H section and method for manufacturing same

Abstract

This H section includes, as a chemical composition, C, Si, Mn, Nb, V, Ti, and N; in which the H section includes, as a metallographic structure, ferrite of 60 area % to less than 100 area %, an average grain size of this ferrite is 1 μm to 30 μm, a thickness of a flange is 20 mm to 140 mm, tensile yield stress is 385 MPa to 530 MPa, and Charpy absorbed energy at −20° C. is 100 J or more.

1 . An H section, wherein
 a steel comprises, as a chemical composition, by mass %,   C: 0.05% to 0.160%,   Si: 0.01% to 0.60%,   Mn: 0.80% to 1.70%,   Nb: 0.005% to 0.050%,   V: 0.05% to 0.120%,   Ti: 0.001% to 0.025%,   N: 0.0001% to 0.0120%,   Cr: 0% to 0.30%,   Mo: 0% to 0.20%,   Ni: 0% to 0.50%,   Cu: 0% to 0.35%,   W: 0% to 0.50%,   Ca: 0% to 0.0050%,   Zr: 0% to 0.0050%,   Al: limited to 0.10% or less,   B: limited to 0.0003% or less, and   a remainder including Fe and impurities, wherein   C, Mn, Cr, Mo, V, Ni, and Cu in the chemical composition satisfy 0.30≤Ceq≤0.48, when Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15,   the steel includes, as a metallographic structure, by area fraction, ferrite of 60% to less than 100%, a mixed structure MA of martensite and austenite being limited to 3.0% or less, and a structure other than the ferrite and the MA being limited to 37% or less,   an average grain size of the ferrite is 1 μm to 30 μm,   a shape is an H-shape and a thickness of a flange is 20 mm to 140 mm, when being seen in a cut section orthogonal to a rolling direction,   at a position of (1/6)F from an end surface of the flange in the width direction, tensile yield stress is 385 MPa to 530 MPa and maximum tensile strength is 490 MPa to 690 MPa, when a length of the flange in a width direction is F, and   at the position of (1/6)F and at a position of (114)t 2  from an outer surface of the flange in a thickness direction, absorbed energy in a Charpy test at −20° C. is 100 J or more, when the thickness of the flange is t 2 .   
     
     
         2 . The H section according to  claim 1 , comprising, as the chemical composition, by mass %,
 Nb: more than 0.02% to 0.050%.   
     
     
         3 . The H section according to  claim 1 , comprising, as the chemical composition, by mass %,
 N: more than 0.005% to 0.0120%.   
     
     
         4 . The H section according to  claim 1 , comprising, as the chemical composition, by mass %,
 Cu: limited to less than 0.03%.   
     
     
         5 . The H section according to  claim 1 , comprising, as the chemical composition, by mass %,
 Al: limited to less than 0.003%.   
     
     
         6 . The H section according to  claim 1 ,
 wherein the thickness of the flange is 25 mm to 140 mm.   
     
     
         7 . A method for manufacturing the H section according to  claim 1 , comprising:
 steelmaking to obtain a molten steel which has the chemical composition according to  claim 1 ;   casting the molten steel after the steelmaking to obtain a steel piece;   heating the steel piece after the casting to 1,100° C. to 1,350° C.;   hot rolling on the steel piece after the heating to form a shape, when being seen in a cut section orthogonal to a rolling direction, into an H-shape under conditions, wherein
 a cumulative rolling reduction at a position of (1/6)F from an end surface of a flange in a width direction within more than 900° C. and 1,100° C. or less is 20% or more, 
 the cumulative rolling reduction at the position within 730° C. to 900° C. is 15% or more, and 
 the rolling ends at 730° C. or more; and 
 cooling a hot rolled material after the hot rolling by natural cooling.