Inside Machines: Process models, advanced control support super steel mill
Extensive system integration and custom advanced control software supports one of only four cold mills in the U.S. coupled to a pickle line. The mill automation system supports a continuous process requiring precision control of motor speeds and integration of auxiliary equipment.
The first major greenfield steel facility built in North America in a very long time is in operation in Columbus, MS. Severstal Columbus, a unit of the Russian Severstal Group, developed the facility to manufacture high-quality steels for use by automakers including BMW, Mercedes, and Honda. Phase I of the new mill, commissioned in 2007, employs 450 people to produce 1.5 million tons of hot-rolled, cold-rolled, and coated steel products each year. Phase II, currently under construction, will double the mill’s capacity. (Six additional photos, tables, and graphics - not appearing in the print and digital edition - appear at the bottom in an ONLINE extra, along with relevant links.)
System integrator TMEIC GE, a global joint venture between Toshiba, Mitsubishi-Electric, and General Electric, supplied the new mill with the constant and adjustable speed drives, motors, control and automation systems, and associated electrical equipment. Projects included the continuous galvanizing line with inline skin pass mill, the standalone temper mill, and the unique and impressive continuous pickle line tandem cold mill.
An article in the Memphis Business Journal in August 2007 quoted Severstal chief commercial officer Mike Wagner as saying, “Our commitment to invest in only the best available technology can easily be seen in our cold mill. The term state-of-the-art is often overused, but it is an accurate term in this case.”
For this very large system integration project, TMEIC GE responsibilities included writing custom software, conducting the factory system test, and supervising the system installation and commissioning. TMEIC GE is also providing ongoing customer support, and is responsible for these system descriptions and illustrations.
The main challenge of the project was its size, said TMEIC GE project manager, Ron Tessendorf. “This was a huge integration project requiring us to work with a variety of suppliers, and to write a lot of custom software,” he said. “In addition, we decided to use a next-generation PLC with new features. The customer required performance guarantees in terms of product quality and production rate, and we met all of these,” he added.
The 659,800-sq-ft cold mill is one of only four in the U.S. coupled to a pickle line, and is the only one capable of producing steel coils up to 72 inches wide, according to the company. A pickle line is a series of acid tanks used to clean the metal strip before it enters the cold mill. Large accumulators located under the tanks store long lengths of strip to allow continuous operation of the cold mill while new coils are being added at the input end.
The pickle line tandem cold mill (PLTCM) is a continuous process requiring precision control of motor speeds and supporting sequencing of auxiliary equipment. The strip is moved through the pickle line at the optimum speed for the pickling process and under precise strip tension control. The pickling process never stops, even when the head of a new coil is automatically welded onto the end of the coil being pickled. In the tandem cold mill, precision reductions are made in strip thickness in each of the five reducing stands powered by 5,000 kW motors.
The total length of the PLTCM line is 1,100 ft and its height is 34 ft. Strip delivery speed is an astounding 4,100 ft per minute. In the cold rolling process, precise control of strip tension, speed, and roll force is required to produce a flat strip. To ensure a perfectly shaped (flat) product, the work rolls are bent and moved horizontally by electrohydraulic actuators under control of the PLCs.
The following list of components illustrates the size of the PLTCM control project: 13,582 digital inputs and outputs, 2,893 analog inputs and outputs, 129 ac motors ranging from 2 hp to 6,700 hp, 116 low-voltage variable speed ac drives, 7 high-power medium-voltage variable speed ac drives, and 34 programmable logic controllers (PLCs) and operator interfaces.
Advanced software for control
The tandem cold mill control system is a good example of the advanced software used on this project. TMEIC GE’s hierarchical automation system, developed over 50 years of steel rolling mill experience, provides control based on physics- based computer process models, called Level 2, which supply references (or setpoints) to, and are tightly integrated with, the Level 1 PLC control. The speed of all the motors is coordinated by the PLCs, which are connected by the control network to the Level 2 computers, as shown in the illustration.
The Level 1 controls accept I/O signals from the mill stands and generate output signals for motor speed, gap control, roll movement, tension, and a host of other machine functions. Some of the major functions performed include:
- Gage control. Automatic gage control (AGC) employs mass flow calculation, thickness measurement, feedforward, feedback, tension, and slip compensation.
- Eccentricity. Eccentricity control dynamically reduces the effect of backup roll irregularities on thickness using Fourier Series Analysis.
- Flatness. Automatic flatness control uses advanced data filtering, parabolic flatness control, tilt control, segmented coolant spray control, and shape maintenance.
- Coordination. Coordinated in-coil adjustments use measured and calculated values to maintain targets.
- Flying gage change. Fast, coordinated, gage control changes minimize strip breaks and mass flow disturbances during rapid changes to products of different thicknesses.
- Micro-tracking. High-speed data acquisition and tracking allows faster and more accurate control action.
Level 2 controls
TMEIC GE’s Level 2 process models are tightly integrated with the Level 1 PLC controls. Process models are dynamic mathematical models based on the fundamental physics of the rolling process. The models generate the PLC setpoints to ensure production of the highest quality product. Model software runs in the supervisory computer shown in the hierarchical automation diagram.
Some of the process models contained in the system include:
- Force, torque, and power models showing the effects of tension and friction
- Deformation resistance models with the material chemistry and work hardening effects on elongation
- Friction models with compensation for coefficient of friction variations
- Strain model showing in-coil strain distribution
- Roll wear model showing the thermal and wear effects on roll diameter profiles
- Interaction models describing the relationship of tension, forward slip, friction, and speed
- The coefficient of friction effect of textured rolls
- Product-dependent transfer functions for Level 1 control
- Roll bending, roll shifting, and IR bending
- Generic flatness interface models.
High-quality cold rolling produces strip metal of reduced thickness and with good surface finish. With this mill automation system, the PLTCM is capable of producing high-quality cold rolled steel 24x7, stopping only for scheduled maintenance. Severstal’s hot mill manager, Dan Lambert, is very pleased with the mill’s performance since startup in August 2007. “There have been zero issues with the reliability or performance of the main mill motors at Severstal…. The design of these machines and the ancillary systems lends to minimal maintenance requirements and what appears to be a long service life. Install it and forget it! This was a very nice approach afforded by the TMEIC motors and installation crew during our construction phase, when there were plenty of other issues on which we had to focus our attention.”
Edited by Renee R. Bassett for Control Engineering, rrobbins3000(at)comcast.net.
ONLINE extra (extra images and two tables not appearing in the print and digital editions)
PLTCM Process Line
Height of the PLTCM
Strip delivery speed
4,100 feet / minute
Digital inputs and outputs, total
Analog inputs and outputs, total
Motors, ac, ranging from 2 hp to 6,700 hp, total
Low voltage variable speed ac drives, total
High-power medium voltage variable speed ac drives, total
Programmable logic controllers (PLC) and operator interfaces
Also see ...
- Videos on motor efficiency and PLC replacement at www.controleng.com/videos
- Channels on Machine Control, System Integration at www.controleng.com/channels
- Machine Safety and AIMing for Automated Vehicles at www.controleng.com/blogs