PLANT AND METHOD FOR PRODUCING FLAT ROLLED PRODUCTS

Abstract

A plant and a method for producing a final strip starting from a slab having a determinate starting thickness, including: at least one heating furnace configured to heat at least the slab to a determinate starting temperature; at least one reversible roughing stand configured to subject the slab to one or more rolling passes in order to obtain an intermediate rolled product; and a continuous rolling train disposed operatively in line with the roughing stand and configured to reduce the thickness of the intermediate rolled product, until the final strip having a determinate final thickness is obtained.

Claims

1. A rolling plant, for producing a final strip starting from a slab having a determinate starting thickness, comprising: at least one heating furnace configured to heat at least said slab to a determinate starting temperature; at least one reversible roughing stand configured to subject said slab to one or more rolling passes in order to obtain an intermediate rolled product; a continuous rolling train disposed operatively in line with said at least one roughing stand and configured to reduce the thickness of said intermediate rolled product, until said final strip having a determinate final thickness is obtained; wherein said rolling train comprises at least one pre-finishing stand disposed at a minimum distance from said roughing stand and able to reduce the thickness of said intermediate rolled product in order to obtain a pre-finished rolled product, and a plurality of finishing stands able to reduce the thickness of said pre-finished rolled product in order to obtain said final strip, and wherein a rapid heating device consisting of selectively activatable elements is interposed between said at least one pre-finishing stand and said plurality of finishing stands, and is configured to heat said pre-finished rolled product so that the temperature of said final strip, in correspondence with the outlet of the last finishing stand of said rolling train, is higher than at least 830? C., even for a final thickness<1.2 mm.

2. The plant as in claim 1, wherein it comprises at least one warehouse configured for the storage of said slabs, said warehouse being disposed substantially upstream of said heating furnace and able to selectively feed at least one of said slabs to said heating furnace.

3. The plant as in claim 1, wherein said rolling train comprises from one to three pre-finishing stands and from five to six finishing stands.

4. The plant as in claim 1, wherein said at least one roughing stand is configured to define said intermediate rolled product having a thickness comprised between 45 mm and 80 mm, wherein said at least one pre-finishing stand is configured to define said pre-finished rolled product having a thickness comprised between 10 mm and 50 mm, and wherein said plurality of finishing stands are configured to define said final strip having a determinate final thickness comprised between about 0.9 mm and about 26 mm.

5. The plant as in claim 1, wherein it comprises at least first descaling means interposed between said heating furnace and said at least one roughing stand, second descaling means interposed between said roughing stand and a first of said pre-finishing stands, and third descaling means interposed between said rapid heating device and a first of said finishing stands.

6. The plant as in claim 1, wherein it comprises a cutting machine interposed between said pre-finishing stand and said rapid heating device.

7. A rolling method, for producing a final strip starting from a slab having a determinate starting thickness, in a rolling plant which comprises: at least one heating furnace configured to heat at least said slab to a determinate starting temperature T1; at least one reversible roughing stand configured to subject said slab to one or more rolling passes in order to obtain an intermediate rolled product; a continuous rolling train disposed operatively in line with said at least one roughing stand and configured to reduce the thickness of said intermediate rolled product, until said final strip having a determinate final thickness is obtained; wherein it provides at least one pre-finishing rolling of said intermediate rolled product, by means of at least one pre-finishing stand of said rolling train disposed at a minimum distance from said roughing stand, so as to reduce the thickness of said intermediate rolled product in order to obtain a pre-finished rolled product, at least one finishing rolling of said pre-finished rolled product, by means of a plurality of finishing stands of said rolling train which are able to reduce the thickness of said pre-finished rolled product, in order to obtain said final strip, and at least one step of heating said pre-finished rolled product, by means of a rapid heating device consisting of selectively activatable elements and interposed between said at least one pre-finishing stand and said plurality of finishing stands, so that the temperature of said final strip, in correspondence with the outlet of the last finishing stand of said rolling train, is higher than at least 830? C., even for a final thickness<1.2 mm.

8. The method as in claim 7, wherein it comprises at least one storage and feeding step in which, by means of a warehouse disposed substantially upstream of said heating furnace, said slabs are stored and fed selectively toward said heating furnace.

9. The method as in claim 7, wherein the thickness of said intermediate rolled product in correspondence with the inlet of said rolling train is comprised in a range from 45 mm to 80 mm.

10. The method as in claim 7, wherein said determinate starting temperature T1 of said slab is lower than or equal to 1200? C.

Description

DESCRIPTION OF THE DRAWINGS

[0077] These and other aspects, characteristics and advantages of the present invention will become apparent from the following description of some embodiments, given as a non-restrictive example with reference to the attached drawings wherein:

[0078] FIGS. 1 and 2 are schematic representations of two types of HSM plants for producing flat rolled products, in accordance with the prior art;

[0079] FIGS. 3 and 4 are graphical representations of the relation between rolling speed and outlet temperature of rolled products with different thicknesses, in accordance with the prior art;

[0080] FIG. 5 is a schematic representation of an embodiment of a new generation HSM plant for producing flat rolled products, in accordance with the present invention;

[0081] FIG. 6 is a graph which relates, for a defined mass flow, the final thickness of a flat rolled product and the rolling speed required for it;

[0082] FIGS. 7 and 8 are graphical representations of the relation between rolling speed and outlet temperature of rolled products with different thicknesses of a new generation HSM plant for producing flat rolled products, in accordance with the present invention.

[0083] We must clarify that in the present description the phraseology and terminology used, as well as the figures in the attached drawings also as described, have the sole function of better illustrating and explaining the present invention, their function being to provide a non-limiting example of the invention itself, since the scope of protection is defined by the claims.

[0084] To facilitate comprehension, the same reference numbers have been used, where possible, to identify identical common elements in the drawings. It is understood that elements and characteristics of one embodiment can be conveniently combined or incorporated into other embodiments without further clarifications.

DESCRIPTION OF SOME EMBODIMENTS OF THE PRESENT INVENTION

[0085] With reference to FIG. 5, this shows a plant 10 in accordance with the present invention, for producing a flat rolled product, for example a final strip P, with a thickness comprised between about 0.9 mm and about 26 mm wound to form a reel, or coil, starting from slabs 50 having a starting thickness comprised between about 150 mm and about 350 mm.

[0086] The plant 10 comprises one or more gas heating furnaces 16, for example of the type known in the sector with the term walking beam, configured to receive and heat to a determinate starting temperature T1 at least one slab 50, supplied even at ambient temperature. Advantageously, at exit from the gas heating furnace 16 the slab 50 has a temperature comprised between about 1100-1150? C. and about 1200? C.

[0087] A warehouse 40 is also part of the plant 10, disposed substantially in line and upstream of the gas heating furnace 16 and configured to store the slabs 50, for example coming from another production site or from another production area of the same factory. The warehouse 40, shown only schematically in FIG. 3, allows to selectively feed at least one slab 50 to the gas furnace 16, according to desired feeding sequences and timings.

[0088] Downstream of the gas heating furnace 16 there are disposed, in sequence, a first water descaling device 20, a vertical or edging stand 21 and a reversible roughing stand 23 configured to subject the slab 50 to a determinate number of passes and reduce its thickness until an intermediate rolled product 51 is obtained. The latter, in an advantageous embodiment, has a thickness comprised between about 45 mm and about 80 mm. By way of example only, at the end of the desired roughing passes, the intermediate rolled product 51 has a temperature ranging from about 1020? C. to about 1120? C.

[0089] In other embodiments, it is not excluded that two roughing stands 23 may be provided, with corresponding vertical stands 21.

[0090] According to another aspect of the present invention, the at least one reversible roughing stand 23 is in turn equipped with descaling means mounted on board and forming an integral part of the stand itself, which are disposed both on the inlet side and also on the outlet side of the stand (not shown in the drawings).

[0091] Downstream of the reversible roughing stand 23 there are disposed, in succession, a second descaling device 24 and a continuous rolling train 25.

[0092] In particular, the continuous rolling train 25 consists of two macro rolling units, a pre-finishing unit comprising two pre-finishing stands 26 and a finishing unit comprising a plurality of finishing stands 31, in this specific case five.

[0093] The continuous rolling train 25 is configured to progressively reduce the thickness of the intermediate rolled product 51 in order to obtain the final strip P, with a minimum thickness of about 1 mm.

[0094] In some embodiments, not shown in the drawings, the plant 10 can also comprise a vertical or edging stand 21, both downstream of the reversible roughing stand 23 as well as upstream of the continuous rolling train 25.

[0095] In general, the number of pre-finishing stands 26 of the rolling train 25 is comprised between one and three, while the number of finishing stands 31 is comprised between five and six and their number and arrangement is chosen as a function of the steel grades, the use of the finished product and the minimum and maximum thicknesses that the final strip P assumes during rolling.

[0096] In the solution according to the present invention, two pre-finishing stands 26 are provided, distanced from the remaining finishing stands 31 of the rolling train 25, so that a pre-finished rolled product 52 having a thickness comprised between about 10 mm and about 50 mm exits from the pre-finishing stands 26.

[0097] Furthermore, the pre-finishing stands 26 are disposed at a determinate distance D from the roughing stand 23, so that the intermediate rolled product 51 is never operatively engaged with both types of stand simultaneously.

[0098] In the example solution shown, downstream of the pre-finishing stands 26 there is disposed a flying shear 27, of the Crop Shear type, to trim the heads and tails of the pre-finished rolled product 52 in order to facilitate its entry into the finishing stands 31 and to reduce the chances of cobble, especially for the production of final strips having a thickness smaller than 3.0 mm.

[0099] Advantageously, since the pre-finished product 52 has a smaller thickness than the corresponding intermediate product 51 of a conventional HSM, the shear 27 can have a smaller size, with benefits in terms of costs, overall dimensions and maintenance.

[0100] The plant 10 according to the present invention also comprises a rapid heating device 28 interposed between the pre-finishing stands 26 and the finishing stands 31 of the continuous rolling train 25.

[0101] Preferably, the rapid heating device 28 comprises, for example, an induction furnace disposed downstream of the flying shear 27 and consisting of elements that can be activated selectively, even independently of each other.

[0102] The rapid heating device 28 is configured to heat, selectively and in an adjustable manner, the pre-finished rolled product 52 before it enters the finishing stands 31.

[0103] The temperature to which the pre-finished rolled product 52 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 830? C. at the outlet of the continuous rolling train 25, and in particular at the outlet of the last finishing stand.

[0104] By way of example only, the temperature to which the pre-finished rolled product 52 is heated, that is, the temperature it has at exit from the rapid heating device 28, reaches a value advantageously comprised between about 1000? C. and about 1100? C.

[0105] This allows to reduce the value of the rolling mass flow MF.sub.L required to obtain the above mentioned optimum temperature of at least 830? C., for example comprised between 830? C. and 900? C., at the outlet of the last finishing stand 31.

[0106] The reduction of the required rolling mass flow MF.sub.L reduces the maximum rolling speed required as a whole from the finishing train 25 to obtain the optimum temperature as above. This allows to avoid, or at least reduce, the so-called speed-up which occurs, during use, during rolling.

[0107] Advantageously, downstream of the rapid heating device 28 and upstream of the finishing stands 31 there is also disposed a third water descaling device 29 which has the function of further cleaning the surface of the pre-finished rolled product of scale before entering the finishing stands 31.

[0108] Therefore, the scale which has formed on the surface of the pre-finished rolled product 52 is effectively removed, thus avoiding qualitative defects on the rolled strip P, such as imprinted scale for example.

[0109] Downstream of the finishing stands 31 there is disposed a cooling device 33 comprising a plurality of showers 34 which can be selectively activated even independently of each other to cool the strip P.

[0110] Furthermore, at exit from the showers 34 there are disposed two winding reels 36, 38 to wind the strip P into coils for its subsequent storage and shipment.

[0111] The solution according to the present invention, thanks to the increase in the temperature of the pre-finished rolled product 52, induced by the rapid heating device 28, allows the finishing stands 31 to carry out greater thickness reductions than in the prior art while guaranteeing the outlet temperature from the last finishing stand of at least 830? C.

[0112] The fact of being able to make large thickness reductions in the continuous rolling train 25 also allows to equip the plant 10 with a single reversible roughing stand 23, considerably reducing the total cost of the plant.

[0113] Another advantage of using only one reversible stand 23 which supplies an intermediate rolled product having a thickness comprised in a range from about 45 mm to about 80 mm, consists in being able to limit the distance between the reversible stand 23 and the continuous rolling train 25, with consequent reduced temperature losses, reduced formation of surface scale and reduction of the overall length of the plant 10. In fact, the plant 10 of the present invention can be a hundred meters shorter than the plants of the prior art, with the same annual production, for example, comprised between about 3 and about 6 million tons per year (Mtpy).

[0114] In addition, thanks to the heating supplied by the rapid heating device 28 in correspondence with the continuous rolling train 25, it is possible to limit the heating of the starting slabs 50 in the gas furnace 16 to a temperature of only 1100/1150-1200? C., with the advantage of consuming less combustible gas and limiting emissions compared to known plants.

[0115] Furthermore, since the slab 50 is heated to a lower temperature than in the prior art, its residence time in the gas furnace 16 will also be shorter than what provided in known plants. This advantageously reduces the production of scale by 25-30% and, therefore, the losses of scale material are reduced by 25-30%, consequently increasing the yield of the furnace 16 itself.

[0116] The present invention also concerns a method for producing a strip P, wound to form a coil, starting from slabs 50 having a starting thickness comprised between about 150 mm and about 350 mm.

[0117] The method provides to heat at least one slab 50 in the gas heating furnace 16 to a temperature of 1100/1150-1200? C. and then feed the latter toward the first descaling device 20.

[0118] Subsequently, the slab 50 is fed toward the edging stand 21 and then toward the reversible roughing stand 23 in correspondence with which it is subjected to some rolling passes that reduce its thickness until the intermediate rolled product 51 is obtained, having a thickness comprised in a range from about 45 mm to about 80 mm. Preferably, the number of rolling passes performed by the reversible roughing stand 23 does not exceed five.

[0119] This reduces the temperature loss of the intermediate rolled product 51 and limits its length, while also reducing the temperature difference between its head and tail.

[0120] Then, the intermediate rolled product 51 is transported to the second descaling device 24, where it is subjected to surface descaling and, subsequently, fed toward the continuous rolling train 25.

[0121] The intermediate rolled product 51 then enters the pre-finishing stands 26, in which it is further reduced in thickness until the pre-finished rolled product 52 is defined, with a thickness comprised between about 10 mm and about 50 mm.

[0122] Thus transformed dimensionally, the pre-finished rolled product 52 is cropped head and tail by the shear 27 and enters the rapid heating device 28 in which it is heated to a temperature such that the final strip P will be at an optimum temperature of at least 830? C. in correspondence with the outlet of the last finishing stand even in the case of thin strips with a thickness comprised between 0.9 mm and 1.2 mm.

[0123] In some embodiments, the heating supplied by the rapid heating device 28 is variable, heating the head of the pre-finished rolled product 52 to a determinate temperature and then increasing the heating supplied thereto in a substantially linear manner so that the body and tail of the final strip P can also exit from the last finishing stand 31 of the continuous rolling train 25 at the optimum temperature of at least 830? C.

[0124] Thanks to the heating supplied by the rapid heating device, it is possible to reduce the value of the rolling mass flow MF.sub.L required to obtain the optimum temperature of at least 830? C., for example comprised between 830? C. and 900? C., at the outlet of the last finishing stand.

[0125] By way of a purely non-limiting example, we refer to the graph in FIG. 6 which shows the final thickness S.sub.F of the final strip P on the abscissas and the rolling speed V.sub.L on the ordinate. Curve A represents the trend of the required rolling mass flow MF.sub.L as a function of the final thickness S.sub.F without the heat input of the rapid heating device 28. Curve B represents the trend of the required rolling mass flow MF.sub.L as a function of the final thickness S.sub.F with the heat input of the rapid heating device 28.

[0126] As evident, the mass flow relating to curve B is lower than the mass flow relating to curve A. In fact, for the same final thickness S.sub.F of the final strip P, the mass flow relating to curve B corresponds to a lower rolling speed V.sub.L than that corresponding to the mass flow relating to curve A.

[0127] The reduction of the rolling mass flow MF.sub.L allows both to carry out the rolling with a reduced rolling speed V.sub.L, preferably lower than 12 m/s, and at the same time to reach the optimum temperature of at least 830? C. at the outlet of the continuous rolling train 25 even for the tail of the final strip P, eliminating the need for the speed up as a tool for reaching the target temperature. An example of this embodiment is schematized graphically in FIG. 7.

[0128] Advantageously, in the absence of speed-up, the rolling speed V.sub.L in the finishing stands 31 is substantially constant and allows both to maintain the temperature of the final strip P constant between its head and tail, 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.

[0129] 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.

[0130] 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.

[0131] 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. 8.

[0132] Furthermore, according to other embodiments, the speed-up can be implemented in combination with the rapid heating device 28, for example to limit the latter's electrical consumption.

[0133] For relatively thick final thicknesses, the speed-up can be implemented keeping the rapid heating device switched off to completely eliminate the latter's electrical consumption.

[0134] It is clear that modifications and/or additions of parts may be made to the plant 10 and to the method for producing flat rolled products as described heretofore, without departing from the field and scope of the present invention, as defined by the claims.

[0135] 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 and plant 10 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.

[0136] In the following claims, the sole purpose of the references in brackets is to facilitate their reading and they must not be considered as restrictive factors with regard to the field of protection defined by the same claims.