EP0190089A1 - Process for treating liquid metals by a calcium-containing cored wire - Google Patents

Abstract

The method comprises the formation of a liquid metal wherein is introduced a coated wire of which the core contains a material consisting of a calcium alloy, said alloy containing at least 75% by mass of calcium and a total contents of at least 5% by mass of NI + Al. Preferred composition: at least 80% of calcium and up to 20% of Ni and/or Al. The tests carried out by means of the device represented in figure 1 have shown that it is possible to effect the introduction of the coated wire (10) into the flowing steel (2) at a rate from 80 to 120g of Ca per ton of flowing steel and per minute without causing an excessive boil. The invention also relates to the coated wire.

Description

The process which is the subject of the invention relates to the treatment of liquid metals and in particular liquid steel by means of a cored wire whose core contains calcium. The term cored wire is used here to designate a product comprising a tubular metallic envelope of great length inside. of which is housed matter in divided or massive form. This cored wire allows this material to be introduced into a metallic bath, avoiding any contact with air or slag, that is to say with better efficiency and in a more reproducible manner.

Such a method of treating metal baths is well known. Thus, European patent application EP 34 994 describes a composite product with tubular casing and core in compacted pulverulent material which is used in particular for the treatment of liquid steel (page 8, lines 13-35). This cored wire comprises a thin steel casing and a pulverulent material core containing calcium.

The use of calcium as a component of the core of cored wires, used for the treatment of liquid steel, has taken an important development in recent years. Indeed, calcium makes it possible to reduce the soluble oxygen content of the steel and at the same time promotes its desulfurization. It also makes it possible to modify the nature and the morphology of the inclusions, such as those of alumina, which are transformed into aluminates of liquid lime. This prevents plugging of the casting nozzles, a particularly troublesome phenomenon for continuous casting and also the formation during hot working of elongated inclusion lines which reduce the ductility across the products obtained. Finally, these modified inclusions are less abrasive with respect to cutting tools for high speed machining.

Thanks to the cored wire, calcium is easily introduced into the bottom of a pocket filled with liquid steel, within which it can thus react particularly effectively. But we do not avoid the violent bubbling of this steel caused by the sudden volatilization of this calcium. Its vapor pressure is indeed about 1.8 atm at 1600 ° C. This boiling, if it is too intense, can disturb the conditions of penetration of the cored wire into the steel bath. At the same time, splashes of liquid steel occur which pass through the slag layer and oxidize on contact with the air before falling. There is then a rise in the contents of 02, N2 and even H2 of the steel obtained.

Experience has shown that if a core material containing unalloyed calcium is used as the core of the cored wire, the rate of introduction of the latter into liquid steel must be limited to approximately 30 to 40 g per tonne of steel. liquid and per minute. As in practice, approximately 125 to 600 g of Ca are introduced per tonne of liquid steel in total, it can be seen that the treatment has a duration of 4 to 15 minutes.

The thickness of the envelope of the cored wire and its internal section must be adjusted as a function of this speed of introduction so that this envelope does not dissolve prematurely before the cored wire has reached the bottom of the steel pocket liquid, the thickness of said envelope increasing when the speed of introduction decreases.

Numerous methods have been proposed with a view to treating metals or liquid alloys, and in particular cast irons and steels, with calcium or other highly reactive elements or compounds. Many compositions comprising calcium or other elements, often in the form of mixtures or alloys, have also been proposed with a view to their use in such methods.

Thus, in European patent application 34 994, a means of introducing Ca in a cored wire is described which consists in using an Si-Ca alloy containing approximately 30% of Ca by mass. It is then observed that the boiling of the liquid steel is reduced and it is possible, in this way, to introduce calcium-filled wire in the form of Si-Ca into the liquid steel, at a speed corresponding to approximately 80 grams per tonne liquid steel per minute without excessive bubbling. The rate of introduction of calcium is thus multiplied by about 2.

The use of the Si-Ca alloy has the serious disadvantage of introducing about twice as much silicon into the steel as calcium. The Si-Ca alloy used in fact contains approximately 60X by mass of Si and experience has shown that it is hardly possible to significantly enrich this Ca alloy beyond 40X. While only about 15X of the Ca introduced into the steel remains fixed there, the Si, on the contrary, is entirely fixed. The steel is therefore enriched with 250 to 1200 ppm of Si depending on the amount of Ca introduced included in the range of 125 to 600 g / t of liquid steel. For many applications, such an addition of Si is very unfavorable in particular for the treatment of steels used for deep drawing. For such steels, the acceptable limit of Si content is of the order of 200 to 300 ppm It is therefore not possible to respect it if an Si-Ca alloy is used as the core of a cored wire introduced into a like steel.

European patent application 30043 describes a process for treating liquid metals, in which a mixture of powders sheathed in a mild steel casing is introduced into the metal bath. This mixture comprises a component "A" consisting of reactive metals such as Mg, Ca, and rare earth metals, and a component "B" comprising Fe, Ni or Mn. The best results are obtained with mixtures comprising substantially equal amounts of each of the two components. The presence of the metal particles of component "B" ensures the dispersion of the metallic vapors given off by metals such as Mg or Ca at high vapor pressure. This reduces the violence of the boiling of the metal bath and the importance of splashes of liquid metal. However, the need to accommodate in the envelope of the cored wire a core comprising a large quantity of component "B", practically inert, increases the cost of the treatment and prolongs its duration. In addition, the difference in density of the two components can cause them to separate during the processing of the mixture and reduce the effectiveness thereof.

German patent 974835 also proposes the deoxidation of cast irons and steels by reactive metals such as Al, Ca, Ti, Mg, Ce. These metals are introduced into the metal bath after a first deoxidation by a ferro alloy. The reactive metal is protected by a sheathing tube made of steel or metal or another alloy steel, according to the needs of metallurgy. No particular solution is proposed to the problem posed by the very vapor pressure. high of metals such as Ca or Mg at the temperature of a bath of liquid steel.

In German patent 1220617 a treatment alloy is proposed which makes it possible to obtain fine grain steels. This alloy contains from 5 to 40X of Ca, 5 to 55X of one or more elements of the group comprising Al, Mn, Ni, Si, possibly small amounts of Ce, Li, Sr, Ba, Mg, the rest being Fe, at a content of 10 to 80%, which increases the density of the alloy so as to promote its penetration into the liquid steel to be treated. As an example, two treatment alloys are indicated: the first contains Fe 15.6%, Mn 12.7X - Ca 20.5% - Si 45.7% - Mg 4.5%. The second contains: Fe 33.5% - Ca. 29.5% - Si 36%. It can be seen that these alloys have a relatively low content of highly reactive elements such as Ca and Mg. By cons they contain silicon at a high content which is unfavorable when this element is not desired.

US Pat. No. 4,094,666 describes a method for refining cast irons and steels using Ca or Mg, with or without the addition of bastnaesite (Ce and La fluocarbonate) under a mild steel sheath. As indicated above, calcium and magnesium both have great efficiency but cause, due to their very high vapor pressure, violent projections when they are introduced into metal baths. The amounts of reactive metals (Ca and Mg) with or without added bastnaesite are between 0.3 and 0.5% of the mass of the steel or cast iron bath to be treated, ie 3 to 5 Kg per tonne. So there are considerable losses.

We looked for the possibility of developing a method for introducing calcium, by flux-cored wire, into a metal bath, in particular into a steel bath, making it possible to carry out such an introduction at high speed, without causing excessive bubbling. causing significant projections above the level of the liquid metal. We therefore sought the possibility of avoiding, during this introduction, of contaminating the liquid metal with elements harmful to its mechanical or other properties, such as silicon for example. We have also sought the possibility of obtaining a yield for introducing calcium as high as possible so as to minimize the cost of treatment. In order to save money we seek also to minimize the quantities of elements possibly associated with calcium to form the core of the cored wire, even if these elements have no harmful effect on the metal bath, so as to minimize the volume of wire thicket necessary for the implementation of calcium.

Finally, we looked for the possibility of treating ferrous metals such as steels or cast iron in such a particularly effective manner, in particular with the aim of deoxidizing and / or desulfurizing them and / or modifying the nature and the morphology. inclusions.

The process for treating metals or alloys in the liquid state, in particular ferrous metals, which is the subject of the invention makes it possible to solve the problem as a whole. It consists of preparing a metal or metallic alloy in the liquid state, preferably subjecting it, beforehand, to deoxidation, then introducing into the metallic bath formed a cored wire whose core contains a material of which the constituent base is an alloy whose calcium content is at least 75% by mass, an alloy also containing at least one metal from the group comprising nickel and aluminum, the total content of this alloy of nickel plus aluminum being at least equal to 5% by mass. This alloy can contain various impurities and / or additional additions. Preferably the calcium content of the alloy is at least 80X by mass, the nickel plus aluminum content in this case being limited to 20% by mass. The alloy may in particular optionally contain up to 15% by mass of silicon and also optionally up to 2% by mass of magnesium. This alloy is preferably used in divided form, that is to say in powder form or even better preferably in granular form, the maximum grain size then being, preferably also, of the order of 2 mm. This alloy may be in the compacted state or not inside the cored wire. This alloy, an essential basic constituent of the material which forms the core of the cored wire, represents at least 30% by mass of the total weight of this material. This material can also, for certain applications, contain another alloy comprising calcium, such as a silico-calcium. It can also contain elements, metallic or not, in alloyed or unalloyed form, making it possible in particular to adjust the composition of the metal bath. Finally, it can contain elements or compounds, metallic or non-metallic, which contribute to the treatment of this metallic bath, in particular in cases where it is made of steel or of cast iron.

It is emphasized that the basic constituent of the material which forms the core of the cored wire is an alloy, according to the composition given above and not a mixture.

Preferably also, in the case of steel, the speed of introduction of the cored wire containing the Ca alloy into the liquid steel is determined so that the amount of Ca introduced per minute is approximately 80 to 120 g of Ca per tonne of liquid steel, the total amount of Ca introduced being from 125 to 600 g per tonne of liquid steel, the introduction being able to take place, for example, in a ladle and / or in a distributor in the case of continuous casting.

The process according to the invention also applies to the preparation of steels having a particular aptitude for stamping. The silicon content of the liquid steel must then be limited to below 300 ppm before introduction of the cored wire, and the composition of the material forming the core thereof must be adjusted, from the point of view of the silicon content, so that the content of the liquid steel, after introduction of the cored wire, does not excessively exceed this content of 300 ppm.

The invention also relates to a cored wire making it possible to treat a liquid metal or alloy, in particular a steel, the core of which preferably contains a material in divided form, that is to say pulverulent or granular, compacted or not, the constituent of which is base is an alloy containing at least 75X by mass of calcium and at least one metal from the group comprising nickel and aluminum, the total content of this alloy of nickel plus aluminum being at least equal to 5% by mass. Certain other interesting characteristics of the cored wire according to the invention have already been given in connection with the description of the process and are not repeated here.

The description and the figures below present, without limitation, an embodiment of the invention.

Figure 1 is a diagram of a device for evaluating the intensity of the boiling of the liquid steel caused by the introduction of a cored wire containing calcium.

Figure 2 is a detail of Figure 1 seen along the arrow (F).

FIG. 3 is a diagram which shows the relationship between the speed of introduction of the cored wire and the intensity of the boiling of the liquid steel.

FIG. 1 represents a pocket (1) containing 83 tonnes of liquid steel (2). This unalloyed steel which has undergone prior deoxidation contains C = 0.12% and Mn = 0.6%. It is covered with a layer of slag (3). At a distance (H) of 300mm from the surface of the slag are located 16 cups, such as (4), made of steel, of approximately 50mm in diameter suspended below a steel disc (5) of 1.2m in diameter itself hanging above the pocket.

Figure 2 shows the ends such as (7) of the fixing rods (6) of the cups which pass through the disc to which they are attached in a distribution as regular as possible. On the right of Figure 1, a device (8) shown schematically comprises rollers (9) which penetrate from top to bottom, substantially vertically, a cored wire (10) in the liquid steel.

Preliminary tests have shown that the intensity of the boiling of the liquid metal, caused by the introduction of the cored wire, should not be such that more than half of the cups (4) receive splashes of metal and / or slag. These preliminary tests also showed that, in the case of a cored wire whose core is unalloyed divided calcium, this boiling intensity is reached for an introduction speed corresponding to 40 g of Ca per tonne of liquid steel and per minute.

Systematic tests are then carried out on castings from an arc furnace producing a steel of type A42, unalloyed, containing C-0.12% and Mn = 0.6%. Figure 3 shows diagrammatically the results obtained. 41 steel castings are tested. Each of these flows weighing 83 tonnes is placed in the pocket (1) of FIG. 1 and then treated with the cored wire (10). This cored wire has a mild steel casing 0.4mm thick and a rectangular section of 16 by 7.5mm. The core of this cored wire is a divided material consisting of a CaNi alloy containing 87Z by mass of Ca and 11% by mass of Ni. The weight per meter of alloy contained in the cored wire is 110 g or 95.7 g of Ca. We see in Figure 3 on the ordinate the velocities of introduction of cored wire in m / min. On the abscissa are given the temperatures of the liquid steel. The result of each test is represented by a cross (x) or a circle (o). The cross (x) corresponds to an excessively intense bubbling of the liquid steel whose projections of steel and slag have reached more than 8 cups (4). The circle (o) corresponds to an acceptable bubbling for which no more than 8 cups have been reached.

We see that the line marked (L) in Figure 3 separates a -lower zone for which, out of 19 tests, only 1 causes too intense bubbling, with an upper zone for which, in 22 tests 9, causes too intense bubbling . This line (L) corresponds to a speed of introduction of the cored wire of 105 m / min, which corresponds for a mass of liquid steel of 83 t to 120 g of Ca / t / min.

A series of tests, similar to those carried out using a Ca Ni alloy, are then carried out using a Ca Al alloy containing 93% by mass of calcium and 5% by mass of aluminum. This alloy is in the divided state, like the Ca Ni alloy which has just been mentioned. These tests show that it is possible to form with this Ca Al alloy the core of a cored wire having dimensional characteristics very close to those of the wire used in the case of the Ca Ni alloy. During these tests, it can be seen that, as in the case of the Ca Ni alloy, it is possible to introduce the cored wire, having for its core the Ca Al alloy whose composition has just been given, in a .bath of A42 type liquid steel, containing 83 tonnes of steel, at a speed corresponding to the introduction of approximately 120 g of calcium per tonne of liquid steel per minute. Under these conditions, no significant projections of liquid steel are observed.

In order to compare the yields of introduction of calcium into the liquid steel bath, that is to say the ratio expressed in X by mass between the quantity of calcium retained in the liquid steel bath and that introduced by means of the cored wire, steel baths are treated under the same operating conditions, each containing 83 tonnes of A42 steel either by the Ca Ni alloy or by the Ca Al alloy described above. In each case the total quantity of calcium introduced is 180 g per tonne, ie 0.0180% by mass. The analyzes carried out show that the amounts of calcium retained in the liquid steel in the distributor are on average 0.0034 X in the case of the Ca Ni alloy and 0.0039% in the case of the Ca Al alloy. The calcium yield is therefore 19% with the Ca Ni alloy and 22% with the Ca Al alloy.

During these same tests, the variation in the sulfur content of the steel bath following the introduction of calcium was also determined. It has been found that this content decreases, on average, by 24 Σ in the case of the introduction of the Ca Ni alloy and by 26 X in that of the introduction of the Ca Al alloy.

By way of comparison, 52 castings of the same steel are treated in the same way by means of a cored wire, with the same characteristics, the core of which is constituted by an SiCa alloy containing by mass 60% of Si and 30% this side.

By establishing a diagram similar to that of FIG. 3, it can be seen that a dividing line can be drawn between a lower zone for which the vast majority of the results do not give rise to too intense bubbling and an upper zone for which this bubbling becomes too violent. This separation line corresponds to a speed of introduction of the cored wire of 120 m / min. This wire containing 180g of SiCa alloy per meter, the maximum rate of Ca introduction is therefore in this case 78g per ton per minute.

The analyzes carried out show that the yield of calcium introduction is, in the case of the Si Ca alloy of 15 X on average. The silicon introduction yield is practically 100%. We see that, already for an introduction of 180 g of calcium per tonne of liquid steel, 360 ppm of silicon is fixed. Such an addition is incompatible with the development of a steel for deep drawing, for which the silicon content must be less than 300 ppm and even, in certain cases, less than 200 ppm.

The invention can also be used, by way of another example, by using a cored wire comprising, as material constituting the core, respectively 50% by mass of Ca-Ni alloy at 90% Ca and 8% Ni and 50% by mass of Ca-Si alloy at 30 × Ca and 60% Si. Such a mixture contains 60% Ca and 30% Si by weight.

To introduce the same amount of calcium into the bath, the mass of product to be added with the cored wire above containing 60% Ca is twice less than that to be added with a cored wire containing Ca-Si at 30% Ca and 60% Si.

Furthermore, the introduction of silicon is itself divided by four.

The process according to the invention can be subject to numerous modifications which do not depart from the field of the invention. This invention also relates to the cored wire. The envelope of this wire can, in particular, be made of steel or any metal compatible with the bath to be treated.

Claims (18)

1) A method of treating metals or alloys, in particular ferrous metals, in which a cored wire comprising a very long metallic tubular casing and a core containing a material is introduced into a metal or liquid alloy bath, characterized in that this material comprises a basic constituent which is an alloy containing at least 75 X by mass of calcium and at least one metal belonging to the group comprising nickel and aluminum, the total content of Al + Ni being at least equal to 5 X mass. 2) Process according to claim 1, characterized in that the basic constituent may contain additional additions and / or various impurities. 3) Method according to claim 1 or 2, characterized in that the basic constituent contains at least 80 X by mass of calcium. 4) Method according to one of claims 1 to 3, characterized in that the total content of Al + Ni is between 5 and 20% by mass. 5) Method according to one of claims 1 to 4, characterized in that the basic constituent contains 0 to 15 X by mass of silicon. 6) Method according to one of claims 1 to 5, characterized in that the basic constituent is in divided form. 7) Method according to one of claims 1 to 6, characterized in that the material of the core of the cored wire is compacted. 8) Method according to one of claims 1 to 7, characterized in that the core of the cored wire contains, in addition to the basic constituent another alloy containing calcium. 9) Method according to one of claims 1 to 7, characterized in that in the case of the treatment of a steel or ferrous alloy the speed of introduction of the cored wire into the metal is determined so that the amount of calcium introduced per minute, or approximately 80 to 120 g per ton of metal or ferrous alloy. 10) Cored wire comprising a thin metallic tubular envelope of great length and a core containing a material, characterized in that this material comprises a basic constituent which is an alloy containing at least 75% by mass of calcium and at least one metal making part of the group comprising nickel and aluminum, the Ni + Al content being at least equal to 5% by mass. 11) Cored wire according to claim 10, characterized in that the total content of Ni + Al is between 5 and 20% by mass. 12) Cored wire according to claim 10 or 11, characterized in that the basic constituent contains 0 to 15% by mass of silicon. 13) Cored wire according to one of claims 10 to 12, characterized in that the basic constituent contains various impurities and / or additional additions. 14) Cored wire according to one of claims 10 to 13, characterized in that it contains, in addition to the basic constituent another alloy containing calcium. 15) Cored wire according to one of claims 10 to 14, characterized in that the material that contains its core further contains elements or compounds, metallic or non-metallic which are not included in the alloy or alloys containing calcium. 16) Cored wire according to one of claims 11 to 15, characterized in that the basic constituent is in divided form. 17) Cored wire according to one of claims 11 to 16, characterized in that the material of the core of the cored wire is compacted. 18) Application of the method according to one of claims 1 to 9 to the preparation of steels having a particular ability to stamping.

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