| Blackbody cavity based continuous temperature measurement sensor realized low-cost and high-accuracy, thus it has been widely used for the temperature measurement of liquid steel in tundish. The sensor which is inserted into molten steel can be damaged in a period of time for the high temperature, corrosion by slag, thermal fatigue and thermal shock, etc. The material property of sensor determines its resistance to damage, i.e. its lifetime. And lifetime index is one of the key factors that decide whether this technology is widely used or not.At present, the continuous temperature measurement sensor was made of Al2O3-C material. Under normal acid slag (CaO/SiO2<1), the lifetime of sensor can be up to24-40hours. But for improving the quality of molten steel, basic slag which has the effect of absorbing inclusion was adopted by the emerging tundish metallurgy technology. And the lifetime of Al2O3-C sensor drops to3.5-9hours under basic slag (CaO/SiO2>2). As the proportion of basic slag application keeps growing, the study on the materials of sensor which is suitable for basic slag became the pressing issue in the research about continuous temperature measurement sensor for liquid steel.In this paper, the mechanism of the sensor’s damage was analyzed firstly. Aimed at the problem of corrosion which leads to the damage, application of new basic slag resisting MgO-C material was proposed, and an ideal composition of this material was deterimed by studying the effects of material components on the corrosion resistance. The grain gradation of material was optimized to further improve the corrosion resistance of this sensor. Anti-oxidants were studied to settle the frangibility to oxidation of MgO-C material. Thermal stress was analyzed to discuss the thermal shock resistance of the sensor. At last, the corrosion resistance and thermal shock resistance of the sensor made of new material were verified by industry trail. The main research work and innovations are as follows:(1) The damage reason of sensor was studied based on the fractal dimension of cross-section profile.Accurate judgment for the damage method of sensor is the precondition of research about sensor material which is suitable for basic slag. The damage methods of sensor would be corrosion and spalling, which usually occur simultaneously at the actual damage process. Therefore it is hard to judge accurately. After analyzing the damage morphology differences between corrosion and spalling, a diagnostic method of sensor damage based on the fractal characteristics of cross-section profile was proposed in this paper. The fractal dimension of cross-section profile of sensor is calculated by box-counting method and then discretized by Chi2algorithm to obtain the thresholds which can classify the damage type correctly. According to this diagnostic method, the damage reason for the sensor made of Al2O3-C material under basic slag is corrosion by slag.(2) Using MgO-C refractory as the sensor material was proposed in this paper, and the effect of material component on the corrosion resistance was studied.To avoid the high aggregate solubility to basic slag caused by using Al2O3-C refractories as sensor material, using MgO-C refractory as the sensor material was first proposed in this paper. The aggregate of MgO-C refractory was MgO-based. On one hand, the using of MgO as aggregate was for the hardly soluble properties of MgO in basic slag, it could slow down the aggregate dissolved in corrosion process. On the other hand, the addition is for suppressing the slag penetration in corrosion process, because it can increase slag viscosity and bring the expansion of spinel reaction.(3) The effect of critical particle size for aggregate on corrosion resistacne of sensor was studied. And the grain gradation of MgO-C material was optimized for the further improvement of the corrosion resistance.The paper first studied the effect of critical particle size for aggregate on corrosion resistance. Corrosion rate and lifetime of sensors made of MgO-C materials with a critical particle size of0.5mm,1.0mm,1.5mm and2.0mm were compared by experiments. And the effect of critical particle size on corrosion resistance was analyzed from the aspects of slag penetration and aggregate dissolution. The analysis result shows that the increase of critical particle size is coming with the rates of slag penetration and aggregate solution going down, thus the corrosion resistance goes up.Based on the boundary condition for grain gradation determined by the above research, an approach to grain gradation optimization for MgO-C material was proposed. In this approach grain gradation optimization model based on close packing theory was founded. And then optimum ratio for materials was concluded by solving the model. The stacking density and the tap density of mixed pug according to the optimum ratio had reached1.46g/cm3and1.70g/cm3individually, both were higher than those according to experimental ratio. And microstructure observation indicated that the dense structure of MgO-C materials according to the optimum ratio is formed by the process of fine particle full filling into the blank of crude particle, which can suppressed the slag penetration and improve the corrosion resistance.(4) Anti-oxidation additive was studied based on effective diffusion coefficient.Against the corrosion resistance decline dues to the easy oxidation of MgO-C material,"anti-oxidants consumption factor"(which indicate the quantity of anti-oxidant that unit quantity of oxygen reacted with) was introduced into shrinking core model to build an oxidation kinetics model for carbon-containing materials. Effective diffusion coefficients of gaseous species in materials with Mg; Al; Si; SiC and B4C additive were calculated, and then the component of composite anti-oxidant was determined. Industry experiment result showed that oxidation rate of material with this composite anti-oxidant was48%less than materials with original anti-oxidant.(5) Thermal shock resistance of MgO-C sensor was researched based on thermal sress finite element analysis.Sensors of three different structures were designed individually according to the practical demands for the cost, corrosion resistance and response rate of sensor. That would be oridinary sensor which was integrally made of MgO-C, erosion-resisting sensor whose slag section was made of ZrO2-C material, and fast response sensor whose measuring section was made of high carbon materials were designed. Against the problem of thermal shock damage dues to the sudden temperature change of sensor after being inserted into molten steel, thermal stress finite element models for MgO-C sensors was founded. And thermal stress in the sensors’wall during thermal shock process was analyzed. The analysis results showed that the maximum thermal stresses of ordinary sensor and the erosion-resisting sensor were3.2MPa and3.4MPa. Both of the maximum stresses were less than the maximum permissible stresses of material, and these structure designs had met the demand for thermal shock resistance. The maximum thermal stress of fast response sensor was6.0MPa. This result had surpassed the maximum permissible stress of material, and the structure design had not satisfied with the thermal shock resistance requirement.(6) The industry trial of MgO-C sensor were carried out and analyzed.The results of industry trial showed that comparing to Al2O3-C sensor, the corrosion rate of MgO-C sensor was23～73%less and the lifetime of MgO-C sensor was33～176%longer to reach8.5～32hours under basic or neutral slag with a CaO/SiO2ratio ranged from1.18to4.51. Moreover, there was no thermal shock damage happened.The above results showed that the continuous temperature measuring sensor made of MgO-C material, which was initially proposed in this paper, can meet the basic slag condition well. The research work is of practical significance to promote the extensive application of blackbody cavity based continuous temperature measurement technology. |
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