High chromium martensitic heat-resistant steel with combined high creep rupture strength and oxidation resistance

Abstract

Provided is martensitic heat-resistant steel for boiler applications with a unique combination of enhanced creep strength and excellent oxidation resistance upon high temperature exposure in steam containing environments contacts (in wt.-%): C: 0.10 to 0.16%, Si: 0.20 to 0.60%, Mn: 0.30 to 0.80%, P≤0.020%, S≤0.010%, Al≤0.020%, Cr: 10.5 to 12.00%, Mo: 0.10 to 0.60%, V: 0.15 to 0.30%, Ni: 0.10 to 0.40%, B: 0.008 to 0.015%, N:0.002 to 0.020%, Co: 1.50 to 3.00%, W: 1.50 to 2.50%, Nb: 0.02 to 0.07%, Ti: 0.001-0.020%, iron and unavoidable impurities. The steel is normalized for about 10 to about 120 minutes at a temperature of 1050-1170° C. and cooled down in air or water to room temperature, and then tempered for at least one hour at a temperature of 750-820° C. The steel has a martensitic microstructure with average δ-ferrite content of less than 5 vol.-%.

1 . A seamless tubular product, made of a steel comprising, in weight percent:
 C: 0.10 to 0.16%,   Si: 0.20 to 0.60%,   Mn: 0.30 to 0.80%,   P≤0.020%,   S≤0.010%,   Al≤0.020%,   Cr: 10.50 to 12.00%,   Mo: 0.10 to 0.60%,   V: 0.15 to 0.30%,   Ni: 0.10 to 0.40%,   B: 0.008 to 0.015%,   N: 0.002 to 0.020%,   Co: 1.50 to 3.00%,   W: 1.50 to 2.50%,   Nb: 0.02 to 0.07%,   Ti: 0.001 to 0.020%,   iron and unavoidable impurities.   
     
     
         2 . The seamless tubular product according to  claim 1 , wherein: B/N≤1.5. 
     
     
         3 . The seamless tubular product according to  claim 1 , wherein, the contents of Mo and W in wt % satisfy:
   1.00%≤Mo+0.5W≤1.50%.
   
     
     
         4 . The seamless tubular product according to  claim 1 , wherein the contents of B, N, and Ti in wt % satisfy:
   B−(11/14)(N−10 −(1/2.45)·(log B+6.81) −(14/48)·Ti)≥0.007.
   
     
     
         5 . The seamless tubular product according to  claim 1 , wherein, the contents of Ni, Co, Mn, C, and N in wt.-% satisfy:
   2.6≤4·(Ni+Co+0.5·Mn)−20·(C+N)≤11.2.
   
     
     
         6 . The seamless tubular product according to  claim 1 , wherein the carbon content is between 0.13 and 0.16%. 
     
     
         7 . The A seamless tubular product according to  claim 1 , wherein the Mo content is between 0.30 and 0.60%. 
     
     
         8 . The seamless tubular product according to  claim 1 , wherein the B content is between 0.0095 and 0.013%. 
     
     
         9 . The seamless tubular product according to  claim 1 , wherein the Ti content is between 0.001 and 0.005%. 
     
     
         10 . The seamless tubular product according to  claim 1 , wherein the product has a microstructure comprising at least 95% of tempered martensite, the balance being delta ferrite. 
     
     
         11 . The seamless tubular product according to  claim 10 , wherein the microstructure comprises at least 98% of tempered martensite. 
     
     
         12 . The seamless tubular product according to  claim 10 , wherein the microstructure is martensitic and free of delta ferrite. 
     
     
         13 . The seamless tubular product according to  claim 1 , which is a seamless tube. 
     
     
         14 . A method of producing the seamless tubular product according to  claim 1 , the method comprising:
 i) casting said steel,   ii) hot forming said steel,   iii) heating and holding said steel for a time between 10 and 120 minutes at a temperature ranging between 1050° C. and 1170° C.,   iv) cooling said steel down to room temperature,   v) reheating and holding said steel up to a tempering temperature TT that is between 750° C. and 820° C. for at least one hour, and   vi) cooling said steel down to room temperature.   
     
     
         15 . The method according to  claim 14 , wherein the cooling iv) and vi) are done using air cooling or water cooling.