EP0018633B1 - Casting flux for the continuous casting of steel - Google Patents

Abstract

A casting powder for the continuous casting of steel on the basis of silicate slags having the chemical analysis -25-45% by wt. of SiO2 20-45% by wt. of CaO -1-16% by wt. of Al2O3 0.1-15% by wt. of MgO -0.1-1.5% by wt. of FeO (20-45.1% by wt. of CaO + MgO) -0.05-10% by wt. of TiO2 3-15% by wt. of Na2O + K2O -0.05-10% by wt. of MnO 0-7% by wt. of B2O3 - 3-10% by wt. of bound F - which may as well include impurities from the raw materials. The slag is premelted, foamed to a bulk weight of 0.5 to 1.4 kg/l, preferably 0.8-1.2 kg/l and coated superficially with a layer of 1.5 to 15% by weight, preferably 2 to 6%, of carbon black and/or graphite with the aid of an appropriate adhesive.

Description

sowie rohstoffbedingten Verunreinigungen.The invention relates to a casting powder for the continuous casting of steel on the basis of silicate slags following analysis values

as well as impurities due to raw materials.

For the continuous casting of steel, casting powder is generally introduced into the continuous casting mold in order to protect the liquid steel flowing into the mold by means of an immersion tube against reoxidation, to dissolve the ascending oxidic inclusions and to produce a well-lubricated lubricating film between the solidifying steel strand and the mold. Mixtures of finely ground, oxidic and carbonate raw materials are mostly used for this purpose. These mixtures can also contain a few percent of graphite to prevent the powder particles from prematurely sintering or melting into lumps when heated (see US Pat. No. 3,964,916).

The mixed continuous casting powders are very uncomfortable to handle, because they are very dusty when they are poured into the mold.

If an attempt is made to remedy this evil by slurrying the raw materials in water and spraying them with a suitable device, whereby small beads are formed from casting powder, the products thus produced inevitably contain more or less large amounts of moisture. However, this has a very disadvantageous effect when the casting powder comes into contact with the molten steel and the molten steel absorbs hydrogen from the moisture of the powder by thermal decomposition.

When using the powders mentioned in practice, there is a common defect: the powders form more or less large lumps on the surface of the bath before a melted slag layer is formed. These lumps attach to the edge of the mold and form the dreaded cinder lines, which create impressions in the surface of the steel strand. The defects on the strand surface have to be removed at great cost either by flaming or grinding.

The invention is therefore based on the object of creating a casting powder, the dreaded slag cords of which do not occur when it is used and therefore the strand surface has a maximum degree of flawlessness.

This problem is solved by a casting powder based on the silicate slags described in the introduction, which is characterized in that it is premelted and foamed and its individual particles ground and sieved to a grain size of up to 5 mm by means of an adhesive with a layer of 1.5 to 15 % By weight of carbon black and / or graphite are coated.

The choice of slag composition within the specified limits depends in individual cases on the special casting conditions, the particular type of continuous casting machine used and the steel melts used which are cast on the continuous casting machine.

• Eine Schlacke der gewünschten Zusammensetzung wird beispielsweise in einem Elektroofen erschmolzen. Die überhitzte Schmelze wird anschließend durch Eingießen in Wasser auf ein Schüttgewicht von 0,5 bis 1,4 kg/I, vorzugsweise 0,8 bis 1,2 kg/I, geschäumt. Das dabei entstehende Granulat wird getrocknet und auf die gewünschte Körnung vermahlen und gesiebt, bis eine Teilchengröße mindestens 95 Gew.% der Teilchen innerhalb 0,05 bis 5 mm, vorzugsweise mindestens 98 Gew.% der Teilchen innerhalb 0,1 bis 2 mm resultiert, bei einem Rest von höchstens 5 Gew.% bzw. höchstens 2 Gew.% Feinstaub. Anschließend wird die vorgeschmolzene, gemahlene und gesiebte Schlacke beispielsweise in einem Mischer mit 1,5 bis 15 Gew.%, vorzugsweise 2 bis 6 Gew.%, Ruß und/oder Graphit unter Zuhilfenahme eines Klebstoffes beschichtet. Es lassen sich dabei organische wie anorganische Kleber einsetzen, wobei sich Kleber auf Basis Polyvinylacetat oder Polyvinylalkohol in wäßriger oder alkoholischer Lösung bzw. als Suspension besonders bewährt haben. Je nach eingesetztem Klebstoff sind hierzu in der Regel etwa 0,3,bis 3 Gew.%, bezogen auf das Schlackengewicht, in Abhängigkeit der eingesetzten Kohlenstoffmengen erforderlich. Bei der bevorzugten Menge von 2 bis 6 Gew.% Ruß und/oder Graphit als Kohlenstoffkomponente für die Beschichtung kommt man beispielsweise mit geringeren Mengen von etwa 0,4 bis 1 Gew.% Klebstoff aus.

The powders according to the invention can be produced as follows:

• A slag of the desired composition is smelted in an electric furnace, for example. The superheated melt is then foamed by pouring it into water to a bulk density of 0.5 to 1.4 kg / l, preferably 0.8 to 1.2 kg / l. The resulting granulate is dried and ground to the desired particle size and sieved until a particle size of at least 95% by weight of the particles within 0.05 to 5 mm, preferably at least 98% by weight of the particles within 0.1 to 2 mm, with a remainder of at most 5% by weight or at most 2% by weight of fine dust. The premelted, ground and sieved slag is then coated, for example in a mixer, with 1.5 to 15% by weight, preferably 2 to 6% by weight, carbon black and / or graphite with the aid of an adhesive. Organic and inorganic adhesives can be used, adhesives based on polyvinyl acetate or polyvinyl alcohol in aqueous or alcoholic solution or as a suspension having proven particularly useful. Depending on the adhesive used, this generally requires about 0.3 to 3% by weight, based on the weight of the slag, depending on the amounts of carbon used. With the preferred amount of 2 to 6 wt.% Carbon black and / or graphite as Carbon components for the coating can be used, for example, with smaller amounts of about 0.4 to 1% by weight of adhesive.

When using the casting powder according to the invention, the otherwise very unpleasant development of smoke when introducing conventional continuous casting powder is avoided. When the powders are melted in the mold, there are no malodorous gases, such as those which often occur with other conventional casting powders, and are a nuisance to the operating personnel. Another great advantage of the casting powder according to the invention is that it does not dust when poured into the mold.

When using the usual mixed continuous casting powder, a very wide zone of a bubbling mass of degassing particles is formed between the melted slag layer above the metal mirror and the unmelted continuous casting powder, some of which are molten, some are sintered together. This boiling above the level of the bath due to the commonly used mixed continuous casting powder also makes it difficult for the operating team to maintain a certain uniform steel level in the mold. The decisive advantage of the powders according to the invention over the products used hitherto can therefore be seen in the fact that when they are fed into the mold they are distributed completely evenly over the metal mirror and the disadvantages described, in particular the formation of the dreaded slag cords, do not occur. Even surprisingly for the person skilled in the art, only a very thin layer of molten casting powder slag forms above the metal level, and the casting powder particles above it appear to remain unchanged. A certain level of the liquid steel in the mold can be exactly maintained, which clearly improves the surface quality of the strand.

Examples of use

1. A steel with the analysis 0.06% C, 0.02% Si and 0.35% Mn was cast on a slab format 1000 x 210 mm at a casting speed of 1.6 m / minute. This steel is a deep-drawing quality for the production of thin sheet metal for car bodies.

For this purpose, a slag was smelted with the following analytical values:

The individual percentages add up to a value above 100%, since, according to analytical practice, the cations were always based on oxygen, although they are not necessarily present in the slag as an oxidic compound, but in particular with regard to calcium, sodium or magnesium, at least partly also as fluoride are or can be present, as indicated by the 4.4% by weight of bound fluorine.

The silicate granules foamed to a bulk density of approx. 1 kg / l were sieved to a grain size of 0.1 to 1.5 mm and with the aid of 0.6% by weight 10% aqueous. Polyvinyl alcohol coated with 4% carbon black.

This continuous casting powder thus produced was used in the casting of the steel mentioned above. 493 t of steel were cast. The casting powder consumption was 200 kg, corresponding to a consumption of 0.4 kg / t poured steel. The slabs produced using the continuous casting powder according to the invention had an excellent surface quality, which was judged solely and very well by the usual criteria. The surfaces of the slabs were particularly free from edge cracks and surface cracks. Except for the sprue, no pores or inclusions were found in the surface of the slab, as are otherwise very common when casting this high-aluminum steel. The spreading was increased significantly due to the excellent block surface compared to the usual quality level.

2. A steel was cast with the following analysis: 0.08% C, 0.5% Si, 1.5% Mn and 1% Cr.

It is a steel quality for heavy plates, from which welded large pipes are made. Slabs with the format 1,600 x 210 mm were poured. The casting speed was 1.2 m / minute.

For this task, a slag was smelted with the following analytical values:

The slag was foamed to a bulk density of 0.9 kg / l, ground to a grain size of 0.1-2 mm and with the aid of 0.8% by weight of 10% aqueous polyvinyl alcohol with 6% by weight of carbon black and graphite coated in equal parts, based on the weight of the silicate slag.

250 t of the above-mentioned steel were cast with this continuous casting powder. The casting powder consumption was 0.45 kg / t of cast steel. The slabs produced in this way had a surface which was better than that achieved with the mixed continuous casting powders which are usually used.

A parallel test using a premelted slag of the same composition, which had been ground to a particle size of less than 0.3 mm and mixed with 3% carbon black and 3% graphite, gave poor slab surfaces. In particular, thick lines of slag appeared in the oscillation marks.

3. A steel was cast with the following analysis: 0.6% C, 0.3% Si and 0.6% Mn.

It is a steel quality from which hard, very thin wires are made. 120 x 120 mm billets were cast. The casting speed was 2 m / minute.

For this task, a slag was smelted with the following analysis:

The slag was foamed to a bulk density of 1.3 kg / l, ground to a grain size of 0.1-1.5 mm and, with the aid of 2.5% by weight of 10% aqueous polyvinyl alcohol, with 12% by weight of a Mixture of carbon black and graphite coated in equal parts.

The billets produced in this way had a much better surface area than those obtained with the mixed continuous casting powders, which was expressed in that the grinding loss in the billets produced with the powder according to the invention was only about 4%, while when using the mixed powders which are usually used a grinding loss of around 7-8% is expected.

Claims (4)

1. A casting powder for the continuous casting of steel based on silicate slags of the following composition

characterized in that the powder is pre-melted and foamed, then ground to a maximum grain size of 5 mm and passed through a sieve, whereupon the grains are coated with a layer of 1.5 to 15 % by weight of soot and/or graphite by means of an adhesive.

2. The casting powder according to claim 1, characterized in that the bulk weight obtained by foaming is between 0.8 and 1.2 kg/I.

3. The casting powder according to claim 1 and/or claim 2, characterized in that the grain size is at most 2 mm and for 98 % by weight of the individual grains, the minimum grain size is 0.1 mm.

4. The casting powder according to anyone or several of claims 1 to 3, characterized in that the individual grains are coated with 2-6 % by weight of soot and/or graphite.