JPH0525586A - Carburizing steel excellent in fatigue characteristic - Google Patents

Abstract

PURPOSE:To prevent the stretching of MnS at the time of rolling and forging, to prevent the formation of intergranular oxide layer and black quenched structure in the surface layer, and to obtain a carburizing steel as a gear blank improved in degree of dedendom fatigue and pitting resistance by regulating the components of a carbon steel to respectively specified values. CONSTITUTION:The steel is a carbon steel having a composition consisting of, by weight, 0.1-<0.3% C, <0.015% Si, 0.30-<1.50% Mn, 0.30-<1.50% Cr, 0.010-<0.030% S, either or both of 0.30-1.00% Mo and <=1.0% Ni, one or more kinds among 0.05-0.30% V, 0.02-0.20% Nb, and 0.01-0.20% Ti, further 0.0010-0.010& Ca and <0.0030% O (where 0.5<=Ca/0<3.0), and the balance Fe with impurities.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gear steel used for carburizing and quenching, and in particular, a carburizing steel for machine structures having high root fatigue strength and high pitting resistance applicable to drive transmission parts such as automobiles. It is about.

[0002]

2. Description of the Related Art Conventionally, mechanical structural parts have been extensively subjected to a surface hardening treatment to improve their bending fatigue strength, surface fatigue strength and wear resistance, and in particular carburizing and quenching, which is highly effective. . However, conventional carburizing steel (for example, JIS-
When SCr420, SCM420, SNCM420) is carburized and quenched, an intergranular oxide layer is formed on the surface of the carburized portion, and an incompletely hardened structure (truthite) is generated, which lowers the surface hardness. . Then, when a grain boundary oxide layer is formed on the surface layer, it acts as a notch, and further a soft incompletely hardened layer is generated, so that fatigue strength and pitting resistance are also deteriorated, and recent automobile quantification and The strength is insufficient for higher output.

Therefore, the applicant of this application has proposed a steel for carburizing for the purpose of preventing the formation of a grain boundary oxide layer and an incompletely hardened layer (JP-A-1-47838), and others. For example, it is disclosed in JP-B-55-32777 and JP-A-60-21359.

[0004]

Generally, a basic study of fatigue of carburizing steel is carried out on a rotating bending fatigue test piece taken from the longitudinal direction of a hot rolled steel bar or a hot forged material.
In this case, fatigue fracture is in the direction perpendicular to the rolling and forging directions. However, in bending fatigue fracture of the root of an actual gear, a force parallel to the rolling and forging direction is often applied, and it has been proposed to prevent the formation of conventional carburizing steel or grain boundary oxide layers or incompletely hardened layers. In the existing carburizing steel, MnS is stretched by rolling and forging, and as a result, the bending fatigue strength of the gear is significantly reduced. That is, stretched MnS that does not appear in conventional basic fatigue research.
However, there is a problem that the adverse effect of is actualized in an actual gear. Further, the same applies to surface fatigue fracture, in which stretched MnS also becomes substantially perpendicular to the surface, and may become the starting point of fatigue fracture together with the grain boundary oxide layer, and the pitting resistance decreases.

The object of the present invention is to control the stretching of MnS,
By suppressing the grain boundary oxidation layer and the incompletely hardened layer, it is intended to solve the above-mentioned problems and to provide a carburizing steel having a high fatigue strength that cannot be reached by conventional carburizing steels.

[0006]

Means for Solving the Problems In the course of research aimed at improving the fatigue properties of carburizing steel, the present inventors have studied MnS in fatigue fracture of actual gears, which has hardly been studied in the past.
We tried to clarify the effect of the fatigue test, and used a T-direction test piece simulating the failure mode of a real gear (a test piece was taken at a right angle to the rolling and forging direction) and a real gear to determine the bending fatigue fracture and the surface. As a result of detailed examination of the fatigue process, the following findings were obtained.

(1) In the case of bending fatigue fracture, the fatigue crack propagates by grain boundary fracture in both the actual gear and the T-direction test piece with the surface grain boundary oxide layer as the origin of the fracture. Furthermore, M
Since the interfacial adhesion strength between nS and base steel is weak and voids exist, the crack propagation speed is accelerated in the presence of drawn MnS as compared with the case where MnS is not present. (2) Even in surface fatigue, pitting often occurs starting from the grain boundary oxide layer of the surface layer, but there is also pitting starting from MnS existing on the surface, and the stretched M
The presence of nS has a very adverse effect on fatigue strength.

(3) The effect of the stretched MnS is remarkable when the grain boundary oxide layer and the incompletely hardened layer on the surface layer are sufficiently suppressed and the strength is further increased. On the other hand, in the case of carburizing steel having many grain boundary oxide layers and incompletely hardened layers, such as the conventional JIS steel that was targeted for relatively low strength,
Compared to the effect of stretched MnS, these grain boundary oxide layers and incompletely hardened layers mainly dominate the fatigue strength.

Based on these findings, the present inventors have found that, in order to improve the bending fatigue strength and surface fatigue strength of carburizing steel for gears, in addition to suppressing MnS stretching, the grain boundary oxide layer and the incompletely hardened layer are formed. It is effective to try to suppress at the same time,
I reached the conclusion. Then, based on the above-mentioned knowledge of high strength, further research and study were conducted, and the following concrete achievement means were clarified, and the present invention was accomplished.

(1) In order to suppress the stretching of MnS, it is necessary to suppress the generation of hard inclusions that become the starting point of fatigue fracture while lowering the deformability. It is effective to use Mn, Ca) S and (Mn, Ca) S + calcium aluminate. (2) In order to change MnS to (Mn, Ca) S and (Mn, Ca) S + calcium aluminate in carburizing steel, it is essential to keep Ca / oxygen at a predetermined ratio and control the reaction system. is there. When Ca / oxygen is less than 0.5, hard CaO.6Al ₂ O ₃ or the like is generated and becomes a starting point of fatigue fracture, and when 3.0 or more, hard CaS is also generated and fatigue strength is lowered. Therefore, Ca / oxygen of 0.5 or more and less than 3.0 is optimal.

(3) In order to prevent the formation of a grain boundary oxide layer and an incompletely hardened layer, Si, Mn and Cr, which are likely to be grain boundary oxidized, are reduced, while Mo and Ni, which hardly undergo grain boundary oxidation, are increased. It was confirmed that the hardenability of the steel material should be increased so that the material can be sufficiently quench-hardened even if the grain boundary oxide layer is generated. The present applicant has already proposed the conditions for suppressing the incompletely hardened layer in the C, Si, Mn, and Cr systems (JP-A-1-47838), but this time, C, S is newly added.
The conditions for i, Mn, Cr, Mo, and Ni were found.

That is, it is possible to suppress the incompletely quenched structure of the tooth base.
Required hardenability D_I'Value has a great influence on the size of the gear.
, Hardenability and equivalent round bar diameter (diameter) and incomplete quenching
As a result of investigating the occurrence state of the structure, C, Si, Mn, C
Hardenability with r, Mo and Ni (D_IInch) and
And the equivalent round bar diameter (φmm) D_I(Inch) ≧ D_I′ (Inch) ＝ 2/3 ・ Equivalent round bar diameter
(Φ mm)^0.65-0.5 ... formula By satisfying the above condition, a completely hardened structure of the surface layer can be achieved.
I found that I could cut it. In this case, D_I= (D_{I c}×
F_Si× F_Mn× F_Cr× F_Mo× F_Ni) Is C, Si, Mn, C
It is an ideal critical diameter due to r, Mo and Ni, and D_{I c}Is basic
Hardenability, F _Si, F_Mn, F_Cr, F_Mo, F_NiIs each element
Calculated value determined by AISI regulations.
Is. Also, the equivalent round bar diameter is the cross section of the gear shown by the diagonal lines in Fig. 1.
It is the equivalent circle diameter equivalent to the area of the part.

Based on these findings, the present inventors suppress the stretching of MnS by rolling and forging, and suppress the generation of incompletely hardened microstructures in the grain boundary oxide layer and the surface layer.
We have developed a carburizing steel for gears that has high bending fatigue strength at the root and excellent pitting resistance. That is,
1st invention in this invention is C: 0.1% by weight%.
Or more and less than 0.3%, Si: less than 0.15%, Mn: 0.
30% or more and less than 1.50%, Cr: 0.30% or more 1.
If less than 50%, Mo: 0.30% or more and 1.00% or less,
Ni: one or more of 1.0% or less, and S: 0.010
% Or more and 0.030% or less, Ca: 0.0010% or more and 0.010% or less, oxygen: less than 0.0030%, but with Ca / oxygen: 0.5 or more and less than 3.0,
V: 0.05% or more and 0.30% or less, Nb: 0.02%
Or more and 0.20% or less, Ti: 0.01% or more and 0.20%
A carburizing steel having excellent fatigue strength, characterized in that it contains one or more of the following and the balance is Fe and impurities:
A second invention of the present invention provides a carburizing steel excellent in fatigue strength, characterized in that the contents of Si, Mn, Cr, Mo, and Ni satisfy the following relational expressions in the method of the first invention. It is provided.

Relational expression: D _I (inch) ≧ 2/3 ・ Equivalent round bar diameter (φmm) ^0.65 -0.5 where D _I = D _IC × F _Si × F _Mn × F _Cr × F _Mo × F _Ni phi = equivalent Marubo径(area equivalent circle equivalent diameter of the gear section portion shown by oblique lines in FIG. 1) D _IC basic hardenability, also _{F Si, F Mn, F Cr} , F Mo, F
_Ni is a multiple of the hardenability of each element and is a calculated value determined by the AISI regulation.

[0015]

The respective constituents of the steel of the present invention will be described below. First, C is an element added to secure the strength required as a carburized part, particularly the core strength, but if it is less than 0.1%, such an effect cannot be sufficiently obtained, and 0.3
%, The toughness decreases and becomes brittle, which makes it difficult to use as a carburizing steel.
Less than%.

Si is an element that has a significant adverse effect on the grain boundary oxidation of carburizing steel, and if the content is 0.15% or more, a grain boundary oxide layer is formed in the carburizing layer, and the material properties of the carburizing steel are significantly deteriorated. Therefore, the content is set to less than 0.15%.
Mn is an element necessary for imparting strength, toughness and hardenability to steel, but if it is 1.50% or more, it becomes a bainite or martensite hard structure upon cooling after hot rolling,
Since it is not suitable for secondary processing such as subsequent cutting, 1.
It is less than 50%. However, if the addition amount of Mn is less than 0.30%, the effect of hardenability is not sufficient, and the content thereof is 0.
30% or more.

Cr contributes to the improvement of mechanical properties, hardenability, and wear resistance of steel, but if this element is also 1.50% or more, it becomes a hard structure of bainite or martensite in cooling after hot rolling. Since it becomes unsuitable for secondary processing such as cutting thereafter, it is set to less than 1.50%. But,
If the addition amount of Cr is less than 0.30%, the effect of hardenability is not sufficient, and the content thereof is 0.30% or more.

Mo and Ni are elements necessary for imparting a predetermined hardenability to steel and improving strength and toughness, and one or more kinds thereof are added. In the present invention, the aforementioned Si, Mn
In order to provide hardenability equivalent to or better than that of conventional steel, Mo is contained in an amount of 0.30% or more under the restrictions on the Cr content and Cr content. However, even if the content exceeds 1.00%, the effect is saturated and the economy is impaired, so the upper limit is made 1.00%. Further, even if Ni is contained in excess of 1.00%, the effect is saturated and the economy is impaired, so the upper limit is made 1.00%.

S is an element necessary for improving the machinability and is contained in an amount of 0.010% or more. But 0.030
%, The amount of inclusions in the steel increases, which adversely affects the cold plastic workability, so the upper limit is 0.030.
%. Ca is an element necessary for suppressing the stretching of MnS. Under the above-mentioned limitation with the oxygen content, 0.0010% or more of Ca is contained in order to give the effect of suppressing the stretching of MnS. However, even if the content exceeds 0.010%, the effect is saturated and the economy is impaired, so the upper limit is set to 0.
010%.

O increases the amount of inclusions in the steel and deteriorates the fatigue strength characteristics such as rotary bending fatigue and surface fatigue, so 0.00
It is less than 30%. For Ca addition, MnS (Mn, C
a) The purpose is to control to S or (Mn, Ca) S + calcium aluminate, and other Ca compounds cannot achieve the object of the present invention. This formation reaction is C
The present inventors have revealed that the a / oxygen ratio is dominant. Under the above-mentioned limitation of Ca and S content, Ca
/ Oxygen ratio less than 0.5 is hard CaO ・ 6Al ₂ O ₃
Etc. are generated and become the starting point of fatigue fracture, and when 3.0 or more, hard CaS is also generated and fatigue strength is lowered. Therefore, Ca / oxygen is set to 0.5 or more and less than 3.0.

V, Ti and Nb form carbonitrides,
It is an element effective in refining carburized crystal grains and can be added arbitrarily. To obtain the effect, V is 0.05
% Or more, Nb 0.02% or more and Ti 0.01%
The above contents are required. However, even if the content of V exceeds 0.30%, the content of Nb exceeds 0.20%, and the content of Ti exceeds 0.20%, the effect is saturated.
30%, 0.30% for Nb, and 0.20% for Ti.

Hardenability D_I(Inch) is the size of the gear
Greatly affected. Required hardenability D _I′ (Inch) ＝ 2 /
3 ・ Equivalent round bar diameter (φmm)^0.65If it is less than -0.5,
An incompletely hardened structure occurs in the surface layer and fatigue characteristics deteriorate.
Because D_I(Inch) ≧ 2/3 ・ Equivalent round bar diameter (φmm)
^0.65-0.5. However, D_I= D_{I c}× F_Si× F_Mn
× F_Cr× F_Mo× F_Ni, D_{I c}Is the basic hardenability, and F_Si,
F_Mn, F_Cr, F_Mo, F_NiIs the multiple of the hardenability of each element, A
It is a calculated value defined by ISI regulations. Also, the equivalent circle
The rod diameter is a circular phase equivalent to the area of the gear cross section shown by the diagonal lines in Fig. 1.
It is the equivalent diameter.

The present invention will be further described below with reference to examples.

[0024]

EXAMPLE Steels having the chemical compositions shown in Table 1 were melted, then ingot-cast, and then slab-rolled and bar-steel rolled to produce a round bar having a diameter of 70 mm (rolling ratio 50). Subsequently, each rolled material was heated to 925 ° C.
After normalizing at 25 mm, a round bar with a diameter of 25 mm x 50 mm is cut out from the direction perpendicular to the rolling direction (T direction), and both ends have a diameter of 25 mm x 85 mm for gripping parts.
The two round bars of No. are aligned by friction welding, the diameter is 15 mm, and the diameter of the central parallel portion is No. 1-No. 7 and No. 9-N
o. No. 13 up to 13 mm. 8 and No. 14 is 12
Machined into rotating bending fatigue specimens in mm. Similarly, for the surface fatigue test piece, a round bar with a diameter of 28 mm x 50 mm is cut out from the direction (T direction) perpendicular to the rolling direction, and two round bars with a diameter of 28 mm x 40 mm are used for gripping parts at both ends by friction welding. Machined into two cylindrical roller pitching fatigue specimens with a matched, test surface diameter of 26 mm. Next, each processed material was carburized and tempered under the conditions of heating at 930 ° C. for 5 hours in a carburizing gas atmosphere → 130 ° C. oil quenching → 180 ° C. × 1 hour tempering. The MnS stretching degree, the grain boundary oxide layer and the incompletely hardened layer depth were measured, and an Ono-type rotary bending fatigue test using a T-direction test piece and a surface fatigue test using roller pitting were performed.

Next, using the steel having the chemical composition shown in Table 2,
After going through the same steps, a 70 mmφ round bar was drawn to 120 mmφ by hot forging, and after normalizing at 925 ° C., it was machined to have a pitch radius of 54 m as shown in FIG.
m, number of teeth 27, module 4, tooth width 9 mm, shaft hole radius 3
A 5 mm (equivalent round bar diameter 10.5 mmφ) test spur gear was prepared. After that, each gear was similarly carburized and hardened, and a one-tooth bending fatigue test was carried out by a hydraulic servo type tensile compression tester.

As shown in Table 3, no. N adding 1 and Ca outside the scope of the present invention
o. 2, No. No. 3 has low bending fatigue strength and surface fatigue strength. Moreover, No. In No. 4, the bending fatigue strength is slightly improved, but the surface fatigue strength is extremely deteriorated. Although Ca was added, its chemical composition was outside the scope of the present invention. 5, No. In No. 6, many grain boundary oxide layers or incompletely hardened layers were generated, and bending fatigue strength and surface fatigue strength were low. No. 7, No. Although 8 contains Ca and other chemical components are within the scope of the present invention, the hardenability D _I (inch) does not reach the required D _I ′ (inch),
Incomplete quenching layer is generated and bending fatigue strength and surface fatigue strength are low.

On the other hand, N within the range of the chemical composition of the present invention.
o. In Nos. 9 to 14, the MnS stretching could be suppressed, and
It also reduces the field oxidation and suppresses the formation of incompletely quenched structure.
The bending fatigue strength and surface fatigue strength are extremely high.
it is obvious. Also, in the actual gear fatigue test, Table 4
As shown in FIG. 15 and
No. in which Ca was added outside the scope of the present invention. 16 is one tooth
Bending fatigue strength is low. Also, although Ca is added, it is chemically
No. in which the component is outside the scope of the present invention. 17 is a grain boundary oxide layer or
A large number of incompletely hardened layers cause low bending fatigue strength
Yes. No. 18 is Ca added, and the chemical composition is within the range of the present invention.
But hardenability D _ID (inch) required_I′
(Inch) is not reached and an incompletely hardened layer is generated.
Tooth bending fatigue strength is low.

On the other hand, N within the chemical composition range of the present invention.
o. In Nos. 19 to 21, the MnS stretching can be suppressed in all cases,
It is clear that the grain boundary oxidation can be reduced and the occurrence of incompletely quenched structure can be suppressed, and the one-tooth bending fatigue strength is remarkably high.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

[Table 3]

[0032]

[Table 4]

[0033]

As described above, according to the present invention, when used in a gear or the like as a drive transmission component of an automobile,
It is possible to achieve high root fatigue strength and pitting resistance. Therefore, it has an excellent effect that it is not necessary to increase the size of the gear or surface-treat it as in the conventional case, and can be applied to a shaft component etc. which is premised on carburization, and its industrial effect is extremely remarkable. There is something.

[Brief description of drawings]

FIG. 1A is a view showing a gear cross section (hatched portion) targeted for calculation of an equivalent round bar diameter, and FIG. 1B is a cross sectional view of a test spur gear.

Claims (2)

[Claims] 1. By weight%, C: 0.1% or more and less than 0.3%, Si: less than 0.15%, Mn: 0.30% or more 1.
Less than 50%, Cr: 0.30% or more and less than 1.50%,
Mo: 0.30% or more and 1.00% or less, Ni: 1.0%
One or more of the following, and S: 0.010% or more 0.03
0% or less, Ca: 0.0010% or more and 0.010% or less, oxygen: less than 0.0030%, but Ca / oxygen:
Including 0.5 or more and less than 3.0, V: 0.05%
Or more and 0.30% or less, Nb: 0.02% or more and 0.20%
Hereinafter, a carburizing steel having excellent fatigue characteristics, characterized by containing at least one of Ti: 0.01% or more and 0.20% or less, and the balance being Fe and impurities. 2. The carburizing steel with excellent fatigue properties according to claim 1, wherein the contents of Si, Mn, Cr, Mo and Ni satisfy the following relational expression. Relational expression: D _I (inch) ≧ 2/3 ・ Equivalent round bar diameter (φmm)
^0.65 −0.5 where D _I = D _IC × F _Si × F _Mn × F _Cr × F _Mo × F _Ni φ = Equivalent round bar diameter (corresponding to the circle equivalent to the area of the gear cross section shown by the diagonal lines in Fig. 1) Diameter) D _IC is basic hardenability, and also F _Si , F _Mn , F _Cr , F _Mo , F
_Ni is a multiple of the hardenability of each element and is a calculated value determined by the AISI regulation.