EP1548141B1 - Machine part and method for manufacturing same - Google Patents

Machine part and method for manufacturing same Download PDF

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Publication number
EP1548141B1
EP1548141B1 EP03798399A EP03798399A EP1548141B1 EP 1548141 B1 EP1548141 B1 EP 1548141B1 EP 03798399 A EP03798399 A EP 03798399A EP 03798399 A EP03798399 A EP 03798399A EP 1548141 B1 EP1548141 B1 EP 1548141B1
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Prior art keywords
hardness
component
nitriding
less
mechanical component
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German (de)
English (en)
French (fr)
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EP1548141B8 (en
EP1548141A1 (en
EP1548141A4 (en
Inventor
Hideki c/o KABUSHIKI KAISHA HONDA MATSUDA
Mitsuo c/o KABUSHIKI KAISHA HONDA TAKASHIMA
Koichiro DAIDO TOKUSHUKO KABUSHIKI KAISHA INOUE
Yutaka DAIDO TOKUSHUKO KUREBAYASHI
Yasushi DAIDO TOKUSHUKO K.K. GIJUTSU MATSUMURA
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Honda Motor Co Ltd
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Honda Motor Co 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
    • C22C38/40—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
    • 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
    • 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
    • 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
    • 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
    • C23C8/40—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 liquids, e.g. salt baths, liquid suspensions
    • C23C8/42—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 liquids, e.g. salt baths, liquid suspensions only one element being applied
    • C23C8/48—Nitriding
    • C23C8/50—Nitriding of ferrous surfaces

Definitions

  • the present invention relates to a mechanical component composed of a steel and surface-hardened by nitriding, and a method of fabricating the same, and in more detail, relates to a mechanical component which is surface-hardened and, at the same time, imparted both with strength and bending straightening property, and a method of fabricating the same.
  • Fabrication of these mechanical components generally involves surface hardening for raising strength such as wear resistance and fatigue property.
  • the surface hardening is typically carried out after a material to be forged composed of a carbon steel or an alloyed steel for manufacturing mechanical structures is hot-forged, annealed typically for normalizing, and further machined into a predetermined geometry desired for various mechanical components. After the surface hardening, the material is finished typically through bend straightening, to thereby given as a final product of the mechanical component.
  • the surface hardening is carried out by nitriding such as salt bath nitriding and gas soft nitriding. It is generally known that the nitriding causes only a small post-processing distortion as compared with that possibly occurs after surface hardening such as cementation, and is recognized as a particularly effective method (as found in Japanese Laid-Open Patent Publication "Tokkaihei" No. 09-324258 , for example).
  • the surface hardening through nitriding may, however in some cases, result in an unallowable level of generated distortion, and as a consequence often needs bend straightening after the surface hardening.
  • the bend straightening is carried out so as to straighten the bend to a level allowable by the product, wherein easiness of the process, or bend straightening property depends on the surface hardness achieved after the surface hardening.
  • the lower the surface hardness becomes the component strength such as wear resistance and fatigue property of the mechanical components degrades. Larger surface hardness is therefore preferable for the purpose of raising component strength of the mechanical components.
  • the mechanical components As described in the above, it is not easy for the mechanical components to harmonize excellent component strength and bend straightening property after being surface-hardened. It is, however, a critical issue to carry out the surface hardening in order to ensure desirable quality of the fabricated mechanical components and to improve the yield ratio of the products, and to improve the component strength and bend straightening property of the mechanical components provided as the products after the surface hardening.
  • the present invention is indeed conceived after considering the above-described problems, and an object thereof resides in providing a mechanical component and a method of fabricating the same, which involve surface hardening by nitriding, and are capable of improving both of component strength and bend straightening property.
  • US-A 6 083 455 relates to a method of manufacturing of nitrided steel parts with high strength, high fatigue strength and excellent bending toughness, such as crank shafts, etc...
  • a mechanical component aimed at solving the above-described problems is composed of a steel and surface-hardened by nitriding according to claim 1, having a Vickers hardness of the surficial portion measured at a reference position corresponded to a 50 ⁇ m depth from the surface of the mechanical component of 340 to 460 HV, having a Vickers hardness of the inner portion being not affected by the nitriding and showing a nearly constant hardness of 190 to 260 HV, and having an effective depth of hardened layer measured from the component surface, where a Vickers hardness of 270 HV is achieved, of 0.3 mm or more.
  • a method of fabricating a mechanical component aimed at solving the above-described problems is a method according to claim 3 of fabricating a mechanical component composed of a steel and surface-hardened by nitriding, wherein the nitriding is carried out so as to adjust Vickers hardness of the surficial portion measured at a reference position corresponded to a 50 ⁇ m depth from the surface of the mechanical component to 340 to 460 HV, so as to adjust Vickers hardness of the inner portion being not affected by the nitriding and showing a nearly constant hardness to 190 to 260 HV, and so as to adjust an effective depth of hardened layer measured from the component surface, where a Vickers hardness of 270 HV is achieved, to 0.3 mm or more.
  • the above-described mechanical component is targeted at those composed of a steel and surface-hardened by nitriding.
  • the nitriding is a treatment allowing nitrogen component to diffuse from the surface towards the inner portion of the mechanical component so as to nitrify the surficial portion of the mechanical component, to thereby raise the hardness of the surficial portion including the component surface.
  • An essential point in the present invention is not only to improve the surface hardness by nitriding, but also to impart the mechanical component, to be provided after the treatment as the product, both with an excellent component strength and bend straightening property. The foregoing paragraphs have already discussed that the surface hardness of the surficial portion of the mechanical component can be improved by the nitriding.
  • Increase in the surface hardness results in increase in the component strength of the mechanical components such as wear resistance and fatigue property.
  • increase in the surface hardness results in decrease in the bend straightening property which indicates degree of process easiness in the bend straightening carried out after the nitriding.
  • the decrease in the bend straightening property results in generation of nonconformities such as micro-cracks in the component surface, and this fails in product making of desirable mechanical components, and is causative of a lowered yield ratio of the products in the manufacturing.
  • the mechanical component of the present invention is therefore such as being surface-hardened by nitriding, and having a Vickers hardness of the surficial portion measured at a reference position corresponded to a 50 ⁇ m depth from the surface of the mechanical component (referred to as "surficial reference position", hereinafter) of 340 to 460 HV, having a Vickers hardness of the inner portion being not affected by the nitriding and showing a nearly constant hardness (simply referred to as "inner portion”, hereinafter) of 190 to 260 HV, and having an effective depth of hardened layer measured from the component surface, where a Vickers hardness of 270 HV is achieved, of 0.3 mm or more.
  • a Vickers hardness at the surficial reference position of less than 340 HV may result in only a small surface hardness, and may fail in making the component useful and excellent in the component strength.
  • a Vickers hardness exceeding 460 HV may result in a large surface hardness, may become more likely to induce nonconformities such as causing micro-cracks during bend straightening, and may fail in making the component useful and excellent in the bend straightening property.
  • a Vickers hardness of the inner portion of less than 190 HV may fail in imparting a desired hardness up to a position of a sufficient depth from the component surface, even if the component is subjected to the nitriding so as to adjust the Vickers hardness at the surficial reference position to a desirable range, and this consequently results in only a small surface hardness, and may fail in making the component useful and excellent in the component strength.
  • a Vickers hardness of the inner portion exceeding 260 HV may result in an excessive increase in the hardness of the surficial portion imparted by the nitriding, even if the component is subjected to the nitriding so as to adjust the Vickers hardness at the surficial reference position to a desirable range, and this makes an amount of increase in the hardness of the surficial portion too large, and consequently results in a large surface hardness, and may fail in making the component useful and excellent in the bend straightening property.
  • the amount of increase in the hardness given by the nitriding decreases in the increasing direction of depth from the component surface towards the inner portion.
  • the rate of decrease in the amount of increase in the hardness can arbitrarily vary depending on species and contents of constituent elements of the steel composing the mechanical components, and on temperature and process time of the nitriding.
  • the mechanical component of the present invention is further given with a condition describing that the effective depth of hardened layer (also simply referred to as "effective hardening depth", hereinafter) measured from the component surface, where a Vickers hardness of 270 HV is achieved, is adjusted to 0.3 mm or more.
  • This condition means moderation of the rate of decrease in the amount of increase in the hardness given by the nitriding, which occurs so as to decrease in the depth-wise direction from the component surface towards the inner portion, and this consequently makes it possible to provide the surficial portion of the mechanical component after the nitriding with a larger hardness over a range from the component surface towards a deeper position. More specifically, an effective depth of hardening from the component surface, where a Vickers hardness of 270 HV is achieved, of less than 0.3 mm may result in a too sharp decrease in the hardness distribution in the depth-wise direction in the surficial portion of the mechanical component, and may sometimes fail in obtaining the surface hardness necessary for making the component useful and excellent in the strength.
  • the mechanical component can be made excellent both in the strength and bend straightening property, by appropriately specifying hardness at the surficial reference position, hardness of the inner portion and effective hardening depth, and by optimizing hardness distribution in the depth-wise direction from the component surface.
  • H ⁇ x H ⁇ ⁇ 0 + H ⁇ ⁇ 1 - H ⁇ ⁇ 0 ⁇ 1 - erf x 2 ⁇ ⁇ Dt
  • H ′ ⁇ 0 C e ⁇ q . ⁇ 254 + 33.8
  • H ′ ⁇ 1 C ⁇ r e ⁇ q . ⁇ 392 + 65.8
  • degree of hardening of the surficial layer is affected by composition of a steel material composing the mechanical component.
  • an effective method may be such as optimizing steel composition effectively contributable to hardness of the inner portion corresponded to the hardness of the surficial portion before the nitriding, and steel composition effectively contributable to the nitriding.
  • chromium equivalence Cr[eq.] defined as Cr eq .
  • compositional components capable of effectively raising hardness at the surficial reference position are found to be Cr, C, Mn and Si, enumerated in a decreasing order of the effect.
  • the constant terms expressing degrees of such effect are measured values determined by extensive measurements.
  • a value of thus-defined Cr[eq.] of less than 0.72% may sometimes fail in raising Vickers hardness at the surficial reference position of the mechanical component to as high as 340 HV or more even after the nitriding, and on the other hand, a value of Cr[eq.] exceeding 1.0% may sometimes fail in suppressing Vickers hardness at the surficial reference position of the mechanical component to as low as 460 HV or less due to an excessive hardening of the surficial portion during the nitriding.
  • carbon equivalence C[eq.], defined as C eq . C + 0.07 ⁇ Si + 0.16 ⁇ Mn + 0.19 ⁇ Cu + 0.17 ⁇ Ni + 0.2 ⁇ Cr , is adjusted to 0.65% or more and 0.86% or less in % by weight.
  • the C[eq.] herein is understood as an index of compositional components capable of effectively raising hardness of the inner portion.
  • Compositional components capable of effectively raising hardness of the inner portion of the mechanical component were found to be C, Cr, Cu, Ni, Mn and Si, enumerated in a decreasing order of the effect.
  • the constant terms expressing degrees of such effect are values determined by extensive measurements, similarly to as described in the above.
  • a value of thus-defined C[eq.] of less than 0.65% may sometimes fail in raising Vickers hardness of the inner portion of the mechanical component to as high as 190 HV or more, and on the other hand, a value of C[eq.] exceeding 0.86% may sometimes fail in suppressing Vickers hardness of the inner portion of the mechanical component to as low as 260 HV or less due to an excessive hardening of the inner portion.
  • the mechanical component is also characterized by having a hardness distribution profile H(x), which is given by plotting, on the H-x plane, Vickers hardness H measured in the depth-wise direction x as viewed from the component surface, fallen in region Z expressed by the equation (1) below:
  • H ⁇ x H ⁇ ⁇ 0 + H ⁇ ⁇ 1 - H ⁇ ⁇ 0 ⁇ 1 - erf x 2 ⁇ ⁇ Dt
  • H ′ ⁇ 0 C e ⁇ q . ⁇ 254 + 33.8
  • H ′ ⁇ 1 C ⁇ r e ⁇ q . ⁇ 392 + 65.8
  • the nitriding is a treatment allowing nitrogen component to diffuse in the depth-wise direction from the component surface.
  • Diffusion equation C(x) expressing diffusion concentration C of the nitrogen component with respect to the depth-wise direction x can generally be given by the equation (2) below:
  • C x C ⁇ 0 + C ⁇ 1 - C ⁇ 0 ⁇ 1 - erf x 2 ⁇ Dt
  • the nitriding is a treatment allowing nitrogen component to diffuse in the depth-wise direction from the component surface so as to harden the surficial portion through nitriding. Diffusion concentration of the nitrogen component at a certain depth from the component surface is, therefore, closely related to a degree of hardness attained at the depth by the nitriding, and allows an approximated substitution.
  • C(x) in the equation (2) is substituted by hardness distribution H'(x) in the depth-wise direction x as viewed from the component surface after the nitriding.
  • H'0 expressing hardness of the inner portion of the mechanical component
  • C1 in the equation (2) is substituted by H'1 expressing hardness at the surficial reference position of the mechanical component, wherein H'1 is supposed to have a value expressing hardness at the surficial reference position of the mechanical component because the hardness of the true surface of the mechanical component cannot be measured.
  • diffusion coefficient D was determined assuming that nitrogen as a diffusing element diffuses in a pure Fe, because the mechanical component is composed of a steel, and content of a major component Fe contained therein is supposed to be at least 50 wt% or more.
  • ⁇ in the equation (1) is a correction diffusion coefficient correcting diffusion coefficient D used in the equation (1).
  • the correction diffusion coefficient ⁇ is used for incorporating any influences of constituent elements other than Fe contained in the steel exerted on the diffusion of N into H'(x).
  • ⁇ is a measured value based on results of extensive hardness measurement.
  • Si and Mn contained in the steel, in particular Si are constituent elements suppressing the diffusion of N. In other words, ⁇ sharply decreases as the contents of Si and Mn increase.
  • optimization of the Si content is, therefore, understood as one essential point in reliably optimizing the hardness distribution in the depth-wise direction of the surficial portion of the mechanical component. It is preferable to adjust the Si and Mn contents so as to make ⁇ fall within a range from 0.3 to 1.6, for example.
  • t represents process time of the nitriding, and a value thereof generally falls within a range from 3.6 ⁇ 10 3 to 18 ⁇ 10 3 seconds.
  • T represents process temperature of the nitriding, and a value thereof generally falls within a range from 500 to 650°C.
  • H'(x) is determined as described in the above.
  • the H'(x) is a function having arbitrary variables of t and T with respect to process conditions of the nitriding, given when a composition of the steel used for composing the mechanical component is uniquely determined.
  • Hardness distribution profile which is obtained by plotting, on the H-x plane, Vickers hardness H measured in the depth-wise direction x as viewed from the component surface, is now given as H(x).
  • H(x) is restricted to reside only in region Z, wherein the region Z is defined as a region in which H'(x) can move on the H-x plane when t is arbitrarily varied from 3.6 ⁇ 10 3 to 18 ⁇ 10 3 and T is arbitrarily varied from 500 to 650, while satisfying a condition that a Vickers hardness of 270 HV is attained at a depth from the component surface of 0.3 mm or more, or in other words, under a condition that a position of 0.3 mm deep from the component surface will show a hardness of 270 HV or more as expressed by H'(0.3 ⁇ 1 -3 ) ⁇ 270.
  • a region of the hardness distribution H(x) as viewed from the surface of the mechanical component restricted to as described in the above makes it possible to reliably optimize the hardness distribution in the depth-wise direction in the surficial portion of the mechanical component, and to impart both of excellent component strength and bend straightening property to the mechanical component.
  • the region of H(x) restricted herein in the region Z means that all requirements on the composition of the steel material for the mechanical component, such as Cr[eq.], C[eq.], Si content and Mn content, are optimized within ranges for general process conditions of the nitriding. This mode of optimization of the composition of the steel material makes it possible to more reliably impart excellent strength and bend straightening property to the mechanical component.
  • nitriding conditions adopted to gas soft nitriding or salt bath nitriding include a process time of 3.6 ⁇ 0 3 to 18 ⁇ 10 3 seconds and nitriding temperature of 500 to 650°C.
  • Other nitriding conditions adopted herein are same as those adopted by the general gas soft nitriding or salt bath nitriding.
  • a nitriding temperature of less than 500°C may sometimes excessively reduce the diffusion of the nitrogen component, and may consequently fail in imparting, by the nitriding, a desired profile of surface hardness in the depth-wise direction to the mechanical component.
  • a nitriding temperature exceeding 650°C may excessively accelerate the diffusion of the nitrogen component, and may sometimes excessively raise the surface hardness than desired.
  • a time less than 3.6 ⁇ 10 3 seconds, or one hour may sometimes fail in imparting, by the nitriding, a desirable surface hardness profile in the depth-wise direction to the mechanical component.
  • a process time of the nitriding exceeding 18 ⁇ 10 3 seconds, or five hours, may sometimes result in a too large surface hardness than desired.
  • Ranges of the process time and process temperature of the nitriding are thus set in consideration of these situations, wherein these ranges can be said as more general ones than those set from viewpoints such as operation efficiency in the manufacture.
  • the conditions for the nitriding are set based on these reasons, and this is consequently successful in imparting excellent strength and bend straightening property to the mechanical component in a more reliable manner.
  • the mechanical component of the present invention is characterized by having the composition as defined by claim 1.
  • C is contained in an amount of 0.35 to 0.5% by weight.
  • C is an element useful for effectively raising the hardness in the inner portion and at the surficial reference position of the mechanical component, wherein a content of 0.35% or more makes the effect more distinct.
  • the content exceeding 0.5% may sometimes result in an excessive effect, and may fail in adjusting the hardness of the surficial layer of the mechanical component to a desired level. It may also be causative of degradation in the machinability when the mechanical component is machined into a desired geometry, for example when a forged material composed of a steel is machined.
  • Si is contained in an amount of 0.01 to 0.3% by weight.
  • Si is used as a deoxidizer element in steel melting, so that it is necessarily contained at least in an amount of 0.01% or more.
  • Si is, however, also a constituent element suppressing the N diffusion in the nitriding. In view of reliably imparting a desired hardness profile to the mechanical component, it is preferable to suppress the content thereof to as low as 0.3% or less.
  • Mn is contained in an amount of 0.6 to 1.8% by weight. Mn is an element useful for effectively raising the hardness of the inner portion and at the surficial reference position, wherein a content of 0.6% or more makes the effect more distinct. On the contrary, the content exceeding 1.8% may sometimes result in generation of bainite during operations such as hot forging and normalizing before the nitriding.
  • Mn is a constituent element suppressing the N diffusion in the nitriding, although to a lesser degree as compared with Si. Also from this point of view, the content of Mn is preferably suppressed to as small as 1.8% or less.
  • Both of Cu and Ni are contained in an amount of 0.01 to 0.5% by weight. Both of Cu and Ni are contained as inevitable impurities in an amount of 0.01% or more, and are useful for effectively raising the hardness of the inner portion of the mechanical component. The content exceeding 0.5% may, however, be disadvantageous from an economical point of view, and may raise cost of the mechanical component, so that the content is adjusted to 0.5% or less.
  • Cr is contained in an amount of 0.01 to 0.5% by weight. Cr is an element useful for effectively raising the hardness of the inner portion and at the surficial reference position of the mechanical component. The content adjusted to 0.01% or more is successful in making the effect more distinct.
  • the content exceeding 0.5% may sometimes result in an excessive effect, and may fail in adjusting the hardness of the surficial layer of the mechanical component to a desired level.
  • Al is contained in an amount of 0.001 to 0.01% by weight.
  • Si also Al is used as a deoxidizer element in steel melting, so that it is necessarily contained at least in an amount of 0.001 % or more.
  • Al may excessively raise the hardness at the surficial reference position of the mechanical component, so that the content thereof is preferably limited to 0.01% or less.
  • N is contained in an amount of 0.005 to 0.025% by weight.
  • N is an element useful for effectively suppressing crystal grain growth of the steel component typically during hot forging, through formation of nitride with Al.
  • the content thereof is preferably set to as much as 0.005% or more, but a content of 0.025% may be enough for the upper limit thereof, because the effect saturates above 0.025%.
  • the mechanical component of the present invention is further characterized by containing any one species, or two or more species of constituent elements with the individual contents, in % by weight, of Pb: 0.30% or less, S: 0.20% or less, Ca: 0.01% or less, Bi: 0.30% or less, Ti: 0.02 or less, Zr: 0.02% or less and Mg: 0.01% or less.
  • Pb, S, Ca and Bi described in the above are elements useful for effectively improving machinability when a forged material composed of a steel is machined into a desired geometry. Without a desirable level of machinability, an excessive machining distortion or the like may generate on the surface of the component during machining, and may fail in reliably imparting a desired level of bend straightening property to the mechanical component.
  • any contents exceeding the above-described upper limits may degrade the hot workability or component strength such as fatigue property of the mechanical component, so that it is preferable to adjust, in % by weight, Pb to 0.30% or less, S to 0.20% or less, Ca to 0.01% or less and Bi to 0.30% or less.
  • Ti, Zr and Mg are known to promote micro-dispersion of MnS and so forth in steel melting, through formation of their oxides.
  • the effect also makes it possible to improve the machinability in the machining, and to micronize a crystal texture of the steel after annealing such as normalizing in the succeeding step of hot forging, for example, and further to reliably impart necessary component strength and bend straightening property to the mechanical component.
  • any contents exceeding the above-described upper limits may saturate the effect, so that it is preferable to adjust, in % by weight, Ti to 0.02% or less, Zr to 0.02% or less, and Mg to 0.01 % or less.
  • crank shaft is a mechanical component used under high-speed rotation and needs a precise control of decentering by bend straightening.
  • the crank shaft can improve its usefulness by being applied with the mechanical component of the present invention which can be made excellent both in strength and bend straightening property.
  • Fig. 1 is a schematic side elevation of a fillet portion of one essential portion of a crank shaft, which is one embodiment of the mechanical component.
  • Fig. 2 is a schematic sectional view of the fillet portion taken along line II-II in Fig. 1 .
  • the fillet portion 1 is understood as a mechanical component 1. Because the crank shaft is formed by assembling separately-manufactured constituents, assumption of the fillet portion as the mechanical component of the present invention will never depart from the spirit of the present invention.
  • the fillet portion 1 is composed of a steel and is nitrided. As shown in Fig.
  • the fillet portion 1 comprises a surficial portion 2 raised in the surface hardness by the nitriding, and an inner portion 3 not affected by the nitriding and showing a nearly constant hardness.
  • the hardness decreases in the depth-wise direction from a component surface 4 towards the inner portion 3.
  • the inner portion 3 is adjusted to have a Vickers hardness of 190 to 260 HV
  • the surficial portion 2 is adjusted to have a Vickers hardness of 340 to 460 HV at the reference position corresponded to a 50 ⁇ m depth from the component surface, and is further adjusted to have an effective depth of hardened layer measured from the component surface 4, where a Vickers hardness of 270 HV is achieved, of 0.3 mm or more.
  • a steel material having a predetermined composition is prepared by melting so as to attain a steel composition necessary for the mechanical component, and is then hot-forged to yield a forged material.
  • the forged material composed of the steel is then thermally refined by annealing such as normalizing, quenching and tempering, and is machined according to a desired geometry of the mechanical component.
  • the mechanical component is subjected to surface hardening by nitriding to so as to improve the strength.
  • the mechanical component is provided as a product.
  • each of the constituents is assumed as the mechanical component, respectively manufactured according to the above-described process flow, and assembled to thereby obtain the mechanical component of a desired geometry.
  • the mechanical component of the present invention is essentially targeted at publicly-known mechanical components such as gear, bearing, shaft, crank shaft and connecting rod, but any of those composed of two or more constituents can be assumed that each of the constituents is understood as the mechanical component of the present invention.
  • the above-described fabrication method is only one example, and the present invention also allows a non-refining process from which the refining by annealing after hot forging is omitted.
  • An essential point is that any fabrication method can be adopted as the fabrication method so far as it involves at least surface hardening by nitriding, which is followed by bend straightening to thereby finish the mechanical component as a product.
  • the nitriding may be carried out by publicly-known method such as salt bath nitriding and gas soft nitriding. Appropriate adjustment of conditions for the nitriding, such as process temperature, process time and flow rate of nitrogen to be supplied to the component surface, makes it possible to achieve a desired depth-wise hardness distribution in the surficial portion of the mechanical component.
  • Table 1 shows also chromium equivalence Cr[eq.] and carbon equivalence C[eq.] of the individual steels composing the individual test pieces.
  • Table 1 still also shows calculated results of H'(0.3 ⁇ 10 -3 ), which are values of Vickers hardness at a 0.3 mm deep from the component surface based on the theoretical formula H'(x) given as the equation (1) in the above.
  • Embodied product 1 0.4 0.05 1.45 0.05 0.05 0.2 0.005 0.023 0.75 0.69 273 Embodied product 2 0.35 0.28 0.65 0.45 0.45 0.48 0.002 0.012 0.85 0.73 312 Embodied product 3 0.5 0.12 1.75 0.15 0.15 0.04 0.008 0.008 0.72 0.85 282 Embodied product 4 0.45 0.1 1.2 0.1 0.1 0.45 0.002 0.02 0.97 0.78 330 Embodied product 5 0.4 0.08 1.48 0.1 0.1 0.2 0.005 0.005 Pb:0.18, S:0.062 0.76 0.72 277 Embodied product 6 0.42 0.1 1.44 0.08 0.08 0.18 0.003 0.003 S:0.121, Ca:0.0025 0.74 0.72 275 Embodied product 7 0.41 0.12 1.45 0.1 0.1 0.21 0.004 0.004 Bi:0.1, S:0.052, Ca:0.0042 0.77
  • test pieces were subjected to fatigue test using an Ono-type rotating-bending fatigue tester, and measured values of fatigue strength (MPa) were used as indices of fatigue property as component strength.
  • test pieces were subjected to three-point bending test using a universal testing machine, wherein measured values of amount of indentation (mm) causative of cracks in the component surface were used as indices of the bend straightening property.
  • Table 2 shows results of these measurements which include Vickers hardness at the surficial reference position (at a position 50 ⁇ m deep from the component surface), Vickers hardness at a position 0.3 mm deep from the component surface (referred to as "effective hardening depth position", hereinafter), fatigue strength as an index of the fatigue property, and amount of indentation as an index of the bend straightening property. It is to be noted that the individual measurements for the sectional hardness, fatigue property and bend straightening property were made using separate test pieces individually fabricated under the same conditions.
  • embodiment products 1 to 10 showed values of Vickers hardness at the surficial reference positon of 340 to 460 HV, Vickers hardness of the inner portion of 190 to 260 HV, and Vickers hardness at the effective hardning depth position of 270 HV or above. They where confirmed to be excellent both in the fatigue property and bend straightening property.
  • the mechanical components excellent both in the strength and bend straightening property are defined as those having a fatigue strength which gives an index of fatigue property of 400 MPa or above, and an amount of indentation which gives an index of bend straightening property of 2 mm or more.
  • the residual portion other than those listed therein is essentially composed of Fe.
  • comparative product 1 showed a hardness of the inner portion of less than 190 HV, and a hardness at the effective hardening depth position is less than 270 HV, although the hardness at the surficial reference position was maintained at 355 HV. This consequently resulted in only an insufficient surface hardness of the surficial portion, and in considerably lowered fatigue strength as compared with that of the embodied products, in other words, this failed in obtaining a desirable level of component strength.
  • comparative product 1 has C[eq.] smaller than that of the embodied products, due to the C content.
  • the present embodiment adopts process conditions (process temperature, process time) of nitriding which fall in general ranges, and this reaches a conclusion that C[eq.] is adjusted to 0.65 or more in view of reliably adjusting hardness at the surficial reference position and at the effective hardening depth position to desirable levels, and raising the hardness of the inner portion, which is necessary for ensuring a sufficient level of the component strength.
  • the C content it is adjusted to 0.35 wt% or more (see embodied product 2).
  • comparative product 8 showed a hardness of the inner portion of less than 190 HV, and showed hardness both at the surficial reference position and at the effective hardening depth position of the desired levels, but a reduction rate of the hardness towards the inner portion was high similarly to comparative product 1, and this consequently resulted in only an insufficient surface hardness of the surficial portion, and in considerably lowered fatigue strength as compared with that of the embodied products.
  • C[eq.] is adjusted to 0.65 or more in view of reliably raising the hardness of the inner portion, which is necessary for ensuring a sufficient level of the component strength, based on the same reason with comparative product 1.
  • Comparative product 3 showed desirable level of hardness values at the surficial reference position and of the inner portion, but showed a hardness at the effective hardening depth position of less than 270 HV, proving a large reduction rate of the hardness towards the inner portion, and this consequently resulted in only an insufficient surface hardness of the surficial portion, and in considerably lowered fatigue strength as compared with that of the embodied products.
  • the reduction rate of the hardness towards the inner portion was excessively increased because the Si content thereof became excessively large as compared with those of the embodied products.
  • the Si content is adjusted to 0.3 wt% or less (see embodied product 2), for example.
  • Comparative product 12 showed a desired hardness for the inner portion, but showed hardness smaller than desired for the surficial reference position and effective hardening depth position. This consequently resulted in only an insufficient surface hardness of the surficial portion, and in considerably lowered fatigue strength as compared with that of the embodied products.
  • Cr[eq.] is adjusted to 0.72 or more in view of reliably raising the surface hardness required for ensuring a sufficient level of the component strength.
  • Comparative product 5 showed a hardness of the inner portion exceeding 260 HV, and a desirable level of hardness at the effective hardening depth position of as high as 270 HV, but also showed a hardness at the surficial reference position exceeding 460 HV. This resulted in an excessively large surface hardness of the surficial portion, an amount of indentation considerably lowered from that of the embodied products, and consequently resulted in only an insufficient bend straightening property. As discussed from the viewpoint of steel composition of comparative product 5, the excessively large surface hardness of the surficial portion was supposed to be ascribable to the Cr[eq.] larger than that of the embodied products, due to the Cr content.
  • Cr[eq.] is therefore adjusted to 1.0 or less in view of reliably obtaining a desired level of surface hardness at the surficial reference position required for ensuring a sufficient level of bend straightening property.
  • Another possible reason for the excessively large surface hardness was also supposed to be the large Cr content, which was ascribable to the large C[eq.] and the hardness of the inner portion larger than desired. It is therefore necessary to limit C[eq.] to as low as 0.86 or less in view of reliably ensuring a sufficient level of bend straightening property. From the viewpoint of Cr content, it is adjusted to 0.5 wt% or less (see embodied product 2).
  • Comparative products 6 and 7 showed desirable levels of hardness of the inner portion and at the effective hardening depth position, but showed hardness values exceeding 460 HV at the surficial reference position. This resulted in an excessively large surface hardness, an amount of indentation considerably lowered from that of the embodied products, and consequently resulted in only an insufficient bend straightening property. From the viewpoint of steel composition, comparative product 7 was supposed to be excessively raised in the surface hardness of the surficial portion due to Cr[eq.] larger than that of the embodied products. It is therefore necessary to limit Cr [eq.] to 1.0 or less in view of reliably obtaining a desired level of hardness at the surficial reference position required for ensuring a sufficient level of bend straightening property.
  • Comparative product 6 was supposed to be excessively raised in the surface hardness due to an excessively large Al content. It is therefore necessary to limit the Al content to 0.01 wt% or less (see embodied product 3) in view of reliably obtaining a desired level of bend straightening property.
  • Comparative product 2 was found to be excellent both in the component strength and bend straightening property, but suppressed in the machinability due to a large C content. For the case where there are demands for an improved machinability, and for both of excellent component strength and bend straightening property, it is therefore necessary to limit the C content to, 0.5 wt% or less (see embodied product 3).
  • Embodied products 5 to 10 contain any one or more of Pb, S, Ca, Bi, Ti, Zr and Mg which are machinability-improving elements. Embodied products 5 to 10 are thus successfully raised in the machinability as compared with the others.
  • Addition of the machinability-improving elements to the steel composition is understood as an effective measure, because suppression of the machinability may sometimes result in the component strength. It was found from comparison, for example, between embodied products 1 and 6 that the both showed an equivalent amount of indentation, but embodied product 6 containing the machinability-improving elements certainly showed a larger fatigue limit.
  • Comparative product 4 showed an excessive generation of bainite due to an excessively large Mn content in the steel composition. Comparative product 4 was found to be inappropriate as a product as early as when it was made into a forged material. It is therefore necessary to limit the Mn content to 1.8 wt% or less.
  • the individual data points in Fig. 3 express results of the measurement of the sectional hardness of the representative test pieces, selected from those of the individual test pieces.
  • the individual curves (broken lines) almost fitted to the individual data points were obtained based on the equation (1). It is obvious that the equation (1) is a good approximate expression based on a better reflection of the measured values of the sectional hardness.
  • the sectional hardness reduces from the component surface towards the inner portion, and reaches plateau at the inner portion. It is to be noted herein that the inner portion is defined by a region 1 mm deep or more from the component surface.
  • Fig. 3 filled plots represent the embodied products, and blank plots represent the comparative products.
  • a region surrounded by the data points of the embodied products is included in region Z defined by using the equation (1). More specifically, further increase in the process temperature and process time of the nitriding for embodied product 1 results in increase in the sectional hardness so as to come closer to that of embodied product 4. On the other hand, decrease in the process temperature and process time of the nitriding for embodied product 4 results in decrease in the sectional hardness so as to come closer to that of embodied product 1.
  • a numerical range of H'1 in the equation (1), expressing hardness at the surficial reference position, is based on Cr[eq.], showing a range of Vickers hardness defined thereby from 348 HV to 458 HV, and on the other hand, a numerical range of H'0, expressing hardness of the inner portion, is based on C[eq.], showing a range of Vickers hardness defined thereby from 199 HV to 252 HV.
  • both of the component strength and bend straightening property are made excellent. It is made possible to obtain desired levels of component strength and bend straightening property by appropriately varying the hardness distribution profile within the region Z.

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Abstract

鋼の素材とする機械部品１は、窒化処理にて表面硬化処理がなされたものであるとともに、該窒化処理による窒化にて表面硬さが高められた表層部２と、窒化の影響が及んでいない略一定硬さを示す内層部３とを有する。そして、内層部３のビッカース硬さを１９０～２６０ＨＶ、表層部２において、部材表面から深さ５０μmに対応した基準位置でのビッカース硬さを３４０～４６０ＨＶ、さらに、ビッカース硬さが２７０ＨＶとされる部材表面４からの有効硬化層深さを０．３ｍｍ以上とする。これにより、窒化処理にて表面硬化処理がなされる鋼を素材とする機械部品において、部品強度および曲がり矯正性をともに優れたものとすることを可能とする機械部品およびその製造方法を提供する。
EP03798399A 2002-09-25 2003-09-11 Machine part and method for manufacturing same Expired - Lifetime EP1548141B8 (en)

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JP2002279833 2002-09-25
JP2002279833 2002-09-25
JP2003037802 2003-02-17
JP2003037802A JP4230794B2 (ja) 2002-09-25 2003-02-17 機械部品およびその製造方法
PCT/JP2003/011612 WO2004029314A1 (ja) 2002-09-25 2003-09-11 機械部品およびその製造方法

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EP1548141A4 EP1548141A4 (en) 2006-02-08
EP1548141B1 true EP1548141B1 (en) 2011-12-21
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RU2556261C2 (ru) * 2013-08-20 2015-07-10 Федеральное государственное бюджетное образовательное учреждение высшего профессионального образования "Ижевский государственный технический университет имени М.Т. Калашникова" Способ изготовления мелкоразмерного режущего инструмента из быстрорежущей стали

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JP4923927B2 (ja) * 2006-09-29 2012-04-25 大同特殊鋼株式会社 クランクシャフトの製造方法
JP2008144200A (ja) * 2006-12-07 2008-06-26 Isuzu Motors Ltd 軟窒化鋼製品の製造方法
JP4821711B2 (ja) * 2007-06-15 2011-11-24 住友金属工業株式会社 軟窒化用鋼材
JP5580517B2 (ja) * 2008-03-31 2014-08-27 本田技研工業株式会社 軟窒化クランクシャフト用素材の製造方法
WO2012163391A1 (en) * 2011-05-27 2012-12-06 Società Bulloneria Europea S.B.E. Spa Process for manufacturing engine starting motor shafts
JP5811303B2 (ja) 2013-03-07 2015-11-11 新日鐵住金株式会社 非調質型軟窒化部品
WO2016098143A1 (ja) * 2014-12-18 2016-06-23 新日鐵住金株式会社 窒化部品の製造方法及び窒化用鋼材
US10837096B2 (en) 2015-09-08 2020-11-17 Nippon Steel Corporation Nitrided steel part and method of production of same
EP3584347B1 (en) * 2017-02-20 2023-12-06 Nippon Steel Corporation Steel sheet
CN112888796A (zh) 2018-10-29 2021-06-01 日本制铁株式会社 氮化部件粗形材及氮化部件
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CN1685073A (zh) 2005-10-19
EP1548141B8 (en) 2012-04-25
EP1548141A1 (en) 2005-06-29
US20060048860A1 (en) 2006-03-09
CN100594249C (zh) 2010-03-17
JP4230794B2 (ja) 2009-02-25
EP1548141A4 (en) 2006-02-08
WO2004029314A1 (ja) 2004-04-08

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