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

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.

T 1(° C.)=850+10×(C+N)×Mn+350×Nb+250×Ti+40×B+10×Cr+100×Mo+100×V  (1)

t ≦2.5 ×t 1  (2)

Here, t1 is obtained by Expression (3) below

t 1=0.001×(( Tf−T 1)× P 1/100) 2 −0.109×(( Tf−T 1)× P 1/100)+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( t 2)=0.0002( T 2−425) 2 +1.18  (4)

Here, T2 represents an overaging treatment temperature, and the maximum value of t2 is set to 400.

2. The manufacturing method of the high-strength cold-rolled steel sheet having excellent local deformability according to claim 1 , wherein the total reduction ratio in a temperature range of lower than T1+30° C. is 30% or less.

3. The manufacturing method of the high-strength cold-rolled steel sheet having excellent local deformability according to claim 1 , wherein the waiting time t second further satisfies Expression (2a) below

t<t1  (2a).

4. The manufacturing method of the high-strength cold-rolled steel sheet having excellent local deformability according to claim 1 , wherein the waiting time t second further satisfies Expression (2b) below

t 1 ≦t≦t 1×2.5  (2b).

5. The manufacturing method of the high-strength cold-rolled steel sheet having excellent local deformability according to claim 1 , wherein the primary cooling is started between rolling stands.

6. The manufacturing method of the high-strength cold-rolled steel sheet having excellent local deformability according to claim 1 , wherein when heating is performed up to the temperature region of Ae3 to 950° C. after the cold rolling, an average heating rate of not lower than room temperature nor higher than 650° C. is set to HR1 (° C./second) expressed by Expression (5) below, and

an average heating rate of higher than 650° C. to the temperature region of Ae3 to 950° C. is set to HR2 (° C./second) expressed by Expression (6) below

HR1≧0.3  (5)

HR 2≦0.5 ×HR 1  (6).

7. The manufacturing method of the high-strength cold-rolled steel sheet having excellent local deformability according to claim 1 , further comprising:

forming a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface.