JPH05156421A - Production of carburization hardened product having high fatigue strength - Google Patents

Abstract

PURPOSE:To produce the carburized product having the high resistance to the fatigue failure at an internal start point by subjecting a steel having a specific component compsn. to a carburization treatment, then hardening the steel to a specific structure and subjecting the steel to a shot peening treatment under specific conditions. CONSTITUTION:The steel materials which contain, by weight%, 0.1 to 0.4% C, <=0.3% Si, 0.02 to 0.08% Al and are incorporated with >=2 kinds of the elements selected from 0.3 to 3.1% Mn, 0 to 6% Ni, 0 to 1.2% Cr, 0 to 1.2% Mo so as to satisfy formula I are subjected to the carburization or carbonitriding treatment to satisfy formula II. In succession, the steel materials are hardened from an austenite single phase region, by which the steel products having 550 to 620 max. hardness HV of the hardened layer and maintaining the residual austenite area rate down to 300mum from the surface without falling down to <=20% are obtd. The steel materials are thereafter subjected to the shot peening treatment under the condition of >=0.6mmA arc height. As a result, the carburization hardened products having the excellent fatigue strength are produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a carburized and hardened product having excellent fatigue strength, and more particularly to automobiles, construction machinery,
The present invention relates to a method for manufacturing a carburized and hardened product capable of enhancing resistance to fatigue fracture originating from an internal point in manufacturing a carburized and hardened product used for various shafts.

[0002]

2. Description of the Related Art Among machine parts such as automobiles, construction machines, various shafts, etc., which are particularly strongly required to have fatigue resistance, surface hardening treatment by shot peening is often used after carburizing and quenching. Many. One of the main mechanisms for improving the fatigue strength by shot peening is the effect of applying surface compressive residual stress.

However, since ordinary carburized and hardened steel has a high surface hardness of about Hv800 by itself, the compressive residual stress imparting effect due to plastic deformation due to shot peening is limited to the vicinity of the outermost surface, In some cases, it did not show sufficient strength against fracture starting from inside the material. It was also found that a more effective compressive residual stress can be obtained by allowing a certain amount of residual austenite phase (residual γ phase) to exist in the carburized layer and subjecting this to work-induced transformation by shot peening. The method of increasing the carbon potential of time has been adopted. However, even in this method, since carbon and nitrogen have a large effect of solid-solution hardening the matrix, the residual γ
Despite the existence of phases, the matrix hardness before shot peening is as high as Hv 700, and the effect of shot peening still does not reach the inside of the material sufficiently, so the strength against fatigue fracture from the surface origin is improved, but the internal In some cases, it did not show sufficient strength for fatigue failure.

[0004]

The present invention has been made in view of such circumstances, and its purpose is to make the effect of applying surface compressive residual stress by shot peening reach deeper inside the material. This is to provide a method of manufacturing a material that is particularly resistant to fatigue fracture originating from the inside of the material.

[0005]

The method of the present invention which has achieved the above object is that C: 0.1 to 0.4% by weight and Si: 0.3% by weight.
% Or less, Al: 0.02 to 0.08%, and M
n: 0.3 to 3.1%, Ni: 0 to 6%, Cr: 0 to 1.2
%, Mo: 0 to 1.2%, two or more kinds selected from the group consisting of so as to satisfy the following formula (1), and the balance of steel and iron inevitable impurities satisfy the following formula (2). By carburizing or carbonitriding as described above, and then quenching from the austenite single-phase region, the maximum hardness of the carburized and hardened layer is Hv: 550 to 620 and 300 μ from the surface.
A steel material with a retained austenite area ratio of 20% or less up to the depth of m is obtained, and then the arc height: 0.
The point is that shot peening is performed under the condition of 6 mmA or more. 6.4% ≦ 2 [Mn] + [Ni] + [Cr] + [Mo] ≦ 8.2% (1) where [] indicates the weight% of each element present in the steel. 0.55% ≤ surface carbon content (wt%) + surface nitrogen content (wt%) ≤ 0.90% (2)

[0006]

The present inventors have studied from various angles how to effectively apply the compressive residual stress by shot peening to the deep inside of the carburized and quenched product. As a result, it was found that compressive residual stress is more likely to occur deeper inside when the amount of residual γ in the carburized layer is sufficiently increased to lower the maximum hardness of the carburized layer. It was also found that it is necessary that a sufficient amount of residual γ be present before the shot peening treatment at all depths at which residual stress is to be imparted. However, if the surface hardness is too low, the fatigue strength of the surface may decrease. However, regarding this point, the maximum hardness of the carburized and hardened layer is Hv: 5
If it is set within the range of 50 to 620, the hardness after shot peening is higher than that of ordinary carburized material in combination with martensite formation due to the work-induced transformation of residual γ.
It was confirmed by experiments that the fatigue strength was not significantly affected by the decrease to Hv100.

By the way, the conventional increase in carbon potential due to carburization and increase in residual γ due to carbonitriding relatively easily form a large amount of residual γ, but the decrease in hardness of the entire matrix becomes insufficient, Therefore, the compressive residual stress imparting effect is not sufficient. Therefore, the present inventors have examined the relationship between the solid solution element and the residual γ, and found that Ni, M
If the total content of n, Cr and Mo is kept within a predetermined range, a large amount of residual γ can be obtained, and the maximum value after carburization is obtained.
It was found that a very low hardness around Hv600 is obtained, and as a result, the compressive residual stress after shot peening is given relatively deep inside the material. This characteristic has resulted in a great improvement in the resistance to fatigue fracture, particularly from the inclusions inside the material as the starting point, thus completing the present invention. The reasons for limiting the chemical composition in the steel material used in the present invention are as follows.

C: 0.1-0.4% C is an essential element for imparting a predetermined core hardness to the carburized parts. For this purpose, it is necessary to add 0.1% or more, but if too much is added, the toughness and machinability will decrease, so it is not possible to add.
Must be 4% or less. Si: 0.3% or less Si is added for the purpose of deoxidizing during melting, but if added excessively, the grain boundary oxide layer becomes deep and the surface strength is lowered, so it should be 0.3% or less.

Al: 0.02 to 0.08% Al is an element that promotes deoxidation during melting and suppresses the growth of austenite crystal grains during carburizing and heating. If it is less than 0.02%, such an effect cannot be obtained. However, if added excessively, the machinability and toughness will be deteriorated, so the content should be 0.08% or less. Next, elements such as Mn, Ni, Cr, and Mo are effective for obtaining a large amount of residual γ as described above, and further, the effect of decreasing the matrix hardness due to the presence of residual γ is extremely large. It is an effective element for achieving the purpose. The inventors have conducted various studies and found that it is necessary for the amounts of these elements to satisfy the relationship represented by the following formula (1) in order to achieve the object of the present invention. Further, these elements are deeply involved in improving the hardenability of the material, and sufficient hardenability is secured within the range that satisfies the formula (1). 6.4% ≦ 2 [Mn] + [Ni] + [Cr] + [Mo] ≦ 8.2% (1) where [] indicates the weight% of each element present in the steel.
The limited range of each element is as follows.

Mn: 0.3 to 3% Mn is required to be 0.3% for deoxidation during melting, but if it is too much, it promotes the grain boundary oxide layer during carburization, so should be 3% or less. .. Ni: 6% or less With respect to Ni, the upper limit is 6% due to economical problems. Cr: 1.2% or less Cr promotes the abnormal carburization layer and promotes the formation of grain boundary carbides, so it is necessary to limit it to 1.2% or less.

Mo: 1.2% or less Since Mo forms carbide when too much, it should be limited to 1.2% or less. The steel material targeted by the present invention has the above elements as basic components and the balance iron and unavoidable impurities, but may contain Nb, V and the like if necessary. The contents when these elements are added are as follows. Nb: 0.005 to 0.2%, V: 0.03 to 0.8% Nb and V are both elements that form carbonitrides and refine the austenite crystal grains during carburizing and heating. To obtain this effect, Nb is 0.005% or more and V is 0.03%.
It is necessary to add more than this. However, for Nb
Even if it is added in an amount of more than 0.2%, the above effect is saturated, and if V is added in an amount of more than 0.8%, toughness and fatigue strength are lowered due to excess carbonitride.

Further, the steel materials to which the present invention is applied include P and S.
Etc. are inevitably contained, but the contents of these elements should be limited as follows. P: 0.03% or less P reduces the grain boundary strength, so it must be suppressed to 0.03% or less. S: 0.03% or less S is an element that improves machinability, but its content is 0.
If it exceeds 03%, the fatigue strength and toughness will decrease, so 0.
The upper limit was 03%.

It is an object of the present invention to improve residual stresses by shot peening, especially inside the material (about 200-400 μm), and increase resistance to fatigue fracture from that depth. Therefore, in the present invention, the maximum hardness of the carburized quench hardening layer is Hv550 ~ 620, 300 from the surface
It is extremely necessary to satisfy the two requirements that the residual γ does not become 20.0% or less anywhere within the depth range of μm. The quantitative meaning of these requirements is
When carburized steel with the maximum hardness specified in 1 above is shot peened under 0.6 mmA or more, the critical depth at which residual stress is effectively applied is 300 regardless of the size of the material.
This is based on the experimental fact that the compressive residual stress due to shot peening can be effectively obtained only when the residual γ is about 20% or more at this depth. The following conditions are necessary to satisfy these two requirements, which will be described in more detail below.

In the present invention, it is necessary to satisfy the following expression (2). 0.55% ≤ surface carbon amount (wt%) + surface nitrogen amount (wt%) ≤ 0.90% (2) The lower the surface carbon amount (hereinafter abbreviated as CP) and the surface nitrogen amount (hereinafter abbreviated as NP), Since the maximum hardness is low, it is easy to satisfy the requirements regarding the maximum hardness. However, if (CP + N.P.) is set too low, the residual γ area ratio will be too low, and it will be difficult to obtain the effect of compressive residual stress, and the increase in hardness due to shot peening will also decrease. Fatigue strength cannot be improved. From this point of view, in the present invention, the lower limit of CP + N.P. Is set to 0.55%.

On the other hand, from the viewpoint of increasing the residual γ (C.
When P. + N.P.) Is excessively high, it becomes difficult to obtain a desired hardness due to solid solution strengthening of the matrix with carbon or nitrogen even if a large amount of residual γ is obtained (see below). In addition, (C + N.P.) Must be suppressed to 0.9% or less because there is a possibility that grain boundary carbon (nitride) precipitates in the carburized layer during the carburizing process to reduce the fatigue strength.

In practicing the present invention, the material interior 300
The carburizing time needs to be adjusted appropriately in order to maintain the residual γ amount in the range up to the μm depth of more than 20%. If the carburizing time is too short, the penetration depth of carbon atoms will be shallow, so even if the desired properties are obtained on the outermost surface, 300 μm
There is a possibility that residual γ will be insufficient inside the m depth and that residual stress will be insufficiently applied to that portion. Therefore, considering this, an appropriate time may be set in consideration of the chemical composition of the sample, the carburizing temperature, and the like.

Regarding the shot peening treatment in the present invention, the shot peening treatment with an arc height of less than 0.6 mmA is insufficient to provide the required residual stress up to a depth of 300 μm, so the shot peening treatment should be performed under a condition of 0.6 mmA or more. is necessary.

The present invention will be described in more detail with reference to the following examples, but the following examples are not intended to limit the present invention, and any change in the design of the present invention can be made without departing from the spirit of the preceding and following paragraphs. It is included in the technical scope.

[0019]

[Examples] Table 1 shows the chemical composition of the test materials. After these test materials were forged, they were normalized and processed into test pieces.

[0020]

[Table 1]

Next, the carburizing treatment was performed under the conditions A to E shown in FIG. 1 and Table 2, or the carbonitriding treatment was performed under the condition shown in FIG. 2 (condition F). Then, shot peening processing was performed by the arc height of three conditions of ac shown in Table 3. The carburizing treatment under the conditions of C and D shown in Table 2, the carbonitriding treatment under the condition of F shown in FIG. 2 and the condition of c shown in Table 3 are outside the range limited by the present invention. ..

[0022]

[Table 2]

[0023]

[Table 3]

Each of the test steels shown in Table 1 was treated under different carburizing conditions and peening conditions, and then fatigue tests were conducted.

Table 4 shows the maximum Vickers hardness of each sample,
Residual γ amount at 30 μm depth from the outermost surface, 30 from the outermost surface
Residual γ amount at 0 μm depth (above values before shot peening), compressive residual stress value at 30 μm depth from the outermost surface, compressive residual stress value at 300 μm depth from the outermost surface,
The fatigue limit values (above values after shot peening) are shown. The fatigue test was carried out by an Ono-type rotary bending fatigue tester. In particular, in order to investigate the fatigue strength when cracks are generated from the inside, a rotating bending fatigue test piece with a gentle notch with a stress concentration factor of 1.10. Further, the residual stress value and the residual γ amount were measured by using a minute portion X-ray diffractometer after removing a necessary amount of the surface layer by a chemical polishing method.

[0026]

[Table 4]

The upper row of Table 4 is an example satisfying all the requirements specified in the present invention.
Relatively large compressive residual stress values were obtained inside the 300 μm region, indicating a good fatigue limit. On the other hand, all the lower rows are comparative examples having some out-of-claim conditions. From these results, it can be considered as follows.

(1) C treatment (CP = 0.45%) or E treatment (short-time carburization) was performed even for No. 1 which had good fatigue strength if proper carburizing and shot peening conditions were selected. When applied, the maximum hardness is within the range specified in the present invention as shown in Table 4, but the amount of residual γ within 300 μm is much lower than the range of the present invention, and as a result, sufficient compression is achieved inside. Residual stress is not obtained and fatigue strength is relatively low. (2) Even if the No. 1 steel is carburized under condition A, if it is C treated with low shot peening strength,
The residual stress imparting effect does not reach the inside, resulting in low fatigue strength. (3) No. 8 is a general-purpose carburizing steel, and No. 9 is an improved steel in which Mo is added to No. 8 to increase residual γ.
The value of [Mn] + [Ni] + [Cr] + [Mo] is 3.40, which is much smaller than the range of the present invention. Comparing No. 8 and No. 9 with the A carburizing treatment, No. 9 is superior in residual stress and fatigue strength, but is significantly inferior to the treated material of the present invention.
When the No. 9 steel is subjected to D treatment (increasing carbon potential) or F treatment (carburizing and nitrifying), the maximum hardness is higher than the range specified by the present invention, although the residual γ increases as much as the steel of the present invention. Since it is much higher, the compressive residual stress at the depth of 300 μm increases more than that of the A treatment, but is insufficient, and the fatigue limit does not improve significantly.

(4) No. 10 contains a larger amount of residual γ-stabilizing element than No. 9, but 2 [Mn] + [Ni]
Since the value of + [Cr] + [Mo] is still less than 6.4% and the residual γ amount is still insufficient, the maximum hardness is also higher than the range specified in the present invention, and as a result, the internal residual stress is increased. It becomes low and the fatigue limit is low. (5) No. 11 is 2 [Mn] + [Ni] + [Cr] +
The value of [Mo] greatly exceeds the range of the present invention, and the maximum hardness is remarkably reduced due to a large amount of residual γ. Therefore, although the residual stress value is high, the fatigue strength is greatly reduced. There is. (6) In No. 12 to 14, one of the chemical compositions is out of the range specified in the present invention, and the maximum hardness of all of them is within the range of Hv550 to 620. Although the residual stress has a good value, the fatigue strength is deteriorated due to factors other than the residual stress, and the fatigue strength is low.

[0030]

EFFECTS OF THE INVENTION The present invention is configured as described above, and in particular, it has become possible to manufacture a carburized product having a strong resistance to fatigue fracture originating from the inside.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an example of a carburizing process pattern.

FIG. 2 is an explanatory diagram showing an example of a carbonitriding treatment pattern.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location // C22C 38/00 301 A 7217-4K 38/06 38/58

Claims (2)

[Claims] 1. C: 0.1-0.4% by weight%, Si: 0.3
% Or less, Al: 0.02 to 0.08%, and M
n: 0.3 to 3.1%, Ni: 0 to 6%, Cr: 0 to 1.2
%, Mo: 0 to 1.2%, containing two or more elements selected from the group consisting of the following formula (1) and the balance iron and unavoidable impurities. By subjecting to a satisfactory carburizing or carbonitriding treatment and then quenching from the austenite single-phase region, the maximum hardness of the carburized and hardened layer is Hv: 550 to 620, and 3 from the surface.
The area ratio of retained austenite up to the depth of 00 μm is 2
A method for producing a high fatigue strength carburized and quenched product, characterized by obtaining a steel material which does not fall below 0%, and then subjecting it to shot peening under the conditions of arc height: 0.6 mmA or more. 6.4% ≦ 2 [Mn] + [Ni] + [Cr] + [Mo] ≦ 8.2% (1) where [] indicates the weight% of each element present in the steel. 0.55% ≤ surface carbon content (wt%) + surface nitrogen content (wt%) ≤ 0.90% (2) 2. The method according to claim 1, further comprising N
b: 0.005 to 0.2% and V: 0.03 to 0.8%. A method for producing a high fatigue strength carburized and quenched product using a steel material containing one or two selected from the group consisting of 0.03 to 0.8%.