APPARATUS FOR MAKING THIN HOT STRIP

Abstract

The invention relates to an apparatus for making a thin hot strip (1) by roll casting in a continuous process, comprising a casting machine (2) in which first a thin slab (3) is cast, at least one rolling train (4, 5) downstream of the casting machine (2) and in which the thin slab (3) is rolled using the primary heat of the casting process, and a plurality of finish-rolling stands (7) that, relative to a travel direction (F) of the strip (1), are arranged in a downstream region (6) and that can roll the strip (1) with work rolls (8) to a final thickness. To improve the manufacture of a thin hot strip in the continuous process, the invention provides that the work rolls (8) of the finish-rolling stands (7) have a low-wear roll surface. Furthermore, the invention relates to a method of making a thin hot strip (1) by roll casting in a continuous process.

Description

The invention relates to an apparatus for making thin hot strip by roll casting in a continuous process, comprising a casting machine in which a thin slab is cast, at least one rolling train downstream of the casting machine and in which the thin slab is rolled using the primary heat of the casting process, and a plurality of finish-rolling stands that, relative to a travel direction of the strip, are in a downstream region and in which the strip can be rolled with work rolls to a final thickness. Furthermore, the invention relates to a method of producing a thin hot strip by roll casting in a continuous process.

Plants of this type are known as thin slab/thin strip roll casting plants under the name CSP plants. They allow an efficient manufacture of hot strips if the rigid connection of is continuous casting plant and rolling train and their temperature control is managed by the overall system.

In EP 0 286 862 A1 and EP 0 771 596 B1, continuous rolling with the casting heat is described. Here, the casting and the rolling process directly follow one another. Cutting the continuous strip is carried out with a shear right before the coiler.

Similar methods for making steel strips by combining the casting and rolling plant are disclosed in EP 0 415 987 B2 and EP 0 889 762 B1.

An alternative technology is the rolling of individual slabs or individual strips. For the discontinuous rolling of strips, casting and rolling are decoupled. The casting speed is usually very slow and the rolling speed takes place independently therefrom at a high level in such a manner that the temperature for the final shaping lies above the minimum temperature.

The manufacture of thin hot strip is of interest for many applications. However, it is disadvantageous that during rolling high work-roll wear occurs that makes it necessary to periodically change work rolls primarily in the downstream rolling train stands, viewed in the travel direction of the strip. Up to now, so-called IC rolls are used at this location in most cases. These are rolls made of a traditional roll material with a high carbide content and interdendritic graphite. These rolls are characterized by uniform wear and are operationally reliable. They are relative insusceptible to thermal stress; in case of cracks, an orientation with respect to the graphite takes place.

However, overall, the wear on these rolls is relatively high. This involves limited rolling times.

Also, generally known are work rolls in rolling stands that have a special low-wear roll surface that is in particular formed by a layer of low-wear material. Such rolls are known as HSS rolls or semi-HSS roll or as PM rolls.

The designation HSS stands for high-speed steel and for a material that has a relatively low carbide content. After casting, a reaustenitization with a subsequent hardening and annealing takes place.

PM rolls are rolls that are manufactured according to the HIP method (hot isostatic pressing). Here, powdered shell material is compressed under high pressure. With the HIP method, a roll core is conventionally pre-fabricated (spheroidal cast iron or forged). Then, the HSS shell material is bonded with the core. For this purpose, the core as well as the powdered shell material is heated in special HIP plants under a very high gas pressure acting all-around and high temperature (approx. 1100° C.) above its yield point and made denser at the same time.

However, up to now, such rolls are not used for finish rolling but only in the upstream end of a rolling stand line. The reason for this is that these rolls are very sensitive, i.e. in case of excessively rolled strip ends or due to other defects, surface cracks or thermal cracks and thus an early failure of the roll can occur that makes the use of them in a finish-rolling stand basically inefficient or even impossible.

Thus, the object of the invention is to develop an apparatus and a method of the manufacture of a thin hot strip of the above-mentioned type in such a manner that an increased productivity and higher efficiency can be achieved.

With respect to the apparatus design, this object is solved according to the invention in that the work rolls of the finishing roll stands have a low-wear roll surface. This is in particular achieved in that the roll surface is formed by a layer of low-wear material.

The low-wear layer can consist in a manner known per se of a powder metal material. In this case, the low-wear layer is preferably produced by the so-called HIP method (hot isostatic pressing method).

The low-wear layer can also consist of a metal-ceramic composite material (so-called “cermet”).

According to the invention, the work rolls are designed as HSS rolls or semi-HSS rolls (high-speed steel rolls).

Advantageously, all the work rolls of the rolling train have low-wear roll surfaces.

The method of making a thin hot strip by roll casting in a continuous process is characterized in that as work rolls of the finish-rolling stands, work rolls with a low-wear roll surface are used, in particular work rolls with a layer of low-wear material.

To ensure a safe rolling process and a high service life of the rolls at least in the area of the finish-rolling stands, the roll gap between the work rolls is opened prior to entry of the strip and, after entry of the strip, the roll gap is adjusted to the target value. The roll gap is preferably set to a value at which rolling without damage is possible at a thickness that can be rolled in a save manner.

Accordingly, at least in the area of the finish-rolling stands, prior to exiting of the strip, the roll gap is set from the target value to a higher value.

Advantageously, the strip is rolled in the finish-rolling stands to a final strip thickness between 0.5 mm and 1.5 mm.

The basic idea of the invention is focused on the previously unknown and unusual fact that for finish rolling a thin hot strip by roll casting in a continuous process, low-wear work rolls are used that are previously known as such, that, however, could not be used due the high roll load during finish rolling.

This is supported or made possible by the described process-related measures that ensure that also during finish rolling, an excessive loading of the work rolls does not take place.

With the continuous rolling provided according to the invention, rolling failures can be avoided so that the use of work rolls with low-wear roll surface is possible also in the finish rolling area. Here, rolling without threading in and threading out takes place over a rather long period (several hours) at the rolling train. At the beginning or the end, the mentioned special measures, such as opening the stands for the safe rolling of greater final thickness, can be taken to keep the work roll load in the finish-rolling stand low. The manufacture losses are relatively small here because the lost lengths are minor compared to the total length of continuously rolled strips.

Under the conditions of the continuous rolling while using low-wear work rolls (in particular HSS rolls, PM rolls), the efficient manufacture of thin strips is particularly advantageous. Less roll changes and thus fewer interruptions during casting take place. Therefore, rolling programs or casting sequences can be extended while achieving better strip shapes because now the work roll wear is lower also in the downstream stands.

The productivity of the overall system can be increased by extending the rolling programs or casting sequences.

Embodiments of the invention are illustrated in the drawing. Therein:

FIG. 1 shows schematically a roll casting plant according to a first embodiment of the invention with roughing train and finishing train; and

FIG. 2 shows a casting plant in an embodiment according to the invention with a compact rolling train as an alternative to FIG. 1.

In FIG. 1, an apparatus for making a thin hot strip 1 is illustrated. Manufacture of the strip is carried out by roll casting in a continuous process. For this purpose, a continuous-casting machine 2 is provided in which first a thin slab 3 is cast. A downstream rolling train 4, 5 is provided, relative to a travel direction F of the slab or the strip. In the rolling train 4, 5, the thin slab 3 is gradually rolled to form the finished strip while using the primary heat of the casting process. To this end, a roughing train 4 and a finishing train 5 are provided. The finishing train 5 has a plurality of finish-rolling stands 7 that, relative to the travel direction F, are in a downstream region 6 of the rolling train 4, 5. The strip 1 is rolled here to a final thickness lying preferably between 0.5 mm and 1.5 mm. The rolling of the strip in the finish-rolling stands 7 is carried out with work rolls 8 supported by backing rolls.

Here it is essential that the work rolls 8 of the finish-rolling stands 7 have a low-wear roll surface. In particular, the roll surface is formed by a layer of low-wear material.

Rolls of this type—as explained above—are known as such in the prior art. However, up to now, they were never used for finish rolling a strip 1.

With respect to the rolls used with wear resistant surface, reference is made to the article of M. Andersson et al “Introduction of enhanced indefinite chill and high-speed steel rolls in European hot strip mills” in “Ironmaking and Steelmaking”, 2004, Vol. 31, No. 5, page 383 ff, and to EP 1 365 869 B1 (in particular paragraph [0014]).

The HIP method is illustrated in more detail in the article of Ch. Willems et al “PM-HIP-Lösungen für Werkzeuge and Formen” in “Stahl”, 1998, issue 6, page 38 to 40.

Metal-ceramic composites, also called cermets, are used as wear-resistant materials in a variety of components due to the combination of high hardness and fracture toughness that is used also in the present case.

Preferably, the low-wear work rolls can be used in all stands of the hot strip train, thus, also in the roughing train 4 (the latter measure is already known as such in the prior art).

Here, the process flow is such that excessively rolled strip ends or general failures and thus damage to the work rolls can be avoided by the following measures:

Threading in with an open roll gap or threading in the strip head can take place at product dimensions that can be generated safely (thicker finished strip). Only after threading in the strip into the finishing stand, is the thickness of the roll gap set to a target thickness that is critical to generate, preferably to a value below 1.5 mm.

Furthermore, continuous rolling for a plurality of coils is provided for the long filet part of the strip.

Also, threading out the strip can take place again with a greater final thickness or open roll gap.

The apparatus shown in FIG. 2 differs from the one according to FIG. 1 substantially only in that here a compact rolling train 4 is provided. However, viewed in the travel direction F, it also has a downstream part 6 in which work rolls 8 with low-wear roll surface are provided. In this plant, a continuous rolling process or, alternatively, a batch mode can be carried out.

In the figures, various mechanisms that are known per se, are illustrated that, however, are not a priority for the invention.

However, in particular to be referenced is an induction furnace 9 and a holding furnace 10 that are operated in a is conventional manner. Furthermore, a cooler 11 is to be mentioned downstream of which a coiler 12 for winding up the finished strip to form coils 13 is provided.

As already mentioned above, the work rolls 8 used in the finish-rolling stands 7 are known as such under the designations HSS rolls, semi-HSS rolls, PM-HIP rolls, or cermet rolls. Other shell materials that are wear resistant are also possible; however, they are usually sensitive.

The length of the rolling program is usually determined by the depth or shape of wear of the work rolls or—associated therewith—by the desired strip shape. Particularly disturbing is a strong strip edge-drop or beads caused for example by high edge wear.

Furthermore, also the roll surface quality as well as the strip surface quality influences significantly the length of the rolling program. Not only considerable roughness but also marks on the roll surface caused for example by excessively rolled strip ends are relevant here.

Specifically in case of thin strips, high wear occurs. Particularly apparent is the wear effect when rolling a width. In conventional hot strip trains, the danger of excessive strip end rolling or other rolling defects is high.

Thus, this technology, namely the thin-strip rolling in a continuous thin-slab plant using low-wear work rolls (also in the last stand) is particularly suitable. Continuous rolling technology prevents rolling defects, and the low-wear rolls improve the quality of the product and allow for an extension of the rolling program length.

The combination of direct charging (casting with subsequent rolling at minimal energy supply) and the extension of the casting sequences make the described technology particularly efficient.

REFERENCE NUMBER LIST

• 1 Hot strip (strip)
• 2 Casting machine

3 Thin slab

• 4 Rolling train
• 5 Rolling train
• 6 Downstream region
• 7 Finish-rolling stand
• 8 Work roll
• 9 Induction furnace
• 10 Holding furnace
• 11 Cooler
• 12 Coiler
• 13 Coil
• F Travel direction

Claims

1. An apparatus for making a thin hot strip by roll casting in a continuous process, the apparatus comprising:

a casting machine in which a thin slab is cast,

at least one rolling train downstream of the casting machine and in which the thin slab is rolled using the primary heat of the casting process, and

a plurality of finish-rolling stands that, relative to a travel direction of the strip are in a downstream region and that can roll the strip with work rolls to a final thickness, the work rolls of the finish-rolling stands each having a low-wear outer surface formed by a layer of powder metallurgical material or a metal-ceramic composite material.

2-3. (canceled)

4. The apparatus according to claim 1, wherein the low-wear layer of powder metallurgical material is produced by the HIP method.

5-6. (canceled)

7. The apparatus according to any one of the claims 1 to 6, wherein all the work rolls of the rolling train have the low-wear roll surface of low-wear powder metallurgical material or a metal-ceramic composite material.

8-12. (canceled)