JPH11131184A - Case hardening steel minimal in heat treatment strain - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a case hardening steel minimal in the occurrence of heat treatment strain due to carburizing and quenching even without constraining of parts at the time of carburizing and correcting the shape after carburizing and quenching, etc. SOLUTION: In the cross section of a rod-like rolled stock, the region occupied by equiaxed crystals is regulated to <=30% by area ratio. It is desirable to regulate the value of a/D to <=0.05 when (a) represent the distance between the center of a central segregation zone in the cross section of the rod-like rolled stock and the center of the cross section and also D represents the diameter of the rolled stock. Further, it is desirable to regulate the value of b/D to <=0.05 when (b) represents the distance between the center of an equiaxed crystal region in the cross section of the rod-like rolled stock and the center of the cross section.

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 a part due to carburizing and quenching and a small dimensional variation.

[0002]

2. Description of the Related Art Steel parts such as gears and shafts are required to be excellent in 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 fatigue resistance is ensured by using low-carbon steel having excellent toughness, and the surface is hardened by carburizing and quenching the low-carbon steel to improve wear resistance.
However, carburizing and quenching is a process of quenching after heating at a high temperature, so that in the cooling step, thermal stress occurs due to the temperature difference between the surface layer and the inside of the part, and the transformation stress due to the volume change accompanying the phase transformation. And heat treatment distortion is inevitably generated. For example, in the case of a substantially cylindrical part such as a gear or a ring part such as a sleeve, the roundness of the outer diameter and the flatness of the end face are deteriorated, and in the case of a shaft part such as a shaft, bending occurs. Or In addition, since carburizing and quenching are generally performed at the final processing stage of a part, the generated heat treatment distortion is left on the part as it is, and high pressure is applied only to specific positions, reducing the durability. It directly affects the performance of components, such as inviting or generating noise or vibration.

For the purpose of minimizing the heat treatment distortion generated during carburizing and quenching, a method of restraining a steel material using a restraining jig during carburizing and quenching, a method of correcting the shape of a part after carburizing and quenching, etc. However, there is a problem that both methods require a great deal of cost and labor. JP-A-2-240249 discloses a method for manufacturing a carburized component that can reduce the occurrence of heat treatment distortion without restraining the component during carburizing and quenching or correcting the shape after carburizing and quenching. However, in this method, after carburizing using a specific structural steel for machine structure,
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 based on carburizing and quenching without constraining parts at the time of carburizing and quenching without modifying the shape after carburizing and quenching. An object of the present invention is to provide a case hardened steel that can reduce heat treatment distortion and that does not require a specific quenching agent or the like during quenching.

[0005]

The case-hardened steel of the present invention which has solved the above-mentioned problems is characterized in that the area occupied by equiaxed crystals in the cross section of the rod-shaped rolled material is 30% or less in area ratio. When the distance between the center of the center segregation zone and the center of the cross section in the cross section of the rod-shaped rolled material is a and the diameter of the rolled material is D, the value of a / D is 0.05 or less. It is desirable that 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 be b.
It is desirable that the value of / D be 0.05 or less.

In the present invention, when calculating the area ratio of the equiaxed crystal region, when there is a region of the branched columnar crystal in which the equiaxed crystal and the columnar crystal are mixed, the above-mentioned branched columnar crystal region is not equiaxed. The area ratio is calculated to be included in the crystal region.

[0007] The case hardening steel of the present invention contains C:
0.01 to 0.50% (meaning by mass%, the same applies hereinafter), S
i: 2.0% or less (excluding 0%), Mn: 0.20
-2.5%, Al: 0.01-1.0%, N: 0.00
It is desirable to contain 3 to 0.03%, the balance being Fe and unavoidable impurities.
i: 0.005 to 0.3% may be contained.

The chemical components of the case hardening steel of the present invention include:
C: 0.01 to 0.50%, Si: 2.0% or less (0%
), Mn: 0.20-2.5%, Al: 0.
01 to 1.0%, B: 0.01% or less (excluding 0%), Ti: 0.005 to 0.3%, N: 0.002 to
It may contain 0.01%, with the balance being Fe and unavoidable impurities.

In order to further improve hardenability, Ni:
0.2 to 4.5%, Cr: 0.2 to 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%, and for the purpose of improving machinability, S: 0.02 to 0.3%
%, Ca: 0.0003 to 0.01%, Pb: 0.3%
The following (excluding 0%), Te: 0.1% or less (excluding 0%), Zr: 0.1% or less (excluding 0%), containing one or more selected from the group consisting of May be.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Based on many experimental data including the examples described later, the present inventors have determined that the area ratio of equiaxed crystals (including branched columnar regions) in the cross section of the rolled material is carburized and quenched. As a result of examining the effect on the heat treatment strain at the time, as shown in the graph of FIG. 1, it was found that the smaller the area ratio of the equiaxed crystal region in the cross section of the rolled material, the smaller the amount of quenching strain. Specifically, it was found that when the area ratio exceeded 30%, the amount of strain generation was extremely poor, and it was found that the proportion of the equiaxed crystal region had to be suppressed to 30% or less.

As described above, the present inventors have conducted various studies on the effects of the cast structure on the heat treatment distortion of the steel for machine structural use from various angles. The present inventors have found that there is a large correlation, and conceived the present invention. In the present invention, in the cross section of the rod-shaped rolled material, the equiaxed crystal region (including the branched columnar crystal region) is defined as an area ratio of 3%.
By suppressing the content to 0% or less, the occurrence of heat treatment distortion 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 later stage of casting. In this case, the content of the hardenability improving elements such as C and Cr is large, and the degree of segregation of the components of the hardenability improving elements is increased microscopically. For this reason, the hardenability partially differs greatly in the equiaxed crystal region, and it is considered that this variation in hardenability causes distortion during carburizing and quenching. Therefore, in the present invention, by reducing the equiaxed crystal region where the micro component segregation is large and the hardenability variation is large, and increasing the columnar crystal region where the micro component segregation is small and the hardenability variation is small, The amount of quenching distortion can be limited to a very small value.

Further, since the center segregation zone formed near the center during solidification in the final stage of casting has considerably higher hardenability improving elements such as C and Cr than its surroundings, the center segregation zone is higher than the surroundings. High hardenability. If this central segregation zone exists at a position away from the center of the steel material, martensite is more likely to be generated in the central segregation zone than in the surroundings during carburizing and quenching, and the amount of transformation expansion increases, resulting in a strain on one side. Resulting in. On the other hand, when the center segregation zone exists near the center, 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 central 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. 0.05 or less is desirable in order to reduce heat treatment distortion during carburizing and quenching.

Further, in the equiaxed crystal region solidified in the final solidification stage, micro segregation of hardenability improving elements such as C and Cr is larger than in the columnar crystal region solidified first. For this reason, in the equiaxed crystal region, variation in hardenability is large, and this variation causes distortion during carburizing and quenching. If the equiaxed crystal region having a large variation in hardenability is located at a position deviated from the center, the equiaxed crystal region becomes large on one side, and one-sided strain occurs during carburizing and quenching. 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, and thus the deformation occurs uniformly in the entire circumference. Therefore, 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 It is recommended to be 0.05 or less in order to reduce heat treatment distortion during carburizing and quenching.

In controlling the equiaxed crystal area to an area ratio of 30% or less, continuous casting is desirably performed while cooling at a relatively high speed.
It is preferable to cool to 0 ° C. at a rate of 4 ° C./min or more. In addition, in the case of continuous casting, when electromagnetic stirring is performed, the equiaxed crystal region is likely to be large. Therefore, it is recommended to limit the electromagnetic stirring as much as possible.

Furthermore, since the center segregation zone is shifted from the center of the rolled material when cooling is performed only from one side surface of the slab, it is desirable to cool the slab as uniformly as possible from the entire circumference. In the case of a slow cooling rate or non-uniform cooling in which only one side of the slab is cooled, the value of the b / D exceeds 0.05 at the center of the equiaxed crystal region because of the deviation from the center of the rolled material. Therefore, it is desirable that the cooling rate be relatively high and that the casting be uniformly cooled from the entire circumference.

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 0.50% C is an effective element for imparting core hardness during carburizing and quenching as a strengthening element and for obtaining a required effective hardened layer depth. It is necessary to contain at least 0.01%, and desirably at least 0.10%. However, if it is too large, the amount of transformation expansion of the core during quenching increases and the heat treatment strain increases, and the machinability and toughness deteriorate. Therefore, the upper limit must be 0.50%. It is desirable that it is 25% or less.

Si: 2.0% or less (excluding 0%) Si not only functions effectively as a deoxidizer at the time of melting but also contributes to the improvement of core hardness as a strengthening element. The upper limit is set to 2.0% because it inhibits carburization and promotes grain boundary oxidation, adversely affecting bending fatigue properties. More preferably, it is 0.8% or less.

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

Al: 0.01 to 1.0% Al is an element effective for suppressing the distortion by suppressing the growth of austenite crystal grains during carburizing heating, and it is necessary to add 0.01% or more. It is. However, 1.0
%, The effect is saturated, so the upper limit was made 1.0%. Preferably it is 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 exhibit this effect, 0.003% or more must be added. However, if the content is excessively larger than 0.03%, cracks are likely to occur during forging or hot working.
%. Preferably it is 0.02% or less.

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

B: 0.01% or less (excluding 0%) B is an element useful for improving hardenability and improving grain boundary strength by adding a small amount. However, the effect is saturated even if it is too much, so the upper limit was made 0.01%.

Ti: 0.005 to 0.3% Ti is effective not only for deoxidation and denitrification of steel but also for refining crystal grains. However, when it is used in combination with B, the effect of fixing N in steel and making B work effectively is also exhibited. In order to exhibit such an effect of adding Ti, 0.005%
The above addition is necessary. However, if too much, coarse Ti
Since a large amount of hard inclusions such as N are generated to deteriorate the bending fatigue characteristics and the rolling fatigue characteristics, the content is set to 0.3% or less. It is preferably at most 0.1%, more preferably at most 0.05%.

N: 0.002 to 0.01% When B is added together with B, if N is too large, it is combined with B and becomes BN, impairing the effect of improving the hardenability of B. Therefore, N should be added in a small amount. Is desirable, and the lower limit is 0.002%
It is desirable to set the upper limit to 0.01%.

In order to further improve hardenability, Ni,
Cr, Mo, and Cu may be added in the following ranges.

Ni: 0.2 to 4.5% Ni is an element effective for securing hardenability of the surface hardened portion to suppress generation of an incompletely hardened structure and improving toughness.
For this purpose, it is necessary to add 0.2% or more. However,
Even if it is too large, the core hardness becomes too high, and the quenching strain is rather increased. Therefore, the upper limit is set to 4.5%. It is more preferably at most 2.0%.

Cr: 0.2 to 6.0% Cr is an element effective for improving hardenability and ensuring the effective hardened layer depth and 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 content is too large, not only does the carburizing property be impaired, but the core hardness also increases too much and the strain increases, so the upper limit was made 6.0%. Preferably 2.0%
It is as follows.

Mo: 0.05 to 1.0% Mo secures the hardenability of the hardened surface to suppress the formation of an incompletely hardened layer, or increases the effective hardened layer depth to harden the core. It is an element useful for improving flexural fatigue strength and toughness by increasing the austenitic crystal grain by further improving the austenitic crystal grain, and therefore, it is desirable to contain 0.05% or more. However,
If it is too large, the core hardness becomes too high, which increases the strain and lowers the machinability. Therefore, the content is preferably 1.0% or less, more preferably 0.5% or less.

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

V and Nb, like Ti, are effective in refining crystal grains and effective in improving toughness. Therefore, V and Nb are desirably contained in the following ranges.

V: 0.03 to 1.5% V is effective for forming carbides to refine crystal grains, and also enhances the stability of the carbides and the rolling resistance by increasing the softening resistance. Since it is effective to improve the strength, it is preferable to contain 0.03%, and 0.20%
The above is more preferable. However, if too much, A
₃ , the A ₁ transformation point is greatly reduced, the austenitization of the core is insufficient, the quenching does not cause quenching, and the hardness is low. Therefore, the upper limit should be 1.5%, and the upper limit should be 1.0%.
% Is more preferable.

Nb: 0.005 to 1.5% Since Nb is effective in refining crystal grains, 0.005%
More preferably, the content is more preferably 0.01% or more. However, the effect is saturated even if it is added excessively, so 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 of them in the following ranges.

S: 0.02 to 0.3% S is an element for improving machinability, and is desirably contained in an amount of 0.02% or more. The upper limit is preferably set to 0.3%, since it has an adverse effect on the fatigue characteristics and impact characteristics in the direction perpendicular to the rolling direction).

Ca: 0.0003 to 0.01% Ca forms sulfide-based inclusions with MnS, thereby spheroidizing the inclusions to improve anisotropy and not deteriorating toughness and bending fatigue strength. Therefore, it is desirable to add 0.0003% or more because machinability can be improved.
0.0008% or more is more desirable. On the other hand, 0.01%
If more than, a large number of coarse composite inclusions are formed and the bending fatigue characteristics and the rolling fatigue characteristics deteriorate, so the upper limit is 0.01%.
And a content of 0.005% or less is desirable.

Pb: 0.3% or less (excluding 0%) Pb is also an element for improving machinability, but when added in excess of 0.3%, bending fatigue strength and rolling fatigue life are significantly reduced. Therefore, the upper limit is set to 0.3%. It is more preferably at most 0.1%.

Te: 0.1% or less (excluding 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 of the grain and the bending fatigue strength. However, if it exceeds 0.1%, the bending fatigue strength is degraded due to an increase in nonmetallic inclusions, so the upper limit was made 0.1%.

Zr: 0.1% or less (excluding 0%) Zr suppresses deformation of MnS during hot rolling and contributes to spheroidization of MnS, thereby improving anisotropy, and improving 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 are generated and the bending fatigue strength is deteriorated.
1%.

When the case hardening steel of the present invention is applied to a part, it may be processed into a part and then carburized and quenched by a usual method. The surface may be strengthened by performing a treatment, performing a soft nitriding treatment, or performing an induction hardening treatment after carburizing, and performing a shot peening process as 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. It is included in the technical range of.

[0043]

EXAMPLE In order to change the ratio of equiaxed crystals at the time of casting a steel material having the chemical composition shown in Table 1 and to change the ratio of the equiaxed crystal at the time of casting, 80% of the steel was used.
Casting was performed by changing the cooling rate to 0 ° C depending on the steel material as follows. That is, steel Nos. 1 to 21 were cooled at 10 ° C./min. No. 22 was cooled at 7 ° C./min. 23
No. 25 to 25 were cast at a cooling rate of 2 ° C./min. still,
No. Nos. 21 and 22 and Nos. In Nos. 24 and 25, only one of the cast slabs was water-cooled in order to investigate the influence of the center segregation zone and the center of the equiaxed crystal region being shifted from the center of the rolled material. In addition, No. Except 23, cast by bending continuous casting,
No. No. 23 was cast by an ingot-making method, and then rolled into a φ80 mm round bar.

Thereafter, a macro test was performed on the cross section of each rolled material, and the proportion of the equiaxed crystals in the cross section of the rolled material was indicated by the area ratio. The measurement of the equiaxed crystal area is performed by about 20% according to the macrostructure test method for steel specified in JIS G 0553.
Corroded for about 30 to 40 seconds in HCl solution, separated into equiaxed crystals and columnar crystals, measured the area ratio of the equiaxed crystal region, and further determined the center of the center segregation zone and the center of the equiaxed crystal region and the center of the cross section. Was measured. All the regions where the equiaxed crystals and the columnar crystals are mixed (branched columnar crystal regions) were all classified into the 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 were hot-forged into a 28 mm-high disc and heated (900 ° C. × 1 hr) → air-cooled normalizing treatment. Was. Thereafter, a ring-shaped test piece as shown in FIG. 3 was cut out, and as shown in FIG.
Carburizing treatment at 8% (920 ° C × 2Hr + 850 ° C × 0.5
After performing (Hr), quenching was performed in oil at 130 ° C., followed by tempering at 170 ° C. × 2 hours. Thereafter, the flatness of the end face and the roundness of the outer diameter were measured at the positions shown in FIG.

After forging the rolled material to φ30 mm,
After performing a normalization process of heating (900 ° C. × 1 hr) → air cooling, it was processed into a φ20 mm × 200 mm shaft type test piece shown in FIG. Further, the carburizing treatment and the quenching / tempering treatment shown in FIG. 4 were performed, and thereafter, 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 22 have an equiaxed crystal area ratio of 30.
% Of the present invention steel having an equiaxed crystal area ratio of more than 30%. As compared with 23 to 25, the flatness of the end face and the roundness of the outer diameter were higher, and the bending of the shaft was less. In addition, comparative steel No. In Nos. 24 and 25, not only the area ratio of the equiaxed crystal was high, but also the deviation of the center segregation zone and the center of the equiaxed crystal was large, and particularly the outer roundness was poor.

No. In Nos. 16 to 20, the area ratio of the equiaxed crystal is 30% or less, and the deviation of the center segregation zone and the center of the equiaxed crystal is small, but the content of C, Si, Mn, Al, N is too large or small. This is an example of the present invention when it is too long. 16
Nos. 18 to 18 have high hardenability due to high contents of C, Si and Mn, respectively. The heat treatment strain is larger than those of the steels of Nos. 1 to 15 of the present invention. No. 19 and 20 are Al,
N content is small, and austenite crystal grains become coarse during carburizing and quenching. The heat treatment strain is larger than those of the steels of Nos. 1 to 15 of the present invention.

No. 21 and 22 have an equiaxed crystal area ratio of 3
The steel of the present invention is 0% or less, and the heat treatment strain is very small as compared with the comparative steel, but the center segregation zone is shifted from the center of the slab in any case. In No. 22, the deviation of the center of the equiaxed crystal was large. Compared with the inventive steels Nos. 1 to 15, the bending of the shaft is large.

[0052]

The present invention is constituted as described above. By controlling the area ratio of the equiaxed crystal zone at the time of casting a steel material, it is desirable to further position the center segregation zone and the center of the equiaxed crystal and to improve the composition of the component. Is a case hardening steel that can reduce heat treatment distortion without restraining parts during carburizing and quenching and without modifying the shape after carburizing and quenching. It is possible to provide case hardening steel that does not require the above.

[Brief description of the 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 view showing a state of a cross section of a steel material.

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

FIG. 4 is an explanatory diagram showing conditions of a carburizing process and a quenching / tempering process adopted in an example.

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

[Explanation of symbols]

 1 columnar crystal 2 equiaxed crystal

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Naoya Kondo 2 Nadahama-Higashi-cho, Nada-ku, Kobe Kobe Steel, Ltd. Inside Kobe Works

Claims (9)

[Claims] 1. A case hardening steel having a small heat treatment strain, wherein the area occupied by equiaxed crystals in the cross section of the rod-shaped rolled material is 30% or less in terms of area ratio. 2. When the distance between the center of the central 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 0.05 or less. The case hardened steel according to claim 1. 3. When the distance between the center of the equiaxed crystal region and the center of the cross section in the cross section of the rod-shaped rolled material is b and the diameter of the rolled material is D, the value of b / D is 0.05 or less. Claim 1
Or the case hardened steel according to 2. 4. C: 0.01 to 0.50% (meaning of mass%, the same applies hereinafter) Si: 2.0% or less (excluding 0%), Mn: 0.20 to 2.5%, The case hardening steel according to any one of claims 1 to 3, which contains 0.01 to 1.0% of Al and 0.003 to 0.03% of N and the balance is Fe and unavoidable impurities. 5. The case hardened steel according to claim 4, further comprising Ti: 0.005 to 0.3% as another element. 6. C: 0.01 to 0.30%, Si: 2.0% or less (excluding 0%), Mn: 0.20 to 2.5%, Al: 0.01 to 1. 0%, B: 0.01% or less (excluding 0%), Ti: 0.005 to 0.3%, N: 0.002 to 0.01%, with the balance being Fe and inevitable impurities The case hardened steel according to any one of claims 1 to 3. 7. Still 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 4 to 6, comprising one or more selected from the group consisting of 0.0%. 8. The composition according to claim 4, further comprising at least one element selected from the group consisting of V: 0.03 to 1.5% and Nb: 0.005 to 1.5%. 8. The case hardened steel according to any one of items 1 to 7. 9. Other elements: S: 0.02 to 0.3%, Ca: 0.0003 to 0.01%, Pb: 0.3% or less (excluding 0%), Te: 0 10. 1% or less (excluding 0%), Zr: 0.1% or less (excluding 0%) It contains one or more selected from the group consisting of: Case hardening steel according to the above.