Heat resistant cast steel, manufacturing method thereof, cast parts of steam turbine, and manufacturing method of cast parts of steam turbine

A method of manufacturing a cast part of a steam turbine, the method comprising:

• melting raw materials,
• refining and degassing the melted raw materials,
• pouring the melted raw materials into a predetermined mold to form a shape,
• performing an annealing treatment at a temperature of 1000 to 1150° C.,
• cooling the annealed raw materials at a cooling rate of 20 to 60° C./hour,
• performing a normalizing treatment at a temperature of 1000 to 1200° C.,
• cooling the normalized raw materials at a cooling rate of 100 to 600° C./hour in a center part of the heat resistant cast steel,
• performing a first stage tempering treatment at a temperature of 500 to 700° C.,
• cooling the first-stage tempered raw materials at a cooling rate of 40 to 100° C./hour in the center part of the heat resistant cast steel,
• performing a second stage tempering treatment at a temperature of 700 to 780° C., and
• cooling the second-stage tempered raw materials at a cooling rate of 20 to 60° C./hour,
• to obtain a cast part comprising, in percent by mass, C:
• 0.05-0.15, Si:
• 0.03-0.2, Mn:
• 0.1-1.5, Ni:
• 0.1-1, Cr:
• 8-8.7, Mo:
• 0.2-1.5, V:
• 0.1-0.3, Co:
• 2.7-5, W:
• 0.1-5, N:
• 0.005-0.03, Nb:
• 0.01-0.2, B:
• 0.004-0.015, Ti:
• 0.01-0.1, and Fe.