Hot-dip galvanized strip steel is a high-performance, high-value-added product produced by cold rolling-annealing-galvanizing with hot-rolled coils as raw materials. Hot-dip galvanized products play an immeasurable and irreplaceable role in reducing corrosion and prolonging the life of steel, saving energy and materials, and are currently widely used in the automotive and home appliance industries.
First, color difference defects
Color difference defects are a general term for a type of defect on the surface of galvanized strip steel, which directly affects the appearance and coating effect of galvanized strip steel. More seriously, they will also hurt the mechanical properties of the strip. From an optical point of view, the color difference defect is due to the abnormal surface roughness of the defect position, resulting in a difference in the reflectivity of the defect area to light. According to the current research status analysis at home and abroad, the factors related to the color difference defects of strip steel are mainly: uneven lateral roughness of the strip steel surface, foreign matter on the zinc layer surface, uneven grain size of the zinc layer, etc.
Second, the edge color difference
The macroscopic morphology of edge color difference defect, the color difference of the galvanized sheet roll edge is located about 5cm from the edge of the strip, and it exists continuously throughout the roll, which is slightly brighter than the normal plate surface position. The handheld roughness meter was used to detect that the roughness Ra of the edge color difference position was 0.56μm, and the roughness Ra of the normal plate surface position was 0.72μm. The color difference of the cold-hardened plate roll edge is also located about 5cm from the edge of the strip, and it exists continuously throughout the roll, which is slightly whiter than the normal plate surface position. The color difference defect cannot be observed by the naked eye due to the thick surface iron oxide scale of the hot-rolled coil. Due to the edge color difference, the defect position of the raw material roll and the finished product roll has a strong correspondence, and it is suspected that the color difference of the galvanized sheet roll edge is caused by the raw material.
Subsequently, the galvanized finished coil with edge color difference and the raw material coil of the same batch were sampled to observe the microscopic morphology of their surface and cross-section. This is the cross-sectional microscopic morphology of the color difference position and the normal position of the galvanized roll edge under an electron microscope. It can be seen that at the edge color difference position, the surface of the galvanized substrate is relatively flat, and the zinc layer thickness is about 7-10μm; at the non-color difference position, the surface of the galvanized substrate has a height fluctuation of 3-4μm, and the zinc layer thickness is about 10-12μm. The zinc layer thickness at the color difference position of the galvanized sheet curling edge is thin and the roughness is low. This is due to the low roughness of the galvanized raw material cold hard coil within 5cm from the edge. During the cooling process of the hot rolling process, the cooling speed of the strip edge is relatively fast, resulting in fine grains, high hardness and strength, and a thin and dense iron oxide scale at the edge of the strip. After pickling, the hot rolled coil with such characteristics is easy to remain with a dense iron oxide scale at the edge. After cold continuous rolling, due to the fine grain strengthening effect of the edge and the lubrication effect of the iron oxide scale, the roughness of the cold hard coil edge is low.
Based on this, corresponding improvement measures are formulated. In the actual production process of hot rolled coils, the uniformity of the lateral temperature is mainly controlled by adjusting the water distribution of the upper header to form a convex water flow distribution form. To control the color difference defects on the edges of galvanized finished products, the edge shielding method on the cooling device was adopted, and different shielding strategies were adopted according to different strip widths, thicknesses, and steel types, which improved the unevenness of the lateral temperature, organization, and stress distribution during the cooling process of hot-rolled coils.
Third, the color difference after grinding
The color difference defect on the surface of the hot-dip galvanized strip after sanding with a sand disc is macroscopic, dark black, and round; the macroscopic morphology after sanding with a sand disc at the position without a color difference. The strips were at various positions after the zinc pot was polished, and there were color difference defects after polishing, indicating that the defect was generated at the position of the zinc pot air knife. Under the same air knife process parameters, the jitter of the strip was observed: when the strip running speed was 60m/min, the strip ran smoothly, basically without jitter, the surface was consistent after polishing, and the color difference defect was not obvious; when the strip running speed was 100m/min, the strip had a slight shake, and the surface color difference defect was obvious after polishing. This shows that the color difference after polishing is highly correlated with the production line running speed and the jitter amplitude of the strip after leaving the zinc pot. During the shaking process, the distance between the upper and lower air knives of the strip will change. Under the same air pressure conditions, the change in the distance between the air knives will cause the thickness of the zinc layer to change. The main reasons for the shaking of the strip after leaving the zinc pot are: (1) poor installation accuracy of the sinking roller, or serious wear of the shaft head and sleeve of the sinking roller after use; (2) poor incoming material pattern; (3) inconsistent blowing pressure on the upper and lower surfaces of the quick cooling wind box in the post-galvanizing cooling section. The following measures are formulated for the hot-dip galvanizing process to control the shaking of the strip: (1) After the sinking roller is processed and before installation, check its surface roughness and whether the groove has chamfers, burrs, scratches, and whether it will affect the installation accuracy. When installing the sinking roller, check whether the center line of the strip between the sinking roller and the top roller of the cooling tower is consistent; (2) Adjust the air volume on the upper and lower surfaces of the moving wind box in the post-galvanizing cooling section to make the baffle opening and pressure of the wind box on the upper and lower surfaces consistent.
Fourth, color difference after stamping
The color difference defect after stamping appears on the stamping parts of hot-dip galvanized steel sheets, showing irregular flakes. The color difference defect after stamping is caused by the damage of the zinc layer on the surface of the galvanized steel sheet during the stamping process, resulting in cracks, internal oxidation of the zinc layer, and color difference defects on the surface of the stamped parts. There are many factors affecting the surface damage of galvanized steel sheet stamping, such as the galvanizing process of the steel sheet, the material of the mold, and the lubrication effect. Measures such as surface strengthening and lubrication of the stamping mold can be taken to prevent the occurrence of surface damage. The main measures are (1) nitriding treatment of the stamping mold surface; and (2) Through tensile extrusion tests, the effects of different lubricants are studied. The study shows that the surface film generated by the lubricant on the friction pair surface can reduce the surface damage of the stamped parts.
Conclusion
(1) The color difference of the galvanized sheet curling edge is caused by the low roughness of the galvanized raw material cold hard roll within 5 cm from the edge. During the cooling process of the hot rolling process, the edge of the strip cools quickly, the grains are fine, the hardness and strength are high, and the iron oxide scale is thin and dense. After pickling, dense iron oxide scale remains on the edge. During the cold rolling process, due to the fine-grain strengthening effect of the edge and the lubrication effect of the iron oxide scale, the roughness of the cold hard coil edge is relatively low. To control the color difference defects of the edge of the galvanized finished product, the edge shielding method on the cooling device is adopted to improve the unevenness of the lateral temperature, structure, and stress distribution during the cooling process of the hot rolled coil, so that the overall surface quality of the cold hard coil and the galvanized coil has been significantly improved.
(2) The color difference defects of the galvanized strip surface after sanding are highly correlated with the production line running speed and the vibration amplitude of the strip after leaving the zinc pot. During the vibration of the strip, the distance between the upper and lower air knives of the strip will change. Under the same air pressure conditions, the change in the air knife distance will cause a change in the thickness of the zinc layer. Therefore, it is necessary to control the vibration of the strip, improve the installation accuracy of the sinking roller, adjust the air volume on the upper and lower surfaces of the moving bellows in the cooling section after plating, and make the baffle opening and pressure of the upper and lower surface bellows consistent.
(3) After stamping, color difference defects appear on stamping parts made of hot-dip galvanized steel sheets. This is because the zinc layer on the surface of the galvanized steel sheet is damaged during the stamping process, resulting in cracks, which causes internal oxidation of the zinc layer, resulting in color difference defects on the surface of the stamping parts. Therefore, it is necessary to perform nitriding treatment on the surface of the stamping die; lubricants are added during the stamping process. The surface film formed by the lubricant on the friction pair surface can reduce the surface damage of the stamping parts.
Post time: Aug-26-2024