What Is a Dip Tube?

Hot-dip galvanized.

Hot-dip plating originated from the ancient Roman era. It started with hot-dip tin plating on iron plates. The United Kingdom and France were the first countries to use this method to produce tin-plated plates. In 1742, the French chemist Melouin first plated molten zinc on steel products. In 1836 French Sorel applied hot-dip plating to production. In 1837, the British Grawford invented the currently widely used solvent hot dip plating process. In 1931, Sendzimir was granted a patent for the "Sendimir" method for the production of continuous galvanized steel by a protective gas reduction method. In 1937, the first continuous hot-dip galvanizing production line was completed in the United States, laying the foundation for the production of modern galvanized steel. In the 1960s, the hot-dip galvanizing process has developed rapidly. Its signs are: the pre-plating treatment was changed from an oxidation furnace to a non-oxidation heating furnace, the thickness of the coating was changed from roll plating to air knife, and the post-plating treatment was changed from bending The stretch straightening machine replaces the multi-roller repeated bending straightening machine. In the 1980s, a series of reforms allowed hot-dip galvanizing to develop rapidly again. ^[1]

The hot-dip galvanizing process generally includes three parts: pretreatment, hot-dip galvanization, and post-treatment of plated parts. According to different pretreatment methods, hot-dip galvanizing can be divided into solvent method and protective gas reduction method.

1. Solvent method

Solvent hot-dip galvanizing is suitable for hot-dip galvanizing of steel strip, steel plate, steel wire, steel pipe, section steel and components. The main features of this process are: before the steel parts are immersed in the zinc solution, the surface must be degreased to remove the oil on the surface of the steel parts, pickled to remove the oxide scale on the surface of the steel parts, and the surface of the steel parts must be coated with a plating agent to protect the surface Contact with the atmosphere to avoid re-oxidation, and then immerse the zinc solution by drying or not drying the surface of the steel to complete the dipping process. The surface flux enters the molten zinc-covered molten zinc for hot-dip galvanizing without drying. This method is called wet hot-dip galvanizing. This process has been gradually phased out because aluminum alloy is not added to the zinc solution, the produced alloy layer is thick and the adhesion is not good, and there are quality problems. Conversely, the steel coated with flux on the surface is sent to a drying furnace (drying temperature is about 250 ) to dry the flux, and then into the zinc liquid for hot-dip galvanizing is dry hot-dip galvanizing. Currently, the hot-dip galvanizing by the flux method is basically a dry hot-dip galvanizing method. The content of aluminum in dry hot-dip galvanizing is also relatively low (usually 0.1% by weight), so the hot-dip galvanized layer produced by solvent hot-dip galvanizing has poor adhesion.

Solvent method for continuous galvanizing of steel strip is a well-known Huilin method. This process requires annealing outside the cold-rolled steel strip line, and then the steel strip enters the production line for multiple pre-treatment steps such as alkali washing, acid washing, water washing, flux coating, and drying. This process is more complicated, and the production cost is higher. The product variety is single, and only thin-gauge steel strips can be produced. The product often has flux defects caused by the decrease in the bonding force between the coating and the substrate caused by the reaction between the flux and the aluminum of the zinc solution. . Its products are mainly used in construction and container manufacturing industries where the quality of galvanized sheet is not high. Before the steel strip enters the zinc liquid, the temperature is about 250 ° C. Therefore, the steel strip must absorb a large amount of heat in the zinc liquid and increase its temperature so as to balance with the temperature of the zinc liquid. This requires a large amount of induction heating. The zinc pot provides sufficient heat for the steel strip, and at the same time increases the dip plating time (about 20s). Compared with the reduction method, the hot dip coating time (3 to 5s) is greatly extended. Therefore, the thickness of the solvent method alloy layer is higher than that of the reduction method. Thickness, resulting in reduced adhesion. This process has been around for many years. Although the investment cost is low, the environment is polluted.

2.Protective gas reduction method

The protective gas reduction method, also known as the Sengimir method, is a method commonly used in modern continuous galvanizing production lines and represents the modern hot-dip galvanizing process. The reduction method hot-dip process uses cold-rolled steel strips and hot-rolled steel strips as hot-dip galvanized raw plates, and performs protective gas recrystallization annealing in the operation line.

The protective gas reduction method is characterized in that steel heat treatment (continuous annealing) and hot-dip galvanizing are connected on the same production line, and a series of operation steps such as alkali washing, acid washing, water washing and flux treatment in the flux method are eliminated. In this method, the steel is first passed through a micro-oxidation furnace that is directly heated by gas or natural gas. At this time, the flame burns off residual oil and emulsion on the surface of the steel, and at the same time, the surface of the steel is oxidized to form a sky-blue oxide film. The steel then enters the reduction furnace. The reduction furnace is hermetically sealed, and a protective gas made by mixing hydrogen and nitrogen is passed in the furnace. The reduction furnace uses radiant tubes for indirect heating or resistance heating. The radiant tube can burn gas, natural gas or heavy oil. The main role of the reduction furnace is twofold: a. Reducing the oxide scale on the steel surface to active sponge iron suitable for hot-dip galvanizing; b. Continuing to heat the steel heated to a certain temperature by the micro-oxidation furnace to complete recrystallization annealing. The steel processed by the reduction furnace is then cooled to a suitable temperature in a protective atmosphere and then enters a hot-dip galvanizing pot. ^[1]