STUDY ON BLOWHOLES IN IRON AND STEEL (III)
Although it is important to eliminate blowholes or pinholes in iron and steel, the mechanism of growth of them has not been yet suffciently studied. As a first step to study this mechanism, vacuum-cutting apparatus was designed and chemical composition of gas in blowholes was studied.
Blowholes caused by CO gas grow in rimmed steel but they do not grow in kilied steel, because they are deoxidized. This limit of blowhole formation can be decided by calculation from the equilibrium constant but experimentally was not obtained to this day. Therefore the relation between formation of blowhloles by CO gas and Si, Mn, Al deoxidation was explained experimentally and the results of experiments were compared with calculation values from the equilibrium constabt.
Blowhole samples were melted in a 100kg basic high-frequency electric furnace. When the temperature of molten steel attained to 1600°C, deoxidation reagents (Si, Mn, Al) were added in different quantities. Some times after deoxidation oxygen gas was lanced into molten steel for 30 seconds and by reaction of C and O, CO gas evolved violently. 25kg of molten steel was immediately tapped to ladle and then poured to 16kg ingot. In succession, oxygen gas was lanced again to the rest molten steel in the furnace for 30 seconds and poured to a 16kg ingot. Accordingly from one melt of 100kg, four ingots were obtained and they were different in Si, Mn, Al and oxygen contents. These ingots were cut lengthwise and the distribution of blowholes were investigated. The results gained from this experiments were as follows.
1) Even if carbon content was as high as compared with usual rimmed steel. blowholes by CO gas were same as rimmed steel and primary and tecondary blowholes exists.
2) When deoxidized by Si, the limit of blowhole. formation was 0.10%Si for 0.33-0.40% C and 0.20% Si for 0.40-0.80% C. This limit was raised by carbon content and corresponds to a curve which was calculated by the equilibrium constant and the melting point.
3) When deoxidized by Al, the limit of blowhole formation was 0.01% Al.
4) When deoxidized by Mn, the blowhole formation was not prevented for weak deoxidation power of Mn.