Steel sheet, member, and methods for producing the same

Patent metadata

Field	Value
Publication number	US-12146204-B2
Application number	US-202017769829-A
Country	US
Kind code	B2
Filing date	Oct 23, 2020
Priority date	Oct 31, 2019
Publication date	Nov 19, 2024
Grant date	Nov 19, 2024

Abstract

The steel sheet of the present invention has a steel microstructure containing, in area fraction, martensite: 20% to 100%, ferrite: 0% to 80%, and another metal phase: 5% or less, in which, on a surface of the steel sheet, a ratio of dislocation density in metal phases at a widthwise edge of the steel sheet to dislocation density in the metal phases at a widthwise center of the steel sheet is 100% to 140%, and, at a thicknesswise center of the steel sheet, a ratio of dislocation density in the metal phases at the widthwise edge of the steel sheet to dislocation density in the metal phases at the widthwise center of the steel sheet is 100% to 140%. The maximum amount of warpage of the steel sheet when the steel sheet is sheared to a length of 1 m in a rolling direction is 15 mm or less.

First claim

The invention claimed is:

1. A steel sheet comprising a steel microstructure containing:
in area fraction, martensite: from 20% to 100%, ferrite: from 0% to 80%, and another metal phase: 5% or less;
in which, on a surface of the steel sheet, a ratio of a dislocation density in metal phases at a widthwise edge of the steel sheet to a dislocation density in the metal phases at a widthwise center of the steel sheet is from 100% to 140%; and
in which, at a thicknesswise center of the steel sheet, a ratio of a dislocation density in the metal phases at the widthwise edge of the steel sheet to a dislocation density in the metal phases at the widthwise center of the steel sheet is from 100% to 140%,
wherein the maximum amount of warpage of the steel sheet when the steel sheet is sheared to a length of 1 m in a rolling direction is 15 mm or less.

2. The steel sheet according to claim 1 , having a chemical composition containing, in mass %,
C: from 0.05% to 0.60%,
Si: from 0.01% to 2.0%,
Mn: from 0.1% to 3.2%,
P: 0.050% or less,
S: 0.0050% or less,
Al: from 0.005% to 0.10%, and
N: 0.010% or less, with the balance being Fe and incidental impurities.

3. The steel sheet according to claim 2 , wherein the chemical composition further contains at least one selected from following groups A to E consisting of:
Group A: in mass %, at least one selected from
Cr: 0.20% or less,
Mo: less than 0.15%, and
V: 0.05% or less;

Group B: in mass %, at least one selected from
Nb: 0.020% or less and
Ti: 0.020% or less;

Group C: in mass %, at least one selected from
Cu: 0.20% or less and
Ni: 0.10% or less;

Group D: in mass %,
B: less than 0.0020%;

Group E: in mass %, at least one selected from
Sb: 0.1% or less and
Sn: 0.1% or less.

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

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

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

7. A method for producing a steel sheet, the method comprising:
a hot rolling step of heating a steel slab having the chemical composition according to claim 2 and then hot-rolling the steel slab; and
an annealing step of holding a hot-rolled steel sheet obtained in the hot rolling step at an annealing temperature equal to or higher than A C1 temperature for 30 seconds or longer, then starting water quenching the hot-rolled steel sheet from a temperature equal to or higher than Ms temperature including water cooling to 100° C. or lower, and reheating the hot-rolled steel sheet to from 100° C. to 300° C.,
wherein, in a region in which a surface temperature of the steel sheet is equal to or lower than (Ms temperature+150° C.) during the water cooling in the water quenching in the annealing step, the steel sheet is restrained from front and back sides of the steel sheet using two rolls such that the following conditions (1) to (3) are satisfied, the two rolls being disposed with the steel sheet interposed therebetween:
(1) a depression amount of each of the two rolls is more than 0 mm and t mm or less, where t is a thickness of the steel sheet;
(2) Rn and rn are from 50 mm to 1000 mm, where Rn and rn are roll diameters of the respective two rolls; and
(3) an inter-roll distance between the two rolls is more than 0 mm and (Rn+rn+t)/16 mm or less.

8. A method for producing a steel sheet, the method comprising:
a hot rolling step of heating a steel slab having the chemical composition according to claim 3 and then hot-rolling the steel slab; and
an annealing step of holding a hot-rolled steel sheet obtained in the hot rolling step at an annealing temperature equal to or higher than A C1 temperature for 30 seconds or longer, then starting water quenching the hot-rolled steel sheet from a temperature equal to or higher than Ms temperature including water cooling to 100° C. or lower, and reheating the hot-rolled steel sheet to from 100° C. to 300° C.,
wherein, in a region in which a surface temperature of the steel sheet is equal to or lower than (Ms temperature+150° C.) during the water cooling in the water quenching in the annealing step, the steel sheet is restrained from front and back sides of the steel sheet using two rolls such that the following conditions (1) to (3) are satisfied, the two rolls being disposed with the steel sheet interposed therebetween:
(1) a depression amount of each of the two rolls is more than 0 mm and t mm or less, where t is a thickness of the steel sheet;
(2) Rn and rn are from 50 mm to 1000 mm, where Rn and rn are roll diameters of the respective two rolls; and
(3) an inter-roll distance between the two rolls is more than 0 mm and (Rn+rn+t)/16 mm or less.

9. A method for producing a steel sheet, the method comprising:
a hot rolling step of heating a steel slab having the chemical composition according to claim 2 and then hot-rolling the steel slab;
a cold rolling step of cold-rolling a hot-rolled steel sheet obtained in the hot rolling step; and
an annealing step of holding a cold-rolled steel sheet obtained in the cold rolling step at an annealing temperature equal to or higher than A C1 temperature for 30 seconds or longer, then starting water quenching the cold-rolled steel sheet from a temperature equal to or higher than Ms temperature including water cooling to 100° C. or lower, and reheating the cold-rolled steel sheet to from 100° C. to 300° C.,
wherein, in a region in which a surface temperature of the steel sheet is equal to or lower than (Ms temperature+150° C.) during the water cooling in the water quenching in the annealing step, the steel sheet is restrained from front and back sides of the steel sheet using two rolls such that the following conditions (1) to (3) are satisfied, the two rolls being disposed with the steel sheet interposed therebetween:
(1) a depression amount of each of the two rolls is more than 0 mm and t mm or less, where t is a thickness of the steel sheet;
(2) Rn and rn are from 50 mm to 1000 mm, where Rn and rn are roll diameters of the respective two rolls; and
(3) an inter-roll distance between the two rolls is more than 0 mm and (Rn+rn+t)/16 mm or less.

10. A method for producing a steel sheet, the method comprising:
a hot rolling step of heating a steel slab having the chemical composition according to claim 3 and then hot-rolling the steel slab;
a cold rolling step of cold-rolling a hot-rolled steel sheet obtained in the hot rolling step; and
an annealing step of holding a cold-rolled steel sheet obtained in the cold rolling step at an annealing temperature equal to or higher than A C1 temperature for 30 seconds or longer, then starting water quenching the cold-rolled steel sheet from a temperature equal to or higher than Ms temperature including water cooling to 100° C. or lower, and reheating the cold-rolled steel sheet to from 100° C. to 300° C.,
wherein, in a region in which a surface temperature of the steel sheet is equal to or lower than (Ms temperature+150° C.) during the water cooling in the water quenching in the annealing step, the steel sheet is restrained from front and back sides of the steel sheet using two rolls such that the following conditions (1) to (3) are satisfied, the two rolls being disposed with the steel sheet interposed therebetween:
(1) a depression amount of each of the two rolls is more than 0 mm and t mm or less, where t is a thickness of the steel sheet;
(2) Rn and rn are from 50 mm to 1000 mm, where Rn and rn are roll diameters of the respective two rolls; and
(3) an inter-roll distance between the two

Assignees

• Jfe Steel Corp

Inventors

• Hirashima Takuya
• Yoshioka Shimpei
• Kaneko Shinjiro
• Yoshimoto Soshi
• HASHIMUKAI Tomohiro

Classifications

• C21D8/02

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

• C21D8/00

  Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment (hardening articles or materials formed by forging or rolling with no further heating beyond that required for the formation C21D1/02) · CPC title

• C22C38/60

  containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur · CPC title

• C22C38/18

  containing chromium · CPC title

• C22C38/16

  containing copper · CPC title