GB2105081A

GB2105081A - Monitor for continuous casting apparatus

Info

Publication number: GB2105081A
Application number: GB08221813A
Authority: GB
Inventors: Darryl Toothill
Original assignee: Davey Loewy Ltd
Current assignee: Davey Loewy Ltd
Priority date: 1981-08-26
Filing date: 1982-07-28
Publication date: 1983-03-16
Also published as: GB2105081B

Abstract

Continuous casting apparatus comprises a horizontal mould and at least one pair of driven withdrawal rolls positioned downstream of the outlet end of the mould. The rolls are rotated in a cyclic manner to dispose an ingot formed in the mould so that, for each cycle, there is a pull period and a push period (Figure 2). The withdrawal cycle is continuously monitored and the information supplied to a computer which is arranged to take corrective action or issue a warning if an abnormal condition is detected. <IMAGE>

Description

SPECIFICATION Continuous casting This invention relates to a method of operating horizontal continuous casting apparatus and in particular to a method of monitoring the withdrawal cycle during which an ingot is displaced from the horizontal continuous casting mould.

It is known for continuous casting apparatus to have a horizontal mould and at least one pair of withdrawal rolls positioned downstream of the outlet end of the mould. In use, at least one of the or each pair of withdrawal rolls is rotated in a cyclic manner to displace an ingot of the mould and, during each complete cycle, there is a pull period and a push period. The withdrawal rolls may be rotated by individual electric motors or by a single motor driving through a gear train and, alternatively, hydraulic motors may be coupled to the withdrawal roll, either directly or through a gear train.

The invention resides in a method of operating horizontal continuous casting apparatus having a horizontal mould and at least one pair of withdrawal rolls positioned downstream of the outlet end of the mould, wherein at least one of the or each pair of withdrawal rolls is rotated in a cyclic manner to displace an ingot formed in the mould, such that each complete cycle includes a pull, a pause, and a push period; the withdrawal cycle is monitored continuously and the monitored information supplied to a computer; and, in the event of the computer detecting an abnormal condition in the progress of the withdrawal cycle, it causes either corrective action to be taken or a warning alarm to be generated.

One of more of the following parameters is monitored (a) the mould temperature using temperature sensing devices, (b) withdrawal force using pressure transducers, in the case of hydraulic motors, or a feedback signal from a controller of an electric motor, and (c) withdrawal roll and ingot movement using a speed or position transducer.

In order that the invention may be more readily understood, it will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a typical known withdrawal pattern of the withdrawal rolls of a horizontal continuous casting apparatus, Figure 2 is a velocity profile of an ingot as it is withdrawn from the mould, Figure 3 is a time-temperature graph showing the characteristics of mould temperature, and Figure 4 is a sectional side elevation of part of a mould showing alternative positions ofthetempera- ture sensing devices.

Referring to Figure 1, at least one of a pair of withdrawal rolls, positioned downstream of the outlet end of the mould of a horizontal continuous casting apparatus, is driven in a cyclic manner such that, for each complete cycle, there will be a pull period, a push period, and a predetermined pause period. The pause period may be between the pull and push periods or it may be between the push period of one cycle and the pull period of the next cycle. In Figure 1 it can be seen that, over each cycle, there is a pull period tl, followed by a pause period t2, followed by a push period t3. This is then followed by a pull period tl of the of the next cycle.

As shown in Figure 2, under normal operating conditions, the velocity profile of the ingot formed in the mould has a part during which the motor and the ingot is accelerated in one direction of rotation during a predetermined maximum time ta until it achieves a maximum velocity at point B. This velocity B is maintained for a period of time t1 - ta td before being rapidly decelerated to zero velocity along line CD in time td. An optional time period t2 then follows during which the motor is stationary, but steps are taken to prevent overshoot due to the system inertia during the previous deceleration period. At the beginning of the time period t3, the motor is reversed and accelerated until it achieves a predetermined angular displacement. At the end of the time period t3, the motor is stopped.

The computer will control the operation, using a - feedback loop comprising any one or a combination of any of the casting parameters referred to above, and, in the event of abnormal conditions being detected, the computing means will slow down or stop the process and will be capable of restarting withdrawal under automatic control. Manual override by the casting operator is always available.

To determine abnormal conditions in the mould temperature, various alternative schemes are available. One of these schemes is illustrated in Figure 4 in which a temperature measuring device P is located in an opening in the mould wall adjacent to that part of the length of the mould from the inlet thereof which is within the length of maximum withdrawal stroke L1. Since this device measures the temperature of the mould as the solidifying shell starts to form during the withdrawal cycle, and the solidifying shell acts as an increasing thermal barrier as its thickness increases, then it can be used to indicate the characteristics of the shell growth in the primary solidification zone of the mould.

The basic characteristics which can be expected to be measured are as shown in Figure 3. Because the withdrawal process is cyclic, and, therefore, a nonsteady state, the resulting temperature profile is osciliatory or sinusoidal and cyclic. Deviations at the points X and Y would not be regarded as abnormal, while the dip at point D is a normal characteristic where a deliberate pause during the start-up phase is made during the withdrawal. This is to allow the material in the mould to consolidate and make sure there are no voids.The fall at point, however, is the typical characteristic of an abnormal condition caused by a skin tear and could be determined by the computer by one of the following methods: (i) by measuring the deviation from a mean temperature over a number of previous consecutive cycles and take action when deviation features an unacceptable level, (ii) by picking and measuring peak temperatures on each cycle and comparing the deviation from a predetermined maximum acceptable rate of temperature decay; this, in fact, is measuring the rate of change of temperature, (iii) by examining for temperature reversal during each cycle, (iv) by comparing mean temperatures during a cycle with predetermined temperature limits, and (v) by comparing the rate of temperature change of a number of cycles with a predetermined acceptable limit.

The above-mentioned checks are made by employing one temperature measuring device P but, as an alternative, also shown in Figure 4, any number of temperature measuring devices may be placed in the mould or along its entire length, thus permitting control based upon a temperature profile along the mould length. Under normal conditions, the position ofthe hottest point in the mould would be at the inlet end adjacent to the primary solidification zone but, under abnormal conditions, it would be noticed that the hottest point would move progressively along the mould wall towards the exit cjftht; mould.

If two temperature measuring devices P and S are used to monitor the temperature differences between two points, P will be in the vicinity of the inlet end of the mould and Swill be downstream beyond the length of the withdrawal stroke L1. Under normal casting cinditions, the temperature at P is higher than that at S because the shell thickness at P is less than it is at S. If a tear occurs, as the tear passes the point S, there is a sudden rapid rise in temperature as molten metal comes into contact with the mould wall and this signal can be used to change the casting conditions.

Because of the cyclic pull/pause nature of the withdrawal process, the temperature measured by the thermocouples will oscillate about a mean value.

The amplitude and/orfrequency will not always be constant but will depend upon such factors as: (i) withdrawal cycle pattern, (ii) characteristics of the temperature sensor, (iii) location of the temperature sensor, (iv) mould material, (v) mould design, and (vi) cast metal characteristics/properties.

When the withdrawal rolls are rotated by one or more hydraulic motors, either as an alternative to, or in addition to, the measurement of the temperature of the mould, the hydraulic motor pressure can be monitored. The torque deveioped by a hydraulic motor is proportional to the pressure across the inlet and outlet ports. If molten metal breaks out from the skin of an ingot being cast, then this change in the resistance of the ingot to withdrawal would result in a drop in torque required to maintain consistent withdrawal. This would be reflected in the change in the pressure drop characteristics across the motor ports which will be detected by pressure transducers.

The distance moved by the ingot is represented by the area under the curve A, B, C, D in the pull part of the cycle shown in Figure 1. Thus, the theoretical pull length can be determined for a known set of cycle parameters. This value can be taken and multiplied by a constant for the machine and ingot section being cast and compared with the actual lengths, measured by a position or speed transducer, of the ingot fron the mould. Any difference exceeding predetermined limits can be regarded as an indication of an abnormal casting condition and this can be used as a reference in addition to, or instead of, the abnormality detecting conditions referred to above.

Should the computing means indicate by one of the methods prescribed above that an abnormal condition has occurred, then this indicates that a partial or a complete breach in the skin of the solidifying ingot has occurred. The situation can only be recovered if the breach occurs in the mould and it is recovered by increasing the duration of the pause/push period of the cycle, i.e. t2 and/or t3 relative to the pull period. This allows a greater time for the molten metal to flow into the breach and weld together the skin tear, thus promoting consolidation and the formulation of a thicker skin able to withstand tearing during subsequent withdrawal.When an abnormal condition arises and continues for more than a predetermined number of cycles, then the t2 and/or t3 period is increased, either as a predetermined function of the current values of t2 and t3 or to some fixed predetermined value. Should the abnormal condition continue for a further predetermined number of cycles, then the operation should be repeated by increasing the value of t2 and/or t3 further in a similar manner. It will be clear that it is preferable to slow down the withdrawal process rather than stop it altogether, since this avoids the formation of surface defects on the ingot.

If, however, after a predetermined number of incremental increases in the t2 and/or t3 period, the abnormal condition still persists, then there is no alternative but to stop the withdrawal process. This can be carried out automatically or an alarm can be given for the operator to manually stop the withdrawal process. When the process is stopped by the computing means, the process can be restarted automatically after a predetermined time interval or it can be arranged that it can only be restarted by the operator.

Claims

1. A method of operating horizontal continuous casting apparatus having a horizontal mould and at least one pair of withdrawal rolls positioned downstream of the outlet end of the mould, wherein at least one of the or each pair of withdrawal rolls is rotated in a cyclic manner to displace an ingot formed in the mould, such that each complete cycle includes a pull, a pause, and a push period; the withdrawal cycle is monitored continuously and the monitored information supplied to a computer; and, in the event of the computer detecting an abnormal condition in the progress of the withdrawal cycle, it causes either corrective action to be taken or a warning alarm to be generated.

2. A method as claimed in claim 1, in which, during the withdrawal cycle, the mould temperature is monitored continuously.

3. A method as claimed in claim 1, in which, during the withdrawal cycle, the withdrawal force is monitored continuously.

4. A method as claimed in claim 1, in which, during the withdrawal cycle, the length of the ingot withdrawn from the mould is monitored continuously.

5. A method as claimed in any preceding claim, in which the corrective action for the abnormal condition is to increase the length of time of the push and/or pause period relative to the length of time of the pull period.

6. A method as claimed in claim 5, in which said corrective action is taken and, if the abnormal condition remains, the length of time of the push and/or pause period relative to the length of time of the pull period is further increased, and, if the abnormal condition is not corrected, then the withdrawal cycle is stopped.

7. A method as claimed in claim 2, in which the mould temperature is measured at the mould wall adjacent to that part of the length of the mould from the inlet thereof which is within the length of maximum withdrawal stroke.

8. A method as claimed in claim 2, in which the mould temperature is measured in the vicinity of the mould inlet and at a position downstream beyond the length of maximum withdrawal stroke.

9. A method as claimed in claim 3, in which the torque developed by the drive means for the or each pair of withdrawal rolls is monitored.

10. Continuous casting apparatus comprising a horizontal mould; at least one pair of withdrawal rolls positioned downstream of the outlet end of the mould; drive means for rotating at least one of each pair of withdrawal rolls in a cyclic manner to displace an ingot formed in the mould; means for continuously monitoring the withdrawal cycle, and a computer arranged to receive and process the monitored information and, in the event of the computer detecting an abnormal condition, to cause corrective action to be taken or a warning alarm to be generated.

11. A method of operating horizontal continuous casting apparatus substantially as hereinbefore described with reference to the accompanying drawings.