Forged part of steel and a method of manufacturing thereof

Abstract

A steel part for the transmission system of an automobile comprising a core made of the following elements0.15% ≦ C ≦ 0.25%; 1.0% ≦ Mn ≦1.6%; 0.4% ≦ Si ≦ 0.9%; 0.001% ≦ Al ≦ 0.1 %; 0.10% ≦ Mo ≦ 0.5%;1% ≦ Cr ≦ 1.5 %; 0 ≦ P ≦ 0.09%; 0 ≦ S ≦ 0.09%; 0% ≦ N ≦ 0.09%; 0 ≦ Nb ≦ 0.06%; 0% ≦ Ni ≦ 1%; 0% ≦ V≦ 0.2%; 0% ≦ Ti ≦ 0.1%; 0% ≦ Cu≦ 1%; 0% ≦ B ≦ 0.008%; 0% ≦ Sn≦ 0.1%; 0% ≦ Ce ≦ 0.1%; 0% ≦ Mg ≦ 0.10%; 0% ≦ Zr ≦ 0.10%; the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of the core of said steel part comprising, by area percentage, at least 90% of Martensite, with a cumulative optional presence of any one or more from Residual Austenite, Pearlite, ferrite and Bainite from 1% to 10% and precipitates of Aluminum and / or Niobium in form of AlN and / or Nb (C,N), wherein the percentage of grains with prior austenite grains size from 3 to 5 is limited to a maximum of 10% of the total grains, such steel part comprising a martensite­deficient layer till the depth of 1mm or less on all its surfaces, such martensite­ deficient layer comprising from 55% to 72% of martensite, 10% to 40% residual austenite and the remainder being any one or more from bainite, ferrite or cementite.

Description

[0001]FORGED PART OF STEEL AND A METHOD OF MANUFACTURING THEREOF The present invention relates to steel suitable for forging mechanical parts of steel for automobiles and particularly to the steel suitable for manufacturing of gear, shafts and other transmission parts for the transmission system of an automobile. Transmission parts such as gears, shafts, differentials and other parts of a transmission system of an automobile works under the conditions of high rotating speed and high load and continuous alternation of rotating speed and load. Hence it is necessary for the transmission parts to have high strength, high hardness and good wear resistance specifically the contact surface of these parts whereas the core of the transmission parts are required to have good durability, and meanwhile, the meshing precision of the transmission parts is required to be high and the working noise is required to be low. Therefore it is mandated for the steel for the transmission system of an automobile to meet two requirements of machinability to facilitate the manufacturing process on the contrary to have high strength and high hardness so that the steel is suitable to be used during the high load and high rotating speed operation and usability. Therefore, intense Research and development endeavors are put in to develop a material that is good in machinability while having high yield strength that is above 1330 MPa with adequate impact toughness. Earlier research and developments in the field of steels for transmission system of the automobiles have resulted in several methods for producing high strength and good formability some of which are enumerated herein for conclusive appreciation of the present invention: WO2020 / 178854 provides a steel composition for high temperature carburizing and a steel article made from the steel composition. The composition comprises: a) 0.11 to 0.3 wt.% of Carbon, b) 1.1 to 1.4 wt. % of Manganese, c) 0.15 to 0.35wt. % of Silicon, d) 1 to 1.3 wt. % of Chromium, e) ≤0.0006 wt. % of Boron, f) 0.04 to 0.05 wt. % of Titanium, g) 0.035 to 0.056 wt. % of Niobium, h) <0.2 wt. % of Nickel, i) <0.06 wt. % of Molybdenum, j) <0.025 wt.% of Sulphur, k) <0.025 wt.% of Phosphorous, l) 0.02 to 0.03 wt. % of Aluminium, m) ≤190 ppm of Nitrogen, and n) the rest is Iron (Fe). However the steel of WO2020 / 178854 is not able to reach sufficient tensile strength and Impact toughness levels. Hence the purpose of the present invention is to solve these problems by making available a steel suitable for mechanical operations for manufacturing a part for the transmission system of an automobile that simultaneously have: ­ a tensile strength greater than or equal to 1530 MPa and preferably above 1570 MPa, ­ a yield strength greater than or equal to 1230 MPa, and preferably above 1250 MPa, ­ an impact toughness of 58J / cm2 or more and preferably 61J / cm2 or more when measured for a KCU type of sample, ­ a striction greater than or more than 50% and preferably more than 52%, Preferably, such steel is suitable for manufacturing of a forged steel parts for the transmission system of an automobile wherein each part can have a cross section up to 150mm *150 mm and the steel is also suitable for other parts of an automobiles such as chassis members. Preferably, a surface hardness 650 Hv or more on a case depth from 0.4mm to 0.6mm. Preferably, a total elongation of more than 12% Another object of the present invention is also to make available a method for the manufacturing of these mechanical parts that is compatible with conventional industrial applications while being robust towards manufacturing parameters shifts. Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention. Carbon is present in the steel of present invention is from 0.15% to 0.25%. Carbon is an element necessary for increasing the strength of the Steel of present invention by producing a low­temperature transformation phases such as Bainite, But Carbon content less than 0.15% will not be able to impart the tensile strength to the steel of present invention. On the other hand, at a Carbon content exceeding 0.25%, the toughness is adversely impacted due to the excessive formation of proeutectoid cementite during the cooling after hot rolling or forging. Further excessive formation of proeutectoid cementite is also detrimental for mechanical operations on the part of the transmission system such as hobbing, lapping, shaping drilling, honing or grinding. The carbon content is advantageously in the range 0.16% to 0.24% and more especially 0.18% to 0.24%. Manganese is added in the present steel from 1.0% to 1.6%. This element is gammagenous. Manganese provides solid solution strengthening and suppresses the ferritic transformation temperature and reduces ferritic transformation rate hence assist in the formation of bainite. An amount of at least 1.0% is required to impart strength as well as to assist the formation of Bainite. But when Manganese content is present more than 1.6% it causes segregation which results in banded microstructure after annealing and this banded microstructure is deferential to the mechanical properties of the steel of present invention. process. The preferred limit for the presence of Manganese is from 1.1% to 1.5% and more preferably from 1.1% to 1.4%. Silicon is present in the steel of present invention from 0.4% to 0.9%. Silicon imparts the steel of present invention with strength through solid solution strengthening and also acts as a deoxidizer. Silicon is a constituent that can retard the precipitation of carbides during cooling after mechanical operation, therefore, Silicon promotes formation of Martensite. But Silicon is also a ferrite former and also increases the Ac3 transformation point which will push the austenitic temperature to higher temperature ranges that is why the content of Silicon is kept at a maximum of 0.9%. Further Silicon higher than 0.9% also enhances segregation.The preferred limit for the presence of Silicon is from 0.5% and 0.8% and more preferably from 0.55% and 0.70%. The content of the Aluminum is from 0.001% and 0.1%. Aluminum removes Oxygen existing in molten steel to prevent Oxygen from forming a gas phase during solidification process. Aluminum also fixes Nitrogen in the steel to form Aluminum nitride to reduce the size of the grains. But the deoxidizing effect saturates for aluminum content more than 0.1%. Aluminum also control the grain size of the present steel by forming AlN. Higher content of Aluminum above 0.1% lead to the occurrence of coarse aluminum­rich oxides that deteriorate machinability and hot forging on steel. The preferred limit for the presence of Aluminium is from 0.01% and 0.09% and more preferably from 0.01 and 0.035% Molybdenum is an essential element and may be present from 0.1 % and 0.5% in the present invention. Molybdenum is added to impart hardenability and hardness to steel by forming Molybdenum based carbides and also promote the formation of Martensite during the carburization and also retard the formation of coarse Niobium carbides or Niobium Carbonitrides. However, the addition of Molybdenum excessively increases the cost of the addition of alloy elements, so that for economic reasons its content is limited to 0.5%. The preferred limit for molybdenum content is from 0.1% and 0.4% and more preferably from 0.15% and 0.35%. Chromium is present from 1% and 1.5% in the steel of present invention. Chromium is an essential element that provide strength to the steel by solid solution strengthening and a minimum of 1% is required to impart the strength but when used above 1.5% increase the hardenability is beyond an acceptable limit due the formation of coarse cementite after cooling thereby impairing the forgeability as well as the ductility of the steel. Chromium addition also decreases the diffusion coefficient of carbon in the austenite same as nickel hence promote the formation of martensite during carburization The preferred limit for the presence of Chromium is from 1.1% and 1.4 % and more preferably from 1.2% and 1.4%. Phosphorus content of the steel of present invention is from 0 % and 0.09%. Phosphorus tends to segregate at the grain boundaries or co­segregate with Manganese. For these reasons, it is recommended to use phosphorus as less as possible. Specifically, content over 0.05% can cause rupture by intergranular interface decohesion which may be detrimental for the fatigue limit. The preferred limit for Phosphorus content is from 0% and 0.05%. Sulphur is contained from 0 % and 0.09%. Sulphur forms MnS precipitates which improve the machinability and assists in obtaining a sufficient machinability. During metal forming processes such as rolling and forming, deformable manganese sulfide (MnS) inclusions become elongated. Such elongated MnS inclusions can have considerable adverse effects on mechanical properties such as striction and impact toughness if the inclusions are not aligned with the loading direction further higher sulphur content is also detrimental for the forgeability of the steel. Therefore, sulfur content is limited to 0.09%. A preferable range the content of Sulphur is 0 % from 0.05% and more preferably from 0% and 0.040%to obtain the best balance from machinability and fatigue limit. Nitrogen is in an amount from 0% and 0.09% in steel of present invention. In the scope of the invention Nitrogen content can be more that 0.011% to promote AlN precipitation; NbCN and Aln precipitation. It seems that NbCN nucleate on AlN precipitate. To have the precipitates a minimum of 0.009% nitrogen is required. Hence the preferred limit for nitrogen is from 0.009% and 0.05% and more preferably 0.009% and 0.04% Niobium is an optional element for the Steel of present invention and may be added up to 0.06% and suitable for forming carbo­nitrides to impart strength of the Steel of present invention by precipitation hardening. Niobium will also impact the size of microstructural components through its precipitation as carbo­nitrides and by retarding the recrystallization during heating process. Thus, finer microstructure formed at the end of the holding temperature and as a consequence after the complete austenitization lead to the hardening of the product. However, Niobium content above 0.06% is not economically interesting as well as forms coarser precipitates which are detrimental for the fatigue properties, impact toughness of the steel and also when the content of niobium is 0.06% or more niobium is also detrimental for steel hot ductility resulting in difficulties during steel casting and rolling. The preferred limit for niobium content is from 0.020 and 0.06% or better from 0.025% to 0.058% and more preferably from 0.025% to 0.055%. Nickel is added to the present invention from 0% and 1% to increase the strength of the steel present invention and to improve toughness specially after Normalizing and carburizing. Nickel is beneficial in improving its pitting corrosion resistance. A minimum of 0.1% is required to get such effects. Nickel is added into the steel composition to decreases the diffusion coefficient of carbon in the austenite thereby promoting the formation of martensite during the Carburization process as well as low temperature phases such as bainite. But the presence of nickel content above 1% lowers the martensite start temperature hence leading to the excessive stabilization of residual austenite thereby having a detrimental impact on tensile strength and yield strength. Further Nickel is also restricted to 1% due to the economic reasons. It is preferred to have nickel from 0.1% and 0.9% in the steel of present invention. Vanadium is an optional element for the present invention and is content is from 0% and 0.2%. Vanadium is effective in enhancing the strength of steel by precipitation strengthening especially by forming carbides or carbo­nitrides. Upper limit is kept at 0.2% due to the economic reasons. The steel of present invention is always Titanium free due to the reason that Titanium forms coarse is an optional element and present upt to 0.1%. Titanium forms titanium nitrides which impart steel with strength, but these nitrides may form during solidification process, therefore have a detrimental effect fatigue limit. Hence the preferred limit for titanium is up to 0.05%. Copper is a residual element and may be present up to 1% due to processing of steel. Till 0.5% copper does not impact any of the properties of steel but over 0.5% the hot workability decreases significantly. Other elements such as Tin, Cerium, Calcium, Bismuth, Magnesium or Zirconium can be added individually or in combination in the proportions Tin ≦0.1%, Cerium ≦0.1%, Magnesium Calcium 0.0010%, Bismuth ≦ ≦ 0.008%and Zirconium ≦ 0.10%. Up to the maximum content levels indicated, these elements make it possible to refine the grain during solidification. The remainder of the composition of the Steel consists of iron and inevitable impurities resulting from processing. The rest of the composition is iron and unavoidable impurities, in particular resulting from the elaboration. More particularly, the composition of the steel part consists of the above­ mentioned elements. The steel part for the transmission system of an automobile has a microstructure comprising, in area %, at least 90% Martensite, and an optional cumulative presence of Residual Austenite, Pearlite, ferrite and bainite from 1% to 10%. Martensite constitutes at least 90% of the microstructure by area fraction. The martensite of the present invention can comprise both fresh and tempered martensite. However, fresh martensite is an optional microconstituent which is limited in the steel at an amount of up to 4%, preferably up to 2% and even better equal to 0%. Fresh martensite may form during cooling after tempering. Tempered martensite is formed from the martensite which forms during the second step of cooling after carburizing annealing and particularly after below Ms temperature and more particularly from Ms­10°C to 20°C.Such martensite is then tempered during the holding at a tempering temperature Temper from 180°C to 250°C. The martensite of the present invention imparts toughness and strength to steel. Preferably, the content of martensite is from 92% to 99% and more preferably from 95% to 99%. The cumulated amount of Residual Austenite, Pearlite, ferrite and bainite from 1% and 10% of the microstructure. The cumulative presence of Residual Austenite, Pearlite, ferrite and bainite does not affect adversely to the present invention till 10% but above 10% the mechanical properties may get impacted adversely. Residual Austenite may impart toughness and ductility to the steel of present invention. Pearlite of the present invention may impart strength and fatigue endurance to steel. Hence the preferred limit for the cumulative presence of ferrite and bainite is kept from 1% to 8% and more preferably from 1% to 5%. Precipitates of Al and Nb are present in the steel according to the invention as AlN and Niobium Carbo­nitrides Nb(C,N) respectively. These precipitates preferably has a size from 20nm to 350nm and more preferably from 25nm to 300nm. Precipitate formation takes place during the annealing process as well as the cooling step. Thereafter, the precipitates of the present invention are responsible for the pinning of the prior Austenite grains during Carburizing process thereby assisting in the formation of Martensite of the martensite deficient layer present invention in targeted amounts. Hence, it is preferred that the prior austenite grain size is from 3 to 15 measured as per the ASTM grain Index. It is more preferable to have prior austenite grain size from 3 to 13 and more preferably from 3 to 12. Further, in accordance with the scope of the present invention, the grains with prior austenite grain size from 3 to 5 can be present in the steel of present up to a maximum of 10% of the total grains because whenever the percentage of grains with prior austenite grain size from 3 to 5 is more than 10% the over total gains become coarse on the final steel product and it is not possible to reach the mechanical properties such as yield strength or impact toughness and preferably from 2% to 9% and more preferably 2% to 8%. In addition to this microstructure in the core of the steel part, it also includes a martensitic­ deficient layer on all the surfaces of the steel part of the transmission system of an automobile up to a depth of 1mm and preferably up to a depth of 0.5mm and showing a martensite percentage from 55% to 72% in area fraction, preferably from 60% to 72% more preferably from 65% to 72%. The martensite deficient layer formed on the surfaces preferably comprises any or all possible martensite kinds and notably fresh martensite, tempered martensite etc. The martensite of the martensite deficient layer preferably has carbon from 0.5% to 0.9% and more preferably from 0.6% to 0.8%. This martensite layer imparts the steel of the invention with a surface hardness of 650 Hv or more which provides the final steel part good resistance against the wear and also impart the precision during the meshing of part with each other during rotary operation of transmission system. This martensite deficient layer is formed during the carburizing done at high temperature. This surface layer also contains residual austenite from 10% to 40%. The austenite presence in the Martensite deficient layer imparts the steel with impact toughness. Whenever the residual Austenite is present over 40%, the steel is not able to reach the surface hardness of 650 Hv or more. The preferable limit for Residual Austenite is from 15% to 35% and more preferably from 20% to 35%. The remaining part of this surface layer comprises optionally of bainite and / or ferrite and / or cementite. A steel part for the transmission system of an automobile according to the invention can be produced by any suitable manufacturing process, with the stipulated process parameters explained hereinafter. A preferred exemplary method is demonstrated herein but this example does not limit the scope of the disclosure and the aspects upon which the examples are based. Additionally, any examples set forth in this specification are not intended to be limiting and merely set forth some of the many possible ways in which the various aspects of the present disclosure may be put into practice. In this preferred embodiment the steel part consider for demonstrating preferred process according to the present invention is a gear. A preferred method consists in providing a semi­finished casting of steel with a chemical composition according to the invention. The casting can be done in any form such as ingots or blooms or billets which is capable of being manufactured or processed into a steel part that can have a cross section up to 150mm*150 mm. For example, the steel having the above­described chemical composition is casted into a billet and then rolled in form of a bar. This bar can act as a semi­finished product for further process steps of manufacturing. Multiple rolling steps may be performed to obtain the desired semi­ finished product. A preferred Semi­finished product has a cross section be from Ø20mmto Ø110mm The semi­finished product after the rolling process can be used directly at a high temperature after the rolling or may be first cooled to room temperature and then reheated for hot forging at a temperature ranging from Ac3 + 30° C to 1300° C wherein Ac3 for the steel sheet is calculated by using the following formula: ^^^^^^^^3 = 910 − 203[^^^^]^(1 / 2) − 15.2[^^^^^^^^] + 44.7[^^^^^^^^] + 104[^^^^] + 31.5[^^^^^^^^] + 13.1[^^^^] − 30[^^^^^^^^]− 11[^^^^^^^^] − 20[^^^^^^^^] + 700[^^^^] + 400[^^^^^^^^] + 400[^^^^^^^^]The temperature of the semi­finished, which is subjected to hot forging, is preferably at least 1150° C and must be below 1300°C because the temperature of the semi­finished product is lower than 1150° C, excessive load is imposed on forging dies and, further, the temperature of the steel may decrease to a Ferrite transformation temperature during finishing forging, whereby the steel will be forged in a state in which transformed Ferrite contained in the structure. Therefore, the temperature of the semi­finished product is preferably sufficiently high so that hot forging can be completed in the austenitic temperature range. Reheating at temperatures above 1300°C must be avoided because they are industrially expensive. A final hot forging finishing temperature, herein after referred as Tforging, must be kept above 830°C to have a structure that is favorable to recrystallization and forging. It is preferred to have final forging to be performed at a temperature greater than Ac3+100°C and preferably above Ac3+200°C because below this temperature the steel bar exhibits a significant drop in forging. The hot forged part is thus obtained in this manner and then this hot forged steel part is cooled to room temperature. The hot forged steel part is then subjected to annealing to reduce the hardness of the steel part for further machining and also impart the steel part of present invention with targeted microstructure and mechanical properties. In the annealing, the hot forged steel part is subjected to heating to reach the soaking temperature TA from 600°C to Ac3 +200°C, the preferred TA temperature is from 625°C to Ac3 +100°, more preferably from 640°C to Ac3 +50°. In the heating step, hot forged steel part is heated from room temperature to soaking temperature TA at a heating rate HR1 from 0.1°C / s to 100°C / s. It is preferred to have HR1 rate from 0.1°C / s to 50°C / s and more preferably from 0.1°C / s to 10°C / s. Then the hot forged steel part is held at the annealing soaking temperature TA during 10 to 10000 seconds and preferably from 10 to 5000 seconds to ensure adequate transformation to Austenite microstructure of the strongly work­hardened initial structure thereby reducing the hardness of the hot forged steel part. It is Then the hot forged steel part is cooled is at a cooling rate CR1 which is more than 1°C / s and preferably more than 2°C / s and more preferably more than 5°C / s to a cooling stop temperature range CS1 from Ms­5°C to 15°C and preferably from Ms­10°C to 20°C and more preferably from Ms­20°C to 20°C, wherein Ms for the steel sheet is calculated by using the following formula Ms (°C) = 539 – 423x %C ­30.4x %Mn ­17.7x %Ni – 12.1x %Cr – 7.5x %Mo – 11x %Si^^^^^^^^3(°^^^^) = 910 − 203^^^^1⁄ 2 + 44.7^^^^^^^^ − 15,2^^^^^^^^ + 31.5^^^^^^^^ + 104^^^^ + 13.1^^^^ − 30^^^^^^^^ − 11^^^^^^^^ −20^^^^^^^^ + 700^^^^ + 400^^^^^^^^ + 400^^^^^^^^Thereafter a forged steel part is obtained which is subjected to at least one mechanical manufacturing operation. Mechanical operation may comprise hobbing, shaping, machining, grinding, honing or any other suitable mechanical operation or manufacturing procedure. The mechanical operations can be performed at room temperature or a higher temperature as desired by condition of specific mechanical operation. The forged steel part is then subjected to carburization to form the martensite deficient layer on all the surfaces of the steel part. In the carburization, the forged steel part is subjected to heating to reach the carburization temperature TZ from 800°C to 1100°C, the preferred TZ temperature is from 850°C to Ac3 +200°C, more preferably from 900°C to 1040°C. In the heating step, forged steel part is heated from room temperature to TZ at a heating rate HR2 from 0.1°C / s to 20°C / s. It is preferred to have HR2 rate from 0.1°C / s to 15°C / s and more preferably from 0.1°C / s to 10°C / s. Then the forged steel part is held at TZ during 10 to 6000 seconds in a Carbon enriched atmosphere having a dew point of from ­15°C to +15°C. The Carburizing treatment is intended to fuse the carbon from the Carbon enriched atmosphere into the surface of the the forged steel part at high temperature which enriched the martensite deficient layer on the surface with Carbon 0.5% to 0.9%of Carbon on surface and preferably from 0.6% to 0.8% of Carbon. Then the forged steel part is cooled is at a cooling rate CR2 which is more than 1°C / s and preferably more than 2°C / s and more preferably more than 5°C / s to a cooling stop temperature range CS2 from Ms­5°C to 25°C and preferably from Ms­5°C to 20°C and more preferably from Ms­ 10°C to 20°C. This carburizing obtain a case hardened depth from 0.2mm to 0.6mm to have a surface hardness of more than 650HV. Thereafter is reheated to a tempering temperature Ttemper from 180°C to 250°C with a heating rate of at least 1°C / s and preferably of at least 2°C / s and held at the temperature Ttemper during 100 s and 6000s.The preferred temperature range for tempering is from 180°C and 240°C and the preferred duration for holding at Ttemper is from 200 s and 5000s to obtain a steel part for the transmission system of an automobile. EXAMPLES The following tests, examples, figurative exemplification and tables which are presented herein are non­restricting in nature and must be considered for purposes of illustration only and will display the advantageous features of the present invention. Forged mechanical part made of steels with different compositions is gathered in Table 1 with the Ac3 and Ms temperatures, where the forged mechanical part is produced according to process parameters as stipulated in Table 2, respectively. Thereafter Table 3 gathers the microstructures of the forged mechanical part obtained during the trials and table 4 gathers the result of evaluations of obtained properties. Table 1 Steel AC3 MsSamplesC Mn Si Al Cr S P N Mo Cu Ni(°C) (°C)I10.22 1.32 0.60 0.016 1.28 0.019 0.012 0,0179 0,246 0,218 0,234 802 376I20.23 1.35 0.61 0.029 1.31 0.017 0.013 0,0170 0,253 0,232 0,252 804 370R10.25 1.19 0.25 0.025 1.10 0.027 0.014 0,0110 0.05 0,222 0,140 781 365R20.23 1.35 0.22 0.026 1.29 0.017 0.011 0,0099 0,249 0,232 0,245 784 374R30.21 1.26 0.27 0.021 1.30 0.029 0.012 0.0119 0.05 0,228 0,151 787 388Table 2 Table 2 gathers the process parameters implemented on semi­finished product made of steels 5 of Table 1. The trials I1 to I2 serve for the manufacture of forged mechanical part according to the invention. This table also specifies the reference forged mechanical parts which are designated in table from R1 to R3. The table 2 is as follows: Trials Reheati Tforging HR1 TA TA CR1 CS1 HR2 TZ TZ time CR2 CS2 Ttemper T temper ng (°C) (°C / s) (°C) time (°C / s) (°C) (°C / s) (°C) (s) (°C / s) (°C) (°C) time (s) Temper (s) ature (°C) I11200 1150 1 650 840 1 25 1.21000 36001 25200 3600I21200 1150 1 890 2460 1 25 1.21000 36001 25200 3600R11200 1150 1 650 2460 1 25 1.21000 36001 25200 3600R21200 1150 1 890 2460 1 25 1.21000 36001 25200 3600R3 1200 1150 1 650 840 1 25 1.2 1000 3600 1 25 200 3600 10 according to the invention; R = reference; underlined values: not according to the invention. Table 3 gathers the results of test conducted in accordance of standards on different microscopes such as Scanning Electron Microscope for determining microstructural composition of both the inventive steel and reference trials and Xray measurements. 5 Table 3 : microstructures of the trials and the presence of Martensite in surface layer Trials Core of Steel part Surface layer of Steel Precipitates PAGS % of grains with part (µm) grain size from 3 to 5 MartensiteBainite RA Martensite RA (%)(%) (%) (%)(%)I1 98 1 1 71 29 AlN 9,16.6I2 98 1 1 70 30 AlN 8,72.1R1 98 1 1 75 25 AlN 9,425R2 98 1 1 70 30 AlN 8,913R3 98 1 1 73 27 AlN 8,513I = according to the invention; R = reference; underlined values: not according to the invention. 0 Table 4 exemplifies the mechanical properties of both the inventive steel and reference steels. In order to determine the tensile strength, tests are conducted in accordance of NF EN ISO 6892­1 standards. Tests to measure the toughness and fatigue for both inventive steel and reference steel are conducted in accordance of EN ISO 148­1 standard KCU specimen with U­notch at toom temperature. The results of the various mechanical tests conducted in 5 accordance with the standards are gathered Table 4 Trials Tensile Strength Yield Strength YS / TS Striction (%) KCU (J / cm²) (MPa) (MPa) I1 1581 1271 0.8 52 70 I2 1579 1252 0.79 50 62 R1 1671 1324 0.79 52.3 56 Trials Tensile Strength Yield Strength YS / TS Striction (%) KCU (J / cm²) (MPa) (MPa) R2 1533 1228 0.8 53.4 77.8 R3 1525 1227 0.8 55.6 82 according to the invention; R = reference; underlined values: not according to the invention.

Claims

CLAIMS 1. A steel part for the transmission system of an automobile comprising a core made of the following elements, expressed in percentage by weight: 0.15% ≦ C ≦ 0.25%;and can contain one or more of the following optional elements0% ≦ Zr ≦ 0.10%; the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of the core of said steel part comprising, by area percentage, at least 90% of Martensite, with a cumulative optional presence of any one or more from Residual Austenite, Pearlite, ferrite and Bainite from 1% to 10% and precipitates of Aluminum and / or Niobium in form of AlN and / or Nb (C,N), wherein the percentage of grains with prior austenite grains size from 3 to 5 is limited to a maximum of 10% of the total grains, such steel part comprising a martensite­deficient layer till the depth of 1mm or less on all its surfaces, such martensite­ deficient layer comprising from55% to 72% of martensite, 10% to 40% residual austenite and the remainder being any one or more from bainite, ferrite or cementite.

2. Steel part for the transmission system of an automobile according to claim 1, wherein the composition includes 0.5% to 0.8% of Silicon.

3. Steel part for the transmission system of an automobile according to claim 1 or 2, wherein the composition includes 0.16% to 0.24% of Carbon.

4. Steel part for the transmission system of an automobile according to anyone of claims 1 to 3, wherein the composition includes 0.1% to 0.4% of Molybdenum.

5. Steel part for the transmission system of an automobile according to anyone of claims 1 to 4, wherein the composition includes 1.1% to 1.4% of Chromium.

6. Steel part for the transmission system of an automobile according to anyone of claims 1 to 5, wherein, the Martensite is from 92% to 99%.

7. Steel part for the transmission system of an automobile according to anyone of claims 1 to 6, wherein, the martensite­ deficient layer comprises from 60% to 72% of martensite, the remainder being any one or more from bainite, residual austenite, ferrite or cementite.

8. Steel part for the transmission system of an automobile according to anyone of claims 1 to 7, wherein, the tensile strength of the steel is at least 1530 MPa.

9. Steel part for the transmission system of an automobile according to anyone of claims 1 to 8, wherein said steel has an impact toughness equal or more than 58J / cm2.

10. A method of production of a steel part for the transmission system of an automobile according to anyone of claims comprising the following successive steps: providing a steel composition according to anyone of claims 1 to 5 in form of semi­finished product; ­ reheating said semi­finished product to a temperature from Ac3 +30°C to 1300°C; ­ hot forging the said semi­finished product in the austenitic range wherein the hot forging finishing temperature Tforging shall be above 830°C to obtain a hot forged part; ­ cooling hot forged part to room temperature;­ thereafter heating the hot forged part at a heating rate HR1 from 0.1°C / s to 100°C / s from room temperature to an annealing soaking temperature TA which is in a range from 600°C to Ac3+200°C; ­ then perform annealing at TA during 10 to 1000 seconds ­ then cooling the hot forged part from TA to cooling stop temperature CS1 from Ms ­5°C to 15°C with a cooling rate CR1 greater than 1°C / s to obtain a forged steel part ­ performing one or more mechanical operations on the said forged steel part; ­ thereafter heating the forged part at a heating rate HR2 from 0.1°C / s to 20°C / s from room temperature to a Carburizing temperature TZ which is in a range from 800°C to 1100°C; ­ then perform Carburizing in a Carbon rich environment having a dew point from ­15°C to +15°C at TZ during 10 to 6000 seconds ­ then cooling the forged part from TZ to cooling stop temperature CS2 from Ms ­5°C to 15°C with a cooling rate CR2 of at least 1°C / s ­ thereafter the forged part is reheated to a tempering temperature Ttemper from 180°C to 250°C with a heating rate of at least 1°C / s and held at Ttemper during 100 s and 6000s and then cooled to room temperature to obtain a steel part for the transmission system of an automobile.

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

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

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

14. Use of a steel part according to anyone of claims 1 to 9 or of a steel part produced according to the method of claims 10 to 13, for the manufacture of structural or safety parts of a vehicle or an engine.

15. Vehicle comprising a part obtained according to claim 14.

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