Regulation Of Microstructure And Mechanical Properties Of C38+N Crankshaft Of Non Quenched And Tempered Steel

The German MC engine technology introduced by a heavy truck manufacturer in China has reached the international advanced level.In the process of localization of the non-quenched and tempered steel C38+N used in the heavy-duty crankshaft,there is a problem that the mechanical properties of the crankshaft are unstable.In this paper,the problems of the tensile specimens in the inner diameter of the shaft of the crankshaft and the large difference in mechanical properties are studied.The raw materials,forging process and controlled cooling process are studied separately.The Marc and Forge finite element software were used to simulate the heating process of C38+N die-cast steel ingot and the die forging process.The continuous cooling curve of C38+N material was determined by Formaster-F? thermal expansion instrument.Gleeble-3800 forging-controlled cooling and heat simulation tests were performed on C38+N to determine the optimal control cooling process.The numerical simulation of the heating process of C38+N die-casting steel ingots shows that the steel ingots are pitted at 850?,heated to 1100? for 0.5 h at 50?/h,and then heated at 50?/h.up to 1200?,after 2-3 hours of heat preservation,the pit can be used to ensure the best heating uniformity of the die-cast steel ingot.The numerical simulation of the forging process of the crankshaft shows that the in-situ segregation position of the die-casting material is forged and flowed to the position of the tensile specimen at the inner diameter of the connecting rod,which is the tensile specimen of the inner diameter of the connecting rod.The main reason for the incompatibility of mechanical properties.The die-casting and rolling material is continuous casting,which can effectively solve the problem of ingot segregation and uneven billet heating.By measuring the C38+N static continuous cooling transition curve(CCT)of non-quenched and tempered steel,it is shown that the C38+N phase transition interval is about 700-500 ?,and the total ferrite can be obtained at a cold rate of less than O.7?/s in this interval.With pearlite structure,the slower the cooling rate,the higher the proportion of ferrite,the faster the cooling rate,the lower the temperature at which the phase transition starts,the finer the structure;the colder speed of the phase change zone exceeding 0.7 ?/s begins to occur.The bulk transformation,to a cooling rate of 2.8?/s,the martensite transformation begins,the critical cooling rate of the whole martensite transformation does not exceed 14?/s.According to the continuous transformation curve of non-quenched and tempered steel C38+N,combined with the actual forging process at the production site to determine the parameters such as deformation and strain value,the Gleeble-3800 thermal simulation test program was designed,and two sets of simulated heating temperatures were set at 1150?and 1200?The cooling rate of the phase change zone of 200-2500?/h 6 groups,a total of 12 groups of control programs.The Vickers hardness and microstructure analysis were performed on the thermal simulation samples.The simulation data analysis shows that the average temperature distribution of the ferrite network in the phase change zone is 1000-1500?/h,and the average size distribution of the sample ferrite network is concentrated,showing good microstructure uniformity;the proportion of ferrite is 12-15%.The sample can obtain good comprehensive mechanical properties required for the crankshaft;the heating at 1150? and the cooling rate of 1000?/h in the phase change zone are the most suitable forging and controlled cooling processes.Using non-tempered steel C38+N continuous casting and optimized forging and controlled cooling technology,the practice of the production site was organized.The mechanical properties and microstructure inspection of the first,middle and rear crankshafts of 100 consecutive crankshafts were passed.The tensile strength of the tensile specimens in the shaft diameter of the connecting rod is stable and qualified,and the mechanical properties such as strength and plasticity are uniform and stable,which verifies the effectiveness of the material improvement and optimized forging and cooling process.