Views: 0 Author: Site Editor Publish Time: 2021-03-29 Origin: Site
Refining to remove oxides
At the end of the secondary refining, in order to obtain high purity molten steel, so-called "bubble floating purification" is required. Here, "bubble floating and purification" means the boiling of molten steel caused by small bubbles near the bottom of the ladle top slag rising through the molten steel. The floating and purification of bubbles accelerates the floating of residual oxides to the ladle top slag.
Many basic prerequisites must be met in order to achieve the best bubble floating and purification and thus obtain high-purity molten steel. First of all, the top slag of the ladle must be greatly reduced. "A lot of reduction of ladle top slag" means that the content of FeO, MnO and SiO2 must be as low as possible. On the other hand, it is necessary to avoid secondary oxidation caused by contact between molten steel and air. The secondary oxidation of molten steel caused by the refractory material of the ladle must also be excluded. In addition, it is necessary to provide the best conditions for the oxides in the steel to float to the top slag of the ladle.
The molten steel is stirred by the floating of weak bubbles, which accelerates the elimination of oxides. The total oxygen content in molten steel decreases with the prolongation of the bubble floatation and purification time. When the bubble floatation and purification time exceeds 10-15min, the total oxygen content will soon be lower than 20ppm.
In the continuous pouring process of molten steel, the final total oxygen content will decrease. Residual oxides are removed in the intermediate tank or during pouring. Obviously, the oxide removal is the least on the elliptical continuous casting machine, the situation is improved on the arc continuous casting machine or the vertical bending continuous casting machine, and the oxide removal effect is the best on the vertical continuous casting machine. Since in most occasions the vertical continuous casting machine will not be used or the arc continuous casting machine has been installed, the length of the vertical section of the installed arc continuous casting machine is about 2.5-3m.
After transforming the arc continuous casting machine with the arc crystallizer into a vertical bending caster with a vertical section length of 2.5m, it is observed that the purity of the product has been improved macroscopically. This kind of transformation will improve the quality of the cast slab. The effect is more obvious.
As far as the vertical caster is concerned, the inclusions can float up to the mold flux in the mold during the entire solidification process. On the contrary, in the arc continuous casting machine, the inclusions are entrained into the solidified shell at a certain bending radius, instead of floating up into the mold. The result of the comparison is that the distribution of oxygen in the cross section of the slab is very different. On the vertical caster, the oxygen distribution along the cross section of the slab is even. For arc casters, there is a high degree of concentration of oxygen in the so-called inclusion zone. This is the result of the inclusion of the aforementioned inclusions to the solidified shell.