FR3064282A1 - STEEL, PROCESS FOR THE MANUFACTURE OF MECHANICAL PARTS IN THIS STEEL, AND PARTS SO MANUFACTURED - Google Patents

Abstract

Hot-formed breakable piece steel, characterized in that its composition consists of, expressed in percentages by weight: 0.15% ≤ C ≤ 0.40%; preferably 0.20% ≤ C ≤ 0.35%; 0.60% ≤ Mn ≤ 1.80%; preferably 0.80% ≤ Mn ≤ 1.60%; Traces ≤ Cr ≤ 1.60%; Traces ≤ Mo ≤ 0.40%; Traces ≤ Ni ≤ 1.50%; - Traces ≤ Cu ≤ 0.80%; 0.02% ≤ V ≤ 0.70%; Traces ≤ Nb ≤ 0.08%; Traces ≤ If ≤ 1.20%; preferably traces ≤ Si ≤ 0.60%; Traces ≤ Al ≤ 0.10%; Traces ≤ B ≤ 0.010%, Traces ≤ Ti ≤ 0.10%; Traces ≤ S ≤ 0.15%; preferably 0.005% ≤ S ≤ 0.15%; Traces ≤ P ≤ 0.10%; Traces ≤ Ca ≤ 0.010%; Traces ≤ Te ≤ 0.030%; Traces ≤ Se ≤ 0.050%; Traces ≤ Bi ≤ 0.10% Traces ≤ Pb ≤ 0.20%; Traces ≤ N ≤ 0.025%; Traces ≤ 0 ≤ 0.008%; the rest being iron and impurities related to the elaboration; and: * V '% = V% + 2 Nb% + Cu% / 5 ≥0.18%; * Bs = 830 - 270C% - 90Mn% - 37Ni% - 70Cr% - 83Mb - 50Cu% - 100V% ≤ Bslim, with Bslim calculated as follows: Bslim = 530 + 330 (V '- 0.18), if 530 + 330 (V '- 0.18) is less than 600; Bslim = 600, if 530 + 330 (V '- 0,18) is greater than or equal to 600. A method of manufacturing a mechanical part from this steel, and mechanical part thus produced.

Description

® FRENCH REPUBLIC

NATIONAL INSTITUTE OF INDUSTRIAL PROPERTY © Publication number: 3,064,282 (to be used only for reproduction orders) (© National registration number: 17 52403

COURBEVOIE © Int Cl ⁸ : C 22 C 38/46 (2017.01), C 21 D 8/00, F16C 7/00

A1 PATENT APPLICATION

©) Date of filing: 23.03.17. © Applicant (s): ASCO INDUSTRIES - FR. (30) Priority: @ Inventor (s): SOURMAIL THOMAS, MAMINSKA KAROLINA and GALTIER ANDRE. (43) Date of public availability of the request: 28.09.18 Bulletin 18/39. ©) List of documents cited in the report preliminary research: Refer to end of present booklet (© References to other national documents ® Holder (s): ASCO INDUSTRIES. related: ©) Extension request (s): (© Agent (s): LAVOIX.

FR 3 064 282 - A1 (34) STEEL, PROCESS FOR THE MANUFACTURE OF MECHANICAL PARTS MADE OF THIS STEEL, AND PARTS THUS MANUFACTURED.

©) Steel for breakable part hot formed, characterized in that its composition consists of, expressed in weight percentages: 0.15% C 0.40%; preferably 0.20% C 0.35%; 0.60% Mn 1.80%; preferably

0.80% Mn 1.60%; Traces Cr 1.60%; Traces Mo

0.40%; Traces Ni 1.50%; - Cu traces 0.80%; 0.02% V

0.70%; Traces Nb 0.08%; Traces If 1.20%; preferably traces Si 0.60%; Traces Al 0.10%; Traces B

0.010%, Traces Ti 0.10%; Traces S 0.15%; preferably 0.005% S 0.15%; Traces P 0.10%; Traces Ca

0.010%; Traces Te 0.030%; Traces Se 0.050%; Traces

Bi 0.10% Traces Pb 0.20%; Traces N 0.025%; Traces

O 0.008%; the remainder being iron and impurities associated with processing; and:

* V '% = V% + 2 Nb% + Cu% / 5 0.18%;

* Bs = 830 - 270C% - 90Mn% - 37Ni% - 70Cr% - 83 MB%

- 50Cu% - 100V% BS | _im , with BS | _im calculated as follows:

BS | _im = 530 + 330 (V - 0.18), if 530 + 330 (V - 0.18) is less than 600;

BS | _im = 600, if 530 + 330 (V -0.18) is greater than or equal to 600.

Method of manufacturing a mechanical part from this steel, and mechanical part thus produced.

Steel, process for manufacturing mechanical parts from this steel, and parts thus manufactured

The present invention relates to the use of a steel of determined composition for the manufacture of a mechanical part, typically a breakable part, and in particular for the manufacture of a breakable rod for an internal combustion engine, and the process for obtaining of this room.

Certain mechanical parts, such as the rods of an internal combustion engine, consist of two separable elements assembled by fixing means such as screws. If, historically, in the case of connecting rods, the two parts were manufactured independently and then machined and assembled, the development of the controlled breaking process has become widespread over the past fifteen years. This process, by which a single piece is now formed, for example by forging, then broken in a controlled manner before reassembly using fixing means, has allowed a considerable simplification of the production lines.

For breakable parts made of steel, as is often the case for modern internal combustion engines, specific grades are used (for example the grade known as C70S6) or have been specially developed for this use.

For example, document EP-B1-0 779 375 describes a process for manufacturing a breakable part and the steel used for this purpose, the latter having a composition specifically adjusted to make the steel suitable for the breaking operation. controlled. Likewise, document EP-B1-1 051 531 applies to the same technical field, and describes a grade designed to improve the performance of the parts produced, in particular by the use of a significant addition of vanadium (0.20- 0.50% by weight).

Other solutions have been developed to offer steels suitable for the manufacture of breakable forgings such as connecting rods. Mention may be made, in particular, of documents EP-B2-0 856 590, EP-B1-1 070 153, EP-A1-1 243 665, US-A-2006/000088, JP-A-2002356743.

Two important points should be noted about all of the steels described in the documents cited above.

First of all, all of the steels in the above documents have a ferrito-pearlitic structure with, in some cases, a controlled ferrite content.

On the other hand, these grades are specifically designed so as not to require heat treatment after forging.

These nuances are thus implemented in a process typically comprising the following steps:

First, we supply a piece obtained by sawing or shearing a rolled bar.

Then the piece is heated to temperatures typically between 1050 and 1280 ° C.

Then the piece thus heated is given the desired shape, by forging.

Then let the room cool down, usually in air.

Then we proceed to a cleaning, for example by sandblasting, and to the machining of the part.

Finally, we proceed to the controlled breaking of the part, and to its reassembly, possibly with finishing machining operations.

In the vast majority of processes, the mechanical characteristics are obtained at the end of cooling, and can moreover be modulated by the use of accelerated cooling (use of fans to obtain forced convection) or slowed down (setting room body) compared to natural cooling in the open air and calm. In very rare cases and on very specific applications, a quenching and tempering heat treatment can be used, to achieve superior mechanical characteristics, but such production ranges are not compatible with the manufacture of breakable rods.

In order to respond to the ever-increasing stresses encountered by mechanical parts in internal combustion engines, special steels for the manufacture of forgings have undergone constant evolution. In the case of ferrito-perlitic steels, the use of vanadium contents of the order of 0.2-0.3% has made it possible to achieve high levels of mechanical characteristics without requiring heat treatment (EP- B1-1 051 531). Thus, for example, on breakable connecting rods for the automobile (of typical section equivalent to a cylinder of 10 to 25 mm in diameter) using the ferrito-pearlitic grades 36MnV4S or 46MnVS6mod (K. Lipp and H. Kauffman, Schmiede- und Sinterschmiede-Werkstoffe für PKW-Pleuel, MTZ, 2011), elastic limits of up to 850 MPa are measured, compared to 600-650 MPa on more conventional materials. This is accompanied, of course, by high values of mechanical strength, but with the counterpart a significant reduction in machinability.

However, nuances of this type have reached their limits for this type of application. A first limitation resides in the fact that the mechanical properties mentioned above can only be obtained on parts of modest dimensions (connecting rods for engines of light vehicles for example), and they decrease rapidly with the increase in the dimension of the components. Thus, the grade 36MnV4S, which makes it possible to obtain a conventional elastic limit Rp ₀₂ of 850 MPa on an automotive connecting rod (K, Lipp and H, Kauffman, Schmiede- und SinterschmiedeWerkstoffe für PKW-Pleuel, MTZ, 2011), has an Rp ₀₂ of 700 MPa on a 55 mm diameter bar. A second limitation is due to the great sensitivity to the cooling conditions of the results obtained with these grades (already reflected in the sensitivity to the dimensions of the part), when they are used to the maximum of their performance. Therefore, if it is possible, in the laboratory, to obtain Rp ₀₂ values reaching 900-940 MPa, these results are difficult to transpose to industrial production.

Current technical solutions for the manufacture of breakable parts, and in particular connecting rods, are therefore today limited to the performance levels mentioned above.

An obvious development at first glance for those skilled in the art would consist, for example, in further increasing the V contents. This would however be ineffective since only vanadium in solid solution before cooling can contribute to hardening by precipitation. However, at contents higher than 0.35-0.40%, it becomes difficult to carry out a complete solution solution of the precipitates containing V, even for the commonly used forging temperatures (of the order of 1250 ° C. ).

The purpose of the invention is to provide a shade and a method of manufacturing breakable parts allowing the limitations outlined above to be overcome, in particular with:

- the possibility of obtaining high mechanical characteristics, resulting in a measurable benefit in fatigue and compaction performance;

- The possibility of obtaining these mechanical characteristics in a robust manner, that is to say of making these characteristics insensitive to fluctuations in the parameters of the shaping process.

To this end, the subject of the invention is a steel for breakable part hot formed, characterized in that its composition consists of, expressed in percentages by weight:

- 0.15% <C <0.40%; preferably 0.20% <C <0.35%;

- 0.60% <Mn <1.80%; preferably 0.80% <Mn <1.60%;

- Traces <Cr <1.60%;

- Traces <Mo <0.40%;

- Traces <Ni <1.50%; preferably traces <Ni <1.0%; better traces <Ni <0.60%;

- Traces <Cu <0.80%;

- 0.02% <V <0.70%;

- Traces <Nb <0.08%;

- Traces <Si <1.20%; preferably traces <Si <0.60%;

- Traces <Al <0.10%;

- Traces <B <0.010%,

- Traces <Ti <0.10%;

- Traces <S <0.15%; preferably 0.005% <S <0.15%;

- Traces <P <0.10%;

- Traces <Ca <0.010%;

- Traces <Te <0.030%;

- Traces <Se <0.050%;

- Traces <Bi <0.10%

- Traces <Pb <0.20%;

- Traces <N <0.025%;

- Traces <O <0.008%;

the remainder being iron and impurities associated with processing; and for which the following relationships are verified:

* V ’= V% + 2 Nb% + Cu% / 5> 0.18%;

* Bs = 830 - 270C% - 90Mn% - 37Ni% - 70Cr% - 83 Mo% - 50Cu% - 100V% <Bsiim, with Bsi _im calculated as follows:

BS | _im = 530 + 330 (V '- 0.18), if 530 + 330 (V' - 0.18) is less than 600;

BS | _im = 600, if 530 + 330 (V '- 0.18) is greater than or equal to 600.

Preferably, Ti> 3.5 N% if B> 0.0010%.

In this case, preferably traces <N <0.010%.

The invention also relates to a steel having the above composition, characterized in that its microstructure comprises at least 60% of a mixture of bainitic ferrite and carbides or residual austenite, the rest of the microstructure comprising at most 40 % of martensite and / or pro-eutectoid ferrite and / or perlite, the presence of pro-eutectoid ferrite and / or perlite being limited to at most 10%.

The invention also relates to a process for manufacturing a mechanical part, characterized in that:

- A steel semi-finished product is hot formed in the austenitic phase, the composition of which is expressed in weight percentages:

* 0.15% <C <0.40%; preferably 0.20% <C <0.35%;

* 0.60% <Mn <1.80%; preferably 0.80% <Mn <1.60%;

* T breeds <Cr <1.60%;

* T breeds <Mo <0.40%;

* Traces <Ni <1.50%; preferably traces <Ni <1.0%; better traces <Ni <0.60%;

* T races <Cu <0.80%;

* 0.02% <V <0.70%;

* T breeds <Nb <0.08%;

* T breeds <Si <1.20%; preferably traces <Si <0.60%;

* Traces <Al <0.10%;

* T races <B <0.010%, * Traces <Ti <0.10%;

* T breeds <S <0.15%; preferably 0.005% <S <0.15%;

* Traces <P <0.10%;

* T breeds <Ca <0.010%;

* T races <Te <0.030%;

* T races <Se <0.050%;

* Traces <Bi <0.10% * T races <Pb <0.20%;

* Traces <N <0.025;

* T breeds <O <0.008%;

the remainder being iron and impurities associated with processing; and for which the following relationships are verified:

* V ’= V% + 2 Nb% + Cu% / 5> 0.18%;

* Bs = 830 - 270C% - 90Mn% - 37Ni% - 70Cr% - 83 Mo% - 50Cu% - 100V% <BS | _irT1 , with BS | _im calculated as follows:

Bsiim = 530 + 330 (V ’- 0.18), if 530 + 330 (V’ - 0.18) is less than 600;

Bsiim = 600, if 530 + 330 (V ’- 0.18) is greater than or equal to 600;

- Cooling is carried out with calm air, or with forced air, or under a hood or in the case of said semi-finished product, preferably at cooling rates greater than or equal to 0.3 ° C / s in l 'range 750-550 ° C and between 0.1 and 5 ° C / s in the range 550-300 ° C;

- At least one tempering of said semi-finished product cooled to a temperature Trev of between 450 and 680 ° C. is carried out for a total duration of 15 min to 10 h.

Preferably, Ti> 3.5 N% if B> 0.0010%.

In this case, preferably traces <N <0.010%.

After cooling after hot forming, it is possible to form the semi-finished product, for example by machining or shaping, which brings the dimensions of the semi-finished product closer to the precise final dimensions of the product without modifying its microstructure.

After the tempering, the semi-finished product can be shaped, for example by machining or shaping, which gives the semi-finished product the precise final dimensions of the product without modifying its microstructure.

One can proceed to a controlled breakage of said mechanical part, if the latter is a breakable mechanical part.

It is possible to carry out at least one additional heat treatment after said income, said additional treatment having an income parameter lower than that of said income.

The invention also relates to a mechanical steel part, characterized in that:

- its composition consists of, expressed in weight percentages:

* 0.15% <C <0.40%; preferably 0.20% <C <0.35%;

* 0.60% <Mn <1.80%; preferably 0.80% <Mn <1.60%;

* T breeds <Cr <1.60%;

* T breeds <Mo <0.40%;

* Traces <Ni <1.50%; preferably traces <Ni <1.0%; better traces <Ni <0.60%;

* T races <Cu <0.80%;

* 0.02% <V <0.70%;

* T breeds <Nb <0.08%;

* T breeds <Si <1.20%; preferably traces <Si <0.60%;

* Traces <Al <0.10%;

* T races <B <0.010%, * Traces <Ti <0.10%;

* T breeds <S <0.15%; preferably 0.005% <S <0.15%;

* Traces <P <0.10%;

* T breeds <Ca <0.010%;

* T races <Te <0.030%;

* T races <Se <0.050%;

* Traces <Bi <0.10% * T races <Pb <0.20%;

* Traces <N <0.025;

* T breeds <O <0.008%;

the remainder being iron and impurities associated with processing; and for which the following relationships are verified:

* V ’= V% + 2 Nb% + Cu% / 5> 0.18%;

* Bs = 830 - 270C% - 90Mn% - 37Ni% - 70Cr% - 83 Mo% - 50Cu% - 100V% <BS | _irT1 , with BS | _im calculated as follows:

Bsiim = 530 + 330 (V ’- 0.18), if 530 + 330 (V’ - 0.18) is less than 600;

Bsiim = 600, if 530 + 330 (V ’- 0.18) is greater than or equal to 600;

and whose microstructure comprises at least 60% of a mixture of bainitic ferrite and carbides or residual austenite, the rest of the microstructure comprising at most 40% martensite and / or pro-eutectoid ferrite and / or perlite , the presence of proeutectoid ferrite and / or perlite being limited to at most 10%.

Preferably, Ti> 3.5 N% if B> 0.0010%.

In this case, preferably traces <N <0.010%.

It can be a breakable piece.

It can be a connecting rod of an internal combustion engine.

As will be understood, the invention first of all consists in preparing a steel of precise composition, giving it a temperature Bs at the start of formation of the bainite relatively low and a relationship linking the contents of V, Nb and Cu ensuring d on the one hand the high performance of the part, and on the other hand a good breakability of the final product by facilitating the breakage of fragile type. The hot shaping and subsequent cooling, which is cooling at a moderate speed provided by calm air, forced air, or placing the semi-finished product under the hood or in the crate, provides the semi-finished product. produces a predominantly bainitic structure, the remainder of which (up to 40% maximum) is essentially martensite with a quantity of proeutectoid ferrite + perlite not exceeding 10%. Precipitation income is then used to adjust the mechanical properties.

In a whole other field than that of breakable parts, the so-called “bainitic” nuances (see documents EP-B1-0 787 812, S. Engineer, H. Justinger, P. Janssen, M. Hàrtel, C. Hampel, F Randelhoff, Int Conf Steel in Cars and Trucks,

Salzburg, Austria, 2011, and H. Roelofs, S. Hasler, M. Lembke, FG Caballero, Int Conf Steel in Cars and Trucks, Salzburg, Austria, 2011) are becoming increasingly popular as a substitute for hardened steels. revenues (in relation to which they avoid heat treatment), or to replace microalloyed ferritoperlitic steels (for example 38MnSiV5) with a performance gain.

It will be recalled that these bainitic grades have suitable compositions so as to obtain, with little or even no cooling control, a predominantly bainitic microstructure in the hot forge. The use of the term "bainitic nuance" in the rest of the document should be understood in this sense, and what is meant by "predominantly bainitic" will be explained below.

In addition to the advantage of eliminating the quenching and tempering heat treatments in certain production lines, these grades generally have higher impact flexural strengths (resilience) than ferritoperlitic grades with the same mechanical strength. In the case of breakable parts, however, this advantage becomes a major drawback, since the controlled breaking operation requires low resistance to impact bending.

Another major drawback of bainitic grades of continuous cooling is that they often have modest values for their Rp ₀₂ / Rm ratio. Thus, it is frequent to obtain Rp ₀ , ₂ of the order of 650-750 MPa, for mechanical strengths, however, of the order of 1100-1200 MPa. This difference in behavior is all the more marked if we consider the true elastic limit, which may be much lower than the conventional limit for bainitic microstructures as obtained after continuous cooling (and this, by comparison with ferrito-pearlitic microstructures).

However, at least for the particular case of breakable rods, one of the dimensioning criteria is, in addition to the resistance to fatigue, the resistance to compression compaction. It is, in a simplified way, to prevent any permanent deformation of the connecting rod during one or a few significant overload (s) but of limited duration. This resistance is generally evaluated over a few cycles only (order of magnitude of ten). As a first approximation, it is correlated with the true elastic limit, so that the use of bainitic shades as defined at present leads to performances far behind those of ferritoperlitic shades.

As a corollary, a second major disadvantage of the low ratio Rp _{0, 2} / Rm of bainitic shades is that it will take to obtain a yield strength similar to that of shades ferritic-pearlitic, increase the Rm significantly, with the consequence of a very strong deterioration of the machinability.

It is therefore clear that the bainitic nuances of continuous cooling, as they are currently used, do not allow to easily envisage their use for the manufacture of a breakable part, and more particularly of a connecting rod for an internal combustion engine. . This solution would indeed have only drawbacks compared to micro-alloy ferrito-pearlitic steels, which is well reflected in the fact that only this microstructure is found on this type of parts as they are currently marketed.

Surprisingly, the inventors concluded that, to overcome the difficulties mentioned above, it was possible to use a shade leading to a predominantly bainitic microstructure in the forge hot, even in the absence of cooling control, or only with a little restrictive control, as allowed by equipment commonly found in blacksmiths, namely a conveyor with or without cover, fans or a cooling box). The invention further consists in modifying the manufacturing process of the part, in order to integrate therein a precipitation income after forging, and in using very specific adjustments of the composition, which are such that spectacular performance gains are obtained. , and largely compensate for the cost of the lengthening of the production which involves the execution of the precipitation income.

We specify here what is meant by "majority bainite" within the meaning of the invention:

- the microstructure comprises a mixture of bainitic ferrite (either conventional, ie germinated at the grain boundaries, or germinated intragranularly, which gives a structure also called "acicular ferrite") and carbides or austenite residual, the whole, commonly known as "bainite" by those skilled in the art, being present at a level of at least 60%;

- the rest of the microstructure, up to a maximum of 40%, consists mainly of martensite in addition;

- on the other hand, the presence of pro-eutectoid ferrite or perlite at contents greater than 10% is prohibited for the total of these two constituents; in fact, in pro-eutectoid ferrite (as present in ferrito-pearlitic shades), vanadium tends to precipitate during processing, so that the hardening potential on tempering is low or even zero; the same goes for the lamellar ferrite present in the perlite, and which is therefore included in the constituents which, taken together, must not exceed 10% of the microstructure.

In other words, the microstructure of the steel must contain at least 60% of bainite in the conventional sense for those skilled in the art and at most 40% of other constituents, among which the pro-eutectoid ferrite and the perlite must not, taken together, represent more than 10% of the microstructure.

According to the invention, the steel has the composition, in weight percent:

- 0.15% <C <0.40%; preferably 0.20% <C <0.35%;

- 0.60% <Mn <1.80%; preferably 0.80% <Mn <1.60%;

- Traces <Cr <1.60%;

- Traces <Mo <0.40%;

- Traces <Ni <1.50%; preferably traces <Ni <1.0%; better traces <Ni <0.60%;

- Traces <Cu <0.80%;

- 0.02% <V <0.70%;

- Traces <Nb <0.08%;

- Traces <Si <1.20%; preferably traces <Si <0.60%;

- Traces <Al <0.10%;

- Traces <B <0.010%,

- Traces <Ti <0.10%;

- Traces <S <0.15%; preferably 0.005% <S <0.15%;

- Traces <P <0.10%;

- Traces <Ca <0.010%;

- Traces <Te <0.030%;

- Traces <Se <0.050%;

- Traces <Bi <0.10%

- Traces <Pb <0.20%;

- Traces <N <0.025%;

- Traces <O <0.008%;

the remainder being iron and impurities associated with processing; and for which the following relationships are verified:

* V ’= V% + 2 Nb% + Cu% / 5> 0.18%.

* Bs = 830 - 270C% - 90Mn% - 37Ni% - 70Cr% - 83 Mo% - 50Cu% - 100V% <BS | _im , with BS | _im calculated as follows:

Bsiim = 530 + 330 (V ’- 0.18), if 530 + 330 (V’ - 0.18) is less than 600;

Bsiim = 600, if 530 + 330 (V ’- 0.18) is greater than or equal to 600.

Preferably, Ti> 3.5 N% if B> 0.0010%.

In this case, preferably traces <N <0.010%.

Said steel is used for the manufacture of a mechanical part, typically of a breakable part, according to a process characterized in that it has at least the following steps:

- A steel of composition conforming to the above description is poured and solidified;

- one or more hot forming (s) is carried out, in the austenitic field, typically by rolling and / or hot forging;

- the product obtained is allowed to cool in air, optionally modulating the cooling rate using standard tools which are available to blacksmiths (box, conveyor with or without hood, fans); this natural cooling, possibly slowed down or accelerated in moderate proportions must give the room a predominantly bainitic structure according to the definition given above; it typically corresponds to cooling rates greater than or equal to 0.3 ° C / s in the range 750-550 ° C è between 0.1 and 5 ° C / s in the range 550-300 ° C;

- we proceed to an income at a temperature Trev between 450 and 680 ° C, for a period of 15 min to 10 h depending on the dimensions of the room and the equipment available, with the double objective of increasing the limit of true elasticity of the part by precipitation while retaining or significantly reducing its mechanical strength, and greatly reducing its resistance to impact bending to make the material breakable; although this is not a priori desirable for economic reasons, this simple income could be replaced by several shorter incomes and / or at lower temperature according to the well-known principles dictating the choice of the parameters of income, without this can be considered as departing from the present invention;

- We proceed to a controlled breaking operation of the part by a conventional process exploiting the low resistance to impact bending of the material.

One or more machining operations can take place at different stages of the implementation described above.

One or more additional heat treatments, each carried out at a temperature T for a period t, may be carried out during the implementation described above, subsequently to precipitation income. . However, the total income parameter of these additional treatments must not exceed the income parameter of precipitation hardening income which allowed the establishment of the mechanical characteristics, so as not to frankly modify them. It is recalled that the “income parameter” is conventionally defined, for each treatment, by M = T (20 + log t). 10 ³ (with T in K and t in h).

The part concerned may be, but is not limited to, a connecting rod for an internal combustion engine.

The invention will be better understood on reading the description which follows.

Table 1 compares the possibilities typically offered by the various ferrito-perlitic solutions mentioned above with those of the present invention.

Shade Rpo, 2 / Rm typical, connecting rod automotive MPa Rpo, 2 / Rm typical, connecting rod weight heavy MPa C70S6 (forged state) 650-700 / 950-1050 520-570 / 900-1000 36MnV4S (forged state) 850-920 / 1100-1200 720-750 / 1000-1100 Invention (forged state then income) > 900 /> 1100 > 900 /> 1100

Table 1: Conventional elastic limits and tensile strengths typical of connecting rods produced according to the prior art and according to the invention

Table 1 above shows the conventional elastic limits Rp ₀ , ₂ and the tensile strengths Rm typically expected on parts of different dimensions (connecting rods for cars on the one hand, connecting rods for heavy goods vehicles on the other hand) , the maximum values for the reference grades being those which are accessible if the cooling following forging is carried out under optimized conditions.

The standard grade C70S6 does not allow an elastic limit of around 650 MPa to be exceeded, even on small parts. The 36MnV4S grade currently has the best mechanical performance (see for example Lipp and Kaufmann, MTZ, 2011, p.70), but nevertheless remains, most typically, limited to an elastic limit of 850 MPa on small piece, which drops to 720-750 MPa on a large piece.

In comparison, the grade used in the invention allows, forged and tempered condition, to reach and exceed largely a conventional yield strength Rp _{0, 2} 900 MPa. However, the conventional elastic limit is, as a first approximation, a good indicator of the service performance of the connecting rods. As shown in Table 1, this is moreover possible much less dependent on the size of the rod than for ferrito-pearlitic grades. In addition, excellent tensile strength is retained, again regardless of the size of the rod.

It should be noted that all of the nuances of the examples in Table 1 have values of KV typically less than 10 J, and therefore corresponds, from this point of view, to use for the manufacture of breakable parts.

We will now justify the choice of ranges of composition for the various elements of the invention. As mentioned, all contents are given as percentages by weight.

The C content is between 0.15 and 0.40%, preferably between 0.20 and

0.35%.

As we have seen, the microstructure required for the invention is mainly bainitic, and this microstructure must be obtained during cooling after hot forming. A C content greater than 0.15% makes it possible to limit the additions necessary for the removal of pro-eutectoid ferrite (Mo for example), the presence of which is not desirable in the context of the invention. However, C having a delaying effect on the bainitic transformation, its content is limited to 0.40% to avoid excessive formation of martensite.

The choice of the preferred range 0.20-0.35% makes it possible to avoid more assuredly both an excessive presence of ferrite (for the low limit) and martensite (for the high limit).

The Mn content is between 0.60 and 1.80%, preferably between 0.80 and

1.60%.

Mn is used, together with Cr, to lower the temperature Bs at the start of bainite formation during continuous cooling. This effect is significantly obtained from 0.60% Mn. However, it is well known that Mn contributes significantly to the formation of segregated bands which, given the contents of C used, are particularly harmful because they can lead, depending on the cooling path, to the formation of martensitic bands of very high hardness. For this reason, the maximum content of Mn is limited to 1.80%. The preferred range of 0.80-1.60% more effectively achieves lowering of Bs and more surely avoids excessive segregation.

The Cr content is between traces and 1.60%.

In the present invention, Cr is used in the same way as Mn, to lower the temperature Bs at the start of bainitic transformation. Within the limits imposed, it can be used in partial or total substitution of part of the Mn and / or all or part of the Ni, provided that the condition on Bsi _im remains verified (this condition being detailed below). The Cr content is limited to 1.60% to limit the problems of segregation. Cr may only be present in traces resulting from processing and, therefore, may not be added voluntarily, if, moreover, the contents of the other elements make it possible to comply with the condition required on BS | _im .

The Mo content is between traces and 0.40%.

Mo’s role in hardenability is well established: it helps prevent the formation of ferrite and perlite, but it does not slow or slow the formation of bainite. It can therefore be added in variable quantities depending on the diameter of the part. The upper limit of 0.40% is established mainly for economic reasons.

The Ni content is between traces and 1.50%; preferably between traces and 1.0%, better between traces and 0.60%.

It can be present only by its introduction by the raw materials as a residual element, or it can be added in sufficient quantity to contribute to the reduction of the temperature BS | _im , in which case it may partially replace Mn, Cr, or Mo. But its content is limited to 1.50% for cost reasons, Ni being expensive and likely to see its price fluctuate strongly on the market.

The Cu content is between traces and 0.80%.

Cu can be used to contribute to the secondary hardening which will communicate to the part its final characteristics, but contents higher than 0.80% are generally to be avoided because they are associated with significant difficulties of hot forming. Its addition is not compulsory, provided that the contents of other elements guarantee a temperature BS | _im sufficiently low and that the relation between V, Nb and Cu which will be seen later is respected.

The V content is between 0.02 and 0.70%.

The addition of V makes it possible on the one hand to contribute to the hardenability of the steel, and on the other hand to control, in cooperation with Cu and Nb if they are present, the secondary hardening which will give the part its characteristics mechanical. A minimum of V is necessary because, as we can see, the only addition of Cu does not allow the verification of the criterion V% + 2 Nb% + Cu% / 5> 0.18% which will be seen later. It is further known that the additions of Nb to obtain hardening by precipitation with tempering are effective only in addition to a minimum of V but not independently. Its addition is moreover limited to 0.70% for technical reasons (absence of effect beyond this content due to the non-re-solution of the precipitates during reheating) and economical (high cost).

The contents of V, Nb and Cu must also verify the following relationship:

V ’= V% + 2 Nb% + Cu% / 5> 0.18%.

This relationship is an important condition of the invention, because it guarantees not only a significant increase in mechanical properties during tempering, but also the obtaining of embrittlement which will lead to the obtaining of a breakable shade with high performance.

The contents of C, Mn, Cr, Ni, Mo, Cu and V expressed in% by weight must, moreover, verify the following relation which determines the temperature Bs (in ° C) at the start of the formation of bainite:

Bs = 830 - 270 C% - 90 Mn% - 37 Ni% - 70 Cr% - 83 Mo% - 50 Cu% - 100 V% <Bs, _im with Bsiim calculated as follows:

Bsiim = 530 + 330 (V '- 0.18) in the case where 530 + 330 (V' - 0.18) is less than 600 (note that the requirement on V 'leads to a low limit value of 530 );

Bsiim = 600 in the case where 530 + 330 (V ’- 0.18) is greater than or equal to 600.

The fineness of the microstructure allows, when this relationship is verified, to obtain mechanical characteristics after forging sufficiently high that the secondary hardening brings them to the objective of the invention.

As will be understood, however, it is possible, within certain limits, to tolerate mechanical characteristics before tempering which are all the weaker as the hardening power of tempering (measured by V ’) is high.

Thus, we correlate the requirement on Bs with hardening power, so that the calculated value Bs must be less than or equal to 530 ° C if the value of V 'is at its minimum, but may be less than or equal to another limit temperature BS | _im , which can go up to 600 ° C., if the hardening power, as expressed by the formula V ′, is higher than its minimum.

Beyond BS | _im = 600 ° C, however, compensation for the weakness of the mechanical properties by tempering becomes difficult, as does the ability to obtain the microstructure of the invention. For this reason, a value of Bs greater than 600 ° C. is not tolerated, regardless of the value of V '.

The Nb content is between traces and 0.08%.

This optional element can be used to refine the austenitic structure after forging or hot rolling. It is also involved in the secondary hardening reaction and can, in the latter, replace a part of the V insofar as the equation defining V ’seen previously, connecting the elements precipitating during tempering, is verified. Its addition is limited to 0.08%, not only because it is difficult to re-dissolve the precipitates formed above this content, but also to limit the formation of primary carbonitrides during solidification.

The Si content is between traces and 1.20%; preferably Si is between traces and 0.60%.

Si is sometimes used to slow the formation of cementite and promote the development of a microstructure with residual austenite at room temperature. In the context of the invention, the residual austenite being destabilized during tempering and therefore not being useful, large additions of Si are not necessary. Silicon can also be used for the deoxidation of liquid metal, as an alternative to aluminum.

The Al content is between traces and 0.10%.

Al is optionally added to ensure the deoxidation of the steel, particularly if the Si content is relatively low. Additions are capped at 0.10% to limit the risk of reoxidation inclusions forming on contact of the metalliquide with air.

The O content is between traces and 0.008%.

The presence of oxygen is limited to 0.008% to avoid deterioration of the mechanical performance of the parts.

The B content is between traces and 0.010%.

This optional element can be used to guarantee the uniformity of the structure (limit the presence of ferrite). In this case, it may be preferable to couple the addition of B with an addition of Ti which captures the nitrogen to form nitrides, and avoids the formation of nitrides of B. Thus all the B is available in solution to play its role of homogenizer of the structure.

The Ti content is between traces and 0.10%.

As we have just said, this optional element can be used mainly for nuances with B. In the case of an elaboration with B (corresponding to B> 0.0010%), preferably N is limited to 100 ppm (0.010 %) and it is ensured that the addition of Ti satisfies the condition Ti%> 3.5 N%.

The S content is between traces and 0.15%.

As is well known, this element can, if necessary, be left at a relatively high level, or even added voluntarily, to improve the machinability of the steel. It is then given a content of 0.005 to 0.15%. Preferably, this significant presence of S can then be accompanied by an addition of Ca up to 0.010%, and / or Te up to 0.030%, and / or Se up to 0.050%, and / or Bi up to 0.10% and / or Pb up to 0.20%.

The P content is between traces and 0.10%.

P can be used for its hardening power and for its embrittling power. However, its content is limited to 0.10%, because these contents can lead to difficulties in preparation, rolling and forging.

The N content is between traces and 0.025%.

In the case of the joint use of B and Ti (that is to say B contents of at least 0.0010% and Ti contents of at least 0.005%), preferably N is limited to 0.010% to limit the formation of boron nitrides.

On the other hand, in the contrary case, it will be possible to accept or impose higher N contents, so as to obtain a more intense precipitation of the V. The N content is naturally limited by the solubility of nitrogen in liquid steel to contents approaching 0.025%.

The other elements contained in the steel according to the invention are iron and impurities resulting from the production, present at usual contents taking into account the raw materials used and the method of production of the liquid steel (use of a converter or an electric arc furnace to obtain the liquid metal, treatment under vacuum or not of the liquid metal ...).

It should be understood that the possible preferential contents of each element taken in isolation are independent of each other. In other words, we can be in a preferential range for one or more of these elements and outside the preferential ranges for the other elements which include one.

Industrially, the part according to the invention can be produced by hot forming a semi-finished product such as a piece, a bar or a bloom, having the composition described above. Typically, this hot forming is hot forging or hot rolling or a succession of such steps. The part according to the invention can also be produced by machining ready-to-use bars, insofar as the method of manufacturing the latter corresponds to the steps of the method described.

In the first case, the process according to the invention involves a hot forming step carried out in the austenitic phase (typically but not exclusively from 1100 to 1300 ° C., as the precise composition of the steel must be taken into account for determine if it is in the austenitic phase at a given temperature but also to ensure the complete solution solution of the elements causing the secondary hardening), typically followed by natural cooling (that is to say in air, without use of a powerful cooling medium such as water, oil, a cryogenic medium such as dry ice or liquid nitrogen).

One of the important points of the invention is the possibility of obtaining relatively constant mechanical characteristics before tempering, regardless of the dimensions of the shaped part, such as its maximum section.

This is notably based on obtaining a homogeneous bainitic microstructure during cooling, which can be carried out naturally, in calm air.

However, if the installations allow it, an adaptation of the cooling may in certain cases be used, in particular to obtain initial mechanical characteristics superior to those which would be obtained by natural cooling with calm air. Supply air cooling may be sufficient to achieve this objective. However, care should be taken that the cooling is not too rapid to the point of causing a massive appearance of martensite which would reduce the proportion of bainite to less than 60%.

Conversely, a slowed-down cooling, in particular by a cover covering the part which has just been shaped, or a crating of the shaped part, can be used in certain cases to ensure a sufficient presence of bainite.

Preferably, this cooling is carried out at speeds greater than or equal to 0.3 ° C / s in the interval 750-550 ° C and taken between 0.1 and 5 ° C / s in the interval 550-300 ° vs.

Optionally, one or more operations are then carried out such as shaping, machining, etc. intended to at least bring the dimensions of the semi-product closer to the final dimensions of the final product, during which the thermal state of the part is not significantly affected and its microstructure is therefore preserved.

The process then involves, according to the invention, a tempering step, at a temperature chosen to optimize hardening by precipitation, which concomitantly leads to a reduction in ductility and makes the part "breakable". Usually, the proportions of the various constituents obtained after cooling after hot shaping are not affected by this income, or are little affected. The microstructural modifications introduced by the income relate very mainly to the nature and the number of the various precipitates which participate in hardening.

This tempering treatment is carried out at a temperature Trev between 450 and 680 ° C, for a period of 15 min to 10 h depending on the dimensions of the room and the equipment available. As an alternative to a simple income, it is possible to carry out several incomes, in the same range of time and temperature, according to the usual and well-known principles dictating the choice of the income parameters, without this being considered to be deviating. of the present invention.

As mentioned above, it is conceivable that other heat treatments may be carried out after the precipitation hardening income. But their parameter of total income should not exceed that of precipitation income.

It should be emphasized that such a hot shaping treatment followed by tempering is completely out of step with current industrial practice, which is based almost exclusively on the use of micro-alloyed ferrito-pearlitic grades, for which a such income would be of no technical interest.

Optionally, one or more operations such as shaping, machining, etc. are then carried out which do not modify the microstructure obtained thanks to the tempering according to the invention, so as to give the part its precise final dimensions if these n have not already been reached at the end of the previous steps. It is possible that, in certain cases, the heat treatments have led to a deformation of the part more or less sensitive, so that this final adjustment of the precise dimensions of the part is necessary, in particular if there is no had machining, shaping or other similar operation since hot forming and subsequent cooling.

We will now present results obtained in the laboratory on steel compositions according to the invention (lnv. 1 to 4), compositions close to these but not in accordance with the invention (Ref. 1 and 2), and compositions of two steels of known grades for the manufacture of connecting rods.

All of the compositions selected are presented below in Table 2, which includes the calculations of the values of Bs and V ’. It should be understood that the complement to 100% of the compositions cited is Fe, the elements not mentioned in this table being present only in the form of traces including those which may only be optionally present in the invention, such as the machinability elements Ca, Te, Se, Bi, Pb which have not been added in the examples considered. The values in bold are those which are not in accordance with the invention.

vs % Yes % Mn % Cr % Or % Cu % Mo % V % Nb % Al % B % Ti % s % P % NOT % 0 % 36MnV4S 0.36 0.50 1.00 0.12 0.10 0.10 0.04 0.32 0.005 0.001 0.0004 0.001 0.079 0.007 0.0180 0.0027 C70S6 0.69 0.16 0.59 0.12 0.09 0.12 0.03 0.004 0.001 0.006 0.0004 0.001 0.059 0.013 0.0080 0.0012 lnv.1 0.32 0.78 1.17 1.10 0.16 0.65 0.11 0.19 0.003 0.013 0.0031 0.029 0.069 0.01 0.0075 0.0009 lnv.2 0.30 0.15 1.08 0.87 0.10 0.16 0.06 0.46 0.005 0.017 0.0031 0.025 0.054 0.012 0.0056 0.0027 lnv.3 0.35 0.15 1.37 0.76 0.10 0.16 0.06 0.35 0.002 0.018 0.0033 0.025 0.0056 0.012 0.0053 0.0021 lnv.4 0.16 0.16 1.40 0.79 0.15 0.29 0.10 0.76 0,000 0.014 0.0029 0.028 0.026 0.011 0.0075 0.0017 Ref. 1 0.30 0.70 1.17 0.51 0.15 0.15 0.10 0.20 0.002 0.025 0.0003 0.002 0.08 0.025 0.0120 0.0015 Ref. 2 0.37 0.38 1.33 0.73 0.15 0.15 0.08 0.12 0.003 0.012 0.0003 0.002 0.051 0.012 0.0136 0.0011

V % BSiim ° c Bs ° C 36MnV4S 0.35 576 590 C70S6 0.03 570 570 Inv. 1 0.33 586 495 lnv.2 0.50 530 528 lnv.3 0.39 570 507 lnv.4 0.82 589 501 Ref. 1 0.23 530 567 Ref. 2 0.16 530 529

Table 2: Compositions of the tested samples, with their calculated values for V ’, Bsiim, and Bs

It will be recalled that the invention requires Bs> Bsi _im and V '> 0.18.

As can be seen, the compositions of the various examples are relatively close, and the contents of the various elements, taken individually, would, for all the examples (except the V of C70S6), comply with the requirements of the invention. It is on compliance or not with at least one of the two conditions on Bs and V 'that the examples not in accordance with the invention and the examples according to the invention are essentially distinguished, which shows the importance of simultaneous compliance with these two conditions.

The results detailed subsequently were obtained using plots of 25 × 25 mm section, to which a heat treatment was applied such that their condition was similar, or even identical, to that expected on a forged connecting rod of mass and dimensions typical of '' an automobile connecting rod: austenitization to at least 1050 ° C followed by cooling in calm air leading to speeds of typically 1 ° C / s between 800 and 550 ° C and 0.35 ° C / s between 550 and 300 ° C. We then determined the optimal tempering conditions to increase the mechanical characteristics and applied the optimized treatment for each grade, consisting of maintaining for 2 h at a temperature of 620 ± 10 ° C. Finally, metdlographic examinations and mechanical tests were carried out in order to assess the expected performance gain thanks to the invention. The results are presented in Table 3, in which B denotes a bainitic microstructure, M denotes martensite which is optionally present in conjunction with the bainite, and F + P denotes a ferrito-pearlitic microstructure.

Microstructure State RPo, ₂ MPa Rm MPa KV J 36MnV4S F + P Forge 780 1057 9 C70S6 P Forge 590 990 7 lnv.1 B (20% M) Forged + Income 1000 1174 4 lnv.2 B Forged + Income 1043 1156 4 lnv.3 B (10% M) Forged + Income 965 1101 5 lnv.4 B Forged + Income 1075 1169 3 Ref. 1 B Forged + Income 868 1018 14 Ref. 2 B Forged + Income 860 1000 14

Table 3: Microstructures and mechanical properties of the samples tested

The values in bold relate to the results considered as not in accordance with the present invention. These nonconforming results relate primarily to the standard 36MnV4S and C70S6 grades, ferrito-perlitic or perlitic type, the limitations of which have already been stressed in terms of elastic limit and lack of significant response to income. Note that for 36MnV4S, the precipitation of V takes place during the development of the microstructure, during the cooling of the part after hot forming. It therefore gives advantageous mechanical properties to this grade, as well as a low resistance to impact rupture (<10 J), which explains the success of this grade and others similar for the manufacture of breakable connecting rods with high performances. Thus, if these standard grades cannot reach the high elastic limit of the grades of the invention, their impact resistance as measured by the quantity KV nevertheless remains in accordance with what may be required for the manufacture of breakable rods.

The other shades not in accordance with the invention (Ref. 1 and 2) shed more light on the latter. Note that these two grades present homogeneous bainitic microstructures in the forged state, which is already a complete break with current practices for the manufacture of connecting rods.

The example Ref.1 presents an insufficiently low value of Bs (Bs = 567 ° C, while, according to the invention, the limit Bsi _im as defined above not to be exceeded would be 530 ° C) with, however, a value of V 'which would be in accordance with the invention (0.23%, for 0.18% at least in the invention). It is noted that consequently, the elastic limit does not meet the requirements of the invention, which aims to provide a minimum of 900 MPa. In this first case, it is too low tensile properties in the forged state which explain this insufficiency, the increase in these tensile properties during tempering, correlated with V ', not being less than that of several examples according to the invention.

Example Ref.2 presents a value of Bs according to the invention (Bs = 529 ° C for a maximum BS | _im of 530), but it has a curing potential, as measured by V ′, which is insufficient. (V '= 0.16%, therefore not in accordance with the invention).

For all of the examples according to the invention, the forged and returned bainitic microstructure is either of the same very essentially bainitic nature as that of Examples Ref. 1 and Ref. 2, or contains 10 or 20% of martensite together with the bainite. But the respect of both B's criteria and V 'can achieve the target mechanical characteristics by the invention which are a Rp _{0, 2} of at least 900 MPa and simultaneously strength Rm resistance of at least 1100 MPa, and have flexural resistance to impact KV in accordance with the requirements of a use for the manufacture of breakable parts, namely at most 12 J. Note, moreover, that it has been possible to demonstrate, by fatigue tests and compaction in the laboratory, that this increase in tensile strength resulted in a very significant increase in fatigue performance and resistance to compaction. The bainitic grades according to the invention make it possible to satisfy the dimensioning criteria of the connecting rods already mentioned, which the bainitic grades considered in the prior art did not allow.

A second advantage of the invention can be underlined by the results, appearing in table 4, of an additional test carried out on example lnv.4. For this test, a heat treatment was carried out on a bar with a diameter of 80 mm such that the microstructure after natural cooling could be considered to be representative of that of a hot forged part. Following this treatment, the optimized income treatment was carried out as already developed on the 25 x 25 mm ² section pieces. Metallographic examinations and mechanical tests were then carried out in the same manner as above.

Section State Microstructure Rpo, 2 (MPa) Rm (MPa) 25 x 25 mm Forged + income B 1075 1169 80mm diameter Forged + income B 1071 1175

Table 4: Results of additional tests on the example lnv.4

As these results show, a second major advantage of the invention is to provide the mechanical characteristics targeted on parts of very different dimensions, while the sensitivity of ferrito-pearlitic grades to this parameter is well known and has already been emphasized. It can therefore be seen that the grade of the present invention can be used, without significant modification of the manufacturing process or of the precise composition of the steel, for the manufacture of breakable parts intended for automobiles, but also for heavy goods vehicles, or large mechanical parts (transformers, marine engines, etc.).

Claims (16)

1, - Steel for breakable part hot formed, characterized in that its composition consists of, expressed in weight percentages: - 0.15% <C <0.40%; preferably 0.20% <C <0.35%; - 0.60% <Mn <1.80%; preferably 0.80% <Mn <1.60%; - Traces <Cr <1.60%; - Traces <Mo <0.40%; - Traces <Ni <1.50%; preferably traces <Ni <1.0%; better traces <Ni <0.60%; - Traces <Cu <0.80%; - 0.02% <V <0.70%; - Traces <Nb <0.08%; - Traces <Si <1.20%; preferably traces <Si <0.60%; - Traces <Al <0.10%; - Traces <B <0.010%, - Traces <Ti <0.10%; - Traces <S <0.15%; preferably 0.005% <S <0.15%; - Traces <P <0.10%; - Traces <Ca <0.010%; - Traces <Te <0.030%; - Traces <Se <0.050%; - Traces <Bi <0.10% - Traces <Pb <0.20%; - Traces <N <0.025%; - Traces <O <0.008%; the remainder being iron and impurities associated with processing; and for which the following relationships are verified: * V ’= V% + 2 Nb% + Cu% / 5> 0.18%; * Bs = 830 - 270C% - 90Mn% - 37Ni% - 70Cr% - 83 Mo% - 50Cu% - 100V% <BS | _irT1 , with BS | _im calculated as follows: Bsiim = 530 + 330 (V ’- 0.18), if 530 + 330 (V’ - 0.18) is less than 600; Bsiim = 600, if 530 + 330 (V ’- 0.18) is greater than or equal to 600. 2, - Steel according to claim 1, characterized in that Ti> 3.5 N% if B> 0.0010%. 3. - Steel according to claim 2, characterized in that traces <N <0.010%. 4, - Steel according to one of claims 1 to 3, characterized in that its microstructure comprises at least 60% of a mixture of bainitic ferrite and carbides or residual austenite, the rest of the microstructure comprising at most 40 % of martensite and / or pro-eutectoid ferrite and / or perlite, the presence of proeutectoid ferrite and / or perlite being limited to at most 10%. 5. - Process for manufacturing a mechanical part, characterized in that: - A steel semi-finished product is hot formed in the austenitic phase, the composition of which is expressed in weight percentages: * 0.15% <C <0.40%; preferably 0.20% <C <0.35%; * 0.60% <Mn <1.80%; preferably 0.80% <Mn <1.60%; * T breeds <Cr <1.60%; * T breeds <Mo <0.40%; * Traces <Ni <1.50%; preferably traces <Ni <1.0%; better traces <Ni <0.60%; * T races <Cu <0.80%; * 0.02% <V <0.70%; * T breeds <Nb <0.08%; * T breeds <Si <1.20%; preferably traces <Si <0.60%; * Traces <Al <0.10%; * T races <B <0.010%, * Traces <Ti <0.10%; * T breeds <S <0.15%; preferably 0.005% <S <0.15%; * Traces <P <0.10%; * T breeds <Ca <0.010%; * T races <Te <0.030%; * T races <Se <0.050%; * Traces <Bi <0.10% * T races <Pb <0.20%; * Traces <N <0.025; * T breeds <O <0.008%; the remainder being iron and impurities associated with processing; and for which the following relationships are verified: * V ’= V% + 2 Nb% + Cu% / 5> 0.18%; * Bs = 830 - 270C% - 90Mn% - 37Ni% - 70Cr% - 83 Mo% - 50Cu% - 100V% <Bsiim, with Bsi _im calculated as follows: BS | _im = 530 + 330 (V '- 0.18), if 530 + 330 (V' - 0.18) is less than 600; BS | _im = 600, if 530 + 330 (V '- 0.18) is greater than or equal to 600; - Cooling is carried out with calm air, or with forced air, or under a hood or in the case of said semi-finished product, preferably at cooling rates greater than or equal to 0.3 ° C / s in l 'range 750-550 ° C and between 0.1 and 5 ° C / s in the range 550-300 ° C; - At least one tempering of said semi-finished product cooled to a temperature Trev of between 450 and 680 ° C. is carried out for a period of 15 min to 10 h. 6. - Method according to claim 5, characterized in that Ti> 3.5 N% if B> 0.0010%. 7. - Method according to claim 6, characterized in that traces <N <0.010%. 8. - Method according to one of claims 5 to 7, characterized in that after cooling after hot shaping is carried out shaping of the semi-product, for example by machining or shaping, which brings the dimensions of the semi-product of precise final dimensions of the product without modifying its microstructure. 9. - Method according to one of claims 5 to 8, characterized in that after the tempering, the semi-finished product is shaped, for example by machining or shaping, which gives the semi-finished product the precise final dimensions. of the product without modifying its microstructure. 10. - Method according to one of claims 5 to 9, characterized in that one proceeds to a controlled breakage of said mechanical part, which is a breakable mechanical part. 11. - Method according to one of claims 5 to 10, characterized in that at least one additional heat treatment is carried out after said income, said additional treatment having an income parameter lower than that of said income. 12. - Mechanical steel part, characterized in that: - its composition consists of, expressed in weight percentages: * 0.15% <C <0.40%; preferably 0.20% <C <0.35%; * 0.60% <Mn <1.80%; preferably 0.80% <Mn <1.60%; * T breeds <Cr <1.60%; * T breeds <Mo <0.40%; * Traces <Ni <1.50%; preferably traces <Ni <1.0%; better traces <Ni <0.60%; * T races <Cu <0.80%; * 0.02% <V <0.70%; * T breeds <Nb <0.08%; * T breeds <Si <1.20%; preferably traces <Si <0.60%; * Traces <Al <0.10%; * T races <B <0.010%, * Traces <Ti <0.10%; * T breeds <S <0.15%; preferably 0.005% <S <0.15%; * Traces <P <0.10%; * T breeds <Ca <0.010%; * T races <Te <0.030%; * T races <Se <0.050%; * Traces <Bi <0.10% * T races <Pb <0.20%; * Traces <N <0.025; * T breeds <O <0.008%; the remainder being iron and impurities associated with processing; and for which the following relationships are verified: * V ’= V% + 2 Nb% + Cu% / 5> 0.18%; * Bs = 830 - 270C% - 90Mn% - 37Ni% - 70Cr% - 83 Mo% - 50Cu% - 100V% <Bsiim, with Bsi _im calculated as follows: BS | _im = 530 + 330 (V '- 0.18), if 530 + 330 (V' - 0.18) is less than 600; BS | _im = 600, if 530 + 330 (V '- 0.18) is greater than or equal to 600; and whose microstructure comprises at least 60% of a mixture of bainitic ferrite and carbides or residual austenite, the rest of the microstructure comprising at most 40% martensite and / or pro-eutectoid ferrite and / or perlite , the presence of proeutectoid ferrite and / or perlite being limited to at most 10%. 13. - Mechanical part according to claim 12, characterized in that Ti> 3.5 N% if B> 0.0010%. 14. - Mechanical part according to claim 13, characterized in that traces <N <0.010%. 15. - Mechanical part according to one of claims 12 to 14, characterized in that it is a breakable part. 16.- Mechanical part according to claim 15, characterized in that it is a connecting rod of an internal combustion engine.