1 INTRODUCTION Continuous casting is an important process in steel making procedure. In this process, steel is poured several times from one vessel into another. As shown in Figure 1.

Fig 1. Part of the production flow of continuous casting machine Molten steel pouring from a ladle into a tundish through ladle shroud. The ladle has a side wall of refractory lining which adapted to be contacted by molten steel. The tundish is an copper alloy intermediate holding container lined with refractory bricks. Ladle shroud is a cylinder with an internal refractory lining. A metal arm connects to the ladle shroud and removes the ladle shroud during ladles changing, this arm is called ladle shroud manipulator arm. Then molten steel pour into the water cooled copper mould. The lock gate and the slide gate control the flow of molten steel between the ladle and the tundish, the slide gate control the flow of molten steel between the tundish and the mould[1]. In the continuous casting process, on top of the molten steel is a layer of slag. Slag is a by-product of the smelting ore process. It is usually a mixture of metal oxides and silicon dioxide. Slag is used to reduce the re-oxidation ratio of the

This work is supported by National Nature Science Foundation under Grant 51178373 and Industry Application Development Research Program of Xi’an city under Grant YF07045 and Application Technology Research Program of Beilin District under Grant GX1206

molten steel. To keep the cleanness of the steel, the slag arises during each step of the continuous casting process must not be transferred into the next vessel. Slag carry over from ladle to tundish has a serious effect on casting operations, with negative impact on yield, quality and safety. During the sequence casting process, slag carry over leads to a build-up of slag in the tundish. This increased the risk of nozzle clogging in the casters, sliver defects in the cast strand, reduced the tundish refractory life and cause an increase in the inclusion content of the steel products.

2 SLAG DETECTION METHOD In the past years various types of methods have been developed for slag detection, such as visual observation, tare weight monitoring, rate of teeming change, electromagnetic methods, opto-electronic monitoring of stream surface, infrared thermal imaging methods, supersonic wave methods, vibration analysis of the ladle shroud, etc. Visually observation detect slag carry over method is highly dependent on the operator experience. The carry over slag amount is unstable before detection. Monitoring the tare weight is easy to be implemented, but its accuracy relies on correct ladle, ladle cover and slag weight estimates. The pure steel and the slag have different density, monitor the variation in the rate of ladle weight change, slag carry over can be found. This method can lead to errors at low teeming rates and false alarms by the movements of the ladle flow control device or the ladle. The main shortcoming of opto-electronic method is that the core of the stream cannot be observed, large amounts of slag can be carried over before detection[2].

Vibration Style Ladle Slag Detection Method based on Discrete Wavelet Decomposition

Dengfeng Chen, Haiyan Xiao, Qichun Ji School of Information and Control Engineering, Xi’an University of Architecture and Technology, Xi'an, Shanxi, 710055

E-mail: chdengf@163.com

Abstract: The purity of molten steel is a crucial factor of steel products' quality. Detection and removal of slag carryover from the molten metal is an important task in steel making procedure, especially in the continuous casting process. Researchers have applied several different slag carryover detection methods to solve the slag carry-over since 1980s, such as electromagnetic coils method of AMEPA, Infrared thermal infrared imaging method, supersonic wave method and vibration method, which are all non-contact methods and some other contact type slag detection methods. In this paper, a non-contact vibration analysis slag detection method is presented. The main principle of this method is based on the vibration variety of ladle shroud between pure molten steel fluid and slag carryover fluid. The experiences of steel making operators have proven the differences of vibration wave in continuous casting process. The signal data gathered from produce field is analyzed by power spectral density (PSD) and discrete wavelet decomposition (DWD). The experiment results are given, the vibration wave feature of slag carryover is distinctive. Which verified the validity of vibration slag detection method. Key Words: Slag Detection, Steel Making, Vibration, Power Spectral Density, Discrete Wavelet Decomposition.

3019978-1-4799-3708-0/14/$31.00 c©2014 IEEE

2.1 Electromagnetic Slag Detection Method The electromagnetic slag detection method is the most widely used detection method which was presented by AMEPA in 1980s. This system uses one concentrically arranged coils located around the ladle shroud as sensor. As shown in Figure 2, an alternating current is fed into the sensor, thereby inducing eddy currents in the molten steel as it flows out through the ladle shroud. The slag has a significantly lower electrical conductivity than molten steel, the eddy currents induced in the slag are smaller than those induced in the steel. The conductivity changes and the electromagnetic field increases slightly when there is any slag carry over in the molten steel. Because of the concentric design of the sensors, a maximum electromagnetic field is generated in the molten steel and a strong signal is achieved. The measuring results are independent of the pouring velocity of the melt. It has been reported by several steel plants that the electromagnetic slag detection method reduce the downgraded slabs attributable to ladle-to-tundish slag carry over significantly.

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1) Electromagnetic field Only steel 2) Electromagnetic field With slag

Fig 2. Electromagnetic slag detection principle[3] In the steel plant environment, all measurement equipment and sensors are expected to meet the highest demands in terms of reliability and robustness. The coil sensors must work in temperatures of up to 800° C and will be easily damaged, this cause the high maintenance charge. And the system installation process is complicated.

2.2 Infrared thermal imaging slag detection method The principle of infrared thermal imaging slag detection method stand in that steel and slag have different features with regard to radiation physics in the infrared spectral range. The schematic arrangement of the infrared thermal imaging technique is shown in Figure 3, thermal imaging techniques has been used to reduce the slag from converter to ladle during the end of tapping. But the total amount of slag inside the molten steel cannot be removed and a large amount of slag is carried into the caster side. In the ladle slag detection with thermal imaging technique, a CCD camera is aimed to take the image of the area of the junction of shroud and tundish layer. The camera is connected to the computer through a frame grabber card. As the density of the slag density is about half of the steel, the slag floats immediately as it enters the tundish through ladle shroud. The floating slag breaks the surface of the molten steel and slag layer and a significant splashing of light takes place. The imaging system detects the sudden splashing of light which is possibly to determine whether the slag carry over has happened without any contact of the molten steel.

Fig 3. Thermal infrared imaging slag detection schematic[4] This method is in real time which of both materials is the current main constituent of the tapping, respectively the deslagging stream coming out of the ladle. Full access of the tapping data is available to the operator for quality control purposes.

2.3 Supersonic wave slag detection method Supersonic wave slag detection is based on the difference of wave reflection between steel and slag[5]. The structure of dip-in style supersonic wave detection system is shown in Figure 4.

Fig 4. Supersonic wave slag detection system schematic[4][6] As shown in Figure 4, the dip-in style supersonic detection system consists of a supersonic impulse generator, a launch probe which radios the supersonic impulse, a reception probe which receives the supersonic impulses, an amplifier and a computer. The launch probe and the reception probe are dipped in the tundish, the reception probe can catch two peak signals reflected from the two side walls when slag doesn’t carry over, supersonic wave will create contacting reflection on slag when slag enters the tundish, and the system will obtain three or more peak signals as shown in Figure 4.

3 Vibration Analysis Slag Detection Method Experienced operators have found when molten steel flows through the ladle shroud into the tundish, it will create a certain extent of vibration, and the vibration is more violent as the steel stream increases. Combined expertise in metallurgy, casting operations, vibration analysis and software engineering to develop a reliable real-time slag detection system. The vibration analysis slag detection method is based on vibration monitoring of the ladle shroud, using an accelerometer attached to the shroud manipulator arm. As is shown in Figure 5, the vibration slag detection system consists of an accelerometer, a signal converter, a controller and a computer. The ladle shroud temperature is highly in casting process. An advanced digital tri-axial accelerometer is attached to the ladle shroud manipulator arm which can collect the vibration signal of ladle shroud as the arm is fixed with the ladle shroud. Signal converter amplifies and converts the vibration signals to digital signals. The vibration signals cause of molten steel and slag flow in the

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shroud are detected by the accelerometer and processed by the computer. If slag carry over occurs, the controller will deal with the ladle removal.

Fig 5. Vibration slag detection system schematic[2][7] The accelerometer is positioned on the manipulator arm and can be used on any of the ladle, the vibration analysis slag detection method appears attractive and preferable to other techniques that require sensors installation on each ladle or in severe environment. Because all components are installed far away from sensitive casting regions, thus guaranteeing long lifespan and durability of the system. The density of slag is about half of melted steel, the vibration caused by slag must be different from pure molten steel. From the vibration signal differences the slag carry over can be detected. Set up a slag alarm threshold points which can activate an alarm and shut the steel flow off when the signal exceeds the threshold. The molten steel in the ladle shroud was assumed to be an incompressible and isothermal Newtonian steady flow. The main governing equations for fluid flow consist of the continuity equation, the Navier–Stokes equations, the turbulence energy, and its dissipation rate equations. Slag, pure steel, and mixture have different specific vibration “pattern” , slag carry over can be detected by pattern recognition.

4 Experiments and Analysis Using the system shown in Figure 5. We can get the data of steel fluid, mixture of steel and slag and slag carry over in time domain, as shown in Figure 6.

1) Steel fluid 2) Mixture of steel and slag 3) Slag carry over

Fig 6. Vibration signals of shroud manipulator arm From Figure 6 we can an overview that the vibration of ladle shroud decreased as the slag flow through ladle shroud increased, the vibration waveform changed distinctly at the time of slag carry over happened.

4.1 Power Spectral Density

Energy spectral density describes how the energy of a signal or a time series is distributed with frequency. Power spectral density (PSD), which describes how the power of a signal or time series is distributed over the different frequencies. Here, power can be the actual physical power, or more often, for convenience with abstract signals, can be defined as the squared value of the signal. The integral of the PSD over a given frequency band computes the average power in the signal over that frequency band. In contrast to the mean-squared spectrum, the peaks in this spectra do not reflect the power at a given frequency. Welch's method is used for estimating the power of a signal at different frequencies: that is, it is an approach to spectral density estimation. The method is based on the concept of using periodograms spectrum estimates, which are the result of converting a signal from the time domain to the frequency domain. Welch's method splits the data into overlapping segments, computes modified periodograms of the overlapping segments, and averages the resulting periodograms to produce the power spectral density estimate. Estimates the power spectral density of the vibration signals using Welch's method, we can get the results in Figure 7.

1) PSD of Figure 6.1

2) PSD of Figure 6.3

Fig 7. PSD of shroud manipulator arm by Welch's method From Figure 7 we can get the conclusion that the vibration frequency of the shroud manipulator arm varied at different phase of continuous casting. The vibration signal frequency of pure steel concentrate in the scale of [60,120] Hz. The vibration signals frequency concentrate in the scale of [10, 60] Hz when slag carry over happened. This result according with the Figure 20.2 in reference [2].

2.4 Discrete Wavelet Decomposition

The Fourier transform is a powerful tool in analyze the frequency components of the signal. However, if we take the Fourier transform over the whole time axis, we cannot get the information of a particular frequency rises at which instant. Short-time Fourier transform (STFT) uses a sliding window to find spectrogram, which gives the information of both time and frequency. But still another problem exists: The length of window limits the resolution in frequency. Wavelet transform seems to be a solution to the problem above. Wavelet transforms are based on small wavelets with limited duration. The translated wavelets locate where we concern. Whereas the scaled-version wavelets allow us to analyze the signal in different scale. Discrete Wavelet Transform (DWT) is any wavelet transform for which the wavelets are discretely sampled. As with other wavelet transforms, a key advantage it has over Fourier transforms is temporal resolution: it captures both frequency and location information (location in time).

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In this paper, the layer is set as 8, the frequency scale is shown in Table 1.

Table 1 Wavelet decomposition frequency scale

Layer No. Frequency scale(Hz) d1 500-1000 d2 250-500 d3 125-250 d4 62.5-125 d5 31.25-62.5 d6 15.63-31.25 d7 7.81-15.63 d8 3.91-7.81

The eight scale layers wavelet decomposition results of Figure 6 is shown in Figure 8.

1) Wavelet decomposition of Steel fluid

2) Wavelet decomposition of Slag carry over

Fig 8. Wavelet Decomposition of Shroud manipulator arm From Figure 8 we can get the conclusion that in the phase of steel fluid, the vibration frequency of shroud manipulator arm centralized in the scale of d3, d4, d5, in the phase of slag carry over the vibration frequency of shroud manipulator arm centralized in the scale of d5, d6, d7. The conclusion is accordance with what we have got from Figure 7.

5 Conclusion An indirect slag detection method based on vibration is presented in this paper, we can get the slag carry over information by detect the vibration of Ladle shroud manipulator arm. The method can decrease the environment sensitivity and requirements to device. From the experiments data we can get the results: 1) Using the PSD method, the frequency of pure steel fluid concentrate in the scale of [60,120] Hz and the frequency of slag carry over concentrate in the scale of [10,60]Hz. 2) Using the Discrete Wavelet Decomposition method, the frequency scale of pure steel fluid centralized in the scale of [31.25-62.5] Hz, [ 62.5-125] Hz, [ 125-250] Hz and the frequency of slag carry over centralized in the scale of [7.81-15.63]Hz, [15.63-31.25]Hz, [31.25-62.5]Hz. 3) The results of PSD method accordance with the results of Discrete Wavelet Decomposition method. The vibration style slag detection method is validity.

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[4] D.Tan et. al., Development of vibration style ladle slag detection methods and the key technologies. Science China-Technological Sciences, Vol.53, No.9, 2378 2387, 2010.

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