High-strength steel sheet excellent in workability and cold brittleness resistance, and manufacturing method thereof

Patent metadata

Field	Value
Publication number	US-9745639-B2
Application number	US-201213489005-A
Country	US
Kind code	B2
Filing date	Jun 5, 2012
Priority date	Jun 13, 2011
Publication date	Aug 29, 2017
Grant date	Aug 29, 2017

Abstract

The invention relates to a steel sheet having a tensile strength of 1180 MPa or more, which excels in workability and cold brittleness resistance. The high-strength steel sheet contains 0.10% to 0.30% of C, 1.40% to 3.0% of Si, 0.5% to 3.0% of Mn, 0.1% or less of P, 0.05% or less of S, 0.005% to 0.20% of Al, 0.01% or less of N, 0.01% or less of O, as well as Fe and inevitable impurities. The steel sheet has: (i) a ferrite volume fraction of 5% to 35% and a bainitic ferrite and/or tempered martensite volume fraction of 60% or more; (ii) a MA constituent volume fraction of 6% or less (excluding 0%); and (iii) a retained austenite volume fraction of 5% or more.

First claim

What is claimed is:

1. A steel sheet, comprising in percent by mass:
carbon (C) in a content of from 0.10% to 0.30%,
silicon (Si) in a content of from 1.40% to 3.0%,
manganese (Mn) in a content of from 0.5% to 3.0%,
phosphorus (P) in a content of 0.1% or less,
sulfur (S) in a content of 0.05% or less,
aluminum (Al) in a content of from 0.005% to 0.20%,
nitrogen (N) in a content of 0.01% or less, and
oxygen (O) in a content of 0.01% or less,
with the remainder including iron (Fe) and inevitable impurities;
the steel sheet having a volume fraction of ferrite of from 5% to 35% and a volume fraction of bainitic ferrite and/or tempered martensite of 60% or more based on the total volume of structures as determined through observation of the structures at a position of a depth one-quarter the thickness of the steel sheet under a scanning electron microscope;
the steel sheet having a volume fraction of a mixed structure (martensite-austenite constituent) of fresh martensite and retained austenite of 1% to 6% based on the total volume of structures as determined through observation of the structures under an optical microscope;
the steel sheet having a volume fraction of retained austenite of 5% or more based on the total volume of structures as determined through X-ray diffractometry of retained austenite; and
the steel sheet having a tensile strength of 1180 MPa or more.

2. The steel sheet according to claim 1 ,
further comprising, as an additional element, at least one element selected from the group consisting of:
chromium (Cr) in a content of from 1.0% or less and
molybdenum (Mo) in a content of from 1.0% or less.

3. The steel sheet according to claim 1 ,
further comprising, as an additional element, at least one element selected from the group consisting of:
titanium (Ti) in a content of 0.15% or less,
niobium (Nb) in a content of 0.15% or less, and
vanadium (V) in a content of 0.15% or less.

4. The steel sheet according to claim 1 ,
further comprising, as an additional element, at least one element selected from the group consisting of:
copper (Cu) in a content of from 1.0% or less and
nickel (Ni) in a content of from 1.0% or less.

5. The steel sheet according to claim 1 ,
further comprising, as an additional element, boron (B) in a content of from 0.005% or less.

6. The steel sheet according to claim 1 ,
further comprising, as an additional element, at least one element selected from the group consisting of:
calcium (Ca) in a content of 0.01% or less,
magnesium (Mg) in a content of 0.01% or less, and
one or more rare-earth elements (REM) in a content of 0.01% or less.

7. A method for manufacturing the steel sheet of claim 1 , the method comprising:
preparing a steel sheet through rolling a steel comprising
carbon (C) in a content of from 0.10% to 0.30%,
silicon (Si) in a content of from 1.40% to 3.0%,
manganese (Mn) in a content of from 0.5% to 3.0%,
phosphorus (P) in a content of 0.1% or less,
sulfur (S) in a content of 0.05% or less,
aluminum (Al) in a content of from 0.005% to 0.20%,
nitrogen (N) in a content of 0.01% or less, and
oxygen (O) in a content of 0.01% or less,
with the remainder including iron (Fe) and inevitable impurities;
soaking the rolled steel sheet at a temperature higher than Ac 1 point by 20° C. or more and lower than the Ac 3 point;
cooling the soaked steel sheet at an average cooling rate of 5° C./second or more to a temperature in the range of from 100° C. to 400° C.; and
holding the cooled steel sheet in a temperature range of from 200° C. to 500° C. for 100 seconds or longer.

8. A method for manufacturing the steel sheet of claim 1 , the method comprising:
preparing a steel sheet through rolling a steel comprising
carbon (C) in a content of from 0.10% to 0.30%,
silicon (Si) in a content of from 1.40% to 3.0%,
manganese (Mn) in a content of from 0.5% to 3.0%,
phosphorus (P) in a content of 0.1% or less,
sulfur (S) in a content of 0.05% or less,
aluminum (Al) in a content of from 0.005% to 0.20%,
nitrogen (N) in a content of 0.01% or less, and
oxygen (O) in a content of 0.01% or less,
with the remainder including iron (Fe) and inevitable impurities;
soaking the rolled steel sheet at a temperature equal to or higher than Ac 3 point;
cooling the soaked steel sheet at an average cooling rate of 50° C./second or less to a temperature in the range of from 100° C. to 400° C.; and
holding the cooled steel sheet in a temperature range of from 200° C. to 500° C. for 100 seconds or longer.

9. The steel sheet according to claim 1 , comprising a volume fraction of ferrite of from 12% to 35%.

10. The steel sheet according to claim 1 , wherein the volume fraction of the martensite-austenite constituent is from 2% to 6%.

11. The steel sheet according to claim 1 , wherein the volume fraction of the martensite-austenite constituent is from 3% to 6%.

12. The steel sheet according to claim 1 , wherein the volume fraction of the martensite-austenite constituent is from 1% to 5%.

13. The steel sheet according to claim 1 , wherein the volume fraction of the martensite-austenite constituent is from 1% to 4%.

14. The steel sheet according to claim 1 , wherein the steel sheet, after being subjected to an austempering time of at least 100 seconds has a volume fraction of ferrite of from 5% to 35% and a volume fraction of bainitic ferrite and/or tempered martensite of 60% or more based on the total volume of structures as determined through observation of the structures at a position of a depth one-quarter the thickness of the steel sheet under a scanning electron microscope;
a volume fraction of a mixed structure (martensite-austenite constituent) of fresh martensite and retained austenite of 6% or less based on the total volume of structures as determined through observation of the structures under an optical microscope;
a volume fraction of retained austenite of 5% or more based on the total volume of structures as determined through X-ray diffractometry of retained austenite; and
having a tensile strength of 1180 MPa or more.

15. The steel sheet according to claim 9 , comprising the volume fraction of ferrite of from 12% to 25%.

16. The steel sheet according to claim 1 , wherein the volume fraction of retained austenite of 11% or more based on the total volume of structures as determined through X-ray diffractometry of retained austenite.

17. The steel sheet according to claim 1 , wherein the volume fraction of retained austenite of 12% or more based on the total volume of structures as determined through X-ray diffractometry of retained austenite.

18. The steel sheet according to claim 1 , wherein the balance between tensile strength and elongation (TS-EL balance) is 18,000 or more.

19. The steel sheet according to claim 1 , comprising a volume fraction of tempered martensite of 65% or more based on the total volume of structures as determined through observation of the structures at a position of a depth one-quarter the thickness of the steel sheet under a scanning electron microscope.

20. The steel sheet according to claim 1 , wherein the volume fraction of retained austenite is 7% or more.

Assignees

• Mizuta Sae
• Futamura Yuichi
• Utsumi Yukihiro
• Kobe Steel Ltd

Inventors

• Mizuta Sae
• Futamura Yuichi
• Utsumi Yukihiro

Classifications

• C22C38/001

  containing N · CPC title

• C22C38/02

  containing silicon · CPC title

• C22C38/005

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

• C22C38/08

  containing nickel {(C22C38/105 takes precedence)} · CPC title

• C22C38/16

  containing copper · CPC title