EP0147258B1 - Method of changing the width of a continuously cast strand without interrupting the casting - Google Patents

Abstract

A method of changing the width of a continuous metal casting of rectangular cross section without interrupting the casting, wherein a length of the continuous casting is cast through a mold defining a cavity of rectangular cross section having a longitudinal axis, the mold having two small side walls determining therebetween the width of the continuous casting, at least one of the side walls being displaceable with respect to the longitudinal axis to change the width, which comprises the improvement of so controlling the displacement of the one side wall in response to the length of the continuous casting cast since the beginning of the width changing operation that a transition zone of changing width of the continuous casting has a predetermined length regardless of any change in the casting speed of the continuous casting during said operation.

Description

The present invention relates to a method for modifying the width of a continuous casting product with rectangular section, such as a slab, without interrupting the casting, by moving at least one small side of the mold by means of motors, such as cylinders, controlled by a programmable controller or a computer.

In all known methods for changing the width on the move, the speed of movement of the short side (s) is predetermined for the entire duration of the movement, so that the length of the variable-width section of the cast product, called the transition section, depends the speed of extraction of the cast product. This is particularly the case of the method described in document EP-A-0.028.766, on the basis of which the preamble of claim 1 is written. It is not possible to obtain a transition section of predetermined length if the extraction speed fluctuates during the change of width. However, to reduce losses and facilitate rolling operations, it is very important to be able to impose a given length on the transition section.

The object of the present invention is to provide a method for changing the width of a continuous casting product, without interrupting the casting, making it possible to produce a transition section of imposed length whatever the evolution of the speed of extraction of the cast product during the change of width.

The process which is the subject of the present invention is characterized in that, before the start of the width changing operation, the variation in width of the ingot mold and the length of the transition section are fixed, and to arrive at the fixed values, during the whole width change operation, the length of the product poured from the start of the operation, and at regular time intervals, the computer calculates the displacements to be imposed on said motors during the following time interval, by means of predetermined relationships linking the displacements due to the motors to the length of the product poured since the start of the operation, possibly corrects the displacements calculated to take account of variations in the speed of extraction of the product poured during the preceding time interval and control said motors so that they carry out the calculated and possibly corrected displacements.

These relationships between the displacements of the motors and the length of the cast product will preferably be linear functions.

In general, several phases will be used to move the short side from its initial position to its final position, and the relationships between the abscissa of any point on the short side and the length of the cast product will be different from phase to phase. 'other. In particular, at the start of the width change operation, it will be possible to tilt the short side relative to its initial position by an angle p corresponding in magnitude and as a sign to the slope of the small face of the transition section, trapezoidal shape, then make it undergo a rotation of large radius and, at the end of the operation, straighten it to bring it into its final position. The intermediate phase could be a rotation around an axis coincident with the intersection of the planes defined by the inner face of the short side when the latter is in its initial and final positions, respectively, or an axis coincident with l intersection of the planes defined by the inner face of the short side when the latter is in the intermediate positions making the angle β with its initial and final positions, respectively, or of an axis parallel to these axes and located in the vicinity of these.

For the implementation of this process, it will be possible, for example, to measure the length of the poured product throughout the width-changing operation, calculate periodically, at times separated by very short time intervals, the distance by which the short side must be moved during the time interval following the instant considered from the length of the product poured from the start of the operation increased by the length cast during the time interval preceding the instant considered , order the displacement of the short side over the calculated distance, then at the next instant compare the actual length of the product poured during the time interval which has just elapsed with the length of the product poured during the previous time interval and if they differ correct by the value corresponding to this difference the new distance of displacement of the short side calculated at this instant.

• La figure 1 est une vue partielle, en coupe, d'une lingotière conçue pour la mise en oeuvre de l'invention;
• La figure 2 est un schéma montrant les phases successives de l'opération d'élergissement de la cavité d'une lingotière à brames suivant le pro cédé de l'invention;
• La figure 3 représente le tronçon de brame produit pendant cet élargissement;
• La figure 4 est un schéma illustrant l'opération de rétrécissement de la cavité d'une lingotière suivant le procédé de l'invention; et
• La figure 5 représente le tronçon de brame produit pendant ce rétrécissement.

The following description refers to the accompanying drawings and illustrate, by way of non-limiting example, some embodiments of the invention. In these drawings:

• Figure 1 is a partial view, in section, of an ingot mold designed for the implementation of the invention;
• FIG. 2 is a diagram showing the successive phases of the operation of enlarging the cavity of a slab ingot mold according to the process of the invention;
• FIG. 3 represents the section of slab produced during this widening;
• Figure 4 is a diagram illustrating the operation of narrowing the cavity of an ingot mold according to the method of the invention; and
• FIG. 5 represents the section of slab produced during this shrinking.

The continuous casting ingot molds used for the production of slabs consist of a frame 10 in the form of a frame supporting four plates forming the sides of the ingot mold. The plates 12 constituting the short sides are normally clamped between the plates 14 constituting the long sides by pressure means, such as jacks or springs. When these means are made inactive, the short sides can be moved to adjust the width of the mold cavity and, consequently, of the slabs produced. To support and move each short side, two screw jacks mounted on the chassis 10 are used, one element of which, screw or nut, is articulated by its end on the outer face of the small side; the other element of the jacks is coupled to an electric servo motor 18 whose rotation is controlled by a programmable automaton, a microprocessor or a computer 20.

In Figure 2, there is shown schematically the successive positions of a small side of the mold which is moved away from the axis XX 'of the cavity of the mold, to widen the latter. When the mold cavity is curved, the axis X'-X represents the trace on the plane of the figure, that is to say on the median plane parallel to the large faces of the mold, of the plane containing the axis of the mold cavity. In what follows, when reference is made to the axis XX ′, it will be, if appropriate, the plane containing this axis. The line AA ₃ represents the lower edge of the mold and the line BB ₃ the level of the free surface of the metal in the mold.

Initially, the short side is in the AB position; it is inclined at an angle a ₁ , with respect to the axis XX ′, to take account of the contraction of the cast product due to its cooling in the mold; this inclination makes it possible to maintain the short side of the mold in contact with the small faces of the product poured practically over the entire height of the mold and, consequently, to reduce the risks of breakthroughs. It has been demonstrated and verified by experience that this inclination is such that the extension of the trace on the short side always passes through a fixed point 0, called the center of conicity, for all the widths of the cast product that we are able to produce industrially.

During the first phase, the short side is brought, by rotation around a fictitious axis 0 ₁ located below the ingot mold, from the initial position AB to the position A ₁ B ₁ such that its plane makes with the perpendicular plane in the plane of the figure and whose trace thereon is B ₁ O an angle β whose tangent is equal to the slope of the small faces of the transition section of the cast product (see Figure 3). This angle depends on the length of the transition section and the variation in width that has been imposed. The position of the axis of rotation 0 ₁ is chosen so as to limit to an acceptable value, for example 1 mm, the clearance between the small side and the small face of the cast product and / or the compression of the latter by the small side, over the entire height of the mold.

During a second phase, the short side is rotated around a second imaginary axis perpendicular to the plane of the figure and passing through 0 to bring it into position A ₂ B ₂ . This axis is in fact the intersection of all the planes defined by the inner face of the short side when it is in the normal casting position, for different widths of the mold.

During the last phase, the short side is pivoted around a third imaginary axis 0 ₂ , located at the level of the free surface of the metal in the ingot mold or in the vicinity thereof, to bring it into the chosen position A ₃ B ₃ . In this position, the short side is inclined at an angle'a ₂ relative to the axis X'X and the plane of its interior face contains the center 0. As in the first phase, the position of the axis O ₂ is chosen to limit the clearance between the short side and the small face of the cast product and / or the compression of the latter to an acceptable value. During the three phases of the operation, the short side is moved according to the length of the cast product. That is to say that the abscissa X _i , counted from the axis X'-X, of any point on the short side is linked to the length 1 of the product poured from the start of the first phase by a relation x _i = f (1), f being a polynomial of degree at most equal to six. In practice we will use whenever possible a linear function of the form x _i , = a1 + b, a and b being parameters different from one phase to another and which can change during the same phase .

The above-mentioned pivotings or rotations are combined with a translation in the plane of the short side. These combined movements on the small side are controlled by the jacks 16, the running of which is controlled by the automaton or the computer 20 which imposes on them laws of linear displacement, the displacements of the two jacks remaining during each phase in a constant ratio which depends on the position chosen for the axis of rotation.

If β is relatively small, for example less than 0.005 rd, we will use the same relation x _i = a ₁ 1 + b ₁ a ₁ and b ₁ being constants, throughout the first phase. If β is larger, it may be necessary to change parameters a and b one or more times during the first phase (a ' ₁ , b' ₁ -a " ₁ , b" ₁ , '...) so that the movement of the short side is relatively slow at the start of this first phase, then faster.

Similarly, in the last phase, we will use constant parameters a ₃ and b ₃ if p is small and, if not, we will change parameters (a ' ₃ , b' ₃ -a " ₃ , b" ₃ . ..) so that the movement of the short side is relatively quick at the start of this phase and then slower.

During the second phase, the parameters a ₂ and b ₂ will remain constant.

For the implementation of the method, the length of the cast product is continuously measured, for example by means of a roller 22 applied to one face of the cast product and associated with an encoder which delivers pulses whose number is proportional to the number of turns of the roller and, therefore, the length of the cast product.

Before any change in width, the operator introduces into the automaton 20 the values imposed for the variation in width and length of the transition section. From these values, which make it possible to define the angle β, the automaton selects in a file the values of the parameters a ₁ , b ₁ - possibly a ' ₁ , b' ₁ , a " ₁ , b" ₁ .. - a ₂ , b ₂ , a ₃ and b ₃ - possibly a'3, b ' ₃ , a " ₃ , b" ₃ ...

The computer or automaton, which is equipped with a computer center, calculates from the moment the order to change the width is given, and at regular and very short time intervals, for example five seconds, the displacements to be imposed on the two cylinders 16 during the interval of time following the instant considered. At time t _n , the automaton calculates the abscissae of a reference point of each of the screws which correspond to the position which the small side will have to reach at time t _{n + i} . For this it replaces 1, in the relation a1 + b selected as a function of the position of the short side or the length of the cast product, by the value measured by the roller 22 from the start of the change in width increased by a length Δ1 _n equal to the length cast during the time interval t _n- t _n-1 . By using the abscissae calculated as the reference value, the servomotors 18 bring the short side to the position thus determined at instantt _{n + 1} .

If the extraction speed of the cast product changes during the time interval t _{n + i} -t _n , the length Δ1 _{n + 1} of the cast product during this interval will not be equal to Δ1 and the position of the short side must be corrected. For this, the automaton will calculate, from 'instant t _{n + 1} , the corrected abscissas, from the real length of the product poured up to this instant, then the differences between the real abscissas and the corrected abscissas and will correct in consequently the new abscissae calculated for the instant t _{n + 2} .

During the second phase, the rotation of the short side can be done around an axis 0 'coincident with the intersection of the planes defined by the inner face of the short side when it is in positions A ₁ B ₁ and A ₂ B ₂ , respectively, or around an axis close to 0 or 0 '.

To reduce the width of the cast product (Figures 4 and 5), the operation is carried out in the same way by tilting, at the start of the operation, the short side towards the inside of the mold with an angle β equal to l 'angle of inclination of the small faces of the transition section of the cast product, then by rotating it around an axis defined as in the previous case and passing through 0 or 0' or close to them and finally by straightening the short side by rotation about an axis O _z located near the level of the free surface of the liquid metal in the mold.

Generally, the two short sides of the ingot mold will be displaced simultaneously and symmetrically with respect to the axis X'X of the ingot mold.

Claims (9)

1. Method of changing the width of a continuously cast strand of rectangular section without interrupting the casting by moving at least one of the narrow sides of the mold by means of motors (16), controlled by a computer (20), characterized in that the mold width variation and the length of the transition section are determined before starting the width changing process and that, in order to achieve the determined values, during the whole width changing process, the length (1) of the product cast is measured from the beginning of the process, and the computer (20) calculates at regular time intervals the displacements to be imposed on said motors during the next time interval, by means of predetermined relations connecting the displacements due to the motors with the length (1) of the product cast from the beginning of the operation, possibly corrects the calculated displacements in order to take into account variations in the withdrawal speed of the product cast during the preceding time interval, and controls said motors so that they achieve the calculated and possibly corrected displacements.

2. Method according to claim 1, characterized in that said relations between the displacements by the motors and the length of the cast product are linear functions.

3. Method according to claim 1 or 2, characterized in that the computer controls said motors so as to slant the narrow side from its initial position toward the inside or the outside of the mold, depending on whether the width of the cast products shall be decreased or increased, by an angle (β) the tangent of which is equal to the inclination of the narrow sides of the cast product in the transition section, at the beginning of width changing process, and to right the narrow side to bring it into its final position, at the end of the process.

4. Method according to claim 3, characterized in that, in a first phase, the narrow side is slanted by pivoting it about an assumed axis (0₁) located slightly under the mold in order to bring it from its initial position into a first intermediate position, then, in a second phase, it is moved into a second intermediate position by pivoting it about a second assumed axis (0, 0') located under the mold approximately at the height of the centre of conicity (0), and, lastly, in a third phase, the narrow side is righted by pivoting it about a third assumed axis (0₂) located at the level of the free metal surface in the mold or close to it, in order to bring it into its final position.

5. Method according to claim 4, characterized in that, during the first phase, the slanting speed of the narrow side increases stepwise from the beginning to the end of this phase.

6. Method according to claim 4 or 5, characterized in that, during the third phase the speed at which the narrow side is righted decreases stepwise from the beginning to the end of this phase.

7. Method according to claim 4, 5 or 6, characterized in that, during the second phase, the narrow side is pivoted about an axis passing through the centre of conicity (0) or in its proximity.

8. Method according to claim 4, 5 or 6, characterized in that, during the second phase, the narrow side is pivoted about an axis (0') which coincides with the intersection of the planes defined by the inner face of the narrow side when it is respectively in the first and the second intermediate position (A_lb_l-A₂B₂) or in the proximity of this intersection.

9. Method according to any of the preceding claims, characterized in that, for calculation of the displacements to be imposed on the motors during the time interval (t_n-t_n+1) following each calculation, the computer takes into account the length of product cast from the beginning of the width change process increased by the length cast during the time interval (t_n-1-t_n) preceding the calculation, and for correction of the calculated value, the computer takes into account the possible difference between the lengths of product actually cast during the time interval preceding the calculation and during the time interval before it.

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