Low alloy steel for geothermal power generation turbine rotor, and low alloy material for geothermal power generation turbine rotor and method for manufacturing the same

Abstract

The invention provides Low alloy steel for geothermal power generation turbine rotor, and low alloy material for geothermal power generation turbine rotor and method for manufacturing the same. A low alloy steel ingot contains from 0.15 to 0.30 % of C, from 0.03 to 0.2 % of Si, from 0.5 to 2.0 % of Mn, from 0.1 to 1.3 % of Ni, from 1.5 to 3.5 % of Cr, from 0.1 to 1.0 % of Mo, and more than 0.15 to 0.35 % of V, and optionally Ni, with a balance being Fe and unavoidable impurities. Performing quality heat treatment including a quenching step and a tempering step to the low alloy steel ingot to obtain a material, which has a grain size number of from 3 to 7 and is free from pro-eutectoid ferrite in a metallographic structure thereof, and which has a tensile strength of from 760 to 860 MPa and a fracture appearance transition temperature of not higher than 40 DEG C.

Description

technical field [0001] The present invention relates to a low alloy steel used primarily in corrosive environments, and in particular the invention is applicable to turbomachinery components such as large geothermal power generation turbine rotors. Background technique [0002] In geothermal power generation, although the steam temperature is as low as about 200°C, the steam contains corrosive gases such as hydrogen sulfide. In view of this fact, high-temperature creep strength required for thermal power generation is unnecessary in a turbine rotor material for geothermal power generation, but corrosion resistance, tensile strength at room temperature, yield strength, and toughness are considered to be important. In such a low temperature range, NiCrMoV steel having excellent toughness containing 3 to 4% by mass of Ni is generally used. However, a steel type containing a large amount of nickel involves a disadvantage of easily causing SCC (Stress Corrosion Cracking). There...

AI Technical Summary

Problems solved by technology

For example, in the case of enlarging 1%CrMoV, the following problems are involved: in the central part of the rotor, the cooling rate is greatly reduced and ferrite is precipitated, resulting in a decrease in toughness; and in the feeder head of the steel ingot C enrichment occurs on the side, resulting in the possibility of quenching cracks caused by water cooling during quenching

In Japanese Patent Application Publication No. 52-30716, Japanese Patent Application Publication No. 55-50430 and Japanese Patent Application Publication No. 61-143523, although the toughness of 1% CrMoV steel has been improved, it has not considered Various problems caused, and there is a concern that the toughness will decrease due to the decrease in the cooling rate

In Japanese Patent Application Laid-Open No. 62-290849, although the reduction in cooling rate due to the increase in size is considered, the problem of C enrichment on the riser side of the steel ingot in the case of manufacturing a large steel ingot is not considered. And there is a concern that the segregation resistance will deteriorate when manufacturing large steel ingots

Images