KR100539994B1 - Improved Unit of Equipments for the High-speed Continuous Casting of Good Quality Thin Steel Slabs - Google Patents

Abstract

The invention relates to a unit of an apparatus for continuous casting, in particular of thin slab and steel slab suitable for high speed, comprising a mold (1) fed by an immersion nozzle (2) and connected to an oscillator (3) driven by a hydraulic servo controller. As regards the shape of the mold and the immersion nozzle and the arrangement between them,

(A1 / S1) / (A2 / S2) = 0.9 to 1.1

Also suitably A1 / S1 = A2 / S2, on the mold horizontal cross section at the meniscus level, A1 is the area enclosed between the immersion nozzle and the large side of the mold, and A2 is enclosed between the immersion nozzle and the small side. The remaining area of the cross section, and S1 and S2 are the sum of the mold outer peripheral lengths corresponding to the respective areas. Moreover, at least inside the mold horizontal cross section at the meniscus level, the distance between the copper plate and the immersion nozzle forming the mold wall is kept constant.

Steel Slab Casting Unit, High Speed Continuous Casting, Steel Slab, Mold, Vibration Unit, Immersion Nozzle

Description

Improved Unit of Equipments for the High-speed Continuous Casting of Good Quality Thin Steel Slabs}

The present invention relates to an improved unit of an apparatus for high speed continuous casting of high quality thin steel slabs.

It is known that continuous casting of steel, so-called "thin slabs", having a thickness of up to 80 mm in particular, has had quality problems, especially in the case of high speed casting of, for example, 4.5 m / min or more.

This problem may cause the following defects on the surface of the slab called shells formed inside the mold.

Longitudinal cracks due to trapping of the casting powder,

Longitudinal and transverse cracks due to the lack of lubricating and insulating films formed by the so-called "slag" which represents the product when the casting powder is dissolved and resolidified,

Longitudinal cracking due to thermal stress, and

Longitudinal cracks due to discontinuous copper cooling surfaces.

This qualitative problem mainly affects special steels and can be at least partially solved by reducing the casting speed, but it involves lowering productivity and thus reducing the economics of the production equipment. Another solution is the so-called "EMBR" (electro-magnetic brake ruler), which can flatten a liquid steel wave that causes a wave (wave) in the meniscus inside the mold by reducing its height. Although electronic devices may be used, such devices are very expensive and only partially solve the above-mentioned problems. However, other problems arise from the geometric and flow conditions that occur inside the mold, thereby shortening the lifespan and casting efficiency of casting nozzles submerged in liquid metal and commonly referred to as "submerged nozzles". Can be.

It is clear that the above-mentioned problems cannot be solved in a systematic and satisfactory state even when operated independently on the mold, the immersion nozzle, and the mold oscillating unit. In continuous casting, these three elements are closely connected to each other to form the actual "casting unit", and an effective solution can only be obtained by operating on the integrated unit as a whole.

It is an object of the present invention to provide a casting unit which makes it possible to overcome the above problems when trying to cast thin slabs at high speed.

The improved casting unit according to the invention generally has not only the features of claim 1 but also the additional features described in the dependent claims in accordance with the particular aspect of the invention.

These and other objects, advantages and features of the casting unit according to the invention will become more apparent from the detailed description of the preferred embodiment of the invention, which is illustrated by way of non-limiting example with reference to the accompanying drawings.

1 is a schematic side view of a casting unit according to the invention;

FIG. 2 shows only the upper part of the mold combined with the immersion nozzle, in the direction of arrow II of FIG.

3A, 3B and 3C show the various parts which must be considered in the geometric relationship that the mold and the immersion nozzle must meet in the casting unit according to the invention, line III- of FIG. 2 at the meniscus level. Same schematic diagram of the section cut along III.

4 is a plan view of the same mold schematically shown for three orthogonal axes.

5A and 5B are two schematic views of the mold of FIG. 4 showing a longitudinal cross sectional view in a plane in which the shell of the cooling system pipe is parallel to the y and z axes of FIG. 4, respectively, and along line B-B of FIG. 5A;

Referring to the drawings, Figure 1 shows a mold 1, a submerged casting nozzle 2, hereinafter referred to as an "immersion nozzle", and a hydraulically driven mold body in accordance with this embodiment so as not to interfere with the casting line. A schematic side view of a casting unit according to the invention with an oscillator 3 fastened to it. FIG. 1 also shows the passage of a liquid steel flow between the immersion nozzle 2 and the cell formed along the copper wall, ie the two “channels” 4 thus formed.

For molds, the main problem with thin slab casting for conventional molds is that when the flow rates of molten steel are the same, decreasing slab thickness increases the contact between the slab surface and the mold wall in unit time, and accordingly The need for lubrication "slag" as described above is increased. In fact, T1 and T2 are the thicknesses of thick and thin steel slabs (T1> T2), W1 and W2 are the thick and thin widths of steel slabs (W1 = W2), respectively, and V1 and V2 are the average of thick and thin slabs, respectively. If casting speed (T1 >> T2, V1 << V2, maintain constant width), a is T1 / T2 ratio (> 1), and if steel flow ratio is the same,

T2 x W2 x V2 = T1 x W1 x V1. (Ⅰ)

Therefore, the area of the thin slab cast in unit time is

2 x (T2 + W2) x V2, which is roughly equal to 2 x W2 x V2 if the thickness of the thin slab is considered negligible for its width. Substituting the product of W2 x V2 into the value obtained from equation (I),

2 x W2 x V2 = 2 x (T1 / T2) x W1 x V1 = a x (2 x W1 x V1). (Ⅱ)

Equation (II) clearly shows that when a> 1, the lubricating slag coating the slab-mold contact surface in unit time is formed in inverse proportion to the thickness, so that the thinner the slab, the thicker it is. On the other hand, the interface in the mold between the molten steel and the cast powder has a small area due to the small thickness in the meniscus region where the slag is formed and due to the immersion nozzle in the central region.

This problem may be partially solved by using cast powders that can improve slag formation, but in known configurations, the immersion nozzles are immediately consumed when infiltrated between the molten slag formed by powder dissolution and the meniscus and the wall surface. It should be taken into account that the required equilibrium with the resulting slag cannot be maintained in the whole meniscus area.

According to the present invention, the thin mold may contain a thick immersion nozzle with sufficient reliability, and the large copper plate has a shape that exactly matches the shape of the immersion nozzle in the horizontal plane around the meniscus level, and hence the central region. At all points, a constant distance is maintained between the immersion nozzle and the wall. Referring to Figures 3A, 3B and 3C, this distance is the ratio A1 / between the area of the interface with the casting powder which is substantially proportional to slag formation and the slab area around the immersion nozzle which is substantially proportional to the slag consumption. S1 (see FIG. 3B) is selected to approximately coincide with A2 / S2 (see FIG. 3C) measured outside of the immersion nozzle area. Therefore, the following formula is

(A1 / S1) / (A2 / S2) = 0.9 to 1.1, suitably = 1.

For example, in the case where the mold is 1300 x 65 mm and has an immersion nozzle 300 mm wide (having a reliable thickness of 60 mm as shown in FIGS. 3B and 3C), the ratio A1 / S1 = A2 The optimal value of / S2 is 30 mm. Such a ratio may be used, for example, to clarify the tendency of the mold shape in the horizontal plane at the meniscus level, once the dimensions of the immersion nozzle and the thickness of the small side have been determined, or the dimensions of the mold shape are known. In this case, it may be used to determine the tendency of the shape of the immersion nozzle, and likewise to ensure a sufficient amount of lubricating slag along the entire mold shape.

This geometry is formed between the immersion nozzle and the cell formed in contact with the copper wall so that the "channel" indicated by reference numeral 4 in FIG. 1 becomes sufficiently large, often causing the powder to become clogged, causing the above problem, This is because it can be prevented from being formed by the acceleration of the flow converging from the side of the small side of the mold in the varnish region, and is also important for the flow of molten steel in the meniscus region.

Suitably the mold used in the casting unit according to the invention has a variable bending in the longitudinal direction, which is the subject of European Patent 0705152 (Italian Patent Application MI 93A001004) in the name of the applicant, in order to properly position the immersion nozzle By providing bending of the slab formed inside the mold with an outlet on an arc-shaped casting guide in addition to the vertical shape while having a substantially infinite bending radius in the upper region, the height of the casting unit is well reduced, and thus the corresponding static load ( reduce the risk of ferrostatic forces and slab bumps. According to the patent application described above, the bending gradually changes in a gradual and uniform manner from the infinite radius of the mold inlet to the bending radius R ₀ (FIG. 1) corresponding to the casting guide, thereby bringing the solidified outer cell of the slab Overstress and the possibility of incomplete contact with the mold copper wall.

In order to solve this technical problem, a unit for cooling the mold plate is particularly important and resists the unique high heat flux by means of thin slabs (average value up to 3 MW / m ² across the cooling surface of the mold). As such, the cooling action is enhanced in the meniscus region to prevent copper cracks, and is sufficiently uniformly cooled throughout the mold to prevent thermal stress of the slab thus formed.

Referring to FIG. 4, dq is the heat transferred by the casting surface through the unit area dA, and taking into account the specific normal heat flux dq _n between the surface of the cast product and the mold,

dq _n = dq / dA [W / m ² ].

This heat flux is a function of the local surface temperature at the hot surface of the copper plate and in turn depends on the distance from the pipe through which the coolant flows.

As can be seen from Fig. 4, Cartesian axis systems x, y, z, in which z is a downward axis toward the bottom of the mold, are used, and f (x, y , z) = 0, the local surface temperature changes locally according to t = t [f (x, y, z)].

The heat flux dq _n should be kept as constant as possible along the horizontal line (excitation line z = z ₀ ) belonging to the mold surface, ie since the temperature t should be kept substantially constant along the horizontal line,

t = t [f (x, y, z ₀ )] = t ₀

This can be obtained according to the invention by keeping the normal distance (Nd) perpendicular to the hot surface from the ideal surface shell (E) at all ends of the cooling pipe (W) at all points of the copper hot surface (Fig. 5a and 5b). Thus, it was found that Nd = constant, and that the optimum constant value should experimentally be in the range of 10 to 25 mm in order to have the above mentioned conditions of the cooling system.

For immersion nozzles, in addition to the above-described dimensional conditions for the mold, they must be designed to allow for optimal operation of the molten steel flow, while taking into account the gradual cell formation and the life of the immersion nozzle itself. In fact, immediately after reducing the slab thickness, problems increase with respect to the liquid motion inside the mold, resulting in standing waves in the meniscus region, which can lead to local reduction of the liquid slag thickness. This may adversely affect the insulation and lubrication of solidified slab cells.

Immersion nozzles for thin slabs, which are also the subject of patent application PCT / IT97 / 00135 (Italian Patent Application MI 96 A 001234) in the name of the applicant, have a low exit energy during casting and high dissipation probability inside the slab liquid volume, In addition to the improvement of the flow guide due to the branch side shape (which prevents the formation of vortex and the clogging of the powder), it also has the geometrical characteristics to realize the improvement of the level controllability in the mold. Moreover, the initial surface inside the immersion nozzle is maintained because the supply is stable, the flow is substantially divided into two flows, and the oxide adhesion is very small, and these good flow conditions also contribute to the degree of external mechanical corrosion in the meniscus region. Decreases.

According to the present invention, the optimum design of the mold-immersion nozzle unit is, in addition to the above-described conditions, the ratio of the standing wave altitude (peak to peak in mm) and the casting speed in m / min is 3.3 as an average value. Never exceed five.

Moreover, the standard deviation measured for the sample signal at template level ML and expressed as stdDEV (ML) is typically included in the following range of values.

stdDEV (ML) = 0.7 to 1.5 mm

Finally, with regard to the oscillator 3, which is an element of the third unit, this should be considered as a very important variable for the surface quality of the slab and the reliability of the continuous casting process. Referring to FIG. 1, this vibrator may be formed of a framework 3a hinged to the floor and driven by the hydraulic servo controller 5. In addition, the framework 3a is integrally formed with a spring set that is hinged to the mold support 3b and fitted to both ends, thereby forming a kind of quadrilateral.

Control flexibility is ensured by vibration parameters relating to the waveform, and program logic controls that can vary the wave amplitude and vibration program between ± 2 and ± 10. The control continuously records the actual value of the casting speed to control the oscillation frequency based on the parameter. The maximum oscillation frequency is obtained at high values such as 480 to 520 strokes / min for 16.7 Hz, the initial natural frequency of the entire dynamic system. This flexibility can be adjusted for all qualities of steel to ensure that the vibration parameters get optimal lubrication and surface quality as a function of casting speed.

Alternatively, the oscillator may be of the so-called "resonant" type, and the mold is mounted directly to the flexible spring without the lever system, so that the inherent nature of the elastic system follows a very precise path with no clearance by the hydraulic servo controller. It vibrates at a frequency close to the frequency.

For the above embodiments, additions and / or modifications are possible by those skilled in the art without departing from the scope of the present invention. In particular, if the above geometrical relationship is satisfied, the mold itself may have, in a vertical plane, a shape different from that disclosed in European Patent No. 0705152 (Italian Patent Application MI 93 A 001004), and the immersion nozzle is patent application PCT / IT97 / It may differ from that disclosed and claimed in 00135 (Italian patent application MI 96 A 001234).

Claims (7)

A continuous casting mold 1 having copper plate walls formed on large sides, a supply nozzle having an immersion outlet or an immersion nozzle 2, and a vibrator 3 driven by a hydraulic servo controller; In units of continuous casting of steel slabs, especially suitable for thin thicknesses and high speeds, At the center of the mold horizontal cross section at the meniscus level, the distance between the immersion nozzle 2 and the copper plate remains constant, An area A1 corresponding to the center of the surface of the mold horizontal cross-section at the meniscus level and surrounded between the large sides of the mold and the immersion nozzle 2 and a mold outer circumference length corresponding to the area A1; The ratio A1 / S1 to the sum S1 is a sum of the residual area A2 of the horizontal cross section of the mold 1 and the mold outer circumference length corresponding to the residual area A2 at the meniscus level ( 0.9 to 1.1 times the ratio with S2) (A2 / S2), A unit of continuous casting of a steel slab, characterized in that the standard distance Nd between each point of the inner surface of the mold walls and the ideal surface shell E of all ends of the cooling pipe W is constant. 2. The unit of claim 1, wherein the ratios A1 / S1 and A2 / S2 are the same. 3. The unit of claim 1, wherein the constant value Nd is in the range of 10 to 25 mm. 4. The ratio of the altitude measured in mm from the apex to the apex of the standing wave formed on the meniscus in the mold, and the casting speed in m / min, is an average value of 3.3. And a unit of continuous casting of steel slab, characterized in that it does not exceed five. The unit of claim 4, wherein the standard deviation measured in the sample signal of the mold level is in the range of 0.7 to 1.5 mm. The quadrilateral lever system (3a, 3b) according to claim 1 or 2, wherein the vibrator (3) has a hydraulic servo controller (5) capable of varying the amplitude of the vibration wave between ± 2 and ± 10 mm. Unit of the apparatus for continuous casting of steel slabs, characterized in that it is formed of 3c). 3. The hydraulic vibrator according to claim 1 or 2, wherein the vibrator (3) is resonant, and the mold (1) is mounted directly to the flexible spring, and is spaced by the hydraulic servo controller at a frequency close to the natural frequency of the elastic system. Unit of the apparatus for continuous casting of steel slabs, characterized in that driven without.

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