High-strength cold-rolled steel sheet with excellent material homogeneity and production method therefor

A production method for a high-strength cold-rolled steel sheet with excellent material homogeneity, the production method comprising:

• continuously casting molten steel to obtain a slab,
• the molten steel having a chemical composition including:
• C:
• 0.15 to 0.25%, by mass %,
• Si:
• 1.2 to 2.2%, by mass %,
• Mn:
• 1.7 to 2.5%, by mass %,
• P:
• 0.05% or less, by mass %,
• S:
• 0.005% or less, by mass %,
• Al:
• 0.01 to 0.10%, by mass %,
• N:
• 0.006% Of less, by mass %,
• Ti:
• 0.003 to 0.030%, by mass %,
• B:
• 0.0002 to 0.0050%, by mass %, and
• Fe and inevitable impurities;
• cooling the slab to 600° C. within 6 hours from a termination of the continuous casting;
• reheating the cooled slab;
• after reheating the slab, hot rolling the slab under the conditions of a hot rolling start temperature of 1,150 to 1,270° C. and a finishing delivery temperature of 850 to 950° C. to produce a hot-rolled steel sheet;
• starting cooling of the hot-rolled steel sheet within 1 second after completion of the hot rolling;
• performing first cooling of the hot-rolled steel sheet to 650° C. or lower at a first average cooling rate of 80° C/s or more;
• performing second cooling of the hot-rolled steel sheet to 585° C. or lower at a second average cooling rate of 5° C/s or more;
• performing coiling of the hot-rolled steel sheet;
• then, after the coiling, performing cold rolling of the hot-rolled steel sheet;
• and
• performing continuous annealing that includes;
• (i) heating to a temperature range of from 800° C. to Ac3 transformation temperature at an average heating rate of 3 to 30° C/s,
• (ii) holding at a first soaking temperature within a temperature range of from 800° C. to the Ac3 transformation temperature for 30 seconds or longer,
• (iii) performing primary cooling to a primary cooling finish temperature of 650° C. or higher at a third average cooling rate of 1° C/s or more,
• (iv) performing cooling from the primary cooling finish temperature to 100° C. or lower at a fourth average cooling rate of 100 to 1,000° C/s, and
• (v) then, after step (iv), holding in a second soaking temperature range of from 100 to 250° C. for 120 to 1,800 seconds.