Patent Description:
Rolling plants known as Hot Strip Mills, or more simply indicated hereafter by the acronym "HSM", are known, designed for the hot production of metal strip starting from slabs typically from about <NUM> to about <NUM> thick.

Two examples of such plants are shown schematically in <FIG>.

These plants comprise heating furnaces <NUM> of the "Walking Beam" type in which the slabs are heated and, in line, one or two reversible roughing stands <NUM>. In the case where they comprise a single roughing stand <NUM> (<FIG>) the stand generally performs from five to seven rolling passes while, in the case where they comprise two roughing stands <NUM> (<FIG>), the first generally performs three rolling passes while the second performs from three to five further rolling passes, in order to obtain an intermediate bar having a thickness comprised between about <NUM> and about <NUM>.

Normally, downstream of the reversible stands <NUM> there is provided a transfer table, for example provided with passive insulated hoods <NUM>, that is, without heating burners, to limit the heat losses of the bar.

Downstream of the transfer table there is a shear <NUM>, normally of the Drum Shear type, sized to cut a rolled product which normally has a thickness comprised between about <NUM> and about <NUM>.

Immediately downstream of the shear <NUM> there is provided a water descaler <NUM> and a continuous rolling train, or compact finishing train <NUM>, having six or seven finishing stands disposed in line and in close succession to each other, an outlet table <NUM>, also called "run-out table", provided with cooling showers <NUM> and two or more winding reels <NUM>, <NUM> (downcoilers) which wind the finished strip to form the reels or coils.

In order for the rolling in the compact finishing train <NUM> to take place in the austenitic range, that is, without phase transformations in the structure of the steel, the strip has to leave the last stand of the finishing train <NUM> at a temperature not lower than <NUM>.

Therefore, the rolling mass flow in the compact finishing train <NUM> has to be set to obtain said optimum temperature of at least <NUM> at the outlet of the last finishing stand.

It is also known that the rolling mass flow is calculated as the product of the thickness of the strip and its rolling speed. Therefore, when a certain rolling mass flow is set, the rolling speed of the strip is determined only by the final thickness of the latter.

A first disadvantage of known plants is that the heating of the thick, or conventional slabs, takes place in heating furnaces which use gas burners to raise the temperature of the product up to about <NUM> - <NUM>. This temperature is necessary because all the temperature losses along the line must be taken into account so that the strip leaves the last rolling stand at a temperature, as we said, of at least <NUM>.

However, the slab heating operation requires long times, for example comprised between <NUM> and <NUM> hours, requiring a very high gas consumption by the burners, with consequences on environmental emissions and production costs.

Furthermore, in the case of heating particular steels, the thermal targets can be even higher, with a consequent increase in both gas consumption and emissions. It should also be added that, in order to differentiate the heating according to the type of steel and the required final quality, it is necessary to wait for the correct heating of the furnace to the desired temperature, higher or lower, and this limits production flexibility, given that it will be necessary to organize production to heat products that are thermally similar to each other, so as to try to optimize the times needed to reach the target temperature of the furnace. Due to this, the delivery times of the finished product are lengthened, which is increasingly required in small batches.

Another disadvantage of conventional HSM plants is that it is necessary to limit the maximum speed of the strip exiting from the finishing train in order to prevent the head of the strip, in the path that goes from the last stand to the winding reel <NUM>, <NUM>, from rising dangerously because of aerodynamic-type effects due to speed. Typically, the maximum speed allowed for the head of the strip on the run-out table is about <NUM>-<NUM>/s; this speed can then be increased after the winding on the winding reel has started.

By head of the strip we conventionally mean the front end of the strip which, in the direction of travel, meets the first stand of the finishing rolling line.

Similarly, by tail of the strip we mean the rear end of the strip which, in the direction of travel, enters the first stand of the finishing rolling line last.

The portion of strip comprised between the head and tail is referred to as the body of the strip.

Because of this speed limitation it may happen that, especially for thin strip, for example with thicknesses of <NUM> or less, it is not possible to reach said optimum temperature of at least <NUM> at the outlet of the last finishing stand.

To prevent this from happening, in known plants, after the head enters the winding reels <NUM>, <NUM> the so-called "speed-up" of the stands of the compact finishing train <NUM> is carried out, in order to make the strip transit faster and thus reduce losses in temperature, allowing the body and tail of the strip to exit from the compact finishing train <NUM> at the optimum temperature not lower than <NUM>.

This type of solution, if applied for example to the production of a rolled product having a final thickness of <NUM>, and as shown schematically in the graph in <FIG>, requires an acceleration of the tail of about <NUM>%, in order to guarantee a minimum temperature of <NUM> at exit from the last stand, since the only active heat input coincides solely with the heating furnace upstream of the line.

However, if one wants to obtain rolled products with rolling thicknesses of less than <NUM>, despite using a speed-up of even <NUM>%, with which the typically limit speed of <NUM> - <NUM>/s is reached, with conventional HSM plants it is not possible to guarantee that the desired minimum temperature of <NUM>, required at exit from the last stand, is maintained, since the temperature loss of the product being rolled is excessive, with consequent and undesirable phase changes of the steel, which affect the quality of the final product.

As shown schematically in the graph in <FIG>, with a traditional HSM plant, in order to produce a <NUM> thick strip, and a limit speed-up as indicated above, the exit temperature from the last rolling stand is around <NUM>, not only for the head but also for the tail of the strip, making a quality production for strip of such a limited thickness substantially impossible.

To overcome these limitations, solutions have been proposed which provide to carry out induction heating immediately in front of the compact finishing train in order to enter with the bar at a higher temperature, but since the heating is carried out before the initial stands, which are the slowest, a greater amount of scale is formed due to the higher temperature for the same time of exposure to the air of the bar being rolled in the initial stands.

Furthermore, the compact finishing train <NUM>, in conventional HSM plants, does not allow to carry out a further high-pressure descaling step inside the train itself, before the winding of the strip.

This means that the scale that forms following the exposure to high temperature air of the bar being rolled in the initial passes, since it cannot be removed, is imprinted into the strip during the rolling in the final passes, with a consequent reduction in the quality of the finished product.

Currently, the need to also produce quality strip with a thin thickness from <NUM> and lower, to a minimum value comprised between <NUM> and <NUM>, is increasingly felt in HSM plants, overcoming the disadvantages of the state of the art. Quality must be understood both in terms of surface quality of the strip and also in terms of the final mechanical characteristics required by the market.

One purpose of the invention is therefore to perfect a method for revamping a Hot Strip Mill plant so that it can also produce thin thicknesses of quality strip, without negatively affecting the productivity of the existing plant, which can reach up to <NUM> or more million tons/year.

Another purpose of the present invention is to carry out said revamping with a reduced economic and operational impact compared to the existing plant.

Another purpose of the present invention is to provide an existing Hot Strip Mill plant, in order to keep the mechanical and geometric properties uniform along the entire length of the coil produced.

The dependent claims describe evolved and perfected aspects of the independent claim.

In accordance with the above purposes, a method according to the present invention is applied for revamping an existing hot strip mill plant for the production of a steel strip starting from a slab having a determinate starting thickness, in which the revamped plant comprises:.

Therefore, the existing plant to be revamped is generally configured as a traditional hot strip mill rolling plant which operates in coil-to-coil mode, that is, of the discontinuous type, in which the rolled product is obtained starting from single slabs, for example with a thickness comprised between about <NUM> and about <NUM>, with all the operational, dimensional and production characteristics that this type of plant entails.

In the existing plant, there are generally disposed in sequence, from upstream to downstream, three heating furnaces, a first water descaling device, a vertical or edging stand for working the edges, which is combined with at least one reversible roughing stand, configured to subject the slab to a determinate number of rolling passes and reduce its thickness until an intermediate rolled product (or bar) is obtained, which normally has a thickness comprised between about <NUM> and about <NUM>. A transfer table follows, for example provided with passive insulated hoods, that is, without heating burners, to limit the heat losses of the bar.

In some hot strip mill plants, at the end of the transfer table there is a coil-box in which the intermediate rolled product is wound into a reel to be subsequently unwound.

Downstream of the transfer table, or of the coil-box, there is a shear, normally of the Drum Shear type, sized in order to cut the intermediate rolled product.

Immediately downstream of the shear there is provided a second water descaling device and a continuous rolling train, or compact finishing train, having six or seven finishing stands disposed in line and in close succession to each other, an outlet table, also called "run-out table", provided with cooling showers, and two or more winding reels (downcoilers) which wind up the finished strip to form the reels or coils.

In accordance with one aspect of the present invention, in order to obtain the desired revamped plant, the method provides at least one step of modifiying the existing compact finishing train, in which at least the first stand is moved away from the remaining stands located downstream of it and brought closer to the reversible roughing stand, at a minimum distance "D" from it, whereby the intermediate rolled product is not operatively engaged with both stands simultaneously.

With this displacement, a first group of stands is configured, called pre-finishing unit, positioned at a predetermined distance "d" from the second group of stands, called finishing unit.

According to the present invention, the modification step provides that the pre-finishing unit comprises from one to two pre-finishing stands and that the finishing unit comprises from five to six finishing stands.

Depending on the initial configuration of the existing plant and the final division of the finishing rolling train to be obtained, it may be necessary to insert a new stand in the pre-finishing unit, in addition to those obtained from the existing train.

For example, if the existing train has seven stands, the first two are moved to the pre-finishing unit, giving rise to a <NUM>+<NUM> configuration.

If, on the other hand, the existing train has six stands, the first stand is moved to the pre-finishing unit and, in order to obtain the <NUM>+<NUM> configuration, a new stand is added to the pre-finishing unit.

In the space comprised between the reversible stand and the pre-finishing unit, the existing passive insulated hoods are kept for the amount necessary to cover part of said distance "D".

If the existing plant was provided with a coil-box, according to the invention it is preferable to remove it in the revamped plant, replacing it with passive insulated hoods.

Advantageously, there is provided at least one step of installing a new rapid heating device, for example an inductor consisting of selectively activatable modules, which is interposed between the pre-finishing unit and the finishing unit, so as to heat the pre-finished rolled product.

According to another aspect of the present invention, the method provides to remove the existing drum shear and to position it, after possible revamping, between the pre-finishing unit and the rapid heating device.

According to another aspect of the present invention, the method provides to position the second existing descaling means in front of the pre-finishing unit.

Advantageously, the method provides to insert new third water descaling means between the rapid heating device and the first stand of the finishing unit, with the function of further cleaning the surface of the pre-finished rolled product of the scale before it enters the finishing unit. In this way, the scale that has formed on the surface of the pre-finished product is removed, thus avoiding quality defects on the rolled strip, such as imprinted scale, for example.

The at least one reversible roughing stand is in turn equipped with descaling means mounted on board and being an integral part of the stand itself, which are disposed both on the inlet side and also on the outlet side of the stand.

The revamped plant according to the method described above operates as indicated below.

The outlet temperature of the slab at exit from the heating furnaces is comprised between about <NUM>-<NUM> and about <NUM>, therefore about <NUM>-<NUM> lower than the original temperature, with consequent benefits in terms of gas consumption, and corresponding costs, atmosphere emissions and scale formation, thanks to the shorter permanence time in the furnace.

If the existing plant is equipped with only one reversible roughing stand, which remains unchanged in the revamped plant, the number of rolling passes is reduced from <NUM> to <NUM>.

If the existing plant is equipped with two reversible roughing stands, which remain unchanged in the revamped plant, the number of rolling passes is equal to <NUM> in the first stand while it is reduced from <NUM>-<NUM> to <NUM>-<NUM> in the second stand.

In both cases, the intermediate rolled product obtained at exit from the roughing stand/s has a thickness comprised between about <NUM> and about <NUM>. By way of example only, at the end of the required roughing passes, the intermediate rolled product has a temperature which ranges from about <NUM> to about <NUM>.

The pre-finishing unit is able to reduce the thickness of the intermediate rolled product, in order to obtain a pre-finished rolled product, for example with a thickness comprised between about <NUM> and about <NUM>.

In the revamped version of the plant, the shear head and tail trims the pre-finished product on a lower thickness therefore, with the same trimmed portion, the weight of rejected material is lower, thus positively affecting the yield of the plant.

In the rapid heating device, the heating can occur, advantageously, up to an outlet temperature therefrom comprised between about <NUM> and about <NUM>, or in any case to a temperature such that, also as a function of the operating and product parameters, the temperature of the final strip, at exit from the last finishing stand, is higher than at least <NUM>.

This advantageous aspect of the solution according to the present invention allows the steel to remain substantially in the austenitic range during rolling in the finishing unit and, therefore, without phase transformations before exiting the last finishing stand.

In this way, it is possible to provide the production of rolled products that are substantially uniform both as regards mechanical properties and also as regards geometric properties, along the entire length of the coil produced.

Furthermore, the installation of the induction heating device between the pre-finishing and finishing stands allows to relieve the upstream gas heating furnace of a portion of the thermal contribution to be given to the slab, thus reducing gas consumption and emissions compared to the same plant before the revamping according to the present invention.

The finishing stands, on the other hand, are configured to reduce the thickness of the pre-finished rolled product, so as to obtain the final strip, for example with a thickness comprised between about <NUM> and about <NUM>.

By making the modifications according to the steps of the method of the present invention to an existing conventional hot strip mill plant, this plant is revamped allowing to produce flat rolled products with a thin thickness from <NUM> and lower, to a minimum value comprised between <NUM> and <NUM>, overcoming the quality, productivity and yield problems that the conventional plant presents in producing such thin thicknesses.

Therefore, by substantially dividing the existing rolling train into two macro pre-finishing and finishing rolling units, and interposing a rapid heating device between them, the starting HSM plant is revamped to produce quality thin thicknesses, without negatively affecting the productivity of the plant itself, which can reach up to <NUM>-<NUM> million tons/year.

With reference to <FIG>, these show two types of HSM plants <NUM>, each obtained from the revamping of corresponding starting operating HSM plants <NUM>, according to the method of the present invention.

The plants <NUM> are revamped with respect to the starting plants <NUM>, for rolling a flat rolled product, for example a final strip P, with a thickness comprised between about <NUM> and about <NUM> wound to form a reel, or coil, starting from slabs <NUM> having a starting thickness comprised between about <NUM> and about <NUM>.

Both the starting plant <NUM> as well as the revamped plant <NUM> comprise one or more gas heating furnaces <NUM>, for example of the type known in the sector with the term "walking beam", configured to receive and heat to a determinate starting temperature at least one slab <NUM>, supplied even at ambient temperature.

In the solutions with the revamped plant <NUM>, at exit from the gas furnace <NUM> the slab <NUM> has a temperature comprised between about <NUM>-<NUM> and about <NUM>, instead of the original <NUM> - <NUM>. The outlet temperature of the slab at exit from the heating furnaces is therefore <NUM>-<NUM> lower than the original one, with consequent benefits in terms of gas consumption, and corresponding costs, atmosphere emissions and scale formation, thanks to the shorter residence time in the furnace.

A warehouse <NUM> can also be part of the plant <NUM>, which cooperates with the gas heating furnace <NUM> and is configured to store the slabs <NUM>, for example coming from another production site or from another production area of the same factory. The warehouse <NUM> allows to selectively feed at least one slab <NUM> to the gas heating furnace <NUM>, according to desired feeding sequences and timings.

In the original type of plant <NUM> shown in <FIG>, and in the corresponding revamped plant <NUM> of <FIG>, downstream of the gas heating furnace <NUM> there are disposed, in sequence, a first water descaling device <NUM>, a vertical or edging stand <NUM> and a reversible roughing stand <NUM> which is configured to subject the slab <NUM> to a determinate number of passes and reduce its thickness until an intermediate rolled product <NUM> is obtained.

In the original type of plant <NUM> shown in <FIG>, and in the corresponding revamped plant <NUM> of <FIG>, downstream of the gas furnace <NUM>, in addition to the descaling device <NUM>, there are provided two reversible roughing stands <NUM> with corresponding vertical stands <NUM>.

The reversible roughing stands are equipped with descaling means mounted on board and forming an integral part of the stands themselves, which are disposed both on the inlet side and also on the outlet side of each stand (not shown in the drawings).

The corresponding layout described heretofore between the original plant <NUM> and the corresponding revamped plant <NUM>, whether of the type with one or two roughing stands <NUM>, highlights the advantageous characteristic of the revamping method according to the present invention, whereby most of the plant remains intact as originally installed, to the advantage of costs, timings and the impact of the revamping intervention.

With the revamped plant <NUM>, the intermediate rolled product <NUM> obtained at exit from the roughing stand/s <NUM> has a thickness comprised between about <NUM> and about <NUM>, instead of the original <NUM> - <NUM>. By way of example only, at the end of the required roughing passes, the intermediate rolled product <NUM> has a temperature which ranges from about <NUM> to about <NUM>.

At this point, in order to carry out the revamping of both types of plant <NUM> shown in <FIG>, the drum shear <NUM> is removed, and the descaler <NUM> and two stands of the compact rolling train <NUM> are disassembled.

The descaler <NUM> and the two stands are displaced toward the reversible stands <NUM> at a determinate distance D therefrom, so that the intermediate rolled product <NUM> is never operatively engaged with both types of stand simultaneously.

In this way, the compact rolling train <NUM> is divided into two macro rolling units, a pre-finishing unit <NUM> and a finishing unit <NUM>, which are deliberately distanced from each other by a predetermined distance "d".

The stands of the finishing unit <NUM> substantially maintain their original installation position, without impacting the revamping intervention.

The revamped rolling train is configured to progressively reduce the thickness of the intermediate rolled product <NUM> in order to obtain the final strip P with a minimum thickness equal to <NUM> - <NUM>.

In the solution according to the present invention, a pre-finished rolled product <NUM> exits from the two pre-finishing stands <NUM> having a thickness comprised between about <NUM> and about <NUM>.

According to the invention, downstream of the pre-finishing unit <NUM> there is disposed, in this specific case, the same drum shear <NUM>, after possible reconditioning, to trim the heads and tails of the pre-finished rolled product <NUM>, in order to facilitate its entry into the stands of the finishing unit <NUM> and to reduce the chances of cobble, especially for the production of final strips having a thickness smaller than <NUM>.

However, it is not excluded that, according to one variant, the shear <NUM> can be replaced with an alternative cutting machine, having a different size and functionality from the drum shear <NUM> originally provided in the plant <NUM>.

The method according to the present invention also comprises positioning a rapid heating device <NUM> interposed between the pre-finishing unit <NUM> and the finishing unit <NUM> of the revamped rolling train.

Preferably, the rapid heating device <NUM> comprises, for example, an induction furnace disposed downstream of the flying shear <NUM> and consisting of elements that can be activated selectively, even independently of each other.

The rapid heating device <NUM> is configured to heat, selectively and in an adjustable manner, the pre-finished rolled product <NUM> before it enters the finishing stands <NUM>.

The temperature to which the pre-finished rolled product is heated is selected, among other parameters, at least as a function of its thickness and the final thickness of the final strip P, so that the latter has an optimum temperature of at least <NUM> at the outlet of the finishing unit <NUM>, and in particular at the outlet of the last finishing stand.

By way of example only, the temperature to which the pre-finished rolled product <NUM> is heated, that is, the temperature it has at exit from the rapid heating device <NUM>, reaches a value advantageously comprised between about <NUM> and about <NUM>, or in any case a temperature such that, also as a function of the operating and product parameters, the temperature of the final strip at exit from the last finishing stand <NUM> is higher than at least <NUM>.

This allows to reduce the value of the rolling mass flow MFL required to obtain the above mentioned optimum temperature of at least <NUM>, for example comprised between <NUM> and <NUM>, at the outlet of the last stand of the finishing unit <NUM>.

The reduction of the rolling mass flow MFL allows both to carry out the rolling with a reduced rolling speed VL, preferably lower than <NUM>/s, and at the same time to reach the optimum temperature of at least <NUM> at the outlet of the continuous rolling train <NUM> even for the tail of the final strip P, eliminating the need for "speed up" as a tool for reaching the target temperature. An example of this embodiment is schematized graphically in <FIG>.

Advantageously, in the absence of speed-up, the rolling speed VL in the finishing stands <NUM> is substantially constant and allows both to keep the temperature of the final strip P between its head and tail constant, and also to choose the most suitable temperature control (for example thermomechanical treatment) as a function of the steel grade and the use of the final strip P.

Another advantage of not performing the speed-up consists in the fact that it allows a high control of both the final shape of the final strip P, for example crown and flatness thereof, which will therefore be advantageously uniform along the entire length of the coil, and also of the mechanical properties of the final strip P which will be advantageously constant and uniform along the entire length of the coil.

This last advantage, which cannot be achieved with plants of the prior art, is of considerable importance, particularly for quality productions such as, for example, final strips P intended for molding.

According to some embodiments, it may be necessary to resort to speed-up in order to be able to increase the productivity of the line when very thin thicknesses are produced, or to achieve very high productivity with other thicknesses. An example of this embodiment is schematized graphically in <FIG>.

In addition to the original plant <NUM>, downstream of the rapid heating device <NUM> and upstream of the finishing unit <NUM> there is also disposed a third water descaling device <NUM>, which has the function of further cleaning the surface of the pre-finished rolled product of scale before entering the finishing stands.

Therefore, the scale that has formed on the surface of the pre-finished product is removed, thus avoiding qualitative defects on the rolled strip, such as imprinted scale for example.

Downstream of the finishing unit <NUM>, the cooling device <NUM> and the showers <NUM> of the original plant <NUM> are kept, to cool the strip P.

Furthermore, at the outlet of the showers <NUM> the two winding reels <NUM>, <NUM> are kept, to wind the strip P into coils for its subsequent storage and shipment.

It is clear that modifications and/or additions of parts may be made to the method for revamping a plant for producing flat rolled products as described heretofore, without departing from the field and scope of the present invention, as defined by the claims.

It is also clear that, although the present invention has been described with reference to some specific examples, a person of skill in the art shall certainly be able to achieve many other equivalent forms of method for revamping a plant for producing flat rolled products, having the characteristics as set forth in the claims and hence all coming within the field of protection defined thereby.

Claim 1:
Method for revamping an existing rolling plant (<NUM>) for producing a final strip (P) starting from a slab (<NUM>) having a determinate starting thickness, the existing rolling plant comprising:
- at least one heating furnace (<NUM>) configured to heat at least said slab (<NUM>) to a determinate starting temperature;
- at least one reversible roughing stand (<NUM>) configured to subject said slab (<NUM>) to one or more rolling passes in order to obtain an intermediate rolled product (<NUM>);
- a compact rolling train (<NUM>) disposed operatively in line with said at least one roughing stand (<NUM>), comprising a plurality of finishing stands (<NUM>) located in line with each other and configured to reduce the thickness of said intermediate rolled product (<NUM>), until said final strip (P) having a determinate final thickness is obtained;
characterized in that in order to obtain a revamped plant (<NUM>), the method provides to operate on said existing plant (<NUM>) by carrying out at least one step of modifying said rolling train (<NUM>), in which at least one initial stand of said rolling train (<NUM>) is moved away from the remaining stands located downstream of it and brought closer to the roughing stand (<NUM>), thus dividing the rolling train (<NUM>) into a pre-finishing unit (<NUM>) and a finishing unit (<NUM>), wherein said pre-finishing unit (<NUM>) is positioned at a minimum distance (D) from said roughing stand (<NUM>) such that the intermediate rolled product is not operatively engaged with both respective stands (<NUM>, <NUM>) simultaneously, and at least one step of installing a rapid heating device (<NUM>), consisting of selectively activatable elements, between said pre-finishing unit (<NUM>) and said finishing unit (<NUM>), in order to heat said pre-finished rolled product (<NUM>), at exit from said pre-finishing unit (<NUM>), so that the temperature of said final strip (P) in correspondence with the outlet of the last stand of said finishing unit (<NUM>) is higher than at least <NUM>, even for a minimum thickness of less than <NUM>.