High-strength heat resisting cast steel, method of producing the steel, and applications of the steel

Abstract

A high-strength heat resisting cast steel which has high creep rupture strength at temperatures of 620° C. or above, high toughness, and good weldability. A method of producing the steel, a steam turbine casing, a main steam valve casing, and a steam control valve casing, each casing being made of that steel, as well as a steam turbine power plant using those components are also provided. The high-strength heat resisting cast steel contains 0.06-0.16% by mass of C, 0.1-1% of Si, 0.1-1% of Mn, 8-12% of Cr, 0.1-1.0% of Ni, 0.7% or less of Mo, 1.9-3.0% of W, 0.05-0.3% of V, 0.01-0.15% of one or more of Nb, Ta and Zr in total, 0.1-2% of Co, 0.01-0.08% of N, and 0.0005-0.01% of B, the balance being Fe and unavoidable impurities.

1 . A high-strength heat resisting cast steel containing 0.06-0.16% by mass of C, 0.1-1% of Si, 0.1-1% of Mn, 8-12% of Cr, 0.1-1.0% of Ni, 0.7% or less of Mo, 1.9-3.0% of W, 0.05-0.3% of V, 0.01-0.15% of one or more of Nb, Ta and Zr in total, 0.1-2% of Co, 0.01-0.08% of N, and 0.0005-0.01% of B, the balance being Fe and unavoidable impurities.  
   
   
       2 . The high-strength heat resisting cast steel according to  claim 1 , wherein Al is 0.0005-0.04% by mass and O is 0.02% or less.  
   
   
       3 . The high-strength heat resisting cast steel according to  claim 1 , wherein in orthogonal coordinates expressed by the relationship between [W/(Mo+0.5W)] and (Co/W), values of [W/(Mo+0.5W)] and (Co/W) are not larger than the values represented by linear lines interconnecting a coordinate point A (1.1, 0.90), a coordinate point B (1.5, 0.55), and a coordinate point C (1.8, 0.55).  
   
   
       4 . The high-strength heat resisting cast steel according to  claim 1 , wherein the steel contains at least one of 1.5% by mass or less of Re, 0.5% or less of Nd, and 1.0% or less of Sr.  
   
   
       5 . The high-strength heat resisting cast steel according to any  claim 1 , wherein creep rupture strength at 620° C. and 10 5  hours is 98 MPa or more, and impact absorbed energy at room temperature is 29.4 J or more.  
   
   
       6 . A method of producing a high-strength heat resisting cast steel, the method comprising the steps of: 
 smelting, in an electric furnace, raw materials with composition containing 0.06-0.16% by mass of C, 0.1-1% of Si, 0.1-1% of Mn, 8-12% of Cr, 0.1-1.0% of Ni, 0.7% or less of Mo, 1.9-3.0% of W, 0.05-0.3% of V, 0.01-0.15% of one or more of Nb, Ta and Zr in total, 0.1-2% of Co, 0.01-0.08% of N, and 0.0005-0.01% of B, the balance being Fe and unavoidable impurities;    deaerating the smelted materials by vacuum ladle refining; and    casting the deaerated materials into a sand mold.    
   
   
       7 . A method of producing a high-strength heat resisting cast steel, the method comprising the steps of: 
 annealing, at 1000-1150° C., cast steel with composition containing 0.06-0.16% by mass of C, 0.1-1% of Si, 0.1-1% of Mn, 8-12% of Cr, 0.1-1.0% of Ni, 0.7% or less of Mo, 1.9-3.0% of W, 0.05-0.3% of V, 0.01-0.15% of one or more of Nb, Ta and Zr in total, 0.1-2% of Co, 0.01-0.08% of N, and 0.0005-0.01% of B, the balance being Fe and unavoidable impurities;    performing thermal normalizing of the annealed steel through steps of heating to 1000-1100° C. and subsequent quick cooling; and    performing successively primary tempering at 550-750° C. and secondary tempering at 670-770° C.    
   
   
       8 . The method of producing the high-strength heat resisting cast steel according to  claim 6 , wherein the steel further contains 0.0005-0.04% by mass of Al and 0.02% or less of O.  
   
   
       9 . A steam turbine casing made of the high-strength heat resisting cast steel according to  claim 1 .  
   
   
       10 . A method of producing a steam turbine casing, wherein the method includes a step of obtaining a casing material by the method of producing the high-strength heat resisting cast steel according to  claim 6 .  
   
   
       11 . A steam turbine comprising a rotor shaft including implanted rotor blades, an inner casing including stator nozzles implanted corresponding to said rotor blades and covering said rotor shaft including said implanted rotor blades, and an outer casing covering said inner casing, 
 wherein said inner casing is constituted by the steam turbine casing according to  claim 9 .    
   
   
       12 . A main steam valve for controlling supply and stop of main steam obtained by a boiler with respect to a steam turbine, 
 wherein a casing of said main steam valve is made of the high-strength heat resisting cast steel according to  claim 1 .    
   
   
       13 . A method of producing a main steam valve for controlling supply and stop of main steam obtained by a boiler with respect to a steam turbine, 
 wherein the method includes a step of obtaining a casing material of said main steam valve by the method of producing the high-strength heat resisting cast steel according to  claim 6 .    
   
   
       14 . A steam control valve for controlling a supply amount of main steam obtained by a boiler through a main steam valve which controls supply and stop of the main steam with respect to a steam turbine, 
 wherein a casing of said steam control valve is made of the high-strength heat resisting cast steel according to  claim 1 .    
   
   
       15 . A method of producing a steam control valve for controlling a supply amount of main steam obtained by a boiler through a main steam valve which controls supply and stop of the main steam with respect to a steam turbine, 
 wherein the method includes a step of obtaining a casing material of said steam control valve by the method of producing the high-strength heat resisting cast steel according to  claim 6 .    
   
   
       16 . A steam turbine power plant including any of a set of a high-pressure steam turbine, an intermediate-pressure steam turbine, and two low-pressure steam turbines connected in tandem, and a set of a high- and intermediate-pressure integral steam turbine and a low-pressure steam turbine, 
 wherein at least one of said high-pressure steam turbine, said intermediate-pressure steam turbine, and said high- and intermediate-pressure integral steam turbine is constituted by the steam turbine according to  claim 11 .    
   
   
       17 . A steam turbine power plant including any of a set of a high-pressure steam turbine, an intermediate-pressure steam turbine, and two low-pressure steam turbines connected in tandem, and a set of a high- and intermediate-pressure integral steam turbine and a low-pressure steam turbine, said power plant further including a main steam valve for controlling supply and stop of main steam obtained by a boiler and a steam control valve for controlling a supply amount of the main steam through a main steam valve, 
 wherein at least one of said main steam valve and said steam control valve is constituted by the main steam valve and the steam control valve according to  claim 12.