Continuously cast slab and manufacturing method therefor

A continuously cast slab for high-strength steel comprising, by mass %,

• C:
• 0.10 to 1.00%,
• Si:
• 0.10 to 2.50%, and
• Mn:
• 0.40 to 5.00%, and
• optionally comprising, by mass %, P:
• 0.100% or less, S:
• 0.0200% or less, Al:
• 0.100% or less, N:
• 0.0100% or less, and O:
• 0.0100% or less, and further optionally contains, by mass %, at least one or two or more elements selected from the group consisting of Ti:
• 0.200% or less, Nb:
• 0.200% or less, V:
• 0.200% or less, Ta:
• 0.10% or less, W:
• 0.10% or less, Cr:
• 1.00% or less, Mo:
• 1.00% or less, Ni:
• 1.00% or less, B:
• 0.0100% or less, Co:
• 1.00% or less, Cu:
• 1.00% or less, Sn:
• 0.200% or less, Sb:
• 0.200% or less, Ca:
• 0.0100% or less, Mg:
• 0.0100% or less, REM:
• 0.0100% or less, Zr:
• 0.100% or less, Te:
• 0.100% or less, Hf:
• 0.10% or less, and Bi:
• 0.200% or less, with a balance being Fe and unavoidable impurities, wherein:
• an average prior austenite grain size ratio (d20/d10) is between 1.0 and 4.0, provided that d10 is an average prior austenite grain size at a position 10 mm below a surface layer of the continuously cast slab and d20 is an average prior austenite grain size at a position 20 mm below the surface layer of the continuously cast slab;
• a microstructure of the continuously cast slab consists of ferrite, pearlite, and a low-temperature transformation phase, the low-temperature transformation phase containing at least one selected from the group consisting of bainite, tempered martensite, quenched martensite, and residual austenite;
• the microstructure at the position 10 mm below the surface layer of the continuously cast slab is such that a total of an area ratio of the ferrite and an area ratio of the pearlite is 80% or more, provided that a total of the area ratio of the ferrite, the area ratio of the pearlite, and an area ratio of the low-temperature transformation phase is 100%;
• and
• the microstructure at the position 20 mm below the surface layer of the continuously cast slab is such that a total of an area ratio of the ferrite and an area ratio of the pearlite is 60% or more, provided that a total of the area ratio of the ferrite, the area ratio of the pearlite, and an area ratio of the low-temperature transformation phase is 100%.