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: 0.70% to 2.00%;

V: 0.01% to 0.20%;

Al: 0.0001% to 0.10%;

Ti: 0.003% to 0.030%;

N: 0.0010% to 0.0200%;

O: 0.0001% to 0.0100%;

Mg: 0.0003% to 0.0050%;

Ni: 0% to 0.50%;

Cr: 0% to 0.50%;

Cu: 0% to 0.50%;

Mo: 0% to 0.30%;

Nb: 0% to 0.010%;

B: 0% to 0.0020%;

Ca: 0% to 0.0050%; and

a remainder of Fe and impurities,

wherein a carbon equivalent C eq obtained by the following Equation 1 is 0.30% to 0.50%;

a Mg-containing oxide having an equivalent circle diameter of 0.005 μm to 0.5 μm at a total number density of 100 pieces/mm 2 to 5000 pieces/mm 2 ;

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

at a strength evaluation portion 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, a fraction of bainite in a steel structure is 80% or more, and an average prior austenite grain size is 70 μm or more; and

at a toughness evaluation portion which is at a ½ position from the surface of the flange in the length direction and at a ¾ position from the surface of the flange in the thickness direction, the average prior austenite grain size in a steel structure is 200 μm or less,

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

wherein 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 ,

wherein the H-section steel includes, as the chemical composition, by mass %, one or more of

Ni: 0.01% to 0.50%,

Cr: 0.01% to 0.50%,

Cu: 0.01% to 0.50%,

Mo: 0.001% to 0.30%,

Nb: 0.001% to 0.010%,

B: 0.0001% to 0.0020%, and

Ca: 0.0001% to 0.0050%.

3. The H-section steel according to claim 1 ,

wherein, at the strength evaluation portion at room temperature, a yield strength or 0.2% proof stress is 450 MPa or more, and a tensile strength is 550 MPa or more; and

at the toughness evaluation portion, a Charpy absorbed energy at a test temperature of 21° C. is 100 J or more.

4. A method of producing an H-section steel, the method comprising:

a refining step which performs deoxidizing to cause a concentration of oxygen in a molten steel to be 0.0020% to 0.0100%, then sequentially adding Ti, Al, and Mg, 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: 0.70% to 2.00%, V: 0.01% to 0.20%, Al: 0.0001% to 0.10%, Ti: 0.003% to 0.030%, N: 0.0010% to 0.0200%, O: 0.0001% to 0.0100%, Mg: 0.0003% to 0.0050%, Ni: 0% to 0.50%, Cr: 0% to 0.50%, Cu: 0% to 0.50%, Mo: 0% to 0.30%, Nb: 0% to 0.010%, B: 0% to 0.0020%, Ca: 0% to 0.0050%, and a remainder of Fe and impurities, and to have a carbon equivalent C eq obtained by the following Equation 2 of 0.30% to 0.50%;

a casting step which casts the molten steel to obtain a steel piece;

a heating step which heats the steel piece to 1100° C. to 1350° C.;

a hot-rolling step which performs rolling on the heated steel piece such that a surface temperature of 850° C. or higher when the rolling is finished, thereby obtaining the H-section steel; and

a cooling step which performs water-cooling the H-section steel after the hot-rolling step;

wherein Al and Mg are added after 1 minute or longer has passed from the addition of the previous element;

wherein in the cooling step, water cooling conditions are controlled so that a cooling rate in a range from 800° C. to 600° C. at a ⅙ position from a surface of a flange in a length direction and at a ¼ position from the surface of the flange in a thickness direction is 2.2° C./s or more and the surface temperature after stopping the water-cooling is recuperated within a temperature range of 300° C. to 700° C.,

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

wherein 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.

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

wherein the H-section steel includes, as the chemical composition, by mass %, one or more of

Ni: 0.01% to 0.50%,

Cr: 0.01% to 0.50%,

Cu: 0.01% to 0.50%,

Mo: 0.001% to 0.30%,

Nb: 0.001% to 0.010%,

B: 0.0001% to 0.0020%, and

Ca: 0.0001% to 0.0050%.

6. The H-section steel according to claim 2 ,

wherein, at the strength evaluation portion at room temperature, a yield strength or 0.2% proof stress is 450 MPa or more, and a tensile strength is 550 MPa or more; and

at the toughness evaluation portion, a Charpy absorbed energy at a test temperature of 21° C. is 100 J or more.