Measures for improving AOD furnace life in refinery branch

　　Abstract: in order to improve the AOD furnace life of MgO CA brick, the characteristics of MgO CA brick and MgO CR brick were compared, the erosion status in different areas of AOD furnace was analyzed, and the erosion in different stages of smelting process was analyzed. The analysis results show that the slag line in AOD furnace is seriously eroded, which is mainly caused by low alkalinity slag. Therefore, through a series of targeted measures such as increasing slag basicity, increasing MgO content in slag, controlling smelting and reduction time, and reducing AOD smelting process temperature, the average furnace life of AOD is increased from about 63 times to more than 68 times.

　　Key words: magnesium calcium brick, slag alkalinity, refractory, AOD furnace life
 

　　1. Foreword

　　AOD (argon oxygen decarburization) furnace is mainly used for smelting stainless steel. Magnesia calcium refractory bricks are used in the furnace lining. The average furnace life has been about 63 times for a long time, resulting in high consumption of refractory materials per ton of steel. Moreover, the AOD furnace life is the embodiment of the overall level of AOD smelting process. Therefore, increasing the AOD furnace life has always been an important index for stainless steel smelting. Since it was put into operation, the AOD furnace technicians of the refining branch have worked together to find out the erosion mechanism of the AOD furnace lining, and taken targeted measures to improve the furnace life.
 

　　2. Characteristics of AOD refining

　　The metallurgical principle of AOD refining technology is to blow the mixed gas of O2 and inert gas (N2, AR) into the furnace, dilute the partial pressure of CO gas produced by decarburization reaction with AR or N2, so as to promote the decarburization reaction and inhibit the oxidation of chromium in molten steel. Due to high refining temperature, long smelting cycle, violent gas stirring, serious steel slag scouring, large periodic change of lining temperature and large change of slag alkalinity, AOD lining working conditions are very poor.

　　2.1 main factors affecting AOD furnace life

　　2.1.1 influence of refractory of furnace lining and masonry and baking process of furnace lining

　　AOD generally uses alkaline refractories. The quality of refractory materials, including refractoriness, high temperature strength, porosity, impurity content, brick size, internal structure, waterproof, etc., seriously affects the furnace life. At the same time, the lining masonry and baking process, such as brick shape design, brick joint size, baking temperature and heating rate, will also have a great impact on the service life of AOD lining.

　　2.2 influence of smelting process and operation

　　AOD smelting process and operation have a great impact on the life of furnace lining, as shown below:

　　(1) High furnace temperature. Studies have shown that when the temperature in the molten pool is above 1700 ℃, the erosion rate of furnace lining refractory will be doubled for every 50 ℃ increase in temperature. When AOD smelts stainless steel, the temperature of decarburization period may reach 1750 ℃, and the lining refractory works at high temperature for a long time. Therefore, the temperature of molten pool and refining time of AOD during decarburization period have a great impact on the service life of lining.

　　(2) The furnace temperature changes greatly. Firstly, AOD production is intermittent. During the period of molten steel, the temperature of furnace lining will drop to about 1300 ℃, while the temperature of furnace lining in molten pool during decarburization period will reach about 1700 ℃, and it will drop to about 1560 ℃ during reduction period; The second air supply elements are air-cooled, and the refractory materials of the furnace lining around the air gun will drop sharply below 900 ℃ during molten steel waiting; Thirdly, in the smelting process, the temperature of the furnace lining is also uneven. The temperature of the furnace lining in the air hole area is higher than that in other areas; Fourth, during stainless steel refining, a large amount of slagging materials, alloys and metals need to be added to the molten pool, which will cause a sharp drop in the lining temperature at the slag line in a short time, which will greatly change the furnace temperature of AOD, resulting in the peeling off of lining refractory and affecting the lining life.

　　(3) The gas and steel flow scour the furnace lining seriously. During the whole operation of AOD, a large amount of gas will be blown into the molten pool. With the continuous development of technology, the oxygen supply intensity and stirring intensity of molten pool continue to improve. This has caused serious erosion to the refractory materials of the molten pool, especially the back wall of AOD has to bear a lot of air and steel flow erosion, which will reduce the AOD furnace life.

　　(4) The basicity of slag fluctuates greatly. AOD furnace lining is not only eroded and scoured by slag, but also has a large fluctuation range of slag alkalinity within a refining cycle, with its value fluctuating within 1.0-4.0. In the early stage of refining, because the slag is not melted, at this time, the alkalinity of the slag is about 1.0 when blowing O2 to raise temperature. In addition, in the reduction period, the SiO2 content in the slag suddenly increases. Under the stirring of inert gas, the SiO2 in the slag will react with MgO and Cao in the alkaline refractories, Low melting point calcium magnesium olivine (Cao • MgO • SiO2) and magnesium rosaxene (3CaO • MgO • 2SiO2) were formed, and the bonding between periclase was destroyed. These low melting point products will soften and fall off during AOD refining, which will reduce the life of furnace lining.

　　It can be seen from the above analysis that due to the characteristics of AOD refining stainless steel, the lining refractory works under the environment of high temperature, large temperature change, serious slag erosion and strong scouring by slag and steel. The working conditions are very bad, so it is very difficult to improve the furnace life.
 

　　3. Measures for increasing AOD furnace life

　　3.1 select high-quality magnesium calcium brick as furnace lining refractory

　　There are generally two types of refractories used by AOD, magnesium chrome brick and magnesium calcium brick. The two kinds of refractories have their own advantages and disadvantages: magnesia chrome brick has high high temperature strength, but its thermal shock resistance and slag corrosion resistance with alkalinity greater than 1.5 are not as good as magnesia calcium brick. Magnesium calcium brick has good high temperature performance, can withstand high temperature above 1750 ℃, and can adapt to large-scale fluctuation of alkalinity of alkaline slag. Compared with magnesia chrome brick, magnesia calcium brick contains more Cao, and Cao can form C2S dense layer with SiO2 in slag, so as to protect the brick from erosion; In addition, Cao has great cowardice at high temperature, which can delay the generation of temperature difference thermal stress cracks; In addition, the price of magnesium calcium brick is lower than that of magnesium chromium brick, and the pollution to the environment is also small. Therefore, at present, MgO Cao bricks are used as AOD furnace lining refractories in the refinery branch. However, the high temperature strength and waterproof performance of magnesia calcium brick are not as good as magnesia chrome brick. In addition, the thickness of furnace lining in tuyere area is increased in view of the rapid erosion of furnace lining in tuyere area.

　　3.2 formulate reasonable oven drying system

　　Formulate a reasonable baking system for AOD furnace lining to slowly raise the furnace bricks to high temperature and ensure that the furnace bricks are thoroughly baked; In case of long-time waiting for molten steel or furnace shutdown for maintenance, the furnace lining shall also be baked to avoid rapid heating of furnace lining refractory in contact with molten steel or excessive temperature drop during long-time waiting for molten steel.

　　3.3 adjustment of smelting process

　　The AOD furnace life is a function of temperature and smelting time when the lining refractory and smelting steel are certain. The temperature of stainless steel smelting at the end of oxidation is generally controlled at about 1700 ℃, and the furnace life is inversely proportional to the smelting time. Therefore, while controlling the furnace temperature and slag basicity, the smelting time should be shortened as much as possible. Control the amount of slag. The greater the amount of slag, the greater the scouring of slag on the furnace lining. The corresponding slag layer thickness also has a great impact on the decarbonization time, heating rate and heat utilization rate. Taking the smelting of 304 stainless steel as an example, the AOD smelting time of refining branch plant from 2012 to 2015 was about 130 minutes; The consumption of lime per ton of steel is 160 kg / ton; The consumption of fluorite per ton of steel is 46 kg / ton; Ferrosilicon consumption is 20 kg / T ~ 23 kg / T. After many technical improvements and equipment transformation, the smelting time of 304 is about 110 minutes at present; The consumption of lime per ton of steel is 125 kg / ton; The consumption of fluorite per ton of steel is 32 kg / ton; Ferrosilicon consumes 16 kg / T ~ 18 kg / T per ton of steel.

　　3.4 adjust slag alkalinity and composition

　　The basicity and composition of slag have great influence on the service life of furnace lining. It is found that with the increase of CaF2 content in slag, the lining life is greatly reduced, while with the increase of magnesium oxide content in slag, the lining life is correspondingly increased. Therefore, the basicity and composition of reduction slag are adjusted. The basicity of slag cao/sio2 is controlled at 2.1, (cao+mgo) / SiO2 is controlled at 2.3, and the addition of CaF2 is strictly controlled to control the proportion of caf2/cao.
 

　　4. Conclusion

　　Based on the above description, the AOD furnace life can be improved by:

　　(1) Formulate a reasonable baking system and ensure continuous production as much as possible after the furnace is put into service.

　　(2) Control the maximum smelting temperature not greater than 1710 ℃. As the corrosion resistance of magnesia calcium brick decreases sharply with the increase of temperature, the temperature in the smelting process must not be greater than 1710 ℃.

　　(3) Control the silicon content of AOD molten iron. At the early stage of AOD smelting, the basicity of slag is greatly reduced due to the oxidation of silicon in steel. At the same time, the reaction speed between lime and silicon dioxide is relatively slow compared with that of silicon oxidation. Therefore, AOD incoming silicon must be controlled below 1.0%.

　　(4) The content of magnesium oxide in slag is 5%-7%. Increasing the content of magnesium oxide in slag can improve the melting kinetic conditions of calcium oxide and promote slagging. At the same time, it can reduce the corrosion of slag on magnesia calcium brick and avoid the transfer of magnesia from magnesia calcium brick to slag.

　　(5) Increase the alkalinity of slag ≥ 1.5. Increasing the basicity of the slag means that the CaO content in the AOD slag is correspondingly increased and the erosion to the furnace lining is reduced.

　　(6) Reduce the amount of slag. In the process of stainless steel smelting, the greater the amount of slag produced, the greater the mechanical and chemical erosion to the lining materials, the greater the melting loss rate of the lining, and the faster the melting loss rate of the slag line. Therefore, reducing slag amount is an important measure to improve AOD furnace life.