Research progress on austenite grain growth and second-phase particle control technology in automotive gear steel

With the implementation of the global two-carbon policy, energy saving and CO₂ emission reduction have become important developmental goals of the automobile manufacturing industry. At present, the combination of high temperature for the automobile gear steel and short carburizing time is the most direct carbon reduction countermeasure for gear production enterprises. However, the problem of abnormally coarsened austenite grains often occurs in the high-temperature carburization of gear steel. With the increase in carburizing temperature, the degree of mixing crystals becomes serious. As a requirement of gear manufacturing enterprises, microalloying is carried out on the carburized gear steel. Upon the addition of microalloying elements, the second-phase particles are precipitated during heating, and the pinning effect is generated to prevent the movement of austenite grain boundaries, thus preventing the abnormal growth of austenite grains. Although the second-phase particles precipitate at the usual carburizing temperature, the partial solid solution of particles appears. In this work, the effects of the heating temperature and holding time on the austenite grain size of gear steel are studied to clarify the mechanisms of complex austenite grain growth and second-phase particle precipitation for the realization of fine austenite grain size after high-temperature carburization. The influence of the contents of microalloying elements (Nb, Al) on the pinning effect, precipitation position, and solution temperature of the second-phase particles (Nb (C, N)/AlN) are also discussed. Austenite grain growth models, critical sizes of austenite grain abnormal growth, and pinning force models of second-phase particles are summarized. The austenite grain growth model is based on the Beck equation, and the most common models are the modified Sellars and Arrhenius models. For the study of the inhibition effect on austenite grains, the pinning force model is used to study the critical size of austenite by modifying the dimensionless constant (A) of the Zener equation mainly through the pinning effect (P_z) produced by all particles on the grain boundary. After the experimental data are obtained, the trend of austenite growth can be predicted accurately by fitting the curve using the mathematical method. The precipitated second-phase particles are generally distributed along the grain boundary. Nb (C, N) particles have a higher solution temperature than AlN particles, so they are more stable at high temperatures. When the temperature exceeds the grain coarsening temperature, the precipitated particles become dissolved or coarsened. The mixed crystal structure generally starts to appear at about 1000 ℃, and adding the appropriate amount of microalloying elements can increase the coarsening temperature.