Causes and effects of iron oxide scale formation

　　Iron oxide scale is a layer of metal oxide attached to the metal surface when the metal is heated, heat treated or processed in a hot state. Due to the difference of metal composition, surface temperature, heating and cooling system, oxygen content of surrounding medium and other factors, the composition and structure of iron oxide scale also vary.

　　Generally speaking, the more active the chemical properties of metals, the higher the temperature, the faster the oxidation rate of metals. The longer the oxidation time, the greater the thickness of iron oxide scale.

　　Iron is a kind of active metal, and various iron oxides and structures are relatively loose. Most of the rolling of steel and the processing of iron products are carried out at higher temperatures. Therefore, the oxidation speed of steel is accelerated and the formation of iron oxide scale on the surface of steel is promoted.

　　Due to the different chemical composition, rolling temperature, cooling rate after rolling and coiling temperature of hot rolled strip, the structure, thickness and properties of iron oxide scale formed on the strip surface are also different.

　　1. Formation of surface scale

　　When the rolled piece runs along the roller table to the finishing mill after rough rolling, the temperature is about 1000 ℃. At this time, a thin layer of iron oxide skin has been formed on the surface of the rolled piece, but it can be removed by the secondary descaling device in front of the finishing mill.

　　In the process of finishing rolling, the strip steel is exposed between stands for a very short time, and the large reduction prevents the formation of thick oxide skin on the surface, and the formed oxide cell film is immediately damaged and washed by cooling water. Therefore, it can be said that the strip steel just rolled from the finishing mill has a high temperature, but the oxide skin on the surface of the strip steel is extremely thin.

　　After laminar cooling on the output roller table, the strip steel is coiled at about 600 ℃ and then cooled slowly. In this process, the surface of the strip steel is oxidized to form iron oxide scale.
 

　　2. Composition and structure of iron oxide scale

　　The composition and structure of the iron oxide scale on the surface of the strip steel vary due to the differences in the chemical composition of the steel, the surface temperature of the strip steel during rolling, the heating temperature, the final rolling temperature, the cooling system and the oxygen content of the surrounding medium.

　　The final rolling temperature of hot-rolled carbon structural steel is generally controlled at about 870 ℃, and the surrounding medium contains a large amount of oxygen, followed by a fairly fast cooling rate, so its iron oxide scale generally has a three-layer structure:

　　·One layer on the surface of the steel is the bainite (FeO and Fe3O4 solid solution)

　　·On the upper floor is Fe3O4

　　·The top layer is Fe2O3.
 

　　3. Factors affecting iron oxide scale on strip steel surface

　　A effect of finish rolling temperature and speed

　　The oxidation process of iron is Fe → FeO → Fe3O4 → Fe2O3. With the increase of temperature, the oxidation rate increases gradually. In the range of 600~800 ℃, the generated iron oxide scale can well hinder the diffusion of iron and oxygen atoms, so the oxidation rate will not continue to increase.

　　When the temperature exceeds 800 ℃, the ability of iron oxide scale to hinder diffusion will be greatly reduced, so the oxidation rate will increase rapidly.

　　As the temperature increases, the oxidation speed increases, so the thickness of iron oxide scale on the surface of the strip steel increases with the temperature.

　　Similarly, high rolling speed can reduce the contact time between steel and air at high temperature, thus reducing the thickness of iron oxide scale. Therefore, in order to reduce the thickness of iron oxide scale, the hot rolled strip should be rolled at the lowest possible temperature and the highest possible rolling speed.

　　From the structure of iron oxide scale, when the finishing rolling temperature is between 700~900 ℃, the formed iron oxide scale contains 80%~90% FeO and 10%~20% Fe3O4. When the temperature is higher than 900 ℃, iron will be oxidized rapidly when there are many oxidizing and oxidizing gases. Fe3O4 can be formed rapidly at high temperature, and a separate layer of Fe2O3 will be formed on the surface of iron scale. When the temperature is lower than 570 ℃, the iron oxide scale is composed of Fe3O4, and the surface is covered with a very thin layer of Fe2O3.

　　Increasing the rolling speed can reduce the thickness of iron oxide scale. However, too high rolling speed will rapidly increase the coiling temperature and cause the transformation of fullerene in iron oxide scale into Fe3O4, which will make it difficult to remove iron oxide scale (pickling) on the surface of strip steel in the future.

　　Therefore, accurate control of rolling speed is an important factor in favor of pickling

　　B effect of cooling rate

　　Generally speaking, there are three layers of iron oxide scale on the surface of hot rolled strip steel: the inner layer near the base iron is ferrite, the middle layer is Fe3O4, and the outer layer is Fe2O3.

　　Among them, the bainite which is conducive to acid pickling is stable above 575 ℃, and FeO in the bainite is unstable below 570 ℃, and it decomposes into Fe3O4 and Fe according to 4feo = fe3o4+fe. When the temperature is further reduced below 300 ℃, the transition will approach zero.

　　If the iron oxide scale layer is rapidly cooled between 570~300 ℃, the fullerene layer will not be decomposed in the future and will be fixed at a lower temperature, so as to obtain a fullerene structure conducive to acid pickling.

　　During slow cooling, the bainite layer in the iron sheet gradually decreases with the cooling speed decreasing. Therefore, when the cooling speed of the strip steel in the cooling area is slow, there is only a small amount or no bainite layer in the iron sheet.

　　In the actual production process, it usually enters the laminar cooling zone for rapid cooling after rolling. In the case of water spraying, the thickness of iron oxide scale increases rapidly. Because the iron oxide scale forms faster in water than in air, the longer it stays in steam atmosphere, the more iron oxide scale will be formed, while the FeO content will decrease. Therefore, it is very important to accurately adjust the cooling rate of the spray section and minimize the residence time in the water.

　　C effect of coiling temperature

　　When the coiling temperature of strip steel is 600~700 ℃, there is no obvious effect on the increase of iron oxide skin thickness. However, with the increase of coiling temperature, Fe3O4 will be generated at the edge and head of the strip steel.

　　Further lowering the coiling temperature has no obvious effect on the thickness of iron oxide scale, but the danger of Fe2O3 appearing at the edge and tail of the strip steel is reduced, and the degree of transformation from fullerene to Fe3O4 is also reduced. When the coiling temperature is reduced from 700 ℃ to 600 ℃, the pickling time is shortened by 10%~20%. In order to control the transformation of bainite, the strip steel should be coiled at a relatively low temperature (500 ℃ ~ 550 ℃), but this will increase the cooling time of the strip steel before coiling, resulting in an increase in the nonuniformity of the iron oxide skin thickness, an increase in Fe3O4 and a decrease in the bainite. Therefore, it is necessary to formulate the optimal coiling temperature to reduce the transformation of bainite in the cooling process of the strip steel and prevent an obvious increase in the iron oxide skin thickness. The practice shows that when coiling at 550 ℃ ~ 590 ℃, the iron oxide scale on the surface of the strip steel is the thinnest, in which the layer of fullerene is thicker and the decomposition of fullerene is the least, so the pickling time can be reduced.
 

　　4. Pickling ability of iron oxide scale on strip steel surface

　　Adhesion strength of iron oxide scale, chemical composition of strip steel, type and degree of mechanical deformation, structure and thickness of iron oxide scale, surface pollution, surface defects, type and composition of pickling agent and working conditions during pickling, etc.

　　In the iron oxide scale, the bainite only exists near the surface of the steel substrate, while the Fe3O4 and Fe2O3 in the outer layer of the iron scale are difficult to dissolve in acid solution. However, due to the existence of cracks and pores in the iron sheet layer (especially after scale breaking and tension correction), the acid solution can reach the metal surface and the fullerene layer through these cracks and pores. With the dissolution of metal iron and fullerene, the adhesion between the iron sheet and the metal is reduced. Under the action of hydrogen generated during the reaction between the acid solution and metal iron, the iron oxide sheet will fall off from the substrate and sink to the bottom of the acid bath. At the same time, the insoluble Fe3O4 and Fe2O3 are also reduced to soluble FeO, so that the iron oxide scale is separated from the surface of the strip steel.

　　Another important factor affecting pickling property is the density of iron oxide scale.

　　The fullerenite has the natural maximum porosity, while the Fe3O4 layer and Fe2O3 layer are dense, which will block all the pores in other oxide layers in the iron sheet, thus hindering the infiltration of acid. Although some cracks may be formed in the strip steel during cooling, it is not guaranteed that the acid will penetrate into the depth of iron oxide scale. Especially for the hot rolled strip produced by modern rolling mill, the thickness of iron sheet is quite stable and its density is quite high. Therefore, in order to improve the pickling ability of iron oxide sheet, it is still very necessary to use scale breaking equipment to increase cracks.