| The drive shaft is an important part of the automotive transmission system,and the drive shaft pipe is the main component of the drive shaft.Its main function is to connect the universal joint and the sliding spline and transmit the power of the engine to the tire.At present,the common steel(480QZ)is widely used in large automobile manufacturing enterprises such as FAW,Heavy Truck and Dongfeng to produce transmission shaft pipes,and the yield strength of the steel is about 480 MPa.Because the strength of the steel is relatively low and the ability of transmitting torque is weak,the mechanical properties of transmission shaft pipe of heavy truck can only be satisfied by increasing the wall thickness of the shaft pipe.This undoubtedly increases the weight of the shaft pipe,which is contrary to the strategic requirements of lightweight and energy saving.On the premise of ensuring the transmission torque,in order to achieve the weight reduction of the shaft pipe,the strength of the raw material must be increased.With the increasing requirement of lightweight,safety and energy saving,it has become an inevitable trend for automotive industry to develop the forming and manufacturing technologies of high strength steel transmission shaft pipes with yield strength of 700 MPa and above,and to replace ordinary strength shaft pipes with high strength ones.At present,researches have been carried out on the deformation law of common strength steel pipe billet in roll forming process,the influence of welding parameters on the weld mechanical properties,and the influence of post weld heat treatment process on structural evolution and mechanical properties of the weld.However,in the process of roll forming,high frequency welding and post weld heat treatment of high strength steel automobile transmission shaft pipe,there are still several critical issues that need to be studied and solved:(1)Deformation law of high strength steel pipe billet in roll forming process.(2)In roll forming process,the intrinsic relationship between roll profiles and the spatial configuration of high strength steel pipe billet.(3)Determination of optimum high frequency welding processing parameters and welding temperature range for high strength steel pipe billet.(4)Effects of post weld heat treatment process parameters on mechanical properties of high strength steel high frequency welded pipe welds.(5)Effects and mechanisms of induction heat treatment cycles on toughness of high frequency welded pipe welds.Focusing on the above-mentioned issues that need to be solved,this thesis carried out the following researches and the research results obtained are as follows:(1)Accounting for the characteristics of large springback deformation of high strength steel,a mathematical model is proposed to describe the deformation path of any point on the curved surface of the pipe billet in forming direction by taking springback into account,namely the trajectory function G(X),the calculation formulas of the springback curve of the pipe billet are derived based on the energy method,and a new method for calculating the spatial configuration of the pipe billet in roll forming process is also established.The above trajectory function and the calculation method are collectively called trajectory function method.The calculation accuracy of the method is verified by comparing with the experimental data.(2)Based on the trajectory function method proposed in this thesis,an optimization method of roll profiles is proposed by taking the maximum edge membrane longitudinal strain of the billet as the constraint condition and according to the optimization criterion of minimum edge stretching.The optimization method is used to optimize the roll forming process of high strength V channel steels,and the number of roll stands required for final V channels was reduced from 8 to 5 on the premise of ensuring product quality.(3)A three-dimensional electromagnetic thermal coupling finite element model of high frequency induction welding temperature field is established,and the high frequency welding process of ?134×4 mm high strength steel automobile transmission shaft pipe is simulated.The calculation accuracy of the model is verified by comparing with the experimental data.Based on the finite element model,the influence of high frequency welding process and geometric parameters on welding temperature is further studied.The high frequency induction welding process of the shaft pipe is simulated by Gleeble simulation test,and the relationship between welding temperature and weld mechanical properties is determined by tensile and impact tests.Finally,a BP neural network with high frequency welding process and geometric parameters as the input layer units and the tensile strength and the impact energy at room temperature of the weld as the output layer units is established.After training,the neural network shows high precision in predicting the strength of high frequency welded pipe welds.(4)The effect of heating rate on austenite transformation temperature of the high strength steel is studied and a reasonable range of post weld heat treatment process parameters is determined.The effects of various process parameters such as heating rate,heating temperature,holding time and cooling rate on the mechanical properties of the weld are studied,and the reasons for changes in weld mechanical properties are analyzed in microstructure,grain size,texture,composition homogenization and grain boundary type aspects.(5)In order to solve the problem of poor toughness of the high frequency welded pipe welds,two induction heat treatment cycles,namely quenching and tempering(Q&T)and similar normalization(SN)are proposed,and the optimum heat treatment parameters of Q&T and SN are determined based on good matching principle of weld strength and toughness.The matching coefficient of the tensile strength of the optimum Q&T(T1=990?,T2=550?)treated welds to that of the base metal is 1.053 and the absorbed energy of the Charpy V-notched specimen at room temperature is increased from 20 J to 36 J.The matching coefficient of the tensile strength of the optimum SN(T1=990?,T3=550?,V=24?/s)treated welds to that of the base metal is 1.012 and the absorbed energy of the Charpy V-notched specimen at room temperature is increased from 20 J to 42 J.The mechanism of weld toughness improvement is revealed in the aspects of microstructure,texture,inclusion,grain size and type of grain boundary. |
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