Deoxidation of 18-8 Stainless Steel Si, Mn and Al

Abstract

Eexperiments are carried out to clarify the deoxidation behaviour of Si, Mn and Al is 18Cr-8Ni stainless steel for which strong deoxidizers are needed. The molten 18Cr-8Ni stainless steel, in which Si (0.3 or 0.7%) and Mn (0.4, 0.8 or 1.6%) have already been added, is casted into the ingot of 10 kg after one minute later of Al addition.The oxygen content, the index of cleanliness, the final polishing surface of the samples taken from the ingot and the destribution of the oxide inclusions are investigated together with the identification of the oxide inclusions.
The following results are obtained.
(1) The oxide inclusions vary with increasing amount of Al addition at low Si (0.3%) content, inclusions change as follows: Mn-chromite (MnO-Cr₂O₃)→Mn-Al-chromite (MnO-(Al, Cr) ₂O₃)→Mnaluminate (MnO-Al₂O₃, at high Mn (1.6%) content)→alumina (Al₂O₃), and at high Si (0.7%) content, Mn-silicate (MnO-Si0₂)→Mn-Al silicate (chiefly 3MnO-Al₂O₃-3Si0₂)→alumina (Al₂0₃).
(2) In the case of 0.01% Al addition, the surface defects decrease in spite of the high oxygen content and the high index of cleanliness. These phenomina are attributed to the formation of liquid oxides, i. e. Mn-Al-chromite, Mn-aluminate or Mn-Al-silicates. But the granular alumina and the alumina cluster, which are formed at higher Al addition, give a harmful effect to the polishing surface.
(3) Therefore, the deoxidation with a suitable amount of Al to form the Mn-aluminate in 18Cr-8Ni stainless steel is one of the ways for decreasing surface defects caused by the oxide inclusions.