EP1144705A2

EP1144705A2 - Method and device for producing coated metal strands, especially steel strips

Info

Publication number: EP1144705A2
Application number: EP00901068A
Authority: EP
Inventors: Fritz-Peter Pleschiutschnigg
Original assignee: SMS Schloemann Siemag AG; Schloemann Siemag AG
Current assignee: SMS Siemag AG
Priority date: 1999-01-20
Filing date: 2000-01-07
Publication date: 2001-10-17
Anticipated expiration: 2020-01-07
Also published as: WO2000043563A2; WO2000043563A3; DE50000212D1; EP1144705B1; AU2106200A; US6565925B1; EP1144705A3; DE19902066A1; ATE219160T1

Abstract

The invention relates to a method for producing coated metal strands, notably steel strips (1), according to which a metal strand (1.) is guided through the bottom of a vessel (3) filled with a molten bath (3.3) of the same or a different composition as the metal strand (1.1). The dwell time of the metal strand (1.1) is selected in accordance with the bath level (3.3.1), the casting speed, the metal strand thickness and the preheating temperature of the metal strand such that the melt which is deposited on the metal strand has a desired thickness which is a multiple of the initial thickness of the metal strand and that said metal strand (1.), on which a partially crystallized layer (3.6) is formed, undergoes a skin pass (2) after delivery from the molten bath (3.3). The skin pass is carried out when the surface temperature of the partially crystallized strand (3.6) is lower than the solidus temperature of the molten bath (3.3) so that at least the surface (1.3) of the partially crystallized layer (3.6) has solidified. According to the invention a controlled thickness of said partially crystallized layer is obtained if between the molten bath (3.4) and the onset of solidification (9.3) on the strip surface (1.3) the crystallized layer (3.6) having a given thickness (1.2) is stripped (20) to a smaller thickness (21).

Description

Method and device for producing coated strands of metal, in particular steel strips

The invention relates to a method and a device for producing coated strands of metal, in particular steel strips, in which a metal strand is passed through the bottom of a vessel filled with melt of the same or different composition as the metal strand, the residence time of the Metal strand depending on the molten pool height, the casting speed, the metal strand thickness and the preheating temperature of the metal strand is selected so that the deposited melt on the metal strand assumes a desired thickness of several times the initial thickness of the metal strand and the metal strand with crystallized layer after exiting the Melt bath experiences a smoothing stitch, which is carried out when the surface temperature of the crystallized strand is lower than the solidus temperature of the melt pool and thus at least the surface of the crystallized layer is solidified.

In a method and device which has become known from EP 0 832 990 A2, a disadvantage in the production of coated metal strands, preferably strips of a steel grade or different steel grades, such as, for example, monomaterial or composite material and here in particular also composite material made of carbon steel, is thinly coated with Fixed stainless steels. It has been shown to be disadvantageous in the case of such inversion casting devices in the case of a crystallized layer with a "pasty surface and pasty core" that the layer adhering to the mother tape is on the one hand already relatively solidified, but on the other hand in their outer zone still has sufficient proportions of liquid phase when entering the smoothing roller pair. The belt then experiences severe hypothermia as it passes through the pair of smoothing rollers, which causes the tendency to crack both along and across the belt direction. This danger increases with higher casting and rolling speeds. The smoothing stitch is therefore carried out there when the surface temperature of the crystallized strand is lower than the solidus temperature of the melt pool and thus at least the surface of the crystallized layer has solidified. Nevertheless, it cannot be ruled out that the thickness of the crystallized layer fluctuates.

The invention is therefore based on the object of providing a method and a device which ensure smoothing of the strip with a strip thickness tolerance of at most 2% without cracking both in the surface and in the interior of the strip while controlling the thickness of the crystallization.

The surprising solution is achieved with regard to the method by the characterizing features specified in claims 1 to 3 and with regard to the device by the features contained in claims 4 to 7.

The main features for producing defect-free, flat-coated strips, for example with a width / thickness ratio greater than 60 and a total thickness of at most 12 mm, preferably 2 to 6 mm, from one material or from composite materials of different metal qualities such as carbon steel as mono- material or carbon steel with a stainless steel coating of at least 5% of the total strip thickness as a composite material and a maximum thickness deviation of 2% between the edge (40 mm from the edge) and the center of the strip are the same with regard to the desired goal Thickness of the crystallized layer in particular characterized in that between the molten bath and the start of solidification on the strip surface, the crystallization is stripped to a smaller thickness with a desired thickness. In the end, only as much material is stripped or carried as is necessary to make the same To achieve the thickness of the crystallized layer over the length of the strand. The excess of the crystallized layer is thus stripped off.

The stripping of the crystallization with a desired thickness to a smaller thickness is advantageously carried out in the device between the molten bath and the start of solidification on the strip surface, the walls of the space accommodating the pair of smoothing rollers and the pair of smoothing rollers themselves being designed to be heat-controlled. Mechanical stripping devices in the form of preferably knives are advantageously suitable for stripping, alternatively a pneumatic stripping device can be used.

In Figures 1 to 3, the invention is shown both in terms of the method and the device. Show it:

Figure 1 is an overall view of the method and its device for smoothing coated strands of metal, preferably steel strips;

FIG. 2 shows a temperature field of the strand between the strip entry into the crystallizer and the pair of smoothing rollers during casting;

FIG. 3 shows a coated strip between the melt pool surface in the crystallizer and the pair of smoothing rollers, detail from FIG. 1.

FIGS. 1 and 2 show the overall view of the method and the device for smoothing coated strands, preferably strips of steel 1, using a pair of smoothing rollers 2. The mother tape 1.1 is in the crystallizer 3, filled with melt 3.3, which is introduced via a melt inlet 3.1, through the nozzles of a bottom inlet device 3.2 with a casting and rolling speed 7.1 of 0.05 to 10 m / s by means of a pair of drive rollers 1.5 below Promoted crystal lisators. The mother tape 1.1, with a temperature of either 20 to 80 ° C before entering the crystallizer 3, begins above the steel meniscus 3.5 at the nozzle outlet 3.2 with the crystallization 3.6 of melt in point 3.6.1 and removes the melt 3.3 from superheating and crystallization energy simultaneous warming. This energy flow 4 from the melt into the mother tape takes place when the mother tape passes through the melt pool 3.3 between the meniscus 3.5 and the bath surface 3.4 over the melt pool height 3.3.1. At the exit 5 of the coated strip 1 from the bath surface 3.4 of the melt pool with a surface roughness 1.3, it has reached a certain thickness 1.2, which is essentially determined by the strip temperature when it enters the crystallizer, the melt temperature and the contact time of the strip with the melt , with which the strip 1 enters the roll gap 2.1 of the pair of smoothing rollers 2.

The strip 1 coated in this way is "doughy" at the exit 5 from the bath 3.4 on the surface (two phases: melt and crystal) and has a surface roughness 1.3 of greater than 2%, which meets the flatness criteria of a strip with a width / thickness Ratio greater than 60 does not do it justice.

When leaving the coated strip 1 from the bath 3.4 with the final thickness 5.1, the solidification runs from the exit 5 to the pair of smoothing rollers 2 and beyond in the crystallized layer, which consists of melt and crystal, from the outside in, ie the energy flow 6 reverses compared to the heat flow 4 in the melt 3.3 um and runs from the inside (middle of the strip) to the outside into the walls 6.1 with a heat-controlled passage. This controlled heat flow can be divided by wall elements 6.2 into zones corresponding to the temperature control of the strip 1 in the casting and rolling direction 7. These device features make it possible to control the heat flow 6 from the belt to the heat transfer controlled walls 6.1 and 6.2, ie to control it or to regulate it depending on the steel quality, the casting speed 7.1 and the position 2.4 of the pair of smoothing rollers 2. To describe and understand the unexpected solution that makes up the invention, it is also necessary to change the temperature fields and thus the phase states of the coated strip 1 in interaction with the heat flows 4.6 and 2.7 from the melt 3.3 into the mother strip 1.1, from the coated strip 1 Check into the walls with a heat-controlled passage 6.1 between the bath surface 3.4 and the pair of smoothing rollers 2 and also from the coated strip 1 in the nip 2.1 of the pair of smoothing rollers 2 via the roller body into the internal cooling 2.5 of the pair of smoothing rollers.

The crystallization 3.6 in the bath 3.3 has on its surface 4.1 a temperature 8 (Tx) which is greater than the solidus temperature and lower than the liquidus temperature (T-Ii> Tx> T-sol) and has a two-phase state , consisting of melt and crystal. This crystallization steadily decreases in temperature from the surface perpendicular to the mother tape 1.1. Functional to the surface profile 4.1 of the crystallization 3.6, the liquidus isotherm 10 runs in the molten bath up to the bath surface 3.4.

When the coated strip 1 emerges from the bath 3.4 at point 5, the melted layer 9.2 of the mother strip 1.1 is largest, which began in the melt bath 3.3 at point 9.1 when the solidus temperature was reached. When this melting of the mother tape begins, the welding between the mother tape 1.1 and the crystallized layer 3.6 begins.

Above the melt with reversal of the energy flow 6, the solidification of the residual melt in the crystallized layer, consisting of the melt and crystal phases, begins from the surface of the strip 1 perpendicular to the strip center and in the surface itself in the direction of the pair of smoothing rollers 2 parallel to the casting and rolling direction 7, ie the surface temperature of the strip decreases steadily from the bath surface 3.4 at point 5 in the direction of the pair of smoothing rollers 2, the solidus temperature passes through in point 9.3 Inlet 2.1.1 of the coated belt 1 into the pair of smoothing rolls 2, where it then assumes a value which is below T-Solidus.

In addition, the thickness 1.2 of the coated strip between the strip surface 3.4 and the start of solidification 9.3, that is to say still in the "pasty" (two phases: melt and crystal) strip surface area 24, is reduced to a smaller thickness 21 by a stripping 20. This reduction must take place before solidification 9.3 in order to obtain a clean and flat belt surface.

This stripping 20 or control of the coating thickness can alternatively exist both with a mechanical stripper 22 (knife), which is made of metal and / or ceramic and can also be cooled, as well as with a pneumatic stripper, which is operated with an oxygen-free gas jet 23 .

In addition, both types of wipers 22 and 23 can be used in succession. In addition, both wipers are in their position, i.e. with their distance to the belt surface 1.3, to the bath surface 3.4 and to each other freely selectable.

To control a desired temperature control of the coated strip 1, the position 2.4 of the pair of smoothing rollers 2, the energy flow 6 into the walls with heat-controlled passageways 6.1 and 6.2 and the pouring and rolling speed 7.1 in the sense of the invention are to be controlled in such a way that the surface temperature of the coated strip 1 before entering the pair of smoothing rollers 2 is below the solidus temperature and thus the coated strip is at least solidified in its surface.

This condition is imperative for a crack-free surface, since the solidified phase has a pronounced expansion behavior without crack formation, especially immediately below the solidification. In contrast to this good elasticity of the material "steel" immediately below the solidification point, T-Solidus, it is known that the deformation limit in the "pasty" area, in the two-phase offers melt / crystal and therefore the avoidance of cracks is very small and is between 0.1 and 0.3% depending on the steel grade.

In the case of so-called inner-crack-sensitive steel grades, steels that are in the "doughy" area with the slightest deformation, i.e. Elongation stresses tend to crack, it is important for the inventive method that the solidification profile 9 is controlled solid / liquid at the phase boundary so that the solidification 9.4 of the coated strand 1 until the exit 2.1.2 and 9A of the roll gap or at the latest to the entrance 2.1.1 and 9B in the roll gap of the pair of smoothing rollers 2 has ended.

These conditions of the coated belt 1 in the pair of smoothing rollers can, at a given casting speed 7.1 with the help of the regulation of the heat flows 6 and 2.7 by means of the wall elements 6.1 and 6.2 and / or the smoothing roller pair 2 with internal cooling 2.5 adjustable in relation to the distance from the casting level 2.4.1 , can be set.

When securing a strip 9.5 which has solidified at least in the surface area in the roll gap 2.1 with the pressed length 2.2, the strip 1 with its rough surface 1.3 can now be rolled or smoothed 1.4 with a decrease in thickness of up to 20% by adjusting the pair of smoothing rollers 2 in the thickness direction 2.3, without surface cracks or internal cracks occurring in the crystallized layer while at the same time ensuring good welding between the mother tape 1.1 and the crystallized layer 3.6. The thus smoothed and flat band 1.4.1 is crack-free in its surface 1.4 and in the interior of its solidified, crystallized layer 3.6. The flatness or the resulting profile of the band 1.4.1 can be a tolerance of max. 2% of the thickness can be set in the transverse and longitudinal directions.

FIG. 3 shows the area of the pair of smoothing rollers 2 in somewhat more detail. The coated strip 1 with its crystallization 3.6 runs in the roll gap 2.1.1 a surface temperature, T-2.1.1 less than T-Solidus (T-2.1.1 <T-sol) and occurs with a controlled reduced temperature T-2.1.2 less than T-2.1.1 (T-2.1.2 < T-2.1.1 <T-sol) out of the roll gap 2.1 at its outlet 2.1.2. The temperature loss in the roll gap should be controlled and kept small. According to the invention, this can be achieved by means of a correspondingly heat-controlled, controlled pair of smoothing rollers 2 with internal cooling 2.5 and heat-controlling layer 2.6 or layers.

For this, the cooling, the materials and the thickness of the rollers 2, their layer structure 2.6 and the choice of different roller materials such as Steel, metals, metal ceramics and / or ceramics can be coordinated.

The entire space 11 above the bath surface 3.4 is controlled in its temperature and atmosphere (nitrogen and / or argon), so that the conditions described above are ensured and oxidation of the bath surface is avoided.

The strip coated in this way is fed directly or indirectly to a further rolling mill 12 and rolling process for producing finished hot strip and / or cold strip both as a mono material and as a composite material with or without an upstream pickling line.

To control, control and / or regulate the temperature field in the coated strip 1 and on the strip surface 1.3 between the melt pool surface 3.4 and the exit of the coated and smoothed strip 1.4.1 from the pair of smoothing rollers 2, measuring devices for temperature detection 2.8 on the inside of the heat-controlled wall elements 6.2 appropriate.

References:

1. Coated tape between the bath surface and the pair of smoothing rollers 1.1 Mother tape, initial metal tape

1.2 Thickness of the coated tape between the bath surface and the pair of smoothing rollers

1.3 rough, coated belt surface

1.4 flat, smoothed belt surface 1.4.1 coated and smoothed belt

1.5 pair of drive rollers below the crystallizer

1.6 Detail, see Fig. 3

2nd pair of smoothing rollers

2.1 Roll gap

2.1.1 Start of the roll gap

2.1.2 End of the roll gap

2.2 pressed length in the roll gap

2.3 Position of the smoothing roller pair in the thickness direction of the belt, roll adjustment in the thickness direction

2.4 Position of the smoothing roller pair in the casting and rolling direction of the strip 2.4.1 Distance of the smoothing roller pair to the bath surface

2.5 Internal cooling of the pair of smoothing rollers

2.6 Coating of the pair of smoothing rollers to control the heat transfer

2.7 Energy flow into the internally cooled pair of smoothing rollers

2.8 Measuring device for determining the surface temperature of the strip

3. Crystallizer

3.1 melt feed

3.2 Nozzle of the floor inlet device

3.3 Melt pool 3.3.1 Melt pool height

3.4 Bath surface

3.5 meniscus

3.6 Crystallization of the melt 3.6.1 Start of the crystallization

4. Energy flow in the melt pool from the melt into the mother tape 4.1 Surface and profile of crystallization in the melt pool

5. Exit of the coated tape from the bath 5.1 final thickness of the crystallized layer

6. Energy flow above the bath surface to the outside with heat-controlled passage

6.1 walls with heat-controlled passage

6.2 Wall elements controlled by heat transfer, independently of each other

7. Casting and rolling direction

7.1 Casting and rolling speed

8. Surface temperature, T-x of crystallization in the molten bath

9. Isotherm of the solidus temperature, solidification profile

9.1 Beginning of the band surface melting and the welding of the mother band and crystallization

9.2 max. Melting zone in the surface of the mother tape

9.3 Start of solidification starting from the strip surface in the direction of the strip center, surface temperature equal to the solidus temperature

9.4 End of solidification, solidification

9.5 solidified belt surface

9.A solidification profile, isotherm of the solidus temperature, solidification by the end of the roll gap at the latest 9.B solidification profile, isotherm of the solidus temperature, solidification up to the entry into the roll gap at the latest

10. Isotherm of the liquidus temperature

11. Temperature and atmosphere controlled room above the melting bath

12. Directly processing rolling mill

20 stripping the crystallization (3.6) with a thickness (1.2 to a smaller thickness (21)

21 smaller thickness of the crystallization (3.6) with a thickness (1.2)

22 mechanical stripping, for example with a knife 22.1 mechanical stripping device, knife 2.2 pneumatic stripping device 3 pneumatic stripping, for example with an oxygen-free gas jet (23.1) 4 "pasty" band surface area (two phases: melt and crystal) between the bath surface (1.3) and the start of solidification (9.3) in the band surface (1.3).

Claims

Claims:

1. A method for producing coated strands of metal, in particular strips (1) made of steel, in which a metal strand (1.) through the bottom of a vessel (3) filled with melt (3.3) of the same or different composition as the metal strand ( 1.1) is carried out, the residence time of the metal strand (1.1) depending on the molten pool height (3.3.1), the casting speed, the metal strand thickness and the preheating temperature of the metal strand being chosen so that the deposited melt on the metal strand has a desired thickness of Assumes several times the initial thickness of the metal strand and the metal strand (1.) with crystallized layer (3.6) experiences a smoothing stitch (2) after emerging from the melt bath (3.3) and that the smoothing stitch is carried out when the surface temperature of the crystallized strand (3.6) is lower than the solidus temperature of the weld pool (3.) and thus at least the surface (1.3) of the crystallized layer (3.6) is solidified, characterized in that between the molten bath (3.4) and the beginning of solidification (9.3) on the strip surface (1.3), a stripping (20) of the crystallization (3.6) with a thickness (1.

2) to a smaller thickness (21).

A method according to claim 1, characterized in that the stripping (20) mechanically (22), for example with a knife (22.1) takes place.

3. The method according to claim 1 and / or 2, characterized in that the stripping (20) is pneumatic (23), for example with an oxygen-free gas jet (23.1).

4. Apparatus for carrying out the method according to claims 1 to 3, comprising a melting vessel, a crystallizer (3), through the bottom of which a mother tape (1.1) forms a meniscus (3.5) in the region of the bottom inlet device (3.2) by means of a pair of drive rollers, a space (11) above the melting bath (3.3) in which a pair of smoothing rollers (2) with a roll gap (2.1) for passing the metal strand (1) with a crystallized layer (3.6) solidified in its surface is arranged, with a stripping (20 ) (3.6) the crystallization with a thickness (1.2) to a smaller thickness (21) between the melting bath (3.4) and the start of solidification (9.3) on the belt surface (1.3) is provided and the walls of the room (11) and the pair of smoothing rollers (2) are each heat-controlled.

5. The device according to claim 4, characterized in that the stripping (20) of the crystallization (3.6) is designed as a mechanical stripping device, knife (22.1).

6. The device according to claim 4 and / or 5, characterized in that the stripping (20) of the crystallization (3.6) is designed as a pneumatic stripping device (23.1). Device according to one of claims 4 to 6, characterized in that the stripping devices (22.1) and / or 23.1) are variably arranged in their position, ie their distance from the bath (3.4) and the bath surface (1.3).