GB1573151A

GB1573151A - Production of metal ingots

Info

Publication number: GB1573151A
Application number: GB815877A
Authority: GB
Original assignee: Sumitomo Metal Industries Ltd
Current assignee: Nippon Steel Corp
Priority date: 1977-02-25
Filing date: 1977-02-25
Publication date: 1980-08-13

Description

(54) IMPROVEMENTS RELATING TO THE PRODUCTION OF METAL INGOTS (71) We, SUMITOMO METAL INDUSTRIES LIMITED, a Japanese Corporation of No. 15, 5-chome, Kitahama, Higashi-ku, Osaka-shi, Osaka-fu, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates in general to ingot making processes in steel manufacturing. More particularly, the present invention pertains to a control of teeming rate in such ingot making processes.

In typical steel manufacturing processes, the molten steel is first received in a ladle and then poured into moulds to form ingots. The ingots thus formed are subjected to further working operations such as rolling operations or forging operations. It has widely been recognized that the quality of the final steel products is in large part dependent on the quality of the ingots and that the quality of the ingots are substantially affected by the process condition in pouring operation of the molten metal, particularly, by the rate of pouring into the mould.

Conventionally, the teeming rate has been adjusted by the use of pouring nozzles in various diameters. However, it has been experienced that the 'conventional method has not been effective to provide stable and desired control of the teeming rate. For example, it has been difficult to maintain a constant rate of rise of the surface of the molten metal which is believed to have influence on the skin thickness of the ingot particularly in the manufacture of rimmed steel.

Further, it has also been difficult to prevent or at least decrease surface defects due to the splashing of molten metal at the initial stage of the pouring process.

In order to eliminate the above problems, proposals have been made to detect or measure the teeming rate in terms of the change in weight of the pouring ladle and control the opening degree of the pouring nozzle in accordance with the results of the measurements.

However, the most serious problem in the above process is that the programmed control is susceptible to unacceptably large errors. For example, when the above mentioned process is put into practice, a so-called "crane scale" is usually employed for measuring the change of the weight of the pouring ladle. The problem herein is that such a crane scale usually has measuring errors which amount to 10 percent of the maximum value of the scale. Therefore, in order to obtain the change in weight of the pouring ladle with an adequate accuracy, it becomes necessary to increase the time interval between two succeeding measuring points.

According to the present invention, an apparatus for making metal ingots comprises a source of molten metal formed with a passage having a gate valve for regulating the flow of molten metal to a mould under the control of an actuator responsive to a secondary rate control signal, the apparatus including a control system comprising a feed forward circuit receiving a primary control signal from a teeming rate programmer and producing the secondary rate control signal, means for measuring the teeming rate and producing a teeming rate signal, a modifying signal circuit for comparing the primary rate control signal and the teeming rate signal and when there is any difference between the two signals producing a modifying signal for modifying the secondary rate control signal.

Since the rate of flow of the molten metal is affected not only by the degree of opening of the gate valve but also by the wear and clogging of the passage and head pressure of molten metal in the ladle, the relationship between the valve opening and the actual teeming rate must always be taken into account. Therefore, in apparatus according to the present invention, the actual measurement of the teeming rate is compared with the programmed rate signal, that is, the primary rate control signal to modify the secondary rate control signal.

In a preferred method of operating the actual teeming rate is calculated. More specifically, the teemed weight is measured at predetermined intervals and the results of the measurements are all taken into account so that the average slope of the change in the teemed weight can be calculated. For the purpose, when the measurement is made by means of a crane scale which measures the weight of the ladle, the calculation is performed in accordance with the following formula: where:

bn is the calculated teeming rate; W1 is an indication of the crane scale; T is the interval between each two measurements; n is the number of measurements subtracted by one.

According to a further feature of the present invention, means is provided for calculating the relationship between the valve opening degree and the rate of teeming and modifying the second rate control signal before it is applied to the actuator means. For the purpose, an assumption may be made that there is a linear relationship between the valve opening degree and the teeming rate and the slope and constant in the linear equation is determined through calculation.

In order that the present invention is more clearly understood, it will further be described by way of embodiments taking reference to the accompanying drawings, in which: Figure 1 is a diagrammatical view of a ladle and a mould provided with teeming rate control means in accordance with one embodiment of the present invention; Figure 2 is a block diagram showing one example of the control means in accordance with the present invention; Figure 3 is a block diagram showing another example of the present invention; Figure 4 is a diagram showing a typical example of the programmed pattern of the teeming rate control; and Figure 5 is a diagram showing the result of rate calculation in accordance with the present invention.

Referring now to the drawings, particularly to Figure 1, there is shown an ingot making device comprising a pouring ladle 1 and a mould 2 located beneath the ladle 1. As is conventional in the art, the ladle 1 is charged with molten metal which is teemed into the mould to make an ingot. To control the rate of pouring or teeming, the ladle 1 is provided with a gate valve 7 of which opening degree is controlled by means of a servo actuator 6.

The ladle 1 is suspended by means of suspending arm 3 of an overhead crane.

In the embodiment, the suspending arms 3 are provided with weight measuring means such as a crane scale 4 for measuring the weight of the ladle 1. The crane scale 4 is connected with teeming rate control means 5. The rate control means 5 receives a weight signal from the crane scale 4 and produces an output which is utilized to control the servo actuator 6.

Referring to Figure 2, the control means 5 comprises a teeming rate programmer 10 in which a teeming rate pattern is programmed in terms of millimeters per second which represents the rising rate of the surface level of molten metal in the mould. The programmer 10 is connected with unit converting means 11 which receives a programmed signal from the programmer 10 and produces a first rate control signal in terms of kilograms per second. The unit converting means 11 is connected with a feed forward means 12 which receives the first rate control signal from the unit converting means 11 and produces a second rate control signal in accordance with the first rate control signal. The second control signal is then applied to the servo actuator 6 which determines the opening degree of the gate valve 7 in accordance with the second control signal.

The control means 5 further includes a teeming rate signal generator 13 which is connected with a teeming rate sensor constituted by the crane scale 4 to receive a weight signal therefrom and produces a teeming rate signal in accordance with equation (1). The teeming rate signal generator 13 is connected through a sampling device 14 with a hold circuit 15. The sampling device 14 is in the form of a switch which is closed at periodical intervals so that the teeming rate signal is allowed to pass therethrough intermittently at predetermined intervals. The hold circuit 15 maintains a previously received teeming rate signal until a next signal is received.

Further, the hold circuit 15 produces an output signal which is the same as that maintained therein and applied to a modifying composition in the form of a signal generator 18.

The modifying signal generator 18 also receives the first rate control signal from the unit converting means 11 and compares the first rate control signal with the teeming rate signal from the hold circuit 15 to produce a modifying signal which corresponds to the difference between the first rate control signal and the teeming rate signal. The modifying signal is applied to the feed forward means 12 to modify the second rate control signal produced therein.

The output of the hold circuit 15 is also applied to unit converting means 16 which converts the signal in kilograms per second from the holding circuit 15 to a corresponding signal in terms of millimeters per second and the latter signal is utilized to effect a time indication of teeming rate at an indicator 17.

Means are also provided for setting the casting weight (referred to in the diagrams as moulding weight program 19) which produces a signal corresponding to a desired ingot weight for each mould 2. The output of the setting means 19 is connected with a stop circuit 20 which also receives the teeming rate signal from the teeming rate signal generator 13. The circuit 20 integrates the teeming rate signal from the device 13 to obtain the weight of the cast metal and, as soon as the weight becomes equal to the weight as determined by the device 19, it produces a stop signal which is applied to the servo actuator 6.

Referring to Figure 4, there is shown a typical pattern of the teeming rate program for rimmed steel. According to the pattern, the gate valve 7 is at first opened to the full open position for the first or leading mould 2 as shown by A so that an adequate passage is established for the molten metal. After a short period, the valve is closed to a restricted position as shown by B. Then, after a certain time interval, the gate valve is gradually opened along the line C to the position D and maintained at the level for a certain period to the point E.

The period B and the gradual opening of the gate valve 7 are required for preventing turbulence at the bottom part of the mould 2 and avoid surface defects (so calls "scab") which may otherwise be produced near bottom side of the ingot.

During the period D, the teeming rate is maintained as high as possible within a limit in which surface defects such as "hair crack" can be avoided.

At the point E, the gate valve 7 is closed by a certain amount to decrease the rate of teeming as shown by F and after a certain period it is further closed to decrease the rate as shown by G. As soon as the weight of the moulded metal reaches a desired value, the gate valve 7 is closed to the restricted position H to complete the teeming operation for the first mould 2. This stepped closing of the gate valve from the period E to H is required to prevent turbulence at the top portion of the poured metal and avoid surface defects which may otherwise be produced on the top surface of the casting.

Thereafter, a second mould 2 is placed beneath the pouring ladle and the gate valve 7 is gradually opened along the fine I to a position J to conduct a second casting operation.

In operation, the programmer 10 produces a signal which varies as shown in Figure 4 and the signal is modified by the unit converting means 11 into the first teeming rate control signal. The first teeming rate control signal is received by the feed forward means 12 to be passed therethrough and applied as the second rate control signal to the servo actuator 6 to determine the opening degree of the valve in accordance with the programmed pattern.

The teeming rate signal generator 13 continuously calculates the rate of teeming and the output therefrom is intermittently passed through the sampling device 14 at predetermined intervals, for example, at every three seconds. Thus, the teeming rate signal from the hold circuit 15 is renewed at the predetermined intervals and applied to the modifying signal generator 18.

The modifying signal generator 18 compares the rate signal from the hold circuit 15 with the first rate control signal from the device 11 and, if there is any difference therebetween, it produces a modifying signal preferably in accordance with the following equation:

wherein C: value of the modifying signal, Ab: the difference between the rate signal from the holding circuit 15 and the first rate control signal proportional gain, time parameter.

The modifying signal thus produced is applied to the feed forward means 12 to modify the second rate control signal. When the calculated weight of the moulded metal reaches the programmed weight which is represented by the signal from the device 19, the stop circuit 20 produces a stop signal to close the gate valve to the restricted position to decrease the teeming rate to a minimum value as shown by H in Figure 4. As soon as a subsequent mould is located beneath the pouring ladle 1, the programmer 10 send a further signal to repeat a subsequent moulding cycle.

One of the most important features of the present invention is in the manner of calculating the rate of teeming. More specifically, the circuit 13 intermittently conducts calculations at time intervals of for example three seconds based on the equation (1) which has previously been referred to. Through the calculations, it is possible to eliminate or at least decrease the effect of possible errors in the crane scale as shown in Figure 5.

Figure 3 shows another embodiment of the present invention in which corresponding parts perform the same functions and are designated by the same reference numeral as in Figure 2. In the embodiment, a valve control signal generator 21 is interposed between the feed forward means 12 and the servo actuator 6. The valve control signal generator 21 functions to make calculations based on a predetermined equation xn=f(bn) which represents the relationship between the teeming rate bn and the valve opening xn. Practically, the relationship may be assumed as being represented by a linear equation bn=pxn+q (3) wherein p and q are constants.

In the embodiment shown in Figure 3, calculating means constituted by a constant modifying current 22 is provided in order to obtain the constants p and q.

The calculating means 22 receives signals from the valve control signal generator 21 and the teeming rate signal generator 13 and conducts a calculation based on the equation

wherein yn=yN-n, y being a constant smaller than one. For example, it may be 0.99.

The calculating means 22 intermittently makes calculations, for example at the time interval of three seconds to obtain the values p and q where the value I is the smallest.

The output of the calculating means 22 is thus considered as representing the constants p and q and the output is then intermittently applied to thinning-out means 23 which permits the signal from the calculating means 22 to pass therethrough once per several times of signal inputs. The signal passed through the thinning-out means 23 is applied to the valve control signal generator 21 to determine the relationship between the teeming rate control signal and the valve control signal. If the output of the calculating means 22 were to be directly applied to the valve control signal generator 21, there would be a possibility of overshooting or hunting. Therefore, it is preferable to provide the thinning-out means 23 as shown, although, in some cases, it may be omitted.

Practically, the calculation of the teeming rate in accordance with the equation (I) does not always represent the actual rate V but there is a time lag which is estimated as being represented by the following equation

Therefore, it is necessary to have the valve control signal from the generator 21 correspondingly modified before the calculation is performed by the calculating means 22. Such modification is recommendably be made in accordance with the equation

Thus, the calculation should be made using the value x" in the place of xn in the equations (3) and (4). According to the control of the present invention, the most desirable operating condition can be attained within a few minutes after starting of teeming operation and the adjustments or modifications performed in previous teeming operation can all be utilized in the succeeding operation.

Although the present invention has been described with reference to an ingot making process in which the molten-metal is poured from a ladle into the mould, it can also be applied to a process wherein molten metal is injected through an opening formed in the bottom of the mould under the control of a gate valve in the same manner as described for the ladle.

WHAT WE CLAIM IS: 1. Apparatus for producing metal ingots comprising a source of molten metal formed with a passage having a gate valve for regulating the flow of molten metal to a mould under the control of an actuator responsive to a secondary rate control signal, the apparatus including a control system comprising a feed forward circuit receiving a primary control signal from a teeming rate programmer and producing the secondary rate control signal, means for measuring the teeming rate and producing a teeming rate signal, a modifying signal circuit for comparing the primary rate control signal and the teeming rate signal and when there is any difference between the two signals producing a modifying signal for modifying the secondary rate control signal.

2. Apparatus in accordance with Claim I in which the teeming rate measuring means includes means for measuring the weight of metal already poured into the mould and means for calculating the teeming rate in accordance with changes in the weight of teemed metal.

3. Apparatus in accordance with Claim 2 in which the weight measuring means operates to measure the weight of a ladle comprising the source of molten metal and the teeming rate calculating means includes means for making calculations in accordance with an equation where:

bn is the calculated teeming rate; W, is an indication of the crane scale; T is the interval of each two measurements; n is the number of measurements subtracted by one.

4. Apparatus in accordance with any one of the preceding claims in which valve control signal generating means is provided between the feed forward circuit and the actuator so as to receive the secondary rate control signal and produce a valve control signal which is applied to the actuator to control the gate valve.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. equation (I) does not always represent the actual rate V but there is a time lag which is estimated as being represented by the following equation Therefore, it is necessary to have the valve control signal from the generator 21 correspondingly modified before the calculation is performed by the calculating means 22. Such modification is recommendably be made in accordance with the equation Thus, the calculation should be made using the value x" in the place of xn in the equations (3) and (4). According to the control of the present invention, the most desirable operating condition can be attained within a few minutes after starting of teeming operation and the adjustments or modifications performed in previous teeming operation can all be utilized in the succeeding operation. Although the present invention has been described with reference to an ingot making process in which the molten-metal is poured from a ladle into the mould, it can also be applied to a process wherein molten metal is injected through an opening formed in the bottom of the mould under the control of a gate valve in the same manner as described for the ladle. WHAT WE CLAIM IS:

1. Apparatus for producing metal ingots comprising a source of molten metal formed with a passage having a gate valve for regulating the flow of molten metal to a mould under the control of an actuator responsive to a secondary rate control signal, the apparatus including a control system comprising a feed forward circuit receiving a primary control signal from a teeming rate programmer and producing the secondary rate control signal, means for measuring the teeming rate and producing a teeming rate signal, a modifying signal circuit for comparing the primary rate control signal and the teeming rate signal and when there is any difference between the two signals producing a modifying signal for modifying the secondary rate control signal.

5. Apparatus in accordance with Claim 4 in which the valve control signal

generating means is means for converting the secondary rate control signal to the valve control signal in accordance with a specific function.

6. Apparatus in accordance with Claim 4 in which the valve control signal generating means is means for converting the secondary rate control signal to the valve control signal in accordance with an equation bpx,+q wherein: bn represents the secondary teeming rate; xn the valve opening; p and q are constants.

7. Apparatus in accordance with Claim 6 in which means is provided for receiving the valve control signal from the valve control signal generating means and teeming rate signal from the teeming rate measuring means to produce constant modifying signals through calculation based on an equation

wherein pn=p"~", y being a constant smaller than one, the calculations being performed to obtain the values p and q where the value becomes the smallest, the constant modifying signals being applied to the valve control signal generating means to modify the constants for the calculation.

8. Apparatus in accordance with Claim 7 in which the calculations for obtaining the constant modifying signals are performed intermittently with predetermined time intervals.

9. Apparatus in accordance with Claim 8 in which thinning-out means is provided between the constant modifying signal generating means and the valve control signal generating means to allow the constant modifying signals to pass therethrough once in a predetermined times of signal inputs thereinto.