Continuous casting method for steel and reduction roll for continuous casting

A continuous casting method for steel, comprising:

• providing a slab at a position at which a center solid phase ratio of the slab is 0.8 or more and which includes after complete solidification;
• providing at least one pair of reduction rolls;
• and
• reducing the slab by the at least one pair of reduction rolls during continuous casting, wherein a slab width to be cast is defined as W (mm) and a slab thickness is defined as t (mm),
• wherein, in the pair of reduction rolls, one reduction roll on a lower side is a flat roll, and the other reduction roll on an upper side is a convex roll formed in a roll outer peripheral shape in a cross-section including a roll rotation axis has a convex shape overhanging outward in a region including a width-direction center position of the slab,
• wherein the convex shape is any one of a curved shape that is convex outward and has no corner portion or a shape that is a combination of a curve that is convex outward and a straight line with a length within 0.25×W and that has no corner portion, in a convex shape defining range of a total length of 0.80×W on both sides in a roll width direction from the width-direction center position,
• wherein a reduction roll radius at the width-direction center position is greater by 0.005×t or more than a reduction roll radius at both ends of the convex shape defining range,
• wherein the roll outer peripheral shape has a straight line parallel to the roll rotation axis at both ends in the width direction, and
• has a concave curve that is smoothly connected to the straight line, has an inflection point where a second-order differential value of a function defining the roll outer peripheral shape is 0, and the second-order differential value is continuous before and after the inflection point, and
• wherein the roll outer peripheral shape within the convex shape defining range forms an arcuate shape in which a length portion of the convex shape defining range is a chord.