In the production of modern stainless steel strips, annealing and pickling units are generally used for pre- and post-cold-rolling treatment. Annealing and pickling are important processes in the production of stainless steel hot-rolled and cold-rolled coils, and their principles and processes have a very important guiding role in actual production. The selection of annealing equipment, the selection of pickling methods, and the determination of pickling conditions directly affect the final quality of stainless steel products.

Stainless steel annealing process

1 . Tropical annealing
The hardness of stainless steel increases after hot rolling, accompanied by the precipitation of carbides. The purposes of stainless steel annealing of different steel types are not the same.

Solution Treatment of Austenitic Stainless Steel
Austenitic stainless steel will precipitate carbides after hot rolling, and the grains will be deformed with processing. The purpose of annealing austenitic stainless steel is to heat the steel to the annealing temperature (1010-1150°C). The precipitated carbides are fully dissolved in the austenite at high temperature, and then rapidly cooled (water cooling and quenching), so that the carbon dissolved in the austenite is kept at room temperature. At the same time, the grain size is adjusted during annealing to achieve the purpose of softening.
Due to the need for rapid cooling to prevent re-precipitation of carbides, only continuous annealing furnaces are used for hot-rolled austenitic stainless steel.

Annealing of ferritic and martensitic stainless steel
Most ferritic stainless steels will produce certain martensite during the cooling process after hot rolling. The main purpose of annealing is to decompose the martensite into ferrite and granular or spherical carbides to achieve the purpose of softening ; In addition, annealing can also make the grains elongated in rolling become equiaxed grains.
Martensitic stainless steel is austenite at high temperature, martensitic transformation occurs during cooling after hot rolling, and high hardness martensite structure is obtained at room temperature. The purpose of annealing is to decompose this martensite into spherical carbides uniformly distributed on the ferrite matrix to soften the steel.
It takes a long time for the precipitation, aggregation and spheroidization of carbides during annealing of martensitic stainless steel and ferritic stainless steel, so the hot-rolled coils of this type of stainless steel are usually annealed in bell furnace. Of course, for single-phase ferritic steel that does not have martensite after hot rolling, it is more reasonable to choose a continuous furnace.

2. cold band annealing
Stainless steel cold strip annealing is divided into two types: intermediate annealing and finished annealing.
The purpose of intermediate annealing is to eliminate work hardening in the cold rolling process, so that the cold-rolled stainless steel strip can be softened by recrystallization annealing for the next rolling process. The final annealing process is generally the same as the intermediate annealing process, and individual products are specially treated according to requirements.
Continuous annealing furnaces are used for cold strip annealing. Generally, a horizontal cold belt annealing and pickling unit is used. For BA boards, it needs to enter the bright annealing production line for processing.

Pickling process of stainless steel strip

The iron oxide scale produced on the surface of stainless steel during hot rolling and heat treatment will have adverse effects on subsequent processing and surface quality, and it needs to be removed in time. As pickling is an important process for removing scale in the production of stainless steel hot-rolled and cold-rolled coils, its principle and process have a very important guiding role for actual production. The choice of pickling method and the determination of pickling conditions directly affect the final quality of stainless steel products.

1. stainless steel oxide scale

Structure and characteristics of stainless steel oxide scale
When stainless steel is heated or rolled, iron oxide scales are formed on the surface. The composition of scale depends on the steel grade and the affinity of iron and other alloying elements for oxygen. The 300 series stainless steel oxide scale contains Fe2O3, Fe3O4, FeO•Cr2O3 and NiO•Cr2O3, Cr2O3 from top to bottom; the 400 series stainless steel oxide scale contains Fe2O3, Fe3O4, FeO•Cr2O3, Cr2O3 from top to bottom.
Stainless steel oxide scale mainly contains Cr2O3 and spinel FeO Cr2O3, usually black, sometimes blue or green. It is octahedral equiaxed crystal system, vitreous luster, conchoidal cross-section, relative density is 3.5-5.21, melting point is as high as 2435°C, hardness is 7.5-8.5, and it is not soluble in inorganic acids such as H2SO4, HCl or HNO3 at a temperature of 80°C .

Removal of stainless steel scale
The main methods for removing scale from stainless steel coils are chemical pickling and mechanical scale breaking.
The mechanical descaling method is used for the pretreatment of hot-rolled coils before pickling. It is divided into descaling roll (repeated bending method) and shot blasting method. Generally, the two are used in combination.
There are two types of chemical pickling methods: acid leaching and acid electrolysis. Acid leaching generally uses acids such as sulfuric acid, nitric acid + hydrofluoric acid in stainless steel pickling. Among them, sulfuric acid has a prominent pickling effect only at higher temperatures, so it is generally used for pickling hot-rolled coils. Acid solution electrolysis is divided into nitric acid electrolysis (passivation), sulfuric acid electrolysis and so on.
In addition, a neutral salt electrolysis section is used for pretreatment before pickling of cold-rolled coils.

2. Pickling Process of Hot Rolled Stainless Steel Strip
The pickling of hot-rolled stainless steel strip is now mostly using sulfuric acid + mixed acid (nitric acid + hydrofluoric acid) process. Before pickling, it is necessary to mechanically break scales to remove part of the iron oxide scale or crisp oxide scale.

Sulfuric acid pickling
During sulfuric acid pickling, the following chemical reactions occur on the strip surface:
Fe2O3 + 3H2SO4 → Fe2(SO4)3+ 3H2O
Fe3O4+4H2SO4 → Fe2(SO4)3+FeSO4+4H2O
FeO + H2SO4 → FeSO4 + H2O
3Cr2O3+4 H2SO4 → Cr2(SO4)3+CrSO4+4 H2O+5/2O2
NiO+ H2SO4 → NiSO4+ H2O (300 series stainless steel)
Various metal oxides in the iron oxide scale are dissolved in sulfuric acid solution to form a water-soluble sulfuric acid compound, thereby removing the iron oxide scale on the surface of the billet. In addition, the iron in the steel reacts with the sulfuric acid solution to produce a large amount of hydrogen, and the expansion pressure generated by the hydrogen peels off the iron oxide scale from the billet. This effect is called mechanical stripping. The stripping effect can speed up the pickling speed and reduce the consumption of sulfuric acid.
For different steel types, the conditions of sulfuric acid pickling are slightly different. The sulfuric acid concentration, metal ion (Me) content and temperature are shown in Table 1.

Mixed acid pickling
After sulfuric acid pickling, part of the oxide layer is removed, and the remaining oxide layer exists in the form of Fe-Cr-Ni oxides. Generally, a mixture of nitric acid + hydrofluoric acid is used to remove these complex components.
When stainless steel products pass through the mixed acid pickling tank, HNO3 chemically reacts with its surface Fe2O3, Fe, Cr, Ni and other substances to generate various soluble metal salts:
Fe2O3＋6HNO3→2Fe(NO3)3＋3H2O
Fe＋4H++＋NO3- ←→ Fe3++＋NO＋2H2O
Cr＋4H++＋NO3- ←→ Cr3++＋NO＋2H2O
3Ni＋8H++＋2NO3- ←→ Ni2++＋2NO＋4H2O (300 series stainless steel)
And HF reacts with various metal ions in the solution to generate some soluble or insoluble metal fluorides:
3HF+Fe3+ → FeF3↓+3H+
2HF+Fe3+ → FeF2++2H+
3HF＋Cr3+ → CrF3↓＋3H+
2HF+Cr3+ → CrF2++2H+
HF＋Ni2+ → NiF++＋H+
Conditions for mixed acid pickling of different steel types

3. Pickling Process of Cold Rolled Stainless Steel Strip

The pickling of cold-rolled stainless steel strips is now mostly using mixed acid (nitric acid + hydrofluoric acid) process. Neutral salt electrolysis and nitric acid electrolysis pretreatment are required before pickling

Neutral salt electrolysis
The neutral salt electrolysis method is a descaling method developed by RUTHNER Company in Austria. The strip steel is electrolyzed in a neutral salt solution such as sodium nitrate (NaNO3) or sodium sulfate (Na2SO4). Neutral salt electrolysis adopts the central conductor method, that is, the strip steel is suspended in the electric field between the cathode and the anode, and the anode and cathode of the strip steel are exchanged continuously. Sodium sulfate (Na2SO4) is mostly used in the electrolyte, and the following electrochemical reactions are carried out in electrolytic pickling:
Anode reaction (the steel plate is the cathode at this time):
H2O – 2e- → 1/2O2↑ + 2H+
Cr2O3 + 5H2O – 6e- → 2CrO42- + 10H+
Cr2O3 + 4H2O – 6e- → Cr2O72- + 8H+
Cr + 4H2O – 6e- → CrO42- + 8H+
Cathodic reaction (the steel plate is the anode at this time):
CrO42- + 4H2O+3e- → Cr(OH)3↓+5OH-
Cr2O72- + 7H2O+6e- → 2Cr(OH)3↓+8OH-
2H2O + 2e- → H2↑+2OH-

During the entire reaction process, Na2SO4 is only used as a conductive medium, responsible for the transfer of electrons, and no chemical reaction itself occurs. Theoretically, the electrolyte is not consumed, and only water is consumed. The biggest advantage of neutral salt electrolysis is that it can dissolve chromium oxide and convert it into water-soluble CrO42- under the action of electric current. In addition, the force formed by the H2 and O2 generated by the reaction escaping from the surface of the strip can strip the oxides on the surface.
The pH value of the electrolyte for neutral salt electrolytic pickling can be controlled between 3 and 10, and the electrolytic current density is generally controlled at 10 to 15A/dm2.

Nitric acid electrolysis (passivation)
The chromium oxide in the iron oxide scale can be easily dissolved by neutral salt electrolysis, and the iron oxide needs to be treated in mixed acid. However, mixed acid can effectively and quickly remove iron oxides on the surface of 300 series austenitic stainless steel. However, for 400 series, especially 410 (martensite), 430 (ferrite) and other stainless steels, the descaling performance is unstable, which will increase the surface roughness of the steel plate and make it dull. Moreover, mixed acid pickling also has the problem of slow dissolution rate in this type of steel, which cannot meet the requirements of continuous production. In order to not only maintain the characteristics of neutral salt electrolysis, but also improve the processing speed of chromium steel, after neutral salt electrolysis, nitric acid electrolysis is added to dissolve and remove residual iron oxides. However, the traditional HNO3+HF mixed acid pickling method is still used in the treatment of austenitic stainless steel.
The principle of nitric acid electrolysis is as follows:
anode:
H2O – 2e- → 1/2O2↑ + 2H+
cathode:
Fe3O4+8H+ + 2e- → 3Fe2+ + 4H2O
NO3-+H2O + 2e- → NO2-+2OH-
The maximum current density of nitric acid electrolysis is 6A/dm2.

Mixed acid pickling
The mixed-acid pickling principle of cold-rolled stainless steel strip is basically the same as that of hot-rolled, and the chromium steel does not pass through the mixed-acid pickling section after being electrolyzed with nitric acid.