Research On Molten Metal Level Measurement Device With Covering Flux Based On Temperature Gradient Analysis And Its Application

Molten metal level which has covering flux is still a measurement problem. Because the existence of covering flux and harsh environment with high temperature and dust, it is difficult to measure the molten metal level effectively by non-contact measuring method, e.g. laser, ultrasonic, microwave, and contact measuring method, e.g. float, electrode. Molten steel level in tundish with this feature is a typical representative of molten metal level measurement. The molten steel level in tundish plays an important role in billet production quality, molten steel yielding rate and production efficiency.A new measuring device based on temperature gradient analysis and computer vision is bringing forward to solve the uneven and thickness time-varying features of slag layer which includes covering flux, slag and inclusion. In summary, some works and innovation are presented as followed:(1) Molten steel level measuring device based on temperature gradient analysisA measuring bar made of refractory is inserted into tundish to get the temperature distribution of molten steel layer, slag layer and air layer. After thermal balance, extremums exist at the interfaces of "air layer-slag layer" and "slag layer-molten steel layer". The measuring device is designed according to temperature gradient analysis and computer vision, which use a CCD camera to acquire the thermal image of measuring bar after being raised from molten steel. Interfaces are located by temperature gradient extremums in thermal images, finally molten steel level can be calculated by geometric equation.The heat transfer model is established in this paper. Numerical calculation proves that temperature gradient extremums existing at the interfaces between air layer, slag layer and molten steel layer. The positions of temperature gradient extremums don’t change within10 seconds after measuring bar being raised from molten steel. The numerical calculation provides the theoretical basis for measuring the molten steel level.(2) Target identification and location of measuring barVeracity of target identification and location of measuring bar influences the interfaces judgment between air layer, slag layer and molten steel layer. The conventional image segmentation method is inapplicable to segment the measuring bar and background. According to the characteristics of measuring bar which the edge is beeline, the Hough transform is used for target locating. The line parameter of original image is calculated in polar coordinate space. Accoring to the pose constraint of measuring bar, line parameters can separate measuring sensor and background at the accuracy rate of98%.(3) Factor analysis of interface locating and the solutionThe influence of interface locating is analyzed when visual distortion engender, attitude and position parameter change. For distortion of the visual system an improved correcting method of monocular vision distortion is put forward in this paper. For parameter changing of measuring sensor, relationship between baseline references and measuring sensor is used to achieve the correction. System error is reduced to2.4mm after correction compare to the value20mm before correction.The cause and effect which "temperature gradient" fake interface influences measuring result are analyzed. Character criterion for fake interface is established aimed at slag adherence, flowing and crusting of slag. By merging the criterion with the image data, accuracy for the interface locating reached about95%.(4) Field experiment and application effect of measurement systemThe measuring system is tested in field for three years; it had worked stably for6months until now. Manual measuring methods is used to validate the measuring results.The experimental results show that the average measuring deviation is4.2mm; maximum measuring deviation is6mm, which meets the requirements of field application and has promotion prospects.At the same time the principle is established for determining key parameters of optical system which lays a foundation for application. According to the principle, the horizontal distance of optic center and measuring stick top, OWA equals415.5mm, and camera obliquity, α equals57.5°. (5) Determination on slag carryover critical level of tundish molten steelMathematical model of slag carryover at the end of casting is established which is proved by numerical calculation and physical simulation. At the same time critical level could be calculated by the nozzle diameter, out flowrate, initial circulation and Froude numbers of outflow. The results showed that the average error between calculation and experiment value is7.2%which lays the foundation of level measurement system on the lowest level control for the next step.Now seven steel companies have signed technical cooperation agreement, and this measurement principle has a certain degree of versatility and it can be extended to other similar molten metal level measurement.