Effect of Trace Magnesium Addition on TiN Inclusions and Microstructure in 20CrMnTi Gear Steel

To enhance the quality of the microtitanium alloy steel, this study is the first to utilize the addition of trace amounts of magnesium in 20CrMnTi gear steel to improve the TiN inclusions and microstructure within this type of steel. Herein, the effect of different magnesium contents (0–50 ppm) on nonmetallic inclusions in steel is taken as a starting point. Simultaneously, the pinning effect of the modified inclusions on the microstructure is also explored. The results indicate that after adding magnesium, the average size of the inclusions decreases from 2.8 to 2.3 μm, and the grain boundary mobility M decreases from 16 to 1.27 × 10 −11 m 4 kJ −1 s −1 . Mg can reduce Ca and Ti in oxide inclusions, forming finer MgAl 2 O 4 particles, thereby refining their size. The formed MgO and MgAl 2 O 4 act as inhomogeneous nucleation sites for nitrides, resulting in smaller size, more uniform distribution, and less harmful TiN. Notably, TiN can provide nucleation sites for MnS. The size and distribution of sulfides are also improved during the modulation of TiN. It is found that the modulated TiN–MgO–MnS microinclusions can be used as austenitic pinning particles. These particles increase the pinning resistance and improve the grain boundary mobility, thus contributing to grain refinement.