Hot-dip galvanized steel varieties, uses, production technology development process, defects. Everything you need is ready for you!

In recent years, because hot-dip galvanized steel plate has good corrosion resistance, good appearance, conducive to follow-up processing, can reduce costs and reduce environmental pollution and other advantages are welcomed by the majority of users. This paper will give you a comprehensive understanding of hot-dip galvanized steel from the aspects of variety, use, development process of production technology and defects.

Hot dip galvanizing is a kind of technology to dip steel, stainless steel, cast iron and other metals into molten liquid metal or alloy to obtain coating. Hot dip galvanizing is the most widely used steel surface treatment method with the best ratio of performance to price in the world. Hot-dip galvanized products play an inestimable and irreplaceable role in reducing corrosion and prolonging life and saving energy and materials of iron and steel. at the same time, coated steel is also a high value-added short-term product that the country supports and gives priority to the development of high value-added short-term products.

At present, China's hot-dip galvanizing industry has entered a new stage of high-speed and sustainable development.

Varieties and uses of hot-dip galvanized steel

Common varieties of hot-dip galvanized steel include galvanized steel sheet and strip, galvanized steel wire and wire mesh, galvanized steel pipe, galvanized steel and structural parts, galvanized pipe fittings (malleable cast iron fittings), galvanized parts, fasteners, galvanized device products, etc. The common coatings are pure zinc coating, zinc aluminum alloy coating (zinc aluminum rare earth, zinc aluminum magnesium, zinc aluminum lead), zinc nickel alloy coating, aluminum zinc alloy coating (aluminum zinc silicon) and so on.

Galvanized products are widely used in construction, communications, electric power, transportation, energy, automobile, light industry (household appliances), agriculture, animal husbandry, fishery, petrochemical and other industries. Their main uses are as follows:

(1) Light industry and household electrical appliances industries: air conditioners, television sets, washing machines, refrigerators, microwave ovens, water heaters and other household appliances and bottom plates, computer bottom plates and shells, civilian chimneys, kitchen utensils, buckets, containers, etc.

(2) Electric power and communication industry: electric power transmission and transformation tower, communication tower, cable armour tape, galvanized low carbon steel wire for communication, galvanized steel wire for steel core aluminum hinge wire, hanger, suspension and fastening galvanized steel strand. Pole crossbar, cable and fastener, all kinds of cable tray.

(3) Construction industry: all kinds of light steel keel for industrial and civil construction, building layer panel, corrugated board, roof grille, galvanized pipe for ventilation, water supply and drainage.

(4) Automobile industry: car body, door, bottom plate, inner plate and so on.

(5) Transportation industry: highway guardrail, highway isolation network, light poles for roads and highways, various indication signs, galvanized structural parts for bridges.

(6) Metallurgical and chemical industries: all kinds of low pressure fluid transportation galvanized pipes, structural frames, all kinds of containers.

(7) Agriculture: light steel keel for sprinkler irrigation pipe, plastic shed, granary, storage and transportation container, agricultural tools, galvanized iron wire for binding.

(8) Fisheries and animal husbandry: wire rope for fishery and net fence for animal husbandry.

The development of continuous hot-dip galvanized steel plate in China began in the 1970s. In 1979, WISCO introduced the first modern continuous hot-dip galvanizing production line and color coated steel plate production line from abroad, marking the beginning of modern steel strip galvanizing technology in China.

Production flow chart of galvanized steel sheet

Due to the strong economic strength and strong technical force of state-owned enterprises, most of the equipment is imported from abroad, the output of units is high, the product positioning is high, and the sales market is mainly for cars and high-grade household appliances, which has developed to more than 70 by 2015. After 2000, the rise of private enterprises, mainly with steel dealers as the backbone, they are familiar with the market, good management, the use of hot-dip galvanizing unit equipment completely localization, low unit production, low investment, low production cost, high efficiency, Mainly for the low-grade building materials market.

By 2008, the production capacity of hot-dip galvanizing steel plate in China has reached the level of developed countries in Europe and America. According to the investigation of relevant departments, the galvanized steel production lines in China are mainly concentrated in the developed areas such as the Yangtze River Delta, the Pearl River Delta and the Bohai Rim Economic Zone. However, affected by the relevant national policies and market situation, on the one hand, the original production line will appear a certain degree of concentration and integration, on the other hand, the focus of the new production line will gradually shift to the central and western regions.

With the development of China's economy, construction, automobile, home appliances and other industries have developed rapidly, and have become the pillar industries of the national economy. The large demand for galvanized steel in construction, automobile, household appliances and other industries has greatly promoted the market development and technological progress of continuous galvanized steel in China.

Application example of Hot galvanized Steel sheet

Hot-dip galvanized strip is used in construction, home appliances, automobile, light industry and other industries, including construction, automobile and household appliances. At present, China not only has the largest construction market in the world, but also the largest manufacturer of automobile and household appliances in the world.

In recent years, although the output, product quality, technology and equipment level of galvanized sheet in China have been greatly improved, there are still the following four problems:

The main results are as follows: 1. The difference of equipment level is large, and there are many low-capacity units.

2. Excess production capacity, weakening downstream demand, insufficient overall operation rate of galvanized sheet and strip enterprises, and serious waste of high configuration units;

3. Private enterprises do not produce according to the standard, and the thickness of the coating is on the low side, resulting in a waste of resources. At the same time, the enterprise environmental protection consciousness is weak, the enterprise environmental protection management is not standard;

4. For high-quality automotive plates, functional household panels and high corrosion resistance building coated steel plates also rely on imports, domestic high-end brands are scarce.

Application Field and galvanizing requirements of galvanized products

Zinc consumption in iron and steel industry is mainly used as galvanized steel products, including galvanized strip, galvanized steel wire rope strand, galvanized welded pipe and galvanized steel structure. The thickness of galvanized products and the thickness of zinc layer will have a great influence on the zinc consumption per ton of steel, and the zinc consumption per ton of galvanized products varies greatly. With the development of industry specifications, the vicious competition phenomenon of reducing the cost of hot-dip galvanized sheet with thin galvanized coating will be gradually reduced.

For zinc demand for galvanized products, the relevant authorities believe that in the future, with the standardization of the galvanizing industry, the total demand for zinc in the iron and steel industry will still increase slightly, such as accelerating with the popularization of steel structure buildings. The amount of galvanized sheet for thick zinc layer building will be increased. Galvanized steel in some middle and high-end areas will gradually replace uncoated steel, zinc consumption in the galvanizing industry will still rise slightly.

Main downstream consumption industries of galvanized products

Development of production Technology of Hot-dip galvanized sheet

The development of production technology of hot-dip galvanized steel plate is as follows:

Development of Unit to High yield Energy

Since the 1990s, the hot-dip galvanizing units have developed to high-yield. There are more than 60 sets of hot-dip galvanizing units with a global production capacity of more than 300000 t / a, with a total production capacity of more than 28 million t / a.

Specialized production of unit

Different types of units are used to produce automobile galvanized sheet and building material galvanized sheet. The unit process, single equipment configuration and furnace selection are more reasonable, so that the investment effect is obvious and the product cost is low.

Emphasis on substrate surface cleaning

In recent years, the automobile plate galvanizing unit basically adopts the American steel union process of alkali electrolytic cleaning and all-radiation tube heating furnace, and even sets up the cleaning section before and after the looper device. In order to reduce the investment, the improved Senjimir process is often used in the building material galvanizing unit, but the cleaning section is set up to make the substrate surface cleaner and the product quality better.

Tower furnace

The investment of tower furnace is about 25% ~ 30% higher than that of horizontal furnace. In the past, when the production capacity of unit is more than 300000 t / a, tower furnace is often used due to the limitation of furnace length, otherwise horizontal furnace is often used.

After the 1990s, the factory pays more attention to the advantages of tower furnace: good gas tightness of furnace, small number of furnace rollers, long service life, low maintenance cost, improvement of plate shape, shortening of furnace length and so on. Many galvanizing units with a production capacity of 250000 t / a or less also use tower furnaces. For example, the hot-dip galvanizing unit with a production capacity of 250000 t / a built by Sollak Iron and Steel Company in France in December 1990, the hot-dip galvanizing unit with a capacity of 250000 t / a built in September 1991 at the Didlang Steel Plant in Luxembourg, and the BHP aluminizing and galvanizing unit in California, USA. Several sets of new hot-dip galvanizing units in China have also adopted tower furnace.

Continuously improving the efficiency of reheating furnace

The heating waste gas is used to preheat the strip to save energy and avoid strip deformation, the continuous furnace sets up different forms of cooling section according to the product performance requirements, and sets up the hot tension adjustment roll to optimize the tension in the strip furnace. The heating section is provided with a high precision anti-heat warping roller, and the heating section is equipped with an induction heater. The heating technology such as drum-pump pulse control burner and multi-stage combustion radiation tube is adopted, which improves the adjusting range of burner and the life of radiation tube, is beneficial to the uniformity of heating, reduces the content of NOx and is beneficial to environmental protection. The design of furnace roll profile and the surface spraying technology of furnace roll are improved.

Continuous improvement of galvanizing equipment

Galvanizing equipment includes air knife, sunk roll, stabilization roll and auxiliary equipment, zinc pot and zinc adding equipment, etc. Over the years, air knife manufacturers have been committed to improving air knives. The air knives produced by several famous professional manufacturers, such as Fangdeng, Austrian Steel Union, Duma and Kohler, have their own characteristics, and the general direction is to enable the air knife to track strip steel automatically. And maintain a stable distance from the strip; The air knife can be assembled and adjusted as a whole, has an accurate positioning mechanism, can realize the dynamic control of the pressure stability in the nozzle area, adopts the double-lip rotary air knife and the better lip clearance curve, and sets up a fast lip cleaning device. Setting up the air knife horizontal direction fast, slow open and accurate reset system, so that the air knife has been greatly improved. The shape, spacing and bearing of the zinc slot on the surface of the sunken roll have also been improved, and the stability and service life have been improved. The zinc pot mostly adopts the melting ditch type ceramic zinc pot, the volume of the zinc pot tends to increase, in order to facilitate the stability of the zinc liquid, the lower edge of the sunken roll of the modern large unit is usually at least 800mm from the bottom of the pot. On the other hand, the volume of the zinc pot of the hot-dip galvanizing unit for the production of automobile plate should be larger. The development of coreless zinc pot is also more mature. SMS also developed unsunk roll zinc pot. According to the principle of magnetohydrodynamic closure formed by electromagnetic moving field, a vertical zinc pot was designed, and the strip passed vertically through the zinc liquid in the zinc pot. There are no corrosive parts in the zinc pot. The technology has completed the industrial pilot phase and has received orders from eastern South Korea to make steel, attracting the attention of people in the industry.

Better leveling effect

The four-roll finishing machine is equipped with two kinds of dry and wet leveling processes, large and small diameter work rolls, and the work roll surface is treated by shot peening or electron beam texturing to meet the different requirements of extension and surface finish. For different kinds of products, good results can be obtained by using different cooperation methods between finishing machine and drawing and straightening machine.

Perfect post-treatment process

According to the different needs of users, passivation treatment, phosphating treatment, alloying treatment or coating organic coating can be used. At present, a better passivator formula is being developed. At present, chromate is mainly used in passivation treatment, but fluoride, phosphate and other activating agents are added to obtain thicker chromate film. When there is fluoride in the passivation solution, the formation of the passivation film can be accelerated and the passivation film can be fine and bright. In the study of inorganic corrosion inhibitors with non-toxic or low toxicity as passivators, A passivation solution was obtained by adding a small amount of molybdate and phosphate to the non-toxic water-soluble acrylic acid resin (that is, ACM), is used to passivate instead of toxic chromate, which can delay the time of white rust in the galvanized layer. Its corrosion resistance is close to the level of chromate passivation. Although the technology is not very mature at present, it is a general trend for chromium-free passivation to replace chromate passivation for environmental protection. The phosphating hot-dip galvanized steel plate can be used on the outer plate of household appliances instead of the traditional galvanized steel plate, which can not only prolong the product life, but also reduce the material cost. The on-line phosphating treatment of hot-dip galvanizing unit has great potential for development. Fingerprint resistance and the application of inorganic lubricating film and organic lubricating film are the research hotspots.

Development of Zinc Alloy coating

Zinc alloy coated steel plate has better properties than galvanized steel plate, so its production proportion increases year by year. Among them, the rapid development of aluminum zinc alloy coating (Galvalume), zinc aluminum alloy coating (Galfan) and zinc iron alloy coating (Galvanneal). Recently, the corrosion resistance of galvanized Al-mg alloy coating (ZAM), is 10 times higher than that of traditional galvanized sheet.

Compared with the general hot-dip galvanized sheet, Al-Zn alloy and Zn-Al alloy coated steel plate has better corrosion resistance, coating and formability, and is suitable for building materials, light industry and other industries. Zinc-iron alloy coated steel plate has better coating property, corrosion resistance and weldability than zinc-aluminum alloy coated steel plate and aluminum-zinc alloy coated steel plate, so it is widely used in automobile industry. In order to adapt to the production of zinc alloy coated steel plate, the up-and-down mobile double plating pot is often used in galvanizing units, so that the types of coatings can be replaced quickly. The coreless zinc pot developed by Yingda Company in 1998 was put into production. This kind of zinc pot has the advantages of low energy consumption and low investment, so it is very suitable for the production of alloy coating. The zinc-ferroalloy diffusion furnace tends to use high frequency induction heating in the heating section and resistance heating in the soaking section, so that the annealing according to the optimization curve can be realized and the iron content in the galvanized coating can be controlled more accurately.

Development of zinc-free flowered steel plate

Zinc-free steel plate can be produced by lead-free galvanization. this coating can reduce intergranular corrosion, so it has a long service life and is beneficial to environmental protection. It is suitable for automobile industry and is suitable to be used as color coated substrate. A few years ago, Europe stopped the production of ordinary zinc flower steel plate, so the production of zinc-free flower steel plate has an obvious development trend.

Development of ultra-deep drawing and high strength steel galvanized sheet

IF steel is used in the production of heavy deep drawing and ultra deep drawing hot dip galvanized sheet for automobile, and there is no outdated section in the continuous furnace, which simplifies the furnace structure and production control. Production enterprises focus on the development of hot-dip galvanized sheet of high strength steel. Usually, the content of Si and mn in high strength steel is high, which is not conducive to the adhesion of zinc layer. In particular, the successful production of galvanized sheet and galvanized sheet of dual-phase steel with low Si content was realized by adding Mo element. The TRIP hot-dip galvanized steel plate obtained by replacing Si with Al is equivalent to the traditional CM nSi TRIP steel, but the addition of Al element brings some problems to the steelmaking and hot rolling process, and this problem is being solved.

Production of ultra-thin coated plate

In recent years, equipment such as air knives have been continuously improved to produce a coating of 25 ~ 30g/m sq. double-sided hot-dip galvanized sheet, which is mostly used in the electrical appliance industry and can replace part of galvanized or partially electroplated tin steel plate.

One-sided hot-dip galvanized sheet is decreasing day by day

The one-side hot-dip galvanizing process is cumbersome and the production cost is high, so it tends to be replaced by one-side galvanized sheet or double-sided differential thickness hot-dip galvanized sheet.

Attach importance to the Development of Hot rolled galvanized sheet

With the attention to energy saving and economical material and the development of thin slab continuous casting and rolling production, more thin specification hot rolled strip can be produced, and the production of hot rolled hot dip galvanized sheet has been paid attention to and developed, and its products are mainly used in the field of construction. As steel structure, cable frame, ventilation duct, granary and so on. At present, hot-rolled hot-dip galvanized sheet accounts for about 8% ~ 9% of the total output of galvanized sheet in the world. Hot-rolled hot-dip galvanizing units have been built in Handan Iron and Steel Co., Ltd., Tangshan Iron and Steel Co., Ltd.

The level of automation control is getting higher and higher.

The whole line is controlled by computer, which can realize the whole line speed, tension, rolling piece alignment, furnace temperature, furnace atmosphere, zinc pot temperature optimization and coating thickness closed-loop control.

Surface appearance defects of hot-dip galvanized steel sheet

Taking into account the production process and finished product packaging of hot-dip galvanized steel plate, hot-dip galvanized steel plate may have some surface defects as follows:

(1) original plate defect

The surface defects of galvanized sheet, such as creases, pits, wave edges, etc., continue to affect the galvanized sheet and become its surface defects. Therefore, before production, producers have to inspect the original plate.

(2) Black spot

a. The purity of the zinc coating is not enough, and the metal impurities form the original battery on the zinc surface, which is beneficial to the hydrogen evolution, and it is easy to produce black spots on the surface of the zinc layer.

b. When the content of copper, iron, arsenic and other impurities in the galvanized layer is high, these impurities appear black after passivation treatment.

c. The surface of the original plate is not adequately treated, the formation of residue, so that some parts can not be galvanized, resulting in problems in the galvanized layer and easy to fall off, resulting in black spots.

d. Leakage plating occurs during galvanizing, and it is easy to form black spots in post-treatment.

e. The zinc plating bath contains residue or small black ash (carbon or carbon compound), adheres to the surface of the zinc layer or lives in the zinc layer, and binds firmly to the zinc layer. Black spots can be seen on the finished galvanized sheet.

f. Wear away the black spots. Caused by rough operation in the process of transportation and stacking.

(3) White rust

The surface of galvanized steel plate is wet or immersed in Rain Water. At a certain temperature, zinc is oxidized to produce white powder. The anticorrosion ability of the surface that produces white rust is greatly reduced, which affects its use.

(4) uneven galvanizing layer

The zinc layer on the surface of galvanized sheet is uneven, the thickness is uneven, and even small particles appear on the surface. There is excess liquid zinc on the surface of the strip coming out of the galvanizing pot, which is blown off with a gas knife (spraying high-pressure gas) to control the thickness of the zinc layer. The gas pressure of the air knife, the distance between the air knife and the strip, the speed of the strip and so on will directly affect the thickness of the zinc layer. If the gas supply of the air knife is unstable or the pressure of the air knife is insufficient, it will cause the accumulation of zinc coating, that is, zinc scar.

(5) inclusion of slag

There are residues or scum (metal or non-metallic inclusions) in the galvanizing bath. Sometimes these slag inclusions are included in the galvanized layer. The industrial model of the Shanghai model ship model mechanical model makes it appear small round spots on the surface, and even make the original plate not stained with zinc. It's easy to cause dew iron spots.

(6) scratches and bruises, etc.

When the galvanized coil passes through the tension straightener, the straightener has surface scratches and scratches caused by foreign bodies.

(7) chromic acid dirt

In order to improve the anticorrosion ability of galvanized steel plate, a very thin passivation layer can be formed on the surface of strip steel, which can be finished by chromic acid treatment. However, if the chromic acid treatment is poor, there is residual liquid, the surface of galvanized sheet will appear yellowish band or stain, or even black spot.

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