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

Patent metadata

Field	Value
Publication number	US-10077486-B2
Application number	US-201414911059-A
Country	US
Kind code	B2
Filing date	Jul 18, 2014
Priority date	Aug 9, 2013
Publication date	Sep 18, 2018
Grant date	Sep 18, 2018

Abstract

A high-strength cold-rolled steel sheet has a composition and a microstructure. The microstructure comprises: ferrite having an average grain size of 5 μm or less and a volume fraction of 3% to 20%, retained austenite having a volume fraction of 5% to 20%, and martensite having a volume fraction of 5% to 20%, the remainder being bainite and/or tempered martensite. The total number of retained austenite with a grain size of 2 μm or less, martensite with a grain size of 2 μm or less, or a mixed phase thereof is 150 or more per 2,000 μm2 of a thickness cross section parallel to the rolling direction of the steel sheet.

First claim

The invention claimed is:

1. A high-strength cold-rolled steel sheet having a composition and a microstructure, the composition comprising:
0.15% to 0.27% C, 0.8% to 2.4% Si, 2.3% to 3.5% Mn, 0.08% or less P, 0.005% or less S, 0.01% to 0.08% Al, and 0.010% or less N on a mass basis, the remainder being Fe and inevitable impurities, and

the microstructure comprising:
ferrite having an average grain size of 5 μm or less and a volume fraction of 3% to 20%, retained austenite having a volume fraction of 5% to 20%, and martensite having a volume fraction of 5% to 20%, the remainder being bainite and/or tempered martensite; and
the total number of retained austenite with a grain size of 2 μm or less, martensite with a grain size of 2 μm or less and a mixed phase thereof with a grain size of 2 μm or less being 150 or more per 2,000 μm 2 of a thickness cross section parallel to the rolling direction of the steel sheet.

2. The high-strength cold-rolled steel sheet according to claim 1 , wherein the composition further contains at least one selected from the group consisting of 0.10% or less V, 0.10% or less Nb, and 0.10% or less Ti on a mass basis.

3. The high-strength cold-rolled steel sheet according to claim 1 , wherein the composition further contains 0.0050% or less B on a mass basis.

4. The high-strength cold-rolled steel sheet according to claim 1 , wherein the composition further contains at least one selected from the group consisting of 0.50% or less Cr, 0.50% or less Mo, 0.50% or less Cu, and 0.50% or less Ni on a mass basis.

5. The high-strength cold-rolled steel sheet according to claim 1 , wherein the composition further contains at least one selected from the group consisting of 0.0050% or less Ca and 0.0050% or less of a REM on a mass basis.

6. A method of manufacturing the high-strength cold-rolled steel sheet according to claim 1 comprising:
preparing a steel slab having the composition according to claim 1 ;
hot-rolling the steel slab to produce hot-rolled steel sheet;
pickling the hot-rolled steel sheet;
cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet;

subjecting the cold-rolled steel sheet to a first annealing, the first annealing comprising:
holding the cold-rolled steel sheet at a first soaking temperature of 800° C. or higher for 30 seconds or more,
cooling the cold-rolled steel sheet from the first soaking temperature to 320° C. to 500° C. at a first average cooling rate of 3° C./s or more,
holding the cold-rolled steel sheet in a first holding temperature range of 320° C. to 500° C. for 30 seconds or more, and
cooling the cold-rolled steel sheet to room temperature;

subjecting the cold-rolled steel sheet to a second annealing, the second annealing comprising:
heating the cold-rolled steel sheet to a second soaking temperature of 750° C. or higher at an average heating rate of 3° C./s to 30° C./s,
holding the cold-rolled steel sheet for 30 seconds or more,
cooling the cold-rolled steel sheet from the second soaking temperature to 120° C. to 320° C. at a second average cooling rate of 3° C./s or more,
heating the cold-rolled steel sheet to a second holding temperature range of 320° C. to 500° C., is held for 30 seconds or more, and
cooling the cold-rolled steel sheet to room temperature.

7. A method of manufacturing the high-strength cold-rolled steel sheet according to claim 2 comprising:
preparing a steel slab having the composition according to claim 2 ;
hot-rolling the steel slab to produce hot-rolled steel sheet;
pickling the hot-rolled steel sheet;
cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet;

subjecting the cold-rolled steel sheet to a first annealing, the first annealing comprising:
holding the cold-rolled steel sheet at a first soaking temperature of 800° C. or higher for 30 seconds or more,
cooling the cold-rolled steel sheet from the first soaking temperature to 320° C. to 500° C. at a first average cooling rate of 3° C./s or more,
holding the cold-rolled steel sheet in a first holding temperature range of 320° C. to 500° C. for 30 seconds or more, and
cooling the cold-rolled steel sheet to room temperature;

subjecting the cold-rolled steel sheet to a second annealing, the second annealing comprising:
heating the cold-rolled steel sheet to a second soaking temperature of 750° C. or higher at an average heating rate of 3° C./s to 30° C./s,
holding the cold-rolled steel sheet for 30 seconds or more,
cooling the cold-rolled steel sheet from the second soaking temperature to 120° C. to 320° C. at a second average cooling rate of 3° C./s or more,
heating the cold-rolled steel sheet to a second holding temperature range of 320° C. to 500° C., is held for 30 seconds or more, and
cooling the cold-rolled steel sheet to room temperature.

8. A method of manufacturing the high-strength cold-rolled steel sheet according to claim 3 comprising:
preparing a steel slab having the composition according to claim 3 ;
hot-rolling the steel slab to produce hot-rolled steel sheet;
pickling the hot-rolled steel sheet;
cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet;

subjecting the cold-rolled steel sheet to a first annealing, the first annealing comprising:
holding the cold-rolled steel sheet at a first soaking temperature of 800° C. or higher for 30 seconds or more,
cooling the cold-rolled steel sheet from the first soaking temperature to 320° C. to 500° C. at a first average cooling rate of 3° C./s or more,
holding the cold-rolled steel sheet in a first holding temperature range of 320° C. to 500° C. for 30 seconds or more, and
cooling the cold-rolled steel sheet to room temperature;

subjecting the cold-rolled steel sheet to a second annealing, the second annealing comprising:
heating the cold-rolled steel sheet to a second soaking temperature of 750° C. or higher at an average heating rate of 3° C./s to 30° C./s,
holding the cold-rolled steel sheet for 30 seconds or more,
cooling the cold-rolled steel sheet from the second soaking temperature to 120° C. to 320° C. at a second average cooling rate of 3° C./s or more,
heating the cold-rolled steel sheet to a second holding temperature range of 320° C. to 500° C., is held for 30 seconds or more, and
cooling the cold-rolled steel sheet to room temperature.

9. A method of manufacturing the high-strength cold-rolled steel sheet according to claim 4 comprising:
preparing a steel slab having the composition according to claim 4 ;
hot-rolling the steel slab to produce hot-rolled steel sheet;
pickling the hot-rolled steel sheet;
cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet;

subjecting the cold-rolled steel sheet to a first annealing, the first annealing comprising:
holding the cold-rolled steel sheet at a first soaking temperature of 800° C. or higher for 30 seconds or more,
cooling the cold-rolled steel sheet from the first soaking temperature to 320° C. to 500° C. at a first average cooling rate of 3° C./s or more,
holding the cold-rolled steel sheet in a first holding temperature range of 320° C. to 500° C. for 30 seconds or more, and
cooling the cold-rolled steel sheet to room temperature;

subjecting the cold-rolled steel sheet to a second annealing, the second annealing comprising:
heating the cold-rolled steel sheet to a second soaking temperature of 750° C. or higher at an average heating rate of 3° C./s to 30° C./s,
holding the cold-rolled steel sheet for 30 seconds or more,
cooling the cold-rolled steel sheet from the second soaking temperature to 120° C. to 320° C. at a second average cooling rate of

Assignees

• Jfe Steel Corp

Inventors

• Takashima Katsutoshi
• Ono Yoshihiko
• Hasegawa Kohei

Classifications

• C21D8/02

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

• C21D1/84

  Controlled slow cooling (cooling-beds for metal rolling B21B43/00) · CPC title

• C22C38/005

  containing rare earths, i.e. Sc, Y, Lanthanides · CPC title

• C22C38/14

  containing titanium or zirconium · CPC title

• C21D8/0263

  following hot rolling · CPC title