STEEL FOR SMALL-CALIBRE WEAPON

Abstract

The present invention relates to a steel composition, to the process for manufacturing same, to the steel blank obtained having a hardness of between 46 and 48 HRC and a resilience KV at 40 C. of at least 40 joules, and to the use thereof for manufacturing a pressure appliance component.

Claims

1. A steel composition essentially comprising: Carbon: 0.28-0.35; Manganese: 0.10-0.60; Silicon: 0.10-0.20; Chromium: 2.80-3.40; Molybdenum: 0.70-1.60; Vanadium: 0.20-0.50; Phosphorus: 0.005; Nickel: 0.10; Aluminium: 0.025; Copper: 0.10; Arsenic+Antimony+Tin: <100 ppm; Sulfur: <10 ppm; Iron: remainder; as weight percentages of the total composition, and also inevitable impurities.

2. A steel composition according to claim 1, essentially comprising: Carbon: 0.28-0.35; Manganese: 0.10-0.20; Silicon: 0.10-0.20; Chromium: 2.80-3.40; Molybdenum: 0.70-1.30; Vanadium: 0.20-0.40; Phosphorus: 0.005; Nickel: 0.10; Aluminium: 0.025; Copper: 0.10; Arsenic+Antimony+Tin: <100 ppm; Sulfur: <10 ppm; Iron: remainder; as weight percentages of the total composition, and also the inevitable impurities.

3. A steel composition according to claim 1, wherein the molybdenum content is between 0.7 and 1.1, as weight percentages of the total composition.

4. A process for manufacturing a steel blank having the composition according to claim 1, the process comprising: a) a step of producing the steel; b) a step of transforming the steel; c) a heat treatment of the steel comprising a tempering treatment at a temperature of at least 530 C. for an overall time of between 2 and 6.

5. A manufacturing process according to claim 4, wherein step c) comprises several tempering treatments, the cumulative times of which correspond to the overall time of said step.

6. A manufacturing process according to claim 4, wherein step c) comprises, before the tempering treatment, quenching at a temperature of at least 900 C.

7. A manufacturing process according to claim 4, wherein step b) consists of a rolling step.

8. A manufacturing process according to claim 4, wherein the production step a) is performed in an electric arc furnace followed by vacuum arc degassing, or via VIM-VAR or VIM-ESR processes.

9. A steel blank obtainable by a process according to claim 4.

10. A steel blank according to claim 9, having hardness between 46 and 48 HRC.

11. A steel blank according to claim 9, having a resilience KV at 40 C. of at least 40 joules.

12. A steel blank according to claim 9, having a mechanical strength Rm of between 1500 and 1600 MPa.

13. A pressure appliance component made from a blank according to claim 9 or from a steel composition according to claim 1.

14. The pressure appliance component according to claim 13, wherein the pressure appliance component is a barrel tube.

15. A steel composition according to claim 1, wherein the Aluminium content is between 0.006 and 0.025, as weight percentages of the total composition.

16. A manufacturing process according to claim 4, wherein the tempering treatment of step c) is at a temperature of between 530 and 550 C. for an overall time of 4 hours.

17. A manufacturing process according to claim 5, wherein step c) comprises two tempering treatments of 2 hours each.

18. A manufacturing process according to claim 6, wherein the quenching is at a temperature of between 900 and 930 C.

19. A manufacturing process according to claim 8, wherein the production step a) is performed in an electric arc furnace followed by vacuum arc degassing with a step of electroslag remelting (ESR) or vacuum arc remelting (VAR).

20. The pressure appliance component according to claim 14, wherein the barrel tube is for a small-caliber weapon.

Description

COMPARATIVE EXAMPLE 1CASTING A (STEEL COMPOSITION WITH SI AND MN CONTENTS OF GREATER THAN 0.2%)

[0067] A standard industrial production of 60 tonnes composed of an electric furnace melt comprising the melt operation per se and also a forced dephosphorization operation, followed by a ladle refining operation to finely adjust the chemical elements and to obtain a good level of deoxidation with degassing treatment at the end of production to ensure desulfurization and also low hydrogen contents (the H.sub.2 content is typically less than 2 ppm and preferably less than 1.5 ppm, in particular about 1.2 ppm), was used to manufacture a 3% CrMoV steel composition with an Si and Mn content of greater than 0.2%. The chemical composition of the steel composition obtained is reported in Table 1 below:

TABLE-US-00001 TABLE 1 Chemical composition in mass % of casting A except (*) in ppm C Mn Si Cr Mo V P Ni Al Cu 0.321 0.529 0.287 2.96 0.841 0.286 0.0044 0.086 0.012 0.038 S* As* Sb* Sn* <10 26 <15 37

[0068] The O.sub.2 content is between 7 and 12 ppm.

[0069] The casting was rolled into bars.

[0070] The mechanical properties obtained after heat treatment at 920 C. for 20 minutes and tempering at 545 C. for 2 hours reach a hardness level of 46 HRC with a relatively fine grain size greater than an ASTM index 10. The resilience KV at 20 C. is 60 joules minimum, the resilience at 40 C. is 37.7 J. The resilience is thus less than 40 J at 40 C.

COMPARATIVE EXAMPLE 2CASTING B (STEEL COMPOSITION WITH SI AND MN CONTENTS OF GREATER THAN 0.2% AND LOW CONTENTS OF P, AS, SB AND SN)

[0071] The casting is obtained via the same process as that of Example 1. The only difference concerns the chemical composition of the steel. It is indicated in Table 2 below.

TABLE-US-00002 TABLE 2 Chemical composition in mass % of casting B except (*) in ppm C Mn Si Cr Mo V P Ni Al Cu 0.312 0.483 0.288 3 0.812 0.278 <0.002 0.052 0.013 0.036 S* As* Sb* Sn* <10 29 <15 18

[0072] The casting was rolled into bars. The resilience values at 40 C. obtained for casting B with a low content of residuals (P, As, Sb and Sn), with heat treatments identical to those performed on casting A, are 38.7 J (mean of 3 values). Thus, a very low value of P, obtained especially by means of a production process particularly followed in an electric furnace by controlled insufflation of oxygen and also in controlling the chemical quality of the metal and non-metal additions, does not make it possible to significantly increase the resilience values at low temperature (40 C.), and similarly very low values of residuals As, Sb and Sn, the sum of which for casting B is 62 ppm. The resilience is thus less than 40 J at 40 C.

EXAMPLE 1CASTING C (COMPOSITION ACCORDING TO THE INVENTION)

[0073] The casting is obtained via the same process as that of Example 1. The only difference concerns the chemical composition of the steel. It is indicated in Table 3 below and corresponds to a composition according to the invention.

TABLE-US-00003 TABLE 3 Chemical composition in mass % of casting C except (*) in ppm C Mn Si Cr Mo V P Ni Al Cu 0.312 0.18 0.115 2.98 0.842 0.278 0.002 0.058 0.015 0.035 S* As* Sb* Sn* 9 24 <15 30

[0074] The casting was rolled into bars.

[0075] The resilience values at 40 C. obtained for casting C with heat treatments identical to those performed on casting A reach 43.3 J on an average of 6 tests. The hardness obtained remains between 46 and 48 HRC. The austenitic grain size also remains very fine with an ASTM index of greater than or equal to 10.

[0076] The increase in resilience is significant compared with castings A and B (Comparative examples 1 and 2) with a gain of the order of 15%.

EXAMPLE 2CASTING D (COMPOSITION ACCORDING TO THE INVENTION)

[0077] The casting is obtained via the same process as that of Example 1. The only difference concerns the chemical composition of the steel. It is indicated in Table 4 below.

TABLE-US-00004 TABLE 4 Chemical composition in mass % of casting D except (*) in ppm C Mn Si Cr Mo V P Ni Al Cu 0.316 0.188 0.193 2.99 0.847 0.275 0.002 0.053 0.014 0.029 S* As* Sb* Sn* 4 26 17 36

[0078] The casting was rolled into bars.

[0079] The resilience values at 40 C. obtained for casting C with heat treatments identical to those performed on casting A reach 43 J on an average of 6 tests. The hardness obtained is between 46 and 48 HRC.

[0080] The increase in resilience is thus also significant compared with castings A and B (Comparative examples 1 and 2) with a gain of the order of 15%. The Si and Mn content (less than 0.20%) thus has a significant impact on the resilience at 40 C.

EXAMPLE 3CASTING E (COMPOSITION ACCORDING TO THE INVENTION)

[0081] The casting is obtained via the same process as that of Example 1. The only difference concerns the chemical composition of the steel. It is indicated in Table 5 below.

TABLE-US-00005 TABLE 5 Chemical composition in mass % of casting E except (*) in ppm C Mn Si Cr Mo V P Ni Al Cu 0.311 0.454 0.132 3.06 0.841 0.287 <0.004 0.046 0.011 0.039 S* As* Sb* Sn* <10 34 <15 26

[0082] The casting was rolled into bars.

[0083] For casting E, the resilience values at 40 C. obtained with heat treatments strictly identical to those performed in Comparative examples 1 and 2 and in Examples 1 and 2 on castings A to D show an average of 41.16 J (average of 6 tests). The hardness range obtained is 46-48 HRC.

[0084] Thus, if the Mn content of the steel composition is greater than 0.200%, the toughness obtained KV at 40 C. is less than those obtained on castings C and D which have Mn contents <0.200%, while at the same time remaining greater than 40 J.

EXAMPLE 4IMPACT OF THE TEMPERING TREATMENT ON THE STRENGTH/TOUGHNESS COMPROMISE OF THE COMPOSITION ACCORDING TO THE INVENTION

[0085] Casting C (Example 1) after its production and rolling into bars underwent a heat treatment at 920 C. for 20 minutes followed by one or more tempering steps at 545 C. for 2 hours.

[0086] The mechanical properties obtained (resilience KV at 40 C. and mechanical strength Rm at room temperature) according to the number of temperings are indicated in Table 6 below.

TABLE-US-00006 TABLE 6 KV at 40 C. and Rm at room temperature according to the number of temperings at 545 C. for 2 hours of casting C Number of temperings Rm (MPa) Average KV (J) X1 1552 42.7 X2 1541 44.1 X3 1530 47.3 X4 1516 46.5

[0087] As illustrated, the more the number of temperings increases, the more the resilience at 40 C. increases, with the exception of the fourth tempering for which a very slight decrease is observed, while at the same time still having a very good level. The fourth tempering treatment gives interesting results, but the mechanical strength is markedly lower and very close to the 46 HRC minimum desired for this application.

[0088] The number of temperings is readily convertible into an equivalent treatment time for a single tempering operation at 545 C. Table 7 below shows that a single tempering treatment with a time corresponding to 2 tempering treatments at 545 C. or 3 tempering treatments at 545 C. gives very similar results.

TABLE-US-00007 TABLE 7 KV at 40 C. and Rm as a function of the tempering time at 545 C. of casting C Tempering time Rm (MPa) Average KV (J) 2 hours 1552 42.7 4 hours 1549 45.3 6 hours 1533 47.5

[0089] Adaptation of the number of temperings or its equivalent in tempering time makes it possible to significantly increase the level of resilience. The gain relative to casting A treated under the standard conditions is 25% to 30% approximately for casting C.

[0090] It should be noted that this improvement results from the combination of a low Si content and a low Mn content (less than 0.2%) with a 3% CrMoV base as shown in Table 8 below.

TABLE-US-00008 TABLE 8 Influence of the number of temperings at 545 C. for 2 hours according to the chemical composition on Rm and KV at 40 C. Average KV at Number of temperings Rm (MPa) 40 C. (J) Casting C X1 1559 43.3 (Example 1) X2 1546 46 X3 1543 47.7 X4 1516 46.5 Casting E X1 1532 41 (Example 3) X2 1524 43.7 X3 1511 44 X4 1516 44.7 Casting A X1 1542 37.7 (Comparative X2 1532 39 example 1) X3 1528 35.7 X4 1516 34.7

[0091] Only casting C makes it possible to pass to a resilience level greater than 45 J with a suitable number of temperings at 545 C. The low content of silicon alone (less than 0.2%: casting E) makes it possible to increase the resilience level to approximately 44 J. It should be noted that in the case of the steel with a high content of Si and Mn (casting A), the number of temperings does not have any influence on the resilience level. The average resilience value even has a tendency to decrease significantly after the third tempering treatment.

EXAMPLE 5IMPACT OF THE QUENCHING TEMPERATURE OF THE HEAT TREATMENT ON A CASTING F ACCORDING TO THE INVENTION: 920 C. VS 960 C.

[0092] Casting F is obtained via the same process as that of Example 1. The only difference concerns the chemical composition of the steel. It is indicated in Table 9 below.

TABLE-US-00009 TABLE 9 Chemical composition in mass % of casting F C Mn Si Cr Mo V 0.30 0.19 0.19 3.1 1.1 0.28

[0093] The resilience value at 40 C. obtained for casting F with a heat treatment with quenching at 920 C. and a single tempering of 2 hours at 545 C. reaches 42 J; whereas, for the same casting F, a heat treatment with quenching at 960 C. and a single tempering of 2 hours at 545 C. leads to a resilience value at 40 C. of 27 J.

[0094] A high quenching temperature, at 960 C., thus degrades the resilience of the steel.