IMPROVED UNIT OF EQUIPMENTS FOR THE HIGH-SPEED CONTINUOUS CASTING OF GOOD QUALITY THIN STEEL SLABS

Abstract: Unit of equipments for the continuous casting of steel slabs, especially low thickness slabs at high speed, comprising a mould (1) fed by a submerged nozzle (2) and connected to an oscillator (3) driven by a hydraulic servocontrol, wherein the following geometrical relation is valid concerning both the mould and the submerged nozzle shapes and their mutual arrangement: (A1/S1)/(A2/S2) = 0.9 DIVIDED 1.1 and preferably A1/S1 = A2/S2, wherein, on the mould horizontal section at the meniscus level, A1 is the area enclosed between submerged nozzle and larger sides of the mould, and A2 is the residual area on said section, between submerged nozzle and smaller sides, S1 and S2 being the total sums of the mould peripheral lengths corresponding to each of said areas. Furthermore, at least in the mould horizontal section at the meniscus level, the distance between submerged nozzle and copper plates forming the mould walls is kept constant.

Description: "EMPROVED UNIT OF EQUIPMENTS FOR THE HIGH-SPEED CONTINUOUS CASTING OF GOOD QUALITY THIN STEEL SLABS" DESCRIPTION The present invention relates to an improved unit of equipments for highspeed continuous casting of good quality thin steel slabs. The continuous casting of the so-called "thin slabs" of steel, up to 80 mm thick, is known to have been so far subject to some quality problems, especially for casting at high speed, e.g. above 4.5 rn/min. Such problems may result in some flaws of the slab surface, the so-called shell which is getting formed in the mould: - longitudinal cracks due to the trapping of casting powders; - longitudinal and transversal cracks due to the lack of the lubricating and insulating film formed by the so-called "slag", a term indicating the product of casting powders being melted and in case re-solidified; - longitudinal cracks due to thermal stresses; and - longitudinal cracks due to the copper cooling surfaces being discontinuous. This quality problems mainly affect special steels and could be at least partially solved by reducing the casting speed, which however would involve a lower productivity and accordingly a reduced plant economicity. Another possible solution could be the use of an electromagnetic device, called "EMBR" (Electro- Magnetic Brake Ruler), capable of flattening, by reducing their height, the liquid steel waves rippling the meniscus inside the mould, but such a device is very expensive and would only partially solve the aforementioned problems. However other problems arise from the geometrical and flow conditions occurring inside the mould, possibly resulting in a life reduction of the casting nozzle (which, dipped in the liquid metal, is usually called "submerged nozzle") and in harmful outcomes for the process efficiency. It is now clear that the aforementioned problems could not be solved in a systematic and satisfying way by independently operating on the mould, on the submerged nozzle and on the mould oscillating unit. These three elements, being moreover accountable for the continuous casting, are so much interconnected to form a real "casting unit", and an effective solution may be found only by operating on the integrated unit as a whole. It is an object of the present invention to provide a casting unit allowing to overcome the aforementioned drawbacks when wanting to cast thin slabs at high speed. The improved unit of casting elements according to the present invention generally has the characteristics of claim 1, as well as, according to specific aspects of the invention, additional characteristics described in the dependent claims. These and other objects, advantages and characteristics of the casting unit according to the present invention will be more evident from the following detailed description of a preferred embodiment thereof, reported by way of non-limiting example with reference to the attached drawings, wherein: Figure 1 shows a diagrammatic side view of a casting unit according to the invention; Figure 2 shows a view of the sole upper part of the mould, combined with the submerged nozzle, in the direction of arrow II of Figure 1; Figs. 3a. 3b, 3c show the same diagrammatic view, in a cross-section, taken along line III-III of Figure 2, at the meniscus level, in order to particularly show the various parts to be considered in the geometrical relation that mould and submerged nozzle must satisfy in the casting unit according to the invention; Figure 4 shows a plan view of the same mould, diagrammatically represented with respect to a tern of Cartesian axes; Figs. 5a and 5b show two diagrammatic views of the mould of Figure 4, wherein the envelope of the cooling system pipes is represented in longitudinal section through a plan parallel to y and z axes of Figure 4, and along line B-B of Figure 5a, respectively. With reference to the drawings, Figure 1 is a diagrammatic view of the casting unit according to the invention, with a mould 1, a dip casting nozzle 2, hereinafter always referred to as "submerged nozzle" and an oscillator 3, which is hydraulically driven and, according to this embodiment, is fastened to the mould body, obviously so as not to interfere with the casting line. Figure 1 also shows the section of passage of the liquid steel flow between submerged nozzle 2 and the shell getting formed along the copper walls, i.e. the two "channels" 4 thus formed. As to the mould, the main problem occurring when casting thin slabs, with respect to the traditional ones, is the fact that, the flow rate of molten steel being the same, a reduction of the slab thickness would involve an increase of the slab surface contacting the mould walls in the time unit, and thus an increasing need of lubricating "slag", as previously defined. In fact, Tl, Wl, VI being the thickness, the width and the average casting speed of a normally thick slab, T2=Tl/a (a>l), W2=W1 and V2»V1 being the equivalent quantities for a thin slab, the steel flow rate being the same, one has: T2»W2*V2 = T1 «W1 »V1 | (I) The area of thin slab cast in the time unit is thus: 2*(T2+W2) » V2 I , equal to about |2»W2 »V2l if the thickness of the thin slab is considered as negligible with respect to its width. By replacing the W2»V2 product with the value resulting from equation (I), one has: I 2 *W2»V2 = 2 « (Tl/T2)*Wl 'Vl = a« (2* Wl 'Vl) I (II) The above equation (II) clearly shows, being a>l, the importance of forming a lubricating slag which coats in the time unit a slab-mould contact surface being in inverse proportion with the thickness and thus the larger the thinner the slab is. On the contrary the interface in the mould between molten steel and casting powders has a smaller area in the meniscus region, where said slag is formed, due to the low thickness and, in the middle region, to the submerged nozzle. Although this problem may be partially solved by using casting powders capable of enhancing the slag formation, it has to be considered that, in the known configurations, the submerged nozzle did not allow to keep, in all the meniscus regions, the required equilibrium between molten slag, formed by powders melting, and slag consumed once penetrated between meniscus and wall. According to the present invention, the thin mould is capable of containing a reliable, i.e. sufficiently thick, submerged nozzle, with its large copper plates having such a profile in the horizontal plane, around the meniscus level, to exactly match the profile of the submerged nozzle in the said horizontal plane, thereby keeping in every point of the middle region a constant distance between the submerged nozzle and the walls. With reference to Figs. 3a, 3b and 3c, such a distance is chosen so that the ratio Al/Sl, i.e. between the area of the interface with the casting powders, virtually proportional to the slag formation, and the area of the slab around the submerged nozzle, virtually proportional to the slag consuming (see Figure 2), is approximately the same as A2/S2, measured outside the submerged nozzle region (see Figure 3c). Thus the equation to be satisfied is: (Al/Sl )/(A2/S2) = 0.9÷1.1, and preferably = 1 For example, for a mould being 1300 x 65 mm and having a submerged nozzle 300 mm wide (with a reliable thickness of 60 mm as indicated in Figs. 3b and 3c), the optimal ratio Al/Sl = A2/S2 is equal to 30 mm. Such a ratio, once e.g. the dimensions of the submerged nozzle and the thickness of the smaller sides have been fixed, may be used for defining the trend of the mould profile in the horizontal plan at the meniscus level, or, the dimensions of the mould profile being known, may be used for determining the profile trend of the submerged nozzle, likewise in order to ensure a well-balanced amount of lubricating slags along the entire mould profile. This geometrical configuration is important also for the flow of molten steel in the meniscus region, since the "channels", indicated with numeral 4 in Figure 1, which are generated between the submerged nozzle and the shell getting formed against the copper walls, will be sufficiently large to prevent the vortex formation due to the acceleration of the streams converging in the middle from the mould smaller sides, in the meniscus region, which vortexes often cause the powders to be trapped, resulting in the aforementioned drawbacks. It should be noted that preferably the mould used in the casting unit according to the invention is the one with a variable bending in longitudinal direction, being the subject matter of European patent 0705152 in the applicant's name, which allows to have a nearly infinite bending radius in the upper region for a better arrangement of the submerged nozzle, while providing the bending of the slab already getting formed inside the mould with an exit on the arc-shaped casting guide other than the vertical, so as to advantageously reduce the height of the casting unit and accordingly the ferrostatic forces and the risk of slab swellings. According to the aforementioned patent application, the bending is graded in a progressive and uniform way from the infinite radius of the mould inlet to the bending radius Ro corresponding to the casting guide (Figure 1), thereby preventing both exceeding stresses on the solidified external shell of the slab and the possibility of an imperfect contact with the copper walls of the mould. In order to solve the technical problems dealt with, the unit for cooling the mould plates is especially important, having to be capable of withstanding the high heat fluxes typical for thin slabs (up to 3 MW/m 2 , average value on the entire cooling surface of the mould), with a cooling being enhanced in the meniscus region in order to prevent copper cracks and nevertheless being sufficiently uniform around the mould to prevent thermal stresses for the slab getting formed. With reference to Figure 4, when considering the specific normal heat flux (dq n between the surface of the casting product and the mould, one has: dq n = dq/dA [W/m 2 ] This heat flux is also a function of the local surface temperature on the hot surface of the copper plates, in turn dependent also upon the distance from the pipes wherein the cooling water flows. By using, as it can be seen in Figure 3, a system of Cartesian axes x, y, z, wherein z is the axis turned downwards or towards the mould bottom, and considering the complex surface formed by the mould f(x,y,z) = 0, the local surface temperature locally varies as t = t [f(x,y,z)j\ Since the heat flux dqn must be kept as constant as possible along a horizontal line (wherein z = zo) belonging to the mould surface, i.e. the temperature t must be kept virtually constant along such a line, whereby: t = t [f(x,y, zo)] = to this is obtained according to the present invention by keeping in every point of the copper hot surface the same normal distance Nd, taken on the perpendicular with respect to the hot surface, from the ideal surface envelope E of all the ends of the cooling pipes W (Fig. 5a, 5b). Thus it has to be Nd = constant, and experimentally it has been found that this optimal constant value must range from 10 to 25 mm in order to have the aforementioned conditions for the cooling system. As for the submerged nozzle, besides the aforementioned dimensional conditions with respect to the mould, it has to be designed so as to allow the optimal behavior of the molten steel flow, while taking into account also the gradual shell formation, as well as the life of the submerged nozzle itself. In fact, it is known that, upon decreasing of the slab thickness, the problems increase concerning the motions of the liquid inside the mould, possibly resulting in the formation of stationary waves in the meniscus region and thus a local reduction of the thickness of the liquid slag, which adversely affects the lubrication and the insulation of the shell of the slab getting solidified. The submerged nozzle for thin slabs, being the subject matter of patent application PCT/IT-97/00135 in the applicant's name, has geometrical characteristics resulting in castings having a low energy in outlet and a high probability of dissipation inside the liquid volume of the slab, improved flow guiding by virtue of the side shape of the submerged portion (thereby preventing the vortex formation and the powder trapping), besides an improved level control in the mould. Furthermore the feed is steady, the flow is substantially split in two streams and the starting surfaces inside the submerged nozzle are preserved, since the oxide deposit is negligible; moreover those good flow conditions result in a reduced extent of external mechanical erosion in the meniscus region. According to the present invention, the optimized design of the mould- submerged nozzle unit, besides the aforementioned conditions, is such that the ratio between the height of stationary wave (from peak to peak in mm) and the casting speed in m/min never exceeds 5, with an average value of 3.3. Furthermore, the standard deviation measured for the sampled signal of mould level (ML), indicated as stdDEV(ML), is usually comprised in the following value range: stdDEV(ML) = 0.7-1.5 mm Finally, as to the third element of the unit, i.e. oscillator 3, it has to be considered too as a critical factor for the surface quality of the slab and the reliability of the continuous casting process. With reference to Figure 1, it may be formed of a framework 3 a being hinged to the floor and driven by a hydraulic servocontrol 5. Framework 3a is also hinged to a mould support 3b, thus forming a kind of quadrilateral together with a set of springs fitted into both ends. The control flexibility is ensured by a program logic control allowing to change the oscillation parameters concerning the wave shape, the wave amplitude between ±2 and ±10 mm, as well as the oscillation program. The control continuously records the actual value of the casting speed so as to control the oscillation frequency based on the previous parameters. Maximum oscillation frequencies have been obtained as high as 480-520 strokes/min, for the first natural frequency of the entire dynamic system of 16.7 Hz. The flexibility is such that the oscillation parameters may be adjusted so as to obtain, for every quality of steel, an optimal lubrication and a surface quality as a function of the casting speed. Alternatively, the oscillator may be of the so-called "resonance" type, the mould being directly mounted upon flexure springs, without no lever system, and oscillated by a hydraulic servocontrol at a frequency close to the natural frequency of the elastic system, with no clearance and thus along an extremely precise path. Possible additions and/or modifications may be made by those skilled in the art to the above described and illustrated embodiment without departing from the scope of the invention. In particular, the mould itself may have in the vertical plane a profile other than the one disclosed in European patent 0705152 and the submerged nozzle may be different than the one disclosed and claimed in application PCT/IT-97/00135, provided that the aforementioned geometrical relations are complied with.

Claims: CLAIMS 1. A unit of equipments for the continuous casting of steel slabs, especially suitable for low thicknesses and high speeds, comprising a mould (1) for continuous casting, defined by copper plate walls on its larger sides, a feeding nozzle with submerged outlet or submerged nozzle (2) and an oscillator (3) driven by hydraulic servocontrol, characterized in that, at least in the middle region of the mould horizontal section at the meniscus level, the distance between submerged nozzle (2) and copper plates is kept constant; that moreover the ratio between the area (Al), corresponding to the middle portion of the surface of the mould horizontal section at the meniscus level, which area is enclosed between the larger sides of the mould and the submerged nozzle (2), and the total sum (SI) of the mould external lengths corresponding to said area (Al) is 0.9÷1.1 times the ratio between the area (A2) of the residual surface of the mould (1) horizontal section at the meniscus level and the total sum (S2) of the mould peripheral lengths corresponding to said area (A2); wherein furthermore the normal distance (Nd) between each point of the inner surface of the mould walls and the ideal surface envelope (E) of all the ends of the cooling pipes (W) is constant. 2. A unit according to claim 1, characterized in that said ratios Al/Sl and A2/S2 are substantially equal. 3. A unit according to claim 1 or 2, characterized in that said constant value Nd ranges from 10 to 25 mm. 4. A unit according to claim 1 or 3, characterized in that the ratio between the height of the stationary wave being formed on the meniscus in the mould, measured from peak to peak in mm, and the casting speed in m/min do not exceed 5, with an average value of 3.3. 5. A unit according to claim 4, characterized in that the standard deviation measured on the sampled signal of mould level ranges from 0.7 to 1.5 mm. 6. A unit according any of the previous claims, characterized in that the oscillator (3) is formed of a quadrilateral lever system (3a, 3b, 3c) with hydraulic servocontrol (5) capable of varying the amplitude of the oscillation wave between ±2 and ±10 mm. 7. A unit according to any of the claims 1-5, characterized in that the oscillator (3) is of the resonance type, wherein the mould (1) is directly mounted upon flexure springs and is driven by a hydraulic servocontrol at a frequency close to the natural frequency of the elastic system, with no clearance.

Inventors: ARVEDI GIOVANNI, BIANCHI ANDREA, MANINI LUCIANO, ARVEDI, GIOVANNI, BIANCHI, ANDREA, MANINI, LUCIANO
Applicants: ARVEDI GIOVANNI, BIANCHI ANDREA, MANINI LUCIANO, ARVEDI, GIOVANNI, BIANCHI, ANDREA, MANINI, LUCIANO
Application year: 1997
Main classification: B22D11
IPC codes: B22D11