Method for the continuous production of a metal strip

Metal deforming – By use of roller or roller-like tool-element – With non-roller metal deforming station

Reexamination Certificate

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C072S008600, C072S039000, C072S203000, C072S205000, C072S365200

Reexamination Certificate

active

06463777

ABSTRACT:

The present invention relates to a method of continuous production on a line of a metal strip such as a steel rolled metal sheet, from a heat formed strip.
It is generally known that producing metal products first requires the production of a coarse product by ingot casting or by continuous casting, a heat forging and/or heat rolling treatment and a cold treatment comprising several steps depending upon the metal nature, for example ferritic or austenitic steel, and upon the quality of the product to be produced.
Usually, the heat formed product is submitted, successively, to a de-scaling treatment for removing scales, to a cold rolling process until a desired thickness has been obtained, and, finally, to finishing treatments.
The cold rolling process is usually carried out in several successive passes, either in two opposite directions on a reversible train, or on several roll stands operating as a tandem.
It is known that, in a rolling mill, the product is driven between two working rolls the spacing of which is less than the rough thickness of the upstream product. A metal flow occurs which is friction driven in the gripping gap between the rolls up to an outlet section the thickness of which substantially corresponds to the spacing between the working rolls, with a progressive speed increase which corresponds to the metal preservation.
During a rolling process, the working rolls tend to be spaced apart one from another and the clearance between the opposite generators must therefore be maintained by applying, between the rolls, a clamping effort, often so-called rolling power.
The rolling power to be exerted in order to obtain a certain thickness reduction rate depends of course upon the diameter of the working rolls and upon the metal composition: common poorly alloyed low carbon steel, stainless steel, alloyed steel, as well as upon the features thereof, more particularly the yield point.
Under the action of the rolling power being applied between their ends, the rolls tend to be bent. Moreover, deformations and collapses of the different elements of the roll stand occur, in particular the columns supporting the clamping means.
Several means are available in a roll stand to compensate for the deformation of the rolls and the stand, in order to keep the thickness regularity and correct the flatness defects by modifying the stress distribution along the backing generators of the working rolls.
In normal operation, these correcting means make it possible to hold the strip qualities on the largest length of the spool, often referred to as “strip body”. However, at the start of a new spool unwinding, the upstream end or “head” thereof must be inserted into the installation, on a sufficient length so as to allow for strip driving. Therefore, the new spool head may be welded onto the downstream head, or “tail”, of the preceding spool. Similarly, at the device output, when the spool being wound has reached its maximum size, the strip has to be sheared so as to release the spool and secure the upstream end of the subsequent strip portion on the winding mandrel.
Reducing the unwinding speed cannot be therefore avoided, more particularly in the cold rolling stands,.such a speed even becoming nil if a single winding core is being used.
As a result, the strip qualities cannot be usually held on the spool head and tail, which must be eliminated.
Moreover, it is necessary, before cold rolling, to eliminate, as much as possible, the oxidation produced scales due to the previously undergone treatments and, to that effect, several known methods can be used, for example shot-blasting or chemical de-scaling, etc.
In the past, the various cold treatments were continuously carried out in separate sections, the product being wound into a spool at the end of each section in order to be transferred to the following section. Nevertheless, these batch processes have the disadvantage that they multiply the spool winding and unwinding processes and that they require intermediary storage steps leading to high costs, because of the indispensable handling devices and the necessary personnel.
Manufacturing processes have been developed for a few years, allowing to eliminate the spool winding at least for some intermediary steps, the spool successively running in at least two treatment sections located on a continuous line. Accumulators are mounted between the successive sections in order to make it possible for them to work at different instantaneous speeds. The spool running step may thereby be slowed down or even stopped in a section, for example in the case of a mishap or when changing spool, whereas the other sections keep on working.
In particular, coupling the de-scaling process with the cold rolling process makes it possible to reduce substantially the above-mentioned disadvantages.
On such a continuous line assembly, rolling is performed in a tandem rolling mill having normally four to five roll stands.
Such an assembly is described, for example, in “Décapage-tandem couplé de Sainte Agathe à Sollac-Florange”, published in “La Revue de Métallurgie”, March 1998.
Until now, the hot forming and treatment methods allowed to produce heat strip spools with a relatively high thickness. In order to obtain the usually required thickness, the thickness reduction rate to be performed was therefore large, generally in the range of 70% to 80% and up to 90% for some steel grades.
The rolling mills adapted to develop the necessary power are very expensive and usually the coupled line assemblies are therefore optimised favouring the tandem rolling mill working that is the bottleneck of the assembly.
More particularly, as just described, the rolling mill must at least slow down at the spool change and, if the running speed decreases, the friction coefficient increases, so does the rolling power to be applied between the working rolls to obtain the required thickness reduction. Moreover, the deformations and collapses within the roll stand increase as well. It has thus been observed that there is a boundary under which rolling cannot be performed with a thickness adjustment.
Usually, one therefore attempts to maintain a relatively high speed, in the order of 300 m/min, for example, so as not to enter a field where a slow speed would make it impossible to hold the thickness quality.
As the strip shearing cannot be avoided for the spool change, at the assembly outlet, it is necessary to minimise, as much as possible, the period of time needed for this operation. To that effect, strip shearing is usually carried out at a relatively high speed using a so-called “flying” shear and two successively actuated winding mandrels are used so that, during the time needed for unloading the spool wound on the first mandrel, the strip can be engaged and starts to wind on the second mandrel, immediately after the shearing step.
Under these conditions, the strip quality is only affected for a low value, 1% or a few %, and on a short distance, which may be limited to a few meters. Thus, the yield per 1000 kg of the assembly remains acceptable. Moreover, the external spool face serving as a wrapping thereof can anyway be more or less deteriorated when being handled and is therefore sacrificed.
However, to optimise the working of the rolling mill, it should be adapted to determine product types and the other sections of the coupled line, more particularly the de-scaling and finishing assemblies, must be provided accordingly.
That is why, until now, the strip producing assemblies on a continuous line were essentially provided for products having a very large capacity, in the order of 1 to 2 million tons a year, for example automotive bodywork sheets.
For special products requiring a more limited capacity, it would be more profitable to perform the rolling operation in successive passes, in one direction and in the opposite direction, on a reversible roll stand.
On the other hand, for hard or very thin products, for example in stainless steel, it is usually preferred to use a small roll rolling mill of the SENDZIMIR type.
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