NON-HEAT-TREATED STEEL SHEET HAVING HIGH YIELD STRENGTH IN WHICH HARDNESS OF A WELDING-HEAT-AFFECTED ZONE AND DEGRADATION OF LOW-TEMPERATURE TOUGHNESS OF THE WELDING-HEAT-AFFECTED ZONE ARE SUPPRESSED

摘要

Provided is a non-heat-treated steel sheet having high yield strength in which hardness of a welding-heat-affected zone and degradation of low-temperature toughness of the welding-heat-affected zone are suppressed. A non-heat-treated steel sheet having high yield strength in which hardness of a welding-heat-affected zone and degradation of low-temperature toughness of the welding-heat-affected zone are suppressed, the non-heat-treated steel sheet characterized by including predetermined components in the steel, the Ceq specified by expression (1) is less than 0.44, the A value specified by expression (2) is 2.5 or greater, the B value specified by expression (3) is 2.37 or greater, the area ratios of the metallographic structure below at a position 1/4 of the thickness of the steel sheet satisfy the values of 80% or more of bainite and 0% to 0.26% of island martensite, and the maximum hardness of the bainite is 270 HV or greater. (1): Ceq = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5. (2): A value = 1.15 × Mn + 2.20 × Mo + 6.50 × Nb. (3): B value = 1.20 × Mn + 0.50 × Ni + 4.25 × Nb.