Method for continuously casting slab

A method for continuously casting a slab, the method comprising a series of:

• a step 1 of casting a slab having a round cross section with a casting mold;
• a step 2 of carrying out reduction on the slab by a plurality of pairs of rolls consisting of cylindrical rolls whose axes are arranged in a horizontal direction until there is no unsolidified portion inside the slab, to form a pair of parallel surfaces on the slab;
• a step 3 of alternatively carrying out reduction on the slab completely solidified after reduction in the step 2, by a pair of cylindrical horizontal rolls whose axes are arranged in the horizontal direction and a pair of cylindrical vertical rolls whose axes are arranged in a vertical direction, to form a pair of parallel surfaces which are vertical to the pair of parallel surfaces formed in the step 2 on the slab, wherein:
• in the step 2, a reduction ratio of the slab by each pair of the plurality of pairs of rolls is more than 0.5% and no more than 3% and a ratio of a width of the unsolidified portion on a cross section of the slab at a reduction position of said each pair of rolls to a width of a part of the slab where the rolls have contact is from 0 to 7.15;
• and
• in the step 3, each of a reduction ratio of the slab by the horizontal rolls and a reduction ratio of the slab by the vertical rolls is from 5.4% to 6.8%.