Method for manufacturing carburized steel component, and carburized steel component

Abstract

To provide a method for manufacturing a carburized steel component, capable of suppressing excess carburization in the edge part of the carburized steel component.SOLUTION: A method for manufacturing a carburized steel member comprises: the first carburization diffusion step of alternately performing a high temperature carburization stage of holding a shaped steel material at a high carburization temperature of 930-1100°C and a high temperature diffusion stage of holding the shaped steel material at the high carburization temperature in a vacuum or inert gas atmosphere while supplying carburization gas into a vacuum carburization furnace; and the second carburization diffusion step after the first carburization diffusion step, of performing the cooling step of cooling the shaped steel material to a low carburization temperature of 800-880°C in the vacuum carburization furnace and alternately performing the low temperature carburization stage of holding the shaped steel material at the low carburization temperature while supplying the carburization gas and the low temperature diffusion stage of holding the shaped steel material in the vacuum or inert gas atmosphere at the low carburization temperature. A carbon concentration Cs21 (mass%) in the flat part of the shaped steel material after the cooling step is lower by 0.08% or more than a carbon concentration Cs3 (mass%) in the flat part of the shaped steel material after a quenching step.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present disclosure relates to a method for manufacturing a carburized steel part and the carburized steel part, and more particularly to a method for manufacturing a carburized steel part that involves vacuum carburizing treatment and the carburized steel part. [Background technology]

[0002] High surface fatigue strength is required for steel parts, such as gears, shafts, and CVT pulleys. These steel parts are manufactured using a surface hardening process. One example of a surface hardening process is vacuum carburizing. Vacuum carburizing includes a heating process, a carburizing process, and a diffusion process. In the heating process, high-frequency induction heating or an electric heater is used to heat the steel preform, the material for the carburized steel part, to a temperature above the austenite transformation temperature. In the carburizing process, a hydrocarbon gas called carburizing gas is introduced to increase the carbon concentration in the surface layer of the steel preform heated to the carburizing temperature. In the diffusion process, the introduction of carburizing gas is stopped, allowing the carbon to diffuse inside the steel preform. The carbon concentration on the surface of the carburized steel part can be adjusted by adjusting the time of the carburizing and diffusion processes.

[0003] When carburized steel parts are manufactured using vacuum carburizing, the carbon concentration at the edge of the part tends to be higher than at the flat part. This makes the edge of the part prone to over-carburization. Over-carburized parts can develop many Cr-enriched regions due to the precipitation of cementite. The formation of many Cr-enriched regions leads to the formation of Cr-depleted regions. During vacuum carburizing, Cr-depleted regions form incompletely hardened structures that can serve as fracture initiation points. Carburized steel parts containing such incompletely hardened structures have reduced surface fatigue strength. Therefore, carburized steel parts with many edges, such as gears, shafts, and CVT pulleys, require the ability to suppress over-carburization.

[0004] Techniques relating to the suppression of excessive carburization at the edges of carburized steel parts have been proposed in Japanese Patent Application Laid-Open No. 2011-117027 (Patent Document 1), Japanese Patent Application Laid-Open No. 2004-115893 (Patent Document 2), Japanese Patent Application Laid-Open No. 2005-350729 (Patent Document 3), and International Publication No. 2014 / 034150 (Patent Document 4).

[0005] The vacuum carburizing method described in Patent Document 1 involves diffusing carbon from the surface layer of a steel preform into the core by a neutral or reducing gas plasma treatment containing an inert gas, typically argon, after the carburizing treatment. Patent Document 1 states that this vacuum carburizing method can reduce the difference in carbon concentration between the edge and flat parts of a carburized steel part.

[0006] The vacuum carburizing method described in Patent Document 2 involves introducing a decarburizing gas into a furnace after the carburizing process and during the initial stage of the diffusion process to decarburize the surface of the steel preform in the furnace. This vacuum carburizing method decarburizes the over-carburized edge portion through the decarburization process. Patent Document 2 states that this reduces or removes cementite in the surface of the steel preform, thereby suppressing over-carburization.

[0007] In the vacuum carburizing method described in Patent Document 3, a carburizing flow rate V1 is supplied in the early stage of the carburizing process, which is sufficiently higher than the theoretical flow rate of carburizing gas required for the carburizing treatment and which does not cause sooting. Furthermore, a diffusion flow rate V2, which is lower than the theoretical flow rate V, is supplied in the later stage of carburizing. Patent Document 3 states that this vacuum carburizing method can significantly reduce the amount of carburizing gas consumed, prevent excessive carburization of the surface of steel shapes, and prevent the formation of cementite, which has an adverse effect on the carburized hardened layer.

[0008] In the vacuum carburizing process described in Patent Document 4, after high-temperature carburizing, the material is cooled to 500°C or below and then reheated and quenched. Patent Document 4 states that this makes excessive carburization harmless and prevents the remaining coarse cementite that adversely affects the carburized hardened layer. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-117027 [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-115893 [Patent Document 3] Japanese Patent Application Laid-Open No. 2005-350729 [Patent Document 4] International Publication No. 2014 / 034150 Summary of the Invention [Problem to be solved by the invention]

[0010] However, other means than those disclosed in Patent Documents 1 to 4 may be used to suppress excessive carburization of the edge portion of the carburized steel part.

[0011] An object of the present disclosure is to provide a method for manufacturing a carburized steel part that can prevent excessive carburization of the edge portion of the carburized steel part, and the carburized steel part. [Means for solving the problem]

[0012] The method of manufacturing a carburized steel component of the present disclosure includes: a first carburizing diffusion process in which steel blanks, which are the raw materials for carburized steel parts, are carburized in a vacuum carburizing furnace; a second carburizing-diffusion step of carburizing the steel shape material in the vacuum carburizing furnace after the first carburizing-diffusion step; a quenching step of cooling the steel material to 300°C or less using a refrigerant after the second carburizing / diffusion step, The first carburization diffusion step includes: a heating step of heating the steel material in the vacuum carburizing furnace to a high carburizing temperature in the range of 930 to 1100°C; a high-temperature soaking step of soaking the steel shape material at the high-temperature carburizing temperature after the heating step; a high-temperature carburizing and diffusion process for performing a carburizing treatment and a diffusion treatment on the steel shape material after the high-temperature soaking process, The high-temperature carburization diffusion process includes: a high-temperature carburizing period in which the steel workpiece is maintained at the high-temperature carburizing temperature while a carburizing gas is supplied into the vacuum carburizing furnace; a high-temperature diffusion period in which the supply of the carburizing gas into the vacuum carburizing furnace is stopped and the steel preform is maintained at the high-temperature carburizing temperature in a vacuum of 10 Pa or less or in an inert gas atmosphere of 100 kPa or less, and The second carburizing and diffusion step is a temperature-reducing step of reducing the temperature of the steel material in the vacuum carburizing furnace to a low-temperature carburizing temperature in the range of 800 to 880°C; a low-temperature soaking step of soaking the steel material at the low-temperature carburizing temperature after the temperature-lowering step; a low-temperature carburizing and diffusion process for performing a carburizing treatment and a diffusion treatment on the steel shape material after the low-temperature soaking process, The low-temperature carburization diffusion process includes: a low-temperature carburizing period in which the steel workpiece is maintained at the low-temperature carburizing temperature while the carburizing gas is supplied into the vacuum carburizing furnace; a low-temperature diffusion period in which the supply of the carburizing gas into the vacuum carburizing furnace is stopped and the steel material is maintained at the low-temperature carburizing temperature in a vacuum of 10 Pa or less or in an inert gas atmosphere of 100 kPa or less, and The carbon concentration C of the flat portion of the steel material after the temperature decreasing process s21 (mass%) is the carbon concentration C of the flat portion of the steel material after the quenching process. s3 (mass%) is more than 0.08% lower.

[0013] The carburized steel components of the present disclosure include: It includes a flat portion and an edge portion, a carburized hardened layer formed on the surface layer of the carburized steel part; a core portion located inside the carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.10~0.25%, Si: 0.02 to 2.00% Mn: 0.30~2.50% P: 0.030% or less, S: 0.060% or less, Cr: 0.30~3.00%, Al: 0.010~0.060%, N: 0.025% or less, Cu: 0-0.50% Ni: 0 to 1.00% Mo: 0-1.00%, V: 0~0.20%, Sn: 0 to 0.10% Sb: 0 to 0.02% Nb: 0 to 0.10% Ti: 0 to 0.10%, and B: 0 to 0.0040%, The balance is Fe and impurities. In the edge surface region, which is a region extending from the surface of the edge to a depth of 80 μm, the area ratio of the Cr-enriched region is 1.00% or less. [Effects of the Invention]

[0014] The manufacturing method of the carburized steel part of the present disclosure can suppress excessive carburization of the edge portion of the carburized steel part. In the carburized steel part of the present disclosure, excessive carburization of the edge portion is suppressed. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a perspective view showing an example of a carburized steel part. [Figure 2] FIG. 2 is a perspective view showing an example of a carburized steel part different from that shown in FIG. [Figure 3] FIG. 3 is a schematic diagram of the cross section CS in FIGS. [Figure 4] FIG. 4 is a heat pattern diagram showing the steps of the method for manufacturing a carburized steel part according to this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0016] The present inventors have conducted research and studies to solve the above-mentioned problems, and as a result have made the following findings. (A) In the vacuum carburizing process, the carburizing and diffusion processes are carried out at 930 to 1100°C. Then, before the quenching process, the temperature is lowered to 800 to 880°C to adjust the carbon concentration of the surface of the steel preform to about 0.8% by mass, and then quenching is carried out. In this case, excessive carburization occurs at the edges of the carburized steel part. (B) On the other hand, in vacuum carburizing, the carbon concentration in the flat areas of the steel preform is reduced by adjusting the carburizing and diffusion process times at 930-1100°C. In this case, even if the temperature is lowered to 800-880°C after the carburizing and diffusion processes, significant over-carburization is unlikely to occur in the edge areas of the steel preform. However, in this case, a sufficient carbon concentration may not be obtained in the flat areas of the carburized steel part. (C) Carburizing and diffusion processes are carried out at 930-1100°C, followed by another process at 800-880°C. In other words, the carburizing and diffusion processes are carried out separately. In this case, Cr does not concentrate at the edge of the steel preform, making excessive carburization unlikely. Furthermore, sufficient carbon concentration is obtained even in the flat areas of the carburized steel part.

[0017] The method for manufacturing a carburized steel part and the carburized steel part of this embodiment, which have been completed based on the above findings, have the following configurations.

[0018] A method for manufacturing a carburized steel component of the first configuration includes: a first carburizing diffusion process in which steel blanks, which are the raw materials for carburized steel parts, are carburized in a vacuum carburizing furnace; a second carburizing-diffusion step of carburizing the steel shape material in the vacuum carburizing furnace after the first carburizing-diffusion step; a quenching step of cooling the steel material to 300°C or less using a refrigerant after the second carburizing / diffusion step, The first carburization diffusion step includes: a heating step of heating the steel material in the vacuum carburizing furnace to a high carburizing temperature in the range of 930 to 1100°C; a high-temperature soaking step of soaking the steel shape material at the high-temperature carburizing temperature after the heating step; a high-temperature carburizing and diffusion process for performing a carburizing treatment and a diffusion treatment on the steel shape material after the high-temperature soaking process, The high-temperature carburization diffusion process includes: a high-temperature carburizing period in which the steel workpiece is maintained at the high-temperature carburizing temperature while a carburizing gas is supplied into the vacuum carburizing furnace; a high-temperature diffusion period in which the supply of the carburizing gas into the vacuum carburizing furnace is stopped and the steel preform is maintained at the high-temperature carburizing temperature in a vacuum of 10 Pa or less or in an inert gas atmosphere of 100 kPa or less, and The second carburizing and diffusion step is a temperature-reducing step of reducing the temperature of the steel material in the vacuum carburizing furnace to a low-temperature carburizing temperature in the range of 800 to 880°C; a low-temperature soaking step of soaking the steel material at the low-temperature carburizing temperature after the temperature-lowering step; a low-temperature carburizing and diffusion process for performing a carburizing treatment and a diffusion treatment on the steel shape material after the low-temperature soaking process, The low-temperature carburization diffusion process includes: a low-temperature carburizing period in which the steel workpiece is maintained at the low-temperature carburizing temperature while the carburizing gas is supplied into the vacuum carburizing furnace; a low-temperature diffusion period in which the supply of the carburizing gas into the vacuum carburizing furnace is stopped and the steel material is maintained at the low-temperature carburizing temperature in a vacuum of 10 Pa or less or in an inert gas atmosphere of 100 kPa or less, and The carbon concentration C of the flat portion of the steel material after the temperature decreasing process s21 (mass%) is the carbon concentration C of the flat portion of the steel material after the quenching process. s3 (mass%) is more than 0.08% lower.

[0019] A method for manufacturing a carburized steel component of the second configuration comprises: 1. A method for manufacturing a carburized steel component of a first configuration, comprising: The treatment time of the low-temperature carburization diffusion step is set to be 0.03 to 0.30 times the treatment time of the high-temperature carburization diffusion step.

[0020] A method for manufacturing a carburized steel part of the third configuration comprises: A method for manufacturing a carburized steel component of a first or second configuration, comprising: The chemical composition of the steel material is, in mass%, C: 0.10~0.25%, Si: 0.02 to 2.00% Mn: 0.30~2.50% P: 0.030% or less, S: 0.060% or less, Cr: 0.30~3.00%, Al: 0.010~0.060%, N: 0.025% or less, Cu: 0-0.50% Ni: 0 to 1.00% Mo: 0-1.00%, V: 0~0.20%, Sn: 0 to 0.10% Sb: 0 to 0.02% Nb: 0 to 0.10% Ti: 0 to 0.10%, and B: 0 to 0.0040%, The balance consists of Fe and impurities.

[0021] The carburized steel parts of the first configuration are It includes a flat portion and an edge portion, a carburized hardened layer formed on the surface layer of the carburized steel part; a core portion located inside the carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.10~0.25%, Si: 0.02 to 2.00% Mn: 0.30~2.50% P: 0.030% or less, S: 0.060% or less, Cr: 0.30~3.00%, Al: 0.010~0.060%, N: 0.025% or less, Cu: 0-0.50% Ni: 0 to 1.00% Mo: 0-1.00%, V: 0~0.20%, Sn: 0 to 0.10% Sb: 0 to 0.02% Nb: 0 to 0.10% Ti: 0 to 0.10%, and B: 0 to 0.0040%, The balance is Fe and impurities. In the edge surface region, which is a region extending from the surface of the edge to a depth of 80 μm, the area ratio of the Cr-enriched region is 1.00% or less.

[0022] The second configuration of carburized steel parts is 1. A carburized steel component of a first configuration, comprising: The chemical composition of the core is, in mass%, Cu: 0.01 to 0.50%, Ni: 0.01 to 1.00%, Mo: 0.01 to 1.00%, V: 0.01 to 0.20%, Sn: 0.01 to 0.10% Sb: 0.01 to 0.02%, Nb: 0.01 to 0.10%, Ti: 0.01 to 0.10%, and B: 0.0001 to 0.0040%.

[0023] The method for manufacturing the carburized steel part and the carburized steel part according to this embodiment will be described below.

[0024] [Shapes of steel materials and carburized steel parts] First, the carburized steel parts and the steel blanks that are the raw materials for the carburized steel parts will be described. The carburized steel part comprises a carburized hardened layer and a core. The carburized hardened layer is formed on the surface of the carburized steel part. The core is the part of the carburized steel part that is deeper than the carburized hardened layer. The carbon introduced into the carburized steel part by vacuum carburizing has not diffused into the core. Therefore, the chemical composition of the core is the same as that of the steel preform.

[0025] Fig. 1 is a perspective view showing an example of a carburized steel part. In Fig. 1, the carburized steel part 100 has a cylindrical shape, as an example. Fig. 2 is a perspective view showing an example of a carburized steel part 100 different from that shown in Fig. 1. The carburized steel part 100 shown in Fig. 2 is assumed to be a gear.

[0026] 1 and 2, the carburized steel component 100 further includes a flat portion 3 and an edge portion 4. The flat portion 3 is a flat region of the carburized steel component 100. The edge portion 4 is a region of the carburized steel component 100 that includes a corner.

[0027] In this specification, the flat portion 3 and the edge portion 4 are defined as follows: With reference to Figures 1 and 2, attention is focused on side 2 of the end face of the carburized steel part 100. A point located at an arbitrary position on side 2 is defined as point Pc. Then, as shown in Figures 1 and 2, a cross section CS is imagined that includes point Pc and is perpendicular to the tangent line at point Pc on side 2.

[0028] FIG. 3 is a schematic diagram of cross section CS in FIGS. 1 and 2. Referring to FIG. 3, on cross section CS, an imaginary point P is assumed to be 1.0 mm deep from an arbitrary point XP on the surface of the carburized steel component 100 in the direction perpendicular to the surface (to the tangent at point XP). A cluster 15 of imaginary points P is indicated by a dashed line in FIG. 3. An imaginary circle with a radius of 1.0 mm is assumed to be centered at imaginary point P. When this imaginary sphere is tangent to the surface of the carburized steel component 100 (i.e., the imaginary circle and the surface of the carburized steel component 100 on cross section CS intersect at a single point), the imaginary point P is designated as point P1. The point of contact between the imaginary circle centered at point P1 and the surface of the carburized steel component 100 is designated as point XP1. The area of ​​the surface of the carburized steel component 100 defined by point XP1 is defined as flat portion 3.

[0029] Furthermore, the portion of the surface of the carburized steel part 100 other than the flat portion 3, that is, the region of the side on the cross section CS in FIG.

[0030] In the edge portion 4, the closer to the boundary with the flat portion 3, the less likely excessive carburization occurs. On the other hand, in the edge portion 4, the farther from the boundary with the flat portion 3 (i.e., the region near point Pc in Figure 3), the more likely excessive carburization occurs.

[0031] The steel preform, which is the raw material for the carburized steel part 100, has the same shape as the carburized steel part 100. Therefore, the steel preform also includes a flat portion 3 and an edge portion 4. The definitions of the flat portion 3 and the edge portion 4 in the steel preform are the same as those of the flat portion 3 and the edge portion 4 in the carburized steel part 100.

[0032] The chemical composition of the steel preform and the chemical composition of the core of the carburized steel part are not particularly limited. It is sufficient to use a steel preform having a known chemical composition for which carburizing treatment is performed. The chemical composition of the steel preform and the chemical composition of the core of the carburized steel part 100 may be, for example, a chemical composition equivalent to that of alloy steel for machine structures as specified in JIS G 4053:2018. The chemical composition of the steel preform and the chemical composition of the core of the carburized steel part 100 are, for example, SCr415, SCr420, SCM415, and the like as specified in JIS G 4053:2018.

[0033] [Example of chemical composition of steel materials] The chemical composition of the steel preform and the chemical composition of the core of the carburized steel part may contain, for example, the following elements: In the following description, "%" regarding the content of an element means mass %.

[0034] C: 0.10 to 0.25% Carbon (C) increases the core hardness of carburized steel parts. If the C content is too low, the above effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. If the C content is too low, it becomes necessary to further increase the C concentration increased by carburization during the vacuum carburizing process. In this case, excessive carburization is likely to occur at the edge of the carburized steel part. On the other hand, if the C content is too high, the core hardness of the carburized steel part will be excessively high, and in this case, the toughness of the carburized steel part will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the C content is 0.10 to 0.25%. The lower limit of the C content is preferably 0.11%, more preferably 0.12%, and even more preferably 0.13%. The upper limit of the C content is preferably 0.24%, more preferably 0.23%, and even more preferably 0.22%.

[0035] Si: 0.02 to 2.00% Silicon (Si) suppresses the precipitation of cementite and the occurrence of excessive carburization. If the Si content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content is too high, soft ferrite will form in the core of the carburized steel part, reducing the hardness of the core even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.02 to 2.00%. The lower limit of the Si content is preferably 0.05%, more preferably 0.08%, and even more preferably 0.10%. The upper limit of the Si content is preferably 1.80%, more preferably 1.10%, and even more preferably 0.50%.

[0036] Mn: 0.30 to 2.50% Manganese (Mn) improves hardenability and increases the hardness of carburized steel parts. If the Mn content is too low, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content is too high, an excessive amount of retained austenite will remain in the carburized steel part, which will reduce the hardness of the carburized steel part even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 0.30 to 2.50%. The lower limit of the Mn content is preferably 0.35%, more preferably 0.40%, and even more preferably 0.45%. The upper limit of the Mn content is preferably 2.20%, more preferably 2.00%, and even more preferably 1.50%.

[0037] P:0.030% or less Phosphorus (P) is an impurity. P segregates at grain boundaries and embrittles them. Therefore, if the P content is too high, the bending fatigue strength and surface fatigue strength of the carburized steel part will decrease even if the contents of other elements are within the ranges specified in this embodiment. Therefore, the P content is 0.030% or less. The P content is preferably as low as possible. However, excessive reduction in the P content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the P content is preferably more than 0%, more preferably 0.001%, even more preferably 0.003%, and even more preferably 0.006%. The upper limit of the P content is preferably 0.025%, more preferably 0.023%, and even more preferably 0.020%.

[0038] S: 0.060% or less Sulfur (S) is an impurity. S improves the machinability of steel. However, if the S content is too high, the fatigue strength (bending fatigue strength and surface fatigue strength) of the carburized steel part will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the S content is 0.060% or less. The S content is preferably as low as possible. However, excessive reduction in the S content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the S content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%. The upper limit of the S content is preferably 0.050%, more preferably 0.040%, and even more preferably 0.030%.

[0039] Cr: 0.30~3.00% Chromium (Cr) improves the hardenability of steel and increases the hardness of carburized steel parts. If the Cr content is too low, these effects cannot be fully achieved. On the other hand, if the Cr content is too high, excessive carburization is likely to occur even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cr content is 0.30 to 3.00%. The lower limit of the Cr content is preferably 0.40%, more preferably 0.50%, and even more preferably 0.60%. The upper limit of the Cr content is preferably 2.70%, more preferably 2.50%, and even more preferably 2.00%.

[0040] Al: 0.010 to 0.060% Aluminum (Al) combines with N to form AlN, which suppresses grain coarsening, thereby increasing the fatigue strength of carburized steel parts. If the Al content is too low, the above effect cannot be fully achieved. On the other hand, if the Al content is too high, excessive coarse oxides are formed, which reduces the fatigue strength of the carburized steel part even if the contents of other elements are within the ranges of this embodiment. Therefore, the Al content is 0.010 to 0.060%. The lower limit of the Al content is preferably 0.012%, more preferably 0.015%, and even more preferably 0.020%. The upper limit of the Al content is preferably 0.050%, more preferably 0.040%, and even more preferably 0.030%.

[0041] N: 0.025% or less Nitrogen (N) combines with Al to form AlN, which inhibits grain coarsening. This increases the fatigue strength of carburized steel parts. Even a small amount of N can achieve this effect to some extent. However, if the N content is too high, the effect saturates. Therefore, the N content is 0.025% or less. The N content is preferably as low as possible. However, excessive reduction in the N content increases production costs. Therefore, taking into consideration normal industrial production, the lower limit of the N content is preferably more than 0%, more preferably 0.001%, even more preferably 0.005%, and even more preferably 0.007%. The upper limit of the N content is preferably 0.023%, more preferably 0.020%, even more preferably 0.015%, and still more preferably 0.011%.

[0042] The balance of the chemical composition of the steel preform and the balance of the chemical composition of the core of the carburized steel part in this embodiment are composed of Fe and impurities. Here, the term "impurities" refers to substances that are mixed in from raw materials such as ore and scrap or from the manufacturing environment during the industrial production of steel, which is the raw material for the steel preform and the carburized steel part, and that are acceptable within a range that does not adversely affect the core of the steel preform and the carburized steel part.

[0043] [Optional element] The chemical composition of the core of the above-mentioned carburized steel part and steel preform may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Cu: 0-0.50%, Ni: 0-1.00%, Mo: 0-1.00%, V: 0-0.20%, Sn: 0-0.10%, Sb: 0-0.02%, Nb: 0-0.10%, Ti: 0-0.10%, and B: 0-0.0040%. All of these elements are optional.

[0044] Cu: 0 to 0.50% Copper (Cu) is an optional element and may not be contained, that is, the Cu content may be 0%. When copper (Cu) is contained, that is, when the Cu content is more than 0%, it improves the hardenability of the steel material and increases the hardness of the carburized steel part. Even if even a small amount of Cu is contained, the above effects can be obtained to some extent. However, if the Cu content is too high, the hot workability of the steel material used to make the steel preform will be reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0 to 0.50%, and when Cu is contained, the Cu content is 0.50% or less. The lower limit of the Cu content is preferably 0.01%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the Cu content is preferably 0.45%, more preferably 0.40%, even more preferably 0.35%, and still more preferably 0.30%.

[0045] Ni: 0 to 1.00% Nickel (Ni) is an optional element and may not be contained, that is, the Ni content may be 0%. When Ni is contained, that is, when the Ni content is more than 0%, Ni increases the toughness and fatigue strength of the carburized steel part. Even if even a small amount of Ni is contained, these effects can be obtained to some extent. However, if the Ni content is too high, the effect saturates and the manufacturing cost increases. Therefore, the Ni content is 0 to 1.00%, and when Ni is contained, it is 1.00% or less. The lower limit of the Ni content is preferably 0.01%, more preferably 0.05%, and even more preferably 0.10%. The upper limit of the Ni content is preferably 0.90%, more preferably 0.80%, even more preferably 0.70%, even more preferably 0.60%, and even more preferably 0.50%.

[0046] Mo: 0 to 1.00% Molybdenum (Mo) is an optional element and may not be contained, that is, the Mo content may be 0%. When contained, that is, when the Mo content is more than 0%, Mo increases the toughness and fatigue strength of the carburized steel part. Even if even a small amount of Mo is contained, these effects can be obtained to some extent. However, if the Mo content exceeds 1.00%, the effect saturates and the production cost increases. Therefore, the Mo content is 0 to 1.00%, and if contained, it is 1.00% or less. The lower limit of the Mo content is preferably 0.01%, more preferably 0.02%, even more preferably 0.10%, and still more preferably 0.25%. The upper limit of the Mo content is preferably 0.70%, more preferably 0.50%, even more preferably 0.30%, and still more preferably 0.25%.

[0047] V: 0 to 0.20% Vanadium (V) is an optional element and may not be contained, that is, the V content may be 0%. When V is contained, if the V content exceeds 0.20%, excessive carburization may occur even if the contents of other elements are within the ranges of this embodiment. Therefore, the V content is 0 to 0.20%. The lower limit of the V content is preferably 0.01%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the Cr content is preferably 0.16%, more preferably 0.14%, and even more preferably 0.12%.

[0048] Sn: 0 to 0.10% Tin (Sn) is an optional element and may not be contained. In other words, the Sn content may be 0%. Sn is a so-called tramp element contained in scrap, etc. If the Sn content is too high, Sn will segregate excessively at grain boundaries. In this case, even if the contents of other elements are within the ranges of this embodiment, the carburized steel part will become embrittled and its fatigue strength will decrease. Therefore, the Sn content is 0 to 0.10%, and if Sn is contained, it is 0.10% or less. The lower limit of the Sn content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the Sn content is preferably 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0049] Sb: 0 to 0.02% Antimony (Sb) is an optional element and may not be contained, that is, the Sb content may be 0%. When Sb is contained, that is, when the Sb content is more than 0%, Sb suppresses grain boundary oxidation and surface cracking during the hot rolling or hot forging process. Even if even a small amount of Sb is contained, the above effects can be obtained to some extent. However, if the Sb content is too high, the toughness and fatigue strength of the carburized steel part will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sb content is 0 to 0.02%, and if contained, it is 0.02% or less. The preferred upper limit of the Sb content is 0.01%.

[0050] Nb: 0 to 0.10% Niobium (Nb) is an optional element and may not be contained, that is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content is more than 0%, Nb forms precipitates and has a pinning effect that suppresses grain coarsening. This increases the toughness and fatigue strength of carburized steel parts. Even if only a small amount of Nb is contained, the above effects can be obtained to some extent. However, if the Nb content is too high, the effect becomes saturated. Therefore, the Nb content is 0 to 0.10%, and if Nb is contained, it is 0.10% or less. The lower limit of the Nb content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the Nb content is preferably 0.09%, more preferably 0.07%, and even more preferably 0.06%.

[0051] Ti: 0 to 0.10% Titanium (Ti) is an optional element and may not be contained, that is, the Ti content may be 0%. When Ti is contained, that is, when the Ti content is more than 0%, Ti forms precipitates and has a pinning effect that suppresses grain coarsening. This increases the toughness and fatigue strength of carburized steel parts. Even if Ti is contained even a small amount, the above effects can be obtained to some extent. However, if the Ti content is too high, the effect becomes saturated. Therefore, the Ti content is 0 to 0.10%, and if contained, it is 0.10% or less. The lower limit of the Ti content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the Ti content is preferably 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0052] B: 0 to 0.0040% Boron (B) is an optional element and may not be contained, that is, the B content may be 0%. When B is contained, that is, when the B content exceeds 0%, B segregates at the grain boundaries to strengthen the grain boundaries and increase the fatigue strength of the carburized steel parts. Even if even a small amount of B is contained, the above effect can be obtained to some extent. However, if the B content exceeds 0.0040%, this effect becomes saturated. Therefore, the B content is 0 to 0.0040%. The lower limit of the B content is preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The upper limit of the B content is preferably 0.0035%, more preferably 0.0030%, and even more preferably 0.0025%.

[0053] The method for manufacturing a carburized steel part of this embodiment using the above-described steel preform will now be described in detail.

[0054] Fig. 4 is a heat pattern diagram showing the steps of the method for manufacturing a carburized steel part of this embodiment. Referring to Fig. 4, the method for manufacturing a carburized steel part of this embodiment includes a first carburization-diffusion step (S1), a second carburization-diffusion step (S2), and a quenching step (S3). That is, in the method for manufacturing a carburized steel part of this embodiment, the carburization-diffusion step is divided into two stages, and then the quenching step is carried out.

[0055] The first carburizing / diffusion step, which is the first of the two separate steps, involves carburizing and diffusing at a high carburizing temperature where the carbon diffusion rate is fast to ensure the desired carbon penetration depth. Furthermore, a high-temperature diffusion period is performed to sufficiently reduce the carbon concentration on the flat surface to prevent Cr-enriched regions from remaining at the edges of the carburized steel part. Meanwhile, the second carburizing / diffusion step, which follows the first carburizing / diffusion step, involves carburizing and diffusing at a low carburizing temperature where the Cr diffusion rate is slow. This prevents excessive carburization at the edges. Each step of the method for manufacturing a carburized steel part according to this embodiment will be described in detail below.

[0056] [First carburizing and diffusion process (S1)] Referring to FIG. 4, the first carburization-diffusion step (S1) includes a heating step (S11), a high-temperature soaking step (S12), and a high-temperature carburization-diffusion step (S13).

[0057] [Heating process (S11)] In the heating step, the steel preform is heated to a high carburizing temperature T1 in a vacuum carburizing furnace. Specifically, the steel preform is first inserted into the vacuum carburizing furnace. The steel preform may be provided by a third party or manufactured by the person performing the vacuum carburizing process. The prepared steel preform may be a steel preform shaped by hot working, or may be a steel preform shaped by cold working. Examples of hot working include hot rolling, hot extrusion, and hot forging. Examples of cold working include cold rolling, cold drawing, and cold forging. The steel preform may be one that has been subjected to machining, typically cutting, after hot working or cold working.

[0058] In the heating step (S11), the steel preform is placed in a vacuum carburizing furnace and heated to the high-temperature carburizing temperature T1 (°C). The pressure inside the vacuum carburizing furnace during the heating step is not particularly limited. Preferably, the pressure inside the vacuum carburizing furnace is 100 Pa or less. In this case, oil and other contaminants adhering to the steel preform are easily removed. If oil and other contaminants are sufficiently removed from the surface of the steel preform before the steel preform is placed in the vacuum carburizing furnace, the atmosphere inside the vacuum carburizing furnace may be an inert gas atmosphere of 100 kPa or less. Here, the inert gas is a gas that does not easily react with the steel preform. Examples of the inert gas include one or more gases selected from the group consisting of nitrogen, hydrogen, helium, and argon. The inert gas atmosphere inside the vacuum carburizing furnace prevents the alloy components in the steel preform from forming oxides. This allows for a faster heating rate during the heating step.

[0059] [High temperature carburizing temperature T1 (℃)] The high-temperature carburizing temperature T1 (°C) is set in the range of 930 to 1100°C. If the high-temperature carburizing temperature T1 is too low, the time required for vacuum carburizing will be long. Furthermore, in the first carburizing diffusion process, Cr concentrated at the edge of the steel preform may not be sufficiently diffused and may remain at the edge. On the other hand, if the high-temperature carburizing temperature T1 (°C) is too high, the time required for vacuum carburizing will be shorter, but the crystal grains will become coarse and the fatigue strength of the carburized steel part will decrease. Therefore, the high-temperature carburizing temperature T1 is set to 930 to 1100°C. The lower limit of the high-temperature carburizing temperature T1 is preferably 940°C, and more preferably 950°C. The upper limit of the high-temperature carburizing temperature T1 is preferably 1080°C, and more preferably 1060°C.

[0060] [High temperature soaking process (S12)] In the high-temperature soaking step (S12), the steel preform after the heating step is soaked at a high-temperature carburizing temperature T1 (°C). The soaking time (holding time) at the high-temperature carburizing temperature T1 is, for example, 10 to 240 minutes. In the high-temperature soaking step, temperature variation in the steel preform is suppressed. Preferably, in the high-temperature soaking step, the temperature variation in the steel preform is suppressed to within 20°C. As described above, by setting the soaking time at the high-temperature carburizing temperature T1 to 10 to 240 minutes, the temperature variation in the steel preform can be suppressed to within 20°C.

[0061] The pressure inside the vacuum carburizing furnace during the high-temperature soaking step may be within the same range as that during the heating step. Preferably, the pressure inside the vacuum carburizing furnace during the high-temperature soaking step is 100 Pa or less. In this case, oil and other substances adhering to the steel preform are easily removed. If oil and other substances are sufficiently removed from the surface of the steel preform before the steel preform is charged into the vacuum carburizing furnace, the atmosphere inside the vacuum carburizing furnace may be an inert gas atmosphere of 100 kPa or less.

[0062] [High-temperature carburizing diffusion process (S13)] After the high-temperature soaking step, a high-temperature carburizing diffusion step (S13) is carried out. In the high-temperature carburizing diffusion step, a high-temperature carburizing period and a high-temperature diffusion period are alternately carried out at least once. The high-temperature carburizing period and the high-temperature diffusion period will be described below.

[0063] [High temperature carburizing period] During the high-temperature carburizing period, the steel preform is maintained at the high-temperature carburizing temperature T1 (°C) while carburizing gas is supplied into the vacuum carburizing furnace. The high-temperature carburizing period refers to the process of supplying carburizing gas into the furnace under vacuum or reduced pressure as described above. In other words, the high-temperature carburizing period begins when the supply of carburizing gas begins into the vacuum or reduced-pressure furnace after the high-temperature soaking process. During the high-temperature carburizing period, the pressure inside the furnace changes depending on the carburizing gas flow rate and the exhaust speed of the vacuum pump.

[0064] The flow rate of the carburizing gas can be set appropriately depending on the desired carbon concentration distribution, surface area, temperature, and chemical composition of the steel material in the steel preform. The exhaust speed of the vacuum pump is adjusted depending on the capacity of the vacuum pump, the inner diameter of the piping, the furnace pressure, and other factors. The furnace pressure during the high-temperature carburizing stage is not particularly limited. A preferred furnace pressure is 1.3 kPa or less. In this case, soot generation is suppressed. Examples of the carburizing gas include acetylene, methane, propane, and ethane. Preferably, the carburizing gas is acetylene.

[0065] The time for the high-temperature carburizing period can be appropriately set depending on the desired carbon concentration distribution in the steel material, the temperature, the chemical composition of the steel material, and the like.

[0066] [High temperature diffusion period] In the high-temperature diffusion period, the supply of carburizing gas to the vacuum carburizing furnace is stopped, and the steel material is held in a vacuum of 10 Pa or less or in an inert gas atmosphere of 100 kPa or less at the high-temperature carburizing temperature T1 (°C). In other words, when the supply of carburizing gas to the vacuum carburizing furnace is stopped, the high-temperature carburizing period transitions to the high-temperature diffusion period.

[0067] If the furnace atmosphere during the high-temperature diffusion period is a vacuum of 10 Pa or less, the alloy components of the steel preform are less likely to oxidize. Also, if the furnace atmosphere during the high-temperature diffusion period is an inert gas atmosphere of 100 kPa or less, the alloy components of the steel preform are less likely to form oxides and nitrides, which are factors that reduce fatigue strength. Therefore, the furnace atmosphere during the high-temperature diffusion period is set to a vacuum of 10 Pa or less, or an inert gas atmosphere of 100 kPa or less.

[0068] The inert gas atmosphere can be maintained by continuously or intermittently evacuating the furnace with a vacuum pump and introducing an inert gas. Continuous evacuation at an inert gas atmosphere of 100 Pa or more makes it easier to purify the furnace atmosphere. Furthermore, when continuously evacuating the furnace with a vacuum pump, if the pressure exceeds 1000 Pa, the load on the vacuum pump increases. Therefore, the preferred furnace atmosphere during the high-temperature diffusion period is an inert gas atmosphere of 100 Pa or more and 1000 Pa or less.

[0069] The holding time in the high-temperature diffusion step is appropriately set depending on the target carbon concentration distribution in the surface layer of the steel material after the high-temperature carburizing stage, and therefore is not particularly limited.

[0070] [Alternating high-temperature carburizing and high-temperature diffusion periods in the high-temperature carburizing and diffusion process] In the high-temperature carburization diffusion step, the above-mentioned high-temperature carburization period and high-temperature diffusion period are alternately carried out at least once.

[0071] Increasing the number of times the high-temperature carburizing period and the high-temperature diffusion period are alternately performed makes it easier to prevent excessive carburization at the edge of the carburized steel part. On the other hand, increasing the number of times the high-temperature carburizing period and the high-temperature diffusion period are alternately performed increases the time for the first carburizing-diffusion step. Therefore, the number of times the high-temperature carburizing period and the high-temperature diffusion period are alternately performed is preferably three or less times, more preferably two times, and even more preferably once.

[0072] [Time for high-temperature carburization and high-temperature diffusion periods] If the high-temperature carburizing time is extended, the carbon diffusion depth in the steel preform increases. As a result, the hardening depth of the carburized steel part tends to increase. On the other hand, if the high-temperature carburizing time is extended, the carbon concentration in the surface layer of the steel preform becomes excessively high, and it becomes more likely that a significant Cr-enriched region will remain at the edge.

[0073] Increasing the time of the high-temperature diffusion period increases the carbon diffusion depth in the steel preform. As a result, the hardening depth of the carburized steel part tends to increase. Furthermore, the carbon concentration in the surface layer decreases due to diffusion, making it difficult for the Cr-enriched region at the edge to remain.

[0074] [Time settings for the high-temperature carburizing and high-temperature diffusion periods in the high-temperature carburizing and diffusion process] The duration of the high-temperature carburizing period and the duration of the high-temperature diffusion period in the high-temperature carburizing diffusion step can be appropriately set so that the surface carbon concentration and the carbon concentration at a predetermined depth from the surface of the steel preform after the temperature-lowering step (S21) and before the low-temperature soaking step (S22) described below will be desired values. The surface layer region from the surface to a predetermined depth is a region that requires a higher hardness than the core. The carbon concentration at a predetermined depth required for the surface layer region to have a predetermined hardness or higher can be determined in advance by performing a vacuum carburizing treatment.

[0075] The carbon concentration of the surface of the flat portion of the steel preform after the temperature-lowering step (S21) and before the low-temperature soaking step (S22) can be obtained by conducting a vacuum carburizing test in which the same steps as in this embodiment are carried out after the temperature-lowering step (S21) and before the low-temperature soaking step (S22), followed by quenching and determining the carbon concentration of the surface of the flat portion of the steel preform. This allows the carbon concentration at a predetermined depth required for the surface region to achieve a predetermined hardness or higher to be determined in advance by performing a carburizing treatment. The times of the high-temperature carburizing period and the high-temperature diffusion period are appropriately set to prevent excessive carburization at the edge of the steel preform after the temperature-lowering step. Instead of an actual vacuum carburizing test, a well-known diffusion simulation can be performed to determine the carbon concentration in the flat surface layer of the steel workpiece after the temperature-lowering step (S21) and before the low-temperature soaking step (S22).

[0076] [Second carburizing diffusion process (S2)] Referring to FIG. 4, the second carburization-diffusion step (S2) includes a temperature-lowering step (S21), a low-temperature soaking step (S22), and a low-temperature carburization-diffusion step (S23).

[0077] [Temperature cooling process (S21)] After the first carburizing diffusion step, in the temperature-reducing step (S21), the temperature of the steel material is reduced from the high-temperature carburizing temperature T1 (°C) to the low-temperature carburizing temperature T2 (°C) in the vacuum carburizing furnace. The pressure in the vacuum carburizing furnace in the temperature-reducing step is not particularly limited. Preferably, the pressure in the vacuum carburizing furnace is a vacuum of 100 Pa or less, or the atmosphere in the vacuum carburizing furnace is an inert gas atmosphere of 100 kPa or less. The cooling rate (temperature-reducing rate) in the temperature-reducing step is not particularly limited. If the cooling rate is reduced, the low-temperature soaking step can be shortened.

[0078] [About low-temperature carburizing temperature T2] The low-temperature carburizing temperature T2 is set to 800 to 880°C. If the low-temperature carburizing temperature T2 is low, the time required for the carburizing process will be long. Furthermore, in the second carburizing diffusion process, ferrite may form in the core of the carburized steel part, which may reduce the fatigue strength of the carburized steel part. On the other hand, if the low-temperature carburizing temperature T2 is too high, a significant Cr-enriched region may form in the edge part of the carburized steel part. Therefore, the low-temperature carburizing temperature T2 is set to 800 to 880°C. The lower limit of the low-temperature carburization temperature T2 is preferably 810°C, more preferably 820°C, and even more preferably 830°C. The upper limit of the low-temperature carburization temperature T2 is preferably 870°C, more preferably 860°C, and even more preferably 850°C.

[0079] [Low temperature soaking process (S22)] In the low-temperature soaking step (S22), the steel material after the temperature-lowering step is soaked at the low-temperature carburizing temperature T2 (°C). The soaking time (holding time) at the low-temperature carburizing temperature T2 (°C) is the time after the furnace temperature has reached within 10°C of the low-temperature carburizing temperature T2. The soaking time is, for example, 3 to 30 minutes. The point in time when the temperature of the steel material has reached within 10°C of the low-temperature carburizing temperature T2 can be determined by simulation and preliminary testing using an actual machine. The low-temperature soaking process suppresses temperature variations in the steel preform. If the soaking time in the low-temperature soaking process is appropriate, temperature variations in the steel preform are small. This suppresses variations in the penetration and diffusion behavior of carbon in the low-temperature carburizing diffusion process described below. For example, the carbon concentration in the surface layer of the steel preform is high in relatively high-temperature regions, and low in relatively low-temperature regions. As a result, Cr-enriched regions are likely to form in regions with high carbon concentrations in the surface layer. The low-temperature soaking process suppresses such variations in carbon concentration. The temperature variation in the steel preform after the low-temperature soaking process is preferably within 3°C. If the soaking time at the low-temperature carburizing temperature T2 is 3 to 30 minutes, the temperature variation in the steel preform after the low-temperature soaking process will be within 10°C.

[0080] The pressure in the vacuum carburizing furnace during the low-temperature soaking step may be within the same range as that during the temperature-lowering step. Preferably, the pressure in the vacuum carburizing furnace during the low-temperature soaking step is set to 100 Pa or less, or the atmosphere in the vacuum carburizing furnace is an inert gas atmosphere of 100 kPa or less.

[0081] [Low temperature carburizing diffusion process (S23)] After the low-temperature soaking step, a low-temperature carburizing diffusion step (S23) is carried out. In the low-temperature carburizing diffusion step, a low-temperature carburizing period and a low-temperature diffusion period are alternately carried out at least once. The low-temperature carburizing period and the low-temperature diffusion period will be described below.

[0082] [Low temperature carburizing period] During the low-temperature carburizing period, the steel preform is maintained at the low-temperature carburizing temperature T2 (°C) while carburizing gas is supplied into the vacuum carburizing furnace. The low-temperature carburizing period refers to the process of supplying carburizing gas into the furnace under vacuum or reduced pressure as described above. In other words, the low-temperature carburizing period begins when the supply of carburizing gas begins into the furnace under vacuum or reduced pressure after the low-temperature soaking process. During the low-temperature carburizing period, the pressure inside the furnace changes depending on the carburizing gas flow rate and the exhaust speed of the vacuum pump.

[0083] The carburizing gas flow rate can be set appropriately depending on the desired carbon concentration distribution, surface area, temperature, and chemical composition of the steel material in the steel preform. The exhaust speed of the vacuum pump is adjusted depending on the vacuum pump capacity, the inner diameter of the piping, and the furnace pressure. The furnace pressure during the high-temperature carburizing stage is not particularly limited. A preferred furnace pressure is 0.2 kPa or less. This suppresses the generation of soot.

[0084] The holding time of the low-temperature carburizing stage can be appropriately set depending on the desired carbon concentration distribution in the steel material, the temperature, the chemical composition of the steel material, and the like.

[0085] [Low temperature diffusion period] In the low-temperature diffusion period, the supply of carburizing gas to the vacuum carburizing furnace is stopped, and the steel material is held in a vacuum of 10 Pa or less or in an inert gas atmosphere of 100 kPa or less at the low-temperature carburizing temperature T2 (°C). In other words, when the supply of carburizing gas to the vacuum carburizing furnace is stopped, the transition from the low-temperature carburizing period to the low-temperature diffusion period occurs.

[0086] If the furnace atmosphere during the low-temperature diffusion period is a vacuum of 10 Pa or less, the alloy components of the steel preform are less likely to oxidize. Also, if the furnace atmosphere during the low-temperature diffusion period is an inert gas atmosphere of 100 kPa or less, the alloy components of the steel preform are less likely to form oxides and nitrides, which are factors that reduce fatigue strength. Therefore, the furnace atmosphere during the low-temperature diffusion period is maintained at a vacuum of 10 Pa or less, or an inert gas atmosphere of 100 kPa or less.

[0087] The inert gas atmosphere can be maintained by continuously or intermittently evacuating the furnace with a vacuum pump and introducing an inert gas. Continuous evacuation at an inert gas atmosphere of 100 Pa or more makes it easier to purify the furnace atmosphere. Furthermore, when continuously evacuating the furnace with a vacuum pump, if the pressure exceeds 1000 Pa, the load on the vacuum pump increases. Therefore, the preferred furnace atmosphere during the low-temperature diffusion period is an inert gas atmosphere of 100 Pa or more and 1000 Pa or less.

[0088] The holding time in the low-temperature diffusion step is appropriately set depending on the target carbon concentration distribution in the surface layer of the steel material after the low-temperature carburizing stage, and therefore is not particularly limited.

[0089] [Alternating low-temperature carburizing and low-temperature diffusion periods in the low-temperature carburizing and diffusion process] In the low-temperature carburization diffusion step, the above-mentioned low-temperature carburization period and low-temperature diffusion period are alternately carried out at least once.

[0090] Increasing the number of times the low-temperature carburizing period and the low-temperature diffusion period are alternately performed reduces the variation in carbon concentration in the surface layer of the carburized steel part. On the other hand, increasing the number of times the low-temperature carburizing period and the low-temperature diffusion period are alternately performed increases the time for the second carburizing-diffusion step. In this case, a significant Cr-enriched region may remain at the edge of the carburized steel part. Therefore, the number of times the low-temperature carburizing period and the low-temperature diffusion period are alternately performed is preferably two or less times, and more preferably once.

[0091] When the low-temperature carburization and diffusion steps are alternately repeated multiple times, heating to 800 to 880°C may be performed during the low-temperature diffusion step. Preferably, heating is performed during the final low-temperature diffusion step. In this case, the Cr-enriched region is likely to diffuse and disappear. The heating may be performed at the beginning of the low-temperature diffusion step or during the low-temperature diffusion step.

[0092] [Time settings for the low-temperature carburizing and low-temperature diffusion periods in the low-temperature carburizing and diffusion process] The duration of the low-temperature carburizing period and the low-temperature diffusion period in the low-temperature carburizing diffusion process can be appropriately set so that the surface carbon concentration of the carburized steel part and the carbon concentration at a predetermined depth from the surface reach the desired values. The carbon concentration at a predetermined depth required for the surface region to reach a predetermined hardness or higher can be determined in advance by performing a vacuum carburizing treatment.

[0093] [Processing time for low-temperature carburizing diffusion process] The processing time of the low-temperature carburization diffusion step is not particularly limited. Preferably, the processing time P2 of the low-temperature carburization diffusion step is 0.03 to 0.30 times the processing time P1 of the high-temperature carburization diffusion step. In other words, P2 / P1 is 0.03 to 0.30. If P2 / P1 is 0.03 or more, the variation in carbon concentration can be further reduced. Furthermore, if P2 / P1 is 0.30 or less, the Cr-enriched region can be further reduced.

[0094] [Quenching process (S3)] After the second carburizing and diffusion step is performed, the quenching step (S3) is performed. In the quenching process, after the low-temperature carburizing diffusion process, the steel material is extracted from the vacuum carburizing furnace. The extracted steel material is then rapidly cooled (quenched) to below 300°C using a refrigerant. This causes the surface layer of the steel material, which has an increased carbon concentration, to transform into martensite, forming a carburized hardened layer.

[0095] The quenching process is a well-known process in vacuum carburizing. A well-known refrigerant may be used in the quenching process. Examples of well-known refrigerants include gas, water, and oil. That is, the quenching method in the quenching process is, for example, gas cooling, water cooling, or oil cooling.

[0096] Before quenching, slow cooling without rapid cooling may be performed, followed by high-frequency induction heating and water quenching. In this case, high-frequency induction heating can reduce the austenite grain size of the steel preform before rapid cooling (quenching). As a result, the strength of the carburized steel part can be increased. Furthermore, slow cooling before high-frequency induction heating can reduce heat treatment distortion. The preferred cooling rate for slow cooling before high-frequency induction heating is 1.0°C / s or less.

[0097] [Carbon concentration in the surface layer of the steel material after the cooling process and the quenching process] In the above-mentioned vacuum carburizing process, the carbon concentration C of the flat part of the steel material after the temperature-lowering process (but before the low-temperature soaking process) s21 (mass%) is the carbon concentration C of the flat part of the steel material after the quenching process. s3 (mass%) is 0.08% or more lower than the carbon concentration C s21and carbon concentration C s3 The difference ΔC between these values ​​is given by the following equation (1): ΔC=C s3 -C s21 ≧0.08 (1)

[0098] If ΔC is 0.08 or more, excessive carburization of the edges of carburized steel parts is suppressed, so ΔC should be set to 0.08 or more.

[0099] The lower limit of ΔC is preferably 0.09, more preferably 0.10, and even more preferably 0.11. If ΔC exceeds 0.50, the time for the first carburization diffusion step becomes excessively long, so the preferred upper limit of ΔC is 0.50.

[0100] [Method for measuring the difference ΔC] The difference ΔC can be measured by the following method. Chips are collected from the flat part of the manufactured carburized steel part to a depth of 0.05 mm from the surface by turning. The collected chips are used to determine the carbon concentration (mass%) by the well-known high-frequency combustion method (combustion-infrared absorption method). The carbon concentration obtained is referred to as the carbon concentration C S3 (% by mass).

[0101] Furthermore, a steel preform having the same chemical composition as the manufactured carburized steel part is subjected to the first carburizing diffusion step (S1) and the temperature-reducing step (S21) under the same manufacturing conditions, and the steel preform is quenched after the temperature-reducing step is completed. Chips are collected from the surface of the quenched steel preform to a depth of 0.05 mm by turning. The collected chips are dissolved in acid to obtain a solution. The carbon concentration (mass%) of the solution is determined by the well-known high-frequency combustion method (combustion-infrared absorption method). The obtained carbon concentration is referred to as the carbon concentration C S21 (% by mass).

[0102] The obtained carbon concentration C s21 (mass%) and carbon concentration C s3 Based on the mass %, the difference ΔC is calculated.

[0103] [How to adjust the difference ΔC] For example, by adjusting the times of the high-temperature carburizing-diffusion process and the low-temperature carburizing-diffusion process, it is possible to satisfy formula (1). Specifically, for example, the time of the high-temperature diffusion period is set longer than the time of the high-temperature carburizing period, thereby lowering the carbon concentration in the flat portion of the steel preform after the temperature-lowering process. Furthermore, the times of the low-temperature carburizing period and the low-temperature diffusion period are shortened, thereby increasing the carbon concentration in the flat portion of the steel preform after the low-temperature carburizing-diffusion process. This allows the difference ΔC to be adjusted to 0.08 or more.

[0104] [Process after quenching process (S3)] In the method for manufacturing a carburized steel part of this embodiment, a well-known tempering step may be carried out after the quenching step (S3). The tempering temperature in the tempering step is, for example, 150 to 250° C. The holding time at the tempering temperature is, for example, 0.5 to 4.0 hours.

[0105] As described above, in the manufacturing method of the carburized steel part of this embodiment, the carburized-diffusion process is divided into two stages, the first carburized-diffusion process and the second carburized-diffusion process, which makes it possible to sufficiently suppress excessive carburization at the edge of the carburized steel part.

[0106] [About the carburized steel parts manufactured by the manufacturing method of the carburized steel parts of this embodiment] The carburized steel part manufactured by the above-described manufacturing method includes a flat portion and an edge portion, as shown in Figures 1 to 3. The carburized steel part also includes a carburized hardened layer formed on the surface and a core portion located inside the carburized hardened layer.

[0107] The carburized case layer is a well-known structure. It is primarily composed of martensite and retained austenite. When observing the microstructure of a cross section perpendicular to the surface of a carburized steel part, those skilled in the art can easily distinguish between the carburized case layer and the core.

[0108] Furthermore, in the carburized steel component, the area ratio of the Cr-enriched region in the edge surface layer region, which is the region from the surface of the edge to a depth of 80 μm, is 1.00% or less.

[0109] [Method for measuring the area ratio of Cr-enriched regions] Here, the area ratio of the Cr-enriched region can be measured by the following method. Specifically, a test piece is taken with the cross section CS shown in Figures 1 to 3 as the observation surface. The test piece is embedded in resin, and the observation surface is polished. In the edge portion of the observation surface, the center position of the length of the surface of the edge portion (point Pc in Figure 3) is identified in the surface layer region of the edge portion, which is the region from the surface to a depth of 80 μm. A region 50 with a width of 250 μm and a depth of 80 μm, centered at the identified center position, is defined as the measurement region 50.

[0110] Using FE-EPMA, elemental analysis is performed on an area wider than the measurement area 50, including the measurement area 50, to determine the distribution of the Cr concentration (mass%) in the measurement area 50. In the EPMA measurement, the acceleration voltage is 15 kV, the probe current is 50 nA, and the beam diameter is 0.5 μm. The entire measurement area 50 is measured vertically, horizontally, and vertically at 0.5 μm intervals using wavelength-dispersive spectroscopy of characteristic X-rays. The element to be analyzed is Cr. A calibration curve is previously obtained to determine the relationship between the X-ray intensity of Cr and the Cr concentration using a substance with a known Cr concentration. The Cr concentration (mass%) at each measurement point is determined from the X-ray intensities obtained at all measurement points in the measurement area 50 and the above-mentioned calibration curve.

[0111] Within the measurement region 50, a region having a Cr concentration 1.6 times or more the Cr content of the core is identified as a Cr-enriched region. The area ratio (%) of the Cr-enriched region is calculated based on the area of ​​the measurement region 50 and the total area of ​​the Cr-enriched region. The total area of ​​the measurement region 50 to be evaluated is set to 12,800 to 35,000 μm 2 In other words, the total area of ​​the measurement regions 50 is within the range of 12800 to 35000 μm using one or more measurement regions 50. 2 So that it becomes.

[0112] As the EPMA analyzer, for example, an Electron Probe X-ray Micro Analyzer (EPMA, product name "JXA-8230") manufactured by JEOL Datum Co., Ltd. can be used.

[0113] If the area ratio of the Cr-enriched region is 1.00% or less, the Cr-enriched region in the surface layer region of the edge is sufficiently small, and therefore, in the carburized steel part, excessive carburization at the edge is sufficiently suppressed. The upper limit of the area ratio of the Cr-enriched region is preferably 0.80%, more preferably 0.60%, even more preferably 0.50%, and still more preferably 0.45%. [Example]

[0114] Steel bars having the chemical compositions shown in Table 1 (Table 1A and Table 1B) were prepared. The prepared steel bars were subjected to cutting and grinding to produce round bars with a diameter of 26 mm and a length of 100 mm. Each round bar had the shape shown in Figure 1, with an edge portion on the end face having a corner angle of 90°.

[0115] [Table 1A]

[0116] [Table 1B]

[0117] The round bars with each test number were subjected to vacuum carburizing and quenching under the conditions shown in Table 2 (Tables 2A to 2C).

[0118] [Table 2A]

[0119] [Table 2B]

[0120] [Table 2C]

[0121] Specifically, the round bar of each test number was heated in a vacuum carburizing furnace reduced to 10 Pa or less while evacuating to the high-temperature carburizing temperature T1 (°C) shown in Table 2A. The heated round bar was then soaked at the high-temperature carburizing temperature T1 (°C) for 60 minutes. In test number 1, nitrogen was intermittently introduced while evacuating to a vacuum, and the pressure was controlled at 100 to 150 Pa. In test numbers 2 to 48, evacuation was performed without introducing nitrogen, and the pressure inside the vacuum carburizing furnace was 10 Pa or less.

[0122] After soaking, acetylene gas was introduced into the vacuum carburizing furnace as the carburizing gas. After soaking, the round bars were subjected to a high-temperature carburizing diffusion process at the high-temperature carburizing temperature T1 (°C) shown in Table 2A. The times (minutes) for the high-temperature carburizing and high-temperature diffusion periods were as shown in Table 2A. The "Number of high-temperature carburizing diffusion cycles" in Table 2A indicates the number of alternating cycles of the high-temperature carburizing and high-temperature diffusion periods.

[0123] The carburizing gas pressure during the high-temperature carburizing period was 1 kPa or less. In test number 1, nitrogen was intermittently introduced while evacuating during the high-temperature diffusion period, and the pressure was controlled at 100 to 150 Pa. In tests number 2 to 48, the pressure during the high-temperature diffusion period was 10 Pa or less.

[0124] After the high-temperature carburizing diffusion process, the temperature was lowered to the low-temperature carburizing temperature T2 (°C) shown in Table 2B. The processing time for the temperature lowering process was 20 minutes or more. After that, the specimen was soaked at the low-temperature carburizing temperature T2 for 10 minutes. In test number 1, nitrogen was intermittently introduced while evacuating to a vacuum, and the pressure was controlled at 100 to 150 Pa. In test numbers 2 to 48, the specimen was evacuated without introducing nitrogen, and the pressure in the latter half of the soaking period was 10 Pa or less.

[0125] After soaking, acetylene gas was introduced into the vacuum carburizing furnace as the carburizing gas. After soaking, the round bars underwent a low-temperature carburizing diffusion process at the low-temperature carburizing temperature T2 (°C) shown in Table 2B. The times (minutes) for the low-temperature carburizing and low-temperature diffusion periods were as shown in Table 2B. The "Number of low-temperature carburizing diffusion cycles" in Table 2B indicates the number of alternating cycles of the low-temperature carburizing and low-temperature diffusion periods.

[0126] The carburizing gas pressure during the low-temperature carburizing period was 1 kPa or less. In test number 1, nitrogen was intermittently introduced while evacuating during the low-temperature diffusion period, and the pressure was controlled at 100 to 150 Pa. In test numbers 2 to 46, the pressure during the low-temperature diffusion period was 10 Pa or less. In test number 47, the low-temperature diffusion period was omitted. In test number 48, the second carburizing diffusion step was not performed. In other words, in test number 48, conventional vacuum carburizing treatment was performed.

[0127] After the second carburizing diffusion process, the round bar was oil quenched using quenching oil at 120°C. After quenching, the round bar was tempered. The tempering temperature was 180°C, and the holding time at the tempering temperature was 2 hours. Carburized steel parts with each test number were manufactured using the above process.

[0128] In addition to the process used to manufacture the carburized steel parts of each test number, a first carburizing-diffusion process was carried out under the conditions in Table 2A, followed by a temperature-lowering process under the same conditions as above. Thereafter, quenching and tempering were carried out under the same conditions as above without carrying out the low-temperature soaking process and the low-temperature carburizing-diffusion process, to manufacture steel preforms of each test number (hereinafter referred to as intermediate steel preforms).

[0129] [Evaluation test] The following evaluation tests were carried out using the carburized steel parts and steel preforms manufactured with each test number. (Test 1) Differential ΔC measurement test (Test 2) Measurement of the area of ​​Cr-enriched regions at the edge (Test 3) Coarse cementite confirmation test Tests 1 to 3 will be explained below.

[0130] [(Test 1) Differential ΔC measurement test] Using the carburized steel parts and intermediate steel preforms of each test number, the difference ΔC for each test number was determined based on the method described above in [Method for measuring difference ΔC]. The obtained difference ΔC is shown in Table 2C.

[0131] [(Test 2) Measurement of the area of ​​Cr-enriched regions at the edge] Using the carburized steel parts with each test number, the area ratio of the Cr-enriched region at the edge was measured according to the method described above in "Method for measuring the area ratio of the Cr-enriched region." The area ratios (%) of the Cr-enriched region obtained are shown in Table 2C.

[0132] [(Test 3) Coarse cementite confirmation test] A test to confirm coarse cementite at the edge of each carburized steel part with each test number was carried out by the following method.

[0133] A test specimen was taken from the cylindrical carburized steel part shown in Figure 1, with the cross-sectional surface (CS) as the observation surface. The cross-sectional surface (CS) was 2 mm x 2 mm or larger. After embedding the test specimen in resin, the cross-sectional surface (CS) was mirror-polished. The mirror-polished cross-sectional surface (CS) was etched with a 3% nital solution to reveal the microstructure. Using a scanning electron microscope (SEM) at 1000x magnification, the edge surface region (corresponding to measurement area 50 in Figure 3), which is the region from the surface of the carburized steel part to a depth of 80 μm, was observed to determine the presence or absence of cementite with a maximum length exceeding 5.0 μm in the edge surface region.

[0134] The results are shown in the "Presence or Absence of Coarse Cementite" column in Table 2C. "Presence" means that one or more cementite particles with a maximum length exceeding 5.0 μm were observed in the surface layer region of the edge portion. "Absence" means that no cementite particles with a maximum length exceeding 5.0 μm were observed in the surface layer region of the edge portion.

[0135] [Evaluation results] Referring to Tables 1 and 2, the manufacturing methods of the carburized steel parts of Test Nos. 1 to 42 were appropriate. Therefore, the area ratio of the Cr-enriched region at the edge of the carburized steel part was 1.00% or less. As a result, no coarse cementite was observed at the edge of the carburized steel part, and the occurrence of excessive carburization was sufficiently suppressed.

[0136] On the other hand, in test number 43, the high-temperature carburizing temperature T1 was too low. As a result, the area ratio of the Cr-enriched region at the edge of the carburized steel part exceeded 1.00%. As a result, coarse cementite was observed at the edge of the carburized steel part, and the occurrence of excessive carburization could not be sufficiently suppressed.

[0137] In test number 44, the low-temperature carburizing temperature T2 was too low. As a result, the area ratio of the Cr-enriched region at the edge of the carburized steel part exceeded 1.00%. As a result, coarse cementite was observed at the edge of the carburized steel part, and the occurrence of excessive carburization could not be sufficiently suppressed.

[0138] In test number 45, the low-temperature carburizing temperature T2 was too high. As a result, the area ratio of the Cr-enriched region at the edge of the carburized steel part exceeded 1.00%. As a result, coarse cementite was observed at the edge of the carburized steel part, and the occurrence of excessive carburization could not be sufficiently suppressed.

[0139] In test number 46, ΔC was less than 0.08. Therefore, the area ratio of the Cr-enriched region at the edge of the carburized steel part exceeded 1.00%. As a result, coarse cementite was observed at the edge of the carburized steel part, and the occurrence of excessive carburization could not be sufficiently suppressed.

[0140] In test number 47, the low-temperature diffusion period of the low-temperature carburizing diffusion process in the second carburizing diffusion process was not performed. As a result, the area ratio of the Cr-enriched region at the edge of the carburized steel part exceeded 1.00%. As a result, coarse cementite was observed at the edge of the carburized steel part, and the occurrence of excessive carburizing could not be sufficiently suppressed.

[0141] In test number 48, the second carburizing diffusion step was not performed. As a result, the area ratio of the Cr-enriched region at the edge of the carburized steel part exceeded 1.00%. As a result, coarse cementite was observed at the edge of the carburized steel part, and the occurrence of excessive carburization could not be sufficiently suppressed.

[0142] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. 1. A method for manufacturing a carburized steel component, comprising: a first carburizing diffusion process in which a steel blank, which is a raw material for the carburized steel part, is carburized in a vacuum carburizing furnace; a second carburizing-diffusion step of carburizing the steel shape material in the vacuum carburizing furnace after the first carburizing-diffusion step; a quenching step of cooling the steel shape material to 300°C or less using a refrigerant after the second carburizing / diffusion step, The first carburization diffusion step comprises: a heating step of heating the steel blank in the vacuum carburizing furnace to a high carburizing temperature in the range of 930 to 1100°C; a high-temperature soaking step of soaking the steel shape material at the high-temperature carburizing temperature after the heating step; a high-temperature carburizing and diffusion process for performing a carburizing treatment and a diffusion treatment on the steel shape material after the high-temperature soaking process, The high-temperature carburization diffusion process includes: a high-temperature carburizing period in which the steel workpiece is maintained at the high-temperature carburizing temperature while a carburizing gas is supplied into the vacuum carburizing furnace; a high-temperature diffusion period in which the supply of the carburizing gas into the vacuum carburizing furnace is stopped and the steel preform is maintained at the high-temperature carburizing temperature in a vacuum of 10 Pa or less or in an inert gas atmosphere of 100 kPa or less, and the high-temperature diffusion period is alternately carried out one or more times; The second carburization diffusion step comprises: a temperature-reducing step of reducing the temperature of the steel material in the vacuum carburizing furnace to a low-temperature carburizing temperature in the range of 800 to 880°C; a low-temperature soaking step of soaking the steel material at the low-temperature carburizing temperature after the temperature-lowering step; a low-temperature carburizing and diffusion process for performing a carburizing treatment and a diffusion treatment on the steel shape material after the low-temperature soaking process, The low-temperature carburization diffusion process includes: a low-temperature carburizing period in which the steel workpiece is maintained at the low-temperature carburizing temperature while the carburizing gas is supplied into the vacuum carburizing furnace; a low-temperature diffusion period in which the supply of the carburizing gas into the vacuum carburizing furnace is stopped and the steel preform is maintained at the low-temperature carburizing temperature in a vacuum of 10 Pa or less or in an inert gas atmosphere of 100 kPa or less, and the low-temperature diffusion period is alternately carried out one or more times; The carbon concentration C of the flat portion of the steel preform after the temperature decreasing step ｓ２１ (mass%) is the carbon concentration C of the flat portion of the steel material after the quenching process. ｓ３ (mass%) is 0.08% or more lower, Manufacturing method for carburized steel parts.

2. 2. A method for manufacturing a carburized steel part according to claim 1, comprising the steps of: The treatment time of the low-temperature carburization diffusion step is set to 0.03 to 0.30 times the treatment time of the high-temperature carburization diffusion step. Manufacturing method for carburized steel parts.

3. 3. A method for manufacturing a carburized steel part according to claim 1 or 2, comprising: The chemical composition of the steel material is, in mass%, C: 0.10-0.25%, Si: 0.02-2.00%, Mn: 0.30-2.50%, P: 0.030% or less, S: 0.060% or less, Cr: 0.30-3.00%, Al: 0.010-0.060%, N: 0.025% or less, Cu: 0 to 0.50%, Ni: 0 to 1.00%, Mo: 0-1.00%, V: 0-0.20%, Sn: 0 to 0.10%, Sb: 0 to 0.02%, Nb: 0 to 0.10%, Ti: 0 to 0.10%, and B: 0 to 0.0040%; The balance consists of Fe and impurities. Manufacturing method for carburized steel parts.

4. A carburized steel part, It includes a flat portion and an edge portion, a carburized hardened layer formed on the surface layer of the carburized steel part; a core portion located inside the carburized hardened layer, The chemical composition of the core is, in mass %, C: 0.10-0.25%, Si: 0.02-2.00%, Mn: 0.30-2.50%, P: 0.030% or less, S: 0.060% or less, Cr: 0.30-3.00%, Al: 0.010-0.060%, N: 0.025% or less, Cu: 0 to 0.50%, Ni: 0 to 1.00%, Mo: 0-1.00%, V: 0-0.20%, Sn: 0 to 0.10%, Sb: 0 to 0.02%, Nb: 0 to 0.10%, Ti: 0 to 0.10%, and B: 0 to 0.0040%; The balance is Fe and impurities. In the edge portion surface layer region, which is a region from the surface of the edge portion to a depth of 80 μm, the area ratio of the Cr-enriched region is 1.00% or less. Carburized steel parts.

5. 5. The carburized steel component of claim 4, The chemical composition of the core is, in mass %, Cu: 0.01 to 0.50%, Ni: 0.01-1.00%, Mo: 0.01-1.00%, V: 0.01-0.20%, Sn: 0.01 to 0.10%, Sb: 0.01-0.02%, Nb: 0.01 to 0.10%, Ti: 0.01 to 0.10%, and B: 0.0001 to 0.0040%; Carburized steel parts.

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