Online Electromagnetic Measurement of Molten Zinc Surface Velocity in Hot Galvanized Process

ZHENG Jincan, LIU Runcong(), WANG Xiaodong()

Center of Materials Science and Optoelectronics Engineering, College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China

The behavior of zinc flow in the zinc bath plays an important role in hot galvanizing process, which has an important influence on the temperature distribution, the composition of zinc coat, the control of air knife, and so on, thus affecting the surface quality of zinc products (surface oxidation, rake slag). However, due to the high temperature, strong activity, opacification of the zinc bath and harsh, complex industrial environment, it is difficult to directly measure the flow behavior of zinc in the zinc bath through conventional methods. In this work, based on the principle of electromagnetic induction, Lorentz force velocimetry (LFV) method was used to measure and analyze the velocity of zinc flow in the bath during the galvanizing process for the first time. The LFV has the characteristics and advantages of non-contact, online and continuous measurement, and can realize the real-time quantitative measurement of molten metal flow by reasonable design and ingenious implementation. The key parameters of LFV, such as the gap between device and molten zinc, penetration depth and geometry of the applied model, were discussed through numerical analysis, the LFV device suitable for the characteristics of zinc plating process was designed, and the in-plant measurement was carried out. The results show that the fluctuation range of zinc flow velocity in the zinc bath is almost 0.13~0.20 m/s, which is within typical range referenced in previous studies. In addition, the flow behavior and flow field characteristics of zinc liquid were analyzed, and these discussions reflect the capacity of zinc slag or ash in the zinc flow at the monitoring position. The work promoted in this study revealed that this LFV method can measure the surface velocity of zinc liquid in real time, on-line and quantitatively, which provides a new way for the velocity monitoring of high temperature liquid metal in metallurgical industry.

ZHENG Jincan, LIU Runcong, WANG Xiaodong. Online Electromagnetic Measurement of Molten Zinc Surface Velocity in Hot Galvanized Process. Acta Metall Sin, 2020, 56(7): 929-936.

Fig.1 Application principle of Lorentz force velocimetry (LFV) (Due to the distribution of magnetic field (B_{0}) in the flowing molten metal, eddy current generated by electromagnetic induction between magnetic field and the molten metal, the Lorentz force (F_{l}) which brakes the molten metal is produced by the induction between the eddy current and the moving liquid (with the velocity of v), and the permanent magnet (PM) is also subjected to a force (F_{m}) in the same magnitude and the opposite direction)

Fig.3 Measurement schematic of molten zinc flow (top view) (I, II, III, IV and V are the test positions; Δx—distance between the two adjacent positions, Δy—distance between the test position and side of the zinc bath; Z+ and Z- are the outward and inward directions of galvanized sheet, respectively)

Fig.4 Schematic of numerical model (The zinc plate (length a, width a and thickness δ) is placed directly below the LFV device with a gap of h, its velocity is v_{s}, and the distance between PM and outer bottom of the LFV device is ζ. P1~P4 represent different center positions of LFV device in numerical simulation)

Fig.5 Effect of δ on calibration results during numerical calculation

Fig.6 Effect of a on calibration results during numerical calculation

Fig.7 Effect of relative position between the LFV device and zinc plate on the results during numerical calculation (The horizontal ordinate x=$\frac{\mathrm{\Delta}y}{\frac{\mathrm{1}}{\mathrm{2}}a}$, the longitudinal coordinates y=$\frac{{f}_{x}}{{f}_{x=\mathrm{6}}}$×100%, in which f_{x} is the Lorentz force with different positions at x=$\frac{\mathrm{\Delta}y}{\frac{\mathrm{1}}{\mathrm{2}}a}$)

Fig.8 Effect of relative position between the LFV device and zinc plate on distribution of induced eddy current density (j) (a) position P1 (b) position P2 (c) position P3 (d) position P4

Fig.9 Numerical calculation data and calibration curve of the relationship between Lorentz force (F_{s}) and v_{s} of zinc plate during numerical calculation

Fig.10 Measurement experiment of molten zinc flow (a) industry measurement positions (I~V represent the center positions of LFV device above the zinc bath respectively, while the device is on the position I in this photo, as shown in Fig.3, Δx=30 mm, Δy=200 mm, and h=50 mm)(b) Lorentz force and velocity of molten zinc in the zinc bath due to the electromagn-etic induction

Fig.11 Flow directions of zinc liquid and the location of the LFV device (α_{II}~α_{Ⅴ} are the angles between the flow direction of zinc liquid and the measurement direction at II~V)

Fig.12 Spectrum analysis of the velocity data of zinc flow in the bath

Fig.13 Turbulence energy spectrum analysis of the velocity data of zinc flow in the bath

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