Annealing involves heating metals to high temperatures and then rapidly cooling them. The process is used to remove stresses; induce softness; alter ductility; impart toughness, electrical, magnetic or other physical properties; refine the crystalline structure; remove gases; or produce a definite micro-structure. In annealing, the temperature of the operation and the rate of cooling depend upon the material being heat-treated and the purpose of the treatment.
J&L Specialty Steel operates a continuous Direct Roll, Anneal and Pickle (DRAP) Line at its Midland plant in Pennsylvania. The unit is designed to process 300 and 400 series stainless steel. With a capacity of 60 tons/hr, it can handle coils from 0.034″ to 0.312″ gauge and 26″ to 62″ width. Line speed can vary from 20 to 150 ft/min. The maximum cold reduction is 62%.
The furnace portion of the line consists of two unfired preheat zones and five top-and bottom-fired heating zones. Temperature control in each zone relies on roof temperature measurement as the process variable, with the output setting the top and bottom firing rates of the fuel/air controllers. Each zone temperature setpoint is generated by the host computer, which contains the furnace model.
Process Dynamics and Conventional PID Controls
The dynamic variables of the process are:
- Transition from one series of stainless to another
- Change in the gauge reduction being applied
- Movement of a weld through the furnace
- The cross-section of the strip
- The emissivity of the strip
- Change in line speed
The line’s conventional PID controls could handle the mission-critical Zone 5 (furnace exit) temperature under steady-state conditions. But variances in process dynamics (listed above) caused upsets from which the PID controls could not recover fast enough. This resulted in processing problems and a need to reprocess certain coils. A particular problem stems from the fact that when a coil is cold-rolled, a length of metal on each side of a butt weld cannot be rolled. When this section of strip moves through a zone, the load could go up or down as much as 62% (the maximum gauge reduction applied) because the weld section is virtually the same length as the length of a zone. A section of strip moves through a zone between 12 and 90 seconds, depending upon line speed.
The Control Objectives
The strip annealing temperature at the furnace exit must be maintained within a narrow band. The excess oxygen in each zone must stay within a narrow band to eliminate one of the causes for oxides forming on the strip. These criteria must be met when going from one type of coil to another.
The QuickStudy™ Solution
Before installing the QuickStudy adaptive process controller (APC) on the furnace, it was used to create process models automatically, off-line, from historical data. During this process it was learned that replacing the five zone temperature controllers with five independent predictive control blocks within the APC could predict each zone temperature closely. The inputs to each block were zone temperature for process variable, with line speed, cross-section and strip weld events as disturbance variables. The outputs went directly to the fuel/air controls of the respective zones. With this established, QuickStudy was put on-line and fine-tuned in the adaptive mode. As a result, zone temperatures are now controlled within 5°F, and coil transitions do not cause firing control problems. This provides better fuel/air control with less excess oxygen.
Speed ramp control was implemented to prevent strip overheating when ramping up, especially during short line stops or slow downs. Ramp-ups occur during startup and during changes to thinner gauges. The objective of this control was to enable faster ramp-ups with tighter temperature control, increasing energy efficiency and throughput. An existing industrial PC running a Windows NT operating system was used as the platform to implement the QuickStudy application. This PC is networked over Ethernet to GE 90-70 PLCs, which control the DRAP line. A 32-bit DDE driver software package loaded on the PC provides read/write access to the furnace PLC. An operator display on the existing workstations provides access to the APC. Logic in the furnace PLC supplies the necessary interlocks and bumpless transfers between the APC and the conventional PID controls.
The QuickStudy furnace control enabled smooth transitioning from coil to coil when:
- Changing from one series of stainless to another
- There was change in the gauge reduction applied
- There was movement of a weld through the furnace
- There was a change in the cross-section of the strip
- There was a change in the emissivity of the strip
- There was a change in line speed
Additionally, the QuickStudy furnace control reduced temperature variability, which resulted in:
- Improved quality
- Reduced reprocessing of coils
- Increased throughput at lower operating costs.
The QuickStudy speed ramp control enabled faster speed ramps and tighter strip temperature control, increasing energy efficiency and throughput.