Investigations On Heat Treatment Of 50CrMnVA And FAS3550 For Automobile Spring Steel Plate

With the rapid development of the automobile industry,higher requirements have been put forward for the development direction of lightweight and high-performance for automobiles.It urgently demanded to increase the strength and extend fatigue life of automotive springs.According to most researchers' studies,controlling the thickness of the decarburization layer and improving the heat treatment process to improve the fatigue life are still the main research directions.In this paper,50CrMnVA and FAS3550 spring steels were investigated.The oxidative decarburization behaviors of two steels under different heating conditions were studied,and an effective decarburization model was established;In order to researche the principles of the micro structure and the relationship between the microstructure and mechanical properties during the heat-treatment,the author used the methods of processing simulation by dilatometer,adjust and control the heat treatment parameters in laboratory.Based on these reseaches,the main results can be obtained as follows:(1)Under the same heat treatment conditions,the anti-decarburization of 50CrMnVA performs worse than FAS3550.The thickness of the decarburized layer of 50CrMnVA increases with the increase of temperature,the decarburized layer thickness of FAS3550 increases first and then decreases with increasing temperature,and reaches the peak decarburized layer thickness at 940?.The optimized heat treatment process temperature is 880??900?,the holding time should be less than 60min for 50CrMnVA;and the optimized heat treatment process temperature is 860??920?,holding time should be less than 40min for FAS3550.The energy of 50CrMnVA oxidation is Q2=172.033kJ/mol,and the energy of FAS3550 oxidation is Q2=109.428kJ/mol;The anti-oxidation of 50CrMnVA is better than that of FAS3550.The theoretical model of decarburized layer has been obtained.(2)The austenite dynamic recrystallization behavior of 50CrMnVA experimental steel shows:the higher deformation temperature and the lower strain rate,the more easily the dynamic recrystallization occurs.In order to obtain a fine uniform austenite structure,a higher deformation temperature is needed to ensure the austenite structure can be completely recrystallized.By analyzing the relationship between flow stress-stain curves,the activation energy of dynamic recrystallization is 315190.3 J/mol.Then the Zener-Hollomon parameter was determined.The constitutive equations of flow stress corresponding to different deformation conditions,the relationship between ?c and ?p,?cand?p with InZ can be obtained.(3)The austenite transformation point of 50CrMnVA is AC1=751?,AC3=790?,and that of FAS3550 is AC1=735?,AC3=790?,were measured by the expansion method;The phase transformation process of steel is from the mixed phase of pearlite and ferrite to bainite and martensite,and the hardenability of 50CrMnVA is worse than FAS3550.(4)The microstructure after oil quenching and medium temperature tempering is troostite.The best heat-treatment process of the two steels is:oil quenching at 920? for 40 min followed by tempering at 390? for water cooling.The tensile strength of 50CrMnVA under this process was 1702MPa,and its yield strength was 1520MPa,the elongation rate was 11%,the area shrinkage rate reached 44%,and the yield ratio reached 0.89.The tensile strength of FAS3550 under this process was 1730MPa,the yield strength was 1580MPa,the elongation rate was 10%,the reduction in area was 44%,and the yield ratio was 0.91.