EP3310936B1 - Steel for small-calibre weapon - Google Patents

Description

The present invention relates to a new steel intended to be used for the manufacture of a small caliber weapon tube, exhibiting good hammering ability and good bursting resistance during excessive inflation of the tube under high pressure.

When using a weapon, its barrel is subjected to strong thermal and mechanical stresses. It is in particular important that the barrel does not burst when firing the weapon, which could injure its user. It is therefore necessary to have weapons with high security and high quality. For this, it is necessary to have available steel having good mechanical characteristics and in particular good resistance to bursting, even at a very low temperature.

• 3,6 - 4,4% Cr, avantageusement 3,8 - 4,2 % ;
• 1,2 - 1,8% Mo, avantageusement 1,4 - 1,6 % et
• 0,42 - 0,5% V, avantageusement 0,44 - 0,48 %.

The requests AT508777 and US 2011 / 0253270A1 describe a family of steels for the manufacture of small caliber weapon tubes with higher contents of main elements than the contents commonly developed for these applications. Thus in particular this steel comprises:

• 3.6 - 4.4% Cr, advantageously 3.8 - 4.2%;
• 1.2 - 1.8% Mo, advantageously 1.4 - 1.6% and
• 0.42 - 0.5% V, advantageously 0.44 - 0.48%.

These documents state that these high contents of chromium and molybdenum have an advantageous effect on the quenching behavior of the material and its properties at high temperature. Indeed according to the comparative example of Figure 1 of the document AT508777 , V320 steel which has a much lower chromium and molybdenum content, does not meet the conditions to serve as a barrel tube from 390 ° C. On the other hand, the steel family described in this document makes it possible to obtain the desired mechanical properties with tempering temperatures above 560 ° C., which thus makes it possible to exhibit high mechanical strengths up to temperatures of 500 ° C. However, the use of a high chromium, molybdenum and vanadium content is expensive. Furthermore, the impact properties of steel are not addressed by this document, and the request AT508777 illustrates in its figure 2 a value in standard impact strength of 30J at - 40 ° C. However, it would be advantageous to be able to have a steel having a better impact value, in particular at least 40J at -40 ° C, while reducing the production costs of this steel. In addition, these high contents of chromium, molybdenum and vanadium require high treatment temperatures for quenching greater than 940 ° C., which can generate an increase in distortions after quenching and accentuates the risk of decarburization.

Japanese demand JP2000-080444 also describes a family of steel for gun barrel application. It is a steel with a lower chromium content than conventional steels 3% Cr but with a higher content of Mo and V. However, the family described only achieves a limited level of HRC hardness at 36 HRC. This level of hardness is very far from the levels required for high-end applications: 46-48 HRC. In addition, the claimed low temperature resilience level is low compared to a standard 3% Cr steel, since the minimum claimed is 16 J for a minimum level of 20 J with 3% Cr.

It would therefore be interesting to be able to have a steel having a better impact strength, in particular at least 40J at -40 ° C, while having the hardness required for high-end applications: 46-48 HRC.

The patent US 2,876,095 describes two families of steel for application of a weapon tube with improved lifetime using the addition of rare earths during the production of the liquid metal. The chromium and molybdenum contents are lower there than those of other steels of the prior art. However, this patent does not describe any particular mechanical properties and in particular does not indicate the mechanical strength when hot or the resilience of the steel.

The patent US 4,622,080 describes a steel for a barrel tube, the manganese content of which is 0.50-1.00%.

The inventors have surprisingly realized that it is possible to obtain a new family of steels which is less expensive than those of the prior art, having a toughness value greater than current steels and in particular at least 40J at -40 ° C, while having a hardness required for high-end applications (46-48 HRC). This family of steels can therefore be used in the manufacture of a high-end small-caliber and therefore high-quality weapon tube having good safety for the user, since this family of steels has good hammering ability. and good resistance to bursting during excessive inflation of the tube under high pressure without being too expensive. This family is characterized by a low Mn content associated with a low Si content, while avoiding the addition of excessively high contents of Cr, Mo and V, particularly expensive elements.

• Carbone : 0,28-0,35 ;
• Manganèse : 0,10- 0,30, de préférence 0,10-0,20 ;
• Silicium : 0,10-0,20 ;
• Chrome : 2,80-3,40;
• Molybdène : 0,70-1,60, de préférence 0,70-1,30 ;
• Vanadium : 0,20-0,50, de préférence 0,20-0,40;
• Phosphore : ≤0,005;
• Nickel : ≤ 0,10;
• Aluminium : ≤0,025, de préférence 0,006 - 0,025 ;
• Cuivre : ≤ 0,10;
• Arsenic + Antimoine + Etain : < 100 ppm;
• Soufre : < 10ppm
• Fer : solde

en pourcentages en poids de la composition totale, ainsi que les impuretés inévitables.The present invention therefore relates to a steel composition consisting of:

• Carbon: 0.28-0.35;
• Manganese: 0.10-0.30, preferably 0.10-0.20;
• Silicon: 0.10-0.20;
• Chrome: 2.80-3.40;
• Molybdenum: 0.70-1.60, preferably 0.70-1.30;
• Vanadium: 0.20-0.50, preferably 0.20-0.40;
• Phosphorus: ≤0.005;
• Nickel: ≤ 0.10;
• Aluminum: ≤0.025, preferably 0.006 - 0.025;
• Copper: ≤ 0.10;
• Arsenic + Antimony + Tin: <100 ppm;
• Sulfur: <10ppm
• Iron: balance

in percentages by weight of the total composition, as well as the inevitable impurities.

In particular the inevitable impurities, in particular in the form of lead (Pb), Bismuth (Bi), magnesium (Mg), cobalt (Co), are kept at the lowest level. These impurities are generally due mainly to the manufacturing process and the quality of the oven.

The steel composition according to the invention comprises a carbon content (C) of between 0.28 and 0.35% by weight relative to the total weight of the composition. Indeed, the carbon content makes it possible to achieve the desired hardness (46-48 HRC) with a minimum of 0.28% while not being detrimental to impact resistance with a maximum of 0.35 %. A higher content would not make it possible to obtain good impact resistance properties at low temperature, the high carbon content raising the ductile / brittle transition temperature to temperatures close to 0 ° C.

The steel composition according to the invention comprises a manganese (Mn) content of between 0.10 and 0.30% by weight relative to the total weight of the composition: a minimum content of 0.10% is essential to ensure deoxidation of the liquid metal in order to obtain less than 15 ppm of oxygen on the product. Furthermore, the Mn content must not be too high to obtain a good level of resilience. Advantageously, the Mn content is between 0.10 and 0.20%, by weight relative to the total weight of the composition. Indeed, a low content of Mn limited to 0.3% appreciably improves the level of resilience at -40 ° C, and a content even more limited to 0.20% improves even more significantly the level of resilience at -40 ° C , all the more with a suitable tempering time, while keeping sufficient mechanical strength. This low Mn content must be accompanied by a low S content in order to avoid any embrittlement by sulfides with low melting point.

The steel composition according to the invention comprises a silicon (Si) content of between 0.10 and 0.20%, by weight relative to the total weight of the composition: in fact, the inventors have noticed that the combination with a low Si content and a low Mn content makes it possible to improve the resilience value at low temperature. However, the Si content must not be below 0.10% in order to ensure sufficient deoxidation during the preparation of the liquid metal.

The steel composition according to the invention comprises a chromium (Cr) content of between 2.80 and 3.40% by weight relative to the total weight of the composition. This content must be at least 2.80% to ensure the high mechanical properties after tempering at a minimum temperature of 530 ° C. This element being expensive it is desirable to limit its addition on an economic level. In addition, it is very likely that beyond 3.40% chromium, there is no significant improvement in mechanical properties. In addition, the limitation to a chromium content of 3.4% makes it possible to carry out a solution treatment for quenching at 920 ° C. This temperature limitation makes it possible to limit the phenomena of decarburization and to minimize the phenomena of distortion on quenching. A maximum temperature of 940 ° C for the quenching operation is indeed desirable in order to limit the magnification of the austenitic grain which negatively affects the level of resilience at low temperature (-40 ° C). Advantageously, the chromium content is between 2.80 and 3.20%, even more advantageously between 2.90 and 3.10%, by weight relative to the total weight of the composition.

The steel composition according to the invention comprises a molybdenum (Mo) content of between 0.70 and 1.60% by weight relative to the total weight of the composition. This content must be at least 0.70% to ensure the high mechanical properties after tempering at a minimum temperature of 530 ° C. This element being expensive it is desirable to limit its addition on an economic level. Advantageously, the molybdenum content is between 0.70 and 1.30% by weight relative to the total weight of the composition. Indeed, this range seems to be the best compromise between the mechanical properties and the cost of the steel obtained. Even more advantageously the molybdenum content is between 0.70 and 1.10%, in particular between 0.80 and 0.90%, by weight relative to the total weight of the composition.

The steel composition according to the invention comprises a vanadium (V) content of between 0.20 and 0.50% by weight relative to the total weight of the composition. Indeed, a small addition of Vanadium makes it possible to control the size of the austenitic grain. A fine grain size improves the resistance to low temperature resilience. However, vanadium is also a fairly expensive element. Therefore, the best compromise between the resilience behavior at low temperature and the cost of the steel obtained is between 0.20 and 0.50% by weight. Advantageously, the vanadium content is between 0.20 and 0.40%, even more advantageously between 0.20 and 0.30%, by weight relative to the total weight of the composition.

The steel composition according to the invention must not comprise more than 0.025% of aluminum (Al) in percentages by weight relative to the total weight of the composition, in order to avoid the formation of alumina detrimental to the desired properties. In an advantageous embodiment, the aluminum content must be greater than 0.006%, in particular 0.008%, by weight relative to the total weight of the composition, in order to ensure sufficient deoxidation of the metal, the silicon content being limited. at 0.200%. Thus, in a particular embodiment, the aluminum content of the composition according to the present invention is between 0.006 and 0.025%, advantageously between 0.008 and 0.025%, by weight relative to the total weight of the composition.

The steel composition according to the present invention has a low residual content in order to limit the risk of embrittlement. Thus a maximum Phosphorus (P) content of 50 ppm, advantageously a maximum of 20 ppm, combined with limited contents of arsenic (As), antimony (Sb) and tin (Sn) with a sum of these three elements less than 100 ppm provides a very good compromise between strength and toughness. In an advantageous embodiment, the content of tin of the steel composition according to the invention is less than 40 ppm. In another advantageous embodiment, the arsenic content of the steel composition according to the invention is less than 40 ppm. In yet another advantageous embodiment, the antimony content of the steel composition according to the invention is less than 20 ppm.

The steel composition according to the invention must not comprise more than 0.10% nickel (Ni) in percentages by weight relative to the total weight of the composition in order to achieve low H ₂ contents. In a particular embodiment, the nickel content of the composition according to the present invention is less than or equal to 0.08%.

The steel composition according to the invention must not comprise more than 0.10% copper (Cu) in percentages by weight relative to the total weight of the composition, in order to avoid weakening the steel. In a particular embodiment, the copper content of the composition according to the present invention is less than or equal to 0.05%.

The steel composition according to the invention must not comprise more than 10 ppm of sulfur (S) in percentages by weight relative to the total weight of the composition in order to avoid any embrittlement by sulfides with low melting point.

• Carbone : 0,28-0,35 ;
• Manganèse : 0,10-0,20 ;
• Silicium : 0,10-0,20 ;
• Chrome : 2,80-3,40;
• Molybdène : 0,70-1,30 ;
• Vanadium : 0,20-0,40;
• Phosphore : ≤0,005;
• Nickel : ≤ 0,10;
• Aluminium : ≤0,025, de préférence 0,006 - 0,025 ;
• Cuivre : ≤ 0,10;
• Arsenic + Antimoine + Etain : < 100 ppm;
• Soufre : < 10ppm
• Fer : solde

en pourcentages en poids de la composition totale, ainsi que les impuretés inévitables.In a particularly advantageous embodiment, the steel composition according to the present invention consists of:

• Carbon: 0.28-0.35;
• Manganese: 0.10-0.20;
• Silicon: 0.10-0.20;
• Chrome: 2.80-3.40;
• Molybdenum: 0.70-1.30;
• Vanadium: 0.20-0.40;
• Phosphorus: ≤0.005;
• Nickel: ≤ 0.10;
• Aluminum: ≤0.025, preferably 0.006 - 0.025;
• Copper: ≤ 0.10;
• Arsenic + Antimony + Tin: <100 ppm;
• Sulfur: <10ppm
• Iron: balance

in percentages by weight of the total composition, as well as the inevitable impurities.

1. a) une étape d'élaboration de l'acier ;
2. b) une étape de transformation de l'acier
3. c) un traitement thermique de l'acier comprenant une trempe à une température d'au moins 900°C et un traitement de revenu à une température d'au moins 530°C, avantageusement comprise entre 530 et 550°C, pendant une durée globale comprise entre 2 et 6 heures, avantageusement pendant une durée globale de 4 heures.

The present invention also relates to a process for manufacturing a steel blank having the steel composition according to the invention, characterized in that it comprises:

1. a) a stage of steel making;
2. b) a steel transformation stage
3. c) a heat treatment of the steel comprising a quenching at a temperature of at least 900 ° C and an tempering treatment at a temperature of at least 530 ° C, advantageously between 530 and 550 ° C, for a period overall between 2 and 6 hours, advantageously for an overall duration of 4 hours.

The method according to the present invention therefore comprises a step a) for producing the steel. This step therefore makes it possible to obtain the steel having the composition according to the present invention. Advantageously step a) of preparation is implemented in an electric arc furnace followed by refining in a pocket with degassing treatment (Vacuum Arc Degassing), possibly with a reflow step under conductive slag (ESR) or under vacuum. (VAR), or by VIM-VAR or VIM-ESR methods.

The development by implementation in an electric arc furnace followed by a pocket refining with degassing treatment is the most economical. It makes it possible to obtain a good inclusive state and low dissolved gas contents, in particular low H ₂ contents. However, a reflow treatment under conductive slag or under vacuum gives similar results. These methods are well known to those skilled in the art.

The method according to the present invention further comprises a step b) of transformation of the steel obtained in step a). Advantageously, step b) consists of a rolling, forging, hammering, stamping or any other means making it possible to correct the steel, even more advantageously it is a rolling step.

The method according to the present invention finally comprises a step c) of thermal treatment of the steel comprising an tempering treatment at a temperature of at least 530 ° C, advantageously between 530 and 550 ° C, in particular 545 ° C , for an overall duration of between 2 and 6 hours, advantageously for an overall duration of 4 hours.

This tempering heat treatment gives the final mechanical properties of the steel blank. The microstructure obtained is of returned martensite type with possibly the presence of some ferritic ranges in very small proportion.

In a particular embodiment, step c) comprises several income treatments, in particular several income treatments of 2 hours each, the added durations of which correspond to the overall duration of said step (that is to say advantageously between 2 and 6 hours, more preferably 4 hours). In an advantageous embodiment, step c) comprises two or three income treatments of 2 hours each (overall duration of 4 and 6 hours respectively), in particular two income treatments of 2 hours each, which therefore corresponds to an income treatment with an overall duration of 4 hours.

Thus, step c) can consist of a single income treatment or of several income treatments. However, in a preferred mode, it consists of a single income treatment.

Stage c) comprises, before the tempering treatment, quenching at a temperature of at least 900 ° C., advantageously between 900 and 930 ° C., more advantageously of 920 ° C., in particular for between 10 and 30 minutes, especially for 20 minutes. It is a standard treatment well known to those skilled in the art.

In yet another particular embodiment, the heat treatment step c) can be followed by a step d) consisting of a nitriding operation, advantageously at a maximum temperature of 545 ° C. This is a step well known to those skilled in the art.

The present invention also relates to a steel blank capable of being obtained by the method according to the invention. This blank is made from steel having the composition according to the present invention and as described above.

Thanks to the heat treatment treatments of the process according to the invention, the steel blank thus obtained has good hammering ability and good ability to withstand bursting during excessive swelling of the tube under high pressure. It has a good compromise between strength and toughness, in particular at low temperature, that is to say at a temperature less than or equal to -40 ° C.

In one embodiment of the present invention, the steel blank according to the invention has a hardness of between 46 and 48 HRC measured according to standard ASTM E18 or equivalent standard.

In another embodiment of the present invention, the steel blank according to the invention has a resilience KV at -40 ° C of at least 40 Joules, advantageously at least 43 Joules, in particular at least minus 44 Joules, more particularly at least 46 Joules, the resilience being measured according to standard NF-EN ISO 148-1 or equivalent standard.

In yet another embodiment of the present invention, the steel blank according to the invention has a mechanical resistance Rm at ambient temperature of between 1500 and 1600MPa, advantageously between 1510 and 1560 MPa, the mechanical resistance being measured according to NF EN ISO 6892-1 or equivalent standard.

The present invention also relates to the use of a steel blank according to the invention or of a steel composition according to the invention for the manufacture of a pressure device element, in particular such as plugs or sleeves, in particular of cylinder head, or tubes of pressure vessels supporting in particular from 4,000 to 10,000 bars, including in particular cannon tubes.

Advantageously, the pressure device element is a barrel tube, in particular of small caliber weapons, more particularly for light weapons, even more advantageously for high-end weapons. The guns thus obtained are of very good quality, and offer very good security for their user.

The invention will be better understood on reading the examples which follow which are given as a non-limiting indication.

In the examples, unless otherwise indicated, all the percentages are expressed by weight, the temperature is expressed in degrees Celsius and the pressure is atmospheric pressure.

In addition, the resilience KV to is measured according to standard NF-EN ISO 148-1, the mechanical resistance Rm is measured according to standard NF EN ISO 6892-1 and the hardness is measured according to standard ASTM E18.

Comparative example 1 - casting A (steel composition with Si and Mn contents greater than 0.2%)

A standard industrial production of 60 tonnes consisting of a melting in an electric oven including the melting operation itself as well as an advanced dephosphorization operation, followed by a ladle refining operation allowing fine adjustment of the chemical elements and obtaining a good level of deoxidation with degassing treatment at the end of production, making it possible to ensure desulfurization as well as low hydrogen contents (the H ₂ content is typically less than 2 ppm and preferably less than 1.5 ppm, in in particular about 1.2 ppm), was used to manufacture a 3% CrMoV steel composition with an Si and Mn content greater than 0.2%. The chemical composition of the steel composition obtained is given in Table 1 below: <u> Table 1 - Chemical composition in% by mass of flow A except (*) in ppm </u> VS mn Yes Cr MB V P Or 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

The O ₂ content is between 7 and 12 ppm.

The casting was rolled into bars.

The mechanical properties obtained after heat treatment at 920 ° C for 20 minutes and returned to 545 ° C for 2 hours reach a hardness level of 46 HRC with a relatively fine grain size greater than an ASTM 10 index. The resilience KV at 20 ° C is 60 Joules minimum, the resilience at -40 ° C is 37.7J. The resilience is therefore less than 40J at -40 ° C.

Comparative example 2 - casting B (steel composition with Si and Mn contents greater than 0.2% and low P, As, Sb and Sn contents)

The casting is obtained by the same process as that of Example 1. The only difference concerns the chemical composition of the steel. It is shown in Table 2 below. <u> Table 2 - Chemical composition in% by mass of flow B except (*) in ppm </u> VS mn Yes Cr MB V P Or 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

The casting was rolled into bars. The resilience values at -40 ° C obtained for flow B with a low residual content (P, As, Sb and Sn), with heat treatments identical to those carried out on flow A, are 38.7J (average of 3 values). Thus, a very low P value, obtained thanks in particular to an elaboration process particularly followed in an electric furnace by a controlled insufflation of oxygen as well as in the control of the chemical quality of metallic and non-metallic additions, does not allow significantly increase the resilience values at low temperature (-40 ° C), as well as very low residual values As, Sb and Sn, the sum of which is for flow B of 62 ppm. The resilience is therefore less than 40J at - 40 ° C.

Example 1 - casting C (composition according to the invention)

The casting is obtained by 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. <u> Table 3 - Chemical composition in% by mass of flow C except (*) in ppm </u> VS mn Yes Cr MB V P Or 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

The casting was rolled into bars.

The resilience values at -40 ° C obtained for flow C with heat treatments identical to those carried out in flow A reach 43.3J 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 greater than or equal to 10.

The increase in resilience is significant compared to flows A and B (comparative examples 1 and 2) with a gain of around 15%.

Example 2 - casting D (composition according to the invention)

The casting is obtained by the same process as that of Example 1. The only difference concerns the chemical composition of the steel. It is shown in Table 4 below. <u> Table 4 - Chemical composition in% by mass of flow D except (*) in ppm </u> VS mn Yes Cr MB V P Or 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

The casting was rolled into bars.

The resilience values at -40 ° C obtained for casting C with heat treatments identical to those carried out on casting A reach 43 J on an average of 6 tests. The hardness obtained is between 46-48 HRC.

The increase in resilience is therefore also significant compared to flows A and B (comparative examples 1 and 2) with a gain of around 15%. The Si and Mn content (less than 0.20%) therefore has a significant impact on the resilience at -40 ° C.

Comparative example 3 - casting E

The casting is obtained by the same process as that of Example 1. The only difference concerns the chemical composition of the steel. It is shown in Table 5 below. <u> Table 5 - Chemical composition in% by mass of flow E except (*) in ppm </u> VS mn Yes Cr MB V P Or 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

The casting was rolled into bars.

For casting E, the resilience values at -40 ° C obtained with heat treatments strictly identical to those carried out in comparative examples 1 and 2 and in examples 1 and 2 on castings A to D have an average of 41.16J (average over 6 trials). The hardness range obtained is 46-48 HRC.

Thus, if the Mn content of the steel composition is greater than 0.200%, the toughness obtained KV at -40 ° C is lower than that obtained on flows C and D having Mn contents <0.200%, while remaining greater than 40J.

Example 4 Impact of the Income Treatment on the Resistance / Tenacity Compromise of the Composition According to the Invention

The casting C (example 1) after its preparation and its rolling into bars underwent a heat treatment at 920 ° C for 20 minutes then one or more steps of tempering at 545 ° C for 2 hours.

The mechanical properties obtained (KV resilience at -40 ° C and mechanical resistance Rm at room temperature) according to the number of tempers are shown in Table 6 below. <u> Table 6 - KV at -40 ° C and Rm at room temperature depending on the number of tempers at 545 ° C for 2 hours of casting C </u> Number of income Rm (MPa) Average KV (J) X1 1552 42.7 X2 1541 44.1 X3 1530 47.3 X4 1516 46.5

As illustrated, the more the number of incomes increases, the more the resilience at -40 ° C increases, with the exception of the fourth income for which there is a very slight decrease while still exhibiting a very good level. The fourth tempering treatment provides interesting results but the mechanical resistance is clearly lower and very close to the minimum 46HRC sought for this application.

The number of tempers is easily convertible into an equivalent treatment time for a single tempering operation at 545 ° C. Table 7 below shows that a single income treatment with a time corresponding to 2 income treatments at 545 ° C or 3 income treatments at 545 ° C gives very similar results. <u> Table 7 - KV at -40 ° C and Rm as a function of the tempering time at 545 ° C of flow C </u> Income time Rm (MPa) Average KV (J) 2 hours 1552 42.7 4 hours 1549 45.3 6 hours 1533 47.5

The adaptation of the number of incomes or its equivalent in time of income makes it possible to significantly increase the level of resilience. The gain compared to casting A treated under standard conditions is from 25% to 30% approximately for casting C.

It should be noted that this improvement results from the combination of a low Si content and low Mn content (less than 0.2%) with a 3% Cr-Mo-V base as shown in Table 8 below. <u> Table 8 - influence of the number of incomes at 545 ° C for 2 hours according to chemical composition on Rm and KV at -40 ° C </u> Number of income Rm (MPa) Average KV at - 40 ° C (J) Casting C (example 1) X1 1559 43.3 X2 1546 46 X3 1543 47.7 X4 1516 46.5 Casting E (example 3) X1 1532 41 X2 1524 43.7 X3 1511 44 X4 1516 44.7 Casting A (comparative example 1) X1 1542 37.7 X2 1532 39 X3 1528 35.7 X4 1516 34.7

Only the casting C allows to pass to a level of resilience greater than 45J with a number of tempering at 545 ° C adapted. The low content of silicon alone (less than 0.2%: casting E) makes it possible to increase the level of resilience up to approximately 44J. It should be noted that in the case of steel with a high content of Si and Mn (casting A), the number of incomes does not influence the level of resilience. The average value in resilience even tends to decrease significantly after the third income treatment.

Example 5 - Impact of the quenching temperature of the heat treatment on a flow F according to the invention : 920 ° C vs. 960 ° C

Casting F is obtained by the same process as that of Example 1. The only difference concerns the chemical composition of the steel. It is shown in Table 9 below. <u> Table 9 - Chemical composition in% by mass of flow F </u> VS mn Yes Cr MB V 0.30 0.19 0.19 3.1 1.1 0.28

The resilience value at -40 ° C obtained for casting F with a heat treatment with quenching at 920 ° C and a single income 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.

A high quenching temperature, at 960 ° C, therefore degrades the resilience of the steel.

Claims (14)

1. Steel composition essentially being constituted by:

   Carbon: 0.28-0.35;

   Manganese: 0.10-0.30, preferably 0.10-0.20;

   Silicon: 0.10-0.20;

   Chromium: 2.80-3.40;

   Molybdenum: 0.70-1.60, preferably 0.70-1.30;

   Vanadium: 0.20-0.50, preferably 0.20-0.40;

   Phosphorus: ≤ 0.005;

   Nickel: ≤ 0.10;

   Aluminium: ≤ 0.025, preferably 0.006-0.025;

   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.
2. Steel composition according to Claim 1, being constituted by:

   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, preferably 0.006-0.025;

   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. Steel composition according to either of Claims 1 and 2, characterized in that the molybdenum content is between 0.7 and 1.1, as weight percentages of the total composition.
4. Process for manufacturing a steel blank having the composition according to any one of Claims 1 to 3, characterized in that it comprises:

   a) a step of producing the steel;

   b) a step of transforming the steel;

   c) a heat treatment of the steel comprising quenching at a temperature of at least 900°C and a tempering treatment at a temperature of at least 530°C, advantageously between 530 and 550°C, for an overall time of between 2 and 6 hours, advantageously for an overall time of 4 hours.
5. Manufacturing process according to Claim 4, characterized in that step c) comprises several tempering treatments, the cumulative times of which correspond to the overall time of said step, advantageously two tempering treatments of 2 hours each.
6. Manufacturing process according to either of Claims 4 and 5, characterized in that the quenching temperature is between 900 and 930°C.
7. Manufacturing process according to any one of Claims 4 to 6, characterized in that step b) consists of a rolling step.
8. Manufacturing process according to any one of Claims 4 to 7, characterized in that the production step a) is performed in an electric arc furnace followed by vacuum arc degassing, optionally with a step of electroslag remelting (ESR) or vacuum arc remelting (VAR), or via VIM-VAR or VIM-ESR processes.
9. Steel blank that may be obtained via a process according to any one of Claims 4 to 8.
10. Steel blank according to Claim 9, characterized in that its hardness is between 46 and 48 HRC.
11. Steel blank according to either of Claims 9 and 10, characterized in that its resilience KV at -40°C is at least 40 joules.
12. Steel blank according to any one of Claims 9 to 11, characterized in that its mechanical strength Rm is between 1500 and 1600 MPa.
13. Use of a blank according to any one of Claims 9 to 12 or of a steel composition according to any one of Claims 1 to 3 for the manufacture of a pressure appliance component.
14. Use according to Claim 13, characterized in that the pressure appliance component is a barrel tube, in particular for small-calibre weapons.