MXPA96001810A - Method and apparatus for the continuous milling of ac materials - Google Patents

Abstract

The present invention relates to a method for the continuous casting of steel plate or sheet comprising emptying liquid steel in an oscillating casting mold through a submerged refractory nozzle, adding a solid molding powder to the upper part of said mold. This means that a portion of said powder melts and forms a layer of liquid slag on the surface of the liquid steel, adding said molding powder to lubricate and reduce the friction of the steel in said mold and also to provide thermal insulation between the molds. liquid phases of steel and slag to the environment, maintain a layer of solid molding powder on the top of said mold with a thickness in the range of 150 mm to 200 mm (6 to 8 inches) on the steel surface liquid, sufficient to effectively thermally insulate the upper part of the mold, to reduce or eliminate the formation of a solidified slag ring, which projects towards the interior of said mold, and therefore to increase the surface quality of said casting plate or sheet

Description

SUMMARY OF THE INVENTION Method for the continuous casting of plates or sheets of steel and similar products in which a layer of molding powder is maintained in the upper part of the casting mold with sufficient thickness to provide greater thermal insulation to the liquid steel phases and slag, whereby the formation of a solidified slag ring in the mold is eliminated and whereby the surface quality of the steel products is considerably improved. The addition of molding powder, according to the invention, also reduces the number of interruptions and consequently considerably increases the availability and productivity of the continuous casting machine.

FIELD OF THE INVENTION The present invention relates to the continuous casting of steel materials, more particularly, with the continuous casting of plates or thin sheets of carbon steel, useful in the manufacture of hot-rolled steel sheet.

The invention provides a method for modifying the heat transfer pattern at the top of the casting mold by maintaining a layer of heat insulating powder of sufficient thickness to homogenize the heat transfer pattern at the top of the casting mold, by which the quality of the steel sheet is greatly improved and is produced with a minimum of longitudinal cracks.

BACKGROUND OF THE INVENTION The surface quality of continuously cast steel plates and sheets, which are subsequently hot rolled, is one of the most important attributes that determine the quality of the sheet produced in the modern steel industry. In the extremely competitive steel industry, the surfaces of the cast product must be essentially free of all defects, particularly in plants that have thin sheet voids or voids to almost final shape. The surface defects are related to several factors, such as trapped additives, liquid and solid inclusions, surface and internal cracks and mold oscillation marks. Many studies have been conducted, looking for the reasons and remedies to correct the defects, particularly those such as longitudinal cracks and marks by oscillation of the mold. The applicant has discovered that one of the factors causing these defects is the formation of a solidified slag ring in the upper part of the casting mold, due to the heat losses from the liquid slag formed by the molten molding powder.

The molding powders provide chemical and thermal insulation to the surface of the liquid steel and control the heat flow and friction between the mold and the initial steel wrap layer formed as the sheet solidifies. The rapid cooling exerted by the mold cooled by water on the liquid steel, produces very pronounced thermal gradients in the solid layer of steel which generates thermal stresses as said layer expands and contracts. Semisolid steel is also subject to mechanical stresses induced by friction of the steel with the mold, including vertical oscillations. Any of these efforts in the initial stage of solidification of the steel can result in cracking on the surface of the sheet.

The molding powder melts to form a liquid slag layer between the steel being solidified and the mold, which controls the friction and also isolates the liquid steel and the molten molding powder in the upper part of the mold. A typical operating practice for powder dosing is described by Rama Bommaraju, "Optimum Selection and Application of Molding Additives for Carbon Steels", presented at the 74th. Steel Conference of the Iron and Steel Society in Washington, DC, on April 14-17, 1991. In this article the author does not recommend the tendency of operators to add large amounts of molding powder and wait until it becomes Red by the temperature to make the next addition. The author concludes that in most cases, the liquid slag layer can be maintained from 6 to 12 mm. This can usually be achieved by maintaining a layer of unreacted molding powder with a minimum thickness of 25 mm. The author acknowledges that the powder also thermally insulates the slag and prevents it from solidifying in the mold, which would cause solidification of the steel meniscus and other problems. The author suggests dropping the dust on the edge of the mold and keeping the area between the wall of the mold and the protective wall of the submerged entry nozzle always covered with a layer of powder of 25 to 50 mm thick, more perhaps another 2 or 3 inches during the start of the emptying machine or during its maintenance.

It has been found that current practices, including the aforementioned addition of powder, is not satisfactory for customers who require higher quality and that these practices are still causing many defects in the products, for example longitudinal cracks and mold oscillation marks. The technical literature in this field is full of discussions and explanations of the phenomena involved in the early stages of solidification of the thin plates of a continuous cast, as well as many proposals and remedies to minimize these quality problems. For example, increase the frequency of the oscillation of the mold, increase the descending speed of the mold, insert a surface of chromium carbide in the upper part of the mold to create a "hot spot" thus decreasing the extension of the solidified ring, etc.

Another proposal to improve the quality of the surface is to apply electromagnetic or magneto-hydrodynamic forces to generate agitation in the liquid steel and consequently to improve the uniformity of heat transfer and solidification.

None of the mentioned techniques have nevertheless been completely successful for the elimination of longitudinal cracks and oscillation marks, and the technical literature accepts the problem of formation of the slag ring as an inevitable consequence of the cooling of the mold. With the method of the present invention it has been found that the solidified slag ring is removed and no longitudinal cracks are detected in the product.

OBJECTIVES AND SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method and apparatus for continuously casting steel to eliminate longitudinal cracks in the surface of the casting plate. Other objects of the invention will be partly obvious and partly signaled later.

The applicant achieves the objective of the invention by providing in the upper part of the casting mold a thermal insulation sufficiently effective to minimize the solidified slag ring that is formed in the processes of the prior art. More particularly, the invention provides a method and apparatus for improving the surface quality of a continuous cast steel product by maintaining a layer of molding powder having a predetermined thickness at the top of the casting mold.

The present invention can be incorporated into a continuous casting system for casting steel plates, and similar products, where an oscillating casting mold is used and molding powder is added for purposes of thermal insulation and lubrication In accordance with the present invention The objectives of the same are achieved by providing a method for the continuous emptying of steel plate, and similar products, which comprises emptying liquid steel in an oscillating casting mold through a submerged refractory nozzle, - adding a solid molding powder to the upper part of said casting mold, whereby a portion of the casting powder melts and forms a layer of liquid slag on the liquid steel in said mold, said casting powder being added to provide friction control between the liquid steel and the mold and also to provide thermal insulation between the liquid phases of the steel and the slag and the environment , - characterized by maintaining a layer of solid molding powder in the upper part of the mold with a thickness in the range between 6 to 8 inches, whereby the thermal insulation in the upper part of the mold is increased and the quality of said plate continuous casting is improved.

BRIEF DESCRIPTION OF THE DRAWINGS In this specification and in the accompanying drawings, some preferred embodiments of the invention have been shown and various alternatives and modifications thereof have been suggested; but it should be understood that these changes and modifications can be made without departing from the spirit of the invention. The suggestions contained herein have been selected and included for purposes of illustration so that others skilled in the art can understand the invention and the principles thereof and can thus modify it in a variety of ways, each of which as best suited to the conditions of a particular use.

Figure 1 is a schematic diagram of a continuous casting system showing the incorporation of the invention in a casting mold of the prior art.

Figure 2 is a schematic diagram illustrating a casting mold of the prior art and the formation of a solidified slag ring, showing in dotted lines the vertical displacement, exaggerated for clarity, of the solid slag ring in relation to the envelope layer of steel that is solidifying.

Figure 3 is a schematic diagram illustrating a casting mold, without the formation of the slag ring, and the molding powder layer, added according to the invention and shown in dotted line.

DETAILED DESCRIPTION OF THE INVENTION With reference to Figure 1, the numeral 10 designates in general form a distributor pot containing liquid steel 12, which is emptied through a submerged entry nozzle (SEN) 14 in a manner known per se. the art. The liquid steel is emptied into a standard continuous cast oscillating mold 16 through appropriately distributed apertures 18 and its solidification begins by the rapid heat transfer to the copper mold which is designed to receive large amounts of heat from the liquid steel towards a cooling fluid, usually water. The mold 16 oscillates in the vertical direction by means of a hydraulic mechanism to separate the solidified plate 17 from the mold while said plate continuously flows downward to be processed in a continuous rolling mill. The molding powder 20 is periodically added to the upper part of the mold 16 to maintain there a layer of solid powder that provides, among other things, insulation between the liquid steel 56 and the environment and also so that there is lubrication between the solid steel. and the mold. A portion of the molding powder 20 melts on contact with the liquid steel to form a layer of liquid slag 22, which provides lubrication between the solid steel 17 and the mold by flowing down into the peripheral zone adjacent the mold walls. .

Following current practice, the solid molding powder is contained by the walls of the mold since the thickness is of the order of 25 mm (1 inch), or at most it reaches 63 mm (2.5 inches) when the operators exceed the thickness currently recommended. In accordance with this invention, a container 24 is attached to the mold by means of suitable fasteners 26 in order to be able to maintain a molding powder thickness 28 of at least 150 mm to 200 mm (6 to 8 inches) above the surface of the liquid steel 56. It is understood that the shape and materials of the container 24 can vary according to the particular circumstances of each continuous casting machine, that the method of adding said powder can be automatic or manual, for example, dropping it from a hopper 30 through some pipe for this purpose, or pneumatically fed from a further point. Also the addition of the powder can be done automatically by determining a thickness of the powder layer and acting on a dosing system that responds to said determination. Another modality of this alternative is to set a predetermined level in the container and add the amount of powder necessary to reach said predetermined level. Any type of container, shape and materials suitable for this purpose, can be adapted to the upper part of the casting mold. The addition of the powder can be done automatically by any suitable metering system controlled by measuring means for determining the thickness of the molding powder layer. The purpose of the thick layer of solid powder at the top of the casting mold is to provide greater thermal insulation between the liquid materials in the mold and the environment. If the powder layer is of a very small thickness, as in current practice, the temperature of the upper portion of the mold is not homogeneous and also causes a solidified slag ring to form which results in known quality problems, which however have not been fully understood by the experts in the art. With reference to Figure 2, where the prior art practice is illustrated, only the upper part of one side of the mold 16 is shown, and a layer of solidified molding powder adhered to the inner face of said mold. A slag ring 51 is formed around the mold in this area probably because steel and slag lose heat to the environment through the solid powder layer 20. As the mold 16 oscillates in a vertical direction, this ring 51 it can touch the meniscus 54 which is the highest point of the solid steel layer 52, which causes defects of cracks and oscillation marks on the plates or sheets produced. The layer of

Claims (7)

1. The steel 52 surrounds the liquid core 56 which solidifies as it advances through the mold. Figure 3 illustrates the same diagram of Figure 2 but with a thick layer of molding powder and the absence of a slag ring. The applicant found that with the thick layer of solid powder, according to this invention, no ring was detected in the mold and that the number of interruptions (ie the number of times the mold operation is interrupted) per one thousand meters of emptying plate has been between 0.18 and 0.0 during the first two months of operation, while in another continuous casting plant this parameter reaches values of an order of magnitude higher. It is to be understood, of course, that the foregoing description is for purposes of illustration only and that many changes can be made in the structure of the system described and in the operating conditions, without departing from the spirit of the invention as defined in the following claims. the continuous casting of steel plate or sheet, comprising emptying liquid steel in an oscillating casting mold through a submerged refractory nozzle; adding a solid molding powder to the top of said casting mold, whereby a portion of said powder melts and forms a layer of liquid slag on the surface of the liquid steel; adding said molding powder to lubricate and reduce the friction of the steel in said mold and also to provide thermal insulation between the liquid phases of the steel and the slag towards the environment; maintain a layer of solid molding powder on top of said mold with a thickness in the range of 150 mm to 200 mm (6 to 8 inches) on the surface of the liquid steel, sufficient to effectively thermally insulate the top of the mold, to diminish or eliminate the formation of a solidified slag ring, which projects towards the interior of said mold, and therefore to increase the surface quality of said continuous casting plate or sheet.
2. 2. Method for the continuous casting of steel plate or sheet, according to claim 1, further comprising periodically determining the thickness of said solid molding powder layer and adding an amount of additional powder with such frequency and so that the The thickness of said layer is maintained in the range of 150 mm to 200 mm (6 to 8 inches) in the upper part of the mold.
3. 3. Method for the continuous casting of steel plate or sheet, according to claim 1, further comprising providing a container in the upper part of the mold and the area surrounding said mold where said mold is emptied; and adding a quantity of solid molding powder in said container to a predetermined level in said container.
4. . Method for the continuous casting of steel plate or sheet, according to claim 3, wherein the addition of the molding powder is done automatically in response to the determination of the thickness of said solid molding powder layer.
5. 5. Apparatus for the continuous casting of steel plate, comprising an oscillating casting mold having an opening in its upper part and an opening in its lower part, a nozzle for emptying liquid steel in said mold in a point close to the opening of the upper part, a container whose walls extend from the mold upwards, attached to said mold around its opening of the upper part of sufficient capacity to maintain a layer of solid molding powder at a predetermined height on the surface of the phase liquid molten slag in said mold to increase the thermal insulation of the upper part of said mold and liquid steel and liquid slag that covers the steel that is solidified in said mold, with respect to the environment surrounding the mold.
6. 6. Apparatus for the continuous casting of steel plate, according to claim 5, wherein the height of the walls of said container are sized to maintain a height of said layer of solid molding powder in the range of 150 mm to 200 mm (6 to 8 inches).
7. 7. Apparatus for the continuous casting of steel plate, according to claim 5, wherein the walls of said container form an integral part of said mold. In testimony of which, I sign this in Mexico City, D.F. on May 15, 1996. By HYLSA, S. A. DE C .V.