JPH05171348A - Carburizing steel excellent in impact fatigue strength - Google Patents

Abstract

PURPOSE:To obtain a carburizing steel excellent in impact fatigue strength by inhibiting the occurrence of abnormal layer at carburizing while keeping the carburizing atmosphere in the conventional state and preventing the occurrence of the impact fatigue fracture originated from the oxides forming in the surface of a carburized layer. CONSTITUTION:The carburizing steel excellent in impact fatigue strength has a composition containing, by weight ratio, 0.15-0.30% C, <=0.15% Si, 0.30-0.60% Mn, <=0.015% P, <=0.020% S, 0.30-0.80% Cr, 0.30-0.80% Mo, <=0.015% Al, 0.0080-0.0200% N, <=0.0015% O, and further 0.02-0.30% Nb and/or 0.02-0.30% V. Further, impact fatigue strength is improved to a greater extent by incorporating 0.50-2.50% Ni, if necessary.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carburizing steel which is used for gears, shafts and the like and has excellent repeated impact fatigue strength.

[0002]

Steel generally used for gears, shafts, etc. is used after carburizing, soft nitriding and induction hardening to improve pitting resistance, scoring resistance, impact fatigue strength and bending fatigue strength. is doing.

However, due to the increase in engine output found in recent automobiles, the SCM4 conventionally used has been used.
The carburizing steels such as SNCM420 and SNCM420 lack the above-mentioned strength and cause a problem that parts are damaged during use. Therefore, by adding an alloy element to a part of the steel to improve the strength of the material, Attempts have been made to improve impact fatigue strength.

[0004]

However, in the above-mentioned method of adding alloy elements, high output and impact fatigue strength are not fully satisfactory, and the cost is high. Therefore, it has been desired to develop a carburizing steel that is inexpensive and has excellent impact fatigue strength.

[0005]

The present invention has been made in view of such drawbacks of conventional steels, and the present inventor has investigated factors that reduce impact fatigue strength. Although the material strength has a great influence on the fatigue strength of carburized parts, it is known that the quality of heat treatment by carburizing also has a large effect on the fatigue strength of parts.

That is, since the carburizing atmosphere contains oxygen gas, it is inevitable in the current general carburizing method to oxidize over the entire surface of the carburized component over ten and several microns. In this way, most of the produced oxide exists in the grain boundaries on the outermost surface of the component. These oxides react with Si, Mn, and Cr contained in steel and oxygen in the carburizing atmosphere to generate the above elements and oxides, and the surface of the carburized layer is accompanied by an incompletely hardened layer. An abnormal carburizing layer is generated.

The formation of such an abnormal carburized layer on the carburized surface lowers the surface hardness of the carburized layer, and the oxide acts as a notch to cause impact fatigue fracture from the surface origin. Then, the inventors of the present invention conducted a study to reduce the abnormal carburization layer from the above viewpoint while keeping the carburizing atmosphere as it is, and as a result, the Si content was reduced to 0.15% or less and the Mn content was reduced to 0%. Carburizing in the austenite region by a normal gas carburizing method using RX gas as a carrier gas and butane as an enriched gas by regulating the Cr content to 0.30 to 0.60% and the Cr content to 0.30 to 0.80%. Time,
It was found that the abnormal carburizing layer was significantly reduced compared to the conventional steel.

In order to improve impact fatigue strength, A
It has been found that reducing the 1 content is extremely effective.
Furthermore, by adding an appropriate amount of Ni, Mo, Nb or V and suppressing impurities such as P and S and gas components such as O during melting and refining, we developed a carburizing steel with excellent properties such as impact fatigue strength and grain size adjustment. It was successful in doing.

That is, the carburizing steel excellent in impact fatigue strength according to claim 1 of the present invention has a weight ratio of C: 0.1.
5 to 0.30%, Si: 0.15% or less, Mn: 0.3
0 to 0.60%, P: 0.015% or less, S: 0.02
0% or less, Cr: 0.30 to 0.80%, Mo: 0.3
0 to 0.80%, Al: 0.015% or less, N: 0.0
080-0.0200%, O: 0.0015% or less, and Nb: 0.02-0.30%, V: 0.0
2 to 0.30% of 1 type or 2 types are contained, and in order to further improve the impact fatigue strength of claim 1, the second aspect is 0.05 to 2.50% of Ni. The gist of the inclusion is that.

Next, the reasons for limiting the components of the steel of the present invention will be explained. C: 0.15 to 0.30% C is an element necessary to secure the hardness of the core portion by carburizing and quenching, and the hardness to secure the fatigue limit required for gears, shafts, etc. HRC30 to In order to obtain 45, the content must be at least 0.15% or more. However, if contained in a large amount, the machinability and the toughness after carburization deteriorate, so the upper limit was made 0.30%.

Si: 0.15% or less Si is effective for deoxidizing molten steel. This utilizes the strength of the affinity with oxygen in the molten steel of Si. However, if the content exceeds 0.15%, it reacts with oxygen in the atmosphere during carburization, and a carburized abnormal layer is easily generated, so the upper limit was made 0.15%.

Mn: 0.30 to 0.60% Mn is effective as a deoxidizing and desulfurizing material for molten steel and has an affinity with oxygen in the atmosphere like Si. Further, Mn is an element that also has an effect of improving hardenability, and at least 0.30% or more is required to obtain these effects.
However, if the content exceeds 0.60%, an abnormal carburizing layer is likely to be formed, so the upper limit was set to 0.60%.

Cr: 0.30 to 0.80% Cr is effective for improving the hardenability and the strength after quenching and tempering, and for carburized parts, the hardness and effectiveness of the carburized layer. It is an element effective for improving the depth, and it is necessary to contain at least 0.30% to obtain these effects. However, like Si and Mn, it has a strong affinity with oxygen and easily causes an abnormal carburizing layer, so the upper limit is 0.8.
It was set to 0%.

Ni: 0.50 to 2.50% Ni is an element effective for improving the hardenability and the strength after quenching and tempering, and at least 0.50% for obtaining these effects. The above contents are required. Preferably, the content is 1.00% or more. But 2.50%
If it is contained in excess of 0.1%, the retained austenite in the abnormal carburized layer becomes excessive, which lowers the surface hardness and deteriorates the machinability, so the upper limit was made 2.50%.

Mo: 0.30 to 0.80% Mo is an element effective for improving hardenability and toughness after quenching and tempering, and hardness of the carburized layer for carburized parts. It is an element that improves the effective hardening depth, and at least 0.30% or more is necessary to obtain these effects. However, if the Mo content is too large, carbides are formed in the carburized layer, the impact fatigue strength is lowered, and the machinability is deteriorated, so the upper limit was made 0.80%.

Al: 0.015% or less Al acts as a deoxidizing agent during melting, and combines with N in molten steel to form AlN, which prevents coarsening of crystal grains during carburization to improve impact fatigue strength. It has the effect of improving, but if it is contained in excess of 0.015%, coarse Al
₂ O ₃ reduces the impact fatigue strength, so the upper limit is set to 0.
It was set to 015%.

Nb: 0.02 to 0.30%, V: 0.0
2 to 0.30% Nb, V is an element that produces carbonitrides and is effective for refining crystal grains during carburization, similar to AlN. To obtain this effect, 0.02% or more is contained. It is necessary, and the lower limit of each is set to 0.02%. However, if the content exceeds 0.30%, it combines with C in the steel and impairs the hardenability, so the upper limits were made 0.30%.

N: 0.0080 to 0.0200% N combines with Al, Nb or V to form a nitride or carbonitride, which prevents coarsening of crystal grains during carburization and improves impact fatigue strength. Have an effect. In order to obtain this effect, it is necessary to contain at least 0.0080% or more, and the lower limit was made 0.0080%. However, even if the content exceeds 0.0200%, the effect is saturated, so the upper limit is 0.002.
It was set to 0%.

O: 0.0015% or less O forms oxides such as Al ₂ O ₃ and SiO ₂ and becomes oxide-based inclusions, so these oxides act as notches to improve impact fatigue strength. Lower. Further, these oxide-based inclusions are also harmful to workability such as machinability, and it is necessary to suppress the content thereof, and the upper limit is 0.0015%.
And

P: 0.015% or less P is an element which easily forms striped segregation and causes the steel to be embrittled by segregating at grain boundaries. It is necessary to reduce it as much as possible, and its upper limit is 0.015. %.

S: 0.020% or less S is mainly present in the form of sulfide and is an element effective for machinability, but if it is present in a large amount, it causes anisotropy of steel,
Since the cleanliness will be impaired, its upper limit is 0.020%
And

[0022]

EXAMPLES Next, the features of the present invention will be clarified by examples in comparison with comparative steels. In Table 1, A to F steels are steels of the present invention, and G to K steels are comparative steels. Among the comparative steels, G steel has a low C content, H steel has a high Si content, I steel has a high Mn and Cr content, and J steel has a high Ni content. Therefore, K steel has a high Al and O content.

[0023]

[Table 1]

Carburizing 0.90% of carbon potential, carburizing temperature 920 ° C. × 3 hours carburizing from the test steel of Table 1 2.
Diffuse for 5 hours, hold at 830 ° C for 30 minutes, oil quench, and then temper at 160 ° C for 90 minutes, and measure abnormal carburizing layer depth, impact fatigue strength, effective hardening depth, core hardness The measurement results are shown in Table 2. The impact fatigue strength was measured by the Matsumura type impact test and was 30K.
The number of repeated breaks at gfcm is shown. For the effective hardening depth, the distance from the surface to the hardness Hv 550 is shown.

[0025]

[Table 2]

As is known from Table 2, Comparative Steel G, which had a low carbon content, had a low impact fatigue strength of 2100 and a core hardness of 321 which was not sufficient. It was Comparative steel H has a high Si content, and comparative steel I is M
Due to the high n and Cr contents, the abnormal carburizing layer is 12 to 13
The impact fatigue strength was as deep as 1 μm and low as 1900. Comparative Steel J has a high Ni content and therefore has a low impact fatigue strength of 2300. Comparative steel K has a high Al, O content, and therefore has an inferior impact fatigue strength of 2300. On the other hand, in the steels A to F which are the steels of the present invention, the abnormal carburizing layer is as shallow as 4 to 8 μm, and the number of repeated ruptures in the impact fatigue test is 2600 to 310 with respect to 1900 to 2300 times of the comparative steel.
It was 0 times and was found to be excellent in impact fatigue strength.

[0027]

INDUSTRIAL APPLICABILITY As described above, the carburizing steel having excellent impact fatigue strength of the present invention reduces the Si content to 0.15% or less and the Mn content to 0.30 to 0.60%. Since the Cr content is regulated to 0.30 to 0.80%, when carburizing in the austenite region by the ordinary gas carburizing method, the abnormal carburizing layer is significantly reduced compared to the conventional steel, and the impact fatigue strength is excellent. Could be In addition, Ni, Mo,
Addition of appropriate amount of Nb or V, etc., and impurity elements such as P and S, gas components such as O and 0.015 when Al is dissolved / refined.
%, The impact fatigue strength and the grain boundary adjusting property were further improved, and the steel of the present invention is extremely effective for strengthening and reducing the weight of parts such as gears and shafts.

Claims (2)

[Claims] 1. A weight ratio of C: 0.15 to 0.30.
%, Si: 0.15% or less, Mn: 0.30 to 0.60
%, P: 0.015% or less, S: 0.020% or less, C
r: 0.30 to 0.80%, Mo: 0.30 to 0.80
%, Al: 0.015% or less, N: 0.0080 to 0.
0200%, O: 0.0015% or less and further N
b: 0.02-0.30%, V: 0.02-0.30%
A carburizing steel having excellent impact fatigue strength, which is characterized by containing one or two of the above. 2. A weight ratio of C: 0.15 to 0.30.
%, Si: 0.15% or less, Mn: 0.30 to 0.60
%, P: 0.015% or less, S: 0.020% or less, N
i: 0.50 to 2.50%, Cr: 0.30 to 0.80
%, Mo: 0.30 to 0.80%, Al: 0.015%
Below, N: 0.0080 to 0.020%, O: 0.00
15% or less, further Nb: 0.02-0.3
0%, V: 0.02 to 0.30%, containing 1 or 2 kinds, carburizing steel with excellent impact fatigue strength.