Apparatus and method for the continuous casting of a long metal product

Abstract

Method for the continuous casting of a metal product (P) comprising the following steps: constantly discharging a determinate amount of liquid metal inside a crystallizer (12) until a determinate upper level (M) is reached, distributing a casting powder on top of said upper level (M), and determining an oscillation of said crystallizer (12) on the basis of oscillation parameters that depend on a frequency (f) and a stroke (H).

Description

[0001] “APPARATUS AND METHOD FOR THE CONTINUOUS CASTING OF A LONG METAL PRODUCT”

[0002] FIELD OF THE INVENTION The present invention concerns an apparatus for the continuous casting of long metal products at high speed, that is, higher than approximately 5.5 m / min, such as billets, blooms or suchlike, and the corresponding casting method in which casting powders having specific chemical-physical characteristics are used.

[0003] This apparatus and method are particularly suitable for casting and rolling processes based on the endless mode, that is, without interruptions between casting and rolling, but can also be used for other casting and rolling modes, such as billet- to-billet or semi-endless for example.

[0004] BACKGROUND OF THE INVENTION

[0005] It is known that in continuous casting apparatuses the core of the casting machine consists of the crystallizer into which the liquid metal is introduced so as to be progressively solidified with the formation of a solid external shell. Known continuous casting apparatuses are characterized by being intended to operate on long products or on flat products.

[0006] The type of products, long or flat, for which casting apparatuses are designed determines very different choices and solutions in terms of crystallizer geometry, speed parameters, lubricant choices, oscillation values, and more.

[0007] So-called long products can be classified, according to their sizes, into categories of products called billets or blooms, which are then rolled to create products such as bars, wire or wire rod. Long products are mainly used in structural, building and mechanical applications. On the contrary, flat products, such as thin slabs, are used in the automotive industry, in the production of household appliances and in other applications that require flat and uniform surfaces. From the point of view of quality and desired performance, these types of products have different and non- equivalent characteristics.

[0008] The main characteristic that distinguishes long products from flat ones (that is, slabs, from which strip, sheets and plates are produced), such as to identify two very distinct categories of products in terms of technology, processes and equipment, is the substantial dimensional similarity between the product’s sides.

[0009] Long products have relatively small and long cross-sections, managing the rapid and uniform solidification of the metal along the entire length of the product. These sections have sides that do not excessively diverge from each other, thus giving rise to shapes such as squares or rectangles, with a ratio of wide sides to narrow sides of less than 3: 1. Possible sections also include rounds, octagons, polygons in general and similar figures.

[0010] Conversely, flat products are characterized by the high dimensional difference between wide and narrow sides, which entails an important redefinition of the casting machine and the process, mainly because the cooling between wide and narrow sides is very different. The thin slab crystallizer has a wider and flatter cross-section, guaranteeing a uniform solidification over a wider surface, which is technically more complex due to the cooling differences between the wide and narrow sides. The technological challenges faced for the two types of products are also different. For long products, the main challenges include managing refractory wear, lubricating the crystallizer and preventing process interruptions. Casting speed is a critical factor in maintaining product quality and process efficiency.

[0011] For flat products, challenges include heat transfer control, surface crack prevention, and slag trapping. The management of the viscosity of the casting powder and the formation of an air layer between the slag film and the crystallizer are crucial for guaranteeing a uniform surface quality.

[0012] Operating parameters also differ between the two sectors. The casting speed for long products can be very high, but has to be balanced with the need to maintain uniform solidification and prevent structural defects. For flat products, the casting speed is generally lower, since more precise control of the cooling and the solidification is needed in order to prevent surface defects and guarantee a uniform product quality.

[0013] The crystallizer is defined by a tubular body, generally monolithic so as to be less subject to play and deformation, which is cooled by means of a forced circulation cooling fluid which indirectly subtracts heat from the liquid metal by means of the heat exchange that occurs between it and the walls of the crystallizer in contact with the cooling fluid. Through this heat exchange, the liquid metal begins to solidify externally, resulting in the formation of a surface skin that thickens as the product approaches the outlet of the crystallizer, determining the shape of the long product.

[0014] At the outlet of the crystallizer, the solidified external shell still contains liquid metal inside it, which progressively continues to solidify along the casting line.

[0015] The thickness of the skin is influenced by the casting speed, which determines the residence time of the metal in the crystallizer and therefore the duration of the heat exchange.

[0016] The casting speed for long products can be very high, but has to be balanced with the need to maintain uniform solidification and prevent structural defects. For flat products, the casting speed is generally lower, since more precise control of the cooling and of the solidification is needed in order to prevent surface defects and guarantee a uniform product quality. To promote the lubrication and heat exchange between the crystallizer and the cast product, it is known to distribute lubricating elements, such as oils or so-called casting powders with suitable chemical-physical characteristics, on the free surface of the liquid steel present in the crystallizer.

[0017] These chemical-physical characteristics - which have to guarantee, in particular, a desired melting speed of the powder, lubrication capacity and uniformity, thermal flow control as well as protection from product re-oxidation and inclusion capture - are mainly: chemical composition, viscosity, granulometry and density, melting and crystallization temperature.

[0018] The thickness of the skin and its regularity are directly correlated to the occurrence of defects on the product or, in the most serious cases, to the adhesion of the skin to the crystallizer’s walls which can lead to the breakage of the skin, with a consequent leakage of liquid metal that causes an interruption of the casting process (so-called break-out).

[0019] For high casting speeds, casting powders having a low viscosity, typically less than approximately 5 Poise, are known, which guarantee a high heat exchange between the solidified skin and the crystallizer’s wall.

[0020] A Poise (P) is the unit of measurement in the CGS system (Centimeter-Gram- Second System or Gauss System) of dynamic viscosity, corresponding in the International System to the Poiseuille (symbol Pl). The conversion is as follows. Poise: g 1 kg

[0021] 1 P - 1 — - - — - 0.1 Pa - s cm ■ s 10 in ■ s

[0022] Poiseuille: kg

[0023] 1 Pl - 1 Pa ■ s - 1 m • s These low- viscosity lubrication powders are known to be extremely aggressive toward the refractories used to cast steel in ingot moulds, leading to their accentuated wear, with the consequent need for frequent interruptions of the casting process and the replacement of the refractories themselves. Frequent interruptions are extremely critical to the economy of the production process, particularly in the case of the endless mode process.

[0024] It is also known that, at low casting speeds, very viscous powders are normally used that conveniently limit the heat exchange and prevent an excessive cooling of the skin itself, as in the field of casting flat products, which notoriously reach maximum casting speeds of 3-4 m / min. High viscosity powders, in fact, are normally less aggressive toward refractories, thus guaranteeing their use for a longer time and fewer process interruptions.

[0025] However, high viscosity powders are considered counterproductive in the formation of an adequate and uniform skin layer in products cast at high speeds. EP 1.027.944 concerns a powder used for the continuous casting of thin slabs that fall within the category of flat products, wherein the thickness, indicated in the order of 150 mm or less, is generally between 1 / 10 and 1 / 20 of the width. It is evident that the problems with cooling a product with such dimensional ratios compared to a square, polygonal, or even rectangular product with maximum ratios of the order of one third are completely different. The powder described in EP ’944 is characterized by an almost equal ratio of calcium oxide to silicon oxide, and by a rather high fluorine content.

[0026] In particular, the powder described in EP’944 has a CaO / SiCh ratio comprised between 0.5 and 1.2, a carbon content between 0.5% and 5%, a LiiO content between 1% and 7%, and a fluoride content between 0.5% and 8%.

[0027] In EP’944, the presence of lithium oxide is considered essential for absorbing the inclusions that are trapped in the molten metal. The presence of fluoride is also considered fundamental in controlling the crystallization of the powder in the crystallizer.

[0028] The technical problem addressed in EP ’944 is mainly that of controlling the heat transfer through the formation of an air layer between slag film and crystallizer walls. Thanks to this, the formation of solid skin is improved, and the appearance of cracks is prevented.

[0029] There is therefore the need to perfect an apparatus for the continuous casting of a long metal product, and a corresponding casting method, that can overcome at least one of the disadvantages of the state of the art.

[0030] One purpose of the present invention is to provide an apparatus and to perfect a method - which works mainly in endless mode - that allow to increase productivity without any impact on the quality of the cast products, reducing any problems of plant downtime to a minimum. Another purpose of the present invention is to provide such an apparatus and mathod capable of casting at high speed long products with any cross-section whatsoever, in particular square, even rounded or connected, round, and polygonal, for example octagonal.

[0031] The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.

[0032] SUMMARY OF THE INVENTION

[0033] The present invention is set forth and characterized in the independent claims.

[0034] The dependent claims describe other characteristics of the present invention or variants to the main inventive idea.

[0035] In accordance with the above purposes and to resolve the technical problem described above in a new and original way, also achieving considerable advantages compared to the state of the prior art, a method for the continuous casting of a long metal product, predominantly in endless mode, but semi-endless or billet-to-billet work modes are not excluded, uses a granular lubricating composition that at the time of supply is in the solid state. This granular lubricating composition, hereafter called casting powder, at the supply temperature is free of any liquid component, in particular oleic, therefore it should not be confused with products such as lubricating oils or oil-powder mixtures widely used in the metallurgical sector, but not useful for the purposes of this invention.

[0036] This casting powder is - in the state of the art - considered a high viscosity powder. The method comprises the following steps:

[0037] - constantly discharging a determinate amount of liquid metal inside a tubular crystallizer until a determinate upper level is reached, called meniscus,

[0038] - distributing a casting powder on top of this upper level,

[0039] - determining, by means of a device called oscillating table, an oscillation of the tubular crystallizer on the basis of oscillation parameters that depend on a frequency and a stroke with which the crystallizer is made to oscillate.

[0040] In accordance with one aspect of the present invention, in order to cast the metal product at a casting speed greater than 5.5 m / min, preferably greater than 7 m / min, more preferably greater than 9 m / min, the casting powder has a viscosity p greater than 5 Poise, preferably greater than 8 Poise, more preferably greater than 10 Poise.

[0041] The Applicant has observed that these viscosity values - contrary to what is known from the state of the art - have a beneficial effect both with regard to the lubricating effect and also with regard to the heat exchange, and therefore the uniformity of the cast product’s skin formation inside the crystallizer, with other technical and technological process parameters being equal, allows to increase the casting speed. For a greater benefit, the crystallizer suitable for casting long products is of the monolithic tubular type, consisting of a plurality of walls, suitable for contact with the solidifying product, which are connected longitudinally and without a break in continuity so as to define a single body. Using a casting powder with such high viscosity values allows to effectively address the technical problems that are encountered in this type of continuous casting processes, such as maintaining a constant casting speed, correct lubrication and reducing process interruptions.

[0042] In accordance with another aspect of the present invention, the casting powder has, moreover, a total carbon content Ctot greater than 14%, preferably greater than

[0043] 15%, more preferably greater than 17%.

[0044] In accordance with another aspect of the present invention, the casting powder has a basicity B (CaO / SiCh - so called IB2) comprised between 0.5 and 0.9, preferably between 0.6 and 0.8, more preferably approximately 0.7.

[0045] In accordance with another aspect of the present invention, the casting powder is free of components containing lithium and fluorine.

[0046] The optimization of the components of the high viscosity casting powder - in coordination with the other parameters - ensures that the powder is absorbed and completely amalgamated into the liquid metal being worked, thus favoring the casting speed. In the absence of this optimization, at least part of the high viscosity powder remains on the surface perimeter.

[0047] In accordance with another aspect of the present invention, the oscillation parameters comprise a Negative Strip Time (NST) value comprised between 0.05s and 0.09s, preferably approximately 0.07s, chosen based on a number of tests which have empirically established the correspondence of NST values comprised in this range, to be able to suitably cast the product at high speeds with viscous powders. In accordance with another aspect of the present invention, the oscillation parameters comprise a Negative Strip Ratio (NSR) value comprised between 20% and 35%, preferably between 23% and 27%, more preferably approximately 25% and possibly no lower than this.

[0048] In accordance with another aspect of the present invention, the oscillation parameters comprise a Mould Lead (ML) value comprised between 2mm and 4mm, preferably between 2.5mm and 3mm.

[0049] It goes without saying that, and in accordance with another aspect of the present invention, the oscillation parameters are controlled by a control unit which executes a management program which, on the basis of input data such as shape and size of the crystallizer or of the cast product, type of metal, desired oscillation parameters, chemical-physical characteristics of the casting powder, supplies the correlated frequency and stroke. The equivalent product sections involved in the present invention preferably range from about 120 mm to 250 mm and can be used to cast all known types of steel grades and also alloyed / microalloyed steels. Some embodiments of the present invention also concern a corresponding apparatus for the continuous casting of a metal product comprising: a preferably monolithic tubular crystallizer, suitable for casting long products, a discharge device for discharging a liquid metal, a delivery device for delivering a casting powder, an oscillator device for oscillating the crystallizer, a plurality of extraction rollers and a control unit.

[0050] In accordance with another aspect of the present invention, the control unit is configured at least to control the discharge device, the delivery device, the oscillator device and the rollers in order to cast the product at a casting speed greater than 5.5 m / min, preferably greater than 7 m / min, more preferably greater than 9 m / min, wherein the casting powder has a viscosity p greater than 5 Poise, preferably greater than 8 Poise, more preferably greater than 10 Poise, it has a total carbon content Ctot greater than 14% and it is free of components containing lithium and fluorine.

[0051] The present invention also concerns the use of a casting powder with the chemical-physical characteristics described above, in a method for the continuous casting of a metal product at a casting speed greater than 5.5 m / min, preferably greater than 7 m / min, more preferably greater than 9 m / min. DESCRIPTION OF SOME EMBODIMENTS

[0052] With reference to fig. 1, a continuous casting apparatus 10 is schematized comprising an ingot mould 11 and a crystallizer 12, associated in a known manner with the ingot mould 11 , of a tubular, monolithic type, suitable for casting long steel products. The apparatus 10 preferably comprises a discharge device 16 for discharging liquid metal into the crystallizer 12 until a determinate upper level, or meniscus M, is reached, and a delivery device 21 for delivering a casting powder inside the crystallizer on top of the meniscus M.

[0053] The crystallizer 12, which is configured to solidify the liquid metal that is introduced inside it and produce a cast product P at outlet, defines a casting line or axis Z along which the solidifying product P transits. For this purpose, the crystallizer 12 is provided with known cooling devices (not shown), also called primary cooling devices, configured to cool the liquid metal in contact with its walls through the circulation of a cooling fluid. By way of example, the crystallizer 12 has a crystallizer length LC, determined along a first linear segment of the casting line Z, comprised between about 500 mm and 1500 mm, and it has, inside it, a substantially conical development narrowing downward moving from the upper inlet zone to the lower outlet zone. The crystallizer 12 has a casting cavity 13 defined by walls 14 and having a cross-section that can have a shape that is square, rounded or connected, round, or polygonal, for example octagonal, the latter according to one and / or the other of the embodiments described in WO 2020 / 261311 in the name of the Applicant, so as to be able to cast corresponding long steel products.

[0054] The cross-section of the casting cavity 13 defines the shape of the cross-section of the cast product P at outlet from the crystallizer 12, with figs. 2a-2b showing the rounded square section (fig. 2a) and the connected square section (fig. 2b) shape thereof. In the example of figs. 2a, 2b the sides are curved with a radius of curvature that can reach about 40% of the length of the side. For example, for a 160 mm side, the radius of curvature could be about 55-56 mm.

[0055] The apparatus 10 comprises an oscillator device 15, associated with the ingot mould 11 , able to make the latter oscillate substantially vertically along the casting line Z with frequency f and total amplitude or stroke H, with the aim of promoting the advance of the long cast product P. The apparatus 10 also comprises a control unit 17, comprising a central processing unit, or CPU, 17a and at least one connected memory unit 17b, able at least to control the oscillator device 15, the discharge device 16 and the delivery device 21.

[0056] The apparatus 10 of fig. 1 also comprises a plurality of guide rollers, also referred to as foot rollers 18, disposed at the outlet end of the crystallizer 12, which form an integral part of the ingot mould 11, a secondary cooling system 19 installed downstream of the ingot mould 11, and guide means 20 for guiding the cast product P along the casting line Z. The foot rollers 18 and the guide means 20 are commanded by the control unit 17 to give a desired casting speed Vc to the cast product P, in a manner known per se.

[0057] As known, the liquid metal that has been produced in the steel mill’s melting furnace is emptied from the ladle to an underlying tundish, and from there it is continuously discharged into the crystallizer 12 until a determinate upper level, or meniscus M, is reached. It is also known that the liquid metal can be discharged from the tundish to the crystallizer through an unguided free jet or through a discharge device 16 whose outlet end is located below the level of the meniscus M (sub entry nozzle or shroud (SEN / SES) - terms known in the steel industry), which has the purpose of isolating the liquid metal from the surrounding air. One of the fundamental conditions in the casting process is to work as much as possible in stationary conditions, in particular in the zone of the meniscus M, in order to prevent disturbances to the meniscus M that are responsible for many of the product’s defects, from cracks to rhomboidity. Reducing the friction force between the cast product P and the internal wall of the crystallizer 12 is another important condition to increase the casting speed Vc and improve the quality of the product itself.

[0058] For this purpose, a casting powder is distributed on top of the meniscus M to minimize friction between the forming skin and the internal walls of the crystallizer 12, and to isolate the free surface of the meniscus M, limiting its oxidation.

[0059] In particular, the casting powder is spread on the metal bath in an appropriate amount and melts in contact with the liquid metal, forming a surface slag that infiltrates the interstice between the casting metal and the internal wall of the crystallizer 12, guaranteeing the lubrication necessary for the flow and promoting the heat exchange, as well as the protection against the re-oxidation of the product and the capture of inclusions.

[0060] The casting powder is a completely solid mixture at supply temperature, comprising elemental carbon selected from amorphous graphite, coke or carbon black, silicates and / or aluminosilicates of alkali and / or alkaline-earth metals, transition metal oxides and other oxides.

[0061] By way of example, the casting powder can comprise a combination of one or more of the following compounds or individual elements: SiO2, CaO, MgO, CaO MgO, AI2O3, Na2O, Na2O+K2O, MnO, Fe2O3, free C, CO2, total C, TiO2, S.

[0062] The Applicant has experimented that there is a strong correlation mainly between the casting speed Vc and the chemical-physical characteristics of the casting powder, but also between the latter and the oscillation parameters of the ingot mould 11 , as described in detail below.

[0063] The chemical-physical characteristics of the casting powder in question are, specifically, viscosity p, total carbon content Ctot, basicity B, and fluorine F and lithium Li content.

[0064] The Applicant has also experimented that in order to cast at a casting speed Vc>5.5 m / min, preferably Vc>7 m / min, more preferably Vc>9 m / min, in stationary conditions obtaining a good quality of the cast product P, also on the subsequent rolled product, the casting powder has to have at least a viscosity p greater than 5 Poise, preferably greater than 8 Poise, more preferably greater than 10 Poise.

[0065] By viscosity p of the casting powder we mean the viscosity value of the liquid phase of the casting powder at a temperature of 1300°C, which substantially corresponds to the temperature of the skin that is forming in the zone of the meniscus M.

[0066] The Applicant has also experimented that the casting powders that perform best are those for which the variation in viscosity p with the temperature is the most significant, that is, those casting powders for which the viscosity p at temperatures around 1500°C, that is, close to the temperature of the meniscus M, is lower than 2 Poise, while the reference viscosity at 1300°C is greater than 5 Poise, preferably greater than 8 Poise, more preferably greater than 10 Poise.

[0067] The lower viscosity p in correspondence with the meniscus M could promote the distribution of the molten powder on the perimeter and the speed with which it moves to reach the periphery of the meniscus, improving uniformity of distribution and therefore lubrication.

[0068] Although it is known that the lubrication capacity and uniformity of a powder is linked to its viscosity p, this result is surprising and completely counter-intuitive because logic would suggest that a higher viscosity tends to slow down the flow of the product and not increase its speed.

[0069] However, a higher viscosity powder manages to guarantee the formation of a lubricant layer with a greater thickness, thus reducing the possibility of any interruption thereof and the associated defect. These values of viscosity p, in some way and in synergistic combination with other factors such as, but not limited to, oscillation of the ingot mould 11, also contribute to the formation of a constant layer of skin at the casting speeds Vc indicated above. This result is of primary importance because, as mentioned, as the casting speed Vc increases, the thickness of the skin of the cast product P, inside the crystallizer 12, has a decreasing value, all other factors being equal.

[0070] The Applicant has also experimented that in order to cast at the above mentioned casting speeds Vc, the casting powder should have a total carbon content Ctot>14%, preferably Ctot>15%, more preferably Ctot>17%. We must clarify that carbon is present both in the form of carbonates and also of free carbon. Carbonates tend to increase, more or less significantly, the melting speed of the casting powder, while free carbon tends to decrease it more or less markedly, depending on the percentage, granulometry and form it is in, since the consumption of the casting powder is not linear with respect to the casting speed Vc, therefore it is better to suitably calibrate the free carbon content in the casting powder with respect to the casting speed Vc range within which work is being carried out. The casting powder’s melting speed is directly correlated to a defect known to the people of skill in the art by the English term “slag rim”. With these total carbon values Ctot and at the casting speeds Vc identified above, this defect does not occur.

[0071] The Applicant has also experimented that, in order to cast at the aforementioned casting speeds Vc, the casting powder has to advantageously have a basicity B, calculated as a weight ratio between CaO and SiCE, comprised between about 0.5 and about 0.9, preferably between about 0.6 and about 0.8, more preferably approximately 0.7. These values are correlated, in particular, with the viscosity p, because silica gives a glassy behavior called “network formers” and therefore increases the viscosity p, while calcium oxide tends to break bonds and reduce the viscosity p.

[0072] The basicity B value can be further optimized / refmed, as well as on the basis of the casting speed Vc, also with respect to the cross-section of the cast product P and with respect to the type of steel being worked.

[0073] The casting powder can also be conveniently characterized by means of an additional basicity index IB4 = (CaO+MgO SiCE+AECE) comprised between 0.5-0.65. The Applicant has in fact experimented that the basicity index IB4, a parameter that takes into account the powder’s main basic and acidic compounds, is strongly correlated to the viscosity p and therefore to the performance of the casting powder being used.

[0074] The Applicant has also experimented that, in order to cast at the aforementioned casting speeds Vc with the casting powder having the aforementioned viscosity p, the powder has to be free of fluorine F and lithium Li.

[0075] It is also known, moreover, that being able to reach high casting speeds is correlated to the optimization of a plurality of technical and technological parameters, thanks to which the liquid metal is partly solidified in the tubular crystallizer 12. The aforementioned parameters mainly concern:

[0076] - the geometric and dimensional characteristics of the crystallizer 12,

[0077] - the rigidity of the crystallizer 12,

[0078] - the cooling modes of the crystallizer 12, on which the ability to remove heat within a predetermined length LC depends,

[0079] - the modes of lubrication of the internal walls of the crystallizer 12 fulfilled by the casting powder.

[0080] In order for the skin not to adhere to the wall of the crystallizer 12 and the cast product P to descend correctly, the casting method provides that the ingot mould 11 is made to oscillate along the first segment of the casting line Z by means of the oscillator device 15 according to the following oscillation parameters:

[0081] - Negative Strip Time (NST), which is the time interval, expressed in seconds, during which the ingot mould 11 drops faster than the cast product P; the NST can be expressed with the following formula:

[0082] 1 AST = — ■ cosh nf

[0083] - Negative Strip Ratio (NSR), which is the ratio, expressed as a percentage, between the NST and the period of oscillation of the ingot mould 11 ; the NSR can be expressed with the following formula: 100 = F2(NST, f) - Mould Lead (ML), which is the excess space of the total stroke H, expressed in meters, that the ingot mould completes with respect to the advance of the product cast during the NST, the ML can be expressed with the following formula:

[0084] ML = H ■ sin(rr ■ f ■ NST) - Vc■ NST = F3(H, f, NST, Vc)

[0085] These parameters, as well as being closely corelated to each other, are correlated to the viscosity p of the casting powder used. In fact, it is precisely during the NST that the desired vertical space is created, corresponding to the ML value, useful for the filtration of the casting powder in the interstice between the casting metal and the internal wall of the crystallizer 12.

[0086] On the basis of these assumptions, the Applicant has observed that in order to cast at the aforementioned casting speeds Vc, the NST has to advantageously be between comprised about 0.05s and about 0.09s, preferably approximately 0.07s.

[0087] The Applicant has observed that the NSR has to advantageously be comprised between about 20% and about 35%, preferably between about 23% and about 27%, more preferably approximately 25% and possibly not below this percentage of the period of each oscillation of the crystallizer 12.

[0088] The Applicant has observed that the ML has to advantageously be comprised between about 2mm and about 4mm, preferably between about 2.5mm and about 3mm.

[0089] The identified values of optimal NST and NSR are functional to obtain a determinate ML for the correct absorption of the casting powder. In fact, a casting powder with higher viscosity p has to correspond to a larger space because a more effective absorption is necessary.

[0090] In operational terms, if the casting speed Vc increases, and the oscillation parameters (f, H) remain the same, the NST consequently decreases. To keep the NST within the indicated ranges, and favorably at approximately 0.07s, it is necessary to act on the stroke H and on the oscillation frequency f of the ingot mould 11.

[0091] Advantageously, the main control parameter is the stroke H, while the frequency f is adjusted accordingly by the automation so as to remain within the appropriate NST, NSR, ML parameters.

[0092] Below are two examples of casting processes in which the casting speed Vc is 8m / min and 9m / min, respectively, and in which it can be observed that by increasing the stroke H the values of all the parameters (NST, NSR, ML) are respected.

[0093] Example 1

[0094] Example 2

[0095] According to some embodiments, the process control is fulfdled by a management program installed in the memory 17b and executed by the CPU 17a. The control unit 17 receives input data such as shape and size of the crystallizer / long product, type of steel, NST, NSR and ML limit values, chemicalphysical characteristics of the casting powder, which are saved in the memory 17b, and executes the management program which, on the basis of the aforementioned input data, supplies output data such as the stroke H and the oscillation frequency f of the ingot mould 11 in order to comply with the aforementioned limit values. The control of the ingot mould’s 12 oscillation is continuously managed by means of a control signal generated by the CPU 17a on the basis of the stroke H and frequency f values calculated.

[0096] As shown in fig. 3, a continuous casting steel plant 100 comprises a first apparatus 10 and a first rolling line 101 located directly in line with the apparatus 10 for rolling, for example in endless mode, the cast long product (co-rolling).

[0097] The plant 100 can also comprise additional apparatuses 10 which define respective casting lines parallel to the first and which feed, in direct hot charge mode, through a common transfer system 102 located downstream of the casting lines, a second rolling line 103.

[0098] Directly upstream of the first rolling line 101 and / or of the second rolling line 103 there can be interposed a heating device 104, for example by induction, for the rapid heating of the billets.

[0099] The plant 100 has a length LI that depends on its configuration (plant components and lay-out) and on the type of products it allows to produce.

[0100] In particular, the plant 100 can have a length LI of about 250m if producing commercial length bars, of about 350-550m for rolled products and of about 600m for long bars.

[0101] It is clear that modifications and / or additions of parts may be made to the apparatus 10 and method as described heretofore, without thereby departing from the field and scope of the present invention, as defined by the claims.

[0102] 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 will be able to achieve other equivalent forms of apparatus and method for the continuous casting of a long metal product, having the characteristics as set forth in the claims and hence all coming within the field of protection defined thereby.

[0103] 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 claims.

Claims

CLAIMS1. Method for the continuous casting of a long metal product (P) at a speed greater than 5.5 m / min, comprising the following steps:- constantly discharging a determinate amount of liquid metal inside a tubular crystallizer (12) until a determinate upper level (M) is reached,- distributing a casting powder on top of said upper level (M),- determining an oscillation of said crystallizer (12) on the basis of oscillation parameters that depend on a frequency (f) and a stroke (H), said method being characterized in that said casting powder has a viscosity p greater than 5 Poise, preferably greater than 8 Poise, more preferably greater than10 Poise, it has a total carbon content Ctot greater than 14% and it is free of components containing lithium and fluorine.

2. Method as in claim 1, characterized in that said casting powder has a total carbon content Ctot greater than 15%, more preferably greater than 17%.

3. Method as in claim 1 or 2, characterized in that said casting powder has a basicity B comprised between 0.5 and 0.9, preferably between 0.6 and 0.8, more preferably approximately 0.7.

4. Method as in any claim hereinbefore, characterized in that said oscillation parameters comprise a Negative Strip Time (NST) value comprised between 0.05s and 0.09s, preferably approximately 0.07s.

5. Method as in any claim hereinbefore, characterized in that said oscillation parameters comprise a Negative Strip Ratio (NSR) value comprised between 20% and 35%, preferably between 23% and 27%, more preferably approximately 25%.

6. Method as in any claim hereinbefore, characterized in that said oscillation parameters comprise a Mould Lead (ML) value comprised between 2mm and4mm, preferably between 2.5mm and 3mm.

7. Method as in any claim hereinbefore, characterized in that said oscillation parameters are controlled by a control unit (17) which executes a management program which, on the basis of input data such as shape and size of said crystallizer (12) or of said cast product (P), type of metal, desired oscillation parameters, chemical-physical characteristics of said casting powder, supplies said correlated frequency (f) and stroke (H).

8. Apparatus for the continuous casting of a long metal product (P) at a speedgreater than 5.5 m / min comprising: a tubular crystallizer (12), a discharge device (16) for discharging a liquid metal, a delivery device (21) for delivering a casting powder, an oscillator device (15) for oscillating said crystallizer (12), a plurality of extraction rollers (18, 20) and a control unit (17), characterized in that said control unit (17) is configured at least to control said discharge device (16), said delivery device (21), said oscillator device (15) and said rollers (18, 20) in order to cast said long metal product (P) to obtain said casting speed greater than 5.5 m / min, wherein said casting powder has a viscosity p greater than 5 Poise, preferably greater than 8 Poise, more preferably greater than 10 Poise, it has a total carbon content Ctot greater than 14% and it is free of components containing lithium and fluorine.

9. Use of a casting powder that has a viscosity p greater than 5 Poise, preferably greater than 8 Poise, more preferably greater than 10 Poise, a total carbon content Ctot greater than 14% and is free of components containing lithium and fluorine in a method for the continuous casting of a metal product (P) at a casting speed (Vc) greater than 5.5 m / min, preferably greater than 7 m / min, more preferably greater than 9 m / min.

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