by Paul Baumgardner, Heinz Diekmann-Moeller and Marshall Fischer
Steel Dynamics of Butler, Indiana, USA, is a new 1.4 million tons per year (tpy) mini mill for flat material serving the Midwestern U.S. markets with hot rolled coils since 1995. Thanks in part to Siemens' drive and automation technology, this rolling mill is a state-of-the-art steel making facility.
Greenfield construction of the facility began in October 1994, with start of production in January of 1996. This was an especially challenging project because of the unusually short delivery and start-up time of just 15 months.
The plant is made up of a 195 ton twin-shell AC electric arc furnace; ladle metallurgy facility; single-strand thin slab caster; tunnel heat furnace; and 6-stand hot strip mill. Phase III involves the installation of a second melting furnace, thin slab caster and tunnel reheat furnace, and a second coiler, enabling the casting plant production to match the potential 2.4 million tons annual capacity of the hot rolling mill. Commissioning for Phase III, which includes Siemens electricals, will begin this December.
In caster level one, the process PLC controls spray water, mill level, stopper rod hydraulics and tundish weight. The drive PLC controls all drive sequencing and control, along with strand tracking, remote adjustable mold, hydraulic and grease systems, and shear control.
The HMI is programmed to allow the operator to control all process-related functions of the caster. From the drive HMI station, the operators have complete control of the caster drive systems, strip tracking, remote adjustable mold and all other drive functions. Trending HMI is programmed to enable long-term archiving of process variables.
The thin strip caster at Steel Dynamics is a DC motor driven machine utilizing Siemens digital drives for the stand control. The auxiliary drives for tundish travel, crop car and shear exit rolls use Siemens digital AC drive technology.
There is also a complete emergency control system comprising relay circuits and 24 VDC battery backup power.
The compact rolling mill has a capacity of 2.4 million tpy with the two casters. The 6-stand mill can roll exit gauges down to 1.0 mm. The main motors, electrical drives, control and automation systems are supplied by Siemens.Following its exit from the tunnel furnace, the thermally equalized slab passes an emergency shear, a high pressure water descaler box, and the six mill stands which include loopers, a runout table with a laminar flow cooling section. Then the rolled strip is wound at the downcoiler.
All six mill stands are each equipped with a 7700 kW AC synchronous motor with cylindrical rotor and air-water cooling - a standard Siemens design for compact rolling mills. Air-cooled modular SIMADYN D_ micro processor controlled cycloconverters supply the variable voltage and frequency required for the highly dynamic and accurate variable speed operation of the drives. All auxiliary variable speed motors use digital AC drive technology to communicate with the mill master via a PROFIBUS data link.
The same microprocessor-based control system is used for the finishing mill speed master to provide the speed references, strip tracking, control of the interstand cooling emergency shear control, interface to the HMI, and the interface to the Level 2 process automation system.
The process automation system, Level 2, of the hot strip mill of the Steel Dynamics CSP plant is engineered from modules of the overall Siemens automation concept for steel plants. It receives the strip/coil schedule and the associated primary data from a plantwide production management system, Level 3. In return, the Level 2 system sends process data that are acquired for each coil produced back to the Level 3 system.Basic automation system controls must be set well before a strip to be rolled enters the first mill stand, thus the required strip thickness and temperature obtained on leaving the last finishing mill stand are achieved without regard to changes in dimensions and rolling orders. In distributing the total strip reduction over the individual finishing mill stands, close attention must be paid not only to the load limits of the stands and drives but also to the technological conditions. Setpoints are then issued to the subordinated closed and open-loop controllers. While the process automation system is responsible for the proper setup to achieve the best strip head and tail performance, the basic automation system ensures constant thickness during the strip run.
Water spray patterns for the run-out table cooling section must be determined prior to the strip head's entry into the cooling section, in order to achieve the required coiling temperature already at the strip head. During the strip run through the mill, effects of strip temperature changes are compensated by turning sprays on and off to achieve not only a constant coiling temperature but also the required final crystalline structure of the steel strip.
The coiler is provided with setpoints for the pinch roll and wrapper roll gaps as well as strip tension that takes strip bending torques into account.
For the entire HSM area, the measured values necessary for production assessment are acquired and entered into operation logs and/or sent to the Level 3 computer for later evaluation, yielding gapless production records.
The success of Steel Dynamics in their quest to produce high quality flat-rolled products at low specific costs in their new mini steel mill at Butler, Indiana, is in part based on state-of-the-art Siemens drive, control and automation systems, on innovative software solutions, and the professional project management services rendered by the Metals and Mining Division. The SDI plant is the third of a series of 18 compact strip mills to date whose electrical and automation systems are or will be supplied by Siemens in an unmatched series of orders.
Use of neural networksThe general objective of the process automation system for the hot strip mill is to calculate the presets for the finishing mill train as accurately as possible to ensure that the tight product tolerances are achieved. These calculations are based on mathematical models that are continually matched to the process conditions by means of auto-adaptation.At Steel Dynamics and other steel and rolling mills, a new tool taken from the field of artificial intelligence has been employed: neural networks. These networks learn from the incoming data and their logical connections. They obtain their experience from the observation of systematically repetitive processes by recognition of continually recurring patterns. They are thus able to support or fully assume the functions of the mathematical models. The pass schedule pre-calculation is solved by parallel connection of a mathematical model and a neural network. Greater production flexibility and closer tolerances are attained as a result. Since the chemical analysis of the material is a direct input parameter of the neural network, no traditional classification of the production spectrum is required for the model adaptation. |