Method for extending the casting cycle for two-roll strip casting, and installation for carrying out said method

A method for extending the casting cycle for the two-roll strip casting of a steel strip, with inline rolling thereof, the steel strip being rolled in at least two successive rolling units of a rolling mill having replaceable working rolls. The casting process is carried out independently from the roll replacement by replacing the working rolls of one rolling unit during the casting, below or above the steel strip. An installation for carrying out the method includes a lifting devices for lifting the working rolls off the steel strip so as not to damage the strip during the replacement of the rolls.

BACKROUND OF THE INVENTION

The invention concerns a method for extending the casting cycle in two-roll strip casting with in-line rolling of steel strip, which is rolled in at least one and preferably two successive rolling units of a rolling mill with changeable work rolls. The invention also concerns an installation for carrying out this method.

As is well known, rolling mill work rolls are subject to wear during operation. Therefore, it is necessary during the rolling operation to regularly change especially the work rolls of the rolling units in the final strip thickness range to prevent wear marks on the final strip from resulting in defects and to prevent the production of a poor strip surface.

In the conventional hot-rolling operation, the work rolls of the last rolling units are changed approximately every three hours of operation. The roll change is carried out in a discontinuous operation, i.e., between the rolling operations.

In the new type of two-roll strip casting with in-line rolling, the rolling temperature is about 1,000-1,200° C., i.e., as high as in conventional hot-rolling practice. However, the strip speed is only 0.5-2.0 m/s, i.e., much lower than in conventional hot-rolling practice, in which rolling is carried out at rolling speeds of about 20 m/s. In this regard, casting cycles of 10 hours or more are strived for in the casting process.

SUMMARY OF THE INVENTION

The objective of the invention is to extend the casting cycle in two-roll strip casting of steel strip with in-line rolling of the steel strip.

In accordance with the invention, this objective is achieved by changing the work rolls of one of the rolling units in the rolling mill above and below the steel strip during casting. In this way, it is possible to continue the casting operation indefinitely, independently of the rolling operation, because the casting cycle is not limited by the limited life of the roll surface of the rolling mill work rolls.

The invention also provides that during the roll change a greater strip thickness is produced with the work rolls of the other rolling unit that is in use than before the roll change, namely, according to the reduction rate of this unit.

In accordance with the invention, it is alternatively provided that during the roll change the same strip thickness is temporarily produced without transition with the work rolls of the rolling unit that is in use as before the roll change, in which case these work rolls take on the total reduction rate of both rolling units. The strip thickness that is being run thus remains unchanged during the roll-changing operation.

To avoid deviations of the strip thickness during the transition phase, the invention provides that the given strip thickness that is being run is controlled for the time being with the work rolls of the rolling unit that is still in use, before the work rolls that are to be changed are released.

To support the work of the rolling unit that is still in use, the invention provides that, during the roll change, the casting process parameters that critically affect the casting thickness are varied, such as casting rate and/or bath level and/or heat dissipation and/or temperature of the liquid steel supply. This makes it possible to vary the casting thickness during the roll change according to the requirements of the rolling unit that is still in use.

The invention also concerns an installation for carrying out the method of the invention, whose rolling units are provided with work rolls that can be changed above and below the steel strip. In accordance with the invention, to avoid damage to the steel strip during the roll change, the upper work rolls are provided with lifting devices, by means of which the upper work rolls can be lifted from the steel strip.

For this purpose, the invention provides that the work rolls can be lifted by the lifting devices and that the rolling mill is provided with lifting rolls for the steel strip before and after the work rolls. It is advantageous for the lifting rolls to be mounted on the free ends of pivoted levers.

Coordinated lifting of the work rolls and the steel strip produces a gap between the steel strip and the work rolls, and this gap ensures contact-free passage of the steel strip between the work rolls that are to be changed.

In this connection, it is advantageous in accordance with a simple mode of operation, if the upper work roll can be lifted together with its associated backup roll.

To facilitate or speed up the roll-changing operation, the invention provides that the work rolls can be supported on guides that can move with them and/or that can be swung in and out.

If the wear of the work rolls to be changed is variable, then it is advantageous if the work rolls can be individually changed. Otherwise, however, it is advantageous to change both work rolls simultaneously in pairs. In this case, the invention provides that the work rolls to be changed are provided with a common extraction device.

The invention is explained in greater detail below with reference to the specific embodiment illustrated in the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The rolling mill illustrated inFIGS. 1 and 2consists of two successive rolling units1and2, each of which is equipped with a stand3and4, respectively, and a pair of rolling mill work rolls5a,5band6a,6b, respectively, with backup rolls7a,7band8a,8b, respectively. The two rolling units have identical designs in all respects. Therefore, for the sake of simplicity, only rolling unit2will be described in detail.

The work rolls6a,6bof this unit are supported on bearing boxes9,10and11,12, respectively, which are supported on guide supports13and14, respectively. Additional guides15,16are mounted as a continuation of these supports for holding the work rolls6aand6bduring the roll change.

The guide supports13,14and thus the work rolls6aand6b, respectively, that are supported on them can be lifted by hydraulic lifting devices17, which act on support housings19and20, respectively, of the guide supports13,14along with the work rolls6aand6b, respectively. Their lifting direction is indicated inFIG. 2by reference numbers21,22.

The steel strip23, which is produced in the two-roll casting installation (not shown), passes through the rolling units1and2supported on lifting rolls24,25, which can be hydraulically lifted and are positioned before and after the work rolls5a,5band6a,6b, respectively, of the rolling units1and2. In the variant illustrated inFIG. 3, the lifting rolls24,25are mounted on the free ends of pivoted levers26and27, respectively.

An extraction device (not shown) for joint extraction of the work rolls6a,6bfrom the area of the rolling line is assigned to the work rolls. The direction of extraction of the work rolls is indicated inFIG. 2by reference number28.

To change the work rolls6aand6bof rolling unit2, the given strip thickness that is being run is first controlled with the work rolls5a,5bof rolling unit1, namely, before the pair of work rolls6a,6bto be changed is released. This mode of operation ensures that the same strip thickness continues to be temporarily produced without transition with rolling unit1as has been previously produced with the two rolling units1and2together, in which case rolling unit1then takes on the total reduction rate of the two rolling units.

The guide supports13and14together with the work rolls6a,6bare then lifted by the lifting devices17. At the same time, the steel strip23is also lifted by the lifting rolls24,25.

In this regard, the lift distance of the work rolls and of the steel strip is adjusted in such a way that, after the work rolls have been lifted above and below the steel strip, a gap is formed between the steel strip and the work rolls. This ensures that during the work roll change, the work rolls do not cause any damage to the surfaces of the strip.

In accordance with the invention, it is also immediately possible to design the lifting devices17in such a way that they act directly on the guide supports13,14of the work rolls6a,6b. In this case, the upper work roll6ais lifted together with its backup roll8a, while the lower work roll6bis lifted by itself without its backup roll.

After the work rolls6aand6bhave been lifted, they are extracted together from the area of the rolling line in the direction of arrow28on the guide supports13,14and on the guides15,16that constitute continuations of the guide supports, and they are then removed to the repair shop. To facilitate this operation, the guides15,16can be moved together with the work rolls. According to requirements, they can also be swung in and out.

The mounting of the new work rolls is carried out in similar fashion but in the opposite order.

Naturally, it is also possible, in accordance with the invention, to change the work rolls individually, and in this case, each work roll can have its own extraction device.

It is also possible to work in such a way during the changing of the work rolls that a greater strip thickness is produced with the work rolls of the rolling unit that is still in use during the roll change, specifically, by using the reduction rate of this unit. This simplifies the control of the rolling mill during the roll-change operation.

Naturally, the method that has been described can also be used to change the work rolls5a,5bof rolling unit1. In addition, it can be used in rolling mills with more than two successive rolling units.