Influence of die-forging deformation on microstructure of 1538MV non-quenched and tempered steel for crankshaft

Non-quenched and tempered steels offer many advantages such as energy saving, emission reduction, simple processing, short production time, and low cost. Demand for energy saving and emission reduction is increasing with the rapid increase of car production and ownership. As a result, the usage of non-quenched and tempered steel in automotive parts has attracted increasing attention. The main problem with non-quenched and tempered steel in actual production is the lack of a hot deformation process and a cooling process that can be used to precisely control the microstructure and properties of the material. The material 1538MV is a type of pearlite+ferrite non-quenched and tempered steel, and its use for building crankshafts has not been studied sufficiently thus far. The main factors affecting the structure of a crankshaft are deformation, final forging temperature, and metal flow and cooling after forging. In this paper, the forging process of 1538MV non-quenched and tempered steel for building a crankshaft was studied by means of numerical simulation, microstructures of the rolled material and finished crankshaft were analyzed, and influence of deformation on the microstructure of the crankshaft was discussed. The results show that the microstructure of a crankshaft forged at a higher temperature and with smaller deformation is coarser than that of a crankshaft made of rolled material. The inhomogeneity of the microstructure is caused by unevenness of temperature and strain distribution during crankshaft deformation. The ferrite content of the crankshaft and the pitch of the pearlite were lower than those of the rolled material, and the bainite structure appeared in a few parts, which indicated that the cooling rate was too fast during phase transformation. Therefore, the cooling process should be optimized further. In addition, the metal flow in the segregation zone during the crankshaft forging process significantly influences the forged microstructure of the crankshaft, which is another cause of the formation of bainite. Therefore, the quality of the rolled material should be strictly controlled. The above results provide directions for improving the quality of the rolled material, optimization of the crankshaft forging process, and optimization of the cooling process after forging.