Continuously cast slab and method for producing the same

Abstract

A continuously cast slab that is unlikely to cause thermal cracking during cooling therefor even if the toughness of the slab is low, and a method for producing the same. Specifically, a continuously cast slab for high-strength steel with features such that the average prior austenite grain size at a position 10 mm from the surface layer of the continuously cast slab is in the range of 0.5 mm to 2.0 mm; and a total of an area ratio of ferrite and an area ratio of pearlite in the microstructure of the slab is 90% or more, and the area ratio of ferrite is less than 5% or 10% or more.

1 . A continuously cast slab for high-strength steel, characterized in that
 an average prior austenite grain size at a position 10 mm from a surface layer of the continuously cast slab is in the range of 0.5 mm to 2.0 mm;   a total of an area ratio of ferrite and an area ratio of pearlite in a microstructure of the slab is 90% or more; and   the area ratio of ferrite is less than 5% or 10% or more.   
     
     
         2 . The continuously cast slab according to  claim 1 , comprising, in mass %:
 C: in a range of 0.10% to 1.00%;   Si: in a range of 0.10% to 2.50%; and   Mn: in a range of 0.40% to 5.00%.   
     
     
         3 . A method for producing a continuously cast slab for high-strength steel, the continuously cast slab preventing thermal cracking due to cooling, comprising
 subjecting the continuously cast slab having the ingredient composition according to claim  2  to the following:   a first cooling step of cooling the continuously cast slab under a cooling condition that a retention time while a cooling temperature of a center of a face at a position 10 mm from a surface layer of the continuously cast slab is in a range of 1200° C. to 1450° C. is 130 seconds or less;   a second cooling step of cooling the continuously cast slab under a cooling condition that an average cooling rate when a surface temperature at the center of the wide face of the continuously cast slab is in a range of 700° C. to 850° C. is 20° C./hr or less; and   a third cooling step of cooling the continuously cast slab under a cooling condition that an average cooling rate when the surface temperature at the center of the wide face of the continuously cast slab is in a range of 500° C. to 700° C. is 10° C./hr or less.