Calcium refining is one of the techniques for improving the quality of steel. It has a profound effect on the size distribution, content, and morphology of inclusions.
Adding calcium can modify the morphology of Al2O3 inclusions to liquid calcium aluminate, which prevents nozzle clogging. Moreover, it can also reduce the plasticity of manganese sulfide inclusions to improve the physical properties of steel.
Thermodynamic calculations provide a good basis for simulating high-temperature material processes. They rely on a set of fundamental postulates that became the laws of thermodynamics and a self-consistent model of the properties of dense fluid systems.
For example, the phosphorus distribution (LP = (pct P)slag/(pct P)Fe) between molten Fe and liquid slag saturated with solid CaO can be predicted by thermodynamic calculations based on modern statistical mechanics and molecular dynamics models over a wide range of pressures and temperatures. The resulting results are in very good agreement with experimental data.
Another example is predicting the deoxidation behavior of new Twinning-Induced Plasticity (TWIP) steels in the liquid solution phase. Calculations using the Modified Quasichemical Model (MQM) can predict more accurate deoxidation equilibria than conventional WIPF or UIPF-based databases. This is critical for avoiding unexpected non-metallic inclusion generation and nozzle clogging. The MQM is an advanced model for describing the chemical composition of dense fluids and provides a much better predictive ability than conventional semi-empirical equations of state.
Many industrial applications require the use of steel with precise chemical and physical specifications. These exacting requirements, often with a range of tolerances, create challenges for steel manufacturers in producing the desired product.
As the most widely used metal, steel has more than 3,500 recognized grades. In order to meet customer demands, high-performance steels are needed with superior properties that include fatigue resistance. In the stress-controlled regime of crack propagation, inclusions and notches play a critical role in determining the limit of fatigue life.
Various techniques can be implemented to reduce the inclusion size and control the morphology of nonmetallic traps in the ferrite lattice. These modifications are aimed at improving the cleanliness of the steel by controlling the sulfur-carbon-nitrogen content, modifying the inclusion morphology through calcium treatment and increasing the alloying element contents. However, these methods are expensive and can only be applied at a limited scale due to the requirement for complex equipment.
Depending on their size, inclusions can change their composition and morphology in steel. Their growth takes place by diffusion of oxygen, deoxidant and agglomeration/coalescence. They collide with the steel matrix or slag/ladle refractory particles and become trapped.
In the present industrial experiment without calcium treatment, the content of large-size inclusions (Sample 5) is relatively high and they cause nozzle blockage during continuous casting and a significant number of surface defects. Incompletely modified CaO*2Al2O3 inclusions with high melting point and Al2O3 are the main constituents of these large-size inclusions.
These large-size inclusions can be classified as class B or 'bulls eye' type, consisting of an undeformed dark aluminate and a surrounding (Ca-Mn)S phase. Class B inclusions are the most frequent inclusions in calcium treated steels. This is because they are able to absorb a lot of sulphide from the slag and melt. These inclusions can also agglomerate and coalesce due to intense bath stirring. These characteristics are very important for the quality of the final steel.
MnS and Al2O3 are two major inclusions that deteriorate steel fatigue life and machinability. To reduce their detrimental effects, they need to be properly controlled. In this study, a Mg-Ca treatment was employed to modify these inclusions in resulfurized special steel. The experimental results showed that Mg-Ca treatment can effectively reduce the number and size of complex inclusions, which consist of an oxide core surrounded by a sulfide outer layer.
To achieve this effect, pure Ca or SiCa wire was injected into the molten steel for calcium treatment in the laboratory. The morphology and composition change of the inclusions were studied by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). The result showed that the modification of solid alumina to liquid alumina was realized through diffusion of molten calcium into the boundary layer of the inclusions.
The results also showed that the effectiveness of calcium treatment was related to the slag refining process, the stirring power and the argon flow rate. The optimum mixing energy was found to be the one that maximized the calcium yield.
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