JP3524229B2 - High toughness case hardened steel machine parts and their manufacturing method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technology for modifying steel machine parts that have undergone case hardening by carburizing and quenching treatment or carburizing and nitriding quenching treatment. The present invention relates to a case hardened steel machine part having excellent toughness as a machine part such as a gear, a shaft and a constant velocity joint, and a manufacturing method thereof.

In the present specification, the description will be given mainly on the application to gears, but the application of the present invention is not limited to gears, of course, and high level by carburizing or carburizing / nitriding quenching. It can be widely applied to various machine structural parts that require surface hardness, impact resistance and fatigue resistance.

[0003]

2. Description of the Related Art In recent years, air pollution caused by exhaust gas emitted from various transportation machines such as automobiles and motorcycles has become a major social problem. To alleviate these problems and reduce fuel consumption. As part of measures to reduce the weight of vehicle bodies, mechanical parts such as gears and shafts are being made smaller and lighter, and along with this, demands for higher strength and higher fatigue strength for these parts are further increasing.

By the way, when strengthening a gear or the like, it is necessary to improve not only fatigue characteristics but also impact characteristics. As measures for improving impact characteristics, for example, Japanese Patent Laid-Open No. 1-2475.
As described in Japanese Patent Publication No. 61, the impact resistance is enhanced by reducing the impurity elements such as P and S as much as possible and containing the alloying elements such as Mo and V, and carburizing or carburizing / nitriding the surface. There is known a method of increasing the surface strength by applying the case hardening treatment. In addition, JP-A-62
JP-A-1843 discloses that the core structure after carburizing treatment is made into a fine ferrite + martensite two-layer structure with a grain size number of 9 or more by adding Mo, Si, or the like to the raw material steel and carburizing. A high toughness carburizing steel having improved impact properties is also disclosed by setting the grain size number of the layer to 9 or more.

However, it is not possible to satisfy the recent demands for higher strength and impact properties by merely reducing the impurity element or adding the alloy element as described above, and even if the method of controlling the grain size is used, It cannot be said that satisfactory performance is necessarily obtained, and it is further pointed out that the cost increases due to the addition of a large amount of expensive alloying elements.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and its purpose is to achieve low cost and wear resistance without adding a large amount of expensive alloying elements. The present invention aims to provide a mechanical component made of a high toughness case hardened steel, which has excellent fatigue properties, impact resistance, and the like, and exhibits excellent performance as a mechanical component such as a gear and a shaft, and a manufacturing method thereof.

[0007]

[Means for Solving the Problems] The structure of a machine part made of a high toughness case-hardening steel according to the present invention, which was able to solve the above problems, has a composition of C: 0.1 to 0.25% (hereinafter, mas unless otherwise specified).
s%) Si: 0.15% or less Mn: 1.20% or more, 2.0% or less P: 0.02% or less S: 0.02% or less Cr: 0.70 or more, 1.50 % Or less Al: 0.015 to 0.060% N: 0.005 to 0.030% Balance: Carburizing and quenching treatment or carburizing and nitriding quenching treatment on a machine part using a steel material composed of Fe and unavoidable impurities The total area ratio of martensite and bainite in the core is 95% or more, the martensite area ratio is 90% or less, and the pro-eutectoid ferrite is not generated in an area ratio of more than 5%. The grain size number of the austenite crystal grains in the nitrided layer is 8 or more, and the amount of retained austenite in the carburized layer or the carburized / nitrided layer up to a depth position of 0.05 mm from the surface satisfies 10 to 50%. In the above-mentioned present invention, Mo: 0.08 as another element in the steel material.
.About.0.6% and / or Ni: 0.2 to 3.75%, and further Cu: 0.3 to 2.0%, or V: 0.03 to 0.5
%, Ti: 0.005-0.1% and Nb: 0.00
At least one element selected from the group consisting of 5 to 0.1% is contained, and Ca: 0.0005 to 0.0
By incorporating 8% and / or Zr: 0.002 to 0.08%, it is possible to further improve the physical properties.

Further, in the above-mentioned high toughness case hardened steel machine parts, after the carburizing and quenching treatment or the carburizing and nitriding quenching treatment, a compressive residual stress is applied to the surface thereof and the surface layer portion is subjected to 30
Those that have been given a compressive residual stress of 0 N / mm ² or more show much better fatigue resistance, and such compressive residual stress is
For example, after the carburizing and quenching treatment or the carburizing / nitriding and quenching treatment, hard particles having a hardness of HRC45 or more and a particle diameter of 0.04 to 1 mm are used for the surface of 60 m /
It can be provided by performing the shot peening treatment at least once at a projection speed of s.

The composition of the manufacturing method according to the present invention is based on the above-mentioned components.
Carburizing of machine parts made of steel that satisfy composition requirements
Alternatively, when quenching after carburizing / nitriding
The hardness of the steel material in the Jominy hardenability curve is
Of machine parts after hardening treatment or carburizing / nitriding hardening treatment
Jominy position J showing hardness equivalent to core strength_eq(M
m) and the surface where proeutectoid ferrite exceeds 5%
Critical cooling rate V that does not occur at product moment_C1(℃ / sec) and Marte
Critical cooling rate V at which the site ratio is 90% _C2(℃ /
Second) from the composition of the steel material
Obtained by the above-mentioned Jominie position J_eq(Mm) below
Cooling rate V obtained by substituting in equation (3)
Is the critical cooling rate V_C1(℃ / sec) and V_C2(℃ / second)
The point is that quenching is performed within the range of
It is something. V_C1= 10^k1…… (1) Where k₁ = 3.62-7.17 [C%]-0.43 [Mn%]-0.64 [Cr%]-1.18
[Mo%]-3.86 [P%]-0.20 [Ni%] V_C2= 10^k2…… (2) Where k₂ = 4.01-5.96 [C%]-0.33 [Mn%]-0.33 [Cr%]-0.66
[Mo%]-9.45 [P%]-0.33 [Ni%] V = 390J_eq ^-1.35 …… (3)

After the carburizing and quenching treatment or the carburizing and nitriding and quenching treatment by the above-mentioned method, the hardness is HRC45 or more and the particle diameter is 0.04 to 1 on the surface.
By using the hard fine particles having a size of mm and performing the shot peening treatment at least once at a projection speed of 60 m / s, it is possible to obtain a high toughness case hardened steel machine component having a further increased surface hardness.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, a machine part made of a steel material having a specified composition as described above is carburized or carburized / nitrided and quenched to obtain a non-carburized or non-carburized / nitrided layer of a core portion. In addition to defining the metallographic structure, it defines the size of austenite grains and the amount of retained austenite in the carburized or carburized / nitrided layer, which results in superior fatigue resistance and durability compared to conventional case-hardened steel parts. We have succeeded in obtaining mechanical parts made of high-strength case-hardened steel that also have impact characteristics.

It has been conventionally considered that in steel materials having similar carbon contents, the higher the hardness of the core after carburizing or carburizing / nitriding quenching, the lower the impact resistance.
However, according to various studies conducted by the present inventors, even if the metal structure of the core is mainly martensite and bainite, if the area ratios thereof are appropriately adjusted,
It has been found that the impact properties are superior to those having a core structure mainly composed of martensite or bainite.

The reason for this is not necessarily clarified, but it is considered as follows. That is, when the core structure is martensite or bainite, since the unit in which the lath direction is aligned in the old austenite crystal grains is large, it is possible to obtain the same impact resistance improving effect as refining the austenite crystal grains. I can't. However, if the core structure is a mixed structure of martensite and bainite, the unit in which the lath direction is aligned in the former austenite grains becomes extremely fine, and is comparable to the mechanical part of the core structure having fine austenite crystal grains. In order to exhibit excellent impact characteristics and to secure such impact characteristics, the core metal structure should be a mixed structure mainly composed of martensite and bainite, and their area ratio should be 95% or more. The fact that the area ratio is 90% or less and the pro-eutectoid ferrite should not be generated in an area ratio of more than 5% was found. The martensite area ratio of 90% or less and the pro-eutectoid ferrite area ratio of more than 5% are not generated, and the remaining portion becomes a bainite structure, and as a result, excellent impact characteristics can be obtained.

On the other hand, regarding the carburized or carburized / nitrided layer, it has been conventionally considered that the surface hardness decreases and the fatigue characteristics deteriorate as the amount of retained austenite increases. However, according to the research conducted by the present inventors, the fatigue characteristics do not always decrease as the retained austenite amount increases, and in order to secure a high level of fatigue characteristics, an appropriate amount in the carburizing or carburizing / nitriding layer is required. The amount of retained austenite in the carburized layer or the carburized / nitrided layer up to a depth of 0.05 mm from the surface should be 10-50.
It was found that the stress caused by cracks generated from the surface at the time of impact fatigue fracture is relaxed and excellent impact fatigue characteristics are exhibited by setting the content in the range of%.

The reason for defining the chemical composition of the steel material used in the present invention and the reason for defining the metal structure after the carburizing and quenching treatment or the carburizing and nitriding quenching treatment will be described in detail below. First of all, the reasons for defining the chemical composition of steel materials are clarified.

C: 0.1 to 0.25% C is an element that is indispensable for ensuring the strength of the core portion as a mechanical part, and if it is less than 0.1%, sufficient strength cannot be obtained. However, if it is contained excessively, the toughness deteriorates, and the machinability and cold forgeability deteriorate and the workability is impaired, so the upper limit is 0.25%. The more preferable content of C is in the range of 0.12 to 0.25%.

Mn: more than 1.2% and 2.0% or less Mn has the effect of increasing the surface carbon concentration during carburizing or carburizing / nitriding, and Ms of the carburizing or carburizing / nitriding layer by the action of Mn itself. It has the effect of lowering the point and increasing the retained austenite in the carburized (or carburized / nitrided) layer after the carburizing (or carburizing / nitriding) quenching treatment, and also effectively acts to deoxidize molten steel. In order to exert such effects effectively, it must be contained in excess of 1.2%, but if it is excessively contained, cold workability and machinability are adversely affected, and the amount of segregation at grain boundaries is also increased. However, since the grain boundary strength is reduced and the impact characteristics are adversely affected by the increase in the content of the alloy, it must be suppressed to 2.0% or less. The more preferable content of Mn is 1.4 to 1.
It is in the range of 8%.

Cr: more than 0.7% and 1.5% or less Cr is an element effective as a hardenability improving element, and in order to exert its effect effectively, it is essential to contain more than 0.7%. However, if it exceeds 1.5%, segregation of Cr or its carbide to the grain boundaries will occur and the grain boundary strength will be reduced, and the toughness will be adversely affected. The more preferable content of Cr is in the range of 1.0 to 1.3%.

Al: 0.015 to 0.06% Al is an element contained in steel as a deoxidizing agent for steel materials, and combines with N in the steel to form AlN, resulting in coarsening of crystal grains. Has the effect of preventing In order to effectively exert such an effect, the content must be 0.015% or more, but the effect is saturated at about 0.06%, and beyond that, it binds with oxygen to form a non-metallic inclusion, resulting in impact. Since it will adversely affect the characteristics, etc., 0.06% was set as the upper limit.

N: 0.005 to 0.030% N combines with Al, V, Ti, Nb, etc. in steel to form a nitride, which has the effect of suppressing coarsening of crystal grains. ,
The effect is effectively exhibited by containing 0.005% or more. However, their effect is about 0.030.
%, And if it is contained more than this amount, the nitride will act as an inclusion and adversely affect the physical properties, so further addition should be avoided.

Si: 0.15% or less Si effectively acts as a strengthening element or a deoxidizing element, but on the other hand, it promotes grain boundary oxidation to deteriorate bending fatigue characteristics and also has an adverse effect on cold forgeability. Therefore, in order to eliminate such obstacles, the content must be suppressed to 0.15% or less, and it is desirable to suppress the content to 0.10% or less particularly when high level bending fatigue characteristics are required. Be done. From such a viewpoint, the more preferable content of Si is in the range of 0.02 to 0.10%.

P: 0.02% or less P segregates at the grain boundaries to lower the toughness, so the upper limit was set to 0.02%. The more preferable content of P is 0.015% or less, and further preferably 0.010% or less.

S: 0.02% or less S forms MnS and contributes to the improvement of machinability. However, when the present invention is applied to gears and the like, not only the impact characteristics of the longitudinal grain but also the impact characteristic of the transverse grain. This is important, and it is necessary to reduce the anisotropy in order to improve the impact characteristics of the cross grain, and for that purpose, the S content must be suppressed to 0.02% or less. The more preferable content of S is 0.017% or less, further preferably 0.0
It is 15% or less.

It is also effective that the steel material used in the present invention contains appropriate amounts of the following elements in addition to the above-mentioned essential elements to further enhance the characteristics as a case hardened steel machine part.

Mo: 0.08 to 0.6% and / or Ni: 0.2 to 3.75% These elements have the same effect in that they have the effect of enhancing hardenability or refining the hardened structure. Mo is an element, especially Mo effectively acts to reduce the incompletely quenched structure and improve the hardenability, and further to improve the grain boundary strength, and Ni further refines the structure after quenching to improve the impact resistance. Contribute. These effects are effectively exhibited by including Mo: 0.08% or more and Ni: 0.2% or more, but the above effect of Mo is saturated at about 0.6%, and the above effect of Ni is Also 3.7
Since it saturates at 5%, any further addition is economically completely useless.

Cu: 0.3 to 2.0% Cu is an element that effectively acts to improve the corrosion resistance, and its effect is effectively exhibited by containing 0.3% or more, but the effect is 2 Since it saturates at 0.0%, further inclusion is useless. If Cu is contained alone, the hot workability of the steel material tends to deteriorate. Therefore, in order to avoid such an adverse effect, Ni having a hot workability improving effect should be used in combination within the above range. Is desirable.

V: 0.03 to 0.5%, Ti: 0.00
5 to 0.1% and Nb: at least one element selected from the group consisting of 0.005 to 0.1% These elements combine with C and N to form carbides and nitrides, and crystal grains Contributes to the improvement of toughness (impact resistance). These effects are effectively exhibited by containing each of the above lower limit values or more, but the effect is saturated near the upper limit value, respectively, so that machinability and cold workability may be adversely affected. Each must be kept below the upper limit.

Ca: 0.0005 to 0.08% and /
Alternatively, Zr: 0.002-0.08% Ca wraps hard inclusions with soft inclusions, and Zr makes MnS spherical, both of which contribute to improvement of machinability and both elements. It has the effect of enhancing the impact properties of the lateral grain by reducing the anisotropy due to the spheroidization of MnS, and these effects are effectively exhibited by containing each of the above lower limit values or more. However, their effects saturate at 0.08% respectively.

The high toughness case hardened steel machine part according to the present invention is obtained by carburizing (or carburizing / nitriding) quenching a machine part made of a steel material satisfying the above composition. In addition to the above component composition requirements, the invention has a major feature in that it defines the metallographic structure and grain size of the core and carburized (or carburized / nitrided) layer after carburizing (or carburizing / nitriding) quenching. Therefore, they will be described in detail below.

The core has a mixed structure of mainly martensite and bainite, and the core has a martensite + bainite area ratio of 95% or more and a martensite area ratio of 90% or less.
In the present invention in which the area ratio of pro-eutectoid ferrite does not exceed 5% , the austenite crystal grains are refined by the conventional method as described above by making the core structure a mixed structure mainly composed of martensite and bainite. The same effect can be obtained, and the impact resistance can be enhanced by improving the toughness of the core portion. These effects can be confirmed by the fact that the fracture surface unit in the impact fracture surface of the core having the above-mentioned mixed structure is extremely fine. Moreover, if the metal structure of the core portion is such a mixed structure, the hardness of the core portion of Hv250 or more can be secured,
The occurrence of case crushing is also suppressed. In addition, case crushing means that a crack occurs at the boundary between the carburized layer (or carburized / nitrided layer) and the core, or at a position slightly closer to the core, and the crack propagates in a direction parallel to the surface. Indicates a phenomenon in which the carburized layer (or carburized / nitrided layer) is connected to the surface and peels off.

In order to effectively exert such an effect of improving the impact characteristics of the core part, the (martensite + bainite) area ratio of the core part is 95% or more and the martensite area ratio is 90%.
% Or less, it is essential not to generate proeutectoid ferrite in an area ratio exceeding 5%. By the way, if the (martensite + bainite) area ratio of the core is less than 95%, the martensite area ratio exceeds 90%, and the pro-eutectoid ferrite area ratio exceeds 5%, the microstructure refinement due to the mixing of two layers becomes insufficient. As a result, the impact characteristic improving effect cannot be obtained.

The metallographic structure of the core of the case-hardened part changes depending on the cooling rate after carburizing (or carburizing / nitriding), and the cooling rate changes depending on the shape and size of the part, the type of quenching agent, and the like. However, suitable conditions for obtaining the core structure as described above will be described later. The area ratio of martensite, bainite, and pro-eutectoid ferrite can be measured by, for example, using a scanning electron microscope, photographing the core structure at 1500 times, and analyzing the image.

Austener of carburized (or carburized / nitrided) layer
Itite crystal grain size number is 8 or more It is known that impact characteristics are improved by refining austenite crystal grains. Therefore, also in the present invention, N, Al, Nb, Ti,
Carburizing (or carburizing / nitriding) when V is contained, but when carburizing (or carburizing / nitriding) is performed at high temperature for a long time, or when carburizing (or carburizing / nitriding) is performed after cold working. The austenite crystal grains in the layer) become coarse and the impact properties are significantly deteriorated. Therefore, in order to prevent the deterioration of impact properties due to the coarsening of crystal grains,
It is an essential requirement that the austenite crystal grains of the carburized (or carburized / nitrided) layer after the carburizing (or carburizing / nitriding) quenching treatment have a grain size number of 8 or more (according to the grain size test method of JIS G 0551).

The amount of retained austenite from the surface of the carburized (or carburized / nitrided) layer to a depth of 0.05 mm
10 to 50% As described above, conventionally, it has been considered that the surface hardness decreases and the fatigue characteristics decrease as the amount of retained austenite in the carburized (or carburized / nitrided) layer increases. However, according to the research conducted by the present inventors, the fatigue characteristics do not always decrease as the retained austenite amount increases, and the impact fatigue properties can be remarkably enhanced if the retained austenite amount is controlled in an appropriate range. I knew That is, the retained austenite in the carburized (or carburized / nitrided) layer has an action of relieving the stress caused by cracks generated from the surface during impact fatigue fracture. It was found that the amount of retained austenite up to a depth of 0.05 mm should be 10% or more. However, if the amount of retained austenite in this portion becomes too large, the surface hardness becomes excessively low, and the effect of improving impact fatigue properties due to a decrease in fatigue strength cannot be effectively exhibited. Therefore, the amount of retained austenite (from the surface to 0. After electropolishing 05 mm, measured by X-ray diffraction) should be kept below 50%. A more preferable range of the amount of retained austenite is 20 to 40%.

As described above, the case-hardening steel machine component according to the present invention has the composition of the steel material, the core portion, and the carburized (or carburized.
By defining the metallographic structure and grain size of the (nitriding) layer, it has excellent impact characteristics and fatigue characteristics, but a case-hardened steel machine part with such characteristics has the following characteristics. It can be easily obtained by adopting the method.

That is, when quenching after carburizing or carburizing / nitriding a machine part made of a steel material satisfying the requirements of the above composition, the hardness of the steel material in the Jominy hardenability curve is the carburizing and quenching treatment. Alternatively, a Jominy position J _eq (mm) indicating hardness corresponding to the core strength of the machined part after carburizing / nitriding and quenching treatment is obtained, and a critical cooling rate V at which proeutectoid ferrite is not formed in an area ratio of more than 5% _C1 (° C / sec) and critical cooling rate V _C2 (° C / sec) at which the martensite area ratio becomes 90% are obtained from the composition of the steel material by the above formulas (1) and (2), and the Jominy position J _The cooling rate V obtained by substituting _eq (mm) into the equation (3) is the critical cooling rate V
_This is a method in which quenching is performed at a cooling rate V that is in the range of _C1 (° C / sec) and V _C2 (° C / sec).

That is, the martensite area ratio of the core is 90
%, It is effective to properly control the cooling rate during quenching after carburizing (or carburizing / nitriding) in order to prevent the generation of proeutectoid ferrite area ratio exceeding 5%. ) 90 during quenching after treatment
Average cooling rate V (° C / sec) of the core between 0 and 300 ° C
However, it is effective to set the pro-eutectoid ferrite area ratio to a critical cooling rate V _C1 (° C./sec) or more that does not exceed 5% and a critical cooling rate V _C2 (° C./sec) or less at which 90% martensite is generated. Become.

By the way, in general, the cooling rate of the case hardening component changes depending on the shape and size of the component, the kind of the quenching agent, etc. Therefore, it is usual to determine an appropriate cooling condition by actual measurement in advance and set the standard cooling condition. However, actually, it is not easy to actually measure the cooling rate inside the components in the quenching tank during operation. Therefore, we thought that it would be possible to correlate the component composition of the machine part with the Jominy hardenability curve and calculate the cooling rate of the core of the carburized (or carburized / nitrided) part from the composition, and study along that line. As a result, the above-mentioned critical cooling rates V _C1 and V _C2 can be calculated by the following k ₁ and k ₂ from the composition of the steel material, and an appropriate cooling rate V can be obtained from the calculated values by the following formula. I knew that. V _C1 ≤ V ≤ V _C2 V _C1 = 10 ^k1 (1) In the formula, k ₁ = 3.62-7.17 [C%]-0.43 [Mn%]-0.64 [Cr%]-1.18
[Mo%]-3.86 [P%]-0.20 [Ni%] V _C2 = 10 ^k2 (2) In the formula, k ₂ = 4.01-5.96 [C%]-0.33 [Mn%]-0.33 [Cr% ] -0.66
[Mo%]-9.45 [P%]-0.33 [Ni%] V = 390J _eq ^-1.35 …… (3)

The method of calculating the cooling rate V will be described below. The calculation method of the cooling rate in the core of the part is as follows. First, as shown in Fig. 1, the hardness (Hc) of the core of the part after carburizing (or carburizing / nitriding) quenching is measured, and the hardness is Hc in the Jominy hardenability curve. Jominy position equivalent to J _eq (mm)
Ask for. On the other hand, the present inventors have found that for each steel material having various composition and size, each Jominy position J _eq and 9
When the relationship between the average cooling rates V between 00 and 300 ° C. was examined, there was a certain correlation between the two as shown in FIG. 2, and “V = 390 J _eq ” as defined by the above equation (3).
^-1.35 ”was established. Then, by substituting the Jominy position J _eq obtained above into this relational expression, it was found that an appropriate cooling rate V of the core portion at the time of quenching can be obtained. That is, "V = 390J _eq ^-1.35 "
Is the average cooling rate (° C) of the core of 900 to 300 ° C at the Jominy position corresponding to the hardness of the core during quenching.
/ Sec).

That is, the measurement of the Jominy hardenability curve
Processing the desired steel material into actual machine parts, and hardening the core after hardening
When the degree Hc is measured, the average cooling rate V of the core during quenching is
And the pro-eutectoid ferrite surface as described above.
Critical cooling rate V with product factor not exceeding 5%_C1And 90% Maru
Critical cooling rate V produced by tensite_C2And the relevant
Based on k1 and k2 calculated from the chemical composition of steel,
As shown in equations (1) and (2), 10^k1, 10^k2Calculated value of
As carburizing (or carburizing / nitriding
Averaged core part cooling speed of 900 to 300 ° C after treatment
Degree V is the above V _C1And V_C2If you control so that it fits in between,
The core structure of the obtained case-hardened steel parts is properly adjusted as described above.
Use a mixed structure consisting mainly of martensite and bainite
Can be done.

The austenite crystal grains in the carburized (or carburized / nitrided) layer are affected by the composition of the steel material and the hardening conditions after carburized (or carburized / nitrided) as described above. In order to sufficiently miniaturize the crystal grains to obtain a grain size number of 8 or more under various quenching conditions, a steel material in which the amounts of N and Al satisfy the above requirements is subjected to high temperature rolling, and then Nb and In the case of a steel material containing Ti, it is carburized (or carburized / nitrided) after low-temperature rolling.
A method of performing quenching treatment or the like may be adopted.

The amount of retained austenite at a depth of 0.05 mm from the surface of the carburized (or carburized / nitrided) layer should be 10 to 50 under the above quenching conditions.
%, The content of alloying elements (Mn, Cr, Ni, Mo), etc., which are retained austenite forming elements, should be properly adjusted, and the carbon concentration in the carburizing (or carburizing / nitriding) treatment atmosphere or A method of properly adjusting the NH ₃ flow rate or the like may be adopted.

Further, in the present invention, after the above carburizing (or carburizing / nitriding) quenching treatment is applied, the surface thereof is subjected to shot peening treatment or the like under appropriate conditions so that the surface layer 3
If a compressive residual stress (measured by X-ray diffraction) of 00 N / mm ² or more is applied, the fatigue characteristics can be further improved without impairing the impact characteristics. Such a modification effect is obtained by using hard fine particles having a particle diameter of 0.04 to 1 mm and an HRC of 45 or more, and at a projection speed of 60 m / s or more.
It is obtained by performing shot peening treatment more than once.

Incidentally, the hardness of shot particles is less than HRC45, the particle diameter is less than 0.04 mm, or the projection speed is 60.
If it is less than m / s, it is difficult to significantly exhibit the above-mentioned compressive residual stress imparting effect by the shot peening treatment.
The hardness of shot particles and the upper limit of the projection speed are not particularly limited, but in view of practicality, the preferable upper limits thereof are HRC.
It is about 65 and about 150 m / s. In addition, since the problem that surface roughness occurs with coarse hard particles having a particle size of more than 1 mm, the particle size of the shot particles should be increased in order to exert the shot peening effect more effectively without causing such a problem. Is preferably about 0.3 to 1.0 mm. The shot peening treatment may be performed once, but may be performed twice or more if necessary.

There is no limitation on the carburizing (or carburizing / nitriding) method employed in the present invention. For example, a conventionally known gas carburizing (or carburizing / nitriding) method, a solid carburizing (or carburizing / nitriding) method. The liquid carburizing (or carburizing / nitriding) method, the plasma carburizing (or carburizing / nitriding) method, the vacuum carburizing (or carburizing / nitriding) method, and the like can all be adopted.

[0046]

EXAMPLES Next, the constitution and effects of the present invention will be described more specifically with reference to examples, but the present invention is not limited by the following examples, and may be adapted to the gist of the preceding and following. It is of course possible to make changes within the scope of implementation, and all of them are included in the technical scope of the present invention.

EXAMPLE 15 Steel materials having the chemical compositions of 1 to 37 shown in Tables 1 and 2 were used.
Melted and cast in a 0 kg vacuum melting furnace, then hot forged to a diameter of 30 mm, and solution heat treated (1250 ° C x 1 hour → air cooling)
After the normalizing treatment (850 ° C. × 1 hour → air cooling), the Charpy impact test piece shown in FIG. 4,
For 6, 10, 29, and 36, after performing melting, hot forging, solution treatment, and normalizing treatment in the same manner as above, the impact bending fatigue test piece shown in FIG. 4 was manufactured by machining. . Then, each of the test pieces was carburized and quenched under the conditions shown in Tables 3 and 4 and FIG. 5, and then subjected to an impact test at room temperature and a Matsumura-type repeated impact bending fatigue test. The hardness of the core of the impact test piece and the impact bending fatigue test piece after carburizing and quenching was measured, and the Jominy position J _eq corresponding to the cooling rate of the core at the time of quenching was obtained based on the Jominy hardenability curve, and cooling was performed. When the speed V is calculated,
J _eq is 5 mm and 5.5 mm, respectively, and the cooling rate V at the time of carburizing and quenching treatment when manufacturing the Charpy impact test piece and the impact bending fatigue test piece is 44 (° C /
Sec) and 39 (° C./sec).

Critical cooling rates V _C1 , V _{C2 at which} 10% or 90% martensite is obtained from the composition of the steel material by the above equations (1) and (2), carburizing temperature, and CP value at the time of carburizing ( Carbon potential), martensite + bainite area ratio after carburizing treatment, grain size number of carburized layer, retained austenite amount of carburized layer, core hardness, impact properties are shown in Tables 3 and 4, and repeated impact bending fatigue test results. Is shown in FIG.

[0049]

[Table 1]

[0050]

[Table 2]

[0051]

[Table 3]

[0052]

[Table 4]

As is clear from Table 1, invention steels 1 to 1
No. 25 satisfies the specified requirements of the present invention in all of the chemical composition, the amount of retained austenite in the carburized layer, the grain size number of the carburized layer, the martensite + bainite area ratio, and the cooling rate V = during carburizing and quenching. 44 (° C / sec) is a critical cooling rate V in which the area ratio of proeutectoid ferrite does not exceed 5%.
_{Since it is not} less than _C1 (° C / sec) and not more than the critical cooling rate V _C2 (° C / sec) at which 90% martensite is formed, 50 J
A high impact value of not less than / cm ² is obtained.

On the other hand, the steel types 26 and 27 in Table 2 show extremely excellent impact properties even though the martensite area ratio and the martensite + bainite area ratio are both outside the specified ranges. However, since the core hardness is HV 250 or less, case crushing is likely to occur, and there is a concern that fatigue strength may decrease. Comparative steel 28,
Although the chemical composition of 29, the grain size number, the amount of retained austenite, etc. satisfy the specified requirements, the martensite area ratio of the core part is out of the specified range due to an improper cooling rate, and a satisfactory impact value is obtained. Not obtained. Comparative steel 30,
In No. 31, since the retained austenite amount exceeds the specified range, there is a high possibility of causing a problem in fatigue strength.

The comparative steels 32 and 33 have C content and Mn content exceeding the specified ranges, respectively, and also have an inadequate cooling rate and insufficient hardenability, so that the martensite area ratio and the martensite + bainite area ratio. Is out of the specified range. Nevertheless, it has very excellent impact characteristics, but on the other hand, since the hardness of the core is extremely low at HV 250 or less, case crushing occurs and fatigue strength is likely to decrease. Although the comparative steels 34 and 35 have appropriate chemical compositions, the impact resistance is low because the grain size number of the carburized layer after carburization is small and out of the specified range. Comparative steel 36,3
In No. 7, the amount of Al and the amount of N are out of the specified ranges, respectively, and the grain size number of the carburized layer is out of the specified range and is small, so the impact properties are poor.

Further, as is clear from FIG. 6, all of the steel types 4, 6 and 10 satisfying all the specified requirements of the present invention have shock bending fatigue characteristics which are far superior to those of the comparative steels 29 and 36. You know that you have. That is, in Comparative Steel 29,
Although the chemical composition satisfies the requirements of the present invention, the martensite area ratio of the core portion after the carburizing treatment is high because the cooling rate V is out of the specified requirement. In Comparative Steel 36, the carburizing after the carburizing treatment is performed. Satisfactory impact bending fatigue properties cannot be obtained because the grain size number of the layer is small and out of the specified requirements of the present invention.

Next, among the Charpy impact test pieces (before carburizing and quenching treatment) shown in Table 1, steel types 1 to 5 were subjected to carburizing / nitriding quenching treatment under the conditions shown in Table 5 and FIG. An impact test was performed on each of the obtained test pieces at room temperature. The results are shown in Table 5. The composition of each steel material and the cooling rate during carburizing / nitriding and quenching are appropriate, and the martensite + bainite area ratio after carburizing / nitriding / quenching, martensite area ratio, carburized layer Since the grain size number and the amount of retained austenite, etc. all satisfy the specified requirements, a high impact value of 50 J / cm ² or more is obtained.

[0058]

[Table 5]

Further, among the Charpy impact test pieces (after the carburizing and quenching treatment) shown in Table 1 above, steel types 1 to 5 were carburized and tempered under the same conditions as shown in FIG. After performing the shot peening treatment under the conditions shown in FIG. 6, an impact test was performed at room temperature in the same manner as above, and the results shown in Table 7 were obtained.

[0060]

[Table 6]

[0061]

[Table 7]

As is clear from Table 7, each of these test materials satisfied the specified requirements of the present invention, and the compressive residual stress of 300 N / mm ² or more was applied to the surface layer portion by the shot peening treatment. It can be seen that the impact value is further increased in each case as compared with the one without shot peening shown in Table 3.

[0063]

EFFECTS OF THE INVENTION The present invention is configured as described above, and specifies the chemical composition of the steel material and determines the metallographic structure of the core after carburizing (or carburizing / nitriding) quenching treatment to a predetermined amount of martensite area. With a mixed structure consisting mainly of martensite + bainite with a certain ratio and carburizing (or carburizing
For the (nitrided) layer, by defining the amount of retained austenite and the grain size number, it has become possible to provide a high toughness case hardened steel machine part having excellent impact strength and impact bending fatigue properties. Further, by subjecting this component to shot peening treatment under more appropriate conditions to impart a compressive residual stress to the surface layer portion, it is possible to obtain a mechanical component having even more excellent impact fatigue characteristics.

Further, according to the method of the present invention, the quenching conditions for obtaining a case-hardened steel machine part satisfying the requirements such as the above-mentioned metallographic structure are determined by the composition of the steel material constituting the machine part and its Jominy firing. Almost accurately can be determined based on the hardenability curve, and standardization of quenching conditions can be promoted.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a Jominy hardenability curve.

FIG. 2 is a graph showing a relationship between a Jominy distance and an average cooling rate between 900 and 300 ° C.

FIG. 3 is a diagram showing a shape of an impact test piece used in an experiment.

FIG. 4 is a diagram showing the shape of a cyclic impact bending fatigue test piece adopted in an experiment.

FIG. 5 is a diagram showing carburizing and quenching / tempering treatment conditions adopted in an experiment.

FIG. 6 is a graph showing the comparison between the experimental results of the example and the comparative example regarding the number of cycles and the cyclic stress in the impact bending fatigue test.

FIG. 7 is a diagram showing carburizing / nitriding quenching treatment conditions adopted in the experiment.

─────────────────────────────────────────────────── ─── Continued Front Page (51) Int.Cl. ⁷ Identification Code FI C21D 9/40 C21D 9/40 A C22C 38/38 C22C 38/38 38/58 38/58 (72) Inventor Yoshio Okada Kanagawa 2 Takaracho, Kanagawa-ku, Yokohama-shi Nissan Motor Co., Ltd. (72) Hideki Usuki 2 Takara-cho, Kanagawa-ku, Yokohama-shi, Kanagawa Nissan Motor Co., Ltd. (72) Houshi Kamada 2 Takara-cho, Kanagawa-ku, Yokohama-shi, Kanagawa Address within Nissan Motor Co., Ltd. (56) Reference JP-A-7-179990 (JP, A) JP-A-5-156421 (JP, A) JP-A-7-97660 (JP, A) JP-A-61-1 210154 (JP, A) JP-A-7-190173 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 6/00 C21D 7/06 C21D 9/00-9 / 44,9 / 50 C23C 8/00-12/02

Claims (9)

(57) [Claims] 1. C: 0.1 to 0.25% (hereinafter, unless otherwise specified, means mass%) Si: 0.15% or less Mn: more than 1.20%, 2.0% or less P: 0 0.02% or less S: 0.02% or less Cr: more than 0.70, 1.50% or less Al: 0.015 to 0.060% N: 0.005 to 0.030% Balance: Fe and inevitable impurities Machine parts made of steel made of steel are carburized and quenched or carburized / nitrided and hardened. The total area ratio of martensite and bainite in the core is 95% or more, and the martensite area ratio is 90% or less. The precipitated ferrite is not generated in an area ratio exceeding 5%, the austenite crystal grains of the carburized layer or carburized / nitrided layer have a grain size number of 8 or more and 0.05 mm from the surface.
A mechanical part made of case-hardened steel with high toughness, characterized in that the amount of retained austenite in the carburized layer or carburized / nitrided layer up to the depth position of is 10 to 50%. 2. A steel material containing Mo: 0.08 as another element.
The high toughness case hardened steel machine part according to claim 1, which contains ˜0.6% and / or Ni: 0.2 to 3.75%. 3. A steel material containing Cu: 0.
The high toughness case hardened steel machine part according to claim 2, which contains 3 to 2.0%. 4. A steel material containing V: 0.0 as another element.
3 to 0.5%, Ti: 0.005 to 0.1% and N
b: containing at least one element selected from the group consisting of 0.005 to 0.1%.
A machine part made of a high toughness case-hardening steel according to any one of 1. 5. A steel material containing Ca: 0.
0005 to 0.08% and / or Zr: 0.00
The mechanical component made of a high toughness case-hardened steel according to any one of claims 1 to 4, which contains 2 to 0.08%. 6. After the carburizing and quenching treatment or the carburizing and nitriding and quenching treatment, a compressive residual stress is applied to the surface thereof.
The high toughness case hardened steel machine component according to any one of claims 1 to 5, wherein the surface layer portion has a compressive residual stress of 300 N / mm ² or more. 7. After the carburizing and quenching treatment or the carburizing and nitriding quenching treatment, hard fine particles having a hardness of HRC45 or more and a particle diameter of 0.04 to 1 mm are used on the surface thereof.
The high toughness case hardened steel machine component according to claim 6, which has been subjected to shot peening treatment at least once at a projection speed of 60 m / s. 8. When quenching after carburizing or carburizing / nitriding a machine part made of a steel material satisfying the requirements of the chemical composition according to any one of claims 1 to 5, a Jominy quenching of the steel material is performed. Jominy position J _eq (m) where the hardness in the property curve shows the hardness corresponding to the core strength of the machine part after carburizing and quenching treatment or carburizing and nitriding quenching treatment.
m) with obtaining the critical cooling rate V _C1 is not generated at the area ratio of the pro-eutectoid ferrite exceeds 5% (° C. / sec) and the critical cooling rate V _C2 martensite area ratio is 90% (° C. /
Sec) is obtained from the composition of the steel material by the following equations (1) and (2), and the cooling rate V obtained by substituting the Jominie position J _eq (mm) into the equation (3) below.
Is the above critical cooling rate V _C1 (° C / sec) and V _C2 (° C / sec)
A method for producing a mechanical component made of case hardened steel having high toughness, characterized in that the metal structure according to any one of claims 1 to 5 is obtained by performing quenching treatment within the range. V _C1 = 10 ^k1 (1) where k ₁ = 3.62-7.17 [C%]-0.43 [Mn%]-0.64 [Cr%]-1.18
[Mo%]-3.86 [P%]-0.20 [Ni%] V _C2 = 10 ^k2 (2) In the formula, k ₂ = 4.01-5.96 [C%]-0.33 [Mn%]-0.33 [Cr% ] -0.66
[Mo%]-9.45 [P%]-0.33 [Ni%] V = 390J _eq ^-1.35 …… (3) 9. After the carburizing and quenching treatment or the carburizing / nitriding and quenching treatment by the method according to claim 8, the surface thereof has a hardness of HRC45 or more and a particle diameter of 0.0.
A method of manufacturing a machine part made of a high toughness case-hardening steel with an increased surface hardness, characterized in that hard pebbles of 4 to 1 mm are used and shot peening is performed at least once at a projection speed of 60 m / s.