Surface-hardened steel and production method for same

Abstract

The present invention addresses the problem of providing a surface-hardened steel that has excellent cold-forgeability and can sufficiently suppress variations in deformation resistance during cold forging. Provided is a surface-hardened steel that has a prescribed chemical composition and has a structure that comprises ferrite and spheroidized carbides. A dispersion index Vθ that represents the dispersibility of the spheroidized carbides satisfies Vθ≤0.80, and, for test pieces taken at 10 arbitrary locations in the rolling direction of a steel material that comprises the surface-hardened steel, the coefficient of variation CV of the Rockwell hardness at a cross-section that is orthogonal to the rolling direction satisfies CV×100≤0.65.

Description

Case hardening steel and its manufacturing method

[0001] The present invention relates to a case-hardening steel having excellent cold forgeability and a method for producing the same.

[0002] Cold forging is sometimes selected as a manufacturing process for drivetrain components such as gears and shafts to reduce manufacturing costs. Case-hardened steel for cold forging is required to have material properties (cold forgeability) such as low deformation resistance and crack resistance from the viewpoints of reducing the load on tools and ensuring the shape of the tools.

[0003] In order to reduce deformation resistance, spheroidizing annealing is generally performed. However, even if the deformation resistance can be sufficiently reduced, if the carbide distribution in the structure after spheroidizing annealing is non-uniform, it may not be possible to sufficiently suppress cold forging cracks.

[0004] Furthermore, the properties required for parts manufactured by cold forging case-hardened steel include wear resistance and fatigue resistance. To ensure these properties, carburizing is sometimes performed appropriately after cold forging. Carburizing directly after cold forging can cause grain coarsening. This is because cold forging causes ferrite to recrystallize finely as the carburizing temperature rises, resulting in the formation of fine austenite grains. Since grain coarsening can lead to reduced part strength and bending during quenching, steel is also required to be resistant to grain coarsening after carburizing.

[0005] Various proposals have been made to address the issues of improving cold forgeability and suppressing grain coarsening. For example, Patent Document 1 proposes that, aiming for excellent cold forgeability, the structure fraction (area fraction) of ferrite and pearlite in the structure before spheroidizing annealing be 85% or more and the average ferrite grain size be 25 μm or less. Furthermore, aiming for suppression of grain coarsening, it has been proposed to finely disperse AlN second phase particles.

[0006] Patent Document 2 proposes that the area ratio of lamellar pearlite in the spheroidized annealed structure be set to 3% or less by limiting the chemical components and the heat treatment method, thereby suppressing the coarsening of crystal grains.

[0007] However, the above-mentioned methods do not take into consideration the influence of carbide distribution in the spheroidized annealed structure, and therefore, it is not possible to suppress non-uniformity of carbide distribution. As a result, there is a problem that cold forging cracking, grain coarsening, etc. cannot be sufficiently suppressed. To address this problem, for example, Patent Document 3 proposes a steel for machine structural use in which the area ratio of ferrite grains in which carbides with a minor axis of 50 nm or more and an aspect ratio of 3 or less are precipitated intragranularly is 90% or more of the entire structure. It is said that this makes it possible to provide a steel for machine structural use that is less likely to crack during cold working and less likely to cause grain coarsening.

[0008] JP 2013-082988 A JP 2010-242209 A Japanese Patent No. 7149131 A

[0009] As described above, the steel for machine structural use of Patent Document 3 can suppress cracking during cold working and grain coarsening, but it cannot sufficiently suppress variations in deformation resistance during cold forging.

[0010] An object of the present invention is to provide a case-hardened steel having excellent cold forgeability and capable of sufficiently suppressing variations in deformation resistance during cold forging, and a method for manufacturing the same.

[0011] In order to solve the above problems, the present invention provides the following case-hardening steel and a manufacturing method thereof.

[0012] (1) In mass%, C: 0.14% to 0.35%, Si: 0.05% to 1.00%, Mn: 0.10% to 0.90%, Cr: 1.30% to 3.50%, Al: 0.020% to 0.200%, Nb: 0.02% to 0.10%, N: 0.0040% to 0.0300%, P: 0% to 0.030%, S: 0% to 0.030%, Ni: 0% to 2.00%, Mo: 0% to 2.00%, B: 0% to 0.0050%, V: 0% to 0.500%, Ti: 0% or more and 0.200% or less, and the balance: Fe and impurities A case-hardening steel having a chemical composition comprising: a structure comprising ferrite and spheroidized carbides; and a dispersion index V representing the dispersion of the spheroidized carbides.θ is V θ ≦0.80, and the coefficient of variation CV of Rockwell hardness in a cross section perpendicular to the rolling direction of test pieces taken from any 10 locations in the rolling direction of the steel material made of the case-hardened steel satisfies CV × 100≦0.65. The case-hardened steel.

[0013] (2) The chemical composition comprises Ni: 0.05% or more and 2.00% or less, Mo: 0.05% or more and 2.00% or less, B: 0.0010% or more and 0.0050% or less, V: 0.010% or more and 0.500% or less, and Ti: 0.020% or more and 0.200% or less. The case-hardened steel according to (1) above, including one or more selected from the group consisting of:

[0014] (3) The dispersion index V θ is an arithmetic mean value of dispersion indices obtained for five 50 μm square images of a cross section of a steel material made of the case-hardened steel parallel to the rolling direction, and the dispersion index obtained for each of the five images of the five images of the five images of the case-hardened steel, and the dispersion index obtained for each of the images is a value obtained by: determining the number n of spheroidized carbides included in the image and the actual coordinates of the center of gravity of each spheroidized carbide in an xy coordinate system with the upper left corner of the image as the origin; arranging the n spheroidized carbides in a lattice pattern at equal intervals within the image and obtaining ideal coordinates for each of the n spheroidized carbides; determining the distance between the ideal coordinate and the nearest actual coordinate for each ideal coordinate; and dividing the arithmetic mean value of the obtained distances by the length of one side of a unit cell, which is the distance between adjacent ideal coordinates.

[0015] (4) The ideal coordinates are expressed as (xj, yj) obtained by xj = j / (N + 1) × 50, yj = j / (N + 1) × 50, where j is an integer from 1 to N, and N is a value obtained by rounding up the decimal point of √n, and the ideal coordinates are set in order from the coordinate closest to the origin of the image. (3) Case hardening steel according to the above.

[0016] (5) The case-hardened steel according to (4), wherein the length (L) of one side of the unit cell is calculated by L = 50 / (N + 1) [μm].

[0017] (6) The case-hardened steel according to any one of (1) to (5), wherein the Rockwell hardness of a cross section perpendicular to the rolling direction of the steel material made of the case-hardened steel is 85 HRB or less.

[0018] (7) A method for producing case-hardened steel according to any one of (1) to (6) above, comprising a step of performing spheroidizing annealing on a rolled steel material having the chemical composition according to (1) or (2) above, wherein in the spheroidizing annealing, the steel material is heated to a holding temperature T1 [°C] that satisfies A1 transformation point≦T1≦(A1 transformation point+40°C), and held for a holding time t [h] that satisfies 1 hour≦t≦6 hours, and cooled at a cooling rate of 5°C / h or more and 40°C / h or less to a temperature T2 [°C] that satisfies (A1 transformation point-100°C)≦T2≦(A1 transformation point-40°C).

[0019] (8) The method according to (7) above, wherein the grain size number of ferrite in the structure of the steel before the spheroidizing annealing treatment is 8.0 or more.

[0020] (9) A method for producing case-hardened steel according to any one of (1) to (6) above, comprising a step of performing spheroidizing annealing on a rolled steel material having the chemical composition according to (1) or (2) above, wherein in the spheroidizing annealing treatment, the steel material is heated to a holding temperature T [°C] that satisfies (A1 transformation point - 30°C) ≦ T ≦ A1 transformation point, and is held for a holding time t [h] that satisfies t ≧ 120 / (T - A1 transformation point + 50).

[0021] (10) The method according to (9) above, wherein the grain size number of ferrite in the structure of the steel before the spheroidizing annealing treatment is 8.0 or more.

[0022] According to the present invention, a case-hardened steel having excellent cold forgeability and capable of sufficiently suppressing variations in deformation resistance during cold forging, and a method for manufacturing the same are provided.

[0023] dispersion index V θ A schematic diagram of the steel material (test piece) for which the dispersion index V is calculated is shown below. θ 1 is a diagram illustrating a method for calculating the dispersion index V θ 1 is a diagram illustrating a method for calculating the dispersion index V θ1 is a diagram illustrating a method for determining the heat treatment conditions for spheroidizing annealing in an example.

[0024] Hereinafter, an embodiment of the case-hardening steel of the present invention will be described.

[0025] (Configuration of Case-Hardening Steel) The case-hardening steel according to this embodiment is a steel that has been subjected to spheroidizing annealing treatment, and can be used as a material for parts that are manufactured by carrying out, for example, a cold forging process and a carburizing process.

[0026] The case-hardening steel according to this embodiment has a predetermined chemical composition described below and a structure consisting of ferrite and spheroidized carbides. In the case-hardening steel according to this embodiment, a dispersion index V θ is V θ The coefficient of variation CV of Rockwell hardness, which is an index showing the variation in Rockwell hardness in a cross section perpendicular to the rolling direction of test pieces taken from any 10 locations in the rolling direction (longitudinal direction) of a steel material made of case-hardened steel, satisfies CV × 100≦0.80.

[0027] The structure of the case-hardening steel according to this embodiment will be described in detail below. In this embodiment, the chemical composition of the steel is expressed in terms of the mass fraction (%) of each element in the steel. That is, the unit of the chemical composition of the steel is mass%.

[0028] (Chemical composition) The case-hardened steel according to this embodiment contains C, Si, Mn, Cr, Al, Nb, and N as essential components. The case-hardened steel according to this embodiment may contain one or more elements selected from P, S, Ni, Mo, B, V, and Ti as optional components. In the case-hardened steel according to this embodiment, the balance other than the essential components and optional components is Fe and impurities. "Impurities" refer to components that are mixed in from the raw materials of the steel (e.g., ore, scrap, etc.), the environment of the manufacturing process, etc., and are not components that are intentionally contained in the steel.

[0029] The essential components of the case-hardening steel according to this embodiment will be described below.

[0030] C: 0.14% or more and 0.35% or less C is an element necessary for ensuring the core strength of steel after carburization. If the mass fraction of C is less than 0.14%, the deep hardness after carburization will be insufficient, resulting in insufficient strength. On the other hand, if the mass fraction of C exceeds 0.35%, the material hardness will increase, resulting in a decrease in workability (e.g., cold forgeability, machinability, etc.) and a decrease in toughness. Therefore, the mass fraction of C is set to 0.14% or more and 0.35% or less. For the same reason, the mass fraction of C is preferably set to 0.14% or more and 0.27% or less.

[0031] Si: 0.05% or more and 1.00% or less Si is an element necessary for deoxidation. If the mass fraction of Si is less than 0.05%, the effect is not sufficiently obtained. On the other hand, if the mass fraction of Si exceeds 1.00%, the material hardness increases, workability decreases, and carburizing treatment may be hindered. Therefore, the mass fraction of Si is set to 0.05% or more and 1.00% or less. For the same reason, the mass fraction of Si is preferably set to 0.15% or more and 0.80% or less.

[0032] Mn: 0.10% or more and 0.90% or less Mn is an element necessary for ensuring hardenability. If the mass fraction of Mn is less than 0.10%, the hardenability effect is not sufficiently obtained. On the other hand, if the mass fraction of Mn exceeds 0.90%, the workability decreases. Therefore, the mass fraction of Mn is set to 0.10% or more and 0.90% or less. For the same reason, the mass fraction of Mn is preferably set to 0.15% or more and 0.70% or less.

[0033] Cr: 1.30% or more and 3.50% or less. Cr is an essential element for obtaining a structure in which spheroidized carbides are uniformly dispersed within the grains through spheroidizing annealing. If the Cr mass fraction is less than 1.30%, the uniform dispersion of spheroidized carbides is insufficient, resulting in reduced cold forgeability, increased variation in deformation resistance, and deterioration of grain size characteristics. Furthermore, hardenability is also insufficient. On the other hand, if the Cr mass fraction exceeds 3.50%, the material hardness increases, reducing workability. It may also hinder carburizing. Therefore, the Cr mass fraction is set to 1.30% or more and 3.50% or less. For the same reason, the Cr mass fraction is preferably set to 1.40% or more and 2.50% or less.

[0034] Al: 0.020% or more and 0.200% or less Al is an element used as a deoxidizer. Furthermore, it combines with N to form AlN, which has the effect of suppressing grain coarsening. If the mass fraction of Al is less than 0.020%, the deoxidizer is insufficient, resulting in a lack of fine nitrides, which in turn leads to a lack of precipitation nuclei for spheroidized carbides, making the carbides more likely to be dispersed unevenly. This results in coarsening of the grains, resulting in a deterioration in toughness, fatigue properties, etc. On the other hand, if the mass fraction of Al exceeds 0.200%, coarse nitrides are formed, resulting in a deterioration in fatigue properties, workability, etc. Therefore, the mass fraction of Al is set to 0.020% or more and 0.200% or less. For the same reason, the mass fraction of Al is preferably set to 0.020% or more and 0.050% or less.

[0035] Nb: 0.02% or more and 0.10% or less Nb forms carbides and / or nitrides, which have the effect of preventing grain coarsening. In particular, nano-sized carbides and / or nitrides finely dispersed in steel inhibit grain growth. If the Nb mass fraction is less than 0.02%, the fine carbides and / or nitrides are insufficient, resulting in a small effect of inhibiting grain coarsening and insufficient toughness, fatigue strength, etc. Furthermore, the precipitation nuclei of spheroidized carbides are insufficient, causing uneven dispersion of carbides. On the other hand, if the Nb mass fraction exceeds 0.10%, the amount of carbides and / or nitrides becomes excessive, resulting in reduced workability. Therefore, the Nb mass fraction is set to 0.02% or more and 0.10% or less. For the same reason, the Nb mass fraction is preferably set to 0.02% or more and 0.08% or less.

[0036] N: 0.0040% or more and 0.0300% or less N precipitates finely as AlN and / or Nb nitrides in steel, preventing grain coarsening. If the mass fraction of N is less than 0.0040%, fine nitrides are insufficient, resulting in grain coarsening and a decrease in toughness, fatigue properties, etc. On the other hand, if the mass fraction of N exceeds 0.0300%, coarse carbonitrides are formed, resulting in a decrease in fatigue properties, workability, etc. Therefore, the mass fraction of N is set to 0.0040% or more and 0.0300% or less. For the same reason, the mass fraction of N is preferably set to 0.0040% or more and 0.0200% or less.

[0037] The optional components of the case-hardening steel according to this embodiment will be described below.

[0038] The case-hardening steel according to this embodiment may contain, as an optional component, one or more selected from P and S. The mass fraction of each optional component may be 0% or may exceed 0%.

[0039] P: 0% or more and 0.030% or less P is an impurity that is mixed in from the raw materials of the steel (e.g., ore, scrap, etc.) and segregates at austenite grain boundaries, reducing toughness such as impact strength and bending strength. Therefore, the mass fraction of P is set to 0% or more and 0.030% or less. The mass fraction of P may be 0% or more, or may be more than 0%. The mass fraction of P may be, for example, 0.001% or more, 0.002% or more, or 0.003% or more.

[0040] S: 0% or more and 0.030% or less S is an impurity that is mixed in from the raw materials of the steel (e.g., ore, scrap, etc.) and forms non-metallic inclusions MnS, which reduce toughness and fatigue strength. Therefore, the mass fraction of S is set to 0% or more and 0.030% or less. The mass fraction of S may be 0% or more, or may be more than 0%. The mass fraction of S may be, for example, 0.001% or more, 0.002% or more, or 0.003% or more.

[0041] For the purpose of improving mechanical properties, the case-hardened steel according to this embodiment may contain one or more optional components (optional component 1) selected from Ni and Mo. The mass fraction of each optional component may be 0% or more than 0%.

[0042] Ni: 0% or more and 2.00% or less If the Ni mass fraction exceeds 2.00%, costs increase. Furthermore, the increased material hardness reduces workability. Therefore, the Ni mass fraction is set to 0% or more and 2.00% or less. The Ni mass fraction may be 0% or more, but if the Ni mass fraction is less than 0.05%, the effect of improving hardenability is small. Furthermore, the effect of improving toughness is also small. Therefore, the Ni mass fraction is preferably set to 0.05% or more and 2.00% or less, and more preferably set to 0.05% or more and 1.80% or less. Ni may be contained as an impurity at a mass fraction less than the above-mentioned lower limit.

[0043] Mo: 0% or more and 2.00% or less If the mass fraction of Mo exceeds 2.00%, costs increase. Furthermore, the increased material hardness reduces workability. Therefore, the mass fraction of Mo is set to 0% or more and 2.00% or less. The mass fraction of Mo may be 0% or more, but if the mass fraction of Mo is less than 0.05%, the effect of improving hardenability is small. Therefore, the mass fraction of Mo is preferably set to 0.05% or more and 2.00% or less, and more preferably set to 0.05% or more and 0.50% or less. Mo may be contained as an impurity in a mass fraction less than the above-mentioned lower limit.

[0044] The case-hardening steel according to this embodiment may contain one or more optional components (optional component 2) selected from B, V, and Ti. The mass fraction of each optional component may be 0% or more than 0%.

[0045] B: 0% or more and 0.0050% or less If the mass fraction of B exceeds 0.0050%, the material hardness increases, which may reduce workability. Therefore, the mass fraction of B is set to 0% or more and 0.0050% or less. The mass fraction of B may be 0% or more, but if the mass fraction of B is less than 0.0010%, the effect of improving hardenability is small. Therefore, the mass fraction of B is preferably set to 0.0010% or more and 0.0050% or less, and more preferably set to 0.0010% or more and 0.0030% or less. B may be contained as an impurity at a mass fraction less than the above lower limit.

[0046] V: 0% or more and 0.500% or less If the mass fraction of V exceeds 0.500%, the amount of carbonitrides will be excessive, which may result in reduced workability. Therefore, the mass fraction of V is set to 0% or more and 0.500% or less. The mass fraction of V may be 0% or more, but if the mass fraction of V is 0.010% or more, a more preferable amount of fine carbides and / or nitrides will be obtained, the effect of suppressing grain coarsening will be improved, and toughness, fatigue strength, etc. will be improved. Therefore, the mass fraction of V is preferably set to 0.010% or more and 0.500% or less, and more preferably set to 0.010% or more and 0.400% or less. V may be contained as an impurity in a mass fraction less than the above lower limit.

[0047] Ti: 0% or more and 0.200% or less If the mass fraction of Ti exceeds 0.200%, the amount of carbonitrides will be excessive, which may result in reduced workability. Therefore, the mass fraction of Ti is set to 0% or more and 0.200% or less. The mass fraction of Ti may be 0% or more, but if the mass fraction of Ti is 0.020% or more, the amount of fine nitrides becomes more preferable, N is fixed, BN formation is suppressed, and hardenability is improved. In addition, the effect of suppressing grain coarsening is improved. Therefore, the mass fraction of Ti is preferably set to 0.020% or more and 0.200% or less, and more preferably set to 0.020% or more and 0.100% or less. Ti may be contained as an impurity in a mass fraction less than the above lower limit.

[0048] The case-hardened steel according to this embodiment may contain one or both of selective component 1 (one or more selected from Ni and Mo) and selective component 2 (one or more selected from B, V, and Ti).

[0049] (Dispersion index V of spheroidized carbide θ The case-hardened steel according to this embodiment is a steel that has been subjected to spheroidizing annealing and has a structure consisting of ferrite and spheroidized carbides. θ is V θ Satisfies ≦0.80. Dispersion index V θ is an index showing the dispersion of spheroidized carbides in the steel structure, and the smaller the dispersion index V θ indicates that the dispersion of spheroidized carbides is more uniform. θ When the value is ≦0.80, a steel having excellent cold forgeability can be obtained.

[0050] dispersion index V θ is the arithmetic mean value of the dispersion index obtained for each of the five images of a cross section of a steel material made of case hardened steel parallel to the rolling direction (longitudinal direction) of the steel material, the cross section being 50 μm square.

[0051] The dispersion index obtained for each image is a value obtained by: determining the number n of spheroidized carbides contained in the image and the actual coordinates of the center of gravity of each spheroidized carbide in an xy coordinate system with the upper left corner of the image as the origin; arranging the n spheroidized carbides in a grid pattern at equal intervals within the image; determining the ideal coordinates of each of the n spheroidized carbides; determining the distance between the ideal coordinate and the nearest actual coordinate for each ideal coordinate; and dividing the arithmetic mean value of the obtained distances by the length of one side of the unit cell, which is the distance between adjacent ideal coordinates.

[0052] dispersion index V θ can be calculated by the following method. θ 2 to 4 are schematic diagrams of a cylindrical steel material (test piece) for determining the dispersion index V θ FIG. 10 is a diagram illustrating a method for calculating

[0053] First, a cross section (L cross section) parallel to the rolling direction at the D / 4 position (D is the diameter of the cylindrical steel) of a steel (test piece) made of case-hardened steel is mirror-polished, and then pyracle corrosion is performed. Next, the L cross section is observed using a scanning electron microscope (SEM), and five arbitrary regions (five fields) of 50 μm square are photographed. Image analysis (including calculation) is performed on each of the images of the five fields to determine the dispersion index for each of the images of the five fields, and the arithmetic average value of these is used to determine the dispersion index V of the target steel. θ The image analysis can be performed using, for example, image analysis software Winroof manufactured by Mitsubishi Corporation.

[0054] In the image analysis of the captured image, first, the number n of spheroidized carbides and the actual coordinates (Xi, Yi) of the center of gravity of each spheroidized carbide are derived for each image. The number n of spheroidized carbides is determined by counting the number of spheroidized carbides in the image of one field of view. At this time, carbides with a circle equivalent diameter of 0.3 μm or less are excluded. The actual coordinates of the center of gravity of each spheroidized carbide are determined using x-y coordinates with the upper left corner of the image of one field of view as the origin, as shown in Figure 2. The center of gravity of each spheroidized carbide is the intersection of the major axis and minor axis of each spheroidized carbide.

[0055] In the image analysis of the captured image, the n captured spheroidized carbides are then arranged in a grid pattern at equal intervals within one field of view of the image, and the ideal coordinates (xj, yj) of each spheroidized carbide are derived for each image. FIG. 3 is a schematic diagram of a case in which spheroidized carbides are arranged in a grid pattern at equal intervals within a 50 μm square field of view of an image. The ideal coordinates (xj, yj) of each spheroidized carbide are expressed as a combination of xj = j / (N+1) × 50 and yj = j / (N+1) × 50. In the above formula, j is an integer from 1 to N, and N = √n. If √n contains a decimal point, the value obtained by rounding up the decimal point is used to obtain N. For example, when n = 500, √500 = 22.4, so N = 23 is obtained by rounding up the decimal point.

[0056] The ideal coordinates of n spheroidized carbides are set within one field of view image, from the first ideal coordinate (x1, y1) closest to the origin to the nth ideal coordinate (xN, yN) farthest from the origin. Specifically, the y1 column, the y2 column, ..., the yN column are arranged in order along the Y axis, with the y1 column closest to the X axis and the yN column furthest from the X axis. The y1 column consists of (x1, y1), (x2, y1), ..., (xN, y1). In the y1 column, (x1, y1), (x2, y1), ..., (xN, y1) are arranged in order along the X axis, with (x1, y1) closest to the Y axis and (xN, y1) furthest from the Y axis. The y2 column consists of (x1, y2), (x2, y2), ..., (xN, y2). In the y2 column, (x1, y2), (x2, y2), ..., (xN, y2) are arranged in order along the X axis, with (x1, y2) closest to the Y axis and (xN, y2) furthest from the Y axis. The yN column consists of (x1, yN), (x2, yN), ..., (xN, yN). In the yN column, (x1, yN), (x2, yN), ..., (xN, yN) are arranged in order along the X axis, with (x1, yN) closest to the Y axis and (xN, yN) furthest from the Y axis.

[0057] In the image analysis of the captured image, the distance di between the ideal coordinate and the actual coordinate is calculated for each of the derived n ideal coordinates of the spheroidized carbides. FIG. 4 is a schematic diagram in which the actual coordinates and the ideal coordinates are superimposed in an image of one field of view. For each ideal coordinate, the distance di between the ideal coordinate and the actual coordinate is calculated. At this time, the actual coordinate with the shortest distance from each ideal coordinate (i.e., the closest actual coordinate) is selected to calculate the distance di. Since duplicate actual coordinates may be selected, there may be actual coordinates of spheroidized carbides that are not used in calculating the distance di. For example, in FIG. 4, if carbide 1 is the spheroidized carbide whose actual coordinates are closest to the ideal coordinates (x1, y1), the distance between the ideal coordinates (x1, y1) and carbide 1 is calculated. Similarly, if the actual coordinates of carbide 1 are closest to the ideal coordinates (x2, y1), the distance between the ideal coordinates (x2, y1) and carbide 1 is calculated. When the actual coordinates of the carbide 3 are closest to the ideal coordinates (x1, yN), the distance between the ideal coordinates (x1, yN) and the carbide 3 is calculated. The distance di can be calculated by the following formula. i is an integer from 1 to n, and n is the number of the above-mentioned spheroidized carbides. Each ideal coordinate is substituted for xj and yj, and the actual coordinates of the spheroidized carbide closest to the target ideal coordinates are substituted for X and Y.

[0058]

[0059] By the above method, the distance d is calculated for each of the n ideal coordinates, and the arithmetic mean value of all these distances is calculated. The value obtained by dividing the calculated arithmetic mean value by the length of one side of the unit cell of the ideal coordinates is the dispersion index V in the image of one field of view. θ The length L of one side of the unit lattice of the ideal coordinates corresponds to the distance between two adjacent ideal coordinates, as shown in Figures 3 and 4, and is calculated as L = 50 / (N + 1) [μm]. The length L of one side of the unit lattice varies depending on the number of carbides confirmed in the image of one field of view, so it is calculated for each field of view. The dispersion index V θ The calculated arithmetic mean value is the "dispersion index V" of the carbide of this embodiment. θ "

[0060] The case hardening steel according to this embodiment has a dispersion index V obtained by the above method. θ But, V θ ≦0.80. Dispersion index V of spheroidized carbides θ If the dispersion index V of the spheroidized carbides is more than 0.80, it can be said that the spheroidized carbides are precipitated non-uniformly. In this case, cracks may occur during cold forging, variations in deformation resistance may occur during cold forging, and the crystal grains may become coarse due to carburization after cold forging. Therefore, the dispersion index V of the spheroidized carbides is θ V θ ≦0.80. θ It is preferable that the dispersion index V is ≦0.70, which further suppresses the variation in deformation resistance and makes it possible to obtain steel with excellent cold forgeability. θ The lower limit of the dispersion index V can be adjusted as appropriate. θ may be, for example, 0.15 or more, or 0.20 or more.

[0061] (Rockwell Hardness Coefficient of Variation CV) In the case-hardened steel according to this embodiment, when the Rockwell hardness of a cross section perpendicular to the rolling direction is measured at a plurality of different positions in the rolling direction (longitudinal direction) of a steel material made of case-hardened steel, the variation in the Rockwell hardness is within a predetermined range. Specifically, the Rockwell hardness coefficient of variation CV obtained by measuring the Rockwell hardness of test pieces taken from 10 arbitrary positions in the rolling direction of a steel material made of case-hardened steel on a cross section perpendicular to the rolling direction satisfies CV × 100≦0.65. By satisfying this condition, the variation in deformation resistance during cold forging can be sufficiently reduced.

[0062] The coefficient of variation CV of Rockwell hardness can be determined by the following method. First, test specimens are cut out and collected from 10 arbitrary locations in the rolling direction of a single steel material (steel bar). The Rockwell hardness (HRB) of the cross section (T surface) perpendicular to the rolling direction of each of the 10 collected test specimens is measured. For each test specimen, the Rockwell hardness is measured at four points on the middle circumference of the T surface, and the average value is taken as the Rockwell hardness of each test specimen. The average value and standard deviation of the Rockwell hardness of sites 1 to 10 (10 test specimens) are calculated, and the standard deviation is divided by the average value, thereby determining the coefficient of variation CV of the Rockwell hardness of the steel material. Rockwell hardness can be measured using a Rockwell hardness tester in accordance with JIS Z 2245:2016.

[0063] In the case-hardening steel according to this embodiment, the coefficient of variation CV of Rockwell hardness obtained by the above method satisfies CV×100≦0.65. When this condition is satisfied, the variation in deformation resistance during cold forging can be sufficiently suppressed. On the other hand, if the value of CV×100 exceeds 0.65, the variation in deformation resistance during cold forging is large. Therefore, CV×100≦0.65 is set. It is more preferable to set CV×100≦0.50. This makes it possible to further reduce the variation in deformation resistance during cold forging. The lower limit of CV×100 can be adjusted as appropriate. The value of CV×100 may be, for example, 0.05 or more, or 0.10 or more.

[0064] (Other Configurations) In the case-hardened steel according to this embodiment, the grain size number of ferrite in the steel structure before spheroidizing annealing is preferably 8.0 or more. If the grain size number of ferrite in the steel structure before spheroidizing annealing is 8.0 or more, a structure with a more uniform carbide distribution can be obtained after spheroidizing annealing. The grain size number is determined by the method described in the Examples below. The upper limit of the grain size number can be adjusted as appropriate. The grain size number may be, for example, 9.5 or less, or 10 or less.

[0065] In the case-hardened steel according to this embodiment, the average value of the Rockwell hardness measured on a cross section (T surface) perpendicular to the rolling direction (longitudinal direction) of the steel material made of the case-hardened steel (the average value of the Rockwell hardness measured at four points on the mid-periphery of the T surface) is preferably 85 HRB or less. If it is 85 HRB or less, the cold forgeability is excellent and the occurrence of cold forging cracks can be suppressed. The lower limit of the average value of the Rockwell hardness can be appropriately adjusted. The average value of the Rockwell hardness may be, for example, 67 HRB or more, or 70 HRB or more.

[0066] (Method for manufacturing case-hardening steel) Hereinafter, a method for manufacturing case-hardening steel according to this embodiment will be described.

[0067] The case-hardened steel according to this embodiment can be manufactured by a method including, for example, a step of performing spheroidizing annealing on a rolled steel material having the above-described predetermined chemical composition (spheroidizing annealing step). The method may also include a step of manufacturing a rolled steel material (rolled steel material manufacturing step).

[0068] The manufacturing process for rolled steel material includes, for example, the following steps: (Step 1) Raw materials are melted in a melting furnace to produce steel material (steel ingot or steel billet) having the above-mentioned chemical composition. (Step 2) The steel material obtained in Step 1 is subjected to a first hot rolling. Specifically, the steel material obtained in Step 1 is heated to 1200°C or higher, rolled, and air-cooled. (Step 3) The steel material obtained in Step 2 is subjected to a second hot rolling. Specifically, the steel material obtained in Step 2 is reheated to a temperature of 960°C or lower, rolled, and air-cooled. In this way, a steel material such as a steel bar is obtained.

[0069] The spheroidizing annealing process includes, for example, the following steps: (Step 4) The rolled steel material having the predetermined chemical composition is heated to a holding temperature T1 [°C] that satisfies the relationship A1 transformation point≦T1≦(A1 transformation point+40°C), and held for a holding time t [h] that satisfies the relationship 1 hour≦t≦6 hours. (Step 5) The steel material subjected to step (4) is cooled at a cooling rate of 5°C / h or more and 40°C / h or less to a slow cooling end temperature T2 [°C] that satisfies the relationship (A1 transformation point−100°C)≦T2≦(A1 transformation point−40°C).

[0070] The case-hardened steel according to this embodiment can be obtained by the above steps. Steps (2) to (5) will be described in more detail below.

[0071] Temperature of the first hot rolling step in step (2): 1200°C or higher By setting the heating temperature to 1200°C or higher, Al nitrides and / or Nb oxides and / or Nb nitrides generated during the solidification stage can be dissolved into the steel material. This allows the effect of Al and / or Nb to be fully achieved in suppressing grain size coarsening. Therefore, the hot rolling temperature in the first hot rolling step is preferably set to 1200°C or higher, and more preferably set to 1250°C or higher. The upper limit of the hot rolling temperature can be adjusted as appropriate. The hot rolling temperature may be, for example, 1300°C or lower, or 1340°C or lower.

[0072] Temperature of the second hot rolling step in step (3): 960°C or less. By setting the heating temperature to 960°C or less, the structure of the steel material having the above-mentioned predetermined chemical composition before the spheroidizing annealing treatment can be made to consist of ferrite and pearlite, and the ferrite grain size number can be set to a grain size equivalent to 8.0 or more in the austenite grain size number according to JIS G0551. This allows for a structure with more uniform carbide distribution to be obtained after the spheroidizing annealing treatment. Steel having such a structure suppresses non-uniform deformation due to cold forging and is less likely to crack. Furthermore, coarsening of the grain size due to the subsequent carburizing treatment is also less likely to occur. Therefore, the hot rolling temperature in the second hot rolling step is preferably set to 960°C or less, and more preferably to 925°C or less. The lower limit of the hot rolling temperature can be adjusted as appropriate. The hot rolling temperature may be, for example, 860°C or more or 820°C or more.

[0073] In steps (4) and (5), spheroidizing annealing is performed under the above-described spheroidizing annealing conditions (hereinafter sometimes referred to as "spheroidizing annealing conditions 1") to obtain a steel material with a structure with a highly uniform carbide distribution. Steel with such a structure is less susceptible to cracking due to the suppression of non-uniform deformation during cold forging. Furthermore, it is less susceptible to grain coarsening during the subsequent carburizing treatment. Therefore, in the spheroidizing annealing treatment, the steel is heated to a holding temperature T1 that satisfies the relationship A1 transformation point ≦ T1 ≦ (A1 transformation point + 40°C) and held for a holding time t that satisfies the relationship 1 hour ≦ t ≦ 6 hours (step (4)). The steel is then cooled at a cooling rate of 5°C / h to 40°C / h to a slow cooling end temperature T2 that satisfies the relationship (A1 transformation point - 100°C) ≦ T2 ≦ (A1 transformation point - 40°C) (step (5)). After cooling to the slow cooling end temperature T2, air cooling may be performed.

[0074] In this embodiment, the A1 transformation point is a temperature calculated by the following formula (1). In formula (1), "Mn", "Si", "Ni", and "Cr" are substituted with the mass fractions (%) of Mn, Si, Ni, and Cr in the case-hardening steel, respectively. A1 transformation point (°C) = 723°C - 14Mn + 22Si - 14.4Ni + 23.3Cr ... (1)

[0075] The above is the manufacturing method of case-hardened steel according to the present embodiment. By manufacturing case-hardened steel using steel having the above-mentioned predetermined chemical composition through the above-mentioned manufacturing process, it is possible to manufacture case-hardened steel according to the present embodiment that can sufficiently suppress variations in deformation resistance during cold forging.

[0076] The spheroidizing annealing treatment can also be performed under the following spheroidizing annealing condition 2 instead of the spheroidizing annealing condition 1 in the above steps (4) and (5). That is, in the spheroidizing annealing treatment, the rolled steel material having the above-mentioned predetermined chemical composition may be heated to a holding temperature T [°C] that satisfies (A1 transformation point - 30°C) ≦ T ≦ A1 transformation point, and held for a holding time t [h] that satisfies t ≧ 120 / (T - A1 transformation point + 50). After the spheroidizing annealing treatment, air cooling (or water cooling) may be performed as appropriate.

[0077] If the holding temperature T is lower than (A1 transformation point - 30°C), the carbides that constitute pearlite or bainite will not be sufficiently spheroidized, which may result in insufficient softening. On the other hand, if the holding temperature T is higher than the A1 transformation point, austenite will be generated, and pearlite or martensite will be generated during subsequent air or water cooling. This may result in increased hardness and reduced workability. If the holding time t is shorter than 120 / (T - A1 transformation point + 50), the carbides that constitute pearlite or bainite will not be sufficiently spheroidized, which may result in insufficient softening. Therefore, the steel is heated to a holding temperature T [°C] that satisfies (A1 transformation point - 30°C) ≦ T ≦ A1 transformation point, and held for a holding time t [h] that satisfies t ≧ 120 / (T - A1 transformation point + 50). The A1 transformation point can be calculated using the above formula (1).

[0078] In the case of case-hardened steel produced by the spheroidizing annealing condition 2, the dispersion state of spheroidized carbides becomes more uniform, and the variation in hardness is also reduced. Therefore, case-hardened steel with reduced variation in deformation resistance can be obtained. Furthermore, case-hardened steel with reduced grain coarsening can be obtained.

[0079] According to the present embodiment, the uniformity of the dispersion of spheroidized carbides is further improved, and the homogeneity of the structure is improved, thereby obtaining a case-hardened steel with excellent cold forgeability that can further suppress variations in deformation resistance. The suppression of variations in deformation resistance also further suppresses the occurrence of cracks during cold forging. Furthermore, even if carburizing treatment is performed directly after cold forging, the crystal grains are less likely to coarsen. Therefore, the case-hardened steel according to this embodiment can be said to be a case-hardened steel that can reduce manufacturing costs due to its excellent workability and also has excellent strength.

[0080] The case-hardening steel of the present invention will be described in more detail below based on examples.

[0081] Steels of examples and comparative examples having the chemical compositions shown in Tables 1A and 1B were prepared, respectively.

[0082] The chemical compositions and A1 transformation points of the steels of the examples and comparative examples are shown in Tables 1A and 1B. The units of the chemical composition of the steels are in mass%, and the balance other than the essential components, optional component 1, and optional component 2 is Fe. The shaded areas in Table 1B are outside the scope of the present invention. Nos. 1 to 22 are example steels having chemical compositions within the scope of the present invention, and are hereinafter referred to as "invention steels." Nos. 22 to 33 are comparative steels having chemical compositions outside the scope of the present invention, and are hereinafter referred to as "comparison steels." The A1 transformation point (°C) is the temperature calculated using the above formula (1).

[0083]

[0084]

[0085] First, 100 kg of each steel having the chemical composition shown in Tables 1A and 1B was melted in a vacuum induction furnace (VIM) to produce a steel ingot (step (1)). Next, in a first hot rolling step, each steel ingot obtained in step (1) was rolled at 1250°C to a diameter of 65 mm (step (2)). Next, in a second hot rolling step, each steel bar obtained in step (2) was heated to 925°C and rolled to a diameter of 25 mm (step (3)). Each steel bar obtained in step (3) was cut to a length of 100 mm to obtain a steel product (cylindrical steel product) of each steel.

[0086] (Evaluation of Ferrite Grain Size Number) The ferrite grain size of the obtained steel material was confirmed before the spheroidizing annealing process. Specifically, an L cross section parallel to the rolling direction at the D / 4 position (D is the diameter of the cylindrical steel material) of the obtained steel material was etched with nital and then observed under a microscope. The ferrite grain size number was then evaluated using the grain size number specified in JIS G 0551.

[0087] (Spheroidizing annealing step) In the spheroidizing annealing step, each steel material was subjected to spheroidizing annealing treatment. Two different types of test materials were produced by two different types of spheroidizing annealing steps. One test material was produced by subjecting each steel material to spheroidizing annealing treatment under spheroidizing annealing condition 1. The other test material was produced by subjecting each steel material to spheroidizing annealing treatment under spheroidizing annealing condition 2.

[0088] (Spheroidizing annealing condition 1) Spheroidizing annealing treatment under spheroidizing annealing condition 1 was carried out under the heat treatment conditions shown in Fig. 5. Fig. 5 is a diagram showing heat treatment conditions that are an example of spheroidizing annealing condition 1. In the spheroidizing annealing treatment under spheroidizing annealing condition 1, each steel material was heated to a holding temperature T1 [°C] over 6 hours, held at that holding temperature for 2 hours, then cooled from the holding temperature T1 to a slow cooling end temperature T2 over 10 hours, and then air-cooled. The holding temperature T1 and the slow cooling end temperature T2 are shown in Tables 2A and 2B.

[0089] (Spheroidizing annealing condition 2) In the spheroidizing annealing treatment under spheroidizing annealing condition 2, the steel material was heated to a holding temperature T that satisfied the relationship (A1 transformation point - 30°C) ≦ T ≦ A1 transformation point, and held for a holding time t that satisfied the relationship t ≧ 120 / (T - A1 transformation point + 50), and then air-cooled. The holding temperature T and holding time t are shown in Tables 3A and 3B.

[0090] For each test material obtained in the spheroidizing annealing process, the hardness, the coefficient of variation of hardness, the carbide dispersion index V θ Tests were conducted to evaluate the coefficient of variation of deformation resistance, the presence or absence of cracks in a cold upsetting test, and the critical temperature for suppressing grain coarsening. The test conditions were as follows:

[0091] (Hardness) The Rockwell hardness was measured for a cross section (T surface) perpendicular to the rolling direction at an arbitrary position of each test material. The Rockwell hardness was measured using a Rockwell hardness tester in accordance with JIS Z 2245:2016. The average value of the Rockwell hardness measured at four points on the middle circumference of the T surface of each test material was taken as the hardness of each test material.

[0092] (Coefficient of variation of hardness CV) Test specimens were cut out from 10 arbitrary different locations (sites 1 to 10) in the rolling direction of each test material, and the Rockwell hardness of the cross section (T surface) perpendicular to the rolling direction of each test specimen was measured by the method described in the above embodiment. The Rockwell hardness was measured using a Rockwell hardness tester in accordance with JIS Z 2245:2016. The average value of the Rockwell hardness measured at four points on the middle circumference of the T surface of each test specimen was taken as the hardness of each test specimen. The coefficient of variation of hardness CV was calculated for each test material from the hardness of the 10 test specimens from sites 1 to 10, the average value of those hardnesses, and the standard deviation.

[0093] (Carbide dispersion index V θ ) By the method described in the above embodiment, the cross section (L cross section) parallel to the rolling direction at the D / 4 position (D is the diameter of the cylindrical steel material) of each test material was photographed by SEM and image analysis was performed to determine the carbide dispersion index V θ asked for.

[0094] (Variation coefficient of deformation resistance) In order to evaluate the variation in deformation resistance during cold forging of each test material, a cold upsetting test was performed to determine the deformation resistance, and the variation coefficient of the obtained deformation resistance was calculated. Specifically, cylindrical test pieces (φ8 × 12 mm) were prepared at 10 arbitrary positions from one test material (the rolling direction of the test material and the longitudinal direction of the test piece were parallel). The test pieces were subjected to a strain rate of 10 s －１ The specimens were subjected to upsetting at a temperature of 100°C, and the deformation resistance was measured. The deformation resistance when the true strain reached 0.9 was taken as the deformation resistance of the test specimen. The deformation resistance of each of 10 test specimens made from one test material was measured, and the coefficient of variation of the deformation resistance was calculated. The variation in the deformation resistance of the test material was evaluated using the value of (coefficient of variation of deformation resistance x 100). If the value of (coefficient of variation of deformation resistance x 100) is 0.85 or less, it can be evaluated that the variation in the deformation resistance is sufficiently suppressed, and if it is 0.65 or less, it is even more preferable.

[0095] (Presence or absence of cracks in cold upsetting test) Cylindrical test pieces measuring φ14 × 21 mm were prepared from the center of the test material (the forging direction of the test material was parallel to the longitudinal direction of the test piece). Four cylindrical test pieces were prepared for each test material. A cold upsetting test was carried out on the test pieces at room temperature. The upsetting speed was 10 mm / min, and the final upsetting rate (the rate of reduction in height of the test piece) was 75%, and the test was carried out four times (number of test pieces n = 4), and the presence or absence of surface cracks on the test pieces after processing was confirmed. If all four test pieces were crack-free, it was rated as "absent", and if even one crack was found, it was rated as "present".

[0096] (Critical Temperature for Suppressing Grain Coarsening) The following test was conducted to evaluate whether grain coarsening was suppressed when cold forging was performed using the case-hardened steel of the present invention and then directly carburizing. Cylindrical test specimens similar to those used in the test for the presence or absence of cracks due to cold upsetting were prepared, and cold upsetting was performed as the cold forging process, with a final upsetting ratio (the reduction in height of the test specimen) of 70%. After upsetting, the test specimens were split in half in the axial direction, and a pseudo-carburizing treatment was performed in which the test specimens were heated and then water-cooled. The pseudo-carburizing treatment was performed by increasing the temperature from 860°C to 1040°C in 20°C intervals. After holding at each temperature for 10.8 ks (3 hours), water-cooling was performed, and the procedure of confirming the grain size was repeated. That is, first, the test piece was heated to 860°C and held for 10.8 ks, and then water-cooled to confirm the grain size. Next, the test piece was heated to 880°C, which is 20°C higher than the temperature, held for the same time, water-cooled, and the grain size was confirmed. This process was repeated up to 1040°C.

[0097] The grain size was confirmed by etching a cross section of a halved test piece with a saturated picric acid solution and observing the grain size under an optical microscope. Grain size numbers of 3.0 or less, as determined by the same evaluation method as for the grain size number of the ferrite, were considered coarse grains, and the heating temperature at which the grain size number first became 3.0 or less was considered the limit temperature for suppressing grain coarsening. A higher limit temperature is preferable because it makes it less likely for grain coarsening to occur due to carburizing. Specifically, a limit temperature of 960°C or higher can be evaluated as being sufficient to suppress grain coarsening, and a limit temperature of 980°C or higher can be evaluated as being even more excellent in suppressing coarsening.

[0098] Tables 2A and B and Tables 3A and B show the heating conditions and test results for spheroidizing annealing. Only the ferrite grain size number is an evaluation of the steel before spheroidizing annealing (SA), while the others are evaluations of the case-hardened steel after spheroidizing annealing. Tables 2A and B show the evaluation results for the steel produced under spheroidizing annealing condition 1, and Tables 3A and B show the evaluation results for the steel produced under spheroidizing annealing condition 2. The ferrite grain size number before SA is common to Tables 2A and B and Tables 3A and B, so it is only shown in Tables 2A and B.

[0099]

[0100]

[0101]

[0102]

[0103] As shown in Tables 2A and 2B and Tables 3A and 3B, all of the invention steels No. 1 to 22 had a carbide dispersion index V θ Furthermore, all of the invention steels manufactured under the spheroidizing annealing condition 2 shown in Table 3A had a dispersion index V θ It was confirmed that the charcoal dispersion was excellent in uniformity.

[0104] With regard to hardness variation, the value of (coefficient of variation of hardness CV x 100) was 0.65 or less for all of the invention steels. With regard to the coefficient of variation of deformation resistance, it was 0.85 or less for all of the invention steels, confirming that by having a coefficient of variation of hardness below a predetermined value, it is possible to further suppress the variation of deformation resistance. Furthermore, the invention steel produced under spheroidizing annealing condition 2 shown in Table 3A had an even smaller coefficient of variation of hardness, 0.50 or less. The coefficient of variation of deformation resistance was also even smaller, 0.65 or less for all of the invention steels, confirming that the variation of deformation resistance was further suppressed.

[0105] Furthermore, none of the invention steels showed cracking in the cold upset test, and the critical temperature for suppressing grain coarsening was 960°C or higher, confirming that they had excellent strength and workability. The invention steel produced under spheroidizing annealing condition 2 had an even higher critical temperature of 980°C or higher, confirming that it could further suppress grain coarsening.

[0106] On the other hand, the comparative steel has a carbide dispersion index V θ The coefficient of variation of the hardness and the temperature coefficient of variation of the hardness were large. Therefore, the coefficient of variation of the deformation resistance was also larger than that of the inventive steel, and the variation in the deformation resistance could not be suppressed. Furthermore, cracks occurred in the cold upset test for all the comparative steels, and sufficient strength was not obtained. The critical temperature for suppressing grain coarsening was also lower than that of the inventive steel, and grain coarsening was more likely to occur.

[0107] The comparative steels No. 30 and No. 31, in which the Cr content exceeds the upper limit value indicated in the above embodiment, exhibited a carbide dispersion index V θ The hardness was high at over 85 HRB, and the coefficient of variation of hardness was also large. The large variation in hardness also led to large variation in deformation resistance, which resulted in cracks. The coefficient of variation of hardness and the dispersion index V of carbides θ It can be said that both of the above conditions must be satisfied. The comparative steel No. 33 has a Cr content within the range of the content indicated in the above embodiment, but the C, Si, and Mn contents exceed the upper limits, resulting in poor cold forgeability. Furthermore, since no Nb was added, the critical temperature for suppressing grain coarsening was low, and grain coarsening could not be suppressed.

Claims

1. In mass%, C: 0.14% to 0.35%, Si: 0.05% to 1.00%, Mn: 0.10% to 0.90%, Cr: 1.30% to 3.50%, Al: 0.020% to 0.200%, Nb: 0.02% to 0.10%, N: 0.0040% to 0.0300%, P: 0% to 0.030%, S: 0% to 0.030%, Ni: 0% to 2.00%, Mo: 0% to 2.00%, B: 0% to 0.0050%, V: 0% to 0.500%, Ti: 0% or more and 0.200% or less, and the balance: Fe and impurities A case-hardening steel having a chemical composition comprising: a structure comprising ferrite and spheroidized carbides; and a dispersion index V representing the dispersion of the spheroidized carbides. θ is V θ ≦0.80, and the coefficient of variation CV of Rockwell hardness in a cross section perpendicular to the rolling direction of test pieces taken from any 10 locations in the rolling direction of the steel material made of the case-hardened steel satisfies CV × 100≦0.

65. The case-hardened steel.

2. The case-hardening steel according to claim 1, wherein the chemical composition includes one or more elements selected from the group consisting of Ni: 0.05% or more and 2.00% or less, Mo: 0.05% or more and 2.00% or less, B: 0.0010% or more and 0.0050% or less, V: 0.010% or more and 0.500% or less, and Ti: 0.020% or more and 0.200% or less.

3. The dispersion index V θ 3. The case-hardening steel according to claim 1 or 2, wherein the dispersion index is an arithmetic mean value of the dispersion indexes obtained for five images of 50 μm square fields of view of a cross section of a steel material made of the case-hardening steel parallel to the rolling direction, and the dispersion index obtained for each of the five images of the five fields of view, and the dispersion index obtained for each of the images is a value obtained by: determining the number n of spheroidized carbides included in the image and the actual coordinates of the center of gravity of each spheroidized carbide in an xy coordinate system with the upper left corner of the image as the origin; arranging the n spheroidized carbides in a lattice pattern at equal intervals within the image and determining ideal coordinates for each of the n spheroidized carbides; determining the distance between the ideal coordinate and the nearest actual coordinate for each of the ideal coordinates; and dividing the arithmetic mean value of the obtained distances by the length of one side of a unit cell, which is the distance between adjacent ideal coordinates.

4. The case-hardening steel according to claim 3, wherein the ideal coordinates are expressed as (xj, yj) calculated by xj = j / (N+1) x 50, yj = j / (N+1) x 50, where j is an integer between 1 and N, and N is the value of √n rounded up to the nearest integer, and the ideal coordinates are set in order starting from the coordinate closest to the origin of the image.

5. The case-hardening steel according to claim 4, wherein the length (L) of one side of the unit cell is calculated by L = 50 / (N + 1) [μm].

6. Case-hardened steel according to claim 1 or 2, wherein the cross section of the steel material made of the case-hardened steel perpendicular to the rolling direction has a Rockwell hardness of 85 HRB or less.

7. A method for producing case-hardening steel as set forth in claim 1 or 2, comprising a step of subjecting rolled steel material having the chemical composition as set forth in claim 1 or 2 to spheroidizing annealing, wherein in the spheroidizing annealing treatment, the steel material is heated to a holding temperature T1 [°C] that satisfies A1 transformation point ≦ T1 ≦ (A1 transformation point + 40°C), held for a holding time t [h] that satisfies 1 hour ≦ t ≦ 6 hours, and cooled at a cooling rate of 5°C / h or more and 40°C / h or less to a temperature T2 [°C] that satisfies (A1 transformation point - 100°C) ≦ T2 ≦ (A1 transformation point - 40°C).

8. The method according to claim 7, wherein the grain size number of ferrite in the structure of the steel before the spheroidizing annealing treatment is 8.0 or more.

9. A method for producing case-hardened steel as set forth in claim 1 or 2, comprising a step of subjecting rolled steel material having the chemical composition as set forth in claim 1 or 2 to spheroidizing annealing, wherein in the spheroidizing annealing treatment, the steel material is heated to a holding temperature T [°C] that satisfies (A1 transformation point - 30°C) ≦ T ≦ A1 transformation point, and is held for a holding time t [h] that satisfies t ≧ 120 / (T - A1 transformation point + 50).

10. The method according to claim 9, wherein the grain size number of ferrite in the structure of the steel before the spheroidizing annealing treatment is 8.0 or more.

Images