JP2005113168A - Steel component for machine structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel component for a machine structure, which is used in a power transmission section in an automatic transmission car using an automatic transmission oil, and has superior pitting resistance. <P>SOLUTION: The steel component for the machine structure has a core part containing, by mass% (hereafter, the same), 0.15-0.25% C, more than 0.35% but 0.7% or less Si, 0.2-1.0% Mn, 1.0-1.5% Cr, 0.30-0.6% Mo, 0.008-0.05% N, and 0.005-0.2% in total of one or more elements selected from the group consisting of Al, Nb and Ti; includes a carbide so as to occupy 5% or more by an area rate on the surface layer within 50μm deep from the steel surface; and has a solid lubrication coating on the steel surface. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a steel part for machine structure, and more particularly, to a steel part for machine structure that exhibits excellent pitting resistance when used in a power transmission part of an automatic vehicle using automatic transmission oil.

  For example, a mechanical structural steel part used in a power transmission section such as a transmission gear of an automobile easily causes so-called pitching damage in which a contact portion is peeled and damaged due to an increase in contact surface pressure. Therefore, as steel parts used for such applications, in order to suppress such damage, Cr case hardening steel and Cr-Mo case hardening steel specified by JIS G 4104, JIS G 4105, JIS G 4103, etc., After forming using Ni-Cr-Mo case-hardened steel, surface hardening treatment such as carburizing treatment or carbonitriding treatment is performed, and a solid lubricating film such as molybdenum disulfide is formed on the surface of the member. I came.

Patent Document 1 proposes to perform high carbon carburizing treatment to further prevent pitting damage and deposit fine carbides on the carburized hardened layer of the steel member.
JP 2002-212672 A

  However, in recent years, as the demand for higher stress in machine structural parts and the reduction in size and weight of steel parts for machine structural use has further increased, the load stress on steel parts has been increasing. It is difficult to achieve the desired pitching strength even when a solid lubricating film is applied to a steel member obtained by carburizing a hardened steel, Cr-Mo case-hardened steel, or Ni-Cr-Mo case-hardened steel. It is becoming.

  Further, the steel member disclosed in Patent Document 1 has a sufficiently high tempering hardness, which is considered to be a controlling factor of the pitching strength, but does not reach the level at which the pitching strength is desired. However, it is difficult to adapt to the severe use environment described above.

  By the way, in an automatic vehicle, an automatic transmission oil having a kinematic viscosity of 3 to 10 cSt at 100 ° C. and a relatively low viscosity is used in the power transmission unit, and the oil film formed on the surface of the steel parts constituting the power transmission unit is thin. In addition, a portion where an oil film is hardly formed also occurs locally. Therefore, the friction between the steel parts is likely to occur in such a place, and as a result, the temperature rises due to the frictional heat and the steel parts tend to soften, so that the pitching strength is likely to be lowered.

  The present invention has been made by paying attention to such circumstances, and its object is to achieve better pitching resistance when employed in a power transmission part of an automatic vehicle using an automatic transmission oil having a relatively low viscosity. An object of the present invention is to provide a mechanical structural steel part (hereinafter, simply referred to as a “steel part”) that exhibits its properties.

  The steel part for machine structure of the present invention that has solved the above-mentioned problems is a steel part for machine structure used in a power transmission part of an automatic vehicle using automatic transmission oil, and the core part is in mass% ( Hereinafter, the same), C: 0.15 to 0.25%, Si: more than 0.35 to 0.7%, Mn: 0.2 to 1.0%, Cr: 1.0 to 1.5%, Mo: 0.30 to 0.6%, N: 0.008 to 0.05%, and one or more selected from the group consisting of Al, Nb, and Ti in a total amount of 0.005 to 0.2 %, And the area ratio of carbide in the steel surface layer within a depth of 50 μm from the surface of the steel part is 5% or more when observed at a magnification of 8000 times with a scanning electron microscope, and the solid lubricating film on the surface of the steel part It is characterized in that is formed. As the solid lubricant film, molybdenum disulfide is preferably formed.

Further, the steel part for mechanical structure of the present invention has a surface density of carbide of 0.5 μm or less in diameter in the steel surface layer portion within a depth of 50 μm from the steel surface when observed with a scanning electron microscope at a magnification of 8000 times. and 6.0 units / 10 [mu] m ² or more, and preferably in which the ratio of the total carbides number accounted diameter 0.5μm number following the carbide in the steel surface layer portion within a depth 50μm from the steel part surface is 80% or more.

  Furthermore, if the amount of nitrogen at a depth of 50 μm is 0.07 to 0.2% from the surface of the steel part, an incompletely quenched structure can be obtained.

  Further, the steel part for machine structure of the present invention has, as another element, Ca: 0.0005 to 0.05%, Zr: 0.01 to 0.15%, Bi: 0.05% or less as the other element. (Excluding 0%), S: 0.12% or less (excluding 0%), Pb: 0.09% or less (excluding 0%), or Mg: 0.02% or less (0% If it is not included, it is preferable because machinability can be improved.

  According to the present invention, when used in a power transmission part of an automatic vehicle in which an automatic transmission oil having a kinematic viscosity at 100 ° C. of 3 to 10 cSt is used, a gear, a sliding part, which exhibits remarkably excellent pitting resistance, Steel parts for machine structures such as shafts can be provided.

  Under the circumstances described above, the present inventors have eagerly obtained a steel part for machine structural use that exhibits superior pitting resistance even when lubricating oil having a relatively low viscosity is used in the power transmission part of an automatic vehicle. I did research. As a result, it was found that the steel core portion satisfying the specified chemical composition should be formed, a large number of carbides should be precipitated on the steel surface layer portion, and a solid lubricating film should be formed on the surface of the steel portion. It was.

  Note that the steel part for machine structure of the present invention exhibits extremely excellent pitting resistance even when an automatic transmission oil having a relatively low viscosity is used in a power transmission part of an automatic vehicle. Machine structural steel parts are assumed to be used in this situation.

  Examples of the vehicle transmission oil include those that comply with the DEXRON standard and the MERCON standard. In particular, in a power transmission unit in which a lubricating oil having a kinematic viscosity of 3 to 10 cSt at 100 ° C. is used, the effect of the present invention is fully achieved. Demonstrated.

  Hereinafter, the reason why the size and number of the carbides and the chemical component composition are determined in the present invention will be described in detail.

  First, the steel part of the present invention is required to have carbides precipitated in an area ratio of 5% or more in a steel surface layer portion having a depth of 50 μm or less from the surface of the steel portion. Thus, by making carbide exist in the steel surface layer portion, the hardness of the steel part can be maintained even in a high temperature environment at the time of use, and high material strength can be secured. In order to obtain such an effect, it is necessary to deposit 5% or more of the carbide in the area ratio as described above, preferably 6% or more, more preferably 7% or more.

  When the amount of carbide precipitation is large, an incompletely hardened structure is generated in the steel matrix, which tends to cause a decrease in strength. Therefore, the carbide is preferably suppressed to an area ratio of 20% or less, more preferably 18% or less, and still more preferably 15% or less.

  The steel component of the present invention is further required to have a solid lubricating film formed on the steel surface. When a solid lubricant film is formed on the surface of a steel member on which carbide is deposited on the surface layer portion, when a solid lubricant film is not formed only by precipitation of carbide, or after a normal carburizing process is performed As a matter of course, an effect far exceeding these additive effects can be obtained. The reason can be considered as follows.

  When a steel part is used (power transmission), a high-temperature environment is generated due to frictional heat with the counterpart steel part, and the material strength of the steel part generally tends to decrease. However, in steel parts in which carbide is precipitated on the steel surface layer portion, high hardness is given by the carbide and the material strength can be maintained at a high level, and as a result, excellent pitting resistance can be ensured. However, if there is a part where the contact surface pressure is extremely high due to the meshing condition with the counterpart gear, etc., the effect of improving the pitting resistance by the above-mentioned carbide is not sufficiently exhibited, and the contact surface pressure is high. It is thought that the pitching life is shortened due to peeling damage from the location.

  On the other hand, the mechanism by which the pitting resistance is improved by the formation of the solid lubricating film is different from the case where the carbide is precipitated. The solid lubricant film formed on the surface of the steel part is worn during use and disappears in the initial stage of use, and the surface of the steel part of the steel part is also worn at the location where the film disappears. Such wear of the solid lubricating coating and the steel part occurs from a portion having a high contact surface pressure generated by meshing with the mating steel part, and as a result, the steel part has a shape along the shape of the mating steel part. This so-called “initial familiarity” is thought to improve the pitching resistance as a result of suppressing the subsequent local load.

  The steel part of the present invention is obtained by performing both the precipitation of the carbide and the formation of the solid lubricating film, but the effect is a phase by the precipitation of the carbide and the formation of the solid lubricating film as will be apparent from the examples described later. It is far more than the additive action.

  The reason for this has not been clarified yet, but when there is no portion with high contact surface pressure, the pitching resistance of the base steel member is considered to dominate the pitching resistance of the steel parts. When the steel parts of the invention are used, the hardness maintenance effect by the carbides is fully exerted in a state where the solid lubricating film is worn away and there is no part with extremely high contact surface pressure, and extremely excellent pitting resistance It is thought that it shows.

Examples of the solid lubricating coating include soft metals such as gold, silver, copper, zinc, and lead, metal oxides such as lead oxide, (double oxide such as CaF ₂ + BaF ₂ + Ag), molybdenum disulfide, Examples thereof include sulfides such as tungsten disulfide, nitrides such as fluoride and boron nitride, and graphite. Among these, a molybdenum disulfide film generally called “deflick coat” is particularly preferable because the above-described effects are easily obtained.

  When the film thickness of the solid lubricant film is too thin, the initial acclimation effect is small. Therefore, the film thickness is preferably 5 μm or more (more preferably 10 μm or more). On the other hand, if the film thickness is too thick, the initial conforming effect is saturated and the adhesiveness is deteriorated. Therefore, the thickness is preferably 30 μm or less, and more preferably 25 μm or less.

  When the influence of the size of the carbide on the characteristics of the steel part was also examined, if the carbide is coarse, the solid solution Cr in the steel easily dissolves in the carbide, and the solid solution Cr in the steel. It was found that the amount was reduced and the improvement in hardenability was suppressed. For this reason, when carburizing and quenching is performed on a steel material with a small amount of solute Cr, an incompletely quenched structure is likely to occur around the carbide, which tends to cause a reduction in strength. Further, fatigue failure due to coarse carbides is likely to be caused.

  Therefore, in order to improve the fatigue characteristics such as bending fatigue while reliably increasing the pitching strength, it is preferable to precipitate a large number of fine carbides on the steel surface layer. Specifically, the following requirements (a) and (b) It is recommended to satisfy.

(A) The surface density of carbides having a diameter of 0.5 μm or less in the steel surface layer within a depth of 50 μm from the steel part surface is 6.0 pieces / μm ² or more, more preferably 8.0 pieces / 10 μm ² or more. .

  (B) The ratio of the number of carbides having a diameter of 0.5 μm or less to the total number of carbides in the steel surface layer part within a depth of 50 μm from the steel part surface is 80% or more. This is because the bending fatigue strength of steel parts for machine structural use can be increased by reducing coarse carbides that cause fatigue failure with a diameter exceeding 0.5 μm. More preferably, it is 90% or more, More preferably, it is 95% or more.

  Furthermore, in order to ensure the hardenability around the carbide and suppress the generation of incompletely hardened structure, nitriding is performed at least part of the high-concentration carburizing process for precipitating carbide on the steel surface layer part, It is effective to increase the nitrogen amount at a depth of 50 μm from the steel part surface of the steel part to 0.07% or more. More preferably, it is 0.08% or more, More preferably, it is 0.09% or more. However, if the amount of nitrogen exceeds 0.2%, CrN is generated and hardenability is lowered, which is not preferable. The amount of nitrogen is more preferably 0.18% or less, and still more preferably 0.16% or less.

  In the present invention, if a steel material having an appropriate chemical component in the core is used, the above requirements can be easily satisfied, the pitting resistance can be improved reliably, and other characteristics to be provided as a steel part can be secured. Hereinafter, the reason for limitation of each component is demonstrated about the chemical component of the steel materials defined by this invention.

Cr: 1.0 to 1.5%
A particularly important element in this steel is Cr. Cr forms carbide in the carburized layer of the steel surface layer portion by high-concentration carburizing treatment, and this carbide increases the softening resistance of the steel portion and contributes to the improvement of the pitching strength. In order to exhibit such an effect effectively, the Cr content in the steel should be 1.0% or more, preferably 1.2% or more. However, if the Cr content is too high, the core becomes hard and the workability becomes poor. Therefore, the Cr content is suppressed to 1.5% or less. Preferably it is 1.45% or less.

C: 0.15-0.25%
C (carbon) is an element necessary for ensuring the core hardness of the steel part for machine structural use, and is contained in an amount of 0.15% or more, preferably 0.18% or more. On the other hand, if the amount of C is too large, the hardness becomes higher than necessary and the machinability and cold forgeability deteriorate, so it is suppressed to 0.25% or less, preferably 0.23% or less.

Si: more than 0.35 to 0.7%
Si is also one of the important elements in the present invention, and has the effect of refining carbides precipitated in the carburized layer of the steel surface layer portion and strengthening the steel matrix by solid solution strengthening these effects effectively. In order to exhibit it, it is contained more than 0.35%, preferably 0.4% or more, more preferably 0.45% or more. However, when the amount of Si becomes excessive, the hardness of the steel material becomes higher than necessary, and the machinability and cold forgeability deteriorate, so it is suppressed to 0.7% or less, preferably 0.6% or less.

Mn: 0.2 to 1.0%
Mn acts as a deoxidizer, reduces oxide inclusions and improves the internal quality of steel parts, improves hardenability and increases the core hardness and hardened layer depth of steel parts, Contributes to improvement. In order to sufficiently exhibit such an action, it is necessary to contain Mn in an amount of 0.2% or more, preferably 0.3% or more. On the other hand, when the amount of Mn becomes excessive, the amount of retained austenite in the steel surface layer portion increases and the hardness of the part surface decreases, so the amount of Mn is 1.0% or less, preferably 0.8% or less.

Mo: 0.30 to 0.6%
Mo is also an important element, and has the effect of promoting the formation of carbides, ensuring the hardenability of the matrix, and suppressing the formation of an incompletely quenched structure. In order to exhibit such an effect effectively, it is contained 0.30% or more, preferably 0.4% or more. On the other hand, if added over 0.6%, the hardness of the core becomes higher than necessary, and the machinability and cold forgeability at the time of machining deteriorate. Preferably it is 0.5% or less.

At least one selected from the group consisting of Al, Nb and Ti: 0.005 to 0.2% in total
Al, Nb, and Ti have the role of increasing the toughness of steel parts by forming nitrides to refine crystal grains. In order to exhibit such an action effectively, it is necessary to contain 0.005% or more in total of at least one selected from the group consisting of Al, Nb and Ti. A more preferable content is 0.010% or more. On the other hand, when the content of the elements is excessive, coarse compounds of these elements are generated, and the strength and cold workability of the parts are lowered. Accordingly, the total amount of these elements must be suppressed to 0.2% or less, and preferably 0.1% or less.

N: 0.008 to 0.05%
N is an element effective for making the crystal grains fine by increasing the above-described Al, Nb, Ti, and nitride, and improving the toughness of the steel part. In order to exhibit such an effect effectively, it is necessary to contain N amount 0.008% or more, preferably 0.010% or more. On the other hand, even if the amount of N becomes excessive, the above effect is only saturated, but rather the drawback that the core becomes brittle appears, so the upper limit is made 0.05%. Preferably it is 0.03% or less.

  The contained elements specified in the present invention are as described above, and the remaining component is substantially Fe, but as an unavoidable impurity brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc., 0.30% or less Cu, 0.25% or less of Ni, 0.002% or less of O (oxygen), etc. are allowed to be included, and within the range not adversely affecting the function of the present invention, As described above, it is possible to further contain other elements.

  That is, the steel part for machine structural use according to the present invention has, as another element, Ca: 0.0005 to 0.05%, Zr: 0.01 to 0.15%, Bi: 0.05% or less as the other element. (Excluding 0%), S: 0.12% or less (excluding 0%), Pb: 0.09% or less (excluding 0%), or Mg: 0.02% or less (0% (Not including) can improve machinability when a steel part is cut into a final part shape.

  By the way, if the content of these selected elements is less than the lower limit, the above-mentioned effects are not sufficiently exhibited, while the above-mentioned effects of each element are saturated near the upper limit, rather, a large amount of coarse composite inclusions are generated. , It may cause a failure such as bending fatigue strength and pitting resistance deterioration.

In order to obtain the steel part of the present invention that satisfies the above requirements, forging, cutting, carburizing treatment, high-concentration carburizing treatment, and solid lubricating coating treatment that forms a solid lubricating coating on the surface may be performed. It can manufacture by adopting the process of following (i)-(iv) using the rolling material obtained by carrying out hot rolling after doing.
(I) After hot forging the rolled material, machining is performed.
(Ii) A high-concentration carburizing treatment is performed to form a hardened surface layer on which the carbide is deposited on the steel part surface.
(Iii) Finish machining.
(Iv) After performing the ground treatment for forming the solid lubricating film, the solid lubricating film is formed.

  In particular, in order to precipitate the carbide specified in the present invention on the steel surface layer and to refine the carbide, the high-concentration carburizing treatment is performed after the carburizing treatment as in Examples described later. In the carburizing treatment and carbide precipitation treatment, it is preferable to adopt the following conditions.

<Carburizing conditions>
(1) Heating temperature (carburizing temperature)
In order to sufficiently carburize steel parts, it is preferable to set the carburizing temperature to 900 ° C. or more. If the temperature is lower than this, it is not preferable because the time required for carburizing becomes longer and the productivity is lowered. More preferably, it is 920 degreeC or more. On the other hand, even if the carburizing temperature is too high, the crystal grains become coarse and the toughness of the steel parts is inferior, so the carburizing temperature is preferably 980 ° C. or lower. More preferably, it is 950 degrees C or less. The holding time at this carburizing temperature (carburizing time) is not particularly limited and may be a time at which a desired carburized layer depth can be obtained, but in terms of cost, within a range of 1 hour to 10 hours. It is good to set appropriately.

(2) Heating atmosphere The heating atmosphere is preferably set to a carbon potential of 0.9% to 1.5%. If the carbon potential in this process is too low, the carbon concentration on the surface of the machine structural steel component will be low, and sufficient carbides cannot be deposited in the next carbide precipitation process, resulting in ensuring the desired pitching strength. It becomes difficult. More preferably, it is 1.0% or more. On the other hand, the upper limit of the carbon potential is not particularly limited, but if it is too high, soot called “sooting” adheres to the surface of the steel part and inhibits carburization. More preferably, it is 1.2% or less.

The carbon potential can be measured by a commonly used method such as a CO ₂ method using an O ₂ sensor method, an infrared analyzer, a dew point measurement method, or a carbon potentiometer using an iron wire, and is particularly called a Cp coil. The most excellent method in terms of measurement accuracy is to leave the iron wire in the furnace atmosphere and perform quantitative analysis by the infrared absorption method using this Cp coil.

(3) Cooling rate After holding for a certain period of time at the carburizing temperature, it is preferable to cool at a cooling rate of 10 ° C / min or more. As a reason, in order to precipitate fine carbides in the carbide precipitation step, which is the next step, after holding for a certain time at the carburizing temperature, the carbonized carbon is not precipitated, but once dissolved in supersaturated state, Ar ₁ point or less However, when the cooling rate is less than 10 ° C./min, there is a time allowance for the carbon in the supersaturated state to diffuse to the crystal grain boundaries, and during precipitation, the carbide precipitates in a network shape and causes pitting, etc. This is because various strengths are reduced. More preferably, it is 15 ° C./min or more. The upper limit of the cooling rate is not particularly limited, but deformation and cracking are likely to occur in the steel part even if it is cooled too rapidly, so the cooling rate is preferably 1200 ° C./min or less, more preferably 600 ° C. / Min or less.

<Carbide precipitation conditions>
As described above, by depositing fine carbides from the carburized layer in which carbon is dissolved in supersaturation in the carburizing step, the pitching strength of the steel part for machine structure can be improved. In addition, since the nitrogen potential can be set high while maintaining the heating temperature in the carbide precipitation step, it is preferable to perform nitridation for ensuring hardenability in the carbide precipitation step. Below, the conditions for carbide precipitation and nitriding are described in detail.

(1) Heating temperature Carbide precipitation is preferably performed within a temperature range of 800 ° C. or higher and 860 ° C. or lower. This is because if the heating temperature exceeds 860 ° C., the solid solubility limit of the carbon amount increases, and the amount of precipitated carbide decreases, more preferably 850 ° C. or less. On the other hand, when the heating temperature is less than 800 ° C., the precipitated carbide is excessively refined and the contribution to the improvement of the pitching strength is reduced, and more preferably 830 ° C. or more.

(2) Heating atmosphere In the carbide precipitation step, it is preferable to deposit carbide in an atmosphere having a carbon potential of 0.7% to 1.1%. This is because when the carbon potential is less than 0.7%, decarburization occurs while maintaining the heating temperature, and the amount of precipitated carbide decreases. A more preferable carbon potential is 0.75% or more. On the other hand, when the carbon potential exceeds 1.1%, the carburization progresses while the heating temperature is maintained, the carbides become coarse, and various strengths such as pitching are not preferable. More preferably, the carbon potential is 1.0% or less. The carbon potential can be measured by the method described above.

  The heating and holding time in this carbide precipitation step is not particularly limited, and may be a time for obtaining a desired carburized layer depth. However, when the component size increases and the holding time is 30 minutes or more. For this, the carbon potential is preferably 0.7% or more and less than or equal to the carbon concentration corresponding to the Acm point in the center. The carburized layer of the steel part while maintaining the heating temperature has a mixed structure of austenite and carbide consisting of the composition of the Acm point of the core steel, but in an atmosphere where the carbon potential exceeds the carbon concentration corresponding to the Acm point, This is because the carburizing atmosphere is strong against austenite in the structure, and the carbides are coarsened to reduce the mechanical strength of the steel parts. It has been found that such a tendency becomes more prominent as the part becomes larger and the heating and holding time becomes longer. It is more preferable that the carbon potential is not more than 0.1% lower than the carbon concentration corresponding to the Acm point in the center. On the other hand, if the carbon potential is less than 0.7%, decarburization from the surface occurs and the amount of precipitated carbide decreases, which is not preferable. More preferably, the content is 0.75% or more.

  Since the nitrogen potential can be set high while maintaining the heating temperature in the carbide precipitation step, the nitriding treatment is preferably performed in the carbide precipitation step. When nitriding is performed in an atmosphere having a nitrogen potential of 0.05% or more, the amount of nitrogen on the surface can be secured, and sufficient quenching can be performed during quenching to suppress the formation of an incompletely quenched structure. The nitrogen potential is preferably 0.08% or more. Further, even if the nitrogen potential exceeds 0.20%, the incompletely quenched structure increases, which is not preferable. The reason for this is not clear, but it is presumed that if the amount of nitrogen is too high, Cr nitride precipitates and the hardenability of the steel matrix decreases. Preferably it is 0.15% or less. As a method for measuring the nitrogen potential, a method in which an iron wire called a Cp coil is left in the furnace atmosphere, and the Cp coil is quantitatively analyzed by an infrared absorption method or the like can be applied.

  As a specific method of nitriding, it is possible to apply a general method such as injecting ammonia while maintaining the heating temperature for a certain period of time. The timing of nitriding is not particularly limited, and in addition to performing nitriding treatment in all steps of the carbide precipitation step, in the case of performing nitriding treatment simultaneously with carbide precipitation in a part of the carbide precipitation treatment step, or in two or more parts The case where the nitriding process is performed separately is also included.

In order to further improve the pitting resistance by precipitating finer carbides in the present invention, it is particularly preferable to perform carbide precipitation / nitriding by the following method. That is, the carbide precipitation process
(I) Heating is performed at a temperature of 810 ° C. or higher and 860 ° C. or lower in an atmosphere having a carbon potential of 0.7% or higher and 1.1% or lower, and at a cooling rate of 10 ° C./minute or higher, After the temperature was lowered to a temperature not lower than 800 ° C. (hereinafter referred to as “carbide precipitation stage (I)”),
(II) Hold at this temperature in an atmosphere with a carbon potential of 0.7% to 1.1% (hereinafter referred to as “carbide precipitation stage (II)”)
As a result, finer carbides can be more uniformly dispersed, and a steel part for mechanical structure having excellent characteristics can be obtained with certainty.

  In the case of performing carbide precipitation in such a process, in the carbide precipitation stage (II), if nitriding is performed simultaneously with carbide precipitation in an atmosphere having a nitrogen potential of 0.05% or more and 0.15% or less, heating is performed. This is desirable because the nitrogen potential can be set high during temperature holding and nitriding can be performed satisfactorily.

  Hereinafter, the process conditions of the said process are explained in full detail.

<Conditions for carbide precipitation stage (I)>
(1) Heating temperature The heating temperature in the carbide precipitation stage (I) is preferably 810 ° C or higher and 860 ° C or lower. When the temperature exceeds 860 ° C., the solid solubility limit of the carbon content increases, and the amount of precipitated carbide decreases, and more preferably 850 ° C. or less. On the other hand, when the temperature is lower than 810 ° C., the precipitated carbide becomes too fine, and the contribution to improving the pitching strength is reduced, and more preferably 830 ° C. or higher.

(2) Heating atmosphere The heating atmosphere in the carbide precipitation stage (I) preferably has a carbon potential of 0.7% to 1.1%. This is because when the carbon potential is less than 0.7%, decarburization occurs while the heating temperature is maintained, and the amount of precipitated carbide decreases. More preferably, the carbon potential is set to 0.75% or more. On the other hand, when the carbon potential exceeds 1.1%, the carburization proceeds while the heating temperature is maintained, the carbides become coarse, and various strengths such as pitting are reduced, which is not preferable. More preferably, it is 1.0% or less. The carbon potential can be measured by the method described above.

  The heating and holding time in the carbide precipitation stage (I) is not particularly limited, and may be a time for obtaining a desired carburized layer depth. However, the part size increases, and the holding time is 30 minutes or more. In this case, the carbon potential is preferably 0.7% or more and less than or equal to the carbon concentration corresponding to the Acm point in the center. The carburized layer of the steel part while maintaining the heating temperature has a mixed structure of austenite and carbide consisting of the composition of the Acm point of the core steel, but in an atmosphere where the carbon potential exceeds the carbon concentration corresponding to the Acm point, This is because the carburizing atmosphere is strong against austenite in the structure, and the carbides are coarsened to reduce the mechanical strength of the steel parts. It has been found that such a tendency becomes more prominent as the part becomes larger and the heating and holding time becomes longer. It is more preferable that the carbon potential is not more than 0.1% lower than the carbon concentration corresponding to the Acm point in the center. On the other hand, if the carbon potential is less than 0.7%, decarburization from the surface occurs and the amount of precipitated carbide decreases, which is not preferable. More preferably, the content is 0.75% or more.

(3) Cooling rate The cooling rate after maintaining the heating temperature is preferably 10 ° C./min or more. When the cooling rate is less than 10 ° C./min, there is a time allowance for the carbon once dissolved in the matrix to be diffused, and the carbide precipitated at the crystal grain boundaries is coarsened. It becomes. The cooling rate is preferably 20 ° C./min or more. The upper limit of the cooling rate is not particularly limited, but it is preferable that the cooling rate is 200 ° C./min or less because deformation and cracking are likely to occur in the steel part even if it is cooled too rapidly.

<Conditions for carbide precipitation stage (II)>
(1) Heating temperature Cooling at the cooling rate in the carbide precipitation stage (I) is not performed to room temperature, but is 800 ° C. or higher and to a temperature range of 10 ° C. or lower than the heating temperature in the previous carbide precipitation stage (I). It is good to hold at this temperature for a certain period of time.

  This is because when the heating temperature is less than 800 ° C., the precipitated carbide is excessively refined and does not contribute much to the improvement of the pitching strength. More preferably, it is set to 830 ° C. or higher. On the other hand, when the temperature is higher than the heating temperature in the carbide precipitation stage (I) by less than 10 ° C., the amount of carbide precipitated during cooling is small, and the effect of improving the pitching strength cannot be obtained. More preferably, it is set to 20 ° C. or lower (however, 800 ° C. or higher) than the heating temperature in the carbide precipitation stage (I).

(2) Heating atmosphere It is preferable to perform nitriding simultaneously with carbide precipitation in the carbide precipitation stage (II) because the nitrogen potential can be set high while maintaining the heating temperature. The nitrogen potential in the nitriding treatment is within the above-described range, that is, preferably 0.05% or more, more preferably 0.08% or more, preferably 0.20% or less, more preferably 0.15% or less. It is good to do. The carbon potential is preferably set in an atmosphere having a carbon potential of 0.7% or more and 1.1% or less, as in the carbide precipitation stage (I). This is because when the carbon potential is less than 0.7%, the amount of carbides that are decarburized and precipitated while maintaining the heating temperature is reduced. Preferably it is 0.75% or more. On the other hand, when the carbon potential exceeds 1.1%, the surface layer is excessively carburized while maintaining the heating temperature, the carbides become coarse, and various strengths such as pitting decrease. Preferably it is 1.0% or less.

  In this case, the nitriding treatment may be performed simultaneously with the precipitation of the carbide in at least a part of the carbide precipitation stage (II).

  The heating and holding time in the carbide precipitation stage (II) is not particularly limited, and may be a time for obtaining a desired carburized layer depth. However, the part size increases, and the holding time is 30 minutes or more. In this case, the carbon potential is preferably 0.7% or more and less than or equal to the carbon concentration corresponding to the Acm point in the center. The carburized layer of the steel part while maintaining the heating temperature has a mixed structure of austenite and carbide consisting of the composition of the Acm point of the core steel, but in an atmosphere where the carbon potential exceeds the carbon concentration corresponding to the Acm point, This is because the carburizing atmosphere is strong against austenite in the structure, and the carbides are coarsened to reduce the mechanical strength of the steel parts. It has been found that such a tendency becomes more prominent as the part becomes larger and the heating and holding time becomes longer. It is more preferable that the carbon potential is not more than 0.1% lower than the carbon concentration corresponding to the Acm point in the center. On the other hand, if the carbon potential is less than 0.7%, decarburization from the surface occurs and the amount of precipitated carbide decreases, which is not preferable. More preferably, the content is 0.75% or more.

  When cooling after holding the heating temperature in the previous carbide precipitation step, it is better to quench to ensure the strength of the parts, and as a quenching method, a general quenching method such as salt quenching or oil cooling is applied. Is possible.

  The steel part for machine structure of the present invention is used in a power transmission part of an automatic vehicle using an automatic transmission oil as described above. Specifically, for example, an output gear (for example, used at a high surface pressure) Output gears), sliding parts, shafts, and the like.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, but may be implemented with appropriate modifications within a range that can meet the purpose described above and below. Any of these may be included in the technical scope of the present invention.

  Steel materials having the component compositions shown in Table 1 were melted in a converter, and bar steel was obtained through partial rolling and bar rolling. Next, after cutting this steel bar, hot forging is performed, and after blanking into a shape close to the part, machining such as gear cutting is performed to obtain a gear having the shape shown in FIGS. 1 (a) and 1 (b). It was. The specifications of the gears are as follows.

Number of teeth: 19
Module: 2.12
Pressure angle: 19 ° 0'0 ”
Twist angle: 26 ° 45'0 "
Tooth width: 33.5mm
Outer diameter: φ52.51mm
Next, the carburizing process of the pattern shown in FIGS. 2 and 4 to 7 show a high-concentration carburizing process for forming a hard layer on the steel surface layer by carburizing and precipitating carbide in the hard layer, and FIG. 3 shows a normal carburizing process. Each treatment was performed under the conditions of holding temperature, holding time, and atmosphere ("Cp" in the figure indicates a carbon potential and "Np" in the figure indicates a nitrogen potential) shown in FIGS. In the high-concentration carburizing treatment shown in FIGS. 2 and 4 to 7, nitriding treatment was simultaneously performed by blowing ammonia into the atmosphere and flowing in part of the treatment process.

  After the above treatment, honing was performed to finish the gear. Some of the plurality of gears finished in this way were subjected to a solid lubricating film treatment.

  The solid lubricating coating was formed by baking coating using molybdenum disulfide, which is usually referred to as “deflick treatment”, after the base treatment of dry honing and luebright treatment.

  A pitching durability test was conducted using the obtained gear (sample). In the pitching durability test, as shown schematically in FIG. 8, it is meshed with the gear on the other side and rotated at a constant input torque (200 N · m), and the pitching resistance is determined by the number of cycles (pitching life) until pitching occurs. Evaluated.

  In addition, the other gear used what gave normal carburizing process (said FIG. 3) to SCr420H prescribed | regulated by JIS. The specifications of the mating gear are as follows.

Number of teeth: 88
Module: 2.12
Pressure angle: 19 ° 0'0 ”
Twist angle: 26 ° 45'0 "
Tooth width: 28mm
Outer diameter: φ214.64mm
As a lubricating oil, ATF oil [Mazda genuine M-III, kinematic viscosity at 100 ° C .: 7.4 cSt] was used.

  The gear after the test is cut as shown in FIG. 1 (c), and the tooth pitch line position 4 [(d) of FIG. 1] on the non-contact side is cut using a scanning electron microscope (SEM). Observation was made to confirm the state of carbide precipitation in the steel surface layer. Observation was performed at a magnification of 8000 times at a position of 25 μm from the steel surface.

  The photographed SEM (scanning electron microscope) photograph was subjected to image analysis, the size and number of carbides at 25 μm from the steel surface were measured, and the area ratio, surface density, and the ratio of fine carbides were determined. In addition, the diameter of the carbide | carbonized_material was calculated | required as a diameter (circle equivalent diameter) of the circle | round | yen which has an area equivalent to the area of each carbide | carbonized_material. Table 2 shows the results of these pitching endurance tests and the results of microscopic observation.

  No. in Table 2 Since 1, 6-9, 17 and 18 satisfy the requirements defined in the present invention, those exhibiting excellent pitting resistance were obtained. That is, the area ratio of the carbide deposited on the steel surface layer is 5% or more, the tempering hardness is high, and the initial running-in effect is exhibited by the solid lubricant film formed on the surface of the steel part, so the pitching life is extremely long. became.

  No. From 7 and 8, it can be seen that in order to obtain better pitting resistance, it is better to deposit more fine carbides on the steel surface layer. No. 1 and No. From the comparison of 9, it can be seen that by slightly nitriding the surface of the steel member, generation of an incompletely hardened structure around the carbide is suppressed, and the pitching life is further increased.

  On the other hand, no. Nos. 2 to 5 and 10 to 16 do not satisfy any of the requirements defined in the present invention, so that they are inferior in pitting resistance or do not have properties as steel parts for machine structures.

  That is, no. No. 2 has a short pitching life, but this shows the initial running-in effect due to the solid lubricant film, but carbide does not precipitate on the steel surface layer and the tempering hardness of the steel itself is low, so the pitching life is short. It seems that it has become shorter.

  No. No. 3 has a specified amount of carbides precipitated on the surface layer of the steel, and the tempering hardness of the steel itself is sufficiently high. In the existing local unevenness | corrugation, it is thought that the crack generate | occur | produced from the part with a high contact surface pressure, and the pitching lifetime was shortened.

  No. No. 5 has a solid lubricating film formed on the surface, but it is considered that the pitching life was shortened because carbide was not sufficiently precipitated on the steel surface layer.

  No. 4 is the above-mentioned No.4. 2 or No. 3, no. Pitching life is shorter than 5. This is because normal carbide carburizing treatment does not precipitate carbide, so the steel itself has a low tempering hardness and no solid lubricating film is formed, so there is no initial running-in effect and peeling damage. Seems to have occurred early.

  No. Nos. 10 to 16 do not satisfy the component composition defined in the present invention, so that they are inferior in pitting resistance or do not have other properties as machine structural steel parts.

  That is, no. No. 10 is excessive in Si. No. 11 is excessive in Mn. Since No. 13 had excessive Cr, the workability was lowered, and the tool life used during blanking was shortened, resulting in an increase in production cost.

  No. In No. 12, since Mn was insufficient, the hardenability of the base material was insufficient, and an incompletely hardened structure was generated.

  No. No. 14 was short of Cr, so the carbides defined in the present invention were insufficient, and the pitching life was shortened.

  No. In No. 15, the tool broke in the gear cutting process because Mo was excessive. No. In No. 16, Mo is insufficient, and the hardenability of the base material is insufficient, and an incompletely quenched structure is generated. Therefore, it is considered that the pitching life is shortened.

(A) is a mechanical design drawing (side view) of the gear used in the embodiment, (b) is the mechanical design drawing (cross-sectional view), (c) is a top view of the tooth portion of the gear, and (d) is ( It is XX sectional drawing of c). It is a figure which shows the high concentration carburizing process pattern employ | adopted in the Example. It is a figure which shows the conventional carburizing process pattern. It is a figure which shows another high concentration carburizing process pattern employ | adopted in the Example. It is a figure which shows another high concentration carburizing process pattern employ | adopted in the Example. It is a figure which shows another high concentration carburizing process pattern employ | adopted in the Example. It is a figure which shows another high concentration carburizing process pattern employ | adopted in the Example. It is a schematic diagram which shows a part of pitching durability test done in the Example.

Explanation of symbols

1 tooth bottom 2 tooth tip 3 pitch circle 4 tooth part pitch line position

Claims (5)

A steel part for machine structure used in a power transmission part of an automatic vehicle using automatic transmission oil,
C: 0.15 to 0.25% (representing mass%, hereinafter the same in the case of component content),
Si: more than 0.35 to 0.7%,
Mn: 0.2 to 1.0%,
Cr: 1.0 to 1.5%,
Mo: 0.30 to 0.6%,
N: 0.008 to 0.05%
And the area ratio of carbides in the steel surface layer part within a depth of 50 μm from the steel part surface includes 0.005 to 0.2% in total of at least one selected from the group consisting of Al, Nb and Ti. A steel part for mechanical structure, characterized in that it is 5% or more when observed at a magnification of 8000 times with a scanning electron microscope, and a solid lubricating film is formed on the surface of the steel part.   The steel part for machine structure according to claim 1, wherein the solid lubricant film is molybdenum disulfide. When observed at a magnification of 8000 times with a scanning electron microscope, the surface density of carbides having a diameter of 0.5 μm or less in the steel surface layer part within a depth of 50 μm from the steel part surface is 6.0 pieces / 10 μm ² or more, 3. The steel part for machine structure according to claim 1, wherein the ratio of the number of carbides having a diameter of 0.5 μm or less to the total number of carbides in the steel surface layer part within a depth of 50 μm from the steel part surface is 80% or more.   The steel part for machine structure according to any one of claims 1 to 3, wherein a nitrogen amount at a depth of 50 µm from the steel part surface is 0.07 to 0.2%. As another element, in the heart,
Ca: 0.0005 to 0.05%,
Zr: 0.01 to 0.15%,
Bi: 0.05% or less (excluding 0%),
S: 0.12% or less (excluding 0%),
Pb: 0.09% or less (not including 0%), or Mg: 0.02% or less (not including 0%)
The steel part for machine structure in any one of Claims 1-4 in which is contained.

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