A calcium addition has been found to improve the fluidity and cleanliness of high alloy steels. The calcium modifies the inclusions and precipitates in the molten steel. This impacts notch elongation and charpy properties of the steel.
It also modifies the shape of inclusions and prevents their formation of high melting point calcium aluminates which tend to clog continuous casting nozzles. This effect is pronounced for both air melt and air melt with remelting sequence.
Steelmakers routinely use calcium additives to improve fluidity and cleanliness in high alloys, such as welded or sensitization prone grades of austenitic stainless steel. The violent agitation that accompanies Ca addition to the liquid metal reduces gas content and yields sounder, less porous cast structures.
To obtain the best results, the molten steel is vacuum degassed before the calcium addition. It is also advisable to utilize protective slag blankets and refractory shrouded or submerged nozzles to prevent reoxidation of the molten steel following the calcium addition.
It is particularly important to maintain the sulfur content below 0.007% and the total oxygen below 100 ppm after the calcium addition to avoid excessive reoxidation. Excessive reoxidation results in the formation of high melting point calcium aluminates and other inclusions such as manganese sulfides. This in turn, leads to a decrease in the impact energy of the steel.
Steel micro-cleanliness (also referred to as inclusion content) is a measure of the number and volume of non metallic materials entrapped within a metal matrix. These inclusions are commonly called sulfides, oxides or silicates.
Inclusion content is measured by a variety of methods including light optical observation, scanning electron microscopy and photomicrographic analysis. The results are compared to standardized charts.
When examining heavy gauge plates, large segregations are typically observed for a number of alloying elements such as chromium, molybdenum and vanadium. The ESR process dramatically reduces these variations in chemical uniformity and microcleanliness. It also helps maintain a low sulfur content which aids in avoiding brittle sulfide stringers that can cause problems in the polishing and texturing stages of the mold steelmaking process. A high level of microcleanliness in mold steel is crucial for optimum toughness levels and excellent polishability. This is achieved through specialized melting techniques and the use of the ESR process.
Inclusions are a significant source of damage and poor properties in steels. The addition of calcium to the molten steel changes their composition and size, improving both the microcleanliness and the mechanical properties of the final product.
Large oxide stringers are the primary cause of inclusion-related fatigue damage in tapered roller bearings, and these can have a significant impact on the life of rolled parts. Today’s clean steels have significantly lower concentrations of oxide stringers, but variations remain between “clean” steel sources.
To maintain high microcleanliness, a combination of techniques is required. These include X-ray tomography, which provides input on the shape of inclusions, and modelling (metiS software), which enables estimation of the area or volume required to observe one inclusion.
It is known that calcium can be added to molten steel to clean it of nodularate sulfides. This is because calcium has a high melting point and rapid vapour pressure at the steel metallurgical operation temperature and can therefore act as an effective agent of agitation in the liquid phase of the molten steel.
Adding calcium to molten steel is most beneficial when the deoxidizing process is performed at a temperature of 1480 to 1800 deg C. This is because calcium contaminates are not readily soluble in steel at temperatures lower than this.
When a sample of 52100 bearing steel was vacuum remelted with the addition of calcium, rotary bend fatigue testing results showed that the microcleanliness of the resulting material was improved compared to the air melt condition. This was confirmed by metallographic and micro-probe investigations which showed that the frequency of D-type globular inclusions was reduced along with their size. Consequently, the bearing life rating was doubled.
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