Patent ID: 9534269
Date: 2017-01-03
CPC Classifications: C21D,C22C

Claim:
1. A method of manufacturing a cold rolled steel sheet comprising: hot-rolling and cold-rolling a steel slab having a composition comprising, in percent by mass, more than 0.015% to less than 0.100% of carbon, less than 0.40% of silicon, 1.0% to 1.9% of manganese, more than 0.015% to 0.05% of phosphorus, 0.03% or less of sulfur, 0.01% to 0.3% of soluble aluminum, 0.005% or less of nitrogen, less than 0.30% of chromium, 0.0050% or less of boron, less than 0.15% of molybdenum, 0.4% or less of vanadium, 0.02% or less of titanium, and 0.18% or less of copper, and satisfying formula (1): wherein [% A] is content (% by mass) of alloying element A, the balance being iron and incidental impurities to form a steel sheet; annealing the steel sheet at an annealing temperature of 750° C. to 830° C.; subjecting the steel sheet to first cooling at an average cooling rate of 3° C./sec to 40° C./sec in a temperature range from the annealing temperature to 480° C.; subjecting the steel sheet to second cooling at an average cooling rate of 8° C./sec to 80° C./sec in a temperature range from 480° C. to Tc (° C.) given by formula (6): wherein [% A]is the content (% by mass) of alloying element A; and subjecting the steel sheet to third cooling at an average cooling rate of 0.3° C./sec to less than 1° C./sec in a temperature range from Tc (° C.) to 200° C., wherein at least one of the first, second and third cooling rates is different from the other cooling rates and the third cooling rate is lower than the second cooling rate, and the second cooling rate is greater than the first cooling rate.