High strength steel sheet, high strength member, and methods for manufacturing the same

Patent metadata

Field	Value
Publication number	US-12392007-B2
Application number	US-202017630243-A
Country	US
Kind code	B2
Filing date	Jul 29, 2020
Priority date	Jul 31, 2019
Publication date	Aug 19, 2025
Grant date	Aug 19, 2025

Abstract

The high strength steel sheet of the present invention has a specific chemical composition, and contains, in terms of area fraction relative to an entire steel microstructure, 30% or more and 100% or less ferrite, 0% or more and 70% or less martensite, and less than 20% in total of pearlite, bainite and retained austenite, and the ferrite contains, in terms of area fraction relative to an entire microstructure, 0% or more and 10% or less non-recrystallized ferrite, with a difference of the area fraction of the non-recrystallized ferrite in the longitudinal direction of the steel sheet of 5% or smaller.

First claim

The invention claimed is:

1. A high strength steel sheet having a tensile strength of 590 MPa or larger and a chemical composition in mass % containing:
C: 0.06% or more and 0.14% or less,
Si: 0.1% or more and 1.5% or less,
Mn: 1.4% or more and 2.2% or less,
P: 0.05% or less,
S: 0.0050% or less,
Al: 0.01% or more and 0.20% or less,
N: 0.10% or less,
Nb: 0.015% or more and 0.060% or less, and
Ti: 0.001% or more and 0.030% or less, optionally containing one or more of following (A) to (C);
(A) one of, or two or more of
Cr: 0.01% or more and 0.15% or less,
Mo: 0.01% or more and less than 0.10%, and
V: 0.001% or more and 0.065% or less,

(B) B: 0.0001% or more and less than 0.002%,
(C) one of or two of
Cu: 0.001% or more and 0.2% or less, and
Ni: 0.001% or more and 0.1% or less,

contents of S, N and Ti satisfying Formula (1) below,
a balance being Fe and an inevitable impurity,
comprising, in terms of area fraction relative to an entire steel microstructure, 30% or more and 100% or less ferrite, 0% or more and 70% or less martensite, and less than 20% in total of pearlite, bainite and retained austenite, and the ferrite containing, in terms of area fraction relative to an entire microstructure, 0% or more and 10% or less non-recrystallized ferrite, with a difference between a maximum value and a minimum value of the area fraction of the non-recrystallized ferrite in a longitudinal direction of the steel sheet of 5% or below:
[% Ti]−(48/14)[% N]−(48/32)[% S]≤0,  Formula (1):

in Formula (1), [% Ti] represents content (mass %) of component element Ti, [% N] represents content (mass %) of component element N, and [% S] represents content (mass %) of component element S.

2. The high strength steel sheet according to claim 1 , having a plating layer on a surface the steel sheet.

3. A high strength member comprising the high strength steel sheet according to claim 1 subjected to at least either forming or welding.

4. A high strength member comprising the high strength steel sheet according to claim 2 subjected to at least either forming or welding.

5. A method for manufacturing a high strength steel sheet according to claim 1 , comprising: a hot rolling process in which a steel slab having the chemical composition is heated at a heating temperature T (° C.) that satisfies Formula (2) below for 1.0 hour or longer, then cooled from the heating temperature down to a rolling start temperature at an average cooling rate of 2° C./sec or faster, then finish rolled at a finisher delivery temperature of 850° C. or higher, then cooled from the finisher delivery temperature down to 650° C. or lower at an average cooling rate of 10° C./sec or faster, and then coiled at 650° C. or lower; and
an annealing process in which the hot rolled steel sheet obtained in the hot rolling process is heated up to an annealing temperature which is A C1 transformation temperature or higher and (A C3 transformation temperature+20° C.) or lower, at an average heating rate from 600° C. to 700° C. of 8° C./sec or slower, held at the annealing temperature for a hold time t (second) that satisfies Formula (3) below, and then cooled:
0.80×(2.4−6700/T)≤log {[% Nb]×([% C]+12/14[% N])}≤0.65×(2.4−6700/T)  Formula (2):

in Formula (2), T represents heating temperature (° C.) of the steel slab, [% Nb] represents content (mass %) of component element Nb, [% C] represents content (mass %) of component element C, and [% N] represents content (mass %) of component element N; and
1500≤(AT+273)×log t< 5000  Formula (3):

in Formula (3), AT represents annealing temperature (° C.), and t represents hold time (second) at the annealing temperature.

6. A method for manufacturing a high strength steel sheet according to claim 1 , comprising: a hot rolling process in which a steel slab having the chemical composition is heated at a heating temperature T (° C.) that satisfies Formula (2) below for 1.0 hour or longer, then cooled from the heating temperature down to a rolling start temperature at an average cooling rate of 2° C./sec or faster, then finish rolled at a finisher delivery temperature of 850° C. or higher, then cooled from the finisher delivery temperature down to 650° C. or lower at an average cooling rate of 10° C./sec or faster, and then coiled at 650° C. or lower;
a cold rolling process in which the hot rolled steel sheet obtained in the hot rolling process is cold-rolled; and
an annealing process in which the cold rolled steel sheet obtained in the cold rolling process is heated up to an annealing temperature which is A C1 transformation temperature or higher and (A C3 transformation temperature+20° C.) or lower, at an average heating rate from 600° C. to 700° C. of 8° C./sec or slower, held at the annealing temperature for a hold time t (second) that satisfies Formula (3) below, and then cooled:
0.80×(2.4−6700/T)≤log {[% Nb]×([% C]+12/14[% N])}≤0.65×(2.4−6700/T)  Formula (2):

in Formula (2), T represents heating temperature (° C.) of the steel slab, [% Nb] represents content (mass %) of component element Nb, [% C] represents content (mass %) of component element C, and [% N] represents content (mass %) of component element N; and
1500≤(AT+273)×log t< 5000  Formula (3):

in Formula (3), AT represents annealing temperature (° C.), and t represents hold time (second) at the annealing temperature.

7. The method for manufacturing a high strength steel sheet according to claim 5 , further comprising a plating process for providing plating, following the annealing process.

8. The method for manufacturing a high strength steel sheet according to claim 6 , further comprising a plating process for providing plating, following the annealing process.

9. A method for manufacturing a high strength member, comprising subjecting the high strength steel sheet manufactured by the method for manufacturing a high strength steel sheet according to claim 5 , to at least either forming or welding.

10. A method for manufacturing a high strength member, comprising subjecting the high strength steel sheet manufactured by the method for manufacturing a high strength steel sheet according to claim 6 , to at least either forming or welding.

11. A method for manufacturing a high strength member, comprising subjecting the high strength steel sheet manufactured by the method for manufacturing a high strength steel sheet according to claim 7 , to at least either forming or welding.

12. A method for manufacturing a high strength member, comprising subjecting the high strength steel sheet manufactured by the method for manufacturing a high strength steel sheet according to claim 8 , to at least either forming or welding.

Assignees

• Jfe Steel Corp

Inventors

• Hirashima Takuya
• Hashimoto Yu
• Kaneko Shinjiro
• Ono Yoshihiko

Classifications

• C21D8/02

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

• C23C2/26

  After-treatment (C23C2/14 takes precedence) · CPC title

• C23C2/28

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

• C23C2/022

  by heating · CPC title

• C23C2/02

  Pretreatment of the material to be coated, e.g. for coating on selected surface areas (C23C2/30 takes precedence) · CPC title