COOLING METHOD OF STEEL MATERIAL, MANUFACTURING METHOD OF STEEL MATERIAL, COOLING DEVICE OF STEEL MATERIAL AND MANUFACTURING FACILITY OF STEEL MATERIAL

摘要

PROBLEM TO BE SOLVED: To provide a technology for manufacturing a steel material having uniform hardness in the thickness direction, by suppressing generation of a martensite structure in a surface layer and hardness increase of the surface layer of the steel material caused thereby.SOLUTION: In cooling method of a steel material, when cooling a steel material after hot rolling or a reheated steel material from a temperature region of Ar 3 points or more to a bainite transformation temperature region, high-pressure air and mist are jetted out simultaneously from separated nozzles to the steel material. The pressure of the high-pressure air is 0.3 MPa or higher, the average particle diameter of the mist is 0.1 mm or smaller, and the water volume density is 10-500 mL/(m-min).SELECTED DRAWING: Figure 1