Process comparison of annealing and pickling line for cold rolled stainless steel

After the cold rolling treatment, the hardness and strength of stainless steel sheet are significantly improved, forming performance is insufficient, and it is difficult to be put into the market, so a series of follow-up treatment should be carried out. Among them, the cold-rolled stainless steel annealing and pickling line is to carry out processing annealing for cold-rolled sheet, remove the strip stress, remove the oxide scale formed during processing annealing, and finally carry out finishing and straightening treatment to improve the surface quality of the strip and obtain satisfactory products. Therefore, it is of great practical significance to study the process comparison of annealing and pickling line for cold-rolled stainless steel.

Cold rolled stainless steel, annealing and pickling

After annealing and pickling of hot rolled stainless steel strip, cold rolling should be carried out in order to meet the corresponding performance and thickness requirements. In the process of cold rolling of stainless steel, work hardening occurs, and the work hardening level will increase with the increase of cold rolling amount. If the temperature of work hardening material is raised to 900-1100 ℃, the deformation stress can be eliminated, and then recrystallization will occur at the elevated temperature, so that the material will be softened. Annealing after cold rolling can be divided into continuous horizontal annealing and vertical bright annealing according to annealing mode, and into intermediate annealing and final annealing according to annealing process.
Process comparison of annealing and pickling line for cold rolled stainless steel

Traditional technology

The traditional cold-rolled stainless steel annealing and pickling line covers the following aspects:

  • ① The entrance area, which consists of uncoiler, shearer, straightener, etc., and is mainly used for uncoiling.
  • ② Welding machine, usually choose narrow lap seam welding machine, choose seam welding rolling process to carry out welding work.
  • ③ The degreasing section consists of four functional sections, including spray washing section, brush washing section and cleaning section machine drying section one by one. The degreasing medium is mainly alkali solution and hot water in rinsing section, and air knife is selected for drying.
  • ④ Loop, horizontal loop including outlet loop, inlet loop, etc.
  • ⑤ For annealing furnace, the horizontal suspension type gas annealing furnace is usually selected to raise the temperature of steel strip to 1050-1150 ℃, so as to eliminate the work hardening and internal stress formed during steel strip rolling. The components include preheating section, heating section, air cooling section and water mist cooling section.
  • ⑥ Pickling section, which is mainly used to eliminate the rust on the surface of the steel strip during annealing in the furnace area. The components include neutral salt electrolysis area, scrubbing area, pickling area, cleaning section and dryer, etc. During the stainless steel annealing and pickling production, HF + HNO3 mixed acid pickling is often used to remove the oxide layer on the surface of the steel strip produced by the annealing process of the hot rolling mill, and at the same time, the stainless steel surface is passivated by pickling to enhance the corrosion resistance of the steel plate.
  • ⑦ The exit section consists of a shearing machine, a coiler and a flattening machine, which are mainly used for winding and winding.

Acid pickling without nitric acid

The difference between this technology and the traditional process is mainly in the pickling section. Nitric acid free pickling is to add HF, H2O2, wetting agent and stabilizer into H2SO4, so as to prepare a mixed acid washing solution with the mechanism of H2SO4, HF and H2O2. At the beginning of preparation, an appropriate amount of Fe3 + should be added to guide the reaction between H2SO4 and iron oxide scale by the oxidation of Fe3 +, so as to make the oxidation through the activation performance of HF The cracks in the iron sheet pass through the acid solution and enter the chromium poor layer under the iron oxide sheet, which is easy to form reaction. It makes the acid react easily with Fe and Cr, and forms a lot of H2. With the help of the stripping effect of H2, iron oxide scale can be eliminated.
The main process flow of nitric acid free pickling is: preheating → alkali leaching → quenching water → rinsing → sulfuric acid pre pickling → rinsing → cleanox pickling → high pressure water spraying → peroxynitric acid passivation → high pressure water spraying → neutralization.
Nitric acid free pickling technology has various advantages:

  • ① Advanced technology in environmental protection, no nitrogen oxide will be formed in pickling process, which can effectively reduce the “three wastes” and harmful substances in waste acid, and it does not contain nitrate which is very difficult to deal with;
  • ② It has a very wide range of applicability, which can deal with a series of steel plates, pipes, etc. such as ferrite, martensite, etc.;
  • ③ Process flow from During the pickling process, only the iron oxide scale will be dissolved, while the base metal will not be greatly affected;
  • ④ The pickling efficiency is good, the surface quality is good, and the service cycle of acid solution is long.

Dalnox brighttm Technology

Compared with the traditional technology, the main difference of dalnox brighttm technology is that it greatly reduces the oxidation of annealing process. Based on this, the pickling process can be carried out without or in a small amount to further improve the surface quality, prevent pickling and reduce the neutralization amount of waste liquid. Select special heat cycle of strip steel, strictly control annealing heating and cooling in each treatment process to achieve the control of oxide film production, especially pay attention to the process with high temperature.
The main process flow of dalnox brighttm technology is as follows:

  • ① The fast heating section starts in the controlled oxidation atmosphere, in which oxide aggregate and oxide film are produced;
  • ② In the annealing stage, if we want to restrict the development of oxide layer, we should realize the metallographic change in the non oxidation atmosphere;
  • ③ Cooling section, cooling rate to prevent carbide precipitation, non oxidizing atmosphere;
  • ④ Scientific and standard regulation of the scale segment of point release;
  • ⑤ A small amount of chemical pickling section reduces surface passivation and environmental impact.

Dalnox brighttm technology has a variety of advantages:

  • ① The production level is about the same as that of the traditional annealing and pickling line;
  • ② Excellent flexibility, whether it is to produce 2D product surface or similar to 2R surface can be applied;
  • ③ Acid pickling link can be reduced or omitted, further reducing management cost and investment cost;
  • ④ Reduce the pollution of ecological environment and further reduce the cost of production and emission treatment;
  • ⑤ It can be applied to both the existing plant transformation and the new plant. As far as the plant transformation is concerned, only the furnace section needs to be transformed, and the pickling section does not need to be changed, so as to promote the acquisition of good economic benefits.

Comparison of the three

The technical maturity, operation cost and environmental protection of traditional process, acid pickling without nitric acid and dalnox brighttm technology are compared: 

  • ① In terms of technical maturity, after a long time of development, the traditional process has been relatively mature and perfect, and all equipment can be independently produced in China, and the product quality can meet the standards; acid pickling without nitric acid has been popularized in China in recent years According to the production practice, the technology is very reliable and mature, and the product quality meets the requirements; dalnox brighttm technology is still in the stage of equipment experiment, even though the experimental production products can reach 2R level, its maturity still needs further research. 
  • ② In terms of operation cost, the operation cost of traditional process is compared with that of acid pickling without nitric acid. The operation cost of the latter is about 5 yuan per ton higher than that of the former. However, there is no clear data on the operation cost of dalnox brighttm technology. However, according to the relevant cost research, the operation cost of production of AISI304 can be reduced by about three layers – four layers compared with the traditional process. 
  • ③ In terms of environmental protection, the traditional process selects neutral salt electrolysis combined acid pickling process, which will form a lot of nitrogen oxides during pickling. After the treatment of these harmful substances, high environmental protection cost will be eliminated; when acid pickling without nitric acid is selected, no nitrogen oxide will be formed during pickling, which can effectively reduce the harmful substances in “three wastes” and waste acid, and it does not contain extremely difficult to treat Because of the reduction of oxide layer, dalnox brighttm technology can save acid pickling process to a certain extent, so it has no significant impact on the environment.

Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry (www.pipelinedubai.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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process comparison of annealing and pickling line for cold rolled stainless steel - Process comparison of annealing and pickling line for cold rolled stainless steel
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Process comparison of annealing and pickling line for cold rolled stainless steel
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After the cold rolling treatment, the hardness and strength of stainless steel sheet are significantly improved, forming performance is insufficient, and it is difficult to be put into the market, so a series of follow-up treatment should be carried out.
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