Forged steel parts and method for manufacturing the same

Abstract

A steel component for an automotive transmission, comprising the following elements, namely, 0.2% ≤ C ≤ 0.35%, 1.0% ≤ Mn ≤ 1.6%, 0.2% ≤ Si ≤ 0.7%, 0.001% ≤ Al ≤ 0.1%, 0.01% ≤ Mo ≤ 0.5%, 0.020% ≤ Nb ≤ 0.06%, 1% ≤ Cr ≤ 1.5%, 0 ≤ P ≤ 0.09%, 0 ≤ S ≤ 0.09%, 0.009% ≤ N ≤ 0.09%, 0 ≤ Ni ≤ 1%, 0 ≤ V ≤ 0.2%, 0 ≤ Ti ≤ 0.1%, 0 ≤ Cu ≤ 1%, 0 ≤ B ≤ 0.008%, 0 ≤ Sn ≤ 0.1%, 0 ≤ Ce ≤ 0.1%, 0 ≤ Mg ≤ 0.10%, 0 ≤ Zr ≤ 0.10%, the remaining composition being composed of iron and inevitable impurities generated by processing, the microstructure of the core of such a steel component containing, in area percentage, at least 90% bainite, with any one or more of 0% - 10% retained austenite, pearlite, ferrite or martensite being optionally present cumulatively, and containing precipitates of aluminum and niobium in the form of AlN and Nb(C,N), such a steel component having a martensite-enriched layer up to a depth of 1 mm on all surfaces of such a steel component, such a martensite-enriched layer containing 85% - 95% martensite, the remainder being any one or more of bainite, retained austenite, ferrite or cementite.

Description

Technical Field

[0001] The present invention relates to a steel suitable for forging mechanical parts of automotive steel, and particularly to a steel suitable for manufacturing gears, shafts and other transmission parts for automotive transmissions.

Background Art

[0002] Transmission parts such as gears, shafts, differential devices and other parts of automotive transmissions operate under conditions of high rotational speed, high load and continuous change of rotational speed and load. Therefore, transmission parts are required to have high strength, high hardness and good wear resistance, especially at the contact surfaces of these parts. On the other hand, the core of the transmission parts is required to have good durability. On the other hand, it is required that the transmission parts have high meshing accuracy and low operating noise. Therefore, the steel for automotive transmissions is obliged to meet two requirements: machinability for facilitating the manufacturing process and, on the contrary, high strength, high hardness and ease of use for being suitable for use during high-load and high-rotational-speed operation.

[0003] Therefore, intensive research and development efforts have been made to develop a material with excellent machinability while having a high yield strength exceeding 1330 MPa and sufficient impact toughness.

[0004] Previous research and development in the field of steel for automotive transmissions has led to several methods for producing high strength and good formability, some of which are listed herein for the ultimate understanding of the present invention.

[0005] US 20070193658 A1 is a steel for machine parts, and its composition is in weight percent: 0.19% ≤ C ≤ 0.25%, 1.1% ≤ Mn ≤ 1.5%, 0.8% ≤ Si ≤ 1.2%, 0.01% ≤ S ≤ 0.09%, trace level ≤ P ≤ 0.025%, trace level ≤ Ni ≤ 0.25%, 1% ≤ Cr ≤ 1.4%, 0.10% ≤ Mo ≤ 0.25%, trace level ≤ Cu ≤ 0.30%, 0.010% ≤ Al ≤ 0.045%, 0.010% ≤ Nb ≤ 0.045%, 0.0130% ≤ N ≤ 0.0300%, optionally including trace level ≤ Bi ≤ 0.10% and / or trace level ≤ Pb ≤ 0.12% and / or trace level ≤ Te ≤ 0.015% and / or trace level ≤ Se ≤ 0.030% and / or trace level ≤ Ca ≤ 0.0050%, with the balance being iron and impurities resulting from manufacturing operations, and its chemical composition is the average value J of five Jominy tests 3m , J 11m , J 15m and J2 5m are such that

[0006]

Number

[0007] WO2020 / 178854 provides a steel composition for high-temperature carburization and a steel article made from this steel composition. The composition contains: a) 0.11 to 0.3 wt% carbon, b) 1.1 to 1.4 wt% manganese, c) 0.15 to 0.35 wt% silicon, d) 1 to 1.3 wt% chromium, e) boron ≤ 0.0006 wt%, f) 0.04 to 0.05 wt% titanium, g) 0.035 to 0.056 wt% niobium, h) nickel < 0.2 wt%, i) molybdenum < 0.06 wt%, j) sulfur < 0.025 wt%, k) phosphorus < 0.025 wt%, l) 0.02 to 0.03 wt% aluminum, m) nitrogen ≤ 190 ppm, and n) the balance is iron (Fe). However, the steel of WO2020 / 178854 cannot reach sufficient levels of tensile strength and impact toughness.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Summary of the Invention

[0009] Therefore, an object of the present invention is to solve these problems by making available steel parts for automotive transmission devices that simultaneously have the following: - A maximum tensile strength of 1600 MPa or more, preferably exceeding 1650 MPa, - A yield strength of 1330 MPa or more, - When measured for KCU-type samples, an impact toughness of 53 J / cm 2 or less, preferably 50 J / cm 2 or less, - A YS / TS ratio of 0.80 or more

[0010] In a preferred embodiment, the steel part according to the present invention exhibits a surface hardness of 650 HV or more from a surface depth of 0.4 mm to 0.6 mm.

[0011] Preferably, such steel is suitable for the manufacture of forged steel parts for automotive transmission devices, and each part can have a cross-section of up to 150 mm × 150 mm. This steel is also suitable for other automotive parts such as chassis members.

[0012] Another object of the present invention is also to make available a manufacturing method for these mechanical parts that is stable with respect to shifts in manufacturing parameters while conforming to conventional industrial applications.

Embodiments for Carrying Out the Invention

[0013] Other features and advantages of the present invention will become apparent from the following detailed description of the present invention.

[0014] The carbon present in the steel of the present invention is 0.2% to 0.35%. Carbon is an element necessary to increase the strength of the steel of the present invention by generating low-temperature transformation phases such as bainite. However, a carbon content of less than 0.2% cannot impart tensile strength to the steel of the present invention. On the other hand, when the carbon content exceeds 0.35%, primary cementite is excessively formed during cooling after hot rolling or forging, which has an adverse effect on toughness. Furthermore, the excessive formation of primary cementite is also harmful to mechanical operations on parts of the transmission, such as hobbling, lapping finish, form drilling, honing, or grinding. The carbon content is preferably in the range of 0.22% to 0.35%, more specifically 0.25% to 0.30%.

[0015] Manganese is added to the base steel in an amount of 1.0% - 1.6%. This element is gammagenous. Manganese provides solid solution strengthening, suppresses the ferrite transformation temperature, reduces the ferrite transformation rate, and thus aids in the formation of bainite. An amount of at least 1.0% is required to impart strength and aid in the formation of bainite. However, when the manganese content is present in an amount greater than 1.6%, it causes segregation that results in a banded microstructure after annealing, and this banded microstructure is deferential to the mechanical properties of the steel of the process of the present invention. The preferred limit of the presence of manganese is 1.1% - 1.5%, more preferably 1.1% - 1.4%.

[0016] Silicon is present in the steel of the present invention in an amount of 0.2% - 0.7%. Silicon imparts strength to the steel of the present invention by solid solution strengthening and also acts as a deoxidizer. Silicon is a component that can retard the precipitation of carbides during cooling after mechanical operations, and thus silicon promotes the formation of bainite. However, silicon is also a ferrite former and raises the Ac3 transformation point, which pushes the austenite temperature into a higher temperature range, which is why the silicon content is maintained at a maximum of 0.7%. Further, silicon above 0.7% also enhances segregation. The preferred limit of the presence of silicon is 0.2 - 0.6%, more preferably 0.22% - 0.4%.

[0017] The aluminum content is 0.001% - 0.1%. Aluminum removes oxygen present in the molten steel and prevents oxygen from forming a gas phase during the solidification process. Aluminum also fixes nitrogen in the steel to form aluminum nitride and reduces the grain size. However, the deoxidation effect saturates when the aluminum content exceeds 0.1%. Aluminum also controls the grain size of the base steel by forming AlN. When the aluminum content is higher and exceeds 0.1%, coarse aluminum-enriched oxides are generated, which reduce the machinability and hot forging properties of the steel. The preferred limit of the presence of aluminum is 0.01% - 0.09%, more preferably 0.01 - 0.035%.

[0018] Molybdenum is an essential element and can be present in the range of 0.01% to 0.5% in the present invention. Molybdenum is added to impart hardenability and hardness to steel by forming molybdenum-based carbides, and also promotes the formation of martensite during carburizing and delays the formation of coarse niobium carbide or niobium carbonitride. However, the addition of molybdenum excessively increases the cost of adding alloying elements, so for economic reasons, its content is limited to 0.5%. The preferred limit of the molybdenum content is 0.03% to 0.4%, more preferably 0.05% to 0.2%.

[0019] Niobium is an essential element for the steel of the present invention at 0.020% to 0.06%, and is suitable for forming carbonitrides to impart strength to the steel of the present invention by precipitation hardening. Niobium also affects the size of the microstructure components through its precipitation as carbonitrides and by retarding recrystallization during the heating process. Thus, at the end of the holding temperature, the resulting microstructure formed after complete austenitization leads to the hardening of the product. However, a niobium content exceeding 0.06% is not economically interesting and forms coarser precipitates that are harmful to the fatigue properties and impact toughness of the steel. Also, when the niobium content is 0.06% or more, niobium is harmful to the hot ductility of the steel, causing difficulties during the casting and rolling of the steel. The preferred limit of the niobium content is 0.025% to 0.058%, more preferably 0.025% to 0.055%.

[0020] Chromium is present in the steel of the present invention at 1% to 1.5%. Chromium is an essential element that imparts strength to steel by solid-solution strengthening and requires at least 1% for strength imparting. However, if used in excess of 1.5%, coarse cementite is formed after cooling, impairing the forging property and ductility of the steel, and the hardenability exceeds the allowable limit. The addition of chromium also reduces the diffusion coefficient of carbon in austenite, similar to nickel, and thus promotes the formation of martensite during carburizing. The preferred limit of the presence of chromium is 1.1% to 1.4%, more preferably 1.1% to 1.3%.

[0021] The phosphorus content in the steel of the present invention is 0% to 0.09%. Phosphorus tends to segregate at grain boundaries or co-segregate with manganese. For these reasons, it is recommended to use as little phosphorus as possible. Specifically, a content exceeding 0.05% can cause cracking due to the peeling of grain boundaries that can be harmful to the fatigue limit. The preferred limit of the phosphorus content is 0% to 0.05%.

[0022] Sulfur is contained in an amount of 0% to 0.09%. Sulfur forms MnS precipitates that improve machinability and help obtain sufficient machinability. During metal forming processes such as rolling and forming, deformable manganese sulfide (MnS) inclusions elongate. Such elongated MnS inclusions can have a significant adverse effect on mechanical properties such as cross-sectional shrinkage and impact toughness when the inclusions are not aligned with the loading direction, and a higher sulfur content is also harmful to the forgeability of the steel. Therefore, the sulfur content is limited to 0.09%. The preferred range of the sulfur content is 0.05% to 0%, more preferably 0% to 0.040%, in order to obtain the best balance between machinability and fatigue limit.

[0023] Nitrogen is present in the steel of the present invention in an amount of 0.009% to 0.09%. Nb(C,N) seems to nucleate on AlN precipitates. A minimum of 0.009% nitrogen is required to obtain Nb(C,N) precipitates. The preferred limit of nitrogen is 0.009% to 0.05%, more preferably 0.009% to 0.04%.

[0024] By adding 0% to 1% of nickel to the present invention, the strength of the steel of the present invention is increased, and in particular, the toughness after normalization and carburization is improved. Nickel is beneficial for improving its pitting corrosion resistance. At least 0.1% is required to obtain such an effect. Adding nickel to the steel composition reduces the diffusion coefficient of carbon in austenite, thereby promoting the formation of low-temperature phases such as martensite and bainite during the carburization process. However, the presence of a nickel content exceeding 1% lowers the martensite start temperature, thus causing excessive stabilization of the retained austenite, thereby having an adverse effect on the tensile strength and yield strength. Furthermore, nickel is limited to 1% for economic reasons. In the steel of the present invention, nickel is preferably 0.1% to 0.9%.

[0025] Vanadium is an optional element of the present invention, and its content is 0% to 0.2%. Vanadium is effective in increasing the strength of steel, particularly by precipitation strengthening by forming carbides or carbonitrides. The upper limit is maintained at 0.2% for economic reasons.

[0026] The steel of the present invention does not always contain titanium because it forms coarse ones and is an optional element, with 0% to 0.1% present. Titanium forms titanium nitride that imparts strength to the steel, but since these nitrides can occur during the solidification process, they have an adverse effect on the fatigue limit. Therefore, the preferred limit of titanium is 0% to 0.05%.

[0027] Copper is a residual element and can be present up to 1% by steel processing. Copper up to 0.5% does not affect any of the properties of the steel, but when it exceeds 0.5%, the hot workability is significantly reduced.

[0028] Other elements such as tin, cerium, calcium, bismuth, magnesium or zirconium can be added individually or in combination in the following weight ratios, i.e., tin ≤ 0.1%, cerium ≤ 0.1%, magnesium ≤ 0.10%, calcium ≤ 0.0010%, bismuth ≤ 0.05%, 0% ≤ boron ≤ 0.008% and zirconium ≤ 0.10%. Up to the maximum content levels shown, these elements enable grain refinement during solidification. The balance of the steel composition consists of iron and inevitable impurities resulting from processing.

[0029] The balance of the composition is iron and inevitable impurities, in particular those resulting from refining. More specifically, the composition of the steel part consists of the elements described above.

[0030] Steel parts for automotive transmissions have a microstructure containing at least 90% bainite in surface fraction or area %, 0% - 10% of any residual austenite, perlite, ferrite and martensite in cumulative presence, and precipitates of Al and Nb in the form of AlN and Nb(C,N).

[0031] Bainite is present in the steel according to the invention as a matrix phase and imparts strength to such steel. Bainite is present in the steel in an area ratio of at least 90%, preferably 90% - 100% in area ratio, more preferably 95% - 100%. Bainite is formed during cooling after normalization. Such bainite may include lath-like bainite without cementite, granular bainite, upper bainite and lower bainite or any other bainite. Lath-like bainite without cementite consists of bainite in the form of laths and contains carbides between these laths with a number N of carbides between laths greater than 0.1 micrometer per unit surface area of 50000 / mm 2 as follows. Carbides are included between these laths. This lath-like bainite microstructure without cementite can impart high strength and impact toughness to the steel of the present invention. Lower bainite consists of bainite in the form of laths and contains fine iron carbide sticks precipitating inside the laths. The lower bainite microstructure can provide elongation and tensile strength to the steel of the present invention.

[0032] Precipitates of Al and Nb are present in the steel according to the invention as AlN and niobium carbonitride Nb(C,N), respectively. These precipitates preferably have a size of 20 nm to 350 nm. The formation of the precipitates occurs during the annealing process and the cooling step. Thereafter, the precipitates of the present invention are involved in the fixation of the prior austenite grains during the carburizing process, thereby assisting in the formation of bainite and martensite in the martensite-enriched layer of the present invention in the target amount. Therefore, the prior austenite grain size is preferably 3 to 12 as measured according to the ASTM grain index. More preferably, it has a prior austenite grain size of 4 to 11, still more preferably 4 to 10.

[0033] The cumulative presence of retained austenite, pearlite, ferrite and martensite does not adversely affect the present invention up to 10%, but the mechanical properties may be adversely affected if it exceeds 10%. Retained austenite can impart toughness and ductility to the steel of the present invention. The martensite of the present invention can impart strength and fatigue durability to the steel. Therefore, the preferred limit of the cumulative presence of ferrite and bainite is maintained at 0% to 8%, more preferably 0% to 4%.

[0034] In addition to this microstructure in the core of the steel part, it also includes a martensite-enriched layer on the entire surface of the steel part of the automobile transmission up to a depth of 1 mm or less, preferably 0.8 mm or less, more preferably 0.5 mm or less, showing a martensite ratio of 85% to 95% by area ratio, preferably 85% to 92%, more preferably 85% to 90%. The martensite-enriched layer formed on the surface preferably includes any or all possible martensite types, particularly fresh martensite, tempered martensite, etc. This martensite layer imparts a surface hardness of 650 Hv or more to the steel of the present invention, which provides good resistance to wear to the final steel part and also imparts accuracy when meshing parts during the rotational operation of the transmission.

[0035] The remaining portion of this surface layer is composed of any one or more of bainite, retained austenite, ferrite, and cementite.

[0036] The steel parts for automotive transmissions according to the present invention can be manufactured by any suitable manufacturing process using the specified process parameters described below.

[0037] Although a preferred exemplary method is demonstrated herein, this example does not limit the scope of the present disclosure and the aspects based on the examples. Further, any example described herein is not intended to be limiting and merely describes some of the many possible ways in which the various aspects of the present disclosure can be implemented.

[0038] In this preferred embodiment, the steel part considered to demonstrate the preferred process according to the present invention is a gear.

[0039] A preferred method is to provide a semi-finished casting of steel having the chemical composition according to the present invention. The casting can be carried out in any form such as an ingot or bloom or billet that can be manufactured or processed into a steel part having a cross-section up to 150 mm × 150 mm.

[0040] For example, the steel having the above chemical composition is cast into a billet and then rolled into the form of a bar. This bar can be used as a semi-finished product for further process steps of manufacturing. A plurality of rolling steps can be carried out to obtain the desired semi-finished product. A preferred semi-finished product has a cross-section of φ20 mm to φ110 mm

[0041] The semi-finished product after the rolling process may be used directly at a high temperature after rolling, or may first be cooled to room temperature and then reheated for hot forging at a temperature in the range of Ac3 + 30°C to 1300°C. The Ac3 of this steel is calculated by dilatometry research.

[0042] The temperature of the semi-finished product to be hot forged is preferably at least 1150 °C, but must be less than 1300 °C. This is because if the temperature of the semi-finished product is less than 1150 °C, an excessive load will be applied to the forging die, and furthermore, there is a risk that the temperature of the steel will drop to the ferrite transformation temperature during finish forging, whereby the steel is forged with transformed ferrite contained in the structure. Therefore, the temperature of the semi-finished product is preferably high enough so that hot forging can be completed in the austenite temperature range. Reheating at temperatures exceeding 1300 °C must be avoided because it is industrially expensive.

[0043] The final finish forging temperature, which is referred to as Tforging in this specification, must be maintained above 830 °C in order to have a structure favorable for recrystallization and forging. It is preferable to perform the final forging at a temperature exceeding Ac3 + 100 °C, preferably exceeding Ac3 + 200 °C, because below this temperature the steel bar shows a significant decline in forging.

[0044] In this way, hot forged parts are obtained, and then these hot forged steel parts are cooled to room temperature.

[0045] Next, the hot forged steel parts are annealed to reduce the hardness of the steel parts for further machining.

[0046] In annealing, the hot forged steel parts are heated to reach a soaking temperature TA of 600 °C to Ac3 + 200 °C, and the preferred TA temperature is 625 °C to Ac3 + 100 °C, more preferably 640 °C to Ac3 + 50 °C.

[0047] In the heating step, the hot forged steel parts are heated from room temperature to the soaking temperature TA at a heating rate HR1 of 0.1 °C / second to 100 °C / second. It is preferable to have an HR1 rate of 0.1 °C / second to 50 °C / second, more preferably 0.1 °C / second to 10 °C / second.

[0048] Subsequently, the hot-forged steel part is held at the annealing soaking temperature TA for 10 to 1000 seconds to ensure proper transformation to an austenite microstructure with a strongly work-hardened initial structure, thereby reducing the hardness of the hot-forged steel part. Next, the hot-forged steel part is cooled at a cooling rate CR1 exceeding 1 °C / second, preferably exceeding 2 °C / second, more preferably exceeding 5 °C / second, to a cooling stop temperature range CS1 of Ms - 5 °C to 15 °C, preferably Ms - 5 °C to 20 °C, more preferably Ms - 10 °C to 20 °C. The Ms of the steel of the present invention is calculated from the following formula. Ms (°C) = 539 - 423×%C - 30.4×%Mn - 17.7×%Ni - 12.1×%Cr - 7.5×%Mo - 11×%Si - Ac3 (°C) = 910 - 203C 1 / 2 +44.7Si - 15.2Ni + 31.5Mo + 104V + 13.1W - 30Mn - 11Cr - 20Cu + 700P + 400Al + 400Ti

[0049] Thereafter, a forged steel part is obtained, which is subjected to at least one mechanical manufacturing operation. The mechanical operations can include hobbling, forming, machining, grinding, honing, or any other suitable mechanical operation or manufacturing procedure. The mechanical operations can be performed at room temperature or at a higher temperature as desired by the conditions of the specific mechanical operation.

[0050] Next, the forged steel part is subjected to carburizing to form a martensite-enriched layer on all surfaces of the steel part and to impart the target microstructure and mechanical properties to the steel part of the present invention.

[0051] In carburizing, the forged steel part is heated to reach a carburizing temperature TZ of 800 °C to 1100 °C. The preferred TZ temperature is 850 °C to 1080 °C, more preferably 900 °C to 1080 °C.

[0052] In the heating step, the forged steel part is heated from room temperature to TZ at a heating rate HR2 of 0.1 °C / second to 20 °C / second. It is preferable to have an HR2 rate of 0.1 °C / second to 10 °C / second, more preferably 0.1 °C / second to 5 °C / second.

[0053] Subsequently, the forged steel part is held at TZ for 10 to 3600 seconds in a carbon-enriched atmosphere having a dew point of -15°C to +15°C. The carburizing treatment is intended to fuse carbon from the carbon-enriched atmosphere onto the surface of the forged steel part at a high temperature that converts the microstructure of the surface of the forged steel part into martensite. Thereby, a martensite-enriched layer is formed on all surfaces of the forged steel part. This enriched martensite layer can be up to a depth of 1 mm or less, preferably up to a depth of 0.8 mm or less, more preferably up to a depth of 0.5 mm or less. Subsequently, the forged steel part is cooled to a cooling stop temperature range CS2 of Ms - 5°C to 15°C, preferably Ms - 5°C to 20°C, more preferably Ms - 10°C to 20°C, at a cooling rate CR2 exceeding 1°C / second, preferably exceeding 2°C / second, more preferably exceeding 5°C / second, to obtain a steel part for an automotive transmission.

[0054] Thereafter, the obtained steel part for an automotive transmission can be reheated to a tempering temperature Ttemper of 150°C to 250°C for 100 to 600 seconds at a heating rate of at least 1°C / second, preferably at least 2°C / second, more preferably at least 10°C / second, optionally. The preferred temperature range for tempering is 180°C to 240°C, and the preferred duration for holding at Ttemper is 200 to 500 seconds.

Example

[0055] The following tests, examples, illustrative examples, and tables presented in this specification are essentially non-limiting and must be considered for illustrative purposes only and are intended to show the advantageous features of the present invention.

[0056] Forged machine parts made of steel having different compositions are summarized in Table 1, and the forged machine parts are manufactured according to the process parameters specified in Table 2, respectively. Subsequently, the microstructures of the forged machine parts obtained during the trial are summarized in Table 3, and the results of evaluating the obtained properties are summarized in Table 4.

[0057]

Table 1

[0058] Table 2 summarizes the process parameters applied to the steel semi-finished products of Table 1. Steels 1 and 2 are useful for the manufacture of forged machine parts according to the invention. This table also identifies the steels for the reference forged machine parts, steels 3 and 4.

[0059] Table 2 is as follows.

[0060] All steels were reheated to a temperature of 1250 °C and subjected to mechanical manufacturing operations.

[0061]

Table 2

[0062] Table 3 summarizes the results of tests and X-ray measurements carried out according to the standard with different microscopes such as scanning electron microscopes to determine the microstructure composition of both the steels of the invention and the reference trials.

[0063]

Table 3

[0064] Table 4 illustrates the mechanical properties of both the steel parts of the invention and the reference steel parts. To determine the tensile strength, tests are carried out according to the NF EN ISO6892-1 standard. The tests for measuring toughness and fatigue are carried out at room temperature according to the EN ISO148-1 standard KCU test specimens with U-notch.

[0065]

Table 4

Claims

1. A steel part for a transmission of an automobile, containing the following elements expressed in weight percent: 0.2%≦C≦0.35%, 1.0%≦Mn≦1.6%, 0.2%≦Si≦0.7%, 0.001%≦Al≦0.1%, 0.01%≦Mo≦0.5%, 0.020%≦Nb≦0.06%, 1%≦Cr≦1.5%, 0≦P≦0.09%, 0≦S≦0.09%, 0.009%≦N≦0.09%, and any of the following elements: 0%≦Ni≦1%, 0%≦V≦0.2%, 0%≦Ti≦0.1%, 0%≦Cu≦1%, 0%≦B≦0.008%, 0%≦Sn≦0.1%, 0%≦Ce≦0.1%, 0%≦Mg≦0.10%, 0%≦Zr≦0.10% the remainder of the composition being made up of iron and unavoidable impurities resulting from processing, and the microstructure of the core of the steel part comprises, in area percent, at least 90% bainite with 0% to 10% of any one or more of retained austenite, pearlite, ferrite or martensite optionally present cumulatively, and aluminum and niobium precipitates in the form of AlN and Nb(C,N), and the steel part has a martensite-rich layer on all surfaces of the steel part to a depth of 1 mm or less, the martensite-rich layer comprising 85% to 95% martensite, with the remainder being any one or more of bainite, retained austenite, ferrite or cementite.

2. 2. The steel part for an automotive transmission according to claim 1, wherein said composition comprises 0.2% to 0.6% silicon.

3. 2. The steel part for an automotive transmission according to claim 1, wherein said composition comprises 0.22% to 0.35% carbon.

4. 2. The steel for leaf springs according to claim 1, wherein said composition comprises 0.03% to 0.4% molybdenum.

5. 2. The steel for leaf springs according to claim 1, wherein said composition comprises 0.025% to 0.058% niobium.

6. 2. The steel part for an automobile transmission according to claim 1, wherein the bainite is 90% to 100%.

7. 2. The automotive transmission steel component according to claim 1, wherein the martensite-enriched layer comprises 85% to 92% martensite, the remainder being any one or more of bainite, retained austenite, ferrite, or cementite.

8. 2. A steel part for a motor vehicle transmission according to claim 1, wherein the ultimate tensile strength of the steel is at least 1600 MPa.

9. The steel has a strength of 53 J / cm ２ 2. A steel part for an automotive transmission according to claim 1, having an impact toughness of:

10. A method for manufacturing a steel part for a transmission of an automobile according to any one of claims 1 to 9, comprising the following successive steps: - providing a steel composition according to any one of claims 1 to 5 in the form of a semi-finished product, - reheating the semi-finished product to a temperature of between Ac3+30°C and 1300°C, - hot forging said semi-finished product in the austenitic range, the hot forging finishing temperature Tforging being greater than 830°C, to obtain a hot forged part; - cooling the hot forged part to room temperature; - thereafter, heating said hot forged part from room temperature to an annealing soaking temperature TA lying in the range of 600°C to Ac3+200°C, at a heating rate HR1 between 0.1°C / s and 100°C / s; - followed by annealing in TA for 10 to 1000 seconds; - then cooling said hot forged part from TA to a cooling stop temperature CS1 of Ms-5°C to 15°C at a cooling rate CR1 greater than 1°C / s to obtain a forged steel part; - performing one or more mechanical operations on said forged steel part; - then heating the forged part from room temperature to a carburizing temperature TZ in the range of 800°C to 1100°C at a heating rate HR2 of 0.1°C / s to 20°C / s; - then carburizing in a carbon-rich environment with a dew point of -15°C to +15°C at TZ for 10 to 3600 seconds; - then cooling said forged part from TZ to a cooling stop temperature CS2 of Ms-5°C to 15°C at a cooling rate CR2 greater than 1°C / s to obtain a steel part for a motor vehicle transmission; A method comprising:

11. 11. The method of claim 10, wherein the TA temperature is between 625°C and Ac3+100°C.

12. 11. The method of claim 10, wherein the temperature TA is between 640°C and Ac3+50°C.

13. 11. The method of claim 10, wherein the temperature TZ is between 850°C and 1080°C.

14. Use of a steel part according to any one of claims 1 to 9 for the manufacture of structural or safety parts of a vehicle or engine.

15. Use of a steel part manufactured according to the method of claim 10 for the manufacture of a structural or safety part of a vehicle or engine.

16. A vehicle comprising a part obtained according to claim 14.