EP3209806B1 - An ultra-high strength thermo-mechanically processed steel - Google Patents

An ultra-high strength thermo-mechanically processed steel Download PDF

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Publication number
EP3209806B1
EP3209806B1 EP15808458.2A EP15808458A EP3209806B1 EP 3209806 B1 EP3209806 B1 EP 3209806B1 EP 15808458 A EP15808458 A EP 15808458A EP 3209806 B1 EP3209806 B1 EP 3209806B1
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Prior art keywords
steel
proportion
ultra
high strength
forging
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German (de)
French (fr)
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EP3209806A1 (en
Inventor
Babasaheb Neelkanth Kalyani
Madan Umakant TAKALE
Prakasam Balachandran DR. GNANA
Rajkumar Prasad DR. SINGH
Abhay Ramchandra CHAUTHAI
Suresh Babu ARANGI
Rajesh Surendra MANE
Kumar DHARMESH
Srinivas Perla
Vinayak Pralhad PAWAR
Shital Shahaji JADHAV
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Saarloha Advanced Materials Private Ltd
Bharat Forge Ltd
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Saarloha Advanced Materials Private Ltd
Bharat Forge Ltd
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    • 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • 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
    • 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
    • C22C38/18—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
    • 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
    • C21D1/30—Stress-relieving
    • 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
    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling

Definitions

  • the present invention relates to ultra-high strength steel for structural components.
  • it relates to the process of making such steel that has a desirable microstructure in the thermo-mechanically processed and differently cooled conditions that delivers high fatigue resistance in service.
  • the steel and the process of its manufacturing technique enables manufacture of components that exhibit bainitic microstructure that impart ultra-high strength ranges with very high fatigue resistance properties.
  • the steel and its method of manufacturing process enables saving in alloying additives compared to hardened and tempered alloy steels and in addition avoid expensive heat treatment operations to achieve the desired range of mechanical properties.
  • the steel developed in the present invention is a suitable replacement for microalloyed steel bars used for structural component development.
  • the steel can be used for applied as the hot rolled and air cooled long products that can be directly used for applications or it can be directly hot forged in open or closed die forging followed by controlled cooling to achieve the desired microstructure and range of mechanical properties.
  • the ultra-high strength steel (UTS in the 1100 to 1420 MPa range) of the invention provides a competitive replacement for quench tempered alloy steels in terms of strength and fatigue, ductility, although its toughness is lower compared to Quenched and Tempered steels but superior to micro alloyed steels.
  • the potential steel bar product can be applied in various industries that include automotive, railways, general engineering, agricultural implements, construction, mining shafts, etc.
  • the steel exhibits unique range of mechanical properties that develops due to the process of invention which involves the steel chemistry and the thermo-mechanical processing followed by controlled cooling.
  • CN101613830A discloses a hot rolled bainitic steel rail and a production process.
  • US2004/149359A1 discloses a method of fabricating a steel forging, and a forging obtained thereby.
  • the present invention relates to innovative composition of a steel and associated thermo-mechanical processing of making it, such that it has desirable microstructure that gives ultra-high strength and fatigue resistance.
  • the fatigue properties of the steel rolled bar of the invention have been found to exhibit greater than 5 million cycles with rotating bending type testing at a stress level of 515MPa. It was also found that the fatigue life of the forged component of the invention is at least five times more than that of the existing alloy steel materials.
  • the present invention describes the steel composition and the entire process of making the steel from melting to thermo-mechanical processing to achieve steel with ultra-high strength and very high fatigue performance. Specifically, the invention relates to an ultra-high strength steel in accordance with claim 1, a process of making an ultra-high strength steel rolled bar in accordance with claim 2, and a process of making forged components in accordance with claim 3.
  • the object of the present Invention is to develop an ultra-high strength steel with high fatigue resistance with thermo-mechanically processed and control cooled steel that is processed with induction or electric arc furnace or Basic oxygen furnace routes followed by secondary refining, vacuum degassing and traditional casting processes such as ingot casting or continuous casting.
  • Another object of the present investigation is to achieve a wide range of mechanical properties within Ultra high strength range by changing the cooling rate post thermo-mechanical processing in air, water, polymer, oil and vermiculite cooling.
  • Another object of the present invention is to propose usefulness of steel in weight reduction by at least 10% as a substitute for heat treated alloy steels or microalloyed steels.
  • Another object of the invention is to minimize the cost of steel manufacturing as compared to alloy steels and savings associated with avoidance of heat treatment in alloy steels.
  • Another object of present invention is to produce ultra-high strength components having fatigue resistance of at least five times compared to regular heat treated or micro alloyed steels.
  • the present invention describes the development of ultra-high strength steel for potential use in light weighting and fatigue resistant component applications.
  • the primary steel is manufactured using induction furnaces, electric arc furnaces (EAF), ladle furnaces or basic oxygen furnaces.
  • the primary steel is suitably alloyed in a secondary refining furnace followed by vacuum degassing to produce the steel of designed chemistry additives shown in Table 1, based on theoretical considerations on alloy design principles.
  • the steel is refined and killed with Aluminium (Al) to get a residual oxygen level to less than 15ppm. This is found to reduce the potential oxide inclusions that have an effect on the fatigue life of the component that may be made using the steel of the present invention.
  • Table 1 Composition of the steel designed for meeting Ultra high strength range and fatigue resistance.
  • the steel is cast through ingot casting preferably with bottom pouring up-hill casting technique or it may be cast as a concast steel product of suitable size.
  • the as-cast ingot or the concast bloom may be hot charged or cold charged in a furnace for further deformation.
  • the initial soaking for hot deformation is done at a temperature between 1280° C and 1220° C.
  • the soaked bloom is subject to hot rolling. Suitable reduction per pass is applied.
  • the material gets deformed easily by forging or directly hot rolling with good surface finish.
  • the hot worked material is suitably hot finished at a temperature between 1000 and 900° C. This is followed by control cooling the deformed steel in any of the cooling medium vermiculite, air, oil, polymer or water.
  • the distortion of the steel is depending on the quenching severity of the media.
  • the steel quenched in water or polymer may require a stress relief temper treatment at a temperature between 200 and 340 °C.
  • the invention discloses development of the steel and its processing where the content, morphology and distribution of the phases ensure the unique range of mechanical properties.
  • the bainitic transformation involves a displacive transformation followed by diffusion of carbon to form carbides that create the bainitic microstructures.
  • the invention discloses the stability of predominantly bainitic microstructure when control cooled in various media such as vermiculite, air, oil, polymer and water quenching which resulted in higher strength.
  • the invention also discloses development of steel which can be made at par with that of processing an air cooled micro alloyed steel but delivers ultra-high strength level and superior fatigue resistance.
  • the primary steel was melted in a 35MT electric arc furnace followed by secondary steel making using a ladle furnace.
  • the entire process flow followed is shown in Figure 1 .
  • the process of the present invention ensures that the steel developed by adopting above mentioned process has Oxygen content less than 15ppm (a condition that is necessary to give ultra-high strength).
  • Secondary steel making followed by vacuum degassing ensures low gas content.
  • One advantage of completely air cooled bainitic structure is that the transformation has a natural tendency to reject hydrogen gas and the steel would be virtually free from hydrogen flaking unlike that in an alloy steel.
  • the steel in the present case was manufactured by continuous casting while ingot casting is also possible. Macro segregation was minimised by having low carbon and sulphur contents.
  • the steel can be hot or cold charged during deformation.
  • the as-cast steel is amenable for hot deformation using hot forging and hot rolling and in this specific case it was directly hot rolled with a reduction ratio more than 4 with a cross section of round corner square.
  • the hot deformation temperature at which thermo-mechanical processing is carried out is between 1280 and 850 °C.
  • the plastic deformation of the steel happens with good plastic flow behaviour and with no surface defects.
  • the finish rolling temperature was maintained in a range between 1000 to 900°C.
  • the steel after thermo-mechanical processing was subject to initially three different cooling rates using vermiculite cooling, air cooling or water quenching. Each of the three cooling methods leads to the formation of predominantly bainitic microstructure.
  • the typical mechanical properties achieved in the 35 MT directly hot rolled steel are shown in Table 3.
  • the corresponding microstructures are shown in Figure 2 .
  • Table 2 Properties of raw material achieved at three cooling conditions.
  • This example shows that the steel is amenable for processing to bainitic transformation after thermo-mechanical processing over a wide range of temperatures. This implies that the steel exhibits bainitic structure with lesser control of cooling rate. This simplifies the manufacturing process in closed die forging where in some cases desired cooling rate is imposed to get a specific microstructure.
  • the present invention confirms that there is no need for forced air cooling or a very controlled cooling in the conveyor after deformation.
  • the steel made using the process of invention was used in making of close die steel component, where excellent consistency in mechanical properties is achieved.
  • the mechanical properties obtained in this study shows ultra-high strength range consistent in a product. Such levels of properties are usually achieved in hardened and tempered steels. In the case of the present invention, there is no need for hardening and tempering and air cooling is sufficient to achieve the desired mechanical properties. It was also observed that the transverse properties as shown in Table 2 significantly improved when the steel was subject to thermo-mechanical processing in a closed die forging operation.
  • the steel bar manufactured using the invented process are easy to forge into complex shaped components.
  • the forging made of such steel had the desired bainitic structure.
  • This ultra-high strength steel contributes to weight reduction of the existing components that enhances fuel efficiency in automotive type applications.
  • the component weight reduction can be achieved by at least 10%.
  • the steel made using the process of invention was used for forging an automotive front axle beam designed for Gross Axle Weight Rating (GAWR) of about 7 tons as shown in Figure 3 .
  • the closed die forging of components made using the steel of the invention can be manufactured using a hammer or a press, and the forging process involves soaking of rolled bar at a temperature range between 1280° C and 1220° C , The Finish forging temperature is maintained within temperature range 1000 to 900°C.
  • the forging process involves preform manufacturing step for blocker forging and finisher forging, the process of preform manufacturing consist of reduce rolling the heated billet and bending some portion using bender tool.
  • the finish forging is followed by cooling the hot forged component in any of the cooling medium selected from a group comprising controlled cooled, air, oil, or polymer.
  • the microstructure of forging component using the ultra-high strength steel is as shown in Figure 4 .
  • the fatigue test results shows at least five times better fatigue life as compared to conventional heat treated steel grades such as AISI 1045, 40Cr4.
  • Axle beams made from the invention of the present steel can be made slimmer than those made with micro alloyed steel grades (30MnVS6+Ti) resulting in weight reduction.
  • the steel shows dense sheaves of bainitic ferrite with nano-carbides.
  • the strengths developed in this invention far surpass the traditional medium carbon microalloyed steel grades used for component fabrication.
  • One key feature of the process of the present invention is that the enhanced strength of the steel disclosed herein is achieved without heat treatment by cooling in air or cooling in other media after thermo-mechanical processing.
  • the mechanical properties of the steel of the invention are comparable to that in the oil hardened and tempered alloy steels and superior to that obtained in ferrite pearlitic medium carbon microalloyed steel.
  • the present steel achieves the strength value merely using thermo-mechanical processes and with suitable cooling medium.
  • the steel of the present invention is comparable with 1% Cr alloy steel in terms of cost but it has mechanical properties which are equivalent to alloy steels with further alloying additive and heat treatment.
  • the enhanced strength enables reduction in weight of the components in range of at least 10 % in case of the traditional medium carbon micro alloyed steel.
  • the improved strength values have resulted in improved fatigue life of components made from the steel of the present invention is at least 5 times when compared with those which were made using traditional micro alloyed steel grade.
  • the steel of the present invention is useful for light-weighting opportunities in applications such as shafts, axle beams, steering knuckles, connecting rods, camshaft, etc.

Description

    Field of the Invention:
  • The present invention relates to ultra-high strength steel for structural components. In particular, it relates to the process of making such steel that has a desirable microstructure in the thermo-mechanically processed and differently cooled conditions that delivers high fatigue resistance in service. The steel and the process of its manufacturing technique enables manufacture of components that exhibit bainitic microstructure that impart ultra-high strength ranges with very high fatigue resistance properties.
  • The steel and its method of manufacturing process enables saving in alloying additives compared to hardened and tempered alloy steels and in addition avoid expensive heat treatment operations to achieve the desired range of mechanical properties. The steel developed in the present invention is a suitable replacement for microalloyed steel bars used for structural component development. The steel can be used for applied as the hot rolled and air cooled long products that can be directly used for applications or it can be directly hot forged in open or closed die forging followed by controlled cooling to achieve the desired microstructure and range of mechanical properties. The ultra-high strength steel (UTS in the 1100 to 1420 MPa range) of the invention provides a competitive replacement for quench tempered alloy steels in terms of strength and fatigue, ductility, although its toughness is lower compared to Quenched and Tempered steels but superior to micro alloyed steels. Thus, the potential steel bar product can be applied in various industries that include automotive, railways, general engineering, agricultural implements, construction, mining shafts, etc. The steel exhibits unique range of mechanical properties that develops due to the process of invention which involves the steel chemistry and the thermo-mechanical processing followed by controlled cooling.
    • Background of the invention:
  • High performance and weight reduction of dynamic components in engineering machines is a major driver for achieving fuel efficiency. In automotive industry, even small weight reduction in dynamic engine components translates into significant fuel efficiency and overall weight savings in the structure. Products such as bars, tubes/pipes, and other solid products such as axles, typically termed as 'long' products, made of steel and produced using minor addition of additives such as Vanadium (V), Niobium (Nb) and Titanium (Ti) and with ferrite pearlite microstructures have long dominated the markets. These materials, which have been competing with alloy steel in terms of achieving the required strength, are produced using thermo-mechanical processes with controlled cooling, which completely avoid subsequent quenching and tempering heat treatment of the component. The micro alloyed steels are known to have ultimate tensile strength (UTS) upto1000 MPa although they have lower impact toughness.
  • CN101613830A discloses a hot rolled bainitic steel rail and a production process.
  • US2004/149359A1 discloses a method of fabricating a steel forging, and a forging obtained thereby.
  • The present invention relates to innovative composition of a steel and associated thermo-mechanical processing of making it, such that it has desirable microstructure that gives ultra-high strength and fatigue resistance. The mechanical properties of the invented steel shows values of ultimate tensile strength (UTS) between 1100 and 1420 MPa; yield stress (YS) between 700 and 1030 MPa; percentage elongation (% E) =10 to 18%; % reduction in area between 32 and 50 %, Charpy V-notch Impact toughness greater than 20 J/cm2 and hardness greater than 330HBW. Higher strengths were obtained with increasing cooling rate after thermo-mechanical processing. The fatigue properties of the steel rolled bar of the invention have been found to exhibit greater than 5 million cycles with rotating bending type testing at a stress level of 515MPa. It was also found that the fatigue life of the forged component of the invention is at least five times more than that of the existing alloy steel materials.
    • Summary of the Invention:
  • The present invention describes the steel composition and the entire process of making the steel from melting to thermo-mechanical processing to achieve steel with ultra-high strength and very high fatigue performance. Specifically, the invention relates to an ultra-high strength steel in accordance with claim 1, a process of making an ultra-high strength steel rolled bar in accordance with claim 2, and a process of making forged components in accordance with claim 3.
    • Objects of the Invention:
  • The object of the present Invention is to develop an ultra-high strength steel with high fatigue resistance with thermo-mechanically processed and control cooled steel that is processed with induction or electric arc furnace or Basic oxygen furnace routes followed by secondary refining, vacuum degassing and traditional casting processes such as ingot casting or continuous casting.
  • Another object of the present investigation is to achieve a wide range of mechanical properties within Ultra high strength range by changing the cooling rate post thermo-mechanical processing in air, water, polymer, oil and vermiculite cooling.
  • Another object of the present invention is to propose usefulness of steel in weight reduction by at least 10% as a substitute for heat treated alloy steels or microalloyed steels.
  • Another object of the invention is to minimize the cost of steel manufacturing as compared to alloy steels and savings associated with avoidance of heat treatment in alloy steels.
  • Another object of present invention is to produce ultra-high strength components having fatigue resistance of at least five times compared to regular heat treated or micro alloyed steels.
    • Brief Description of Figures:
    • Figure 1 Manufacturing method process flow of ultra-high strength steel.
    • Figure 2 Microstructure of the steel made at various cooling conditions showing the bainitic microstructure obtained at various processing conditions
    • Figure 3: Automotive front axle beam manufactured using ultra-high strength steel grade.
    • Figure 4 Microstructure of the forging with invented ultra-high strength steel.
    Detailed Description of the Invention
  • The present invention describes the development of ultra-high strength steel for potential use in light weighting and fatigue resistant component applications.
  • The primary steel is manufactured using induction furnaces, electric arc furnaces (EAF), ladle furnaces or basic oxygen furnaces. The primary steel is suitably alloyed in a secondary refining furnace followed by vacuum degassing to produce the steel of designed chemistry additives shown in Table 1, based on theoretical considerations on alloy design principles. The steel is refined and killed with Aluminium (Al) to get a residual oxygen level to less than 15ppm. This is found to reduce the potential oxide inclusions that have an effect on the fatigue life of the component that may be made using the steel of the present invention. Table 1: Composition of the steel designed for meeting Ultra high strength range and fatigue resistance. [Composition in % w/w]
    Elements C Mn Si S P Cr Ni Mo V Al B Ti Nb O
    Min 0.10 1.2 0.5 0.8 0 0.05 - - 0.010 0.05
    Max 0.25 2.5 1.7 0.03 0.02 1.4 0.4 0.1 0.1 0.02 0.003 0.035 0.10 0.0015
  • The steel is cast through ingot casting preferably with bottom pouring up-hill casting technique or it may be cast as a concast steel product of suitable size. The as-cast ingot or the concast bloom may be hot charged or cold charged in a furnace for further deformation. The initial soaking for hot deformation is done at a temperature between 1280° C and 1220° C. The soaked bloom is subject to hot rolling. Suitable reduction per pass is applied. The material gets deformed easily by forging or directly hot rolling with good surface finish. The hot worked material is suitably hot finished at a temperature between 1000 and 900° C. This is followed by control cooling the deformed steel in any of the cooling medium vermiculite, air, oil, polymer or water. The distortion of the steel is depending on the quenching severity of the media. The steel quenched in water or polymer may require a stress relief temper treatment at a temperature between 200 and 340 °C.
  • The invention discloses development of the steel and its processing where the content, morphology and distribution of the phases ensure the unique range of mechanical properties. The bainitic transformation involves a displacive transformation followed by diffusion of carbon to form carbides that create the bainitic microstructures.
  • The invention discloses the stability of predominantly bainitic microstructure when control cooled in various media such as vermiculite, air, oil, polymer and water quenching which resulted in higher strength.
  • The invention also discloses development of steel which can be made at par with that of processing an air cooled micro alloyed steel but delivers ultra-high strength level and superior fatigue resistance.
  • In one typical embodiment, the primary steel was melted in a 35MT electric arc furnace followed by secondary steel making using a ladle furnace. The entire process flow followed is shown in Figure 1. The process of the present invention ensures that the steel developed by adopting above mentioned process has Oxygen content less than 15ppm (a condition that is necessary to give ultra-high strength). Secondary steel making followed by vacuum degassing ensures low gas content. One advantage of completely air cooled bainitic structure is that the transformation has a natural tendency to reject hydrogen gas and the steel would be virtually free from hydrogen flaking unlike that in an alloy steel. The steel in the present case was manufactured by continuous casting while ingot casting is also possible. Macro segregation was minimised by having low carbon and sulphur contents. The steel can be hot or cold charged during deformation. The as-cast steel is amenable for hot deformation using hot forging and hot rolling and in this specific case it was directly hot rolled with a reduction ratio more than 4 with a cross section of round corner square.
  • The hot deformation temperature at which thermo-mechanical processing is carried out is between 1280 and 850 °C. The plastic deformation of the steel happens with good plastic flow behaviour and with no surface defects. The finish rolling temperature was maintained in a range between 1000 to 900°C. The steel after thermo-mechanical processing was subject to initially three different cooling rates using vermiculite cooling, air cooling or water quenching. Each of the three cooling methods leads to the formation of predominantly bainitic microstructure. The typical mechanical properties achieved in the 35 MT directly hot rolled steel are shown in Table 3. The corresponding microstructures are shown in Figure 2. Table 2: Properties of raw material achieved at three cooling conditions.
    Type of cooling Direction UTS (MPa) YS (MPa) %E %RA CVN (J) Hardness HB
    Vermiculite Cooled Longitudinal 1140 767 14.8 45 12.41 332
    Transverse 1108 762 10.2 25 10.45
    Air cooled Longitudinal 1137 761 17.8 46 17.6 340
    Transverse 1178 784 12 31 11.3
    Water quenched Longitudinal 1195 861 16 30 20.58 352
    Transverse 1189 850 9.8 29 9.15
  • This example shows that the steel is amenable for processing to bainitic transformation after thermo-mechanical processing over a wide range of temperatures. This implies that the steel exhibits bainitic structure with lesser control of cooling rate. This simplifies the manufacturing process in closed die forging where in some cases desired cooling rate is imposed to get a specific microstructure. The present invention confirms that there is no need for forced air cooling or a very controlled cooling in the conveyor after deformation. The steel made using the process of invention was used in making of close die steel component, where excellent consistency in mechanical properties is achieved.
  • The mechanical properties obtained in this study shows ultra-high strength range consistent in a product. Such levels of properties are usually achieved in hardened and tempered steels. In the case of the present invention, there is no need for hardening and tempering and air cooling is sufficient to achieve the desired mechanical properties. It was also observed that the transverse properties as shown in Table 2 significantly improved when the steel was subject to thermo-mechanical processing in a closed die forging operation.
  • The steel bar manufactured using the invented process, are easy to forge into complex shaped components. The forging made of such steel had the desired bainitic structure. This ultra-high strength steel contributes to weight reduction of the existing components that enhances fuel efficiency in automotive type applications. The component weight reduction can be achieved by at least 10%.
  • In an extended study, the steel made using the process of invention was used for forging an automotive front axle beam designed for Gross Axle Weight Rating (GAWR) of about 7 tons as shown in Figure 3. The closed die forging of components made using the steel of the invention can be manufactured using a hammer or a press, and the forging process involves soaking of rolled bar at a temperature range between 1280° C and 1220° C , The Finish forging temperature is maintained within temperature range 1000 to 900°C. The forging process involves preform manufacturing step for blocker forging and finisher forging, the process of preform manufacturing consist of reduce rolling the heated billet and bending some portion using bender tool. The finish forging is followed by cooling the hot forged component in any of the cooling medium selected from a group comprising controlled cooled, air, oil, or polymer. The microstructure of forging component using the ultra-high strength steel is as shown in Figure 4.
  • It should be noted that the steps of forging process disclosed in the example above are specific to the axle beam type components. Forging process for other types of components manufactured using the steel of the invention may vary depending on component geometry.
  • The mechanical properties and fatigue testing of the beam was carried out. The mechanical properties and the performance of the steel are as given in Table 3. The test results shown superior fatigue properties of steel of the present invention as compared to the conventional quench tempered medium carbon steel AISI 1045 and alloy steel DIN 40Cr4. Table 3: Typical mechanical property performance of the steel of the invention after forging into components.
    Type of cooling YS (MPa) UTS (MPa) %E %RA Impact (J) Hardness HB
    Air Cooled 700-800 1100-1200 13-18 35-45 20J 331-352
    Oil Quenched 890-980 1300-1380 10-15 32-50 23J 412-425
    Water Quenched 980-1030 1350-1420 12-17 34-50 27J 417-438
  • The fatigue test results shows at least five times better fatigue life as compared to conventional heat treated steel grades such as AISI 1045, 40Cr4. Axle beams made from the invention of the present steel can be made slimmer than those made with micro alloyed steel grades (30MnVS6+Ti) resulting in weight reduction.
    • Benefits of the steel of the invention over other steel grades:
  • The steel shows dense sheaves of bainitic ferrite with nano-carbides. The strengths developed in this invention far surpass the traditional medium carbon microalloyed steel grades used for component fabrication.
  • One key feature of the process of the present invention is that the enhanced strength of the steel disclosed herein is achieved without heat treatment by cooling in air or cooling in other media after thermo-mechanical processing. The mechanical properties of the steel of the invention are comparable to that in the oil hardened and tempered alloy steels and superior to that obtained in ferrite pearlitic medium carbon microalloyed steel.
  • Unlike alloy steels which obtain ultra high strength levels after quenching and low temperature tempering, the present steel achieves the strength value merely using thermo-mechanical processes and with suitable cooling medium. The steel of the present invention is comparable with 1% Cr alloy steel in terms of cost but it has mechanical properties which are equivalent to alloy steels with further alloying additive and heat treatment. The enhanced strength enables reduction in weight of the components in range of at least 10 % in case of the traditional medium carbon micro alloyed steel. The improved strength values have resulted in improved fatigue life of components made from the steel of the present invention is at least 5 times when compared with those which were made using traditional micro alloyed steel grade. In the case of automotive components, the steel of the present invention is useful for light-weighting opportunities in applications such as shafts, axle beams, steering knuckles, connecting rods, camshaft, etc.
  • In summary, the present invention has the following aspects and advantages:
    1. 1. A robust chemistry of the steel was developed from as-cast ingot or concast product which can be hot forged or hot rolled followed by controlled cooling in media such as vermiculite cooling, air cooling, oil cooling or polymer show ferrite-carbidic bainitic microstructure that exhibits ultra high strength levels with good ductility and reasonable toughness.
    2. 2. The steel has a composition as specified in claim 1. The Oxygen is maintained at a level of less than 15 ppm to ensure cleanliness for the ultra-high strength range.
    3. 3. The steel which is thermo-mechanically processed and can be subject to a wide variety of cooling condition to give bainitic structure namely vermiculite cooling, air cooling, oil cooling, polymer quenching or water quenching exhibit bainitic structure and a wide variety of properties in the ultra high strength range with good ductility and reasonable toughness.
    4. 4. The steel could be manufactured in large tonnages and present study has resulted in a 35 MT electric arc furnace steel making furnace followed by continuous casting and it could be hot forged and hot rolled without defects at a temperature range between 850 and 1280°C. The steel developed was extensively studied for mechanical properties in as-rolled, as-forged and closed die forging condition followed by air cooling where excellent Ultra high strength- ductility properties could be achieved.
    5. 5. The components like front axle beam have been analysed for fatigue performance. The steel shows at least five times the fatigue life of the corresponding alloy steel component. Thus, the steel developed has a potential opportunity to replace expensive alloy steels processed by quenching and tempering treatment and produce lightweight component with better strength to weight ratio. Weight reduction of structural component of at least 10 % can be achieved.

Claims (7)

  1. An ultra-high strength steel of present invention characterised in that the said steel comprises in weight percentage:
    carbon in the proportion of 0.1 to 0.25%,
    manganese in the proportion of 1.2 to 2.5%,
    silicon in the proportion of 0.5 to 1.7 %,
    chromium in the proportion of 0.8 to 1.4 %,
    molybdenum in the proportion of 0.05 to 0.1%,
    niobium in the proportion of 0.05 to 0.10%,
    titanium in the proportion of 0.01 to 0.035%,
    and the residual elements less than 0.4%, wherein the residual elements are:
    nickel in the proportion of less than 0.4%,
    vanadium in the proportion of less than 0.1%,
    sulphur in the proportion of less than 0.03%,
    phosphorus in the proportion of less than 0.02%,
    wherein the steel further comprises in weight percentage:
    aluminium in the proportion of less than 0.02%,
    boron less than 30 ppm,
    oxygen in an amount of less than 15 ppm,
    and balance iron and any unavoidable impurities.
  2. A process of making an ultra-high strength steel rolled bar, the said process comprising the steps of:
    - melting the steel of the chemical composition disclosed in claim 1, followed by secondary refining and vacuum degassing, wherein said secondary refining comprises the step of killing with aluminium to bring the oxygen level in said steel to under 15 ppm,
    - casting of said steel through ingot casting or continuous casting into a bloom,
    - hot charging the as-cast ingot or the continuously cast bloom, or alternatively, cold-charging them in a furnace,
    - soaking said ingot or said bloom at a temperature range between 1280 °C and 1220 °C,
    - subjecting said soaked bloom to hot rolling,
    - carrying out reduce rolling,
    - finish rolling at a temperature range 1000 to 900°C,
    - control cooling the hot rolled steel in any of the cooling medium selected from a group comprising vermiculite, air, oil, polymer, or water.
  3. A process of making forged components with an ultra-high strength steel, characterised in that said process comprises the steps of:
    - heating and soaking of rolled bar made using the process disclosed in claim 2 at a temperature range between 1280 °C and 1220 °C,
    - hot forging of component,
    - finish forging the component at a temperature range 1000 to 900°C,
    - cooling the finish forged component in any of the cooling medium selected from a group comprising controlled cooled, air, oil, or polymer.
  4. A process of making an ultra-high strength steel as claimed in claims 1 and 2, characterised in that melting in the step of casting is carried out using an induction furnace or an electric arc furnace or a basic oxygen furnace.
  5. A process of making an ultra-high strength steel as claimed claim 4, characterised in that the step of thermo-mechanical processing is carried out by a technique selected from a group comprising hot rolling, or closed die forging, or extrusion.
  6. A process of making forged components as claimed in claim 3, characterised in that the step of hot forging of component comprises the step of preform manufacturing for blocker forging and finisher forging, followed by cooling.
  7. A process of making forged components as claimed in claim 6, characterised in that the step of preform manufacturing comprises the steps of reduce rolling and bending.
EP15808458.2A 2014-10-21 2015-10-21 An ultra-high strength thermo-mechanically processed steel Active EP3209806B1 (en)

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WO2018163189A1 (en) * 2017-03-10 2018-09-13 Tata Steel Limited Hot rolled steel product with ultra-high strength minimum 1100mpa and good elongation 21%
CN109986011A (en) 2018-01-02 2019-07-09 通用电气公司 Forge head, forging apparatus and increasing material manufacturing system
WO2019180492A1 (en) 2018-03-23 2019-09-26 Arcelormittal Forged part of bainitic steel and a method of manufacturing thereof
US20230323493A1 (en) 2020-09-07 2023-10-12 Arcelormittal Forged part of steel and a method of manufacturing thereof
CN113444978B (en) * 2021-06-29 2023-02-21 钢铁研究总院 Preparation method of ultrahigh-strength steel
CN113528763A (en) * 2021-07-20 2021-10-22 苏州雷格姆海洋石油设备科技有限公司 Production process of ultrahigh-strength large-wall-thickness underwater Christmas tree wellhead connector forge piece
CN116384159B (en) * 2023-05-29 2023-08-22 北京科技大学 Method and system for continuous casting process temperature simulation and macroscopic tissue prediction

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FR2741632B1 (en) * 1995-11-27 1997-12-26 Ascometal Sa STEEL FOR MANUFACTURING A FORGED PART HAVING A BATH STRUCTURE AND METHOD FOR MANUFACTURING A PART
JPH09241750A (en) * 1996-03-06 1997-09-16 Hino Motors Ltd Production of case hardening boron steel gear small in heat treating strain
FR2847910B1 (en) * 2002-12-03 2006-06-02 Ascometal Sa METHOD FOR MANUFACTURING A FORGED STEEL PIECE AND PART THUS OBTAINED
CN101613830B (en) * 2008-06-27 2012-08-29 鞍钢股份有限公司 Hot rolled bainite steel rail and production process
JP5402711B2 (en) * 2010-02-17 2014-01-29 新日鐵住金株式会社 Steel product having carbonitriding layer and method for producing the same
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