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
  • C22C38/40—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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/30—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for crankshafts; for camshafts
• • 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • 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
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
  • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
  • C23C8/24—Nitriding
  • C23C8/26—Nitriding of ferrous surfaces

Definitions

• the invention relates to steel, and to a forged product manufactured therewith and characterized by a totally ferritic-pearlitic microstructure without bainite and, accordingly, by a high strength, a particularly high fatigue strength, and good machinability
• microalloyed steels in hot-forged products are economically sensible as the pieces require no hardening and tempering.
• a problem with these microsteels is a modest strength, especially a low yield strength, when compared to tempered alloy steels.
• the highest strength microalloyed forging steel is 46MnVS6, having a minimum yield limit of 580 MPa.
• the strength of air cooled microalloyed steels can be improved by increased alloying, for example by adding manganese and/or chromium.
• increased alloying creates a problem as a result of the formation of bainite in the microstructure of steels.
• An objective in such steels is to obtain a totally ferritic-pearlitic microstructure capable of providing the desired properties. Even small amounts of bainite in the microstructure undermine mechanical properties by decreasing yield strength and toughness, especially elongation and reduction of area in tensile test, as well as by degrading machinability.
• bainite An element particularly active with regard to the development of bainite is molybdenum, which in such steels must be considered an impurity and the concentration of which must be often limited to as low as not more than 0,03...0,04 wt-%. Occasionally, in the microstructure is formed not only bainite but also untempered martensite, the effect of which on the above properties is even more deteriorating than that of bainite.
• bainitic steels are a steel as set forth in the published EP application 00850178.5 , which is cooled in air.
• a problem in this case is also a poor and uneven machinability and an increase of alloy content. Since attaining a totally bainitic structure by air cooling without retaining a constant temperature is difficult in practice, in the microstructure usually formed untempered martensite as well. In order to attain even a moderate toughness and machinability, it is generally necessary that the pieces be also annealed for softening hard martensite regions, which increases costs even more.
• an object of the invention is to provide a steel alloy having improved machinability combined with high yield stress and toughness.
• a steel of the invention is characterized by what is set forth in the characterizing clause of claim 1.
• the simultaneous use of silicon alloying and molybdenum alloying results economically a high-strength steel, having a structure which is purely ferritic-pearlitic with neither bainite nor martensite nor drawbacks inflicted thereby.
• chromium concentrations within the range of appr. 0,5 wt-% to appr. 1,5 wt-% enables an improvement in the nitriding properties of steel.
• the chromium concentration is preferably within the range of appr. 0,5 wt-% to appr. 0,8 wt-%.
• the chromium alloying causes development of chromium nitrides in the ferritic diffusion layer of a nitrided surface, thus increasing the layer's hardness.
• 1 wt-% of chromium provides a hardness of about 600 HV ( K-E. Thelning, Steel and its Heat Treatment, Butterworths 1975 pp. 86-87 ). This hardness in the diffusion layer is sufficient even for highly demanding mechanical engineering.
• titanium and vanadium also provide activities similar to that of chromium, thus enhancing the latter's effect.
• the diffusion layer in chromium-alloyed steel has a hardness which is several hundred HV degrees higher, which provides a solid base for an extremely hard compound layer present in the outermost surface. Vanadium and titanium alloying enhances this effect.
• a surface layer nitrided as described is more resistant to a surface pressure without a risk of cracking, which makes it possible to use the steel e.g. for gears and shafts.
• a hard diffusion layer means usually also a greater consolidated layer thickness for improved fatigue strength. This is favourable, for example in nitrided crankshafts.
• the invention relates also to a method of manufacturing ferritic-pearlitic hot forged products having a high yield and fatigue strength, said method being characterized by what is set forth in the characterizing clause of the independent claim 13.
• Still another object of the invention is a forged product, which is characterized by what is set forth in the characterizing clause of the independent claim 16.
• a yet further object of the invention is a method of manufacturing ferritic-pearlitic hot rolled steel bars having a high yield and fatigue strength, said method being characterized by what is set forth in the characterizing clause of the independent claim 19.
• the invention relates also to a hot rolled steel bar, which is characterized by what is set forth in the characterizing clause of the independent claim 21.
• Table 1 Chemical composition of test steels and reference steels in weight percentage Steel C Si Mn P S Cr Ni Mo V Ti Als N Ref. 1 0,37 0,60 1,31 0,008 0,034 0,13 0,09 0,02 0,11 0,025 0,018 0,011 Ref.
• Re yield strength [MPa]
• AS elongation [%]
• Z reduction of area [%]
• KCU2 notch impact strength with 2 mm U-notch bar [J/cm2]
• X sum value representing bainite formation [%]
• KCU2 Microstructure Austenite grain size Ref. 1 555 18 48 46 F + P Ref.
• Raising the carbon concentration to higher than 0,4 wt-% increases strength, but the effect on tensile strength is lesser than on yield strength.
• a lower carbon concentration e.g. 0,15...0,25 wt-%, it is possible to establish a higher yield ratio, which is beneficial in some cases.
• the inventive steel has a fatigue strength which is better than that of standard quenched and tempered and micro-alloy steels, as disclosed in the publication EP 0572246B1 .
• the silicon alloying particularly reduces the tendency to bainite formation as evident by comparing a prior known reference alloy (Ref. 2) with test alloys 2...5 of the invention (Tables 1...3), the sum expression thereof giving the value X of higher than 3,3 wt-%.
• Table 2 also shows how the yield strength, elongation, reduction of area and impact strength of a bainite containing reference alloy (Ref. 2) are distinctly weaker than those of test alloys containing more silicon.
• Manganese and chromium increase strength, but add to the risk of bainite formation at high concentrations.
• An alloying element with a particularly powerful strengthening effect and at the same time promoting bainite formation is molybdenum.
• molybdenum alloying together with silicon alloying, has been utilized for increasing strength without drawbacks resulting from bainite.
• the inventive steel tolerates 0,06 wt-% of molybdenum with no problems, but at large dimensions the cooling rate is slower and higher molybdenum concentrations (e.g. 0,1...0,2 wt-%) are possible without a risk of bainite.
• Vanadium is an effective precipitation hardener. Provided that hot working temperatures are not overly high, vanadium is also functional as a grain-size growth inhibitor. At rather high concentrations, higher than 0,3 wt-%, the use of vanadium is uneconomical and, in addition, toughness is reduced by vanadium. For these reasons it is in some cases advisable to omit vanadium completely.
• Nitrogen is an effective hardener, either as such or together with vanadium.
• high concentrations those higher than 0,03...0,04 wt-%, may nevertheless degrade the surface quality of a hot-rolled bar.
• Niobium functions as a precipitation hardener the same way as vanadium.
• Titanium nitrides are capable of withstanding, without dissolving, extremely high temperatures, even higher than 1200 C, which is why a minor addition of titanium is preferred especially in hot forging to inhibit an excessive growth of grain size and to improve toughness.
• Oversized additions of titanium result in a structure developing large primary TiN particles, which precipitate as early as during solidification and which are ineffective in terms of grain growth and which, by functioning as a crack initiator, undermine toughness and fatigue strength.
• they deteriorate machinability especially at higher cutting speeds.
• For the maximum machinability Ti should be kept lower than 0,008 wt-%.
• the inventive steel is highly suitable for such a process by virtue of its minor tendency to bainite formation.
• steels of the invention covers hot-forged products, for example parts of automotive engines, such as crankshafts, connecting rods and pistons.
• such steels are especially applicable for parts of a vehicular chassis, such as suspension arms, steering arms, front axle beams, etc.
• Chromium-alloyed steel in particular, is highly applicable for nitrided components, such as crankshafts, gear wheels and pinions.
• steels of the invention can be used directly in hot rolled condition without forging or heat treatment.
• steels can replace steel bars heat-treated by hardening and tempering.
• Intended applications include vehicular parts and machine components, for example drive shafts, steering components, fasteners, etc.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Steel (AREA)
• Heat Treatments In General, Especially Conveying And Cooling (AREA)

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Citations (13)

• 2005
  • 2005-03-09 EP EP05101801A patent/EP1700925B1/de not_active Expired - Lifetime
  • 2005-03-09 AT AT05101801T patent/ATE442464T1/de not_active IP Right Cessation
  • 2005-03-09 DE DE602005016522T patent/DE602005016522D1/de not_active Expired - Lifetime

Patent Citations (14)

Non-Patent Citations (7)

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