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The improvement in mixing conditions in a vacuum refining unit plays an important role in enhancing the purity and decarburization of molten steel. Mixing time is an important index to evaluate the operation efficiency of a metallurgical reactor. However, in water models, the effect of salt tracer dosages on the measured mixing time in a vacuum reactor is not clear. In this study, a water model of a Single Snorkel Refining Furnace (SSRF) was established to study the effect of salt solution tracer dosages on the mixing time of monitor points. The experimental results show that, in some areas at the top of the ladle, the mixing time decreases first and then increases when increasing the tracer dosage. Numerical simulation results show that, when the tracer dosage increases, the tracer flows downwards at a higher pace from the vacuum chamber to the bottom of the ladle. This may compensate for the injection time interval of large dosage cases. However, the mass fraction of the KCl tracer at the right side of the bottom is the highest, which indicates that there may be a dead zone. For the dimensionless concentration time curves and a 99% mixing time, at the top of the vacuum chamber, the curve shifts to the right side and the mixing time decreases gradually with the increase in tracer dosage. At the bottom of the ladle, with the increase in tracer dosage, the peak value of the dimensionless concentration time curve is increased slightly. The mixing time of the bottom of the ladle decreases significantly with the increase in tracer dosage. However, in the dead zone, the mixing time will increase when the tracer dosage is large. At the top of the ladle, the effect of the tracer dosage is not obvious. The mixing time of the top of the ladle decreases first and then increases when increasing the tracer dosage. In addition, the mixing time of the top of the ladle is the shortest, which means that sampling at the top of the ladle in industrial production cannot represent the entire mixing state in the ladle.

Many experimental methods such as transmission electron microscopy (TEM) and X-ray energy dispersive spectroscopy were used to study the precipitates of low carbon steel with niobium addition, the austenite/carbonitride equilibrium in Fe-Nb-C-N system was performed in this paper. The equilibrium volume fraction, chemical driving force of carbonitride precipitation fraction of each element in austenite, and carbonitride at different temperature were calculated. The results show that the equilibrium mole fraction of carbonitride increases, mole fraction of each element dissolved in austenite are basically precipitated fully at low temperature, the chemical driving force is 2.072 Â 10 3 J Á mol À1 at 10508C. It is found that Nb(CN) precipitated on grain boundaries by using TEM. The volume fraction of Nb(CN) precipitates increased with the temperature decrease, which is in good agreement with the value calculated by the model.

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The inclusion type, and the relationship between inclusions and the total oxygen, and the ways of decreasing the inclusion count and controlling the inclusion evolution are studied. The results are shown as follows: when the slag composition is controlled in an appropriate range, the slag melting point is low and the ability of deoxidation and inclusion absorption are good. The total oxygen is proportional to the count of 0–6 μm inclusions in the unit area. The mixing of the ladle and the removal of inclusion were promoted by optimising the ladle field. By Mg or Ca treatment, the size of Al2O3 inclusions can be reduced. But the large size CaO inclusions may be brought into steel by the Ca treatment. This paper is part of a Themed Issue on Recent developments in bearing steels.