抄録
The hot ductility of ultra-low carbon steels and 0.2 % plain carbon steels containing copper and/or tin has been investigated at elevated temperatures ranging from ferrite region to lower austenite region and at various strain rates. Ultra-low carbon steels generally exhibit good ductility regardless of copper and tin additions except for the lower austenite temperature range, where the addition of tin decreases the ductility slightly. The plain carbon steel containing 1.0 % copper also exhibits good ductility, except for a ductility trough around 1050 K and at 10-2 s-1. Recrystallization results in the improvement of ductility above 1180 K and at 10-2s-1. The plain carbon steel containing 0.2 % tin shows good ductility at 200 s-1, but exhibits a ductility trough in the lower austenite temperature range below 1 s-1. At the strain rate of 10-2s-1, the embrittlement takes place most severely at 1080 K, which corresponds to the transition temperature from austenite to ferrite. The embrittlement with intergranular fracture occurs in the specimen with low reduction in area. Initial cracking is observed at grain boundaries without proeutectoid ferrite. The addition of tin can prevent grain boundary migration or dynamic recrystallization by its grain boundary segregation, which leads to decrease in ductility.
