REFRACTORY STEEL MATERIAL WITH EXCELLENT WELDED-JOINT TOUGHNESS AND PROCESS FOR PRODUCING THE SAME

摘要

A refractory steel material giving a welded joint having such excellent toughness that the welded joint has high high-temperature proof strength at 700-800°C, which is a supposed fire temperature, and suffers no embrittlement even when exposed to the supposed fire temperature; and a process for producing the steel material. The refractory steel material is obtained by heating a steel billet to 1,150-1,300°C, subsequently subjecting it to hot working or hot rolling in which the final temperature is 880°C or higher, subjecting the worked or rolled steel material to accelerated cooling under such conditions that the cooling rate in a position where cooling is slowest is at least 2 °C/sec, stopping this accelerated cooling in a temperature region where the temperature of the surface of the steel material reaches 350-600°C, and then allowing the steel material to cool. The refractory steel material has a composition which contains, in terms of mass%, 0.005-0.03%, excluding 0.03%, carbon, 0.01-0.50% silicon, 0.05-0.40% manganese, 1.50-5.00% chromium, 0.05-0.50% vanadium, and 0.001-0.005% nitrogen and is reduced in the contents of nickel, copper, molybdenum, boron, phosphorus, sulfur, and oxygen.