High strength martensitic stainless steel excellent in carbon dioxide gas corrosion resistance and sulfide stress-corrosion cracking resistance

Abstract

The present invention provides a martensitic stainless steel in which specified elements in a steel composition are limited. The martensitic stainless steel can have high strength of 0.2% proof stress of 860 MPa or more and excellent carbon dioxide gas corrosion resistance and sulfide stress-corrosion cracking resistance by limiting the steel composition of specified elements and defining Mo content in the steel by relationships with IM values as well as by forming microstructure of the steel with main tempered martensite, carbide precipitated during tempering, and intermetallic compounds such as a Laves phase, a σ phase and the like. As a result the martensitic stainless steels of the present invention can be applied to practical steels, which can be widely used in oil well tubes and the like under environment including carbon dioxide gas, hydrogen sulfide, chlorine ions or two or more of them, in wide fields.

1 . A high strength martensitic stainless steel excellent in carbon dioxide gas corrosion resistance and sulfide stress-corrosion cracking resistance and having 0.2% proof stress of 860 MPa or more, which comprises: 
 including, by mass %, C: 0.005-0.04%, Si: 0.5% or less, Mn: 0.1-3.0%, P: 0.04% or less, S: 0.01% or less, Cr: 10-15%, Ni: 4.0-8%, Mo: 2.8-5.0%, Al: 0.001-0.10%, N, 0.07% or less, Ti: 0-0.25%, V: 0-0.25%, Nb: 0-0.25%, Zr: 0-0.25%, Cu: 0-1%, Ca: 0-0.005%, Mg: 0-0.005%, La: 0-0.005%, and Ce: 0-005%, and the balance of Fe and impurities; and    satisfying the expression (1) given below wherein the microstructure mainly comprises tempered martensite, carbide precipitated during tempering, and intermetallic compounds such as Laves phase, σ phase and the like finely precipitated during tempering,      Mo≧=2.3−0.89 Si+32.2 C  (1)    where the symbols of the respective elements in the expression (1) show the content (mass %) of each element.    
   
   
       2 . A high strength martensitic stainless steel excellent in carbon dioxide gas corrosion resistance and sulfide stress-corrosion cracking resistance and having 0.2% proof stress of 860 MPa or more according to  claim 1 , comprising one or more selected from a group consisting of Ti: 0.005-0.25%, V: 0.005-0.25%, Nb: 0.005-0.25%, and Zr: 0.005-0.25%.  
   
   
       3 . A high strength martensitic stainless steel excellent in carbon dioxide gas corrosion resistance and sulfide stress-corrosion cracking resistance and having 0.2% proof stress of 860 MPa or more according to  claim 1 , comprising Cu: 0.05-1%.  
   
   
       4 . A high strength martensitic stainless steel excellent in carbon dioxide gas corrosion resistance and sulfide stress-corrosion cracking resistance and having 0.2% proof stress of 860 MPa or more according to  claim 1 , comprising one or more selected from a group consisting of Ca: 0.0002-0.005%, Mg: 0.0002-0.005%, La: 0.0002-0.005%, and Ce: 0.0002-0.005%.  
   
   
       5 . A high strength martensitic stainless steel excellent in carbon dioxide gas corrosion resistance and sulfide stress-corrosion cracking resistance and having 0.2% proof stress of 860 MPa or more, which comprises: 
 the compositions defined in  claim 1;  and 
 being subjected to tempering in which (20+log t)(T+273) satisfies 13500-17700 when, after quenching the steel at a quenching temperature of 880° C.-1000° C., a range of a tempering temperature is set to 450° C.-620° C., a tempering temperature is set to T (° C.) and tempering time is set to t (hour).  
   
   
   
       6 . A high strength martensitic stainless steel excellent in carbon dioxide gas corrosion resistance and sulfide stress-corrosion cracking resistance and having 0.2% proof stress of 860 MPa or more, which comprises: 
 the compositions defined in  claim 2;  and 
 being subjected to tempering in which (20+log t)(T+273) satisfies 13500-17700 when, after quenching the steel at a quenching temperature of 880° C.-1000° C., a range of a tempering temperature is set to 450° C.-620° C., a tempering temperature is set to T (° C.) and tempering time is set to t (hour).  
   
   
   
       7 . A high strength martensitic stainless steel excellent in carbon dioxide gas corrosion resistance and sulfide stress-corrosion cracking resistance and having 0.2% proof stress of 860 MPa or more, which comprises: 
 the compositions defined in  claim 3;  and 
 being subjected to tempering in which (20+log t)(T+273) satisfies 13500-17700 when, after quenching the steel at a quenching temperature of 880° C.-1000° C., a range of a tempering temperature is set to 450° C.-620° C., a tempering temperature is set to T (° C.) and tempering time is set to t (hour).  
   
   
   
       8 . A high strength martensitic stainless steel excellent in carbon dioxide gas corrosion resistance and sulfide stress-corrosion cracking resistance and having 0.2% proof stress of 860 MPa or more, which comprises: 
 the compositions defined in  claim 4;  and 
 being subjected to tempering in which (20+log t)(T+273) satisfies 13500-17700 when, after quenching the steel at a quenching temperature of 880° C.-1000° C., a range of a tempering temperature is set to 450° C.-620° C., a tempering temperature is set to T (° C.) and tempering time is set to t (hour).