KR20000053199A - Improved unit of equipments for the high-speed continuous casting of good quality thin steel slabs - Google Patents

Abstract

PURPOSE: A casting unit is provided to allow to overcome the aforementioned drawbacks when wanting to cast thin slabs at high speed. CONSTITUTION: Unit of equipments for the continuous casting of steel slabs, especially low thickness slabs at high speed, comprises 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

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

Continuous casting of so-called "thin sheets" of metal, which has a thickness of 80 mm or less in the past, has had a quality problem, especially for high speed casting of, for example, 4.5 m / min or more.

This problem may lead to the following defects of the thin plate surface called shells which are formed inside the mold.

Longitudinal cracks due to trapping of casting powder,

Longitudinal and transverse cracks, due to the lack of lubrication and insulation film formed by the so-called "slag" indicating the result of the cast powder when dissolved and resolidified,

Longitudinal cracks due to thermal stress,

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 involve low productivity and thus a reduction in the economics of the production equipment. Another solution is the so-called "EMBR" (electro-magnetic brake ruler), which can flatten liquid steel waves that cause waves in the meniscus inside the mold by reducing their height. It is also possible to use an electronic device called, but such a device is very expensive and only partially solves the above-mentioned problems. However, other problems arise from the geometric and flow conditions occurring inside the mold, which can lead to detrimental effects on the process life and process efficiency of casting nozzles submerged in liquid metal and called "submerged nozzles". Can be.

It is clear that the above-mentioned problems cannot be solved in a systematic and satisfactory state by operating independently on the mold, the immersion nozzle, and the mold oscillating unit. These three elements responsible for continuous casting 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.

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

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 lines of FIG. 2 at the meniscus level to illustrate the various parts that must be considered in the geometric relationship that the mold and immersion nozzle must meet within the casting unit according to the invention. Same schematic diagram of the section cut along III-III).

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

5A and 5B are two of the molds of FIG. 4, in which the sheath of the cooling system pipe is shown in a plane parallel to the y and z axes of FIG. 4 and in a longitudinal section along the line BB of FIG. 5A, respectively. schematic.

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 sheets at high speeds.

The improved casting unit according to the invention generally has the properties of claim 1 as well as the additional properties described in the dependent claims according to a particular aspect of the invention.

These and other objects, effects and properties of the casting unit according to the present invention will become more apparent from the detailed description of suitable embodiments, which is described in a manner not limited to the examples shown in the accompanying drawings.

1 is a schematic side view of a casting unit according to the invention, which unit has a mold 1, a locked casting nozzle 2 and an oscillator 3, the locked casting nozzle 2 being referred to hereinbelow " Immersion nozzle ", wherein the vibrator 3 is hydraulically driven and tied to the mold body so as not to interfere with the casting line in this embodiment. FIG. 1 also shows a cross section of a liquid steel flow path between a shell formed along a copper wall and an immersion nozzle 2, ie two “channels” 4 formed as above.

In molds, the main problem that arises in thin sheet casting with respect to traditional molds is that, when the flow rate of molten steel is constant, the decrease in sheet thickness is due to the increase in the sheet surface contacting the mold wall in unit time and accordingly It is accompanied by an increase in the need for one "slag" lubrication. T1, W1, and V1 are the thickness, width, and average casting speed of thin sheets of conventional thickness, respectively, and T2 = T1 / a (a> 1), W2 = W1, and V2 >> V1 are the equilibrium for thin sheets. When the value in the state, in order for the steel flow rate to be the same,

T2, W2, V2 = T1, W1, V1. (Ⅰ)

Therefore, the casting area of the thin sheet in unit time is

It is 2 * (T2 + W2) * V2, and if the thickness of a thin plate is negligible compared with the width, it will become approximately 2 * W2 * V2. Substituting the result from formula (I) into the value of W2 and V2,

It becomes 2 * W2 * V2 = 2 * (T1 / T2) * W1 * V1 = a * (2 * W1 * V1). (Ⅱ)

Equation (II) shows the importance of lubricating slag formation, which is inversely proportional to thickness when a> 1, i.e. thinner thin plates, coat a wide sheet-mold contact surface for a unit of time. On the other hand, the contact surface in the mold between the molten steel and the cast powder has a small area due to the small thickness in the meniscus region in which 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 the immersion nozzles in known configurations are characterized in that, in the entire meniscus region, dissolved slag formed by powder dissolution and convex surface It should be taken into account that seeping between the wall and the wall does not maintain the required equilibrium between the spent slag.

According to the invention, a thin mold is capable of receiving a reliable, ie sufficiently thick, immersion nozzle, the nozzle having a large copper cross section in the horizontal plane that exactly matches the cross section of the immersion nozzle in the horizontal plane around the meniscus level. Plates, thereby maintaining a constant gap between the immersion nozzle and the wall at all points in the central area. According to FIGS. 3A, 3B and 3C, this spacing is A1 / S1, ie the contact area with the casting powder which is substantially proportional to the slag formation, and the immersion nozzle circumferential lamination substantially proportional to the slag consumption in FIG. The ratio between the areas is chosen to approximately match A2 / S2 measured outside the immersion nozzle area in FIG. 3C. So the formula,

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

For example, for a 1300 x 65 mm mold with a 300 mm wide immersion nozzle with a reliable thickness of 60 mm, as shown in Figures 3b and 3c, the optimum ratio of A1 / S1 = A2 / S2 The value is 30 mm. This ratio may be used to define the tendency of the mold cross section in the horizontal plane at the meniscus level, for example when the dimensions of the immersion nozzle and the small side thickness are fixed, or the dimensions of the known mold cross section. Can also be used to determine the tendency of the immersion nozzle cross section, to likewise ensure a well-balanced amount of lubricating slag along the entire mold cross section.

This geometry is also important for the flow of molten steel in the meniscus region, which means that the channel 4 in FIG. 1 created between the immersion nozzle and the shell formed opposite the copper wall is the meniscus region. As it will be large enough to prevent vortex formation due to acceleration of flow converging from the mold small side within the vortex, which may cause the powder to become clogged and cause the aforementioned disadvantages.

Suitably the molds used in the casting unit according to the invention are those having variable bending in the longitudinal direction, which is the subject of European Patent 0705152 in the name of the applicant, whereby the molds have outlets on arc casting guides other than vertical. While providing the bending of the thin plate already formed in the die, it can have an almost infinite bending radius in the upper region for a good arrangement of the immersion nozzles, whereby the height of the casting unit and, consequently, the ferrostatic force and the risk of thin sheet bumps. Advantageously reduces. According to the aforementioned patent application, the bending is graded in a gradual and uniform manner from the infinite radius of the mold inlet to the bending radius R ₀ corresponding to the casting guide of FIG. 1, whereby the bending on the solidified outer shell of the sheet To prevent overstress and the possibility of incomplete contact with the mold copper wall.

In order to solve this technical problem, the mold plate cooling unit is particularly important and must be able to withstand the high heat flux typical of thin sheets reaching an average value of 3 MW / m ² over the entire mold cooling surface and to prevent copper cracks. Despite its improvement in the meniscus area, it has a sufficiently uniform cooling around the mold to prevent thermal stress of the thin plates to be formed.

According to FIG. 4, considering 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, which in turn depends on the distance from the pipe through which the coolant flows.

As can be seen from Fig. 3, in the x, y, and z axes, the z axis uses a Cartesian axis system facing downward, that is, the bottom of the mold, and f (x, y, z ) = 0, the local surface temperature changes locally with t = t [f (x, y, z)].

The heat flux dq _n should be kept as constant as possible along the z = z ₀ horizontal line 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 achieved according to the invention by maintaining the same standard spacing Nd from the ideal surface ridges E at all ends of the cooling pipe W of FIGS. 5A and 5B at all points of the same high temperature surface, the standard The spacing Nd is obtained on a vertical line with respect to the hot surface. Therefore, it is known that Nd must be a constant and that the optimum constant value must be empirically 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 molten steel flow behavior, while also taking into account the gradual shell formation and the life of the immersion nozzle itself. At the same time as the sheet thickness decreases, problems increase with respect to the liquid motion inside the mold, which can lead to the formation of standing waves in the meniscus region and thereby a local reduction in the thickness of the liquid slag, which causes May adversely affect insulation and lubrication.

Immersion nozzles for thin sheets, also the subject of patent application PCT / IT97 / 00135 in the applicant's name, have geometrical properties that allow castings to have low emission energy and high dissipation probability inside the thin liquid volume, whirling and powder clogging Improved flow guidance by immersion side geometry, as well as improved level control within the mold. Moreover, the supply is steady, the flow is split in two, and the starting surface inside the immersion nozzle is maintained, since the oxide precipitation is negligible, and furthermore, these good flow conditions are external mechanical within the meniscus region. Reduces the extent of corrosion.

According to the present invention, the optimum design of the mold-immersion nozzle unit, in addition to the above-mentioned conditions, the ratio of the altitude (in mm) and the casting speed (in m / min) between the peaks and valleys of the standing wave does not exceed 5 and the average value is 3.3 To have.

Furthermore, the standard deviation measured for the template water level (ML) sample signal, denoted as stdDEV (ML), is typically included in the range below.

stdDEV (ML) = 0.7-1.5 mm

Finally, with reference to the third element of the unit, the oscillator 3, this should be considered sufficiently as a critical parameter for the quality of the sheet surface and the reliability of the continuous casting process. According to FIG. 1, it may be formed of a frame 3a hinged to the floor and driven by the hydraulic servo controller 5. The frame 3a is also hinged to the mold support 3b to form a kind of quadrilateral integrally with a set of springs fitted inside both ends.

Control flexibility is ensured not only by the vibration program, but also by the program logic control which can change the vibration parameters related to wave amplitude and wave shape between ± 2 and ± 10. The control continuously records the actual value of the casting speed, in order to control the vibration period based on the variable. The maximum vibration period is obtained at a value equal to 480-520 strokes / min for the first natural vibration period of the entire dynamic system of 16.7 Hz. Flexibility is such that for all steels the vibration parameters can be adjusted to obtain surface conditions and optimum lubrication as a function of casting speed.

Optionally, the vibrator may be of the so-called "resonant" type, the mold mounted directly to the bending spring without the lever system, and by the hydraulic servo control at a frequency close to the natural frequency of the elastic system, there is no gap and thus very It vibrates along a precise path.

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, the mold itself may have a cross section different from that disclosed in European Patent 0705152, in the vertical plane, if it satisfies the aforementioned geometric relationship, and the immersion nozzle may differ from that disclosed and claimed in patent application PCT / IT97 / 00135.

Claims (7)

In particular comprising a mold (1) for continuous casting, partitioned by copper plate walls on the opposite side, a supply nozzle having an immersion nozzle (2) or an immersion outlet, and a vibrator (3) driven by a hydraulic servo controller. In the unit of the apparatus for continuous casting of steel sheet, suitable for thin thickness and high speed, At least at the center of the mold horizontal cross section at the meniscus level, the spacing between the immersion nozzle 2 and the copper plate remains constant, Furthermore, the sum of the area A1 corresponding to the center of the mold horizontal cross-sectional surface at the meniscus level and enclosed between the opposite side of the mold and the immersion nozzle 2 and the mold outer length corresponding to the area A1 (S1). ) Ratio is 0.9 to 1.1 times the ratio of the total area S2 of the remaining area A2 of the horizontal section of the mold 1 to the remaining area A2 at the meniscus level, A unit of the apparatus for continuous casting of steel sheets, characterized in that the standard spacing Nd between each point of the inner surface of the mold wall and the ideal surface envelope E of all ends of the cooling pipe W is constant. The unit according to claim 1, wherein the ratios (A1 / S1, A2 / S2) are the same. The unit according to claim 1 or 2, wherein the constant value (Nd) has a range of 10 to 25 mm. 4. The ratio of the altitude measured in mm between the peaks and valleys of standing waves formed on the inner meniscus of the mold and the casting speed in m / min has an average value of 3.3 and is greater than 5. The unit, characterized in that not. 5. The unit of claim 4, wherein the standard deviation measured on the mold water level sample signal ranges from 0.7 to 1.5 mm. The quadrilateral lever system 3a according to any one of claims 1 to 5, wherein the vibrator 3 has a hydraulic servo controller 5 that can vary the amplitude of the vibration wave between ± 2 and ± 1.5 mm. , 3b, 3c). 6. The vibrator (3) according to any one of the preceding claims, wherein the vibrator (3) is resonant and the mold (1) is mounted directly to the bend spring, and to the hydraulic servo controller at a frequency close to the natural frequency of the elastic system. Driven by and having no gap.