JP7323791B2 - Carburized gear steel, carburized gear, and method for manufacturing carburized gear - Google Patents

Description

TECHNICAL FIELD The present invention relates to a carburized gear steel, a carburized gear, and a method for manufacturing a carburized gear.

Gears used in construction machinery, industrial machinery, etc. are generally carburized and quenched after machining in order to achieve both precise dimensional accuracy and strength.
Among these gears, gears that are used without being sealed from the outside are used while being in contact with moisture or the like that enters from the environment, causing corrosion and thinning. On the other hand, in recent years, the miniaturization of gears has progressed with the aim of reducing weight.
As a result, the impact of thinning due to corrosion due to the environment has increased and is starting to become a problem, so gears with excellent corrosion resistance are required.

Regarding the technical development of conventional gears, for example, Patent Document 1 discloses a technique of providing gears with excellent corrosion resistance by performing heat treatment in the order of carburizing, nitriding, and induction hardening.

Japanese Patent No. 6314648

The technique disclosed in Patent Document 1 requires complicated heat treatment and does not provide a realistic technique in terms of production efficiency.

An object of the present invention is to provide a carburized gear steel and a method for manufacturing a carburized gear that can produce a carburized gear with excellent corrosion resistance when gas carburizing and quenching is performed to produce a carburized gear, and a carburized gear with excellent corrosion resistance. It is to be.

Means for solving the problems of the present invention include the following aspects.
<1> % by mass,
C: 0.17-0.22%,
Si: 0.50 to 1.00%,
Mn: 0.85-1.20%,
Cr: 0.60 to 1.00%,
Mo: 0.01 to 0.40%,
S: 0.001 to 0.050%,
N: 0.005 to 0.020%,
Al: 0.001 to 0.100%,
Nb: 0.001 to 0.050%,
O: 0.005% or less,
Carburized gear steel consisting of P: 0.05% or less and the balance: Fe and impurities.
<2> % by mass,
Ni: 0.1 to 3.0%,
Cu: 0.05-1.0%,
Co: 0.05 to 3.0%,
W: 0.05 to 1.0%,
V: 0.01 to 0.3%,
Ti: 0.005-0.3% and B: 0.0005-0.005%
Carburized gear steel according to <1> above, containing one or more elements selected from the group consisting of:
<3> % by mass,
Pb: 0.09% or less,
Bi: 0.0001 to 0.5%,
Ca: 0.0003-0.01%,
Mg: 0.0005-0.01%,
Zr: 0.0005 to 0.05%,
Te: 0.0005 to 0.1%, and rare earth element: 0.0005 to 0.005%
The carburized gear steel according to <1> or <2> above, containing one or more elements selected from the group consisting of:
<4> The content of Mn, Fe, and Cr in the oxide present on the surface after gas carburizing is any one of <1> to <3> that satisfies the following formula (1): Carburized gear steel as described.
10%≦Mn/(Fe+Mn+Cr)×100≦100% (1)
(The symbol of the element in formula (1) represents the content in mass % of the element in the oxide.)
<5> Machining the carburized gear steel according to any one of <1> to <4> into a gear-shaped member;
a step of gas carburizing the gear-shaped member;
A method of manufacturing a gear having
<6> At a position deeper than 3 mm from the surface, in mass%,
C: 0.17-0.22%,
Si: 0.50 to 1.00%,
Mn: 0.85-1.20%,
Cr: 0.60 to 1.00%,
Mo: 0.01 to 0.40%,
S: 0.001 to 0.050%,
N: 0.005 to 0.020%,
Al: 0.001 to 0.100%,
Nb: 0.001 to 0.050%,
O: 0.005% or less,
P: 0.05% or less, and the balance: Fe and impurities,
A carburized gear in which the contents of Mn, Fe, and Cr in oxides present on the surface satisfy the following formula (1).
10%≦Mn/(Fe+Mn+Cr)×100≦100% (1)
(The symbol of the element in formula (1) represents the content in mass % of the element in the oxide.)
<7> At a position deeper than 3 mm from the surface, in % by mass,
Ni: 0.1 to 3.0%,
Cu: 0.05-1.0%,
Co: 0.05 to 3.0%,
W: 0.05 to 1.0%,
V: 0.01 to 0.3%,
Ti: 0.005-0.3% and B: 0.0005-0.005%
The carburized gear according to <6> above, containing one or more elements selected from the group consisting of:
<8> At a position deeper than 3 mm from the surface, in % by mass,
Pb: 0.09% or less,
Bi: 0.0001 to 0.5%,
Ca: 0.0003-0.01%,
Mg: 0.0005-0.01%,
Zr: 0.0005 to 0.05%,
Te: 0.0005 to 0.1%, and rare earth element: 0.0005 to 0.005%
The carburized gear according to <6> or <7>, containing one or more elements selected from the group consisting of:

INDUSTRIAL APPLICABILITY According to the present invention, a carburized gear steel capable of producing a carburized gear having excellent corrosion resistance when gas carburizing and quenching is performed to produce a carburized gear, a carburized gear manufacturing method, and a carburized gear having excellent corrosion resistance are provided. be done.

An embodiment, which is an example of the present invention, will be described in detail below.
In this specification, a numerical range represented by "-" means a range including the numerical values before and after "-" as lower and upper limits.
Moreover, "%" means "% by mass" with respect to the content of elements in the chemical composition.

(steel for carburizing gears)
First, the circumstances leading to the present invention will be described.
The inventors of the present invention conducted extensive research on methods for improving the corrosion resistance of gears after gas carburizing and quenching. As a result, it was found that the composition of oxides formed by gas carburizing has an effect on corrosion resistance. Therefore, the present inventors further investigated the influence of the chemical composition of the steel on the method of obtaining the oxide composition that improves the corrosion resistance. As a result, the steel ingredients were C: 0.17 to 0.22%, Si: 0.50 to 1.00%, Mn: 0.85 to 1.20%, Cr: 0.60 to 1.00%. , Mo: 0.01 to 0.40%, S: 0.001 to 0.050%, N: 0.005 to 0.020%, Al: 0.001 to 0.100%, Nb: 0.001 ~ 0.050%, O: 0.005% or less, P: 0.05% or less, and the balance: Fe and impurities, the Mn concentration in the oxide after gas carburizing increases and becomes high It was found that corrosion resistance can be obtained.

In a gas carburizing atmosphere, iron is not oxidized, but alloying elements such as Mn and Cr are selectively oxidized. The oxide formed thereby changes to Mn ₃ O ₄ or MnCr ₂ O ₄ and is affected by the amounts of Mn and Cr added. As a result of investigation, corrosion resistance was improved when an oxide with a high Mn content was formed on the surface.

Next, reasons for limiting the chemical composition of the steel according to this embodiment will be described.

C: 0.17-0.22%
The C content affects the hardness of the non-carburized portion of the gear. The C content is made 0.17% or more in order to secure the required hardness. From the viewpoint of ensuring the hardness of the non-carburized portion, the lower limit of the C content is preferably 0.18% or more, more preferably 0.19% or more.
On the other hand, if the C content is too high, the hardness of the non-carburized portion after carburization increases and the strength against impact decreases, so the C content is made 0.22% or less. A preferable upper limit of the C content is 0.21% or less, more preferably 0.20% or less.

Si: 0.50-1.00%
Si is an element that affects oxides formed by gas carburizing. In order to obtain an oxide with excellent corrosion resistance, the Si content should be within the range of 0.50 to 1.00%.
From the viewpoint of obtaining an oxide with excellent corrosion resistance by gas carburizing, the lower limit of the Si content is preferably 0.55% or more, more preferably 0.60% or more. Similarly, the upper limit of the Si content is preferably 0.90% or less, more preferably 0.80% or less.

Mn: 0.85-1.20%
Mn is an element that affects oxides formed by gas carburizing. In order to obtain an oxide with excellent corrosion resistance, the Mn content should be within the range of 0.85 to 1.20%.
From the viewpoint of obtaining an oxide with excellent corrosion resistance, the lower limit of the Mn content is preferably 0.88% or more, more preferably 0.90% or more. Similarly, the upper limit of Mn content is preferably 1.15% or less, more preferably 1.10% or less.

Cr: 0.60-1.00%
Cr is an element that affects oxides formed by gas carburizing. In order to obtain oxides with excellent corrosion resistance, the Cr content should be within the range of 0.60 to 1.00%.
From the viewpoint of obtaining an oxide with excellent corrosion resistance, the lower limit of the Cr content is preferably 0.65% or more, more preferably 0.70% or more. Similarly, the upper limit of the Cr content is preferably 0.95% or less, more preferably 0.90% or less.

Mo: 0.01-0.40%
Mo is an element that affects oxides formed by gas carburizing. In order to obtain oxides with excellent corrosion resistance, the Mo content should be within the range of 0.01 to 0.40%.
From the viewpoint of obtaining an oxide with excellent corrosion resistance, the lower limit of the Mo content is preferably 0.10% or more, more preferably 0.15% or more. Similarly, the upper limit of Mo content is preferably 0.35% or less, more preferably 0.30% or less.

S: 0.001 to 0.050%
S forms MnS in the steel, which improves the machinability of the steel. In order to obtain machinability at a level that enables cutting of parts (gears), an S content equivalent to that of general steel for machine structural use is required. For the above reasons, the S content should be within the range of 0.001 to 0.050%.
From the viewpoint of improving the machinability of steel, the lower limit of the S content is preferably 0.005% or more, more preferably 0.010% or more.
On the other hand, if the S content is excessively high, coarse MnS is formed and the mechanical performance of the gear is impaired, so the upper limit of the S content is preferably 0.040% or less, more preferably 0.030% or less is.

N: 0.005-0.020%
N has a crystal grain refining effect by forming a compound with Al, Ti, V, Cr, etc., so it is necessary to contain 0.005% or more.
However, if the N content exceeds 0.020%, the compound becomes coarse and the crystal grain refining effect cannot be obtained. For the above reasons, the N content should be within the range of 0.005 to 0.020%.
From the viewpoint of obtaining the crystal grain refining effect, the preferable lower limit of the N content is 0.006% or more, more preferably 0.007% or more.
On the other hand, from the viewpoint of suppressing embrittlement due to coarsening of the compound, the upper limit of the N content is preferably 0.018% or less, more preferably 0.015% or less.

Al: 0.001-0.100%
Al is an element that is effective in deoxidizing steel, and is an element that combines with N to form nitrides and refine grains. This effect is insufficient if the Al content is less than 0.001%. On the other hand, if the Al content exceeds 0.100%, the nitride becomes coarse and embrittled.
From the viewpoint of obtaining the crystal grain refining effect, the lower limit of the Al content is preferably 0.004% or more, more preferably 0.007% or more.
On the other hand, from the viewpoint of suppressing embrittlement due to coarsening of nitrides, the upper limit of the Al content is preferably 0.080% or less, more preferably 0.050% or less.

Nb: 0.001-0.050%
Nb is an element that affects oxides formed by gas carburizing. In order to obtain oxides with excellent corrosion resistance, the Nb content should be within the range of 0.001 to 0.050%.
From the viewpoint of obtaining an oxide excellent in corrosion resistance, the lower limit of the Nb content is preferably 0.005% or more, more preferably 0.010% or more. Similarly, the upper limit of the Nb content is preferably 0.045% or less, more preferably 0.040% or less.

O: 0.005% or less O forms oxides in steel and acts as inclusions to lower the fatigue strength, so the O content is preferably limited to 0.005% or less.
From the viewpoint of suppressing the formation of oxides in the steel, the upper limit of the O content may be 0.003% or less, or 0.002% or less. Since the smaller the O content, the lower limit of the O content is 0%.

P: 0.05% or less P segregates at austenite grain boundaries during heating before quenching, thereby reducing fatigue strength. Therefore, it is preferable to limit the P content to 0.05% or less. The upper limit of the P content may be 0.04% or less, or may be 0.03% or less. Since the smaller the P content, the lower limit of the P content is 0%.
However, removing P more than necessary increases the manufacturing cost. Therefore, the practical lower limit of the P content is usually about 0.004% or more.

In the steel according to the present embodiment, in order to enhance the hardenability or grain refinement effect, a part of Fe is replaced with Ni, Cu, Co, W, V, Ti, and B. may contain one or more. The lower limit when these elements are not contained is 0%.

Ni: 0.1-3.0%
Ni is an effective element for imparting necessary hardenability to steel. If the Ni content exceeds 3.0%, the amount of retained austenite increases after quenching and the hardness decreases. Therefore, the Ni content is set to 3.0% or less.
From the viewpoint of suppressing a decrease in hardness due to retained austenite after quenching, the upper limit of the Ni content may be 2.0% or less, or 1.8% or less.
On the other hand, from the viewpoint of imparting hardenability to steel, the lower limit of the Ni content may be 0.1% or more, or may be 0.3% or more.

Cu: 0.05-1.0%
Cu is an element effective in improving the hardenability of steel. If the Cu content exceeds 1.0%, the hot ductility is lowered. Therefore, the Cu content is set to 1.0% or less. When Cu is contained to obtain the above effect, the lower limit of the Cu content may be 0.05% or more, or 0.1% or more.

Co: 0.05-3.0%
Co is an element effective in improving the hardenability of steel. When the Co content exceeds 3.0%, the effect is saturated. Therefore, the Co content is set to 3.0% or less. When Co is contained to obtain the above effect, the lower limit of the Co content may be 0.05% or more, or 0.1% or more.

W: 0.05-1.0%
W is an element effective in improving the hardenability of steel. When the W content exceeds 1.0%, the effect is saturated. Therefore, the W content is set to 1.0% or less. When W is contained to obtain the above effect, the lower limit of the W content may be 0.05% or more, or may be 0.1% or more.

V: 0.01-0.3%
V is an element that forms fine compounds with C and N in steel and brings about grain refining effect. If the V content exceeds 0.3%, the compound becomes coarse and the crystal grain refining effect cannot be obtained. Therefore, the V content is set to 0.3% or less. When V is contained to obtain the above effect, the lower limit of the V content may be 0.01% or more, or 0.15% or more.

Ti: 0.005-0.3%
Ti is an element that forms fine compounds with C and N in steel and brings about the effect of refining crystal grains. When the Ti content exceeds 0.3%, the effect saturates. Therefore, the Ti content is set to 0.3% or less. When Ti is contained to obtain the grain refinement effect, the lower limit of the Ti content may be 0.005% or more, or may be 0.010% or more.
On the other hand, the upper limit of the Ti content may be 0.25% or less, or 0.2% or less, from the viewpoint of suppressing a decrease in machinability due to an increase in hardness.

B: 0.0005 to 0.005%
B has a function of suppressing the grain boundary segregation of P. B also has the effect of improving intergranular strength and intragranular strength and the effect of improving hardenability, and these effects improve the fatigue strength of steel. When the B content exceeds 0.005%, the effect saturates. Therefore, the content of B is set to 0.005% or less. When B is contained to obtain the above effect, the lower limit of the B content may be 0.0005% or more, or 0.0010% or more.
On the other hand, the upper limit of the B content may be 0.0045% or less, or 0.004% or less, from the viewpoint of suppressing the occurrence of cracks by improving hardenability.

The chemical composition (steel composition) of the steel according to the present embodiment further includes one selected from the group consisting of Pb, Bi, Ca, Mg, Zr, Te and rare earth elements (REM) instead of part of Fe, or You may contain 2 or more types. The lower limit when these elements are not contained is 0%.

Pb: 0.09% or less Pb is an element that adversely affects the environment, so it is desirable not to contain it. On the other hand, Pb is a machinability-enhancing element, and exhibits the effect of enhancing the tool life by promoting breakage of the steel material during cutting, promoting fragmentation of chips, and melting at the tool contact surface. When Pb is added to improve machinability, the Pb content should be 0.09% or less in consideration of environmental impact. When Pb is contained to obtain the above effect, the lower limit of the Pb content may be 0.01% or more.

Bi: 0.0001 to 0.5%
Bi is an element that improves machinability by finely dispersing sulfides. However, if the Bi content is excessive, the hot workability of the steel deteriorates and hot rolling becomes difficult, so the Bi content is made 0.5% or less. When Bi is contained to obtain the above effect, the preferable lower limit may be 0.0001% or more, or 0.001% or more.
From the viewpoint of suppressing deterioration of the hot workability of steel, the upper limit of Bi may be 0.4% or less, or 0.3% or less.

Ca: 0.0003-0.01%
Ca is an element that is effective in deoxidizing steel and reduces the content of Al ₂ O ₃ in oxides. However, when the Ca content exceeds 0.01%, a large amount of coarse oxides containing Ca appear, which causes a decrease in fatigue life. Therefore, the Ca content should be 0.01% or less. When Ca is contained to obtain the above effects, the preferable lower limit of the Ca content may be 0.0003% or more, or 0.0005% or more.
On the other hand, from the viewpoint of suppressing the generation of coarse oxides containing Ca, the upper limit of the Ca content may be 0.008% or less, or 0.006% or less.

Mg: 0.0005-0.01%
Mg is a deoxidizing element and produces oxides in steel. Furthermore, Mg-based oxides formed by Mg tend to act as nuclei for crystallization and/or precipitation of MnS. Moreover, the sulfide of Mg turns MnS into spheroids by becoming a composite sulfide of Mn and Mg. Thus, Mg is an effective element for controlling the dispersion of MnS and improving the machinability.
However, when the Mg content exceeds 0.01%, a large amount of MgS is produced, and the machinability of the steel deteriorates. Therefore, when Mg is included to obtain the above effect, the Mg content must be 0.01% or less. The upper limit of the Mg content may be 0.008% or less, or 0.006% or less.
On the other hand, when Mg is contained to obtain the above effect, the preferable lower limit of the Mg content may be 0.0005% or more, or 0.001% or more.

Zr: 0.0005-0.05%
Zr is a deoxidizing element and produces an oxide. Furthermore, Zr-based oxides formed by Zr tend to act as nuclei for crystallization and/or precipitation of MnS. Thus, Zr is an effective element for controlling the dispersion of MnS and improving the machinability. However, when the Zr content exceeds 0.05%, the effect is saturated. Therefore, when Zr is included to obtain the above effect, the Zr content is set to 0.05% or less. From the viewpoint of suppressing the deterioration of the mechanical properties of the gear, the upper limit of the Zr content may be 0.04% or less, or 0.03% or less.
On the other hand, when Zr is contained to obtain the above effect, the preferable lower limit of the Zr content may be 0.0005% or more, or 0.001% or more.

Te: 0.0005 to 0.1%
Te improves the machinability of the steel because it promotes spheroidization of MnS. When the Te content exceeds 0.1%, the effect is saturated. Therefore, the Te content is set to 0.1% or less. From the viewpoint of suppressing deterioration of the mechanical properties of the gear, the upper limit of the Te content may be 0.08% or less, or 0.06% or less.
On the other hand, when Te is contained to obtain the above effect, the preferable lower limit of the Te content may be 0.0005% or more, or 0.001% or more.

Rare earth elements: 0.0005 to 0.005%
Rare earth elements (REMs) are elements that form sulfides in steel, and the sulfides serve as MnS precipitation nuclei, thereby promoting the formation of MnS and improving the machinability of steel. If the total content of rare earth elements exceeds 0.005%, the sulfides become coarse and reduce the fatigue strength of the steel. Therefore, the total content of rare earth elements should be 0.005% or less. From the viewpoint of suppressing deterioration of the mechanical properties of the gear, the upper limit of the total content of rare earth elements may be 0.004% or less, or 0.003% or less.
On the other hand, when the above effects are obtained by including rare earth elements, the lower limit of the total content of rare earth elements may be 0.0005% or more, or 0.001% or more.

The rare earth elements referred to in this specification are 15 elements from atomic number 57 lanthanum (La) to atomic number 71 lutetium (Lu) in the periodic table, plus yttrium (Y) and scandium (Sc). It is a general term for 17 elements. The content of rare earth elements means the total content of one or more of these elements.

The steel according to this embodiment contains the above-described alloy components, and the balance is Fe and impurities. Elements other than the alloying components described above are permitted as impurities in the steel from the raw materials and manufacturing equipment as long as the amount of the inclusion does not affect the properties of the steel.

(Manufacturing method for carburized gear steel)
Manufacturing conditions for the carburized gear steel according to the present embodiment will be described.
The method for manufacturing the carburized gear steel according to the present embodiment is not particularly limited as long as it can manufacture the carburized gear steel having the chemical composition described above.
First, molten steel having the above chemical composition is obtained in the refining process, and casting is performed. At this time, secondary refining may be performed in order to adjust the components and improve the cleanliness of the steel. After that, bar or wire rolling is performed to obtain a desired shape. You may perform blooming rolling before these. In order to improve workability into a gear shape, normalizing or annealing may be performed.
Regarding the rolling method, the heating temperature before rolling is preferably 1150° C. or higher in order to obtain a homogeneous structure without coarse grains. On the other hand, from the viewpoint of durability of the heating furnace, the heating temperature is preferably 1350° C. or less. In addition, in order to obtain a homogeneous structure, the reduction in cross-sectional area due to rolling is 20% or less, and the average cooling rate between 800 ° C. and 300 ° C. during rolling is 0.1 ° C./sec or more and 3.0 ° C./sec or less. is preferably controlled to The structure after rolling is preferably a mixed structure of ferrite and pearlite or a mixed structure of ferrite and bainite, and the hardness after rolling is preferably 200 or less in terms of Vickers hardness.

(carburized gear)
The carburized gear steel according to the present embodiment is suitable as a material for manufacturing carburized gears by gas carburizing and quenching, and it is particularly preferable to manufacture carburized gears by gas carburizing and quenching. For example, the carburized gear steel according to the present embodiment is used to form a gear shape by machining such as cutting, and then gas carburizing and quenching is performed to manufacture a carburized gear. As a result, a carburized gear with excellent corrosion resistance can be manufactured.
For example, after cutting the carburized gear steel according to the present embodiment into a gear shape, it is held at 900 to 1000 ° C. in an atmosphere with a carbon potential of 0.7 to 1.2 for 1 to 30 hours, and then heated at 50 to 140 ° C. Gas carburizing treatment is performed under the conditions for oil quenching. Next, by performing tempering under the condition of holding at 120 to 180° C. for 0.5 to 3 hours, the non-carburized portion has the same chemical composition as the carburized gear steel according to the present embodiment and has excellent corrosion resistance. Gears can be manufactured.
Although it varies depending on the required depth of the part, the C content in the region (carburized portion) at a depth of about 0.5 to 3 mm from the surface due to gas carburizing is greater than the C content in the non-carburized portion. Strength can be improved. From this point of view, the C content in the carburized portion is preferably 0.6 to 0.9%. In the case of the carburized gear steel according to the present embodiment, a portion deeper than 3 mm from the surface is normally a non-carburized portion regardless of the part (use of the gear).

In addition, the present inventors produced a carburized gear under the above conditions using the carburized gear steel according to the present embodiment, and subjected the produced carburized gear to JISH8502 "neutral salt spray cycle test". Using a combined cycle tester (CCT tester) used in some cases, the atmosphere was adjusted to maintain a humidity of 50% and a temperature of 20°C, and the test piece was stored in the same atmosphere for 72 hours. Pure water was used for humidity adjustment. After storage, the tooth flank portion of the gear test piece (the portion formed into a tooth shape by cutting) was observed to measure the rust area ratio.
The rust area ratio is obtained as follows.
Take a picture of the appearance of the tooth surface, trim only the tooth surface from the photograph, and save it in JPEG format. After that, using image editing software (for example, Adobe Photoshop), the number of pixels of the entire evaluation surface and the number of pixels of the image cut other than the rusted portion (that is, the number of pixels of the rusted portion) are calculated from the histogram, and the rusted area is calculated. A ratio S is obtained as S=B/A×100.
S: Rust area ratio (%)
A: The number of pixels on the entire evaluation surface B: The number of pixels on only the rusted portion Corrosion resistance was evaluated by judging that the corrosion resistance was excellent when the calculated rust area ratio was less than 5%.
As a result of measuring the Mn concentration in the oxide on the surface, excellent corrosion resistance was obtained when Mn/(Fe+Mn+Cr)×100 exceeded 10%. In addition, the element symbol in the above formula means the mass % of each element in the oxide. The upper limit of Mn/(Fe+Mn+Cr)×100 is 100%. The lower limit of Mn/(Fe+Mn+Cr)×100 in the oxide on the surface may be 15% or 20%. The upper limit of Mn/(Fe+Mn+Cr)×100 may be 90% or 80%.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to Examples. It should be noted that these examples do not limit the present invention.

Various steel ingots having the chemical compositions shown in Table 1 were hot forged to a diameter of 28 mm. The heating temperature before forging was 1250°C.
After forging, the steel was held at 950° C. for 1 hour for complete austenitization, and then normalized under the condition of standing to cool.

(Production of carburized gear)
Next, in order to evaluate the corrosion resistance of the carburized gear, a 30 mm wide spur gear having a module of 2, 16 teeth, and an inner diameter of φ18 mm was produced by cutting. Then, the prepared gear is held at 930° C. for 5 hours in an atmosphere with a carbon potential of 0.8, gas carburized under the conditions of oil quenching at 130° C., and then held at 150° C. for 2 hours. It was tempered under the conditions.

(evaluation)
To investigate oxides on the surface, one tooth is cut from the carburized gear, and a thin film sample is taken from the center of the tooth surface (center in both the tooth trace direction and the tooth profile direction) using a Hitachi High-Technologies FIB (FB2100). bottom. Elemental analysis was performed on the thin film sample using an EDS (energy dispersive X-ray spectroscope) in a TEM (JEM2100) manufactured by JEOL Ltd. Elemental analysis was performed at a magnification of 50,000 times, with the area containing the surface oxide as the measurement range. After the measurement, quantitative analysis was performed using software Analysis Station to calculate Mn/(Fe+Mn+Cr). Quantitative analysis was performed in % by mass with Ratio as quantitative correction and simple quantitative determination as quantitative mode.
In addition, when the C content in the region (carburized portion) at a depth of 50 μm from the surface was measured, the C content was higher than the C content in the non-carburized portion, and the C content in the carburized portion was 0.65 to 0.90. %Met.

Then, the corrosion resistance of the manufactured carburized gear was evaluated. Carburized gears were subjected to JISH8502 "neutral salt spray cycle test" using Suga Test Instruments Co., Ltd.'s combined cycle tester (CYP-90) in an atmosphere of 50% humidity and 20°C. The test piece was stored in the same atmosphere for 72 hours. Pure water was used for humidity adjustment. After storage, the tooth flank portion of the gear test piece (the portion formed into a tooth shape by cutting) was observed to measure the rust area ratio. For the rust area ratio, take a picture of the appearance of the tooth surface, trim only the tooth surface from the photograph, save it in JPEG format, and then use image editing software to cut the number of pixels on the entire evaluation surface and the areas other than the rusted area. The number of pixels in the resulting image (that is, the number of pixels in the rust-generating portion) was calculated from the histogram, and the rust area ratio S was determined as S=B/A×100.
S: Rust area ratio (%)
A: Number of pixels on the entire evaluation surface B: Number of pixels on only the rusted portion

The measurement results are shown in the "corrosion resistance evaluation" column of Table 1. "A" indicates that the rust area ratio was less than 3%. "◯" indicates that the rusted area ratio is 3% or more and less than 5%, "Δ" indicates that the rusted area ratio is 5% or more and less than 8%, and "×" indicates that the rusted area ratio was 8% or more.
In Table 1, "-" for the content of each component means that the element is not included (not intentionally added).

Test No. of an invention example in which the range of chemical components is within the range of the present invention. 1 to 19 had good corrosion resistance. On the other hand, Test No. of the comparative example in which the range of chemical components is outside the scope of the present invention. Nos. 20-29 did not provide good corrosion resistance.

Claims (7)

in % by mass,
C: 0.17-0.22%,
Si: 0.50 to 1.00%,
Mn: 0.85-1.20%,
Cr: 0.60 to 1.00%,
Mo: 0.01 to 0.40%,
S: 0.001 to 0.050%,
N: 0.005 to 0.020%,
Al: 0.001 to 0.100%,
Nb: 0.001 to 0.050%,
O: 0.005% or less,
Carburized gear steel consisting of P: 0.05% or less and the balance: Fe and impurities. in % by mass,
Ni: 0.1 to 3.0%,
Cu: 0.05-1.0%,
Co: 0.05 to 3.0%,
W: 0.05 to 1.0%,
V: 0.01 to 0.3%,
Ti: 0.005-0.3% and B: 0.0005-0.005%
2. The carburized gear steel according to claim 1, containing one or more elements selected from the group consisting of: in % by mass,
Pb: 0.09% or less,
Bi: 0.0001 to 0.5%,
Ca: 0.0003-0.01%,
Mg: 0.0005-0.01%,
Zr: 0.0005 to 0.05%,
Te: 0.0005 to 0.1%, and rare earth element: 0.0005 to 0.005%
3. The carburized gear steel according to claim 1, which contains one or more elements selected from the group consisting of: a step of machining the carburized gear steel according to any one of claims 1 to 3 into a gear-shaped member;
The gear-shaped member is held in an atmosphere of 900 to 1000° C. with a carbon potential of 0.7 to 1.2 for 1 to 30 hours, and subjected to gas carburizing under the conditions of oil quenching at 50 to 140° C., and then , tempering under conditions of holding at 120 to 180 ° C. for 0.5 to 3 hours, so that the content of Mn, Fe, and Cr in the oxide present on the surface satisfies the following formula (1); ,
A method for manufacturing a carburized gear having
10%≦Mn/(Fe+Mn+Cr)×100≦100% (1)
(The symbol of the element in formula (1) represents the content in mass % of the element in the oxide.) At a position deeper than 3 mm from the surface, in mass%,
C: 0.17-0.22%,
Si: 0.50 to 1.00%,
Mn: 0.85-1.20%,
Cr: 0.60 to 1.00%,
Mo: 0.01 to 0.40%,
S: 0.001 to 0.050%,
N: 0.005 to 0.020%,
Al: 0.001 to 0.100%,
Nb: 0.001 to 0.050%,
O: 0.005% or less,
P: 0.05% or less, and the balance: Fe and impurities,
A carburized gear in which the contents of Mn, Fe, and Cr in oxides present on the surface satisfy the following formula (1).
10%≦Mn/(Fe+Mn+Cr)×100≦100% (1)
(The symbol of the element in formula (1) represents the content in mass % of the element in the oxide.) At a position deeper than 3 mm from the surface, in % by mass,
Ni: 0.1 to 3.0%,
Cu: 0.05-1.0%,
Co: 0.05 to 3.0%,
W: 0.05 to 1.0%,
V: 0.01 to 0.3%,
Ti: 0.005-0.3% and B: 0.0005-0.005%
6. The carburized gear according to claim 5 , containing one or more elements selected from the group consisting of At a position deeper than 3 mm from the surface, in % by mass,
Pb: 0.09% or less,
Bi: 0.0001 to 0.5%,
Ca: 0.0003-0.01%,
Mg: 0.0005-0.01%,
Zr: 0.0005 to 0.05%,
Te: 0.0005 to 0.1%, and rare earth element: 0.0005 to 0.005%
7. The carburized gear according to claim 5 or 6, containing one or more elements selected from the group consisting of: