JP3469443B2 - Hardened steel with excellent fatigue properties - Google Patents

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a case hardened steel for machine structure which is used after being carburized or carbonitrided.
The present invention relates to a case hardening steel which does not cause coarsening of crystal grains and has excellent rolling fatigue life and rotating bending fatigue life.

[0002]

2. Description of the Related Art Conventionally, steel for machine structural use, which requires rolling fatigue life and rotary bending fatigue life, is often carburized or carbonitrided before use. In order to improve fatigue strength, it is necessary to strengthen the matrix and stabilize the crystal grains, but since carburizing or carbonitriding treatment is held at high temperature for a long time, the crystal grains tend to become coarse and the crystal grains become coarse. If it is turned into fatigue, the fatigue strength is significantly deteriorated. Therefore, as in the invention of Japanese Patent Laid-Open No. 55-20024, Al and N have been conventionally used.
Although many carbonitrides of b have tried to stabilize the crystal grains, none have tried to improve the fatigue strength by stabilizing the crystal grains and strengthening the matrix, and none have obtained sufficient fatigue properties. .

[0003]

DISCLOSURE OF THE INVENTION According to the present invention, by adding 0.10 to 0.30% of Ti to a conventional steel for machine structure, a large number of Ti carbides are dispersed in the steel to coarsen the crystal grains. And to suppress the structural change during rolling fatigue, thereby providing a steel having improved rolling fatigue life and rotational bending fatigue strength.

[0004]

The means of the present invention for solving the above-mentioned problems is the weight% in the invention of claim 1.
C: 0.10 to 0.45%, Si: 0.05 to 0.
50%, Mn: 0.2 to 1.5%, Ti: 0.10
0.30%, N: <0.01%, balance Fe and unavoidable impurities, Ti carbide is finely dispersed in steel, excellent in rolling fatigue life and rotating bending fatigue life. It is made of steel.

According to the second aspect of the present invention, C: 0.
10 to 0.45%, Si: 0.05 to 0.50%, M
n: 0.2 to 1.5%, Ti: 0.10 to 0.30%,
N: <0.01%, more optionally Cr:
0.15-2.0%, Mo: 0.03-1.5%, N
i: A rolling fatigue life and a rotating bending fatigue life, which are characterized by containing at least one of 0.1 to 3.0%, balance Fe and unavoidable impurities, and finely dispersing Ti carbide. Excellent hardened steel.

According to the third aspect of the present invention, C: 0.
10 to 0.45%, Si: 0.05 to 0.50%, M
n: 0.2 to 1.5%, Ti: 0.10 to 0.30%,
N: <0.01%, B: 0.0005 to 0.0050%
And a balance of Fe and inevitable impurities in which Ti carbide is finely dispersed, which is a case hardening steel excellent in rolling fatigue life and rotating bending fatigue life.

According to the fourth aspect of the invention, C: 0.
10 to 0.45%, Si: 0.05 to 0.50%, M
n: 0.2 to 1.5%, Ti: 0.10 to 0.30%,
N: <0.01%, B: 0.0005 to 0.0050%
And optionally, Cr: 0.15 to 2.0
%, Mo: 0.03 to 1.5%, Ni: 0.1 to 3.0
%, At least one kind of Fe, and the balance Fe and unavoidable impurities, and finely dispersed Ti carbide, which is a case hardening steel excellent in rolling fatigue life and rotating bending fatigue life.

According to the present invention, as described above, the Ti content is 0.10 to
By adding 0.30%, a large number of Ti carbides are dispersed in the steel, the coarsening of crystal grains is prevented during carburization, and the matrix is dispersed and strengthened to suppress or delay the structural change of the matrix, It improves rolling fatigue life and rotating bending fatigue strength.

The reasons for limiting the composition ratio of the present invention will be described. C
Is an element that significantly improves the hardenability, and is required to be at least 0.10% in order to obtain the strength and impact value required for case hardening steel with respect to the mechanical properties of parts. Also,
If the C content exceeds 0.45%, the workability is remarkably reduced, so the C content is set to C: 0.10 to 0.45%.

Si: Si is an element effective in retarding the structural change during rolling fatigue and in hardenability, but 0.05
If it is less than 0.5%, the deoxidizing effect is not sufficient, and if it exceeds 0.50%, the workability is remarkably reduced.
And

Mn: an element effective in the hardenability of steel, but if it is less than 0.2%, the hardenability is insufficient, and if it exceeds 1.5%, the workability is deteriorated, so that 0.2 to 1. 5%.

Ti: An element that finely disperses in the form of Ti carbide in steel, improves rolling fatigue life, and suppresses coarsening of crystal grains during quenching.
Most of them are Ti nitrides and have no effect of Ti. Also,
If it exceeds 0.30%, the workability decreases, so the content is made 0.10 to 0.30%.

N: N is not necessary in the steel of the present invention, and Ti nitride harmful to the fatigue strength increases as the amount of N increases, so N: <0.01%.

B: B is an element that remarkably improves the hardenability of steel by adding a very small amount, but 0.0005%
If it is less than 0.005%, the effect is not sufficient, and if it exceeds 0.005%, on the contrary, the hardenability is lowered and the toughness is deteriorated. Therefore, the content is set to 0.0005 to 0.005%.

Cr, Mo, Ni: An element effective in improving hardenability and rolling fatigue life, which can be selectively added, but if it is too small, it has no effect, and if it is too large, the effect is saturated. Therefore, Cr: 0.15 to 2.0%, Mo:
At least one of 0.03 to 1.5% and Ni: 0.1 to 3.0% is used.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Test steels (Comparative Examples 1 to 9 and Examples 1 to 11) having the chemical composition shown in Table 1 were melted in a 100 kg vacuum melting furnace and hot-forged to 65φ. After normalizing and normalizing, thrust type life test piece (60mmφ × 4.5mm
T) and rotary bending fatigue test piece (8.0 mm x 17 mm
It processed into L) and used for the rolling fatigue life test and the rotating bending fatigue test piece.

The steel compositions of the embodiments of the present invention are shown in Examples 1 to 11 of Table 1. The examples correspond to steels whose carbon content is based on S20C steel, Ti
Of about 0.10% to 0.30%. In Example 1, 0.108% of Ti was added to S20C, and in Example 2, 0.155% of Ti was added to S20C.
In Example 3, 0.192% of Ti was added to S20C. Example 4 is the one in which Cr is added to Example 2, Example 5 is the one in which Mo is added to Example 2, Example 6 is the one in which Ni is added to Example 2, and Example 7 is the one in Example 2. C
r and Mo are added. Example 8 is the same as Example 2.
Si and Mn are reduced and B is added. Example 9 is an example in which Cr is added to Example 8, Example 10 is an example in which Mo is added, and Example 11 is an example in which Ni is added. Further, Al is added and contained as a deoxidizing material during melting of each steel.

[0018]

EXAMPLES First, the test conditions for the rolling fatigue life test are shown.
The various component composition materials shown in Table 1 were rolled into a 65 mmφ steel bar, processed into 60 mmφ × 4.5 mmT, and 930 ° C. × 6.
Carburizing, quenching, and tempering treatments were performed for a time, and a thrust type rolling life test was performed. The test condition is the maximum Hertzian stress of 5.24.
The evaluation was made by the ratio of the number of repetitions with GPa and the number of stress repetitions of 1800 cpm and a cumulative failure probability of 10% (L ₁₀ ).

Next, the test conditions for the rotary bending fatigue test will be shown. The various composition materials shown in Table 1 were rolled into 20 mmφ steel bars, processed into smooth test pieces of parallel part 8.0 mmφ × 17 mmL, carburized and tempered at 930 ° C. × 6 hours, and subjected to a rotating bending fatigue test. Went one piece. The test conditions were evaluated by the ratio of 10 ⁷ times fatigue time strength (fatigue limit strength) at 3000 rpm.

[0020]

[Table 1]

Table 1 shows the components of the steel used in this experiment.
Comparative Example 1 is S20C steel, Comparative Examples 2, 3, and 4 are steels containing Cr, Mo, and Ni in Comparative Example 1, respectively, and Comparative Example 5 is Comparative Example 1 containing Cr and Mo. It is steel. Comparative Example 6 is a steel in which Si and Mn of Comparative Example 1 are reduced and B is added, and Comparative Examples 7, 8 and 9 are steels in which Cr, Mo and Ni are added to Comparative Example 6, respectively.

On the other hand, Examples 1 to 11 are steels to which 0.108 to 0.192% of Ti is added, as compared with Comparative Examples.
It is an example showing an embodiment of the present invention.

The test piece produced above was tested using a rolling fatigue tester and a rotary bending fatigue tester, and a cumulative failure rate of 1
Comparison of stress repetition number of 0% (L ₁₀₎ Example 1 (S2
(0C steel) was set to 1 or the fatigue strength of Comparative Example 1 at 10 ⁷ times fatigue time strength was set to 1, and the life ratio and the fatigue strength ratio were evaluated and shown in Table 2.

[0024]

[Table 2]

From Table 2, it can be seen that in Examples 1 to 3 in which Ti is added to Comparative Example 1, rolling fatigue life and rotating bending fatigue life are improved. Further, in Comparative Examples 2, 3, 4, and 5 in which Cr, Mo, and Ni are added, Examples 4, 5, 6, and 7 in which 0.15% of Ti is added are rolling fatigue life and rotation, respectively. It can be seen that the bending fatigue life is improved. Similarly to Comparative Examples 6, 7, 8 and 9 which are B steels, Examples 8, 9, 10 in which 0.15% of Ti was added.
It can be seen that No. 11 has improved rolling fatigue life and rotating bending fatigue life, respectively.

As a result of the above test, it is understood that the rolling fatigue life and the rotational bending fatigue life of the example are improved by adding Ti, as compared with the comparative example. Further, in the example, the crystal grains at the time of carburization are also stable as compared with the comparative example,
In addition, fine precipitates of Ti carbide were confirmed.

[0027]

As described above, the steel of the present invention has T
By adding i of 0.01 to 0.30%, Ti carbide is finely dispersed in the steel, the coarsening of crystal grains is prevented, and the matrix is strengthened by dispersion strengthening. It has an excellent effect of improving the bending fatigue life. Further, by adding B in an extremely small amount, the hardenability of steel is remarkably improved, and by further selectively adding Cr, Mo, and Ni, the hardenability and rolling fatigue life are further improved.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-63-62859 (JP, A) JP-A-61-104065 (JP, A) JP-A-4-99846 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) C22C 38/00 301 C22C 38/14 C22C 38/50 C22C 38/54

Claims (4)

(57) [Claims] 1. C: 0.10 to 0.45% by weight,
Si: 0.05 to 0.50%, Mn: 0.2 to 1.5
%, Ti: 0.10 to 0.30%, N: <0.01%, balance Fe and inevitable impurities, and Ti carbide finely dispersed in steel. Hardened steel with excellent life and rotating bending fatigue life. 2. C: 0.10 to 0.45% by weight,
Si: 0.05 to 0.50%, Mn: 0.2 to 1.5
%, Ti: 0.10 to 0.30%, N: <0.01%, and optionally, Cr: 0.15 to 2.0%,
Mo: 0.03 to 1.5%, Ni: 0.1 to 3.0%, at least one kind or more, balance Fe and unavoidable impurities, and Ti carbide finely dispersed in steel. Hardened steel with excellent rolling fatigue life and rotating bending fatigue life. 3. C: 0.10 to 0.45% by weight,
Si: 0.05 to 0.50%, Mn: 0.2 to 1.5
%, Ti: 0.10 to 0.30%, N: <0.01%,
B: 0.0005 to 0.0050%, balance Fe and unavoidable impurities, Ti carbide is finely dispersed in steel, and is excellent in rolling fatigue life and rotating bending fatigue life. Baked steel. 4. C: 0.10 to 0.45% by weight,
Si: 0.05 to 0.50%, Mn: 0.2 to 1.5
%, Ti: 0.10 to 0.30%, N: <0.01%,
B: 0.0005 to 0.0050%, and further optionally Cr: 0.15 to 2.0%, Mo: 0.03 to
1.5%, Ni: 0.1 to 3.0% of at least one kind, balance Fe and unavoidable impurities, Ti carbide is finely dispersed in steel, and rolling fatigue life is characterized. And case hardening steel with excellent rotary bending fatigue life. [0001]