Steel for vacuum carburization having excellent grain size characteristics

Abstract

Provided is a steel for vacuum carburization which is capable of obtaining, by vacuum carburization, a carburized steel component having excellent grain size characteristics and excellent toughness and flexural fatigue. A steel for vacuum carburization, which contains, in mass%, 0.15 to 0.30% of C, 0.05 to 2.00% of Si, 0.10 to less than 0.50% of Mn, 1.3 to 2.5% of Cr, 0.020 to 0.100% of Nb, 0.020 to 0.100% of Al, 0.0040 to 0.0300% of N, optionally one or more elements selected from the group consisting of 0.020 to 0.200% of Ti, 0.0050% or less of B and 0.01 to 0.50% of V, optionally one or more elements selected from the group consisting of 1.0% or less of Ni and 1.0% or less of Mo, optionally one or two elements selected from the group consisting of 1.0% or less of Ni and 1.0% or less of Mo, and the balance of Fe and unavoidable impurities, in the inevitable impurities, P is 0.030% or less, S is 0.030% or less, and Cu is 0.30% or less, and satisfies the formula A: 0.12 < = 3 [Al] + 4 [Nb] < = 0.50, and the formula B: 0.12 < [N] / [Al] < 0.70.

Description

Technical Field

[0001] The present invention relates to steel for vacuum carburizing, and is a case-hardening steel having excellent grain size characteristics suitable for vacuum carburizing. Background Art

[0002] Gas carburizing is a common method for carburizing steel parts after forming steel into parts and then carburizing them. Recently, vacuum carburizing has also been adopted. Vacuum carburizing involves heating steel in a vacuum, introducing a carburizing gas to carburize the steel, and then performing a diffusion treatment in the vacuum.

[0003] Compared with gas carburizing, which is a common method at present, vacuum carburizing has the following advantages.

[0004] 1) Because the treatment is carried out in a vacuum, no grain boundary oxide layer can be seen on the surface of the steel, which can avoid the decline of various strengths.

[0005] 2) Since carburizing can be performed at high temperatures, carburizing can be performed quickly.

[0006] However, although vacuum carburizing has the above-mentioned effects, carburized parts manufactured by vacuum carburizing at high temperatures tend to suffer from coarsening of crystal grains in the carburized layer, especially on the outermost surface. Such coarse crystal grains reduce toughness and bending fatigue strength.

[0007] Conventionally, as carburized parts carburized by vacuum carburizing, for example, the following have been proposed.

[0008] Patent Document 1 (Japanese Patent Application Laid-Open No. 2007-291486) proposes a carburized part obtained by forming carburizing steel into a part shape and carburizing it by vacuum carburizing. The carburizing steel has an alloy composition containing, in mass%, 0.1-0.3% C, 0.5-3.0% Si, 0.3-3.0% Mn, 0.03% or less P, 0.03% or less S, 0.01-1.00% Cu, 0.01-3.00% Ni, 0.3-1.0% Cr, 0.20% or less Al, and 0.05% or less N, with the balance being inevitable impurities and Fe, and satisfying the condition of [Si%]+[Ni%]+[Cu%]-[Cr%]>0.5.

[0009] Patent document 2 (WO2020 / 202406) proposes a carburized part, wherein the composition in the depth region of 1.5 mm or more from the surface is as follows, in mass %, C: 0.10-0.40%, Si: 0.10-3.00%, Mn: 0.50-3.00%, Cr: 0.30-3.00%, Al: 0.010-0.050%, N: 0.003-0.030%, S: 0.003-0.030%, P: 0.030% or less, Mo: 0-3.00%, B: 0-0.0050%, Nb: 0-0.100%, Ti: 0-0.100%, V: 0-0.30%, Ni: 0-0.40%, In: 0-0.02%, Cu: 0-0.20%, Bi: 0-0.300%, Pb: 0-0.50%, and REM: 0-0.020%, with the balance being Fe and impurities. The Vickers hardness at a depth of 1.5 mm from the surface is 200-400 HV. In the area from the surface to a depth of 0.10 mm, the C content is 0.60-1.20% by mass%, the fraction of quenched structure is 99.00% or more by area, the fraction of grain boundary cementite is 0.50% or less by area, and the fraction of incompletely quenched structure is 0.50% or less by area.

[0010] Patent document 3 (Japanese Patent Application Publication No. 2022-55308) proposes a carburizing steel having the following composition: C: 0.10-0.35%, Si: 0.50% or less, Mn: 0.30-1.50%, Cr: 1.10-2.00%, P: 0.02% or less, S: 0.05% or less, Al: 0.01-0.05% and N: 0.030% or less, the balance being Fe and inevitable impurities, the cementite fraction at a depth of 0.05 mm from the surface being 5% or less, the hardness at a depth of 0.05 mm from the surface being HV600 or greater, the austenite grain size at a depth of 0.05 mm from the surface being No. 5 grain size or greater, and the hardness at a depth of 0.10 mm from the surface being HV650 or greater.

[0011] Prior art literature

[0012] Patent Literature

[0013] Patent Document 1: Japanese Patent Application Laid-Open No. 2007-291486

[0014] Patent Document 2: WO2020 / 202406

[0015] Patent Document 3: Japanese Patent Application Laid-Open No. 2022-55308 Summary of the Invention

[0016] However, Patent Documents 1 and 2 do not specify the grain size of austenite, and therefore the toughness is not fully considered, resulting in some variations in quality and cannot be considered sufficient.

[0017] Furthermore, in Patent Document 1, Mn is added to improve hardenability. However, if the Mn content is 0.50% or more, the grains of the ferrite-pearlite structure formed by normalizing or annealing tend to be reduced to less than 15 μm, thereby causing coarsening of the grains during carburizing.

[0018] Furthermore, Patent Document 3 attempts to suppress coarse grains mainly by carburizing, but does not consider the components that form pinning particles, and does not evaluate the core hardness that affects toughness, and therefore cannot be considered sufficient.

[0019] Therefore, an object of the present invention is to provide a case hardening steel which stably suppresses the generation of coarse grains in vacuum carburizing at high temperatures where coarse grains are likely to form on the surface of the carburized layer, has excellent grain size characteristics, and is also excellent in toughness and bending fatigue, and a case hardening steel part using the same.

[0020] Therefore, the inventors have conducted intensive research and found that by specifying the chemical composition and appropriately adjusting the Nb, Al and N content, a vacuum carburizing steel with finer grains can be obtained. In addition, by vacuum carburizing this vacuum carburizing steel at a temperature below 1050°C, carburized steel parts with an austenite grain size of 6 or more in terms of grain size number can be obtained.

[0021] According to the present invention, the following aspects are provided.

[0022] [Method 1]

[0023] A steel for vacuum carburizing, comprising:

[0024] In mass %,

[0025] C: 0.15-0.30%,

[0026] Si: 0.05-2.00%,

[0027] Mn: 0.10 to less than 0.50%,

[0028] Cr: 1.3-2.5%,

[0029] Nb: 0.020~0.100%,

[0030] Al: 0.020~0.100%,

[0031] N: 0.0040~0.0300%,

[0032] As needed, any one or two or more selected from the group consisting of Ti: 0.020 to 0.200%, B: 0.0050% or less, and V: 0.01 to 0.50%,

[0033] As needed, either or both of Ni: 1.0% or less and Mo: 1.0% or less.

[0034] The balance Fe and unavoidable impurities,

[0035] Among the inevitable impurities, P: 0.030% or less, S: 0.030% or less, Cu: 0.30% or less,

[0036] Among them, satisfy

[0037] Formula A: 0.12≤3[Al]+4[Nb]≤0.50, and

[0038] Formula B: 0.12<[N] / [Al]<0.70

[0039] (Wherein, the % values ​​of the corresponding chemical components are substituted into [Al], [Nb], and [N]).

[0040] [Method 2]

[0041] The steel for vacuum carburizing according to aspect 1 contains, in mass %, any one or two or more of 0.020% to 0.200% of Ti, 0.0050% or less of B, and 0.01% to 0.50% of V.

[0042] [Method 3]

[0043] The steel for vacuum carburizing according to aspect 1 or 2 contains, in mass %, either or both of Ni: 1.0% or less and Mo: 1.0% or less.

[0044] [Method 4]

[0045] A carburized steel part is a carburized steel part using the vacuum carburizing steel according to any one of aspects 1 to 3, wherein the hardness at a depth of 5.0 mm from the surface is 200 to 500 Hv.

[0046] [Method 5]

[0047] A carburized steel part is a carburized steel part that is vacuum carburized at 1050°C or lower using the vacuum carburizing steel according to any one of embodiments 1 to 3, wherein the austenite grain size at a depth of 0.05 mm from the surface is 6 or greater in grain size number.

[0048] [Method 6]

[0049] A carburized steel part, comprising: a carburized steel part that has been vacuum carburized at 1050°C or lower using the vacuum carburizing steel according to any one of embodiments 1 to 3, wherein the austenite grain size at a depth of 0.05 mm from the surface is 6 or greater in terms of grain size number, and the austenite grain size at a depth of 5.0 mm from the surface is 6 or greater in terms of grain size number.

[0050] [Method 7]

[0051] A carburized steel part, comprising: a carburized steel part that has been vacuum carburized at 1050°C or lower using the vacuum carburizing steel according to any one of embodiments 1 to 3, wherein the austenite grain size at a depth of 0.05 mm from the surface is 6 or greater in grain size number, and the austenite grain size at a depth of 5.0 mm from the surface is 7 or greater in grain size number.

[0052] [Method 8]

[0053] A carburized steel part is the carburized steel part according to any one of aspects 5 to 7, wherein the hardness at a depth of 5.0 mm from the surface is 200 to 500 Hv.

[0054] The carburized steel parts using the steel of the present invention that have been vacuum carburized at 1050°C or below have a grain size number of 6 or above for the austenite grain size at a depth of 0.05 mm from the surface, and a grain size number of 6 or above for the austenite grain size at a depth of 5.0 mm from the surface, which can stably suppress the generation of coarse grains. In addition, the carburized steel parts using the steel of the present invention have a 25 J / cm 2 The impact value is above 500MPa, so the toughness is excellent. In addition, it can ensure a 1.0×10 7 Therefore, according to the present invention, it is possible to obtain case-hardened steel having excellent grain size characteristics, toughness, and bending fatigue, and carburized steel parts using the same. DETAILED DESCRIPTION

[0055] Before explaining the embodiment of the present invention, the chemical composition of the case hardening steel for vacuum carburizing of the present invention will be explained. % in the chemical composition is mass %.

[0056] C：0.15～0.30％

[0057] C is a component that increases the hardness of the raw material. If the C content is less than 0.15%, the core hardness after carburizing decreases, resulting in insufficient strength. On the other hand, if the C content is too high, the raw material hardness increases excessively, workability decreases, and core toughness decreases. From this point of view, the C content is 0.30% or less, preferably 0.25% or less.

[0058] Si: 0.05-2.00%

[0059] Si is a useful deoxidizer and also increases the hardness of the raw material. Insufficient Si results in inadequate deoxidation, so the Si content should be at least 0.05%, preferably at least 0.30%. On the other hand, excessive Si increases the hardness of the raw material, reducing workability. Therefore, the Si content should be 2.00% or less, preferably 0.80% or less.

[0060] Mn: 0.10 to less than 0.50%

[0061] Mn is a component useful for improving hardenability. Therefore, the Mn content is 0.10% or more, preferably 0.15% or more. On the other hand, excessive Mn content reduces machinability and tends to cause grain coarsening during carburizing. Therefore, the Mn content is less than 0.50%, preferably 0.40% or less.

[0062] Cr：1.3～2.5％

[0063] Cr is a useful component for improving hardenability and material hardness. Too little Cr reduces hardenability and strength. Therefore, the Cr content is 1.3% or higher, preferably 1.5% or higher. On the other hand, too much Cr reduces workability as material hardness increases. Therefore, the Cr content is 2.5% or lower, preferably 2.2% or lower.

[0064] Nb: 0.020~0.100%

[0065] Nb is a component that generates fine carbonitrides and is effective in suppressing grain coarsening. If the Nb content is too low, fine carbonitrides are insufficient, and the effect of suppressing grain coarsening is minimal, which can lead to reduced toughness and 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 becomes excessive, reducing workability. Therefore, the Nb content is set to 0.100% or less, preferably 0.080% or less.

[0066] Al：0.020～0.100％

[0067] Al is a useful component as a deoxidizing agent and also forms fine nitrides, inhibiting grain coarsening. Too little Al leads to insufficient fine nitrides, causing grain coarsening, which in turn reduces toughness and fatigue properties. Therefore, the Al content should be at least 0.020%. On the other hand, too much Al increases the amount of alumina-based oxides, reducing fatigue properties and workability. Therefore, the Al content should be 0.100% or less, preferably 0.050% or less.

[0068] N：0.0040～0.0300％

[0069] Nitrogen is a component that forms fine carbonitrides and is effective in suppressing grain coarsening. If the N content is too low, fine carbonitrides are insufficient, grains tend to coarsen, and toughness and fatigue properties tend to decline. Therefore, the N content is set to 0.0040% or higher. If the N content is too high, coarse carbonitrides form, which reduces fatigue properties and also tends to reduce workability. Therefore, the N content is set to 0.0300% or lower, preferably 0.0200% or lower.

[0070] margin

[0071] The balance is Fe and inevitable impurities. Therefore, the reasons for specifying the upper limits of P, S, and Cu that may be contained in steel as inevitable impurities are described below.

[0072] P: 0.030% or less

[0073] P is contained in steel as an unavoidable impurity, but if too much P is present, toughness will decrease due to grain boundary segregation. Therefore, the P content is set to 0.030% or less.

[0074] S: 0.030% or less

[0075] S is contained in steel as an unavoidable impurity. However, if S is too high, it forms MnS, which reduces toughness and fatigue strength. Therefore, the S content is set to 0.030% or less.

[0076] Cu: 0.30% or less

[0077] Cu is contained in steel as an unavoidable impurity, but excessive Cu content reduces hot workability. Therefore, the Cu content is set to 0.30% or less.

[0078] Formula A: 0.12≤3[Al]+4[Nb]≤0.50

[0079] The value of formula A is obtained as 3[Al]+4[Nb]. For [Al] and [Nb], substitute the % value of the corresponding chemical component. Formula A is an indicator of toughness and fatigue properties. If the value of formula A is less than 0.12, the pinning particles are insufficient and the grains are coarsened, thereby reducing toughness and fatigue properties. On the other hand, if the value of formula A is higher than 0.50, the amount of carbonitrides is excessive, so the workability is reduced. Therefore, formula A is 0.12≤3[Al]+4[Nb]≤0.50, and preferably 0.15≤3[Al]+4[Nb]≤0.45.

[0080] Formula B: 0.12<[N] / [Al]<0.70

[0081] The value of Formula B is calculated as [N] / [Al]. Substitute the % values ​​of the corresponding chemical components for [N] and [Al]. Formula B is an indicator of toughness and fatigue properties. If the ratio of Al to N increases, AlN coarsens, which reduces pinning force and, consequently, reduces toughness and fatigue properties. On the other hand, if the ratio of Al to N is too low, there is a shortage of fine nitrides, resulting in coarsening of grains, which reduces toughness and fatigue properties. Therefore, Formula B is 0.12<[N] / [Al]<0.70.

[0082] Next, the reasons for specifying the optional components (optional components) that can be added in the present invention will be explained. As steel components of the present invention, one or more selected from the group consisting of Ti, B, and V may be added within the ranges specified below. Furthermore, as steel components of the present invention, one or both of Ni and Mo may be added within the ranges specified below.

[0083] Ti: 0.020~0.20%

[0084] Ti is an optional component that helps ensure a sufficient amount of fine nitrides. If Ti is too low, insufficient fine nitrides can be achieved, and nitrogen cannot be fixed, forming BN and other substances, reducing hardenability. Furthermore, if Ti is too low, the effect of suppressing grain coarsening is minimal. Therefore, when adding Ti, the amount is preferably set to 0.020% or more. On the other hand, if Ti is excessive, the amount of carbonitrides becomes excessive, reducing workability. Therefore, when adding Ti, the amount is preferably set to 0.20% or less, and more preferably 0.16% or less.

[0085] B: 0.0050% or less

[0086] B is an optional component that increases the hardness of the raw material. However, if it is excessive, the workability decreases as the raw material hardness increases. Therefore, when B is added, the amount of B added is preferably 0.0050% or less.

[0087] V：0.01～0.50％

[0088] V is an optional component useful for ensuring the amount of carbonitrides. If the V content is too low, fine carbonitrides are insufficient, and the effect of suppressing grain coarsening is reduced, resulting in insufficient toughness and fatigue strength. Therefore, when adding V, the V content is preferably set at 0.01% or more. On the other hand, excessive V content results in excessive carbonitrides, which can reduce workability. Therefore, the V content is preferably 0.50% or less, and more preferably 0.30% or less.

[0089] Ni: 1.0% or less

[0090] Ni is an optional component that increases the hardness of the raw material. If added excessively, costs increase, and workability decreases as the raw material hardness increases. Therefore, when adding Ni, it is preferable to keep the Ni content to 1.0% or less.

[0091] Mo: 1.0% or less

[0092] Mo is an optional component that increases the hardness of the raw material. If added excessively, costs increase, and workability decreases as the raw material hardness increases. Therefore, when adding Mo, the Mo content is preferably kept at 1.0% or less.

[0093] Hardness at a depth of 5.0 mm from the surface: 200 to 500 Hv

[0094] If the internal hardness of a carburized steel part is too low, the bending fatigue strength will be low. On the other hand, if the internal hardness of a carburized steel part is too high, the toughness will be low. Therefore, in the carburized state, the hardness at a depth of 5.0 mm from the steel surface is 200 to 500 Hv in Vickers hardness.

[0095] Austenite grain size at a depth of 0.05 mm from the surface of carburized steel parts: No. 6 or above in grain size number

[0096] Austenite grain size at a depth of 5.0 mm from the surface of carburized steel parts: No. 6 or above in grain size number

[0097] If the austenite grain size is coarse, toughness and fatigue properties are all reduced. Therefore, at any depth of 0.05mm as the depth of the carburized layer and 5.0mm as the depth of the non-carburized layer, it is desired that the austenite grain size not coarsen. Therefore, the preferred grain size of the carburized layer is more than No. 6. In addition, the preferred grain size of the non-carburized layer is more than No. 6, more preferably more than No. 7.

[0098] 100 kg of steel, each having the chemical composition and balance of Fe and inevitable impurities described in Table 1 for Inventive Steels Nos. 1-12 and Comparative Steels Nos. 1-6, was melted in a vacuum induction melting furnace (VIM) to produce a steel ingot. The ingot was then forged and drawn at 1250°C into 32 mm round bars, held at 925°C for 1 hour, and then air-cooled for normalization. The ingot was then processed into test specimens.

[0099]

Table 1

[0100]

[0101] Each of the prepared test pieces was carburized in a vacuum carburizing furnace at 930° C., 1000° C., or 1050° C. under the following conditions, and the grain size of the microstructure was evaluated.

[0102] <930℃ carburizing>

[0103] After soaking at 930°C for 40 minutes, carburization was performed at 930°C for 55 minutes (acetylene atmosphere, pressure 150 Pa). Diffusion was then performed at 930°C in a vacuum (5 Pa or less) for 100 minutes. After that, the steel was held at 880°C for 40 minutes, quenched, and then tempered at 180°C for 1.5 hours.

[0104] <1000℃ carburizing>

[0105] After soaking at 1000°C for 40 minutes, carburization was performed at 1000°C for 25 minutes (acetylene atmosphere, pressure 150 Pa). Diffusion was then performed at 1000°C in a vacuum (5 Pa or less) for 65 minutes. After that, the steel was quenched after being held at 880°C for 40 minutes, and then tempered at 180°C for 1.5 hours.

[0106] <1050℃ carburizing>

[0107] After soaking at 1050°C for 40 minutes, carburization was performed at 1050°C for 15 minutes (acetylene atmosphere, pressure 150 Pa). Diffusion was then performed at 1050°C in a vacuum (5 Pa or less) for 50 minutes. After that, the steel was held at 880°C for 40 minutes, quenched, and then tempered at 180°C for 1.5 hours.

[0108] The carburized test pieces were subjected to microstructure observation (grain observation), hardness after carburization (Vickers hardness test), pendulum impact test, and notched rotary bending test. The results are shown in Table 2.

[0109]

Table 2

[0110]

[0111] Underlining indicates departures from the present invention.

[0112] (1) Hardness after carburizing (0.10 mm from the surface, 5.0 mm from the surface)

[0113] The test piece was cut perpendicular to the rolling direction, and the cut surface was surface-ground. The hardness was then measured using a Vickers hardness tester (load 300 gf) at a predetermined depth, for example 0.10 mm, from the surface in accordance with JIS Z 2244 (2020).

[0114] (2) Microstructure

[0115] The test piece was cut through the center parallel to the rolling direction, polished, and etched with saturated picric acid. The specimen was then observed using an optical microscope at positions 0.05 mm and 5.0 mm from the surface (observation field: 10 mm x 10 mm).

[0116] Regarding the austenite grain size, the prior austenite grain size was measured in accordance with JIS G 0551 (2020) and the grain size number was determined. That is, if there was a grain size number lower than 6, it was determined that coarse grains had occurred.

[0117] (3) Pendulum impact test

[0118] The resulting steel components were processed into test specimens and evaluated using a pendulum impact test based on JIS Z 2242 (2018). The pendulum impact test was conducted at room temperature (23±5°C) using a 10 mm square 10RC notch (the carburized layer was only on the notch and notch surface, and the notch bottom had an R value of 0.8).

[0119] (4) Notch rotation bending test

[0120] The obtained steel member was processed into a test piece and evaluated using a notched rotating bending test based on JIS Z 2274 (1978). In the Ono type rotating bending fatigue test, a notched rotating bending fatigue test piece with a parallel portion of φ8 mm (notch bottom: R0.8) was used.

[0121] The invention steel No. 1 to 12 of the present invention was vacuum carburized at 1000°C or less. The austenite grain size at a depth of 0.05 mm from the surface was 6 or more, and the austenite grain size at a depth of 5.0 mm from the surface was 7 or more. Therefore, it was confirmed that the formation of coarse grains can be stably suppressed. In addition, the carburized steel parts of the invention steel No. 1 to 12 have a 25 J / cm 2 The impact value is above 500MPa, so the toughness is excellent. In addition, it can ensure a 1.0×10 7 Therefore, by vacuum carburizing these inventive steels, carburized steel parts with excellent grain size characteristics, toughness, and bending fatigue resistance can be obtained.

[0122] Compared with Steel 1, Mn is excessive, Nb is insufficient, and the values ​​deviate from those of Formulas A and B. Therefore, the grains are likely to coarsen, the austenite grain size at a depth of 0.05 mm is numbered 4, and the toughness and fatigue properties are reduced.

[0123] Compared with Steel 2, Cr and Al are too low, deviating from the value of Formula B. Therefore, the hardness is also low, and the grains are easily coarsened, so the austenite grain size at a depth of 0.05 mm is numbered 5, and the toughness and fatigue properties are reduced.

[0124] Compared with Steel 3, the Cr content is too low, and the value deviates from Formula A. Therefore, the hardness is also low, the austenite grain size at a depth of 0.05 mm is numbered 4, and the toughness and fatigue properties are reduced.

[0125] Compared with Steel 4, Mn is excessive, Cr and Nb are insufficient, and the values ​​deviate from Formula A. Therefore, the austenite grain size at a depth of 0.05 mm is numbered 3, and toughness and fatigue properties are reduced.

[0126] Compared with Steel 5, Al is excessive and deviates from the value of Formula B. Therefore, if AlN coarsens, not only does the pinning force decrease, but the austenite grain size at a depth of 0.05 mm is numbered 4 and the austenite grain size at a depth of 5 mm is numbered 5, and the toughness and fatigue properties also decrease.

[0127] Comparative Example 6 has too little Nb, and the austenite grain size number at depths of 0.05 mm and 5 mm is 5. In addition to the coarsening of the grains, the interior is also too hard, resulting in reduced toughness and fatigue properties.

Claims

1. A steel for vacuum carburizing, wherein: In terms of mass %, it contains: C：0.15～0.30％； Si: 0.05~2.00%; Mn: 0.10 to less than 0.50%; Cr：1.3～2.5％； Nb: 0.020~0.100%; Al：0.020～0.100％； N：0.0040～0.0300％； As needed, any one or more selected from the group consisting of Ti: 0.020-0.200%, B: 0.0050% or less, and V: 0.01-0.50%; As needed, either or both of Ni: 1.0% or less and Mo: 1.0% or less; Balance: Fe and inevitable impurities, Among the inevitable impurities, P: 0.030% or less, S: 0.030% or less, Cu: 0.30% or less, and satisfy: Formula A: 0.12≤3[Al]+4[Nb]≤0.50, and Formula B: 0.12<[N] / [Al]<0.70, Wherein, the % values ​​of the corresponding chemical components are substituted into [Al], [Nb], and [N].

2. The steel for vacuum carburizing according to claim 1, wherein: In terms of mass%, it contains any one or two or more of Ti: 0.020 to 0.200%, B: 0.0050% or less, and V: 0.01 to 0.50%.

3. The steel for vacuum carburizing according to claim 1, wherein: In terms of mass%, it contains either or both of Ni: 1.0% or less and Mo: 1.0% or less.

4. The steel for vacuum carburizing according to claim 1, wherein: In mass %, it contains: Any one or two or more of Ti: 0.020-0.200%, B: 0.0050% or less, and V: 0.01-0.50%; and Either one or both of Ni: 1.0% or less and Mo: 1.0% or less.

5. A carburized steel part, which is a carburized steel part using the vacuum carburizing steel according to any one of claims 1 to 4, wherein: The hardness at a depth of 5.0 mm from the surface is 200 to 500 Hv.

6. A carburized steel part, which is a carburized steel part that has been vacuum carburized at 1050°C or lower using the vacuum carburizing steel according to any one of claims 1 to 4, wherein: The austenite grain size at a depth of 0.05 mm from the surface is 6 or larger in grain size number.

7. A carburized steel part, which is a carburized steel part that has been vacuum carburized at 1050°C or lower using the vacuum carburizing steel according to any one of claims 1 to 4, wherein: The austenite grain size at a depth of 0.05 mm from the surface is 6 or larger in terms of grain size number, and the austenite grain size at a depth of 5.0 mm from the surface is 6 or larger in terms of grain size number.

8. A carburized steel part, which is a carburized steel part that has been vacuum carburized at 1050°C or lower using the vacuum carburizing steel according to any one of claims 1 to 4, wherein: The austenite grain size at a depth of 0.05 mm from the surface is grain size number 6 or greater, and the austenite grain size at a depth of 5.0 mm from the surface is grain size number 7 or greater.

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