Hot-dip galvanizing, also known as hot-dip galvanizing, is a method by which steel components are immersed in molten zinc to obtain a metal coating. In recent years, with the rapid development of high-voltage power transmission, transportation, and communications, the protection requirements for steel parts have become higher and higher, and the demand for hot-dip galvanizing has also increased.

Protection performance of hot dip galvanized layer

The thickness of the electroplated zinc layer is usually 5 ~ 15um, while the hot-dip galvanized layer is generally above 35um, and even up to 200um. The hot-dip galvanizing has good covering ability, the coating is dense, and no organic matter is included. As we all know, the mechanism of zinc’s resistance to atmospheric corrosion is mechanical protection and electrochemical protection. Under atmospheric corrosion conditions, the zinc layer has Zn0, Zn (oH) 2 and basic zinc carbonate protective film to slow down the corrosion of zinc to a certain extent. The protective film (also called white rust) will be damaged and a new film will be formed.

When the zinc layer is seriously damaged and endangers the iron substrate, zinc produces electrochemical protection to the substrate. The standard potential of zinc is -0.76V, and the standard potential of iron is -0.44V. When zinc and iron form a microbattery, zinc is dissolved as an anode, iron It is protected as a cathode. Obviously, the hot-dip galvanizing has better atmospheric corrosion resistance to the base metal iron than electro-galvanizing.

Hot galvanized layer formation process

The process of forming a hot-dip galvanized layer is the process of forming an iron-zinc alloy between the iron substrate and the outermost pure zinc layer. The iron-zinc alloy layer is formed on the surface of the workpiece during hot dip plating, which makes the iron and pure zinc layer very Good combination, the process can be simply described as: when the iron workpiece is immersed in the molten zinc solution, the solid solution of zinc and a iron (body center) is first formed on the interface.

This is a kind of crystal formed by dissolving zinc atoms in the solid state of the metal matrix iron. The two metal atoms are fused and the attraction between the atoms is relatively small. Therefore, when zinc reaches saturation in the solid melt, the two element atoms of zinc and iron diffuse with each other, and the zinc atoms diffused (or infiltrated) in the iron matrix migrate in the matrix lattice, gradually forming an alloy with iron, and diffusing The iron in the molten zinc solution forms an intermetallic compound FeZn13 with zinc and sinks into the bottom of the hot-dip galvanizing pot, which is zinc slag. When the workpiece is removed from the zinc dipping solution, a pure zinc layer is formed on the surface, which is hexagonal crystal. Its iron content is not more than 0.003%.

Hot galvanizing process

Galvanized steel coil for roofing panels

1. Process

Workpiece → degreasing → water washing → acid washing → water washing → dipping auxiliary plating solvent → drying preheating → hot galvanizing → finishing → cooling → passivation → rinse → drying → inspection

2. Process description

(1) Degreasing: chemical degreasing or water-based metal degreasing cleaning agent can be used to degrease until the workpiece is completely wetted by water.

(2) Pickling: H2S0415%, thiourea 0.1%, 40 ~ 60 ℃ or HC125%, urotropine 3 ~ 5g / L, 20 ~ 40 ℃ can be used for pickling. The addition of corrosion inhibitor can prevent the matrix from over-corrosion and reduce the amount of hydrogen absorbed by the iron matrix. At the same time, the fog inhibitor is added to suppress the escape of acid mist. Poor degreasing and pickling treatment will result in poor adhesion of the coating, zinc plating or zinc layer fall off.

(3) Dip plating aid: also known as solvent, it can keep the workpiece with certain activity before immersion plating to avoid secondary oxidation, so as to enhance the combination of the plating layer and the substrate. NH4C1100-150g / L, ZnC12150-180g / L, 70 ~ 85 ℃, 1 ~ 2min. And add a certain amount of surfactant.

(4) Drying and preheating: In order to prevent the workpiece from being deformed due to the rapid temperature rise during immersion plating, and to remove residual moisture, to prevent the occurrence of zinc explosion and the zinc liquid to splash, preheating is generally 80 ~ 140 ℃. But now generally anti-explosion agent is added.

(5) Hot-dip galvanizing: It is necessary to control the temperature of the zinc solution, the time of immersion plating and the speed of the work pieces drawn from the zinc solution. The lead-out speed is generally 1.5 m / min. If the temperature is too low, the fluidity of the zinc solution is poor, the coating is thick and uneven, it is easy to produce sagging, and the appearance quality is poor. Wrinkle phenomenon occurs, strong adhesion, thin coating, good appearance, and high production efficiency; but the temperature is too high, the iron loss of the workpiece and the zinc pot is serious, a large amount of zinc slag is produced, affecting the quality of the zinc dipping layer and easily causing color difference to make the surface color Unsightly, high zinc consumption.

The thickness of the zinc layer depends on the temperature of the zinc solution, the immersion time, the steel material and the composition of the zinc solution. In addition, galvanized alloy is also very important. In order to prevent high temperature deformation of the workpiece and reduce zinc slag caused by iron loss, general manufacturers use 450 ~ 470 ℃ for 0.5 ~ 1.5min. Some factories use higher temperatures for large workpieces and cast iron parts, but to avoid the temperature range of the peak iron loss. However, we recommend adding an alloy with iron removal function and lower eutectic temperature to the zinc solution and lower the galvanizing temperature to 435-445 ℃.

Galvanized steel coil for roofing panels

(6) Finishing: The finishing of the workpiece after plating is mainly to remove the residual zinc and zinc tumors on the surface, which is completed by using a special galvanized vibrator.

(7) Passivation: The purpose is to improve the surface corrosion resistance of the workpiece, reduce or prolong the appearance of white rust, and maintain a good appearance of the coating. All are passivated with chromate, such as Na2Cr20780 ~ 100g / L, sulfuric acid 3 ~ 4ml / L, but this passivation liquid seriously affects the environment, it is best to use chromium-free passivation.

(8) Cooling: generally water cooling, but the temperature should not be too low or too high, generally not lower than 30 ℃ not higher than 70 ℃.

(9) Inspection: The appearance of the coating is bright, detailed, without sagging and wrinkling. Thickness inspection can use coating thickness gauge, the method is relatively simple. The thickness of the coating can also be converted from the amount of zinc adhesion. For the bonding strength, a bending press can be used to bend the sample 90 ~ 180 °, and there should be no cracks and coating falling off. It can also be tested by hammering, and the salt spray test and copper sulfate corrosion test are done in batches.

The formation and control of zinc ash and zinc slag

Zinc ash and zinc slag not only seriously affect the quality of the zinc immersion layer, causing the coating to be rough and producing zinc nodules. Moreover, the cost of hot dip galvanizing is greatly increased. Generally, the zinc consumption per 1t workpiece is 40 ~ 100kg. If the zinc dust is serious, the zinc consumption will be as high as 140 ~ 200kg. The control of zinc slag is mainly to control the temperature and reduce the scum produced by the oxidation of the surface of the zinc liquid, so it is more necessary to use alloys with iron removal and anti-oxidation functions and use small thermal conductivity, high melting point, small specific gravity, and not with zinc liquid .The reaction can not only reduce the heat loss but also prevent the oxidation of ceramic beads or glass balls. This kind of ball is easy to be pushed away by the workpiece and has no adhesion to the workpiece.

Galvanized steel coil for roofing panels

For the formation of zinc slag in the zinc solution, the main reason is that the iron content dissolved in the zinc solution exceeds the solubility at this temperature. The zinc-iron alloy with extremely poor fluidity is formed. The zinc content in the zinc slag can be as high as 94%, which is hot plating The key to the high cost of zinc.

It can be seen from the solubility curve of iron in zinc solution: at different temperatures and different holding times, the amount of iron dissolved or iron loss is different. At around 500 ℃, the iron loss increases sharply with heating and holding time, almost in a linear relationship. Below or above the range of 480 ~ 510 ℃, the iron loss increases slowly with time. Therefore, people call 480 ~ 510 ℃ malignant dissolution zone.

In this temperature range, the zinc solution has the most serious corrosion to the workpiece and the zinc pot, and the iron loss exceeds 560 ℃, and the iron loss increases significantly. Above 660 ℃, zinc is a destructive attack on the iron matrix, and the zinc slag will increase sharply, and the plating cannot be performed . Therefore, the current plating is mostly carried out in the range of 430 ~ 450 ℃.

The existence of the zinc layer brings some difficulties to the welding of hot-dip galvanized steel sheets. The main problems are: weld cracks and increased porosity, zinc evaporation and soot, oxide inclusions, and galvanized layer melting and destruction . Among them, welding cracks, porosity and slag inclusion are the main problems.

Welding crack

During the welding process, the molten zinc floats on the surface of the molten pool or at the root of the weld. Since the melting point of zinc is much lower than that of iron, the iron in the molten pool crystallizes first, and liquid zinc will penetrate into the grain boundary of the steel, resulting in weaker intercrystalline bonding. Moreover, the intermetallic brittle compounds Fe32n10 and FeZn10 are easily formed between zinc and iron, which further reduces the plasticity of the weld metal. Therefore, under the action of welding residual stress, it is easy to crack along the grain boundary and form cracks.

Galvanized steel coil for roofing panels

Factors affecting crack sensitivity:

(1) Thickness of zinc layer: The zinc layer of galvanized steel is thin and the crack sensitivity is small, while the zinc layer of hot-dip galvanized steel is thick and the crack sensitivity is large.

(2) Workpiece thickness: the greater the thickness, the greater the welding restraint stress and the greater the crack sensitivity.

(3) Bevel gap: The greater the gap, the greater the crack sensitivity.

(4) Welding method: crack sensitivity is small when welding by manual arc welding, while crack sensitivity is higher when welding by CO2 gas shielded welding.

Ways to prevent cracks:

(1) Before welding, open the groove V, Y or X-shaped groove at the welding place of galvanized sheet, use oxyacetylene or sandblasting to remove the galvanized layer near the groove, and the control gap should not be too large, generally 1.5 mm or so.

(2) Select welding materials with low Si content. Welding wire with low S content should be used for gas shielded welding, and titanium type and titanium calcium type electrode should be used for manual welding.

Stomata

The zinc layer near the groove is oxidized (formed as Zno) and evaporated under the action of arc heat, and evaporates white smoke and steam, so it is easy to cause porosity in the weld. The greater the welding current, the more serious the evaporation of zinc, and the greater the stomata sensitivity. When welding with titanium-type and titanium-calcium-type electrodes, it is not easy to produce pores in the medium current range. When welding with cellulose-type and low-hydrogen electrode, stomata are likely to occur under both small current and large current. In addition, the electrode angle should be controlled as much as possible within the range of 300 ~ 70.

Galvanized steel coil for roofing panels

Zinc evaporation and soot

When welding galvanized steel plates by arc welding, the zinc layer near the molten pool is oxidized to Zn0 and evaporated by the action of arc heat, forming a large amount of smoke. The main component of this smoke is Zno, which has a great stimulating effect on workers’ respiratory organs. Therefore, good ventilation measures must be taken when welding. Under the same welding specification, the amount of fumes generated when welding with titanium oxide electrodes is lower, while the amount of fumes generated when welding with low hydrogen electrodes is larger.

Galvanized steel coil for roofing panels

Oxide slag

When the welding current is small, the ZnO formed during heating is not easy to escape, and it is easy to cause Zn0 slag inclusion. ZnO is relatively stable, its melting point is 1800 ℃, large Zn0 slag inclusions have a very adverse effect on the weld plasticity. When using titanium oxide electrode, ZnO is finely and uniformly distributed, and has little effect on plasticity and tensile strength. When using cellulose or hydrogen electrode, the Zno in the weld is larger and more, and the weld performance is poor.

Hot-dip galvanized sheet is hot-rolled galvanized sheet, cold-dip galvanized sheet is cold-rolled galvanized sheet. In terms of material, the composition of the two galvanized sheets is basically similar, the difference is that cold-galvanized sheet is better than hot-dip galvanized sheet The thickness is thinner, the surface source quality is better, the process is more complicated, and the price is higher.

From the technical point of view, the steel is sent from the billet to hot rolling and rolled into a plate of a certain thickness, such as a thickness of about 10mm.If the user does not have higher requirements on the surface, thickness and mechanical strength of the plate, the finished product can be processed. If it is galvanized or the like, it is directly sold as a hot plate. If there are higher requirements for the board, the hot-rolled board is sent to the cold-rolling line for reprocessing, after pickling, annealing, and rolling again, to obtain thinner, smoother surface, and better mechanical properties cold rolled plate.

Product Package Details

1. Hot-dip galvanizing generally has a thick zinc layer, about 10um or more, and has a strong anti-corrosion ability, so it is a commonly used galvanizing process. The electroplated zinc is very thin, about 3-5um, the surface of the hot-dip plating is rough and bright, and the severe one has zinc flowers, and the electroplating is smooth and gray (fouling). The galvanized steel plate produced by the electroplating method has good processing performance but the coating is thin, and the corrosion resistance is not as good as the hot-dip galvanized steel plate.

2. Cold galvanized is electro-galvanized, and the amount of galvanizing is very small. Only the outer tube wall is galvanized (hot-plated-both internal and external), and its corrosion resistance is only 10-50g / m2. The galvanizing amount of hot-dip galvanizing cannot be too small, generally the minimum is 50 ~ 60g / m2 on both sides, the maximum can reach 600g / m2, the galvanized layer of the electrogalvanized sheet can be very thin, the minimum is 15g / m2, and the maximum is 100g / m2 or so. In addition, electro-galvanized zinc can be plated on only one side, or galvanized on both sides, hot galvanized must be galvanized on both sides.

3. Chemical industry generally has more electro-galvanized, suitable for small parts; hot-dip galvanized is generally used for power equipment and components, suitable for large parts and equipment.

Product Package Details

4. Hot-dip galvanized sheet and electro-galvanized sheet are fundamentally different in plating structure. There is a slightly brittle compound layer between the pure zinc coating of the hot-dip galvanized sheet and the steel strip substrate. Most of the pure zinc coating forms zinc flower when crystallized, and the coating is uniform and non-porous. The zinc atoms of the electroplated zinc layer are only deposited on the surface of the steel strip, and they are attached to the surface of the steel strip by physical action. There are many holes to dry, which is easy to cause pitting corrosion due to corrosive media, so the hot-dip galvanized sheet is better than the electro-galvanized sheet More resistant to corrosion.

5.The heat treatment process of hot-dip galvanized and electro-galvanized sheets is also completely different. Hot-dip galvanized sheets generally use cold-hardened plates as raw materials, which are continuously annealed and hot-dip galvanized on the galvanizing line.After the steel belt is heated within a short time It is also cooled, so the strength and plasticity are affected to a certain extent, and its stamping performance is worse than that of the cold-rolled sheet after the same cold-hardened sheet is degreased and annealed by a professional production line. Electrogalvanized sheet uses cold-rolled sheet as the raw material, which basically guarantees the same processing performance of cold-rolled sheet, but its complicated technological process also increases production costs.

Galvanized-steel-coil-for-roofing-sheet

In short, the production cost of hot-dip galvanized sheet is lower, and the application range is wider, and it has also become the main variety in the galvanized sheet market.

6. Compared with electro-galvanized steel sheet, hot-dip galvanized sheet has the characteristics of thick zinc layer, good corrosion resistance and low production cost.

7.The surface quality of electro-galvanized sheet is more excellent, the coating layer is evenly distributed, and its weldability, paintability and stamping formability are better than hot-dip galvanized sheet; single-sided, double-sided or poor-thickness coating can be easily switched during production . However, in the electrogalvanizing process, zinc sends zinc ions in the electroplating solution to the steel plate by current. The thicker the coating, the more power is consumed and the cost is higher.

Galvanized sheet refers to a steel sheet coated with a layer of zinc on the surface. Galvanizing is a frequently used economical and effective anticorrosion method, and about half of the world’s zinc production is used in this process. Hot-dip galvanized steel sheet has ideal corrosion resistance, forming, coating and other comprehensive properties, as well as low cost and good appearance.

Hot dip galvanized steel

Galvanized sheet is mainly used in construction, light industry, automobile, agriculture, animal husbandry, fishery and commerce. The construction industry is mainly used to manufacture anti-corrosion industrial supplies and panels, roof grilles, balcony panels, window sills, newsstands, warehouses, rolling doors, heaters, rainwater pipes, etc. used in civil construction houses; and the light industry industry uses it to manufacture household appliance shells , Civil chimneys, kitchen utensils, etc., the automotive industry is mainly used to manufacture corrosion-resistant parts of cars, etc .; agriculture, animal husbandry and fishery are mainly used as food storage and transportation, meat and aquatic products frozen processing appliances, etc .; commercial use is mainly used as materials Storage and transportation, packaging utensils, etc. Household appliances such as refrigerators, washing machines, switch cabinets, air conditioners, microwave ovens, toasters, copiers, vending machines, electric fans, vacuum cleaners, etc. will also use galvanized sheets.

Package

The packaging of galvanized sheet can be divided into two types: galvanized sheet cut to length and galvanized sheet with coil. It is usually packed in iron sheet, lined with moisture-proof paper, and strapped on the bracket with an iron waist. The strapping should be firm to prevent the internal galvanized sheets from rubbing against each other.

Product Package Details

Hot dip galvanized steel sheet classification

The galvanized sheet is divided according to the layout: there are zinc flower galvanized sheet and non-zinc galvanized sheet according to layer thickness: 80G zinc, 100G zinc, 120G zinc, 180G zinc, 275G zinc can be divided into the following according to production and processing methods class:

(1) Hot-dip galvanized steel sheet. The steel sheet is immersed in the molten zinc bath, and a layer of zinc thin steel sheet is adhered on the surface. At present, it is mainly produced by a continuous galvanizing process, that is, a coil of steel sheet is continuously immersed in a zinc-melted plating tank to make a galvanized steel sheet;

(2) Alloyed galvanized steel plate. This kind of steel plate is also made by hot dipping method, but it is heated to about 500 ℃ immediately after leaving the trough to make it form an alloy film of zinc and iron. This galvanized sheet has good paint adhesion and weldability;

(3) Electro-galvanized steel sheet. The galvanized steel sheet manufactured by the electroplating method has good workability. However, the coating is thin, and the corrosion resistance is not as good as the hot-dip galvanized sheet;

(4) Single-sided and double-sided galvanized steel sheets. Single-sided galvanized steel sheet, that is, products that are galvanized on only one side. In welding, painting, anti-rust treatment, processing, etc., it has better adaptability than double-sided galvanized sheet. In order to overcome the shortcomings of uncoated zinc on one side, there is a galvanized sheet coated with a thin layer of zinc on the other side, that is, a double-sided galvanized sheet;

(5) Alloy, composite galvanized steel plate. It is a steel plate made of zinc and other metals such as aluminum, lead, zinc, etc. alloy or even composite plating. This steel plate has both excellent anti-rust performance and good coating performance.

In addition to the above five types, there are color galvanized steel sheets, printed coating galvanized steel sheets, polyvinyl chloride laminated galvanized steel sheets, etc. But currently the most commonly used hot dip galvanized sheet.

Production Line

Basic requirements for appearance quality

Colour:

1. The galvanized layer is slightly silvery gray with light yellow tone, and the crystal is uniform and fine;

2. The galvanized layer passivated by chromate is divided into four types according to different processes. Its typical appearance is as follows: bright: bright, clear, sometimes with light blue tone bleaching: clear, slightly rainbow-colored rainbow: rainbow Color dark: grass green, olive green, tan, black, etc.

No defects are allowed:

1. No local coating;

2. Rainbow color, the color is purple-red after passivation;

3. More water marks or chromate residues;

4. Orange peel, blisters or more severe stripes;

5. Dendritic and spongy crystals;

6. Excessive corrosion or mechanical rubbing;

7. Erasable passivation film or partial passivation film.

Allow defects:

1. Due to the different materials, processing methods and surface conditions of the workpiece, the coating has a slightly uneven color and gloss;

2. Slight water marks and chromate residues;

3. The passive film has slight local abrasions and spot mechanical abrasions;

4. The screws or rivets and other fasteners are slightly darker in the holes or percussion parts;

5. The workpiece color after dehydrogenation is slightly dark;

6. There is slight roughness on the sharp edge of the workpiece that does not affect the assembly;

7. Slight fixture printing;

8. The coating at the welding seam of the weldment is dark or has small black spots;

9. For non-through holes with a diameter greater than 8mm at a depth exceeding one diameter, or for a through hole with a diameter not greater than 6mm at a depth exceeding two diameters, no plating is allowed;

10. On the premise of not affecting the assembly and performance, the plating surface of the workpiece that is partially electroplated is allowed to expand.

Galvanized-steel-coil-for-roofing-sheet

Galvanized amount

Standard value of galvanized amount: galvanized amount is a commonly used effective method to indicate the thickness of zinc layer of galvanized sheet. The unit of galvanization is g / m2.

Galvanized steel sheet index (unit: g / m2)

JISG3302 code: Z12, Z18, 222, 225, 227, 235, 243, 250, Z60

The galvanized amount is: 120, 180, 220, 250, 270, 350, 430, 500, 600

ASTMA525: A40, A60, G60, G90, G115, G140, G165, G185, G210

The galvanized amounts are: 122,183, 183,275, 351, 427, 503, 564, 640

Mechanical behavior

(1) Tensile test: generally refers to the tensile test, which can measure a series of strength indexes and plastic indexes of the material. Strength usually refers to the ability of a material to resist elastic deformation, plastic deformation, and fracture under external force. Plasticity refers to the ability of a metal material to deform plastically under load without damage. Commonly used plasticity indicators are elongation and shrinkage.

Performance indicators: Generally speaking, only the galvanized sheets for structural, drawing and deep drawing have tensile performance requirements. Among them, the galvanized sheet for structure requires yield point, tensile strength and elongation, etc .; for stretch, only elongation is required.

(2) Bending test: The bending test is mainly used to determine the flexural strength of brittle and low plastic materials (such as cast iron, high carbon steel, tool steel, etc.) and can reflect the deflection of the plastic index.

Requirements: It is generally required that after the galvanized sheet is bent 180 degrees, the outer surface must not have the zinc layer detached, and the sheet base must not be cracked or broken.

Calculation formula

Calculation formula of galvanized sheet: M = 7.85 * length * width thickness + calculated weight of zinc layer (lookup table) (density of steel = 7.85 g / cm3).

In the production of color-coated steel plates, in order to meet the needs of different use occasions, the materials are divided into many types, such as hot-dip aluminum-zinc color-coated steel plates, cold-rolled substrate color-coated steel plates, electro-galvanized color-coated steel plates, etc. What are the differences?

1. Cold-rolled substrate color coated steel plate

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Color plates produced from cold-rolled substrates have a smooth and beautiful appearance, and have the processing performance of cold-rolled plates; however, any small scratches on the surface coating will make the cold-rolled bases, buildings or factories made of printed color coated steel plates , Often has a long service life when washed by rain, otherwise, it will be affected by the use of sulfur dioxide gas, salt and dust. Therefore, when designing, the greater the inclination of the roof, the less likely it is to accumulate dust and other dirt, and the longer the service life; those areas or parts that are not frequently washed by rain should be washed regularly with water. The board is exposed to the air, so that red rust is quickly generated at the exposed iron. Therefore, such products can only be used for temporary isolation measures that are not demanding and used as interior materials.

2. Hot-dip galvanized color coated steel plate

6 inch corrugated roofing sheets

According to the requirements, hot-dip aluminum-zinc steel plates can also be used as color-coated substrates (55% AI-Zn and 5% AI-Zn). The color-coated steel plates have light weight, beautiful appearance and good corrosion resistance, and can be directly processed. Provides a new type of raw materials for the construction industry, shipbuilding industry, vehicle manufacturing industry, furniture industry, electrical industry, etc., which has a good effect of replacing wood with steel, saving energy, preventing pollution, etc., using galvanized steel plate as the color printing color of the substrate In addition to zinc protection, the coated steel plate has an organic coating on the zinc layer to protect the steel plate from rusting and has a longer service life than the galvanized steel plate.

3. Electro-galvanized color coated steel plate

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Using electro-galvanized sheet as the substrate, the product obtained by baking with organic coating is electro-galvanized color coated steel sheet. Due to the thin zinc layer of electro-galvanized sheet, the zinc content is usually 20 / 20g / m2. Steel plate, color plate. Color-coated steel plate is a product made of cold-rolled steel plate and galvanized steel plate as the substrate, after surface pretreatment, coating with a continuous method (roller coating method), baking and cooling.

Each color-coated steel plate product of different materials has its own advantages and uses. When users buy products, they should first clearly understand their actual needs, so that when they buy products, they can provide manufacturers with accurate data to recommend products. .

1. The impact of the natural environment (different regions)

The selection of color-coated steel plates in different regions mainly considers the local wind and snow loads and corrosive environment. The wind and snow load is mainly considering the safety factor of the building. Although it is related to the mechanical properties of the color-coated steel plate, it is more related to the shape and thickness of the profiled steel plate and the connection between the plates.
It is relatively difficult to quantitatively describe the corrosion environment in an area, and there are many factors affecting the corrosion of color-coated steel plates. Such as: temperature, humidity, temperature difference between day and night, sunshine time and sunshine intensity, rainfall and acidity of rain, wind direction and speed, corrosive sediments, etc.

Elements of anti-corrosion effect of color-coated steel plate

They are coating type, coating thickness, coating supplier, coating type, coating thickness, pretreatment type, steel plate thickness, coating structure, etc.

corrugated roofing sheets

Impact of construction design on corrosion

Installation or processing quality: it mainly reflects whether the coating is damaged during processing, whether it is processed when the coating is brittle at low temperature, the steel plate is damaged during installation, and the structural welding iron chips fall on the coating during installation. Cleaning the roof panel, etc., will cause corrosion of the material.
From so many introductions to the corrosion factors of color-coated steel plates, as well as the feedback of our users’ usage and the analysis of the results of exposure tests, our recommendations for use in different regions are as follows:

In plateaus with strong ultraviolet rays (southwest and northwest) and low latitudes (guangdong and south), consider color-coated steel plates with coatings with superior durability (PVDF coating, HDP coating, SMP coating, etc.). For high-humidity areas (most parts of the southern coast), coatings and coatings must be made of materials with excellent corrosion resistance, and the coating thickness and coating thickness must at least meet the requirements of national standards.

6 inch corrugated roofing sheets

The use of color-coated steel plates at the seaside requires thick-coated color-coated steel plates or relatively dense coating varieties, because the chloride ion in seawater has a strong penetration ability, it can directly corrode through thin coatings or non-dense coatings The substrate coating, of course, the substrate must be galvanized or aluminum-zinc-plated material. Where the seawater can splash, it is not recommended to use color-coated steel plates.

In an industrial environment, the impact on color coating is mainly acid rain. These areas are concentrated in Hunan, Chongqing, Jiangxi and other areas. In connection with color coating using high zinc layer galvanized substrates, the color coating still needs to be thick and dense. In other areas that do not belong to the above-mentioned areas, the choice of using color-coated steel plates is relatively large, as long as they meet the national standards for color coating.

2. The impact of the use environment (different industries)

The use of color-coated steel plates in different industries is analyzed in two aspects. One is the outer plate of color-coated steel plates. Under normal circumstances, it is sufficient to choose according to different regions, but also to consider the impact of small environments, that is, whether there is corrosion in the vicinity The medium falls on the building envelope, such as the gas emitted by the boiler, sulfur dioxide dust, etc. This should be considered in the design of the height of the chimney and the main local wind direction.

In addition, there is also the tightness of the plant, it is also possible that the high temperature and humidity in the plant
Corrosive gas comes out and meets cold air outdoors to condense on the color-coated steel plate. This phenomenon should be avoided. It will accelerate the generation of local corrosion; Steel plate) or back paint (single-layer board). What needs to be explained here is that the inner panel of the roof is relatively easier to corrode than the inner panel of the wall. If workers can work in this environment, it is possible to use color-coated steel plates. However, the quality of the indoor environment or the investment of environmental protection devices will directly affect the service life of the painted steel plate.

corrugated metal roofing sheets

Non-ferrous metal smelters (copper, zinc, aluminum, lead, etc.) are the most challenging years for color-coated steel plates. However, reasonable selection of materials, design, and environmental protection investment can ensure or increase the service life of color-coated steel plates. Replacement or maintenance can be done within 10 years. Iron and steel plants, power plants, etc. also produce corrosive media, but color-coated steel plates have long-term experience.

The machinery manufacturing industry (automobiles, steam turbines, machine tools, etc.) will basically not have a negative impact on the color coating corrosion, but it should be noted that these industries may have heat treatment furnaces, boilers, etc., which will also affect the local environment. Corrosion of coated steel plates, especially at processing damages or trims.

The industry of warehousing and logistics is developing rapidly, and the use of light steel is the most. It does not corrode the color-coated steel plate itself. Its selection of color-coated steel plate mainly considers the impact of the environment. Generally, such projects are concentrated in places with developed industries and heavy traffic , That is, the industrial environment and coastal areas.

The appearance and durability of public and civil buildings are the primary considerations for color-coated steel plates. Therefore, it is very important that the coating is resistant to discoloration, powdering, and the surface of the coating is complete and delicate. There are many people in public and civil buildings concerned, and they are close. Therefore, the choice of paint is generally PVDF or HDP, and light colors are mostly selected.

Roll forming is the most commonly used method for the production of building roofing panels and wall panels with color coated boards. Roll forming machines use a series of differently shaped rolls on a rack to press the steel rolls into the required shape.

Color-coated steel sheet is pressed into various shapes according to the strength and elongation of the steel sheet, coating characteristics, the model of the forming machine and the maintenance status.

Color coated galvanized steel coil

In order to improve the on-site performance of the color plate, especially the deformation area of ​​the steel plate, the following requirements should be observed:

(1) Adopting a large bending center radius design can meet the deep shape requirements of architectural design.

(2) Roll design meets the deformation of the steel plate from the middle to the side.

(3) The rolling design is matched with the strength of the steel plate. For example, for high-strength steel plates, the deformation of the same shape may require more deformation passes.

(4) Low-strength steel plates should be selected for complex shapes to allow greater design margin.

(5) Maintain the rolling equipment so that it has the required horizontality and verticality, and sufficient rotating roller clearance to avoid scratching the coating and cleaning the rollers to avoid damage to the topcoat.

Color steel plate

(6) The flexible primer and topcoat system are used to increase the performance of the steel plate during bending.

Although roll forming is the most commonly used method for color-coated sheet forming, for the production of roof panels and wall panels, blanking or other accessories may be required. For the same reason, the largest bend radius is needed to improve the performance of the bending part.

It is also very important to avoid scratches on the surface of the coating during the blanking process. Measures to protect the coating surface from scratches can be taken with a stamper protective film. Proper mold clearance should be maintained during pressing.

Embossing is a type of color-coated sheet molding. The embossing process needs to ensure that the plating and coating are not damaged. It needs a sufficiently low embossing sharpness and depth to ensure that the durability of the color-coated sheet is not affected.

Other forming methods for color coated plates include stamping, slotting and perforating. Many of the influencing factors discussed previously, such as die clearance and coating flexibility, are as important as the above forming methods.

Hot-dip galvanizing, also called hot-dip galvanizing, is a method of obtaining a metallic coating by immersing steel components in molten zinc. With the rapid development of high-voltage power transmission, transportation, and communications in recent years, the requirements for protection of iron and steel parts have become higher and higher, and the demand for hot-dip galvanizing has also increased.

Protective properties of hot-dip galvanized layer

Usually the thickness of the galvanized layer is 5-15um, while the hot-dip galvanized layer is generally above 65um, even as high as 100um. Hot-dip galvanizing has good covering ability, dense plating and no organic inclusions. As we all know, the mechanism of zinc’s resistance to atmospheric corrosion includes mechanical protection and electrochemical protection. Under the condition of atmospheric corrosion, there are protective films of Zn0, Zn (OH) 2 and basic zinc carbonate on the surface of zinc layer, which can slow down the corrosion of zinc to a certain extent. If the protective film (also called white rust) is damaged, a new layer will be formed.

When the zinc layer is severely damaged and the iron matrix is ​​endangered, zinc generates electrochemical protection for the matrix. The standard potential of zinc is -0.76V, the standard potential of iron is -0.44V. When zinc and iron form a microbattery, zinc is dissolved as an anode, and iron is dissolved. Protected as a cathode. Obviously, the hot-dip galvanizing ability of the base metal iron is better than that of electro-galvanizing.

Formation process of hot-dip galvanized layer

The process of hot-dip galvanizing layer formation is the process of forming an iron-zinc alloy between the iron substrate and the outermost pure zinc layer. The surface of the workpiece is formed with an iron-zinc alloy layer during hot dip plating, which makes the iron and pure zinc layer very Good combination, the process can be simply described as follows: when an iron workpiece is immersed in molten zinc liquid, zinc and a iron (body-centered) solid solution are first formed on the interface. This is a crystal formed by dissolving zinc atoms in the solid state of the base metal iron. The two metal atoms are fused and the attraction between the atoms is relatively small.

Therefore, when zinc reaches saturation in the solid solution, the two elements of zinc and iron diffuse into each other, and the zinc atoms diffused into (or infiltrated into) the iron matrix migrate in the matrix of the matrix, gradually form an alloy with iron, and diffuse The iron in the molten zinc solution forms an intermetallic compound FeZn13 with zinc, which sinks into the bottom of the hot-dip galvanizing pot, which is zinc slag. When the workpiece is removed from the zinc bath, a pure zinc layer is formed on the surface, which is a hexagonal crystal with an iron content of not more than 0.003%.

application

Crafting process

Workpiece degreasing → water washing → pickling → water washing → dip assisting plating solvent → drying and preheating → hot galvanizing → finishing → cooling → passivation → rinsing → drying → inspection

Process description

(1) Skim

Chemical degreasing or water-based metal degreasing cleaning agents can be used to remove the oil until the workpiece is completely wetted with water.

(2) Pickling

It can be pickled with H2S0415%, thiourea 0.1%, 40 ~ 60 ℃ or HC120%, hexamethylenetetramine 1 ~ 3g / L, 20 ~ 40 ℃. Adding a corrosion inhibitor can prevent over-corrosion of the substrate and reduce the hydrogen absorption of the iron substrate. Poor degreasing and pickling treatment will cause poor adhesion of the coating, and zinc will not be plated or the zinc layer will fall off.

(3) Dip plating agent

Also called bonding agent, it can keep the workpiece active before immersion plating to enhance the bonding between the coating and the substrate.

NH4C115% ~ 25%, ZnC122.5% ~ 3.5%, 55 ~ 65 ℃, 5 ~ 10min. In order to reduce the volatilization of NH4C1, glycerin can be added appropriately.

(4) Drying and preheating

In order to prevent the workpiece from being deformed due to the sharp rise in temperature during immersion plating, and to remove the residual moisture, to prevent the occurrence of zinc explosion, which will cause the zinc liquid to splash, the preheating is generally 120 ~ 180 ° C.

(5) Hot-dip galvanizing

It is necessary to control the temperature of the zinc solution, the immersion time and the speed of removing the workpiece from the zinc solution. Too low temperature, poor liquidity of zinc liquid, thick and uneven coating, easy to produce sagging, poor appearance quality; high temperature, good liquidity of zinc liquid, zinc liquid easily detached from the workpiece, reducing the occurrence of sagging and wrinkling, adhesion Strong, thin coating, good appearance, and high production efficiency; but the temperature is too high, the workpiece and zinc pot iron damage is serious, a large amount of zinc slag is generated, which affects the quality of the zinc immersion layer, the zinc consumption is large, and even can not be applied.

At the same temperature, the immersion plating time is long and the plating layer is thick. When the same thickness is required at different temperatures, high temperature dip plating takes longer.

In order to prevent high-temperature deformation of workpieces and reduce zinc slag due to iron loss, general manufacturers use 450 ~ 470 ℃, 0.5 ~ 1.5min. Some factories use higher temperatures for large workpieces and cast iron, but avoid the temperature range of the peak iron loss. In order to improve the fluidity of the hot-dip plating solution at a lower temperature, prevent the coating from being too thick, and improve the appearance of the coating, 0.01% to 0.02% of pure aluminum is often added. Aluminum should be added in small quantities.

(6) Finishing

The finishing of the workpiece after plating is mainly to remove the residual zinc and zinc knobs on the surface, either by vibration or manual methods.

(7) Passivation

The purpose is to improve the surface corrosion resistance of the workpiece surface, reduce or prolong the appearance of white rust, and maintain the good appearance of the coating. All are passivated with chromate, such as Na2Cr20780 ~ 100g / L, sulfuric acid 3 ~ 4ml / L.

(8) Cooling

It is generally water-cooled, but the temperature should not be too low to prevent the workpiece, especially the casting, from cracking in the matrix structure due to chilling and shrinking.

(9) Inspection

The appearance of the coating is bright, detailed, and free from sagging and wrinkling. Coating thickness gauge can be used for thickness inspection. The method is relatively simple, and the thickness of the coating can also be obtained through conversion of zinc adhesion. The bonding strength can be bent with a bending press, and the sample is bent at 90 ~ 180 °. There should be no cracks and coatings falling off, and it can also be tested by hammering.

Control of zinc ash and zinc slag

application

(1) Formation of zinc ash and zinc slag

Zinc ash and zinc slag not only seriously affect the quality of the zinc coating, cause the coating to be rough, and produce zinc knobs. Moreover, the cost of hot-dip galvanizing is greatly increased. Normally, the workpiece consumes 80 ~ 120kg zinc per 1t of plating. If the zinc ash and zinc slag are serious, its zinc consumption will reach 140 ~ 200kg. Zinc-carbon control is mainly to control the temperature and reduce the dross produced by the surface oxidation of the zinc solution.

Some manufacturers in China are covered with refractory sand and charcoal. Foreign countries use ceramics or glass balls with small thermal conductivity, high melting point, small specific gravity, and no reaction with zinc liquid, which can reduce heat loss and prevent oxidation. Such balls are easily pushed away by the workpiece and are non-sticky to the workpiece Attached effect.

For the formation of zinc slag in the zinc solution, the zinc-iron alloy with extremely poor fluidity is formed when the iron content dissolved in the zinc solution exceeds the solubility at this temperature. The key to high zinc costs.

From the solubility curve of iron in zinc solution, it can be seen that the amount of dissolved iron, ie, the amount of iron loss, is different at different temperatures and different holding times. When the temperature is around 500 ° C, the iron loss increases sharply with the heating and holding time, and it is almost linear. Below or above the range of 480 ~ 510 ℃, iron loss increases slowly with time. Therefore, people call 480 ~ 510 ℃ the malignant dissolution zone. In this temperature range, the zinc solution has the most severe corrosion on the workpiece and the zinc pot. The iron loss has increased significantly above 560 ° C. Above 660 ° C, zinc is a destructive attack on the iron substrate. . Therefore, plating is currently performed in two regions of 450 to 480 ° C and 520 to 560 ° C.

(2) Control of the amount of zinc slag

To reduce zinc slag, we must reduce the iron content in the zinc solution, which is to start from the factors that reduce the dissolution of iron:

a. As far as possible, the material of zinc pot should be welded with carbon steel and low silicon content. The high carbon content will accelerate the corrosion of the iron pot by the zinc liquid, and the high silicon content can also promote the corrosion of the zinc liquid to the iron. At present, 08F high-quality carbon steel plates are mostly used. It contains 0.087% carbon (0.05% ~ 0.11%), silicon ≤ 0.03%, and contains elements such as nickel and chromium that can inhibit iron from being etched.

Do not use ordinary carbon steel, otherwise the zinc consumption is large and the life of the zinc pot is short. It has also been proposed to use silicon carbide to make a molten zinc bath. Although the iron loss can be solved, the modeling process is also a problem.

b. Always slag. First raise the temperature to the upper limit of the process temperature so that the zinc slag is separated from the zinc liquid, and then drop below the process temperature. After the zinc slag sinks to the bottom of the tank, it is fished with a spoon. Plating parts that have fallen into the zinc solution must be salvaged in time.

c. To prevent the iron in the flux from being brought into the zinc bath with the workpiece, the red-brown iron-containing compound will be generated after the flux is used for a certain period of time, and it should be filtered off regularly. It is better to maintain the pH of the flux at about 5.

d. Less than 0.01% of aluminum in the plating solution will accelerate the formation of slag. Appropriate amount of aluminum will not only improve the fluidity of the zinc solution, increase the brightness of the coating, but also help reduce zinc slag and zinc ash. A small amount of aluminum floating in the liquid is beneficial to reduce oxidation, too much affects the quality of the coating, and produces defects.

e. Heating and heating should be uniform to prevent explosion and local overheating.

Color steel plates are often used in construction projects. It is indispensable today, but it is worth mentioning that there are many types of color steel plates and their thicknesses have individual differences. Many people do not understand the specific specifications. , How to choose the thickness of the color steel plate is even more difficult to start, then how to choose the thickness of the color steel plate?

Generally speaking, the color steel plate is divided into two layers, and the size depends on the customer’s requirements. In most cases, 0.476MM color steel plate is used for the lower layer, 950-type tongue and groove board is used for the wall surface, and 0.376 color steel plate is used for the inside and outside. Of course, there are special circumstances, and some mobile homes are galvanized steel with a thickness of 0.35–0.4 mm.

If it is an outdoor simple color steel plate movable room, the color steel plate wall plate uses a 0.4 cm thick galvanized plate, the zinc content of the galvanized is 60-80 grams, and the foam material for the sandwich filler can be used.

corrugated roofing sheets

Standard information on the thickness of color steel plates used in international standard purification plants, operating rooms, etc. Wall mobile rooms should use a thickness of 0.6 cm or more, and the zinc content of galvanized should be over 110 grams. The purpose is to prevent rust and fill the color steel plates. It is forbidden to use polyvinyl chloride, that is, foam board, and the fire rating has national standards.

The outer layer of the wall is made of G550 high strength color aluminized zinc steel plate, and the thickness of the substrate is not less than 0.60mm. The inner layer is made of high-strength aluminum-zinc plated steel, and the thickness of the substrate is not less than 0.50mm. The thickness error is ± 0.02mm. The wall insulation material is ultra-fine glass wool.

Color steel plate coating weight: the amount of aluminized zinc ≥180g / m2 (front and back);

Wall panel: AZ185 hot-dip aluminized zinc steel plate;

Paint coating: 20um thick fluorocarbon topcoat on the front of the outer panel, 5um thick epoxy primer;

Organic coating: PVDF fluorocarbon-polyester coating system is selected;

15um thick polyester finish on the front of the inner panel, 5um thick epoxy primer;

20um thick polyester topcoat on the back, 5um thick epoxy primer;

Color steel plate thickness (substrate): outer plate ≥0.6;

5um thick polyester finish on the back, 5um thick epoxy primer.

corrugated metal roofing sheets

The polystyrene color steel sandwich panel is composed of two layers of color-coated steel sheets inside and outside, and is coated with adhesive glue on the steel sheet by an automatic composite molding machine, and thin-wall channel steel is embedded on both sides of the sheet. And blanks made of sheet.

Machine plate specifications: L × 1150 × 50mm, L × 1150 × 75mm, L × 1150 × 100mm, color steel plate manual plate specifications: L × 985 × 50 color steel plate thickness: 0.426mm, 0.476mm, 0.50mm, 0.60mm , Polystyrene density ≥18kg / m3 Thermal conductivity: 0.041w / mk Combustion performance: B1 level (flammability), the maximum operating temperature is 100.

In the production process of color steel plate, the thickness dimension must be carried out according to certain specifications. Among them, the thickness of the color steel plate is required to be between 50-250mm. If it exceeds the specified range, it is regarded as a substandard product.

The requirements for the size of the color steel plate are: the width is generally 1150mm, and the length can be changed according to user needs. The thickness and size of the color steel plate largely determines the role and function of the color steel plate, so it is necessary to strictly control the production process.

Color-coated sheet is made of cold-rolled steel sheet and galvanized steel sheet as the substrate. After surface pretreatment (degreasing, cleaning, chemical conversion treatment), coating is applied in a continuous method (roller coating method), and baked and cooled The product.

The color-coated steel plate refers to the color-coated steel plate, and the color-coated steel plate is a steel plate with an organic coating. Color steel plate is divided into veneer, color steel composite plate, floor bearing plate and so on. So what is the difference between the two, the color steel plate manufacturers will tell you what are the specific differences between them.

6 inch corrugated roofing sheets

The specific differences are as follows:

1. During the refining process, the substrate of the color-coated board is a steel strip used for coating. The front and back of the color-coated board are very important. Usually, the two surfaces of the color-coated board have color, coating properties, and High quality requirements. Color-coated board has strong hardness and flexibility. Its coating is resistant to mechanical effects such as scratches, friction, collision, and press-in. The coating of the color-coated board and the substrate are deformed together without damage. .

From the end of production, color-coated boards have an interval between the performance of the original coating and the time it must be overhauled to maintain its protective effect on the substrate. Under the influence of use and over time, it will gradually deteriorate, and a major repair must be performed to make it more durable.

corrugated metal roofing sheets

2. The color steel plate is based on cold-rolled steel plate, and after degreasing and phosphating, it is hot-dip galvanized on both sides under the protection of nitrogen, and a chemical conversion film that is easy to adhere firmly to the primer is applied. He was painted with an epoxy primer, and finally a chemical-resistant color finish.

The color plate is a color steel plate, which is coated with a layer of polymer material on the steel plate, which is divided into one side and two sides. Most of the single side has a pattern on the surface.

The above two major points are the differences between the color coated plate and the color steel plate. Although the two names are similar, the production method and the scope of application are different, so we must choose our own according to our actual situation when choosing to use it. We need to learn to distinguish between the required product and the type of building used in different boards. Only boards that meet the requirements for use can play a good effect.