Composition and structure of iron oxideThe composition and structure of the iron oxide on the surface of the strip varies with its chemical composition, rolling temperature, heating and final rolling temperature, cooling system and oxygen content in the turnover medium.
The thermal analysis of the ferric oxygen system showed that the oxidation process of iron was ferric -- ferric oxide (23.26% oxygen content) ferric oxide (27.64% oxygen content)-- ferric oxide (30.04% oxygen content). These phases are the iron oxide solid solution, the rich phase (close to ferric oxide), ferric oxide, ferric oxide and oxide solid solution.
It can be seen from the graph of the state of ferric oxygen that when the stable boundary of each phase is in the state of equilibrium, it can also be seen that when the temperature is greater than 575 degrees, the futensite is stable and decomposes into magnetic iron oxide and metallic iron at a lower temperature. :If the sheet iron oxide cools rapidly between 570 and 300 degrees, the fullerite phase has no time to decompose and is fixed at lower temperatures. In the case of slow cooling, the fusiform phase is only a small amount or not at all. When heated to more than 1100 degrees, especially when oxygen content is limited, water vapor or carbon oxide content is high, ferric oxide will be reduced to ferric oxide, that is,
strip steel is treated at high temperature, and the oxidized iron sheet generated on its surface is a mixture of various phases.
If the surface of the steel plate is oxidized at a temperature greater than 570 degrees and has excess oxygen, which is then cooled fairly quickly, then generally the iron sheet will be composed of three layers, one of which is directly attached to the surface of the steel is the futensite (FeO and Fe,O4 solid solution), and another layer is Fe,O4 and FeO3 at the top. As the final rolling temperature of hot-rolled carbon structure is generally controlled at about 870 "degrees, the circulating medium contains a large amount of oxygen, followed by a fairly rapid cooling rate, so the iron oxide sheet is generally the three-layer structure mentioned above.
Usually iron contains other elements besides iron atoms. For example, silicon steel contains very high silicon atoms, while stainless steel and heat-resistant steel contains a lot of chromium atoms and nickel atoms. Even ordinary carbon structure contains a small amount of carbon, silicon, manganese, phosphorus, sulfur and other elements. In this case, it's not just the iron ions that are diffusing, it's the ions of other elements that are also diffusing. The ions of these elements can all be synthesized with oxidation. Therefore, in addition to iron oxide, iron oxide also contains some other elements of oxidation, matter. For example, according to the spectral analysis of stainless steel oxide film containing 15-16% chromium, there are 23.1% FeO, 70%CrO3 and 2.38% SiO2o implied in the oxide film.
Meanwhile, the diffusion rate of atoms of any two elements is different.
For example, the diffusion rate of silicon and chromium atoms is much slower than that of iron atoms. Therefore, their oxides are often close to the base iron part and even become a single layer of oxide under the skin of iron oxide.