JPS62196360A - Case-hardening steel for carburizing and parts for machine structural use made therefrom - Google Patents

Abstract

PURPOSE:To improve fatigue strength and pitting resistance of the titled steel and steel parts to be obtained, by specifying the amounts of components in the steel. CONSTITUTION:One or more kinds among <=0.30% S, by weight 0.35% Pb, <=0.01% Ca, and <=0.05% Te are further incorporated to the titled steel consisting of 0.1-0.5% C, >0.05-0.15% Si, 2.0-5.0% Mn, <=1.5% Cr, 0.008-0.025% N, <0.015% sol.Al, and the balance Fe with accompanying impurities. Moreover, the steel with the above composition is subjected to forming into parts for machine structural use of prescribed shape and then formed into the titled carburizing-hardened parts such as power transmission shafts, gears, etc. If necessary, one or more kinds among 0.01-0.14% Nb, 0.05-0.5% Mo, and 0.1-2.0% Ni are further added to the steel with the above composition.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a case hardening steel for carburizing that has excellent pitting resistance and fatigue strength, and mechanical structural parts made from the same. More specifically, the present invention generates sufficient retained austenite in the surface layer by carburizing and quenching, and due to the action of this retained austenite, a long pitting life,
This invention relates to case-hardening steel for carburizing and mechanical structural parts made from it, which ensure high fatigue strength.

(Prior Art) Power transmission parts for industrial machinery, construction machinery, automobiles, etc. are generally subjected to surface hardening treatment in order to ensure sufficient pitting resistance and fatigue strength.

Although there are various methods for surface hardening, carburizing and quenching still dominates today because it significantly improves the above-mentioned properties of steel. Pitting resistance due to carburizing and quenching,
One of the main reasons for the improvement in fatigue strength is the presence of compressive residual stress in the surface layer due to the difference in hardness between the surface and the inside.

Therefore, based on this idea, for example, Japanese Patent Application Laid-Open No. 119760/1983 has been proposed as a steel that improves the compressive residual stress in the surface layer after carburizing or carbonitriding and quenching. In other words, in JP-A-56-TTQ76O, the basic idea is to secure the difference between surface hardness and internal hardness and generate compressive residual stress by specifying the proportions of C% nn5Cr, Mo, and the amount of old addition. It has become. However, if the carbon potential during carburizing or carbonitriding is increased to 1.0% or more, network-like pro-eutectoid cementite is generated on the surface, which significantly deteriorates the properties.
Therefore, there is a natural upper limit to the surface hardness (llv 800-900), and for use as a structural member, there is naturally a lower limit to the internal hardness (llv 200-400). Therefore, there is a limit to the improvement in surface compressive residual stress based on the idea of JP-A-56-119760.

(Problems to be Solved by the Invention) The purpose of the present invention is to solve the above-mentioned problems of the prior art. The purpose of the present invention is to provide case-hardened steel and mechanical structural parts made from it.

As already mentioned, it is a well-known fact that compressive residual stress in the surface layer improves pitting life and fatigue strength.
Furthermore, it is well known that retained austenite in the surface layer has the effect of improving pitting life, as shown in ``Practical Carburizing and Quenching'' by Takeshi Naio, published by Nikkan Kogyo Shimbun, pp. 164-169. (However, there is no clear theory as to the cause.) Therefore, a heat treatment method for adjusting the amount of retained austenite, that is, a so-called carbonitriding method in which the temperature is lowered at the end of carburization and N1 gas is added, as shown in the above-mentioned literature, has been proposed. However, this method N113 gas and Nl!
Since the cost of the three-gas addition device is high, not only is the cost significantly higher than in the case of carburizing alone, but it is also very difficult to adjust the amount of retained austenite produced.

Therefore, a specific object of the present invention is to provide a case hardening steel that stably produces sufficient residual austenite, and a mechanical structural part made from the case hardening steel.

(Means for Solving the Problems) Therefore, the present inventors conducted extensive research and found that the larger the amount of retained austenite in the surface layer (up to 0.1 mm below the surface), the greater the Due to the stress applied during the use of mechanical structural parts, the structurally unstable residual austenite is induced to transform into martensite, and this generates compressive residual stress, which improves pitting life and fatigue strength. Furthermore, in order to stably generate a sufficient amount of retained austenite, it is necessary to adjust the chemical composition of the steel, especially the amount of Mn, Nil. It has been found that adjusting the amount of Al is most effective.

That is, the present inventors have determined that the residual austenite (I) in the surface layer (0.111 m below the surface) is preferably 15% by volume or more and 70% by volume to ensure high pitting life and high fatigue strength.
For this purpose, the Mn amount is 2.0 to 5.0.
%, the amount of N is 0.008 to 0.025%. Al view is 0.
It has been found that it is necessary to make it less than 0.015%.

Therefore, as a result of further investigation, as mentioned above, Mn
Addition of 2.0% or more promotes the deterioration of machinability and the formation of an abnormal carburized layer, so it is also essential to add an element that improves machinability and reduce Si, which is an element that promotes an abnormal carburized layer. I found out.

Here, the gist of the present invention is in weight %, c: o, i ~ 0.5%, Si: more than 0.05% 0.15%
Below, Mn: 2.0 to 5.0%, Cr: 1.5%
Hereinafter, N: 0.008 to 0.025%, S: 0.30% or less, I'b: 0.35
% or less, Ca: 0.01% or less, and Te: 0.05
% or less, as well as M, -2, Nb: 0.01 to 0.14%, Mo: 0.05 to 0.
5%, and one or more of Ni:O, 1 to 2.0%, and limits sol and 8Q to less than 0.015%, and the remainder is F.
The present invention is a machine structural part obtained by carburizing and quenching a carburizing case hardening steel having a composition consisting of E and inevitable impurities, and a machine structural part manufactured from the same.

Here, the mechanical structural parts include power transmission shafts, gears, constant velocity joints, steering parts, and the like. Further, the carburizing and quenching operation itself is well known and is not limited to a specific operation.

(Function) Next, the reason why the compositions of the case-hardening steel for carburizing and the steel for machine structural parts, which are the objects of the present invention, are limited as described above will be described below.

In this specification, unless otherwise specified, "%"
” is “% by weight”.

C: C is a basic component to ensure strength as a structural component, and as a case hardening steel, the center hardness after carburizing and tempering must be at least HIIC25, and for this purpose, the amount of C is It is necessary to contain at least 0.001%.

On the other hand, if more than 0.5% is added, cold forgeability and machinability will be significantly deteriorated, so the lower limit is set at 0.1% and the upper limit is set at 0.5%.
It was set at 5%.

Si: St is usually added as a deoxidizing agent. However, Si
is an element that is very easily oxidized, and thus promotes internal oxidation of the surface layer during carburizing, and has a strong tendency to generate an incompletely quenched layer, that is, an abnormal carburized layer, on the surface. As is well known, extremely abnormal carburized layers (internal oxidation and incompletely quenched layers) reduce pitting life and fatigue strength.
In the steel of the present invention, it is necessary to control the amount of Si added, which is most harmful to the abnormal carburized layer. By the way, since Mn also has the strongest tendency to do so next to Si, as described above, in the case where the amount of Mn added is as large as 2.0% or more in the present invention, this tendency is exacerbated.

Therefore, in the present invention where Mn is 2.0% or more, in order to suppress the carburized abnormal layer to the extent that it does not affect the pitting life and fatigue strength, it is necessary to regulate Si to 0°15% or less. be. However, if the amount is less than 0.05%, not only will the effect not improve any further, but deoxidation will become insufficient and oxide inclusions will increase, which will actually reduce pitting life and fatigue strength. Upper limit 0.15
%, and the lower limit was set to exceed 0.05%. Preferably 0.06
~0.12%.

Mn is usually added to ensure hardenability, but in the present invention, it is also an extremely important element that promotes the formation of retained austenite in the surface layer after carburizing and quenching. . As already mentioned, retained austenite improves pitting life and fatigue strength, but to ensure the minimum amount of retained austenite required for this purpose, Mn should be reduced to 2°0.
It is necessary to add more than %. Additionally, if it is added in excess of 5.0%, the amount of retained austenite will increase more than necessary.
Not only does the pitting life and fatigue strength deteriorate due to the decrease in surface hardness, but the machinability also deteriorates drastically.
In the present invention, the lower limit of 1N addition is set to 2.0% and the upper limit is set to 5.0%.

Preferably it is 2.0 to 3.0%.

Cr: C not only improves hardenability, but also improves carburizability and is effective in improving wear resistance after carburizing. It is desirable to add However, if it is added in an amount exceeding 1.5%, there is a tendency for excessive carburization to occur, and a network of cementite is formed in the surface layer, which significantly reduces pitting life and fatigue strength, so the upper limit was set at 1.5%.

N: Similar to tan, solid solution N has a large effect of increasing retained austenite after carburizing and quenching, which is effective in improving pitting life and fatigue strength. You need to be gentle enough to enter. Furthermore, by combining with 8Q to form AQN, it has the effect of reducing IB>98Q (described later), which suppresses the formation of retained austenite.

Therefore, N is generally an element contained as an unavoidable impurity, but due to the above-mentioned effects, in the present invention, these effects are fully exhibited and the pitting life,
Nl required to improve fatigue strength is 0.008%
, preferably 0.010%, but if it is added in excess of 0.025%, retained austenite a becomes too large and the desired purpose cannot be achieved. More preferably, it is 0.013 to 0.020%.

Note that the typical N content of case hardened steel produced under today's standard operating conditions is 0.006-0.012%.

S SPb, Ca, Te These elements are all machinability improving elements and are effective in improving machinability when cutting before carburizing treatment.

According to the present invention, as already mentioned, the addition of Mn, which deteriorates machinability, is large, so it is essential to add these machinability-improving elements, but the amount and number of elements to be added depend on the machinability. It is preferable to decide accordingly. However, if the content exceeds 0.30% for S and 0.35% for Pb, the strength, toughness, and pitting life will be significantly reduced, and it is difficult to add Ca in excess of 0.01% due to steelmaking technology. In addition, when Te exceeds 0.05%, hot workability decreases rapidly, so S is used as an element to improve machinability.
The upper limit of each of SPb, Ca, Te is 0.30%
, 0.35%, 0.01%, and 0.05%.

sol. Contrary to Mn, Q tends to reduce the amount of residual austenite formed in the surface layer after carburizing and quenching, and therefore tends to reduce pitting life and fatigue strength. However, so
When the QI is less than 0.015%, this tendency becomes almost negligible.

Hard again? a AQ is more easily oxidized than Si and therefore promotes the formation of an abnormal carburized layer, but s
ol, 8 (When Hit is less than 0.015%, this tendency is negligible, so in the present invention, so
1 and 8Q were set to less than 0.015%. Preferably 0.0
It is 10% or less.

In the present invention, as a preferable embodiment of the target steel, it further contains at least [species] of Nb, Mo, and Ni, all of which improve the carburizability of the steel and improve the durability of the steel. This improves pitching properties and fatigue strength. Below, the reason and effect of each addition will be explained in detail.

Nb: As already mentioned, in the present invention, sol. Since Alffl is regulated to be lower than usual, the effect of suppressing coarsening of austenite grains during carburizing by MN is small. Therefore, depending on the carburizing temperature, for example, when processing at 930° C. or higher, it is necessary to suppress coarsening of austenite grains using NbN or NbC. For this,
If desired, at least 0.01% Nb is required,
Even if it is added in excess of 0.14%, the effect of suppressing austenite grain coarsening will be saturated, so the lower limit is set at 0.01% and the upper limit is set at 0.
゜14%.

hO: Mo is an optionally added element that improves the hardenability and carburizability of steel. Also. Al, Si, Mn, Cr
Unlike Fe, it is less likely to be oxidized than Fe, so it has the effect of suppressing the formation of an abnormal carburized layer. In order to exhibit these effects, at least 0.05% is required, but since it is an expensive element, considering that it is added in combination with other hardenability improving elements such as Mn and Cr, 0 as case hardening steel.
It is not a good idea to add more than 50%. Therefore, the lower limit was set to 0.05% and the upper limit was set to 0.50%.

Ni: Ni is also an optionally added element, and is an element that improves the hardenability and toughness of steel. Furthermore, like Mo, it suppresses the formation of an abnormal carburized layer. In order to fully exhibit these effects, it is necessary to add at least 0.1%, but since it is an expensive element and also has the effect of inhibiting carburization, it is not preferable to add more than 2.0%. Therefore, the lower limit is 0.1%
, the upper limit was set at 2.0%.

Next, in another aspect of the present invention, the case-hardened steel having the composition as described above is formed into a predetermined shape by suitable means such as forging, pressing, or cutting, and then carburized and quenched to produce a mechanical structural part. That is to say.

As mentioned above, although the steel according to the present invention has a composition that tends to generate retained austenite in the surface layer after quenching,
As a premise, in a preferred embodiment, C, which has a large influence on the formation of retained austenite, is in the surface layer from yO27 to
It needs to be 0.9%. Therefore, the present invention stipulates that carburizing and quenching be performed. In other words, the carburizing and quenching in the present invention reduces the c1 degree in the surface layer (surface area 0°1m below the surface) from 0.7 to 0.
．． The specific conditions are not particularly limited as long as it is set to 9%. Note that carburizing itself is well known. In addition, stress or plastic deformation is generally applied to the surface layer of mechanical structural parts during use, resulting in residual austenite being induced to transform into martensite, introducing compressive residual stress, and reducing pitting resistance. , improving fatigue strength.

Next, the present invention will be explained in more detail with reference to Examples.

Example 39 types of steel having the chemical composition shown in Table 1 were melted,
After forming the ingot, it was rolled into a bar with a diameter of 60 mm to prepare a test material. A plurality of smooth rotating bending fatigue test pieces (parallel part diameter: 10 mm) and roller type rolling fatigue test pieces (outer diameter: 30 mm) were prepared from these materials by machining.

Next, these test pieces were tested at 920°C x 4 hours on the same occasion.
After performing conventional gas carburizing (carbon potential 0°9%), it was quenched with hot quench oil at 150°C.

After carburizing and quenching, the Ono type rotary bending fatigue test was performed on the rotary bending fatigue test piece, and repeated stress was applied up to 10 times.
Create an S-N curve as shown in Figure 1, and create the fatigue limit (σ
, 4) was obtained. In addition, for the transfer fatigue test piece, a roller contact type transfer fatigue test (no lubrication, slip rate 20%, surface pressure 250 Jf/mn+") was conducted, and the test results were organized on Weibull probability paper as shown in Figure 2. Then, the 50% probability of failure (t, s.) was determined.

Note that both FIGS. 1 and 2 show the results for steel type 2.

Furthermore, 0.1 mm of the smooth portion of the rotary bending fatigue test piece before the test was removed from the surface by chemical polishing, and the amount of retained austenite (ra) was measured by X-ray diffraction. 71% of the tone measured in this way σ1. are also listed in Table 1.

In Table 1, steel grades 1 to 1lk122 are steels according to the present invention, steel grades 23 to 27 are steel grades 33 to 35 in terms of Si content, and steel grades 28 to 32 are steel grades 33 to 35 in terms of in content. is sol.

In terms of Q content, Steel Inserts 36-39 are comparative steels that are outside the scope of the present invention in terms of N content.

As is clear from the test results in Table 1, the steel of the present invention has a σ8 of 85 kgf/ in the rotating bending fatigue test.
mn+” or higher, and 50% in the transfer fatigue test.
Probability of damage. All of them were more than 26 x 10b times, which is superior to the comparative steel.

In addition, the amount of retained austenite γ8 is 18 to 66 in all cases.
%, and due to the effect of retained austenite, σ1. This suggests that the results have improved.

Furthermore, in order to clarify the influence of the amount of Si, we investigated steel types Nll~3.11k123~ with substantially the same amount of components other than the St group.
σ for 27. bird,. are shown in Figures 3 and 4.

In addition, in order to clarify the influence of -nl, σIIT for steel types 4 to 8 and 28 to 32, which have substantially the same content of components other than Mn, is taken. are shown in FIGS. 5 and 6, sol. Al
To clarify the effect of quantity, sol. σ0 and s for steel types Na16 to 18 and floors 33 to 35, which have substantially the same amount of components other than Al. In FIG. 7 and FIG. 8, in order to clarify the influence of Nl, σ8 is set for steel types 19 to 22 and holes 36 to 39, which have substantially the same amount of components other than N. The ninth
As shown in FIG.

From FIG. 3 to FIG. 10, it is clear that σHs is only within the scope of the present invention.
Lg. is clearly superior.

This shows that the present invention significantly improves fatigue strength.

[Brief explanation of drawings]

FIGS. 1 to 10 are graphs collectively showing data of the embodiments of the present invention.

Claims (4)

[Claims] (1) In weight%, C: 0.1 to 0.5%, Si: more than 0.05% 0.15%
Below, Mn: 2.0 to 5.0%, Cr: 1.5% or less,
Contains N: 0.008 to 0.025%, and further contains one or two of S: 0.30% or less, Pb: 0.35% or less, Ca: 0.01% or less, and Te: 0.05% or less. species, and sol. Limit Al to less than 0.015%, with the remainder F
A case hardening steel for carburizing having a composition consisting of E and incidental impurities. (2) In weight%, C: 0.1 to 0.5%, Si: more than 0.05% 0.15%
Below, Mn: 2.0 to 5.0%, Cr: 1.5% or less,
Contains N: 0.008 to 0.025%, and further contains one or two of S: 0.30% or less, Pb: 0.35% or less, Ca: 0.01% or less, and Te: 0.05% or less. species, and sol. Limit Al to less than 0.015%, with the remainder F
A machine structural part made of case hardened steel for carburizing, which is formed by forming a steel having a composition consisting of E and incidental impurities into a machine structural part of a predetermined shape, and then carburizing and quenching the steel. (3) In weight%, C: 0.1 to 0.5%, Si: more than 0.05% 0.15%
Below, Mn: 2.0 to 5.0%, Cr: 1.5% or less,
Contains N: 0.008 to 0.025%, and further contains one or two of S: 0.30% or less, Pb: 0.35% or less, Ca: 0.01% or less, and Te: 0.05% or less. species, and Nb: 0.01-0.14%, Mo: 0.05-0.5
0%, and one or more of Ni: 0.1 to 2.0%, and sol. Limit Al to less than 0.015%, with the remainder F
A case hardening steel for carburizing having a composition consisting of E and incidental impurities. (4) In weight%, C: 0.1 to 0.5%, Si: more than 0.05% 0.15%
Below, Mn: 2.0 to 5.0%, Cr: 1.5% or less,
Contains N: 0.008 to 0.025%, and further contains one or two of S: 0.30% or less, Pb: 0.35% or less, Ca: 0.01% or less, and Te: 0.05% or less. species, and Nb: 0.01-0.14%, Mo: 0.05-0.5
0%, and one or more of Ni: 0.1 to 2.0%, and sol. Limit Al to less than 0.015%, with the remainder F
A machine structural part made of case hardened steel for carburizing, which is formed by forming a steel having a composition consisting of E and incidental impurities into a machine structural part of a predetermined shape, and then carburizing and quenching the steel.