不完全球状化組織を有する中炭素鋼の冷間鍛造性

抄録

Cold forging has been widely used for forming of machine structural parts. Steels have been usually softened by carbide spheroidizing treatment before cold forging. In recent years, it has been demanded to shorten or to eliminate spheroidizing because it has usually needed many hours. The cold forgeability of medium carbon steels with insufficiently spheroidized microstructure made by shortened spheroidizing was studied. The cold forgeability was evaluated by cold upsetting test in which the end surfaces of cylindrical and flanged specimens were frictionally constrained. The effect of strain path on the cold forgeability was also investigated by measuring the surface strains both parallel and circumferential at the midheigh of the specimens during upsetting.
The cold cracks initiated from voids formed at cementites just below bulged free surface in insufficiently spheroidized steels. On the other hand, the surface micro-cracks generated in proeutectoid ferrite, where strain concentrated, in the as-rolled steel with ferrite-pearlite microstructure. The decrease in cementite length caused the better cold forgeability in the insufficiently spheroidized steels. Fracture limit decreased with the increase in strain-path slope in the circumferential strain versus axial strain diagram, namely the shift in strain path from homogeneous compression to plain strain condition. The more improvement in cold forgeability by shortened spheroidizing was observed in the specimens with larger strain-path slope. This result means that the cold forgeability of insufficiently spheroidized steels can be improved by the suitable design of the strain path during cold forging.