Ferritic heat-resistant steel and method for producing it

Abstract

The invention provides a ferritic heat-resistant steel having excellent high-temperature oxidation resistance, especially excellent steam oxidation-resistant characteristics. In high-Cr ferritic heat-resistant steel, ultra-fine oxide particles having a size of not larger than 1 μm are formed just below the oxide films and formed on the steel base, whereby the adhesiveness between the films and the base is enhanced. The ferritic heat-resistant steel contains Cr in an amount of from 8.0 to 13.0% by weight, and at least one of Rh and Ir in a total amount of from 0.3 to 5.0% by weight.

What is claimed is:  
     
         1 . Ferritic heat-resistant steel capable of forming an oxide film on its surface during use and having good steam oxidation-resistance, which is characterized in that ultra-fine oxide particles having a diameter of not larger than 1 micron are formed in and/or around the interface between the steel base and the oxide film formed thereon, to thereby increase the adhesiveness between the oxide film and the steel base.  
     
     
         2 . Ferritic heat-resistant steel as claimed in  claim 1 , which contains from 8.0 to 13.0% by weight of Cr and contains at least one of Ti and Y added thereto in a total amount of from 0.01 to 0.30% by weight.  
     
     
         3 . Ferritic heat-resistant steel as claimed in  claim 1  or  2 , which has a composition comprising from 8.0 to 13.0% (by weight—the same shall apply herein) of Cr; at least one of from 0.02 to 0.18% of C, from 0.1 to 1.0% of Si, from 0.05 to 1.5% of Mn, from 0 to 0.5% of Ni, from 0 to 4.0% of W, from 0 to 2.0% of Mo, provided that W+2Mo≦4%, from 0.10 to 0.50% of V, from 0.02 to 0.14% of Nb, from 0 to 0.1% of N, from 0 to 0.010% of B and not larger than 0.010% of 0; at least one of Ti and Y in an amount of 0.01%≦Ti+Y≦0.30%; and a balance of Fe and inevitable impurities.  
     
     
         4 . Ferritic heat-resistant steel having good steam oxidation-resistance and high long-term creep strengh, which contains at least one of Co, Rh, Ir, Pd and Pt in a total amount of not larger than 5.0% by weight.  
     
     
         5 . Ferritic heat-resistant steel as claimed in  claim 1 , which contains Cr in an amount of from 8.0 to 13.0% by weight and at least one of Rh and Ir in a total amount of from 0.3 to 5.0% by weight.  
     
     
         6 . Ferritic heat-resistant steel as claimed in  claim 5 , which contains at least one of Rh and Ir in an amount of from 0.3 to 5.0% (by weight—the same shall apply herein) of Rh and from 0.6 to 5.0% of Ir and in a ratio of 0.3%≦Rh+(1/2)Ir≦5.0%.  
     
     
         7 . Ferritic heat-resistant steel as claimed in  claim 5  or  6 , of which the lath structure is made fine and the martensite phase is reinforced by at least one of Rh and Ir added thereto.  
     
     
         8 . Ferritic heat-resistant steel as claimed in any one of  claims 5  to  7 , which comprises from 0.06 to 0.18% (by weight—the same shall apply herein) of C, from 0 to 1.0% of Si, from 0 to 1.5% of Mn, not larger than 0.030% of P, not larger than 0.015% of S, from 8.0 to 13.0% of Cr, from 0 to 4.0% of W, from 0 to 2.0% of Mo, provided that W+2Mo≦4.0%, from 0.030 to 0.14% of Nb, from 0.10 to 0.50% of V, from 0 to 0.10% of N, from 0 to 0.030% of B, not larger than 0.010% of 0, and from 0 to 0.050% of sol. Al; at least one of Rh and Ir in a total amount of from 0.3 to 5.0%; and a balance of Fe and inevitable impurities.  
     
     
         9 . Ferritic heat-resistant steel having steam oxidation resistance, which contains Cr in an amount of from 8.0 to 13.0% by weight, and at least one of Pd and Pt in a total amount of from 0.3 to 5.0% by weight.  
     
     
         10 . Ferritic heat-resistant steel as claimed in  claim 9 , which contains at least one of Pd and Pt in an amount of from 0.3 to 5.0% (by weight—the same shall apply herein) of Pd and from 0.3 to 5.0% of Pt and in a ratio of 0.3%≦Pd+Pt.≦5.0%.  
     
     
         11 . Ferritic heat-resistant steel as claimed in any of  claim 9  or  10 , which comprises from 0.06 to 0.18% (by weight—the same shall apply herein) of C, from 0 to 1.0% of Si, from 0 to 1.5% of Mn, not larger than 0.030% of P, not larger than 0.015% of S, from 8.0 to 13.0% of Cr, from 0 to 4.0% of W, from 0 to 2.0% of Mo, provided that W+2Mo≦4.0%, from 0.030 to 0.14% of Nb, from 0.10 to 0.50% of V, from 0 to 0.10% of N, from 0 to 0.030% of B, not larger than 0.010% of 0, and from 0 to 0.050% of sol. Al; at least one of Pd and Pt in a total amount of from 0.3 to 5.0%; and a balance of Fe and inevitable impurities.  
     
     
         12 . A method for producing ferritic heat-resistant steel of any one of  claims 1  to  4 , which comprises heating steel at a temperature not lower than 1250° C., subjecting it to plastic working, such as forging, rolling or the like, then immediately keeping it at a temperature falling between 1000 and 1150° C. for 1 hour or longer, and thereafter rapidly cooling it to a temperature not higher than its martensitic transformation-finishing point thereby making it have a martensitic texture, and then heating and tempering it at a temperature falling between 650 and 800° C.