METHOD FOR HOT-ROLLING A SLAB AND HOT-ROLLING MILL

Abstract

The invention relates to a method for hot-rolling a slab (1), especially a steel slab, the slab (1) being subjected to at least two shaping steps at different temperatures in a hot-rolling mill (2), the slab (1) being cooled between two such shaping steps. In order to prevent ferrite from forming too soon during the hot-rolling, the slab (1) is cooled such that the lateral end sections (3, 4) of the slab (1) are cooled with a lower cooling efficiency than a center section (5) of the slab (1). The invention further relates to a hot-rolling mill (2) for hot-rolling a slab.

Description

The invention concerns a method for hot rolling a slab, especially a steel slab, wherein the slab is subjected to at least two deformations at different temperatures in a hot rolling mill, and wherein the slab is cooled between two such deformations. In addition, the invention also concerns a hot rolling mill for hot rolling a slab.

The hot rolling of a slab is sufficiently well known in the prior art. A preferred method consists in the use of multiphase hot rolling, with thermomechanical rolling (TM process) being a special method of this type.

A typical application of this method is the production of hot-rolled steel strip and plate made of microalloyed steels.

The goal of this method is to achieve a high degree of homogeneity of the material used to produce the hot-rolled slab, i.e., the material properties should be constant throughout the volume of the material. To achieve this goal, the temperature homogeneity in the slab during the hot rolling process, i.e., the temperature homogeneity in the intermediate slab, must be as high as possible, for this leads to uniform properties in the finished product.

If the individual rolling operations in the hot rolling occur in several temperature phases, as is typical in thermomechanical rolling, the individual rolling phases are separated by pauses, in which the slab is uniformly cooled to a new, lower temperature. As soon as the target temperature has been reached, the next rolling phase begins. In this regard, it is technologically advantageous for the intermediate slabs to have a uniform temperature.

During the waiting time, the lateral regions of the slab cool faster than the core. This results in cold edges. These cold edges can cause problems during the subsequent rolling passes. Furthermore, they can impair the properties of the finished product. Since the waiting times between the rolling phases increase with increasing slab thickness, this problem is encountered especially often in the case of thick intermediate slabs.

Therefore, the objective of the present invention is to further develop a method of the aforementioned type and a corresponding rolling mill for carrying out this method, which eliminate the disadvantages described above. This is intended to make it possible to maintain high quality of the intermediate slab and the finished product. In addition, it should be possible to carry out the hot rolling process in an optimal way after the first slab cooling.

The objective of the invention with respect to the method is achieved by virtue of the fact that active cooling of the slab is carried out in such a way that the lateral regions of the slab are cooled with lower cooling efficiency than the center region of the slab.

The slab is preferably cooled by applying a cooling medium, such that the lateral regions of the slab are at least partly shielded from the cooling medium.

The slab can be cooled in an intermediate cooling stand or on a cooling line.

By controlling the active cooling, it is also possible to ensure that undesired transformation of austenite to ferrite does not occur during the active cooling phase.

In this connection, the hot rolling of the slab is preferably a multiphase rolling process, especially a thermomechanical rolling process.

The proposed hot rolling mill for hot rolling a slab has at least two hot rolling stands, such that at least one cooling station in which the slab can be cooled is arranged either between the hot rolling stands or upstream of the roughing stand. The hot rolling mill is characterized by the fact that the cooling station has means by which the amount of cooling capacity applied to the slab can be varied along the width of the slab in such a way that the lateral regions of the slab are cooled with lower cooling efficiency than the center region of the slab.

In accordance with a preferred embodiment, said means consist of shielding elements designed to shield the lateral regions of the slab from cooling medium.

It is also possible for said means to consist of a cooling spray bar for applying cooling medium to the slab, such that the width of the jet of cooling medium can be adjusted over the width of the slab.

The cooling station is preferably an intermediate cooling stand or a cooling line.

In accordance with the invention, therefore, in the hot rolling process, an active intermediate cooling is carried out, which is designed in such a way that the lateral regions (edges) of the slab are not cooled along with the center. After the active cooling and after an equalization period, a slab is obtained that has a more uniform temperature distribution than a conventionally cooled slab, regardless of whether the cooling occurs actively or passively (in air).

The hot rolling mill is characterized by the fact that the cooling operation can be controlled, possibly by means of an integrated temperature computing unit, in such a way that the temperature can be prevented from falling below the ferrite precipitation temperature, so that possible undesired ferrite formation can be prevented.

Finally, the hot rolling mill is characterized by the fact that the proper cooling width is obtained by adjusting it by electrical or mechanical means.

Specific embodiments of the invention are illustrated in the drawings.

FIG. 1 is a schematic side view of a hot rolling mill, which shows only two hot rolling stands with a cooling station arranged between them.

FIG. 2 is a schematic view along sectional line A-B in FIG. 1, showing the upper half of the cooling station.

FIG. 3 is an alternative solution in a schematic view along sectional line A-B in FIG. 1, showing the upper half of the cooling station.

FIG. 1 is a sketch of a hot rolling mill 2, in which a slab 1 can be rolled in a plurality of hot roiling stands 8, 9, of which only two are shown here. During rolling, the slab is conveyed in rolling direction W and rolled in a way that is already well known. In the rolling process, the slab is subjected to thermomechanical working.

Accordingly, the slab 1 is subjected to a thickness reduction in a first hot rolling stand 8. Downstream of the stand 8, the slab 1 is cooled by passing it through a cooling station 7, which in the present case is designed as an intermediate cooling stand.

Downstream of the cooling station 7, the slab 1 is subjected to another rolling in a hot rolling stand 9, but now at a lower temperature than in rolling stand 8.

The slab 1 is cooled in the cooling station 7 by spraying it with a cooling medium, here in the form of water. Cooling spray bars 12, which in themselves are already well known, are used for this purpose.

FIG. 2 shows how the cooling of the slab 1 is carried out in accordance with a first embodiment, in which the slab 1 passes in rolling direction W under jets of cooling medium 6 sprayed from the cooling spray bar 12 and is thus cooled (the slab is also cooled by being sprayed from below by a cooling spray bar, although this is not shown in the drawing). In order to reduce the level of cooling that occurs in the lateral regions 3, 4 of the slab 1, shielding plates 10 and 11 are present, which are pushed so far towards the center of the slab in the direction of the double arrows that a sufficiently large lateral region of the slab is shielded from direct contact with cooling medium 6, while the center region 5 of the slab is exposed to the full cooling effect of the cooling medium.

The extent to which the lateral regions 3, 4 of the slab 1 are to be shielded can be influenced by choosing the position of the shielding plates 10 and 11, so that all told a homogeneous temperature distribution over the width of the slab 1 can be ensured by the choice of the specified parameters. The width and position of the slab are known from the last centering operation of the slab before the cooling station 7 and can be transferred by the level 2 control system. The shielding plates 10 and 11 are adjusted and positioned accordingly.

Nevertheless, relatively fast and thus economical hot rolling production is possible, since the active cooling in the cooling station 7 allows rapid cooling of the slab 1, so that the subsequent hot rolling operation in stand 9 can take place soon.

FIG. 3 shows another embodiment of the present invention, in which a cooling spray bar 12 is used that covers a sufficient width of the slab 1. However, the spray bar 12 is provided with slide plates 10′, 11′ for shielding the marginal regions of the spray bar 12 and, in particular, the nozzles located there, so that the stream of cooling medium can be adjusted to the desired width. To this end, the slide plates 10′, 11′ can be moved in the direction of the double arrow. The effect is the same as in the solution illustrated in FIG. 2. The lateral regions 3, 4 of the slab 1 receive less cooling medium, while the center region 5 receives the maximum amount of cooling.

The edges of the slab can thus be shielded during the active cooling either by “trays” that can be extended (as in the cooling line)—see the solution according to FIG. 2—or by activating only cooling spray bars whose spray pattern is specifically configured for a specific slab width—see solution according to FIG. 3.

Naturally, other technical realizations of the concept of the invention are also possible.

The problem of cold edges of the transfer slab 1 is thus avoided, and a homogeneous temperature of the slab over its cross section is obtained.

LIST OF REFERENCE NUMBERS

• 1 slab
• 2 hot rolling mill
• 3 lateral region of the slab
• 4 lateral region of the slab
• 5 center region of the slab
• 6 cooling medium
• 7 cooling station (intermediate cooling stand)
• 8 hot rolling stand
• 9 hot rolling stand
• 10 shielding means
• 10′ shielding means
• 11 shielding means
• 11′ shielding means
• 12 cooling spray bar
• W rolling direction

Claims

1. A method for hot rolling a slab (1), especially a steel slab, wherein the slab (1) is subjected to at least two deformations at different temperatures in a hot rolling mill (2), wherein the slab (1) is actively cooled between two such deformations, and wherein the cooling of the slab (1) is carried out in such a way that the lateral regions (3, 4) of the slab (1) are cooled with lower cooling efficiency than the center region (5) of the slab (1), wherein the cooling operation is controlled by means of an integrated temperature computing unit in such a way that the temperature can be prevented from falling below the ferrite precipitation temperature, so that undesired ferrite formation is prevented, such that two slide plates (10′, 11′) that shield the nozzles of a cooling spray bar (12) are moved in a horizontal direction transversely to the rolling direction (W) in such a way that a well-defined delivery width of the stream of cooling medium from the cooling spray bar (12) is obtained.

2. A method in accordance with claim 1, wherein the slab (1) is cooled by applying a cooling medium (6), such that the lateral regions (3, 4) of the slab (1) are at least partly shielded from the cooling medium (6).

3. A method in accordance with claim 1, wherein the slab (1) is cooled in an intermediate cooling stand (7).

4. A method in accordance with claim 1, wherein the slab (1) is cooled on a cooling line.

5. A method in accordance with claim 1, wherein the hot rolling of the slab (1) is a multiphase rolling process, especially a thermomechanical rolling process.

6. A hot rolling mill (2) for hot rolling a slab (1), especially a steel slab, such that the hot rolling mill (2) has at least two hot rolling stands (8, 9), such that at least one cooling station (7) in which the slab (1) can be cooled is arranged between the hot rolling stands (8, 9) or upstream of the roughing stand, such that the cooling station (7) has means (10, 11; 10′, 11′, 12) by which the amount of cooling capacity applied to the slab (1) can be varied along the width of the slab (1) in such a way that the lateral regions (3, 4) of the slab (1) are cooled with lower cooling efficiency than the center region (5) of the slab (1), and such that the means (10′, 11′, 12) consist of a cooling spray bar (12) for applying cooling medium (6) to the slab (1), wherein the width of the stream of cooling medium (6) can be adjusted (10′, 11′) over the width of the slab (1) by virtue of the fact that two slide plates (10′, 11′) that shield the nozzles of the cooling spray bar (12) are arranged in such a way that they can be moved in a horizontal direction transversely to the rolling direction (W), so that a well-defined delivery width of the stream of cooling medium from the cooling spray bar (12) is obtained.

7. A hot rolling mill (2) in accordance with claim 6, wherein the cooling station (7) is an intermediate cooling stand.

8. A hot rolling mill (2) in accordance with claim 6, wherein the cooling station (7) is a cooling line.

9. A hot rolling mill (2) in accordance with claim 6, wherein the proper cooling width is obtained by adjusting it by electrical or mechanical means.