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What Is Galvanized Steel?

Jul. 14, 2020
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Galvanized steel refers to the ordinary carbon construction steel after galvanizing can effectively prevent steel corrosion and rust to prolong the service life of steel. Among them, galvanizing is divided into electro-galvanizing and hot-dip galvanizing. Generally used in building exterior walls, such as glass curtain wall, marble curtain wall, aluminum curtain wall as pillars and stressed materials, or used in outdoor telecommunications towers, highways, and other open-air construction steel called galvanized steel, which is divided into electro galvanized and thermal Dip galvanized.

Classification

Hot-dip galvanized steel

Hot dip galvanized steel pipe is to make molten metal and iron substrate react to produce an alloy layer so that the substrate and the coating are combined. Hot-dip galvanizing is to pickle the steel pipe first, in order to remove the iron oxide on the surface of the steel pipe, after pickling, wash through the ammonium chloride or zinc chloride aqueous solution of ammonium chloride and zinc chloride mixed aqueous solution tank, and then send into Hot dip plating bath. Hot-dip galvanizing has the advantages of uniform coating, strong adhesion, and long service life. The hot-dip galvanized steel pipe substrate and the molten plating solution undergo complex physical and chemical reactions to form a corrosion-resistant zinc-iron alloy layer with a compact structure. The alloy layer is integrated with the pure zinc layer and the steel pipe substrate. Therefore, its corrosion resistance is strong.

Cold galvanized pipe

The cold-galvanized pipe is electro-galvanized, the amount of galvanized is very small, only 10-50g/m2, its own corrosion resistance is much different than that of hot-dip galvanized pipe. Regular galvanized pipe manufacturers, in order to ensure quality, most do not use electro-galvanized (cold plating). Only those small enterprises with small scale and outdated equipment use electro-galvanized, of course, their prices are relatively cheaper. At present, the Ministry of Construction has officially eliminated cold-dip galvanized pipes with backward technology. In the future, cold-dip galvanized pipes are not allowed to be used as water or gas pipes. The galvanized layer of the cold-dip galvanized steel pipe is an electroplated layer, and the zinc layer and the steel pipe substrate are layered independently. The zinc layer is relatively thin, and the zinc layer simply adheres to the steel pipe substrate and is easy to fall off. Therefore, its corrosion resistance is poor. In newly built houses, the use of cold-galvanized steel pipes as water supply pipes is prohibited.

Galvanized Steel Water Pipe

Galvanized Steel Water Pipe

Development History

It is often said that the use of galvanized pipes and galvanized pipes are now galvanized pipes.

The production process of Galvanized Steel Water Pipe has the following production steps

a. Round steel preparation; b. Heating; c. Hot rolled piercing; d. Head cutting; e. Pickling; f. Grinding; g. Lubrication; h. Cold rolling; i. Skim; j. Solution heat treatment; k. Straightening; l. Cut the tube; m. Pickling; n. Product testing.

Technical requirements for galvanized pipes

1. Brand and chemical composition

The grade and chemical composition of the steel for galvanized steel pipes shall comply with the grade and chemical composition of the steel for black pipes as specified in GB 3092.

2. Manufacturing method

The manufacturing method of the black tube (furnace welding or electric welding) is selected by the manufacturer. Galvanizing adopts a hot-dip galvanizing method.

3. Thread and pipe joint

a: Galvanized steel pipe with thread delivery, the thread should be made after galvanizing. The thread should comply with YB 822.

b: Steel pipe joints should comply with YB 238; malleable cast iron pipe joints should comply with YB 230.

4. Mechanical properties The mechanical properties of steel pipes before galvanizing should meet the requirements of GB 3092.

5. Uniformity of galvanized layer Galvanized steel pipe should be tested for the uniformity of galvanized layer. Steel pipe samples are continuously immersed in copper sulfate solution.

6. Cold bending test The galvanized steel pipe with a nominal diameter of not more than 50mm shall be subjected to cold bending test. The bending angle is 90°, and the bending radius is 8 times the outer diameter. No filler is used during the test, and the welding seam of the sample should be placed outside or above the bending direction. After the test, there should be no cracks on the sample and the zinc layer flaking off.

7. Hydrostatic test The hydrostatic test should be carried out on the black tube and the hydrostatic test can also be replaced by eddy current testing. The test pressure or eddy current inspection comparative sample size shall comply with the provisions of GB 3092. The mechanical properties of steel are important indicators to ensure end-use performance (mechanical properties) of steel. It depends on the chemical composition of the steel and the heat treatment system. In the steel pipe standard, according to different use requirements, the tensile properties (tensile strength, yield strength or yield point, elongation), hardness and toughness indicators, as well as the high and low-temperature properties required by users are specified.

①Tensile strength (σb): The maximum force (Fb) that the specimen withstands when it is broken during the tensile process, and the stress (σ) obtained from the original cross-sectional area (So) of the specimen is called the resistance Tensile strength (σb) in N/mm2 (MPa). It represents the maximum ability of metal materials to resist destruction under the action of tension. In the formula: Fb--the maximum force that the sample bears when it is broken, N (Newton); So--the original cross-sectional area of the sample, mm2.

② Yield point (σs): the metal material with yield phenomenon, the stress when the sample does not increase during the stretching process (keep constant) and can continue to extend, is called the yield point. If the force decreases, the upper and lower yield points should be distinguished. The unit of yield point is N/mm2 (MPa). Upper yield point (σsu): the maximum stress before the sample yields and the force first decreases; lower yield point (σsl): the minimum stress in the yield stage when the initial transient effect is not counted. In the formula: Fs--the yield force (constant) during the tensile process of the sample, N (Newton) So--the original cross-sectional area of the sample, mm2.

③ Elongation after breaking: (σ) In the tensile test, the percentage of the length that the gauge length increases after the sample is broken and the length of the original gauge length is called elongation. Expressed by σ, the unit is %. In the formula: L1--the gauge length after the sample is broken, mm; L0--the original gauge length of the sample, mm.

④Section shrinkage: (ψ) In the tensile test, the percentage of the maximum reduction in the cross-sectional area at the reduced diameter of the specimen after it is broken and the original cross-sectional area is called the section shrinkage. Expressed in ψ, the unit is %. In the formula: S0--the original cross-sectional area of the sample, mm2; S1--the minimum cross-sectional area of the reduced diameter after the sample is broken, mm2.

⑤ Hardness index: The ability of metal materials to resist the depression of hard objects on the surface is called hardness. According to different test methods and application scope, the hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, Shore hardness, microhardness, and high-temperature hardness. For pipes, Brinell, Rockwell and Vickers hardness are commonly used.

A. Brinell hardness (HB): Use a certain diameter steel ball or cemented carbide ball to press into the sample surface with the specified test force (F), remove the test force after the specified holding time, and measure the pressure on the sample surface Mark diameter (L). The Brinell hardness value is the quotient obtained by dividing the test force by the indented spherical surface area. Expressed by HBS (steel ball), the unit is N/mm2 (MPa).

Elements that affect the performance of galvanized steel pipes

(1) Carbon; the higher the carbon content, the higher the hardness of the steel, but the worse its plasticity and toughness.

(2) Sulfur; it is harmful debris in steel. Steel with high sulfur content is easy to be brittle when it is processed at high temperature under pressure, usually called thermal brittleness.

(3) Phosphorus; can significantly reduce the plasticity and toughness of steel, especially at low temperatures, this phenomenon is called cold brittleness. In high-quality steel, sulfur and phosphorus should be strictly controlled. But from another aspect, the low-carbon steel contains high sulfur and phosphorus, which can make it easy to cut, which is beneficial to improve the machinability of the steel.

(4) Manganese; can improve the strength of steel, weaken and eliminate the adverse effects of sulfur, and can improve the hardenability of steel. High alloy steel with high manganese content (high manganese steel) has good wear resistance And other physical properties.

(5) Silicon; can increase the hardness of steel, but the plasticity and toughness decrease and the steel used in electrical engineering contains a certain amount of silicon, which can improve the soft magnetic properties.

(6) Tungsten; can improve the red hardness and thermal strength of steel, and can improve the wear resistance of steel.

(7) Chromium; can improve the hardenability and wear resistance of steel, can improve the corrosion resistance and oxidation resistance of steel.

In order to improve the corrosion resistance of steel pipes, general steel pipes (black pipes) are galvanized. Galvanized steel pipes are divided into hot-dip galvanized steel and electric steel zinc. The thickness of hot-dip galvanized steel is thick, and the cost of electro galvanizing is low, so there are galvanized steel pipes.