The excessive surface roughness of low-carbon steel galvanized coils after hot-dip galvanizing is typically closely related to the condition of raw materials, process parameter control, and equipment condition. Improving surface roughness requires a synergistic approach involving source control, process optimization, and equipment maintenance to form a systematic solution.
The surface condition of the raw material is the primary factor affecting coating roughness. If the low-carbon steel substrate surface has rust, scale, or metal particles embedded during rolling, the zinc-iron alloy will preferentially grow at these defects during hot-dip galvanizing, forming localized protrusions. For example, tiny metal particles remaining on the steel surface can cause abnormal thickening of the zinc layer due to the "point effect," forming a volcano-like granular roughness. Therefore, strict pretreatment of the substrate is necessary before galvanizing, using pickling, sandblasting, or electrolytic cleaning to remove surface impurities and ensure the substrate surface cleanliness meets process requirements. Simultaneously, controlling rolling process parameters avoids periodic indentations caused by roll surface damage, reducing microscopic defects on the substrate surface.
The composition and temperature control of the zinc bath are core aspects of process optimization. Excessive iron content in molten zinc will generate ζ and δ1 phase zinc dross. These particles, suspended in the molten zinc, are easily carried out by the strip steel and embedded in the coating surface, creating roughness. Regular removal of zinc dross and control of the molten zinc temperature are necessary to avoid changes in iron solubility due to temperature fluctuations. For example, excessively high molten zinc temperature accelerates the iron-zinc reaction, increasing the amount of zinc dross generated; excessively low temperature leads to poor fluidity of the molten zinc and uneven coating thickness. Furthermore, adding an appropriate amount of aluminum can form a dense alumina protective film, reducing zinc oxidation and zinc ash production, but the aluminum content must be strictly controlled to avoid the formation of aluminum-iron compounds, which increases coating brittleness. By adjusting the composition and temperature of the molten zinc, the impact of zinc dross particles on coating roughness can be significantly reduced.
The cooling process plays a decisive role in the surface quality of the coating. If the cooling rate is too fast after the galvanized part is removed from the molten zinc, uneven shrinkage of the zinc layer can easily lead to cracks and roughness; if the cooling rate is too slow, the zinc layer will continue to grow, forming coarse grains. For example, using a segmented cooling method, first lowering the coating temperature with air cooling, and then controlling the final cooling rate with water cooling or mist cooling, can prevent micro-cracks or orange-peel-like roughness on the coating surface. Simultaneously, it is essential to ensure the cleanliness of the cooling medium to prevent impurities from adhering to the coating surface and causing secondary contamination.
Air knife control is a key device for adjusting coating thickness. The air knife uses high-pressure gas to blow away excess zinc liquid from the strip surface, forming a uniform coating. If the distance between the air knife and the strip is too large or the angle is incorrect, it will lead to uneven zinc liquid blowing, resulting in locally excessively thick coatings and roughness. The position and angle of the air knife need to be calibrated regularly to ensure that the air knife is parallel to the strip and at an appropriate distance. At the same time, the air knife pressure must be controlled stably to avoid periodic changes in coating thickness caused by pressure fluctuations. For example, excessively high air knife pressure can blow through the coating, creating exposed iron defects, while excessively low pressure can cause coating buildup and roughness.
The shape of the incoming sheet and the control of the rolling force also affect the uniformity of the coating. If the strip steel has defects such as edge waviness, center waviness, or warping, the zinc liquid will flow unevenly on the strip surface during hot-dip galvanizing, easily resulting in localized areas of excessively thick or thin coatings. The strip steel needs to be straightened using a finishing mill or tension leveler to improve its shape quality. Simultaneously, the uniformity of rolling force must be controlled to avoid strip thickness deviations caused by rolling force fluctuations, which in turn affect the uniformity of coating thickness. For example, excessive rolling force may cause edge stretching deformation of the strip steel, resulting in edge thickening.
Equipment maintenance and adherence to process discipline are fundamental to ensuring surface quality. The stability of the zinc pot mechanical system must be checked regularly to prevent strip steel vibration in the zinc liquid from causing coating thickness fluctuations. At the same time, the cleaning equipment must be maintained rigorously to ensure effective cleaning of the strip steel surface and prevent residual acid or impurities from affecting coating adhesion. Furthermore, process parameter control standards must be strictly followed to prevent operators from arbitrarily adjusting key parameters such as zinc liquid temperature and air knife pressure, ensuring the stability of the production process.
Improving the surface roughness of low-carbon steel galvanized coils after hot-dip galvanizing requires a multi-pronged approach, including raw material pretreatment, zinc bath composition control, cooling process optimization, precise air knife adjustment, plate shape correction, equipment maintenance, and strict adherence to process discipline. Through systematic process control and equipment maintenance, the surface quality of the coating can be significantly improved, meeting the dual requirements of high-end applications for both product appearance and performance.
