High-strength steel sheet having excellent room-temperature formability and warm formability, and warm forming method thereof

Patent metadata

Field	Value
Publication number	US-9657381-B2
Application number	US-201214238599-A
Country	US
Kind code	B2
Filing date	Aug 15, 2012
Priority date	Aug 17, 2011
Publication date	May 23, 2017
Grant date	May 23, 2017

Abstract

This high-strength steel plate has a component composition including, by mass %, C: 0.02-0.3%, Si: 1-3%, Mn: 1.8-3%, P: 0.1% or less, S: 0.01% or less, Al: 0.001-0.1%, N: 0.002-0.03%, the rest consisting of iron and impurities. Said steel plate has a microstructure including, in terms of area ratio relative to the entire microstructure, each of the following phases: bainitic ferrite: 50-85%; retained γ: 3% or greater; martensite+the aforementioned retained γ: 10-45%; and ferrite: 5-40%. The C concentration (CγR) in the aforementioned retained austenite is 0.3-1.2 mass %, part or all of the N in the aforementioned component composition is solid solution N, and the amount of said solid solution N is 30-100 ppm.

First claim

The invention claimed is:

1. A steel sheet, comprising,
by mass percent:
C: 0.02 to 0.3%;
Si: 1.0 to 3.0%;
Mn: 1.8 to 3.0%;
P: 0.1% or less, including 0%;
S: 0.01% or less, including 0%;
Al: 0.001 to 0.1%;
N: 0.01 to 0.03%; and
a remainder comprising iron and impurities, and
having a microstructure comprising phases of, by area ratio to an entire microstructure:
bainitic ferrite: 50 to 85%;
retained austenite: 3% or more;
martensite and a retained austenite in total: 10 to 45%; and
ferrite: 5 to 40%,
wherein C concentration (CγR) in the retained austenite is from 0.3 to 1.2 mass percent,
part or all of N in the composition is dissolved, and an amount of the dissolved N is from 30 to 100 ppm, and
wherein a dislocation density in the entire microstructure is 5×10 15 m −2 or less.

2. The steel sheet of claim 1 , wherein the composition further comprises, by mass percent:
Cr: 0.01 to 3.0%,
Mo: 0.01 to 1.0%,
Cu: 0.01 to 2.0%,
Ni: 0.01 to 2.0%,
B: 0.00001 to 0.01%,
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%, and
REM: 0.0001 to 0.01% of an element.

3. The steel sheet of claim 1 , wherein the bainitic ferrite is present in an amount of 60 to 85%.

4. The steel sheet of claim 1 , wherein the bainitic ferrite is present in an amount of 70 to 85%.

5. The steel sheet of claim 1 , wherein the martensite and a retained austenite in total are present in an amount of 12 to 45%.

6. The steel sheet of claim 1 , wherein the martensite and a retained austenite in total are present in an amount of 16 to 45%.

7. The steel sheet of claim 1 , wherein the ferrite is present in an amount of 10 to 35%.

8. The steel sheet of claim 1 , wherein the ferrite is present in an amount of 15 to 30%.

9. The steel sheet of claim 1 , wherein C concentration (CγR) in the retained austenite is from 0.4 to 0.9 mass percent.

10. The steel sheet of claim 1 , wherein the retained austenite is present in an amount of 5% or more.

11. The steel sheet of claim 1 , wherein the retained austenite is present in an amount of 10% or more.

12. The steel sheet of claim 1 , wherein the dislocation density is 4×10 15 m −2 or less.

13. The steel sheet of claim 1 , wherein the dislocation density is 3×10 15 m −2 or less.

14. A warm forming method of a high-strength steel sheet, the method comprising heating the steel sheet of claim 1 to a temperature of from 100 to 250° C., and then forming the steel sheet within 3600 sec.

Assignees

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

Inventors

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

Classifications

• C22C38/04

  containing manganese · CPC title

• C21D2211/002

  Bainite · CPC title

• C22C38/02

  containing silicon · CPC title

• C21D8/0226

  Hot rolling · CPC title

• C22C38/001

  containing N · CPC title