H-section steel and method of producing the same

1. An H-section steel comprising, as a chemical composition, by mass %:

C: 0.05% to 0.16%;

Si: 0.01% to 0.50%;

Mn: 0.80% to 2.00%;

Ni: 0.05% to 0.50%;

V: 0.01% to 0.20%;

Al: 0.005% to 0.100%;

Ti: 0.005% to 0.030%;

N: 0.0010% to 0.0200%;

S: 0.002% to 0.02%;

Mg: 0.0005% to 0.005;

Cr: 0% to 0.50%;

Cu: 0% to 0.50%;

Mo: 0% to 0.20%;

Nb: 0% to 0.05%;

B: 0% to 0.0020%, and

a remainder consisting of Fe and impurities,

wherein C eq obtained by the following Equation 1 is 0.35% to 0.50%,

a thickness of a flange is 100 mm to 150 mm,

an area fraction of bainite in a steel structure at a strength evaluation position which is at a ⅙ position from a surface of the flange in a length direction and at a ¼ position from the surface in a thickness direction is 80% or more,

yield strength or 0.2% proof strength is 450 MPa or more, and tensile strength is 550 MPa or more and 680 MPa or less at the strength evaluation position,

an average austenite grain size in a steel structure at a toughness evaluation position which is at a ½ position from the surface of the flange in the length direction and at a ¾ position from the surface in the thickness direction is 150 μm or less, and

(Mg, Mn)S having a particle size of 0.005 μm to 0.5 μm is included at a density of 1.0×10 5 pieces/mm to 1.0×10 7 pieces/mm 2 ,

the (Mg, Mn)S includes, by mass %, 20% to 80% of Mn, 20% to 80% of Mg, and a remainder, and a ratio of S with respect to a total mass of S and O in the remainder is, by mass %, 50% to 100%,

C eq =C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15  Equation 1

here, C, Mn, Cr, Mo, V, Ni, and Cu represent the amount of each element contained by mass % and the amount of an element not contained is 0.

2. The H-section steel according to claim 1 , comprising, as the chemical composition, by mass %,

one of or two or more of

Cr: 0.01% to 0.50%,

Cu: 0.01% to 0.50%,

Mo: 0.001% to 0.20%,

Nb: 0.001% to 0.05%, and

B: 0.0001% to 0.0020%.

3. A method of producing an H-section steel comprising:

forming (Mg, Mn)S by adding Mn, Mg, and Al to a molten steel and adjusting a chemical composition of the molten steel to include, by mass %, C: 0.05% to 0.16%, Si: 0.01% to 0.50%, Mn: 0.80% to 2.00%, Ni: 0.05% to 0.50%, V: 0.01% to 0.20%, Al: 0.005% to 0.100%, Ti: 0.005% to 0.030%, N: 0.0010% to 0.0200%, S: 0.002% to 0.02%, Mg: 0.0005% to 0.005%, Cr: 0% to 0.50%, Cu: 0% to 0.50%, Mo: 0% to 0.20%, Nb: 0% to 0.05%, B: 0% to 0.0020%, and a remainder consisting of Fe and impurities, and have C eq obtained by the following Equation 2 of 0.35% to 0.50%;

casting the molten steel to obtain steel pieces;

heating the steel piece to 1100° C. to 1350° C.;

performing a rough rolling on the heated steel pieces using a roughing mill and forming the steel pieces into an H-section steel;

performing a reverse rolling on the H-section steel using an intermediate rolling mill;

performing a finish rolling on the H-section steel using a finishing mill so that a rolling finish temperature reaches a surface temperature of 800° C. or higher;

water-cooling the H-section steel; and

recuperating a temperature of the H-section steel so that the surface temperature is within a temperature range of 300° C. to 700° C.,

wherein in the forming of the (Mg, Mn)S, the concentration of O in the molten steel when the Mg is added is 50 ppm or less, and

the reverse rolling in the performing of the reverse rolling is controlled rolling,

C eq =C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15  Equation 2

here, C, Mn, Cr, Mo, V, Ni, and Cu represent the amount of each element contained by mass % and the amount of an element not contained is 0.

4. The method of producing an H-section steel according to claim 3 ,

wherein the steel includes, as the chemical composition, by mass %,

one of or two or more of

Cr: 0.01% to 0.50%,

Cu: 0.01% to 0.50%,

Mo: 0.001% to 0.20%,

Nb: 0.001% to 0.05%, and

B: 0.0001% to 0.0020%.