Study on the Rimmed Ingot Which Evolved Gas under Reduced Pressure

Abstract

Rimmed steels containing 0.03-0.08% C were teemed into slab molds for 8-16t ingot, which had been set in the vacuum tanks without sealing covers. Immediately after teeming, the tanks were sealed and evacuated. As the pressure decreased, the rimming action became extremely stronger, but in the succeeding stage gas evolution became less and less and ceased at 1 mm Hg after 6-8 minutes. And then vacuum was broken and further solidification ensued in the atmosphere showing an appearance close to killed steel ingots.
In the case of vacuum break at such a high pressure as 10mm Hg, ingot tops became crowned. In order to prevent this phenomenon about 0.3kg/t of aluminum was added at the final stage of degassing.
Changes of chemical compositions caused by the vacuum treatment were closely examined on the samples taken out from molten steel in the mold immediately after the eeming as well as after the vacuum treatment and later was verified by the check analysis on cold sheets. The equilibrium level of the C-O relation, which was close to atm after teeming shifted following the reaction of carbon monoxide formation, to that of 0.1 to 0.01 atm. Although decrease of carbon and oxygen contents occurs also in the solidification of rimmed steel in the atmosphere, it was very much emphasized by the vacuum treatment. The maximum decrease of carbon was about 0.003%.Further more carbon decreased greatly by injecting oxygen or by adding mill scale into molten steel during vacuum treatment. For example, C-content was decreased 0.067-0012% by injection of oxygen for 10 minutes and in this case a lance pipe with a half inch dia. was consumed 4 meters in length. On the contrary the C-addition to molten steel containing 0.033% after this treatment resulted in a very low carbon steel with oxygen content only 0.08%.The final value of oxygen with no carbon addition was 0.03-0.05%.
It was noticed that no manganese loss took place during solidification, while the manganese loss was about 0.05% in conventional ingots.
The structure of 8 t vacuum degassed ingots was considerably similar to that of capped ingots, but superior to the latter, since it showed much less primary holes and thicker solid layers in the rim zone. The segregation pattern was remarkably improved in comparison with the conventional rimmed ingots of the same size and from the same heat. A remarkable improvement of segregation was noticed in 16 t ingots.
Coinciding to the results of oxygen analysis, (Fe, Mn) O inclusions were greatly reduced both in number and size.Sulphide inclusions were uniformly distributed and very small in size.
Mechanical properties of cold sheets showed the remarkable merits of vacuum treatment. Drawing quality was very much improved and became considerably uniform in the longitudinal direction of the cold coils.
To summarize the results shown above, this vacuum degassing process is recommendable for making ingots having cleaner and better quality than conventional capped ingots and a surface as good as rimmed ingots. From the view point of productivity, this process is instrumental for producing very low carbon steel ingots from molten steel higher in C content and consequently for cutting down the smelting time in steel making furnace. This process is also effective to remove a great disadvantage of large remmed steel ingots which tend to show too much segregation.