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Abstract
The demands made on the surface quality and thickness of steel strip have
increased in recent years. At the same time, operators have had to concentrate on
maintaining high annual production rates. To balance these needs, plants have to
be equipped with advanced electrical systems featuring dedicated control
functions.
This is a study made on the general automation and control system generally used
in major Steel Industries of the world. This study deals only with the products and
services provided by ABB Ltd.This Study includes the general description on
Automation & Control System of Steel Industry, Drives, AC800 Power
Electronics Controller, Communication Protocols & Case Study on Coil Car
Pushing.
When procuring electrical equipment for a plant, consideration needs to be given
not only to the first-time cost of the equipment but also to the total cost over its
lifetime. This has to take into account factors such as efficiency, energy
consumption, spare parts and maintenance. The industry‟s preference in the past
for adjustable speed DC drives, which easily achieve a good torque and speed
response, is giving way to a trend towards AC drives. This has come about as a
result of modern electronic converters offering the same speed accuracy and fast
torque response,
but with the added plus that the AC motors allow a major cost saving due to their
simpler construction and high reliability, even in harsh environments, and easier
maintenance.
It is not possible to define a unique control strategy for a continuous processing
line that will take account of all the different drive combinations in the various line
configurations; this is particularly true in the case of the process section.
Nevertheless, it can be done for some of the motor drives.
This arrangement, known as indirect tension control, ensures that the required strip
speed and tension are maintained. Inother words, a bridle not assigned the function
of a speed master acts as an indirecttension-controlled drive.
I could successfully understand the whole system of automation and control in a
metal industry. I understood the products and services provided by ABB Ltd and I
was even fortunate to learn the manufacturing, assembly & testing procedures of
those products.
INTRODUCTION
The demands made on the surface quality and thickness of steel strip have increased in recent
years. At the same time,operators have had to concentrate on maintaining high annual production
rates.To balance these needs, plants have to be equipped with advanced electrical systems
featuring dedicated control functions.
This is a study made on the general automation and control system generally used in major Steel
Industries of the world, amongst them is ABB Ltd.
New developments in AC drive technology, including Direct Torque Control,are at the heart of
advanced electrical systems developed by ABB for stainless steel treatment lines. Among the
system features are operator stations for automated plant control and efficient management of the
production data. Powerful software functions not only enable steel producers to control the
quantity of produced material more precisely but also provide valuable information about the
quality of the finished steel strip.
INTRODUCTION TO METAL
INDUSTRY
What is Metal ?
Greek Metallon, a word of unknown origin, has a range of meanings, including 'mine' (the
original sense) and 'mineral' as well as 'metal.' These were carried over into Latin Metallum, but
by the time the word reached English, via Old French metal, 'metal' was all that was left.
Mettle is a variant spelling of metal, used to distinguish its metaphorical senses. Closely related
to medal, which etymologically means 'something made of metal.'
Metal : a substance that is usually shiny, a good conductor of heat and electricity, and can be
made into wire, or hammered into sheets. Gold, silver, iron, copper, lead, tin
or aluminum are metals” . All metals can be classified as either Ferrous or Nonferrous.
How metals are manufactured ?
Industries in the Primary Metal Manufacturing subsector smelt and/or refine ferrous and
nonferrous metals from ore,or scrap, using electrometallurgical and other process metallurgical
techniques. Establishments in this subsector also manufacture metal alloys and superalloys by
introducing other chemical elements to pure metals. The output of smelting and refining, usually
in ingot form, is used in rolling, drawing, and extruding operations to make sheet, strip, bar, rod,
or wire, and in molten form to make castings and other basic metal products.
Non-Ferrous Metals
These are metals which do not contain any iron. They are not magnetic and are usually more
resistant to corrosion than ferrous metals.
Example aluminium, copper, lead. zinc and tin.
Ferrous Metals
These are metals which contain iron. They may have small amounts of other metals or other
elements added, to give the required properties.
Rolling Mill
Rolling: A process of working on metals to flatten or spread ,by passing them through rotating
rolls.
Mill: A machine for grinding or crushing
Rolling Mill: Machine where metal is rolled in to sheets and bars.
Hot Rolling
Hot rolling is a hot working metalworking process where large pieces of metal, such as slabs or
billets, are heated above their recrystallization temperature and then deformed between rollers to
form thinner cross sections. Hot rolling produces thinner cross sections than cold rolling
processes with the same number of stages. Hot rolling, due to recrystallization, will reduce the
average grain size of a metal while maintaining an equiaxed microstructure where as cold rolling
will produce a hardened microstructure.
Hot Rolling Process
A slab or billet is passed or deformed between a set of work rolls and the temperature of the
metal is generally above its recrystallization temperature, as opposed to cold rolling, which takes
place below this temperature. Hot rolling permits large deformations of the metal to be achieved
with a low number of rolling cycles. As the rolling process breaks up the grains, they
recrystallize maintaining an equiaxed structure and preventing the metal from hardening. Hot
rolled material typically does not require annealing and the high temperature will prevent
residual stress from accumulating in the material resulting better dimensional stability than cold
worked materials.
Hot rolling is primarily concerned with manipulating material shape and geometry rather than
mechanical properties. This is achieved by heating a component or material to its upper critical
temperature and then applying controlled load which forms the material to a desired specification
or size.
Hot Rolling Applications
Hot rolling is used mainly to produce sheet metal or simple cross sections such as rail road bars
from billets.
Mechanical properties of the material in its final 'as-rolled' form are a function of:
material chemistry,
reheat temperature,
rate of temperature decrease during deformation,
rate of deformation,
heat of deformation,
total reduction,
recovery time,
recrystallisation time, and
subsequent rate of cooling after deformation.
Types of hot rolling mill
Prior to continuous casting technology, ingots were rolled to approximately 200 millimetres (7.9
in) thick in a slab or bloom mill. Blooms have a nominal square cross section, whereas slabs are
rectangular in cross section.Slabs are the feed material for hot strip mills or plate mills and
blooms are rolled to billets in a billet mill or large sections in a structural mill.
The output from a strip mill is coiled and, subsequently, used as the feed for a cold rolling mill or
used directly by fabricators. Billets, for re-rolling, are subsequently rolled in either a merchant,
bar or rod mill.
Merchant or bar mills produce a variety of shaped products such as angles, channels, beams,
rounds (long or coiled) and hexagons. Rounds less than 16 millimetres (0.63 in) in diameter are
more efficiently rolled from billet in a rod mill.
COLD ROLLING
Cold rolling is a metalworking process in which metal is deformed by passing it through rollers
at a temperature below its recrystallization temperature. Cold rolling increases the yield strength
and hardness of a metal by introducing defects into the metal's crystal structure. These defects
prevent further slip and can reduce the grain size of the metal, resulting in Hall-Petch hardening.
Cold rolling is most often used to decrease the thickness of plate and sheet metal.
HISTORY
Dates back to 1859. Initially cold rolling developed in the area of profile mills.
Later with the development of wider mills,cold flat rolling developed to what it is to-day.
Purpose of cold rolling was more to achieve mechanical properties than required end thickness.
To-day‟s cold rolling produces tailor made products to suit individual end product requirement.
Physical metallurgy of cold rolling
Cold rolling is a method of cold working a metal. When a metal is cold worked, microscopic
defects are nucleated throughout the deformed area. These defects can be either point defects (a
vacancy on the crystal lattice) or a line defect (an extra half plane of atoms jammed in a crystal).
As defects accumulate through deformation, it becomes increasingly more difficult for slip, or
the movement of defects, to occur. This results in a hardening of the metal.
If enough grains split apart, a grain may split into two or more grains in order to minimize the
strain energy of the system. When large grains split into smaller grains, the alloy hardens as a
result of the Hall-Petch relationship. If cold work is continued, the hardened metal may fracture.
During cold rolling, metal absorbs a great deal of energy. Some of this energy is used to nucleate
and move defects (and subsequently deform the metal). The remainder of the energy is released
as heat.
While cold rolling increases the hardness and strength of a metal, it also results in a large
decrease in ductility. Thus metals strengthened by cold rolling are more sensitive to the presence
of cracks and are prone to brittle fracture.
A metal that has been hardened by cold rolling can be softened by annealing. Annealing will
relieve stresses, allow grain growth, and restore the original properties of the alloy. Ductility is
also restored by annealing. Thus, after annealing, the metal may be further cold rolled without fracturing.
Degree of cold work
Cold rolled metal is given a rating based on the degree it was cold worked. "Skin-rolled" metal
undergoes the least rolling, being compressed only 0.5-1% to harden the surface of the metal and
make it more easily workable for later processes. Higher ratings are "quarter hard," "half hard"
and "full hard"; in the last of these, the thickness of the metal is reduced by 50%.
Cold rolling is a common manufacturing process. It is often used to form sheet metal. Beverage
cans are closed by rolling, and steel food cans are strengthened by rolling ribs into their sides.
Rolling mills are commonly used to precisely reduce the thickness of strip and sheet metals.
Types Of Cold Rolling Mill
Reversible Mill
o 20Hi
o 6Hi
o 4Hi
o 2Hi
Non-reversible Mill
o Tandem Mill
o Skin Pass Mill
COMPONENTS
Pay-off Reel
The incoming coil which is the raw-material to be processed is loaded.This is a rotating mandrel
which may be electrically or hydraulically driven.
Tension Reels
Where the strip after reduction in each pass is wound.Tension reels are electrically driven from
Drives and maintain a constant tension in the strip for proper winding.
Mill Stand
Contains set of rotating rolls where reduction takes
place.These rolls are rotated at constant speed and hydraulic
pressure to get the desired thickness.
Consists of a hydraulic mandrel with segmented
construction.
After coil loading,mandrel is expanded to hold
the coil tight.
Is driven by electric motors, either AC or DC.
Is required to provide a constant preset tension
throughout the coil.
Screwdown (Hydraulic or Electric)
To impart the necessary pressure on the rolls which work on the strip.
PROCESSING LINE
Hot rolled coils needs to be surface cleaned before they can be taken for cold rolling. Cold
Rolled metal will not have the necessary chemical and surface properties desired for various end
products for which they will be used. Processing lines is the generic name for a machine which
runs the rolled metal through a process which imparts the necessary surface or chemical
qualities. Depending on the type of process,a processing line may be positioned ahead or after
cold rolling.
Main processes are:
Annealing
Pickling
Galvanizing
Tinning
Electrolytic cleaning
Tension levelling cum inspection lines
Cut to length/ Slitting lines
Some times annealing is carried out as a separate process through Batch Annealing Furnaces.
Main processing lines
Strip processing lines alter the characteristics,appearance and/or dimensions of flat-rolled
products. Typical examples are the galvanizing line, which coats the steel with a layer of
corrosion-resistant zinc, the colour coating line, which applies a layer of paint, and the slitting
line, which cuts wide coils into narrow strips. Except for those lines with a shearing section at
the exit end, most coil processing lines can be described as continuous coil-to-coil operations.
This means that coils of metal are brought to the line entry, uncoiled, fed continuously
throughout the treatment process, and recoiled at the exit.
Continuous operating lines
To ensure that the quality goals are achieved, the process sections have to operate at constant
speed and the process has to be supervised from beginning to end. After preparation of the coil,
eg by removing any damaged outer wraps, the strip is fed into the line. One of the first operations
to be performed is the welding of the incoming coil to the tail end of the coil being processed.
This is a prerequisite for continuous operation, and requires a strip storage device known as the
entry looper. The entry looper, in effect a buffer between the entry and the process area, stores
enough strip to keep the processing section operating during the welding. As soon as the looper
has emptied, the entry section accelerates to a preselected overspeed to provide more strip to
refill it.
The main functions of the exit section are strip rewinding and coil discharging. These are made
possible by another looper, which stores the strip coming from the processing section. Also, the
exit section is capable of working at overspeed to compensate for the excess strip stored in the
exit looper during stops in this section.
Annealing and pickling line
The annealing and pickling line (APL) is one of the plants requiring a constant material
processing time. To remove the hardness caused by rolling, the strip is first run through the
annealing section of the APL. During the annealing process the lattice of the steel is stressrelieved
and its structure rearranged. Annealing can be performed in a continuous process in
which the strip is passed through a furnace with different heating zones that raise it to an exactly
defined temperature and afterwards through cooling zones that gradually cool it down to its exit
temperature of about 80 °C (higher temperatures cause the line to be stopped to prevent possible
damage to mechanical equipment further along). The temperatures in the heating zones are
varied according to the type of steel being treated and the strip gauge and width. After being
annealed the strip is passed through the pickling section to give the material a clean, bright
surface. This section consists of tanks containing electrolytic, electrochemical and mixed acid
solutions. Table 2 gives details, including the running speeds and annealing data, of a new
APL installed recently by ABB at Baoyong Special Steel in Ningbo, China . Drive control
strategy It is not possible to define a unique control strategy for a continuous processing line
that will take account of all the different drive combinations in the various line configurations;
this is particularly true in the case of the process section. Nevertheless, it can be done for some of
the motor drives.
Normally, it is necessary to isolate the strip tensions in the various sections from each other in
order to stop one section from influencing another. This is accomplished
by means of speed-controlled bridle rolls. Each section has a master bridle which determines the
reference speed; a speed pilot in the entry and exit sections controls the overspeed for the looper
operation during stops (eg, for coil welding and finishing operations). When these operations
have been completed the speed is adapted again to the process. Normally, there is one bridle
operating in underspeed mode (feedbackward regulation) and another in overspeed mode
(feedforward regulation), in each case referred to the master bridle of the process. This
arrangement, known as indirect tension control, ensures that the required strip speed and tension
are maintained. In other words, a bridle not assigned the function of a speed master acts as an
indirect tension-controlled drive. Very precise control of the strip tension is necessary to avoid
strip breakage in critical areas. Direct tension control, with load cells mounted directly on the
rolls , guarantees this.
Usually, the speed control of a master bridle is based on load sharing between the two drives of
the bridle. The advantage of this configuration over the solution with one drive as the speed
master and the other speed-controlled is that the stability is better during acceleration
and deceleration and differences in the roll diameter are compensated for at constant speed.
Indirect tension control with compensation of acceleration and losses is normally used for the
coiler and looper. Thus, in the entry and exit section only one bridle is designated the speed
master. If there is a side trimmer in the exit section it may have (with respect to the strip
direction) one bridle before and one after the side trimmer, the latter acting as master so as to
ensure constant speed at the side trimmer.
There is no particular rule for the process section. In general, the speed master should be behind
the most critical part (eg, the furnace). If the line has only one process, the speed master will be
next to the exit of the process. If there is a stretch leveler in the section, the leveler itself should
be the master.
Electrical solutions
When procuring electrical equipment for a plant, consideration needs to be given not only to the
first-time cost of the equipment but also to the total cost over its lifetime. This has to take into
account factors such as efficiency, energy consumption, spare parts and maintenance. The
industry‟s preference in the past for adjustable speed DC drives, which easily achieve a
good torque and speed response, is giving way to a trend towards AC drives. This has come
about as a result of modern electronic converters offering the same speed accuracy and fast
torque response, but with the added plus that the AC motors allow a major cost saving due to
their simpler construction and high reliability, even in harsh environments, and easier
maintenance.
Direct torque control
Direct Torque Control (DTC) [1, 2, 3] is the motor control platform launched by ABB in 1994 as
the universal solution for LV drive applications and recently adapted for MV applications. This
technology is also used to control the induction motors delivered to the new annealing and
pickling line of Baoyong Special Steel in Ningbo, China.
Unlike traditional vector control, in which the parameters affecting the voltage and frequency
(eg, the motor current and flux) are measured indirectly and a pulse encoder has to constantly
provide new data to obtain a real degree of accuracy, DTC allows fast and flexible control of the machine without encoder feedback. Also, the variables used in flux vector control are controlled
by a modulator, which delays the responsiveness of the motor to changes in torque and speed.
DTC onthe other hand uses advanced motor theory to calculate the torque directly without
the need for a modulator; the control variables are the stator flux and the motor torque.
When DTC open-loop drives are installed, high dynamic performance (speed accuracy and
torque control) is possible in many cases without having to use a tachometer. Where a higher
accuracy is required, closed-loop DTC drives are employed, but the feedback device may be
less accurate and therefore cheaper than the one used in traditional flux vector drives as the speed
error and not the rotor position is known by the drive. In processing lines such as the APL
described, the main motors used to transport material (in the bridles, loopers, uncoilers, coilers)
are fitted with pulse generators. The control variables in DTC are:
• Stator flux
• Torque, calculated on the basis of the flux and stator current
• Comparison of the flux amplitude and torque deviation with given references;
the information this provides is sufficient to determine the optimum voltage vector at each
instant The high precision of the mathematical motor model makes speed feedback unnecessary.
Combining high-speed signal processing with the advanced mathematical model has produced a
25 μs high-performance control loop that ensures accurate torque control and low oscillation
levels. The resulting very fast torque response makes the DTC AC drive twice as fast as flux
vector AC drives and at least ten times faster than open-loop AC drives with scalar control.
Other benefits in the torque control area include very precise torque control at low speeds, even
down to zero, and full torque at zero speed. Measurements of shaft torque (with a torque ramp
from 100% to –100 % at zero speed) for different drive controls are shown in . With DTC the
dynamic speed accuracy is at least eight times better than with open-loop AC drives, and static
speed control accuracy is twice as good as with the existing general-purpose AC drives .
Automation systems
Modern automation systems based on an open system architecture provide userfriendly, reliable
tools that support the operator in his daily work. Such systems feature a combination of field
controls and higher-level information that makes it easy to interchange data between the Open
Control System (OCS) and the Manufacturing Execution System (MES) . By combining these
concepts, a plant automation system evolves with capabilities that extend from single motor
control to overall plant control.
OCS operator stations
Advant OCS operator stations have direct access to a database in which all the data related to the
processing line is stored. 6 Located at the entry and exit pulpits of the line, the stations manage
alarm reports and information arriving from each section, allowing the status of the plant to be
kept under control. For example, the general starting conditions, motor torque and motor speed
can be viewed and preset from these stations.
Strip tracking is one of the main functions provided by Advant OCS . It as assists the operator
with routine work by keeping track of the coil welding so that the position of the strip inside the
line and the amount of coil threaded in the entry section and rewound at the exit are always known.
Standard ABB solution
Programmable logic controllers manage the exchange of signals between the
different process sections. The current standard ABB solution for
a strip processing line consists of two PLCs (AC450RMC)
dedicated to applications in the metallurgical sector. A wide
choice of standardized functions and ready-made software
modules makes it easy to find reliable solutions that meet
customers‟ needs.
The first multi-CPU AC450 controls the entry section, the
tracking and the presetting functions, while the second PLC
interfaces with the process and exit sections.To relieve the
CPU load of the PLCs some functions are implemented on
the motor drives; these incorporate the majority of the
application software for motor control. The large drive systems
are, in fact, linked through a fast, dedicated fieldbus(AF100)
via a control unit called the Application Controller (APC).
Softwarerunning on the APC includes modules for
speed control, current control and tensioncontrol. Remote I/O
devices communicate through the AF100 with the overriding
control CPU.
The standard overall control system function covers the
generation of all sequences,velocity and acceleration
references for the drives, and the signals for starting and
stopping the line. Application specific modifications are made
according to the project requirements.
Manufacturing Execution System
Quality control depends not only on accurate control of the technological parameters of the strip
but also on overall control of the production process. The necessary coordination is achieved by
means of Manufacturing Execution System (MES) functions, being divided into operator
functions and process functions.
Operator functions
These functions are as follows:
• Order management, giving the list of coils to be worked and detailing for each coil its
dimensional data, main characteristics (coil code, steel grade identification for furnace and
pickling, customer code) and required final characteristics.
• Line preset management , comprising a set of data used to set the line up before starting
production; preparations for all the electrical and mechanical devices are based on the order
data. Coil data given by the order management and line preset functions assigned to the coil
constitute the preset data sent to the OCS for correct coil processing.
Coil reporting , with displays and print-outs of data on worked coils. The main displays are the
quality product report (thickness, flatness, elongation data) and the technological product
report (furnace, pickling, thickness, flatness, elongation distribution data for the process
technology engineer).
Production reporting, showing the number of coils produced and the work shifts in the plant
(production reports can be displayed on a shift, daily and monthly basis). Reporting of the plant
time distribution (how long the plant has been in operation and how long at standstill) and the
pickling consumption is also possible.
• Maintenance reporting, showing the actual operating time of the mechanical and electrical
equipment.
Process functions
These functions are automatically activated by the system whenever a message is received or
something occurs in the plant. 10
• Material tracking, allowing monitoring of the position of the coil in each section of the line.
• Data acquisition, for collecting information from the OCS about the uncoiler and recoiler,
tension and process sections, as well as for archiving in the system database.
APL automation systems normally make use of mathematical models that control the processing
area with high precision and have a direct effect on the overall strip quality. In the case of the
furnace, for example, the mathematical model uses the line speed, type of steel, strip width
and thickness as information when converting the annealing curve characteristics into working
parameters. A model may also be provided for the pickling area, for example to precisely control the acid dosing needed to obtain a clean, bright surface.
DRIVES
In electrical engineering, a drive is an electronic device to provide power to a motor or servo. A
Drive (motor controller) is a device or group of devices that serves to govern in some
predetermined manner the performance of an electric motor. A motor controller might include a
manual or automatic means for starting and stopping the motor, selecting forward or reverse
rotation, selecting and regulating the speed, regulating or limiting the torque, and protecting
against overloads and faults. There are in general three types of dirves : Standard Control Drive,
System Control Drive, Motion Control Drive.
Many industrial applications are dependent upon motors (or machines), which range from the
size of one's thumb to the size of a railroad locomotive. The motor controllers can be built into
the driven equipment, installed separately, installed in an enclosure along with other machine
control equipment or installed in motor control centers. Motor control centers are multicompartment
steel enclosures designed to enclose many motor controllers. It is also common for
more than one motor controller to operate a number of motors in the same application. In this
case the controllers communicate with each other so they can work the motors together as a
team.
The most basic is the Standard Drive. ABB manufactures standard drive control by the name of
ACS800.
STANDARD DRIVE CONTROL (ACS800)
Genrally ACS800 are vertically installed and there is free space above and below the unit. The
design of the cabin or cabinet of ACS800 ensures that there is sufficient cool air in the cabinet to
compensate for the power losses.
In ACS800 if the supply network is floating (IT network) both grounding screws are removed
otherwise it may lead to accident or damage the unit. Here the motor cables are three phase
cables and shielded type. Motor cable are routed away from control wires and the power supply
cable to avoid electromagnetic interference. For this kind of drive motor must be a three-phase
induction motor and suitable for frequency converter use.
The control of drive may be done by a desktop or control display panel. The drive controls the
speed, frequency, torque, power etc.
There are two start-up methods between which the user can select: Run the Start-up
Assistant, or perform a limited start-up. Standard ID Run needs to be performed during the drive
start-up. (ID Run is essential only in applications which require the ultimate in motor control
accuracy.) The ID Run (STANDARD or REDUCED) should be selected if:
- The operation point is near zero speed, and/or Operation at torque range above the motor nominal torque within a wide speed range.