WO2002044435A1

WO2002044435A1 - Steel for carburization and carburized gear

Info

Publication number: WO2002044435A1
Application number: PCT/JP2001/010523
Authority: WO
Inventors: Ichie Nomura; Tomoya Kato; Makoto Sumida; Yukio Ito; Masazumi Oonishi; Hideo Aihara; Masahiko Mitsubayashi; Tadashi Eriguchi
Original assignee: Aichi Steel Corporation; Toyota Motor Corporation
Priority date: 2000-12-01
Filing date: 2001-11-30
Publication date: 2002-06-06
Also published as: JP4136656B2; JPWO2002044435A1

Abstract

A steel for carburization which has excellent suitability for cold forging; and a carburized gear having excellent low-cycle fatigue strength. The steel for carburization consists of 0.10 to 0.30 wt.% carbon, up to 0.50 wt.% silicon, 0.50 to 1.50 wt.% manganese, up to 0.030 wt.% phosphorus, up to 0.030 wt.%, sulfur, 0.85 to 2.00 wt.% chromium, up to 0.35 wt.% molybdenum, 0.0010 to 0.0050 wt.% boron, 0.11 to 0.30 wt.% aluminum, 0.0080 to 0.0250 wt.% nitrogen, 0.01 to 0.10 wt.% niobium, 0.01 to 0.10 wt.% titanium, and iron and unavoidable impurities as the remainder, provided that the steel satisfies Hd = 83C wt.% + 5.5Mn wt.% + 4.0Cr wt.% + 10.5Mo wt.% + 12 and Hd ≥ 60∑C wt.% + 12.5, and that effective Al wt.% = Al wt.% - 2( N wt.% - 0.30Ti wt.%) ≥ 0.1.

Description

Description Carburizing steel and carburizing gear

The present invention relates to a carburized steel and a carburized gear using the same, and more particularly, to a carburized steel excellent in cold forging formability and a carburized gear excellent in low cycle fatigue strength. Background art

Conventionally, gears used for automobiles, industrial machines, etc. are made of alloy steels for machine structures such as JIS SMnC, SCr, SCM, SNCM, etc., which are subjected to hot forging, warm forging, or cold forging. After being machined and formed, it is provided with a surface hardening treatment (carburized quenching, induction hardening, soft nitriding, etc.) to improve wear resistance and fatigue strength.

Japanese Patent Publication No. 7-100840 discloses a gear made of steel to which a large amount of alloying elements such as Mo is added, which provides low cycle fatigue strength (tooth surface fatigue strength such as spalling fracture and impact tooth root). Is known to be improved. However, in addition to the addition of expensive alloying elements, there are drawbacks such as a significant increase in cost due to deterioration of cold forgeability and machinability. Therefore, as disclosed in JP-A-10-152746 and JP-A-11-71654, a small amount of B is added to reduce the alloying component added to the material and compensate for the decrease in hardenability due to this. Carburizing steel has been proposed to be added to achieve both workability and strength.

However, the inventions described in these two publications do not sufficiently study the prevention of the formation of troostite in the gear, particularly in the carburized layer, and the stability of austenite crystal grains during carburization, and do not provide excellent low cycle fatigue strength. .

In this context, the term “truth” refers to an imperfect quenching layer observed on the gear surface when gas carburizing is usually performed (oxygen in the carburizing gas atmosphere diffuses from the gear surface and is contained in the material). Microstructure formed due to reduced hardenability due to lack of solid solution alloying elements such as Si, Mn, Cr, etc. in the surrounding area to form oxides with alloying elements such as i, Mn, Cr, etc. Unlike), it refers to fine pearlite that precipitates mainly along the austenite grain boundaries of the carburized layer during carburizing and quenching, and when present in large amounts, is a major factor in lowering low cycle fatigue strength.

The present invention has been made in view of the above circumstances, and has as its object to provide a carburizing steel excellent in cold forging formability and a carburized gear excellent in low cycle fatigue strength. Disclosure of the invention

The present inventors have studied carburizing steel having excellent cold forgeability and carburized gear having excellent low cycle fatigue strength, and as a result, have completed the present invention.

That is, the carburizing steel of the present invention contains: C: 0.10 to 0.30% by weight, Si: 0.50% by weight or less, Mn: 0.50 to: I. 50% by weight, P: 0.30% % By weight, S: 0.003% by weight or less, Cr: 0.85 to 2.00% by weight, Mo: 0.35% by weight or less, B: 0.0010 to 0.0050% by weight, A1 : 0.11 to 0.30% by weight, N: 0.0080 to 0.0250% by weight, Nb: 0.01 to 0.10% by weight, Ti: 0.01 to 0.10% by weight The remainder consists of Fe and unavoidable impurities,

Hd = 83C wt% + 5.5Mn wt% + 4. OCr wt% + 10.5Mo wt% + 12 In the relational expression, Hd≥6 O ^ TC wt% + 12.5, and is effective A 1% by weight = A 1% by weight_2 (N% by weight-0.30Ti% by weight) ≥0.1.

C improves the case hardening depth and internal hardness after carburizing and quenching, and greatly affects the improvement of low cycle fatigue strength. However, when the content of C is less than 0.1% by weight, it takes a long time to carburize to obtain the case-hardening depth, so that the cost is high, and the internal strength is also reduced, and the low cycle fatigue strength of the gear is greatly reduced. I do. On the other hand, if it exceeds 0.30% by weight, the internal hardness after carburizing and quenching increases, and the toughness of the gear teeth decreases significantly, resulting in a decrease in low cycle fatigue strength. In addition, the cold workability and machinability, which are the manufacturing processes of gears, deteriorate, and the service life of dies and tools deteriorates, resulting in a significant cost increase. The lower limit of the C content is preferably 0.13% by weight, more preferably 0.19% by weight, and the upper limit of the C content is preferably 0.27% by weight. It is more preferably 0.24% by weight.

Although Si is an element added as a deoxidizing agent, it is an element that forms a solid solution in ferrite and strengthens it, and particularly affects cold workability. If the content of Si exceeds 0.50% by weight, carburization may be impaired. The content of Si is preferably 0.35% by weight or less, more preferably 0.15% by weight or less. The lower limit is usually 0.03% by weight.

The above Mn is an element added as a deoxidizer, but in order to improve the hardenability of the material, it improves the case hardening depth and internal hardness after carburizing and quenching, and greatly improves low cycle fatigue strength. Affect. However, if the Mn content is less than 0.50% by weight, the formation of trussite in the carburized diffusion layer becomes remarkable, the internal hardness is also reduced, and the low cycle fatigue strength of the gear is greatly reduced. On the other hand, if the content exceeds 1.50% by weight, the transformation temperature of the pearlite during soft annealing decreases, and the hardness of the pearlite portion exceeds Hv 300, thereby deteriorating the cold workability and machinability of the gear. However, the life of molds and tools is shortened, resulting in significant cost increase. The lower limit of the Mn content is preferably 0.8% by weight, more preferably 1.0% by weight, and the upper limit of the Mn content is preferably 1.5% by weight, more preferably 1. 4% by weight.

P is an element that forms a solid solution in ferrite and strengthens it, and in particular, is an element that deteriorates cold workability. If the P content exceeds 0.5% by weight, the austenite grain boundaries during carburization are biased, and the grain boundary strength of the carburized layer decreases. The content of P is preferably 0.015% by weight or less, more preferably 0.012% by weight or less. The lower limit is usually 0.002% by weight.

If a large amount of S is contained, it is deflected to the austenite grain boundaries during carburization, which lowers the grain boundary strength of the carburized layer. It is also an element that reduces cold workability because it forms MnS, a compound with Mn, but is also an element that improves the machinability of gears. The lower limit of the S content is preferably 0.005% by weight, more preferably 0.008% by weight, and the upper limit of the S content is preferably 0.002% by weight, more preferably It is 0.015% by weight.

The above Cr improves the hardenability of the material, thereby improving the case hardening depth and internal hardness after carburizing and quenching, and greatly affecting the improvement in low cycle fatigue strength. In particular, B is added In the case of steels that are used, it is easy to generate a true style that reduces the low cycle fatigue strength in the carburized diffusion layer, but there is a remarkable effect to prevent the generation. If the Cr content is less than 0.85% by weight, the formation of troostite in the carburized layer becomes remarkable, while if it exceeds 2.00% by weight, Cr carbides precipitate at the austenite grain boundaries during carburizing, and the carburized layer is formed. The grain boundary strength is lowered, which is not preferable. The lower limit of the Cr content is preferably 0.9% by weight, more preferably 1.0% by weight, and the upper limit of the Cr content is preferably 1.5% by weight, more preferably 1. The Mo content of 3% by weight improves the hardenability of the material, improves the case hardening depth and internal hardness after carburizing and quenching, and is also effective in improving low cycle fatigue strength. However, if the Mo content exceeds 0.35% by weight, the hardness increases remarkably during the production of the gear, the cold workability and machinability deteriorate, and the life of dies and tools is shortened. The cost will be high. In addition, in steels to which B is added, the presence of a large amount of Mo tends to generate troostite in the carburized layer. The content of Mo is preferably 0.25% by weight or less, more preferably 0.20% by weight or less. The lower limit is usually 0.005% by weight. The above B precipitates at the austenite grain boundaries of the carburized layer to improve the grain boundary strength of the carburized layer and to improve the internal hardenability after carburizing and quenching without increasing the material hardness during cold forging. Is an important element. However, if the content exceeds 0.0050% by weight because the effect of improving the hardenability of the carburized layer is small, it becomes easy to form troostite in the carburized layer, and the effect of improving the hardenability is only saturated. In addition, the hot forgeability or cold forgeability deteriorates, and if it is less than 0.0010% by weight, the internal hardenability during carburizing and quenching decreases, which is not preferable. The content of B is preferably 0.0010 to 0.0035% by weight, more preferably 0.0015 to 0.0030% by weight.

Although A1 is an element added as a deoxidizing agent, it reacts with N in steel to form A1N, and has an action of preventing austenite crystal grains from being coarsened during carburizing heating. Effective A 1% by weight = A 1% by weight_2 (N% by weight-0.30Ti% by weight) ≥0.1 and if the content of A1 is less than 0.11% by weight, In the case of a gear having a high cold working rate, abnormal growth of austenite crystal grains during carburization is observed, and the content of A1 exceeds 0.3% by weight. If this is the case, the incompletely quenched layer on the surface of the gear during carburization becomes deep, which is not preferable. If the effective A content of 1% by weight is less than 0.1% by weight, the hardenability decreases, and the amount of truss in the carburized layer becomes conspicuous. The content of A1 is preferably from 0.11 to 0.20% by weight, and more preferably from 0.11 to 0.15% by weight.

The content of A 1 N formed is preferably 200 to 600 ppm, more preferably 250 to 550 ppm, and still more preferably 300 to 500 ppm. Although 1N precipitates, if the N content is less than 0.0080% by weight, in a cold-worked gear, the amount of precipitated A1N decreases and the austenite crystal grains during carburization become coarse. On the other hand, if it exceeds 0.0250% by weight, the amount of dissolved B decreases, and the hardenability decreases. The lower limit of the N content is preferably 0.008% by weight, more preferably 0.012% by weight, and the upper limit of the N content is preferably 0.025% by weight, more preferably 0.002% by weight. Duru.

The above-mentioned Nb reacts with N or C to generate Nb (C, N), which has an effect of preventing austenite crystal grains from being coarsened during carburizing heating. If the Nb content is less than 0.01% by weight, the above effects are not exhibited. If it exceeds 0.10% by weight, the effect of preventing austenite crystal grains from being coarsened during carburizing heating is saturated, resulting in high cost. I don't like it. The Nb content is preferably 0.01 to 0.07% by weight, more preferably 0.01 to 0.05% by weight.

The Ti reacts with N to form TiN, which has the effect of preventing austenite crystal grains from becoming coarse during carburizing heating. If the Ti content is less than 0.01% by weight, the above effects are not exhibited, and if it exceeds 0.10% by weight, the effect of preventing the austenite crystal grains from becoming coarse during carburizing heating is saturated, and the cost is reduced. It is undesirably high. The content of Ti is preferably 0.01 to 0.07% by weight, more preferably 0.01 to 0.055% by weight.

The above formula Hd = 83 C wt% + 5.5 Mn wt% + 4. OC r wt% + 10.5 Mo wt% + 12 is the magnitude of the internal hardness after carburizing and quenching of the carburizing steel having the above composition. The greater the value, the higher the internal hardness after carburizing and quenching. When the Hd thus obtained satisfies the above formula, Hd≥60 "wt% + 12.5, it is possible to increase the martensite ratio to 90% or more after carburizing, quenching and tempering.

In addition, when the above-described carburizing steel is processed into a predetermined shape before carburizing and quenching, the material is appropriately soft-annealed and has a hardness of Hv 170 or less (preferably ΗV 165 or less, (Preferably Hv 160 or less) ferrite + pearlite structure, and the hardness of the pearlite structure is Hv 300 or less (preferably Hv 295 or less, Hv 290 or less is more preferable. This improves the cold forgeability of the carburizing steel.

The carburized gear of the present invention is manufactured by using the steel for carburization described above, has a case hardening depth of 0.5 mm or more from the surface and has a hardness of Hv 513, and has a hardening depth within the case hardening depth. The characteristic is that the area ratio is 5% or less.

The method of working the carburized gear is not particularly limited. Usually, any one of hot forging, warm forging, and cold forging is performed, followed by machining and carburizing and quenching. In this case, the hardness of the surface layer is usually represented by Vickers hardness, and the case depth of hardness Hv 513 is preferably 0.6 mm or more from the surface, more preferably 0.65 mm or more. More preferably, it is 0.7 mm or more. However, the upper limit is usually 1.5 mm, more preferably 1.2 mm. If the case hardening depth is less than 0.5 mm, the fracture starts from the inside of the gear, which is not preferable. On the other hand, if the case hardening depth is 0.5 mm or more, the fatigue strength is remarkably excellent. The method for measuring the hardness of the gear will be described in Examples.

Fracture of the above gears due to low cycle fatigue can be caused by tooth surface fatigue delamination caused by plastic deformation inside the tooth surface due to high contact stress on the tooth surface, and high bending stress on the tooth root due to high bending stress on the tooth root. Root impact fatigue fracture and force occur due to plasticity and increased root surface stress (see Fig. 1). In general, carburized and quenched gears have a moderate C concentration distribution from 0.8% of the surface C concentration to the inside of the tooth (uncarburized portion). Since the strength and toughness of the (uncarburized part) are important, the suppression of troostite formation in the carburized layer and the austenitic crystal grains caused by the toughness In addition to stability, the quenched structure (martensite ratio) inside the tooth due to internal strength is important.

The “area fraction of truss evening light” is the depth from the surface after carburizing and quenching.

It means the average area fraction in a region up to 0.5 mm, preferably 4.7% or less, more preferably 4.5% or less. If it exceeds 5%, the fatigue strength is reduced, and there is a possibility that peeling may occur on the tooth surface of the gear (the part where the teeth contact each other when the gear is engaged).

The internal hardness of the pitch portion of the carburized gear may be Hv 350 or more, preferably Ην 370 or more, and more preferably Hv 390 or more. If it is less than 350, tooth surface fatigue strength is undesirably reduced.

Further, the internal hardness of the root portion of the carburized gear may be Hv 300 or more, preferably Hv 330 or more, more preferably Hv 360 or more. If Hv is less than 300, the root impact fatigue strength is undesirably reduced. In addition, the above-mentioned "tooth root part" means an R part which goes from the pitch circle part of the tooth toward the root of the tooth.

As a preferred combination of the internal hardness of the pitch portion and the root portion, the pitch portion is Hv 350 or more, and the root portion is Hv 300 or more, more preferably, the pitch portion is Hv 370 or more, and the root portion is Hv. 330 or more, more preferably the pitch part is HV 390 or more, and the tooth root part is Hv 360 or more.

Further, as a preferable combination of the internal hardness of the pitch portion, the internal hardness of the root portion, and the area ratio of the troostite, (1) the hardness of the pitch portion is Hv 350 or more, and the hardness of the root portion is Hv 300 or more, and the area ratio of the trousers is 5% or less. (2) More preferably, the hardness of the pitch portion is 370 or more, and the hardness of the root portion is 30 or more, and the area ratio of the trousers. (3) More preferably, the hardness of the pitch portion is Hv 390 or more, and the hardness of the root portion is Hv 360 or more, and the troostite area ratio is 4.7% or less, ( 4) Particularly preferably, the hardness of the pitch portion is Hv400 or more, the hardness of the root portion is Hv390 or more, and the area ratio of the trousers is 4.5% or less.

In the case of producing a carburized gear using the above-mentioned carburizing steel, a steel having the above structure and properties can be obtained by performing normal carburizing, quenching and tempering. In particular, the organization The gear diameter is equivalent to a round bar, and the outer diameter is preferably 40 mm or less, more preferably 30 mm or less, and even more preferably 25 mm or less. The quenching medium is preferably 200 and the following oil quenching is preferred. However, the carburizing atmosphere may be any one of gas carburizing, carburizing and nitriding, and vacuum carburizing.

According to the carburizing steel of the present invention, it has a specific elemental composition, and Hd = 83C weight% + 5.5Mn weight% + 4.01 "weight% + 10.5 Mo weight% + 12 By setting Hd ≥ 60 C wt% + 12.5, a carburized and quenched structure with a martensite ratio of 90% or more can be obtained, and abnormal growth of austenite crystal grains can be prevented. In addition, when processing into a predetermined shape before carburizing and quenching, by applying appropriate softening annealing, the cold forging formability can be improved, and the steel can be easily processed. It can be used to easily manufacture gears, shafts, etc. used for automobiles, industrial machines, etc. Further, according to the carburized gear of the present invention, the stress of the tooth root portion, which is an effect of the cold forged gear, is obtained. Higher strength and resistance in high torque region due to ease of concentration and to obtain a flow line along the tooth profile Excellent in low cycle fatigue strength (耐歯 surface fatigue strength and 耐歯 original fatigue strength). BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory view showing the form of low cycle fatigue fracture of a gear.

FIG. 2 is an explanatory diagram showing a heat pattern for carburizing, quenching and tempering a bevel gear. FIG. 3 is an explanatory diagram showing a low cycle fatigue test of a bevel gear. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described specifically with reference to examples.

1. Production and evaluation of carburizing steel

(1) Hardness and effective A 1% by weight obtained from the composition of carburizing steel

Steels A to S having the chemical compositions shown in Tables 1 to 3 (A to H were used as the inventive steels, I to W were used as comparative steels. Of the comparative steels, Q 1 to 33 steel and R 1 to 20 steel were It corresponds to Japanese Unexamined Patent Publication No. Hei 10-15 2746 and Japanese Unexamined Patent Publication No. Hei 11-71654. However, S steel is equivalent to JIS standard SNCM420, which is often used for gears requiring low cycle fatigue strength. ) Were melted in a vacuum melting furnace.

Tables 4 and 5 show the calculated Hd

C wt% + 5.5Mn wt% + 4. OC r wt% + 10.5Mo wt% + 12 and Hd ₂ = 60 "C wt% + 12.5 and these

showed that. Tables 6 and 7 show the calculation results of 1% by weight of effective A. In Tables 4 to 7, those whose constituent element content is out of the range of the present invention are indicated by an asterisk beside the numerical value.

Table 5

Η し. nd- | πα ·, one Ηα min (wt%) (wt%) (wt%) (wt%) Calculated value Calculated value Calculated value

Q22 0.28 0.36 1.85-44.62 44.25 0.37

Q23 0.23 1.10 0.95 0.39 * 40.94 41.27 -0.33

Q24 0.25 0.86 0.56 * 0.15 43.82 42.50 1.32

Q25 0.17 0.45 * 0.35 *-31.56 37.24 -5.68

Q26 0.35 1.10 0.35 *-48.50 48.00 0.50

Q27 0.25 2.15 * 0.65 *-47.18 42.50 4.68 Ratio Q28 0.08 * 0.45 * 0.05 *-21.32 29.47 -8.15

Q29 0.18 0.29 * 0.68 *-45.61 37.96 7.65

Q30 0.23 1.15 0.55 *-43.19 41.27 1.92

Q31 0.20 0.86 2.15 *-41.93 39.33 2.60

Q32 0.21 0.82 0.76 * ― 36.98 40.00 -3.02

Q33 0.22 0.95 1.15 0.18 41.98 40.64 1.34

R1 0.21 0.70 0.70 * 0.17 37.87 40.00 -2.13

R2 0.18 0.71 0.65 * 0.17 35.23 37.96 -2.73

R3 0.17 0.68 0.81 * 0.17 34.88 37.24 -2.36

R4 0.12 1.19 0.11 * 0.16 30.63 33.28 -2.65

R5 0.21 0.11 * 1.46 0.17 37.66 40.00 -2.34

R6 0.27 0.50 0.50 * 0.17 40.95 43.68 -2.73 Comparison R7 0.21 0.60 0.60 * 0.17 37.24 40.00 -2.76

R8 0.18 0.35 * 0.35 * 0.65 * 36.49 37.96 -1.47

R9 0.18 0.70 0.70 * 0.30 34.78 37.96 -3.18

R10 0.19 0.68 0.68 * 0.40 * 36.47 38.65 -2.18

R11 0.19 0.71 0.25 * 0.17 34.46 38.65 -4.19

R12 0.22 1.21 1.51 0.17 44.74 40.64 4.10

R13 0.21 0.65 0.75 * 0.17 37.79 40.00 -2.21 14 0.18 0.60 0.77 * 0.17 35.11 37.96 -2.85

R15 0.19 0.77 0.76 * 0.71 * 42.50 38.65 3.85

R16 0.18 0.60 0.77 * 0.16 35.00 37.96 -2.96

R17 0.17 0.60 0.78 * 0.22 34.84 37.24 -2.40

R18 0.16 0.15 0.07 * 0.17 28.17 36.50 -Β.33 Steel R19 0.17 0.15 0.07 * 0.17 29.00 37.24 -8.24

R20 0.20 0.31 0.31 * 0.78 * 39.74 39.33 0.41

S 0.21 0.60 0.56 * 0.20 37.07 40.00 -2.93

T 0.21 1.03 0.89 0.15 42.00 41.30 0.70

U 0.17 0.72 0.91 0.16 38.70 40.00 -1.30

V 0.31 * 0.86 1.29 0.22 49.93 45.91 4.02

W 0.17 1.80 * 0.21 * 0.01 36.96 37.24 -0.28

〇AV

Ward AI N Nb Ti

Steel

Min (Weight <½) (Weight%) (Weight%) AI% -2 (N% -0.30Ti%)

Q22 0.035 * 0.0050 * 0030 0.004 * 0.03

Q23 0.005 * 0.0050 * 0030 0.004 * 0.00

Q24 0.07 * 0.0050 * 0030 0.004 * 0.06

Q25 0.165 0.0050 * n 030 0.004 * 0.16

Q26 0.079 * 0.0077 * 0034 0.003 * 0.07

Q27 0.088 * 0.0035 * 0.002 * 0.09

Q28 0.135 0.0045 * 0.005 * 0.13 ratio Q29 0.093 * 0.0017 * 0.001 * 0.09

Q30 0.074 * 0.0034 * 0045 0.009 * 0.07

Q31 0.078 * 0.0054 * 0.002 * 0.07

Q32 0.035 * 0.0045 * Lord 0.035 0.05

Q33 0.025 * 0.0055 * | [DQ ― 0.01

R1 0.091 * 0.0141 0029 0.042 0.09

R2 0.051 * 0.0041 * 0.002 * 0.04

R3 0.045 * 0.0023 * 0041 one 0.04

R4 0.044 * 0.0045 * ― 0.04

R5 0.041 * 0.0065 * 0003 * 0.002 * 0.03

R6 0.011 * 0.0038 * 0001 * 0.031 0.02 Comparison R7 0.091 * 0.0141 n fiq 5 0.048 0.09

R8 0.041 * 0.0061 * 0.002 * 0.03

R9 0.025 * 0.0055 * 0011 0.001 * 0.01

R10 0.055 * 0.0032 * 0.011 0.06

R11 0.054 * 0.0044 * 0044 0.012 0.05

R12 0.042 * 0.0044 * ― 0.03

R13 0.049 * 0.0050 * 0.002 * 0.04

R14 0.005 * 0.0210 ― 0.02

R15 0.033 * 0.0045 * ― 0.05

R16 0.053 * 0.0033 * 0.001 * 0.001 * 0.05

R17 0.055 * 0.0043 * 0.001 * 0.001 * 0.05

Steel R18 0.041 * 0.0042 * 0.003 * 0.030 0.03

R19 0.041 * 0.0035 * 0.03

R20 0.024 * 0.0150 -0.01

S 0.031 * 0.0099 0.01

T 0.121 0.0205 0.019 0.021 0.09

U 0.141 0.0130 0.031 0.041 0.14

V 0.091 * 0.0133 0.029 0.031 0.10

W 0.139 0.0134 0.033 0.022 0.15

(2) Production and evaluation of carburizing steel

The ingots A to P, Ql, R1, and S were heated at 1200 ° C or higher for 0.5 hours, and then hot forged at a temperature of 1000 to 1200 ° C to produce round bars with a diameter of 30 mm. . This was annealed from 900 ° C to 600 ° C at a cooling rate of 75 tZ to obtain carburizing steel. Among the carburizing steels obtained above, the carburizing round bars of A to H, 〇, P and S have the hardness (hardness of the base and pearlite) and the cold workability (70% (Deformation resistance and critical working ratio) were measured by the following methods. Table 8 shows the results.

The substrate hardness was measured at a load of 10 kg using a Vickers hardness tester (model: AVK_C2, manufacturer: AKAS HI). The hardness of the pearlite portion was measured by measuring the micro Vickers hardness under a load of 10 g.

Deformation resistance was measured by loading a test piece (without notch) with a diameter of 10 mm and a height of 15 mm using a 100 t universal test device (model: RH_100, manufacturer name: manufactured by Shimadzu Corporation) at a load cell moving speed of ImmZ. The compression load at 70% upsetting was measured, and the deformation resistance obtained by using the deformation resistance measurement method by the end face constrained compression proposed by the Japan Society for Technology of Plasticity is shown in Table 8. (1980) pp. 529-532

) o

The limit processing rate is determined by performing an end face restraint test on the above test piece using the above equipment, applying a load cell at a moving speed of lmm / min, and limiting the upsetting rate when the end (circumferential portion) cracks. (See Plasticity and Forming No. 241, (1981) p. 139-144, published by the Japan Society for Technology of Plasticity). Table 8

Ward Hardness Cold force

Steel Base hardness / \ light hardness Deformation resistance Limit processing rate

Minute (Hv) (Hv) (Mpa) (%)

A 155 281 970 74

B 158 289 982 75 pcs C 148 255 918 77

Departure D 159 254 972〉 80 Description E 156 243 998 79 Steel F 159 279 979 76

G 157 285 977 73

H 156 287 970 74

0 173 267 1105 72 Ratio P 159 307 998 68

Early Father S 171 321 1081 66

Steel V 171 261 1089 70

W 158 305 934 66 2. Manufacture and evaluation of carburized gears

Of the carburizing steels obtained above, A to N, Q 1, Shaku 1 and 5 to N, the carburizing round bar was cold forged with a module of 4.8 and a number of teeth of 10 A bevel gear with a pitch circle diameter of 48.8 mm was manufactured by carburizing, quenching and tempering with the heat pattern shown in Fig. 2 and further grinding the inner diameter and the like. The bevel gear was set in a hydraulic fatigue tester as shown in Fig. 3, and a bevel gear fatigue test was performed as follows. A jig having a curvature equivalent to that of the actual gear was fabricated on the test gear, and a complete one-sided fatigue test was performed. Using the acoustic emission, the crack generation time was regarded as the life, and the tooth surface or root origin was determined by magnetic particle flaw detection. Table 9 shows the results.

Next, the case hardening depth and the internal hardness at the pitch portion and the root portion of the bevel gear were measured, and further, the troostite structure within the case hardening depth was observed and the austenite crystal grains were mixed. Table 9 also shows the results. The case hardening depth was measured with a load of 300 g using a micro Vickers hardness tester (model: MVK-E, manufacturer name: AKASH I). The internal hardness was measured at a load of 10 kg using a Vickers hardness tester (model: AVK-C2, manufacturer: AKASH I). For the observation of the troostite structure, the area ratio of troostite was determined by the following method using an image analyzer (model: LUZEX-IIIU, manufacturer name: manufactured by NI RECO). After the nail corrosion, the area ratio of the black corroded portion of the troostite structure was calculated. Austenitic crystal grains were observed using an optical microscope (model: BX60M, manufacturer name, manufactured by OLYMPUS).

¾: kak Table 9

Low ¹ crew J Labor test Carburizing Thermal material AA

Ward Pi? Portion Pitch portion Root portion Root portion Remarks

5 rfl ^ A7l + — 7 ÷ +

Min Origin of fracture Case hardening depth Internal hardness Case hardening depth Internal hardness

Strength Area ratio jLj

(mm; (Hv) (mm) (Hv)

A 84.5 N Tooth surface 0.83 415 0.80 406 1.9% Sizing ―

B 84.1 KN Tooth surface 0.83 414 0.77 399 2.6% Sizing ―

This C 80.9 N Tooth surface 0.79 403 0.67 373 4.4% One-shot sizing D 80.1 KN Root 1.01 463 0.96 450 2.8% Sizing-clear E 80.3 KN Root 0.94 444 0.85 420 1.9¾ Sizing-Steel F 84.4 KN Root 0.93 440 0.89 430 4.2% Sizing ―

G 84.7 KN Tooth surface 0.83 416 0.75 394 2.2% Sizing ―

H 81.0KN Tooth 0 79 403 0.74 390 ≤1%

I 65 Fine tooth surface 0.59 347 0.49 302 2.6% Sizing out of range

J 75.5 N Tooth surface 0.80 398 0.67 358 7.8% Sizing out of Mn range

K 76.5KN Tooth surface 0.85 421 0.84 408 5.8% Sizing out of Mo range

76.0KN Tooth surface 0.87 426 0.81 400 6.3% Mixed Cr, out of AI range, effective AK0.1

M 73.1 KN ≤1% sizing Cr out of range

Ratio Root 0.96 450 0.94 434

N 77.9 N Root 0.94 445 0.87 417 4.2¾ Mixed N Out of range

O 64.2 N Root ∞ 516 ≥2.0 500 1.7% Sizing out of range

Comparison P 85.1 KN Tooth surface 0.84 418 0.80 397 ≤1% Sizing out of Mn range

Q1 75.Knitting tooth surface 0.85 388 0.73 347 1 1.2% Sizing Hd,-Hd ₂ 0 0, out of Nb'Ti range

R1 76.2 N Tooth surface 0.89 398 0.81 367 8.2% Mixed grain Hd, -Hd ₂ <0, Effective Al <0.1, Out of AI steel

S 76.6 N Root 0.84 416 0.56 416 ≤1% Mixed SNCM420

T 76.6KN Tooth surface 0.89 424 0.85 408 5.8% Mixed grain effective AK0.1

u 75.7KN Tooth surface 0.80 407 0.70 370 6.8% Sizing Hd, -Hd ₂ <0

V 67.1 KN Root ∞ 518 ∞ 51 1 1.0% Mixed C, AI out of range, effective AK0.1 w 76 Fine tooth surface 0.83 413 0.74 380 7.4% Sized Hd,-Hd, 0, out of Cr.Mn range

Although it was good at Hv 267, the substrate hardness was high at Hv 173, and the deformation resistance in cold workability was higher than 1000 MPa. The P steel had an Mn content outside the range of the present invention and had a good base hardness of Hv 159, but had a high pearlite hardness and a low deformation resistance in cold workability of less than 100 OMPa. However, the critical processing rate was also inferior. S steel did not contain B, Nb and Ti, and was inferior in the base material hardness and pearlite hardness, and the cold workability was further inferior to the O and P steels in the critical working ratio. Steel V had a C content outside the range of the present invention and had a good pearlite hardness of Hv 171.However, the base hardness was high at Hv 171 and the deformation resistance in cold workability exceeded 1000 MPa. Was expensive. The W steel had a Mn content outside the range of the present invention and had a good base hardness of Hv 158, but had a high pearlite hardness of Hv 305 and was inferior to the limit workability.

On the other hand, the steels A to H of the steels of the present invention were excellent in the base material hardness and the pearlite hardness, had a deformation resistance of 1000 MPa or less even in cold workability, and had a critical work ratio exceeding 70%. In particular, steel C has low deformation resistance, and steel D has a critical working ratio of over 80%, showing excellent cold workability. The steels A to H of the steel of the present invention correspond to Hd in Table 4.

And 1% by weight of effective A in Table 6 was within the scope of the present invention.

In Table 9, in steel I, the C content was out of the range of the present invention, the internal hardness of the pitch portion was low, and the low cycle fatigue strength was low due to the decrease in tooth surface strength. The content of Mn in the steel J and the content of Cr in the steel K are out of the range of the present invention, respectively, and 5% or more of troostite precipitates in the carburized layer, and the tooth surface strength is reduced. The cycle fatigue strength was reduced. In L steel, mixed grains were generated in the austenite crystal grains after carburization. This results in gear strain and low cycle fatigue strength variation. In the M steel, since the Cr content was higher than the range of the present invention, the root flexural strength decreased and the low cycle fatigue strength decreased. In N steel, since the N content was below the range of the present invention, austenite grains after carburization were mixed. Steel 劣 had poor low cycle fatigue strength. In the case of No. 91 and No. 1 steels, a large amount of trussite precipitated to lower the tooth surface strength and lower the low cycle fatigue strength. Steel S had excellent internal hardness at the pitch and roots, and the area ratio of trussite was 1% or less, but mixed grains occurred. In T and U steels, a large amount of trussite precipitates and reduces tooth surface strength did. In steel V, the content of C is higher than the range of the present invention, and the effective A 1% by weight is low, so that mixed grains are generated and the toughness is reduced, so that the tooth root strength is reduced and low cycle fatigue is caused. The strength has decreased. In W steel, since the Cr content is lower than the range of the present invention and the H di-Hd ₂ value is out of the range of the present invention, a large amount of trussite precipitates, the tooth surface strength is reduced, and the cycle is low. The fatigue strength has been reduced.

I steel except the C content is outside the range of the present invention, Hd i-Hd ₂ values and effective A 1 wt% is within the scope of the present invention. The J steel had an effective A of 1% by weight within the range of the present invention except that the Mn content and the Hd Hd ₂ value were outside the range of the present invention. The K steel had an H di-H d ₂ value and an effective A of 1% by weight within the range of the present invention except that the content of Mo was outside the range of the present invention. L steel C r, the content of A 1, Hd - H d ₂ values and effective A 1 wt% were outside the scope of the present invention. M steels except the content of C r is outside the range of the present invention, Hd i-Hd ₂ values and effective A 1 wt% were within the limits of the present invention. N steel except the content of A 1 and N are outside the scope of the present invention, Hd丄_ｰHd ₂ values and effective A 1 wt% is within the scope of the present invention. Q 1 steel except the content and HDI-Hd ₂ value of C r, N b and T i is outside the range of the present invention, effective A 1 wt% is within the scope of the present invention. The length of steel 1 ", the content of A1, the Hd i -Hd ₂ value and the effective A 1% by weight were out of the range of the present invention. The S steel was Cr, A 1, N, B, Nb and Ti content, Hd

The values and 1% by weight of effective A were outside the scope of the present invention. The T steel had an effective A content of 1% by weight, and the U steel had an H di-Hd ₂ value outside the range of the present invention. Steel V was out of the scope of the present invention in the content of C and A1 and the effective A1% by weight. The W steel had Cr and Mn contents and Hd ₁ —Hd ₂ values outside the range of the present invention.

On the other hand, in the steels A to H of the present invention, the area ratio of trussite was small, and the austenite crystal grains were also sized. The 300 times strength also exceeded 80KN, showing excellent low cycle fatigue strength.

The Ql-33 steel and the R1-20 steel are those described in JP-A-117-1654 and JP-A-10-15746, respectively. , Q2 to 5, 7, 9 to 11, 13 to: 15, 20, 21, 23, 32 steel and R 1 to 11, 13, 14, 14, 16 to 19 steel are shown in Table 4. Hc ^ -Hd ₂ values in Tables 6 and 7 and Tables 6 and 7 Effective A in 1% by weight is outside the scope of the present invention. Q 6, 8, 12, 16~19 , 25, 28 steel effective A 1 wt% is more than 0.1 but, HDI-Hd ₂ value takes a negative value. In addition, Q22, 24, 26, 27 steel and R12, 15, 20 steel are Hd

However, 1% by weight of effective A is less than 0.1.

It should be noted that the present invention is not limited to the above embodiments, and various embodiments can be made within the scope of the present invention depending on the purpose and application.

Claims

The scope of the claims

1. C: 0.10 to 0.30 wt%, Si: 0.50 wt% or less, Mn: 0.3 wt%

50-1.50% by weight, P: 0.30% by weight or less, S: 0.30% by weight or less, Cr: 0.85-2.00% by weight, Mo: 0.35% by weight or less, B: 0.000-10-0. 0050% by weight, A1: 0.11 to 0.30% by weight, N: 0.008 0 to 0.0250% by weight, Nb: 0.01 to 0.10% by weight, T i: 0.01 to 0.10% by weight, with the balance being Fe and unavoidable impurities

Hd (hardness) = 83C wt% + 5.5Mn wt% + 4. OCr wt% + 10.5Mo wt% + 12 In the relational expression, Hd ≥ 60C wt% + 12.5, and is effective 1% by weight of A = 1% by weight of A-2 (% by weight of N-0.30% by weight of Ti) ≥0.1.

2. The case hardening depth which is manufactured by using the carburizing steel according to claim 1 and which has a hardness of Hv 513 by carburizing, quenching and tempering is 0.5 mm or more from the surface and within the case hardening depth. A carburized gear wherein the area ratio of troostite is 5% or less.

3. The carburized gear according to claim 2, wherein the internal hardness of the pitch portion of the carburized gear is Hv 350 or more, and the internal hardness of the root portion is Hv 300 or more.