Metal treatment – Process of modifying or maintaining internal physical... – With casting or solidifying from melt
Reexamination Certificate
1999-04-07
2001-08-28
King, Roy (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
With casting or solidifying from melt
C148S321000, C164S417000, C164S476000, C266S103000, C266S113000
Reexamination Certificate
active
06280542
ABSTRACT:
The invention relates to a method for the manufacture of a steel strip, whereby molten steel is cast in a continuous casting machine into a slab and, while making use of the casting heat, is conveyed through a furnace apparatus, is roughed in a roughing apparatus, and finish-rolled in a finishing apparatus into a steel strip of a desired finished thickness, and to an apparatus for use therewith.
Such method is known from European patent application EP 0 666 122.
The invention is particularly suitable for application to a thin slab of a thickness less than 150 mm, preferably less than 100 mm, more preferably in a thickness range between 40 and 100 mm.
In EP-0 666 122 a method is disclosed whereby, following homogenisation in a tunnel furnace apparatus, a continuously cast thin steel slab is rolled in a number of hot-rolling steps, that is in the austenitic range, into a strip with a thickness less than 2 mm.
In order to achieve such a finished thickness with rolling apcaratuses and rolling trains which can be realized in practice, it is proposed to reheat the steel strip at least after the first mill stand, preferably by means of an induction furnace.
Located between the continuous casting machine and the tunnel furnace apparatus is a shearing apparatus with which the continuously cast thin slab can be cut into pieces of roughly equal length, which pieces are homogenized in the tunnel furnace apparatus at a temperature of approx. 1050° C. to 1150° C. After leaving the tunnel furnace apparatus the pieces can if desired be cut again into half slabs with a weight corresponding to the coil weight of the coil to be manufactured. Every half slab is rolled to a strip of the desired finished thickness and subsequently coiled by means of a coiling apparatus set up after the rolling apparatus.
EP-A-0 306 075 relates to a continuous process for the manufacture of a ferritically rolled steel strip and to an apparatus for performing the process. According to this publication a thin slab, thickness less than 100 mm, is cast in a continuous casting machine, hot rolled in the austenitic region, cooled into the ferritic region and subsequently coiled. In the method there is a continuous flow of steel from the continuous casting machine to the coiling apparatus for coiling the ferritically rolled steel strip.
DE-A-19 520 832 relates to a method and an apparatus for the manufacture of steel strip having as-cold-rolled properties. The object of the invention of DE-A-19 520 832 is to provide a method that does not require a reheating step in the austenitic region. DE-A-19 520 832 proposes to a single roughing step without reheating followed by cooling of the strip into the ferritic region and subsequent ferritic rolling in a temperature range of between 850 and 600° C. In the method of this publication, the steel strip is manufactured on a coil-by-coil basis. The object of the invention is to create a method of the known type which offers more possibilities, and moreover with which steel strip can be manufactured in a more efficient manner. To this end the method in accordance with the invention is characterized in that
a. for the manufacture of a ferritically rolled steel strip, the slab is rolled in the roughing apparatus in the austenitic range and after the rolling in the austenitic range is cooled to a temperature whereby the steel has essentially a ferritic structure, and the strip, the slab or a part of the slab is rolled in the finishing apparatus at speeds essentially corresponding to the entry speed into the finishing apparatus and the subsequent thickness reductions and in at least one stand of the finishing apparatus is rolled in the ferritic range;
b. for the manufacture of an austenitically rolled steel strip, the strip leaving the roughing apparatus is heated to or held at a temperature in the austenitic range and is rolled in the finishing apparatus essentially in the austenitic range to the finished thickness and, following that rolling, is cooled down to a temperature in the ferritic range;
and the ferritically or austenitically rolled strip after reaching the desired finished thickness is cut to portions of the desired length which are subsequently coiled.
In this context a strip is taken to be a slab reduced in thickness, both before and after reaching the finished thickness.
Preferably the method is carried out as an endless or semi-endless process.
The invention is based on a plurality of new and inventive notions.
One new notion is that it is possible to apply the method with which according to known prior art only hot-rolled steel strip is manufactured, in such a way, that with it, besides an austenitically rolled steel strip, a ferritically rolled steel strip with the properties of a cold-rolled steel strip can also be obtained while making use of essentially the same means.
This opens up the possibility of manufacturing a wider range of steel strips in an apparatus of itself known more particularly of manufacturing with it steel strips which have a considerably higher added value on the market. In addition, as explained in the following, the method produces a particular advantage in the case of the rolling of a ferritic strip.
A second new notion is based on the insight that considerable advantages can be obtained with a method whereby not a coil-by-coil manner of manufacture is employed but whereby in a semi-endless or endless process one or more slabs are rolled into a strip of the desired finished thickness. A semi-endless process is to be understood as a process whereby from a single slab a plurality of coils, preferably more than three, more preferably more than five coils of usual coil dimension are rolled to the finished thickness in a continuous process in at least the finishing apparatus. In an endless rolling process slabs, or after the roughing apparatus, strips, are connected to each other such that in the finishing apparatus an endless rolling process can be performed whereby in the semi-endless and in the endless process there is no material connection between the steel in the continuous casting machine on the one hand and the steel being rolled in the finishing apparatus on the other hand.
The starting point for the conventional manner of manufacturing steel strip is a hot-rolled coil which is also manufactured with the known method in EP 0 666 112 by cutting a slab into portions of the desired coil weight. Normally this kind of hot-rolled coil has a weight of between 16 to 30 tonnes. This method of manufacture has a serious drawback. One drawback is that in the case of great width/thickness ratio of the steel strip obtained, the shape control, in other words the variation of thickness across the width of the strip, is very difficult to control. The shape control is in particular a problem when the strip runs in and out of the finishing apparatus. Because of the discontinuity in the material flow more in particular the associated discontinuity in the tension and the temperature variation in the strip, the head and tail of the hot-rolled steel to be rolled behave differently from the middle portion in the rolling apparatus. In practice advanced forward- and self-adapting control methods and numerical models are used to attempt to keep the head and the tail having a poor shape as short as possible. Despite these measures, a head and tail must still be rejected with each coil and this can mount up to several tens of meters in length in which the variation in thickness is a factor of four or more higher than the allowed value.
In the installations currently in use width/thickness ratio of the austenitically rolled strip of approximately 1200-1400 are considered to be the practically achievable maximum: any greater width/thickness ratio leads to a too long head and tail before a stable situation is reached, and so to high rejection.
On the other hand, because of the materials efficiency in processing both austenitically or hot-rolled and cold-rolled steel strip, there is a need for a greater width with an unchanged or decreasing thickness. Width/thickness ratios
Cornelissen Marcus Cornelis Maria
Den Hartog Huibert Willem
Groot Aldricus Maria
Corus Technology BV
Coy Nicole
King Roy
Stevens Davis Miller & Mosher LLP
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