EP1062065B1 - Method and device for controlling the thickness profile of a metal strip resulting from continuous casting between two loose moulds - Google Patents

Description

TECHNICAL AREA

The invention relates to a method and a device for obtaining a strip. thin metal having a controlled and constant thickness profile by casting continuous between movable molds, for example between blocks, between ribbons, between blocks and ribbons or preferably between cooled cylinders. It mainly concerns aluminum and its alloys.

STATE OF THE ART AND PROBLEM POSED

Continuous casting facilities between movable molds include generally an oven supplying liquid metal, using a chute, a injection device uniformly distributing said liquid metal in the air gap located between the movable molds. This device comprises at least one injector having one end down the width of the strip to be poured and the lips of which have a shape adapted to that of the molds; in general it also includes a tray injector located upstream of said injector. Such a casting makes it possible to obtain strips typically having a thickness between 2 and 20 mm.

During continuous casting of very wide strips, generally greater than 1 m, between movable molds we seek to obtain a strip having a thickness given and a constant thickness profile adapted to the use which will then be made of the band. Usually this profile should have a slight bulge while the two edges of the strip must be of equal and constant thickness; this bulge, calculated as the ratio of the thickness difference between the center and the edges on the thickness in the center, is generally of the order of 0.5 to 1%.

It is essential to master this profile well and to keep it constant which is all the more difficult to obtain when the thickness of the strip is low, in particular for thinner strips less than 5 mm thick.

Indeed the transformation tools give an absolute value tolerance which, in the case of a thin strip, corresponds to an increase in the tolerance relative value; the objective is to obtain the lowest possible relative tolerance, this therefore implies maintaining absolute tolerance within limits all the more strict that the thickness of the thin strip is small.

By way of illustration, the relative tolerance to be obtained on the crown of a strip is generally ± 0.2%, which translates to a 5mm thick strip having a convex of 0.7%, i.e. 35 µm, with a tolerance (± 0.2%) of ± 10 µm, while for a 2.5mm thick strip having a convex of 0.7%, i.e. 17.5µm, the tolerance of ± 0.2% becomes ± 5µm more difficult to hold.

The thickness of the strip is given by the dimension of the resulting air gap mainly forces exerted by the cylinders on the metal being solidification and solidified and by the speed of casting.

The prerequisite for obtaining a constant thickness is to have a metal vein liquid uniform in speed and temperature in the injection device so as to obtain a solidification front as uniform as possible in the air gap. We realize this uniformity using known actuators that are the forces exerted by the cylinders on metal, casting speed, lubrication or cooling of cylinders, as is well known in rolling. This is for example described in US patents 4,550,767 (Alcoa), 4,751,958 (Hunter), 5,452,827 (Eckert).

American patent US 3799410 describes an injection device provided with deflectors fixed positioned so as to produce a uniform distribution of liquid metal at the level of the injector and uniform solidification of the metal over the entire width of the bandaged.

As for the regulation of the thickness profile (for example the bulging, the thickness of the edges ...) of the thin strip, the usual means used to obtain it consist in act on the distribution of the forces applied to the metal by the cylinders, to be modulated the cooling of the cylinders to modify their bulging and / or to adapt the lubrication applied during casting.

To illustrate these different means, patent EP 0407978 (Hunter), US 4944342 (Lauener) describe for example actions on the cooling of cylinders to modify the thermal bulge, patent EP 0228038 (Alcoa) describes actions on the couples and the efforts to control the position of the forehead solidification, patent WO 95/23662 (Davy Mc Kee) describes obtaining a uniform coating, patent EP 0081848 (Hazelet) describes, in a flow between ribbons, actions on maintaining support and the shape of the support rollers the band. So we see that in all these processes we act on the solid metal or during solidification to control the thickness profile of the strip.

These means are insufficient to obtain a regular thickness profile during casting and difficult to implement. They might be sufficient if the front of solidification was perfectly uniform; however the mere presence of obstacles in the flow of liquid, which is generally the case, introduces disturbances which inevitably lead to imperfections of said front solidification.

The plaintiff therefore sought to improve the regularity and the control of the profile. thick by simple means making it possible to replace, or complete the usual means, said thickness profile having to remain within tolerances requested by the subsequent transformer which are all the more difficult to adhere to as the strip is thin.

DESCRIPTION OF THE INVENTION

The invention is a method of continuous casting between movable molds of a strip. metallic comprising a liquid metal supply of a device injection system comprising at least one injector, and optionally a tank injector located upstream of said injector, said liquid metal being distributed by the downstream end of said injector in the air gap of the movable molds. characterized in that the solidification front of the liquid metal is deformed between the molds movable to obtain the desired strip thickness profile.

The solidification front of the liquid metal represents the interface separating the phase liquid from the solid phase and is located in the air gap.

The plaintiff thus sought to act only on the liquid metal to supplement, or replace, the usual means of checking the regularity of the thickness profile whereas the usual means, as has already been said, do not were interested in solid metal or solidification.

The invention therefore consists in imposing a speed distribution and a profile thermal at the outlet of the liquid metal injection device such as the deformation of the solidification front obtained is adapted to the thickness profile of tape sought.

It is remarkable to note that by acting simply on the thermal and / or the flow of liquid metal in the injection device, comprising a injector and optionally an injector tank, we have a means sufficiently effective to control the strip thickness profile.

Previously it was on the contrary sought to regularize the front as much as possible solidification because it was a prerequisite for obtaining a good thickness profile of tape. For this purpose we can for example see in document US 4751958 (Hunter), previously cited, that the injector has spacers intended to regulate the flow of liquid metal.

As already mentioned, the injection device may include a tank injector, located upstream and integral with the injector, whose function is to ensure a first distribution of the liquid metal, that of the injector being to complete this distribution and bring the liquid metal into the air gap.

The injector tank receiving the liquid metal generally comprises a bottom and two side walls.

The injector generally comprises a bottom, two side walls and a wall upper facing the bottom; the side walls can be convergent, parallel or divergent; the bottom and the top wall are generally converging or parallel. Its downstream end is profiled to introduce the liquid metal between the movable molds or the cylinders and distribute it there over their entire width.

To carry out the deformation of the solidification front, at least one of the following means.

First of all at least one mobile baffle, generally flat and in one or several elements, can be placed in the injector tank and block transversely and at least partially the stream of liquid metal; he is advantageous according to the invention that on each side of the baffle there is a space separating it from the side walls of the injector tank. Its width represents between 40 and 95% of the width of the injector tank.

It can be moved or oriented in several directions. So she can slide vertically or be moved transversely increasing the lateral space at one of its ends while reducing the space accordingly lateral located at the other end; this movement allows to deform the front of solidification. Its side ends can be moved independently from each other in the vertical and / or horizontal direction. The chicane can still be moved around horizontal, vertical or oblique axes but also in front or behind, that is to say parallel to the direction of the current of liquid metal. The displacement can also combine all these movements.

When the baffle consists of several elements, each of these can be moved vertically and independently of each other.

These simple movements allow you to modify the profiles thermal and velocity of liquid metal veins and thus deform the forehead solidification in the desired way, and all the better as the baffle leaves a passage at each of its lateral ends in addition to the passage lower, to control the strip thickness profile.

To make the baffle more effective, it is generally planned to adapt it to the shape of the injector tank; this can be easily achieved when the baffle is in many elements. The injector tank can advantageously be of dimensions reduced compared to the width of the injector.

It is also possible to provide for the installation of at least one orientable deflector, of preferably shaped like an airplane wing, in the injector between the bottom and the upper side. As before, it allows you to modify the flow therefore the thermal path of the liquid metal; but we prefer to use it in complement of the baffle because its implementation is less easy.

We can also plan to install means to locally modify the temperature of liquid metal veins and thus deform the front of solidification, for example insulations or mobile heat shields or heating and / or cooling means installed on or in the walls of the injection device (injector tank or preferably injector) or in liquid metal. These means may include a plurality of resistors electrical and / or also a plurality of heat and / or coolant fluid tubes, liquid or gaseous, distributed over the surface of the injection device or embedded in its walls and supplied individually.

Separate or combined use of a mobile baffle, deflectors orientable or thermal means capable of locally modifying the temperature and / or speed of liquid metal streaks, while being simple allow deformation of the solidification front and control of effectively the regularity of the strip thickness profile to be obtained.

As a result, by judiciously deforming the solidification front, it is possible to unexpectedly compensate for irregularities in setting or continuous casting operation between movable molds.

So for example when continuously examining the strip thickness profile thin shows an insufficient thickness of one of its edges, we can increase the lateral space corresponding to this bank by moving transversely the chicane, without having to find the cause of this defect and remedy it, so to roll back the solidification front locally and quickly.

When the bulge of the strip is too pronounced, we can either raise the chicane or only pick up some of its central elements when it is in several elements, either limiting thermal losses by reinforced insulation or possibly heat in the zone corresponding to the thickest part Of the band.

• une chicane mobile, en un seul ou plusieurs éléments, barrant transversalement et au moins partiellement le flux de métal liquide dans le bac d'injecteur,
• au moins un déflecteur orientable, de préférence profilé comme une aile d'avion, dans l'injecteur,
• des moyens permettant de modifier localement la température du métal liquide dans le dispositif d'injection.

The invention also relates to a device making it possible to obtain the deformation of the solidification front, that is to say a device for continuous casting between movable molds comprising a supply of liquid metal including at least one injector, optionally an injector tank located upstream of said injector and at least one of the following means:

• a movable baffle, in one or more elements, blocking transversely and at least partially the flow of liquid metal in the injector tank,
• at least one orientable deflector, preferably profiled like an airplane wing, in the injector,
• means for locally modifying the temperature of the liquid metal in the injection device.

The baffle can be fitted at each of its ends with movable shutters allowing to modify the dimensions of the lateral space.

Displacements of the chicane or one of its elements, and / or the orientation baffle or deflectors and / or local temperature control can be automated and controlled according to the faults observed and the characteristics of the liquid metal feed.

Figure 1 shows a liquid metal supply for performing the method according to the invention and comprising a mobile baffle; this power supply is seen in section in (a) and in plan in (b), when the said chicane mobile is in a single element and is seen from the front in (c) when the said baffle mobile has several vertically adjustable elements.

FIG. 2 represents an example of the result obtained with the method according to the invention.

In fig. 1 the supply of liquid metal, the level of which is seen in (7), includes a chute (1) for supplying liquid metal, an injector tank (2) with a baffle (3) movable in several directions, according to the invention, for deforming the solidification front and an injector (4) having profiled outlet lips (5) adapted to the movable molds between which they introduce the liquid metal. We see that on each side of the chicane (3) there is a space (6) separating it from the edges of the injector tank (2). In Figure 1 (c) we sees different elements (31 ... 35) vertically movable one relative to the other constituting the mobile baffle (3); dotted lines have been illustrated (32a, 34a) which can be taken respectively for example by the said elements (32, 34).

FIG. 2 represents the evolution of the strip thickness profile over time, continuously measured with an X-ray thickness gauge, when the solidification front by modifying the position of a mobile baffle like that in Figure 1. We see that at time t the edges of the strip are completely asymmetrical with a tilting (noted: BASC), which is the difference in thickness between both banks, 166µm too large and a bulge (noted: BOMB) of 1.24% also too large and completely offset to the right. Playing only on the supply of liquid metal by moving the baffle on sees that in half an hour the tilt and the off-center bulge too great have been corrected to finally bring them respectively to less than 55µm and to about 0.5%, values difficult to obtain otherwise.

Claims (7)

1. Process for continuous casting of a metal strip between mobile molds comprising a liquid metal feed to an injection device comprising at least one injector (4), and optionally an injector tank (2) on the upstream side of the said injector (4), said liquid metal being distributed through the downstream end of said injector (4) into the air gap between the mobile molds, characterized in that the liquid metal solidification front is deformed between the mobile molds during casting by applying a speed distribution and a temperature profile at the outlet of said device as a function of the results of a continuous examination of the strip thickness profile, so as to obtain the required strip thickness profile.
2. Process according to claim 1, characterized in that the solidification front is deformed by at least one of the following means:

   displacement or rotation of a mobile baffle (3) comprising one or several elements that can be moved vertically, being placed in the injector tank (2) and transversally blocking the flow of the liquid metal;

   swivelling of at least one deflector, preferably profiled like an aircraft wing, located in the injector (4);

   local modification of the temperature of liquid metal streams in the injection device making use of means fitted on or embedded in the walls of the said injection device, or in direct contact with the liquid metal.
3. Process according to claim 2, characterized in that the mobile baffle (3), comprising one or several elements, occupies only part of the width of the liquid metal stream in the injector tank (2) and preferably between 40 and 95% of this width.
4. Process according to either of claims 2 or 3, characterized in that the mobile baffle (3) is moved vertically, transversally, forwards or backwards with respect to the liquid metal flow and/or about vertical, horizontal or inclined axes, and/or that elements of the mobile baffle are moved vertically.
5. Process according to any one of claims 2 to 4, characterized in that the temperature of the liquid metal is modified locally by insulation or mobile thermal screens and/or heating and/or cooling means installed on or in the walls of the injector tank (2) or preferably the injector (4), or in direct contact with the liquid metal.
6. Continuous casting device between cooled mobile molds for a thin metal strip, suitable for embodying the process according to any one of claims 1 to 5, comprising at least an injector (4), optionally an injector tank (2) located on the upstream side of said injector (4) and at least one of the following means:

   a mobile baffle (3) comprising one or several elements that can be moved vertically, and transversally and at least partially blocking the flow of the liquid metal in the injector tank (2),

   at least one swivelling deflector, preferably profiled like an aircraft wing, in the injector (4),

   means of locally modifying the temperature of the liquid metal in the injection device, said means being fit to be set in action during casting as a function of the results of a continuous examination of the strip thickness profile during casting.
7. Device according to claim 6, characterized in that the mobile baffle (3) can be moved vertically, transversally, parallel to the liquid metal flow and/or swiveled with respect to vertical, horizontal or inclined axes and does not occupy the entire width of said flow.

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