High-strength steel sheet and method for producing the same

Patent metadata

Field	Value
Publication number	US-9322088-B2
Application number	US-201314440118-A
Country	US
Kind code	B2
Filing date	Dec 6, 2013
Priority date	Dec 12, 2012
Publication date	Apr 26, 2016
Grant date	Apr 26, 2016

Abstract

Provided is a high-strength steel sheet having a tensile strength of 1180 MPa or more. The high-strength steel sheet has a specific chemical composition, includes a microstructure meeting all conditions (1) to (4) as measured by specific methods at a position corresponding to one-fourth the thickness of the steel sheet, has excellent workability and low-temperature toughness, and has a tensile strength of 1180 MPa or more. (1) The microstructure includes ferrite in a volume fraction of 5% to 35% and at least one of bainitic ferrite and tempered martensite in a volume fraction of 50% or more relative to the entire microstructure. (2) The microstructure includes MA constituent in a volume fraction of 20% or less (excluding 0%) relative to the entire microstructure, where the MA constituent is a mixed phase of fresh martensite and retained austenite. (3) The microstructure includes retained austenite in a volume fraction of 5% or more relative to the entire microstructure. (4) The microstructure has an amount of stacking faults in the retained austenite of 10.0×10 −3 (nm/nm 2 ) or less.

First claim

The invention claimed is:

1. A high-strength steel sheet having a tensile strength of 1180 MPa or more, the steel sheet comprising in a chemical composition:
C in a content of 0.10% (in mass percent, hereinafter the same for chemical compositions) to 0.30%;
Si in a content of 1.40% to 3.0%;
Mn in a content of 0.5% to 3.0%;
P in a content of 0.1% or less (excluding 0%);
S in a content of 0.05% or less (excluding 0%);
Al in a content of 0.005% to 0.20%;
N in a content of 0.01% or less (excluding 0%); and
O in a content of 0.01% or less (excluding 0%),
with the remainder consisting of iron and inevitable impurities,
wherein a microstructure at a position corresponding to one-fourth the thickness of the steel sheet meets all of the following conditions (1) to (4):
(1) the microstructure comprises ferrite in a volume fraction of 5% to 35% and bainitic ferrite and/or tempered martensite in a volume fraction of 50% or more relative to the entire microstructure, where the volume fractions of ferrite and bainitic ferrite and/or tempered martensite are determined by scanning electron microscopic observation;
(2) the microstructure comprises MA constituent in a volume fraction of 20% or less (excluding 0%) relative to the entire microstructure, where the MA constituent is a mixed structure of fresh martensite and retained austenite, and where the volume fraction of MA constituent is determined by optical microscopic observation;
(3) the microstructure comprises retained austenite in a volume fraction of 5% or more relative to the entire microstructure, where the volume fraction of retained austenite is determined by X-ray diffractometry; and
(4) the microstructure has an amount of stacking faults in the retained austenite of 10.0×10 −3 (nm/nm 2 ) or less, where the amount is determined by transmission electron microscopic observation.

2. The high-strength steel sheet according to claim 1 , further comprising, on or over a surface thereof:
an electrogalvanized layer;
a hot-dip galvanized layer; or
a hot-dip galvannealed layer.

3. The high-strength steel sheet according to claim 1 , further comprising, in the chemical composition, at least one element selected from any of the following groups (A) to (E):
(A) at least one element selected from the group consisting of:
Cr in a content of 1.0% or less (excluding 0%); and
Mo in a content of 1.0% or less (excluding 0%);
(B) at least one element selected from the group consisting of:
Ti in a content of 0.15% or less (excluding 0%);
Nb in a content of 0.15% or less (excluding 0%); and
V in a content of 0.15% or less (excluding 0%);
(C) at least one element selected from the group consisting of:
Cu in a content of 1.0% or less (excluding 0%); and
Ni in a content of 1.0% or less (excluding 0%);
(D) B in a content of 0.0050% or less (excluding 0%); and
(E) at least one element selected from the group consisting of:
Ca in a content of 0.0100% or less (excluding 0%);
Mg in a content of 0.0100% or less (excluding 0%); and
REM in a content of 0.0100% or less (excluding 0%).

4. A method for producing the high-strength steel sheet according to claim 1 , the steel sheet having a tensile strength of 1180MPa or more, the method comprising:
preparing a steel sheet having the chemical composition;
subjecting the steel sheet sequentially to soaking in a temperature range from (Ac 1 point +20° C.) to lower than Ac 3 point, cooling down to a temperature range from 100° C. to 500° C. at an average cooling rate of 5° C/second or more, holding in the temperature range from 100° C. to 500° C. for 100 seconds or longer, and cooling down to room temperature; and
reheating and holding the steel sheet in a temperature range from 50° C. to 200° C. for 20 seconds or longer.

5. A method for producing the high-strength steel sheet according to claim 1 , the steel sheet having a tensile strength of 1180MPa or more, the method comprising:
preparing a steel sheet having the chemical composition;
subjecting the steel sheet sequentially to soaking in a temperature range from Ac 3 point to 950° C., cooling down to a temperature range from 100° C. to 500° C. at an average cooling rate of 50° C/second or less, holding in the temperature range from 100° C. to 500° C. for 100 seconds or longer, and cooling down to room temperature; and
reheating and holding the steel sheet in a temperature range from 50° C. to 200° C. for 20 seconds or longer.

Assignees

• Kobe Steel Ltd

Inventors

• Mizuta Sae
• Utsumi Yukihiro

Classifications

• C22C38/34

  with more than 1.5% by weight of silicon · CPC title

• C21D2211/002

  Bainite · CPC title

• C22C38/54

  with boron · CPC title

• C22C38/02

  containing silicon · CPC title

• C21D2211/001

  Austenite · CPC title