The influence of cooling rate and chemical composition on phase transformation and hardness of C70S6 steel were studied by Gleeble-3800 thermal simulation testing machine and box type electric furnace. The results showed that when the cooling rate was between 0.3 and 5 °C/s, the transformation products of two experimental steels were mainly composed of ferrites, pearlite and sorbite. The pearlite content gradually decreased with the cooling rate increasing, while the sorbite content increased and the ferrite content changed little. Both the ferrite and pearlite transformation starting temperature and ending temperature decreased with the cooling rate increasing. Besides, the hardness increased with the cooling rate. At the same cooling rate, the phase transformation temperature increased slightly with the carbon equivalent decreasing, and the pearlite content increased, while the hardness decreased. The hardness of C70S6 steel was reduced by cooling rate decreasing. However, it was difficult to realize the method of decreasing the hardness by adjusting the cooling rate in the case of higher carbon equivalent. Therefore, in order to obtain an appropriate hardness, the Ceq must be controlled. And a Ceq=0.83% was recommended.
The relationship between microstructure and mechanical properties of 36MnVS4 steel was studied. Different prior austenite grain sizes were obtained by austenitizing at 850, 950 and 1050 °C, respectively, and different ferrite contents were obtained by different cooling rates. Austenitizing temperature mainly influenced the grain size. With the austenitizing temperature increasing, grain size increased and the phase transformation starting temperature increased. Also, the strength increased and the plasticity and toughness decreased. Cooling rate mainly influenced the microstructure percentage. With the cooling rate increasing, ferrite percentage decreased and pearlite percentage increased. And meanwhile, the strength increased and ductility and toughness decreased. Microstructure had a significant influence on fracture splitting properties. With the grain size increasing, fracture splitting properties were markedly improved. With the ferrite percentage increasing and pearlite percentage decreasing, fracture splitting properties were worsened.
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