1. Converter refractories damaged by physical erosion:

The mechanism of physical erosion is mainly manifested in several forms, such as bubble counterattack, water hammer erosion, pit melting loss, etc. The air blown into the molten pool enters the molten pool in the form of bubbles. When the bubbles break away from the gas supply element, the phenomenon of impacting the refractory backseat around the gas supply element is called "bubble counterattack". The higher the flow rate of bottom blow, the higher the counterattack frequency, the greater the energy, and the more serious the erosion of resistant materials. Water hammer scour refers to the phenomenon of molten steel flowing when the bubbles leave the gas supply element and scour the refractory material around the gas supply element. The higher the gas flow rate, the more serious the erosion of molten steel caused by water hammer phenomenon. Due to the erosion of gas and molten steel, pits are formed around the gas supply element. The deeper the pit, the worse the convective heat transfer and the worse the erosion.

2. Bessemer refractory damaged by thermal stress:

The main feature of the high-pressure repeat blowing process is that the flow rate of the bottom blowing gas is greatly adjustable, and the strength of the bottom gas supply is large, especially the high pressure and large flow gas stirring is used near the end. The impact of the process is that the stress is highly concentrated, the refractories on the working surface of the gas supply element, especially the refractories around the air outlet, are in direct contact with high temperature molten steel (1600 ~ 1700℃), and are affected by extremely high temperature molten steel and the continuous outflow of cold air, resulting in a greater temperature gradient than under low pressure and small flow conditions. According to the research on the temperature field of Mg-carbon gas supply element under the condition of gas supply intensity of 0.2m3/(min·t), the temperature gradient changes greatly only within a small range around the air outlet (about 6mmx20mm), while the temperature gradient changes little in other places, which means that the stress height is concentrated in a small range around the air outlet. Due to the large change of gas flow and the rapid change of temperature caused by the process of steel production, the urgent cooling and heating effect around the air outlet increases, and the increase of thermal stress is an important factor that is prone to crack. In order to meet the requirements of high pressure reblowing process, it is necessary to overcome the influence of increasing thermal stress and improve the thermal shock resistance of the material.

The internal and external REDOX reactions of magnesium-carbon materials are as follows:

MgO(s)+C(s)=Mg(g)+CO(g)

2C(s)+O2(g)=2CO

At steelmaking temperature, the internal reaction between magnesium oxide and carbon leads to the damage of the MG-carbon breathable element itself, but the blowing air has a cooling effect on the breathable element and the surrounding refractory material, and the temperature of the MG-carbon material is reduced, which slows down the damage caused by the above reaction. For the external reaction of carbon and oxygen, even if it is an inert gas, its oxygen content can not be ignored. When the gas supply intensity increases, the oxygen content in the air stream also increases, intensifying the oxidation of magnesium-carbon materials and stainless steel snorkel.

3. Metallurgical molten liquid and refractory reaction effect:

The oxidation of silicon, manganese, carbon and other elements in the smelting pool in the early and middle stages of converter reblowing steelmaking, or the oxides SiO2, MnO2 and FeO in the late stage of blowing have an erosion effect on the MgO-C gas supply elements, especially in the late stage of blowing, the FeO content in the liquid steel and slag increases, and FeO is easy to react with C to accelerate the graphite oxidation in MgO-C materials. In addition, FeO invaded along the grain boundary of magnesite and formed (Mg, Fe)O solid solution by interaction with magnesium oxide. The melting temperature of FeO was 1380℃, and the invasion of FeO significantly reduced the formation temperature of the liquid phase.

The microstructure analysis shows that the MgO-C refractory-work face does not hang slag, the graphite (especially the graphite around the air outlet) is damaged, the metal aluminum disappears, but the bare magnesium oxide particles remain. 6mm away from the working face, the metal aluminum still disappeared, leaving only space, but the graphite is basically intact, indicating that the temperature here has been greatly reduced; About 10mm away from the working face, the structure is relatively dense, X-ray diffraction analysis confirmed the formation of MA spinel; A circular "appearance" appears in the refractory matrix near the stainless steel tube. Electron probe analysis confirmed that the "appearance" was metal aluminum. The presence of metal aluminum indicates that the temperature of this part of the refractory is greatly reduced, indicating that when the air flow through the airway, the stainless steel pipe and the surrounding refractory have a sharp cooling effect.

In addition, clogging of bottom-blown gas supply system is also an important factor affecting the life of re-blown ventilation elements. In production practice, the phenomenon of clogging of ventilation elements occurred many times due to the unreliable control system within a short time after furnace opening.

At present, there are some problems such as low bottom blowing strength (domestic converter is less than 0.1m3/(min·1)), easy clogging of bottom blowing gun, poor bottom blowing effect in the middle and late stages of furnace service, which increase the cost (large consumption of deoxidizer and alloy), long refining time, increase of non-metallic inclusions, high oxygen content of molten steel at the end of blowing, and it is difficult to adapt to the situation of increasing phosphorus content of molten iron.

4. The development trend of refractory materials for converter:

The converter is developing in the modern direction of large, automatic, efficient, environmental protection and longevity, and some advanced technology (slag splashing furnace protection, repeated blowing, dynamic control, slide slag blocking, dry dust removal, slag retention + double slag process, etc.) is also widely used. Compared with the usual bottom-blown uniform gas supply, the flow ratio between the two branches of the bottom-blown asymmetric gas supply is 2.5:1 ~ 4:1. By forming eccentric reflux in the melt pool, the mixing time of the melt pool can be shortened, and the efficiency of reblowing can be improved. The mixing time of the melt pool in converter steelmaking and hot metal dephosphorization pretreatment can be shortened by 19.2% and 63% respectively, and the mechanical wear of the bottom blown permeable element can be reduced by 50%.

Due to the carburizing of MgO-C refractory to stainless steel snorkel during use, the melting point of the snorkel will be reduced and the service cycle will be shortened. By applying the protective layer α-Al2O3 powder and aluminum chelate compound on the surface of the stainless steel pipe, the C reaction in the stainless steel pipe and the surrounding MgO-C brick can be prevented, so that the thickness of the carburizing layer of the stainless steel pipe can be reduced, and its service life can be improved.