High-strength cold rolled steel sheet and method for manufacturing the same

Patent metadata

Field	Value
Publication number	US-2020071802-A1
Application number	US-201816493166-A
Country	US
Kind code	A1
Filing date	Mar 8, 2018
Priority date	Mar 13, 2017
Publication date	Mar 5, 2020
Grant date	—

Abstract

A high-strength cold rolled steel sheet having mechanical characteristics having a tensile strength of not less than 780 MPa, a yield ratio of not more than 70%, and a small in-plane anisotropy of a tensile characteristicis obtained by hot rolling a steel slab comprising by mass % C: 0.07 to 0.12%, Si: not more than 0.7%, Mn: 2.2 to 2.8% and Ti and Nb: 0.02 to 0.08% in total, and cold rolling the sheet, followed by continuous annealing to form a steel texture comprised of ferrite having an area ratio of 40 to 80% with respect to the whole texture, and a second phase constituted by tempered martensite, fresh martensite and bainite, wherein the total area ratio of the bainite and the tempered martensite to the second phase is 50 to 80%, and the aspect ratio of the fresh martensite is in the range of 1.0 to 1.5.

First claim

1 . A high-strength cold rolled steel sheet characterized by having:
a chemical composition comprising C: 0.07 to 0.12 mass %, Si: not more than 0.7 mass %, Mn: 2.2 to 2.8 mass %, P: not more than 0.1 mass %, S: not more than 0.01 mass %, Al: 0.01 to 0.1 mass %, N: not more than 0.015 mass %, one or two selected from Ti and Nb: 0.02 to 0.08 mass % in total, and the residue being Fe and inevitable impurities; a steel texture comprising ferrite having an area ratio of 40 to 80% with respect to the whole texture, and a second phase constituted by tempered martensite, fresh martensite and bainite, wherein the total area ratio of the bainite and the tempered martensite to the second phase is 50 to 80%, and the aspect ratio of the fresh martensite is in the range of 1.0 to 1.5; and mechanical characteristics having a tensile strength of not less than780 MPa, a yield ratio of not more than 70%, an absolute value of not more than 30 MPa as in-plane anisotropy ΔYS of yield stress defined according to the following equation (1), and an absolute value of not more than 30 MPa as in-plane anisotropy ΔTS of tensile strength defined according to the following equation (2):
|Δ YS |=( YS L −2× YS D +YS C )/2  (1)

|Δ TS |=( TS L −2× TS D +TS C )/2  (2)

, wherein YS L and TS L represent yield stress and tensile strength, respectively, in the rolling direction, YS C and TS C represent yield stress and tensile strength, respectively, in a direction perpendicular to the rolling direction, and YS D and TS D represent yield stress and tensile strength, respectively, in a direction of 45° with respect to the rolling direction.

2 . The high-strength cold rolled steel sheet according to claim 1 , wherein the average particle size of carbide in the bainite is not more than 0.3 μm, and the average particle size of the fresh martensite is not more than 1.0 μm.

3 . The high-strength cold rolled steel sheet according to claim 1 , wherein the high-strength cold rolled steel sheet further contains one or two or more selected from Cr: 0.05 to 1.0 mass %, Mo; 0.05 to 1.0 mass % and V: 0.01 to 0.1 mass %, in addition to the chemical composition.

4 . The high-strength cold rolled steel sheet according to claim 1 , wherein the high-strength cold rolled steel sheet further contains B: 0.0003 to 0.005 mass % in addition to the chemical composition.

5 . A method for manufacturing a high-strength cold rolled steel sheet, comprising hot rolling a steel slab having chemical composition comprising C: 0.07 to 0.12 mass %, Si: not more than 0.7 mass %, Mn: 2.2 to 2.8 mass %, P: not more than 0.1 mass %, S: not more than 0.01 mass %, Al: 0.01 to 0.1 mass %, N: not more than 0.015 mass %, one or two selected from Ti and Nb: 0.02 to 0.08 mass % in total, and the residue being Fe and inevitable impurities, cold rolling the sheet, and conducting continuous annealing to manufacture a high-strength cold rolled steel sheet, characterized in that
the continuous annealing includes soaking treatment for holding in a temperature range of Ac 3 −30° C. to Ac 3 +50° C. for not less than 60 seconds, primary cooling from the soaking temperature to not higher than 650° C. at an average cooling rate of 2 to 5° C./s, primary retention in a temperature range of 650 to 550° C. for 15 to 60 seconds, then secondary cooling from the retention temperature to a temperature range of not higher than 350° C. at an average cooling rate of 10 to 25° C./s, and secondary retention in a temperature range of 350 to 250° C. for 300 to 500 seconds, followed by tertiary cooling to thereby confer: a steel texture comprised of ferrite having an area ratio of 40 to 80% with respect to the whole texture, and a second phase constituted by tempered martensite, fresh martensite and bainite, wherein the total area ratio of the bainite and the tempered martensite to the second phase is 50 to 80%, and the aspect ratio of the fresh martensite is in the range of 1.0 to 1.5; and mechanical characteristics having a tensile strength of not less than 780 MPa, a yield ratio of not more than 70%, an absolute value of not more than 30 MPa as in-plane anisotropy ΔYS of yield stress defined according to the following equation (1), and an absolute value of not more than 30 MPa as in-plane anisotropy ΔTS of tensile strength defined according to the following expression (2):
|Δ YS |=( YS L −2× YS D +YS C )/2  (1)

|Δ TS |=( TS L −2× TS D +TS C )/2  (2)

wherein YS L and TS L represent yield stress and tensile strength, respectively, in the rolling direction, YS C and TS C represent yield stress and tensile strength, respectively, in a direction perpendicular to the rolling direction, and YS D and TS D represent yield stress and tensile strength, respectively, in a direction of 45° with respect to the rolling direction.

6 . The high-strength cold rolled steel sheet according to claim 2 , wherein the high-strength cold rolled steel sheet further contains one or two or more selected from Cr: 0.05 to 1.0 mass %, Mo: 0.05 to 1.0 mass % and V: 0.01 to 0.1 mass %, in addition to the chemical composition.

7 . The high-strength cold rolled steel sheet according to claim 2 , wherein the high-strength cold rolled steel sheet further contains B: 0.0003 to 0.005 mass % in addition to the chemical composition.

8 . The high-strength cold rolled steel sheet according to claim 3 , wherein the high-strength cold rolled steel sheet further contains B: Q.0003 to 0.005 mass % in addition to the chemical composition.

9 . The high-strength cold rolled steel sheet according to claim 4 , wherein the high-strength cold rolled steel sheet further contains B: 0.0003 to 0.005 mass % in addition to the chemical composition.

10 . The method for manufacturing a high-strength cold rolled steel sheet according to claim 5 , wherein the steel slab further contains one or two or more selected from Cr: 0.05 to 1.0 mass %, Mo: 0.05 to 1.0 mass % and V: 0.01 to 0.1 mass %, in addition to the chemical composition.

11 . The method for manufacturing a high-strength cold rolled steel sheet according to claim 5 , wherein the steel slab further contains B: 0.0003 to 0.005 mass % in addition to the chemical composition.

12 . The method for manufacturing a high-strength cold rolled steel sheet according to claim 10 , wherein the steel slab further contains B: 0.0003 to 0.005 mass % in addition to the chemical composition.

Assignees

• Jfe Steel Corp

Inventors

• Hirashima Takuya
• Kawamura Kenji
• Ono Yoshihiko
• Honda Yuma

Classifications

• C21D8/02

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

• C22C38/04 Primary

  containing manganese · CPC title

• C21D8/0236

  Cold rolling · CPC title

• C22C38/002

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

• C22C38/14

  containing titanium or zirconium · CPC title