EP0141760B2 - Process for the treatment of steel by calcium to increase the cold-workability and to decrease the silicium content - Google Patents

Abstract

The invention concerns the production of steels which have an isotropic structure, are very well adapted to cold forming particularly by pressing and which have a very low silicon content. The method comprises taking a deoxidated liquid steel with a low silicon content and placing in it a cored wire containing a divided material, which is a mixture of silico-calcium and granular calcium in a specific quantity and specific proportions. The method is applicable to non-alloyed or slightly alloyed steels and is particularly suitable for a continuous casting installation.

Description

The process which is the subject of the invention relates to steels whose inclusions, thanks to an appropriate treatment, remain globular after rolling which which gives the state of employment particularly advantageous properties such as a great ability to stamping or cold stamping.

It is known to call upon the preparation of such steel for an addition of calcium to the liquid steel before casting it.

The introduction of this element into liquid steel presents particular difficulties due to its low density and its very high oxidability.

A particularly effective method for carrying out this introduction is that described in European patent application EP 34 994 page 8, lines 13-35.

It consists in using a silico-calcium containing 30% calcium; this alloy is used in the form of a powder which is wrapped in a thin acer sheath and compacted in situ. The composite product thus produced, commonly called cored wire, is introduced by unwinding from a coil at the heart of the bath of liquid steel to be treated. This avoids any oxidation of the active element, calcium, and it acts directly on the metal bath with a high and reproducible yield.

The steels thus treated have improved properties in many fields: isotropy, ductility, machinability. In addition, this addition of calcium makes the inclusions present in the liquid metal liquid and therefore avoids the risk of clogging of the continuous casting nozzles.

However, this method cannot be used when it is desired to prepare certain steels having a particular aptitide for cold forming and more particularly for deep drawing.

Such steels must have a very low silicon content. The limit not to be exceeded is most often 20 to 30 thousandths percent of silicon. In practice, the silico-calcium used contains approximately 30% of Ca, 60% of Si and 10% of Fe + various impurities.

The weight of this silico-calcium which must be introduced into the liquid steel to obtain the full effectiveness of the calcium is approximately 0.5 to 1.5 kg per ton of liquid steel.

This addition therefore results in a silicon enrichment of the liquid steel of 30 to 90 thousandths percent, and this with a yield of practically 100%.

Tests have shown that it is undesirable to use silico-calcium containing more than 30% of calcium. Indeed, these alloys in the ground state are unstable and liable to explosion. Similar risks of explosion exist in the case of ground calcium which contains large proportions of extremely oxidizable and unstable fines.

We know the possibility of developing pure calcium, in the form of grains and free of fines. Such a product can in particular be obtained by the process described in international PCT application WO81 / 01811. We can therefore consider using it for the development of very low silicon steels.

However, these products have the disadvantage of a high cost which significantly increases the cost of refining such steels.

We therefore sought the possibility of developing, under much more favorable cost conditions, calcium steels having a very low silicon content so as to obtain particularly advantageous properties such as in particular cold deformability.

The process for the preparation of a steel having a great aptitude for cold forming according to the invention consists in preparing a liquid steel calmed with aluminum having a silicon content of less than 5 thousandths% by mass and then introducing in this liquid steel a cored wire which contains a divided material. This divided material comprises at least two components: the first of these components is metallic calcium in grains of which the content of particles smaller than the meshes of the sieve of 150 mesh does not exceed 2 to 3% by weight of said component, the second is silico-calcium containing in% by mass Ca 25 to 35, Si 50 to 70, Fe and impurities 5 to 15, the mass ratio K, in the divided material, between the first component and the second being between 0.1 and 3 and preferably between 0.3 and 23, the quantity of silicon introduced not exceeding 300 g per tonne of steel treated.

Most often, the envelope of the cored wire is made of steel. Most often, about 125 to 600 g of calcium are introduced into the liquid steel, by means of this cored wire, per ton of steel treated.

One can in particular, after calming the liquid steel with aluminum, treat it with a basic reducing slag containing for example fluorspar to desulfurize it.

The cored wire advantageously comprises a core of divided material which is in the compacted state inside the envelope, the latter having at least two parallel flattened zones, facing one another. Preferably the grain calcium used is obtained by the process which is the subject of international PCT application WO 81/01811.

This process consists of melting the starting calcium and then passing it in the divided state through a purification bath, then, after settling, spraying it by passage through a vibrating orifice and finally solidifying the drops obtained in grains.

The invention applies particularly well to the continuous casting of steels. In this case, the injection of the cored wire for the above treatment can be carried out either in a bag or in a distributor.

The process according to the invention most often applies to the production of unalloyed or weakly alloyed steels.

In general, the method according to the invention makes it possible to carry out particularly economical addition of calcium in a steel, at the desired concentration to obtain globularization of the inclusions, while keeping the silicon content below a limit level, generally fixed by a standard.

A steel is thus obtained which, while having the characteristics specific to steels treated with calcium, in particular good isotropy of mechanical properties and excellent machinability, also exhibits excellent aptitude for cold deformation and more particularly stamping.

Thanks to the process according to the invention, it is possible to significantly reduce the cost of developing these steels. For this, we first determine for each type of processing the total amount of calcium that must be introduced into the steel, using a cored wire, taking into account that the reaction yield of calcium thus introduced is 15 to 20%.

The quantities of silicon that can be introduced into the steel are also determined without exceeding the maximum acceptable limit.

This limit is most often around 20 to 30 thousandths percent. Insofar as the production of steel made it possible to obtain a steel made it possible to obtain a liquid steel with a very low silicon content, for example less than 5 thousandths percent, it is possible to accept an increase of the silicon content of the steel during the introduction of the cored wire into the liquid steel of about 15 to 30 thousandths percent according to the specifications which the steel must meet. It is therefore possible to introduce into the steel, taking into account its initial silicon content and the limit fixed by the specification, 150 to 300 g of silicon per tonne of liquid steel. When this silicon is introduced in the form of a common type of calcium calcium, containing approximately in mass% Si 60, Ca 30 and Fe + impurities 10, it can be seen that the total amount of calcium calcium to be introduced is from 250 to 500 g per tonne of liquid steel. The corresponding amount of calcium thus introduced is 75 to 150 g per ton. It is enough to mix with calcium silica the calcium supplement in grain allowing to reach the quantity of total calcium which it acts to introduce. We can thus minimize the cost of developing such steels. In fact, on the one hand, a quantity of silico-calcium is used as high as possible, the cost price per unit of calcium introduced is particularly low, and, on the other hand, the use of pure calcium instead of silico-calcium for the complement reduces the total mass and the corresponding volume of divided matter to be introduced.

This means that, with equal cross-section and compaction rate, a shorter length of cored wire will have to be introduced into the bath of liquid steel, which will also contribute to reducing costs. this introduction will most often be made in a layer or even in a distributor in the case of continuous casting. The steel will have been deoxidized beforehand under conditions such that its silicon content is lowered to a level generally less than 5 thousandths percent and in general of the order of 1 to 3 thousandths percent.

The invention also relates to a cored wire for the implementation of the method described above comprising a metal envelope inside which is enclosed a divided material based on calcium and silicon. This divided material comprises at least two components, the first being metallic calcium in grains of which the content of particles smaller than the meshes of the sieve of 150 mesh does not exceed 2 to 3% by weight of said component, the second being of silico-calcium containing in mass% Ca 25 to 35%, Si 50 to 70%, Fe and impurities 5 to 15%, the ratio k between the contents of the first and second components being between 0.1 and 3 and preferably 0, 3 and 2.

Advantageously, the envelope of the cored wire according to the invention is made of steel.

Advantageously also the divided material enclosed in the envelope is in the compacted state. A particularly advantageous solution for producing the cored wire is constituted by a wire whose envelope has at least two flattened parallel zones opposite one another.

The attached figure makes it possible to better understand the possibilities of the method according to the invention and its mode of implementation.

This figure gives the ordinate the enrichment of the metal with silicon by injecting the cored wire as a function of the average calcium content of the mixture used.

• - 450 g/tonne pour la courbe (1)
• - 300 g/tonne pour la courbe (2)
• - 150 g/tonne pour la courbe (3)

This figure has 3 curves (1) (2) and (3) which each correspond to a constant amount of calcium added to the ton of steel.

• - 450 g / ton for the curve (1)
• - 300 g / ton for the curve (2)
• - 150 g / ton for the curve (3)

• Si 60% - Ca 30% - Fe et impuretés 10%

These mixtures are composed of pure calcium in grains free of fines (absence of grains whose diameter is less than 100 microns) and of a ground silico-calcium which contains in% by mass:

• If 60% - Ca 30% - Fe and impurities 10%

On the abscissa is given the total Ca content in% by mass of the calcium / silico-calcium mixtures at each point of the curves. This content can be calculated from the ratio K between the quantities of pure calcium and silico-calcium contained in the mixtures at each point. Curve (4) represents the variation in total calcium content of the mixtures as a function of the K ratio.

The value K = O corresponds to a divided material composed only of calcium calcium with 30% calcium. This value defines the starting points (5), (6) and (7) of each of the curves (1), (2) and (3).

At each of these points the divided matter is constituted respectively by 1.5-1 and 0.5 kg of silico-calcium without addition of pure calcium. The corresponding quantities of silicon which will be introduced into the liquid steel are: 900 g - 600 g and 300 g of silicon per tone. The introduction yield of this silicon being practically 100% it is seen that these additions of silico-calcium enrich the steel in silicon respectively at these points by 90, 60 and 30 thousandths percent. Such enrichments are not acceptable if one wishes to limit the final silicon content to a level for example less than 30 thousandths percent or even less than 25 or 20 thousandths percent.

The curves in the figure show that it is possible, by moderate enrichment in pure calcium of the divided matter, to lower the silicon content to the desired level. It can be seen, for example, that a mixture of divided matter having a ratio K of 0.6, that is to say in which the mass of pure calcium is equal to 60% of the mass of silico-calcium, makes it possible to divide by three silicon enrichment all other things being equal. Such a mixture contains 56% of Ca instead of 30% in the case of silico-calcium alone, and the mass necessary to treat a ton of liquid steel does not represent more than 53.6% of the initial mass.

In many cases it suffices to enrich the divided matter in a relatively small way to obtain the desired result.

For the implementation of the invention, divided materials are used, the K ratio of which can vary as required between 0.1 and 3. In practice, most often, we limit ourselves to K ratios of between 0.2 and 2. It is preferable to seek the use of K ratios as low as possible according to the aim sought so as to minimize the cost of the addition.

The example below describes, without limitation, an embodiment of the method according to the invention.

1) A steel for stamping is produced in known manner by means of an LD converter which, after pouring into a pocket, with dolomite coating, has the following composition in% by mass:

Calming in the ladle is carried out with aluminum without the addition of ferro-silicon. This steel is treated in the ladle with a basic slag consisting of a mixture of alumina lime and fluorspar with agitation by blowing argon through a porous plug placed in the bottom of the ladle. After 10 minutes of blowing, the composition of the steel is as follows in% by mass:

A cored wire with a steel casing is then introduced into the liquid steel, the divided material of which is a silico-calcium containing in% by mass Ca = 30, Si = 60, Fe and impurities 10. The amount of silico-calcium introduced is 1.2 kg (corresponding to 0.36 kg of calcium) per tonne of liquid steel. After 3 minutes of argon blowing, the steel is poured into ingots which are then transformed into slabs. The average composition of these is then as follows in% by mass:

2) A steel for stamping is also produced by the process according to the invention, for which the specification imposes a silicon content of less than 0.020%. After preparation with the LD converter then calming with aluminum and treatment with a basic slag with blowing of argon as in the first case, the following composition is obtained:

A cored wire with a steel casing is then introduced into the liquid steel, the divided material of which is a mixture of silico-calcium of the same composition as for the first preparation, and of pure calcium in grains. The ratio K is equal to 1.33, which corresponds to a divided material whose average calcium content is 70%.

The amount of total calcium added is the same as in the first case, that is to say 360 g / t of liquid steel to be treated. This time the divided matter mixture contains 294 g of pure calcium in grains, 221 g of silico-calcium containing only 132 g of silicon. The mass of mixture to be introduced is reduced to 515 g / t instead of 1200 g / t, which significantly reduces the length of the cored wire.

After 3 minutes of argon blowing, the steel is poured into ingots which are transformed into slabs, the average composition of which is as follows in% by mass:

The steel obtained by the process according to the invention therefore conforms to the specification relating to the silicon content. It has a substantially isotropic structure, a very good drawing ability.

Many alternative embodiments can be made to the method according to the invention which do not depart from the field thereof.

In particular, the invention makes it possible to determine in each case the optimal amounts of calcium and of silico-calcium to be used to obtain a drawing steel under the most economical conditions.

Claims (11)

1. A process for producing a steel having a high level of suitability for cold shaping, comprising preparing a liquid steel which is killed with aluminium, with a silicon content of less than 5 thousandths of a percent by mass, then introducing into said liquid steel a filled wire which contains a divided material, characterised in that said divided material comprises at least two components, the first being metallic calcium in grain form in which the proportion of particles which are smaller than mesh size 150 mesh does not exceed 2 to 3% by weight of said component, the second being silico-calcium containing in percent by mass: Ca 25 to 35; Si 50 to 70; Fe and impurities 5 to 15, the ratio K between the amounts of the first and second components being between 0.1 and 3 and preferably between 0.3 and 2, the amount of silicon introduced not exceeding 300 g per tonne of steel treated.

2. A process according to claim 1 characterised in that the casing of the filled wire is of steel.

3. A process according to claim 1 or claim 2 characterised by introducing into the steel by means of the filled wire an amount of calcium which is between 125 and 600 g per tonne of steel treated.

4. A process according to one of claims 1 to 3 characterised in that after killing of the liquid steel with aluminium, it is treated with a basic reducing slag prior to introduction of the filled wire.

5. A process according to one of claims 1 to 4 characterised in that the filled wire comprises a core portion of compacted divided material within the casing, and that said casing has at least two parallel flattended zones which are disposed facing each other.

6. A process according to one of claims 1 to 5 characterised in that the calcium in grain form, which constitutes one of the components of the divided material, is obtained by fusion of the starting calcium, passing said molten calcium in a divided state through a purification bath, settlement of said calcium, atomisation of said settled calcium by passing it through a vibrating orifice and finally solidification in grain form of the drops of calcium which are formed in that way.

7. A process according to one of claims 1 to 6 characterised in that the operation of introducing the filled wire is effected in the distributor or in the ladle of a continuous casting installation.

8. A filled wire for carrying out the process according to one of claims 1 to 6 comprising a metal casing within which is enclosed a calcium and silicon-based divided material, characterised in that said divided material comprises at least two components, the first being metallic calcium in grain form in which the proportion of particles which are smaller than mesh size 150 mesh does not exceed 2 to 3% by weight of said component, the second being silico-calcium containing in percent by mass: Ca 25 to 35, Si 50 to 70, Fe and impurities 5 to 15, the ratio K between the amounts of the first and second components being between 0.1 and 3 and preferably 0.3 and 2.

9. A filled wire according to claim 8 characterised in that the casing of the filled wire is of steel.

10. A filled wire according to claim 8 or claim 9 characterised in that the divided material contained in the casing is in the compacted state.

11. A filled wire according to one of claims 8 to 10 characterised in that the casing has at least two parallel flattened zones which are disposed facing each other.

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