Ferritic heat-resistant steel and method for producing the same

A method for producing a ferritic heat-resistant steel, having a chemical composition consisting of, in % by mass:

• C:
• 0.03 to 0.15, Si:
• 0 to 0.8, Cr:
• 8.0 to 11.5, Mo:
• 0.2 to 1.5, W:
• 0.4 to 3.0, B:
• 0.002 to 0.010, V:
• 0.1 to 0.4, Nb:
• 0.02 to 0.12, Co:
• 0 to less than 2.0, N:
• 0.02 to 0.10 with the balance of iron and inevitable impurities, an optional content of Ta not more than 0.5, an optional content of Al not more than 0.020, and optional content of Ni not more than 0.4, and having a tempered microstructure and a structure including prior austenite grains in which structure at least one of internal strain or internal stress induced by martensitic transformation is more relaxed than prior austenite grains of ferritic heat-resistant steel heat treated in a solution heat treatment step, a normalizing step and a tempering heat treatment step prescribed for heat treatment conditions for ferritic heat-resistant steel according to the ASME boiler and pressure vessel code or equivalent codes, comprising:
• (a) a solution heat treatment step in which a steel material made up of the ferritic heat-resistant steel is subjected to solution heat treatment at an austenitization temperature,
• (b) a normalizing step cooling the steel material made up of the ferritic heat-resistant steel from the austenitization temperature down to a martensite-untransformed austenite two-phase state temperature in a range of 260° C. to 400° C. at which the steel material is partly transformed into martensite, wherein the two-phase state temperature is lower than the martensitic transformation start temperature (Ms) and higher than the martensitic transformation finish temperature (Mf),
• (c) a step in which the steel material is heated from the two-phase state temperature up to an intermediate tempering heat treatment temperature, wherein the intermediate tempering heat treatment temperature is higher than the martensitic transformation start temperature (Ms) and lower than a second and final tempering heat treatment temperature,
• (d) a step in which the steel material is once cooled down to a temperature lower than the martensitic transformation finish temperature (Mf) thereby transforming the remaining untransformed austenite phase into martensite, and
• (e) a step in which the steel material is subjected to final tempering heat treatment at the second and final tempering heat treatment temperature higher than a temperature at which the steel material made up of the ferritic heat-resistant steel is used.