The production of steel with lower nitrogen content requires stringent process control in steelmaking processes such as LD, ladle furnace as well as in casting operations. This paper focuses on the parameters that can contribute to the nitrogen pickup in the LD steelmaking process before the tapping operations. At Tata Steel, which utilizes the thin-slab caster route with slag splashing technology, the average final nitrogen in steel at the end blow is 33 ppm, with the average nitrogen pickup around 15 ppm from vessel to caster. The denitrogenization of liquid steel during the secondary steelmaking process is feasible only under vacuum treatment and is limited to a critical level, below which it is not possible to reduce even under vacuum. The critical value of nitrogen content in the steel was generally found to be higher than 45 ppm in plant practice. Hence, a study was undertaken to control the nitrogen in the LD process. Various process parameters that can contribute to the higher turndown nitrogen were analyzed and a possible mechanism for higher final nitrogen was proposed.
This paper reviews the methodology applied on the example of the phosphorus and sulfur balance in all stages of the iron- and steelmaking process. By introduction of the conversion factors for each stage, the highs and lows of the operation process can be identified and optimized. The practical aspects of LD-slag recycling and utilization in integrated steelmaking operations will be discussed.
Considering a dimensionless parameter L/L0, where L0 is the liquid steel bath height, this paper will critically review both the theoretical and practical aspects of these depth of penetration equations, and recommend the most feasible equation(s) to determine the lance gap to use during the oxygen blow. Further, the effects of the penetration depth, like total surface of the penetration cavity, which is the reaction surface for direct reaction of oxygen with the metal bath, and droplet generation and size, which is the reaction surface for reaction of Fe droplets with slag FeO, will be discussed.
The chemical behavior of manganese in oxygen steelmaking is complex because the element is readily oxidized in conditions found in steelmaking, and the stability of its oxide is a function of temperature and slag chemistry. This results in distinct oxidation and reversion stages during the steelmaking cycle. Recently, a multi-zone model (developed by the authors) of oxygen steelmaking has successfully predicted the plant behavior of manganese but also highlighted how the generation of droplets and their temperature play an important role in the kinetics of the process. This new understanding should result in improved control of this important alloying element.
High phosphorus levels in steel result in products with poor mechanical properties, erode profit margins, lower flexibility in downstream processing and disrupt delivery schedules. A robust advanced analytics model was developed for finding optimum process parameters resulting in low end blow phosphorus and high phosphorus partitioning in an LD shop. Best patterns were determined for oxygen blowing, material charging sequence and bottom stirring. This model, along with a machine learning approach, can suggest real-time operating recipes to the operators resulting in a massive 13% increase in the current success rate of the heats complying with the aim phosphorus level.
Dynamic tundish superheat can be obtained from continuous temperature measurement, in combination with liquidus prediction; however, quantifying the accuracy of the liquidus prediction is difficult without practical measurement. A new liquidus prediction and sensor measuring liquidus within the tundish have been devised and tested, substantially increasing confidence in the values obtained. Combining, in the same system, liquidus and continuous temperature measurements, delivered through wireless technology, with the integrated liquidus prediction provides an economical, accurate and reliable dynamic superheat value, enabling steelmakers to optimize casting superheat and move toward automated casting practices. Results have been compared to existing liquidus predictions.