Highly formable high-strength steel sheet, warm working method, and warm-worked automobile part

Patent metadata

Field	Value
Publication number	US-10544489-B2
Application number	US-201113988382-A
Country	US
Kind code	B2
Filing date	Nov 16, 2011
Priority date	Nov 18, 2010
Publication date	Jan 28, 2020
Grant date	Jan 28, 2020

Abstract

A high-strength steel sheet has a chemical composition including 0.05% to 0.3% of C, 1% to 3% of Si, 0.5% to 3% of Mn, 0% to 0.1% of P, 0.001% to 0.1% of Al, and 0.002% to 0.03% of N, in mass percent; further includes iron and impurities; and has a structure including 50% to 90% of bainitic ferrite, 5% to 20% of retained austenite (γR), a total of 10% to 50% of martensite and the retained austenite, and 0% to 40% of polygonal ferrite, in area percent based on the entire structure. The retained austenite has a carbon content (CγR) of 0.5% to 1.2% by mass, an average equivalent circle diameter of 0.2 to 2 μm, and an average aspect ratio (maximum diameter/minimum diameter) of less than 3.0. The high-strength steel sheet excels both in elongation and deep drawability while having a strength of 980 MPa or more.

First claim

The invention claimed is:

1. A steel sheet, comprising:
C in a content of 0.05% to 0.3%;
Si in a content of 1% to 3%;
Mn in a content of 0.5% to 3%;
P in a content of 0% to 0.1%;
S in a content of 0% to 0.01%;
Al in a content of 0.001% to 0.1%; and
N in a content of 0.002% to 0.03%,
in mass percent,
wherein:
the steel sheet further comprises iron and impurities;
the steel sheet has a structure comprising:
bainitic ferrite in a content of 50% to 65%;
retained austenite in a content of 5% to 20%;
martensite in a content of 11% or more;

martensite and the retained austenite in a total content of 16% to 50%; and
polygonal ferrite in a content of 0% to 40%,

in area percent based on the entire structure; and
the retained austenite has a carbon content (CγR) of 0.5% to 1.2% in mass percent, an average equivalent circle diameter of 0.2 to 2 μm, and an average aspect ratio of less than 3.0, where an aspect ratio of a retained austenite grain is expressed as a ratio of a maximum diameter to a minimum diameter of the grain.

2. The steel sheet of claim 1 , further comprising, in mass percent, at least one element selected from the group consisting of:
Cr in a content of 0.01% to 3%;
Mo in a content of 0.01% to 1%;
Cu in a content of 0.01% to 2%;
Ni in a content of 0.01% to 2%; and
B in a content of 0.00001% to 0.01%.

3. The steel sheet of claim 1 , further comprising, in mass percent, at least one element selected from the group consisting of:
Ca in a content of 0.0005% to 0.01%;
Mg in a content of 0.0005% to 0.01%; and
a rare-earth element (REM) in a content of 0.0001% to 0.01%.

4. The steel sheet of claim 2 , further comprising, in mass percent, at least one element selected from the group consisting of:
Ca in a content of 0.0005% to 0.01%;
Mg in a content of 0.0005% to 0.01%; and
a rare-earth element (REM) in a content of 0.0001% to 0.01%.

5. A method of warm-working a steel sheet, the method comprising:
heating the steel sheet of claim 1 up to a temperature of 200° C. to 400° C.; and
working the steel sheet within 3600 seconds after the heating.

Assignees

• Kakiuchi Elijah
• Murakami Toshio
• Hata Hideo
• Asai Tatsuya
• Mizuta Naoki
• Kobe Steel Ltd

Inventors

• Kakiuchi Elijah
• Murakami Toshio
• Hata Hideo
• Asai Tatsuya
• Mizuta Naoki

Classifications

• C21D8/0426

  Hot rolling · CPC title

• C22C38/38 Primary

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

• C21D2211/008

  Martensite · CPC title

• C22C38/001

  containing N · CPC title

• C22C38/16

  containing copper · CPC title