Metal treatment – Process of modifying or maintaining internal physical... – With casting or solidifying from melt
Reexamination Certificate
1999-09-03
2003-03-18
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
With casting or solidifying from melt
C148S602000, C148S603000, C148S657000, C148S658000
Reexamination Certificate
active
06533876
ABSTRACT:
The invention relates to a process for producing a steel strip or sheet, in which liquid steel is cast in a continuous-casting machine to form a thin plate and, while making use of the casting heat, is fed through a furnace device, is roughed in a roughing stand to a pass-over thickness and is rerolled in a finishing rolling stand to form a steel strip or sheet of the desired final thickness, and to a device which is suitable for use in such a process.
Where the following text refers to a steel strip, this is also to be understood as including a steel sheet. A thin plate is understood to mean a plate whose thickness is less than 150 mm, preferably less than 100 mm.
A process of this kind is known from European Patent Application 0 666 122.
This Patent Application describes a process in which a continuously cast thin steel plate, after being homogenized in a tunnel furnace device, is rolled in a number of hot-rolling steps, i.e. in the austenitic field, to form a strip having a thickness of less than 2 mm.
In order to achieve such a final thickness using rolling devices and rolling trains which can be realized in practice, it is proposed to reheat the steel strip, preferably by means of an induction furnace, at least after the first rolling mill stand.
A separating device is positioned between the continuous-casting machine and the tunnel furnace device, which device is used to cut the continuously cast thin plate into pieces of approximately equal length, which pieces are homogenized in the tunnel furnace device at a temperature of approx. 1050° C. to approx. 1150° C. After leaving the tunnel furnace device, the pieces may if desired be cut again into half-plates which have a weight which corresponds to the coil weight of the wound coil to which the steel strip is wound downstream of the rolling device.
The object of the invention is to provide a process of the known type which offers more options and with which, moreover, steel strip or sheet can be produced in a more efficient way. To this end, the process according to the invention is characterized in that
a. to produce a ferritically rolled steel strip, the strip, the plate or a part thereof is fed without interruption at least from the furnace device, at speeds which essentially correspond to the speed of entry into the roughing stand and the following reductions in thickness, from the roughing stand to a processing device which is disposed downstream of the finishing rolling stand, the strip coming out of the roughing stand being cooled to a temperature at which the steel has an essentially ferritic structure;
b. to produce an austenitically rolled steel strip, the strip coming out of the roughing roll is brought to or held at a temperature in the austenitic range, and in the finishing rolling stand it is rolled to the final thickness essentially in the austenitic field and is then cooled, after this rolling, to the ferritic field.
In this context, strip is understood to mean a plate of reduced thickness.
In the conventional method for producing ferritic, or cold-rolled, steel strip, the starting point is a hot-rolled roll of steel, as is also produced using the known method from EP 0,666,112. A hot-rolled roll of steel of this kind usually has a weight of between 16 and 30 tonnes. In this case, the problem arises that it is very difficult, with large width/thickness ratios of the steel strip obtained, to control the dimensions of the strip, i.e. the thickness profile over the width of the strip and over the length of the strip. Owing to the discontinuity in the stream of material, the top and tail of the hot-rolled strip behave differently from the central part in the rolling device. Controlling the dimensions represents a problem above all during entry and exit of the hot-rolled strip into and out of the finishing rolling stand for ferritic or cold rolling. In practice, advanced forwards and self-adapting control systems and numerical models have been used in an attempt to keep the top and tail, which have incorrect dimensions, as short as possible. Nevertheless, every roll has a top and tail which is to be rejected and may amount to up to a few tens of meters in length.
In the installations currently used, a width/thickness ratio of about 1200-1400 is regarded as the maximum which can be achieved in practice: a grater width/thickness ratio leads to an excessively long top and tail before reaching a stable situation, and hence to excessive levels of scrap.
On the other hand, with a view to efficiency of materials when working a hot-rolled or cold-rolled steel strip, there is a need for a greater width with an identical or reduced thickness. Width/thickness ratios of 2000 or more are desired in the market, but cannot be achieved in practice with the known process, for the reasons described above.
The process according to the invention makes it possible to rough the steel strip, at any rate from the furnace device, in an uninterrupted or continuous process in the austenitic field, to cool it to the ferritic field and to roll it in the ferritic field to give the final thickness.
A much simpler feedback control has proven sufficient for controlling the dimensions of the strip.
The invention also makes use of the insight that it is possible to employ the process with which, according to the prior art, only hot-rolled steel strip is produced, in such a manner, while making use of essentially the same means, that this process can also be used to obtain, in addition to an austenitically rolled steel strip, a ferritically rolled steel strip as well, having the properties of a cold-rolled steel strip.
This opens up the possibility of using a device which is known per se to produce a wider range of steel strips, and more particularly to produce steel strips which have a considerably higher added value on the market. In addition, the process yields a particular advantage when rolling a ferritic strip according to step a, as will be explained in the following text.
The invention also makes it possible to achieve a number of other important advantages, as will be described in the following text.
When carrying out the process according to the invention, it is preferred for the roughing to take place in the austenitic field, as soon as possible downstream of the furnace device in which the plate is homogenized at temperature. Furthermore, it is preferred to select a high rolling speed and reduction. In order to obtain constant properties for the steel, it is necessary to prevent the plate, or at least an excessive part thereof, from passing into the two-phase field in which the austenitic and ferritic structures exist next to one another. After leaving the furnace device, the homogenized austenitic plate cools most rapidly at the side edges. It has been found that cooling takes place primarily over an edge part of the plate which has a width which is comparable to the current thickness of the plate or strip. By rolling the strip shortly after it leaves the furnace, and preferably with a considerable reduction, the extent of the cooled edge part is limited. It is then possible to produce a strip of the correct strip shape and with constant, predictable properties over virtually the entire width.
The virtually homogeneous temperature distribution over the width, together with the thickness of the plate, provides the additional advantage of a broader working range within which the invention can be employed. Since it is undesirable to carry out rolling in the two-phase field, the working range with regard to the temperature is limited on the underside by the temperature of that part of the plate which first passes into the two-phase field, i.e. the edge region. In the conventional process, the temperatures of the central part is then still fat above the transition temperature at which austenite begins to change into ferrite. In order nevertheless to be able to exploit the higher temperature of the central part, it is proposed in the prior art to reheat the edges. Using the invention, this measure is not necessary, or at least is necessar
Cornelissen Marcus Cornelis Maria
Den Hartog Huibert Willem
Groot Aldricus Maria
Combs Morillo Janelle
Corus Staal
Stevens Davis Miller & Mosher LLP
Wyszomierski George
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