EP0286862B1 - Process for manufacturing a steel strip - Google Patents

Abstract

The invention relates to a process and an installation for manufacturing a steel strip with a thickness of 2 to 25 mm. A steel strand (10) with a thickness of 40 to 50 mm is cast in an oscillating open die (2) at a rate of 5 to 20 m/min. The steel strand emerging from the open die, which is not yet completely solidified, is compressed to such an extent that the inner walls of the already solidified strand shell weld together. After the cooling of the steel strand, the thickness of which has been reduced in this way, to 1000 to 1200 DEG C, the strip is rolled out in at least one pass with a degree of deformation of 5 to 85%. <IMAGE>

Description

The invention relates to a method for producing a steel strip with a thickness of at least 2 mm by deforming a steel strand with a liquid core cast in a continuous mold, the steel strand in a funnel-shaped oscillating continuous mold with a casting speed of 5-20 m / min. cast to a thickness of 40 - 50 mm and then rolled out of the casting heat.

In a known method of this type (special print from "Steel and Iron", (1986), No. 23, pages 27/33), the casting speed should only be a maximum of 6 m / min. be.

In a known method for producing a steel strip (PATENT ABSTRACTS OF JAPAN, Volume 8, No. 210 (M-328) (1647), September 26, 1984; JP-A-59 97 747 (A)), this is made from a continuous mold steel strand emerging with parallel walls, not yet solidified, with its less than 30 mm thick, already hardened strand shells pressed together until the thickness of the steel strand is twice the thickness of the already hardened strand shell. Through this known measure, which is used in continuous casting to achieve void-free strands (AT-PS 18 72 51), however, only strips with a thickness of 20 to 50 mm are due to the relatively large thickness of the solidified strand shells of the steel strand emerging from the mold reached. Such tapes are suitable for a large number of applications however too thick. However, in conventional cold rolling mills, strips with a thickness of 20 to 50 mm cannot be rolled down to the desired minimum thickness of approximately 2 mm.

The processes used in practice for the production of hot-rolled steel strip, even with a thickness of less than 20 mm, are generally based on a continuous cast slab, which, after it has completely solidified, is heated again to the rolling temperature and reduced in thickness in several passes and rolled out to form the strip. Because of the considerable thickness of the slabs used, up to nine roll stands are required for this. Plants for the application of such a method entail high investment costs because, on the one hand, the plants require a correspondingly large continuous casting plant, on the other hand, they require a multi-stand hot strip mill, the roughing stands of which have to be correspondingly compact due to the thickness of the slabs to be rolled out.

In order to reduce this effort, it has already been proposed to start with cast strips with a thickness of 20 to 65 mm in the production of thin strips (DE-OS 32 41 745). The preliminary strip is produced in the continuous casting plant in a conventional manner by cooling it and dividing it into pieces of suitable length after complete solidification and winding it up into a bundle. In preparation for the subsequent rolling out into the thin strip, the bundle is heat-treated in an intermediate storage furnace and set to a uniform temperature. Rolling out takes place in several passes. Because of the necessary intermediate storage furnace and the large number of roll stands for rolling out, the effort for rolling thin strips is still great. Another disadvantage with such a rolling out Heating the tape consists in scaling the tape on the surface. This makes perfect hot rolling difficult.

It is therefore an object of the invention to provide a method with which high-quality steel strips with a thickness of 2 to 25 mm can be produced in a simple manner.

This object is achieved in the generic method according to the invention in that the steel strand emerging from the continuous mold, which has not yet solidified, is compressed in such a way that the steel strand is welded to a thickness of less than 25 mm, in particular 10 to 20, by welding the inner walls of the strand shell which has already solidified mm, is reduced in thickness.

The reduced-thickness strand is preferably rolled into a strip in at least one pass with a degree of deformation of 5 to 85%, the reduction in cross section causing stretching. It has proven to be advantageous if the reduced-thickness strand is cooled to a temperature in the range from 1000 to 1200 ° C. by spraying a cooling medium onto the strand surface before rolling.

By using the measures according to the invention, one obtains a dense, void-free strand, the thickness of which is significantly reduced compared to the cast steel strand, which also has a fine-grained cast structure over its entire cross section, as is otherwise only the case with a continuously cast steel strand in the surface near the edge occurs. Because of the small thickness of the steel strand emerging from the funnel-shaped, in particular oscillating mold, and the subsequent reduction in thickness of the steel strand which has not yet solidified, steel strands with a thickness which can be substantially less than 25 mm are readily obtained.

In the invention, the various parameters for casting the steel strand on the one hand and for rolling out the compressed steel strand on the other hand are coordinated with one another in such a way that the strand shells are sufficiently resistant to both compress the steel strand without breakthroughs and also utilize the casting heat to obtain the desired, possibly high degree of deformation to be able to withstand when rolling to the final dimension (for example to a desired minimum thickness of about 2 mm). The use of the method according to the invention for the production of steel strips is inexpensive, since the production method does not require large plants with holding furnaces and many roll stands with the corresponding energy requirements.

According to a preferred embodiment of the method according to the invention, the steel strand should be given a profile corresponding to the thickness profile of the finished strip when compressed. This measure is important for the method according to the invention, because basic profile changes are not possible in a simple manner when a strip is rolled out.

To avoid surface defects, it is particularly advantageous if, according to a further proposal of the invention, the cast steel strand has a rectangular cross-sectional shape with convexly rounded narrow sides or an oval cross-sectional shape. This shape ensures that a strand shell with a uniform thickness is formed during solidification, so that when pressed together of the steel strand after it emerges from the mold in the forming device, neither cracks in the edge region nor irregularities on the surface of the strand occur.

entzogen. Dabei sollte die Kühlung so intensiv sein, daß die Innenoberflächentemperatur der Kokille unter 400°C, insbesondere zwischen 200 und 400°C bleibt. In diesem Fall ist gewährleistet, daß auch nach dem Zusammendrücken des gegossenen Stahlstrangs ein ausreichend dicker Stahlstrang für die Weiterverarbeitung erhalten wird. Eine Strangschale der genannten Dicke ist auch widerstandsfähig genug, um die beim Zusammendrücken des Gußstranges im Material auftretenden Kräfte ohne Bildung von Rissen aushalten zu können.It has proven to be particularly advantageous if the casting speed v _g and / or the cooling intensity of the mold is controlled in such a way that the cast steel strand has a strand shell with a thickness of 5 to 10 mm when it emerges from the mold. This can be achieved if the cooling effective length 1 _k (m) of the mold (distance between the lower edge of the mold and the casting level) is dimensioned such that the condition 0.05. v _g (m / min) less than or equal to 1 _k less than or equal to 1 m is satisfied, the value 0.05 being a dimensionally dependent component. Under this condition, the solidifying strand is on average sufficient heat, for example

withdrawn. The cooling should be so intensive that the inner surface temperature of the mold remains below 400 ° C, in particular between 200 and 400 ° C. In this case it is ensured that a sufficiently thick steel strand is obtained for further processing even after the cast steel strand has been compressed. A strand shell of the mentioned thickness is also resistant enough to withstand the forces occurring in the material when the cast strand is compressed without the formation of cracks.

With regard to a sufficiently high casting speed, it has proven to be advantageous if the cast steel strand is reduced in thickness by up to 75% immediately after it leaves the mold. A larger reduction in the strand thickness at this point should be avoided, because otherwise when pulling out the Strands from the deformation device such large tensile forces must be applied that there may be cracks in the strand surface, which can not be welded again with subsequent further deformation of the strand.

In order to keep the effect of the pull-out forces of the driven pull-out rollers within tolerable limits, taking into account the distance of the start of the engagement area of the pull-out roller pair from the lower edge of the mold and the pull-out speed, care should be taken to ensure that the elongation (ε) stressing the strand shells is not yet compressed strand area does not exceed 1%. This can be achieved especially with pressure rollers whose diameter is between 0.5 and 1 m, and at a distance of less than 0.5 m from the beginning of the engagement area from the lower edge of the mold. In addition, the effect of the extraction forces can also be reduced by reducing the surface friction through lubrication or surface design.

When the not yet solidified steel strand of 40 to 50 mm is compressed, steel strands are obtained which have a thickness of 10 to 20 mm. Such thin strips can already be used for some applications, without the need for substantial deformation during rolling. A degree of deformation of about 5% to improve the surface quality (skin pass) can be sufficient. After appropriate cooling, the strips, which are still slightly deformed in this way, can be coiled.

Since in the method according to the invention no temperature compensation of the cast strand material is carried out before the rolling process, it is particularly important that the Steel strand is cooled so that it receives a uniform temperature across the cross section. Since the forming behavior of the steel strongly depends on the temperature, areas with different thicknesses arise when the pre-strip is unevenly distributed on the finished strip. For this reason, it is provided according to a further proposal of the invention to continuously measure the thickness profile of the rolled strip behind the roll stand, to compare the measured value with a target profile and, if deviations of the actual profile from the target profile are determined, the coolant supply via the Adjust the width of the strip in front of the roll stand arranged coolant nozzles accordingly.

In this way it is possible to keep the temperature of the cast strand constant within very small limits and to set a uniform temperature over its width, so that a thickness profile that is dimensionally accurate across the width is obtained after the rolling process.

Further advantageous embodiments of the method according to the invention and the system according to the invention are characterized in the claims.

Fig. 1

eine Anlage zur Durchführung des Verfahrens in Seitenansicht
und

Fig. 2

ein Detail der Anlage gemäß Fig. 1 im Bereich zwischen einer Kokille zum Stranggießen und einer Verformungseinrichtung in zur Fig. 1 vergrößerter Darstellung.

The invention is explained in more detail below with reference to a drawing schematically illustrating an exemplary embodiment of a system for producing a steel strip. In detail show:

Fig. 1

a plant for performing the method in side view
and

Fig. 2

1 in the area between a mold for continuous casting and a deformation device in FIG. 1 enlarged view.

Molten steel flows from a tundish 1 into an oscillating mold 2, which consists of a funnel-shaped upper part and a lower part with cooling walls arranged in parallel, the spacing of which is selected according to the thickness of the strand to be cast. Immediately at the mold outlet there is a deformation device 3, by means of which the strand is compressed to a thickness of less than 25 mm, in particular 10-20 mm. The deformation device consists, for example, of cooled plates which continue the broad sides of the mold 2 or a corresponding arrangement of driven pull-out rollers 3a which can be adjusted relative to one another by means of hydraulic cylinders 3b for compressing the cast strand 10. In the area of the pull-out rollers 3a, support rollers determining the profile should be assigned to the narrow sides. The diameter d of the pull-out rollers 3a should be between 0.5 m and 1 m, while the distance D of the beginning of the area of attack from the lower edge of the mold 2 should be less than 0.5 m. If a strip profile with elevation, for example in the middle of the strip, is required for further processing, the bales of the pull-out rollers can have a corresponding contour. A strip with such a profile promotes further processing, for example in a cold rolling mill. However, pull-out rollers with a different profile, for example those with bottle-shaped bales, can also be used.

The pull-out rollers 3a are followed by a cooling device 4, which can consist of ribbed or lattice-shaped cooled plates 4a. In such a cooling device, cooling liquid is sprayed from nozzles 4b between the rods or grids of the plates 4a onto the solidifying strand shell.

The cooling device 4 is immediately followed by at least one roll stand 5 which rolls out the cast strand 13. In the rolling stand 5, the thickness of the compressed strand is reduced by 5-85%, that is, in the case of a 10 mm strand, the thickness is reduced to a minimum thickness of about 2 mm using the casting heat.

A thickness measuring device 12 is arranged behind the first roll stand 5 and determines the thickness profile of the rolled strand 10a over its entire width. The cooling device 4 arranged in front of the roll stand 5 is controlled as a function of the thickness profile determined at this point.

Downstream of the thickness measuring device 12, deflection rollers 6, a further roll stand 7, a further cooling device 8, a pair of scissors 9 and a reel 11 can be provided for winding up the rolled strip 10b.

Claims (9)

1. A process for the production of a steel strip having a thickness of at least 2 mm by the deformation of a steel strand cast having a liquid core in a cooled mould for continuous casting, the steel strand being cast to a thickness of 40-50 mm in a funnel-shaped oscillating cooled mould for continuous casting at a casting speed of 5-20 m/min and then being rolled from the casting heat, characterized in that the not yet completely solidified steel strand emerging from the cooled mould for continuous casting is so squeezed that the thickness of the steel strand is reduced to below 25 mm, more particularly 10 to 20 mm, accompanied by the welding of the inner walls of the already solidified strand shell.
2. A process according to claim 1, characterized in that the steel strand, whose thickness is reduced, is rolled out into a strip in at least one pass with a 5 to 85% degree of deformation.
3. A process according to claim 2, characterized in that the steel strand, whose thickness is reduced, is cooled to a temperature in the range of 1000 to 1200 °C by spraying a cooling agent on to the surface of the strand before rolling.
4. A process according to claim 3, characterized in that by squeezing the steel strand is given a section corresponding to the cross-section of the finished strip.
5. A process according to one of claims 1 to 4, characterized in that a steel strand is cast of rectangular cross-sectional shape with convexly rounded narrow sides or with an oval cross-sectional shape.
6. A process according to one of claims 1 to 5, characterized in that the casting speed and/or the cooling intensity of the mould are so controlled that when the steel strand emerges from the mould it has a solidified strand shell in a thickness of 5 to 10 mm.
7. A process according to one of Claims 1 to 6, characterized in that casting is performed with an effectively cooled mould length 1_k which satisfies the condition 1 m greater than/equal to 1_k greater than/equal to 0.05 v_g, with v_g (m/min) as the casting speed.
8. A process according to one of Claims 1 to 7, characterized in that after it emerges from the mould the thickness of the steel strand is reduced by up to 75%.
9. A process according to one of Claims 1 to 8, characterized in that the cross-section of the strip is measured downstream of the first roll stand, the measured value being compared with a required cross-section, and if the actual cross-section differs from the required cross-section the supply of coolant to the coolant nozzles disposed upstream of the roll stand is readjusted.

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