JPH1112684A - Case hardening steel for cold forging - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a case hardening steel for cold forging, capable of expediting spheroidizing treatment and reducing manufacturing costs, excellent in cold forgeability, and improved, if necessary, in wear resistance. SOLUTION: In this steel, the area ratio of (ferrite + pearlite) is regulated to >=75%, and the average grain size of ferrite and that of pearlite are regulated to <=40 μm and <=30 μm, respectively. The chemical composition of this steel is regulated, if necessary. Simultaneously, the deformation resistance coefficient σeq, working limit coefficient owing to crack UL, and wear resistance coefficient F (wear), which are represented, respectively, as functions of alloying elements, are regulated so that they satisfy the following (1), (2), (3): (1) σeq=C+0.5Si+0.2 Mn+3.0P+0.3Cr+0.3Mo+0.1V+0.1Ti+0.1Nb<1.0; (2) UL=C+0.2Si+0.2Mn+0.2 Cr+0.2Mo+2.0P+2.0S+0.1V+0.1Ti+0.1Nb<0.7; (3) F (wear) = Si+0.2Cr+0.4 Mo+V>0.4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a spheroidizing annealing process which can be rapidly performed, has excellent cold forgeability before surface hardening treatment such as carburizing or carbonitriding, and has a high degree of susceptibility after surface hardening. The present invention relates to a case hardening steel for cold forging that can exhibit high wear resistance.The case hardening steel for cold forging according to the present invention is used for forming a case, such as a gear, a joint, a bearing, a shaft, or a cylinder. It is manufactured through cold forging and can be used for machine structural parts that require high wear resistance after surface hardening.

[0002]

2. Description of the Related Art Various types of mechanical structural parts described above are manufactured by JI.
S-standard steels SCr420, SCM420, etc. are widely used as raw materials. After subjecting them to softening heat treatment such as spheroidizing annealing, they are cold forged and formed into a predetermined shape (machined if necessary). In addition, a surface hardening treatment such as a carburizing treatment or a carburizing / nitriding treatment is performed to secure a target wear resistance.

[0003] From the viewpoint of reducing the manufacturing cost in such a part manufacturing process, reduction in the cost of spheroidizing annealing before cold forging (that is, reduction in the time of spheroidizing annealing) and reduction in the life of a mold during cold forging are described. Items such as reduction of steel deformation resistance and improvement of steel deformability for the purpose of improvement are required.

[0004] Further, with the trend toward higher output power of engines and smaller and lighter parts, the load stress tends to increase, and the parts used so far cannot sufficiently meet such demands. In fact, the realization of parts with even higher wear resistance is desired.

[0005] Under such circumstances, various improved techniques have been proposed. First, as a technique aimed at speeding up the spheroidizing annealing process, for example, Japanese Patent Application Laid-Open No. 60-9832 discloses a method of manufacturing a wire or a steel material which achieves a rapid spheroidizing annealing process by appropriately defining rolling conditions. A method has been proposed. However, the spheroidizing annealing process cannot be sufficiently accelerated only by specifying the rolling conditions.

On the other hand, regarding reduction of deformation resistance during cold forging, for example, Japanese Patent Application Laid-Open No. 2-299241 discloses a carburizing steel in which deformation resistance is suppressed by regulating the carbon equivalent of steel. Japanese Patent Application Laid-Open No. 7-310118 discloses a case hardening steel in which the cold forgeability is improved by reducing Si, Mn, Cr and the like. However, defining the carbon equivalent or reducing the chemical composition is
From the viewpoint of ensuring good cold forgeability, it can be said to be an effective technique, but it is not possible to improve abrasion resistance, which is one of the important characteristics for high strength of components.

[0007] Regarding the improvement of wear resistance, a method for producing wear-resistant steel with improved wear resistance by applying carburizing cooling → spheroidizing annealing → carburizing and quenching is disclosed in Japanese Patent Laid-Open No. 5-594.
No. 28. However, this method not only complicates the process and increases the production cost, but also
It is not possible to improve the cold forgeability, which is a basic required property.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to speed up the spheroidizing annealing process and reduce the manufacturing cost. An object of the present invention is to provide a case hardening steel for cold forging which has a good cold forgeability and also has improved wear resistance as required.

[0009]

Means for Solving the Problems The case hardening steel for cold forging according to the present invention which can solve the above problems has an area ratio of (ferrite + pearlite) of 75% or more and an average grain size of ferrite. The gist is that the diameter is 40 μm or less and the average particle diameter of pearlite is 30 μm or less.

Although the object of the present invention can be achieved only by adjusting the microstructure of case hardened steel as described above, the specific chemical composition of the case hardened steel of the present invention is as follows: C: 0.3 % Or less (not including 0%) Si: 0.3% or less (not including 0%) Mn: 1.5% or less (not including 0%) P: 0.02% or less (including 0%) S: 0.02% or less (excluding 0%) Al: 0.06% or less (excluding 0%) N: 0.03% or less (excluding 0%) Cr: 3% or less (excluding 0%) Mo: 1.5% or less (excluding 0%) V: 1.5% or less (excluding 0%) Containing at least one member selected from the group consisting of: And the rest is F
e and those which are unavoidable impurities are preferred. It is also effective to include Ti: 0.1% or less (excluding 0%) and / or Nb: 0.1% or less (excluding 0%) in this steel material.

In order to more effectively achieve the effects of the present invention, the deformation resistance coefficient .sigma.eq and the crack generation rate coefficient UL expressed as a function of the alloy element in each of the above-mentioned chemical component compositions are as follows. Equations (1) and (2)
It is preferable that the above formula be satisfied. By satisfying these requirements, the cold forgeability can be further improved. σeq = C + 0.5Si + 0.2Mn + 3.0P + 0.3Cr + 0.3Mo + 0.1V + 0.1Ti + 0.1Nb <1.0 ... (1) UL = C + 0.2Si + 0.2Mn + 0.2Cr + 0.2Mo + 2.0P + 2.0S + 0.1V + 0.1Ti + 0.1Nb <0.7… (2)

[0012] Further, in those having the above chemical composition, the wear resistance coefficient F (wea
It is preferable that r) satisfies the following expression (3), and by satisfying these requirements, the wear resistance can be improved. F (wear) = Si + 0.2Cr + 0.4Mo + V> 0.4 (3) In the case hardened steel of the present invention, if necessary, Cu: 1.0% or less (0 %: Ni: 2.5% or less (excluding 0%), Ca: 0.01% or less (excluding 0%) and / or Zr: 0.08% or less (including 0%) No), Pb: 0.3% or less (excluding 0%), B: 0.005% or less (excluding 0%), etc., to further improve the properties as case hardening steel. Can be.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present inventors have studied from various angles to achieve the above object. As a result, it has been found that if the microstructure of the case hardening steel for cold forging is appropriately adjusted as described above, the above object can be achieved brilliantly, and the present invention has been completed. First, the reason for defining the microstructure as described above in the present invention will be described.

In order for the case hardened steel to exhibit excellent cold forgeability, it is necessary to reduce the deformation resistance during cold forging.
In the present invention, from such a viewpoint, the average particle size of ferrite before the spheroidizing annealing treatment is specified to be 40 μm or less, and the area ratio of (ferrite + pearlite) is specified to be 75% or more. By satisfying these requirements, cementite can be uniformly dispersed in ferrite after spheroidizing annealing,
Thereby, the deformation resistance at the time of cold forging can be reduced and the life of the mold can be improved. The area ratio of the above (ferrite + pearlite) is preferably 80% or more, and more preferably 83% or more. The structure other than (ferrite + pearlite) is a single phase of bainite or martensite or a composite structure thereof. The definition of the ferrite particle size is as follows. Average particle size of ferrite = (major axis of ferrite part + minor axis)
/ 2

On the other hand, while ensuring good cold forgeability,
In order to shorten the spheroidizing annealing time, the average particle size of the pearlite before the spheroidizing annealing needs to be 30 μm or less. By setting the average particle size of pearlite to 30 μm or less, (1) promote the dispersion of cementite at the ferrite / cementite interface, (2) increase the carbon concentration in pearlite, and leave the fine nuclei of cementite during heat treatment. Prevent the generation of recycled perlite,
It is considered that the effects such as the above are exhibited, and thereby the spheroidizing annealing treatment is promoted. The average particle size of the pearlite is the average particle size of the pearlite portion surrounded by ferrite, and the measurement site is a cross section of the rolled material. The definition of the average particle size of pearlite is as follows. Average particle diameter of pearlite = (Large diameter + short diameter of pearlite part)
/ 2

The area ratio of (ferrite + pearlite) is measured, for example, by randomly observing the structure in five visual fields using an optical microscope (magnification: about 400) and analyzing the area of (ferrite + pearlite) by image analysis. Measure the rate. The method of measuring the average particle diameter of ferrite and the average particle diameter of pearlite is, for example, by randomly observing the structure of five visual fields using an optical microscope (magnification: about × 400),
The average particle size of ferrite and the average particle size of pearlite are measured at 10 locations per visual field, and the average values are further averaged to obtain measured values of the average particle size of ferrite and the average particle size of pearlite.

In order to adjust the microstructure of the case hardening steel for cold forging as described above, the heating temperature before rolling is 1100 ° C. or less, the rolling reduction is 30% or more, and the rolling finishing temperature is 950 ° C.
Rolling may be performed as follows, and the operation may be performed with the cooling rate after the rolling finish set to 60 ° C./sec or less. Next, a preferable chemical composition in the case hardening steel of the present invention will be described.

C: 0.3% or less (excluding 0%) C is a useful element as a strengthening element for securing the core hardness of the component. The lower limit is set to 0.3% because the lowering and the toughness are caused. The preferred upper limit of the C content is 0.25%, and more preferably 0.20% or less. In order to effectively exhibit the above effects, it is preferable that C is contained at 0.05% or more.

Si: 0.3% or less (excluding 0%) Si is an element useful as a deoxidizing agent at the time of melting, and also effectively acts to improve the wear resistance of steel. However, if the content exceeds 0.3%, the deformation resistance during cold forging is increased, so the upper limit is set to 0.3%.
The Si content should preferably be kept below 0.15%, more preferably below 0.10%. In order to effectively exhibit the above effects, it is preferable that Si is contained at 0.01% or more.

Mn: 1.5% or less (excluding 0%) Mn is an element useful as a deoxidizing agent during melting.
If it exceeds 5%, the deformation resistance during cold forging increases, so the upper limit is set to 1.5%. The preferred upper limit of the Mn content is 1.2%. In order to effectively exhibit the above effects, it is preferable that Mn is contained in an amount of 0.2% or more.

P: 0.02% or less (including 0%) Since P is an element which increases the deformation resistance during cold forging and lowers the deformation resistance of steel, it ensures good cold forgeability. For this purpose, it should be suppressed to 0.02% or less. From the viewpoint of further improving cold forgeability, P is 0.0
It is preferably at most 15%, more preferably at most 0.0%.
Should be kept below 10%.

S: 0.02% or less (excluding 0%) S is an element useful for forming MnS and improving machinability, but when contained in excess of 0.02%, Deformability during cold forging decreases. From the viewpoint of improving the cold forgeability, S is preferably set to 0.015% or less, more preferably, to 0.010% or less. Further, in order to effectively exert the above-mentioned effect, S is set to be not more than 0.
It is preferable to contain 001% or more.

Al: 0.06% or less (excluding 0%) Al also effectively acts as a deoxidizing component at the time of smelting, and combines with N in steel to form AlN and coarsen crystal grains. However, when the content exceeds 0.06%, the above-mentioned effect is saturated. Therefore, the upper limit is set to 0.06%. Further, in order to effectively exhibit the above effects, Al is 0.00%.
It is preferable to contain 5% or more.

N: 0.03% or less (excluding 0%) N combines with Al in steel (and also binds when V, Ti, Nb is added) in the steel to form nitrides, The effect of suppressing the coarsening of the grains is exhibited, but since this effect eventually reaches saturation, the upper limit is set to 0.03%. N
The content should preferably be kept below 0.02%, more preferably below 0.015%. In order to effectively exhibit the above effects, it is preferable that N is contained at 0.001% or more.

Cr, Mo and V can be said to be the same element from the viewpoint of improving wear resistance, and their actions are described in detail below.

Cr: 3% or less (excluding 0%) Cr is an element effective for improving the wear resistance of steel, but when it exceeds 3%, in addition to lowering the cold forgeability, The upper limit is set to 3% because of impairing the carburizing property. Cr
The content should preferably be 2.5% or less, more preferably 2.0% or less, from the viewpoint of improving cold forgeability. In order to effectively exhibit the above effects, C
r is preferably contained at 0.2% or more.

Mo: 1.5% or less (not including 0%) Like Cr, Mo is an element effective for improving the wear resistance of steel. Since the forgeability is reduced, the upper limit is set to 1.5%. The Mo content should preferably be 1.0% or less, more preferably 0.5% or less, from the viewpoint of improving cold forgeability. In order to effectively exhibit the above effects, Mo is preferably contained at 0.01% or more.

V: 1.5% or less (excluding 0%) V is a very effective element for improving the wear resistance of steel, but since this effect eventually reaches saturation, the upper limit is set at 1%. 0.5%. The V content should preferably be 1.0% or less, more preferably 0.5% or less, from the viewpoint of improving cold forgeability. In order to effectively exhibit the above effects, V is preferably contained at 0.01% or more.

The preferred chemical components in the case hardening steel of the present invention are as described above, and the balance is iron and unavoidable impurities. If necessary, the following elements may be further contained in appropriate amounts as other elements. It is possible to further improve the properties as hardened steel.

Ti: 0.1% or less (excluding 0%) and / or Nb: 0.1% or less (excluding 0%) Ti and Nb reduce the crystal grain size to an average of ferrite and pearlite. It is an effective element to reduce the particle size,
In any case, when the content exceeds 0.1%, the machinability decreases, so the upper limit is set to 0.1%. From the viewpoint of improving the cold forgeability, their contents should preferably be 0.05% or less. In order to effectively exhibit the above effects, it is preferable that these elements are contained in an amount of 0.001% or more.

Cu: 1% or less (excluding 0%) Cu is an element effective for improving corrosion resistance, but since this effect eventually reaches saturation, the upper limit is set to 1%.
In order to exhibit the above effects, it is preferable that Cu is contained in an amount of 0.2% or more. However, when Cu is added alone, hot workability deteriorates. Therefore, when Cu is added, it is preferable to add Ni having an effect of improving hot workability to the same extent as the amount of Cu.

Ni: not more than 2.5% (not including 0%) In addition to the above-mentioned effects, Ni effectively works to improve the toughness by refining the structure after carburizing, and provides stable core hardness. Although it is an effective element for securing, the effect is saturated at 2.5%, so the upper limit is set to 2.5%. The preferable upper limit of the Ni content is 2.0%. In order to effectively exhibit the above effects, it is preferable that Ni is contained in an amount of 0.2% or more.

Ca: 0.01% or less (excluding 0%)
And / or Zr: 0.08% or less (excluding 0%) These elements contribute to the improvement of the forging property during cooling by spheroidizing MnS and improving the anisotropy.
Exceeds 0.01% and Zr exceeds 0.08%, the effect is saturated. These preferred upper limits are set at 0.
008% and 0.06% in Zr. Further, in order to effectively exhibit the above effects, 0.0005% or more of Ca and Z
It is preferable that the content of r is 0.001% or more.

Pb: 0.3% or less (excluding 0%) Pb is an element effective for improving machinability, but if it exceeds 0.3%, its effect is saturated. It was set at 3%. A preferred upper limit of the Pb content is 0.25%.
Further, in order to effectively exert the above effect, Pb is set to 0.0
It is preferable to contain 1% or more.

B: 0.005% or less (excluding 0%) B is an element effective for improving hardenability, but 0.005%
Exceeds 0.005, the effect is saturated.
%. A preferable upper limit of the B content is 0.0045%.
It is. In order to effectively exhibit the above effects, it is preferable that B is contained at 0.0005% or more.

The present inventors have also studied from another angle to further enhance the cold forgeability of the spheroidized annealing material. As a result, it was found that the deformation resistance coefficient σeq and the crack initiation rate factor UL were expressed as functions of the main alloying elements, and if these satisfied the following equations (1) and (2), respectively, The resistance can be suppressed to less than 700 N / mm ² , and the working ratio before cracking (hereinafter, referred to as “the cracking working ratio”) can be increased, thereby further improving the cold forgeability. I figured out what I can do. Further, as is apparent from the following equations (1) and (2), it is understood that V, Ti, and Nb have a relatively small effect of lowering the cold forgeability. σeq = C + 0.5Si + 0.2Mn + 3.0P + 0.3Cr + 0.3Mo + 0.1V + 0.1Ti + 0.1Nb <1.0 ... (1) UL = C + 0.2Si + 0.2Mn + 0.2Cr + 0.2Mo + 2.0P + 2.0S + 0.1V + 0.1Ti + 0.1Nb <0.7… (2)

On the other hand, the present inventors also studied from the viewpoint of enhancing the wear resistance after the surface treatment, and found that the wear resistance coefficient F (wear) as an index indicating the wear resistance was a major alloy element. It has been found that the wear resistance can be improved if the wear resistance coefficient F (wear) satisfies the following equation (3). Further, as is apparent from the following equation (3), it is understood that V greatly contributes to the improvement of abrasion resistance similarly to Si. F (wear) = Si + 0.2Cr + 0.4Mo + V> 0.4… (3)

[0038]

EXAMPLES Next, the structure and operation and effect of the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples, and the present invention is applicable to the above and following points. Of course, the present invention can be implemented with modifications as far as possible, and all of them are included in the technical scope of the present invention.

Reference Example 1 A steel grade having a chemical composition shown in Table 1 below was used,
After rolling to m, it was used as a test piece for investigating the processing time spent for spheroidizing annealing. At this time, a steel material having the same chemical composition as shown in Table 1 was used as a comparative test piece, which was rolled to φ35 mm, and then subjected to a solution treatment (1200 ° C. × 1 hr → air cooling) (No. 7 in Table 3 described later). To 12).

[0040]

[Table 1]

These test pieces were subjected to a spheroidizing treatment shown in FIG. 1 and the degree of spheroidizing structure specified in JIS G 3539 was No. 1 to No. The heat treatment time of 3 was defined as a spheroidization treatment time (see Table 2 below).

[0042]

[Table 2]

The processing time required for the spheroidization is shown in Table 3 below together with the microstructure of the steel material before the spheroidization.
Pearlite having an average particle size of 30 μm or less (No. 1)
6), the spheroidization time was shorter than that of pearlite having an average particle size of more than 30 μm (Nos. 7 to 12).

[0044]

[Table 3]

Example 1 Using steel types (G to I) having the chemical composition shown in Table 4 below, after rolling to φ35 mm, performing the spheroidizing annealing treatment shown in FIG. 30mm
A test piece for examining the deformation resistance and a test piece for examining the cracking processing rate shown in FIG. 2 were produced. At this time, various comparative test pieces were prepared by using the same steel types (GI) as the chemical composition shown in Table 4 and by the following methods (1) to (3) (No. 4 in Table 5 described later). To 12).

(1) Rolling to φ35 mm → normalizing treatment (1200 ° C. × 1 Hr → air cooling), then performing spheroidizing annealing treatment as shown in FIG. 1, and φ20 mm × 30 mm by machining
A test piece for examining the deformation resistance and a test piece for examining the crack occurrence reduction rate shown in FIG.
6). (2) Rolling to φ35mm → normalizing treatment (900 ° C x 1
(Hr → impact cooling), the spheroidizing annealing treatment shown in FIG. 1 is performed, and a test piece for measuring the deformation resistance of φ20 mm × 30 mm and a test piece for investigating the crack generation processing rate shown in FIG. (Nos. 7 to 9 in Table 5 described later). (3) Rolling to φ35mm → normalizing treatment (1200 ° C x
1Hr → impact cooling), the spheroidizing annealing treatment shown in FIG. 1 is performed, and a test piece of φ20 mm × 30 mm for deformation resistance investigation and a test piece for crack generation processing rate investigation shown in FIG. 2 are machined. (Nos. 10 to 12 in Table 5 described later).

[0047]

[Table 4]

Using these test pieces, the deformation resistance and the cracking processing rate by a cold pressure test were examined. After the spheroidizing annealing, an Ogoshi abrasion test specimen shown in FIG. 3 was prepared, and an Ogoshi abrasion test was performed after carburizing, quenching and tempering shown in FIG. In carrying out this wear test, a quenched and tempered material of bearing steel SUJ2 was used as a mating material. The test conditions were dry, final load: 6.3 kgf, friction distance: 2
00 mm, friction speed: 3 m / sec. The microstructure of the test piece before the spheroidizing annealing treatment is shown in Table 5 below together with the spheroidizing annealing conditions (spheroidizing treatment temperature and spheroidizing treatment time). The results of the above tests are shown in Table 6 below together with the preferred requirements of the deformation resistance coefficient σeq, the crack generation rate coefficient UL, and the wear resistance coefficient F (wear).

[0049]

[Table 5]

[0050]

[Table 6]

As is clear from these results, 1
3 to 3 are examples satisfying the microstructure (area ratio of ferrite + pearlite, average particle size of ferrite, and average particle size of pearlite) defined in the present invention, and all of them have a crack generation processing rate of 60% or more. In addition, it can be seen that the specific wear amount is equivalent to that of the comparative steel and does not significantly decrease.

On the other hand, no. Samples Nos. 4 to 6 are comparative examples in which the chemical components satisfy the preferred composition of the present invention, but the average particle size of ferrite and the average particle size of pearlite are larger than the ranges specified in the present invention. The generated processing rate has decreased. No. 7 to 9 are comparative examples in which the chemical composition satisfies the preferred composition of the present invention, but the area ratio of (ferrite + pearlite) is smaller than the range specified in the present invention. Is declining. In addition, No. Those having a chemical composition satisfying the preferred composition of the present invention, the average particle diameter of ferrite and the average particle diameter of pearlite are larger than the ranges specified in the present invention, and (ferrite + pearlite) 2) is a comparative example in which the area ratio is smaller than the range specified in the present invention, and the cracking processing rate is reduced.

Example 2 No. 3 in Table 7 Nos. 1 to 20 and Nos. A steel specimen having a chemical composition of 21 to 32 was used, and after being rolled to φ35 mm, the spheroidizing annealing treatment shown in FIG. 1 was performed, and a test piece for deformation resistance investigation of φ20 mm × 30 mm was machined. Test specimens for investigating the crack generation processing rate shown in the table were prepared. At this time, in Table 8, No. The same steel composition as the chemical composition shown in 21, 25 and 31 was used, and the following (1) to (3)
Various test pieces were also prepared by the method of No. 33 (Nos. 33 to 41 in Table 10 described later).

(1) Rolling to φ35 mm → normalizing treatment (1200 ° C. × 1 Hr → air cooling), then performing spheroidizing annealing treatment shown in FIG. 1, and φ20 mm × 30 mm by machining
A test piece for examining the deformation resistance and a test piece for examining the cracking processing rate shown in FIG. 2 were prepared (No. 33 in Table 10 below).
~ 35). (2) Rolling to φ35mm → normalizing treatment (900 ° C x 1
(Hr → impact cooling), the spheroidizing annealing treatment shown in FIG. 1 is performed, and a test piece for measuring the deformation resistance of φ20 mm × 30 mm and a test piece for investigating the crack generation processing rate shown in FIG. It was prepared (Nos. 36 to 38 in Table 10 below). (3) Rolling to φ35mm → normalizing treatment (1200 ° C x
1Hr → impact cooling), the spheroidizing annealing treatment shown in FIG. 1 is performed, and a test piece of φ20 mm × 30 mm for deformation resistance investigation and a test piece for crack generation processing rate investigation shown in FIG. 2 are machined. It was prepared (Nos. 39 to 41 in Table 10 described later).

[0055]

[Table 7]

[0056]

[Table 8]

Using these test pieces, the deformation resistance and the cracking working rate by a cold pressure test were examined. After the spheroidizing annealing, the Ogoshi wear test specimen shown in FIG.
After the carburizing quenching and tempering treatments shown in (A) and (B), an Ogoshi type abrasion test was performed in the same manner as in Example 1. The microstructures of the test pieces before the spheroidizing annealing treatment are shown in Tables 9 and 10 below together with the spheroidizing annealing conditions (spheroidizing treatment temperature and spheroidizing treatment time). The results of the above tests are shown in Tables 11 and 12 below together with the preferred requirements of the deformation resistance coefficient σeq, the crack generation rate coefficient UL, and the wear resistance coefficient F (wear).

[0058]

[Table 9]

[0059]

[Table 10]

[0060]

[Table 11]

[0061]

[Table 12]

As is apparent from these results, N in Table 7
o. Those having 1 to 20 microstructures (area ratio of ferrite + pearlite, average particle size of ferrite and average particle size of pearlite) defined in the present invention, and deformation resistance coefficient σeq, cracking working rate UL and It satisfies the wear resistance coefficient F (wear) and has a processing rate of 80
%, The deformation resistance UL is less than 700 N / mm ² , and the cracking working rate UL is 60% or more, indicating that the steel has excellent cold forgeability. The specific wear amount is also 1.
It is very small, that is, less than 0 × 10 ⁻⁷ mm ² / kg, and it can be seen that it has excellent wear resistance. On the other hand, No. 2
Samples Nos. 1 to 41 lack any of the requirements (or preferred requirements) specified in the present invention, and are inferior in at least one of the properties as described below.

(A) No. 21 satisfies the microstructure and the preferred chemical composition defined in the present invention, but the deformation resistance coefficient σeq is larger than the range specified in the present invention, and the influence on the mold life reduction is small. , The deformation resistance is slightly higher than 700 N / mm ² .

(B) No. Sample No. 22 satisfies the microstructure defined in the present invention, but has a C content larger than the preferred chemical composition of the present invention, and the deformation resistance coefficient σeq is larger than the range specified in the present invention. , The effect on the reduction of the mold life is small, but the deformation resistance is 700 N /
than mm ² is slightly higher.

(C) No. No. 23 satisfies the microstructure and the preferred chemical composition defined in the present invention, but has a deformation resistance coefficient σeq and a crack generation rate coefficient UL.
Is larger than the preferred range of the present invention,
The effect on the reduction of the mold life is small, but the deformation resistance is 70%.
It is slightly higher than 0 N / mm ² , and the crack generation rate is slightly lowered.

(D) No. 24 satisfy the microstructure defined in the present invention, but have a Mn content greater than the preferred chemical composition of the present invention, and have a deformation resistance coefficient σeq
And since the crack generation rate coefficient UL is larger than the preferred range of the present invention, the influence on the reduction of the mold life is small, but the deformation resistance is slightly higher than 700 N / mm ² and the crack generation rate is slightly higher. Is declining.

(E) No. 25, which satisfies the microstructure defined by the present invention and the preferred chemical composition, but has a slightly smaller cracking processing rate because the cracking processing rate coefficient UL is larger than the preferred range of the present invention. Is declining.

(F) No. No. 26 satisfies the microstructure specified in the present invention, but has an S content larger than the preferred chemical composition of the present invention, and has a crack generation processing rate coefficient U
Since L is also larger than the preferred range of the present invention, the crack generation processing rate is slightly reduced.

(G) No. 27 and 28 satisfy the microstructure and the preferred chemical composition defined in the present invention, however, since the wear resistance coefficient F (wear) is smaller than the preferred range of the present invention, the specific wear amount But 5.2 each
^{× 10 -7 mm 2 /kg,3.1×10 -7 mm} 2 / kg and has become large, the wear resistance deteriorated.

(H) No. No. 29 satisfies the preferred chemical composition of the present invention, but the deformation resistance coefficient σeq is larger than the range specified in the present invention, and the influence on the mold life reduction is small, but the deformation resistance is 700 N / m
m ² , and the wear resistance coefficient F
Since (wear) is smaller than the preferred range of the present invention, the specific wear amount is as large as 2.0 × 10 ⁻⁷ mm ² / kg, and the wear resistance is reduced.

(I) No. 30 satisfies the microstructure defined in the present invention, but has an S content larger than the preferred chemical component composition of the present invention, and has a crack generation processing rate coefficient U
Since L is also larger than the preferred range of the present invention, the crack generation processing rate is slightly reduced, and the wear resistance coefficient F (wear) is smaller than the preferred range of the present invention. Specific wear amount is 2.0 × 10 ^-7 mm ² / k
g, and the wear resistance is reduced.

(J) No. No. 31 satisfies the microstructure and the preferred chemical composition defined in the present invention, but has a deformation resistance coefficient σeq and a crack generation rate coefficient UL.
Is larger than the preferred range of the present invention,
The effect on the reduction of the mold life is small, but the deformation resistance is 70%.
It is slightly higher than 0 N / mm ² , and the crack generation rate is slightly lowered.

(K) No. No. 32 satisfies the microstructure specified in the present invention, but the content of Mn and P is larger than the preferred chemical composition of the present invention, and the deformation resistance coefficient σeq and the cracking processing rate coefficient UL are the same as those of the present invention. , The deformation resistance is high, the cracking rate is low, and the wear resistance coefficient F (wear)
Is smaller than the preferred range of the present invention,
The specific wear amount was as large as 3.2 × 10 ⁻⁷ mm ² / kg, and the wear resistance was reduced.

(L) No. No. 33 satisfies the preferred chemical composition of the present invention, but the average particle size of ferrite and the average particle size of pearlite are larger than the ranges specified in the present invention, and the deformation resistance coefficient σeq is in the preferable range of the present invention. 820N, the deformation resistance is 820N
/ Mm ² , and the crack generation rate is also low.

(M) No. No. 34 satisfies the preferred chemical composition of the present invention, but the average particle size of ferrite and the average particle size of pearlite are larger than the ranges specified in the present invention, and the cracking reduction coefficient UL is smaller than that of the present invention. Since it is larger than the preferable range, the crack generation processing rate is reduced.

(N) No. 35 satisfies the preferred chemical composition of the present invention, but the average particle diameter of ferrite and the average particle diameter of pearlite are larger than the ranges specified in the present invention, and the deformation resistance coefficient σeq and the cracking processing rate coefficient Since UL is larger than the preferred range of the present invention, the deformation resistance is as high as 822 N / mm ² , and the crack generation processing rate is also reduced.

(O) No. No. 36 satisfies the preferred chemical composition of the present invention, but the area ratio of (ferrite + pearlite) is smaller than the range specified in the present invention, and the deformation resistance coefficient σeq is larger than the preferable range in the present invention. Therefore, the deformation resistance is as high as 815 N / mm ² , and the cracking processing rate is also reduced.

(P) No. 37 satisfies the preferred chemical composition of the present invention, but the (ferrite + pearlite) area ratio is smaller than the range specified in the present invention, and the crack generation rate factor UL is smaller than the preferable range in the present invention. , The cracking rate is reduced.

(Q) No. No. 38 satisfies the preferred chemical composition of the present invention, but the area ratio of (ferrite + pearlite) is smaller than the range specified in the present invention, and the deformation resistance coefficient σeq and the crack generation rate coefficient UL are less than the range specified in the present invention. Since it is larger than the preferred range of the invention, the deformation resistance is as high as 819 N / mm ² , and the crack generation processing rate is also reduced.

(R) No. 39 satisfy the preferred chemical composition of the present invention, but have an average ferrite particle size,
Since the average particle size of pearlite and the area ratio of (ferrite + pearlite) are out of the ranges specified in the present invention, and the deformation resistance coefficient σeq is larger than the preferable range in the present invention, the deformation resistance is 813 N / mm ^2. And the crack generation rate is also reduced.

(S) No. 40, which satisfies the preferred chemical composition of the present invention, has an average particle size of ferrite and an average particle size of pearlite larger than the ranges specified in the present invention, and has a cracking reduction rate UL of the present invention. Since it is larger than the preferable range, the crack generation processing rate is reduced.

(T) No. 41 satisfies the preferred chemical composition of the present invention, but has an average ferrite particle size,
Since the average particle size of pearlite and the area ratio of (ferrite + pearlite) are out of the ranges specified in the present invention, and the deformation resistance coefficient σeq and the crack generation rate coefficient UL are larger than the preferable ranges of the present invention, The deformation resistance is as high as 816 N / mm ² , and the crack generation processing rate is also low.

[0083]

According to the present invention, the spheroidizing annealing process can be speeded up to reduce the manufacturing cost, and is excellent in cold forgeability and, if necessary, wear resistance. Thus, it is possible to provide a case hardened steel having improved hardness.

[Brief description of the drawings]

FIG. 1 is a diagram showing a heat pattern of a spheroidizing annealing treatment.

FIG. 2 is an explanatory view showing a test piece for investigating a crack generation rate.

FIG. 3 is an explanatory view showing an Ogoshi-type wear test piece.

FIG. 4 is a view showing a heat pattern of a carburizing quenching / tempering process.

Claims (10)

[Claims] An area ratio of (ferrite + pearlite) is 75% or more, and an average particle size of ferrite is 40 μm.
m, and the average particle size of pearlite is 30 μm or less. 2. Chemical composition: C: 0.3% or less (excluding 0%: hereinafter, means mass% unless otherwise specified) Si: 0.3% or less (excluding 0%) Mn : 1.5% or less (not including 0%) P: 0.02% or less (including 0%) S: 0.02% or less (not including 0%) Al: 0.06% or less (0%) N: 0.03% or less (excluding 0%), and Cr: 3% or less (excluding 0%) Mo: 1.5% or less (excluding 0%) V: 1.5% or less (excluding 0%) containing at least one member selected from the group consisting of:
The case hardened steel according to claim 1, which is e and unavoidable impurities. 3. The steel material according to claim 1, further comprising Ti:
0.1% or less (excluding 0%) and / or Nb:
The case hardening steel according to claim 2, which contains 0.1% or less (excluding 0%). 4. A deformation resistance coefficient σeq and a crack initiation rate coefficient UL expressed as a function of an alloy element satisfy the following equations (1) and (2), respectively.
Or the case hardened steel according to 3. σeq = C + 0.5Si + 0.2Mn + 3.0P + 0.3Cr + 0.3Mo + 0.1V + 0.1Ti + 0.1Nb <1.0 ... (1) UL = C + 0.2Si + 0.2Mn + 0.2Cr + 0.2Mo + 2.0P + 2.0S + 0.1V + 0.1Ti + 0.1Nb <0.7… (2) 5. The case hardening steel according to claim 2, wherein a wear resistance coefficient F (wear) expressed as a function of an alloying element satisfies the following expression (3). F (wear) = Si + 0.2Cr + 0.4Mo + V> 0.4… (3) 6. The steel material further comprises Cu: 1 as another element.
The case hardening steel according to any one of claims 2 to 5, which contains not more than 0% (not including 0%). 7. The steel material further comprises Ni:
The case hardened steel according to any one of claims 2 to 6, which contains 2.5% or less (excluding 0%). 8. The steel material further comprises Ca:
0.01% or less (not including 0%) and / or Z
The case hardening steel according to any one of claims 2 to 7, which contains r: 0.08% or less (excluding 0%). 9. The steel material further comprises Pb:
The case hardening steel according to any one of claims 2 to 8, which contains 0.3% or less (excluding 0%). 10. The steel according to claim 1, wherein said steel further contains B: 0.
The case hardening steel according to any one of claims 2 to 9, which contains 005% or less (excluding 0%).