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Continuous hot-dip galvanizing surface control technology of foreign cold-rolled strip steel

    In recent years, developed countries have made a series of technological advancements in the production technology of hot-dip galvanized sheets for automobiles. The chemical composition of the steel matrix, the rolling quality of the steel sheet surface, the uniformity of the thickness of the hot-dip galvanized coating, the optimal phase structure and the alloying A series of technical problems such as process control have been basically solved. At present, a large number of hot-dip galvanized sheets are used as interior and exterior panels of automobiles.
    Cleaning usually includes lye brushing and electrolytic degreasing. It can remove impurities such as iron filings and oil stains on the surface of cold-rolled steel sheets, which is essential for the production of galvanized products with high surface quality and high coating adhesion requirements. The cleaning section of the existing new hot-dip galvanizing unit includes lye dipping, lye scrubbing, electrolytic cleaning, hot water scrubbing, hot water rinsing, cleaning squeezing and cleaning and drying, which can remove the residual oil and carbon residue on the surface of the strip. The amount is controlled in a small range to meet the cleaning quality requirements of hot-dip galvanized automobile outer panels. In addition, a brush roller with Si02 abrasive particles is also used to grind the surface of the strip steel during cleaning, which can enhance the adhesion of the hot-dip galvanized alloyed (GA) product coating. In order to further ensure the surface cleanliness of the strip steel, South Korea’s Pohang Steelers No. 4 hot-dip galvanizing line adopts a dual cleaning system. In addition to the normal pre-cleaning before the inlet looper, after the strip comes out of the inlet looper, it will be electrolytically cleaned again. The cleaning section is usually arranged before the inlet looper, and can also be arranged after the inlet looper to achieve constant speed cleaning.
    Continuous hot-dip galvanizing of strip steel adopts online continuous annealing. Annealing process is the key to the production of high-strength steel hot-dip galvanized products such as dual-phase steel. It has a great influence on the enrichment of Mn, Si and other elements on the surface of the steel sheet, which in turn will affect the coatability and surface quality of high-strength steel . The annealing process of galvanized steel sheet usually includes preheating, heating, soaking and cooling. The use of heating exhaust gas to preheat the hot strip can not only save fuel, but also avoid the deformation of the strip caused by the rapid heating speed of the strip, which is very beneficial to the plate shape of thin-gauge products. The cooling process of some units adopts the sub-cooling method of slow cooling first and then fast cooling, which can not only avoid the poor plate shape caused by the excessive cooling speed, but also meet the cooling speed requirements of high-strength steel such as duplex steel. Some steel mills also set up an equalization section after the cooling section. The purpose is to provide a buffer area between the galvanizing and cooling sections, so that the strip steel has a better plate shape and a more uniform temperature into the galvanizing pot to increase Galvanizing quality.
    The surface roughness of the strip steel is mainly obtained by controlling the surface roughness, rolling load and rolling tension of the roll in the smoothing process and smoothing the hot-dip galvanized sheet. The leveling process parameters mainly include rolling force, roll bending force, leveling entrance tension and leveling exit tension. In terms of improving the shape of the plate and improving the surface quality of the strip, the four-roller is significantly better than the two-roller. At present, Japan has developed a four-roll leveler with work rolls that can be replaced with different roll diameters, which can better control the shape and surface roughness of hot-dip galvanized products of different specifications and different steel types. Zinc sticking on the smoothing roll has always been a problem in the production of high-quality galvanized products. At present, a more effective solution is to add a cleaning agent to the smoothing demineralized water. In terms of roll surface texturing, shot peening texturing and electric spark texturing are the most widely used. Laser texturing is rarely used due to its long cycle and high cost.