STEEL COMPOSITION

Abstract

The present invention relates to a steel composition carburizable and/or nitritable, comprising, in percentages by weight of the total composition: Carbon: 0.05-0.40, preferably 0.10-0.30; Chromium: 2.50-5.00, preferably 3.00-4.50; Molybdenum: 4.00-6.00; Tungsten: 0.01-1.80, preferably 0.02-1.50; Vanadium: 1.00-3.00, preferably 1.50-2.50; Nickel: 2.00-4.00; Cobalt: 2.00-8.00, preferably 3.00-7.00; Iron: balance as well as the inevitable impurities, optionally further comprising one or more of the following elements: Niobium: 2.00; 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 1.00-3.50; and the carbon+nitrogen content being in the range 0.05-0.50.

It further relates to the method of production thereof, the steel blank obtained and a mechanical device comprising the latter.

Claims

1. A steel composition carburizable and/or nitritable comprising, in percentages by weight of the total composition: Carbon: 0.05-0.40; Chromium: 2.50-5.00; Molybdenum: 4.00-6.00; Tungsten: 0.01-1.80; Vanadium: 1.00-3.00; Nickel: 2.00-4.00; Cobalt: 2.00-8.00; Iron: balance as well as the inevitable impurities, optionally further comprising one or more of the following elements: Niobium: 2.00; Nitrogen: 0.50; Silicon: 0.70; Manganese: 0.70; Aluminum: 0.15; the combined niobium+vanadium content being in the range 1.00-3.50; and the carbon+nitrogen content being in the range 0.05-0.50.

2. The steel composition as claimed in claim 1, wherein it comprises, in percentages by weight of the total composition: Carbon: 0.10-0.30; Chromium: 3.00-4.50; Molybdenum: 4.00-6.00; Tungsten 0.02-1.50; Vanadium: 1.50-2.50; Nickel: 2.00-4.00; Cobalt: 3.00-7.00; Silicon: 0.05-0.50; Manganese: 0.05-0.50; Iron: balance as well as the inevitable impurities, optionally further comprising one or more of the following elements: Niobium: 2.00; Nitrogen: 0.20; Aluminum: 0.10; the combined niobium+vanadium content being in the range 1.00-3.50; and the carbon+nitrogen content being in the range 0.05-0.50.

3. The steel composition as claimed in claim 1, wherein it comprises at most 1 wt % of inevitable impurities relative to the total weight of the composition.

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, wherein the tungsten content is in the range 0.03-1.40 in percentages by weight of the total composition.

6. The steel composition as claimed in claim 1, wherein, after thermochemical treatment, followed by heat treatment, it has a surface hardness greater than or equal to 64 HRC.

7. The steel composition as claimed in claim 1, wherein, after thermochemical treatment, followed by heat treatment, it has a martensitic structure having a residual austenite content below 10% and that is free from ferrite and pearlite.

8. The steel composition as claimed in claim 6, wherein the heat treatment comprises solution heat treatment at a temperature between 1090 C.-1160 C. followed by quenching optionally with cooling to a temperature below 40 C. and several tempering operations, at a temperature greater than or equal to 475 C.

9. A method for producing a steel blank having the composition as claimed in claim 1, wherein it comprises: a) a steelmaking step; b) a step of processing the steel; c) a thermochemical treatment; d) and a heat treatment.

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

11. The production method as claimed in claim 9, wherein step d) comprises solution heat treatment at a temperature between 1090 C.-1160 C., followed by holding at this temperature until there is complete austenitization optionally with cooling to a temperature below 40 C., and several tempering operations, at a temperature greater than or equal to 475 C.

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

13. The production method as claimed in claim 9, wherein the steelmaking step a) is carried out by a conventional production process of arc furnace refining and electro slag remelting (ESR), or by a VIM-VAR process, optionally with a step of electro slag remelting (ESR) and/or vacuum arc remelting (VAR), or by powder metallurgy.

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

15. (canceled)

16. A mechanical device, formed from steel having the composition as claimed in claim 1.

17. The mechanical device according to claim 16, which is a bearing or a gear train.

18. The steel composition as claimed in claim 5, wherein the tungsten content is in the range 0.04-1.30 in percentages by weight of the total composition.

19. The production method as claimed in claim 10, wherein step c) consists of a treatment of carburization.

20. The production method as claimed in claim 11, wherein the tempering operations are carried out at a temperature greater than or equal to 500 C.

21. The production method as claimed in claim 11, wherein the heat treatment is carried out at a temperature between 1100 C.-1150 C.

Description

[0146] FIG. 1 shows the profile of surface hardness (microhardness in HV0.5 as a function of depth in the steel (in mm) of two examples according to the invention (grades B and C) and of a comparative example (grade A) according to application WO2015/082342 having the composition shown in Table 1 below as well as of a comparative example 50 NiL (0.12% C-4% Cr-4.2% Mo-3.4% Ni-1% V), obtained after carburization and heat treatment comprising the following steps: (1) heating to 1150 C., (2) holding for 15 min at 1150 C. for austenitization, (3) cooling under neutral gas at a pressure of 2 bar, (4) a period at room temperature, (5) cooling to 75 C. for 2 hours, and (6) 3 tempering operations at 550 C. for grade C and 560 C. for grades A and B for 1 hour each.

[0147] FIG. 2 shows the profile of surface hardness (microhardness in HV0.5 as a function of depth in the steel (in mm) from example 2 (grade C) according to the invention having the composition shown in Table 1 below as well as of a comparative example 50 NiL (0.12% C-4% Cr-4.2% Mo-3.4% Ni-1% V), obtained after carburization and heat treatment comprising the following steps: (1) heating to 1100 C., (2) holding for 15 min at 1100 C. for austenitization, (3) cooling under neutral gas at a pressure of 2 bar, (4) a period at room temperature, (5) cooling to 75 C. for 2 hours, and (6) 3 tempering operations at a temperature of 475 C. or 500 C. or 550 C. or 575 C. for grade C or of 560 C. for the comparative example 50 NiL for 1 hour each.

[0148] FIG. 3 shows the profile of surface hardness (microhardness in HV0.5 as a function of depth in the steel (in mm) from example 2 (grade C) according to the invention having the composition shown in Table 1 below as well as of a comparative example 50 NiL (0.12% C-4% Cr-4.2% Mo-3.4% Ni-1% V), obtained after carburization and heat treatment comprising the following steps: (1) heating to 1150 C., (2) holding for 15 min at 1150 C. for austenitization, (3) cooling under neutral gas at a pressure of 2 bar, (4) a period at room temperature, (5) cooling to 75 C. for 2 hours, and (6) 3 tempering operations at a temperature of 475 C. or 500 C. or 550 C. or 575 C. for grade C or of 560 C. for the comparative example 50 NiL for 1 hour each.

EXAMPLES 1 AND 2

[0149] Three laboratory casts each of about 110 kg (two examples according to the invention: example 1 and example 2 and a comparative example according to application WO2015/082342: comparative example 1) were produced by the VIM-VAR process according to the composition shown in Table 1 below:

TABLE-US-00001 TABLE 1 Element C Si Mn Ni Cr Mo W Comparative Min. 0.18 0.10 0.10 3.00 3.80 4.80 3.00 example 1: Max. 0.20 0.20 0.20 3.20 4.00 5.20 3.20 GRADE A Example 1: Min. 0.18 0.10 0.10 3.00 3.80 4.80 1.10 GRADE B Max. 0.20 0.20 0.20 3.20 4.00 5.20 1.30 Example 2: Min. 0.18 0.14 0.180 3.05 3.90 5.00 0.10 GRADE C Max. 0.20 0.16 0.220 3.09 4.00 5.20 0.20 Element Al V Co Cu N.sub.2 O.sub.2 S P Comparative Min. 0.03 2.00 4.90 example 1: Max. 0.07 2.20 5.20 <0.10 <0.01 <0.002 <0.001 <0.005 GRADE A Example 1: Min. 0.03 2.00 4.90 GRADE B Max. 0.07 2.20 5.20 <0.10 <0.01 <0.002 <0.001 <0.005 Example 2: Min. 0.03 2.10 5.00 GRADE C Max. 0.05 2.30 5.40 <0.10 <0.01 <0.002 <0.001 <0.005

[0150] These three compositions are very similar. The main difference is in the content of W.

[0151] These three laboratory casts were transformed into bar with a diameter of 40 mm by a hot forging process under a 2000 T press. Bars with a diameter of 30 mm were machined from the bar and carburized.

[0152] The carburized bars were treated by (1) heating to 1100 C. or 1150 C., (2) holding for 15 min at this temperature for austenitization, (3) cooling under neutral gas at a pressure of 2 bar, (4) a period at room temperature, (5) cooling to 75 C. for 2 hours, and (6) 3 tempering operations at a temperature between 475 C. and 560 C. for 1 hour each.

[0153] The profiles of surface hardness in HV obtained, measured according to standard ASTM E384, are compared in FIGS. 1 to 3 with those obtained with steel 50 NiL (0.12% C-4% Cr-4.2% Mo-3.4% Ni-1% V) that underwent the same treatment of austenitization and cooling to room temperature and then cold, and 3 tempering operations at 560 C.

[0154] The compositions according to the invention having a low W content have higher levels of hardness, of the order of 860 HV, corresponding to 66 HRC. It should also be noted that lowering the W content relative to the prior art does not significantly affect the level of hardness of the base metal, which is of the order of 540 HV, corresponding to 51 HRC.

[0155] The steel having the composition according to the invention (low W content) therefore makes it possible to obtain higher levels of hardness with a heat treatment limited to 1150 C. relative to that of the prior art with a higher W content.

[0156] It should also be pointed out that a tempering temperature of 500 C. is particularly advantageous since the level of hardness reaches 66-67 HRC (with solution heat treatment at 1100 C. and 1150 C.) (FIGS. 2 and 3).

[0157] At 575 C., the results are still very advantageous with values above 64 HRC after solution heat treatment at only 1150 C. (FIG. 3).