Manufacturing method of a high-strength cold-rolled steel sheet having excellent local deformability

摘要

This high-strength cold-rolled steel sheet contains, in mass %, C: 0.02% to 0.20%; Si: 0.001% to 2.5%; Mn: 0.01% to 4.0%; P: 0.001% to 0.15%; S: 0.0005% to 0.03%; Al: 0.001% to 2.0%; N: 0.0005% to 0.01%; and O: 0.0005% to 0.01%; in which Si+Al is limited to less than 1.0%, and a balance being composed of iron and inevitable impurities, in which an area ratio of bainite in a metal structure is 95% or more, at a sheet thickness center portion being a range of ⅝ to ⅜ in sheet thickness from the surface of the steel sheet, an average value of pole densities of the {100}<011> to {223}<110> orientation group is 4.0 or less, and a pole density of the {332}<113> crystal orientation is 5.0 or less, and a mean volume diameter of crystal grains in the metal structure is 7 μm or less.

主权项

1. A manufacturing method of a high-strength cold-rolled steel sheet having excellent local deformability, comprising:
on a steel billet containing: in mass %, C: not less than 0.02% nor more than 0.20%; Si: not less than 0.001% nor more than 2.5%; Mn: not less than 0.01% nor more than 4.0%; P: not less than 0.001% nor more than 0.15%; S: not less than 0.0005% nor more than 0.03%; Al: not less than 0.001% nor more than 2.0%; N: not less than 0.0005% nor more than 0.01%; and O: not less than 0.0005% nor more than 0.01%; in which Si +Al is limited to less than 1.0%, 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+30° C. nor higher than T1+200° C. determined by Expression (1) below; 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 primary cooling in such a manner that a waiting time t second satisfies Expression (2) below; setting an average cooling rate in the primary cooling to 50° C./second or more and performing the primary cooling in a manner that a temperature change is in a range of not lower than 40° C. nor higher than 140° C.; performing cold rolling at a reduction ratio of not less than 30% nor more than 70%; performing heating and holding for 1 to 300 second/seconds in a temperature region of Ae3 to 950° C.; performing secondary cooling at an average cooling rate of not less than 10° C./s nor more than 200° C./s in a temperature region of Ae3 to 500° C.; and performing an overaging heat treatment in which holding is performed for not shorter than t2 seconds satisfying Expression (4) below nor longer than 400 seconds in a temperature region of not lower than 350° C. nor higher than 500° C.
T1(° C.)=850+10×(C+N)×Mn+350×Nb+250×Ti+40×B+10×Cr+100×Mo+100×V  (1)t≦2.5×t1  (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  (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
log(t2)=0.0002(T2−425)2 +1.18  (4) Here, T2 represents an overaging treatment temperature, and the maximum value of t2 is set to 400.