PROCESS FOR PRODUCTION OF THICK STEEL SHEET

摘要

Disclosed is a process for producing a thick steel sheet in which the coarsening of a structure of a surface layer part of a mother material and a structure of a thicknesswise center part of a welding-heat-affected part can be prevented, and which is applicable as a steel for a large-size structure to which high heat input welding is to be applied, and in which both the mother material and the welding-heat-affected part have excellent toughness. A steel having such a chemical composition that the contents of C, Mn, Nb, Ti and N fulfill specified requirements is cooled at a cooling rate at the thicknesswise center part of 0.1°C/s or more in a temperature range from 1100 to 1300°C and is then casted to produce a cast piece that has TiN having an equivalent circle diameter of 0.02 to 0.05 µm formed on the surface layer part thereof at a density of 7.0 × 104 particles/mm2 or more and also has TiN having an equivalent circle diameter of 0.05 to 0.2 µm formed on the thicknesswise center part thereof at a density of 5.0 × 104 particles/mm2 or more. The cast piece is heated under predetermined conditions, roughly rolled, subjected to a finishing rolling at a temperature ranging from Ar3 to 880°C inclusive at a cumulative rolling reduction of 40% or more, and then subjected to accelerated cooling at a temperature ranging from Ar3 to 550°C inclusive at a cooling rate of 5°C/s or more on thicknesswise average.