EP0449771B2 - 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 liquid Metal in the inside insulated in the upper area Chill molds in an automatic continuous caster with an upstream casting furnace and a Gutter system, which all molds dining at the same level with metal Distribution trough includes, in the below one Inner ring area a direct contact the mold with the metal-preventing gas cushion maintain and in its area oil is injected. The invention further relates to a Application of the procedure.

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

The most important element of a continuous casting machine are the molds which are used in conventional Process the cross section of the cast Determine strands. A casting machine is, ever according to the number of strands cast, with correspondingly many, lowerable approach floors equipped, which is firmly connected to a casting table are.

This flows during continuous casting molten metal, at least when switched on at least of a filter, through a gutter system from the casting furnace into the casting machine, where it goes into the individual molds is distributed.

While the chill molds slowly with the Fill the melt, the metal begins to dry Solidify approach floors. The approach floors are then cooled and with a lowered such a speed that the Solidus line of the solidified metal always within of the mold frame remains. The strands, their solidification is accelerated by water cooling, grow down to the same extent as that Approach floors can be lowered. Within the given The length of a strand is the casting process uninterrupted.

One of the main disadvantages of usual Continuous casting is that the level controlled separately in each individual mold must be and that long molds are required are. The resulting secondary effects lead to a lower surface quality.

Therefore, the so-called Hot top procedures have been developed at which the metal in an all molds on the same Level feeding distribution trough (hot top) flows. The level control devices of all individual molds can be omitted and by a central Control body to be replaced, which is a quieter Metal surface and a simplified casting process allowed.

The usual hot top casting process is through the formation of a gas cushion with automatic Lubrication for a semi-continuous casting process has been further developed, in which a direct contact between the liquid metal and the mold thanks to the air cushion and an oil film is prevented 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 Hof 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 process of the type mentioned above, wherein a circular air cushion underneath of the yard top is formed. This is a slight overpressure necessary. The setting of the Overpressure is done manually, using a screw.

Air and oil are supplied in the same Area but separate.

Other improvements to the process are lately especially in the direction of the so-called Air slip method, as in US Pat. No. 4 598,763. The upper The interior of a mold is made with an open-pore one Graphite ring designed. Air and oil can be mixed or separately via the pores of the graphite ring be led into the mold interior. Graphite is self-lubricating, the oil is not in primarily as a lubricant, but as a pore filler admitted. Water is only below the Sprayed graphite rings.

With a graphite ring, through which air and Oil, can be a very mild, so beneficial cooling be achieved. The use of a graphite ring has the disadvantage, however, for automation the corresponding casting process is expensive and to be complicated.

The object of the present invention is to achieve this based on a method and an apparatus of the to create the kind mentioned above, which a further automation of the hot top casting process allow.

Regarding the procedure, the task solved according to the invention in that a common Main line with distribution lines Air or an inert gas with the same low Overpressure leads to all molds, and the relative pressure between one in function of over one Level probe measured metal height program calculated Target value and one in the main line actual value measured with a transmitter the program-controlled control and Monitoring serves by using the control function of a processor by emitting a signal for the actuator of a common pressure control valve is fulfilled.

The inert gases are, for example, nitrogen and or argon is used. For cost reasons However, air is usually used, which is why in the following, for the sake of simplicity, the designation Air also includes inert gases.

The height of the metal surface can be adjusted with a Level probe measured in a known design but also with a laser sensor. The in actual pressure measured in the main line shows of the large diameter with small pressure drops no fluctuations.

The one that changes considerably depending on the weather External pressure should not affect the casting process. According to a preferred embodiment the invention is therefore the influence of the variable Outside pressure automatically with known means balanced by using a commercially available differential pressure gauge is used.

At the start of pouring, there is no liquid metal in the mold, which resists gas flow opposes. With a flow meter a first higher value is set. If that's on it liquid metal directed into the mold the gas outlet opening reached, the gas flow rate drops because of the gradually increasing metallostatic resistance. If the gas flow falls below a second, lower value, the Giesstsch will lower after a short time with the approach floors for the cast Strands triggered. Without metal there is an air flow of 12 - 15 Nl / min as the first value, as the second value for triggering the lowering about 8 - 10 Nl / min set. A few seconds, appropriate about 5 sec after reaching this second value begins to lower the casting table. The flow control takes place with the relative pressure, the difference between the target and Actual value for the pressure in the main line.

Because of the low gas flow through the distribution lines branching off the main line their length doesn't matter, everyone Chill molds are supplied under the same conditions.

The supply of all molds with the same In contrast, quantity of oil was previously only guaranteed if the individual, leading to the mold Oil lines from Qel main line all the same length were. This is no longer a requirement today with known means can all molds per time, regardless of line resistance, the same Amount of oil can be added.

The oil necessary for lubrication is preferred in pulses in the area of the gas cushion sprayed. This allows the oil to work with higher pressure be injected without reducing overall consumption gets too high.

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

The pressure in the gas cushion may be a certain one Do not exceed the maximum value, otherwise they will form gas bubbles in the metallic melt. Of the However, pressure of the gas cushion may also be a certain one do not fall below the minimum value, otherwise the molten metal can enter the gas supply channels penetration. The minimum and the maximum Value for the pressure in the gas cushion linear to the respective metallostatic pressure in the mold. The minimum pressure, which was not below may correspond to a function the density ρ, the acceleration due to gravity g, the metal level above the gas outlet openings, the Interfacial tension of the melt in the area Isolation mold and the surface tension of the Melt in the area of the gas cushion. The maximum Pressure in the gas cushion, which did not exceed is a function of the density of the Melt ρ, gravitational acceleration g and the Depth of undercut of 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 Processor triggering pressure control valve, and that the target pressure is calculated via a program as a function of the metal height (H ₁ ) measured via a level probe.

The setpoint is calculated based on the for example measured with a laser sensor Metal level determined.

The distribution lines branching off the main line to the molds, for example made of rubber or a plastic an outside one. reinforcing and protective Metal mesh.

The main line for the gas supply has expedient an inner diameter of 5 - 10 cm. The branching distribution lines are preferred directly, without secondary lines, to the molds. The main line is preferably oversized, i.e. the sum of the cross section of all Distribution lines are significantly below that 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. With the cross section is always here and otherwise the inner cross-section is meant.

So between the minimum and maximum permissible Pressure in the gas cushion is a relatively larger one The margin remains the bottom of the Mold-insulating layer preferably undercut. As an optimal value for this Undercut has turned out to be about 10 mm, which better enables a stable gas cushion to build. Although the undercut can take any geometrical shape this preferably as a tapered bevel.

As a level measuring device for determining the everywhere in the gutter system and in the molds same metal level, a removable laser sensor used.

The application of the method according to the invention lies primarily in automation the start-up and pouring end as well as quality control during the stationary phase of 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 is 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 schematic diagram shown in Fig. 1 of the known hot top continuous casting includes in essentially a gutter system 10, made of refractory Material existing hot tops 12, too Called hot heads, molds 14, cast strands 16 and a casting table 18.

The gutter system 10, in which the Metal with the same level in all channels towards of arrow 20 flows includes a distribution trough 22. This serves as a reservoir for liquid Metal. The individual gutters go in Grooves 24 of the hot top 12 over. According to the arranged molds 14, the grooves 24 also in the transverse direction and go above the molds 14 in holes through the hot top 12 over. This ensures that the metal level only must be measured at one point. This Level is inside the whole caster equal to the measurement tolerances.

On the one lowered in the direction of arrow 26 Casting table 18 are one of the number of molds 14 corresponding number of approach floors 28 arranged.

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

The hot top 12 conducts, as shown in FIG. 1, the molten metal 30 via grooves 24 in the Chill molds 14. The Hot Top 12 is made of fireproof Insulating material.

The three-ring mold 14 has a ring-shaped inner insulation in the upper inner area 32, which is the contact of the molten Metal 30 with the upper region of the mold 14 prevented.

The insulation 32 has one in the lower region undercut bevel 34. The one bevel fireproof material existing insulation ring 32 is placed on the mold 14 by means of a pressure plate 36 pressed. An O-ring, not shown, is guaranteed the tightness between the mold 14 and the Isolation ring 32.

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

A middle mold ring 46 contains one through the lower mold ring 38 bounded annular Oil chamber with outlet channels 50, which immediately below the inclined surface 34 of the insulation ring 32 open out. The oil chamber 48 will fed via radial channels, not shown, which from the lower ring 38 or from the middle Ring 46 recessed and by the other Ring are limited.

An upper mold ring 52 includes an annular one Air chamber 53 with radial, not shown Branch channels between the middle and the upper mold ring.

The air gets a little overpressure, in the range of about 45 mbar, immediately below the bevel 34 of the insulation 32 in the Mold interior directed. This creates a ring-shaped one Air cushion 54. This alleviates the cold shock the one hitting the mold 14 molten metal 30.

Air and oil occur in the same area, in the ring-shaped Air or gas cushion 54, from, in the present Case separately.

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

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 H ₁ , increased by the interfacial and surface tension, and must not exceed the depth H for the reasons mentioned above.

3, the metallostatic pressure is plotted as a function of the metal level H ₁ . The metallostatic pressure p is calculated as follows: p = ρgH 1 , where ρ is the alloy 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 measured for optimal casting conditions Values are plotted on curve A, which slightly above the theoretical curve C lies. 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 2nd (Fig. 2).

Fig. 3 can be used in practice for a given metal level the one to be used read optimal pressure. This lies, as already mentioned, at or just below 50 mbar.

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

The transducer 66 becomes a controlled variable passed to a processor 70. There is the Actual pressure corresponding control variable with one of a control variable calculated for a computer 72 for the target pressure depending on the metal level compared. If there is a relative pressure, i.e. a pressure difference between the target and actual pressure, the solves Processor a signal called manipulated variable from which on the actuator 74 of the pressure control valve 64 acts and this depending on the sign and the absolute value of the determined Δp changed. The actuator 74 can be a stepper motor, for example or be a DC motor.

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.

With a dashed line 76 is the regulator pressure, in the present case in stationary normal operation 45 mbar, specified. The dotted line 78 shows the pressure curve after 3 m length in one Main pipe with 6 mm inside diameter.

From Fig. 5 it can be clearly seen that with less Pressure p in the main line of air flow V is larger.

Fig. 6 shows that the air flow Q of the The sum of all air losses corresponds. The air flow is determined via 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),
• Q₅: 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 (e.g. cracks),
• Q ₅ : blistering when the air cushion pressure 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 (H₁) 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 (V₁), the first value (V_A) preferably being 12 - 15 Nl/min at a set pressure of approximately 45 mbar, the second value (V₁) being approximately 8 - 10 Nl/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 (g), the acceleration due to gravity (g), the metal level (H₁), 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 , and in that the desired value is calculated by a program as a function of the metal level (H₁) measured by a level probe.
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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