Surface-hardened steel and production method for the same

Abstract

To provide a surface-hardened steel that has excellent cold-forgeability and can sufficiently suppress variations in deformation resistance during cold forging.SOLUTION: A surface-hardened steel contains, in mass%, C: 0.14% or more to 0.35% or less, Si: 0.05% or more to 1.00% or less, Mn: 0.10% or more to 0.90% or less, P: 0.030% or less, S: 0.030% or less, Cr: 1.30% or more to 3.50% or less, Al: 0.020% or more to 0.200% or less, Nb: 0.02% or more to 0.10% or less, and N: 0.0040% or more to 0.0300% or less, the balance being Fe and impurities, the steel being spheroidized annealed and having a structure of ferrite and spheroidized carbides, wherein a dispersion index Vθ representing dispersibility of the spheroidized carbides satisfies Vθ≤0.80, and wherein, for test pieces taken at 10 arbitrary locations in the rolling direction of the steel material, a coefficient of variation CV of Rockwell hardness at a cross-section orthogonal to the rolling direction satisfies CV×100≤0.65.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a case-hardening steel having excellent cold forgeability. [Background technology]

[0002] Cold forging is sometimes chosen to reduce manufacturing costs when manufacturing drivetrain parts such as gears and shafts. Case-hardened steel for cold forging is required to have material properties (cold forgeability) such as low deformation resistance and crack resistance in order to ensure the load on the tool and the shape.

[0003] In order to reduce the 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 the cold forging process 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 annealing, 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 second-phase AlN 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 account the influence of carbide distribution in the spheroidized annealed structure, and therefore cannot suppress non-uniform carbide distribution. As a result, there is a problem in that cold forging cracking and grain coarsening 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 precipitate 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. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-082988 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-242209 [Patent Document 3] Patent No. 7149131 Summary of the Invention [Problem to be solved by the invention]

[0009] As mentioned 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 application is to provide a case-hardened steel that has excellent cold forgeability and in which variations in deformation resistance during cold forging are sufficiently suppressed. [Means for solving the problem]

[0011] The means for solving the above problems are as follows.

[0012] (1) In mass %, C: 0.14% or more and 0.35% or less, Si: 0.05% or more and 1.00% or less, Mn: 0.10% or more and 0.90% or less, P: 0.030% or less, S: 0.030% or less, Cr: 1.30% or more and 3.50% or less, Al: 0.020% or more and 0.200% or less, Nb: 0.02% or more and 0.10% or less, and N: 0.0040% or more and 0.0300% or less and the balance being Fe and impurities, Spheroidized annealed, with a structure of ferrite and spheroidized carbide, Dispersion index V indicates the dispersibility of spheroidized carbides θ But V θ ≦0.80, A case-hardened steel characterized in that the coefficient of variation CV of Rockwell hardness in a cross section perpendicular to the rolling direction of test pieces taken at any 10 locations in the rolling direction of the steel satisfies CV × 100 ≦ 0.65.

[0013] (2) In place of a part of the Fe, 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-hardening steel according to (1) above, characterized in that it contains one or more selected from the group consisting of:

[0014] (3) The dispersion index V θ is an arithmetic average value of dispersion indices obtained by photographing five 50 μm square images of a cross section of a steel material parallel to the rolling direction, and obtaining the dispersion indices for each of the five images of the five images of the five images, The dispersion index calculated for each image is 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 are calculated, The ideal coordinates of n spheroidized carbides are determined when the n spheroidized carbides are arranged in a lattice pattern at equal intervals within the image, determining a distance between the ideal coordinate and the closest actual coordinate for each ideal coordinate; The case hardening steel according to (1) or (2), characterized in that the value is obtained by dividing the arithmetic mean value of the distance obtained by the length of one side of the unit cell, which is the distance between adjacent ideal coordinates.

[0015] (4) The ideal coordinates are expressed as (xj, yj) calculated by xj = j / (N+1) × 50 and yj = j / (N+1) × 50, where j is an integer between 1 and N, and N is the value of √n rounded up to the nearest whole number. The case-hardening steel according to (3) above, characterized in that the coordinates are set in order from the coordinate closest to the origin of the image.

[0016] (5) The case-hardening steel according to (4) above, characterized in that 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 (1) or (2) above, characterized in that the Rockwell hardness of the cross section perpendicular to the rolling direction is 85 HRB or less.

[0018] (7) A method for manufacturing case hardening steel according to (1) or (2) above, The rolled steel material is heated to a holding temperature T1 [°C] that satisfies the A1 transformation point ≦ T1 ≦ (A1 transformation point + 40°C), and held for a holding time t [h] that satisfies 1 hour ≦ t ≦ 6 hours, A method for manufacturing case-hardened steel, characterized by carrying out a spheroidizing annealing process in which the steel is 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) A method for producing case-hardened steel according to (7) above, characterized in that the grain size number of ferrite in the structure of the steel material before the spheroidizing annealing step is 8.0 or more.

[0020] (9) A method for manufacturing case-hardened steel according to (1) or (2) above, A method for manufacturing case-hardened steel, characterized by carrying out a spheroidizing annealing process in which rolled 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).

[0021] (10) A method for producing case-hardened steel according to (9) above, characterized in that the grain size number of ferrite in the structure of the steel material before the spheroidizing annealing step is 8.0 or more. [Effects of the Invention]

[0022] According to one aspect of the embodiment, it is possible to provide a case-hardened steel having excellent cold forgeability in which the variation in deformation resistance during cold forging is sufficiently suppressed. [Brief explanation of the drawings]

[0023] [Figure 1] A schematic diagram of the steel material (test piece) for determining the dispersion index Vθ is shown. [Figure 2] FIG. 10 is a diagram illustrating a method for determining a dispersion index Vθ. [Figure 3] FIG. 10 is a diagram illustrating a method for determining a dispersion index Vθ. [Figure 4] FIG. 10 is a diagram illustrating a method for determining a dispersion index Vθ. [Figure 5] FIG. 2 is a diagram showing the heat treatment conditions for spheroidizing annealing in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0024] (Configuration of case hardening steel of embodiment) The case-hardened steel of this embodiment will be described. The case-hardened steel of this embodiment is, but is not limited to, a steel material after spheroidizing annealing, and can be used as a material for parts manufactured by carrying out, for example, a cold forging process and a carburizing process. The case-hardened steel of this embodiment has a predetermined chemical composition described below, has a structure of spheroidized annealed ferrite and spheroidized carbides, and has a dispersion index V that indicates the dispersion of the spheroidized carbides. θ V θ The hardness coefficient of variation CV, which is an index showing the variation in Rockwell hardness in cross sections perpendicular to the rolling direction of test pieces taken at any 10 positions in the rolling direction of the steel material, satisfies CV×100≦0.80, and the hardness coefficient of variation CV, which is an index showing the variation in Rockwell hardness in cross sections perpendicular to the rolling direction of test pieces taken at any 10 positions in the rolling direction of the steel material, satisfies CV×100≦0.65. Each component will be described in more detail below. In this embodiment, the chemical composition of the steel is expressed in mass fraction (%).

[0025] (chemical composition) C: 0.14% or more and 0.35% or less C is an element necessary for ensuring the core strength of steel after carburization. However, if C is less than 0.14%, the deep hardness after carburization will be insufficient, resulting in insufficient strength. On the other hand, if C exceeds 0.35%, the material hardness will increase, reducing workability (cold forgeability, machinability) and toughness. Therefore, C is set to 0.14% or more and 0.35% or less. Furthermore, for the same reason, C is more preferably set to 0.14% or more and 0.27% or less.

[0026] Si: 0.05% or more and 1.00% or less Silicon is an element necessary for deoxidation. However, if the Si content is less than 0.05%, this effect is not fully achieved. On the other hand, if the Si content exceeds 1.00%, the material hardness increases, reducing workability and even hindering carburizing. Therefore, the Si content is set to 0.05% or more and 1.00% or less. Furthermore, for the same reason, it is more preferable that the Si content be set to 0.15% or more and 0.80% or less.

[0027] Mn: 0.10% or more and 0.90% or less Mn is an element necessary for ensuring hardenability. However, if the Mn content is less than 0.10%, the hardenability effect is not sufficiently obtained. On the other hand, if the Mn content exceeds 0.90%, workability is reduced. Therefore, the Mn content is set to 0.10% or more and 0.90% or less. Furthermore, for the same reason, the Mn content is more preferably set to 0.15% or more and 0.70% or less.

[0028] P:0.030% or less P is an unavoidable impurity contained in scrap, but it segregates at austenite grain boundaries and reduces toughness such as impact strength and bending strength. Therefore, the P content is set to 0.030% or less.

[0029] S: 0.030% or less S is an unavoidable impurity contained in scrap, but it forms non-metallic inclusions, MnS, which reduces toughness and fatigue strength. Therefore, the S content is set to 0.030% or less.

[0030] 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. At 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. Hardenability is also insufficient. On the other hand, at Cr levels exceeding 3.50%, the material hardness increases, reducing workability. It may also impede carburizing. Therefore, the Cr content is set to 1.30% to 3.50%. For the same reasons, a Cr content of 1.40% to 2.50% is even more preferable.

[0031] 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 Al content is less than 0.020%, there is a lack of deoxidizer and fine nitrides, which results in a lack of precipitation nuclei for spheroidized carbides, making the carbides more likely to be dispersed unevenly. This leads to coarsening of grains and a deterioration in toughness and fatigue properties. On the other hand, if the Al content exceeds 0.200%, coarse nitrides are formed, which deteriorates fatigue properties and workability. Therefore, the Al content is set to 0.020% or more and 0.200% or less. For the same reason, it is more preferable to set the Al content to 0.020% or more and 0.050% or less.

[0032] Nb: 0.02% or more and 0.10% or less Nb forms carbides or nitrides, which have the effect of preventing grain coarsening. In particular, nano-sized carbides and nitrides finely dispersed in steel inhibit grain growth. If the Nb content is less than 0.02%, the fine carbides and nitrides are insufficient, the effect of inhibiting grain coarsening is small, and toughness and fatigue strength are insufficient. Furthermore, the precipitation nuclei of spheroidized carbides are insufficient, causing uneven dispersion of carbides. On the other hand, if the Nb content exceeds 0.10%, the amount of carbides and nitrides becomes excessive, resulting in reduced workability. Therefore, the Nb content is set to 0.02% or more and 0.10% or less. For the same reason, it is more preferable to set the Nb content to 0.02% or more and 0.08% or less.

[0033] N: 0.0040% or more and 0.0300% or less N precipitates finely in steel as AlN or Nb nitrides, preventing grain coarsening. If the N content is less than 0.0040%, fine nitrides are insufficient, resulting in grain coarsening and reduced toughness and fatigue properties. On the other hand, if the N content exceeds 0.0300%, coarse carbonitrides are formed, reducing fatigue properties and workability. Therefore, the N content is set to 0.0040% or more and 0.0300% or less. For the same reason, it is more preferable to set the N content to 0.0040% or more and 0.0200% or less.

[0034] The case-hardening steel of this embodiment basically contains the above elements, with the balance being iron (Fe) and impurities. Here, the impurities refer to components that are mixed in from ore or scrap used as raw materials for the steel, or from the manufacturing process environment, etc., and are not components that are intentionally contained in the steel material.

[0035] The optional components of the chemical composition of case-hardening steel will now be explained. For the purpose of improving mechanical properties, one or more elements selected from Ni and Mo shown below may be further contained in place of a portion of the iron (Fe) in the basic chemical composition (optional component 1). However, since the inclusion of these elements is not essential, the content may be 0%. They may also be contained as impurities in amounts less than the ranges described below.

[0036] Ni: 0.05% or more and 2.00% or less If Ni is less than 0.05%, the effect of improving hardenability is small. Furthermore, the effect of improving toughness is also small. If Ni is more than 2.00%, costs increase. Furthermore, the increase in material hardness reduces workability. Therefore, Ni is preferably 0.05% or more and 2.00% or less. Furthermore, for the same reason, Ni is more preferably 0.05% or more and 1.80% or less.

[0037] Mo: 0.05% or more and 2.00% or less If the Mo content is less than 0.05%, the effect of improving hardenability is small. If it exceeds 2.00%, costs increase. Furthermore, the increased material hardness reduces workability. Therefore, the Mo content is preferably 0.05% or more and 2.00% or less. Furthermore, for the same reason, it is more preferable to limit the Mo content to 0.05% or more and 0.50% or less.

[0038] Furthermore, as another optional component, one or more selected from B, V, and Ti shown below may be further contained in place of a portion of iron (Fe) (optional component 2). However, since the inclusion of these elements is not essential, the content may be 0%. Furthermore, they may be contained as impurities in amounts less than the ranges described below.

[0039] B: 0.0010% or more and 0.0050% or less If B is less than 0.0010%, the effect of improving hardenability is small. If it exceeds 0.0050%, the material hardness increases, which may reduce workability. Therefore, B is preferably 0.0010% or more and 0.0050% or less. Furthermore, for the same reason, B is more preferably 0.0010% or more and 0.0030% or less.

[0040] V: 0.010% or more and 0.500% or less When V is 0.010% or more, more preferable amounts of fine carbides and nitrides are obtained, the effect of suppressing grain coarsening can be improved, and toughness and fatigue strength are improved. When V exceeds 0.500%, the amount of carbonitrides becomes excessive, which may reduce workability. Therefore, V is preferably 0.010% or more and 0.500% or less. Furthermore, for the same reason, V is more preferably 0.010% or more and 0.400% or less.

[0041] Ti: 0.020% or more and 0.200% or less When Ti is 0.020% or more, the amount of fine nitrides becomes more preferable, N is fixed, the formation of BN is suppressed, and hardenability is improved. Furthermore, the effect of suppressing grain coarsening can be improved. If Ti exceeds 0.200%, the amount of carbonitrides becomes excessive, which may result in a decrease in workability. Therefore, Ti is preferably 0.020% or more and 0.200% or less. Furthermore, for the same reason, Ti is more preferably 0.020% or more and 0.100% or less.

[0042] (Dispersion index of spheroidized carbide V θ ) The case-hardened steel of this embodiment is a spheroidized annealed steel and has a structure of ferrite and spheroidized carbides. The spheroidized carbides have a dispersion index V θ but V θ ≦0.80 The dispersion index V θ As mentioned above, V is an index showing the dispersion of spheroidized carbides in the steel structure, and a smaller value indicates higher uniformity of carbide dispersion. θWhen the value is ≦0.80, a steel with excellent cold forgeability can be obtained.

[0043] Dispersion index V θ The dispersion index V can be calculated using the following method. θ The schematic diagram of the cylindrical steel material (test piece) used to calculate the dispersion index V is shown in Figures 2 to 4. θ The figure below explains how to calculate the dispersion index. 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 the steel (test piece) is mirror-polished, and then pyracle corrosion is performed. A scanning electron microscope (SEM) is used to photograph five randomly selected areas (five fields) of 50 μm square on the L cross section. Image analysis and calculations are performed on each of the five images to determine the dispersion index for each field, and the arithmetic mean value is taken as the dispersion index V of the target steel. θ Let's say.

[0044] In the image analysis of the captured image, the number n of spheroidized carbides and the actual coordinates (Xi, Yi) of the center of gravity of each carbide are first derived. Image analysis can be performed using Winroof, an image analysis software manufactured by Mitsubishi Corporation. The number n of spheroidal carbides is determined by counting the number of spheroidal carbides within the field of view. Carbides with a circle-equivalent diameter of 0.3 μm or less are excluded. The actual coordinates of the center of gravity of each carbide are determined using xy coordinates with the upper left corner of the image in one field of view as the origin, as shown in Figure 2. The center of gravity of a carbide is the intersection of the long and short diameters of the carbide.

[0045] Next, image analysis software is used to derive the ideal coordinates (xj, yj) of each carbide when the n photographed carbides are uniformly arranged within one field of view. Figure 3 shows a schematic diagram of carbides arranged in a grid pattern at equal intervals within a 50 μm square field of view. The ideal coordinates (xj, yj) of each carbide are: xj=j / (N+1)×50, yj=j / (N+1)×50 In the above formula, j is an integer between 1 and N. N = √n, and if √n contains a decimal point, the value rounded up to the nearest integer is used as N. For example, if n = 500, √500 = 22.4, so the decimal point is rounded up to N = 23. Ideal coordinates are set within one field of view image, starting with (x1, y1) closest to the origin as the first coordinate, up to the nth coordinate, for n carbides. Specifically, ideal coordinates are set in order from (x1, y1) to (xN, y1) in the y1 column, then (x1, y2) to (xN, y2) in the y2 column, and so on up to the nth coordinate.

[0046] Next, the distance di between the derived n ideal coordinates and the actual coordinates is calculated for each. Figure 4 shows a schematic diagram in which the actual coordinates and ideal coordinates in one field of view are superimposed. The distance di of the actual coordinates for each ideal coordinate is calculated. At this time, the actual coordinate with the shortest distance is selected for each ideal coordinate to calculate the distance. Since duplicate actual coordinates may be selected, there may be actual coordinates of carbides that are not used in calculating the distance. For example, in Figure 4, if the carbide with the closest actual coordinates for ideal coordinate (x1, y1) is carbide 1, the distance to carbide 1 is calculated. Similarly, for ideal coordinate (x2, y1), if the actual coordinates of carbide 1 are closest, the distance to carbide 1 is calculated. If the actual coordinates of carbide 3 are closest to ideal coordinate (x1, yN), the distance to carbide 3 is calculated. The distance di can be calculated using the following formula. i is an integer from 1 to n, and n is the number of carbides mentioned above. Substitute the ideal coordinates for xj and yj, and substitute the actual coordinates of the carbide that are closest to the ideal coordinates of the target for X and Y.

[0047]

number

[0048] Using the above method, the distance di is calculated for each of the n ideal coordinates, and the arithmetic mean value of all the distances is calculated. The value obtained by dividing the calculated mean value by the length of one side of the unit cell of the ideal coordinates is the dispersion index V in one field of view. θThe length L of one side of the unit lattice in the ideal coordinates corresponds to the distance between two adjacent ideal coordinates 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 one field of view, so it is calculated for each field of view. The dispersion index V for each of the five fields of view is θ The arithmetic mean value of the dispersion index V of the carbide of this embodiment is calculated. θ "

[0049] The case hardening steel of this embodiment has a dispersion index V obtained by the above method. θ But V θ Satisfies ≦0.80. Dispersion index of spheroidized carbides V θ If the dispersion index V of the spheroidized carbides is greater 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. Furthermore, V θ It is preferable that the ratio is ≦0.70, and this makes it possible to further suppress the variation in deformation resistance and obtain steel with excellent cold forgeability.

[0050] (Coefficient of variation of hardness CV) In the case-hardening steel of this embodiment, when the hardness of a cross section perpendicular to the rolling direction is measured at a plurality of different positions in the longitudinal direction (rolling direction) of the rolled steel material, the variation in the hardness is within a predetermined range. Specifically, the coefficient of variation CV of hardness obtained by measuring the hardness at a plurality of positions in the direction parallel to the rolling direction of the steel material is CV×100≦0.65 By satisfying this condition, the variation in deformation resistance during cold forging can be sufficiently reduced.

[0051] The coefficient of variation (CV) of hardness can be determined using the following method. First, test pieces are cut out from a single steel material (steel bar) at 10 random locations in the direction parallel to the rolling direction. The hardness (Rockwell hardness HRB) of the cross section (T surface) perpendicular to the rolling direction of each of the 10 test pieces is measured. Hardness is measured at four points around the middle of the T surface, and the average value is taken as the hardness of that test piece. The average value and standard deviation of the hardness of parts 1 to 10 (10 test pieces) are calculated, and the standard deviation is divided by the average value to determine the coefficient of variation (CV) of the hardness (Rockwell hardness) of that steel material.

[0052] In the case-hardening steel of this embodiment, the coefficient of variation CV of hardness obtained by the above method satisfies CV×100≦0.65. When this condition is satisfied, the variation in deformation resistance during cold forging of the steel material 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. Furthermore, it is more preferable that CV×100≦0.50 is set. The variation in deformation resistance during cold forging can be further reduced.

[0053] (Other configurations) Furthermore, the case-hardened steel of this embodiment preferably has a ferrite grain size number of 8.0 or more before spheroidizing annealing. If the ferrite grain size number is 8.0 or more, a structure with a more uniform carbide distribution can be obtained after the spheroidizing annealing process. The grain size number can be determined by the method described in the Examples below.

[0054] Furthermore, the case-hardened steel of this embodiment preferably has an average Rockwell hardness (average value measured at four points on the mid-periphery of the T-plane) of 85 HRB or less, measured on a cross section (T-plane) perpendicular to the rolling direction of the steel material. If the average value is 85 HRB or less, the cold forgeability is excellent and the occurrence of cold forging cracks can be suppressed.

[0055] (Case hardening steel manufacturing method) A method for manufacturing the case-hardened steel of this embodiment will be described. For example, the case-hardened steel of this embodiment can be manufactured by the following steps. (1) Raw materials are melted in a melting furnace to obtain steel (steel ingot or steel billet) having the above chemical composition. (2) The first heating and rolling are carried out. Specifically, the steel material is heated to 1200°C or higher, hot-rolled, and air-cooled. (3) The second heating and rolling are carried out. Specifically, the steel material is reheated to a temperature of 960°C or less, rolled, and then air-cooled to obtain a steel material such as a steel bar. (4) The steel is subjected to the spheroidizing annealing process. A1 transformation point ≦ T1 ≦ (A1 transformation point + 40°C) Heat to a holding temperature T1 [℃] that satisfies the above. 1 hour ≤ t ≤ 6 hours The value is held for a holding time t[h] that satisfies the following. (5) The spheroidizing annealing process continues. (A1 transformation point - 100°C) ≦ T2 ≦ (A1 transformation point - 40°C) The temperature is cooled at a rate of 5°C / h to 40°C / h to a temperature T2 [°C] that satisfies the above. The case-hardened steel of this embodiment can be obtained by the above steps. Steps (2) to (5) will be explained in more detail.

[0056] Temperature of the first heating and rolling process in step (2): 1200°C or higher By setting the heating temperature to 1200°C or higher, it is possible to dissolve Al nitrides, Nb oxides, and Nb nitrides that are generated during the solidification stage into the steel material. This allows the effect of Al and Nb to be fully utilized in suppressing grain size coarsening. Therefore, it is recommended to perform processing at a rolling temperature of 1200°C or higher, preferably 1250°C or higher, in the first rolling step.

[0057] Temperature of the second heating and rolling process in step (3): 960°C or less By setting the heating temperature to 960°C or lower, the steel material having the chemical composition of this embodiment can have a ferrite and pearlite structure before the next spheroidizing annealing process, and the ferrite grain size number can be set to a grain size equivalent to JIS G0551 austenite grain size number 8.0 or higher. This allows for a structure with a more uniform carbide distribution to be obtained after the next spheroidizing annealing process. Steel with such a structure suppresses non-uniform deformation during cold forging and is less likely to crack. Furthermore, coarsening of the grain size during the subsequent carburizing process is also less likely to occur. Therefore, it is preferable to set the rolling temperature in the second rolling process to 960°C or lower.

[0058] By performing spheroidizing annealing under the above conditions as the spheroidizing annealing process in steps (4) and (5) (hereinafter also referred to as "spheroidizing annealing condition 1"), a steel material with a highly uniform carbide distribution can be obtained. 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 process, the steel is heated to a holding temperature T1 that satisfies the relationship A1 transformation temperature ≦ T1 ≦ (A1 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 temperature - 100°C) ≦ T2 ≦ (A1 transformation temperature - 40°C) (step (5)). After cooling to the slow cooling end temperature T2, the steel is air-cooled.

[0059] Here, in this embodiment, the above-mentioned A1 transformation point is a temperature calculated by the following formula (1): where Mn, Si, Ni, and Cr are substituted with the mass fractions of the chemical composition of the case-hardening steel. A1 transformation point (℃) = 723℃ - 14Mn[%] + 22Si[%] - 14.4Ni[%] + 23.3Cr[%] (1)

[0060] The above is the method for manufacturing case-hardened steel according to the present embodiment. By manufacturing case-hardened steel according to the above manufacturing steps using steel having the above-described chemical composition, it is possible to manufacture case-hardened steel according to the present embodiment in which variation in deformation resistance during cold forging is sufficiently suppressed.

[0061] The spheroidizing annealing step can also be carried out by the following spheroidizing annealing condition 2 instead of the above steps (4) and (5). (A1 transformation point - 30°C) ≦ T ≦ A1 transformation point Heat to a holding temperature T [℃] that satisfies the above. t≧120 / (T-A1 transformation point + 50) After the spheroidizing annealing, the material may be air-cooled (or water-cooled) as appropriate.

[0062] If the holding temperature T is lower than (A1 transformation point - 30°C), the carbides that make up pearlite and bainite will not be sufficiently spheroidized, potentially resulting in insufficient softening. On the other hand, if the holding temperature is higher than the A1 transformation point, austenite will form, and pearlite or martensite will form during subsequent air or water cooling. This will increase hardness and potentially reduce workability. If the holding time t is shorter than 120 / (T - A1 transformation point + 50), the carbides that make up pearlite and bainite will not be sufficiently spheroidized, potentially resulting in insufficient softening. Therefore, the material should 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). The A1 transformation point can be calculated using the above formula (1).

[0063] In the case of case-hardened steel produced by the spheroidizing annealing condition 2 method, the dispersion state of spheroidized carbides becomes even more uniform, and the variation in hardness is also reduced. As a result, case-hardened steel with reduced variation in deformation resistance can be obtained. Furthermore, case-hardened steel with reduced grain coarsening can be obtained.

[0064] According to the present embodiment described above, 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 and reduced variation in deformation resistance. The reduced variation in deformation resistance also reduces the occurrence of cracks during cold forging, resulting in a case-hardened steel. Furthermore, a case-hardened steel that is less likely to coarsen grains even when subjected to direct carburizing after cold forging can be provided. Therefore, the case-hardened steel of this embodiment can be said to be a case-hardened steel that is excellent in workability, thereby reducing manufacturing costs, and also has excellent strength. [Example]

[0065] The following will be described in more detail with reference to examples. Test materials having the chemical compositions shown in Table 1 were prepared and subjected to evaluation tests. The evaluation tests included the ferrite grain size number, hardness, coefficient of variation of hardness, and 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.

[0066] Table 1 shows the chemical compositions and A1 transformation points of the test materials in the examples and comparative examples. The chemical composition of the steel test materials is in mass %, with the remainder being Fe. Note that the shaded areas in Table 1 are outside the range of the composition of the present invention. Nos. 1 to 22 are test materials that serve as examples and have chemical compositions within the range of the present invention, and are hereinafter also referred to as "invention steels." Nos. 22 to 33 are test materials that serve as comparative examples and have chemical compositions outside the range, and are hereinafter also referred to as "comparison steels." The A1 transformation point (°C) is the temperature calculated using the above-mentioned formula (1).

[0067] [Table 1]

[0068] First, each steel having the chemical composition shown in Table 1 was melted in a 100 kg vacuum induction furnace (VIM) to produce a steel ingot (step (1)), which was then rolled to a diameter of 65 mm at 1250°C in the first heating and rolling step (step (2)), and then heated to 925°C and rolled in the second heating and rolling step (step (3)). The resulting steel bar was cut to a length of 100 mm to obtain a test material for each steel.

[0069] (Evaluation of ferrite grain size number) The ferrite grain size of the obtained specimens was confirmed before the spheroidizing annealing process. Specifically, the L cross section parallel to the rolling direction at the D / 4 position of the obtained specimens 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.

[0070] (Spheroidizing annealing process) Each test material was subjected to a spheroidizing annealing process. Two types of test material were prepared: one was subjected to the spheroidizing annealing condition 1 described above, and the other was subjected to the spheroidizing annealing condition 2.

[0071] (Spheroidizing annealing condition 1) Condition 1 was performed under the conditions shown in Figure 5. Figure 5 shows the heat treatment conditions performed as spheroidizing annealing condition 1 shown in the embodiment. The test material was heated to holding temperature T1 [°C] over 6 hours and held at that holding temperature for 2 hours. Then, it was cooled from holding temperature T1 to slow cooling end temperature T2 over 10 hours, and then air-cooled. T1 and T2 for each test material are shown in Table 2.

[0072] (Spheroidizing annealing condition 2) For spheroidizing annealing under condition 2, the test material was heated to a holding temperature T that satisfied the above-mentioned condition (A1 transformation point - 30°C) ≦ T ≦ A1 transformation point, and held for a holding time t that satisfied the condition t ≧ 120 / (T - A1 transformation point + 50). The test material was then air-cooled. The holding temperature T and holding time t for each test material are shown in Table 3.

[0073] The hardness, coefficient of variation of hardness, and carbide dispersion index V θ The test conditions are as follows: 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.

[0074] (Hardness) The Rockwell hardness of the steel specimen was measured at an arbitrary position on a cross section (T surface) perpendicular to the rolling direction. The average value measured at four points on the center circumference of the T surface was taken as the hardness of the specimen.

[0075] (Coefficient of variation of hardness CV) Test pieces were cut out from the steel bar of the test material at 10 different arbitrary positions in the rolling direction, and the Rockwell hardness of the cross section (T surface) perpendicular to the rolling direction of the test piece was measured using the method described in the embodiment. The average value measured at four points on the middle circumference of the T surface of the test piece was taken as the hardness of each test piece. The coefficient of variation CV of hardness for each test material was calculated from the hardness of the 10 test pieces from positions 1 to 10, the average hardness value, and the standard deviation.

[0076] (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.

[0077] (coefficient of variation 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 coefficient of variation of the deformation resistance was calculated. Specifically, cylindrical test pieces (φ8 × 12 mm) were prepared at 10 random locations from a steel bar of 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 -1The specimens were upset at 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 that specimen. The deformation resistance of each of 10 specimens made from one specimen was determined, and the coefficient of variation of the deformation resistance was calculated. The variation in the deformation resistance of the specimen 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 deformation resistance is sufficiently suppressed, and if it is 0.65 or less, it is even more preferable.

[0078] (Cracks during cold upset testing) A cylindrical test piece measuring φ14 x 21 mm was 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 the height of the test piece) was 75%. This 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 checked. If all four test pieces were crack-free, it was rated as "absent", and if even one crack was present, it was rated as "present".

[0079] (Critical temperature for suppressing grain coarsening) The following test was conducted to evaluate whether grain size coarsening was suppressed when the case-hardening steel of the present invention was subjected to cold forging followed by direct carburizing. Cylindrical test specimens similar to those used in the test for cracking due to cold upsetting were prepared and subjected to cold forging with a final upsetting ratio (the percentage reduction in the height of the test specimen) of 70%. After upsetting, the test specimens were split in half axially and subjected to a pseudo-carburizing process in which the specimens were heated and then water-cooled. The pseudo-carburizing process was performed by increasing the temperature from 860°C to 1040°C in 20°C increments. Each temperature was held for 10.8 ks (3 hours), followed by water-cooling, and the process of confirming the grain size was repeated. Specifically, the test specimens were first heated to 860°C, held at that temperature for 10.8 ks, water-cooled, and the grain size was confirmed. Then, the test specimens were heated to 880°C, a temperature 20°C higher than the previous temperature, held for the same time, water-cooled, and the grain size was confirmed. This process was repeated up to 1040°C.

[0080] The grain size was confirmed by etching a cross section of a halved specimen in a saturated picric acid solution and observing the grain size under an optical microscope. Grain size numbers of 3.0 or less, using the same evaluation method as for the ferrite grain size number described above, were considered coarse grains, and the heating temperature at which the grain size number first fell below 3.0 was considered the limit temperature for suppressing grain coarsening. A higher limit temperature is preferable, as 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 sufficient suppression of grain coarsening, and a limit temperature of 980°C or higher can be evaluated as even better suppression of coarsening.

[0081] Tables 2 and 3 show the heating conditions and test results for the above spheroidizing annealing. Only the ferrite grain size number is an evaluation of the test material before spheroidizing annealing (SA), while the others are evaluations of the case-hardened steel test material after spheroidizing annealing. Table 2 shows the evaluation results for steel produced under spheroidizing annealing condition 1, and Table 3 shows the evaluation results for steel produced under spheroidizing annealing condition 2. The ferrite grain size number before SA is common to Tables 2 and 3, so it is only shown in Table 2.

[0082] [Table 2]

[0083] [Table 3]

[0084] From the results in Tables 2 and 3, the test materials No. 1 to 22, which are the inventive steels, all have a carbide dispersion index V θ The dispersion index V was 0.80 or less, confirming that the uniformity of the dispersion of spheroidized carbides was excellent. θ was 0.70 or less, and it was confirmed that the carbide dispersion was excellent in uniformity.

[0085] Regarding hardness variation, the value of (coefficient of variation of hardness CV x 100) was 0.65 or less for all of the steels of the invention. Looking at the coefficient of variation of deformation resistance, it was 0.85 or less for all of the steels of the invention, confirming that by having a coefficient of variation of hardness below a predetermined value, the variation of deformation resistance could be further suppressed. Furthermore, for the steels of the invention produced under spheroidizing annealing condition 2 in Table 3, the coefficient of variation of hardness was even smaller, at 0.50 or less. The coefficient of variation of deformation resistance was also even smaller, at 0.65 or less for all of the steels of the invention, confirming that the variation of deformation resistance was further suppressed.

[0086] Furthermore, none of the inventive steels showed cracking in cold upset tests, and the critical temperature for suppressing grain coarsening was 960°C or higher, confirming that they had excellent strength and workability. The inventive steel produced under spheroidizing annealing condition 2 had an even higher critical temperature of 980°C or higher, confirming that it was possible to further suppress grain coarsening.

[0087] On the other hand, the comparative steel has a carbide dispersion index V θ The coefficient of variation of the hardness and grain size was large. As a result, 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.

[0088] The comparative steels Nos. 30 and 31, in which the Cr content exceeds the upper limit value shown in the embodiment, have a carbide dispersion index V θ The hardness was 0.70 or less, but the hardness was high at over 85HRB, and the coefficient of variation of hardness was also large. The large variation in hardness also led to large variation in deformation resistance, and cracks occurred. The coefficient of variation of hardness and the dispersion index of carbides V θ It can be said that both of these conditions must be satisfied. In the comparative steel No. 33, the Cr content is within the range of the content indicated in the embodiment, but the C, Si, and Mn contents exceed the upper limit values, resulting in poor cold forgeability. In addition, since Nb is not added, the critical temperature for suppressing grain coarsening is low, and coarsening could not be suppressed.

Claims

1. In mass%, C: 0.14% or more and 0.35% or less, Si: 0.05% or more and 1.00% or less, Mn: 0.10% or more and 0.90% or less, P: 0.030% or less, S: 0.030% or less, Cr: 1.30% or more and 3.50% or less, Al: 0.020% or more and 0.200% or less, Nb: 0.02% or more and 0.10% or less, and N: 0.0040% or more and 0.0300% or less and the balance being Fe and impurities, Spheroidized annealed, with a structure of ferrite and spheroidized carbide, Dispersion index V, which indicates the dispersibility of spheroidized carbides θ But, V θ ≦0.80, Case-hardened steel characterized in that the coefficient of variation CV of Rockwell hardness in a cross section perpendicular to the rolling direction of test pieces taken at any 10 locations in the rolling direction of the steel satisfies CV × 100 ≦ 0.

65.

2. Instead of a part of the Fe, 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-hardening steel according to claim 1, characterized in that it contains one or more selected from the group consisting of:

3. The dispersion index V θ is an arithmetic average value of dispersion indices obtained by photographing five 50 μm square images of a cross section of a steel material parallel to the rolling direction, and obtaining the images of the five 50 μm square images, The dispersion index calculated for each image is 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 are determined, The ideal coordinates of n spheroidized carbides are determined when the n spheroidized carbides are arranged in a lattice pattern at equal intervals within the image; determining a distance between the ideal coordinate and the closest actual coordinate for each ideal coordinate; The case hardening steel according to claim 1 or 2, characterized in that the value is obtained by dividing the arithmetic mean value of the distance obtained by the length of one side of a unit cell, which is the distance between adjacent ideal coordinates.

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

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

6. 3. The case-hardened steel according to claim 1, wherein the Rockwell hardness of the cross section perpendicular to the rolling direction is 85 HRB or less.

7. The method for manufacturing case hardening steel according to claim 1 or 2, The rolled steel material is heated to a holding temperature T1 [°C] that satisfies the A1 transformation point ≦ T1 ≦ (A1 transformation point + 40°C), and is held for a holding time t [h] that satisfies the 1 hour ≦ t ≦ 6 hours, A spheroidizing annealing process is performed to cool the steel 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.). A method for producing case-hardened steel.

8. The method for manufacturing case-hardened steel according to claim 7, characterized in that the grain size number of ferrite in the structure of the steel material before the spheroidizing annealing step is 8.0 or more.

9. The method for manufacturing case hardening steel according to claim 1 or 2, A method for producing case-hardened steel, characterized in that a spheroidizing annealing process is carried out in which a steel material that has been subjected to rolling processing 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).

10. The method for manufacturing case-hardened steel according to claim 9, characterized in that the grain size number of ferrite in the structure of the steel material before the spheroidizing annealing step is 8.0 or more.

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