Steel sheet, member, and method for producing them

Patent metadata

Field	Value
Publication number	US-12480175-B2
Application number	US-202117915696-A
Country	US
Kind code	B2
Filing date	Mar 25, 2021
Priority date	Mar 31, 2020
Publication date	Nov 25, 2025
Grant date	Nov 25, 2025

Abstract

A steel sheet having a tensile strength (TS) of 780 MPa or more and less than 1180 MPa, high LME resistance, and good weld fatigue properties. The steel sheet has a specific chemical composition and a specific steel microstructure. Crystal grains containing an oxide of Si and/or Mn in a region within 4.9 μm in a thickness direction from a surface of the steel sheet have an average grain size in the range of 3 to 10 μm, the lowest Si concentration L Si and the lowest Mn concentration L Mn in the region within 4.9 μm in the thickness direction from the surface of the steel sheet and a Si concentration T Si and a Mn concentration T Mn at a quarter thickness position of the steel sheet satisfy a specified formula.

First claim

The invention claimed is:

1 . A steel sheet having a chemical composition comprising, by mass %:
Si: 0.3% to 2.0%; Mn: 1.0% or more and less than 2.70%; C: 0.12% to 0.40%; P: 0.05% or less; S: 0.02% or less; Al: 0.01% to 2.0%; N: 0.01% or less; optionally at least one selected from the group consisting of Nb: 0.50% or less, Cr: 1.0% or less, Mo: 0.50% or less, B: 0.005% or less, and Ti: 0.05% or less; optionally at least one selected from the group consisting of Cu, Ni, Sn, As, Sb, Ca, Mg, Pb, Co, Ta, W, REM, Zn, V, Sr, Cs, Hf, and Zr, in a total amount of 0.1% or less; optionally an equivalent carbon content Ceq in a range of 0.458% or more and less than 0.659%; and the remainder being Fe and incidental impurities, wherein the steel sheet has a steel microstructure including a bainitic ferrite area fraction in a range of 10% to 35%, a fresh martensite area fraction in a range of 2% to 15%, a retained austenite area fraction in a range of 5% to 20%, and a ferrite area fraction in a range of 45% to 70%, a total fraction of the fresh martensite and the retained austenite adjacent to the ferrite is 90% or less of a total area fraction of the fresh martensite and the retained austenite, crystal grains containing an oxide of at least one of Si and Mn in a region within 4.9 μm in a thickness direction from a surface of the steel sheet have an average grain size in a range of 3 to 10 μm, the following formula (1) is satisfied:
L Si +L Mn ≤( T Si +T Mn )/4  (1)

where L Si is a lowest Si concentration and L Mn is a lowest Mn concentration in the region within 4.9 μm in the thickness direction from the surface of the steel sheet, and T Si is a Si concentration and T Mn is a Mn concentration at a quarter thickness position of the steel sheet, and
the steel sheet has a tensile strength (TS) in a range of 780 MPa or more and less than 1180 MPa.

2 . The steel sheet according to claim 1 , wherein the steel sheet comprises a soft layer with a thickness in a range of 1.0 to 50.0 μm in the thickness direction from the surface of the steel sheet, the soft layer being a region with hardness corresponding to 65% or less of hardness at a quarter thickness position from the surface of the steel sheet.

3 . The steel sheet according to claim 2 , wherein the steel sheet comprises a hot-dip galvanized layer or a hot-dip galvannealed layer on the surface of the steel sheet.

4 . The steel sheet according to claim 1 , wherein the steel sheet comprises a hot-dip galvanized layer or a hot-dip galvannealed layer on the surface of the steel sheet.

5 . A member produced by performing at least one of forming and welding on the steel sheet according to claim 4 .

6 . A member produced by performing at least one of forming and welding on the steel sheet according to claim 1 .

7 . A member produced by performing at least one of forming and welding on the steel sheet according to claim 2 .

8 . A member produced by performing at least one of forming and welding on the steel sheet according to claim 3 .

9 . A method for producing a steel sheet according to claim 1 , the method comprising:
a hot-rolling step of hot-rolling a steel slab with the chemical composition at a cumulative strain in a range of 0.10 to 0.80 in final two rolling stages followed by coiling at a coiling temperature in a range of 470° C. to 800° C.; a cold-rolling step of cold-rolling a hot-rolled steel sheet formed in the hot-rolling step; an annealing step of holding a cold-rolled steel sheet formed in the cold-rolling step at a dew-point temperature in a range of −50° C. to 0° C. and at an annealing temperature in a range of 750° C. to 900° C., cooling the cold-rolled steel sheet to a cooling stop temperature in a range of 150° C. to 340° C., and bending and unbending the cold-rolled steel sheet three to eight times in total with a roller having a radius in a range of 100 to 1000 mm while cooling from the annealing temperature to the cooling stop temperature; a reheating step of reheating the steel sheet after the annealing step to a reheating temperature in a range of 350° C. to 600° C. and holding the reheating temperature; and optionally a plating step of performing hot-dip galvanizing or hot-dip galvannealing on the steel sheet after the reheating step.

10 . A method for producing a steel sheet according to claim 1 , the method comprising:
a hot-rolling step of hot-rolling a steel slab with the chemical composition at a cumulative strain in a range of 0.10 to 0.80 in final two rolling stages followed by coiling at a coiling temperature in a range of 470° C. to 800° C.; a cold-rolling step of cold-rolling a hot-rolled steel sheet formed in the hot-rolling step; an annealing step of holding a cold-rolled steel sheet formed in the cold-rolling step at a dew-point temperature in a range of −50° C. to 0° C. and at an annealing temperature in a range of 750° C. to 900° C., cooling the cold-rolled steel sheet to a cooling stop temperature in a range of 350° C. to 500° C., and bending and unbending the cold-rolled steel sheet three to eight times in total with a roller having a radius in a range of 100 to 1000 mm while cooling from the annealing temperature to the cooling stop temperature; a plating step of performing hot-dip galvanizing or hot-dip galvannealing on the steel sheet after the annealing step; and a reheating step of cooling the steel sheet after the plating step to a cooling stop temperature in a range of 50° C. to 350° C., reheating the steel sheet to a reheating temperature exceeding the cooling stop temperature and in a range of 300° C. to 500° C., and holding the reheating temperature.

11 . A method for producing a member, the method comprising the step of performing at least one of forming and welding on the steel sheet produced by the method for producing a steel sheet according to claim 9 .

12 . A method for producing a member, the method comprising the step of performing at least one of forming and welding on the steel sheet produced by the method for producing a steel sheet according to claim 10 .

13 . A steel sheet having a chemical composition comprising, by mass %:
Si: 0.3% to 2.0%; Mn: 1.0% or more and less than 2.70%; C: 0.12% to 0.40%; P: 0.05% or less; S: 0.02% or less; Al: 0.01% to 2.0%; N: 0.01% or less; optionally at least one selected from the group consisting of Nb: 0.50% or less, Cr: 1.0% or less, Mo: 0.50% or less, B: 0.005% or less, and Ti: 0.05% or less; optionally at least one selected from the group consisting of Cu, Ni, Sn, As, Sb, Ca, Mg, Pb, Co, Ta, W, REM, Zn, V, Sr, Cs, Hf, and Zr, in a total amount of 0.1% or less; optionally an equivalent carbon content Ceq in a range of 0.458% or more and less than 0.659%; and the remainder being Fe and incidental impurities, wherein the steel sheet has a steel microstructure including a bainitic ferrite area fraction in a range of 10% to 35%, a fresh martensite area fraction in a range of 2% to 15%, a retained austenite area fraction in a range of 5% to 20%, and a ferrite area fraction in a range of 45% to 70%, a total fraction of the fresh martensite and the retained austenite adjacent to the ferrite is 90% or less of a total area fraction of the fresh martensite and the retained austenite, crystal grains containing an oxide of at least one of Si and Mn in a region within 15.0 μm in a thickness direction from a surface of the steel sheet have an average grain size in a range of 3 to 10 μm, the following formula (1) is satisfied:
L Si +L Mn ≤( T Si +T Mn )/4  (1)

where L Si is a lowest Si concentration and L Mn is a lowest

Assignees

• Jfe Steel Corp

Inventors

• Yang Lingling
• Kawasaki Yoshiyasu
• Minami Hidekazu
• Nakagaito Tatsuya
• TERASHIMA Shotaro
• Yamamoto Shunsuke
• Hoshino Katsuya
• Takeda Yuki

Classifications

• C21D8/02

  during manufacturing of plates or strips (C21D8/12 takes precedence) · CPC title

• C23C2/28

  Thermal after-treatment, e.g. treatment in oil bath · CPC title

• C23C2/29

  Cooling or quenching · CPC title

• C23C2/40

  Plates; Strips · CPC title

• C23C2/06

  Zinc or cadmium or alloys based thereon · CPC title