A steel plate with a chemical composition comprising, on a mass percent basis: C: 0.04% to 0.12%; Si: 0.03% to 0.70%; Mn: 0.30% to 2.30%; P: 0.020% or less; S: 0.0050% or less; Nb: 0.005% to 0.060%; Ti: 0.005% to 0.100%; Al: 0.001% to 0.100%; Ca: 0.0005% to 0.0200%; O: 0.01% or less; and N: 0.01% or less, and optionally one or two or more selected from the group consisting of Cu: 2.00% or less; Cr: 1.50% or less; Mo: 1.00% or less; V: 0.30% or less; B: 0.0100% or less; W: 0.50% or less; Mg: 0.0200% or less; and REM: 0.0500% or less, the remainder being Fe and incidental impurities, wherein Ceq defined by the following formula and a thickness t [mm] of the steel plate satisfy 0.0004t + 0.25 ≤ Ceq ≤ 0.55, a steel microstructure has an average grain size of 20 μm or less at a center of a thickness of the steel plate, and a concentration of Mn, a concentration of P, and a concentration of S in a segregation zone at the center of the thickness of the steel plate satisfy the following formula : Ceq = C + Mn / 6 + Cr + Mo + V / 5 + Cu + Ni / 15 Mn * + 20 P * + 50 S * ≤ 15 wherein each element symbol in the formula denotes the corresponding element content, in mass%, and is 0 in the absence of the element, and each element symbol in the formula denotes the concentration, in mass%, of the corresponding element in the segregation zone at the center of the thickness of the steel plate and is 0 in the absence of the element, wherein the steel plate has a yield strength of 300 MPa or more and a tensile strength of 400 MPa or more at the center of the thickness of the steel plate in a tensile test high strength and an impact absorbed energy of 60 J or more at the center of the thickness of the steel plate in a Charpy impact absorption test at -40°C, wherein yield strength, tensile strength and impact absorbed energy are measured according to the methods disclosed in the description. C: 0.04% to 0.12%; Si: 0.03% to 0.70%; Mn: 0.30% to 2.30%; P: 0.020% or less; S: 0.0050% or less; Nb: 0.005% to 0.060%; Ti: 0.005% to 0.100%; Al: 0.001% to 0.100%; Ca: 0.0005% to 0.0200%; O: 0.01% or less; and N: 0.01% or less, and optionally one or two or more selected from the group consisting of Cu: 2.00% or less; Cr: 1.50% or less; Mo: 1.00% or less; V: 0.30% or less; B: 0.0100% or less; W: 0.50% or less; Mg: 0.0200% or less; and REM: 0.0500% or less, the remainder being Fe and incidental impurities, wherein Ceq defined by the following formula and a thickness t [mm] of the steel plate satisfy 0.0004t + 0.25 ≤ Ceq ≤ 0.55, a steel microstructure has an average grain size of 20 μm or less at a center of a thickness of the steel plate, and a concentration of Mn, a concentration of P, and a concentration of S in a segregation zone at the center of the thickness of the steel plate satisfy the following formula : Ceq = C + Mn / 6 + Cr + Mo + V / 5 + Cu + Ni / 15 Mn * + 20 P * + 50 S * ≤ 15 wherein each element symbol in the formula denotes the corresponding element content, in mass%, and is 0 in the absence of the element, and each element symbol in the formula denotes the concentration, in mass%, of the corresponding element in the segregation zone at the center of the thickness of the steel plate and is 0 in the absence of the element, wherein the steel plate has a yield strength of 300 MPa or more and a tensile strength of 400 MPa or more at the center of the thickness of the steel plate in a tensile test high strength and an impact absorbed energy of 60 J or more at the center of the thickness of the steel plate in a Charpy impact absorption test at -40°C, wherein yield strength, tensile strength and impact absorbed energy are measured according to the methods disclosed in the description. <EOS>