JPH04202743A - Steel for carburizing excellent in impact fatigue strength - Google Patents

Abstract

PURPOSE:To offer a steel for carburizing excellent in impact fatigue strength by constituting it of a compsn. contg. specified wt.% of C, Si, Mn, P, S, Ni, Cr, Mo, Al, N and O and the balance Fe. CONSTITUTION:A steel for carburizing is constituted of a compsn. contg., by weight, 0.15 to 0.30% C, <=0.15% Si, 0.30 to 0.60% Mn,<=0.015% P, <=0.02% S, 1.00 to 2.50% Ni, 0.30 to 0.80% Cr, 0.15 to 0.80% Mo, <=0.15% Al, <=90ppm N and <=15ppm O and the balance Fe with impurity elements. Furthermore, 0.03 to 0.10% Nb is incorporated therein. In this way, the toughening of parts such as gears, shafts or the like and their lightening can be attained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a carburizing steel having excellent impact fatigue strength and used for gears, shafts, etc.

[Prior art] Carburizing steel generally used for gears, shafts, etc.
Impact fatigue strength and wear resistance are improved by carburizing or nitriding. However, due to the increase in engine output seen in recent automobiles, the conventionally used SCM
420 and SNCM420 lack impact fatigue strength,
Problems have arisen in which parts break during use.

In addition, there are problems such as the increase in the size of parts, and a carburizing steel with higher impact fatigue strength is required. For this reason, attempts have been made to improve the impact fatigue strength by adding alloying elements to improve the strength of the material itself.

[Problems to be Solved by the Invention] However, the method of adding alloying elements described above does not provide enough impact fatigue strength for high output, and the resulting steel is expensive. It has been desired to develop a carburizing steel that is inexpensive and has excellent impact fatigue strength.

[Means for Solving the Problems] The present invention was made in view of the drawbacks of conventional steels, and the inventor investigated factors that reduce impact fatigue strength.

Although material strength has a large influence on the fatigue strength of carburized parts,
It is known that the quality of heat treatment by carburizing has a large effect on the fatigue strength of parts.

That is, the carburizing atmosphere in the carburizing process contains oxygen gas, and the carburizing temperature is 2! In the current general carburizing method, it is unavoidable that oxygen in the atmosphere oxidizes the entire surface of the carburized part over ten-odd microns or more.

Most of the oxides thus generated are present at the grain boundaries on the outermost surface of the component. These oxides are produced by the reaction of Sl, Mn, and Cr contained in the steel with oxygen in the carburizing atmosphere at high temperatures to produce oxides of the alloying elements mentioned above.

The formation of these oxides on the carburized surface also causes adverse effects on the surface hardness of the carburized layer, but what is important is that these oxides act as notches and cause a decrease in impact fatigue strength.

Therefore, from the above-mentioned viewpoint, the present inventors conducted research to obtain a high impact strength carburizing steel that reduces the abnormal carburizing layer while keeping the carburizing atmosphere as it was before, and found that the Si content was reduced to 0. Lowering the Mn content to 15% or less and reducing the Mn content to 0.3
0 to 0.60%, (-, r(7'1 content 0.30
It was found that by adjusting the value to ~0.8096, when carburizing in the austenitic region by the normal gas carburizing method using RX gas as a carrier gas and butane gas as an enriched gas, the abnormal carburized layer was significantly reduced compared to conventional steel. I did. It has also been found that reducing the content of A1 and N is extremely effective in improving impact fatigue strength. Furthermore, by adding appropriate amounts of N1, Mo, Nb, etc., and suppressing impurities such as P and gas components such as ○ during melting and refining, we succeeded in developing a carburizing steel with excellent impact fatigue strength and grain size control. .

That is, the carburizing steel of the present invention with excellent impact fatigue strength is
As the first invention steel, the weight ratio is C1015~0.30
%, Si; 0.15% or less, Mn; 030-0.60%
, P, 0.015% or less, S; 002% or less, Ni; 1
．． 00~2.50%, Cr:0°30~0.80, Mo
;0.15-0.80%, AI.

0.015% or less, N: 90pp or less, O: 15pp
m or less, with the remainder consisting of Fe and impurity elements. In addition, in order to further improve the impact fatigue strength of the first invention steel, the second invention steel has Nboo, o 3
The gist is that the content is 0.010%.

[Function] The steel of the present invention has a Si content lowered to 0.15% or less, a Mn content of 0.30 to 0.60%, and a C content of 0.30 to 0.80%. Because of this adjustment, when carburized in the austenitic region by the normal gas carburizing method, the abnormal carburized layer is significantly reduced compared to conventional steel.Also, Al;
．． 015% or less, N, 90ppm or less,
Impact fatigue strength has been significantly improved.

Next, the reasons for limiting the composition of the steel of the present invention will be explained.

C: 0.15'=0, 309°C is an element necessary to ensure core hardness through carburizing and quenching. gear,
In order to obtain a hardness of lRc 30 to 45 to ensure the fatigue strength required for shafts, etc., at least o, 1
It is necessary to contain more than 55g. However, if it is contained in a large amount, the machinability and impact resistance after carburizing will deteriorate, so the upper limit was limited to 0.30%.

Si: 0.15% or less S] is effective for deoxidizing molten steel. This takes advantage of the strong affinity of Si with oxygen in molten steel, but if the content exceeds 0.15C3, it will react with oxygen in the atmosphere during carburizing, forming an abnormal carburized layer. 6Mn: O-30 to 0.6 with the upper limit set to 015c3o to make it easier to generate
09°Mn is effective for deoxidizing and desulfurizing molten steel, and like Si, it also has an affinity for oxygen in the carburizing atmosphere. Mn is an element that is effective in improving hardenability, but when the content is 0.30
% or less, it is difficult to obtain the necessary hardenability;
If it exceeds 60%, an abnormal carburized layer tends to occur, so the content was limited to 0.30 to 0.60%.

Cr: 0.30-0.80% Cr is effective for improving hardenability and strength after quenching and tempering, and for carburized parts, it improves the hardness of the carburized layer and the effective carburization depth. However, it is difficult to obtain the necessary hardenability when the content of 1 is below 0.30%, so the lower limit was set at 0°30%. However, Si, Mn
Similarly, it has a strong affinity with oxygen, and carburized abnormal layers are likely to occur. Therefore, the upper limit was set to 080% from the viewpoint of ensuring strength as a carburized part and reducing the carburized abnormal layer.

Ni: 1.00 to 2.50% Ni is an effective element for improving hardenability and toughness after hardening, quenching, and tempering. It is added in an appropriate amount depending on the hardenability and strength required in the present invention. However, the Ni content to exhibit the expected high strength is 1,000
The lower limit was %.

If the Ni content exceeds 2.50%, residual austenite in the carburized layer during carburization becomes excessive, reducing surface hardness and deteriorating machinability. Furthermore, since Ni is an expensive element, the upper limit was set at 2.50% from the viewpoint of economy.

Mo; 0.15 to 0.80% Mo is effective for improving hardenability and toughness after quenching and tempering, and for carburized parts, it improves the hardness of the carburized layer and the effective carburization depth. In the present invention, it is added in an appropriate amount depending on the required hardenability, strength, and carburizability. However, the lower limit of the Mo content to exhibit the expected high strength was set at 0.15%. When the Mo content increases, carbides are formed in the carburized layer and impact fatigue strength decreases,
Since the machinability deteriorates, the upper limit was set at 0.80%.

Al: 0.015% or less Al acts as a deoxidizing agent during melting, and also combines with N in molten steel to generate AIN, which prevents grain coarsening during carburizing and improves impact fatigue strength. There are 6 but 0.
If the content exceeds 0.015%, impact fatigue strength deteriorates due to coarsening of AIN, so the upper limit of 5 was set at 0.015%.

N: 90 ppm or less N combines with AI to form AIN, which has the effect of preventing grain coarsening during carburizing and improving impact fatigue strength. However, if the content exceeds 9 oppm, the impact fatigue strength will deteriorate due to coarsening of AIN, so the upper limit should be set at 90 ppm.
○: 15 prpa or less O produces oxides such as Al2O, 5i02, etc., and these oxides act as notches, reducing impact fatigue strength. In addition, ○ is an element that deteriorates the pitting resistance of gears, etc., and forms oxide inclusions that are harmful to machinability such as machinability, so it is necessary to suppress its content, and the upper limit is 15p1)- And so.

P: 0.015% or less P is an element that tends to form striped segregation in steel and makes steel brittle by segregating at grain boundaries, so the upper limit is set to 0.015% to eliminate this effect. Limited.

S ; o , o 2% or less S mainly exists in the form of sulfide, and is an effective element for machinability, but if present in large amounts, it will cause anisotropy in the steel and impair cleanliness. Therefore, the upper limit was set to 002%.

Nb: 0.03 to 0.10% Nb forms carbonitrides, and like AIN, it is an element that is effective in refining grains during carburizing. Containment is required, and the lower limit is o, o39
It was set at 6. However, if the content exceeds 0.10%, it will combine with C in the steel and impair hardenability, so the upper limit was set at 0.10%.

[Example 2] Next, the characteristics of the steel of the present invention will be clarified by comparing it with conventional steel and comparative steel through examples.

Table 1 shows the chemical composition of these test steels.6 In Table 1, steels A to H are the steels of the present invention, and steels ■ to Q are comparison steels. Of the comparative steels, ■ steel has a low C content, J steel has a high 81 content, K steel has a high Mn content, L steel has a high Ni content, and M steel has a high Ni content. Ni
Low content, N steel has high C content and N content, Q steel has high A1 and N content, P#l
is high in Mn content and A1 content, Q steel is S1
Carbon potential is 0.9 using the sample steel shown in Table 1 as material.
Carburizing was carried out under carburizing treatment conditions of 0% and carburizing temperature of 930°C x 3 hours. Then, it was held at 850°C for 20 minutes, oil quenched, and then tempered at 160°C for 90 minutes.
The carburized abnormal layer depth, impact fatigue strength, effective carburized depth, and core hardness were measured, and the measurement results are shown in Table 2.

The impact fatigue strength was measured by a pine wood type impact fatigue test, and was expressed as the number of repetitions at break at 30 kgcm. Regarding the effective carburizing depth, the distance from the surface to a hardness of Hv550 is shown.

(Left below) Table 2 As is known from Table 2, the comparative steel ■, which had a low carbon content, had a low impact fatigue strength of 1200, and a core hardness of 372, meaning that it had sufficient core strength. Hardness could not be obtained. Comparative Steel J had a high Si content and Comparative Steel had a high Mn content, so both had deep carburized abnormal layers of 12 μm and 10 μm and low impact fatigue strength of 1000 to 1400. The comparative steel has a high N1 content, so it has an inferior impact fatigue strength of 1900,
Comparative steel M had a low N1 content, so sufficient impact fatigue strength was not obtained. Comparative steel N has high C content and high N content, and comparative steel ○ has high AI content and high N content, so both are inferior in impact fatigue strength.

Comparison steel P has a high Mn content, comparison! Since Q has a high S1 content, the carburized abnormal layer is deep at 18 and 20μ, and the impact fatigue strength is also inferior at 100 and 1200. In contrast, the steel of the present invention A~)
The steel has an extremely thin carburized abnormal layer of 4 to 6 μm, and the number of rupture cycles in the impact fatigue test is also lower than that of Comparative Example f) 10.
00 to 1900 times, and 2200 to 2800 times, and it was found that the impact fatigue strength was excellent. In addition, the carburization depth is 0.94 to 1.34 mm, and the core hardness is 421 to 458, giving satisfactory results in terms of carburization depth and core hardness. Obtained.

[Effects of the Invention] As explained above, the carburizing steel with excellent impact fatigue strength of the present invention has a Si content reduced to 0.015% or less, a Mn content of 0.30 to 0.602°, and a Cr content. The amount is 030~oso9. When the steel is carburized in the austenite region using the normal dregs carburizing method, the abnormal carburized layer is significantly reduced compared to conventional steel, resulting in excellent impact fatigue strength. Furthermore, by adding appropriate amounts of N1, Mo, Nb, etc., and suppressing impurities such as P and gas components such as ○ during melting and scouring, it has excellent properties in impact fatigue strength and particle size control, and At: 0.01511'. L゛ノ下N;90p
Since the impact fatigue strength was suppressed to below prLJ, we succeeded in further improving the impact fatigue strength, and the steel of the present invention is extremely useful for strengthening and reducing the weight of parts such as gears and shafts.

Claims (2)

[Claims] (1) Weight ratio of C: 0.15 to 0.30%, Si: 0.
15% or less, Mn; 0.30-0.60%, P; 0.0
15% or less, S: 0.02% or less, Ni: 1.00-2
．． 50%, Cr; 0.30-0.80, Mo; 0.15
~0.80%, Al; 0.015% or less, N; 90pp
A carburizing steel with excellent impact fatigue strength, characterized by containing 15 ppm or less of O, and 15 ppm or less of O, with the remainder consisting of Fe and impurity elements. (2) Weight ratio of C: 0.15 to 0.30%, Si: 0.
15% or less, Mn; 0.30-0.60%, P; 0.0
15% or less, S: 0.02% or less, Ni: 1.00-2
．． 50%, Cr; 0.30-0.80, Mo; 0.15
~0.80%, Al; 0.015% or less, N; 90pp
m or less, contains O; 15 ppm or less, and further contains Nb; 0
．． 1. A carburizing steel with excellent impact fatigue strength, characterized by containing 0.03 to 0.10%, with the remainder consisting of Fe and impurity elements.