The new technology of annealing and pickling cold-rolled stainless steel and its advantages
According to the European standard EN10088/2, the cold-rolled stainless steel strip currently supplied is usually divided into two types of surfaces:
1) 2D/2B surface: the surface after cold rolling, annealing, pickling, and leveling;
2) 2R surface: the surface after cold rolling, bright annealing (BA), and leveling.
2D/2B surface, usually produced by horizontal high-yield continuous annealing and pickling line, the output can reach 150t/h, the annealing of the steel strip is carried out in an oxidizing atmosphere (combustion gas mixture). The oxide layer produced during annealing must be removed, and then the passivation state is restored. This process is usually completed by electrolytic descaling and chemical pickling. Chemical pickling generally uses a mixed acid solution (nitric acid and hydrofluoric acid). The resulting large amounts of pollutants, such as nitrogen oxides, nitrates in wastewater and sludge, require special treatment processes.
2R surface, produced on a vertical bright annealing line. The strip steel is annealed in a hydrogen-nitrogen mixture to control the dew point, thereby preventing the oxidation of the strip surface without the need for pickling. However, the vertical furnace limits the productivity, and currently, the maximum can only reach about 20t/h-25t/h. Due to its mirror characteristics and good appearance, 2R surface products are more popular than 2D/B.
By combining the main advantages of the two processes, innovative process technology has been developed and applied to the annealing and pickling treatment of cold-rolled stainless steel strips.
1 Process and technology
The globalization of the market is driving steel manufacturers to seek new processes and technologies to reduce production costs, increase competitiveness, improve product quality, and at the same time reduce the environmental impact of the production process. In pursuit of this goal, a new stainless steel strip production technology that can obtain an enhanced surface quality close to 2R has been developed, while maintaining the same production capacity and production cost as the traditional annealing and pickling line. Its further advantages are It can reduce the environmental impact of the chemical pickling process.
Compared with the traditional annealing and pickling line, the new process can drastically reduce the oxidation of the strip in the annealing section. Therefore, chemical pickling treatment can also be eliminated or reduced in use, so as to obtain a good surface quality, save the cost of pickling treatment and reduce the amount of waste liquid that needs to be neutralized. Controlling the formation of the oxide layer is through a special strip heat treatment cycle and precise control of the amount of oxidation in the annealing atmosphere of each process step (heating and cooling section), especially in the high-temperature process section. The key steps of this process technology include the following sections:
1) The rapid heating section under a controlled oxidizing atmosphere, where an oxide crystal nucleus and a thin oxide film are formed;
2) Annealing section, complete the transformation of metallurgy (to obtain the required mechanical properties, grain size, carbon solid solution, etc.), the annealing is performed in a non-oxidizing atmosphere (nitrogen) to limit the growth of the oxide layer;
3) In the cooling section under a non-oxidizing atmosphere, the cooling rate can avoid the precipitation of carbides;
4) Effective electro-removal of scales;
5) A slight chemical pickling section can reduce the impact on the environment and produce passivated surfaces. To get the best surface, this step can be eliminated.
2 Research and development
Starting from the existing stainless steel continuous annealing and pickling production process and the manufacturer’s technical know-how, the initial research and development are carried out in the processing facility in the laboratory. Later, the experimental verification of the new process was carried out on a specially designed and constructed continuous annealing experimental unit. The surface characteristics and pickling tests of the annealed sample plates were also carried out.
(1) Annealed
Research on the oxidation phenomenon of AISI 304 stainless steel under traditional annealing and new process, results show that in the initial heating section (850℃～950℃) of annealing under the new process, the reduction of oxidation is achieved by adopting a higher air-fuel ratio than the traditional process. Obtained, thereby forming a protective thin oxide layer in a controlled atmosphere. Annealing at a higher temperature (usually a large amount of oxidation is formed at this stage), by using a non-oxidizing atmosphere to prevent the growth of oxides, similarly, in the cooling section, using a non-oxidizing atmosphere also prevents further oxide production.
(2) Pickling
The pickling section of the traditional cold-rolled annealing pickling line is usually divided into two parts. An electrolysis section dissolves the oxide layer, and a chemical section (mixed nitric acid and hydrofluoric acid pool or ecological salt pool) removes residual oxide scale and restores the passivated surface by dissolving the inner reaction layer.
Because the new process reduces the generation of oxide layers, the thin oxide layer can be easily removed by simple electrolytic descaling in the traditional electrolysis section. After electrolytic treatment, the surface of the strip steel is non-oxidized, showing a surface quality close to 2R.
Finally, the slight chemical treatment (low temperature, low hydrofluoric acid content, short treatment time) ensures that even in the non-standard working environment of the annealing and pickling line (such as the transition period of the furnace caused by changes in production output and incoming materials, Production line speed reduction, etc.) can also achieve complete pickling. Because the oxidation phenomenon is greatly reduced and the surface reactivity of the steel strip is reduced, the final slight chemical pickling greatly reduces material loss and pickling time compared with the traditional process.
(3) Pilot plant
The annealing test line built by can continuously process 0.4mm～2.0mm thick and 300mm wide strip steel at a maximum unit speed of 10m/min. The unit is modular in design and can replicate different annealing cycles and furnace atmospheres. The refractory materials used can withstand temperatures up to 1400°C in the heating zone and are equipped with process control instruments. When the steel strip reaches the second zone of the furnace, the temperature reaches the highest, and at this time it is heated indirectly by resistance heating or radiant tubes. A large number of experiments were carried out on this unit, dedicated to seeking the best process conditions, proportionally increasing standards, and defining the corresponding industrial production process technology.
3 Application areas
Using this technology, all kinds of stainless steel (austenite, ferritic, and duplex stainless steel) can obtain surface quality close to bright annealing (BA) and standard 2B～2D. In the annealing process, only about 20% of the total energy that needs to be transferred to the steel strip is provided by indirect heating systems (such as resistance heating or radiant tube heating).
4 Cost evaluation
Cost analysis is based on local energy and material prices, taking into account the following production costs:
A) Energy (fuel) and process gas consumption in the annealing section;
B) Electricity and chemical consumption in the electrolysis section and the light chemical pickling section;
C) Waste and waste gas treatment;
D) Direct labor cost of factory operation;
E) Metal loss (yield rate) due to annealing and pickling;
Based on the above production costs, compared with the traditional annealing and pickling process, the new process can reduce the production cost of AISI 304 by about 35% to 40%.
5 Main advantages
In short, the main advantages of the new process technology are:
1) It has the same production capacity as the traditional annealing and pickling line;
2) High flexibility, whether it is the surface of the 2D product or the surface close to 2R;
3) Reduce or even eliminate chemical pickling, thereby saving equipment investment and management costs;
4) Reduce the impact on the environment, thereby reducing the cost of discharge and waste neutralization;
5) Better product surface quality;
6) This technology is suitable for both new factories and existing factory renovations. For the transformation of the existing plant, only the furnace section needs to be modified without any changes to the pickling section. The only reason is that the oxidation of the strip is reduced, thereby increasing the output and gaining profit.