The formation of iron oxide scale in continuous casting is the result of a chemical reaction and physical diffusion between iron elements on the billet surface and oxidizing media. It causes cost waste and quality risks in steel production, directly impacting the entire process from continuous casting and rolling to subsequent processing.
Harmful Effects of Iron Oxide Scale:
In conventional continuous casting production, the amount of iron oxide scale generated is approximately 0.5% to 1.0% of the billet output. In some poorly controlled production lines, it can reach over 4.5‰. For example, before the optimization of the No. 3 and No. 4 continuous casting machines at a certain steel company, iron oxide scale led to a decrease in steel recovery rate. Based on an annual output of 2.5 million tons of billets and a billet price of 2100 yuan/ton, the annual economic loss corresponding to metal loss reached 10.5 million yuan.
To remove excessive iron oxide scale, subsequent processes such as flame cleaning and mechanical grinding are required, which not only consumes additional energy and manpower but also further reduces the effective thickness of the billet, lowering the yield.
During continuous casting, iron oxide scale tends to adhere unevenly to the billet surface, forming a “scale indentation” defect: undried scale is pressed into the matrix during subsequent rolling, peeling off upon cooling to form pits and scale; after blistering and cracking, the secondary oxide layer easily forms scattered shallow pits during finish rolling, and the defect becomes more pronounced after pickling.
During the rolling stage, the large difference in thermal expansion coefficients between the iron oxide scale and the matrix easily leads to cracks during heating and rolling, exacerbating the “scale” defect; furthermore, scale can embed into the matrix, which cannot be removed by normal pickling, directly causing product degradation.
In subsequent processing stages, such as with low-carbon wire drawing, residual iron oxide scale can cause stress concentration during drawing, leading to wire breakage; harmful elements in the scale (such as sulfur and phosphorus) can also reduce the steel’s corrosion resistance, affecting product lifespan.
Formation of Iron Oxide Scale:
Under high-temperature environments, iron elements on the surface of the cast billet undergo a series of reactions with oxidizing media such as oxygen and water vapor, forming a multi-layered iron oxide scale. The main reaction equations include: The main components of the iron oxide scale exhibit a regular distribution with temperature changes: above 570℃, a loose oxide layer dominated by FeO is formed, where oxygen atoms easily diffuse, leading to continued and intensified oxidation; below 570℃, FeO undergoes a eutectoid reaction to generate a mixed product of α−Fe+Fe3O4, and the dense Fe3O4 layer can slow down subsequent oxidation. A typical iron oxide scale consists of three layers from the outside in: Fe2O3, Fe3O4, and FeO. The bonding strength between each layer and the substrate varies, creating potential for subsequent spalling and indentation defects.
| Oxide layer | physical appearance | Thickness percentage | Structural Characteristics | Growth Mechanism |
| Outer layer | Fe2O3 | 5%-10% | Dense, brittle | At low temperatures, O2 further reacts with Fe3O4 to form FeO, resulting in a thin layer with weak bonding to the inner layers. |
| Middle layer | Fe3O4 | 10%-20% | Dense structure, high hardness | During the mid-oxidation stage, FeO reacts with O2 to form FeO, which can block oxygen diffusion and provide some protection. |
| Inner layer | FeO | 70%-85% | Porous and porous, with many defects | At high temperatures, FeO reacts rapidly with O2 to form FeO, and is the main contributor to the thickening of the iron oxide scale. |
Factors Affecting Iron Oxide Scale Formation:
Temperature Influence: The surface temperature of the cast billet is the core factor affecting the oxidation rate. Oxidation is insignificant below 700℃, begins to intensify at 900℃, increases sharply at 1000℃, and the oxidation loss at 1200℃ is approximately nine times that at 900℃. High temperatures not only accelerate the chemical reaction rate but also cause growth stress and thermal stress within the iron oxide scale, leading to cracks and blistering.
Steel Composition: Elements such as carbon, silicon, nickel, copper, and sulfur in steel promote iron oxide scale formation, while manganese, aluminum, and chromium can slow down the oxidation process. For example, Q235B steel with a low-Si system (Si content ≤0.08%) has a significantly higher iron oxide scale formation than high-Si steels due to its weak matrix oxidation resistance; when the carbon content is high (close to 0.2%), it easily vaporizes at high temperatures, generating gas and causing the iron oxide scale to blister and crack.
Environmental Media: During continuous casting, water mist in the secondary cooling zone is the main source of oxidizing agents. Water vapor partial pressure affects the phase composition of the iron oxide scale; high water vapor partial pressure easily leads to the formation of an FeO-dominated oxide layer.
Process Parameters: Unstable casting speed and uneven secondary cooling water distribution can cause fluctuations in the billet surface temperature. Excessive localized warming increases the amount of iron oxide scale formation. Rapid temperature rise during heating causes a sharp increase in surface stress on the iron oxide scale, exacerbating blistering and peeling.
FAQ
With the widespread adoption of “continuous casting and rolling” (such as ESP or endless rolling) technology, what specific impact does iron oxide scale have on the hot charging and hot delivery process?
Answer: In traditional continuous casting processes, the billet is cooled and then reheated for surface cleaning; however, in endless casting and rolling, the billet enters the rolling mill directly.
Stringent Requirements: If the iron oxide scale generated in the continuous casting section is too thick or has too strong adhesion, it will be directly drawn into the rolling interface, causing severe surface indentations or cracks.
Control Strategies: Modern industry focus is on maintaining an extremely thin and dense protective oxide layer in the secondary cooling zone through “micro-oxidation control technology” to prevent deep oxidation and ensure the surface quality of directly rolled products.
High-speed casting is key to increasing production capacity; will this exacerbate iron oxide scale formation?
Challenge: High-speed casting means higher surface temperatures for the billet in the secondary cooling zone. According to the article, the oxidation rate increases geometrically above 1000°C.
Countermeasures: To balance high production capacity and low losses, the industry’s focus has shifted to air-mist cooling. This method is more uniform than traditional water cooling and can quickly break the vapor film, allowing the billet surface to rapidly cross the intense oxidation temperature zone while maintaining casting speed, thereby suppressing scale thickness.
