JPH02125842A - Cemented case hardening steel having excellent fatigue characteristics - Google Patents

Abstract

PURPOSE:To suppress the generation of an abnormal carburizing layer and to provide the title steel with high fatigue resistance by reducing the contents of Si, Mn and Cr in a case hardening steel and adding optimum amt. of Nb thereto. CONSTITUTION:The compsn. of a case hardening steel is formed with, by weight, 0.10 to 0.30% C, <=0.15% Si, <=0.60% Mn, <=0.90% Cr, 0.30 to 2.00% Mo, 0.010 to <=0.100% Nb and the balance Fe with inevitable impurities. If required, one or more kinds among <=1.00% Cu, <=3.50% Ni, 0.010 to 0.100% Al, 0.01 to 0.30% V, 0.010 to 0.100% Ti and 0.0003 to 0.0050% B are furthermore incorporated thereto. The reduction of Si, Mn and Cr is effective principally for preventing the oxidation of austenitic grain boundaries and Nb improves the quenchability of austenite in a carburizing layer to prevent the generation of an imperfect quenching layer. In this way, the case hardening steel having excellent fatigue characteristics can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to carburized case-hardened steel used for mechanical parts such as gears and shafts.

(Prior art) Machine parts that require high strength, such as gears and shafts used in automobiles, civil engineering and construction machinery, and various other machines, have traditionally been manufactured using case-hardened steel (defined as alloy steel for machine structures by JIS). 5Cr-420.SCM-420.SNCM-
420, etc.), which are machined into a predetermined shape, and then subjected to carburizing, quenching, and tempering.

However, when gears manufactured using the above-mentioned case-hardened steel are used in recent high-output engines, fatigue failure may occur due to tooth root fatigue, tooth surface fatigue, bending fatigue, etc. The development of hardened steel is strongly desired.

(Problems to be Solved by the Invention) The purpose of the present invention is to solve the above problems by strictly controlling the chemical components in steel, and to achieve high An object of the present invention is to provide case hardening steel having fatigue strength.

(Means for Solving the Problem) In the course of research aimed at improving the fatigue properties of case-hardened alloy steel, the present inventors conducted a detailed study on fatigue strength fracture cases of carburized gears, shafts, etc. As a result, the following findings were obtained. That is, (a) Fatigue fracture is caused by abnormal carburization layer (
This is caused by oxidation of austenite grain boundaries and incompletely quenched structure. The depth of this abnormal carburized layer needs to be suppressed to 3 μm or less.

(b) The abnormal carburized layer can be suppressed by (1) adjusting the Si, Mn, and Cr contents in the case hardening steel to within appropriate ranges, and (2) adding an appropriate amount of Nb.

(c) Reduction of Sr, Mn and Cr is mainly effective in preventing oxidation of austenite grain boundaries, and Nb improves the hardenability of austenite in the carburized layer and prevents the formation of an incompletely hardened layer. .

The present invention has been made based on the above findings, and its gist lies in the following carburized case hardening steel.

1% by weight, C: 0.10-0.30%, Si: 0.
15% or less, Mn: 0.60% or less, Cr: 0.90%
Below, Mo: 0.30-2.00%, Nb: 0.010
Carburized case-hardened steel with excellent fatigue properties characterized by containing ~0.100% and the balance consisting of Fe and unavoidable impurities, or in addition to the above components, Cu: 1.00%
% or less, Ni: 3.50% or less, A! :0.010~0
．． 100%, V: 0.01-0.30%, Ti: 0.
010-0.100%, B: 0.0003-0.00
50% of the carburized case-hardened steel of the present invention is the conventional JIS SCr42
0. Compared to SCM-420, SNC stand-420, etc., Si.

A major feature of this steel is that it contains a suitable amount of Nb in addition to keeping the contents of Mn and Cr low, and it can be further divided into the following two types of steel.

■ Si is 0.05% or less, Mn is 0.35% or less, C
Steel whose r is 0.10% or less.

■ Si exceeds 0.05 to 0.15%, Mn is 0
．． Steel having a Cr content of more than 35% to 0.60% and a Cr content of more than 0.10% to 0.90%.

In the above steel (2), Si, Mn, and Cr are kept particularly low, so that an abnormal carburized layer does not occur at all, and the fatigue strength and static bending strength are further improved.

The steel (2) has a relatively high content of Si, Mn, and Cr to improve hardenability and has a composition suitable for thick-walled or large-diameter parts.

In addition, both ■ and ■ are Cu, Ni, Aj2
, (2), one or more of Ti and B may be contained within the above range. In addition, it goes without saying that the content of each component can be selected within the range of the above-mentioned gist, apart from the above-mentioned categories (1) and (2).

(Function) The present invention reduces the abnormal carburization layer generated during carburization by strictly controlling the chemical composition of the steel, thereby realizing case hardening steel having excellent fatigue properties.

The reasons for limiting the chemical composition will be explained below.

C: C is an element necessary to impart a certain static strength to the steel, but on the other hand, it reduces toughness.

Particularly in case-hardened steel that undergoes carburizing treatment, a balance between static strength and toughness is required, and in order to obtain the minimum static strength, 0.
10% or more is required.

However, if it exceeds 0.30%, the toughness will drop sharply, so the upper limit is set at 0.30%.

Si: Si is an element necessary for deoxidizing steel and imparting a certain static strength. However, because Si has a strong bond with oxygen, case hardening steel that undergoes carburizing treatment
During the carburizing process, Si oxides are generated at grain boundaries, causing grain boundary embrittlement. For example, in gears, tooth root fatigue, tooth surface fatigue,
Decreases bending fatigue strength, etc. In particular, if it exceeds 0.15%, the formation of Si oxide is significant, and in order to improve the fatigue properties that are the objective of the present invention, in addition to limiting the Mn, Cr, and Nb contents described below, it is necessary to % or less is an essential requirement. Further, by keeping the Sl content particularly low, to 0.05% or less, the occurrence of abnormal carburized layers can be further reduced, and fatigue strength and static bending strength can be improved.

Mn: Like Si, Mn is an element necessary for deoxidizing steel, and is also effective in improving the hardenability of steel.

However, due to the strong bonding force with oxygen, Mn oxides are generated at grain boundaries during carburizing, especially when the concentration exceeds 0.60%.
Significant oxide formation. Therefore, in order to improve fatigue strength, which is the objective of the present invention, in addition to limiting the Si content described above and the Cr and Nb content described below, it is necessary to limit the Mn content to 0.60% or less. .

In addition, by keeping Mn particularly low to 0.35% or less, it is possible to further reduce the occurrence of abnormal carburized layers.
It is possible to improve fatigue strength and static bending strength.

Cr: Cr is an element effective in imparting hardenability to steel. Further, it is often added to case hardening steel to improve carburizability. However, like the aforementioned Si and Mn, it has a strong bonding force with oxygen, so it forms Cr oxides at the grain boundaries during carburizing, which embrittles the grain boundaries and reduces fatigue strength. In particular, when the content exceeds 0.90%, the formation of Cr oxide becomes significant. In order to improve the fatigue properties that are the object of this invention, in addition to limiting the content of Si, Mn, and Nb (described later), it is necessary to limit the content of C to 0.90% or less. By suppressing the amount of carbon to a particularly low value of 0.10% or less, the occurrence of abnormal carburized layers can be further reduced, and fatigue strength and static bending strength can be improved.

Mo: Mo is necessary to give steel a certain hardenability and improve static strength and toughness. In the present invention, under the above-mentioned limitations regarding the contents of St, Mn and Cr, Mo is contained in an amount of 0.30% or more in order to provide hardenability equivalent to or better than that of conventional steel. However, 2.
Even if the content exceeds 0.00%, the effect will be saturated and economic efficiency will be impaired, so the upper limit is set at 2.00%.

Nb: In the present invention, Nb is an important element for preventing the formation of an abnormal carburized layer, especially an incompletely hardened layer on the surface. In addition to limiting the SiXMn and Cr contents described above, by including Nb, the occurrence of an abnormal carburized layer can be effectively suppressed. In order to exhibit this effect, it is necessary to add 0.010% or more. However, if the content exceeds 0.100%, the crystal grains of the steel will become coarse, the toughness will deteriorate, and the machinability during machining will be impaired.

The case hardening steel according to the present invention includes the above-mentioned C, Si, Mn,
Although the main point is to contain predetermined amounts of Cr, Mo, and Nb, the effects of the layer can be exhibited by adding appropriate amounts of the following elements.

Cu: Cu is an effective element for increasing the hardenability and static strength of steel. In order to exhibit this effect, it may be added as appropriate, but if the content exceeds 1000%, hot workability will deteriorate. Also, the static strength decreases, so 1
．． The upper limit is 00%.

Ni: Like Cu, Ni is an element effective in imparting a certain hardenability to steel and increasing static strength. Further, since Ni improves the toughness of steel, it can be optionally added to ensure predetermined hardenability and toughness. Furthermore, Nl
can improve the hardenability of the carburized part within the above-mentioned limited ranges of C, Si, Mn, Cr, Mo, and Nb contents. Particularly in the case of large gears, it is possible to prevent the formation of incompletely hardened layers and improve fatigue properties. but,
Even if the content exceeds 3.50%, the effect will be saturated and economic efficiency will be impaired, so the upper limit is set at 3.50%.

(2): v is an element effective in precipitating carbonitrides in steel and increasing the high-temperature strength of steel. For example, when used as a gear in a high-output engine, in order to achieve its hot static strength, it is necessary to It is necessary to contain 1% or more of O. However, if the content exceeds 0.30%, hot workability deteriorates, so the upper limit is set at 0.30%.

Al: Al has the effect of refining high-temperature crystal grains and improving toughness. In order to exhibit its effect, 0.0
It is necessary to contain 10% or more.

However, if the content exceeds 0.100%, the cleanliness of the steel will deteriorate, impairing the machinability, and will also coarsen the crystal grains of the steel, reducing the toughness, so the upper limit is set at 0.100%.

Ti: Similar to Al, Ti refines the crystal grains of steel and improves the toughness of the steel. In order to fully exhibit its effect, it is necessary to contain it in an amount of 0.010% or more. On the other hand, if the content exceeds 0.100%, the cleanliness of the steel decreases, resulting in poor machinability, as well as coarsening the crystal grains of the steel, resulting in a decrease in toughness, so the upper limit is set at 0.100%.

BIB is an element effective in improving the hardenability of steel and increasing its static strength. In order to exhibit this effect, the content must be 0.0003% or more. However, if the content exceeds 0.0050%, it will coarsen the grains of the steel and reduce toughness.
is the upper limit.

(Example) In a vacuum melting furnace with a melting capacity of 150 kg, 1 in Table 1 (above
An ingot is obtained by melting a carburized case-hardened steel having the components shown in Table 1 (invention steel corresponding to (1) above), Table 1 2 (invention steel corresponding to (1) above), and Table 1 3 (comparative steel), This ingot was heated to 1250°C for 1 hour and forged to a diameter of 1100mm.
A forged and drawn material with a diameter of 30n+m was made.

A test piece was created from the above forged material, and using this test piece (
A) gear fatigue test, (b) rotating bending test, (C) static bending test, and (d) Charpy impact test were conducted to investigate the fatigue properties of the inventive steel and comparative steel.

(al gear fatigue test) A forged and drawn material with a diameter of 100 mm was heated to 925°C for 5 hours, then air cooled and normalized, and then machined to a pitch radius of 33 mm, number of teeth 33, and module 2. 0
．． A test spur gear 1 with a face width of 20 mm was created. This spur gear l has a carbon potential of 1.0. Carburizing temperature 925゛C1
After carburizing for 6 hours, quenching and tempering were performed.
Furthermore, shot peening treatment (shot particle size 0.6 mm)
, a projection speed of 47 m/s, and a projection time of 15+n1n).

The thus manufactured spur gear 1 was subjected to a gear fatigue test to examine tooth root fatigue strength and tooth surface fatigue using a power circulation gear fatigue tester. At the same time, the abnormal depth of carburization and the austenite grain size of the carburized area were measured.

In addition, tooth root fatigue strength is evaluated by the strength (fatigue limit) that does not cause fracture at 107 rotations, and tooth surface fatigue is evaluated at 107 rotations without fracture.
Evaluation was made based on the degree of damage to the tooth surface of the gear that did not break due to rotation.

The test results are shown in Table 2, 1 (inventive steel corresponding to item (■) above),
The results are shown in the gear fatigue test column of Table 2, 2 (inventive steel corresponding to item 1 above) and Table 2, 3 (comparative steel).

Looking at the tooth base fatigue strength, the present invention steel (1 to 320 m),
And steel in which Si, Mn, Cr, and Nb (hereinafter abbreviated as four elements) are within the scope of the present invention among comparative steels (
No, 38.39.44.45.46.47.48.4
No. 9 steel) has an improvement of about 30 to 40% compared to other steels. Among them, 5in (1, (15% or less, Mn: 0.35
% or less, Cr: steel suppressed to 0.10% or less (Nα1~1
In steel No. 6), no abnormal carburized layer was generated and the fatigue strength was improved by about 50%. Regarding tooth surface fatigue, the steel of the present invention and the comparative steel containing four elements within the range of the present invention had no damage at all.

In addition, as is clear from Table 2 1, Table 2 2, and Table 2 3, the abnormal immersion layer, which greatly affects tooth root fatigue strength and tooth surface fatigue, is was 3 μm or less for comparative steels within the range defined by the present invention.

In particular, Si: 0.05% or less, Mn: 0.35% or less,
Cr: steel suppressed to 0.10% or less (steel with Nα1 to 16)
No abnormal carburized layer occurred at all. In this way, in order to improve tooth root fatigue strength and tooth surface fatigue, Si
, Mn, Cr, and Nb must be contained within a predetermined range. In addition, Nα37 class in which Nb exceeds the upper limit of the present invention, and N in which Al is outside the range of the present invention even if the four elements are within the present invention range.

45, No., 46 steel, N with Ti exceeding the upper limit (148
In both steels with Nα49 and B outside the upper limit, the austenite A becomes coarse and the toughness decreases.

(b) Rotary bending test A forged and drawn material with a diameter of 30 mm was heated to 925°C for 1 hour and then normalized by air cooling to create a notched Ono type rotary bending fatigue strength test piece 2 as shown in Figure 2. . This test piece 2 was prepared under carburizing conditions at a carbon potential of 1.0. After carburizing at a carburizing temperature of 925°C and carburizing time of 6 hours, quenching and tempering were performed. After further shot peening treatment, the rotary bending fatigue strength was examined using an Ono type rotary bending fatigue tester. Rotational bending fatigue strength was evaluated based on the stress (fatigue limit) that does not cause breakage after 107 rotations.

The test results are shown in Table 2, 1 (inventive steel corresponding to item (■) above),
The results are shown in the rotary bending fatigue strength columns of Table 2, 2 (inventive steel corresponding to item 1 above) and Table 2, 3 (comparative steel).

As is clear from the diagram, the rotating bending fatigue strength of the steels of the present invention (No. 1 to 32) and comparative steels in which four elements are within the range defined by the present invention (Nα3B, 39, 44
, 4546.47.48.49) is found to be about 30 to 40% better than other comparative steels. Among them, steels with Si: 0.05% or less, Mn: 0.35% or less, Cr: O, 10% or less (steels with grades 1 to 16)
It is recognized that this is improved by 50 to 60%.

(C) Static bending test A forged and drawn material with a diameter of 30IIla+ was heated to 925°C for 1 hour and cooled in air to create a static bending test piece 3 as shown in FIG. This test piece was tested at carbon potentials of 1 and 0. After carburizing at a carburizing temperature of 925° C. and a carburizing time of 6 hours, quenching and tempering were performed. After further shot peening treatment, static bending strength was investigated at a strain rate of 10-''/s. This static bending strength was evaluated by crack initiation load.

The test results are shown in Table 2, 1 (inventive steel corresponding to item (■) above),
The results are shown in the static bending strength columns of Table 2, 2 (inventive steel corresponding to item 1 above) and Table 2, 3 (comparative steel).

As is clear from the diagram, Nα of the inventive steel and comparative steel
3B, 44, 45, 46, 47.4B, 49, 50.5
Steels outside the standard No. 1 have achieved the target value, but in particular, No. 1 with lower C than the lower limit, No. 38 steel, No. 1 with higher Cu,
The static bending strength of steel No. 44, steel No. 47.50, and steel No. 51 with an MO lower than the lower limit was considerably lower than the H straight 1900 kgf. In addition, Si: 0.05% or less, M
The static bending strength of steel with n: 0.35% or less and Cr: 0.10% or less (No. 1 to 16 steel) is 2000k.
It exceeds gf.

(d) Charpy impact test A forged material with a diameter of 30InI11 and a length of 2m was heated to 925°C.
After heating for a period of time, it was air cooled, normalized, and cut to a diameter of 25 mm.

After heating this at 925°C for 1 hour and water quenching,
Tempering treatment was performed at 70°C for 1 hour, and JIS No. 3 (2+
amU notch) was processed into a Charpy test piece. An impact test was conducted using this test piece at room temperature.

The test results are shown in Table 2, 1 (inventive steel corresponding to item (■) above),
They are shown in Table 2, 2 (inventive steel corresponding to item 1) and Table 2, 3 (comparative steel).

From these tables, even if the four elements are outside the range of the invention steel (steel No. 33-37.40-43, 50.51) and the four elements are within the range of the invention steel, Nα39 steel with a high C value, AI! is out of range
45 and Nα46 steel, No. 4 with Ti exceeding the upper limit
The impact values of steel No. 8 and steel No. 49, in which B was too high, were only 30 to 50% lower than the steel of the present invention.

(Hereinafter, blank space) (Effects of the invention) As described above, the case hardened gold steel of the present invention contains Si, Mn,
Cr content is lower than normal case hardening steel and appropriate amount of Nb
Because it contains high fatigue strength, it is a case hardening steel that is most suitable as a material for modern machine parts that require high fatigue strength.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a spur gear for testing, FIG. 2 is a diagram of an Ono type rotating bending fatigue strength test piece, and FIG. 3 is a diagram of a static bending test piece.

Claims (4)

[Claims] (1) In weight%, C: 0.10-0.30%, Si: 0
．． 15% or less, Mn: 0.60% or less, Cr: 0.90
% or less, Mo: 0.30-2.00%, Nb: 0.01
0 to 0.100%, and the balance consists of Fe and unavoidable impurities. Carburized case hardened steel with excellent fatigue properties. (2) In weight%, C: 0.10-0.30%, Si: 0
．． 15% or less, Mn: 0.60% or less, Cr: 0.90
% or less, Mo: 0.30-2.00%, Nb: 0.01
0 to 0.100%, further Cu: 1.00% or less, Ni: 3.50% or less, Al: 0.010 to 0.10
0%, V: 0.01~0.30%, Ti: 0.010~
0.100%, B: 0.0003-0.0050% 1
A carburized case-hardened steel with excellent fatigue properties characterized by containing at least 100% of Fe and the remainder consisting of Fe and unavoidable impurities. (3) In weight%, C: 0.10-0.30%, Si: 0
．． 05% or less, Mn: 0.35% or less, Cr: 0.10
% or less, Mo: 0.30-2.00%, Nb: 0.01
0 to 0.100%, with the balance consisting of Fe and unavoidable impurities, or in addition to the above components, Cu: 1
．． 00% or less, Ni: 3.50% or less, Al: 0.01
0-0.100%, V: 0.01-0.30%, Ti:
0.010-0.100%, B: 0.0003-0.0
A carburized case-hardened steel with excellent fatigue properties, characterized by containing at least one type of 0.050%, with the balance consisting of Fe and unavoidable impurities. (4) In weight%, C: 0.10-0.30%, Si: 0
．． Over 0.05% and up to 0.15%, Mn: over 0.35% and up to 0.60%, Cr: over 0.10% and up to 0.90%
up to, Mo: 0.30-2.00%, Nb: 0.010
~0.100%, with the balance consisting of Fe and unavoidable impurities, or in addition to the above components, Cu:1.
00% or less, Ni: 3.50% or less, Al: 0.010
~0.100%, V:0.01~0.30%, Ti:0
．． 010-0.100%, B: 0.0003-0.00
A carburized case-hardened steel with excellent fatigue properties, characterized by containing 50% of one or more elements, and the remainder consisting of Fe and unavoidable impurities.