Basic knowledge of hot dip galvanized steel sheet and steel strip

Basic knowledge of hot dip galvanized steel sheet and steel strip

First, the advantages of hot galvanized sheet

Hot dip galvanizing and its alloy is one of the most common, effective and economical processes for resisting atmospheric corrosion of steel materials.

Hot dip galvanized steel plate has ideal corrosion resistance, forming, coating and other comprehensive properties as well as low cost, good appearance and other advantages, mainly used in construction, automobile and household appliances industries.

The electrochemical potential of hot-dip galvanized layer is more negative than that of steel material, and the coating has excellent electrochemical protection performance. The coating is thick and dense, the coating has strong bonding force with the steel matrix, and the durability is good. The service life of 600g/m2 hot-dip galvanized steel can reach more than 30 years. Hot dip galvanized steel layer in the process of use without maintenance, hot plating process is simple, strong adaptability to the shape and size of steel, in addition to plate steel, hot plating process is also widely used in steel pipe, steel wire and structural parts, and high productivity. Compared with other protective coatings economically, galvanized coating has a strong competitiveness, especially for long-term use of parts, the cost of hot-dip galvanized coating is much lower than that of paint coating.

Two, the use of hot dip galvanized steel plate

The most important application fields of hot-dip galvanized steel plate abroad are construction, automobile and electrical three industries, which occupy the majority of the consumption proportion of hot-dip galvanized steel plate. However, the domestic situation is somewhat different. In addition to construction and automobile industry, light industry is also a big user, and agriculture, animal husbandry, fishery and business also need a large amount.

Three, galvanized brand and technical requirements

1. Name the brand of galvanized steel plate and steel strip

Common brands are DX51D+Z, DX52D+Z, DC51D+Z, DC52D+Z, S350GD+Z and so on. The grade of galvanized steel plate or steel strip is composed of five parts: product use code, steel grade code, steel type characteristics, hot dip galvanized code (D), +, and total type code of coating. The specific grade naming method is as follows:

A) Galvanized sheet for cold forming:

Brand examples: DX51D+Z(or ZFAZ), DC51D+Z(or ZFAZ), DD52D+Z (or ZFAZ) DX: the first letter D represents the flat steel for cold forming, the second letter if X, represents the rolling state of the substrate is not specified; If the second letter is C, it means that the substrate is cold rolled. If the second letter is D, it means that the substrate is specified as a hot-rolled substrate.

51~57: two-digit number indicates steel grade number;

A D after two digits is the code name of hot dip galvanizing;

Z represents pure zinc plating, ZF represents galvanized iron alloy and AZ represents aluminum-zinc plating.

So: DX51D+Z means that the product is used for cold forming, flat steel, not specified substrate state, steel grade serial number 51, pure zinc coating, hot plating products.

B) Examples of naming other galvanized brands

S350GD+Z: S represents structural steel, 350 represents minimum yield strength of 350Mpa, G represents steel type characteristics are not specified, D represents hot dip galvanized, +Z represents zinc coating.

HX340LAD+ZF: indicates that the product is used as a high strength flat steel for cold forming. The minimum yield strength is 340Mpa without specifying the state of the substrate. The steel type is high strength and low alloy steel, zinc-iron alloy coating, or hot plating.

HC340/590DPD+Z: indicates that the product is used for cold forming, high strength flat steel, the specified substrate is cold rolled substrate, the specified minimum yield strength is 340Mpa, the specified minimum tensile strength is 590Mpa, the steel type is dual-phase steel, hot plated with pure zinc coating.

Code for surface quality

Normal grade surface FA

The higher surface FB

Advanced surface FC

Surface treatment

(1) Chromic acid passivation (C) and chromic acid-free passivation (C5)

The surface treatment can reduce the white rust on the surface of the product during transportation and storage, and should be limited to blunt E-Zn-6%-Al-3%-Mg alloy plated steel plate (ZAM) without chromium passivation treatment.

ZAM alloy coated steel plate is a new type of zinc base alloy coating with the highest corrosion resistance. The corrosion resistance of the zinc coating is 18 times that of the traditional zinc coating (Zn-0.2% Al) in the laboratory cycle test. The reason is that the addition of magnesium changes the microstructure of the coating, and magnesium can inhibit the formation of non-protective corrosion products such as zinc carbonate and zinc oxide for a long time in the corrosive environment. The corrosion products of zinc coatings containing magnesium have excellent effect of inhibiting cathode reaction. ZAM alloy coated steel plate is mainly used in the construction industry.

Five, galvanized plate corrosion resistance principle

1. Basic principles of corrosion

The definition of metal corrosion is the physical and chemical interaction between metal and its environment, which leads to the change of metal properties and often damages the metal functional environment or the process system of making products. Metal corrosion includes chemical corrosion and electrochemical corrosion.

A Chemical corrosion

Metal is destroyed by direct chemical reaction with surrounding medium, no electric current is generated. It occurs in dry atmospheres or in anhydrous organic liquids. Including high temperature oxidation, high temperature vulcanization, high temperature carburizing, decarburization, high temperature hydrogen corrosion.

B electrochemical corrosion

Electrochemical corrosion is the corrosion of metal by electrode reaction in humid environment, in which electrons are gained and lost and current is generated. The corrosion of most metals in most natural environments is electrochemical corrosion.

2. Corrosion resistance of galvanized steel plate

In neutral or weakly acidic (PH> 5.2) under the atmospheric environment of galvanized steel plate after corrosion of the corrosion products are non-soluble compounds (zinc hydroxide, zinc oxide and zinc carbonate), these products will precipitate out in the form of dense thin layer, generally up to 8μm, this film has a certain thickness and is not easy to dissolve in water, adhesion and strong, Therefore, it can play a barrier role in isolating the atmosphere and galvanized sheet to prevent further development of corrosion.

In the protection of galvanized layer is damaged, the surface of the steel part is exposed to the atmospheric environment, at this time, zinc and iron form a micro battery, zinc potential is significantly lower than the potential of iron, zinc as an anode on the steel substrate sacrificial anode protection effect, prevent the corrosion of steel plate. If the surface of the steel plate is too exposed for the electrolyte to cover the damaged surface, the iron will quickly corrode as if it were unprotected.

Vi. Production process of continuous hot dip galvanizing

1. Typical continuous hot-dip galvanized steel strip production process (Meilang Method)

2. Quality requirements of hot dip galvanizing on the original plate

a) Annealing: The original hot-dip galvanized plate is usually cold-rolled steel strip, and the cold-rolled steel strip can complete the following functions through annealing: first, the rolling stress can be eliminated in the annealing furnace to improve the mechanical properties of the steel strip; The second is to clean the strip surface, rolling oil on the surface of the steel strip, etc

Hexavalent chromium components harmful to human health in chemical film squadron;

(2) Chromic acid passivation + oiling (CO) and chrome-free passivation + oiling (CO5)

This finish can further reduce white rust on the surface of the product during transportation and storage.

(3) Phosphating (P) and phosphating + oiling (PO)

The surface treatment can reduce white rust during transportation and storage and improve the formability of the steel sheet.

(4) Fingerprint resistant film (AF) and non-chromium fingerprint resistant film (AF5)

Surface treatment can reduce the product during transportation and storage surface rust, no chromium passivation treatment, should be limited to passivation film squadron harmful to human health of the hexavalent chromium components.

(5) Oiling treatment (O)

To reduce the production of white rust in storage and transportation, the antirust oil is generally not used as rolling oil and stamping oil for subsequent processing.

(6) Do not deal with (U)

It must be indicated in the contract when ordering. In this case, the surface of the steel plate and steel belt is easy to form white rust and black spots during transportation and storage. The user should be careful when choosing.

Four, the classification of galvanized sheet

1, according to the galvanized substrate classification

(1) common carbon steel galvanized

Continuous hot dip galvanizing of steel strip is mostly made of ordinary mild steel, whose carbon content is between 0.10% and 0.20%. The characteristics of galvanizing are very similar to those of industrial pure iron. Carbon and silicon are the most influential elements in the hot-dip galvanizing of ordinary mild steel. Generally speaking, the higher the carbon content in steel, the more intense the zinc-iron reaction, the greater the iron loss, and the thicker the alloy coating is generated, making the coating brittle, plastic, coating adhesion is very poor. In addition, the high carbon content in the steel will also affect the surface quality of the coating, often appear iron leakage, but also easy to form zinc nodules.

With the development of the automobile industry, more and more high requirements have been put forward for the cold-working formability of common carbon steel such as deep drawing. The carbon content of steel is expected to be reduced as far as possible, for example, the carbon content of general stamping steel is less than 0.10%, and the carbon content of common steel is not big

At 0.08% (e.g., 08Al), the carbon content of high-quality stamped steel is no more than 0.04% (e.g., K08Al), while the carbon content of ultra-deep stamped steel (e.g., IF steel without interstitial atoms) is no more than 0.005%.

(2) In order to meet the needs of weight reduction, energy saving and safety of the automobile industry, one of the main users of hot-dip galvanized steel plate, the hot-dip galvanized steel plate used by automobiles as the base plate of high-strength steel plate has been rapidly developed. At present, the high strength steel plates that have been developed or are being developed abroad include IF high strength steel, baking hardening steel, low alloy high strength steel, double phase steel and phase change induced plastic steel, carbon manganese high strength steel, martensitic steel and so on, and they can reach the strength level.

IF no gap atom deep drawing steel; IS isotropic steel; Mild steel; BH baked hardened steel; CMn carbon manganese high strength steel; HSLA high strength low alloy steel; DP two-phase steel; CP multiphase steel; TRIP phase change induced plastic steel; MART martensitic steel

2, according to the coating classification

(1) Classification by zinc flower coating

A large zinc flower hot galvanized sheet

This is one of the most original production of galvanized steel plate, as we all know, zinc ore and lead ore total symbiosis, in zinc smelting, it is impossible to remove all lead impurities, the content of 0.05% or more can form large zinc flowers. Each zinc flower, its crystallization center is raised, the edge is concave, its surface is uneven appearance, which is unfavorable to its later processing and coating. In addition, because the solid solution of lead in zinc is very low, it will precipitate at the grain boundary during cooling, resulting in stress and electrochemical corrosion at the grain boundary, so that the galvanized layer produces intergranular corrosion and reduces its corrosion resistance. Therefore, in recent years, people gradually have the understanding of this, the galvanized sheet with large zinc flowers will gradually be eliminated.

B small zinc flower hot galvanized sheet

Because the zinc layer is thicker on the edge of zinc flower crystallization and the core part is thinner, the surface of large zinc flower galvanized sheet is uneven and not suitable for coating or coating, because the coating area will still emerge the outline of zinc flower and affect the appearance of the steel plate. In addition, because of the poor corrosion resistance of large zinc coating, it is inclined to use small zinc coating with uniform appearance. Therefore, by changing the cooling condition when the steel strip is drawn from the zinc pot, the crystal state of the zinc layer can be adjusted, so as to obtain small zinc flowers. For large-scale steel strip continuous hot dip galvanizing production line, small zinc flowers are often obtained by spraying water vapor or water air mixture onto the surface of liquid zinc layer. When a large amount of fine water drops on the surface of the zinc liquid layer, it increases the number of crystal nuclei and accelerates the solidification of the zinc liquid layer, shortens the time of crystal growth, and thus obtains fine zinc flower crystals (diameter below 2-3mm).

C polished zinc flower galvanized sheet

According to the need can also be the small zinc galvanized steel plate for finishing treatment, will eliminate the zinc flowers to obtain a smooth zinc layer, this galvanized steel strip is commonly used as the substrate of coating steel plate.

D zinc-free hot galvanized sheet

In order to improve the coating performance of galvanized steel plate and meet the requirements of automobile manufacturing industry, the zinc flower-free galvanized steel plate can be obtained by removing the lead from the zinc pot in the process of hot-dip galvanizing. This so-called zinc-free hot-dip galvanized steel plate is actually a hot-dip galvanized steel plate with a diameter of zinc flowers less than 1mm.

(2) coating thickness and differential thickness

A double-sided galvanized sheet of equal thickness

B Double-sided galvanized sheet of unequal thickness (differential thickness)

C single-side hot-dip galvanized steel plate

One side of the steel plate galvanized and the other side is not galvanized, it is to meet the needs of automotive materials and appear. This galvanized steel plate is economically advantageous and can save a lot of zinc raw material. The non-galvanized side of the single-side galvanized steel plate is conducive to surface painting and welding after phosphating treatment, which is used for the exterior of the automobile panel; The galvanized side is used inside the car panel to solve the corrosion protection problem. Because there are many problems in hot plating technology of single-sided galvanized sheet, it is not widely used at present.

(3) other functional coatings

A Color galvanized steel plate

Color galvanized steel plate refers to the surface of galvanized plate, without chemical treatment such as passivation dyeing or coating, natural color, such as blue, red, green, yellow and other colors of galvanized products, corrosion resistance is better. For example, when nickel and titanium are added to zinc solution, different impregnation and cooling conditions can produce different colors of yellow, purple and blue surfaces. When manganese and copper are added to the zinc solution, iridescent coatings can be formed under the right conditions.

B Fingerprint resistant hot dip galvanized steel plate

The polished plate rolled by the woolly polishing work roller is supported by fingerprint resistant organic coating, which is favored by electrical manufacturers for its excellent corrosion resistance, fingerprint resistance coating adhesion and weldability. At present, in some developed countries and regions, there are many manufacturers of fingerprint resistant products. Can also be through the existing passivation device, drying device for transformation, and add a set of simple cooling system, using extrusion dry method to produce fingerprint-resistant galvanized products.

(4) coating composition

A Pure zinc hot dip Galvanized Steel Sheet (GI)

Pure zinc hot dip galvanized steel plate, as the name suggests, does not add any element to the zinc pot as an alloy coating in hot dip galvanized. In fact, a small amount of aluminum and a small amount of lead are often added to the zinc pot in order to meet the technological needs of pure zinc hot plating. Under normal circumstances, zinc ingot contains a small amount of lead, about 0.003%-1.75%, the solubility of lead in zinc liquid is about 1.2% at 450-455℃, so a small amount of lead has no effect on the formation of Fe-Zn alloy layer. But it can reduce the melting point of zinc. The low melting point of zinc liquid makes the zinc liquid layer on the surface of galvanized steel plate solidify for a long time when it is put out of the zinc pot, so as to form larger zinc crystals and produce large zinc flowers hot-dip galvanized steel plate.

B alloyed galvanized steel plate (GA)

Alloyed galvanized steel plate is a steel plate in the production line of pure zinc hot dip galvanized steel strip. After the steel strip is pulled out from the zinc pot and entered the alloying furnace for alloying treatment, the pure galvanized layer on the steel surface is transformed into Zn-Fe alloy layer. Alloyed galvanized steel has excellent formability, weldability and coating property besides excellent corrosion resistance. It can meet the requirements of automobile industry and electric appliance industry.

C 55% Al-Zn alloy Coated Steel Plate (GL)

Commonly known as aluminum plated zinc steel plate, commercial name Galvalume, plated alloy composition

55%Al-43.4%Zn-1.6%Si. 55%Al-Zn alloy coated steel plate has better corrosion resistance to atmosphere, water and soil than ordinary pure zinc coated steel plate. In atmospheric environment, the corrosion resistance of 55%Al-Zn alloy coated steel plate is 2 to 6 times that of ordinary pure zinc coated steel plate, and the electrochemical protection of shear edge is better than that of aluminum coated steel plate. In addition,

55%Al-Zn alloy coated steel plate also has better high-temperature oxidation resistance, better thermal radiation and thermal reflection resistance, its surface zinc flowers fine, luster beautiful, it is used in the construction industry and electrical industry.

D Zn -- 5%Al -- RE alloy Plated Steel Plate (ZA)

Commodity name Galfan, in atmospheric environment, its corrosion resistance is. % / times that of ordinary pure zinc coated steel plate, and its comprehensive corrosion resistance, including bare plate corrosion resistance, cutting part sacrificial anode protection, corrosion resistance after processing and coating after the ideal. The weldability and stamping formability of the alloy coated steel plate are excellent, among which the processing property is almost comparable to that of galvanized steel plate. Zn-5% Al-Re alloy coated steel plate can completely replace ordinary pure zinc hot coated steel plate, widely used in construction, automobile, electrical and other industries.

E Zn -- 6%Al -- 3%Mg alloy Plated Steel Plate (ZAM)

ZAM alloy coated steel plate is a new type of zinc base alloy coating with the highest corrosion resistance. The corrosion resistance of the zinc coating is 18 times that of the traditional zinc coating (Zn-0.2% Al) in the laboratory cycle test. The reason is that the addition of magnesium changes the microstructure of the coating, and magnesium can inhibit the formation of non-protective corrosion products such as zinc carbonate and zinc oxide for a long time in the corrosive environment. The corrosion products of zinc coatings containing magnesium have excellent effect of inhibiting cathode reaction. ZAM alloy coated steel plate is mainly used in the construction industry.

Five, galvanized plate corrosion resistance principle

1. Basic principles of corrosion

The definition of metal corrosion is the physical and chemical interaction between metal and its environment, which leads to the change of metal properties and often damages the metal functional environment or the process system of making products. Metal corrosion includes chemical corrosion and electrochemical corrosion.

A Chemical corrosion

Metal is destroyed by direct chemical reaction with surrounding medium, no electric current is generated. It occurs in dry atmospheres or in anhydrous organic liquids. Including high temperature oxidation, high temperature vulcanization, high temperature carburizing, decarburization, high temperature hydrogen corrosion.

B electrochemical corrosion

Electrochemical corrosion is the corrosion of metal by electrode reaction in humid environment, in which electrons are gained and lost and current is generated. The corrosion of most metals in most natural environments is electrochemical corrosion.

2. Corrosion resistance of galvanized steel plate

In neutral or weakly acidic (PH> 5.2) under the atmospheric environment of galvanized steel plate after corrosion of the corrosion products are non-soluble compounds (zinc hydroxide, zinc oxide and zinc carbonate), these products will precipitate out in the form of dense thin layer, generally up to 8μm, this film has a certain thickness and is not easy to dissolve in water, adhesion and strong, Therefore, it can play a barrier role in isolating the atmosphere and galvanized sheet to prevent further development of corrosion.

In the protection of galvanized layer is damaged, the surface of the steel part is exposed to the atmospheric environment, at this time, zinc and iron form a micro battery, zinc potential is significantly lower than the potential of iron, zinc as an anode on the steel substrate sacrificial anode protection effect, prevent the corrosion of steel plate. If the surface of the steel plate is too exposed for the electrolyte to cover the damaged surface, the iron will quickly corrode as if it were unprotected.

Vi. Production process of continuous hot dip galvanizing

1. Typical continuous hot-dip galvanized steel strip production process (Meilang Method)

2. Quality requirements of hot dip galvanizing on the original plate

a) Annealing: The original hot-dip galvanized plate is usually cold-rolled steel strip, and the cold-rolled steel strip can complete the following functions through annealing: first, the rolling stress can be eliminated in the annealing furnace to improve the mechanical properties of the steel strip; The second is to clean the strip surface, the rolling oil and other dirt on the surface of the steel strip through the heating process of volatilization, combustion and removal, so that the steel strip has a clean surface, and the steel strip sealed into the zinc pot for hot dip galvanizing; Third, at the same time of completing the annealing process, a layer of micro oxide film on the surface of the steel strip is reduced to a pure iron layer by hydrogen in the furnace, so as to prepare a surface state with strong adhesion for hot dip galvanizing; Fourth, after annealing and reduction, the temperature of the steel strip entering the zinc pot can be accurately controlled in the annealing furnace by fast cooling and slow cooling, so that the plating process of the steel strip can be completed at the optimal temperature of zinc plating. Five is to maintain or improve galvanized steel strip shape.

Seven, galvanized plate common defects

1. Defects of the substrate: some defects will occur on the surface of the steel strip in the process of hot rolling and cold rolling, and in the follow-up hot-dip galvanizing

The surface quality of galvanized layer will be affected. Common substrate defects are of the following types.

a) Sandwich (Shell) -- Distributed along the rolling direction (WR represents rolling direction), zinc is covered on the surface of the steel plate in a cortical shape, without forming an alloy layer, and there are often non-metallic inclusions or oxide iron sheet on the steel substrate below the zinc layer;

b) Blowholes -- straight strip deposits with a raised appearance, which is a manifestation of discontinuity within the steel strip;

c) Holes - discontinuous holes, which are the result of broken materials. The thinner the steel strip, the more obvious this is.

The main reason is that there is serious mechanical damage on the surface of steel strip before rolling

Become;

d) Scale defects -- These defects are fine pits or small projections where the coating has sunk.

There are two reasons for this: oxidation pits and scars left over during hot rolling, failure to eliminate them during cold rolling, or not pickling during cold rolling

The shape and distribution density of the oxide skin are not the same as that of the foot.

e) Starting material scratches -- there are two forms of scratches on the surface of the board, (1) hot rolling, pickling and cold rolling scratches. The defects are mostly trench stripes along the rolling direction, and the galvanized layer is usually caused by alloy layer

Fast growth leads to thicker coating; (2) Thermal abrasion, which is short and dense in the running direction of the steel strip

The striate eminence of the set;

f) Rust and coolant stain -- Rust on the raw material will result in a rough, pitted surface on the surface of the galvanized layer. The emulsion residue is the thermal decomposition residue of the emulsion on the surface of cold rolled steel strip

Dark spot like. This defect occurs in the residue of red rust or emulsion on cold rolled steel strip. The emulsion residue is

Because the strip is not cleaned or not blown dry during cold rolling. ;

g) Cold rolling marks -- bumps or pits that appear periodically;

h) Edge/ central waves -- Edge and central waves of the plate are caused by defects in the profile of the substrate, and also by improper crown and position of idlers on hot-dip galvanizing lines. Edge waves can also be cut by edge too thick, edge blade

Improper spacing adjustment or contact between the edge of the steel strip and the fixed member, such as the guide;

i) Roll uplift (ridge), also known as convex edge, is a reflection of the uneven thickness of the original plate along the transverse. Hot rolling defects cannot be completely eliminated by cold rolling;

j) stretcher strain marks -- defects on the surface of steel plate or strip, such as slip line and "orange peel", which damage the appearance due to aging.

2. Defects caused by hot dip galvanizing process

a) Furnace roll pimples -- This defect is a pit or sheet indentation in the plating, attributed to the furnace

The surface of the roller has sediment adhesion, which is sometimes taken away by the steel belt, resulting in bumps or pits on the surface of the steel belt.

b) Flakes of scale -- have the appearance of patches, which are coated with thicker flakes and are later

Cracking and shedding will occur during processing. This defect is caused by the oxide coating formed on the surface of the steel strip during the stay in the annealing furnace entering the zinc pan with the steel strip, and the adhesive patch of the oxide coating also adhered to the zinc liquid. It is also possible that the oxidized skin on the surface of the zinc liquid in the furnace nose is embedded in the zinc layer.

c) Dross -- Dross is an unevenly distributed inclusions formed longitudinally in galvanized coating

The zinc ash at the nose of the furnace or the fine granular zinc slag from the steel belt;

d) Ribs or coaling sag of the coating -- the ribs or coaling sag of the coating are not streaks along the width of the steel strip

Regular thickening is usually caused by high aluminium content (greater than 0.25%) in the zinc solution or by low temperature of the steel brought into the zinc pan. This defect can be eliminated by alloying.

3. Defects formed during post-processing

a) Chromate passivation defects -- brownish yellow marks on the surface of zinc layer after chromate passivation treatment. They may appear throughout the width and length of the steel strip, and appear as stripes, elongates or irregular spots. The defect is caused by inadequate squeezing of chromate solution due to surface wear of extrusion roll or insufficient extrusion pressure after chromate passivation.

b) White rust -- uneven grayish-white powder on the surface of the galvanized layer, which may cover the entire surface of the steel sheet in extreme cases. White rust is the corrosion product on the surface of the zinc layer caused by moisture.