Vacuum-cured carburized parts

Abstract

In high-temperature, reduced-pressure carburizing, where coarse crystal grains tend to form on the surface of the carburized layer, the objective is to provide a reduced-pressure carburized component that stably suppresses the formation of coarse crystal grains, exhibits excellent grain size characteristics, and suppresses coarse cementite at the edges, thereby providing a reduced-pressure carburized component with superior toughness. [Solution] A vacuum carburized part having an edge portion of 150° or less, an area ratio of grain boundary carbides in the edge portion of 5% or less, and prior austenite grain size at a depth of 0.05 mm from the surface having a grain size number of 6 or higher.

Description

Technical Field

[0001] This application relates to a vacuum carburized part in which the surface layer of the part is carburized by vacuum carburizing treatment.

Background Art

[0002] In steel parts, conventionally, carburizing and quenching has been used as a case hardening method in which steel is heated in the austenite region, carbon is then introduced and diffused, and then quenched to harden the surface.

[0003] For example, in a depth region of 1.5 mm or more from the surface, the component composition is, in mass%, C: 0.10 to 0.40%, Si: 0.10 to 3.00%, Mn: 0.50 to 3.00%, Cr: 0.30 to 3.00%, Al: 0.010 to 0.050%, N: 0.003 to 0.030%, S: 0.003 to 0.030%, P: 0.030% or less, Mo: 0 to 3.00%, B: 0 to 0.0050%, Nb: 0 to 0.100%, Ti: 0 to 0.100%, V: 0 to 0.30%, Ni: 0 to 0.40%, In: 0 to 0.02%, Cu: 0 to 0.20%, Bi: 0 to 0.300%, Pb: 0 to 0.50%, and REM: 0 to 0.020%, the balance being Fe and impurities, the Vickers hardness at a depth of 1.5 mm from the surface is 200 to 400 HV, and in the depth region from the surface to 0.10 mm, the C content is, in mass%, 0.60 to 1.20%, the fraction of the quenched structure is 99.00% or more in area ratio, the fraction of grain boundary cementite is 0.50% or less in area ratio, and the fraction of the incompletely quenched structure is 0.50% or less in area ratio, and the Vickers hardness at a depth of 0.10 mm from the surface is 700 HV or more. A carburized part having these characteristics has been proposed (see Patent Document 1).

[0004] Furthermore, a carburizing steel has been proposed for use in carburizing parts, having a composition comprising C: 0.10-0.35 mass%, Si: 0.50 mass% or less, Mn: 0.30-1.50 mass%, Cr: 1.10-2.00 mass%, P: 0.02 mass% or less, S: 0.03 mass% or less, Al: 0.01-0.05 mass%, and N: 0.030 mass% or less, with the remainder being Fe and unavoidable impurities; a cementite fraction of 5% or less at a depth of 0.05 mm from the surface; a hardness of HV600 or higher at a depth of 0.05 mm from the surface; an austenite grain size of 5 or higher at a depth of 0.05 mm from the surface; and a hardness of HV650 or higher at a depth of 0.10 mm from the surface (see Patent Document 2).

[0005] Furthermore, a vacuum carburizing steel has been proposed for use in carburized parts, containing, by mass%, C: 0.10-0.30%, Si: 1.41-2.50%, Mn: 1.40-3.00%, P: 0.030% or less, S: 0.060% or less, Cr: 0.01-0.59%, Al: 0.010-0.100%, N: 0.0030-0.0300%, Mo: 0-0.20%, Cu: 0-0.20%, Ni: 0-0.40%, Nb: 0-0.10%, Ti: 0-0.100%, and B: 0-0.0030%, with the remainder being Fe and impurities, having a Si-Cr ratio of 0.90 or more, and a Si-0.8×Mn ratio of 0.50 or less (see Patent Document 3). [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2020 / 202406 [Patent Document 2] Japanese Patent Publication No. 2022-55308 [Patent Document 3] Japanese Patent Publication No. 2019-7063 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] Vacuum carburizing is a method of carburizing a material by heating it in a reduced-pressure atmosphere below atmospheric pressure and introducing a carburizing gas into the furnace to bring it into contact with the high-temperature material. Unlike gas carburizing, which involves impregnation with gas under pressure, vacuum carburizing is less prone to grain boundary oxidation. However, vacuum carburizing has a different carburizing mechanism than gas carburizing, and controlling the carbon concentration distribution and carbide precipitation state is difficult. Therefore, simply switching from gas carburizing is not easy, and the surface during carburizing becomes highly carbon-concentrated. Consequently, the edges of the carburized parts tend to be excessively carburized due to the overlap of diffusion fields, making it easy for carbides to form at grain boundaries. In addition, high-temperature vacuum carburizing tends to generate coarse crystal grains even on the surface of the carburized layer (the surface layer of flat parts). Since the strength decreases due to coarse carbides and coarse crystal grains, there is a demand for increased strength.

[0008] However, neither Patent Document 1 nor Patent Document 2 mentioned above evaluates the cementite fraction (area fraction) in the edge portion, which is prone to excessive carburization, and therefore lacks countermeasures against excessive carburization of the edge portion. Furthermore, although Patent Document 3 focuses on cementite in the edge portion, it does not control the grain size characteristics in any way, and the mechanical properties are still prone to deterioration.

[0009] The problem that this invention aims to solve is to provide a vacuum-carburized component with excellent toughness by providing a vacuum-carburized component that can stably suppress the generation of coarse crystal grains in high-temperature vacuum carburizing, where coarse crystal grains tend to be generated on the surface of the carburized layer, and which has excellent crystal grain size characteristics and can suppress coarse cementite at the edge. [Means for solving the problem]

[0010] The first means for solving the problems of the present invention is a vacuum carburized part having an edge portion of 150° or less, an area ratio of grain boundary carbides in the edge portion of 5% or less, and a prior austenite grain size at a depth of 0.05 mm from the surface having a crystal grain size number of 6 or higher. In other words, it is a vacuum-carburized part with fine flat surfaces and a low area ratio of carbides at the edges.

[0011] The second method is a vacuum carburized part described in the first method, using vacuum carburized steel that satisfies formula A, with the following composition in mass%, C: 0.10-0.30%, Si: 0.05-2.00%, Mn: less than 0.10-0.50%, Cr: 1.3-2.5%, Nb: 0.020-0.100%, Al: 0.020-0.100%, N: 0.0030-0.0300%, the remainder being Fe and unavoidable impurities, wherein the unavoidable impurities contain P: 0.030% or less, S: 0.030% or less, and Cu: 0.30% or less. Formula A: 6.500≦4[C]+8.9[Cr]+12[Mn]-17[Si]≦13.900. However, in equation A, substitute the mass percentage value of the content of the component corresponding to the element symbol in [ ] into the brackets [ ].

[0012] The third method involves, in mass%, C: 0.10~0.30%, Si: 0.05~2.00%, Mn: less than 0.10~0.50%, Cr: 1.3~2.5%, Nb: 0.020~0.100%, Al: 0.020~0.100%, N: 0.0030~0.0300%, and optionally Ni: 0~1.00%, Mo: 0~1.00%. The vacuum carburized part described in the first method uses vacuum carburized steel that satisfies formula A, containing one or more of Ti: 0-0.20%, V: 0-0.50%, and B: 0-0.0050%, with the remainder being Fe and unavoidable impurities, wherein the unavoidable impurities contain P: 0.030% or less, S: 0.030% or less, and Cu: 0.30% or less. Formula A: 6.500≦4[C]+8.9[Cr]+12[Mn]-17[Si]≦13.900. However, in equation A, substitute the mass percentage value of the content of the component corresponding to the element symbol in [ ] into the brackets [ ]. [Effects of the Invention]

[0013] The vacuum-cured carburized parts of the present invention can suppress the formation of grain boundary carbides at edges with an angle of 150° or less, and can also suppress abnormal grain growth on the surface. Furthermore, the carburized steel parts using the steel of the present invention have a curing capacity of 30 J / cm². 2Since the impact value is as described above, it has excellent toughness. Therefore, according to the present invention, the grain boundary carbides in the edge portion of the vacuum carburized part are suppressed, and the crystal grain size characteristics are excellent, so that a carburized steel part with excellent toughness can be obtained.

Brief Description of the Drawings

[0014] [Figure 1] Figure 1 is a schematic diagram of the shape of a test piece for evaluating a 90° edge portion. (a) is a plan view, (b) is a front view, and (c) is a right side view. [Figure 2] Figure 2 is a graph showing the relationship between the impact value and the parameter of Formula A in the example using the inventive steel and the comparative example using the comparative steel. [Figure 3] Figure 3 is a graph showing the relationship between the crystal grain size and Formula A in the example using the inventive steel and the comparative example using the comparative steel. [Figure 4] Figure 4 is a plan view of a schematic diagram of the shape of a test piece for evaluating a 60° edge portion and a 120° edge portion.

Modes for Carrying Out the Invention

[0015] Prior to the embodiments of the present invention, the reasons for defining the area ratio of grain boundary carbides in the edge portion of the vacuum carburized part, the prior austenite grain size at a depth of 0.05 mm from the surface, the suitable component range for the steel used in the vacuum carburized part, the index of Formula A, etc. will be explained.

[0016] Area ratio of grain boundary carbides in the edge portion of the vacuum carburized part at 150° or less: 5% or less When the grain boundary carbides are excessive, the mechanical properties deteriorate. Therefore, the area ratio of grain boundary carbides in the edge portion of the vacuum carburized part at 150° or less is set to 5% or less. Preferably, the area ratio of grain boundary carbides in this edge portion is 3% or less.

[0017] Prior austenite grain size at a depth of 0.05 mm from the surface of the vacuum carburized part: No. 6 or more in terms of crystal grain size number When the prior austenite grain size of the vacuum carburized part coarsens, the mechanical properties deteriorate. Therefore, the prior-austenite grain size at a depth of 0.05 mm from the surface of the flat part shall be No. 6 or more in terms of the crystal grain size number defined in JIS (Japanese Industrial Standards) G0551. More preferably, the crystal grain size number is No. 8 or more.

[0018] Next, the components of steel suitable for the vacuum carburized part of the present invention will be sequentially described. The % of the elemental components is by mass, and the balance of the steel is Fe and inevitable impurities.

[0019] C: 0.10 to 0.30% C is a component that increases the material hardness. If C is too little, the core hardness after carburization will decrease, resulting in insufficient strength of the part. From this perspective, C is preferably 0.10% or more, and more preferably, C is 0.15% or more. On the other hand, if C is too much, the material hardness will increase too much, resulting in a decrease in workability or a decrease in core toughness. Therefore, C is preferably 0.30% or less.

[0020] Si: 0.05 to 2.00% Si is a component that increases the material hardness and is also a useful component as a deoxidizer. If Si is too little, deoxidation will be insufficient. From these perspectives, Si is preferably 0.05% or more, and more preferably 0.30% or more. If Si is too much, the material hardness will increase too much, resulting in a decrease in workability and an increase in ferrite in the non-carburized layer. Therefore, Si is preferably 2.00% or less, and more preferably 0.70% or less.

[0021] Mn: 0.10 to 1.50% Mn is a useful component for quenching. If it is too little, the hardenability will be insufficient. From this perspective, Mn is preferably 0.10% or more. If Mn is too much, the machining property will decrease, and the crystal grains are likely to coarsen during carburization. Therefore, Mn is preferably 1.50% or less, and more preferably 0.5% or less.

[0022] Cr: 1.30~2.50% Cr is a component that increases hardenability and strength, as well as material hardness. Therefore, the Cr content is preferably 1.30% or more, and more preferably 1.50% or more. If the Cr content is excessive, the material hardness will increase, and the workability will decrease. Also, if the Cr content is excessive, a large amount of grain boundary carbides will be generated, and these carbides are likely to remain after the completion of vacuum carburizing. Therefore, the Cr content is preferably 2.5% or less, and more preferably 2.3% or less.

[0023] Nb: 0.02~0.10% Nb is a component that generates fine carbonitrides and suppresses grain coarsening. Therefore, if there is insufficient Nb, there will be insufficient fine carbonitrides, and the grain coarsening suppression effect will be reduced, resulting in insufficient toughness and fatigue strength of the part. For this reason, it is preferable that the Nb content be 0.02% or more. If there is too much Nb, the amount of carbonitrides will be excessive, which tends to reduce machinability. For this reason, it is preferable that the Nb content be 0.10% or less, and more preferably 0.08% or less.

[0024] Al: 0.020~0.100% Al is a component that generates fine nitrides and is also a useful component as a deoxidizing agent. If the amount of Al is insufficient, deoxidation tends to be inadequate, and the lack of fine nitrides can easily lead to grain coarsening, which can reduce toughness and fatigue properties. Therefore, it is preferable that the amount of Al be 0.020% or more. If the amount of Al is excessive, the amount of alumina oxide increases, which tends to reduce fatigue properties and workability. Therefore, it is preferable that the amount of Al be 0.100% or less, and more preferably 0.050% or less.

[0025] N: 0.0030~0.0300% N is a useful component for suppressing grain coarsening by forming fine carbonitrides. A deficiency in N leads to a lack of fine carbonitrides, making grain coarser. Therefore, the N content is preferably 0.0030% or more, and more preferably 0.0040% or more. If N is in excess, coarse carbonitrides will be formed, reducing fatigue properties and workability. Therefore, the N content is preferably 0.0300% or less, and more preferably 0.0020% or less.

[0026] The steel components used in the vacuum carburized parts of the present invention may further contain one or more of the following: Ni: 0-1.00%, Mo: 0-1.00%, Ti: 0-0.20%, V: 0-0.50%, and B: 0-0.0050%. The reasons for adding these will now be explained.

[0027] Ni: 0~1.00% Ni is an ingredient that can increase the hardness of a material when added, but if there is too much Ni, the material hardness will increase excessively, reducing processability and increasing costs. Therefore, when adding Ni, it is preferable to keep the amount to 1.00% or less.

[0028] Mo: 0~1.00% Mo is an ingredient that can increase the hardness of a material when added, but if there is too much Mo, the material hardness will increase excessively, reducing processability and raising costs. Therefore, when adding Mo, it is preferable to keep the amount to 1.00% or less.

[0029] Ti: 0~0.20% Ti is a component that forms carbonitrides. If the Ti content exceeds 0.20%, carbonitrides become excessive, which reduces processability. Therefore, when adding Ti, it is preferable to keep the amount below 0.20%.

[0030] V: 0~0.50% V is a component that forms carbonitrides. If the amount of V exceeds 0.50%, the carbonitrides become excessive, which reduces processability. Therefore, when adding V, it is preferable to keep the amount below 0.50%.

[0031] B: 0~0.0050% B is an ingredient that increases the hardness of the material. If B is added in excess, the hardness of the material will increase, and the processability will decrease. Therefore, when adding B, it is preferable that the amount is 0.0050% or less.

[0032] The remaining steel components are Fe and unavoidable impurities. We will now describe the more preferable cases for P, S, and Cu among these unavoidable impurity components.

[0033] P:0.030% or less P is a component that can be present in steel as an unavoidable impurity. Since P reduces toughness due to grain boundary segregation, it is more preferable that the amount of P be 0.030% or less.

[0034] S: 0.030% or less S is a component that can be present in steel as an unavoidable impurity. Since S forms MnS, which leads to a decrease in toughness and fatigue strength, it is more preferable that its content be 0.030% or less.

[0035] Cu: 0.30% or less Cu is a component that can be present in steel as an unavoidable impurity. Since Cu can lead to a decrease in hot workability, it is more preferable that its content be 0.30% or less.

[0036] Formula A: 6.500≦4[C]+8.9[Cr]+12[Mn]-17[Si]≦13.900 Formula A is an indicator related to the occurrence of coarse grains, and if the value of Formula A is too low, coarse grains will occur. From this viewpoint, the lower limit of the value of Formula A is preferably 6.500 or higher, and more preferably 8.000 or higher.

[0037] However, if the value in equation A is excessive, and if Mn and Cr are also excessive, a large amount of grain boundary carbides will be generated during carburizing (carburizing stage). As a result, more carbides than specified will remain after the vacuum carburizing is completed. If C is excessive, the concentration gradient of C between the surface and the interior after the carburizing stage will be gentle, which will hinder the diffusion of C, making it easier for grain boundary carbides to remain. From this perspective, the preferred upper limit for equation A is 13.900.

[0038] Embodiments of the present invention are described below. First, 100 kg each of the steels described in Table 1 (Invention Steels 1-16) and Table 2 (Comparative Steels 1-8), with the remainder being Fe and unavoidable impurities, were melted in a vacuum induction melting furnace (VIM) to obtain steel ingots. These were then forged into 32 mm diameter round bars at 1250°C, normalized by holding at 925°C for 1 hour and then air-cooling, and finally processed into test pieces. Note that the light gray areas in Table 2 indicate samples outside the preferred range.

[0039] (Regarding the test specimen) Test specimens were taken from the middle circumference of round bars forged from each inventive steel and comparative steel. For edge evaluation, test specimens with the shape shown in Figure 1 were prepared from these parts.

[0040] [Table 1]

[0041] [Table 2]

[0042] (Regarding vacuum carburizing treatment and evaluation) The test specimens prepared as described above were subjected to vacuum carburizing in a vacuum carburizing furnace under reduced pressure. First, for Examples 1 to 16, the inventive steels 1 to 16 were used as the steel type, and vacuum carburizing was performed under either 1000°C or 1050°C carburizing conditions. After that, the microstructure (grain size) of the carburized test specimens was evaluated.

[0043] Comparative Examples 1 to 10 are comparative examples in which comparative steels 1 to 8 and inventive steels 2 and 4 were used as the steel grades and carburized under the following conditions. Comparative Examples 1 to 8 were carburized under either a 1000°C carburizing or 1050°C carburizing condition, Comparative Example 9 was carburized under a 900°C carburizing condition, and Comparative Example 10 was carburized under a 930°C carburizing condition.

[0044] Vacuum carburizing is a carburizing method that directly introduces hydrocarbon-based carburizing gases such as acetylene (C2H2) and propane under reduced pressure. The stage in which carbon penetrates from the surface of the steel material is called the "carburizing stage," and the stage in which carbon diffuses from the surface of the steel material into the interior without introducing carburizing gas, thereby reducing the surface carbon concentration, is called the "diffusion stage." The procedures for each stage are described below.

[0045] (Carburizing at 1000℃) Heat to 1000°C for 40 minutes. →Carburizing treatment at 1000℃ for 10-40 minutes → Diffusion treatment at 1000℃ for 40-80 minutes →Maintain at 880℃ for 40 minutes → Hardening → Temper at 180℃ for 1.5 hours The conditions for the carburizing treatment (carburizing phase) are an acetylene gas atmosphere at a pressure of 150 Pa, and the conditions for the diffusion treatment (diffusion phase) are a vacuum (5 Pa or less). Furthermore, the carburizing treatment also includes treatment patterns in which carburizing and diffusion are repeated alternately two or more times.

[0046] (Carburizing at 1050℃) Heat to 1050°C for 40 minutes. →Carburizing treatment at 1050℃ for 8-30 minutes → Diffusion treatment at 1050℃ for 20-70 minutes →Maintain at 880℃ for 40 minutes → Hardening → Temper at 180℃ for 1.5 hours The conditions for carburizing are an acetylene gas atmosphere at a pressure of 150 Pa, and the conditions for diffusion are a vacuum (5 Pa or less). Furthermore, the carburizing treatment also includes treatment patterns in which carburizing and diffusion are repeated alternately two or more times.

[0047] (900°C carburizing) Carburizing conditions for Comparative Example 9 Heat to 900°C for 40 minutes. →Carburizing treatment at 900℃ for 110-160 minutes → Diffusion treatment at 900℃ for 150-200 minutes →Maintain at 880℃ for 40 minutes → Hardening → Temper at 180℃ for 1.5 hours The conditions for carburizing are an acetylene gas atmosphere at a pressure of 150 Pa, and the conditions for diffusion are a vacuum (5 Pa or less). Furthermore, the carburizing treatment also includes treatment patterns in which carburizing and diffusion are repeated alternately two or more times.

[0048] (930°C carburizing) Carburizing conditions for Comparative Example 10 Soak at 930°C for 40 minutes. →Carburizing treatment at 930℃ for 65-110 minutes → Diffusion treatment at 930°C for 100-150 minutes →Maintain at 880℃ for 40 minutes → Hardening → Temper at 180℃ for 1.5 hours The conditions for carburizing are an acetylene gas atmosphere at a pressure of 150 Pa, and the conditions for diffusion are a vacuum (5 Pa or less). Furthermore, the carburizing treatment also includes treatment patterns in which carburizing and diffusion are repeated alternately two or more times.

[0049] Regarding grain boundary carbides (area ratio) Grain boundary carbides in flat areas and edges can be observed as follows: The procedure is described below using the observation of an edge with a 90° apex angle (90° edge) as an example. First, a cross-section perpendicular to the longitudinal direction is taken at a position of 25 mm within the 50 mm length of the test specimen. After mirror polishing of both the flat portion and the 90° edge, picral etching is performed, and four fields of view are captured using a scanning electron microscope (SEM) at an observation magnification of 1000x, covering the range from the surface to a depth of 0.10 mm. The area ratio of carbides is then measured using image analysis (binarization). Since the edge is more prone to excessive carburization, more grain boundary carbides are visible than in the flat portion, so observing the edge can significantly evaluate its properties. The results for grain boundary carbides at the edge are shown in Tables 3 and 4. In Table 4, those outside the range defined by this invention are shown in light gray.

[0050] Furthermore, observation of edges with a 60° apex angle (60° edge) and 120° edges can be performed by preparing a test specimen as shown in Figure 4 (plan view). A 50mm x 50mm upper-plane test specimen was cut with a perpendicular cross section so that the 60° edge and 120° edge sections were formed, as shown in Figure 4. After mirror polishing, picral etching was performed, and four fields of view were captured using a scanning electron microscope (SEM) at an observation magnification of 1000x, covering the range from the surface to a depth of 0.10mm. The area ratio of carbides was then measured using image analysis (binarization). The results for grain boundary carbides at each edge are shown in Tables 3 and 4. In Table 4, those outside the range defined by the present invention are shown in light gray.

[0051] (Grain size characteristics) For the microstructure's grain size characteristics, the specimen was cut parallel to its longitudinal direction through its center, polished, and then subjected to saturated picric acid etching. The surface of the flat portion was then observed using an optical microscope at a position 0.05 mm from the surface (the observation field was 1 mm × 1 mm). For the evaluation of austenite grain size, the prior austenite grain size was measured according to JIS G0551 (2020), and the grain size number was determined. If the average grain size number was less than 6, it was determined that abnormal grain growth had occurred. The results are shown in Tables 3 and 4. In Table 4, those outside the range defined by this invention are shown in light gray.

[0052] (Charpy impact test specimen) The toughness of the test specimens was evaluated by a Charpy impact test based on JIS Z2242 (2018). The Charpy impact test was conducted at room temperature (23±5℃), and 10mm 10RC notched specimens (carburized only in the notched area and notched surface) were used. The results are shown in Tables 3 and 4. In Table 4, specimens that do not exhibit the effects of the present invention are shown in light gray.

[0053] [Table 3]

[0054] [Table 4]

[0055] In Examples 1 to 16 of the present invention, as shown in Table 3, the carbide area ratio at the edges is low, and the prior austenite grain size number in the flat areas is 6 or higher, indicating fine vacuum carburization, therefore 30 J / cm 2 This demonstrates that vacuum-treated carburized parts with excellent toughness, as shown above, can be obtained. Furthermore, as shown in the graph in Figure 2 illustrating the relationship between impact value and Equation A, all values ​​within the numerical range of Equation A have an impact value of 30 J / cm². 2 The above is good and it has excellent toughness.

[0056] Comparative Example 1 has a low Cr content and a low value for Equation A, which makes it prone to the formation of coarse grains. Furthermore, the coarsening of the prior austenite grains due to carburizing results in inferior toughness. In Comparative Example 2, the excess Cr content makes it easy for intergranular carbides to form, resulting in an overestimation of the value of Equation A. The large area ratio of carbides at the edges leads to an excess of carbides, which reduces the mechanical properties and results in poor toughness. In Comparative Example 3, C was excessive and the value of Equation A was also exceeded, resulting in a large and excessive area ratio of carbides at the edges. Consequently, the mechanical properties were reduced, and the toughness was poor. In Comparative Example 4, there is an excess of Si and an underestimation of the value of Equation A, and the prior austenite grains have become coarser due to carburizing, resulting in inferior toughness. In Comparative Example 5, the amount of Mn was excessive, and the value of Equation A was also exceeded. As a result, the area ratio of carbides at the edges became excessively large, leading to a decrease in mechanical properties and poor toughness. In Comparative Example 6, the amount of Si was insufficient, causing the value of Equation A to exceed the limit, and the area ratio of carbides at the edges became excessively large, resulting in reduced mechanical properties and inferior toughness. In Comparative Example 7, the value of Equation A is too low, and the toughness is inferior because the prior austenite grains have become coarser due to carburizing. In Comparative Example 8, in addition to the low value of formula A, the amount of Al is also low, resulting in coarsening of the prior austenite grains due to carburizing, and thus inferior toughness. Although Comparative Example 9 shares the same steel composition as Example 2, the different carburizing temperature resulted in a significantly larger and excessive proportion of carbides at the edges, leading to reduced mechanical properties and inferior toughness. Although Comparative Example 10 shares the same steel composition as Example 4, the different carburizing temperature resulted in a significantly larger and excessive proportion of carbides at the edges, leading to reduced mechanical properties and inferior toughness.

Claims

1. It has an edge portion of 150° or less, and the area ratio of grain boundary carbides in the edge portion is 5% or less. The prior austenite grain size at a depth of 0.05 mm from the surface is 6 or higher in terms of grain size number. Reduced pressure carburized parts.

2. In mass percent, The composition is as follows: C: 0.10-0.30%, Si: 0.05-2.00%, Mn: less than 0.10-0.50%, Cr: 1.3-2.5%, Nb: 0.020-0.100%, Al: 0.020-0.100%, N: 0.0030-0.0300%, with the remainder being Fe and unavoidable impurities. In the case of unavoidable impurities, P: 0.030% or less, S: 0.030% or less, Cu: 0.30% or less, A vacuum carburized component according to claim 1, using vacuum carburized steel that satisfies formula A. Formula A: 6.500≦4[C]+8.9[Cr]+12[Mn]-17[Si]≦13.

900. However, in formula A, substitute the mass percentage value of the content of the component corresponding to the element symbol in [ ] into the brackets [ ].

3. In mass percent, C: 0.10–0.30%, Si: 0.05–2.00%, Mn: less than 0.10–0.50%, Cr: 1.3–2.5%, Nb: 0.020–0.100%, Al: 0.020–0.100%, N: 0.0030–0.0300% Furthermore, it contains one or more of the following optional components: Ni: 0-1.00%, Mo: 0-1.00%, Ti: 0-0.20%, V: 0-0.50%, B: 0-0.0050%. The remainder consists of Fe and unavoidable impurities. In the case of unavoidable impurities, P: 0.030% or less, S: 0.030% or less, Cu: 0.30% or less, A vacuum carburized component according to claim 1, using vacuum carburized steel that satisfies formula A. Formula A: 6.5≦4[C]+8.9[Cr]+12[Mn]-17[Si]≦13.

9. However, in formula A, substitute the mass percentage value of the content of the component corresponding to the element symbol in [ ] into the brackets [ ].

Images