Characteristics and Formation Mechanism of Complex TiN Inclusions in 20CrMnTi Gear Steel

Abstract

In this paper, in order to study the effect of oxides on the formation of complex TiN inclusions in 20CrMnTi steel, based on the two end quenching specimens with different components, the characteristics of complex TiN inclusions and oxides were observed and analyzed, and the formation mechanism of complex TiN inclusions was discussed. The results indicate the compositions of core oxides are regularly distributed in the MgAl₂O₄–TiO_x composition line for different complex TiN inclusions, and the difference is the TiO_x content. The complex TiN inclusions can be divided into two types, Type 1 and Type 2, based on the compositions of core oxides. For Type 1, TiO_x content of core oxides are below 52%, while for Type 2, TiO_x content of core oxides are over 52%. The core oxides of Type 1 have bigger size at the range of 1–3 µm, and lower nucleation capability for TiN inclusions. The core oxides of Type 2 have smaller size at the range of 0.5–2 µm, and higher nucleation capability for TiN inclusions. For Type 1, the MgAl₂O₄–TiO_x oxides preexisting in molten steel are the heterogeneous nucleation cores of TiN inclusions during solidification. For Type 2, the tiny oxides newly formed during solidification are the nucleation cores of TiN inclusions. By controlling the characteristics of oxides, the characteristics of TiN inclusions can be controlled, which provides a new idea for the control of TiN inclusions in 20CrMnTi gear steel.