ES2349399T3 - STEEL COMPOSITIONS, METHOD FOR OBTAINING SAME AND PARTS MADE FROM SAME. - Google Patents

Abstract

Steel composition characterized in that it comprises, expressed in percentages by weight: 5 C 0.30-1.00% Yes 3.50-4.50% Mn 7.00-10.00% Ni 3.50-4.50% Cr 20.00-27.00% 10 W 0.50-4.50% Nb 0.-2.50% Co 0-1.00% N 0.20-0.50%, 15 the complement being constituted by iron and for inevitable impurities.

Description

The present invention relates to a steel intended more particularly for the realization of mechanical parts that can be used in severe conditions of use, in nuclear installations but also in industries such as agrifood, energy or mechanical construction.

In fact, when some mechanical parts must be subjected to tensions of use that require a strong resistance to wear by abrasion or erosion, a strong resistance to corrosion and a good behavior against heat up to 900ºC, the only materials capable of being suitable Under such conditions are cobalt-based alloys. Moreover, it can be noted that even when required only two of these qualities, there is at present no satisfactory alternative to these alloys.

However, cobalt-based alloys pose both economic and technical problems. Indeed, cobalt is an expensive raw material whose sources of supply are few. On the other hand, this element is highly undesirable in the nuclear environment since it forms long-lived isotopes that are released by the wear of the pieces and are one of the main sources of radiation exposure by people working in these installations. Therefore, the cobalt content must be limited to the maximum in steels or alloys that are used in particular in this industry for the realization of valve seats.

US Patent No. 3,912,503 describes stainless steel compositions resistant to wear and corrosion. They comprise a maximum of 0.12% by weight of carbon, 7 to 13% by weight of manganese, 3 to 5% by weight of silicon, 12 to 19% by weight of chromium, 4 to 12% by weight of nickel, 0, 03 to 0.3% by weight of nitrogen, the complement being constituted by iron. These compositions have a coefficient of friction close to that of cobalt-based alloys but have a hardness at room temperature of only 200 Vickers, hardness that falls to 150 Vickers from 200 ° C. In addition, its ductility is much greater to allow a use for the realization of mechanical parts subjected to strong tensions. 30

4,814,140 US patent describes the same type of steel US Patent No. 3,912,503 and improves the scuffing load. The claimed compositions comprise a maximum of 0.25% by weight of carbon, 2 to 7% by weight of

manganese, 1 to 5% by weight of silicon, 12 to 20% by weight of chromium, 2 to 7.75% by weight of nickel, 0.35% by weight of maximum nitrogen, the complement being constituted by iron. But the mechanical properties, which include ductility, hardness at room temperature and hot, are similar to those of the compositions of US Patent No. 3,912,503 and these are therefore also insufficient for the intended applications.

Therefore, there is a need for a cobalt-based alloy metal substitution material that contains the smallest possible amount, even nothing, but which presents the association of the characteristics of this type of alloy, namely a resistance to abrasion and high wear, good corrosion resistance and good hardness at room temperature and hot for temperatures between 300 and 900C, improving them if possible.

Therefore, the present invention is essentially intended to make such a material available.

To this end, a first object of the invention is constituted by a composition 15 of steel comprising, expressed in percentages by weight:

C 0.30-1.00%

Yes 3.50-4.50%

Mn 7.00-10.00% 20

Ni 3.50-4.50%

Cr 20.00-27.00%

W 0.50-4.50%

Nb 0-2.50%

Co 0-1.00% 25

N 0.20-0.50%,

the complement being constituted mainly by iron and by inevitable impurities.

In a preferred embodiment of the invention, the steel composition 30 comprises, expressed in percentages by weight:

C 0.50-0.80%

Yes 3.50-4.50%

Mn 7.00-10.00%

Ni 3.50-4.50%

Cr 24.00-26.00% 5

W 0.50-1.50%

Nb 0-2.50%

Co 0-0.50%

N 0.20-0.50%

 10

the complement being constituted mainly by iron and by inevitable impurities.

Among the unavoidable impurities, sulfur and phosphorus that should be maintained at the lowest levels compatible with the manufacturing process or the raw materials used will be mentioned in particular. fifteen

The excellent properties observed for the steel compositions according to the invention are obtained thanks to the precise balancing of the elements.

Carbon is present in a relatively high content of 0.30 to 1.00% by weight. This allows, in particular, to obtain the necessary hardness for the mechanical parts, in particular thanks to the formation of carbide-type precipitates that reinforce the structure of the material. A good hardness allows, among other things, to obtain a wear-resistant steel. Depending on the compositions and the heat treatment applied, the hardnesses obtained with the steels according to the invention are between 350 and 550 Vickers.

In a preferred embodiment of the invention, the carbon content of the 25 steels according to the invention is comprised between 0.40 and 0.50% by weight, when the parts to be made do not need a hardness and wear resistance very high. In another preferred embodiment of the invention, the carbon content of the steels according to the invention is comprised between 0.50 and 0.60% by weight, when the pieces to be made need a bit more hardness and wear resistance. high. In another preferred embodiment of the invention, the carbon content is between 0.70 and 0.80% by weight for applications where the hardness and wear resistance must be high. Therefore, it is found that these two can be adjusted

characteristics of steel varying this carbon content, within certain predefined limits.

Silicon is present in a content of 3.50 to 4.00% by weight. It plays a specific role in the improvement of friction and resistance to seizing due to the formation of an adherent oxide film on the surface of the pieces. It also has a function that favors corrosion resistance.

Nickel is a gammogen element that participates in the balancing of the composition. It is present in a content of 3.50 to 4.50% by weight and assures steel compositions good corrosion resistance.

Manganese is present in a content of 7.00 to 10.00% by weight. Its function is similar to that of nickel.

Chromium is present in an amount of 20.00 to 27.00% by weight, preferably 24 to 26% by weight. It also plays an important role in the good corrosion resistance of steels according to the invention. A part of the chromium present in the composition may eventually be replaced by molybdenum 15 on the basis of 1% molybdenum with 3% chromium.

Tungsten is present in an amount from 0.50 to 4.50% by weight, preferably 0.50 to 1.50% by weight. It allows in particular to obtain a good hot behavior of the compositions according to the invention.

Niobium is present in an amount of 0 to 2.5% by weight. It is a carburgenic element 20 that contributes to the hardness of the steel class and at the same time presenting a favorable action with respect to the hot behavior of the steel compositions.

Nitrogen is present in an amount of 0.20 to 0.50% by weight in the compositions according to the invention. It contributes to the balance of the composition due to its gammagenic influence, and to the hardness due to its participation in the formation of niobium carbonitrides in particular.

Cobalt is an undesirable element whose content is limited to a maximum of 1% by weight, preferably 0.5% by weight. Its content will be limited as much as technically possible and will be, particularly preferably, reduced to the state of traces. 30

A second object of the invention is constituted by a method of preparing mechanical parts characterized in that it comprises the following steps:

a) constitute a charge intended to obtain a chemical composition according to any of the variants set forth above,

b) prepare said load;

c) forming a semi-finished part of said mechanical part by molding,

d) machining said mechanical part from semi-finished, 5

e) as an option, perform a heat treatment using said mechanical part.

In a preferred embodiment, a method of preparing mechanical parts is characterized in that it comprises the following steps:

 10

a- constitute a charge intended to obtain a chemical composition according to any of the variants set forth above,

b- melt said load in a crucible,

c- centrifuging a semi-finished part of said mechanical part, and

d- machining said mechanical part from semi-finished, 15

e- as an option, perform a heat treatment using said mechanical part.

This centrifugal molding process is traditionally used to form cobalt-based alloys, since it has great advantages. It consists of progressively introducing the mass of molten metal into a heated jacket in 20 rotation, in such a way that a very thin layer of metal, of the order of one tenth of a millimeter is deposited in each turn inside the jacket. Each layer therefore solidifies independently of the previously deposited layers, which allows to obtain a semi-finished one of a fine and isotropic structure, without microporosities. The finished parts obtained after the semi-finished machining are exempt from 25 rechupes, are very resistant and have a very good surface condition.

Therefore the present inventors have found in a completely surprising way that it was perfectly possible to mold by centrifugation the mechanical parts to be made in the compositions according to the invention.

However, it is clear that these mechanical parts can be molded in static casting, in particular for large parts for which centrifugation is not possible due to the limited size of the heated jacket. It is also possible to perform the lost wax molding.

As for the melting of steel compositions, this can be accomplished by any known method, in particular an arc furnace.

In particular, the steel compositions according to the invention can be found in three different forms depending on their thermal state. This is the state called "raw casting", the state of solution solution at temperatures of 1,050 to 1,150 5 ° C and the state of stability obtained by practicing one or more stabilization treatments of 700 to 900 ° C on a piece in the raw state of casting or in a state of solution solution.

The thermal state depends essentially on the conditions in which the part will be used. Thus, for hot use, an aging stabilization treatment will preferably be carried out, which comprises a solution followed by slow cooling in a fluid such as air. This treatment allows the formation of precipitates in the steel matrix that will no longer vary in hardness or dimensions.

A third object of the invention is constituted by the mechanical parts 15 made in the compositions and / or with the help of the procedures described above. In particular, for example, valve seats, gates, friction rings and discs, gas turbine compounds, punches and hot-work cutting tools, knives and shear blades They work at high temperature. twenty

ESSAYS

The symbols used in the continuation of the description have the following meanings:

Rm = maximum resistance

Rp0.2 = conventional elastic limit at 0.2% deformation,

A = elongation,

Z = strictness 30

HV = Vickers hardness

HRC = Rocwell hardness.

All the percentages mentioned are percentages by weight.

The different tests have been carried out on a steel composition according to the invention called ES steel class whose composition is as follows:

C 0.50% 5

Yes 3.50%

Mn 9.00%

Ni 4.00%

Cr 26.00%

W 1.20% 10

Nb 1.50%

Co 0.12%

N 0.25%,

the complement being constituted by iron. fifteen

The tests have been carried out on steel samples that are in the three thermal states defined above, namely the raw casting state, the state of solution solution and the stabilized state.

The reference material of the prior art selected for comparison is a CoCr29W5 cobalt-based alloy, the composition of which is as follows:

C 1.10%

Cr 29.00%

W 5.00%

 25

the complement being constituted by cobalt.

1. Mechanical characteristics at room temperature

By means of usual tests well known to those skilled in the art, the respective mechanical characteristics at room temperature have been determined of three samples of the ES steel class in different thermal states and of a cobalt-based alloy sample. The results are grouped in table 1.

Table 1

 ES steel class according to the invention CoCr29W5

 BC1) MS2) S3) S3)

 Hardness (HV)

 340 320 460 450

 Rm (MPa)

 820 850 900 900

 Rp0.2 (MPa)

 550 600 680 650

 TO (%)

 5 8 2 2

 Z (%)

 8 10 4 2.5

1) gross state of laundry

2) status of solution 5

3) stabilized state

It is found that the mechanical characteristics at room temperature of the steel compositions according to the invention are completely similar to those obtained with cobalt-based alloys such as CoCr29W5. 10

2. Hot hardness

Stabilized samples of the ES and CoCr29W5 steel class have been tested for temperatures between 100 and 600 ° C. Results 15 are grouped in table 2.

Table 2

 Hardness (HRC)

 ES steel class CoCr29W5

 At 100 ° C

 42 40

 At 200 ° C

 38.5 37.5

 At 300 ° C

 35 35

 At 400 ° C

 31 32

 At 500 ° C

 27 28.5

 At 600 ° C

 23.5 25.5

3. Corrosion behavior

Three samples of the ES steel class and a CoCr29W5 sample have been subjected to a salt spray test according to NFX 41-002. The results, which have been grouped in table 3, are indicated in the form of a number that corresponds to the aspect of the material at the end of the test:

- 10 corresponds to a material without any trace of corrosion,

- 9 corresponds to a material that has some scattered bites, and

- 8 corresponds to a material that has some "points" around the 10 bites.

Table 3

 ES steel class according to the invention CoCr29W5

 Exhibithion time

 BC1) MS2) S3) S3)

 24 hours

 9 10 10 10

 56 hours

 8 10 9 10

 288 hours

 8 10 9 10

 fifteen

1) gross state of laundry

2) status of solution

3) stabilized state

It is found that also in this case the corrosion behavior 20 achieved by the compositions according to the invention is of the same level as that obtained with the cobalt-based alloy.

4. Friction and wear resistance tests

 25

Tests have been carried out on stabilized samples of the ES and CoCr29W5 steel class according to ASTM G99-95a, with the help of a pin-disk type tribometer, under a maximum load of 10 N, dry and with a linear speed

of contact of 0.1 m / s for a total distance traveled of 1,008 m. The results are grouped in table 4.

Table 4

 5

 Steel class Es CoCr29W5

 Coefficient of friction

 Test 1 0.705 0.684

 Essay 2

 0.710 0.625

 Essay 3

 0.702 0.703

 Half

 0.707 0.671

 Wear (10-3mm3)

 Trial 1 5 10

 Essay 2

 5 9

 Essay 3

 6 13

 Half

 5.33 10.66

Initially it is found that the coefficient of friction of the steel classes according to the invention is similar to that of CoCr29W5. On the contrary, the wear rate of these same compositions is twice lower than that of CoCr29W5, which is completely surprising. 10

The great reproducibility of the tests will also be verified when they refer to the steel classes according to the invention, contrary to what is produced with the cobalt-based alloy. This characteristic can be advantageous in particular for the manufacturer when it is necessary to provide the characteristics of a part or of a semi-finished product, since then it is not necessary to carry out several tests to determine the coefficient of friction and / or the rate of wear.

In addition to the qualities just mentioned, it will also be noted that the steels according to the invention are non-magnetic for the crude states of casting and placed in solution, which allows their use for certain sensitive parts, such as submarine parts. twenty

It is evident that the embodiments of the invention described above have been provided purely indicative and not at all limiting, and that the person skilled in the art can easily make numerous modifications without departing from the scope of the invention.

Claims (12)

Claims

1. Composition of steel characterized by comprising, expressed in percentages by weight:

 5 C 0.30-1.00% Yes 3.50-4.50% Mn 7.00-10.00% Ni 3.50-4.50% Cr 20.00-27.00% 10 W 0.50-4.50% Nb 0-2.50% Co 0-1.00% N 0.20-0.50%,  fifteen the complement being constituted by iron and by inevitable impurities.

2. Steel composition according to claim 1, characterized in that it comprises 24 to 26% by weight of chromium.

 twenty

3. Steel composition according to one or another of claims 1 or 2, characterized in that it comprises from 0.50 to 1.50% tungsten.

4. Steel composition according to any of claims 1 to 3, characterized in that it comprises from 3.50 to 4.00% by weight of silicon. 25

5. Steel composition according to any of claims 1 to 4, characterized in that it comprises from 0.40 to 0.50% by weight of carbon.

6. Steel composition according to any of claims 1 to 4, 30 characterized in that it comprises from 0.50 to 0.60% by weight of carbon.

7. Steel composition according to any of claims 1 to 4,

characterized in that it comprises from 0.70 to 0.80% by weight of carbon.

8. Steel composition according to claim 1, characterized in that it comprises, expressed in percentages by weight:

 5 C 0.50-0.80% Yes 3.50-4.50% Mn 7.00-10.00% Ni 3.50-4.50% Cr 24.00-26.00% 10 W 0.50-1.50% Nb 0-2.50% Co 0-0.50% N 0.20-0.50%,  fifteen the complement being constituted by iron and by inevitable impurities.

9. Mechanical parts manufacturing procedure, characterized in that it comprises the following stages:

 twenty a- constituting a charge intended to obtain a chemical composition according to any of claims 1 to 8, b- elaborate said load, c- forming a semi-finished part of said mechanical part by molding, d- machining said mechanical part from semi-finished, 25 e- as an option, perform a heat treatment using said mechanical part.

10. Method of manufacturing mechanical parts according to claim 9, characterized in that it comprises the following steps:

 30

a- constituting a charge intended to obtain a chemical composition according to any of claims 1 to 8,

b- melt said load in a crucible,

c- centrifuging a semi-finished part of said mechanical part, and

d- machining said mechanical part from semi-finished,

e- as an option, perform a heat treatment using said mechanical part.

11. Steel parts having a composition according to any one of claims 1 to 8.

12. Steel parts according to claim 11, characterized in that they are obtained by a method according to one or the other of claims 9 or 10.