Patent ID: 11859260
Assignee: NIPPON STEEL CORPORATION
Field: Materials, metallurgy (Chemistry)
Classification: CPC C  B | IPC B  C

Claim 2:
3. A method for manufacturing the steel sheet according to claim 1, the method comprising:
a heating process of heating a slab having said chemical composition at 1000° C. or higher;
a hot-rolling process of hot-rolling the slab such that a rolling finishing temperature is 950° C. or lower to obtain a hot-rolled steel sheet;
a stress application process of applying a stress to the hot-rolled steel sheet after the hot-rolling process such that an absolute value of a residual stress σs on the surface is 165 MPa to 400 MPa;
a cold-rolling process of cold-rolling the hot-rolled steel sheet after the stress application process such that a cumulative rolling reduction RCR is 70% to 90% to obtain a cold-rolled steel sheet;
an annealing process of heating the cold-rolled steel sheet such that an average heating rate in a range from 300° C. to a soaking temperature T1° C. that satisfies following Expression (2) is 1.5° C./sec to 10.0° C./sec and holding the heated steel sheet at the soaking temperature T1° C. for 30 seconds to 150 seconds for annealing; and
a cooling process of cooling the heated steel sheet after the annealing process to a temperature range of 550° C. to 650° C. such that an average cooling rate in a range from the soaking temperature T1° C. to 650° C. is 1.0° C./sec to 10.0° C./sec and further cooling the cooled steel sheet to a temperature range of 200° C. to 490° C. such that an average cooling rate is 5.0° C./sec to 500.0° C./sec,

1275−25×ln(σs)−4.5×RCR≤T1≤1275−25×ln(σs)−4×RCR  (2).