Patent ID: 11913088
Assignee: KOBE STEEL, LTD.
Field: Materials, metallurgy (Chemistry)
Classification: CPC C  B | IPC B  C

Claim 6:
7. A method for manufacturing the high-strength steel sheet of claim 1, the method comprising, in this order:
after heating a steel material comprising, in mass percentage, the C in a range of from 0.10 to 0.35%; Si+Al in a range of from 0.5 to 3.0%; Mn in a range of from 1.0 to 3.0%; P in a range of from more than 0 to 0.05%; and S in a range of from more than 0 to 0.01%, then subjecting the steel material to hot rolling, cooling, and coiling the steel material after the hot rolling is completed, and then subjecting the steel material to pickling and cold rolling,
heating the steel material to a temperature T1 of an Ac3 transformation point of the steel or more and 950° C. or less and holding the steel material for a time t1 of 5 seconds or more and 1800 seconds or less in this temperature range for austenitization;
cooling from a rapid cooling start temperature T2 of 700° C. or more to a cooling stop temperature T3a in a temperature range of 300° C. or more and 500° C. or less at an average cooling rate CR2 of 10° C./sec or more;
performing retention for a time t3 of 10 seconds or more and less than 300 seconds at an average cooling rate of 10° C./sec or less in a temperature range of 300° C. or more and 500° C. or less;
cooling from a retention end temperature T3b of 300° C. or more to a cooling stop temperature T4 in a temperature range of 100° C. or more and 300° C. or less at an average cooling rate CR3 of 10° C./sec or more; and
heating from the cooling stop temperature T4 to a reheating temperature T5 that satisfies the following requirement in a temperature range of 300° C. or more and 500° C. or less and performing holding for a time t5 of 350 seconds or more and 1800 seconds or less in the temperature range of the reheating temperature T5,
wherein a difference between the reheating temperature T5 and an average temperature of the cooling stop temperature T3a and the retention end temperature T3b being 50° C. or less.