Effect of total oxygen on the nonmetallic inclusion of gear steel

It is an important symbol of the metallurgical quality level of special steel for inclusion controlling, which can improve the service performance of special steel to a greater extent. As a typical steel grade, gear steel, in the special steel field, is also required strictly in controlling of inclusions. It is known that total oxygen content can reflect the level of inclusions to some extent. Since the 1980s, ultralow oxygen has become a direction for the development of special steel. To guarantee controlling of nonmetallic inclusions and determine a reasonable control target of total oxygen content in the gear steel, the effect of total oxygen content on nonmetallic inclusions in gear steel was studied. In this study, three kinds of Mn–Cr-system gear steels with different oxygen content were selected as research objects. The number, distribution, and size of nonmetallic inclusions in these gear steels were studied using an Aspex scanning electron microscope (Aspex SEM), the extreme value method, and fatigue test. The relationship between inclusions and the total oxygen content of gear steel was obtained. Under the experimental condition, with the decrease in total oxygen content, the density of the number of oxide inclusions decreases continuously, among which 5–10 μm small inclusions decrease most obviously. In contrast, the number density of large inclusions above 10 μm does not change obviously. Moreover, the results of the extreme value method and fatigue test show that when total oxygen mass fraction is high (0.0013%), the size of maximum oxide inclusion in the steel is relatively large, which is more than 10 μm higher than the inclusion in 0.0010% or 0.0005% total oxygen steel. Simultaneously, when total oxygen mass fraction is low (≤0.0010%), the change of total oxygen mass fraction (0.0010% and 0.0005%) has little effect on the maximum inclusion size in steel.