JP5405325B2 - Differential gear and manufacturing method thereof - Google Patents

Description

The present invention relates to a differential gear and a manufacturing method thereof. Specifically, the bending fatigue strength in the so-called “low to medium cycle range”, that is, “bending at a repetition rate of about 10 ³ to 10 ⁴ cycles or less when repeated impact load is applied so as to give plastic deformation”. The present invention relates to a differential gear having a higher strength at which fatigue fracture occurs than a conventional carburized and quenched differential gear and a method for manufacturing the same. More specifically, by performing a two-stage quenching process in which decarburization during hot forging and tempering treatment is performed, and after the tempering treatment, contour induction induction quenching is performed, in a “low to medium cycle range” than before. The present invention relates to an induction-quenched differential gear capable of improving the bending fatigue strength of the steel and a manufacturing method thereof.

  The differential gears, which are the driving parts of automobiles, are made of alloy steels (steel materials) for mechanical structures such as SCr420 and SCM420, which are standardized in JIS, and are formed into a predetermined shape by hot forging. In order to ensure the bending fatigue strength of the steel sheet, it is manufactured by subjecting it to a surface hardening treatment. As the surface hardening treatment for the above-mentioned differential gear, in general, a “carburizing and quenching treatment” as shown in Non-Patent Document 1 is used. Has been used.

  On the other hand, in recent years, from the viewpoint of responding to the environment, such as improving the fuel efficiency of automobiles and reducing exhaust gas, there has been an increasing demand for weight reduction of automotive parts, and further improvement in bending fatigue strength is desired for differential gears. ing.

However, in order to ensure the bending fatigue strength at the tooth root portion of the differential gear, when the above-mentioned “carburizing and quenching treatment”, especially “gas carburizing and quenching treatment” is performed,
[1] Formation of “grain boundary oxide layer”
[2] Embrittlement of the hardened layer on the surface due to the high carbon martensite structure,
Since it is difficult to avoid the above, it is not possible to sufficiently meet the above demand.

  Thus, although Non-Patent Document 1 discusses materials based on the premise of “carburizing and quenching”, it is difficult to avoid the problems [1] and [2] described above only by changing these materials. Again, it cannot be said that it is sufficient to improve the bending fatigue strength of the tooth root portion of the differential gear to which an impact load is applied.

  From such a background, in order to improve the fatigue strength against shock load applied to the tooth root portion of the differential gear, that is, the bending fatigue strength in the “low to medium cycle range”, a surface that replaces the above-mentioned “carburizing and quenching treatment”. As a curing method, a technique using “induction hardening” has been proposed.

  Specifically, in Non-Patent Document 2, “repeated quenching treatment” is performed by using S45C, which is a medium carbon steel, as a raw material, and after performing so-called “tempering treatment” of “quenching-tempering”, further performing contour quenching by induction quenching. It is disclosed that the bending fatigue strength of a gear subjected to "" is higher than the bending fatigue strength of a gear subjected to conventional carburizing and quenching treatment using SCM420 as a raw material.

Matsushima et al .: R & D Kobe Steel Engineering Reports, Vol. 50, no. 1 (2000), P.I. 57-60 Horikawa et al .: Proceedings of the Japan Society of Materials Science / Academic Lecture, Vol. 46, (1997) pp. 281-282

  Non-Patent Document 2 discloses that the S45C gear subjected to contour quenching by induction hardening after the tempering treatment has higher bending fatigue strength than the conventional carburized and quenched SCM420 gear. Induction hardening is a technique in which heating is performed rapidly and for a short time by induction heating using high frequency, and then quenched using a cooling medium. Therefore, when contour quenching by induction quenching is performed, the transformation point changes depending on the heating rate and the steel component, and thus it is considered that an incompletely quenched structure may occur. When an incompletely quenched structure that is softer than the martensite structure exists, it may be unavoidable that the incompletely quenched structure becomes a stress concentration limit and fatigue failure occurs. For this reason, the technique proposed in Non-Patent Document 2 does not necessarily provide an effect of improving the bending fatigue strength in the “low to medium cycle range”.

  Furthermore, since the influence of the prior austenite crystal grain size is not disclosed in the Non-Patent Document 2, it is unclear how much the refinement of the prior austenite crystal grain size contributes to the improvement of bending fatigue strength. there were.

  The present invention has been made in view of the above-described present situation, and an object thereof is to provide a differential gear and a method of manufacturing the same that greatly improve the bending fatigue strength in the “low to medium cycle range”.

  The present inventors conducted various investigations and studies in order to improve the bending fatigue characteristics of the differential gear in the “low to medium cycle range”.

  As a result, first, tempering treatment was performed using the alloy steel for mechanical structure SCM440 as a raw material, and after that, induction hardening was performed so that the old austenite crystal grain size of the hardened layer became fine, and "repetitive quenching treatment" was performed, and further It was confirmed that the bending fatigue strength in the “low to medium cycle region” was improved if the generation of incompletely quenched structure could be suppressed when induction hardening was performed.

  Next, in order to improve the induction hardenability and suppress the formation of an incompletely hardened structure, the present inventors made a study from the viewpoint of materials and made various studies on the chemical composition of steel.

  As a result, the following basic component design concepts (a) to (d) were obtained.

(A) Since Si is an element that raises the A ₃ transformation point as the content increases, it causes a reduction in induction hardenability. Therefore, the Si content should be low.

  (B) Cr has the effect of stabilizing cementite. For this reason, Cr, which is one of the elements that increase the hardenability in the case of normal quenching, does not sufficiently dissolve cementite when the content is high in the case of induction hardening with a short heating time. It may cause a decrease in hardenability. Therefore, the Cr content should be low.

  (C) In order to prevent the formation of an incompletely quenched structure when induction-quenched, it is preferable to increase the contents of C, Mn, Mo, and B and utilize the effect of induction hardening by these elements.

(D) In order to secure the hardenability improvement effect of B, it is preferable to contain Ti in order to fix N. However, when the Ti content is too large, the amount of C in the steel material is reduced. Since the ratio of ferrite increases, the A ₃ transformation point of the steel material may be raised, and on the contrary, induction hardenability may be reduced. For this reason, it is good to adjust content of Ti to an appropriate range.

  Therefore, in order to perform a basic test based on the above basic component design concept, the present inventors performed tempering treatment on a four-point bending specimen prepared by melting various steels, and further, Induction quenching was performed so that the prior austenite crystal grain size became fine, and "repetitive quenching treatment" was performed to evaluate the bending fatigue strength.

  As a result, it was possible to obtain a bending fatigue strength higher by 40% or more based on the bending fatigue strength when the steel corresponding to SCr420 was carburized and quenched and tempered.

  Therefore, based on the above basic test results, the present inventors further produced a differential gear by hot forging, performed the same processing on the produced differential gear, and evaluated with actual parts.

  As a result, although it cannot be said that the steel equivalent to SCr420 is 40% or more of bending fatigue strength when carburizing and tempering steel, bending fatigue strength is improved by about 30% even in the differential gear which is an actual part. I was able to.

  Therefore, the present inventors investigated the cause of the bending fatigue strength improvement effect in actual parts being smaller than the bending fatigue strength improvement effect in the basic test.

  As a result, the surface layer of the differential gear subjected to contour induction hardening and tempering after the tempering treatment did not occur in the case of the differential gear manufactured by the conventional carburizing and quenching treatment in a high carbon potential atmosphere. “Decarburization” has occurred, and in particular, the total decarburization layer depth (hereinafter also referred to as “DM-T”) generated in the surface layer portion of the differential gear is compared with the bending fatigue strength of the actual differential gear. It became clear that it had an influence.

  Therefore, studies have been made to suppress the decarburization in the differential gear surface layer, and the following knowledge has been reached.

  (E) Although decarburization that occurred in the differential gear surface layer is estimated to have occurred in either the hot forging process, the tempering process after forging, or the contour induction hardening process, the contour induction hardening process is Since the heat treatment is rapid and short, decarburization hardly occurs.

  (F) In order to suppress decarburization in the state heated to the austenite single phase region at the time of hot forging, it is effective to perform heating at a low temperature for a short time. Moreover, what is necessary is just to let the two-phase structure | tissue temperature area | region of austenite and a ferrite pass as short as possible in order to suppress the decarburization in the cooling process after hot forging.

  (G) In the tempering treatment step, decarburization occurs if heat treatment is performed in an air atmosphere. In order to suppress decarburization in the tempering treatment step, the carbon potential of the heating atmosphere at the time of quenching in the tempering treatment step may be adjusted to be equal to or higher than the C content of steel.

  (H) However, decarburization may occur even if suppression of decarburization in the hot forging step and suppression of decarburization in the tempering treatment step after forging are attempted.

(I) This unique phenomenon is affected by the oxide scale formed on the surface of the forged product in the hot forging process, and even if it is heated by controlling the carbon potential of the heating atmosphere during quenching, the oxide scale Becomes an oxygen supply source and combines with C on the material side to form CO ₂ , thereby reducing the amount of C in the surface layer portion of the forged product, resulting in ferrite decarburization.

  (J) Therefore, in order to suppress decarburization in the tempering treatment step performed after hot forging, carbon oxide in the heating atmosphere at the time of quenching after removing the oxide scale formed on the surface in the hot forging stage What is necessary is just to adjust potential to the same level or more as C content of steel.

  And, if decarburization at the differential gear surface layer portion is suppressed by the above method, the bending fatigue strength by the differential gear of the actual part that has undergone contour induction hardening and tempering treatment after the tempering treatment is the result of the basic test. Similarly, it was found that the bending fatigue strength of a differential gear obtained by carburizing and tempering steel corresponding to SCr420 is improved by 40% or more.

  The present invention has been completed based on the above findings, and the gist of the present invention resides in the differential gear shown in the following (1) and (2) and the method for manufacturing the differential gear shown in (3).

(1) The dough steel is in mass%, C: 0.35 to 0.45%, Si: 0.10% or less, Mn: 0.50 to 1.0%, P: 0.015% or less, S: 0.030% or less, Cr: 0.05 to 0.15%, Mo: 0.15 to 0.25%, Al: 0.01 to 0.05%, N: 0.010% or less, O (Oxygen): 0.0020% or less, B: 0.0010 to 0.0030% and Ti: 0.010 to 0.045%, the balance being steel having a chemical composition consisting of Fe and impurities, And the differential gear characterized by satisfying the following conditions (A) to (D).
(I) Hardened layer depth: 0.80 to 1.50 mm,
(B) Old austenite average particle diameter of the hardened layer: 12 μm or less,
(C) Residual stress at the position of 50 μm from the surface in the tooth root part: −700 MPa or less,
(D) Value of ΔHV represented by [(Vickers hardness of surface layer part) − (Vickers hardness of core part)]: 10-40.

(2) The differential gear described in (1) above, wherein the following conditions (e) and (f) are further satisfied.
(E) Decarburized layer depth of the surface layer part: 0.015 mm or less,
(F) C content of surface layer portion: 0.35 to 0.50%.

(3) A method of manufacturing a differential gear, characterized in that the steel material having the chemical composition of the differential gear cloth described in (1) is processed in the order of the following steps <1> to <8>.
Step <1>: The steel material is heated at a temperature in the temperature range of 1000 to 1200 ° C. and at a heating time of 10 minutes or less.
Step <2>: The heated steel material is hot forged at a forging end temperature of 900 ° C. or higher and formed into a differential gear shape.
Step <3>: The formed differential gear-shaped product is cooled to 300 ° C. at an average cooling rate of 1 to 20 ° C./s from the hot forging end temperature.
Step <4>: The oxide scale generated on the surface of the differential gear-shaped product is removed.
Step <5>: The differential gear-shaped product from which the oxide scale has been removed is heated to 820 to 1000 ° C. in an atmosphere having a carbon potential of 0.40 to 0.50%, and then quenched.
Step <6>: The quenched differential gear product is tempered at a temperature of 200 ° C. or lower.
Step <7>: The tempered differential gear-shaped product is further heated at a frequency of 40 to 60 kHz, preheated so that the root surface temperature of the differential gear reaches 600 to 700 ° C., and then released to the atmosphere. It cools and this heat processing is performed so that a root surface temperature may be 950-1050 degreeC by heating time 0.2-0.5 s, and contour induction hardening is performed.
Process <8>: The differential gear-shaped product subjected to contour induction hardening is tempered at a temperature of 200 ° C. or lower.

  The “impurities” in the remaining “Fe and impurities” refers to those mixed from ore, scrap, or the environment as raw materials when industrially producing steel materials.

  “Hardened layer depth” refers to the distance from the surface to a position where the Vickers hardness is 550, and “hardened layer” refers to the region from the surface to the above “hardened layer depth”.

  “Negative sign” in the residual stress indicates a compressive residual stress.

  The “total decarburization layer depth” refers to “DM-T” in the measurement method using a microscope defined in “Method for measuring the decarburization layer depth of steel” of JIS G 0558 (2007).

  “The amount of C in the surface layer portion” refers to the average amount of C at a position from the surface to a depth of 100 μm.

  The heating temperature of a steel material refers to the temperature on the steel material surface. Similarly, the heating temperature of the differential gear shape product also indicates the temperature at the surface of the differential gear shape product. The hot forging end temperature indicates the temperature at the surface of the material to be forged, and the tempering temperature indicates the set temperature of the tempering furnace.

  An average cooling rate refers to the value calculated | required from the surface temperature of the differential gear shape goods which complete | finished hot forging.

  The invention relating to the differential gear shown in the above (1) and (2) and the invention relating to the manufacturing method of the differential gear shown in (3) are respectively referred to as “present invention (1)” to “present invention (3)”. " Also, it may be collectively referred to as “the present invention”.

  The bending fatigue strength in the “low to medium cycle range” of the differential gear of the present invention is greatly improved compared to the bending fatigue strength of a differential gear subjected to conventional carburizing and quenching treatment. For this reason, the differential gear of the present invention is suitable for use as an automobile differential gear which is shocking and may be subjected to a relatively large load. This differential gear can be easily manufactured by the manufacturing method of the present invention.

It is a figure explaining the shape of the differential gear used in the Example. In an Example, it is a figure explaining the conditions of the contour induction hardening-tempering performed to the differential gear shape goods. It is a figure which shows the example typically about a differential durability test apparatus. It is a figure which shows typically the example about the differential unit in a differential durability test apparatus. Note that “pinion gear” and “side gear” in the figure correspond to “differential gears”.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of each element means “mass%”.

(A) Chemical composition of dough steel C: 0.35 to 0.45%
C has the effect | action which ensures the intensity | strength of steel, and the effect | action which ensures the hardened layer hardness after induction hardening. However, if the content is less than 0.35%, the desired effect due to the above action cannot be obtained. On the other hand, if the content of C exceeds 0.45%, the hardened layer formed by induction hardening causes brittle fracture, so that the toughness decreases. Therefore, the content of C is set to 0.35 to 0.45%. In addition, in order to acquire the said effect stably, it is preferable that the minimum of C content shall be 0.38%.

Si: 0.10% or less Si is an element contained as an impurity. Si increases the A ₃ transformation point, reducing the high-frequency hardenability. Moreover, since the A ₃ transformation point rises with increasing content of Si, the temperature of the two-phase region of the hot decarburization prone austenite and ferrite in the cooling process after the forging spread, the content of Si When it is high, decarburization tends to occur. In particular, when the Si content exceeds 0.10%, the induction hardenability is deteriorated and decarburization after hot forging is remarkably generated. Therefore, the Si content is set to 0.10% or less. The smaller the content of Si, the better.

Mn: 0.50 to 1.0%
Mn is an element that improves induction hardenability. However, when the content of Mn is less than 0.50%, the desired effect due to the above action cannot be obtained. On the other hand, even if Mn is contained exceeding 1.0%, the above effect is saturated and the cost is increased. Therefore, the content of Mn is set to 0.50 to 1.0%. In order to obtain the above effect stably while keeping the alloy cost low, the lower limit of the Mn content is preferably 0.55%.

P: 0.015% or less P lowers the toughness of the hardened layer by induction hardening. In particular, when the content exceeds 0.015%, the toughness of the hardened layer is significantly reduced. Therefore, the content of P is set to 0.015% or less. The P content is preferably 0.010% or less.

S: 0.030% or less S is an element contained as an impurity. Further, S, when added, combines with Mn to form MnS, and has an effect of improving machinability, particularly chip disposal. However, if the S content increases and the amount of MnS produced increases too much, even if the machinability is improved, the fatigue strength is reduced. In particular, when the S content exceeds 0.030%. The fatigue strength is significantly reduced. Therefore, the content of S is set to 0.030% or less. The upper limit of the S content is preferably 0.020%.

Cr: 0.05 to 0.15%
Cr, like C and Mn, has the effect of improving the hardenability of the steel and improving the strength. However, when the Cr content is less than 0.05%, a sufficient effect cannot be obtained. On the other hand, since Cr has an action of stabilizing cementite, in the case of induction hardening, when the content is high, the cementite does not sufficiently dissolve, and on the contrary, the hardenability is lowered. If the content of exceeds 0.15%, the induction hardenability will be significantly reduced. Therefore, the Cr content is set to 0.05 to 0.15%.

Mo: 0.15-0.25%
Mo, like C and Mn, has the effect of increasing the hardenability of the steel and improving the strength. Mo also has the effect of increasing the temper softening resistance. However, when the Mo content is less than 0.15%, the strength improvement effect and the temper softening resistance improvement effect cannot be obtained stably. On the other hand, even if the Mo content exceeds 0.25%, the cost is increased. Therefore, the Mo content is set to 0.15 to 0.25%.

Al: 0.01 to 0.05%
Al has a deoxidizing action like Si. Furthermore, Al combines with N in the steel to form AlN, and this AlN has the effect of preventing crystal grain coarsening when induction hardening is performed. However, when the Al content is less than 0.01%, it is not possible to expect a deoxidation effect or an effect of preventing grain coarsening when induction hardening is performed by the formation of AlN. On the other hand, Al is an element that raises the A ₃ transformation point and lowers the induction hardenability. When the Al content increases and exceeds 0.05%, the induction hardenability is significantly reduced. Therefore, the Al content is set to 0.01 to 0.05%.

N: 0.010% or less N is an element contained as an impurity. N is an element having a large affinity with B, Al, Ti and the like, and when it is combined with B to form BN, it becomes difficult to obtain the effect of enhancing the induction hardenability of B, which will be described later. If the content increases and exceeds 0.010%, the effect of improving the induction hardenability by B cannot be ensured by BN formation. Furthermore, when the amount of solute N in the steel increases, the hot deformability decreases, and particularly when the N content exceeds 0.010%, the hot deformability decreases remarkably. Therefore, the N content is set to 0.010% or less. Note that the content of N as an impurity in the steel is preferably reduced as much as possible.

O (oxygen): 0.0020% or less O is an element contained as an impurity. O combines with elements in steel to form oxides, leading to a decrease in strength, particularly a decrease in fatigue strength. In particular, when the O content exceeds 0.0020%, the amount of oxide formed increases and MnS coarsens, resulting in a significant decrease in fatigue strength. Therefore, the content of O is set to 0.0020% or less. The content of O as an impurity in the steel is preferably 0.0015% or less.

B: 0.0010 to 0.0030%
B has the effect | action which improves induction hardenability, The effect is remarkable when content of B is 0.0010% or more. However, even if it contains B exceeding 0.0030%, the above-mentioned effect is saturated and the cost is increased. Therefore, the content of B is set to 0.0010 to 0.0030%.

  Even when B is contained in an amount in the above range, induction hardenability cannot be improved when BN is formed by combining B with N present as an impurity in steel. Therefore, as described above, in order to exhibit the effect of improving the induction hardenability of B, it is necessary to reduce the content of N present as an impurity in the steel.

Ti: 0.010 to 0.045%
Inclusion of B improves induction hardenability when B is not a compound but exists alone. Therefore, when B is combined with N to form a nitride, the effect of improving hardenability by B cannot be expected. For the above reason, 0.010% of Ti having a greater affinity for N than B and a stronger nitride forming ability is contained. However, even if Ti is contained in an amount exceeding 0.045%, not only the effect of fixing N is saturated, but a large amount of coarse TiN is generated. When the content of Ti is too large, since the C content in the steel is the proportion of ferrite is increased to decrease, increasing the A ₃ transformation point of the steel material, rather deteriorating the high-frequency hardenability. Therefore, the fatigue characteristics may be reduced. Therefore, the Ti content is set to 0.010 to 0.045%. In order to ensure the effect of improving the hardenability of B, the lower limit of the Ti content is preferably 0.015%.

(B) Characteristics of the hardened layer on the surface of the differential gear The differential gear of the present invention in which the material steel has the chemical composition described in the above section (A) has the hardened layer on the surface as described in It must meet the conditions of d).

(I) Hardened layer depth: 0.80 to 1.50 mm,
(B) Old austenite average particle diameter of the hardened layer: 12 μm or less,
(C) Residual stress at the position of 50 μm from the surface in the tooth root part: −700 MPa or less,
(D) Value of ΔHV represented by [(Vickers hardness of surface layer part) − (Vickers hardness of core part)]: 10-40.

  Hereinafter, each of the above (a) to (d) will be described.

(I) Hardened layer depth:
The depth of the hardened layer, which is the distance from the surface of the differential gear to the position where the Vickers hardness (hereinafter also referred to as “HV hardness”) is 550, greatly affects the bending fatigue strength. When the hardened layer depth is less than 0.80 mm, the hardened layer depth is too shallow and the crack growth life is shortened, so that the bending fatigue strength is not improved. On the other hand, when the depth of the hardened layer exceeds 1.50 mm, the thermal energy input at the time of contour induction hardening becomes large, so that the crystal grains of the hardened layer cannot be made fine, and the toughness of the hardened layer is thus lowered. Thus, the bending fatigue strength cannot be increased. Therefore, the hardened layer depth was defined as 0.80 to 1.50 mm. The lower limit of the hardened layer depth is preferably 0.90 mm. The upper limit of the hardened layer depth is preferably 1.40 mm.

(B) Old austenite average particle diameter of the hardened layer:
When the prior austenite average particle diameter of the hardened layer formed by contour induction hardening exceeds 12 μm, the toughness of the hardened layer is reduced, so that the bending fatigue strength cannot be increased. Therefore, the prior austenite average particle diameter of the hardened layer was defined as 12 μm or less. The upper limit of the prior austenite average particle size of the hardened layer is preferably 10 μm. The lower limit of the prior austenite average particle size of the hardened layer is not particularly required, but is industrially limited to about 1.0 μm.

(C) Residual stress at the position of 50 μm from the surface in the tooth root part:
Contour induction hardening causes compressive residual stress (negative residual stress) near the surface of the differential gear. However, in the tooth root portion of the differential gear where bending fatigue strength is particularly required, the residual stress at the position of 50 μm from the surface (hereinafter also referred to as “σr (50)”) is greater than −700 MPa (absolute value). When the residual stress is small), the effect of improving the bending fatigue strength is poor. Therefore, σr (50) is defined as −700 MPa or less at the root portion. The upper limit of σr (50) is preferably −750 MPa. Although the lower limit of σr (50), which is a residual stress, does not need to be specified in particular, about −2000 MPa is an industrial limit.

(D) Value of ΔHV:
In order to improve the bending fatigue strength of the differential gear, it is necessary to increase the toughness while increasing the hardness of the hardened layer formed by induction hardening. When the ΔHV value represented by [(Vickers hardness of the surface layer portion) − (Vickers hardness of the core portion)] is less than 10, the hardness of the hardened layer on the surface and the hardness of the differential gear core portion hardly change. Therefore, improvement in bending fatigue strength cannot be expected. On the other hand, in the case of production by industrial induction hardening, it is difficult for the value of ΔHV to exceed 40. Therefore, the value of ΔHV was defined as 10-40. The lower limit of ΔHV is preferably 12.

  In the differential gear of the present invention, it is more desirable that the surface layer further satisfies the following conditions (e) and (f).

(E) Decarburized layer depth of the surface layer part: 0.015 mm or less,
(F) C content of surface layer portion: 0.35 to 0.50%.

  Hereinafter, each of the above (e) and (f) will be described.

(E) Total decarburized layer depth in the surface layer:
Decarburization generated in the differential gear surface layer has a great influence on the bending fatigue strength. When the total decarburized layer depth (DM-T) of the surface layer part exceeds 0.015 mm in the measuring method by a microscope specified in JIS G 0558 (2007), the toughness of the martensite of the hardened layer as the surface layer part is improved. However, since the strength is reduced, the effect of improving the bending fatigue strength in the “low to medium cycle range” is reduced. Therefore, it is preferable that the total decarburized layer depth of the surface layer portion is 0.015 mm or less. The upper limit of DM-T is preferably 0.012 mm. In addition, the lower limit of DM-T does not need to be specified, and the smaller the better.

(F) C amount in the surface layer:
In order to improve the bending fatigue strength of the differential gear, it is necessary to increase both the hardness and toughness of the hardened layer formed by induction hardening. When the amount of C in the surface layer is less than 0.35%, the toughness of the hardened layer can be increased, but the hardness is insufficient and improvement in bending fatigue strength cannot be expected. On the other hand, when the amount of C in the surface layer portion exceeds 0.50%, the hardness of the hardened layer can be sufficiently secured, but the toughness is lowered and the effect of improving the bending fatigue strength is reduced. Therefore, the C content in the surface layer portion is preferably 0.35 to 0.50%. The lower limit of the amount of C in the surface layer is preferably 0.38%. Further, the upper limit of the amount of C in the surface layer portion is preferably 0.48%.

(C) About manufacturing conditions:
The manufacturing condition of the differential gear described in detail below is one of methods for economically realizing the differential gear of the present invention on an industrial scale, and the technical scope of the differential gear itself is as follows. It is not defined by this manufacturing condition.

  For the differential gear according to the present invention, for example, the treatments described in the following steps <1> to <8> are sequentially performed on a steel material manufactured using the steel having the chemical composition of the dough described in (A). It can be manufactured by applying.

  In addition, about manufacture of the steel materials before performing the process of process <1>, it is not necessary to specify the conditions in particular.

(C-1) Heat treatment in step <1>:
In the step <1>, the steel material having the chemical composition of the dough described in the item (A) is heated at T ° C., which is a temperature range of 1000 to 1200 ° C., for a heating time within 10 minutes.

  In step <1>, which is a heat treatment, first, heating to T ° C., which is a temperature in the temperature range of 1000 to 1200 ° C., is the removal during heating that occurs in the steel material before forming into a predetermined differential gear shape. This is for suppressing charcoal and suppressing an increase in extreme deformation resistance during hot forging.

  When the heating temperature is less than 1000 ° C., the decarburization suppression effect is remarkable, but the deformation resistance of hot forging becomes too high, so excessive for hot forging to be formed into a predetermined differential gear shape Costly increase may be required.

  On the other hand, when the heating temperature exceeds 1200 ° C., the deformation resistance during hot forging becomes low, but it becomes difficult to suppress decarburization and is formed on the surface of the differential gear-shaped product after hot forging. In some cases, the thickness of the oxide scale increases, making it difficult to remove the oxide scale.

  Further, even when heating to T ° C., which is a temperature in the temperature range of 1000 to 1200 ° C., it may be difficult to suppress decarburization if the holding time exceeds 10 min.

  The lower limit of T ° C. as the heating temperature is more preferably 1050 ° C. from the viewpoint of reducing the forging load during hot forging (improving die life). The upper limit of the heating temperature T ° C is more preferably 1100 ° C from the viewpoint of suppressing decarburization.

  If the heating temperature reaches T ° C., hot forging may be started immediately after that. Although it does not prescribe | regulate especially about a heating means, it is preferable to use the high frequency induction heating which can be rapidly heated, or an electrically-heating method from a viewpoint of decarburization suppression, for example.

(C-2) Hot forging process in step <2>:
In step <2>, the heated steel material is hot forged at a forging end temperature of 900 ° C. or higher and formed into a differential gear shape.

  When the hot forging end temperature is lower than 900 ° C., the forging load increases and cracks due to poor ductility of the material may occur, resulting in poor molding. The lower limit of the end temperature of hot forging formed into the shape of the differential gear is more preferably 950 ° C.

  In addition, depending on the shape of a part to be formed by hot forging, a case where a temperature rise accompanying processing heat generation becomes large is assumed. If the hot forging end temperature exceeds 1200 ° C. due to the temperature rise accompanying the heat generated by the processing, it may be difficult to suppress decarburization. Therefore, the upper limit of the end temperature of hot forging formed into the shape of the differential gear is preferably 1200 ° C.

(C-3) Cooling treatment of the differential gear shape product formed by hot forging in step <3>:
In step <3>, the formed differential gear-shaped product is cooled to 300 ° C. at an average cooling rate of 1 to 20 ° C./s from the hot forging end temperature.

  When the average cooling rate in the temperature range from the temperature to 300 ° C. is less than 1 ° C./s after the end of hot forging, it stays for a long time in the temperature range where the two-phase structure of austenite and ferrite is likely to cause decarburization. Therefore, it may be difficult to suppress decarburization.

  On the other hand, after the end of hot forging, when the average cooling rate in the temperature range from that temperature to 300 ° C. is 20 ° C./s or more, martensite is generated and so-called “burning cracks” are formed in the differential gear shape product. May occur.

  The lower limit temperature of the temperature range for cooling at an average cooling rate of 1 to 20 ° C./s is sufficient to be up to 300 ° C., and the cooling rate in the temperature range below 300 ° C. is not limited. . For this reason, it may be determined as appropriate from, for example, air cooling (cooling), forced air cooling, mist cooling, etc. in consideration of manufacturing equipment and productivity.

(C-4) Removal treatment of oxide scale generated on the surface of the differential gear-shaped product in step <4>:
In step <4>, the oxide scale formed on the surface of the differential gear-shaped product is removed. This is because, in the state where the oxide scale is not removed, even if the heating at the time of quenching is carried out by controlling the atmosphere, the oxide scale serves as an oxygen supply source and combines with C on the material side to form CO _2. This is because the amount of C in the surface layer portion of the dynamic gear-shaped product is reduced, and it may be difficult to suppress ferrite decarburization.

  The means for removing the oxide scale generated on the surface of the differential gear-shaped product is not particularly specified. What is necessary is just to perform it by appropriate means, such as machining, such as cutting, shot blasting, and pickling.

(C-5) Quenching of the differential gear-shaped product from which the oxidized scale of step <5> has been removed:
In step <5>, the differential gear-shaped product from which the oxide scale has been removed is heated to 820 to 1000 ° C. in an atmosphere having a carbon potential of 0.40 to 0.50%, and then quenched.

  In the quenching process, when the carbon potential of the heating atmosphere is less than 0.40%, the carbon potential of the atmosphere is low relative to the C content of the dough described in the above (A), and thus decarburization occurs. There is. On the other hand, when the carbon potential of the heating atmosphere exceeds 0.50%, decarburization can be suppressed, but the carbon potential of the atmosphere is high with respect to the C content of the dough described in the above (A) item. The so-called “carburizing phenomenon” in which C enters from the atmosphere occurs, and as the amount of C in the surface layer portion of the differential gear-shaped product increases, it becomes difficult to ensure the toughness of the hardened layer, and brittle fracture is likely to occur. In some cases, the effect of improving the bending fatigue strength may be poor.

  In addition, when the heating temperature in the said atmosphere is less than 820 degreeC, the surface hardened layer does not become a uniform hard martensite structure, but ferrite remains in the microstructure after quenching, or after quenching Even if they all become martensite, fluctuations in the C concentration occur. As a result, since it is not possible to suppress the occurrence of an incompletely hardened structure by contour induction hardening described later, the bending fatigue strength may be reduced.

  On the other hand, when the heating temperature in the above atmosphere exceeds 1000 ° C., the microstructure after the quenching treatment is a uniform martensite structure, but since the prior austenite grain size becomes coarse, it is formed by contour induction hardening described later. It becomes difficult to refine the prior austenite average particle size of the hardened layer to 12 μm or less, and as a result, the bending fatigue strength may be lowered.

  By performing quenching under the above conditions, the amount of C in the surface layer portion of the differential gear is the above-mentioned characteristic (to), specifically, “the amount of C in the surface layer portion: 0.35 to 0.50%” Characteristics can be easily provided.

(C-6) Tempering the differential gear-shaped product in step <6>:
In step <6>, the tempered differential gear-shaped product is tempered at a temperature of 200 ° C. or lower for the purpose of preventing so-called cracking after quenching.

(C-7) Contour induction hardening of the differential gear-shaped product in step <7>:
In step <7>, the tempered differential gear-shaped product is further heated at a frequency of 40 to 60 kHz, preheated so that the tooth bottom surface temperature of the differential gear is 600 to 700 ° C., and thereafter It is left to cool to the atmosphere, and this heat treatment is performed so that the root surface temperature becomes 950 to 1050 ° C. with a heating time of 0.2 to 0.5 s, and contour induction hardening is performed.

  When performing contour induction hardening, it is necessary to perform quenching by heating to a predetermined temperature in a short time. However, when the differential gear-shaped product is heated from room temperature, the differential gear-shaped product cannot be heated in a short time to the quenchable austenite temperature range. Therefore, in order to help increase the temperature of the differential gear-shaped product during the main heating, it is necessary to perform pre-heat treatment for heating the differential gear-shaped product in advance.

  The reason for setting the frequency to 40 to 60 kHz is that the tooth tip temperature does not rise when the frequency is less than 40 kHz, whereas the tooth bottom temperature does not rise when the frequency exceeds 60 kHz.

  The reason for performing the pre-heat treatment so that the root surface temperature becomes 600 to 700 ° C. is that if the bottom surface temperature is lower than 600 ° C., the effect of pre-heating cannot be obtained. This is because the deformation of the differential gear-shaped product becomes large during quenching.

  The reason for setting the heating time for the main heating to 0.2 to 0.5 s is that if the heating time for the main heating is less than 0.2 s, it is difficult to control the induction hardening, and heating cannot be performed. This is because if it exceeds 0.5 s, a hardened layer along the contour cannot be formed.

  The reason for this heating so that the root surface temperature is 950 to 1050 ° C. is that austenitization is not sufficient when the root surface temperature is lower than 950 ° C., whereas the crystal grains are coarse when the root surface temperature is higher than 1050 ° C. This is because of

  After performing the above heat treatment on the differential gear-shaped product, quenching is performed by water cooling, oil cooling, or the like.

  Contour induction hardening under the above conditions facilitates the above-mentioned property (b), specifically, the property of “old austenite average particle size of the hardened layer: 12 μm or less” on the surface hardened layer of the differential gear. Can be provided.

  Note that the conditions of the contour induction hardening may be adjusted according to the shape of the part, etc., the frequency may be fixed and the high frequency heating may be performed, or the frequency may be changed during the heating.

(C-8) Tempering the differential gear-shaped product in step <8>:
In step <8>, the differential gear-shaped product subjected to contour induction hardening is tempered at a temperature of 200 ° C. or lower.

  By performing tempering under the above conditions, the above-mentioned characteristics (c) and (d) are applied to the hardened surface layer of the differential gear, specifically, “residual stress at the position of 50 μm from the surface in the tooth base portion: − 700 MPa or less ”and“ [(Vickers hardness of surface layer portion) − (Vickers hardness of core portion)] ”ΔHV value: 10 to 40” can be easily provided.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

Example 1
Steels A to E having the chemical composition shown in Table 1 were melted in a vacuum furnace to produce a 150 kg steel ingot.

  Steel A in Table 1 suppresses induction hardenability degradation by lowering the content of Si and Cr to 0.05% and 0.10%, respectively, of the components of alloy steel for machine structural use SCM440. In addition, the steel is improved in induction hardenability by adding B. Steel B, like Steel A, suppresses the decrease in induction hardenability by lowering the contents of Si and Cr to 0.08% and 0.10%, respectively, with respect to the components of alloy steel SCM440 for machine structural use. , And the content of C is increased to 0.51% and B is further added. Steel C is a steel in which Mo is not added to the components of alloy steel for mechanical structure SCM440, and steel D is The steel is a steel in which the content of C is reduced to 0.21% with respect to the components of the alloy steel for mechanical structure SCM440 and Mo is not added. Is a steel whose content is reduced to 0.21%. Among the above, steel D and steel E are steels corresponding to SCr420 and SCM420, which were produced in order to compare the characteristics in carburizing and quenching treatment, which is a conventional surface hardening treatment technique.

  Each steel ingot was heated to 1250 ° C. and then hot forged into a round bar having a diameter of 34 mm. The cooling after hot forging was allowed to cool in the atmosphere.

  For steels A to C, a round bar with a diameter of 21 mm was produced by machining from the center of the round bar with a diameter of 34 mm obtained by hot forging. Various round rods with a diameter of 21 mm shown in Table 2 were prepared. After heating to a heating temperature of 90 ° C. and maintaining soaking for 90 minutes, oil quenching was performed. After oil quenching, tempering was further performed at a heating temperature of 180 ° C. and a soaking time of 120 minutes.

  A rectangular parallelepiped having a cross section of 13 mm × 13 mm and a length of 100 mm is cut out from the central portion of the round bar having a diameter of 21 mm subjected to the tempering treatment, and then the length direction of one surface of the rectangular parallelepiped. A semicircle notch with a radius of 2 mm was provided at the central portion to prepare a 4-point bending test piece.

  A part of the above four-point bending test piece was subjected to induction hardening in which the semicircular notch was subjected to induction heating by changing the heating time under a condition where the output was 50 kW and then water cooling. Table 2 also shows details of the induction hardening conditions.

  Next, the induction-quenched 4-point bending test piece was tempered under the conditions of a heating temperature of 180 ° C. and a soaking time of 60 minutes, and the induction hardening property and the prior austenite grain size of the hardened layer were investigated.

  Induction hardenability is determined by JIS Z 2244 (2009) after embedding in a resin and polishing so that the cross-section of the four-point bending test piece after induction hardening and tempering can be examined at the site where the semicircular notch is provided. The Vickers hardness test was conducted by the method prescribed in the above. That is, a Vickers hardness test was performed with a test force of 2.94 N to obtain a hardness distribution, and a distance (mm) from a notch bottom, which is the surface of the cured layer, to a position where the HV hardness is 550 was obtained. In the following, the distance from the notch bottom (that is, the surface of the hardened layer) to the position where the HV hardness is 550 is referred to as “hardened layer depth”.

  Further, in the cross section of the part provided with the semicircular cutout of the test piece, the test force is set at 2.94 N and the position at 0.03 mm from the cutout bottom by the method defined in JIS Z 2244 (2009). HV hardness and HV hardness of the central part are measured, and each measured value is determined as Vickers hardness of the surface layer part and Vickers hardness of the core part, [ΔHV = (Vickers hardness of the surface layer part) − (Vickers of the core part) [Delta] HV expressed by (hardness)].

  The prior austenite grain size of the hardened layer was determined by embedding in a resin so that the cross section at the site where the semicircle notch of the four-point bending test piece after induction hardening and tempering was provided was mirror-polished and then surface-active. Corrosion with a saturated aqueous solution of picric acid added with an agent causes the prior austenite grain boundaries to appear, and using an optical micrograph at a magnification of 400 times taken arbitrarily for five fields from the surface to the above-mentioned “hardened layer depth” Then, the “average section length” L was determined by a cutting method, and “1.128 × L” was defined as the prior austenite average particle diameter of the hardened layer.

  In addition, in the case of test number 8 in Table 2 where induction hardening and tempering were not performed, polishing was performed by embedding in a resin so that the cross section at the site where the semicircle notch of the four-point bending test piece was provided could be investigated. After that, the Vickers hardness test was conducted with the test force set to 2.94 N by the method specified in JIS Z 2244 (2009), and the HV hardness at the position of 0.03 mm from the notch bottom and the HV hardness at the center were determined. Each measured value is expressed as [ΔHV = (Vickers hardness of the surface layer portion) − (Vickers hardness of the core portion)] as Vickers hardness of the surface layer portion and Vickers hardness of the core portion. Asked. In addition, using an optical micrograph at a magnification of 400 times taken arbitrarily for five fields from a surface to a depth of 0.5 mm, an “average section length” L is obtained by a cutting method, and similarly “1.128 × The value of “L” was defined as the prior austenite average particle diameter.

  On the other hand, for steel D and steel E, a rectangular parallelepiped having a cross section of 13 mm × 13 mm and a length of 100 mm is directly cut out from the center of the 34 mm diameter round bar obtained by hot forging, Thereafter, a semicircle notch having a radius of 2 mm was provided at a central portion in the length direction of one surface of the rectangular parallelepiped to prepare a four-point bending test piece.

  Next, the carbon potential was maintained at 940 ° C. for 180 minutes under a carbon potential of 0.8%, then cooled to 870 ° C. and further maintained for 60 minutes, followed by carburizing and quenching under oil quenching conditions. After carburizing and quenching, the steel was further tempered under the conditions of a heating temperature of 180 ° C. and a soaking time of 60 minutes, and then the hardened layer depth and ΔHV were investigated in the same manner as described above. Using an optical micrograph of 400 times magnification arbitrarily taken for 5 fields for the area up to “sa”, the “average section length” L is obtained by a cutting method, and the value of “1.128 × L” is set to the old value of the cured layer. The austenite average particle diameter was used.

  Moreover, the value of σr (50) which is a residual stress at a position of 50 μm from the surface of the semicircular notch bottom using a total of 16 conditions of four-point bending test pieces of steels A to E subjected to the above various treatments. Was measured. The measurement is performed by electrolytic polishing to a depth of 50 μm from the surface, the intensity of the diffracted X-ray is measured at the depth of 50 μm, and the relationship between the half-value width of the peak intensity obtained by the measurement and the peak center position. From this, the residual stress was determined.

  Furthermore, the bending fatigue strength was investigated using the 4-point bending test piece of the total 16 conditions of steel AE which performed said various process.

In the bending fatigue strength test, the stress ratio was 0.1 and the distance between the fulcrums was 45 mm, and the crack initiation strength at the number of repetitions of 1 × 10 ⁴ was evaluated as the bending fatigue strength.

  In addition, on the basis of the bending fatigue strength when the steel D corresponding to SCr420 was carburized and quenched and tempered, the target was to improve it by 40% or more.

  Table 2 also shows the results of the above tests. Table 2 also shows the improvement rate from the value when the bending fatigue strength of test number 15 is used as a reference value.

  Table 2 reveals that test numbers 1 to 7 have good bending fatigue strength, whereas test numbers 8 to 14 have low bending fatigue strength.

And from this Table 2, after heating in the temperature range of 820 to 1000 ° C., performing tempering treatment of quenching and tempering, and further performing induction quenching and tempering,
-Hardened layer depth 0.80-1.50mm,
-The prior austenite average particle size of the hardened layer is 12 μm or less,
-Residual stress at a position of 50 μm from the surface is −700 MPa or less,
A value of ΔHV represented by [(Vickers hardness of surface layer portion) − (Vickers hardness of core portion)] is 10 or more,
Then, it can be seen that a bending fatigue strength higher by 40% or more than that can be obtained on the basis of the bending fatigue strength of the carburized steel D.

(Example 2)
Steels F to H having the chemical compositions shown in Table 3 were melted in a vacuum furnace to produce a 150 kg steel ingot, and evaluation was performed on the actual part shape.

  Steel F in Table 3 is a steel whose chemical composition is within the range defined by the present invention, and is a steel corresponding to Steel A in Table 1 described above. On the other hand, the steel G is a steel of a comparative example in which C among the constituent elements is out of the content range defined in the present invention. Steel H is a steel of a comparative example in which C, Si, P, Cr, Mo, Ti, N, and B among the constituent elements deviate from the content range defined in the present invention.

  Of the steels of the above comparative examples, steel H corresponds to SCr420 described in JIS G 4053 (2008), and corresponds to steel D in Table 1 described above.

  Each steel ingot was heated to 1250 ° C. and then hot forged into a round bar having a diameter of 34 mm. The cooling after hot forging was allowed to cool in the atmosphere.

  Next, with regard to Steel F and Steel G, the above-mentioned round bar having a diameter of 34 mm obtained by hot forging was heated at 870 ° C. for 60 minutes to form an austenite single phase structure, and then allowed to cool in the atmosphere. On the other hand, for steel H, the above-mentioned round bar having a diameter of 34 mm obtained by hot forging was heated at 925 ° C. for 60 minutes to form an austenite single phase structure and allowed to cool in the atmosphere.

  A test piece having a diameter of 28 mm and a length of 34.3 mm was cut out from the center of the cooling material having a diameter of 34 mm by machining, and hot forging was performed under the conditions shown in Table 4 using this test piece as a material. 1 was formed into the shape of a differential gear having an outer diameter of 53.5 mm, an inner diameter of 18 mm, and a thickness of 18 mm.

  Specifically, in Test No. 17, Test No. 20 and Test No. 21, hot forging was performed so that the forging end temperature would be 1000 ° C. after performing a heat treatment for 3 min with a heating temperature T ° C. of 1100 ° C. And it shape | molded in the shape of the differential gearwheel.

  In Test No. 18 and Test No. 19, after heating for 5 minutes with a heating temperature T ° C. of 1100 ° C., hot forging was performed so that the forging end temperature would be 1000 ° C. to obtain a differential gear shape. Molded.

  In Test No. 17, after completion of hot forging, the sample was cooled to 300 ° C. at an average cooling rate of 10 ° C./s, and then allowed to cool in the atmosphere to cool to room temperature.

  In test number 18, after the hot forging was completed, it was cooled to 300 ° C. at an average cooling rate of 0.1 ° C./s, and then allowed to cool in the air to cool to room temperature.

  In Test No. 19 and Test No. 20, after completion of hot forging, the sample was cooled to 300 ° C. at an average cooling rate of 2 ° C./s, then allowed to cool in the atmosphere and cooled to room temperature.

  In test number 21, after completion of hot forging, the steel was cooled to 300 ° C. at an average cooling rate of 0.2 ° C./s, and then allowed to cool in the atmosphere to cool to room temperature.

Next, for the differential gear-shaped products of test number 17, test number 18, test number 20 and test number 21, an oxide scale formed on the surface during hot forging is used as a steel ball having an HV hardness of 400 to 500. Was removed by shot blasting under conditions of a projection time of 15 min and a projection pressure of 0.2 to 0.4 MPa ( ^{2 to 4} kgf / cm ² ). For the differential gear shape product of test number 19, the oxide scale generated on the surface was not removed.

  After the above-described treatment, the differential gear-shaped products of test numbers 17 to 20 were heated to 870 ° C. in an atmosphere adjusted to the carbon potential shown in Table 4 and held soaked for 90 minutes, and then oil quenching was performed.

  After oil quenching, tempering was further performed at a heating temperature of 180 ° C. and a soaking time of 120 minutes.

  The differential gear-shaped products of test numbers 17 to 20 subjected to the above treatment were subjected to contour induction hardening and tempering under the conditions shown in FIG. A water-soluble quenching coolant was used for cooling during induction quenching.

  For the differential gear-shaped product of test number 21, after removing the oxidized scale generated on the surface by shot blasting, holding at 940 ° C. for 180 minutes under the condition of carbon potential of 0.8%, the temperature was increased to 870 ° C. After cooling and holding for an additional 60 minutes, carburizing and quenching was performed under conditions for oil quenching. After carburizing and quenching, the steel was further tempered under the conditions of a heating temperature of 180 ° C. and a soaking time of 60 minutes to finish a differential gear.

  About each differential gear produced as mentioned above, the total decarburized layer depth (DM-T) by the measuring method by a microscope, a hardened layer depth (henceforth "d" may be hereafter), and a hardened layer. Old austenite average particle diameter, ΔHV expressed by [residual stress (σr (50)) at the position of 50 μm from the surface in the root portion], [(Vickers hardness of surface layer portion) − (Vickers hardness of core portion)] Fatigue strength was evaluated along with investigation and measurement of the amount of C in the surface layer portion of the hardened layer. However, the fatigue strength of the differential gear No. 19 was not evaluated.

  First, it was embedded in a resin and polished so that the longitudinal cross section at the tooth root portion of the differential gear could be investigated, then corroded with 3% nitric acid alcohol solution (nitral), and DM- as defined in JIS G 0558 (2007). T was investigated.

  Also, after embedding and polishing in resin so that the longitudinal section at the tooth root part of the differential gear can be investigated, the test force is set to 2.94N and the Vickers hardness test is performed according to the method specified in JIS Z 2244 (2009). To obtain the hardness distribution, and the distance d from the tooth root surface to the position where the HV hardness is 550 was obtained. Further, in the longitudinal section of the tooth root portion, the HV hardness at the position of 0.03 mm from the surface and the HV hardness at the center portion are set to 2.94 N by the method defined in JIS Z 2244 (2009). Measure each measured value as the Vickers hardness of the surface layer part and the hardness of the core part, and obtain ΔHV represented by [ΔHV = (Vickers hardness of the surface layer part) − (Vickers hardness of the core part)]. It was.

  Furthermore, after embedding in a resin and mirror polishing so that the longitudinal section of the hardened layer at the root part of the differential gear can be observed, it is corroded with a saturated aqueous solution of picric acid added with a surfactant, and the former austenite grain boundary appears. Then, using an optical micrograph of magnification 400 times taken arbitrarily for 5 fields of the cured layer having a distance from the surface of up to d, an “average section length” L was determined by a cutting method, and “1.128 × L” From this, the prior austenite average particle size of the hardened layer was calculated.

  Further, the amount of C in the surface layer portion is embedded in a resin so that the longitudinal section at the tooth root portion of the differential gear can be investigated and polished, and then the wavelength dispersion type EPMA device in the center direction of the gear is in a range of 100 μm from the surface. It was determined by measuring with a calibration curve using

  In addition, the surface is polished to a depth of 50 μm from the surface by electrolytic polishing, the intensity of the diffracted X-ray is measured at the depth of 50 μm, and the relationship between the half-value width of the peak intensity obtained by the measurement and the peak center position The value of σr (50), which is the residual stress at the position of 50 μm from the surface, was measured at the root portion of the differential gear.

  For the evaluation of fatigue strength, a differential durability test apparatus was used, and the rotational speed on the input side was set to 12 rpm.

  FIG. 3 schematically shows an example of the differential durability test apparatus. FIG. 4 schematically shows an example of the differential unit in the differential durability test apparatus.

  Hereinafter, the mechanism of the differential durability test apparatus will be described.

  First, torque input from the input shaft 1 of the differential durability test apparatus is input to the differential case 4 constituting the differential unit 3 via the ring gear 2. The torque input from the input shaft 1 means a set value of the motor.

  Next, the rotation input to the differential case 4 is transmitted to the pinion gear 6 and the pair of side gears 7 meshed with the pinion gear 6 via the pinion shaft 5, and from the pair of side gears 7 to the output shaft 8. Is output.

  Note that one end of the output from the differential case 4 constituting the differential unit 3 is a fixed shaft 8 ′, and the differential case 4 is configured to be differential with the rotation of the output shaft 8. That is, when the above-described pinion gear 6 is rotated, the paired side gears 7 can be rotated relative to each other, that is, differentially. The “pinion gear 6” and the “side gear 7” in the figure are “differential gears”. It corresponds to.

From the relationship between the torque obtained in the test using the differential durability test apparatus and the number of repetitions, the breaking torque at the number of repetitions of 1 × 10 ⁴ was calculated, and the calculated breaking torque was evaluated as the bending fatigue strength.

  The bending fatigue strength is a first target that the bending fatigue strength in the case of test number 21 in which the steel H corresponding to SCr420 is carburized and quenched and tempered is 30% or more. The second goal was to improve more than%.

  Table 5 shows the results of the above tests.

  From Table 5, in the case of the differential gear of test number 17 that satisfies the conditions defined in the present invention, the bending fatigue strength is 2880 N · m, and steel H corresponding to SCr420 described in JIS G 4053 (2008) is used. It is 40% or more higher than the bending fatigue strength of the differential gear No. 21 carburized and quenched and tempered, and has excellent bending fatigue strength in the second target “low to middle cycle range”. it is obvious.

  Further, in the case of the differential gear of test number 18, the bending fatigue strength is improved by 30% or more than the bending fatigue strength of the differential gear of test number 21 which is carburized and tempered using steel H corresponding to SCr420. The first target has a bending fatigue strength. However, a decarburized layer has occurred, and DM-T exceeds 0.030 mm, which exceeds the upper limit specified in the invention, so that it has not reached the 40% improvement of the second target.

  On the other hand, in the case of the differential gear of test number 20, since the C content of the steel G as a dough is as high as 0.51% and exceeds the upper limit specified in the present invention, the toughness of the hardened layer is reduced. The bending fatigue strength is not improved as compared with the differential gear of test number 21 carburized and tempered using steel H corresponding to SCr420.

  The bending fatigue strength in the “low to medium cycle range” of the differential gear of the present invention is greatly improved compared to the bending fatigue strength of a differential gear subjected to conventional carburizing and quenching treatment. For this reason, the differential gear of the present invention is suitable for use as an automobile differential gear which is shocking and may be subjected to a relatively large load. This differential gear can be easily manufactured by the manufacturing method of the present invention.

1: Input shaft
2: Ring gear
3: Differential unit
4: Differential case
5: Pinion shaft
6: Pinion gear
7: Side gear
8: Output shaft
8 ': Fixed shaft

Claims (3)

The dough steel is in mass%, C: 0.35 to 0.45%, Si: 0.10% or less, Mn: 0.50 to 1.0%, P: 0.015% or less, S: 0 0.030% or less, Cr: 0.05 to 0.15%, Mo: 0.15 to 0.25%, Al: 0.01 to 0.05%, N: 0.010% or less, O (oxygen) : 0.0020% or less, B: 0.0010 to 0.0030% and Ti: 0.010 to 0.045%, the balance being steel having a chemical composition composed of Fe and impurities, and A differential gear characterized by satisfying the conditions (a) to (d).
(I) Hardened layer depth: 0.80 to 1.50 mm
(B) Old austenite average particle diameter of the hardened layer: 12 μm or less (c) Residual stress at the position of 50 μm from the surface at the root part: −700 MPa or less (d) [(Vickers hardness of the surface layer part) − (core part ΔHV value represented by Vickers hardness)]: 10 to 40 The differential gear according to claim 1, further satisfying the following conditions (e) and (f):
(E) Total decarburized layer depth of surface layer: 0.015 mm or less (f) C content of surface layer: 0.35 to 0.50% A method for manufacturing a differential gear, comprising: treating the steel material having the chemical composition of the cloth of the differential gear according to claim 1 in the order of the following steps <1> to <8>.
Step <1>: The steel material is heated at a temperature in the temperature range of 1000 to 1200 ° C. and at a heating time of 10 minutes or less.
Step <2>: The heated steel material is hot forged at a forging end temperature of 900 ° C. or higher and formed into a differential gear shape.
Step <3>: The formed differential gear-shaped product is cooled to 300 ° C. at an average cooling rate of 1 to 20 ° C./s from the hot forging end temperature.
Step <4>: The oxide scale generated on the surface of the differential gear-shaped product is removed.
Step <5>: The differential gear-shaped product from which the oxide scale has been removed is heated to 820 to 1000 ° C. in an atmosphere having a carbon potential of 0.40 to 0.50%, and then quenched.
Step <6>: The quenched differential gear product is tempered at a temperature of 200 ° C. or lower.
Step <7>: The tempered differential gear-shaped product is further heated at a frequency of 40 to 60 kHz, preheated so that the root surface temperature of the differential gear reaches 600 to 700 ° C., and then released to the atmosphere. It cools and this heat processing is performed so that a root surface temperature may be 950-1050 degreeC by heating time 0.2-0.5 s, and contour induction hardening is performed.
Process <8>: The differential gear-shaped product subjected to contour induction hardening is tempered at a temperature of 200 ° C. or lower.

Images