High-strength cold-rolled steel sheet excellent in uniform elongation and method for manufacturing same
A high-strength cold-rolled steel sheet excellent in uniform elongation, including in percent by mass: 0.10-0.28% of C; 1.0-2.0% of Si; and 1.0-3.0% of Mn, and the structures of the same having the space factors below to the entire structure: 30-65% of bainitic ferrite; 30-50% of polygonal ferrite; and 5-20% of residual austenite.
1. A high-strength cold-rolled steel sheet excellent in uniform elongation, comprising in percent by mass (which is hereinafter applied similarly to chemical components):
0.10-0.28% of C;
1.0-2.0% of Si; and
1.0-3.0% of Mn,
wherein the structure of the high-strength cold-rolled steel sheet is:
40-65% of bainitic ferrite;
30-50% of polygonal ferrite; and
9-17% of residual austenite;
and wherein the product of its tensile strength (TS) in MPa and total elongation (EL) in % (TS×L) is 23,000 or more.
2. A high-strength cold-rolled steel sheet according to claim 1 , further comprising, as other element, at least one element selected from a group consisting of:
0.10% or less (not including 0%) of Nb;
1.0% or less (not including 0%) of Mo;
0.5% or less (not including 0%) of Ni; and
0.5% or less (not including 0%) of Cu.
3. A high-strength cold-rolled steel sheet according to claim 1 , further comprising, as other element, 0.003% or less (not including 0%) of Ca and/or 0.003% or less (not including 0%) of REM.
4. A high-strength cold-rolled steel sheet according to claim 1 , further comprising, as other element, 0.1% or less (not including 0%) of Ti and/or 0.1% or less (not including 0%) of V.
5. A high-strength cold-rolled steel sheet excellent in uniform elongation of claim 1 ,
wherein the structure of the high-strength cold-rolled steel sheet is:
40-65% of bainitic ferrite;
30-50% of polygonal ferrite;
9-16% of residual austenite; and
less than 5% of martensite.
6. A high-strength cold-rolled steel sheet excellent in uniform elongation of claim 1 ,
wherein the structure of the high-strength cold-rolled steel sheet is:
40-65% of bainitic ferrite;
30-50% of polygonal ferrite;
9-15% of residual austenite; and
less than 5% of martensite.
7. A high-strength cold-rolled steel sheet excellent in uniform elongation of claim 1 ,
wherein the structure of the high-strength cold-rolled steel sheet is:
40-65% of bainitic ferrite;
30-50% of polygonal ferrite;
9-14% of residual austenite; and
less than 5% of martensite.
8. A high-strength cold-rolled steel sheet excellent in uniform elongation of claim 1 , having a cold rolled thickness of less than 3 mm.
9. A high-strength cold-rolled steel sheet excellent in uniform elongation of claim 1 ,
wherein the total amount of pearlite, bainite and martensite is 5% or less.
10. A high-strength cold-rolled steel sheet excellent in uniform elongation, comprising in percent by mass (which is hereinafter applied similarly to chemical components):
0.10-0.28% of C;
1.0-2.0% of Si; and
1.0-3.0% of Mn,
wherein the structure of the high-strength cold-rolled steel sheet is:
40-65% of bainitic ferrite;
30-50% of polygonal ferrite; and
9-17% of residual austenite;
wherein the product of its tensile strength (TS) in MPa and total elongation (EL) in % (TS×EL) is 23,000 or more, and the product of its tensile strength (TS) in MPa and uniform elongation (u-EL) in % (TS×u-EL) is 14,700 or more; and
wherein the cold-rolled steel sheet is produced by a method comprising:
(a) hot rolling the steel sheet,
(b) cold rolling the hot rolled steel sheet,
(c) heating the cold rolled steel sheet to a temperature which is equal to or higher than the A 3 transformation point (A 3 ), and cooling the heated steel sheet at an average cooling rate of 1-10° C./sec to a temperature Tq, expressed by the formula (1) below
A 3 −250(° C.)< Tq<A 3 −20(° C.) (1),
and then
(d) quenching the cooled steel sheet down into a bainitic transformation temperature range at an average cooling rate of 11° C./sec or faster.
11. A high-strength cold-rolled steel sheet excellent in uniform elongation of claim 10 ,
wherein the structure of the high-strength cold-rolled steel sheet is:
40-65% of bainitic ferrite;
30-50% of polygonal ferrite;
9-16% of residual austenite; and
less than 5% of martensite.
12. A high-strength cold-rolled steel sheet excellent in uniform elongation of claim 10 ,
wherein the structure of the high-strength cold-rolled steel sheet is:
40-65% of bainitic ferrite;
30-50% of polygonal ferrite;
9-15% of residual austenite; and
less than 5% of martensite.
13. A high-strength cold-rolled steel sheet excellent in uniform elongation of claim 10 ,
wherein the structure of the high-strength cold-rolled steel sheet is:
40-65% of bainitic ferrite;
30-50% of polygonal ferrite;
9-14% of residual austenite; and
less than 5% of martensite.
14. A high-strength cold-rolled steel sheet excellent in uniform elongation of claim 10 , having a cold rolled thickness of less than 3 mm.
15. A high-strength cold-rolled steel sheet excellent in uniform elongation of claim 10 ,
wherein the total amount of pearlite, bainite and martensite is 5% or less.