Heat resistant steel having excellent toughness for use in thermonuclear reactor, and manufacturing method therefor

Mo and Nb which have been used so far for improvement of creep strength of a heat resistant steel are replaced with W and Ta for lowering radio-activation, and the content thereof and amount of other alloy elements are optimized to attain both of high temperature creep strength and reduction of radio-activity. Among factors giving influences on the stability of the performance or on embrittlement of the steel containing W and Ta during long time use, trace amount of B, Nb and Mo at a level of impurities causes the largest aging degradation of toughness and, next to them, P, Cu, Sn, Sb and As have significant undesired effects. Since the embrittlement during use can be prevented by strictly controlling the impurity elements, chemical ingredients are restricted to manufacture a heat resistant steel having excellent toughness for use in a thermonuclear reactor. Since high temperature strength, especially, high temperature strength at creep rupture is greatly improved, and excellent performance, especially, stability in the toughness for a long period of time is attained, it is extremely useful as a material for a first wall of a thermonuclear reactor. (N.H.)