EFFECT OF COPPER AND TIN ON HOT DUCTILITY OF ULTRA-LOW AND 0.2-PERCENT CARBON-STEELS

The hot ductility of ultra-low carbon steels and 0.2% plain carbon ste els containing copper and/or tin has been investigated at elevated tem peratures 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 d uctility slightly. The plain carbon steel containing 1.0% copper also exhibits good ductility, except for a ductility trough around 1050 K a nd at 10(-2)s(-1). Recrystallization results in the improvement of duc tility above 1180 K and at 10(-2)s(-1). The plain carbon steel contain ing 0.2% tin shows good ductility at 200s(-1), but exhibits a ductilit y trough in the lower austenite temperature range below 1s(-1). At the strain rate of 10(-2)s(-1), the embrittlement takes place most severe ly at 1080 K, which corresponds to the transition temperature from aus tenite to ferrite. The embrittlement with intergranular fracture occur s in the specimen with low reduction in area. Initial cracking is obse rved 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 ductili ty.