The present disclosure relates to a nitriding part rough-shaped material and a nitrided part.
Background technology
[0002]
　Machine parts used in automobiles, ships, industrial machines, etc. may be subjected to nitriding treatment in order to improve fatigue strength. In addition to high fatigue strength, nitrided parts may be required to have correctability for correcting deformation during nitriding. The higher the surface hardness, the better the fatigue strength, and the lower the surface hardness, the better the correctability. Both characteristics are in a trade-off relationship. For example, Patent Document 1 discloses a technique for achieving both fatigue strength and correctability.
[0003]
　Specifically, Patent Document 1 describes fatigue strength and correction by optimizing the steel composition and controlling the hardness distribution of the nitrided layer after nitriding and the hardness of the core portion that is not affected by nitriding. A technique for achieving both sex is disclosed.
　Generally, by subjecting steel to preheat treatment such as quenching and tempering or normalizing before the nitriding treatment, the correctability and fatigue strength after the nitriding treatment are improved. In particular, if the quenching and tempering treatment is performed before nitriding and then the nitriding treatment is performed, the correctability and fatigue strength are improved as compared with the case where the steel as it is hot forged is subjected to the nitriding treatment.
　Patent Document 2 discloses a technique for achieving both fatigue strength and correctability after nitriding by performing quenching and tempering treatment before nitriding. Specifically, in Patent Document 2, by controlling the structure of the steel so that the mixed structure of tempered martensite and bainite is the main component, both fatigue strength and correctability can be achieved at the same time.
[0004]
　　Patent Document 1: Japanese Patent Application Laid-Open No. 2004-162161
　　Patent Document 2: WO2017-056896
Outline of the invention
Problems to be solved by the invention
[0005]
　The technique described in Patent Document 1 described above controls the hardness distribution of the nitrided layer after the nitriding treatment and the hardness of the core portion which is not affected by nitriding by optimizing the steel component. However, since the steel structure has not been optimized, it cannot be said that both fatigue strength and correctability can be achieved at a sufficiently high level.
[0006]
　Further, the technique described in Patent Document 2 achieves both fatigue strength and correctability at a high level. On the other hand, from the viewpoint of manufacturability of machine parts, in addition to the effects described in Patent Document 2, it is more desirable if the rough material before the nitriding treatment has good machinability.
　The nitrided crankshaft, which is a nitrided component described in Patent Document 2, assumes a crankshaft having a small crank journal diameter, and the entire component is mainly a tempered structure, and there is no difference between the surface layer structure and the internal structure. There is room for improvement in machinability in the rough-shaped material before nitriding.
　In particular, a portion having a diameter or width in the range of 60 to 130 mm is a portion to be cut (particularly deep hole drilling), and therefore machinability is required.
[0007]
　Therefore, the subject of the present disclosure is to obtain a nitrided part having excellent fatigue strength and correctability after nitriding treatment as well as machinability (particularly deep hole workability) in a portion having a diameter or width in the range of 60 to 130 mm. It is an object of the present invention to provide a nitriding part rough-shaped material and a nitriding part obtained by nitriding the nitriding part rough-shaped material and having excellent fatigue strength and correctability.
Means to solve problems
[0008]
　The above problem is solved by the following means.
<1> A
rough-shaped nitrided part having a portion having a diameter or width in the range of 60 to 130 mm, in terms of
mass%,
C: 0.35 to 0.45%,
Si: 0.10 to 0.50%. ,
Mn: 1.5 to 2.5%,
P: 0.05% or less,
S: 0.005 to 0.100%,
Cr: 0.15 to 0.60%,
Al: 0.001 to 0. 080%,
N: 0.003 to 0.025%,
Mo: 0 to 0.50%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
Ti: 0 to 0.050%,
A chemical composition containing Nb: 0 to 0.050%,
Ca: 0 to 0.005%,
Bi: 0 to 0.30%, and
V: 0 to 0.05%
, with the
balance being Fe and impurities. Have and
　In the portion where the diameter or width of the rough-formed nitrided part is in the range of 60 to 130 mm, the structure at a depth of 14.5 mm from the surface is the total area ratio of tempered martensite and tempered bainite: 70 to 100%. , Residual austenite: 0 to 5%, balance: ferrite and pearlite, and
　the area of ​​the structure at a depth of 15 mm or more from the surface in the portion where the diameter or width of the rough-formed nitrided part is in the range of 60 to 130 mm.
　Coarse nitride parts , in proportion, total of tempered martensite and tempered bainite: 0-50%, retained austenite: 0-5%, balance: ferrite and pearlite .
<2>　
　Containing one or more of Mo: over 0 to 0.50%, Cu: over 0 to 0.50%, and Ni: over 0 to 0.50% in mass% <1> The nitriding part rough shape material described in.
<3> The
　rough nitriding component according to <1> or <2>, which contains one or two types of Ti: exceeding 0 to 0.050% and Nb: exceeding 0 to 0.050% in mass%. Material.
<4>
　Containing one or more of Ca: over 0 to 0.005%, Bi: over 0 to 0.30%, and V: 0 to 0.05% in mass% <1> The rough-shaped material for nitrided parts according to any one of <3>.
<5>
　Nitrided component according to any one of <1> to <4> Nitrided component made of the rough shape material.
　In the part where the diameter or width of the nitrided part is in the range of 60 to 130 mm, the structure at a depth of 0.5 mm from the surface is the total of tempered martensite and tempered bainite: 70 to 100%, retained austenite: 0-5%, balance: ferrite and pearlite, where
　the diameter or width of the nitrided part is in the range of 60-130 mm, the structure at a depth of 15 mm or more from the surface is tempered martensite and tempered in area ratio. Total bainite: 0 to less than 50%, retained austenite: 0 to 5%, balance: ferrite and pearlite, at a
　depth of 0.05 mm from the surface at sites where the diameter or width of the nitrided part is in the range of 60 to 130 mm. A
　nitrided part having a Bainite hardness of 350-550 HV at the position .
<6>
　A single or a plurality of parts having a diameter or width in the range of 60 to 130 mm of the nitrided part, having an L / D of 8 or more and a depth L of 60 mm or more, which is the ratio of the depth L to the diameter D. It has holes, and
　50% or more of the total length in the depth direction of the holes is the area ratio, the total of tempered martensite and tempered bainite: 0 to less than 50%, retained austenite: 0 to 5%, balance: ferrite. The nitrided component according to <5>, which passes through a portion having a structure which is pearlite.
The invention's effect
[0009]
　According to the present disclosure, a nitrided part capable of obtaining a nitrided part having excellent fatigue strength and correctability after nitriding treatment as well as machinability (particularly deep hole workability) in a portion having a diameter or width in the range of 60 to 130 mm. Rough-shaped material and nitrided parts thereof It is possible to provide a nitrided part having excellent fatigue strength and correctability obtained by nitriding the rough-shaped material.
A brief description of the drawing
[0010]
FIG. 1 is a schematic view showing an Ono-type rotary bending fatigue test piece collected from a round bar produced in an example.
FIG. 2 is a schematic view showing a 4-point bending test piece collected from a round bar produced in an example.
FIG. 3 is a schematic view showing the cross section of a round bar and the positional relationship between the hole and the evaluation portion when the diameter of the round bar is 55 mm or 65 mm in the evaluation of the characteristics of the hole.
FIG. 4 is a schematic view showing a cross section of a round bar and a positional relationship between the hole and the evaluation portion when the diameter of the round bar is 80 mm in the evaluation of the characteristics of the hole.
FIG. 5 is a schematic view showing a cross section of a round bar and a positional relationship between the hole and the evaluation portion when the diameter of the round bar is 100 mm in the evaluation of the characteristics of the hole.
FIG. 6 is a schematic view showing the cross section of a round bar and the positional relationship between the hole and the evaluation portion when the diameter of the round bar is 140 mm in the evaluation of the characteristics of the hole.
FIG. 7 is a schematic view showing an example of a crankshaft (crankshaft).
Mode for carrying out the invention
[0011]
　Hereinafter, embodiments that are an example of the present disclosure will be described in detail.
[0012]
　In addition, in this specification, "%" notation of the content of each element of a chemical composition means "mass%".
　The content of each element in the chemical composition may be referred to as "elemental amount". For example, the content of C may be expressed as the amount of C.
　The numerical range represented by using "-" means a range including the numerical values ​​before and after "-" as the lower limit value and the upper limit value.
　The numerical range when "exceeding" or "less than" is added to the numerical values ​​before and after "to" means a range in which these numerical values ​​are not included as the lower limit value or the upper limit value.
　The term "process" is included in this term not only as an independent process but also as long as the intended purpose of the process is achieved even if it cannot be clearly distinguished from other processes.
[0013]
　The "position at a depth of 14.5 mm from the surface of the rough shape material of the nitrided part" is also referred to as a surface layer portion of the rough shape material of the nitrided part.
　The "position at a depth of 15 mm or more from the surface of the rough-shaped material of the nitrided part or the nitrided part" is also referred to as the inside.
　The "position at a depth of 0.5 mm from the surface of the nitrided component" is also referred to as a surface layer portion of the nitrided component.
[0014]
　The rough-shaped martensite material according to the present embodiment is a rough-shaped martensite material
　having a portion having a diameter or width in the range of 60 to 130 mm, has a
　predetermined chemical composition, and has
　the diameter or the diameter of the rough-shaped material for the nitrided part. In the part with a width of 60 to 130 mm, the structure at a depth of 14.5 mm from the surface is the total area ratio of tempered martensite and tempered bainite: 70 to 100%, retained austenite: 0 to 5%, Remaining: Ferrite and pearlite, in the
　part where the diameter or width of the rough-formed nitrided part is in the range of 60 to 130 mm, the structure at a depth of 15 mm or more from the surface is the area ratio of tempered martensite and tempered bainite. Total: 0 to less than 50%, retained austenite: 0 to 5%, balance: ferrite and pearlite.
[0015]
　Due to the above configuration, the nitriding part rough-shaped material according to the present embodiment has fatigue after nitriding treatment as well as machinability (particularly deep hole workability) in a portion having a diameter (maximum diameter) or a width in the range of 60 to 130 mm. It is a rough-shaped material for nitrided parts that can obtain nitrided parts with excellent strength and correctability. Then, the nitriding component rough-shaped material according to the present embodiment is subjected to nitriding treatment to obtain a nitrided component having excellent fatigue strength and correctability in a portion having a diameter or width in the range of 60 to 130 mm.
　Such a rough-shaped nitrided part material according to the present embodiment was found based on the following findings.
[0016]
　Nitriding parts A portion of the rough shape material having a diameter or width in the range of 60 to 130 mm is provided with a higher level of machinability (particularly deep hole workability) while achieving both fatigue strength and correctability after nitriding. For this purpose, the structure near the surface layer portion that most contributes to fatigue strength and correctability may be used. Further, the internal structure that does not affect the fatigue strength and correctability but affects the machinability during deep hole drilling may be a different structure.
　For example, in the technique described in Patent Document 2, the structure of the nitrided part is mainly tempered martensite and tempered bainite (hereinafter, also referred to as “hardened structure”), and there is no difference between the surface layer structure and the internal structure. .. On the other hand, by using ferrite and pearlite (hereinafter, also referred to as "non-quenched structure"), which have excellent machinability, in the internal structure, it is possible to make a part that is particularly excellent in chip control during deep hole drilling. Is possible.
[0017]
　Therefore, the present inventors have a structure in which the vicinity of the surface layer portion of the nitrided part has excellent fatigue strength and correctability when the quenching and tempering process performed in the normal production process of the nitrided part is performed, and the nitrided part has a structure. We investigated a technology that creates a structure in which the inside has excellent machinability (especially deep hole workability). As a result, the inventors obtained the following findings (a) to (c).
[0018]
(A) If the surface layer portion of the steel has a hardened structure and the internal structure has a non-quenched structure, a nitrided part having excellent fatigue strength, correctability and machinability (particularly deep hole workability) can be obtained. Can be done.
(B) One of the requirements for the surface layer of steel to have a hardened structure and the internal structure to have a non-quenched structure is that the diameter and thickness of the part to be deep-perforated are within a certain range. Is to control.
(C) Another requirement for the surface layer of steel to have a hardened structure and the internal structure to have a non-quenched structure is to control the hardenability of the rough-formed nitrided parts within a certain range. Is.
[0019]
　Next, the inventors examined the conditions for improving the nitriding property and the deep hole workability by using various steels having a difference in structure between the surface layer portion and the inside of the steel. As a result, the inventors obtained the following findings (d) to (e).
(D) Fatigue strength and correctability may not be sufficiently improved only by using the surface layer structure of steel as the main body of the hardened structure. In order to sufficiently increase the fatigue strength and correctability, it is necessary to increase the amount of Mn and suppress the amount of Cr in an appropriate range.
(E) If the internal structure of the steel is mainly a non-quenched structure, the chip controllability is improved, but the cutting resistance may not be reduced due to the formation of coarse cementite. In order to effectively reduce the cutting resistance while the internal structure is mainly a non-quenched structure, it is necessary to reduce the amount of C to a certain amount or less in order to reduce the volume fraction of cementite.
[0020]
　Based on the above findings, the nitriding part rough-shaped material according to the present embodiment has machinability (particularly deep hole workability) in a portion having a diameter or width in the range of 60 to 130 mm, as well as fatigue strength and correction after nitriding treatment. It has been found that it is a rough-shaped material for nitrided parts with excellent properties. Then, it is found that the nitriding component rough-shaped material according to the present embodiment can be subjected to nitriding treatment to obtain a nitrided component having excellent fatigue strength and correctability in a portion having a diameter or width in the range of 60 to 130 mm. It was issued.
　The obtained nitrided member is suitable for use as a machine part of an automobile, an industrial machine, a construction machine, and the like.
[0021]
　Hereinafter, the details of the nitriding part rough-shaped material according to the present embodiment will be described.
[0022]
[Chemical composition]
　The chemical composition of the rough-formed nitrided parts according to the present embodiment contains the following elements. In the description of the chemical composition, the rough-formed nitrided part and the nitrided part are also referred to as "steel material".
[0023]
(Essential element)
　C: 0.35 to 0.45%
　carbon (C) enhances the hardness and fatigue strength of steel materials. If the amount of C is too low, the above effect cannot be obtained. On the other hand, if the amount of C is too high, the cutting resistance of the non-quenched structure increases and the machinability decreases. Therefore, the amount of C is 0.35 to 0.45%. The lower limit of the amount of C is preferably 0.36%, more preferably 0.38%. The upper limit of the amount of C is preferably 0.43%, more preferably 0.42%, further preferably 0.41%, and particularly preferably 0.40%.
[0024]
　Si: 0.10 to 0.50%
　silicon (Si) is dissolved in ferrite to strengthen the steel material (solid solution strengthening). If the amount of Si is too low, the above effect cannot be obtained. On the other hand, if the amount of Si is too high, softening during tempering is excessively suppressed and machinability deteriorates. Therefore, the amount of Si is 0.10 to 0.50%. The lower limit of the amount of Si is preferably 0.13%, more preferably 0.15%, still more preferably 0.27% or more. The upper limit of the amount of Si is preferably 0.45%, more preferably 0.40%, and even more preferably 0.35%.
[0025]
　Mn: 1.5 to 2.5%
　manganese (Mn) enhances the hardenability of the structure and makes the structure of the surface layer a hardenable structure. As a result, the hardness and fatigue strength of the nitrided layer (surface layer portion) of the nitrided component are increased. If the amount of Mn is too low, the above effect cannot be obtained. On the other hand, if the amount of Mn is too high, the hardenability of the steel is excessively increased, so that the inside becomes a hardened structure, and the machinability and correctability deteriorate. Therefore, the amount of Mn is 1.5 to 2.5%. The lower limit of the amount of Mn is preferably 1.60%, more preferably 1.70%, and even more preferably 1.75%. The upper limit of the amount of Mn is preferably 2.4%, more preferably 2.3%, and even more preferably 2.2%.
[0026]
　P: 0.05% or less
　Phosphorus (P) is an impurity. P segregates at the grain boundaries and causes grain boundary embrittlement cracking. Therefore, it is preferable that the amount of P is as low as possible. Therefore, the upper limit of the amount of P is 0.05% or less. The upper limit of the P content is preferably 0.02% or less.
　Note that P is an element that does not have to be contained, and the lower limit of the amount of P is 0%. However, the lower limit of the amount of P may be, for example, more than 0% (preferably 0.003%) from the viewpoint of suppressing an increase in the cost of removing P.
[0027]
　S: 0.005 to 0.100%
　sulfur (S) combines with Mn in the steel material to form MnS and enhances the machinability of the steel material. If the amount of S is too low, the above effect cannot be obtained. On the other hand, if the amount of S is too high, coarse MnS is formed and the fatigue strength of the steel material is lowered. Therefore, the amount of S is 0.005 to 0.100%. The lower limit of the amount of S is preferably 0.010%, more preferably 0.015%, and even more preferably 0.020%. The upper limit of the amount of S is preferably 0.080%, more preferably 0.070%, and even more preferably 0.060%.
[0028]
　Cr: 0.15 to 0.60%
　chromium (Cr) combines with N introduced into the steel material by the nitriding treatment to form CrN in the nitriding layer and strengthens the nitriding layer. If the amount of Cr is too low, the above effect cannot be obtained. On the other hand, if the amount of Cr is too high, the nitrided layer is excessively hardened and the correctability is deteriorated. In addition, the machinability also deteriorates. Therefore, the amount of Cr is 0.15 to 0.60%. The lower limit of the amount of Cr is preferably 0.20%, more preferably 0.25%, and more preferably 0.30%. The upper limit of the amount of Cr is preferably 0.55%, more preferably 0.50%.
[0029]
　Al: 0.001 to 0.080%
　aluminum (Al) is a deoxidizing element for steel. On the other hand, if the amount of Al is too high, fine nitrides are formed, the steel is excessively hardened, and the correctability is deteriorated. Therefore, the amount of Al is 0.001 to 0.080%. The lower limit of the amount of Al is preferably 0.005%, more preferably 0.010%. The upper limit of the amount of Al is preferably 0.060%, more preferably 0.050%, and even more preferably 0.040%.
[0030]
　N: 0.003 to 0.025%
　nitrogen (N) dissolves in the steel material to increase the strength of the steel material. If the amount of N is too low, the above effect cannot be obtained. On the other hand, if the amount of N is too high, bubbles are generated in the steel material. It is preferable that the generation of bubbles is suppressed because the bubbles become defects. Therefore, the amount of N is 0.003 to 0.025%. The lower limit of the amount of N is preferably 0.005. The upper limit of the N content is preferably 0.020%, more preferably 0.018%.
[0031]
　Remaining part: Fe and impurities
　are mixed in from ore, scrap, manufacturing environment, etc. as a raw material when a steel material is industrially manufactured, and are mixed in the rough-formed nitrided parts according to the present embodiment. It means something that is acceptable as long as it does not adversely affect it. Specifically, the following elements are allowed as impurities.
Pb: 0.09% or less
W: 0.1% or less
Co: 0.1% or less
Ta: 0.1% or less
Sb: 0.005% or less
Mg: 0.005% or less
REM: 0.005% or less
[0032]
(Arbitrary element)
　The nitriding component rough-shaped material according to the present embodiment may contain one or more of Mo, Cu and Ni. The group consisting of Mo, Cu and Ni has the effect of increasing the strength of the nitrided parts. The lower limit of the contents of Mo, Cu and Ni is 0%.
[0033]
　Mo: When 0 to 0.50%
　molybdenum (Mo) is contained, the strength of the steel material is increased by increasing the hardenability of the steel. As a result, the fatigue strength of the steel material increases. However, if the amount of Mo is excessively large, the effect is saturated and the cost of the steel material increases. Therefore, the amount of Mo is 0 (or more than 0) to 0.50%. The lower limit of the amount of Mo is preferably 0.03%, more preferably 0.05%. The upper limit of the amount of Mo is preferably 0.40%, more preferably 0.30%, and even more preferably 0.20%.
[0034]
　Cu: 0 to 0.50%
　copper (Cu), when contained, dissolves in ferrite to increase the strength of the steel material. Therefore, the fatigue strength of the steel material is increased. However, if the amount of Cu is excessively large, it segregates at the grain boundaries of the steel during hot forging and induces hot cracking. Therefore, the amount of Cu is 0 (or more than 0) to 0.50%. The lower limit of the amount of Cu is preferably 0.05%, more preferably 0.10%. The upper limit of the amount of Cu is preferably 0.30%, more preferably 0.20%.
[0035]
　Ni: 0 to 0.50%
　nickel (Ni), when contained, dissolves in ferrite to increase the strength of the steel material. Therefore, the fatigue strength of the steel material is increased. Ni further suppresses hot cracking caused by Cu when the steel material contains Cu. However, if the amount of Ni is too large, the effect is saturated and the manufacturing cost is high. Therefore, the amount of Ni is 0 (or more than 0) to 0.50%. The lower limit of the amount of Ni is preferably 0.05%, more preferably 0.10%. The upper limit of the amount of Ni is preferably 0.30%, more preferably 0.20%.
[0036]
　The rough-formed nitrided part material according to the present embodiment may contain one or two types of Ti and Nb. The group consisting of Ti and Nb has an effect of preventing coarsening of austenite crystal grains. The lower limit of the contents of Mo, Ti, and Nb is 0%.
[0037]
　Ti: 0 to 0.050%
　titanium (Ti) combines with N to form TiN, and suppresses coarsening of crystal grains during hot forging and quenching and tempering. However, if the amount of Ti is too high, TiC is generated and the variation in hardness of the steel material becomes large. Therefore, the amount of Ti is 0 (or more than 0) to 0.05%. The lower limit of the amount of Ti is preferably 0.005%, more preferably 0.010%. The upper limit of the amount of Ti is preferably 0.04%, more preferably 0.03%.
[0038]
　Nb: 0 to 0.050%
　niobium (Nb) combines with N to form NbN, which suppresses coarsening of crystal grains during hot forging and quenching and tempering. Nb further delays recrystallization during hot forging and quenching and tempering, and suppresses coarsening of crystal grains. However, if the amount of Nb is too high, NbC is generated and the variation in hardness of the steel material becomes large. Therefore, the amount of Nb is 0 (or more than 0) to 0.050%. The lower limit of Nb is preferably 0.005%, more preferably 0.010%. The upper limit of the amount of Nb is preferably 0.040%, more preferably 0.030%.
[0039]
　The nitriding component rough-shaped material according to the present embodiment may contain one or more of Ca, Bi and V. The lower limit of the contents of Ca, Bi and V is 0%.
[0040]
　Ca: 0 to 0.005%
　calcium (Ca), when contained, enhances the machinability of steel materials. However, if the amount of Ca is too high, coarse Ca oxide is generated and the fatigue strength of the steel material is lowered. Therefore, the amount of Ca is 0 (or more than 0) to 0.005%. The lower limit of the amount of Ca for stably obtaining the above effect is preferably 0.0001%, more preferably 0.0003%. The upper limit of the amount of Ca is preferably 0.003% or less, and more preferably 0.002%.
[0041]
　Bi: 0 to 0.30%
　bismuth (B), when contained, enhances the machinability of the steel material. However, if the amount of Bi is too high, the hot workability deteriorates. Therefore, the amount of Bi is 0 (or more than 0) to 0.30%. The lower limit of the amount of Bi for stably obtaining the above effect is preferably 0.05%, more preferably 0.10%. The upper limit of the amount of Bi is preferably 0.25% or less, and more preferably 0.20%.
[0042]
　　V: 0 to 0.05%
　vanadium (V) is precipitated at the interface between ferrite and austenite during diffusion transformation of steel. Further, since precipitation also proceeds when the steel is tempered and then tempered, the non-quenched structure is hardened and the machinability is deteriorated. Therefore, the amount of V needs to be limited to 0 (or more than 0) to 0.05% or less. The upper limit of the amount of V is preferably 0.03%, more preferably 0.02%.
　It is necessary to reduce the content of V, which is often contained in the practical nitriding part rough material (and the nitriding part). However, from the viewpoint of reducing the manufacturing cost, the lower limit of the amount of V may be more than 0% (or 0.001%).
[0043]
　[Structure of Surface Layer of Rough Shaped Material of Nitride Parts]
　The rough shape of nitrided parts according to the present embodiment is a member obtained by roughly forming a steel material into a nitrided part shape by hot forging and then quenching and tempering. The nitriding component rough-shaped material according to the present embodiment has a diameter or width in the range of 60 to 130 mm in order to improve fatigue characteristics and correctability after the nitriding treatment in a portion having a diameter or width in the range of 60 to 130 mm. The structure of the surface layer affected by nitriding at the site is defined as the quenching and tempering structure. If the structure from the surface of the rough-shaped material of the nitrided part to a depth of 15 mm is controlled, the target structure appears on the surface layer portion after cutting.
[0044]
　Specifically, in a portion where the diameter or width of the rough nitride part is in the range of 60 to 130 mm, the structure at a depth of 14.5 mm from the surface is the sum of tempered martensite and tempered bainite in terms of area ratio. 70 to 100%, retained austenite: 0 to 5%, balance: ferrite and pearlite. Fatigue characteristics and correctability of nitrided parts after nitriding are improved.
[0045]
　The lower limit of the total area ratio of tempered martensite and tempered bainite is preferably 80%, more preferably 85%.
　On the other hand, the upper limit of the total area ratio of tempered martensite and tempered bainite may be as high as 100%.
[0046]
　The area ratio of the retained austenite may be 0%, and if it is 5% or less, it does not affect the fatigue characteristics and correctability of the nitrided parts after the nitriding treatment.
　The lower limit of the area ratio of retained austenite may exceed 0% or be 1%.
　The upper limit of the area ratio of retained austenite is preferably 3%, more preferably 2%.
[0047]
　The total area ratio of the remaining "ferrite and pearlite" may be 0%, and if it is 30% or less, it is preferable because it does not easily affect the fatigue characteristics and correctability of the nitrided parts after the nitriding treatment.
[0048]
[Internal Structure of Rough Shaped Material of Nitride Part]
　The rough shape of the nitrided part according to the present embodiment is in a portion having a diameter or width in the range of 60 to 130 mm. In order to improve the machinability of the nitrided parts after the nitriding treatment, it is necessary to make the majority of the internal structure that is not affected by the nitriding treatment into a non-quenched structure.
[0049]
　Specifically, in a portion where the diameter or width of the rough nitride part is in the range of 60 to 130 mm, the structure at a depth of 15 mm or more from the surface is subjected to the area ratio, and the total of tempered martensite and tempered bainite: 0. ~ 50%, retained austenite: 0-5%, balance: ferrite and pearlite. As a result, the machinability (particularly deep hole workability) of the nitrided part after the nitriding treatment is improved.
[0050]
　The lower limit of the total area ratio of tempered martensite and tempered bainite may be 0%, and if it is less than 50%, it is unlikely to affect the machinability (particularly deep hole workability) of the nitrided part after the nitriding treatment.
　The lower limit of the total area ratio of tempered martensite and tempered bainite may exceed 0% and may be 5% or 10%.
　On the other hand, the upper limit of the total area ratio of tempered martensite and tempered bainite is preferably 40%, more preferably 35%, still more preferably 30%, and particularly preferably 20%.
[0051]
　The area ratio of the retained austenite may be 0%, and if it is 5% or less, it does not affect the machinability (particularly deep hole workability) of the nitrided part after the nitriding treatment.
　The lower limit of the area ratio of retained austenite may exceed 0% or be 1%.
　The upper limit of the area ratio of retained austenite is preferably 3%, more preferably 2%.
[0052]
　The total area ratio of the remaining "ferrite and pearlite" is over 50 to 100%.
　The lower limit of the total area ratio of the remaining "ferrite and pearlite" is preferably 60%, more preferably 65%, still more preferably 70%, and particularly preferably 80%.
　The upper limit of the total area ratio of the remaining "ferrite and pearlite" may be as high as 100%.
[0053]
　The nitriding treatment is performed in a temperature range of A1 point or less of the steel, and the internal structure of the rough-shaped material of the nitrided part is taken over by the internal structure of the nitrided part as it is.
[0054]

　The nitriding component according to the present embodiment is a nitriding component made of the rough-formed nitriding component according to the present embodiment. Specifically, it is a nitrided part that has been subjected to a cutting process to form a predetermined shape on the rough-shaped material of the nitrided part and then subjected to a nitriding treatment.
　The nitrided component according to the present embodiment satisfies the following characteristics (1) to (3).
　(1) In the portion where the diameter or width of the nitrided part is in the range of 60 to 130 mm, the structure at a depth of 0.5 mm from the surface is the total area ratio of tempered martensite and tempered bainite: 70 to 100%. Residual austenite: 0-5%, balance: ferrite and pearlite.
　(2) In the portion where the diameter or width of the nitrided part is in the range of 60 to 130 mm, the structure at a depth of 15 mm or more from the surface is the total area ratio of tempered martensite and tempered bainite: 0 to less than 50%. Residual austenite: 0-5%, balance: ferrite and pearlite.
　(3) The Vickers hardness at a position of 0.05 mm deep from the surface is 350 HV or more and less than 550 HV in a portion where the diameter or width of the nitrided part is in the range of 60 to 130 mm.
[0055]
　As described above, the nitrided component according to the present embodiment is a nitrided component having excellent fatigue strength and correctability as well as machinability (particularly deep hole workability).
[0056]
[Structure of Surface
　Layer of Nitrided Part] In the nitrided part according to the present embodiment, a nitrided layer is formed on the surface layer because the nitriding part rough-shaped material is subjected to nitriding treatment. The thickness of the nitrided layer is, for example, 0.1 to 1.0 mm.
　Then, the nitrided component according to the present embodiment has a nitrided layer in a portion having a diameter or width in the range of 60 to 130 mm in order to improve fatigue characteristics and correctability in the portion having a diameter or width in the range of 60 to 130 mm. The structure is preferably a hardened structure.
　Specifically, in a portion where the diameter or width of the nitrided part is in the range of 60 to 130 mm, the structure at a depth of 0.5 mm from the surface is subjected to the area ratio, and the total of tempered martensite and tempered bainite: 70 to 100. %, Residual austenite: 0-5%, balance: ferrite and pearlite.
[0057]
　The lower limit of the total area ratio of tempered martensite and tempered bainite is preferably 80%, more preferably 85%.
　On the other hand, the upper limit of the total area ratio of tempered martensite and tempered bainite may be as high as 100%.
[0058]
　The area ratio of retained austenite may be 0%, and if it is 5% or less, it does not affect the fatigue characteristics and correctability of nitrided parts.
　The lower limit of the area ratio of retained austenite may exceed 0% or be 1%.
　The upper limit of the area ratio of retained austenite is preferably 3%, more preferably 2%.
[0059]
　The total area ratio of the remaining "ferrite and pearlite" may be 0%, and if it is 30% or less, it is preferable because it does not easily affect the fatigue characteristics and correctability of the nitrided parts.
[0060]
　In the part where the diameter or width of the nitrided part is in the range of 60 to 130 mm, if the area ratio of the structure located at a depth of 0.5 mm from the surface satisfies the above regulation, the part closer to the surface is more baked. The organization naturally meets the above provisions because it is easy to enter.
[0061]
[Internal structure of nitriding component]
　The nitrided component according to the present embodiment has an internal structure that is not affected by the nitriding treatment in order to improve machinability in a portion where the diameter or width of the nitriding component is in the range of 60 to 130 mm. It is necessary that the majority of the tissue is non-quenched tissue.
　Specifically, in the portion where the diameter or width of the nitrided part is in the range of 60 to 130 mm, the area ratio of the structure at a position of 15 mm or more from the surface is the total of tempered martensite and tempered bainite: 0 to less than 50%. , Residual austenite: 0-5%, balance: ferrite and pearlite. As a result, machinability (particularly deep hole workability) is improved in a portion where the diameter or width of the nitrided part is in the range of 60 to 130 mm.
[0062]
　The lower limit of the total area ratio of tempered martensite and tempered bainite may be 0%, and if it is less than 50%, it is unlikely to affect the machinability (particularly deep hole workability) of nitrided parts.
　The lower limit of the total area ratio of tempered martensite and tempered bainite may exceed 0% and may be 5% or 10%.
　On the other hand, the upper limit of the total area ratio of tempered martensite and tempered bainite is preferably 40%, more preferably 35%, still more preferably 30%, and particularly preferably 20%.
[0063]
　The area ratio of the retained austenite may be 0%, and if it is 5% or less, it does not affect the machinability (particularly deep hole workability) of the nitrided part.
　The lower limit of the area ratio of retained austenite may exceed 0% or be 1%.
　The upper limit of the area ratio of retained austenite is preferably 3%, more preferably 2%.
[0064]
　The total area ratio of the remaining "ferrite and pearlite" is over 50 to 100%.
　The lower limit of the total area ratio of the remaining "ferrite and pearlite" is preferably 60%, more preferably 65%, still more preferably 70%, and particularly preferably 80%.
　The upper limit of the total area ratio of the remaining "ferrite and pearlite" may be as high as 100%.
[0065]
[Vickers hardness of the surface layer portion of the nitrided component]
　The nitrided component according to the present embodiment has a surface layer portion of the nitrided component in order to improve fatigue characteristics and correctability in a portion having a diameter or width in the range of 60 to 130 mm. Vickers hardness needs to be appropriate. If the hardness near the surface is low, a sufficiently high fatigue strength cannot be obtained. On the other hand, if the hardness near the surface is too high, the correctability deteriorates. Therefore, the Vickers hardness of the surface layer portion of the nitrided component is set to 350 to 550 HV.
　Specifically, the Vickers hardness at a position where the diameter or width of the nitrided part is in the range of 60 to 130 mm and the depth is 0.05 mm from the surface is 350 to 550 HV.
　The lower limit of the Vickers hardness of the surface layer portion of the nitrided component is preferably 370 HV, more preferably 380 HV.
　The upper limit of the Vickers hardness of the surface layer portion of the nitrided component is preferably 520 HV, more preferably 500 HV.
[0066]
[Through Hole of Nitrided Part]
　The nitrided member according to the present embodiment may have one or more holes in a portion where the diameter or width of the nitrided part is in the range of 60 to 130 mm. The holes are provided, for example, by drilling.
　The hole is, for example, a through hole having an L / D ratio of a depth L to a diameter D of 8 or more (preferably 8 to 50) and a depth L of 60 mm or more (preferably 60 to 250 mm).
　Drilling a hole of this shape is difficult to cut, and the structure of the part to be drilled is a ferrite and pearlite structure with relatively little tempered martensite and tempered bainite, which are inferior in machinability, and excellent machinability. It is advantageous that there are many.
　Therefore, 50% or more (preferably 60%, more preferably 70%) of the total length of the holes in this shape in the depth direction is the total area ratio of tempered martensite and tempered bainite: 0 to less than 50%. Residual austenite: 0-5%, balance: ferrite and pearlite. It is preferable to pass through a site having a structure.
　That is, for example, among the structures of the portion through which the drill penetrates, the structure of 50% or more of the total length in the depth direction of the hole may be a structure mainly composed of ferrite and pearlite.
　The preferred mode of the structure mainly composed of ferrite and pearlite is the same as the preferred mode of the structure at a depth of 15 mm or more from the surface of the nitrided component.
[0067]
　Here, the structure of the hole is evaluated by the structure around the hole. Specifically, it is evaluated by the following method.
　First, the depth of the hole is divided into ten equal parts in the depth direction, and ten regions are defined. In each region, the hole is longitudinally traversed along the depth direction, and a field of view taken at a random position within 200 μm in depth from the surface (wall surface) of the hole on the vertical cross section is defined as the field of view to be inspected.
　From one or more test visual fields , a field of view is selected so that the test area for each area is 0.2 mm 2 or more, and a photograph is taken at an appropriate magnification at which the tissue can be observed. Obtain the area ratio of the tissue for each area from the photograph taken. The length of the hole that satisfies the regulation of the area ratio of the structure (the length in the depth direction of the hole) is the region of each region of the surface (wall surface) of the hole that satisfies the regulation of the area ratio of the tissue described above. Is multiplied by 1/10 of the length of the hole. Such an evaluation is performed for all holes, and the ratio of the sum of the lengths satisfying the regulation of the area ratio of the structure to the total length in the depth direction of the holes is obtained.
[0068]
　It should be noted that the nitrided component has a plurality of through holes, and the holes and the portion having the holes have a symmetrical shape, or the portion composed of repeating the same shape has holes having the same shape. For multiple holes that can be reasonably estimated to have the same tissue around the hole, such as when, the tissue around the hole is evaluated for only one of them, and the others. The area ratio of the tissue around the hole may be considered to be the same as the evaluation result.
[0069]
[Area ratio of structure and Vickers hardness]
　The area ratio of structure and Vickers hardness of the rough-formed nitrided parts and the nitrided parts according to the present embodiment are measured according to the methods described in Examples described later. Will be done.
[0070]
[Manufacturing Method]
　Hereinafter, an example of a method for manufacturing a rough-formed nitrided part and a nitrided part according to the present embodiment will be described.
[0071]
　The method for manufacturing a nitriding part according to the present embodiment includes a steel material preparation step, a molding step, a quenching tempering step, a cutting process, and a nitriding process. The nitriding part rough shape material according to the present embodiment includes a steel material preparation step, a molding step, and a quenching and tempering step.
　Each process will be described below.
[0072]
[Steel Material Preparation Step]
　A molten steel satisfying the chemical composition of the steel of the nitriding part rough-shaped material according to the present embodiment is manufactured. Using the manufactured molten steel, slabs (slabs, blooms) are made by a general continuous casting method. Alternatively, molten steel is used to make an ingot by the ingot formation method. Billets are manufactured by hot working slabs or ingots. The hot working may be hot rolling or hot forging. Further, the billet is heated, rolled, and cooled under general conditions to produce a steel bar, which is used as a material for nitrided parts.
[0073]
　[Molding Step] The
　manufactured steel bar is hot-forged to be molded into a rough-shaped nitrided part having a portion having a diameter or width in the range of 60 to 130 mm. If the heating temperature of hot forging is too low, the forging device will be overloaded. On the other hand, if the heating temperature is too high, the scale loss is large. Therefore, the preferred heating temperature is 1000 to 1300 ° C.
[0074]
　The preferred finishing temperature for hot forging is 900 ° C. or higher. This is because if the finishing temperature is too low, the burden on the mold will increase. On the other hand, the preferred upper limit of the finishing temperature is 1250 ° C.
[0075]
[Quenching and tempering treatment] A
　quenching and tempering treatment is performed on the rough-formed nitrided parts after hot forging. At this time, the quenching temperature is at least the A3 point represented by the equation (1) and at least 1000 ° C. The tempering temperature is 570 ° C. or higher and is A1 point or lower represented by the equation (2). The tempering time is preferably 30 minutes or more.
　A3 = 910-203C + 44.7Si-30Mn-11Cr (1)
　A1 = 723-10.7Mn + 29.1Si-16.9Ni + 16.9Cr (2
　) In the formulas (1) and (2), the element symbols are each. Indicates the element content (mass%).
[0076]
　Since the structure immediately before quenching is austenite single phase, the quenching temperature must be A3 point or higher. If the quenching temperature is too high, the hardenability is enhanced, and the inside may be hardened to deteriorate the machinability. Therefore, the quenching temperature is preferably 950 ° C. or lower. The quenching temperature is more preferably 920 ° C. or lower, further preferably 900 ° C. or lower.
　By quenching, the surface structure of the crude material becomes mainly martensite and bainite. When such a structure is nitrided as it is, precipitation of alloy nitride is promoted, the surface layer portion is excessively hardened, and the correctability is deteriorated. In order to suppress the precipitation of alloy nitrides in martensite and bainite by the tempering treatment, it is preferable to temper at a temperature of 570 ° C. or higher. The tempering temperature is more preferably 590 ° C. or higher, and even more preferably 600 ° C. or higher. On the other hand, in order to suppress the reverse transformation during tempering, the tempering temperature needs to be A1 point or less.
[0077]
　Through the above steps, the rough-formed nitrided parts according to the present embodiment can be obtained.
[0078]
[Cutting process] The
　obtained rough shape of the nitrided part is cut to obtain a predetermined shape of the nitrided part.
[0079]
　[Nitriding process]
　Nitriding process is performed on the machined nitriding part. In this embodiment, a well-known nitriding treatment is adopted. The nitriding treatment is, for example, gas nitriding, salt bath nitriding, ion nitriding and the like. Gas introduced into the furnace during nitridation, NH 3 may be only, NH 3 and N 2 and / or H 2 may be a mixture containing a. Further, the carburizing gas may be contained in these gases to carry out the soft nitriding treatment. Therefore, the term "nitriding" as used herein also includes "soft nitriding".
[0080]
　When performing gas nitrocarburizing treatment, for example, in an atmosphere in which an endothermic modified gas (RX gas) and ammonia gas are mixed at a ratio of 1: 1, the soaking temperature is set to 550 to 630 ° C. and the soaking heat is equalized for 1 to 3 hours. do it.
[0081]
　The nitrided parts manufactured by the above manufacturing process have excellent properties of fatigue strength and correctability as well as machinability (particularly deep hole workability).
[0082]
[Applications of
　Nitriding Parts ] Nitriding parts are suitable for parts such as crankshafts, various machine sliding parts (camshafts, bearings, etc.), dies for molding steel products (press forming dies, pipe making plugs, etc.). Applicable.
　When the nitrided component is a crankshaft, specifically, from the viewpoint of obtaining the surface layer portion and the internal structure, the crank journal diameter (maximum diameter) is 60 to 130 mm (preferably 60 to 120 mm, more preferably 65 to 100 mm). (See FIG. 7) is preferred.
　If the crankshaft diameter is too small, both the surface layer and the inside tend to have a tempered structure (mainly tempered martensite and tempered bainite), and there is no difference between the surface layer and the inside. On the other hand, if the crankshaft has an excessively large crank journal diameter, both the surface layer portion and the inside have a structure mainly composed of ferrite and pearlite, and the structure tends to have no difference between the surface layer portion and the inside.
　Therefore, the nitrided component is preferably a crankshaft having the crank journal diameter (maximum diameter) of 60 to 130 mm (preferably 60 to 120 mm, more preferably 65 to 100 mm).
　Similarly, as the nitriding component rough shape material, a crankshaft rough shape material having a diameter (maximum diameter) of 60 to 130 mm (preferably 60 to 120 mm, more preferably 65 to 100 mm) of a portion corresponding to the crank journal is preferable.
　Here, in FIG. 7, 10 is a crankshaft (crankshaft), 12 is a crank journal, 14 is a crank pin, 16 is a crank arm, and 18 is a balance weight.
　In the crankshaft, the crank journal corresponds to as an example of "a portion having a diameter or width in the range of 60 to 130 mm".
Example
[0083]
　Hereinafter, the present disclosure will be described in more detail with reference to examples. However, each of these examples does not limit this disclosure.
[0084]
　First, a 300 kg ingot of steels C, E and H having the chemical compositions shown in Table 1 and a 50 kg ingot of A, B, D, F, G and I to U were produced using a vacuum melting furnace.
[0085]
[table 1]

[0086]
　In the "A1" and "A3" columns in Table 1, the A1 point (° C.) defined by the formula (1) and the A3 point (° C.) defined by the formula (2) are described, respectively.
[0087]
　The ingot of each mark was heated to 1250 ° C. The heated ingot was hot forged to produce a steel bar having a diameter φ shown in Table 2. Using steel bars as a material, heat treatment was performed to simulate the production of rough-shaped nitrided parts. First, heating at 1200 ° C. and air cooling were performed to reproduce the hot forging process. Subsequently, the air-cooled round bar is heat-treated (quenched) under the conditions described in the heat treatment column of the first stage in Table 2, cooled to 150 ° C. or lower, and then the second stage in Table 2 is used. The heat treatment (tempering treatment) was performed under the conditions described in the heat treatment column.
　Through the above steps, a round bar as a rough-shaped material for nitrided parts was produced.
[0088]
The
　following tests were carried out using the round bars of each test number.
[0089]
[Measurement of tissue area ratio and Vickers hardness]
　A sample whose surface is the cross section of the round bar (cross section cut in the direction orthogonal to the longitudinal direction of the round bar) after the two-step heat treatment of test numbers 1 to 30. Was collected. The Vickers hardness (HV) based on JIS Z 2244 (2009) was measured at any 7 points of the collected sample at a depth (surface layer portion) of 14.5 mm from the surface (outer peripheral surface) of the round bar. .. The test force was 9.8 N. The average value of the obtained seven Vickers hardnesses was defined as the Vickers hardness of the surface layer portion.
[0090]
　The sample after measuring the Vickers hardness of the surface layer was corroded with nital containing 3% by mass of nitric acid to reveal a structure. Then, seven optical microscope photographs at a magnification of 200 times were taken centering on the position where the hardness was measured (surface layer portion), and the area ratios of tempered martensite, tempered bainite, ferrite and pearlite were obtained from image analysis.
[0091]
　For the same sample, the volume fraction of retained austenite was measured using an XRD (X-ray diffractometer). Irradiate X-rays with a spot size of φ1.0 mm centered at a depth of 14.5 mm from the surface (outer peripheral surface) of the round bar, and use the volume fraction of the obtained retained austenite as the volume fraction of the retained austenite on the surface layer. Was defined as.
[0092]
　Residual austenite is contained in tempered martensite and tempered bainite. Therefore, the value obtained by subtracting the area ratio of retained austenite measured by XRD from the total area ratio of tempered martensite and tempered bainite measured from the optical micrograph is the true total area ratio of tempered martensite and tempered bainite. And said.
[0093]
　In the same method, the Vickers hardness and the area ratio of the structure at a depth of 15 mm or more (inside) from the surface (outer peripheral surface) of the round bar were also measured. Specifically, the measurement was performed as follows.
[0094]
　When the radius of the round bar is R (mm), the depth is 15 (mm), the depth is 15+ (R-15) / 4 × 1 (mm), and the depth is 15+ from the surface (outer peripheral surface) of the round bar. Vickers hardness (HV) at 3 points each near 5 positions of (R-15) / 4 × 2 (mm), 15+ (R-15) / 4 × 3 (mm) and depth R (mm) ) Was measured. The test force was 9.8 N. The average value of the obtained 15 points of Vickers hardness was defined as the internal hardness.
[0095]
　The sample after measuring the Vickers hardness inside was corroded with nital containing 3% by mass of nitric acid to reveal the structure. Then, a photomicrograph at a magnification of 200 times was taken centering on the position where the hardness was measured, and the area ratios of tempered martensite, tempered bainite, ferrite and pearlite at each depth position were obtained from image analysis.
[0096]
　Furthermore, the volume fraction of retained austenite was measured using XRD with respect to the sample whose Vickers hardness was measured. X-rays having a spot size of φ1.0 mm were irradiated around the position where the hardness was measured, and the volume fraction of the obtained retained austenite was defined as the area ratio of the retained austenite inside.
　The value obtained by subtracting the area fraction of retained austenite measured by XRD from the total area ratio of tempered martensite and tempered bainite measured from the optical micrograph was taken as the total area ratio of tempered martensite and tempered bainite.
　Then, the total area ratio of the tempered martensite and the tempered bainite and the average value of the area ratio of the retained austenite of the obtained 15 points were defined as the internal hardness.
[0097]
[Preparation of Ono-type rotary bending fatigue test piece and 4-point bending test piece] From
　the round bar of each test number, a plurality of Ono-type rotary bending fatigue test pieces shown in FIG. 1 were collected. The length L1 in the figure was 80 mm, and the diameter D1 was φ12 mm. The radius of curvature R1 of the notch at the center of the test piece was 3 mm, and the diameter R3 of the cross section of the test piece at the bottom of the notch was φ8 mm. At this time, the center of the Ono-type rotary bending fatigue test piece was set to a depth of 10 mm from the surface of the round bar. That is, the notched bottom of the Ono-type rotary bending fatigue test piece corresponds to a depth of 6 to 14 mm from the surface of the round bar.
[0098]
　Further, a 4-point bending test piece shown in FIG. 2 was collected from the round bar of each test number. The length L2 of the 4-point bending test piece was 180 mm, and the diameter D2 was φ12 mm. The radius of curvature R2 of the notch at the center of the test piece was 3 mm, and the diameter R4 of the cross section of the test piece at the bottom of the notch was φ8 mm. At this time, the center of the 4-point bending test piece was set to be 10 mm deep from the surface of the round bar. That is, the notch bottom of the 4-point bending test piece corresponds to a depth of 6 to 14 mm from the surface of the round bar.
[0099]
　The collected Ono-type rotary bending fatigue test piece and 4-point bending test piece were subjected to soft nitriding treatment at 580 ° C. × 2.5 h. As the processing gas, ammonia gas and RX gas were introduced into the furnace at a flow rate of 1: 1. Then, after 2.5 hours, the test piece was taken out from the heat treatment furnace and rapidly cooled with oil at 100 ° C.
[0100]
　Through the above steps, an Ono-type rotary bending fatigue test piece and a 4-point bending test piece as nitrided parts were produced.
[0101]
[Measurement of area ratio of nitrided layer (surface layer part) and internal structure]
　Using a part of the Ono type rotary bending fatigue test piece after nitriding of each test number, the nitrided layer (surface layer part) of the fatigue test piece. The area ratio of the nearby tissue was calculated. A sample for observing the structure was prepared so that the cross section of the notch bottom of the fatigue test piece could be observed, and after corroding with nital to reveal the structure, the sample was used for observing the structure. When an arbitrary point on the surface of the circle in the cross section is set as 0 °, the position at a depth of 0.5 mm from the surface is set at four positions of 0 °, 90 °, 180 °, and 270 °. The area ratio of the centered structure was measured in the same manner as described above. The average value of the area ratios of the four structures was defined as the area ratio of the structure of the nitrided layer.
[0102]
　On the other hand, since the area ratio of the internal structure of the fatigue test piece is not affected by the nitriding treatment and is the same as the area ratio of the internal structure of the round bar as the nitriding member rough shape material, the measurement is omitted.
[0103]
[Measurement of Vickers hardness of the nitrided layer (surface layer portion)]
　The Vickers hardness of the surface layer portion of the nitrided layer was determined using the test piece used for measuring the area ratio of the structure of each nitrided layer. Specifically, the Vickers hardness (HV) based on JIS Z 2244 (2009) was measured at any five points near a position at a depth of 0.05 mm from the surface. The test force was 2.9 N. The average value of the obtained five Vickers hardnesses was defined as the Vickers hardness of the nitrided layer (surface layer portion).
[0104]
[Ono-type rotary bending fatigue test (fatigue strength (MPa))] The
　Ono-type rotary bending fatigue test was carried out using the above-mentioned nitriding-treated Ono-type rotary bending fatigue test piece. A rotary bending fatigue test according to JIS Z2274 (1978) was carried out in an air atmosphere at room temperature (25 ° C.). The test was carried out under double swing conditions at a rotation speed of 3000 rpm. The highest stress among the test pieces that did not break up to 1.0 × 10 7 repetitions was defined as the fatigue strength (MPa) of the test number. When the fatigue strength was 550 MPa or more, it was judged that the fatigue strength was excellent.
[0105]
　[4-Point Bending Test (Bending Straightness (Correctable Strain Amount (με)))]
　A 4-point bending test was carried out at room temperature and in the air using the above-mentioned nitriding-treated 4-point bending test piece. The distance between the fulcrums (the axial distance of the test piece between the fulcrum closest to the end of the test piece and the fulcrum closest to the fulcrum) was set to 51 mm. The pushing speed was 0.5 mm / min. In order to measure the amount of strain on the notch bottom of the test piece, a strain gauge was attached to the center of the notch bottom in parallel with the axial direction of the test piece. When the pushing stroke is increased at the above pushing speed and the increment of the strain gauge value when the pushing stroke increases by 0.01 mm becomes 2400 με or more, the strain amount immediately before the crack is assumed to occur. It was defined as the correctable strain amount (με). When the amount of strain that can be corrected was 15,000 με or more, it was evaluated that the bending correctability was excellent.
[0106]
[Drill life evaluation test]
　The round bar after quenching and tempering of each test number was cut to a length of 100 mm. Of the round bars cut, those with a diameter larger than 65 mm are surfaced by cutting and removing one side surface and the side surface opposite to that side surface by 10 mm in width (length in the radial direction of the round bar). A test piece having two surfaces perpendicular to the bottom surface of the rod and parallel to each other and having a cross-sectional height (length between the two parallel surfaces) having a barrel shape of 60 mm, 80 mm, or 120 mm was prepared (Fig.). 3 to 6).
　Among the cut round bars having a diameter of 55 mm or 65 mm, a test piece having a cross-sectional height of 45 mm or 55 mm was prepared with the width to be cut and removed set to 5 mm (see FIGS. 3 to 6).
　Then, the machinability was evaluated on the surface of the test piece that had been surfaced.
　The drill used was a high-speed steel φ5 mm drill, and the feed during cutting was 0.15 mm / rev and the rotation speed was 1000 rpm. Further, at the time of cutting, the water-soluble emulsion was lubricated at 10 L / min by external lubrication. Under this condition, a hole having a depth of 50 mm was drilled in the test piece having a cross-sectional height of 60 mm or more, and the number of holes until the hole could not be drilled was defined as the number of holes that could be drilled. A hole with a depth of 40 mm is drilled in a test piece with a cross-sectional height of 55 mm or less, and the value obtained by multiplying the number of holes until it becomes impossible to drill by 0.8 is rounded off to the nearest whole number. bottom. The total number of holes was 216 holes. If the drill is broken, makes an abnormal noise, or the current value rises (more than twice the average value of the second hole), it is judged that drilling is not possible.
[0107]
[Evaluation of hole characteristics (drill penetration structure)] The
　structure through which the hole passes was determined as follows. In the following, in terms of area ratio, the total of tempered martensite and tempered bainite: 0 to less than 50%, retained austenite: 0 to 5%, and the balance: ferrite and pearlite are described as non-quenched structures. In order for 50% of the total length in the depth direction of the holes to pass through the non-quenched structure, when the total length of the holes is 40 mm, 20 mm of the total length may pass through the non-quenched structure. The non-quenched structure increases as the distance from the center of the round bar increases.
　Therefore, if the non-quenched structure is located 10 mm away from the center of the round bar, 50% or more of the total length of the holes passes through the non-quenched structure. That is, if the diameter of the round bar is 55 mm, the structure at 17.5 mm from the surface is evaluated, and if the diameter of the round bar is 65 mm, the structure at 22.5 mm from the surface is evaluated. FIG. 3 shows the positional relationship between the cross section of the test piece, the hole, and the evaluation portion (that is, the structure determination position) when the diameter of the round bar is 55 mm or 65 mm.
　Similarly, when the total length of the holes is 50 mm, 25 mm of the holes may pass through the non-quenched structure. Therefore, if the diameter of the round bar is 80 mm, it is evaluated whether the structure is 27.5 mm from the surface, and if the diameter of the round bar is 100 mm or 140 mm, the structure at 35 mm is a non-quenched structure. FIGS. 4 to 6 show the positional relationship between the cross section of the test piece, the hole, and the evaluation portion (that is, the structure determination position) when the diameter of the round bar is 80 mm, 100 mm, or 140 mm, respectively.
　For each test number, the structure at the above positions was analyzed to determine if 50% or more of the total length in the depth direction of the holes passed through the non-quenched structure. Then, when it passed, it was evaluated as Y, and when it did not pass, it was evaluated as N.
　In FIG. 3, D indicates the diameter of the round bar (55 mm or 65 mm). In FIGS. 3 to 6, H indicates a hole, SJP indicates a structure determination position, and NQS indicates a region that can be regarded as a non-quenched structure when the structure determination position is a non-quenched structure.
[0108]
　The test results are shown in Tables 2 to 3 below. The "structure fraction" in Table 3 means the fraction of each structure constituting the steel. "Fatigue strength" means the fatigue strength (MPa) obtained in the Ono type rotary bending test, "strain amount" means the correctable strain amount (με), and "drill drilling number" means the drill life evaluation test. The number of holes obtained is used.
　The abbreviations in Tables 2 to 3 are as follows.
・ Φ: Round bar diameter (mm)
・ TMA + TBA + Residual γ: Total area ratio of tempered martensite, tempered bainite and retained austenite (%)
・ α + PA: Total area ratio of ferrite and pearlite (%)
・ Residual γ: Area ratio of retained austenite (%)
・ Hardness: Vickers hardness (Hv)
・ Nitriding layer hardness : Vickers hardness (Hv) of the nitrided layer (surface layer) of nitrided parts
[0109]
[Table 2]

[0110]
[Table 3]

[0111]
　[Test Results] With
　reference to Table 3, in Test Nos. 1 to 17, the chemical composition and the fine structure of steel are within the scope of the present disclosure. It can be seen that those with these test numbers have a fatigue strength of 550 MPa or more, a correctable strain amount of 16558 με or more, and a drill drilling number of 160 holes or more, and have both fatigue strength, correctability, and machinability.
[0112]
　On the other hand, in the case of "Comparative Examples" of test numbers 18 to 30 which deviate from the provisions of the present disclosure, the chemical composition and the structure of the steel are outside the scope of the present disclosure, and the target performance is obtained. No. Specifically, it is as follows.
　In test number 18, the amount of C was excessive, the number of drill holes was small, and the machinability deteriorated.
　In test number 19, the amount of V was excessive, and the bend correctability was deteriorated.
　In test number 20, the amount of C was low, and the fatigue strength deteriorated.
　Test No. 21 is an example in which Mn is small, and both the surface layer portion and the internal structure of the rough material and the nitrided portion have a non-quenched structure (structure mainly composed of ferrite and pearlite), and the hardness and fatigue of the nitrided layer. The strength has deteriorated.
　In test number 22, the amount of Mn was excessive, the correctability was deteriorated, the number of drill holes was small, and the machinability was deteriorated.
　In test number 23, the amount of Cr was excessive, the correctability was deteriorated, the number of drill holes was small, and the machinability was deteriorated.
　Test No. 24 is an example in which Mn is small, and both the surface layer portion and the internal structure of the rough material and the nitrided portion have a non-quenched structure (structure mainly composed of ferrite and pearlite), and the hardness and fatigue of the nitrided layer. The strength has deteriorated.
[0113]
　In test numbers 25, 27 and 29, the diameter of the test piece (round bar) is small, and both the surface layer portion and the internal structure of the rough material and the nitrided portion have a quenching structure (tempering martensite and tempered bainite-based structure). ), The number of drill holes was small, and the machinability deteriorated.
　In test numbers 26, 28 and 30, the diameter of the test piece (round bar) is large, and both the surface layer portion and the internal structure of the rough material and the nitrided portion have a non-quenched structure (structure mainly composed of ferrite and pearlite). , Fatigue strength has deteriorated.
[0114]
　The embodiments of the present disclosure have been described above. However, the embodiments described above are merely examples for carrying out the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented within a range that does not deviate from the gist thereof.
[0115]
　The entire disclosure of Japanese Patent Application No. 2018-202914 is incorporated herein by reference in its entirety.
　All documents, patent applications, and technical standards described herein are to the same extent as if the individual documents, patent applications, and technical standards were specifically and individually stated to be incorporated by reference. Incorporated herein by reference.
The scope of the claims
[Claim 1]
A rough-shaped nitrided part having a portion having a diameter or width in the range of 60 to 130 mm, in terms of
mass%,
C: 0.35 to 0.45%,
Si: 0.10 to 0.50%,
Mn: 1.5 to 2.5%,
P: 0.05% or less,
S: 0.005 to 0.100%,
Cr: 0.15 to 0.60%,
Al: 0.001 to 0.080%,
N: 0.003 to 0.025%,
Mo: 0 to 0.50%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
Ti: 0 to 0.050%,
Nb: 0 It contains ~ 0.050%,
Ca: 0 to 0.005%,
Bi: 0 to 0.30%, and
V: 0 to 0.05%
,
and has a chemical composition in which the balance is Fe and impurities.
　In the portion where the diameter or width of the nitriding part rough material is in the range of 60 to 130 mm, the structure at a depth of 14.5 mm from the surface is the total area ratio of tempered martensite and tempered bainite: 70 to 100%. , Residual austenite: 0-5%, balance: ferrite and pearlite,
　In the portion where the diameter or width of the rough-formed nitrided part is in the range of 60 to 130 mm, the structure at a depth of 15 mm or more from the surface is the total area ratio of tempered martensite and tempered bainite: 0 to less than 50%. , Residual austenite: 0-5%, balance: ferrite and pearlite,
　nitriding part crude material.
[Claim 2]
　The first aspect of claim 1, wherein one or more of Mo: over 0 to 0.50%, Cu: over 0 to 0.50%, and Ni: over 0 to 0.50% in mass% are contained. Rough shape material for nitrided parts.
[Claim 3]
　The rough-formed nitrided part according to claim 1 or 2, which contains one or two kinds of Ti: more than 0 to 0.050% and Nb: more than 0 to 0.050% in mass%.
[Claim 4]
　Claims 1 to 3 containing one or more of Ca: more than 0 to 0.005%, Bi: more than 0 to 0.30%, and V: 0 to 0.05% in mass%. The rough-shaped material for nitrided parts according to any one of the above items.
[Claim 5]
　A nitrided part made of the rough-shaped material of the nitrided part according to any one of claims 1 to 4, wherein
　the diameter or width of the nitrided part is in the range of 60 to 130 mm, and the depth is from the surface. The structure at the position of 0.5 mm is the total of tempered martensite and tempered bainite: 70 to 100%, retained austenite: 0 to 5%, balance: ferrite and pearlite, and
　the diameter or width of the nitrided part is In the region of 60 to 130 mm, the structure at a depth of 15 mm or more from the surface is the total of tempered martensite and tempered bainite: 0 to less than 50%, residual austenite: 0 to 5%, balance: A
　nitrided component which is ferrite and pearlite and has a bainite hardness of 350 to 550 HV at a depth of 0.05 mm from the surface in a portion where the diameter or width of the nitrided component is in the range of 60 to 130 mm
　.
[Claim 6]
　A single or multiple holes having an L / D of 8 or more and a depth L of 60 mm or more, which is the ratio of the depth L to the diameter D, are provided in a portion where the diameter or width of the nitrided part is in the range of 60 to 130 mm. and,
　more than 50% of the total length of the depth direction of the hole, an area ratio, the total of tempered martensite and tempered bainite: 0 to less than 50%, residual austenite: 0-5%, balance: ferrite and pearlite The nitrided component according to claim 5, which passes through a portion having a certain structure.