In the continuous casting process of ultra-low carbon steel, carbonaceous materials in the protective slag penetrate into the surface of the cast billet, increasing the carbon content on the surface. Because the protective slag contains a certain amount of carbonaceous material, it easily causes carburization on the surface of the billet and carbon increase in the molten steel within the copper mould tube. A carbon-rich layer exists between the slag layer and the sintered layer, which is the main reason for carburization and carbon increase because:
Figure 1. Carbon content of each protective slag layer on ultra-low carbon molten steel.
(1) The carbon-rich layer is close to the meniscus of the molten steel, making it easy to contact the molten steel and the billet.
(2) The carbon-rich layer has non-sintering characteristics, making it easy to mix with the slag and molten steel.
(3) The carbon-rich layer has a very high carbon content, providing a strong mass transfer driving force.
The main measures to control the carbon increase in molten steel caused by the protective slag are:
(1) Using exothermic start-up slag. Start-up slag melts rapidly in the initial stage of casting and provides heat to the meniscus region, forming a sufficiently thick liquid slag layer together with the subsequently added ultra-low carbon steel protective slag. This significantly reduces the carbon increase in ultra-low carbon steel caused by start-up casting.
(2) Reducing the free carbon content in the protective slag. This is the simplest and most effective way to avoid carbon increase in ultra-low carbon steel. Typically, the initial carbon content of the protective slag in ultra-low carbon steel crystallizers is controlled below 2%.
Figure 2. Microscopic view of the carbon-rich layer
(3) Use carbon black-based carbonaceous materials. Carbon black-based carbonaceous materials have low combustion temperature and fast combustion speed, which is beneficial to increase the thickness of the liquid slag layer and reduce the carbon content in the enriched carbon layer and molten slag layer. In addition, the particle size of carbon black-based carbonaceous materials is very small, which has a strong ability to separate the base material and to hinder the flow and aggregation of the melt. When the original carbon content is very low, this is beneficial to slow down the melting rate of the protective slag and avoid the molten slag layer being too thick. Using fast-burning activated carbon significantly reduces the carbon content of the billet.
(4) Appropriately increase the viscosity of the protective slag. Due to the poor thermal conductivity of ultra-low carbon steel, the billet drawing speed is generally relatively slow, and the molten slag layer thickness is reduced. Once the operation is unstable, the molten steel is easy to contact the enriched carbon layer and increase the carbon content. Appropriately increasing the viscosity of the protective slag reduces slag consumption, and the liquid slag layer will thicken. At the same time, due to the increase in molten slag viscosity, the mass transfer rate of carbon in the molten slag layer to the molten steel will be greatly reduced.
(5) Add an oxidant to the protective slag. Adding appropriate amounts of oxidants such as MnO2 to the protective slag can promote carbon oxidation in the slag, effectively inhibiting the carbon content of the enriched carbon layer and slag layer. Furthermore, the fluxing effect of MnO2 can thicken the molten slag layer.
(6) Using carbon-free protective slag. Using nitrides with similar crystalline structures to graphite, such as BN, Si3N4, and Cr2N, to replace carbonaceous materials, and using carbon-free protective slag, can prevent carbon enrichment in ultra-low carbon steel. BN is the most commonly used, but its price is very high, and due to the generation of B2O3 and the release of N2, bubbling and expansion often occur on the slag surface.
(7) Stable continuous casting operation. Ultra-low carbon steel is a clean steel, which not only easily accumulates carbon but also has a strong tendency to absorb oxygen. To stabilize process factors such as tundish pouring speed, billet pulling speed, and crystallizer vibration frequency, and to prevent fluctuations in the molten steel level, a system of adding protective slag frequently but in small amounts each time should be adopted to improve the heat insulation effect of the protective slag and maintain a stable slag layer thickness. At the same time, attention should be paid to the protection of the steel flow, which is not only to prevent carbon increase in the billet, but also to prevent secondary oxidation.
FAQ
Why must protective slag contain carbon? What are the consequences of not adding carbon?
A: Carbon (such as graphite and carbon black) mainly acts as a melting rate regulator in protective slag.
Function: Carbon particles coat the surface of the slag powder, preventing low-melting-point materials from prematurely contacting and melting, thus forming a stable layered structure.
Consequences: If no carbon is added and there are no substitutes, the protective slag will melt instantly, leading to an excessively thick slag layer, uncontrolled slag consumption, and loss of lubrication balance, easily causing sticking and leaking steel. Therefore, the difficulty in controlling carbon addition lies in ensuring a moderate melting rate while minimizing the transfer of carbon to the molten steel.
In which region of the copper mould tube does carbon addition in molten steel mainly occur? What is its physical mechanism?
A: Carbon addition mainly occurs in the meniscus region of the crystallizer.
Physical Mechanism: There are two main pathways. First, there is direct contact: carbon particles in the incompletely melted slag powder directly contact the molten steel surface and dissolve. Second, there is indirect diffusion: carbon in the slag layer diffuses into the molten steel at the slag-steel interface through chemical potential difference.
Key point: Fluctuations in the crystallizer surface (wave flow) are the main cause of increased carbon content. Fluctuations cause the molten steel to continuously absorb carbon-containing slag powder, leading to a sharp increase in local carbon content.
Besides reducing carbon content, what other technical means can reduce carbon gain?
A: In addition to changing the protective slag formulation, process optimization is also crucial:
Electromagnetic stirring/braking (EMBR): Suppress turbulence in the molten steel within the crystallizer using electromagnetic force, maintaining a stable slag-steel interface and reducing slag entrapment.
Use carbon-free/low-carbon substitutes: Use exothermic agents (such as aluminum powder, silicon powder) or other non-carbon skeleton materials to adjust the melting rate.
Optimize nozzle design: Improve the outlet angle of the submerged entry nozzle (SEN) to reduce the kinetic energy flowing towards the meniscus and weaken surface ripples.
