Continuous steel casting method

A continuous steel casting method, wherein, during continuous casting of steel, a gap between strand support rolls facing each other with a strand in a rectangular shape interposed therebetween is increased toward a downstream side in a casting direction to thereby bulge the strand having an unsolidified layer thereinside such that a thickness between long-side surfaces of the strand increases within the range of 0.1% or more and 10% or less of the thickness of the strand inside a mold, and wherein, when the long-side surfaces of the bulged strand are rolled by a plurality of guide rolls,

• a total rolling reduction and a reduction gradient satisfy formulas (1) and (2) below when a solid phase fraction in a central portion of the strand is within the range of 0.2 or more and less than 0.9, and the total rolling reduction and the reduction gradient satisfy formulas (3) and (4) below when the solid phase fraction in the central portion of the strand is within the range of 0.9 or more:
• 0.5≤Rt1/D0≤1.0, (1)
• 0.5≤Rg1≤3.0, (2)
• 0.2≤Rt2/D0≤1.0, (3)
• 0.1≤Rg2≤1.5, (4)
• where Rt1:
• the total rolling reduction (mm) of the strand when the solid phase fraction is within the range of 0.2 or more and less than 0.9, D0:
• the amount of bulging (mm) of the strand, Rg1:
• the reduction gradient (mm/m) of the strand when the solid phase fraction is within the range of 0.2 or more and less than 0.9, Rt2:
• the total rolling reduction (mm) of the strand when the solid phase fraction is within the range of 0.9 or more, and Rg2:
• the reduction gradient (mm/m) of the strand when the solid phase fraction is within the range of 0.9 or more.