Spot sales of high zinc coated galvanized steel pipe
Galvanized steel pipe is divided into cold galvanized steel pipe and hot-dip galvanized steel pipe. Cold galvanized steel pipe has been banned, and the latter is also advocated by the state to be used temporarily. In the 1960s and 1970s, developed countries in the world began to develop new pipes, and galvanized pipes were banned one after another. China's Ministry of construction and other four ministries and commissions have also made it clear that galvanized pipes have been banned as water supply pipes since 2000. Galvanized pipes are rarely used in cold water pipes in new communities, and galvanized pipes are used in hot water pipes in some communities. Hot dip galvanized steel pipe is widely used in fire fighting, electric power and expressway. Hot dip galvanized steel pipes are widely used in construction, machinery, coal mining, chemical industry, electric power, railway vehicles, automobile industry, highways, bridges, containers, sports facilities, agricultural machinery, petroleum machinery, exploration machinery and other manufacturing industries.
Welded steel pipe with hot-dip or electro galvanized coating on the surface of galvanized steel pipe. Galvanizing can increase the corrosion resistance of steel pipes and prolong their service life. Galvanized pipe is widely used. In addition to being used as pipeline pipe for water transmission, gas, oil and other general low-pressure fluids, it is also used as oil well pipe and oil transmission pipe in petroleum industry, especially in offshore oil fields, oil heater, condensate cooler and coal distillation oil washing exchanger of chemical coking equipment, trestle pipe pile, support frame pipe of mine tunnel, etc. Hot dip galvanized pipe is to make molten metal react with iron matrix to produce alloy layer, so as to combine the matrix and coating. Hot dip galvanizing is to pickle the steel pipe first. In order to remove iron oxide on the surface of the steel pipe, after pickling, it is cleaned in ammonium chloride or zinc chloride aqueous solution or ammonium chloride and zinc chloride mixed aqueous solution tank, and then sent to the hot dip galvanizing tank. Hot dip galvanizing has the advantages of uniform coating, strong adhesion and long service life. The matrix of hot-dip galvanized steel pipe has complex physical and chemical reactions with the molten plating solution to form a corrosion-resistant zinc ferroalloy layer with compact structure. The alloy layer is integrated with pure zinc layer and steel pipe matrix, so it has strong corrosion resistance. Cold galvanized pipe is electro galvanized. The amount of galvanized is very small, only 10-50g / m2. Its corrosion resistance is much different from that of hot-dip galvanized pipe. In order to ensure the quality, most regular galvanized pipe manufacturers do not use electro galvanizing (cold plating). Only those small enterprises with small scale and old equipment use electro galvanizing, of course, their price is relatively cheap. The Ministry of construction has officially announced that cold galvanized pipes with backward technology shall be eliminated and shall not be used as water and gas pipes. The galvanized layer of cold galvanized steel pipe is an electroplating layer, and the zinc layer is separated from the steel pipe substrate. The zinc layer is thin, and the zinc layer is simply attached to the steel pipe matrix, which is easy to fall off. Therefore, its corrosion resistance is poor. In new houses, it is forbidden to use cold galvanized steel pipes as water supply pipes.
Nominal wall thickness (mm): 2.0, 2.5, 2.8, 3.2, 3.5, 3.8, 4.0, 4.5.
Coefficient parameters (c): 1.064, 1.051, 1.045, 1.040, 1.036, 1.034, 1.032, 1.028.
Note: the mechanical property of steel is an important index to ensure the final service performance (mechanical property) of steel, which depends on the chemical composition and heat treatment system of steel. In the steel pipe standard, according to different service requirements, the tensile properties (tensile strength, yield strength or yield point, elongation), hardness and toughness indexes, as well as the high and low temperature properties required by users are specified.
Grade of steel: q215a; Q215B; Q235A; Q235B。
Test pressure value / MPA: d10.2-168.3mm is 3Mpa; D177.8-323.9mm is 5MPa
National standard and dimension standard of galvanized pipe
GB / t3091-2015 welded steel pipe for low pressure fluid transportation
Straight seam welded steel pipe (GB / t13793-2016)
GB / t21835-2008 welded steel pipe dimensions and weight per unit length
The common use of galvanized pipe is that the iron pipe used for gas and heating is also galvanized pipe. As a water pipe, galvanized pipe produces a large amount of rust in the pipe after several years of use. The yellow water not only pollutes the sanitary ware, but also mixed with bacteria breeding on the unsmooth inner wall. Corrosion causes high content of heavy metals in the water and seriously endangers human health.
The process flow is as follows: black tube - alkali washing - water washing - acid pickling - rinsing with clean water - Leaching additives - drying - hot dip galvanizing - External blowing - internal blowing - air cooling - water cooling - passivation - water rinsing - Inspection - weighing - warehousing.
1. Brand and chemical composition
The grade and chemical composition of steel for galvanized steel pipe shall comply with the grade and chemical composition of steel for black pipe specified in GB / t3091.
2. Manufacturing method
The manufacturing method of black pipe (furnace welding or electric welding) shall be selected by the manufacturer. Hot dip galvanizing method shall be adopted for galvanizing.
3. Thread and pipe joint
(a) For galvanized steel pipes delivered with threads, the threads shall be turned after galvanizing. The thread shall comply with Yb 822.
(b) Steel pipe joints shall comply with Yb 238; Malleable cast iron pipe joints shall comply with Yb 230.
4. Mechanical properties the mechanical properties of steel pipes before galvanizing shall comply with the provisions of GB 3091.
5. Uniformity of galvanized coating galvanized steel pipes shall be tested for uniformity of galvanized coating. The steel pipe sample shall be continuously immersed in copper sulfate solution for 5 times and shall not turn red (copper plating color).
6. Cold bending test: galvanized steel pipe with nominal diameter not greater than 50mm shall be subject to cold bending test. The bending angle is 90 °, and the bending radius is 8 times of the outer diameter. During the test without filler, the weld of the sample shall be placed on the outside or upper part of the bending direction. After the test, the sample shall be free of cracks and spalling of zinc layer.
7. Hydrostatic test the hydrostatic test shall be conducted in the black pipe, or eddy current flaw detection can be used instead of hydrostatic test. The test pressure or the size of the comparison sample for eddy current flaw detection shall comply with the provisions of GB 3092. The mechanical property of steel is an important index to ensure the final service performance (mechanical property) of steel,
① Tensile strength (σ b): the maximum force (FB) borne by the specimen during tension, divided by the original cross-sectional area (so) of the specimen（(σ), Called tensile strength(σ b) , in N / mm2 (MPA). It represents the maximum ability of metal materials to resist failure under tension. Where: FB -- the maximum force borne by the sample when it is broken, n (Newton); So -- original cross-sectional area of sample, mm2.
② Yield point (σ s) : for metal materials with yield phenomenon, the stress when the specimen can continue to elongate without increasing (keeping constant) the stress during the tensile process, which is called the yield point. If the stress decreases, the upper and lower yield points shall be distinguished. The unit of yield point is n / mm2 (MPA). Upper yield point(σ Su): the maximum stress before the yield stress of the sample decreases for the first time; Lower yield point(σ SL): the minimum stress in the yield stage when the initial instantaneous effect is not considered. Where: FS -- yield stress (constant) of the sample during tension, n (Newton) so -- original cross-sectional area of the sample, mm2.
③ Elongation after fracture:( σ) In the tensile test, the percentage of the length increased by the gauge length of the sample after breaking to the original gauge length is called elongation. with σ Expressed in%. Where: L1 -- gauge length after sample breaking, mm; L0 -- original gauge length of sample, mm.
④ Reduction of area:(ψ) In the tensile test, the percentage between the maximum reduction of the cross-sectional area at the reduced diameter and the original cross-sectional area after the specimen is broken is called the reduction of area. with ψ Expressed in%. Where: S0 -- original cross-sectional area of sample, mm2; S1 -- minimum cross-sectional area at the reduced diameter after sample breaking, mm2.
⑤ Hardness index: the ability of metal materials to resist the indentation surface of hard objects is called hardness. According to different test methods and application scope, hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, shore hardness, microhardness and high temperature hardness. Brinell, Rockwell and Vickers hardness are commonly used for pipes.
Brinell hardness (HB): press a steel ball or cemented carbide ball with a certain diameter into the sample surface with the specified test force (f), remove the test force after the specified holding time, and measure the indentation diameter (L) on the sample surface. Brinell hardness number is the quotient obtained by dividing the test force by the spherical surface area of the indentation. Expressed in HBS (steel ball), unit: n / mm2 (MPA).
(1) Carbon; The higher the carbon content, the higher the hardness of steel, but the worse its plasticity and toughness
(2) Sulfur; It is a harmful impurity in steel. Steel with high sulfur content is easy to embrittle during pressure processing at high temperature, which is usually called thermal embrittlement
(3) Phosphorus; It can significantly reduce the plasticity and toughness of steel, especially at low temperature. This phenomenon is called cold brittleness. In high-quality steel, sulfur and phosphorus should be strictly controlled. On the other hand, low carbon steel contains high sulfur and phosphorus, which can make it easy to cut, which is beneficial to improve the machinability of steel
(4) Manganese; It can improve the strength of steel, weaken and eliminate the adverse effects of sulfur, and improve the hardenability of steel. High alloy steel (high manganese steel) with high manganese content has good wear resistance and other physical properties
(5) Silicon; It can improve the hardness of steel, but the plasticity and toughness decrease. Electrical steel contains a certain amount of silicon, which can improve the soft magnetic properties
(6) Tungsten; It can improve the red hardness and thermal strength of steel, and improve the wear resistance of steel
(7) Chromium; It can improve the hardenability and wear resistance of steel, and improve the corrosion resistance and oxidation resistance of steel
In order to improve the corrosion resistance of steel pipe, the general steel pipe (black pipe) is galvanized. Galvanized steel pipe is divided into hot-dip galvanizing and electric steel zinc. The hot-dip galvanizing layer is thick and the cost of electric galvanizing is low, so there is galvanized steel pipe.