Explanation of the process of improving the corrosion resistance of the steel plate surface
The performance of steel plate is usually different because of the service conditions, but the prerequisite for the corrosion resistance of steel plate surface has higher requirements.
The quality of steel plate mainly includes shape, dimension, precision, shape and internal quality. The chemical composition, metallurgical quality and processing technology of steel will have an impact on the use of steel plate. This paper studies the effect of improving the surface corrosion resistance of steel plate by improving the rolling process.
Surface structure analysis
The thickness, composition and structure of oxide film on the surface of steel plate with blistering phenomenon were analyzed by metallography, and then the results were obtained.
- (1) When the steel plate is subjected to a large temperature change, if the thickness of the oxide film is larger, it means that the oxide film on the surface of the steel plate will have a larger internal stress, resulting in fracture.
- (2) Under the condition of high temperature, oxide film will be formed on the surface of steel plate. If the temperature is higher, the thickness of oxide film will be correspondingly larger. According to the investigation, the relationship between the surface temperature of steel plate and the amount of oxidation is shown in the figure.
If the oxidation amount of the steel surface is 1, and the temperature at this time is 90°C, then when 1000°C, the oxidation amount of the steel sheet surface is 2, 1100°C, the oxidation amount of the steel sheet is 3.5, and at 1300°C, The oxidation amount on the surface of the steel sheet is 7; the higher the final rolling temperature, the more oxidation on the surface of the steel sheet.
Under normal conditions, there will be an oxide layer on the surface of the hot-rolled steel sheet, the thickness of which is mainly between 10μm and 50μm, and the surface of the steel sheet is mainly distributed in a sequence, the order is the outer layer Fe2O3, the middle layer Fe3O4 , The inner layer of FeO, and the inner layer of FeO is in a relatively stable state.
In the process of heating the steel plate, the three oxide phases formed on the surface of the steel plate are determined by the oxygen content; because the contact medium on the surface of the steel plate is relatively large, the oxygen content is relatively rich, resulting in a large amount of solids in Fe. It melts and becomes Fe2O3, while the middle layer of the steel plate becomes Fe3O4, and the inner layer becomes FeO.
During the hot rolling process of the steel sheet, the y-Fe2O3, Fe3O4, and FeO produced during heating and cooling are all cubic crystals, and the Fe3O4 is relatively denser, which can effectively avoid further oxidation.
Cooling process analysis
The process implemented by the iron and steel production enterprise is controlled cooling process. In order to ensure that the steel plate can meet the standard to the greatest extent, the weak cooling process is mainly adopted, and the cooling bed temperature is usually above 1000 ℃. When the weak cooling process is applied to the steel plate, the uneven cooling around and the uneven surface temperature are the important reasons for the surface blistering One of the reasons.
(1) Weak cooling process leads to uneven cooling of steel plate surface.
The water piercing device of steel rolling plant is composed of several small cooling rings. When the steel plate passes through each cooling ring, the steel plate will contact with the air. In this process, the steel plate will have water and attachments. If the water contains oil, this situation will be more serious. Under the action of gravity, the water carried out by the steel plate will lead to the uneven distribution on the surface of the steel plate, which directly leads to the bubbling on the surface of the steel plate.
(2) The cooling effect produced by cooling is not good.
The cooling water pressure of rolling mill is low, usually only about 0.4MPa, which is not enough to break the steam film on the surface of steel plate, and it is difficult to remove the secondary oxide film on the surface of steel plate, resulting in the appearance of new oxide film in the process of steel re tempering; the temperature drop caused by water cooling is very small, which is obtained by repeated tests in the whole water piercing section The surface temperature drop is usually only 30 ℃.
In this link, only the surface of the steel plate is cooled after water penetration, and the surface temperature of the steel plate will rise rapidly after water exit, and the thicker the steel plate is, the higher the reddening temperature will be. Therefore, blistering of thicker steel plate will be more common.
Application of improved rolling process to improve corrosion resistance of steel plate surface
In order to solve the bubbling problem on the surface of steel plate properly, through many tests, the following ways can be used to solve the problem.
Reduce tapping temperature
When the billet is discharged from the furnace, properly reducing the tapping temperature can effectively reduce the starting rolling temperature from 1100 ℃ to 1050 ℃, so as to alleviate the corrosion of steel plate.
Reducing the two stage rolling temperature
In the first stage of finishing rolling, the billet is equipped with a pre threading water piping system, which can reduce the two-stage rolling temperature to 900 ℃～ 950 ℃, which can not only effectively improve the performance of steel plate, but also effectively reduce the temperature of cooling bed on steel plate.
Adjustment of controlled cooling process
Through the adjustment of water pressure, flow rate, acceleration and other parameters in the process of controlled cooling and the improvement of nozzle equipment and water quality, the reddening temperature is reduced and the corrosion resistance of steel plate is improved.
The surface temperature drop is enhanced
In the process of controlled cooling, the temperature drop of the steel plate is 30 ℃ under the original weak controlled cooling process. After adjusting the controlled cooling process, the temperature drop of the steel plate can be about 80 ℃; after the steel plate enters the cooling bed, the temperature will gradually decrease from the original 1000 ℃ to 950 ℃.
Under the condition that the metallographic structure is in strict accordance with the international standard, the oxidation phenomenon on the surface of steel plate can be obviously reduced.
The surface quality is improved
The metallographic structure of the steel plate after rapid cooling is inspected. The results are shown in Fig. 3. It can be found that the thickness of oxide film on the surface of the steel plate decreases from 42um to about 16 μ M. Under the normal working condition of rapid cooling process, the bubbling phenomenon of steel plate can be effectively solved. Due to the integrity of the surface oxide film, the corrosion resistance of the steel plate has been effectively improved.
- ① The main causes of surface blistering of hot rolled steel plate are high temperature of upper cooling bed, insufficient cooling intensity and uneven cooling.
- ② The original weak cooling process can not provide good temperature drop, cooling uniformity and other conditions for the steel plate, so the oxide film on the surface of the steel plate is too thick, which is very easy to crack. This situation is a key factor of blistering and easy to rust on the surface of hot rolled steel plate.
- ③ By improving the rolling process, reducing the tapping temperature, the two-stage rolling temperature and the reddening temperature, the cooling bed temperature of the steel plate is adjusted below 1000 ℃, so that the corrosion resistance of the steel plate surface is greatly enhanced.
Author: Chen Yuanhao
Source: China Stainless Steel Plate Manufacturer – Yaang Pipe Industry (www.steeljrv.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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