Effect Of Q&P Process On Microstructures And Mechanical Properties For Steel 60Si2MnNiMo

Quenching and partitioning(Q & P) is a heat treatment processe which involves quenching of the Si-Mn serise steels at a temperature between martensite start(Ms) and martensite finish(Mf) temperature, followed by the carbon partitioning treatment at a temperature the same as or higher than the quench temperature to produce a compound microstructures which contain martensite and a considerable fraction of stable retained austenite. Thus a steel with high strength and good ductility and toughness was produced.In this thesis, the Q & P heat treatment processe has been applied to 60Si2 MnNiMo steel.The effects of microstructure characteristics and mechanical properties under different austenitizing temperature and austenitizing time, quench temperature and quench time,partition temperature and partition time were systematically investigated by means of OM,XRD, SEM and tensile testing machine, impact testing machine and etc. The TRIP effect of remain austenite during the deformation process was discussed. All these studies are prefer to provide theoretical support for the optimization of heat treatment process and microstructures and properties of 60Si2 MnNiMo steel.The obtained main results are as follows.The microstructures of 60Si2 MnNi Mo steel contain(F+M+AR) were gained at a austenitizing temperature of 820℃ followed by Q & P process. The yield strength(σ0.2)and tensile strength(σb) of 60Si2 MnNi Mo steel were 1273 MPa and 1488 MPa,respectively. While the ductility(δ) and toungness(Ak) were 10% and 24 J, respectively.With increasing at the austenitizing temperature, the ductility and tounghness of60Si2 MnNiMo steel were decrease slightly while the strength and hardeness increase because of the volune fraction of remain austenite decrease. With increasing in the holding time at 900℃, the volune fraction of remain austenite increase at first and then decrease slightly which lead to the strength and hardeness of 60Si2 MnNiMo steel decrease at first and then increase, while the ductility, tounghness and product of strength and elongation are opposite. The degree of adaptability of strength, ductility and toughness are the best in the holding time of 20 min.At a low quench temperature, the final microstructure of 60Si2 MnNiMo steelcontains vsat martenite and a small amount of remain austenite that result in high strength and hardeness, low ductility and toughness. With increasing at the quench temperature, the strength and hardeness of 60Si2 MnNiMo steel decrease at first and then increase because of the change of remain austenite fraction. When the QT is 220℃, the volume fraction of remain austenite reach 30% which lead to the best adaptability degree of strength, ductility and toughness for 60Si2 MnNiMo steel. However, the hardeness is slightly low. The comprehensive mechanical properties decrease when the QT is above 220 ℃. The influence of quench time on final microstructure is not significant. The volume fraction of remain austenite range from 23%~26%. With increasing in the quench time, the strength decreases, the hardeness decreases at first and then increases while the ductility increases.Partition temperature and partition time make significant effects on the microstructure and properties of 60Si2 MnNi Mo steel. With increasing at the partition temperature, the strength and hardeness of 60Si2 MnNiMo steel decrease while the ductility and toughness were opposite because of the increasing fraction of remain austenite. With increasing in the partition time at 400℃, the volume fraction of remain austenite increases, and approach to 30% in 300 s. Simultaneously, the M-A and a small amount of carbides were observed. The strength and hardeness of 60Si2 MnNiMo steel decrease with the increasing time while the ductility and toughness were opposite.The tensile and impact fracture morphologies of test samples were observed by SEM.It was found that the fracture mechanisms is ductile fracture with many toughness cavities in the fracture surface under a austenitizing temperature of 820℃. With increasing at the austenitizing temperature, the fracture mechanisms were ranged from quasi cleavage fracture to transgranular cleavage fracture. A local intergranular fracture was found at a low QT or PT. The steel will present good ductilily and toughness with many toughness cavities when increasing the partition temperature or partition time. The volune fraction of AR was obviously decreased after deformation. This indicated that a part of AR changed into martenite induced by stress or strain during the transformation, which improved the strength, ductility and toughness of 60Si2 MnNi Mo steel.