Effects of manganese and carbon on the HAZ micro structural evolution in 500㎫ grade titanium oxide steels were investigated. Micro structural evolution primarily depends on super cooling . When cooled at 3˚C/s in 0.15%C-1.5%Mn steels, grain boundary and Widmanstatten ferrite formed at 640and 620˚C, respectively, followed by competitive formation of acicular ferrite and upper bainite inside of grain at 580˚C. With an increase of manganese, degree of super cooling increased while critical cooling rate for the formation of grain boundary ferrite decreased. Consequently, the amount of acicular ferrite in HAZ was decreased in 2.0%Mn after initial increase in 1.0 and 1.5%Mn. Therefore, aptimum super cooling should be maintained to accelerate acicular ferrite formation in titanium oxide steels. Low carbon steel, 0.11%C-1.5%Mn, showed larger amount of acicular ferrite than higher carbon steel because of effectiveness of diffusionless transformation in low caron steel. |