EP1097248A1

EP1097248A1 - Case hardened steel with high tempering temperature, method for obtaining same and parts formed with said steel

Info

Publication number: EP1097248A1
Application number: EP99926549A
Authority: EP
Inventors: Philippe Dubois
Original assignee: Aubert and Duval SA
Current assignee: Aubert and Duval SA
Priority date: 1998-06-29
Filing date: 1999-06-28
Publication date: 2001-05-09
Anticipated expiration: 2019-06-28
Also published as: FR2780418A1; EP1097248B1; US6699333B1; DK1097248T3; FR2780418B1; DE69901345D1; AR019175A1; DE69901345T2; WO2000000658A1; CA2335911C; ES2175985T3; ATE216739T1; CA2335911A1; BR9912226A

Abstract

The invention concerns a case hardened steel composition comprising, expressed by weight: 0.06 to 0.18 % C; 0.5 to 1.5 % Si; 0.2 to 1.5 % Cr; 1 to 3.5 % Ni; 1.1 to 3.5 % Mo; and, as the case may be: not more than 1.6 % Mn and/or not more than 0.4 % V, and/or not more than 2 % Cu and/or not more than 4 % Co, the rest consisting of iron and residual impurities, said composition contents in Ni, Mn, Cu, Co, Cr, Mo and V, expressed by weight, satisfying the following relationships: 2.5 ≤ Ni + Mn + 1.5 Cu + 0.5 Co ≤ 5 (1); 2.4 ≤ Cr + Mo + V ≤ 3.7 (2). The invention also concerns a method for making case hardened and treated steel parts, made with said compositions.

Description

High tempering case hardening steel, process for obtaining it and parts formed therefrom

The present invention relates to a composition of case-hardening steel, parts formed with this steel, as well as a method for manufacturing parts made from this steel.

Case hardening is a surface thermochemical treatment generally intended to obtain parts combining good core ductility and a hard, hardened and wear resistant surface.

Many applications require the use of a steel having good resistance to softening at operating temperatures. By way of example, mention may be made of pinions, bearings and shafts of gearboxes for helicopters or for vehicles intended for automobile competition, pinions, camshafts and other parts used in distribution systems of heat engines , fuel injectors and compressors.

The cementing steels usually used for these applications are, in particular, 17CrNiMo6, 16NiCr6, 14NiCr12, 10NiCrMo13, 16NiCrMo13 or 17NiCrMo17. These steels can be used up to operating temperatures in the region of 130 ° C, but have neither a softening resistance nor a hot hardness of the cemented layer sufficient for operating temperatures exceeding 190 ° C.

US Patent No. 3,713,905 issued to T.V. Philip and R.L. Vedder on January 30, 1973 describes the properties obtained for a steel whose chemical composition, as a percentage by weight, is as follows:

0.07-0.8% of C, at most 1% of Mn, 0.5-2% of Si, 0.5-1.5% of Cr, 2-5% of Ni,

0.65-4% Cu, 0.25-1.5% Mo, at most 0.5% V, the balance being iron. The tensile and resilience values obtained with this steel are compatible for the envisaged applications, on the other hand the tempering resistance and the hot hardness of the cemented layer are insufficient for the abovementioned applications and for operating temperatures up to 220 ° C.

US Patent No. 4,157,258 issued to T.V. Philip and R.W. Krieble on June 5, 1979 describes a steel whose chemical composition in percentage by weight is as follows:

0.06-0.16% C 0.2-0.7% Mn 0.5-1.5% Si 0.5-1.5% Cr 1 5-3% Ni

1-4% Cu 2.5-4% Mo ≤ 0.4% V <0.05% P ≤ 0.05% S

≤ 0.03% of N ≤ 0.25% of Al ≤ 0.25% of Nb ≤ 0.25% of Ti ≤ 0.25% of Zr

≤ 0.25% Ca, the balance being iron. This steel presents a good compromise between the characteristics of traction and resilience. The cemented layer allows a tempering temperature up to about 260 ° C. The maximum operating temperature is around 230 ° C. However, none of the cementation steel compositions of the prior art makes it possible to achieve a tempering temperature of the cemented layer of up to 350 ° C., as well as good hot hardness for operating temperatures of up to 'at 280 ° C, while retaining satisfactory core characteristics.

However, a need for such steels currently exists in many fields. Regarding, for example, the manufacture of gear parts for helicopters, the regulations stipulate that a helicopter must be able to operate for thirty minutes after losing the oil in its gearbox following an incident. This assumes that the materials used to manufacture these gears have undergone tempering at a minimum temperature of around 280 ° C.

In the field of thermal engines, designers are moving towards an increase in the operating temperatures of engine components and related components such as gearboxes, in order to increase yields and / or simplify heat extraction circuits . However, depending on the location of the parts in these organs, the operating temperatures can reach up to 280 ° C., which imposes a minimum tempering temperature of 330 ° C. to guarantee the stability of the properties in use.

The main object of the present invention is therefore to provide a cementation steel composition making it possible to achieve all of the above characteristics.

A first object of the invention is thus a composition of case-hardening steel comprising, expressed by weight,

0.06 to 0.18% C, 0.5 to 1.5% Si, 0.2 to 1.5% Cr, 1 to 3.5% Ni, 1.1 to 3.5% of Mo, and, where appropriate, at most 1.6% of Mn, and / or at most 0.4% of V, and / or at most 2% of Cu, and / or at most 4% of Co, the remainder consisting of iron and residual impurities, the contents of this composition in Ni, Mn , Cu, Co, Cr, Mo and V, expressed by weight, satisfying the following relationships:

2.5 ≤ Ni + Mn + 1.5 Cu + 0.5 Co ≤ 5 (1)

2.4 <Cr + Mo + V <3.7 (2)

Sulfur is preferably limited to 0.010% and phosphorus to 0.020% by weight for high-end applications, but higher contents are however acceptable for other applications, insofar as they do not cause reduction of the ductility, toughness and fatigue resistance properties of steel.

Elements such as aluminum, cerium, titanium, zirconium, calcium, niobium, which serve either to deoxidize or to refine the grain size are preferably limited to 0.1% by weight each.

With regard to the main elements of the composition, it is generally observed that the low contents of carbon, silicon, molybdenum, chromium and vanadium, as well as the high contents of manganese, nickel, cobalt and copper allow improve the ductility and toughness properties of steel.

Conversely, the high contents of carbon, silicon, molybdenum, chromium and vanadium as well as the low contents of manganese, nickel, cobalt and copper make it possible to improve the resistance to tempering of steel. The role of carbon is essentially to contribute to obtaining hardness, tensile strength and hardenability. For carbon contents of less than 0.06% by weight, the hardness and the tensile strength obtained at the core of the case-hardened and treated parts are insufficient. In practice, the minimum tensile strength sought is approximately 1000 MPa, or approximately 320 HV (Vickers hardness). The higher the carbon content, the higher the hardness, tensile strength and hardenability increases but, at the same time, resilience and toughness decrease. It is for this reason that the carbon content is limited to a maximum value of 0.18% by weight.

The most advantageous range for the compromise between tensile strength and toughness is 0.09-0.16% by weight of carbon. However, the 0.06-0.12% and 0.12-0.18% ranges are also of interest for applications requiring different levels of core hardness.

Silicon contributes to a large extent to the resistance to tempering of this steel and its minimum content is 0.5% by weight. In order to avoid the formation of delta ferrite and to maintain sufficient toughness, the silicon content is limited to a maximum of 1.5% by weight. The optimal range is 0.7-1.3% by weight, but the range 1.3-3.5% is also interesting.

Chromium contributes in part to the hardenability of the core and to the good resistance to tempering of the cemented layer, its minimum content is 0.2% by weight. To avoid embrittlement of the cemented layer by excess of networked carbides, the chromium content must be limited to a maximum value of 1.5% by weight. The optimal range is 0.5-1.2%, but the 0.2-0.8% and 0.8-1.5% ranges are also attractive. Molydbene plays a role identical to that of chromium, and it also makes it possible to maintain a high hot hardness, in particular by the formation of intragranular carbides in the cemented layer. Its minimum content is 1.1% by weight. However, its embrittling effect on this steel leads to limiting its maximum content to 3.5% by weight. The optimal range is 1.5-2.5%, but the ranges 1, 1-2.3% and 2.3-3.5% are also interesting.

Vanadium helps limit grain magnification during the case hardening and processing cycles. Because of its embrittling effect and its influence on the formation of ferrite, its content must be limited to a maximum value of 0.4% by weight. The optimal range is 0.15-0.35% but the ranges 0.05-0.25% and 0.25-0.4% are also interesting. Manganese, nickel and copper are gamma elements necessary to balance the chemical composition, avoid the formation of ferrite and limit the temperature of the α γ transformation points. They also greatly contribute to increasing the hardenability, resilience and toughness but, in too high a content, they deteriorate the income resistance, the hot hardness and the wear resistance and increase the amount of residual austenite in the layer. case-hardened.

Manganese is therefore limited to a maximum of 1.6% by weight. The optimal range is 0.2-0.7% by weight, but the range 0.7-1.5% is also interesting. Likewise, nickel is limited to the range 1-3.5% by weight, the optimal range is 2-3%, but the ranges 1-2% and 2-3.5% are also interesting. Finally, copper is limited to a maximum of 2% by weight, the optimal range is 0.3-1.1%, but the range 1.1-2% can also be interesting. Cobalt contributes to the income resistance of the steel and makes it possible to lower the transformation point on heating. Its effect is noticeable even at low contents. For high contents this element, by its gammagenic character, stabilizes the residual austenite in the cemented layer. The maximum limit is 4% by weight, contents of less than 1.5% by weight being recommended.

A second object of the invention is a method of manufacturing cemented and treated parts comprising the following operations: a - constitution of a charge intended to obtain a composition in accordance with the present invention, as described above, b - fusion of said charge in an arc furnace, c - thermomechanical heating and transformation of the ingot, d - heat treatment for homogenizing the structure and refining of the grain, e - carburizing, and f - heat treatment for use.

The steel according to the invention can be obtained by conventional production techniques but, to obtain better results in resilience, tenacity and fatigue, it is recommended to carry out a reflow by a consumable electrode, either under slag (ESR) or under reduced pressure (VAR), following melting in the arc furnace.

To further increase these performances, it is also possible to carry out the first fusion by induction under reduced pressure (VIM) and to continue with a reflow by consumable electrode.

The ingots obtained by any one of the preceding routes undergo a reheating at temperatures of approximately 1100 ° C. to homogenize the structure, followed by thermomechanical transformations aiming to confer on the product produced in this alloy a sufficient rate of wrinkling which one prefer greater than or equal to 3 (step c of the method according to the invention). Lower working rates may however be allowed for large parts. These thermomechanical transformations are based on conventional procedures, such as rolling, forging, stamping or spinning.

Several alternative embodiments are possible with regard to step d of the method according to the invention. The processed products can simply be softened at a temperature below the critical point (AC-i), or annealed at a temperature above the critical point (AC-i), which then assumes a sufficiently slow start of cooling.

When looking for the best possible characteristics, it is however preferable to carry out a normalization from a temperature above the critical point (AC ₃ ), followed by air cooling and a softening income. at a temperature below the critical point (ACi).

As an indication, the critical point temperature (AC-i) is generally in the range from 700 to 800 ° C, while the critical point temperature (AC ₃ ) is generally in the range from 900 to 980 ° C. The case hardening, step e of the process according to the invention, can be carried out using conventional means, the case hardening cycle being to be defined by the skilled person as a function of the depth hardening sought, in a completely conventional manner. One can in particular use a low pressure process.

With regard to stage f of the heat treatment of the use of the parts, numerous alternative embodiments are possible. It is possible to go directly from the case temperature to the austenitization temperature, then to soak the parts, but it is preferable to allow the parts to cool to room temperature after case hardening, then to heat them up to the temperature austenitization, above the critical point (AC ₃ ) before soaking. The austenitization temperature range is, for information, 900-1050 ° C.

The best characteristics of traction, resilience, toughness of the core and surface hardness of the cemented layer are obtained by carrying out an oil quenching after austenitization, but a good compromise of these same characteristics can be achieved by carrying out a gas quenching which has the advantage of reducing the deformation of the parts during this operation and therefore of minimizing subsequent machining.

In order to obtain the maximum values of hardness of the cemented layer, and of resilience and toughness of the sub-layer, it is preferable to carry out tempering at the lowest possible temperature, compatible with the temperature of use. A difference of 50 ° C. between tempering temperature and use temperature is more particularly preferred, the tempering temperature possibly reaching up to 350 ° C.

In the case of the production of this steel in large quantities, the continuous casting technique can be used in order to reduce the production costs and we must then expect a lowering of the characteristics of ductility, resilience and toughness, especially.

A third object of the invention is constituted by the case-hardened and treated parts produced with the case-hardening steel according to the invention and which exhibit, at ambient temperature, a hardness with a core close to 320 to 460 HV, an resilience ISO V d '' at least 50 Joules, and more particularly from 70 to 150 Joules, a toughness close to 100 MPaVm, a surface hardness of the cemented layer close to 750 HV, and which, at 250 ° C, has a surface hardness of the cemented layer close to 650 HV. These parts can advantageously be manufactured by means of the manufacturing method according to the invention, but also by any other method chosen according to the final application. The embodiments of the invention which follow show that the combination of the elements carbon, manganese, silicon, chromium, nickel, molybdenum, vanadium, copper and cobalt, in the proportions by weight indicated above, results in a steel having simultaneously excellent characteristics of hardness, traction, resilience, transition of resilience and toughness of the core, associated with excellent resistance to tempering and excellent hardnesses of the hardened layer up to operating temperatures of 280 ° C. Examples

The symbols used in the following have the following meanings: Rm = maximum resistance

R in, ₂ = conventional elastic limit at 0.2% deformation

As _d = elongation in% based on 5 d (d = specimen diameter) Z = necking HV = Vickers hardness

HRC = Rockwell hardness

KV = Breaking energy in impact bending on V-notched test piece. The examples are supplemented by the figures in the accompanying drawing plates, in which:

FIG. 1 represents the variations in microhardness as a function of the depth for two samples, the preparation of which is described in example 1,

FIG. 2 represents the variations in microhardness as a function of the depth for two samples, the preparation of which is described in example 2, FIG. 3 represents the variations in microhardness as a function of the depth for two samples, the preparation of which is described in example 3,

FIG. 4 represents the variations in microhardness as a function of the depth for two samples, the preparation of which is described in example 4,

• Figure 5 shows the variations in microhardness as a function of depth for two samples whose preparation is described in Example 5, • Figure 6 shows the variations in microhardness as a function of depth for two samples whose preparation is described in Example 6,

FIG. 7 represents the variations in microhardness as a function of the depth for three samples, the preparation of which is described in Example 8.

Example 1

A 35 kg ingot was produced in the chemical composition indicated in percentage by weight below, in accordance with the indications of the present invention:

C 0.15% If 1.11% Mn 0.43% Cr 0.92% Ni 2.51%

Mo 1.96% V 0.28% the remainder consisting of iron and residual impurities.

This ingot was produced by arc fusion, it was then homogenized at high temperature to give a uniform structure, then it was forged. The forged products were slowly cooled in the oven. They have been standardized in order to dissolve carbides, to homogenize the austenitic structure and to refine the grain.

Bars resulting from this invention were austenitized at 940 ° C., soaked in oil, passed through the cold in a cryogenic enclosure regulated at -75 ° C., then returned to a temperature of 250 ° C.

The mechanical characteristics obtained are indicated in the following table:

Other samples of this steel were cemented using a low pressure process at a temperature of around 900 ° C for 8 hours, then austenitized at 940 ° C, passed through the cold in a cryogenic chamber regulated at -75 ° C and finally returned to temperatures between 150 and 350 ° C. The surface hardnesses of the cemented layer and the core hardnesses obtained for different tempering temperatures are indicated in the following table:

Hardness measurements on polished sections were also carried out in order to determine the hardness gradient in the cemented layer. Figure 1 shows the results obtained for tempering temperatures of 150 ° C and 350 ° C. Example # 2

C 0.146% If 1.12% Mn 1% Cr 0.92% Ni 1.54%

Mo 1, 97% V 0.284% the remainder consisting of iron and residual impurities.

This ingot was produced by arc fusion and was then homogenized at high temperature to obtain a uniform structure, then it was forged. The forged products were slowly cooled in the oven. They have been standardized in order to dissolve the carbides, to homogenize the austenitic structure and to refine the grain.

Bars from these treatments were austenitized at 940 ° C, soaked in oil, cold passed in a cryogenic chamber regulated to -75 ° C, then returned to a temperature of 250 ° C.

The mechanical characteristics obtained are indicated in the following table:

Other samples of this steel were cemented using a low pressure process at a temperature of around 900 ° C for 8 hours, then austenitized at 940 ° C, passed through the cold in an enclosure cryogenic regulated at -75 ° C and finally returned to temperatures between 150 and 350 ° C.

The surface hardnesses of the cemented layer and the core hardnesses obtained for different tempering temperatures are indicated in the following table:

Hardness measurements on polished sections were also carried out in order to determine the hardness gradient in the cemented layer. Figure 2 shows the results obtained for tempering temperatures of 150 ° C and 350 ° C.

Example 3

C 0.14% If 1.49% Mn 0.98% Cr 0.914%

Ni 1.53% Mo 1.99% V 0.284% Cu 0.801% the remainder consisting of iron and residual impurities.

This ingot was produced by arc fusion, it was then homogenized at high temperature to obtain a uniform structure, then it was forged. The forged products were slowly cooled in the oven. they have have been standardized to dissolve carbides, homogenize the austenitic structure and refine the grain.

The mechanical characteristics obtained are indicated in the following table:

Hardness measurements on polished sections were also carried out in order to determine the hardness gradient in the cemented layer. Figure 3 shows the results obtained for tempering temperatures of 150 ° C and 350 ° C. Example 4

C 0.11% If 0.52% Mn 0.49% Cr 0.99% Ni 1.23%

Mo 1.96% Co 3.96% the remainder consisting of iron and residual impurities.

This ingot was produced by arc fusion, it was then homogenized at high temperature to obtain a uniform structure, then it was forged. The forged products were slowly cooled in the oven. They have been standardized in order to dissolve the carbides, to homogenize the austenitic structure and to refine the grain.

The mechanical characteristics obtained are indicated in the following table:

Hardness measurements on polished sections were also carried out in order to determine the hardness gradient in the cemented layer. Figure 4 shows the results obtained for tempering temperatures of 150 ° C and 350 ° C.

Example 5

C 0.12%

If 0.52%

Mn 1.47% Cr 0.54%

Ni 1.05% Mo 3% V 0.01% the remainder consisting of iron and residual impurities. This ingot was produced by arc fusion, it was then homogenized at high temperature to obtain a uniform structure, then it was forged. The forged products were slowly cooled in the oven. they have have been standardized in order to dissolve the carbides, to homogenize the austenitic structure and to refine the grain.

Bars from these treatments were austenitized at 960 ° C, soaked in oil, passed through the cold in a cryogenic chamber regulated at -75 ° C, then returned to a temperature of 250 ° C.

The mechanical characteristics obtained are indicated in the following table:

Other samples of this steel were cemented using a low pressure process at a temperature of around 900 ° C for 8 hours, then austenitized at 960 ° C, passed through the cold in a cryogenic chamber regulated at -75 ° C and finally returned to temperatures between 150 and 350 ° C. The surface hardnesses of the cemented layer and the core hardnesses obtained for different tempering temperatures are indicated in the following table:

Hardness measurements on polished sections were also carried out in order to determine the hardness gradient in the cemented layer. Figure 5 shows the results obtained for tempering temperatures of 150 ° C and 300 ° C. Example 6

C 0.12% If 0.71% Mn 1.57% Cr 1.02% Ni 1.01%

Mo 2.02% V 0.01% the remainder consisting of iron and residual impurities.

The mechanical characteristics obtained are indicated in the following table:

Other samples of this steel were cemented using a low pressure process at a temperature of around 900 ° C for 8 hours, then austenitized at 960 ° C, passed through the cold in an enclosure cryogenic regulated at -75 ° C and finally returned to temperatures between 150 and 350 ° C.

Hardness measurements on polished sections were also carried out in order to determine the hardness gradient in the cemented layer. Figure 6 shows the results obtained for tempering temperatures of 150 ° C and 300 ° C.

Example 7

A 1000 kg ingot was prepared in accordance with the present invention, its chemical composition, expressed as a percentage by weight, being as follows:

C 0.14%

If 1, 12%

Mn 0.44% Cr 0.95%

Ni 2.52% Mo 1, 93% V 0.27% Cu 0.88% the rest being made up of iron and residual impurities.

This ingot was obtained by partial pressure induction melting (VIM), then reflow by consumable electrode, it was then reheated to high temperature, in order to homogenize the structure, then it was laminated. to end up with 90 mm diameter cylindrical bars. These bars have undergone a standardization treatment, in order to dissolve the carbides, homogenize the austenitic structure and refine the grain size.

Samples taken from these bars were cemented using a low pressure process at a temperature of around 900 ° C for 8 hours, the samples intended to characterize the core properties underwent an identical thermal cycle, but in a neutral atmosphere , so as not to modify their chemical composition.

All of the samples were then austenitized at 940 ° C, quenched in oil, passed through the cold in a cryogenic chamber regulated at -75 ° C and returned to a temperature of 300 ° C.

The mechanical characteristics obtained are indicated in the following table:

The test carried out according to ASTM E 399-90 on a 20 mm thick CT specimen led to a KQ tenacity of 107 MPaVm.

The evolution of the surface hardness of the cemented layer as a function of the tempering temperature is indicated in the table below:

The following table indicates the evolution of the surface hardness of the cemented layer as a function of the test temperature, on a sample which has undergone tempering at 300 ° C.

Example 8 (comparative)

Similar samples were machined from 16NiCrMo13 steel and case-hardened under the same conditions as those described in Example 7. All of the samples were then austenitized at 825 ° C and quenched in oil.

Hardness measurements on polished sections were carried out in order to determine the hardness gradient in the cemented layer. Figure 7 shows the results obtained for tempering temperatures of 150 ° C, 200 ° C and 300 ° C.

The preceding eight examples show, on the one hand, that the steels according to the invention exhibit an excellent compromise between the characteristics of traction, resilience and toughness and, on the other hand, that the cemented layer has a high resistance to tempering. , as well as high values of hot hardness, significantly higher than those obtained with traditional case hardening steels.

It goes without saying that the embodiments of the invention which have been described above have been given for purely indicative and in no way limitative, and that numerous modifications can be easily made by those skilled in the art without for any as well go beyond the scope of the invention.

Claims

1. Composition of case-hardening steel comprising, expressed by weight,

0.06 to 0.18% of C,

0.5 to 1.5% Si,

0.2 to 1.5% of Cr, 1 to 3.5% of Ni,

1.1 to 3.5% of Mo, and, where appropriate, at most 1.6% of Mn, and / or at most 0.4% of V, and / or at most 2% of Cu, and / or at most 4% Co, the remainder consisting of iron and residual impurities, the contents of this composition in Ni, Mn, Cu, Co, Cr, Mo and V, expressed by weight, satisfying the following relationships: 2 , 5 <Ni + Mn + 1.5Cu-t-0.5Co <5 (1)

2.4 <Cr + Mo + V <3.7 (2)

2. A cementation steel composition according to claim 1 comprising, expressed by weight

0.09 to 0.16% of C, 0.7 to 1.3% of Si,

0.5 to 1.2% of Cr,

2 to 3% of Ni,

1.5 to 2.5% of Mo,

0.2 to 0.7% of Mn, 0.15 to 0.35% of V,

0,

3 to 1.1% of Cu, and, where appropriate, at most 1.5% of Co, the remainder consisting of iron and residual impurities, the contents of this composition in Ni, Mn, Cu, Co, Cr, Mo and V, expressed by weight, satisfying the following relationships:

2.5 <Ni + Mn + 1.5 Cu + 0.5 Co <5 (1)

2.4 <Cr + Mo + V <3.7 (2) 3. Composition of case-hardening steel according to one of claims 1 or 2, further comprising at most 0.020% by weight of P and at most 0.010% by weight of S.

4. A cementation steel composition according to any one of claims 1 to 3, further containing at most 0.1% by weight of each element Al, Ce, Ti, Zr, Ca, Nb.

5. A method of manufacturing cemented and treated parts, comprising the following operations: a - constitution of a charge intended to obtain a chemical composition according to any one of claims 1 to 4, b - melting of said charge in a furnace arc, c - reheating and hot transformation of the ingot, d - heat treatment for homogenization of the structure and refinement of the grain, e - case hardening, and f - heat treatment for use.

6. The manufacturing method according to claim 5, wherein the melting in an arc furnace (step b) is followed by remelting by a consumable electrode.

7. The manufacturing method according to claim 6, wherein the melting in an arc furnace (step b) is carried out by induction under reduced pressure.

8. The manufacturing method according to any one of claims 5 to 7, wherein step d comprises normalization at a temperature higher than that of the critical point AC ₃ , air cooling and a softening income at a temperature lower than that of the critical point AC _T.

9. The manufacturing method according to any one of claims 5 to 8, wherein step e is carried out according to a low pressure process.

10. The manufacturing method according to any one of claims 5 to 9 wherein step f comprises cooling to room temperature, then reheating to 900-1050 ° C, quenching with oil or gas, and a returned to temperatures up to 350 ° C.

11. Steel part having a composition according to any one of claims 1 to 4.

12. Steel part according to claim 11, characterized in that it is obtained by a process according to any one of claims 5 to 10.