GB2334793A

GB2334793A - Controlling a continuous casting process

Info

Publication number: GB2334793A
Application number: GB9904065A
Authority: GB
Inventors: Leslie Wright
Original assignee: Kvaerner Metals Continuous Casting Ltd
Current assignee: Kvaerner Metals Continuous Casting Ltd
Priority date: 1998-02-25
Filing date: 1999-02-24
Publication date: 1999-09-01
Also published as: GB9803810D0; GB9904065D0

Abstract

Molten material is fed to a casting location, defined by surfaces of a pair of rollers, a roller and melt bath, or moving belts, to form a product strand, eg metal strip. To improve the process, particularly under transient conditions such as start up, shut down and gauge changes, when delays and wastage can occur, a control method is used. The actual load applied to the location defining surfaces, ie pressure 206 in the case of hydraulic roll loading cylinder 50, is compared with an intended reference value 204 and a first signal 220 generated. Similarly, the actual position 222 of the surfaces is compared with an intended reference value 224 and a second signal 228 generated. These two signals at least partially determine the production speed of motor 230. The drawing shows reference profiles 202, 226 and 234 for position load and speed that can be applied during the production start up condition.

Description

IMPROVEMENTS IN AND RELATING TO CASTING This invention concerns improvements in and relating to casting, particularly the control of casting and with particular emphasis on the control of casting processes during transient conditions during roll casting.

Roll casters, as illustrated in Figures la to ld, generally consist of a pair of rollers provided with a gap between them through which the material to be cast is withdrawn. The rollers are rotated in an opposing direction and molten material is fed to behind the gap of the rollers via a casting nozzle.

Roll casting is generally broken down into two different configurations, tip fed casters and pool fed casters. In a pool fed caster molten material is introduced from the nozzle to a pool of molten material retained in the space defined by the rollers before the gap. In a tip fed caster the molten material is fed practically straight from the nozzle to the gap. Typical tip and pool fed caster arrangements are illustrated in Figures la to ld.

In any caster operation, and particularly in roll casters, there is a need for careful control of the various parameters defining the product obtained.

A number of system variables exist, including, roller gap / gauge, casting load, strip reduction, casting speed, melt level, tip set back, strip tension, metal feed temperature, roll friction and metal level in either the head box or melt pool for the caster.

A number of the variables are either set by the product required, or are very poorly suited to variation in an on-line manner to provide a control function.

Casters therefore to date have been operated in one of two control modes.

In load control mode, the pressure applied to the cylinders is maintained at a given constant value. In general, as the load is constant, other variables in the system give rise to variation in gap between the rollers. To maintain a strict gauge in the product, therefore, it is necessary to vary the casting speed or the length of the melt to roll contact.

In the other control mode, position control, the system measures the variation and position of the cylinder, and controls the pressure or other variables in an attempt to maintain a constant gap between the rolls. Position control is the system generally used on production scale casters.

Whilst the two existing control systems have been adopted and used in practice, in each case there are significant inadequacies in the performance of the control philosophy.

These limitations are particularly apparent during the transient conditions such as those experienced during start up and shut down of the caster or when there is a fundamental change to the casting process such as occurs during downgauging.

Prior art control regimes give rise to high scrap losses and a decreased time in which the caster is in production of acceptable cast product.

Examples of compensations for such inadequacies include, for example, on aluminium casters during start up, allowing the metal to drain through the refractory launders until a steady temperature is obtained in the head box as otherwise a constant gap start cannot compensate for the wildly varying temperatures. Similarly during the casting of steel using a pool fed caster7 the unpredictability of the rate of rise of the melt pool during start up has led to empirically produced speed profiles for starting up the caster. These often lead to high loads having to be applied so as to be on the conservative slow side of the ideal conditions to avoid breakout.

The present invention aims to provide a control method and control apparatus for casters which reduce scrap losses and increase metal in caster times.

According to a first aspect of the invention we provide a method of controlling a synchronous casting operation during at least a portion of the time of that casting operation, the casting process involving providing molten material to a casting location to form a product strand, the position of the surfaces defining the casting location, relative to one another, being controlled by a load, the method including a) comparing the load applied to and / or by positioning means for the casting location defining surfaces with an intended load to generate a first signal; b) comparing the position of the casting location defining surfaces with an intended position to generate a second signal; c) controlling the production speed for the strand according to a third signal; wherein the third signal is at least partially dependant on the first and second signals.

In this way a control regime based around load and position information is used to control the casting speed and hence the parameters of the overall casting process.

Synchronous casting operations include those in which the casting location defining surfaces are provide by a pair of rollers, by a roller in contact with a melt bath and where they are provided by moving belts, for instance of non-circular profile. Hereinafter rollers is taken to include the above mentioned other synchronous arrangements.

Preferably the position of the rollers relative to one another is controlled by a hydraulic system. One or both rollers in a pair may be moveable. Preferably the load is controlled by a servo valve.

Preferably the control system is provided with a load profile.

Preferably the load profile generates a load reference signal which is used in controlling the load applied to the rollers, most preferably through an applied load signal. Preferably the load applied to the rollers gives rise to a load feedback signal. Preferably the applied load signal is a function of the load reference signal and the load feedback signal.

Preferably the applied load signal is adjusted according a load error signal. The load error signal may be generated by the subtraction of the load feedback signal from the load reference signal.

The first signal may be a function of a load profile, and more preferably of a load reference signal arising from the load profile. The first signal may be a function of load feedback signal arising from the load applied to the rollers. The first signal may be a function of a load error signal, most preferably where the load error signal is a function of the load reference signal and load feedback signal. In such circumstances the load error signal may be generated by the subtraction of the load feedback signal from the load reference signal.

The second signal may be a function of a position profile and more preferably a position reference signal generated by the position profile. The second signal may be a function of a position feedback signal arising from the position of the rollers. Preferably the second signal is a function of a position error signal, most preferably where the position error signal is a function of the position reference signal and the position feedback signal. It is particularly preferred in such case that the position error signal be generated by the subtraction of the position feedback signal from the position reference signal.

The first and second signals may be combined to form their contribution to the generation of the third signal. The first and second signals may be combined using an on-line process model. The model may be used to determine the best change in speed to minimise the variation from the desired position profile and / or the desired load profile.

Casting speed may be controlled by the speed of a motor. The motor may control the rotation speed of one or both of the rollers. The motor may control the speed of each roller directly. The motor may be controlled by the third signal directly. The speed of the motor may give rise to a speed feedback signal.

The third signal is preferably a function of one or more inputs besides the first and second signals. The control system may be provided with a speed profile. The speed profile may give rise to a speed reference signal. The third signal may be a function of the speed reference signal. The speed reference signal may include a operator trim input. The third signal may also be a function of the speed feedback signal. It is particularly preferred that the third signal be a function of the first signal, second signal, speed reference signal and speed feedback signal.

Preferably the casting operation is controlled according to the method for at least a part of the start up procedure and/or for at least a part of the shut-down procedure and / or also for changing of the casting gauge.

During start up, preferably the method is used from the point at which the introduction of metal between the rollers causes the load on the rollers to exceed a threshold value. The threshold may be an initial value of a load profile.

Preferably the method is used, during start-up, until after steady state, for instance position and/or speed and/or load variations within determined ranges occur. Preferably the method commences with the load threshold being crossed and ends after steady state conditions are reached.

During start-up, before the threshold load level is reached, preferably the roller positions are maintained constant. In this way a fixed gap is presented to the molten material during this part of the procedure.

During start-up the load profile may apply an initially high load level, the level being reduced to the preferred levels for steady state casting.

During shutdown, preferably the method is used until a point at which the amount of metal between the rollers decreases to a point where the load on the rollers drops below a threshold value. The threshold may be a final value of a load profile.

Preferably the method is used, during shutdown, from steady state conditions, for instance conditions in which position and/or speed and/or load variations within determined ranges occur. Preferably the method commences under steady state conditions and ends after the load drops below a threshold value.

During shutdown the gap between the rollers, controlled by their relative position, may be gradually varied, most preferably decreased, from a steady state value to a reduced gap.

During shutdown the load profile may be increased from the preferred levels for steady state casting to higher load level.

On tip fed casters it is preferred that as the gap is varied then the position of the tip is changed to maintain the ideal gap between the roll surfaces and the tip Too close and there is the problem of rapid tip wear or roll marking. Too great a clearance can lead to tip breakage or metal leakage. This movement may be accomplished using a mechanism detailed in Patent Number EP 0516663 B1, the contents of which are incorporated herein with regard to the features, option, possibilities and manner in which this movement is provided.

The tip position, and hence the roll to tip gap, may also be modified with fixed roll gap depending on the casting conditions. Preferably for transient conditions a smaller clearance is used, with a larger clearance used in steady state conditions. Increased productivity may be achieved in this way.

Preferably the control system aims to follow a pre-determined load profile during start-up and/or shut-down. Preferably the control system adjusts the cylinder positions and/or the casting speed, and more preferably both, to follow this load profile. Preferably the control system aims to follow a predetermined position profile during the start-up and/or shutdown. Preferably the control system adjusts the casting speed to follow this position profile.

Preferably the variation of the load and/or position are restricted to within bands. Alternatively or additionally the rate of change of load and/or rate of change of position and/or rate of change of speed may be restricted to within bands.

Preferably the transition from transient to steady state operations and/or the transition from steady state to transient operating conditions is achieved in a bumpless manner.

The molten material is most preferably molten metal, including aluminium or steel. The metal is preferably introduced to the rollers through a nozzle. The method may employ tip or pool fed casting.

According to a second aspect of the invention we provide synchronous casting apparatus comprising molten metal supply means, a casting location defined by casting surfaces, the position of the casting surfaces relative to one another being controlled by load applying means, the apparatus being provided with i) an applied load detector, intended load information and a comparator for the applied load and intended load, the comparator producing a first signal; ii) a casting surface position detector, intended position information and a comparator for the position and intended position, the comparator producing a second signal; iii) control means for adjusting the casting speed, according to a third signal, the third signal being a function of the first and second signals.

According to a third aspect of the invention we provide control apparatus for a synchronous casting process, the casting process/apparatus having a casting location defined by casting surfaces, the position of the casting surfaces relative to one another being controlled by the application of a load, the control apparatus comprising : i) an applied load detector, intended load information and a comparator for the applied load and intended load, the comparator producing a first signal; ii) a casting surface position detector, intended position information and a comparator for the position and intended position, the comparator producing a second signal; iii) processing means for generating a third signal, the third signal being a function of the first and second signals, the third signal being provided to control means in use to adjust the casting speed.

The second and/or third aspect of the invention may include any of the features and options set out elsewhere in this document, including means for their implementation.

Various embodiments of the invention and its operation will now be described, by way of example only, and with reference to the accompanying drawings in which: Figures la to ld illustrate typical structures for tip fed and pool fed casters; Figure 2 illustrates a load control mode of caster operations; Figure 3 illustrates a position control mode of caster operation; Figure 4 illustrates a first control system according to the present invention; Figure 5 illustrates a second control system according to the present invention; and Figure 6 schematically illustrates the input and output for a control system according to the present invention.

Figures la to ld shows the two principal modes in which roller casters are fed with molten material. Figure la illustrates a tip fed caster (1) in which a nozzle (3) feeds molten material (5) between opposing rollers (7, 9). The rollers are respectively rotated in the directions indicated by arrows A.

The minimum spacing between the rollers (7, 9) defines the gap X. The portion of the roller periphery for which the molten material is in contact with it, from point (11) to point (13) defines the melt to roller contact length.

During casting, the material is drawn away from the product side of the caster at a measurable speed.

The use of tip feed casters in various orientations is possible, Figure ib.

The second type of roller caster, illustrated in Figure lc, is a pool fed caster (21). The pool fed caster is similarly provided via nozzle (23) with molten material (25) . The molten material once again passes between opposing rollers (27, 29) and the minimum spacing of these defines the gap (31). Unlike a tip fed caster, however, the molten material is introduced significantly behind the gap (31). The periphery of the rollers (27, 29) and end plates not shown maintain a pool (33) of molten material behind the gap (31).

The rollers are rotated in an equivalent manner to the tip fed caster system. Again a variety of orientations are possible, Figure id.

Whichever type of roller caster is employed, one of two control regimes for the caster is provided according to the prior art.

The first potential regime, load control, is illustrated schematically in Figure 2. In this system, the cylinder (50) providing the load to the rollers is maintained at a constant load by a servo valve (52). The servo valve is controlled according to the load reference (54) which provides a pressure reference (56). The pressure reference and pressure feedback (58) are combined to control the servo valve operation. In load control roller systems, which have generally only been used for casting trials, variations in the operating conditions give rise to a varying gap generated by the constant load.

This variation in gap produces position feedback signal (62) which is compared with position reference signal (64) generated by the desired gap value (66).

Given the variation in gap arising, to maintain the strip gauge for the product, the casting speed and length of melt to roll contact can be varied. In an on-line manner, variation of length of melt to roll contact is not an effectively variable parameter and as a consequence casting speed is used to maintain the strip gauge at the desired level. In the control system, therefore, the speed reference value (70) is used to drive motor (72) according to controller (74). The speed feedback signal (76) is combined with the speed reference value (70) to provide control and maintain strip gauge.

The more commonly employed system on production scale roller casters is the use of constant position control to maintain the gauge accuracy required. A typical control system of this type is illustrated in Figure 3.

Once again, the load applied to the rollers is controlled by roll loading cylinder (50) operated at a pressure controlled by the servo valve (52) . The position of the roll loading cylinder (50) is determined to give position feedback signal (100) and this is compared with a position reference signal (102) obtained from the desired reference (104). The arising position error signal (106) is used to control the servo valve and adjust the position accordingly.

As a result of this operation, the roll loading cylinder (50) provides a pressure feedback output signal (110) which is compared with the load reference value (112) set by the operator and resulting in a pressure reference (114). The comparison of the signals gives a pressure error (118) and this is used to adjust the casting speed so as to maintain the load applied to the roll loading cylinders (50) within operating parameters. The pressure error signal (118) is monitored by the controller (121) in conjunction with a speed reference value (120) and a speed feedback signal (122). The casting speed is raised or lowered as a result to maintain the load reference within an acceptable range.

Varying the speed changes the time in which the strip is in contact with the roll and for which the rolls try to maintain a given loading as a result. As speed is increased, the position of the final solidification approaches the roll nip so that the gauge and the load are reduced. A similar effect can be achieved by moving the solidification of the metal so as to change the length of the contact arc between the metal and the roller by either moving the tip in or out on a tip fed caster, or by adjusting the metal level in the pool of a pool fed caster. However, these processes cannot be done on a dynamic basis to achieve tight control of the load.

Both of the load control and position control systems face significant problems during transient conditions, for instance, those encountered during start up and shut down of casting.

An embodiment of the present invention particularly suited to provide optimum control during start up is illustrated in Figure 4. The control philosophy uses steady state control of gap control by position with the load trimmed by speed variation for steady state casting, but provides an alternative control regime during transient operations. This particular system is concerned with transient conditions during start up and is described in that context.

During start up of the system, the aim is to run a set load profile, dependent on the gauge of the material being cast.

This reference load profile (200) is used to provide a load reference at a point in time (202) which in turn generates a pressure reference (204). The pressure reference (204) and pressure feedback signal (206) from the roll loading cylinder (50), when available, is used to determine a pressure error (207) and this in turn is used to control operation of the servo valve (52!. During start up, therefore, the system aims to achieve the desired load profile and apply it to the rolled loading cylinder (50) . The pressure error signal (207) is also used to modify the speed using signal (220).

The position of the roll loading cylinder (50) gives rise to a position feedback signal (222) which is compared with a position reference signal (224) defined by a gap reference value (223) corresponding to a point on a gap reference profile (226) over time. The position error signal (228) arising from this comparison is used with the pressure error signal (220) in controlling the casting speed. Once again, casting speed is controlled by a motor (230) receiving signals from a controller (232). The signals are dependent upon a speed profile (234) which provides a speed reference value (236) at any point in time, the position error signal (228), pressure error signal (220) and a speed feedback signal (238).

The provision of the gap position feedback signal (228) into the speed ensures that the gap will trend towards the final casting gauge desired for the product during the transient operation. The pressure error signal (220) provided ensures that the rate of change of load and the total change of load applied to the cylinders during the transience is limited. In this way bumpless change over between the control modes for transient and steady state conditions is possible.

The set load profile predetermined for the transient conditions may accommodate the load error itself or the extent of the load error may be corrected using the casting speed to a varying degree dependent upon the rate of change of the strip gauge and other factors.

Using such a system on start up the caster rollers will be rotated to a predetermined speed according to speed profile (234) and with a set gap according to gap profile (226). The gap is thus initially provided according to a position control system. As metal enters the roller system however, a load is generated and measured. At this stage, the system immediately switches to the control philosophy illustrated in Figure 4 with load control being provided using a combination of speed and gap to determine corrections with the gap error being the dominant actuator.

Once a load is detected the speed profile (234) starts to ramp up to the estimated casting speed for the desired steady state conditions. The control mode ensures that over a period of time it will slowly trend to the strip gauge required using the position error feedback (228) into the speed control.

The load profile (220) providing load reference signals (202) is adopted during start up to start with a relatively high load so as to prevent break out of the molten metal from the pool or nozzle due to unevenness of the metal sum. The load profile (200) is gradually ramped down, however, to meet the preferred load levels for steady state operation whilst ensuring that the gap variation is minimised. The provision of the pressure error (220) into the speed base variation, however, ensures that the rate of change of gap is limited during this period.

As illustrated in Figure 5 the shut down control regime functions in a similar manner but in this case the load (302) and gap reference (326) profiles are different. As shut down progresses the load profile (302) increases and the gap reference profile (326) decreases so as to ensure shut down without break out of the molten metal.

On tip casters as the gap is varied then the position of the tip also needs to be changed if the ideal gap between the roll surfaces and the tip is to be maintained. Too close and there is the problem of rapid tip wear or roll marking. Too great a clearance can lead to tip breakage or metal leakage.

The tip position and hence the roll to tip gap can also be modified with a fixed roll gap depending on the casting conditions. Transient conditions may use a smaller clearance with a larger clearance used in steady state conditions to give increased productivity.

Figure 6 provides an overview of the control procedure with the various input values received by the control system and the output values arising being illustrated in relation to a head box fed roll caster with monitoring. The width feedback signal can form part of the load correction by contributing to the pressure reference signal as illustrated in Figure 4.

A particular problem with tip fed casters is that the load feedback signal may not be sufficient for control itself unless it can be determined that any loss in load is due to the sump position being too close to the roll nip, and not due to localised freezing in the tip of the nozzle. It is possible to start the cast with a high metal temperature in such circumstances to ensure that freezing is not a problem and then use the transient control as defined. As an alternative, however, a detector can be used to assess the strip completeness leaving the roller assembly and a rule based system based on the casting conditions used to determine whether the defect is due to freezing or bleeding. A further control signal can be applied to determine the control action to be made, depending on the type of defect detected.

Claims

CLAIMS: 1. A method of controlling a synchronous casting operation during at least a portion of the time of that casting operation, the casting process involving providing molten material to a casting location to form a product strand, the position of the surfaces defining the casting location, relative to one another, being controlled by a load, the method including a) comparing the load applied to and / or by positioning means for the casting location defining surfaces with an intended load to generate a first signal; b) comparing the position of the casting location defining surfaces with an intended position to generate a second signal;

c) controlling the production speed for the strand according to a third signal; wherein the third signal is at least partially dependant on the first and second signals.
2. A method according to claim 1 in which the control system is provided with a load profile and the load profile generates a load reference signal which is used in controlling the load applied to the rollers.
3. A method according to claim 2 in which the load applied to the rollers gives rise to a load feedback signal and the applied load signal is a function of the load reference signal and the load feedback signal.
4. A method according to claim 3 in which the applied load signal is adjusted according a load error signal, the load error signal being generated by the subtraction of the load feedback signal from the load reference signal.
5. A method according to any preceding claim in which the first signal is a function of a load profile, such as a load reference signal arising from the load profile.
6. A method according to any preceding claim in which the first signal is a function of load feedback signal arising from the load applied to the rollers.
7. A method according to any preceding claim in which the first signal is a function of a position profile, such as a position reference signal generated by the position profile.
8. A method according to any preceding claim in which the second signal is a function of a position feedback signal arising from the position of the rollers.
9. A method according to any preceding claim in which the first and second signals are combined to form their contribution to the generation of the third signal.
10. A method according to claim 9 in which the first and second signals are combined using an on-line process model, the model being used to determine the best change in speed to minimise the variation from the desired position profile and / or the desired load profile.
11. A method according to any preceding claim in which the third signal is a function of one or more inputs besides the first and second signals, the control system being provided with a speed profile, the speed profile giving rise to a speed reference signal, the third signal being a function of the speed reference signal.
12. A method according to any preceding claim in which the method is used during start up, the method being used from the point at which the introduction of metal between the rollers causes the load on the rollers to exceed a threshold value.
13. A method according to claim 12 in which the method is used until after steady state, for instance position and/or speed and/or load variations within determined ranges, occurs.
14. A method according to claim 12 or claim 13 in which during start-up, before the threshold load level is reached, preferably the roller positions are maintained constant.
15. A method according to claim 12 or claim 13 or claim 14 in which during start-up the load profile is apply at an initially high load level, the level subsequently being reduced to the preferred levels for steady state casting.
16. A method according to any preceding claim in which during shutdown, the method is used until a point at which the amount of metal between the rollers decreases to a point where the load on the rollers drops below a threshold value.
17. A method according to claim 16 in which during shutdown the gap between the rollers, controlled by their relative position, is gradually decreased from a steady state value to a reduced gap.
18. A method according to claim 16 or claim 17 in which during shutdown the load profile is increased from the preferred levels for steady state casting to higher load level.
19. A method according to any preceding claim in which the control system aims to follow a pre-determined load profile during start-up and/or shut-down.
20. A method according to claim 19 in which the control system adjusts the cylinder positions and/or the casting speed, and more preferably both, to follow the load profile.
21. A method according to any preceding claim in which the control system aims to follow a pre-determined position profile during the start-up and/or shut-down.
22. A method according to claim 21 in which the control system adjusts the casting speed to follow this position profile.
23. A synchronous casting apparatus comprising molten metal supply means, a casting location defined by casting surfaces, the position of the casting surfaces relative to one another being controlled by load applying means, the apparatus being provided with

i) an applied load detector, intended load information and a comparator for the applied load and intended load, the comparator producing a first signal;

ii) a casting surface position detector, intended position information and a comparator for the position and intended position, the comparator producing a second signal;

iii) control means for adjusting the casting speed, according to a third signal, the third signal being a function of the first and second signals.
24. Control apparatus for a synchronous casting process, the casting process/apparatus having a casting location defined by casting surfaces, the position of the casting surfaces relative to one another being controlled by the application of a load, the control apparatus comprising :

i) an applied load detector, intended load information and a comparator for the applied load and intended load, the comparator producing a first signal;

ii) a casting surface position detector, intended position information and a comparator for the position and intended position, the comparator producing a second signal;

iii) processing means for generating a third signal, the third signal being a function of the first and second signals, the third signal being provided to control means in use to adjust the casting speed.