CN1237120A - Modified device for equipment for high-speed continuous casting of thin, high-quality flat steel ingots - Google Patents

Abstract

Device for an apparatus for continuous casting of slabs, particularly small-thickness slabs, suitable for casting at high speed, comprising a mould (1), the mould (1) being fed through a submerged nozzle (2) and being connected to a vibration device (3), the vibration device (3) being driven by a hydraulic servo-control, wherein the following geometrical relations are established in relation to the shape of the mould and the submerged nozzle and to the arrangement between them, (A1/S1)/(A2/S2) =0.9 ÷ 1.1 and, preferably, A1/S1= A2/S2, wherein A1 is the area enclosed between the submerged nozzle and the larger side of the mould in horizontal section at the level of the meniscus surface, A2 is the area of the rest of the section, between the submerged nozzle and the smaller side of the mould, s1 and S2 are the sum of the lengths of the mold peripheries corresponding to the above two areas, respectively. Further, the distance between the submerged nozzle and the copper plate constituting the side wall of the mold is kept constant at least in the horizontal section of the mold at the level of the meniscus surface.

Description

Improved plant for high-speed continuous casting of thin, high-quality flat steel ingots

The present invention relates to an improved arrangement of equipment for high-speed continuous casting of thin high-quality flat steel ingots.

The so-called "thin flat steel ingot" has a thickness of less than 80mm. So far, continuous casting of this "thin flat steel ingot" has encountered some quality problems, especially when casting at a high speed, such as a casting speed of 4.5m This is especially the case above /min.

These quality problems can lead to some cracks on the surface of the flat steel ingot, which is what we usually call the shell, which is formed in the mold: -- due to the encapsulation of the mold powder to form a longitudinal Cracks; - longitudinal and transverse cracks due to lack of lubrication and barrier films formed by so-called "slag", where the term "slag" refers to the product obtained after melting and re-solidification of casting powder; - longitudinal cracks due to thermal stress; and - longitudinal cracks due to discontinuity of the copper cooling surface.

These quality problems mainly affect special steels and can be solved at least partly by reducing the casting speed, which however leads to a reduction in productivity and thus the economics of the plant. Another possible solution is to use an electromagnetic device called an "EMBR" (Electromagnetic Brake Ruler), which smooths steel fluctuations by reducing the height of the steel fluctuations that make The meniscus surface in the mold is corrugated, but such an electromagnetic device is expensive and only partially solves the above-mentioned problems. And other problems due to geometrical conditions and flow conditions in the mold may shorten the life of the sprue (the sprue is immersed under the liquid metal, often called "submerged sprue"), and also affect the process. efficiency has detrimental effects.

From this it is clear that measures for the casting mold, the submerged spout and the mold vibration device alone cannot solve the above-mentioned problems satisfactorily and systematically. These three parts are closely related to each other, forming an actual "ingot casting device", and these three parts are effective for continuous casting, so that it is only possible to find a method if the whole device is taken as a whole. effective solution.

It is an object of the present invention to provide a casting device which overcomes the disadvantages mentioned above when casting thin flat steel ingots at high speeds.

The improved assembly of cast parts proposed according to the present invention generally has the features described in claim 1, while according to particular aspects of the invention, the assembly of cast parts also has the features described in the dependent claims additional features.

These and other objects of the casting device proposed according to the present invention emerge from the following detailed description of a preferred embodiment of the present invention, by way of non-limiting example with reference to the accompanying drawings , advantages and features will become more apparent in the attached figure:

Fig. 1 is a schematic side view showing a casting device proposed according to the present invention; Fig. 2 is a view showing the top of the mold itself and the submerged casting hole obtained by observing the direction of the arrow II shown in Fig. 1; Figures 3a, 3b, and 3c are schematic diagrams showing the same cross-section obtained along line III-III in Figure 2, where line III-III is at the height of the meniscus surface, and these cross-sectional diagrams are for detailed It clearly shows the multiple parts considered in the geometric relationship that the mold and the submerged casting gate in the ingot casting device proposed according to the present invention must satisfy the geometric relationship; Fig. 4 is a plan view representing the same mold, in Cartesian three The mold is shown schematically in the coordinate system; Figures 5a and 5b are two diagrams showing the mold shown in Figure 4, in longitudinal section through a plane view parallel to the Y and Z axes in Figure 4 The outer envelope surface of the cooling system duct is shown and also shown through a cross-sectional view taken along line B-B in Fig. 5a.

Referring to the accompanying drawings, Fig. 1 is a schematic diagram of a casting device according to the present invention, wherein there is a casting mold 1, a sprue 2 submerged in the liquid surface and a vibrating device 3, and the sprue 2 submerged in the liquid surface is often referred to as It is a "submerged casting port", and according to this embodiment, the vibrating device 3 is hydraulically driven and fixedly connected with the mold body, obviously, this can not hinder the casting process. Fig. 1 also shows the section where molten steel flows between the submerged casting hole 2 and the shell formed along the copper side wall, that is, two "channels" 4 are formed.

As for the casting mould, compared to conventional casting devices, when casting thin flat steel ingots, the main problem that arises is that the flow rate of the molten steel is the same, reducing the thickness of the flat steel ingot will increase in unit time with The surface of the ingot in contact with the mold walls, thereby increasing the need for lubricating "slag", which has been defined above. In fact, T1, W1, V1 are the thickness, width and average casting speed of a flat steel ingot of common thickness, and for a thin flat steel ingot, the corresponding parameters are T2=T1/a(a>1), W2=W1 And V2>>V1, the flow rate of molten steel is the same, then:

T2·W2·V2=T1·W1·V1 (I)

Therefore, the area of the thin steel ingot cast per unit time is:

2·(T2+W2)·V2, if the thickness of the thin steel ingot is negligible relative to its width, then 2·(T2+W2)·V2 is approximately equal to 2·W2·V2. Substituting the values derived from equation (I) for W2, V2, then:

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

Since a > 1, the above equation (II) clearly shows the importance of forming a lubricating slag to cover the contact surface between the ingot and the mold per unit time, the contact surface between the ingot and the mold and the surface of the ingot The thickness is inversely proportional, the thinner the steel ingot is, the larger the contact surface between the steel ingot and the casting mold per unit time is. On the contrary, due to the small thickness and the existence of the submerged casting hole in the middle area, in the mold, in the meniscus surface area, the interface between the molten steel and the mold powder has only a small area, and the above-mentioned slag It is at this interface that it is formed.

Although this problem can be partly solved by using mold powders which enhance the formation of slag, in known constructions it has to be taken into account that the submerged sprue cannot remain molten over all the meniscus surface area The balance between the molten slag formed by the melting of the mold powder and the consumed slag due to its infiltration between the meniscus surface and the inner wall.

According to the invention, the thin casting mold is capable of accommodating a solid submerged sprue, that is to say a sufficiently thick submerged sprue, around the level of the above-mentioned meniscus surface, in the horizontal plane, the size of the mold The profile of the copper plate exactly matches the profile of the spout in the same horizontal plane, so that the distance between the sprue and the inner wall remains constant everywhere in the central region. Referring to Figures 3a, 3b and 3c, the value of this distance mentioned above is chosen so that the ratio A1/S1 is approximately the same as A2/S2, A1/S1 is the area of the interface with the mold powder and the flat steel ingot around the submerged casting hole The ratio of the area, in which the area of the interface with the mold powder is actually proportional to the formation of slag, and the area of the flat steel ingot around the submerged casting mouth is actually proportional to the consumption of slag (see Figure 2), The ratio A2/S2 is metered outside the submerged gate area (see Figure 3c). The equation that needs to be satisfied is thus:

(A1/S1)/(A2/S2)=0.9÷1.1, and preferably=1

For example, for a 1300 x 65mm mold with a 300mm wide submerged opening (as shown in Figures 3b and 3c, with a reliable thickness of 60mm), the optimal ratio is A1/S1=A2 /S2 is equal to 30mm. For example, once the size of the submerged sprue and the thickness of the smaller side are determined, this ratio can be used to determine the shape of the mold profile in the horizontal plane at the height of the meniscus surface, or, if the mold is known, The size of the contour, this ratio can be used to determine the contour shape of the submerged casting hole, the purpose of doing this is also to ensure a good balance of the amount of lubricating slag along the entire mold contour.

This geometry is also important for the flow of molten steel in the area of the meniscus surface, since the aforementioned "channels" will be large enough to prevent the The eddy currents formed, in the meniscus surface area, these eddy currents often cause the mold powder to be wrapped up and cause the above-mentioned defects, wherein the above-mentioned "channel" is represented by the number 4 in Figure 1, which is formed by the submerged casting port. and the area between the shell formed against the copper inner wall.

It should be noted that the casting mold used in the casting device according to the invention preferably has a varying degree of curvature in its longitudinal direction, which is the subject matter of the applicant's European patent 0705152, in order to better place the submerged casting hole in The upper region of the mold has an almost infinite radius of curvature, while the curved ingot already formed inside the mold has an outlet which is provided on a circular rather than vertical casting guide, which can be easily Advantageously, the height of the casting device is reduced and the risk of hydrostatic pressure and billet heaving is correspondingly reduced. According to the aforementioned patent application, the curvature of the mold is distributed in a gradual and uniform manner from an infinite radius at the entrance of the mold to a radius of curvature Ro corresponding to the leading part of the casting (Fig. 1), This avoids both undue stress on the shell of the solidified ingot and the possibility of imperfect contact with the inner copper walls of the mould.

In order to solve the technical problems involved, the means for cooling the casting formwork are of particular importance, which must be able to withstand the high heat fluxes (over the entire cooling surface of the casting mold) which are common for thin flat steel ingots. Its average value can be as high as 3MW/m ² ), in the meniscus surface area, the cooling device has an enhanced cooling effect to prevent copper cracking, however, in order to prevent the thermal stress of the formed steel ingot, the cooling effect around the mold Still even enough.

Referring to Figure 4, when considering the nominal specific heat flux (dq _n ) between the surface of the casting and the mold, there should be:

dq _n =dq/dA[W/m ² ]

The heat flux is also a function of the local surface temperature on the hot surface of the copper plate, which in turn depends on the distance from the cooling tube in which the cooling water flows.

As can be seen in Figure 3, the Cartesian coordinate system X, Y, Z is adopted, where the Z axis is directed downwards or towards the bottom of the mold, and it is also taken into account that f(x,y,z)=0 is satisfied by For a complex surface formed by a mold, the local surface temperature changes locally according to t=t[f(x,y,z)].

Because the heat flux dq _n must be kept as constant as possible along a horizontal line on the mold surface (where Z=Z ₀ ), that is, the temperature t must be kept as a constant along such a horizontal line, thus:

t=t[f(x,y,z ₀ )]=t ₀

According to the invention, this is done by maintaining the same vertical distance Nd between each point on the hot copper surface and the ideal envelope E (Fig. 5a, 5b) of the ends of all cooling tubes W , where Nd is measured perpendicular to the hot surface. Then there must be a Nd=constant, and in order for the cooling system to meet the aforementioned conditions, it can be found through experiments that the optimum value of this constant must be in the range of 10 to 25mm.

As for the submerged sprue, in addition to its aforementioned dimensional conditions relative to the mold, it must be designed to optimize the flow of molten steel, taking into account the gradual formation of the shell and the submerged sprue itself. life. In fact, it is known that when the thickness of the slab is reduced, the problems related to the movement of the liquid in the mold increase, which can lead to the formation of standing waves in the meniscus surface area which can lead to the thickness of the liquid slag Local reduction, which would have a detrimental effect on the lubrication and insulation of the shell of the solidifying slab.

A submerged casting gate for thin flat steel ingots is the subject of the applicant's patent application PCT/IT-97/00135, which submerged casting for thin flat steel The sprue has geometrical features that allow casting with low energy at the exit and high probability of power loss inside the liquid part of the slab, which improves the flow (thus preventing The formation of the eddy current and the encapsulation of the mold powder). Further, the feed is stable, since the deposition of oxides is negligible, so that the liquid stream exits in roughly two streams and the starting surface inside the submerged spout is protected; and those good flow conditions make The degree of external mechanical erosion in the meniscus surface area is reduced.

According to the present invention, in addition to the aforementioned conditions, the optimal design for the mold-submerged sprue arrangement is such that the ratio between the height of the standing wave (measured in mm peak-to-peak) and the casting speed does not exceed 5, which The average value is 3.3, where the casting speed is measured in m/min.

Further, for the sampling signal of the mold liquid level (ML), the measured standard deviation is represented by Std DEV (ML), and its value is usually in the following range:

Std DEV(ML)=0.7-1.5mm

Finally, the third component of the ingot casting plant, vibrating device 3, must also be considered as a key factor for the surface quality of the ingot and the stability of the continuous casting process. Referring to FIG. 1 , the vibrating device can be constituted by a frame 3 a hinged on the ground and driven by a hydraulic servo control mechanism 5 . The frame 3a is also hinged to the mold supports 3b so as to form a quadrilateral structure and has a set of springs fixed at both ends thereof.

The flexibility of control is obtained through programmable logic control, which can change the vibration parameters related to the waveform. The amplitude of the fluctuation is between ±2 and ±10mm, and the same is true for the vibration program. The control system continuously records the actual value of the casting speed, thereby controlling the vibration frequency on the basis of the previous parameters. Since the first natural frequency of the entire power system is 16.7 Hz, the maximum vibration frequency is as high as 480-520 reciprocations/min. The flexibility mentioned above is such that for each quality of steel, the optimum lubrication and surface quality can be obtained by adjusting the vibration parameters as a function of the casting speed.

In another way, the vibration device can be of the so-called "resonance" type, the mold is directly mounted on the bending spring, with a position system (Without no lever system), through a hydraulic servo control mechanism to make it close to the elastic system Vibrates at a frequency of natural frequency, and has gaps so that it moves along a very precise path.

Those skilled in the art may make some additions and/or modifications to the illustrated embodiments above without departing from the scope of the present invention. In particular, as long as the aforementioned geometrical relationships are observed, the casting mold itself can have other profiles in the vertical plane than those described in European Patent 0705152, and the submerged casting gate can also be compared to that described in patent application PCT/IT-97/ The submerged spout disclosed and claimed in 00135 is different.

Claims (7)

1. device that is used for the equipment of direct casting cast slab, be particularly suitable for little thickness and high-speed, comprise a mold (1) that is used for direct casting, feeding outlet and a vibrating device that drives by the hydraulic servo controlling organization (3) with the outlet of diving or latent casting nozzle (2), wherein mold (1) is that sidewall by copper coin limits in its bigger side, it is characterized in that at least the zone line in the mold horizontal cross-section at the height and position place that is positioned at the falcate surface, the distance of diving between casting nozzle (2) and the copper coin remains constant; And, area (A1) and be area (A2) and corresponding to 1.1 times of 0.9 ÷ of the ratio between the summation (S2) of the mold peripheral length of this area (A2) corresponding to the ratio between the summation (S1) of the mold peripheral lengths of this area (A1), wherein area (A1) is corresponding to the mid portion in the mold horizontal cross-section of falcate apparent height position, this area is the area that is surrounded between a bigger side of mold and latent casting nozzle (2), and area (A2) is the area at the remainder of mold (1) horizontal cross-section of falcate apparent height position; Further, wherein the vertical range (Nd) between the desirable enveloping surface (E) of all ends of the every bit on the mold inner wall surface and each cooling tube (W) is a constant.

2. device as claimed in claim 1 is characterized in that above-mentioned ratio A 1/S1 and A2/S2 are basic equating.

3. device as claimed in claim 1 or 2 is characterized in that above-mentioned its scope of steady state value Nd is 10 to 25mm.

4. as claim 1 or 3 described devices, it is characterized in that in the mold that the ratio of the height of formed standing wave and casting speed can not surpass 5 on the falcate surface, its mean value is 3.3, wherein the height of standing wave measures with mm from peak to peak, and casting speed is measured with m/min.

5. device as claimed in claim 4 is characterized in that the sampled signal of mould level is measured its number range of standard deviation of gained between 0.7 to 1.5mm.

6. as each described device in the claim of front, it is characterized in that vibrating device (3) is by four push and pull system (3a, 3b, 3c) constitute, this vibrating device (3) has hydraulic servo controlling organization (5), this hydraulic servo controlling organization (5) can ± 2 and ± change the amplitude of vibration and wave motion between the 10mm.

7. as any one described device among the claim 1-5, it is characterized in that vibrating device (3) is a resonance type, wherein mold (1) is directly installed on the bending spring very close to each otherly, drives by the frequency of a hydraulic servo controlling organization with the intrinsic frequency that approaches elastic system.

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