THE FORMATION OF INTRAGRANULAR FERRITE PLATES IN MEDIUM-CARBON STEELS FOR HOT-FORGING AND ITS EFFECT ON THE TOUGHNESS

Mechanism of the formation of intragranular ferrite plates in medium-carbon steels for hot-forging were investigated and the toughness of the steels with the microstructure consisting of intragranular ferrite plates were evaluated. The formation of intragranular ferrite plates was clarified to be enhanced by the addition of both vanadium and nitrogen in the steels continuously cooled after heated at 1250 degrees C. It was observed that the vanadium nitrides precipitating on manganese sulfide particles acted as nucleation sites of intragranular ferrite plates. It is well known that vanadium carbide precipitates in ferrite matrix holding the Baker-Nutting orientation relationship with respect to ferrite matrix in the steels with low nitrogen content and that the lattice mismatch of the interface between vanadium carbide and ferrite matrix is small. In the case of steels with high vanadium and high nitrogen, vanadium nitrides precipitating in austenite matrix during cooling are thought to act as nucleation sites of intragranular ferrite plates. It may, therefore, be concluded that the factor governing the formation of intragranular ferrite plates is the presence of precipitates which can develop coherent, low energy interphase boundaries with ferrite in austenite. The toughness of medium-carbon steels with tensile strength higher than 1000 MPa were shown to be improved through the formation of intragranular ferrite plates.