Continuously cast slab and method for producing the same

Abstract

A continuously cast slab that can prevent thermal cracking during cooling therefor even if the toughness of the slab is low, and a method for producing the same. Specifically, provided is a continuously cast slab for high-strength steel with features such that the average prior austenite grain size at a position of 10 mm from the surface layer of the continuously cast slab is in the range of 0.5 mm to 2.0 mm; and in the microstructure of the slab, the total area ratio of bainite and ferrite is 90% more, and the area ratio of ferrite is 0%, or 3% 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 a range of 0.5 mm to 2.0 mm;   a total of an area ratio of bainite and an area ratio of ferrite in a microstructure of the slab is 90% or more; and   the area ratio of ferrite is 0%, or 3% or more.   
     
     
         2 . The continuously cast slab according to  claim 1 , comprising, in mass %:
 C: in a range of 0.10% to 0.40%,   Si: in a range of 0.10% to 2.50%, and   Mn: in a range of 1.00% 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 wide 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 between 25° C./hr and 40° C./hr, or 50° C./hr or more; 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 15° C./hr or more.