Heat resistant ferritic steel

Abstract

There is provided a heat resistant ferritic steel, excellent in the weld crack resistance of the HAZ and creep strength. A high-Cr heat resistant ferritic steel is characterized by consisting of, by mass %, Si: more than 0.1% and not more than 1.0%, Mn: 2.0% or less, Co: 1 to 8%, Cr: 7 to 13%, V: 0.05 to 0.4%, Nb: 0.01 to 0.09%, either one or both of Mo and W: 0.5 to 4% as a total, B: 0.005 to 0.025%, Al: 0.03% or less, and N: 0.003 to 0.06%, and containing C in an amount satisfying Expression (1), the balance being Fe and impurities, and O, P and S as impurities being such that O: 0.02% or less, P: 0.03% or less, and S: 0.02% or less, respectively,0.005≦C≦(−5 / 3)×B+0.085  (1)in which C and B represent the content of each element (mass %). Furthermore, the high-Cr heat resistant ferritic steel may contain one or more kinds selected from the group consisting of Nd, Ta, Ca and Mg.

Description

TECHNICAL FIELD[0001]The present invention relates to a heat resistant ferritic steel excellent in the high-temperature strength and weld crack resistance in a weld heat-affected zone, which is used as members for high temperature services, such as thermal power generation boilers.BACKGROUND ART[0002]In recent years, in the thermal power generation, efforts are being made toward higher temperatures and higher pressure steam conditions in order to improve thermal efficiency. Also a new plant will be opened under ultra super critical pressure conditions of 650° C. and 350 atmospheric pressures. Heat resistant terrific steels are widely used because they are inexpensive compared to austenitic stainless steels and have small coefficients of thermal expansion, which is the advantage for heat resistant steels.[0003]For heat resistant ferritic steels, efforts have been made to increase strength in order to cope with severer steam conditions in the future. In Patent Document 1 and Patent Do...

AI Technical Summary

Benefits of technology

[0021]Hence, the inventors have found that in order to improve the creep strength in the HAZ and also to ensure excellent weldability, optimizing the contents of C and B is effective as follows. The heat resistant ferritic steel. in accordance with the present invention. sets a target value of rupture time for the creep strength in the HAZ of not less than three times, and preferably not less than five times that of conventional steels.

[0024]However, it has been found that when B is added in an amount larger than the amount at which the creep strength improving effect is obtained, the susceptibilities of the solidification cracking in a weld metal and of the liquation cracking in its HAZ increase.

Problems solved by technology

For this reason, the problem is that the advantage of steels whose strength is increased cannot be adequately made use of.

For this reason, the problem is that when the steel is used as thick wall members in main steam pipes for boilers, pressure vessels and the like, this steel does not have sufficient weldability (weld crack resistance).

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