Method of continuous casting of steel and apparatus therefor

Abstract

 A method of continuous casting and an apparatus therefor are disclosed in which steel cast is drawn by being guided along a single path including a curved reforming section (1) with the drive force and the brake force applied to the steel cast upstream and downstream of the section respectively to subject the steel cast to a compressive force at the section. The brake force downstream of the section is controlled in such a way that the drive force of drive rollers for feeding the steel cast upstream of the section does not exceed but approaches as near as possible to an upper critical value of the drive force determined not to cause any slip between the drive rollers and the steel cast.

Description

[0001] The present invention relates to a method of continuous casting of steel and an apparatus therefor, or more in particular to a method and an apparatus for compression continuous casting of steel in which steel cast is drawn along a path including a curved reforming section and the driving force is applied to the steel cast upstream of the reforming section while the braking force is applied thereto downstream of the reforming section.

[0002] Compression continuous casting of steel of this type is disclosed in Japanese Patent Publications No. 34025/70, No. 39227/71, No. 9972/79, No. 9973/79, No. 10925/79 and No. 51664/80, and Japanese Patent Applications Laid-Open No. 128358/80 and No. 133855/80.

[0003] In conventional methods of compression continuous casting, the steel drive torque or steel pressing force of the drive rollers upstream of the reforming section is controlled in such a manner as to apply a predetermined compressive force to the steel cast at the steel reforming section under given casting conditions. As disclosed in Japanese Patent Application Laid-Open No. 133855/80, for example, the torque of the drive rollers is increased progressively with advancement of the steel cast. In view of the basic arrangement of the continuous casting such that stable operation is achieved by speed control of the drive rollers, however, it is desirable to maintain the drive torque of the drive rollers constant on the one hand and to maintain the compressive force as large and stable as possible on the other hand.

[0004] An object of the present invention is to provide a method of continuous casting in which the casting speed determined by the casting conditions is maintained stable and the drive rollers are driven with a torque which is as near to the critical torque as possible, but not so large as to cause the drive rollers to slip.

[0005] Another object of the present invention is to provide an apparatus for performing the above-mentioned method.

[0006] According to the present invention, the above-mentioned objects are achieved by a method of continuous casting and an apparatus therefor in which the brake force of brake rollers downstream of the curved reforming section is controlled in such a way that the drive torque of motors for driving the drive rollers for feeding the steel cast to the curved reforming section does not exceed but approaches as near to a critical target value as possible without causing a substantial slip between the drive rollers and the steel cast.

[0007] In this way, the casting speed is stabilized and the compressive force is maintained at a high value within a predetermined range. In this case, it is necessary to take into consideration the slip of the drive rollers which may be caused by variations of frictional resistance between the steel cast and the drive rollers. Such a slip, if caused, changes the casting speed. According to a preferred embodiment of the present invention, at least one drive roller is monitored, and if it slips, the brake force of the brake rollers is reduced, and when the slip ceases, the brake force of the brake rollers is increased not to exceed but to approach the critical target value as near as possible.

[0008] In the case where a brake roller slips, on the other hand, the compressive force decreases, and if the slip is considerable, the casting speed changes momentarily. The decrease in the brake force due to a slip is larger than the reduction of the brake force which is provided for eliminating the slip. According to another preferred embodiment of the present invention, therefore, the drive roller and the brake roller are monitored for a slip, and if any of the rollers slips, the brake force of the brake roller is reduced, and when the slip ceases, the brake force therof is increased again.

[0009] The above and other objects, features and advantages will be more clearly understood from the following description of the embodiments of the present invention with reference to the accompanying drawings, in which:

Fig. 1 is a block diagram showing the construction of an apparatus for performing a method according to the present invention;

Fig. 2 is a block diagram showing a circuit of the brake control device shown in Fig. 1; and

Figs. 3a to 3e are graphs of measurement data obtained in an actual application using the method of the present invention, in which Fig. 3a shows the drawing speed of the steel cast, Fig. 3b shows a target drive torque, Fig. 3c shows the drive current of the drive motor and Figs. 3d and 3e show the control current of the brake motor.

[0010] In Fig. 1, guide rollers including drive rollers and brake rollers are divided into a drive group upstream of a curved reforming section 1 and brake groups I and II downstream thereof. The drive force of motors (not shown) coupled to the drive rollers (black circles) of the drive group and the brake force of motors (not shown) coupled to the brake rollers (black circles) of the brake groups I and II are controlled by the brake control device 2.

[0011] The slip, if any, of the drive group is detected by slip sensors 3₁ and 3₂, while the slip of the brake group I is detected by slip sensors 3₃ and 3₄, and that of the brake group II is detected by slip sensors 3₅ and 3₆. Each of these slip sensors is supplied with a signal generated by a tacho-generator coupled to one of the drive or brake rollers and a signal generated by a tacho-generator coupled to one of non-drive guide rollers. The two signals are smoothed and compared with each other by the slip sensor, and if the rotational speed of the drive roller is higher than that of the non-drive guide roller by more than a predetermined value, or if the rotational speed of the non-drive roller is higher than that of the brake roller by more than a predetermined value, then the slip sensor generates a signal of high level "1", which signal is applied through an OR gate to the brake control device 2.

[0012] A steel cast sensor 4₁ is arranged at a predetermined position in the brake group I, so that when the leading end of the steel cast entering the particular region reaches that position, it is detected and a signal of high level "1" is generated. This "1" signal is applied to the brake control device 2 as a brake start signal for the brake group I. This is in order not to start the braking operation of the brake group I before the steel cast enters the region of the brake group I by more than a certain distance (hereinafter called the effective brake distance) in view of the fact that a sufficient braking effect does not act on the steel cast unless the steel cast enters the region of the brake group I by more than the effective brake distance. Similarly, the region of the brake group II includes a steel sensor 4₂ for generating a signal of high level "1" when the steel cast enters the region by the effective brake distance. This "1" signal is applied to the brake control device 2 as a brake start signal for the brake group II.

[0013] A configuration of the brake control device 2 is shown in Fig. 2. The brake control device 2 is impressed with a target value indicative of the critical drive torque or motor current which is sufficiently large to maintain the speed of the motors (drive group) determined by the casting conditions, and provides the steel cast portion of the curved reforming section with a desired compressive force but not so large as to cause any slip between the steel cast and the drive rollers or the brake rollers. The brake control device 2 is also supplied with an actual drive torque or motor current for driving the drive rollers. The difference (error) between the critical drive torque and the actual drive torque is computed at the device 2 and an error signal representing the difference is applied to a slow ramp circuit 2a. The slow ramp circuit 2a includes a limiter LMC1 and an integrator INT1. The portion of the error signal above the positive limit which is predetermined by taking into consideration the characteristics of the apparatus and the operating conditions thereof and the portion thereof lower than the negative limit similarly predetermined are cut off by the limiter LMC1 so that an error signal confined within the two critical limits is applied to the integrator INT1. The integrator I_NT1 integrates this error signal by a comparatively long predetermined time constant so that sharp variations of the error signal level are absorbed, thereby producing a stable signal level with moderate variations, if any.

[0014] The error signal T thus stabilized is applied to brake force proportional distributors 2b_l and 2b₂ including operational amplifiers. The distributors 2bi and 2b₂ are supplied, through a switch SWla, with the load share or ratio A of the brake torque or motor current of the motors for the brake group I and, through a switch SW2a, with the load share or ratio B of the brake torque or motor current of the motors for the brake group II. When the input T is applied to the distributors 2b_l and 2b₂, the distributors 2b_l and 2b₂ produce outputs

·T and

·T respectively. The output of the distributor 2b_l makes up a brake control signal for controlling the brake current for the motors for the brake group I, while the output of the distributor 2b₂ provides a brake control signal for controlling the motors for the brake group II. These signals are applied to the slow ramp circuits 2c₁ and 2c₂ respectively, which are constructed similarly to the slow ramp circuit 2a and include a limiter for cutting off those portions of the brake current for each brake group which exceed a predetermined critical value, above which a slip may occur, and an integrator for smoothing the brake control current to minimize the unfavorable effect on the casting speed control. The control signal thus smoothed is applied through the switches SWlb and SW2b to brake currnt control loops 2d_l and 2d₂ respectively. The drive speed control loops 2e_l and 2e₂ are provided for driving the motors for the brake groups I and II under non-braking conditions, and are supplied with a speed indication signal for indicating the rotational speed through the switches SWlc and SW2c respectively.

[0015] The integrators of the slow ramp circuits 2c_l and 2c₂ are connected with a brake reducing circuit 2f, which includes a brake cancel level setter DCV and a switch SW3. When this switch SW3 is closed, a signal corresponding to the setting of the setter DCV is applied to the integrators of the circuits 2c₁ and 2c₂. Each of the integrators operates in such a manner as to suspend the integration of a first input from the corresponding limiter and integrate a second input from the setter DCV negatively. As a result, the output of. the integrators is rapidly reduced at a rate corresponding to the second input, so that the indication value of the brake current (generation current) to the brake current loops 2d_l and 2d₂ is reduced thereby to lower the brake torque of the brake motor. This switch SW3 is closed when the output of the OR gate is high at "1", namely, when one of the drive rollers or brake rollers slips, and is opened when the output of the OR gate is reduced to "0" after the slip. When the switch SW3 opens, the integrators of the circuits 2ci and 2c₂ resume the integration of the first input from the limiters, so that the output thereof increases at a rate corresponding to the first input.

[0016] The switches SWla to SWlc are operatively interlocked with each other; the switches SWla and SWlb being normally open, the switch SWlc being normally closed. When the output of the steel cast sensor 4₁ (Fig. 1) increases to high level of "1" indicating the detection of steel cast, the switches la and lb are closed and the switch SWlc is opened. The switches SW2a to SW2c are also operatively interlocked with each other in such a manner that when the output of the steel cast sensor 4₂ increases to high level of "1" indicating the detection of steel cast, the switches SW2a and SW2b are closed while the switch SW2c is opened. In Fig. 2, SW4a and SW4b designate normally-open switches for controlling the drive of the steel cast at the time of drawing thereof.

[0017] Now, the operation of the brake control device 2 will be explained with reference to Figs. 1 and 2. Immediately before the steel cast enters the braking region of the brake group I, a torque setting of the motors of the drive motor group predetermined as mentioned above according to the casting conditions and the actual torque or current value of the motors are applied to the brake control device 2. Also, the switches SW4a and SW4b are closed. Since the steel cast sensors 4₁ and 4₂ have not yet detected the steel cast, the switches SWlc and SW2c are closed while the switches SWlb and SW2b are open, and the motors of the brake groups I and II are activated as motors at a speed corresponding to a speed indication.

[0018] When the steel cast enters the braking region of the brake group I and the steel cast sensor 4₁ is turned on, the switches SWla and SWlb are closed and the switch SWlc is opened. Since the switch SW2a is still open, B = 0, and the output

·T=T of the distributor 2b_l is applied to the slow ramp circuit 2c_l, so that the torque or dynamic braking current limitted by the limiter thereof is aplied to the brake current control group 2d_l. As a result, the motors of the brake group I are switched to dynamic braking, thus starting the brake control as controlling the brake torque toward the target brake torque T determined as above-mentioned or toward a maximum brake torque determined by the limiter of the circuit 2c_l, when the target torque T is higher than the brake critical value.

[0019] When the steel cast enters the braking region of the brake group II, the switches SW2a and SW2b are closed and the switch SW2c is opened. The distributor 2b_l produces an output of

·T and the distributor 2b₂ produces an output of

·T. With these outputs as a target, both the brake groups I and II exert the braking force on the steel cast. In any case, the braking force does not exceed the above-mentioned upper critical value, in view of the fact that this force is controlled to a target value lower than the upper critical value in consideration of the slip of the drive motors. The limiter and the integrator prevent a sudden change of the total torque applied to the steel cast, and therefore the speed of the motors for the drive group is not changed greatly. Depending on the surface conditions of the steel cast, a slip may occur, and when such a slip occurs, the switch SW3 is closed, so that the output of the integrators of the slow ramp circuits 2c_l and 2c₂ is sharply reduced and the braking dynamic current of the brake groups I and II is decreased thereby to sharply reduce the braking force. The slip is rapidly eliminated. Also, the switch SW3 is opened, and the outputs of the integrators of the circuits 2ci and 2c₂ are restored at a slow rate by the difference between the braking current target value and actual braking current, thus increasing the braking force.

[0020] Before the last steel cast passes the drive zone of the drive group, the switches SW4a and SW4b are opened, while the switches SWlb and SW2b are opened and the switches SWlc and SW2c are closed, with the result that the motors of the brake groups I and II are driven as motors at the designated speed. The outputs of the steel cast sensors 4₁ and 4₂ and the output of the OR gate are applied to a control board and a drive group motor control system.

[0021] The motor torque and the motor current are proportional to each other and, in the above description of the embodiment, the motor torque has the same meaning as the motor current.

[0022] Measurement data obtained by controlling the braking operation by the above-mentioned controlling method as shown in Figs. 3a to 3b. Fig. 3a shows the drawing speed of the steel cast, Fig. 3b shows a drive torque setting of the motor for the drive group, Fig. 3c shows an energizing current (corresponding to torque) of the motors for the drive group, and Figs. 3d and 3e show a drive current (+) for speed control and a generation current (corresponding to the braking torque) for braking operation of the motors of the brake groups I and II respectively. These drawings share a time axis or abscissa.

[0023] Comparison of Figs. 3c, 3d and 3e shows that the energization current (torque) of the motors of the drive group undergoes only a small change after starting of braking by the group I and that the motors of the brake groups I and II perform a predetermined braking operation in stable manner.

Claims

1. A method of compression continuous casting in which steel cast is drawn along a path including a curved reforming section (1) with a drive force applied to the steel cast upstream of said curved reforming section and a brake force applied thereto downstream of said curved reforming section, thereby subjecting said steel cast to a compressive force at said curved reforming section, wherein said brake force downstream of said curved reforming section is controlled in such a manner that the drive force of drive rollers for feeding the steel cast upstream of said curved reforming section does not exceed but approaches as near as possible an upper critical value of the drive force determined so as not to cause any slip between said drive rollers and said steel cast.

2. A method according to Claim 1, comprising the steps of monitoring a slip between the steel cast and at least one of the drive rollers upstream of said curved reforming section and reducing the brake force downstream of said curved reforming section when a slip occurs and increasing the brake force when the slip disappears so that said drive force again approaches said upper critical value.

3. A method according to Claim 2, further comprising:

the step of dividing the portion downstream of the curved reforming section into a plurality of braking regions (I, II) and predetermining the ratio of braking force to be shared by each of said regions, and

the step of distributing the control amount of brake force among said regions according to said ratio in order to control the brake force of each region in accordance with the brake force to be shared thereby.

4. A method according to Claim 3, further comprising:

the step of controlling the brake force of each of said regions by smoothing the control amount of the brake force distributed among the regions in order not to cause a sudden change of the brake force.

5. An apparatus for compression continuous casting in which steel cast is drawn by along a path including a curved reforming section (1) with the drive force applied to the steel cast upstream of said curved reforming section and the brake force applied thereto downstream of said section, thereby subjecting said steel cast to a compressive force at said section, said apparatus comprising:

means (3₁, 3₂) for detecting a slip between a drive roller for driving said steel cast upstream of said section and the steel cast;

means (2a) for comparing the actual drive torque of drive motors for driving said drive rollers with a predetermined setting of drive torque and geneating a control signal for controlling the brake force downstream of said section in accordance with the difference between said drive torque and said setting in such a manner that said drive torque of said drive motors approaches said setting, and

means (2₅) for reducing the level of said control signal thereby to reduce the brake force downstream of said section in response to the detection of a slip by said slip detector means.

6. An apparatus according to Claim 5, in which said control signal generator means includes first means (LMC1) for preventing said control signal from exceeding a predetermined value and second means (INT1) for smoothing said control signal.

7. An apparatus according to Claim 6, further comprising:

means (2b_l, 2b₂) for dividing the portion of said apparatus downstream of said section into a plurality of brake regions (I, II) and distributing said control signal among said regions according to a predetermined ratio of the brake force to be shared by each of said regions; and

means (2c_l, 2c₂) provided for each of said regions for generating a region control signal for controlling the brake force of said regions in response to said distributed control signal.

8. An apparatus according to Claim 7, in which

said region control signal generator means includes third means (SLMCl, SLMC2) for preventing said region control signal from exceeding a predetermined value and fourth means (SINT1, SINT2) for smoothing said region control signal.

9. An apparatus according to Claim 5, further comprising:

second slip detector means (3₃, 3₄, 3₅, 3₆) for detecting a slip between said steel cast and a drive roller for driving said steel cast while applying brake force thereto downstream of said section; and

means (OR) for causing said means for reducing the brake force to operate in response to the detection of a slip by said second slip detector means.

Drawing