Effect of La Content on Inclusion Morphology in Automobile Sulfur‐Containing Gear Steel SCr420H

Abstract

The inclusion morphology of sulfur‐containing gear steel with different La addition (0, 0.08, 0.5, and 1%) is investigated in laboratory experiments. The La and S concentrations in steel melt significantly decrease with the holding time after La addition increases, leading to different types of inclusions with specific shapes. Inclusion transformation routes with the holding time increased under different La addition are summarized as follows: 1) 0%La addition: cluster‐like Al2O3 + cluster‐like MnS → irregular block‐like MgAl2O4 + cluster‐like MnS; 2) 0.08%La addition: cluster‐like LaAlO3 + spherical‐like La‐Al‐O‐S‐(MnS) → spherical‐like La‐Al‐O‐S‐(MnS) + spherical‐like La‐Al‐O‐S‐(MgO)‐(MnS) + cluster‐like MnS; 3) 0.5%La addition: flower‐like La‐O + spherical‐like La‐O‐S → spherical‐like La‐O‐S; and 4) 1%La addition: flower‐like La‐O + spherical‐like La‐O‐S → spherical‐like La‐O‐S + cubic‐like La‐O‐S‐N‐As. The results of hot compression suggest that La‐containing inclusions are less prone to the shape change than MnS; moreover, microcracks are formed at the surface of La‐containing inclusions. The nanoindentation test demonstrates that the hardness of La‐O‐S is higher than MnS. The high La addition has a remarkable effect on refractory degradation. For 0.5% and 1% La, dissolved La reacts with the MgAl2O4 phase in the MgO‐MgAl2O4 dual‐phase refractory and changes into LaAlO3.