The most effective ways to prevent nozzle clogging during continuous casting of steel are inclusion count reduction, inclusion modification by calcium addition and proper tundish geometry/ nozzle design/ refractories.
The use of three types of calcium alloys can not only modify inclusions but also influence their size distribution in liquid steel. In this study, the effect of calcium content on inclusion modification was studied by analyzing the change rule of the area density distribution of inclusions before and after arsenic removal treatment.
The violent agitation that accompanies Ca addition to liquid steel reduces the gas content of the metal. This makes it possible to improve the fluidity of low alloy cast irons and to produce sounder, less porous cast structures.
It also helps to break up inter dendritic Al2O3 galaxies into fine Type III inclusions that are unlikely to clog continuous caster nozzles. Inclusion analysis by SEM-EDS has shown that the calcium treatment also modifies the shape of these inclusions, making them less detrimental to mechanical properties in the final steel.
The formation of these inclusions depends on the sulphur and oxygen levels in the liquid steel bath. The inclusion window for calcium addition can be calculated from these factors, allowing the steel manufacturer to maximize the potential benefits of the process.
The population, size, distribution and morphology of inclusions in steel are influenced by deoxidants, conditions of the liquid steel, solidification and casting processes. They have detrimental effects on the quality of continuously cast steels.
Harmful oxide and sulphide inclusions are often of dendritic shape and tend to rapidly cluster due to surface tension on the inclusion-liquid steel interface. Their sharp corners and edges increase internal stress in the metal matrix and reduce ductility, toughness and fatigue strength of the steel.
Calcium treatment efficiently modifies such inclusions to large, isotropic and spherical calcium aluminate and calcium silicates which have low melting points. This modification of the inclusion morphology is known as inclusion morphology control. The non-wetting properties of these inclusions also impede their ability to clog the continuous caster nozzles.
Sulfide stringers clog the submerged entry nozzle (SEN) during casting and disrupt normal steel production. Calcium treatment modifies the shape, size and chemistry of inclusions in liquid steel to prevent their accumulation in the SEN.
The chemistry and oxygen concentration in the steel are critical for successful inclusion modification. Obtaining an optimum calcium addition window, which is the range of residual calcium content in the molten steel where all oxides become liquid and no solid sulphides form, requires tight control of the steel chemistry, oxygen, total sulfur and aluminum content as well as accurate measurement of slag activity.
The low density of calcium makes it float on the surface of the molten steel, which allows it to be added deep in the ladle to avoid reoxidation by air and slag and to prolong the time that it is in contact with the molten steel. This enables inclusion modification and reduces directional anisotropy.
Ca treatment greatly modifies the composition, size and morphology of oxides, sulfides and silicates already present in the liquid steel. This greatly reduces directional anisotropy and allows for improved through thickness ductility, especially in the quenched zone.
However, in order to achieve these benefits, calcium addition has to be performed under strict conditions. If the molten steel is not deoxidized to a sufficient degree, oxygen and sulfur will react with the added calcium, consuming it and reducing the inclusion modification effect. This is why it is important to ensure that the slag has a low oxygen content and that air aspiration and excessive argon stirring do not occur during the Ca injection process. This way, the contact between Ca vapour and molten steel is maximized.
The presence of calcium influences the equilibrium between the inclusions and oxides. As a consequence, the size and morphology of the inclusions change. They become globular rather than long stringers or pancake inclusions and can thus be easily removed to the slag without damaging the steel during casting.
In this way, calcium addition significantly reduces reoxidation in the ladle and downstream continuous cast process. This is because the chemical composition of the deoxidant elements (e.g. aluminium, sulphur and silicon) and their activities in the steel are modified by calcium treatment.
This is reflected by the lowering of the calcium range, defined as the gap between the steel liquidus temperature and the optimum concentration of inclusions for castability. The effect is particularly evident when aluminium and sulphur concentrations are high, as is the case in the investigated steel.
English
Write a Message