Effect of Ce on inclusions and mechanical properties of 20MnTiB cold heading steel

Industrialized tests are carried out to produce 20MnTiB cold heading steel with different Ce contents. Vickers hardness tester, tensile tester, impact tester, optical microscope (OM), and scanning electron microscope (SEM) are used to study the deterioration of inclusions in the steel and the changes of microstructure and mechanical properties of hot-rolled wire rod after the addition of Ce, and to analyze the application mechanism of Ce is analyzed. The results show that the S content in the molten steel decreases, and the cleanliness is significantly improved after adding 25ppm Ce. The inclusions consist of large-elongated Al2O3·MgO·CaO·CaS composite inclusions deteriorate into small-sized, globular CeAlO3·MgO·CaO·CaS composite inclusions. Concurrently, the long strips of MnS inclusions disappear. After thermodynamic calculations, the order of precipitation of different Ce inclusions at 1839 K is CeAlO3＞Ce2O3＞Ce2O2S＞CeO2＞Ce3S4＞Ce2S3＞CeS, from which it is judged that when the Ce content is 0.0025%, the most probable inclusions are CeAlO3. As 20MnTiB cold heading steel added B, Ti, and other hardenability elements, in the hot rolling process, if the process control is not appropriate, the wire rod is easy to produce bainite organization. As a result, its mechanical strength is on the high side, the problem of occasional cracking in the late cold heading, and the cold heading mold will be worn out seriously. And after the addition of Ce, the microstructure of the wire rod is refined, and the proportion of ferrite is increased, while the granular bainite organization is alleviated and its size is further reduced. Ferrite is a soft and tough phase, and bainite is a reinforcing phase. The reduction of granular bainite organization and the increase of ferrite percentage fraction contribute to the reduction of strength and hardness. As for the wire rod, when its hardness and strength are low, it is beneficial to improve the cold heading performance of the wire rod, which means that after adding Ce, the cold heading performance of the wire rod is improved to a certain extent. On this basis, it is found that the ambient temperature impact toughness of the wire rod increased significantly from 31.7 J to 52.3 J with the addition of Ce, which is an increase of 65.0%., and the significant increase in the impact performance is more beneficial to the improvement of the cold heading performance of the wire rod. The reduction in hardness and mechanical strength and the significant increase in impact properties of the rare-earth microalloyed hot-rolled wire rod are beneficial for their cold heading performance. The research results can provide technical and theoretical support for the further development of new rare earth microalloyed cold heading steels.