JP2001303173A - Steel for carburizing and carbo-nitriding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce steel for carburizing and carbo-nitriding in which bending fatigue strength and facial fatigue strength are simultaneously improved. SOLUTION: This steel has a composition containing, by weight, 0.10 to 0.30% C, 0.01 to 0.15% Si, 0.30 to 1.50% Mn, 0.01 to 0.90% Ni, 0.30 to 1.50% Cr, 0.40 to 1.00% Mo, 0.00 to 0.50% Cu, 0.010 to 0.035% Al, 0.00 to 0.50% V, 0.000 to 0.035% Nb, 0.000 to 0.050% Ti, 0.0000 to 0.0030% B, <=0.035% P, <=0.0015% O and 0.0050 to 0.0250% N, and the balance Fe with inevitable impurity elements and in which the parameter represented by Mi/3+Mo is >=0.60%, and the content of retained austenite is also controlled to <=35%, by which surface hardness of >=700 HV is secured. Alternatively, the steel moreover contains, as machinability improving elements, one or more kinds selected from 0.005 to 0.035% S, 0.01 to 0.09% Pb, 0.04 to 0.20% Bi, 0.002 to 0.030% Te, 0.01 to 0.20% Zr, 0.0001 to 0.0100% Ca and 0.015 to 0.100% Sb.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for carburizing or carbonitriding applied to gears and shafts requiring high fatigue strength in both bending fatigue strength and surface fatigue strength.

[0002]

2. Description of the Related Art As a conventional technique, as a measure for improving fatigue strength, an element which is more easily oxidized than Fe in order to reduce a grain boundary oxide layer of a surface hardened layer which causes a fatigue crack.
Ni, Mo, etc., elements that reduce i, Mn, Cr, etc., and are less oxidizable than Fe, and provide surface compressive residual stress by shot peening and technology for adjusting hardenability and mechanical properties, and the propagation of fatigue cracks There is a technique to suppress the noise.

[0003] In recent years, gears and shafts have been required to be further reduced in size and weight and to have a higher stress load in order to cope with a reduction in the weight of automobiles and industrial machines and an increase in engine output. On the other hand, in the steel in which the contents of Ni and Mo are simply improved, the bending fatigue strength is improved, but there is a problem that the surface fatigue strength is not sufficient.

[0004]

The problem to be solved by the present invention is to improve the bending fatigue strength and the surface fatigue strength simultaneously by adjusting the chemical composition of steel in view of the above problems. It is.

[0005]

As a means for solving the above-mentioned problems, the Ni content is determined by setting an appropriate ratio of the Ni and Mo contents instead of simply improving the Ni and Mo contents. Is controlled to an appropriate range, thereby improving the bending fatigue strength by increasing the content of Ni and Mo, controlling the retained austenite to 35% or less, and improving the surface fatigue by adjusting the surface hardness to 700 HV or more. It is to be.

[0006] Specifically, C =
0.10 to 0.30%, Si = 0.01 to 0.15%,
Mn = 0.30-1.50%, Ni = 0.01-0.9
0%, Cr = 0.30-1.50%, Mo = 0.40-
1.00%, Cu = 0.00 to 0.50%, Al = 0.
010-0.035%, V = 0.00-0.50%,
Nb = 0.000-0.035%, Ti = 0.000-
0.050%, B = 0.0000-0.0030%,
P 2 = 0.035% or less, O 2 = 0.0015% or less,
N = 0.0050-0.0250%, the balance being Fe and unavoidable impurity elements.
By controlling the parameter represented by / 3 + Mo to be 0.60% or more and controlling the amount of retained austenite to 35% or less, a surface hardness of 700 HV or more is ensured, and further, an element for improving machinability. And S = 0.005 in weight% as an element that does not significantly impair the fatigue properties.
0.035%, Pb = 0.01 to 0.09%, Bi =
0.04 to 0.20%, Te = 0.002 to 0.030
%, Zr = 0.01 to 0.20%, Ca = 0.0001
~ 0.0100%, Sb = 0.015 to 0.100%,
It is an object of the present invention to provide a carburizing and carbonitriding steel containing one or more of the above.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention have been working to improve the fatigue life of chromium molybdenum steel specified in JIS (Japanese Industrial Standard), which is currently used, with a rotational bending fatigue limit of 15%.
Above improvements, improved roller pitting fatigue life L ₅₀ life 5 times or more, has been developing as a target value, in the conventional as described above techniques, problem pitting fatigue strength of those bending fatigue strength to improve is hardly improved Faced with the point, it was the history of forming the present invention.

First, an example in which the problems of the prior art have been confirmed will be described. Table 1 shows the chemical components of the steel used in the confirmation experiment. Here, a high-strength steel according to the prior art is a steel having a reduced Si content and an increased Ni and Mo content in order to improve the bending fatigue strength and to reduce the grain boundary oxide layer depth according to the conventional technology. JIS Chrome Molybdenum Steel 1
And JIS chrome molybdenum steel 2 is chromium molybdenum steel specified by JIS (Japanese Industrial Standards).
All of these steels are actual furnace steels.

[0009]

[Table 1]

Each of these materials was machined by machining to obtain three test pieces having the shape shown in FIG. 1 for evaluating the static bending strength, and a test piece having the shape shown in FIG. 2 for evaluating the rotational bending fatigue hardness. Six pieces and five test pieces having a shape shown in FIG. 3 were prepared for evaluating the pitting fatigue strength. All of these test pieces were carburized and quenched and tempered under the conditions shown in FIG. Next, the static bending strength, bending fatigue strength and pitting fatigue strength of these test pieces were evaluated.

As shown in FIG. 5, the static bending strength of the test piece was set at 100 mm between the spans around the notch as shown in FIG.
A load was applied at a strain rate of 10 ⁻² / S using a mR pushing jig, and the maximum load at the time of breakage was defined and evaluated by static bending strength. The bending fatigue strength was evaluated by performing a normal Ono-type rotary bending fatigue test. The pitting fatigue strength was evaluated using a roller pitting fatigue tester shown in FIG. Where 1
Is a test piece which has been carburized and quenched and tempered, 2 is a load roller, 3 and 4 are meshing gears, 5 is a bearing, 6 is a coupling, 7 is a transmission belt, and 8 is a motor. FIG. 7 shows the shape of the load roller of the roller pitching fatigue tester. Table 2 shows the conditions of the roller pitting fatigue test. The test piece was sampled before the test to examine the amount of retained austenite and the surface hardness of each steel type one by one.

[0012]

[Table 2]

Table 3 shows the measurement results of the static bending strength.

[Table 3]

Thus, the static bending strength of the high-strength steel according to the prior art is 5978 N to 6145 N, and the static bending strength of JIS chrome molybdenum steel 1 is 4650 N to 47 N.
It can be seen that the static bending strength of 70N and JIS chrome molybdenum steel 2 is higher than 4589N to 4729N.
This is because the toughness of the carburized layer and the core was improved by increasing the Ni and Mo contents, and the grain boundary oxide layer was reduced by reducing the Si content. FIG. 8 shows the results of the rotating bending fatigue test. This indicates that the fatigue limit of the high-strength steel according to the conventional technique is 960 MPa, which is higher than the fatigue limit of JIS chrome molybdenum steel 1 of 815 MPa and the fatigue limit of JIS chrome molybdenum steel 2 of 784 MPa. This shows that, similarly to the static bending strength, the toughness of the carburized layer and the core was improved by increasing the Ni and Mo contents.
This is because the grain boundary oxide layer was reduced by reducing the i content.
Therefore, 15% or more of the fatigue limit of JIS chromium molybdenum steel, which is the target value in the prior art, that is, 940
It can be seen that improvement over MPa is possible. FIG. 9 shows the relationship between the rotational bending fatigue limit and the static bending strength. The rotational bending fatigue limit is higher as the static bending strength is higher, but the effect is saturated at a constant static bending strength, and the rotational bending fatigue limit, which is the target value, is reduced to 940.
It has been found that the static bending strength needs to be 5400 N or more in order to make it MPa or more.

FIG. 10 shows the results of a roller pitting fatigue test. Fatigue life is evaluated by the L ₅₀ life. Thus, even the fatigue life of the high-strength steel according to the prior art is 4.08.
× 10 ^6, which is far below the target value of 15.5 × 10 ⁶ .

Therefore, the prior art Ni and M
In the high-strength steel according to the prior art in which the o content is improved, the bending fatigue strength is good but the pitting fatigue strength is hardly improved, and the improvement of the bending fatigue strength and the improvement of the pitting fatigue strength as the object of the present invention is achieved. It was once again confirmed that balancing was impossible. When the amount of retained austenite and the hardness of the surface of the test piece, which were investigated before the test, were confirmed in order to elucidate the factors, as shown in Table 4, the amount of retained austenite was 3 in the conventional high-strength steel.
5.5% of JIS chrome molybdenum steel 1
9.5% and more than 16.5% of that of JIS chromium molybdenum steel 2, the surface hardness considered to be the most important factor for the improvement of pitting fatigue life, compared to 661 HV of the high strength steel according to the prior art, It was found to be lower than 766 HV of JIS chrome molybdenum steel 1 and 753 HV of JIS chrome molybdenum steel 2. That is,
It was found that even if the content of Ni and Mo was increased to improve the static bending strength and the bending fatigue strength, the improvement in pitting fatigue strength could not be expected if the surface hardness was reduced.

[0017]

[Table 4]

Therefore, in the present invention, by adjusting the chemical composition of steel based on the above findings, the following items (1) and (2) are affected for the purpose of achieving both of the following items. The effects of chemical components were investigated.

Item (1): Regarding the improvement of the rotating bending fatigue strength, the static bending strength is set to 5400 N or more, which is the same level as that of the conventional high-strength steel. Item (2): Regarding the improvement of the pitting fatigue strength, the surface hardness is set to 700 HV or more.

Table 5 shows the chemical compositions of the inventive steel and the comparative steel used for evaluating these items.

[0021]

[Table 5]

Here, the first invention steel is an invention steel corresponding to claim 1, and the second invention steel is an invention steel corresponding to claim 2. These steels were melted in a high-frequency vacuum melting furnace, and the melted steel ingot was heated to 1250 ° C. and then heated for 30 m.
It was forged to mφ and further normalized at 925 ° C. From these materials, one specimen having the shape shown in FIG. 11 was measured by machining to measure the amount of retained austenite and the surface hardness, and the shape shown in FIG. 1 was used to evaluate the static bending strength. Were prepared three by three. All of these test pieces were carburized and quenched and tempered under the conditions shown in FIG. After that, the test pieces after carburizing, quenching and tempering shown in FIG. 11 were first measured for the amount of retained austenite on the surface at four points every 90 ° by X-ray, and the average value thereof was obtained. Next, cut perpendicular to the longitudinal direction, 90 °
The hardness at a position of 50 μm from the surface was measured with a micro Vickers hardness tester at each of the four locations, and the average value was determined as the surface hardness. The static bending strength was evaluated by the above-described test method, and an average value of three pieces was obtained. Table 6 shows the results.

[0023]

[Table 6]

As a result of analyzing these results by a statistical method such as a multiple regression analysis including past data, a relationship represented by the following equation 1 was found for the effect of the static bending strength and the chemical components. About 1/3 of that of Mo content
It was found that the Ni / 3 + Mo content was N
Static bending strength (N) = − 17 × C content−30 × Si content + 56 × Mn content + 323 × Ni content + 1038 × Mo content + constant (Equation 1) ) And arranged the results in Table 6.

FIG. 12 shows static bending strength and Ni / 3 + Mo.
Shows the relationship between parameters. From this, Ni / 3 + Mo
It can be seen that in the range where the parameter is 0.60 wt% or more, the static bending strength is 5400 N or more. Therefore, it was found that the condition of item (1) was satisfied if the Ni / 3 + Mo parameter was in a range of 0.60 wt% or more, and this was regarded as one of the claims of the present invention. On the other hand, as for the surface hardness, as a result of the confirmation experiment, it was assumed that there was a strong correlation with the amount of retained austenite, and FIG. 13 shows the relationship between the surface hardness and the amount of retained austenite.
From this, it was found that if the amount of retained austenite was 35% or less, the surface hardness was 700 HV or more, satisfying the requirement of item (2), which was regarded as one of the claims of the present invention. The amount of retained austenite is said to be higher as the Ni content is higher, but FIG. 1 shows the relationship between the retained austenite amount and the Ni content in order to grasp the quantitative effect. From this, it was found that when the Ni content was 0.90 wt% or less, the retained austenite amount was 35% or less, and this was regarded as one of the claimed items of the present invention.

From these results, as described in the claims of the steel of the present invention, when the Ni / 3 + Mo parameter is set to 0.60% or more, the static bending strength is set to 5400 N or more in order to improve the rotational bending fatigue strength. And by setting the Ni content to 0.90 wt% or less, the amount of retained austenite is reduced by 35% in order to improve the pitting fatigue strength.
It has been found that the surface hardness can be limited to 700 HV or more.

Although the present invention has been made based on the above research results,
Next, the reasons for limiting the chemical components of the present invention will be described. The chemical composition of the gear steel may be adjusted to various ranges in consideration of its use environment, that is, the size of the gear, the load strength, and the conditions of carburizing or carbonitriding. As described above, the inventors claimed that the effects of the present invention could be obtained even in the range of the chemical components shown in Table 6, and claimed the components.

C: 0.10 to 0.30 wt% C is required to secure the core hardness required for the gear.
It is necessary to add at least 0.10 wt% or more. However, the excessive addition increases the hardness of the core too much and degrades the toughness of the core. To avoid this, it is necessary to limit the upper limit to 0.30 wt%. Therefore, the addition amount of C is set in the range of 0.10 to 0.30 wt%.

Si: 0.01-0.15 wt% Si is an element necessary for deoxidizing steel, and is at least 0.1%.
It is necessary to add at least 01 wt%. However, Si is also an element that promotes grain boundary oxidation, which is considered to be a starting point of fatigue crack generation, and its upper limit needs to be limited to 0.15 wt%. Therefore, the addition amount of Si is 0.01 to 0.15.
wt% range.

Mn: 0.30 to 1.50 wt% Mn is an element necessary for securing hardenability, and it is necessary to add at least 0.30 wt% or more. However, excessive addition thereof causes the steel material before carburizing to be too hard, thereby deteriorating cold forgeability and machinability. To avoid this, it is necessary to limit the upper limit to 1.50 wt%. Therefore, the added amount of Mn is 0.30-1.
The range was 50 wt%.

P: not more than 0.035 wt% P is an element that segregates at the austenite grain boundaries and weakens the grain boundaries, thereby reducing toughness and fatigue strength.
If the content exceeds 5 wt%, such adverse effects become remarkable.
Therefore, the content of P is limited to 0.035 wt% or less.

Ni: 0.01 to 0.90 wt% Ni is an important element together with Mo in the steel of the present invention. That is, as described above, Ni is an element that improves the toughness of the carburized layer and the core by being combined with Mo. In order to exert the effect, it is necessary to add 0.01 wt% or more. However, as described above, when the content exceeds 0.90 wt%, the surface hardness is reduced by increasing the retained austenite. To avoid this, it is necessary to limit the upper limit to 0.90 wt%. Therefore, the addition amount of Ni is 0.01 to 0.90 w.
t% range.

Cr: 0.30 to 1.50 wt% Cr is an element necessary for ensuring hardenability, and it is necessary to add at least 0.30 wt% or more. However, the excessive addition deteriorates cold forgeability and machinability due to the steel material before carburizing becoming too hard. In order to avoid this, it is necessary to limit the upper limit to 1.50 wt%. Therefore, the added amount of Cr is 0.30-1.
The range was 50 wt%.

Mo: 0.40-1.00 wt% Mo is an important element together with Ni in the steel of the present invention. That is, Mo is an element that improves the toughness of the carburized layer and the core by being combined with Ni. In order to exert the effect, it is necessary to add 0.40% by weight or more.

However, if the content exceeds 1.00 wt%, the steel material before carburizing becomes too hard, thereby deteriorating cold forgeability and machinability. In addition, since Mo is a relatively expensive element, excessive addition of Mo is necessary. Is not desirable from an economic point of view. To avoid this, it is necessary to limit the upper limit to 1.00 wt%. Therefore, the amount of Mo added is 0.1.
The range was 40 to 1.00 wt%.

Cu: 0.00 to 0.50 wt% Cu may not be added positively in some cases.
Is an element for which precipitation hardening can be expected in a relatively high temperature range of 400 to 600 ° C. Therefore, it should be added when severe usage conditions where the temperature of the tooth surface or rolling surface rises significantly are assumed, or when it is used in a high temperature environment near jet propulsion machines or turbines such as aircraft materials. In order to exhibit this effect, it is necessary to add at least 0.01 wt% or more. However, its excessive addition increases hot brittleness and inhibits carburization. To avoid this, the upper limit is 0.50w
It must be limited to t%. Therefore, the addition amount of Cu is set in the range of 0.00 to 0.50 wt%.

Al: 0.010-0.035 wt% Al is an element that combines with N to form AlN and has the effect of reducing the austenite crystal grain size. Contributes to the improvement of toughness. At least 0.010 wt.
% Or more is required. However, its excessive addition promotes the formation of Al ₂ O ₃ inclusions harmful to fatigue strength. To avoid this, it is necessary to limit the upper limit to 0.035 wt%. Therefore, the amount of Al added is 0.1.
It was in the range of 010 to 0.035 wt%.

V: 0.00 to 0.50 wt% V may not need to be positively added, but V forms carbide even at a relatively low temperature near the carburizing temperature, and the hardness due to them. Is an element that can be expected to improve the quenching property at the same time. Therefore, if such an effect is required, it should be added, and at least 0.01 wt% or more is required to exhibit this effect. However, its excessive addition degrades the toughness of the carburized layer, is undesirable from an economical point of view because V is an expensive element, and the steel material before carburizing becomes too hard, resulting in cold forging. Deterioration of workability and machinability. To avoid this, the upper limit is 0.50 wt%
It is necessary to limit to. Therefore, the added amount of V is 0.1.
The content was in the range of 00 to 0.50 wt%.

Nb = 0.000-0.035 wt% Nb may not need to be added positively in some cases.
Is an element that combines with C and N in steel to form carbonitride and is effective in refining the austenite grain size like AlN. Through the refining, it improves the toughness of the carburized layer and the core. To contribute. Therefore, if such an effect is required, it should be added, and in order to exhibit this effect, at least 0.001 wt% or more must be added. However, its excessive addition coarsely forms and precipitates carbonitrides and impairs the toughness of the carburized layer. To avoid this, it is necessary to limit the upper limit to 0.050 wt%. Therefore, the added amount of Nb is 0.000 to 0.03.
The range was 5 wt%.

Ti: 0.000 to 0.050 wt% Ti may not be positively added in some cases.
Is an element that combines with C and N in steel to form carbonitrides and has the effect of refining the austenite grain size, similar to AlN. Through the refinement, the toughness of the carburized layer and the core is improved. To contribute.

Ti is also an element that prevents N in steel from combining with B, which will be described later, to form BN, thereby deteriorating the effect of improving the hardenability of B. Therefore, if such an effect is required, it should be added, and in order to exhibit this effect, at least 0.001 wt% or more must be added. However, if a large amount is added, large TiN may be generated and may become a starting point of fatigue fracture. Therefore, it is necessary to limit the upper limit to 0.050 wt%. Therefore, the addition amount of Ti is 0.000 to 0.050 wt.
%.

B: 0.0000 to 0.0030 wt% B may not be added positively, but B is hardened without deteriorating the cold forgeability and machinability of the steel material before carburizing. It is an element that improves the properties. B is also an element that segregates at the crystal grain boundary in preference to P and prevents embrittlement of the crystal grain boundary due to segregation of P. Therefore, if such an effect is required, it should be added, and in order to exhibit this effect, it is necessary to add at least 0.0001 wt% or more. However, even if 0.0050 wt% or more is added, the effect is saturated and the hot workability is deteriorated, so it is necessary to limit the upper limit to 0.050 wt%. Therefore, the addition amount of B is 0.0000 to 0.00.
The range was 30 wt%.

O: 0.0015 wt% or less O is an element existing as an oxide-based inclusion in steel and impairing the fatigue strength. Therefore, the upper limit of O is specified as 0.0015 wt% or less.

N: 0.0050 to 0.0250 wt% N combines with Al and Nb to form AlN and NbN, and is an element effective in refining the austenite crystal grain size. It contributes to improving the toughness of the layer and the core. In order to exhibit the effect, it is necessary to add at least 0.0050 wt% or more. But,
In order to improve the hardenability by adding B, it is preferable that the amount is as small as possible, and excessive addition causes generation of bubbles on the surface of the steel ingot during solidification and deterioration of the forgeability of the steel material. In order to avoid this, it is necessary to limit the upper limit to 0.0250 wt%. Therefore, the addition amount of N is 0.0050 to
The range was 0.0250 wt%.

Ni / 3 + Mo parameter: 0.60 w
t / 3 or more Ni / 3 + M with 1/3 of Ni content and Mo content added
The o parameter is the most important parameter in the steel of the present invention. That is, Ni and Mo are elements that improve the toughness of the carburized layer and the core, respectively, as described in the respective sections. In order to obtain a strength steel level of 940 MPa or more, the Ni / 3 + Mo parameter is set to 0.60 wt% or more, and the static bending strength is set to 54%.
It is necessary to improve it to more than 00N. Therefore, Ni /
The 3 + Mo parameter was 0.60 wt% or more.

Retained austenite amount: 35% or less As described above, when the retained austenite amount exceeds 35%, the surface hardness may be 700 HV or less. Therefore, the amount of retained austenite was specified to be 35% or less.

Surface hardness: 700 HV or more As described above, if the surface hardness is less than 700 HV, the pitting fatigue strength cannot be improved. Therefore, the surface hardness was specified to be 700 HV or more.

S: 0.005 to 0.035 wt% S is mostly present as sulfide-based inclusions in steel, and is effective for improving machinability in parts formed by cutting such as gears. Element. To do so, at least 0.
It is necessary to add 005 wt% or more. However, excessive addition thereof causes a reduction in fatigue strength. To avoid this, it is necessary to limit the upper limit to 0.035 wt%. Therefore, the addition amount of S is 0.005 to 0.0
The range was 35 wt%.

Pb: 0.01 to 0.09 wt% Pb is an element effective for improving machinability in a part formed by cutting like a gear like S. For that purpose, addition of at least 0.01 wt% is necessary. However, excessive addition is an element that causes a reduction in fatigue strength. If the content is 0.10 wt% or more, the handling of Pb is subject to legal regulations such as dust collectors and methods. To avoid this, the upper limit is 0.09 wt%
It is necessary to limit to. Therefore, the addition amount of Pb is set in the range of 0.01 to 0.09 wt%.

Bi: 0.04 to 0.20 wt% Bi, like S and Pb, Bi is an element effective for improving machinability in a part formed by cutting like a gear.
For that purpose, it is necessary to add at least 0.04 wt% or more. However, its excessive addition reduces toughness. To avoid this, the upper limit is 0.20 wt
It is necessary to limit to%. Therefore, the added amount of Bi is set in the range of 0.04 to 0.20 wt%.

Te: 0.002 to 0.030 wt% Te is an element that increases the interfacial energy between the sulfide-based oxide and the parent phase, Fe, makes the shape spindle-shaped, and improves machinability. For that, at least 0.002wt
% Or more is required. However, its excessive addition causes hot embrittlement. In order to avoid this, it is necessary to limit the upper limit to 0.030 wt%. Therefore,
The amount of Te added was in the range of 0.002 to 0.030 wt%.

Zr: 0.01 to 0.20 wt% Zr is an element for improving machinability. For that purpose, addition of at least 0.01 wt% is necessary. However, its excessive addition reduces toughness. To avoid this, it is necessary to limit the upper limit to 0.20 wt%. Therefore, the addition amount of Zr is 0.01 to 0.20.
wt% range.

Ca: 0.0001 to 0.0100 wt% Ca is an element that improves machinability. For that purpose, it is necessary to add at least 0.0001 wt% or more. However, its excessive addition reduces toughness. To avoid this, it is necessary to limit the upper limit to 0.0100 wt%. Therefore, the amount of Ca added is 0.00
The range was from 0.01 to 0.0100 wt%.

Sb: 0.015 to 0.100 wt% Sb is an element that forms a solid solution in the matrix and embrittles steel, thereby improving machinability. Further, Sb has an effect of reducing the grain boundary oxidation depth, and contributes to the improvement of the fatigue strength. For that purpose, it is necessary to add at least 0.015 wt% or more. However, its excessive addition reduces toughness. To avoid this, the upper limit is 0.10
It is necessary to limit to 0 wt%. Therefore, the addition amount of Sb is set in the range of 0.015 to 0.100 wt%.

[0055]

Next, the present invention will be described in more detail with reference to specific examples. Table 7 shows the chemical compositions of invention steels produced in an actual furnace based on the above findings and comparative steels for comparison with the invention steels.

[0056]

[Table 7]

Here, the invention steel 1 is a boron-free steel.
Invention Steel 2 is a boron-added steel and was melted in an actual furnace. The comparative steel is the same as the steel shown in Table 1. The high-strength steel according to the prior art is a steel in which the contents of Ni and Mo are increased according to the prior art in order to improve the bending fatigue strength.
S Chromium molybdenum steel 1 and JIS Chromium molybdenum steel 2 are chromium molybdenum steels specified in JIS (Japanese Industrial Standards). Invention Steel 1 and Invention Steel 2
As in the confirmation experiment described above, three test pieces having the shape shown in FIG. 1 were used to evaluate the static bending strength, and six test pieces having the shape shown in FIG. 2 were used to evaluate the rotational bending fatigue strength. In order to evaluate the pitting fatigue strength, five test pieces having the shape shown in FIG. 3 were produced. All of these test pieces were carburized and quenched and tempered under the conditions shown in FIG. 4 to evaluate static bending strength, rotary bending fatigue strength and pitting fatigue strength. These evaluation methods are the same as those in the confirmation experiment.

Table 8 shows the measurement results of the static bending strengths of the inventive steel 1 and the inventive steel 2, together with the results of the confirmation experiment for the comparative steel described above.

[0059]

[Table 8]

From this, the static bending strength of Invention Steel 1 was 548.
It was 9N to 5599N, and the static bending strength of Invention Steel 2 was 5792N to 5832N, all of which were higher than the target value of 5400N. Therefore, the static bending strength of the steel of the present invention is determined by the comparison steel JIS.
It was higher than chromium molybdenum steel and was confirmed to be at the same level as high strength steel according to the prior art. In FIG.
The results of the rotary bending fatigue test of Invention Steel 1 and Invention Steel 2 were as follows:
The results are shown together with the results of the confirmation experiment on the comparative steel described above. Thus, the rotational bending fatigue limit of Invention Steel 1 is 955MP.
a, and the rotational bending fatigue limit of Invention Steel 2 is 960 M
Pa was found to have a rotational bending fatigue limit of 940 MPa or more, which is the target value. Therefore, the rotational bending fatigue strength of the steel of the present invention was determined by the comparative steel JI.
S It was higher than the chromium molybdenum steel, and was confirmed to be at the same level as the high-strength steel according to the prior art. Table 9 shows the measurement results of the amount of retained austenite and the surface hardness of the roller pitting fatigue test specimens of Invention Steel 1 and Invention Steel 2, together with the results of the confirmation experiment for the comparative steel described above.

[0061]

[Table 9]

From the results, the amount of retained austenite of Invention Steel 1 was 22.8% and the surface hardness was 746 HV.
Inventive steel 2 had a retained austenite content of 26.3% and a surface hardness of 720 HV, in all cases, a residual austenite content of 35% or less and a surface hardness of 700 HV or more,
It was confirmed that the amount of retained austenite was smaller and the surface hardness was higher than that of the comparative high-strength steel according to the prior art. FIG. 16 shows the results of the pitting fatigue tests of Invention Steel 1 and Invention Steel 2 together with the results of the above-described confirmation experiments on the comparative steel.

[0063] From this, the inventive steels 1 L ₅₀ life 16.3
The _L50 life of invention steel 2 is 2 × 10 ⁶ .
68 × 10 ⁶ , both of which are the target values of 15.5.
It was found to be about 5 to 8 times better than any of the comparative steels at × 10 ⁶ or more. This is because these inventive steels have a high surface hardness, and the toughness of the carburized layer and the core is improved by increasing the contents of Ni and Mo.

Therefore, according to the present invention, it was confirmed that the surface fatigue strength as well as the bending fatigue strength can be simultaneously improved by adjusting the chemical composition of the steel as intended.

[0065]

As described above, according to the present invention, the surface fatigue strength as well as the bending fatigue strength can be improved only by adjusting the chemical composition of steel, and the problems to be solved by the invention can be solved. Therefore, as an effect of the present invention, even in the current manufacturing process, it is possible to reduce the size and weight of the carburized gear, and to achieve a higher output even with the same shape and dimensions. And contributes widely to reducing costs and improving reliability.

[Brief description of the drawings]

FIG. 1 shows the shape of a test piece for evaluating static bending strength.

FIG. 2 shows the shape of a test piece for evaluating rotational bending fatigue strength.

FIG. 3 shows a test piece shape for evaluating pitting fatigue strength.

FIG. 4 shows carburizing-tempering conditions.

FIG. 5 is an outline of a static bending strength evaluation test.

FIG. 6 is an outline of a roller pitching fatigue tester.

FIG. 7 shows the shape of a load roller of a roller pitching fatigue tester.

FIG. 8 shows the results of a rotating bending fatigue test.

FIG. 9 shows the relationship between the rotational bending fatigue limit and static bending strength.

FIG. 10 shows the results of a roller pitting fatigue test.

FIG. 11 is a test piece shape for evaluating the amount of retained austenite and the surface hardness.

FIG. 12 shows the relationship between static bending strength and Ni / 3 + Mo parameter.

FIG. 13 shows the relationship between surface hardness and the amount of retained austenite.

FIG. 14 shows the relationship between the retained austenite amount and the Ni content.

FIG. 15 shows the results of a rotary bending fatigue test of Invention Steel 1 and Invention Steel 2 and Comparative Steel.

FIG. 16 shows the results of a roller pitting fatigue test of Invention Steel 1 and Invention Steel 2 and Comparative Steel.

Claims (2)

[Claims] 1. C = 1.10 to 0.30% by weight
%, Si = 0.01 to 0.15%, Mn = 0.30 to
1.50%, Ni = 0.01-0.90%, Cr = 0.
30-1.50%, Mo = 0.40-1.00%, Cu
= 0.00-0.50%, Al = 0.10-0.03
5%, V = 0.00 to 0.50%, Nb = 0.000
０．０0.035%, Ti = 0.000 to 0.050%, B
= 0.00000-0.0030%, P = 0.035
%, O = 0.0015% or less, N = 0.005
0 to 0.0250%, the balance being Fe and unavoidable impurity elements, and a parameter indicated by Ni / 3 + Mo (hereinafter, referred to as Ni / 3 + Mo parameter) is 0.60% or more, and A steel for carburizing and carbonitriding, characterized in that the surface hardness is maintained at 700 HV or more by controlling the amount of retained austenite to 35% or less. 2. An element which improves machinability and which does not significantly impair fatigue properties, is expressed as S = 0.
005-0.035%, Pb = 0.01-0.09%,
Bi = 0.04-0.20%, Te = 0.002-0.
030%, Zr = 0.01 to 0.20%, Ca = 0.0
001 to 0.0100%, Sb = 0.015 to 0.10
The steel for carburizing and carbonitriding according to claim 1, wherein the steel contains one or more of 0%.