METAL STRIP PRODUCTION PROCESS AND PLANT

Description

Field of the invention

[0001] This invention refers to a process and related plant for the production of metal strip using thin slabs produced by a mold.

[0002] The most relevant state of the art for the subject- matter of claim 1 and claim 5 is given in document EP- A 0 286 862. This document discloses a process and an installation for manufacturing a steel strip with a thickness of 2 to 25 mm. A steel strand with a thickness of 40 to 50 mm is cast in an oscillating open die at a rate of 5 to 20 m/min. The steel strand emerging from the open die, which is not yet completely solidified, is compressed to such an extent that the inner walls of the already solidified strand shell weld together. After the cooling of the steel strand, the thickness of which has been reduced in this way, to 1000 to 1200 DEG C, the strip is rolled out in at least one pass with a degree of deformation of 5 to 85%.

State of the art

[0003] According to the state the art, numerous types of systems are known for the production of metal strip, in particular of steel strip. Such plants envisage the use of molds to produce a slab that undergoes a set of reductions of its thickness, combined in some cases with other types of machining and surface treatments according to the various types of processes used.

[0004] A strip production process, for example, envisages first of all prerolling of the slab with the core still liquid, that is to say the so-called soft reduction, immediately at the outfeed of the mold. The slab, whose size and thickness are already close to that of a strip, is then deviated from a transport trajectory towards a horizontal trajectory along which it passes through a set of successive millstands to bring its thickness to the desired value, for example such as to permit winding in rolls. As the slab at the outfeed of a mold is still very thick, in relation to the final thickness to be obtained at the end of the production line, numerous millstands are required. As passing through each stand with the related thickness reduction causes a drop in temperature of the strip, a heating furnace is always present, ahead of the mill train, that increases the temperature of the slab so that, along the train, the temperature of the material never drops below the recrystallization point Ar₃, and so that rolling always takes place in the austenitic range. Plants of the known art also envisage inter-stand inductors to heat the strip being rolled, also to guarantee that austenitic steel is obtained. However, in this way, the metal strip production plants tend to be very long because the slabs produced by the known type molds, also called crystallizers, are very thick at the outfeed of the mold; as a result the plant must comprise numerous intermediate devices that considerably increase its total length.

[0005] Considerable efforts have been dedicated to constructing molds able to produce thinner slabs in order to reduce the number of thickness reduction millstands along the strip production line with the advantage of reducing the overall length of the line and strip production energy costs.

[0006] At the moment, in known type strip production plants, the minimum outfeed thickness of the slab that can be produced, in the case of more higher performance devices, is between 50 and 60 mm.

[0007] There are no known molds able to produce slabs with a thickness of less than 50 mm in a condition such as to permit soft reduction at the outfeed because, with such thickness values, the slab would leave the mold with the core completely solid. As a result, a slab with a solidified core cannot be soft reduced which is an essential condition to improve quality, in particular internal quality, of the strip produced.

[0008] Italian patent document UD93A000083 describes, for example, a device and the related process of controlled prerolling with liquid core of slabs exiting a mold that contributed to improving the structural qualities of the strip. However, such device is able to preroll slabs of a thickness such as to still require a global production plant of big dimensions. Also, on this plant, the mold used does not make it possible to work at sufficiently high casting speeds and, consequently, does not make it possible, without avoiding the creation of defects, high level rates of flow of liquid steel from the discharge nozzle.

[0009] Therefore, there is a need to construct a metal strip production plant and related process using extra-thin slabs able to overcome the aforesaid drawbacks.

Summary of the invention

[0010] The main aim of this invention is to construct an ultra-compact plant with related process for continuous production of hot-rolled steel strip from extra-thin slabs obtained through high speed continuous casting.

[0011] Another purpose of the invention is that of obtaining a hot-rolled strip, with a thickness of between 1,5 to 5 mm, having a fine grain structure distributed so regularly as to already possess the characteristics of a high quality cold-rolled material.

[0012] A further purpose is to construct a high throughput endless plant that reduces investments and operating costs compared with a conventional plant to produce the same strip thickness values.

[0013] The slabs, produced using an innovative mold, are of very reduced thickness, between 25 and 32 mm, preferably 28 mm and are characterized at the same time by a still liquid core on exiting from the mold, such as to permit advantageous further reduction of thickness to 22 mm.

[0014] Advantageously, accurate dimensioning of the parts of the casting chamber of a mold is such as to guarantee a suitable space so that the jets of liquid steel from the discharge nozzle do not give rise to the undesirable phenomenon of remelting of the skin that forms inside its inner surface, especially at a distance from the discharge nozzle where the area of the section of the jet is higher.

[0015] Therefore, this invention proposes to solve the foregoing problems and to achieve the aforesaid purposes, defining a hot-rolled metal strip production process wherein there is provided a mold, a soft reduction device located close to the outfeed section of a mold, a first rolling device, a path rerouting device which can be activated at least during predetermined periods of time, at least a horizontal pulling device, a descaling device and one or more rolling stands arranged in a compact group, the process including the following stages:

a) casting a thin slab with exit from a mold at a speed of between 6 and 16 m/min, having the narrow sides.between.25 and 32 mm, and a core in which the steel is liquid,

b) soft-reducing the slab using said soft reduction device,

c) performing a first rolling operation on the solidified slab to obtain a prestrip using such first rolling device,

d) performing an operation of scale removal on the prestrip using said descaling device,

performing a plurality of further rolling operations using said one or more millstands on the prestrip, obtaining at the end of these a strip with a thickness of between 1,5 and 5 mm.

[0016] Such process is applied, according to another aspect of this invention, using a hot-rolled metal strip production plant that, according to claim 5, includes a mold, a soft reduction device located close to the outfeed section of a mold, a first rolling device suitable to produce a prestrip, a path rerouting device which can be activated at least during pre-established periods of time from a vertical trajectory to a horizontal trajectory, at least a horizontal pulling device, a descaling device and one or more rolling stands arranged in a compact group,
wherein the plane of the infeed section of the mold is arranged at a height from the horizontal trajectory of less than 7 in and a first one of the one or more rolling stands (20', 20", 20''') is arranged at a distance less than 6 m from the vertical extrados plane of the mold.

[0017] Due to the particular mold used, an ultra-thin slab is cast of a thickness that is much lower than that which can be obtained with known molds, in the range 25 and 32 mm, as indicated above, and in which the core remains liquid also in the zones closest to the side parts.

[0018] Therefore, the entire slab can subsequently undergo soft reduction obtaining, advantageously, refining of the structure of the core that is solidifying, with reduction of internal porosity and elimination of the phenomenon of central segregation.

[0019] With the process and plant according to this invention:

• closing of the liquid cone, i.e. reaching of the so-called "kissing point", takes place at a short distance below the mold due to the very reduced thickness of the slab cast; as a result, soft reduction is relatively short with a consequent saving of space;
• immediately on exiting the soft reduction section, a first solidified core or hard reduction is performed as thickness is reduced with the product still rather hot, low rolling forces are required, with simplification of the millstand, for example a tandem stand, with reduction of costs and also of dimensions; advantageously, such stand also performs the function of puller;
• following hard reduction, the product is in hybrid slab/strip form but is in any case very thin and still at a high enough temperature to endow the product with sufficient ductility that it can form a loop when passing from the vertical direction, of casting, to the horizontal direction of rolling.

[0020] According to the above, first of all the height of the casting plane is reduced in relation to the horizontal plane of rolling, with a value of less than 7 m, and also the total length of the plant with therefore reduced impact on civil works, such as height of the shed, foundations, etc. This promotes lower investment and operating costs in relation to a known art plant.

[0021] Advantageously, soft reduction is carried out in a controlled manner so as to permit suitable management of the closing point of the liquid cone also during transitory phases tied to variations that may take place in casting parameters in relation to on-stream working conditions. This guarantees constantly excellent quality of the slab in all working conditions.

[0022] The liquid cone is closed advantageously in the section between the outfeed section from the mold and the center distance between the first rollers underneath; rolling is therefore performed on the completely solidified product, downstream of or at the limit position of the "kissing point". Such first rolling provides a further contribution to quality, in particular internal, of the product as it closes the interdendritic paths between grains through compacting of the structure. Advantageously, the first rolls also act as pinch rolls for the thin slab.

[0023] The high speed of product casting in the form of ultra-thin slab and direct connection with the mill train, which transform it from slab to strip starting from a lower thickness than that of known casting processes and also the compact design of the plant guarantee that the product is always in the austenitic range during hot rolling.

[0024] Advantageously, compact arrangement of the millstands does not require the presence of a heating furnace ahead of the mill train and of inductors located in an intermediate position between the millstands in order to raise the temperature of the product processed. In this way, at the end of the first part of the production line, the strip has been reduced to a thickness of between 1,5 and 3 mm in less space, with reduced consumption of energy and lower plant costs in that fewer millstands are required and installation of heating furnaces is also avoided.

[0025] Lastly, another advantage lies in the fact that the strip obtained at the end of the entire casting line is already of final thickness, permitting use without the need for further treatment on a cold-rolling lines.

[0026] The dependent claims describe the preferred embodiments of the invention.

Short description of the figures

[0027] Further features and advantages of the invention will become apparent by means of the detailed description of preferred embodiments, given by way of not limitative examples of a metal-strip production plant, illustrated with the aid of the following figures wherein:

Fig. 1 shows a longitudinal section of the plant of the invention;

Fig. 2 shows a 3D view of a first embodiment of a component of the plant of Fig. 1;

Fig. 3a shows a section along the plane y-z of the component of Fig. 2;

Fig. 3b shows a plan view of the component of Fig. 2;

Fig. 4a shows a section along the plane y-z of another embodiment of the component of Fig. 2;

Fig. 4b shows a plan view of the embodiment of the component of Fig. 4a;

Fig. 5 shows an enlargement of a detail of the component of Fig. 2;

Fig. 6 shows a 3D view of another embodiment of a component of the plant of the invention;

Fig. 7a shows a section along the plane y-z of the component of Fig. 6;

Fig. 7b shows a plan view of the component of Fig. 6;

Fig. 8a shows a section along the plane y-z of an embodiment of the component of

Fig. 6;

Fig. 8b shows a plan view of the embodiment of the component of Fig. 8a;

Fig. 9 shows an enlargement of a detail of the plan view of Fig. 7b.

Detailed description of preferred embodiments of the invention

[0028] Referring to Fig. 1, a metal strip production plant including the following is shown:

• a mold 15 for the production of extra-thin slabs with liquid core,
• a soft reduction device 16, located close to the outfeed section of a mold,
• a first rolling device 17 able to produce a prestrip, that also acts as slab puller;
• a rerouting device 18, 18' of the not prerolled slab from a vertical trajectory to a horizontal trajectory,
• two horizontal pinch rolls 22' and 22",
• a scale remover 19,
• a compact group of one or more millstands 20', 20", 20"', advantageously in the number of three.

[0029] Advantageously, the mold 15 produces a very thin slab, with thickness of the narrow walls of between 25 and 32 mm, with a central bulge and a core in which the steel is still liquid.

[0030] With reference to Figures 2 to 5, which defines a system of orthogonal reference axes x, y, z, a first embodiment of a mold is shown, with a longitudinal plane of symmetry X parallel to the reference axes x-z, and a transversal plane of symmetry Y parallel to the reference axes y-z, including two wide plates 1, 2 that, coupled together, define a through cavity 3 or casting chamber.

[0031] The surface of such cavity is defined by two wide walls 4, 5, reciprocally arranged so that they face onto opposite sides in relation to the plane of symmetry X, and by two narrow walls 6, 7 generally flat, rectangular and parallel, arranged horizontally to the plane of symmetry X.

[0032] The wide and narrow walls form four longitudinal edges in the zones where they are joined.

[0033] The wide walls 4, 5 are concave with the concave area facing towards the longitudinal plane of symmetry X. The concave area of the walls 4, 5 is such as to define a lenticular shaped section of the casting chamber.

[0034] Advantageously, considering a section transversal and orthogonal to the plane X, as illustrated partially in Fig. 5, the circumference arc tangent of the wide walls 4, 5 at the point where they meet the respective corner form, with a plane parallel to the plane of symmetry X, an angle ? with a value of between around 1 °and 19°.

[0035] This range of values of the angle ? guarantees low stress on the skin and limited deformation in the zone of the narrow walls with minimum possibilities of tearing of the skin with consequent leakage of the liquid steel, known as "break-out" phenomenon. An elliptical rather than lenticular section would result in higher radii of curvature and therefore higher angles.

[0036] In the mold according to the invention, defining as "I" the distance between the narrow walls and defining as "f" the deflection of the concave wide walls 4, 5 of the casting chamber, the following values are advantageously obtained at the infeed section:

• "I" is between 750 and 2000 mm;
• "f" is between 15 and 100 mm.

[0037] Advantageously, such mold, or lenticular crystallizer, ensures that the corners are not affected by excessive pivoting during prerolling, avoiding the appearance of cracks or other defects in these zones.

[0038] The lenticular shape makes it possible to maintain the core liquid also very close to the end sides of the slab and therefore permits subsequent soft reduction of the entire slab cast.

[0039] Also, the volume of liquid steel in the mold is sufficient to dampen the fluid-dynamic turbulence generated by the discharge nozzle.

[0040] A first advantageous variant of this first embodiment of the mold provides a steel infeed section larger than the outfeed section close to the lower end of the mold. In particular, the radius of curvature of the circumference arc on the transversal plane has a value that increases in linear fashion between the infeed section and a predefined intermediate section that corresponds to the terminal section of the casting chamber, before the outfeed section of the steel.

[0041] This guarantees a suitable distance of the discharge nozzle submerged by the wide walls that avoid occurrence of undesirable solidification bridges.

[0042] In this case, such cavity or casting chamber 3 passes through in a longitudinal direction with generating lines converging towards the exit. In the section illustrated in Fig. 3a , a cone-shaped narrowing is visible.

[0043] Advantageously, such mold envisages a slope of the wide walls of such casting chamber 3, converging towards such intermediate section defined by an angle a of between 0°and 7° such angle a being measured as a slope in relation to the plane X and in projection on the plane of symmetry Y.

[0044] Close to the lower end of the mold, there is a section 10 with parallel generating lines, of predetermined length and in any case shaped like the casting chamber, to permit insertion and extraction of the dummy bar used to start the continuous casting process from inside this.

[0045] On the other hand, a second variant of the first embodiment of the mold envisages infeed and exit sections of the mold of the same size, as illustrated in the section of Fig. 4a. In this case, the radius of curvature of the circumference arc on the transversal plane has the same value at each point of the longitudinal extension of the cavity between the infeed section and outfeed section of the steel.

[0046] Referring to Figures 6 to 9, in which a set of orthogonal reference axes x, y and z is also defined, a second embodiment of the mold of the plant of the invention is represented, also with a longitudinal plane of symmetry X parallel to the reference axes x-z, and a transversal plane of symmetry Y parallel to the reference axes y-z.

[0047] This mold has a casting chamber or through cavity 3' whose surface is defined by two plates 1', 2' with wide walls 4', 5', arranged reciprocally as facing on opposite sides in relation to the plane of symmetry X and by two elements or narrow walls 6', 7' reciprocally parallel and placed between such wide walls, and arranged orthogonally to the plane of symmetry X.

[0048] Also in this case, the walls 4', 5', 6', 7' form four longitudinal corners in the zone where they are joined.

[0049] The wide walls 4', 5' of the plates 1', 2' are characterized, unlike the first embodiment, by a double curve, one concave and one convex, facing towards the longitudinal plane of symmetry X.

[0050] Advantageously, considering a cross-section orthogonal to the plane X, as partially illustrated in Fig. 9, each of the wide walls 4', 5' includes:

• a central concave zone 11 with the concave area facing towards the plane of symmetry X, having the shape of a circumference arc,
• two flat side zones 12, parallel to the plane of symmetry X, each at one end of the central concave zone 11,
• two connection zones 13, having convex surfaces, between the circumference arc of the central zone 11 and the side zones 12, with the convexity facing towards the plane of symmetry X.

[0051] Referring to the left half of the semi-section, the following points are identified in Fig. 9:

• the point P is the point of intersection of the straight section R of the flat zone of the walls with convex joining section S;
• the point P' is the point at which the concave area of the wide plate 2' also known as point of inflection changes;
• the point P" is the point of intersection of the concave zone 11 with the transversal plane of symmetry Y parallel to the reference axes y and z.

[0052] At this point, the following values are defined:

• length "d": is the distance between the point P and the point P' measured in projection on the plane of symmetry X parallel to the reference axes x and z;
• length "I": is the distance between the point P and its symmetrical in relation to the plane Y measured in projection on the plane of symmetry X; "I/2" is therefore the distance between the point P and the point P", of maximum concavity, measured in projection on the plane of symmetry X;
• height "h": is the distance between the point P and the point P' measured in projection on the plane of symmetry Y;
• height "f": is the distance between the point P and the point P" measured in projection on the plane of symmetry Y; such value is also called deflection or lateral semi-widening.

[0053] Advantageously, this mold has the following values at the infeed section:

• I is between 750 and 1500 mm;
• f is between 15 and 100 mm.

[0054] Furthermore, also in the infeed section, the relationship d/I and the relationship h/f are between 10% and 60%.

[0055] A first advantageous variant of this second embodiment of the mold provides a casting chamber 3' with an infeed section of the steel larger than the outfeed section. In particular, the value of the radius of curvature of the concave zone 11, when this has the shape a circumference arc, on the transversal plane, increases linearily between the infeed section and an intermediate section that corresponds to the terminal section of the casting chamber, before the outfeed section of the steel; the value of the radius of curvature of the connection zone 13 decreases linearily between the infeed section and such intermediate section.

[0056] In this case such cavity or casting chamber 3' passes through in a longitudinal direction with generating lines converging towards the exit as far as the intermediate section. In the section illustrated in Fig. 7a, a cone-shaped narrowing is visible.

[0057] In this variant, deflection f has a maximum value in the infeed section of the mold and a minimum value at the outfeed section of the casting chamber.

[0058] Advantageously, such mold envisages a slope of the wide walls of such chamber converging towards such intermediate section, defined by an angle a of between 0° and 7° such angle a being measured as a slope in relation to the plane X and in projection on the plane of symmetry Y.

[0059] Furthermore, this particular configuration of the casting chamber allows the liquid steel to come into contact with the side zones 12 with flat faces, as far as the edges, and therefore prevents complete solidification of the slab in such zone despite a lower distance between such faces compared with that of the known art.

[0060] Below such intermediate section of the mold, there is a final section 10', not sloping and with parallel generating lines of pre-established length that is geometrically the same as such intermediate section and permits insertion and extraction of the shaped head of the dummy bar used to start casting.

[0061] Also, the reduced slope of the wide walls 4', 5' of the casting chamber defined by the angle a makes it possible to avoid the undesirable phenomenon of remelting of the skin formed as the liquid steel poured into the chamber by the discharge nozzle does not cause turbulence in the zone of feeding of the mold close to the walls, guaranteeing optimal flow of the discharged steel.

[0062] The narrow walls 6', 7', preferably flat and rectangular, are advantageously mobile and able to move close to or retract from each other permitting adjustment of the width of the slab. They can also modify their taper that is to say more precisely, they can be more inclined towards the inside close to the outfeed section of the steel so as to reduce the width of the outfeed section, making it possible in this way to avoid problems tied to shrinkage during solidification in the mold.

[0063] A second variant of the second embodiment of the mold of the plant of the invention envisages, on the other hand, infeed and exit sections of the same size, as illustrated in the section of Fig. 8a. In this case, the radii of curvature maintain the same value at each point of the longitudinal extension of the casting chamber between the infeed section and the outfeed section of the steel. In this variant, the aforesaid intermediate section coincides with the outfeed section of the steel.

[0064] It is particularly important that the geometry of the mold, in any of the aforementioned four variants, guarantees a suitable volume of liquid steel in the mold so that, at the outfeed from this, the slab still has an appropriately-sized liquid core. The particular shape makes it possible to maintain the core liquid also very close to the end sides of the slab. In this way, soft reduction will be carried out successfully. Advantageously, the presence of liquid steel in the side zones and the fact that in such narrow zones complete solidification has not occurred prevents occurrence of undesirable cracks and guarantees effective melting of the lubrication powders.

[0065] Using one of these molds, a thin slab with a thickness of between 20 and 50 mm is cast at a speed of between 6 and 16 m/min.

[0066] Close to the outfeed section of the mold, there is a prerolling device 16 including a rollerbed or set of idle transversal rollers 16' shaped so as to modify the transit section of the slab and to perform a gradual action of flattening of the convex or bulging surface, as on exiting from the crystallizer, so as to bring the slab to a rectangular section. Such action of recovery of the swelling involves compression of the slab with liquid core until a thickness equal to the width of the narrow sides of the outfeed section of the mold is reached.

[0067] Advantageously, such rollers 16' can be located at a closer distance so as to obtain, on exit from the rollerbed, a slab of lower thickness compared with that exiting from the crystallizer, and also linearized: basically a reduction of thickness is carried out on a slab that still has a liquid core, i.e. soft reduction. According to the invention, at the end of soft reduction, the slab has been reduced to a thickness of between 10 and 25 mm.

[0068] Advantageously, soft reduction acts in a controlled manner (dynamic soft reduction) so as to permit correct control of the closing point of the liquid cone also during transitory periods tied to possible variations in the casting parameters in relation to on-stream working conditions. This guarantees constantly excellent quality of the slab in all working conditions.

[0069] The set of rollers 16' cooperates with an integrated direct cooling system and also performs the function of containing and guiding the slab.

[0070] Immediately downstream of set of rollers, there is a first rolling device 17 including two cylinders 17', 17" that perform the double function of extracting the slab from the mold and of further reducing its thickness applying a suitable flattening force on this. More particularly, such cylinders 17', 17" flatten the slab downstream or, at the most, at the position of the closing point of the liquid cone, also known as kissing point; in this way the cylinders 17', 17" perform their action on the completely solidified slab and then performing an effective rolling operation also called hard reduction.

[0071] In accordance with the above, at the end of the hard reduction process, the slab exits with a thickness of between 5 and 24 mm thereby defining a product, such prestrip, very close to the final thickness of the strip to be produced. Such rolling provides a further contribution to quality, in particular internal, of the product as it closes the interdendritic paths between grain and grain through compacting of the structure.

[0072] In order also to manage the start-up phase of the casting process, which is a transitory phase, a rerouting device 18 is provided that includes a curved opening rollerbed located immediately below the two cylinders 17', 17". The curved rollerbed 18, 18' is also necessary to permit guidance and insertion of the head of the dummy bar inside the mold. When casting is started, the slab pulled by the dummy bar does not have a liquid core so its thickness cannot be reduced through soft reduction and the two cylinders 17', 17" do not exert their action. Therefore, the first section of slab cast has a thickness equal to that on exit from mold along the entire line as far as the entrance of the millstand 20', and in this first short phase is defined by convention as "thick" slab. The curved rollerbed 18, 18' is scaled so that it can apply sufficient force to bend the "thick" slab. The guide rollers 23 of the curved rollerbed 18, 18' are idle and the support of the rollerbed is maintained in the active position by suitable hydraulic jacks 21, 21'. Both the lower part 18 and the upper part 18' of the rollerbed are hinged so that they can be rotated when it is necessary to free the trajectory followed by the prestrip in steady state operating conditions and when, in emergency conditions such as for example jamming, it is necessary to unload all the material in the pit. The positions of the rollerbed 18, 18' free of the strip are shown with thin lines in figure 1.

[0073] As mentioned above, at the end of the transitory phase of start-up, casting is carried out at duty cycle and the two parts 18, 18' of the rollerbed are in the open position allowing the prestrip to form a loop of variable length (not illustrated), i.e. a path that first of all descends and then returns up and ascends before following the horizontal path. The presence along the casting line of a loop permits various advantages:

a) that of decoupling the rolling train from casting and of managing any changes in speed between the millstands and the mold;

b) that of reducing cooling of the prestrip as there is less heat exchange between the support and guide rollers.

[0074] Along the path followed by the strip, after the curve following the rollerbed, there are two pinch rolls 22' and 22". The function of the pair of pinch rolls 22', 22" is to pull the "thick" slab in the initial transitory phase and the prestrip after start-up. Pinch roll 22', arranged further ahead, has the roller 25 of larger diameter in order to:

a) straighten the point of the "thick" slab in the starting phase and

b) provide a suitable supporting surface for the loop formed by the prestrip during the on-stream process.

[0075] Downstream of the descaling device 19, there are three millstands, for example of the four type, of limited pitch, that is to say having a center distance of less than 1600 mm, forming a compact group. Ahead of the first stand, there is a water type rotating scale remover 19. The fact that the millstands are close to each other makes it possible to reduce losses of heat of the material when passing from one stand to the next and to bring it out of the train at a temperature that is still above 850°C.

[0076] Therefore, the advantage inherent in this compact arrangement of the millstands 20', 20", 20'" is that of maintaining the temperature of the strip being rolled above the recrystallization point Ar₃, thus avoiding the need for heating furnaces ahead of the train. Furthermore, this is also made possible because losses of heat of the prestrip along the path are reduced.

[0077] Advantageously, the end millstands are fitted on rails 26 and can be moved using hydraulic jacks 27', 27" so as to open the compact group and to take action in the case of jamming of the strip. The scale remover is advantageously fitted on a carriage 28 so that it can be traversed in relation to the rolling line in order to create the space necessary to open the millstands.

[0078] With the particular arrangement of plant components, the plane of the infeed section of the mold is at a height in relation to the horizontal rolling line of less than 7 m, preferably between 4,0 and 6,5 m. The length of the casting machine, as far as the end of the curved deviation section, is therefore lower than in plants of the known art.

[0079] After the rolling operations the strip undergoes laminar cooling with water, is cut to size using shears and is wound in rolls of a weight of around 30 tons on at least one reel, preferably two.

[0080] With the plant according to this invention it is possible to obtain the finished product in a very reduced space. In fact, casting of very thin slabs using the mold 15, in its various embodiments, makes it possible to cast an initial product, that is to say the slab, at high speed and already with a thickness very close to that of the finished product, i.e. the strip. Advantageously, the thin slabs obtained have a thickness at the outfeed of between 25 and 32 mm, with a casting speed of between 6 and 16 m/min. All this promotes a considerable reduction in the number of machines of the plant involved in the plant/process which results in a considerable saving in initial investment and energy costs and of the waster used in the cooling systems.

[0081] A preferred embodiment of the plant of the invention is provided with a height of the casting plane in relation to the horizontal plane of the strip in the millstands 20', 20", 20'" equal to around 4,0 m. In particular, the distance between the plane of the infeed section of the mold and the centre distance of the cylinders 17', 17" of the first rolling device 17 is equal to around 2,6 m; and the distance between the height of the meniscus inside the mold and the closing point of the liquid cone, or kissing point, inside the slab during prerolling is around 2,8 m at a casting speed of 10 m/min and around 1,6 m at a speed of 8 m/min.

[0082] Advantageously, the first stand 20' of the millstands 20', 20", 20'" is arranged at a distance from the vertical extrados plane of the mold of less than 6 m, preferably 4,5 m, with a minimum distance between such first stand and the scale remover 19 equal to around 2,5 m. With such compactness of the plant, the prestrip arrives advantageously at the first stand 20' at high temperature, at least equal to 1170°C, and with a feed speed equal to the casting speed, without interruptions on the production line.

Claims

1. Hot-rolled metal strip production process wherein there is provided a mold (15), a soft reduction device (16) located close to the outfeed section of said mold, a first rolling device (17), a path rerouting device (18) which can be activated at least during predetermined periods of time, at least a horizontal pulling device (22', 22"), a descaling device (19) and one or more rolling stands (20', 20", 20"') arranged in a compact group, the process including the following stages:

a) casting a thin slab with exit from said mold (15) at a speed of between 6 and 16 m/min, having the narrow sides between 25 and 32 mm, and a core in which the steel is liquid,

b) soft reducing the slab using said soft reduction device (16),

c) performing a first rolling operation on the solidified slab to obtain a prestrip using said first rolling device (17),

d) performing an operation of scale removal on the prestrip using said descaling device (19),

e) performing a plurality of further rolling operations using said one or more millstands (20', 20", 20"') on the prestrip, obtaining at the end of these a strip with a thickness of between 1,5 and 5 mm.

2. Process according to claim 1, wherein a first of the further rolling operations starts at a distance less than a 6 m from the vertical extrados plane of the mold.

3. Process according to claim 2, wherein the rolling operations are performed using three millstands.

4. Process according to claim 3, wherein after the rolling operations the strip undergoes laminar cooling with water, is cut to size and is wound in rolls on at least one reel.

5. Hot-rolled metal strip production plant including a mold (15), a soft reduction device (16) located close to the outfeed section of a mold, a first rolling device (17) suitable to produce a prestrip, a path rerouting device (18) which can be activated at least during pre-established periods of time from a vertical trajectory to a horizontal trajectory, at least a horizontal pulling device, a descaling device (19) and one or more rolling stands (20', 20", 20"') arranged in a compact group, wherein the plane of the infeed section of the mold is arranged at a height from the horizontal trajectory of less than 7 m and a first one of said one or more rolling stands (20', 20", 20''') is arranged at a distance less than 6 m from the vertical extrados plane of said mold.

6. Production plant according to claim 5, wherein the soft reduction device (16) is in upright position.

7. Production plant according to claim 6, including a set of idle transversal rollers (16') shaped so as to modify the transit section of the slab and to perform a gradual flattening action of the surface of the slab, so as to obtain a rectangular section.

8. Production plant according to claim 7 wherein the rerouting device (18) has a radius of curvature of between 1 and 2 m.

9. Production plant according to claim 8 wherein the rerouting device includes an rollerbed (18', 18") suitable to be opened.

Ansprüche

1. Produktionsprozess für heiß gewalzte Metallbänder, wobei eine Form (15), eine Soft-Reduction- bzw. Weichreduktions-Vorrichtung (16), die nahe dem Abzugsabschnitt der Form angeordnet ist, eine erste Walzvorrichtung (17), eine Pfadumleitvorrichtung (18), die zumindest während vorbestimmter Zeitperioden aktiviert werden kann, zumindest eine horizontale Zugvorrichtung (22', 22"), eine Entzunderungsvorrichtung (19) und ein oder mehrere Walzgerüste (20', 20", 20"') vorgesehen sind, die in einer kompakten Gruppe angeordnet sind, wobei der Prozess die folgenden Stufen umfasst:

a) Gießen einer dünnen Bramme mit einem Austritt von der Form (15) mit einer Geschwindigkeit zwischen 6 und 16 m/min, deren schmale Seiten zwischen 25 und 32 mm liegen und die einen Kern aufweist, innerhalb dem der Stahl flüssig ist,

b) Weichreduzieren der Bramme unter Verwendung der Weichreduktionsvorrichtung (16),

c) Ausführen eines ersten Walzbetriebsablaufs an der erstarrten Bramme, um ein Vorband zu erhalten, unter Verwendung der ersten Walzvorrichtung (17),

d) Ausführen eines Betriebsablaufs zur Zunderentfernung an dem Vorband unter Verwendung der Entzunderungsvorrichtung (19),

e) Ausführen einer Mehrzahl weiterer Walzbetriebsabläufe unter Verwendung des einen oder der mehreren Walzwerke (20', 20", 20"') an dem Vorband, wobei am Ende von diesen ein Band mit einer Dicke zwischen 1,5 und 5 mm erhalten wird.

2. Prozess nach Anspruch 1,
wobei ein erster der weiteren Walzbetriebsabläufe unter einer Distanz von weniger als 6 m von der vertikalen Rückenebene der Form startet.

3. Prozess nach Anspruch 2,
wobei die Walzbetriebsabläufe unter Verwendung von drei Walzwerken ausgeführt werden.

4. Prozess nach Anspruch 3,
wobei nach den Walzbetriebsabläufen das Band einer laminaren Kühlung mit Wasser ausgesetzt wird, auf Größe geschnitten wird und in Rollen auf zumindest eine Bandspule gewickelt wird.

5. Produktionsanlage für heiß gewalzte Metallbänder, mit einer Form (15), einer Weichreduktionsvorrichtung (16), die nahe dem Abzugsabschnitt einer Form angeordnet ist, einer ersten Walzvorrichtung (17), die geeignet ist, ein Vorband zu produzieren, einer Pfadumleitvorrichtung (18), die zumindest während vorher festgesetzter Zeitperioden von einer vertikalen Trajektorie zu einer horizontalen Trajektorie aktiviert werden kann, zumindest einer horizontalen Zugvorrichtung, einer Entzunderungsvorrichtung (19) sowie einem oder mehreren Walzgerüsten (20', 20", 20"'), die in einer kompakten Gruppe angeordnet sind,
wobei die Ebene des Beschickungsabschnittes der Form auf einer Höhe von der horizontalen Trajektorie von weniger als 7 m angeordnet ist und ein erstes des einen oder der mehreren Walzgerüste (20', 20", 20"') unter einer Distanz von weniger als 6 m von der vertikalen Rückenebene der Form angeordnet ist.

6. Produktionsanlage nach Anspruch 5,
wobei sich die Weichreduktionsvorrichtung (16) in einer aufrechten Position befindet.

7. Produktionsanlage nach Anspruch 6,
mit einem Satz von nicht angetriebenen Transversalwalzen (16'), die so geformt sind, dass sie den Transitquerschnitt der Bramme modifizieren und eine allmähliche Abflachungswirkung der Oberfläche der Bramme ausführen, um so einen rechtwinkligen Querschnitt zu erhalten.

8. Produktionsanlage nach Anspruch 7,
wobei die Umleitvorrichtung (18) einen Krümmungsradius zwischen 1 und 2 m besitzt.

9. Produktionsanlage nach Anspruch 8,
wobei die Umleitvorrichtung ein Walzenbett (18', 18") aufweist, das zum Öffnen geeignet ist.

Revendications

1. Procédé de production d'une bande métallique laminée à chaud, dans lequel on fournit un moule (15), un dispositif de réduction douce (16) situé près de la section de sortie dudit moule, un premier dispositif de laminage (17), un dispositif de réacheminement de trajectoire (18) qui peut être activé au moins pendant des laps de temps prédéterminés, au moins un dispositif de traction horizontal (22', 22"), un dispositif de décalaminage (19) et un ou plusieurs supports de laminage (20', 20", 20"') disposés selon un groupe compact, le procédé comprenant les étapes suivantes :

a) le coulage d'une plaque fine qui sort dudit moule (15) à une vitesse comprise entre 6 et 16 m/min, dont les côtés étroits mesurent entre 25 et 32 mm, et un noyau dans lequel l'acier est liquide,

b) la réduction douce de la plaque, en utilisant ledit dispositif de réduction douce (16),

c) la réalisation d'une première opération de laminage sur la plaque solidifiée pour obtenir une pré-bande en utilisant ledit premier dispositif de laminage (17),

d) la réalisation d'une opération de décalaminage sur la pré-bande en utilisant ledit dispositif de décalaminage (19),

e) la réalisation d'une pluralité d'autres opérations de laminage en utilisant lesdits un ou plusieurs cages de laminoirs (20', 20", 20''') sur la pré-bande, en obtenant à la fin de celles-ci une bande ayant une épaisseur comprise entre 1,5 et 5 mm.

2. Procédé selon la revendication 1, dans lequel une première opération parmi les opérations de laminage ultérieures commence à moins de 6 m du plan extrados vertical du moule.

3. Procédé selon la revendication 2, dans lequel les opérations de laminage sont réalisées en utilisant trois cages de laminoir.

4. Procédé selon la revendication 3, dans lequel, après les opérations de laminage, la bande subit un refroidissement laminaire avec de l'eau, est coupée à la taille et est enroulée en rouleaux sur au moins une bobine.

5. Système de production de bande métallique laminée à chaud comprenant un moule (15), un dispositif de réduction douce (16) situé près de la section de sortie d'un moule, un premier dispositif de laminage (17) adapté pour produire une pré-bande, un dispositif de réacheminement de trajectoire (18) qui peut être activé au moins pendant des laps de temps préétablis à partir d'une trajectoire verticale vers une trajectoire horizontale, au moins un dispositif de traction horizontal, un dispositif de décalaminage (19) et un ou plusieurs supports de laminage (20', 20", 20''') disposés dans un groupe compact, dans lequel le plan de la section d'entrée du moule est disposé à une hauteur par rapport à la trajectoire horizontale de moins de 7 m et un premier desdits supports de laminage (20', 20", 20"') est disposé à une distance inférieure à 6 m du plan extrados vertical dudit moule.

6. Système de production selon la revendication 5, dans lequel le dispositif de réduction douce (16) est dans une position verticale.

7. Système de production selon la revendication 6, comprenant un ensemble de rouleaux transversaux libres (16') façonnés de façon à modifier la section de transit de la plaque et à réaliser une action d'aplatissement progressive de la surface de la plaque, de façon à obtenir une section rectangulaire.

8. Système de production selon la revendication 7, dans lequel le dispositif de réacheminement (18) a un rayon de courbure entre 1 et 2 m.

9. Système de production selon la revendication 8, dans lequel le dispositif de réacheminement comprend un chemin de roulement (18', 18 ") adapté pour être ouvert.

Drawing