EP1550736A1 - Element carbure et trempe et son procede de production - Google Patents

Element carbure et trempe et son procede de production Download PDF

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Publication number
EP1550736A1
EP1550736A1 EP02790874A EP02790874A EP1550736A1 EP 1550736 A1 EP1550736 A1 EP 1550736A1 EP 02790874 A EP02790874 A EP 02790874A EP 02790874 A EP02790874 A EP 02790874A EP 1550736 A1 EP1550736 A1 EP 1550736A1
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Prior art keywords
carburized
quenching
hardened member
production method
hardened
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German (de)
English (en)
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EP1550736A4 (fr
Inventor
Takao Aisin Aw Co. Ltd. TANIGUCHI
Kazumasa Aisin Aw Co. Ltd. Tsukamoto
Koji AISIN AW CO. LTD. OOBAYASHI
Tomoki DAIDO STEEL CO. LTD. R&D Lab. HANYUDA
Y. DAIDO STEEL CO. LTD. R&D Lab. KUREBAYASHI
H. Nippon Steel Co. Muroranworks Kanisawa
Seiji NIPPON STEEL CO. MURORAN WORKS ITOH
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Aisin AW Co Ltd
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Aisin AW Co Ltd
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    • 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
    • C23C8/20—Carburising
    • C23C8/22—Carburising 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
    • C23C8/80—After-treatment

Definitions

  • the present invention relates to a carburized and hardened member that is excellent in fatigue strength and dimensional accuracy, and a production method for the member.
  • carburized and hardened members subjected to a carburizing and quenching process are often used in order to increase the surface hardness and the toughness.
  • Conventional carburized and hardened members are normally produced by forming a case hardening steel (JIS: SCM420H, SCR420H, SNCM220) or the like into a desired shape, and then gas-carburizing the steel in a carburizing atmosphere, and then quenching it in an oil or the like.
  • JIS Japanese Industrial Standard SCM420H, SCR420H, SNCM220
  • One of goals regarding the carburized and hardened members is to further improve the post-carburizing and quenching process strength and, at the same time, improve the dimensional accuracy by reducing or suppressing the hardening strain.
  • the present invention has been accomplished in view of the aforementioned problems of the conventional art. It is an object of the present invention to provide a carburized and hardened member that allows strength enhancement while sufficiently reducing the hardening strain, and a production method for the carburized and hardened member.
  • a first aspect of the present invention is a carburized and hardened member production method characterized in: that an alloy steel which contains Fe as a main component and contains 0.10 to 0.50 wt.% of C and 0.50 to 1.50 wt.% of Si and whose hardenability J based on an end quenching test is in a range of 35 to 50 (at 12.5 mm) is used as a raw material; and that after the material is formed into a member of a desired shape, a carburized layer is formed by performing a carburizing process in an oxidation inhibitive atmosphere; and that after the carburizing process, a quenching process is performed in such a condition that cooling is monotonously performed from a pearlite transformation point (A1 point) to a martensite transformation start point (Ms point), and such a condition that a severity of quenching H is in a range of 0.01 to 0.08 (cm -1 ).
  • the aforementioned hardenability J based on an end quenching test is a value acquired by an end quenching test method prescribed in JIS: G0561 (generally termed "Jominy end quench test method"). Furthermore, the indication of (at 12.5 mm) means that the value of hardenability J is a value of hardenability J regarding a position of 12.5 mm from the water cool-side end surface of a rod-like test piece in the Jominy end quench test method.
  • a specific alloy of which the C content and the Si content and the hardenability J are within the specific ranges is used as a raw material.
  • the quenching process is performed so as to fulfill the aforementioned conditions of monotonous cooling and the aforementioned condition of specific severity of quenching H. That is, only after the material characteristics and the production conditions are fulfilled, it becomes possible to provide a carburized and hardened member in which the strength is enhanced while the hardening strain is sufficiently reduced.
  • the setting of the C content within the range of 0.1 to 0.50 wt.% makes it possible to secure an appropriate toughness and an appropriate strength of a non-carburized portion (internal portion) after the carburizing and quenching process. If the C content is less than 0.1 wt.%, the aforementioned effect is not sufficient. If the C content exceeds 0.50 wt.%, the pre-quenching hardness becomes excessively high, thus creating a possibility of increased processing cost and reduced toughness. Furthermore, due to increased structural transformation rate of the interior of the non-carburized portion following the carburizing and quenching process, transformation stress increases, and due to great quenching strain, the component part accuracy may degrade.
  • the member positively contains Si as a component, and the content thereof is 0.50 to 1.50 wt.%.
  • the carburizing process is performed in an oxidation inhibitive atmosphere. Therefore, it becomes possible to achieve improved plane fatigue strength, improved hardenability, improved resistance to temper softening, etc. while reducing the intergranular oxidation, which is likely to occur at the time of the carburizing process.
  • the Si content is less than 0.50 wt.%, the aforementioned improvement effect is small; in particular, there is a problem of reduction of intergranular oxidation preventative effect at the time of the carburizing process. Conversely, if the Si content is greater than 1.50 wt.%, the improvement effect becomes saturated, and uniform austenitization prior to quenching is difficult. In order to prevent or curb degradations in the plastic processability, the cutting processability and the formability of the material, it is preferable that the Si content be less than or equal to 0.70 wt.%. Therefore, a preferable range of the Si content is a range greater than 0.50 wt.% and less than or equal to 0.70 wt.%.
  • the hardenability J of the material is limited within the range of 35 to 50 (at 12.5 mm). Therefore, excellent hardening effect can be achieved even if the range of the severity of quenching H is limited to the aforementioned range. If the hardenability J is less than 35, it becomes impossible to achieve sufficient hardening effect on the carburized layer and the non-carburized portion (internal portion) in the quenching process following the carburizing process, and it is therefore impossible to achieve a desired strength enhancement. Therefore, it is preferable that the hardenability J be greater than or equal to 38. If the hardenability J exceeds 50, the structural transformation rate of the internal portion, that is, the non-carburized portion, rises, so that the transformation stress increases and the hardening strain becomes more likely.
  • hardenability J If the hardenability J is higher, the hardness prior to the carburizing and quenching process is correspondingly higher, so that processability, such as the plastic processability prior to the carburizing process, the cutting processability, etc., degrades. Therefore, in order to prevent such degradation of workability, it is preferable that hardenability J be less than or equal to 45.
  • the severity of quenching H is limited within the range of 0.01 to 0.08 (cm -1 ). If the alloy having the specific amount of carbon and having the hardenability is used, it becomes possible to substantially prevent or reduce the hardenability strain at the time of hardening process and therefore secure excellent dimensional accuracy.
  • the severity of quenching H is less than 0.01 (cm -1 ), it is impossible to achieve sufficient hardening effect on the carburized layer and the non-carburized portion (internal portion) in a hardening process following the carburizing process as in the case where the hardenability J is less than 35. Therefore, desired strength enhancement cannot be accomplished. If the severity of quenching H is greater than 0.08 (cm -1 ), the transformation stress increases due to, particularly, increased structural transformation rate of the internal portion, that is, the non-carburized portion, and therefore the hardening strain is likely to occur, as in the case where the hardenability J is greater than 50.
  • the quenching process is performed under the condition that the cooling monotonously occurs from the A1 point to the Ms point, in addition to the condition of the range of severity of quenching H.
  • the term "monotonously" herein means that re-heating is not performed during the cooling process, that is, there is no rise of the material temperature during the cooling. Therefore, examples of the case where the condition of monotonous cooling is fulfilled include a case where the material temperature continues to fall, and a case where if the temperature stops falling during the process, the temperature remains constant and never rises, and then starts falling again. Furthermore, changes in the cooling rate are allowable.
  • the monotonous cooling condition it is possible to select a cooling condition such that the cooling does not enter a region of a nose of an S curve indicated in an isothermal transformation diagram within the carburized portion. This selection secures sufficient martensite transformation.
  • the present invention provides a carburized and hardened member in which the strength is enhanced while the hardening strain is sufficiently reduced, as the invention comprises the aforementioned C content, the Si content, the hardenability J, the carburizing process in an oxidation inhibitive atmosphere, and the quenching process that fulfills the condition of the monotonous cooling and the condition of the specific severity of quenching H. If any one of these elements is absent, the intended object cannot be achieved.
  • the present inventors have discovered this through many experiments.
  • a second aspect of the present invention is a carburized and hardened member produced by the above-described production method, characterized in that a surface hardness of the carburized layer is in a range of 700 to 900 Hv, and an internal hardness of a non-carburized portion located inward of the carburized layer is in a range of 250 to 450 Hv.
  • This carburized and hardened member is produced by adopting the above-described production method and by adjusting the component range processing condition so as to restrict the surface hardness of the carburized layer and the internal hardness of the non-carburized portion within the aforementioned ranges. Therefore, it becomes possible to secure a static strength (tensile strength, flexural strength, torsional strength, etc.) and a dynamic strength (plane fatigue strength, bending fatigue strength, torsion fatigue strength, etc.) in a region from the surface to the internal portion (core portion), with respect to the distribution of stress applied to the member which results from the operating stress caused on the member by load applied to the member and the stress concentrated adjacent to the surface of the member due to bumps and dips, holes, etc. of the member.
  • a static strength tensile strength, flexural strength, torsional strength, etc.
  • a dynamic strength plane fatigue strength, bending fatigue strength, torsion fatigue strength, etc.
  • the surface hardness of the carburized layer is less than 700 Hv, a conceivable problem is that strength cannot be secure corresponding to the stress concentration adjacent to the surfaces of the member. Another conceivable problem is insufficient abrasion resistance in outermost surface. If the surface hardness is greater than 900 Hv, production of carbide, such as cementite and the like, in the surface layer is conceivable. Therefore, a conceivable problem is insufficient strength and, more particularly, reduced toughness.
  • the internal hardness of the non-carburized portion is less than 250 Hv, the problem of insufficient strength and, more particularly, insufficient static strength, can be considered. If the internal hardness is greater than 450 Hv, the following problem is possible, taking the rate of transformation of structure into consideration. That is, when a hardening process is performed so as to secure 450 Hv, a great transformation stress occurs, which causes a great hardening strain and therefore makes a factor of degradation in component parts accuracy.
  • the carburizing process be performed in a reduced-pressure atmosphere having a reduced pressure of 1 to 30 hPa. Therefore, it becomes possible to easily provide the oxidation inhibitive atmosphere through pressure reduction, and therefore sufficiently prevent intergranular oxidation at the time of carburization.
  • the value of the reduced pressure of the reduced-pressure atmosphere being less than 1 hPa is excessive for substantial prevention of oxidation. If such value of the reduced pressure is required, the device for the pressure reduction needs to have high capability for pressure reduction, and creates a problem of cost increase. If the value of the reduced pressure is higher than 30 hPa, the oxidation preventing effect degrades, and furthermore, other problems, such as production of soot in the carburizing furnace, and the like, occur.
  • the carburizing process be performed in an atmosphere containing an inert gas as a main component.
  • an inert gas examples include nitrogen gas, argon gas, etc.
  • the carburizing process be performed so that a surface carbon amount in the carburized layer becomes 0.6 to 1.5 wt.% (claim 4).
  • the surface carbon concentration in the carburized layer affects the surface hardness of the carburized and hardened member. If the surface carbon amount in the carburized layer is less than 0.6 wt.%, there occurs a problem of insufficient surface hardness. If the surface carbon amount is greater than 1.5 wt.%, the precipitation of carbide becomes great so that the hardenability of the base remarkably degrades and the surface hardness becomes insufficient.
  • intergranular oxidation progressing from a surface of the raw material be at most 3 ⁇ m. That is, it is preferable to restrict the intergranular oxidation to 3 ⁇ m or less from the surface by adjusting the oxidation inhibitive atmosphere, the heating temperature, the heating time, etc., at the time of carburization.
  • the intergranular strength decreases if an intergranular oxide (portion) is produced. Therefore, if intergranular oxidation reaches a depth beyond 3 ⁇ m, there is a danger of reduced abrasion resistance due to insufficient strength of the member, reduced hardness, etc. Furthermore, at the time of intergranular oxidation, surrounding alloy elements are also taken up into the intergranular oxide due to chemical reactions. Therefore, the hardenability-improving elements in the carburized and hardened layer around intergranular oxides are taken up and consumed by the intergranular oxides, thereby forming regions where additives are depleted, around the intergranular oxide layer. Therefore, the hardenability of the carburized and hardened layer becomes insufficient. Hence, there is a danger of causing insufficient hardness and insufficient strength.
  • the raw material have a surface compression residual stress of 300 to 800 MPa. That is, it is preferable to set the surface compression residual stress to at least 300 MPa by adjusting the composition of the raw material, the oxidation inhibitive atmosphere for the carburization, the heating temperature, the heating time, etc. Therefore, the tensile stress near the surface can be reduced by the compression residual stress near the surface of the member. In particular, the dynamic strength (planer fatigue strength, bending fatigue strength, torsional fatigue strength) can be improved. If the surface compression residual stress is greater than 800 MPa, it is necessary to increase the cooling rate during the quenching process beyond a limit in order to increase the amount of martensite. Therefore, great hardening strain occurs, and therefore a dimensional accuracy of the member cannot be secured.
  • the surface compression residual stress can be produced by forming the martensite via the quenching process of the carburized layer, and creating a compression stress field due to volume expansion involved in the transformation.
  • the amount of martensite produced is small, that is, if the amount of retained austenite is great, or if the troostite structure is great in amount, it is impossible to form a sufficient compression residual stress field. Therefore, the reduction of the retained austenite (specifically, to 25% or less) and the reduction of the troostite structure (specifically, to 10% or less) are effective in view of enhancement of compression residual stress effect.
  • the absorption of volume expansion at the time of martensite transformation does not considerably contribute to enhancement of the surface compression residual stress if the amount of martensite is small.
  • the compression residual stress can be increased by performing a surface process, such as shot peening, after the quenching process.
  • a surface process such as shot peening
  • turning the retained austenite into martensite by the shot peening process is more advantageous in increasing the compression residual stress.
  • quenching be performed with the severity of quenching H being in said range during a transition from a temperature in an austenite region to 300°C. Therefore, sufficient quenching effect can be achieved. If the severity of quenching H in a cooling process from the temperature of the austenite region to 300°C is less than 0.01 (cm -1 ), the quenching will be insufficient. Thus, desired hardened structure and characteristic cannot be achieved, and the strength of the member will be insufficient. If the severity of quenching H in a cooling process from the temperature of the austenite region to 300°C is greater than 0.08 (cm -1 ), the quenching will be excessive, so that the structure transformation stress and the thermal stress will increase. Therefore, there is a possibility of increased hardening strain and degraded component part accuracy.
  • quenching be accomplished by gas cooling. Therefore, it becomes relatively easy to secure the aforementioned severity of quenching H.
  • the quenching by gas cooling use an inert gas. Therefore, a safety can be secured during the quenching.
  • the inert gas be a nitrogen gas.
  • the adoption of nitrogen gas as the aforementioned inert gas is preferable in view of cost, ease of handling, availability at the time of mass-production operation, etc.
  • a retained austenite area rate of the carburized layer preferably is at most 25%. If the retained austenite area rate is greater than 25%, structural transformation from retained austenite into martensite occurs in association with changes in temperature and operating stress during a working process after the carburizing and quenching process, or during the use of the member. Due to the stress of the transformation, strain occurs, and the component parts accuracy will likely degrade. It is more preferable that the retained austenite area rate be 20% or less.
  • the retained austenite area rate can be reduced by other manners. For example, the area rate can be reduced by forcibly turning the retained austenite into martensite via shot peening or the like.
  • a troostite structure area rate of a surface layer of the carburized layer be at most 10%.
  • the troostite is a slack-quenched structure formed in the carburized layer after the carburizing and quenching process, and has a low hardness. Therefore, if the troostite structure area rate is greater than 10%, low-strength troostite will reduce the strength of the component part.
  • an internal structure of the carburized and hardened member be bainite. More specifically, it is desirable that the area rate of bainite in a sectional structure be at least 50%. Unlike the case of martensite, transformation of bainite progresses while iron atoms forming a lattice partially diffuse. Therefore, the strain associated with transformation is less in bainite than in martensite. Furthermore, bainite has a greater hardness than pearlite, which is produced if the cooling rate is lower. Thus, bainite appropriately enhances the strength of the internal non-carburized layer.
  • the carburized and hardened member be a carburized toothed gear.
  • the toothed gears require various strict conditions. The excellent characteristics achieved by the above-described production method are very effective for the toothed gears.
  • Step 11 to Steel 14 having chemical compositions shown in Table 1, after being melt-formed in an arc furnace, were hot-rolled into round bars having a diameter of 150 mm and a diameter of 32 mm.
  • the round bars were normalized by keeping them at 925°C for an hour and then air-cooling them.
  • Steel 11 and Steel 12 are steel grades having new compositions developed in the example.
  • Steel 13 and Steel 14 are steel grades corresponding to case hardening steels SCM420 and SNCM 815 according to JIS.
  • a hardenability J was determined by conducting a Jominy end quenching method according to JIS: G0561.
  • Results are shown in Table 1. This characteristic is a characteristic of a raw material irrelevant to the production method described below.
  • Steel grade Component element (wt%) Hardenability J C Si Mn S Ni Cr Mo B Ti Mb A1 N 11 0.16 0.56 0.38 0.012 0.96 1.47 0.01 0.0022 0.044 0.05 0.013 0.006 38 12 0.18 0.75 0.35 0.009 0.71 2.22 0.01 0.0018 0.035 0.03 0.019 0.005 42 13 0.2 0.21 0.78 0.011 0.02 1.01 0.17 - - - 0.027 0.015 25 14 0.15 0.25 0.47 0.009 4.34 0.83 0.27 - - - 0.04 0.018 37
  • Steels 11 and 12 are alloy steels that are applicable as a raw material in the present invention in view of material quality and hardenability J.
  • the hardenability J and the Si content are outside their respective ranges according to the present invention.
  • the Si content is outside the range according the present invention.
  • Steels 11 to 14 were formed into round bar test pieces (not shown) of 25 mm in diameter and 50 mm in length, and were also formed into rotating bending fatigue test pieces 1 having a shape as shown in FIG 1.
  • test spur gears 4 Normalized materials of 150 mm in diameter were machined into test spur gears 4 having a pitch radius of 54 mm, 27 teeth, a module of 4, a facewidth of 9 mm, a shaft hole radius of 35 mm (an equivalent round bar diameter of 10.5 mm ⁇ ) as shown in FIG 2.
  • test pieces and the gears produced from Steels 11, 12 and 14 were subjected to low-pressure carburization (vacuum carburization) and gas quenching under the conditions of "Process 1" shown in Table 2.
  • test pieces produced from Steel 13 were gas-carburized and oil-quenched under the conditions of "Process 2" shown in Table 3.
  • the severity of quenching H after the carburization is 0.05 (cm -1 ) as shown in Table 2, and the elements of the production method of the present invention are included.
  • test pieces prepared as described above were subjected to the following tests.
  • a hardness distribution (internal hardness) of a cross section was investigated using a Vickers hardness meter.
  • the surface layer hardness (surface hardness) of each carburized member was measured at a position of 0.02 mm from the surface.
  • the troostite area rate was measured by image analysis of scanning electron micrographs.
  • the surface carbon concentration was measured at a position of 50 ⁇ m from the surface via an X-ray macroanalyzer.
  • the retained austenite area rate was measured at a surface of the member using a Co-K ⁇ ray in an X-ray diffraction apparatus.
  • the surface residual stress was measured by a half value breadth midpoint method, using an Fe-K ⁇ ray in an X-ray stress meter.
  • Process 1 Step Temperature Time Atmosphere Pressure Severity of Quenching H Carburizing 930°C 2 h Acetylene 20 mbar - Diffusion 930°C 1 h Acetylene 20 mbar - Thermal uniforming 850°C 0.5 h Acetylene 20 mbar - Quenching - - Nitrogen 8 bar 0.05 cm -1 Tempering 150°C 2 h Atmosphere Atmospheric - Process 2 Step Temperature Time Atmosphere Pressure Severity of Quenching H Carburizing 930°C 3 h Mixed gas of CO, H 2 , N 2 , etc.
  • the specimen "Steel 13 + Process 2" had a lower surface layer hardness and a lower central portion hardness than any one of the specimens "Steel 11, 12 + Process 1".
  • the specimen "Steel 14 + Process 1" had a surface layer hardness and a central portion hardness that are approximately equal to those of the specimens "Steel 11, 12 + Process 1", but had a greater retained austenite area rate and a smaller surface residual stress. Correspondingly, the member was inferior in the plane fatigue strength.
  • the gear accuracy and the dimensional accuracy were evaluated as described below.
  • Tooth space heights were measured all round the circumference of each gear, and a value obtained by subtracting a minimum value from a maximum value was determined as a tooth space runout.
  • a ball was placed in two tooth spaces of gears facing each other, and an outer periphery thereof was measured via a dedicated OBD measuring device.
  • OBD measurement circumferential directions were two perpendicular directions (X, Y), and upper, intermediate and lower sites (three sites) (A, B, C) were defined in the direction of facewidth, as indicated in FIGS. 2a and 2b.
  • OBD ellipse an absolute value of the difference in OBD in the two perpendicular directions was determined.
  • As an OBD taper a difference between an upper OBD and a lower OBD in the direction of facewidth was determined.
  • the alloy steel it is appropriate to make a setting such that the alloy steel contains Fe as a main component and, as subsidiary components, 0.12 to 0.22 wt.% of C, 0.5 to 1.5 wt.% of Si, 0.25 to 0.45 wt.% of Mn, 0.5 to 1.5 wt.% ofNi, 1.3 to 2.3 wt.% of Cr, 0.001 to 0.003 wt.% of B, 0.02 to 0.06 wt.% of Ti, 0.02 to 0.12 wt.% ofNb, and 0.005 to 0.05 wt.% of Al.
  • N 106 ⁇ C(wt.%) + 10.8 ⁇ Si(wt.%) + 19.9 ⁇ Mn(wt.%) + 16.7 ⁇ Ni(wt.%) + 8.55 ⁇ Cr(wt.%) + 45.5 ⁇ Mo(wt.%) + 28
  • N is 87.6 and 93.4, respectively, whereas in Steel Grades 13, 14, not included in the present invention in terms of the ranges of components, N is greater than 95. If N is greater than 95, the hardness of the steel in the rolled state or the hardness of the steel in the normalized state remarkably increases, so that neither required machine workability nor required cold workability can be achieved. Therefore, if productivity is highly valued, it is necessary to control the composition of the steel so that the component parameter N is less than or equal to 95.
  • the composition of the steel is set so that no ferrite is produced in a range of cooling rate greater than or equal to 12°C/sec. (hereinafter, referred to as "upper limit cooling rate), in order to ensure that the sufficient hardening of the carburized layer can be achieved even by gas cooling. If ferrite is produced although the cooling rate is greater than or equal to 12°C/sec., it is impossible to accomplish the sufficient production of martensite in the carburized layer by gas cooling, leading to insufficient hardness.
  • the composition of the steel is set so that if the cooling rate is less than or equal to 0.1°C/sec., no bainite is produced. If bainite is produced even though the cooling rate is less than or equal to 0.1°C/sec., the hardening reaches the internal layer portion, which is not affected by the carburized layer. Thus, strain increases.
  • the setting is made so that no bainite is produced if the cooling rate is less than 0.1°C/sec., production of bainite is sufficiently prevented or reduced in an actual range of annealing cooling rate, so that a highly workable structure with a large amount of ferrite and pearlite can be provided. Therefore, if the rate of cooling from austenite is within a range corresponding to the annealing state, that is, a state where the material is air-cooled or let stand to cool, the material is provided with a hardness that is sufficiently low to improve the workability. Thus, the working prior to the carburizing and quenching process becomes easier.
  • an internal layer portion can be provided with a structure in which bainite is major if the cooling rate is set at 0.1 to 10°C/sec. It is particularly desirable to select such a composition that the cooling at 3°C/sec. will provide a structure mainly formed by bainite.
  • steels indicated in Table 6 (Steels 21 to 24 and Steels 31 to 38) were melted and formed into ingots, which were bloom-rolled and bar-rolled to produce round bars of 70 mm in diameter.
  • test pieces and the gears were processed separately by three different production methods (Processes 3 to 5).
  • Process 3 is characterized by gas carburization and oil quenching.
  • steel is carburized and quenched and then tempered in a carburizing gas atmosphere in the manner of heating at 930°C for 5 hours ⁇ diffusion at 850°C for 1 hour ⁇ oil-quenching at 130°C ⁇ tempering at 180°C for 1 hour.
  • the severity of quenching H in this case is 0.15 (cm -1 ).
  • Process 4" is characterized by vacuum carburization and gas cooling. In this process, steel is carburized and quenched and then tempered in the manner of heating at 930°C for 5 hours ⁇ diffusion at 850°C for 1 hour ⁇ nitrogen gas cooling ⁇ tempering at 180°C for 1 hour.
  • the severity of quenching H in this case is 0.05 (cm -1 ).
  • Process 5" is similar to Process 4, except that the nitrogen gas cooling in Process 4 is changed to oil quenching at 130°C.
  • the severity of quenching H in this case is 0.15 (cm -1 ).
  • test pieces and the gears processed by the above-described process were subjected to measurements, tests, and the like as in Example 1.
  • Steel Grades 31 to 34 had a slack quenched structure due to intergranular oxidation formation at the time of gas carburization, and therefore exhibited low surface hardness and low strengths. Furthermore, since oil cooling causes rapider quenching and greater non-uniformity in cooling than gas cooling, the variation in precision due to hardening strain increased.
  • each of Steel Grades 21 to 24 exhibited a high surface hardness and an appropriate value of internal hardness, and reduced strain. Thus, it is apparent that high strengths and low strains were achieved.
  • this example also indicates that it is possible to increase the strength while sufficiently reducing the hardening strain in the members if a specific alloy steel having a C content, an Si content and hardenability J within the aforementioned specific ranges is used as a raw material, and is subjected to a carburizing process in an oxidation inhibitive atmosphere, thereby forming a carburized layer, and then the steel is quenched under the condition of the specific severity of quenching H.
  • the alloy steel it is appropriate to make a setting such that the alloy steel contains Fe as a main component and, as subsidiary components, 0.1 to 0.5 wt.% of C, 0.5 to 1.0 wt.% of Si, 0.3 to 1.0 wt.% of Mn, 0.1 to 1.0 wt.% of Cr, 0.003 to 0.015 wt.% of P, 0.005 to 0.03 wt.% of S, 0.01 to 0 ⁇ 06 wt.% of Al, and 0.005 to 0.03 wt.% ofN, and at least one of 0.3 to 1.3 wt.% of Mo and 0.1 to 1.0 wt.% of Ni.
  • at least one species selected from the group consisting of at most 0.01% by weigh of Ca, at most 0.01% by weight of Mg, at most 0.05% by weight ofZr and at most 0.1% by weight of Te may be contained.

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EP2138591A1 (fr) * 2008-06-09 2009-12-30 Marco Masnari Utilisation d'un procédé de traitement sur des moulins à café et moulin à café ainsi traité
KR101185060B1 (ko) 2012-03-13 2012-09-21 동우에이치에스티 주식회사 자동변속기용 에뉼러스 기어 열처리 방법
EP2284287A4 (fr) * 2008-10-08 2015-05-20 Aisin Aw Co Procédé de production d une pièce cémentée et pièce d acier
US9617632B2 (en) 2012-01-20 2017-04-11 Swagelok Company Concurrent flow of activating gas in low temperature carburization
US10156006B2 (en) 2009-08-07 2018-12-18 Swagelok Company Low temperature carburization under soft vacuum

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WO2004059029A1 (fr) * 2002-12-25 2004-07-15 Aisin Aw Co., Ltd. Élément cémenté et trempé et son procédé de fabrication
DE102005061946B4 (de) * 2004-12-27 2013-03-21 Nippon Steel Corp. Einsatzgehärteter Stahl mit hervorragender Zahnoberflächendauerfestigkeit, diesen verwendendes Zahnrad, und Verfahren zur Herstellung desselben
WO2006118243A1 (fr) * 2005-04-28 2006-11-09 Aisin Aw Co., Ltd. Composant cemente trempe par induction
JP4876668B2 (ja) * 2006-03-29 2012-02-15 アイシン精機株式会社 鋼部材の熱処理方法
US7550048B2 (en) * 2006-12-15 2009-06-23 Tenneco Automotive Operating Company Inc. Method of manufacture using heat forming
DE102010048209C5 (de) 2010-10-15 2016-05-25 Benteler Automobiltechnik Gmbh Verfahren zur Herstellung eines warmumgeformten pressgehärteten Metallbauteils
CN102803539B (zh) * 2010-12-08 2014-12-03 新日铁住金株式会社 面疲劳强度优异的气体渗碳钢部件、气体渗碳用钢材以及气体渗碳钢部件的制造方法
US9389155B1 (en) * 2013-03-12 2016-07-12 United Technologies Corporation Fatigue test specimen
CN104384887A (zh) * 2014-09-19 2015-03-04 马鞍山邦斯科自动化科技有限公司 一种提高铰刀使用寿命的铰刀制造方法
JP6191630B2 (ja) * 2015-01-15 2017-09-06 トヨタ自動車株式会社 ワークの製造方法
CN105525252B (zh) * 2015-12-22 2017-12-22 中车戚墅堰机车车辆工艺研究所有限公司 一种盘类渗碳淬火齿轮的变形矫正方法及其专用工装
KR20180099877A (ko) 2016-03-08 2018-09-05 아이신에이더블류 가부시키가이샤 강 부품, 기어 부품 및 강 부품의 제조 방법

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Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP2138591A1 (fr) * 2008-06-09 2009-12-30 Marco Masnari Utilisation d'un procédé de traitement sur des moulins à café et moulin à café ainsi traité
EP2284287A4 (fr) * 2008-10-08 2015-05-20 Aisin Aw Co Procédé de production d une pièce cémentée et pièce d acier
US10156006B2 (en) 2009-08-07 2018-12-18 Swagelok Company Low temperature carburization under soft vacuum
US10934611B2 (en) 2009-08-07 2021-03-02 Swagelok Company Low temperature carburization under soft vacuum
US9617632B2 (en) 2012-01-20 2017-04-11 Swagelok Company Concurrent flow of activating gas in low temperature carburization
US10246766B2 (en) 2012-01-20 2019-04-02 Swagelok Company Concurrent flow of activating gas in low temperature carburization
US11035032B2 (en) 2012-01-20 2021-06-15 Swagelok Company Concurrent flow of activating gas in low temperature carburization
KR101185060B1 (ko) 2012-03-13 2012-09-21 동우에이치에스티 주식회사 자동변속기용 에뉼러스 기어 열처리 방법

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WO2003056054A1 (fr) 2003-07-10
US20050173026A1 (en) 2005-08-11
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EP1550736A4 (fr) 2005-07-06
JPWO2003056054A1 (ja) 2005-05-12
CN1539026A (zh) 2004-10-20

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