Studies On Coating Uniformity Improvement Of Continuous Hot Dip Galvanizing Strip

Gas-jet wiping has been the main method to control the final film thickness inmodern continuous hot-dip galvanizing process instead of rolling coating method,with the development of coating technologies. In the process mentioned above, thehigh speed flat gas-jet discharged from the nozzle slot located above the zinc potimpinges on the strip, as the same time, the molten liquid of zinc or Zn-Al alloyadhered on the strip dragged by the moving strip at a velocity from the zinc pot werewiped. The excessive zinc mass were removed and the desired thickness was obtained.The thickness and uniformity of coating is an important index of quality evaluation tohot dip galvanizing strip and the coating quality control has made great progress inrecent years, while a lot of high-end products also rely on import as uneven coatingthickness problem, especially the edge over-coating et al is still the bottlenecks todeveloping galvanizing technology.Taken the gas-jet wiping process as the research object, numerical simulationmethod and experiment were explored to analyze its formation mechanism based onthe edge over coating defect happened in the process mentioned above and thecorresponding measures of improvement were presented. The detail as follows:In the experiment part, the visualized gas flow on the strip and impinging jetpressure distributions along the width of strip was carried out by gas impact plateexperiment. Experimental results show that airflow of strip edge occur lateraldeflection and the wall pressure in the strip edge is lower than those in center whichreducing the wiping ability of edge and the edge over-coating easily happened.In the simulation part, a three-dimensional model for gas-jet wiping process wasconducted through the software of Fluent. The flow characteristics pressure on thestrip was analyzed under different working condition. As a result, the airflow deflectsnear the strip edge and the pressure decreases. The simulation data is well agreed withthe experiment result. An attempt was made to add edge baffle near the edge of stripin order to prevent the non-uniform pressure distribution and the baffle was optimizedwith coating thickness calculation model. The results show that strip edge pressureand coating thickness can be significantly influenced by baffle edge shape andthickness close to the strip edge. The maximum pressure values and the minimumcoating thickness on strip edge which is close to that in center is gotten at baffle edgeangle of80°~90°. Strip edge pressure and coating thickness increases and decreasesrespectively with the increase of baffle thickness and the maximum pressure valuesand the minimum coating thickness on strip edge is obtained when the thickness ofbaffle is2mm. As an overall result, edge over-coating is substantially avoided whenthe baffle thickness is2mm and the baffle edge angle is80°~90°. In addition, theincreased edge openmouthed size of wedge lip air knife was proposed and the edge pressure attenuation can be effectively reduced. The edge coating thinning rateincreases linearly with the linear growth of wedge openmouthed size. The uniformityof the coating is improved when the wedge openmouthed size is from1.18mm to1.22mm which the parallel section is1.10mm.