Steel

Abstract

Provided is a steel material having excellent machinability, excellent bending fatigue strength and surface fatigue strength even after vacuum carburizing treatment, etc., and capable of suppressing heat treatment deformation. The chemical composition of the steel material provided in this embodiment contains, by mass%, C: 0.18-0.25%, Si: 0.70-2.00%, Mn: 0.70-1.50%, S: 0.005-0.050%, N: 0.0050-0.0200%, Al: 0.001-0.100%, O: 0.0050% or less, and P: 0.030% or less, with the remainder being Fe and impurities, and satisfying Formula (1) and Formula (2) in the specification. In addition, the microstructure of the cross section and the longitudinal section contains ferrite, with the balance being pearlite and / or bainite, the arithmetic mean of the area fraction of ferrite is 50 to 70%, the standard deviation of the area fraction of ferrite is 4.0% or less, and the average ferrite grain size ratio is 2.00 or less.

Description

Technical Field

[0001] The present disclosure relates to a steel material, and more particularly, to a steel material suitable as a raw material for a machine structural component manufactured by performing a vacuum carburizing treatment.

[0002] In this specification, vacuum carburizing treatment also includes vacuum carbonitriding treatment. In addition, in this specification, vacuum carburizing treatment means a process including a vacuum carburizing step (including a vacuum carbonitriding step) and a quenching step after the vacuum carburizing step. Background Art

[0003] Mechanical structural parts are represented by gears and shafts of automobiles and construction vehicles, etc. Mechanical structural parts are represented by alloy steel materials for mechanical structures such as SCr420, SCM420, and SNCM420 specified in JIS G 4053 (2016).

[0004] These steel materials are made into mechanical structural parts, for example, through the following manufacturing process. The steel material is subjected to forging (hot forging or cold forging) and / or cutting processing to produce an intermediate product of a desired shape. The intermediate product is subjected to heat treatment (quenching and tempering, carburizing or carbonitriding treatment, etc.) to adjust the hardness and microstructure of the intermediate product. Through the above manufacturing process, mechanical structural parts are manufactured.

[0005] As described above, in the manufacturing process of machine structural parts, steel materials are sometimes subjected to cutting processing. Therefore, the steel materials used as the blanks of machine structural parts are required to have high machinability.

[0006] In recent years, in order to improve the fuel efficiency of automobiles and construction vehicles, the weight reduction and miniaturization of machine structural parts have been progressing. Therefore, excellent bending fatigue strength and surface fatigue strength are required for machine structural parts.

[0007] As a method for improving the bending fatigue strength and surface fatigue strength of mechanical structural parts, vacuum carburizing is known. In vacuum carburizing, a hardened layer (carburized layer or carbonitrided layer) is formed on the surface of the mechanical structural parts. The hardened layer improves the bending fatigue strength and surface fatigue strength of the mechanical structural parts.

[0008] On the other hand, when vacuum carburizing (vacuum carburizing and vacuum carbonitriding) is performed, mechanical structure parts are easily deformed. In this specification, the deformation of mechanical structure parts during vacuum carburizing is referred to as heat treatment deformation. Due to heat treatment deformation, the shape of mechanical structure parts is distorted. The shape distortion of mechanical structure parts can cause noise and vibration in automobiles and construction vehicles during operation. Therefore, a steel material that can suppress heat treatment deformation when vacuum carburizing is performed is required.

[0009] The technology for suppressing heat treatment deformation is disclosed in Japanese Patent Publication No. 2016-191151 (Patent Document 1), Japanese Patent Publication No. 2018-028130 (Patent Document 2), Japanese Patent Publication No. 2007-291486 (Patent Document 3), and Japanese Patent Publication No. 2010-150566 (Patent Document 4).

[0010] The carburized part disclosed in Patent Document 1 contains, by mass%, C: 0.10-0.30%, Si: 0.16-1.40%, Mn: 1.40-3.00%, P: 0.030% or less, S: 0.060% or less, Cr: 0.01-0.29%, Al: 0.010-0.300%, and N: 0.003-0.030%, with the remainder being Fe and impurities. The surface of the carburized part has a flat portion and an edge portion. The carbon concentration of the surface layer region of the flat portion from the surface of the flat portion to a depth of 0.05 mm is 0.70-0.89%, and the carbon concentration of the surface layer region of the edge portion from the surface of the edge portion to a depth of 0.05 mm is 1.20% or less. In addition, the depth of the grain boundary oxide layer is 1 μm or less, and the Vickers hardness of the core is 260 or more. Therefore, according to Patent Document 1, the carburized component of Patent Document 1 is excellent in bending fatigue strength even if it is a carburized component having a shape including an edge portion.

[0011] The carburized part disclosed in Patent Document 2 contains, by mass%, C: 0.10-0.30%, Si: 0.16-1.40%, Mn: 1.40-3.00%, P: 0.030% or less, S: 0.060% or less, Cr: 0.01-0.29%, Al: 0.010-0.100%, and N: 0.003-0.030%, with the remainder being Fe and impurities. The surface of the carburized part has a flat portion and an edge portion. The carbon concentration of the surface layer region of the flat portion from the surface of the flat portion to a depth of 0.05 mm is 0.70-0.89%, and the carbon concentration of the surface layer region of the edge portion from the surface of the edge portion to a depth of 0.05 mm is 1.20% or less. In addition, the Vickers hardness from the surface of the flat portion to a depth of 0.3 mm is 650 or more, the depth of the grain boundary oxide layer is 1 μm or less, and the Vickers hardness of the core is 260 or more. Therefore, according to Patent Document 2, the carburized component of Patent Document 2 is excellent in bending fatigue strength even if it is a carburized component having a shape including an edge portion.

[0012] The carburized component disclosed in Patent Document 3 has the following alloy composition: in terms of mass%, it contains C: 0.1-0.3%, Si: 0.5-3.0%, Mn: 0.3-3.0%, P: 0.03% or less, S: 0.03% or less, Cu: 0.01-1.00%, Ni: 0.01-3.00%, Cr: 0.3-1.0%, Al: 0.20% or less, and N: 0.05% or less, and the remainder is inevitable impurities and Fe, which satisfies the condition of [Si%]+[Ni%]+[Cu%]-[Cr%]>0.5. In addition, the carburized component is obtained by carburizing by vacuum carburizing. Therefore, according to Patent Document 3, the carburized component of Patent Document 3 has a carbon concentration of more than 0.6% at the lowest surface carbon concentration without reducing the toughness of the edge portion, and there is no part with low strength due to insufficient carburization.

[0013] Patent document 4 discloses a steel material for vacuum carburizing or vacuum carbonitriding having the following chemical composition: in terms of mass%, it contains C: 0.10-0.25%, Si: 0.35-1.5%, Mn: 0.4-1.5%, P: 0.025% or less, S: 0.015-0.05%, Cr: 0.50-2.0%, Al: 0.010-0.050% and N: 0.012-0.025%, the remainder being Fe and impurities, O (oxygen) of which is 0.0012% or less and Ti of which is 0.003% or less, and satisfies formulas (1) to (3). Here, formula (1) is 910-203×C 0.5 +44.7×Si≤860, formula (2) is 2.0≤(0.31×C 0.5 )×(0.7×Si+1.00)×(3.33×Mn+1.00)×(2.16×Cr+1.00)≤3.5, and formula (3) is 0.2×(S / Mn)+P≤0.030. In addition, on the cross section parallel to the length direction, the major diameter of the inclusion is set to L (μm) and the minor diameter is set to W (μm). When the specified conditions are met, (πLW / 4) 0.5 The maximum equivalent circle diameter of the indicated oxides and other inclusions is 35 μm or less. According to Patent Document 4, the steel material of Patent Document 4 reduces the unevenness of heat treatment strain during quenching by adjusting the C and Si contents, thereby improving the surface fatigue strength and bending fatigue strength.

[0014] Prior art literature

[0015] Patent Literature

[0016] Patent Document 1: Japanese Patent Application Publication No. 2016-191151

[0017] Patent Document 2: Japanese Patent Application Publication No. 2018-028130

[0018] Patent Document 3: Japanese Patent Application Publication No. 2007-291486

[0019] Patent Document 4: Japanese Patent Application Publication No. 2010-150566 Summary of the invention

[0020] Problem that the invention aims to solve

[0021] Patent Documents 1 to 4 all disclose techniques for improving fatigue strength, but do not disclose any techniques for suppressing heat treatment deformation.

[0022] An object of the present disclosure is to provide a steel material having excellent machinability, excellent bending fatigue strength and surface fatigue strength after vacuum carburizing, and capable of suppressing heat treatment deformation after vacuum carburizing.

[0023] Solutions for solving problems

[0024] Chemical composition of the steel material of this embodiment

[0025] In terms of mass%, it contains

[0026] C: 0.18~0.25%,

[0027] Si: 0.70-2.00%,

[0028] Mn: 0.70-1.50%,

[0029] S: 0.005~0.050%,

[0030] N: 0.0050~0.0200%、

[0031] Al: 0.001~0.100%,

[0032] O: 0.0050% or less, and

[0033] P: 0.030% or less,

[0034] The balance is Fe and impurities, and satisfies equations (1) and (2),

[0035] In the cross section of the steel material which is perpendicular to the longitudinal direction and is a circle with a radius R,

[0036] When the center position of the cross section and eight R / 2 positions arranged at 45° intervals around the center of the cross section at a distance of R / 2 from the center of the cross section in the radial direction are defined as nine cross section observation positions,

[0037] The microstructure at each cross-sectional observation position contains ferrite, with the remainder being pearlite and / or bainite.

[0038] The arithmetic mean of the area fraction of ferrite at the nine cross-sectional observation positions is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less,

[0039] Among the average grain sizes of ferrite at the nine cross-sectional observation positions, the ratio of the largest average grain size to the smallest average grain size is 2.00 or less,

[0040] On a longitudinal section which is a section parallel to the longitudinal direction of the steel material and includes the central axis of the steel material,

[0041] When three central axis positions arranged at an R / 2 pitch on the central axis and six R / 2 positions arranged at a distance of R / 2 from each central axis position in the radial direction are defined as nine longitudinal cross-sectional observation positions,

[0042] The microstructure at the observation position of each longitudinal section contains ferrite, and the balance is pearlite and / or bainite.

[0043] The arithmetic mean of the area fraction of ferrite at the nine longitudinal cross-sectional observation positions is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less.

[0044] Among the average grain sizes of ferrite at the nine longitudinal cross-sectional observation positions, the ratio of the largest average grain size to the smallest average grain size is 2.00 or less.

[0045] Si / Mn≥1.00 (1)

[0046] 1-(0.5C+0.03Si+0.06Mn+0.01Cr+0.05Mo)<0.800 (2)

[0047] Here, the content (mass %) of the corresponding element is substituted for each element symbol in formula (1) and formula (2). When the corresponding element is not contained, its element symbol is substituted with "0".

[0048] Effects of the Invention

[0049] The steel material provided by the present disclosure has excellent machinability, has excellent bending fatigue strength and surface fatigue strength after vacuum carburizing treatment, and can suppress heat treatment deformation after vacuum carburizing treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1This is a graph showing the relationship between the F2 (=1-(0.5C+0.03Si+0.06Mn+0.01Cr+0.05Mo)) value and the maximum deformation ratio (%).

[0051] Figure 2 It is a schematic diagram for explaining the cross-sectional observation position at which microstructure observation is performed on the cross section perpendicular to the longitudinal direction of the steel material according to the present embodiment.

[0052] Figure 3 It is a schematic diagram for explaining a longitudinal cross-section observation position for performing microstructure observation on a longitudinal cross-section parallel to the longitudinal direction and including the central axis of the steel material according to the present embodiment.

[0053] Figure 4 Schematic diagram of the banded tissue.

[0054] Figure 5 This is a diagram showing an example of the heating pattern in the vacuum carburizing step and the quenching step.

[0055] Figure 6 This is a top view of the Ono-type rotation bending test piece produced in the example.

[0056] Figure 7 This is a diagram showing an example of the heating pattern in the gas carburizing step and the quenching step.

[0057] Figure 8 This is a plan view of a test piece for a roller pitting fatigue test produced in an example.

[0058] Fig. 9 Schematic diagram for explaining a roller pitting fatigue test.

[0059] Fig.10 This is a front view of a large roll test piece produced in Examples.

[0060] Fig.11A This is a perspective view of a gear simulation test piece produced in the example.

[0061] Fig. 11B for Fig.11A A three-dimensional view of the through hole in the. DETAILED DESCRIPTION

[0062] The present inventors have investigated and studied a steel material which has excellent machinability, has excellent bending fatigue strength and surface fatigue strength when subjected to vacuum carburizing to form a machine structural component, and can suppress heat treatment deformation after vacuum carburizing.

[0063] The present inventors have studied a steel material having excellent machinability and excellent bending fatigue strength and surface fatigue strength after vacuum carburizing treatment from the viewpoint of chemical composition.

[0064] The research results show that when the chemical composition of steel contains C: 0.18-0.25%, Si: 0.70-2.00%, Mn: 0.70-1.50%, S: 0.005-0.050%, N: 0.0050-0.0200%, Al: 0.001-0.100%, O: 0.0050% or less, P: 0.030% or less, Mo: 0-0.50%, Nb: 0-0.0 When the steel contains 0% to 50%, Cr: 0 to 0.60%, Ti: 0 to 0.020%, Cu: 0 to 0.50%, Ni: 0 to 0.80%, V: 0 to 0.30%, Mg: 0 to 0.0035%, Ca: 0 to 0.0030%, and rare earth elements: 0 to 0.0050%, and the balance is Fe and impurities, the steel has excellent machinability and may have excellent bending fatigue strength and surface fatigue strength after vacuum carburizing treatment.

[0065] The present inventors also believed that, provided that the contents of the elements in the chemical composition are within the above ranges and the following formula (1) is satisfied, excellent bending fatigue strength can be obtained after vacuum carburizing.

[0066] Si / Mn≥1.00 (1)

[0067] Here, the symbol of each element in formula (1) is substituted by the content (mass %) of the corresponding element.

[0068] In the steel material of the present embodiment, when the ratio of the Si content to the Mn content is 1.00 or more, that is, when the formula (1) is satisfied, the inclusions are soft MnO—SiO 2 The inclusions undergo vitrification during hot working (hot rolling), elongate, and break, becoming fine. Therefore, the coarse inclusions that reduce the bending fatigue strength can be reduced, and the bending fatigue strength can be improved.

[0069] The present inventors have also studied the means of suppressing heat treatment deformation in vacuum carburizing. The present inventors have paid attention to the microstructure of steel. If the microstructure at each part of the steel is as uniform as possible, specifically, if the phase structure unevenness and grain unevenness of the microstructure at each part of the steel are suppressed, the unevenness of the martensitic phase transformation during vacuum carburizing and quenching can be suppressed. As a result, heat treatment deformation can be suppressed. For this reason, the present inventors have studied the phase structure and crystal grain size at each part of the steel.

[0070] The inventors first focused on the microstructure unevenness on the cross section perpendicular to the longitudinal direction of the steel material. In order to quantify the microstructure unevenness on the cross section, the observation position of the microstructure on the cross section, that is, the cross section observation position, is defined as follows.

[0071] When the cross-sectional radius of the steel material is set to R, the center position of the cross section and eight R / 2 positions arranged at 45° intervals around the center of the cross section, which are positions R / 2 away from the center of the cross section in the radial direction, are defined as nine cross-sectional observation positions.

[0072] The present inventors investigated and studied the microstructure at each cross-sectional observation position. As a result of the study, it was found that when the microstructure at the cross-sectional observation position satisfies the following conditions, heat treatment deformation after carburizing treatment is suppressed.

[0073] (1) The microstructure at each cross-sectional observation position contains ferrite, with the remainder being pearlite and / or bainite.

[0074] (2) The arithmetic mean of the area fraction of ferrite at nine observation positions of the cross section is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less.

[0075] (3) Among the average grain sizes of ferrite at nine cross-sectional observation positions, the ratio of the largest average grain size to the smallest average grain size is 2.00 or less.

[0076] However, it was found that even steel materials having the above chemical composition and satisfying the above microstructure still cannot sufficiently suppress heat treatment deformation, and in particular, cannot sufficiently suppress noise and vibration during operation of automobiles and construction vehicles. Therefore, the present inventors conducted further research.

[0077] As a result, the following was found. In order to suppress noise and vibration during operation, it is effective to suppress the heat treatment deformation of the steel material in three dimensions. As described above, if the phase structure and grain heterogeneity of the microstructure of the cross section of the steel material are suppressed, the heat treatment deformation of the steel material in the direction perpendicular to the length direction can be suppressed.

[0078] However, if only the unevenness of the microstructure of the cross section of the steel material is suppressed, the heat treatment deformation is limited to suppressing two-dimensionally. That is, even if the unevenness of the microstructure of the cross section of the steel material is suppressed, there is still a situation where the microstructure of the cross section parallel to the length direction of the steel material and including the central axis of the steel material, that is, the longitudinal section, is uneven. In this case, the heat treatment deformation is uneven. As a result, the noise and vibration during operation cannot be fully suppressed.

[0079] Therefore, the inventors of the present invention paid attention to the microstructural nonuniformity of the longitudinal section of the steel material in addition to the microstructural nonuniformity of the cross section of the steel material. In addition, in order to quantify the microstructural nonuniformity on the longitudinal section, the observation position of the microstructure on the longitudinal section, i.e., the longitudinal section observation position, was defined as follows.

[0080] Three central axis positions arranged at a pitch of R / 2 on the central axis of the steel material and six R / 2 positions arranged at a distance of R / 2 from each central axis position in the radial direction are defined as nine longitudinal cross-sectional observation positions.

[0081] The inventors investigated and studied the microstructures at various longitudinal section observation positions and found that when the microstructure at the cross-sectional observation position satisfies the above conditions and the microstructure at the longitudinal section observation position satisfies the following conditions, heat treatment deformation after gas carburizing treatment is sufficiently suppressed.

[0082] (4) The microstructure at each longitudinal cross-sectional observation position contains ferrite, with the remainder being pearlite and / or bainite.

[0083] (5) The arithmetic mean of the area fraction of ferrite at nine longitudinal cross-sectional observation positions is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less.

[0084] (6) Among the average grain sizes of ferrite at nine locations where the longitudinal cross section is observed, the ratio of the largest average grain size to the smallest average grain size is 2.00 or less.

[0085] However, even if the steel material has the above chemical composition and the microstructure at the cross-sectional and longitudinal cross-sectional observation positions satisfies the above conditions (1) to (6), heat treatment deformation may not be sufficiently suppressed. Therefore, the present inventors conducted further studies.

[0086] Here, the present inventors paid attention to the martensitic transformation after vacuum carburizing and conducted detailed studies on the mechanism of martensitic transformation during vacuum carburizing and quenching.

[0087] The inventors first attempted to suppress heat treatment deformation by using a steel material having the above chemical composition to make the martensitic transformation timing at each part of the mechanical structural component as uniform as possible. Specifically, the inventors attempted to suppress heat treatment deformation by suppressing the microstructural inhomogeneity at each part of the steel material (cross-sectional observation position, longitudinal section observation position) and suppressing the inhomogeneity of the Ms point at each part as much as possible.

[0088] However, the inventors of the present invention have found that even if the microstructural inhomogeneity at each part of the steel material of the above chemical composition is suppressed, the timing of martensitic transformation at each part of the steel material will always have slight deviations, and it is extremely difficult to make each part undergo martensitic transformation at the same time. Specifically, it was found that when the time of rapid cooling in vacuum carburizing is divided into minute time periods, even if the microstructural inhomogeneity at the cross-sectional observation position and the longitudinal section observation position of the steel material is suppressed to the limit, a minute time band in which a part where martensitic transformation has occurred (hereinafter referred to as "martensitic transformation part") and a part where martensitic transformation has not occurred (hereinafter referred to as "martensitic non-transformed part") are mixed in the steel material will always be produced.

[0089] It is considered that the microstructure of the steel material changes as follows during vacuum carburizing.

[0090] When the quenching time (quenching time) is divided into micro-times, first, a part of the steel material starts the martensitic transformation. Then, as time progresses, the martensitic transformation progresses from the center to the surface. In other words, the martensitic transformation does not start from the surface of the steel material, but from the inside of the steel material.

[0091] After vacuum carburizing treatment, the carbon concentration of the steel surface becomes higher than the carbon concentration inside the steel. Therefore, the Ms point of the steel surface is lower than the Ms point inside the steel. In addition, even if the Ms point at each part inside the steel can be made uniform, the cooling rate of each part will not be exactly the same due to the shape of the steel. Therefore, when the quenching time is divided into small time periods, the martensitic phase transformation starts from the part with a fast cooling rate inside the steel in each part of the steel. Therefore, in the quenching process of gas carburizing treatment, a small time band in which the martensitic phase transformation part and the martensitic non-phase transformation part are mixed is inevitably generated.

[0092] Based on the above understanding, the inventors no longer focus on making the timing of martensitic transformation as similar as possible to suppress heat treatment deformation, but instead study means of suppressing heat treatment deformation based on the premise that a tiny time band in which the martensitic transformation part and the martensitic non-transformed part are mixed during vacuum carburizing treatment must exist.

[0093] During the quenching process, the martensite non-transformed portion is softer than the martensite transformed portion. In addition, the volume of the martensite transformed portion having a body-centered cubic lattice structure is larger than that of the martensite non-transformed portion having a face-centered cubic lattice structure. Therefore, during the quenching process, when a part of the steel undergoes martensite transformation and a martensite transformed portion and a martensite non-transformed portion are mixed, strain is generated in the martensite non-transformed portion. It is believed that this strain causes heat treatment deformation.

[0094] To this end, the inventors of the present invention believe that, based on the existence of a tiny time band in which a martensitic transformation portion and a martensitic non-transformed portion exist in a mixed state during the vacuum carburizing process, if the strength of the martensitic non-transformed portion at the time point when the martensitic transformation portion is generated can be kept high, the strain generation in the martensitic non-transformed portion can be suppressed, and as a result, heat treatment deformation can be suppressed.

[0095] Therefore, the inventors have conducted further research on how to maintain high strength of the martensite non-transformed portion when the martensite transformation portion is generated during the quenching process of vacuum carburizing. In order to improve the strength of the martensite non-transformed portion in the temperature range where the martensite transformation portion is generated, it is effective to appropriately contain an element that strengthens the martensite non-transformed portion in the temperature range where the martensite transformation portion is generated in the steel material having the above chemical composition.

[0096] The inventors of the present invention believe that, in the above chemical composition, C, Si, Mn, Cr and Mo are effective elements for increasing the strength of the martensitic non-transformed portion in the temperature range where the martensitic transformation portion is generated. For this reason, the relationship between these elements and the heat treatment deformation during the quenching process of gas carburizing treatment was further studied. The results show that when the steel material of the above chemical composition also satisfies the following formula (2), the heat treatment deformation is significantly suppressed.

[0097] 1-(0.5C+0.03Si+0.06Mn+0.01Cr+0.05Mo)<0.800 (2)

[0098] Here, the symbol of each element in formula (2) is substituted by the content (mass %) of the corresponding element. When the corresponding element is not contained, the symbol of the element is substituted by "0".

[0099] Define F2=1-(0.5C+0.03Si+0.06Mn+0.01Cr+0.05Mo). Figure 1 This is a graph showing the relationship between the F2 value and the maximum deformation ratio (%) in a steel material whose content of each element in the chemical composition is within the above range and whose microstructures at the cross-sectional observation position and the longitudinal section observation position satisfy the above conditions. The maximum deformation ratio is an indicator of heat treatment deformation. The larger the maximum deformation ratio, the greater the heat treatment deformation of the steel material. The maximum deformation ratio is obtained by the method described below.

[0100] See also Figure 1In steel materials whose contents of each element in the chemical composition are within the above range and whose microstructures at the cross-sectional observation position and the longitudinal section observation position satisfy the above conditions (1) to (6), as F2 decreases, the maximum deformation ratio decreases. Moreover, when F2 is less than 0.800, the maximum deformation ratio decreases significantly. That is, the maximum deformation ratio relative to F2 has an inflection point near F2 = 0.800. Therefore, when F2 is less than 0.800, the heat treatment deformation of the steel material during carburizing and quenching can be fully suppressed.

[0101] In summary, the inventors of the present invention have found that, in the steel having the above-mentioned chemical composition, the uneven microstructure at the cross-sectional observation position and the longitudinal cross-sectional observation position is suppressed, thereby suppressing the uneven timing of the martensitic transformation during quenching to a certain extent, and a tiny time band in which the martensitic transformation part and the martensitic non-transformed part are mixed will be produced during the quenching process. Based on this premise, by making F2 less than 0.800, it is possible to have excellent machinability, excellent bending fatigue strength and excellent surface fatigue strength after vacuum carburizing treatment, and the heat treatment deformation after vacuum carburizing treatment can be fully suppressed.

[0102] The steel material provided in this embodiment completed based on the above findings has the following configuration. [1]

[0104] A steel material, the chemical composition of which contains, in mass %,

[0105] C: 0.18~0.25%,

[0106] Si: 0.70-2.00%,

[0107] Mn: 0.70-1.50%,

[0108] S: 0.005~0.050%,

[0109] N: 0.0050~0.0200%、

[0110] Al: 0.001~0.100%,

[0111] O: 0.0050% or less, and

[0112] P: 0.030% or less,

[0113] The balance is Fe and impurities, and satisfies equations (1) and (2),

[0114] In the cross section of the steel material which is perpendicular to the longitudinal direction and is a circle with a radius R,

[0115] When the center position of the cross section and eight R / 2 positions arranged at 45° intervals around the center of the cross section at a distance of R / 2 from the center of the cross section in the radial direction are defined as nine cross section observation positions,

[0116] The microstructure at each cross-sectional observation position contains ferrite, with the remainder being pearlite and / or bainite.

[0117] The arithmetic mean of the area fraction of ferrite at the nine cross-sectional observation positions is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less,

[0118] Among the average grain sizes of ferrite at the nine cross-sectional observation positions, the ratio of the largest average grain size to the smallest average grain size is 2.00 or less,

[0119] On a longitudinal section which is a section parallel to the longitudinal direction of the steel material and includes the central axis of the steel material,

[0120] When three central axis positions arranged at an R / 2 pitch on the central axis and six R / 2 positions arranged at a distance of R / 2 from each central axis position in the radial direction are defined as nine longitudinal cross-sectional observation positions,

[0121] The microstructure at each longitudinal section observation position contains ferrite, with the remainder being pearlite and / or bainite.

[0122] The arithmetic mean of the area fraction of ferrite at the nine longitudinal cross-sectional observation positions is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less.

[0123] Among the average grain sizes of ferrite at the nine longitudinal cross-sectional observation positions, the ratio of the largest average grain size to the smallest average grain size is 2.00 or less.

[0124] Si / Mn≥1.00 (1)

[0125] 1-(0.5C+0.03Si+0.06Mn+0.01Cr+0.05Mo)<0.800 (2)

[0126] Here, the content (mass %) of the corresponding element is substituted for each element symbol in formula (1) and formula (2). When the corresponding element is not contained, the element symbol is substituted with "0". [2]

[0128] According to the steel material described in [1],

[0129] The chemical composition further contains one or more elements selected from the group consisting of the following elements to replace a portion of the Fe:

[0130] Mo: 0.50% or less,

[0131] Nb: 0.050% or less,

[0132] Cr: 0.60% or less

[0133] Ti: 0.020% or less,

[0134] Cu: 0.50% or less,

[0135] Ni: 0.80% or less,

[0136] V: 0.30% or less,

[0137] Mg: 0.0035% or less,

[0138] Ca: 0.0030% or less, and

[0139] Rare earth elements: less than 0.0050%.

[0140] The steel material of the present embodiment will be described in detail below. "%" regarding an element means mass % unless otherwise specified.

[0141] [Chemical composition of steel]

[0142] The chemical composition of steel contains the following elements.

[0143] C: 0.18~0.25%

[0144] Carbon (C) increases the strength of steel materials. If the C content is less than 0.18%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the ranges of the present embodiment.

[0145] On the other hand, when the C content exceeds 0.25%, the hardenability becomes too high even if the contents of other elements are within the range of the present embodiment. In this case, the hardness of the mechanical structural component after vacuum carburization becomes too high. As a result, the machinability of the mechanical structural component is significantly reduced.

[0146] Therefore, the C content is 0.18 to 0.25%. The preferred lower limit of the C content is 0.19%, more preferably 0.20%, and more preferably 0.21%. The preferred upper limit of the C content is 0.24%, more preferably 0.23%, and more preferably 0.22%.

[0147] Si: 0.70~2.00%

[0148] Silicon (Si) improves the hardenability of steel and increases the strength of steel. Si also increases the tempering softening resistance of the hardened layer when the steel is used as a mechanical structural component. Therefore, the surface fatigue strength of the mechanical structural component is improved. When the Si content is less than 0.70%, even if the content of other elements is within the range of this embodiment, the above effects cannot be fully obtained.

[0149] On the other hand, when the Si content exceeds 2.00%, even if the contents of other elements are within the range of the present embodiment, the hardenability becomes too high. Therefore, the hardness of the steel material after vacuum carburization becomes high. Therefore, the machinability of the steel material is significantly reduced.

[0150] Therefore, the Si content is 0.70 to 2.00%. The preferred lower limit of the Si content is 0.71%, more preferably 0.72%, and more preferably 0.75%. The preferred upper limit of the Si content is 1.90%, more preferably 1.70%, more preferably 1.50%, more preferably 1.47%, and more preferably 1.45%.

[0151] Mn: 0.70~1.50%

[0152] Manganese (Mn) improves the hardenability of steel and improves the bending fatigue strength and surface fatigue strength of machine structural parts. When the Mn content is less than 0.70%, the above effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0153] On the other hand, when the Mn content exceeds 1.50%, the steel material becomes too hard even if the contents of other elements are within the ranges of the present embodiment. In this case, the machinability of the steel material decreases.

[0154] Therefore, the Mn content is 0.70 to 1.50%. The preferred lower limit of the Mn content is greater than 0.70%, more preferably 0.75%, and more preferably 0.80%. The preferred upper limit of the Mn content is less than 1.50%, more preferably 1.45%, more preferably 1.40%, and more preferably 1.35%.

[0155] S: 0.005~0.050%

[0156] Sulfur (S) combines with Mn to form MnS. MnS improves the machinability of steel. When the S content is less than 0.005%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0157] On the other hand, when the S content exceeds 0.050%, even if the contents of other elements are within the range of the present embodiment, MnS is excessively formed. In this case, the bending fatigue strength and surface fatigue strength of the machine structural component are reduced.

[0158] Therefore, the S content is 0.005 to 0.050%. The preferred lower limit of the S content is 0.010%, more preferably 0.013%, and more preferably 0.015%. The preferred upper limit of the S content is less than 0.050%, more preferably 0.035%, and more preferably 0.025%.

[0159] N: 0.0050~0.0200%

[0160] Nitrogen (N) combines with Al and Nb to form AlN and NbN. AlN and NbN can inhibit grain coarsening during the heating process of vacuum carburizing through the pinning effect. When the N content is less than 0.0050%, even if the content of other elements is within the range of this embodiment, the above effect cannot be fully obtained.

[0161] On the other hand, when the N content exceeds 0.0200%, even if the contents of other elements are within the ranges of the present embodiment, flaws are likely to occur on the surface of the produced slab or ingot in the steelmaking process.

[0162] Therefore, the N content is 0.0050 to 0.0200%. The preferred lower limit of the N content is 0.0100%, more preferably 0.0120%, and more preferably 0.0130%. The preferred upper limit of the N content is less than 0.0200%, more preferably 0.0190%, more preferably 0.0180%, and more preferably 0.0150%.

[0163] Al: 0.001~0.100%

[0164] Aluminum (Al) deoxidizes steel. Al also combines with N to form AlN. AlN can inhibit grain coarsening during vacuum carburizing heating through the pinning effect. When the Al content is less than 0.001%, the above effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0165] On the other hand, when the Al content exceeds 0.100%, even if the contents of other elements are within the range of the present embodiment, the formation of coarse Al oxides is promoted. Coarse Al oxides reduce the bending fatigue strength of machine structural parts.

[0166] Therefore, the Al content is 0.001 to 0.100%. The preferred lower limit of the Al content is 0.010%, more preferably 0.020%, more preferably 0.025%, more preferably 0.027%, and more preferably 0.030%. The preferred upper limit of the Al content is 0.090%, more preferably 0.070%, more preferably 0.050%, more preferably 0.045%, more preferably 0.040%, and more preferably 0.035%.

[0167] O (oxygen): 0.0050% or less

[0168] Oxygen (O) is an impurity. O combines with other elements in the steel to form coarse oxide inclusions. Coarse oxide inclusions reduce the bending fatigue strength of mechanical structural parts. When the O content is greater than 0.0050%, the bending fatigue strength of the mechanical structural parts will be significantly reduced even if the contents of other elements are within the range of this embodiment.

[0169] Therefore, the O content is 0.0050% or less. The upper limit of the O content is preferably 0.0040%, more preferably 0.0030%, more preferably 0.0020%, and further preferably 0.0015%.

[0170] The O content is preferably as low as possible. However, excessive reduction of the O content will increase production costs. Therefore, considering the case of normal industrial production, the preferred lower limit of the O content is greater than 0%, more preferably 0.0001%, more preferably 0.0005%, and more preferably 0.0010%.

[0171] P: 0.030% or less

[0172] Phosphorus (P) is an impurity. P segregates at grain boundaries and reduces grain boundary strength. When the P content is greater than 0.030%, even if the contents of other elements are within the range of this embodiment, P will segregate excessively at grain boundaries and reduce grain boundary strength. As a result, the bending fatigue strength and surface fatigue strength of mechanical structural parts will decrease.

[0173] Therefore, the P content is 0.030% or less. The upper limit of the P content is preferably 0.025%, more preferably 0.020%, and further preferably 0.015%.

[0174] The P content is preferably as low as possible. However, excessive reduction of the P content will increase production costs. Therefore, considering the usual industrial production, the preferred lower limit of the P content is greater than 0%, more preferably 0.001%, more preferably 0.005%, and more preferably 0.010%.

[0175] The balance of the chemical composition of the steel provided in this embodiment is Fe and impurities. Here, impurities refer to substances that are mixed from ore, waste or manufacturing environment as raw materials when industrially manufacturing steel, and are allowed to exist within a range that has no adverse effect on the steel of this embodiment. The impurities mentioned here are B, Pb, W, Sb, Bi, Co, Ta, Sn, In, Zr, Te, Se and Zn. The total content of impurities other than O and P is less than 0.01%. It should be noted that among the above impurities, the B content is less than 0.0003%.

[0176] [About optional elements]

[0177] The chemical composition of the steel material of the present embodiment may further contain one or more elements selected from the group consisting of the following elements in place of a portion of Fe:

[0178] Mo: 0.50% or less,

[0179] Nb: 0.050% or less,

[0180] Cr: 0.60% or less,

[0181] Ti: 0.020% or less,

[0182] Cu: 0.50% or less,

[0183] Ni: 0.80% or less,

[0184] V: 0.30% or less,

[0185] Mg: 0.0035% or less,

[0186] Ca: 0.0030% or less, and

[0187] Rare earth elements: 0.0050% or less These elements are optional elements, and all of them improve the bending fatigue strength and surface fatigue strength of machine structural parts.

[0188] Mo: 0.50% or less

[0189] Molybdenum (Mo) is an optional element and may not be contained. That is, the Mo content may be 0%. When contained, Mo improves the hardenability of the steel and improves the bending fatigue strength and surface fatigue strength of mechanical structural parts. As long as Mo is contained in a small amount, the above effects can be obtained to a certain extent.

[0190] However, when the Mo content exceeds 0.50%, the steel material becomes too hard even if the contents of other elements are within the ranges of the present embodiment. In this case, the machinability of the steel material decreases.

[0191] Therefore, the Mo content is 0 to 0.50%, and when contained, it is 0.50% or less (that is, greater than 0 and 0.50% or less).

[0192] The preferred lower limit of the Mo content is 0.01%, more preferably 0.02%, more preferably 0.05%, and more preferably 0.10%. The preferred upper limit of the Mo content is less than 0.50%, more preferably 0.45%, more preferably 0.40%, and more preferably 0.35%.

[0193] Nb: 0.050% or less

[0194] Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, Nb combines with C and / or N to form Nb precipitates (NbC, NbN, Nb(CN), etc.). Nb precipitates can inhibit grain coarsening during gas carburizing treatment by pinning effect, similarly to AlN. Therefore, the bending fatigue strength and surface fatigue strength of mechanical structural parts are improved. As long as Nb is contained in a small amount, the above-mentioned effect can be obtained to a certain extent.

[0195] However, when the Nb content exceeds 0.050%, even if the contents of other elements are within the range of the present embodiment, the Nb precipitates will coarsen. In this case, the grain coarsening during the gas carburizing treatment cannot be fully suppressed. Therefore, the bending fatigue strength and surface fatigue strength of the mechanical structural parts are reduced.

[0196] Therefore, the Nb content is 0 to 0.050%, and when contained, it is 0.050% or less (that is, greater than 0 and 0.050% or less).

[0197] The preferred lower limit of the Nb content is 0.001%, more preferably 0.010%, more preferably 0.015%, more preferably 0.020%, and more preferably 0.025%. The preferred upper limit of the Nb content is less than 0.050%, more preferably 0.045%, more preferably 0.040%, and more preferably 0.035%.

[0198] Cr: 0.60% or less

[0199] Chromium (Cr) is an optional element and may not be contained. That is, the Cr content may be 0%. When contained, Cr improves the hardenability of the steel and improves the bending fatigue strength and surface fatigue strength of mechanical structural parts. The above effects can be achieved to a certain extent as long as Cr is contained in a small amount.

[0200] However, when the Cr content is greater than 0.60%, even if the contents of other elements are within the range of this embodiment, excessive carburization is likely to occur in the surface layer of the mechanical structural component during vacuum carburization. In this case, coarse cementite is generated at the grain boundary. Therefore, the bending fatigue strength of the mechanical structural component decreases.

[0201] Therefore, the Cr content is 0 to 0.60%, and when contained, it is 0.60% or less (i.e., greater than 0 and less than 0.60%). The preferred lower limit of the Cr content is 0.01%, more preferably 0.05%, and more preferably 0.10%. The preferred upper limit of the Cr content is less than 0.60%, more preferably 0.55%, more preferably 0.50%, more preferably 0.45%, and more preferably 0.40%.

[0202] Ti: 0.020% or less

[0203] Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When contained, Ti, like Nb, will form Ti precipitates (TiC, TiN, Ti (CN), etc.). Ti precipitates will inhibit grain coarsening during gas carburizing treatment through the pinning effect. Therefore, the bending fatigue strength and surface fatigue strength of mechanical structure parts are improved. As long as Ti is contained in a small amount, the above-mentioned effect can be obtained to a certain extent.

[0204] However, when the Ti content exceeds 0.020%, the Ti precipitates will coarsen even if the contents of other elements are within the range of the present embodiment. In this case, the grain coarsening during the gas carburizing treatment cannot be fully suppressed. Therefore, the bending fatigue strength and surface fatigue strength of the mechanical structural parts are reduced.

[0205] Therefore, the Ti content is 0 to 0.020%, and when contained, it is 0.020% or less (that is, greater than 0 and 0.020% or less).

[0206] The preferred lower limit of the Ti content is 0.001%, more preferably 0.005%, and further preferably 0.010%. The preferred upper limit of the Ti content is 0.019%, further preferably 0.017%, and further preferably 0.015%.

[0207] Cu: 0.50% or less

[0208] Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When contained, Cu improves the hardenability of the steel and improves the bending fatigue strength and surface fatigue strength of mechanical structural parts. As long as Cu is contained in a small amount, the above effects can be obtained to a certain extent.

[0209] However, when the Cu content is greater than 0.50%, even if the contents of other elements are within the range of this embodiment, the steel will become too hard. In this case, the machinability of the steel decreases.

[0210] Therefore, the Cu content is 0 to 0.50%, and when contained, it is 0.50% or less (that is, greater than 0 and 0.50% or less).

[0211] The preferable lower limit of the Cu content is 0.01%, more preferably 0.05%, and further preferably 0.10%. The preferable upper limit of the Cu content is 0.45%, more preferably 0.40%, further preferably 0.30%, and further preferably 0.25%.

[0212] Ni: 0.80% or less

[0213] Nickel (Ni) is an optional element and may not be contained. That is, the Ni content can be 0%. When contained, Ni improves the hardenability of the steel and increases the bending fatigue strength and surface fatigue strength of mechanical structural components. As long as a small amount of Ni is contained, the above effects can be obtained to a certain extent.

[0214] However, when the Ni content is greater than 0.80%, even if the contents of other elements are within the range of this embodiment, the steel will become too hard. In this case, the machinability of the steel decreases.

[0215] Therefore, the Ni content is 0 to 0.80%, and when contained, it is 0.80% or less (that is, greater than 0 and 0.80% or less).

[0216] The preferable lower limit of the Ni content is 0.01%, more preferably 0.05%, and further preferably 0.10%. The preferable upper limit of the Ni content is 0.70%, more preferably 0.60%, further preferably 0.40%, and further preferably 0.20%.

[0217] V: 0.30% or less

[0218] Vanadium (V) is an optional element and may not be contained. That is, the V content can be 0%. When contained, like Nb, V forms V precipitates (such as VC, VN, V(CN), etc.). The V precipitates can inhibit grain coarsening during gas carburizing treatment through the pinning effect. Therefore, the bending fatigue strength and surface fatigue strength of mechanical structural components are improved. As long as a small amount of V is contained, the above effects can be obtained to a certain extent.

[0219] However, when the V content is greater than 0.30%, even if the contents of other elements are within the range of this embodiment, the steel will become too hard. In this case, the machinability of the steel decreases.

[0220] Therefore, the V content is 0 to 0.30%, and when V is contained, it is 0.30% or less (that is, greater than 0 and 0.30% or less).

[0221] The preferred lower limit of the V content is 0.01%, more preferably 0.03%, and further preferably 0.04%. The preferred upper limit of the V content is 0.20%, further preferably 0.15%, and further preferably 0.10%.

[0222] Mg: 0.0035% or less

[0223] Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, Mg deoxidizes the steel in the same manner as Al. In this case, the formation of coarse oxides is suppressed. Therefore, the bending fatigue strength and surface fatigue strength of mechanical structural parts are improved. As long as Mg is contained in a small amount, the above-mentioned effects can be obtained to a certain extent.

[0224] However, when the Mg content exceeds 0.0035%, even if the contents of other elements are within the range of the present embodiment, the formation of coarse Mg oxides in the steel material is promoted. In this case, the limit working rate during hot working decreases.

[0225] Therefore, the Mg content is 0 to 0.0035%, and when contained, it is 0.0035% or less (that is, greater than 0 and 0.0035% or less).

[0226] The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0003%, and further preferably 0.0005%. The preferred upper limit of the Mg content is 0.0030%, further preferably 0.0028%, further preferably 0.0025%, and further preferably 0.0020%.

[0227] Ca: 0.0030% or less

[0228] Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, Ca refines the sulfides in the steel. Ca also promotes the spheroidization of the sulfides in the steel. Therefore, the bending fatigue strength and surface fatigue strength of the mechanical structural parts are improved. As long as Ca is contained in a small amount, the above effects can be achieved to a certain extent.

[0229] However, when the Ca content exceeds 0.0030%, even if the contents of other elements are within the range of the present embodiment, coarse Ca oxides are generated in the steel material. In this case, the bending fatigue strength and surface fatigue strength of the machine structural component decrease.

[0230] Therefore, the Ca content is 0 to 0.0030%, and when contained, it is 0.0030% or less (that is, greater than 0 and 0.0030% or less).

[0231] The preferred lower limit of the Ca content is 0.0001%, more preferably 0.0002%, more preferably 0.0005%, more preferably 0.0007%, and more preferably 0.0010%. The preferred upper limit of the Ca content is 0.0025%, more preferably 0.0022%, and more preferably 0.0020%.

[0232] Rare earth elements (REM): 0.0050% or less

[0233] Rare earth elements (REM) are optional elements and may not be contained. That is, the REM content may be 0%. When contained, REM will dissolve in the sulfide in the steel material and inhibit the elongation of MnS. As a result, the bending fatigue strength and surface fatigue strength of mechanical structural parts are improved. As long as REM is contained in a small amount, the above effects can be obtained to a certain extent.

[0234] However, when the REM content exceeds 0.0050%, coarse oxides are generated even if the contents of other elements are within the range of the present embodiment. In this case, the bending fatigue strength and surface fatigue strength of the machine structural component are reduced.

[0235] Therefore, the REM content is 0 to 0.0050%, and when contained, it is 0.0050% or less (that is, greater than 0 and 0.0050% or less).

[0236] The preferred lower limit of the REM content is 0.0001%, more preferably 0.0010%, and further preferably 0.0020%. The preferred upper limit of the REM content is 0.0045%, further preferably 0.0040%, further preferably 0.0035%, and further preferably 0.0030%.

[0237] REM in this specification refers to one or more elements selected from the group consisting of scandium (Sc) with an atomic number of 21, yttrium (Y) with an atomic number of 39, and lanthanum (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71 in the lanthanide series. The REM content in this specification refers to the total content of these elements.

[0238] [About formula (1)]

[0239] Assuming that the contents of each element are within the range of the present embodiment, the chemical composition of the steel material of the present embodiment also satisfies the formula (1).

[0240] Si / Mn≥1.00 (1)

[0241] Here, the symbol of each element in formula (1) is substituted by the content (mass %) of the corresponding element.

[0242] Define F1 = Si / Mn. Si and Mn will generate MnO-SiO during the deoxidation process. 2 .MnO-SiO 2 The melting point of is about 1250°C. Therefore, it is liquid in the melt before solidification, but becomes solid in the steel billet after solidification and becomes a vitrified soft inclusion.

[0243] The inclusions are elongated and broken during hot working (hot rolling) and become finer. Therefore, the bending fatigue strength of mechanical structural parts is improved. 2 , it is necessary to properly control the ratio of Si to Mn. This index is F1.

[0244] The larger F1 is, the more the bending fatigue strength of the mechanical structure parts manufactured using the steel material of this embodiment as a blank is improved. Moreover, when F1 is 1.00 or more, the bending fatigue strength is improved compared with SCM420H specified in JIS G 4052 (2016). Therefore, when F1 satisfies formula (1), that is, when F1 is 1.00 or more, the bending fatigue strength of the mechanical structure parts manufactured using the steel material of this embodiment is sufficiently improved, provided that the content of each element is within the range of this embodiment and F2 satisfies formula (2).

[0245] The preferred lower limit of F1 is 1.05, more preferably 1.07, and more preferably 1.10. The upper limit of F1 is not particularly limited. However, considering the content of each element in the chemical composition of this embodiment, the preferred upper limit of F1 is 2.10, more preferably 2.00, and more preferably 1.70.

[0246] [About the microstructure of steel]

[0247] The microstructure of the steel material according to the present embodiment contains ferrite, with the balance being pearlite and / or bainite, and the area fraction of ferrite is 50 to 70%.

[0248] When the area fraction of ferrite is less than 50%, the area fraction of pearlite and / or bainite in the steel is too high. In this case, the hardness of the steel is excessively increased. As a result, the machinability of the steel is reduced.

[0249] On the other hand, when the area fraction of ferrite exceeds 70%, the crystal grain size tends to become uneven during gas carburizing, and therefore, excessive heat treatment deformation occurs during gas carburizing.

[0250] When the area fraction of ferrite is 50 to 70% and the balance other than ferrite in the microstructure is pearlite and / or bainite, the machinability of the steel material is sufficiently improved. In addition, heat treatment deformation during gas carburizing can be suppressed.

[0251] In the steel material of the present embodiment, the microstructure at each cross-sectional observation position and each longitudinal cross-sectional observation position contains ferrite at an area fraction of 50 to 70%, and the remainder is pearlite and / or bainite.

[0252] The preferred lower limit of the area fraction of ferrite at each observation position is 52%, more preferably 55%, and further preferably 57%. The preferred upper limit of the area fraction of ferrite at each observation position is 68%, further preferably 65%, and further preferably 63%.

[0253] [About the microstructure unevenness on the cross section of steel]

[0254] The steel material of the present embodiment also sufficiently suppresses microstructural nonuniformity in a cross section perpendicular to the longitudinal direction of the steel material, that is, in a cross section. This point will be described below.

[0255] Figure 2 Schematic diagram of a cross section perpendicular to the longitudinal direction of the steel material of this embodiment. Figure 2 , the steel cross section CS is a circle with a radius R. On the cross section CS, the center position C1 of the cross section CS and eight R / 2 positions C2 to C9 arranged at 45° intervals around the center of the cross section CS, which are positions R / 2 from the center position C1 of the cross section CS in the radial direction, are defined as nine "cross section observation positions" C1 to C9.

[0256] The microstructures at cross-sectional observation positions C1 to C9 satisfy the following (A) and (B).

[0257] (A) The arithmetic mean of the area fraction of ferrite at cross-sectional observation positions C1 to C9 is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less.

[0258] (B) Among the average grain sizes of ferrite at cross-sectional observation positions C1 to C9, the ratio of the largest average grain size to the smallest average grain size is 2.00 or less.

[0259] (A) and (B) are described in detail below.

[0260] [About (A)]

[0261] As described in (A) above, in the steel material of the present embodiment, the arithmetic mean of the area fraction of ferrite at the cross-sectional observation positions C1 to C9 is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less.

[0262] Since the standard deviation of the area fraction of ferrite is 4.0% or less, the phase fraction variation of the microstructure at each cross-sectional observation position C1 to C9 is sufficiently suppressed. Therefore, during the gas carburizing process, the occurrence timing variation of the martensitic transformation at each cross-sectional observation position C1 to C9 can be suppressed.

[0263] When the standard deviation of the area fraction of ferrite at the cross-sectional observation positions C1 to C9 exceeds 4.0%, the phase fraction at each cross-sectional observation position C1 to C9 is highly non-uniform. In this case, heat treatment deformation during gas carburizing cannot be sufficiently suppressed.

[0264] Therefore, the standard deviation of the area fraction of ferrite at the cross-sectional observation positions C1 to C9 is 4.0% or less.

[0265] The preferred upper limit of the standard deviation of the area fraction of ferrite is 3.8%, more preferably 3.5%, and more preferably 3.0%. The lower limit of the standard deviation of the area fraction of ferrite is not particularly limited. The preferred lower limit of the standard deviation of the area fraction of ferrite is 0.1%, more preferably 0.5%, more preferably 1.0%, and more preferably 1.5%.

[0266] [About (B)]

[0267] The ratio of the maximum average grain size to the minimum average grain size among the average grain sizes of ferrite at the cross-sectional observation positions C1 to C9 is referred to as “ferrite average grain size ratio.” The ferrite average grain size ratio on the cross section is defined by the following formula.

[0268] Ferrite average grain size ratio = (maximum value of ferrite average grain size at C1 to C9) / (minimum value of ferrite average grain size at C1 to C9)

[0269] In the steel material of the present embodiment, the average ferrite grain size ratio at the cross-sectional observation positions C1 to C9 is less than 2.00. In this case, the average grain size unevenness of the ferrite at each cross-sectional observation position C1 to C9 is fully suppressed. That is, the ferrite particles at each position are concentrated. Therefore, the uneven martensitic transformation during the carburizing process can be fully suppressed. Therefore, the heat treatment deformation of the steel material during the carburizing process can be suppressed.

[0270] When the ferrite average grain size ratio is greater than 2.00, the ferrite grains at the observation positions C1 to C9 of each cross section are uneven. In this case, the heat treatment deformation of the steel material during gas carburizing cannot be fully suppressed. Therefore, the ferrite average grain size ratio is 2.00 or less.

[0271] The preferred upper limit of the ferrite average particle size ratio is 1.95, more preferably 1.90, and more preferably 1.80. The lower limit of the ferrite average particle size ratio is not particularly limited. The preferred lower limit of the ferrite average particle size ratio is 1.10, more preferably 1.20, more preferably 1.30, and more preferably 1.40.

[0272] [About the microstructure unevenness on the longitudinal section of steel]

[0273] The steel material of this embodiment has microstructural inhomogeneity not only in the cross section but also in the longitudinal section which is parallel to the longitudinal direction of the steel material and includes the central axis of the steel material. Since the microstructural inhomogeneity of the steel material of this embodiment is fully suppressed not only in the cross section but also in the longitudinal section, it is possible to fully suppress heat treatment deformation occurring in three dimensions. The suppression of microstructural inhomogeneity on the longitudinal section is described below.

[0274] Figure 3 Schematic diagram of a longitudinal section of a steel material according to the present embodiment, which is a cross section parallel to the longitudinal direction and including the central axis. Figure 3 On the longitudinal section LS of the steel material, three center axis positions L1 to L3 arranged at an interval of R / 2 on the center axis CL1 of the steel material and six R / 2 positions L4 to L9 arranged at a distance of R / 2 from each center axis position in the radial direction are defined as nine "longitudinal section observation positions" L1 to L9.

[0275] The microstructures at the above-mentioned 9 longitudinal cross-sectional observation positions L1 to L9 satisfy the following (C) and (D).

[0276] (C) The arithmetic mean of the area fraction of ferrite at the longitudinal cross-sectional observation positions L1 to L9 is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less.

[0277] (D) Among the average grain sizes of ferrite at the longitudinal cross-sectional observation positions L1 to L9, the ratio of the largest average grain size to the smallest average grain size (ferrite average grain size ratio) is 2.00 or less.

[0278] (C) and (D) are described in detail below.

[0279] [About (C)]

[0280] As described in (C) above, in the steel material of the present embodiment, the arithmetic mean of the area fraction of ferrite at the longitudinal cross-sectional observation positions L1 to L9 is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less.

[0281] Since the standard deviation of the area fraction of ferrite is 4.0% or less, the phase fraction variation of the microstructure at each longitudinal section observation position L1 to L9 is sufficiently suppressed. Therefore, during the gas carburizing process, the occurrence timing variation of the martensitic transformation at each longitudinal section observation position L1 to L9 can be suppressed.

[0282] When the standard deviation of the area fraction of ferrite at the longitudinal cross-sectional observation positions L1 to L9 exceeds 4.0%, the phase fraction at each longitudinal cross-sectional observation position L1 to L9 is highly non-uniform. In this case, heat treatment deformation during gas carburizing cannot be sufficiently suppressed.

[0283] Therefore, the standard deviation of the area fraction of ferrite at the longitudinal cross-sectional observation positions L1 to L9 is 4.0% or less.

[0284] The preferred upper limit of the standard deviation of the area fraction of ferrite is 3.8%, more preferably 3.5%, and more preferably 3.0%. The lower limit of the standard deviation of the area fraction of ferrite is not particularly limited. The preferred lower limit of the standard deviation of the area fraction of ferrite is 0.1%, more preferably 0.5%, more preferably 1.0%, and more preferably 1.5%.

[0285] [About (D)]

[0286] The ratio of the maximum average grain size to the minimum average grain size among the average grain sizes of ferrite at the longitudinal cross-sectional observation positions L1 to L9 is referred to as “ferrite average grain size ratio.” The ferrite average grain size ratio on the longitudinal cross section is defined by the following formula.

[0287] Ferrite average grain size ratio = (maximum value of ferrite average grain size at L1 to L9) / (minimum value of ferrite average grain size at L1 to L9)

[0288] In the steel material of the present embodiment, the average ferrite grain size ratio at the longitudinal section observation positions L1 to L9 is less than 2.00. In this case, the unevenness of the average ferrite grain size at each longitudinal section observation position L1 to L9 is fully suppressed. That is, the ferrite particles at each position are concentrated. Therefore, the uneven martensitic transformation during the carburizing process can be fully suppressed. Therefore, the heat treatment deformation of the steel material during the carburizing process can be suppressed.

[0289] When the ferrite average grain size ratio is greater than 2.00, the ferrite grains at each longitudinal section observation position L1 to L9 are uneven. In this case, the heat treatment deformation of the steel during gas carburizing cannot be fully suppressed. Therefore, the ferrite average grain size ratio at the 9 longitudinal section observation positions L1 to L9 is less than 2.00.

[0290] The preferred upper limit of the ferrite average particle size ratio is 1.95, more preferably 1.90, and more preferably 1.80. The lower limit of the ferrite average particle size ratio is not particularly limited. The preferred lower limit of the ferrite average particle size ratio is 1.10, more preferably 1.20, more preferably 1.30, and more preferably 1.40.

[0291] [Observation method of microstructure at each observation position, measurement method of ferrite area fraction and ferrite average grain size ratio]

[0292] The method of observing the microstructure at the cross-sectional observation positions C1 to C9 and the longitudinal cross-sectional observation positions L1 to L9 of the steel material of the present embodiment, and the method of measuring the area fraction of ferrite and the average ferrite grain size ratio are as follows.

[0293] [Observation method of microstructure of cross-section CS]

[0294] The method for observing the microstructure of the cross section CS is as follows. Samples including the cross section observation positions C1 to C9 are collected from the steel material. The surface of the sample surface corresponding to the cross section CS is used as the observation surface. On the observation surface, the observation field including the cross section observation position is set to 0.5 mm×1.0 mm.

[0295] After the observation surface of the sample was polished, the observation surface was etched with 3% nital (nitric acid etchant), and the observation field (0.5 mm×1.0 mm) of the observation surface after etching was observed with an optical microscope at 100 times magnification.

[0296] The contrast of each phase, such as ferrite, pearlite, and bainite, varies in the observation field. Specifically, in the observation field, ferrite is observed to be white, and bainite and pearlite are observed to be black compared to ferrite. Therefore, ferrite can be easily distinguished from other phases (pearlite and bainite). Ferrite is determined based on the contrast.

[0297] [About (A)]

[0298] Calculate the area (μm) of ferrite in each observation field (each cross-sectional observation position). 2 ). Using the area of ​​ferrite and the area of ​​the observation field, the area fraction (%) of ferrite in each observation field (each cross-sectional observation position) was calculated.

[0299] [Method for obtaining the arithmetic mean of the ferrite area fraction]

[0300] The arithmetic mean of the ferrite area fractions (%) in nine observation fields (cross-sectional observation positions) is defined as the arithmetic mean (%) of the ferrite area fractions at nine cross-sectional observation positions C1 to C9.

[0301] [Method for obtaining the standard deviation of the ferrite area fraction]

[0302] The standard deviation (%) of the ferrite area fraction at the nine cross-sectional observation positions C1 to C9 is calculated from the ferrite area fraction (%) in the nine observation fields (cross-sectional observation positions). The standard deviation here is the sample standard deviation.

[0303] [About (B)]

[0304] The area (μm) of each ferrite grain observed in each of the above-mentioned observation fields (each cross-sectional observation position C1 to C9) was measured. 2 ). The arithmetic mean of the area of ​​each ferrite particle is obtained. The circle equivalent diameter of the arithmetic mean of the obtained area is defined as the average particle size (μm) of the ferrite at each cross-sectional observation position C1 to C9. Here, the circle equivalent diameter refers to the diameter (μm) of a circle of the same area as the arithmetic mean of the area of ​​the ferrite particles.

[0305] The average grain size of ferrite at the 9 cross-sectional observation positions C1 to C9 was determined. Among these average grain sizes of ferrite, the maximum average grain size (μm) and the minimum average grain size (μm) of ferrite were determined. The ratio of the determined maximum average grain size to the minimum average grain size (ferrite average grain size ratio) was determined.

[0306] [Observation method of microstructure of longitudinal section LS]

[0307] The method for observing the microstructure of the longitudinal section LS is as follows. Samples containing the longitudinal section observation positions L1 to L9 are collected from the steel. The surface of the sample surface corresponding to the longitudinal section LS is used as the observation surface. On the observation surface, the observation field containing the longitudinal section observation position is set to 0.5 mm × 1.0 mm. More specifically, the 0.5 mm length of the observation field is the radial direction of the steel, and 1.0 mm is the length direction of the steel.

[0308] After grinding the observation surface of the sample, etch the observation surface with 3% nital (nitric acid ethanol etching solution). Observe the observation field (0.5mm×1.0mm) of the observation surface after etching with an optical microscope at 100 times. Determine each phase in the observation field in the same way as the microstructure observation of the cross section CS.

[0309] [About (C)]

[0310] In the phase determined by the above method, the area (μm) of ferrite in each observation field (each longitudinal section observation position) was calculated. 2 ). The area fraction (%) of ferrite in each observation field (each longitudinal cross-sectional observation position) was calculated using the area of ​​ferrite and the area of ​​the observation field.

[0311] [Method for obtaining the arithmetic mean of the area fraction of ferrite]

[0312] The arithmetic mean of the ferrite area fractions (%) in nine observation fields (longitudinal cross-sectional observation positions) is defined as the arithmetic mean of the ferrite area fractions (%) at nine longitudinal cross-sectional observation positions L1 to L9.

[0313] [Method for obtaining the standard deviation of the area fraction of ferrite]

[0314] From the ferrite area fraction (%) in the nine observation fields (longitudinal cross-sectional observation positions), the standard deviation (%) of the ferrite area fraction at the nine longitudinal cross-sectional observation positions L1 to L9 was calculated.

[0315] [About (D)]

[0316] The area (μm) of each ferrite grain observed in each of the above-mentioned observation fields (each of the longitudinal cross-sectional observation positions L1 to L9) was measured. 2 The arithmetic mean of the area of ​​each ferrite grain is calculated. The circle equivalent diameter of the obtained arithmetic mean of the area is defined as the average grain size (μm) of ferrite at each longitudinal cross-sectional observation position L1 to L9.

[0317] The average grain size of ferrite at the 9 longitudinal cross-sectional observation positions L1 to L9 was determined. Among these average grain sizes of ferrite, the maximum average grain size (μm) and the minimum average grain size (μm) of ferrite were determined. The ratio of the determined maximum average grain size to the minimum average grain size (ferrite average grain size ratio) was determined.

[0318] [About formula (2)]

[0319] The chemical composition of the steel material of the present embodiment also satisfies the following formula (2).

[0320] 1-(0.5C+0.03Si+0.06Mn+0.01Cr+0.05Mo)<0.800 (2)

[0321] Here, the symbol of each element in formula (2) is substituted by the content (mass %) of the corresponding element. When the corresponding element is not contained, the symbol of the element is substituted by "0".

[0322] In the steel material of the present embodiment, the microstructure of not only the cross section CS but also the longitudinal section LS is uniform. However, even if the microstructure of the cross section observation positions C1 to C9 in the cross section CS satisfies (A) and (B), and the microstructure of the longitudinal section observation positions L1 to L9 in the longitudinal section LS satisfies (C) and (D) to make the microstructure uniform, as described above, during the quenching process of the vacuum carburizing treatment, a micro time band in which the martensitic phase transformation part and the martensitic phase non-transformation part are mixed will inevitably be generated. When the heat treatment strain of the martensitic phase non-transformation part in this micro time band is large, heat treatment deformation will occur. For this reason, the steel material of the present embodiment also satisfies formula (2).

[0323] Definition: F2 = 1-(0.5C + 0.03Si + 0.06Mn + 0.01Cr + 0.05Mo). F2 is an indicator of the amount of heat treatment deformation of steel when the steel is subjected to gas carburizing treatment. Among the elements in the above chemical composition, C, Si, Mn, Cr and Mo contained in F2 will increase the strength of the martensite non-transformed part in the micro time band where the martensite transformation part and the martensite non-transformed part coexist during quenching.

[0324] See also Figure 1 As F2 decreases, the maximum deformation ratio decreases, and the heat treatment deformation decreases. Moreover, when F2 is less than 0.800, the maximum deformation ratio decreases significantly. That is, the maximum deformation ratio relative to F2 has an inflection point near F2 = 0.800.

[0325] Therefore, on the premise that the contents of each element are within the range of the present embodiment, when F2 also satisfies the formula (2), that is, when F2 is less than 0.800, the heat treatment deformation of the steel material during the gas carburizing treatment can be sufficiently suppressed.

[0326] The preferred upper limit of F2 is 0.799, more preferably 0.797, and more preferably 0.795. The lower limit of F2 is not particularly limited. However, considering the upper limit of the content of each element in the chemical composition of the present embodiment, the preferred lower limit of F2 is 0.765, more preferably 0.770, and more preferably 0.775. The numerical value of F2 is the value rounded to the fourth decimal place.

[0327] The steel material of the present embodiment having the above structure has the content of each element in the chemical composition within the range of the present embodiment, and F1 and F2 satisfy the formula (1) and the formula (2), and the microstructure at the cross-sectional observation positions C1 to C9 and the longitudinal cross-sectional observation positions L1 to L9 is within the range of the present embodiment. Therefore, the steel material of the present embodiment has excellent machinability after hot working. In addition, when the steel material of the present embodiment is subjected to vacuum carburizing treatment, the mechanical structural parts have excellent bending fatigue strength and surface fatigue strength, and can fully suppress heat treatment deformation.

[0328] [About the microstructure of steel]

[0329] The steel material of this embodiment is a so-called as-rolled material. Therefore, in the above-mentioned observation field of the longitudinal section observation position L1 to L9 of the steel material of this embodiment, a so-called banded structure can be observed. Here, the banded structure refers to a well-known microstructure, such as Figure 4 As shown, the ferrite (ferrite band) F extending in the longitudinal direction of the steel material and the non-ferrite (non-ferrite band) NF extending in the longitudinal direction of the steel material are alternately stacked in the radial direction. The non-ferrite is pearlite and / or bainite.

[0330] [Use of Steel]

[0331] As described above, the steel material of this embodiment is suitable as a material for machine structural parts. The steel material of this embodiment is particularly suitable for use in gears or shafts for automobiles, construction machinery, industrial machinery, and the like.

[0332] [Method for manufacturing steel materials]

[0333] An example of a method for manufacturing the steel material of the present embodiment will be described. The method for manufacturing the steel material described below is an example for manufacturing the steel material of the present embodiment. Therefore, the steel material having the above-mentioned structure may also be manufactured by other manufacturing methods other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of the method for manufacturing the steel material of the present embodiment.

[0334] An example of a method for producing a steel material according to the present embodiment includes the following steps.

[0335] (Process 1) Process of preparing blank (blanket preparation process)

[0336] (Step 2) Step of hot working the blank to produce a steel material (hot working step)

[0337] Each step is described below.

[0338] [(Process 1) Blank preparation process]

[0339] In the billet preparation process, a billet of the steel material of the present embodiment is prepared. Specifically, a molten steel having a chemical composition in which the content of each element is within the range of the present embodiment, and F1 satisfies the formula (1), and F2 satisfies the formula (2) is manufactured. The refining method is not particularly limited, and a known method may be used. For example, converter refining (primary refining) is performed on molten iron manufactured by a known method. The molten steel tapped from the converter is subjected to a known secondary refining. In the secondary refining, the content of alloy elements in the molten steel is adjusted to manufacture molten steel having a chemical composition in which the content of each element is within the range of the present embodiment, and F1 satisfies the formula (1), and F2 satisfies the formula (2).

[0340] Using the molten steel produced by the above refining method, a billet is produced by a known casting method. For example, an ingot can be produced from the molten steel by an ingot casting method. In addition, a bloom or billet can be produced from the molten steel by a continuous casting method. By the above method, a billet (ingot, bloom or billet) is produced. When a continuous casting method is used, pressure can be applied to the solidifying billet.

[0341] [(Process 2) Hot working process]

[0342] In the hot working step, the billet (ingot, bloom or billet) prepared in the billet preparation step is hot worked to produce the steel material of the present embodiment. The shape of the steel material is not particularly limited, and may be, for example, a steel bar or a wire. In the following description, as an example, the case where the steel material is a steel bar is described. However, even if the steel material is in a shape other than a steel bar, it can be produced by the same hot working step.

[0343] The thermal processing step includes the following steps.

[0344] (Process 21) Initial rolling process

[0345] Heating temperature: 1250~1300℃

[0346] Holding time: more than 10 hours

[0347] (Process 22) Finishing rolling process

[0348] Heating temperature: 1150~1200℃

[0349] Holding time: 1.5 to 3.0 hours

[0350] Final temperature: 950~1000℃

[0351] (Step 23) Temperature maintenance step

[0352] Average cooling rate at 900-800°C: 0.05°C / sec or less

[0353] (Step 24) Cooling Step

[0354] Average cooling rate at 800-300°C: 0.10-1.00°C / sec

[0355] Each step is described below.

[0356] [(Process 21) Initial rolling process]

[0357] In the initial rolling process, the billet is hot rolled to produce small billets. Specifically, in the initial rolling process, the billet is hot rolled (initial rolling) by a primary rolling mill to produce small billets. When a tandem rolling mill is arranged downstream of the primary rolling mill, the tandem rolling mill can also be used to hot roll the small billets after initial rolling to produce small billets of smaller size. In the tandem rolling mill, a horizontal rolling mill with a pair of horizontal rolls and a vertical rolling mill with a pair of vertical rolls are alternately arranged in a row. In summary, in the initial rolling process, the billet is made into small billets by a primary rolling mill, or by a primary rolling mill and a tandem rolling mill.

[0358] The conditions in the initial rolling process are as follows.

[0359] Heating temperature: 1250~1300℃

[0360] Holding time: more than 10 hours

[0361] The heating temperature in the heating furnace in the initial rolling process is 1250-1300°C. The holding time at the heating temperature (1250-1300°C) is more than 10 hours. When the heating temperature in the heating furnace in the initial rolling process is 1250-1300°C and the holding time at the heating temperature (1250-1300°C) is more than 10 hours, the solidification segregation in the billet generated in the billet preparation process can be fully alleviated under the premise of meeting other manufacturing conditions. Therefore, the standard deviation of the area fraction of ferrite at each cross-sectional observation position C1-C9 and each longitudinal cross-sectional observation position L1-L9 is less than 4.0%. The upper limit of the holding time at the heating temperature is not particularly limited. However, considering the production cost, the preferred upper limit of the holding time at the heating temperature is 30 hours.

[0362] It should be noted that the billet produced through the initial rolling step is naturally cooled (air cooled) to room temperature before the finish rolling step.

[0363] It should be noted that the area reduction ratio in the initial rolling step is 30% or more. Here, the area reduction ratio (%) is defined by the following formula.

[0364] Sectional reduction ratio (%) = (1-area of ​​the cross section of the steel material after initial rolling (the cross section perpendicular to the length direction) / area of ​​the cross section of the billet before initial rolling (the cross section perpendicular to the length direction)) × 100

[0365] When the cross-sectional reduction rate in the initial rolling process is 30% or more, and provided that other manufacturing conditions are satisfied, the standard deviation of the area fraction of ferrite at each cross-sectional observation position C1 to C9 and each longitudinal cross-sectional observation position L1 to L9 is 4.0% or less.

[0366] [(Process 22) Finishing rolling process]

[0367] In the finish rolling process, first, the billet cooled to room temperature is heated in a heating furnace, and then hot-rolled in a continuous rolling mill to produce a steel bar as a steel material.

[0368] The conditions in the finish rolling process are as follows.

[0369] Heating temperature: 1150~1200℃

[0370] Holding time: 1.5 to 3.0 hours

[0371] Final temperature: 950~1000℃

[0372] [Heating temperature and holding time]

[0373] The heating temperature in the heating furnace in the finishing rolling process is 1150-1200°C. The holding time at the heating temperature (1150-1200°C) is 1.5-3.0 hours. When the heating temperature of the heating furnace in the finishing rolling process is 1150-1200°C and the holding time at the heating temperature (1150-1200°C) is 1.5-3.0 hours, the temperature unevenness in the steel (steel bar) can be fully suppressed under the premise of meeting other manufacturing conditions. Therefore, the average ferrite grain size ratio at the cross-sectional observation positions C1-C9 and the longitudinal cross-sectional observation positions L1-L9 is less than 2.00.

[0374] [Final temperature]

[0375] In the finishing rolling process, hot rolling (finishing rolling) is carried out using a tandem rolling mill with multiple rolling mills arranged in a row. In the hot rolling performed by the tandem rolling mill, the temperature of the steel at the exit side of the rolling mill that finally presses the steel is defined as the final temperature (°C). It should be noted that the steel temperature refers to the surface temperature of the steel.

[0376] The final temperature is 950-1000°C. When the final temperature is 950-1000°C, the austenite grain size variation in the steel (steel bar) is sufficiently suppressed, provided that other manufacturing conditions are met. Therefore, when the austenite phase is transformed into ferrite through the temperature holding step and cooling step described later, the variation in the average grain size of the ferrite is sufficiently suppressed. Therefore, the average ferrite grain size ratio at the cross-sectional observation positions C1-C9 and the longitudinal section observation positions L1-L9 is 2.00 or less.

[0377] [(Step 23) Temperature maintaining step]

[0378] In the temperature holding step, the temperature of the steel material is held after the finish rolling step and before the cooling step. The conditions in the temperature holding step are as follows.

[0379] Average cooling rate at steel temperature of 900-800°C: 0.05°C / sec or less

[0380] After the finishing rolling process, the average cooling rate at a steel material temperature of 900 to 800°C is suppressed to 0.05°C / second or less. For example, after the finishing rolling process, the average cooling rate of the steel material at a steel material temperature of 900 to 800°C is controlled to 0.05°C / second or less using a slow cooling hood, a heat preservation hood or a temperature holding furnace.

[0381] When the average cooling rate at a steel temperature of 900 to 800°C is less than 0.05°C / second, the temperature unevenness in the axial direction (length direction) of the steel can be suppressed, provided that other manufacturing conditions are met. Therefore, the unevenness in the timing of the ferrite transformation in the axial direction of the steel can be suppressed. Therefore, the uneven growth of ferrite particles in the axial direction (longitudinal section) of the steel can be particularly suppressed. Specifically, the following mechanism works.

[0382] In the steel after the finishing rolling process, as the temperature of the steel drops, austenite gradually transforms into ferrite. In the range of 900-800°C, if there is temperature unevenness in the axial direction of the steel, after the finishing rolling process, ferrite generated in the earlier stage and ferrite generated in the later stage are mixed. In this case, the ferrite particles generated in the early stage tend to become coarser than the ferrite particles that undergo phase transformation in the later stage. As a result, the unevenness of ferrite particles becomes larger, especially in the axial direction (longitudinal section) of the steel.

[0383] When the average cooling rate is high at a steel material temperature of 900 to 800° C., the temperature variation in the axial direction (longitudinal section) of the steel material becomes large. Therefore, the average ferrite grain size ratio at the longitudinal section observation positions L1 to L9 becomes large.

[0384] For this reason, in the present embodiment, the average cooling rate at a steel temperature of 900 to 800°C is controlled to be less than 0.05°C / second. In this case, the temperature unevenness in the axial direction (longitudinal section) of the steel can be suppressed. Therefore, the deviation in the timing of the formation (phase transformation) of ferrite in the longitudinal section of the steel is suppressed. As a result, under the premise of satisfying other manufacturing conditions, the average ferrite grain size ratio at the longitudinal section observation positions L1 to L9 can be less than 2.00.

[0385] [(Step 24) Cooling Step]

[0386] In the cooling process, the temperature of the steel material after the temperature holding process is cooled. The conditions in the cooling process are as follows.

[0387] Average cooling rate at steel temperature of 800-300°C: 0.10-1.00°C / sec

[0388] The steel material having a steel material temperature of 800 to 300°C is cooled at an average cooling rate of 0.10 to 1.00°C / second. When the average cooling rate at a steel material temperature of 800 to 300°C is set to 0.10 to 1.00°C / second, and under the premise that other manufacturing conditions are satisfied, the arithmetic mean of the area fraction of ferrite at the cross-sectional observation positions C1 to C9 and the longitudinal cross-sectional observation positions L1 to L9 is 50 to 70%.

[0389] Through the above manufacturing process, the steel material of this embodiment having the above-mentioned structure can be manufactured. It should be noted that, as described above, the steel material of this embodiment is a rolled material.

[0390] The steel material of this embodiment is suitable as a blank for mechanical structural parts manufactured by vacuum carburizing. Among them, the steel material of this embodiment can be subjected to other surface hardening heat treatments other than vacuum carburizing to be made into mechanical structural parts. Other surface hardening heat treatments refer to, for example, quenching and tempering, high-frequency quenching and tempering, nitriding treatment (nitriding quenching and tempering), etc.

[0391] [About mechanical structural parts]

[0392] The machine structural parts can be used, for example, in automobiles and construction vehicles, etc. The machine structural parts are, for example, gears and shafts used in steering mechanisms.

[0393] The machine structural component using the steel material of the present embodiment as a blank is manufactured by a known manufacturing method. For example, the machine structural component is manufactured by the following method.

[0394] An example of a method for manufacturing a machine structural component includes the following steps.

[0395] Thermal processing

[0396] Cutting process

[0397] Heat treatment process

[0398] Hereinafter, each step will be described.

[0399] [Hot working process]

[0400] In the hot working process, the steel material of this embodiment is subjected to hot working. The hot working is, for example, well-known hot forging. The heating temperature in the hot working process is, for example, 1000 to 1300° C. The steel material after hot working is naturally cooled (air cooled). If necessary, the steel material after natural cooling can be subjected to annealing treatment.

[0401] [Cutting process]

[0402] The steel material after the hot working process is subjected to a cutting process to produce an intermediate product of a predetermined shape. During this cutting process, the steel material is required to have high machinability. In the cutting process, known cutting processes are performed. Through cutting, it is possible to produce mechanical structural parts with a precise shape that is difficult to produce only by hot working processes.

[0403] [Heat treatment process]

[0404] The intermediate product after cutting is subjected to heat treatment. Here, "heat treatment" includes the well-known vacuum carburizing treatment and the well-known tempering process. It should be noted that, as mentioned above, the vacuum carburizing treatment also includes the vacuum carbonitriding treatment.

[0405] The vacuum carburizing process includes a vacuum carburizing process and a quenching process. In the vacuum carburizing process, it is a technical matter known to those skilled in the art that the surface hardness, core hardness, and surface carbon concentration of the mechanical structural component can be appropriately adjusted by appropriately adjusting known conditions.

[0406] Hereinafter, as an example of the heat treatment process, a well-known vacuum carburizing process will be described. It should be noted that those skilled in the art will appreciate that the well-known vacuum carbonitriding process can also be performed by the same process as the vacuum carburizing process.

[0407] [Vacuum carburizing and quenching treatment]

[0408] The vacuum carburizing and quenching treatment includes a vacuum carburizing step and a quenching step. The vacuum carburizing step and the quenching step are described below.

[0409] [Vacuum carburizing process]

[0410] Figure 5 This is a diagram showing an example of the heating pattern in the vacuum carburizing step S10 and the quenching step S20. Figure 5 The vertical axis is the treatment temperature (°C) during vacuum carburizing treatment, and the horizontal axis is the time (minutes). Figure 5 The vacuum carburizing step S10 includes a heating step S0, a soaking step S1, a carburizing step S2 and a diffusion step S3.

[0411] In the heating step S0, the intermediate product placed in the furnace is heated to the carburizing temperature Tc. At this time, the pressure in the furnace is not more than 10 Pa. The carburizing temperature Tc in the heating step S0 is, for example, 900 to 1100°C.

[0412] In the soaking step S1, the intermediate product is held at the carburizing temperature Tc for a predetermined time (holding time t1) to perform soaking treatment. The holding time t1 at the carburizing temperature Tc in the soaking step S1 is, for example, 5 to 120 minutes. The pressure in the furnace in the soaking step S1 may be 10 Pa or less, or a nitrogen atmosphere of 1000 Pa or less may be achieved by simultaneously introducing nitrogen gas and exhausting the gas by a vacuum pump.

[0413] In the carburizing step S2, the intermediate product is held at the carburizing temperature Tc for a predetermined time (holding time t2). The holding time t2 at the carburizing temperature Tc in the carburizing step S2 may be adjusted appropriately. The holding time t2 at the carburizing temperature Tc is, for example, 20 to 60 minutes.

[0414] A known carburizing gas is used as the carburizing gas in the carburizing step S2. The carburizing gas is, for example, a hydrocarbon gas such as acetylene, propane, or ethylene.

[0415] The carburizing gas pressure in the carburizing step S2 is set to a predetermined pressure depending on the type of carburizing gas. When acetylene is used as the carburizing gas, the carburizing gas pressure is, for example, 10 to 1000 Pa. When propane is used as the carburizing gas, the carburizing gas pressure is, for example, 200 to 3000 Pa.

[0416] In the diffusion step S3, the intermediate product is held at the carburizing temperature Tc for a predetermined time (holding time t3). Here, the holding time t3 at the carburizing temperature Tc in the diffusion step S3 can be adjusted appropriately. The holding time t3 at the carburizing temperature Tc is, for example, 40 to 90 minutes.

[0417] In order to remove the residual gas in the carburizing step, the pressure in the furnace in the diffusion step S3 may be 100 Pa or less. Alternatively, the nitrogen atmosphere may be 1000 Pa or less by simultaneously introducing nitrogen and evacuating with a vacuum pump.

[0418] [Quenching process]

[0419] The intermediate product after the vacuum carburizing step S10 is subjected to a quenching step S20. After the vacuum carburizing step S10, a known cooling method may be used for the cooling method to reach the quenching temperature Ts in the quenching step S20. The cooling method may be, for example, air cooling under vacuum conditions, or gas cooling or other methods. When cooling under vacuum conditions is performed, for example, natural cooling may be performed at a pressure of less than 100 Pa. When gas cooling is performed, an inert gas such as nitrogen and / or helium may be used as the cooling gas.

[0420] The quenching process S20 includes a soaking process S4. The soaking process S4 maintains the intermediate product after the vacuum carburizing process S10 at the quenching temperature. In the quenching process S20, after the soaking process S4, the intermediate product is quenched by rapid cooling. The quenching temperature Ts is not particularly limited, and is, for example, 800 to 880°C. The holding time t4 at the quenching temperature Ts is not particularly limited, and is, for example, 10 to 80 minutes. The atmosphere during the holding process at the quenching temperature Ts is not particularly limited, and is, for example, a nitrogen atmosphere. The pressure in the furnace can be below atmospheric pressure, or, for example, below 400 hPa. The cooling method in the quenching treatment is oil cooling or water cooling. Specifically, the intermediate product maintained at the quenching temperature is immersed in a cooling bath to which oil or water as a cooling medium is added for rapid cooling. The temperature of the oil or water as a cooling medium is, for example, 60 to 200°C. In addition, deep cryogenic treatment can be implemented as needed.

[0421] [Tempering process]

[0422] The intermediate product after the quenching step is subjected to a known tempering step. The tempering temperature is, for example, 100 to 200° C. The holding time at the tempering temperature is, for example, 90 to 150 minutes.

[0423] [Other processes]

[0424] As required, the intermediate product after the above-mentioned tempering process can be further subjected to grinding or shot peening. In the case of grinding, cutting is applied to give the steel a shape. By performing cutting, a more precise shape can be given to the steel. In addition, in the case of shot peening, compressive residual stress is introduced into the surface portion of the intermediate product after vacuum carburizing. Compressive residual stress inhibits the occurrence and development of fatigue cracks. Therefore, the bending fatigue strength and surface fatigue strength of mechanical structure parts are improved. Shot peening can be carried out by a known method. Shot peening uses, for example, particles with a diameter of less than 0.7 mm, and it is desired to be carried out under conditions where the arc height is more than 0.4 mm.

[0425] Example 1

[0426] Steel materials having the chemical compositions shown in Table 1 were prepared. In addition, the steel grade number A corresponds to SCM420H specified in JIS G4052 (2016).

[0427] [Table 1]

[0428] Table 1

[0429]

[0430] The "-" in Table 1 indicates that the corresponding element content is 0% in the significant figures (to the smallest digit) specified in the embodiment. In other words, it indicates that the corresponding element content is 0% after rounding off the mantissa in the significant figures (to the smallest digit) specified in the above embodiment.

[0431] For example, the Mo content specified in this embodiment is specified with a value accurate to the second decimal place. Therefore, the steel grade number C in Table 1 indicates that the measured Mo content is 0% after rounding off to the third decimal place.

[0432] In addition, the Nb content specified in this embodiment is specified with a value accurate to the third decimal place. Therefore, the steel grade number A in Table 1 indicates that the measured Nb content is 0% after rounding off to the fourth decimal place.

[0433] It should be noted that rounding means that when the number of digits (mantissa) after the specified minimum digit is less than 5, it will be discarded, and when it is greater than 5, it will be rounded up.

[0434] The steel material was produced by the following method. Molten steel was continuously cast to produce a cast ingot (bloom) as a billet. The bloom as the billet was subjected to a hot working process under the conditions shown in Table 2.

[0435] [Table 2]

[0436] Table 2

[0437]

[0438] The temperature described in the "Heating Temperature (°C)" column of the "Initial Rolling Process" and "Finishing Rolling Process" columns is the heating temperature (°C). The time described in the "Holding Time (hours)" column of the "Initial Rolling Process" and "Finishing Rolling Process" columns is the holding time (hours) at the heating temperature. The temperature in the "Final Temperature (°C)" column of the "Finishing Rolling Process" column is the steel material temperature (surface temperature of the steel material) (°C) at the exit side of the last rolling mill in the continuous rolling mill in the finishing rolling process. The speed described in the "Cooling Rate (°C / sec)" column of the "Temperature Holding Process" column is the average cooling rate (°C / sec) at a steel material temperature of 900 to 800°C. The speed described in the "Cooling Rate (°C / sec)" column of the "Cooling Process" column is the average cooling rate (°C / sec) at a steel material temperature of 800 to 300°C.

[0439] In the initial rolling process of the hot working process, the manufactured large square billet is heated at the heating temperature and holding time shown in Table 2. Then, the large square billet is initially rolled by an initial rolling mill to manufacture small square billets. The manufactured small square billet is water-cooled to room temperature (25°C). The cross-section of the small square billet perpendicular to the length direction is a rectangle of 162mm×162mm. The heating temperature and holding time in the initial rolling process are shown in Table 2. It should be noted that the cross-sectional shrinkage rate in the initial rolling process is above 30% under all manufacturing conditions.

[0440] The billet after the initial rolling process was subjected to a finishing rolling process under the conditions shown in Table 2 to produce a steel material (steel bar) with a diameter of 50 mm. Specifically, the billet was heated at the heating temperature (°C) and the holding time (hours) shown in the finishing rolling process column of Table 2. The heated billet was subjected to finishing rolling to produce a steel bar. The final temperature (°C) at this time is shown in Table 2.

[0441] The temperature holding process is performed on the steel (steel bar) after the finish rolling process. Manufacturing conditions a to i are: for steel with a steel temperature of 900 to 800°C, the average cooling rate is adjusted to 0.05°C / second or less by using a slow cooling hood. On the other hand, manufacturing condition j is: for steel with a steel temperature of 900 to 800°C, no slow cooling hood is used and natural cooling is performed. Therefore, the average cooling rate at a steel temperature of 900 to 800°C is greater than 0.05°C / second.

[0442] After the temperature holding step, a cooling step is performed. Specifically, the average cooling rate (°C / sec) at a steel material temperature of 800 to 300°C in each manufacturing condition is shown in the cooling step column of Table 2.

[0443] The steel material having a temperature of 300° C. or less is naturally cooled (air cooled) to room temperature. Through the above manufacturing process, steel materials (steel bars) of test numbers 1 to 22 as shown in Table 3 are manufactured. It should be noted that test number 1 is an example using SCM420H as a reference steel, and the steel material having the chemical composition of SCM420H adopts manufacturing condition a, which is one of the conventionally used manufacturing methods.

[0444] [Table 3]

[0445] Table 3

[0446]

[0447] [Evaluation test]

[0448] The following items were obtained for the steel materials (steel bars) of the respective test numbers produced through the above-described production process.

[0449] (A1) Microstructure observation of cross section

[0450] (A11) Arithmetic mean of the area fraction of ferrite in the cross section

[0451] (A12) Standard deviation of the area fraction of ferrite in the cross section

[0452] (A13) Average ferrite grain size ratio of cross section

[0453] (B1) Microstructure observation of longitudinal section

[0454] (B11) Arithmetic mean of the area fraction of ferrite in the longitudinal section

[0455] (B12) Standard deviation of the area fraction of ferrite in the longitudinal section

[0456] (B13) Ferrite average grain size ratio in longitudinal section

[0457] In addition, the following evaluation tests were performed on the steel materials of each test number.

[0458] (C1) Machinability evaluation test

[0459] (C2) Bending fatigue strength evaluation test

[0460] (C3) Surface fatigue strength evaluation test

[0461] (C4) Heat treatment deformation evaluation test

[0462] The details are described below.

[0463] [(A1) Microstructure observation of cross section]

[0464] Nine samples including cross-sectional observation positions C1 to C9 were collected from the steel material of each test number. The surface corresponding to the cross-sectional area CS of each sample was used as the observation surface. On the observation surface, the observation field including the cross-sectional observation position was set to 0.5 mm×1.0 mm.

[0465] After polishing the observation surface of the sample, the observation surface was etched with 3% nital (nitric acid ethanol etching solution). The observation field (0.5 mm×1.0 mm) of the etched observation surface was observed with an optical microscope at 100 times magnification. In the observation field, the phase was determined by contrast.

[0466] The observed phases are shown in the "Phase" column of the "Cross-section" column of Table 3. In the "Phase" column of the "Cross-section" column of Table 3, "○" is recorded when the microstructure at all cross-section observation positions contains ferrite and the balance is pearlite and / or bainite. In all test numbers, the microstructure of the cross section contains ferrite and the balance is pearlite and / or bainite.

[0467] [(A11) Arithmetic mean of the area fraction of ferrite in the cross section]

[0468] Calculate the area (μm) of ferrite in each observation field (each cross-sectional observation position). 2 ). Using the area of ​​ferrite and the area of ​​the observation field, the area fraction (%) of ferrite in each observation field (each cross-sectional observation position) was calculated.

[0469] The arithmetic mean of the ferrite area fraction (%) in the 9 observation fields (cross-sectional observation positions) is defined as the arithmetic mean of the ferrite area fraction (%) at the 9 cross-sectional observation positions C1 to C9. The obtained arithmetic mean of the ferrite area fraction is shown in the "Ferrite Area Fraction (%)" column of the "Cross Section" column of Table 3.

[0470] [Standard deviation of the area fraction of ferrite in the (A12) cross section]

[0471] From the ferrite area fraction (%) in the 9 observation fields (cross-sectional observation positions), the standard deviation (%) of the ferrite area fraction at the 9 cross-sectional observation positions C1 to C9 was calculated. The obtained standard deviation is shown in the "Standard deviation (%) of the ferrite area fraction" column in the "Cross-sectional" column of Table 3.

[0472] [(A13) Ferrite average grain size ratio in cross section]

[0473] In addition, the area (μm) of each ferrite grain observed in each of the above-mentioned observation fields (each cross-sectional observation position C1 to C9) was measured. 2 ). The arithmetic mean of the area of ​​each ferrite grain is calculated at each cross-sectional observation position C1 to C9. The circle equivalent diameter of the obtained arithmetic mean of the area is defined as the average grain size (μm) of the ferrite at each cross-sectional observation position C1 to C9.

[0474] The average particle size of ferrite at the 9 cross-sectional observation positions C1 to C9 is determined. And among these average particle sizes of ferrite, the maximum average particle size (μm) and the minimum average particle size (μm) of ferrite are determined. The ratio of the determined maximum average particle size to the minimum average particle size (ferrite average particle size ratio) is determined. The obtained ferrite average particle size ratio is shown in the "Ferrite particle size ratio" column of the "Cross-sectional" column of Table 3.

[0475] [(B1) Observation of microstructure of longitudinal section]

[0476] Nine samples including longitudinal section observation positions L1 to L9 were collected from the steel material of each test number. The surface corresponding to the longitudinal section LS on the surface of each sample was used as the observation surface. On the observation surface, the observation field including the longitudinal section observation position was set to 0.5 mm×1.0 mm.

[0477] After polishing the observation surface of the sample, the observation surface was etched with 3% nital (nitric acid ethanol etching solution). The observation field (0.5 mm×1.0 mm) of the etched observation surface was observed with an optical microscope at 100 times magnification. In the observation field, the phase was determined by contrast.

[0478] The observed phases are shown in the "Phase" column of the "Longitudinal Section" column of Table 3. In the "Phase" column of the "Longitudinal Section" column of Table 3, when the microstructure at all the longitudinal section observation positions contains ferrite, and the balance is pearlite and / or bainite, it is recorded as "○". In all test numbers, the microstructure of the longitudinal section contains ferrite, and the balance is pearlite and / or bainite.

[0479] [(B11) Arithmetic mean of the area fraction of ferrite in the longitudinal section]

[0480] Calculate the area (μm) of ferrite in each observation field (each longitudinal section observation position). 2 ). The area fraction (%) of ferrite in each observation field (each longitudinal cross-sectional observation position) was calculated using the area of ​​ferrite and the area of ​​the observation field.

[0481] The arithmetic mean of the ferrite area fraction (%) in 9 observation fields (longitudinal section observation positions) is defined as the arithmetic mean of the ferrite area fraction (%) at 9 longitudinal section observation positions L1 to L9. The obtained arithmetic mean of the ferrite area fraction is shown in the "Ferrite Area Fraction (%)" column of the "Longitudinal Section" column of Table 3.

[0482] [(B12) Standard deviation of the area fraction of ferrite in the longitudinal section]

[0483] From the ferrite area fraction (%) in the 9 observation fields (longitudinal section observation positions), the standard deviation (%) of the ferrite area fraction at the 9 longitudinal section observation positions L1 to L9 was calculated. The obtained standard deviation is shown in the "Standard deviation (%) of the ferrite area fraction" column of the "longitudinal section" column of Table 3.

[0484] [(B13) Ferrite average grain size ratio in longitudinal section]

[0485] In addition, the area (μm) of each ferrite grain observed in each of the above-mentioned observation fields (each of the longitudinal cross-sectional observation positions L1 to L9) was measured. 2). The arithmetic mean of the area of ​​each ferrite grain is calculated at each longitudinal section observation position L1 to L9. The circle equivalent diameter of the obtained arithmetic mean of the area is defined as the average grain size (μm) of the ferrite at each longitudinal section observation position L1 to L9.

[0486] The average grain size of ferrite at the 9 longitudinal section observation positions L1 to L9 is determined. And among these ferrite average grain sizes, the maximum average grain size (μm) and the minimum average grain size (μm) of ferrite are determined. The ratio of the determined maximum average grain size to the minimum average grain size (ferrite average grain size ratio) is determined. The obtained ferrite average grain size ratio is shown in the "Ferrite Grain Size Ratio" column of the "Longitudinal Section" column of Table 3.

[0487] [Evaluation test]

[0488] [(C1) Machinability evaluation test]

[0489] The machinability evaluation test is carried out in the following manner. A heat treatment simulating hot forging and a constant temperature annealing treatment are performed on a steel bar with a diameter of 50 mm. Specifically, the steel bar is heated at 1200°C and kept at 1200°C for 30 minutes. Then, the steel bar is naturally cooled to room temperature. Next, it is heated at 950°C and kept at 950°C for 1 hour. Next, after being kept at 650°C for 2 hours, it is naturally cooled to room temperature. The steel bar after natural cooling is subjected to mechanical processing (cutting processing) to produce a test piece for machinability evaluation with a diameter of 45 mm and a length of 400 mm.

[0490] Peripheral turning was performed on the test pieces of each test number to evaluate the tool life. Specifically, peripheral turning was performed on the test pieces of each test number under the following conditions. The cutting tool used was an uncoated superhard alloy equivalent to P20 specified in JIS B 4053 (2013). The cutting speed was 250 m / min, the feed rate was 0.35 mm / rev, and the cutting depth was 1.0 mm. Water-soluble cutting oil was used during turning.

[0491] The outer peripheral turning process was performed for 20 minutes under the above-mentioned cutting conditions. Then, the flank wear amount (mm) of the cutting tool was measured.

[0492] The obtained flank wear amount (mm) is shown in the "Wear amount (mm)" column of Table 3. When the flank wear amount (mm) is less than 0.25 mm, it is judged that the machinability of the steel material is high. When the obtained flank wear amount (mm) is 0.25 mm or more, it is judged that the machinability of the steel material is low.

[0493] [(C2) Bending fatigue strength evaluation test]

[0494] The steel bars (50 mm diameter) of each test number were processed into Figure 6 The following is an intermediate product of the Ono-type rotating bending test piece used for bending fatigue strength evaluation. Figure 6 The values ​​in represent the dimensions (in mm). Figure 6 The "φ" in the figure indicates the diameter. "R1" indicates that the curvature radius of the notch bottom is 1 mm.

[0495] Specifically, the steel material (steel rod with a diameter of 50 mm) of each test number is heated at a heating temperature of 1200°C and a holding time of 30 minutes. Then, hot working (hot forging) is performed in a manner such that the final temperature is above 950°C to produce a steel rod with a diameter of 35 mm. The steel rod with a diameter of 35 mm is machined (cutting) to form an intermediate product of the Ono-type rotary bending test piece. The diameter of the cross section of the intermediate product at the bottom of the notch is 8 mm. The intermediate product is carburized (gas carburizing quenching and tempering or vacuum carburizing quenching and tempering) to produce the following: Figure 6 The Ono-type rotary bending test piece is shown.

[0496] The test piece of Test No. 1 was subjected to gas carburizing and tempering, which is one of the carburizing treatment methods generally used for steel having the chemical composition of SCM420H.

[0497] On the other hand, vacuum carburizing quenching and tempering were performed on the test pieces of test numbers 2 to 22. The conditions of the gas carburizing treatment and vacuum carburizing treatment performed were as follows.

[0498] [Gas carburizing and tempering: Test No. 1]

[0499] Figure 7 1 is a diagram showing an example of a heating pattern for gas carburizing treatment (gas carburizing step and quenching step). Figure 7 Gas carburizing and tempering were performed under the conditions shown. In the gas carburizing, a gas carburizing step S30 and a quenching step S20 were performed.

[0500] Specifically, in the gas carburizing step S30, the test piece is subjected to a heating step S0, a carburizing step S2, and a diffusion step S3. In the heating step S0, the round bar of test number 1 is heated to a carburizing temperature Tc: 950°C. In the carburizing step S2, in an atmosphere with a carbon potential Cp2 of 0.80%, the carburizing temperature Tc: 950°C is set, and the holding time t2: 240 minutes.

[0501] In the diffusion step S3, in an atmosphere with a carbon potential Cp3 of 0.80%, the carburizing temperature Tc was set to 950°C and the holding time t3 was set to 60 minutes.

[0502] After the diffusion step S3, the quenching step S20 is performed. In the quenching step S20, the soaking step S4 is performed. After the furnace is cooled to 850°C, in the soaking step S4, the quenching temperature Ts is set to 850°C and the holding time t4 is set to 30 minutes. Then, quenching is performed using 130°C oil.

[0503] After quenching, the test piece was tempered at a tempering temperature of 180°C and a holding time of 120 minutes. After the holding time, the test piece was air-cooled.

[0504] By the above-described gas carburizing method, the C concentration on the surface of the steel material (round bar) was adjusted to 0.80 mass %.

[0505] [Vacuum carburizing, quenching and tempering]

[0506] The test pieces of test numbers 2 to 22 were subjected to Figure 5 The vacuum carburizing treatment and tempering shown in the figure. Specifically, the pressure in the furnace is maintained below 10 Pa. In the heating step S0, the round bars of each test number are heated to the carburizing temperature Tc: 950°C. After the heating step S0, the soaking step S1 is implemented. In the soaking step S1, the steel material (round bar) is maintained at the carburizing temperature Tc: 950°C for a holding time t1: 60 minutes.

[0507] After the soaking step S1, the carburizing step S2 is performed. In the carburizing step S2, acetylene is supplied to the vacuum carburizing furnace as the carburizing gas. The carburizing gas pressure in the carburizing step S2 is maintained below 1 kPa. In the carburizing step S2, the holding time t2 at the carburizing temperature Tc: 950°C is 40 minutes. The carburizing gas pressure in the diffusion step S3 is maintained below 5 hPa. In the diffusion step S3, the holding time t3 at the carburizing temperature Tc: 950°C is 70 minutes.

[0508] In the soaking step S4 after the diffusion step S3, the steel material temperature was furnace cooled to 850°C, and then the test piece was soaked at a quenching temperature Ts: 850°C for a holding time t4: 30 minutes. Then, quenching was performed using 130°C oil.

[0509] After quenching, the test piece was tempered at a tempering temperature of 180°C and a holding time of 120 minutes. After the holding time, the test piece was air-cooled.

[0510] By the above vacuum carburizing method, the C concentration on the surface of the steel material (round bar) was adjusted to 0.80 mass %.

[0511] The Ono-type rotating bending fatigue test was conducted on the Ono-type rotating bending test pieces after carburizing (gas carburizing quenching and tempering or vacuum carburizing quenching and tempering). A plurality of test pieces were prepared for each test number. The fatigue test was conducted by varying the stress applied to each test piece and repeating the test 10 million times (10 7 The maximum stress without fracture was taken as bending fatigue strength (MPa). In the Ono type rotating bending fatigue test, the rotation speed was set to 3000 rpm and the stress ratio was set to cyclic alternation.

[0512] The test piece using the steel material of test number 1 was used as the reference steel. The ratio of the bending fatigue strength of each test number to the bending fatigue strength of the reference steel was defined as the bending fatigue strength ratio. That is, the bending fatigue strength ratio (%) was calculated by the following formula.

[0513] Bending fatigue strength ratio (%) = (bending fatigue strength of each test number (MPa) / bending fatigue strength of reference steel (MPa)) × 100

[0514] The obtained bending fatigue strength ratio (%) is shown in the "Bending fatigue strength ratio (%)" column of Table 3. When the obtained bending fatigue strength ratio is 120% or more, it is judged that sufficient bending fatigue strength can be obtained. On the other hand, when the bending fatigue strength ratio is less than 120%, it is judged that the bending fatigue strength is low.

[0515] [(C3) Surface fatigue strength evaluation test]

[0516] The steel bars (50 mm diameter) of each test number were processed into Figure 8 The intermediate product of the test piece for roller pitting fatigue test used in the surface fatigue strength evaluation test shown. Figure 8 The numerical value in the figure indicates the size (in mm). The "φ" in the figure indicates the diameter.

[0517] Specifically, the steel material of each test number (steel rod with a diameter of 50 mm) is heated at a heating temperature of 1200°C and a holding time of 30 minutes. Then, hot working (hot forging) is performed at a final temperature of 950°C or above to produce a steel rod with a diameter of 35 mm. The steel rod with a diameter of 35 mm is machined (cutting) to form an intermediate product of a test piece for roller pitting fatigue test. The intermediate product of test number 1 is subjected to gas carburizing treatment and tempering under the above conditions. The intermediate products of test numbers 2 to 22 are subjected to vacuum carburizing treatment and tempering under the above conditions. Through the above steps, the following are produced: Figure 8 The test piece for the roller pitting fatigue test shown (small roller test piece).

[0518] Fig. 9 Figure 1 is a schematic diagram of the roller pitting fatigue test. Fig. 9As shown, the large roller test piece 100 is pressed against the small roller test piece 200 by the surface described later, and the small roller test piece 200 is rotated. The small roller test piece 200 is a test piece for roller pitting fatigue test made by the method of the above test piece. The large roller test piece has Fig.10 Shape shown. Fig.10 The numerical value in represents the size (unit: mm). "R700" in the figure means that the radius of curvature of the outer peripheral surface is 700 mm.

[0519] The large roll test piece 100 was a product that was subjected to gas carburization treatment under the same conditions as the small roll test piece 200 of test number 1 as the reference steel and then surface polished using steel having a chemical composition equivalent to SCM420H specified in JIS G 4053 (2016). The diameter of the large roll test piece 100 was 130 mm.

[0520] In the roller pitting fatigue test, the large roller test piece 100 is pressed against the small roller test piece 200 with various surface pressures of Hertz stress. The circumferential speed directions of the two roller test pieces at the contact portion are in the same direction, and the slip ratio is -40% (the circumferential speed of the contact portion of the large roller test piece 100 is 40% greater than that of the small roller test piece 200), and the test is carried out by rotating it under these conditions. The ATF (lubricating oil for AT) provided as a lubricant to the above-mentioned contact portion has an oil temperature of 90°C, and the maximum surface pressure of the contact stress between the large roller test piece 100 and the small roller test piece 200 is 4000MPa. The test is completed 20 million times (2.0×10 7 For each test number, repeat 2.0 × 10 7 After the test, the highest stress without pitting corrosion was taken as the surface fatigue strength (MPa).

[0521] The test piece using the steel material of test number 1 was used as the reference steel. The ratio of the surface fatigue strength of each test number to the surface fatigue strength of the reference steel was defined as the surface fatigue strength ratio. That is, the surface fatigue strength ratio (%) was calculated by the following formula.

[0522] Surface fatigue strength ratio (%) = (Surface fatigue strength of each test number (MPa) / Surface fatigue strength of reference steel (MPa)) × 100

[0523] The obtained surface fatigue strength ratio (%) is shown in the "Surface fatigue strength ratio (%)" column of Table 3. When the obtained surface fatigue strength ratio is 125% or more, it is judged that sufficient surface fatigue strength can be obtained. On the other hand, when the surface fatigue strength ratio is less than 125%, it is judged that the surface fatigue strength is low.

[0524] [(C4) Heat treatment deformation evaluation test]

[0525] The following are made from the steel material (steel bar with a diameter of 50 mm) of each test number: Fig.11A Gear simulation test piece shown. Specifically, the steel material (steel bar with a diameter of 50 mm) of each test number was heated at a heating temperature of 1200°C and a holding time of 30 minutes. Then, hot working (hot forging) was performed at a final temperature of 950°C or above to produce a steel bar with a diameter of 35 mm. The steel bar with a diameter of 35 mm was machined (cutting) to produce a gear simulation test piece before carburizing treatment (gas carburizing treatment, vacuum carburizing treatment).

[0526] Fig.11A The numerical values ​​with "mm" in the figure represent dimensions (unit: mm). "φ" in the figure represents diameter. The gear simulation test piece has a frustum shape. The gear simulation test piece has a circular upper surface with a diameter of 22 mm and a circular lower surface with a diameter of 34 mm. The gear simulation test piece has a cylindrical through hole TH including a center axis CL2. The diameter (inner diameter) of the through hole TH is 15 mm, and the center axis of the through hole TH coincides with the center axis of the gear simulation test piece.

[0527] The inner diameter (diameter) of the through hole TH in the longitudinal direction of the prepared gear simulation test piece before carburization treatment was measured by a three-dimensional measuring machine. As the three-dimensional measuring machine, a CNC three-dimensional measuring machine (trade name: Crysta-Apex) manufactured by Mitutoyo Corporation was used.

[0528] Specifically, if Fig. 11B As shown, from the upper end to the lower end in the length direction of the through hole TH, a total of 16 inner diameters are measured at 1.0 mm intervals within the range of 1.0 to 16.0 mm from the upper end. In addition, from the upper end to the lower end in the length direction of the through hole TH, the inner diameters at the positions 0.5 mm from the upper end and 16.5 mm from the upper end are measured. That is, the inner diameter of the through hole TH is measured at 18 measurement positions in the length direction of the through hole TH. In addition, at each measurement position, a total of 18 locations ( Fig. 11B Therefore, in the through hole TH, 18 measurement positions×18 locations=324 inner diameters are measured.

[0529] The gear simulation test piece after the above inner diameter measurement is subjected to carburizing treatment (gas carburizing quenching and tempering or vacuum carburizing quenching and tempering) under the above carburizing treatment conditions to produce a gear simulation test piece after carburizing treatment. Specifically, the gear simulation test piece of test number 1 is subjected to gas carburizing treatment and tempering under the above conditions. The gear simulation test pieces of test numbers 2 to 22 are subjected to vacuum carburizing treatment and tempering under the above conditions. For the gear simulation test pieces of each test number after carburizing treatment, the inner diameter of the through hole TH is measured by the same method as the method for measuring the inner diameter of the through hole TH of the gear simulation test piece before carburizing treatment.

[0530] [Maximum deformation ratio of heat treatment]

[0531] At each point P1 to P18 of each measurement position of the through hole TH, the value obtained by subtracting the inner diameter (μm) after carburizing treatment from the inner diameter (μm) before carburizing treatment was used as the heat treatment deformation at each point P1 to P18 of each measurement position. In each test number, the maximum heat treatment deformation was obtained among a total of 324 measurement results.

[0532] The ratio of the maximum heat treatment deformation of each test number to the maximum heat treatment deformation of the reference steel (test number 1) was defined as the “maximum deformation ratio.” That is, the maximum deformation ratio (%) was calculated by the following formula.

[0533] Maximum deformation ratio (%) = (maximum heat treatment deformation of each test number (mm) / maximum heat treatment deformation of the reference steel (mm)) × 100

[0534] The obtained maximum deformation ratio (%) is shown in the "Maximum deformation ratio (%)" column of Table 3. When the obtained maximum deformation ratio is less than 90%, it is judged that the maximum deformation ratio is small. On the other hand, when the maximum deformation ratio is greater than 90%, it is judged that the maximum deformation ratio is large.

[0535] [Heat treatment deformation difference ratio]

[0536] In each test number, the maximum heat treatment deformation amount and the minimum heat treatment deformation amount were obtained from a total of 324 measurement results. The value obtained by subtracting the minimum heat treatment deformation amount from the obtained maximum heat treatment deformation amount was defined as the deformation amount difference (μm).

[0537] The ratio of the deformation difference of each test number to the deformation difference of the reference steel (test number 1) is defined as the deformation difference ratio. That is, the deformation difference ratio is calculated by the following formula.

[0538] Deformation difference ratio (%) = (deformation difference of each test number (μm) / deformation difference of reference steel (μm)) × 100

[0539] The obtained deformation amount difference ratio (%) is shown in the "Deformation amount difference ratio (%)" column of Table 3. When the obtained deformation amount difference ratio is 90% or less, it is judged that the deformation amount difference ratio is small. On the other hand, when the deformation amount difference ratio is greater than 90%, it is judged that the deformation amount difference ratio is large.

[0540] When the maximum deformation ratio and the deformation difference ratio are both 90% or less, the heat treatment deformation is judged to be sufficiently suppressed in three dimensions. When the maximum deformation ratio and / or the deformation difference ratio is greater than 90%, the heat treatment deformation is judged to be insufficiently suppressed.

[0541] [Test results]

[0542] The test results are shown in Table 3. Referring to Table 3, in the steels of test numbers 4 to 6, the contents of each element in the chemical composition are appropriate, and F1 and F2 satisfy equations (1) and (2). In addition, the manufacturing conditions of the steels of test numbers 4 to 6 are also appropriate. Therefore, the microstructure of the steel is appropriate. Specifically, the microstructure of the cross section of test numbers 4 to 6 is a structure containing ferrite, with the remainder being pearlite and / or bainite. In addition, the arithmetic mean of the area fraction of ferrite is 50 to 70%, the standard deviation of the area fraction of ferrite is 4.0% or less, and the average ferrite grain size ratio is 2.00 or less. In addition, the microstructure of the longitudinal section of test numbers 4 to 6 is a structure containing ferrite, with the remainder being pearlite and / or bainite. In addition, the arithmetic mean of the area fraction of ferrite is 50 to 70%, the standard deviation of the area fraction of ferrite is 4.0% or less, and the average ferrite grain size ratio is 2.00 or less.

[0543] Therefore, in the cutting performance evaluation test, the flank wear is less than 0.25 mm, and the cutting performance is high. In addition, the bending fatigue strength ratio is more than 120%, and the surface fatigue strength ratio is more than 125%, and both the bending fatigue strength and the surface fatigue strength are excellent. In addition, the maximum deformation ratio and the deformation difference ratio during heat treatment are less than 90%, and the heat treatment deformation is fully suppressed in three dimensions.

[0544] On the other hand, in test numbers 2 and 3, the holding time of the initial rolling process was less than 10 hours. Therefore, the standard deviation of the area fraction of ferrite in the cross section and the longitudinal section of the steel material was greater than 4.0%. As a result, the difference ratio of the deformation amount in the heat treatment was greater than 90%, and the heat treatment deformation was not fully suppressed.

[0545] In test numbers 7 and 8, the heating temperature in the initial rolling process was too low. Therefore, the standard deviation of the area fraction of ferrite in the cross section and the longitudinal section exceeded 4.0%. As a result, the difference ratio of the deformation amount in the heat treatment exceeded 90%, and the heat treatment deformation was not sufficiently suppressed.

[0546] In test numbers 9 and 10, the heating temperature in the finish rolling process was too low. Therefore, the average ferrite grain size ratio between the cross section and the longitudinal section was greater than 2.00. As a result, the deformation amount difference ratio during the heat treatment was greater than 90%, and the heat treatment deformation was not sufficiently suppressed.

[0547] In test numbers 11 and 12, the holding time of the finish rolling process was too short. Therefore, the average ferrite grain size ratio of the cross section to the longitudinal section was greater than 2.00. As a result, the deformation amount difference ratio during the heat treatment was greater than 90%, and the heat treatment deformation was not sufficiently suppressed.

[0548] In test numbers 13 and 14, the final temperature of the finish rolling process was too high. Therefore, the average ferrite grain size ratio of the cross section to the longitudinal section was greater than 2.00. As a result, the deformation amount difference ratio during the heat treatment was greater than 90%, and the heat treatment deformation was not sufficiently suppressed.

[0549] In test numbers 15 and 16, the final temperature of the finish rolling process was too low. Therefore, the average ferrite grain size ratio of the cross section to the longitudinal section was greater than 2.00. As a result, the deformation amount difference ratio during the heat treatment was greater than 90%, and the heat treatment deformation was not sufficiently suppressed.

[0550] In test numbers 17 and 18, the cooling rate in the cooling process was too slow. Therefore, the arithmetic mean of the area fraction of ferrite in the cross section and the longitudinal section was greater than 70%. Therefore, the difference ratio of the deformation amount in the heat treatment was greater than 90%, and the heat treatment deformation was not sufficiently suppressed.

[0551] In test numbers 19 and 20, the cooling rate in the cooling process was too fast. Therefore, the arithmetic mean of the area fraction of ferrite in the cross section and the longitudinal section was less than 50%. Therefore, the flank wear amount was more than 0.25 mm. As a result, the machinability of the steel material was low.

[0552] In test numbers 21 and 22, the cooling rate in the temperature holding step was too fast. Therefore, in the microstructure of the longitudinal section, the ferrite average grain size ratio was greater than 2.00. Therefore, the deformation amount difference ratio in the heat treatment was greater than 90%, and the heat treatment deformation was not sufficiently suppressed.

[0553] Example 2

[0554] In the same manner as in Example 1, steel materials having the chemical compositions shown in Table 4 were prepared.

[0555] [Table 4]

[0556] Table 4

[0557]

[0558] Steel materials were produced by the following method: Steel materials (steel bars) of test numbers 1 to 35 shown in Table 5 were produced in the same manner as in Example 1 using production conditions b in Table 2 for molten steel.

[0559] [Table 5]

[0560] Table 5

[0561]

[0562] [Evaluation test]

[0563] The steel material (steel bar) manufactured by the above manufacturing process was subjected to the same measurement and evaluation test as in Example 1 in the same manner as in Example 1. In the (C2) bending fatigue strength evaluation test, the (C3) surface fatigue strength evaluation test, and the (C4) heat treatment deformation evaluation test, the steel material of test number 1 in Table 1 was used as the reference steel.

[0564] [Test results]

[0565] The test results are shown in Table 5. Referring to Table 5, the chemical composition of the steel materials of test numbers 1 to 18 has appropriate content of each element, F1 satisfies formula (1), and F2 satisfies formula (2). In addition, the manufacturing conditions of the steel materials of test numbers 1 to 18 are also appropriate. Therefore, the microstructure of the steel materials is appropriate. Specifically, the microstructure of the cross section of test numbers 1 to 18 is composed of ferrite, pearlite and / or bainite, the arithmetic mean of the area fraction of ferrite is 50 to 70%, the standard deviation of the area fraction of ferrite is 4.0% or less, and the average ferrite grain size ratio is 2.00 or less. The microstructure of the longitudinal section of test numbers 1 to 18 is composed of ferrite, pearlite and / or bainite, the arithmetic mean of the area fraction of ferrite is 50 to 70%, the standard deviation of the area fraction of ferrite is 4.0% or less, and the average ferrite grain size ratio is 2.00 or less. Therefore, the flank wear is less than 0.25 mm, and the cutting performance is high. In addition, the bending fatigue strength ratio is 120% or more, and the surface fatigue strength ratio is 125% or more, and both the bending fatigue strength and the surface fatigue strength are excellent. In addition, the maximum deformation ratio and the deformation difference ratio during heat treatment are 90% or less, and the heat treatment deformation is fully suppressed.

[0566] On the other hand, F2 of Test Nos. 19 to 23 was too high. Therefore, the maximum deformation ratio during heat treatment exceeded 90%, and heat treatment deformation was not sufficiently suppressed.

[0567] F1 of test numbers 24 and 25 was too low. Therefore, the bending fatigue strength ratio was less than 120%, and the bending fatigue strength was low.

[0568] The C content of Test No. 26 was too high. Therefore, the flank wear amount was 0.25 mm or more, and the machinability was low.

[0569] The Si content of test number 27 is too low. Therefore, F1 does not satisfy the formula (1). Therefore, the bending fatigue strength ratio is less than 120%, and the surface fatigue strength ratio is less than 125%. As a result, the bending fatigue strength and the surface fatigue strength are low.

[0570] Test No. 28 has too low Si and Mn contents. Therefore, the bending fatigue strength ratio is less than 120%, and the surface fatigue strength ratio is less than 125%. As a result, both the bending fatigue strength and the surface fatigue strength are insufficient.

[0571] The Si content in Test No. 29 was too high. Therefore, the flank wear amount was 0.25 mm or more, and the machinability was low.

[0572] The Mn content of test number 30 was too low. Therefore, the bending fatigue strength ratio was less than 120%, and the surface fatigue strength ratio was less than 125%. As a result, the bending fatigue strength and the surface fatigue strength were low.

[0573] The Mn content of test number 31 was too high. Therefore, the flank wear amount was 0.25 mm or more, and the machinability was low. In addition, F1 did not satisfy the formula (1). Therefore, the bending fatigue strength ratio was less than 120%, and the bending fatigue strength was insufficient.

[0574] The Mn content of Test No. 32 was too high. Therefore, the flank wear amount was 0.25 mm or more, and the machinability was low.

[0575] The Cr content of Test No. 33 was too high. Therefore, the bending fatigue strength ratio was less than 120%, and the bending fatigue strength was low.

[0576] The Mo content of Test No. 34 was too high. Therefore, the flank wear amount was 0.25 mm or more, and the machinability was low.

[0577] The Nb content of Test No. 35 was too high. Therefore, the bending fatigue strength ratio was less than 120%, and the bending fatigue strength was low.

[0578] The embodiments of the present disclosure are described above. However, the above embodiments are only examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above embodiments, and the above embodiments can be appropriately modified and implemented without departing from the spirit of the present disclosure.

Claims

1. A steel material, the chemical composition of which contains, by mass%, C：0.18～0.25％、 Si: 0.70 - 2.00%, Mn: 0.70 - 1.50%, S：0.005～0.050％、 N：0.0050～0.0200％、 Al：0.001～0.100％、 O: 0.0050% or less, and P: 0.030% or less, the balance being Fe and impurities, and satisfying Formula (1) and Formula (2). On a cross-section perpendicular to the length direction of the steel material and being a circular cross-section with a radius R, when defining the center position of the cross-section and eight positions of R / 2 spaced at 45° intervals around the center of the cross-section, which are positions at a distance of R / 2 from the center of the cross-section in the radial direction, as nine cross-section observation positions, the microstructure at each cross-section observation position contains ferrite, and the balance is pearlite and / or bainite. The arithmetic average of the area fractions of ferrite at the nine cross-section observation positions is 50 - 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less. Among the average grain diameters of ferrite at the nine cross-section observation positions, the ratio of the maximum average grain diameter to the minimum average grain diameter is 2.00 or less. On a longitudinal section parallel to the length direction of the steel material and including the central axis of the steel material, when defining three central axis positions spaced at R / 2 intervals on the central axis and six positions of R / 2 spaced at a distance of R / 2 from each central axis position in the radial direction as nine longitudinal section observation positions, the microstructure at each longitudinal section observation position contains ferrite, and the balance is pearlite and / or bainite. The arithmetic average of the area fractions of ferrite at the nine longitudinal section observation positions is 50 - 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less. Among the average grain diameters of ferrite at the nine longitudinal section observation positions, the ratio of the maximum average grain diameter to the minimum average grain diameter is 2.00 or less. Si / Mn ≥ 1.00 (1) 1 - (0.5C + 0.03Si + 0.06Mn + 0.01Cr + 0.05Mo) < 0.800 (2) Herein, each element symbol in Formula (1) and Formula (2) is substituted with the content of the corresponding element by mass%, and when the corresponding element is not contained, the element symbol is substituted with "0".

2. The steel material according to claim 1, wherein the chemical composition further contains one or more elements selected from the group consisting of the following elements to replace a part of the Fe: Mo: 0.50% or less, Nb: 0.050% or less, Cr: 0.60% or less, Ti: 0.020% or less, Cu: 0.50% or less, Ni: 0.80% or less, V: 0.30% or less, Mg: 0.0035% or less, Ca: 0.0030% or less, and rare earth elements: 0.0050% or less.

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