Abstract: A method of manufacturing a hot rolled steel sheet for a square column for building structural members includes a hot rolling step, a cooling step, and a coiling step performed on a steel to form a hot rolled steel sheet, wherein the steel has a composition containing, in terms of % by mass, C: 0.07 to 0.18%, Mn: 0.3 to 1.5%, P: 0.03% or less, S: 0.015% or less, Al: 0.01 to 0.06%, N: 0.0006% or less, and the balance being Fe and unavoidable impurities.

Abstract: A method of manufacturing a hot rolled steel sheet for a square column for building structural members includes a hot rolling step, a cooling step, and a coiling step performed on a steel to form a hot rolled steel sheet, wherein the steel has a composition containing, in terms of % by mass, C: 0.07 to 0.18%, Mn: 0.3 to 1.5%, P: 0.03% or less, S: 0.015% or less, Al: 0.01 to 0.06%, N: 0.0006% or less, and the balance being Fe and unavoidable impurities.

Abstract: A hot rolled steel sheet for a square column for building structural members, the hot rolled steel sheet having a composition of, in terms of % by mass, C: 0.07 to 0.18%, Mn: 0.3 to 1.5%, P: 0.03% or less, S: 0.015% or less, N: 0.006% or less, Al: 0.01 to 0.06%, and the balance being Fe and unavoidable impurities, and having a microstructure that includes a primary phase constituted of ferrite and a second phase constituted of pearlite or pearlite and bainite, wherein a second phase frequency defined by equation (1) below is 0.20 to 0.42 and a mean crystal grain diameter of the primary phase and the second phase together is 7 to 15 ?m Second phase frequency=(Number of second phase grains intersecting line segments of particular length)/(Number of primary phase grains and second phase grains intersecting line segments of particular length)??(1).

Abstract: A hot rolled steel sheet for a square column for building structural members, the hot rolled steel sheet having a composition of, in terms of % by mass, C: 0.07 to 0.18%, Mn: 0.3to 1.5%, P: 0.03% or less, S: 0.015% or less, N: 0.006% or less, Al: 0.01 to 0.06%, and the balance being Fe and unavoidable impurities, and having a microstructure that includes a primary phase constituted of ferrite and a second phase constituted of pearlite or pearlite and bainite, wherein a second phase frequency defined by equation (1) below is 0.20 to 0.42 and a mean crystal grain diameter of the primary phase and the second phase together is 7 to 15 ?m Second phase frequency=(Number of second phase grains intersecting line segments of particular length)/(Number of primary phase grains and second phase grains intersecting line segments of particular length) ??(1).

Abstract: A low yield ratio and high-strength hot rolled steel sheet having a composition containing, on a mass percent basis, 0.03% to 0.10% C, 0.10% to 0.50% Si, 1.4% to 2.2% Mn, 0.005 % to 0.10% Al, 0.02% to 0.10% Nb, 0.001% to 0.030% Ti, 0.05% to 0.50% Mo, 0.05% to 0.50% Cr, and 0.01% to 0.50% Ni, in which Moeq preferably satisfies the range of 1.4% to 2.2%; and a microstructure including a main phase that contains bainitic ferrite having an average grain size of 10 ?m or less and a secondary phase that contains massive martensite having an aspect ratio of less than 5.0 in an area ratio of 1.4% to 15%.