Steel Sheet, Member, and Methods for Producing Same

This is the U.S. National Phase application of PCT/JP2023/006926 filed Feb. 27, 2023 which claims priority to Japanese Patent Application No. 2022-078347 filed May 11, 2022, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.

The present invention relates to a steel sheet, a member made of the steel sheet, and methods for producing them.

In recent years, from the viewpoint of global environmental conservation, improvement of fuel efficiency in automobiles has been an important issue. Thus, there has been an active movement to reduce the weight of automobile bodies by increasing the strength and reducing the thickness of steel sheets used as materials for automotive body parts.

Furthermore, a social demand for improvement of crash safety of automobiles is further increased. Thus, there is a demand for the development of a steel sheet with high strength and enhanced crashworthiness when a vehicle collides while traveling (hereinafter referred to simply as crashworthiness).

For example, Patent Literature 1 discloses, as such a steel sheet serving as a material of an automobile body part, a high-strength steel sheet with high stretch flangeability and enhanced crashworthiness, which has a chemical composition containing, on a mass percent basis, C: 0.04% to 0.22%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.05% or less, S: 0.01% or less, Al: 0.01% to 0.1%, and N: 0.001% to 0.005%, the remainder being Fe and incidental impurities, and which is composed of a ferrite phase as a main phase and a martensite phase as a second phase, the martensite phase having a maximum grain size of 2 μm or less and an area fraction of 5% or more.

Patent Literature 2 discloses a high-strength hot-dip galvanized steel sheet with high coating adhesion and formability having a hot-dip galvanized layer on the surface of a cold-rolled steel sheet, which has a surface layer ground off with a thickness of 0.1 μm or more and is pre-coated with 0.2 g/mor more and 2.0 g/mor less of Ni, wherein the cold-rolled steel sheet contains, on a mass percent basis, C: 0.05% or more and 0.4% or less, Si: 0.01% or more and 3.0% or less, Mn: 0.1% or more and 3.0% or less, P: 0.04% or less, S: 0.05% or less, N: 0.01% or less, Al: 0.01% or more and 2.0% or less, Si+Al>0.5%, the remainder being Fe and incidental impurities, has a microstructure, on a volume fraction basis, 40% or more ferrite as a main phase, 8% or more retained austenite, two or more of three types of martensite [1], [2], and [3] as specified below including martensite [3], 1% or more bainite, and 0% to 10% pearlite, the three types of martensite [1], [2], and [3] being, on a volume fraction basis, martensite [1]: 0% or more and 50% or less, martensite [2]: 0% or more and less than 20%, and martensite [3]: 1% or more and 30% or less, and having a hot-dip galvanized layer containing less than 7% Fe and the remainder composed of Zn, Al, and incidental impurities, on the surface of the steel sheet, and has TS×EL of 18000 MPa·% or more and TS×λ of 35000 MPa·% or more, wherein TS denotes tensile strength (MPa), EL denotes total elongation percentage (%), and A denotes hole expansion ratio (%), and a tensile strength of 980 MPa or more (when martensite [1]:C concentration (CM1) is less than 0.8%, hardness Hv1 satisfies Hv1/(−982.1×CM1+1676×CM1+189)≤0.60, when martensite [2]:C concentration (CM2) is 0.8% or more, the hardness Hv2 satisfies Hv2/(−982.1×CM2+1676×CM2+189)≤0.60, and when martensite [3]:C concentration (CM3) is 0.8% or more, the hardness Hv3 satisfies Hv3/(−982.1×CM3+1676×CM3+189)≥0.80.

Patent Literature 3 discloses a high-strength hot-dip galvanized steel sheet that has a chemical composition composed of, on a mass percent basis, C: 0.15% or more and 0.25% or less, Si: 0.50% or more and 2.5% or less, Mn: 2.3% or more and 4.0% or less, P: 0.100% or less, S: 0.02% or less, and Al: 0.01% or more and 2.5% or less, the remainder being Fe and incidental impurities, and that has a steel sheet microstructure having, on an area fraction, a tempered martensite phase: 30% or more and 73% or less, a ferrite phase: 25% or more and 68% or less, a retained austenite phase: 2% or more and 20% or less, and other phases: 10% or less (including 0%), the other phases being a martensite phase: 3% or less (including 0%) and bainitic ferrite phase: less than 5% (including 0%), the tempered martensite phase having an average grain size of 8 μm or less, the retained austenite phase having a C concentration of less than 0.7% by mass.

At present, however, only steel sheets with a tensile strength (hereinafter also referred to as TS) of 590 MPa are used for impact energy absorbing members of automobiles exemplified by front side members and rear side members.

Thus, to increase absorbed energy at the time of impact (hereinafter also referred to as impact absorbed energy), it is effective to improve yield stress (hereinafter also referred to as YS). However, a steel sheet with higher TS and YS typically has lower press formability and, in particular, lower ductility, flangeability, bendability, and the like. Thus, when such a steel sheet with higher TS and YS is applied to the impact energy absorbing members of automobiles, not only press forming is difficult, but also the member cracks in an axial compression test simulating a collision test. In other words, the actual impact absorbed energy is not increased as expected from the value of YS. Thus, the impact energy absorbing members are currently limited to steel sheets with a TS of 590 MPa.

Actually, it also cannot be said that the steel sheets disclosed in Patent Literature 1 to Patent Literature 3 have a TS of 1180 MPa or more, high YS, high press formability (ductility, flangeability, and bendability), and fracture resistance characteristics (bending fracture characteristics and axial compression characteristics) at the time of compression.

Aspects of the present invention have been developed in view of such circumstances and aim to provide a steel sheet with a tensile strength TS of 1180 MPa or more, high yield stress YS, high press formability (ductility, flangeability, and bendability), and fracture resistance characteristics (bending fracture characteristics and axial compression characteristics) at the time of compression, together with an advantageous method for producing the steel sheet.

Aspects of the present invention also aim to provide a member made of the steel sheet and a method for producing the member.

The term “steel sheet”, as used herein, includes a galvanized steel sheet, and the galvanized steel sheet is a hot-dip galvanized steel sheet (hereinafter also referred to as GI) or a hot-dip galvannealed steel sheet (hereinafter also referred to as GA).

The tensile strength TS is measured in the tensile test according to JIS Z 2241 (2011).

The phrase “with high yield stress YS, high press formability (ductility, flangeability, and bendability), and fracture resistance characteristics (bending fracture characteristics and axial compression characteristics) at the time of compression” refers to satisfying the following.

The phrase “high yield stress YS” means that YS measured in the tensile test according to JIS Z 2241 (2011) satisfies the following formula (A) or (B) depending on TS measured in the tensile test.

The phrase “high ductility” means that the total elongation (E1) measured in the tensile test according to JIS Z 2241 (2011) satisfies the following formula (A) or (B) depending on TS measured in the tensile test.

The phrase “high flangeability” refers to a limiting hole expansion ratio (λ) of 30% or more as measured in the hole expansion test according to JIS Z 2256 (2020).

The phrase “high bendability” refers to a bending angle (α) of 80 degrees or more at the maximum load measured in a bending test according to the VDA standard (VDA 238-100) defined by German Association of the Automotive Industry.

The phrase “good bending fracture characteristics” refers to a stroke (S) of 26.0 mm or more at the maximum load measured in a V-VDA bending test.

The phrase “good axial compression characteristics” means that, after an axial compression test, fracture (appearance crack) occurs at three or less positions in the regions of R=5.0 mm and 200 mm of lower two bending ridge line portions in() (see regions Cx in).

E1, λ, and α described above are characteristics indicating formability at the time of press forming of a steel sheet. On the other hand, the V-VDA bending test is a test simulating the deformation and fracture behavior of a bending ridge line portion in a collision test, and the stroke at the maximum load (S) measured in the V-VDA bending test is a characteristic indicating the resistance to cracking of a member.

To achieve the above objects, the present inventors have conducted extensive studies.

As a result, it has been found that a steel sheet with a tensile strength TS of 1180 MPa or more, high YS, high press formability (ductility, flangeability, and bendability), and fracture resistance characteristics (bending fracture characteristics and axial compression characteristics) at the time of compression is produced when the steel sheet has a base steel sheet with an appropriately adjusted chemical composition, the base steel sheet of the steel sheet has a steel microstructure in which the area fraction of bainitic ferrite: 3.0% or more and 20.0% or less, the area fraction of tempered martensite (excluding retained austenite): 40.0% or more and 90.0% or less, the area fraction of retained austenite: more than 3.0% and 15.0% or less, the concentration of carbon in retained austenite: 0.60% by mass or more and 1.30% by mass or less, the area fraction of fresh martensite: 10.0% or less (including 0.0%), and the density of carbides in tempered martensite: 8.0 particles/μmor less, the amount of diffusible hydrogen in the base steel sheet is 0.50 ppm by mass or less, a V-VDA bending test is performed to a maximum load point, a crack in an L cross section in an overlap region of a V-bending ridge line portion and a VDA bending ridge line portion has a length of 400 μm or less, in a region formed from each position on a starting line present from a starting point of a bending peak on an outside of a VDA bend to a position of 50 μm in a thickness direction up to a position of 50 μm on each side of the starting line perpendicular to the starting line, and, with respect to the average grain size of bainitic ferrite in the thickness direction, the ratio of the average grain size before processing to the average grain size after the processing (a change due to processing: average grain size before processing (nm)/average grain size after processing (nm)) is 5.0 or less.

Aspects of the present invention have been accomplished on the basis of these findings after further consideration.

Aspects of the present invention can be summarized as follows:

[1]A steel sheet including a base steel sheet, wherein the base steel sheet has a chemical composition containing, on a mass percent basis,

[2] The steel sheet according to [1], wherein the base steel sheet has a chemical composition further containing, on a mass percent basis, at least one selected from

[3] The steel sheet according to [1] or [2], including a galvanized layer as an outermost surface layer on one or both surfaces of the steel sheet.

[4] The steel sheet according to any one of [1] to [3], wherein

[5] The steel sheet according to any one of [1] to [4], including a metal coated layer formed on the base steel sheet on one or both surfaces of the steel sheet.

[6]A member including the steel sheet according to any one of [1] to [5].

[7]A method for producing a steel sheet including:

[8] The method for producing a steel sheet according to [7], including a galvanizing step of performing a galvanizing treatment on the steel sheet to form a galvanized layer on the steel sheet after the first cooling step and before the second cooling step.

[9] The method for producing a steel sheet according to [7] or [8], wherein the annealing in the annealing step is performed in an atmosphere with a dew point of −30° C. or more.

[10] The method for producing a steel sheet according to any one of [7] to [9], including a metal coating step of performing metal coating on one or both surfaces of the steel sheet to form a metal coated layer after the pickling step and before the annealing step.

[11]A method for producing a member, including a step of subjecting the steel sheet according to any one of [1] to [5] to at least one of forming and joining to produce a member.

Aspects of the present invention provide a steel sheet with a tensile strength TS of 1180 MPa or more, high yield stress YS, high press formability (ductility, flangeability, and bendability), and fracture resistance characteristics (bending fracture characteristics and axial compression characteristics) at the time of compression.

Furthermore, a member including a steel sheet according to aspects of the present invention as a material has high strength and enhanced crashworthiness and can therefore be extremely advantageously applied to an impact energy absorbing member or the like of an automobile.

Aspects of the present invention are described on the basis of the following embodiments.

A steel sheet according to aspects of the present invention is a steel sheet including a base steel sheet, wherein the base steel sheet has a chemical composition containing, on a mass percent basis, C: 0.050% or more and 0.400% or less, Si: more than 0.75% and 3.00% or less, Mn: 2.00% 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: 0.010% or more and 2.000% or less, and N: 0.0100% or less, with the remainder being Fe and incidental impurities, the base steel sheet has a steel microstructure in which an area fraction of bainitic ferrite: 3.0% or more and 20.0% or less, an area fraction of tempered martensite: 40.0% or more and 90.0% or less, an area fraction of retained austenite: more than 3.0% and 15.0% or less, a concentration of carbon in retained austenite: 0.60% by mass or more and 1.30% by mass or less, an area fraction of fresh martensite: 10.0% or less, and a density of carbides in tempered martensite: 8.0 particles/μmor less, an amount of diffusible hydrogen in the base steel sheet is 0.50 ppm by mass or less, a V-VDA bending test is performed to a maximum load point, in an L cross section, a crack has a length of 400 μm or less, in a region formed from each position on a starting line present from a starting point of a bending peak on an outside of a VDA bend to a position of 50 μm in a thickness direction up to a position of 50 μm on each side of the starting line perpendicular to the starting line, with respect to an average grain size of bainitic ferrite in the thickness direction, a ratio of the average grain size before processing to the average grain size after the processing is 5.0 or less, and the steel sheet has a tensile strength of 1180 MPa or more.

The steel sheet may have a galvanized layer as an outermost surface layer on one or both surfaces of the steel sheet. A steel sheet with a galvanized layer may be a galvanized steel sheet.

First, the chemical composition of a base steel sheet of a steel sheet according to an embodiment of the present invention is described. The unit in the chemical composition is “% by mass” and is hereinafter expressed simply in “%” unless otherwise specified.

C is an element effective in forming appropriate amounts of fresh martensite, tempered martensite, bainitic ferrite, and retained austenite and ensuring a tensile strength TS of 1180 MPa or more and high YS. A C content of less than 0.050% results in an increase in the area fraction of ferrite and makes it difficult to achieve a TS of 1180 MPa or more. This also reduces YS.

On the other hand, a C content of more than 0.400% results in an excessive increase in the concentration of carbon in retained austenite. This greatly increases the hardness of fresh martensite formed by deformation-induced transformation when a steel sheet is punched in a hole expansion test or is subjected to V-bending in a V-VDA test, and subsequently promotes void formation and crack growth, resulting in undesired λ and S.

Thus, the C content is 0.050% or more and 0.400% or less. The C content is preferably 0.100% or more. The C content is preferably 0.300% or less.

Si: More than 0.75% and 3.00% or Less