High-strength steel sheet and method for manufacturing the same
A high-strength steel sheet having a tensile strength TS of 1320 MPa or more and excellent workability. The high-strength steel sheet has a chemical composition containing, by mass %, C: 0.20% or more and 0.40% or less, Si: 0.5% or more and 2.5% or less, Mn: more than 2.4% and 5.0% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.01% or more and 0.5% or less, N: 0.010% or less, and Fe and inevitable impurities. A microstructure of the steel sheet includes, in terms of area fraction with respect to the whole steel sheet microstructure, lower bainite in an amount of 40% or more and less than 85%, martensite including tempered martensite in an amount of 5% or more and less than 40%, retained austenite in an amount of 10% or more and 30% or less, and polygonal ferrite in an amount of 10% or less (including 0%).
1. A high-strength steel sheet having a chemical composition comprising:
C: more than 0.30% and 0.40% or less, by mass %;
Si: 0.5% or more and 2.5% or less, by mass %;
Mn: 3.8% or more and 5.0% or less, by mass %;
P: 0.1% or less, by mass %;
S: 0.01% or less, by mass %;
Al: 0.01% or more and 0.5% or less, by mass %;
N: 0.010% or less, by mass %; and
Fe and inevitable impurities,
wherein:
the steel sheet has a microstructure including, in terms of area fraction with respect to the whole steel sheet microstructure,
lower bainite in an amount of 40% or more and less than 85%,
martensite including tempered martensite in an amount of 5% or more and less than 40%,
retained austenite in an amount of 10% or more and 30% or less, and having an average crystal grain diameter in a range of 1.6 μm or more an 2.0 μm or less, and
polygonal ferrite in an amount of 10% or less (including 0%),
the steel sheet has
a tensile strength of 1320 MPa or more,
(tensile strength×total elongation) of 18000 MPa·% or more, and
(tensile strength×hole expansion ratio) of 40000 MPa·% or more.
2. The high-strength steel sheet according to claim 1 , wherein an average C content in the retained austenite in the steel sheet microstructure is 0.60 mass % or more.
3. The high-strength steel sheet according to claim 2 , wherein the chemical composition further comprises one or more groups of elements selected from the following groups A to C:
Group A:
one or more selected from:
V: 1.0% or less, by mass %,
Mo: 0.5% or less, by mass %, and
Cu: 2.0% or less, by mass %,
Group B:
one or both selected from:
Ti: 0.1% or less, by mass %, and
Nb: 0.1% or less, by mass %, and
Group C:
B: 0.0050% or less.
4. A method for manufacturing a high-strength steel sheet according to claim 3 , the method comprising:
performing hot rolling and cold rolling on a steel sheet having the composition of claim 3 ,
annealing the cold-rolled steel sheet in a temperature range in which an austenite single phase is formed for a holding time of 15 seconds or more and 1000 seconds or less, and cooling the annealed steel sheet at an average cooling rate of 3° C/s or more to a first temperature range of (Ms temperature - 100° C.) or higher and lower than the Ms temperature, and
heating the cooled steel sheet to a second temperature range of 300° C. or higher, (Bs temperature - 50° C.) or lower, and 400° C. or lower, and holding the heated steel sheet in the second temperature range for 15 seconds or more and 1000 seconds or less.
5. The method for manufacturing a high-strength steel sheet according to claim 4 , wherein the hot rolling includes:
rough rolling in which a rolling reduction of a first pass of the rough rolling is 10% or more and 15% or less and
finish rolling in which a rolling reduction of a first pass of the finish rolling is 10% or more and 15% or less.
6. A method for manufacturing a high-strength steel sheet according to claim 2 , the method comprising:
performing hot rolling and cold rolling on a steel sheet having the composition of claim 2 ,
annealing the cold-rolled steel sheet in a temperature range in which an austenite single phase is formed for a holding time of 15 seconds or more and 1000 seconds or less, and cooling the annealed steel sheet at an average cooling rate of 3° C/s or more to a first temperature range of (Ms temperature - 100° C.) or higher and lower than the Ms temperature, and
heating the cooled steel sheet to a second temperature range of 300° C. or higher, (Bs temperature - 50° C.) or lower, and 400° C. or lower, and holding the heated steel sheet in the second temperature range for 15 seconds or more and 1000 seconds or less.
7. The method for manufacturing a high-strength steel sheet according to claim 6 , wherein the hot rolling includes:
rough rolling in which a rolling reduction of a first pass of the rough rolling is 10% or more and 15% or less and
finish rolling in which a rolling reduction of a first pass of the finish rolling is 10% or more and 15% or less.
8. The high-strength steel sheet according to claim 1 , wherein the chemical composition further comprises one or more groups of elements selected from the following groups A to C:
Group A:
one or more selected from:
V: 1.0% or less, by mass %,
Mo: 0.5% or less, by mass %, and
Cu: 2.0% or less, by mass %,
Group B:
one or both selected from:
Ti: 0.1% or less, by mass %, and
Nb: 0.1% or less, by mass %, and
Group C:
B: 0.0050% or less.
9. A method for manufacturing a high-strength steel sheet according to claim 8 , the method comprising:
performing hot rolling and cold rolling on a steel sheet having the composition of claim 8 ,
annealing the cold-rolled steel sheet in a temperature range in which an austenite single phase is formed for a holding time of 15 seconds or more and 1000 seconds or less, and cooling the annealed steel sheet at an average cooling rate of 3° C/s or more to a first temperature range of (Ms temperature - 100° C.) or higher and lower than the Ms temperature, and
heating the cooled steel sheet to a second temperature range of 300° C. or higher, (Bs temperature - 50° C.) or lower, and 400° C. or lower, and holding the heated steel sheet in the second temperature range for 15 seconds or more and 1000 seconds or less.
10. The method for manufacturing a high-strength steel sheet according to claim 9 , wherein the hot rolling includes:
rough rolling in which a rolling reduction of a first pass of the rough rolling is 10% or more and 15% or less and
finish rolling in which a rolling reduction of a first pass of the finish rolling is 10% or more and 15% or less.
11. A method for manufacturing a high-strength steel sheet according to claim 1 , the method comprising:
performing hot rolling and cold rolling on a steel sheet having the composition of claim 1 ,
annealing the cold-rolled steel sheet in a temperature range in which an austenite single phase is formed for a holding time of 15 seconds or more and 1000 seconds or less, and cooling the annealed steel sheet at an average cooling rate of 3° C/s or more to a first temperature range of (Ms temperature - 100° C.) or higher and lower than the Ms temperature, and
heating the cooled steel sheet to a second temperature range of 300° C. or higher, (Bs temperature - 50° C.) or lower, and 400° C. or lower, and holding the heated steel sheet in the second temperature range for 15 seconds or more and 1000 seconds or less.
12. The method for manufacturing a high-strength steel sheet according to claim 11 , wherein the hot rolling includes:
rough rolling in which a rolling reduction of a first pass of the rough rolling is 10% or more and 15% or less and
finish rolling in which a rolling reduction of a first pass of the finish rolling is 10% or more and 15% or less.