EP0092477A1 - Process and apparatus for casting hollow steel ingots - Google Patents

Abstract

1. Process for producing a hallow steel ingot (15), utilising in a conventional way an ingot mould (1) placed on a bottom-casting base (2) fed by at least one bottom outlet (5), and also involving, in the centre of the ingot mould (1), the installation of a vertical cylindrical core comprising an outer cylindrical sleeve (6) made of sheet metal and a hollow inner mandrel (8) which are separated from on another by a uniform gab (13), through which flows continuously a cooling stream consisting of a gas or a mist or of the mixture of a gas and a mist, descending along the axis of the hollow mandrel (8) and rising again along the sleeve (6) in the said uniform gab (13), the temperature of the steel, measured in a ladle just before it is poured into the ingot mould, being at most equal to 1,5908C, this process being characterised both in that the said vertical cylindrical core is completely metallic, in that the feed rate of liquid steel to the ingot mould (1) at each bottom outlet (5) is at most equal to 20 (twenty) cm/s, in that the rising speed of the steel in the ingot mould is at most equal to 14 (fourteen) cm/min, and in that the mandrel (8) can be removed intact after the ingot (15) has solidified, whilst the sleeve (6) remains adhering to the ingot (15) and not welded to the latter.

Description

The present invention relates to the manufacture of hollow steel ingots, and is particularly advantageous for casting large hollow ingots then used as blanks for forging large hollow parts.

The traditional technique for producing hollow forgings, such as rings, tubes, containers intended for mid _s pressure, etc ..., consists in subjecting to a conventional ingot, therefore full, the following successive operations: blooming, crushing, drilling, drawing on mandrel, waxing.

• 1 - Les opérations de forgeage se trouvent simplifiées, car le bloomage, l'écrasement et le perçage deviennent inutiles et sont supprimés;
• 2 - La mise au mille, c'est à dire le rapport entre le poids en kilogrammes du lingot de forge constituant l'ébauche de forgeage et le poids en tonnes de la pièce finie après forgeage, se trouve notablement réduite ;
• 3 - Le taux de ségrégation du carbone et des impuretés est diminué grâce à une durée de solidification du lingot creux beaucoup plus courte que celle d'un lingot classique plein ;
• 4 - Si l'évacuation de la chaleur par le centre creux du lingot est satisfaisante, la ségrégation résiduelle du carbone et des impuretés se localise au voisinage de la mi-épaisseur du lingot creux, puis du produit fini, de telle sorte que la surface intérieure de ce dernier est exempte de toute ségrégation. C'est là un avantage important, car cette surface est celle qui est la plus sollicitée en service et elle doit, par conséquent, présenter partout une analyse homogène du métal de base. C'est ainsi par exemple, qu'elle est souvent recouverte d'acier inoxydable, dans le cas de soudures de cuves nucléaires, ou de pièces creuses utilisées en pétrochimie.

We have sometimes thought of using hollow ingots, but this technique has so far been little developed. However, it has important advantages over the traditional technique:

• 1 - The forging operations are simplified, since blooming, crushing and drilling become unnecessary and are eliminated;
• 2 - The setting to the mile, that is to say the ratio between the weight in kilograms of the forging ingot constituting the forging blank and the weight in tonnes of the finished part after forging, is significantly reduced;
• 3 - The rate of segregation of carbon and impurities is reduced thanks to a solidification time of the hollow ingot much shorter than that of a full solid ingot;
• 4 - If the evacuation of the heat by the hollow center of the ingot is satisfactory, the residual segregation of carbon and impurities is localized in the vicinity of the mid-thickness of the hollow ingot, then of the finished product, so that the surface interior of the latter is free from any segregation. This is an important advantage, because this surface is the one that is the most stressed in service and it must, therefore, present everywhere a homogeneous analysis of the base metal. For example, it is often covered with stainless steel, in the case of nuclear tank welds, or hollow parts used in petrochemicals.

• a) Les difficultés pour obtenir une bonne tenue du noyau.
• b) Les difficultés de démoulage du noyau.
• c) Les difficultés pour évacuer les calories communiquées au noyau pendant la coulée et la solidification du lingot.

Despite these multiple advantages of forging a hollow part from a hollow ingot, this technique has been very little developed, essentially because of the great difficulty of producing a hollow ingot. This indeed requires placing a core in the mold, and many problems relating to this core appear, among which it is necessary to quote :

• a) Difficulties in obtaining good behavior of the core.
• b) Difficulties in removing the core from the mold.
• c) Difficulties in removing the calories communicated to the core during the casting and solidification of the ingot.

Several solutions have indeed been proposed, among which there may be mentioned, for example, that which is described in patent No. 79-09210 (or 2,422,459), and which consists in using a cylindrical refractory core sandwiched between two metal tubes, the assembly being internally cooled by a gas stream. In the one just mentioned, the cylindrical refractory core is opposed to the rapid evacuation of heat by the gas stream, which is unfavorable.

The object of the present invention is to be able to produce a hollow ingot ensuring an energetic evacuation of the heat by the core during the solidification of the ingot, and avoiding the drawbacks mentioned above in a) and b).

To this end, the present invention firstly relates to a method for manufacturing a hollow steel ingot, traditionally using an ingot mold placed on a source casting base, supplied by at least one source output, and further comprising, in the center of the mold, the installation of a vertical cylindrical core, entirely metallic, this process being characterized both in that this core, comprising an outer cylindrical sheet metal jacket and a hollow inner mandrel, separated one of the other by a regular interval, is traversed permanently by a cooling current constituted by a gas, or by a fog, or by the mixture of a gas and a fog, descending along the axis of the hollow mandrel and rising along the jacket in said regular interval, in that the temperature of the steel measured in the pocket just before its casting in an ingot mold is at most equal to 1590 ° C., in that the feeding speed of the liquid steel ingot mold, each source output is at most equal to 20 (twenty) centimeters per second, in that the upward speed of the steel in the ingot mold is at most equal to 14 (fourteen) centimeters per minute, and in that the mandrel can be removed intact after solidification of the ingot, while the liner remains adherent to the ingot, and not welded to the latter, without risk of breakthrough of the liner, and without risk of harmful cracks forming in the ingot.

The optimum temperature for casting the steel is chosen according to the liquidus of the grade of steel to be cast. On another side, the source and rising output speeds of the steel in the ingot mold are chosen taking into account the geometry of the assembly formed by the ingot mold and by the core.

As a core cooling current, the invention excludes the use of any liquid, but not that of droplets constituting a mist or a suspension in a gas.

The cooling element of the core is preferably chosen, but not necessarily, from the following bodies: ordinary air, carbon dioxide, water vapor, water mist.

The invention presents a possibility of modulating the flow rate and the nature of the cooling element, with a view to achieving variable, more or less vigorous cooling, which is optimal at each instant of the solidification of the ingot, for example decreasing during solidification of the ingot according to a very precise variation law for each type of ingot. The invention thus gives complete control of the position, in the thickness of the ingot, of the end of solidification front.

In practice, the core is placed in the mold before introducing the molten metal, and this is then poured from bottom to top between the core and the mold.

The method according to the invention can also be applied under vacuum, if necessary.

The invention also relates to a device for manufacturing a hollow steel ingot, applying the method mentioned above, conventionally comprising an ingot mold placed on a source casting base, fed by at least one outlet from source, and further comprising a vertical cylindrical core disposed in the center of the mold, this device being characterized in that the vertical cylindrical core consists of: a cylindrical sheet metal jacket, consumable, with a thickness of between 4 and 20 millimeters, closed at its lower part by a metal bottom resting on the base, and a reusable hollow metal mandrel, introduced in the center of said jacket, providing with it a regular interval, and resting on said metal bottom by means of wedges leaving free passage between them; and in that said core is traversed by a cooling stream of gas or mist descending in the center of the mandrel, passing between said wedges and rising in the interval formed between the mandrel and the cylindrical jacket.

According to a preferred characteristic of the invention, the thickness of the cylindrical sheet liner is between 5 and 12 mil limiters.

The previous device according to the invention may include exothermic or insulating plates forming weights, arranged at the upper level of the liquid metal, and attached on the one hand to the internal wall of the ingot mold, and on the other hand to the cylindrical jacket of the core. .

One of the main advantages of the method and of the device according to the invention is constituted by the significant evacuation of heat through the core, during the entire solidification of the ingot, because the cylindrical jacket of the core is only one metal sheet, good conductor of heat, the internal face of which is in direct contact with the cooling current, and that the core has no refractory part, therefore no insulating part opposing the transfer of heat.

This transfer of heat through the nucleus is moreover complex: on the one hand, the cylindrical jacket blushes and radiates on the cooled mandrel, and on the other hand the calories heating the mandrel and the jacket are removed, by conductivity and by convection, by the cooling current. It is moreover important, and characteristic of the invention, that the mandrel is effectively cooled by its bore, which is traversed by the cooling current as soon as it enters the core.

Another notable advantage of the invention is its flexibility and speed of response. Indeed, thanks to the small thickness of the cylindrical jacket, it is possible to quickly vary the rate of heat dissipation by the core by varying either the flow rate of the cooling current or its nature, for example by adding water mist to a pre-existing compressed air stream. On the one hand indeed, it is necessary to cool the interior of the ingot vigorously to move away from its internal surface the zone of end of solidification which contains the segregations. But on the other hand, it is nevertheless necessary to avoid solidifying too quickly, to "freeze", the skin of the internal wall of the ingot, under penalty of running the risk of spotting, which are very long cracks due to thermal shock of great amplitude. The modulation of the cooling rate for the core, thanks to the invention, makes it possible to carry out a program for cooling the ingot according to its nature and according to its dimensions, ensuring optimal cooling at all times of the solidification of the ingot.

Thus, in cases where the core must be of small diameter compared to the dimensions of the ingot, it is recommended to use rather a mist than a gas, at least during a good part of the beginning solidification of the ingot, because the cooling effect of a mist is more marked than that of the same mass of gas.

Another advantage of the process according to the invention is that the casting conditions are such that the cylindrical sheet liner incurs no risk of breakthrough.

An advantage of the device according to the invention is that, if the sheet metal jacket is to be renewed at each pouring, the central mandrel, solid and well cooled on its two faces, internal and external, can be reused a certain number of times.

Another advantage of the device according to the invention is that, at the end of solidification, while the sheet liner remains adherent to the solidified ingot, the central mandrel can be extracted without any difficulty, since a regular free interval separates it from the liner over its entire height.

Finally, an advantage of gas or mist cooling is that there is no risk of explosion, unlike water cooling.

In order to clearly understand the invention, an embodiment of the method and the device according to the invention will be described below, by way of nonlimiting example.

This is the source casting of a hollow polygonal forge ingot, 24 sections, weighing 86 tonnes, with an average diameter equal to 2,500 millimeters, and a total height equal to 2,960 millimeters, including 400 mm of counterweight.

• C = 0,16 % ; Si = 0,25 % ; Mn = 1,35 % ; Ni = 0,70 % ; Cr = 0,17 % ; Mo = 0,50 % ; le restant étant constitué par du Fer, avec quelques éléments résiduels.

The composition of the steel is as follows:

• C = 0.16%; If = 0.25%; Mn = 1.35%; Ni = 0.70%; Cr = 0.17%; Mo = 0.50%; the remainder being made up of Iron, with some residual elements.

The single figure 1 represents a vertical section through the axis of the entire device and the ingot.

The ingot mold 1 is placed on the source base 2, through which the liquid metal will arrive from the casting mother 3 not shown, by a channel 4 and two orifices such as 5, 180 millimeters in diameter.

The ingot mold 1 is made of cast iron; it has a taper of about 40 mm per meter with respect to the vertical axis, and its large section is located at the top.

In the middle of base 2, the jacket 6 is made of mild steel sheet 10 mm thick and 1080 mm inside diameter, provided at its lower part with a metallic bottom 7 of the same thickness, by which it rests in the middle of the base 2.

The hollow mandrel 8 has an outside diameter of 980 mm and an inside diameter of 360 mm. It is significantly higher than the jacket 6. It is 50 mm apart. It is made of mild steel. It rests on shims such as 9, themselves placed on the bottom 7 of the jacket 6, and leaving between them free spaces, not visible in the figure. The mandrel 8 must be properly centered relative to the sleeve 6, so that the range of ^5, 0 mm is observed around the mandrel.

Through the upper orifice 10 of the mandrel 8, a pipe 11 introduces compressed air, with or without water mist. This compressed cooling air travels throughout the interior 12 of the mandrel 8, passes between the shims such as 9, and rises in the gap 13 existing between the jacket 6 and the mandrel 8, to exit in the open air annularly at 14 .

The compressed air flow introduced at 10 is normally 125 Nm3 / min. This air flow is here kept constant throughout the duration of the solidification of the ingot, since this adjustment has been determined so that the end of solidification is located at mid-thickness of the hollow ingot. If you want to change the position of the end of solidification front, you just have to adjust the cooling air flow, or even add a little water vapor or water mist to the air.

At the upper level 16 of the ingot 15 are arranged exothermic plates 17, 18, some 17, being leaned against the internal wall of the ingot mold 1, the others 18, being leaned against the jacket 6 of the core. These exothermic plates 17, 18 form a counterweight for the head of the ingot 15. The part immersed in the steel of the exothermic plates 17, 18 has a height of 400 millimeters.

• La température de l'acier en poche juste avant la coulée dans la mère de coulée est de 1580° C.

The source casting conditions in the present example are as follows:

• The temperature of the steel in the ladle just before pouring into the mother is 1580 ° C.

The speed of exit of the liquid steel through the two source exits 5, on entering the ingot mold 1, is approximately 11 centimeters per second.

The upward speed of ingot mold steel is maintained around 9 centimeters per minute.

The duration of the source casting of this 86 ton ingot is limited slightly longer than 35 minutes.

With such source casting conditions, in the device which has just been described, the jacket 6 is not pierced, while on the other hand escaping any formation of harmful cracks in the ingot. Finally, we are entirely in control of the position, in the thickness of the ingot, of the end of solidification front, thanks to an appropriate adjustment, for each type of ingot, of variations in flow rate and in the nature of the gas or core cooling mist, depending on the evolution of the ingot solidification.

It is understood that it is possible, without departing from the scope of the invention, to imagine variants and refinements of details, as well as to envisage the use of equivalent means.

Claims (7)

1. Method for manufacturing an ingot (15) of hollow steel, conventionally using an ingot mold (1) placed on a base (2) for source casting, supplied by at least one source outlet (5), and further comprising, in the center of the mold (1), the installation of a vertical cylindrical core, entirely metallic, this process being characterized both in that this core, comprising an outer cylindrical jacket (6) made of sheet metal and a hollow internal mandrel (8), separated from each other by a regular interval (13), is continuously traversed by a cooling current constituted by a gas, or by a fog, or by the mixture of a gas and a fog, descending along the axis of the hollow mandrel (8) and rising along the jacket (6) in said regular interval (13), in that the temperature of the steel measured in the pocket just before its casting in the mold is at most equal to 1590 ° C., in that the speed of supply of the mold (1) in liquid steel, each he source output (5) is at most equal to 20 (twenty) centimeters per second, in that the upward speed of the steel in the mold is at most equal to 14 (fourteen) centimeters per minute, and in that the mandrel (8) can be removed intact after solidification of the ingot (15) while the jacket (6) remains adherent to the ingot (15), and not welded to the latter. 2. Method according to claim 1, characterized in that the cooling element of the core is chosen from the following bodies: ordinary air, carbon dioxide, water vapor, water mist. 3. Method according to any one of claims 1 and 2, characterized in that the flow rate and the nature of the cooling current of the core are modulated so that the cooling effect is optimal at each instant of the solidification of the ingot. 4. Method according to any one of claims 1 to 3, using source casting, characterized in that the core (6), (7), (8), (9) is placed in the ingot mold before introducing the molten metal, and that it is then poured from bottom to top between the core and the ingot mold (1). 5. Device for manufacturing a hollow steel ingot, applying any of the methods according to claims 1 to 4, comprising an ingot mold (1) placed on a source casting base (2), fed by at least a source output (5), and further comprising a vertical cylindrical core placed in the center of the mold, this device being characterized in that the vertical cylindrical core consists of: a cylindrical jacket (6) made of sheet metal, consumable, with a thickness of between 4 and 20 millimeters, closed at its lower part by a metal bottom (7) resting on the base (2); and of a hollow, reusable metal mandrel (8), introduced in the center of said jacket (6), providing with it a regular interval (13), and resting on said metal bottom (7) by means of shims (9 ) leaving free passage between them; and in that said core is traversed by a cooling stream of gas or mist descending in the center of the mandrel (8), passing between said wedges (9) and rising in the gap (13) formed between the mandrel (8) and the cylindrical jacket (6). 6. Device according to claim 5, characterized in that the thickness of the cylindrical jacket (6) is between 5 and 12 millimeters. 7. Device according to any one of claims 5 and 6, characterized in that plates (17) (18) exothermic or insulating forming weights, are arranged at the upper level (16) of the liquid metal, and leaned on the one hand ( 17) to the internal wall of the mold (1) and secondly (18) to the cylindrical jacket (6) of the core.

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