EP0371840A1 - Free cutting mild steel and process for manufacturing same - Google Patents

Abstract

The present invention relates to a mild steel for bar turning with high machinability performance. It is characterized by a content of C <= 0.25%, a content of Mn between 0.8 and 1.5%, a content of P <= 0.1%, a content of S between 0, 15 and 0.40%, an Si content of between 0.05 and 0.40%, expressed in% by weight. On the other hand, the manganese sulfide inclusions are, after metal working, surrounded by a layer of plastic oxides of average composition SiO2: 35 to 45%; Al2O3: 10 to 20%; CaO: 15 to 25%; MnO: 10 to 20%. The invention also relates to a method of producing this shade, in which the liquid metal, after addition of silicon and manganese, is stirred in the presence of a slag of CaO composition: 20 to 55%; SiO2: 35 to 65%; Al2O3: 15 to 40%; with <IMAGE> close to 1.

Description

The present invention relates to the field of mild steels for bar turning with improved cutting performance.

Free-cutting steels are intended for the machining of various parts by means of quick-cutting machine tools, such as lathes. One of the main qualities required for this type of steel is to cause the least possible wear of the cutting tool. Indeed, low wear allows high cutting speeds, going hand in hand with high productivity of the machine, or a long life of the cutting tool.

The composition of steels for bar turning is conventionally chosen so that within them, the presence of numerous inclusions of manganese sulfide is observed, the role of which is to improve the machinability performance, in particular by facilitating chip fragmentation and limiting wear of cutting tools. This is the reason why bar turning steels contain high sulfur (0.1 to 0.3% and more by weight) and manganese (up to 1.5%) contents.

When particularly high machinability is desired, it is possible to use a mild steel for free cutting taken, for example, from standard NF-A-35561 among the grades S250, S250Pb, S300 or S300Pb. These nuances are characterized:
- by very low tolerated silicon contents (less than 0.05% and preferably less than 0.02%), to avoid the presence of hard inclusions of silica which would deteriorate the cutting tool;
- by the possible addition to the metal of a significant amount of lead (0.20 to 0.30%) whose role is to accentuate the lubricating effect of inclusions of manganese sulfide during cutting.

This addition of lead has serious drawbacks, however. First of all, it is difficult to carry out the addition and dissolution of lead in the liquid steel in a sufficiently homogeneous manner. Due to its high density, it tends to accumulate in the bottom of metallurgical vessels. Above all, its particularly low vaporization temperature causes, upon its introduction into the liquid metal, the formation of fumes of highly toxic lead oxides. The protection of operators requires the use of a smoke capture system at the metal processing station, as well as medical supervision of the personnel ensuring the operation of the installation.

It is therefore understood that it would be very desirable to replace lead, in this type of steel grade, with a material having equivalent cutting properties, without manufacturing disadvantages.

Elements such as bismuth could be substituted for lead. However, their very high cost makes their use difficult to envisage in the mass production of steels for bar turning.

The object of the invention is to provide a mild steel for bar turning whose machinability performance is at least equal to that of lead grades of the S250Pb and S300Pb type without the addition of this element being necessary.

To this end, the subject of the invention is a mild steel for bar turning, characterized in that:
- its composition is C ≦ 0.25%; Mn = 0.8-1.5%; P ≦ 0.1%; S = 0.15-0.40%; Si = 0.05-0.40%, expressed in weight%.
- And in that the manganese sulfide inclusions are, after metal working, surrounded by a layer of plastic oxides of average composition of the SiO₂ type: 35 to 45%; Al₂O₃: 10 to 20%; CaO: 15 to 25%; MnO: 10 to 20%, also expressed in% by weight.

Advantageously, the carbon content is less than or equal to 0.16%, and even preferably to 0.09% which is the most commonly accepted content for mild free cutting steels. It can also contain calcium, in a content of 5 to 50 ppm, and / or tellurium in a content of 5 to 200 ppm.

voisin de 1. Optionnellement, l'addition de silicium est accompagnée de l'addition de 0,1 à 0,3 kg d'aluminium par tonne d'acier pour faciliter l'obtention de la composition recherché pour les oxydes.A subject of the invention is also a method for producing such a steel in the liquid state for free cutting, characterized in that, after addition of silicon and manganese to the metal bath, the latter is stirred in a metallurgical container in the presence of a slag whose composition, expressed in weight percentages, is CaO: 20 to 55%; SiO₂: 35 to 65%; Al₂O₃: 15 to 40%; with

close to 1. Optionally, the addition of silicon is accompanied by the addition of 0.1 to 0.3 kg of aluminum per ton of steel to facilitate obtaining the desired composition for the oxides.

As will be understood, the invention makes it possible to obtain a mild steel for bar turning with high machinability performance without having to add to the liquid metal lead or an element with an expensive or polluting equivalent function. In wrought metal, the manganese sulfide inclusions are thus enveloped by a layer with high plasticity, consisting of oxides whose average composition is within a well-defined range. This range corresponds to the plastic domain of the CaO-SiO₂-Al₂O₃ ternary diagram, the oxide MnO being assimilated to the oxide CaO.

In terms of weight composition, the grades according to the invention are distinguished from mild steels for bar turning S250 and S300 conventionally used (and defined by standard NF-A-35561) by the presence of silicon in significant quantities. As we have seen previously, silicon (and, in general, strong deoxidizers such as aluminum) captures dissolved oxygen in the metal to form hard inclusions, the presence of which in significant quantities in steels for bar turning can be prohibitive. This is why the standards concerning the most frequent free cutting steels impose very low silicon levels.

Going against the ideas generally accepted and enshrined in standards, the inventors have shown that significantly large quantities of silicon could, under certain conditions, be present in a mild steel for bar turning with high machinability, without, however, the properties product use is adversely affected. For certain applications, these new silicon grades even prove to be even more efficient than conventional lead-enriched grades.

This incorporation of silicon into the metal must imperatively go hand in hand with a method of production which allows:
- eliminating a large part of the inclusions formed during the introduction of silicon into the liquid steel, in particular the inclusions of pure silica;
- to transform the remaining oxidized inclusions into plastic inclusions, whose behavior during the solidification of the metal and subsequent wrought work is similar to that of lead in lead steel grades for free cutting.

The possible incorporation of 5 to 200 ppm of tellurium is intended to limit the hot deformability of sulfides. This makes it possible to prevent the sulphides from taking an exaggeratedly elongated shape after the wrought, which would significantly deteriorate the isotropy of the mechanical properties of the metal. This role can also be ensured by calcium. However, this item should only be added in quantities limited and determined, to prevent the oxide inclusions from having too high a CaO content which would be detrimental to their deformability.

The development of these shades can be carried out in the following manner. When metal is poured into a metallurgical container, for example a processing ladle, the additions of silicon, carbon, manganese, sulfur and possibly tellurium necessary for obtaining the desired composition for the metal are carried out. Are also added mineral materials such as lime, wollastonite and alumina, necessary for obtaining a slag whose composition is in the range: CaO: 20 to 55%; SiO₂: 35 to 65%; Al₂O₃: 15 to 40%. On the other hand, the ratio of CaO and SiO₂ contents in this slag, which expresses its basicity index, should preferably be close to 1. In order to obtain this slag composition by means of additions as standardized as possible, it it is preferable to pour the metal into the treatment bag, leaving only a small amount of slag from the primary production furnace (converter or electric furnace) to enter the latter. This can be achieved by means of known devices for retaining slag in the oven, or by means of transferring the metal from pocket to pocket. The liquid steel is then stirred in the treatment bag, for a sufficient time (30 minutes or more), with the aim of eliminating as much as possible the inclusions of pure silica which have been formed during the deoxidation of the metal. On the other hand, this mixing makes it possible to put the liquid metal in thermodynamic equilibrium with the slag, so that the remaining oxidized inclusions have the desired composition, defined above. This stirring is carried out by known means, such as blowing gas or applying an electromagnetic field to the metal. During this production, the compositions of the metal and the slag are checked, and any necessary corrections are made.

The deoxidation of the metal can be carried out partially with aluminum, at the rate of an addition of 0.1 to 0.3 kg per tonne of liquid metal, which accompanies the addition of silicon. This makes it possible to reduce the amount of inclusions of pure silica formed at this time, and to facilitate obtaining the composition targeted for the oxides.

If an addition of calcium proves useful to more easily obtain the desired composition for oxide inclusions, or if it is desired to limit the plasticity of the sulfides, this addition must be carried out towards the end of the preparation, preferably by insufflation. of powder or unwinding of cored wire. The usual amount added is 150 g of calcium per tonne of steel.

The ingot or continuous casting of the metal is then carried out according to the same methods as for the conventional grades of steel for bar turning, as well as the subsequent working and metallurgical treatment which makes it possible to obtain a product ready for processing.

By way of example, the lifetime of a high-speed steel tool, during the machining of a mild free-cutting steel according to the invention, of composition 0.1% of C, 0.97% of Mn , 0.06% P, 0.30% S, 0.17% Si, 70 ppm Te, 9 ppm Ca, and without Pb, is 60 minutes for a cutting speed of 150m / min. These results are similar to those recorded during the machining of a mild lead steel for bar turning S300 Pb.

On the other hand, for cutting speeds greater than 200 m / min, a P30 carbide tool allows, for an equal tool life, to triple the cutting speed during the machining of steel according to the invention which has just been cited, with respect to the machining of an S300 Pb steel.

Of course, without departing from the invention it is possible to add to the claimed shades elements not mentioned, the presence of which does not call into question the plasticity of the inclusions obtained. On the other hand, the addition of silicon and the formation of plastic oxidized inclusions can not only replace an addition of lead, as we have just described, but also intervene in addition to such an addition. The amounts of lead added could then be lower than those usually used.

Likewise, variations in the production and casting of liquid metal, as well as in the first transformations of the metal, can be imagined. For example, the shading operations can be carried out under vacuum, in order to reduce the contents of dissolved gas in the metal. The main thing is that the addition of elements and the modifications made to the method of preparation do not contradict the obtaining of the weight composition of the metal and of the properties of the inclusions in accordance with the claimed invention.

Claims (8)

1) Mild steel for bar turning, characterized
- in that its composition expressed in weight percentages is:
C ≦ 0.25%; Mn: 0.8 to 1.5%; P ≦ 0.1%; S: 0.15 to 0.40%; If: 0.05 to 0.40%
- And in that the manganese sulfide inclusions are, after metal working, surrounded by a layer of plastic oxides of the following average composition, expressed in weight percentages, SiO₂: 35 to 45%; Al₂O₃: 10 to 20%; CaO 15-25%; MnO: 10 to 20% 2) Mild steel for free cutting according to claim 1, characterized in that its carbon content by weight is less than or equal to about 0.16%. 3) Mild steel for free cutting according to claim 1 or 2, characterized in that its calcium content is from 5 to 50 ppm approximately. 4) Mild steel for free cutting according to one of claims 1 to 3, characterized in that its tellurium content is from 5 to 200 ppm approximately. 5) Mild steel for turning according to one of claims 1 to 4, characterized in that its lead content is from 0.05 to 0.30% approximately. 6) Mild steel free cutting according to any one of claims 1,3,4, or 5, characterized in that its carbon content by weight is less than or equal to about 0.09%. 7) Process for the preparation in the liquid state of a mild steel for bar turning, the weight composition of which corresponds to that defined in one of claims 1 to 6, characterized in that: - 1) silicon and manganese are added to the metal bath - 2) and the liquid metal is stirred in a metallurgical container in the presence of a slag of composition, expressed in percentages by weight:
CaO: 20 to 55%; SiO₂: 35 to 65%; Al₂O₃: 15 to 40%, with

close to 1. 8) Method according to claim 7, characterized in that the addition of silicon to the metal bath is accompanied by the addition of 0.1 to 0.3 kg of aluminum per ton of steel.