Austenitic Heat Resistant Alloy and Method for Producing Same

Abstract

Provided is an austenitic heat resistant alloy having a chemical composition consisting of, in mass %: C: 0.02 to 0.12%; Si: 2.0% or less; Mn: 3.0% or less; P: 0.030% or less; S: 0.015% or less; Cr: 20.0% or more and less than 28.0%; Ni: more than 35.0% and 55.0% or less; Co: 0 to 20.0%; W: 4.0 to 10.0%; Ti: 0.01 to 0.50%; Nb: 0.01 to 1.0%; Mo: less than 0.50%; Cu: less than 0.50%; Al: 0.30% or less; N: less than 0.10%; Mg: 0 to 0.05%; Ca: 0 to 0.05%; REM: 0 to 0.50%; V: 0 to 1.5%; B: 0 to 0.01%; Zr: 0 to 0.10%; Hf: 0 to 1.0%; Ta: 0 to 8.0%; Re: 0 to 8.0%; and the balance: Fe and impurities, wherein a shortest distance from a center portion to an outer surface portion of a cross section of the alloy is 40 mm or more, the cross section being perpendicular to a longitudinal direction of the alloy, an austenite grain size number at the outer surface portion is −2.0 to 4.0, an amount of Cr which is present as a precipitate satisfies [CrPB/CrPS≤10.0], and [YSS/YSB≤1.5] and [TSS/TSB≤1.2] are satisfied at a normal temperature.

1 . An austenitic heat resistant alloy having a chemical composition consisting of, in mass %:
 C: 0.02 to 0.12%;   Si: 2.0% or less;   Mn: 3.0% or less;   P: 0.030% or less;   S: 0.015% or less;   Cr: 20.0% or more and less than 28.0%;   Ni: more than 35.0% and 55.0% or less;   Co: 0 to 20.0%;   W: 4.0 to 10.0%;   Ti: 0.01 to 0.50%;   Nb: 0.01 to 1.0%;   Mo: less than 0.50%;   Cu: less than 0.50%;   Al: 0.30% or less;   N: less than 0.10%;   Mg: 0 to 0.05%;   Ca: 0 to 0.05%;   REM: 0 to 0.50%;   V: 0 to 1.5%;   B: 0 to 0.01%;   Zr: 0 to 0.10%;   Hf: 0 to 1.0%;   Ta: 0 to 8.0%;   Re: 0 to 8.0%; and   the balance: Fe and impurities, wherein   a shortest distance from a center portion to an outer surface portion of a cross section of the alloy is 40 mm or more, the cross section being perpendicular to a longitudinal direction of the alloy,   an austenite grain size number at the outer surface portion is −2.0 to 4.0,   an amount of Cr which is present as a precipitate obtained by an extraction residue analysis satisfies a following formula (i), and   mechanical properties at a normal temperature satisfy following formula (ii) and formula
   Cr PB /Cr PS ≤10.0   (i)
 
   YS S /YS B ≤1.5   (ii)
 
   TS S /TS B ≤1.2   (iii)
 
   where meaning of each symbol in the formulas is as follows:   Cr PB : amount of Cr which is present at center portion as precipitate obtained by extraction residue analysis   Cr PS : amount of Cr which is present at outer surface portion as precipitate obtained by extraction residue analysis   YS B : 0.2% proof stress at center portion   YS S : 0.2% proof stress at outer surface portion   TS B : tensile strength at center portion   TS S : tensile strength at outer surface portion.   
     
     
         2 . The austenitic heat resistant alloy according to  claim 1 , wherein the chemical composition contains one or more elements selected from a group consisting of, in mass %:
 Mg: 0.0005 to 0.05%;   Ca: 0.0005 to 0.05%;   REM: 0.0005 to 0.50%;   V: 0.02 to 1.5%;   B: 0.0005 to 0.01%;   Zr: 0.005 to 0.10%;   Hf: 0.005 to 1.0%;   Ta: 0.01 to 8.0%; and   Re: 0.01 to 8.0%.   
     
     
         3 . The austenitic heat resistant alloy according to  claim 1 , wherein
 10,000-hour creep rupture strength at 700° C. in the longitudinal direction at the center portion is 100 MPa or more.   
     
     
         4 . A method for producing an austenitic heat resistant alloy, the method comprising the steps of:
 performing hot working on an ingot or a cast piece having the chemical composition according to  claim 1 ; and   thereafter performing heat treatment where the ingot or the cast piece is heated to a heat-treatment temperature T (° C.) ranging from 1100 to 1250° C., is held for 1000 D/T to 1400 D/T (min), and is cooled with water,   wherein symbol “D” denotes a maximum value (mm) of a linear distance between an arbitrary point on an outer edge of a cross section of the alloy and another arbitrary point on the outer edge, the cross section being perpendicular to a longitudinal direction of the alloy.   
     
     
         5 . The method for producing an austenitic heat resistant alloy according to  claim 4 , wherein
 in the step of performing the hot working, the working is performed one or more times in a direction substantially perpendicular to the longitudinal direction.   
     
     
         6 . The austenitic heat resistant alloy according to  claim 2 , wherein
 10,000-hour creep rupture strength at 700° C. in the longitudinal direction at the center portion is 100 MPa or more.   
     
     
         7 . A method for producing an austenitic heat resistant alloy, the method comprising the steps of:
 performing hot working on an ingot or a cast piece having the chemical composition according to  claim 2 ; and   thereafter performing heat treatment where the ingot or the cast piece is heated to a heat-treatment temperature T (° C.) ranging from 1100 to 1250° C., is held for 1000 D/T to 1400 D/T (min), and is cooled with water,   wherein symbol “D” denotes a maximum value (mm) of a linear distance between an arbitrary point on an outer edge of a cross section of the alloy and another arbitrary point on the outer edge, the cross section being perpendicular to a longitudinal direction of the alloy.   
     
     
         8 . The method for producing an austenitic heat resistant alloy according to  claim 7 , wherein
 in the step of performing the hot working, the working is performed one or more times in a direction substantially perpendicular to the longitudinal direction.