JP3409275B2 - Case hardened steel with little heat treatment distortion - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a case-hardening steel having a small heat treatment distortion, and more particularly to a case-hardening steel having a small change in shape of parts due to carburizing and quenching and a small variation in dimensions.

[0002]

2. Description of the Related Art Steel parts such as gears and shafts are required to have excellent fatigue resistance and wear resistance because bending stress and high surface pressure are repeatedly applied during use. Therefore, a method has been adopted in which the fatigue resistance is secured by using a low carbon steel having excellent toughness and the surface is hardened by carburizing and quenching the low carbon steel to improve the wear resistance.
However, since carburizing and quenching is a process of rapidly cooling after heating at a high temperature, thermal stress caused by the temperature difference between the surface layer part and the internal part of the component occurs in the cooling process, and the transformation stress due to the volume change accompanying the phase transformation occurs. When it occurs, heat treatment strain is inevitably generated. For example, in the case of a substantially cylindrical part such as a gear or a ring-shaped part such as a sleeve, the roundness of the outer diameter or the flatness of the end face becomes poor, and in the case of a shaft-shaped part such as a shaft, bending occurs. Or Moreover, since carburizing and quenching is generally performed at the final processing stage of the part, the heat treatment strain generated is left as it is in the part, and a high pressure is applied only to a specific position to reduce the durability. It directly influences the performance of parts such as inviting or generating noise or vibration.

Therefore, for the purpose of minimizing the heat treatment strain generated during carburizing and quenching, a method of restraining the steel material with a restraining jig during carburizing and quenching, a method of correcting the shape of the part after the carburizing and quenching, etc. However, each method has a problem that it requires a great deal of cost and labor. Japanese Unexamined Patent Publication (Kokai) No. 2-240249 discloses a method of manufacturing a carburized part that can reduce the occurrence of heat treatment distortion without constraining the part during carburizing and quenching or modifying the shape after carburizing and quenching. However, in this method, after carburizing using a mechanical structural steel of a specific component,
It was necessary to perform quenching using a specific quenching agent.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and is achieved by carburizing and quenching without constraining parts during carburizing and quenching or modifying the shape after carburizing and quenching. It is an object of the present invention to provide a case-hardening steel which can reduce heat treatment distortion and which does not require a specific hardening agent or the like at the time of hardening.

[0005]

The case-hardening steel of the present invention which has solved the above-mentioned problems is that the area occupied by equiaxed crystals in the cross section of a rod-shaped rolled material is 30% or less in area ratio, and When the distance between the center of the axial zone and the center of the cross section is b and the diameter of the rolled material is D, the value of b / D is 0.05 or less, and the chemical composition is C: 0.01 to 0.30% (meaning mass%, the same applies hereinafter), Si: 2.0% or less (not including 0%), Mn: 0.20 to less than 2.5%, Al: 0.01
Is 1.0%, N: 0.003 to 0.03%, and the balance is Fe and inevitable impurities, and the center of the center segregation zone in the cross section of the rod-shaped rolled material. It is desirable that the value of a / D is 0.05 or less, where a is the distance between the center and the center of the cross section and D is the diameter of the rolled material.

In the present invention, in calculating the area ratio of equiaxed crystal regions, when there is a region of branched columnar crystals in which equiaxed crystals and columnar crystals coexist, the branched columnar crystal regions are equiaxed. The area ratio is calculated by including it in the crystal region.

The case-hardening steel of the present invention may further contain Ti: 0.005 to 0.3% as another element.

The chemical composition of the case-hardening steel of the present invention is as follows:
C: 0.01 to 0.30%, Si: 2.0% or less (0%
Not included), Mn: 0.20 to less than 2.5%, Al:
0.01-1.0%, B: 0.01% or less (not including 0%), Ti: 0.005-0.3%, N: 0.002
.About.0.01%, and the balance may be Fe and unavoidable impurities.

Further, for the purpose of improving hardenability, Ni:
0.2-4.5%, Cr: 0.2-6.0%, Mo:
One or more selected from the group consisting of 0.05 to 1.0% and Cu: 0.2 to 1.0% may be contained, and V: 0.03 to 1 for the purpose of improving toughness. 0.5% and / or Nb: 0.005 to 1.5% may be contained, and S: 0.02 to 0.3 for the purpose of improving machinability.
%, Ca: 0.0003 to 0.01%, Pb: 0.3%
Contains at least one selected from the group consisting of the following (not including 0%), Te: 0.1% or less (not including 0%), Zr: 0.1% or less (not including 0%). You may let me.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Based on a large amount of experimental data including the examples described below, the present inventors have found that the area ratio of equiaxed crystals (including branched columnar crystal regions) in the cross section of the rolled material is carburized and quenched. When the influence on the heat treatment strain at the time of examination was investigated, it was found that the quenching strain amount was smaller as the area ratio of the equiaxed crystal region in the cross section of the rolled material was smaller as shown in the graph of FIG. Specifically, it was found that when the area ratio exceeds 30%, the amount of strain generation becomes extremely poor, and it is necessary to suppress the proportion of equiaxed crystal regions to 30% or less.

As described above, the inventors of the present invention have conducted various studies on the influence of the cast structure on the heat treatment strain of the steel for machine structural use. The present invention has been found to have a large correlation, and in the present invention, an equiaxed crystal region (including a branched columnar crystal region) in an area ratio of 3 in a cross section of a bar-shaped rolled material is used in the present invention.
By suppressing the content to 0% or less, the occurrence of heat treatment strain during carburizing and quenching is suppressed as much as possible.

As shown in FIG. 2, it is known that columnar crystals 1 are formed in the early stage of casting and equiaxed crystals 2 are formed in the latter stage of casting. However, the content of hardenability improving elements such as C and Cr is large, and the degree of component segregation of the above hardenability improving elements is microscopically increased accordingly. Therefore, the hardenability is largely different in the equiaxed crystal region, and it is considered that the variation in the hardenability is a cause of strain during carburizing and quenching. Therefore, in the present invention, by reducing the equiaxed crystal region of large hardenability variation large micro component segregation, by increasing the small columnar crystal region of micro component segregation small hardenability variation, The amount of quenching strain is very small and can be limited.

Further, when the equiaxed crystal region having a large variation in hardenability, which is considered to be the cause of distortion during carburizing and quenching, exists at a position deviated from the center, the equiaxed crystal region becomes large on one side, and carburizing and quenching At times, strain that is biased to one side occurs. On the other hand, when the center of the equiaxed crystal exists near the center of the rolled material, the deformation occurs uniformly in the entire circumferential direction, so that the deformation occurs uniformly in the entire circumference. Therefore, in the present invention,
Further, as shown in FIG. 2, when the distance between the center of the equiaxed crystal region in the cross section of the rod-shaped rolled material and the center of the cross section is b and the diameter of the rolled material is D, the value of b / D is 0. By controlling the content to be not more than 0.05, the heat treatment strain during carburizing and quenching is reduced.

Further, since the hardenability-improving elements such as C and Cr are considerably higher in the central segregation zone formed near the central portion during solidification at the final stage of casting, the central segregation zone is higher than the surrounding area. High hardenability. When this center segregation zone exists at a position away from the center of the steel, martensite is more likely to be generated in the center segregation zone than in the surrounding area during carburizing and quenching, and the transformation expansion amount is also large, resulting in strain on one side. Resulting in. On the other hand, when the central segregation zone exists near the center, the deformation occurs uniformly in the entire circumferential direction or the thickness direction. Therefore, as shown in FIG. 2, when the distance between the center of the center segregation zone in the cross section of the rod-shaped rolled material and the center of the cross section is a and the diameter of the rolled material is D, the value of a / D is 0. A value of 0.05 or less is desirable for reducing the heat treatment strain during carburizing and quenching.

In order to control the equiaxed crystal region to have an area ratio of 30% or less, it is desirable to carry out continuous casting while cooling at a relatively high speed, and about 80% from the start of casting.
It is preferable to cool up to 0 ° C. at a rate of 4 ° C./min or more. Further, in continuous casting, if magnetic stirring is performed, the equiaxed crystal region tends to be large, so it is recommended to limit electromagnetic stirring as much as possible.

Further, the center segregation zone deviates from the center of the rolled material when, for example, only one side surface of the cast piece is cooled. Therefore, it is desirable that the center segregated zone be uniformly cooled from the entire circumference of the cast piece. Further, the center of the equiaxed crystal region is deviated from the center of the rolled material and the value of b / D exceeds 0.05 when the cooling rate is slow or in the case of nonuniform cooling such that only one side of the slab is cooled. Therefore, it is desirable to make the cooling rate relatively fast and to uniformly cool the entire circumference of the slab.

Regarding the chemical components, it is desirable that at least C, Si, Mn, Al, and N are contained in the following ranges, and the balance is Fe and inevitable impurities.

C: 0.01 to 0.30% C is an element that imparts core hardness during carburizing and quenching as a strengthening element and is an element effective in obtaining a necessary effective hardened layer depth. It is necessary to contain 0.01% or more, and preferably 0.10% or more. However, if the amount is too large, the transformation expansion amount of the core portion during quenching increases, the heat treatment strain increases, and the machinability and toughness deteriorate, so the upper limit must be 0.30%, and It is preferably 25% or less.

Si: 2.0% or less (not including 0%) Si acts effectively as a deoxidizing material during melting, and contributes to the improvement of core hardness as a strengthening element. The upper limit was set to 2.0% because it inhibits carburizing properties and promotes grain boundary oxidation to adversely affect bending fatigue properties. It is more preferably 0.8% or less.

Mn: 0.20 to less than 2.5% Mn is an element that acts effectively as a deoxidizing material and also enhances hardenability to enhance surface layer and core hardness and contributes to improvement in fatigue strength. Is. 0.2 in order to effectively exert those effects
Must be contained at 0% or more. It is more preferably 0.3% or more. On the other hand, if the amount is too large, the material becomes too hard and the cold pressure property and machinability deteriorate, and the transformation expansion amount of the core part during quenching increases and the heat treatment strain increases, so the upper limit was made less than 2.5%. . It is preferably 1.6% or less.

Al: 0.01 to 1.0% Al is an element effective in suppressing strain by suppressing the growth of austenite crystal grains during carburizing and heating, and addition of 0.01% or more is necessary. Is. However, 1.0
Even if added in excess of%, this effect saturates, so the upper limit was made 1.0%. It is preferably 0.04% or less.

N: 0.003 to 0.03% N is an element that combines with Al to form AlN and suppresses the growth of austenite crystal grains. In order to exert this effect, it is necessary to add 0.003% or more. However, if the content exceeds 0.03% and is contained excessively, cracking easily occurs during forging or hot working, so the upper limit is 0.03.
%. It is preferably 0.02% or less.

For the purpose of further improving the hardenability,
In addition to the above components, B may be contained, and in this case, it is desirable to further limit N content and use Ti in combination.

B: 0.01% or less (not including 0%) B is an element useful for improving the hardenability and adding the grain boundary strength by adding a small amount. However, if too much, the effect is saturated, so the upper limit was made 0.01%.

Ti: 0.005 to 0.3% Since Ti is effective in deoxidizing and denitrifying steel, and is also effective in refining crystal grains, Ti is contained regardless of whether B is contained or not. However, when used in combination with B, it also exerts the effect of fixing N in the steel and making B work effectively. In order to exert such Ti addition effect, 0.005%
The above additions are necessary. However, if too much, coarse Ti
A large amount of hard inclusions such as N is generated to deteriorate bending fatigue characteristics and rolling fatigue characteristics, so the content is made 0.3% or less. It is preferably 0.1% or less, and more preferably 0.05% or less.

N: 0.002 to 0.01% When added together with B, if too much N is combined with B to form BN, which hinders the effect of improving the hardenability of B, so add a small amount of N. Is desirable, the lower limit is 0.002%
It is desirable that the upper limit be 0.01%.

For the purpose of further improving the hardenability, Ni,
You may add Cr, Mo, and Cu in the following ranges.

Ni: 0.2-4.5% Ni is an element effective in securing the hardenability of the surface hardened part, suppressing the formation of an incompletely hardened structure and improving the toughness.
For this purpose, it is necessary to add 0.2% or more. However,
If the amount is too large, the hardness of the core portion becomes too high, and rather the quenching strain increases, so the upper limit was made 4.5%. It is more preferably 2.0% or less.

Cr: 0.2 to 6.0% Cr is an element effective for improving the hardenability and ensuring the effective hardened layer depth and the core hardness during carburizing and quenching. For that purpose, addition of 0.2% or more is necessary, and 0.3% or more is desirable. On the other hand, if the amount is too large, not only the carburizing property is hindered, but also the hardness of the core portion increases too much and the strain increases, so the upper limit was made 6.0%. Preferably 2.0%
It is the following.

Mo: 0.05% to 1.0% Mo secures the hardenability of the surface hardened part to suppress the formation of an incompletely hardened layer, and deepens the effective hardened layer depth to make the core hardened. Since it is an element useful for improving the bending fatigue strength and toughness by increasing the grain size and further refining the austenite crystal grains, it is desirable to contain 0.05% or more. However,
If the amount is too large, the hardness of the core portion becomes too high to increase the strain and reduce the machinability. Therefore, the content is preferably 1.0% or less, and more preferably 0.5% or less.

Cu: 0.2 to 1.0% Cu is an element effective in improving hardenability and improving toughness, and is also effective in improving corrosion resistance. In order to fully exert this effect, it is desirable to contain 0.2% or more. However, if the amount is too large, hot cracking is likely to occur and the hot workability is hindered. Therefore, the upper limit is preferably 1.0%, and more preferably 0.6% or less.

Further, V and Nb are effective in refining the crystal grains and are effective in improving the toughness like Ti, so that it is desirable to contain V and Nb in the following ranges.

V: 0.03 to 1.5% V is effective for forming carbides and refining the crystal grains, and also improves stability of carbides and softening resistance to improve rolling contact fatigue. Since it is effective for improving strength, it is preferable to contain 0.03%, and 0.20%
The above is more preferable. However, if too much, A of the core part
₃ , the A ₁ transformation point is significantly lowered, the austenitization of the core is insufficient, quenching does not occur due to quenching, and the hardness becomes low, so the upper limit should be 1.5%, 1.0
It is more preferable that the content is not more than%.

Nb: 0.005 to 1.5% Nb is effective in refining crystal grains, so 0.005%
It is desirable to contain the above, and more desirably 0.01% or more. However, since the effect is saturated even if added excessively, the addition amount may be 1.5% or less, more preferably 1.0% or less.

S, Ca, Pb, Te and Zr are all machinability improving elements, and it is recommended to add each in the following range.

S: 0.02 to 0.3% S is a machinability improving element, and it is desirable that S is contained in an amount of 0.02% or more. The upper limit is preferably set to 0.3% because it adversely affects fatigue properties and impact properties in the direction perpendicular to the rolling direction).

Ca: 0.0003 to 0.01% Ca forms sulfide-based inclusions with MnS, so that the inclusions are spheroidized to improve anisotropy and not deteriorate toughness and bending fatigue strength. Since it is possible to improve machinability, it is desirable to add 0.0003% or more,
It is more preferably 0.0008% or more. On the other hand, 0.01%
If it exceeds 0.1%, a large number of coarse composite inclusions are generated, and the bending fatigue characteristics and rolling contact fatigue characteristics deteriorate, so the upper limit is 0.01%.
The content of 0.005% or less is desirable.

Pb: 0.3% or less (not including 0%) Pb is also a machinability improving element, but if added in excess of 0.3%, bending fatigue strength and rolling fatigue life will be significantly reduced. Therefore, the upper limit is set to 0.3%. It is more preferably 0.1% or less.

Te: 0.1% or less (not including 0%) Te forms Mn-Te and coexists around MnS, suppresses deformation of MnS during hot rolling, and contributes to spheroidization of MnS. By doing so, the machinability is improved without deteriorating the toughness and bending fatigue strength of the cross grain. However, if it exceeds 0.1%, the bending fatigue strength is deteriorated due to the increase of non-metallic inclusions, so the upper limit was made 0.1%.

Zr: 0.1% or less (not including 0%) Zr suppresses deformation of MnS during hot rolling and contributes to spheroidization of MnS, thereby improving anisotropy, toughness and bending. Improves machinability without deteriorating fatigue strength.
However, if it exceeds 0.1%, a large amount of non-metallic inclusions such as ZrO ₂ is generated and the bending fatigue strength is deteriorated.
It was set to 1%.

When the case-hardening steel of the present invention is applied to a part, carburizing and quenching may be carried out by a usual method after processing the part. However, carburizing treatment by gas, vacuum, plasma, etc. or carbonitriding may be performed. The surface may be strengthened by performing a treatment, a soft nitriding treatment, or an induction hardening treatment after carburizing, and a shot peening treatment if necessary.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following Examples are not intended to limit the present invention, and any modification of the design can be made in view of the gist of the preceding and the following. Are included in the technical scope of.

[0043]

EXAMPLE A steel material having the chemical composition shown in Table 1 was melted, and in order to change the proportion of equiaxed crystals at the time of casting, 80 times from the start of casting.
Casting was performed by changing the cooling rate up to 0 ° C. depending on the steel material as follows. That is, Steel Nos. 1 to 20 were cooled at 10 ° C./min, and No. No. 21 was cooled at 7 ° C./min, and No. 21 was cooled. 22
No. 24 to 24 were cast at a cooling rate of 2 ° C./min. still,
No. 15, 21 and No. In Nos. 23 and 24, in order to investigate the influence of the center segregation zone and the center of the equiaxed crystal region being deviated from the center of the rolled material, only one of the slabs was water-cooled. In addition, No. Other than 22, cast by bending continuous casting,
No. No. 22 was cast by the ingot making method and then rolled into a round bar of φ80 mm.

Thereafter, a macro test was conducted on the cross section of each rolled material, and the proportion of equiaxed crystals in the cross section of the rolled material was shown by the area ratio. The equiaxed crystal region is measured by about 20% according to the method for testing the macrostructure of steel specified in JIS G 0553.
Corroded in HCl solution for about 30-40 seconds, separated into equiaxed and columnar crystals, and measured the area ratio of equiaxed crystal regions. Was measured. In addition, all regions (branched columnar crystal regions) in which equiaxed crystals and columnar crystals coexist were classified as equiaxed crystal regions. Table 2 shows the measurement results
Shown in.

After cutting each of these steel materials to a length of 200 mm, they are hot-forged and upset into a disk having a height of 28 mm, and heating (900 ° C. × 1 Hr) → air-cooling normalizing treatment is performed. It was Then, a ring-shaped test piece as shown in FIG. 3 was cut out, and as shown in FIG.
Carburizing at 8% (920 ℃ × 2Hr + 850 ℃ × 0.5
After Hr), quenching was performed in oil at 130 ° C., followed by tempering at 170 ° C. × 2 Hr. After that, the flatness and the outer diameter roundness of the end face were measured at the position shown in FIG.

Further, after forging the above-mentioned rolled material to φ30 mm,
After performing normalizing treatment of heating (900 ° C. × 1 Hr) → air cooling, it was processed into a shaft type test piece of φ20 mm × 200 mm shown in FIG. Further, the carburizing treatment and the quenching / tempering treatment shown in FIG. 4 were performed, and then the bending of the shaft was measured at the position shown in FIG. The measurement results are also shown in Table 2.

[0047]

[Table 1]

[0048]

[Table 2]

No. 1 to 15 have an equiaxed area ratio of 30
% Or less and the deviation of the equiaxed crystal center is small (b / D
≦ 0.05) The steel of the present invention, the equiaxed crystal area ratio exceeds 30% and / or the deviation of the equiaxed crystal center is large (b / D>
0.05) Comparative Steel No. Compared with Nos. 21 to 24, the flatness of the end face and the roundness of the outer diameter were high, and the bending of the shaft was small. Comparative steel No. 21 has an equiaxed area ratio of 30
% Or less, the deviation of the equiaxed crystal center is large, and the heat treatment strain is large as compared with the steel of the present invention. In addition, comparative steel No. In Nos. 23 and 24, not only the area ratio of equiaxed crystals was high, but also the deviation of the central segregation zone and the center of equiaxed crystals was large, and the roundness of the outer shape was particularly bad.

No. In Nos. 16 to 20, the area ratio of equiaxed crystals is 30% or less, and the deviation of the central segregation zone and the center of equiaxed crystals is small, but the content of C, Si, Mn, Al, N is too large or small. It is an example of the present invention in the case of passing by, and No. 16
Nos. 18 to 18 have high contents of C, Si, and Mn, respectively, and therefore have high hardenability, and the heat treatment strain is large as compared with the steels of the present invention. No. In Nos. 19 and 20, the contents of Al and N were small, the austenite crystal grains became coarse during carburizing and quenching, and the heat treatment strain was large as compared with the steels of the present invention.

No. No. 15 is the steel of the present invention in which the area ratio of equiaxed crystal is 30% or less, and the deviation of the center of the equiaxed crystal is small, but the central segregation zone is displaced from the center of the slab, and No. 15 1
The bending of the shaft is larger than that of the steels of the present invention Nos. 14 to 14 of the present invention.

[0052]

The present invention is constructed as described above, and it is desirable to further control the center segregation zone and the component composition by controlling the area ratio of the equiaxed crystal region and the position of the equiaxed crystal center during steel casting. Is a case hardening steel that can reduce heat treatment distortion without restraining parts during carburizing and quenching or modifying the shape after carburizing and quenching. It is now possible to provide case-hardening steel that does not require the like.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the area ratio of equiaxed crystals and heat treatment strain (flatness of end faces).

FIG. 2 is an explanatory diagram showing a state of a cross section of a steel material.

FIG. 3 is an explanatory view showing the shape of the ring-shaped test piece used in the example and the measurement position of heat treatment strain.

FIG. 4 is an explanatory view showing conditions of carburizing treatment and quenching / tempering treatment adopted in the examples.

FIG. 5 is an explanatory diagram showing the shape of the axial test piece used in the example and the measurement position of heat treatment strain.

[Explanation of symbols]

1 columnar crystals 2 equiaxed

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Naoya Kondo 2 Nadahamahigashi-cho, Nada-ku, Kobe-shi Kamido Steel Works, Ltd. Kobe Steel Works (56) Reference JP-A-56-80367 (JP, A) JP Heihei 3-264614 (JP, A) JP-B 61-2458 (JP, B1) JP-B 7-56046 (JP, B2) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00 -38/60

Claims (8)

(57) [Claims] 1. C: 0.01 to 0.30% (meaning mass%; the same applies hereinafter ) Si: 2.0% or less (not including 0%), Mn: 0.20 to less than 2.5% , Al: 0.01 to 1.0%, N: 0.003 to 0.03% , the balance Fe and unavoidable impurities, and the area occupied by equiaxed crystals in the cross section of the bar-shaped rolled material is the area. The ratio is 30% or less, and when the distance between the center of the equiaxed crystal region and the center of the cross section is b and the diameter of the rolled material is D, the value of b / D is 0.05 or less. Case hardening steel with little heat treatment distortion characterized by the following. 2. When the distance between the center of the center segregation zone in the cross section of the bar-shaped rolled material and the center of the cross section is a and the diameter of the rolled material is D, the value of a / D is 0.05 or less. The case-hardening steel according to claim 1. 3. A Ti Still other elements: from 0.005 to 0.3% of those that contain claim 1 or 2 hardened steel according to. 4. C: 0.01 to 0.30%, Si: 2.0% or less (not including 0%), Mn: 0.20 to less than 2.5%, Al: 0.01 to 1 0.0%, B: 0.01% or less (not including 0%), Ti: 0.005-0.3%, N: 0.002-0.01%, balance Fe and unavoidable impurities The area occupied by equiaxed crystals in the cross section of the rolled rod is
Is 30% or less, and the center of the equiaxed crystal region is
Let b be the distance from the center of the cross section and D be the diameter of the rolled material.
When, the value of b / D is 0.05 or less.
Case hardening steel with little heat treatment distortion. 5. Center segregation zone in cross section of rolled bar
The distance between the center of the cross section and the center of the cross section is a, the diameter of the rolled material
The value of a / D is 0.05 or less, where d is D.
Case-hardening steel according to item 4. 6. As other elements, Ni: 0.2 to 4.5%, Cr: 0.2 to 6.0%, Mo: 0.05 to 1.0%, Cu: 0.2 to 1 The case-hardening steel according to any one of claims 1 to 5, which contains at least one selected from the group consisting of 0.0%. 7. V Still other elements: 0.03 to 1.5%, Nb: 0.005 to 1.5 are those containing at least one selected from from the group consisting% claim 1 The case-hardening steel according to any one of to 6. 8. As other elements, S: 0.02-0.3%, Ca: 0.0003-0.01%, Pb: 0.3% or less (not including 0%), Te: 0 10. 1% or less (not including 0%), Zr: 0.1% or less (not including 0%) One or more kinds selected from the group consisting of any one of claims 1 to 7. Case hardening steel described in.