Mold cover powder for continuous casting of steel,...

Details Mold cover powder for continuous casting of steel,... Mold cover powder for continuous casting of steel,...

1998-10-13

2001-12-11

Elve, M. Alexandra (Department: 1725)

Specialized metallurgical processes, compositions for use therei

Compositions

Solid treating composition for liquid metal or charge

C075S305000, C075S307000, C075S328000, C075S329000

Reexamination Certificate

active

06328781

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to the continuous casting of steel. More precisely, it relates to the field of slags or cover powders which are deposited on the surface of the steel in the continuous casting mould, for the purpose of preventing the metal from being reoxidized by ambient air and being cooled by radiation, of trapping the non-metallic inclusions which have settled out, and for lubricating the walls of the mould while the cast product is being extracted.
DESCRIPTION OF THE RELATED ART
It will be recalled that these cover powders are composed of a basis powder, comprising especially oxides such as silica, lime, alumina and magnesia, and of various additives. Among these, mention may be made of sodium oxide, fluorspar, carbonates, etc. These powders are deposited on the surface of the liquid steel in the mould in order to form a layer of a few cm in depth. Near the powder/metal interface, the powder becomes liquid, enabling it to infiltrate between the wall of the mould and the solidifying skin of the cast product, and thus to perform its lubricant role. As the powder becomes consumed, it is replenished by hand or by the use of automatic devices, such as the one described in document FR 2,635,029. In this latter case, it is preferable for the powder not to have too fine a particle size, so as to reduce the risk of the pipes conveying it into the mould becoming clogged up. Thus, powders are very often used whose particles consist of hollow spheres of relatively coarse average particle size (greater than 100 &mgr;m) which are manufactured by atomization. Even if, strictly speaking, these materials can no longer really be termed pulverulent materials, they too will be designated in the rest of the text by the term “powder”, as those practised in the art are wont to do.
For the purpose of reducing the rate of melting of the powder, and therefore the rapidity with which it is consumed, free carbon is mixed with its constituents, this being in the form of graphite or channel black for example. The free-carbon contents (as opposed to the combined-carbon contents included in other constituents of the powder, such as carbonates) are generally of the order of a few % by weight. It has been observed that part of this carbon passes from the powder into the liquid metal, therefore causing its carbon content to increase. In the most common cases, this increase does not impair the quality of the cast product. However, in recent years there has been a significant increase in the requirements with regard to ultra-low-carbon steels, that is to say those having carbon contents below 50 ppm, or even less. At this requirement level, the approximately 4 to 10 ppm recarburization of the liquid metal, which is usually observed when powder containing even only 1 to 2% by weight of free carbon is used, can no longer be neglected. It would therefore be highly advantageous to make available to the steelmaker cover powders which no longer lead to recarburization of the metal, or to significantly less recarburization than with the usual powders, but which would nevertheless preserve sufficiently slow melting while at the same time remaining at a reasonable cost level.
In document FR 2,314,000 it has been proposed to use powders having no free carbon, in which the latter is replaced by particles of metal nitrides, such as boron, silicon, manganese, chromium, iron, aluminium, titanium and zirconium nitrides. Preferably, the nitride content is between 2 and 10% by weight of the powder. This content must therefore be relatively high in order for such a carbon-free powder to have properties equivalent to those of the usual carbon-containing powders. However, the presence of a large quantity of nitride runs the risk of causing an appreciable uptake of nitrogen by the cast steel. Now, the applications of ultra-low-carbon steels quite often require the nitrogen content also to be kept at very low levels (less than 30 ppm, for example), and this nitrogen uptake may also be as troublesome as the carbon uptake which it was desired to avoid. Moreover, the average particle size of these powders was relatively fine, and therefore not very suitable for dispensing them automatically. Finally, nitrides are expensive compounds, the addition of which in large quantities raises the cost of the powder appreciably. For these reasons, it seems that these powders have not been the subject of extensive industrial development.
SUMMARY OF THE INVENTION
The object of the invention is to provide steel-makers, especially those casting steels having an ultra-low carbon content and possibly an ultra-low nitrogen content, cover powders which do not lead to unacceptable recarburization and renitriding of the metal, while at the same time maintaining satisfactory preservation properties and a reasonable cost.
For this purpose, the subject of the invention is a mould cover powder for continuous casting of steel, especially ultra-low-carbon steel, of the type including a basis powder and particles of at least one metal nitride, characterized in that its free-carbon content (%C
free
) lies between 0 and 1% by weight, in that it is manufactured by atomization and in that it is in the form of granules of diameter lying between 20 and 800 &mgr;m.
According to one embodiment of the invention, the said nitride is silicon nitride and its weight content (%Si
3
N
4
) is equal to:
%Si
3
N
4
=0.5−0.28×%C
free
±0.10
As will be understood, the invention consists in using, as a compound for controlling the rate of melting of the powder, no longer carbon alone or a nitride alone at high contents, but a mixture of carbon and one or more metal nitrides, especially silicon nitride, or possibly one or more nitrides alone but always at relatively low contents. This is made possible by the fact that the powder is manufactured by an atomization process.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT


REFERENCES:
patent: 4038067 (1977-07-01), Takeuchi
patent: 5876482 (1999-03-01), Debiesme et al.
patent: 2314000A (1977-01-01), None
patent: 2201108A (1988-08-01), None
Patent Abstracts of Japan; vol. 004, No. 190 (C-037), Dec. 26, 1980 & JP-A-55128526 (Nippon Kokan KK; Others; 01), Oct. 4, 1980.

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