A steel sheet with a tensile strength of 550 MPa or more before and after a post weld heat treatment comprising, as chemical components, by mass: C: 0.02 to 0.07%; Si: 0.1 to 0.4%; Mn: 1.2 to 2%; P: more than 0% and 0.02% or less; S: more than 0% and 0.005% or less; Cu: 0.1 to 0.7%; Al: 0.01 to 0.08%; Ni: 0.45 to 0.85%; Mo: 0.01 to 0.25%; Nb: 0.015 to 0.05%; Ti: 0.005 to 0.025%; Ca: 0.0005 to 0.003%; and N: 0.001 to 0.01%; optionally further comprising, as another element, at least one selected from the group consisting of: Cr: 0.001% or more and 0.2% or less; V: 0.0001% or more and 0.02% or less; and B: 0.0001% or more and 0.001% or less, with the balance being iron and inevitable impurities, wherein, an estimated precipitation amount P 0 being 1.50 or more which is obtained from amounts of Nb, Mo and C and is represented by formula below, and the metallographic structure consists of: ferrite: 60 area% or more; bainite: 4 area% or more; and other structures like pearlite, island martensite:10 area% or less, relative to an entire structure, P 0 = 340 × 0.6 × Mo + 22 + Nb × C 1 in the formula , C 1 is obtained by formulae or below, and [ ] represents a content of each element by mass% in the formulae to , when C ≥ 12 / 95 × Mo + Nb C 1 = 12 / 95 × Mo + Nb when C < 12 / 95 × Mo + Nb C 1 = C and wherein when a precipitation amount P 1 is calculated, by formula below, from an average interval λ in μm between carbides containing at least one selected from the group consisting of Nb and Mo, wherein the average interval λ is calculated on the basis of the formula (c) provided in the description, wherein the tensile strength and the microstructure refer to a position at 1/4 of the sheet thickness, and wherein the tensile strength was measured on a test piece as defined in ASTM A370 in a direction orthogonal to the rolling direction, a solid solution equivalent A satisfies a value of 0.50 or more, the solid solution equivalent A obtained from the estimated precipitation amount P 0 and the precipitation amount P 1 and represented by formula below: P 1 = 0.7 / λ A = P 0 − P 1 C: 0.02 to 0.07%; Si: 0.1 to 0.4%; Mn: 1.2 to 2%; P: more than 0% and 0.02% or less; S: more than 0% and 0.005% or less; Cu: 0.1 to 0.7%; Al: 0.01 to 0.08%; Ni: 0.45 to 0.85%; Mo: 0.01 to 0.25%; Nb: 0.015 to 0.05%; Ti: 0.005 to 0.025%; Ca: 0.0005 to 0.003%; and N: 0.001 to 0.01%; Cr: 0.001% or more and 0.2% or less; V: 0.0001% or more and 0.02% or less; and B: 0.0001% or more and 0.001% or less, with the balance being iron and inevitable impurities, wherein, an estimated precipitation amount P 0 being 1.50 or more which is obtained from amounts of Nb, Mo and C and is represented by formula below, and the metallographic structure consists of: ferrite: 60 area% or more; bainite: 4 area% or more; and other structures like pearlite, island martensite:10 area% or less, relative to an entire structure, ferrite: 60 area% or more; bainite: 4 area% or more; and other structures like pearlite, island martensite:10 area% or less, relative to an entire structure, when C ≥ 12 / 95 × Mo + Nb C 1 = 12 / 95 × Mo + Nb when C < 12 / 95 × Mo + Nb C 1 = C and wherein when a precipitation amount P 1 is calculated, by formula below, from an average interval λ in μm between carbides containing at least one selected from the group consisting of Nb and Mo, wherein the average interval λ is calculated on the basis of the formula (c) provided in the description, wherein the tensile strength and the microstructure refer to a position at 1/4 of the sheet thickness, and wherein the tensile strength was measured on a test piece as defined in ASTM A370 in a direction orthogonal to the rolling direction, a solid solution equivalent A satisfies a value of 0.50 or more, the solid solution equivalent A obtained from the estimated precipitation amount P 0 and the precipitation amount P 1 and represented by formula below: P 1 = 0.7 / λ A = P 0 − P 1 <EOS>