Steel parts

Abstract

This invention provides a steel component that can suppress crack formation during cold forging and possess high core strength, even when cold forging is performed without spheroidizing annealing. [Solution] The steel part of this embodiment is provided with a cylindrical portion, and the chemical composition of the core of the cylindrical portion is, in mass%, C: 0.30~0.60%, Si: 0.01~0.10%, Mn: 0.30~2.00%, P: less than 0.050%, S: 0.001~0.050%, Al: 0.001~0.200%, N: less than 0.020%, Cr: 0.01~0.40%, O: 0.0030% or less, and the remainder being Fe and impurities, the Vickers hardness of the hardened layer is 500 HV or more, the total area ratio of ferrite and pearlite in the core is 90% or more, the pearlite area ratio is 40% or more, and the arithmetic mean WP of the widths of a plurality of pearlite grains intersecting a 150 μm line segment perpendicular to the axial direction of the cylindrical portion and the standard deviation σP of said widths satisfy equation (1). σP / WP<0.80 (1)

Description

[Technical Field]

[0001] This disclosure relates to steel components. [Background technology]

[0002] Steel parts such as shafts and gears used in automobile parts, industrial machinery parts, and construction machinery parts are manufactured by plastically deforming the steel material. Plastic deformation mainly consists of hot forging and cold forging. In hot forging, the steel material is heated to a high temperature in the austenite range and then forged. At high temperatures, the strength of the steel material decreases. Therefore, compared to cold forging, hot forging can impart large plastic deformation to the steel material with a lower load. On the other hand, in the process of cooling the steel material to room temperature after plastic deformation at high temperature, the steel material shrinks due to thermal deformation and changes shape due to phase transformation. Therefore, the shape accuracy of the steel material (intermediate part) after hot forging is low. As a result, a large amount of steel material has to be removed in the next machining process, which increases costs.

[0003] On the other hand, cold forging involves forging steel at room temperature. Cold forging is less prone to thermal distortion. Therefore, the shape of the steel material (intermediate part) after cold forging can be made closer to that of the final part. This allows for reduced machining costs.

[0004] However, since cold forging is performed at room temperature, it is necessary to suppress the occurrence of cracks in the steel during cold forging. In other words, the steel must have excellent cold forgeability. To obtain excellent cold forgeability, spheroidizing annealing is usually performed on the steel before cold forging. In spheroidizing annealing, the steel is held for a long time in the temperature range just below the Ac1 point, and then slowly cooled. This breaks down the layered cementite in the pearlite of the steel, causing the cementite to become spheroidal. As a result, the hardness of the steel decreases, and its cold forgeability increases.

[0005] However, spheroidizing annealing increases manufacturing costs. Therefore, in order to reduce manufacturing costs associated with reducing man-hours, there is a need for steel materials that suppress crack formation during cold forging even when spheroidizing annealing is omitted.

[0006] The steel material disclosed in Japanese Patent Publication No. 2005-133153 (Patent Document 1) contains, by mass%, C: 0.05~0.3%, Si: 0.01~0.35%, Mn: 0.2~2%, P: 0.001~0.02%, S: 0.001~0.02%, and also contains at least one selected from Ni: 0.01~2%, Cr: 0.01~2%, and Mo: 0.01~0.5%, and further contains Ti: 0.005~0.02%, Al: 0.020~0.1%, and N: 0.005~0.02%, with the remainder being Fe and unavoidable impurities, and further satisfies Al / (N-Ti / 3.4)≧4 for Ti, Al, and N. Patent Document 1 states that this steel material has excellent cold forging properties by satisfying the above formula. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2005-133153 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, even if cold forging is performed by omitting spheroidizing annealing using means other than those disclosed in Patent Document 1, the occurrence of cracks during cold forging may still be suppressed. Furthermore, steel parts such as shafts and gears are generally subjected to surface hardening treatments. Examples of surface hardening treatments include high-frequency induction hardening, carburizing (including carbonitriding), and tempering (quenching and tempering). Surface hardening treatments create a hardened layer on the surface of steel parts. This increases the surface fatigue strength of the steel parts. Recently, there has been a demand for miniaturization and higher rotational speeds in steel components that transmit rotational power, such as shafts and gears. To achieve higher rotational speeds, high strength is required not only in the hardened layer of the steel component but also in the core. Therefore, steel components are required to have two conflicting characteristics: excellent cold forgeability that suppresses crack formation during cold forging even when cold forging is performed without spheroidizing annealing, and high strength in the core.

[0009] The purpose of this disclosure is to provide a steel part that can suppress crack formation during cold forging and has high strength in the core, even when cold forging is performed without spheroidizing annealing. [Means for solving the problem]

[0010] The steel components disclosed herein are It has a cylindrical section with a circular cross-section perpendicular to the axial direction, The aforementioned cylindrical portion is A hardened layer formed on the surface, It comprises a core portion located inside the aforementioned hardened layer, The chemical composition of the core is, in mass%, C: 0.30~0.60%, Si: 0.01~0.10%, Mn: 0.30~2.00%, P: Less than 0.050% S: 0.001~0.050%, Al: 0.001~0.200%, N: Less than 0.020% Cr: 0.01~0.40%, and, Contains O: 0.0030% or less, The remainder consists of Fe and impurities. The Vickers hardness of the hardened layer is 500 HV or higher. In the core portion, the total area ratio of ferrite and pearlite is 90% or more, and the area ratio of pearlite is 40% or more. In the core part, the arithmetic mean value WP of the widths of a plurality of pearlite grains intersecting a 150-μm line segment perpendicular to the axial direction of the cylindrical part and the standard deviation σP of the widths of the plurality of pearlite grains satisfy Equation (1). σP / WP < 0.80 (1)

[0011] The steel component of the present disclosure includes a cylindrical part having a circular cross-section perpendicular to the axial direction, The cylindrical part includes a hardened layer formed on the surface layer and a core part inside the hardened layer, The chemical composition of the core part is, in mass%, C: 0.30 to 0.60%, Si: 0.01 to 0.10%, Mn: 0.30 to 2.00%, P: less than 0.050%, S: 0.001 to 0.050%, Al: 0.001 to 0.200%, N: less than 0.020%, Cr: 0.01 to 0.40%, and O: 0.0030% or less, and contains further contains one or more selected from the group consisting of Group 1 or Group 2, the balance consists of Fe and impurities, the Vickers hardness of the hardened layer is 500 HV or more, In the core part, the total area ratio of ferrite and pearlite is 90% or more, and the pearlite area ratio is 40% or more, In the core part, the arithmetic mean value WP of the widths of a plurality of pearlite grains intersecting a 150-μm line segment perpendicular to the axial direction of the cylindrical part and the standard deviation σP of the widths of the plurality of pearlite grains satisfy Equation (1). [Group 1] Mo: 0.20% or less, V: 0.150% or less, Nb: 0.050% or less, Ti: 0.050% or less, Cu: 0.40% or less, Ni: 0.40% or less, Sn: 0.100% or less, and, B: Select one or more from the group consisting of 0.0050% or less. [Group 2] Ca: 0.0050% or less, One or more selected from the group consisting of Mg: 0.0050% or less. σP / WP<0.80 (1) [Effects of the Invention]

[0012] In the steel parts of this disclosure, even if spheroidizing annealing is omitted and cold forging is performed, the occurrence of cracks during cold forging can be suppressed, and the core has high strength. [Modes for carrying out the invention]

[0013] The inventors first investigated, from the perspective of chemical composition, steel parts that can sufficiently suppress crack occurrence even when cold forging is performed without spheroidizing annealing. As a result, the chemical composition of the core of the steel part, which corresponds to the chemical composition of the raw steel material, is as follows in mass%, C: 0.30~0.60%, Si: 0.01~0.10%, Mn: 0.30~2.00%, P: less than 0.050%, S: 0.001~0.050%, Al: 0.001~0.200%, N: less than 0.020%, Cr: 0.01~0.40%, O: 0.0030% or less, Mo: 0~0.20%, V: 0~ The inventors believe that if the chemical composition consists of 0.150%, Nb: 0-0.050%, Ti: 0-0.050%, Cu: 0-0.40%, Ni: 0-0.40%, Sn: 0-0.100%, B: 0-0.0050%, Ca: 0-0.0050%, Mg: 0-0.0050%, and the remainder being Fe and impurities, then crack formation can be sufficiently suppressed even if cold forging is performed without spheroidizing annealing.

[0014] Therefore, the inventors further investigated, from the perspective of microstructure, means to sufficiently suppress crack occurrence and increase the strength of the core even when cold forging is performed without spheroidizing annealing before cold forging.

[0015] The inventors investigated the mechanism of cracking during cold forging. As a result, the following was discovered.

[0016] Cracking during cold forging is caused by ductile fracture. In ductile fracture, voids are formed in areas of localized strain distribution differences within the steel material of the steel part. These voids expand or connect with other voids, causing the crack to propagate. Repeated expansion and connection of these cracks leads to fracture.

[0017] In the case of the chemical composition described above, a microstructure consisting mainly of ferrite and pearlite is formed. Generally, pearlite is harder than ferrite. When cold forging is performed on such steel, strain concentrates near the interface between ferrite and pearlite grains, causing localized stress concentration in the ferrite. As a result, the ferrite is thought to deform excessively, leading to the formation of voids. Furthermore, if there is a large variation in the pearlite grains, excessive deformation occurs locally in some of the ferrite. In this case as well, voids are thought to be formed.

[0018] Taking the above into consideration, the inventors conducted further investigations. As a result, they found that in the microstructure of steel with the above chemical composition, increasing the pearlite area ratio results in pearlite-dominant deformation during cold forging, suppressing excessive deformation in ferrite. Furthermore, the inventors found that excessive deformation in ferrite can also be suppressed by suppressing the variation in pearlite grains.

[0019] Based on the above findings, the inventors conducted further investigations. As a result, they found that in a core having the above-described chemical composition, if the total area ratio of ferrite and pearlite is 90% or more, the pearlite area ratio is 40% or more, and the arithmetic mean WP of the widths of multiple pearlite grains intersecting a 150 μm line segment perpendicular to the axial direction of the cylindrical part of the steel component, and the standard deviation σP of the widths of the multiple pearlite grains satisfy equation (1), then crack formation can be sufficiently suppressed even if cold forging is performed without spheroidizing annealing. Furthermore, it was found that high strength can be obtained when the microstructure of the core is the structure described above. σP / WP<0.80 (1)

[0020] The steel component of this embodiment was completed based on the above-described technical concept and has the following configuration.

[0021] The steel component of the first configuration is It has a cylindrical section with a circular cross-section perpendicular to the axial direction, The aforementioned cylindrical portion is A hardened layer formed on the surface, It comprises a core portion located inside the aforementioned hardened layer, The chemical composition of the core is, in mass%, C: 0.30~0.60%, Si: 0.01~0.10%, Mn: 0.30~2.00%, P: Less than 0.050% S: 0.001~0.050%, Al: 0.001~0.200%, N: Less than 0.020% Cr: 0.01~0.40%, and, Contains O: 0.0030% or less, The remainder consists of Fe and impurities. The Vickers hardness of the hardened layer is 500 HV or higher. In the core portion, the total area ratio of ferrite and pearlite is 90% or more, and the area ratio of pearlite is 40% or more. In the core portion, the arithmetic mean WP of the widths of multiple pearlite grains intersecting the 150 μm line segment perpendicular to the axial direction of the cylindrical portion, and the standard deviation σP of the widths of the multiple pearlite grains satisfy equation (1). σP / WP<0.80 (1)

[0022] The steel component of the second configuration is It has a cylindrical section with a circular cross-section perpendicular to the axial direction, The aforementioned cylindrical portion is A hardened layer formed on the surface, It comprises a core portion located inside the aforementioned hardened layer, The chemical composition of the core is, in mass%, C: 0.30~0.60%, Si: 0.01~0.10%, Mn: 0.30~2.00%, P: Less than 0.050% S: 0.001~0.050%, Al: 0.001~0.200%, N: Less than 0.020% Cr: 0.01~0.40%, and, Contains O: 0.0030% or less, Furthermore, it contains one or more selected from the group consisting of Group 1 or Group 2, The remainder consists of Fe and impurities. The Vickers hardness of the hardened layer is 500 HV or higher. In the core portion, the total area ratio of ferrite and pearlite is 90% or more, and the area ratio of pearlite is 40% or more. In the core portion, the arithmetic mean WP of the widths of multiple pearlite grains intersecting the 150 μm line segment perpendicular to the axial direction of the cylindrical portion, and the standard deviation σP of the widths of the multiple pearlite grains satisfy equation (1). [Group 1] Mo: 0.20% or less V: 0.150% or less, Nb: 0.050% or less, Ti: 0.050% or less, Cu: 0.40% or less, Ni: 0.40% or less, Sn: 0.100% or less, and, B: Select one or more from the group consisting of 0.0050% or less. [Group 2] Ca: 0.0050% or less, One or more selected from the group consisting of Mg: 0.0050% or less. σP / WP<0.80 (1)

[0023] The third component of the steel is the second component of the steel, The chemical composition of the core contains the first group.

[0024] The fourth component of the steel is a steel component of the second or third component, The chemical composition of the core contains the second group.

[0025] The steel components of this embodiment will now be described. In the following description, unless otherwise specified, percentages related to elements refer to mass percentages.

[0026] [Configuration of steel components in this embodiment] The steel component of this embodiment includes a cylindrical portion. The cylindrical portion is a member that has a circular cross-section perpendicular to the axial direction and extends in the axial direction. The axial length of the cylindrical portion is not particularly limited. The cylindrical portion may be a solid cylinder, a hollow cylinder, a central shaft, or a hollow disc. A toothed profile may be formed on the outer circumference of the cylindrical portion. The steel component may also include members other than the cylindrical portion. Such members may, for example, have a polygonal cross-section perpendicular to the axial direction, or have an asymmetric shape. Examples of the steel component of this embodiment include shafts and gears.

[0027] The steel component further comprises a hardened layer and a core in the cylindrical portion. The hardened layer is formed on the surface of the cylindrical portion. The hardened layer may be formed on the entire surface of the cylindrical portion or on a part of the surface of the cylindrical portion. The hardened layer is formed by applying a surface hardening treatment to the raw material (intermediate product after cold forging), which is the material for the steel part. The surface hardening treatment is one or more of the following: high-frequency induction hardening, carburizing, and tempering. In this specification, carburizing treatment also includes carbonitriding treatment. The carburizing treatment may be gas carburizing or vacuum carburizing.

[0028] The hardened layer is, for example, a carburized hardened layer or a quenched hardened layer. The carburized hardened layer is formed by carburizing treatment. The quenched hardened layer is formed by high-frequency induction hardening treatment or tempering treatment. The microstructure of the hardened layer consists mainly of hard structures. Here, hard structures refer to martensite and / or bainite. Furthermore, "mainly" means that the area ratio of hard structures is 70% or more. It is common technical knowledge to those skilled in the art that the hardened layer will have these structures.

[0029] The core is the region inside the hardened layer. Furthermore, in the cylindrical portion, the surface area of ​​the portion where the hardened layer has not formed is also considered part of the core. The hardened layer and the core differ in hardness and microstructure. The hardened layer is harder than the core. Furthermore, in the microstructure of the hardened layer, the area ratio of hard tissue is 70% or more, while in the microstructure of the core, the area ratio of ferrite and pearlite is 90% or more. A person skilled in the art can easily distinguish between the hardened layer and the core.

[0030] [Features of the steel parts of this embodiment] The steel component of this embodiment satisfies the following characteristics. (Feature 1) The chemical composition of the core is as follows (by mass%): C: 0.30-0.60%, Si: 0.01-0.10%, Mn: 0.30-2.00%, P: less than 0.050%, S: 0.001-0.050%, Al: 0.001-0.200%, N: less than 0.020%, Cr: 0.01-0.40%, O: 0.0030% or less, M The composition is as follows: o: 0-0.20%, V: 0-0.150%, Nb: 0-0.050%, Ti: 0-0.050%, Cu: 0-0.40%, Ni: 0-0.40%, Sn: 0-0.100%, B: 0-0.0050%, Ca: 0-0.0050%, Mg: 0-0.0050%, and the remainder consists of Fe and impurities. (Feature 2) The Vickers hardness of the hardened layer is 500 HV or higher. (Feature 3) In the core, the total area ratio of ferrite and pearlite is 90% or more, and the area ratio of pearlite is 40% or more. (Feature 4) In the core, the arithmetic mean WP of the widths of multiple pearlite grains intersecting a 150 μm line segment perpendicular to the axial direction of the cylindrical part, and the standard deviation σP of the widths of the multiple pearlite grains, satisfy equation (1). σP / WP<0.80 (1) Features 1 through 4 are explained below.

[0031] [(Feature 1) Regarding chemical composition] The chemical composition of the core of the steel component in this embodiment contains the following elements:

[0032] C: 0.30~0.60% Carbon (C) enhances the hardenability of steel, thereby increasing the strength of steel parts manufactured using steel as a material. If the C content is less than 0.30%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the carbon content exceeds 0.60%, the strength of the steel material used for the steel parts will increase excessively, even if the content of other elements is within the range of this embodiment. As a result, the cold forgeability of the steel material will decrease. Therefore, the C content is 0.30-0.60%. The preferred lower limit for the C content is 0.32%, more preferably 0.34%, and even more preferably 0.36%. The preferred upper limit for the C content is 0.55%, more preferably 0.50%, and even more preferably 0.45%.

[0033] Si: 0.01~0.10% Silicon (Si) deoxidizes steel during the steelmaking process in the manufacturing of steel materials. If the Si content is less than 0.01%, the above effect cannot be fully obtained. On the other hand, if the Si content exceeds 0.10%, the strength of the steel material will increase excessively, even if the content of other elements is within the range of this embodiment. As a result, the cold forgeability of the steel material will decrease. Therefore, the Si content is 0.01 to 0.10%. The preferred lower limit for the Si content is 0.02%, and more preferably 0.03%. The preferred upper limit for the Si content is 0.09%, and more preferably 0.08%.

[0034] Mn: 0.30~2.00% Manganese (Mn) is A c3 The point is reduced to increase the amount of pearlite produced. Mn further enhances the strength of ferrite through solid solution strengthening. If the Mn content is less than 0.30%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mn content exceeds 2.00%, coarse Mn sulfides are produced in excess. As a result, even if the content of other elements is within the range of this embodiment, the cold forgeability of the steel material decreases. Therefore, the Mn content is 0.30-2.00%. The preferred lower limit of the Mn content is 0.35%, more preferably 0.40%, even more preferably 0.45%, and even more preferably 0.50%. The preferred upper limit for the Mn content is 1.90%, more preferably 1.80%, and even more preferably 1.70%.

[0035] P: Less than 0.050% Phosphorus (P) is an unavoidable impurity. In other words, the P content is greater than 0%. If the P content is 0.050% or more, even if the content of other elements is within the range of this embodiment, P will excessively segregate at the grain boundaries, reducing the grain boundary strength. As a result, the cold forgeability of the steel material will decrease. Therefore, the P content is less than 0.050%. A low phosphorus (P) content is preferable. However, excessively reducing the P content increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit for the P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the P content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0036] S: 0.001~0.050% Sulfur (S) combines with manganese (Mn) to form Mn sulfides. Mn sulfides improve the machinability of steel. If the S content is less than 0.001%, the above effect cannot be fully obtained. On the other hand, if the sulfur content exceeds 0.050%, even if the content of other elements is within the range of this embodiment, excessive coarse Mn sulfides will be formed. These coarse Mn sulfides become the starting point for cracks during cold forging. As a result, the cold forgeability of the steel material decreases. Therefore, the sulfur content is between 0.001% and 0.050%. The preferred lower limit for the S content is 0.002%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit for the S content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0037] Al: 0.001~0.200% Aluminum (Al) deoxidizes steel. If the Al content is less than 0.001%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Al content exceeds 0.200%, coarse Al-based inclusions will form even if the content of other elements is within the range of this embodiment. These coarse Al-based inclusions become the starting points for cracks during cold forging. As a result, the cold forgeability of the steel material decreases. Therefore, the Al content is between 0.001% and 0.200%. The preferred lower limit for the Al content is 0.003%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Al content is 0.150%, more preferably 0.100%, and even more preferably 0.080%. In this embodiment, the Al content refers to the total Al (Total-Al) content.

[0038] N: Less than 0.020% Nitrogen (N) is an unavoidable impurity. In other words, the N content is greater than 0%. N combines with Al, Ti, B, etc. to form nitrides. If the N content is 0.020% or more, even if the content of other elements is within the range of this embodiment, an excess of coarse nitrides will be formed. Coarse nitrides become the starting point for cracks during cold forging. Therefore, the cold forgeability of the steel material decreases. Therefore, the N content is less than 0.020%. A low nitrogen (N) content is preferable. However, excessively reducing the N content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit for the N content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the N content is 0.018%, more preferably 0.016%, and even more preferably 0.014%.

[0039] Cr: 0.01~0.40% Chromium (Cr) enhances the hardenability of steel and increases the hardness of steel parts after quenching. If the Cr content is less than 0.01%, the above effects cannot be fully obtained. On the other hand, if the chromium content exceeds 0.40%, the lamellar spacing in pearlite becomes excessively narrow. In this case, the hardness of pearlite increases excessively, and the hardness difference between pearlite and ferrite becomes excessively large. As a result, the cold forgeability of the steel decreases. Therefore, the Cr content is 0.01-0.40%. The preferred lower limit for the Cr content is 0.02%, and more preferably 0.03%. The preferred upper limit for the Cr content is 0.35%, and more preferably 0.30%.

[0040] O: 0.0030% or less Oxygen (O) is an unavoidable impurity. In other words, the O content is greater than 0%. O combines with other elements in the steel to form oxides. If the O content exceeds 0.0030%, even if the content of other elements is within the range of this embodiment, coarse oxides will be excessively formed. Coarse oxides become the starting point for cracks during cold forging. Therefore, the cold forgeability of the steel decreases. Therefore, the O content is 0.0030% or less. A low oxygen content is preferable. However, excessively reducing the oxygen content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit for oxygen content is 0.0001%, and more preferably 0.0002%. The preferred upper limit for the O content is 0.0027%, more preferably 0.0024%, and even more preferably 0.0020%.

[0041] The remainder of the chemical composition of the core of the steel part according to this embodiment 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 during the industrial production of the steel part, and are acceptable within a range that does not adversely affect the steel part according to this embodiment.

[0042] [About Optional Elements] The chemical composition of the core of the steel component in this embodiment may further contain one or more substances selected from the following groups, Group 1 and Group 2. [Group 1] Mo: 0.20% or less V: 0.150% or less, Nb: 0.050% or less, Ti: 0.050% or less, Cu: 0.40% or less, Ni: 0.40% or less, Sn: 0.100% or less, and, B: Select one or more from the group consisting of 0.0050% or less. [Group 2] Ca: 0.0050% or less, One or more selected from the group consisting of Mg: 0.0050% or less. These elements are arbitrary elements. The following describes each element.

[0043] [Group 1 (Mo, V, Nb, Ti, Cu, Ni, Sn and B)] Mo, V, Nb, Ti, Cu, Ni, Sn, and B are optional elements and may not be present. All of these elements increase the strength of steel parts manufactured using steel as the raw material.

[0044] Mo: 0.20% 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 present, Mo enhances the hardenability of steel, thereby increasing the strength of steel parts manufactured using steel as a material. Even a small amount of Mo will provide some degree of the above effect. On the other hand, if the Mo content exceeds 0.20%, the strength of the steel becomes excessively high, even if the content of other elements is within the range of this embodiment. As a result, the cold forgeability of the steel decreases. Therefore, the Mo content is 0-0.20%, and if present, the Mo content is 0.20% or less. The preferred lower limit for the Mo content is 0.01%, more preferably 0.05%, and even more preferably 0.08%. The preferred upper limit for the Mo content is 0.18%, more preferably 0.15%, and even more preferably 0.12%.

[0045] V:0.150% 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 produces V precipitates such as carbides and carbonitrides. These V precipitates enhance the strength of steel parts manufactured using steel as a material through precipitation strengthening. Even if only a small amount of V is present, the above effect can be obtained to some extent. However, if the V content exceeds 0.150%, coarse V precipitates are formed. These coarse V precipitates become the starting points for cracks during cold forging. Therefore, even if the content of other elements is within the range of this embodiment, the cold forgeability of the steel material decreases. Therefore, the V content is between 0 and 0.150%, and if present, the V content is 0.150% or less. The preferred lower limit for the V content is 0.001%, more preferably 0.010%, and even more preferably 0.030%. The preferred upper limit for the V content is 0.130%, more preferably 0.100%, and even more preferably 0.080%.

[0046] Nb: 0.050% or less Niobium (Nb) is an optional element and does not need to be included. In other words, the Nb content may be 0%. When Nb is present, that is, when the Nb content is greater than 0%, Nb forms Nb precipitates such as carbides, nitrides, and carbonitrides. These Nb precipitates enhance the strength of steel parts manufactured from steel materials through precipitation strengthening. Even a small amount of Nb can provide some degree of the above effect. However, if the Nb content exceeds 0.050%, coarse Nb precipitates will form. These coarse Nb precipitates become the starting points for cracks during cold forging. Therefore, even if the content of other elements is within the range of this embodiment, the cold forgeability of the steel material will decrease. Therefore, the Nb content is 0-0.050%, and if present, the Nb content is 0.050% or less. The preferred lower limit for the Nb content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit for the Nb content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0047] Ti: 0.050% 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 forms Ti precipitates such as carbides and nitrides. These Ti precipitates enhance the strength of steel fittings manufactured using steel as a material through precipitation strengthening. Even if only a small amount of Ti is present, the above effect can be obtained to some extent. However, if the Ti content exceeds 0.050%, coarse Ti precipitates will form. These coarse Ti precipitates become the starting points for cracks during cold forging. Therefore, even if the content of other elements is within the range of this embodiment, the cold forgeability of the steel material will decrease. Therefore, the Ti content is between 0 and 0.050%, and if present, the Ti content is 0.050% or less. The preferred lower limit for the Ti content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Ti content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0048] 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%. When present, i.e., when the Cu content is greater than 0%, Cu enhances the hardenability of steel, thereby increasing the strength of steel parts manufactured using steel as a material. Even a small amount of Cu will provide some degree of the above effect. However, if the Cu content exceeds 0.40%, the strength of the steel becomes excessively high, even if the content of other elements is within the range of this embodiment. As a result, the cold forgeability of the steel decreases. Therefore, the Cu content is 0-0.40%, and if present, the Cu content is 0.40% or less. The preferred lower limit for the Cu content is 0.01%, more preferably 0.02%, and even more preferably 0.04%. The preferred upper limit for the Cu content is 0.37%, more preferably 0.34%, and even more preferably 0.30%.

[0049] Ni: 0.40% or less Nickel (Ni) is an optional element and does not need to be included. In other words, the Ni content may be 0%. When present, i.e., when the Ni content is greater than 0%, Ni enhances the hardenability of steel and increases the strength of steel parts manufactured using steel as a material. Even a small amount of Ni will provide some degree of the above effect. However, if the Ni content exceeds 0.40%, the strength of the steel becomes excessively high, even if the content of other elements is within the range of this embodiment. As a result, the cold forgeability of the steel decreases. Therefore, the Ni content is 0-0.40%, and if present, the Ni content is 0.40% or less. The preferred lower limit for the Ni content is 0.01%, more preferably 0.02%, and even more preferably 0.04%. The preferred upper limit for the Ni content is 0.35%, more preferably 0.30%, and even more preferably 0.25%.

[0050] Sn: 0.100% or less Tin (Sn) is an optional element and does not need to be included. In other words, the Sn content may be 0%. When sn is present, that is, when the sn content is greater than 0%, sn suppresses grain coarsening and increases the strength of the steel. 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 0.100%, excessive segregation of Sn occurs, causing the steel to become brittle. Therefore, even if the content of other elements is within the range of this embodiment, the cold forgeability of the steel decreases. Therefore, the Sn content is between 0 and 0.100%, and if present, the Sn content is 0.100% or less. The preferred lower limit for the Sn content is 0.001%, more preferably 0.002%, and even more preferably 0.004%. The preferred upper limit for the Sn content is 0.090%, more preferably 0.080%, and even more preferably 0.070%.

[0051] B: 0.0050% or less Boron (B) is an optional element and does not need to be included. In other words, the B content may be 0%. When present, i.e., when the B content is greater than 0%, B enhances the hardenability of steel. As a result, it increases the strength of steel parts manufactured using steel as a material. Even a small amount of B can provide some degree of the above effect. However, if the B content exceeds 0.0050%, coarse B nitrides are formed. These coarse B nitrides become the starting points for cracks during cold forging. Therefore, even if the content of other elements is within the range of this embodiment, the cold forgeability of the steel material will decrease. Therefore, the B content is 0-0.0050%, and if present, the B content is 0.0050% or less. The preferred lower limit for the B content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit for the B content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%.

[0052] [Group 2: Ca and Mg] The chemical composition of the core of the steel component in this embodiment may further include one or more elements selected from the group consisting of Ca and Mg, in place of a portion of Fe. These elements are arbitrary and all refine the Mn sulfide in the steel, thereby improving the cold forgeability of the steel.

[0053] Ca:0.0050% or less Calcium (Ca) is an optional element and does not need to be included. In other words, the Ca content may be 0%. When calcium is present, i.e., when the calcium content is greater than 0%, the calcium refines the manganese sulfides. This improves the cold forgeability of the steel. Even a small amount of calcium can provide some degree of this effect. However, if the Ca content exceeds 0.0050%, coarse Ca oxides will be formed, even if the content of other elements is within the range of this embodiment. Coarse Ca oxides become the starting point for cracks during cold forging. As a result, the cold forgeability of the steel material decreases. Therefore, the Ca content is between 0 and 0.0050%, and if present, the Ca content is 0.0050% or less. The preferred lower limit for the Ca content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The preferred upper limit for the Ca content is 0.0040%, and more preferably 0.0030%.

[0054] Mg: 0.0050% or less Magnesium (Mg) is an optional element and does not need to be included. In other words, the Mg content may be 0%. When Mg is present, that is, when the Mg content is greater than 0%, Mg refines Mn sulfides. This improves the cold forgeability of the steel. Even a small amount of Mg can provide some degree of this effect. However, if the Mg content exceeds 0.0050%, coarse Mg oxides will be formed, even if the content of other elements is within the range of this embodiment. Coarse Mg oxides become the starting point for cracks during cold forging. As a result, the cold forgeability of the steel material decreases. Therefore, the Mg content is between 0 and 0.0050%, and if present, the Mg content is 0.0050% or less. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The preferred upper limit for the Mg content is 0.0040%, and more preferably 0.0030%.

[0055] [(Feature 2) Regarding the Vickers hardness of the hardened layer] In the steel component of this embodiment, the Vickers hardness of the hardened layer is further 500 HV or higher. The preferred lower limit for the Vickers hardness of the hardened layer is 520 HV or higher, and more preferably 550 HV or higher. There is no particular upper limit to the Vickers hardness of the hardened layer. For example, the upper limit to the Vickers hardness of the hardened layer may be 850 HV or 900 HV.

[0056] [Regarding the measurement method of Vickers hardness of the hardened layer] The Vickers hardness of the hardened layer is measured by the following method. A test specimen is taken from the cylindrical portion of a steel part, containing the central axis of the cylinder, having a cross-section parallel to the central axis, and including the surface of the cylinder. The surface of the test specimen is designated as the measurement surface. On the measurement surface, the Vickers hardness (HV) is determined at five arbitrary measurement points located 50 μm deep from the surface of the cylinder, in accordance with JIS Z-1 (2020). The test force is 9.8 N. The arithmetic mean of the five obtained Vickers hardnesses is taken as the Vickers hardness (HV) of the hardened layer. [(Feature 3) Regarding the microstructure of the core] In the steel component of this embodiment, the total area ratio of ferrite and pearlite in the core of the cylindrical portion is 90% or more, and the pearlite area ratio is 40% or more.

[0057] As mentioned above, when cold forging is performed without spheroidizing annealing, it becomes necessary to suppress the occurrence of cracks during cold forging. Cracks that occur during cold forging are mainly caused by ductile fracture. In ductile fracture, strain concentrates at the interface between ferrite and pearlite in the steel, creating voids. Multiple voids that are created connect, causing cracks to occur and propagate.

[0058] In the microstructure of the core of the cylindrical steel part of this embodiment, the total area ratio of ferrite and pearlite is 90% or more, and furthermore, the area ratio of pearlite is 40% or more. By increasing the total area ratio of ferrite and pearlite in the microstructure, the occurrence of cracks during cold forging can be suppressed. Furthermore, by increasing the area ratio of pearlite in the microstructure to 40% or more, the proportion of pearlite in the deformation during cold forging can be increased, thereby suppressing excessive deformation of ferrite during cold forging. If the pearlite area ratio is 40% or more, the strength of the core is further increased.

[0059] If other tissues besides ferrite and pearlite are present in the core microstructure, the remaining tissues other than ferrite and pearlite consist of, for example, hard tissues (bainite and / or martensite).

[0060] The preferred lower limit for the total area ratio of ferrite and pearlite is 91%, more preferably 93%, and even more preferably 95%. The total area ratio of ferrite and pearlite may be 100%.

[0061] The preferred lower limit for the perlite area ratio is 42%, more preferably 44%, and even more preferably 46%. The preferred upper limit for the perlite area ratio is 90%, more preferably 85%, and even more preferably 80%.

[0062] [Method for measuring ferrite area ratio and pearlite area ratio] The ferrite area ratio and pearlite area ratio in the core are measured by the following method.

[0063] A test specimen is taken from the cylindrical portion of the steel part, having a cross-section that includes the central axis of the cylinder and is parallel to the central axis, and also including the surface of the cylinder. Of the surface of the test specimen, this cross-section is designated as the observation surface. On the observation surface, the observation field is defined as a rectangular area with a center point 2.0 mm deep from the surface of the cylinder, and an area of ​​120 μm in the axial direction and 160 μm in the radial direction. Five observation fields are selected.

[0064] Within the observation field, each tissue (ferrite, pearlite, and hard tissues (bainite and martensite)) differs in contrast and morphology. Therefore, each tissue is identified as follows based on its contrast and morphology: White, polygonal grains without internal structure are identified as ferrite. A structure in which bright white areas and dark black areas are mixed in a striped pattern is identified as pearlite. A structure with narrow plate-like or needle-like structures, accompanied by black precipitates inside or at the boundaries of these narrow plate-like or needle-like structures, is identified as hard tissue. In this embodiment, it is not necessary to strictly distinguish between bainite and martensite in the microstructure of the steel material.

[0065] Determine the total area of ​​ferrite and the total area of ​​pearlite in each of the five observation fields. Based on the total area of ​​ferrite, the total area of ​​pearlite, and the total area of ​​the five observation fields, calculate the ferrite area percentage (%) and the pearlite area percentage (%). The ferrite area percentage (%) is rounded to the nearest integer by rounding the obtained value to the first decimal place. The pearlite area percentage (%) is rounded to the nearest integer by rounding the obtained value to the nearest integer by rounding the obtained value to the first decimal place.

[0066] [(Feature 4) Regarding the perlite particle width in the core and the variation in perlite particle width] In the steel component of this embodiment, the arithmetic mean WP of the widths of multiple pearlite grains intersecting a 150 μm line segment perpendicular to the axial direction of the cylindrical portion, and the standard deviation σP of the widths of the multiple pearlite grains, satisfy equation (1). σP / WP<0.80 (1)

[0067] Here, a perlite grain refers to a continuous region of perlite tissue. When adjacent perlite colonies and pseudo-perlites are in contact with each other, they are considered a single perlite grain. Pseudo-perlite refers to perlite in which fragmented lamellar cementite and / or spheroidized cementite are arranged in a dotted pattern.

[0068] If the width WP (μm) of the pearlite grains and the standard deviation σP (μm) of the pearlite grain widths satisfy equation (1), then the variation in pearlite grain width in the core is sufficiently small. In this case, localized stress concentration during cold forging caused by variations in pearlite grain size can be alleviated, and uniform deformation can be promoted. As a result, even if cold forging is performed without spheroidizing annealing, the occurrence of cracks can be sufficiently suppressed.

[0069] F1 is defined as follows: F1 = σP / WP A preferred upper limit for F1 is 0.78, more preferably 0.75, and even more preferably 0.70. The lower limit of F1 is not particularly limited. For example, the lower limit of F1 could be 0.10, or for example, 0.20.

[0070] [Method for measuring perlite grain width WP and standard deviation σP] The width WP (μm) and standard deviation σP (μm) of pearlite grains in the core of a cylindrical steel part are measured by the following method.

[0071] In the five observation fields selected using the above-described [Method for Measuring Ferrite Area Ratio and Pearlite Area Ratio], pearlite grains are identified based on the contrast and morphology described above. In each observation field, a line segment is drawn perpendicular to the axial direction of the cylindrical portion. The length of the line segment is set to 150 μm. Pearlite grains overlapping the line segment are identified. At this time, the position where the line segment is drawn is selected so that there are five or more pearlite grains overlapping the line segment. For each pearlite that overlaps with a line segment, the length of the overlapping portion is defined as the width (μm) of that pearlite. The arithmetic mean of the widths of all pearlites obtained from the five observation fields is defined as the width WP (μm) of the pearlite grain. In this case, the width WP (μm) is the value obtained by rounding the second decimal place of the arithmetic mean to the first decimal place.

[0072] Furthermore, the sample standard deviation of the widths of all pearlite grains obtained from the five observation fields is defined as the standard deviation σP(μm). In this case, the standard deviation σP(μm) is the first decimal place obtained by rounding the second decimal place of the calculated value. Note that σP / WP is the second decimal place obtained by rounding the third decimal place of the obtained value.

[0073] [Effects of the steel parts of this embodiment] The steel component of this embodiment satisfies features 1 to 4. Therefore, even if spheroidizing annealing is omitted or cold forging is performed during manufacturing, the occurrence of cracks can be sufficiently suppressed in the steel component of this embodiment. Furthermore, high strength can be obtained in the core.

[0074] [Applications of the steel parts of this embodiment] The steel components of this embodiment can be widely applied as rotating bodies such as shafts and gears to automobile parts, industrial machinery parts, and construction machinery parts.

[0075] [Method for manufacturing steel parts of this embodiment] An example of a manufacturing method for the steel component of this embodiment will be described. The steel component having the above-described structure may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a manufacturing method for the steel component of this embodiment.

[0076] An example of a method for manufacturing steel parts according to this embodiment includes the following steps. (Process 1) Steelmaking process (Process 2) Blooming rolling process (Process 3) Finish rolling process (Process 4) Cold forging process (Step 5) Surface hardening treatment process (Process 6) Machining process The following describes each step.

[0077] [(Process 1) Steelmaking Process] In the steelmaking process, molten steel having a chemical composition satisfying characteristic 1 described above 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 refined in a converter (primary refining). The molten steel tapped from the converter is then refined using a well-known secondary refining method. Through these steps, molten steel having a chemical composition satisfying characteristic 1 is produced. Using the produced molten steel, blooms (cast slabs) are manufactured by a continuous casting method.

[0078] [(Process 2) Blossom rolling process] In the bract rolling process, a bract rolling mill is used to hot-roll (bract rolling) the bloom to produce a billet. If a continuous rolling mill with multiple rolling stands arranged in a row is located downstream of the bract rolling mill, the billet may be further hot-rolled using the continuous rolling mill to further reduce its size. The heating temperature in the bract rolling process can be within a well-known temperature range. For example, the heating temperature is 1100 to 1300°C. The billet produced in the bract rolling process is allowed to cool to room temperature (air-cooled) before the finish rolling process.

[0079] [(Process 3) Finish Rolling Process] In the finishing rolling process, hot rolling (finishing rolling) is performed on the billet to produce the steel material that will be used for the steel parts of this embodiment. Specifically, the billet after the bloc rolling process is heated in a heating furnace. The heating temperature is, for example, 1000 to 1300°C. After heating, hot rolling (finishing rolling) is performed on the billet using a finishing rolling mill in which multiple rolling stands are arranged in a row. Through the above manufacturing process, the steel material that will be used for the steel parts of this embodiment is produced.

[0080] [Regarding conditions during the manufacturing process] In the manufacturing process described above, the following conditions 1 to 4 must be met. (Condition 1) The solidification and cooling rate CR1 at a depth of 1 / 4 of the short side of the bloom, from the bloom surface corresponding to the long side in a cross-section perpendicular to the longitudinal direction of the bloom during casting, shall be 8.0°C / min or higher. (Condition 2) The finishing rolling temperature (FT) should be kept below 850°C. (Condition 3) The average cooling rate CR2 from the finishing rolling temperature FT until the steel surface temperature reaches 500°C shall be less than 0.05 to 1.00°C / second. (Condition 4) The FA defined by formula (A) shall be 5.0 or greater. FA=10000×CR1 / {(CR2×60)×FT} (A) Conditions 1 through 4 will be explained below.

[0081] [(Condition 1) Regarding the solidification and cooling rate CR1] In the steelmaking process, the average cooling rate from the liquidus temperature to the solidus temperature within the temperature range of steel during casting is defined as the solidification cooling rate CR1 (°C / min). In the steelmaking process, the solidification cooling rate CR1 at a depth of 1 / 4 of the short side of the bloom, from the bloom surface corresponding to the long side of the cross section perpendicular to the longitudinal direction of the bloom during casting, is defined as 8.0°C / min or higher.

[0082] If the solidification and cooling rate CR1 is too slow, the crystal grains will become coarse, and the variation in crystal grain size will increase. In this case, the pearlite grain width PW and the standard deviation σP of the pearlite grain width may not satisfy equation (1). Therefore, the solidification and cooling rate CR1 should be set to 8.0°C / min or higher.

[0083] [Method for measuring the solidification cooling rate CR1] The solidification and cooling rate CR1 can be determined by the following method: Take a specimen from a rectangular cross-section perpendicular to the longitudinal direction of a bloom produced by continuous casting, including a position 1 / 4 of the way down from the long side of the bloom surface to the short side of the bloom. More specifically, take a specimen from the cross-section that has a 5 mm × 5 mm observation area on its surface, centered on a position 1 / 4 of the way down from the width center of the long side of the bloom surface to the short side of the bloom. One side of the observation area is parallel to the long side of the bloom, and the other pair of sides of the observation area is parallel to the short side of the bloom. In the observation area, the spacing between 10 dendrite secondary arms is measured, and the arithmetic mean is taken as λ² (μm). Using the measured dendrite secondary arm spacing λ² (μm), the solidification and cooling rate CR1 (°C / min) is calculated using the following formula. CR1 = (λ² / 770) -1 / 0.41

[0084] [(Condition 2) Regarding the finishing rolling temperature FT] In the finishing rolling process, the surface temperature of the steel material at the exit of the rolling stand that last rolled the steel material down in the continuous rolling mill is defined as the finishing rolling temperature FT (°C). The finishing rolling temperature FT (°C) can be measured using a thermometer installed at the exit of the rolling stand. If the finishing rolling temperature FT is 850°C or higher, the rolling temperature in the finishing rolling process is too high. In this case, the variation in pearlite grains will increase. Therefore, the pearlite grain width PW and the standard deviation of the pearlite grain width σP may not satisfy equation (1). For this reason, the finishing rolling temperature should be set to less than 850°C. The lower limit of the finishing rolling temperature FT is, for example, 750°C.

[0085] [(Condition 3) Regarding the average cooling rate CR2] The average cooling rate from the finishing rolling temperature FT until the surface temperature of the steel reaches 500°C is defined as the average cooling rate CR2 (°C / second). In this embodiment, the average cooling rate CR2 is set to 0.05 to less than 1.00°C / second.

[0086] If the average cooling rate CR2 is too slow, the variation in pearlite grains will increase. As a result, the pearlite grain width PW and the standard deviation σP may not satisfy equation (1). Also, the ferrite area ratio may increase, and the pearlite area ratio may become excessively small. On the other hand, if the average cooling rate CR2 is too fast, the difference in hardness between ferrite and pearlite in the steel material before cold forging will become excessively large. As a result, the cold forgeability of the steel material will decrease. Therefore, the average cooling rate CR2 should be set to less than 0.05 to 1.00°C / second.

[0087] [(Condition 4) Regarding FA] In the manufacturing process of this embodiment, the FA defined by formula (A) is further set to 5.0 or higher. FA=10000×CR1 / {(CR2×60)×FT} (A) Even if conditions 1 to 3 are met, if FA is less than 5.0, the variability of pearlite grains will increase. As a result, the pearlite grain width PW and the standard deviation σP of the pearlite grain width may not satisfy equation (1). Furthermore, the ferrite area ratio may increase, causing the pearlite area ratio to become excessively small. Therefore, FA should be 5.0 or greater. Note that FA is the value of the calculated value rounded to the first decimal place.

[0088] Through the above manufacturing process, the steel material that is the raw material for the steel parts of this embodiment is produced.

[0089] [(Process 4) Cold Forging Process] Cold forging is performed on the manufactured steel material to produce raw materials (intermediate products). In this embodiment, spheroidizing annealing is not required for the steel material before cold forging. In this embodiment, even if spheroidizing annealing is omitted, the occurrence of cracks during cold forging can be sufficiently suppressed.

[0090] [(Step 5) Surface hardening treatment process] A surface hardening treatment is performed on the raw material after cold forging. The surface hardening treatment is one or more of the well-known high-frequency induction hardening, carburizing, and tempering treatments. The conditions for the surface hardening treatment may be any well-known conditions that result in a hardness of 500 HV or more of the hardened layer. Adjusting the conditions for the surface hardening treatment so that the hardness of the hardened layer is 500 HV or more is a well-known technique that can be appropriately set by those skilled in the art. A hardened layer is formed on the surface of the steel part by the surface hardening treatment process.

[0091] (Process 6) Machining process Machining is optional. If performed, the raw material after the surface hardening treatment is ground or cut to adjust the shape of the steel part.

[0092] The steel parts of this embodiment are manufactured through the above manufacturing process. [Examples]

[0093] The effects of one embodiment of the steel component of this embodiment will be further explained in detail by the following examples. The conditions in the following examples are just one example of conditions adopted to confirm the feasibility and effects of the steel component of this embodiment. Therefore, the steel component of this embodiment is not limited to this one example of conditions.

[0094] Steel materials having the chemical compositions shown in Table 1 (Tables 1A and 1B) were manufactured by the following method.

[0095] [Table 1A]

[0096] [Table 1B]

[0097] Blooms were produced by continuous casting using molten steel. The solidification and cooling rate CR1 (°C / min) was determined using a portion of the produced blooms based on the method described in [Method for Measuring Solidification and Cooling Rate CR1] above. The obtained solidification and cooling rates CR1 are shown in the "CR1 (°C / min)" column of the "Condition 1" column in Table 2.

[0098] [Table 2]

[0099] The manufactured bloom was subjected to a bract rolling process to produce billets. The bloom heating temperature during the bract rolling process was 1100-1300°C. Using the billets, a finish rolling process was performed to produce steel bars (steel bars) with a diameter of 60 mm. The heating temperature during the finish rolling process was 1100-1300°C. The finish rolling temperature FT(°C) during the finish rolling process is shown in the "FT(°C)" column of the "Condition 2" column in Table 2. The average cooling rate CR2 during the finish rolling process is shown in the "CR2(°C / sec)" column of the "Condition 3" column in Table 2. The FA is shown in the "FA" column of the "Condition 4" column in Table 2.

[0100] The manufactured steel bars were cut to a length of 80 mm. After cutting, a through-hole with a diameter of 20 mm was drilled into the center of the steel bar. The central axis of the through-hole was coaxial with the central axis of the steel bar.

[0101] A 30mm diameter raw material was produced by cold forging a steel bar with a through-hole formed in it, without spheroidizing annealing. The diameter of the through-hole in the raw material was approximately 15mm. Surface hardening treatment was performed on the raw material. Specifically, high-frequency induction hardening was performed as the surface hardening treatment. In high-frequency induction hardening, the raw material was heated using a high-frequency heating device under the conditions of 100kW power and 50kHz frequency, and then rapidly cooled. Specifically, jet cooling using cooling water was performed. Through the above manufacturing process, multiple cylindrical steel parts, including the hardened layer and core, were produced for each test number.

[0102] [Evaluation Test] The following tests were performed on the steel parts for each test number. (Test 1) Vickers hardness measurement test of the hardened layer (Test 2) Measurement test of ferrite area ratio and pearlite area ratio (Test 3) Measurement test of perlite particle width PW and standard deviation σP (Test 4) Cold forging performance evaluation test (Test 5) Strength evaluation test of the core of a steel component The following explains Exams 1 through 5.

[0103] [(Test 1) Vickers hardness measurement test of the hardened layer] The Vickers hardness (HV) of the hardened layer was determined based on the method described in [Regarding the Measurement Method of the Hardened Layer] above. The obtained Vickers hardness (HV) is shown in the "Hardness of Hardened Layer (HV)" column of Table 3. In all test numbers, the Vickers hardness of the hardened layer was 500 HV or higher.

[0104] [Table 3]

[0105] [(Test 2) Measurement test of ferrite area ratio and pearlite area ratio] Based on the method described in [Measurement Method for Ferrite Area Ratio and Pearlite Area Ratio] above, the ferrite area ratio (%) and pearlite area ratio (%) of the core of the cylindrical steel part for each test number were determined. The obtained ferrite area ratio is shown in the "F Area Ratio (%)" column of Table 3. The pearlite area ratio (%) is shown in the "P Area Ratio (%)" column of Table 3. The total area ratio (%) of ferrite and pearlite is shown in the "F+P Area Ratio (%)" column of Table 3. Note that if the total area ratio of ferrite and pearlite is less than 100%, the remaining microstructure for all test numbers was a hard microstructure (bainite and / or martensite).

[0106] [(Test 3) Measurement test of perlite particle width PW and standard deviation σP] Based on the method described in [Measurement Method for Pearlite Grain Width WP and Standard Deviation σP] above, the pearlite grain width WP (μm) and standard deviation σP (μm) in the core of the cylindrical portion of the steel part for each test number were determined. The obtained width WP is shown in the "WP (μm)" column of Table 3. The obtained standard deviation σP is shown in the "σP (μm)" column of Table 3. Also, F1 is shown in the "F1" column of Table 3.

[0107] [(Test 4) Cold Forging Performance Evaluation Test] The outer surface of each cylindrical steel part for each test number was visually inspected to check for cracks. If cracks were visually observed across the entire outer surface, it was judged that the cold forgeability was poor (indicated as "B (Bad)" in the "Cold Forgeability" column in Table 3). On the other hand, if no cracks were visually observed across the entire outer surface, it was judged that excellent cold forgeability was obtained (indicated as "E (Excellent)" in the "Cold Forgeability" column in Table 3).

[0108] [(Test 5) Strength evaluation test of the core of a steel component] The following core strength evaluation tests were conducted using the steel components of each test number. First, the tensile strength of each steel component with the specified test number was determined by the following method. A tensile test specimen with a parallel section diameter of 4 mm and a gauge length of 20 mm was taken from the center of the thickness of the steel component. The center of the thickness from which the tensile test specimen was taken was the core. Using the tensile test specimen, a tensile test was performed at room temperature and in air in accordance with JIS Z2241 (2022) to determine the tensile strength.

[0109] Next, for each test number, a steel component different from the one from which the tensile test specimen was taken was prepared. The maximum value of the Mises stress generated in the steel component during rotation was calculated, assuming that the steel component was rotated axially at 40,000 rpm. Specifically, the Mises stress during rotation of the steel component was determined using equations (A) to (C). Radial stress σr: σr={(3+ν) / 8}·(a 2 +b 2 -r 2 -a 2 b 2 / r 2 )·ρω2 (A) Circumferential stress σθ: σθ={(3 + ν) / 8}·{a 2 + b 2 -(1 + 3ν) / (3 + ν)·r 2 + a 2 b 2 / r 2}·ρω 2 (B) Mises stress σeq: σeq = √{(σr 2 + σθ 2 +(σr – σθ) 2 ) / 2} (C) Here, r in the formula is the radius (mm) at an arbitrary position, a is the outer radius (mm), b is the inner radius (mm), ρ is the density of the steel part, ω is the angular velocity (rad / second), and ν is the Poisson's ratio (dimensionless). In the above calculation, ρ = 7.8×10 -9 (t / mm 3 ) was used, and ν = 0.3 was used. Also, with r = b, the maximum value of the Mises stress was calculated.

[0110] The obtained Mises stress was compared with the tensile strength. When the Mises stress was higher than the tensile strength, it was judged that the core strength was low (indicated as "B (Bad)" in the "Core Strength" column in Table 3). On the other hand, when the Mises stress was below the tensile strength, it was judged that excellent core strength was obtained (indicated as "E (Excellent)" in the "Core Strength" column in Table 3).

[0111] [Evaluation Results] Referring to Tables 1 to 3, in Test Nos. 1 to 14, Features 1 to 4 were satisfied. Therefore, even when spheroidizing annealing was omitted and cold forging was carried out, the occurrence of cracks during cold forging could be suppressed. Furthermore, in the core strength evaluation test, the Mises stress was below the tensile strength, and excellent core strength was obtained.

[0112] On the other hand, in test number 15, the carbon content was low. As a result, the pearlite area ratio was low. Furthermore, F1 did not satisfy equation (1). Therefore, spheroidizing annealing was omitted and cold forging was performed, resulting in cracking during cold forging. In addition, in the core strength evaluation test, the Mises stress was higher than the tensile strength, and sufficient core strength could not be obtained.

[0113] In test number 16, the carbon content was high. As a result, cold forging was performed without spheroidizing annealing, and cracks occurred during cold forging.

[0114] In test number 17, the Si content was high. As a result, spheroidizing annealing was omitted and cold forging was performed, which resulted in cracking during cold forging.

[0115] In test number 18, the Mn content was low. As a result, the pearlite area ratio was low. Furthermore, F1 did not satisfy equation (1). Therefore, spheroidizing annealing was omitted and cold forging was performed, resulting in cracking during cold forging. In addition, in the core strength evaluation test, the Mises stress was higher than the tensile strength, and sufficient core strength could not be obtained.

[0116] In test number 19, the Mn content was high. As a result, cold forging was performed without spheroidizing annealing, and cracks occurred during cold forging.

[0117] In test number 20, the P content was high. As a result, spheroidizing annealing was omitted and cold forging was performed, which resulted in cracking during cold forging.

[0118] In test number 21, the sulfur content was high. As a result, cold forging was performed without spheroidizing annealing, and cracks occurred during cold forging.

[0119] In test number 22, the Al content was high. As a result, spheroidizing annealing was omitted and cold forging was performed, which resulted in cracking during cold forging.

[0120] In test number 23, the nitrogen content was high. As a result, cold forging was performed without spheroidizing annealing, and cracks occurred during cold forging.

[0121] In test number 24, the chromium content was high. As a result, cold forging was performed without spheroidizing annealing, which led to cracking during the cold forging process.

[0122] In tests 25 and 26, the solidification and cooling rate CR1 was too slow. As a result, F1 did not satisfy equation (1). Consequently, spheroidizing annealing was omitted and cold forging was performed, resulting in cracking during the cold forging process.

[0123] In tests 27 and 28, the finish rolling temperature FT was too high. As a result, F1 did not satisfy equation (1). Consequently, spheroidizing annealing was omitted and cold forging was performed, resulting in cracking during cold forging.

[0124] In tests 29 and 30, the average cooling rate CR2 was too slow. As a result, F1 did not satisfy equation (1). Consequently, spheroidizing annealing was omitted and cold forging was performed, resulting in cracking during the cold forging process.

[0125] In tests 31 and 32, FA did not satisfy equation (A). Therefore, F1 did not satisfy equation (1). As a result, when cold forging was performed without spheroidizing annealing, cracks occurred during cold forging.

[0126] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

Claims

1. Steel parts, It has a cylindrical section with a circular cross-section perpendicular to the axial direction, The aforementioned cylindrical portion is A hardened layer formed on the surface, It comprises a core portion located inside the aforementioned hardened layer, The chemical composition of the core is, in mass%, C: 0.30-0.60%, Si: 0.01 to 0.10%, Mn: 0.30-2.00%, P: Less than 0.050% S: 0.001-0.050%, Al: 0.001-0.200%, N: Less than 0.020% Cr: 0.01-0.40%, and, O: Contains 0.0030% or less, The remainder consists of Fe and impurities. The Vickers hardness of the hardened layer is 500 HV or higher. In the aforementioned core portion, the total area ratio of ferrite and pearlite is 90% or more, and the area ratio of pearlite is 40% or more. In the core portion, the arithmetic mean WP of the widths of a plurality of pearlite grains intersecting a 150 μm line segment perpendicular to the axial direction of the cylindrical portion, and the standard deviation σP of the widths of the plurality of pearlite grains satisfy equation (1). Steel parts. σP / WP<0.80 (1)

2. Steel parts, It has a cylindrical section with a circular cross-section perpendicular to the axial direction, The aforementioned cylindrical portion is A hardened layer formed on the surface, It comprises a core portion located inside the aforementioned hardened layer, The chemical composition of the core is, in mass%, C: 0.30-0.60%, Si: 0.01 to 0.10%, Mn: 0.30-2.00%, P: Less than 0.050% S: 0.001-0.050%, Al: 0.001-0.200%, N: Less than 0.020% Cr: 0.01-0.40%, and, O: Contains 0.0030% or less, Furthermore, it contains one or more selected from the group consisting of Group 1 or Group 2, The remainder consists of Fe and impurities. The Vickers hardness of the hardened layer is 500 HV or higher. In the aforementioned core portion, the total area ratio of ferrite and pearlite is 90% or more, and the area ratio of pearlite is 40% or more. In the core portion, the arithmetic mean WP of the widths of a plurality of pearlite grains intersecting a 150 μm line segment perpendicular to the axial direction of the cylindrical portion, and the standard deviation σP of the widths of the plurality of pearlite grains satisfy equation (1). Steel parts. [Group 1] Mo: 0.20% or less V: 0.150% or less, Nb: 0.050% or less, Ti: 0.050% or less, Cu: 0.40% or less, Ni: 0.40% or less, Sn: 0.100% or less, and, B: One or more selected from the group consisting of 0.0050% or less. [Group 2] Ca: 0.0050% or less, One or more selected from the group consisting of Mg: 0.0050% or less. σP / WP<0.80 (1)

3. A steel part according to claim 2, The chemical composition of the core contains the first group, Steel parts.

4. A steel part according to claim 2 or claim 3, The chemical composition of the core contains the second group, Steel parts.

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