Manufacturing method of a high-strength cold-rolled steel sheet having excellent uniform elongation and hole expandability

1. A manufacturing method of a high-strength cold-rolled steel sheet having excellent uniform elongation and hole expandability, comprising:

on a steel billet containing:

in mass %,

C: 0.01 to 0.4%;

Si: 0.001 to 2.5%;

Mn: 0.001 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,

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 pre-cold rolling 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 less than 40° C. nor more than 140° C.;

performing cold rolling at a reduction ratio of not less than 30% nor more than 70%;

performing heating up to a temperature region of 700 to 900° C. and performing holding for not shorter than 1 second nor longer than 1000 seconds;

performing post-cold rolling primary cooling down to a temperature region of 580 to 750° C. at an average cooling rate of 12° C./second or less;

performing post-cold rolling secondary cooling down to a temperature region of 350 to 500° C. at an average cooling rate of 4 to 300° C./second; 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)

wherein, C, N, Mn, Nb, Ti, B, Cr, Mo, and V each represent the content of the element (mass %),

t≦ 2.5× t 1  (2)

wherein, 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)

wherein, 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)

wherein, 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 uniform elongation and hole expandability according to claim 1 , further comprising:

after performing the pre-cold rolling primary cooling, performing pre-cold rolling secondary cooling down to a cooling stop temperature of 600° C. or lower at an average cooling rate of 10 to 300° C./second before performing the cold rolling, and performing coiling at 600° C. or lower to obtain a hot-rolled steel sheet.

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

4. The manufacturing method of the high-strength cold-rolled steel sheet having excellent uniform elongation and hole expandability according to claim 1 , wherein

the waiting time t second further satisfies Expression (2a) below,

t<t 1  (2a).

5. The manufacturing method of the high-strength cold-rolled steel sheet having excellent uniform elongation and hole expandability according to claim 1 , wherein

the waiting time t second further satisfies Expression (2b) below,

t 1≦ t≦t 1×2.5  (2b),

6. The manufacturing method of the high-strength cold-rolled steel sheet having excellent uniform elongation and hole expandability according to claim 1 , wherein post-hot rolling primary cooling is started between rolling stands.

7. The manufacturing method of the high-strength cold-rolled steel sheet having excellent uniform elongation and hole expandability according to claim 1 , wherein

when the heating is performed up to the temperature region of 700 to 900° 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 a temperature region of 700 to 900° C. is set to HR2 (° C./second) expressed by Expression (6) below,

HR 1≧0.3  (5)

HR 2≦0.5× HR 1  (6).

8. The manufacturing method of the high-strength cold-rolled steel sheet having excellent uniform elongation and hole expandability according to claim 1 , further comprising:

performing hot-dip galvanizing on the surface.

9. The manufacturing method of the high-strength cold-rolled steel sheet having excellent uniform elongation and hole expandability according to claim 8 , further comprising:

performing an alloying treatment at 450 to 600° C. after performing the hot-dip galvanizing.