steel

Abstract

A steel material is provided that has excellent machinability, excellent bending fatigue strength and surface fatigue strength even after gas carburizing treatment, and can suppress heat treatment deformation. The chemical composition of the steel material according to this embodiment includes, by mass%, C: 0.20-0.25%, Si: 0.40-0.70%, Mn: 0.50-0.90%, Cr: 1.00-2.00%, 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 balance being Fe and impurities, and satisfies formula (1) in the specification. Furthermore, the microstructure of the cross section and the longitudinal section contains ferrite, with the balance being pearlite and / or bainite, 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 application relates to a steel material, and more particularly to a steel material suitable for forming a blank of a machine structural component manufactured by subjecting it to a gas carburizing treatment.

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

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

[0004] These steel materials are manufactured into mechanical structural components, for example, through the following manufacturing process. The steel material is subjected to forging (hot or cold forging) and / or cutting processes to produce an intermediate product of the desired shape. The intermediate product is then heat treated (quenching and tempering, carburizing, or carbonitriding) to adjust its hardness and microstructure. Through these manufacturing processes, mechanical structural components are manufactured.

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

[0006] In recent years, the trend toward lighter and smaller machine components has been toward improving fuel efficiency in automobiles, construction vehicles, and other vehicles. Consequently, these components are required to exhibit superior bending and surface fatigue strength.

[0007] Gas carburizing and gas carbonitriding are known methods for improving the bending fatigue strength and surface fatigue strength of mechanical structural components. Gas carburizing and gas carbonitriding form a hardened layer (carburized layer or carbonitrided layer) on the surface of the mechanical structural component. This hardened layer improves the bending fatigue strength and surface fatigue strength of the mechanical structural component.

[0008] However, when gas carburizing (gas carburizing and gas carbonitriding) is performed, mechanical structural components are susceptible to deformation. In this specification, deformation of mechanical structural components during gas carburizing is referred to as heat treatment deformation. Due to heat treatment deformation, the shape of mechanical structural components is deformed. This deformation of the shape of mechanical structural components causes noise and vibration during the operation of automobiles and construction vehicles. Therefore, steel materials that can suppress heat treatment deformation during gas carburizing are required.

[0009] Japanese Patent Application Laid-Open No. 9-137266 (Patent Document 1) and Japanese Patent Application Laid-Open No. 11-50191 (Patent Document 2) propose techniques for suppressing heat treatment deformation.

[0010] The steel material disclosed in Patent Document 1 comprises, in mass %, C: 0.03-0.40%, Mn: 2.0% or less, Si: 2.0% or less, Al: 0.015-0.06%, N: 0.005-0.03%, P: 0-0.030%, and the balance: Fe and unavoidable impurities. The chemical composition of this steel material is also adjusted so that the quenching start temperature in quenching performed after surface hardening of a part formed using this steel material is set to T A When T A It is not less than T1 (=788-117×[C]+29×[Si]-14×[Mn]) and not more than T2 (=900-387×[C]+63×[Si]-18×[Mn]), and Heq1 (=[C]+0.12×[Si]+0.13×[Mn]) is not less than 0.33.

[0011] In Patent Document 1, the above-mentioned configuration allows the area fraction of proeutectoid ferrite in the core structure of a machine structural component after carburization to be 20 to 80%, thereby suppressing heat treatment deformation of the steel material.

[0012] However, in the machine structural component disclosed in Patent Document 1, the bending fatigue strength of the machine structural component may be low.

[0013] The carburized shaft-shaped component disclosed in Patent Document 2 contains, by mass%, C: 0.10-0.35%, Si: 0.02-0.50%, Mn: 0.30-1.80%, S: 0.005-0.15%, Al: 0.015-0.040%, Nb: 0.005-0.040%, N: 0.0060-0.0200%, P: 0.025% or less, Ti: 0.01% or less, O: 0.0025% or less, and further contains one or more of Cr: 0.40-1.80%, Mo: 0.02-1.0%, Ni: 0.1-3.5%, and V: 0.03-0.5%, with the remainder being iron and unavoidable impurities. In this carburized shaft-shaped component, a total of 80 particles / 100 μm are dispersed. 2 The above Nb(CN), AlN or composite precipitates of Nb(CN) and AlN have a diameter of 0.1 μm or less, and the austenite grain size is No. 8 or more.

[0014] In Patent Document 2, the formation of coarse grains during carburizing is suppressed, and as a result, heat treatment deformation of the steel material is suppressed.

[0015] In the carburized shaft-shaped component of Patent Document 2, it is believed that heat treatment deformation is suppressed to a certain extent during gas carburization. However, in order to suppress noise and vibration during operation of automobiles and construction vehicles, steel materials that can suppress heat treatment deformation by other means are also available.

[0016] International Publication No. 2014 / 038548 (Patent Document 3), Japanese Patent Application Laid-Open No. 2013-108144 (Patent Document 4), and Japanese Patent Application Laid-Open No. 2013-151719 (Patent Document 5) propose technologies for suppressing heat treatment deformation by means different from those of Patent Documents 1 and 2.

[0017] The steel material disclosed in Patent Document 3 contains, by mass%, 0.20-0.30% C, 0.10-1.50% Si, 0.10-1.20% Mn, 0.030% or less P, 0.030% or less S, 1.30-2.50% Cr, 0.30% or less Cu, 0.008-0.300% Al, 0.0030% or less O, and 0.0020-0.0300% N, with the balance being Fe and unavoidable impurities. The martensitic transformation start temperature (Ms point) of this steel material is 460°C or less. Furthermore, when the hardness at a position 1.5 mm away from the quenched end of the steel material measured by the Jomini top quenching method is defined as J1.5, the hardness at a position 9 mm away from the quenched end is defined as J9, and the hardness at a position 11 mm away from the quenched end is defined as J11, (J9 / J1.5) is 0.70 to 0.85, and (J11 / J1.5) is 0.67 to 0.78.

[0018] In the steel material disclosed in Patent Document 3, the Ms point is set low and the hardness is adjusted to a predetermined range, thereby suppressing heat treatment deformation of the steel material.

[0019] The steel material disclosed in Patent Document 4 contains, by mass%, 0.10-0.25% C, 0.01-0.10% Si, 0.40-1.00% Mn, 0.003-0.050% S, 1.60-2.00% Cr, 0-0.10% Mo, 0.025-0.050% Al, and 0.0100-0.0250% N, with the balance being Fe and impurities. Among the impurities, P, Ti, and O (oxygen) are respectively 0.025% or less for P, 0.003% or less for Ti, and 0.0020% or less for O. Furthermore, in this steel material, fn (=Cr+2×Mo) is 1.82-2.10. Furthermore, in this steel material, the difference between the maximum and minimum values ​​of the Ms point measured at 17 locations on a cross section perpendicular to the longitudinal direction of the steel material is 10 or less.

[0020] The steel disclosed in Patent Document 5 contains, by mass%, 0.1-0.3% C, 0.01-0.6% Si, 0.4-1.0% Mn, 0.003-0.05% S, 0.80-2.00% Cr, 0-0.50% Mo, 0.01-0.05% Al, and 0.010-0.025% N, with the balance being Fe and impurities. Among the impurities, P, Ti, and O are 0.025% or less, 0.003% or less, and 0.002% or less, respectively. Furthermore, the difference between the maximum and minimum values ​​of the Ms point measured at 17 locations on a cross section perpendicular to the longitudinal direction of the steel is 10 or less. The microstructure of the steel consists of ferrite and pearlite and / or bainite. Furthermore, the ratio of the maximum to minimum average ferrite grain size on the cross section is 2.0 or less.

[0021] In Patent Documents 4 and 5, the variation of Ms points in the cross section of the steel material is suppressed, thereby suppressing the heat treatment deformation of the steel material.

[0022] Prior art literature

[0023] Patent Literature

[0024] Patent Document 1: Japanese Patent Application Laid-Open No. 9-137266

[0025] Patent Document 2: Japanese Patent Application Laid-Open No. 11-50191

[0026] Patent Document 3: International Publication No. 2014 / 038548

[0027] Patent Document 4: Japanese Patent Application Laid-Open No. 2013-108144

[0028] Patent Document 5: Japanese Patent Application Laid-Open No. 2013-151719 Summary of the Invention

[0029] Problems to be solved by the invention

[0030] However, it is also possible to improve the bending fatigue strength and the surface fatigue strength and suppress the heat treatment deformation by means other than those disclosed in Patent Documents 3 to 5.

[0031] The object of the present application is to provide a steel material having excellent machinability, excellent bending fatigue strength and surface fatigue strength after gas carburizing treatment, and capable of suppressing heat treatment deformation after gas carburizing treatment.

[0032] Solutions for solving problems

[0033] The steel material of the present application has the following constitution.

[0034] A type of steel,

[0035] Its chemical composition contains in mass %

[0036] C: 0.20~0.25%,

[0037] Si: 0.40-0.70%,

[0038] Mn: 0.50~0.90%,

[0039] Cr: 1.00-2.00%,

[0040] S: 0.005~0.050%,

[0041] N: 0.0050~0.0200%,

[0042] Al: 0.001~0.100%,

[0043] O: 0.0050% or less, and

[0044] P: 0.030% or less,

[0045] The remainder is Fe and impurities, and satisfies formula (1),

[0046] On a circular cross section of a radius R that is a cross section perpendicular to the longitudinal direction of the steel material,

[0047] When the center position of the cross section and eight positions of the R / 2 position arranged at 45° intervals around the center of the cross section at a distance R / 2 from the center of the cross section in the radial direction are defined as nine cross section observation positions,

[0048] The microstructure at each cross-sectional observation position contains ferrite, with the remainder being pearlite and / or bainite.

[0049] 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.

[0050] 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.

[0051] On a longitudinal cross section including the central axis of the steel material, which is a cross section parallel to the longitudinal direction of the steel material,

[0052] When three central axis positions arranged at an R / 2 pitch on the central axis and six R / 2 positions arranged at R / 2 intervals from each central axis position in the radial direction are defined as nine longitudinal cross-sectional observation positions,

[0053] The microstructure at the observation position of each longitudinal section contains ferrite, with the remainder being pearlite and / or bainite.

[0054] 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.

[0055] Among the average grain sizes of the 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.

[0056] 1-(0.5C+0.03Si+0.06Mn+0.01Cr+0.05Mo)<0.800 (1)

[0057] In formula (1), the content (mass %) of the corresponding element is substituted for each element symbol. If the corresponding element is not contained, "0" is substituted for the element symbol.

[0058] Effects of the Invention

[0059] The steel material of the present application has excellent machinability, and after being subjected to gas carburizing treatment, has excellent bending fatigue strength and surface fatigue strength, and can suppress heat treatment deformation after gas carburizing treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 This is a graph showing the relationship between the F1 (=1-(0.5C+0.03Si+0.06Mn+0.01Cr+0.05Mo)) value and the maximum deformation ratio (%).

[0061] Figure 2 This is a schematic diagram of a cross section perpendicular to the longitudinal direction of the steel material according to this embodiment.

[0062] Figure 3 This is a schematic diagram of a longitudinal cross section, which is a cross section parallel to the longitudinal direction and including the central axis of the steel material according to the present embodiment.

[0063] Figure 4 Schematic diagram of the banded tissue.

[0064] Figure 5 This is a diagram showing an example of heating patterns in the gas carburizing step and the quenching step.

[0065] Figure 6 This is a top view of the Ono-type rotary bending test piece produced in the examples.

[0066] Figure 7 This is a plan view of a test piece for a roller pitting fatigue test produced in an example.

[0067] Figure 8 Schematic diagram for explaining a roller pitting fatigue test.

[0068] Figure 9 This is a front view of a large roller test piece produced in Examples.

[0069] Figure 10A This is a perspective view of a gear simulation test piece produced in the examples.

[0070] Figure 10B for Figure 10A A three-dimensional view of the through hole in the . DETAILED DESCRIPTION

[0071] The present inventors have conducted research and studies on a steel material having excellent machinability, excellent bending fatigue strength and surface fatigue strength when manufactured into a machine structural component through gas carburizing, and capable of suppressing heat treatment deformation after gas carburizing.

[0072] The present inventors first studied a steel material having excellent machinability from the viewpoint of chemical composition and, further, excellent bending fatigue strength and surface fatigue strength after gas carburizing treatment.

[0073] The results of the study suggest that if the chemical composition contains C: 0.20-0.25%, Si: 0.40-0.70%, Mn: 0.50-0.90%, Cr: 1.00-2.00%, 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: 0-0.3 0%, Nb: 0-0.050%, Ti: 0-0.020%, Cu: 0-0.50%, Ni: 0-0.80%, V: 0-0.30%, Mg: 0-0.0035%, Ca: 0-0.0030%, and rare earth elements: 0-0.0050%, with the balance being Fe and impurities, steel has excellent machinability and may have excellent bending fatigue strength and surface fatigue strength after gas carburizing treatment.

[0074] The present inventors have also studied means for suppressing heat treatment deformation after gas carburizing. The present inventors have focused on the microstructure of the steel. If the microstructure at each location in the steel is as uniform as possible, specifically, if the unevenness of the phase structure and the unevenness of the grains in the microstructure at each location in the steel are suppressed, the unevenness of the timing of the martensitic transformation during gas carburizing and quenching can be suppressed. As a result, heat treatment deformation can be suppressed. To this end, the present inventors have studied the phase structure and grain size of each location in the steel.

[0075] The present inventors first focused on the microstructural nonuniformity in the cross section perpendicular to the longitudinal direction of the steel material. To quantify the microstructural nonuniformity in the cross section, the observation position of the microstructure in the cross section, i.e., the cross section observation position, was defined as follows.

[0076] When the radius of the cross section 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 section observation positions.

[0077] The present inventors investigated and studied the microstructure at various cross-sectional observation positions and found that heat treatment deformation after gas carburizing treatment is suppressed if the microstructure at the cross-sectional observation position satisfies the following conditions.

[0078] (1) The microstructure at each cross-sectional observation position contains ferrite, with the remainder being pearlite and / or bainite.

[0079] (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.

[0080] (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.

[0081] However, even steel materials having the aforementioned chemical composition and satisfying the aforementioned microstructure still cannot sufficiently suppress heat treatment distortion, and in particular, noise and vibration during operation of automobiles and construction vehicles, etc., cannot sometimes be sufficiently suppressed. Therefore, the present inventors have conducted further research.

[0082] As a result, the following findings were discovered: Three-dimensionally suppressing heat treatment deformation in steel is effective for reducing noise and vibration during operation. As described above, suppressing the unevenness of the phase structure and grain size in the microstructure of the steel's cross section can suppress heat treatment deformation in a direction perpendicular to the steel's longitudinal direction.

[0083] However, simply suppressing microstructural variations in the cross section of the steel material only limits the suppression of heat treatment deformation to two dimensions. That is, even if microstructural variations in the cross section of the steel material are suppressed, microstructural variations may still occur in the longitudinal section of the steel material, parallel to the longitudinal direction and including the central axis of the steel material. In this case, variations occur in the heat treatment deformation. As a result, noise and vibration during operation cannot be fully suppressed.

[0084] To this end, the present inventors have focused not only on the microstructural heterogeneity of the cross-section of a steel material, but also on the microstructural heterogeneity of the longitudinal section of the steel material. Furthermore, in order to quantify the microstructural heterogeneity on the longitudinal section, the observation position of the microstructure on the longitudinal section, i.e., the longitudinal section observation position, is defined as follows.

[0085] Nine longitudinal cross-sectional observation positions are defined as three central axis positions arranged at R / 2 intervals on the central axis of the steel material and six R / 2 positions arranged radially at R / 2 intervals from each central axis position.

[0086] The present inventors investigated and studied the microstructure at various longitudinal cross-sectional observation positions and found that heat treatment deformation after gas carburizing is sufficiently suppressed if the microstructure at the cross-sectional observation position satisfies the above conditions and further the microstructure at the longitudinal cross-sectional observation position satisfies the following conditions.

[0087] (4) The microstructure at each longitudinal cross-section observation position contains ferrite, with the remainder being pearlite and / or bainite.

[0088] (5) The arithmetic mean of the area fraction of ferrite at nine observation positions of the longitudinal cross section is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less.

[0089] (6) Among the average grain sizes of ferrite at nine locations observed in the longitudinal cross section, the ratio of the largest average grain size to the smallest average grain size is 2.00 or less.

[0090] However, even steel materials having the above chemical composition and having a microstructure satisfying the above conditions (1) to (6) in cross-sectional and longitudinal section observation may not be able to sufficiently suppress heat treatment deformation.

[0091] Here, the present inventors focused on the martensitic transformation after gas carburizing and conducted detailed studies on the mechanism of martensitic transformation during gas carburizing and quenching.

[0092] Patent Documents 3 to 5 all suppress heat treatment deformation by preventing the martensite transformation timing from varying across the steel during gas carburizing and quenching. Specifically, Patent Document 3 suppresses significant variations in the martensite transformation timing across the steel by setting the Ms point to 460°C or lower. Patent Documents 4 and 5 suppress significant variations in the martensite transformation timing across the steel by suppressing variations in the Ms point across the steel.

[0093] To this end, the present inventors first attempted to use a steel material having the above-mentioned chemical composition and, similarly to Patent Documents 3 to 5, to suppress heat treatment deformation by aligning the timing of martensitic transformation in various parts of the steel material. Specifically, they attempted to suppress microstructural variations in various parts of the steel material (cross-sectional observation positions and longitudinal section observation positions) and to minimize variations in the Ms point in each part, thereby suppressing heat treatment deformation.

[0094] However, the present inventors have discovered that even if microstructural variations in various parts of a steel material having the aforementioned chemical composition are suppressed, the timing of martensitic transformation in various parts of the steel material will inevitably deviate slightly, making it extremely difficult to achieve martensitic transformation at the same timing in each part. Specifically, the inventors discovered that when the time of rapid cooling during gas carburizing is divided into small time periods, even if microstructural variations at cross-sectional and longitudinal cross-sectional observation positions of the steel material are suppressed to the maximum extent, in the steel material, there will inevitably be small periods where portions that undergo martensitic transformation (hereinafter also referred to as "martensitic transformation portions") and portions that do not undergo martensitic transformation (hereinafter also referred to as "martensitic non-transformed portions") coexist.

[0095] It is considered that the microstructural changes of the steel material during vacuum carburizing occur as follows.

[0096] When the quenching time (quenching time) is divided into small periods, martensitic transformation begins initially in a portion of the steel's interior. Then, over time, martensitic transformation progresses from the center toward the surface. In other words, martensitic transformation begins not at the steel's surface but within the steel itself.

[0097] Gas carburizing increases the carbon concentration on the steel's surface compared to the steel's interior. Consequently, the Ms point on the steel's surface is lower than the Ms point inside the steel. Furthermore, even if the Ms point could be made uniform across the steel's interior, the cooling rate at each location would not be exactly the same due to the steel's shape. Therefore, if the quenching time is divided into micro-intervals, martensitic transformation begins at the location within the steel with the fastest cooling rate. Consequently, during gas carburizing quenching, micro-intervals inevitably occur where portions of the steel that have undergone martensitic transformations mix with portions that have not undergone martensitic transformations.

[0098] Based on the above insights, the present inventors did not try to make the martensitic transformation timing as consistent as possible to suppress heat treatment deformation as in the technical concepts disclosed in Patent Documents 3 to 5. Instead, they studied means of suppressing heat treatment deformation on the premise that there must be a small period of time during gas carburizing where the martensitic transformation part and the martensitic non-transformed part are mixed.

[0099] During quenching, the untransformed martensite portion is softer than the transformed martensite portion. Furthermore, the transformed martensite portion, which has a body-centered cubic lattice structure, is larger in volume than the untransformed martensite portion, which has a face-centered cubic lattice structure. Therefore, during quenching, when a portion of the steel undergoes martensite transformation, resulting in a mixture of the transformed martensite portion and the untransformed martensite portion, strain is generated in the untransformed martensite portion. This strain is believed to cause heat treatment deformation.

[0100] To this end, the inventors of the present invention believe that, under the premise that there is a small period of time in which the martensitic phase transformation part and the martensitic phase non-transformation part are mixed during the gas carburizing treatment, if the strength of the martensitic phase non-transformation part at the time when the martensitic phase transformation part is generated can be maintained high, the occurrence of strain in the martensitic phase non-transformation part can be suppressed, and as a result, heat treatment deformation can be suppressed.

[0101] To this end, the present inventors have further studied means for maintaining high strength of the untransformed martensite portion when the martensite transformation portion is formed during quenching in a gas carburizing process. In order to improve the strength of the untransformed martensite portion in the temperature range where the martensite transformation portion is formed in the steel material having the above-mentioned chemical composition, it is effective to appropriately contain an element that strengthens the untransformed martensite portion in the temperature range where the martensite transformation portion is formed.

[0102] The present inventors believe that, in the above chemical composition, C, Si, Mn, Cr, and Mo are effective elements for increasing the strength of the untransformed martensite portion in the temperature range where the martensite transformation portion forms. Therefore, they further investigated the relationship between these elements and the amount of heat treatment deformation during quenching in gas carburizing. They found that, in the steel material having the above chemical composition, heat treatment deformation is significantly suppressed by further satisfying the following formula (1).

[0103] 1-(0.5C+0.03Si+0.06Mn+0.01Cr+0.05Mo)<0.800(1)

[0104] In formula (1), the content (mass %) of the corresponding element is substituted for each element symbol. If the corresponding element is not contained, "0" is substituted for the element symbol.

[0105] Defined as F1=1-(0.5C+0.03Si+0.06Mn+0.01Cr+0.05Mo). Figure 1 This graph shows the relationship between the F1 value and the maximum deformation ratio (%) for steel materials whose chemical composition contains each element within the above ranges and whose microstructural variations in cross-sectional and longitudinal sections meet the above conditions. The maximum deformation ratio is an indicator of heat treatment deformation. A larger maximum deformation ratio indicates greater heat treatment deformation of the steel material. The maximum deformation ratio is calculated using the method described below.

[0106] Reference Figure 1 In steel materials whose chemical composition contains the elements within the above ranges and whose microstructures at the cross-sectional and longitudinal sections satisfy the above conditions (1) to (6), the maximum deformation ratio decreases as F1 decreases. Moreover, when F1 is less than 0.800, the maximum deformation ratio decreases significantly. That is, the maximum deformation ratio has an inflection point with respect to F1 around F1 = 0.800. Therefore, if F1 is less than 0.800, the heat treatment deformation of the steel material during carburizing and quenching can be sufficiently suppressed.

[0107] As described above, in the steel having the above-mentioned chemical composition, the unevenness of the microstructure at the cross-sectional observation position and the longitudinal cross-sectional observation position is suppressed, thereby suppressing the unevenness of the timing of the occurrence of the martensitic phase transformation during quenching to a certain extent, and during quenching, there will always be a small period where the martensitic phase transformation part and the martensitic phase non-transformed part are mixed. Based on this premise, the present inventors found that by making F1 less than 0.800, excellent machinability is achieved, excellent bending fatigue strength and excellent surface fatigue strength are achieved after gas carburizing treatment, and heat treatment deformation after gas carburizing treatment can be fully suppressed.

[0108] The steel material according to the present embodiment, which was completed based on the above findings, has the following configuration. [1]

[0110] A type of steel,

[0111] Its chemical composition contains in mass %

[0112] C: 0.20~0.25%,

[0113] Si: 0.40-0.70%,

[0114] Mn: 0.50~0.90%,

[0115] Cr: 1.00-2.00%,

[0116] S: 0.005~0.050%,

[0117] N: 0.0050~0.0200%,

[0118] Al: 0.001~0.100%,

[0119] O: 0.0050% or less, and

[0120] P: 0.030% or less,

[0121] The remainder is Fe and impurities, and satisfies formula (1),

[0122] On a circular cross section of a radius R that is a cross section perpendicular to the longitudinal direction of the steel material,

[0123] When the center position of the cross section and eight positions of the R / 2 position arranged at 45° intervals around the center of the cross section at a distance R / 2 from the center of the cross section in the radial direction are defined as nine cross section observation positions,

[0124] The microstructure at each cross-sectional observation position contains ferrite, with the remainder being pearlite and / or bainite.

[0125] 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.

[0126] 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.

[0127] On a longitudinal cross section including the central axis of the steel material, which is a cross section parallel to the longitudinal direction of the steel material,

[0128] When three central axis positions arranged at an R / 2 pitch on the central axis and six R / 2 positions arranged at R / 2 intervals from each central axis position in the radial direction are defined as nine longitudinal cross-sectional observation positions,

[0129] The microstructure at the observation position of each longitudinal section contains ferrite, with the remainder being pearlite and / or bainite.

[0130] 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.

[0131] Among the average grain sizes of the 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.

[0132] 1-(0.5C+0.03Si+0.06Mn+0.01Cr+0.05Mo)<0.800 (1)

[0133] In formula (1), the content (mass %) of the corresponding element is substituted for each element symbol. If the corresponding element is not contained, "0" is substituted for the element symbol. [2]

[0135] The steel material according to [1], wherein

[0136] The chemical composition further contains one or more elements selected from the group consisting of the following elements in place of a portion of the Fe:

[0137] Mo: 0.30% or less,

[0138] Nb: 0.050% or less,

[0139] Ti: 0.020% or less,

[0140] Cu: 0.50% or less,

[0141] Ni: 0.80% or less,

[0142] V: 0.30% or less,

[0143] Mg: 0.0035% or less,

[0144] Ca: 0.0030% or less, and

[0145] Rare earth elements: less than 0.0050%.

[0146] Hereinafter, the steel material of this embodiment will be described in detail. Unless otherwise specified, "%" related to an element represents mass %.

[0147] The chemical composition of the steel material of this embodiment contains the following elements.

[0148] C: 0.20~0.25%

[0149] Carbon (C) increases the strength of steel. If the C content is less than 0.20%, the above-mentioned effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment.

[0150] On the other hand, if the C content exceeds 0.25%, the hardenability will become too high even if the contents of other elements are within the range of this embodiment. In this case, the hardness of the steel material after gas carburizing treatment will increase. As a result, the machinability of the steel material will decrease.

[0151] Therefore, the C content is 0.20 to 0.25%.

[0152] The lower limit of the C content is preferably 0.21%, more preferably 0.22%.

[0153] The upper limit of the C content is preferably 0.24%, more preferably 0.23%.

[0154] Si: 0.40-0.70%

[0155] Silicon (Si) improves the hardenability of steel and increases its strength. Si also increases the resistance to temper softening of the hardened layer of mechanical structural components. Consequently, the surface fatigue strength of mechanical structural components is enhanced. If the Si content is less than 0.40%, the aforementioned effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment.

[0156] On the other hand, if the Si content exceeds 0.70%, even if the contents of other elements are within the ranges of this embodiment, the grain boundary oxide layer after gas carburizing treatment becomes too deep, which reduces the bending fatigue strength of the machine structural component.

[0157] Therefore, the Si content is 0.40 to 0.70%.

[0158] The lower limit of the Si content is preferably 0.41%, more preferably 0.42%, more preferably 0.45%, more preferably 0.47%, and even more preferably 0.50%.

[0159] The upper limit of the Si content is preferably 0.69%, more preferably 0.67%, further preferably 0.65%, and further preferably 0.63%.

[0160] Mn: 0.50~0.90%

[0161] Manganese (Mn) improves the hardenability of steel and increases the bending fatigue strength and surface fatigue strength of machine structural parts. If the Mn content is less than 0.50%, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment.

[0162] On the other hand, if the Mn content exceeds 0.90%, even if the contents of other elements are within the ranges of this embodiment, the formation of Mn oxides on the surface of the mechanical structural component during gas carburizing will be promoted. In this case, the grain boundary oxide layer of the mechanical structural component becomes too deep, thereby reducing the bending fatigue strength of the mechanical structural component. The Mn oxides formed on the surface of the mechanical structural component also cause surface-originating spalling. If surface-originating spalling occurs, the surface fatigue strength of the mechanical structural component will be reduced.

[0163] Therefore, the Mn content is 0.50 to 0.90%.

[0164] The lower limit of the Mn content is preferably 0.51%, more preferably 0.52%, further preferably 0.55%, and further preferably 0.60%.

[0165] The upper limit of the Mn content is preferably 0.89%, more preferably 0.87%, and even more preferably 0.85%.

[0166] Cr: 1.00~2.00%

[0167] Chromium (Cr) improves the hardenability of steel and increases the bending fatigue strength and surface fatigue strength of machine structural parts. If the Cr content is less than 1.00%, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment.

[0168] On the other hand, if the Cr content exceeds 2.00%, the steel material becomes too hard even if the contents of other elements are within the ranges of this embodiment. In this case, the machinability of the steel material decreases.

[0169] Therefore, the Cr content is 1.00 to 2.00%.

[0170] The lower limit of the Cr content is preferably 1.10%, more preferably 1.20%, more preferably 1.40%, more preferably 1.60%, more preferably 1.70%, and still more preferably 1.80%.

[0171] The upper limit of the Cr content is preferably less than 2.00%, more preferably 1.95%, and even more preferably 1.90%.

[0172] S: 0.005~0.050%

[0173] Sulfur (S) bonds with Mn to form MnS. MnS improves the machinability of steel. If the S content is less than 0.005%, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment.

[0174] On the other hand, when the S content exceeds 0.050%, even if the contents of other elements are within the ranges of this embodiment, excessive MnS is formed. In this case, the bending fatigue strength and surface fatigue strength of the machine structural component are reduced.

[0175] Therefore, the S content is 0.005 to 0.050%.

[0176] The lower limit of the S content is preferably 0.007%, more preferably 0.010%, further preferably 0.013%, and further preferably 0.015%.

[0177] The upper limit of the S content is preferably 0.049%, more preferably 0.045%, more preferably 0.040%, more preferably 0.035%, and even more preferably 0.025%.

[0178] N: 0.0050~0.0200%

[0179] Nitrogen (N) bonds with Al and Nb to form AlN and NbN. AlN and NbN suppress grain coarsening during heating during gas carburizing through their pinning effect. When the N content is less than 0.0050%, the aforementioned effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment.

[0180] 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 during the steelmaking process.

[0181] Therefore, the N content is 0.0050 to 0.0200%.

[0182] The lower limit of the N content is preferably 0.0100%, more preferably 0.0120%, and even more preferably 0.0130%.

[0183] The upper limit of the N content is preferably less than 0.0200%, more preferably 0.0190%, further preferably 0.0180%, and further preferably 0.0150%.

[0184] Al: 0.001~0.100%

[0185] Aluminum (Al) deoxidizes steel. Al also bonds with nitrogen to form AlN. AlN suppresses grain coarsening during heating during gas carburizing through its pinning effect. If the Al content is less than 0.001%, the aforementioned effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment.

[0186] On the other hand, when the Al content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, the formation of coarse Al oxides is promoted. Coarse Al oxides reduce the bending fatigue strength of machine structural parts.

[0187] Therefore, the Al content is 0.001 to 0.100%.

[0188] The lower limit of the Al content is preferably 0.020%, more preferably 0.025%, further preferably 0.027%, and further preferably 0.030%.

[0189] The upper limit of the Al content is preferably 0.090%, more preferably 0.070%, more preferably 0.050%, more preferably 0.045%, more preferably 0.040%, and still more preferably 0.035%.

[0190] O (oxygen): 0.0050% or less

[0191] Oxygen (O) is an impurity. O bonds with other elements in the steel to form coarse oxide inclusions. Coarse oxide inclusions reduce the bending fatigue strength of mechanical structural components. When the O content exceeds 0.0050%, the bending fatigue strength of mechanical structural components is significantly reduced, even if the contents of other elements are within the ranges of this embodiment.

[0192] Therefore, the O content is set to 0.0050% or less.

[0193] The O content is preferably as low as possible. However, excessively reducing the O content increases production costs. Therefore, considering typical industrial production, the preferred lower limit of the O content is greater than 0%, more preferably 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%.

[0194] The upper limit of the O content is preferably 0.0040%, more preferably 0.0030%, further preferably 0.0020%, further preferably 0.0015%.

[0195] P: 0.030% or less

[0196] Phosphorus (P) is an impurity. P segregates at grain boundaries, reducing grain boundary strength. If the P content exceeds 0.030%, even if the contents of other elements are within the ranges of this embodiment, excessive P segregation at grain boundaries reduces grain boundary strength. As a result, the bending fatigue strength and surface fatigue strength of mechanical structural components decrease.

[0197] Therefore, the P content is set to 0.030% or less.

[0198] The P content is preferably as low as possible. However, excessively reducing the P content increases production costs. Therefore, considering typical industrial production, the preferred lower limit of the P content is greater than 0%, more preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%.

[0199] The upper limit of the P content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.

[0200] The balance of the chemical composition of the steel according to this embodiment is Fe and impurities. Impurities are substances that enter the steel from raw materials such as ore and scrap, or from the manufacturing environment during industrial steel production. These substances are permitted as long as they do not adversely affect the steel according to this embodiment. The impurities mentioned here include 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 0.01% or less. It should be noted that among the above impurities, the B content is 0.0003% or less.

[0201] About optional elements

[0202] The chemical composition of the steel material of this embodiment may further contain one or more elements selected from the group consisting of the following elements in place of a portion of Fe:

[0203] Mo: 0.30% or less,

[0204] Nb: 0.050% or less,

[0205] Ti: 0.020% or less,

[0206] Cu: 0.50% or less,

[0207] Ni: 0.80% or less,

[0208] V: 0.30% or less,

[0209] Mg: 0.0035% or less,

[0210] Ca: 0.0030% or less, and

[0211] Rare earth elements: less than 0.0050%.

[0212] These elements are optional elements, and all of them improve the bending fatigue strength and surface fatigue strength of machine structural parts.

[0213] Mo: 0.30% or less

[0214] Molybdenum (Mo) is an optional element and may not be contained. That is, the Mo content may be 0%.

[0215] When Mo is contained, it improves the hardenability of steel and increases the bending fatigue strength and surface fatigue strength of mechanical structural parts. The above effects can be achieved to a certain extent even if Mo is contained in a small amount.

[0216] However, if the Mo content exceeds 0.30%, the steel material becomes too hard even if the contents of other elements are within the ranges of this embodiment. In this case, the machinability of the steel material decreases.

[0217] Therefore, the Mo content is 0 to 0.30%, and when contained, it is 0.30% or less (that is, greater than 0 and 0.30% or less).

[0218] The lower limit of the Mo content is preferably 0.01%, more preferably 0.02%, further preferably 0.05%, and further preferably 0.10%.

[0219] The upper limit of the Mo content is preferably less than 0.30%, more preferably 0.25%, further preferably 0.20%, and further preferably 0.15%.

[0220] Nb: 0.050% or less

[0221] Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%.

[0222] When Nb is present, it bonds with C and / or N to form Nb precipitates (NbC, NbN, Nb(CN), etc.). Like AlN, Nb precipitates suppress the coarsening of grains during gas carburizing through the pinning effect. Therefore, the bending fatigue strength and surface fatigue strength of mechanical structural components are improved. The above-mentioned effects can be achieved to a certain extent by including Nb in a small amount.

[0223] However, if the Nb content exceeds 0.050%, even if the contents of other elements are within the ranges of this embodiment, Nb precipitates will coarsen. In this case, grain coarsening during gas carburizing cannot be fully suppressed. Consequently, the bending fatigue strength and surface fatigue strength of mechanical structural components are reduced.

[0224] 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).

[0225] The lower limit of the Nb content is preferably 0.001%, more preferably 0.010%, more preferably 0.015%, more preferably 0.020%, and even more preferably 0.025%.

[0226] The upper limit of the Nb content is preferably less than 0.050%, more preferably 0.045%, further preferably 0.040%, and further preferably 0.035%.

[0227] Ti: 0.020% or less

[0228] Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%.

[0229] When Ti is contained, it forms Ti precipitates (TiC, TiN, Ti(CN), etc.) in the same manner as Nb. Ti precipitates suppress the coarsening of grains during gas carburizing through the pinning effect. Therefore, the bending fatigue strength and surface fatigue strength of mechanical structural components are improved. As long as Ti is contained in a small amount, the above-mentioned effects can be achieved to a certain extent.

[0230] However, if the Ti content exceeds 0.020%, Ti precipitates will coarsen even if the contents of other elements are within the ranges of this embodiment. In this case, grain coarsening during gas carburizing cannot be fully suppressed. Consequently, the bending fatigue strength and surface fatigue strength of the machine structural component are reduced.

[0231] 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).

[0232] The lower limit of the Ti content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%.

[0233] The upper limit of the Ti content is preferably 0.019%, more preferably 0.017%, and even more preferably 0.015%.

[0234] Cu: 0.50% or less

[0235] Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%.

[0236] When contained, Cu improves the hardenability of steel and increases the bending fatigue strength and surface fatigue strength of mechanical structural parts. The above effects can be achieved to a certain extent even with a small amount of Cu.

[0237] However, when the Cu content exceeds 0.50%, the steel material becomes too hard even if the contents of other elements are within the ranges of this embodiment. In this case, the machinability of the steel material decreases.

[0238] 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).

[0239] The preferred lower limit of the Cu content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit of the Cu content is 0.45%, more preferably 0.40%, even more preferably 0.30%, and even more preferably 0.25%.

[0240] Ni: 0.80% or less

[0241] Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%.

[0242] When Ni is contained, it improves the hardenability of steel and increases the bending fatigue strength and surface fatigue strength of mechanical structural parts. The above effects can be achieved to a certain extent even if Ni is contained in a small amount.

[0243] However, when the Ni content exceeds 0.80%, the steel material becomes too hard even if the contents of other elements are within the ranges of this embodiment. In this case, the machinability of the steel material decreases.

[0244] Therefore, the Ni content is 0 to 0.80%, and when Ni is contained, it is 0.80% or less (that is, greater than 0 and 0.80% or less).

[0245] The lower limit of the Ni content is preferably 0.01%, more preferably 0.05%, and even more preferably 0.10%.

[0246] The upper limit of the Ni content is preferably 0.70%, more preferably 0.60%, further preferably 0.40%, and further preferably 0.20%.

[0247] V: 0.30% or less

[0248] Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%.

[0249] When V is contained, V and Nb can form V precipitates (VC, VN, V (CN) etc.) in the same manner. V precipitates suppress the coarsening of grains in gas carburizing treatment by the pinning effect. Therefore, the bending fatigue strength and the surface fatigue strength of mechanical structure parts are improved. As long as V is contained in a small amount, the above-mentioned effect can be obtained to a certain extent.

[0250] However, when the V content exceeds 0.30%, the steel material becomes too hard even if the contents of other elements are within the ranges of this embodiment. In this case, the machinability of the steel material decreases.

[0251] 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).

[0252] The lower limit of the V content is preferably 0.01%, more preferably 0.03%, and even more preferably 0.04%.

[0253] The upper limit of the V content is preferably 0.20%, more preferably 0.15%, and even more preferably 0.10%.

[0254] Mg: 0.0035% or less

[0255] Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%.

[0256] When Mg is included, it deoxidizes the steel in the same way as Al. This suppresses the formation of coarse oxides. Consequently, the bending fatigue strength and surface fatigue strength of mechanical structural components are improved. Even a small amount of Mg can achieve these effects to a certain extent.

[0257] However, when the Mg content exceeds 0.0035%, even if the contents of other elements are within the ranges of this embodiment, the formation of coarse Mg oxides in the steel material is promoted. In this case, the maximum working rate during hot working decreases.

[0258] Therefore, the Mg content is 0 to 0.0035%, and when Mg is contained, it is 0.0035% or less (that is, greater than 0 and 0.0035% or less).

[0259] The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.

[0260] The upper limit of the Mg content is preferably 0.0030%, more preferably 0.0028%, further preferably 0.0025%, and further preferably 0.0020%.

[0261] Ca: 0.0030% or less

[0262] Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%.

[0263] When Ca is present, it refines the sulfides in steel. It also promotes the spheroidization of sulfides in steel. Consequently, it improves the bending fatigue strength and surface fatigue strength of mechanical structural components. Even a small amount of Ca can achieve these effects to a certain extent.

[0264] However, if the Ca content exceeds 0.0030%, even if the contents of other elements are within the ranges of this embodiment, coarse Ca oxides will form in the steel material, which will reduce the bending fatigue strength and surface fatigue strength of the machine structural component.

[0265] 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).

[0266] The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0002%, more preferably 0.0005%, more preferably 0.0007%, and even more preferably 0.0010%.

[0267] The upper limit of the Ca content is preferably 0.0025%, more preferably 0.0022%, and even more preferably 0.0020%.

[0268] Rare earth elements (REM): 0.0050% or less

[0269] Rare earth elements (REM) are optional elements and do not need to be included. That is, the REM content can be 0%. When included, REM dissolves in the sulfides in the steel, suppressing the extension of MnS. As a result, the bending fatigue strength and surface fatigue strength of mechanical structural components are improved. Even a small amount of REM can achieve these effects to a certain extent.

[0270] However, if the REM content exceeds 0.0050%, even if the contents of other elements are within the ranges of this embodiment, coarse oxides will be generated, which will reduce the bending fatigue strength and surface fatigue strength of the machine structural component.

[0271] 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).

[0272] The lower limit of the REM content is preferably 0.0001%, more preferably 0.0010%, and even more preferably 0.0020%.

[0273] The upper limit of the REM content is preferably 0.0045%, more preferably 0.0040%, further preferably 0.0035%, further preferably 0.0030%.

[0274] REM as used herein refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, which are members of the lanthanide series. The REM content used herein refers to the total content of these elements.

[0275] [About the microstructure of steel]

[0276] The microstructure of the steel material of the present embodiment contains ferrite, with the balance being pearlite and / or bainite, and the area fraction of ferrite is 50 to 70%.

[0277] If 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 increases excessively, resulting in a decrease in the machinability of the steel.

[0278] On the other hand, when the area fraction of ferrite exceeds 70%, the crystal grain size tends to become uneven during gas carburizing, thereby causing excessive heat treatment distortion during gas carburizing.

[0279] When the area fraction of ferrite is 50-70% and the balance other than ferrite in the microstructure is pearlite and / or bainite, the machinability of the steel material is sufficiently improved. Furthermore, heat treatment distortion during gas carburizing can be suppressed.

[0280] In the steel material of the present embodiment, the microstructure at each cross-sectional observation position and each longitudinal cross-sectional observation position contains 50 to 70% of ferrite by area fraction, with the remainder being pearlite and / or bainite.

[0281] The lower limit of the area fraction of ferrite at each observation position (each cross-sectional observation position, each longitudinal cross-sectional observation position) is preferably 52%, more preferably 55%, and even more preferably 57%.

[0282] The upper limit of the area fraction of ferrite at each observation position (each cross-sectional observation position, each longitudinal cross-sectional observation position) is preferably 68%, more preferably 65%, and even more preferably 63%.

[0283] [About the unevenness of the microstructure in the cross section of steel materials]

[0284] In the steel material of this embodiment, microstructural nonuniformity is sufficiently suppressed 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.

[0285] Figure 2 This is a schematic diagram of a cross section perpendicular to the longitudinal direction of the steel material of this embodiment. Figure 2 The cross section CS of the steel material is a circle with a radius R. In this 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 at a distance 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.

[0286] The microstructures at cross-sectional observation positions C1 to C9 satisfy the following (A) and (B).

[0287] (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.

[0288] (B) Among the average grain sizes of ferrite at cross-sectional observation positions C1 to C9, the ratio of the maximum average grain size to the minimum average grain size is 2.00 or less.

[0289] Hereinafter, (A) and (B) will be described in detail.

[0290] [About (A)]

[0291] As described in (A) above, in the steel material of this embodiment, 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.

[0292] The standard deviation of the ferrite area fraction is 4.0% or less, thus sufficiently suppressing variations in the microstructural phase fraction at cross-sectional observation positions C1 to C9. Consequently, variations in the timing of martensitic transformation at cross-sectional observation positions C1 to C9 during gas carburizing can be suppressed.

[0293] If the standard deviation of the area fraction of ferrite at cross-sectional observation positions C1 to C9 exceeds 4.0%, the phase fraction at each cross-sectional observation position C1 to C9 will vary greatly. In this case, heat treatment distortion during gas carburizing cannot be sufficiently suppressed.

[0294] Therefore, the standard deviation of the area fraction of ferrite at the cross-sectional observation positions C1 to C9 is 4.0% or less.

[0295] The upper limit of the standard deviation of the area fraction of ferrite is preferably 3.8%, more preferably 3.5%, and even more preferably 3.0%. The lower limit of the standard deviation of the area fraction of ferrite is not particularly limited.

[0296] The lower limit of the standard deviation of the area fraction of ferrite is not particularly limited, but the lower limit of the standard deviation of the area fraction of ferrite is preferably 0.1%, more preferably 0.5%, more preferably 1.0%, and even more preferably 1.5%.

[0297] [About (B)]

[0298] The ratio of the maximum average grain size to the minimum average grain size of ferrite at cross-sectional observation positions C1 to C9 is referred to as the “ferrite average grain size ratio.” The ferrite average grain size ratio on the cross section is defined by the following formula.

[0299] 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)

[0300] In the steel material of this embodiment, the average ferrite grain size ratio at cross-sectional observation positions C1 to C9 is 2.00 or less. In this case, the variation in the average ferrite grain size at each cross-sectional observation position C1 to C9 is sufficiently suppressed. In other words, the ferrite grains at each position are concentrated. Consequently, the uneven occurrence of martensitic transformation during carburizing can be suppressed. Consequently, heat treatment deformation of the steel material during carburizing can be suppressed.

[0301] When the ferrite average grain size ratio exceeds 2.00, the ferrite grains at observation positions C1 to C9 of each cross section are uneven. In this case, heat treatment deformation of the steel material during gas carburizing cannot be suppressed.

[0302] Therefore, the ferrite average grain size ratio is 2.00 or less.

[0303] The upper limit of the ferrite average grain size ratio is preferably 1.95, more preferably 1.90, and even more preferably 1.80.

[0304] The lower limit of the ferrite average grain size ratio is not particularly limited, but is preferably 1.10, more preferably 1.20, further preferably 1.30, and further preferably 1.40.

[0305] [About the Microstructure Unevenness in the Longitudinal Section of Steel Materials]

[0306] In the steel material of this embodiment, microstructural variations are sufficiently suppressed not only in the cross-section described above, but also in the longitudinal section, which is a section parallel to the longitudinal direction of the steel material and includes the central axis of the steel material. In the steel material of this embodiment, microstructural variations are sufficiently suppressed not only in the cross-section but also in the longitudinal section, thereby sufficiently suppressing three-dimensional heat treatment deformation. The suppression of microstructural variations in the longitudinal section will be described below.

[0307] Figure 3 This is a schematic diagram of a longitudinal cross section of the steel material according to this embodiment, which is a cross section parallel to the longitudinal direction and including the central axis. Figure 3 In the longitudinal section LS of the steel material, three center axis positions L1 to L3 arranged at an R / 2 interval on the center axis CL1 of the steel material, and six R / 2 positions L4 to L9 arranged at positions R / 2 away from each center axis position in the radial direction are defined as nine "longitudinal section observation positions" L1 to L9.

[0308] In the microstructures at the aforementioned nine longitudinal cross-sectional observation positions L1 to L9, the following (C) and (D) are satisfied.

[0309] (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.

[0310] (D) Among the average grain sizes of ferrite at longitudinal cross-sectional observation positions L1 to L9, the ratio of the maximum average grain size to the minimum average grain size (ferrite average grain size ratio) is 2.00 or less.

[0311] Hereinafter, (C) and (D) will be described in detail.

[0312] [About (C)]

[0313] As described in (C) above, in the steel material of this 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.

[0314] The standard deviation of the ferrite area fraction is 4.0% or less, thus sufficiently suppressing variations in the microstructural phase fraction at each longitudinal cross-sectional observation position L1 to L9. Consequently, variations in the timing of martensitic transformation at each longitudinal cross-sectional observation position L1 to L9 during gas carburizing can be suppressed.

[0315] If the standard deviation of the area fraction of ferrite at longitudinal cross-sectional observation positions L1 to L9 exceeds 4.0%, the phase fraction at each longitudinal cross-sectional observation position L1 to L9 will vary greatly. In this case, heat treatment deformation during gas carburizing cannot be sufficiently suppressed.

[0316] 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.

[0317] The upper limit of the standard deviation of the area fraction of ferrite is preferably 3.8%, more preferably 3.5%, and even more preferably 3.0%.

[0318] The lower limit of the standard deviation of the area fraction of ferrite is not particularly limited, but the lower limit of the standard deviation of the area fraction of ferrite is preferably 0.1%, more preferably 0.5%, more preferably 1.0%, and even more preferably 1.5%.

[0319] [About (D)]

[0320] The ratio of the maximum average grain size to the minimum average grain size among the average grain sizes of ferrite at longitudinal cross-sectional observation positions L1 to L9 is referred to as the “ferrite average grain size ratio.” The ferrite average grain size ratio on the longitudinal cross section is defined by the following formula.

[0321] 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)

[0322] In the steel material of this embodiment, the average ferrite grain size ratio at longitudinal cross-sectional observation positions L1 to L9 is 2.00 or less. In this case, the variation in the average ferrite grain size at each longitudinal cross-sectional observation position L1 to L9 is sufficiently suppressed. In other words, the ferrite grains at each position are concentrated. Consequently, the uneven occurrence of martensitic transformation during carburizing can be suppressed. Consequently, heat treatment deformation of the steel material during carburizing can be suppressed.

[0323] If the ferrite average grain size ratio exceeds 2.00, the ferrite grains at each longitudinal cross-sectional observation position L1 to L9 will be uneven. In this case, heat treatment deformation of the steel during gas carburizing cannot be suppressed. Therefore, the ferrite average grain size ratio at the nine longitudinal cross-sectional observation positions L1 to L9 is set to 2.00 or less.

[0324] The upper limit of the ferrite average grain size ratio is preferably 1.95, more preferably 1.90, and even more preferably 1.80.

[0325] The lower limit of the ferrite average grain size ratio is not particularly limited, but is preferably 1.10, more preferably 1.20, further preferably 1.30, and further preferably 1.40.

[0326] [Observation Method of Microstructure at Each Observation Position, and Measurement Method of Ferrite Area Fraction and Ferrite Average Grain Size Ratio]

[0327] The method for observing the microstructure at cross-sectional observation positions C1 to C9 and longitudinal cross-sectional observation positions L1 to L9 of the steel material according to the present embodiment, and the method for measuring the area fraction of ferrite and the average ferrite grain size ratio are as follows.

[0328] [Observation method of microstructure of cross-section CS]

[0329] The microstructure of cross section CS was observed as follows. Samples were collected from the steel material, including observation positions C1 to C9. The surface of the sample corresponding to cross section CS was designated as the observation surface. Within the observation surface, the observation field, including the cross section observation position, was set to 0.5 mm x 1.0 mm.

[0330] The observation surface of the sample was polished, and then etched with 3% nital (Nital etching solution), and the observation field (0.5 mm x 1.0 mm) of the etched observation surface was observed using an optical microscope at 100x magnification.

[0331] Within the observation field, the contrast of each phase—ferrite, pearlite, and bainite—varies. Specifically, within the observation field, ferrite appears white, while bainite and pearlite appear darker than ferrite. Therefore, ferrite can be easily distinguished from the other phases (pearlite and bainite). Ferrite is identified based on contrast.

[0332] [About (A)]

[0333] 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 determined.

[0334] [Method for calculating the arithmetic mean of the ferrite area fraction]

[0335] 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.

[0336] [Method for calculating the standard deviation of the area fraction of ferrite]

[0337] The standard deviation (%) of the ferrite area fraction at nine cross-sectional observation positions C1 to C9 was calculated from the ferrite area fraction (%) in nine observation fields (cross-sectional observation positions). The standard deviation here refers to the sample standard deviation.

[0338] [About (B)]

[0339] 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. 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. Here, the circle-equivalent diameter refers to the diameter (μm) of a circle having the same area as the arithmetic mean of the area of ​​the ferrite grains.

[0340] Calculate the average ferrite grain size at nine cross-sectional observation locations C1 to C9. Then, determine the maximum average grain size (μm) and minimum average grain size (μm) of these ferrite grain sizes. Calculate the ratio of the determined maximum average grain size to the minimum average grain size (ferrite average grain size ratio).

[0341] [Observation method of microstructure of longitudinal section LS]

[0342] The microstructure of longitudinal section LS is observed as follows. Samples are collected from the steel material, including longitudinal section observation positions L1 to L9. The surface of the sample corresponding to longitudinal section LS is designated as the observation surface. Within the observation surface, the observation field encompassing the longitudinal section observation position is set to 0.5 mm x 1.0 mm. More specifically, the 0.5 mm length of the observation field corresponds to the radial direction of the steel material, and the 1.0 mm length corresponds to the longitudinal direction of the steel material.

[0343] After polishing the observation surface of the sample, etch it with 3% nital (Nital). Observe the observation field (0.5 mm x 1.0 mm) of the etched observation surface using a 100x optical microscope. Identify each phase in the observation field using the same method as for the microstructural observation of the cross-section CS.

[0344] [About (C)]

[0345] 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 ). Using the area of ​​ferrite and the area of ​​the observation field, the area fraction (%) of ferrite in each observation field (each longitudinal cross-sectional observation position) was determined.

[0346] [Method for calculating the arithmetic mean of the area fraction of ferrite]

[0347] 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.

[0348] [Method for calculating the standard deviation of the area fraction of ferrite]

[0349] From the ferrite area fraction (%) in the nine observation fields (vertical 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.

[0350] [About (D)]

[0351] The area (μm) of each ferrite grain observed in each of the above-mentioned observation fields (each longitudinal cross-sectional observation position L1 to L9) was measured. 2 The arithmetic mean of the area of ​​each ferrite grain was calculated. The circle-equivalent diameter of the obtained arithmetic mean of the area was defined as the average grain size (μm) of the ferrite at the observation positions L1 to L9 of the longitudinal cross section.

[0352] Calculate the average ferrite grain size at nine longitudinal cross-sectional observation locations L1 to L9. Then, determine the maximum average grain size (μm) and the minimum average grain size (μm) of these ferrite grain sizes. Calculate the ratio of the determined maximum average grain size to the minimum average grain size (ferrite average grain size ratio).

[0353] [About formula (1)]

[0354] As described above, in the steel material of this embodiment, the microstructure is made as uniform as possible not only in the cross-section CS but also in the longitudinal section LS. Furthermore, in the steel material of this embodiment, by satisfying equation (1), the generation of heat treatment strain in the untransformed martensite portion is suppressed, assuming that a small period of time during quenching occurs where the martensite transformed portion and the untransformed martensite portion coexist. As a result, sufficient machinability can be maintained, as well as sufficient bending fatigue strength and sufficient surface fatigue strength in machine structural components, while heat treatment deformation can be sufficiently suppressed. Equation (1) is explained below.

[0355] The chemical composition of the steel material of the present embodiment is based on the premise that the contents of the elements in the chemical composition are within the above-mentioned ranges and further satisfies the following formula (1).

[0356] 1-(0.5C+0.03Si+0.06Mn+0.01Cr+0.05Mo)<0.800 (1)

[0357] In formula (1), the content (mass %) of the corresponding element is substituted for each element symbol. If the corresponding element is not contained, "0" is substituted for the element symbol.

[0358] In the steel material of this embodiment, the microstructure is uniform not only in the cross-section CS but also in the longitudinal section LS. However, even if the microstructure at cross-sectional observation positions C1 to C9 in the cross-sectional view CS satisfies (A) and (B), and the microstructure at longitudinal observation positions L1 to L9 in the longitudinal view LS satisfies (C) and (D), during quenching in the gas carburizing process, as described above, a microscopic period in which a mixture of martensite transformed portions and untransformed portions of martensite are inevitably generated. If the heat treatment strain in the untransformed portions of martensite is high in these microscopic periods, heat treatment distortion will occur. To this end, the steel material of this embodiment further satisfies equation (1).

[0359] Defined as F1 = 1 - (0.5C + 0.03Si + 0.06Mn + 0.01Cr + 0.05Mo). F1 is an indicator of the amount of heat treatment deformation of steel during gas carburizing. Among the elements in the above chemical composition, the C, Si, Mn, Cr, and Mo contained in F1 particularly enhance the strength of the untransformed martensite during the minute period of time when the martensite transformed portion and the untransformed portion coexist during quenching.

[0360] Reference Figure 1 As F1 decreases, the maximum deformation ratio decreases, and the heat treatment deformation decreases. Moreover, when F1 is less than 0.800, the maximum deformation ratio decreases significantly. In other words, the maximum deformation ratio relative to F1 has an inflection point around F1 = 0.800.

[0361] Therefore, under the premise that the contents of each element are within the range of the present embodiment and F1 satisfies the formula (1), that is, when F1 is less than 0.800, heat treatment deformation of the steel material during gas carburizing can be sufficiently suppressed.

[0362] The preferred upper limit of F1 is 0.799, more preferably 0.797, and even more preferably 0.795. The lower limit of F1 is not particularly limited. However, considering the upper limit of the content of each element in the chemical composition of this embodiment, the preferred lower limit of F1 is 0.765, more preferably 0.770, and even more preferably 0.775. The numerical value of F1 is the value obtained by rounding off to the fourth decimal place.

[0363] The steel material of this embodiment having the above-described structure has the chemical composition of each element within the range of this embodiment, and F1 satisfies equation (1). The microstructure at positions C1 to C9 observed in cross section and positions L1 to L9 observed in longitudinal section is within the range of this embodiment. Therefore, the steel material of this embodiment exhibits excellent machinability after hot working. Furthermore, when the steel material of this embodiment is subjected to gas carburizing, mechanical structural components exhibit excellent bending fatigue strength and surface fatigue strength, and heat treatment deformation can be sufficiently suppressed.

[0364] [About the microstructure of steel]

[0365] The steel material of this embodiment is a so-called as-rolled material. Therefore, in the steel material of this embodiment, a so-called banded structure is observed in the above-mentioned observation field of the longitudinal section observation positions L1 to L9. Here, the banded structure is a well-known microstructure, such as Figure 4The structure shown in FIG. 1 is a structure in which ferrite (ferrite bands) F extending in the longitudinal direction of the steel material and non-ferrite (non-ferrite bands) NF extending in the longitudinal direction of the steel material are alternately layered in the radial direction. The non-ferrite is pearlite and / or bainite.

[0366] [Use of Steel]

[0367] The steel material of this embodiment is suitable for the material of machine structural parts as described above. The steel material of this embodiment is particularly suitable for use in gears or shafts for automobiles, construction machinery, industrial machinery, and the like.

[0368] [Method for manufacturing steel materials]

[0369] An example of a method for manufacturing the steel material of this embodiment will be described. The method described below is an example for manufacturing the steel material of this embodiment. Therefore, the steel material having the above-described structure can also be manufactured using other manufacturing methods besides the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing the steel material of this embodiment.

[0370] An example of a method for producing a steel material according to the present embodiment includes the following steps.

[0371] (Process 1) Process of preparing blank (Blank preparation process)

[0372] (Step 2) Hot working the blank to produce a steel material (hot working step)

[0373] Hereinafter, each step will be described.

[0374] [(Process 1) Blank preparation process]

[0375] In the billet preparation process, a billet of the steel material of the present embodiment is prepared. Specifically, a molten steel is produced in which the content of each element in the chemical composition is within the range of the present embodiment and F1 satisfies the formula (1). The refining method is not particularly limited, and a known method can be used. For example, the molten iron produced by the known method is subjected to refining using a converter (primary refining). The molten steel tapped from the converter is subjected to known secondary refining. In the secondary refining, the content of the alloying elements in the molten steel is adjusted to produce 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).

[0376] Molten steel produced by the above-mentioned refining method is used to produce billets using known casting methods. For example, the molten steel can be used to produce ingots using an ingot casting method. Alternatively, the molten steel can be used to produce blooms or billets using a continuous casting method. Billets (ingots, blooms, or billets) are produced using the above methods. When using a continuous casting method, reduction can be applied to the billet during solidification.

[0377] [(Process 2) Hot working process]

[0378] 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 this embodiment. The shape of the steel material is not particularly limited, and examples include steel bars or wire rods. The following description uses a steel bar as an example. However, steel materials in shapes other than steel bars can also be produced using the same hot working steps.

[0379] The thermal processing process includes the following steps.

[0380] (Process 21) Initial rolling process

[0381] Heating temperature: 1250~1300℃

[0382] Holding time: more than 10 hours

[0383] (Process 22) Finishing Rolling Process

[0384] Heating temperature: 1150~1200℃

[0385] Holding time: 1.5 to 3.0 hours

[0386] Final temperature: 950~1000℃

[0387] (Step 23) Temperature maintenance step

[0388] Average cooling rate at 900-800°C: 0.05°C / s or less

[0389] (Process 24) Cooling process

[0390] Average cooling rate at 800-300°C: 0.10-1.00°C / s

[0391] Hereinafter, each step will be described.

[0392] [(Process 21) Blooming process]

[0393] In the initial rolling process, the billet is hot rolled to produce small square billets. Specifically, in the initial rolling process, the billet is hot rolled (initial rolling) using a initial rolling mill to produce small square billets. When a continuous rolling mill is provided downstream of the initial rolling mill, the continuous rolling mill can also be used to hot roll the initial rolled small square billets to produce small-sized small square billets. In the continuous rolling mill, a horizontal rolling mill having a pair of horizontal rollers and a vertical rolling mill having a pair of vertical rollers are alternately arranged in a row. As described above, in the initial rolling process, the billet is produced into small square billets using a initial rolling mill, or using a initial rolling mill and a continuous rolling mill.

[0394] The conditions in the blooming step are as follows.

[0395] Heating temperature: 1250~1300℃

[0396] Holding time: more than 10 hours

[0397] 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. If 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 on 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 becomes 4.0% or less. 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.

[0398] Note that the billet produced by the blooming step is naturally cooled (air-cooled) to room temperature before the finish rolling step.

[0399] The reduction in area during the initial rolling process is 30% or more. Here, the reduction in area (%) is defined by the following formula.

[0400] Sectional reduction (%) = (1-area of ​​the cross section of the steel material after initial rolling (a cross section perpendicular to the longitudinal direction) / area of ​​the cross section of the billet before initial rolling (a cross section perpendicular to the longitudinal direction)) × 100

[0401] If the reduction of area in the initial rolling process is 30% or more, 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 becomes 4.0% or less, assuming that other manufacturing conditions are satisfied.

[0402] [(Process 22) Finishing Rolling Process]

[0403] In the finish rolling process, a billet cooled to room temperature is first heated in a heating furnace and then hot rolled in a tandem rolling mill to produce a steel bar.

[0404] The conditions in the finish rolling process are as follows.

[0405] Heating temperature: 1150~1200℃

[0406] Holding time: 1.5 to 3.0 hours

[0407] Final temperature: 950~1000℃

[0408] [Heating temperature and holding time]

[0409] The heating temperature in the heating furnace during the finishing rolling process is 1150-1200°C. The holding time at the heating temperature (1150-1200°C) is 1.5-3.0 hours. If the heating temperature of the heating furnace during 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 within the steel material (steel bar) can be fully suppressed on the premise of satisfying 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 becomes less than 2.00.

[0410] [Final temperature]

[0411] In the finishing rolling process, hot rolling (finishing rolling) is performed using a tandem mill with multiple rolling mills arranged in a row. In hot rolling using a tandem mill, the temperature of the steel at the exit of the rolling mill where the steel is finally rolled is defined as the final temperature (°C). Note that the steel temperature refers to the surface temperature of the steel.

[0412] The final temperature is 950-1000°C. When the final temperature is 950-1000°C, variations in austenite grain size within the steel material (steel bar) are sufficiently suppressed, provided that other manufacturing conditions are met. Therefore, during the transformation from austenite to ferrite in the temperature holding and cooling steps described later, variations in the average ferrite grain size are sufficiently suppressed. Consequently, the average ferrite grain size ratio at cross-sectional observation positions C1-C9 and longitudinal section observation positions L1-L9 is 2.00 or less.

[0413] [(Step 23) Temperature Maintaining Step]

[0414] 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.

[0415] Average cooling rate at steel temperature of 900-800°C: 0.05°C / s or less

[0416] After the finish rolling process, the average cooling rate at a steel material temperature of 900-800°C is suppressed to 0.05°C / second or less. For example, after the finish rolling process, the steel material temperature is 900-800°C, and the average cooling rate is suppressed to 0.05°C / second or less using a slow cooling hood, a heat preservation hood, or a temperature holding furnace.

[0417] If the average cooling rate at a steel temperature of 900-800°C is less than 0.05°C / second, the temperature unevenness in the axial direction (longitudinal direction) of the steel can be suppressed, provided that other manufacturing conditions are met. Therefore, the unevenness in the timing of ferrite transformation in the axial direction of the steel can be suppressed. Therefore, in particular, the uneven growth of ferrite grains in the axial direction (longitudinal section) of the steel can be suppressed. Specifically, the following mechanism plays a role.

[0418] In steel after the finish rolling process, austenite gradually transforms into ferrite as the steel temperature decreases. Within the steel temperature range of 900-800°C, if there is temperature unevenness in the axial direction of the steel, ferrite formed earlier and ferrite formed later will be mixed after the finish rolling process. In this case, the ferrite grains formed earlier are more likely to become coarser than those transformed later. As a result, the unevenness of the ferrite grains becomes particularly large in the axial direction (longitudinal cross-section) of the steel.

[0419] 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 increases. Therefore, the average ferrite grain size ratio at longitudinal section observation positions L1 to L9 increases.

[0420] To this end, in this embodiment, the average cooling rate at a steel temperature of 900-800°C is suppressed to 0.05°C / second or less. This reduces temperature unevenness in the axial direction (longitudinal cross-section) of the steel. Consequently, variations in the timing of ferrite formation (phase transformation) along the longitudinal cross-section of the steel are suppressed. As a result, provided that other manufacturing conditions are met, the average ferrite grain size ratio at longitudinal cross-section observation positions L1-L9 can be reduced to 2.00 or less.

[0421] [(Step 24) Cooling Step]

[0422] In the cooling process, the temperature of the steel material is cooled after the temperature holding process. The conditions in the cooling process are as follows.

[0423] Average cooling rate at steel temperature of 800-300°C: 0.10-1.00°C / second

[0424] 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, the arithmetic mean of the area fraction of ferrite at positions C1 to C9 observed in the cross section and positions L1 to L9 observed in the longitudinal section is 50 to 70%, assuming that other manufacturing conditions are satisfied.

[0425] The steel material of this embodiment having the above-mentioned structure can be manufactured through the above-mentioned manufacturing process. It should be noted that, as mentioned above, the steel material of this embodiment is a rolled material.

[0426] The steel material of this embodiment is suitable for use as a blank for mechanical structural parts manufactured by gas carburizing. However, the steel material of this embodiment can also be subjected to other surface hardening heat treatments other than gas carburizing to form mechanical structural parts. Other surface hardening heat treatments include, for example, quenching and tempering, high-frequency quenching and tempering, and nitriding (nitriding, quenching, and tempering).

[0427] [Regarding mechanical structural components]

[0428] Mechanical structural parts are used in, for example, automobiles and construction vehicles, etc. Mechanical structural parts are, for example, gears and shafts used in steering mechanisms.

[0429] The machine structural component using the steel material of this embodiment as a blank is manufactured by a known manufacturing method. For example, the machine structural component is manufactured by the following method.

[0430] An example of a method for manufacturing a machine structural component includes the following steps.

[0431] Thermal processing

[0432] Cutting process

[0433] Heat treatment process

[0434] Hereinafter, each step will be described.

[0435] [Hot working process]

[0436] In the hot working process, the steel material of this embodiment is subjected to hot working. For example, the hot working process is known as hot forging. The heating temperature in the hot working process is, for example, 1000 to 1300°C. After the hot working, the steel material is naturally cooled (air cooled). The naturally cooled steel material may also be annealed as needed.

[0437] [Cutting process]

[0438] After the hot working process, the steel material is subjected to a cutting process to produce an intermediate product of a predetermined shape. This cutting process requires high machinability of the steel material. This cutting process employs known cutting techniques. Cutting enables the production of precise mechanical structural components that are difficult to produce using hot working alone.

[0439] [Heat treatment process]

[0440] The intermediate product after cutting is subjected to heat treatment. Here, "heat treatment" includes the well-known gas carburizing treatment and the well-known tempering process. It should be noted that, as mentioned above, gas carburizing treatment also includes gas carbonitriding treatment.

[0441] Gas carburizing includes a gas carburizing step and a quenching step. During the gas carburizing, known conditions can be appropriately adjusted to adjust the surface hardness, core hardness, and surface carbon concentration of the mechanical structural component, which is a technical matter known to those skilled in the art.

[0442] Hereinafter, a well-known gas carburizing treatment will be described as an example of a heat treatment process. It should be noted that, as is well known to those skilled in the art, a well-known gas carbonitriding treatment is also carried out in the same process as the gas carburizing treatment.

[0443] [Gas carburizing treatment]

[0444] Gas carburizing treatment includes a gas carburizing step and a quenching step. The gas carburizing step and the quenching step are described below.

[0445] [Gas carburizing process]

[0446] Figure 5 It is a diagram showing an example of the heating pattern in the gas carburizing step S10 and the quenching step S20. Figure 5 The vertical axis is the treatment temperature (°C) during gas carburizing treatment, and the horizontal axis is the time (minutes). Figure 5 The gas carburizing step S10 includes a heating step S0, a carburizing step S1 and a diffusion step S2.

[0447] In the heating step S0, the intermediate product placed in the furnace is heated to a carburizing temperature Tc. The carburizing temperature Tc in the heating step S0 is, for example, 830 to 1100°C.

[0448] In the carburizing step S1, the intermediate product is held in an atmosphere of a predetermined carbon potential Cp1 and at a carburizing temperature Tc for a predetermined time (holding time t1). The carbon potential Cp1 in the carburizing step S1 is, for example, 0.5 to 1.2%, and the holding time t1 at the carburizing temperature Tc is, for example, 60 minutes or more.

[0449] In the diffusion step S2, the atmosphere is held at the carburizing temperature Tc for a predetermined time (holding time t2) in an atmosphere having a predetermined carbon potential Cp2. The carbon potential Cp2 in the diffusion step S2 is, for example, 0.5 to 1.2%, and the holding time t2 at the carburizing temperature Tc is, for example, 30 minutes or longer. The carbon potential Cp2 in the diffusion step S2 is preferably lower than the carbon potential Cp1 in the carburizing step S1.

[0450] [Quenching process]

[0451] The intermediate product after the gas carburizing step S10 is subjected to a quenching step S20. In the quenching step S20, A r3 After the intermediate product after the gas carburizing step S10 is maintained at a quenching temperature Ts above 30°C, the intermediate product is quenched and quenched. The holding time t3 at the quenching temperature Ts is not particularly limited, and is, for example, 30 to 60 minutes. The quenching temperature Ts is preferably lower than the carburizing temperature Tc. 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, and is quenched. The temperature of the oil or water as the cooling medium is, for example, room temperature to 200°C. In addition, cryogenic treatment can also be performed as needed.

[0452] [Tempering process]

[0453] 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.

[0454] Through the above steps, the steel material of this embodiment is used as a blank to manufacture a machine structural component.

[0455] [Other processes]

[0456] Alternatively, the intermediate product after the tempering step may be further subjected to grinding or shot peening as required. When grinding is performed, cutting is performed to give the steel a shape. By performing cutting, the steel can be given a precise shape. In addition, when shot peening is performed, compressive residual stress can be introduced into the surface portion of the intermediate product after gas carburizing. Compressive residual stress can inhibit the generation and development of fatigue cracks. Therefore, the bending fatigue strength and surface fatigue strength of the mechanical structure components are improved. Shot peening can be performed by a known method. For example, it is desirable to use shot peening particles with a diameter of less than 0.7 mm, and to perform the shot peening under conditions where the arc height is more than 0.4 mm.

[0457] Example 1

[0458] Steel materials having the chemical compositions shown in Table 1 were prepared. Note that steel grade number A corresponds to SCr420H specified in JIS G 4052 (2016). Furthermore, a steel material having a chemical composition corresponding to SCM420H specified in JIS G 4052 (2016) was used as a reference steel material.

[0459] [Table 1]

[0460] Table 1

[0461]

[0462] The "-" in Table 1 means that the corresponding element content is 0% of the significant figures (numbers rounded to the smallest digit) specified in the embodiment. In other words, it means that the corresponding element content is 0% when the decimal point of the significant figures (numbers rounded to the smallest digit) specified in the embodiment is rounded off.

[0463] For example, the Mo content specified in this embodiment is specified by the numerical value up to the second decimal place. Therefore, in the steel grade number A in Table 1, the measured Mo content is expressed as o The content is 0% when rounded to the third decimal place.

[0464] In addition, the Nb content specified in this embodiment is specified by a numerical value to the third decimal place. Therefore, the steel grade number A in Table 1 indicates that the measured Nb content is 0% when rounded to the fourth decimal place.

[0465] It should be noted that rounding means that if the last digit (mantissa) of the specified minimum digit is less than 5, it is discarded; if it is 5 or more, it is rounded up.

[0466] The steel material was produced by the following method: Molten steel was continuously cast to produce a billet (bloom). The bloom was subjected to a hot working process under the conditions shown in Table 2.

[0467] [Table 2]

[0468] Table 2

[0469]

[0470] The temperature recorded in the "Heating Temperature (°C)" column of the "Initial Rolling Process" and "Finishing Rolling Process" columns is the heating temperature (°C). The time recorded 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 temperature (surface temperature of the steel) (°C) at the exit side of the rolling mill in the final rolling process of the continuous rolling mill in the finishing rolling process. The speed recorded in the "Cooling Rate (°C / Sec)" column of the "Temperature Holding Process" column is the average cooling rate (°C / Sec) when the steel temperature is 900-800°C. The speed recorded in the "Cooling Rate (°C / Sec)" column of the "Cooling Process" column is the average cooling rate (°C / Sec) when the steel temperature is 800-300°C.

[0471] 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 using a 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 longitudinal 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 in the initial rolling process is 30% or more under any manufacturing conditions.

[0472] After the initial rolling process, the billet was subjected to a finish 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 holding time (hours) shown in the Finishing Rolling Process column of Table 2. The heated billet was then finish rolled to produce a steel bar. The final temperature (°C) at this time is shown in Table 2.

[0473] The steel (steel bar) after the finish rolling process is subjected to a temperature holding process. In manufacturing conditions a to i, the steel material at a temperature of 900-800°C is adjusted to an average cooling rate of 0.05°C / second or less by using a slow cooling hood. On the other hand, in manufacturing condition j, the steel material at a temperature of 900-800°C is naturally cooled without using a slow cooling hood. Therefore, the average cooling rate at a steel temperature of 900-800°C is greater than 0.05°C / second.

[0474] After the temperature holding step, a cooling step is performed. Specifically, the average cooling rate (°C / second) at a steel material temperature of 800 to 300°C under various manufacturing conditions is shown in the cooling step column of Table 2.

[0475] Steel materials with a temperature of 300°C or less were naturally cooled (air cooled) to room temperature. The above manufacturing process produced the steel materials (steel bars) of Test Nos. 1 to 31 shown in Table 3. Test No. 31 is an example using SCM420H as the reference steel, and used Manufacturing Condition a, one of the commonly used manufacturing methods for steel materials having the chemical composition of SCM420H.

[0476] [Table 3]

[0477] Table 3

[0478]

[0479] [Evaluation test]

[0480] The following items were obtained for the steel materials (steel bars) of each test number produced through the above-described production process.

[0481] (A1) Microstructure observation of cross section

[0482] (A11) Arithmetic mean of the area fraction of ferrite in the cross section

[0483] (A12) Standard deviation of the area fraction of ferrite in the cross section

[0484] (A13) Ferrite average grain size ratio in cross section

[0485] (B1) Observation of the microstructure of the longitudinal section

[0486] (B11) Arithmetic mean of the area fraction of ferrite in the longitudinal section

[0487] (B12) Standard deviation of the area fraction of ferrite in the longitudinal section

[0488] (B13) Ferrite average grain size ratio in longitudinal section

[0489] Furthermore, the following evaluation tests were performed on the steel materials of each test number.

[0490] (C1) Machinability evaluation test

[0491] (C2) Bending fatigue strength evaluation test

[0492] (C3) Surface fatigue strength evaluation test

[0493] (C4) Heat treatment deformation evaluation test

[0494] The following is a detailed description.

[0495] [(A1) Microstructure observation of cross section]

[0496] Nine samples were collected from each steel material with each test number, including cross-sectional observation positions C1 to C9. The surface corresponding to cross-sectional view CS of each sample was used as the observation surface. The observation field within the observation surface, including the cross-sectional observation position, was set to 0.5 mm x 1.0 mm.

[0497] After polishing the observation surface of the sample, etch the observation surface with 3% nital (Nital etching solution). Observe the observation field (0.5 mm x 1.0 mm) of the etched observation surface using an optical microscope at 100x magnification. Phases are determined by contrast within the observation field.

[0498] The observed phases are shown in the "Phase" column of the "Cross-Section" column in Table 3. In the "Phase" column of the "Cross-Section" column in Table 3, "○" indicates that the microstructure at all cross-sectional observation positions contained ferrite, with the balance being pearlite and / or bainite. In all test numbers, the cross-sectional microstructure contained ferrite, with the balance being pearlite and / or bainite.

[0499] [(A11) Arithmetic mean of the area fraction of ferrite in the cross section]

[0500] 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 determined.

[0501] The arithmetic mean of the ferrite area fraction (%) in the nine observation fields (cross-sectional observation positions) is defined as the arithmetic mean (%) of the ferrite area fraction at the nine cross-sectional observation positions C1 to C9. The resulting arithmetic mean of the ferrite area fraction is shown in the "Ferrite Area Fraction (%)" column of the "Cross-sectional" column in Table 3.

[0502] [(A12) Standard deviation of the area fraction of ferrite in the cross section]

[0503] The standard deviation (%) of the ferrite area fraction at nine cross-sectional observation positions C1 to C9 was calculated from the ferrite area fraction (%) in the nine observation fields (cross-sectional observation positions). The obtained standard deviation is shown in the "Standard Deviation (%) of Ferrite Area Fraction" column in the "Cross-sectional" column of Table 3.

[0504] [(A13) Ferrite average grain size ratio in cross section]

[0505] Furthermore, 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 at each cross-sectional observation position C1 to C9 is calculated. 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.

[0506] The average ferrite grain size at nine cross-sectional observation locations C1 to C9 was determined. Among these average ferrite grain sizes, the maximum average grain size (μm) and the minimum average grain size (μm) were determined. The ratio of the determined maximum average grain size to the minimum average grain size (ferrite average grain size ratio) was calculated. The obtained ferrite average grain size ratio is shown in the "Ferrite Grain Size Ratio" column under the "Cross-Section" column in Table 3.

[0507] [(B1) Observation of microstructure of longitudinal section]

[0508] Nine samples were collected from each steel material with each test number, including longitudinal cross-sectional observation positions L1 to L9. The surface of each sample corresponding to longitudinal cross-sectional view LS was used as the observation surface. The observation field of the observation surface, including the longitudinal cross-sectional observation position, was set to 0.5 mm x 1.0 mm.

[0509] After polishing the observation surface of the sample, etch the observation surface with 3% nital (Nital etching solution). Observe the observation field (0.5 mm x 1.0 mm) of the etched observation surface using an optical microscope at 100x magnification. Phases are determined by contrast within the observation field.

[0510] The observed phases are shown in the "Phase" column of the "Longitudinal Section" column in Table 3. In the "Phase" column of the "Longitudinal Section" column in Table 3, "○" indicates that the microstructure at all longitudinal section observation positions contained ferrite, with the balance being pearlite and / or bainite. In all test numbers, the microstructure of the longitudinal section contained ferrite, with the balance being pearlite and / or bainite.

[0511] [(B11) Arithmetic mean of the area fraction of ferrite in the longitudinal section]

[0512] Calculate the area (μm) of ferrite in each observation field (each longitudinal section 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 longitudinal cross-sectional observation position) was determined.

[0513] The arithmetic mean of the ferrite area fraction (%) in the nine observation fields (vertical cross-section observation positions) was defined as the arithmetic mean (%) of the ferrite area fraction at the nine longitudinal cross-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 Cross-Section" column in Table 3.

[0514] [(B12) Standard Deviation of the Area Fraction of Ferrite in the Longitudinal Section]

[0515] The standard deviation (%) of the ferrite area fraction at nine longitudinal cross-sectional observation positions L1 to L9 was calculated from the ferrite area fraction (%) in the nine observation fields (longitudinal cross-sectional observation positions). The obtained standard deviation is shown in the "Standard Deviation (%) of Ferrite Area Fraction" column in the "Longitudinal Cross-Section" column of Table 3.

[0516] [(B13) Ferrite average grain size ratio in longitudinal section]

[0517] Furthermore, 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 at each longitudinal cross-sectional observation position L1 to L9 is calculated. 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 cross-sectional observation position L1 to L9.

[0518] The average ferrite grain size at nine longitudinal cross-section observation positions L1 to L9 was determined. Among these ferrite average grain sizes, the maximum average grain size (μm) and the minimum average grain size (μm) were determined. The ratio of the determined maximum average grain size to the minimum average grain size (ferrite average grain size ratio) was determined. The obtained ferrite average grain size ratio is shown in the "Ferrite Grain Size Ratio" column under the "Longitudinal Cross-Section" column in Table 3.

[0519] [Evaluation test]

[0520] [(C1) Machinability evaluation test]

[0521] The machinability evaluation test is carried out using the following method. 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 maintained at 1200°C for 30 minutes. Then, the steel bar is naturally cooled to room temperature. Further, it is heated at 950°C and maintained at 950°C for 1 hour. Further, it is maintained at 650°C for 2 hours and then 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.

[0522] The test pieces for each test number were subjected to peripheral turning to evaluate tool life. Specifically, the test pieces for each test number were subjected to peripheral turning 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 depth of cut was 1.0 mm. A water-soluble cutting oil was used during turning.

[0523] The outer periphery turning process was performed for 20 minutes under the above-mentioned cutting conditions, and then the flank wear amount (mm) of the cutting tool was measured.

[0524] The obtained flank wear (mm) is shown in the "Wear (mm)" column of Table 3. When the flank wear (mm) is less than 0.25 mm, the steel material is judged to have high machinability. When the obtained flank wear (mm) is 0.25 mm or more, the steel material is judged to have low machinability.

[0525] [(C2) Bending fatigue strength evaluation test]

[0526] The steel bars (50 mm diameter steel bars) 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"φ" in the figure indicates the diameter. "R1" means the curvature radius of the cut bottom is 1 mm.

[0527] Specifically, the steel material of each test number (steel bar 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 bar with a diameter of 35 mm. The steel bar 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 cut is 8 mm. The intermediate product is carburized (gas carburizing and tempering) to produce Figure 6 The carburizing conditions for the Ono type rotary bending test piece are as follows.

[0528] [Gas carburizing and tempering]

[0529] The test piece was heated at 950°C for 240 minutes in an atmosphere with a carbon potential Cp1 of 0.8%. Next, it was heated at 950°C for 60 minutes in an atmosphere with a carbon potential Cp2 of 0.8%. It was then heated at 850°C for 30 minutes and oil-cooled with 130°C oil. The oil-cooled test piece was tempered at 180°C for 120 minutes. After the hold time, it was air-cooled.

[0530] 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 %.

[0531] The Ono type rotating bending fatigue test was performed using carburized Ono type rotating bending test pieces. For each test number, multiple test pieces were prepared. The stress applied to each test piece was changed and the fatigue test was performed. The fatigue test was repeated 10 million times (10 7 The maximum stress without fracture was taken as the bending fatigue strength (MPa). In the Ono type rotating bending fatigue test, the rotation speed was 3000 rpm and the stress ratio was cyclic alternating.

[0532] 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 using the following formula.

[0533] Bending fatigue strength ratio (%) = (bending fatigue strength of each test number (MPa) / bending fatigue strength of reference steel (MPa)) × 100

[0534] The obtained bending fatigue strength ratio (%) is shown in the "Bending fatigue strength ratio (%)" column of Table 3. If the obtained bending fatigue strength ratio is 110% or higher, it is judged that sufficient bending fatigue strength is obtained. On the other hand, if the bending fatigue strength ratio is less than 110%, it is judged that the bending fatigue strength is low.

[0535] [(C3) Surface fatigue strength evaluation test]

[0536] The steel bars (50 mm diameter steel bars) of each test number were processed into Figure 7 The following is an intermediate product of a roller pitting fatigue test specimen used in a surface fatigue strength evaluation test. Figure 7 The numerical values ​​in the figure represent the dimensions (unit: mm). The "φ" in the figure represents the diameter.

[0537] Specifically, the steel material (50 mm diameter steel bar) 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 35 mm diameter steel bar. The 35 mm diameter steel bar was machined (cutting) to produce an intermediate product for the roller pitting fatigue test specimen. The intermediate product was carburized (gas carburizing and tempering) under the above-mentioned carburizing conditions to produce Figure 7 The following is a test piece for a roller pitting fatigue test (small roller test piece).

[0538] Figure 8 Schematic diagram of roller pitting fatigue test. Figure 8 As shown, the large roller test piece 100 is pressed against the small roller test piece 200 by the surface pressure described later, while 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 Figure 9 The shape shown. Figure 9 The values ​​in the figure represent dimensions (in mm). "R700" in the figure indicates that the radius of curvature of the outer surface is 700 mm.

[0539] Large roller test piece 100 was made of steel having a chemical composition equivalent to SCM420H specified in JIS G 4053 (2016), and was surface-polished after gas carburizing under the same conditions as small roller test piece 200. Large roller test piece 100 had a diameter of 130 mm.

[0540] In the roller pitting fatigue test, the large roller test piece 100 is pressed against the small roller test piece 200 with surface pressures of various Hertzian stresses. The circumferential speed directions of the two roller test pieces at the contact portion are set to the same direction, and the slip ratio is set to -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) to rotate for the test. The oil temperature of the ATF (lubricating oil for AT) supplied as lubricating oil to the above-mentioned contact portion is 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 4000 MPa. The test cutoff number is set to 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).

[0541] 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 using the following formula.

[0542] Surface fatigue strength ratio (%) = (surface fatigue strength of each test number (MPa) / surface fatigue strength of reference steel (MPa)) × 100

[0543] The obtained surface fatigue strength ratio (%) is shown in the "Surface fatigue strength ratio (%)" column of Table 3. If the obtained surface fatigue strength ratio is 120% or more, it is judged that sufficient surface fatigue strength can be obtained. On the other hand, if the surface fatigue strength ratio is less than 120%, it is judged that the surface fatigue strength is low.

[0544] [(C4) Heat treatment deformation evaluation test]

[0545] The steel bars (50 mm diameter steel bars) of each test number were used to make Figure 10A The gear simulation test pieces shown are shown. Specifically, steel materials (50 mm diameter steel bars) of each test number were heated at 1200°C for 30 minutes. They were then hot-worked (hot forged) at a final temperature of 950°C or higher to produce 35 mm diameter steel bars. The 35 mm diameter steel bars were then machined (cut) to produce gear simulation test pieces before gas carburizing.

[0546] Figure 10AThe numerical values ​​with "mm" indicate dimensions (unit: mm). "φ" in the figure indicates diameter. The gear simulation test piece has a truncated cone 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 containing a central axis CL2. The diameter (inner diameter) of the through hole TH is 15 mm, and the central axis of the through hole TH coincides with the central axis of the gear simulation test piece.

[0547] The inner diameter (diameter) of the through hole TH of the prepared gear simulation test piece before carburization treatment at various positions in the longitudinal direction was measured using a three-dimensional measuring machine. A CNC three-dimensional measuring machine (trade name: Crysta-Apex) manufactured by Mitutoyo Corporation was used as the three-dimensional measuring machine.

[0548] Specifically, if Figure 10B As shown, from the upper end toward the lower end of the through hole TH in the longitudinal direction, a total of 16 inner diameters are measured at positions with a pitch of 1.0 mm in the range of 1.0 to 16.0 mm from the upper end. Furthermore, from the upper end toward the lower end of the through hole TH in the longitudinal direction, the inner diameters at 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 longitudinal direction of the through hole TH. Furthermore, at each measurement position, a total of 18 positions ( Figure 10B Therefore, in the through hole TH, the inner diameters of 18 measurement positions×18 locations=324 points are measured.

[0549] The gear simulation test piece, after measuring the inner diameter, was subjected to gas carburizing and tempering under the same conditions as those used for the gas carburizing and tempering of the Ono-type rotating bending fatigue test piece. The inner diameter of the through-hole TH of the gear simulation test piece after carburizing was measured using the same method as that used to measure the inner diameter of the through-hole TH of the gear simulation test piece before carburizing.

[0550] [Maximum deformation ratio during heat treatment]

[0551] At each of the measurement positions P1 to P18 of the through-hole TH, the value obtained by subtracting the inner diameter (μm) after carburizing from the inner diameter (μm) before carburizing was used as the heat treatment deformation at each of the measurement positions P1 to P18. For each test number, the maximum heat treatment deformation was determined from the measurement results of a total of 324 points.

[0552] The ratio of the maximum heat treatment deformation of each test number to the maximum heat treatment deformation of the reference steel was defined as the "maximum deformation ratio." That is, the maximum deformation ratio (%) was calculated using the following formula.

[0553] Maximum deformation ratio (%) = (maximum heat treatment deformation of each test number (mm) / maximum heat treatment deformation of reference steel (mm)) × 100

[0554] The obtained maximum deformation ratio (%) is shown in the "Maximum deformation ratio (%)" column of Table 3. If the obtained maximum deformation ratio is 90% or less, the maximum deformation ratio is judged to be small. On the other hand, if the maximum deformation ratio is greater than 90%, the maximum deformation ratio is judged to be large.

[0555] [Heat treatment deformation difference ratio]

[0556] In each test number, the maximum heat treatment deformation and the minimum heat treatment deformation were determined from the measurement results of a total of 324 points. The value of subtracting the minimum heat treatment deformation from the obtained maximum heat treatment deformation was defined as the deformation difference (μm).

[0557] The ratio of the deformation difference of each test number to the deformation difference of the reference steel is defined as the deformation difference ratio. That is, the deformation difference ratio is calculated using the following formula.

[0558] Deformation difference ratio (%) = (deformation difference of each test number (μm) / deformation difference of reference steel (μm)) × 100

[0559] The obtained deformation difference ratio (%) is shown in the "Deformation difference ratio (%)" column of Table 3. If the obtained deformation difference ratio is 90% or less, the deformation difference ratio is judged to be small. On the other hand, if the deformation difference ratio is greater than 90%, the deformation difference ratio is judged to be large.

[0560] If the maximum deformation ratio and the deformation difference ratio are both 90% or less, it is judged that heat treatment deformation can be sufficiently suppressed three-dimensionally. If the maximum deformation ratio and / or the deformation difference ratio are greater than 90%, it is judged that heat treatment deformation cannot be sufficiently suppressed.

[0561] [Test results]

[0562] The test results are shown in Table 3. Referring to Table 3, the chemical composition of the steel materials of test numbers 5 to 9 is suitable in the content of each element, and F1 satisfies formula (1). Furthermore, the manufacturing conditions of the steel materials of test numbers 5 to 9 are also suitable. Therefore, the microstructure of the steel materials is suitable. Specifically, the microstructure of the cross section of test numbers 5 to 9 is a structure containing ferrite, with the remainder being pearlite and / or bainite. Furthermore, 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. Furthermore, the microstructure of the longitudinal section of test numbers 5 to 7 is a structure containing ferrite, with the remainder being pearlite and / or bainite. Furthermore, 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.

[0563] As a result, in machinability evaluation tests, flank wear was less than 0.25 mm, demonstrating high machinability. Furthermore, the bending fatigue strength ratio was 110% or greater, and the surface fatigue strength ratio was 120% or greater, demonstrating excellent bending and surface fatigue strengths. Furthermore, the maximum deformation ratio and deformation difference ratio during heat treatment were 90% or less, demonstrating that heat treatment deformation was fully suppressed three-dimensionally.

[0564] On the other hand, in test number 1, the Si content is too low. Furthermore, F1 does not satisfy formula (1). Furthermore, the holding time of the initial rolling process is less than 10 hours. Therefore, the standard deviation of the area fraction of ferrite in the cross section and longitudinal section is greater than 4.0%. As a result, the bending fatigue strength and surface fatigue strength are low. Furthermore, the maximum deformation ratio and deformation difference ratio during heat treatment are greater than 90%, and heat treatment deformation cannot be fully suppressed.

[0565] In Tests 2 to 4, the holding time in the initial rolling process was less than 10 hours. Consequently, the standard deviation of the ferrite area fraction in the cross-section and longitudinal sections exceeded 4.0%. Consequently, the difference in deformation during heat treatment exceeded 90%, failing to adequately suppress heat treatment deformation.

[0566] In Tests 10 to 12, the heating temperature in the initial rolling process was too low. Consequently, the standard deviation of the ferrite area fraction in the cross-section and longitudinal sections exceeded 4.0%. Consequently, the difference in deformation during heat treatment exceeded 90%, failing to adequately suppress heat treatment deformation.

[0567] In Tests 13 to 15, the heating temperature in the finish rolling step was too low. Consequently, the average ferrite grain size ratio in the cross section to the longitudinal section exceeded 2.00. As a result, the difference in deformation during heat treatment exceeded 90%, and heat treatment deformation could not be sufficiently suppressed.

[0568] In Tests 16 to 18, the hold time in the finish rolling process was too short. Consequently, the average ferrite grain size ratio between the cross-section and the longitudinal section exceeded 2.00. As a result, the difference in deformation during heat treatment exceeded 90%, failing to adequately suppress heat treatment deformation.

[0569] In Tests 19 to 21, the final temperature of the finish rolling process was too high. Consequently, the average ferrite grain size ratio between the cross-section and the longitudinal section exceeded 2.00. As a result, the difference in deformation during heat treatment exceeded 90%, and heat treatment deformation could not be fully suppressed.

[0570] In Tests 22 to 24, the final temperature of the finish rolling process was too low. Consequently, the average ferrite grain size ratio between the cross-section and the longitudinal section exceeded 2.00. As a result, the difference in deformation during heat treatment exceeded 90%, and heat treatment deformation could not be sufficiently suppressed.

[0571] In Tests 25 to 27, the cooling rate during the cooling step was too slow. Consequently, the arithmetic mean of the area fraction of ferrite in the cross-section and longitudinal sections exceeded 70%. Consequently, the difference in deformation during heat treatment exceeded 90%, and heat treatment deformation could not be sufficiently suppressed.

[0572] In Tests 28 and 29, the cooling rate during the cooling process was too high. Consequently, the arithmetic mean of the ferrite area fraction in the cross-section and longitudinal sections was less than 50%. Consequently, the flank wear was greater than 0.25 mm. Consequently, the steel's machinability was low.

[0573] In Test No. 30, the cooling rate during the temperature holding step was too fast. As a result, the ferrite average grain size ratio in the longitudinal cross-section microstructure exceeded 2.00. Consequently, the deformation difference ratio during heat treatment exceeded 90%, and heat treatment deformation could not be fully suppressed.

[0574] Example 2

[0575] In the same manner as in Example 1, steel materials having the chemical compositions shown in Table 4 were prepared.

[0576] [Table 4]

[0577] Table 4

[0578]

[0579] Steel materials were produced as follows: Using the production conditions b in Table 2 for molten steel, steel materials (steel bars) of test numbers 1 to 32 shown in Table 5 were produced in the same manner as in Example 1.

[0580] [Table 5]

[0581] Table 5

[0582]

[0583] [Evaluation test]

[0584] The steel materials (steel bars) produced in the above-described manufacturing process were subjected to the same measurement and evaluation tests as in Example 1 using the same methods as in Example 1. Note that the reference steels listed in Table 1 were used as reference steels in the (C2) bending fatigue strength evaluation test, the (C3) surface fatigue strength evaluation test, and the (C4) heat treatment deformation evaluation test.

[0585] [Test results]

[0586] The test results are shown in Table 5. Referring to Table 5, the chemical composition of the steel materials of test numbers 1 to 17 has appropriate content of each element, and F1 satisfies formula (1). Furthermore, the manufacturing conditions of the steel materials of test numbers 1 to 17 are also appropriate. Therefore, the microstructure of the steel materials is appropriate. Specifically, the microstructure of the cross section of test numbers 1 to 17 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 17 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 machinability is high. Furthermore, the bending fatigue strength ratio is 110% or more, and the surface fatigue strength ratio is 120% or more, indicating excellent bending fatigue strength and surface fatigue strength. Furthermore, the maximum deformation ratio and deformation difference ratio during heat treatment are 90% or less, indicating that heat treatment deformation is fully suppressed.

[0587] On the other hand, in test numbers 18 to 23, F1 was too high. Therefore, the maximum deformation ratio during heat treatment exceeded 90%, and heat treatment deformation could not be sufficiently suppressed.

[0588] In test number 24, the C content was too high. Therefore, the flank wear amount was 0.25 mm or more, and the machinability was low.

[0589] The Si content in test number 25 was too low. Therefore, the surface fatigue strength ratio was less than 120%, and the surface fatigue strength was low.

[0590] Test No. 26 has an excessively high Si content, and therefore has a bending fatigue strength ratio of less than 110%, resulting in low bending fatigue strength.

[0591] Test No. 27 has an excessively low Mn content. Therefore, the bending fatigue strength ratio is less than 110%, and the surface fatigue strength ratio is less than 120%. As a result, the bending fatigue strength and surface fatigue strength are low.

[0592] Test No. 28 has an excessively high Mn content. Therefore, the bending fatigue strength ratio is less than 110%, and the surface fatigue strength ratio is less than 120%. As a result, the bending fatigue strength and surface fatigue strength are low.

[0593] The Cr content in test number 29 was too low. Therefore, the bending fatigue strength ratio was less than 110%, and the surface fatigue strength ratio was less than 120%. As a result, both the bending fatigue strength and the surface fatigue strength were insufficient.

[0594] The Cr content in test number 30 was too high. Therefore, the flank wear amount was 0.25 mm or more, and the machinability was low.

[0595] The Mo content in test number 31 was too high. Therefore, the flank wear amount was 0.25 mm or more, and the machinability was low.

[0596] The Nb content in test number 32 was too high. Therefore, the bending fatigue strength ratio was less than 110%, and the bending fatigue strength was low.

[0597] The above describes the embodiments of the present application. However, the above embodiments are merely examples for implementing the present application. Therefore, the present application is not limited to the above embodiments, and the above embodiments may be appropriately modified and implemented without departing from the scope of the present invention.

Claims

1. A steel material, Its chemical composition contains in mass % C：0.20～0.25％、 Si: 0.40-0.70%, Mn: 0.50~0.90%, Cr：1.00～2.00％、 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 remainder is Fe and impurities, and satisfies formula (1), On a circular cross section of a radius R that is a cross section perpendicular to the longitudinal direction of the steel material, When the center position of the cross section and eight positions of the R / 2 position arranged at 45° intervals around the center of the cross section at a distance R / 2 from the center of the cross section in the radial direction are defined as nine cross section observation positions, The microstructure at each cross-sectional observation position contains ferrite, with the remainder being pearlite and / or bainite. 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. 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. On a longitudinal cross section including the central axis of the steel material, which is a cross section parallel to the longitudinal direction of the steel material, 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, The microstructure at the observation position of each longitudinal section contains ferrite, with the remainder being pearlite and / or bainite. 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. 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. 1-(0.5C+0.03Si+0.06Mn+0.01Cr+0.05Mo)<0.800 (1) in, In formula (1), the content of each element in mass % is substituted for each element symbol. If the corresponding element is not contained, "0" is substituted for the element symbol.

2. A steel material, Its chemical composition contains in mass % C：0.20～0.25％、 Si: 0.40-0.70%, Mn: 0.50~0.90%, Cr：1.60～2.00％、 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 remainder is Fe and impurities, and satisfies formula (1), On a circular cross section of a radius R that is a cross section perpendicular to the longitudinal direction of the steel material, When the center position of the cross section and eight positions of the R / 2 position arranged at 45° intervals around the center of the cross section at a distance R / 2 from the center of the cross section in the radial direction are defined as nine cross section observation positions, The microstructure at each cross-sectional observation position contains ferrite, with the remainder being pearlite and / or bainite. 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. 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. On a longitudinal cross section including the central axis of the steel material, which is a cross section parallel to the longitudinal direction of the steel material, 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, The microstructure at the observation position of each longitudinal section contains ferrite, with the remainder being pearlite and / or bainite. 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. 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. 1-(0.5C+0.03Si+0.06Mn+0.01Cr+0.05Mo)<0.800 (1) in, In formula (1), the content of each element in mass % is substituted for each element symbol. If the corresponding element is not contained, "0" is substituted for the element symbol.

3. The steel material according to claim 1 or 2, wherein: The chemical composition further contains one or more elements selected from the group consisting of the following elements in place of a portion of the Fe: Mo: 0.30% or less, Nb: 0.050% 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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