Method and apparatus for continuous casting of steel materials

Abstract: Method of continuously casting steel slabs and the like wherein a layer of melting powder is maintained at the top portion of the casting mold with sufficient thickness so as to provide an increased thermal insulation to the liquid layers of steel and slag, whereby the formation of a solidified slag rim in the mold is eliminated and the surface quality of the steel products is considerably improved. The melting powder addition according to the invention also reduces the number of break-outs and consequently considerably increases the availability and productivity of the caster.

Description: The present invention relates to the continuous casting of steel materials, more particularly, continuous casting of carbon steel thin slabs useful for fabrication 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 so as to homogenize the heat transfer pattern at the top portion of the casting mold, whereby the quality of the steel sheet is greatly improved with a minimum number of longitudinal cracks. BACKGROUND OF THE INVENTION The surface quality of continuously cast steel slabs, which are subsequently rolled to steel sheet is one of the most important attributes that determine the sheet product quality in the modern steel industry. In the extremely competitive steel industry, the cast surfaces have to be essentially free of all defects, particularly in plants having near net shape and thin slab casting. Surface defects are related to a number of factors, such as entrapped flux, solid and liquid inclusions, surface and subsurface cracking, and mold oscillation marks. Many studies have been made seeking the reasons and remedies to correct particularly such defects as longitudinal cracks and mold oscillation marks. Applicant has discovered that one of the factors causing these defects is the formation of a solidified rim of slag at the top portion of the casting mold, due to the heat loss to the mold and to the environment of the liquid slag formed by the melting mold powder. Mold powders provide chemical and thermal insulation to the molten steel surface and control the heat flow and friction between the mold and the initial steel shell formed as the slab undergoes solidification. The rapid cooling exerted by the water-cooled mold on the liquid steel in order to promote solidification thereof, results in steep temperature gradients in the solid shell generating thermal strains as the shell expands and contracts. Also the semisolid steel is subjected to mechanically induced stresses by the friction of the steel with the mold (including vertical oscillations). Any of these stresses and strains at the initial solidification of the shell may result in crack formation on the slab surface. Mold powder melts to form a layer of liquid slag between the solidifying steel and the mold, which controls friction and also insulates the liquid steel and molten powder slag at the top of the mold. Typical operating practice for addition of mold powder is described by Rama Bommaraju, "Optimum selection and application of Mold fluxes for carbon steels", presented at the 74th Steelmaking Conference of the Iron and Steel Society, in Washington, D.C., on April 14-17, 1991. In this article the author discourages the tendency of operators to add large quantities of mold powder and wait until it turns red-hot and then make the next addition. The author concludes that in most instances, the liquid layer of slag should be maintained above 6 to 12 mm. This can usually be achieved by maintaining a minimum of 25 mm thickness of unreacted solid powder layer. The author recognizes that the powder also thermally insulates the slag and prevents it from freezing in the mold which may cause freezing of the steel meniscus and other problems. The author suggests to cascade the powder over the edge of the mold and to maintain the area between the mold wall and the shroud of the Submerged Entry Nozzle (SEN) always covered with a 25 to 50 mm (1-2 inches) thick powdered layer, plus perhaps another 25 to 50 mm (1-2 inches) during start-up or tundish or tube changes. It has been found that the current practices, including the aforementioned mold powder addition, is not satisfactory to increasingly demanding customers and that such practices are still causing many defects in the products, as for example longitudinal cracks and oscillation marks. The technical literature in this field is full of discussions and explanation of the phenomena involved in the early solidification stages of continuously cast slabs, as well as including many proposals and countermeasures to minimize such quality problems. For example it has been proposed to increase the mold oscillation frequency, to increase the downward speed of the mold, to insert an area of chromium carbide at the top area of the mold in order to create a "hot spot" thus decreasing the extension of such solidified rim, etc. Another proposal to improve the surface quality is to apply electromagnetic or magneto-hydrodynamic forces to generate stirring of the liquid steel and consequently to improve the uniformity of heat transfer and solidification. None of the above techniques however has been completely successful in the elimination of longitudinal cracks and oscillation marks, and the technical literature accepts the problem of slag rim formation as an unavoidable consequence of the mold cooling. With the method of the present invention it has been found that the solidified slag rim is eliminated and the no longitudinal cracks have been detected in the product. A prior art that is also of interest is that disclosed in EP-A-0 063 823 that is directed to the continuous manufacture of steel slab by means of an oscillating mold and is concerned with the thickness P (mm) of a molten powder pool on the surface of the steel melt in the mold, as well as m (mm) the distance of descent of the oscillating mold during a negative strip period T (min) wherein the descending speed of the mold is greater than the withdrawal speed of the casting. Additionally V the withdrawal speed of the casting (m/min); and a is the rippling amplitude of the molten steel surface in the mold; are taken into account. All of these values are controlled such that P ≧ m - 1000 x V x T + a . This prior art does not take into account the parameter of any non-molten powder that may exist above the molten powder. The problem solved by this prior art is one concerned with a "powder rim" or "slag bear" that occurs on the inner wall of the mold at the upper portion of the pool layer. The problem, it is stated, is that this adhering substance moves up and down with the oscillations of the mold and when the thickness of the molten pool of powder is small this powder rim or slag bear comes into touch with the upper end of the solidified shell of steel. The object of this prior invention is to avoid that this slag rim comes into contact with the upper end of the solidified shell of steel and this is achieved by carrying out the process according to the formula given. A further prior art that is of interest is that disclosed in US-A-2 825 947 in which molten metal from a tundish passes downwardly via an outlet to the entrance of a water cooled mold. Between the outlet from the tundish and the entrance to the mold is an enclosed gap and the molten metal passes freely downwardly centrally of the enclosed gap such that there is a space between the flowing stream of molten metal and the internal wall of the enclosed gap. Plastic refractory material is introduced into said gap to form a layer on top of the molten metal in the mold. The enclosed gap has a window to permit the layer of plastic refractory material covering the meniscus at the top of the metal in the mold to be seen. The plastic refractory works down between the meniscus at the metal line and the cooled mold wall. There it builds up slightly on the mold wall where the plastic refractory will appear darker to the eye. This build up of refractory material on the mold wall prevents the first thin skin of molded metal from contacting the mold wall. From Fig. 2 of this prior art it is clear that there is a build up of a blanket of plastic refractory material that floats at all times on top of the metal in the mold while molten metal continuously passes through the blanket. It is stated that the plastic refractory must always have a low viscosity, preferably under 100 poises and can be 15.24 cms (6 inches) or more thick. OBJECTS AND SUMMARY OF THE INVENTION It is an object of the present invention to provide a method and apparatus for continuous casting of steel to eliminate longitudinal cracks in the cast slab surface. Other objects of the invention will be in part obvious and in part pointed out hereinafter. The invention is as defined in the accompanying claims of which method Claim 1 has been divided into a two part form based on the assumption that EP-A-0 063 823 is the nearest state of the art and apparatus Claim 6 has been divided into a two part form based on the assumption that US-A-2 825 947 is the nearest state of the art. The present invention achieves the afore said objective by providing that the top portion of the casting mold is sufficiently thermally insulated to minimize the solidified slag rim formed in the prior art. More particularly this invention provides method and apparatus to improve the surface quality of continuously cast steel product by maintaining a layer of melting powder having a predetermined thickness at the top portion of the casting mold. The present invention can be usefully incorporated in a continuous casting system for casting of steel slabs, and the like, where an oscillating casting mold is used and melting powder is added for thermal insulation and lubrication. According to the present invention the objects thereof are achieved by providing a method for continuous casting of steel slabs and the like, comprising pouring liquid steel into an oscillating casting mold through a submerged refractory nozzle; adding a solid mold powder to the top portion of said casting mold, whereby a portion of said mold powder is melted and forms a layer of liquid slag above said liquid steel in said mold; said mold powder being added to provide control of friction of said steel and said mold and also to provide thermal insulation between the liquid phases of steel and slag and the environment; characterized by maintaining a layer of solid mold powder at the top portion of said mold with a thickness in the range from 6 to 8 inches, whereby the thermal insulation at the top portion of said mold is increased and the quality of said continuously cast slabs is improved. BRIEF DESCRIPTION OF THE DRAWINGS. In this specification and in the accompanying drawings, some preferred embodiments of the invention are shown and described and various alternatives and modifications thereof have been suggested; but it is to be understood that these changes and modifications can be made within the scope of the invention. The suggestions herein are selected and included for purposes of illustration in order that others skilled in the art will more fully understand the invention and the principles thereof and will thus be enabled to modify it in a variety of forms, each as may be 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 rim, (showing in dotted outline the vertical displacement, exaggerated for purposes of clarity, of the solid slag rim relative to the solidifying steel shell). Figure 3 is a schematic diagram illustrating a casting mold (without formation of the slag rim) and the layer of mold powder, added according to the invention, being shown in dotted outline. DETAILED DESCRIPTION OF THE INVENTION. Referring to figure 1, numeral 10 generally designates a tundish holding liquid steel 12 ready to be cast through a submerged entry nozzle (SEN) 14 in a manner known in the art. The liquid steel is poured into a standard oscillating continuous casting mold 16 through suitably distributed openings 18 and begins its solidification by the rapid heat transfer to the copper mold which is designed to take large amounts of heat from said liquid steel to a cooling fluid, normally water. The mold 16 oscillates in the vertical direction by means of hydraulic drives in order to disengage the solidified slab 17 from the mold while said slab is continuously flowing downwards to be further processed in a continuous rolling mill. Mold powder 20 is periodically added at the top of mold 16 in order to maintain a layer of solid powder thereon to provide insulation between the liquid steel 56 and the environment and also to, provide lubrication between the solid steel 17 and the mold, among other purposes. A portion of the mold powder 20 is melted by contact with the liquid steel forming a layer of liquid slag 22, which provides the lubrication between the solid steel 17 and the mold by flowing downwardly in the peripherical zone adjacent to the mold walls. Following the current practice, the solid mold powder is contained by the mold walls since the depth of the powder layer is in the order of 25 mm (or 1 inch), at most it reaches 63.5 mm (2.5 inches) when the operators exceed the currently recommended thickness. According to this invention, the mold itself may be of sufficient height so as to contain said solid powder layer with a thickness in the range of 15.24 to 20.32 cm (6 to 8 inches). Alternatively, a conventional mold may be modified by a container 24 fastened to the top of the mold by suitable means 26 in order to hold at least a depth 28 in the range of 15.24 to 20.32 cm (6 to 8 inches) of powder at the top of the liquid steel 56. It is understood that the shape and materials of the container 24 may vary according to the particular circumstances of each casting machine, and that the method of addition of said powder may be manual or automatic. One can periodically determine the thickness of said layer of solid mold powder sufficiently often, and add more mold powder as needed, such that the thickness of said layer is maintained in the desired range of 15.24 to 20.32 cm (6 to 8 inches) at the top of said mold. For example it can be cascaded from a nearby bin 30 or through suitable piping. Also the addition of mold powder can be made automatic by determining the thickness of powder layer and acting on a dosification system in response to said determination. Another embodiment of this alternative is to set a predetermined level in the container and adding the amount of powder necessary to reach said predetermined level. Any type of container of suitable shape, and material can be adapted to the top of the casting mold. The powder addition can be made automatically by a suitable dosification system controlled by and/or responding to a suitable measuring means for determination of the thickness of said powder layer. The purpose of the thick layer of solid powder at the top of the casting mold is to provide an increased thermal insulation between the liquid materials in the mold and the environment. If the layer is of a small thickness as in the current practice, the temperature of the top portion of the mold is not homogeneous and also causes a solidified slag rim formation resulting in quality problems known, but not understood, by those skilled in this art. Referring to figure 2, where the prior art practice is illustrated, only the top portion of one side of mold 16 is shown, a layer of solidified mold powder is adhered to the inner face of said mold. A slag rim 51 is formed around the mold in this layer of solid slag 50, probably because the steel and slag lose heat to the environment through the layer of solid powder 20. As the mold 16 oscillates in the vertical direction, this rim 51 may touch the meniscus 54 which is the top part of the solidified shell of steel 52, causing defects in the product slab as cracks and oscillation marks. The shell 52 surrounds the liquid core 56 which becomes solid as it advances through the mold. Figure 3 illustrates the same diagram of figure 2 but with a thick layer of mold powder and the absence of said slag rim. The applicant found that with the thick solid powder layer according to the invention, no rim was detected in the mold and that the number of break-outs (meaning the number of times when the steel cast operation in the mold is interrupted) per each thousand of meters cast has been about 0.18 and 0.0 during the first two months of operation while in another similar steel making plant this parameter reached values of one order of magnitude higher. This invention is particularly important in thin slab continuous casting processes, because the surface quality of the thin steel slab produced is typically of greater significance in the end product made from such slabs (such as automotive sheet steel). Conventional slabs made by continuous casting typically have a cross section of 200-250 mm thick by 800-1700 mm wide. Thin slabs in contrast have a thickness of about 50 mm or even less. It is of course to be understood that the foregoing description is intended to be illustrative only and that numerous changes can be made in the structure of the system described and its operating conditions without departing from the appended claims.

Claims: A method for continuous casting of a steel slab (17); comprising pouring liquid steel (12) into an oscillating casting mold (16); adding a solid mold powder (20) to the top portion of said casting mold (16), whereby a portion of said mold powder is melted and forms a layer of liquid slag (22) above the liquid steel (56) in said mold; said mold powder being added to lubricate and reduce friction of said steel in said mold and also to provide thermal insulation between the liquid phases (56) of steel and slag and the environment; characterized in that the liquid steel (12) is poured into said oscillating casting mold (16) through a submerged refractory nozzle (14); and that the method further comprises maintaining a layer of solid mold powder (20) at the top portion of said mold with a thickness (28) in the range of at least 15.24 to 20.32 cm (6 to 8 inches), sufficient to effectively thermally insulate the top portion of said mold to diminish or eliminate formation of an inwardly protruding solidified slag rim (51) and thereby increase the surface quality of said continuously cast slabs. A method for continuous casting of a steel slab (17) according to Claim 1, further comprising periodically determining the thickness of said layer of solid mold powder sufficiently often, and adding more mold powder as needed, such that the thickness of said layer is maintained in said range at the top of said mold. A method for continuous casting of a steel slab (17) according to Claim 1 or Claim 2, further comprising providing a container (24) at the top of said mold (16) and surrounding the area of said mold where said steel is cast and adding an amount of solid mold powder in said container up to a predetermined level in said container. A method for continuous casting of a steel slab (17) according to Claim 2 or Claim 3 when appended to Claim 2, wherein said addition of mold powder is made automatically in response to said determination of the thickness of said layer of solid mold powder. A method for continuous casting of a steel slab according to Claim 1, further comprising a mold (16) of sufficient height so as to contain said solid powder layer with a thickness in the range of at least 15.24 to 20.32 cm (6 to 8 inches). Apparatus for continuous casting of a steel slab (17), comprising a cooling mold (16) having a top opening and a bottom opening; characterized by the cooling mold (16) being an oscillating mold; a nozzle (14) for pouring liquid steel (12) into said mold (16) at a point proximal to said top opening, and an upwardly extending container wall (24) attached to the top portion of said mold (16) around said top opening sized sufficient to maintain a layer of solid mold powder (20) at a height (28) of at least 15.24 to 20.32 cm at the top portion of said mold (16) in order to increase the thermal insulation of the top portion of said mold and of the liquid steel (56) and liquid slag (22) covering steel solidifying in said mold with respect to the environment surrounding said mold. Apparatus for continuous casting of a steel slab according to Claim 6, wherein the height (28) of said container wall (24) is adapted to maintain the height of said layer of solid powder (26) in the range of at least 15.24 to 20.32 cm (6 to 8 inches). Apparatus for continuous casting of a steel slab (17) according to Claim 6, wherein said container wall forms an integral part of said mold (16). A method for continuous casting of a steel slab (17) according to any of Claims 1 to 5, wherein said steel (17) is cast in a thin slab of about 5.08 cm (two inches) or less. A method for continuous casting of a steel slab (17) according to Claim 9, wherein said thin steel slab has a near net shape to the final rolled steel sheet end product.

Inventors: PEDROZA-CONTRERAS MIGUEL ANGEL, PEDROZA-CONTRERAS, MIGUEL ANGEL
Applicants: HYLSA SA, HYLSA S.A. DE C.V.
Application year: 1996
Main classification: B22D11
IPC codes: B22D11