15CrMoG: GB5310 steel (corresponding to 1Cr-1 / 2Mo and 11 / 4Cr-1 / 2Mo-Si steels widely used in countries around the world), its chromium content is higher than 12CrMo steel, so it has a temperature of 500-550 ℃Higher heat resistance. When the temperature exceeds 550 ° C, its thermal strength is significantly reduced. When it is operated at 500-550 ° C for a long time, no graphitization occurs, but carbide spheroidization and redistribution of alloy elements will occur, which will cause the heat of the steel. The strength is reduced, and the steel has good resistance to slackening at 450 ° C. Its pipe making and welding process performance is good. It is mainly used as high and medium pressure steam pipes and headers with steam parameters below 550 ℃, and superheater tubes with tube wall temperature below 560 ℃. Its chemical composition is C0.12-0.18, Si0.17-0.37, Mn0.40-0.70, S≤0.030, P≤0.030, Cr0.80-1.10, Mo0.40-0.55; strength level σs≥ in the tempering state 235, σb≥440-640 MPa; plasticity δ≥21. This small diameter steel pipe is mainly used for low temperature superheater / reheater, wall-mounted superheater and other components.