Patent ID: 11946111
Assignee: JFE STEEL CORPORATION
Field: Materials, metallurgy (Chemistry)
Classification: CPC C | IPC C

Claim 12:
13. A method for producing a steel sheet according to claim 2, the method comprising:
producing a hot-rolled steel sheet by heating a steel slab having a composition that contains, in terms of mass %
C: 0.048% or more and 0.200% or less,
Si: 0.40% or less,
Mn: 1.50% or more and 3.00% or less,
P: 0.001% or more and 0.100% or less,
S: 0.0001% or more and 0.0200% or less,
Al: 0.001% or more and 1.000% or less,
N: 0.0005% or more and 0.0100% or less,
at least one element selected from:
Cr: 0.01% or more and 1.00% or less,
Nb: 0.001% or more and 0.100% or less,
V: 0.001% or more and 0.100% or less,
Ti: 0.001% or more and 0.100% or less,
B: 0.0001% or more and 0.0100% or less,
Mo: 0.01% or more and 0.50% or less,
Cu: 0.01% or more and 1.00% or less,
Ni: 0.01% or more and 1.00% or less,
As: 0.001% or more and 0.500% or less,
Sb: 0.001% or more and 0.200% or less,
Sn: 0.001% or more and 0.200% or less,
Ta: 0.001% or more and 0.100% or less,
Ca: 0.0001% or more and 0.0200% or less,
Mg: 0.0001% or more and 0.0200% or less,
Zn: 0.001% or more and 0.020% or less,
Co: 0.001% or more and 0.020% or less,
Zr: 0.001% or more and 0.020% or less, and
REM: 0.0001% or more and 0.0200% or less, and

the balance being Fe and unavoidable impurities;
rough-rolling the heated steel slab; in subsequent finish-rolling, hot-rolling the rough-rolled steel slab under conditions of a finish-rolling inlet temperature of 1020° C. or higher and 1180° C. or lower, a rolling reduction in a final pass of the finish rolling of 5% or more and 15% or less, a rolling reduction in a pass before the final pass of 15% or more and 25% or less, and a finish-rolling delivery temperature of 800° C. or higher and 1000° C. or lower; after the hot-rolling, cooling the hot-rolled steel sheet under a condition of an average cooling rate of 5° C./s or more and 90° C./s or less; coiling the cooled steel sheet under a condition of a coiling temperature of 300° C. or higher and 700° C. or lower;

producing a heat-treated steel sheet by heating the hot-rolled steel sheet obtained in the method under conditions of a maximum attained temperature of a T1 temperature or higher and a T2 temperature or lower and an average heating rate of 50° C./s or less in a temperature range of 450° C. to [T1 temperature—10° C.]; and, after the heating, performing cooling and pickling; and
re-heating the heat-treated steel sheet to a temperature equal to or higher than the T1 temperature; and cooling the re-heated steel sheet under a condition of an average cooling rate of 3° C./s or more in a temperature range of [T1 temperature—10° C.] to 550° C., wherein a dew point in a temperature range of 600° C. or higher is −40° C. or lower.