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

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.