Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
1998-10-02
2001-08-21
Nguyen, Nam (Department: 1722)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S476000, C164S437000, C164S335000, C164S337000, C164S133000
Reexamination Certificate
active
06276437
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for the manufacture of formable steel strip comprising the steps of forming in the mould of a continuous casting machine liquid steel into a thin slab having a thickness of less than 150 mm homogenizing in a homogenizing furnace and rolling the slab in the austenitic region using the casting heat to obtain an intermediate slab using the casting heat, if desired cooling the intermediate slab to a temperature where a substantial portion of the steel is transformed into the ferritic region, and rolling said intermediate slab to the strip either in the austenitic or ferritic region.
BACKGROUND OF THE INVENTION
Such method is disclosed in EP-A-0541754. The method has particular advantages because it can be performed in a continuous or seni-continuous manner leading among other things to a better material efficiency and a more efficient use of the equipment. However, an important disadvantage of the method is that until now it is until now not well suited for the manufacture of high quality steel such as interstitial free steel or other formable steel with high surface quality and high degree of freedom of internal defects. The source of most of these problems are the processes in the mould of the continuous casting machine. The processes are particularly complex due to the high width to thickness ratio of the mould and the high casting speed, in the region of 6 m/min leading to vigorous flows in the mould.
Another embodiment of a prior art method is disclosed in EP-A-0666122. The proposed method therein comprises the steps of continuous casting of a thin slab, homogenizing the slab in a reheating furnace and subsequently rolling the slab in the austenitic region to a desired final thickness of e.g. 2 mm.
Still another embodiment of a prior art method is disclosed in FR-A-2675411. The proposed apparatus therein consists of a tun dish for the continuous casting of a molten metal, more especially steel, which is used between a ladle and a mould. It is characterized by the presence of a lower chamber, which is fed from the ladle, and a higher chamber which chambers are connected by an inclined tunnel. Means are provided for evacuating the atmosphere in the higher chamber.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method with which it is possible to manufacture in a continuous or semi-continuous manner high quality formable steel strip starting from a thin cast slab.
This object is achieved with a method that, according to the invention is characterized in that the liquid steel is fed from a ladle into a first atmospheric chamber of a vacuum tun dish also comprising a second chamber hydraulically connected by a conduit to the first chamber in which second chamber a low pressure is preserved and the steel is conveyed from the second, low pressure or vacuum chamber, through an exit port therein into the mould.
The invention is particularly suitable for use in such methods as are described in among others EP-0306076, EP-0329220, EP-0370575, EP-0504999, EP-0541574, NL-1000693, NL-1000694 and NL-1000696, the contents of which are considered to be incorporated in this description by this reference.
It is known to cast steel into thin slabs, thinner than 150 mm, and preferably thinner than 100 mm in order to limit subsequent processing stages. Up to now the quality achieved by casting thin slabs has been low. In particular the steel is susceptible to ageing, has moderate to poor forming properties and is ridden with inclusions. These and other problems are described in the publication New Steel, May 1994, page 22 et seq.
The invention breaks with the deep rooted prejudice that high quality thin-cast steel cannot be manufactured economically. Advantages of the method are further elaborated and explained in the following.
When using an atmospheric tundish for casting thin slabs with a thickness smaller than 150 mm, more usually with a thickness between 40 and 100 mm, the flow rate of the steel through an entry nozzle from the tundish to the mould is high because of the high casting rate such as 6 m/min. A ratio of 1:100 in these two rates is not exceptionally high. The high entry rate into the mould in the known method causes turbulences whereby molten steel is driven up along the narrow sidewalls of the mould. This makes the meniscus of the molten steel higher on the narrow sidewalls of the mould than in the middle. The meniscus is covered with a layer of molten casting powder. Molten steel being driven up causes the casting powder to flow to the lowest point, i.e. around the middle part of the mould. As a result, the effect of the casting powder on the heat transfer from the thin slab to the surroundings and to the cooled walls of the mould is not equal around the circumference of the mould.
This leads to increased oxide growth in places with higher than desired temperatures and to increased deformation resistance in those places of the thin slab with lower temperature. The thin slab then displays surface defects and shape deviations both of which can no longer be remedied during subsequent processing of the thin slab, in particular in the case of continuous or semi-continuous processes, whereby the steel thin slab is rolled out from the casting heat.
The effects of driving up and also asymmetry occur to a great extent in a tundish for casting thin slabs. With the method in accordance with the invention it is possible to control better any turbulences occurring and asymmetry and instability of the flow in the mould no longer occurs. Consequently it is possible to control better the shape and quality of the cast thin slab and the strip manufactured from it.
For constructional reasons it is sometimes desirable to design the mould in a curved shape joining the radius of curvature of the roller table of the thin slab continuous casting machine. With the method and apparatus in accordance with the invention it is possible to use a submerged nozzle curved to match the curved shape of the mould in the case of such a machine.
The submerged entry nozzle used in combination with a vacuum tundish is no longer bound to strict limits either on shape or on dimensions. Both the entry opening and the exit opening of the entry nozzle can have a desired shape better adapted to their purpose. There is also a high degree of freedom of choice in terms of shape and dimensions of the internal cross-section of the body of the submerged nozzle i.e. that part running between the two openings.
As described the impulse of flow of molten steel from the conventional submerged nozzles causes a depression in the meniscus. In order to reduce the magnitude of the depression a preferred embodiment of the method in accordance with the invention is characterized in that the liquid steel is conveyed from the second chamber to the mould through an entry nozzle with an internal cross-sectional area of more than 5% and preferably of more than 10% of the cross-sectional area of the mould.
In the case of casting thin slabs thinner than 150 mm the conventional casting rate, i.e. the rate at which the slab leaves the mould, is approximately 6 m/min. In accordance with this embodiment of the invention the exit rate of the molten steel from the submerged nozzle is less than 100 m/min. The greater freedom of choice in the dimensions of the submerged nozzle even makes it possible to make the exit opening of the submerged nozzle greater than 10% of the cross-section of the mould, and consequently further reduce the impulse of flow. It has been found possible to achieve a virtually flat meniscus.
A very important advantage of the possibility to choose the size of the entry opening and the exit opening of the submerged nozzle within broad limits is the possibility of increasing the casting rate for casting thin slab of continuous casting machines and consequently increasing the production capacity. The outflow opening as well as the body may be made smaller while retaining the shape to match the shape of the mould used so that the contour of the outflow
Corus Staal BV
Lin I.-H.
Nguyen Nam
Stevens Davis Miller & Mosher LLP
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