Austenitic stainless steel

Abstract

An austenitic stainless steel, which comprises by mass %, C: 0.04 to 0.18%, Si≦1.5%, Mn≦2.0%, Ni: 6 to 30%, Cr: 15 to 30%, N: 0.03 to 0.35%, sol. Al≦0.03% and further contains one or more elements selected from Nb≦1.0%, V≦0.5% and Ti≦0.5%, with the balance being Fe and impurities, and among the impurities P≦0.04%, S≦0.03%, Sn≦0.1%, As≦0.01%, Zn≦0.01%, Pb≦0.01% and Sb≦0.01%, and satisfy the conditions P1=S+{(P+Sn) / 2}+{(As+Zn+Pb+Sb) / 5}≦0.06 and 0.2≦P2=Nb+2(V+Ti)≦1.7−10×P1 has high strength and excellent resistance to cracking due to grain boundary embrittlement in the welded portion during the use at high temperatures. Therefore, the said steel can be suitably used as materials for constructing machines and equipment, such as power plant boilers, which are to be used at high temperatures for a long period of time.

Description

TECHNICAL FIELD[0001]The present invention relates to an austenitic stainless steel. More particularly, the present invention relates to a high strength austenitic stainless heat resistant steel which is to be used in constructing high temperature machines and equipment, such as power plant boilers, and is excellent in resistance to cracking at weld heat affected zone due to grain boundary embrittlement during use at high temperatures.BACKGROUND ART[0002]In recent years, the operation conditions for power plant boilers and the like have become higher temperature and higher pressure ones on a worldwide scale in consideration of environmental deterioration. Therefore, these austenitic stainless heat resistant steels which are to be used as materials for superheater tubes and reheater tubes are required to have increased high temperature strength.[0003]In such a technological background, various austenitic stainless steels have been disclosed.[0004]For example, the Patent Document 1 di...

AI Technical Summary

Benefits of technology

[0054]The austenitic stainless steels of the present invention have high strength and excellent resistance to cracking due to grain boundary embrittlement in welded portions during use at high temperatures. Consequently, they can be used as materials for constructing machines and equipment, such as power plant boilers, which are to be used at high temperatures for a prolonged period of time.Best Modes for Carrying Out the Invention

[0055]In the following, the reasons for restricting the contents of the component elements of the austenitic stainless steels in the present invention are described in detail. In the following description, the symbol “%” for the content of each element means “% by mass”.

[0057]C is an element having an austenite-stabilizing effect and at the same time it forms fine intragranular carbonitrides with N and thereby it contributes toward improvements in high temperature strength. In order to obtain the above-mentioned effects, it is necessary that the content of C be not less than 0.04%. However, when the C content is excessive, in particular when it exceeds 0.18%, C causes over precipitation of fine intragranular carbonitrides during use at high temperatures. Thereby this inhibits the intragranular deformation and causes stress concentration at grain boundary to increase the susceptibility to cracking due to grain boundary embrittlement in the coarse-grained HAZ. In addition, it sometimes forms a large amount of intergranular carbides upon exposure to weld thermal cycles or during use at high temperatures causes the formation of Cr-depleted layers in the vicinity of grain boundaries, which leads to deterioration of corrosion resistance. Therefore, the content of C is set to 0.04 to 0.18%. The lower limit of the C content is preferably 0.05% and the upper limit thereof is preferably 0.13%.

[0058]In the case of the present invention (2) according to which the Ni content is 6 to 13% and the Cr content is 15 to 25%, the content of C is preferably 0.05 to 0.15%. The content range of C in this case is more preferably 0.07 to 0.13%.

[0059]In the case of the present invention (3) according to which the Ni content is more than 13% to not more than 30% and the Cr content is 15 to 30%, the C content range of 0.04 to 0.15% is preferable out of the above-mentioned range of 0.04 to 0.18%.

[0061]Si is an element having a deoxidizing effect. It is also effective in increasing corrosion resistance and oxidation resistance at high temperatures. However, when the content thereof is excessive, in particular at a content level exceeding 1.5%, it deteriorates the stability of the austenite phase, thus creep strength and toughness are deteriorated. Therefore, the content of Si is set to not more than 1.5%. The content of Si is preferably not more than 1.0%.

Problems solved by technology

While these austenitic stainless heat resistant steels are generally used at high temperatures after welding fabrication, they encounter the problem of an occurrence of cracking in the weld heat affected zone (hereinafter referred to as “HAZ”) after a long period of use at high temperatures.