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The surface of strip steel is oxidized iron the mechanism of pickling and the factors influencing pickling(三)

- Aug 20, 2018 -

The surface of strip steel is oxidized iron, the mechanism of pickling and the factors influencing pickling(三)


      Factors influencing the oxidizing of strip steel surface.

    Influence of final rolling temperature and speedThe oxidation process of iron is Fe--FeO--FeO--FeO, and the oxidation rate gradually increases as the temperature increases. At a temperature range of 600-800 degrees, the resulting iron oxide is a good barrier to the diffusion of iron and oxygen atoms, so the rate of oxidation does not continue to increase. When the temperature exceeds 800 degrees, the ability of iron oxide to prevent diffusion will be greatly reduced, so the oxidation speed will increase rapidly. Due to the increase of temperature, oxidation and speed, the thickness of iron oxide on the strip surface increases with the increase of oxidation temperature per unit time. In the same way, high rolling speed reduces the time for the strip to contact air at high temperatures, thus reducing the thickness of the oxide sheet. Therefore, in order to reduce the thickness of the oxide sheet, hot rolled strip should be rolled at the lowest temperature and the highest rolling speed possible.

    In terms of the structure of iron oxide, when the final rolling temperature is between 700-900 degrees, the formed iron oxide contains 80%-90% FeO, 10%-20% Fe and O4. In temperature greater than 900 degrees, oxide or oxide gas is large, the iron will quickly be oxidation, FeO, can rapid forming at high temperature, the oxidized iron except Fe3O4 FeO will not appear, and began to form in the metal surface Fe, m1, the surface is covered with a thin layer of Fe2O3o improve the rolling speed can reduce the thickness of the tin oxide, however, too high speed will make coiling temperature increase rapidly, and cause oxidative austenite transformed into FeO, rich in iron, after to clear the iron oxide on the strip surface (acid) work difficult. Therefore, accurate control of rolling speed is an important factor of favorable pickling.The results show that the temperature of final rolling should be 850 degrees to obtain the oxidized iron sheet with the best pickling time.

     The effect of speed

      We know that the general hot rolled strip steel surface oxidation iron has three layers, close to the lining of the base iron for rich austenite, intermediate for FeO4, outer Fe203, which is advantageous to the pickling rich austenite in over 575 degrees is stable, below 570 degrees when rich FeO in the austenite is not stable, and according to the four FeO - = = = - FeO, + Fe, when the temperature further reduced to 300 degrees, the shift will approach zero.If the iron oxide cortex cools rapidly between 570 and 300 degrees, the rich layer will not be able to decompose and be fixed at lower temperatures, resulting in a structure conducive to pickling the rich.In the case of slow cooling, the rich layer in the iron sheet gradually decreases as the cooling rate slows down. 

     Therefore, when the strip steel cools slowly in the cooling area, the rich layer in the iron sheet only exists in a small amount or is completely absent.In general, high-pressure water is sprayed on the output roller to speed up the cooling rate of the strip. In the case of water spraying, the thickness of iron oxide increases rapidly. Since oxidized iron forms faster in water than in air, the longer it stays in the vapor atmosphere, the more oxidized iron forms, while the content of FeO decreases. Therefore, it is necessary to accurately regulate the cooling speed in water sprinkler section and reduce the residence time in water as much as possible.

    The influence of coiling temperature

     When the coil temperature of strip is between 600-700 degrees, it has no obvious influence on the increase of thickness of sheet metal layer.However, the higher the coiling temperature, the Fe2O3 will be generated at the edge and end of the strip, which is not desirable.

     Further reduction of coiling temperature had no effect on the thickness of iron oxide, but the risk of Fe,O appearing on the edge and tail of the strip decreased, and the degree of transformation of the fusite to Fe,O decreased. Pickling time can be reduced by 10-15% when the coiling temperature is reduced from 700 to 600 degrees. 

     Strip should be rich in order to control the austenite transformation, at relatively low temperatures, such as volume from 500-550 degrees, but this will result in the coiling strip before water, cold time increased, leading to the increase of iron oxide thickness nonuniformity, FeO, will grow in quantity, rich austenite decreases, therefore, have to find the best strip coiling temperature for rolling mill, in order to reduce the strip rich after cooling austenite transformation, prevent the tin thickness increased significantly. The results showed that the iron oxide layer on the strip was the thinnest at 550-590 degree coiling, and the rich layer was thicker, and the decomposition of the rich body was the least, so the pickling time could be reduced.

    Other factors

    In order to avoid the transformation of the fullert, the cooling rate of the coil must be accelerated, especially at 350 to 500 degrees. In recent years, all newly built hot rolling mills have produced a considerable increase in coil weight, with the consequent extension of coil cooling time. 

    For example, the volume weight increased from 4-6t to 20-25t, and the cooling time increased from 1.5-2t days and nights to 3.5-5 days and nights. In this way, the fullerenes will be completely transformed, making the pickling work difficult. In order to improve the structure of the sheet metal, some countries now put the coil into the water container to reduce the temperature of the coil to less than 350 degrees. As it turns out, this is good for faster pickling. The only disadvantage of this method is that the inner and outer parts of the coil cool abruptly, while the middle part cools slowly, resulting in uneven metal structure.

    In the pickling test, it is important to note the effect of roll roughness. When rough and damaged rolls are used, the pickling time is usually increased by about 50% at the edge of the strip.To sum up, large rolling speed, smooth roll, low final rolling temperature and coiling temperature, and high cooling rate can have a favorable influence on the pickling of hot rolled strip.