Forged component of steel and method of manufacturing thereof

Abstract

Steel used for forging mechanical parts contains the following elements: 0.04%≦C≦0.28%; 1.2%≦Mn≦2.2%; 0.3%≦Si≦1.2%; 0.5%≦Cr≦1.5%; 0.01%≦Ni≦1%; 0%≦S≦0.06%; 0%≦P≦0.02%; 0%≦N≦0.015%; 0%≦Al≦0.1%; 0.03%≦Mo≦0.5%; 0%≦Cu≦0.5%; 0.04%≦Nb≦ 0.15%; 0.01%≦Ti≦0.1%; 0%≦V≦0.5%; 0.0015%≦B≦0.004%; the remaining component consists of iron and unavoidable impurities caused by processing, and the microstructure of the steel has the following composition by area fraction: 55% to 85% martensite, 20% to 45% self-tempered martensite, 0% to 10% retained austenite, and wherein the cumulative amount of self-tempered martensite and martensite is at least 90%.

Description

[0001] This invention relates to steel suitable for forging mechanical parts for automobiles.

[0002] Automotive components need to meet two conflicting requirements: ease of forming and strength. However, in recent years, due to global environmental concerns, a third requirement has been added: improved fuel efficiency. Therefore, automotive components must now be made from highly formable materials to meet the standard of easy assembly of complex automotive components, while simultaneously improving strength for vehicle engine crashworthiness and durability, and reducing vehicle weight to improve fuel efficiency.

[0003] Therefore, extensive research and development efforts have been undertaken to reduce the amount of materials used in automobiles by increasing the strength of materials. Conversely, increasing the strength of steel reduces its formability, thus necessitating the development of materials with high strength, high impact toughness, and high formability.

[0004] Early research and development in the field of high strength and high impact toughness have yielded several methods for producing high strength and high impact toughness steels, some of which are listed herein for the purpose of understanding the certainty of the invention:

[0005] US7314532 is a high-strength forged component comprising a base phase microstructure and a second phase microstructure and containing the following: C: 0.41% to 0.6%, Si+Al: 0.5% to 3%, Mn: 0.5% to 3%, P: 0.15% or less (excluding 0%), S: 0.02% or less (including 0%), wherein the base phase microstructure comprises 30% or more ferrite with respect to the space factor of the whole microstructure, the second phase microstructure comprises retained austenite and bainite and / or martensite, the content of retained austenite relative to the whole microstructure is represented by the following expression (1), the average grain diameter d of the second phase microstructure is 5 μm or less, and the space factor of the rough portion with an average grain diameter of (1.5 × d) or greater contained in the second phase microstructure is 15% or less, 0 × [C] < [V γR ]<150×[C]-(1), where [V γR [] represents the space factor of retained austenite relative to the whole microstructure, and [C] represents the C content (mass%) in the forged part. However, the steel of US7314532 cannot reach the tensile strength.

[0006] WO2016 / 063224 claims a steel having the following chemical composition in weight percentages: 0.1 ≤ C ≤ 0.25%, 1.2 ≤ Mn ≤ 2.5%, 0.5 ≤ Si ≤ 1.7%, 0.8 ≤ Cr ≤ 1.4%, 0.05 ≤ Mn ≤ 0.1, 0.05 ≤ Nb ≤ 0.10, 0.01 ≤ Ti ≤ 0.03%, 0 < Ni ≤ 0.4%, 0 < V ≤ 0.1%, 0 < S ≤ 0.03%, 0 < P ≤ 0.02%, 0 < B ≤ 30 ppm, 0 < O ≤ 15 ppm, and the balance being less than 0.4%. However, in terms of mechanical properties, the tensile strength is less than 1200 MPa and the yield strength is never higher than 800 MPa.

[0007] Therefore, based on the above disclosure, an object of the present invention is to provide a bainitic steel for hot-forged mechanical components such that a tensile strength higher than 1300 MPa and an impact toughness of 38 J at 20°C in KCV can be obtained.

[0008] Therefore, an object of the present invention is to solve these problems by enabling the availability of a bainitic steel suitable for hot forging that simultaneously has the following:

[0009] - An ultimate tensile strength greater than or equal to 1300 MPa, and preferably higher than 1400 MPa,

[0010] - An impact toughness greater than or equal to 38 J at 20°C, and preferably greater than 40 J at 20°C,

[0011] - A yield strength to tensile strength ratio of 0.8 or less, and preferably 0.75 or less.

[0012] In a preferred embodiment, the steel plate according to the present invention may also exhibit a yield strength greater than or equal to 800 MPa, and preferably higher than 850 MPa.

[0013] Preferably, such a steel is suitable for manufacturing forged steel components with a cross-section of 10 mm to 100 mm, such as connecting rods, pitman arms, and steering knuckles, having no significant hardness gradient between the skin and the heart of the forged component.

[0014] Another object of the present invention is also to enable the availability of a method for manufacturing these mechanical components that is compatible with conventional industrial applications and robust to variations in manufacturing parameters.

[0015] Carbon is present in the steel of the present invention at a concentration of 0.04% to 0.28%. Carbon imparts strength to the steel through solid solution strengthening, and carbon is a gammagenous element, thus delaying the formation of ferrite. Carbon is an element that influences the martensitic transformation initiation temperature (Ms). Martensite transformed at low temperatures combined with self-tempered martensite transformed at high temperatures (particularly below Ms) exhibits better strength and ductility. A minimum of 0.04% carbon is required to achieve a tensile strength of 1300 MPa, but if carbon is present at a concentration higher than 0.28%, the ductility and machinability of the final product deteriorate due to the formation of cementite. The carbon content is advantageously in the range of 0.08% to 0.25% to simultaneously obtain high strength and high ductility, and more preferably 0.09% to 0.22%.

[0016] Manganese is added to the steel of the present invention at a concentration of 1.2% to 2.2%. Manganese provides hardenability to the steel. It allows for a reduction in the critical cooling rate at which a martensitic transformation can be achieved in continuous cooling without any prior transformation. A minimum content of 1.2% by weight is required to obtain the desired martensitic microstructure and also to stabilize the austenite. However, above 2.2%, manganese has a negative impact on the steel of the present invention because the retained austenite can transform into bainite and MA islands, and these relative properties are detrimental. Furthermore, manganese forms sulfides such as MnS. If the shape and distribution are well controlled, these sulfides can increase machinability. If not, they can have a very detrimental effect on impact toughness. The preferred limit for manganese is 1.4% to 2.1%, and more preferably 1.5% to 1.9%.

[0017] Silicon is present in the steel of the present invention at a concentration of 0.3% to 1.2%. Silicon imparts strength to the steel of the present invention through solid solution strengthening. Silicon reduces the formation of cementite nucleation because it hinders the precipitation and diffusion-controlled growth of carbides by forming a Si-rich layer around the precipitate nuclei. Consequently, the austenite is rich in carbon, which reduces the driving force during the bainite transformation. Therefore, the addition of Si slows down the overall bainite transformation kinetics, which leads to an increase in martensite formation. Silicon also acts as a deoxidizer. A minimum of 0.3% silicon is required to impart strength to the steel of the present invention and to provide a delay for bainite formation under continuous cooling. A concentration greater than 1.2% increases the activity of carbon in the austenite, thereby promoting its transformation into proeutectoid ferrite, which may degrade strength and also produce excessive retained austenite at the end of cooling. The preferred limit for silicon is 0.3% to 1%, and more preferably 0.3% to 0.9%.

[0018] Chromium is present in the steel of the present invention at a concentration of 0.5% to 1.5%. Chromium is an essential element for the formation of martensite and for imparting toughness to the steel of the present invention. The addition of chromium promotes a uniform and finer martensitic microstructure over a temperature range from Ms to room temperature. A minimum content of 0.5% chromium is required to produce the target martensitic microstructure, but the presence of 1.5% or more chromium causes segregation. A chromium concentration of 0.7% to 1.4%, and more preferably 0.8% to 1.3%, is advantageous.

[0019] Nickel is included in the form of 0.01% to 1%. Its addition contributes to the hardenability and toughness of the steel. Nickel also helps to lower the bainite initiation temperature. However, for economic feasibility, its content is limited to 1%. Preferably, it contains 0.01% to 0.8%, and more preferably 0.01% to 0.7% nickel.

[0020] Sulfur is included in the content from 0% to 0.06%. Sulfur forms MnS precipitates that improve machinability and contribute to achieving sufficient machinability. During metal forming processes such as rolling and forging, deformable manganese sulfide (MnS) inclusions become elongated. If such elongated MnS inclusions are not aligned with the load direction, the inclusions can have a considerable adverse effect on mechanical properties such as tensile strength and impact toughness. Therefore, the sulfur content is limited to 0.06%. The preferred range for sulfur content is 0.03% to 0.04%.

[0021] Phosphorus is an optional component of the steel of the present invention and is present in amounts from 0% to 0.02%. Phosphorus reduces weldability and hot ductility, particularly due to its tendency to segregate at grain boundaries or co-segregate with manganese. For these reasons, its content is limited to 0.02%, and preferably less than 0.015%.

[0022] Nitrogen is present in the steel of the present invention at an amount of 0% to 0.015%. Nitrogen forms nitrides with Al, Nb, and Ti, which prevents the austenitic structure of the steel from coarsening during hot forging and enhances its toughness. When the Ti content is 0.01% to 0.03% and the Ti / N ratio is <3.42, effective use of TiN to pin austenite grain boundaries is achieved. Using stoichiometric excess nitrogen content causes an increase in the size of these particles, which not only reduces the efficiency of pinning austenite grain boundaries but also increases the likelihood that TiN particles will act as fracture initiation sites.

[0023] Aluminum is an optional element in the steel of the present invention. Aluminum is a strong deoxidizer and also forms precipitates dispersed in the steel as nitrides that prevent the growth of austenite grains. However, the deoxidizing effect saturates for aluminum contents exceeding 0.1%. Contents greater than 0.1% may result in the formation of coarse, aluminum-rich oxides that deteriorate tensile properties, and particularly impact toughness. Preferably, the aluminum content is 0% to 0.06%, and more preferably 0% to 0.05%.

[0024] Molybdenum is an optional element that may be present in the present invention at a concentration of 0.03% to 0.5%. Molybdenum forms Mo2C precipitates that increase the yield strength of the steel of the present invention. Molybdenum also has a significant effect on the hardenability of the steel. Such an effect is only feasible with a minimum of 0.03% molybdenum. Excessive addition of molybdenum increases alloying costs and will enhance the MA composition formed by retained austenite. Furthermore, if the Mo content is too high, segregation problems may occur. Therefore, for the present invention, molybdenum is limited to 0.5%. For the steel of the present invention, the preferred limit is 0.03% to 0.3%, and more preferably 0.03% to 0.1%.

[0025] Copper is a residual element from the electric arc furnace steelmaking process and must always be kept below 0.5%, preferably reduced to 0. Above this value, hot workability is significantly reduced.

[0026] Niobium is an optional element that can be present in the steel of the present invention at a concentration of 0.04% to 0.15%. The addition of niobium increases the hardenability of the steel by strongly delaying the diffusion transformation when in a solid solution. Niobium can also be used synergistically with boron, preventing boron from precipitating as boron carbides along grain boundaries due to the preferential precipitation of niobium carbides. Furthermore, niobium is known to slow recrystallization and austenite grain growth kinetics both in the form of a solid solution and precipitates. The combined effect of austenite grain size and hardenability helps refine the final martensitic microstructure, thereby increasing the strength and toughness of parts manufactured according to the present invention. It cannot be added to a concentration higher than 0.15% by weight to prevent coarsening of niobium precipitates, which can act as nuclei for ductile fracture and for ferrite transformation.

[0027] Titanium is an optional element that can be present in amounts from 0.01% to 0.1%. Titanium prevents boron from forming nitrides. Titanium precipitates in steel as nitrides or carbon-nitrides that effectively pin austenite grain boundaries and thus limit austenite grain growth at high temperatures. Since the martensite grain size is closely related to the austenite grain size, the addition of titanium is effective in improving toughness. Such an effect cannot be obtained at titanium contents less than 0.01%, and the effect tends to saturate at contents greater than 0.1%, only increasing the alloy cost. Furthermore, the presence of coarse titanium nitrides formed during curing is detrimental to impact toughness and fatigue properties. Therefore, the presence of titanium is preferably from 0.01% to 0.03%.

[0028] Vanadium is an optional element and is present in amounts ranging from 0% to 0.5%. Vanadium is effective in enhancing the strength of steel by forming carbides or carbon-nitrides, and for economic reasons, the upper limit is 0.5%. The preferred limit for vanadium is 0% to 0.1%.

[0029] Boron content ranges from 0.0015% to 0.004%. Boron is typically added in very small amounts, as even a few ppm can cause significant microstructural changes. At this level of addition, boron has no effect on the bulk due to the very low boron / iron atom ratio (typically <0.00005), and therefore does not cause solid solution hardening or precipitation strengthening. In fact, boron segregates strongly at austenite grain boundaries, where for large grain sizes, boron atoms may be as numerous as iron atoms. This segregation causes a delay in the formation of ferrite and pearlite, which promotes a martensitic microstructure during cooling and thus increases the strength of such steels after the austenite decomposes at moderate cooling rates. To allow and exhibit this effect, it is recommended to add B in amounts of 0.0015% or greater. Higher boron content causes a rapid deterioration in the low-temperature toughness of such steels, therefore its upper limit is set at 0.004%.

[0030] Other elements such as tin, cerium, magnesium, or zirconium may be added individually or in combination in the following proportions by weight: tin ≤ 0.1%, cerium ≤ 0.1%, magnesium ≤ 0.010%, and zirconium ≤ 0.010%. Up to the maximum content levels shown, these elements allow for grain refinement during solidification. The remaining composition of steel consists of iron and unavoidable impurities resulting from processing.

[0031] The microstructure of the steel plate, expressed as an area fraction, comprises:

[0032] Martensite comprises 55% to 85% of the steel of the present invention. Martensite is the matrix phase of the steel of the present invention. Martensite provides the steel with tensile strength and other mechanical properties. To achieve a tensile strength of 1300 MPa, a minimum of 55% martensite is required. A content of 60% to 85%, and preferably 65% ​​to 80%, of martensite is advantageous. Martensite is formed during a second cooling step, particularly at Ms-150°C to room temperature. The martensite of the present invention comprises fresh martensite and stress-regenerated martensite.

[0033] Self-tempered martensite is present in the steel of the present invention at a concentration of 20% to 45%. Self-tempered martensite is an essential microstructure component of the steel of the present invention. Self-tempered martensite imparts impact toughness and ductility to the steel of the present invention. A minimum of 20% self-tempered martensite is required to achieve impact toughness, but tensile strength decreases whenever the self-tempered martensite content exceeds 45%. Therefore, the preferred content of self-tempered martensite is 25% to 40%, and more preferably 30% to 40%. The self-tempered martensite of the steel of the present invention is formed by self-tempering of the martensite obtained at the end of the first cooling step during the second cooling step through an exothermic reaction due to the formation of martensite during cooling.

[0034] The distinction between self-tempered martensite and other martensite was determined by using LePera etching to expose the two phases, followed by SEM observation of the self-tempered carbides. For example, in Figure 1 In the figure, the number 10 indicates that the carbides are clearly visible as tiny white spots in the self-tempered martensite, and the number 20 indicates that there are no carbides in the martensite.

[0035] The cumulative amount of martensite and self-tempered martensite is at least 90%, and preferably 95%, to simultaneously ensure tensile strength and impact toughness. The martensite of the present invention imparts tensile strength, while the self-tempered martensite imparts toughness. Whenever the cumulative amount is less than 90%, the presence of soft phases, such as retained austenite, is enhanced, which is detrimental to both tensile strength and toughness.

[0036] Retained austenite may be present in steel at 0% to 10% and must be kept as low as possible. Retained austenite up to 10% is harmless to the target properties, but when present at levels above 10%, it adversely affects tensile strength. Preferably, it has 0% to 5%, and more preferably 0% to 2% retained austenite.

[0037] Apart from the microstructures described above, the microstructures of mechanically forged parts do not contain microstructure components such as bainite, pearlite, and cementite.

[0038] The mechanical components according to the invention can be produced by any suitable hot forging process, such as drop forging, pressure forging, upsetting forging and rolling forging, according to the process parameters described below.

[0039] This document describes preferred exemplary methods, but these examples do not limit the scope of this disclosure or the aspects on which the examples are 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 practiced.

[0040] A preferred method includes providing a semi-finished casting of steel having the chemical composition according to the invention. The casting can be completed in any form, such as an ingot or bloom or billet, capable of being forged into mechanical parts having a cross-sectional diameter of 30 mm to 100 mm.

[0041] For example, steel with the above chemical composition can be cast into a large billet, and then rolled into a bar, which will serve as a semi-finished product. Several rolling operations can be performed to obtain the desired semi-finished product.

[0042] The semi-finished product after the casting process can be used directly at high temperature after rolling, or it can be cooled to room temperature first and then reheated in the temperature range of Ac3+30℃ to 1300℃ for hot forging.

[0043] 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. Furthermore, during finish forging, the steel temperature may drop to the ferrite transformation temperature, resulting in the steel being forged in a state where transformed ferrite is present in the microstructure. Therefore, the temperature of the semi-finished product is preferably high enough that hot forging can be completed within the austenitic temperature range. Reheating at temperatures above 1300°C must be avoided, as it is industrially expensive.

[0044] The final forging temperature (hereinafter referred to as T-forging) must be maintained above 950°C to achieve a microstructure favorable for recrystallization and forging. It is preferable to perform the final forging at a temperature greater than Ac3+50°C, and more preferably above Ac3+100°C, because below this temperature, the steel sheet exhibits a significant decrease in forging performance.

[0045] Therefore, hot-forged parts are obtained in this way, and then the hot-forged steel parts are cooled in a two-step cooling process.

[0046] In the two-step cooling process of hot-forged parts, the hot-forged parts are cooled at different cooling rates within different temperature ranges.

[0047] In the first cooling step, the hot-forged part is cooled from T-forging to a temperature range of 750°C to 1250°C (also referred to herein as T1) at an average cooling rate of 0.2°C / s to 10°C / s. The part can optionally be held at T1 for up to 3600 seconds. During this first cooling step, the average cooling rate from T-forging to T1 preferably has a range of 0.2°C / s to 8°C / s, and more preferably 0.2°C / s to 2°C / s.

[0048] Subsequently, a second cooling step begins, in which the hot-forged component is cooled from T1 to a temperature referred to herein as T2 at an average cooling rate of 0.1°C / s to 10°C / s and in the range of Ms-150°C to room temperature. During the second cooling step, the cooling from T1 to T2 is preferably maintained at an average cooling rate of 1.0°C / s to 5.0°C / s. This second cooling step is present to promote the transformation of austenite to martensite and to allow the already formed martensite to self-temper, thereby reducing the possibility of austenite remaining in the final microstructure. This average cooling rate is also chosen to ensure uniform cooling across the cross-section of the hot-forged component.

[0049] After completing the second cooling step, the forged mechanical parts were obtained.

[0050] The obtained forged mechanical parts may optionally be tempered from 100°C to 200°C / second over a period of 5 to 3600 seconds, and preferably from 125°C to 200°C.

[0051] For all cooling steps, for the steel of this invention, the Ms temperature is calculated using the following formula:

[0052] Ms=539-423C-30Mn-18Ni-12Cr-11Si-7Mo

[0053] The element content is expressed as a weight percentage. Example

[0054] The following tests, embodiments, graphical examples, and tables presented herein are non-limiting in nature and must be considered for illustrative purposes only, and will demonstrate advantageous features of the invention.

[0055] Table 1 summarizes the forged machine parts made of steels with different compositions, wherein the forging machine parts were produced according to the process parameters specified in Table 2. Subsequently, Table 3 summarizes the microstructure of the forged machine parts obtained during the test, and Table 4 summarizes the evaluation results of the obtained properties.

[0056] Table 1 - Composition

[0057] steel sample C Mn Si Cr Ni S P N Al Mo Nb Ti V B 1 0.18 1.77 0.77 1.11 0.46 0.015 0.01 0.012 0.020 0.08 0.07 0.027 0 0 <![CDATA[ 2 ]]> <![CDATA[ 0.42 ]]> <![CDATA[ 0.80 ]]> <![CDATA[ 0.19 ]]> 1.05 <![CDATA[ 0 ]]> 0.007 0.01 <![CDATA[ 0.050 ]]> 0.032 0.19 <![CDATA[ 0 ]]> <![CDATA[ 0.003 ]]> 0 0 <![CDATA[ 3 ]]> <![CDATA[ 0.50 ]]> <![CDATA[ 0.90 ]]> 0.25 1.10 0.07 0.007 0.01 0.008 0.030 0 <![CDATA[ 0 ]]> <![CDATA[ 0 ]]> 0.12 0 <![CDATA[ 4 ]]> <![CDATA[ 0.30 ]]> 1.20 0.25 <![CDATA[ 0.35 ]]> 0.20 0.015 0.01 0 0.030 0.12 <![CDATA[ 0 ]]> 0.05 0 <![CDATA[ 0.005 ]]>

[0058] Table 2 - Process Parameters

[0059] Table 2 summarizes the process parameters implemented on semi-finished products made of the steels in Table 1 after reheating at 1280°C and then hot forging. Steel compositions I1 to I3 are used to manufacture forging machine parts according to the invention. The table also illustrates reference forging machine parts designated as R1 to R3 in the table. Table 2 also shows a list of Ms and Ac3.

[0060]

[0061] I = according to the present invention; R = reference; underlined value: not according to the present invention.

[0062] Table 3 - Microstructure

[0063] Table 3 illustrates the results of tests performed according to standards on different microscopes, such as scanning electron microscopes, used to determine the microstructure of both the inventive steel and the reference steel, in terms of area fraction.

[0064] The results are recorded in this article:

[0065]

[0066] I = according to the present invention; R = reference; underlined value: not according to the present invention.

[0067] Table 4 - Characteristics

[0068] Table 4 illustrates the mechanical properties of the invention steel and the reference steel. Tensile strength and yield strength were determined according to NF ENISO 6892-1. Impact toughness of the invention steel and the reference steel was measured at 20°C on V-notch standard KCV specimens according to ENISO 148-1.

[0069] The results of various mechanical tests conducted according to the standards were summarized.

[0070]

[0071] I = according to the present invention; R = reference; underlined value: not according to the present invention.

Claims

1. A steel for forging machine parts, expressed as a percentage by weight, comprising the following elements: 0.04%≤C≤0.28%； 1.4%≤Mn≤2.1%; 0.3%≤Si≤1.2%; 0.5%≤Cr≤1.5%； 0.01%≤Ni≤1%; 0%≤S≤0.06%； 0%≤P≤0.02%； 0%≤N≤0.015%； And it can contain one or more of the following optional elements: 0%≤Al≤0.1%； 0.03%≤Mo≤0.5%; 0%≤Cu≤0.5%; 0.04%≤Nb≤0.15%; 0.01%≤Ti≤0.1%; 0%≤V≤0.5%； 0.0015%≤B≤0.004%； The remaining portion consists of iron and unavoidable impurities caused by processing. The steel comprises, by area fraction, the following microstructure: 55% to 85% fresh martensite and stress-regenerated martensite, 20% to 45% self-tempered martensite, 0% to 10% retained austenite, and wherein the cumulative amount of self-tempered martensite, fresh martensite, and stress-regenerated martensite is at least 90%. in, The steel has an ultimate tensile strength of 1300 MPa or greater and a tensile strength of 38 J / cm at 20°C or greater. 2 Impact toughness; yield strength greater than or equal to 800 MPa.

2. The steel for forging mechanical parts according to claim 1, wherein the composition comprises 0.3% to 0.9% silicon.

3. The steel for forging mechanical parts according to claim 1 or 2, wherein the composition comprises 0.08% to 0.25% carbon.

4. The steel for forging mechanical parts according to claim 1 or 2, wherein the composition comprises 0% to 0.06% aluminum.

5. The steel for forging mechanical parts according to claim 1 or 2, wherein the composition comprises 0.7% to 1.4% chromium.

6. The steel for forging mechanical parts according to claim 1 or 2, wherein, The content of fresh martensite and stress-regenerated martensite is 60% to 85%.

7. The steel for forging mechanical parts according to claim 1 or 2, wherein the cumulative presence of self-tempered martensite, fresh martensite and stress-regenerated martensite is at least 95%.

8. A method for producing steel forged mechanical parts, comprising the following sequential steps: - Provided in the form of 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, wherein the forging temperature T is above 950°C to obtain the hot-forged component; - The hot-forged part is cooled in a two-step cooling process, wherein in the first step, the hot-forged part is cooled from the forging temperature T to a temperature T1 ranging from 780°C to 1250°C at an average cooling rate of 0.2°C / s to 10°C / s. - Subsequently, in step two, the hot-forged component is cooled from T1 to a temperature T2 ranging from Ms-150°C to room temperature at an average cooling rate of 0.1°C / s to 10°C / s to obtain a forged mechanical component.

9. The method according to claim 8, wherein in the cooling step one, the hot-forged component is cooled from T-forging to T1 at an average cooling rate of 0.2°C / sec to 8°C / sec.

10. The method according to claim 8 or 9, wherein in the second step of cooling, the hot-forged component is cooled from T1 to T2 at an average cooling rate of 1.0°C / second to 5.0°C / second.

11. The method according to claim 8 or 9, wherein the tempering is performed in the range of 100°C to 200°C.

12. The method of claim 8, wherein the hot-forged component is held at T1 for up to 3600 seconds.

13. The use of the steel according to any one of claims 1 to 7 or the forged machine parts produced by the method according to any one of claims 8 to 12 for the manufacture of structural or safety components of vehicles or engines.

14. A vehicle comprising the component obtained according to claim 13.

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