JP2024095320A - Processed material for nitrided component and nitrided component - Google Patents

Abstract

To provide a processed material for nitrided component which can be manufactured by omitting or simplifying quenching and tempering, and has excellent machinability, whose bending in nitriding is suppressed, and by which a nitrided component having excellent bending fatigue properties is obtained, and to provide the nitrided component.SOLUTION: A processed material for nitrided component includes a shaft part having a diameter of 10 mm or more, has a chemical composition, by mass%, of C: 0.35 to 0.60%, Si: 0.03 to 0.35%, Mn: 1.00 to 2.10%, P: 0.050% or less, S: 0.010 to 0.095%, Cr: more than 0.60 to 1.20%, Al:0.001 to 0.080%, and N: 0.0040 to 0.0250%, and the balance consisting of Fe and impurities, and has a fine structure made of ferrite and pearlite, or pearlite at the depth 5 mm position from the surface of the shaft part, where a ratio of a solid-solution Cr amount for the total Cr amount at the depth 5 mm position from the surface of shaft part is 0.10 to 0.75.SELECTED DRAWING: None

Description

This disclosure relates to base materials for nitrided parts and nitrided parts.

Some mechanical structural parts used in automobiles, ships, industrial machines, etc. are repeatedly subjected to high bending stress. In order to provide these parts with the necessary fatigue strength, various surface hardening heat treatments may be performed. In the case of parts that require high bending fatigue strength, sliding properties, corrosion resistance, and small distortion, nitriding treatment may be used as a surface hardening heat treatment.
An example of a method for manufacturing a nitrided part is as follows. First, a steel material is hot forged to produce a steel preform. The produced steel preform is heat treated as necessary to optimize the microstructure. It is then machined (cutting, etc.) to form a shape close to the final product. The machined steel preform is then subjected to a nitriding treatment to increase the strength of the surface layer. After nitriding, finishing processes such as polishing are performed, or bending is straightened. Through these steps, a nitrided part is manufactured.

The compound layer (part of the nitride layer) that is formed on the surface by nitriding improves the sliding characteristics of the parts by reducing the coefficient of friction and suppressing wear. The compound layer also has the effect of increasing the corrosion resistance of the parts, and nitriding is sometimes performed on parts for the purpose of increasing their corrosion resistance. In this specification, mechanical structural parts (such as crankshafts) that have been subjected to nitriding are referred to as nitrided parts.

Non-tempered nitrided parts are desired. From the viewpoint of manufacturing costs and reducing _CO2 emissions during manufacturing, it is desirable to omit or simplify the quenching and tempering processes.

A technology has been disclosed that relates to nitrided parts in which quenching and tempering is omitted or simplified.

For example, Patent Document 1 describes how high bending fatigue strength and bending straightening properties can be obtained by optimizing the steel composition and specifying the hardness near the surface after nitriding and the thickness of the compound layer.

Patent Document 2 describes how, by optimizing the steel composition and appropriately controlling impurities, high bending fatigue strength and surface fatigue strength after nitriding can be obtained while suppressing deformation during nitriding.

International Publication No. 2014/136348 JP 2012-158812 A

Nitrided parts that are subjected to repeated bending stresses require high bending fatigue strength. The bending fatigue strength of nitrided parts is governed by the fatigue properties of the nitrided layer. For example, when a nitrided part is large, such as a crankshaft, and the gradient of bending stress near the surface of the nitrided part is gentle, hardening only the very outermost layer is not enough to suppress internal fracture and does not sufficiently improve fatigue strength. If quenching and tempering is omitted or simplified, it is not possible to increase the hardness of the core, and therefore it may not be applicable to parts with a gentle gradient of bending stress near the surface of the part.
In order to obtain the necessary core hardness even if quenching and tempering of the nitrided parts is omitted, it is necessary to add a large amount of alloying elements. However, alloying elements such as Mn and Cr, which have the effect of increasing the core hardness, contribute to hardening of the nitrided layer by forming nitrides with nitrogen, but also have the effect of increasing the deformation amount of the part by forming nitrides and expanding the nitrided layer. Therefore, if a large amount of alloying elements such as Mn and Cr is added to obtain the necessary core hardness even if quenching and tempering of the nitrided parts is omitted or simplified, the amount of deformation during nitriding will be large, and the shape of the part may not be within the allowable range.

The technique disclosed in Patent Document 1 does not mention suppression of deformation after nitriding.
The technology disclosed in Patent Document 2 is able to suppress deformation after nitriding. However, since this technology is optimal for parts that require extremely high machinability, such as gears, the hardness of the nitrided part base material shown in the examples is 210 HV or less, and high fatigue strength cannot be obtained when the gradient of bending stress near the surface of the nitrided part is gradual.

The objective of the present disclosure is to provide a base material for nitrided parts that can be manufactured without or with simplified quenching and tempering, has excellent machinability, is suppressed from bending during nitriding, and can produce nitrided parts with excellent bending fatigue properties, as well as to provide a nitrided part that is suppressed from bending during nitriding and has excellent bending fatigue properties.

The above problems are solved by the following means.
<1> A shaft portion having a diameter of 10 mm or more,
The chemical composition, in mass%, is
C: 0.35 to 0.60%,
Si: 0.03 to 0.35%,
Mn: 1.00 to 2.10%,
P: 0.050% or less,
S: 0.010 to 0.095%,
Cr: more than 0.60 to 1.20%,
Al: 0.001 to 0.080%,
N: 0.0040 to 0.0250%, the balance being Fe and impurities;
The shaft portion has a microstructure of ferrite and pearlite or pearlite at a depth of 5 mm from the surface thereof,
The base material for nitrided parts, wherein the ratio of the amount of dissolved Cr to the amount of total Cr at a depth of 5 mm from the surface of the shaft portion is 0.10 to 0.75.
<2> Including a shaft portion having a diameter of 10 mm or more,
The chemical composition, in mass%, is
C: 0.35 to 0.60%,
Si: 0.03 to 0.35%,
Mn: 1.00 to 2.10%,
P: 0.050% or less,
S: 0.010 to 0.095%,
Cr: more than 0.60 to 1.20%,
Al: 0.001 to 0.080%,
N: 0.0040 to 0.0250%, and further containing one or more selected from the group consisting of the following first group, second group, and third group, with the balance being Fe and impurities;
The shaft portion has a microstructure of ferrite and pearlite or pearlite at a depth of 5 mm from the surface thereof,
The base material for nitrided parts, wherein the ratio of the amount of dissolved Cr to the amount of total Cr at a depth of 5 mm from the surface of the shaft portion is 0.10 to 0.75.
[First Group]
Cu: 0.40% or less,
Ni: 0.40% or less,
V: 0.20% or less, and Mo: 0.30% or less [Group 2]
Sn: 0.10% or less, and Ca: 0.0050% or less [Group 3]
<3> The nitrided part base material according to <2>, wherein the chemical composition contains the first group.
<4> The base material for nitrided parts according to <2> or <3>, wherein the chemical composition contains the second group.
<5> The base material for nitrided parts according to any one of <2> to <4>, wherein the chemical composition contains the third group.
<6> The shaft portion has a diameter of 10 mm or more,
A component having a core portion that is a portion deeper than 1.5 mm from a surface, and a nitride layer that is present on the outside of the core portion,
The chemical composition of the core is, in mass%,
C: 0.35 to 0.60%,
Si: 0.03 to 0.35%,
Mn: 1.00 to 2.10%,
P: 0.050% or less,
S: 0.010 to 0.095%,
Cr: more than 0.60 to 1.20%,
Al: 0.001 to 0.080%,
N: 0.0040 to 0.0250%, optionally containing one or more selected from the group consisting of the following first, second and third groups, with the balance being Fe and impurities;
The shaft portion has a microstructure of ferrite and pearlite or pearlite at a depth of 5 mm from the surface thereof,
A nitrided component, wherein the ratio of the amount of dissolved Cr to the amount of total Cr at a depth of 5 mm from the surface of the shaft portion is 0.10 to 0.75.
[First Group]
Cu: 0.40% or less,
Ni: 0.40% or less,
V: 0.20% or less, and Mo: 0.30% or less [Group 2]
Sn: 0.10% or less, and Ca: 0.0050% or less [Group 3]
Ti: 0.100% or less

According to the present disclosure, a base material for nitrided parts is provided that can be manufactured by eliminating or simplifying quenching and tempering, and that can produce nitrided parts that have excellent machinability, are suppressed from bending during nitriding, and have excellent bending fatigue properties, as well as nitrided parts that are suppressed from bending during nitriding and have excellent bending fatigue properties.

FIG. 2 is a side view of an Ono-type rotating bending fatigue test piece used in the bending fatigue test of the examples. FIG. 2 is a schematic diagram of a test piece used in evaluating the amount of bending in the examples. FIG. 4 is a schematic diagram illustrating a method for measuring the amount of bending in the examples.

An embodiment that is an example of the present disclosure will be described.
In this disclosure, a numerical range expressed using "to" means a range that includes the numerical values before and after "to" as the lower and upper limits. However, when the numerical values before and after "to" are followed by "more than" or "less than," the numerical range does not include these numerical values as the lower or upper limit.
The content of an element in a chemical composition may be expressed by adding "amount" to the element symbol (for example, C amount, Si amount, etc.).
With regard to the contents of elements in chemical compositions, "%" means "mass %".
When the content of an element in a chemical composition is described as "0-", this means that the element does not necessarily need to be contained.
The term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

The inventors of the present disclosure have investigated a method for obtaining high fatigue strength even when quenching and tempering is omitted or simplified when manufacturing nitrided parts that have a gentle stress gradient when bending stress is applied, and further for reducing the amount of deformation during nitriding, and have obtained the following findings and speculations.

(a) In order to reduce the amount of deformation during nitriding even when a large amount of alloying elements is contained, a steel containing a large amount of elements such as Mn and Cr is used, and these elements are made to enter a solid solution state during cooling after hot forging and contribute to hardening, and during nitriding, these elements are controlled to a form that has little effect on deformation.
(b) When an alloying element dissolved in the parent phase precipitates as a nitride, which has a lower density than the parent phase, its volume expands, which causes deformation of the part. Since cementite has a lower density than the parent phase, if the alloying element is concentrated in the cementite in advance, the volume change is small even if the element precipitates as a nitride, and deformation of the part is suppressed.
(c) In order to concentrate elements such as Cr in cementite, heat treatment can be performed after hot forging. If the heat treatment after hot forging is low-temperature annealing performed at or below the _A1 point, it will emit less _CO2 than quenching and tempering. If residual stress is present in the part after hot forging, this low-temperature annealing will release the residual stress, and the amount of deformation during the subsequent nitriding treatment will be reduced.
(d) If the alloying elements are concentrated too much in cementite, the amount of solid solution in the matrix decreases too much, reducing the amount of hardening during nitriding and deteriorating the fatigue properties.
The preform for a nitrided part and the nitrided part according to the present disclosure have been discovered based on the above findings and speculations. Note that in the present disclosure, the preform for a nitrided part means a steel material for manufacturing a nitrided part through cutting, nitriding, etc., and has a shape that resembles the nitrided part to be finally manufactured.

[Materials for nitrided parts]
The nitrided part base material according to the present disclosure includes a shaft portion having a diameter of 10 mm or more,
The chemical composition, in mass%, is
C: 0.35 to 0.60%,
Si: 0.03 to 0.35%,
Mn: 1.00 to 2.10%,
P: 0.050% or less,
S: 0.010 to 0.095%,
Cr: more than 0.60 to 1.20%,
Al: 0.001 to 0.080%,
N: 0.0040 to 0.0250%, optionally containing one or more elements (optional elements) selected from the first to third groups described below, with the balance being Fe and impurities.
In addition, at a depth of 5 mm from the surface of the shaft portion, the shaft portion has a microstructure of ferrite and pearlite or a microstructure of pearlite,
Furthermore, at a depth of 5 mm from the surface of the shaft portion, the ratio of the amount of dissolved Cr to the total amount of Cr is 0.10 to 0.75.

(shape)
The nitrided part preform according to the present disclosure has a shaft portion with a diameter of 10 mm or more, and typically has a long shape along the shaft portion. The shape of the nitrided part obtained by cutting and nitriding the nitrided part preform can be selected according to the application. The diameter of the shaft portion depends on the final product, but may be, for example, 15 to 160 mm from the viewpoint of ease of manufacture. The nitrided part to be finally manufactured is not particularly limited, but examples thereof include crankshafts and camshafts.

(Chemical Composition)
The chemical composition of the nitrided part preform according to the present disclosure will now be described.
The chemical composition of the nitrided part base material according to the present disclosure and the chemical composition of the core of the nitrided part are the same as the chemical composition of the steel material used as the raw material. In the following description, the nitrided part base material according to the present disclosure may be referred to as the "steel base material," and the steel material used to manufacture the nitrided part base material according to the present disclosure may be referred to as the "steel material of the present disclosure" or simply as the "steel material."
The steel material and steel preform of the present disclosure contain the following elements.

C: 0.35 to 0.60%
Carbon (C) increases the core hardness of the nitrided part and increases the bending fatigue strength of the nitrided part manufactured using the nitrided part preform as a material. C is also used to form cementite to dissolve nitride-forming elements. If the C content is less than 0.35%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges disclosed herein.
On the other hand, if the C content exceeds 0.60%, the machinability of the nitrided part preform decreases even if the contents of other elements are within the ranges of the present disclosure.
Therefore, the C content is 0.35 to 0.60%.
The lower limit of the C content is preferably 0.37%, more preferably 0.40%, and further preferably 0.43%.
The upper limit of the C content is preferably 0.57%, more preferably 0.55%, and further preferably 0.53%.

Si: 0.03 to 0.35%
Silicon (Si) dissolves in ferrite to increase the bending fatigue strength of nitrided parts made from the nitrided part preform. If the Si content is less than 0.03%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges disclosed herein.
On the other hand, if the Si content exceeds 0.35%, the machinability of the nitrided part preform decreases even if the contents of other elements are within the ranges disclosed in this disclosure.
Therefore, the Si content is 0.03 to 0.35%.
The lower limit of the Si content is preferably 0.05%, more preferably 0.08%, and further preferably 0.10%.
The upper limit of the Si content is preferably 0.32%, and more preferably 0.30%.

Mn: 1.00 to 2.10%
Manganese (Mn) increases the core hardness of the nitrided parts and improves the fatigue strength. In the nitriding process, the hardness of the nitrided layer is increased by forming nitrides, which also improves the fatigue strength. Mn also combines with S to form MnS, improving the machinability of the preform for nitrided parts. If the Mn content is less than 1.00%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of the present disclosure.
On the other hand, if the Mn content exceeds 2.10%, even if the contents of other elements are within the ranges of the present disclosure, a compound layer of sufficient thickness cannot be obtained, and sliding properties and corrosion resistance may not be obtained. Therefore, the Mn content is 1.00 to 2.10%.
The lower limit of the Mn content is preferably 1.15%, more preferably 1.20%, and further preferably 1.25%.
The upper limit of the Mn content is preferably 2.05%, more preferably 2.00%, and further preferably 1.95%, and may be 1.90%.

P: 0.050% or less Phosphorus (P) is an element contained in steel as an impurity. If the P content exceeds 0.050%, the toughness of the steel deteriorates. Therefore, the P content is 0.050% or less.
The P content may be as small as possible. The upper limit of the P content is preferably 0.045%, and more preferably 0.040%.

S: 0.010 to 0.095%
Sulfur (S) combines with Mn to form MnS, which improves the machinability of the nitrided part preform. If the S content is less than 0.010%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges disclosed herein.
On the other hand, if the S content exceeds 0.095%, even if the contents of other elements are within the ranges of the present disclosure, coarse MnS is formed. In this case, the bending fatigue strength of the nitrided part made from the nitrided part preform decreases. Therefore, the S content is 0.010 to 0.095%.
The lower limit of the S content is preferably 0.015%, more preferably 0.020%, and further preferably 0.025%.
The upper limit of the S content is preferably 0.090%, more preferably 0.085%, further preferably 0.080%, and may be 0.070%.

Cr: more than 0.60 to 1.20%
Chromium (Cr) increases the core hardness of the nitrided parts and improves the fatigue strength. In addition, in the nitriding process, the hardness of the nitrided layer is increased by forming nitrides, which also improves the fatigue strength. If the Cr content is 0.60% or less, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of the present disclosure.
On the other hand, if the Cr content exceeds 1.20%, even if the contents of other elements are within the ranges of the present disclosure, a compound layer of sufficient thickness cannot be obtained, and the sliding properties and corrosion resistance may not be obtained. Furthermore, the hardening depth during nitriding is also shallow, so that the required fatigue strength may not be obtained. Therefore, the Cr content is more than 0.60 to 1.20%.
The lower limit of the Cr content is preferably 0.65%, and more preferably 0.70%.
The upper limit of the Cr content is preferably 1.10%, more preferably 1.00%, further preferably 0.95%, further preferably 0.90%, and further preferably 0.85%.

Al: 0.001 to 0.080%
Aluminum (Al) deoxidizes steel during the steelmaking process in the manufacturing process of steel material. If the Al content is less than 0.001%, the above effect cannot be sufficiently obtained. On the other hand, if the Al content exceeds 0.080%, even if the contents of other elements are within the ranges of the present disclosure, excessive amounts of Al oxides are generated in the steel material. In this case, the machinability of the preform for nitrided parts is reduced.
Therefore, the Al content is 0.001 to 0.080%.
The lower limit of the Al content is preferably 0.002%, more preferably 0.005%, and further preferably 0.010%.
The upper limit of the Al content is preferably 0.060%, more preferably 0.050%, and further preferably 0.040%.

N: 0.0040 to 0.0250%
Nitrogen (N) increases the core hardness and improves fatigue strength of nitrided parts. If the N content is less than 0.0040%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges disclosed herein.
On the other hand, if the N content exceeds 0.0250%, even if the contents of other elements are within the ranges of the present disclosure, bubbles due to nitrogen gas may be generated in the nitrided component, which may reduce the bending fatigue strength.
Therefore, the N content is 0.0040 to 0.0250%.
The lower limit of the N content is preferably 0.0060%, and more preferably 0.0080%.
The upper limit of the N content is preferably 0.0230%, and more preferably 0.0200%.

The balance of the chemical composition of the steel material of the present disclosure consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment when the steel material is industrially manufactured, and are not intentionally contained, but are tolerated to the extent that they do not adversely affect the steel material of the present disclosure.

[Optional Elements]
The chemical composition of the steel material of the present disclosure may further contain one or more elements selected from the first, second and third groups (Ti) in place of a portion of Fe. The optional elements will be described below.
In the present disclosure, in order to increase the bending fatigue strength of the steel, one or more elements selected from the following first group may be contained in place of a portion of Fe.
(First group)
Cu: 0.40% or less Ni: 0.40% or less V: 0.20% or less Mo: 0.30% or less

Cu: 0.40% or less Copper (Cu) is an optional element and may not be contained. In other words, the Cu content may be 0%. Cu dissolves in ferrite to increase the core hardness, thereby improving the bending fatigue strength. If even a small amount of Cu is contained, the above effect can be obtained to a certain extent.
However, if the Cu content exceeds 0.40%, even if the contents of other elements are within the ranges of the present disclosure, segregation may occur at grain boundaries of the steel material, resulting in hot cracking, during the manufacturing process of the steel material or during the hot working process in the manufacturing process of a base material for nitrided parts using the steel material as a material.
Therefore, the Cu content is not more than 0.40%.
The lower limit of the Cu content is preferably more than 0%, more preferably 0.01%, and further preferably 0.05%.
The upper limit of the Cu content is preferably 0.35%, and more preferably 0.30%.

Ni: 0.40% or less Nickel (Ni) is an optional element and may not be contained. In other words, the Ni content may be 0%. Ni dissolves in ferrite to increase the core hardness, thereby improving bending fatigue strength. Ni also suppresses the occurrence of hot cracks caused by Cu when the steel material contains Cu. If even a small amount of Ni is contained, the above effects can be obtained to a certain extent.
However, if the Ni content exceeds 0.40%, the above effects become saturated and the production cost increases.
Therefore, the Ni content is 0.40% or less.
The lower limit of the Ni content is preferably more than 0%, more preferably 0.01%, and further preferably 0.05%.
The upper limit of the Ni content is preferably 0.35%, and more preferably 0.30%.

Vanadium (V) is an optional element and may not be contained. In other words, the V content may be 0%.
When V is contained, that is, when the V content is more than 0%, V increases the core hardness and the hardness of the nitrided layer of the nitrided part, thereby increasing the bending fatigue strength of the nitrided part. Even if even a small amount of V is contained, the above effects can be obtained to a certain extent.
However, if the V content exceeds 0.20%, the amount of deformation during nitriding increases, the hardness of the raw material becomes excessively high, and the machinability deteriorates. Therefore, the V content is 0 to 0.20%.
The lower limit of the V content is preferably more than 0%, and more preferably 0.01%.
The upper limit of the V content is preferably 0.15%, and more preferably 0.10%.

Mo: 0.30% or less Molybdenum (Mo) is an optional element and may not be contained. In other words, the Mo content may be 0%.
When Mo is contained, that is, when the Mo content is more than 0%, Mo increases the core hardness and the hardness of the nitrided layer of the nitrided part, thereby increasing the bending fatigue strength of the nitrided part. Even if even a small amount of Mo is contained, the above effects can be obtained to some extent.
However, if the Mo content exceeds 0.30%, the amount of deformation during nitriding increases, the hardness of the raw material becomes excessively high, and the machinability deteriorates.
Therefore, the Mo content is 0 to 0.30%.
The lower limit of the Mo content is preferably 0.05%, and more preferably 0.10%.
The upper limit of the Mo content is preferably 0.25%, and more preferably 0.20%.

The chemical composition of the nitrided part preform according to the present disclosure may further contain one or more elements selected from the following second group in place of a portion of Fe in order to improve the machinability of the steel.
(Group 2)
Ca: 0.0050% or less Sn: 0.10% or less

Ca: 0.0050% or less Calcium (Ca) is an optional element and does not have to be contained. In other words, the Ca content may be 0%. When Ca is contained, it improves the machinability of the steel material. However, if the Ca content is too high, coarse Ca oxides are generated and the fatigue strength of the steel material decreases. Therefore, the Ca content is 0 to 0.0050%.
In order to stably obtain the above effects, the lower limit of the Ca content is preferably 0.0001%, and the lower limit of the Ca content is preferably 0.0003%.
The upper limit of the Ca content is preferably 0.0040%, and more preferably 0.0035%.

Sn: 0.10% or less Tin (Sn) is an optional element and may not be contained. In other words, the Sn content may be 0%. When Sn is contained, that is, when the Sn content is more than 0%, Sn improves the chip disposability during cutting processing, thereby improving the machinability of the steel material. If even a small amount of Sn is contained, the above effect can be obtained to a certain extent.
However, if the Sn content exceeds 0.10%, the toughness of the steel material may deteriorate, so the Sn content is set to 0 to 0.10%.
The lower limit of the Sn content is preferably 0.01%, and more preferably 0.02%.
The upper limit of the Sn content is preferably 0.09%, and more preferably 0.07%.

The steel material of the present disclosure may further contain Ti as a third group element in place of a portion of Fe in order to increase the toughness of the steel.
(Group 3)
Ti: 0.100% or less Titanium (Ti) is an optional element and may not be contained. In other words, the Ti content may be 0%.
When Ti is contained, that is, when the Ti content exceeds 0%, Ti refines the grain size and increases the toughness of the steel material. Even if even a small amount of Ti is contained, the above effect can be obtained to some extent.
However, if the Ti content exceeds 0.100%, coarse Ti carbo-nitrides are formed, which may deteriorate the fatigue properties. Therefore, the Ti content is 0 to 0.100%.
The lower limit of the Ti content is preferably more than 0%, and more preferably 0.003%.
The upper limit of the Ti content is preferably 0.050%, and more preferably 0.030%.

(Method of measuring chemical composition)
The chemical composition of the steel material, steel preform, or nitrided part of the present disclosure can be measured by a known component analysis method in accordance with JIS G0321:2017. Specifically, chips are collected from the inside of the steel material or the like at a depth of 1 mm or more from the surface using a drill. The collected chips are dissolved in acid to obtain a solution. ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) is performed on the solution to perform elemental analysis of the chemical composition. The C content and S content are determined by a known high-frequency combustion method (combustion-infrared absorption method). The N content is determined by a known inert gas fusion-thermal conductivity method.

The content of each element is determined by rounding off the measured value based on the significant figures specified in this disclosure to the lowest digit of the content of each element specified in this disclosure. Rounding off means rounding down if the fraction is less than 5, and rounding up if the fraction is 5 or more. For example, the C content of the steel material in this disclosure is specified as a value to two decimal places. Therefore, the C content is determined as a value to two decimal places obtained by rounding off the measured value to two decimal places.

Similarly, the contents of other elements other than the C content of the steel material disclosed herein shall be the measured values rounded off to the lowest digit specified in this disclosure to obtain the contents of those elements.

(Microstructure)
Microstructure: Ferrite + Pearlite, or Pearlite The nitrided part raw material according to the present disclosure has a microstructure consisting of ferrite and pearlite (a mixture of ferrite and pearlite) or pearlite at a depth of 5 mm from the surface of the shaft portion.
The nitrided part preform according to the present disclosure contains an appropriate amount of alloying elements to provide high core hardness, and does not require quenching to obtain the required core hardness. Low-temperature annealing is performed to optimize the Cr content in the cementite, but since the low-temperature annealing is performed below the transformation point of the steel, the microstructure of the steel remains the same as it was when hot forged.

When steel having the chemical composition specified in this disclosure is cooled by air cooling or wind cooling without using accelerated cooling such as water cooling after hot forging, the microstructure of the steel becomes ferrite + pearlite or pearlite. More specifically, the shaft portion of the preform for nitrided parts has a microstructure consisting of ferrite + pearlite or pearlite at a depth of 5 mm from the surface. By having such a microstructure, machinability is improved when manufacturing nitrided parts. Note that even when cooled by air cooling or wind cooling, if the structure in this area contains bainite, the alloy elements are excessive, and machinability is deteriorated. Note that a specific method for observing the microstructure will be explained in the examples.

Ratio of dissolved Cr content to total Cr content (dissolved Cr content/total Cr content): 0.10 to 0.75
The nitrided part preform according to the present disclosure contains a large amount of alloy elements to ensure the core hardness of the nitrided part, and therefore, if the preform is nitrided as is, the amount of deformation during nitriding will be large. In order to suppress deformation, Cr is concentrated in the cementite by a heat treatment such as low-temperature annealing between hot forging and nitriding, and the amount of dissolved Cr in the ferrite is optimized. Specifically, at a depth of 5 mm from the surface of the shaft part of the nitrided part preform, the ratio of the amount of dissolved Cr to the total amount of Cr (amount of dissolved Cr/total amount of Cr) is set to a range of 0.10 to 0.75. If the ratio of the amount of dissolved Cr to the total amount of Cr is too large, the amount of deformation during nitriding will be large. The preferred upper limit of the ratio of the amount of dissolved Cr to the total amount of Cr is 0.70, and more preferably 0.68. If the ratio of the amount of dissolved Cr to the total amount of Cr is too small, the fatigue strength will deteriorate. The preferred lower limit of the ratio of the amount of dissolved Cr to the total amount of Cr is 0.20, and more preferably 0.30.

[Method of manufacturing steel material]
An example of a method for manufacturing a steel material for producing a nitrided component preform according to the present disclosure includes the following steps.
(Step 1) Material preparation step (Step 2) Hot working step Each step will be explained below.

(Step 1) Material Preparation Step In the material preparation step, a material for the steel material of the present disclosure is prepared.
Specifically, molten steel satisfying the above-mentioned chemical composition is produced. The refining method is not particularly limited, and any known method may be used. For example, molten iron produced by a known method is subjected to refining in a converter (primary refining). The molten steel tapped from the converter is subjected to a known secondary refining. In the secondary refining, alloy elements are added to the molten steel to adjust the composition, and molten steel having the above-mentioned chemical composition is produced.

The molten steel produced by the above-mentioned refining method is used to manufacture materials using known casting methods. For example, the molten steel is used to manufacture ingots using an ingot casting method. The molten steel may also be used to manufacture blooms or billets using a continuous casting method. Materials (ingots, blooms, or billets) are manufactured using the above methods.

(Step 2) Hot Working Step The produced material is hot worked to produce steel material.
In the hot working step, one or more hot working steps are usually performed. When multiple hot working steps are performed, the first hot working step may be, for example, rolling using blooming or hot forging, and subsequent hot working steps may be rolling using a continuous rolling mill. The continuous rolling mill includes multiple rolling stands arranged in a row.
The steel material after hot working is cooled to room temperature. A billet may be produced by rough rolling and rolling using a continuous rolling mill, and then the billet may be reheated and further subjected to finish rolling using a continuous rolling mill to produce a steel material of a desired size. Alternatively, the steel material may be produced from the material only by hot forging. The heating temperature of the material during hot working is not particularly limited, but is, for example, 1100 to 1300°C.

The above steps produce the steel material for manufacturing the nitrided part base material according to the present disclosure.

[Method of manufacturing base material for nitrided parts]
An example of a method for producing a nitrided component preform according to the present disclosure includes the following steps.
(Step 3) Steel material forming process (Step 4) Cr concentration process in cementite

(Step 3) Steel preform forming step In the steel preform forming step, a steel material having the above-mentioned chemical composition is formed into a preform for nitrided parts (steel preform) by, for example, hot forging.
Specifically, first, a steel material having the above-mentioned chemical composition is heated. The heating temperature is, for example, 1000 to 1300°C. The heated steel material is hot forged and formed into a steel preform having a predetermined shape. The formed steel preform is air-cooled. The cooling rate during air cooling may be appropriately adjusted using a fan.

(Step 4) Step of concentrating Cr in cementite The hot forged steel preform is subjected to a heat treatment for concentrating Cr in the parent phase in cementite. Specifically, the steel preform is heated at a predetermined temperature for a predetermined time, and then cooled to room temperature. The heating temperature is, for example, 600 to 650°C, and the heating time is, for example, 30 to 300 minutes. This heat treatment concentrates Cr in cementite, so that the amount of deformation during nitriding to manufacture a nitrided part from the steel preform can be reduced.
Any treatment may be used as long as it can concentrate a predetermined amount of Cr in cementite and bring the ratio of the amount of dissolved Cr to the total amount of Cr within the range of 0.10 to 0.75.

[Nitriding parts]
The nitrided part according to the present disclosure includes a nitrided layer formed on the surface layer and a core part located inside the nitrided layer, and the chemical composition of the core part is the same as that of the steel material and the preform for nitrided part according to the present disclosure. In the present disclosure, the surface layer of the nitrided part means a region from the surface to a depth of 1.5 mm, and the core part means a region deeper than 1.5 mm from the surface. The depth of nitrogen penetrating from the surface by the nitriding process is expected to be about 1.5 mm at maximum. In other words, the surface layer of the nitrided part here refers to a region into which nitrogen can penetrate, and the core part refers to a region that is not reached by the penetration of nitrogen by the nitriding process. The nitrided part according to the present disclosure has a core part that is a portion deeper than 1.5 mm from the surface, and a nitrided layer that exists outside the core part (surface layer).

The chemical composition and microstructure of the core of the nitrided component according to the present disclosure are the same as those of the base material for nitrided components according to the present disclosure described above, and therefore will not be described here.
When steel is nitrided, a layer called a compound layer, which is mainly composed of iron nitride, is formed on the surface, and a layer called a diffusion layer, in which the matrix is reinforced with solid-solution N or alloy nitride, is formed immediately below the compound layer. In the present disclosure, the nitride layer includes both the compound layer and the diffusion layer, and the N content of the compound layer is, for example, 2 to 11%, and the N content of the diffusion layer is, for example, 0.0300 to 0.6000%.

[Manufacturing method of nitrided parts]
The nitrided component according to the present disclosure can be manufactured, for example, by using the above-mentioned preform for a nitrided component. An example of a method for manufacturing the nitrided component according to the present disclosure using the above-mentioned preform for a nitrided component as a material includes the following steps.
(Step 5) Machining step (Step 6) Nitriding step Each step will now be described.

In the machining process, the nitrided part preform is machined to shape the nitrided part preform into a shape close to the shape of the final product. Specifically, the shape of the nitrided part preform is shaped by known cutting and/or grinding.

(Step 6) Nitriding Step In the nitriding step, the steel blank after the machining step is subjected to nitriding. Examples of nitriding include gas nitriding, gas soft nitriding, salt bath soft nitriding, plasma nitriding, etc. The gas used in the nitriding step may be only _NH3 , or may be a well-known mixed gas containing _NH3 , _N2 , _H2 , _CO2 , and various hydrocarbons.
The temperature and time of the nitriding treatment may be any conditions as long as the necessary hardened layer is formed on the surface. The nitriding temperature may be 530 to 620° C., and the nitriding time may be 0.5 to 10 hours.
The cooling method after nitriding may be water cooling, oil cooling, or furnace cooling.
The nitrided component according to the present disclosure can be manufactured by the above manufacturing process.

The base material for nitrided parts according to the present disclosure will be explained in more detail with reference to examples. The conditions in the following examples are examples of conditions adopted to confirm the feasibility and effectiveness of the base material for nitrided parts and nitrided parts according to the present disclosure. Therefore, the base material for nitrided parts and nitrided parts according to the present disclosure are not limited to the following examples.

[Steel manufacturing]
Steel materials A to P having the chemical compositions shown in Table 1 (the balance being Fe and impurities) were produced by the following methods.
A 50 kg ingot was produced using a vacuum melting furnace. The ingot was heated to 1250°C, and then hot processing was performed to produce a steel material. Specifically, hot forging was performed to produce a steel bar having a cross section perpendicular to the axial direction of 75 mm x 75 mm. The steel bar was heated again to 1250°C, and then hot forged into a steel bar having a cross section of 60 mm in diameter, and allowed to cool to room temperature. The steel material (steel bar) was produced by the above manufacturing process.

[Manufacturing of steel blanks]
Using each of the produced steel materials as a raw material, steel preforms were produced by the following method.

To simulate the steel material forming process in step 3 described above, the steel material was subjected to a simulated hot forging process in which it was held at 1,150°C for 60 minutes and then air-cooled to room temperature.

The steel bars after the hot forging simulation treatment were subjected to low-temperature annealing treatment in which they were held at 600 to 700°C for 1.0 to 4.0 hours and then cooled in the furnace to concentrate Cr in the cementite.
Through the above manufacturing process, a steel preform simulating a preform for nitrided parts was manufactured.

[evaluation]
The following evaluations (Tests 1 to 5) were carried out on each of the produced steel preforms and test pieces that were prepared by cutting and nitriding each of the steel preforms to simulate nitrided parts.
(Test 1) Observation of microstructure of steel preform (Test 2) Evaluation of machinability of steel preform (Test 3) Analysis of Cr concentration in cementite of steel preform (Test 4) Evaluation of bending fatigue strength (Test 5) Evaluation of bending amount during nitriding Each test will be described below.

(Test 1) Microstructural Observation of Steel Preforms Block-shaped test pieces for microstructural observation were cut out from a cross section perpendicular to the longitudinal direction of each steel preform of test number so that the center of the test surface was located 5 mm deep from the surface, embedded in resin, and mirror-polished. The polished surface was etched with nital, and one spot in a region with a visual field area of 1.56 ^mm2 was observed at a magnification of 200 times using an optical microscope.
None of the structures of the test numbers contained martensite or bainite, and were composed of ferrite and pearlite, or pearlite only.

(Test 2) Machinability evaluation of steel materials (hardness test)
The Vickers hardness of the microstructure observation test piece of each test number was measured. A Vickers hardness test was performed in any five positions on the mirror-polished surface in accordance with JIS Z 2244:2009. The test force was 9.8 N. The arithmetic average value of the five hardness values obtained was defined as the Vickers hardness of the test number. When the Vickers hardness was 300 HV or less, it was determined that sufficient machinability was obtained.

(Test 3) Analysis of Cr concentration in cementite of steel material (calculation of dissolved Cr amount/total Cr amount)
A test piece for electrolytic analysis having a diameter of 5 mm and a length of 40 mm was prepared from a position 5 mm deep from the surface of a cross section perpendicular to the axial direction of each steel blank (a steel bar having a diameter of 60 mm) of each test number. The test piece was electrolyzed at a current density of 20 mA/ ^cm2 in a 10% acetylacetone-1% tetramethylammonium chloride-methanol electrolyte. The solution after electrolysis was suction filtered with a Crypore filter having a mesh size of 0.2 μm. The amount of Cr in the resulting residue was analyzed using a high-frequency inductively coupled plasma emission spectrometry device, and the amount of Cr was considered to be the amount of Cr concentrated in the cementite. The value obtained by subtracting the amount of Cr concentrated in the cementite from the total amount of Cr was considered to be the amount of dissolved Cr.
Then, the mass ratio of the amount of dissolved Cr to the total amount of Cr contained in the steel material (amount of dissolved Cr/total amount of Cr) was calculated.

(Test 4) Bending fatigue strength evaluation An Ono-type rotating bending fatigue test piece as shown in Fig. 1 was prepared from the radial center position (R/2 position) of a cross section perpendicular to the axial direction of the steel blank (steel bar with a diameter of 60 mm) of each test number. The numbers in Fig. 1 indicate dimensions (unit: mm). "φ" in Fig. 1 means diameter. The same applies to Figs. 2 and 3. "R3" means that the radius of curvature at the notch bottom is 3 mm.

Specifically, the steel blanks of each test number were machined (cut) to process them into intermediate Ono-type rotating bending fatigue test pieces. The intermediate pieces were then subjected to a nitriding process to produce the Ono-type rotating bending fatigue test pieces shown in Figure 1. The nitriding conditions were as follows: In the nitriding process, the intermediate pieces were held at 570°C for three hours in an atmosphere of 1:1 RX gas (endothermic transformation gas) and ammonia gas. After holding, the intermediate pieces were oil-cooled. Through the above process, Ono-type rotating bending fatigue test pieces simulating nitrided parts were produced.

Ono-type rotating bending fatigue tests were conducted using Ono-type rotating bending fatigue test pieces of each test number. A number of Ono-type rotating bending fatigue test pieces were prepared for each test number. Fatigue tests were conducted by changing the stress applied to each test piece, and the highest stress at which the test piece did not break after 10 million cycles ( ¹⁰⁷ cycles) was taken as the bending fatigue strength (MPa). In the Ono-type rotating bending fatigue test, the rotation speed was 3000 rpm, and the stress ratio was reversed. The obtained bending fatigue strengths are shown in the "Bending fatigue strength (MPa)" column of the "Nitriding parts" column in Table 2. It was determined that a sufficient fatigue strength was obtained when the fatigue strength was 540 MPa or more.

(Test 5) Evaluation of bending amount during nitriding A test piece for bending measurement, measuring φ15×100 mm as shown in Fig. 2, was prepared from the radial center position (R/2 position) of a cross section perpendicular to the axial direction of the steel preform (steel bar with a diameter of 60 mm) of each test number. The test piece was subjected to a nitriding treatment in which it was held at 570°C for 3 hours in an atmosphere of RX gas and ammonia gas at a ratio of 1:1, and then cooled with oil.
The amount of bending of the test piece after nitriding was measured by a contact type three-dimensional measuring machine. Specifically, as shown in FIG. 3, a reference circle was obtained from the cross section at three positions, 10 mm from both ends of the test piece and at the midpoint in the longitudinal direction, and the center of the reference circle was obtained. The distance between the line (A1) connecting the center coordinates of the reference circle at the 10 mm position from both ends and the center coordinate (A0) of the reference circle at the midpoint in the longitudinal direction was regarded as the amount of bending. The method of obtaining the reference circle will be described. At the position where the reference circle is to be obtained, the coordinates of eight points were measured at 45° pitches on the circumference. The coordinates of each of the eight points were used to approximate a perfect circle and regarded as the reference circle at that position. When the amount of bending was 2.0 μm or less, it was considered that bending could be sufficiently suppressed.

The heat treatment conditions after the simulated hot forging process and the evaluation results of tests 2 to 5 are shown in Table 2.

In the test numbers that satisfied the requirements of the present disclosure, the bending fatigue strength was 540 MPa or more and the bending amount was 2.0 μm or less.
Test No. 1 had an insufficient C content and thus had insufficient bending fatigue strength.
Test No. 4 had an excessively high C content and was poor in machinability.
Test No. 6 had an insufficient Mn content and thus had insufficient bending fatigue strength.
Test No. 8 had an excessively high Mn content and was poor in machinability.
Test No. 9 had an insufficient Cr content and had insufficient bending fatigue strength.
Test No. 11 had an excessively high Cr content and was bent to an excessively large extent.
In test numbers 17 and 18, heat treatment was not performed, so Cr was not concentrated in cementite, the dissolved Cr amount/total Cr amount was high, and the amount of bending was too large.
In test number 19, the heat treatment temperature was too high, so that Cr was too concentrated in the cementite, the dissolved Cr amount/total Cr amount was a low value, and the bending fatigue strength was insufficient.

Claims (6)

A shaft portion having a diameter of 10 mm or more,
The chemical composition, in mass%, is
C: 0.35 to 0.60%,
Si: 0.03 to 0.35%,
Mn: 1.00 to 2.10%,
P: 0.050% or less,
S: 0.010 to 0.095%,
Cr: more than 0.60 to 1.20%,
Al: 0.001 to 0.080%,
N: 0.0040 to 0.0250%, the balance being Fe and impurities;
The shaft portion has a microstructure of ferrite and pearlite or pearlite at a depth of 5 mm from the surface thereof,
The base material for nitrided parts, wherein the ratio of the amount of dissolved Cr to the amount of total Cr at a depth of 5 mm from the surface of the shaft portion is 0.10 to 0.75. A shaft portion having a diameter of 10 mm or more,
The chemical composition, in mass%, is
C: 0.35 to 0.60%,
Si: 0.03 to 0.35%,
Mn: 1.00 to 2.10%,
P: 0.050% or less,
S: 0.010 to 0.095%,
Cr: more than 0.60 to 1.20%,
Al: 0.001 to 0.080%,
N: 0.0040 to 0.0250%, and further containing one or more selected from the group consisting of the following first group, second group, and third group, with the balance being Fe and impurities;
The shaft portion has a microstructure of ferrite and pearlite or pearlite at a depth of 5 mm from the surface thereof,
The base material for nitrided parts, wherein the ratio of the amount of dissolved Cr to the amount of total Cr at a depth of 5 mm from the surface of the shaft portion is 0.10 to 0.75.
[First Group]
Cu: 0.40% or less,
Ni: 0.40% or less,
V: 0.20% or less, and Mo: 0.30% or less [Group 2]
Sn: 0.10% or less, and Ca: 0.0050% or less [Group 3]
Ti: 0.100% or less The base material for nitrided parts according to claim 2, wherein the chemical composition contains the first group. The base material for nitrided parts according to claim 2, wherein the chemical composition contains the second group. The base material for nitrided parts according to claim 2, wherein the chemical composition contains the third group. A shaft portion having a diameter of 10 mm or more,
A component having a core portion that is a portion deeper than 1.5 mm from a surface, and a nitride layer that is present on the outside of the core portion,
The chemical composition of the core is, in mass%,
C: 0.35 to 0.60%,
Si: 0.03 to 0.35%,
Mn: 1.00 to 2.10%,
P: 0.050% or less,
S: 0.010 to 0.095%,
Cr: more than 0.60 to 1.20%,
Al: 0.001 to 0.080%,
N: 0.0040 to 0.0250%, optionally containing one or more selected from the group consisting of the following first, second and third groups, with the balance being Fe and impurities;
The shaft portion has a microstructure of ferrite and pearlite or pearlite at a depth of 5 mm from the surface thereof,
A nitrided component, wherein the ratio of the amount of dissolved Cr to the amount of total Cr at a depth of 5 mm from the surface of the shaft portion is 0.10 to 0.75.
[First Group]
Cu: 0.40% or less,
Ni: 0.40% or less,
V: 0.20% or less, and Mo: 0.30% or less [Group 2]
Sn: 0.10% or less, and Ca: 0.0050% or less [Group 3]
Ti: 0.100% or less