Forged steel component and method for manufacturing same

Abstract

The steel part for the automobile transmission system comprises the following elements: 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 invention relates to a steel component, the remainder consisting of iron and inevitable impurities resulting from processing, the microstructure of the core of said steel component comprising, in area percentage: at least 90% bainite, with 0% to 10% cumulative optionally present from any one or more of retained austenite, pearlite, ferrite or martensite, and at least one of AlN and Nb (C, N), with the remainder consisting of iron and inevitable impurities resulting from the processing. The invention relates to a steel component comprising precipitates of aluminum and niobium in the form of N, N), such a steel component having a martensite-rich layer up to a depth of 1 mm or less on all surfaces of the steel component, such martensite-rich layer comprising 85% to 95% martensite, the remainder being from any one or more of bainite, retained austenite, ferrite or cementite.

Description

Technical Field

[0001] The present invention relates to steel suitable for forging mechanical steel parts for automobiles, and in particular to steel suitable for manufacturing gears, shafts and other transmission parts for automobile transmission systems. Background Art

[0002] The transmission components of the automobile transmission system, such as gears, shafts, differentials and other components, work under high speed and high load and under the conditions of continuous alternation of speed and load. Therefore, it is necessary to make the transmission components have high strength, high hardness and good wear resistance, especially the contact surfaces of these components, and the core of the transmission components needs to have good durability, and at the same time, the meshing accuracy of the transmission components needs to be high, and the working noise needs to be low. Therefore, it is stipulated that the steel for automobile transmission system meets two requirements, one is machinability, so as to facilitate the manufacturing process, and on the contrary, it has high strength and high hardness, so that the steel is suitable for use and availability during high load and high speed operation.

[0003] Therefore, a great deal of research and development effort has been invested in developing materials which are good in processability while having a high yield strength above 1330 MPa and sufficient impact toughness.

[0004] Earlier research and development in the field of automotive driveline steels has resulted in several methods for producing high strength and good formability, some of which are listed herein for a clear understanding of the present invention:

[0005] US20070193658A1 is a steel for mechanical components, wherein its composition by weight percentage is: 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, 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%; the balance is iron and impurities resulting from production operations, the chemical composition being adjusted so that the average value of five Jominy tests J 3m , J 11m , J 15m and J 25m To make: α=|J 11m -J 3m×14 / 22-J 25m ×8 / 22|≦2.5HRC; and β=J 3m -J 15m ≦9HRC. Method for producing a mechanical component using the steel and a mechanical component produced in this manner. However, the steel of US20070193658A1 cannot achieve sufficient tensile strength and impact toughness levels.

[0006] WO2020 / 178854 provides a steel composition for high temperature carburizing and a steel product made of the steel composition. The composition comprises: a) 0.11 wt% to 0.3 wt% carbon, b) 1.1 wt% to 1.4 wt% manganese, c) 0.15 wt% to 0.35 wt% silicon, d) 1 wt% to 1.3 wt% chromium, e) ≤0.0006 wt% boron, f) 0.04 wt% to 0.05 wt% titanium, g) 0.035 wt% 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 wt% to 0.03 wt% aluminum, m) ≤190 ppm nitrogen, and n) the remainder is iron (Fe). However, the steel of WO2020 / 178854 cannot achieve sufficient levels of tensile strength and impact toughness. Summary of the invention

[0007] The object of the present invention is therefore to solve these problems by making available a steel component for automotive transmission systems having simultaneously:

[0008] - an ultimate tensile strength greater than or equal to 1600 MPa, and preferably higher than 1650 MPa,

[0009] - a yield strength greater than or equal to 1330 MPa,

[0010] - an impact toughness of 53 J / cm2 or less, and preferably 50 J / cm2 or less, when measured on a sample of KCU type,

[0011] -YS / TS ratio equal to 0.80 or greater.

[0012] In a preferred embodiment, the steel component according to the invention exhibits a surface hardness of 650 HV or more from a surface depth of 0.4 mm to 0.6 mm.

[0013] Preferably, such a steel is suitable for producing forged steel components for automotive drive trains, wherein the components may have a cross-section of up to 150 mm*150 mm, and also for other automotive components such as chassis components.

[0014] Another object of the invention is also to make available a method for manufacturing these mechanical parts that is compatible with conventional industrial applications and at the same time robust to variations in manufacturing parameters. DETAILED DESCRIPTION

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

[0016] Carbon is present in the steel of the present invention at 0.2% to 0.35%. Carbon is an element required to improve the strength of the steel of the present invention by producing low temperature transformation phases such as bainite, but a carbon content of less than 0.2% will not be able to impart tensile strength to the steel of the present invention. On the other hand, at a carbon content exceeding 0.35%, toughness is adversely affected due to excessive formation of proeutectoid cementite during cooling after hot rolling or forging. In addition, excessive formation of proeutectoid cementite is also harmful to mechanical operations such as hobbing, polishing, form drilling, honing or grinding of components of the transmission system. The carbon content is advantageously in the range of 0.22% to 0.35%, and more particularly 0.25% to 0.30%.

[0017] Manganese is added to the steel of the present invention at 1.0% to 1.6%. This element is a gamma-phase generating element (gammagenous). Manganese provides solid solution strengthening and suppresses the ferrite transformation temperature and reduces the ferrite transformation rate, thus contributing to the formation of bainite. An amount of at least 1.0% is required to impart strength and contribute to the formation of bainite. However, when the manganese content is present at more than 1.6%, it causes segregation, which results in a banded microstructure after annealing, and the banded microstructure is detrimental to the mechanical properties of the steel of the present invention. The preferred limit of 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 0.2% to 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 delay the precipitation of carbides during cooling after mechanical operation, and therefore, silicon promotes the formation of bainite. But silicon is also a ferrite former and also increases the Ac3 transformation point, which pushes the austenite temperature to a higher temperature range, which is why the silicon content is kept at a maximum of 0.7%. In addition, silicon above 0.7% also enhances segregation. The preferred limit of silicon presence is 0.2% to 0.6%, and more preferably 0.22% to 0.4%.

[0019] The content of aluminum is between 0.001% and 0.1%. Aluminum removes oxygen present in the molten steel to prevent it from forming a gas phase during the solidification process. Aluminum also fixes nitrogen in the steel to form aluminum nitride, thereby reducing the size of the grains. But for aluminum contents greater than 0.1%, the deoxidation effect is saturated. 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 presence of coarse aluminum-rich oxides that deteriorate the workability and hot forging of the steel. The preferred limit of aluminum presence is between 0.01% and 0.09%, and more preferably between 0.01% and 0.035%.

[0020] Molybdenum is an essential element and may be present in the present invention at 0.01% to 0.5%. Molybdenum is added to impart hardenability and hardness to the steel by forming molybdenum-based carbides, and also promotes the formation of martensite during carburizing, and also delays the formation of coarse niobium carbides or niobium carbonitrides. However, the addition of molybdenum excessively increases the cost of adding alloying elements, so that for economic reasons, its content is limited to 0.5%. The preferred limit of the molybdenum content is 0.03% to 0.4%, and more preferably 0.05% to 0.2%.

[0021] Niobium is an essential element of 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 will also affect the size of the microstructural components by its precipitation as carbonitrides and by delaying recrystallization during the heating process. Therefore, the microstructure formed at the end of the holding temperature and therefore after complete austenitization leads to hardening of the product. However, niobium contents above 0.06% are not economically attractive, and form coarser precipitates that are detrimental to the fatigue properties and impact toughness of the steel, and also when the niobium content is 0.06% or more, niobium is also detrimental to the hot ductility of the steel, thereby causing difficulties during steel casting and rolling. The preferred limit of the niobium content is 0.025% to 0.058%, and more preferably 0.025% to 0.055%.

[0022] Chromium is present in the steel of the present invention at 1% to 1.5%. Chromium is an essential element that provides strength to the steel by solid solution strengthening, and a minimum of 1% is required to impart strength, but when used above 1.5%, the hardenability is increased beyond acceptable limits due to the formation of coarse cementite after cooling, thereby compromising the forgeability and ductility of the steel. Like nickel, chromium addition also reduces the diffusion coefficient of carbon in austenite, thus promoting the formation of martensite during carburization. The preferred limit of chromium presence is 1.1% to 1.4%, and more preferably 1.1% to 1.3%.

[0023] The phosphorus content of the steel according to the invention is 0% to 0.09%.

[0024] 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. In particular, a content above 0.05% may cause fracture due to intergranular interface decohesion forces, which may be detrimental to fatigue limit. The preferred limit of phosphorus content is 0% to 0.05%.

[0025] Sulfur is included at 0% to 0.09%. Sulfur forms MnS precipitates that improve machinability and helps to obtain sufficient machinability. During metal forming processes such as rolling and forming, deformable manganese sulfide (MnS) inclusions become elongated. If such elongated MnS inclusions are not aligned with the load direction, the inclusions may have a significant adverse effect on mechanical properties such as contraction and impact toughness. In addition, higher sulfur contents are also detrimental to the forgeability of 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 obtain the best balance between machinability and fatigue limit.

[0026] Nitrogen is present in the steel of the present invention in an amount of 0.009% to 0.09%. It appears that Nb(C,N) nucleates and precipitates on AlN precipitates. In order to obtain Nb(C,N) precipitates, a minimum of 0.009% nitrogen is required. The preferred limit of nitrogen is 0.009% to 0.05%, and more preferably 0.009% to 0.04%.

[0027] Nickel is added to the present invention at 0% to 1% to increase the strength of the steel of the present invention and improve toughness, especially after normalizing and carburizing. Nickel is beneficial in improving its pitting resistance. A minimum of 0.1% is required to obtain such an effect. Nickel is added to the steel composition to reduce 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% reduces the martensite starting temperature, thereby causing excessive stabilization of retained austenite, which has a deleterious effect on tensile strength and yield strength. In addition, nickel is also limited to 1% for economic reasons. In the steel of the present invention, it is preferred to have 0.1% to 0.9% nickel.

[0028] Vanadium is an optional element of the present invention and is present in an amount of 0% to 0.2%. Vanadium is effective in increasing the strength of steel by precipitation strengthening, particularly by forming carbides or carbonitrides. For economic reasons, the upper limit is kept at 0.2%.

[0029] The steel of the present invention always contains no titanium for the following reasons: Titanium is an optional element and is present in the range of 0% to 0.1%. Titanium forms coarse titanium nitrides which give strength to the steel, but these nitrides may form during the solidification process and thus have a deleterious effect on the fatigue limit. Therefore, the preferred limit of titanium is 0% to 0.05%.

[0030] Copper is a residual element and may be present up to 1% due to the processing of the steel. Up to 0.5% copper does not affect any properties of the steel, but above 0.5%, hot workability is significantly reduced.

[0031] Other elements such as tin, cerium, calcium, bismuth, magnesium or zirconium may be added individually or in combination in the following weight proportions: 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 make it possible to refine the grains during solidification. The remainder of the steel composition consists of iron and unavoidable impurities resulting from processing.

[0032] The remainder of the composition is iron and unavoidable impurities, in particular impurities resulting from processing. More particularly, the composition of the steel component consists of the above elements.

[0033] The steel component for automotive transmission has a microstructure comprising, by surface fraction or area %, at least 90% of bainite, 0% to 10% of retained austenite, pearlite, ferrite and martensite, optionally present cumulatively, and precipitations of Al and Nb in the form of AlN and Nb(C,N).

[0034] Bainite is present in the steel according to the present invention as a matrix phase and imparts strength to such steel. Bainite is present in the steel in an area fraction of at least 90%, and preferably 90% to 100%, and more preferably 95% to 100%. Bainite is formed during cooling after normalizing. Such bainite may include lath bainite, granular bainite, upper bainite and lower bainite or any other bainite without cementite. Lath bainite without cementite consists of bainite in the form of laths and contains carbides between these laths, so that the number N of inter-lath carbides greater than 0.1 micrometers per unit surface area is less than or equal to 50000 / mm 2 This lath-shaped bainite structure without cementite can impart high strength and impact toughness to the steel of the present invention. The lower bainite is composed of bainite in the form of laths and containing fine iron carbide rods precipitated inside the laths. The lower bainite structure can provide elongation and tensile strength to the steel of the present invention.

[0035] 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. Precipitate formation occurs during the annealing process as well as during 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 target amount of bainite and martensite in the martensite-rich layer of the invention. Therefore, it is preferred that the original austenite grain size measured according to the ASTM grain index is 3 to 12. More preferably, it has a original austenite grain size of 4 to 11, and more preferably 4 to 10.

[0036] The cumulative presence of up to 10% of retained austenite, pearlite, ferrite and martensite does not adversely affect the present invention, but above 10%, the mechanical properties may be adversely affected. 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 resistance to the steel. Therefore, the preferred limit of the cumulative presence of ferrite and bainite is maintained at 0% to 8%, and more preferably 0% to 4%.

[0037] In addition to this microstructure in the core of the steel component, it also includes such a rich martensite layer on all surfaces of the steel component of the automotive transmission system: the depth is up to 1 mm or less, and the depth is preferably up to 0.8 mm or less, and more preferably 0.5 mm or less, and shows a martensite percentage of 85% to 95%, preferably 85% to 92%, more preferably 85% to 90% by area fraction. The rich martensite layer formed on the surface preferably contains any or all possible martensite species, and in particular fresh martensite, tempered martensite, etc. This martensite layer gives the steel of the present invention a surface hardness of 650 Hv or more, which provides good resistance to wear to the final steel component, and also gives precision during the meshing of the components with each other during the rotation operation of the transmission system.

[0038] The remainder of the surface layer comprises any one or more of bainite, retained austenite, ferrite and cementite.

[0039] The steel component for automotive transmission system according to the present invention can be produced by any suitable manufacturing process with the noted process parameters explained below.

[0040] Preferred exemplary methods are described herein, but this example does not limit the scope of the present disclosure and the aspects on which the example is based. In addition, any examples set forth in this specification are not intended to be limiting, but merely set forth some of the many possible ways in which various aspects of the present disclosure can be put into practice.

[0041] In the preferred embodiment, the steel component considered for illustrating the preferred process according to the present invention is a gear.

[0042] The preferred method comprises providing a semi-finished casting of a steel having a chemical composition according to the invention. The casting may be completed in any form such as an ingot or bloom or billet that can be manufactured or processed into a steel component that may have a cross-section of up to 150 mm*150 mm.

[0043] For example, a steel having the above chemical composition is cast into a billet and then rolled in the form of a rod. The rod can serve as a semi-finished product for further processing steps in manufacturing. Multiple rolling steps can be performed to obtain the desired semi-finished product. The cross-section of the preferred semi-finished product is to

[0044] The semi-finished product after the rolling process can be used directly at high temperature after rolling or can be first 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 the steel of the invention is calculated by dilatometry studies.

[0045] The temperature of the semi-finished product subjected to hot forging is preferably at least 1150°C and must be lower than 1300°C, because the temperature of the semi-finished product is lower than 1150°C, an excessive load is imposed on the forging die, and in addition, the temperature of the steel may drop to the ferrite transformation temperature during fine forging, whereby the steel will be forged in a state containing transformed ferrite in the structure. Therefore, the temperature of the semi-finished product is preferably high enough so that the hot forging can be completed in the austenitic temperature range. Reheating at a temperature higher than 1300°C must be avoided because it is industrially expensive.

[0046] The final finishing forging temperature (hereinafter referred to as Tforging) must be kept above 830°C to have a structure that is conducive to recrystallization and forging. It is preferred that the final forging be carried out at a temperature greater than Ac3+100°C, and preferably greater than Ac3+200°C, because below this temperature the steel bar exhibits a significant forging drop.

[0047] Thus, a hot forged component is obtained in this way, which is then cooled to room temperature.

[0048] The hot forged steel component is then subjected to annealing to reduce the hardness of the steel component for further machining.

[0049] In annealing, the hot forged steel part is subjected to heating to reach an soaking temperature TA of 600°C to Ac3+200°C, preferably a TA temperature of 625°C to Ac3+100°C, more preferably 640°C to Ac3+50°C.

[0050] In the heating step, the hot forged steel part is heated from room temperature to a soaking temperature TA at a heating rate HR1 of 0.1 to 100°C / sec. The HR1 rate is preferably 0.1 to 50°C / sec, and more preferably 0.1 to 10°C / sec.

[0051] Then, the hot forged steel part is kept at the annealing soaking temperature TA for 10 seconds to 1000 seconds to ensure that the initial structure of the strong work hardening is fully transformed into the austenite microstructure, thereby reducing the hardness of the hot forged steel part. Then, the hot forged steel part is cooled to a cooling stop temperature range CS1 of Ms-5°C to 15°C, preferably Ms-5°C to 20°C, and more preferably Ms-10°C to 20°C at a cooling rate CR1 greater than 1°C / second, and preferably greater than 2°C / second, and more preferably greater than 5°C / second. Ms of the steel of the present invention is calculated by the following formula:

[0052] Ms(℃)=539-423×%C-30.4×%Mn-17.7×%Ni-12.1×%Cr-7.5×%Mo-11×%SiAc3(℃)=910-203C 1 / 4 +44.7Si-15,2Ni+31.5Mo+104V+13.1W-30Mn-11Cr-20Cu+700P+400Al+400Ti

[0053] Thereafter, a forged steel component is obtained which has been subjected to at least one mechanical manufacturing operation. The mechanical operation may include hobbing, forming, machining, grinding, honing or any other suitable mechanical operation or manufacturing process. The mechanical operation may be carried out at room temperature or at a higher temperature as desired according to the specific mechanical operation conditions.

[0054] The forged steel component is then subjected to carburization to form a martensite-rich layer on all surfaces of the steel component and also to impart the target microstructure and mechanical properties to the steel component of the present invention.

[0055] In carburizing, the forged steel component is subjected to heating to reach a carburizing temperature TZ of 800 to 1100° C. A preferred TZ temperature is 850 to 1080° C., more preferably 900 to 1080° C.

[0056] In the heating step, the forged steel part is heated from room temperature to TZ at a heating rate HR2 of 0.1 to 20°C / sec. The HR2 rate is preferably 0.1 to 10°C / sec, and more preferably 0.1 to 5°C / sec.

[0057] Then, the forged steel part is kept at TZ for 10 seconds to 3600 seconds in a carbon-rich atmosphere with a dew point of -15°C to +15°C. The carburizing treatment is intended to fuse carbon from the carbon-rich atmosphere into the surface of the forged steel part at high temperature, which will transform the microstructure of the surface of the forged steel part into martensite. Thereby, a rich martensite layer is formed on all surfaces of the forged steel part. The rich martensite layer can be up to a depth of 1 mm or less, and preferably up to a depth of 0.8 mm or less, and more preferably up to a depth of 0.5 mm or less. Then, 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, and more preferably Ms-10°C to 20°C at a cooling rate CR2 greater than 1°C / second, and preferably greater than 2°C / second and more preferably greater than 5°C / second to obtain a steel part for an automotive transmission system.

[0058] Thereafter, the obtained steel component for automotive transmission system may be optionally reheated to a tempering temperature T temper of 150°C to 250°C within a time of 100 to 600 seconds at a heating rate of at least 1°C / second, and preferably at least 2°C / second, and more preferably at least 10°C / second. The preferred temperature range for tempering is 180°C to 240°C, and the preferred duration for holding at T temper is 200 to 500 seconds.

[0059] Example

[0060] The following tests, examples, graphical illustrations and tables presented herein are non-limiting in nature and must be considered solely for purposes of illustration and will demonstrate the advantageous features of the present invention.

[0061] Table 1 summarizes forged machine parts made of steels with different compositions, wherein the forged machine parts were respectively produced according to the process parameters as noted in Table 2. Thereafter, Table 3 summarizes the microstructures of the forged machine parts obtained during the tests, and Table 4 summarizes the evaluation results of the obtained properties.

[0062] Table 1

[0063] steel C Mn Si Al Mo Nb Cr S P N Cu Ni Ti AC3 Ms 1 0.284 1.230 0.251 0.025 0.043 0.027 1.180 0.029 0.014 0.011 0.228 0.156 0 778 361 2 0.288 1.280 0.277 0.025 0.162 0.050 1.240 0.029 0.015 0.011 0.222 0.167 0 781 356 3 0.283 1.240 0.239 0.023 0.047 0 1.167 0.027 0.014 0.011 0.222 0.155 0.002 777 362 4 0.290 1.270 0.255 0.023 0.055 <![CDATA[ 0.079 ]]> 1.200 0.027 0.013 0.011 0.220 0.146 0 776 357

[0064] Table 2

[0065] Table 2 summarizes the process parameters applied to the semi-finished products made from the steels of Table 1. Steels 1 and 2 were used to produce forged machine parts according to the invention. The table also specifies reference forged machine part steels, which are Steels 3 and 4.

[0066] Table 2 is as follows:

[0067] All steels were reheated to a temperature of 1250°C and subjected to machining operations.

[0068]

[0069] I = according to the invention; R = reference; underlined values: not according to the invention.

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

[0071] Table 3: Microstructure of steel samples and presence of martensite in the martensite layer on the surface

[0072]

[0073]

[0074] I = according to the invention; R = reference; underlined values: not according to the invention.

[0075] Table 4 illustrates the mechanical properties of both the inventive and reference steel parts. To determine the tensile strength, tests were performed according to the NF EN ISO 6892-1 standard. KCU test specimens were tested at room temperature with a U-notch according to the EN ISO 148-1 standard to measure toughness and fatigue.

[0076] Table 4

[0077]

[0078] I = according to the invention; R = reference; underlined values: not according to the invention.

Claims

1. A steel component for an automobile transmission system, comprising the following elements expressed in weight percentage: 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 can include one or more of the following optional 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 is composed of iron and unavoidable impurities caused by processing, the microstructure of the core of the steel component comprises, by area percentage, at least 90% of bainite, and 0% to 10% cumulatively optionally present precipitations of aluminum and niobium in the form of AlN and Nb(C,N), such a steel component has a martensite-rich layer up to a depth of 1 mm or less on all surfaces of the steel component, such martensite-rich layer contains 85% to 95% of martensite, and the remainder is from any one or more of bainite, retained austenite, ferrite or cementite.

2. The steel component for an automotive transmission system according to claim 1, wherein the composition contains 0.2% to 0.6% silicon.

3. The steel component for an automotive transmission system according to claim 1 or 2, wherein the composition contains 0.22% to 0.35% carbon.

4. The steel for leaf spring according to any one of claims 1 to 3, wherein the composition contains 0.03% to 0.4% of molybdenum.

5. The steel for leaf springs according to any one of claims 1 to 4, wherein the composition contains 0.025% to 0.058% niobium.

6. The steel component for automobile transmission system according to any one of claims 1 to 5, in, The bainite content is 90% to 100%.

7. The steel component for automobile transmission system according to any one of claims 1 to 6, in, The martensite-rich layer comprises 85% to 92% martensite, with the remainder being any one or more of bainite, retained austenite, ferrite or cementite.

8. The steel component for automobile transmission system according to any one of claims 1 to 7, in, The ultimate tensile strength of the steel is at least 1600 MPa.

9. The steel component for an automotive transmission system according to any one of claims 1 to 8, wherein the impact toughness of the steel is equal to or less than 53 J / cm2.

10. A method for producing a steel component for an automotive transmission system according to any one of claims 1 to 4, comprising the following steps in sequence: - 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 Ac3+30°C to 1300°C; - hot forging the semi-finished product in the austenitic range to obtain a hot forged component, wherein the hot forging final forging temperature Tforging should be higher than 830°C; - Cooling the hot forged component to room temperature; - thereafter, heating the hot forged component from room temperature to an annealing soaking temperature TA in the range of 600° C. to Ac3+200° C. at a heating rate HR1 of 0.1° C. / s to 100° C. / s; - then annealing at TA for a period of 10 to 1000 seconds; - then cooling the 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 / sec to obtain a forged steel part; - performing one or more mechanical operations on the forged steel component; - thereafter, heating the forged component 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; - carburizing is then carried out at TZ in a carbon-rich environment with a dew point of -15°C to +15°C for a period of 10 seconds to 3600 seconds; - The forged component is then cooled from TZ to a cooling stop temperature CS2 of Ms-5°C to 15°C at a cooling rate CR2 greater than 1°C / sec to obtain a steel component for an automotive transmission system.

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

12. The method according to any one of claims 11 or 12, wherein the temperature TA is 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 1080°C.

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

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

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