Continuous Caster - Classical Control Design Virtual Laboratory

Summary

Continuous Casters are a common means by which molten liquid metal is solidified.

The process results in a long bar of metal, with a rectangular cross section, being drawn out of the bottom of the mould while a stream of molten metal is poured into the top. The creation of this moulded bar requires a number of processes to be run in unison and the failure to balance any one could lead to a spectacular, dangerous and expensive accident.

An important issue with the delivery of the molten steel is that the level of steel in the mould should be kept constant to a high degree of accuracy.

This (virtual) laboratory allows students to gain exposure to this real world control design problem.

Figure 4.1: Screenshot of Program
Figure 4.1: Screenshot of Program

The Physical Apparatus

A Continuous Caster is a common means by which molten liquid metal is solidified. The molten metal is poured into a mould which is a rectangular container open at each end (see Figure 4.2).

Figure 4.2: Mould and Bloom
Figure 4.2: Mould and Bloom

The mould, which has a rectangular cross section, starts a run by being closed off at the bottom to build up the molten steel, this is accompanied by intense cooling using water jets. When the metal in the mould starts to solidify it is drawn slowly out from the bottom of the mould as a solid piece. The metal drawn out from the bottom of the mould starts to leave a space at the top of the mould, this space is continually filled by molten metal poured into it. The process results in a long bar of metal, with a rectangular cross section, being drawn out of the bottom of the mould while a stream of molten metal is poured into the top. In steel mills the bar would be drawn out for a number of metres by which time it would be completely solidified. This solid bar would then be cut into manageable pieces called blooms which can then be further processed, e.g. by rolling or forging.

As you can imagine, this process requires a number of processes to be run in unison and the failure to balance any one could lead to a spectacular, dangerous and expensive accident. An example would be if solidifying steel was drawn out too quickly from the mould. This would expose the molten metal in the mould which would then flow out.

We can accept that the cooling system and withdrawal mechanisms have been properly tuned and work safely so we will look at the system controlling the pouring of the molten metal into the mould.
As used in the BHP Steel mill in Newcastle in the 1990s (see Figure 4.4) the continuous caster comprises a reservoir of steel (the Tundish) directly above the mould (see Figure 4.3). The flow of the molten steel is controlled by a Slide Gate Valve (SGV). An important issue with the delivery of the molten steel is that the level of steel in the mould be kept constant to a high degree of accuracy. If the level of steel is allowed to fluctuate too widely then contamination of the steel can occur from flux material, e.g. slag, being embedded into the surface of the steel. The removal of these impurities is an expensive extra process. The impurities are also capable of causing structural weakness in the products made from the blooms.

Figure 4.3: Continuous Caster Schematic with Slide Gate Valve
Figure 4.3: Continuous Caster Schematic with Slide Gate Valve

Figure 4.4: Overall View of the Continuous Caster at BHP
Figure 4.4: Overall View of the Continuous Caster at BHP

The liquid steel enters the mould via a submerged entry nozzle. The nozzle has an outer refractory lining to deal with the extreme conditions, however the flux material on the surface of the molten steel eventually destroys this lining. In order to maximize the nozzle life the mould set point is periodically changed.

This (virtual) laboratory allows students to gain exposure to a real world control problem. This kind of problem is clearly beyond the reach of a normal teaching environment. However, the virtual apparatus allows students to experience this real world problem. Note that the real system is actually extremely critical to operate a “breakout” of the steel from the caster can cause millions of dollars of damage and can threaten human life. Thus, to experiment on a real machine of this type in industry requires management approval at the highest level. The laboratory accurately reflects a real life control system design exercise carried out on a continuous caster at BHP’s Newcastle steel works [1].

This application of control was motivated by a desire to improve the quality attributes of the cast material. Post implementation audits [2] showed a very significant improvement in quality with an estimated saving to the company of US$1.5m annually. Thus the payback period for the control system design and implementation was only 1 or 2 months. Photos of the real system are shown in Figures 4.4, 4.5 and 4.6.

Figure 4.5: The Steel is Pouring Through the Slide Gate Valve at BHP
Figure 4.5: The Steel is Pouring Through the Slide Gate Valve at BHP

Figure 4.6: The Steel Continuously Cast Steel Passing Through the Secondary Cooling Chamber
Figure 4.6: The Steel Continuously Cast Steel Passing Through the Secondary Cooling Chamber

Prerequisites

Although this laboratory relates to a (dangerous) real industrial problem, the design was actually relatively straightforward. Thus students need only have a familiarity with classical control ideas e.g. simple linear modelling, transfer functions, PID design and feedforward control.

Learning Objectives

The learning objectives of this virtual laboratory are:

  • Exposure to a realistic industrial control problem
  • Exploration of simple tradeoffs in feedback systems
  • Input disturbance compensation
  • P, I and PI control design
  • Simple stability issues
  • Feedforward control

References

  1. S.F. Graebe, G.C. Goodwin and G. Elsley, “Control Design and Implementation in Continuous Steel Casting'’, IEEE Control Systems Magazine, Vol 15, No. 4, August 1995.
  2. M.S. Jenkins, B.G. Thomas, W.C. Chen and R.B. Mahapatra, “Investigation of Strand Surface Defects using Mold Instrumentation and Modelling'’, 77th Steelmaking Conference, Chicago, IL, Iron and Steel Society, Warrendale, PA, 1994, pp. 337-345.