| With development of lightweight and high-performance in automobile industry,especially trucks developed towards the goal of high speed and heavy load,there is an urgent need to improve the strength of automobile spring steel,as well as prolong their fatigue life.50CrMnVA and 50CrMnMoVA spring steel were investigated based on fully understanding their application environment,chemical composition system and microstructure and properties requirements.The decarburization sensitivity the static continuous cooling transformation rules of two steels under different heating conditions and the recrystallization of 50CrMnMoVA steel under different deformation conditions were studied.In addition,the optimum heat treatment processes of the two steels were obtained to improve their microstructure and mechanical properties.The main contents are as follows.(1)50CrMnMoVA steel possesses better performance of anti-decarbuzation than that of 50CrMnVA steel under the same heating and cooling conditions.The two experimental steels may decarburize at 700?.The decarburization layer depth of 50CrMnVA reaches its peak at 1000?.However,when the temperature increases,due to the oxidation rate is faster than decarburization rate,the decarburization rate was slow down.When it.comes to 50CrMnMoVA steel,its depth of decarburization layer will increase with temperature.Additionally,with the increasing of holding time,the decarburization layer depth of two steels will increase linearly.And the decarburization layer depth will drop dramatically with the increase of cooling rate.When the cooling rate is higher than 1?/s,both of two steels do not occur ferrite decarburization.(2)The flow stress curves of 50CrMnMoVA steel deformed at elevated temperatures are obtained by researching on its austenite dynamic recrystallization behavior.The influences of deformation parameters on the dynamic recrystallization and flow stress were also studied.Additionally,the activation energy Q and the parameter Z have been calculated.The modified mathematical model of deformation resistance of 50CrMnMoVA steel was obtained through calculating stress constitutive equation coefficient under different deformation degree.Also,the linear relationship between the critical strain and Z parameters was obtained by calculating and counting the deformation data through ln?-? curve analysis.(3)The static recrystallization of 50CrMnMoVA steel was accelerated with the increasing of deformation temperature,prolonging of the interval time and the increasing of strain rate through researching on 50CrMnMoVA steel austenite static recrystallization behavior.When strain-induced precipitations accured,the static recrystallization will suppress and softening rate curve may appear platform.Also,the static recrystallization activation energy of 50CrMnMoVA steel and its static recrystallization kinetics model was obtained.(4)The static continuous cooling transformation rules of the two steels by thermal expansion and metallographic method.The stability of overcooled Austenite of 50CrMnMoVA steel is higher than that of 50CrMnVA steel.The results indicated that the full martensitic microstructure could be obtained when the cooling rate is no less than 2?/s.(5)The optimized heat treatment process was obtained with considering the best match between strength and plasticity.The mechanical property of 50CrMnVA steel can make a significant improvement when it was heated at 900? with holding for 45 min followed by oil quenching,then tempered at 400? for 120min.Its yield strength,tensile strength,elongation and shrinkage reach 1743 MPa,1560 MPa,10.97%and 40.98%,respectively.The mechanical property of 50CrMnMoVA steel can make a significant improvement when it was heated at 880? with holding for 45 min followed by oil quenched,then tempered at 400? for 120min.Its yield strength,tensile strength,elongation and shrinkage reach 1729 MPa,1560 MPa,11.63%and 47.02%,respectively.Compared the two steels,the strength of 50CrMnMoVA steel is just the same as that of 50CrMnVA steel but the plasticity of 50CrMnMoVA steel is much better than that of 50CrMnVA steel.Finally,the research results show that through appropriate hot working and heat treatment,two experimental steels can possess good comprehensive mechanical properties and can achieve even exceed the expected targets. |
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