Deoxidation and Inclusions in Semi-Killed Steel Ingots

Studies on semi-killed steel ingots-III

Abstract

Numerous black spots are usually observed on sulphur prints along the skin layers of semi-killed steel ingots, which probably may develop into surface defects under severe processing of semi-killed steel products.
Using 10 ton slab ingots, investigations were carried out into the effects of deoxidizers on these black spots.
The main results obtained are as follows:
(1) The substances of these black spots consist of crowds of sulphides to be accompanied with blow holes and of large silicates, the greater part of which contains some sulphides.
(2) These black spots can be reduced by using aluminium as the main deoxidizer instead of silicon, although this result may be also attained by the decrease of sulphur content of molten steel.
(3) The large silicates have mnch relations with ladle refractories. When silicon is used as deoxidizer, it can catch a greater amount of oxygen, as the temperature of molten steel drops during the period from tapping to solidifying, whereas in case aluminium is used, the content of residual aluminium in molten steel cannot be expected to be high because of its strong deoxidizing power to combine with more oxygen while cooling. It is presumed, therefore, that in order to get the same condition of blow holes in ingots, the oxygen content of molten steel in ladle should be lower, and accordingly the ladle refractories are the less damaged when aluminium is used as deoxidizer than when silicon is used.