EP0403332A1 - Resulfurized austenitic stainless steel with improved machinability - Google Patents

Abstract

Resulfurized austenitic stainless steel with improved machinability, characterized in that its composition by weight is as follows:- Carbon less than or equal to 0.15%- Silicon less than or equal to 2%- Manganese less than or equal to 2%- Molybdenum less than or equal to 3%- Nickel between 7 and 12%- Chromium between 15 and 25%- Sulfur between 0.10 and 0.40%- Calcium greater than or equal to 30 10⁻⁴%- Oxygen greater than or equal to 70 10 ⁻⁴%- The ratio of the content of calcium and oxygen Ca/O being between 0.2 and 0.6.

Description

The present invention relates to a resulfurized austenitic stainless steel with improved machinability.

Such an austenitic steel is known from JP-A-160785. This patent deals with a steel which can be machined and deformed under cold conditions and which, in weight composition, in particular, has a sulfur content of less than 0.030%, calcium and oxygen contents respectively in the ranges 10-300 ppm and 30-300 ppm , from 0.8 to 5% of copper and from 0.01 to 0.25% of lead.

In this austenitic stainless steel, oxygen and calcium are introduced, which makes it possible to transform hard inclusions into inclusions based on calcium oxide. The improvement in machinability is generated by the introduction into the composition of a variable amount of lead.

It is well known that austenitic stainless steels are difficult to machine, largely because of their low thermal conductivity, resulting in poor flow of the heat produced at the tip of a cutting tool and rapid deterioration of the material. 'tool and their high hardenability locally inducing areas of high hardness.

One way to improve machinability is the introduction of the lead element, in particular in a proportion of 0.01 to 0.25%. This element has the disadvantages of being difficult to dissolve homogeneously in a molten bath, and, because of its high density, of having a tendency to accumulate in the bottom of metallurgical vessels. In addition, it forms phases with a low melting point deteriorating the hot deformability.

In FR-A-2,542,761 which describes a process manufacturing high machinability steel, it is specified that another cause of the difficulty in machining stainless steels is the fact that they contain inclusions of hard oxides such as for example alumina or chromite which damage the cutting tools.

One way to reduce the harmfulness of hard oxide inclusions is to introduce one or more alkaline earth compounds into the steel, in order to replace a good proportion of hard inclusions with inclusions of calcium-based oxides, for example. It is specified on the one hand, that a certain quantity of sulfur combined with hard inclusions reduces its harmfulness, the sulfur content being generally less than 0.5 10⁻⁴%, and on the other hand, that a Another way to reduce the harmfulness of inclusions is to reduce their quantity thanks to good deoxidation and good decantation of the molten bath during the production of steel.

In the documents mentioned above, the machinability of the steel is improved:
- by introducing lead as a lubricant,
- by the introduction of oxygen or calcium to reduce hard inclusions to inclusions based on alkaline earth compounds.
- by reducing the number of hard inclusions by deoxidation of the molten bath, during production.

The present invention relates to a resulfurized austenitic steel with improved unisability containing on the one hand, sulfur for the creation of a manganese and chromium sulfide having lubricating properties and, on the other hand, a deter mined with oxygen and calcium introduced in the form of lime silicoaluminate, in order to create, in number, specific inclusions improving machinability.

Austenetic stainless steel is characterized by the following composition expressed in weight percentages:
- Carbon less than or equal to 0.15%
- Silicon less than or equal to 2%
- Manganese less than or equal to 2%
- Molybdenum less than or equal to 3%
- Nickel between 7 and 12%
- Chrome between 15 and 25%
- Sulfur between 0.10 and 0.40%
- Calcium greater than or equal to 30 10⁻⁴%
- Oxygen greater than or equal to 70 10⁻⁴%
-The ratio of calcium and oxygen Ca / O content being between 0.2 and 0.6.

Calcium is introduced, during production, into the molten bath, by addition of silicocalcium under control of the oxygen contents.

In a preferred composition of the invention, the austenitic steel comprises sulfur in a proportion of between 0.15 and 0.35%. Sulfur forms with manganese and, to a lesser extent, with chromium, a manganese sulphide and chromium (Mn, Cr) S which generates in the form of inclusions a hot lubrication of the cutting tool during the machining and steel.

In another form of the invention, the value of the ratio of the contents of calcium and oxygen elements is between 0.3 and 0.5.

The range of values of the Ca / O ratio is determined by machinability measurements of the various steels having the basic composition of the steel according to the invention and the contents of calcium and oxygen are varied.

In particular, the manganese sulfide and chromium inclusions are coated with a phase of lime silicoaluminate of the anorthite and / or pseudowollastonite type to form associated inclusions. The creation of this type of inclusion is made possible by the introduction of calcium compounds into the liquid bath under control of determined oxygen contents.

In addition, the associated inclusions have a form factor of between 3 and 6. The form factor is determined by the ratio of the length to the width of the inclusion, the value of the form factor being a criterion for measuring the quality of steel machinability.

• La figure 1 représente le diagramme de com­position ternaire CaO-Al₂O₃-SiO₂ situant la composi­tion pondérale de l'anorthite de la pseudowollastonite et de gehlenite.
• La figure 2 représente une image en coupe d'une inclusion associée.
• La figure 3 est un diagramme représentant les valeurs du critère d'usinabilité VB30/0,3 en fonction de la variation de concentration d'oxygène.
• La figure 4 est un diagramme représentant les valeurs du critère d'usinabilité VB30/0,3 en fonction de la variation de concentration de soufre.
• La figure 5 représente, dans un exemple d'acier donné à titre de comparaison (acier selon l'invention sans soufre), un diagramme représentant les valeurs VB30/0,3 en fonction du rapport des teneurs Ca/O.

The tests described below and the appended figures will make the invention better understood.

• FIG. 1 represents the CaO-Al₂O₃-SiO₂ ternary composition diagram locating the weight composition of the anorthite of pseudowollastonite and of gehlenite.
• Figure 2 shows a sectional image of an associated inclusion.
• FIG. 3 is a diagram representing the values of the machinability criterion VB30 / 0.3 as a function of the variation in oxygen concentration.
• FIG. 4 is a diagram representing the values of the machinability criterion VB30 / 0.3 as a function of the variation in sulfur concentration.
• FIG. 5 represents, in an example of steel given for comparison (steel according to the invention without sulfur), a diagram representing the values VB30 / 0.3 as a function of the ratio of the Ca / O contents.

The present invention relates to a resulfurized austenitic stainless steel alloy whose machinability is improved by the creation of associated inclusions, lime silicoaluminate / manganese sulfide and chromium.

The alloy includes, by weight:
- Carbon less than or equal to 0.15%
- Silicon less than or equal to 2%
- Manganese less than or equal to 2%
- Molybdenum less than or equal to 3%
- Nickel between 7 and 12%
- Chrome between 15 and 25%
- sulfur between 0.10 and 0.40%
- oxygen greater than or equal to 70 ppm
- Calcium greater than or equal to 30 ppm.

The beneficial role of sulfur on machinability is well known. Sulfur generates manganese sulfide inclusions also containing chromium in the steel.

The addition of sulfur, or even, for example, selenium, makes it possible to improve the machinability of austenitic stainless steels but to the detriment of other properties such as, for example, a reduction in corrosion resistance and in hot and deformability. cold.

Despite the unfavorable effect of a reduction in the resistance to corrosion caused by sulfur, tests on the machinability of austenitic steels have been directed towards the introduction, into resulfurized steel, of lime silicoaluminate oxides. These oxides do not deteriorate the corrosion resistance.

The lime silico-aluminate oxides are created during the production of steel thanks to the introduction of calcium, preferably, in the form of a wire filled with silicocalcium, in the molten bath under control of the oxygen contents.

According to the invention, the vast majority of the oxides are linked to the sulfides and form, with the inclusion of sulfides, associated inclusions, the sulfide being placed inside the oxide inclusions. These sulfides are manganese sulfides, however, also containing chromium.

The lime silicoaluminate oxides in their chemical composition are preferably anorthite or pseudowollastonite, (the chemical composition of which is shown in the ternary diagram of FIG. 1), the majority of said oxides being anorthite. These oxides can also contain a little MnO.

The lime silicoaluminate oxides formed around the sulphides are malleable, low-melting oxides which can easily deform during rolling. During the machining of steel, due to the high cutting temperatures, these inclusions play a lubricating role at the interface-steel to be machined-cutting tool, thus leading to reduced wear of cutting tools and better surface appearance of the machined parts.

The research carried out led to the definition of a composition of high-machinability steel from a base whose composition was given above and having essentially:
- a sulfur content of 0.15 to 0.35%.
- a calcium content greater than or equal to 30 ppm on the finished product,
- an oxygen content greater than or equal to 70 ppm on the finished product,
- a ratio of the Ca / O element contents of between 0.3 and 0.5.
It follows:
- The presence of inclusions of lime silico-aluminates, preferably anorthite (majority) or pseudowollastonite (minority) oxides, generally coating manganese and chromium sulphides.

Figure 2 is a sectional image of a steel according to the invention containing an associated inclusion 2 of lenticular shape composed of lime silicoaluminate 3 coating inclusions of sulfide 4 of manganese and chromium.

Clearly, there is no formation of calcium sulfide, sulfide recognized as harmful, for the machinability of steels and corrosion resistance.

The following tests illustrate in a comparative manner the qualities of machinability of the steel according to the invention.

Turning tests with a carbide tool (P20 ISO reference) were carried out. For a cutting depth of 1.5 mm, an advance of 0.25 mm / revolution, several cutting speeds are fixed. For each cutting speed, the tool is disassembled every 4 min to measure the wear and tear. Thus, for several speeds V1, V2, V3 ..., a curve giving the wear and tear of the tool as a function of time, this machining is plotted. Cutting speed, leading to a wear and tear of 0.3 mm in 30 min, can thus be determined for each production, and be taken as a reference under the criterion VB30 / 0.3.

Table 1 below gives some results obtained on steels whose basic composition is: C: 0.05%, Si: 0.5%, Mn: 1.8%, Ni: 8.6%, Cr: 17%, Mo: 0.2%, S: 0.3%, but whose calcuim and oxygen contents vary. In addition, for each preparation, the average surface and the average form factor (length / width) of the sulfide inclusions and of the associated oxide / sulfide inclusions are given. TABLE I STEEL N ° CA (ppm) O (ppm) Ca | O VB 30 / 0.3 (m / min) SURFACE ASSOCIATED INCLUSIONS (mm) ² FORM FACTOR RELATED INCLUSIONS SULFURIZED SURFACE (mm) ² SULFIDE FORM FACTOR 1 5 94 0.05 245 2.4 1.8 39.3 3.1 2 5 70 0.07 250 2.1 2.1 31.1 3.2 3 45 120 0.38 300 24.9 4.5 31.4 3.1 4 43 105 0.41 295 41.9 5 21.7 2.8 5 40 96 0.42 303 39.8 5.4 31.4 3.1 6 47 102 0.46 300 39.2 4.8 27.4 3.4 7 35 73 0.48 308 45.5 5.6 35.3 3.7

Steels N ° 1 and N ° 2 constitute references and do not contain lime silicoaluminate.

The associated inclusions are very few, very small and slightly deformed.

Steels No. 3 to 7 corresponding to a composition according to the invention. The values of VB30 / 0.3 are higher by about 20%. The mean surface and the mean form factor of the sulfide inclusions of steels No. 1 and No. 2 on the one hand and of steels No. 3 to No. 7 do not differ significantly.

On the other hand, the associated inclusions of the steels according to the invention (Nos. 3 to 7) have a much larger surface and are much more deformed therefore much more deformable.

Thus, these associated inclusions of lime silico-aluminates coating the inclusions of manganese sulfide and chromium are at the origin of the significant increase in the value of the machinability criterion VB 30 / 0.3.

Likewise, turning tests with a TiN-coated carbide tool were carried out. These tools are increasingly used by machinists. For a feed of 0.25 mm / revolution, a pass of 1.5 mm and a speed of 340 m / min, the wear and tear of the tool was measured as a function of time. Table II below gives some values obtained on steels No. 1, 4, 5 and 6 from the preceding table. TABLE II ACEIR N ° Ca (ppm) O (ppm) Ca / O Time for wear 0.15 mm (min) Wear after 30 min of cutting (mm) 1 5 94 0.05 5 0.27 4 43 105 0.41 22 0.16 5 40 96 0.42 24 0.16 6 47 102 0.46 24 0.17

In these tests, the criteria used to compare the steels are on the one hand the time leading to an undercut wear of 0.15 mm under the cutting conditions given above and on the other hand the measurement of wear in skin after 30 minutes of cutting.

Thus, compared with reference steel No. 1 not corresponding to a composition according to the invention, the tests carried out on steels No. 4, 5 and 6 show that the cutting time before wear of 0.15 mm is multiplied by a factor of 4 and the wear, measured on the tool after 30 minutes of cutting, is reduced by approximately 60%.

This improvement is linked to the associated inclusions described, and introduced into the steel according to the invention.

FIG. 3 presents a diagram giving the variations of the machinability criterion VB30 / 0.3 as a function of the oxygen concentration in a series of measurements corresponding on the one hand to the production of the steels according to the invention and, on the other hand, the development of calcium-free steel.

FIG. 4 represents a diagram giving the variations of the machinability criterion VB30 / 0.3 as a function of the sulfur concentration in a series of measurements corresponding on the one hand to the preparation according to the invention and, on the other hand , to the development of calcium-free steel.

Figures 3 and 4 show first of all that the evolution of grades of resulfurized austenitic stainless steels towards high oxygen contents or high sulfur contents does not make it possible to improve machinability significantly (criterion VB 30 / 0.3). On the other hand, steels according to the invention constitute a separate population, on the diagrams of FIGS. 3 and 4, with high machinability criteria.

By way of comparison, the same machinability tests were carried out using the machinability criterion VB 30 / 0.3 on austenitic steels whose basic composition is as follows:
C: 0.06%, Si: 0.45%, Mn: 0.6%, Ni: 8.6%, Cr: 18%, Mo: 0.2%, S: 0.02%.

These steels contain little or no sulfur compared to the steel according to the invention.

The tests focused on the variation of calcium and oxygen, using the same procedure for introducing lime silicoaluminate as in the preparation of the steels according to the invention.

Table III below gives the values of VB 30 / 0.3 of several steels as a function of the calcium and oxygen content and of the value of the ratio of element concentrations. TABLE III STEEL N ° Ca (ppm) O (ppm) Ca | O VB 30 | 0.3 (m / min) 8 2 57 0.04 168 9 6 123 0.05 160 10 32 79 0.4 200 11 43 118 0.36 200 12 26 47 0.55 155 13 17 117 0.15 172 14 32 129 0.25 180

Steels N ° 8 and N ° 9 contain little or no calcium and are the reference steels for these measurements. The oxide inclusions are of the polyphase silicate and chromite type.

The evolution towards high oxygen contents alone does not lead to an improvement in the machinability (comparison of the values of VB 30 / 0.3 between steels N ° 8 and 9).

Steels No. 10 and No. 11 having:
- a calcium content greater than 30 ppm on the finished product,
- an oxygen content greater than 70 ppm on the finished product,
- A ratio of the Ca / O element contents of between 0.30 and 0.50 have only inclusions of oxides of the anorthite type. Note, during machining, an increase in the values of the machinability criterion VB 30 / 0.3 as shown in example in FIG. 5.

Steel No. 12 has low calcium and oxygen contents and a very high Ca / O ratio. Machinability remains poor. The inclusions analyzed chemically are of the gehlenite type (Figure 1).

Steel No. 13 has an oxygen content corresponding to that of the composition of the steel according to the invention without sulfur but a calcium content and a lower Ca / O ratio. The machinability is not significantly improved.

Steel No. 14 has contents corresponding to those of the composition of the steel according to the invention without sulfur, but a Ca / O ratio below 0.30. The improvement in machinability is appreciable but remains far below that of steels N ° 10 and N ° 11.

The comparison of the VB 30 / 0.3 values in Tables I and III shows the importance of the effect of the anorthite inclusions alone and of the effect of the inclusions contained in the resulfurized steel according to the invention.

The subject of the present invention is a resulfurized austenitic stainless steel, the machinability of which is improved thanks to the creation of associated inclusions of oxides of lime silico-aluminate / sulfide (Mn, Cr) S type.

The characteristics allowing to obtain an improved unisability are:
- a sulfur content of between 0.10 and 0.40% and preferably between 0.15 and 0.35%
- a calcium content greater than or equal to 30 ppm,
- an oxygen content greater than or equal to 70 ppm,
- A ratio of the calcium and oxygen Ca / O content between 0.2 and 0.6 and preferably between 0.3 and 0.5.

These results lead to the presence of associated and deformed inclusions of oxides coating sulfide inclusions. The oxides are lime silico-aluminates, preferably of the anorthite and pseudowollastonite type, the chemical compositions of which are determined in the ternary diagram CaO-SiO2-Al203 in FIG. 1. These associated inclusions have a surface and a form factor (length / width) important. The high deformability of the inclusions and their lubricating effect at the cutting tool / chip interface allowing improved machinability.

Claims (8)

1. Resulfurized austenitic stainless steel with improved machinability, characterized in that its weight composition is as follows:
- Carbon less than or equal to 0.15%
- Silicon less than or equal to 2%
- Manganese less than or equal to 2%
- Molybdenum less than or equal to 3%
- Nickel between 7 and 12%
- Chrome between 15 and 25%
- Sulfur between 0.10 and 0.40%
- Calcium greater than or equal to 30 10⁻⁴%
- Oxygen greater than or equal to 70 10⁻⁴%
- The ratio of calcium and oxygen Ca / O content is between 0.2 and 0.6. 2. Stainless steel according to claim 1, characterized in that it comprises sulfur in a proportion of between 0.15 and 0.35%. 3. Stainless steel according to claims 1 and 2, characterized in that it contains inclusions of sulfide, manganese and chromium (Mn, Cr) S. 4. Stainless steel according to claim 1, characterized in that the value of the ratio of the contents of element calcium and oxygen is between 0.3 and 0.5. 5. Stainless steel according to claims 1 and 4, characterized in that it contains inclusions of lime silicoaluminate of the anorthite and / or pseudowollastonite type. 6. Stainless steel according to claims 1 to 5, characterized in that the sulfide, manganese and chromium inclusions are coated with a lime silico-aluminate phase of the anorthite and / or pseu-dowollastonite type to form associated inclusions. 7. Stainless steel according to claim 6, characterized in that the associated inclusions are generated by the addition of calcium introduced into the molten bath in the form of a wire filled with siliocalcium. 8. Stainless steel according to claim 6, characterized in that the inclusions have a form factor of between 3 and 6.

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