Effects of rare earth elements on inclusions, microstructure and impact toughness of spring steel

Spring steel is an indispensable basic part manufacturing material in mechanical equipment, which is widely used in transportation tools such as automobiles, railways, construction machinery and other industries. The effects of rare earth (RE) on inclusions, microstructure, carbides and impact toughness of 50CrV spring steel were studied by means of optical microscope, scanning electron microscope, transmission electron microscope and impact tester. The results show that the addition of rare earth can modify the main MnS and Al2O3 inclusions in steel to form spherical or near-spherical rare earth inclusions, and the size of inclusions is significantly reduced. In the quenched structure, the size of martensite slab becomes smaller and the structure is refined. After tempering, the carbide changes from sheet to ellipse, which makes it easier for spring steel to form stress concentration cracks under stress. VC becomes finer and more dispersed, which leads to better pinning of grain boundaries and smaller grain sizes. The impact toughness of 50CrV-RE spring steel increased from 21 to 41 J by 95% due to the improvement of inclusions, microstructure and carbides by rare earth. It is possible to improve the strength and toughness of spring steel for the improvement of railway running speed and the development of vehicle lightweight in China.