Dual phase steel sheet and manufacturing method thereof

1. A dual phase steel sheet comprising:

in mass %,

C: 0.01 to 0.1%;

Mn: 0.2 to 3%;

Al: 0.04 to 1.5%;

Ti: 0.015 to 0.2%;

Si: 0 to 0.5%;

Nb: 0 to 0.06%;

Cu: 0 to 1.2%;

Ni: 0 to 0.6%;

Mo: 0 to 1%;

V: 0 to 0.2%;

Cr: 0 to 2%;

W: 0 to 0.5%;

Mg: 0 to 0.01%;

Ca: 0 to 0.01%;

REM: 0 to 0.1%;

B: 0 to 0.002%;

P: 0.01% or less;

S: 0.005% or less;

N: 0.01% or less,

in which [Ti]−48/14×[N]−48/32×[S]≧0% is satisfied and when Ex.C (%)=[C]−12/48×{[Ti]+48/93×[Nb]−48/14×[N]−48/32×[S]} is set, 0.001≦Ex.C (%)/fsd (%)≦0.01 is satisfied, and

a balance comprising Fe and impurities, wherein

at the position of ¼ thickness of a sheet thickness, a microstructure is a dual phase with its main phase composed of polygonal ferrite precipitation-strengthened by carbide of Ti and its second phase composed of 1 to 10% in area fraction (fsd (%)) of low-temperature transformation products dispersed plurally, and

an average crystal diameter of the low-temperature transformation product is 3 to 15 μm and an average value of a distance of closest approach between the low-temperature transformation products is 10 to 20 μm.

2. The dual phase steel sheet according to claim 1 , comprising:

in mass %,

Si: 0.02% to 0.5%.

3. The dual phase steel sheet according to claim 1 , comprising:

one or two or more of

in mass %,

Nb: 0.005 to 0.06%;

Cu: 0.02 to 1.2%;

Ni: 0.01 to 0.6%;

Mo: 0.01 to 1%;

V: 0.01 to 0.2%;

Cr: 0.01 to 2%; and

W: 0.01 to 0.5%.

4. The dual phase steel sheet according to claim 1 , comprising:

one or two or more of

in mass %,

Mg: 0.0005 to 0.01%;

Ca: 0.0005 to 0.01%; and

REM: 0.0005 to 0.1%.

5. The dual phase steel sheet according to claim 1 , comprising:

in mass %,

B: 0.0002 to 0.002%.

6. The dual phase steel sheet according to claim 1 , wherein

galvanizing is performed on its surface.

7. A manufacturing method of a dual phase steel sheet according to claim 1 , the method comprising:

on a slab containing:

in mass %,

C: 0.01 to 0.1%;

Mn: 0.2 to 3%;

Al: 0.04 to 1.5%;

Ti: 0.015 to 0.2%;

Si: 0 to 0.5%;

Nb: 0 to 0.06%;

Cu: 0 to 1.2%;

Ni: 0 to 0.6%;

Mo: 0 to 1%;

V: 0 to 0.2%;

Cr: 0 to 2%;

W: 0 to 0.5%;

Mg: 0 to 0.01%;

Ca: 0 to 0.01%;

REM: 0 to 0.1%;

B: 0 to 0.002%;

P: 0.01% or less;

S: 0.005% or less;

N: 0.01% or less,

in which [Ti]−48/14×[N]−48/32×[S]≧0% is satisfied and when Ex.C (%)=[C]−12/48×{[Ti]+48/93×[Nb]−48/14×[N]−48/32×[S]} is set, 0.001≦Ex.C (%)/fsd (%)≦0.01 is satisfied, and

a balance comprising Fe and impurities, performing heating to a temperature SRT min (° C.) or higher, which is defined by Expression (1) below, and then in hot rolling, performing rough rolling at a reduction ratio of 20% or more in a temperature zone of not lower than 1050° C. nor higher than 1150° C. for at least one pass, and then starting finish rolling within150 seconds in a temperature zone of 1000° C. or higher and lower than 1080° C., and completing finish rolling with the total reduction ratio for plural passes of not less than 75% nor more than 95% in a temperature zone of not lower than an Ar3 transformation point temperature +50° C. nor higher than 1000° C.; and

within 3 seconds, performing cooling down to lower than the Ar3 transformation point temperature at an average cooling rate of 15° C./sec or more, and next performing cooling down to a temperature zone of higher than 600° C. at an average cooling rate of 10° C./sec or less for a time period of 1 second or longer and shorter than 100 seconds, and next performing cooling down to a temperature zone of 350° C. or lower at a cooling rate of 15° C./sec or more, and performing coiling,

SRT min =10780/{5.13−log([Ti]×[C])}−273  Expression (1).

8. The manufacturing method of the dual phase steel sheet according to claim 7 , further comprising:

in the hot rolling, performing rough rolling at a reduction ratio of 20% or more in a temperature zone of not lower than 1050° C. nor higher than 1150° C. for plural passes, wherein

the total reduction ratio of the rough rolling is not less than 60% nor more than 90%.

9. The manufacturing method of the dual phase steel sheet according to claim 7 , further comprising:

performing cooling down to a temperature zone of 100° C. or lower and performing coiling.

10. The manufacturing method of the dual phase steel sheet according to claim 7 , wherein

in the performing the cooling down to the temperature zone of higher than 600° C. at an average cooling rate of 10° C./sec or less for a time period of 1 second or longer and shorter than 100 seconds, when a total cumulative diffusion length L total of Ti in ferrite is expressed by Expression (3) below by adding up a diffusion length L of Ti in ferrite expressed by Expression (2) below for an infinitesimal time a Δt/sec from a cooling completing temperature to coiling, 0.15≦L total ≦0.5 is satisfied,

L=√D ( T+ 273) t   Expression (2),

L total =Σ√( D ( T+ 273)Δ t )  Expression (3),

Here, D(T+273) is a volume diffusion coefficient at T° C. t is a diffusion time period, D(T) is expressed by Expression (4) below using a diffusion coefficient D0 of Ti, an activation energy Q, and a gas constant R,

D ( T )= D 0×Exp(− Q/R ·( T+ 273))  Expression (4).

11. The manufacturing method of the dual phase steel sheet according to claim 7 , wherein

in the performing the cooling down to the temperature zone of higher than 600° C. at an average cooling rate of 10° C./sec or less for a time period of 1 second or longer and shorter than 100 seconds, a steel sheet is immersed in a galvanizing bath to galvanize its surface.

12. The manufacturing method of the dual phase steel sheet according to claim 11 , further comprising:

on a galvanized dual phase steel sheet, performing an alloying treatment in a temperature range of 450 to 600° C.