Rolled h-shaped steel and manufacturing method thereof

Abstract

Rolled H-shaped steel is characterized in that a top 5% average value of Mn concentrations in a most embrittled portion in a flange is 1.6 times or less an Mn concentration at a position of 1/6 in a flange width direction from an end face in the flange width direction and 1/4 in a flange thickness direction from a face of a flange positioned on a side opposite to that of a web, and a top 5% average value of Mn concentrations in a central segregation portion dispersed in a region 15 mm or more apart from a center of the flange width toward one end face or both end faces in the flange width direction and within 2 mm from a flange surface layer in the thickness direction is not less than 1.1 times nor more than 1.6 times the Mn concentration at the position of 1/6 in the flange width direction from the end face in the flange width direction and 1/4 in the flange thickness direction from the face of the flange positioned on the side opposite to that of the web.

1 - 4 . (canceled) 
     
     
         5 . Rolled H-shaped steel, comprising a chemical composition composed of:
 in mass %,   C: 0.01 to 0.25%;   Si: 0.05% to 0.50%;   Mn: 0.40 to 2.50%;   P: 0.050% or less;   S: 0.050% or less;   N: 0.020% or less;   Cu: 0.70% or less;   Ni: 0.70% or less;   Cr: 0.50% or less;   V: 0.12% or less;   Mo: 0.30% or less;   Nb: 0.08% or less;   Ti: 0.05% or less;   Al: 0.07% or less;   REM: 0.010% or less;   Ca: 0.0050% or less; and   the balance: Fe and inevitable impurities, wherein:   a top 5% average value of Mn concentrations in a most embrittled portion in a flange is 1.6 times or less an Mn concentration at a position of 1/6 in a flange width direction from an end face in the flange width direction and 1/4 in a flange thickness direction from a face of a flange positioned on a side opposite to that of a web; and   a top 5% average value of Mn concentrations in a central segregation portion dispersed in a region along 15 mm or more from a center of the flange width toward one end face or both end faces in the flange width direction and within 2 mm from a flange surface layer in the thickness direction is not less than 1.1 times nor more than 1.6 times the Mn concentration at the position of 1/6 in the flange width direction from the end face in the flange width direction and 1/4 in the flange thickness direction from the face of the flange positioned on the side opposite to that of the web.   
     
     
         6 . A manufacturing method of rolled H-shaped steel being a manufacturing method of manufacturing the rolled H-shaped steel according to  claim 5 , comprising heating a steel billet having a rectangular cross section to 1100 to 1350° C., and sequentially performing a rough rolling step, an intermediate rolling step, and a finish rolling step, wherein:
 to a rolling mill which performs the rough rolling step, a plurality of calibers of three or more to shape a material to be rolled are provided; 
 at least one of the plurality of calibers is a split-creating caliber provided to a pair of upper and lower rolls and formed with projections which create splits vertically with respect to a width direction of the material to be rolled; and 
 in a subsequent stage of the split-creating caliber, a shaping caliber which sequentially bends divided parts formed by the split-creating caliber is provided. 
 
     
     
         7 . The manufacturing method of the rolled H-shaped steel according to  claim 6 , wherein
 a tip angle of the projections formed in the split-creating caliber is 40° or less.   
     
     
         8 . The manufacturing method of the rolled H-shaped steel according to  claim 6 , wherein a length H of the split created by the projection, a thickness T of the steel billet having the rectangular cross section, and a width F of a flange of the rolled H-shaped steel formed by the finish rolling step satisfy the following expression (1),
     H≥ 0.5 F− 0.5 T    (1).
   
     
     
         9 . The manufacturing method of the rolled H-shaped steel according to  claim 7 , wherein
 a length H of the split created by the projection, a thickness T of the steel billet having the rectangular cross section, and a width F of a flange of the rolled H-shaped steel formed by the finish rolling step satisfy the following expression (1),
     H≥ 0.5 F− 0.5 T    (1).