Hot Galvanizing Process Technology Innovation

   In recent years, hot dip galvanizing has made significant progress in the process, equipment and product development, and the U.S. steel joint method has become the mainstream in the process. Strip continuous hot dip galvanizing production line is developing towards high speed, high output and wide and thick specifications. In terms of products, it mainly develops towards high-corrosion-resistant coated plates, high-strength galvanized plates, and environmentally friendly zinc-free steel plates.
  Innovation of hot galvanizing equipment technology
  Introduction of vertical annealing furnace. The U.S. Steel Union method replaced the modified Sendzimir method and introduced a vertical annealing furnace. The biggest characteristic of the vertical annealing furnace is: it occupies less space, which can greatly shorten the length of the hot-dip galvanizing unit; good temperature control, long running distance of the strip in the furnace, large annealing temperature control adjustment margin, continuous annealing effect is good, can be produced Extra deep drawing, high strength hot galvanized products.
  Vertical annealing furnace is composed of heating section, soaking section, heat preservation section and cooling section. The preheating section does not adopt the method of directly preheating hot steel with combustion gas, but uses the combustion exhaust gas from the heating section to heat the protective gas through the heat exchanger. Then it is drawn out by the circulating fan, and the hot protective gas is forced to be blown to the surface of the strip steel through the gas spray blow box with spray holes. The shielding gas whose temperature becomes lower is drawn out from the side slot of the wind box by a fan, then heated by a heat exchanger, and the strip steel is cyclically sprayed to preheat the strip steel to about 150℃-200℃. This method not only saves fuel by about 7%-8%, but also reduces the thermal shock to the first turning roller of the heating section. It can avoid the deformation of the steel strip caused by the rapid heating of the steel strip while extending the life of the roller. It is very beneficial to the shape of thin products. The heating section mainly adopts the indirect heating method of W-shaped radiant tube, which is beneficial to the cleaning of the strip surface. The radiant tubes are staggered on both sides of the strip to ensure that the strip is heated evenly. The main burner of the radiant tube is a drum-pump type, which has precise combustion control and high combustion efficiency, and is equipped with a heat exchanger, which can preheat the combustion air to about 500°C. This burner can not only reduce fuel consumption, but also reduce NOx emissions. In order to reduce the direct heat radiation from the radiant tube to the furnace rolls and reduce the hot buckling of the steel strip, this section is equipped with thermal insulation panels, and some manufacturers are also equipped with a roll crown control system at the top sets of furnace top rolls in this section. The maximum furnace temperature in this section is about 960℃. When using radiant tube heating, usually the heating section and the soaking section are integrated, and there is no longer a partition wall in the middle. There is no strict limit for heating and soaking except for the number of radiant tubes. The cooling section is a rapid cooling chamber. In order to obtain a higher cooling rate, high-concentration hydrogen can be used as the cooling medium, combined with optimized high-speed jet cooling, the maximum cooling rate can reach 130-160℃/s.
   Galvanizing equipment is constantly improved. Galvanizing equipment includes air knife, sinking roller, stabilizer roller and auxiliary equipment, zinc pot and zinc adding equipment, etc. Air knife manufacturers continue to improve the air knife, forming Fang Deng, VAI, Duma, Kohler and other famous professional manufacturers. Each air knife has its own characteristics. The general direction is to automatically track the steel strip to maintain a stable distance between the air knife and the steel strip; the air knife can be assembled and adjusted as a whole with an accurate positioning mechanism; the nozzle area can achieve dynamic control of constant pressure; Double-lip rotary air knife; better lip clearance curve; setting remote control pneumatic quick lip cleaning device; setting air knife horizontal fast, slow opening and accurate reset system; using air knife nitrogen blowing technology to produce hot-dip galvanizing for automobiles board. Most of the zinc pots use molten ditch ceramic zinc pots. The volume of the zinc pot tends to increase, which is conducive to the stability of the zinc solution. Usually the lower edge of the sinking roller is at least 1800mm away from the bottom of the pot. , More than 320-350 tons, and set two sets of automatic zinc ingot device to ensure that the zinc level of the zinc pot is stable.
  Zinc layer annealing furnace heating method. The zinc layer is annealed so that the zinc layer on the surface of the strip contains 8%-10% zinc-iron thickened alloy layer. A zinc layer annealing furnace with a zinc layer alloying device is provided above the zinc pot. There are heating section, heat preservation section and cooling section in the annealing furnace. In order to alloy the iron element in the galvanizing of the steel strip, the steel strip must be heated to 500-600℃ and kept warm for a period of time to obtain a better alloying effect. From the perspective of the zinc layer annealing process, the shorter the heating time, the more favorable the alloying process. The direct heating of gas is limited by the structure of the equipment itself. The heating time cannot be very short. At the same time, the flame directly sprays the surface of the strip steel, which is detrimental to the quality of the coating surface. The induction heating method can complete the heating process in 1 second or less, which is not only beneficial to the zinc layer annealing process, but also can reduce the height of the overall equipment and even the workshop, and the induction heating control is convenient and the response is fast. Therefore, the heating method has evolved from direct gas heating to induction heating.
  Perfect finisher and tension leveler. The finisher can obtain good strip straightness by controlling the rolling force or elongation. The finishing machine has developed from two rolls to four rolls, and two dry and wet finishing processes are provided. Wet finishing of the strip steel is beneficial to improve the surface quality and work roll life. In order to prevent the problem of zinc sticking to the rollers in the polishing machine area, the polishing machine is equipped with a high-pressure water cleaning system for the cleaning of work rolls and support rolls. The maximum pressure of the cleaning water is 140 bar. Wet finishing uses two kinds of wet finishing agents, desalinated water and finishing solution. When producing alloyed products, desalinated water is usually used for wet smoothing. When producing high roughness and requiring pure zinc plate, special finishing liquid is used as the wet finishing medium. The finishing machine adopts large and small diameter work rolls, which are adapted to the special needs of mild steel and hard steel. The roller surface of the work roll is textured to meet the roughness requirements of the strip surface of the automobile plate or high-end home appliance plate. To produce 800MPa high-strength hot-dip galvanized sheet, the finishing machine can choose a maximum rolling force of 1,000 tons and a maximum elongation of 2%.
   Chromium-free passivation post-treatment technology. According to different needs of users, a variety of post-treatment methods are used. The most widely used before 2006 is passivation with hexavalent chromate, but because it is a highly toxic substance, the European Parliament and the Council issued the ROHS directive in 2003. From 2006 From July 1st, it is prohibited to use in electrical and electronic equipment products newly put on the market. In order to meet the demand for environmentally friendly hot-dip galvanized surface-treated steel plates, the development and production of chromium-free hot-dip galvanized passivation-coated steel plates have gradually received attention. In the process of searching for chromate substitutes, there are more foreign studies on molybdate, tungstate, silicate, titanium salt, zirconium-containing solution, cobalt-containing solution, rare earth metal salt solution, Cr3+ solution, phosphate Passivation and other inorganic salt passivation, there are also tannic acid, phytic acid, acrylic acid, epoxy resin, organic silane and other organic passivation.
  Advanced cooling technology. In order for the hot dip galvanizing unit to produce baked hardened steel, dual-phase steel and other products, the unit needs to have high-speed cooling capability, and the cooling rate must be 50°C/s or higher. The cooling technology used in the hot dip galvanizing annealing furnace is mainly jet cooling and high-speed jet cooling. High-speed jet cooling is to spray the protective gas in the furnace to the surface of the steel strip at a high speed to achieve the purpose of quickly cooling the steel strip in the cooling section and reducing the temperature difference of the steel strip surface. Because jet cooling has the characteristics of self-stabilizing convergence, that is, if the temperature of the strip is uneven, the temperature difference between the high temperature part and the gas is large, and the cooling is fast; otherwise, the temperature difference between the low strip temperature part and the gas is small, and the cooling is slow, so that the final To achieve the purpose of uniform temperature across the board. The high-speed jet cooling developed by Nippon Steel & Sumitomo Corporation of Japan is divided into 5 sections in the strip width direction, and 5 baffles are provided, which cooperate with the scanning thermometer on the exit side to control the temperature of the strip horizontal plate to make it uniform. JFE developed high-speed jet cooling combined with roll cooling technology to form a high-speed jet cooling plus roll cold composite technology (RGCC). The main difference between Nippon Steel’s high-speed jet cooling technology and JFE is the nozzle. Nippon Steel uses cylindrical nozzles, JFE uses narrow-slot nozzles, and adopts countermeasures such as lateral movement of the nozzle to effectively reduce gas recirculation to ensure uniform cooling in the width direction of the strip. Compared with the narrow slit nozzle of JFE, the cylindrical nozzle achieves the same cooling capacity and requires less energy. Compared with other cooling methods, the product produced by high-speed jet cooling technology has fewer defects, stronger shape control ability, and the initial temperature of cooling can be higher, about 675 ℃, which is conducive to improving the performance of the steel plate and is more suitable for the high-speed and stable operation of the thin plate.