Shaped Steel Excellent in Fire Resistance and Producing Method Therefor

Abstract

A shaped steel, such as H-shaped steel, excellent in fire resistance, where a flange portion has 50% or more of a ratio of strength, 80% or less of yield ratio, and 100 J or more of impact strength of Charpy impact test at 0° C., wherein the ratio of strength=(proof stress of 0.2% at 600° C.)/(yield strength at room temperature), and a producing method thereof are provided. The shaped steel comprises in mass percent (%): C: 0.03-0.15; Mo: 0.1-0.6; V≦0.35; and N: 0.002-0.012, and the balance being iron and residual impurities, wherein (x) a mol fraction of precipitate of alloy carbides and alloy carbonitrides at 600° C. is 0.3% or more, and (y) the ratio of the mol fraction of precipitate of alloy carbides and alloy carbonitrides at 300° C. to the mol fraction of precipitate of alloy carbides and alloy carbonitrides at 600° C. is 2.0 or less.

1 . A shaped steel, comprising in mass percent (%):
 C: 0.03-0.15;   Mo: 0.1-0.6;   V≦0.35; and   N: 0.002-0.012,   
     and the balance being iron and residual impurities, wherein
 (x) a mol fraction of precipitate of alloy carbides and alloy carbonitrides at 600° C. is 0.3% or more, and 
 (y) a ratio of the mol fraction of precipitate of alloy carbides and alloy carbonitrides at 300° C. to the mol fraction of precipitate of alloy carbides and alloy carbonitrides at 600° C. is 2.0 or less. 
 
   
   
       2 . The shaped steel according to  claim 1 , further comprising:
 Si: 0.05-0.50;   Mn: 0.4-2.0; and   Al≦0.01,   
     wherein the amount of V ranges from 0.04 to 0.35 in mass %, a flange portion of the shaped steel has 50% or more of a ratio of strength, 80% or less of yield ratio, and 100 J or more of impact strength of Charpy impact test at 0° C., wherein the ratio of strength=(proof stress of 0.2% at 600° C.)/(yield strength at room temperature). 
   
   
       3 . The shaped steel according to  claim 1  or  2 , further comprising one or more of following elements in mass percent (%):
 Ti: 0.005-0.020;   Nb≦0.06%;   Cr≦0.7;   Ni≦1.0; and   Cu≦1.0.   
     wherein the content of V ranges from more than 0.20 to 0.35 in mass %. 
   
   
       4 . A method for producing a shaped steel according to  claim 1  by hot rolling after reheating a casted steel, comprising steps of:
 (a) reheating the casted steel to 1100-1300° C.;   (b) hot rolling the casted steel to form the shaped steel; and   (c) after terminating the hot rolling, naturally cooling or rapidly cooling then naturally cooling the shaped steel.   
   
   
       5 . The method according to  claim 4 , wherein the step of hot rolling includes water-cooling the shaped steel at least once to 700° C. or less as measured at the surface of a flange portion of the shaped steel and then rolling in the heat returning process. 
   
   
       6 . The method according to  claim 5 , wherein the step of rapid cooling is performed at an average cooling rate of 0.5 to 5.0° C./s.