Steel sheet, member, and methods for manufacturing the same

Patent metadata

Field	Value
Publication number	US-12565690-B2
Application number	US-202117800650-A
Country	US
Kind code	B2
Filing date	Feb 24, 2021
Priority date	Feb 28, 2020
Publication date	Mar 3, 2026
Grant date	Mar 3, 2026

Abstract

A steel sheet has a specific chemical composition and a steel microstructure including, in terms of area fraction, ferrite: 60% or more and 85% or less, bainite: 3% or more and 15% or less, retained austenite: 3% or more and 15% or less, fresh martensite: 3% or more and 15% or less, and the remainder: 5% or less. Cementite particles are present in the retained austenite, a ratio of an area fraction of the cementite particles in the retained austenite to an area fraction of the retained austenite is 5% or more and 25% or less, and the steel sheet has a tensile strength of 590 MPa or more and less than 780 MPa.

First claim

The invention claimed is:

1 . A steel sheet comprising:
a chemical composition containing, by mass %, C: 0.05% or more and 0.18% or less, Si: 0.01% or more and 2.0% or less, Al: 0.01% or more and 2.0% or less, a total of Si and Al: 0.7% or more and 2.5% or less, Mn: 0.5% or more and 2.3% or less, P: 0.1% or less, S: 0.02% or less, and N: 0.010% or less, and optionally containing at least one selected from Cr, V, Mo, Ni, and Cu in a total amount of 1.0% or less, optionally containing at least one selected from Ti: 0.20% or less and Nb: 0.20% or less, optionally containing B: 0.005% or less, optionally containing at least one selected from Ca: 0.005% or less and REM: 0.005% or less, optionally containing at least one selected from Sb: 0.05% or less and Sn: 0.05% or less, with the balance being Fe and incidental impurities; and
a steel microstructure including, in terms of area fraction, ferrite: 60% or more and 85% or less, bainite: 3% or more and 15% or less, retained austenite: 3% or more and 15% or less, fresh martensite: 3% or more and 15% or less, and the remainder: 5% or less,
wherein cementite particles are present in the retained austenite, a ratio of an area fraction of the cementite particles in the retained austenite to an area fraction of the retained austenite is 5% or more and 25% or less, and
the steel sheet has a tensile strength of 590 MPa or more and less than 780 MPa.

2 . The steel sheet according to claim 1 , wherein the cementite particles in the retained austenite have an average major axis of 30 nm or more and 400 nm or less.

3 . The steel sheet according to claim 1 , further comprising a hot-dip galvanized layer or a hot-dip galvannealed layer on a surface of the steel sheet.

4 . The steel sheet according to claim 1 , wherein the steel microstructure includes, in terms of area fraction, ferrite: 70% or more and 85% or less.

5 . The steel sheet according to claim 2 , further comprising a hot-dip galvanized layer or a hot-dip galvannealed layer on a surface of the steel sheet.

6 . A member obtained by subjecting the steel sheet according to claim 1 to at least one of forming and welding.

7 . A member obtained by subjecting the steel sheet according to claim 2 to at least one of forming and welding.

8 . A member obtained by subjecting the steel sheet according to claim 3 to at least one of forming and welding.

9 . A member obtained by subjecting the steel sheet according to claim 5 to at least one of forming and welding.

10 . A method for manufacturing the steel sheet according to claim 1 , the method comprising:
hot-rolling and cold-rolling a slab having the chemical composition; subsequently performing holding at an annealing temperature of 700° C. or higher and 950° C. or lower for 30 seconds or more and 1000 seconds or less; performing cooling from the annealing temperature to a cooling stop temperature of 150° C. or higher and 420° C. or lower at an average cooling rate of 10° C./s or higher; subsequently performing first holding under conditions in a temperature range of 380° C. or higher and 420° C. or lower for 10 seconds or more and 500 seconds or less; and further performing second holding under conditions of a temperature X in ° C. and a holding time Y in seconds that satisfy formulae 1 to 3 below:
10000≤(273+ X )(12+log Y )≤11000  Formula 1:

440≤ X≤ 540  Formula 2:

Y≤ 200  Formula 3.

11 . The method for manufacturing the steel sheet according to claim 10 , wherein an average heating rate from a holding temperature in the first holding to the temperature X° C. in the second holding is 3° C./s or higher.

12 . The method for manufacturing the steel sheet according to claim 10 , wherein an average heating rate from a holding temperature in the first holding to the temperature X° C. in the second holding is 10° C./s or higher.

13 . The method for manufacturing the steel sheet according to claim 10 , comprising, between the first holding and the second holding or after completion of the second holding, forming a hot-dip galvanized layer or a hot-dip galvannealed layer on a surface of the steel sheet.

14 . The method for manufacturing the steel sheet according to claim 11 , comprising, between the first holding and the second holding or after completion of the second holding, forming a hot-dip galvanized layer or a hot-dip galvannealed layer on a surface of the steel sheet.

15 . The method for manufacturing the steel sheet according to claim 12 , comprising, between the first holding and the second holding or after completion of the second holding, forming a hot-dip galvanized layer or a hot-dip galvannealed layer on a surface of the steel sheet.

16 . A method for manufacturing a member, the method comprising a step of subjecting a steel sheet manufactured by the method for manufacturing the steel sheet according to claim 10 to at least one of forming and welding.

17 . A method for manufacturing a member, the method comprising a step of subjecting a steel sheet manufactured by the method for manufacturing the steel sheet according to claim 11 to at least one of forming and welding.

18 . A method for manufacturing a member, the method comprising a step of subjecting a steel sheet manufactured by the method for manufacturing the steel sheet according to claim 12 to at least one of forming and welding.

19 . A method for manufacturing a member, the method comprising a step of subjecting a steel sheet manufactured by the method for manufacturing the steel sheet according to claim 13 to at least one of forming and welding.

20 . A method for manufacturing a member, the method comprising a step of subjecting a steel sheet manufactured by the method for manufacturing the steel sheet according to claim 14 to at least one of forming and welding.

21 . A method for manufacturing a member, the method comprising a step of subjecting a steel sheet manufactured by the method for manufacturing the steel sheet according to claim 15 to at least one of forming and welding.

Assignees

• Jfe Steel Corp

Inventors

• Wada Yusuke
• Nakagaito Tatsuya
• TERASHIMA Shotaro
• Yang Lingling
• Yokota Takeshi
• Yamamoto Shunsuke
• Takeda Yuki

Classifications

• C23C2/40

  Plates; Strips · CPC title

• C22C38/06

  containing aluminium · CPC title

• C22C38/04 Primary

  containing manganese · CPC title

• C22C38/02

  containing silicon · CPC title

• C22C38/002

  containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60 · CPC title