JP4835367B2 - Carburized parts or carbonitrided parts - Google Patents

Description

  The present invention relates to a carburized part or a carbonitrided part. Specifically, the present invention relates to a carburized part or a carbonitrided part excellent in bending fatigue resistance and pitting resistance, including automobile gears.

  Gears used in automobile transmissions are generally tempered after surface hardening treatment such as carburizing quenching or carbonitriding quenching from the viewpoint of improving the bending fatigue strength of the tooth root and the pitching strength of the tooth surface. It is manufactured with shot peening.

The “carburizing and quenching” described above is a process of quenching after intruding and diffusing C in a high temperature austenite region of Ac ₃ point or higher.

  “Carbon nitriding and quenching” is a treatment in which C and N are simultaneously penetrated and diffused and then hardened. Compared with carburizing and quenching, it has a feature that high surface hardness and high temper softening resistance can be obtained. is there.

  In recent years, automobiles are required to be lightweight and high torque. For this reason, parts used for automobiles including the gears described above require higher bending fatigue strength and pitching strength than ever before.

  In response to these requirements, when the surface hardness after carburizing or carbonitriding is low, bending fatigue strength and pitching strength may be low.

  The bending fatigue strength can be improved by performing shot peening after carburizing or carbonitriding and applying compressive residual stress to the surface of the component. However, when the surface roughness is increased by performing shot peening, the pitching strength may be reduced due to occurrence of a portion where the surface pressure is locally increased when the gears are engaged.

  Furthermore, when a so-called “incompletely hardened layer” exists after carburizing or carbonitriding, bending fatigue strength and pitching strength may be low because many pearlite structures with low strength exist on the surface. there were.

  For this reason, there is a great demand for developing carburized parts or carbonitrided parts having excellent bending fatigue resistance and pitting resistance.

  Accordingly, in order to meet the above-described demand, for example, Patent Document 1 and Patent Document 2 propose “steel for carburizing or carbonitriding” and “steel member”, respectively.

  That is, Patent Document 1 discloses a steel that is shot peened after carburizing or carbonitriding, and in terms of weight percent, C = 0.10 to 0.30%, Si = 0.40 to 1.50%, Mn = 0. .30 to 2.40%, Al = 0.01 to 0.050%, N = 0.0050 to 0.0250%, and if necessary, S = 0.005 to 0.035%, Pb = 0 0.01 to 0.09%, Bi = 0.04 to 0.20%, Te = 0.002 to 0.030%, Zr = 0.01 to 0.20%, Ca = 0.0001 to 0.0100 %, Or “the steel for carburizing or carbonitriding” consisting of Fe and unavoidable impurity elements.

Moreover, in patent document 2, a core part is the mass%, C: 0.10-0.30%, Si: 0.15-1.0%, Mn: 0.20-1.0%, Cr: 1.0-2.0%, Mo: 0.05-0.6%, and A1: 0.005-0.05%, Nb: 0.005-0.05%, Ti: 0.00. 1 or 2 or more elements selected from the group consisting of 005 to 0.1%, and N: 0.008 to 0.05%, and the austenite grain size number after carburizing and tempering is The surface density of the carbide of 0.5 μm or less in the surface layer portion within 50 μm from the surface is 6.0 pieces / 10 μm ² or more, and the number of carbides of 0.5 μm or less in the total number of carbides Excellent pitting resistance such as gears, sliding parts, shafts, etc. used at high surface pressure, with a ratio of 80% or more Steel components for machine structures that have both good workability and fatigue strength, and “steel members” used as tools having excellent wear resistance are disclosed.

JP 2000-273574 A JP 2002-212672 A

  The technique disclosed in Patent Document 1 described above is a technique for improving compressive residual stress and surface hardness while reducing the surface roughness due to shot peening by adjusting the chemical composition of steel. . However, by simply adjusting the chemical composition, it has not always been possible to provide parts with characteristics that can cope with the recent reduction in weight and torque of automobiles, in particular, high pitching strength. .

  The technique disclosed in Patent Document 2 has a good pitching life as evaluated in a state in which rough skin due to shot peening after finish polishing is removed, as shown in the examples. However, some actual parts are used in a state in which the rough surface remains as shot peened. Therefore, even with the above technique, the pitching life of the actual parts is not necessarily improved.

  Accordingly, an object of the present invention is to provide a carburized part or a carbonitrided part excellent in bending fatigue resistance and pitching resistance, including automobile gears.

  In order to solve the above-mentioned problems, the present inventors have provided the necessary strength as a carburized part or a carbonitrided part, and then improved the surface fatigue property of the part in order to improve the bending fatigue resistance and pitting resistance. As a result of various studies, the following findings (a) to (c) were obtained.

  (A) Since the yield strength increases as the surface hardness of the part increases, generation of microcracks at the beginning of bending fatigue and generation of microcracks at the beginning of pitching can be suppressed. In order to provide the carburized part or the carbonitrided part with good bending fatigue resistance and pitting resistance, the minimum value of the surface hardness of the part (hereinafter also referred to as “HV (min)”) is Vickers hardness. Now it should be more than a specific value.

  The “minimum value of surface hardness” in the above-mentioned Vickers hardness of carburized parts or carbonitrided parts is defined by cutting the part at right angles to the machining direction, mirror-polishing the surface as the test surface, and JIS Z 2244 According to "Vickers hardness test-test method" in (2003), the minimum value when 100 points are arbitrarily measured at a position of 0.03 mm from the surface. However, the term “perpendicular to the working direction” means, for example, that the process is perpendicular to the rolling direction when the process is rolling, and that the process is perpendicular to the forging axis when the process is forging.

  (B) As the surface roughness of the part increases, a large surface pressure is generated locally, and pitching is likely to occur. In order to provide the carburized component or the carbonitrided component with good pitching resistance, the maximum height roughness Rz (hereinafter also referred to as “Rz (max)”) of the roughness curve of the component is used. What is necessary is just to be below a specific value.

  The above-mentioned “maximum height of roughness curve Rz” of carburized parts or carbonitrided parts is “geometric characteristic specification (GPS) of product—surface property: contour curve” in JIS B 0601 (2001). In accordance with "Method-Terminology, Definition and Surface Property Parameters", the surface roughness was measured by 3 mm each in parallel with the processing direction with reference to any 100 points on the surface of the part using a stylus type surface roughness meter. It refers to the maximum value of the maximum height roughness Rz. However, “parallel to the processing direction” means, for example, that the processing is parallel to the rolling direction when the processing is rolling, and that the processing is parallel to the forging axis when the processing is forging.

  (C) Bending fatigue and pitching are more likely to occur as the depth of the incompletely hardened layer of the part increases. In order to provide the carburized part or the carbonitrided part with good bending fatigue resistance and pitting resistance, the maximum value of the incompletely hardened layer of the part (hereinafter also referred to as “T (max)”) is used. What is necessary is just to be below a specific value.

  The “incompletely hardened layer” of carburized parts or carbonitrided parts is obtained by cutting the part at right angles to the machining direction, mirror-polishing the surface as the test surface, and then corroding with nital for 0.5 to 2 seconds. The part that is deeply corroded when observed at a magnification of 1000 times so as to include the outermost surface of the component by an optical microscope. In addition, the “maximum value of the incompletely hardened layer” of the carburized part or the carbonitrided part is defined as “from the top surface of the part in each of the“ incompletely hardened layer ”observed arbitrarily at 100 fields with an optical microscope at a magnification of 1000 times. The largest value among the maximum distances.

  As described above, “perpendicular to the processing direction” means, for example, that the processing is a rolling process at a right angle to the rolling direction, and that the processing is a forging process, that is at a right angle to the forging axis. Point to.

  The present invention has been completed based on the above findings, and the gist thereof resides in the carburized parts or carbonitrided parts shown in the following (1) and (2).

(1) Dough is mass%, C: 0.10 to 0.30%, Si: 0.05 to 1.0%, Mn: 0.35 to 1.2 %, P: 0.030% or less , S: 0.005 to 0.050%, Cr: 1.21 to 3.0%, Al: 0.010 to 0.050%, N: 0.0050 to 0.0250%, the balance being It is a steel of chemical composition consisting of Fe and impurities. The minimum value of the surface hardness is 750 or more in terms of Vickers hardness, the maximum value of the roughness curve, the maximum value of the roughness Rz is 6 μm or less, and the maximum incompletely hardened layer A carburized part or a carbonitrided part having a value of 15 μm or less.
Here, the incompletely hardened layer means that the part is cut at right angles to the machining direction, the surface is mirror-polished as a test surface, and then corroded with nital for 0.5 to 2 seconds. The portion that is deeply corroded when observed at a magnification of 1000 times so as to include.

  (2) The above-mentioned steel containing one or more of Mo: 1.0% or less, V: 0.250% or less, and Nb: 0.070% or less, instead of part of Fe The carburized component or carbonitrided component according to (1).

  As already mentioned, the “minimum value of surface hardness” in Vickers hardness means that a part is cut at right angles to the machining direction, and the surface is mirror-polished as the surface to be tested, according to JIS Z 2244 (2003). Based on "Vickers hardness test-test method", it refers to the minimum value when 100 points are arbitrarily measured at a position of 0.03 mm from the surface.

  “Maximum height of roughness curve Rz maximum value” means “product geometric property specification (GPS) -surface property: contour curve method—terminology, definition and surface property” in JIS B 0601 (2001). The maximum height roughness Rz obtained by measuring 3 mm each in parallel with the machining direction with a stylus type surface roughness meter based on an arbitrary 100 points on the part surface in accordance with the “parameter” Points to the value.

  Furthermore, the “incompletely hardened layer” means that a part is cut at right angles to the machining direction, and the surface is mirror-polished as a test surface and then corroded with nital for 0.5 to 2 seconds. When observed at a magnification of 1000 times so as to include the outermost surface, it refers to a portion that is deeply corroded, and the “maximum value of an incompletely hardened layer” was arbitrarily observed at 100 magnifications at a magnification of 1000 times with an optical microscope. It indicates the largest value among the maximum distances from the outermost surface of the part in each “incompletely hardened layer”.

  Hereinafter, the inventions related to the carburized parts or carbonitrided parts of (1) and (2) above are referred to as “present invention (1)” and “present invention (2)”, respectively. Also, it may be collectively referred to as “the present invention”.

  Since the carburized or carbonitrided parts of the present invention have good bending fatigue strength and pitching strength, they can be used as parts such as gears used in automobile transmissions.

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

(A) Chemical composition of the steel of the dough:
C: 0.10 to 0.30%
C is an essential element for ensuring the strength of the carburized part or the carbonitrided part, and a content of 0.10% or more is necessary. However, when the C content increases, the hardness increases and the machinability is lowered. In particular, when the content exceeds 0.30%, the machinability is significantly lowered as the hardness is increased. Therefore, the content of C is set to 0.10 to 0.30%.

  In addition, when much better machinability is required, the C content is preferably 0.10 to 0.25%.

Si: 0.05-1.0%
Si has an action of improving hardenability and a deoxidizing action. Si also has the effect of increasing the tempering softening resistance and improving the pitching strength. In order to obtain these effects, a content of 0.05% or more is necessary. However, when the Si content increases, the hardness increases and the machinability deteriorates. In particular, when the content exceeds 1.0%, the machinability decreases as the hardness increases. Moreover, when the Si content increases, Si is selectively oxidized by a small amount of H ₂ O or CO ₂ contained in the carburizing gas, and Si oxide is formed. The formation of this Si oxide leads to a deficiency in the surface portion of Si, which is a hardenability improving element, so that the depth of the incompletely hardened layer is increased. In particular, the Si content is 1. If it exceeds 0%, the depth of the incompletely hardened layer will increase significantly. Therefore, the Si content is set to 0.05 to 1.0%. A more preferable Si content is 0.05 to 0.80%.

Mn: 0.35 to 1.5%
Mn has an action of improving hardenability and a deoxidizing action. In order to obtain these effects, a content of 0.35% or more is necessary. However, when the Mn content increases, the hardness increases and the machinability is lowered. In particular, when the content exceeds 1.5%, the machinability is significantly lowered as the hardness is increased. Moreover, when the Mn content increases, the depth of the incompletely hardened layer increases as in the case of Si. In particular, when the Mn content exceeds 1.5%, The increase in depth becomes significant. Therefore, the content of Mn is set to 0.35 to 1.5%. The Mn content is more preferably 0.4 to 1.2%.

P: 0.030% or less P is an impurity contained in the steel and has the effect of strengthening ferrite, but segregates at the grain boundaries and embrittles the steel. In particular, its content is 0.030%. If it exceeds 1, the embrittlement of the steel becomes significant. Therefore, the content of P is set to 0.030% or less. In order to suppress the embrittlement of the steel, the P content is preferably 0.020% or less.

S: 0.005 to 0.050%
S combines with Mn to form MnS and has the effect of improving machinability. However, if the content is less than 0.005%, it is difficult to obtain the above effect. On the other hand, if the S content exceeds 0.050%, coarse MnS is formed, and hot forgeability, cold forgeability, bending fatigue strength, and pitching strength are reduced. Therefore, the content of S is set to 0.005 to 0.050%.

  In addition, when much better hot forgeability, cold forgeability, bending fatigue strength, and pitching strength are required, the S content is preferably 0.005 to 0.030%.

Cr: 0.70 to 3.0%
Cr has the effect of improving hardenability. In order to obtain this effect, a content of 0.70% or more is necessary. However, when the Cr content increases, the hardness increases and the machinability is lowered. In particular, when the content exceeds 3.0%, the machinability is significantly lowered as the hardness is increased. Moreover, when the Cr content is increased, Cr oxide or Cr nitride is generated on the surface during carburizing or carbonitriding, and the depth of the incompletely hardened layer is increased. If it exceeds 3.0%, the depth increase of the incompletely hardened layer becomes remarkable. Therefore, the content of Cr is set to 0.70 to 3.0%.

  In addition, when much better machinability is required and it is necessary to further reduce the depth of the incompletely hardened layer, the Cr content is 0.70 to 1.70%. It is preferable to do.

Al: 0.010 to 0.050%
Al has a deoxidizing action. Al combines with N to form AlN to refine crystal grains and strengthen steel. However, when the Al content is less than 0.010%, it is difficult to obtain the above effect. On the other hand, when the Al content is excessive, the machinability is lowered due to the formation of hard and coarse Al ₂ O ₃ , and the bending fatigue strength and the pitching strength are also lowered. In particular, when the Al content exceeds 0.050%, the machinability, bending fatigue strength, and pitching strength are significantly reduced. Therefore, the content of Al is set to 0.010 to 0.050%. In addition, preferable Al content is 0.020 to 0.040%.

N: 0.0050 to 0.0250%
N has the effect of reducing the crystal grains by forming nitrides and improving the bending fatigue strength. In order to obtain this effect, the N content needs to be 0.0050% or more. However, if the N content is excessive, coarse nitrides are formed and the toughness is reduced. In particular, if the N content exceeds 0.0250%, the toughness is significantly reduced. Therefore, the N content is set to 0.0050 to 0.0250%. In addition, preferable N content is 0.0100 to 0.0250%.

  For the above reasons, the chemical composition of the steel of the carburized part or carbonitrided part according to the present invention (1) contains the elements from C to N in the above-mentioned range, and the balance consists of Fe and impurities. Stipulated.

  It should be noted that the chemical composition of the steel of the carburized part or carbonitrided part according to the present invention is replaced with a part of Fe in the present invention (1) in order to ensure even better bending fatigue strength and pitching strength. In addition, one or two or more of Mo: 1.0% or less, V: 0.250% or less, and Nb: 0.070% or less may be contained.

  That is, at least one or more of Mo, V, and Nb described above may be contained in place of a part of Fe in the dough steel of the present invention (1).

  Hereinafter, the above elements will be described.

Mo: 1.0% or less Mo is an element that improves hardenability, and improves surface hardness, hardened layer depth, and core hardness (dough hardness) after carburizing or carbonitriding. , Has an effect of increasing bending fatigue strength and pitching strength. However, when the Mo content increases, the hardness increases and the machinability decreases, and particularly when the Mo content exceeds 1.0%, the machinability decreases as the hardness increases. Therefore, the content when Mo is contained is set to 1.0% or less.

  In addition, in order to acquire the above-mentioned effect of Mo reliably, it is desirable to make the content 0.05% or more. Therefore, the more desirable Mo content for improving the bending fatigue strength and the pitching strength is 0.05 to 1.0%. In addition, when much better machinability is required, the upper limit of the Mo content is preferably 0.50%.

V: 0.250% or less V binds to C and N, precipitates as fine carbides, nitrides, and carbonitrides, refines crystal grains, and improves bending fatigue strength and pitting strength. . Furthermore, the fine carbides, nitrides, and carbonitrides of V described above also have an effect of improving the hardened layer depth. However, when the V content increases, the precipitated carbides, nitrides and carbonitrides become coarse, and on the contrary, the bending fatigue strength and the pitting strength are lowered. In particular, when the content exceeds 0.250%. In addition, the bending fatigue strength and the pitching strength are significantly reduced. Therefore, the content when V is contained is set to 0.250% or less.

  In order to surely obtain the effect of V described above, the content is preferably set to 0.030% or more. Therefore, the more desirable V content for improving the bending fatigue strength and the pitching strength is 0.030 to 0.250%. In order to stably precipitate fine carbides and carbonitrides of V, the V content is more preferably 0.050% or more. Moreover, in order to suppress precipitation of coarse carbides and carbonitrides, it is more preferable to set the upper limit of V to 0.150%. Therefore, when more bending fatigue strength and pitching strength are required, the V content is more preferably 0.050 to 0.150%.

Nb: 0.070% or less Nb combines with C and N, precipitates as fine carbides, nitrides, and carbonitrides, refines crystal grains, and improves bending fatigue strength and pitching strength . However, when the Nb content increases, surface flaws are likely to occur during hot rolling or hot forging. In particular, when the content exceeds 0.070%, the surface during hot rolling or hot forging. The occurrence of wrinkles becomes significant. Therefore, the content when Nb is contained is set to 0.070% or less. In addition, in order to acquire the effect of above-mentioned Nb reliably, it is preferable that the content shall be 0.010% or more. Therefore, the more desirable Nb content for improving the bending fatigue strength and the pitching strength is 0.010 to 0.070%.

  In addition, said Mo, V, and Nb can be contained only in one of them, or 2 or more types of composites.

  For the above reasons, the chemical composition of the steel of the carburized part or carbonitrided part according to the present invention (2) is replaced with a part of Fe in the steel of the present invention (1), Mo: 1.0 % Or less, V: 0.250% or less, and Nb: 0.070% or less.

(B) Surface properties of parts:
Minimum surface hardness (HV (min)): 750 or more in terms of Vickers hardness The higher the surface hardness of carburized parts or carbonitrided parts, the higher the yield strength. Therefore, the occurrence and pitching of microcracks at the beginning of bending fatigue. The occurrence of initial microcracks can be suppressed. And by making the minimum value of the surface hardness of a component into 750 or more by a Vickers hardness, a high bending fatigue strength and a large pitching strength can be provided.

  For example, if the part is a “gear”, if the HV (min) is 750 or more in terms of Vickers hardness, the surface hardness of the tooth root surface where stress is concentrated is large, so that tooth root bending fatigue is suppressed. In addition, since the surface hardness of the tooth surface portion is high, it is possible to suppress pitting fatigue.

  The upper limit of the HV (min) Vickers hardness is that if the surface hardness is too high, it is difficult to correct when shaft parts such as shafts need to be corrected after carburizing and quenching, or Since there is a problem that it is difficult to polish when finishing polishing is required, it is preferably about HV1000.

  As already stated, the "minimum surface hardness" in Vickers hardness is the part cut at right angles to the machining direction, and the surface is mirror-polished as the test surface, and JIS Z 2244 (2003) In accordance with “Vickers hardness test—test method”, it refers to the minimum value when 100 points are arbitrarily measured at a position of 0.03 mm from the surface. Further, the term “perpendicular to the working direction” means, for example, that the process is perpendicular to the rolling direction when the process is rolling, and that the process is perpendicular to the forging axis when the process is forging.

Maximum height of roughness curve Maximum value of roughness Rz (Rz (max)): 6 μm or less By reducing the surface roughness of carburized parts or carbonitrided parts, it is possible to prevent the occurrence of large surface pressure locally. Pitching is less likely to occur. And by setting the maximum value of the maximum height roughness Rz of the roughness curve of the component to 6 μm or less, a high pitching strength can be provided stably. If Rz (max) is 4 μm or less, a higher pitching strength can be provided more stably.

  If Rz (max) is too small, the area of metal-to-metal contact may increase depending on the operating conditions and seizure may occur. Therefore, the lower limit of Rz (max) should be about 0.5 μm. preferable.

  As already mentioned, “maximum value of roughness curve maximum height Rz” means “product geometric property specification (GPS) -surface property: contour curve method—term, definition in JIS B 0601 (2001). The maximum height roughness obtained by measuring each 3 mm parallel to the machining direction with reference to an arbitrary 100 points on the surface of the part with a stylus type surface roughness meter in accordance with It refers to the maximum value of Rz. “Parallel to the processing direction” means, for example, that the processing is parallel to the rolling direction when the processing is rolling, and that the processing is parallel to the forging axis when the processing is forging.

Maximum value of incompletely hardened layer (T (max)): 15 μm or less An incompletely hardened layer is formed by a small amount of H ₂ O or CO ₂ contained in the carburizing gas during carburizing or carbonitriding. Elements that are easily oxidized, such as Mn and Cr, are selectively oxidized, and these hardenability improving elements are generated by the lack of the component surface. And the depth of this incompletely hardened layer becomes large, especially when it exceeds 15 μm, bending fatigue and pitching tend to occur. In other words, by setting the maximum value of the incompletely hardened layer of the component to 15 μm or less, it is possible to stably provide a high bending fatigue strength and a large pitching strength.

  In addition, since it is better that the incompletely hardened layer is not present, the lower limit of T (max) is preferably 0.

  As already mentioned, the “incompletely hardened layer” means that a part is cut at right angles to the machining direction, and the surface is mirror-polished as a test surface and then corroded for 0.5 to 2 seconds. In this case, the portion corrodes deeply when observed at a magnification of 1000 times so as to include the outermost surface of the part. In addition, the “maximum value of the incompletely hardened layer” of the carburized part or the carbonitrided part is defined as “from the top surface of the part in each of the“ incompletely hardened layer ”observed arbitrarily at 100 fields with an optical microscope at a magnification of 1000 times. The largest value among the maximum distances.

  For the above reasons, the carburized part or carbonitrided part according to the present invention has a surface property that the minimum value of the surface hardness is 750 or more in terms of Vickers hardness, and the maximum value of the roughness curve has a maximum value of the roughness Rz. It was specified that the maximum value of the incompletely hardened layer was 6 μm or less and 15 μm or less.

In addition, for the carburized parts or carbonitrided parts in which the steel of the fabric has the chemical composition described in the above (A), the surface properties are
・ The minimum value of surface hardness is 750 or more in Vickers hardness,
-Maximum height of roughness curve The maximum value of roughness Rz is 6 μm or less,
・ Maximum value of incompletely hardened layer is 15μm or less,
In order to achieve this, for example, the following <a> to <c> are taken into consideration, carburizing quenching or carbonitriding quenching, and then shot peening.

  <a> A homogenized material having a chemical component in the range defined in the item (A) and having as little central segregation as possible is used.

  First, oxide-forming elements, especially Si, Mn, and Cr increase the incompletely hardened layer as already described. For this reason, it is necessary to strictly control the content of these elements within the range previously defined in the item (A) so as to ensure hardenability and reduce the incompletely hardened layer.

  Next, when forming an internal gear by forging, the center segregation part of the material may correspond to the tooth surface of the inner tooth, and if the central segregation part has a significant Mn concentration, Problems such as an incomplete hardened layer may occur.

  Therefore, it is preferable to use a homogenized material with as little central segregation as possible.

  In order to reduce the center segregation, in the case of continuous casting, a method in which an equiaxed crystal zone is generated by electromagnetic stirring and a component that easily segregates, such as C, is dispersed. However, in the method using electromagnetic stirring, the center segregation may not be sufficiently reduced, and electromagnetic stirring may not be used in ingot casting.

  Therefore, in order to reduce the center segregation, it is preferable to perform a solution treatment at a high temperature for a long time after casting so that the components in the slab or ingot are homogenized. From the viewpoint of productivity, yield, etc., it is preferable that the temperature of the solution treatment is 1250 to 1350 ° C. and the time is 6 to 30 hours.

  <B> The quenching solution is stirred so that the cooling in the carburizing quenching and the carbonitriding quenching becomes uniform.

  When the quenching liquid (cooling medium) is stirred at the time of quenching, the quenching liquid in the quenching tank has a more uniform temperature distribution as compared with the case where stirring is not performed, so that the parts are uniformly cooled. Therefore, a decrease in surface hardness due to quenching failure can be suppressed, and high surface hardness can be obtained stably.

  <C> The processing conditions for shot peening are adjusted so that the surface roughness is reduced.

  By performing shot peening after carburizing and quenching, a compressive residual stress is imparted to the surface of the component, and the surface hardness is improved by work hardening, thereby having high bending fatigue resistance. However, when shot peening that is too strong is applied, the surface roughness increases, and a large surface pressure is generated locally, which may reduce the pitching strength.

  Therefore, it is preferable to perform the shot peening process under the condition that the surface roughness can be kept small while applying the compressive residual stress. The conditions are that the shot particle size and hardness are 0.1 to 0.8 mm and Rockwell C hardness 55 to 65, respectively, the projection speed is 80 to 100 m / second, and the projection time is 30 to 120 seconds. Preferably there is.

  Hereinafter, the present invention will be described in more detail with reference to examples.

Steels 1, 3, 5, and 7 having the chemical composition shown in Table 1 were melted by a 180 kg vacuum melting furnace to produce ingots.

Steels 1 to 3 and 5 in Table 1 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, steel 7 is a comparative steel in which the contents of Si and Mn deviate from the conditions defined in the present invention.

  Each ingot was subjected to a solution treatment and homogenized, followed by hot forging.

  That is, the ingot of steel 1 is divided into 1/3 and 2/3 by volume, and for the solution with 2/3 by volume, solution treatment (furnace cooling after holding at 1250 ° C. for 24 hours ( (Hereinafter referred to as “solution treatment A”), and then homogenized, then hot forged, and for those whose volume is 1/3, hold at 1200 ° C. for 5 hours and cool in the furnace After performing solution treatment (hereinafter referred to as “solution treatment B”) and homogenizing, hot forging was performed.

The ingots of steels 2 , 3, 5 and 7 were subjected to hot forging after being homogenized by solution treatment at the size as cast. Specifically, with respect to the ingot steel 2, 3 and 5, after homogenization solution treated A, subjected to hot forging was or, for ingot steel 7, the solution After performing processing B and homogenizing, hot forging was performed.

  Of the two types of solution treatments, the solution treatment A is a treatment at a higher temperature for a longer time than the solution treatment B, so that segregation of the ingot is further reduced. For this reason, the ingot that has undergone the solution treatment A is more homogenized as a hot work material than the ingot that has undergone the solution treatment B.

  In hot forging, three types of round bars having a diameter of 20 mm and a length of 1000 mm, a diameter of 30 mm, a length of 1000 mm, a diameter of 140 mm, and a length of 1000 mm were produced. The hot forging was allowed to cool in the atmosphere.

  Next, after carrying out the normalization which hold | maintains at 90 degreeC for 1 hour and cools to each said round bar with a diameter of 20 mm, from the center part, it is parallel to a forge axis | shaft and the notched Ono type | formula shown in FIG. A rotating bending fatigue test piece was cut out.

  Moreover, after performing the normalization which hold | maintains at 900 degreeC for 1 hour, and cools to each round bar with a diameter of 30 mm, the roller pitching small roller test piece shown in FIG. Cut out.

  Furthermore, after carrying out the normalization which hold | maintains at 900 degreeC for 3 hours and cools to each round bar with a diameter of 140 mm, the roller pitching large roller test piece shown in FIG. Cut out.

  In addition, the unit of the dimension in said cut-out test piece shown in FIGS. 1-3 is all "mm".

  The finishing symbols “▽”, “▽▽” and “▽▽▽” in the above figures are “triangular symbols” indicating the surface roughness in the explanatory table 1 of JIS B 0601 (1982).

  Further, “G” added to “▽▽▽” means an abbreviation of a processing method indicating “grinding” defined in JIS B 0122 (1978). Similarly, “E (paper finish)” means an abbreviation of a processing method indicating “polishing” with “paper file”.

  Each test piece was subjected to the treatment of “carburizing and quenching-tempering” or “carbonitriding and quenching-tempering” under the conditions shown in FIGS.

  In the case of “carburizing quenching-tempering” (hereinafter referred to as “carburizing A”) shown in FIG. 4 and in the case of “carbonitriding quenching-tempering” (hereinafter referred to as “carbonitriding A”) shown in FIG. Each of the treatments was performed with a space of 50 mm or more between the test pieces so that “carburization” and “carbonitriding” were performed as uniformly as possible. In addition, the Ono-type rotating bending fatigue test piece with a notch and the roller pitching small roller test piece were processed in a suspended state by machining a hole for suspension and passing a wire through it. The roller pitching large roller test piece was placed on a jig on a wire mesh in a flat state and processed. Further, the quenching after “carburizing” or “carbonitriding” is performed so that the quenching is uniformly performed by putting the test piece into the quenching oil whose temperature is uniformly stirred at 100 ° C. did.

  On the other hand, in the case of “carburization quenching-tempering” (hereinafter referred to as “carburizing B”) shown in FIG. 6 and “carbonitriding quenching-tempering” (hereinafter referred to as “carbonitriding B”) shown in FIG. In some cases, the respective treatments were performed in a narrow state where the distance between the test pieces was less than 50 mm. As in the case of the processes shown in FIGS. 4 and 5, the notched Ono type rotating bending fatigue test piece and the roller pitching small roller test piece are machined with a wire for passing through a hole for suspension. Processed in a lowered state. The roller pitching large roller test piece was placed on a jig on a wire mesh in a flat state and processed. However, quenching after “carburizing” or “carbonitriding” was performed by putting a test piece into a quenching oil having a temperature of 100 ° C. without stirring.

  Uniformly on the outer periphery of the roller pitching small roller test piece subjected to the above-mentioned “carburizing quenching-tempering” or “carbonitriding quenching-tempering” treatment, under either condition A or B shown in Table 2 After performing shot peening, surface hardness, surface roughness and incompletely hardened layer as surface properties were investigated.

  Similarly, the Ono-type rotating bending fatigue test piece with notch, the roller pitching small roller test piece, and the roller pitching large roller test piece subjected to the above-mentioned “carburizing quenching-tempering” or “carbonitriding quenching-tempering” treatment. After the shot peening was performed uniformly on the outer peripheral portion of the sheet under the condition A or B shown in Table 2, it was finished and subjected to an Ono rotary bending fatigue test and a roller pitching test. However, the portion where “˜” (“waveform symbol”) of each of the above-described test pieces shown in FIGS. 8 to 10 is not subjected to finishing, and the surface finished by shot peening is left as it is.

<1> Investigation of surface properties:
<1-1> Surface hardness investigation:
Roller pitching finished by shot peening After traversing the central part of φ26mm of the small roller, it is filled with resin so that the surface becomes the surface to be tested, and then mirror-polished, then “Vickers hardness in JIS Z 2244 (2003)” In accordance with “Test-Test Method”, the Vickers hardness at an arbitrary 100 points at a position of 0.03 mm from the surface of the test piece is measured with a test force of 0.9807 N, and the Vickers hardness is measured from the minimum value. The “minimum value of surface hardness (HV (min))” was obtained.

  Further, an “average value of surface hardness” (hereinafter referred to as “HV (ave)”) obtained by averaging the Vickers hardness at the 100 points was also obtained.

<1-2> Surface roughness investigation:
In order to remove shot peening grains and dust adhering to the surface after shot peening, use a roller pitching small roller that is ultrasonically washed in water with neutral detergent, then washed with water and dried with warm air. Surface roughness survey was conducted.

  That is, in accordance with JIS B 0601 (2001) “Product Geometric Specification (GPS)-Surface Properties: Contour Curve Method-Terminology, Definitions and Surface Properties Parameters”, a stylus type surface roughness meter is used to perform shots. Each of the roller pitching small rollers that have been subjected to ultrasonic cleaning, water washing, and hot air drying after peening is measured by measuring 3 mm in the axial direction on the basis of any 100 points in the circumferential direction of the φ26 mm central surface. In this case, the maximum height roughness Rz was determined, and the “maximum value (Rz (max)) of the maximum height roughness Rz of the roughness curve” was determined from the maximum value.

  Further, an “average value of the maximum height roughness Rz of the roughness curve” (hereinafter referred to as “(Rz (ave)”)) obtained by averaging the maximum height roughness Rz obtained with reference to the 100 points is also obtained. It was.

<1-3> Investigation of incompletely hardened layer:
After traversing the central part of φ26mm of the roller pitching small roller finished by shot peening, it is filled with resin so that the surface becomes the surface to be examined, mirror-polished, then corroded with nital for 0.5 to 2 seconds, Using an optical microscope, arbitrarily observe 100 fields at a magnification of 1000 times so as to include the outermost surface of the test piece, and measure the maximum distance from the outermost surface of the test piece of the “incompletely hardened layer” in each case. From the maximum value, the “maximum value of incompletely hardened layer (T (max))” was determined.

  Further, the “average value of the maximum distance of the incompletely hardened layer” obtained by averaging the maximum distances from the outermost surface of the test piece of the “incompletely hardened layer” in the 100 fields of view (hereinafter referred to as “(T (ave)”). .)

<2> Ono type rotating bending fatigue test:
The Ono type rotating bending fatigue test was carried out at room temperature, in the atmosphere and at a rotational speed of 3000 rpm using 10 notched Ono type rotating bending fatigue test pieces with notches shown in FIG. 8 finished after shot peening.

In the evaluation, the maximum strength at which the number of repetitions was 10 ⁷ times and no fracture occurred was defined as the rotational bending fatigue strength. The target of the rotational bending fatigue strength is 850 MPa or more, and the rotational bending fatigue strength is excellent when the rotational bending fatigue strength is 850 MPa or more.

<3> Roller pitching test:
Roller pitting tests, using a roller pitting small roller test pieces and the roller pitting large roller test piece shown in processed FIGS finish after shot peening performed at test conditions described below, the repetition rate is 10 ⁷ times, The maximum strength at which no pitting with a long side of 1 mm or more occurred was defined as the pitching strength. The pitching strength target is 3000 MPa or more, and when the pitching strength is 3000 MPa or more, the pitching strength is excellent.

・ Slip rate: 40%
・ Rotation speed: 1000rpm
Lubrication: Manual transmission lubricating oil with an oil temperature of 90 ° C. was sprayed at a rate of 2.0 liters / minute onto the contact portion between the roller pitching small roller test piece and the roller pitching large roller test piece.
However, the “slip ratio” is {(V2−V1) / V1} where “V1” is a tangential speed on the surface of the roller pitching small roller test piece and “V2” is a tangential speed on the surface of the roller pitching large roller test piece. The value calculated by x100.

  In addition, the unit of the dimension in the above-mentioned test piece shown in FIGS. 8-10 is "mm".

  The finishing symbols “▽”, “▽▽”, and “▽▽▽” in the above figures are the same as those in FIGS. 1 to 3, respectively, and indicate the surface roughness in JIS B 0601 (1982). "Triangle sign".

  Further, “G” added to “▽▽▽” means an abbreviation of a processing method indicating “grinding” defined in JIS B 0122 (1978).

  Furthermore, “˜” is a “waveform symbol”, which means that it is a fabric, that is, it remains a surface finished by shot peening.

  Table 3 summarizes the above test results.

  11 to 22 show the influence of HV (min), HV (ave), Rz (max), Rz (ave), T (max), and T (ave) on the rotational bending fatigue strength and the pitching strength.

  That is, FIGS. 11 and 12 show the effects of HV (min) and HV (ave) on the rotational bending fatigue strength, respectively.

  FIGS. 13 and 14 show the effects of HV (min) and HV (ave) on the pitching strength, respectively.

  FIGS. 15 and 16 show the influences of Rz (max) and Rz (ave) on the rotational bending fatigue strength, respectively.

  FIGS. 17 and 18 show the influences of Rz (max) and Rz (ave) on the pitching strength, respectively.

  19 and 20 show the effects of T (max) and T (ave) on the rotational bending fatigue strength, respectively.

  21 and 22 show the effects of T (max) and T (ave) on the pitching strength, respectively.

11 to 22, the test numbers 1 to 3 and 5 shown as examples of the present invention in Table 3 and the test numbers 6 to 12 and 14 shown as comparative examples are respectively referred to as “invention examples (1 to 1). 3, 5 ) "and" Comparative Examples (6-12, 14 ) ". Moreover, an arrow was attached to the line of the rotational bending fatigue strength of 850 MPa and the pitching strength of 3000 MPa, and described as “target”.

  From FIG. 11 and FIG. 12, it is clear that the rotational bending fatigue strength has a better correlation with HV (min) than with HV (ave).

  Also, from FIG. 13 and FIG. 14, it is clear that the pitching intensity has a better correlation with HV (min) than with HV (ave).

  From FIG. 15 and FIG. 16, it is clear that the rotational bending fatigue strength has a better correlation with Rz (max) than with Rz (ave).

  Also, from FIG. 17 and FIG. 18, it is clear that the pitching intensity has a better correlation with Rz (max) than with Rz (ave).

  It is clear from FIGS. 19 and 20 that the rotational bending fatigue strength has a better correlation with T (max) than with T (ave).

  Also, from FIG. 21 and FIG. 22, it is clear that the pitching intensity has a better correlation with T (max) than with T (ave).

Further, from Table 3, in the case of Test Nos. 1 to 3 and 5 that satisfy the conditions specified in the present invention, the rotational bending fatigue strength and the pitching strength are 850 MPa or more and 3000 MPa or more, respectively. It is clear that it has bending fatigue strength and pitching strength.

On the other hand, in the case of the test numbers 6 to 8 and the test numbers 10 to 12 and 14 of the comparative examples that deviate from the conditions specified in the present invention, both the pitching strength and the rotational bending fatigue strength are also tested. In the case of No. 9, the pitching strength does not reach the target of the present invention.

  That is, in Test No. 6, HV (min) is as low as 710 in terms of Vickers hardness, Rz (max) is large as 7.48 μm, and T (max) is also large as 18 μm. Therefore, rotational bending fatigue strength and pitching strength are high. Are as low as 810 MPa and 2600 MPa, respectively, and have not reached the target of the present invention.

  In Test No. 7, HV (min) is Vickers hardness of 725, which is lower than the value specified in the present invention, so that the rotational bending fatigue strength and the pitching strength are as low as 835 MPa and 2800 MPa, respectively, and have reached the target of the present invention. Absent.

  In test number 8, HV (min) is 645 in terms of Vickers hardness, which is lower than the value specified in the present invention, and Rz (max) and T (max) are specified in the present invention as 6.86 μm and 16 μm, respectively. Is greater than the value to be For this reason, the rotational bending fatigue strength and the pitching strength are as low as 790 MPa and 2500 MPa, respectively, and have not reached the target of the present invention.

  In Test No. 9, since Rz (max) is 8.90 μm, which is larger than the value specified in the present invention, the pitching strength is as low as 2650 MPa and does not reach the target of the present invention.

  In test number 10, HV (min) is 707 in terms of Vickers hardness, which is lower than the value specified in the present invention, and Rz (max) and T (max) are specified in the present invention as 8.35 μm and 22 μm, respectively. Is greater than the value to be For this reason, the rotational bending fatigue strength and the pitching strength are as low as 820 MPa and 2400 MPa, respectively, and do not reach the target of the present invention.

  In Test No. 11, Rz (max) and T (max) are larger than the values specified in the present invention, 7.48 μm and 23 μm, respectively. Therefore, the rotational bending fatigue strength and the pitching strength are as low as 800 MPa and 2450 MPa, respectively. The goal of the present invention has not been reached.

  Test No. 12 has an HV (min) of 745 in terms of Vickers hardness, which is lower than the value specified in the present invention, and Rz (max) is 8.21 μm, which is larger than the value specified in the present invention. Fatigue strength and pitching strength are as low as 840 MPa and 2600 MPa, respectively, and the target of the present invention is not reached.

  In Test No. 14, the contents of Si and Mn in Steel 7 are 2.52% and 3.20%, respectively, which are higher than the values specified in the present invention. Moreover, HV (min) is Vickers hardness 720, which is lower than the value specified in the present invention, and in addition, Rz (max) and T (max) are 7.66 μm and 35 μm, respectively, which are specified in the present invention. Bigger than. For this reason, the rotational bending fatigue strength and the pitching strength are as low as 750 MPa and 2300 MPa, respectively, and do not reach the target of the present invention.

  Since the carburized or carbonitrided parts of the present invention have good bending fatigue strength and pitching strength, they can be used as parts such as gears used in automobile transmissions.

It is a figure which shows the shape as cut out from the round bar of the Ono type | formula rotation bending fatigue test piece with a notch used in the Example. It is a figure which shows the shape as cut out from the round bar of the roller pitching small roller test piece used in the Example. It is a figure which shows the shape as cut out from the round bar of the roller pitching large roller test piece used in the Example. It is a figure which shows one condition of "the carburizing quenching-tempering" in an Example. It is a figure which shows one condition of "carbonitriding quenching-tempering" in an Example. It is a figure which shows another one condition of "the carburizing quenching-tempering" in an Example. It is a figure which shows another one condition of "carbonitriding quenching-tempering" in an Example. It is a figure which shows the shape of the Ono type | formula rotation bending fatigue test piece with a notch used in the Ono type | formula rotation bending fatigue test of the Example. It is a figure which shows the shape of the roller pitching small roller test piece used by the roller pitching test of the Example. It is a figure which shows the shape of the roller pitching large roller test piece used by the roller pitching test of an Example. It is a figure which arranges and shows rotational bending fatigue strength in an example by HV (min) which is the minimum value of surface hardness. It is a figure which arranges and shows rotational bending fatigue strength in an example by HV (ave) which is an average value of surface hardness. It is a figure which arranges and shows pitching strength in an example by HV (min) which is the minimum value of surface hardness. It is a figure which arranges and shows pitching strength in an example by HV (ave) which is an average value of surface hardness. It is a figure which arranges and shows rotational bending fatigue strength in an example by Rz (max) which is the maximum value of maximum height roughness Rz of a roughness curve. It is a figure which rearranges and shows the rotation bending fatigue strength in an Example by Rz (ave) which is an average value of the maximum height roughness Rz of a roughness curve. It is a figure which arranges and shows pitching intensity in an example by Rz (max) which is the maximum value of maximum height roughness Rz of a roughness curve. It is a figure which arranges and shows pitching intensity in an example by Rz (ave) which is an average value of maximum height roughness Rz of a roughness curve. It is a figure which arranges and shows the rotation bending fatigue strength in an example by T (max) which is the maximum value of an incomplete hardened layer. It is a figure which rearranges and shows the rotation bending fatigue strength in an Example by T (ave) which is the average value of the maximum distance of an incomplete hardened layer. It is a figure which arranges and shows pitching strength in an example by T (max) which is the maximum value of an incomplete hardened layer. It is a figure which arranges and shows pitching strength in an example by T (ave) which is an average value of the maximum distance of an incomplete hardened layer.

Claims (2)

Dough is mass%, C: 0.10 to 0.30%, Si: 0.05 to 1.0%, Mn: 0.35 to 1.2 %, P: 0.030% or less, S: 0.005 to 0.050%, Cr: 1.21 to 3.0%, Al: 0.010 to 0.050%, N: 0.0050 to 0.0250%, the balance being Fe and impurities The minimum value of surface hardness is 750 or more in terms of Vickers hardness, the maximum value of roughness height Rz is 6 μm or less, and the maximum value of incompletely hardened layer is 15 μm. A carburized part or a carbonitrided part characterized by:
Here, the incompletely hardened layer means that the part is cut at right angles to the machining direction, the surface is mirror-polished as a test surface, and then corroded with nital for 0.5 to 2 seconds. The portion that is deeply corroded when observed at a magnification of 1000 times so as to include.   The dough steel contains one or more of Mo: 1.0% or less, V: 0.250% or less, and Nb: 0.070% or less, instead of part of Fe. Carburized or carbonitrided parts as described.

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