Steel Sheet, Member, and Methods for Producing Same

This is the U.S. National Phase application of PCT/JP2023/006924 filed Feb. 27, 2023 which claims priority to PCT/JP2022/019991 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.

Automotive steel sheets have been reinforced to achieve both the reduction of COemissions due to an improvement of fuel efficiency by reducing the thickness and weight of steel sheets used in automobile bodies and an improvement of crash safety. Furthermore, new laws and regulations are continuously introduced. Thus, for the purpose of increasing the strength of an automobile body, high-strength steel sheets, particularly high-strength steel sheets with a tensile strength (hereinafter also referred to simply as TS) of 780 MPa or more, are increasingly applied to main structural members and reinforcing members (hereinafter also referred to as automobile frame structural members or the like) to be assembled to frames of automobile cabins. Furthermore, high-strength steel sheets used for frame structural members or the like of automobiles are required to have high member strength during press forming. To increase the strength of parts, for example, it is effective to increase the yield ratio (hereinafter also referred to simply as YR) obtained by dividing the yield stress (hereinafter also referred to simply as YS) of a steel sheet by TS. This increases the impact absorbed energy in case of a vehicle collision (hereinafter also referred to simply as impact absorbed energy). Furthermore, among frame structural members and the like of automobiles, for example, crash boxes and the like, have bent portions. From the perspective of press formability, therefore, a steel sheet with high bendability is preferably applies to such parts. Furthermore, from the perspective of anti-rust performance of an automobile body, a steel sheet serving as a material of an automobile body parts is often galvanized. Thus, the development of a hot-dip galvanized steel sheet with high press formability and enhanced crashworthiness in addition to high strength has been desired.

For example, Patent Literature 1 discloses, as such a steel sheet serving as a material of automobile body parts, 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 containing, 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 λ 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.

Patent Literature 4 discloses a hot-dip galvannealed steel sheet having a hot-dip galvannealed layer on the surface of the steel sheet, wherein the steel sheet has a chemical composition of, on a mass percent basis, C: 0.03% or more and 0.35% or less, Si: 0.005% or more and 2.0% or less, Mn: 1.0% or more and 4.0% or less, P: 0.0004% or more and 0.1% or less, S: 0.02% or less, sol. Al: 0.0002% or more and 2.0% or less, and N: 0.01% or less, the remainder being Fe and impurities, the concentrated portion average interval is 1000 μm or less at a depth of 50 μm from the surface of the steel sheet, the concentrated portion average interval being an average interval in the direction perpendicular to the rolling direction of a concentrated portion in which Mn and/or Si spread in the rolling direction is concentrated, the number density of cracks with a depth of 3 μm or more and 10 μm or less on the surface of the steel sheet is 3/mm or more and 1000/mm or less, the steel sheet has a steel microstructure containing, on an area percent basis, bainite: 60% or more, retained austenite: 1% or more, martensite: 1% or more, and ferrite: 2% or more and less than 20%, and having a superhard phase average interval, which is the average closest distance of martensite and retained austenite, of 20 μm or less, and the hot-dip galvannealed steel sheet has mechanical characteristics with a tensile strength (TS) of 780 MPa or more.

Incidentally, although a steel sheet with a tensile strength TS (hereinafter also referred to simply as TS) of more than 590 MPa has been applied to a structural member of an automobile exemplified by a center pillar, only a steel sheet with a TS of 590 MPa is applied to an impact energy absorbing member of an automobile exemplified by a front side member or a rear side member.

Thus, to increase absorbed energy in case of a collision (hereinafter also referred to as impact absorbed energy), it is effective to improve the yield stress YS (hereinafter also referred to simply as YS) and the yield ratio YR (hereinafter also referred to simply as YR). However, a steel sheet with higher YS and YR 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 4 have a TS of 780 MPa or more, high YS and YR, high press formability (ductility, flangeability, and bendability), and fracture resistance characteristics (bending fracture characteristics and axial compression characteristics) in case of a collision.

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 780 MPa or more, high yield stress YS and yield ratio YR, high press formability (ductility, flangeability, and bendability), and fracture resistance characteristics (bending fracture characteristics and axial compression characteristics) in case of a collision, and a 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 “high yield stress YS and yield ratio YR” 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 (El) 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” means that R (critical bending radius)/t (thickness) measured in the V-bending test according to JIS Z 2248 (2014) satisfies the following formulae (A) or (B) depending on TS.

The phrase “good axial compression characteristics” means that the critical spacer thickness (ST) in a U-bending+tight bending bending test satisfies the following formula (A) or (B) depending on TS.

The phrase “good axial compression characteristics” means that the stroke at the maximum load (SFmax) measured in a V-bending+orthogonal VDA bending test satisfies the following formulae (A) or (B) depending on TS.

The phrase “good axial compression characteristics” means that, after an axial compression test, fracture (appearance crack) occurs at one 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).

The phrase “good bending fracture characteristics” means that the critical spacer thickness (ST) in the U-bending+tight bending bending test satisfies the formula (A) or (B) depending on TS, and the stroke at the maximum load (SFmax) measured in the V-bending+orthogonal VDA bending test satisfies the formula (A) or (B) depending on TS.

The El (ductility), A (stretch flangeability), and R/t (bendability) are characteristics indicating the ease of forming a steel sheet during press forming (the degree of freedom of forming for press forming without cracking). On the other hand, the U-bending+tight bending test is a test simulating the deformation and fracture behavior of a vertical wall portion in a collision test, and the critical spacer thickness (ST) measured in the U-bending+tight bending test is a measure indicating the resistance to cracking of a steel sheet and a member of an automobile body in case of a collision (crashworthiness for absorbing impact energy without fracture).

The V-bending+orthogonal 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 (SFmax) measured in the V-bending+orthogonal VDA bending test is a measure indicating the resistance to cracking of an energy absorbing member.

As a result of extensive studies to achieve the objects, the present inventors have found the following.

(1) A TS of 780 MPa or more can be ensured with specified components by controlling the area fraction of tempered martensite to 10.0% or more, decreasing an island-like hard second phase (martensite+retained austenite) in contact with a ferrite grain boundary, and increasing the ratio of an isolated fine island-like hard second phase (martensite+retained austenite) in a ferrite grain.

(2) High YS and YR can be ensured with specified components by controlling the area fraction of tempered martensite to 10.0% or more and decreasing an island-like hard second phase (martensite+retained austenite) in contact with a ferrite grain boundary.

(3) Ductility (correlated with stretch formability, which is one mode of press formability) can be improved with specified components by controlling the area fraction of ferrite to 20.0% or more.

(4) Flangeability correlated with stretch flangeability, which is one mode of press formability, can be improved with specified components by controlling the area fraction of fresh martensite to 15.0% or less, the area fraction of retained austenite to 3.0% or less, and the area fraction of tempered martensite to 10.0% or more, and increasing the ratio of an isolated fine island-like hard second phase (martensite+retained austenite) in a ferrite grain.

(5) Bendability, which is one mode of press formability, can be improved with specified components by controlling the area fraction of fresh martensite to 15.0% or less, the area fraction of retained austenite to 3.0% or less, and the area fraction of tempered martensite to 10.0% or more, and increasing the ratio of an isolated fine island-like hard second phase (martensite+retained austenite) in a ferrite grain.

(6) The formation of hard fresh martensite by deformation-induced transformation of retained austenite during primary processing, such as punching or press forming, and the void formation and crack growth in a subsequent test can be suppressed at Si: 0.75% by mass or less and with specified components by controlling the area fraction of retained austenite to 3.0% or less. Furthermore, the critical spacer thickness (ST) measured in a U-bending+tight bending test simulating the deformation and fracture behavior of a vertical wall portion in a collision test, and the stroke at the maximum load (SFmax) measured in a V-bending+orthogonal VDA bending test simulating the deformation and fracture behavior of a bending ridge line portion in a collision test, which are measures of the crashworthiness of a steel sheet and a member of an automobile body in case of a collision, can be improved by controlling the area fraction of tempered martensite to 10.0% or more and increasing the ratio of an isolated fine island-like hard second phase (martensite+retained austenite) in a ferrite grain.

The present disclosure is based on these findings. The gist of the present disclosure is 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 chemical composition further contains, on a mass percent basis, at least one element 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 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 780 MPa or more, high yield stress YS and yield ratio YR, high press formability (ductility, flangeability, and bendability), and fracture resistance characteristics (bending fracture characteristics and axial compression characteristics) in case of a collision.