Abstract: A high strength, low alloy, heat resistant steel having excellent weldability has an average crystal grain diameter of at most 110 &mgr;m and consists essentially of, by mass %: C: 0.03-0.15%, Si: at most 1%, Mn: at most 2%, P: at most 0.03%, S: at most 0.03%, Ni: at most 0.5%, Cu: at most 0.5%, Cr: 1.8-2.8%, V: 0.1-0.3%, Nb: 0.01-0.08%, Mo: 0.05-0.35%, W: 1.2-1.8%, Ti: 0.001-0.05%, B: 0-0.02%, Al: at most 0.1%, O: at most 0.1%, N: in an amount satisfying the formula [%N]≦[%Ti]+5[%B]+0.004, and a remainder of unavoidable impurities.

Abstract: An alloy having an excellent corrosion resistance and workability and being suitable particularly for seamless tubes of the industrial waste incineration boiler. The alloy consists essentially of, in weight %,:______________________________________ up to 0.05% C, up to 0.5% Si, up to 0.5% Mn, up to 0.01% P 20-25% Cr, 8-12% Mo, more than 0.5% and up to 1.0% Nb, more than 15% and up to 20% Fe, up to 0.4% Al, up to 0.1% in total of rare earth metals, up to 0.01% Ca, up to 0.01% Mg, up to 0.01% B, ______________________________________and the balance Ni and incidental impurities, wherein the Fe content and the Nb content are defined so as to satisfy the following formula (a):Fe(%).gtoreq.4.times.Nb(%)+12.