High-strength cold-rolled steel sheet having excellent stretch flangeability and precision punchability and manufacturing method thereof

A manufacturing method of a high-strength cold-rolled steel sheet having excellent stretch flangeability and precision punchability, comprising:

• on a steel billet containing:
• in mass %,
• C:
• greater than 0.01% to 0.4% or less;
• Si:
• not less than 0.001% nor more than 2.5%;
• Mn:
• not less than 0.001% nor more than 4%;
• P:
• 0.001 to 0.15% or less;
• S:
• 0.0005 to 0.03% or less;
• Al:
• not less than 0.001% nor more than 2%;
• N:
• 0.0005 to 0.01% or less;
• and
• a balance being composed of iron and inevitable impurities,
• performing first hot rolling in which rolling at a reduction ratio of 40% or more is performed one time or more in a temperature range of not lower than 1000° C. nor higher than 1200° C.;
• setting an austenite grain diameter to 200 μm or less by the first hot rolling;
• performing second hot rolling in which rolling at a reduction ratio of 30% or more is performed in one pass at least one time in a temperature region of not lower than a temperature T1 determined by Expression (1) below +30° C. nor higher than T1+200° C.;
• setting the total reduction ratio in the second hot rolling to 50% or more;
• performing final reduction at a reduction ratio of 30% or more in the second hot rolling and then starting pre-cold rolling cooling in such a manner that a waiting time t second satisfies Expression (2) below;
• setting an average cooling rate in the pre-cold rolling cooling to 50° C./second or more and setting a temperature change to fall within a range of not less than 40° C. nor more than 140° C.;
• performing cold rolling at a reduction ratio of not less than 40% nor more than 80%;
• performing heating up to a temperature region of 750 to 900° C. and performing holding for not shorter than 1 second nor longer than 300 seconds;
• performing post-cold rolling primary cooling down to a temperature region of not lower than 580° C. nor higher than 750° C. at an average cooling rate of not less than 1° C./s nor more than 10° C./s;
• performing retention for not shorter than 1 second nor longer than 1000 seconds under the condition that a temperature decrease rate becomes 1° C./s or less;
• and
• performing post-cold rolling secondary cooling at an average cooling rate of 5° C./s or less.
• T1(° C.)=850+10×(C+N)×Mn+350×Nb+250×Ti+40×B+10×Cr+100×Mo+100×V (1)
• Here, C, N, Mn, Nb, Ti, B, Cr, Mo, and V each represent the content of the element (mass %).
• t≦2.5×t1 Expression (2)
• Here, t1 is obtained by Expression (3) below.
• t1=0.001×((Tf−T1)×P1/100)2−0.109×((Tf−T1)×P1/100)+3.1 Expression (3)
• Here, in Expression (3) above, Tf represents the temperature of the steel billet obtained after the final reduction at a reduction ratio of 30% or more, and P1 represents the reduction ratio of the final reduction at 30% or more.