High-strength steel for welded structures excellent in high temperature strength and method of production of the same

Abstract

The present invention provides a high-strength steel for welded structures excellent in strength at a high temperature of a temperature range of 600° C. to 800° C. and a method of production of the same, in particular 490 MPa class high-strength steel for welded structures excellent in high temperature strength containing C: 0.005% to less than 0.040%, Si: 0.5% or less, Mn: 0.1 to less than 0.5%, P:0.02% or less, S: 0.01% or less, Mo: 0.3 to 1.5%, Nb: 0.03 to 0.15%, Al: 0.06% or less, and N: 0.006% or less and in accordance with need one or more of Cu, Ni, Cr, V, Ti, Ca, REM, and Mg, having a weld crack susceptible formulation P CM defined as P CM =C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B of 0.15% or less, substantially not containing B, and a remainder Fe and unavoidable impurities, the microstructure being mainly composed of a mixed structure of ferrite and bainite, and the fraction of bainite being 20 to 90%.

1 . A 490 MPa class high-strength steel for welded structures excellent in high temperature strength, comprising as steel compositions, by wt %, 
 C: 0.005% to less than 0.040%,    Si: 0.5% or less,    Mn: 0.1 to less than 0.5%,    P: 0.02% or less,    S: 0.01% or less,    Mo: 0.3 to 1.5%,    Nb: 0.03 to 0.15%,    Al: 0.06% or less, and    N: 0.006% or less,    having a weld crack susceptible formulation P CM  defined as P CM =C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B of 0.15% or less, substantially not containing B, and a remainder of Fe and unavoidable impurities, the microstructure mainly composed of a mixed structure of ferrite and bainite, and the percentage of bainite being 20 to 90%.    
   
   
       2 . A 490 MPa class high-strength steel for welded structures excellent in high temperature strength as set forth in  claim 1 , further comprising by wt % at least one of 
 Cu: 0.05 to 1.0%,    Ni: 0.05 to 1.0%,    Cr: 0.05 to 1.0%,    V: 0.01 to 0.1%,    Ti: 0.005 to 0.025%,    Ca: 0.0005 to 0.004%,    REM: 0.0005 to 0.004%, and    Mg: 0.0001 to 0.006%    
   
   
       3 . A 490 MPa class high-strength steel for welded structures excellent in high temperature strength as set forth in  claim 1  or  2 , wherein an average circle equivalent diameter of the prior austenite of a cross-section parallel to the rolling direction at a ¼ thickness position is 120 μm or less.  
   
   
       4 . A method of production of a 490 MPa class high-strength steel for welded structures excellent in high temperature strength comprising the steps of; reheating semi-finished products or cast products comprised of the steel compositions as set forth in  claim 1  or  2  to a range of 1100 to 1250° C., rolling it at a temperature of 850° C. or more with a cumulative amount of reduction at 1100° C. or less of 30% or more, and cooling it by air cooling or accelerated cooling from a temperature of 800° C. or more to a temperature of 650° C. or less.