EP0449771B1 - Controlled feeding of molten metal into the moulds of an automatic continuous casting plant - Google Patents

Abstract

Process for the removal of peroxides from a vegetable material which has been bleached with the aid of hydrogen peroxide and is dry, which consists in subjecting the said material solely to a heat treatment at a temperature equal to or greater than 60 DEG C in a confined atmosphere so that its weight content of water remains at its initial value during the said treatment.

Description

The invention relates to a method and a device for feeding molten metal into the molds which are internally insulated in the upper region of an automatic continuous casting installation with an upstream casting furnace and a pouring channel system which comprises a distribution trough which feeds all molds at the same level with metal, the lower one in the In the inner ring area, a gas cushion preventing direct contact of the mold with the metal is maintained and oil is injected into its area. The invention further relates to an application of the method.

In the continuous casting process, metals are cast in the form of bars or bolts several meters long, which are used as primary material for various subsequent processing steps, such as for pressing, rolling or forging.

The most important element of a continuous casting machine are the molds, which determine the cross section of the cast strand in conventional processes. Depending on the number of cast strands, a casting machine is equipped with a corresponding number of lowerable approach floors, which are firmly connected to a casting table.

During the continuous casting, the molten metal flows, possibly with at least one filter switched on, through a channel system from the casting furnace into the casting machine, where it is distributed into the individual molds.

As the molds slowly fill with the melt, the metal begins to solidify on the dried start-up floors. The approach floors are then cooled and lowered at such a speed that the solidus line of the solidified metal always remains within the mold frame. The strands, the solidification of which is accelerated by water cooling, grow downwards to the same extent as the approach floors are lowered. The casting process is uninterrupted within the specified length of a strand.

One of the main disadvantages of conventional continuous casting processes is that the level in each individual mold has to be checked separately and that long molds are required. The resulting secondary effects lead to a lower surface quality.

For this reason, the so-called hot top process was developed a long time ago, in which the metal flows into a distribution trough (hot top) that feeds all molds. The level control devices of all individual molds can be omitted and replaced by a central control element, which allows a quieter metal surface and a simplified casting process.

The usual hot top casting process has been further developed by the formation of a gas cushion with automatic lubrication to a semi-continuous casting process, in which a direct contact between the liquid metal and the mold is prevented thanks to the air cushion and an oil film in the top area.

• - Oberflächensegregationen und versteckte Kaltschweissstellen werden durch die milderen Kühlungsbedingungen weitgehend verhindert.
• - Segregation und Ausfliessen von Schmelze durch kleine Oeffnungen des bereits erstarrten Metallmantels werden durch die niedriger ausgebildete Kokille verhindert.
• - Reibung und Ausbrüche werden verhindert, weil die Kontaktoberfläche zwischen dem Metall und der Kokille wegen des Gaskissens kürzer und das Schmiermittel besser verteilt ist.

The compressed air to form the gas cushion is introduced in the upper part of the mold, below an internal insulation. The following additional advantages can be achieved with a gas cushion compared to the usual hot top casting process, especially in combination with an oil film:

• - Surface segregation and hidden cold welds are largely prevented by the milder cooling conditions.
• - Segregation and outflow of melt through small openings in the already solidified metal jacket are prevented by the lower mold.
• - Friction and breakouts are prevented because the contact surface between the metal and the mold is shorter due to the gas cushion and the lubricant is better distributed.

US Pat. No. 4,157,728 describes a hot top continuous casting method of the type mentioned above, an annular air cushion being formed underneath the hot top. A slight overpressure is necessary for this. The overpressure is set manually using a screw.

Air and oil are supplied in the same area, but separately.

Further improvements in the method have recently been sought, in particular in the direction of the so-called air slip method, as described in US Pat. No. 4,598,763. The upper interior of a mold is covered with an open-pore graphite ring. Air and oil can be mixed or fed separately through the pores of the graphite ring into the mold interior. Graphite is self-lubricating, the oil is not added primarily as a lubricant, but as a pore filler. Water is only sprayed on below the graphite ring.

With a graphite ring, through which air and oil flow, very mild, i.e. advantageous, cooling can be achieved. However, the use of a graphite ring has the disadvantage of being complex and complex for automating the corresponding casting process.

The present invention has for its object to provide a method and a device of the type mentioned, which allow further automation of the hot top casting process.

With regard to the method, the object is achieved according to the invention in that a common main line with distribution lines leads air or an inert gas with the same, low overpressure into all molds, and the relative pressure between a setpoint value, which is calculated as a function of the metal height measured via a level probe and an actual value measured in the main line with a transmitter is used for program-controlled control and monitoring, in that the control function is fulfilled by means of a processor by emitting a signal for the actuator of a common pressure control valve.

For example, nitrogen and / or argon are used as inert gases. However, air is generally used for reasons of cost, which is why the term air also includes inert gases in the following for the sake of simplicity.

The height of the metal surface can be measured with a level probe known per se, but also with a laser sensor. The actual pressure measured in the main line shows no fluctuations due to the large diameter with small pressure losses.

The external pressure, which changes considerably depending on the weather, should not influence the casting process. According to a preferred embodiment of the invention, the influence of the variable external pressure is therefore automatically compensated for by known means by using a commercially available differential pressure meter.

At the start of casting, there is no liquid metal in the mold that opposes the gas flow resistance. A first higher value is set with a flow meter. When the liquid metal that is then passed into the mold reaches the gas outlet opening, the gas flow rate drops because of the gradually increasing metallostatic resistance. If the gas flow falls below a second, lower value, the lowering of the casting table with the approach floors for the cast strands is triggered after a short time. Without metal, an air flow of 12 - 15 NI / min is reached as the first value, the second value for triggering the lowering via the flow rate difference is about 8 - 10 NI / min. A few seconds, expediently about 5 seconds, after this second value has been reached, the casting table begins to lower. The flow is controlled with the relative pressure, the difference between the setpoint and actual value for the pressure in the main line.

Because of the low gas flow through the distribution lines branching off from the main line, their length is irrelevant; all molds are supplied under the same conditions.

On the other hand, the supply of all molds with the same amount of oil was previously only guaranteed if the individual oil lines leading to the molds were all of the same length from the main oil line. This is no longer a requirement today, with known means the same amount of oil can be supplied to all molds per unit of time, regardless of the line resistance.

The oil required for lubrication is preferably injected in pulses into the area of the gas cushion. This means that the oil can be injected at a higher pressure without the overall consumption becoming too high.

The outlet channels for the gas and the oil can be separate or combined into one channel.

The pressure in the gas cushion must not exceed a certain maximum value, otherwise gas bubbles will form in the metallic melt. However, the pressure of the gas cushion must also not fall below a certain minimum value, otherwise the molten metal can penetrate into the gas supply channels. The minimum and maximum values for the pressure in the gas cushion are linear to the respective metallostatic pressure in the mold. The minimum pressure, which must not be fallen below, corresponds to a function of the density p, the acceleration due to gravity g, the metal level above the gas outlet openings, the interfacial tension of the melt in the insulation / mold area and the surface tension of the melt in the gas cushion area. The maximum pressure in the gas cushion, which must not be exceeded, is a function of the density of the melt p, the acceleration due to gravity g and the depth of the undercut of the insulation.

With regard to the device, the object is achieved according to the invention in that it has a main line for the gas supply with a servo pressure valve and a transmitter on the system side and a computer comparing the actual pressure control variables of the transmitter and the control variable of the setpoint value, a manipulated variable for the actuator of the Pressure control valve triggering processor includes.

The setpoint is determined arithmetically on the basis of the metal level measured, for example, with a laser sensor.

The distribution lines branching off from the main line to the molds consist, for example, of rubber or a plastic with an external, reinforcing and protective metal fabric.

The main line for the gas supply expediently has an inside diameter of 5-10 cm. The branching distribution lines preferably lead directly to the molds without secondary lines. The main line is preferably oversized, i.e. the sum of the cross section of all distribution lines is substantially below the cross section of the main line, preferably at least 20%. It has already been mentioned that the distribution lines do not have to be the same length. The cross-section here and the rest always means the inner cross-section.

In order that there remains a relatively greater margin between the minimum and maximum permissible pressure in the gas cushion, the lower edge of the insulation layer projecting above the mold is preferably undercut. The optimal value for this undercut has been found to be about 10 mm, which makes it easier to form a stable gas cushion. Although the undercut can take on any geometrical shape, it preferably runs as a cone-shaped bevel.

An optionally removable laser sensor is expediently used as the level measuring device for determining the same metal level everywhere in the trough system and in the molds.

The application of the method according to the invention lies primarily in the automation of the start-up and casting end as well as the quality control during the stationary phase of the continuous casting.

• - Fig. 1 eine perspektivische Teilansicht einer Hot Top-Giessmaschine,
• - Fig. 2 einen teilweisen Vertikalschnitt durch den Kokillenbereich einer Hot Top-Giessmaschine,
• - Fig. 3 Kurven für den metallostatischen Druck in Funktion des Metallstandes,
• - Fig. 4 eine automatische Druckregelung,
• - Fig. 5 den Durchfluss von Luft während des Giessens, und
• - Fig. 6 den Durchfluss von Luft und die Luftverluste.

The invention is explained in more detail with reference to the exemplary embodiments shown in the drawing, which are also the subject of dependent claims. They show schematically:

• 1 is a partial perspective view of a hot top casting machine,
• 2 shows a partial vertical section through the mold area of a hot top casting machine,
• 3 curves for the metallostatic pressure as a function of the metal level,
• 4 shows an automatic pressure control,
• 5 shows the flow of air during casting, and
• 6 shows the flow of air and the air losses.

The basic sketch shown in FIG. 1 of the hot top continuous casting known per se essentially comprises a pouring channel system 10, hot tops 12 made of refractory material, also called hot heads, molds 14, cast strands 16 and a casting table 18.

The trough system 10, in which the metal flows at the same level in all channels in the direction of the arrow 20, comprises a distribution trough 22. This serves as a reservoir for liquid metal. The individual gutters merge into grooves 24 of the hot top 12. Corresponding to the arranged molds 14, the grooves 24 also run in the transverse direction and merge into bores through the hot top 12 above the molds 14. This ensures that the metal level only has to be measured at one point. This level is the same in the entire casting machine within the measurement tolerances.

A number of approach floors 28 corresponding to the number of molds 14 are arranged on the casting table 18, which is lowered in the direction of the arrow 26.

2 shows a hot top 12, a mold 14 and a cast metal strand 16 in detail.

As shown in FIG. 1, the hot top 12 conducts the molten metal 30 into the molds 14 via grooves 24. The hot top 12 is made of fire-resistant insulating material.

The mold 14 consisting of three rings has an annular inner insulation 32 in the upper inner region, which prevents the contact of the molten metal 30 with the upper region of the mold 14.

The insulation 32 has an undercut bevel 34 in the lower region. The insulation ring 32, which is made of a refractory material, is pressed onto the mold 14 by means of a pressure plate 36. An O-ring, not shown, ensures the tightness between the mold 14 and the insulation ring 32.

The inner surface of a lower mold ring 38 determines the diameter of the strand 16. Water 44 is sprayed onto the strand 16 via channels 42 from the ring-shaped water reservoir 40.

A middle mold ring 46 contains an annular oil chamber delimited by the lower mold ring 38 with outlet channels 50, which open out directly below the inclined surface 34 of the insulation ring 32. The oil chamber 48 is fed via radial channels, not shown, which are recessed from the lower ring 38 or from the middle ring 46 and are delimited by the respective other ring.

An upper mold ring 52 contains an annular air chamber 53 with radial puncture channels (not shown) between the middle and the upper mold ring.

The air is conducted into the mold interior with a slight excess pressure, in the range of approximately 45 mbar, immediately below the bevel 34 of the insulation 32. This creates an annular air cushion 54. This alleviates the cold shock of the molten metal 30 striking the mold 14.

Air and oil emerge in the same area, in the annular air or gas cushion 54, in the present case separately.

Between the molten metal 30 and the solidified part 56 of the strand 16, between the liquidus surface L and the solidus surface S, a pasty area 58 with a mixture of liquid and solid phase is formed.

The vertical distance between the common level 60 of the molten metal 30 in the channel system 10, the grooves 24 and the mold 14 and the transition from the bevel 34 of the insulation ring 32 to the mold 14, in the region of the air outlet channels, is referred to as the metal level H ₁ . The metal level H ₁ is in the range of 200 mm. The insulation 32 has a chamfer depth H ₂ of approximately 10 mm. The sum of H ₁ + H ₂ is denoted by H.

The pressure in the air cushion 54 must not fall below the metallostatic pressure in the depth H1, increased by the interface and surface tension, and must not exceed the depth H for the reasons mentioned above.

• p ⁼ pgH₁,

wobei p die von der Legierung und der Temperatur abhängige Dichte des schmelzflüssigen Metalls ist und g der ortskonstanten Erdbeschleunigung entspricht. Die nach dieser Formel berechneten Werte sind in Fig. 3 auf der Kurve C eingetragen.3, the metallostatic pressure is plotted as a function of the metal level H ₁ . The metallostatic pressure p is calculated as follows:

• p ⁼ pgH ₁ ,

where p is the alloy-dependent and temperature-dependent density of the molten metal and g corresponds to the constant gravitational acceleration. The values calculated according to this formula are plotted on curve C in FIG. 3.

The values measured for optimal casting conditions are plotted on curve A, which is slightly above the theoretical curve C. The distance is about 2 mbar.

Finally, curve B shows the values for the onset of blistering. Theoretically, bubble formation begins when, in the above formula, with the addition of the interfacial and surface tension mentioned above, H is substituted for H ₁ in the above formula, where H = H ₁ + H ₂ (Fig. 2).

Fig. 3 can be used in practice to read the optimum pressure to be used for a given metal level. As already mentioned, this is at or just below 50 mbar.

Fig. 4 shows a main line 62 of the compressed air supply, which is passed through a pressure control valve 64. After branching off to a transmitter 66 for the actual pressure, distribution lines 68 leading from the main line 62 to the molds branch off. The number of distribution lines 68 corresponds to the number of molds in the casting machine, for example up to 36.

A control variable is passed from the transmitter 66 to a processor 70. There, the control variable corresponding to the actual pressure is compared with a control variable calculated by a computer 72 for the target pressure dependent on the metal level. If there is a relative pressure, i.e. a pressure difference between the setpoint and actual pressure, the processor triggers a signal called the manipulated variable, which acts on the actuator 74 of the pressure control valve 64 and changes it depending on the sign and absolute value of the detected Ap. The actuator 74 can be a stepper motor or a DC motor, for example.

With this automatic pressure control, a target value dependent on the metal level H ₁ (FIG. 2) is continuously calculated, which is compared with the actual value of the air supply. The pressure in the air cushion is automatically adapted to a changed metal level by changing the pressure in the main line 62.

The air flow V shown in FIG. 5 per unit of time and mold is plotted as a function of the casting time t. At the start of casting t ₁ , the air flow rate V _{A is} relatively high. With the onset of the supply of liquid metal and increasing metal level, the air flow drops relatively steeply. When the air volume V ₁ is reached, a signal for lowering the casting table is triggered with a delay of about 5 seconds. In the present case, a cold run K occurs shortly after the minimum target value V _s of about 2 to 3 mbar has been reached. Due to poor strand quality, air can escape between the mold and your strand. After a short time, the quality is normal, the air flow drops again to the minimum setpoint V _s . At the end of casting, at time t ₂ , the metal level in the mold drops, and the air flow V rises correspondingly steeply. When V ₂ is reached, a signal for the end of casting is triggered.

The regulator pressure, in the present case in stationary normal operation 45 mbar, is indicated by a dashed line 76. The dotted line 78 shows the pressure curve after a length of 3 m in a main line with an inner diameter of 6 mm.

5 that the air flow V is greater at a lower pressure p in the main line.

Fig. 6 shows that the air flow Q corresponds to the sum of all air losses. The air flow is determined by a flow meter 80.

The losses between the flow meter and the mold, in lines, couplings, filters, valves, pressure regulators, etc. are denoted by Q ₁ , the losses in the mold itself by Q ₂ .

• - Q₃: Undichtigkeiten zwischen dem Isolationsring 32 und der Kokille 14,
• - Q₄: Undichtigkeiten des Isolationsrings 32 (z.B. Risse),
• - Qs: Blasenbildung, wenn der Druck des Luftkissens über dem maximal zulässigen Druck liegt,
• - Q₆: Reaktion der Luft mit der Schmelze und/oder dem Schmiermittel,
• - Q₇: Undichtigkeiten zwischen der Kokille und dem gegossenen Strang (Oberflächenrauhigkeit des Strangs, Zustand der Kokillenwand).

The following air losses occur in the area of the air cushion 54:

• Q ₃ : leaks between the insulation ring 32 and the mold 14,
• Q ₄ : leaks in the insulation ring 32 (for example cracks),
• - Qs: blistering when the pressure of the air cushion is above the maximum allowable pressure,
• Q ₆ : reaction of the air with the melt and / or the lubricant,
• - Q ₇ : leaks between the mold and the cast strand (surface roughness of the strand, condition of the mold wall).

The losses Q ₁ to Q ₄ are due to the condition of the system; they must be negligibly small in the case of functional systems.

The air losses Q ₅ and in particular Q ₇ allow conclusions to be drawn about the quality of the cast strand.

Of course, as mentioned, other gases, in particular nitrogen or argon, can be used instead of the air listed in the examples. The essential features of the invention are not affected by this, although the loss Q ₆ disappears.

Claims (10)

1. Process for feeding molten metal (30) into the moulds (14), internally insulated in the upper region, of an automatic continuous casting plant comprising an upstream pouring furnace and a runner system (10) which includes a distributing trough (22) feeding all of the moulds (14) with metal to the same level (60), a gas cushion (54) preventing direct contact between the mould (14) and the molten metal (30) being maintained in the region situated below an inner ring (32) and oil being injected into this region, characterised in that a common main (62) with distributing lines (68) leads air or an inert gas at the same slight excess pressure into all of the moulds (14) and the relative pressure between a desired value program-calculated as a function of the metal level (H_i) measured by a level probe and an actual value measured in the main (62) by a measuring transducer (66) serves for the program-controlled control and monitoring, the controller function being performed by means of a processor (70) delivering a signal for the actuator (74) of a common pressure control valve (64).

2. Process according to claim 1, characterised in that the variable external pressure is compensated for.

3. Process according to claim 1 or claim 2, characterised in that, at the start of casting, without molten metal (30), the air flow rate (V) per mould (14) reaches a first higher value (V_A) and, with flowing metal (30), lowering of the casting plate (18) with the starting bases (28) is triggered shortly after the air flow rate (V) falls below a second lower value (V_i), the first value (V_A) preferably being 12 - 15 NI/min at a set pressure of approximately 45 mbar, the second value (Vi) being approximately 8 - 10 NI/min and the delay after the second value is reached being approximately 5 sec.

4. Process according to one of claims 1 to 3, characterised in that the minimum and the maximum pressure in the gas cushion (54) are set as a function of the metallostatic pressure, the minimum pressure being a function of the density of the melt (p), the acceleration due to gravity (g), the metal level (Hi), the interface strain of the melt (30) in the region of the insulation (32) and the mould (14) and the surface tension of the melt (30) in the region of the gas cushion (54), and the maximum pressure being a function of the density of the melt (30), the acceleration due to gravity (g) and the depth (H₂) of the bevel (34) of the insulating ring (32).

5. Process according to one of claims 1 to 4, characterised in that the same quantity of oil is supplied to all of the moulds (14) per unit of time irrespective of the line resistance and the oil is preferably injected in pulses into the region of the gas cushion (54).

6. Device for carrying out the process according to one of claims 1 to 5, comprising internally insulated moulds (14) of an automatic continuous casting plant, an upstream pouring furnace and a runner system (10) which includes a distributing trough (22), characterised in that it includes a main (62) for the gas supply with a servo-control valve (64) and a measuring transducer (66) at the plant side and a processor (70) comparing the actual pressure controlled variables of the measuring transducer (66) and the controlled variable of the desired pressure and triggering a manipulated variable for the actuator (74) of the pressure control valve (64) at the computer side.

7. Device according to claim 6, characterised in that connecting lines (68) leading exclusively directly to the moulds (14) with an inner diameter of preferably 5 - 10 cm branch off from the main (62), the sum of the cross section of all of the connecting lines (68) being substantially below the cross section of the main (62), preferably at least 20 %.

8. Device according to claim 6 or claim 7, characterised in that an upper insulating ring (32) of the moulds (14) is provided on its underside with an undercut, preferably a bevel (34) with a depth (H₂) of approximately 10 mm.

9. Device according to one of claims 6 to 8, characterised in that a likewise removable laser sensor is provided for measuring the metal level (60) identical throughout.

10. Application of the process according to one of claims 1 to 5 for automating the starting and end of casting and for quality control during the stationary phase of continuous casting.

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