A steel plate comprising, as a chemical composition, by mass%: C: 0.020% to 0.080%; Si: 0.01% to 0.50%; Mn: 0.50% to 1.60%; Nb: 0.001% to 0.100%; N: 0.0010% to 0.0100%; Ca: 0.0001% to 0.0050%; P: 0.030% or less; S: 0.0025% or less; Ti: 0.005% to 0.030%; Al: 0.010% to 0.040%; O: 0.0040% or less; Mo: 0% to 2.00%; Cr: 0% to 2.00%; Cu: 0% to 2.00%; Ni: 0% to 2.00%; W: 0% to 1.00%; V: 0% to 0.200%; Zr: 0% to 0.0500%; Ta: 0% to 0.0500%; B: 0% to 0.0020%; REM: 0% to 0.0100%; Mg: 0% to 0.0100%; Hf: 0% to 0.0050%; Re: 0% to 0.0050%; and a remainder: Fe and impurities, wherein the following formula (i) is satisfied, Ceq represented by the following formula (ii) is 0.30 to 0.50, a metallographic structure in a thickness middle portion includes, by area%, 0% to 80% of polygonal ferrite and one or two selected from acicular ferrite and bainite, a remainder is an M-A phase, and an effective grain size is 15.0 μm or less, a metallographic structure in a surface layer that is a range of 1.0 mm in a thickness direction from a surface includes, by area%, a total of 95% or more of one or two selected from acicular ferrite and bainite, and a remainder is an M-A phase, and a maximum hardness in the surface layer is 250 HV0.1 or less, wherein the area% of polygonal ferrite, the area% of acicular ferrite and bainite, and the effective grain size are determined according to the methods described in the description, and the maximum hardness in the surface layer is evaluated by a Vickers hardness test according to JIS Z 224:2009 in which the test force is set to 0.98 N (0.1 kgf), 0.05 ≤ Mo + Cr + Cu + Ni ≤ 2.00 Ceq = C + Mn / 6 + Ni + Cu / 15 + Cr + Mo + V / 5 here, each element symbol in the formulae represents an amount (mass%) of each element contained in steel and is regarded as zero in a case where a corresponding element is not contained. C: 0.020% to 0.080%; Si: 0.01% to 0.50%; Mn: 0.50% to 1.60%; Nb: 0.001% to 0.100%; N: 0.0010% to 0.0100%; Ca: 0.0001% to 0.0050%; P: 0.030% or less; S: 0.0025% or less; Ti: 0.005% to 0.030%; Al: 0.010% to 0.040%; O: 0.0040% or less; Mo: 0% to 2.00%; Cr: 0% to 2.00%; Cu: 0% to 2.00%; Ni: 0% to 2.00%; W: 0% to 1.00%; V: 0% to 0.200%; Zr: 0% to 0.0500%; Ta: 0% to 0.0500%; B: 0% to 0.0020%; REM: 0% to 0.0100%; Mg: 0% to 0.0100%; Hf: 0% to 0.0050%; Re: 0% to 0.0050%; and a remainder: Fe and impurities, wherein the following formula (i) is satisfied, Ceq represented by the following formula (ii) is 0.30 to 0.50, a metallographic structure in a thickness middle portion includes, by area%, 0% to 80% of polygonal ferrite and one or two selected from acicular ferrite and bainite, a remainder is an M-A phase, and an effective grain size is 15.0 μm or less, a metallographic structure in a surface layer that is a range of 1.0 mm in a thickness direction from a surface includes, by area%, a total of 95% or more of one or two selected from acicular ferrite and bainite, and a remainder is an M-A phase, and a maximum hardness in the surface layer is 250 HV0.1 or less, wherein the area% of polygonal ferrite, the area% of acicular ferrite and bainite, and the effective grain size are determined according to the methods described in the description, and the maximum hardness in the surface layer is evaluated by a Vickers hardness test according to JIS Z 224:2009 in which the test force is set to 0.98 N (0.1 kgf), 0.05 ≤ Mo + Cr + Cu + Ni ≤ 2.00 Ceq = C + Mn / 6 + Ni + Cu / 15 + Cr + Mo + V / 5 here, each element symbol in the formulae represents an amount (mass%) of each element contained in steel and is regarded as zero in a case where a corresponding element is not contained. <EOS>