Raw materials for nitrided parts and nitrided parts

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

Technical Field

[0001] The present disclosure relates to a blank for nitride parts and nitride parts.

Background Art

[0002] Among the mechanical structure parts used in automobiles, ships, industrial machines, etc., there are parts to which high bending stress is repeatedly applied. In order to provide the fatigue strength required for those parts, various surface hardening heat treatments may be performed. In the case of parts that require high bending fatigue strength, sliding characteristics, corrosion resistance, and small strain, nitriding treatment may be used as the surface hardening heat treatment. An example of a method for manufacturing nitride parts is as follows. First, a steel material used as a raw material is hot forged to produce a steel blank. The manufactured steel blank is subjected to a heat treatment for optimizing the microstructure as needed. Then, it is machined (such as cutting) to a shape close to the final product. Nitriding treatment is performed on the machined steel blank to increase the strength of the surface layer. After nitriding, finishing such as polishing or bending correction is performed. Through such processes, nitride parts are manufactured.

[0003] The compound layer (a part of the nitride layer) formed on the surface by nitriding treatment improves the sliding characteristics of the parts by reducing the friction coefficient and suppressing wear. The compound layer also has the effect of enhancing the corrosion resistance of the parts, and in some cases, the parts are subjected to nitriding treatment for the purpose of enhancing the corrosion resistance of the parts. In this specification, a mechanical structure part (such as a crankshaft) subjected to nitriding treatment is referred to as a nitride part.

[0004] Non-quenching is desired for nitride parts. From the viewpoints of manufacturing cost and reduction of CO2 emissions during manufacturing, it is desirable to omit or simplify quenching and tempering.

[0005] Techniques related to nitride parts with quenching and tempering omitted or simplified have been disclosed.

[0006] For example, Patent Document 1 describes that 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.

[0007] Patent Document 2 describes how, by optimizing the steel composition and appropriately suppressing impurities, high bending fatigue strength and surface fatigue strength after nitriding can be obtained while suppressing deformation during nitriding. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] International Publication No. 2014 / 136348 [Patent Document 2] Japanese Patent Publication No. 2012-158812 [Overview of the project] [Problems that the invention aims to solve]

[0009] Nitrided parts subjected to repeated bending stress require high bending fatigue strength. The bending fatigue strength of a nitrided part is governed by the fatigue properties of the nitrided layer. For example, when a nitrided part is large, such as a crankshaft, and the bending stress gradient near the surface of the nitrided part is gentle, simply hardening the very surface layer is insufficient to suppress internal failure and adequately improve fatigue strength. Omitting or simplifying quenching and tempering may not increase the hardness of the core, and may therefore not be applicable to parts with a gentle bending stress gradient near the surface. To obtain the required core hardness of nitrided parts without quenching and tempering, a large amount of alloying elements must be added. However, while alloying elements such as Mn and Cr, which have the effect of increasing core hardness, contribute to the hardening of the nitrided layer by forming nitrides with nitrogen, they also have the effect of increasing the amount of deformation of the part by forming nitrides and causing the nitrided layer to expand. Therefore, if a large amount of alloying elements such as Mn and Cr is included to obtain the required core hardness without quenching or simplifying the quenching and tempering of nitrided parts, the amount of deformation during nitriding will increase, and the shape of the part may not fall within the acceptable range.

[0010] The technology disclosed in Patent Document 1 does not address the suppression of deformation after nitriding. The technology disclosed in Patent Document 2 can suppress deformation after nitriding, but since it is an optimal technology for parts that require extremely high machinability, such as gears, the hardness of the raw material for nitrided parts shown in the examples is 210 HV or less, and high fatigue strength cannot be obtained when the bending stress gradient near the surface of the nitrided part is gentle.

[0011] The object of this disclosure is to provide a raw material for nitrided parts that can be manufactured by omitting or simplifying quenching and tempering, has excellent machinability, suppresses bending during nitriding, and yields nitrided parts with excellent bending fatigue characteristics, as well as nitrided parts that suppress bending during nitriding and have excellent bending fatigue characteristics. [Means for solving the problem]

[0012] The above problems will be solved by the following means. <1> Including a shaft portion with a diameter of 10 mm or more, The chemical composition is expressed in mass percent. C: 0.35~0.60%, Si: 0.03~0.35%, Mn: 1.00~2.10%, P: 0.050% or less, S: 0.010~0.095%, Cr: over 0.60~1.20%, Al: 0.001~0.080%, It is a raw material in the form of a bar with N: 0.0040 to 0.0250%, and the balance consisting of Fe and impurities. At a position 5 mm deep from the surface of the shaft portion, it has a microstructure composed of ferrite and pearlite, or pearlite. A raw material for a nitrided part, where at a position 5 mm deep from the surface of the shaft portion, the ratio of the dissolved Cr amount to the total Cr amount is 0.10 to 0.75. <2> It includes a shaft portion with a diameter of 10 mm or more. The chemical composition 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 0.2% and further contains one or more selected from the group consisting of the following Group 1, Group 2, and Group 3, and the balance is a raw material in the form of a bar consisting of Fe and impurities. At a position 5 mm deep from the surface of the shaft portion, it has a microstructure composed of ferrite and pearlite, or pearlite. A raw material for a nitrided part, where at a position 5 mm deep from the surface of the shaft portion, the ratio of the dissolved Cr amount to the total Cr amount is 0.10 to 0.75. [Group 1] Cu: 0.40% or less Ni: 0.40% or less V: 0.20% or less, and One or more selected from the group consisting of Mo: 0.30% or less [Group 2] Sn: 0.10% or less, and One or two selected from the group consisting of Ca: 0.0050% or less [Group ] Ti: 0.100% or less <3> The raw material for a nitrided part according to <2>, wherein the chemical composition contains Group 1. <4> The raw material for nitride components according to <2> or <3>, wherein the chemical composition contains the second group. <5> The raw material for nitride components according to any one of <2> to <4>, wherein the chemical composition contains the third group. <6> including 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 the surface and a nitride layer existing outside the core portion, <07]] the chemical composition of the core portion is, by 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%, and optionally contains one or more selected from the group consisting of the following first group, second group, and third group, and the balance consists of Fe and impurities, at a depth of 5 mm from the surface of the shaft portion, it has a microstructure composed of ferrite and pearlite, or pearlite, a nitride component in which the ratio of the amount of dissolved Cr to the total amount of Cr is 0.10 to 0.75 at a depth of 5 mm from the surface of the shaft portion. [First group] Cu: 0.40% or less, V: 0.20% or less, and one or more selected from the group consisting of Mo: 0.30% or less [Second group] Sn: 0.10% or less, and one or two selected from the group consisting of Ca: 0.0050% or less [Third group] Ti: 0.100% or less

Advantages of the invention

[0013] According to this disclosure, a raw material for nitrided parts is provided that can be manufactured by omitting or simplifying quenching and tempering, and that nitrided parts can be obtained that have excellent machinability, suppressed bending during nitriding, and excellent bending fatigue characteristics, as well as nitrided parts that have suppressed bending during nitriding and excellent bending fatigue characteristics. [Brief explanation of the drawing]

[0014] [Figure 1] This is a side view of the Ono-type rotary bending fatigue test specimen used in the bending fatigue test of the example. [Figure 2] This is a schematic diagram of the test specimen used in the evaluation of the amount of bending in the example. [Figure 3] This is a schematic diagram illustrating the method for measuring the amount of curvature in the example. [Modes for carrying out the invention]

[0015] An example of an embodiment of this disclosure will be described. In this disclosure, a numerical range represented by "~" means a range that includes the numbers before and after "~" as the lower and upper limits. However, if the numbers before and after "~" are preceded by "greater than" or "less than", the numerical range means a range that does not include those numbers as the lower or upper limit. The elemental content in a chemical composition is sometimes expressed by adding the "amount" to the element symbol (for example, C amount, Si amount, etc.). In chemical composition, the percentage "%" indicates "mass percent". When the chemical composition of an element is described as "0~", it means that the element does not need to be included. The term "process" includes not only independent processes, but also any process that cannot be clearly distinguished from other processes, as long as its intended purpose is achieved.

[0016] The inventors of this disclosure investigated methods to obtain high fatigue strength even when quenching and tempering are omitted or simplified when manufacturing nitrided parts with a gentle stress gradient when bending stress is applied, and to further reduce the amount of deformation during nitriding, and obtained the following findings and inferences.

[0017] (a) In order to reduce the amount of deformation during nitriding even when a large amount of alloying elements are included, steel containing a large amount of elements such as Mn and Cr should be used, and during cooling after hot forging, these elements should be in a solid solution state that contributes to hardening, and during nitriding, these elements should be controlled to a form that has little effect on deformation. (b) When alloying elements dissolved in the matrix precipitate as nitrides, which have a lower density than the matrix, their volume expands, causing deformation of the part. Since cementite has a lower density than the matrix, if alloying elements are concentrated in the cementite beforehand, the volume change will be small even if those elements precipitate as nitrides, and deformation of the part will be suppressed. (c) To concentrate elements such as Cr in cementite, heat treatment should be performed after hot forging. If the heat treatment after hot forging is low-temperature annealing performed below the A1 point, the amount of CO2 emissions will be less than that of quenching and tempering. If residual stress is present in the part after hot forging, this low-temperature annealing will release the residual stress, reducing the amount of deformation during the subsequent nitriding treatment. (d) If the alloying elements are concentrated too much in the cementite, the amount of solid solution in the matrix decreases too much, reducing the amount of hardening during nitriding and degrading the fatigue properties. The raw materials for nitrided parts and nitrided parts relating to this disclosure were discovered based on the above knowledge and inferences. In this disclosure, "raw materials for nitrided parts" refers to steel materials used to manufacture nitrided parts through cutting, nitriding, etc., and which have a shape that mimics the ultimately manufactured nitrided part.

[0018] [Materials for nitrided parts] The nitride component material relating to this disclosure includes a shaft portion with a diameter of 10 mm or more. The chemical composition is expressed in mass percent. C: 0.35~0.60%, Si: 0.03~0.35%, Mn: 1.00~2.10%, P: 0.050% or less, S: 0.010~0.095%, Cr: over 0.60~1.20%, Al: 0.001~0.080%, N is 0.0040-0.0250%, and optionally contains one or more elements (arbitrary elements) selected from the first to third groups described later, with the remainder consisting of Fe and impurities. Furthermore, at a depth of 5 mm from the surface of the shaft, it has a microstructure consisting of ferrite and pearlite, or pearlite. Furthermore, at a depth of 5 mm from the surface of the shaft, the ratio of the amount of solid-solution Cr to the total amount of Cr is 0.10 to 0.75.

[0019] (shape) The nitrided component profile according to this disclosure has a shaft portion with a diameter of 10 mm or more, and typically has an elongated shape along the shaft portion. The shape of the nitrided component obtained by cutting and nitriding the nitrided component profile can be selected according to the application of the nitrided component. The diameter of the shaft portion depends on the final product, but may be 15 to 160 mm, for example, from the viewpoint of ease of manufacture. The nitrided component to be finally manufactured is not particularly limited, but examples include crankshafts and camshafts.

[0020] (chemical composition) The chemical composition of the nitride component material related to this disclosure is described below. The chemical composition of the nitrided component material and the chemical composition of the core of the nitrided component are the same as the chemical composition of the steel material used as the base material. In the following description, the nitrided component material according to this disclosure may be referred to as "steel material," and the steel material used in the manufacture of the nitrided component material according to this disclosure may be referred to as "steel material of this disclosure" or simply "steel material." The steel materials and steel profiles of this disclosure contain the following elements.

[0021] C: 0.35~0.60% Carbon (C) enhances the core hardness of nitrided parts and increases the bending fatigue strength of nitrided parts manufactured using raw materials for nitrided parts. Furthermore, C is used to form cementite, which is necessary for solid solution of nitride-forming elements. If the C content is less than 0.35%, the above effects cannot be fully obtained, even if the content of other elements is within the scope of this disclosure. On the other hand, if the carbon content exceeds 0.60%, the machinability of the raw material for nitrided parts will decrease, even if the content of other elements is within the scope of this disclosure. Therefore, the C content is 0.35-0.60%. The preferred lower limit for the C content is 0.37%, more preferably 0.40%, and even more preferably 0.43%. The preferred upper limit for the C content is 0.57%, more preferably 0.55%, and even more preferably 0.53%.

[0022] Si: 0.03~0.35% Silicon (Si) is dissolved in ferrite to increase the bending fatigue strength of nitrided parts made from nitrided component profiles. If the Si content is less than 0.03%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this disclosure. On the other hand, if the Si content exceeds 0.35%, the machinability of the raw material for nitrided parts decreases, even if the content of other elements is within the range of this disclosure. Therefore, the Si content is 0.03 to 0.35%. The preferred lower limit for the Si content is 0.05%, more preferably 0.08%, and even more preferably 0.10%. The preferred upper limit for the Si content is 0.32%, and more preferably 0.30%.

[0023] Mn: 1.00~2.10% Manganese (Mn) increases the core hardness of nitrided parts and improves fatigue strength. In addition, fatigue strength is improved by increasing the hardness of the nitrided layer through the formation of nitrides during the nitriding process. Furthermore, Mn combines with sulfur to form MnS, which improves the machinability of the raw material for nitrided parts. If the Mn content is less than 1.00%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this disclosure. On the other hand, if the Mn content exceeds 2.10%, even if the content of other elements is within the range of this disclosure, a compound layer of sufficient thickness may not be obtained, and sliding properties and corrosion resistance may not be achieved. Therefore, the Mn content is 1.00 to 2.10%. The preferred lower limit of the Mn content is 1.15%, more preferably 1.20%, and even more preferably 1.25%. The Mn content has a preferred upper limit of 2.05%, more preferably 2.00%, even more preferably 1.95%, and may also be 1.90%.

[0024] P:0.050% or less Phosphorus (P) is an element found in steel as an impurity. If the P content exceeds 0.050%, the toughness of the steel deteriorates. Therefore, the P content should be 0.050% or less. The P content can be as low as possible. The preferred upper limit for the P content is 0.045%, and more preferably 0.040%.

[0025] S: 0.010~0.095% Sulfur (S) combines with Mn to form MnS, which improves the machinability of the raw material for nitrided parts. If the S content is less than 0.010%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this disclosure. On the other hand, if the S content exceeds 0.095%, coarse MnS is formed even if the content of other elements is within the range of this disclosure. In this case, the bending fatigue strength of nitrided parts made from nitrided material decreases. Therefore, the S content is 0.010 to 0.095%. The preferred lower limit for the S content is 0.015%, more preferably 0.020%, and even more preferably 0.025%. The preferred upper limit for the S content is 0.090%, more preferably 0.085%, even more preferably 0.080%, and may also be 0.070%.

[0026] Cr: More than 0.60~1.20% Chromium (Cr) increases the core hardness of nitrided parts and improves fatigue strength. In addition, fatigue strength is also improved in the nitriding process by increasing the hardness of the nitrided layer through the formation of nitrides. If the Cr content is 0.60% or less, the above effects cannot be fully obtained, even if the content of other elements is within the range of this disclosure. On the other hand, if the Cr content exceeds 1.20%, even if the content of other elements is within the range of this disclosure, a compound layer of sufficient thickness may not be obtained, and sliding properties and corrosion resistance may not be achieved. Furthermore, the hardening depth during nitriding will also be shallower, which may result in the inability to obtain the required fatigue strength. Therefore, the Cr content is between 0.60% and 1.20%. The preferred lower limit for the Cr content is 0.65%, and more preferably 0.70%. The preferred upper limit for the Cr content is 1.10%, more preferably 1.00%, even more preferably 0.95%, even more preferably 0.90%, and even more preferably 0.85%.

[0027] Al: 0.001~0.080% Aluminum (Al) deoxidizes steel during the steelmaking process in the manufacturing of steel materials. 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 content of other elements is within the range of this disclosure, an excessive amount of Al oxide will be generated in the steel material. In this case, the machinability of the raw material for nitrided parts will decrease. Therefore, the Al content is between 0.001% and 0.080%. The preferred lower limit for the Al content is 0.002%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Al content is 0.060%, more preferably 0.050%, and even more preferably 0.040%.

[0028] N: 0.0040~0.0250% Nitrogen (N) increases the core hardness of nitrided components and improves fatigue strength. If the N content is less than 0.0040%, the above effects cannot be fully obtained, even if the content of other elements is within the scope of this disclosure. On the other hand, if the N content exceeds 0.0250%, even if the content of other elements is within the range of this disclosure, bubbles due to nitrogen gas may form in the nitrided component, which may reduce the bending fatigue strength. Therefore, the N content is 0.0040 to 0.0250%. The preferred lower limit for the N content is 0.0060%, and more preferably 0.0080%. The preferred upper limit for the N content is 0.0230%, and more preferably 0.0200%.

[0029] The remainder of the chemical composition of the steel material disclosed herein consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are introduced during the industrial manufacture of the steel material from raw materials such as ore, scrap, or the manufacturing environment, and are not intentionally included, and are acceptable to the extent that they do not adversely affect the steel material disclosed herein.

[0030] [Optional Elements] The chemical composition of the steel materials disclosed herein may further contain one or more elements selected from Group 1, Group 2, and Group 3 (Ti) in place of a portion of Fe. The following describes the optional elements. This disclosure further indicates that, in order to increase the bending fatigue strength of the steel, a portion of Fe may be replaced with one or more substances selected from the following Group 1. (Group 1) Cu: 0.40% or less Ni: 0.40% or less V: 0.20% or less Mo: 0.30% or less

[0031] Cu: 0.40% or less Copper (Cu) is an optional element and does not need to be included. In other words, the Cu content may be 0%. Cu improves bending fatigue strength by dissolving in ferrite and increasing the core hardness. Even a small amount of Cu content will provide the above effect to some extent. However, if the Cu content exceeds 0.40%, even if the content of other elements is within the scope of this disclosure, segregation at the grain boundaries of the steel may occur during the manufacturing process of the steel material, or during the hot working process in the manufacturing process of the raw material for nitrided parts made from steel, causing hot cracking. Therefore, the copper content is 0.40% or less. The preferred lower limit of the Cu content is greater than 0%, more preferably 0.01%, and even more preferably 0.05%. The preferred upper limit for the Cu content is 0.35%, and more preferably 0.30%.

[0032] Ni: 0.40% or less Nickel (Ni) is an optional element and may not be present at all. In other words, the Ni content may be 0%. Ni improves bending fatigue strength by dissolving in ferrite and increasing the core hardness. Furthermore, when the steel material contains Cu, Ni suppresses the occurrence of hot cracks caused by Cu. Even a small amount of Ni will provide some of the above effects. However, if the Ni content exceeds 0.40%, the above effects saturate, and manufacturing costs increase. Therefore, the Ni content is 0.40% or less. The preferred lower limit for the Ni content is greater than 0%, more preferably 0.01%, and even more preferably 0.05%. The preferred upper limit for the Ni content is 0.35%, and more preferably 0.30%.

[0033] V: 0.20% or less Vanadium (V) is an optional element and does not need to be present. In other words, the V content may be 0%. When V is present, that is, when the V content is greater than 0%, V increases the core hardness and hardness of the nitrided layer of the nitrided component, and increases the bending fatigue strength of the nitrided component. Even if only a small amount of V is present, the above effects can be obtained to some extent. However, if the V content exceeds 0.20%, the deformation during nitriding increases, the hardness of the raw material becomes excessively high, and the machinability deteriorates. Therefore, the V content is between 0 and 0.20%. The preferred lower limit for the V content is greater than 0%, and more preferably 0.01%. The preferred upper limit for the V content is 0.15%, and more preferably 0.10%.

[0034] Mo: 0.30% or less Molybdenum (Mo) is an optional element and does not need to be included. In other words, the Mo content may be 0%. When Mo is present, that is, when the Mo content is greater than 0%, Mo increases the core hardness and hardness of the nitrided layer of the nitrided component, thereby increasing the bending fatigue strength of the nitrided component. Even if only a small amount of Mo is present, 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 0-0.30%. The preferred lower limit for the Mo content is 0.05%, and more preferably 0.10%. The preferred upper limit for the Mo content is 0.25%, and more preferably 0.20%.

[0035] The chemical composition of the nitride component material relating to this 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

[0036] Ca: 0.0050% or less Calcium (Ca) is an optional element and may not be present. In other words, the Ca content may be 0%. When Ca is present, it improves the machinability of the steel. However, if the Ca content is too high, coarse Ca oxides are formed, reducing the fatigue strength of the steel. Therefore, the Ca content is typically between 0% and 0.0050%. The preferred lower limit for Ca content to stably obtain the above effects is 0.0001%, and the preferred lower limit for Ca content is 0.0003%. The preferred upper limit for the Ca content is 0.0040%, and more preferably 0.0035%.

[0037] Sn: 0.10% or less Tin (Sn) is an optional element and does not need to be present. In other words, the Sn content may be 0%. When Sn is present, that is, when the Sn content is greater than 0%, Sn improves the machinability of the steel material by improving chip evacuation during cutting. Even if only a small amount of Sn is present, the above effect can be obtained to some extent. However, if the Sn content exceeds 0.10%, the toughness of the steel may deteriorate. Therefore, the Sn content is between 0 and 0.10%. The preferred lower limit for the Sn content is 0.01%, and more preferably 0.02%. The preferred upper limit for the Sn content is 0.09%, and more preferably 0.07%.

[0038] The steel material of this disclosure may further contain Ti as a third group in place of some of the Fe in order to enhance the toughness of the steel. (Group 3) Ti:0.100% or less Titanium (Ti) is an optional element and does not need to be included. In other words, the Ti content may be 0%. When Ti is present, that is, when the Ti content is greater than 0%, Ti refines the grain size and increases the toughness of the steel. Even a small amount of Ti will provide some degree of the above effect. However, if the Ti content exceeds 0.100%, coarse Ti carbonitrides may form, potentially degrading the fatigue properties. Therefore, the Ti content is between 0 and 0.100%. The preferred lower limit for the Ti content is greater than 0%, and more preferably 0.003%. The preferred upper limit for the Ti content is 0.050%, and more preferably 0.030%.

[0039] (Method for measuring chemical composition) The chemical composition of the steel materials, steel profiles, or nitrided parts disclosed herein can be measured by well-known component analysis methods in accordance with JIS G0321:2017. Specifically, chips are collected from the inside of the steel material, etc., 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. Elemental analysis of the chemical composition is performed on the solution using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry). The C and S content is determined by the well-known high-frequency combustion method (combustion-infrared absorption method). The N content is determined by the well-known inert gas melting-thermal conductivity method.

[0040] Furthermore, the content of each element shall be rounded to the least significant digit of the measured value, based on the significant figures specified in this disclosure. Rounding means truncating if the fraction is less than 5, and rounding up if the fraction is 5 or greater. For example, the carbon content of the steel material in this disclosure is specified to two decimal places. Therefore, the carbon content shall be the value obtained by rounding the third decimal place of the measured value to two decimal places.

[0041] Similarly, for the carbon content of the steel materials disclosed herein, the content of other elements shall be determined by rounding the measured value to the minimum number of decimal places specified in this disclosure.

[0042] (microstructure) Microstructure: Ferrite + Pearlite, or Pearlite The nitride component material relating to this 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 component material according to this disclosure contains an appropriate amount of alloying elements to achieve high core hardness, and therefore does not require quenching to obtain the necessary core hardness. Low-temperature annealing is performed to optimize the amount of Cr in the cementite, but since low-temperature annealing is performed below the transformation point of steel, the microstructure of the steel material remains the same as that in the as-hot forging state.

[0043] When steel with the chemical composition defined in this disclosure is hot forged and then cooled by air cooling or wind cooling without using accelerated cooling such as water cooling, the microstructure of the steel becomes ferrite + pearlite or pearlite. More specifically, the shaft portion of the raw material for nitrided parts has a microstructure consisting of ferrite + pearlite or pearlite at a depth of 5 mm from the surface. Having such a microstructure results in good machinability when manufacturing nitrided parts. However, if bainite is present in the microstructure of this area even when cooled by air cooling or wind cooling, it means that there is an excess of alloying elements, and the machinability deteriorates. The specific method of observing the microstructure will be explained in the examples.

[0044] Ratio of dissolved chromium to total chromium (dissolved chromium / total chromium): 0.10~0.75 The raw material for nitrided parts according to this disclosure contains a large amount of alloying elements to ensure the core hardness of the nitrided part, so if it is nitrided as is, the amount of deformation during nitriding will be large. To suppress deformation, between hot forging and nitriding, Cr is concentrated in the cementite by heat treatment such as low-temperature annealing, and the amount of solid-solution Cr in the ferrite is optimized. Specifically, at a depth of 5 mm from the surface of the shaft portion of the raw material for nitrided parts, the ratio of solid-solution Cr to total Cr (solid-solution Cr amount / total Cr amount) is set to the range of 0.10 to 0.75. If the ratio of solid-solution Cr to total Cr amount is too large, the amount of deformation during nitriding will be large. The preferred upper limit of the ratio of solid-solution Cr to total Cr amount is 0.70, and more preferably 0.68. If the ratio of solid-solution Cr to total Cr amount is too small, the fatigue strength will deteriorate. The preferred lower limit of the ratio of solid-solution Cr to total Cr amount is 0.20, and more preferably 0.30.

[0045] [Methods for manufacturing steel materials] An example of a method for manufacturing steel materials for producing nitrided component materials related to this disclosure includes the following steps. (Process 1) Material preparation process (Process 2) Hot working process The following describes each step.

[0046] (Process 1) Material preparation process In the material preparation process, the materials for the steel material of this disclosure are prepared. Specifically, molten steel satisfying the aforementioned chemical composition is produced. The refining method is not particularly limited, and any well-known method may be used. For example, molten iron produced by a well-known method is subjected to refining in a converter (primary refining). The molten steel tapped from the converter is subjected to a well-known secondary refining. In secondary refining, alloying elements are added to the molten steel to adjust its composition and produce molten steel having the aforementioned chemical composition.

[0047] The raw material is manufactured using molten steel produced by the aforementioned refining method, and a well-known casting method. For example, an ingot is manufactured using molten steel by the ingot-making method. Alternatively, a bloom or billet may be manufactured using molten steel by the continuous casting method. The raw material (ingot, bloom, or billet) is manufactured by the above method.

[0048] (Process 2) Hot working process Steel materials are manufactured by hot working the manufactured raw materials. In a hot working process, one or more hot working steps are typically performed. When multiple hot working steps are performed, the first hot working step may be, for example, rolling using bract rolling or hot forging, while subsequent hot working steps may be rolling using a continuous rolling mill. A continuous rolling mill is equipped with multiple rolling stands arranged in a row. The steel material is cooled to room temperature after hot working. Billets may be produced by rough rolling and rolling using a continuous rolling mill, and then the billets may be reheated and further finished rolling using a continuous rolling mill to produce steel material of the desired size. Alternatively, steel material may be produced from the raw material solely by hot forging. The heating temperature of the raw material during hot working is not particularly limited, but is, for example, 1100 to 1300°C.

[0049] Through the above process, steel materials for manufacturing nitride component raw materials related to this disclosure are produced.

[0050] [Manufacturing method for raw materials for nitrided parts] An example of a method for manufacturing a nitrided component material relating to this disclosure includes the following steps: (Process 3) Steel profile forming process (Step 4) Cr enrichment process in cementite

[0051] (Process 3) Steel profile forming process In the steel profile forming process, steel materials having the aforementioned chemical composition are used to form nitrided component profiles (steel profiles). One example of the forming method is hot forging. Specifically, first, a steel material having the aforementioned chemical composition is heated. The heating temperature is, for example, 1000 to 1300°C. The heated steel material is hot-forged to form a steel profile of a predetermined shape. The formed steel profile is then air-cooled. The cooling rate during air cooling may be adjusted as appropriate using a fan.

[0052] (Step 4) Cr enrichment process in cementite A heat treatment is performed on the steel profile after hot forging to concentrate the chromium in the matrix phase into the cementite. Specifically, the steel profile is heated at a predetermined temperature for a predetermined time, and then cooled to room temperature. The heating temperature is, for example, 600-650°C, and the heating time is, for example, 30-300 minutes. This heat treatment concentrates the chromium in the cementite, thereby reducing the amount of deformation during nitriding when manufacturing nitrided parts from the steel profile. Any treatment is acceptable as long as it concentrates a predetermined amount of Cr into the cementite and brings the ratio of solid-solution Cr to the total amount of Cr within the range of 0.10 to 0.75.

[0053] [Nitrided components] The nitrided component according to this disclosure comprises a nitrided layer formed on the surface and a core portion located inside the nitrided layer, the chemical composition of the core portion being the same as the chemical composition of the steel material and the raw material for nitrided components according to this disclosure as described above. In this disclosure, the surface of the nitrided component means the region from the surface to a depth of 1.5 mm, and the core portion means the region deeper than 1.5 mm from the surface. The maximum depth to which nitrogen penetrates from the surface due to nitriding is expected to be about 1.5 mm. In other words, the surface of the nitrided component refers to the region to which nitrogen can penetrate, and the core portion refers to the region to which nitrogen penetration due to nitriding does not reach. The nitrided component according to this disclosure has a core portion which is deeper than 1.5 mm from the surface and a nitrided layer which is located outside the core (surface).

[0054] The chemical composition and microstructure of the core of the nitrided component relating to this disclosure are the same as those of the raw material for the nitrided component relating to this disclosure described above, so a detailed explanation is omitted here. When steel is nitrided, a layer called a compound layer, mainly composed of iron nitride, is formed on the surface, and directly beneath it, a layer called a diffusion layer is formed, in which the matrix phase is reinforced with solid-solution nitrogen or alloy nitride. In this disclosure, the nitrided layer includes both the compound layer and the diffusion layer, the nitrogen content of the compound layer is, for example, 2 to 11%, and the nitrogen content of the diffusion layer is, for example, 0.0300 to 0.6000%.

[0055] [Manufacturing method for nitrided components] The nitrided component relating to this disclosure can be manufactured, for example, using the aforementioned nitrided component material. An example of a method for manufacturing the nitrided component relating to this disclosure using the aforementioned nitrided component material includes the following steps. (Process 5) Machining process (Step 6) Nitriding process The following describes each step.

[0056] (Process 5) Machining process In the machining process, the raw material for nitrided parts is machined to shape it to a form close to the final product shape. Specifically, well-known cutting and / or grinding processes are performed to shape the raw material for nitrided parts.

[0057] (Step 6) Nitriding process In the nitriding process, the steel material after the machining process is subjected to nitriding. Examples of nitriding methods include gas nitriding, gas soft nitriding, salt bath soft nitriding, and plasma nitriding. The gas used in the nitriding process may be NH3 alone, or it may be a well-known mixed gas containing NH3, N2, H2, CO2, and various hydrocarbons. The temperature and time of the nitriding treatment can be any conditions that allow the necessary hardened layer to form on the surface. The nitriding temperature may be 530-620°C, and the nitriding time may be 0.5-10 hours. The cooling method after nitriding may be water cooling, oil cooling, or furnace cooling. By the above manufacturing process, the nitrided component according to this disclosure can be manufactured. [Examples]

[0058] The materials for nitrided parts according to the present disclosure will be described in more detail by the examples below. The conditions in the following examples are just one example of conditions adopted to confirm the feasibility and effectiveness of the materials for nitrided parts and nitrided parts according to the present disclosure. Therefore, the materials for nitrided parts and nitrided parts according to the present disclosure are not limited to the following examples.

[0059] [Steel manufacturing] Steel materials A to P having the chemical compositions shown in Table 1 (the remainder being Fe and impurities) were manufactured by the following methods. A 50kg ingot was manufactured using a vacuum melting furnace. After heating the ingot to 1250°C, hot working was performed to produce steel material. Specifically, hot forging was carried out to create a steel bar with a cross-section perpendicular to the axial direction of 75mm x 75mm. The steel bar was heated again to 1250°C, then hot-forged into a steel bar with a cross-section of 60mm in diameter, and allowed to cool to room temperature. Through the above manufacturing process, steel material (steel bar) was produced.

[0060] [Table 1]

[0061] [Manufacturing of steel profiles] Using each of the manufactured steel materials as raw materials, steel profiles were produced in the following manner.

[0062] To simulate the steel profile forming process described in step 3 above, a hot forging simulation was performed on the steel material, which was held at 1150°C for 60 minutes and then air-cooled to room temperature.

[0063] After a simulated hot forging process, the steel bars were subjected to a low-temperature annealing treatment, which involved holding them at 600-700°C for 1.0-4.0 hours followed by furnace cooling, thereby enriching the cementite with chromium (Cr). Through the above manufacturing process, a steel profile material that simulates a nitrided component profile material was produced.

[0064] [evaluation] The following evaluations (Tests 1 to 5) were performed on each manufactured steel profile and on test pieces that simulated nitrided parts by cutting and nitriding each steel profile. (Test 1) Observation of the microstructure of steel profiles (Test 2) Evaluation of machinability of steel profiles (Test 3) Analysis of Cr concentration in cementite of steel profiles (Test 4) Evaluation of bending fatigue strength (Test 5) Evaluation of bending amount during nitriding The following describes each test.

[0065] (Test 1) Observation of the microstructure of steel profiles For each test number, a block-shaped test specimen for microstructural observation was cut from a cross-section perpendicular to the longitudinal direction of the steel profile, with the center of the examination surface located 5 mm from the surface. This specimen was then embedded in resin and mirror-polished. The polished surface was etched with Nital, and an optical microscope was used to examine the microstructure with a field of view of 1.56 mm². 2 We observed one location in that region at a magnification of 200x. None of the test samples contained martensite or bainite, and were composed of ferrite and pearlite, or pearlite alone.

[0066] (Test 2) Machinability evaluation of steel profiles (hardness test) The Vickers hardness of microstructure observation specimens for each test number was measured. Vickers hardness tests were performed at five arbitrary locations on the mirror-polished surface, in accordance with JIS Z 2244:2009. The test force was set to 9.8 N. The arithmetic mean of the five hardnesses obtained was defined as the Vickers hardness for the test number. A Vickers hardness of 300 HV or less was considered to indicate sufficient machinability.

[0067] (Test 3) Analysis of Cr concentration in cementite of steel profiles (calculation of solid solution Cr amount / total Cr amount) For each test number, a 5mm diameter, 40mm long electrolytic analysis specimen was prepared from a 5mm depth from the surface of a cross-section perpendicular to the axial direction of the steel profile (60mm diameter steel bar). The specimen was subjected to a 10% acetylacetone-1% tetramethylammonium chloride-methanol electrolyte at a current density of 20mA / cm².2 Electrolysis was performed. The solution after electrolysis was filtered by suction using a Clipore filter with 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 spectrometer, and this amount of Cr was considered to be the amount of Cr concentrated in the cementite. The amount of solid-solution Cr was considered to be the total amount of Cr minus the amount of Cr concentrated in the cementite. Then, the mass ratio of the amount of solid-solution chromium to the total amount of chromium contained in the steel profile material (amount of solid-solution chromium / total amount of chromium) was calculated.

[0068] (Test 4) Evaluation of bending fatigue strength For each test number, an Ono-type rotary bending fatigue test specimen was prepared from the midpoint of the radius (R / 2 position) of the cross-section perpendicular to the axial direction of the steel profile (60 mm diameter steel bar), as shown in Figure 1. The numbers in Figure 1 indicate dimensions (in mm). In Figure 1, "φ" means diameter. The same applies to Figures 2 and 3. "R3" means that the radius of curvature of the notch bottom is 3 mm.

[0069] Specifically, the steel profiles for each test number were machined (cut) to produce intermediate specimens for the Ono-type rotary bending fatigue test. Nitriding treatment was then performed on these intermediate specimens to produce the Ono-type rotary bending fatigue test specimens shown in Figure 1. The conditions for the nitriding treatment were as follows: In the nitriding treatment, the intermediate specimens were held at 570°C for 3 hours in an atmosphere of RX gas (endothermic modified gas) and ammonia gas in a 1:1 ratio. After holding, the intermediate specimens were oil-cooled. Through these steps, Ono-type rotary bending fatigue test specimens simulating nitrided parts were produced.

[0070] Ono-type rotary bending fatigue tests were performed using Ono-type rotary bending fatigue test specimens for each test number. Multiple Ono-type rotary bending fatigue test specimens were prepared for each test number. Fatigue tests were conducted by varying the stress applied to each specimen, and the results were obtained for 10 million cycles (10 7After several cycles, the highest stress at which fracture did not occur was defined as the bending fatigue strength (MPa). In the Ono rotary bending fatigue test, the rotation speed was set to 3000 rpm, and the stress ratio was set to bidirectional. The obtained bending fatigue strengths are shown in the "Bending Fatigue Strength (MPa)" column under the "Nitrided Components" column in Table 2. A fatigue strength of 540 MPa or higher was considered sufficient.

[0071] (Test 5) Evaluation of bending amount during nitriding A φ15 × 100 mm test specimen for bending measurement, as shown in Figure 2, was prepared from the radial center position (R / 2 position) of the cross-section perpendicular to the axial direction of each test number's steel profile (60 mm diameter steel bar). The test specimens were subjected to nitriding treatment, held at 570°C for 3 hours in an atmosphere of RX gas and ammonia gas in a 1:1 ratio, followed by oil cooling. The amount of curvature of the nitrided specimen was measured using a contact-type 3D measuring machine. Specifically, as shown in Figure 3, a reference circle was determined from the cross-section at three locations: 10 mm from both ends of the specimen and at the midpoint in the longitudinal direction. The center of this reference circle was determined, and the distance between the line (A1) connecting the center coordinates of the reference circles at 10 mm from both ends and the center coordinates of the reference circle at the midpoint in the longitudinal direction (A0) was considered to be the amount of curvature. The method for determining the reference circle is described below. At the location where the reference circle was to be determined, the coordinates of eight points on the circumference were measured at 45° intervals. An approximate circle was obtained from the coordinates of each of the eight points and used as the reference circle at that location. If the amount of curvature was 2.0 μm or less, it was considered that the curvature could be sufficiently suppressed.

[0072] Table 2 shows the heat treatment conditions after the simulated hot forging process, as well as the evaluation results of tests 2-5.

[0073] [Table 2]

[0074] In the test numbers that met the requirements of this disclosure, the bending fatigue strength was 540 MPa or higher, and the bending amount was 2.0 μm or less. Test number 1 had an insufficient carbon content and inadequate bending fatigue strength. Test number 4 had an excessive C content and poor machinability. Test number 6 had an insufficient Mn content and therefore inadequate bending fatigue strength. Test number 8 had an excessive Mn content and poor machinability. Test number 9 had an insufficient chromium content and inadequate bending fatigue strength. Test number 11 had an excessive Cr content and too much curvature. In tests 17 and 18, because no heat treatment was performed, Cr was not concentrated in the cementite, resulting in a high ratio of solid-solution Cr to total Cr, and thus excessive bending. In test number 19, the heat treatment temperature was too high, resulting in excessive concentration of chromium in the cementite. This led to a low ratio of dissolved chromium to total chromium, and consequently, insufficient bending fatigue strength.

Claims

1. Including a shaft portion with a diameter of 10 mm or more, The chemical composition is expressed in mass percent. C: 0.35-0.60%, Si: 0.03-0.35%, Mn: 1.00-2.10%, P: 0.050% or less, S: 0.010-0.095%, Cr: more than 0.60 to 1.20%, Al: 0.001-0.080%, N: 0.0040-0.0250%, with the remainder being Fe and impurities, which constitute the raw material. At a depth of 5 mm from the surface of the shaft portion, it has a microstructure consisting of ferrite and pearlite, or pearlite. A nitrided component material wherein, at a depth of 5 mm from the surface of the shaft portion, the ratio of the amount of solid-solution Cr to the total amount of Cr is 0.10 to 0.

75.

2. Including a shaft portion with a diameter of 10 mm or more, The chemical composition is expressed in mass percent. C: 0.35-0.60%, Si: 0.03-0.35%, Mn: 1.00-2.10%, P: 0.050% or less, S: 0.010-0.095%, Cr: more than 0.60 to 1.20%, Al: 0.001-0.080%, The material is composed of N: 0.0040 to 0.0250%, and further contains one or more elements selected from the groups consisting of the following groups 1, 2, and 3, with the remainder being Fe and impurities. At a depth of 5 mm from the surface of the shaft portion, it has a microstructure consisting of ferrite and pearlite, or pearlite. A nitrided component material wherein, at a depth of 5 mm from the surface of the shaft portion, the ratio of the amount of solid-solution Cr to the total amount of Cr is 0.10 to 0.

75. [Group 1] Cu: 0.40% or less, Ni: 0.40% or less, V: 0.20% or less, Mo: One or more types selected from the group consisting of 0.30% or less. [Group 2] Sn: 0.10% or less, Ca: One or two types selected from the group consisting of 0.0050% or less [Group 3] Ti: 0.100% or less

3. The nitrided component material according to claim 2, wherein the chemical composition contains the first group.

4. The nitrided component material according to claim 2, wherein the chemical composition contains the second group.

5. The nitrided component material according to claim 2, wherein the chemical composition contains the third group.

6. Including a shaft portion with a diameter of 10 mm or more, A component having a core portion which is deeper than 1.5 mm from the surface, and a nitrided layer which is located outside the core portion, The chemical composition of the core is, in mass%, C: 0.35-0.60%, Si: 0.03 to 0.35%, Mn: 1.00-2.10%, P: 0.050% or less, S: 0.010-0.095%, Cr: more than 0.60 to 1.20%, Al: 0.001-0.080%, N: 0.0040 to 0.0250%, optionally containing one or more elements selected from the following groups 1, 2, and 3, with the remainder consisting of Fe and impurities. At a depth of 5 mm from the surface of the shaft portion, it has a microstructure consisting of ferrite and pearlite, or pearlite. A nitrided component in which the ratio of the amount of solid-solution Cr to the total amount of Cr at a depth of 5 mm from the surface of the shaft portion is 0.10 to 0.

75. [Group 1] Cu: 0.40% or less, Ni: 0.40% or less, V: 0.20% or less, Mo: One or more types selected from the group consisting of 0.30% or less. [Group 2] Sn: 0.10% or less, Ca: One or two types selected from the group consisting of 0.0050% or less [Group 3] Ti: 0.100% or less

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