STEEL HAVING HIGH MECHANICAL PROPERTIES AND MANUFACTURING PROCESS THEREOF

Abstract

A steel having high mechanical properties, characterized in that it has the following composition by weight: 12% to 25 % Nickel; 7.4% to 20 % Cobalt; 3% to 11% Molybdenum; 0.2% to 2.21% addition elements, the remainder being iron, the structure of the material including a combination of fine grains and ultrafine grains, the so-called fine grains having a grain size of between 1.2 micrometers and 3 micrometers and the so-called ultrafine grains having a grain size of between 0.2 and 1 micrometer, the proportion of ultrafine grains being between 55 % and 65 %, and a process for manufacturing the steel.

Claims

1. A steel having high mechanical properties wherein it has the following composition by weight: 12 % to 25 % Nickel; 7.4 % to 20 % Cobalt; 3 % to 11 % Molybdenum; 0.2 % to 2.21 % addition elements, the remainder being iron, the structure of the material comprising a combination of fine grains and ultrafine grains, the fine grains having a grain size of between 1.2 micrometers and 3 micrometers, and the ultrafine grains having a grain size of between 0.2 micrometer and 1 micrometer, the proportion of ultrafine grains being between 55 % and 65 %.

2. The steel having high mechanical properties according to claim 1, wherein it has the following composition: 12 % to 25 % Nickel; 7.4 % to 20 % Cobalt; 3 % to 11 % Molybdenum; 0.15 % to 1.6 % Titanium; 0.05 % to 0.2 % Aluminum; 0 % to 0.1% of at least one of Silicon and Manganese; 0 % to 0.08 % of at least one of Nitrogen and Oxygen; 0 % to 0.03 % Carbon; 0 % to 0.01 % of at least one of Sulfur and Phosphorus, the remainder being iron.

3. The steel having high mechanical properties according to claim 2, wherein it has the following composition: 15.7 % Nickel; 7.4 % Cobalt; 4.8 % Molybdenum; 0.6 % Titanium; 0.05 % to 0.2 % Aluminum; 0 % to 0.1 % of at least one of Silicon and Manganese; 0 % to 0.08 % of at least one of Nitrogen and Oxygen; 0 % to 0.03 % Carbon; 0 % to 0.01 % of at least one of Sulfur and Phosphorus, the remainder being iron.

4. A process for manufacturing a steel having high mechanical properties according to claim 1, wherein the process comprises the following steps: preparing a steel powder having the desired composition, for example via gas atomisation, and having a grain size of between 5 and 100 micrometers; mechanical milling with a planetary ball mill until a powder is obtained having a grain size distribution associating fine grains having a grain size between 1.2 micrometers and 3 micrometers and ultrafine grains having a grain size between 0.2 micrometer and 1 micrometer, the proportion of ultrafine grains being between 55 % and 65 %; mixing the powders obtained; sintering the powder mixture using SPS flash sintering technology, to obtain a block of steel.

5. The process for manufacturing a steel having high mechanical properties according to claim 4, wherein the mechanical milling is performed on a planetary mill with a support disc of having a diameter of approximately 800 mm, the rotating speed of the support disc being between 50 rpm and 350 rpm, whereas the rotating speed of the bowls is between -50 rpm and -350 rpm, the ratio between the mass of balls arranged in each bowl and the mass of powder in said bowl being between 4 and 10.

6. The process for manufacturing a steel having high mechanical properties according to claim 4, wherein the flash sintering cycle comprises: a temperature rise to austenitizing temperature (higher than 820° C.) at a heating rate of between 25° C./minute and 200° C./minute; a temperature hold for a time of 5 minutes to 20 minutes at austenitizing temperature; cooling to ambient temperature at a cooling rate of between 25° C./minute and 200° C./minute.

7. The process for manufacturing a steel having high mechanical properties according to claim 6, wherein the axial stress applied to the block throughout the entire duration of the cycle is between 100 kilo Newtons and 1000 kilo Newtons.

8. The process for manufacturing a steel having high mechanical properties according to claim 4, wherein, after the sintering operation, ageing heat treatment is carried out of tempering type.

9. The process for manufacturing a steel having high mechanical properties according to claim 8, wherein the heat treatment consists of bringing the block of steel to a temperature of 480° C. for 3 hours.

Description

[0089] Two examples of materials of the invention will be described with reference to the appended Figures in which:

[0090] [FIG. 1] shows the tensile stress-strain curve for a steel according to a first embodiment of the invention, compared with a reference material;

[0091] [FIG. 2] is a micrograph showing the structure of the grains of the steel according to this first embodiment;

[0092] [FIG. 3] gives the tensile stress-strain curve for a steel according to a second embodiment of the invention, compared with a reference material;

[0093] [FIG. 4] is a micrograph showing the structure of the grains of the steel according to this second embodiment.

EXAMPLE 1

[0094] Steel having high mechanical properties of Maraging M300 type but without tempering treatment.

[0095] This first example concerns a material prepared from powders having a grain size distribution (before milling) centred around 37 .Math.m. The chemical composition in weight percent of these powders is the following: [0096] 15.70 % nickel, [0097] 7.40 % cobalt, [0098] 4.80 % molybdenum, [0099] 0.60 % titanium, [0100] 0.05 to 0.20 % aluminum [0101] silicon content less than or equal to 0.10 %, [0102] manganese content less than or equal to 0.10 %, [0103] phosphorus content less than or equal to 0.01 %, [0104] sulfur content less than or equal to 0.01 %, [0105] carbon content less than or equal to 0.03 %, [0106] nitrogen content less than or equal to 0.08 %, [0107] oxygen content less than or equal to 0.08%, [0108] remainder being iron.

[0109] The powders were milled with a planetary mill under conditions of frictional type, at different rotation speeds ω and Ω of 250 rpm and -250 rpm, respectively, for a milling time of less than 4 hours.

[0110] The milled powders were consolidated by flash sintering of Spark Plasma Sintering (SPS) type under uniaxial pressure of at least 70 MPa (Megapascals) at a sintering temperature lower than 950° C. and with a rate of temperature rise varying from 25° C./minute to 200° C./minute.

[0111] Sintering was followed by a hold at austenitizing temperature, still under uniaxial pressure of at least 70 MPa, and for a time of between 5 and 20 minutes.

[0112] The block was gradually cooled down to ambient temperature at a cooling rate varying from 25° C./minute to 200° C./minute.

[0113] FIG. 1 shows tensile tests on this non-treated block of steel compared with a reference block of steel of Maraging M300 type that had not been subjected to ageing treatment i.e. a block of steel of Maraging type obtained by casting and subjected to austenitizing at 820° C. for 2 hours followed by quenching, but not subjected to tempering.

[0114] Curve 1 corresponds to the block of steel of the invention, curve 2 to the reference block of steel.

[0115] It can be seen that for the block of the invention (curve 1) the ultimate tensile nominal stress is 1260 MPa, whereas for the reference block (curve 2) this ultimate tensile nominal stress is 1090 MPa. Ultimate stress is therefore increased by 15.6 %, which imparts this block with strength close to that of Maraging M200 without the need to apply tempering treatment.

[0116] Of more interest, it is observed from the curves that true plastic strain up to ultimate tensile stress (ε.sub.pEM) for the steel of the invention (curve 1) is 380 % higher than that of the reference steel (curve 2).

[0117] In the steel of this embodiment of the invention, the microstructure is of 100 % martensitic type. The microstructure in the non-treated state is composed of fine and ultrafine microstructures of the starting milled powders.

[0118] More specifically, the structure is composed of about 40 % fine grains of size 1.8 ±0.3 micrometers (.Math.m) and about 60 % ultrafine grains of size 0.6 ±0.2 .Math.m.

[0119] FIG. 2 is a micrograph of this structure allowing viewing of these fine structures (circle referenced 5) and ultrafine structures (circle referenced 6).

[0120] The material is formed of agglomerates of fine grains composed on average of 32±5 homogeneously distributed grains. The spacing between the agglomerates of fine grains is between 8 and 14 micrometers.

EXAMPLE 2

[0121] Steel having high mechanical properties of Maraging M300 type after receiving tempering treatment.

[0122] The second example concerns a steel that is the same as in the first example but which, after preparation, has received ageing treatment of tempering type.

[0123] This ageing heat treatment (or structural hardening) of tempering type was performed by bringing the steel to a temperature of 480° C. for 3 hours.

[0124] FIG. 3 shows tensile testing of this treated block of steel compared with a reference block of steel of Maraging M300 type which had also been subjected to ageing treatment.

[0125] Curve 3 corresponds to the block of steel of the invention, curve 4 to the reference block of steel.

[0126] It can be seen in FIG. 3 that the steel of the invention (curve 3) exhibits an ultimate tensile nominal stress of 1890 MPa, which is close to the strength of a steel of M270 type.

[0127] Of more interest, true plastic strain up to ultimate tensile stress (ε.sub.pEM) for the steel of the invention (curve 3) is 300 % higher than that of the reference steel (curve 4) which is a Maraging steel prepared with the conventional approach of casting or forging type.

[0128] In the steel of this embodiment of the invention, the microstructure is of martensitic type with avec 10 % reverted austenite. The microstructure in the treated state maintains the fine and ultrafine microstructures of the starting milled powders.

[0129] More specifically, the structure is composed of about 40% fine grains of size 1.6 ±0.4 .Math.m and about 60 % ultrafine grains of size 0.8 ±0.2 .Math.m.

[0130] FIG. 4 is a micrograph of this structure allowing viewing of these fine structures (circle referenced 5) and ultrafine structures (circle referenced 6).

[0131] The material is composed of agglomerates of fine grains on average composed of 25±2 homogeneously distributed fine grains. The spacing between the agglomerates of fine grains is between 9 and 15 micrometers.