Steel forged parts and methods for manufacturing the same

Abstract

To provide a steel for hot forging of machine parts that has a tensile strength of 1300 MPa or more and an impact toughness of 38 J or more at 20°C, and a method for manufacturing said parts. [Solution] In weight percent, 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%≦T The steel contains i ≤ 0.1%, 0% ≤ V ≤ 0.5%, and 0.0015% ≤ B ≤ 0.004%, with the remaining composition consisting of iron and unavoidable impurities, and the microstructure of the steel contains 55% to 85% martensite, 20% to 45% auto-tempered martensite, and 0% to 10% retained austenite in area fraction, with the cumulative amount of auto-tempered martensite and martensite being at least 90%.

Description

Technical Field

[0001] The present invention relates to a steel suitable for forging mechanical parts of automotive steel.

Background Art

[0002] Automotive parts are required to meet two conflicting requirements, namely, ease of forming and strength. In recent years, due to considerations for the global environment, a third requirement of fuel consumption improvement has also been imposed on automobiles. Therefore, automotive parts must now be manufactured from materials with high formability in order to meet the criteria of ease of fitting in complex automotive assemblies, while at the same time improving the strength for engine crashworthiness and durability of automobiles while reducing the weight of the automobiles to improve fuel consumption.

[0003] Therefore, intensive research and development is underway to reduce the amount of materials used in automobiles by increasing the strength of the materials. Conversely, increasing the strength of steel reduces formability, so it is necessary to develop materials with high strength, high impact toughness, and high formability.

[0004] Previous research and development in the field of high strength and high impact toughness has led to several methods for manufacturing high strength and high impact toughness steels, some of which are listed herein for the final evaluation of the present invention.

[0005] US7314532 has a basic phase structure and a second phase structure, contains C: 0.41 to 0.6%, Si + Al: 0.5 to 3%, Mn: 0.5 to 3%, P: 0.15% or less (not including 0%), S: 0.02% or less (including 0%), the basic phase structure relates to the space ratio, contains 30% or more of ferrite with respect to the whole structure, the second phase structure contains retained austenite and bainite and / or martensite, the content ratio of retained austenite is represented by the following expression (1) with respect to the whole structure, the average grain size d of the second phase structure is 5 μm or less, and the space ratio of the coarse part of (1.5 × d) or more in the average grain size contained in the second phase structure is 15% or less, 0 × [C][V γR<150×[C]-(1), where in the formula, [V γR means the volume fraction of retained austenite with respect to the entire structure, and [C] means the C content (mass%) in the forged part. However, the steel of US7314532 cannot achieve the tensile strength.

[0006] WO2016 / 063224 claims a steel containing the following chemical composition in weight percent, namely, 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 less than 0.4% of residual elements. However, in terms of mechanical properties, the tensile strength is less than 1200 MPa, and the yield strength does not exceed 800 MPa.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] Therefore, in light of the above publications, an object of the present invention is to provide a bainitic steel for hot forging of mechanical parts that enables obtaining a tensile strength exceeding 1300 MPa and an impact toughness of 38 J at 20 °C in KCV.

[0009] Thus, an object of the present invention is to solve these problems by making available a bainitic steel suitable for hot forging that simultaneously has the following. - An ultimate tensile strength of 1300 MPa or more, preferably exceeding 1400 MPa, - Impact toughness of 38 J or more at 20°C, preferably exceeding 40 J at 20°C. - The ratio of yield strength to tensile strength is 0.8 or less, preferably 0.75 or less.

[0010] In one preferred embodiment, the steel sheet according to the present invention can also exhibit a yield strength of 800 MPa or more, preferably exceeding 850 MPa.

[0011] Preferably, such steel is suitable for manufacturing forged steel parts with a cross-section between 10 mm and 100 mm, such as connecting rods, pitman arms, and steering knuckles, where there is no significant hardness gradient between the skin and the center of the forged part.

[0012] Another objective of the present invention is to make available a method for manufacturing these machine parts that is stable to shifts in manufacturing parameters while being compatible with conventional industrial applications. [Means for solving the problem]

[0013] Carbon is present in the steel of this invention at a concentration of 0.04% to 0.28%. Carbon imparts strength to the steel through solid solution strengthening, and because carbon is gamma-ray emitting, it delays ferrite formation. Carbon is an element that affects the martensitic transformation onset temperature (Ms). Martensite transformed at low temperatures exhibits better strength and ductility, especially in combination with auto-tempered martensite transformed at high temperatures below Ms. A minimum of 0.04% carbon is required to achieve a tensile strength of 1300 MPa, but if carbon is present above 0.28%, it degrades the ductility as well as the machinability of the final product due to cementite formation. The carbon content is advantageously in the range of 0.08% to 0.25%, and more preferably between 0.09% and 0.22%, in order to obtain both high strength and high ductility simultaneously.

[0014] Manganese is added to the steel in an amount between 1.2% and 2.2%. Manganese imparts hardenability to the steel. Manganese allows martensitic transformation to decrease the critical cooling rate obtained by continuous cooling without pre-transformation. A minimum amount of 1.2% by weight is required to obtain the desired martensitic microstructure and also stabilizes the austenite. However, above 2.2%, retained austenite may transform into bainite as well as island MA, and since these phases adversely affect the aforementioned properties, manganese has a negative effect on the steel of the present invention. Furthermore, manganese forms sulfides such as MnS. If the shape and distribution of these sulfides are well controlled, machinability can be improved. Otherwise, sulfides can have a very adverse effect on impact toughness. The preferred limit for manganese is between 1.4% and 2.1%, and more preferably between 1.5% and 1.9%.

[0015] Silicon is present in the steel of the present invention in an amount between 0.3% and 1.2%. Silicon imparts strength to the steel of the present invention through solid solution strengthening. Silicon reduces cementite nucleation by inhibiting carbide precipitation and diffusion-limited growth by forming a Si-enriched layer around the precipitation nuclei. Thus, austenite is enriched in carbon, reducing the driving force during bainite transformation. As a result, the addition of Si slows the overall bainite transformation rate, which leads to increased martensite formation. Silicon also acts as a deoxidizing agent. A minimum of 0.3% silicon is required to impart strength to the steel of the present invention and to slow bainite formation under continuous cooling. Amounts exceeding 1.2% increase the activity of carbon in the austenite, promoting transformation to proterite ferrite, which degrades strength and results in too much retained austenite at the end of cooling. The preferred limit for silicon is between 0.3% and 1%, more preferably between 0.3% and 0.9%.

[0016] Chromium is present in the steel of the present invention in an amount between 0.5% and 1.5%. Chromium is an essential element for generating martensite and conferring toughness to the steel of the present invention. The addition of chromium promotes a homogeneous and finer martensite microstructure in the temperature range between Ms and room temperature. A minimum of 0.5% chromium is required to produce the target martensite microstructure, but the presence of chromium content above 1.5% causes segregation. Having chromium between 0.7% and 1.4%, more preferably between 0.8% and 1.3%, is advantageous.

[0017] Nickel is present in amounts between 0.01% and 1%. Nickel is added to contribute to the hardenability and toughness of the steel. Nickel also helps to lower the bainite onset temperature. However, its content is limited to 1% from an economic standpoint. It is preferable to have nickel between 0.01% and 0.8%, more preferably between 0.01% and 0.7%.

[0018] Sulfur content is between 0 and 0.06%. Sulfur forms MnS precipitates that improve machinability, helping to achieve sufficient machinability. During metal forming processes such as rolling and forging, deformable manganese sulfide (MnS) inclusions become elongated. Such elongated MnS inclusions can have a significant adverse effect on mechanical properties such as tensile strength and impact toughness if the inclusions are not aligned in the direction of the load. Therefore, the sulfur content is limited to 0.06%. The preferred range for sulfur content is 0.03% to 0.04%.

[0019] Phosphorus is an optional component of the steel of the present invention, between 0% and 0.02%. Phosphorus tends to segregate, particularly at grain boundaries, or co-segregate with manganese, thus reducing spot weldability and hot ductility. For these reasons, its content is limited to 0.02%, preferably lower than 0.015%.

[0020] Nitrogen is present in the steel of the present invention in an amount between 0% and 0.015%. Nitrogen forms nitrides together with Al, Nb and Ti, which prevent the coarsening of the austenite structure of the steel during hot forging and enhance its toughness. When the Ti content is between 0.01% and 0.03% and the Ti / N ratio is <3.42, efficient use of TiN for fixing the austenite grain boundaries is achieved. Using a chemically excessive nitrogen content increases the size of these particles, which not only makes the efficiency of fixing the austenite grain boundaries worse, but also increases the probability that TiN particles act as fracture initiation sites.

[0021] Aluminum is an optional element for the steel of the present invention. Aluminum is a strong deoxidizer and also forms precipitates dispersed in the steel as nitrides that prevent austenite grain growth. However, the deoxidation effect saturates at an aluminum content exceeding 0.1%. A content exceeding 0.1% may lead to the formation of coarse aluminum-enriched oxides that deteriorate the tensile properties, especially the impact toughness. It is preferable to have aluminum between 0 and 0.06%, more preferably between 0 and 0.05%.

[0022] Molybdenum is an optional element that can be present in the present invention at 0.03% - 0.5%. Molybdenum forms Mo2C precipitates, which increase the yield strength of the steel of the present invention. Molybdenum also clearly affects the hardenability of the steel. Such an effect can only be realized with a minimum of 0.03% molybdenum. Adding an excessive amount of molybdenum increases the alloying cost and promotes the formation of the MA component from retained austenite. Also, if the Mo content is too high, segregation problems may appear. Therefore, in the present invention, molybdenum is limited to 0.5%. Its preferred limit for the steel of the present invention is between 0.03% and 0.3%, more preferably between 0.03% and 0.1%.

[0023] Copper is a residual element resulting from the electric arc furnace steelmaking process and must always be reduced to less than 0.5%, preferably to 0. Exceeding this value significantly reduces the hot workability.

[0024] Niobium is an optional element that can be present in the steel of the present invention at 0.04% to 0.15%. When niobium is in solid solution, it is added to increase the hardenability of the steel by significantly delaying the diffusion transformation. Niobium can also be used in a synergistic effect with boron, and due to the preferential precipitation of niobium carbonitride, it prevents boron from precipitating into boride along the grain boundaries. Also, niobium is known to slow down the recrystallization and austenite grain growth rate in both solid solution and precipitate. The combined effect on austenite crystal grain size and hardenability helps to refine the final martensite microstructure, thereby enhancing the strength and toughness of the parts manufactured according to the present invention. To prevent coarsening of niobium precipitates that can act as nuclei for ductile damage and ferrite transformation, niobium cannot be added up to a content higher than 0.15% by weight.

[0025] Titanium is an optional element that can be present at 0.01% to 0.1%. Titanium prevents boron from forming nitrides. Titanium precipitates in the steel as nitrides or carbonitrides that can efficiently fix austenite grain boundaries and 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 when the titanium content is less than 0.01%, and the effect tends to saturate at a content exceeding 0.1%, with only the alloy cost increasing. Furthermore, the generation of coarse titanium nitride during solidification is adverse to impact toughness and fatigue properties. Therefore, the presence of titanium is preferably between 0.01% and 0.03%.

[0026] Vanadium is an optional element and exists between 0% and 0.5%. Vanadium is effective in increasing the strength of the steel by forming carbides or carbonitrides, and its upper limit is 0.5% for economic reasons. The preferred limit of vanadium is between 0% and 0.1%.

[0027] The boron content ranges from 0.0015% to 0.004%. Since boron only causes significant microstructural changes at concentrations of a few ppm, it is usually added in very small amounts. At this level, the ratio of boron atoms to iron atoms is very low (generally <0.00005), and it does not affect the bulk, thus not resulting in solid solution hardening or precipitation strengthening. In fact, boron strongly segregates at austenite grain boundaries, where, for larger grain sizes, boron atoms can be as numerous as iron atoms. This segregation promotes the martensite microstructure during cooling, thus delaying ferrite and pearlite formation, which increases the strength of such steel after austenite decomposition at moderate cooling rates. To enable and demonstrate this effect, it is recommended to add more than 0.0015% boron. Since higher boron content drastically deteriorates the low-temperature toughness of such steel, its upper limit is set at 0.004%.

[0028] Other elements such as tin, cerium, magnesium, or zirconium may be added individually or in combination in the following weight ratios: tin ≤ 0.1%, cerium ≤ 0.1%, magnesium ≤ 0.010%, and zirconium ≤ 0.010%. These elements, up to the indicated maximum content levels, allow for grain refinement during solidification. The remainder of the steel composition consists of iron and unavoidable impurities resulting from processing.

[0029] The microstructure of the steel sheet includes the following in terms of area percentage:

[0030] The martensite content in the steel of the present invention is 55% to 85%. Martensite is the matrix phase of the steel of the present invention. Martensite imparts tensile strength and other mechanical properties to the steel. A minimum of 55% martensite is required to achieve a tensile strength of 1300 MPa. It is advantageous to have martensite between 60% and 85%, preferably between 65% and 80%. The martensite is formed in a second step, particularly during cooling between Ms-150°C and room temperature. The martensite of the present invention includes fresh martensite and stress-relived martensite.

[0031] The steel of the present invention contains auto-tempered martensite between 20% and 45%. Auto-tempered martensite is an essential fine component of the steel of the present invention. Auto-tempered martensite imparts impact toughness and ductility to the steel of the present invention. A minimum of 20% auto-tempered martensite is required to obtain impact toughness, but tensile strength always decreases when the amount of auto-tempered martensite exceeds 45%. Therefore, the preferred presence of auto-tempered martensite is between 25% and 40%, and more preferably between 30% and 40%. The auto-tempered martensite of the steel of the present invention is obtained when the martensite obtained at the end of the first cooling step is self-tempered during the second cooling step, and because martensite is generated, an exothermic reaction continues during cooling.

[0032] The distinction between auto-tempered martensite and regular martensite is confirmed by using LePera etching to reveal both phases, and then observing the carbides in the auto-tempered material using SEM. For example, in Figure 1, reference numeral 10 indicates auto-tempered martensite in which carbides are clearly visible as small white dots, while reference numeral 20 indicates martensite in which carbides are absent.

[0033] The cumulative amount of martensite and auto-tempered martensite is at least 90%, preferably 95%, in order to simultaneously ensure tensile strength and impact toughness. The martensite of the present invention imparts tensile strength, and the auto-tempered martensite imparts toughness. Whenever the cumulative presence is less than 90%, the presence of soft phases such as retained austenite increases, which adversely affects both tensile strength and toughness.

[0034] Retained austenite can be present in steel at a concentration of 0% to 10%, and should be kept to the minimum possible. Up to 10% retained austenite does not adversely affect the target properties, but if it exceeds 10%, it adversely affects the tensile strength. It is preferable to have 0% to 5%, more preferably 0% to 2%, of retained austenite.

[0035] The microstructure of forged machine parts does not contain microstructure components such as bainite, pearlite, and cementite, in addition to the microstructure described above.

[0036] The machine parts according to the present invention can be manufactured by any suitable hot forging process, such as drop forging, press forging, upsetting forging, and rotary forging, in accordance with the specified process parameters described below.

[0037] While preferred exemplary methods are shown herein, these examples do not limit the scope of the disclosure or the embodiments on which the examples are based. Furthermore, none of the embodiments described herein are intended to be limiting, but merely describe some of the many possible ways in which various embodiments of this disclosure may be carried out.

[0038] A preferred method comprises providing a semi-finished casting of steel having the chemical composition according to the present invention. The casting can be carried out in any form such as an ingot, bloom, or billet that can be forged into machine parts having a cross-sectional diameter between 30 mm and 100 mm.

[0039] For example, steel having the above chemical composition is cast into a bloom and then rolled into a bar that functions as a semi-finished product. Several rolling operations can be performed to obtain the desired semi-finished product.

[0040] The semi-finished products after the casting process can be used directly at high temperatures after rolling, or they can be cooled to room temperature first and then reheated for hot forging at temperatures ranging from Ac3+30°C to 1300°C.

[0041] The temperature of the semi-finished product after hot forging is preferably 1150°C and must be lower than 1300°C. This is because a temperature lower than 1150°C can place an excessive load on the forging die, and furthermore, the temperature of the steel may drop to the ferrite transformation temperature during finish forging, resulting in the steel being forged with transformed ferrite in its structure. Therefore, it is preferable that the temperature of the semi-finished product be sufficiently high so that hot forging can be completed within the austenite temperature range. Reheating to temperatures above 1300°C should be avoided because it is industrially costly.

[0042] In this specification, the final finish forging temperature, referred to as T forging, must be maintained above 950°C in order to have a structure favorable for recrystallization and forging. It is preferable to perform the final forging at a temperature above Ac3 + 50°C (because below this temperature the steel sheet shows a significant decrease in quality during forging), preferably above Ac3 + 100°C.

[0043] In this way, a hot-forged part is obtained, and then this hot-forged steel part is cooled in a two-step cooling process.

[0044] In a two-step cooling process for hot-forged parts, the hot-forged parts are cooled at different temperatures and at different cooling rates.

[0045] In cooling step 1, 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 / sec to 10°C / sec. The part may optionally be held at T1 for up to 3600 seconds. During this cooling step 1, the average cooling rate from T-forging to T1 is preferably in the range of 0.2°C / sec to 8°C / sec, more preferably in the range of 0.2°C / sec to 2°C / sec.

[0046] Subsequently, a second cooling step begins (the hot-forged part is cooled from T1 to a temperature in the range of Ms-150°C to room temperature, referred to herein as T2, at an average cooling rate of 0.1°C / sec to 10°C / sec). During cooling step 2, the cooling between T1 and T2 is preferably maintained at an average cooling rate of 1.0°C / sec to 5.0°C / sec. Such a second cooling step promotes the transformation of austenite to martensite and auto-tempers the already formed martensite, reducing the possibility of austenite remaining in the final microstructure. This average cooling rate (rat) is also selected to ensure homogeneous cooling across the cross-section of the hot-forged part.

[0047] After the second cooling step is complete, the forged machine parts are obtained.

[0048] The resulting forged machine parts can be tempered at any temperature from 100°C to 200°C / second, preferably from 125°C to 200°C / second, for a period of 5 to 3600 seconds.

[0049] For all cooling processes, the Ms temperature is calculated for the steel using the following formula. Ms=539-423C-30Mn-18Ni-12Cr-11Si-7Mo In the formula, the elemental content is expressed as a weight percentage. [Brief explanation of the drawing]

[0050] [Figure 1] Reference numeral 10 indicates auto-tempered martensite in which carbides are clearly visible as small white dots, while reference numeral 20 indicates martensite in which carbides are absent. [Examples]

[0051] The following tests, examples, illustrative diagrams, and tables presented herein are not restrictive in nature and should be considered for illustrative purposes only, as they illustrate the advantageous features of the present invention.

[0052] Table 1 summarizes forged machine parts made from steel of different compositions, where each forged machine part is manufactured according to the process parameters specified in Table 2. Subsequently, Table 3 summarizes the microstructure of the forged machine parts obtained during testing, and Table 4 summarizes the evaluation results of the obtained properties.

[0053] [Table 1]

[0054] Table 2 summarizes the process parameters applied to semi-finished products made from the steel shown in Table 1 after reheating at 1280°C and subsequent hot forging. Steel compositions I1-I3 are useful for the manufacture of forged machine parts according to the present invention. This table also specifies the reference forged machine parts designated as R1-R3 in the table. Table 2 also shows the tabular representation of Ms and Ac3.

[0055] [Table 2]

[0056] Table 3 illustrates the results of tests performed according to standards for different microscopes, such as scanning electron microscopy, to determine the microstructure of both the steel of the present invention and the reference steel in terms of area fraction.

[0057] The results are specified herein.

[0058] [Table 3]

[0059] Table 4 illustrates the mechanical properties of both the steel of the present invention and the reference steel. Tensile tests are performed according to the NF EN ISO6892-1 standard to determine the tensile strength and yield strength. Impact toughness tests are performed on standard KCV specimens with V-notches at 20°C according to EN ISO148-1 for both the steel of the present invention and the reference steel.

[0060] The results of various mechanical tests conducted in accordance with the standards will be compiled.

[0061] [Table 4]

Claims

1. Steel for forging machine parts, containing the following elements expressed in weight percentages: 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% It includes and may contain one or more of the following 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 composition consists of iron and unavoidable impurities resulting from processing, and the microstructure of the steel contains, in area fraction, 55% to 85% martensite, 20% to 45% auto-tempered martensite, and 0% to 10% retained austenite, with the cumulative amount of auto-tempered martensite and martensite being at least 90%.

2. A steel for forging machine parts according to claim 1, wherein the composition contains 0.3% to 0.9% silicon.

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

4. A steel for forging machine parts according to any one of claims 1 to 3, comprising aluminum in a composition of 0% to 0.06%.

5. A steel for forging machine parts according to any one of claims 1 to 4, wherein the composition comprises 1.4% to 2.1% manganese.

6. A steel for forging machine parts according to any one of claims 1 to 5, wherein the composition comprises 0.7% to 1.4% chromium.

7. A steel for forging machine parts according to any one of claims 1 to 6, wherein the martensite content is 60% to 85%.

8. A steel for forging machine parts according to any one of claims 1 to 7, wherein the presence of auto-tempered martensite and the cumulative presence of martensite is at least 95%.

9. The plate has an ultimate tensile strength of 1300 MPa or more and a tensile strength of 38 J / cm² at 20°C. ２ A steel having the above impact toughness, for forging machine parts according to any one of claims 1 to 8.

10. A method for manufacturing forged steel machine parts, comprising the following series of steps: - A step of providing the steel composition according to any one of claims 1 to 6 in the form of a semi-finished product. - A step of reheating the semi-finished product to a temperature of Ac3 + 30°C to 1300°C. - A process of hot forging the semi-finished product in the austenite range where the forging temperature exceeds 950°C to obtain a hot-forged part. - A two-step cooling process for hot-forged parts, in step 1, the hot-forged parts are cooled from the forging temperature T to a temperature T1 in the range of 780 to 1250°C at an average cooling rate of 0.2°C / sec to 10°C / sec, and the hot-forged parts are held for an arbitrary maximum of 3600 seconds. - Subsequently, in step 2, the hot-forged part is cooled from T1 to a temperature T2 in the range of Ms-150°C to room temperature at an average cooling rate of 0.1°C / sec to 10°C / sec, thereby obtaining a forged machine part. This is a two-step cooling process. Methods that include...

11. The method according to claim 10, wherein in cooling step 1, the hot-forged part is cooled from T-forging to T1 at an average cooling rate of 0.2°C / second to 8°C / second.

12. The method according to claim 10 or 11, wherein in cooling step 2, the hot-forged part is cooled from T1 to T2 at an average cooling rate of 1.0°C / sec to 5.0°C / sec.

13. The method according to any one of claims 10 to 12, wherein tempering can be performed in the range of 100°C to 200°C.

14. Use of steel plates according to any one of claims 1 to 9 or forged machine parts manufactured by the method described in claims 10 to 13 for manufacturing structural or safety components of a vehicle or engine.

15. A vehicle comprising the part obtained according to claim 15.

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