Simultaneously enhancing the toughness and hardenability of Cr–V series spring steel based on multi-microelement coupling theory

The uniformity of microstructure and performance is crucial for large-sized quenched steel, necessitating the materials to have excellent hardenability. Grain refinement is an effective strategy to simultaneously enhance both the strength and toughness of the materials. However, if austenite grains are refined during the quenching process, the increase in grain boundary density deteriorates the hardenability of the materials, thereby adversely affecting the uniformity of the microstructure and performance of the materials. Based on the theory of multiple micro elements coupling, a coupled thermodynamic model was constructed to analyze the solution-precipitation behavior of multielement microalloys in the Ti-Nb-V-B-N-C system, and a new composition of spring steel was designed on this basis. The effects of multiple micro elements coupling on the microstructure of spring steel were studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD) . Combined with the results of thermodynamic analysis, it was discovered that during quenching, Ti and Nb form precipitates at higher temperatures. These precipitates constrain the migration of austenite grain boundaries during the quenching holding stage, thereby refining the grains. "During tempering, Ti and Nb enhance the tempering stability of the materials and delay the decomposition of martensite. Jominy end-quench tests and mechanical performance test results indicate that the newly designed spring steel S2, based on the theory of multiple micro elements coupling, simultaneously possesses superior strength-toughness and hardenability .