Vacuum carburizing steel and vacuum carburized component having excellent impact resistance and bending fatigue properties even under excess carburizing

Abstract

To provide a component that enables suppression of strength degradation in an excess carburized portion, including degradation of impact resistance and bending fatigue characteristics.SOLUTION: A vacuum carburizing steel containing, in mass%, C: 0.15-0.30%, Si: 0.70-1.20%, Mn: 0.90-3.00%, Cr: 0.50-1.50%, Mo: 0.10-1.00%, Al: 0.020-0.050%, Nb: 0.020-0.100%, and N: 0.0100-0.0300%, the balance being Fe and unavoidable impurities, wherein P and S among the unavoidable impurities satisfy P: 0.030% or less and S: 0.030% or less, and formulas A, B, C, and D: A: 0.12≤3[Al]+4[Nb]≤0.50, B: 0.20≤[N] / [Al]≤0.80, C: 5.0≤3[Si] / 0.5[Cr], D: 1.95≤-2[Si]+3[Mn]+[Cr]+2[Mo], are all satisfied.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to steels for vacuum carburizing in which the impact resistance and bending fatigue strength obtained by carburizing are hardly reduced even when the steels are over-carburized, and to vacuum-carburized parts made from these steels. [Background technology]

[0002] Vacuum carburizing is sometimes used instead of gas carburizing as a carburizing process for steel and steel parts. Vacuum carburizing involves heating in a vacuum, introducing a carburizing gas to carburize the material, and then performing a diffusion process in a vacuum again. Carburizing in a vacuum is said to have the advantage of reducing the formation of grain boundary oxide layers on the steel surface and allowing for rapid carburizing at high temperatures.

[0003] However, vacuum carburized parts are generally prone to the formation of coarse grains in the surface layer, which can lead to a decrease in impact strength. This is because carburized parts manufactured by vacuum carburizing at high temperatures are prone to coarsening of crystal grains in the carburized layer, especially in the outermost surface. These coarse grains can reduce bending fatigue strength.

[0004] Furthermore, in vacuum-carburized parts, the carbon concentration at the edges of the part tends to be higher than at the flat parts, which means that the edges tend to be over-carburized (the carbon concentration is excessive) compared to the flat parts, which can significantly reduce the impact value and bending fatigue strength. The reason why the edges of carburized parts tend to be over-carburized is thought to be because when carburized parts are manufactured using vacuum carburizing, they contain carbon at concentrations higher than necessary. Furthermore, coarse cementite and retained austenite tend to form in the hardened structure of the over-carburized parts, which can result in insufficient bending fatigue strength and hardness.

[0005] Therefore, the following proposals have been made so far regarding carburized parts that have been carburized by vacuum carburizing.

[0006] In the 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. A carburized part characterized by this has been proposed (see Patent Document 1).

[0007] Also, a high-strength carburized steel in which a carburized quenched layer is formed on the surface of a steel material containing 0.30 ≤ C ≤ 0.60 mass%, 2.00 < Si ≤ 4.00 mass%, 0.10 ≤ Mn ≤ 1.50 mass%, 0.50 ≤ Ni ≤ 2.50 mass%, 0.10 ≤ Cr ≤ 2.00 mass%, 0.05 ≤ Mo ≤ 1.00 mass%, and 0.05 ≤ V ≤ 0.50 mass%, the balance being Fe and inevitable impurities, wherein the carbon concentration X on the surface of the carburized quenched layer is 0.6 ≤ X ≤ 0.7 mass%, and the carburized depth δ is 0.8 mm ≤ δ ≤ 1.2 mm, the segregation ratio a of Si is 1.00 ≤ a ≤ 1.30, and the residual γ amount of the steel material is 10 vol% or less has been proposed (see Patent Document 2).

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

[0009] Vacuum carburizing, which involves heating, can cause the crystal grains on the outermost surface to coarsen, which can lead to a decrease in bending fatigue strength. The edges of vacuum-carburized parts are also prone to over-carburization, containing more carbon than necessary, which can lead to a decrease in bending fatigue strength and a decrease in impact resistance. This is because, while the carburizing gas diffuses from one direction on the surface at flat parts, the carburizing gas diffuses from multiple directions at edges, making carburization more likely than at flat parts.

[0010] Although there have been attempts to prevent over-carburization, which reduces toughness, by controlling the C concentration, as in Patent Document 2, this does not address the over-carburized portions.

[0011] The inventors have found that these conventional techniques are still insufficient in preventing the deterioration of bending fatigue strength, and as a result of investigating the factors that cause the deterioration of bending fatigue strength, they have newly discovered that the initial defect size (granular fracture surface unit) has an influence.

[0012] Therefore, the present invention aims to provide parts that can reduce the deterioration in strength (deterioration in impact resistance and bending fatigue properties) caused by an increase in retained austenite and an increase in grain boundary carbides in the structure of the over-carburized parts when over-carburizing inevitably occurs at the edges of parts during vacuum carburizing.The purpose of the present invention is to optimize the structure of the over-carburized parts, refine the crystal grains in the surface and core, reduce grain boundary carbides in the surface, and suppress ferrite in the core. [Means for solving the problem]

[0013] The inventors, taking into consideration that in impact tests, cracks are generated at the grain boundaries on the surface of the carburized layer and propagate towards the core, firstly, in order to ensure the impact value of the normally carburized part, it is useful to make it difficult for cracks to occur at the grain boundary fracture surface on the surface of the carburized layer and to make it difficult for cracks to propagate in the non-carburized layer. Therefore, it is effective to refine the surface grains of the carburized layer and reduce grain boundary carbides, and in the core to refine the grains and suppress ferrite.

[0014] Furthermore, it was found that even in the case of over-carburization, reducing grain boundary carbides is effective in preventing a decrease in fatigue strength, in order to ensure the impact strength of over-carburized parts.

[0015] Furthermore, when examining bending fatigue strength, it was newly discovered that initial cracks originate at the grain boundaries on the surface of the carburized layer. In other words, it was found that a single grain boundary fracture unit acts as an initial crack. Therefore, in order to improve fatigue strength, it was found that it is effective to reduce the size of the initial crack (grain boundary fracture unit) and the amount of grain boundary carbide. It was also found that by refining the grains in the over-carburized area and reducing the amount of grain boundary carbide, the initial crack becomes smaller and the decrease in fatigue strength is suppressed.

[0016] Furthermore, the amount of retained austenite in over-carburized areas is large, which reduces surface hardness and is therefore disadvantageous from the perspective of bending fatigue strength. From this perspective, too, reducing the initial crack size suppresses early fracture. This in turn increases surface hardness through work hardening of the retained austenite or transformation into work-induced martensitic steel during repeated stress loading, which is advantageous for fatigue strength.

[0017] Therefore, after extensive research, the inventors have come up with the following ideas in order to obtain an appropriate structure that reduces initial cracks and grain boundary carbides: specify the chemical composition of the steel for vacuum carburizing before carburizing (the non-carburized layer after carburizing) and specify an appropriate component composition; specify formulas A and B to utilize pinning particles (Al nitrides, Nb carbonitrides) (optimize Al, Nb, N) to refine the grains of the surface and core; specify formula D to optimize the balance of elements that increase hardenability (Cr, Mn, Mo) and elements that promote ferrite precipitation (Si) to suppress core ferrite; and specify formula C to increase the amount of Si and reduce the amount of Cr to reduce surface grain boundary carbides.

[0018] The first means for solving the problem is to provide, in mass %, C: 0.15~0.30%, Si: 0.70 to 1.20% Mn: 0.90-3.00%, Cr: 0.50~1.50%, Mo: 0.10 to 1.00%, Al: 0.020~0.050%, Nb: 0.020~0.100%, N: Contains 0.0100 to 0.0300% The balance is Fe and unavoidable impurities, The P and S in the unavoidable impurities are P: 0.030% or less, S: 0.030% or less, Furthermore, the steel for vacuum carburizing satisfies all of the following formulas A to D. Formula A: 0.12≦3[Al]+4[Nb]≦0.50, Formula B: 0.20≦[N] / [Al]≦0.80, Formula C: 5.0≦3[Si] / 0.5[Cr], Formula D: 1.95≦-2[Si]+3[Mn]+[Cr]+2[Mo], However, substitute the percentage values ​​of the corresponding chemical components for [Si][Mn][Cr][Mo][Al][Nb][N] in formulas A to D.

[0019] The composition of the non-carburized layer after vacuum carburizing corresponds to the composition of the steel for vacuum carburizing of the present invention before carburizing treatment.

[0020] The second means includes, in addition to the chemical components described in the first means, the following optional components in mass %: Ni: 0.02 to 2.00% Ti: 0.020 to 0.200%, V: 0.010~0.500%, B: Contains 0.0005 to 0.0050% of one or more types, The balance is Fe and unavoidable impurities, The P and S in the unavoidable impurities are P: 0.030% or less, S: 0.030% or less, Furthermore, the steel for vacuum carburizing satisfies all of the following formulas A to D. Formula A: 0.12≦3[Al]+4[Nb]≦0.50, Formula B: 0.12≦[N] / [Al]≦0.70, Formula C: 5.0≦3[Si] / 0.5[Cr], Formula D: 1.95≦-2[Si]+3[Mn]+[Cr]+2[Mo], However, substitute the percentage values ​​of the corresponding chemical components for [Si][Mn][Cr][Mo][Al][Nb][N] in formulas A to D.

[0021] The third means is a steel part that has been vacuum-carburized using the steel for vacuum carburizing according to any one of the first and second means, The grain size of the carburized surface layer is No. 7.5 or more and there is no mixed grain. The area of ​​grain boundary carbides in the surface layer of the over-carburized part is 5.0% or less, The area of ​​grain boundary carbides in the surface layer of the normally carburized part is 3.0% or less, The grain size of the non-carburized layer is No. 7.0 or more and there is no mixed grain. The structure of the non-carburized layer is martensite or martensite and bainite. It is a vacuum carburized steel part.

[0022] The grain size number in the present invention is evaluated according to a method in accordance with Japanese Industrial Standards (JIS) G0551. The term "duplex grain" as used herein refers to a structure in which regions where grains have grown larger than other regions by 3 or more in terms of grain size number, occupy 20% or more of the area, according to JIS G0551. A martensite structure or a martensite and bainite structure means that ferrite is not precipitated. [Effects of the Invention]

[0023] In parts using the steel for vacuum carburizing of the present invention, the structure of the over-carburized area is optimized, the crystals in the surface and core are refined, the grain boundary carbides in the surface are reduced, and the ferrite in the core is suppressed. Therefore, carburized steel parts in a vacuum carburized state using the steel for vacuum carburizing of the present invention have an impact value of 35 J / cm in the normally carburized area. 2 The fatigue strength is 480 MPa or more, and the impact value and fatigue strength of the over-carburized area are both 90% or more of those of the normally carburized area, reducing the deterioration in strength (deterioration of impact resistance and bending fatigue properties) of the over-carburized area. [Brief explanation of the drawings]

[0024] [Figure 1] 1(a) is an overall view showing the shape of a Charpy impact test piece used in the present invention, and FIG. 1(b) is an enlarged view of the shape of part A. [Figure 2] FIG. 1 is a diagram showing the shape of a rotating bending fatigue test piece used in the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0025] Prior to describing the embodiments of the present invention, we will explain the chemical components of the case-hardening steel for vacuum carburizing of the present invention and the reasons for defining formulas A to D. Note that % of the chemical components is mass %.

[0026] C: 0.15~0.30%, C is an element that increases the hardness of the material. If the C content is less than 0.15%, the core hardness after carburizing will decrease, resulting in insufficient strength. On the other hand, if there is too much C, the material hardness will increase too much, reducing workability, and the core hardness after carburizing will be too high, resulting in a decrease in impact value. From these perspectives, the C content is set to 0.30% or less.

[0027] Si: 0.70 to 1.20% Silicon is an element that suppresses excessive carburization at the edges of carburized steel parts. If there is insufficient silicon, the above effect cannot be obtained, so 0.70% or more silicon is required. On the other hand, if there is too much silicon, soft ferrite forms in the core, resulting in insufficient fatigue strength. Therefore, the silicon content is set to 1.20% or less.

[0028] Mn: 0.90-3.00%, Mn is a component useful for improving hardenability. Therefore, the Mn content is set to 0.90% or more. On the other hand, if the Mn content is excessive, the strength after hot working becomes too high, and machinability deteriorates. Therefore, the Mn content is set to 3.00% or less. Preferably, the Mn content is 1.50% or less.

[0029] Cr: 0.50~1.50%, Cr is useful for improving hardenability and material hardness, and is an ingredient that increases fatigue strength. Therefore, the Cr content is set to 0.50% or more. On the other hand, if there is too much Cr, excessive carburization tends to occur at the edges of carburized parts. Therefore, the Cr content is set to 1.50% or less.

[0030] Mo: 0.10 to 1.0% Mo is an element that increases the hardness of the material. If there is too little Mo, the hardenability will be insufficient, which will lead to the precipitation of ferrite in the non-carburized layer and a lack of impact value. From these perspectives, Mo content is set to 0.10% or more. On the other hand, adding too much Mo increases costs and reduces workability as the material hardness increases. Therefore, Mo content is set to 1.0% or less.

[0031] Al: 0.020 to 0.050% Al is a useful component as a deoxidizer, and also suppresses the coarsening of crystal grains by forming fine nitrides. If the Al content is too low, there will be a shortage of fine nitrides, which will cause the crystal grains to become coarse, resulting in poor fatigue properties. Therefore, the Al content is set to 0.020% or more. On the other hand, if the Al content is too high, the amount of alumina-based oxides will increase, which will reduce fatigue properties and workability. Therefore, the Al content is set to 0.050% or less.

[0032] Nb: 0.020~0.100%, Nb is a component that generates fine carbonitrides and is useful for suppressing grain coarsening. If the Nb content is too low, the amount of fine carbonitrides will be insufficient, and the effect of suppressing grain coarsening will be small, which will tend to result in insufficient fatigue strength. Therefore, the Nb content is set to 0.020% or more. On the other hand, if the Nb content is too high, the amount of carbonitrides will be excessive, which will result in reduced workability. Therefore, the Nb content is set to 0.100% or less. Preferably, the Nb content is 0.080% or less.

[0033] N: 0.0100~0.0300%, N is a component that forms fine carbonitrides and is useful for suppressing the coarsening of crystal grains. If the N content is too low, there will be a shortage of fine carbonitrides, which will lead to coarsening of crystal grains and therefore poor fatigue properties. Therefore, the N content is set to 0.0040% or more. If the N content is too high, coarse carbonitrides will be formed, which will lead to poor fatigue properties and a decrease in workability. Therefore, the N content is set to 0.0300% or less. Preferably, the N content is 0.0250% or less.

[0034] The balance is Fe and unavoidable impurities. Next, the reason for specifying the upper limits of P and S contained as unavoidable impurities in steel will be explained.

[0035] P:0.030% or less P is sometimes contained in steel as an unavoidable impurity, but if there is an excessive amount of P, grain boundary segregation causes insufficient fatigue properties, so the P content is set to 0.030% or less.

[0036] S: 0.030% or less S may be contained in steel as an unavoidable impurity, but if there is an excessive amount of S, the fatigue strength will be insufficient due to the formation of MnS. Therefore, the S content is set to 0.030% or less.

[0037] Next, the reasons for specifying the components that can be selectively added in the present invention will be explained. As the steel components of the present invention, one or more of Ni, Ti, V and B may be intentionally added within the ranges specified below.

[0038] Ni: 0.02 to 2.00% Ni is an element that increases the hardness of the material. However, adding too much increases the cost, and the increased hardness of the material reduces workability. Therefore, when Ni is added, the amount is set to 2.00% or less.

[0039] Ti: 0.020 to 0.200% When added, Ti forms fine nitrides and fixes N, improving hardenability and suppressing grain coarsening. However, adding too much Ti results in an excessive amount of carbonitrides, which reduces workability. Therefore, when Ti is added, the amount should be 0.200% or less.

[0040] V: 0.010 to 0.500% When added, V forms fine carbonitrides and has the effect of suppressing grain coarsening, thereby contributing to toughness and fatigue strength. However, if excessive V is added, the amount of carbonitrides becomes excessive, resulting in a decrease in workability. Therefore, when V is added, the amount should be 0.500% or less.

[0041] Next, the reasons why the steel for vacuum carburizing of the present invention is required to satisfy formulas A to D will be described.

[0042] Formula A: 0.12≦3[Al]+4[Nb]≦0.50 must be satisfied. The value of formula A is calculated by 3[Al] + 4[Nb]. Substitute the percentage value of the corresponding chemical component for [Al] and [Nb]. Formula A is an index mainly related to fatigue properties. If the value of formula A is less than 0.12, the number of pinning particles will be insufficient and the crystal grains will become coarse, resulting in insufficient fatigue properties and a decrease in impact value. On the other hand, if the value of formula A exceeds 0.50, the amount of carbonitrides will be excessive, resulting in a decrease in workability. Therefore, the formula A is set to 0.12≦3[Al]+4[Nb]≦0.50. Preferably, the formula A satisfies 0.15≦3[Al]+4[Nb]≦0.45.

[0043] Formula B: 0.20<[N] / [Al]<0.80 must be satisfied. The value of formula B is calculated by [N] / [Al], where [N][Al] is substituted with the percentage value of the corresponding chemical component. Formula B is an index mainly related to fatigue properties. If the AlN grains become coarse due to an increase in the ratio of Al to N, the pinning force decreases, resulting in insufficient fatigue properties and a decrease in impact value. On the other hand, if the ratio of Al to N becomes too low, there will be a shortage of fine nitrides, which will cause the crystal grains to become coarse, resulting in insufficient fatigue properties and a decrease in impact value. Therefore, the formula B is set to satisfy 0.20<[N] / [Al]<0.80.

[0044] Formula C: 5.0≦3[Si] / 0.5[Cr] must be satisfied Si is an element that suppresses over-carburization, but if there is an excess, fatigue strength will be insufficient. On the other hand, Cr is an element that increases fatigue strength, but if there is an excess, over-carburization will occur easily. Therefore, formula C was established as an index for the ratio of Si to Cr. If a large amount of grain boundary carbide precipitates in the carburized layer, the impact value and fatigue strength will decrease, so the value of formula C must be 5.0 or more. It is desirable that formula C satisfy the condition 5.0≦3[Si] / 0.5[Cr≦12.0.

[0045] Formula D:1.95≦-2[Si]+3[Mn]+[Cr]+2[Mo] If ferrite precipitates in the non-carburized layer, the impact strength tends to decrease. Therefore, to suppress core ferrite, we have defined Formula D as an index to optimize the balance between elements that increase hardenability (Cr, Mn, Mo) and elements that promote ferrite precipitation (Si), and we have decided that the value must be 1.95 or greater. It is desirable that Formula D satisfy the following: 1.95≦-2[Si]+3[Mn]+[Cr]+2[Mo]≦5.00.

[0046] (Regarding the manufacturing process of the test material) For the steels having the chemical compositions shown in Inventive Steels Nos. 1 to 20 in Table 1 and Comparative Steels Nos. 1 to 10 in Table 2, with the balance being Fe and unavoidable impurities, 100 kg of each was melted in a vacuum induction melting furnace (VIM) to obtain steel ingots. These were then forged into φ32 round bars at 1250°C, held at 925°C for 1 hour, and then normalized by air cooling.

[0047] [Table 1]

[0048] [Table 2]

[0049] The obtained forged and drawn materials were roughly machined into test pieces to prepare Charpy impact test pieces with the shape shown in Figure 1 and rotary bending test pieces with the shape shown in Figure 2. Next, these test pieces were vacuum carburized under two conditions described below to obtain (a) normal carburization and (b) excessive carburization, and then finish machined.

[0050] (a) Normal carburizing conditions: After soaking at 950°C for 40 minutes, the specimen was carburized at 950°C (in an acetylene gas atmosphere at a pressure of 150 Pa) for 85 minutes. This was followed by a diffusion treatment at 950°C in a vacuum (5 Pa or less) for 71 minutes. The specimen was then held at 880°C for 40 minutes, after which it was quenched and further tempered at 180°C for 1.5 hours. (b) Over-carburizing conditions: After soaking at 950°C for 40 minutes, the specimen was carburized at 950°C (in an acetylene gas atmosphere at a pressure of 150 Pa) for 154 minutes. This was followed by a diffusion treatment at 950°C in a vacuum (5 Pa or less) for 22 minutes. The specimen was then held at 880°C for 40 minutes, quenched, and tempered at 180°C for 1.5 hours.

[0051] Using the Charpy impact test specimens (normally carburized and over-carburized) and rotating bending test specimens (normally carburized and over-carburized) prepared as described above, we measured the grain size, area ratio of grain boundary carbides, surface C concentration, presence or absence of ferrite, Charpy impact value, fatigue strength, etc. Details of the evaluation items and evaluation methods are described below.

[0052] (Evaluation items, evaluation methods) (1) Calculation of particle size number Each test piece is cut and polished, and then the grain boundaries are corroded with saturated picric acid. Five fields of view of the cross section are then observed under an optical microscope, and the grain size number is calculated using the comparative method specified in JIS G 0551. The presence or absence of mixed grains is also determined from the observation results in accordance with JIS D 0551.

[0053] (2) Area ratio of grain boundary carbides Each test piece is cut and polished, and then the structure is corroded in a picral solution. The cross section is then observed in three fields using an optical microscope, and the area ratio of carbides is calculated using image analysis software.

[0054] (3) Calculation of surface C concentration Each test piece is cut and polished, and then the carbon concentration is analyzed using EPMA, and the average carbon concentration on the surface (average value from the outermost surface to a depth of 30 μm) is calculated.

[0055] (4) Presence or absence of ferrite A Charpy impact test piece is cut and polished, and then the structure is corroded with a nital solution. The cross section is then observed under an optical microscope to determine whether or not ferrite is present. The steel of the present invention has a martensite structure or a martensite and bainite structure, and does not contain ferrite.

[0056] (5) Impact value measurement Using a Charpy impact test, the impact value (average value of n=3) of the Charpy impact test piece at room temperature is measured, and the notch and notch face are tested on the carburized test piece. Next, when the impact value of the normally carburized piece is set to 100%, the ratio [%] of the impact value of the over-carburized piece is calculated as follows: (Impact value of over-carburized product) / (Impact value of normal carburized product) x 100

[0057] (6) Fatigue strength measurement Using the bending fatigue test specimen shown in Figure 2, measurements are made using the Ono-type rotating bending test (rotation speed: 3000 rpm), and the test is performed on a carburized test specimen only at the notched portion. Fatigue strength is calculated based on JIS Z2273-1978 using a 1.0 x 10 7 Calculate the cycle fatigue strength. When the fatigue strength of a normally carburized product is taken as 100%, the ratio [%] of the fatigue strength of an over-carburized product is calculated as follows: (Fatigue strength of over-carburized product) / (Fatigue strength of normal carburized product) x 100

[0058] Measurements were carried out for each of the evaluation items (1) to (6) for the invention steels 1 to 20 and the comparative examples 1 to 10. The results are shown in Tables 3 to 7. The underlined parts of the comparative steels indicate that they are outside the ranges and effects defined by the present invention.

[0059] [Table 3]

[0060] [Table 4]

[0061] [Table 5]

[0062] [Table 6]

[0063] [Table 7]

[0064] The carburized steel parts in the vacuum carburized state using the invention steels No. 1 to 20 had fine grains in the carburized layer and the uncarburized core layer as specified, with no mixed grains, and the area of ​​grain boundary carbides was below the specified level, with no ferrite precipitated. As shown in Table 7, the impact value of the normally carburized area was 35 J / cm 2 The fatigue strength was 480 MPa or more. The impact value of the over-carburized area was 90% or more of that of the normally carburized area, and the fatigue strength of the over-carburized area was also 90% or more of that of the normally carburized area. Thus, carburized steel parts using the vacuum carburized steel of the present invention have reduced strength deterioration in the over-carburized areas, and even when over-carburized, carburized steel parts can be obtained that maintain good impact resistance and bending fatigue properties.

[0065] Comparative Steel 1 has insufficient Si and Mn, resulting in values ​​outside the ranges specified in Formula A and Formula C. As a result, the impact value is low with normal carburization, and the impact value drops even more with over-carburization. The fatigue strength is also low with normal carburization, and the fatigue strength drops even more with over-carburization. Comparative steel 2 has insufficient Mn and Mo and excessive Si and Cr, resulting in values ​​outside the ranges specified in formulas C and D. Therefore, the impact value is low with normal carburization, and the impact value drops even more with over-carburization. The fatigue strength is also low with normal carburization, and the fatigue strength drops even more with over-carburization. Comparative steel 3 has insufficient Mn, Cr, and Mo and excessive Si, which deviates from the values ​​given by formula D. As a result, the impact value was low with normal carburization. Also, the fatigue strength decreased significantly with excessive carburization. Comparative steel 4 has insufficient Mn and N and excessive Cr, resulting in an out-of-range value in formula C. Therefore, excessive carburization significantly reduces the impact value and fatigue strength. Comparative steel 5 has insufficient Mn and Mo, and the values ​​obtained are outside the ranges given by formulas C and D. Therefore, the impact value is low with normal carburization, and the impact value drops even more with over-carburization, and the fatigue strength also drops significantly with over-carburization. Comparative steel 6 has too little Mn and too much Si and Mo, and the values ​​are outside the range of formula B. Therefore, the impact value is low with normal carburization, and the impact value drops even more with over-carburization. The fatigue strength is also low with normal carburization, and the fatigue strength drops even more with over-carburization. Comparative steel 7 has an insufficient amount of Mn, which does not satisfy formula D. As a result, the impact value in normal carburization is low. Comparative steel 8 has insufficient Si, Mn, and Al and excessive Mo, so that formulas B and D do not hold. Therefore, the impact value is low with normal carburizing, and the impact value drops even more with over-carburizing, and the fatigue strength is also low with normal carburizing, and the fatigue strength drops even more with over-carburizing. Therefore, the impact value is low with normal carburizing, and the impact value drops even more with over-carburizing, and the fatigue strength is also low with normal carburizing, and the fatigue strength is also low with over-carburizing, and the fatigue strength is also low with normal carburizing, and the fatigue strength drops even more with over-carburizing. Comparative Steel 9 has insufficient Cr and Mo and excessive Si, which deviates from formula D. As a result, the impact value in normal carburization is reduced. Comparative Steel 10 has excessive amounts of Si and Mo, and does not satisfy formula C. As a result, excessive carburization significantly reduces the impact value and fatigue strength.

Claims

1. In mass%, C: 0.15-0.30%, Si: 0.70-1.20%, Mn: 0.90-3.00%, Cr: 0.50-1.50%, Mo: 0.10-1.00%, Al: 0.020-0.050%, Nb: 0.020-0.100%, N: 0.0100 to 0.0300%; The balance consists of Fe and unavoidable impurities, The P and S in the unavoidable impurities are P: 0.030% or less, S: 0.030% or less, Furthermore, the vacuum carburizing steel satisfies all of the following conditions: Formula A: 0.12≦3[Al]+4[Nb]≦0.50; Formula B: 0.20≦[N] / [Al]≦0.80; Formula C: 5.0≦3[Si] / 0.5[Cr]; and Formula D: 1.95≦-2[Si]+3[Mn]+[Cr]+2[Mo]. However, the percentage values ​​of the corresponding chemical components are substituted for [Si][Mn][Cr][Mo][Al][Nb][N] in formulas A to D.

2. In addition to the chemical components of claim 1, the optional components are, in mass %: Ni: 0.02-2.00%, Ti: 0.020-0.200%, V: 0.010-0.500%, B: 0.0005 to 0.0050% of one or more kinds, The balance consists of Fe and unavoidable impurities, The P and S in the unavoidable impurities are P: 0.030% or less, S: 0.030% or less, Furthermore, the vacuum carburizing steel satisfies all of the following conditions: Formula A: 0.12≦3[Al]+4[Nb]≦0.50; Formula B: 0.20≦[N] / [Al]≦0.80; Formula C: 5.0≦3[Si] / 0.5[Cr]; and Formula D: 1.95≦-2[Si]+3[Mn]+[Cr]+2[Mo]. However, the percentage values ​​of the corresponding chemical components are substituted for [Si][Mn][Cr][Mo][Al][Nb][N] in formulas A to D.

3. A steel part that has been vacuum carburized using the steel for vacuum carburizing according to claim 1 or 2, The grain size of the surface of the carburized layer is No. 7.5 or more and there is no mixed grain. The area of ​​grain boundary carbides in the surface layer of the over-carburized part is 5.0% or less, The area of ​​grain boundary carbides in the surface layer of the normally carburized part is 3.0% or less, The grain size of the non-carburized layer is No. 7.0 or more and there is no mixed grain, The structure of the non-carburized layer is martensite or martensite and bainite. Vacuum carburized steel parts.

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