Wrought steel part and method for manufacturing thereof

Abstract

A steel component for an automotive transmission system includes a core made of the following elements: 0.15%≦C≦0.25%; 1.0%≦Mn≦1.6%; 0.4%≦Si≦0.9%; 0.001%≦Al≦0.1%; 0.10%≦Mo≦0.5%; 1%≦Cr≦1.5%; O≦P≦0.09%; O≦S≦0.09% ; 0%≦N≦0.09%; 0≦Nb≦0.06%; 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%; Remaining components Composed of iron and unavoidable impurities caused by processing, the microstructure of the core of the steel component comprises at least 90% martensite by area percentage, and optionally accumulates 1% to 10% of any one or more of retained austenite, pearlite, ferrite, and bainite, as well as precipitates of aluminum and / or niobium in the form of AlN and / or Nb(C,N), wherein the percentage of original austenite grains with a size of 3 to 5 is limited to a maximum of 10% of the total grains. Such a steel component comprises a martensite depletion layer with a depth of 1 mm or less on all its surfaces, such martensite depletion layer comprising 55% to 72% martensite, 10% to 40% bainite, and the remainder being any one or more of retained austenite, ferrite, or cementite.

Description

Technical Field

[0001] This invention relates to steel suitable for forging automotive steel mechanical parts, and particularly to steel suitable for manufacturing gears, shafts and other transmission components for automotive transmission systems. Background Technology

[0002] Automotive transmission components, such as gears, shafts, differentials, and other parts, operate under conditions of high speed and high load, as well as constantly alternating speed and load. Therefore, transmission components must possess high strength, high hardness, and good wear resistance, especially the contact surfaces, while the core of these components requires good durability. Simultaneously, high meshing accuracy and low operating noise are also required. Therefore, steel used in automotive transmission systems must meet two requirements: firstly, it must be machinable to facilitate the manufacturing process; secondly, it must possess high strength and high hardness to ensure its usability during high-load and high-speed operation.

[0003] Therefore, a great deal of research and development effort has been invested in developing a material with good machinability, a high yield strength of over 900 MPa, and sufficient impact toughness.

[0004] Early research in the field of automotive transmission steel has yielded various methods for producing high-strength steel with good formability. This article lists some of these methods to clearly understand the present invention:

[0005] WO2020 / 178854 provides a steel composition for high-temperature carburizing and steel articles made from the steel composition. The composition comprises: 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) ≤0.0006 wt.% boron, f) 0.04 to 0.05 wt.% titanium, g) 0.035 to 0.056 wt.% niobium, h) <0.2 wt.% nickel, i) <0.06 wt.% molybdenum, j) <0.025 wt.% sulfur, k) <0.025 wt.% phosphorus, l) 0.02 to 0.03 wt.% aluminum, m) ≤190 ppm nitrogen, and n) the balance being iron (Fe). However, WO2020 / 178854 steel cannot achieve sufficient tensile strength and impact toughness levels. Summary of the Invention

[0006] Therefore, the object of the present invention is to solve these problems by providing a steel suitable for mechanical operation for manufacturing components for automotive transmission systems, which simultaneously possesses:

[0007] - Tensile strength greater than or equal to 1050 MPa, preferably greater than 1100 MPa.

[0008] - Yield strength greater than or equal to 960 MPa, preferably greater than 1000 MPa.

[0009] - When measuring KCU type samples, impact toughness of 60 J / cm2 or higher, preferably 65 J / cm2 or higher, is required.

[0010] - A reduction of area greater than or exceeding 55%, preferably exceeding 58%.

[0011] Preferably, such steel is suitable for manufacturing forged steel components for automotive transmission systems, wherein the cross-section of each component can be up to 150 mm x 150 mm, and the steel is also suitable for other automotive components, such as chassis components.

[0012] Preferably, the surface hardness at a depth of 0.4 mm to 0.6 mm in the surface hardened layer is 650 Hv or greater.

[0013] Preferably, the total elongation exceeds 15%.

[0014] Another object of the present invention is to provide a method for manufacturing these mechanical parts, which is compatible with conventional industrial applications and robust to changes in manufacturing parameters. Detailed Implementation

[0015] Other features and advantages of the invention will become apparent from the following detailed description of the invention.

[0016] Carbon is present in the steel of the present invention at a concentration of 0.15% to 0.25%. Carbon is an essential element for improving the strength of the steel of the present invention by generating low-temperature phase transformation phases such as bainite, but a carbon content below 0.15% will not impart tensile strength to the steel of the present invention. On the other hand, at carbon contents exceeding 0.25%, toughness is adversely affected due to the excessive formation of proeutectoid cementite during cooling after hot rolling or forging. Furthermore, the excessive formation of proeutectoid cementite is also detrimental to mechanical operations on transmission system components, such as gear hobbing, grinding, gear shaping, drilling, honing, or grinding. The carbon content is advantageously in the range of 0.16% to 0.24%, and more particularly in the range of 0.18% to 0.24%.

[0017] Manganese is added to the steel of the present invention at a concentration of 1.0% to 1.6%. This element is an austenite-forming element. Manganese provides solid solution strengthening and inhibits the ferrite transformation temperature, as well as reducing the ferrite transformation rate, thereby contributing to bainite formation. At least 1.0% is required to impart strength and contribute to bainite formation. However, when the manganese content is present at a concentration greater than 1.6%, it causes segregation, which results in a banded microstructure after annealing, and this banded microstructure is detrimental to the mechanical properties of the steel of the present invention. The preferred limit for the presence of manganese is 1.1% to 1.5%, and more preferably 1.1% to 1.4%.

[0018] Silicon is present in the steel of the present invention at a concentration of 0.4% to 0.9%. Silicon imparts strength to the steel of the present invention through solid solution strengthening and also acts as a deoxidizer. Silicon is a component that can delay carbide precipitation during cooling after mechanical operation; therefore, silicon promotes martensite formation. However, silicon is also a ferrite-forming element and also raises the Ac3 phase transformation point, which will push the austenite temperature to a higher temperature range, which is why the silicon content is kept at a maximum of 0.9%. Furthermore, silicon concentrations above 0.9% also enhance segregation. The preferred limit for the presence of silicon is 0.5% to 0.8%, and more preferably 0.55% to 0.70%.

[0019] The aluminum content is from 0.001% to 0.1%. Aluminum removes oxygen present in the molten steel to prevent oxygen from forming a gaseous phase during solidification. Aluminum also fixes nitrogen in the steel to form aluminum nitride, thereby reducing grain size. However, the deoxidation effect saturates for aluminum contents exceeding 0.1%. Aluminum also controls the grain size of the steel of the present invention by forming AlN. Higher aluminum contents above 0.1% lead to the formation of coarse aluminum-rich oxides, thereby deteriorating the machinability and hot forging properties of the steel. The preferred limit for the presence of aluminum is from 0.01% to 0.09%, and more preferably from 0.01% to 0.035%.

[0020] Molybdenum is an essential element and can be present in the present invention at a concentration of 0.1% to 0.5%. The addition of molybdenum imparts hardenability and hardness to the steel by forming molybdenum-based carbides, and also promotes martensite formation during carburizing while delaying the formation of coarse niobium carbide or carbonitride. However, excessive addition of molybdenum increases the cost of alloying element addition, therefore, for economic reasons, its content is limited to 0.5%. The preferred limit for molybdenum content is 0.1% to 0.4%, and more preferably 0.15% to 0.35%.

[0021] Chromium is present in the steel of the present invention at a concentration of 1% to 1.5%. Chromium is an essential element for providing strength to steel through solid solution strengthening, and a minimum of 1% is required to impart strength. However, when used at concentrations above 1.5%, the increase in hardenability exceeds acceptable limits due to the formation of coarse cementite upon cooling, thereby impairing the forgeability and ductility of the steel. Like nickel, the addition of chromium also reduces the diffusion coefficient of carbon in austenite, thereby promoting martensite formation during carburizing. The preferred concentration of chromium is 1.1% to 1.4%, and more preferably 1.2% to 1.4%.

[0022] The phosphorus content of the steel of the present invention is 0% to 0.09%.

[0023] Phosphorus tends to segregate at grain boundaries or co-segregate with manganese. For these reasons, it is recommended to use phosphorus as little as possible. Specifically, contents exceeding 0.05% may lead to fracture due to depolymerization at grain boundaries, which can be detrimental to fatigue limit. The preferred limit for phosphorus content is 0% to 0.05%.

[0024] Sulfur is included in the content from 0% to 0.09%. Sulfur forms MnS precipitates, which improve machinability and contribute to achieving sufficient machinability. During metal forming processes such as rolling and forming, deformable manganese sulfide (MnS) inclusions are elongated. If the inclusions are not aligned with the loading direction, such elongated MnS inclusions can have a considerable adverse effect on mechanical properties such as reduction of area and impact toughness. Furthermore, a higher sulfur content is detrimental to the forgeability of the steel. Therefore, the sulfur content is limited to 0.09%. The preferred range of sulfur content is 0% to 0.05%, and more preferably 0% to 0.040%, to achieve the optimal balance between machinability and fatigue limit.

[0025] Nitrogen is present in the steel of the present invention in an amount of 0% to 0.09%. Within the scope of the present invention, the nitrogen content can be greater than 0.011% to promote AlN precipitation; NbCN and AlN precipitation. It appears that NbCN nucleates on the AlN precipitates. For precipitates to be present, a minimum of 0.009% nitrogen is required. Therefore, the preferred limit of nitrogen is 0.009% to 0.05%, and more preferably 0.009% to 0.04%.

[0026] Niobium is an optional element in the steel of the present invention and can be added at a maximum of 0.06%, and is suitable for forming carbonitrides to impart strength to the steel of the present invention through precipitation strengthening. Niobium also affects the size of the microstructure components by its precipitation as carbonitrides and by recrystallization during the delayed heating process. Thus, a finer microstructure is formed at the end of the holding temperature, and subsequently, the product hardens after complete austenitization. However, niobium contents above 0.06% are not economically attractive, and the formation of coarser precipitates is detrimental to the fatigue properties and impact toughness of the steel. Furthermore, when the niobium content is 0.06% or higher, niobium is also detrimental to the hot ductility of the steel, leading to difficulties during steel casting and rolling. The preferred limit for niobium content is 0.020% to 0.06%, or more preferably 0.025% to 0.058%, and more preferably 0.025% to 0.055%.

[0027] Nickel is added to the present invention at 0% to 1% to improve the strength and toughness of the steel, especially after normalizing and carburizing. Nickel is beneficial for improving its resistance to pitting corrosion. A minimum of 0.1% is required to achieve this effect. Adding nickel to the steel composition reduces the diffusion coefficient of carbon in austenite, thereby promoting the formation of martensite and low-temperature phases such as bainite during the carburizing process. However, the presence of nickel content above 1% lowers the martensitic transformation initiation temperature, leading to excessive stabilization of the retained austenite, which adversely affects tensile strength and yield strength. Furthermore, nickel is limited to 1% for economic reasons. The steel of the present invention preferably contains 0.1% to 0.9% nickel.

[0028] Vanadium is an optional element in this invention, and its content is from 0% to 0.2%. Vanadium is effective in strengthening steel by precipitation, particularly by forming carbides or carbonitrides. For economic reasons, the upper limit is kept at 0.2%.

[0029] The steels of this invention are typically titanium-free because titanium forms coarse precipitates. Titanium is an optional element and is present at a maximum of 0.1%. Titanium forms titanium nitrides, which impart strength to the steel, but these nitrides can form during solidification, thus adversely affecting the fatigue limit. Therefore, the preferred limit for titanium is a maximum of 0.05%.

[0030] Copper is a residual element and can be present at levels as high as 1% due to steel processing. Up to 0.5% copper will not affect any properties of steel, but above 0.5%, hot workability will decrease significantly.

[0031] Other elements such as tin, cerium, calcium, bismuth, magnesium, or zirconium may be added individually or in combination in the following proportions: by weight, tin ≤0.1%, cerium ≤0.1%, magnesium ≤0.10%, calcium ≤0.0010%, bismuth ≤0.05%, boron ≤0.008%, and zirconium ≤0.10%. These elements, up to the maximum content levels shown, enable grain refinement during solidification. The remaining portion of the steel composition consists of iron and unavoidable impurities resulting from processing.

[0032] The remainder of this composition consists of iron and unavoidable impurities, particularly those resulting from smelting. More specifically, steel components are composed of the aforementioned elements.

[0033] Steel components for automotive transmission systems have a microstructure comprising, by area %: at least 90% martensite, and optionally 1% to 10% of retained austenite, pearlite, ferrite, and bainite.

[0034] Martensite constitutes at least 90% of the microstructure by area fraction. The martensite of the present invention may comprise fresh martensite and tempered martensite. However, fresh martensite is an optional microcomponent, and its amount in the steel is limited to a maximum of 4%, preferably a maximum of 2%, and even more preferably equal to 0%. Fresh martensite can be formed during cooling after tempering. Tempered martensite is formed from martensite formed during a second cooling step after carburizing annealing, particularly below the Ms temperature, and even more particularly between Ms -10°C and 20°C. Such martensite is then tempered at a tempering temperature Ttemper of 180°C to 250°C during holding. The martensite of the present invention imparts toughness and strength to the steel. Preferably, the martensite content is 92% to 99%, and more preferably 95% to 99%.

[0035] The accumulated amount of retained austenite, pearlite, ferrite, and bainite accounts for 1% to 10% of the microstructure. The accumulation of retained austenite, pearlite, ferrite, and bainite does not adversely affect the invention up to 10%, but above 10%, mechanical properties may be adversely affected. Retained austenite can impart toughness and ductility to the steel of the present invention. Pearlite can impart strength and fatigue durability to the steel of the present invention. Therefore, the preferred limit for the accumulation of ferrite and bainite is maintained at 1% to 8%, and more preferably 1% to 5%.

[0036] Al and Nb precipitates exist in the steel according to the invention as AlN and niobium carbonitride Nb(C,N), respectively. The size of these precipitates is preferably 20 nm to 350 nm, more preferably 25 nm to 300 nm. The formation of the precipitates occurs during the annealing process and the cooling step. Thereafter, the precipitates of the invention are responsible for pinning the original austenite grains during the carburizing process, thereby contributing to the formation of the martensite-depleted layer of the invention in the target amount. Therefore, it is preferred that the original austenite grain size, measured according to ASTM grain size grades, is 3 to 15. More preferably, the original austenite grain size is 3 to 13, and even more preferably 3 to 12. Furthermore, according to the scope of the invention, grains with a primary austenitic grain size of 3 to 5 may be present in the steel of the invention at a maximum of 10%, preferably 2% to 9%, and more preferably 2% to 8% of the total grains, because whenever the percentage of grains with a primary austenitic grain size of 3 to 5 exceeds 10%, the overall grain size on the final steel product becomes coarser and mechanical properties such as yield strength or impact toughness cannot be achieved.

[0037] In addition to the microstructure of the steel component core, it also includes a martensite-depleted layer with a depth of up to 1 mm, preferably up to 0.5 mm, on all surfaces of the automotive transmission system steel components, exhibiting a martensite percentage of 55% to 72%, preferably 60% to 72%, and more preferably 65% ​​to 72% by area. The martensite-depleted layer formed on the surface preferably contains any or all possible types of martensite, particularly fresh martensite, tempered martensite, etc. The martensite in the martensite-depleted layer preferably has a carbon content of 0.5% to 0.9%, more preferably 0.6% to 0.8%. This martensite layer imparts a surface hardness of 650 Hv or higher to the steel of the present invention, which provides good wear resistance to the final steel components and also imparts precision during component meshing during rotational operation of the transmission system. This martensite-depleted layer is formed during carburizing at high temperatures.

[0038] The surface layer also contains 10% to 40% bainite. The presence of bainite in the martensite-depleted layer imparts surface hardness and impact toughness to the steel. The preferred bainite content is 15% to 35%, and more preferably 20% to 35%.

[0039] The remainder of the surface layer contains retained austenite and / or ferrite and / or cementite.

[0040] The steel components for automotive transmission systems according to the present invention can be produced by any suitable manufacturing method, wherein the specified process parameters are described below.

[0041] This document presents a preferred exemplary method, but this example does not limit the scope of this disclosure or the aspects on which the example is based. Furthermore, any examples set forth in this specification are not intended to be limiting, but merely illustrate some of the many possible ways in which various aspects of this disclosure can be put into practice.

[0042] In this preferred embodiment, the steel component intended to present the preferred method according to the invention is a gear.

[0043] A preferred method includes providing a steel semi-finished casting having the chemical composition according to the invention. Casting can be carried out in any form, such as steel ingots or large or small billets, which can be manufactured or processed into steel parts with cross-sections up to 150 mm x 150 mm.

[0044] For example, steel with the above chemical composition is cast into small billets and then rolled into bars. These bars can serve as semi-finished products for further manufacturing processes. Multiple rolling steps can be performed to obtain the desired semi-finished product. Preferred semi-finished products have a cross-section of Ø20 mm to Ø110 mm.

[0045] The semi-finished product after rolling can be used directly at high temperature after rolling, or it can be cooled to room temperature first and then heated to hot forge in the temperature range of Ac3+30℃ to 1300℃, wherein Ac3 of the steel plate is calculated using the following formula:

[0046]

[0047] The temperature of the semi-finished product undergoing hot forging is preferably at least 1150°C and must be below 1300°C. This is because a temperature below 1150°C would place excessive load on the forging die, and furthermore, the steel temperature might drop to the ferrite transformation temperature during final forging, resulting in the steel being forged with a microstructure containing transformed ferrite. Therefore, the temperature of the semi-finished product is preferably high enough to allow hot forging to be completed within the austenitic temperature range. Reheating at temperatures above 1300°C must be avoided, as it is industrially expensive.

[0048] The final hot forging temperature (hereinafter referred to as Tforging) must be maintained above 830°C to obtain a microstructure that is favorable for recrystallization and forging. Final forging is preferably performed at a temperature above Ac3+100°C, and more preferably above Ac3+200°C, because below this temperature, the steel bar exhibits a significant decrease in forging performance.

[0049] Therefore, hot-forged parts are obtained in this way, and then the hot-forged steel parts are cooled to room temperature.

[0050] The hot-forged steel component is then subjected to annealing to reduce its hardness for further machining, and also to impart the desired microstructure and mechanical properties to the steel component of the present invention.

[0051] During annealing, the hot-forged steel part is heated to a homogenization temperature TA of 600°C to Ac3+200°C, preferably 625°C to Ac3+100°C, more preferably 640°C to Ac3+50°C.

[0052] In the heating step, the hot-forged steel part is heated from room temperature to a homogenization temperature TA at a heating rate HR1 of 0.1°C / second to 100°C / second. Preferably, the HR1 rate is 0.1°C / second to 50°C / second, and more preferably 0.1°C / second to 10°C / second.

[0053] The hot-forged steel component is then held at the annealing homogenization temperature TA for 10 to 10,000 seconds, preferably 10 to 5,000 seconds, to ensure that the initially work-hardened microstructure is fully transformed into an austenitic microstructure, thereby reducing the hardness of the hot-forged steel component. The hot-forged steel component is then cooled at a cooling rate CR1 greater than 1°C / second, preferably greater than 2°C / second, more preferably greater than 5°C / second, to a cooling stop temperature range CS1 of Ms -5°C to 15°C, preferably Ms -10°C to 20°C, more preferably Ms -20°C to 20°C, wherein Ms of the steel plate is calculated using the following formula:

[0054]

[0055] The forged steel component is then obtained and subjected to at least one mechanical manufacturing operation. This mechanical operation may include gear hobbing, gear shaping, machining, grinding, honing, or any other suitable mechanical operation or manufacturing process. The mechanical operation may be performed at room temperature or higher, depending on the conditions required for the specific mechanical operation.

[0056] The forged steel component is then subjected to carburizing to form a martensitic depletion layer on all surfaces of the steel component.

[0057] In carburizing, the forged steel part is heated to a carburizing temperature TZ of 800°C to 1100°C, preferably 850°C to Ac3+200°C, more preferably 900°C to 1040°C.

[0058] 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. Preferably, the HR2 rate is 0.1°C / second to 15°C / second, more preferably 0.1°C / second to 10°C / second.

[0059] The forged steel component is then held at TZ for 10 to 6000 seconds in a carbon-rich atmosphere with a dew point of -15°C to +15°C. The carburizing process aims to melt carbon from the carbon-rich atmosphere into the surface of the forged steel component at high temperature, resulting in a martensitic depleted layer on the surface rich in 0.5% to 0.9% surface carbon, preferably 0.6% to 0.8%. The forged steel component is then cooled at a cooling rate CR2 greater than 1°C / s, preferably greater than 2°C / s, more preferably greater than 5°C / s, to a cooling stop temperature range CS2 of Ms-5°C to 25°C, preferably Ms-5°C to 20°C, more preferably Ms-10°C to 20°C. This carburizing yields a surface-hardened layer depth of 0.2 mm to 0.6 mm, achieving a surface hardness exceeding 650 HV.

[0060] Subsequently, the material is reheated to a tempering temperature Ttemper of 275°C to 600°C at a heating rate of at least 1°C / second, preferably at least 2°C / second, and held at Ttemper for 100 seconds to 6000 seconds. The preferred tempering temperature range is 300°C to 575°C, and the preferred holding time at Ttemper is 200 seconds to 5000 seconds, to obtain steel components for automotive transmission systems.

[0061] Example

[0062] The tests, embodiments, illustrative diagrams and tables presented herein are non-limiting in nature and must be considered for illustrative purposes only, and will present advantageous features of the invention.

[0063] The forged mechanical parts made of steels with different compositions, along with the Ac3 and Ms temperatures, are summarized in Table 1, whereby the forged mechanical parts were produced according to the process parameters specified in Table 2. Subsequently, Table 3 summarizes the microstructure of the forged mechanical parts obtained during the experiment, and Table 4 summarizes the evaluation results of the obtained properties.

[0064] Table 1

[0065]

[0066] Table 2

[0067] Table 2 summarizes the process parameters implemented on the semi-finished products made from the steel in Table 1. Tests I1 to I2 were used to manufacture forging machinery parts according to the invention. The table also specifically lists reference forging machinery parts, which are designated R1 to R3 in the table. Table 2 is as follows:

[0068]

[0069] I = According to the present invention; R = Reference; Underlined values: not in accordance with the present invention.

[0070] Table 3 summarizes the results of tests and X-ray measurements performed according to standards on different microscopes, such as scanning electron microscopes, to determine the microstructure composition of both the invention steel and the reference test.

[0071] Table 3: Microstructure of the experiment and presence of martensite in the surface layer

[0072]

[0073] I = According to the present invention; R = Reference; Underlined values: not in accordance with the present invention.

[0074] Table 4 illustrates the mechanical properties of the invention steel and the reference steel. Tensile strength was determined according to NF EN ISO 6892-1. Toughness and fatigue tests for both the invention steel and the reference steel were performed at room temperature using KCU specimens with a U-notch, according to EN ISO 148-1. The results of various mechanical tests performed according to the standards are summarized.

[0075] Table 4

[0076]

[0077] I = According to the present invention; R = Reference; Underlined values: not in accordance with the present invention.

Claims

1. A steel component for an automotive transmission system, comprising a core made of the following elements: (expressed as a percentage by weight) 0.15%≦C≦0.25%； 1.0%≦Mn≦1.6%; 0.4%≦Si≦0.9%; 0.001%≦Al≦0.1%； 0.10%≦Mo≦0.5%; 1%≦Cr≦1.5%； 0≦P≦0.09%； 0≦S≦0.09%； 0%≦N≦0.09%； And it can contain one or more of the following optional elements: 0≦Nb≦0.06%; 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 portion consists of iron and unavoidable impurities caused by processing. The microstructure of the core of the steel component comprises at least 90% martensite by area percentage, and optionally accumulates 1% to 10% of any one or more of retained austenite, pearlite, ferrite, and bainite, as well as precipitates of aluminum and / or niobium in the form of AlN and / or Nb(C,N), wherein the percentage of original austenite grains with a size of 3 to 5 is limited to a maximum of 10% of the total grains. Such a steel component comprises a martensite depletion layer with a depth of 1 mm or less on all its surfaces, such martensite depletion layer comprising 55% to 72% martensite, 10% to 40% bainite, and the remainder being any one or more of retained austenite, ferrite, or cementite.

2. The steel component for an automotive transmission system according to claim 1, wherein the component comprises 0.5% to 0.8% silicon.

3. The steel component for an automotive transmission system according to claim 1 or 2, wherein the component comprises 0.16% to 0.24% carbon.

4. The steel component for an automotive transmission system according to any one of claims 1 to 3, wherein the component comprises 0.1% to 0.4% molybdenum.

5. The steel component for an automotive transmission system according to any one of claims 1 to 4, wherein the component comprises 1.1% to 1.4% chromium.

6. The steel component for an automotive transmission system according to any one of claims 1 to 5, wherein, The martensite content is 92% to 99%.

7. The steel component for an automotive transmission system according to any one of claims 1 to 6, wherein, The martensite depletion layer comprises 60% to 72% martensite, with the remainder being any one or more of bainite, retained austenite, ferrite, or cementite.

8. The steel component for an automotive transmission system according to any one of claims 1 to 7, wherein, The tensile strength of the steel is at least 1050 MPa.

9. The steel component for an automotive transmission system according to any one of claims 1 to 8, wherein the steel has an impact toughness equal to or less than 90 J / cm².

10. A method for producing a steel component for an automotive transmission system according to any one claim, comprising the following sequential steps: - Provided as a semi-finished product the steel composition according to any one of claims 1 to 5; - Reheat the semi-finished product to a temperature of Ac3+30°C to 1300°C; - The semi-finished product is hot-forged in the austenitic range to obtain a hot-forged part, wherein the final hot-forging temperature Tforging should be higher than 830°C; - Cool the hot-forged parts to room temperature; - The hot forged part is then heated from room temperature to annealing temperature TA at a heating rate HR1 of 0.1°C / sec to 100°C / sec, wherein the annealing temperature TA is in the range of 600°C to Ac3+200°C. - Then anneal under TA for 10 to 1000 seconds; - The hot forged part is then cooled from TA to a cooling stop temperature CS1 of Ms-5°C to 15°C at a cooling rate CR1 greater than 1°C / second to obtain a forged steel part. - Perform one or more mechanical operations on the forged steel component; - The forged part is then heated from room temperature to carburizing temperature TZ at a heating rate HR2 of 0.1°C / s to 20°C / s, wherein the carburizing temperature TZ is in the range of 800°C to 1100°C. - Then carburizing was carried out at TZ for 10 to 6000 seconds in a carbon-rich environment with a dew point of -15°C to +15°C; - Then the forging is cooled from TZ to a cooling stop temperature CS2 of Ms-5°C to 15°C at a cooling rate of at least 1°C / second CR2; - The forged part is then reheated to a tempering temperature of 275°C to 600°C at a heating rate of at least 1°C / second, held at Ttemper for 100 seconds to 6000 seconds, and then cooled to room temperature to obtain steel parts for automotive transmission systems.

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

12. The method according to any one of claims 11 or 12, wherein the temperature TA is from 640°C to Ac3+50°C.

13. The method according to any one of claims 10 to 12, wherein the temperature TZ is from 850°C to Ac3+200°C.

14. Use of the steel component according to any one of claims 1 to 9 or the steel component produced by the method according to claims 10 to 13 in the manufacture of structural or safety components of a vehicle or engine.

15. A vehicle comprising the component obtained according to claim 14.

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