Modification of non-metallic inclusions in 55SiCr high stress spring steel by rare earth Ce

55SiCr high stress spring steel is the main raw material used in the manufactures of vehicle valve, suspension and braking springs. The fatigue failure caused by large-sized compounded inclusions and the anisotropy and the sensitivity of hydrogen-induced crack caused by strip MnS in the steel are the important aspects to reduce the service life and performance of the springs. The modification process of inclusions in this steel by rare earth Ce and its mechanism, along with the influence of Ce content on inclusions were studied through high-temperature tube furnace smelting experiments combined with SEM-EDS detections and thermodynamic calculations. The research results show that after adding Ce, a large number of small-sized rare earth inclusions are generated in the steel first. As the reaction progresses, the number of inclusions gradually decreases and most of them are removed. A proper amount of rare earth can purify the molten steel. Moreover, the Ce content has a great influence on the characteristics of inclusions. When it is added 0.02%, the inclusions in steel are mainly of modified MnS, Ce-O-S and Ce-S inclusions with sizes 1-3 μm, spherical or ellipsoid in geometry. When the Ce addition is increased to 0.26%, the MnS inclusions disappear, generating a large number of angular and irregular Ce-O and Ce-O-S inclusions, which will adversely affect the fatigue properties of the steel. It is pointed out that MnS is modified by Ce through three ways, outside inwards, side inwards and inside outwards. To make the inclusions harmless in steel, an appropriate addition amount of rare earth is suggested to be 0.01%-0.02%, at which the mass percent of Ce in steel will reach to 0.009%-0.014%.