EP0123632B2 - Process for the production of steels with high machinability - Google Patents

Abstract

The invention relates to the methods for the preparation of steels with high machinability, comprising globular inclusions which allow for high cutting speeds by means of carbide tools. The method according to the invention consists of introducing into a liquid steel which has been deoxidized carefully with aluminum, and desulfurized, additions of calcium and sulfur, carried out by a cored wire, the contents obtained being from 20 to 100 ppm calcium and 150 to 500 ppm sulfur. The steels prepared under this method offer a particular aptitude for high speed machining with the aid of carbide cutting tools.

Description

The process which is the subject of the invention relates to steels with high machinability.

It is well known that the machinability of steels depends in particular on the nature and the morphology of the inclusions present in the metal. They are mainly oxides and sulfides. Oxides have an unfavorable action on the cutting tool, on the other hand, sulphides can play a favorable role as a lubricant.

In the case of machining at moderate cutting speed using fast cutting steel tools, it is the sulphides which play an essential role and steels are used whose sulfur contents can vary from 0.07 to 0, 37%.

In the case of machining at high cutting speed using carbide tools, the high sulfur contents have no particularly favorable effect. On the other hand, it has been found that the inclusions of oxides are particularly harmful, because they cause wear of the cutting tool. It is possible to reduce the novelty of these inclusions by known means. We can, in particular, reduce the amount through good deoxidation and good decantation. We can also make these inclusions, generally based on alumina, globular by the addition of alkaline earth elements such as calcium, or other elements. We can finally arrange for these remaining globular inclusions to contain a certain amount of combined sulfur which reduces their harmfulness. In this case, the sulfur content is generally not greater than that which is usually present in the steel, that is to say less than 500 ppm (parts per million by mass), and generally of the order from 150 to 500 ppm. In this interval, it is often sought to target a narrower sulfur content range, for a given use and grade, which presents serious difficulties.

More generally, experience has shown that it is difficult to produce reproducible steels with a low content of inclusions, these inclusions being made little harmful by their globular shape and by the presence of small amounts of combined sulfur.

These difficulties are due in particular to the fact that it is difficult to properly control the sulfur contents of the steel after deoxidation, that it is also not easy to control with precision the possible additions of sulfur carried out steel, and finally that the yield of calcium additions as an agent making it possible to make globular inclusions lacks reproducibility.

We looked for the possibility of developing a process for developing steels with high machinability which are particularly suitable for high speed machining using carbide (s) cutting tools, by adding sulfur and calcium, under conditions making it possible to obtain a high efficiency of the combined action of calcium and sulfur as well as an excellent reproducibility of the results, while regulating the sulfur content within narrow ranges of composition, this content not exceeding not the upper limit of sulfur content commonly accepted in steels without voluntary addition of sourre.

In particular, the possibility of developing a method of introducing calcium and sulfur into liquid steel was investigated, which makes it possible to adjust with great precision the quantities of calcium supplied to the steel in metallic form, as well as the corresponding quantities of sulfur, in order to obtain, from the machinability point of view, in a reproducible manner, optimal results.

The process which is the subject of the invention provides a particularly advantageous solution to the problem which arises.

It consists in developing by conventional fusion a non-alloyed, or alloyed, or stainless steel, then in carrying out an addition of aluminum in order to lower the oxygen content of the steel below 100 ppm and in carrying out a desulfurization pushed by a basic slag in order to lower the sulfur content of the steel below 100 ppm, then to carry out, in cored wire, additions of calcium and sulfur in order to reach in the steel a content of calcium from 20 to 100 ppm, and a sulfur content from 150 to 500 ppm, the addition of aluminum having been carried out so that the residual content of the dissolved aluminum steel is between 150 and 500 ppm. The oxygen content is preferably lowered below 50 ppm and, preferably also, the desulfurization is carried out until a sulfur content of less than 50 ppm is obtained. The calcium and sulfur additions can be carried out either successively, the calcium being introduced first, or simultaneously.

The addition of calcium is carried out by means of a cored wire advantageously containing a calcium alloy in grains or powder, such as a silicocalcium.

The addition of sulfur is carried out by means of a cored wire advantageously containing flower of sulfur or a sulphide.

In the case of simultaneous addition of calcium and sulfur, it is possible to use several cored wires, or a single cored wire containing both calcium and sulfur in the desired proportions.

The process makes it possible in particular to obtain repoductively steels for which the difference between the sulfur content obtained and the targeted content does not exceed ± 40 ppm.

The process according to the invention makes it possible, thanks to the very precise additions of calcium and sulfur thus carried out, to obtain finely distributed globular inclusions which confer reproducible manner on steel, high machinability. These steels are particularly suitable for high speed machining using carbide cutting tools.

• on élabore da façon conventionnelle un acier, tel qu'un acier allié ou non allié de type courant;
• on effectue en fin d'élaboration, une désoxydation de cet acier au moyen d'aluminium, dont la quantité est déterminée pour obtenir une teneur résiduelle en aluminium dissout dans l'acier d'environ 150 à 500 ppm, cette teneur résiduelle visée étand d'autant plus forte, à l'intérieur de ces limites, que la teneur en carbone est plus faible. La désulfuration est ensuite effectuée par exemple au moyen d'un laitier basique qui peut être constitué par exemple par de la chaux ou par une composition alumino-calcique. Pour permettre une désulfuration très poussée, il faut brasser le métal liquide au contact du laitier, par exemple par insufflation d'un gaz neutre à travers ce métal, ou par tout autre moyen. Les teneurs finales en soufre et oxygène doivent être, de préférence, inférieures à 50 ppm pour chacun de ces éléments.

In detail, the method according to the invention can advantageously be implemented in the following way:

• steel is produced in a conventional manner, such as alloy or non-alloy steel of the standard type;
• at the end of production, this steel is deoxidized by means of aluminum, the quantity of which is determined to obtain a residual content of aluminum dissolved in the steel of approximately 150 to 500 ppm, this residual content referred to as d 'The stronger, within these limits, the lower the carbon content. The desulphurization is then carried out for example by means of a basic slag which may consist, for example, of lime or of an aluminum-calcium composition. To allow very thorough desulfurization, the liquid metal must be stirred in contact with the slag, for example by blowing a neutral gas through this metal, or by any other means. The final sulfur and oxygen contents should preferably be less than 50 ppm for each of these elements.

The calcium is then introduced into the liquid steel by means of a cored wire such as, for example, that described in FR 2 476 542. This wire consists of an envelope, generally made of mild steel. , a few tenths of a mm thick, which surrounds the core which contains the calcium in the divided state in the form of metal or alloy. The introduction of the cored wire is preferably carried out at a relatively rapid speed in general of the order of 1 to a few meters per second. This speed is adjusted as a function of the content of the calcium-filled wire per unit of length and of the quantity to be introduced, so that the duration of this introduction does not exceed a few minutes. The wire is made to penetrate from top to bottom through the metal bath, at an angle preferably close to 90 ° relative to the horizontal. It is thus possible to make the calcium penetrate very deep into the liquid steel, which considerably increases the efficiency of the addition. The amount of calcium thus introduced into the liquid steel bath, in the form of metal or alloy, is preferably between 150 and 600 g / t, which allows, after reduction of the oxides still present in the metal , obtaining a calcium content preferably between 20 and 80 ppm. The liquid steel bath thus added with calcium is preferably homogenized by stirring before the addition of sulfur. This latter addition is carried out by means of a cored wire containing either sulfur in bloom or a sulphide such as iron or manganese sulphide, in pulverulent or granular form. The envelope is generally made of mild steel a few tenths of a mm thick, as in the case of the addition of calcium. The introduction of sulfur is carried out like that of calcium at relatively high speed.

It is sought to obtain in liquid steel, a sulfur content of between 150 and 500 ppm, preferably between 150 and 300 ppm. The introduction yield is generally greater than 90% thanks to the use of cored wire, which makes it possible to adjust with great precision the addition of sulfur.

In practice, by making sulfur flower additions, it is possible to accept a yield of the order of 95%.

After the sulfur has been introduced, the steel is cast either in ingots or by means of a continuous casting installation. Maximum care must be taken to avoid reoxidation of the liquid steel jets during this casting operation.

The examples below describe, without limitation, two modes of production of a steel with high machinability by the process according to the invention:

Example 1

• 1°) Elaboration de l'acier par des modes habituels dans un four à arc de 80 t, à partir de ferrailles, avec fusion oxydante, soufflage d'oxygène, déphosphoration, décrassage et recarbura- tion.
• 2°) Coulée du métal en poche magnésie, une partie du manganèse pouvant être ajoutée dans la poche sous forme de ferro-manganèse. Désoxydation par addition d'aluminium dans le jet de coulée (1,5 kg/t soit 120 kg). Mise en place sur le métal d'un laitier de chaux (8 kg/t de poudre de chaux anhydre, soit 640 kg).

It is proposed to apply the process according to the invention to the preparation of a steel corresponding to the AISI 1045 standard and containing in% by mass:

• 1 °) Elaboration of the steel by usual methods in an arc furnace of 80 t, from scrap, with oxidizing fusion, oxygen blowing, dephosphorization, scrubbing and recarburization.
• 2 °) Pouring of metal into a magnesia pocket, part of the manganese can be added to the pocket in the form of ferro-manganese. Deoxidation by addition of aluminum in the casting jet (1.5 kg / t or 120 kg). Placement on the metal of a lime slag (8 kg / t of anhydrous lime powder, or 640 kg).

• 3°) Brasage du métal par l'argon pendant 20 minutes. Réglage de la composition par addition de fonte et ferro manganèse.

From the start of the casting, mixing of the metal by blowing argon. Taking of a steel sample 1 minute after the end of casting. The composition of the steel is then in% by mass:

• 3 °) Brazing of the metal with argon for 20 minutes. Adjustment of the composition by addition of cast iron and ferro manganese.

• 4°) Introduction dans le métal d'un fil fourré contenant 180 g au mètre de silico-calcium à 31 % en masse de calcium. Ce fil est introduit à la vitesse de 120 m/minute, soit 6,7 kg/minute de calcium pendant 3 minutes, soit une addition de 0,25 kg de calcium par tonne d'acier liquide.

The following composition is then obtained, in% by mass:

• 4 °) Introduction into the metal of a cored wire containing 180 g per meter of silico-calcium at 31% by mass of calcium. This wire is introduced at the speed of 120 m / minute, or 6.7 kg / minute of calcium for 3 minutes, or an addition of 0.25 kg of calcium per tonne of liquid steel.

Maintaining light mixing of the steel liquid with argon for 3 minutes after the end of the injection.

• 5°) Resulfuration par injection dans l'acier, après les 3 minutes de brassage léger suivant l'addition de calcium, d'un fil fourré contenant 135 g au mètre de fleur de soufre, la vitesse d'injection est de 90 m/minute et l'introduction de soufre dure 1 minute et 20 secondes; soit une addition totale de 16,2 kg de soufre ou 200 ppm.
• 6°) Le métal est coulé en ronds de 223 mm de diamètre par coulée continue rotative, en passant préalablement dans un répartiteur comportant un revêtement basique. La composition finale du produit coulé est la suivante:
  
• 7°) Ces ronds sont laminés en tubes mécaniques de 180 mm de diamètre extérieur et de 20 mm d'épaisseur.

A sample taken after these 3 minutes has the following composition:

• 5 °) Resulfurization by injection into steel, after 3 minutes of light stirring following the addition of calcium, of a cored wire containing 135 g per meter of flower of sulfur, the injection speed is 90 m / minute and the introduction of sulfur lasts 1 minute and 20 seconds; or a total addition of 16.2 kg of sulfur or 200 ppm.
• 6 °) The metal is cast in rounds of 223 mm in diameter by continuous rotary casting, passing beforehand through a distributor having a basic coating. The final composition of the cast product is as follows:
  
• 7 °) These rounds are rolled into mechanical tubes 180 mm outside diameter and 20 mm thick.

The tubes thus obtained have a machinability, by means of carbide cutting tools, much greater than that of common steels of the same composition.

• un acier A élaboré selon la procédure qui vient d'êcre décrite;
• un acier B élaboré normalement dans le même four à arc de 80t avec des matières premières analogues mais n'ayant été ni resulfuré, ni traité par le calcium au moyen de fil fourré. La teneur en S de 0,018/0,025% a été obtenue directement par brassage modéré et de plus courte durée avec un laitier moins riche en chaux (300 kg de chaux ajouté en poche après coulée en poche au lieu de 640 kg).

This gain in machinability is illustrated in the single figure which compares, for the same reference analysis indicated above (standard AISI 1045):

• a steel A produced according to the procedure which has just been described;
• a steel B normally produced in the same 80t arc furnace with similar raw materials but which has neither been resulfurized nor treated with calcium by means of cored wire. The S content of 0.018 / 0.025% was obtained directly by moderate brewing and of shorter duration with a slag less rich in lime (300 kg of lime added to the bag after pouring into the bag instead of 640 kg).

The duration in minutes on the abscissa along the axis T was shown for a frontal wear of tools of 0.4 mm and on the ordinate along the axis V the cutting speed in meters per minute.

The curves A and B of the single figure thus give for each steel corresponding to the normalized state the cutting speed in meters per minute which allows a determined cutting time corresponding to a frontal wear of the tool of 0.4 mm. This is a dry run test carried out with an ISO-P30 carbide tool, the feed being 0.4 mm per revolution and the depth of the pass 2 mm.

The higher the cutting speed for a given service life, the greater the machinability of the steel.

The effectiveness of the proposed processing technique is thus measured.

Example 2

The same steel as in Example 1 is produced under similar conditions, but by carrying out the final additions of calcium and sulfur using a cored wire containing a mixture of the flower of the coufre and of calcium-silicon at 30 % by mass of calcium.

This mixture contains 20% sulfur and 80% silico-calcium. This cored wire weighs 170 g per meter. It is introduced at 120 m per minute for 4 minutes, giving results similar to those of Example 1.

Claims (11)

1. Process for the production of a steel with high machinability in which a non-alloyed or alloyed or stainless steel is produced by conventional fusion, characterised in that, after conventional production of the steel, aluminium is added in order to reduce the oxygen content of the steel below 100 ppm and advanced desulphurisation is effected with a basic slag to reduce the sulphur content of the steel below 100 ppm, then in that additions of calcium and sulphur are made with a cored wire to give the steel a calcium content of 20 to 100 ppm and a sulphur content of 150 to 500 ppm, the addition of aluminium having been made in such a way that the residual content of dissolved aluminium in the steel is between 150 and 500 ppm.

2. Process according to claim 1, characterised in that the deoxidation is carried out so as to reduce the oxygen content of the steel below 50 ppm.

3. Process according to one of claims 1 or 2, characterised in that desulphurisation is carried out so as to reduce the sulphur content of the steel below 50 ppm.

4. Process according to one of claims 1, 2 or 3, characterised in that the additions of calcium and of sulphur are made successively, the calcium being added first.

5. Process according to claim 4, characterised in that the addition of calcium is effected by means of a cored wire containing an alloy based on granular calcium such as a silicocalcium.

6. Process according to claim 5 or 4, characterised in that the addition of sulphur is made by means of a cored wire containing sulphur as flowers or as sulphide.

7. Process according to one of claims 1 to 3, characterised in that calcium and sulphur are added simultaneously by means of at least one cored wire.

8. Process according to one of claims 1 to 7, characterised in that the addition of sulphur is between 150 and 300 ppm.

9. Process according to one of claims 1 to 7, characterised in that the sulphur content is adjusted within the range of between 150 and 500 ppm with such precision that the discrepancy between the obtained content and the desired content does not exceed ± 40 ppm.

10. Process according to one of claims 1 to 9, characterised in that the phases of oxygen content reduction, desulphurisation, addition of calcium and of sulphur are carried out in a ladle.

11. Process according to one of claims 1 to 10, characterised in that the metal is cast by continuous casting.

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