Steel purity plays a critical role in determining the final mechanical properties of a product. Non-metallic inclusions are problematic and must be controlled at each stage of the steelmaking process.
Calcium treatment is an effective method for improving steel cleanliness by modifying the composition and shape of inclusions in molten steel. Thermodynamic commercial software can be used to optimize the process for maximum results.
Steel cleanliness is a major quality requirement for many steel grades and applications. Clean steel can be defined as low concentration of inclusions below the tolerable limit and absence of large (macro) inclusions. The most important factors for achieving clean steel are effective deoxidation in the ladle, control of inclusion population during ladle treatment and transport, and slag chemistry.
In this process, strong oxide-forming elements like aluminum, silicon and manganese take part in deoxidation reactions to reduce oxygen content of the steel. They can form pure oxide like solid SiO2 or Al2O3 or a liquid complex oxide with iron.
Calcium also participates in deoxidation forming pure CaO or Ca-Cr-MnOliq. Besides, it has the ability to transform solid alumina clusters in the slag into liquid aluminate and prevent nozzle clogging. SEM-EDS analysis of the re-melted samples showed that the number of single MnS inclusions decreased significantly after Ca treatment, while the size and aspect ratio of complex inclusions improved.
A significant proportion of inclusions are harmful sulphide minerals such as MgS or AlS. These are difficult to control in terms of their chemistry, size and shape. Inclusion modification methods are important tools for ensuring that these are kept to a minimum.
Thermodynamic calculations show that mass transfer and solid state diffusion within the inclusion are not rate limiting for micro-inclusions at steelmaking temperatures. However, practical limitations are imposed by kinetic reactions between the slag and steel and the homogenization of steel composition in the course of deoxidation which take only a few seconds.
Inclusions are modified in the same way as oxidation and deoxidation by calcium treatment. For example, manganese sulphides are transformed to varying degrees of Ca-Mn sulphide, silicates are eliminated and complex globular CaO-Al2O3-SiO2 inclusions are formed frequently surrounded by a sulphide rim. These changes are often represented on ternary diagrams but ternary plots have the disadvantage that they are limited to three composition variables and require careful interpretation using user-defined rules.
Desulfurization is an important step to reduce sulphur oxides emissions and improve fuel efficiency. It can be done by VAR, ESR or by oxidative (or sulphiting) desulfurization (ODS).
The latter method is more efficient since it reduces the overall sulfur content of the feedstock by converting thiophenic sulfur compounds into sulfoxides and sulfones. This is particularly useful for reducing the sulfur content of heavy fractions of crude oil such as heavies, and is complementary to hydrodesulfurization of flue gas.
Calcium treatment has been shown to be an effective method for desulfurizing Al-killed steels, transforming existing solid Al2O3 and spinel inclusions into liquid calcium aluminates and modifying their morphology for improved castability. This is especially beneficial during hot deformation, where labile inclusions are the prime sites for damage initiation.
Heterogeneous nucleation is a major phenomenon in nature and technology and it controls many aspects of metal processing including crystallization, grain growth, alloy formation and decomposition. The aim of this research is to develop methods that can induce and control heterogeneous nucleation in the steel melt by the addition of tailor made reactivity or seed alloys.
Grain refinement is an important aspect in the optimization of the process because it increases strength and toughness, while reducing the corrosion susceptibility of a steel. In particular, large inclusions are the dominant cause of surface cracking during casting whereas smaller grains can lead to poor ductility.
The morphology, composition, number and size distribution of inclusions were analyzed in samples of Al-Ti complex deoxidized calcium treated steel with different aluminum content using FE-SEM and energy dispersive X-ray spectroscopy (EDS). Sample 1 had larger crystals and more MnS precipitated on oxides than sample 2. The inclusions in the first sample had a more uniform morphology, lower density and higher melting point than those in the second.
English
Write a Message