Carburized parts and their manufacturing method

Abstract

To provide a carburized part with excellent low cycle fatigue strength and a method of manufacturing the same.SOLUTION: A core contains, in mass%, C: 0.10-0.30%, Si: 0.03-0.80%, Mn: 0.40-1.30%, P: 0.005-0.020%, S: 0.003-0.060%, Cr: 0.10-2.00%, Al: 0.050% or less, N: 0.0030-0.0300%, and O: 0.0020% or less, the remainder is Fe and impurities, and a ratio of the grain boundary P concentration Pb, which is P content of the grain boundary at a depth of 50 μm below the surface of the part, to the steel P concentration Pm, which is the P content of the core, Pb / Pm is 30.00 or less, the Vickers hardness at a depth of 0.10 mm below the surface of the part is 650 HV or more, and the Vickers hardness at a depth of 1.5 mm below the surface of the part is 300 HV or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a carburized part and a method for manufacturing the same. [Background technology]

[0002] In recent years, in order to improve the fuel efficiency of automobiles, there has been an increasing demand for smaller and lighter mechanical structural parts such as gear components, which has led to a demand for improved strength of the parts. In particular, gear components such as automotive differential gears and transmission gears are often subjected to impact loads when the vehicle suddenly starts or stops or runs over a bump in the road. As a result, such gear components can be damaged by low-cycle fatigue, which leads to fracture after an extremely low number of cycles, such as several tens to several thousand. Therefore, parts used in these applications are required to have improved strength, especially strength against low-cycle fatigue failure.

[0003] Many of the above-mentioned parts are manufactured by machining steel material into a specified shape and then subjecting it to carburizing and quenching. Most of the steel used in this case is alloy steel for machine structures as specified in JIS G 4053:2008. Generally, case-hardened steels such as SCr420 and SCM420 are used to ensure the toughness of the core of the part, while carburizing and quenching followed by low-temperature tempering at around 180°C give the surface of the part a tempered martensite structure with approximately 0.8% C, improving bending fatigue strength and wear resistance.

[0004] Various gear steels and gear components, as well as manufacturing methods for them, have been proposed with the aim of improving low-cycle bending fatigue strength. Low-cycle fatigue failure of gear components occurs at the grain boundaries in the surface layer. For this reason, for example, gear steels with reduced impurity elements such as P and S to strengthen the austenite grain boundaries in the carburized hardened layer, and gear components with improved impact resistance due to refined austenite grain size and manufacturing methods for them have been proposed.

[0005] For example, Patent Document 1 proposes that after carburizing and quenching, the entire steel is reheated to the austenite region and then quenched, thereby refining the austenite grain size in the carburized hardened layer and thereby increasing impact resistance. In addition, Patent Document 1 proposes adding Ni, Mo, and B to enhance the effect of grain boundary strengthening.

[0006] Furthermore, Patent Document 2 proposes a carburizing and quenching method that includes a first step of carburizing a material, a second step of cooling the material to below the austenitizing temperature after the first step, a third step of rapidly heating the carburized surface and interior of the material cooled in the second step to just above the austenitizing temperature, and a fourth step of quenching the material following the third step, thereby achieving an austenite grain size of #10 or more and preventing the segregation of P, S, carbides, etc., which reduce grain boundary strength.

[0007] Furthermore, Patent Document 3 proposes a steel material in which the P content of the steel material is set to more than 0.015% and 0.030% or less, and further proposes a steel material and a manufacturing method thereof in which the amount of grain boundary segregation per unit volume is reduced by refining the austenite grain size through induction hardening. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 8-92690 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-048292 [Patent Document 3] Japanese Patent Application Publication No. 9-241798 Summary of the Invention [Problem to be solved by the invention]

[0009] However, the above-mentioned conventional techniques have the following problems. Patent Document 1 mentions that grain boundary strength is improved by reheating and quenching to refine the crystal grains and by short-time heat treatment. However, in Patent Document 1, the reheating and quenching after carburizing is performed by oil quenching. As a result, the P concentration in the grain boundaries of the resulting part increases during oil quenching, which may result in a decrease in grain boundary strength. In other words, the part described in Patent Document 1 is insufficient to significantly improve low-cycle fatigue strength compared to conventional parts.

[0010] Patent Document 2 mentions that short-time heat treatment improves the grain boundary strength of the resulting part. However, in Patent Document 2, oil quenching is used for cooling during quenching, which increases the P concentration in the grain boundaries of the resulting part, potentially reducing the grain boundary strength. In other words, the part described in Patent Document 2 is insufficient to significantly improve the low-cycle fatigue strength compared to conventional parts.

[0011] Patent Document 3 mentions that the amount of grain boundary segregation per unit volume is reduced by grain refinement through induction hardening. However, the technology of Patent Document 3 does not sufficiently reduce the grain boundary P concentration, and does not achieve a significant improvement in low-cycle fatigue strength compared to conventional methods.

[0012] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a carburized part having excellent low-cycle fatigue strength and a method for manufacturing the same. [Means for solving the problem]

[0013] As a result of intensive research conducted by the present inventors to solve the above problems, the present inventors have discovered the following new findings.

[0014] (a) In order to increase the low-cycle bending fatigue strength of carburized parts, it is effective to reduce the ratio of the P concentration at the grain boundaries in the surface layer (up to a depth of 50 μm from the surface of the part after carburization), which is the starting point of fatigue fracture (hereinafter referred to as the grain boundary P concentration), to the P concentration in the core (hereinafter referred to as the steel P concentration), to 30.00 or less.

[0015] (b) To increase the low-cycle bending fatigue strength of carburized parts, it is effective for the Vickers hardness to be 650 HV or higher at a depth of 0.10 mm below the surface, the Vickers hardness to be 300 HV or higher at a depth of 1.5 mm below the surface, and for the surface structure to be composed of tempered martensite and retained austenite.

[0016] (c) In order to reduce the grain boundary P concentration / steel P concentration to 30.00 or less, it is important to perform induction hardening on carburized parts with a specified steel composition in such a way that the following formulas (1) and (2) are satisfied.

[0017] 1000 <T1···(1) 50 <y ···(2) In the above formulas (1) and (2), T1 is the hardening temperature (°C) on the surface of the part, and y is the average cooling rate (°C / sec) from the maximum heating temperature to 500°C during the induction hardening process.

[0018] Here, the above finding (c) will be explained in more detail. In the academic field of grain boundary segregation, it is generally known that in simple compositions such as Fe-P or Fe-C, the amount of grain boundary segregation tends to decrease at higher temperatures and increase at lower temperatures. However, it was not clear whether a similar tendency for grain boundary segregation to occur at different temperatures was observed in practical steels, such as carburized parts and carburizing steels, which contain large amounts of alloying elements in addition to the grain boundary segregating elements.

[0019] Therefore, the present inventors conducted extensive research using the practical steel and found that the higher the quenching temperature, the lower the grain boundary P concentration, and that even if the quenching temperature is increased, the grain boundary P concentration in the austenite grain boundaries increases if the average cooling rate to 500°C is slower than 50°C / sec.

[0020] In addition, carburized parts are generally subjected to low-temperature tempering at around 180°C for about two hours after quenching, but low-temperature tempering does not result in any change in the grain boundary P concentration because the volume diffusion of P is slow. In other words, we found that in order to reduce the grain boundary P concentration / steel P concentration, it is important to increase the temperature during the quenching process and to rapidly cool the steel by water cooling.

[0021] However, raising the carburizing temperature and water cooling are industrially difficult. This is because the carburizing process takes a long time and requires a long time to heat up, and maintaining a high temperature during the carburizing process causes coarsening of austenite grains, resulting in a decrease in fatigue strength. Furthermore, carburizing is inherently difficult because it is difficult to heat the material to temperatures above 1000°C and to perform water cooling. Therefore, the inventors focused on "induction hardening," which is performed after the carburizing process and is thought to be industrially feasible for quenching from a high temperature and water cooling with a sufficiently fast cooling rate.

[0022] According to the research of the present inventors, the grain boundary P concentration at the austenite grain boundaries in a steel material is reduced by heating to a high temperature exceeding 1000°C in the induction hardening process. Furthermore, by setting the average cooling rate during quenching from a high temperature exceeding 1000°C to more than 50°C / s, the increase in the grain boundary P concentration at the austenite grain boundaries during quenching can be suppressed, and as a result, high grain boundary strength can be maintained. Furthermore, water cooling is more preferable for quenching, as it is relatively easy to achieve an average cooling rate exceeding 50°C / s. If quenching from a high temperature is not performed in this induction hardening process after carburizing, or if the average cooling rate is slowed, the grain boundary P concentration at the austenite grain boundaries will increase and the grain boundary strength will decrease.

[0023] The present invention was achieved as a result of further detailed studies based on the above findings, and the gist of the present invention is as follows.

[0024] (1) A carburized part according to one aspect of the present invention has a core composition, in mass%, of: C: 0.10~0.30%, Si: 0.03 to 0.80% Mn: 0.40 to 1.30% P: 0.005~0.020%, S: 0.003~0.060%, Cr: 0.10~2.00%, Al: 0.050% or less, N: 0.0030 to 0.0300%, and O: 0.0020% or less and the balance being Fe and impurities, The P content of the core is the steel P concentration P m The grain boundary P concentration P, which is the amount of P at the grain boundary at a depth of 50 μm below the surface of the part, b The ratio P b / P m is less than or equal to 30.00, The Vickers hardness at a depth of 0.10 mm below the surface of the part is 650 HV or more, The Vickers hardness at a depth of 1.5 mm below the surface of the part is 300 HV or more. (2) The carburized part of (1) further comprises the core having a composition of, in mass %, Mo: 0-1.00%, V: 0~0.50%, Cu: 0-0.50% Ni: 0 to 1.00% Bi: 0 to 0.10% Nb: 0 to 0.100%, Ti: 0 to 0.200%, and B: 0 to 0.005% may contain one or more of the above.

[0025] (3) A method for manufacturing a carburized component according to one aspect of the present invention is the method for manufacturing a carburized component described in (1) above, comprising the steps of: The composition is, in mass%, C: 0.10~0.30%, Si: 0.03 to 0.80% Mn: 0.40 to 1.30% P: 0.005~0.020%, S: 0.003~0.060% or less, Cr: 0.10~2.00%, Al: 0.050% or less, N: 0.003~0.030% or less, O: 0.0020% or less a forming step of forming a steel material containing the above-mentioned and the remainder being Fe and impurities into a part shape; A carburizing process in which carburizing is performed at a carburizing temperature of 870°C to 1050°C, followed by cooling from the austenite region; an induction hardening process for performing induction hardening under conditions that satisfy the following formulas (1) and (2); Next, a tempering process of tempering at 130 to 200 ° C. It has. 1000 <T1 ···(1) 50 <y ···(2) In the above formulas (1) and (2), T1 is the hardening temperature (°C) on the surface of the part, and y is the average cooling rate (°C / sec) from the maximum heating temperature to 500°C during the induction hardening process. (4) The method for producing a carburized part according to (3) above, wherein the composition is in mass %: Mo: 0-1.00%, V: 0~0.50%, Cu: 0-0.50% Ni: 0 to 1.00% Bi: 0 to 0.10% Nb: 0 to 0.10% Ti: 0 to 0.20%, and B: 0 to 0.0050% may contain one or more of the above.

[0026] (5) The carburized parts of (1) or (2) above may be machine structural parts. [Effects of the Invention]

[0027] The carburized parts according to this embodiment have excellent low-cycle fatigue strength, and a method for manufacturing such parts can be provided. Therefore, the carburized parts of the present invention are suitable for use as gears in automobiles and industrial machinery, particularly in machines powered by electric motors. [Brief explanation of the drawings]

[0028] [Figure 1] Figure 1 shows a cantilever fatigue test piece, with dimensions in the figure in millimeters. [Figure 2A] FIG. 2A is a diagram showing the position where an Auger test specimen is taken from a cantilever fatigue test specimen. [Figure 2B] Figure 2B shows an Auger test piece, with dimensions in mm. DETAILED DESCRIPTION OF THE INVENTION

[0029] A carburized part and a method for manufacturing the same according to one embodiment of the present invention will be described in detail below. Note that "%" for the content of each component element means "% by mass" unless otherwise specified.

[0030] The carburized part according to this embodiment (hereinafter, sometimes simply referred to as the "part") has a core (hereinafter, sometimes simply referred to as the "core") which is the center of the part in the depth direction, and a hardened layer located on the surface of the part. Here, the core refers to the area where carbon did not penetrate during carburizing and the area where the structure did not transform to martensitic form during induction hardening. In other words, the core is the area where the chemical composition and metal structure did not change, or the change was negligible, despite the carburizing and induction hardening processes, and has the same chemical composition as the base material of the part. The composition of the core can also be said to be the composition at a depth of 1.5 mm from the surface of the part, for example.

[0031] [Chemical composition] The chemical composition of the core of the carburized component of this embodiment will be described. Normally, the components of the core of a carburized component are the same as the components of the component's material (steel). In other words, the chemical composition of the core described below can also be considered the chemical composition of the component's material.

[0032] C: 0.10 to 0.30% Carbon (C) has the effect of improving the hardenability of steel and increasing the hardness of the core. This can increase the low-cycle fatigue strength of carburized parts. If the C content is less than 0.10%, this effect cannot be obtained. On the other hand, if the C content exceeds 0.30%, the machinability and cold forgeability of the steel material will decrease. Therefore, the C content is 0.10 to 0.30%. The preferred lower limit of the C content is 0.15% or more. The preferred upper limit of the C content is 0.28% or less.

[0033] Si: 0.03% to 0.80% Silicon (Si) has the effect of deoxidizing steel. Si also has the effect of increasing the hardenability of steel and, further, the effect of increasing the strength of steel through solid solution strengthening. These factors increase the hardness of the core and the low-cycle fatigue strength of carburized parts. If the Si content is less than 0.10%, this effect cannot be obtained. On the other hand, if the Si content exceeds 0.80%, the machinability and cold workability of the steel material decrease. Therefore, the Si content is 0.03 to 0.80%. The preferred lower limit of the Si content is 0.20% or more. The preferred upper limit of the Si content is 0.50% or less.

[0034] Mn: 0.40 to 1.30% Manganese (Mn) has the effect of deoxidizing steel. Furthermore, Mn has the effect of increasing the hardenability and strength of steel, thereby increasing the core hardness and low-cycle fatigue strength of carburized parts. If the Mn content is less than 0.40%, this effect cannot be obtained. On the other hand, if the Mn content exceeds 1.30%, the machinability and cold workability of the steel material decrease. Therefore, the Mn content is 0.40 to 1.30%. The preferred lower limit of the Mn content is 0.60% or more. The preferred upper limit of the Mn content is 1.00% or less.

[0035] P: 0.005 to 0.020% Phosphorus (P) is an impurity. P segregates at austenite grain boundaries during carburization, reducing the grain boundary strength of the carburized layer. This reduction in grain boundary strength reduces low-cycle fatigue strength. In this embodiment, optimizing the conditions of the induction hardening process reduces the concentration of P segregated at grain boundaries during carburization, thereby reducing the grain boundary P concentration / steel P concentration. However, if the P content of the raw material is excessively high, optimizing the conditions of the induction hardening process to reduce the grain boundary P concentration / steel P concentration may not be enough to reduce the grain boundary P concentration to a level that contributes to improving low-cycle fatigue strength. Therefore, the P content is set to 0.020% or less. If the P content is 0.020% or less, the P content is low not only in the core but also in the surface layer, thereby increasing the toughness of the surface layer and suppressing the occurrence of intergranular cracking. As a result, low-cycle fatigue strength is improved. Therefore, the P content is set to 0.020% or less. A lower P content is preferable. However, excessive reduction of P leads to increased costs for dephosphorization. Therefore, in consideration of the economic efficiency of refining, the P content is set to 0.005% or more, preferably 0.006% or more.

[0036] S: 0.003% to 0.060% Sulfur (S) is an impurity. S remains at grain boundaries and has the effect of reducing the grain boundary strength of the carburized layer. S also forms coarse MnS at grain boundaries, reducing low-cycle fatigue strength. Therefore, the S content is 0.060% or less. The preferred upper limit of the S content is 0.030% or less, and more preferably 0.015% or less. The S content should be as low as possible. However, excessive reduction of the S content leads to increased costs for desulfurization. Therefore, taking into consideration the economic efficiency of refining, the S content is set to 0.003% or more. The S content is preferably 0.005% or more.

[0037] Cr: 0.10~2.00% Chromium (Cr) has the effect of increasing the hardenability of steel. This increases the core hardness and the low-cycle fatigue strength. If the Cr content is less than 0.10%, this effect cannot be fully achieved. On the other hand, if the Cr content exceeds 2.00%, the machinability and cold workability of the steel material decrease. Therefore, the Cr content is 0.10 to 2.00%. The preferred lower limit of the Cr content is 0.50% or more. The preferred upper limit of the Cr content is 1.50% or less.

[0038] Al: 0.050% or less Aluminum (Al) has the effect of deoxidizing steel. Furthermore, Al combines with N in steel to form AlN, which has the effect of suppressing the coarsening of austenite grains during carburizing. This can improve the low-cycle fatigue strength of carburized parts. If the Al content exceeds 0.050%, the coarsening of austenite grains cannot be suppressed due to the coarsening of inclusions, and low-cycle fatigue strength may deteriorate. Therefore, the Al content is 0.050% or less. The preferred upper limit of the Al content is 0.035% or less. The lower the Al content, the better. There is no particular limit to the lower limit of the Al content, but it may be 0.005% or more to enjoy the deoxidizing effect.

[0039] N: 0.0030 to 0.0300% Nitrogen (N) combines with Ti, Al, V, and Nb in steel to form nitrides and carbonitrides, thereby suppressing the coarsening of austenite grains during carburizing. This can improve the low-cycle fatigue strength of carburized parts. If the N content is less than 0.0030%, this effect cannot be obtained. On the other hand, if the N content exceeds 0.0300%, the above effect saturates. Therefore, the N content is 0.0030 to 0.0300%. The preferred upper limit of the N content is 0.0200% or less. The preferred lower limit of the N content is 0.0035% or more.

[0040] O: 0.0020% or less Oxygen (O) is an element that is inevitably contained and segregates at grain boundaries, making intergranular embrittlement more likely to occur. O is also an element that is likely to form hard oxide-based inclusions in steel, which are the cause of brittle fracture. In order to prevent such intergranular embrittlement and brittle fracture, the O content is set to 0.0020% or less. A preferred upper limit of the O content is 0.0018% or less. The O content is preferably as low as possible. There is no particular limitation on the lower limit of the O content, but it may be, for example, 0.0001% or more.

[0041] The chemical composition of the core of the carburized part of this embodiment contains the above elements, with the remainder consisting of Fe and impurities. Here, impurities refer to elements that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment during industrial production of carburized parts, and include elements that are not intentionally added. The term "impurities" used here refers to elements that are acceptable within a range that does not adversely affect the carburized part of this embodiment.

[0042] [Optional elements] The core of the carburized part of this embodiment may further contain one or more elements selected from the group consisting of Mo, V, Cu, Ni, Bi, Nb, Ti, and B in place of a portion of Fe. These elements are optional elements. In other words, the carburized part of this embodiment can solve the problem without including the optional elements exemplified below. Therefore, the lower limit of the content of the elements exemplified below is 0%.

[0043] Mo: 0 to 1.00% or less Molybdenum (Mo) is an optional element and may not be contained. When Mo is contained, it has the effect of increasing the hardenability of steel, thereby increasing the core hardness and the low-cycle fatigue strength of carburized parts. Mo also has the effect of increasing the toughness of the carburized layer. Even if even a small amount of Mo is contained, these effects can be obtained. However, if the Mo content exceeds 1.00%, these effects saturate and the raw material cost increases. Therefore, the Mo content is 0 to 1.00%. To stably obtain the above effects, the preferred lower limit of the Mo content is 0.03% or more. The preferred upper limit of the Mo content is 0.50% or less.

[0044] V:0~0.50% or less Vanadium (V) is an optional element and does not necessarily need to be contained. When V is contained, V bonds with C and N in the steel to form V carbonitride (V(CN)), which can suppress coarsening of austenite grains during carburizing. This can improve the low-cycle fatigue strength of carburized parts. This effect can be achieved even if even a small amount of V is contained. However, if the V content exceeds 0.50%, carburization resistance decreases. Therefore, the V content is 0 to 0.50%. To stably obtain the above effect, the preferable lower limit of the V content is 0.01% or more. The preferable upper limit of the V content is 0.48% or less.

[0045] Cu: 0 to 0.50% Copper (Cu) is an optional element and does not necessarily need to be contained. When Cu is contained, it has the effect of increasing the hardenability of the steel, thereby increasing the core hardness and the low-cycle fatigue strength of the carburized part. This effect can be obtained even if even a small amount of Cu is contained. On the other hand, if the Cu content exceeds 0.50%, the hot workability decreases. Therefore, the Cu content is 0 to 0.50%. To stably obtain the above effect, the preferable lower limit of the Cu content is 0.01% or more. The preferable upper limit of the Cu content is 0.48% or less.

[0046] Ni: 0 to 1.00% Nickel (Ni) is an optional element and may not be included. When Ni is included, Ni has the effect of increasing the hardenability of steel, thereby increasing the core hardness. This can improve the low-cycle fatigue strength characteristics of carburized parts. Ni also has the effect of increasing the toughness of the carburized layer. Even if even a small amount of Ni is included, these effects can be obtained. However, if the Ni content exceeds 1.00%, the amount of retained austenite in the surface layer after carburizing increases, reducing the surface hardness and, as a result, the low-cycle bending fatigue strength may decrease. Therefore, the Ni content is 0 to 1.00%. To stably obtain the above effects, the preferred lower limit of the Ni content is 0.01% or more. The preferred upper limit of the Ni content is 0.80% or less.

[0047] Bi: 0 to 0.10% Bismuth (Bi) is an optional element and does not necessarily need to be contained. When Bi is contained, Bi can improve the machinability of the steel. This effect can be obtained even if even a small amount of Bi is contained. However, if the Bi content exceeds 0.10%, the above effect saturates. Therefore, the Bi content is 0 to 0.10%. To stably obtain machinability, the preferred lower limit of the Bi content is 0.001% or more. The preferred upper limit of the Bi content is 0.08% or less.

[0048] Nb: 0 to 0.100% Niobium (Nb) is an optional element and does not necessarily need to be contained. When Nb is contained, Nb bonds with C and N in the steel to form Nb carbonitride (Nb(CN)), which can suppress coarsening of austenite grains during carburizing. This can improve the low-cycle fatigue strength of carburized parts. This effect can be achieved even if even a small amount of Nb is contained. However, if the Nb content exceeds 0.100%, carburization resistance decreases. Therefore, the Nb content is 0 to 0.100%. To stably obtain the above effect, the preferable lower limit of the Nb content is 0.001% or more. The preferable upper limit of the Nb content is 0.060% or less.

[0049] Ti: 0 to 0.200% Titanium (Ti) is an optional element and may not be included. When Ti is included, Ti bonds with C and S in the steel to form fine TiC and TiS, thereby suppressing the coarsening of austenite grains during carburizing. This increases the low-cycle fatigue strength of carburized parts. Even a small amount of Ti can achieve this effect. However, if the Ti content exceeds 0.200%, TiC coarsens and the toughness of the steel decreases. In this case, the low-cycle fatigue strength of the carburized parts decreases. Therefore, the Ti content is 0 to 0.200%. To stably obtain the above effects, the preferred lower limit of the Ti content is 0.001% or more. The preferred upper limit of the Ti content is 0.150% or less.

[0050] B: 0 to 0.005% Boron (B) is an optional element and does not necessarily need to be contained. When B is contained, B has the effect of increasing the hardenability of steel, thereby increasing the core hardness and the low-cycle fatigue strength of carburized parts. Even if even a small amount of B is contained, these effects can be obtained. However, if the B content exceeds 0.005%, these effects saturate. Therefore, the B content is 0 to 0.005%. To stably obtain the above effects, the preferred lower limit of the B content is 0.0001% or more. The preferred upper limit of the B content is 0.003% or less.

[0051] Examples of elements that may be mixed into parts as impurities include Pb, Ca, Mg, W, Sb, Co, Ta, and REM. Even when these elements are contained, the present invention can be implemented without problems and the effects of the present invention can be enjoyed as long as the contents are, respectively, 0.10% or less of Pb, 0.001% or less of Ca, 0.001% or less of Mg, 0.10% or less of W, 0.005% or less of Sb, 0.10% or less of Co, 0.10% or less of Ta, and 0.001% or less of REM.

[0052] Next, the surface hardness, core hardness, grain boundary P concentration, and metal structure of the carburized part of this embodiment will be described.

[0053] [Surface hardness: 650HV or more] For parts, the Vickers hardness (surface hardness) at a depth of 0.10 mm from the surface shall be 650 HV or more. The hardness of the depth region (surface layer) from the surface of a part to a depth of 0.10 mm affects the low-cycle fatigue strength of the part. In other words, the higher the hardness of the surface layer, the smaller the amount of plastic strain in the surface layer and the higher the low-cycle fatigue strength. However, if the Vickers hardness at a depth of 0.10 mm from the surface is less than 650 HV, the amount of plastic strain in the surface layer increases, reducing the low-cycle fatigue strength and impairing the wear resistance. Therefore, the surface layer hardness is 650 HV or more. In this embodiment, the "depth of 0.10 mm from the surface" is located within the hardened layer.

[0054] Since the component in this embodiment is a carburized component, the component surface has a hardened layer (for example, a region extending from the component surface to a depth of approximately 1.0 mm) formed thereon, which is a region where carbon penetrates during the carburizing process and a region where the structure has been transformed into martensitic through induction hardening. However, as described above, the region of hardness that affects low-cycle fatigue strength is the depth region (surface layer) extending from the component surface to a depth of 0.10 mm, and therefore, in this embodiment, the Vickers hardness in this region is specified. In this embodiment, the Vickers hardness at a depth of 0.10 mm from the surface is specified as a representative surface layer hardness. However, since the hardness generally increases toward the component surface in carburized components, naturally, the hardness is 650 HV or higher at locations shallower than the 0.10 mm depth.

[0055] [Core hardness: 300HV or more] In parts, the Vickers hardness (core hardness) at a depth of 1.5 mm from the surface must be 300 HV or higher. The internal region (core) at a depth of 1.5 mm or more from the surface of the part is an area with no change in chemical composition or metal structure, despite having undergone carburizing and induction hardening treatments, and has a component composition equivalent to that of the part's base material (base material). If this core hardness is low, fracture will occur from within the part, and low-cycle fatigue strength will decrease. Therefore, the core hardness must be 300 HV or higher.

[0056] The Vickers hardness in this embodiment refers to the Vickers hardness (HV) in accordance with JIS Z 2244:2009 "Vickers hardness test - Test method."

[0057] The surface hardness and core hardness can be calculated by the following method. For both surface and core hardness, the carburized part after induction hardening is first cut perpendicular to the main axis or longitudinal direction, and the resulting cross section is mirror-polished. Surface hardness is then measured at three arbitrary points on the surface equivalent to a depth of 0.10 mm (100 μm) from the part surface (perpendicular to the part surface) with a measuring load of 300 gf. Core hardness is measured at three arbitrary points on the surface equivalent to a depth of 1.5 mm from the part surface with a measuring load of 300 gf. The surface and core hardness are obtained by calculating the average of the Vickers hardness values ​​obtained for each.

[0058] [Grain boundary P concentration P b (%) / Steel P concentration P m (%): 30.00 or less In parts, the amount of P in the core (steel P concentration P m , mass%), the amount of P at the grain boundary at a depth of 50 μm below the surface of the part (grain boundary P concentration P b , mass%) ratio P b / P m is 30.00 or less. Grain boundary P concentration P b The lower the P concentration in the steel, the higher the grain boundary strength and the lower the low cycle fatigue strength. m and grain boundary P concentration P b It is known that there is a correlation between the P concentration in steel and m If is reduced, the grain boundary P concentration P b However, there was a limit to how much improvement in low-cycle fatigue strength could be achieved by simply reducing the amount of P in the material. Therefore, we investigated ways to further improve the properties and found that by controlling the conditions during induction hardening, which will be described later, the grain boundary P concentration P bIn other words, the carburized part of this embodiment can significantly reduce the grain boundary P concentration compared to a conventional carburized part with an equivalent steel P content. b / P m When the P content is 30.00 or less, the grain boundary strength is higher and the low cycle fatigue strength is significantly improved compared to conventional carburized parts with the same P content of steel. b / P m is preferably 26.00 or less. b Since the lower the better, P b / P m However, a realistically achievable level is P b / P m may be 2.00 or more, or 8.00 or more, or 12.00 or more, or 15.00 or more.

[0059] In addition, the grain boundary P concentration P directly affects the properties of carburized parts. b From the viewpoint of improving the grain boundary strength, the grain boundary P concentration P b Therefore, the grain boundary P concentration P b is preferably 0.45 mass % or less, and more preferably 0.35 mass % or less.

[0060] Grain boundary P concentration P b (% by mass) can be determined by Auger analysis as shown below. First, Auger test pieces with the shape shown in Figure 2 are prepared from the surface of the carburized part. Each of the obtained test pieces is placed in an Auger electron spectrometer ("PHI-700", ULVAC-PHI, FE type) and fractured by cooling inside the device. Spectral analysis is then performed on the grain boundary fracture surface up to a depth of 50 μm from the carburized surface. Since there is a certain degree of variation in the grain boundary concentration, concentration measurements are performed at at least seven grain boundaries, and the grain boundary P concentration P is calculated by calculating the average of the obtained concentrations. b To calculate the grain boundary concentration, a narrow scan is used, narrowing the measurement energy range and increasing the number of integration times to 10 for each measurement. The grain boundary P concentration P is calculated from the relative sensitivity coefficients of the elements detected by the above method.b The test conditions for Auger analysis are as shown in Table 3.

[0061] [Metal structure] The metal structure of the surface layer of the carburized part of this embodiment is primarily a tempered martensite structure. Specifically, it is primarily composed of tempered martensite and retained austenite. The tempered martensite structure is an acicular metal structure, and it is difficult to measure the proportion of structures other than tempered martensite and retained austenite. Even if structures other than tempered martensite and retained austenite are clearly present, the effects of the present invention can be achieved as long as the proportion of structures other than tempered martensite and retained austenite is 20% or less. Preferably, the proportion of structures other than tempered martensite and retained austenite is 10% or less.

[0062] The carburized part of this embodiment has been described above, but the above-described surface hardness, core hardness, grain boundary P concentration, and metallographic structure can be obtained by subjecting a steel material having the above-described chemical composition to heat treatment under the conditions described below, for example.

[0063] [Manufacturing method] The manufacturing method according to this embodiment includes a forming step in which steel having the above chemical composition is used as a material and formed into the shape of a part such as a gear, a carburizing step in which the resulting formed body (intermediate product) is carburized and quenched, an induction hardening step in which the carburized formed body is induction hardened, and a tempering step in which it is tempered at 130 to 200° C. However, the manufacturing method for a carburized part according to this embodiment is not limited to this mode.

[0064] [Molding process] First, a steel material satisfying the above-mentioned chemical composition is produced. For example, molten steel having the above-mentioned chemical composition is produced, and a cast piece (slab or bloom) is produced by continuous casting using the molten steel. Alternatively, an ingot may be produced by ingot casting using the molten steel. Next, the cast piece or ingot is hot worked to produce a billet. The billet is hot worked to produce a steel bar or wire rod. The hot working may be hot rolling or hot forging.

[0065] In order to homogenize the grain size of the structure after hot working, normalizing in accordance with JIS B 6911:2010 "Normalizing and annealing of steel" or isothermal annealing (IA) for the purpose of reducing the material hardness may be performed before the processing described below. The structure after hot working or normalizing is a mixed structure of ferrite + pearlite or ferrite + pearlite + bainite, and the average Vickers hardness is preferably 130 to 220 HV.

[0066] The produced steel bar or wire rod is cold forged or machined to produce an intermediate product of a desired shape. Machining includes, for example, cutting and drilling. The shape of the intermediate product is formed by a well-known method. For example, if the part is a gear, it is machined by broaching or the like.

[0067] [Carburizing process] The manufactured intermediate product is carburized at a carburizing temperature of 870°C to 1050°C and cooled from the austenite region. The carburizing method may be vacuum carburizing or gas carburizing. Nitriding is not performed in this embodiment. The carburizing conditions may be those commonly used and well-known. Furthermore, if the product is reheated to the austenite region to a depth of 1.5 mm below the surface in the subsequent induction hardening process, the cooling in the carburizing process may be quenching or slow cooling. Furthermore, if induction hardening cannot be performed immediately after the carburizing process, so-called aging cracks may occur due to residual stresses generated during the carburizing heat treatment. In such cases, low-temperature tempering at 130°C to 200°C is desirable. Furthermore, in this embodiment, no other treatments, such as graphitization, are performed between the carburizing and induction hardening processes.

[0068] [High-frequency hardening process] After the carburizing process, induction hardening is carried out once or more under conditions that satisfy the following formulas (1) and (2), and then a tempering process is carried out at 130 to 200°C.

[0069] 1000 <T1 ···(1) 50 <y ···(2) In the above formulas (1) and (2), T1 is the hardening temperature (°C) on the surface of the part, and y is the average cooling rate (°C / sec) from the maximum heating temperature to 500°C during the induction hardening process.

[0070] The reasons for specifying the quenching temperature T1 during induction hardening and the average cooling rate y will be explained below.

[0071] <Quenching temperature T1: Over 1000°C> The higher the induction hardening temperature T1, the higher the grain boundary P concentration P b However, if the quenching temperature is less than 1000°C, the grain boundary P concentration P b exceeds 0.8 at%, and P b / P m Therefore, the reduction in the temperature becomes insufficient, making it difficult to sufficiently improve the low-cycle fatigue strength. Therefore, the quenching temperature T1 during induction hardening is above 1000°C. It is preferably 1020°C or higher. More preferably, it is 1030°C or higher. The upper limit of the quenching temperature T1 may be equal to or lower than the liquidus temperature, which is the upper limit of the austenite single-phase region. However, in practice, in order to suppress coarsening of the austenite grain size, the upper limit of the quenching temperature T1 is preferably about 1100°C.

[0072] <Average cooling rate: more than 50℃ / sec> The faster the average cooling rate from the maximum heating temperature to 500°C during the induction hardening process, the more the diffusion of P on the grain boundaries during cooling is suppressed, and the increase in the grain boundary P concentration can be avoided. To achieve this effect, the average cooling rate from the maximum heating temperature to 500°C is set to more than 50°C / sec. Therefore, in this embodiment, even if the hardening temperature T1 is sufficiently increased, the grain boundary P concentration P b Even if the P concentration can be sufficiently reduced, if the average cooling rate is 50°C / sec or less, P diffusion occurs actively during cooling, causing the grain boundary P concentration to increase again, resulting in a decrease in low-cycle fatigue strength. b / P m In order to sufficiently reduce the hardening temperature T1, it is important to achieve both a higher quenching temperature T1 and a faster average cooling rate. The temperature range for controlled cooling is from the maximum heating temperature to 500°C because the temperature range in which P diffusion becomes active is above 500°C. Therefore, in this embodiment, the average cooling rate up to 500°C during induction hardening is specified using 500°C as a guideline.

[0073] The average cooling rate for water and oil cooling varies depending on the dimensions of the part. For example, for a small part with a diameter of about 20 mm, the average cooling rate to 500°C is about 30°C / sec with oil cooling and about 150°C / sec with water cooling. For a large part with a diameter of about 95 mm, the average cooling rate to 500°C is about 10°C / sec with oil cooling and about 50°C / sec with water cooling. For parts smaller than 20 mm, even oil cooling can result in a cooling rate of over 50°C / sec, but water cooling is preferable to ensure an average cooling rate of consistently over 50°C / sec to 500°C.

[0074] Furthermore, the induction hardening treatment may be performed once or multiple times. By performing the induction hardening treatment multiple times, it is possible to refine the crystal grains. When the induction hardening treatment is performed multiple times, it is sufficient that the final treatment satisfies the above conditions, and the other treatments may be performed under general conditions.

[0075] The carburized component of this embodiment can be manufactured by the above steps.

[0076] The carburized part and its manufacturing method according to this embodiment have been described above. According to this embodiment, the grain boundary P concentration can be reduced and grain boundary strength can be improved by performing optimal heat treatment, without reducing the P concentration in the steel, which increases manufacturing costs. As a result, it is possible to provide a carburized part with significantly improved low-cycle fatigue strength compared to conventional parts. Therefore, the carburized part according to this embodiment is suitable for use as, for example, gears in automobiles and industrial machinery, especially in machines powered by electric motors. [Example]

[0077] Next, an example of the present invention will be described. However, the conditions in the example are an example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions. The present invention can employ various conditions as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0078] (Manufacturing steel for carburized parts) Steels a to z having the chemical compositions shown in Table 1 were melted in a 50 kg vacuum melting furnace to produce molten steel, which was then cast into ingots. Steels a to o in Table 1 are steels having the chemical compositions specified in the present invention. On the other hand, steels p to z are comparative steels in which at least one element does not meet the chemical composition specified in the present invention. In Table 1, underlines indicate compositions outside the range of the present invention, and blank spaces indicate that no alloying elements were intentionally added. In addition, of the compositions of steels a to z shown in Table 1, the components other than those shown in Table 1 (the balance) are Fe and impurities. Here, the chemical compositions of steels a and b correspond to SCM420 specified in JIS G 4052:2008.

[0079] The obtained ingot was hot forged into a steel bar with a diameter of 60 mm. In the hot forging, the ingot was first held at a temperature between 1100°C and 1200°C in a heating furnace for 5 hours, and then forged into the aforementioned steel bar. After forging, it was allowed to cool in the atmosphere. Thereafter, the steel bar (vacuum melted material) was subjected to a normalizing treatment. The heating temperature in the normalizing treatment was 925°C, and the holding time was 2 hours. Thereafter, the steel bar was allowed to cool in the atmosphere.

[0080] (Creating test specimens) After normalizing, the 60 mm diameter steel bars were machined to prepare cantilever fatigue test specimens with the shape shown in Figure 1. Each test specimen was taken from a position of R / 2 (R: radius of the steel bar). When preparing cantilever fatigue test specimens from the steel bars, the test specimens were taken so that their longitudinal direction coincided with the longitudinal direction of the steel bar. However, all four steels (test numbers 28 to 31) from steel w to z had high hardness (HV 230 or more) after normalizing due to their chemical composition, and had poor cold workability. Therefore, these were judged to be unsuitable for processing, and no further processing was carried out.

[0081] (Carburizing treatment) The prepared test pieces were subjected to gas carburizing and quenching treatment under the following two conditions. <1> Aiming for a Cp value of 0.8%: 930°C x 80 min (Cp value 1.0%) → 930°C x 60 min (Cp value 0.8%) → 830°C x 30 min (Cp value 0.8%) → Oil cooling (oil temperature: 80°C) <2> Aiming for 1.0% Cp value: 930℃ x 80 min (1.0% Cp value) → 930℃ x 60 min (1.0% Cp value) → 830℃ x 30 min (1.0% Cp value) → Oil cooling (oil temperature: 80℃)

[0082] The cooling rate to 500°C during oil quenching was 30°C / sec. The test numbers and carburizing conditions are shown in Table 2. EPMA measurements confirmed that the surface carbon concentration after carburizing was approximately 0.65 to 0.80%. In this example, induction hardening could not be performed within 24 hours of carburizing and quenching, so low-temperature tempering at 200°C for 120 minutes was performed after carburizing and quenching. Note that if induction hardening is performed immediately after carburizing and quenching, this low-temperature tempering does not need to be performed.

[0083] (High frequency hardening treatment) The gas carburized and quenched test pieces were subjected to induction hardening once under the conditions (IH conditions) in Table 2. The heating hold time during induction heating was set to 10 seconds or less to prevent the growth of gamma grains and decarburization. When water cooling was used, the average cooling rate from the maximum heating temperature (quenching temperature) during quenching to 500°C was 100°C / second. When oil cooling was used, the average cooling rate was 30°C / second.

[0084] (Tempering treatment) The test pieces after induction hardening were tempered at a temperature of 200°C for 120 minutes.

[0085] Using the above manufacturing process, carburized parts (cantilever fatigue test specimens) were produced with test numbers 1 to 27. In all test specimens, the metal structure of the carburized surface layer was composed of tempered martensite and retained austenite, with no remaining structure being identified or comprising less than 10% by area.

[0086] (Evaluation test) (Hardness measurement) The cantilever fatigue test specimens were cut perpendicular to their longitudinal direction and the exposed cross sections were mirror-polished. Then, hardness measurements were taken at three locations on the cross sections, 0.10 mm below the surface of the specimen, and the average value was calculated as the Vickers hardness (surface hardness) at 0.10 mm below the surface. The measurement load was 300 gf. Furthermore, on the same cross section, hardness was measured at three locations 1.5 mm below the surface, and the average value was calculated as the Vickers hardness (core hardness) at 1.5 mm below the surface.

[0087] (Grain boundary P concentration measurement) Auger test specimens with the shape shown in Figure 2B were taken from the surface of the cantilever fatigue test specimens that had been treated as described above, at the location shown in Figure 2A. Specifically, the 3.7 x 18 mm back surface of the notch of the Auger test specimen encompassed the 20 x 100 mm surface of the cantilever fatigue test specimen, and the center of the 3.7 x 18 mm back surface of the Auger test specimen (the surface opposite the notch) was aligned with the center of the 20 x 100 mm surface of the cantilever fatigue test specimen. Note that all values ​​in Figures 2A and 2B are in mm. Details of the Auger test conditions are shown in Table 3. Next, each Auger test specimen was placed in an Auger electron spectrometer (PHI-700, ULVAC-PHI, FE type) and cooled inside the device until fractured. Spectroscopic analysis was then performed on the grain boundary fracture surface within a 50 μm depth range from the carburized surface. The sample temperature during cooling was approximately -120°C. Since there was some variation in the grain boundary concentration, measurements were taken at seven grain boundaries and the average value was calculated. To calculate the grain boundary concentration, a narrow scan was used, narrowing the measurement energy range and increasing the number of measurements to 10. The grain boundary P concentration (at%) was calculated from the relative sensitivity coefficients of the elements detected by the above method and converted to mass%. The grain boundary P concentration converted to mass% is shown in Table 2.

[0088] (Low cycle bending fatigue test) Low-cycle bending fatigue strength was measured by a cantilever bending fatigue test using a tension-compression hydraulic servo fatigue testing machine (Shimadzu Corporation, Servopulsar "EHF-UM50kN-10L"). The shape of the cantilever fatigue test specimen is shown in Figure 1. As shown in Figure 1, the cantilever fatigue test specimen measured 20 x 14(10) x 210 mm, with a notch of R2.0 located 110 mm from the end. The force point was located 10 mm from the end. The dimensions in Figure 1 are in mm. The central axis of the cantilever fatigue test specimen was coaxial with the central axis of the steel bar.

[0089] Using the above cantilever fatigue test specimen, a fatigue test shown in Table 4 was carried out at room temperature in an air atmosphere. The test was carried out under pulsating load control, and the number of times to fracture was measured. In this example, assuming application to gear parts, evaluation was carried out according to the following criteria. First, cantilever fatigue test specimens were prepared using a steel that met the SCM420 standard of JIS G 4053:2016, following a typical manufacturing process: normalizing, specimen processing, eutectoid carburizing in a gas carburizing furnace, and low-temperature tempering. The 100-cycle fracture strength of these standard specimens (test numbers 26 and 27) was used as the standard. A fatigue limit of the target test number was deemed a pass (◯) if it exceeded this standard by 15% or more, and a fail (×) if it exceeded this standard by less than 15%. Because it is known that reducing the P content in steels also reduces the grain boundary P concentration, test number 26 was used as the standard for the 100-cycle fracture strength of steels a and c to z, which have a P concentration of 0.010% or more, and test number 27 was used as the standard for the 100-cycle fracture strength of steel b, which has a P concentration of less than 0.010%. The breaking strength after 100 cycles of test number 26 was 4800N, and the breaking strength after 100 cycles of test number 27 was 5200N.

[0090] [Table 1]

[0091] [Table 2]

[0092] [Table 3]

[0093] [Table 4]

[0094] (Test results) The test results are shown in Table 2. Test Nos. 1 to 15 had chemical compositions, surface hardness, and core hardness that were within the ranges of the present invention, and therefore the steel bars after normalizing had sufficient machinability, and as carburized parts, the grain boundary P concentration was low and excellent low-cycle bending fatigue properties were obtained.

[0095] On the other hand, in test number 16, the P content of the steel component of the part was excessive. b / P m Although the value was within the range of the present invention, the grain boundary P concentration could not be sufficiently reduced, which resulted in a decrease in the grain boundary strength, and as a result, the low cycle fatigue strength did not achieve the target.

[0096] In test number 17, the amount of S in the steel components of the part was excessive, and the target low cycle fatigue strength was not achieved due to the coarsening of sulfides.

[0097] In test number 18, the Al content in the steel components of the part was excessive, and the target low cycle fatigue strength was not achieved due to the coarsening of inclusions.

[0098] In test number 19, the carbon content of the steel component of the part was insufficient, and the core hardness was not within the target range, so fatigue fracture occurred from an internal origin, and the low-cycle fatigue strength did not achieve the target.

[0099] In test number 20, the Si content of the steel components of the part was insufficient, and the core hardness was not within the target range, so fatigue fracture occurred at an internal origin, and the low-cycle fatigue strength did not achieve the target.

[0100] In test number 21, the Mn content of the steel component of the part was insufficient, and the core hardness was not within the target range, so fatigue fracture occurred at an internal origin, and the low-cycle fatigue strength did not achieve the target.

[0101] In test number 22, the Cr content of the steel components of the part was insufficient, and the core hardness was not within the target range, so fatigue fracture occurred at an internal origin, and the low-cycle fatigue strength did not achieve the target.

[0102] In test number 23, the induction hardening temperature was outside the range, and the hardening temperature was too low, resulting in a high grain boundary P concentration, which reduced the grain boundary strength and prevented the low cycle fatigue strength from achieving the target.

[0103] In test number 24, the induction hardening temperature was outside the range, and the hardening temperature was too low, resulting in a high grain boundary P concentration, which reduced the grain boundary strength and prevented the low cycle fatigue strength from achieving the target.

[0104] In test number 25, the cooling rate during induction hardening was outside the range, and the grain boundary P concentration increased during cooling, resulting in a decrease in grain boundary strength, and as a result, the target low-cycle fatigue strength was not achieved.

[0105] Test No. 26 is a reference test piece for the 100-cycle rupture strength of Steel A and Steels C to Z, and Test No. 27 is a reference test piece for the 100-cycle rupture strength of Steel B. Both Test Nos. 26 and 27 are comparative examples in which induction hardening was not performed after carburizing.

[0106] Comparing Test No. 1 and Test No. 26, both of which used the same steel a (P concentration: 0.015%), we found that the grain boundary P concentration was sufficiently reduced in Test No. 1, where induction hardening was performed under appropriate conditions, and that the low-cycle fatigue strength was significantly improved. Similarly, in Test No. 2 and Test No. 27, where induction hardening was performed under appropriate conditions, the grain boundary P concentration was sufficiently reduced, and the low-cycle fatigue strength was significantly improved. Furthermore, because Steel b has a relatively low P concentration, the grain boundary P concentration in Test No. 27, which used Steel b, was reduced to a certain extent even without induction hardening. However, in Test No. 1, which used Steel a (a comparative example) with a high P concentration, despite its high P concentration, the induction hardening of the present invention under appropriate conditions reduced the grain boundary P concentration compared to Test No. 27, which used Steel b with a low P concentration. In other words, according to the present invention, the grain boundary P concentration can be significantly reduced by optimizing the heat treatment conditions without excessively reducing the P content of the raw material.

[0107] In addition, in test numbers 28 to 31, the Si content, Mn content, Cr content, or Mo content of the steel components of the parts was excessive, and the hardness after normalizing was high, so the machinability and cold workability were reduced. It was determined that the workability of the steel materials was poor, and no further tests were conducted. [Industrial Applicability]

[0108] As described above, the use of the carburized steel parts of the present invention, which have excellent low-cycle bending fatigue strength, enables significant miniaturization and weight reduction of gears, such as differential gears and transmission gears for automobiles. As a result, it becomes possible to improve the fuel efficiency of automobiles and reduce CO2 emissions. Furthermore, by using the manufacturing process of this embodiment, it is possible to reduce the grain boundary P concentration without reducing the P content of the steel to the utmost limit in the steelmaking process, resulting in significant cost savings. Therefore, the effects of the present invention are extremely significant, and the present invention has great industrial applicability.

Claims

1. The composition of the core is, in mass%, C: 0.10-0.30%, Si: 0.03-0.80%, Mn: 0.40 to 1.30%, P: 0.005-0.020%, S: 0.003-0.060%, Cr: 0.10-2.00%, Al: 0.050% or less, N: 0.0030 to 0.0300%, and O: 0.0020% or less and the balance being Fe and impurities, The P concentration P of the steel material is the P amount of the core ｍ The grain boundary P concentration P, which is the amount of P at the grain boundary at a depth of 50 μm below the surface of the part, ｂ The ratio P ｂ / P ｍ is 30.00 or less, The Vickers hardness at a depth of 0.10 mm below the surface of the part is 650 HV or more, A carburized part characterized in that the Vickers hardness at a depth of 1.5 mm below the surface of the part is 300 HV or more.

2. Furthermore, the composition of the core portion is, in mass %, Mo: 0-1.00%, V: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 1.00%, Bi: 0-0.10%, Nb: 0 to 0.100%, Ti: 0 to 0.200%, and B: 0 to 0.005% 2. The carburized part according to claim 1, wherein the carburized part contains one or more of the following:

3. 2. A method for manufacturing a carburized component according to claim 1, comprising the steps of: The composition is, in mass %, C: 0.10-0.30%, Si: 0.03-0.80%, Mn: 0.40 to 1.30%, P: 0.005-0.020%, S: 0.003 to 0.060% or less, Cr: 0.10-2.00%, Al: 0.050% or less, N: 0.003 to 0.030% or less, O: 0.0020% or less a forming step of forming a steel material containing the above-mentioned and the remainder being Fe and impurities into a part shape; a carburizing step in which carburizing is performed at a carburizing temperature of 870°C to 1050°C, followed by cooling from the austenite region; an induction hardening step of performing induction hardening treatment under conditions that satisfy the following formulas (1) and (2); Next, a tempering process of tempering at 130 to 200 ° C. A method for manufacturing a carburized part, comprising: 1000<T1...(1) 50<y...(2) In the above formulas (1) and (2), T1 is the hardening temperature (°C) on the surface of the part, and y is the average cooling rate (°C / sec) from the maximum heating temperature to 500°C during the induction hardening process.

4. The composition is, in mass %, Mo: 0-1.00%, V: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 1.00%, Bi: 0-0.10%, Nb: 0 to 0.10%, Ti: 0 to 0.20%, and B: 0 to 0.0050% 4. The method for manufacturing a carburized part according to claim 3, wherein the carburized part contains one or more of the following:

5. 3. The carburized part according to claim 1 or 2, which is a machine structural part.

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