| Non-quenched and tempered(NQ&T)steel has many advantages such as energy saving,environmental protection and low cost,which has been widely used in crankshaft production instead of traditional quenched and tempered steel.Crankshaft is the important part of automobile engine,and its service conditions is very bad,which requires enough strength and high impact toughness.Now,due to the lack of strict control of raw material quality,forming technology and cooling system after forging,the fatigue life of crankshaft produced by NQ&T steel is often low.The control of comprehensive quality of crankshaft is to realize the purpose of "shape control and quality control".Therefore,the control of crankshaft quality is a systematic engineering.In order to promote the application of NQ&T steel in automobile and machinery manufacturing industries,in this paper,the basic characteristics of raw material,the crankshaft forming technology,the microstructure and property evolution during the forging process are systematically studied.Combined with numerical simulation technology,crankshaft "shape control and quality control" technology were optimized.The high-temperature deformation behavior was researched to provide theoretical basis for optimization design of forging.The material parameters of Arrhenius constitutive equation with hyperbolic sine were calculated by linear regression method.The improved constitutive eqation compensated by strain of 4degree polynomial fit could predict flow stress more exact,and the coefficient is 0.9857.The hot deformation conditions satisfying the requirement of crankshaft grain sizeare determined as the temperature is 1130 ?,the strain rate higher than 0.1 s-1 and the minimum parameter is 2.3 E+12.The mathematical expressions of average grain size of dynamic recrystallized austenite D and Z parameters were obtained by data fitting.The phase transformation of steel was researched.Ferrite and pearlite,bainite and martensite transformation occurs successively during the cooling process.With the increase of cooling rate,the initial temperature of transformation decreased gradually.The cooling rate is less than 1 ? ·s-1,only ferrite and pearlite transformation occurs.Under a cooling rate of 1 ?·s-1 or above,bainite transformation occurs.Under a cooling rate of 5 ?·s-1 or above,martensite transformation occurs.Through the study of crankshaft forming technology,the characteristics of crankshaft forming technology are defined.The corresponding deformation parameters in crankshaft forming process are obtained by multi-field coupling simulation.The results show that the metal flow intense during crankshaft forming and the distribution of temperature,stress and strain are not uniform.The microstructure evolution during the forging process was researched to provide theoretical basis for the quality control of raw material,the optimization of forging process and the formulation of cooling system.The results show that the microstructure of the crankshaft is coarser than the raw material due to the higher forging temperature and less deformation.The temperature and deformation distribution uneven during forging process leads to the uneven microstructure of crankshaft.The rapid cooling rate in the phase transition zone had a significant influence on the microstructure of crankshaft.The cooling process should be controlled sectionally.The reason of abnormal bainite microstructure in crankshaft is investigated.The results show that the austenite grain was coarsened,the ferrite content was reduced,the quantity of nano vanadium precipitates was increased,the size was tended to be fine and the distribution was dispersed.These microstructures characteristics lead to the fine grain strengthening and precipitation strengthening,and furthermore,result in the increasing of strength and hardness,the reducing of plasticity.The effects of forging and cooling technological parameters on the microstructure were studied by thermal simulation tests.To get refined grains,the critical value of deformation must be reached.With the decreasing of deformation temperature,the austenite grain refined.The austenite grain refined with the increasing of strain rate.The austenite grain size decreased with the increasing of cooling rate,but the pearlite morphology distortion,which can increase the strength,but decrease the toughness,In addition,fast cooling rate can also cause abnormal microstructure formation.The forming process of 465 type crankshaft is optimized by numerical simulation technology.In order to ensure full filling of the carvity and non-overloaded of the forging equipment,the finish forging deformation of the crankshaft should be reasonable,and the temperature should not be too low.The optimized deformation temperature of the 465 type crankshaft was 1130 ? and the deformation of the balance block was 12%.Segmental control cooling system is adopted and the technology transplanted to the forging site..Through the technical experiment,the fatigue-limit bending moment of crankshaft is 883.33 N·m,and the fatigue-limit bending moment increased 6.3%.The confidence level of the fatigue-limit bending moment is 95%,and the error is less than 5%. |
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