Martensitic Creep Resistant Steel Strengthened by Z-Phase

Abstract

The present invention relates to steel alloys having a martensitic or martensitic-ferritic structure and comprising Z-phase (CrXN) particles, where X is one or more of the elements V, Nb, Ta, and where the Z-phase particles have an average size of less than 400 nm. The alloy comprises by wt % the following components: 9 to 15% Cr, 0.01-0.20% N, C in an amount less than 0.1%, one or more of: 0.01-0.5% V, 0.01-1% Nb, 0.01-2% Ta, and a balance being substantially iron and inevitable impurities. The invention further relates to a method of manufacturing such a steel alloy, a component comprising such a steel alloy, and to the use of such a steel alloy for high temperature components.

1 . A steel alloy comprising by wt % the following components:
 9 to 15% Cr,   0.01-0.20% N,   C in an amount less than 0.1%,   one or more of: 0.01-1% Nb and 0.01-2% Ta, and   a balance being substantially iron and inevitable impurities,   
     the steel alloy optionally further comprising one or more of the following components:
 Co in an amount less than 8%, 
 one or more of Mo and W in an amount less than 4%, 
 one or more of Mn, Ni and Cu in an amount less than 3%, 
 Si in an amount less than 2%, and 
 B in an amount less than 0.04%, 
 
     said steel alloy having a martensitic or martensitic-ferritic structure and comprising Z-phase (CrXN) particles which precipitate either during heat treatment as a part of the manufacturing process or during service exposure, where X is one or more of the elements Nb, Ta, and where the Z-phase particles have an average size of less than 100 nm even after long-term exposure for more than 100,000 hours at temperatures above 600° C. 
   
   
       2 . A method of manufacturing a steel alloy according to  claim 1 , said method comprising the steps of:
 melting the components,   molding the melt to form a material,   subjecting the molded material to a solution temperature between 1000° C. and 1300° C., and   subjecting the molded material to a tempering treatment at a temperature between 500° C. and 850° C.,   
     wherein the duration of the solution and tempering treatments depends on the size of the component being manufactured and on predefined requirements for toughness and tensile strength. 
   
   
       3 . A high temperature component comprising a steel alloy according to  claim 1 . 
   
   
       4 . Use of a steel alloy according to  claim 1  for high temperature components. 
   
   
       5 . Use of a steel alloy according to  claim 4  for components in power generation boilers or turbines, nuclear power generation facilities, jet engines, or components in chemical industry.