JPH06299287A - Shot peening treatment type high fatigue strength case hardening steel - Google Patents

Abstract

PURPOSE:To develop case hardening steel excellent in the effect of shot peening treatment and high in strength or the like by limiting the size and number of sulfide inclusions in carbon steel having a specified compsn. to the specified value or below. CONSTITUTION:Steel having a compsn. contg., by weight, 0.1 to 0.3% C, 0.05 to 0.5% Si, 0.4 to 1.5% Mn, <0.02% P, <0.02% S, 0.2 to 1.5% Cr, 0.015 to 0.05% sol.Al, 0.005 to 0.02% total N2 content and <0.0010% total o, content, in which the amounts of impurities of Ti, Nb, Zr or the like intruded from the outer part are all regulated to <=0.005%, and contg., as necessary, one or >=two kinds among 0.1 to 0.5% Cu, 0.2 to 2.5% Ni and 0.1 to 0.5% Mo is used. The number of metal sulfides having 100mum<2> size contained per mm<2> of the area to be examined is regulated to <=5 pieces, and shot peening treatment is executed to produce the case hardening steel improved in fatigue strength.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a shot peening type high fatigue strength case hardening steel used for various machine structural parts such as gears.

[0002]

2. Description of the Related Art It has been conventionally known that reduction of inclusions is effective for improving fatigue strength in case hardening steel for machine structures such as gears. For example, Japanese Patent Laid-Open No. 2-2
No. 70935 discloses that shot peening is performed by limiting the oxygen content to 0.0010% or less and reducing the number of oxide inclusions and nitride inclusions having a diameter of 20 μm or more in steel to 14 or less per 1 g of steel. It is disclosed that when applied, the rotary bending fatigue strength is improved.

Further, in JP-A-59-74262, JP-A-63-60257, and JP-A-63-118052, sulfur is contained from the viewpoint of improving fatigue strength by reducing sulfide inclusions. It is disclosed to limit the amount to 0.005-0.020% or less.

Further, in JP-A-1-306545, from the viewpoint of improving the fatigue strength by controlling the shape of sulfide inclusions, the sulfur content is limited to 0.030% or less, and the sulfide inclusions are limited. It is disclosed to limit the aspect ratio to 5 or less.

[0005]

Among the above-mentioned conventional techniques, Japanese Patent Laid-Open No. 2-270935 discloses an improvement in fatigue strength, but it is still insufficient. In addition, JP-A-59-
Although the reduction of the sulfur content shown in Japanese Patent Publication No. 74262, etc. is certainly effective in improving the fatigue strength, it also has the effect of reducing the machinability of steel, so that it is industrially new. There are drawbacks such as causing problems. Further, the limitation of the aspect ratio of the sulfide-based inclusions disclosed in JP-A-1-306545 is that the sulfide-based inclusions elongated in the form of a string in the steel lower the lateral fatigue strength. It is effective, and therefore it is certainly effective for improving the fatigue strength of spur gears, etc., where the fatigue strength can be evaluated almost unambiguously only with the lateral fatigue strength. When the lateral fatigue strength alone cannot be evaluated, the effect is not always clear.

The present invention has been made in view of the above circumstances, and is a shot peening type high fatigue strength case hardening steel capable of obtaining sufficient fatigue strength when applied to various mechanical structural parts such as gears. The purpose is to provide.

[0007]

Means and Action for Solving the Problems
% By weight, C: 0.1 to 0.3%, Si: 0.05
~ 0.5%, Mn: 0.4 to 1.5%, P: 0.02%
Hereinafter, S: 0.02% or less, Cr: 0.2 to 1.5%,
Sol. Al: 0.015 to 0.05%, Total
N: 0.005-0.020%, Total O: 0.
Containing less than 0010% and containing 0.005% or less of Ti, Nb, and Zr, and the balance Fe and unavoidable impurities, and sulfide inclusions having an area of 100 μm ² or more are 5 per 1 mm ^{2 of the} detected area. Provided is a shot-peening-processed high fatigue strength case-hardening steel characterized in that the number is less than or equal to the number of pieces.

Secondly, in% by weight, in% by weight, C:
0.1-0.3%, Si: 0.05-0.5%, Mn:
0.4-1.5%, P: 0.02% or less, S: 0.02
% Or less, Cr: 0.2 to 1.5%, Sol. Al: 0.
015-0.05%, Total N: 0.005-
0.020%, Total O: 0.0010% or less, and all of Ti, Nb, and Zr mixed are 0.00
5% or less, Cu: 0.1-0.5%, Ni:
0.2-2.5%, Mo: 0.1-0.5%, one or more of them are contained, the balance Fe and unavoidable impurities, and an area of 100 μm ² or more of sulfide-based inclusions Provided is a shot-peening-processed high fatigue strength case-hardening steel which is characterized in that the number of test pieces is 5 or less per 1 mm ^{2 of the} test area.

The inventors of the present application have conducted research to obtain a shot-peening-processed case hardening steel having high fatigue strength and sufficient characteristics as various mechanical structural parts such as gears.
It was found that the composition of steel should be specified in a specific range and the number of sulfide inclusions having a certain size or more should be strictly limited. The present invention having the above configuration is based on such knowledge.

The present invention will be specifically described below.

First, the reasons for limiting the component composition will be described. In addition, hereinafter, "%" means "% by weight". (1) About C C is an element necessary to secure the hardness of the core part by carburizing and quenching, and in order to obtain HRC ₂₅ or more necessary to secure the fatigue strength required for gears and shafts. Must be added at least 0.1% or more. However, addition of a large amount impairs impact properties and machinability after carburization, so the upper limit must be set to 0.3%. Therefore, the C content is specified in the range of 0.1 to 0.3%. (2) About Si Si is an element necessary to secure the deoxidizing effect. However, if it is less than 0.05%, this effect cannot be secured. On the other hand, if it exceeds 0.5%, the occurrence of an abnormal carburization layer is promoted during carburization. Therefore, the Si content should be 0.05
To 0.5%. (3) Mn Mn is an element added to improve the deoxidizing effect, the desulfurizing effect, and the hardenability. However, if it is less than 0.4%, these effects cannot be secured. On the other hand, if it exceeds 1.5%, the impact properties after carburization and the workability of steel are impaired. Therefore,
The Mn content is specified in the range of 0.4 to 1.5%. (4) About P P is a typical element contained in steel, but it is an element that lowers the grain boundary strength and lowers the fatigue strength of steel, and its effect is remarkable when it exceeds 0.02%. Therefore, the P content is limited to 0.02% or less. (5) About S Although S is also a typical element contained in steel, it produces sulfide-based inclusions to lower the fatigue strength, and its effect is 0.
It is remarkable when it exceeds 02%. Therefore, the S content is 0.02
% Or less. (6) About Cr Cr is necessary to improve the hardenability and the strength after quenching and tempering, and is 0.2% for improving the hardness of the carburized layer of the carburized parts and the effective carburized depth. It is necessary to add above. However, if it exceeds 1.5%, excessive carburization is caused, which deteriorates hardenability and machinability. Therefore, the Cr content is specified in the range of 0.2 to 1.5%. (7) Sol. Al (Acid-Soluble Al) Al acts as a deoxidizer during melting of steel and combines with N during carburization to form AlN, which suppresses the growth of crystal grains. However, if it is less than 0.015%, these effects cannot be secured. On the other hand, if it exceeds 0.05%, a large amount of oxide-based inclusions are generated and the fatigue strength is deteriorated. Therefore, Sol. The range of Al content is 0.015 to 0.05
%. (8) About Total N N has an effect of preventing crystal grain coarsening during carburization by forming AlN by combining with Al. However, if it is less than 0.005%, this effect is small, and if it exceeds 0.020%, the toughness is impaired. Therefore, the content of Total N is 0.005
To 0.020%. In addition, Total
N includes all solid solution N and N in the compound. (9) About Total O O forms oxide inclusions and adversely affects the fatigue strength of gears and the like, and as will be apparent from Examples described later,
There is a tendency for the size of sulfide-based inclusions to become coarse, and the effect becomes significant when the content exceeds 0.0010%. Therefore, T
The total O content is limited to 0.0010% or less.
Note that Total O includes both solid solution O and O in the compound. (10) Ti, Nb and Zr Ti combines with N in steel to form TiN. This T
iN adversely affects the fatigue strength as well as oxide inclusions. Further, Nb and Zr may also generate high hardness inclusions. And, such an influence is 0.
It becomes obvious when it exceeds 005%. Therefore, Ti, Nb,
All of Zr are limited to 0.005% or less.

In addition to these components, one or more of Cu, Ni and Mo may be contained within the following range. From the viewpoint of further improving the properties of steel, it is desirable to add one or more of these. (11) Cu Cu is an element effective in preventing the formation of the abnormal carburization layer.
However, if less than 0.1%, this effect is small, and
If it exceeds 0.5%, the carburizing property decreases. Therefore, when Cu is added, its content is specified in the range of 0.1 to 0.5%. (12) Ni Ni is an element that improves the hardenability and carburizing property of steel, and also has the action of suppressing the occurrence of a carburized abnormal layer. In order to exert these effects, it is necessary to add at least 0.2%. Further, since Ni is an expensive element, if it exceeds 2.5%, the cost will increase and it is not a good idea. Therefore, Ni
When added, the content is specified within the range of 0.2 to 2.5%. (13) About Mo Like Mo, Mo is an element that improves the hardenability and carburizing property of steel, and also has the action of suppressing the occurrence of an abnormal carburizing layer. In order to exert these effects, at least 0.1
% Addition is required. Moreover, since Mo is also an expensive element, if it exceeds 0.5%, the cost will increase and it is not a good idea. Therefore, when Mo is added, its content should be 0.
It is specified in the range of 1 to 0.5%.

Next, sulfide inclusions in steel will be described. In the present invention, regarding sulfide inclusions in steel, attention is paid only to sulfide inclusions having an area of 100 μm ² or more. This is because sulfide-based inclusions of this size or larger have a substantial adverse effect on fatigue strength. The reason why the number of existing sulfide inclusions is limited to 5 or less per 1 mm ^{2 of the} test area is that the number of sulfide-based inclusions exceeds 5 per 1 mm ² of the test area, as will be apparent from the examples described later. This is because the fatigue strength of gears and the like decreases, and further, by limiting the number in this way, an effect on rolling contact fatigue characteristics and pitting resistance characteristics can be expected.

[0014]

EXAMPLES 4 to A to D having the chemical components shown in Table 1
After casting various types of steel and making ingots, test gears were prepared. In Table 1, A and C are steels of the present invention, and B and C are comparative steels out of the composition range of the present invention. Table 1 shows the chemical composition values of these steels in% by weight.

[0015]

[Table 1] Next, the present invention steel A (O: 0.
0009%) and comparative steel B (O: 0.0)
For the test gears (020%), shot peening was performed under the conditions of two types (arc height values 0.45A and 0.30C) shown in Table 2 and a gear fatigue test was performed.
The gear fatigue test was carried out on a gear having 28 teeth and a module 2.5 using a power circulation type gear fatigue tester.

[0016]

[Table 2] As a result of investigating the effect of oxygen reduction on the fatigue life, the results shown in FIG. 1 were obtained. FIG. 1 is a graph showing the fatigue test results as a relationship between the load torque (kgf · m) and the fatigue life (cycle).

As shown in FIG. 1, when the shot peening was not performed, the fatigue limits of the invention steel A and the comparative steel B were 22.5 kgf · m and 22.0 kgf · m, respectively. The strength was strong, and even if the Total O content was reduced from 0.0020% (Steel B) to 0.0009% (Steel A), the effect was hardly recognized.

Further, among the shot peening conditions,
When shot peening with an arc height value of 0.45 A is performed, the fatigue limit of the invention steel A is 32.5 kgf · m, whereas the comparative steel B is 31.0 kgf · m, and the fatigue limit of the steel A is slightly higher. However, the improvement in fatigue strength due to the low oxygen content is only about 5%.

In the shot peening conditions, when the strong shot peening with the arc height value of 0.30 C was applied, the fatigue limit was 38.5 kgf · m for the invention steel A and 33.0 kgf · m for the comparative steel B. It was confirmed that the fatigue strength was remarkably higher than the above, and that the fatigue strength was improved by about 17% due to the low oxygen content. .

As a result of examining the fatigue fracture form of the test gear, fatigue cracks are generated from the surface when shot peening is not applied, whereas fracture is generated from the inside when strong shot peening is applied. It was confirmed that

Further, as a result of a detailed examination of the fracture surface of the gear which was destroyed from the inside by applying strong shot peening with a scanning electron microscope, the fracture surfaces of the invention steel A and the comparative steel B are shown in FIGS. 2 and 3, respectively. It came to be shown in. In each figure, (a) is 150 times, and (b) is 500 times. From these figures, coarse inclusions did not exist at the fracture starting point of low oxygen steel A, but coarse sulfide inclusions were observed at the fracture starting point of steel B.

Next, the sulfide inclusions of the test pieces of the steels A to D shown in Table 1 were observed, and image processing was performed on these to measure the size distribution of the sulfide inclusions. Table 3 shows the number of sulfide-based inclusions having an area of 100 μm ² or more existing in steel per 1 mm ^{2 of the} tested area. The fatigue limit (kgf ・
Table 3 also shows m).

[0023]

[Table 3] Further, FIG. 4 shows the relationship between the number of sulfide-based inclusions having an area of 100 μm ² or more in the steel per 1 mm ^{2 of the} detected area and the Total O content in the steel.

As is clear from Table 3 and FIG. 4, the lower the Total O content in the steel, the area in the steel 100 μ
There is a tendency that the number of sulfide-based inclusions having a size of m ² or more per 1 mm ^{2 of the} tested area is small, and it is effective to reduce the oxygen content in steel in order to reduce coarse sulfide-based inclusions. I know there is.

This is considered to be due to the following reasons. In general, the lower the oxygen concentration in steel, the more finely divided oxide-based inclusions that remain in the steel. Since the sulfide-based inclusions in the steel form with oxides as nuclei, the smaller the oxygen concentration of the steel, the greater the amount of fine sulfides. As a result, in steels having the same S concentration in steel and different oxygen concentrations, the lower the oxygen concentration in the steel, the smaller the oxide and sulfide inclusions become than the steel having the higher oxygen concentration in the steel. There is a tendency. Therefore, if the oxygen concentration in the steel is low, coarse sulfide-based inclusions are reduced.

Next, the fatigue limit (kgf · m) in a gear fatigue test when shot peening is not applied and when strong shot peening (0.30 C) is applied, and a sulfide system having an area of 100 μm ² or more in steel Area to be inspected for inclusion 1
The relationship with the number per mm ² was grasped. The relationship is shown in FIG.

From FIG. 5, the area in steel is 100 μm ^2.
When the number of sulfide-based inclusions per 1 mm ^{2 of the} tested area is reduced, the fatigue limit is almost unchanged when shot peening is not performed, but the arc height of 0.
It was confirmed that the fatigue limit was remarkably improved when shot peening was performed at 30C.

As is clear from Table 3, FIG. 4 and FIG. 5, if the number of coarse sulfide inclusions having an area of 100 μm ² or more in the steel is 5 or less per 1 mm ^{2 of the} test area, the arc height Fatigue strength is remarkably improved when shot peening is performed at 0.30C.

As is clear from the inclusion photograph of FIG. 3, the size of the sulfide-based inclusion, which is the starting point of fisheye fracture, is much larger than the area of 100 μm ² , and therefore the fatigue strength and It can be said that it is preferable to define the number of inclusions having a larger size as a condition closely related to each other. However, for that purpose, a wider test area is required, which may substantially make the inspection by image processing difficult. Therefore, in the present invention, as a range in which the fatigue strength can be improved to a practical level, the number of inclusions having an area of 100 μm ² or more is 5 or less per 1 mm ^{2 of the} tested area.

[0030]

As described above, according to the present invention, the chemical composition and composition of steel are defined, and the size and number of sulfide-based inclusions in the steel are limited to improve the effect of shot peening. Provided is a shot peening type high fatigue strength case hardening steel which can be remarkably increased and which is inexpensive and can obtain sufficient fatigue strength when applied to various machine structural parts such as gears.

[Brief description of drawings]

FIG. 1 is a diagram showing the results of gear fatigue tests of the present invention steel and comparative steel.

FIG. 2 is a metallographic photograph showing a result of observing a fracture starting point of a gear of the invention steel, which is subject to fisheye fracture, with a scanning electron microscope.

FIG. 3 is a metallographic photograph showing a result of observing a fracture starting point of a gear in which the fish eye of the comparative steel is fractured by a scanning electron microscope.

FIG. 4 is a diagram showing the relationship between the oxygen content in steel and the number of sulfide-based inclusions having an area of 100 μm ² or more per 1 mm ^{2 of the} tested area.

FIG. 5 is a diagram showing the relationship between the number of sulfide-based inclusions having an area of 100 μm ² or more per 1 mm ^{2 of the} tested area and the fatigue limit.

Claims (2)

[Claims] 1. By weight%, C: 0.1 to 0.3%, S
i: 0.05 to 0.5%, Mn: 0.4 to 1.5%,
P: 0.02% or less, S: 0.02% or less, Cr: 0.
2 to 1.5%, Sol. Al: 0.015 to 0.05
%, Total N: 0.005-0.020%, To
tal O: T containing 0.0010% or less and mixed in
All of i, Nb, and Zr are 0.005% or less, the balance is Fe and unavoidable impurities, and the number of sulfide inclusions having an area of 100 μm ² or more is 5 or less per 1 mm ^{2 of the} test area. Shot peening type fatigue hardening case hardening steel. 2. C: 0.1-0.3% by weight, S
i: 0.05 to 0.5%, Mn: 0.4 to 1.5%,
P: 0.02% or less, S: 0.02% or less, Cr: 0.
2 to 1.5%, Sol. Al: 0.015 to 0.05
%, Total N: 0.005-0.020%, To
tal O: T containing 0.0010% or less and mixed in
i, Nb, and Zr are all set to 0.005% or less, and Cu: 0.1 to 0.5%, Ni: 0.2 to 2.5%,
Mo: One or more of 0.1 to 0.5% is contained, the balance is Fe and unavoidable impurities, and the area is 100.
Shot peening type high fatigue strength case hardening steel, characterized in that the number of sulfide-based inclusions having a size of μm ² or more is 5 or less per 1 mm ^{2 of the} test area.