Effect of heat input on microstructure and toughness of rare earth-contained C–Mn steel

Abstract

Gleeble-1500D thermal simulator was used to simulate the thermal cycle of different welding processes in C–Mn steel. The toughness of steel matrix and heat-affected zone (HAZ) was investigated, and the microstructure and inclusion were characterized as well. The results showed that the welding process has a great influence on the microstructure in HAZ. When t8/5 (the cooling time from 800 to 500 °C) value of the welding process is less than 111 s, the microstructure in HAZ is mainly bainite/Widmanstatten in spite of the addition of rare earth. However, when t8/5 value is more than 250 s, there are a lot of acicular ferrites in the steel containing rare earth, while the main microstructures are grain boundary ferrite and bainite/Widmanstatten in the steel without rare earth. The impact toughness of the HAZ at ambient temperature first decreases and then increases with the increase in t8/5 value. The impact toughness is the worst when t8/5 value is 111 s. Rare earth can improve the impact performance of HAZ at ambient temperature, especially when t8/5 value of the welding process is 445 s. After the rare earth treatment, the cooling rate of forming acicular ferrite is about 0.5–7.5 °C/s. Acicular ferrite can form even during the welding process with larger t8/5 value up to 600 s in rare earth-treated steel.