EP0603598A1 - Method for manufacturing a steel strip by casting an ingot and subsequently rolling - Google Patents

Abstract

In order to control the course of a strip produced by continuous casting, subsequent compression of the ingot (strand) (2) until the ingot shells (2a, 2b) bond together, and subsequent hot rolling, the position of the edges of the strip after hot rolling (4a, 4b) is detected and the profile of the ingot during compression (3a, 3b) is altered as a function of this position, while maintaining the roll gap unchanged, to ensure that the strip (5) runs in a straight line. The single figure is intended for the abstract. <IMAGE>

Description

The invention relates to a method for producing a steel strip, in particular with a thickness of 2-25 mm, by casting a strand in a cooled, oscillating continuous mold, by squeezing the continuous mold with solidified strand shells and a liquid core, in particular in a thickness of 40- 50 mm, leaving strand until at least for the welding of the strand shells and subsequent hot rolling of the strand in the casting heat, in particular up to a strip thickness of 2-25 mm.

With such a method known from EP 0 286 862 A1, high-quality steel strips of small thickness can be produced with comparatively little effort. In this method, the squeezing is carried out by means of a pair of pinch rollers immediately downstream of the mold. The strand is then rolled down to a strip thickness of 2-25 mm with a degree of deformation of 5-85%.

In this process, the heat transfer during cooling of the strand in the continuous mold can lead to the formation of Strand shells come with locally different thicknesses. If such a strand is squeezed together by the crushing rollers arranged downstream of the mold until the strand shells are welded together, a strand can be formed which not only has a different temperature over its width, but also a different thickness and structure. These local differences across the strand width cause different deformation resistances of the material and, when rolled down with the pinch rollers, cause the strip emerging from the roll gap to appear wavy across the width in the longitudinal direction due to fibers of different lengths. If the material on the opposite band edges has different lengths, this leads to the band adopting a saber-shaped course, that is to say deviating from the straight running. Belts deviating from the straight run lead to processing difficulties in the downstream processing units, such as the high-forming roll stand and the reel.

In order to control the straight running of rolled strips, it is known from US Pat. No. 3,491,562 to determine the deviation of the strip running from the straight running behind a rolling stand and, in the event of a deviation, to set the gap profile of the upstream rolling stand in order to correct the strip running. This correction of the tape run inevitably leads to a change in the tape thickness over the bandwidth. When rolling a strip with a low degree of deformation, the different strip thickness may be acceptable; at much higher degrees of deformation up to 85%, this type of correction of the strip run would result in unacceptable thickness differences across the strip width, in particular also because of that Subsequent high-forming rollers a predetermined strip profile is required as an inlet profile. This known method of correcting the tape run is therefore not suitable for a method of the type mentioned at the outset.

The invention has for its object to provide a correction option for the strip travel for a method for producing a steel strip of the type mentioned.

This object is achieved in a method of the type mentioned at the outset in that, given a roll gap profile which is predetermined over the strip width, the strip travel of the hot-rolled strip is corrected in the event of a deviation from straight running by changing the extruded profile when squeezing.

In contrast to the prior art of the generic type, the method according to the invention for correcting the strip travel does not intervene on the deformation unit which determines the strip profile and behind which any misalignment is determined, but on the upstream deformation unit. This is advantageous because, on the one hand, the engagement does not adversely affect the belt profile and, on the other hand, the engagement can be carried out with comparatively small deformation forces on the upstream deformation unit, ie where the strand is squeezed together. The invention is based on the idea that all that is required for the correction of the strip travel is to specify a changed profile for the deformation unit which gives the strip profile. Whether that lopsided in the area between the crushing of the strand and the rolling down Belt run is irrelevant, because in the end it is only a question of the belt running straight after being rolled down and having the profile required for further processing over the entire width.

mit SD als Dicke der Strangschale bei Erreichen der Quetschzone und QS als Dicke des Stranges.When squeezing together, the thickness profile of the strand can be set within certain limits very precisely and without great effort, since here the strand still has a liquid core, or the inside of the shell is still doughy. In order to obtain a homogeneous and dense strand, which is important for setting a certain thickness profile over the strip width during rolling and for a straight strip run, the following boundary conditions should be observed:

$\text{2 SD - 3 mm <QS <2 x SD - 0.5 mm}$

with SD as the thickness of the strand shell when the pinch zone is reached and QS as the thickness of the strand.

eingehalten wird. Gute Ergebnisse hat man mit einem Durchmesser für die Quetschrollen DQ = 200-500 mm erzielt. Ausreichend für das Verschweißen der Strangschalen und für die gewünschte Gefügeverdichtung ist eine Quetschkraft QK = 50-600 kN.The strand emerging from the mold is squeezed together preferably by at least one pair of rollers with driven rollers. In order to avoid on the one hand the action of impermissibly high tensile forces on the strand and on the other hand to prevent the strand from slipping due to slag-related different slippage across the strand width with the disadvantage that the strip is fed to the subsequent rolling stand at an angle, it is provided according to one embodiment of the invention that in the case of squeeze rollers used for squeezing together with a diameter DQ and a squeezing force QK acting on these squeeze rollers, the boundary condition for the torque M of these squeeze rollers:

$\text{0.2 DQ x QK <= M <= 1.2 DQ x QK}$

is observed. Good results have been achieved with a diameter for the squeeze rolls DQ = 200-500 mm. A squeezing force QK = 50-600 kN is sufficient for welding the strand shells and for the desired structural compaction.

If the strand is squeezed together in several steps, it is provided according to a further embodiment of the invention that the adjustment of the strand profile correcting the strip travel takes place on the last pinch rollers.

There are several ways of determining the setpoint / actual value deviation of the strip run of the hot-rolled strip. Thereafter, the tape edges can be detected by means of conventional measuring devices, such as rolls that scan the tape edges or contactless sensors, such as inductive sensors, or laser distance measuring devices. In order to maintain a short positioning time, the measuring location should be as close as possible to the hot rolling location. However, it must be so far from the hot rolling location that a deviation can be measured using simple means. It is particularly advantageous if the strip edges are detected in several layers lying one behind the other in the strip running direction. If the tape is guided in the sheet, the tape run can also be determined by determining the radius of the sheet on both tape edges. This method of measurement is based on the knowledge that the tape is firmly clamped in front of and behind the sheet, but is freely guided in between, so that different lengths of tape are responsible for the misalignment Result in arc radii.

The correction of a slanting tape generally leads to different gap widths for the two edges of the strand in the pinch roller gap when squeezed together.

To regulate the strip travel after the roll stand, the gap of the pinch rollers arranged in front of the roll stand is adjusted according to the invention. The result is that the nip roller gap on the right and left edge takes on different values. It has now been shown that when an extremely asymmetrical pinch roller gap is set, the position of the strand between the pinch rollers and the mold outlet can also be influenced, even if the drive of the pinch rollers prevents the rollers from slipping. This can also have repercussions on the strand part still in the mold. This can result in canting of the strand in the mold, which leads to the partial lifting of the strand shell, in particular from the walls of the narrow sides of the mold. In this case, the lifting of the strand shell from the mold wall leads to a one-sided poor heat transfer between the strand shell and the mold wall and thus to an uneven shell growth of the lifted shell shell. In extreme cases, this can lead to strand breakdown. As a rule, however, there will only be an uneven heat distribution in the strand shell and thus an irregular shell thickness. To avoid these disadvantages and to achieve a strand with a uniform structure, the invention provides that the control of the pinch rollers, which are dependent on the rear position values of the strip measured on the roll stand, a second control is superimposed, which takes place as a function of the temperature or the heat flow density of the mold narrow sides. This temperature-dependent control ensures uniform shell growth, which is crucial for setting a dense strand with a temperature that is uniform over the width. According to the invention, the heat flow density measured in the narrow sides should be set between 500 and 1,500 kW / m². If the heat flow density falls below 500 kW / m², there is a risk of a strand shell breakthrough, so that the casting must be temporarily interrupted when such a value occurs.

The invention is explained in more detail below with reference to a drawing, which shows a system for producing a strip in a schematic illustration in a side view.

The system consists of an oscillating continuous mold 1 with cooled walls. A strand 2 with solidified strand shells 2a, 2b and liquid core 2c emerges from this continuous mold 1. The strand 2 is squeezed between a pair of pinch rollers 3a, 3b to such an extent that the strand shells 2a, 2b each having a thickness SD in the inlet area of the pair of pinch rollers 3a, 3b at least weld to one another on the inside. From the pinch rollers 3a, 3b, the strand 2 enters a roll stand with two rollers 4a, 4b, which give the emerging strip 5 a profile that is predetermined over the strip width. The band 5 is guided freely in the sheet and possibly passed over guide rollers 6a, 6b through an oven 7 to compensate for the temperature losses and further from one or more high-deformation stands 8a, 8b reshaped.

Such a system without furnace 7 is state of the art (EP 0 286 862 A1). With this system, a steel strip 5 with a thickness of up to 2 mm can be produced. The steel billet 2 is turned from the mold 1 at a speed of 2-20 m / min. subtracted in a thickness of 40-50 mm. Between the pinch rollers, it is squeezed to below 30 mm, in particular 10-25 mm. In the roll stand 4a, 4b, it is rolled down to the final dimension of 2-25 mm with a degree of deformation of> - 20%, but primarily 30%.

The special feature of the system according to the invention is that the strip travel is measured in the strip running direction behind the roll stand 4a, 4b, that is to say it is determined whether the strip 5 runs wrong.

In the exemplary embodiment, the measurement is carried out via the radii of curvature R 1, R 2 of the two strip edges or via a distance measurement of the strip edges from fixed locations arranged at the same distance when the strip is at the desired position. Depending on this measurement result, a setting is made on the pinch rollers 3a, 3b by increasing or decreasing the distance between the pinch rollers 3a, 3b at one end, depending on the desired strip travel correction. However, the limit condition 2 SD - 3 mm <QS <2 SD - 0.5 mm (SD = thickness of the strand shell 2a or 2b; QS = distance of the pinch rollers 3a, 3b) is observed.

With this setting of the strip run, the knowledge is exploited that when a steel strand is rolled, the change in length of the rolled strip is inter alia from the strand thickness in front of the roll gap is dependent, in the sense that an increase in the supply of the material to be deformed in front of the roll gap leads to an extension of the corresponding area of the rolled strip. Applied to the exemplary embodiment, this means that the material available for the edge with the radius R 1 must be reduced, that is to say the distance between the pinch rollers 3a, 3b on the side assigned to this web edge is reduced, or the material available for the belt edge with the radius R is increased, that is to say the The distance between the pinch rollers 3a, 3b on the side assigned to this band edge must be increased.

Claims (6)

Method for producing a steel strip (5), in particular with a thickness of 2-25 mm, by casting a strand (2) in a cooled, oscillating continuous mold (1), by squeezing (3a, 3b) the continuous mold (1) with solidified strand shells (2a, 2b) and liquid core (2c), in particular in a thickness of 40-50 mm, leaving strand (2) until at least welding of the strand shells (2a, 2b) and subsequent hot rolling (4a, 4b) of the strand (2) in the casting heat to a thickness of 2-25 mm,
characterized in that in the case of a roll gap predetermined over the strip width, the strip travel of the hot-rolled strip (5) is corrected in the event of a deviation from straight running by changing the profile of the strand (2) when squeezing together (3a, 3b). Method according to claim 1,
characterized in that when squeezing (3a, 3b) the strand (2) with a thickness (SD) of each strand shell (2a, 2b) for the thickness (QS) of the strand (2) the boundary condition 2 SD - 3 mm <QS <2 SD - 0.5 mm is observed. The method of claim 1 or 2,
characterized in that when driven for crimping (3a, 3b), driven Pinch rollers (3a, 3b) with a diameter (DQ) and a pinch force (QK) acting on these pinch rollers (3a, 3b) for the torque (M) of these pinch rollers (3a, 3b) the boundary condition $\text{0.2 x DQ x QK <= M <= 1.2 DQ x QK}$

is observed. Method according to one of claims 1 to 3,
characterized in that in the case of a plurality of pairs of pinch rollers (3a, 3b) used for squeezing the strand (2), the profile of the strand (2) correcting the strip travel is carried out on the last pinch rollers (3a, 3b). Method according to one of claims 1 to 4,
characterized in that for the correction of the strip run the setpoint / actual value deviation is determined in at least one plane immediately behind the hot rolling location - in the case of several passes the first pass. Method according to one of claims 1 to 4,
characterized in that the radii (R₁, R₂) of the sheet are determined at the strip edges in a strip (5) freely guided in the sheet after hot rolling for the correction of the strip run.

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