Steel H-shape for low temperature service and manufacturing method therefor

1. A steel H-shape comprising, by mass %,

C: 0.05% to 0.13%,

Mn: 0.80% to 2.00%,

Nb: 0.011% to 0.060%,

Ti: 0.005% to 0.025%,

O: 0.0005% to 0.0100%.

V: 0% to 0.08%,

Cu: 0% to 0.40%,

Ni: 0% to 0.70%,

Mo: 0% to 0.10%,

Cr: 0% to 0.20%,

Si: limited to 0.50% or less,

Al: limited to 0.008% or less,

Ca: limited to 0.0010% or less,

REM: limited to 0.0010% or less,

Mg: limited to 0.0010% or less,

N: limited to 0.0120% or less, and

a remainder including Fe and impurities,

wherein a CEV obtained b the following Expression (1) is 0.40 or less,

wherein at a ¼ position from an outer side across a thickness of a flange and a ⅙ position from an outer side across a flange width, a sum of an area ratio of one or both of ferrite and bainite is 90% or more, and an area ratio of a hard phase consisting of one or both of MA and pseudo-pearlite is 10% or less,

wherein an effective grain size is 20.0 μm or less, and a grain size of the hard phase is 10.0 μm or less,

wherein Charpy absorbed energy at −40° C. is 60 J or greater,

wherein 30 pieces/mm 2 or more Ti oxides having an equivalent circle diameter ranging from 0.01 to 3.0 μm are included, and

wherein a thickness of the flange is 12 to 50 mm,

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

where, C, Mn, Cr, Mo, V, Ni, and Cu each indicate an amount of the element by mass %.

2. The steel H-shape according t comprising, by mass %, one or two or more selected from the group consisting of

V: 0.01% to 0.08%,

Cu: 0.01% to 0.40%,

Ni: 0.01% to 0.70%,

Mo: 0.01% to 0.10%, and

Cr: 0.01% to 0.20%.

3. A method of manufacturing the steel H-shape according to claim 1 , the method comprising:

melting a steel including the same chemical composition as that of the steel H-shape according to claim 1 ;

casting the steel obtained through the melting to obtain a slab;

heating the slab to a temperature ranging from 1,100° C. to 1,350° C., and then performing hot rolling at a finishing temperature ranging from (Ar 3 -30)° C. to 900° C. to obtain a steel H-shape; and

performing an accelerated cooling of the steel H-shape, in which inner and outer surfaces of a flange are subjected to water cooling at a cooling rate exceeding 15° C./sec,

wherein in the melting, Ti is added after oxygen concentration of a molten steel immediately before addition of the Ti is adjusted to a range from 0.0015 to 0.0110 mass %, and

wherein in the accelerated cooling, the water cooling is performed such that a cooling stop temperature at a ⅙ position from an outer side across a flange width of the steel H-shape is 300° C. or lower at a surface temperature, and a maximum temperature of the surface temperature after recuperating is 350° C. to 700° C.

4. A method of manufacturing the steel H-shape according to claim 2 , the method comprising:

melting a steel including the same chemical composition as that of the steel H-shape according to claim 2 ; casting the steel obtained through the melting to obtain a slab; heating the slab to a temperature ranging from 1 , 100 ° C. to 1 , 350 ° C., and then performing hot rolling at a finishing temperature ranging from (Ar 3 -30)° C. to 900° C. to obtain a steel H-shape; and

performing an accelerated cooling of the steel H-shape, in which inner and outer surfaces of a flange are subjected to water cooling at a cooling rate exceeding 15° C./sec,

wherein in the melting, Ti is added after oxygen concentration of a molten steel immediately before addition of the Ti is adjusted to a range from 0.0015 to 0.0110 mass %, and

wherein in the accelerated cooling, the water cooling is performed such that a cooling stop temperature at a ⅙ position from an outer side across a flange width of the steel H-shape is 300° C. or lower at a surface temperature, and a maximum temperature of the surface temperature after recuperating is 350° C. to 700° C.