Abstract
Following the investigation described in the former report (Tetsu-to-Hagane, 48 (1962) 2, p.117), further experiments were conducted with small test ingots and also 10t slab ingots concerning the effect of degree of deoxidation and ladle refractories upon black spots appearing on the sulphur prints along the skin layers of semi-killed steel ingots. The mechanism of occurrence of the black spots tentatively referred to in the former report was confirmed.
The main results obtained are as follows: -
(1) The number of these black spots can be reduced by intensifying the degree of deoxidation, the effect of which will be more noticeable when aluminium is used as deoxidizer than when silicon is used. Intensification of the degree of deoxidation, however, is not favorable because of the resultant large pipe cavities which are produced in semi-killed steel ingots.
(2) At the same ratio of added silicon against added aluminum and at the same degree of deoxidation, the number of black spots becomes smaller when high-alumina bricks are used for ladle refractories than when silicious bricks. Roseki bricts. are used.
(3) To prevent the appearance of the black spots when high-alumina ladle refractories are used, a little aluminum must be added together with silicon.
(4) High-alumna bricks are, however, very expensive and liable to increase the ladle skull owing to their high thermal capacity. Moreover, it is difficult to remove this skull for lack of glassy layer between bricks and skull. Therefore, there are still many problems unsolved to put this brick into practical use.
(5) Consequently, on the present stage of development, the use of aluminium as the main deoxidizer for the special semi-killed steel products that must be subject to severe cold processing, as mentioned in the former report, is the most desirable method for preventing appearance of these black spots, and hence, for preventing occurrence of surface defects.
