EP0736610B1 - Resulfurized austenitic stainless steel with improved machinability, suitable for use in the field of high-speed cutting and on the automatic lathe - Google Patents

Abstract

Austenitic stainless steel contains in wt.%- less than 0.1C; less than 2Si; less than 2Mn; 7-12Ni; 15-25Cr; 0.01-0.55 S; 1-5Cu; less than 0.0035Ca; less than 0.007 O with the ratio of Ca to O being 0.2-0.6.

Description

The present invention relates to an austenitic stainless steel resulfurized with improved machinability, used in particular in the field machining at very high cutting speed and the field of bar turning.

For a person skilled in the art, by large-scale machining is meant speed of austenitic stainless steels, the use of speeds cutting speeds greater than 500 m / min.

The speeds which can be used on a steel are, for example, determined by turning tests with tools comprising coated carbide inserts, tests designated Vb _{15 / 0.15} , which consist in determining the speed at which the wear in clearance is 0.15 mm after 15 minutes of machining. Beyond this speed, it is not conceivable to machine without risk, below industrial practice is possible.

It is known from European Patent No. 403 332 a resulfurized austenitic steel with improved machinability. This document describes a process in which it is proposed, to improve the machinability, to introduce into a steel having the following general composition: carbon less than 0.15%, silicon less than 2%, manganese less than 2%, molybdenum less than 3%, nickel between 7 and 12%, chromium between 15 and 25%, an amount of sulfur in a proportion between 0.1 and 0.4%, associated with calcium and oxygen in contents respectively greater than 30.10 ^-4 % and 70. 10 ^-4 %, the contents of calcium and oxygen satisfying the Ca / O relationship between 0.2 and 0.6.

In this document, the aim sought is training, with the manganese and in a lower proportion, with chromium, of a manganese and chromium sulfide (Mn, Cr) S which generates under the form of specific inclusions solid lubrication of the tool cutting during machining operations.

It is also specified that sulfur has an unfavorable effect on corrosion resistance and that, despite this, an orientation chosen is the introduction, in a resulfurized steel, of inclusions of lime silicoaluminate oxides most often associated with sulfide inclusions.

Such an austenitic steel has good machinability properties in the field of conventional cutting speeds, i.e. less than 500 m / min while shooting. Steel has inclusions associated composed of oxides of the silicoaluminate type which coat preferably sulfide inclusions. These inclusions are more large and more deformable than the sulphide inclusions alone. The effect of the so-called solid lubrication of the cutting tool is thereby improved. However, the steel described in the cited document has a disadvantage. Indeed, sulfur significantly reduces the properties of steel from the point of view of cold deformation with appearance of tapures for example in stretching or drawing.

The object of the invention is to propose a steel with machinability improved which can be used, on the one hand in the field of very high speed machining, with cutting speeds in turning which can exceed 700 m / min, and, on the other hand in the bar turning area with productivity greater than 30% at those obtained with a resulfurized austenitic stainless steel ordinary.

The subject of the invention is an austenitic stainless steel resulfurized with improved machinability usable in particular in the field high-speed cutting machining and bar turning, such as as defined in claim 1.

• la teneur en soufre est comprise entre 0,20 et 0,40% et, de préférence, entre 0,25 et 0,35%.
• la teneur en cuivre est comprise entre 1,2 et 3% et, de préférence, entre 1,4 et 1,8%.
• la composition comprend en outre moins de 3% de molybdène.

The preferred features of the invention are:

• the sulfur content is between 0.20 and 0.40% and preferably between 0.25 and 0.35%.
• the copper content is between 1.2 and 3% and preferably between 1.4 and 1.8%.
• the composition also comprises less than 3% molybdenum.

The following description and the attached figures, all given to by way of nonlimiting example, will make the invention well understood.

Figure 1 shows the wear curves in steel clearance resulfurized stainless steels, either copper-free or without inclusion aluminosilicate of lime and of a resulfurized steel according to the invention, these steels being machined at very high cutting speed.

Figure 2 presents the hardening curves of a steel resulfurized without copper and a steel according to the invention.

The austenitic stainless steel according to the invention has the following composition by weight: carbon less than 0.1%, silicon less than 2%, manganese less than 2%, nickel, from 7 to 12%, chromium, from 15 to 25% , sulfur, 0.10 to 0.55%, copper, 1 to 5%, calcium greater than 35.10 ^-4 %, oxygen greater than 70.10 ^-4 %, the ratio of calcium content to oxygen content being between 0.2 and 0.6.

This steel is in the field of so-called resulfurized steels whose content in sulfur and the calcium and oxygen contents in a ratio determined ensure said steels good machinability at speeds cutting less than 500 m / min.

In the use of the steel according to the invention, in the field of machining at very high cutting speed, machinability is improved by the joint action of a large number of inclusions, sulfides of manganese and aluminosilicate oxides of lime from the contribution of calcium and oxygen and by the presence of copper. Copper limits effort required to form the chip. Because of this property, the temperature at the tip of the tool remains at a level bearable for this one. Under these conditions, the many manganese sulfide inclusions and aluminosilicate oxides of hot fully assume their role as lubricant in combination solid to delay tool wear. In the steel according to the invention, manganese sulfides are very little substituted for chromium because a manganese content adapted to the sulfur content, their malleability and therefore their effectiveness when cutting improved. Sulfur can be partially replaced by selenium and / or tellurium.

The resulfurized steel according to the invention, which can preferably be used in the field of machining known as high cutting speeds, by the presence of a large number of malleable inclusions at low point fusion of sulfide and associated or unassociated oxide, and by the presence of a copper content according to the invention, ensures on the one hand, machining at exceptional cutting speeds and secondly, conservation of the life of the cutting tool.

In a comparative machinability test at very high cutting speed, that is to say at more than 500 m / min, a TiN-coated carbide tool is used. A comparison was made of the evolution of wear in the tool clearance during the machining of three resulfurized steels referenced A, B and C. The steels A and B are reference resulfurized steels, steel A not containing calcium or oxygen in suitable proportion, steel B not containing copper in its composition. Steel C, according to the invention, in this application example, comprises in its composition, 1.5% copper, 44.10 ^-4 % calcium and 118 10 ^-4 % oxygen. The compositions of reference steels A and B and of steel C according to the invention are presented in table 1 below. Steel VS Yes Mn Mo Or Cr S Cu It O AT 0.048 0.42 1.50 0.29 8.05 17.03 0.30 1.5 10 53 B 0.051 0.38 1.49 0.29 8.03 17.05 0.30 0.5 51 110 VS 0.050 0.43 1.50 0.31 8.10 17.04 0.30 1.5 44 118 D 0.049 0.45 1.48 0.28 8.02 17.11 0.39 1.5 14 57 E 0.052 0.39 1.51 0.30 8.07 17.03 0.30 1.5 62 134

The test consists of a turning operation, without lubrication, with an advance of 0.25 mm / revolution, a depth of 1.5 mm pass and a cutting speed of 700 m / min. The tool is regularly dismantled for measurement of draft wear. The The resulting curves are shown in Figure 1.

Reference steels A and B are unsuitable for this type of machining. After only a few minutes of turning, the tools machining these steels are destroyed, that is to say that either their wear and tear is greater than 0.15 mm, or their edge is collapsed. It is therefore not thinkable of using, for the machining of these steels, such speeds of chopped off. On the other hand, with steel C according to the invention, the coated tool is still in working condition after 20 minutes of turning, which allows, with conventional carbide coated tools to work industrially at such cutting speeds. This is due to the combined presence, in the composition of steel, of a large quantity of sulfur, malleable oxides with low melting point and a optimal copper content.

In the use of the steel according to the invention, in the field of machining is improved by the presence of copper during from the manufacture of bars, then by the action of sulfide inclusions of manganese and aluminosilicate oxides of lime during machining. Copper reduces hardening, as shown in the figure 2 on which the reference steel B is again compared and steel C according to the invention. This low hardenability leads to obtaining less hard drawn bars, in particular at the surface.

The effect of inclusions is then added to promote chip shearing and lubricate the tool / metal interface.

In a test for the production of turned parts, the productivity of two resulfurized steels referenced D and E was compared. The reference D steel is a resulfurized steel which does not contain calcium or oxygen in its composition. in suitable proportion, and the steel E, according to the invention, in this exemplary embodiment, comprises in its composition, 1.5% copper, 62.10 ^-4 % calcium and 134.10 ^-4 % oxygen.

Surprisingly, the action in combination of the three elements copper, oxygen, calcium, generates a particular effect on improved machinability, unpredictable when these elements are introduced into the composition two by two or separately.

The compositions of reference steel D and of steel E according to the invention are described in Table 1.

The test consists in the production, from a drawn bar with a diameter of 5 mm, of a piece 50 mm long comprising essentially turning, with variable depth of pass from 0.5 to 1.5 mm. Table 2 below presents the results of a bar turning test on a single spindle cam lathe with solid carbide tools and whole oil lubrication. The values in table 2 represent the number of parts with good quality machining before changing the tools. Productivity Steel D Steel E 1.82 pieces / min 3,200 8,000 2.30 piece / min 1,500 3,200

In optimized cutting conditions for a steel of reference, we can produce 2.5 times more parts with steel according to invention, before having to change the tools. Conversely, with a 30% higher productivity on the steel according to the invention, the duration of life is identical.

In another bar turning test, the same two steels D and E are compared on a simple cutting operation, consisting to produce axes of 4 mm in diameter from a wire rod sectioned on a torch machine. Productivity has been improved 28% with steel E according to the invention in comparison with steel Reference D not containing calcium or oxygen in suitable proportion.

Claims (3)

1. Resulphurized austenitic stainless steel having improved machinability, used especially in the high-cutting-speed machining field and in the screw-machining field, and containing, in its weight composition, the following elements:

   Carbon < 0.15%

   Silicon < 2%

   Manganese < 2%

   Nickel from 7 to 12%

   Chromium from 15 to 25%

   Sulphur > 0.10%, optionally replaced in part with selenium and/or tellurium

   Calcium > 30 x 10^-4%

   Oxygen > 70 x 10^-4%,

      the balance consisting of iron and residual impureties,
   the ratio of the calcium content to the oxygen content being between 0.2 and 0.6,
   characterized in that in its weight composition:

   carbon < 0.1%,

   sulphur ≤ 0.55%,

   copper from 1 to 5%,

   calcium > 35 x 10^-4%.
2. Steel according to Claim 1, characterized in that the copper content is between 1.2 and 3%.
3. Steel according to Claim 1, characterized in that the copper content is between 1.4 and 1.8%.

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