STEEL COMPOSITION

Abstract

The present invention relates to a steel composition comprising, in percentages by weight of the total composition: Carbon: 0.06-0.20 preferably 0.08-0.18; Chromium: 2.5-5.0, preferably 3.0-4.5; Molybdenum: 4.0-6.0; Tungsten: 0.01-3.0; Vanadium: 1.0-3.0, preferably 1.5-2.5; Nickel: 2.0-4.0; Cobalt: 9.0-12.5, preferably 9.5-11.0; Iron: remainder
as well as the inevitable impurities,
optionally further comprising one or more of the following elements: Niobium: 2.0; Nitrogen: 0.50, preferably 0.20; Silicon: 0.70, preferably 0.05-0.50; Manganese: 0.70, preferably 0.05-0.50; Aluminum: 0.15, preferably 0.10;
the combined niobium+vanadium content being in the range 1.0-3.5; and the carbon+nitrogen content being in the range 0.06-0.50.

It further relates to method of manufacture thereof, the steel blank obtained and a mechanical device or an injection system comprising same.

Claims

1. A steel composition comprising, in percentages by weight of the total composition: Carbon: 0.06-0.20; Chromium: 2.5-5.0; Molybdenum: 4.0-6.0; Tungsten: 0.01-3.0; Vanadium: 1.0-3.0; Nickel: 2.0-4.0; Cobalt: 9.0-12.5; Iron: remainder as well as the inevitable impurities, optionally further comprising one or more of the following elements: Niobium: 2.0; Nitrogen: 0.50; Silicon: 0.70; Manganese: 0.70; Aluminum: 0.15; the combined niobium+vanadium content being in the range 1.0-3.5; and the carbon 30 nitrogen content being in the range 0.06-0.50.

2. The steel composition as claimed in claim 1, comprising, in percentages by weight of the total composition: Carbon: 0.06-0.20; Chromium: 3.0-4.5; Molybdenum: 4.0-6.0; Tungsten 0.01-3.0; Vanadium: 1.5-2.5; Nickel: 2.0-4.0; Cobalt: 9.5-12.5; Iron: remainder as well as the inevitable impurities, optionally further comprising one or more of the following elements: Niobium: 2.0; Nitrogen: 0.20; Silicon: 0.70; Manganese: 0.70; Aluminum: 0.10; the combined niobium+vanadium content being in the range 1.0-3.5; and the carbon+nitrogen content being in the range 0.06-0.50.

3. The steel composition as claimed in claim 1, comprising at most 1 wt % of inevitable impurities.

4. The steel composition as claimed in claim 1, wherein the inevitable impurities are selected from titanium, sulfur, phosphorus, copper, tin, lead, oxygen and mixtures thereof.

5. The steel composition as claimed in claim 1, which is carburizable and/or nitridable.

6. The steel composition as claimed in claim 1, which has, after a thermochemical treatment, followed by a heat treatment, a surface hardness above 67 HRC.

7. The steel composition as claimed in claim 1, which has, after a thermochemical treatment, followed by a heat treatment, a martensitic structure having a residual austenite content below 0.5 wt % and free from ferrite and pearlite.

8. The steel composition as claimed in claim 6, wherein the thermal treatment comprises a solution treatment at a temperature between 1090 C.-1160 C. followed by quenching optionally with cooling and several tempering operations at a temperature between 475 C. and 530 C.

9. A method of making a steel blank having the composition as claimed in claim 1, comprising: a) a steelmaking step; b) a step of transformation of the steel; c) a thermochemical treatment; d) and a heat treatment.

10. The method of making as claimed in claim 9, wherein step c) consists of a treatment of carburizing or of nitriding or of carbonitriding or of carburizing and then nitriding.

11. The method of making as claimed in claim 9, wherein step c) consists of a carburizing treatment allowing carbon enrichment of the surface leading to a final surface carbon content of at least 1 wt %.

12. The method of making as claimed in claim 9, wherein step d) comprises a solution treatment at a temperature between 1090 C.-1160 C. followed by holding at this temperature until completion of austenitization optionally with cooling to a temperature below 40 C., and several tempering operations at a temperature between 475 C. and 530 C.

13. The method of making as claimed in claim 9, wherein step b) consists of a step of rolling, forging and/or extrusion.

14. The method of making as claimed in claim 9, wherein the steelmaking step a) is carried out by a conventional steelmaking process in an arc furnace and with refining and remelting under conductive slag (ESR, electroslag remelting), or by a VIM or VIM-VAR process, optionally with a step of remelting under conductive slag (ESR, electroslag remelting) and/or under vacuum (VAR), or by powder metallurgy such as gas atomization and compaction by hot isostatic pressing (HIP).

15. A steel blank obtainable by a method as claimed in claim 9.

16. (canceled)

17. A mechanical device made of steel having the composition as claimed in claim 1.

18. An injection system made of steel having the composition as claimed in claim 1.

19. A mechanical device as claimed in claim 17 which is a transmission component.

20. A mechanical device as claimed in claim 17 which is a transmission component, a bearing, or a gear train.

Description

1ST SERIES OF EXAMPLES

[0173] Seven laboratory heats of about 9 kg each (6 examples according to the invention and a comparative example with a composition similar to that in U.S. Pat. No. 8,157,931: comparative example 1) were produced by the VIM process according to the composition shown in Table 1 below (in wt % relative to the total weight of the composition), the remainder being Fe:

TABLE-US-00001 TABLE 1 Element C Ni Cr Mo V W Co Si Mn Al N Example 1: 0.18 3.1 3.9 5.1 2.1 1.18 10.0 0.2 0.18 0.023 0.005 GRADE A Example 2: 0.20 3.1 3.9 5.1 2.2 2.96 10.1 0.18 0.21 0.02 0.009 GRADE B Example 3: 0.16 3.1 3.9 5.1 2.1 1.19 10.0 0.21 0.18 0.02 0.009 GRADE C Example 4: 0.16 3.0 4.0 5.1 2.1 2.92 10.1 0.22 0.25 0.016 0.005 GRADE D Example 5: 0.16 3.1 3.9 5.0 2.1 0.01 10.0 0.123 0.2 0.042 0.005 GRADE E Example 6: 0.17 3.1 4.0 5.2 2.2 0.01 12.4 0.17 0.2 0.038 0.006 GRADE F Comparative 0.14 3.1 2.1 2.7 1.2 1.32 10.0 0.222 0.16 0.022 0.004 example 1: GRADE G

[0174] The Nb content is below the limit of detection. Nb<0.005% for all the examples.

[0175] These compositions are very similar, with the exception of comparative example 1. The notable main differences between comparative example 1 and example 1 relate to the content of V, Mo and Cr.

[0176] These laboratory heats were transformed into bars with a diameter of 40 mm by hot forging with a 2000 T press. Rods with a diameter of 20 mm were machined from the bar and carburized.

[0177] The carburized rods were treated by (1) a solution treatment at 1100 C. or 1150 C., (2) holding at this temperature for 15 min for austenitization, (3) cooling under neutral gas at a pressure between 2 and 6 bar (210.sup.5 and 610.sup.5 Pa), (4) a period at room temperature, (5) cooling to 70 C. for 2 hours, and (6) 3 tempering operations at a temperature of 500 C. for 1 hour each.

[0178] The profiles of surface hardness in HV measured according to standard ASTM E384 published in August 2017 for examples 1 to 6 and comparative example 1 are presented in Tables 2 and 3.

TABLE-US-00002 TABLE 2 (solution treatment at 1100 C.) Core Hardness material at depth Surface Example hardness of 1 mm hardness Example 1: GRADE A 522 888 936 Example 2: GRADE B 485 863 927 Example 3: GRADE C 542 890 938 Example 4: GRADE D 495 878 934 Example 5: GRADE E 554 880 942 Example 6: GRADE F 567 927 976 Comparative example 1: 576 835 847 GRADE G

TABLE-US-00003 TABLE 3 (solution treatment at 1150 C.) Core Hardness material at depth Surface Example hardness of 1 mm hardness Example 1: GRADE A 550 888 949 Example 2: GRADE B 543 888 943 Example 3: GRADE C 603 933 957 Example 4: GRADE D 552 904 957 Example 5: GRADE E 612 934 940 Example 6: GRADE F 627 936 988 Comparative example 1: 585 868 878 GRADE G

[0179] For all the chemical compositions except comparative example 1, the surface hardness after carburizing exceeds 920 HV for a temperature of solution treatment of 1100 C. and exceeds 930 HV for a temperature of solution treatment of 1150 C. The hardness at a depth of 1 mm is always above 860 HV for a temperature of solution treatment of 1100 C. and is always above 880 HV for a temperature of solution treatment of 1150 C. for all the examples except comparative example 1 (effect of the lack of alloying elements).

[0180] The hardnesses of the base materials are all below 650 HV.

2ND SERIES OF EXAMPLES

[0181] 2 heats of 100 kg each (one example according to the invention and a comparative example 2) were produced by the VIM process according to the composition shown in Table 4 below (in wt % relative to the total weight of the composition), the remainder being Fe:

TABLE-US-00004 TABLE 4 Element C Ni Cr Mo V W Co Si Mn Al N Example 7: 0.06 3.2 3.9 4.8 2.1 1.1 10.2 0.16 0.14 GRADE H Comparative 0.05 3.1 3.8 5.0 2.1 2.8 10.0 0.17 0.14 example 2: GRADE I

[0182] These laboratory heats were transformed into bars with a diameter of 40 mm by hot forging with a 2000 T press. Rods with a diameter of 20 mm were machined from the bar and carburized.

[0183] The carburized rods were treated by the same method as for the first test series apart from the solution treatment, which was carried out at 1100 C. and the triple tempering, which was carried out at 525 C. for 1 hour. Table 5 below gives the results of the toughness tests performed on test specimens CT10 according to standard ASTM E399-17 published in February 2018.

TABLE-US-00005 TABLE 5 Toughness Mechanical Example (MPa .Math. m) strength (MPa) Example 7 44-60 1400-1700 Comparative example 2 35 1500

[0184] Comparative example 2 has delta ferrite after heat treatment, at a low level but sufficient to decrease the toughness properties.

[0185] Example 7, very close to comparative example 2 at the level of its composition apart from W, does not have delta ferrite and makes it possible to obtain toughness values almost doubled relative to comparative example 2 while maintaining good mechanical strength (Rm) of about 1500 MPa, which was determined according to standard ASTM E399-17 published in February 2018, equivalent to a core hardness of 450 HV according to standard ASTM E384 published in August 2017.