Hammered steel part and a method for producing it

Abstract

A steel for hammering and making mechanical parts containing the following elements in percentage by weight:\n0.2% ≦ C ≦ 0.5 %; 0.8% ≦ Mn ≦ 1.5 %; 0 .4% ≦ Si ≦ 1 %; 0.15% ≦ V ≦ 0.6%; 0.01% ≦ Nb ≦ 0.15%; 0.01% ≦ Cr ≦ 0.5 %; 0.01% ≦ P ≦ 0.05%; 0.04% ≦ S ≦ 0.09%; 0.01% ≦ N ≦ 0.025%\nwhich may also contain one or more of the following optional elements:\n0 % ≦ Al ≦ 0.05 %; 0% ≦ Mo ≦ 0.5%; 0.01% ≦ Ni ≦ 0.5%; 0% ≦ Ti ≦ 0.2%; 0% ≦ B ≦ 0.008%; 0% ≦ Cu ≦ 0.5%\nThe rest of the composition consists of iron and unavoidable impurities resulting from processing. The microstructure of that steel consists of 50% to 90% pearlite, 10% to 40% ferrite, with optional acicular ferrite of 0% to 2%, and a niobium equivalent of 80% or more.

Description

Hammer-forged steel part and a method for producing it The present invention relates to a ferritic-pearlitic steel suitable for hammering steel parts used in automobiles. Mechanical parts for automobiles, especially for internal combustion engines, are generally manufactured by hammer forging. Hammer forged materials inherently face the problem of being unable to fulfill the dual requirements of sufficient impact toughness and high yield strength on the one hand, while simultaneously meeting the automotive industry's requirements for engine parts. Another additional and mandatory requirement for these materials is that they must have good machinability, and in particular, fracture toughness, in order to be used for the production of mechanical parts in internal combustion engines, such as crankshafts, camshafts, connecting rods, and the like. Therefore, much research and development efforts have been devoted to developing a material that has favorable machinability and at the same time has high yield strength, i.e., above 750 MPa, and also has sufficient impact toughness. Previous research and development in the field of steels for forging mechanical parts for internal combustion engines has led to several methods for producing high strength and desirable machinability, some of which are enumerated in the present text to provide a better understanding of the present invention: Document US20100186855 is a patent application in which the invention relates to a steel and a method of manufacturing high-strength, breakable machine components comprising at least two breakable parts. The steel and method are characterized in that the chemical composition of the steel (expressed as a percentage by weight) is as follows: 0.40% ≦ C ≦ 0.60%; 0.20% ≦ Si ≦ 1.00%; 0.50% ≦ Mn ≦ 1.50%; 0% ≦ Cr ≦ 1.00%; 0% ≦ Ni ≦ 0.50%; 0% ≦ Mo ≦ 0.20%; 0% ≦ Nb ≦ 0.050%; 0% ≦ V ≦ 0.30%; 0% ≦ Al ≦ 0.05%; 0.005% ≦ N ≦ 0.020% The remainder consists of iron and impurities and residual materials associated with casting. The steel of document US20100186855 is capable of achieving a yield strength of 750 MPa but is unable to provide impact toughness. EP2246451 is a patent application for hot forging a micro-alloyed steel and a hot rolled steel which are highly desirable in terms of fracture toughness and machinability and can be used as separate steel components for fracture toughness and a component made of hot forged and micro-alloyed steel. However, the steel of EP2246451 is not capable of providing sufficient impact toughness. Therefore, in view of the documents detailed in the previous lines, the object of the invention is to provide a steel for hot hammering of mechanical parts such as connecting rods that allows achieving a yield strength of at least 750 MPa, a tensile strength of at least 1030 MPa and an impact toughness of less than or equal to 5 J at ambient temperature and using samples with a V-shaped notch. Therefore, the aim of the present invention is to solve these problems by providing a ferrite-pearlite steel suitable for hot forging that simultaneously possesses the following properties: - a yield strength greater than or equal to 750 MPa and preferably greater than 770 MPa, - an ultimate tensile strength greater than or equal to 1030 MPa and preferably higher than 1040 MPa, - an impact toughness less than or equal to 5 J and preferably less than 4.5 J at ambient temperature, - An ultimate elongation greater than or equal to 12.0%. Such steels are preferably suitable for the production of forged steel parts with cross-sections up to 50 mm in diameter, such as crankshafts, connecting rods and camshafts, without a significant hardness gradient between the shell and core of the forged part. Another object of the present invention is to provide a method for producing these mechanical parts that is compatible with common industrial applications and at the same time efficient in response to changes in production parameters. Carbon is present in the steel of the present invention in the range of 0.2% to 0.5%. Carbon gives the steel strength by forming pearlite and also provides sufficient toughness by limiting the formation of ferrite. Carbon also forms precipitates in combination with vanadium and niobium in the form of carbides or carbo-nitrides. A minimum of 0.2% of carbon is required to achieve a tensile strength of 1030 MPa with the formation of a minimum of 50% pearlite, but if carbon is present in amounts higher than 0.5%, the tensile strength after hot forging reaches above 1200 MPa, with a significant increase in the risk of the formation of secondary hard phases such as ferrite, bainite and acicular martensite, which are detrimental to the machinability of the hot forged part. The carbon content is advantageously in the range of 0.3% to 0.5% and more particularly between 0.35% and 0.45%. Manganese is present in the present steel in amounts between 0.8% and 1.5%. Manganese provides hardness in the steel. Manganese is added to the steel to reduce the ferrite-pearlite transformation temperature, which results in a smaller microstructure and in particular a lower cementite interlayer spacing in the pearlite and a lower pearlite colony size. It is preferred that the manganese content is between 0.9% and 1.3%, and more preferably between 0.95% and 1.15%. Silicon is present in the steel of the present invention in amounts between 0.4% and 1%. Silicon in the steel of the present invention provides strength and toughness through solid solution strengthening. Silicon also acts as a deoxidizer. The preferred content is between 0.5% and 0.9%, and in particular 0.6% and 0.75%, in the steel of the present invention. Vanadium is a key element for the present invention and its content is between 0.15% and 0.6%. Vanadium is effective in improving the strength of steel through precipitation hardening and in particular by forming carbides or carbo-nitrides. The lower limit is 0.15% and is required to ensure a yield strength of 750 MPa. The upper limit is kept at 0.6% because above 0.6%, the effect of vanadium is not particularly beneficial in increasing tensile strength and yield strength. Furthermore, excessive precipitation of vanadium causes a reduction in elongation. The preferred limit for vanadium is between 0.2% and 0.5% and more preferably between 0.25% and 0.45%. Niobium is present in the steel of the present invention in the range of 0.01% to 0.15%. In this invention, niobium begins to form precipitates in the austenite region at temperatures above 900°C, which limits the kinetics of austenite grain size growth and also the formation of nitrides and carbo-nitrides, such as vanadium, at temperatures below 900°C. This leads to an improvement in the yield strength of the steel of the present invention. This element cannot be added at a content higher than 0.15% by weight to avoid coarsening of the niobium precipitates, which can act as nuclei for ferrite transformation, leading to excessive ferrite in the newly formed microstructure, thereby reducing the tensile strength and yield strength beyond the desired limit. In addition, a content of 0.15% or more of niobium is also detrimental to the hot forging of the steel and leads to problems during the forming and rolling of the steel. The preferred range for niobium is between 0.02% and 0.12%, and more preferably between 0.02% and 0.1%. Chromium is present in the steel of the present invention in a range between 0.01% and 0.5%. The addition of chromium refines the interlayer spacing of the pearlite because chromium reduces the carbon diffusion coefficient in the austenite. However, the presence of a chromium content of more than 0.5% carries the risk of forming hard phases and segregation. Chromium above 0.5% also increases the hardness beyond an acceptable limit. The preferred limit for chromium is between 0.05% and 0.3%. More preferably, it is between 0.05% and 0.2%. The phosphorus content of the steel of the present invention is between 0.01% and 0.05%. A minimum of 0.01% by weight of phosphorus is necessary to ensure a good fracture toughness. However, it is not recommended to use a phosphorus content higher than 0.05% by weight as this is detrimental to the fatigue strength and may lead to failure through loss of adhesion at the intergranular interface. The preferred limit for the phosphorus content is between 0.01% and 0.025%. Sulfur is present in amounts between 0.04% and 0.09%. Sulfur forms MnS precipitates which improve machinability and help in achieving adequate machinability. During metal forming processes such as rolling and hammering, deformable manganese sulfide inclusions elongate. Such elongated MnS inclusions, if not aligned with the loading direction, can have significant adverse effects on mechanical properties such as elongation and impact toughness. Therefore, the sulfur content is limited to 0.09%. A preferred range for sulfur content is 0.060% to 0.085% to achieve the best balance between machinability and fatigue strength. Nitrogen is present in the steel of the present invention in an amount between 0.01% and 0.025%. Nitrogen is added to promote the precipitation of vanadium and niobium in the form of nitrides or carbo-nitrides. During cooling after hammering, nitrogen traps vanadium and niobium to form nitrides and carbonitrides. A minimum amount of nitrogen of 0.01% is required to form nitrides or carbonitrides to significantly increase the precipitation hardening of the steel and thereby improve the yield strength. However, a nitrogen content of more than 0.025% leads to the risk of forming gas porosity within the material during the solidification process of the steel. Nitrogen also has the ability to form nitrides with aluminum, which limits the kinetics of austenitic grain growth. The small austenite grain size results in a low effective ferrite and pearlite grain size and higher yield strength, while at the same time, due to the pearlite content, maintaining impact toughness at ambient temperature below 5 KV(J). Aluminum is a residual element for the steel of the present invention and is added to deoxidize the steel and also to form dispersed precipitates in the steel in the form of nitrides which prevent the growth of austenite grains. However, the deoxidizing effect reaches saturation for aluminum contents above 0.05%. A content exceeding 0.05% can lead to the occurrence of coarse aluminum-rich oxides which reduce fatigue strength and machinability. For the present invention, it is suitable to limit the aluminum content to 0.05% and preferably 0.03%. Molybdenum is an optional element and can be present in the range of 0% to 0.5% in the present invention. Molybdenum is added to provide hardness properties. The preferred range for molybdenum content is between 0% and 0.2%, and more preferably between 0% and 0.1%. Nickel is an optional element for the present invention and is present in the range of between 0.01% and 0.5%. Nickel is added to the steel composition to refine the interlayer spacing of pearlite because nickel, like chromium, reduces the diffusion coefficient of carbon in austenite. It is preferred that the presence of nickel be limited to 0.2% to be economically viable, so the preferred range is between 0.01% and 0.2%. Titanium is an optional element and is present in the composition in the range of 0% to 0.2%. Titanium should be added in the lowest possible amount because it retains small amounts of nitrogen in solid solution and is thus available for precipitation with niobium and vanadium to provide strength in the steel of the present invention. Titanium forms titanium nitrides which strengthen the steel but these nitrides may form during the solidification process and therefore have an adverse effect on machinability and fatigue strength. Therefore, the preferred range for titanium is between 0% and 0.1% and more preferably between 0% and 0.05%. Boron is an optional element that can be present in the range of 0% and 0.008%. Boron has no role in steel intended for mechanical parts. Boron has a pronounced effect on hardness and may result in a completely ferrite or pearlite microstructure at the end of the hammering process. Copper is a residual element and may be present in the composition up to 0.5% due to the processing of the steel. Up to 0.5%, copper has no effect on the properties of the steel, but beyond 0.5%, hot workability is significantly reduced. Other elements such as tin, cerium, magnesium or zirconium can be added separately or in combination in the following weight ratios: Tin ≦ 0.1%, Cerium ≦ 0.1%, Magnesium ≦ 0.010% and Zirconium ≦ 0.010% Up to the maximum content levels mentioned, these elements allow for grain refinement during solidification. The remainder of the steel composition consists of iron and unavoidable impurities resulting from the process. The microstructure of steel is as follows: Ferrite is a critical microstructural constituent of the steel of the present invention. Ferrite is present in the range of 10% and 40% by area fraction in the steel of the present invention. Ferrite of the present invention includes both intergranular and intragranular niobium and vanadium precipitates in the form of carbides, nitrides and / or carbo-nitrides which contribute to the strength of the steel of the present invention. Ferrite also contributes to the elongation of the steel of the present invention. A minimum of 10% ferrite is required to ensure an elongation of at least 12.0% while achieving a strength of 1030 MPa, but if ferrite exceeds 40%, the desired strength is no longer achieved and the impact toughness increases beyond the desired limit, leading to poor fracture cleavage. Ferrite is formed during the cooling stage and after hot hammering. The preferred range for ferrite is between 15% and 40% in a preferred embodiment according to the invention, it is preferred that the ferrite content is between 25% and 40%, and more preferably between 25% and 35% when the carbon content is between 0.2 and 0.4%. In another preferred embodiment, when the carbon content is between 0.4% and 0.5%, the preferred ferrite content is between 15% and 35%. Pearlite is present in the present steel in the range of between 50% and 90% by area fraction. Pearlite is considered a hard phase compared to ferrite and provides strength to the steel of the present invention. The pearlite of the steel of the present invention has a two-phase interlayer structure consisting of alternating layers of ferrite and cementite, in which the pearlite ferrite is strengthened by intergranular and intragranular precipitations of niobium and vanadium in the form of carbides, nitrides and / or carbo-nitrides. Pearlite is formed during the cooling stage after hammering. However, when pearlite is present in an amount exceeding 90%, an adverse effect on the machinability of the steel is observed. It is preferred that pearlite be between 60% and 90%, and more preferably between 60% and 85%. In a preferred embodiment according to the invention, the preferred perlite content is between 50% and 75%, more preferably between 60% and 75%, when the carbon content is between 0.2 and 0.4%. In another preferred embodiment, the preferred perlite content is between 75% and 90%, more preferably between 75% and 85%, when the carbon content is between 0.4 and 0.5%. The steel of the invention may optionally contain between 0% and 2% acicular ferrite. Acicular ferrite is not intended to be part of the invention but is a residual microstructure resulting from the processing of the steel. The acicular ferrite content should be kept as low as possible and should not exceed 2%. To achieve the desired mechanical properties, especially yield strength and tensile strength, the niobium equivalent must be 80% or more, i.e. the amount of niobium present in the form of carbides, nitrides and / or carbonitrides must be equivalent to at least 90% of the nominal niobium content present in the steel. It is preferred that the niobium equivalent is higher than 90% and more preferably higher than 95%. In addition, the steel of the present invention in preferred embodiments may have a vanadium equivalent of at least 60%, i.e. the amount of vanadium present in the form of carbides, nitrides and / or carbo-nitrides is equivalent to at least 60% of the nominal vanadium content present in the steel. When such vanadium equivalent is achieved, the mechanical properties, particularly tensile strength and yield strength, are improved. In addition to the microstructure mentioned above, the microstructure of the mechanically hammered part is free of microstructural components such as bainite, martensite, and reverted martensite. A mechanical part according to the invention can be obtained by any suitable hot forging process, such as impact forging, compression forging, horizontal forging, and rolling forging, and in accordance with the process components described below. An exemplary and preferred method is described below, but the example does not limit the scope of this disclosure and the aspects upon which the examples are based. Furthermore, all examples provided in this application are not intended to be exhaustive, but merely to illustrate some of the many ways in which various aspects of the present disclosure may be practiced. A preferred method involves providing a semi-finished casting of steel with a chemical composition according to the invention. The casting can be made in any form, such as ingots or billets, which can be hammered into pieces with a cross-section of up to 50 mm. For example, steel with the chemical composition mentioned above is cast into an ingot and then rolled into a bar. This bar can be considered as a semi-finished product for hammering. Several rolling steps can be performed to achieve the desired semi-finished product. To prepare for hammering operations, the semi-finished product can be used directly after rolling at a high temperature or it can be first cooled and then heated for hot hammering. The semi-finished product is reheated at a temperature between 1150 and 1300°C. The semi-finished product is then subjected to hot forging at above 950°C and preferably below 1280°C, preferably between 1000 and 1280°C and more preferably at a forging temperature between 1050 and 1280°C. If the reheating temperature of the semi-finished product is lower than 1150°C, excessive load will be placed on the forging dies during the subsequent forging operation and in addition the temperature of the steel may drop below the ferrite transformation onset temperature. Metallurgical transformation under pressure may result in a significant change in the resulting microstructure for a given cooling rate or chemical composition. As a result, the resulting microstructure and mechanical properties will be quite different from those intended. Therefore, the temperature of the semi-finished product should preferably be high enough to allow hot forging to be completed in the austenitic temperature range. Reheating at temperatures above 1300°C should be avoided as they are industrially expensive and may lead to the formation of liquid zones which affect the forging ability of the steel. A final finish hammering temperature (FFT) must be maintained above 950°C to achieve a structure that is susceptible to recrystallization and hammering. It is necessary to perform the final hammering at a temperature above 950°C because below this temperature, the steel sheet will show a significant reduction since hammering will occur below the non-recrystallization temperature of the steel. The malleability of the steel is severely degraded below the non-recrystallization temperature. This can lead to problems with the final dimensions of the hammered part as well as poor surface finish and can even lead to cracking or complete failure of the hammered part. After hot forging, a hot forged steel part is obtained, and then the hot forged steel part is cooled in a three-stage cooling process. In the first cooling stage, the hot forged steel is cooled at an average cooling rate of 3°C / s or less, preferably 2.5°C / s or less, and more preferably 2.0°C / s or less, from the final forged temperature to a temperature range of between 775 and 875°C, referred to herein as T1. The preferred T1 temperature range is between 775 and 825°C. During this stage, precipitation strengthening also occurs, forming precipitates of niobium and vanadium, nitrides, carbides, and / or carbo-nitrides. The hot forged steel can optionally be held at the T1 temperature range for 600 seconds or less. After T1, a second cooling stage begins, in which the hot forged part is cooled from T1 at an average cooling rate of between 0.5°C / s and 2.1°C / s, more preferably between 0.6°C / s and 2.0°C / s, to a temperature range of between 430 and 530°C, referred to herein as T2. The preferred T2 temperature range is between 475 and 525°C. During this stage, austenite transforms into ferrite and pearlite, and vanadium forms precipitates in the form of carbides, nitrides or carbonitrides. In the third stage, the hot forging is brought from T2 to ambient temperature where the average cooling rate is maintained at 5°C / s or less, preferably below 4°C / s, and more preferably below 2°C / s. These average cooling rates are selected to provide homogeneous cooling across the entire cross-section of the hot forging. After completing the third cooling stage, the hammer-finished mechanical part is obtained. Examples The following experiments, examples, illustrations and tables are non-limiting in nature and should be considered merely as exemplary representations that illustrate the advantageous features of the present invention. The hammer-forged mechanical part made of steels with different compositions is shown in Table 1, where the mechanical part was produced in accordance with the processing parameters detailed in Table 2. Table 3 shows the microstructures of the hammer-forged mechanical part during the tests, and Table 4 contains the results of the property evaluations obtained. Table 1 Steel sample C Mn Si V Nb Cr PSN Al Mo Ni Ti A 0.39 1.07 0.70 0.327 0.084 0.145 0.015 0.067 0.0175 0.004 0.018 0.078 0.002 B 0.45 1.10 0.68 0.278 0.029 0.149 0.014 0.064 0.0180 0.007 0.036 0.069 0.002 C 0.43 1.09 0.67 0.276 0.080 0.148 0.015 0.064 0.0192 0.007 0.037 0.069 0.002 D 0.41 1.07 0.70 0.330 0.029 0.145 0.014 0.070 0.0161 0.005 0.017 0.075 0.002 E 0.71 0.55 0.16 0.040 0.002 0.136 0.010 0.059 0.0121 0.003 0.022 0.081 0.002 F 0.46 1.20 0.58 0.140 0.002 0.226 0.009 0.047 0.0121 0.007 0.010 0.173 0.002 Underlined indicators do not correspond to the invention. Table 2 Table 2 contains the processing parameters performed on the semi-finished product made from the steels of Table 1. Tests I1 and I5 were performed to produce hammer-forged mechanical parts according to the invention. This table also specifies the reference hammer-forged mechanical parts, which are designated R1 to R3 in the table. Table 2 is as follows: Tests Steel Sample Reheating Temperature (°C) FFT (°C) CR1 (°C / s) T1 (°C) CR2 (°C / s) T2 (°C) CR3 (°C / s) T3 (°C) I1 A 1280 1150 2 800 1.0 500 2 20 I2 A 1280 1200 2 800 0.6 500 2 20 I3 B 1280 1150 2 800 0.6 500 2 20 I4 C 1280 1200 2 800 0.6 500 2 20 I5 D 1280 1150 2 800 1.9 500 2 20 R1 E 1280 1150 2 800 0.9 500 2 20 R2 F 1280 1100 2 800 0.9 500 2 20 R3 B 1280 1150 2 800 2.3 500 2 20 I: In accordance with the invention, R: Reference, underlined indicators do not comply with the invention. Table 3 Table 3 contains the results of the tests carried out according to the standards on the various microstructures such as scanning electron microscopy to determine the microstructures of both the innovative and reference steels in terms of area fraction. The measurement of the vanadium and niobium equivalents was carried out based on an electrolytic extraction followed by an optical emission spectroscopic analysis. The selective extraction of the precipitates was carried out with an electrolyte made of lithium chloride and salts of salicylic acid diluted in methanol. Methanol is preferred to prevent oxidation and to ensure an efficient filtration. The steel samples were exposed to a current density that allowed only the matrix to dissolve. After this electrolytic treatment, the resulting solution was filtered on a 200 nm polycarbonate membrane. Afterwards, the acid mineralization was carried out on the filter and the solution was then analyzed by ICP-OES. The results are as follows: Experiments Pearlite (%) Ferrite (%) Acicular Ferrite (%) Niobium Equivalent (%) I1 69 31 0 100 I2 67 33 0 100 I3 83 17 0 100 I4 80 20 0 99.2 I5 79 19 2 95.5 R1 97 3 0 0 R2 92 8 0 0 R3 88 7 5 92.4 I: In accordance with the invention, R: Reference, underlined indicators do not comply with the invention. Table 4 Table 4 contains the mechanical properties of both the innovative and reference steels. To determine the tensile strength and yield strength, strength / strength tests were carried out in accordance with the NF EN ISO 6892-1 criteria. Tests aimed at measuring the impact toughness of both the innovative and reference steels were carried out in accordance with the EN ISO 148-1 standard for the DVM specimen with a V-shaped notch at ambient temperature. The results of various mechanical tests performed in accordance with the standards can be seen below: Tests YS (MPa) UTS (MPa) Impact toughness (J) T El (%) I1 832 1075 3.1 13.6 I2 806 1067 2.3 14.1 I3 786 1076 4.7 15.1 I4 777 1048 3.3 14.7 I5 924 1155 2.6 12.5 R1 696 1016 5.9 14.2 R2 706 1017 9.3 15.9 R3 906 1172 4.1 11.6 I: In accordance with the invention, R: Reference, underlined indicators do not comply with the invention.

Claims

CLAIMS 1. A steel for forging mechanical parts comprising of the following elements, expressed in percentage by weight: 0.2% ≦ C ≦ 0.5 %; 0.8% ≦ Mn ≦1.5 %; 0 .4% ≦ Si ≦ 1 %; 0.15% ≦ V ≦ 0.6%; 0.01% ≦ Nb ≦ 0.15%; 0.01% ≦ Cr ≦ 0.5 %; 0.01% ≦ P ≦ 0.05%; 0.04% ≦ S ≦ 0.09%; 0.01% ≦ N ≦ 0.025%; and can contain one or more of the following optional elements 0 % ≦ Al ≦ 0.05 %; 0% ≦ Mo ≦ 0.5%; 0.01% ≦ Ni ≦ 0.5%; 0% ≦ Ti ≦ 0.2%; 0% ≦ B ≦ 0.008%; 0% ≦ Cu ≦ 0.5%; 0% ≦ Ce ≦ 0.1%; 0% ≦ Sn ≦ 0.1%; 0% ≦ Mg ≦ 0.010% and 0% ≦ Zr ≦ 0.010%. the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel comprising 50% to 90% of Pearlite, 10% to 40% of Ferrite, with an optional presence of acicular ferrite between 0% and 2%, a niobium equivalent of 80% or more, wherein niobium equivalent is the amount of niobium present as carbides, nitrides and / or carbo-nitrides is equivalent to at least 80% of the nominal niobium content present in the steel, said sheet having a total elongation of at least 12%, an ultimate tensile strength of 1030 MPa or more, and a yield strength of 750MPa or more, wherein the tensile strength and yield strength tensile are measured according to NF EN ISO 6892-1 standard, and said sheet having an impact toughness equal to or less than 5 J measured at room temperature in accordance with NF EN ISO 148-1 standard on a V-notched standard DVM specimen .

2. Steel for forging mechanical parts according to claim 1, wherein the composition includes 0.5% to 0.9% of Silicon.

3. Steel for forging mechanical parts according to claim 1 or 2, wherein the composition includes 0.3% to 0.5% of Carbon.

4. Steel for forging mechanical parts according to anyone of claim 1 to 3, wherein the composition includes 0.9% to 1.3% of Manganese.

5. Steel for forging mechanical parts according to anyone of claim 1 to 4, wherein the composition includes 0.05% to 0.3% of Chromium.

6. Steel for forging mechanical parts according to anyone of claim 1 to 5, wherein the composition includes 0.2% to 0.5% of Vanadium.

7. Steel for forging mechanical parts according to anyone of claim 1 to 6 wherein the composition includes 0.02% to 0.12% of Niobium.

8. Steel for forging mechanical parts according to anyone of claims 1 to 7 , wherein, the Niobium equivalent is between 90 and 100%.

9. Steel for forging mechanical parts according to anyone of claims 1 to 8 , wherein, the Vanadium equivalent is between 60 and 100%, wherein Vanadium equivalent is the amount of Vanadium present as carbides, nitrides and / or carbo-nitrides is equivalent to at least 60% of the nominal Vanadium content present in the steel.

10. Steel for forging mechanical parts according to anyone of claims 1 to 9 , wherein, the Pearlite is between 60% and 90%.

11. Steel for forging mechanical parts according to anyone of claims 1 to 10 , wherein, the Ferrite is between 10% and 40%.

12. A method of production forged mechanical parts of steel comprising the following successive steps: ­ providing a steel composition according to anyone of claims 1 to 9 in form of semi-finished product; ­ reheating said semi-finished product to a temperature between 1150°C and 1300°C; ­ hot forging the said semi-finished product in the austenitic range wherein the finishing hot forging finishing temperature shall be above 950°C to obtain a hot forged part; ­ cooling hot forged part in a three-step cooling, wherein o in step one the hot forged part is cooled at an average cooling rate CR1 of 3°C / s or less from the hot forging finishing temperature to a temperature T1 between 775 and 875°C, o in step two, the hot forged part is cooled at an average cooling rate CR2 between 0.5°C / s and 2.1°C / s from T1 to a temperature T2 between 430 and 530°C, o in step three, the hot forged part is cooled at an average cooling rate CR3 of 5°C / s or less from T2 to room temperature to obtain a forged mechanical part.

13. A method according to claim 12, wherein in the step one of cooling the hot forged part is cooled at an average cooling rate less than 2.5°C / s from the finishing hot forging temperature to a T1 temperature range between 775°C and 825°C .

14. A method according to claim 12 or 13, wherein in the step two of cooling the hot forged part is cooled at an average cooling rate between 0.6°C / s and 2.0°C / s from T1 to a T2 temperature range between 475°C and 525°C.

15. A method according to anyone of claims 12 to 14, wherein in step three the hot forged part is cooled at a cooling rate of 4°C / s or less from T2 to room temperature.

16. Use of a steel according to anyone of claims 1 to 11 or of a forged mechanical part produced according to the method of claims 12 to 15, for the manufacture of structural or safety parts of a vehicle or an engine.

17. Vehicle comprising a part obtained according to claim 16.