Development Of Novel Fast Response Blackbody Cavity Sensor For Continuously Measuring The Temperature Of Molten Steel

Blackbody cavity sensor has solved the problem of continuously measuring the temperature of molten steel in metallurgical measurement field.And it has achieved outstanding results of wide application in dozens of steel factories at home and abroad.However,its response time,as one of the key technical characteristics for the sensor,is long.This is the outstanding problem of the technology that has a long respond lag up to 5?6min,which has difficulty in satisfying the demand of response speed of continuously measuring the temperature in some conditions,such as the casting start stage,ladle changing and refining furnace.The root cause of the response lag is the slow heat transfer that was resulted from the present sensor's thickness of 20?30mm.The thick wall structure is a comprehensive selection from applying environment of the sensor and properties of the present material.Under the measuring environment with high temperature and strong corrosion,Al₂O₃-C refractory has been the present best practical material for the sensor.But the low strength and high porosity of this material result in that the sensor has to adopt the thick wall structure.These make the response lag become an inevitable problem of the present sensor.The main work of this dissertation is to solve the response lag problem of continuously measuring the molten steel temperature.This work has proposed that a new sensor material of cermet is adopted to manufacture a blackbody cavity sensor which had a fast response characteristic of thin wall?about 3mm?.In order to solve the high cost problem of the cermet's application,the cermet is used as the cavity for measuring the temperature,which is compounded with a structural support of Al₂O₃-C refractory for manufacturing the novel sensor.The main works include:a)a cermet,the material for the sensor with thin wall,has been developed.It has the properties of thermal shock resistance and corrosion resistance.b)The miniaturization cavity for the temperature measurement has been designed on the basis of accuracy and cost.c)The novel fast response sensor has been manufactured,and then it is tested by application experiments.The main research works and innovations of this dissertation are as follows:?1?Development of a sensor material for the fast response structure of thin wallIn order to solve the conflict between the thin wall of the sensor and its demand of strong thermal shock resistance and long corrosion lifetime,a cermet has been developed which is used as the sensor material.First,the raw materials with little linear coefficient expansion and elastic modulus,including Mo,W and ZrO₂,are selected for reducing the heat stress.Second,the metal phase is used to form semi-continuous network in the ceramic matrix and establishes the network for bridging the crack.Besides,the plastic deformation of the metal network will occur and consume the crack propagation energy.These can seal the crack propagation in the metal phase network as soon as possible,which improves the ability of resisting the thermal shock fracture.Meanwhile,the low corrosion rate characteristic of the ceramic phase that cannot be wetted by the molten steel is used to form a ceramic skeleton layer.This ceramic layer can prevent the penetration of the molten steel into the cermet and slow melting corrosion of the metal phase,which will reduce the corrosion rate of the cermet.Consequently,the cermet,which has good thermal shock resistance?not damaged by the insertion of molten steel about 5 cycles?and good corrosion resistance?its corroded thickness is only 0.8mm after the 20h corrosion in molten steel?,has been developed.?2?Design of miniaturization cavity for temperature measurement of the fast response sensor on the basis of the accuracy and costThe novel sensor requires that the temperature measurement cavity of the cermet tube forms an online blackbody cavity independently and rapidly,which can achieve the rapid and accuracy temperature measurement of molten steel.Otherwise,the fast response may be impacted by the slow heat transfer of the structural support,and the fast heat transfer advantage of the thin wall cermet tube cannot come true.Thus,on the basis of studying the heat transfer,a model for calculating the effective emissivity of the temperature measurement cavity is established.Combining the influence of the size on temperature distribution of the cavity,the work calculates the effect of cavity size on the effective emissivity of the cavity,and designs the theoretical miniaturization length of the measurement cavity.According to the theoretical result,the length of the measurement cavity is determined through experimental studying.?3?Manufacture of the novel sensor and application experiment.a)It is a problem of manufacture how to improve the compound bonding structure of the novel sensor,mainly depending on the curing and firing of the Al₂O₃-C refractory support.This work has investigated the physical chemical reaction mechanism of manufacture process for improving the strength and reliability,which provides theoretical basis for making and optimizing the manufacture processes.The curing and firing convert the phenolic resin binder from liquid to solid,and then from the solid phenolic resin to glassy carbon,respectively.In essence,these are used to control the physical and chemical reactions of the binder during the curing and firing through controlling the heating rate and temperature.Variations of the heat,weight and structure of the binder are examined for investigating the physical and chemical reactions.Besides,the intensity of the dehydration-condensations of curing and pyrolysis and shrinkage reactions of firing and their influence on the material's properties are analyzed.And then,the corresponding heating strategy under different temperatures is determined.b)The novel sensor,which is used to continuously measure the molten steel temperature,has improved the temperature response speed remarkably.The field experiments show that the novel sensor reduces the response time from 5-6min to about 54-56s,which improves the dynamic response characteristic of continuous measurement of the molten steel temperature basically.This satisfies the demand of casting start stage that needs response time about 90s,which can help solve the problem that the present casting start stage only adopts a disposable thermocouple to discontinuously measure temperature and its value has some scatter.Besides,this can lay the foundations for rapid monitoring of temperature anomaly during ladle changing and continuously measuring the molten steel temperature in refining furnace and converter.