JP5503170B2 - Case-hardened steel with excellent maximum grain reduction characteristics - Google Patents

Description

  The present invention relates to a case-hardened steel useful for producing steel parts used for surface hardening treatment such as carburizing treatment in automobiles, construction machines, and other industrial machines.

  Case-hardened steel is used to manufacture parts that require wear resistance, high fatigue strength, and the like in automobiles, construction machines, and other industrial machines. SCr, SCM, SNCM, etc. are frequently used as case hardening steel. After these case hardening steels are formed into parts using machine processing such as forging and cutting, surface hardening such as carburizing, nitriding, carbonitriding, etc. Steel parts are manufactured by processing and then polishing as necessary. In recent years, in order to reduce the manufacturing cost of steel parts, it is required to reduce the load caused by cutting. For example, the amount of cutting after forging is reduced by switching the cutting process to forging or changing hot forging to warm or cold forging to increase dimensional accuracy. However, if the surface hardening treatment is performed after warm forging or cold forging, the crystal grains are likely to be coarsened.

  As a technique for preventing crystal grain coarsening during the surface hardening treatment, for example, Patent Documents 1 to 3 disclose a technique using Nb. Patent Document 1 proposes that 0.005 to 0.20% of Nb is added, and Nb in the steel combines with C, N and Al to form extremely fine precipitates, preventing coarsening. It is explained that it is effective. Patent Document 2 proposes that 0.001 to 0.1% of Nb is added, and Nb combines with C and N in the steel and precipitates such as Nb carbide, Nb nitride, and Nb carbonitride. In order to finely disperse the precipitate in the steel, it is effective for preventing the coarsening of the crystal grains, and in order to finely disperse the precipitate, after heating at a temperature at which these precipitates can be dissolved, It is explained that the austenite region should be cooled quickly. Patent Document 3 explains that Nb is added in an amount of about 0.015 to 0.050%, and that all the carbides of Nb are once dissolved to promote fine and uniform precipitation and suppress the growth of crystal grains. Yes.

However, if the amount of Nb is less than 0.05% by the method of once dissolving Nb precipitates as in Patent Documents 1 to 3, a sufficient amount of precipitates cannot be obtained, and the effect of preventing crystal grain coarsening is reduced. There was a bug to do. Therefore, in Patent Document 4, 0.05 to 0.30% of Nb is added. In Patent Document 4, it is said that the coarse precipitates produced during casting must be once dissolved in order to produce fine precipitates effective for preventing the coarsening of crystal grains, and the heating temperature (T1) before the batch rolling is performed. ; ° C.) and heating time (t; minutes) must satisfy 4000 ≦ (T1 + 273) × log ₁₀ (t × 60) (in Patent Document 4, 4000 ≦ (T1 + 273) × Although it is described as log ₁₀ (t), 4000 ≦ (T1 + 273) × log ₁₀ (t × 60) is correct when calculated backward from the calculation result in the column of Example.

JP-A-9-78184 Japanese Patent Application Laid-Open No. 2000-63943 JP 2007-39732 A JP 2007-162128 A

  By making it like patent document 4, many fine Nb precipitates can be precipitated and a crystal grain coarsening prevention characteristic can be improved. An object of the present invention is to improve the technique of Patent Document 4 and to control the crystal grains to a higher degree.

  In each of Patent Literatures 1 to 3 and Patent Literature 4, Nb precipitates are once solid-dissolved to finely precipitate them. However, according to this technique, when Nb is small, the number of fine Nb precipitates is insufficient (Patent Documents 1 to 3). By adding Nb in a predetermined amount or more, fine Nb precipitates can be secured, and coarsening of crystal grains can be prevented (Patent Document 4).

  However, as a result of further studies by the present inventors, according to the technique of Patent Document 4, the coarsening of most crystal grains is prevented, but there are crystal grains that are specifically enlarged in a very small part. (Mixed grain) was found. As a result of investigation on the cause, if even a small amount of coarse Nb precipitate remains, the coarse Nb precipitate grows Ostwald during heating in the surface hardening treatment, and the surrounding fine Nb precipitate disappears. It was found that the crystal grains were specifically enlarged in some areas. Therefore, when further studies were conducted with the aim of a technique (a technique capable of preventing mixed grains) that can reduce the largest crystal grain while dispersing a large number of fine Nb precipitates to prevent the coarsening of the crystal grains, rather, If the amount of Nb added is reduced to the necessary minimum while preventing even a slight solid solution of Nb precipitates, a sufficient amount of fine Nb precipitates exhibiting the prevention of grain coarsening can be obtained, In addition, the inventors have found that coarse Nb precipitates can be remarkably reduced to prevent the mixed grain phenomenon, and the present invention has been completed.

That is, the case-hardened steel according to the present invention is C: 0.1 to 0.3% (meaning mass%. When% is used for the content of chemical components, hereinafter, the same means mass%. ), Si: 1.5% or less (not including 0%), Mn: 2% or less (not including 0%), Cr: 2.5% or less (not including 0%), and Nb: 0. It contains 01-0.05%, the balance consists of iron and inevitable impurities, and satisfies the formula (1). Such a case-hardened steel is excellent in the reduction characteristics of the maximum crystal grains.
A / [Nb] ≦ 0.7 (1)
(In the formula, A represents the area ratio (%) of Nb-based inclusions having an area of 20 μm ² or more. [Nb] represents the Nb content (mass%) in the steel)

  In the case-hardened steel, Mo: 2.0% or less (not including 0%), B: 0.005% or less (not including 0%), Cu: 0.1% or less (not including 0%) ), Ni: 3% or less (excluding 0%), etc. may be added as appropriate. The inevitable impurities include, for example, P: 0.03% or less (not including 0%), S: 0.03% or less (not including 0%), Al: 0.06% or less (including 0%) N), N: 0.05% or less (not including 0%).

The case-hardened steel of the present invention is obtained by heating the steel material having the above-mentioned chemical composition at an average speed of 100 ° C./h or more, rolling it in pieces, and then re-heating it to a temperature of 850 to 1050 ° C. The heating temperature T1 (° C.) and the heating time t (minute) of the above-mentioned block rolling satisfy the formulas (2) and (3).
1100 ≦ T1 ≦ 1350 (2)
(T1 + 273) × log ₁₀ (t × 60) <4000 (3)
The present invention also includes a steel material that is cold-worked and then subjected to a surface hardening heat treatment.

  According to the present invention, the amount of Nb added is narrowed down to the minimum necessary so as to prevent even a slight solid solution of Nb precipitates, so that the coarse Nb-based inclusions that cause Ostwald growth are significantly reduced. And the reduction characteristics of the maximum crystal grains of the case-hardened steel can be enhanced.

FIG. 1 is a conceptual diagram showing the structure of a conventional case-hardened steel. FIG. 2 is a conceptual diagram showing an example of the structure of the case hardening steel of the present invention. FIG. 3 is a view showing a heat pattern when the steel material obtained in the experimental example is annealed. FIG. 4 is a view showing a heat pattern when gas carburizing the steel material obtained in the experimental example. FIG. 5 is a diagram showing a heat pattern when vacuum carburizing the steel material obtained in the experimental example. FIG. 6 is a diagram showing the relationship between the left side (A / [Nb]) of equation (1) and the grain size number of the largest crystal grain.

  The present invention is directed to a steel material suitable for surface hardening after being processed into a part shape, that is, case-hardened steel. And Nb is added in order to prevent a crystal grain coarsening at the time of the heating in a surface hardening process. Nb is useful for forming fine inclusions (such as carbides and carbonitrides) and preventing crystal grains from becoming coarse during heating in the surface hardening treatment. In order to effectively exhibit this effect, Nb is set to 0.01% or more, preferably 0.015% or more, and more preferably 0.020% or more. Conventionally, with this amount of Nb addition, the effect of preventing the coarsening of crystal grains is insufficient. However, as described later, in the present invention, Nb after casting is prevented from forming a solid solution thereafter, so that a small amount of Nb is added. Even if it is added, it can be used effectively and a sufficient effect of preventing coarsening of crystal grains can be exhibited.

  By the way, when Nb increases, coarse Nb-based inclusions tend to remain. According to the study by the present inventors, it has been found that even if a large amount of fine Nb-based inclusions are produced, mixed grains are produced if even a small amount of coarse Nb-based inclusions remain. FIG. 1 is a conceptual diagram for explaining the mixed grain formation mechanism. FIG. 1 (a) shows a steel structure before heating, and FIG. 1 (b) shows a steel structure after heating. . In the steel structure before heating, as shown in FIG. 1A, a large number of fine Nb-based inclusions 1 exist, and one coarse Nb-based inclusion exists. When this steel is heated, coarse Nb-based inclusions 2 are Ostwald-grown, and the surrounding fine Nb-based inclusions 1 are absorbed. As a result, the fine Nb-based inclusions 1 are sparse around the coarse Nb-based inclusions 2 and the growth of crystal grains cannot be suppressed. Therefore, coarse crystal grains (abnormal grains) 12 are generated in the place where the coarse Nb-based inclusions 2 are present, and mixed grains are generated (FIG. 1B).

  Therefore, in the present invention, in order to eradicate coarse Nb-based inclusions, the amount of Nb added is limited to the necessary minimum. FIG. 2A shows a steel structure when the Nb addition amount is reduced to a necessary minimum, and FIG. 2B shows the steel structure after the steel structure is heated for surface hardening treatment. As shown in FIG. 2A, when the amount of Nb added is reduced to the necessary minimum, a steel structure in which the fine Nb-based inclusions 1 are almost formed can be formed. If such a steel structure is used, as shown in FIG. 2 (b), the disappearance of fine Nb-based inclusions 1 can be prevented, and the effect of preventing grain coarsening is shown throughout the steel. Particles (mixed particles) can be prevented. The Nb addition amount is 0.05% or less, preferably 0.045% or less, and more preferably 0.040% or less.

  The case-hardened steel of the present invention further includes C: 0.1 to 0.3%, Si: 1.5% or less (not including 0%), Mn: 2% or less (not including 0%), and Cr : 2.5% or less (excluding 0%) is contained as an essential element. The reason for adding each component is as follows.

  C is an important element for securing the core hardness necessary for a part, and if it is small, the static strength as a part is insufficient due to insufficient hardness. On the contrary, when C is too much, it becomes too hard, and forgeability and machinability deteriorate. Therefore, C is 0.1 to 0.3%, preferably 0.12 to 0.28%. More preferably, the content is 0.15 to 0.25%.

  Si is an effective element for ensuring the hardness of the surface layer of the surface-hardened treated part in order to suppress the hardness reduction during the tempering process. However, as the addition amount increases, the deformation resistance of the material increases and the forgeability decreases. Therefore, Si is 1.5% or less, preferably 0.03 to 1.3%, more preferably 0.1 to 1.0%.

  Mn acts as a deoxidizer, reduces the amount of oxide inclusions and enhances the internal quality of the steel, and significantly improves the hardenability during quenching after surface hardening (such as carburizing) have. However, as the Mn increases, striped segregation becomes prominent, resulting in large variations in material and adversely affecting cold workability. Therefore, Mn is 2% or less, preferably 0.1 to 1.5%, more preferably 0.3 to 1.0%.

  Cr, like Mn, has the effect of significantly improving the hardenability during quenching after surface hardening (such as carburizing). In addition, Cr is effective in improving wear resistance because it has the effect of improving the hardness of the carbide by dissolving in the carbide. Therefore, an appropriate amount is added to sliding parts such as gears and bearings. However, if it is contained excessively, the hardness of the material becomes too high and the machinability and forgeability become poor. Therefore, Cr is 2.5% or less, preferably 0.3 to 2.0%, more preferably 0.6 to 1.5%.

  The case hardening steel of this invention may contain the other component as needed. Examples of other components include Mo, B, Cu, Ni, and the like. These may be added alone or in appropriate combination. Preferred addition amounts and reasons for addition of Mo, B, Cu, Ni, etc. are as follows.

Mo: 2.0% or less (excluding 0%)
Mo has an effect of remarkably improving the hardenability at the time of quenching after surface hardening treatment (such as carburizing), and is effective in improving the impact strength. However, if it is added excessively, the material hardness increases, so that the machinability becomes poor. Therefore, Mo is 2.0% or less, preferably 0.01 to 1.0%, more preferably 0.1 to 0.5%.

B: 0.005% or less (excluding 0%)
B may be added in a small amount because it has the effect of significantly improving the hardenability of the steel material and has the effect of strengthening the grain boundaries and increasing the impact strength. However, if added excessively, nitrides are produced and cold and hot workability are reduced. Therefore, B is 0.005% or less, preferably 0.0001 to 0.004%, more preferably 0.001 to 0.003%.

Cu: 0.1% or less (excluding 0%)
Since Cu is an element that is less likely to be oxidized than Fe, it improves the corrosion resistance of the steel material. However, when it adds excessively, the hot ductility of steel materials will fall. Therefore, Cu is 0.1% or less, preferably 0.01 to 0.05%, more preferably 0.01 to 0.03%.

Ni: 3% or less (excluding 0%)
Ni is an element that improves the corrosion resistance of the steel together with Cu, and may be added alone, but it is desirable to add it in combination with Cu. Ni also has the effect of improving the impact resistance of the steel material. However, when it adds excessively, the manufacturing cost of steel materials will rise. Therefore, Ni is 3% or less, preferably 0.01 to 2.5%, more preferably 0.03 to 2.0%.

  In the case-hardened steel of the present invention, the components other than the above (remainder) are usually iron and inevitable impurities. The inevitable impurities mean impurities mixed in from raw materials (main raw materials, auxiliary raw materials, etc.) and manufacturing equipment, and examples thereof include P, S, Al, and N. Preferred amounts of P, S, Al, N, etc. are as follows.

P: 0.03% or less (excluding 0%)
P is preferably as small as possible because it segregates at the grain boundaries and reduces the impact characteristics of the part. Therefore, P is, for example, 0.03% or less, preferably 0.02% or less, and more preferably 0.015% or less.

S: 0.03% or less (excluding 0%)
It is preferable that S is reduced as much as possible in order to combine with Mn to produce MnS inclusions and reduce the fatigue strength and impact strength of the part. Therefore, S is, for example, 0.03% or less, preferably 0.025% or less, and more preferably 0.020% or less. Note that S may contribute to improvement in machinability. Therefore, S may be, for example, 0.001% or more, preferably 0.005% or more, and more preferably 0.010% or more.

Al: 0.06% or less (excluding 0%)
Al acts as a deoxidizer during melting and reduces the amount of oxide inclusions to increase the internal quality of the steel, but the remaining Al that cannot be removed as slag is coarse and hard non-metallic inclusions ( It acts as an impurity, for example, by generating Al ₂ O ₃ ) and reducing fatigue characteristics. Therefore, the less Al remaining in the steel, the better. For example, 0.06% or less, preferably 0.04% or less, and more preferably 0.02% or less.

N: 0.05% or less (excluding 0%)
When N is large, coarse Nb-based inclusions are generated and impact strength is lowered, and the hardness and deformation resistance of the steel material are increased, and forgeability is lowered. Accordingly, N is preferably as small as possible, for example, 0.05% or less, preferably 0.04% or less, and more preferably 0.03% or less. Note that N cannot be completely removed, but the remaining N (for example, 0.005% or more, preferably 0.010% or more, more preferably 0.015% or more) is a fine Nb-based material. Used to generate inclusions.

In the case hardening steel of the present invention, as described above, the amount of Nb is suppressed to the minimum necessary, and this is utilized without making it as solid as possible. By doing in this way, while ensuring a fine Nb-type inclusion, a coarse Nb-type inclusion can be reduced significantly. By significantly reducing coarse Nb-based inclusions, the maximum crystal grains can be reduced and mixed grains can be prevented. Also, cold forgeability can be improved. In addition, since the amount of coarse Nb-based inclusions decreases as the Nb addition amount decreases, the allowable amount of coarse Nb-based inclusions is determined according to the Nb addition amount. That is, the case-hardened steel of the present invention satisfies the formula (1).
A / [Nb] ≦ 0.7 (1)
(In the formula, A represents the area ratio (%) of Nb-based inclusions having an area of 20 μm ² or more. [Nb] represents the Nb content (mass%) in the steel)

Formula (1) means that Nb-based inclusions having an area of 20 μm ² or more are positioned as coarse inclusions, and the amount thereof is reduced. The reason why the area is 20 μm ² is that the Nb-based inclusions require a certain size or more for Ostwald growth. The left side (A / [Nb]) of the preferred formula (1) is 0.6 or less, particularly 0.5 or less. Note that the lower limit of the left side (A / [Nb]) of equation (1) is not particularly limited, but even if it is too small, the control only becomes difficult and the effect is saturated. Therefore, the lower limit of the left side (A / [Nb]) of the formula (1) may be, for example, 0.01 or more (particularly 0.1 or more).

The case-hardened steel of the present invention can be produced by processing (rolling or the like) a slab whose component has been adjusted to the above range, without causing Nb-based inclusions to dissolve as much as possible. For example, when producing a wire (such as a steel bar), a block rolling, a hot rolling or the like is performed, but the solid solution of Nb is prevented during the heating. Although the heating temperature of the partial rolling and hot rolling is appropriately set according to the steel type, when heating to a temperature of 1100 ° C. or higher and performing the partial rolling , the Nb-based inclusions are easily dissolved. It is recommended to strictly control the heating time t when heating to 1100 ° C. or higher and performing batch rolling .

The case-hardened steel of the present invention is often manufactured by heating to a temperature of 1100 to 1350 ° C., then performing batch rolling, then reheating to a temperature of 850 to 1050 ° C. and then hot rolling. Therefore, it is recommended to shorten the heating time of the partial rolling as much as possible. For example, it is desirable that the heating temperature T1 (° C.) and the heating time t (minute) of the partial rolling satisfy the expression (3).
(T1 + 273) × log ₁₀ (t × 60) <4000 (3)
The left side of (3) ((T1 + 273) × log ₁₀ (t × 60)) may be even smaller, for example, 2000 or less, particularly 0 or less.

  In order to prevent Nb from being dissolved as much as possible, it is desirable to quickly heat to a temperature of 1100 to 1350 ° C. The average heating rate at the time of heating to 1100 to 1350 ° C. (the time required for heating from room temperature to the maximum temperature and the heating rate determined by the rising temperature therebetween) is, for example, 100 ° C./h or more, preferably 200 C / h or more (for example, about 200 to 300 ° C./h), particularly 250 ° C./h or more.

  The case-hardened steel obtained as described above has the shape of appropriate parts (for example, gears, shafts, continuously variable transmission (CVT) pulley, constant velocity joint (CVJ), bearing, etc.) by cold working or the like. Then, surface hardening treatment (carburizing, nitriding, carbonitriding, etc., in particular carburizing or carbonitriding) can be used to make steel parts.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Experimental example 1
A slab having the components shown in Table 1 was prepared by melting in a converter, and the slab was heated to a predetermined temperature and then subjected to block rolling to obtain a billet having a size of 155 mm × 155 mm × 10 m. The obtained billet was reheated and hot-rolled (steel rolling) to produce a steel bar having a diameter of 46 mm. Table 2 shows the heating conditions before the block rolling and the heating conditions before the hot rolling (bar rolling). Tables 3 and 4 show the heating rates before the batch rolling.

Each characteristic of the steel bar obtained in Experimental Example 1 was evaluated as follows.
1) Cold forgeability (70% press)
The steel bar was heated at a temperature of 760 ° C. for 5 hours, then cooled to 680 ° C. over 8 hours, and then furnace-cooled (spheroidizing annealing or softening annealing. FIG. 3). A cylindrical test piece having a diameter of 15 mm and a height of 22.5 mm was cut out from the annealed steel bar, and the test piece was compressed in the height direction (processing rate: 70%). The surface of the test piece was observed with a stereomicroscope (magnification 20 times) to confirm the presence or absence of cracks, and the cold forgeability was evaluated based on the following criteria.
Good: No cracking Bad: Cracking

2) Area ratio of Nb inclusions with an area of 20 μm ² or more (A)
A sample of the L cross section (cross section including the axis) was cut out from the t / 4 (t is the diameter of the steel bar) position of the steel bar and polished. The polished surface was measured with EPMA (Electron Probe Microanalyzer). The composition of inclusions having an area of 20 μm ² or more was examined, and inclusions having an Nb content of 5% by mass or more were defined as Nb inclusions, and the area ratio was calculated. The measurement conditions of EPMA are as follows.

EPMA analyzer: JXA-8100 type electron probe microanalyzer (manufactured by NEC Corporation)
Analyzer (EDS): SystemSix (manufactured by Thermo Fisher Scientific)
Acceleration voltage: 15 kV
Operating current: 4nA
Measurement area: 100 mm ² or more Observation magnification: 200 times

3) Mixed grain prevention properties (size number of the largest crystal grain)
After the steel bar was annealed as shown in FIG. 3, it was cut to create a cylindrical body (diameter 15 mm × height 22.5 mm), and 75% compression at a stroke speed of 18 spm (average strain speed of 8.5 sec ⁻¹ ). Processing was performed to produce a test piece.

  The obtained test piece was subjected to gas carburizing conditions shown in FIG. 4 (carburizing condition: temperature 950 ° C., time 70 minutes, carburizing gas: propane gas, carbon potential 0.8. Diffusion period condition: temperature 850 ° C., time 60 minutes, Carburizing gas: propane gas, carbon potential 0.8, quenching conditions: 80 ° C., oil cooling) or vacuum carburizing conditions (vacuum carburizing period conditions: temperature 950 ° C., time 29 minutes), carburizing gas: acetylene gas. Periodic conditions: temperature 860 ° C., time 49 minutes, carburizing gas: acetylene gas, carbon potential 0.8, quenching conditions: 60 ° C., oil cooling).

The grain size number Gh of the prior austenite grains at the location where the equivalent strain was 0.7 was determined according to JIS G0551. More specifically, the average grain size number was determined by a counting method (Appendix 3). Further, the grain size number of the coarsest crystal grain in the observation field of view 800 μm × 800 μm was determined by a comparative method, and this was set as the maximum crystal grain size number.
The results are shown in Tables 3-4.
Moreover, the left side (A / [Nb]) of Formula (1) is calculated and shown in Tables 3-4. Further, the relationship between the left side (A / [Nb]) and the maximum crystal grain is shown in FIG.

No. When Nb is excessive as shown in 48 to 49, even if the manufacturing conditions V to VI are adopted and the solution treatment is sufficient, the maximum crystal grain cannot be reduced, and a mixed grain state is generated. .
On the other hand, when Nb addition amount is narrowed down to the necessary minimum and manufacturing conditions I to IV are employed to prevent even a slight solid solution of Nb precipitates (No. 1 to 43), fine Nb-based intervening Coarse Nb-based inclusions can be made extremely small while securing the material, and the left side (A / [Nb]) of formula (1) can be made small, so that the crystal grains can be prevented from becoming coarse and the maximum crystal grains can be made small. Yes (can prevent mixed grains).

  When the amount of Nb is insufficient (Nos. 44 to 47), the average crystal grains become coarse. Moreover, although the area ratio A itself of a coarse inclusion becomes small, since the Nb amount itself is small, the left side (A / [Nb]) of Formula (1) becomes large. Therefore, the maximum crystal grain cannot be reduced.

  The case-hardened steel of the present invention is excellent in workability (especially cold forgeability) and can prevent mixed grains after the surface hardening treatment. Therefore, steel parts (for example, gears, shafts) in automobiles, construction machinery, and other industrial machines. , Continuously variable transmission (CVT) pulley, constant velocity joint (CVJ), bearing, etc.).

1 Fine Nb-based inclusions 2 Coarse Nb-based inclusions 11 Fine crystal grains 12 Coarse crystal grains

Claims (6)

C: 0.1 to 0.3% (meaning mass%. When using% for the content of chemical components, hereinafter, the same shall mean mass%),
Si: 1.5% or less (excluding 0%),
Mn: 2% or less (excluding 0%),
Cr: 2.5% or less (excluding 0%), and Nb: 0.01 to 0.05%
The balance is made of iron and inevitable impurities, satisfies the formula (1), and the maximum grain size number of the prior austenite grains is 5.2 or more. Case-hardened steel with excellent properties.
A / [Nb] ≦ 0.7 (1)
(In the formula, A represents the area ratio (%) of Nb-based inclusions having an area of 20 μm ² or more. [Nb] represents the Nb content (mass%) in the steel) The case-hardened steel according to claim 1, wherein the inevitable impurities include P, S, Al, and N, and the contents thereof are as follows.
P: 0.03% or less (excluding 0%)
S: 0.03% or less (excluding 0%)
Al: 0.06% or less (excluding 0%)
N: 0.05% or less (excluding 0%)   Furthermore, the case hardening steel of Claim 1 or 2 containing Mo: 2.0% or less (excluding 0%).   Furthermore, B: 0.005% or less (0% is not included) The case hardening steel in any one of Claims 1-3.   Furthermore, skin hardening in any one of Claims 1-4 containing at least 1 type selected from Cu: 0.1% or less (excluding 0%) and Ni: 3% or less (not including 0%). steel.   A steel material subjected to surface hardening heat treatment after cold working the case-hardened steel according to any one of claims 1 to 5.

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