Galvanized coils often face challenges during processing, including insufficient surface smoothness, uneven roughness, color variations, limited formability, and fluctuations in mechanical properties. Finishing, as a key process, systematically addresses these issues through physical and mechanical means, providing a stable foundation for subsequent processing.
Surface smoothness is the primary challenge in galvanized coil processing. During hot-dip galvanizing, the strip steel is prone to deformation such as waviness and warping due to temperature changes and mechanical stress. Residual zinc dross or particles on the surface further exacerbate unevenness. Finishing, through low-reduction rolling, utilizes the rigid pressure of the rolls to eliminate localized deformation, flattening surface dross particles and partially correcting waviness, ultimately resulting in a smooth and flat sheet. This process not only improves appearance quality but also provides a uniform substrate for subsequent coating and printing processes.
Roughness control directly affects the processing adaptability of galvanized coils. Different applications have different surface roughness requirements: coating processes require moderate roughness to enhance coating adhesion, while deep drawing requires lower roughness to reduce friction. Finishing processes adjust the surface roughness of the work rolls to unify the original unevenness to a target range. For example, using roughened rolls can create micron-level pits on the surface, preserving lubrication channels while avoiding slippage caused by excessive smoothness, thus balancing paintability and formability.
Color uniformity is a crucial indicator of galvanized coil quality. Spangle-free products are prone to color differences due to variations in coating thickness, while spangle-rich products suffer from inconsistent gloss due to crystallization differences. Finishing processes smooth out surface unevenness, making the coating thickness more uniform, while blurring the bumps formed by spangle crystallization, eliminating localized reflective differences. For color-coated substrates, the blurred spangles after finishing can be completely covered by the coating, avoiding appearance defects caused by exposed spangles and improving the finished product's quality.
Formability optimization is one of the core values of finishing processes. Galvanized coils are prone to slip lines (i.e., Lüders lines) during deep drawing and stretching processes, caused by uneven localized plastic deformation due to the yield plateau. Finishing, through micro-cold rolling, eliminates or weakens the yield plateau, expanding the range of plastic deformation and allowing for uniform elongation of the material during processing. For example, finishing galvanized coils used in automotive parts can prevent surface defects generated during stamping, improving yield.
Mechanical property control is an extended function of finishing. By controlling rolling force and reduction rate, finishing can fine-tune the yield strength and tensile strength of galvanized coils, bringing them closer to target values. Simultaneously, the shape stability of the finished sheet is enhanced, reducing subsequent processing deformation caused by residual stress. For example, galvanized coils used in construction can maintain long-term flatness after finishing, avoiding creep caused by gravity or temperature changes.
Finishing also indirectly improves the corrosion resistance of galvanized coils. Uniform surface roughness reduces the retention of moisture and corrosive media, while the blurred zinc spangle structure reduces localized differences in electrochemical corrosion. Furthermore, the finished sheet surface is more conducive to the uniform adhesion of passivation solutions and fingerprint-resistant coatings, forming a dense protective film and extending service life.
Finishing processes address key issues such as flatness, roughness, color, formability, and mechanical properties, giving galvanized coils wider processing adaptability and higher quality stability. Whether in high-end manufacturing industries like automobiles and home appliances, or basic sectors like construction and hardware, finishing processes have become a core procedure for enhancing the added value of galvanized coils.
