WO2020090999A1 - Élément en acier nitruré, et procédé et appareil pour produire un élément en acier nitruré - Google Patents

Élément en acier nitruré, et procédé et appareil pour produire un élément en acier nitruré Download PDF

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WO2020090999A1
WO2020090999A1 PCT/JP2019/042887 JP2019042887W WO2020090999A1 WO 2020090999 A1 WO2020090999 A1 WO 2020090999A1 JP 2019042887 W JP2019042887 W JP 2019042887W WO 2020090999 A1 WO2020090999 A1 WO 2020090999A1
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steel member
nitrided steel
depth
layer
furnace
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PCT/JP2019/042887
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English (en)
Japanese (ja)
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泰 平岡
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パーカー熱処理工業株式会社
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Publication of WO2020090999A1 publication Critical patent/WO2020090999A1/fr

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/06—Surface hardening
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
    • C21D1/76—Adjusting the composition of the atmosphere
    • 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
    • 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
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/32—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for gear wheels, worm wheels, or the like

Definitions

  • the present invention relates to a nitrided steel member and a method and an apparatus for manufacturing the nitrided steel member. More specifically, the present invention relates to a nitrided steel member having excellent fatigue resistance, which is useful for gears for automobile transmissions, crankshafts, and the like, and a manufacturing method and manufacturing apparatus for the nitrided steel member.
  • a compound layer that is an iron nitride is formed on the surface, and a hardened layer called a diffusion layer is formed inside.
  • the hardened layer is usually made of an alloy nitride such as Si or Cr as a base material component.
  • the atmosphere in the gas nitriding furnace is also controlled in order to control the thickness (depth) of each of these two layers and / or the type of iron nitride on the surface. It is controlled appropriately. Specifically, the nitriding potential (K N ) in the gas nitriding furnace is appropriately controlled.
  • the volume fraction (type of iron nitride) of ⁇ 'phase (Fe 4 N) and ⁇ phase (Fe 2-3 N) in the compound layer formed on the surface of the steel material is controlled through the control.
  • the fatigue resistance is improved by forming the ⁇ ′ phase rather than the ⁇ phase (Yasu Hiraoka, Yoichi Watanabe, Akitake Ishida: heat treatment, 55, No. 1, Page 1-2: Non-Patent Document 1).
  • a nitrided steel member having improved bending fatigue strength and surface fatigue by forming a ⁇ 'phase is also provided (Japanese Patent Laid-Open No. 2013-221203: Patent Document 1).
  • the nitriding treatment when the nitriding treatment is performed at a temperature higher than the eutectoid transformation point (about 590 ° C) of the Fe-N binary alloy, a compound layer is formed on the surface, and if it is then rapidly cooled, a nitrogen-containing martensite structure is formed under the A cured layer containing is formed.
  • the nitriding treatment in the temperature range is called an nitriding treatment in distinction from the conventional nitriding treatment.
  • the austenite in the structure near the surface is stabilized, and most of the austenite remains even after the rapid cooling. Therefore, the strain after the heat treatment is about the same as the nitriding treatment.
  • the stabilized austenite is reheated to a temperature of 250 to 300 ° C. to be transformed into a hard martensite structure.
  • STKM-13C mechanical carbon steel pipe defined by JIS G 3445
  • JIS G 3445 is nitrified at 640 ° C for 90 minutes, then rapidly cooled, and then reheated at 280 ° C for 90 minutes to obtain 800 austenite near the surface. It is cured up to 900 HV (Patent No. 6228403: Patent Document 3).
  • the structure of the compound layer on the surface after the nitriding treatment is a structure in which ⁇ 'is a solid solution in ⁇ , but when reheated at 280 ° C for 90 min, the compound layer mainly composed of the ⁇ '' phase is present on the surface. Is formed.
  • JIS-SPCC a kind of cold rolled steel sheet
  • a compound layer is formed on the surface, and a quenching layer thereafter forms a hardened layer having a nitrogen martensite structure underneath.
  • Fatigue failure of mechanical parts occurs from notches where high load stress is applied, such as at the root of gears.
  • a stress distribution corresponding to the shape and the load environment occurs only in the surface layer region (from the surface to the inside of the predetermined depth). Therefore, it is desired to harden only the surface layer region so as not to impair the toughness and machinability of the steel material.
  • the diffusion layer is not sufficiently hardened, and sufficient improvement in fatigue strength has not been realized (this is considered to be because it is higher than the temperature range of the present invention described below. ). Furthermore, in the technique disclosed in Non-Patent Document 3, the hardened layer is too thick and the thermal strain / transformation strain is large, and it is not suitable for hardening the surface layer region.
  • the inventor of the present invention repeatedly conducted diligent studies and various experiments, and controlled the nitriding temperature and the nitriding potential with high accuracy after limiting the configuration of the processing furnace, whereby the fatigue strength in which the surface layer region was desirably hardened. It has been found that a nitrided steel member having excellent heat resistance can be manufactured.
  • An object of the present invention is to provide a nitrided steel member whose surface layer region is desirably hardened, and a manufacturing method and a manufacturing apparatus for manufacturing such a nitrided steel member.
  • the present invention is a nitrided steel member having a carbon steel or a low alloy steel as a parent phase, the surface of which is provided with a hardened layer having an austenite structure containing 1.0% or more of nitrogen by mass%, and a lower part of the hardened layer. And a diffusion layer in which nitrogen is diffused in the matrix, the hardened layer has a thickness of 2 ⁇ m to 50 ⁇ m from the surface of the nitrided steel member, and the diffusion layer is the nitrided steel.
  • the hardness of the diffusion layer at a depth of 100 ⁇ m from the surface of the nitrided steel member is more than the hardness at a depth of 2 mm from the surface of the nitrided steel member. It is a nitrided steel member characterized by being larger than 100 HV.
  • the hardened layer having an austenite structure containing 1.0% or more of nitrogen is limited to a thickness of 2 ⁇ m to 50 ⁇ m from the surface of the nitrided steel member, heat treatment strain / transformation strain is small. Further, since the hardness of the diffusion layer at a depth of 100 ⁇ m from the surface of the nitrided steel member is 100 HV or more than the hardness at a depth of 2 mm from the surface of the nitrided steel member, the hardened layer is thin, Sufficient strength can be guaranteed.
  • the present invention is a nitrided steel member having a carbon steel or a low alloy steel as a mother phase, comprising a compound layer having an ⁇ phase on the surface side, and the compressive residual stress on the surface of the compound layer is ⁇ 200 MPa.
  • the hardened layer having an austenite structure containing 1.0% or more of nitrogen in mass% is provided below the compound layer, and nitrogen is diffused into the matrix below the hardened layer.
  • a diffusion layer wherein the hardened layer has a thickness of 2 ⁇ m to 50 ⁇ m from the surface of the nitrided steel member, and the diffusion layer extends from the surface of the nitrided steel member to a depth of more than 100 ⁇ m.
  • the hardness of the diffusion layer at a depth of 100 ⁇ m from the surface of the nitrided steel member is 100 HV or more than the hardness at a depth of 2 mm from the surface of the nitrided steel member. It is a member.
  • the hardened layer having an austenite structure containing 1.0% or more of nitrogen is limited to a thickness of 2 ⁇ m to 50 ⁇ m from the surface of the nitrided steel member, heat treatment strain / transformation strain is small. Further, since the hardness of the diffusion layer at a depth of 100 ⁇ m from the surface of the nitrided steel member is 100 HV or more than the hardness at a depth of 2 mm from the surface of the nitrided steel member, the hardened layer is thin, Sufficient strength can be guaranteed.
  • a compressive residual stress of ⁇ 200 MPa or more (200 MPa or less in absolute value) is present on the surface of the compound layer (such a structure was first realized by the nitriding method described later).
  • the occurrence of fatigue cracks is suppressed, and high fatigue strength can be exhibited.
  • the “compound layer having the ⁇ phase” means a state in which the volume of the ⁇ phase contained in the compound layer is 60% or more.
  • the lower limit (upper limit in terms of absolute value) of the compressive residual stress was set to ⁇ 200 MPa, which is the minimum value at which the effect of improving fatigue strength confirmed by the inventors of the present application was confirmed by the time of filing of the present application ( This is because it is the maximum value in terms of absolute value (using a circulation type processing furnace described later, using S45C steel as a mother phase, processing temperature: 640 ° C., nitriding potential: 0.17, processing time: 2 hours, It has been confirmed that it can be obtained under the processing conditions).
  • the present invention is a nitrided steel member having a carbon steel or a low alloy steel as a mother phase, the surface side of which is provided with a hardened layer having an austenite structure containing 1.0% or more of nitrogen by mass%, and the hardening
  • a compound layer having a ⁇ 'phase having a thickness of 10 ⁇ m or less is provided on the surface of the layer as a whole or locally, and a diffusion layer in which nitrogen is diffused in the matrix is provided below the hardened layer.
  • the hardened layer has a thickness of 2 ⁇ m to 50 ⁇ m from the surface of the nitrided steel member, and the diffusion layer extends from the surface of the nitrided steel member to a depth of more than 100 ⁇ m.
  • the hardness of the diffusion layer at a depth of 100 ⁇ m from the surface of the nitrided steel member is 100 HV or more greater than the hardness at a depth of 2 mm from the surface of the steel member.
  • the hardened layer having an austenite structure containing 1.0% or more of nitrogen is limited to the surface of the nitrided steel member or the thickness of 2 ⁇ m to 50 ⁇ m, heat treatment strain / transformation strain is small. Further, since the hardness of the diffusion layer at a depth of 100 ⁇ m from the surface of the nitrided steel member is 100 HV or more than the hardness at a depth of 2 mm from the surface of the nitrided steel member, the hardened layer is thin, Sufficient strength can be guaranteed.
  • a compound layer having a ⁇ ′ phase of 10 ⁇ m or less is formed on the surface of the hardened layer (such a compound layer was first realized by a nitriding method described later). ), The gradient of hardness and residual stress is continuously formed, and the fatigue strength can be improved. It is considered that this is because the ⁇ ′ phase and the ⁇ phase have the same face-centered cubic lattice structure (fcc structure), and therefore the interface between the ⁇ ′ phase and the ⁇ phase has high compatibility.
  • the upper limit of 10 ⁇ m is because that value is the maximum thickness confirmed by the inventors of the present invention by the time of filing of the present application (using a circulating treatment furnace described later, the low carbon steel S25C It has been confirmed that the mother phase can be obtained under the treatment conditions of treatment temperature: 660 ° C., nitriding potential: 0.13, treatment time: 2 hours).
  • ⁇ ′ phase and the ⁇ phase have a face-centered cubic lattice structure (fcc structure), they are superior in toughness to the ⁇ phase, which is a dense hexagonal lattice structure (hcp structure), and can be applied to a member to which an impact load is applied. It is even more suitable for application.
  • fcc structure face-centered cubic lattice structure
  • hcp structure dense hexagonal lattice structure
  • carbon steel having a carbon content of 0.25% by mass or more can be used as the matrix phase.
  • a low alloy steel having a carbon content of 0.1% or more by mass% and a chromium content of 0.4% or more by mass% can be used as the parent phase.
  • SCr420 or SCM415 can be used.
  • the present invention includes a circulation type processing furnace having a guide tube and a stirring fan, and during the nitriding treatment, the temperature range in the circulation type treatment furnace is controlled to 610 ° C. to 660 ° C.
  • the nitriding potential in the circulation type processing furnace is controlled in the range of 0.06 to 0.3.
  • a hardened layer having an austenite structure containing 1.0% or more of nitrogen is provided on the surface, and a diffusion layer in which nitrogen is diffused in the matrix is provided below the hardened layer. It has a thickness of 2 ⁇ m to 50 ⁇ m from the surface of the steel member, the diffusion layer extends from the surface of the nitrided steel member to a depth of more than 100 ⁇ m, and the depth of 2 ⁇ m from the surface of the nitrided steel member. It is possible to manufacture a nitrided steel member characterized in that the hardness of the diffusion layer at a depth of 100 ⁇ m from the surface of the nitrided steel member is 100 HV or more than the hardness in the above.
  • a hardened layer having an austenite structure containing 1.0% or more of nitrogen in mass% is provided below the layer, and a diffusion layer in which nitrogen is diffused in the matrix is provided below the hardened layer.
  • the hardened layer has a thickness of 2 ⁇ m to 50 ⁇ m from the surface of the nitrided steel member, and the diffusion layer extends to a depth of more than 100 ⁇ m from the surface of the nitrided steel member. It is also possible to manufacture a nitrided steel member characterized in that the hardness of the diffusion layer at a depth of 100 ⁇ m from the surface of the nitrided steel member is 100 HV or more greater than the hardness at a depth of 2 mm from the surface of the. it can.
  • a hardened layer having an austenite structure containing 0.0% or more is provided, and a diffusion layer in which nitrogen is diffused in the matrix is provided further below the hardened layer, and the hardened layer is formed from the surface of the nitrided steel member.
  • the diffusion layer has a thickness of 2 ⁇ m to 50 ⁇ m, extends to a depth of more than 100 ⁇ m from the surface of the nitrided steel member, and has a hardness at a depth of 2 mm from the surface of the nitrided steel member. It is also possible to manufacture a nitrided steel member characterized in that the hardness of the diffusion layer at a depth of 100 ⁇ m from the surface of the nitrided steel member is 100 HV or more.
  • the apparatus for producing a nitrided steel member according to the present invention is configured so that, for example, ammonia gas and ammonia decomposition gas are introduced into the circulation type processing furnace.
  • the manufacturing apparatus in order to control the nitriding potential, can perform control such that the amount of ammonia decomposition gas introduced into the furnace is constant and the amount of ammonia gas introduced is changed. preferable.
  • the hardened layer having an austenite structure containing 1.0% or more of nitrogen is limited to a thickness of 2 ⁇ m to 50 ⁇ m from the surface of the nitrided steel member, heat treatment strain / transformation strain is small. Further, since the hardness of the diffusion layer at a depth of 100 ⁇ m from the surface of the nitrided steel member is 100 HV or more than the hardness at a depth of 2 mm from the surface of the nitrided steel member, the hardened layer is thin, Sufficient strength can be guaranteed.
  • FIG. 3 is a cross-sectional micrograph of a nitrided steel member according to the first embodiment of the present invention. It is a figure which shows the analysis result by the EBSD method of the nitrided steel member of FIG. 7 is a cross-sectional micrograph of a nitrided steel member according to the second embodiment of the present invention. It is a figure which shows the analysis result by the EBSD method of the nitrided steel member of FIG. 7 is a sectional micrograph of a nitrided steel member according to a third embodiment of the present invention. It is a table which shows the example of an experiment about hardness distribution. It is a graph which shows the example of an experiment about hardness distribution.
  • FIG. 1 is a schematic view of an apparatus for manufacturing a nitrided steel member according to an embodiment of the present invention. It is a schematic sectional drawing of a circulation type processing furnace (horizontal type gas nitriding furnace). It is a graph which shows an example of gas introduction control. It is a graph which shows an example of gas introduction control. It is a figure which shows the form of an Ono-type rotary bending fatigue test piece.
  • FIG. 1 is a cross-sectional micrograph of a nitrided steel member 110 according to the first embodiment of the present invention.
  • the nitrided steel member 110 of the present embodiment is provided with a hardened layer 111 having an austenite structure containing 1.0% or more of nitrogen on the surface, and is provided below the hardened layer 101 in the matrix.
  • the diffusion layer 112 in which nitrogen is diffused is provided.
  • the base phase (base material) of the present embodiment is carbon steel having a carbon content of 0.45% by mass. (What is visible above the surface is the polishing plate, not the constituent elements of the nitrided steel member. The same applies to FIGS. 3 and 5.)
  • the phase distribution of the nitrided steel member 110 can be analyzed by using the EBSD method and X-ray diffraction together. Specifically, as shown in FIG. 2, it can be seen by the EBSD method that the first layer on the surface has the fcc crystal phase. Then, by using X-ray diffraction together, it can be confirmed that the fcc crystal phase of the first layer on the surface is an austenite phase ( ⁇ phase).
  • the hardened layer 111 has a thickness of about 20 ⁇ m from the surface of the nitrided steel member 100, which is in the range of 2 ⁇ m to 50 ⁇ m.
  • the diffusion layer 112 extends from the surface of the nitrided steel member 110 to a depth exceeding 100 ⁇ m.
  • the hardness of the diffusion layer 112 at a depth of 100 ⁇ m from the surface of the nitrided steel member 110 (for example, about 290 HV) is more than the hardness at a depth of 2 mm from the surface of the nitrided steel member 110 (for example, about 190 HV). It is larger than 100 HV.
  • the nitrided steel member 110 of the present embodiment is subjected to a nitriding treatment under the treatment conditions of a treatment temperature: 640 ° C., a nitriding potential: 0.12, and a treatment time: 2 hours using a circulation type treatment furnace described later, It can be manufactured by being rapidly cooled.
  • a treatment temperature 640 ° C.
  • a nitriding potential 0.12
  • a treatment time 2 hours using a circulation type treatment furnace described later
  • the hardened layer 111 having an austenite structure containing 1.0% or more of nitrogen is limited to a thickness of 2 ⁇ m to 50 ⁇ m from the surface of the nitrided steel member 110.
  • Small heat treatment strain / transformation strain since the hardness of the diffusion layer 112 at a depth of 100 ⁇ m from the surface of the nitrided steel member 110 is 100 HV or more higher than the hardness at a depth of 2 ⁇ m from the surface of the nitrided steel member 110, the hardened layer 111 becomes thin. Nevertheless, sufficient strength can be guaranteed.
  • FIG. 3 is a cross-sectional photomicrograph of the nitrided steel member 120 of the second embodiment of the present invention.
  • the nitrided steel member 120 of the present embodiment is provided with a compound layer 123 having an ⁇ phase on the surface thereof, and austenite containing 1.0% or more of nitrogen in mass% is provided below the compound layer 123.
  • a hardened layer 121 having a texture is provided, and further below the hardened layer 121, a diffusion layer 122 in which nitrogen is diffused in a matrix is provided.
  • the base phase (base material) of the present embodiment is carbon steel having a carbon content of 0.45% by mass.
  • the phase distribution of the nitrided steel member 120 can also be analyzed by using the EBSD method and X-ray diffraction together. Specifically, as shown in FIG. 4, the hcp crystal phase, the fcc crystal phase, and the bcc crystal phase can be identified by the EBSD method. By using X-ray diffraction together, it can be confirmed that the hcp crystal phase is the ⁇ phase and the fcc crystal phase is the austenite phase ( ⁇ phase).
  • the compound layer 123 has a thickness of about 12 ⁇ m from the surface of the nitrided steel member 120, and the compressive residual stress (residual stress value) on the surface of the compound layer 123 is ⁇ 200 MPa.
  • the compressive residual stress (residual stress value) can be measured by X-ray diffraction as described later.
  • the hardened layer 121 has a thickness below the compound layer 123 of about 20 ⁇ m, which is in the range of 2 ⁇ m to 50 ⁇ m.
  • the diffusion layer 122 extends from the surface of the nitrided steel member 120 to a depth exceeding 100 ⁇ m.
  • the hardness of the diffusion layer 122 at a depth of 100 ⁇ m from the surface of the nitrided steel member 120 is more than the hardness at a depth of 2 mm from the surface of the nitrided steel member 120 (for example, about 190 HV). It is larger than 100 HV.
  • the nitrided steel member 120 of the present embodiment is subjected to a nitriding treatment under the treatment conditions of a treatment temperature: 640 ° C., a nitriding potential: 0.17, and a treatment time: 2 hours using a circulation type treatment furnace described later, It can be manufactured by being rapidly cooled.
  • a treatment temperature 640 ° C.
  • a nitriding potential 0.17
  • a treatment time 2 hours using a circulation type treatment furnace described later
  • the hardened layer 121 having an austenite structure containing 1.0% or more of nitrogen is limited to a thickness of 2 ⁇ m to 50 ⁇ m from the surface of the nitrided steel member 120, and therefore heat treatment is performed. Strain / transformation strain is small. Further, since the hardness of the diffusion layer 122 at a depth of 100 ⁇ m from the surface of the nitrided steel member 120 is 100 HV or more higher than the hardness at a depth of 2 ⁇ m from the surface of the nitrided steel member 120, the hardened layer 121 becomes thin. Nevertheless, sufficient strength can be guaranteed.
  • FIG. 5 is a cross-sectional photomicrograph of the nitrided steel member 130 according to the third embodiment of the present invention.
  • the nitrided steel member 130 of the present embodiment is provided with a hardened layer 131 having an austenite structure containing 1.0% or more of nitrogen in mass% on the surface side, and locally on the surface of the hardened layer 131.
  • the compound layer 133 having a ⁇ ′ phase having a thickness of about 0 to 3 ⁇ m is provided.
  • a diffusion layer 132 in which nitrogen is diffused in the matrix is provided below the hardened layer 131.
  • the base phase (base material) of the present embodiment is carbon steel having a carbon content of 0.45% by mass.
  • the phase distribution of the nitrided steel member 130 can also be analyzed by using the EBSD method and X-ray diffraction together. Specifically, the hardened layer 131 and the diffusion layer 132 are distinguished by the EBSD method, and it can be confirmed by X-ray diffraction that the compound layer 133 is in the ⁇ 'phase.
  • the thickness of the compound layer 133 is 10 ⁇ m or less.
  • the hardened layer 131 has a thickness of about 20 ⁇ m from the surface of the nitrided steel member 130, which is in the range of 2 ⁇ m to 50 ⁇ m.
  • the diffusion layer 132 extends from the surface of the nitrided steel member 130 to a depth exceeding 100 ⁇ m. Then, the hardness of the diffusion layer 132 at a depth of 100 ⁇ m from the surface of the nitrided steel member 130 (for example, about 290 HV) is more than the hardness at a depth of 2 mm from the surface of the nitrided steel member 130 (for example, about 190 HV). It is larger than 100 HV.
  • the nitrided steel member 130 of the present embodiment is subjected to a nitriding treatment under the treatment conditions of a treatment temperature of 640 ° C., a nitriding potential of 0.13, and a treatment time of 2 hours using a circulation type treatment furnace described later, It can be manufactured by being rapidly cooled.
  • a nitriding treatment under the treatment conditions of a treatment temperature of 640 ° C., a nitriding potential of 0.13, and a treatment time of 2 hours using a circulation type treatment furnace described later, It can be manufactured by being rapidly cooled.
  • the compound layer 133, the cured layer 131, and the diffusion layer 132 can be clearly distinguished from each other.
  • the hardened layer 131 having an austenite structure containing 1.0% or more of nitrogen is limited to a thickness of 2 ⁇ m to 50 ⁇ m from the surface of the nitrided steel member 130, heat treatment is performed. Strain / transformation strain is small. Further, since the hardness of the diffusion layer 132 at a depth of 100 ⁇ m from the surface of the nitrided steel member 200 is 100 HV or more than the hardness at a depth of 2 ⁇ m from the surface of the nitrided steel member 130, the hardened layer 131 becomes thin. Nevertheless, sufficient strength can be guaranteed. (Range of nitrogen concentration of hardened layer)
  • the nitrogen concentration of the hardened layers 111, 121, 131 is the result of considering the stability of the austenite structure at room temperature. That is, by containing 1.0% or more of nitrogen, most of the austenite phase is stabilized at room temperature when quenched, that is, martensite transformation does not occur during quenching. As a result, the strain is extremely small as compared with the case where martensitic transformation occurs during rapid cooling. (Cured layer thickness range)
  • the thickness of the hardened layers 111, 121 and 131 basically, the thicker the fatigue strength, the better. However, depending on the load environment of the nitrided steel members 110, 120, and 130, even if the thickness is further increased, there is a case where there is no further effect of improving the fatigue strength (the effect is saturated). Specifically, the stress distribution in the notch may differ depending on the shape of the nitrided steel members 110, 120, 130 and the load environment. Therefore, the thickness of the hardened layers 111, 121, 131 can be appropriately selected depending on the shapes of the nitrided steel members 110, 120, 130 and the load environment.
  • the manufacturing conditions satisfying the condition that "the hardness of the diffusion layer at a depth of 100 ⁇ m from the surface of the nitrided steel member is 100 HV or more greater than the hardness at a depth of 2 mm from the surface of the nitrided steel member". : 610 ° C. to 660 ° C., nitriding potential: 0.06 to 0.3), the thickness of the hardened layers 111, 121 and 131 is 2 to 50 ⁇ m.
  • the lower limit is 2 ⁇ m. It has a value. (Condition of hardness of diffusion layer)
  • the nitrided steel members 110, 120, 130 of this embodiment are characterized in that not only the hardened layers 111, 121, 131 but also the diffusion layers 112, 122, 132 have sufficient hardness.
  • FIGS. 6A and 6B show hardness distributions of JIS-S45C steels (carbon steels) subjected to nitriding treatment for 1.5 hours at various temperatures shown in the drawings and then rapidly cooled. There is.
  • FIGS. 6C and 6D show the hardness distribution of each test piece of JIS-SCM415 steel (Cr-Mo steel) which was subjected to nitrification treatment for 1.5 hours at various temperatures shown in the figure and then rapidly cooled. Shows.
  • the surface hardness after nitriding is generally acquired at a depth of 50 ⁇ m from the surface.
  • the hardness at the depth position of 100 ⁇ m from the surface is the evaluation target in order to avoid the influence of the hardened layers 111, 121, 131 having the austenite structure.
  • the hardness at a depth of 2 mm from the surface is defined as an evaluation target for the internal structure that is not affected by nitriding.
  • the nitriding potential K N is defined by the following equation (2).
  • K N P NH3 / P H2 3/2 ⁇ ⁇ ⁇ (2)
  • P NH3 is the partial pressure of ammonia in the furnace
  • P H2 is the partial pressure of hydrogen in the furnace.
  • the nitriding potential K N is well known as an index showing the nitriding ability of the atmosphere in the gas nitriding furnace.
  • the reaction of formula (3) mainly occurs in the furnace, and the nitriding reaction of formula (1) can be almost ignored in terms of quantity. Therefore, the nitriding potential can be calculated if the concentration of ammonia in the furnace consumed in the reaction of the equation (3) or the concentration of hydrogen gas generated in the reaction of the equation (3) is known. That is, hydrogen and nitrogen generated are 1.5 mol and 0.5 mol, respectively, from 1 mol of ammonia. Therefore, if the ammonia concentration in the furnace is measured, the hydrogen concentration in the furnace can be known, and the nitriding potential can be calculated. You can Alternatively, if the hydrogen concentration in the furnace is measured, the ammonia concentration in the furnace can be known, and the nitriding potential can be calculated.
  • the ammonia gas flown into the gas nitriding furnace is circulated in the furnace and then discharged to the outside of the furnace. That is, in the gas nitriding treatment, the fresh (new) ammonia gas is constantly introduced into the furnace with respect to the existing gas in the furnace, so that the existing gas is continuously discharged to the outside of the furnace (pushed out by the supply pressure). ..
  • the flow rate of the ammonia gas introduced into the furnace is small, the gas residence time in the furnace becomes long, so that the amount of the decomposed ammonia gas increases and the nitrogen gas generated by the decomposition reaction is increased. + The amount of hydrogen gas increases.
  • the flow rate of ammonia gas introduced into the furnace is large, the amount of ammonia gas that is not decomposed and discharged outside the furnace will increase, and the amount of nitrogen gas + hydrogen gas generated in the furnace will decrease. To do.
  • FIG. 7 is a schematic view showing a manufacturing apparatus for manufacturing a nitrided steel member according to an embodiment of the present invention.
  • the manufacturing apparatus 1 of the present embodiment includes a circulation type processing furnace 2, and uses only two types of gas, ammonia and ammonia decomposition gas, as the gas to be introduced into the circulation type processing furnace 2. ing.
  • the ammonia decomposition gas is also called AX gas, and is a mixed gas of nitrogen and hydrogen in a ratio of 1: 3.
  • ammonia and ammonia decomposition gas Only three types of nitrogen gas can be selected.
  • Fig. 8 shows an example of a cross-sectional structure of the circulation type processing furnace 2.
  • a cylinder 202 called a retort is arranged in a furnace wall (also called a bell) 201, and a cylinder 204 called an inner retort is arranged inside the cylinder 202.
  • the introduction gas supplied from the gas introduction pipe 205 passes around the object to be treated and then passes through the space between the two cylinders 202 and 204 by the action of the stirring fan 203, as shown by the arrow in the figure.
  • Circulate. 206 is a gas hood with a flare
  • 207 is a thermocouple
  • 208 is a lid for cooling work
  • 209 is a fan for cooling work.
  • the circulation type processing furnace 2 is also called a horizontal gas nitriding furnace, and its structure itself is known.
  • the processed product S is carbon steel or low alloy steel, and is, for example, a crankshaft or a gear which is an automobile part.
  • a furnace opening / closing lid 7 a stirring fan 8, a stirring fan drive motor 9, and an atmospheric gas concentration detecting device 3 are provided.
  • a nitriding potential controller 4, a programmable logic controller 30, and a furnace introduction gas supply unit 20 are provided.
  • the stirring fan 8 is arranged in the processing furnace 2 and rotates in the processing furnace 2 to stir the atmosphere in the processing furnace 2.
  • the stirring fan drive motor 9 is connected to the stirring fan 8 and rotates the stirring fan 8 at an arbitrary rotation speed.
  • the atmosphere gas concentration detection device 3 is composed of a sensor capable of detecting the hydrogen concentration or the ammonia concentration in the processing furnace 2 as the furnace atmosphere gas concentration.
  • the detection body of the sensor communicates with the inside of the processing furnace 2 via the atmosphere gas pipe 12.
  • the atmospheric gas pipe 12 is formed in a path that directly connects the sensor main body of the atmospheric gas concentration detection device 3 and the processing furnace 2, and the in-furnace gas waste pipe connected to the exhaust gas combustion decomposition device 41 on the way. 40 is connected.
  • the atmospheric gas is distributed between the discarded gas and the gas supplied to the atmospheric gas concentration detection device 3.
  • the atmospheric gas concentration detection device 3 is adapted to output an information signal including the detected concentration to the nitriding potential controller 4 after detecting the atmospheric gas concentration in the furnace.
  • the nitriding potential controller 4 has an in-furnace nitriding potential calculating device 13 and a gas flow rate output adjusting device 30.
  • the programmable logic controller 31 also includes a gas introduction amount control device 14 and a parameter setting device 15.
  • the in-furnace nitriding potential calculation device 13 is configured to calculate the nitriding potential in the processing furnace 2 based on the hydrogen concentration or the ammonia concentration detected by the in-furnace atmosphere gas concentration detection device 3. Specifically, a calculation formula of the nitriding potential programmed according to the actual gas introduced into the furnace is incorporated, and the nitriding potential is calculated from the value of the atmospheric gas concentration in the furnace.
  • the parameter setting device 15 is composed of, for example, a touch panel, and can set and input the total flow rate of the gas introduced into the furnace, the gas type, the processing temperature, the target nitriding potential, and the like. Each setting parameter value that has been set and input is transmitted to the gas flow rate output adjusting means 30.
  • the gas flow rate output adjusting means 30 sets the nitriding potential calculated by the in-furnace nitriding potential calculating device 13 as the output value, sets the target nitriding potential (set nitriding potential) as the target value, and outputs the ammonia gas and the ammonia decomposition gas.
  • the control is carried out with each introduced amount as an input value. More specifically, it is possible to perform control such that the amount of ammonia decomposition gas introduced into the furnace is constant and the amount of ammonia gas introduced into the furnace is changed.
  • the output value of the gas flow rate output adjusting means 30 is transmitted to the gas introduction amount control means 14.
  • the gas introduction amount control means 14 sends a control signal to each of the first supply amount control device 22 for ammonia gas and the second supply amount control device 26 for ammonia decomposition gas in order to realize the introduction amount of each gas. It has become.
  • the in-reactor introduction gas supply unit 20 of the present embodiment includes a first in-reactor introduction gas supply unit 21 for ammonia gas, a first supply amount control device 22, a first supply valve 23, and a first flow meter 24. ,have. Further, the in-reactor introduction gas supply unit 20 of the present embodiment includes a second in-reactor introduction gas supply unit 25 for ammonia decomposition gas (AX gas), a second supply amount control device 26, and a second supply valve 27. , And a second flow meter 28.
  • AX gas ammonia decomposition gas
  • the ammonia gas and the ammonia decomposition gas are mixed in the furnace introduction gas introduction pipe 29 before entering the processing furnace 2.
  • the first furnace introduction gas supply unit 21 is formed of, for example, a tank filled with the first furnace introduction gas (ammonia gas in this example).
  • the first supply amount control device 22 is formed by a mass flow controller and is interposed between the first in-furnace introduced gas supply unit 21 and the first supply valve 23.
  • the opening degree of the first supply amount control device 22 changes according to the control signal output from the gas introduction amount control means 14. Further, the first supply amount control device 22 detects the supply amount from the first in-furnace introduction gas supply part 21 to the first supply valve 23, and sends an information signal including the detected supply amount to the gas introduction control means 14. It is designed to output.
  • the control signal can be used for correction of the control by the gas introduction amount control means 14 or the like.
  • the first supply valve 23 is formed by an electromagnetic valve that switches between open and closed states according to a control signal output by the gas introduction amount control means 14, and is provided between the first supply amount control device 22 and the first flow meter 24. It is installed.
  • the second-furnace-introduced-gas supply unit 25 is formed of, for example, a tank filled with the second-furnace-introduced gas (in this example, an ammonia decomposition gas).
  • the second supply amount control device 26 is formed by a mass flow controller and is interposed between the second in-furnace introduced gas supply unit 25 and the first supply valve 27.
  • the opening degree of the first supply amount control device 26 changes according to the control signal output from the gas introduction amount control means 14.
  • the third supply amount control device 26 detects the supply amount from the second in-furnace introduction gas supply unit 25 to the second supply valve 27, and sends an information signal including the detected supply amount to the gas introduction control means 14. It is designed to output.
  • the control signal can be used for correction of the control by the gas introduction amount control means 14 or the like.
  • the second supply valve 27 is formed by an electromagnetic valve that switches between open and closed states in accordance with a control signal output by the gas introduction amount control means 14, and is provided between the second supply amount control device 26 and the second flow meter 28. It is installed.
  • the object S to be processed is put into the circulation type processing furnace 2 and the circulation type processing furnace 2 is heated to a desired processing temperature. Then, a mixed gas of ammonia gas and ammonia decomposition gas, or only ammonia gas is introduced into the processing furnace 2 from the in-furnace introduction gas supply unit 20 at a set initial flow rate.
  • This set initial flow rate can also be set and input in the parameter setting device 15, and is controlled by the first supply amount control device 22 and the second supply amount control device 26 (both mass flow controllers).
  • the stirring fan drive motor 9 is driven to rotate the stirring fan 8 to stir the atmosphere in the processing furnace 2.
  • the in-reactor nitriding potential calculation device 13 of the nitriding potential controller 4 calculates the in-reactor nitriding potential (initially, the value is extremely high (because hydrogen does not exist in the furnace), but decomposition of ammonia gas (hydrogen generation)). Becomes lower as the value of the target nitriding potential advances), it is determined whether or not it is below the sum of the target nitriding potential and the reference deviation value. This reference deviation value can also be set and input in the parameter setting device 15.
  • the nitriding potential controller 4 causes the gas introduction amount control means 14 to introduce the introduced gas amount in the furnace. Control of.
  • the in-reactor nitriding potential calculator 13 of the nitriding potential controller 4 calculates the in-reactor nitriding potential based on the input hydrogen concentration signal or ammonia concentration signal. Then, the gas flow rate output adjusting means 30 sets the nitriding potential calculated by the in-reactor nitriding potential calculation device 13 as an output value, sets the target nitriding potential (set nitriding potential) as a target value, and introduces the amount of introduced gas in the furnace.
  • PID control is performed with the input value of. Specifically, in the PID control, control is performed so that the amount of ammonia decomposition gas introduced into the furnace is constant and the amount of ammonia gas introduced into the furnace is changed. In the PID control, each setting parameter value set and input by the parameter setting device 15 is used. For this setting parameter value, for example, different values are prepared depending on the value of the target nitriding potential.
  • the gas flow rate output adjusting means 30 controls the amount of each introduced gas in the furnace as a result of the PID control. Specifically, the gas flow rate output adjusting means 30 determines the flow rate of each gas, and the output value is transmitted to the gas introduction amount control means 14.
  • the gas introduction amount control means 14 sends control signals to the first supply amount control device 22 for ammonia gas and the second supply amount control device 26 for ammonia decomposition gas in order to realize the introduction amount of each gas.
  • the in-furnace nitriding potential can be controlled stably near the target nitriding potential.
  • the nitrification process of the object S can be performed with extremely high quality.
  • FIGS. 9A and 9B An example of the above control is shown in FIGS. 9A and 9B.
  • the amount of ammonia decomposed gas introduced into the furnace is constant, and the amount of ammonia gas introduced into the furnace is feedback-controlled little by little in the vicinity of 40 (l / min).
  • the nitriding potential is controlled to 0.17 with high accuracy.
  • the cooling process after the nitrification process in the manufacturing apparatus 1.
  • the workpiece S is removed from the furnace while the heating temperature is maintained after the nitrification processing in the manufacturing apparatus 1. It is necessary to transport it to a quenching device (for example, an oil tank) and then quench it.
  • a quenching device for example, an oil tank
  • the austenite structure stabilized by 1.0% or more nitrogen becomes brownite (a lamellar structure of ferrite phase and ⁇ 'phase) when the cooling rate is slow, and the hardness and the fatigue strength decrease.
  • brownite a lamellar structure of ferrite phase and ⁇ 'phase
  • the oil cooling it is possible to sufficiently maintain the austenite structure in the case of general parts.
  • the residual stress on the surface is measured by an X-ray residual stress measuring method by the sin 2 ⁇ method with respect to the parallel part (RD direction) of the test piece, using a minute part X-ray residual stress measuring device (AutoMATE manufactured by Rigaku Co., Ltd.). Was done. More specifically, it was performed under the conditions shown in Table 1.
  • the stress constant of the ⁇ phase was ⁇ 611 MPa / deg.
  • Example 1 the treatment temperature was 640 ° C., the nitriding potential was 0.12, and the treatment time was 2 hours. As a result, a hardened layer having an austenite structure was obtained on the surface with a thickness of 22 ⁇ m.
  • the difference ( ⁇ HV) between the hardness of the diffusion layer at a depth of 100 ⁇ m from the surface and the hardness at a depth of 2 mm from the surface was 116 HV (> 100 HV). Also, the fatigue strength performance was sufficient.
  • Example 2 the treatment temperature was 640 ° C., the nitriding potential was 0.13, and the treatment time was 2 hours.
  • a compound layer having a ⁇ ′ phase (60% or more in volume ratio) on the surface was 2 ⁇ m, and a hardened layer having an austenite structure was 22 ⁇ m below the compound layer.
  • the difference ( ⁇ HV) between the hardness of the diffusion layer at a depth of 100 ⁇ m from the surface and the hardness at a depth of 2 mm from the surface was 112 HV (> 100 HV). Also, the fatigue strength performance was sufficient.
  • Example 3 after the nitrification treatment at the treatment temperature of 640 ° C., the nitriding potential of 0.17, and the treatment time of 2 hours, the oil cooling was performed. As a result, a compound layer having an ⁇ -phase (60% or more by volume ratio) on the surface was obtained in a thickness of 12 ⁇ m, and a hardened layer having an austenite structure was obtained in a thickness of 20 ⁇ mm below the compound layer.
  • the difference ( ⁇ HV) between the hardness of the diffusion layer at a depth of 100 ⁇ m from the surface and the hardness at a depth of 2 mm from the surface was 116 HV (> 100 HV). Further, the residual stress value on the surface was -200 MPa, and the fatigue strength was sufficient.
  • Example 4 after the nitrification treatment at the treatment temperature of 640 ° C., the nitriding potential of 0.22, and the treatment time of 2 hours, the oil cooling was performed. As a result, a compound layer having an ⁇ -phase (60% or more by volume ratio) on the surface was obtained in a thickness of 21 ⁇ m, and a hardened layer having an austenite structure was formed thereunder with a thickness of 13 ⁇ m.
  • the difference ( ⁇ HV) between the hardness of the diffusion layer at a depth of 100 ⁇ m from the surface and the hardness at a depth of 2 mm from the surface was 112 HV (> 100 HV). Further, the surface residual stress value was ⁇ 311 MPa, and the fatigue strength performance was also sufficient.
  • Example 5 the treatment temperature was 640 ° C., the nitriding potential was 0.3, and the treatment time was 2 hours. As a result, a compound layer having an ⁇ -phase (60% or more by volume) on the surface was obtained in a thickness of 30 ⁇ m, and a hardened layer having an austenite structure in a thickness of 10 ⁇ mm was obtained under the compound layer.
  • the difference ( ⁇ HV) between the hardness of the diffusion layer at a depth of 100 ⁇ m from the surface and the hardness at a depth of 2 mm from the surface was 115 HV (> 100 HV). Further, the surface residual stress value was -438 MPa, and the fatigue strength performance was also sufficient.
  • the treatment temperature was 640 ° C.
  • the nitriding potential was 0.17
  • the treatment time was 2 hours.
  • oil cooling was performed, and further, reheating treatment was performed at 250 ° C. for 2 hours.
  • a compound layer having an ⁇ phase on the surface side (a mixture of ⁇ ′ phases was also observed) was 11 ⁇ m, and a hardened layer having an austenite structure was formed in a thickness of 18 ⁇ m below the compound layer.
  • the difference ( ⁇ HV) between the hardness of the diffusion layer at a depth of 100 ⁇ m from the surface and the hardness at a depth of 2 mm from the surface is 94 HV ( ⁇ 100 HV), and the residual stress value on the surface is 4 MPa (> -200 MPa). Yes (there was a tensile residual stress), and the fatigue strength performance was insufficient as compared with each example.
  • the treatment temperature was 640 ° C.
  • the nitriding potential was 0.17
  • the treatment time was 2 hours
  • the oil cooling was performed
  • the reheating treatment was further performed at 200 ° C. for 1 hour.
  • a compound layer having an ⁇ phase on the surface side (a mixture of ⁇ ′ phases was also observed) was 12 ⁇ m
  • a hardened layer having an austenite structure was formed in a thickness of 19 ⁇ m below the compound layer.
  • the difference ( ⁇ HV) between the hardness of the diffusion layer at a depth of 100 ⁇ m from the surface and the hardness at a depth of 2 mm from the surface was 102 HV (> 100 HV), but the residual stress value on the surface was ⁇ 59 MPa (> ⁇ 200 MPa), and the fatigue strength performance was insufficient as compared with each example.
  • the treatment temperature was 700 ° C. (> 660 ° C.)
  • the nitriding potential was 0.1
  • the treatment time was 1.5 hours
  • the oil cooling was performed, followed by the reheating treatment at 280 ° C. for 2 hours. Carried out.
  • a hardened layer having a nitrogen martensite structure (not an austenite structure) was obtained on the surface in a thickness of 40 ⁇ m.
  • the difference ( ⁇ HV) between the hardness of the diffusion layer at a depth of 100 ⁇ m from the surface and the hardness at a depth of 2 mm from the surface was 20 HV ( ⁇ 100 HV).
  • the fatigue strength performance was insufficient.
  • the treatment temperature was 570 ° C. ( ⁇ 610 ° C.)
  • the nitriding potential was 0.25
  • the treatment time was 3.5 hours.
  • a 10 ⁇ m ⁇ ′ phase-rich compound layer was obtained on the surface, but a layer corresponding to the cured layer was not obtained.
  • the difference ( ⁇ HV) between the hardness of the diffusion layer at a depth of 100 ⁇ m from the surface and the hardness at a depth of 2 mm from the surface was 129 HV (> 100 HV), but the fatigue strength performance was compared with each example. Was insufficient.

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Abstract

L'invention concerne un élément en acier nitruré qui a une matrice qui est formée d'un acier au carbone ou d'un acier faiblement allié, et qui est caractérisé en ce qu'il a une couche durcie dans la surface, ladite couche durcie ayant une structure austénitique qui contient 1,0 % en masse ou plus d'azote, et en ce qu'il y a une couche de diffusion en dessous de la couche durcie, ladite couche de diffusion étant obtenue par diffusion d'azote dans la matrice. Cet élément en acier nitruré est également caractérisé en ce que : la couche durcie a une épaisseur de 2 à 50 µm à partir de la surface de l'élément en acier nitruré ; la couche de diffusion s'étend jusqu'à une profondeur supérieure à 100 µm à partir de la surface de l'élément en acier nitruré ; et la dureté de la couche de diffusion à une profondeur de 100 µm à partir de la surface de l'élément en acier nitruré est supérieure à la dureté à une profondeur de 2 mm à partir de la surface de l'élément en acier nitruré de 100 HV ou plus.
PCT/JP2019/042887 2018-11-02 2019-10-31 Élément en acier nitruré, et procédé et appareil pour produire un élément en acier nitruré WO2020090999A1 (fr)

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