Fire Resistant High Strength Rolled Steel Material and Method of Production of The Same

Abstract

A fire resistant high strength rolled steel material superior in fire resistance and toughness used for a structural member of a building etc. containing, by mass %, C: 0.005% to less than 0.04%, Mn: 0.8 to 1.7%, Si: 0.05 to less than 0.4%, Nb: 0.02 to 1%, Ti: 0.005 to 0.02%, N: 0.005% or less, B: 0.0003 to 0.003%, and Al: 0.005% to 0.03%, and a balance of Fe and unavoidable impurities, and having the ratio Ti/N of 2 to 8, the value C-Nb/7.74 of 0.02% or less, and having the ratio of the 0.2% yield strength at 600° C. to the yield strength at room temperature of 0.50 or more.

1 . A fire resistant high strength rolled steel material containing, by mass %,
 C: 0.005% to less than 0.04%,   Mn: 0.8 to 1.7%,   Si: 0.05 to less than 0.4%,   Nb: 0.02 to 1%,   Ti: 0.005 to 0.02%,   N: 0.005% or less,   B: 0.0003 to 0.003%,   Al: 0.005% to 0.03%,   
     and the balance of Fe and unavoidable impurities, and having the ratio Ti/N of 2 to 8, the value C—Nb/7.74 of 0.02% or less by mass %, and having the ratio of the 0.2% yield strength at 600° C. to the yield point strength (when the yield point is unclear, 0.2% yield strength) at room temperature of 0.50 or more. 
   
   
       2 . A fire resistant high strength rolled steel material as set forth in  claim 1 , further containing, by mass %, one or more of
 Cr: 0.4% or less,   Cu: 1% or less, and   Ni: 0.7% or less.   
   
   
       3 . A method of production of a fire resistant high strength rolled steel material comprising preparing a cast slab containing, by mass %,
 C: 0.005% to less than 0.04%,   Mn: 0.8 to 1.7%,   Si: 0.05 to less than 0.4%,   Nb: 0.02 to 1%,   Ti: 0.005 to 0.02%,   N: 0.005% or less,   B: 0.0003 to 0.003%, and   Al: 0.005% to 0.03%,   
     and the balance of Fe and unavoidable impurities, and having the ratio Ti/N of 2 to 8, the value C—Nb/7.74 of 0.02% or less by mass %, heating this to a temperature region of 1250 to 1350° C., then starting rolling and rolling to give a cumulative reduction rate at 1000° C. or less of 30% or more to thereby give the ratio of a 0.2% yield strength at 600° C. and a yield point strength (when yield point is unclear, a 0.2% yield strength) at room temperature of 0.50 or more. 
   
   
       4 . A method of production of a fire resistant high strength rolled steel material as set forth in  claim 3 , wherein said cast slab further contains, by mass %, one or more of
 Cr: 0.4% or less,   Cu: 1% or less, and   Ni: 0.7% or less.   
   
   
       5 . A method of production of a fire resistant high strength rolled steel material comprising preparing a cast slab containing, by mass %,
 C: 0.005% to less than 0.04%,   Mn: 0.8 to 1.7%,   Si: 0.05 to less than 0.4%,   Nb: 0.02 to 1%,   Ti: 0.005 to 0.02%,   N: 0.005% or less,   B: 0.0003 to 0.003%, and   Al: 0.005% to 0.03%,   
     and a balance of Fe and unavoidable impurities, and having the Ti/N of 2 to 8, the value C—Nb/7.74 of 0.02% or less by mass %, heating this to a temperature region of 1250 to 1350° C., then starting rolling, and cooling, after the end of said rolling, at 800 to 500° C. in temperature range by a 0.1 to 10° C./sec average cooling rate to thereby give a ratio of 0.2% yield strength at 600° C. and yield point strength (when yield point is unclear, 0.2% yield strength) at room temperature of 0.50 or more. 
   
   
       6 . A method of production of a fire resistant high strength rolled steel material as set forth in  claim 5 , wherein said cast slab further contains, by mass %, one or more of
 Cr: 0.4% or less,   Cu: 1% or less, and   Ni: 0.7% or less.