Fire-resistant steel material superior in weld heat affected zone reheat embrittlement resistance and low temperature toughness and method of production of same

Abstract

The present invention provides a fire-resistant steel material superior in weld heat affected zone reheat embrittlement resistance and low temperature toughness when welded by large heat input and exposed to fire and a method of production of the same, that is, a material containing C: 0.012 to 0.050%, Mn: 0.80 to 2.00%, Cr: 0.80 to 1.90%, and Nb: 0.01 to less than 0.05%, restricting Cu to 0.10% or less, containing suitable quantities of Si, N, Ti, and Al, restricting the contents of Mo, B, P, S, and O, and having a balance of Fe and unavoidable impurities, having contents of C, Mn, Cr, Nb, and Cu satisfying −1200C−20Mn+30Cr−330Nb−120Cu≧−80, having a steel structure as observed by an optical microscope of an area fraction of 80% or more of a ferrite phase, and having a balance of the steel structure of a bainite phase, martensite phase, and mixed martensite-austenite structure.

1 . A fire-resistant steel material superior in weld heat affected zone reheat embrittlement resistance and low temperature toughness characterized by containing, by mass %,
 C: 0.012% to 0.050%,   Si: 0.01% to 0.50%,   Mn: 0.80% to 2.00%,   Cr: 0.80% to 1.90%,   Nb: 0.01% to 0.05%,   N: 0.001% to 0.006%,   Ti: 0.010% to 0.030%, and   Al: 0.005% to 0.10%,   
       further limiting the contents of Cu, Mo, B, P, S, and O to:
 Cu: 0.10% or less, 
 Mo: less than 0.01%, 
 B: less than 0.0003%, 
 P: less than 0.02%, 
 S: less than 0.01%, and 
 less than 0.01%, and 
 having a balance of Fe and unavoidable impurities, 
 having contents of C, Mn, Cr, Nb, and Cu [mass %] satisfying
   −1200C−20Mn+30Cr−330Nb−120Cu≧−80,
 
 
 
       having a steel structure as observed by an optical microscope of an area fraction of 80% or more of a ferrite phase, and having a balance of the steel structure of a bainite phase, martensite phase, and mixed martensite-austenite structure. 
     
     
         2 . A fire-resistant steel material superior in weld heat affected zone reheat embrittlement resistance and low temperature toughness as set forth in  claim 1  characterized by further containing, by mass %, one or both of
 V: 0.40% or less and 
 Ni: 1.00% or less. 
 
     
     
         3 . A fin-resistant steel material superior in weld heat affected zone reheat embrittlement resistance and low temperature toughness as set forth in  claim 1  characterized by further containing, by mass %,
 Zr: 0.010% or less, 
 Mg: 0.005% or less, 
 Ca: 0.005% or less, 
 Y: 0.050% or less, 
 La: 0.050% or less, and 
 Ce: 0.050% or less. 
 
     
     
         4 . A method of production of a fire-resistant steel material superior in weld heat affected zone reheat embrittlement resistance and low temperature toughness characterized by heating a steel slab having the steel compositions as set forth in  claim 1  to 1150 to 1300° C. temperature, then hot working or hot rolling it at 800° C. to 900° C. temperature by a reduction ratio of 50% or more, then allowing it to cool. 
     
     
         5 . A method of production of a fire-resistant steel material superior in weld heat affected zone reheat embrittlement resistance and low temperature toughness characterized by applying the method of production as set forth in  claim 4 , then treating the steel material at a 400° C. to less than 650° C. temperature range for 5 minutes to 360 minutes for tempering heat treatment.