Basic knowledge of galvanized sheet

There is a big market for galvanized steel plate in our country, and its consumption is also a large amount, among which a considerable number of galvanized steel plate (weight) should be imported every year. The common imported galvanized sheet is hot galvanized sheet, electrogalvanized sheet and galvanized lacquered sheet. Imported galvanized sheet, there are high quality (first grade) galvanized sheet, there are also non-high quality galvanized sheet, that is, the so-called "second grade". In these different quality goods, there may be external claims; On the other hand, the import duty of the good quality steel is not the same as that of the non - good quality steel. In the daily inspection or customs clearance work often encounter the situation of declaring for non-superior quality. The entry-exit inspection and quarantine department, which inspects the quality of commodities, should attach importance to quality control, promote market economy and trade, and safeguard national interests and tariff interests.

At present, there is no current national standards or producer's standards or international standards in our country for the inspection and identification of non-high quality of galvanized steel plate. Generally speaking, the inspection and identification of non-quality galvanized steel plate is based on its similar quality standards. Therefore, in the practice of steel inspection, it is of great significance to explore and master the correct inspection methods and inspection scales to determine the quality grade of goods, and summarize a set of effective experience methods in practice.

one Standard for quality inspection of galvanized steel plates

According to production and processing methods, galvanized steel plate can be divided into the following categories [1] : hot dip galvanized steel plate (commonly known as galvanized iron sheet), electric galvanized steel plate, single-sided or double-sided difference thickness galvanized steel plate, alloy composite galvanized steel plate, in addition to the above several, there are color galvanized steel plate (commonly known as color plate), printing coated galvanized steel plate, PVC laminated galvanized steel plate, etc. In addition, according to the purpose can be divided into general use, roof, building lateral plate, structure, corrugated board, stretching and deep drawing galvanized steel plate.

The quality requirements of high quality galvanized sheet include dimensions, appearance, amount of galvanizing, chemical composition, shape, mechanical properties and packaging.

1. packing

It is divided into two kinds: galvanized sheet cut into fixed length and galvanized sheet packing with coil. Generally, it is packed in iron sheet, lined with moisture-proof paper and bound with iron kidneys. The binding is firm to prevent the internal galvanized sheet from rubbing with each other

2. Specification and size

The recommended standard thicknesses, lengths and widths of galvanized sheets and their allowable deviations are specified in the relevant product standards (as and below). In addition, the width and length of the board, the width of the volume can also be determined according to user requirements.

3. appearance

Surface state: Because of the different treatment methods in the plating process, the surface state of galvanized plate is different, such as ordinary zinc flowers, fine zinc flowers, flat zinc flowers, zinc flowers and phosphating surface. The galvanized sheet and galvanized coil cut to a fixed length shall not have defects affecting the use (detailed below), but the coil is allowed to have some abnormal parts such as welding parts.

4. Galvanizing amount

Galvanized quantity standard value: galvanized quantity is a generally used effective method to express the thickness of galvanized sheet zinc layer. There are two kinds of galvanized quantity the same on both sides (equal thickness galvanized) and zinc quantity different on both sides (differential thickness galvanized). The unit of galvanized quantity is g/m2.

5. Mechanical property

(1) Tensile test: Generally speaking, only structural, tensile and deep tensile galvanized sheet has tensile performance requirements.

(2) Bending test: it is the main project to measure the process performance of sheet metal. But the standard of various countries on the requirements of galvanized sheet is not consistent. Generally, after the galvanized sheet is bent 180o, the outer surface shall not be separated from the zinc layer, and the substrate shall not be cracked and fractured.

6. Chemical composition

The requirements for the chemical composition of galvanized substrate are different in different countries. If Japan does not require it, the United States does. Inspection of finished products is not generally carried out.

7. Plate shape

There are two indicators to measure the shape of the plate, namely flatness and sickle bend. The flatness of the plate and the maximum allowable value of the sickle bend are specified.

The main foreign standards related to galvanized sheet are listed below for reference [4,5] :

JIS G3302 galvanized steel sheet

Electric galvanized steel sheet and steel strip

General Requirements for Hot Dip Galvanized Sheet Steel

Commercial Grade Hot Dip Galvanized Sheet Steel

ASTM 527 Bite Molding grade Hot Dip galvanized sheet steel

ASTM 528 Deep drawing grade Hot Dip Galvanized Sheet Steel

Hot dip galvanized sheet steel for roofing and siding

ASTM A444 Hot Dip Galvanized Sheet Steel for Ditches

ASTM A446 Structural Grade Hot Dip Galvanized sheet steel

1. Hot dip galvanized steel plate

Continuous hot dip galvanizing is widely used in the production process of hot dip galvanized steel. The process of continuous hot dip galvanizing is simple and can be described as:

The original plate; Pretreatment of plating (cleaning and annealing); Hot dip galvanizing; Post-plating treatment (such as zinc flower treatment, alloy treatment and chemical treatment, etc.); Finished product

* Advantages of hot dip galvanizing

1, low processing cost: the cost of hot dip galvanized rust prevention is lower than that of other paint coating;

2, durable: in the suburban environment, the standard thickness of hot dip galvanized rust prevention, can be maintained for more than 50 years without repair; In urban or offshore areas, the standard hot-dip galvanized rust protection layer can be maintained for 20 years without repair;

3, good reliability: galvanized layer and steel is metallurgical combination, become a part of the steel surface, so the durability of the coating is more reliable;

4. The toughness of the coating is strong: the galvanized layer forms a special metallurgical structure, which can withstand mechanical damage during transportation and use;

5, comprehensive protection: every part of the plating can be plated with zinc, even in the depression, sharp corners and hidden places can be fully protected;

6, save time and effort: galvanizing process is faster than other coating construction method, and can avoid the time required to brush the above picture on the site after installation;

l has double cathodic protection performance and good shielding protection, salt spray test up to 9500 hours, corrosion resistance is better than hot dip galvanized and zinc rich paint. It is the best material to replace hot dip zinc and hot spray zinc (aluminum).

l can be used alone, as a heavy anti-corrosion coating, can also be used as a base coating with other supporting coating combination, its comprehensive anti-corrosion life can be increased by 1.8 ~ 2.5 times, up to 30 ~ 50 years.

Zinc content is very high (zinc content in dry film is more than 96%), the product does not contain any lead, cadmium and other heavy metals, does not contain toluene, xylene, methyl chloride, methyl ethyl ketone and other organic solvents, zinc and dry film coating can be in direct contact with drinking water.

Zinc plus dry film contains more than 96% pure zinc enough to guarantee excellent cathodic protection for steel plate. As long as 9500 hours of salt spray experiment proved: even in the extremely harsh erosion environment, zinc coating can still sacrifice itself and form protection on the surface of steel, its protection principle is similar to hot dip plating, corrosion protection is better than hot dip zinc.

Passive protection: screen protection

When zinc is oxidized, a zinc salt layer is slowly formed on the zinc coating surface to provide barrier protection. In turn, the binder in zinc addition provides an additional layer of barrier protection, which slows zinc oxidation. So the corrosion resistance of zinc is much better than that of zinc spray (aluminum), hot dip galvanizing and other zinc rich coatings.

Thus, zinc plus effectively combines the galvanizing properties of the zinc layer with the barrier protection of the organic coating. Excellent anti-rust effect on steel.

Hot dip galvanizing, also known as hot dip galvanizing, is to melt the zinc ingot at high temperature, put some auxiliary materials in, and then dip the metal structure into the galvanized groove, so that the metal component is attached to a layer of zinc layer. The advantage of hot dip galvanized is that its anticorrosion ability is strong, the adhesion and hardness of galvanized layer is better. The disadvantage is that the price is high, the need for a lot of equipment and site, the steel structure is too large to be put into the galvanized groove, the steel structure is too thin, hot plating and easy to deformation.

Traditional cold galvanized has been considered electroplating, is the use of electrolytic principle in some metal surface plating on a thin layer of other metal or alloy process, electroplating has too many defects, electroplating production to use a large number of strong acid, strong base, salt and organic solvents and other chemicals, in the process of operation will emit a large number of toxic and harmful gases, such as safety management

If it is not done well, poisoning, burning, combustion and explosion can easily occur, so electroplating will eventually be replaced by another kind of cold galvanizing.

This cold galvanizing is one of the things I talk about below. It is a simple galvanizing process. The principle is that it plays a cathodic protection role on steel in the process of electrochemical corrosion, so it can prevent the diffusion of rusty parts. Rust prevention effect due to the zinc powder content of the coating and the thickness of the coating and change, Shanghai Luobalu rich zinc coating Co., Ltd. production and sales of cold galvanized coating is this principle, Luobalu cold galvanized dry film zinc content of more than 96%, is a single component coating, simple to use, convenient construction, but also a kind of environmental protection coating. Brush coating, roller coating, gas spray, airless spraying, etc. It can be used in Bridges, iron towers, airports, ships, steel structure factories, hot dip galvanizing plants, machinery and other metal structural parts that need corrosion prevention and rust prevention. It is the best material to replace hot spray zinc aluminum and hot dip galvanizing.

Considering the production process and finished product packaging and other factors, hot-dip galvanized steel plate may have some surface defects as follows:

(1) Defects of the original plate

Surface defects of galvanized original plate, such as creases, pits and wavy edges, continue to affect galvanized plate and become surface defects. Therefore, before production, the manufacturer must check the original plate.

(2) Dark spots [3]

a. The purity of galvanized layer is not enough, metal impurities on the surface of zinc form galvanic cells, which are easy to produce black spots on the surface of zinc layer when it is favorable for hydrogen evolution.

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

c. The surface of the original plate is not sufficiently treated with dirt, forming the residue, so that some parts can not be plated with zinc, leading to problems in the galvanized layer and easy to fall off, resulting in black spots.

d. When galvanized, there is leakage plating, which is easy to form black spots in post-processing.

e. The zinc solution contains residue or small black ash (carbon or carbon compounds), which adheres to the surface or resides in the zinc layer and is firmly combined with the zinc layer. Black spots can be seen on the finished galvanized sheet.

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

(3) white rust

The surface of galvanized steel plate is soaked in moisture or rain. At a certain temperature, zinc is oxidized and white powder is produced. The corrosion resistance of the surface producing white rust is greatly reduced, which affects its use.

(4) Uneven galvanized layer

Galvanized sheet surface zinc layer uneven, uneven thickness, and even surface small particles. The reason is that when hot dip galvanized, the air knife is not evenly pumped, or there is excess zinc liquid on the surface of the air knife and the strip steel coming out of the galvanized pot. The excess zinc liquid is blown off with the air knife (spraying high pressure gas), so as to control the thickness of the zinc layer. The gas pressure of air knife, the distance between air knife and strip steel, and the speed of strip steel will directly affect the thickness of zinc layer. If the gas supply of the air knife is unstable or the pressure of the air knife is insufficient, the accumulation of galvanized layer will be caused, that is, zinc scar.

(6) slag inclusion

There is residue or scum (metal or non-metallic inclusions) in galvanized liquid, sometimes these slag inclusions in galvanized layer, Shanghai model ship model mechanical model industrial model to make its surface appear small round spots, even the original plate with zinc, it is easy to cause iron exposure point.

(7) Scratches and bruises, etc

Galvanized coil after tension straightener, straightener has foreign body caused by surface scratches, bruises, etc.

(8) chromic acid fouling

In order to improve the anti-corrosion ability of galvanized steel plate, a very thin passivation layer is formed on the surface of strip steel, which can be completed by chromic acid treatment, namely chroming treatment. But if the chromic acid treatment is poor, there is residual liquid, galvanized sheet surface will appear light yellow belt or stain, to black spots.

2. Electric galvanized steel plate

The difference between electrogalvanized sheet and hot galvanized sheet is that there is no zinc flower on the surface, the surface is not bright, and the amount of galvanized is low.

Similar to the production process of hot dip galvanized steel plate, the production process of electric galvanized steel plate is also continuous plating method:

Pretreatment (cleaning)  galvanized  plating post-processing cleaning purpose is to remove grease, iron oxide on the surface of the steel strip and all dirt, keep the strip surface is very clean. The quality of electrogalvanized sheet depends on the washing quality before electroplating to a large extent.

The main purpose of electrogalvanizing post treatment is to improve the surface coating performance and corrosion resistance of galvanized sheet, so as to prolong the service life of galvanized sheet. Post-plating treatment mainly includes phosphating treatment and chroming treatment. In addition to the defects of the original board, any improper operation of a process will cause quality problems. General surface defects are as follows [6] :

1. Pinhole: refers to the fine holes on the surface of the coating that have been chiseled similar to the needle tip. Although their density and distribution are not the same, they are generally similar in size and shape when observed under a magnifying glass. Pinholes are usually defects caused by the adsorption of hydrogen bubbles during electroplating. In the electroplating reaction, a large number of bubbles are produced. If these bubbles are not eliminated in time, they will remain between the strip surface and the anode. The anode and the strip surface are covered by gas layer and affect the conduction, resulting in the local plating on the strip surface.

2. Pitting: irregular pits on the surface of the coating, characterized by shape, size and depth. Pitting is generally due to substrate defects or foreign body adhesion in the plating process.

3. Burr (or rough) : refers to the surface of the coating protruding and prickly feeling of foreign bodies, usually characterized by plating to the upper or high current density area is more significant. The thickness of zinc layer is controlled by controlling current density and strip speed, that is, increasing current density and decreasing strip speed can improve the thickness of zinc layer. But this is a certain limit, when the speed of the strip is very low, the current density is high, the coating surface will become rough, but affect the quality of the coating.

4. Bubbling: The raised vesicles on the surface of the coating, characterized by varying size, density, and separation from the substrate. Generally in zinc alloy, aluminum alloy coating is more obvious.

5. Peeling (or peeling) : Cracked or non-cracked defects in the coating and substrate peeling. It is usually caused by poor treatment before plating.

6. Spots: refers to the surface of the coating stains, dark spots and other defects. It is caused by poor deposit of metal ions in plating process, foreign body adhesion or passivation solution cleaning after passivation treatment.

7, Yin and Yang surface: refers to the coating surface of local brightness or uneven color defects, in most cases in similar products show a certain regularity.

8. Local no coating: there are defects of missing plating, exposed iron spots or spots on the surface of the coating with different areas and shapes (except process regulations).

In addition to the above surface defects, the coating surface sometimes has damage, abrasion, white rust, roll marks, pits, creases, black spots, unwashed salt marks, water marks, erasable or brown or brown passivation film and dendritic, spongy and striped coating defects. All these defects will have a certain impact on the use of galvanized sheet.

(5) Quality: The galvanized quantity, mechanical properties, chemical composition and other indicators are inspected. If one or some of these indicators are found to be inconsistent with the nominal value of the goods label, it can be considered as unqualified products and can be identified as non-excellent quality goods.

Synthesize the above five factors, appraise the quality of goods in a balanced way.

Three. Check operation

The inspection of galvanized steel plate, for products produced according to different standards, should refer to the specific product label or contract standards and requirements. In the process of inspection, comprehensive consideration of the above mentioned quality requirements.

1. Methods for identifying non-premium grade products

In the actual inspection process, it is found that some indicators such as size, weight, plate shape, galvanized amount, chemical composition, mechanical properties, etc. can generally meet the standard requirements indicated on the goods label, so these indicators have little influence on the inspection and identification of non-high quality grade, not the main influencing factor. Surface appearance is an important basis for identification. In fact, the identification of non - quality grade galvanized steel plate is largely caused by surface appearance defects. The packaging should be analyzed in detail. Some goods are in standard packaging, but there are many surface appearance defects after unpacking, while some goods are in simple packaging, or packaging damage, or even bare packaging. When unpacking, except the head or tail (about 10-30 meters), there are no defects harmful to use on the surface of the goods.

2. Evaluation of the degree of use of harmful defects

The harmful surface appearance defects described here are for use. If there is more serious leakage plating, in this case, the loss of anti-corrosion protection of the substrate, easy to cause the substrate rust, thus affecting the use of. Standards in various countries

For the coil plate, it is allowed to include a number of abnormal parts and welding parts. National standards are not very clear about specific defects unless specified in the order contract.

There are two implications for the degree of use of harmful surface appearance defects. One is the primary and secondary position of a certain surface appearance defect in identified defects, that is, whether it is the main defect or not. The second is the magnitude of a defect degree, that is, whether the defect is serious or minor. For example, leakage plating, peeling (or peeling), white rust, rust, black spots, damage, abrasion, etc., are the main defects, while roller marks, burrs, poor galvanized, yin-yin-yin-surface, creases, pits, twig coating, striate coating, etc., are less important defects. For another example, if a roll of galvanized sheet has appearance defects only in the range of 10-30 meters at the head and end, but not in other parts, or if only a few parts in a batch of goods have defects, then relatively speaking, the batch of goods can not be considered as non-quality grade.

Therefore, in addition to the understanding of the production process of galvanized steel plate to analyze and judge the appearance of defects, but also have a comprehensive view of the whole batch of goods, according to the above five aspects to specifically measure the scale of the judgment. In addition, due to the relative differences in product quality between countries, this is also a possible factor.

3. Expression of test results

As mentioned above, at present, there is no publicly released standard of second-level galvanized steel plate at home and abroad, that is to say, there is no clear standard for inspection and quarantine. Therefore, in the inspection and quarantine certificate can not make a decision on whether the second grade, only according to the quality of the grade (grade one) standard boundary definition. After the completion of appearance inspection, mechanical property testing, chemical composition analysis and other work, make quality assessment of the inspection results: for qualified inspection, list the inspection results in the certificate and give the evaluation opinion of "conforming to the provisions of certain standard"; If the result is found inconsistent with the standard in the inspection, the certificate will give the evaluation opinion of "X item does not meet the provisions of X standard", or "for non-high quality grade goods". When goods are mixed and it is difficult to make a qualitative opinion on whether the whole batch of goods is good or bad, it can only be the inspection situation and inspection results of good and bad goods.