Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/06—Surface hardening
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/26—Methods of annealing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
  • C21D1/76—Adjusting the composition of the atmosphere
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/02—Hardening by precipitation
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/28—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for plain shafts
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/32—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for gear wheels, worm wheels, or the like
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• 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.
• the transmission parts 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.
• US20070193658A1 is a steel for mechanical components, wherein the composition thereof is, in percentages by weight: 0.19% ⁇ C ⁇ 0.25%; 1.1% ⁇ Mn ⁇ 1.5%; 0.8% ⁇ Si ⁇ 1.2%; 0.01% ⁇ S ⁇ 0.09%; trace levels ⁇ P ⁇ 0.025%; trace levels ⁇ Ni ⁇ 0.25%; 1% ⁇ Cr ⁇ 1.4%; 0.10% ⁇ Mo ⁇ 0.25%; trace levels ⁇ Cu ⁇ 0.30%; 0.010% ⁇ Al ⁇ 0.045%; 0.010% ⁇ Nb ⁇ 0.045%; 0.0130% ⁇ N ⁇ 0.0300%; optionally trace levels ⁇ Bi ⁇ 0.10% and/or trace levels ⁇ Pb ⁇ 0.12% and/or trace levels ⁇ Te ⁇ 0.015% and/or trace levels ⁇ Se ⁇ 0.030% and/or trace levels ⁇ Ca ⁇ 0.0050%; the balance
• 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.
• 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.
• 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.
• Carbon is present in the steel of present invention is from 0.2% to 0.35%. 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 0.2% will not be able to impart the tensile strength to the steel of present invention.
• 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 hobbling, lapping, shaping drilling, honing or grinding.
• the carbon content is advantageously in the range 0.22% to 0.35% and more especially 0.25% to 0.30%.
• 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 cause 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 0.2% to 0.7%. 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 Bainite.
• 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 0.7%.Further Silicon higher than 0.7% also enhances segregation.
• the preferred limit for the presence of Silicon is from 0.2% to 0.6% and more preferably from 0.22% to 0.4%.
• the content of the Aluminum is from 0.001% to 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 controls the grain size of the present steel by forming AlN. Higher content of Aluminum above 0.1% lead to the occurrence of coarse aluminum­ rich oxides that deteriorate machinability and hot forging on steel.
• the preferred limit for the presence of Aluminium is from 0.01% to 0.09% and more preferably from 0.01 to 0.035%
• Molybdenum is an essential element and may be present from 0.01 % to 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.
• Niobium is an essential element for the Steel of present invention from 0.020% 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, microstructure formed at the end of the holding temperature and as a consequence after the complete austenitization lead to the hardening of the product.
• 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% to 1.4 % and more preferably from 1.1% to 1.3%.
• Phosphorus is content of the steel of present invention is from 0 % to 0.09%. Phosphorus tends to segregate at the grain boundaries or co­segregate with Manganese.
• 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% to 0.05%.
• Sulphur is contained from 0 % to 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% to 0.040%to obtain the best balance between machinability and fatigue limit.
• Nitrogen is in an amount from 0.009% % and 0.09% in steel of present invention. It seems that Nb(C,N) precipitated nucleate on AlN precipitates. To obtain the Nb(C,N) precipitates, a minimum of 0.009% nitrogen is required.
• the preferred limit for nitrogen is from 0.009% to 0.05% and more preferably from 0.009% to 0.04%
• Nickel is added to the present invention from 0% to 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.
• Nickel is also restricted to 1% due to the economic reasons. It is preferred to have nickel from 0.1% to 0.9% in the steel of present invention. Vanadium is an optional element for the present invention and is content is from 0% to 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 from 0% 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 from 0% 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 following proportions by weight: Tin ⁇ 0.1%, Cerium ⁇ 0.1%, Magnesium ⁇ 0.10%, Calcium ⁇ 0.0010%, Bismuth ⁇ 0.05%, 0% ⁇ Boron ⁇ 0.008% and Zirconium ⁇ 0.10%. Up to the maximum content levels 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 above­mentioned elements.
• the steel part for the transmission system of an automobile has a microstructure comprising, in surface fractions or area%, of at least 90% bainite, an optional cumulative presence of Residual Austenite, Pearlite, ferrite and martensite from 0% to 10% and the precipitates of Al and Nb in form of AlN and Nb(C,N).
• Bainite is present in the steel according to the invention as a matrix phase and imparts strength to such steel. Bainite is present in the steel at least 90% by area fraction and preferably from 90% to 100% by area fraction and more preferably from 95% to 100%.
• Such bainite may include Cementite­Free Lath­Like Bainite, granular bainite, Upper bainite and Lower bainite or any other bainite.
• the cementite­ free Lath­Like bainite is consisting of bainite in the form of laths and including, between these laths, carbides such that the number N of inter­lath carbides larger than 0.1 micrometers per unit of surface area is less than or equal to 50000/mm 2 .
• This cementite­free lath­like bainite structure can confer to the steel of present invention high strength as well as impact toughness.
• the lower bainite is consisting of bainite in the form of laths and including, fine iron carbides stick which are precipitated inside the laths.
• the lower bainite structure can provide the steel of present invention with elongation and tensile strength.
• 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 have a size from 20nm to 350nm. Precipitate formation takes place during the annealing process as well as the cooling step.
• 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 Bainite and Martensite of the martensite enriched layer present invention in targeted amounts.
• the prior austenite grain size is from 3 to 12 measured as per the ASTM grain Index. It is more preferable to have prior austenite grain size from 4 to 11 and more preferably from 4 to 10.
• the cumulative presence of Residual Austenite, Pearlite, ferrite and martensite 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.
• Martensite of the present invention may imparts strength and fatigue endurance to steel.
• the preferred limit for the cumulative presence ferrite and bainite is kept from 0% to 8% and more preferably from 0% to 4%.
• this microstructure in the core of the steel part also includes a martensitic­enriched layer on all the surfaces of the steel part of the transmission system of an automobile up to a depth of 1mm or less and preferably up to a depth of 0.8mm or less and more preferably 0.5mm or less and showing a martensite percentage from 85% to 95% in area fraction, preferably from 85% to 92% more preferably from 85% to 90%.
• the martensite enriched layer formed on the surfaces preferably comprises any or all possible martensite kinds and notably fresh martensite, tempered martensite etc.
• 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.
• the remaining part of this surface layer comprises of anyone or more from bainite, residual austenite, ferrite and 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.
• the steel part considered 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.
• 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.
• a preferred Semi­finished product has a cross section be from ⁇ 20mmto ⁇ 110mm
• 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.
• the Ac3 for the present steel is calculated by a dilatometry study.
• 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 finishing forging 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.
• 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°C.
• 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.
• the hot forged steel part is held at the annealing soaking temperature TA during 10 to 1000 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 Ms­5°C to 20°C and more preferably from Ms­10°C to 20°C.
• 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 Ms­5°C to 20°C and more preferably from Ms­10°C to 20°C.
• Mechanical operation may comprise hobbling, 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 enriched layer on all the surfaces of the steel part and also impart the steel part of present invention with targeted microstructure and mechanical properties
• 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 1080°C , more preferably from 900°C to 1080°C.
• This enriched martensite layer can be up to a depth of 1mm or less and preferably up to a depth of 0.8 mm or less and more preferably up to a depth of 0.5mm or less.
• 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 15°C and preferably from Ms­5°C to 20°C and more preferably from Ms­10°C to 20°C to obtain a steel part for the transmission system of an automobile.
• the obtained steel part for the transmission system of an automobile may optionally be reheated to a tempering temperature Ttemper from 150°C to 250°C with a heating rate of at least 1°C/s and preferably of at least 2°C/s and more of at least 10°C/s during 100 s to 600s.
• the preferred temperature range for tempering is from 180°C to 240°C and the preferred duration for holding at Ttemper is from 200 s to 500s.
• Table 1 Forged mechanical part made of steels with different compositions is gathered in Table 1, 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 Table 2
• Table 2 gathers the process parameters implemented on semi­finished product made of steels of Table 1.
• the steels 1 and 2 serve for the manufacture of forged mechanical part according to the invention.
• This table also specifies the steels for reference forged mechanical parts which are steels 3 and 4.
• 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.
• Table 4 exemplifies the mechanical properties of both the inventive steel parts and reference steel parts.
• tests are conducted in accordance of NF EN ISO 6892­1 standards.
• Tests to measure the toughness and fatigue are conducted in accordance of EN ISO 148­1 standard KCU specimen with U­ notch at toom temperature.

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• 2022
  • 2022-06-28 EP EP22743571.6A patent/EP4547881A1/en active Pending
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  • 2022-06-28 CN CN202280095685.5A patent/CN119137304A/zh active Pending
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