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

摘要

PROBLEM TO BE SOLVED: To provide a non-heat-treated steel sheet having high yield strength in which degradation of low-temperature toughness of a weld heat-affected zone and hardness of the weld heat-affected zone are suppressed.SOLUTION: There is provided the non-heat-treated steel sheet having high yield strength in which degradation of low-temperature toughness of a weld heat-affected zone and hardness of the weld heat-affected zone are suppressed. The non-heat-treated steel sheet is characterized in that: it includes predetermined components in the steel; a Ceq specified by expression (1) is less than 0.44; an A value specified by expression (2) is 2.50 or greater; a B value specified by expression (3) is 2.37 or greater; 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. Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5...(1) A value=1.15×Mn+2.20×Mo+6.50×Nb...(2) B value=1.20×Mn+0.50×Ni+4.25×Nb...(3)SELECTED DRAWING: None