Abstract
Effects of various factors on large inclusions (greater than 0.05 mm in dia.) in plain carbon steel billets were investigated, and following results were obtained.
1) The numbers of large inclusions in Si killed steel billets are decreased with decreasing total oxygen content or Mn/Si ratio, with increasing amount of carbon or soluble Al in the billets or amount of Ti added to low Mn steel; whereas the numbers of entrapped scum are increased with decreasing Mn/Si ratio and addition of Ti to low Mn steel.
2) The numbers of large inclusions are increased with addition of slag powder into mould if a submerged nozzle is not used. It was found by EPMA identification that they were originated from slag powder. The numbers of large inclusions are decreased by use of a air-tight submerged nozzle and slag powder.
3) Large Mn-silicate inclusions are occasionaly detected in Al killed steel billets in the case of open nozzle and oil casting.
4) Sodium is detected in mould scum when glass powder is added into the tundish.
5) The numbers of large inclusions and ratio of very large inclusions (greater than 0.5 mm in dia.) are decreased with increasing billet's size from 80 mm square to 160 mm square, but slightly increased with increasing from 160 mm square to 260×370 mm.
6) The effects of these factors are discussed from the view point of a) cleanliness of molten steel in tundish, b) air oxidation of pouring stream to mould, c) entrapment of mould scum, d) possibilities of floatation and coagulation of large inclusions during solidification process.
