Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
1998-12-04
2001-07-17
Lin, Kuang Y. (Department: 1722)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S415000, C164S444000, C164S487000
Reexamination Certificate
active
06260605
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention concerns the continuous casting of metals, especially steel.
2. Description of the Related Art
The continuous casting operation consists schematically, as is known, in pouring a molten metal into an ingot mould mainly consisting of a tubular element without a bottom, defining a passageway for the cast metal. The walls of the mould, made of copper or, more generally of a copper alloy, are energetically cooled by circulating water. The product already solidified externally over a thickness of several centimeters is continuously extracted from the mould. The solidification then progresses towards the centre of the product and is completed during the descent of the product downstream of the ingot mould in the so-called “secondary cooling” zone under the effect of water spray lines. The product obtained, the bloom, billets or slab, is next cut to length then rolled before being shipped to the customer or transformed in situ into bars, wires, sections, plates, sheets, etc..
Surface defects or defects under the surface of the products obtained by the continuous casting of steel often lead to scrapping as they are not well tolerated by the rolling operation or are even amplified by this operation which may go as far as intolerably degrading the metallurgical quality of the rolled products.
During casting, the molten metal, fed into the ingot mould via a nozzle, forms a solid film when it comes into contact with the cooled walls of the ingot mould. This film is driven downwards during the extraction of the product by jerky movements punctuated by the vertical oscillations of the ingot mould and, simultaneously, its thickness increases due to continued heat extraction via the walls of the ingot mould. Therefore, a new film of solid metal is continuously created at the level of the free surface of the metal in the ingot mould, this film solidifying over the complete perimeter of the inner wall of the ingot mould and thus comprising a solid ring liable to contract due to the cooling to which it is submitted during its descent in the ingot mould.
The contraction of the ring is augmented as heat extraction increases and by the natural tendency of the cast metal to contract during cooling, for example by change of solid phase at end of solidification as is the case especially for the 0.1% carbon steel or stainless steel AISI 304 grades.
This peripheral contraction tends to separate the solidified skin from the wall of the ingot mould and therefore leads to a reduction in the heat exchange as the contact of the said skin with the cold walls is degraded. This separation is generally unequal over the perimeter of the solidified skin and is a source of surface defects in the product finally obtained.
To avoid or limit these defects, a specific technique not yet industrialised, known as vertical continuous casting, consists in placing a hot-top element made of a heat-insulating refractory material above the cooled metal walls of the ingot mould and in maintaining, during casting, the free surface of the metal bath at the level of the said hot-top element (French patent No. 2000365). Thus, the molten metal does not solidify in contact with the hot-top element, the first solidified skin starting to form only from the upper edges of the cooled metal wall. As these edges are located at a sufficient distance below the disturbed zone near to the free surface, the creation and the growth of the solid skin is achieved continuously always at same level in the ingot mould, in a calm environment from a hydrodynamic viewpoint, in the region where the ferrostatic pressure exerted by the weight of the liquid metal located above opposes the tendency of the first solidified skin to separate from the cold wall of the ingot mould.
In this technique, an improvement, known by document EP-A-O 620 062, consists in injecting, into the ingot mould, at the level of the said hot-top element and at least just at the interface between the said hot-top element and the cooled metal walls, an inert gas under pressure. This gas injection, made via a thin annular slot made between the said walls and the hot-top element, forms jets perpendicular to the walls and directed towards the liquid metal which shear any solidified skins which may have formed in contact with the refractory hot-top element in order to ensure that solidification effectively starts exactly at the upper edge of the cooled walls.
Although this technique in principle enables the appearance of certain surface defects on the finished product to be reduced, it does not however solve the problems concerning the adaptation of the casting process to the various families of steel grades which can be continuously cast to take into account the specificities of each of the grades concerning their thermomechanical behaviour during solidification.
SUMMARY OF THE INVENTION
The aim of this invention is to solve these problems and more especially to enable, in the vertical continuous casting technique, control and easy adaptation of the heat flux extraction conditions especially in the zone where solidification starts.
With these targets in mind, the subject of the invention is a continuous metal casting process where an ingot mould including energetically cooled metal walls surmounted by a hot-top element made of a heat insulating material is used and, during casting, the free surface of the molten metal contained in the ingot mould is kept at the level of the said hot-top element, and a gas under pressure is injected around the complete periphery of the ingot mould at the level of the said hot-top element and at least at the interface between this hot-top element and the cooled walls. According to the invention, this process is characterised in that the said injected gas is a gas or a gaseous mix having an adjustable thermal expansion capacity to adjust, according to the composition of the cast metal alloy and the casting conditions, the density of the heat flux, which is extracted from the said metal alloy in the zone where it starts to solidify, to a predetermined value specific to the cast alloy.
Thus, the process according to the invention offers the possibility of easily adapting, according to needs, the heat flux density extracted from the cast metal to the level where the solidified skin forms, in particular to the composition of the said metal, especially the grade for casting steels.
Indeed, the inventors observed during casting tests made by injecting an inert gas, such as argon or helium, at the interface between the hot-top element and the cooled metal walls, that the heat flux density was strongly influenced by the thermal expansion capacity of the gas. Thus, for the casting of a 0.8% carbon steel in an ingot mould the cooled walls of which were made of an uncoated copper alloy and with a casting speed of 1.5 m/min, the extracted heat flux density over the first 40 millimeters from the upper edge of the metal walls was around 5 MW/m
2
when the temperature of the injected argon was around 500° C., and was only 4.2 or even 3.2 MW/m
2
when the temperature of the injected argon was around 100° C. During another test conducted with an ingot mould where the upper faces of the cooled walls were covered with a 1.5 mm layer of nickel, for casting a 0.09% carbon steel, and with a casting speed of 2 m/min, the extracted heat flux was 5.5 MW/m
2
for an injected argon temperature of 500° C., and only 3.5 MW/m
2
for an argon temperature of 100° C.
These high differences in the extracted heat flux value could not be explained by the sole influence of the temperature of the gas on the cast metal which has a temperature of around 1600° C. in the upper section of the ingot mould. A hypothesis put forward by the inventors is that this difference results, on the one hand, from the stirring effect of the liquid steel caused by the injected gas in the direct vicinity of the upper edge of the cooled metal walls, where solidification initiates and, on the other hand, and in a predominant manner, by the influence
Burty Marc
Jolivet Jean-Marc
Perrin Eric
Salaris Cosimo
Spiquel Jacques
Cole Thomas W.
Lin Kuang Y.
Nixon & Peabody LLP
Ugine-Savoie Imphy
LandOfFree
Facility and method for the continuous casting of metals does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Facility and method for the continuous casting of metals, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Facility and method for the continuous casting of metals will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2470399