| With the rapid development of national defense industry, high and new technology industries and key field in economic construction, the demand for non-magnetic steel rises fast. As typical high manganese non-magnetic steel, Fe-Mn alloy has a relatively simple production process, low cost and stable property so that it has good market prospect. The works of this thesis was carried out intergrating with the NSFC project of Formation Mechanism of Annealing Twins in Non-magnetic High Manganese Austenitic Steel. The hot deformation behavior and the effects of heating temperature and cooling procedure on microstructure and macro-hardnesss of50Mn18Cr4V (designated as N18) and30Mn20A14V (designated as N20) non-magnetic steel were investigated. The microstructure and property evolution during hot rolling and heat treatment were analyzed. The deformation mechanism during tensile test was also investigated. The main original works of this thesis are presented as follows:(1) Flow stress curves of N18and N20steel were obtained by high temperature uniaxial compression tests. The hot deformation behavior and microstructure evolution during high temperature compression process were studied with constitutive equations for N18and N20steel being established. The impact factors of high temperature deformation resistance for high manganese non-magnetic steel were analyzed and a deformation resistance model was also established. The experimental results show that N18steel has higher hot deformation activation energy and more difficulty in dynamic recrystallization compared with N20steel; Thermal cracking and flow instability of N18steel exist at low and high strain rates, respectively, when the temperature of hot deformation is relatively low.(2) The variation of austenite grain size and macro-hardness for N18and N20steel at different heating temperature were analyzed, while the effect of cooling procedure on microstructure evolution and macro-hardness of N18and N20steel was investigated by high temperature uniaxial compression tests. The experimental results show that grain size of both N18and N20steel increases rapidly after holding10minutes, when the heating temperature exceeds1100℃and1000℃, respectively. Especially when the heating temperature is higher than1200℃, N20steel has abnormal grain growth. Besides, the macro-hardness of N18and N20steel decreases with increasing heating temperature. Cooling rate after hot deformation within a range of relatively high temperature can influence the final state of austenite crucially and fast cooling makes macro-hardness of the steel raised.(3) The microstructure evolution of N18and N20steel during hot rolling was investigated. The macro-hardness, impact toughness and tensile property which is just for N20hot rolled steel, were also examined. The experimental results show that annealing twins contribute to refine austenite grain. Strain induced precipitation of V(C, N) makes the dynamic recrystallization for N18steel more difficult with decreasing finish rolling temperature. As for the impact energy of N18and N20steel, neither one is high enough. The impact energy of N20steel decreases slightly with the temperature decreasing and the inclusion of AlN does harm to the impact toughness of N20steel.(4) The microstructure evolution of N18and N20steel during heat treatment was investigated. The deformation behavior was analyzed through tensile tests after solution and aging treatment of the steels. The permeability of N18and N20steel subjected to aging treatment is also examined. The experimental results show that austenite can easily transform to martensite for N18steel at high soaking temperature and cooling rate. At the soaking temperature of1050℃, N18steel has fine microstructure of crystallization and high strength. The strength can be further raised by subsequent aging with partial loss of elongation. For N20steel, solution treatment at high temperature makes the strength reduced and the ductile reformed dramatically. The plastic deformation mechanism of N18and N20steel is dislocation gliding and mechanical twinning. Both of them have low permeability and show good non-magnetism after aging treatment. |
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