Continuous cast slab and producing method therefor

1. A continuous cast steel slab comprising, by mass %,

C: 0.01-0.3%,

Si: 0.05-0.5%,

Mn: 0.4-2%,

P: 0.03% or less,

S: 0.03% or less,

Al: 0.005-0.03%,

Ni: 0.2-2%,

O:0.006% or less,

N: 0.006% or less; and

a balance of Fe and inevitable impurities;

wherein a structure in the steel in a region within at least a depth of 2 mm from a broad surface of the slab is composed of ferrite and pearlite, and an equivalent circular diameter of ferrite grains in the region is equal to or shorter than 30 μm.

2. The continuous cast steel slab according to claim 1 , further comprising,

by mass %,

Cu: 0.2-2%, and

Cr: 0.2-2%.

3. The continuous cast steel slab according to claim 1 , further comprising,

by mass %,

Ti: 0.005-0.02%,

Nb: 0.005-0.04%, and

V: 0.005-0.04%.

4. A method for producing a continuous cast steel slab, the method comprising: casting continuously a molten steel comprising by mass %,

C: 0.01-0.3%,

Si: 0.05-0.5%,

Mn: 0.4-2%,

P: 0.03% or less,

S: 0.03% or less,

Al: 0.005-0.03%,

Ni: 0.2-2%,

O: 0.006% or less,

N: 0.006% or less, and

a balance of Fe and inevitable impurities,

using a vertical-bending or a bow continuous casting machine;

cooling down a surface of the cast steel slab to 550° C. or lower between a mold outlet and a straightening zone of the vertical-bending or the bow continuous casting machine; and

thereafter reheating between the mold outlet and the straightening zone of the vertical-bending or the bow continuous casting machine to 850° C. or higher to straighten the cast steel slab.