Austenite grain size is of great significance to stabilize the hardenability of steel, reduce the deformation of automobile gear after heat treatment and improve the brittle fracture resistance of carburized steel. Because the coarse grain in automobile gear steel increases the carbon concentration of carburized layer, which leads to the increase of brittleness, the decrease of bending strength and the easy peeling of tooth surface. If mixed crystals appear, the heat treatment deformation between teeth may lose rules and cannot be matched.
At present, the grain size grade of automotive gear steel in China generally requires grade 5 ~ 8. With the development of high temperature vacuum carburizing technology, there will be higher requirements for the stability of austenite grain of automotive gear steel.
Grain refinement is mainly ensured by adding an appropriate amount of Al or a small amount of Nb. The steel with aluminum grain refinement ensures the established composition from the metallurgical aspect, which is only the basis of fine grain. Whether the grain of the actual product can be refined depends on whether the forging, rolling and heat treatment processes after steelmaking comply with the dissolution of AlN.
Therefore, when coarse grains or mixed grains appear in production, it depends not only on whether the aluminum content in the steel (generally controlled at 0.020% ～ 0.055%) and the matching nitrogen content (0.010% ～ 0.018%) are appropriate, but also on whether the corresponding process is reasonable.