JP2016194156A - Carbonitrided component and method of manufacturing carbonitrided component - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carbonitrided component having a shape including an edge part and excellent in temper softening resistance and bending fatigue strength, and a method of manufacturing the same.SOLUTION: There is provided a carbonitrided component 100 which is excellent in bending fatigue strength and in which: a core part has a composition containing, by mass%, C:0.10 to 0.25%, Si:0.03 to 0.49% Mn:0.30 to 1.50%, P:0.030% or less, S:0.060% or less, Cr:0.90 to 3.00%, Al:0.100% or less and N:0.030% or less and the balance Fe with inevitable impurities; a flat part 3 has a particle boundary oxidation layer depth of 1 μm or less; a flat part surface area has carbon concentration of 0.70% to 0.89%, a nitrogen concentration of 0.20% to 0.70% and a deposit density of carbide and/or nitride of 0.5/μmor more; and edge part surface area has a maximum length of the deposit of carbide and/or nitride of 5.0 μm or less.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a carbonitrided part made of a steel material subjected to carburizing and nitriding and a method for manufacturing the same.

Conventionally, as a machine part such as a gear and a pulley for a belt type continuously variable transmission (CVT), there is a carburized part made of a carburized steel material.
Examples of the carburizing process include a gas carburizing process and a vacuum carburizing process (see, for example, Patent Documents 1 to 5). The vacuum carburizing process has the following effects compared to the gas carburizing process. That is, since the carburizing temperature can be increased, carburized parts having a predetermined carbon concentration can be obtained in a short time. Moreover, since the grain boundary oxidation accompanying a carburizing process can be suppressed, it is easy to obtain a carburized part having high bending fatigue strength. Furthermore, since the carbon yield is high, the amount of carbon dioxide emission can be suppressed.

  Moreover, there exists a component which carbonitrided the steel material as a mechanical component (for example, refer patent document 6 and patent document 7).

JP 2014-77198 A International Publication No. 2009/131202 JP-A-4-21757 JP 2007-291486 A JP 2007-308772 A JP 2011-184768 A JP 2014-185379 A

However, the carburized parts carburized using the vacuum carburizing process tend to have a higher carbon concentration at the edge portion than at the flat portion. For this reason, the edge part of the carburized component tends to be excessively carburized containing carbon at a higher concentration than necessary. Coarse cementite, which is the starting point of fracture, tends to remain in the hardened structure of the excessively carburized portion, so that the carburized parts subjected to vacuum carburizing treatment may have insufficient bending fatigue strength.
In particular, in carburized parts having a surface shape with many edge portions such as gears and pulleys for CVT, deterioration of bending fatigue strength due to excessive carburization of the edge portions due to vacuum carburizing treatment has been a problem.

In addition, carburized parts made of steel that has been subjected to vacuum carburizing treatment have a low resistance to temper softening, so that sufficient surface fatigue strength may not be obtained.
When the carbonitriding process is performed on the steel material, the temper softening resistance can be improved as compared with the case where only the carburizing process is performed.

However, even a carbonitrided part obtained by subjecting a steel material to carbonitriding may have insufficient tempering softening resistance, and it has been required to further improve tempering softening resistance.
Further, in the carburized component and the carbonitrided component having a shape including the conventional edge portion, it has been required to further improve the bending fatigue strength.
The present invention has been made in view of the above circumstances, and an object thereof is to provide a carbonitrided component having a shape including an edge portion and excellent in temper softening resistance and bending fatigue strength and a method for manufacturing the same. .

In order to solve the above-mentioned problems, the present inventor diligently studied the temper softening resistance and bending fatigue strength of carbonitrided parts. As a result, it has excellent temper softening resistance and bending even with a shape including an edge portion by having a surface layer with appropriate carbon concentration and nitrogen concentration and sufficiently containing fine carbide and / or nitride precipitates. The inventors have found that fatigue strength can be obtained, and arrived at the carbonitrided parts of the present invention.
The gist of the present invention is as follows.

[1] The core is mass%,
C: 0.10 to 0.25%,
Si: 0.03 to 0.49%,
Mn: 0.30 to 1.50%,
P: 0.030% or less,
S: 0.060% or less,
Cr: 0.90 to 3.00%,
Al: 0.100% or less and N: 0.030% or less,
The balance has a chemical composition consisting of Fe and impurities,
The surface has a flat part and an edge part,
The depth of the grain boundary oxide layer in the flat part is 1 μm or less, and the flat part surface layer region from the surface of the flat part to the depth of 0.05 mm has a carbon concentration of 0.70% or more and 0.89% or less. The nitrogen concentration is 0.20% or more and 0.70% or less, and the precipitate density of carbide and / or nitride is 0.5 piece / μm ² or more,
Bending fatigue strength characterized in that the maximum length of the carbide and / or nitride precipitate is 5.0 μm or less in the edge surface layer region from the surface of the edge to a position having a depth of 0.05 mm Excellent carbonitriding parts.

[2] The core part is further in mass%,
Nb: 0.100% or less,
The carbonitrided part according to [1], containing at least one selected from the group consisting of Ti and 0.100% or less.
[3] The core part is further mass%,
Mo: 0.50% or less,
Cu: 0.5% or less,
Ni: The carbonitrided component according to [1] or [2], containing at least one selected from the group consisting of 0.50% or less.

[4] A method for producing a carbonitrided component according to any one of [1] to [3],
[1] to [3] having the chemical composition according to any one of steps, and manufacturing a steel material having a surface including a vertex portion, an edge portion, and a flat portion;
A step of performing nitriding treatment and quenching treatment after vacuum carburizing treatment of the steel material;
The vacuum carburizing treatment includes a carburizing step of carburizing the steel material at a carburizing temperature of 980 to 1100 ° C, a diffusion step of diffusing carbon that has entered the steel material while maintaining the carburizing temperature, A cooling step of cooling the steel material having a surface temperature after the diffusion step of the carburizing temperature to 500 ° C. at a cooling rate of 3.0 to 100 ° C./sec, and a ratio of carburizing time to diffusion time (carburizing time / Diffusion time) is 0.20 or more and 0.45 or less.

In the carbonitrided part of the present invention, the Cr content in the core part is 0.90% or more, the Si content is 0.49% or less, and the carbon concentration in the surface region of the flat part is 0.70% or more and 0.00. 89% or less, nitrogen concentration is 0.20% or more and 0.70% or less, carbide and / or nitride precipitate density is 0.5 / μm ² or more, and carbide and / or nitridation in the edge layer region The maximum length of the precipitate of the product is 5.0 μm or less. Therefore, it is possible to provide a carbonitrided part having a shape including an edge portion and having excellent temper softening resistance and bending fatigue strength.

  In the carbonitriding component manufacturing method of the present invention, a steel material containing Cr and Si having a surface including an edge portion in the above content is vacuum carburized so as to have a sufficiently high carbon concentration, and at a sufficiently fast cooling rate. The temperature between the carburizing temperature and 500 ° C. is cooled, and the steel material after cooling is subjected to nitriding treatment and quenching treatment. For this reason, the carbonitrided part of the present invention having excellent temper softening resistance and bending fatigue strength can be obtained.

It is the perspective view which showed an example of the carbonitriding components of this invention. It is sectional drawing which showed a part of carbonitriding components shown in FIG. 1, and is the schematic diagram which showed the cross section CS shown in FIG. It is a side view of the Ono type rotation test piece used in the example.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
In order to solve the above problems, the present inventor has intensively studied the bending fatigue strength of carbonitrided parts. As a result, when Cr nitride and / or MnSi nitride deposited on the surface layer of the carbonitrided part is present at the grain boundary, the hardenability is lowered and the bending fatigue strength of the carbonitrided part is lowered. I understood that.
Then, this inventor examined paying attention to the chemical composition of the steel materials vacuum-carburized, and the precipitate deposited on the surface layer of the carbonitriding component. As a result, the following findings (1) to (4) were obtained.

(1) By setting the Cr content in the steel material to 0.90% or more, precipitation in the crystal grains of carbide and nitride due to carburizing and nitriding is promoted and hardenability is ensured. And the bending fatigue strength is improved.
(2) By making Si content in steel materials 0.49% or less, the fall of the bending fatigue strength by having too much Si content can be suppressed.

(3) A steel material containing Cr and Si having a shape including an edge portion in the above content is vacuum carburized so as to have a sufficiently high carbon concentration, and a carburizing temperature between 500 ° C. and a sufficiently high cooling rate. By cooling, it is possible to precipitate fine cementite and suppress a decrease in bending fatigue strength due to excessive carburization of the edge portion, and to precipitate a sufficient number of fine carbides in the crystal grains.
(4) The temper softening resistance is improved by nitriding the cooled steel material. Also, in nitriding treatment, fine nitrides are precipitated in the crystal grains using carbides in the steel after cooling as the precipitation sites, so carbonitriding that suppresses the precipitation and growth of nitrides on the grain boundaries of the surface layer. Parts are obtained.

Hereinafter, the carbonitriding component of the present invention completed based on the above knowledge and the method for manufacturing the carbonitriding component will be described in detail.
The carbonitrided component of the present embodiment is obtained by subjecting a steel material having an apex portion, a flat portion, and an edge portion to vacuum carburizing treatment, nitriding treatment, and quenching treatment.
The surface of the carbonitrided component of the present embodiment includes a vertex portion, an edge portion, and a flat portion. Of the surface of the carbonitrided component, excessive carburization is likely to occur at the apex portion and the edge portion. The apex has a large surface area relative to the steel volume. For this reason, it is easy for carbon to enter by carburizing treatment, and it is difficult to diffuse in the depth direction. Therefore, excessive carburization tends to occur at the apex portion. However, the apex portion is a portion where a large load is not applied when actually used. Therefore, it is not necessary for the apex portion to evaluate the degree of excessive carburization. Therefore, the object which suppresses excessive carburization by this invention is an edge part. Hereinafter, the vertex part, the edge part, and the flat part in the present embodiment will be described with reference to the drawings.

FIG. 1 is a perspective view showing an example of a carbonitriding component of the present invention. A carbonitriding component 100 shown in FIG. 1 is a four-point bending test piece having a vertex portion, an edge portion, and a flat portion on the surface. Here, the vertex part, the edge part, and the flat part in the carbonitriding component 100 shown in FIG. 1 will be described.
The entire carbonitrided component 100 shown in FIG. 1 has a quadrangular prism shape, and a notch portion is formed at the approximate center in the length direction.

  First, a vertex part is demonstrated among the surfaces of the carbonitriding component 100 shown in FIG. A point existing at an arbitrary position on the part surface is defined as Pa. Assuming a virtual sphere with a radius of 1.0 mm centered on the point Pa, the volume of the portion where the virtual sphere and the carbonitriding part overlap is V, and the area of the part of the carbonitriding part included in the virtual sphere is S To do. A portion where the parameter represented by V / S is 0.223 or less is defined as a vertex. In Pa shown in FIG. 1, V / S is 0.222, which is a vertex portion.

  Next, attention is paid to the surface portion (edge surface portion) of the side 2 of the cutout portion. A point existing at an arbitrary position on the side 2 in the edge surface portion is defined as a point Pc. Then, as shown in FIG. 1, a cross section CS perpendicular to the side 2 at the point Pc is assumed.

  2 is a cross-sectional view showing a part of the carbonitriding component shown in FIG. 1, and is a schematic view showing a cross-section CS shown in FIG. As shown in FIG. 2, a virtual point P having a depth of 1.0 mm in the vertical direction from the surface is assumed from an arbitrary point XP on the surface of the carbonitrided component in the cross section CS. An aggregate of the virtual points P is indicated by a dotted rectangle in FIG. Assuming a virtual sphere with a radius of 1.0 mm centered on the virtual point P, when the virtual sphere contacts the surface of the carbonitrided part (one intersection), the virtual point P is P1, and the virtual point centered on P1 The contact point between the sphere and the surface of the carbonitrided part is designated XP1. A portion defined by the point XP1 is defined as a flat portion (a portion indicated by reference numeral 3 in FIG. 1, “flat portion” in FIG. 2).

  As described above, the point P1 and the point XP1 paired with the point P1 are defined, and the flat portion is defined by the point XP1. Then, a part other than the flat part and the apex part, that is, a point XP2 on the surface of the carbonitriding component other than the point XP1 and the point Pa is defined as an edge part (for example, the side indicated by reference numeral 2 in FIG. "Edge part").

  Further, the “core portion” of the carbonitrided component in the present embodiment means that the concentration of C and N, which are chemical components of the steel material subjected to vacuum carburizing and nitriding, varies (increases) depending on the carbonitriding of the steel. No, it refers to the part deeper than the surface layer of carbonitriding parts. Specifically, the inside of the carbonitrided part having a minimum depth from the surface of the carbonitrided part (depth in the vertical direction from the surface) of 2.0 mm or more is defined as a core part.

"Chemical composition of steel (core)"
The core of the carbonitrided component of the present embodiment has the following chemical composition. The chemical composition of the core is the same as the chemical composition of the steel material before the vacuum carburizing process. “%” Of the element content means “% by weight”.
(C: 0.10 to 0.25%)
Carbon (C) increases the core hardness of the carbonitrided component. If the C content is too low, the above effect cannot be obtained. On the other hand, if the C content is too high, the core hardness becomes too high and the toughness is lowered, so that the bending fatigue strength is lowered. Therefore, the content of C is set to 0.10 to 0.25%.

(Si: 0.03-0.49%)
Si increases the hardenability and temper softening resistance of the steel material, and increases the bending fatigue strength and the surface fatigue strength. However, if the Si content is too high, MnSi nitride precipitates, which rather deteriorates the hardenability of the steel material and decreases the bending fatigue strength. Therefore, to obtain the above effect, the Si content is set to 0.03 to 0.49%. The Si content is preferably 0.10 to 0.40%.

(Mn: 0.30 to 1.50%)
Mn has an effect of improving hardenability and is an effective element for increasing bending fatigue strength. However, if the Mn content is less than 0.30%, the above effects cannot be obtained sufficiently. On the other hand, if the Mn content exceeds 1.50%, the retained austenite increases excessively, and the bending fatigue strength decreases. Therefore, the content of Mn is set to 0.30 to 1.50%. Further, the Mn content is more preferably 0.70 to 1.30%.

(P: 0.030% or less)
P is an element that easily segregates at the grain boundaries and embrittles the grain boundaries. For this reason, when P content exceeds 0.030%, bending fatigue strength and surface fatigue strength will be reduced. Therefore, the P content is 0.030% or less. In addition, P is inevitably contained in the steel material, and if it is attempted to reduce the P content, the manufacturing cost increases. Therefore, a preferable lower limit of the P content is 0.003%, and a more preferable lower limit is 0.006%.

(S: 0.060% or less)
S is an element contained as an impurity. When there is much S content, coarse MnS will be easy to produce and bending fatigue strength will fall. If the S content exceeds 0.060%, the desired bending fatigue strength cannot be obtained. Therefore, the content of S is set to 0.060% or less. The upper limit with preferable S content is 0.030%. However, when the S content is reduced to less than 0.003%, the manufacturing cost increases. Therefore, the preferable lower limit of the S content is 0.003%.
Further, S combines with Mn to form MnS, thereby improving the machinability. However, if the S content is less than 0.006%, it is difficult to obtain an effect of improving the machinability.

(Cr: 0.90 to 3.00%)
Cr is an element effective in increasing bending fatigue strength and surface fatigue strength because it promotes the precipitation of carbides and nitrides by carburizing and nitriding, and has the effect of increasing hardenability and temper softening resistance. is there. However, when the Cr content is less than 0.90%, these effects are hardly obtained. On the other hand, if the Cr content exceeds 3.00%, the bending fatigue strength decreases due to excessive carburization. Therefore, the content of Cr is set to 0.90 to 3.00%. The Cr content is preferably 1.00 to 2.00%.

(Al: 0.100% or less)
Al has a deoxidizing action and has an effect of increasing hardenability and temper softening resistance. However, when the Al content is less than 0.010%, these effects are hardly obtained. Therefore, the Al content is preferably 0.010% or more. On the other hand, Al tends to form hard oxide inclusions, and if the Al content exceeds 0.100%, the bending fatigue strength decreases. Therefore, the Al content is set to 0.100% or less.

(N: 0.030% or less)
Nitrogen (N) is inevitably contained in the steel material. N combines with Al to form AlN and refines the crystal grains. However, when N content is too high, the forgeability of steel materials will fall. Therefore, the N content is 0.030% or less. The upper limit with preferable N content is 0.020%, More preferably, it is 0.018%. If it is attempted to reduce the N content, the manufacturing cost increases. Therefore, the minimum with preferable N content shall be 0.003%.

  The balance of the chemical composition of the core portion of the carbonitrided component of the present embodiment is composed of Fe and impurities. Here, the impurities mean impurities mixed from raw ore, scrap, or a manufacturing environment when a steel material before carburizing to be a carbonitrided part is industrially produced.

The steel material in the present embodiment may further contain Nb and / or Ti instead of a part of Fe. Nb and Ti are elements contained arbitrarily.
(Nb: 0 to 0.100% or less)
Nb combines with N and / or C in steel to produce fine carbides, nitrides, or carbonitrides, and to suppress grain growth during surface hardening heat treatment (vacuum carburizing and nitriding) There is. If Nb is contained even a little, the above effect can be obtained to some extent. On the other hand, when Nb is contained exceeding 0.100%, the coarsening suppression effect is saturated. Therefore, the upper limit of the Nb content is set to 0.100%. The upper limit of the Nb content is preferably 0.080%, more preferably 0.045%.

(Ti: 0 to 0.100% or less)
Ti combines with N and / or C in steel to produce fine carbides, nitrides, or carbonitrides, and to suppress grain growth during surface hardening heat treatment (vacuum carburizing and nitriding) There is. If Ti is contained even a little, the above effect can be obtained to some extent. On the other hand, when the Ti content is excessive, coarse nitrides and oxides are generated, and the toughness of the steel material is lowered. Therefore, the Ti content is 0.100% or less, preferably 0.050% or less.

  The steel material in the present embodiment may further contain one or more selected from the group consisting of Mo, Cu, and Ni instead of a part of Fe. These elements are elements contained arbitrarily. All of these elements increase toughness.

(Mo: 0 to 0.50% or less)
Mo increases the hardenability of the steel material and increases the bending fatigue strength of the steel material. If Mo is contained even a little, the above effect can be obtained to some extent. On the other hand, even if it contains Mo exceeding 0.50%, not only the effect is saturated, but the strength of the steel material after hot working becomes too high, and the machinability of the steel material is lowered. Therefore, the Mo content is 0.50% or less. The Mo content is preferably 0.40% or less.

(Cu: 0 to 0.5% or less)
Cu suppresses excessive carburization of the steel material and further increases the toughness of the steel material. If Cu is contained even a little, the above effect can be obtained to some extent. On the other hand, when Cu content is too high, the hot workability of steel materials will fall. Therefore, the Cu content is 0.5% or less. The said effect is acquired notably that Cu content is 0.1% or more. The Cu content is preferably 0.3% or less.

(Ni: 0 to 0.50% or less)
Ni suppresses excessive carburization of the steel material and further increases the toughness of the steel material. If Ni is contained even a little, the above effect can be obtained to some extent. On the other hand, even if Ni exceeds 0.50%, the effect is saturated and the manufacturing cost of the steel material increases. Therefore, the Ni content is 0.50% or less. The said effect is acquired notably that Ni content is 0.10% or more. The Ni content is preferably 0.20% or less.

“Carbon and nitrogen concentration of carbonitrided parts”
In the carbonitrided component of this embodiment, the flat portion surface layer region from the surface of the flat portion to a position having a depth of 0.05 mm has a carbon concentration of 0.70% or more and 0.89% or less, and a nitrogen concentration of 0.1%. It is 20% or more and 0.70% or less.

The carbon concentration and the nitrogen concentration in the surface region of the flat part can be measured by the following methods, respectively.
The carbon concentration and the nitrogen concentration in the cross section of the flat portion surface layer region are analyzed by EPMA (Electron Microphone Mouth Analyzer), and the average values of the carbon concentration and the nitrogen concentration from the surface to a depth of 0.05 mm are calculated.
Note that the carbon concentration and nitrogen concentration in the surface area of the flat portion were set to the concentration from the surface to a depth of 0.05 mm because the influence of surface contamination on the evaluation of the degree of carbonitriding was suppressed and carbonitriding of the surface layer was performed. This is because the degree of the above is evaluated with high accuracy.

  The carbon concentration in the surface area of the flat portion needs to be 0.70% or more in order to increase the surface hardness of the carbonitrided component and increase the bending fatigue strength. On the other hand, if the carbon concentration in the surface region of the flat part exceeds 0.89%, the carbon concentration in the surface region of the edge part becomes too high, and coarse cementite is likely to precipitate, and the bending fatigue strength of the edge part decreases. For this reason, the carbon concentration of the flat portion surface layer region is set to 0.70% or more and 0.89% or less. The carbon concentration in the surface region of the flat part is preferably 0.75% or more. The carbon concentration in the surface region of the flat part is preferably 0.85% or less.

  The nitrogen concentration in the surface area of the flat portion needs to be 0.20% or more in order to improve the temper softening resistance and increase the surface hardness of the carbonitrided component. On the other hand, if the nitrogen concentration in the surface region of the flat part exceeds 0.70%, the amount of retained austenite having a low hardness remaining after the quenching treatment increases, and the hardness of the carbonitrided component decreases. For this reason, the nitrogen concentration of the flat portion surface layer region is set to 0.20% or more and 0.70% or less. The nitrogen concentration in the flat surface layer region is preferably 0.25% or more. The nitrogen concentration in the flat surface layer region is preferably 0.65% or less.

  The carbon concentration and the nitrogen concentration in the surface area of the flat portion can be controlled by adjusting the conditions in the vacuum carburizing process and the nitrogen process (and quenching process) performed after the vacuum carburizing process.

“Carbide and / or nitride precipitates”
In the carbonitrided component of the present embodiment, the carbide and / or nitride precipitate density in the flat portion surface layer region is 0.5 / μm ² or more. When the precipitate density is less than 0.5 / μm ² , the carbide and / or nitride precipitates are insufficient, and the carbonitrided parts are insufficient in hardness and / or bending fatigue strength. The precipitate density is preferably 0.7 / μm ² or more in order to improve the hardness and / or bending fatigue strength of the carbonitrided part.

  Further, in the carbonitrided component of the present embodiment, the maximum length of carbide and / or nitride precipitates in the edge surface layer region from the edge surface to a depth of 0.05 mm is 5.0 μm or less. . If the maximum length of the precipitate exceeds 5.0 μm, the precipitate of carbide and / or nitride is coarse, and the bending fatigue strength of the carbonitrided part is insufficient. The maximum length of the precipitate is preferably 3.0 μm or less in order to improve the bending fatigue strength of the carbonitrided part.

(Depth of grain boundary oxide layer in flat part: 1 μm or less)
Since the carbonitriding component of this embodiment has been subjected to vacuum carburizing, there are few grain boundary oxide layers formed by the carbonitriding. The grain boundary oxide layer is preferably as small as possible in order to reduce the incompletely quenched structure. The incompletely quenched structure causes a decrease in the fatigue strength of the carbonitrided part, and the degree of the decrease in the fatigue strength increases as the incompletely quenched structure increases. Therefore, the upper limit of the grain boundary oxide layer depth of the flat portion of the carbonitrided part is set to 1 μm.
In this embodiment, the grain boundary oxide layer depth of the flat part of the carbonitrided component means the maximum depth from the surface where the black oxide continuously extending from the surface of the flat part toward the inside reaches. To do.

"Production method"
Next, an example is given and demonstrated about the manufacturing method of the carbonitriding components of this embodiment.
First, a steel material that satisfies the above-described chemical composition and has a vertex portion, a flat portion, and an edge portion is manufactured.
In this embodiment, for example, molten steel having the above chemical composition is manufactured, and a slab (slab or bloom) is manufactured by a continuous casting method. Instead of slabs, ingots (steel ingots) may be produced by the ingot-making method using molten steel having the above chemical composition.
Next, the billet (steel piece) is manufactured by hot working the slab or ingot. Thereafter, the billet is hot-worked into a bar or wire. The hot working may be hot rolling or hot forging.

Next, cold forging and / or machining is performed on the manufactured steel bar or wire to obtain a steel material having a predetermined shape having a vertex portion, a flat portion, and an edge portion.
Examples of the machining include cutting and drilling. The shape of the steel material having the apex portion, the flat portion, and the edge portion is determined according to the use of the carbonitriding component, and can be formed by a known method.

Next, in this embodiment, a vacuum carburizing process, a nitriding process, and a quenching process are performed on the steel material having the apex part, the flat part, and the edge part. In the present embodiment, a case where the nitriding process also serves as a quenching process will be described as an example.
In the vacuum carburizing treatment, first, a heating process is performed in which the steel material is heated to the carburizing temperature in a furnace whose pressure is reduced to 10 Pa or less, for example. Next, a soaking step is performed in which the steel material is soaked at the carburizing temperature. Subsequently, a carburizing process is performed in which carburizing gas is introduced into the furnace and the steel material is carburized at a predetermined carburizing gas pressure and carburizing temperature. Thereafter, a diffusion process for diffusing carbon that has entered the steel material into the steel material while maintaining the carburizing temperature and a cooling process for cooling the steel material are performed in this order.

The soaking time in the soaking step is preferably 5 to 120 minutes, and more preferably 30 to 60 minutes. The pressure in the furnace in the soaking process may be 100 Pa or less, or may be a nitrogen atmosphere of 1000 Pa or less by simultaneously introducing nitrogen gas and evacuating with a vacuum pump.
As the type of carburizing gas used in the carburizing process, known ones used in vacuum carburizing treatment can be used, and for example, hydrocarbon gases such as acetylene, propane, and ethylene can be used.

  The carburizing gas pressure in the carburizing step is preferably set within a predetermined range according to the type of carburizing gas because the ease of sooting and the likelihood of occurrence of carburizing unevenness differ depending on the type of gas. For example, when the carburizing gas is acetylene, the carburizing gas pressure is preferably 10 to 1000 Pa. When the carburizing gas is propane, the carburizing gas pressure is preferably 200 to 3000 Pa.

  The carburizing temperature is in the range of 980 to 1100 ° C. When the carburizing temperature is 980 ° C. or higher, a carbonitrided part having a predetermined carbon concentration can be manufactured in a short time. The carburizing temperature is preferably 1020 ° C or higher. In addition, if the carburizing temperature is too high, elements that improve the hardenability such as Mn and / or Cr are released into the vacuum, so the temperature is set to 1100 ° C. or lower. The carburizing temperature is preferably 1080 ° C. or lower.

The processing time in the carburizing step and the diffusion step is such that the ratio of the processing time in the carburizing step (carburizing time) to the processing time in the diffusion step (diffusion time) (carburizing time (min) / diffusion time (min)) is 0.20 or more. Within a range of 0.45 or less, it is appropriately determined according to the chemical composition of the steel material, the carbon concentration of the target flat surface layer region, the core hardness, and the grain boundary oxide layer depth.
If the ratio between the carburizing time and the diffusion time is less than 0.20, the precipitate density of carbide and / or nitride in the surface region of the flat portion is insufficient. By setting the ratio of the carburizing time and the diffusion time to 0.20 or more, the carbon concentration of the surface layer can be made sufficiently high, and the carbonitrided parts with sufficiently high carbide and / or nitride precipitate density in the surface region of the flat part Is obtained. The ratio between the carburizing time and the diffusion time is preferably 0.25 or more.

  The ratio of carburizing time and diffusion time is 0.45 or less. In the present embodiment, as will be described later, since the steel material after the diffusion process is rapidly cooled at a cooling rate of 3.0 ° C./sec or more, the diffusion of carbon in the steel material being cooled is suppressed. When the ratio of the carburizing time and the diffusion time is 0.45 or less, the diffusion time can be sufficiently secured, so that even if the steel is rapidly cooled after the diffusion process, the carbon can be sufficiently diffused in the steel material, and the carburizing time is Excess carburization due to being too long can be prevented. The ratio between the carburizing time and the diffusion time is preferably 0.40 or less, and more preferably 0.35 or less.

  The pressure in the furnace in the diffusion process is preferably 10 Pa or less in order to remove residual gas in the carburization process. When introducing nitrogen gas and evacuating with a vacuum pump at the same time, a nitrogen atmosphere of 1000 Pa or less may be used.

In the cooling step, the steel material after the diffusion step is cooled at a cooling rate of 3.0 to 100 ° C./sec. When the cooling rate is 3.0 ° C./sec or more, precipitation of coarse carbides is suppressed, and fine carbides can be sufficiently precipitated in the crystal grains. Further, when the cooling rate is 3.0 ° C./sec or more, fine cementite can be precipitated, and a decrease in bending fatigue strength due to excessive carburization of the edge portion can be suppressed. Therefore, the cooling rate is set to 3.0 ° C./sec or more, preferably 4 ° C./sec or more.
The cooling rate is set to 100 ° C./sec or less in order to reduce heat treatment strain.

  In the present embodiment, the cooling rate in the cooling step means the average value of the cooling rate (° C./second) when the surface temperature of the steel material after the diffusion step is between the carburizing temperature and 500 ° C.

As a cooling method in the cooling step, a known method capable of cooling at the above-described cooling rate can be used. For example, it may be allowed to cool, gas cooling, or other methods. Also good. In the present embodiment, gas cooling is preferably used because cooling can be easily performed at a cooling rate of 3.0 ° C./sec or more.
When gas cooling is used in the cooling step, it is preferable to use an inert gas as the cooling gas. For example, nitrogen gas and / or helium gas is preferably used as the inert gas, and it is particularly preferable to use nitrogen gas that is inexpensive and easily available. By using an inert gas as the cooling gas, oxidation of the steel material can be prevented.

Next, a nitriding treatment that also serves as a quenching treatment is performed on the steel material that has been vacuum carburized.
Specifically, for example, a steel material that has been vacuum carburized is placed in a furnace, heated to a quenching temperature in a nitrogen atmosphere, an ammonia-containing gas is then introduced into the furnace, and the steel material is quenched in an ammonia-containing atmosphere ( Nitriding is performed by holding at a nitriding temperature, and then cooling is performed (quenching treatment).

  The ammonia-containing atmosphere at the time of nitriding may be a mixed gas atmosphere containing nitrogen gas, ammonia gas, and gas generated by decomposition of ammonia. The preferable nitriding gas pressure in the furnace in the nitriding step is not more than atmospheric pressure, more preferably not more than 400 hPa. In the present embodiment, the flow rate of the ammonia-containing gas is adjusted so that the nitrogen concentration in the flat portion surface layer region becomes a predetermined concentration. For example, when the nitriding test is performed at a predetermined ammonia gas flow rate, and the nitrogen concentration in the flat portion surface layer region falls outside the desired range, the ammonia gas flow rate is changed so that the nitrogen concentration in the flat portion surface layer region is 0.1. The ammonia gas flow rate is determined so as to be 20 to 0.70%. The ammonia gas flow rate has a strong correlation with the furnace volume. For example, the ammonia gas flow rate in a furnace having a volume of 2000 liters is 3 liters / minute or more and less than 20 liters / minute. Preferably it is 5 liters / minute or more and 15 liters / minute or less.

The quenching temperature is preferably in the range of 780 to 880 ° C, and more preferably in the range of 820 to 870 ° C.
The holding time (nitriding time) in the quenching treatment is preferably 30 to 240 minutes in order to obtain a carbonitrided component having a nitrogen concentration in the flat surface layer region of 0.20% or more and 0.70% or less, More preferably, it is 120 minutes.
As a cooling method in the quenching treatment, a known method such as oil cooling or water cooling can be used. When using oil cooling as the cooling method, the temperature of the quenching oil is preferably in the range of 60 to 160 ° C.

In the present embodiment, each condition in the nitriding process that also serves as a vacuum carburizing process and a quenching process (soaking time, type of carburizing gas, carburizing gas pressure, carburizing temperature, processing time in the carburizing process, processing time in the diffusion process, cooling The cooling rate in the process, quenching temperature (nitriding temperature), type of ammonia-containing gas, holding time in quenching treatment (nitriding time, etc.), the chemical composition of the steel material, the carbon concentration and nitrogen of the target flat surface area It is determined according to the concentration, the density and size of the carbide and / or nitride precipitates in the surface region of the flat portion and the surface region of the edge portion.
Specifically, the above-mentioned conditions in the vacuum carburizing treatment and / or nitriding treatment may be determined using simulation, or the vacuum carburizing treatment test and / or nitriding treatment test is performed, Determine each of the above conditions so that the carbon concentration and the nitrogen concentration are within the predetermined concentration range, and the fine surface carbide layer and / or nitride precipitate are sufficiently contained in the flat surface layer region and the edge surface layer region. Also good.

  In the present embodiment, the case where the nitriding process that also serves as the quenching process is performed has been described as an example, but the quenching process may be performed after the nitriding process.

In the present embodiment, a tempering process may be performed as necessary after performing a nitriding process that also serves as a quenching process.
When performing the tempering treatment, the tempering temperature is preferably in the range of 150 to 200 ° C, and more preferably in the range of 160 to 190 ° C.
The holding time at the tempering temperature is preferably 10 to 180 minutes.
Through the above steps, the carbonitrided component according to the present embodiment is manufactured.

Hereinafter, the present invention will be described specifically by way of examples.
The molten steel which has the chemical composition of the steel A-steel X shown in Table 1 was manufactured, and the ingot was manufactured using the manufactured molten steel. Next, the ingot was hot forged into a bar steel. Next, cold forging and machining were performed on the manufactured steel bar to manufacture a round bar having a diameter of 35 mm.

“Preparation of 4-point bending fatigue test piece”
As shown in FIG. 1, as a steel material having a surface including a vertex portion, a flat portion, and an edge portion by machining a round bar having each chemical composition of Steel A to Steel X manufactured as described above. A plurality of 4-point bending fatigue test pieces were formed. The four-point bending fatigue test piece was a square in cross section with a height and width of 13 mm, and the length was 100 mm. At the center in the length direction of the 4-point bending fatigue test piece, a notch having a semicircular cross-sectional shape was formed. The radius at the cutout of the semicircle was 2 mm.

  Thereafter, a four-point bending fatigue test piece having each chemical composition of Steel A to Steel X was subjected to vacuum carburizing treatment and nitriding treatment also serving as a quenching treatment under the following conditions. The carbonitrided parts (four-point bending fatigue test pieces) of Examples 1 to 11 were obtained.

"Examples 1 to 22 and Comparative Examples 1 to 11"
A four-point bending fatigue test piece as a steel material was heated to a carburizing temperature shown in Table 2 in a furnace depressurized to 10 Pa or less, and soaked for 60 minutes at the carburizing temperature. Subsequently, acetylene gas was introduced into the furnace as the carburizing gas, and a carburizing process was performed in which the steel material was carburized at the carburizing gas pressure and the processing time in the carburizing process shown in Table 2.

  Next, while maintaining the carburizing temperature, a diffusion step of diffusing carbon that had entered the steel material in the treatment time shown in Table 2 was performed. Thereafter, the steel material was gas-cooled with nitrogen gas at a cooling rate shown in Table 2 for 10 minutes, so that the temperature of the steel material was reduced from the carburizing temperature to 500 ° C. or less, and the vacuum carburizing treatment was completed.

  Subsequently, the vacuum carburized steel was heated to a quenching temperature of 860 ° C. in a nitrogen atmosphere of 100 to 160 Pa and held for 10 minutes. Thereafter, nitrogen gas is introduced until the furnace pressure reaches 300 hPa, ammonia gas is supplied into the furnace at a flow rate shown in Table 2, and the steel material is quenched in an ammonia-containing atmosphere with the furnace pressure kept constant at 300 Pa. Nitriding was performed by holding at (nitriding temperature) for 60 minutes. The furnace volume was 2000 liters. Thereafter, the steel material was oil-quenched using a quenching oil at 120 ° C., and the nitriding treatment also serving as a quenching treatment was completed.

  Subsequently, the steel material subjected to nitriding treatment that also serves as a quenching treatment is heated to a tempering temperature of 170 ° C. and subjected to a tempering treatment for 2 hours, and carbonitrided parts of Examples 1 to 22 and Comparative Examples 1 to 11 Got.

"Measurement of carbon and nitrogen concentrations in the surface area of the flat part"
The carbon concentration and the nitrogen concentration of the cross section in the flat portion surface layer region of the carbonitrided parts (four-point bending fatigue test pieces) of Examples 1 to 22 and Comparative Examples 1 to 11 were analyzed by EPMA (Electron Microphone Mouth Analyzer). The average values of the carbon concentration and the nitrogen concentration from the surface to a position having a depth of 0.05 mm were calculated. The results are shown in Table 3.

“Observation of precipitates in the surface area of the flat part”
The carbonitrided parts (four-point bending fatigue test pieces) of Examples 1 to 22 and Comparative Examples 1 to 11 were cut perpendicularly to the surface of the flat part, and filled with resin so that the cut surface became the observation surface, and observed. The surface was mirror-polished to obtain a test piece. Next, the observation surface is subjected to nital etching, and observed with a scanning electron microscope (SEM) set to 5000 times, and the number of precipitates from the surface of the flat portion to a position having a depth of 0.05 mm is obtained. The density of carbide and / or nitride precipitates contained in the region was calculated. The results are shown in Table 3.

"Observation of precipitates on the surface layer of the edge"
The carbonitrided parts (four-point bending fatigue test pieces) of Examples 1 to 22 and Comparative Examples 1 to 11 were cut perpendicularly to the surface of the edge portion, embedded in the resin so that the cut surface became the observation surface, and observed. The surface was mirror-polished to obtain a test piece. Next, the observation surface was etched with nital, and observed with a scanning electron microscope (SEM) set to 2000 times from the surface of the edge part to a depth of 0.05 mm, with the largest cementite length. Identified. Then, the maximum distance between any two points at the cementite interface having the maximum length is measured, and this is determined as the maximum length of the carbide and / or nitride precipitates contained in the edge surface layer region. did. The results are shown in Table 3. “> 5” in Table 3 means that the maximum length of the carbide and / or nitride precipitates in the surface layer region of the edge portion exceeds 5.0 μm.

"Grain boundary oxide layer depth"
The carbonitrided parts (four-point bending fatigue test pieces) of Examples 1 to 22 and Comparative Examples 1 to 11 were cut so as to be substantially perpendicular to the surface of the edge part, and the cut surface was mirror-polished to 1000 times It was observed with a set optical microscope. Then, the black oxide continuously extending in a streak from the surface to the inside is observed, and the maximum depth from the surface where the streak black oxide reaches is measured as the grain boundary oxide layer depth. did.
As a result, no streaky black oxide was observed in any of the carbonitrided parts of Examples 1 to 22 and Comparative Examples 1 to 11, and the grain boundary oxide layer depth was 0 μm.

"4-point bending fatigue test"
A 4-point bending fatigue test was carried out using the carbonitrided parts (4-point bending fatigue test pieces) of Examples 1 to 22 and Comparative Examples 1 to 11.
A servo-type fatigue tester was used for the 4-point bending fatigue test. The distance between the fulcrums of the 4-point bending fatigue test piece was 45 mm. The maximum load stress was 1373 MPa, and the stress ratio between the maximum load stress and the minimum load stress was 0.1. The frequency was 10 Hz. Then, the breaking strength when the number of repeated stress loads was 1 × 10 ⁴ was evaluated as 4-point bending fatigue strength (MPa). The results are shown in Table 3.

"Hardness of surface of flat part (tempering softening resistance test)"
The carbonitrided parts (four-point bending fatigue test pieces) of Examples 1 to 22 and Comparative Examples 1 to 11 were heated to a tempering temperature of 300 ° C. and subjected to an additional tempering treatment for 2 hours.
Then, about each 4-point bending fatigue test piece which performed the additional tempering process, the hardness of the position of depth 0.05mm from the surface of a flat part was measured based on JISZ2244 with the Vickers hardness meter. The results are shown in Table 3.

"Preparation of Ono type rotating bending fatigue test piece"
As a steel material having a shape having a flat portion and an edge portion from R / 2 portion of a round bar having chemical compositions of steel A to steel X used for the production of a four-point bending fatigue test piece, the shape shown in FIG. A plurality of Ono-type rotating bending fatigue test pieces were collected. The R / 2 portion is a portion that bisects the center and the outer periphery of the round bar cross section (circular shape). The shape of the rotating bending fatigue test piece conformed to JIS Z2274 (1974). The rotating bending fatigue test piece had a circular cross section, and a parallel portion having a diameter of 10 mm was formed at the center. In the center of the parallel part, an annular semicircular groove having a radius R1 = 1 mm was formed. Each numerical value in FIG. 3 indicates a dimension (unit: mm).

  Thereafter, the Ono rotary bending fatigue test piece having the chemical composition of steel A to steel X is subjected to nitriding treatment that also serves as vacuum carburizing treatment and quenching treatment in the same manner as the above four-point bending fatigue test piece. The carbonitriding parts (Ono type rotating bending fatigue test pieces) of Examples 1 to 22 and Comparative Examples 1 to 11 were obtained.

"Ono type rotating bending test"
For the carbonitrided parts (Ono type rotating bending fatigue test pieces) of Examples 1 to 22 and Comparative Examples 1 to 11, the Ono type rotating bending fatigue test was performed at room temperature (25 ° C.) in accordance with JIS Z2274 (1974). In the inside, it implemented on the conditions of the rotation speed 3400rpm. The maximum stress (MPa) that did not break when the number of repeated stress additions was 1 × 10 ⁷ was evaluated as Ono-type rotary bending fatigue strength (MPa). The results are shown in Table 3.

As shown in Table 3, in Examples 1 to 22, the carbon concentration in the surface region of the flat portion is 0.70% to 0.89%, the nitrogen concentration is 0.20% to 0.70%, carbide and / or Alternatively, the nitride precipitate density was 0.5 pieces / μm ² or more, and the maximum length of the carbide and / or nitride precipitates in the surface layer region of the edge portion was 5.0 μm or less. And as shown in Table 3, in Examples 1-22, the surface of the high flat part which has high 4-point bending fatigue strength of 1000 MPa or more and high Ono-type rotational bending fatigue strength of 500 MPa or more, and HV660 or more It had a hardness of.

On the other hand, since the comparative example 1 has little C content in steel materials, the 4-point bending fatigue strength became low. Since the comparative example 2 had much C content in steel materials, four-point bending fatigue strength became low.
Since the comparative example 3 had much Si content in steel materials, Ono type | formula rotation bending fatigue strength became low.
Since the comparative example 4 had little Mn content in steel materials, 4 point | piece bending fatigue strength and Ono type | formula rotation bending fatigue strength became low.

Since the comparative example 5 had much P content in steel materials, four-point bending fatigue strength became low.
In Comparative Example 6, since the Cr content in the steel material was small, the density of carbide and / or nitride precipitates contained in the surface layer region of the flat portion was insufficient, and the Ono-type rotary bending fatigue strength was low.

In Comparative Example 7, the carbon concentration in the surface region of the flat part is high, and the maximum length of the carbide and / or nitride precipitates in the surface region of the edge part exceeds 5.0 μm, so the four-point bending fatigue strength is low. became.
In Comparative Example 8, since the carbon concentration in the flat surface layer region is low, the precipitate density of carbide and / or nitride contained in the flat surface layer region is insufficient, and the four-point bending fatigue strength, Ono-type rotary bending fatigue strength, The hardness of the surface of the flat part was lowered.

In Comparative Example 9, the cooling rate in the vacuum carbonitriding process was less than 3.0 ° C./second. For this reason, the maximum length of the carbide and / or nitride precipitates in the surface layer region of the edge portion exceeded 5.0 μm, and the 4-point bending fatigue strength was lowered.
In Comparative Example 10, since the ammonia gas flow rate was large and the nitrogen concentration in the surface region of the flat part was high, the Ono type rotary bending fatigue strength was low.
In Comparative Example 11, since the ammonia gas flow rate is small and the nitrogen concentration in the flat surface layer region is low, the density of carbide and / or nitride precipitates contained in the flat surface layer region is insufficient, and the temper softening resistance is low. The hardness of the surface of the flat part was lowered.

100 Carbonitriding parts 2 Sides 3 Flat part 11, 12 CS cross section

Claims (4)

The core is mass%,
C: 0.10 to 0.25%,
Si: 0.03 to 0.49%,
Mn: 0.30 to 1.50%,
P: 0.030% or less,
S: 0.060% or less,
Cr: 0.90 to 3.00%,
Al: 0.100% or less and N: 0.030% or less,
The balance has a chemical composition consisting of Fe and impurities,
The surface has a flat part and an edge part,
The depth of the grain boundary oxide layer in the flat part is 1 μm or less, and the flat part surface layer region from the surface of the flat part to the depth of 0.05 mm has a carbon concentration of 0.70% or more and 0.89% or less. The nitrogen concentration is 0.20% or more and 0.70% or less, and the precipitate density of carbide and / or nitride is 0.5 piece / μm ² or more,
Bending fatigue strength characterized in that the maximum length of the carbide and / or nitride precipitate is 5.0 μm or less in the edge surface layer region from the surface of the edge to a position having a depth of 0.05 mm Excellent carbonitriding parts. The core part is further mass%,
Nb: 0.100% or less,
2. The carbonitrided part according to claim 1, comprising at least one selected from the group consisting of Ti and 0.100% or less. The core part is further mass%,
Mo: 0.50% or less,
Cu: 0.5% or less,
The carbonitriding component according to claim 1 or 2, comprising at least one selected from the group consisting of Ni and 0.50% or less. It is a manufacturing method of the carbonitriding component as described in any one of Claims 1-3,
A step of producing a steel material having the chemical composition according to any one of claims 1 to 3 and having a surface including a vertex portion, an edge portion, and a flat portion,
A step of performing nitriding treatment and quenching treatment after vacuum carburizing treatment of the steel material;
The vacuum carburizing treatment includes a carburizing step of carburizing the steel material at a carburizing temperature of 980 to 1100 ° C, a diffusion step of diffusing carbon that has entered the steel material while maintaining the carburizing temperature, A cooling step of cooling the steel material having a surface temperature after the diffusion step of the carburizing temperature to 500 ° C. at a cooling rate of 3.0 to 100 ° C./sec, and a ratio of carburizing time to diffusion time (carburizing time / Diffusion time) is 0.20 or more and 0.45 or less.

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