| Blowing argon at the bottom of ladle is a secondary refining technique with low cost,simple operation and good refining effect,and the purging plug is an important functional component to realize blowing argon into molten steel.At present,slit type purging plug is widely used in China’s iron and steel enterprises.During the"soft blowing"argon treatment stagein the later refining stage,the removal efficiency of inclusions in molten steel is generally low,which is not suitable for the smelting of clean steel and high clean steel.There are a large number of interconnected pores in the porous plug,and a large number of small and uniform bubbles can be formed when gas enters into molten steel through the porous plug,which has a positive effect on improving the purity of molten steel.As a typical porous material in particles-packing type,the size,shape and size distribution of pores in the material are particularly complex due to the different size and shape of aggregate particles,molding factors and various physical and chemical reactions in the process of heat treatment.Therefore,it is of great significance to discuss the key factors affecting the pore structure characteristics of porous purging materials in particles-packing type,reveal the influence of pore structure parameters on the permeability performance,and realize the adjustable and controllable permeability performance of porous purging materials from the point of particles stacking.In addition,the purging plug is in direct contact with molten steel in the service process,which needs to bear the great gravity of molten steel and the scouring force of steel flow and air flow.Therefore,the purging plug should have both high mechanical strength and good gas permeability at high temperature.How to balance the mechanical strength and permeability of the material is particularly critical.Based on this,the research contents of this paper mainly include:from the point of aggregate/matrix ratio,aggregate particle size and aggregate shape,the pore structure parameters of porous purging materials in particles-packing type are controlled,and the permeability and mechanical properties of porous materials areinvestigated;From the point of binding phase,the effects of the introduction of high-temperature functional phase on the mechanical strength,pore structure parameters and air permeability of porous porous materials were investigated by in-situ formation of lamellar calcium hexaaluminate and lanthanum hexaaluminate at the interface of aggregate particles,and then the related mechanisms were discussed.In order to prepare porous purging materials with higher strength at lower heat treatment temperature,corundum based porous materials were prepared by metal direct oxidation bonding process.The influence of the amount of Al powder and its composite introducing with Si powder on the microstructure,phase composition,mechanical strength at room temperature and high temperature and gas permeability behavior of porous materials were investigated.The pressure-drop curves of porous purging materials were fitted to the Forchheimer’s equation,and the correlation between gas permeability coefficient（k1 and k2）and pore structure parameters were evaluated by using grey correlation theory.Finally,the bubble formation behavior of gas entering water through porous purging material is investigated by using physical water model.The main conclusions are as follows:（1）The pores originated from the stacking of coarse aggregates are connected with each other in the porous purging materials in particles-packing type,and these pores can be used as the gas flow channel.The pore structure parameters of corundum porous purging materials can be controlled by adjusting the ratio of matrix and particle size.The ratio of matrix mainly affects the apparent porosity,the surface fractal dimension of pores,and pore size and volume fraction of pores originated from coarse aggregates stacking,while particle size has more significant effect on the size and fractal dimension of pores.An appropriate increase of matrix content can increase the bonding degree between aggregate particles and improve the mechanical strength of porous purging materials.The mechanical strength of corundum porous materials reaches the highest at a matrix content of 17 wt%,and then decreases with further increasing the matrix content.The cold modulus of rupture of porous purging materials increases with the decrease of aggregate size,while the cold compressive strength does not change much.（2）Shape modification of corundum coarse aggregates（1-0.5 mm）was conducted in an intensive mixer,and the speed and processing time are the key factors affecting the shape-modification degree of the aggregates.After the shape-modification process,the roundness of the aggregates increases,the aspect ratio decreases and the stacking density increases.The application of shape-modified aggregates can reduce the apparent porosity of porous materials,improve the bonding degree between coarse aggregates,and then improve the cold modulus of rupture and cold compressive strength of the porous materials.The use of shape-modified aggregates can improve the structural stability of the pores from the stacking of coarse aggregates in porous materials with increased size and volume fraction,and reduce the surface fractal dimension of pores.These can help to reduce the flow resistance of gas in porous materials and improve the permeability coefficients.（3）The mechanical strength of corundum porous purging materials can be improved by the in-situ formation of lanthanum hexaaluminate and calcium hexaaluminate at the bonding interface,and the in-situ formation of lanthanum hexaaluminate has more significant effect on the mechanical strength and permeability coefficient.The strengthening mechanisms of in-situ formation of lanthanum hexaaluminate in corundum porous purging materials are mainly due to activation sintering,grain refinement of alumina and the deflection and branching of cracks caused by the lamellarfunctional phase.An appropriate amount of in-situ formation calcium hexaaluminate can bridge the micro-cracks at the interface between aggregate and bonding phase,and improve the mechanical strength of porous purging materials.However,the in-situ formation calcium hexaaluminate always hinders the sintering of porous purging materials,and the improvement of mechanical strength is not obvious.The results show that proper amount of lanthanum hexaaluminate and calcium hexaaluminate in corundum porous materials can increase pore size and relative volume fraction of the pores from the stacking of aggregates,reduce the fractal dimension of pores,and improve the permeability coefficients of gas in porous purging materials.（4）Corundum based porous purging materials in particles-packing type withhigh strength were prepared by direct metal oxidation process at lower heat treatment temperature.When pure Al powder is added,the metal Al powder in porous purging materials will form alumina hollow shell structure in situ after high-temperature heat treatment.This structure is not conducive to improving the mechanical strength of porous materials,but also blocking the pores from the stacking of aggregate,increasing the complexity of gas flow channel,and then reducing the gas permeability coefficients.The addition of Al/Si composite powder can reduce the volume fraction of small pores（≤7.84μm）,increase the size and volume fraction of pores from the stacking of aggregate in porous purging materials,reduce the resistance of gas flow,and significantly increase the gas permeability coefficients of porous purging materials.（5）The correlation coefficients between pore structure parameters and gas permeability coefficients k1 and k2 are analyzed by grey correlation theory.The results show that:the surface fractal dimension and apparent porosity are the key factors affecting the gas permeability coefficient k1;the complexity of pore structure has more influence on k2 than apparent porosity;An increased gas permeability coefficient k2 can be obtained for the porous purging materials with larger median diameter,higher volume fraction of pores from the stacking of aggregates and smaller surface fractal dimension.Under the condition of viscous flow,the prediction models of gas flow rate in porous purging materials are as follows:when considering the compressibility of gas:（?）,when ignoring the compressibility of gas:（?）.（6）The physical water model study of porous purging materials shows that a large number of bubbles with uniform size can be formed when gas enters into water through porous purging materials.With the increase of gas flow rate,more bubbles are formed with larger size.At the same gas flow rate,more bubbles with smaller size are formed for the porous purging materials with smaller pore size.With the increase of gas flow rate,the bubble coalescence behavior is more significant. |
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