SUBJECTS
COVERED
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ROLLING MILL CONFIGURATION - The history of the
rolling process is presented commencing with its early origins and following through to
the modern continuous strip production techniques. Different rolling configurations for
cold and hot rolling mills are also discussed. The basic mill configurations, two and four
high mills, cluster and six high mills are compared. Different mill housing designs are
examined in terms of the relationship between the measured rolling load and the forces
applied to the mill housing. These relationships are fundamental to the measurement and
control of mill forces. Rollstack alignment, coilers and their geometry and
instrumentation technology are other items presented.
AN INTRODUCTION TO THE METALLURGY OF SHEET PRODUCTS -
An holistic knowledge of rolling theory, mill operating practices and product metallurgy
is desirable for engineers, operations and technical personnel charged with the
responsibility of optimising mill performance and product quality. The final material
properties are a function of pre-rolling processing, the rolling process and post-rolling
processing. The development of material properties through the various manufacturing
stages, including rolling, are considered. The main sheet products are considered: steel,
aluminium and copper/ brass.
INTRODUCTION TO ROLLING CONCEPTS -
This introductory lecture introduces a range of important physical concepts whose
understanding is critical to an appreciation of the rolling process. The notation, units
and symbols employed in the IAS course and published information are also described so
that all delegates have a common “language” in which to converse. The various
phenomena occurring in the rollgap deformation zone between the work rolls are explained
in detail and include: yield stress behaviour of plastic deformation of common materials,
physical properties, frictional effects, surface interactions between the strip and the
work roll, definition of forward slip. Finally a summary of the characteristic differences
between the different types rolling is presented, covering hot, cold, tinplate, temper and
foil mills.
ROLLING THEORY - The equations
describing the two dimensional deformation of the metal in the rollgap and their use in
predicting force, torque and slip are presented. This lecture focuses on the circular arc
models, providing a brief derivation the model developed by Orowan in 1943. This model
employs the iterative solution of the roll deformation as described by the Hitchcock
(1935). The contributions of the elastic compression and recovery regions and
inhomogeneous deformation in rollgap modelling are included. The lecture also presents a
simplified graphical model which highlights the effect of changes in tension, yield
stress, friction and strip thickness on the roll force and slip. Typical pressure
distributions for difference rolling scenarios and expected model accuracy conclude the
presentation.
ROLLGAP SENSITIVITY COEFFICIENTS -
Rollgap sensitivity analysis allows the use of a linear model to represent a non-linear
rolling process. A rollgap sensitivity is a single number representing the change in one
rolling condition with respect to another. A computer model can generate a sensitivity
coefficient, which will be applicable for the simulated rolling condition. The sensitivity
can be used to estimate for example the change in roll force cause by a change in entry
thickness or the change in entry thickness required to achieve a specified roll force
change. Alternatively a sensitivity coefficient could be measured on a rolling mill and
used for future reference on a particular product.
MODELLING OF THIN STRIP, FOIL AND TEMPER ROLLING -
This lecture extends on the material covered in the rolling theory lecture, examining
those circumstances where the circular arc model fails to adequately predict the strip
deformation. Alternative models are examined and the non-circular arc model proposed by
Fleck et al. is presented. A comparison of the circular arc and non-circular arc models
highlights their differences. A number of particular rolling scenarios including skin pass
rolling, asymmetric rolling foil and temper rolling are covered. The impact of lubrication
and friction on foil rolling and surface roughness in temper rolling are studied. The
lecture concludes with a brief coverage of roll touching or kissing.
ROLLGAP MODELLING FOR HOT ROLLING - The
trends towards tighter tolerances for width, thickness and finish temperature of hot
rolled strip, and towards thinner finish thicknesses, have driven the development of more
accurate rollgap models for online use. These new models are replacing older, less
sophisticated models which were generally tabular in form, schedule specific and heavily
dependent on adaption for effective performance. The influence of temperature,
recrystallisation, precipitation and phase transformations complicate the modelling
effort. The key issues to be addressed are considered and developments in the modelling of
hot yield stress, friction, slip and strip temperature are reviewed. An indication of the
expected performance that can be achieved in the strip quality parameters with these new
models is provided.
SIMPLIFIED MODELS FOR ROLLGAP ANALYSIS
- This lecture outlines a unified approach to the derivation of simplified models for
predicting the major rollgap variables, namely specific roll force and torque, forward
slip, drive motor power and contact arc length. Equations for the elastic zone
contributions to the roll force are included. These models are relevant to a wide range of
ferrous and non-ferrous rolling conditions including hot reversing and finishing mills,
cold mills and tinplate mills. Typical model accuracies are provided and the numerical
algorithms involved in solving the implicit equations described.
MILL THERMAL ANALYSIS - The thermal
flows within the mill impact upon shape and profile via the thermal expansion of the roll,
on the strip properties via the strip temperature change, the lubricant via the heat
generated during rolling and thermal fatigue of the rolls. Mill thermal analysis examines
the rollgap heat flows which allows the derivation of strip temperature models for the
rollgap and between the stands. Particular attention is given to the interstand behaviour
of hot steel and aluminium and runout table strip cooling. This often requires the two
dimensional analysis of the temperature distribution in hot rolling mills.
ROLL THERMAL CAMBER ANALYSIS - The
development of the roll thermal expansion or camber is studied. Experimental measurements
on roll temperature are used to study the internal heat flows which occur in the roll. The
solution of the roll temperature field and subsequent thermal expansion are described for
the case of continuous steady state rolling. Simple algorithms to predict the thermal
camber behaviour are described. The lecture moves on to show how the development of
thermal camber over time for both continuous and batch rolling can be predicted. The
response of thermal camber to a single spray, a situation which occurs in shape spray
control systems is highlighted. The lecture finishes with practical issues in the
measurement of parameters required for the prediction of thermal camber.
INTRODUCTION TO STRIP PROFILE, SHAPE AND FLATNESS
CONCEPTS - This lecture gives an introduction to definitions and causes of
thickness profile, strip shape and flatness of strip. A parametric representation of
measured flatness and thickness profile is described. The origins of thickness profile and
shape in both the hot mill and cold mill regimes are discussed. Apart from the typical
rollgap interaction affecting these defects, other causes and effects due to friction,
lubrication, sprays, product qualities, mill alignment and temperature are important.
Causes of flatness defects that may not necessarily be manifest in a shapemeter display
but affect final product quality can be critical and difficult to diagnose. Shape
parameter sensitivities to roll force, roll bending and thermal camber are important for
calculating controller gains for shape control systems.
SCHEDULING OF HOT REVERSING MILLS -
While modelling of rolling is well covered in the literature there is little published
about the strategies behind the calculation of pass thicknesses and drafts to satisfy mill
equipment constraints and strip flatness and profile requirements, or the calculation of
pass speeds and spray patterns to achieve the required strip surface quality and last pass
exit temperature. Several drafting strategies are compared, and the normalised drafting
strategy is considered in some detail, as it is a robust, versatile and practical method
for online use. A limited discussion of width drafting strategy is provided. Some results
from an online scheduling algorithm based on a normalised drafting strategy are presented.
TANDEM COLD MILL SETUP STRATEGIES -
The objective of the mill setup strategy is to provide a consistent and reliable
prediction of threading references which, in conjunction with the experience and skill of
the roller, will achieve the most efficient rolling operation. Constraints for the
scheduling algorithms are defined. There are two major components - the strategies for
scheduling tensions, thickness reductions and last stand roll force and the calculation of
actuator references based on models and physics. In particular, a tension strategy based
on mill exit thickness, a reduction strategy based on a normalised reduction technique and
last stand roll force based on specific force is described. A calculation of control
actuator references for threading of tandem cold mills to minimise off-thickness material
and threading delays can significantly effect the efficiency of operation. Schedule
dependent controller gains are calculated to improve thickness and tension control.
HOT FINISHING MILL SETUP STRATEGIES -
While modelling of rolling is well covered in the literature there is little published
about the strategies behind the calculation of stand thicknesses and reductions,
interstand tensions or the calculation of mill exit speed to satisfy strip temperature
requirements. Different thickness reduction algorithms are compared and an algorithm for
interstand tensions is provided. The influence of temperature on the setup model
calculations for a hot finishing mill is considered. A number of algorithms for
calculating the runout table spray quantity for steel rolling are presented.
ROLLING MILL ACTUATORS - The
operation and performance of a rolling mill is influenced by many factors, a key one being
the design, speed, accuracy and robustness of the actuators. An introduction to AC and DC
drive motors, hydraulic and electric screw rollgap position actuators, roll bending
actuators and electric, hydraulic and pneumatic interstand loopers in hot finishing mills
are presented. The merits of each type of actuator are discussed, with typical response
times and design limitations.
A MODEL FOR FLATNESS AND PROFILE ANALYSIS -
The modelling of strip flatness and profile is described in terms of its component models.
These include the three dimensional deformation in the rollgap, the rollgap entry and exit
tension stress analysis, the roll stack deflection and the interstand creep. One important
aspect of shape and profile development is the tension stress feedback mechanism which
occurs in the rollgap. This feedback loop is a predominant mechanism in the development
and control of shape and profile. After development of the model a number of model
applications are studied including the control of profile and shape in hot and cold
rolling tandem mills and the impact of spread and thermal camber on shape and profile.
STRIP WIDTH CONTROL - A conceptual
design of width control systems, typically employed at the roughing and finishing stands
of steel hot strip mills, is described. The lecture focuses on the many physical phenomena
present in rolling which modify the final strip width. These include: spread in flat
rolling, edging pass efficiencies, slab end effects in reversing mills, width changes due
to interstand creep and bending stresses and thermal expansion behaviour. Typical
practices and the control of slab end effects by dynamic edger position control is
addressed.
DESIGN OF SINGLE STAND THICKNESS CONTROLLERS
- The principle concepts and design issues of the various thickness controllers employed
on single stand mills are presented, through the development of process and control
models. The strengths and weaknesses of each controller are compared. Model based time
delay compensation for faster tuning of conventional feedback controllers with feedback
path time delay is explained. Peripheral issues such as mill stretch, speed effects and
tension interactions are discussed also. This lecture is an excellent introduction for the
more challenging issue of designing thickness and tension controls for tandem mills. The
thickness controllers described are also used on tandem cold mills and hot mills.
DESIGN OF INTEGRATED CONTROL SYSTEMS FOR TANDEM MILLS - Many of the
concepts and issues in the design of thickness and tension controls for tandem mills are
not intuitive. Unlike in single stand mills, there is scope for loop coupling and
interaction. The design of an integrated, non-interacting tension and thickness control
scheme is presented for both cold mills and hot mills. The controller design is developed
from a consideration of the uncontrolled tandem mill response to a range of disturbance
types, and the response to the use of different rollgap and motor actuators for control
action. Other issues examined are the use of schedule dependent controller gains and the
potential interaction of multiple thickness controllers.
SHAPE CONTROL SYSTEMS - The online
measurement of strip shape which began in the early 1960's has seen extensive development
in the area of shape control as shape has become one on the important strip quality
parameters. The design of shape control systems commences with an examination of the major
shape disturbances and the different types of actuators available. Roll bending, roll
crossing, roll side shifting, hydraulically modified rolls, segmented rolls and cold and
hot sprays are all used for shape control. The sensitivity of strip shape to these
actuators and the common forms of disturbance form the fundamental basis for the design of
the shape control system. Other important aspects covered include shape measurement and
signal processing. An examination of the performance of different levels of automation
concludes this lecture.
PROFILE CONTROL STRATEGIES - The
lecture starts with a discussion of the physical phenomena which affect the thickness
profile. The mathematical representation of typical thickness profiles is discussed and
common methods of measurement described. The design of automation systems for profile
control is reviewed and the characteristics of currently available measurement and
actuator systems described. The mathematical formulation of a vector based profile setup
model algorithm is developed which has many important advantages over the systems
installed at most mills. Typical performance of online thickness profile models and
control systems is given as well as a discussion of practical constraints and future
directions for further improvements.
ADAPTION OF ROLLING MILL MODELS -
There are a number of model coefficients that vary over time as mill conditions change or
are not accurately known for every product. If these are not addressed adequately, the
model errors lead to operating difficulties. The concept of adaption, or model learning is
discussed using simple algorithms for achieving enhanced on-line model performance. A
number of more recent techniques, such as Kalman Filtering and Neural Networks are
introduced. Many practical aspects of model adaption schemes for reversing and tandem
mills and methods of tuning adaption filters for optimum performance are presented.
Ensuring that the number of adaption parameters is minimised and the parameters are not
interdependent is of prime importance.
MILL VIBRATION PHENOMENA - Mill
vibration has become an increasingly important operational issue in both cold and hot
rolling mills as mill size and speed have increased. Torsional, third octave and fifth
octave chatter, common vibration modes which occur in cold and hot mills, are studied. For
each of the three modes of vibration the underlying mechanisms involved are discussed. For
third octave chatter a stability criteria for the critical rolling speed is presented.
This allows the effect of the major parameters on the critical speed to be determined.
Methods of suppressing third octave chatter are examined including the design, development
and testing of the vibration inhibiting piston. The current understanding of the mechanism
and methods for suppression of fifth octave chatter are then presented.
LUBRICATION & COOLING -
Lubrication and cooling remains a highly specialised field in the rolling industry. This
lecture provides an overview of the major principles of lubrication and cooling, their
impact upon the automation of rolling mills and some of the practical aspects of this
subject. It commences examining factors which impact upon the selection of the lubricant
and describes the different modes of lubrication. Differences between water based
emulsions, dispersions, stabilised dispersions and petroleum based solutions are
highlighted. It also examines the design of roll coolant application and solution systems.
CASE STUDY IN REVERSING COLD MILL AUTOMATION
- This lecture discusses the issues involved in automating single stand rolling mills,
highlighting the areas of similarity and difference with tandem cold mills. The strategy
for selecting suitable tensions and reductions is described. A discussion is included on
the importance of selecting the correct winding tensions to avoid coil collapse and
inter-wrap sticking during batch annealing in downstream processing. The components of a
state-of-the-art dynamic thickness and flatness control are summarised. A discussion on
the relative merits of different types of measurement transducers is included. Finally,
notes on the design of performance reporting and fault diagnosis systems are provided
ROLLGAP MODELING FOR
HOT ROLLING - Developments in yield stress,
friction, slip and strip temperature models for hot rolling. The influence of temperature,
recrystallisation, precipitation and phase transformations on yield stress is also
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