Ferritic heat resistant steel

Abstract

Heat-resistant steel excellent in high-temperature long-term creep strength and creep-fatigue strength is provided. This heat-resistant steel comprises C: 0.01 to 0.13%, Si: 0.15 to 0.50%, Mn: 0.2 to 0.5%, P: not higher than 0.02%, S; not higher than 0.005%, Cr: exceeding 8.0% but lower than 12.0%, Mo: 0.1 to 1.5%, W: 1.0 to 3.0%, V: 0.1 to 0.5%, Nb: 0.02 to 0.10%, sol. Al: not higher than 0.015%, N: 0.005 to 0.070%, Nd: 0.005 to 0.050% and B: 0.002 to 0.015%, with the balance Fe and impurities; the content of Ni is lower than 0.3%, the content of Co is lower than 0.3% and the content of Cu is lower than 0.1% among the impurities; the steel contains Nd inclusions at a Nd inclusion density of not lower than 10000 inclusions / mm3. This steel may contain, in addition to the above-specified components, one or more elements of Ta, Hf, Ti, Ca and Mg.

Description

TECHNICAL FIELD [0001] The present invention relates to ferritic heat resistant steel. More specifically, it relates to ferritic heat resistant steel excellent in high-temperature long-term creep strength and creep-fatigue strength. The heat resistant steel of the invention is suited for use as heat exchanger tubes, steel plates for pressure vessels, turbine members and the like which are used under high-temperature and high-pressure environments in boilers, nuclear power plant facilities, chemical industry facilities and so forth. BACKGROUND OF THE INVENTION [0002] Heat resistant steels used in high-temperature and high-pressure environments in boilers, nuclear power plant facilities, chemical industry facilities and the like are generally required to have high-temperature creep strength, creep-fatigue strength, corrosion resistance and oxidation resistance. [0003] High-Cr ferritic steels are superior in strength and corrosion resistance at temperatures of 500 to 650° C. in low all...

AI Technical Summary

Benefits of technology

[0013] When the life of the PP type strain wave form mentioned above is compared with the life of the CP type strain wave form, the life of the CP type strain wave form causing creep damages is shorter. Generally, the lives of heat resistant steels used in boilers, nuclear power plants and chemical plants under high-temperature and high-pressure environments are estimated by carrying out a creep-fatigue test in the total strain range of 0.4 to 1.5%.

[0026] Temper embrittlement is generally known as a phenomenon caused by segregation of S. Temper embrittlement results when martensite is tempered in a certain temperature range around 600° C. and a minute amount of Mo is known to be effective in reducing that phenomenon.

[0029] If the segregation of the liberated S produces adverse influences on the creep-fatigue characteristics, as mentioned above, it is expected that the creep-fatigue strength may possibly be increased by incorporating, in addition to Mn, an element capable of more firmly trapping S.

[0035] As a result, it was revealed that the steel containing Nd, in combination with a minute amount of Cu, Ni or Co, showed a creep-fatigue life of about 4000 cycles and thus improved in creep-fatigue characteristics, as compared with the steel containing no Nd but, when compared with the steel containing only Nd, the creep-fatigue life was markedly inferior.

[0037] (3) Addition of Nd and immobilizing S by Nd oxide or in the form of composite inclusions comprising Nd oxide and Nd sulfide and further immobilizing S partly as MnS, it becomes possible to markedly improve the creep-fatigue strength. That effect is significant when the Nd inclusion density is not lower than 10000 / mm3. The “Nd inclusions” is a term collectively referring to the above-mentioned “Nd oxide” and “composite inclusions comprising Nd oxide and Nd sulfide”.

Problems solved by technology

However, the inventions disclosed in these documents are not intended to improve the creep-fatigue strength of the heat resistant steels.

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