PURPOSE: To improve the strength of a steel having a specified composition contg. C, Ni, Cr, Si Mn and one or more among Nb, Ti, Zr and Ta at high temp. without deteriorating the workability by adjusting the austenite grain size of the steel.

CONSTITUTION: This steel having high strength at high temp. and contg. a carbide forming element is obtd. by adjusting the austenite grain size of a steel having a composition consisting of 0.06W0.30% C, 9W45% Ni, 15W30% Cr, ≤1.0% Si, ≤2.0% Mn, one or more among Nb, Ti, Zr and Ta as carbide forming elements and the balance essentially Fe while satisfying equations I WVI to a large grain size prescribed by JIS 3W5 by a suitable soln. heat treatment. To the composition may be added ≤0.1% one or more among Ca, Mg, Y and Se besides said elements.