Continuous casting method of steel

A method of continuous casting of steel, the method comprising:

• dispensing molten steel into a continuous casting mold and solidifying the molten steel to form a slab including a solidified shell and an unsolidified central layer;
• intentionally bulging wide side surfaces of the slab having there inside the unsolidified layer with a total intentional bulging amount of 3 to 10 mm by increasing stepwise toward a downstream side in a casting direction a roller gap of a plurality of pairs of slab support rollers disposed in a curved continuous casting machine or a vertical-bending continuous casting machine;
• and
• after bulging the wide side surfaces of the slab, performing rolling reduction on the wide side surfaces of the slab in a soft reduction zone in which the roller gap of a plurality of pairs of slab support rollers is reduced stepwise toward the downstream side in the casting direction;
• wherein
• the wide side surfaces of the slab undergo rolling reduction at a rolling reduction speed of 0.3 to 2.0 mm/min with a total rolling reduction amount smaller than or equal to the total intentional bulging amount in the soft reduction zone, and
• a solid phase fraction at a center of a thickness of the slab is smaller than 0.2, or is greater than or equal to a flow limit solid phase fraction and not greater than 1.0 in a reformation zone in which a shape of the slab in the casting direction is reformed from a circular arc shape into a linear shape.