WO1987000462A1

WO1987000462A1 - Process and device for casting metal strip directly from the molten mass

Info

Publication number: WO1987000462A1
Application number: PCT/CH1986/000101
Authority: WO
Inventors: Wilfried Heinemann; Hans Gloor; Hans-Joachim Guentherodt; Heinz Breitenstein; Peter Reimann
Original assignee: Concast Standard Ag
Priority date: 1985-07-21
Filing date: 1986-07-21
Publication date: 1987-01-29
Also published as: EP0230436A1; CH667022A5

Abstract

When casting metal strip directly from the molten mass on to a moving cooling body with the aid of a nozzle, at least one of the freely-selectable casting process parameters (P, d1, u) is so adjusted, for a given nozzle design, that the melt meniscus which is formed between the inlet edge (3) of the nozzle (1) and the surface of the cooling body (2) is located in the region (T-T2) for the position of the phase triple-point (T) on the surface of the cooling body. Preferably the position of the triple point (T) is adjusted by means of the gas pressure (P) at which the molten mass is pressed out of the nozzle (1) on to the cooling body. In addition, the triple-point position can be adjusted by a controlled regulation of the distance between the inlet edge (3) of the nozzle (1) and the surface of the cooling body, i.e. by altering the meniscus. For casting iron-containing melts a cooling body (1) having an iron-based surface has been found to be advantageous.

Description

METHOD AND DEVICE FOR FOUNDING METAL STRIPS DIRECTLY FROM THE MELT

The invention relates to a method for casting metal strips directly from the melt onto the surface of a moving heat sink, according to the preamble of patent claim 1, and to a device for carrying out this method.

In the production of metal sheets and metal strips, in particular steel sheets, a multi-stage production process is selected according to the current state of the art for the main part of the material produced. The large-scale industrial production of steel sheets takes place, for example, in two main stages, of which the first stage consists in the casting of the metal melts on a continuous casting installation, for example into slabs. These intermediate products are fed to a rolling process directly or after they have completely cooled, in which case they have to be reheated to the annealing temperature and the rolling process is generally carried out in several steps.

The entire expenditure for the pouring of the liquid metal into the intermediate products as well as the subsequent heating up for the rolling and the rolling process itself are associated with a considerable investment in plant and energy. Since a large proportion of the metal consumption takes place in the form of metal sheets, various efforts in the past have been aimed at simplifying the production of metal sheets in such a way that the metal sheets can be produced in a single process step.

For example, methods and devices for the direct casting of molten metal onto moving heat sinks are known, a cooling drum or a moving cooling belt being generally used. The surface of the cooling drum. the molten metal is fed to the cooling belt via a discharge element similar to a nozzle. The most common process parameters for the casting process include the nozzle geometry, the speed of movement of the heat sink surface relative to the nozzle outlet and the thermal conductivity of the heat sink surface. The cooling rate of the potted material adhering to the heat sink surface must be matched to the casting speed according to known proposals. Attempts to use this process on an industrial scale have so far failed to optimally design the control of the process parameters and to keep them stable over a long period of time. Rather, the processes have been scaled up for special purposes, e.g. used for the production of very thin films or tapes with special properties. Wide application, e.g. for the production of masses, these processes have so far not been found.

So that the casting of large quantities of molten metal into metal strips is technically possible, the mastery of the various process parameters is an essential requirement. Attempts to technically master these parameters in their interdependency have so far only been attempted to some extent without leading to usable results. Approaches in the context of known methods lead, on the one hand, to very thin cast metal sheets, from which the heat can be removed relatively quickly during their contact with the heat sink surface, and, on the other hand, to enlarged heat sinks, for example drums with a considerable diameter or very long heat sink belts that are led through cooling areas. The efforts in the context of previous approaches to improving the cooling performance have essentially focused on cooling the heat sinks from the side opposite the casting surface, generally from the inside. A coolant, preferably water, is generally used for this purpose. The cooling surface is made of a good heat-conducting material, preferably copper or copper alloys for price reasons, so that the heat from the casting surface of the heat sink is conducted as effectively as possible to the strongly cooled opposite side of the heat sink. Nevertheless, this type of heat conduction is relatively slow and is fundamentally limited by the heat conduction properties of the heat sink material. In a hint and in general form, the possibility of evaporative cooling has already been pointed out, such as by external contact of the heat sink with water or some other liquid medium which brings about cooling by evaporation (US Pat. No. 4,142,571). Experience has shown that the removal of heat by liquid evaporation directly in the casting area is less suitable for practical use for practical and control reasons.

It is an object of the present invention to provide a method and a device which facilitate the production of metal strips directly from the melt using a moving heat sink in such a way that an improved consideration of the various process parameters and their interdependency process control that can be used in production technology. According to the invention, this object is achieved by the features defined in patent claims 1 and 5.

By suitable control of individual process parameters, e.g. of the casting pressure that is exerted on the melt in the pouring nozzle, the melt meniscus, which forms between the inlet edge of the nozzle and the surface of the heat sink, can be adjusted so that the phase triple point lies on the heat sink surface is coming. This condition has proven to be particularly advantageous for the production of tapes with as uniform a quality as possible. As a result of this measure, cast bodies can be selected according to different criteria than was previously the case. In particular, this leads to the use of cast body material which is easier to control and which has better mechanical properties than the copper which has hitherto been used almost exclusively for this purpose.

Details of the invention are explained in more detail below with reference to preferred exemplary embodiments with the aid of the drawings. Show it:

Fig. 1 shows the schematic diagram of a pouring nozzle directed onto the surface of a heat sink and

Fig. 2 shows a device for performing the method.

FIG. 1 shows the basic dimensions and flow conditions when a molten metal flows out of a nozzle 1 onto the surface of a moving heat sink 2. The heat sink moves in direction of the arrow at a speed u with respect to the nozzle outlet, ie to the right in FIG. 1 . The gap width of the nozzle in the direction of the heat sink movement is th. The rear edge 3 of the nozzle is at a distance d1 from the heat sink surface. The thickness of the resulting band-shaped material is d2. The melt emerges from the nozzle at a speed v, a pressure P existing in the interior of the nozzle. The density of the melt is "ro", while "sigma" means its surface tension.

As further indicated in FIG. 1, a meniscus forms in the area of the inlet edge of the nozzle, which is shown in a circle with the radius R for the sake of simplicity.

The outer surface of the heat sink 2 consists of a good heat-conducting material and is forced-cooled, for example with water. The forced cooling can be carried out from the inside of the heat sink or from the top. Both types of cooling can also be provided together. Under the influence of the cooling and the relatively high speed u of the moving heat sink 2, the solidification of the cast material begins at point T. Starting from this point, the wall thickness of the solidified material increases to the final thickness d2.

According to the known teaching, in addition to the speed of movement u of the heat sink, the nozzle gap width and the pressure under which the melt is pressed through the nozzle opening, the thermal conductivity of the heat sink surface is of crucial importance for the success of the casting process rens or for the quality of cast products.

According to the prevailing teaching, thermal conductivity is a decisive process parameter for the heat balance in the heat sink due to the high thermal load. Upon closer examination of the facts, it has surprisingly been found that, in order to achieve perfect casting qualities, in particular perfect surface qualities, it is less the thermal conductivity that is to be regarded as a decisive process parameter than the wettability of the surfaces directly involved in the casting process. In particular, it was found, for example, that under the test conditions given below, the use of copper as the casting surface, despite its good thermal conductivity, did not lead to perfect casting products. The test conditions were: d1 = 0.2 mm do = 0.35 mm

P = 0.2 bar u = 20-25 m / s d2 = 30-40 µm

(Protective gas atmosphere) The surfaces of these products were uneven. Frequent demolitions also occurred.

Replacing the heat sink material with a heat sink made of an iron alloy surprisingly led to completely flawless products under otherwise identical test conditions.

An examination of the process parameters and the physical relationships gave the following surprising result:

The meniscus with the radius R forming in the area of the leading edge 3 of the nozzle 1 obviously represents an important criterion for the uniform and controlled formation of the band. Inside the meniscus, the start of solidification, ie the position of the triplet point T, is fixed / liquid ⁽ Schme Ize) / gas phase decisive. Closer investigations have shown that the position of the tip point T along the heat sink surface, in relation to the nozzle outlet do, has a significant influence on the surface quality of the cast product, that is to say of the cast strips. It has been shown that the triple point T should lie on the surface of the heat sink with a view to the undisturbed shell growth. Transferred to the present 1 means that shifting the triple point from T to T1 would result in a deterioration in quality, while the range between T and T2 represents the ideal range:

Further theoretical investigations and test series have now shown that the position of the trip point T is not particularly influenced by the thermal conductivity of the heat sink material. The wettability of the heat sink surface due to the cast melt and the instantaneous temperature in the area of the applied melt have a much stronger influence. The temperature could be increased by massive external cooling, even when end of a poorer heat conductor can be kept as a heat sink. Further series of tests in this direction showed that the combination of the melt and the heat sink actually has a decisive influence on the position of the trip point es T.

In detail, the Meni is skusform, in accordance with the present assumption Figure 1 thus the radius R is a function of surface tension of the melt and the thermal conductivity of the gas phase in the meniscus region, thus a ^'s function of the heat conduction between the melt and the gas atmosphere in the range of the distance d1. The jump in heat conduction at the transition of the melt from the gas atmosphere within the distance d1 to the heat sink surface is also of influence. One consequence of the strong influence of the wettability of the heat sink surface is that an ideal material pairing between the surface of the heat sink and the melt, more precisely its surface tension or viscosity, is important.

The findings from the described observations provide the surprising result that, in the case of a predetermined nozzle geometry, the shape of the meniscus Material pair cooling surface / melt can be adjusted. This relationship is of different importance for the different material pairings. The decisive factors here are the amorphous / crystalline forms of solidification sought and the corresponding change in viscosity during cooling. To achieve this goal, namely the positioning of the point in the desired area, the parameters are controlled or regulated via the speed and / or the pressure P during the casting process. Since, for purely practical reasons, it is expedient to keep the speed u constant so that the cast tapes can be pulled off or rolled up at the same speed, the cooling body surface / melt remains with the given material pairing and gas atmosphere as an essential control parameter for influencing the position of the trip point T, the pressure P which is exerted on the melt within the nozzle body 1.

For the production of steel strips, it has proven expedient to set the triple point T within a tolerance range, which is a certain amount on both sides by the pro ection of the gap width do on the cooling, with an optimal setting of the parameters mentioned body surface extends. The tolerance range for setting the trip point T is 5 x do, measured on the heat sink surface.

In a further development of the described method, it has proven to be particularly advantageous not only to use the pressure P to adjust the meniscus, but at least one further parameter. The distance d1 is of crucial importance for the geometric shape of the meniscus. In view of the desired automatic tracking of the process parameters in the widest possible control range, it has proven to be particularly advantageous that Keeping the distance d1 between the nozzle edge and the heat sink surface also adjustable, preferably controlled or regulated. The basic setting of the distance d1 is adjusted to the desired value by means of suitable adjusting means on the nozzle body 1, for example by means of a linear motor or suitable controllable lifting elements performed.

The control signals required for the adjustment of the nozzle 1 are supplied by a computer 10, which detects the controlled variable on the basis of a thickness measurement with the aid of a thickness probe 11 and calculates the control deviation and, from this, the correcting variable, taking into account a predetermined guide variable. Instead of a thickness measuring probe 11, another indicator element can also be used to record the strip thickness on the cast strip. For example, optical measuring methods or other quickly responding and highly sensitive thickness measuring elements can be used.

The heat sink 1 may be arcs Trained as a drum example else, which is stored in a ^'shown unspecified stand or housing. Storage and training of the drum are such that the surface of the heat sink compared to the stationary nozzle body reaches a speed of the order of 20 m / s, for example. The band 4 forming from the melt 3 on the drum surface is detached from the drum surface in a suitable manner and then led away from the casting device, for example wound up into a bundle.

The largest part of the free heat sink surface, between the detachment area of the cast strip and the nozzle position, is wetted by a fluid coolant according to FIG. A liquid, for example water, is preferably used as the coolant, which is guided through suitable flow channels in a closed housing that is largely sealed off from the drum surface. A drive 5 provided for the heat sink 2 is preferably connected to a computer 10, so that the precise rotational speed of the heat sink 2 is taken into account as a controlled variable in the control device 11 together with other specified parameters for controlling the pressure and temperature of the coolant can.

In a modification of the exemplary embodiment described, the surface of the heat sink 1 can also be forcedly sprayed or wetted with a coolant instead of in a flowing coolant bath with the aid of nozzles and other devices.

The described external cooling, ie the cooling of the heat sink surface which serves as the casting surface, enables a particularly effective and rapid dissipation of the considerable amount of heat on the drum surface. As already explained above, the heat conduction through the wall of the cooling body does not play the decisive role, as was previously assumed.

In particular, contamination by foreign bodies or elements that change the wettability of the heat sink surface would have a disruptive, if not devastating, effect on the smooth running of the casting process described. In the example, therefore, a cleaning element 20 is arranged in the inlet area just before the nozzle body, which keeps the surface of the cooling body clean. This can be designed, for example, as a roller brush.

The support of the nozzle on the heat sink surface can be done via an air cooler element 30. A prestressing spring 31 or a similar element can build up a prestress with respect to the air cushion, so that the contact pressure can be selected or adjusted with respect to the heat sink surface. With the help of an additionally arranged measuring device 32, the exact height of the nozzle opening relative to the surface of the heat sink surface is determined and the pressure P1 for the air-kissed element 30 is set accordingly, so that the nozzle spacing relative to the surface of the heat sink 2 remains constant. The control function is achieved by a control device (not shown in FIG. 2) which is connected to the lifting element 8.

Instead of an air cushion, a corresponding cushion made of any other suitable gas or fluid can also be used.

The knowledge that the thermal conductivity of the heat sink is of less critical importance than previously assumed has a particularly advantageous effect for the construction of a device for carrying out the method. It has therefore proven to be extremely advantageous to use copper or iron alloys instead of the copper surfaces generally used in the past, which have a somewhat poorer thermal conductivity, but better strength properties. For example, cast iron pipes made of iron or iron alloys are less subject to the risk of deformation, as is the case for the relatively soft copper. Not only can you use purely priced, stable castings, but the entire casting process can be controlled more easily and precisely. Overall, this results in more stable conditions not only with regard to the wettability of the casting surface but also with regard to the manageability of the casting process. Furthermore, a significant improvement in the service life for the casting surface can be achieved, so that the service life of these components could be extended considerably. Because of the great influence of the wettability of the heat sink surface by the melt, it is advantageous to use different heat sinks which are matched to the melt with regard to optimal wettability for different melt compositions. In such cases, it is therefore expedient to store the heat sinks interchangeably and to insert the optimal one into the casting device from a supply of different heat sinks. In addition, the material of the nozzle body can also influence the casting process in those areas which are wetted by the outflowing melt.

The following basic considerations apply to the function of the control device 10 with reference to FIG. 1:

The distance d1 is related to the surface tension ~> of the melt and the pressure P for a given melt flow velocity v and a given speed u of the surface of the heat sink 2 relative to the nozzle outlet via the following relationship:

)

and thus:

4 <r do \ ^{d 1 «} ^ {-dT- I

Therein, as shown in the figures, ~ mean the surface tension and g the density of the melt, do the width of the nozzle opening in the direction of movement of the heat sink, and d2 the thickness of the strip to be produced. Taking this connection into account, the process parameters are monitored and adjusted with the aid of the control device 10, taking into account metallurgical aspects, in accordance with the rules set out above.

With the described method and the described device, not only amorphous but also particularly advantageously crystalline structures can be produced.

Claims

PATENT CLAIMS

1. Method for casting metal strips directly from the melt using a pouring nozzle directed onto the surface of a moving heat sink, characterized in that, given the nozzle geometry, at least one of the freely selectable casting process parameters (P, d1, u) goes to that end it is regulated that the melt meniscus, which is between the leading edge (3) of the. Forms nozzle (1) and - the surface of the heat sink (2), to an area (T - T2) for the position of the phase trip point (T) on the heat sink surface.

2. The method according to claim 1, characterized in that the triple point (T) is set within a range (T - T2) on the heat sink surface which extends at most over five times the nozzle width (do).

3. The method according to claim 1, characterized in that the position of the triple point (T) is regulated via the casting pressure (P) with which the melt from the nozzle (1) is pressed onto the cooling surface.

4. The method according to claim 3, characterized in that an additional control of the tri point point position by controlled adjustment of the distance between the inlet edge (3) of the nozzle (1) and the heat sink surface and thus by changing the Meniscus is made.

5. Apparatus for performing the method according to claim 1, with at least one casting nozzle (1) and a moving relative to the casting nozzle heat sink (2), characterized in that a control device (10) for the delivery of Gi essdruck-speci fi rule pressure control si gna len to the pouring nozzle (1) is formed at least on the basis of measurement signals which are supplied by a thickness measuring probe (11) aligned with the cast strip.

6. The device according to claim 5, characterized in that the heat sink is interchangeably mounted and can be exchanged for further heat sinks with different wettability.

7. The device according to claim 5, for casting iron-containing melts, characterized in that a heat sink (1) is provided with an iron-based surface.

8. The device according to claim 5, characterized in that the control device (10) having means for delivering distance Verstellsignalen of distance-adjusting means ⁽⁸⁾ due to the ckenmesssonde of Di (11) measuring signals supplied is formed, wherein the distance-adjustment ¬ device influences the distance between the inlet edge (3) of the pouring nozzle (1) and the heat sink surface.