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

To provide a non-heat-treated steel plate having high yield strength in which hardness of a weld heat-affected zone and degradation of low-temperature toughness of the weld heat-affected zone are suppressed. The steel wire rod of the present disclosure includes predetermined components in steel, wherein Ceq defined by the formula (1) below is less than 0.44, an A value defined by the formula (2) below is 2.50 or more, and a B value defined by the formula (3) below is 2.37 or more, and wherein area ratios of metal structures in a 1/4 position of a thickness of the steel plate satisfy bainite: 80% by area or more, and martensite-austenite constituent: 0% by area or more and 0.26% by area or less, and wherein a maximum hardness of the bainite is 270 HV or more. $\mathrm{Ceq}=\mathrm{C}+\mathrm{Mn}/6+\left(\mathrm{Cu}+\mathrm{Ni}\right)/15+\left(\mathrm{Cr}+\mathrm{Mo}+\mathrm{V}\right)/5\mathop{\mathrm{image}}$$\mathrm{A\; value}=1.15\times \mathrm{Mn}+2.20\times \mathrm{Mo}+6.50\times \mathrm{Nb}\mathop{\mathrm{image}}$$\mathrm{B\; value}=1.20\times \mathrm{Mn}+0.50\times \mathrm{Ni}+4.25\times \mathrm{Nb}\mathop{\mathrm{image}}$