Bainite steel forged parts and manufacturing method thereof

Abstract

Providing steel for forging machine parts. The present invention includes, in weight percent, 0.15%≦C≦0.22%, 1.6%≦Mn≦2.2%, 0.6%≦Si≦1%, 1%≦Cr≦1.5%, 0.01%≦Ni≦1%, 0%≦S≦0.06%, 0%≦P≦0.02%, 0%≦N≦0.013%, and optionally 0%≦Al≦0.06%, 0.03%≦Mo≦0.1%, 0%≦Cu≦0.5%, 0.01%≦Nb≦0.15%, 0.01%≦Ti≦0.03%, 0%≦V≦0.08%, A steel for forging machine parts, having 0.0015%≦B≦0.004%, the balance consisting of iron and unavoidable impurities, and having a microstructure with an area fraction of retained austenite and martensite-austenite islands between 1% and 20%, the balance being at least 80% bainite, in which the proportion of bainite grain boundaries having a misorientation angle of 59.5° is at least 7%, and between 0% and 10% martensite is optionally present.

Description

[Technical Field]

[0001] The present invention relates to a bainitic steel suitable for forging steel mechanical parts for automobiles. [Background technology]

[0002] Automotive parts must satisfy two contradictory requirements, namely, ease of formability and strength, but in recent years, due to considerations of the global environment, a third requirement of improved fuel efficiency has also been imposed on automobiles. Therefore, automotive parts must now be manufactured from highly formable materials to meet standards for ease of fit into complex automotive assemblies, while at the same time improving the strength of the vehicle's engine for impact resistance and durability while reducing vehicle weight to improve fuel efficiency.

[0003] Therefore, vigorous research and development efforts are being made to reduce the amount of material used in automobiles by increasing the strength of materials. Conversely, as the strength of steel increases, its formability decreases, so there is a need to develop materials that have high strength, high impact toughness, and high formability as well.

[0004] Previous research and development in the field of high strength and high impact toughness has resulted in several methods for producing high strength and high impact toughness steels, some of which are listed herein for a clear understanding of the present invention.

[0005] US2013 / 0037182 claims a bainitic steel for manufacturing machine parts, having a chemical composition in weight percent of 0.05% ≤ C ≤ 0.25%, 1.2% ≤ Mn ≤ 2%, 1% ≤ Cr ≤ 2.5%, 0 < Si ≤ 1.55, 0 < Ni ≤ 1%, 0 < Mo ≤ 0.5%, 0 < Cu ≤ 1%, 0 < V ≤ 0.3%, 0 < Al ≤ 0.1%, 0 < B ≤ 0.005%, 0 < Ti ≤ 0.03%, 0 < Nb ≤ 0.06%, 0 < S ≤ 0.1%, 0 < Ca ≤ 0.006%, 0 < Te ≤ 0.03%, 0 < Se ≤ 0.05%, 0 < Bi ≤ 0.05%, 0 < Pb ≤ 0.1%, with the balance of the steel parts being iron and impurities resulting from processing. The steel of US2013 / 0037182 cannot achieve a yield strength of 800 MPa or more. Furthermore, this steel does not have an impact toughness value (KCU) of 70 J·cm at 20 °C. -2 The steel does not have an impact toughness value (KCU) of 70 J·cm at 温度20 °C.

[0006] WO2016 / 063224 claims a steel having a chemical composition in weight percent of 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 residual elements are less than 0.4%. However, in terms of mechanical properties, the tensile strength is less than 1200 MPa, the yield strength does not exceed 800 MPa, and the impact toughness in CVN is about 20 J.

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 publication, the object of the present invention is to provide a steel sheet having a tensile strength of more than 1100 MPa and an impact toughness of 70 J·cm at 20°C in DVM. -2 The present invention aims to provide a bainite steel for hot forging of machine parts, which makes it possible to obtain

[0009] Therefore, the object of the present invention is to Ultimate tensile strength of 1100 MPa or more, preferably above 1150 MPa 70 J·cm at 20°C -2 Impact toughness above Yield strength of 800 MPa or more, preferably above 850 MPa The object of the present invention is to solve these problems by providing a bainitic steel suitable for hot forging, which simultaneously has the above properties.

[0010] In a preferred embodiment, the steel sheet according to the invention may also have a yield strength to tensile strength ratio of 0.72 or greater.

[0011] Preferably, such steel is suitable for the manufacture of forged steel parts with a cross section between 30 mm and 100 mm, such as crankshafts, pitman arms, steering knuckles, etc., without a significant hardness gradient between the surface and the center of the forged part.

[0012] Another object of the present invention is also to provide a method for manufacturing these machine parts that is robust to shifts in manufacturing parameters, yet compatible with conventional industrial applications.

[0013] Carbon is present in the steel of the present invention in an amount of 0.15% to 0.22%. Carbon imparts strength to the steel through solid solution strengthening, and carbon retards the formation of ferrite through gamma formation. Carbon is an element that influences the bainite transformation start temperature (Bs) and the martensitic transformation start temperature (Ms). Bainite transformed at low temperatures exhibits a better strength / ductility combination than bainite transformed at high temperatures.

[0014] A minimum of 0.15% carbon is required to reach a tensile strength of 1100 MPa, but if carbon is present in excess of 0.22%, it reduces the ductility and machinability and weldability of the final product. The carbon content is advantageously in the range of 0.15% to 0.20% to simultaneously obtain high strength and high ductility.

[0015] The steel of the present invention contains manganese added between 1.6% and 2.2%. Manganese imparts hardenability to the steel. This reduces the critical cooling rate at which bainite or martensitic transformation can be achieved with continuous cooling without prior transformation. Manganese promotes bainite transformation at low temperatures. A manganese content of at least 1.6% by weight is required to obtain the desired bainite structure and stabilize austenite. However, manganese in excess of 2.2% has a detrimental effect on the steel of the present invention because it coarsens the retained austenite after bainite transformation and increases the likelihood of it transforming to martensite or martensite islands during the third stage of cooling. These phases adversely affect the desired properties. Furthermore, manganese forms sulfides such as MnS. These sulfides can improve machinability if their shape and distribution are properly controlled. Otherwise, they can have a significant detrimental effect on impact toughness.

[0016] Silicon is present in the steel of the present invention between 0.6% and 1%. Silicon imparts strength to the steel of the present invention through solid solution strengthening. Silicon reduces the formation of cementite nucleation because it inhibits carbide precipitation and diffusion-limited growth by forming a Si-rich layer around the precipitation nuclei. Thus, the austenite becomes carbon-rich, reducing the driving force during bainite transformation. As a result, the addition of Si slows the overall bainite transformation rate and increases the retained austenite content. Silicon addition can result in the development of cementite-free bainite, which generally exhibits a higher combination of strength and ductility than conventional upper and lower bainite transformed over the same temperature range. Additionally, silicon acts as a deoxidizer. A minimum of 0.6% silicon is required to impart strength to the steel of the present invention and provide cementite-free bainite under continuous cooling. Amounts of silicon greater than 1% may increase the activity of carbon in austenite to promote its transformation to proeutectoid ferrite, reducing strength, but also excessively restrict the extent of the bainite transformation, resulting in excess retained austenite at the end of the bainite transformation and, as a result, excess martensite and martensite islands at the end of cooling.

[0017] Chromium is present in the steel of the present invention at between 1% and 1.5%. Chromium is an essential element for the formation of bainite and also for promoting the stabilization of austenite. The addition of chromium promotes a uniform, fine bainite structure in the temperature range between Bs + 30°C and Bs + 50°C. While a minimum of 1% chromium is required to produce the desired bainite structure, chromium contents of 1.5% or greater promote the formation of martensite from retained austenite in the temperature range between Ms and Ms + 60°C. Another reason for limiting chromium content levels below 1.5% is that segregation occurs above 1.5% chromium.

[0018] 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 start temperature. However, due to economic feasibility, its content is limited to 1%.

[0019] Sulfur is included between 0% and 0.06%. Sulfur improves machinability and forms MnS precipitates, which help achieve satisfactory machinability. During metal forming processes such as rolling and forging, deformable manganese sulfide (MnS) inclusions become elongated. These elongated MnS inclusions can significantly adversely affect mechanical properties such as tensile strength and impact toughness if the inclusions are not aligned with the load direction. Therefore, the sulfur content is limited to 0.06%. The preferred range of sulfur content is 0.03% to 0.04%.

[0020] Phosphorus is an optional element of the steel of the present invention, and its content is between 0% and 0.02%. Phosphorus reduces spot weldability and hot ductility due to its tendency to segregate or co-segregate with manganese, especially at grain boundaries. For these reasons, the phosphorus content is limited to 0.02%, and preferably less than 0.015%.

[0021] Nitrogen is present in the steel of the present invention in an amount between 0% and 0.013%. Nitrogen forms nitrides with Al, Nb, and Ti, preventing the austenite structure of the steel from coarsening during hot forging and increasing its toughness. Effective use of TiN to pin austenite grain boundaries is achieved when the Ti content is between 0.01% and 0.03% with a Ti / N ratio of <3.42. Using a nitrogen content in excess of the stoichiometric value 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.

[0022] 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 inhibit austenite grain growth. However, when the aluminum content exceeds 0.06%, the deoxidizing effect saturates. A content greater than 0.06% can lead to the formation of coarse aluminum-rich oxides that reduce tensile properties, especially impact toughness.

[0023] In the present invention, molybdenum is present between 0.03% and 0.1%. Molybdenum forms Mo2C precipitates, which increase the yield strength of the steel of the present invention. Molybdenum also has a clear effect on the hardenability of the steel. Solute molybdenum substantially inhibits the growth of bainite laths, making them finer. This effect can only be achieved with a minimum of 0.03% molybdenum. Excessive addition of molybdenum increases alloying costs and promotes the formation of martensite islands from retained austenite. Furthermore, if the Mo content is too high, segregation problems may occur. Therefore, in the present invention, molybdenum is limited to 0.1%.

[0024] Copper is a residual element produced in the electric arc furnace steelmaking process and should be reduced to 0%, but always below 0.5%, as above this value hot workability is significantly reduced.

[0025] Niobium is present in the steel of the present invention at between 0.04% and 0.15%. Niobium is added to enhance the hardenability of the steel by strongly retarding diffusional transformations in solid solution. Niobium can also be used synergistically with boron, preventing boron from precipitating as boron carbides along grain boundaries, resulting in the preferential precipitation of niobium carbonitrides. Furthermore, niobium is known to slow the rate of recrystallization and austenite grain growth in both solid solution and precipitates. This combined effect on austenite grain size and hardenability helps refine the final bainite structure, thereby improving the strength and toughness of components manufactured according to the present invention. To prevent coarsening of niobium precipitates, which can adversely affect ductility and act as nuclei for ferrite transformation, niobium cannot be added at a concentration greater than 0.15 wt.%.

[0026] Titanium is present between 0.01% and 0.03%. Titanium prevents boron from forming nitrides. Titanium precipitates in steel as nitrides or carbonitrides, effectively pinning austenite grain boundaries and limiting austenite grain growth at high temperatures. Because bainite packet size is closely related to austenite grain size, titanium addition is effective in improving toughness. This effect is not achieved at titanium contents below 0.01%, and tends to saturate at contents above 0.03%, while alloy cost increases. Furthermore, the formation of coarse titanium nitrides during solidification is detrimental to impact toughness and fatigue properties.

[0027] Vanadium is an optional element present between 0% and 0.08%. Vanadium is effective in increasing the strength of steel by forming carbides or carbonitrides, and the upper limit is 0.08% for economic reasons.

[0028] Boron ranges from 0.0015 to 0.004%. Boron is usually added in very small amounts because only a few ppm can cause significant structural changes. At this level of addition, the ratio of boron atoms per iron atom is very low (typically <0.00005), so boron is ineffective in the bulk and therefore does not provide solid solution hardening or precipitation strengthening. In fact, boron segregates strongly at austenite grain boundaries, and at large grain sizes, boron atoms can outnumber iron atoms. This segregation delays the formation of ferrite and pearlite and promotes bainitic or martensitic structures during cooling, thus improving the strength of such steels after austenite decomposition at moderate cooling rates. To achieve this effect, B additions of 0.0015% or more are desirable. If not adequately protected by additions of Nb and / or Mo, boron carbides M can form at austenite grain boundaries at temperatures <950°C. 23 Precipitation of (B,C)6 may occur. 23Some researchers consider (B,C)6 a ferrite precursor because, when sufficiently large, it promotes ferrite nucleation at incoherent interfaces. The effect of unbound boron is clearly stronger than that of boron trapped in carbides. Therefore, to obtain bainitic or martensitic structures at moderate cooling rates, boron must be maintained unbound. The best hardenability is obtained in low-carbon steels with boron contents ranging from 15 to 30 ppm, up to 0.2%. The low-temperature toughness of such steels decreases rapidly with increasing boron content, so the upper limit is set at 0.004%.

[0029] Other elements such as tin, cerium, magnesium or zirconium can be added individually or in combination in the following weight proportions: tin≦0.1%, cerium≦0.1%, magnesium≦0.010% and zirconium≦0.010%. Up to the maximum content levels indicated, these elements allow for grain refinement during solidification. The remainder of the steel composition consists of iron and unavoidable impurities resulting from processing.

[0030] The microstructure of the steel plate includes:

[0031] The retained austenite and martensite-austenite islands constituent are present cumulatively in an amount between 1% and 20% and are essential components of the present invention. Preferentially, the amount of retained austenite and martensite islands is advantageously between 5% and 20%. The retained austenite provides ductility and the martensite-austenite islands provide strength to the steel of the present invention. The retained austenite and martensite-austenite islands are formed during cooling stages 2 and 3 from prior austenite that remained untransformed during cooling stage 2.

[0032] In the steel of the present invention, bainite constitutes 80% or more of the microstructure by area, and advantageously, more than 85% bainite. In the present invention, the microstructural composition of bainite has 7% or more, preferably more than 9%, of misoriented bainite grain boundaries with a misorientation angle of 59.5°. These misoriented bainite grains impart impact toughness to the steel of the present invention. Bainite of the present invention is formed during cooling, particularly in stage 2 between 470°C and Ms. However, bainite formed in the upper bainite zone above 470°C is coarse bainite that cannot have more than 7% misoriented bainite grains due to its coarse size. Therefore, to avoid the formation of coarse bainite, faster cooling rates are preferred between T1 and T2, particularly between T1 and 470°C. This is illustrated in Figure 1, which shows the microstructure of Test I1 according to the present invention, and Figure 2 shows the microstructure of Test R1, not according to the present invention. In comparison with the bainite of Figure 1, where the bainite according to the invention is designated by the reference numeral 20, Figure 2 contains less than 80% bainite by area and also contains coarse bainite, designated by the reference numeral 10 in Figure 2. Furthermore, Figure 3 shows a comparison of the presence of misoriented bainite grain boundaries with a misorientation angle of 59.5° between the steel of the invention and the reference steel. The curve designated by the reference numeral 1 in Figure 3 is the curve for test I1, which contains 9.6% misoriented bainite grain boundaries with a misorientation angle of 59.5°, whereas the curve designated by the reference numeral 2 in Figure 3 is the curve for test R1, which contains 4% misoriented bainite grain boundaries with a misorientation angle of 59.5°.

[0033] The steel of the present invention contains trace amounts of martensite, up to a maximum of 10%. Martensite is not intended to be part of the present invention, but is formed as a residual microstructure from processing the steel. The martensite content should be kept as low as possible and should not exceed 10%. Martensite up to a constituent percentage of 10% imparts strength to the steel of the present invention, but the presence of martensite above 10% reduces the machinability of the steel part.

[0034] In addition to the above microstructure, the microstructure of the machine forged part does not include microstructural constituents such as pearlite and cementite. [Brief explanation of the drawings]

[0035] [Figure 1] 1 shows the microstructure of test I1 according to the invention. [Figure 2] 1 shows the microstructure of test R1 not according to the invention. [Figure 3] 1 shows a comparison of the presence of misoriented bainite grain boundaries at a misorientation angle of 59.5° between the steel of the invention and the reference steel. DETAILED DESCRIPTION OF THE INVENTION

[0036] The machine part according to the invention can be manufactured by any suitable hot forging process, such as drop forging, press forging, upset forging and roll forging, according to the described process parameters explained below.

[0037] While preferred exemplary methods are set forth herein, the examples do not limit the scope of the disclosure and the aspects on which the examples are based. Furthermore, any examples set forth herein are not intended to be limiting, but merely to describe some of the many possible ways in which various aspects of the disclosure may be implemented.

[0038] A preferred method is to provide a semi-finished product casting of a steel having a chemical composition according to the invention, which may be 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 may be cast into a bloom and then rolled into a bar to form the semi-finished product. Several rolling operations may be performed to obtain the desired semi-finished product.

[0040] After the casting process, the semi-finished products can be used directly at high temperatures after rolling, or they can be first cooled to room temperature and then reheated for hot forging. The semi-finished products are reheated to temperatures between 1150°C and 1300°C.

[0041] The temperature of the semi-finished product subjected to hot forging is preferably at least 1150°C and must be less than 1300°C. This is because a temperature lower than 1150°C places excessive strain on the forging die, and furthermore, the temperature of the steel drops to the ferrite transformation temperature during finish forging, which may result in the steel being forged with transformed ferrite in the structure. Therefore, the temperature of the semi-finished product is preferably high enough to complete the hot forging in the austenite temperature range. Reheating at temperatures above 1300°C is industrially expensive and should be avoided.

[0042] The final forging temperature must be above 915°C, which is preferable for obtaining a structure favorable for recrystallization and forging. The final forging temperature must be above 915°C because steel plates show significant deterioration during forging below this temperature. This results in a hot-forged part, which is then cooled in a three-stage cooling process.

[0043] In the three-stage cooling process of hot forged parts, the hot forged parts are cooled at different cooling rates between different temperature ranges.

[0044] In cooling stage 1, the hot forged part is cooled from finish forging to a temperature range between Bs+50°C and Bs+30°C, also referred to as T1 in this specification, at an average cooling rate between 0.2°C / s and 10°C / s, and the hot forged part can be held for a time period between 0 and 3600 seconds, and during this cooling stage 1, it is preferable to have an average cooling rate between 0.2°C / s and 2°C / s from a temperature range between 750°C and 780°C to T1.

[0045] Thereafter, a second stage of cooling is initiated from the temperature range T1, where the hot forged part is cooled at an average cooling rate of 0.40°C / s to 2.0°C / s from the temperature range T1 to a temperature between Ms + 60°C and Ms, also referred to herein as T2. Further, during the second stage of cooling, the cooling from T1 to the temperature range between 470°C and 450°C is preferably maintained at an average cooling rate of 1.0°C / s to 2.0°C / s to promote the transformation of austenite to bainite and reduce the possibility of martensite formation.

[0046] In a third stage, the hot forged part is brought from the temperature range between T2 to room temperature, and the average cooling rate during the third stage is maintained at less than 0.8°C / sec, preferably less than 0.5°C / sec, and more preferably less than 0.2°C / sec, selected to provide uniform cooling across the cross section of the hot forged part.

[0047] After the third stage of cooling is completed, the forged machine part is obtained.

[0048] For all stages of cooling, the Bs and Ms temperatures of the steel of the present invention are determined according to the following formula: Bs=962-288C-84Mn-81Si-6Ni-95Mo-153Nb+108Cr 2 -269Cr Ms=539-423C-30Mn-18Ni-12Cr-11Si-7Mo is calculated using where the element contents are expressed as weight percent. [Example]

[0049] The following tests, examples, figurative illustrations and tables presented herein are non-limiting in nature and should be considered for illustrative purposes only, illustrating advantageous features of the present invention.

[0050] The forged machine parts manufactured from steels of different compositions are summarized in Table 1, where the forged machine parts are manufactured according to the processing parameters listed in Table 2, respectively. Table 3 then summarizes the microstructures of the forged machine parts obtained during the tests, and Table 4 summarizes the evaluation results of the obtained properties.

[0051] Table 1

[0052] [Table 1]

[0053] Table 2 Table 2 summarizes the processing parameters carried out on semi-finished products made from the steels of Table 1 after reheating between 1150°C and 1300°C and then finish hot forging above 915°C. Steel compositions I1 to I3 are useful for the manufacture of forged machine parts according to the invention. The table also lists reference forged machine parts, designated R1 to R3 in the table. Table 2 also lists Bs and Ms, which are defined as follows for the steels of the invention and the reference steels:

[0054] Bs(℃)=962-288C-84Mn-81Si-6Ni-95Mo-153Nb+108Cr 2 -269Cr Ms(℃)=539-423C-30Mn-18Ni-12Cr-11Si-7Mo where the element contents are expressed as weight percent.

[0055] Table 2 is as follows:

[0056] [Table 2]

[0057] Table 3 Table 3 illustrates the results of tests carried out according to various microscopy standards, such as scanning electron microscopy, to determine the microstructure of both the steel of the invention and the reference steel on the basis of area fraction. The measurement of the fraction of misoriented grain boundaries was carried out by EBSD, in which the relative frequency of bainite grains was measured in the misorientation profile.

[0058] The results are documented here.

[0059] [Table 3]

[0060] Table 4 Table 4 illustrates the mechanical properties of both the inventive and reference steels. To determine the tensile strength, yield strength tensile tests were carried out according to the NF EN ISO 6892-1 standard. Tests to measure the impact toughness of both the inventive and reference steels were carried out according to EN ISO 148-1 on U-notched standard DVM specimens at 20°C.

[0061] The results of various mechanical tests carried out in accordance with the standards are summarized in Table 4.

[0062] [Table 4]

Claims

1. A forged steel machine part comprising the following elements, expressed in weight percentage: 0.15% ≤ C ≤ 0.22%, 1.6% ≤ Mn ≤ 2.2%, 0.6% ≤ Si ≤ 1%, 1% ≤ Cr ≤ 1.5%, 0.01% ≤ Ni ≤ 1%, 0% ≤ S ≤ 0.06%, 0% ≤ P ≤ 0.02%, 0% ≤ N ≤ 0.013% It includes any of the following elements 0% ≤ Al ≤ 0.06%, 0.03% ≤ Mo ≤ 0.1%, 0% ≤ Cu ≤ 0.5%, 0.04% ≤ Nb ≤ 0.15%, 0.01% ≤ Ti ≤ 0.03%, 0% ≤ V ≤ 0.08%, 0.0015% ≤ B ≤ 0.004% It can contain one or more of the following: Forged steel machine parts, wherein the remaining composition consists of iron and unavoidable impurities arising from processing, the structure on the surface of the forged steel consists of a microstructure, the microstructure includes the cumulative presence of retained austenite and island martensite-austenite between 1% and 20%, the remaining microstructure consists of at least 80% bainite and any martensite between 0% and 10%, the total of retained austenite and island martensite-austenite, bainite, and martensite (if present) is 100%, and the proportion of bainite grain boundaries having a misorientation angle of 59.5° is at least 7%.

2. The forged steel machine part according to claim 1, wherein the composition contains 0.7% to 1% silicon.

3. The forged steel machine part according to claim 1 or 2, wherein the composition contains 0.15% to 0.2% carbon.

4. A forged steel machine part according to any one of claims 1 to 3, wherein the composition comprises 0% to 0.05% aluminum.

5. A forged steel machine part according to any one of claims 1 to 4, wherein the composition contains 1.6% to 1.9% manganese.

6. A forged steel machine part according to any one of claims 1 to 5, wherein the composition contains 1.1% to 1.5% chromium.

7. The forged steel machine part according to any one of claims 1 to 6, wherein the bainite content is 85% or more.

8. A forged steel machine part according to any one of claims 1 to 7, wherein the total amount of retained austenite and island martensite-austenite is between 1% and 15%.

9. The forged steel machine part according to any one of claims 1 to 8, wherein the steel plate has an ultimate tensile strength of 1100 MPa or more and a yield strength of 800 MPa or more.

10. The forged steel machine part according to claim 9, wherein the steel has an ultimate tensile strength of 1150 MPa or more and a yield strength of 850 MPa or more.

11. The aforementioned steel plate has a load of 70 J / cm². ２ A forged steel machine part according to any one of claims 1 to 10, having the above impact toughness.

12. The aforementioned steel plate has a load capacity of 90 J / cm². ２ A forged steel machine part according to any one of claims 10 or 11, having the above impact toughness.

13. A method for manufacturing steel forging machine parts, comprising the following sequential steps - A step of providing steel having the composition described in 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 between 1150°C and 1300°C. - A step of hot forging the semi-finished product in the austenite region, wherein the hot forging finish temperature is set to over 915°C to obtain a hot forged part, - A three-stage cooling procedure for a hot-forged part, wherein in stage 1, the hot-forged part is cooled from the hot-forging finish temperature to a temperature range T1 between Bs + 50°C and Bs + 30°C at a cooling rate of 0.2°C / sec to 10°C / sec, and the hot-forged part can be held for any time between 0 seconds and 3600 seconds. - Subsequently, in step 2, the hot-forged part is cooled from the temperature range between T1 to the temperature range between T2, which is between Ms + 60°C and Ms, at an average cooling rate of 0.40°C / sec to 2°C / sec. Next, in step 3, the hot-forged part is cooled from the temperature range between T2 to room temperature at an average cooling rate of less than 0.8°C / second to obtain a forging machine part. Includes, A method for manufacturing the hot-forged part, wherein the microstructure and the proportion of bainite grain boundaries of the hot-forged part are as described in any one of claims 1, 7, and 8.

14. The method according to claim 13, wherein in cooling stage 1, the hot-forged part is cooled from a temperature range of 780°C to 750°C to a temperature range of T1 at an average cooling rate of 0.2°C / second to 2°C / second, and the hot-forged part can be held for any time between 0 seconds and 3600 seconds.

15. The method according to claim 13 or 14, wherein in cooling step 2, the hot-forged part is cooled from a temperature range between T1 to a temperature range between 470°C and 450°C at an average cooling rate between 1.0°C / sec and 2.0°C / sec.

16. The method according to any one of claims 13 to 15, wherein in step 3, the hot-forged part is cooled from the temperature range between T2 to room temperature at a cooling rate of less than 0.5°C / second.

17. Use of forged steel machine parts according to any one of claims 1 to 12 or forged machine parts manufactured according to any one of claims 13 to 16 for the manufacture of vehicle structural or safety parts or engines.

18. A method for manufacturing a vehicle, comprising the step of using a part obtained according to any one of claims 13 to 16.

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