Thus, 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 even more preferably 3.10% or less, even further more preferably 3.00% or less.P: 0.001% or more and 0.100% or less

[0078] P is an element that has a solid-solution strengthening effect and increases TS and YS of a steel sheet. To produce such an effect, the P content is preferably 0.001% or more.

[0079] On the other hand, when the P content is more than 0.100%, P may segregate at a prior-austenite grain boundary and embrittle the grain boundary. Thus, in a V-bending test, void formation and crack growth may occur along the prior-austenite grain boundary, and desired R / t may not be achieved. Thus, the P content is preferably 0.001% or more and 0.100% or less. The P content is more preferably 0.002% or more, 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, even more preferably 0.020% or less.S: 0.0001% or more and 0.0200% or less

[0080] S is present as a sulfide in steel. In particular, when the S content is more than 0.0200%, void formation and crack growth may occur from the sulfide in a V-bending test, and desired R / t may not be achieved. Thus, the S content is preferably 0.0200% or less. The S content is more preferably 0.0080% or less. The S content is even more preferably 0.0050% or less, even further more preferably 0.0030% or less.

[0081] On the other hand, due to constraints on production technology, the S content is preferably 0.0001% or more. The S content is even more preferably 0.0003% or more, even further more preferably 0.0005% or more.Al: 0.005% or more and 2.000% or less

[0082] Al promotes ferrite transformation during annealing and in a cooling process after annealing. That is, Al is an element that affects the area fraction of ferrite. An Al content of less than 0.005% may result in a decrease in the area fraction of ferrite and a decrease in ductility.

[0083] On the other hand, an Al content of more than 2.000% may result in an excessive increase in the area fraction of ferrite and make it difficult to achieve a TS of 980 MPa or more. Furthermore, YS and YR may be decreased. Thus, 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 even more preferably 0.020% or more, even further more preferably 0.030% or more.

[0084] The Al content is more preferably 1.000% or less. The Al content is even more preferably 0.800% or less, even further more preferably 0.500% or less.N: 0.0100% or less

[0085] N is present as a nitride in steel. In particular, when the N content is more than 0.0100%, void formation and crack growth may occur from the nitride in a V-bending test, and desired R / t may not be achieved. Thus, 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, even further more preferably 0.0040% or less.

[0086] The N content may have any lower limit but is preferably 0.0005% or more due to constraints on production technology. The N content is more preferably 0.0010% or more, even more preferably 0.0015% or more.Sb: 0.200% or less (including 0%)

[0087] Sb is a useful element that can segregate on the surface of a base steel sheet during annealing and improve coatability and chemical convertibility. Thus, 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, even further more preferably 0.008% or more.

[0088] On the other hand, an Sb content of more than 0.200% may result in saturation of the effects of improving coatability and chemical convertibility and a decrease in bendability and crack propagation resistance in a steel sheet. Thus, 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 further more preferably 0.012% or less. The Sb content is even further more preferably 0.011% or less.Sn: 0.200% or less (including 0%)

[0089] Like Sb, Sn is a useful element that can segregate on the surface of a base steel sheet during annealing and improve coatability and chemical convertibility. Thus, the Sn content may be 0% but is preferably 0.002% or more. The Sn content is more preferably 0.003% or more.

[0090] On the other hand, a Sn content of more than 0.200% may result in saturation of the effects of improving coatability and chemical convertibility and a decrease in bendability and crack propagation resistance in a steel sheet. Thus, 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, even further more preferably 0.004% or less.

[0091] Although a suitable base chemical composition of a base steel sheet according to an embodiment of the present invention has been described above, a base steel sheet according to an embodiment of the present invention has a chemical composition containing the base components with the remainder other than the base components containing iron (Fe) and incidental impurities. A base steel sheet according to an embodiment of the present invention preferably has a chemical composition that contains the base components and the remainder composed of Fe and incidental impurities.

[0092] A base steel sheet according to an embodiment of the present invention may contain at least one selected from the following optional components in addition to the base components. Provided that the following optional components are contained in an amount equal to or lower than the upper limit amount described below, the advantages of the present invention can be achieved, and the lower limits are not particularly defined. The following optional elements, when contained below the appropriate lower limits described later, are contained as incidental impurities.

[0093] At least one selected from 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, 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 lessNb: 0.200% or less

[0094] Nb increases TS and YS by forming fine carbide, nitride, or carbonitride during hot rolling or annealing. To produce such an effect, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more.

[0095] On the other hand, a Nb content of more than 0.200% may result in a large number of coarse precipitates or inclusions. A coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, when Nb is contained, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.Ti: 0.200% or less

[0096] Like Nb, Ti increases TS and YS by forming fine carbide, nitride, or carbonitride during hot rolling or annealing. To produce such an effect, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more.

[0097] On the other hand, a Ti content of more than 0.200% may result in a large number of coarse precipitates or inclusions. A coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.V: 0.200% or less

[0098] Like Nb or Ti, V increases TS and YS by forming fine carbide, nitride, or carbonitride during hot rolling or annealing. To produce such an effect, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. The V content is even more preferably 0.010% or more, even further more preferably 0.030% or more.

[0099] On the other hand, a V content of more than 0.200% may result in a large number of coarse precipitates or inclusions. A coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, when V is contained, the V content is preferably 0.200% or less. The V content is more preferably 0.060% or less.B: 0.0100% or less

[0100] B is an element that segregates at an austenite grain boundary and thereby enhances hardenability. B is also an element that controls the formation and grain growth of ferrite during cooling after annealing. To produce such an 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, even further more preferably 0.0007% or more.

[0101] On the other hand, a B content of more than 0.0100% may result in cracking in the inner portion of a base steel sheet during hot rolling in the process of producing the base steel sheet. Furthermore, during bending deformation (as a structural member), the internal crack may act as a starting point of cracking and make it difficult to ensure the bendability at an end portion of a base steel sheet. Thus, when B is contained, the B content is preferably 0.0100% or less. The B content is more preferably 0.0050% or less.Cr: 1.000% or less

[0102] Cr is an element that increases hardenability, and the addition of Cr forms an appropriate amount of tempered martensite and increases TS and YS. To produce such an effect, the Cr content is preferably 0.0005% or more. The Cr content is more preferably 0.010% or more. Cr is even more preferably 0.030% or more, even further more preferably 0.050% or more. The Cr content is more preferably 0.100% or more, even more preferably 0.150% or more.

[0103] On the other hand, a Cr content of more than 1.000% may result in an increase in the area fraction of fresh martensite and a decrease in bendability (as a structural member) and particularly make it difficult to ensure the bendability at an end portion of a base steel sheet. Thus, when Cr is contained, the Cr content is preferably 1.000% or less. The Cr content is more preferably 0.800% or less, even more preferably 0.700% or less.Ni: 1.000% or less

[0104] Ni is an element that increases hardenability, and the addition of Ni forms a large amount of tempered martensite and increases TS and YS. To produce such an 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, even further more preferably 0.060% or more.

[0105] On the other hand, a Ni content of more than 1.000% may result in an increase in the area fraction of fresh martensite and a decrease in bendability (as a structural member) and particularly make it difficult to ensure the bendability at an end portion of a base steel sheet. Thus, 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, even further more preferably 0.400% or less.Mo: 1.000% or less

[0106] Mo is an element that increases hardenability, and the addition of Mo forms a large amount of tempered martensite and increases TS and YS. To produce such an 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, even further more preferably 0.150% or more.

[0107] On the other hand, a Mo content of more than 1.000% may result in an increase in the area fraction of fresh martensite and a decrease in bendability (as a structural member) and particularly make it difficult to ensure the bendability at an end portion of a base steel sheet. Thus, 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, even further more preferably 0.400% or less. The Mo content is more preferably 0.350% or less, even more preferably 0.300% or less.Cu: 1.000% or less

[0108] Cu is an element that increases hardenability, and the addition of Cu forms a large amount of tempered martensite and increases TS and YS. To produce such an effect, the Cu content is preferably 0.005% or more. The Cu content is more preferably 0.008% or more, even more preferably 0.010% or more. The Cu content is more preferably 0.020% or more. The Cu content is even more preferably 0.100% or more, even further more preferably 0.150% or more.

[0109] On the other hand, a Cu content of more than 1.000% may result in an excessive increase in the area fraction of fresh martensite. Furthermore, a large number of coarse precipitates or inclusions may be formed. In such a case, excessively formed fresh martensite and a coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, when Cu is contained, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.200% or less.Ta: 0.100% or less

[0110] Like Ti, Nb, and V, Ta increases TS and YS by forming fine carbide, nitride, or carbonitride during hot rolling or annealing. Furthermore, Ta partially dissolves in Nb carbide or Nb carbonitride and forms a complex precipitate, such as (Nb, Ta) (C, N). This suppresses coarsening of the precipitate and stabilizes precipitation strengthening. This further improves TS and YS. To produce such an effect, the Ta content is preferably 0.001% or more. The Ta content is more preferably 0.002% or more, even more preferably 0.004% or more.

[0111] On the other hand, a Ta content of more than 0.100% may result in a large number of coarse precipitates or inclusions. In such a case, an excessively coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, when Ta is contained, the Ta content is preferably 0.100% or less. The Ta content is more preferably 0.090% or less, even more preferably 0.080% or less. The Ta content is more preferably 0.030% or less, even more preferably 0.010% or less.W: 0.500% or less

[0112] W is an element that increases hardenability, and the addition of W forms a large amount of tempered martensite and increases TS and YS. To produce such an effect, the W content is preferably 0.001% or more. The W content is more preferably 0.010% or more, even more preferably 0.030% or more.

[0113] On the other hand, a W content of more than 0.500% may result in an increase in the area fraction of fresh martensite and a decrease in bendability (as a structural member) and particularly make it difficult to ensure the bendability at an end portion of a base steel sheet. Thus, when W is contained, the W content is preferably 0.500% or less. The W content is more preferably 0.450% or less, even more preferably 0.400% or less. The W content is even further more preferably 0.300% or less.Mg: 0.0200% or less

[0114] Mg is an element that is effective in spheroidizing the shape of an inclusion of sulfide, oxide, or the like and improving the flangeability and bendability of a base steel sheet. To produce such an effect, the Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0005% or more, even more preferably 0.0010% or more. The Mg content is even further more preferably 0.0030% or more.

[0115] On the other hand, a Mg content of more than 0.0200% may result in a large number of coarse precipitates or inclusions. In such a case, an excessively coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, when Mg is contained, the Mg content is preferably 0.0200% or less. The Mg content is more preferably 0.0180% or less, even more preferably 0.0150% or less. The Mg content is even further more preferably 0.0100% or less.Zn: 0.0200% or less

[0116] Zn is an element that is effective in spheroidizing the shape of an inclusion and improving the flangeability and bendability of a base steel sheet. To produce such an effect, the Zn content is preferably 0.0010% or more. The Zn content is more preferably 0.0020% or more, even more preferably 0.0030% or more.

[0117] On the other hand, a Zn content of more than 0.0200% may result in a large number of coarse precipitates or inclusions. In such a case, an excessively coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, when Zn is contained, the Zn content is preferably 0.0200% or less. The Zn content is more preferably 0.0180% or less, even more preferably 0.0150% or less.Co: 0.0200% or less

[0118] Like Zn, Co is an element that is effective in spheroidizing the shape of an inclusion and improving the flangeability and bendability of a base steel sheet. To produce such an effect, the Co content is preferably 0.0010% or more. The Co content is more preferably 0.0020% or more, even more preferably 0.0030% or more.

[0119] On the other hand, a Co content of more than 0.0200% may result in a large number of coarse precipitates or inclusions. In such a case, an excessively coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, when Co is contained, the Co content is preferably 0.0200% or less. The Co content is more preferably 0.0180% or less, even more preferably 0.0150% or less.Zr: 0.1000% or less

[0120] Like Zn and Co, Zr is an element that is effective in spheroidizing the shape of an inclusion and improving the flangeability and bendability of a base steel sheet. To produce such an effect, the Zr content is preferably 0.0010% or more.

[0121] On the other hand, when the Zr content exceeds 0.1000%, in such a case, an excessively coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, 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, even further more preferably 0.0100% or less.Ca: 0.0200% or less

[0122] Ca is present as an inclusion in steel. A Ca content of more than 0.0200% results in a large number of coarse inclusions. In such a case, an excessively coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, 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, even more preferably 0.0018% or less. The Ca content may have any lower limit and is preferably 0.0005% or more. Due to constraints on production technology, the Ca content is more preferably 0.0010% or more.

[0123] 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

[0124] Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are elements effective in improving the flangeability and bendability of a base steel sheet. To produce such an effect, each of the Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM contents is preferably 0.0001% or more.

[0125] On the other hand, Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM contents each exceeding 0.0200% and / or an As content of more than 0.0500% may result in a large number of coarse precipitates or inclusions. In such a case, an excessively coarse precipitate or inclusion may act as a starting point of a void or a crack during bending deformation and make it difficult to ensure bendability required as a structural member. Thus, when at least one of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM is contained, each of the Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM contents is preferably 0.0200% or less, and the As content is preferably 0.0500% or less.

[0126] The Se content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Se content is even further more preferably 0.0010% or more, even further more preferably 0.0050% or more. The Se content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0127] The Te content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Te content is even further more preferably 0.0010% or more, even further more preferably 0.0050% or more. The Te content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0128] The Ge content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Ge content is more preferably 0.0010% or more, even more preferably 0.0050% or more. The Ge content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0129] The As content is more preferably 0.0010% or more, even more preferably 0.0015% or more. The As content is even further more preferably 0.0050% or more. The As content is more preferably 0.0400% or less, even more preferably 0.0300% or less.

[0130] The Sr content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Sr content is even further more preferably 0.0010% or more, even further more preferably 0.0050% or more. The Sr content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0131] The Cs content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Cs content is even further more preferably 0.0010% or more, even further more preferably 0.0050% or more. The Cs content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0132] The Hf content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Hf content is even further more preferably 0.0010% or more, even further more preferably 0.0050% or more. The Hf content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0133] The Pb content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Pb content is even further more preferably 0.0010% or more, even further more preferably 0.0050% or more. The Pb content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0134] The Bi content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Bi content is even further more preferably 0.0010% or more, even further more preferably 0.0050% or more. Bi is more preferably 0.0180% or less, even more preferably 0.0150% or less. The Bi content is even further more preferably 0.0100% or less.

[0135] REM is more preferably 0.0005% or more, even more preferably 0.0008% or more. The REM content is more preferably 0.0010% or more, even more preferably 0.0020% or more. REM is more preferably 0.0180% or less, even more preferably 0.0150% or less. The REM content is even further more preferably 0.0100% or less.

[0136] The term "REM", as used herein, refers to scandium (Sc) with an atomic number of 21, yttrium (Y) with an atomic number of 39, and lanthanoids from lanthanum (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71.

[0137] The term "REM concentration", as used herein, refers to the total content of one or two or more elements selected from the REM.

[0138] The REM is preferably, but not limited to, Sc, Y, Ce, or La.

[0139] A steel sheet to be used as a base material of a structural member according to the present invention may be a cold-rolled steel sheet that is a base steel sheet, or may be a hot-dip galvanized steel sheet (GI) or a hot-dip galvannealed steel sheet (GA) in which a coated layer is formed on a base steel sheet.Coated Layer

[0140] In the present invention, a coated layer is formed on a base steel sheet (on the surface of the base steel sheet), and the coated layer may be provided on only one surface of the base steel sheet or may be provided on both surfaces thereof.

[0141] That is, in the present invention, a base steel sheet may be provided, and a coated layer (such as a galvanized layer or an aluminum coated layer) may be formed on the base steel sheet.

[0142] The term "coated layer (galvanized layer)", as used herein, refers to a coated layer containing Zn as a main component (Zn content: 50.0% or more), for example, a hot-dip galvanized layer or a hot-dip galvannealed layer.

[0143] The hot-dip galvanized layer is preferably composed of, for example, Zn, 20.0% or less by mass of Fe, and 0.001% or more by mass and 1.0% or less by mass of Al. The hot-dip galvanized layer may optionally contain one or two 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% or more by mass and 3.5% or less by mass. The Fe content of the hot-dip galvanized layer is more preferably less than 7.0% by mass. The remainder other than the above elements is incidental impurities.

[0144] The hot-dip galvannealed layer is preferably composed of, for example, 20.0% or less by mass of Fe and 0.001% or more by mass and 1.0% or less by mass of Al. The hot-dip galvannealed layer may optionally contain one or two 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% or more by mass and 3.5% or less by mass. The Fe content of the hot-dip galvannealed layer is more preferably 7.0% or more by mass, even more preferably 8.0% or more by mass. The Fe content of the hot-dip galvannealed layer is more preferably 15.0% or less by mass, even more preferably 12.0% or less by mass. The remainder other than the above elements is incidental impurities.

[0145] Furthermore, the coating weight per side of a coated layer (galvanized layer) is preferably, but not limited to, 20 g / m 2< or more and 80 g / m 2< or less.

[0146] The coating weight of a coated layer (galvanized layer) is measured as described below.

[0147] That is, a treatment liquid is prepared by adding 0.6 g of a corrosion inhibitor for Fe ("Ibit 700BK" (registered trademark) manufactured by Asahi Chemical Co., Ltd.) to 1 L of 10% by mass aqueous hydrochloric acid. A steel sheet (galvanized steel sheet) as a specimen is then immersed in the treatment liquid to dissolve a coated layer (galvanized layer). The mass loss of the specimen due to the dissolution is then measured and is divided by the surface area of a base steel sheet (the surface area of a coated portion) to calculate the coating weight (g / m 2< ).

[0148] The thickness of a steel sheet according to an embodiment of the present invention is preferably, but not limited to, 0.5 mm or more, more preferably 0.6 mm or more, even more preferably 0.8 mm or more. The steel sheet preferably has a thickness of 2.3 mm or less, more preferably 1.6 mm or less.

[0149] Next, the bendability of a base steel sheet in a top sheet portion according to an embodiment of the present invention will be described.

[0150] Regarding the bendability, R (critical bending radius) / t (sheet thickness) measured by a 90-degree V-bending test according to JIS Z 2248 (2022) is preferably 2.0 or less. High bendability means good fracture resistance characteristics during collision deformation.[Production Method (Method for Producing Structural Member and Method for Producing Steel Sheet)]

[0151] First, according to an embodiment of the present invention, a method for producing a steel sheet having a base steel sheet used as a base material of a structural member will be described.

[0152] The method for producing a steel sheet used as a base material according to the present invention includes a hot rolling step of hot-rolling a steel slab into 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, when necessary, a coating step of subjecting the steel sheet to a galvanizing treatment. The galvanizing treatment is, for example, a hot-dip galvanizing treatment or a galvannealing treatment.

[0153] A suitable example of a method for producing a steel sheet used for a base material of a structural member according to an embodiment of the present invention will be described.

[0154] The method for producing a base steel sheet according to the present invention includes a hot rolling step of hot-rolling a steel slab with the chemical composition described above at a finish rolling temperature of 820°C or more, an annealing step of heating the steel sheet after the hot rolling step and annealing the steel sheet at an annealing temperature of 750°C or more and 900°C or less for a soaking time of 20 seconds or more in an atmosphere with a dew-point temperature of -10°C or more under the conditions satisfying the formulae (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 reheating and holding temperature range of the cooling stop temperature or more and 440°C or less and holding the steel sheet for 10 seconds or more, a surface layer strain introduction step of applying a tension of 19.6 N / mm 2< or more to the steel sheet after the first holding step in the reheating and holding temperature range, and a second holding step of holding the steel sheet after the surface layer strain introduction step in the reheating and holding temperature range for 10 seconds or more, or further a cold rolling step of cold-rolling the steel sheet after the hot rolling step and before the annealing step at a rolling reduction ratio of 20% or more and 80% or less to produce a cold-rolled steel sheet. 2400 ≤ Y ≤ 20000 Y = T − Ac 1 × t 1 / 2 + T − Ac 1 × t 2

[0155] In the formula (3), T denotes the annealing temperature (°C), t1 denotes the time (s) from 650°C to the annealing temperature T during heating in the annealing step, t2 denotes the soaking time (s), Ac1 (°C) denotes 727.0 - 32.7 x [%C] + 14.9 x [%Si] + 2.0 x [%Mn], [%C] denotes the C content of the steel sheet, [%Si] denotes the Si content of the steel sheet, and [%Mn] denotes the Mn content of the steel sheet.

[0156] In the present invention, a steel material (steel slab) may be produced by any melting method and may be produced by any known melting method using a converter, an electric arc furnace, or the like. A steel slab (slab) is preferably produced by a continuous casting method to prevent macrosegregation but may also be produced by an ingot casting method, a thin slab casting method, or the like. Furthermore, in addition to a known method of producing a steel slab, then temporarily cooling the steel slab to room temperature, and then heating the steel slab again, it is also possible without problems to apply an energy-saving process, such as hot direct rolling or direct hot charge rolling, in which a hot piece is charged into a furnace as it is without being cooled to room temperature or is subjected to slight heat retention and is then immediately rolled.(Hot Rolling Step)

[0157] When a slab is heated, from the perspective of dissolution of carbide and reduction of rolling force, the slab heating temperature is preferably 1100°C or more. Furthermore, to prevent an increase in scale loss, the slab heating temperature is preferably 1300°C or less. The slab heating temperature is the temperature of the slab surface. Furthermore, although a slab is formed into a sheet bar by rough rolling under normal conditions, when the heating temperature is lowered, from the perspective of preventing troubles during hot rolling, the sheet bar is preferably heated with a bar heater or the like before finish rolling.Finish rolling temperature: 820°C or more

[0158] A low finish rolling temperature results in an increase in rolling load. A low finish rolling temperature also results in an increase in the rolling reduction ratio of austenite in an unrecrystallized state, growth of an abnormal microstructure elongated in the rolling direction, and a decrease in the ductility, flangeability, and bendability of a final material. Thus, the finish rolling temperature is 820°C or more. The finish rolling temperature is preferably 830°C or more, more preferably 850°C or more. The finish rolling temperature is preferably 1080°C or less, more preferably 1050°C or less.

[0159] Furthermore, although the coiling temperature after hot rolling is not particularly limited, it is necessary to consider a case where the ductility, flangeability, and bendability of the final material are degraded. Thus, the coiling temperature after hot rolling is preferably 300°C or more. The coiling temperature after hot rolling is preferably 700°C or less.

[0160] During hot rolling, sheets after rough rolling may be joined and continuously subjected to finish rolling. A sheet after rough rolling may be once coiled. Furthermore, to reduce the rolling force during hot rolling, part or all of the finish rolling may be subjected to lubrication rolling. The lubrication rolling is also effective in making the shape and the material quality of a steel sheet uniform. The friction coefficient during the lubrication rolling is preferably 0.10 or more and 0.25 or less.(Pickling Step)

[0161] A hot-rolled steel sheet thus produced may be pickled. Pickling can remove oxides on the surface of a steel sheet and can therefore be performed to ensure high chemical convertibility and coating quality of a high-strength steel sheet as the final product. Furthermore, pickling may be performed once or may be divided into a plurality of times.(Cold Rolling Step)

[0162] A pickled sheet after hot rolling or a hot-rolled steel sheet produced as described above is cold-rolled as required. In cold rolling, after hot rolling, a pickled sheet may be cold-rolled as it is or may be cold-rolled after heat treatment. Optionally, a cold-rolled steel sheet after cold rolling may be pickled.

[0163] The cold rolling is performed, for example, by multipass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling.Rolling reduction ratio of cold rolling as required: 20% or more and 80% or less

[0164] In cold rolling, the rolling reduction ratio (cumulative rolling reduction ratio) of the cold rolling is preferably, but not limited to, 20% or more and 80% or less. When the rolling reduction ratio of the cold rolling is less than 20%, coarsening and non-uniformity of the steel microstructure are likely to occur in the annealing step, and the final product may have a decrease in TS and bendability. On the other hand, a rolling reduction ratio of more than 80% in the cold rolling tends to result in a steel sheet with a poor shape and, in a case where galvanizing is performed in the subsequent step, may result in an uneven galvanized coating weight.(Annealing Step)

[0165] In an embodiment of the present invention, a steel sheet after the hot rolling step (in a case where cold rolling is not performed) or after the cold rolling step (in a case where cold rolling is performed) is preferably heated and annealed at an annealing temperature of 750°C or more and 900°C or less for a soaking time of 20 seconds or more in an atmosphere with a dew-point temperature of -10°C or more under the conditions satisfying the formulae (2) and (3). 2400 ≤ Y ≤ 20000 Y = T − Ac 1 × t 1 / 2 + T − Ac 1 × t 2

[0166] In the formula (3), T denotes the annealing temperature (°C), t1 denotes the time (s) from 650°C to the annealing temperature T during heating in the annealing step, and t2 denotes the soaking time (s).Annealing temperature: 750°C or more and 900°C or less

[0167] An annealing temperature of less than 750°C results in an insufficient formation ratio of austenite during heating in a two-phase region of ferrite and austenite. Thus, the area fraction of ferrite increases excessively after annealing, and desired TS and YS cannot be achieved.

[0168] On the other hand, an annealing temperature of more than 900°C results in a decrease in ductility.

[0169] Thus, the annealing temperature is preferably 750°C or more and 900°C or less.

[0170] The annealing temperature is more preferably 880°C or less. The annealing temperature is even more preferably 870°C or less. The annealing temperature is more preferably 780°C or more, even more preferably 800°C or more.

[0171] The annealing temperature is the highest temperature (soaking temperature) reached in the annealing step.Soaking time: 20 seconds or more

[0172] A soaking time of less than 20 seconds results in an insufficient proportion of austenite formed during heating in a two-phase region of ferrite and austenite. Thus, the area fraction of ferrite increases excessively after annealing, and TS and YS cannot be achieved. Thus, the soaking time is preferably 20 seconds or more. The soaking time is more preferably 30 seconds or more, even 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, more preferably 800 seconds or less. The annealing time is even more preferably 600 seconds or less, even further more preferably 250 seconds or less.

[0173] The term "soaking time" refers to the holding time in the temperature range of (annealing temperature - 40°C) or more and the annealing temperature or less. That is, the soaking time includes, in addition to the holding time at the annealing temperature, the residence time in the temperature range of (annealing temperature - 40°C) or more and the annealing temperature or less in the heating and cooling before and / or after reaching the annealing temperature.

[0174] The annealing may be performed twice or more but is preferably performed once from the perspective of energy efficiency.Dew-point temperature of atmosphere of annealing step (annealing atmosphere): -10°C or more

[0175] In an embodiment of the present invention, the dew-point temperature of the atmosphere in the annealing step (annealing atmosphere) is preferably -10°C or more. Annealing at a dew-point temperature of -10°C or more in the annealing atmosphere in the annealing step can promote a decarburization reaction and more deeply form a surface soft layer. The dew-point temperature of the annealing atmosphere in the annealing step is more preferably -5°C or more, even more preferably 0°C or more, most preferably +5°C or more. Although the dew-point temperature of the annealing atmosphere in the annealing step may have any upper limit, to suitably prevent the oxidation of the surface of an Fe-based electroplated layer and to improve the adhesiveness of coating when a galvanized layer is provided, the dew-point temperature of the annealing atmosphere in the annealing step is preferably 30°C or less. The dew-point temperature of the annealing atmosphere in the annealing step is more preferably 25°C or less, even more preferably 20°C or less. 2400 ≤ Y ≤ 20000 Y = T − Ac 1 × t 1 / 2 + T − Ac 1 × t 2

[0176] In the formula (3), T denotes the annealing temperature (°C), t1 denotes the time (s) from 650°C to the annealing temperature T during heating in the annealing step, and t2 denotes the soaking time (s).

[0177] t1 denotes the time (s) from 650°C to the annealing temperature (soaking temperature) T during heating in the annealing step, t2 denotes the soaking time (holding time) (s) in the annealing step, and Ac1 denotes the Ac1 temperature (°C).

[0178] In the present invention, annealing is preferably performed under conditions that satisfy the formulae (2) and (3). Y of less than 2400 in the formula (3) may result in a surface soft layer defined in the present invention below 20 µm. On the other hand, Y of more than 20000 may result in a surface soft layer defined in the present invention exceeding (0.1 x t - 3800 x [Sb] - 1900 x [Sn]) µm. Thus, Y in the formula (3) is 2400 or more and 20000 or less.

[0179] Y is preferably 9000 or more, more preferably 12000 or more. Y is preferably 19000 or less, more preferably 18000 or less.

[0180] t1 is preferably 30 s or more. t1 is preferably 80 s or less.

[0181] The Ac1 temperature (°C) is calculated by the following formula: wherein [%C] denotes the C content of the steel sheet, [%Si] denotes the Si content of the steel sheet, and [%Mn] denotes the Mn content of the steel sheet.(Cooling Step)Cooling to cooling stop temperature of less than 100°C

[0182] The average cooling rate is 10°C / s or more and 50°C / s or less, and the dew-point temperature of atmosphere is - 20°C or less (preferred requirement)

[0183] In the cooling step, a steel sheet after the annealing step is preferably cooled to a cooling stop temperature of less than 100°C.

[0184] At this time, the cooling start temperature can be 750°C or more and 900°C or less.

[0185] The cooling stop temperature is more preferably 80°C or less, even more preferably 60°C or less.

[0186] The cooling stop temperature is more preferably 5°C or more, even more preferably 15°C or more.

[0187] The average cooling rate in the cooling step is preferably 10°C / s or more and 50°C / s or less. In this cooling step, a steel microstructure defined in the present invention can be formed.

[0188] The dew-point temperature of the atmosphere in the cooling step is preferably -20°C or less. An atmosphere with a dew-point temperature of more than -20°C may result in a large variation in the thickness of a surface soft layer in the in-plane direction of the steel sheet, and the tensile strength defined in the present invention may not be achieved. Thus, the dew-point temperature of the atmosphere in the cooling step is preferably -20°C or less.

[0189] The average cooling rate (°C / s) is calculated 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).(First Holding Step (Reheating and Holding Step))

[0190] In the first holding step (reheating and holding step), preferably, a steel sheet is reheated to a reheating and holding temperature range of the cooling stop temperature or more and 440°C or less and is held for 10 seconds or more.

[0191] Reheating the steel sheet to the reheating and holding temperature range of the cooling stop temperature or more and 440°C or less and holding the steel sheet for 10 seconds or more facilitate the formation of bainitic ferrite and tempered martensite defined in the present invention. The reheating and holding temperature range is more preferably 420°C or less, even more preferably 400°C or less.

[0192] The holding time is more preferably 20 seconds or more, even more preferably 30 seconds or more.

[0193] The holding time is more preferably 100 seconds or less, even more preferably 80 seconds or less.(Surface Layer Strain Introduction Step)

[0194] In the surface layer strain introduction step, a tension of 19.6 N / mm 2< or more is preferably applied to a steel sheet after the first holding step in the reheating and holding temperature range.

[0195] The tension is calculated by dividing the total value of the loads (N) of load cells at the left and right of a roll through which the steel sheet passes while contacting the roll by the cross-sectional area of the steel sheet (= sheet thickness (mm) x sheet width (mm)) (mm 2< ). The load cells should be arranged parallel to the direction of the tension.

[0196] The load cells are preferably disposed at a position of 200 mm from both ends of the roll. The length of the roll to be used preferably ranges from 1500 to 2500 mm.

[0197] The tension is preferably 21.5 N / mm 2< or more, more preferably 23.5 N / mm 2< or more. The tension is preferably 147 N / mm 2< or less, more preferably 98 N / mm 2< or less. The tension is even more preferably 39.2 N / mm 2< or less.(Second Holding Step)

[0198] In the second holding step, a steel sheet after the surface layer strain introduction step is preferably held in the reheating and holding temperature range for 10 seconds or more. The holding time in the second holding step is more preferably 15 seconds or more, even more preferably 20 seconds or more. The holding time is preferably 60 seconds or less, more preferably 50 seconds or less.(Coating Step (Hot-Dip Galvanizing Step, Hot-Dip Galvannealing Step))

[0199] In the present invention, in a coating step after the annealing step, a steel sheet can be subjected to a galvanizing treatment to produce a galvanized steel sheet.

[0200] The galvanizing treatment is, for example, a hot-dip galvanizing treatment or a hot-dip galvannealing treatment.

[0201] In the hot-dip galvanizing treatment, preferably, a steel sheet is immersed in a galvanizing bath at 440°C or more and 500°C or less, and the coating weight is then adjusted by gas wiping or the like. The hot-dip galvanizing bath is not particularly limited, provided that the galvanized layer has the composition described above, and is preferably, for example, a coating bath having a composition with an Al content of 0.10% or more by mass and 0.23% or less by mass and with the remainder being composed of Zn and incidental impurities.

[0202] In the hot-dip galvannealing treatment, after the hot-dip galvanizing treatment performed in the manner described above, the hot-dip galvanized steel sheet is preferably heated to an alloying temperature of 450°C or more and 600°C or less to perform an alloying treatment.

[0203] An alloying temperature of less than 450°C may result in a low Zn-Fe alloying speed and make alloying difficult. On the other hand, an alloying temperature of more than 600°C results in transformation of non-transformed austenite into pearlite and makes it difficult to achieve a TS of 980 MPa or more. The alloying temperature is more preferably 500°C or more, even more preferably 510°C or more. The alloying temperature is more preferably 570°C or less.

[0204] The coating weight of each of the hot-dip galvanized steel sheet (GI) and the hot-dip galvannealed steel sheet (GA) preferably ranges from 20 to 80 g / m 2< per side. The coating weight can be adjusted by gas wiping or the like.

[0205] Furthermore, the steel sheet thus produced may be further subjected to temper rolling.

[0206] A rolling reduction ratio of more than 2.00% in the temper rolling may result in an increase in yield stress and a decrease in dimensional accuracy when the steel sheet is formed into a member. Thus, the rolling reduction ratio of the temper rolling is preferably 2.00% or less. The lower limit of the rolling reduction ratio in the temper rolling is preferably, but not limited to, 0.05% or more from the perspective of productivity. The temper rolling may be performed with an apparatus coupled to an annealing apparatus for each step (on-line) or with an apparatus separated from the annealing apparatus for each step (offline). The number of times of temper rolling may be one or two or more. Provided that the elongation percentage can be similar to that of temper rolling, rolling with a leveler or the like may be used.

[0207] Although the other conditions of the production method are not particularly limited, from the perspective of productivity, a series of treatments, such as the annealing, hot-dip galvanizing, and alloying treatment of galvanization, are preferably performed in a continuous galvanizing line (CGL), which is a hot-dip galvanizing line. After the hot-dip galvanizing, wiping can be performed to adjust the coating weight. The conditions of coating and the like other than the above-described conditions can be the same as in a usual hot-dip galvanizing method.

[0208] Next, a method for producing a structural member according to an embodiment of the present invention will be described.

[0209] Fig. 2 illustrates an example of a process of producing a structural member using a steel sheet having a base steel sheet as a base material. A structural member 1 may have a hat-shaped part (fabricated part 10) composed of a top sheet portion, a vertical wall portion, and a flange portion, and a back sheet part (reinforcing part 11). The hat-shaped part illustrated in Fig. 2(a) is fabricated by press forming, bending, roll forming, extrusion, or other ordinary methods for forming a thin sheet material. As illustrated in Fig. 2(b), the hat-shaped part (fabricated part 10) and the back sheet part (reinforcing part 11) are fabricated into an integral part (structural member 1) by a joining method, such as spot welding (see reference sign A in the drawing), laser welding, bolt fastening, or adhesive joining, or assembly by caulking or the like.

[0210] Next, as illustrated in Fig. 2(c), an integral part (structural member 1) having a hat-shaped part and a back sheet part is further assembled with an end fixing jig 12 to fabricate a hollow cross-section part 13. It should be noted that 40 mm, 40 mm, and 20 mm in Fig. 2(a) are examples of the lengths of the top sheet portion, the vertical wall portion, and the flange portion in the cross-sectional shape, respectively, but the lengths of the top sheet portion, the vertical wall portion, and the flange portion are not limited to these lengths.

[0211] The back sheet part is joined to the hat-shaped part to form a closed cross section, thereby suppressing opening of the cross section of the hat-shaped part, and is a reinforcing member for stably and largely deforming the structural member. The back sheet part can be, for example, a cold-rolled steel sheet or a galvanized steel sheet. The back sheet part may have any tensile strength and ductility and is preferably a steel sheet with a tensile strength of 590 MPa or more.

[0212] In the above examples, treatments in the annealing step, the cooling step, the first holding step, the surface layer strain introduction step, and the second holding step are performed in a state in which the base steel sheet is a steel sheet. However, in the present invention, it is not necessary to perform the treatments in the heat treatment steps from the annealing step to the second holding step in the state of the steel sheet. After a base steel sheet is press-formed into the shape of a structural member, the treatments in the heat treatment steps from the annealing step to the second holding step may be performed.

[0213] Although a structural member according to the present invention is a structural member with good collision characteristics particularly when used as an automotive structural member, the application is not limited thereto. A structural member according to the present invention can be used as a structural member of a building structure or a structural member of a machine structure. For example, by using it as a strength member, such as a pillar or a beam for a building structure, it can be a strong structural member with high impact resistance at the time of occurrence of an earthquake. Furthermore, when used as a structural member of a machine structure, it is possible to obtain a strong structural member with high impact resistance when any impact load is applied to the machine structure.EXAMPLES<Method for Producing Steel Sheet>

[0214] Slabs containing the chemical components shown in Table 1 were produced by melting in a converter and a continuous casting method. The steel slabs were heated to 1200°C and, after the heating, were subjected to hot rolling composed of rough rolling and finish rolling at a finish rolling temperature of 900°C to form hot-rolled steel sheets. The hot-rolled steel sheets were then pickled and cold-rolled at a cold-rolling reduction ratio in the range of 30% to 60% to produce cold-rolled steel sheets with a thickness of 1.6 mm. The cold-rolled steel sheets were then subjected to an annealing treatment and a cooling treatment. Some of the steel sheets were subjected to a hot-dip galvanizing treatment or a hot-dip galvannealing treatment. Table 2 shows the conditions in the annealing step, the cooling step, the coating step during the cooling step, the reheating and holding step (first holding step), the surface layer strain introduction step, and the second holding step. In Table 2, the type of coating step GA is a condition for the hot-dip galvannealing treatment, GI is a condition for the hot-dip galvanizing treatment, and CR is a condition under which a base steel sheet was produced without coating treatment. [Table 1]Steel gradeChemical composition (% by mass)OthersAc1 (°C)CSiMnPSAlNSbSnA0.1250.562.880.0960.00120.0300.00320.0100--737B0.1150.532.730.0180.01950.0370.00650.01100.0030-737C0.1200.352.960.0080.00100.0320.0027---734D0.1410.663.210.0090.00090.0380.00380.0090--739E0.1330.602.890.0090.00110.5000.00320.0080--737F0.0200.502.820.0100.00150.0280.00490.0090--739G0.1283.202.750.0120.00180.0350.00320.0060--776H0.1250.541.200.0140.00140.0270.00420.0060--733I0.1180.572.720.0100.00080.0320.00450.0010-Ti: 0.040737J0.1150.632.650.0090.00090.0350.00300.0100-Ti: 0.028, B: 0.0022738K0.1240.592.620.0100.00120.0460.00330.0050-Nb: 0.018, Ti: 0.022, B: 0.0014737L0.1210.452.510.0110.00120.0290.00320.00300.0010Nb: 0.035, Ti: 0.015, B: 0.0012, Cr: 0.580735M0.1370.693.120.0090.00080.0320.00350.0010-Nb: 0.012, Ti: 0.023, B: 0.0015739N0.1650.572.520.0110.00090.0390.00220.0080-Nb: 0.025, Ti: 0.025, B: 0.0010, Cr: 0.52073500.0750.612.550.0140.00150.0340.00510.0010-Nb: 0.035, Ti: 0.020, B: 0.0015, Cr: 0.440739P0.1150.732.530.0120.00250.0350.00330.00400.0020Nb: 0.040, Ti: 0.010, B: 0.0014, Cr: 0.680739Q0.1190.072.500.0150.00380.0540.00380.0070-Nb: 0.015, Ti: 0.015, B: 0.0020, Cr: 0.350729R0.1210.433.350.0180.00200.0420.00290.0010-Nb: 0.025, Ti: 0.015, B: 0.0012, Cr: 0.550736S0.1160.582.150.0120.00140.0310.00260.0090-Nb: 0.020, Ti: 0.020, B: 0.0015, Cr: 0.600736T0.1250.602.630.0100.00120.0320.00350.01100.0010V: 0.055737U0.1300.552.780.0090.00120.0450.00320.0060-Cu: 0.180737v0.1130.502.720.0100.00100.0380.00190.0010-Cr: 0.590736W0.1090.182.950.0080.00090.0520.00260.00900.0030Ni: 0.150732X0.1210.452.760.0090.00230.0410.0037--Mo: 0.200735Y0.1180.322.720.0110.00120.0350.00220.0030-Nb: 0.032, Ta: 0.007733Z0.0980.512.980.0100.00050.0520.00250.0005-Ta: 0.008737AA0.1550.482.480.0150.00290.0490.00440.0110-W: 0.090734AB0.1220.562.810.0040.00180.0300.00530.0005-Mo: 0.0050737AC0.1170.622.720.0090.00200.0320.00340.0030-Zn: 0.0060738AD0.1390.472.320.0110.00150.0380.00320.0005-Co: 0.0080734AE0.1100.532.730.0160.00090.0250.00270.0030-Zr: 0.0030737AF0.1000.582.820.0120.00160.0420.00380.0010-Ca: 0.0018738AG0.1170.492.780.0450.00090.0280.00320.0005-Se: 0.0075736AH0.1120.532.730.0180.00780.0370.00650.01100.0030Te: 0.0140737AI0.1270.282.680.0270.00070.0320.00270.0020-Ge: 0.0060732AJ0.1180.452.710.0160.00320.0390.0071--As: 0.0210735AK0.0910.603.180.0090.00170.0340.00350.0005-Sr: 0.0070739AL0.1220.522.690.0070.00210.0190.00390.00800.0010Cs: 0.0100736AM0.0990.413.290.0100.00360.0300.00310.01000.0020Hf: 0.0070736AN0.1150.472.750.0210.00190.0310.0033--Pb: 0.0100736AO0.1170.502.720.0130.00110.0420.00350.0005-Bi: 0.0050736AP0.1160.482.770.0110.00150.0350.00420.0010-REM: 0.0040736AQ0.1000.342.190.0130.00120.0340.00310.01000.0010Nb: 0.180, Ti: 0.180, V: 0.180, B: 0.0075, Cr: 0.950, Ni: 0.960, Mo: 0.950, Cu: 0.900, Ta: 0.095, W: 0.450, Mg: 0.0170, Zn: 0.0180, Co: 0.0180, Zr: 0.0930, Ca: 0.0180, Se: 0.0190, Te: 0.0185, Ge: 0.0190, As: 0.0400, Sr: 0.0180, Cs: 0.0185, Hf: 0.0185, Pb: 0.0190, Bi: 0.0190, REM: 0.0190733BA0.0520.753.020.0200.00060.0110.00360.0130--743BB0.3940.891.500.0160.00170.0210.00410.0110--730BC0.2200.022.590.0050.00150.0120.00380.0140--725BD0.0893.032.740.0180.00120.0340.00450.0110--775BE0.2930.651.460.0170.00050.0100.00260.0130--730BF0.1180.452.700.0160.00320.0050.0071---735BG0.1310.602.890.0090.00111.9800.00320.0080--737BH0.1190.452.710.0160.00320.0390.0100---735BI0.1250.552.880.0110.00120.0300.00320.0360--737BJ0.1130.532.730.0180.00780.0370.0065-0.0730-737- The remainder other than those described above is composed of Fe and incidental impurities. [Table 2] No.Steel gradeAnnealing stepCooling stepFirst holding stepSurface layer strain introduction stepSecond holding stepCoating stepAnnealing temperature T (°C)Heating t1 (s)Annealing (soaking) t2 (s)Annealing dew-point temperature (°C)Formula (3) Y (-)Cooling stop temperature (°C)Holding temperature (°C)Holding time (s)Average tension (N / mm 2< )Holding temperature (°C)Holding time (s)TypeAlloying temperature (°C)1A850481001014010403503028.434025GA5002C8404590-511901353402527.432020CR-3D850341001013029162702031.426040GA5204E83077701010050313106027.431060GA5105F88076801016587323806026.536010GA4906G8903590012256343407037.236010GI-7H8804570513564364105030.439060GA5108A7006516015-7126363706031.436010GI-9A860511004366353303026.533025GA51010A8504396-2013276303403025.533020GA54011A820515201452433505021.635040GA49012A81580170-1016377483307019.632050GA51013A84080801012358173605030.445010GI-14A86037801512114173405028.420010GA52015A8505490-513219303609032.336010GA52016B82053100510551343303032.335040GA49017I85077801513382363904035.339010GA51018J8507170-511824533403039.234010GA52019K8005670106176603706027.435060GI-20L8505170511005373304038.233010GA51021M8105680207664363503034.337010GI-22N8307680011194173307038.233010GI-2308406390512304183405033.333010GA51024P84064702010284563604030.436020GI-25Q86069901016291323309036.332010GA52026R8304990510746333806033.340040GI-27S83041120-5131865533010031.433010GA51028T8507870012305473304024.532040GA52029U8305280209911593706024.537050GI-30v8404390011574153305037.231050GA51031W85063110-1016694243607031.435010GA51032X8505590-513481593507025.535010GA49033Y83064100-512758302905032.328030GA51034Z8306570-59496433605039.238010GA48035AA840411151514357533502019.635020GA51036AB8506370011472283002024.530020GI-37AC8407290512871583403031.433030GA52038AD8204980208977113306037.233010GI-39AE85057901013419483606036.334010GA52040AF85070110516238404206037.242020CR-41AG8406470010604354004028.438010GA50042AH82053100510538343303032.335040GA49043AI83076802011519423804032.838010GA50044AJ85073901014514263504038.234010GA51045AK81065120010778283603037.238020CR-46AL82054100010650263106020.629010GA51047AM8406490-512633573305021.632010GA50048AN8503880011311233509028.433010GA51049AO83077702010192243307035.331010GA50050AP85061110516031433304036.335040GI-51AQ83059902011570473203027.431040GA50052A8606880-1114020173802026.538010GA53053A84055120-515190383805024.537010GA51054A84052120515036163206030.431020GA50055BA8006312008709323603034.338020GA52056BB7907310008139353804031.437020GA53057BC85062110517585293305031.432015GA51058BD8007280102933313508033.333020GA51059BE8804570513873364105030.439060GA51060BF82054100010764263106020.629010GA51061BG83077701010043313106027.431060GA51062BH85073901014518263504038.234010GA51063BI850481001014028403503028.434025GA50064BJ82053100510542343303032.335040GA490 <Method for Producing Member>

[0215] A blank for a hat-shaped structural member was cut out from these steel sheets.

[0216] That is, a material was prepared so that a hat-shaped member after forming has a constant cross section with an axial length of 200 mm and was press-formed into the hat-shaped member. Fig. 2 illustrates the cross-sectional shape of the hat-shaped member. It was formed so that the top sheet portion and the vertical wall portions had a length of 40 mm, the flange portions had a length of 20 mm, and all the bending ridge line portions between the top sheet portion and the vertical wall portions and between the vertical wall portions and the flange portions had an inner bending R of 5 mm. Subsequently, the hat-shaped member and a back sheet member were joined to each other by spot welding so that the flange portions of the hat-shaped member and the back sheet part were joined to each other, thereby producing a structural member having a hollow cross section. The weld pitch was 45 mm.

[0217] The back sheet member is a cold-rolled steel sheet with a thickness of 1.6 mm and a tensile strength of 1180 MPa and is a reinforcing member that is joined to the hat-shaped part to form a closed cross section, thereby suppressing opening of the cross section of the hat-shaped part and effectively and stably causing large deformation in an axial crushing test described later.

[0218] To stabilize deformation in an axial crushing test for evaluating collision performance, one end portion of the structural member in the axial direction of the specimen was joined by arc welding to a jig for fixing an end cross section.<Surface Soft Layer Measurement Method>

[0219] Measurement is performed on a surface soft layer as described below. A steel sheet test specimen was cut out from the top sheet portion of the hat-shaped structural member, a cross section in the thickness direction (L cross section) perpendicular to the longitudinal direction of the hat-shaped structural member was smoothed by wet grinding, and measurement was then performed with a Vickers hardness tester at a load of 9.8 x 10 -2< N at intervals of 1 µm from a position of 1 µm in the sheet thickness direction from the steel sheet test specimen surface to a position of 120 µm in the sheet thickness direction. The measurement was then performed at intervals of 20 µm up to the center of the sheet thickness. A region where the hardness decreased to 84% or less relative to the hardness at the quarter thickness was defined as a soft layer (surface soft layer), and the thickness of the region in the sheet thickness direction was defined as the thickness of the soft layer.<Microstructure Measurement Method>

[0220] The microstructure at the half thickness of a surface soft layer and the microstructure at the quarter thickness of a base steel sheet were measured by the method described above.<Tensile Test>

[0221] The tensile test was performed in accordance with JIS Z 2241 (2011). That is, a JIS No. 5 specimen was taken from the top sheet portion of the hat-shaped structural member. The test specimen was subjected to a tensile test at a crosshead speed of 10 mm / min to measure TS and YS.<90-Degree V-Bending Test>

[0222] The V (90-degree) bending test was performed in accordance with JIS Z 2248 (2022).

[0223] A 100 mm x 35 mm test specimen was taken from the top sheet portion of the hat-shaped structural member by shearing and end grinding. Bending radius R: change in 0.1 to 0.5 mm pitch Test method: die support, punch indentation Forming load: 10 ton Test speed: 30 mm / min Holding time: 5 s Bending direction: direction (C) perpendicular to rolling direction

[0224] Evaluation was performed three times to calculate R / t by dividing the minimum bending radius (critical bending radius) R at which no cracking occurred in any case by the thickness t of the base steel sheet. A crack with a length of 200 µm or more was determined to be a crack using a stereomicroscope manufactured by Leica at a magnification of 25 times.<Axial Crushing Test>

[0225] Fig. 3 is an explanatory view of an axial crushing test method performed to evaluate the collision performance of a structural part (hollow cross-section part) 13 fabricated based on the present embodiment. The axial crushing test was performed with an Autograph bending tester. The end fixing jig 12 was installed on a flat floor surface of a non-movable portion 20 of the bending tester in such a direction that the axial direction of the specimen was the movable direction of the bending tester. A punch 22 with a flat surface was attached to a movable portion 21 of the bending tester such that the flat surface of the punch 22 was perpendicular to the movable direction of the tester (see the reference sign X in Fig. 3), and the movable portion 21 of the bending tester was moved to crush the specimen (the hollow cross-section part 13).

[0226] In the axial crushing test, the movable speed of the punch was 10 mm / min, and the movable stroke of the punch was 100 mm. In the test, three members to which each material was applied were tested. The load applied to the punch during the crushing test was measured, the maximum load at the time of initial buckling was collected, and the average value of the three members was calculated as the maximum load N3 average. The maximum load N3 average of 220 kN or more was determined to be good, and the maximum load N3 average of less than 220 kN was determined to be poor.

[0227] To evaluate the collision fracture characteristics of a steel sheet itself in the axial crushing test, the fracture state was checked. Fig. 4 shows examples of fracture portions after the axial crushing test of the structural parts to which materials of Experiment No. 10 (Comparative Example) and Experiment No. 3 (Example) are applied.

[0228] The fractures shown in Fig. 4 are bending fractures of the base material (steel sheet itself), the comparative example of Fig. 4(a) shows a fracture form in which the fracture progresses in the base material and becomes large cracking, and the example of Fig. 4(b) shows a fracture form of minor cracking in which the base material is bent and fractured but the fracture is limited to partial cracking of the bent and deformed portion.

[0229] In the fracture states of Fig. 4, an arrow in Fig. 4(a) indicates the position of large cracking in the base material bending, and an arrow illustrated in Fig. 4(b) indicates the position of minor cracking in the base material bending. In the present example, regarding the bending fracture of the base material that could be confirmed by visual observation of the appearance, a fracture portion with a fracture portion length of 20 mm or more was determined to be large cracking, a fracture portion with a fracture portion length of less than 20 mm was determined to be minor cracking, and the number of fracture portions was counted. The determination of the fracture and the number count were performed for all the three members, and the total value was calculated as the total number of fractures. One or more large cracks or five or more minor cracks were determined to be poor, and the other was determined to be good.

[0230] Table 3 shows the results of the present example.

[0231] Nos. 5 to 9, 11, and 13 are comparative examples in which the tensile strength of the base steel sheet of the top sheet portion is less than 980 MPa.

[0232] Comparative Example Nos. 9, 10, and 52 have a thin surface soft layer, a high R / t in the bending test, and therefore have large cracking at two or more locations in the axial crushing test.

[0233] On the other hand, Examples have smaller total numbers of fractures than Comparative Example Nos. 9, 10, and 52 and have better bending fracture resistance characteristics of the base material in case of a collision. Furthermore, it can be seen that Examples have higher YS and maximum loads than Comparative Example Nos. 5 to 9, 11, and 13.

[0234] It can also be seen that Example Nos. 1 to 4, 12, 16, 19 to 29, 31 to 34, 36 to 38, 42, 43, 45 to 51, and 53 to 61 have a thick surface soft layer and an R / t of 1.5 or less, exhibiting particularly high bendability, and the total number of fractures is therefore suppressed to two or less minor cracks, indicating that these examples have much better bending fracture resistance characteristics.

[0235] As described above, according to the present embodiment, an automotive structural part with good collision characteristics can be fabricated, which combines a high collision load achieved by increasing the yield strength even at the same tensile strength level through the formation of a controlled material microstructure in the base steel sheet, and stable collision deformability achieved by suppressing collision fracture through the formation of a controlled microstructure in the surface layer.Reference Signs List

[0236] 1structural member 10fabricated part 11reinforcing part 12end fixing jig 13hollow cross-section part 20non-movable portion of bending tester 21movable portion of bending tester 22punch

Examples

examples

[0214]Slabs containing the chemical components shown in Table 1 were produced by melting in a converter and a continuous casting method. The steel slabs were heated to 1200°C and, after the heating, were subjected to hot rolling composed of rough rolling and finish rolling at a finish rolling temperature of 900°C to form hot-rolled steel sheets. The hot-rolled steel sheets were then pickled and cold-rolled at a cold-rolling reduction ratio in the range of 30% to 60% to produce cold-rolled steel sheets with a thickness of 1.6 mm. The cold-rolled steel sheets were then subjected to an annealing treatment and a cooling treatment. Some of the steel sheets were subjected to a hot-dip galvanizing treatment or a hot-dip galvannealing treatment. Table 2 shows the conditions in the annealing step, the cooling step, the coating step during the cooling step, the reheating and holding step (first holding step), the surface layer strain introduction step, and the second holding step. In Table 2,...

Claims

1. A structural member comprising a steel sheet having a base steel sheet, wherein the structural member has a cross-sectional shape that includes a top sheet portion extending in a width direction, and two vertical wall portions extending from both ends in a width direction of the top sheet portion in a direction different from a direction of extension of the top sheet portion, the base steel sheet of the top sheet portion has, in a surface layer, a surface soft layer with a Vickers hardness that is 84% or less of a Vickers hardness at a quarter thickness, the surface soft layer has a thickness satisfying the following formula (1), and the structural member has a tensile strength of 980 MPa or more and 1.8 GPa or less, 20 ≤ X ≤ 0.1 × t − 3800 × Sb − 1900 × Sn wherein X denotes the thickness of the surface soft layer (µm), t denotes a thickness of the base steel sheet (µm), and [Sb] and [Sn] denote an Sb content and a Sn content (% by mass) of the base steel sheet, respectively.

2. The structural member according to claim 1, wherein in the base steel sheet of the top sheet portion, as microstructures in the surface soft layer, ferrite has an area fraction of 60.0% or more, among microstructures other than ferrite, an area fraction of fresh martensite divided by a total area fraction of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less, and retained austenite has an area fraction of 3.0% or less, and as microstructures at a quarter thickness of the base steel sheet, ferrite has an area fraction of 55.0% or less (including 0.0%), bainitic ferrite and tempered martensite (excluding retained austenite) have a total area fraction of more than 40.0%, and fresh martensite has an area fraction of 10.0% or less (including 0.0%).

3. The structural member according to claim 1 or 2, wherein the base steel sheet of the top sheet portion has a chemical composition containing, on a mass percent basis, 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 composed of Fe and incidental impurities.

4. The structural member according to claim 3, wherein the base steel sheet of the top sheet portion further contains, as the chemical composition, on a mass percent basis, at least one element selected from 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, 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 structural member according to any one of claims 1 to 4, wherein the base steel sheet of the top sheet portion has an R / t value of 2.0 or less as measured by a 90-degree V-bending test according to JIS Z 2248, wherein R denotes a critical bending radius (µm), and t denotes a thickness (µm) of the base steel sheet.

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