Interaction Between Microporous MgO Refractories And Functional Oxides,Molten Steel And Slag In Tundish

Tundish is the last metallurgical vessel that molten steel flows through before solidification in the continuous casting process,providing the last opportunity for the purification of liquid steel.Magnesia based refractories are widely used in tundish and theoretically have great adsorption capacity for oxide inclusions in steel.However,the traditional dense magnesia-based refractories possess high thermal conductivity,which lead to large heat loss.In order to meet the requirement of energy saving,the microporous magnesia-based refractory with great thermal insulation,slag resistance and purifying liquid steel function were prepared from magnesite by in-situ decomposition and adding pore-forming agent.Considering that long-time contact between tundish refractory,slag and molten steel,the corrosion mechanism of microporous magnesia in tundish slag and the adsorption mechanism of oxide inclusions in steel by microporous magnesia were studied.On this basis,the microporous magnesia based castable was prepared with microporous magnesia as aggregates.The interaction between microporous magnesia-based castable and 38Cr Mo Al steel bearing 0.876 wt%[Al]was studied.Furthermore,the interaction between functional oxides（Ce Al O₃,Al₂O₃,CA₆ and so on）and molten steel was also investigated.Based on the above studies,the following conclusions are obtained:（1）The chemical stability of intergranular phases in the microporous magnesia has a significant effect on its corrosion resistance.During the penetration of slag into refractories,the intergranular phase can dissolve into slag,damaging the integrity of microporous magnesia structure.After adding nano-sized Al₂O₃,spinel was formed at the grain boundaries of microporous magnesia.However,the chemical stability of spinel in the slags with high basicity,low Al₂O₃ content or low Mg O content was poor,leading to serious corrosion of microporous magnesia.The microporous magnesia added with0.75 wt%nano-sized Zr O₂ showed great slag resistance.（2）The microporous magnesia has great chemical adsorption on oxide inclusions（Al₂O₃,Si O₂,Ti O₂）in steel.The interaction between microporous magnesia and oxide inclusions in steel at high temperature mainly includes three stages:1)dissolution stage,where oxide inclusions dissolve into the intergranular liquid phase and penetrate microporous magnesia;2)reaction stage,where oxide inclusions react with magnesia grains when the liquid phase around grains reaches equilibrium,and the inclusions directly contacted with magnesia grains may also react with Mg O;3)post-reaction stage,where the oxide inclusions continue to react with the previous reaction products when the magnesia grains are completely transformed into corresponding solid products.（3）After holding for 30 min,the total oxygen content in the 38Cr Mo Al steel sample for the test of microporous magnesia-based castable was only 42.2%of that for the test of fused magnesia-based castable.The micro-sized pores with high specific surface and reaction activity in the microporous magnesia-based castable can promote the formation of Mg O·Al₂O₃ spinel layer.Once the dense and continuous spinel layer is formed,it will serve as a metallurgical isolation layer to protect the molten steel,and inhibit the further reaction between molten steel and refractory.（4）The total oxygen content in the 38Cr Mo Al steel sample after reacted with the Ce Al O₃ refractory was only 22.9%of that with microporous magnesia.The total oxygen content in the X70（0.040 wt%[Al]）steel sample after reacted with the Ce Al O₃refractory was 70.9%of that with the microporous magnesia.However,the Ce O₂refractory caused serious oxidation of molten steel.Ce Al O₃ is promising to be used as functional refractory for improving the cleanliness of molten steel and reducing the loss of Al elements in steel.（5）The total oxygen content in the 38Cr Mo Al steel sample after reacted with the CA₆ refractory was only 24.0%of that with the microporous magnesia.The total oxygen content in the X70 steel sample after reacted with the CA₆ refractory was 30.4%of that with the microporous magnesia.CA₆ is promising to be used as functional refractory for improving the cleanliness of molten steel.