H-section steel and method of producing the same

Abstract

An H-section steel has a predetermined chemical composition, an Nb content and a B content satisfy Nb+125×B≧0.075%, a thickness of a flange is 12 mm to 40 mm, and in a thickness center portion and ¼ flange width portion of the flange, a metallographic structure of a cross-section perpendicular to a rolling direction contains pearlite having a distribution density of 3.2×10 −3 units/μm 2 or lower and the remainder is formed of ferrite and bainite.

1 . An H-section steel, wherein
 a chemical composition thereof includes, in terms of mass %:   C: 0.010% to 0.014%;   Si: 0.05% to 0.50%;   Mn: 0.8% to 2.0%;   Cu: 0.01% or more and less than 0.60%;   Ni: 0.01% or more and less than 0.50%;   Al: more than 0.005% and 0.040% or less;   Ti: 0.001% to 0.025%;   Nb: 0.010% to 0.070%;   N: 0.001% to 0.009%;   O: 0.0005% to 0.0035%;   B: more than 0.0003% and 0.0015% or less;   V: 0% to 0.10%;   Mo: 0% to 0.10%;   Cr: 0% or more and less than 0.20%;   Zr: 0 to 0.030%;   Hf: 0 to 0.010%;   REM: 0 to 0.010%;   Ca: 0 to 0.0050%; and   a remainder including Fe and an impurity,   wherein an Nb content and a B content satisfy Expression 1,   a thickness of a flange is 12 mm to 40 mm, and   in a thickness center portion and ¼ flange width portion of the flange, a metallographic structure of a cross-section perpendicular to a rolling direction contains a pearlite having a distribution density of 3.2×10 −3  units/μm 2  or lower and the remainder consists of a ferrite and a bainite,
     Nb+ 125× B≧ 0.075%  (Expression 1),
 
   wherein Nb and B in Expression 1 are amounts of the elements in terms of mass %.   
     
     
         2 . The H-section steel according to  claim 1 ,
 wherein a tensile strength is 460 MPa to 550 MPa.   
     
     
         3 . The H-section steel according to  claim 1  or  2 ,
 wherein the chemical composition includes, in terms of mass %, at least one of: 
 V: 0.01% to 0.10%; 
 Mo: 0.01% to 0.10%; and or 
 Cr: 0.01% or more and less than 0.20%. 
 
     
     
         4 . The H-section steel according to  claim 1  or  2 ,
 wherein the chemical composition includes, in terms of mass %, at least one of: 
 Zr: 0.001% to 0.030%; or 
 Hf: 0.001% to 0.010%. 
 
     
     
         5 . The H-section steel according to  claim 1  or  2 ,
 wherein the chemical composition includes, in terms of mass %, at least one of: 
 REM: 0.0001% to 0.010%; or 
 Ca: 0.0001% to 0.0050%. 
 
     
     
         6 . A method of producing an H-section steel, the method comprising:
 a heating process of heating a steel slab having the chemical composition according to  claim 1  or  2  to 1100° C. to 1350° C.;   a hot rolling process of performing a hot rolling on the steel slab into an H-section steel; and   an air cooling process of performing an air cooling on the H-section steel,   wherein the hot rolling process includes a rough rolling process of rolling the steel slab by using a roughing mill, an intermediate rolling process of performing a reverse rolling by using an intermediate rolling mill, and a finish rolling process of performing a rolling by using a finishing mill,   wherein during the reverse rolling of the intermediate rolling process, a controlled rolling in which a rolling is performed while the H-section steel is cooled by using a water cooling apparatus provided on a front surface and a rear surface of the intermediate rolling mill is performed, and   in the finish rolling process, the rolling is performed in one or more passes while a surface temperature of a flange is in a range of 770° C. to 870° C., and   a thickness of the flange is 12 mm to 40 mm.