WO2011111269A1

WO2011111269A1 - Carburized steel component having excellent low-cycle bending fatigue strength

Info

Publication number: WO2011111269A1
Application number: PCT/JP2010/070516
Authority: WO
Inventors: 修司小澤; 学久保田; 修加田; 元裕西川; 高志田中; 典正常陰
Original assignee: 新日本製鐵株式会社; 山陽特殊製鋼株式会社
Priority date: 2010-03-10
Filing date: 2010-11-11
Publication date: 2011-09-15
Also published as: KR20120012837A; CN103382538A; CN102471842A

Abstract

Disclosed is a carburized steel component having excellent low-cycle bending fatigue strength, which is obtained by carburizing a steel material, which contains 0.1-0.6% by mass of C, 0.01-1.5% by mass of Si, 0.3-2.0% by mass of Mn, 0.02% by mass or less of P, 0.001-0.15% by mass of S, 0.001-0.03% by mass of N, 0.001-0.06% by mass of Al and 0.005% by mass or less of O, with the balance substantially made up of iron and unavoidable impurities, and then tempering the steel material. The carburized steel component is characterized by having a surface hardness from HV 550 to HV 800 and a core hardness from HV 400 to HV 500.

Description

Carburized steel parts with excellent low cycle bending fatigue strength

The present invention relates to a carburized steel part excellent in low cycle bending fatigue strength.

Gears such as mechanical structural parts, differential gears, transmission gears, and carburized shafts with gears have low cycle fatigue (fatigue in the range of several hundred to several thousand cycles) due to the sudden start and stop of the vehicle. ) May be damaged. In particular, improvement of low cycle fatigue strength is further desired for differential gears and transmission gears.
Conventionally, the steel parts are made of case-hardened steel with C: around 0.2% such as JIS SCr420, SCM420, etc. as steel materials, thereby ensuring the toughness of the core, carburizing and quenching, and low temperature around 150 ° C. By tempering, the surface has a tempered martensite structure with C: around 0.8%, and the high cycle bending fatigue strength and wear resistance are enhanced.
As a steel part having low cycle bending fatigue strength, Patent Document 1 contains C: 0.1 to 0.3%, B: 0.005% or less, Si: 0.3% or less, P : Carburized parts limited to 0.03% or less and core hardness: HV350 or more are disclosed.
In Patent Document 2, the sum of plastic deformation resistance and grain boundary strength calculated from the component composition is limited to C: 0.15 to 0.3%, Si: 0.5% or less, and P: 0.01% or less. The case hardening steel which raised low cycle fatigue strength by making more than a fixed value is disclosed.
In Patent Document 3, C: 0.1 to 0.3%, B: 0.001 to 0.005%, Si: 0.5% or less, P: 0.03% or less, The hardness of the core part: A carburized gear excellent in low cycle fatigue strength of HV300 or more is disclosed.
In Patent Document 4, C is 0.15 to 0.3%, B is 0.0003 to 0.005%, Si is 0.03 to 0.25%, and P is limited to 0.02% or less. And the carburized component which improved the low cycle impact fatigue characteristic by making the value relevant to the core hardness calculated from the component composition more than a fixed value is disclosed.
In Patent Document 5, C: 0.1 to 0.4%, Si: 1.0% or less, Mn: more than 1.5 to 3%, P: 0.03% or less, S: 0.03% or less Cr: 0.3 to 2.5%, Al: 0.005 to 0.050%, Ti: 0.003% or less, O: 0.0015% or less, N: 0.025% or less, balance is inevitable A carbonitrided bearing steel comprising impurities and Fe, having a surface hardness of 58 HRC or more after carbonitriding or subsequent secondary quenching and tempering, and a surface retained austenite amount of 20 to 50% is disclosed. .
In Patent Document 6, C: 0.1 to 0.4%, Si: 0.02 to 1.3%, Mn: 0.3 to 1.8%, S: 0.001 to 0.15%, Al: 0.001 to 0.05%, N: 0.003 to 0.020%, P: 0.025% or less, O: 0.0025% or less, and Cr: 1.8% or less Mo: 1.5% or less, Ni: 3.5% or less, B: 0.006% or less, V: 0.5% or less, Nb: 0.04% or less, Ti: 0.2% or less In a steel material containing seeds or two or more kinds, the balance being iron and inevitable impurities, the projection core hardness Hp-core (= Hcore / (1-t / r) [ Hcore: core hardness, t: effective hardened layer depth, r: radius of the damaged part or half of the thickness of the damaged part]) is HV390 or higher and has excellent low cycle fatigue characteristics. It has been hardened steel disclosure.
In Patent Document 7, C: 0.1 to 0.4%, Si: 0.5% or less, Mn: 1.5% or less, P: 0.03% or less, S: 0.03% or less, Cr : 0.3-2.5%, Mo: 0.1-2.0%, V: 0.1-2.0%, Al: 0.050% or less, O: 0.0015% or less, N: 0.025% or less, V + Mo: 0.4 to 3.0%, balance Fe and inevitable impurities, steel subjected to carburizing quenching and tempering treatment, and surface layer C concentration after tempering treatment is 0.6 to Hydrogen embrittlement with 1.2%, surface hardness of HRC 58 or more and less than 64, and the proportion of fine V-type carbides having a particle size of less than 100 nm among the V-type carbides dispersed and precipitated on the surface layer is 80% or more A case-hardened steel excellent in mold surface fatigue strength is disclosed.
However, in any of the carburized steel parts, the low cycle bending fatigue strength does not reach the level of low cycle bending fatigue strength required today.

JP-A-8-92690 Japanese Patent Laid-Open No. 10-259450 International Publication WO02 / 44435 JP 2004-238702 A Japanese Patent Laying-Open No. 2005-042188 JP 2007-332438 A JP 2008-280583 A

The technologies disclosed in Patent Documents 1 to 7 cannot answer the improvement in low cycle bending fatigue strength that is required today. Then, this invention makes it a subject to provide the carburized-steel components which the low cycle bending fatigue strength improved notably compared with the conventional low cycle bending fatigue strength.

In order to solve the above-mentioned problems, the present inventors diligently conducted a low cycle bending fatigue test by changing the component composition and carburizing characteristics of steel materials in a wide range and systematically. As a result, the following findings (a) to (d) were obtained.
(A) In order to increase the low cycle bending fatigue strength, it is optimal to set the surface hardness to HV550 to HV800. Within that range, it is effective to reduce the surface hardness.
(B) (b1) In order to increase the low cycle bending fatigue strength, it is optimal to set the core hardness to HV400 to HV500. Within that range, it is effective to increase the core hardness. Moreover, (b2) C: 0.6% or less is preferable as the core hardness is increased.
Conventionally, it is said that when C exceeds 0.3%, the toughness is lowered and the low cycle bending fatigue strength is lowered, but the present inventors (b3) that the toughness is lowered depends on the amount of C. Instead, it was found that the core hardness exceeded HV500, and 0.6% where the core hardness exceeded HV500 was the upper limit of C.
(C) (c1) In order to increase the low cycle bending fatigue strength, it is effective to increase Si within a range of 0.01 to 1.5%.
Conventionally, Si has been recommended to be 0.5% or less because it forms a grain boundary oxide layer at the time of carburizing and causes a decrease in strength.
However, the present inventors have (c2) the effect of the grain boundary oxide layer on the low cycle bending fatigue strength, if any, which is extremely small, an increase in Si, a decrease in surface hardness, and / or a core. It has been found that it is effective in increasing the club hardness.
(D) When P is reduced as much as possible and B is added, the effects (a) to (c) are further improved.
This invention was made | formed based on the said knowledge, The summary is as follows.
(1) In mass%,
C: 0.1 to 0.6%
Si: 0.01 to 1.5%,
Mn: 0.3 to 2.0%,
P: 0.02% or less,
S: 0.001 to 0.15%,
N: 0.001 to 0.03%,
Al: 0.001 to 0.06%, and
O: contains 0.005% or less,
A steel part in which the balance is substantially iron and inevitable impurities, carburized and then tempered,
A carburized steel part excellent in low cycle bending fatigue strength, characterized by having a surface hardness of HV550 to HV800 and a core hardness of HV400 to HV500.
(2) The carburized steel part having excellent low cycle bending fatigue strength according to (1), wherein the low cycle bending fatigue strength is 20 kN or more.
(3) The steel material further comprises B: 0.0002 to 0.005% by mass%, and is excellent in low cycle bending fatigue strength as described in (1) or (2) above Carburized steel parts.
(4) The low cycle bending fatigue strength according to any one of (1) to (3), wherein the steel further contains Cr: 1.20 to 3.0% by mass. Excellent carburized steel parts.
(5) The low cycle bending fatigue strength according to any one of (1) to (4), wherein the steel material further contains Ti: 0.01 to 0.2% by mass%. Excellent carburized steel parts.
(6) The steel material is further in mass%, Mo: less than 0.1%, Cu: less than 0.1%, and Ni: less than 0.1% as one or more kinds as unavoidable components. The carburized steel part having excellent low cycle bending fatigue strength according to any one of (1) to (5), wherein the carburized steel part is contained.
(7) The steel material is one type of Mo: 0.1 to 1.5%, Cu: 0.1 to 2.0%, and Ni: 0.1 to 5.0% in mass%. Alternatively, the carburized steel part having excellent low cycle bending fatigue strength according to any one of the above (1) to (5), comprising two or more kinds.
(8) The steel material further contains one or two kinds of Nb: 0.01 to 0.2% and V: 0.03 to 0.2% by mass%. The carburized steel part having excellent low cycle bending fatigue strength according to any one of (1) to (7).
(9) The steel material is one type of Ca: 0.0002 to 0.005%, Zr: 0.0003 to 0.005%, and Mg: 0.0003 to 0.005% by mass%. Alternatively, the carburized steel part having excellent low cycle bending fatigue strength according to any one of the above (1) to (8), comprising two or more kinds.
(10) The carburized steel part having excellent low cycle bending fatigue strength according to any one of (1) to (9), wherein the carburized steel part is a differential gear or a transmission gear.

By using the carburized steel parts excellent in low cycle bending fatigue strength of the present invention, gears such as differential gears for automobiles can be greatly reduced in size and weight, and as a result, the fuel efficiency of the automobile is improved, and , CO ₂ emission can be reduced.

FIG. 1 is a diagram showing a low cycle bending fatigue test piece and a low cycle bending fatigue test method.
FIG. 2 is a diagram showing the effect of compressive residual stress (MPa) on 500 cycle bending fatigue strength (kN).
FIG. 3 is a diagram showing the influence of the grain boundary oxide layer depth (μm) on the 500 cycle bending fatigue strength (kN).
FIG. 4 is a diagram showing the influence of surface hardness (HV) on 500 cycle bending fatigue strength (kN).
FIG. 5 is a diagram showing the influence of core hardness (HV) on 500 cycle bending fatigue strength (kN).

Hereinafter, the carburized steel part excellent in the low cycle bending fatigue strength of the present invention will be described in detail.
First, the reason for limiting the component composition of the steel material used in the present invention (the present steel material) will be described. Hereinafter,% related to the component composition means mass%.
C: 0.1 to 0.6%
C is an element that imparts hardness to the core portion of the steel part subjected to carburizing and quenching and increases the low cycle bending fatigue strength. The core structure is a hardened structure mainly composed of martensite, and the hardened martensite becomes harder as the amount of C increases.
When the core hardness is the same, if the amount of C is large, the yield ratio increases due to dispersion strengthening of fine carbides. In order to obtain the effect of addition, C is made 0.1 to 0.6%.
In order to increase the core hardness to HV450 or more in order to increase the low cycle bending fatigue strength, C is preferably 0.2% or more, and more preferably more than 0.3%. In addition, from the viewpoint of machinability, C is preferably 0.4% or less.
In order to increase the fatigue strength of case-hardened steel, it is effective to apply compressive residual stress. In carburizing and quenching of case-hardened steel, the core of C: around 0.2% first expands in the martensitic transformation, and then the carburized layer of C: around 0.8% expands in the martensitic transformation. Compressive stress remains near the surface of the steel part.
Usually, in case-hardened steel, if the amount of C is increased as in the present invention, the difference in the amount of C between the core and the carburized layer decreases, the difference in expansion of the martensitic transformation decreases, and the residual compressive stress decreases. As a result, it is presumed that the fatigue strength of steel parts is reduced.
Therefore, the present inventors investigated the effect of compressive residual stress (MPa) on 500 cycle bending fatigue strength (kN). The result is shown in FIG. As shown in FIG. 2, it has been found that there is no influence of compressive residual stress on 500 cycle bending fatigue strength.
Si: 0.01 to 1.5%
Si is an element effective for deoxidation of steel materials, and is an element effective for increasing temper softening resistance. Furthermore, Si is an element that enhances hardenability, increases the core hardness of the steel part after carburizing and quenching, and contributes to the improvement of low cycle bending fatigue strength.
If it is less than 0.01%, the effect of addition is insufficient. On the other hand, if it exceeds 1.5%, the carburizing property is inhibited, so Si is made 0.01 to 1.5%.
When a general gas carburizing method with a carbon potential of 0.7 to 1.0 is adopted for the carburizing treatment, Si increases the activity of C in the steel material in the range of 0.5 to 1.5%. Therefore, it is an element effective for further improving the low cycle bending fatigue strength because it acts to suppress the surface hardness. Therefore, Si is preferably 0.5 to 1.5%.
Conventionally, when Si is carburized, it forms a grain boundary oxide layer and causes a decrease in strength, so it has been recommended to limit it to 0.5% or less. This is an analogy based on the conventional knowledge that if the amount of Si is limited, the depth of the grain boundary oxide layer can be reduced and the bending fatigue strength in the high cycle region can be increased.
Therefore, the present inventors investigated the influence of the grain boundary oxide layer depth (μm) on the 500 cycle bending fatigue strength (kN). The result is shown in FIG. As shown in FIG. 3, it has been found that the depth of the grain boundary oxide layer does not affect the 500 cycle bending fatigue strength.
Mn: 0.3 to 2.0%
Mn is an element effective for deoxidation of steel materials, and is an element that improves the hardenability of steel materials, increases the core hardness of steel parts after carburizing and quenching, and contributes to the improvement of low cycle bending fatigue strength. .
If it is less than 0.3%, the effect of addition is insufficient. On the other hand, if it exceeds 2.0%, the effect of addition is saturated, so Mn is set to 0.3 to 2.0%. Preferably, it is 0.8 to 1.5%.
P: 0.02% or less P is an impurity, and segregates at austenite grain boundaries during carburizing, causing grain boundary fracture and reducing low cycle bending fatigue strength. Therefore, P is limited to 0.02% or less. Preferably it is 0.01% or less.
S: 0.001 to 0.15%
S is an element that forms MnS in the steel and contributes to improvement of machinability. If it is less than 0.001%, the effect of addition is insufficient. On the other hand, if it exceeds 0.15%, the effect of addition is saturated and segregates at the grain boundary to cause grain boundary embrittlement. 0.001 to 0.15%. Preferably it is 0.01 to 0.1%.
N: 0.001 to 0.03%
N is an element that forms a nitride or carbonitride that binds to Al, Ti, Nb, V, and the like in the steel material and has an effect of suppressing coarsening of crystal grains.
If it is less than 0.001%, the effect of addition is insufficient. On the other hand, if it exceeds 0.03%, the effect of addition is saturated, so N is made 0.001 to 0.03%. Preferably it is 0.003 to 0.008%.
Al: 0.001 to 0.06%
Al is an element added for the purpose of deoxidation of steel materials. If it is less than 0.001%, the effect of addition is insufficient. On the other hand, if it exceeds 0.06%, the effect of addition is saturated, so Al is made 0.001 to 0.06%. Preferably, the content is 0.01 to 0.04%.
O: 0.005% or less O is inevitably contained, and segregates at the grain boundaries to easily cause grain boundary embrittlement, and forms hard oxide inclusions that cause brittle fracture in the steel material. It is an element that is easy to do. In order to prevent grain boundary embrittlement and brittle fracture, O is made 0.005% or less. Preferably it is 0.002% or less.
The steel material of the present invention contains B for further improving the low cycle bending fatigue strength (20 kN or more).
B: 0.0002 to 0.005%
B is an element that suppresses the grain boundary segregation of P and increases the grain boundary strength, intragranular strength, and hardenability, and contributes to the improvement of low cycle bending fatigue strength (20 kN or more).
If it is less than 0.0002%, the effect of addition is insufficient. On the other hand, if it exceeds 0.005%, the effect of addition is saturated, so B is set to 0.0002 to 0.005%. Preferably it is 0.0005 to 0.003%.
The steel material of the present invention further contains Cr in order to improve hardenability and further improve low cycle bending fatigue strength.
Cr: 1.20 to 3.0%
Cr is an element that improves the hardenability of the steel material, increases the core hardness of the steel part after carburizing and quenching, and contributes to the improvement of low cycle bending fatigue strength. If it is less than 1.20%, the effect of addition is insufficient. On the other hand, if it exceeds 3.0%, the effect of addition is saturated, so Cr is set to 1.20 to 3.0%. Preferably it is 1.50 to 2.5%.
The steel according to the present invention contains Ti in order to prevent the crystal grains from becoming coarse and the low cycle fatigue strength from deteriorating during high temperature carburizing.
Ti: 0.005 to 0.2%
Ti is an element that generates fine TiC and / or TiS in a steel material.
Due to the presence of TiC and / or TiS, austenite grains can be stably refined in high-temperature carburization at a carburizing temperature of 980 ° C. or higher, or for a long time of carburizing for 10 hours or longer. Deterioration of fatigue strength can be prevented.
Ti is an element that combines with N in the steel material to generate TiN, prevents precipitation of BN, and contributes to securing solid solution B.
If the content is less than 0.005%, the effect of addition is insufficient. On the other hand, if the content exceeds 0.2%, a large amount of TiN-based precipitates are precipitated and the rolling fatigue characteristics are deteriorated. 005 to 0.2%. Preferably it is 0.01 to 0.1%.
In the steel material of the present invention, Mo, Cu, and Ni inevitably mixed are limited to less than 0.1%. Preferably, it is limited to 0.05% or less, more preferably 0.01% or less.
Mo, Cu, and Ni are elements that have the effect of increasing the hardenability and increasing the low cycle bending fatigue strength, and contain one or more of the required amounts of Mo, Cu, and Ni. May be.
Mo: 0.1 to 1.5%
Mo is an element that improves the hardenability of the steel material, increases the core hardness of the steel part after carburizing and quenching, and contributes to the improvement of low cycle bending fatigue strength. If it is less than 0.1%, there is no effect. On the other hand, if it exceeds 1.5%, the effect of addition is saturated, so Mo is made 0.1 to 1.5%. Preferably it is 0.3 to 1.2%.
Cu: 0.1 to 2.0%
Cu is an element that improves the hardenability of the steel material, increases the core hardness of the steel part after carburizing and quenching, and contributes to the improvement of low cycle bending fatigue strength. If it is less than 0.1%, the effect of addition is insufficient. On the other hand, if it exceeds 2.0%, the effect of addition is saturated, so Cu is made 0.1 to 2.0%. Preferably it is 0.3 to 1.5%.
Ni: 0.1-5.0%
Ni is an element that improves the hardenability of the steel material, increases the core hardness of the steel part after carburizing and quenching, and contributes to the improvement of low cycle bending fatigue strength. If it is less than 0.1%, there is no effect. On the other hand, if it exceeds 5.0%, the effect of addition is saturated. Preferably, it is 0.5 to 3.5%.
The steel according to the present invention may further contain one or two of Nb and V in order to prevent crystal grains from becoming coarse during low temperature carburization and deterioration of low cycle fatigue strength.
Nb: 0.01 to 0.2%
Nb is an element that generates Nb carbonitride in steel. Because of the presence of Nb carbonitrides, austenite grains can be stably refined in high-temperature carburization with a carburizing temperature of 980 ° C. or higher and long-term carburizing with a carburizing time of 10 hours or longer. Strength deterioration can be prevented.
If it is less than 0.01%, the effect of addition is insufficient. On the other hand, if it exceeds 0.2%, the machinability deteriorates, so Ti is made 0.01 to 0.2%. Preferably it is 0.02 to 0.1%.
V: 0.03-0.2%
V is an element that generates V carbonitride in the steel material. Because of the presence of V carbonitride, austenite grains can be stably refined in high-temperature carburization at a carburizing temperature of 980 ° C. or higher, or in long-term carburizing for a carburizing time of 10 hours or longer, so low cycle fatigue Strength deterioration can be prevented.
If it is less than 0.03%, the effect of addition is insufficient. On the other hand, if it exceeds 0.2%, the machinability deteriorates, so V is set to 0.03 to 0.2%. Preferably it is 0.05 to 0.1%.
The steel material of the present invention may contain a required amount of one or more of Ca, Zr, and Mg for improving machinability.
Ca: 0.0002 to 0.005%
Ca is an element that lowers the melting point of oxides in steel materials. The low melting point oxide is softened by the temperature rise in the cutting environment, and improves the machinability of the steel material.
If it is less than 0.0002%, there is no effect of addition. On the other hand, if it exceeds 0.005%, a large amount of CaS is generated and the machinability of the steel material is lowered. %. Preferably it is 0.0008 to 0.003%.
Zr: 0.0003 to 0.005%
Zr is an element that deoxidizes a steel material to generate an oxide, and also generates a sulfide. Sulfide contributes to the improvement of machinability in cooperation with MnS. Since Zr-based oxides serve as nuclei for MnS crystallization / precipitation, Zr is also an effective element for controlling the dispersion of MnS.
Zr is added in an amount exceeding 0.003% for spheroidizing MnS. Conversely, in order to finely disperse MnS, 0.003 to 0.005% is added.
From the viewpoint of production quality stability (component yield, etc.), it is practically preferable to add 0.0003 to 0.005% Zr for finely dispersing MnS. If it is less than 0.0003%, there is almost no effect of adding Zr.
Mg: 0.0003 to 0.005%
Mg is an element that deoxidizes a steel material to generate an oxide, and also generates a sulfide. Sulfide contributes to the improvement of machinability in cooperation with MnS.
Mg-based oxides serve as nuclei for crystallization / precipitation of MnS, and sulfides become composite sulfides of Mn and Mg, thereby suppressing deformation of composite sulfides and making them spheroidized. Is an element effective for controlling the dispersion of MnS.
If it is less than 0.0003%, there is no effect of addition. On the other hand, if it exceeds 0.005%, a large amount of MgS is generated and the machinability of the steel material is lowered. %. Preferably it is 0.0008 to 0.003%.
Next, the reason for specifying the surface hardness and the core hardness in steel parts obtained by carburizing and quenching the steel of the present invention and then tempering will be described.
Surface hardness: HV550 to HV800
The present inventors investigated the influence of surface hardness (HV) on 500 cycle bending fatigue strength (kN) in the range of surface hardness HV500 to HV800. The result is shown in FIG.
From FIG. 4, it can be seen that the lower the surface hardness in the range of surface hardness HV500 to HV800, the lower the low cycle bending fatigue strength.
As a result of verifying the fracture surface of the damaged product, (i) if the surface hardness is high, a crack of the brittle fracture surface occurs from the surface and propagates rapidly, but (ii) if the surface hardness is low, the crack is Even if it occurs from the surface, it has been found that since the incidence of brittle fracture surfaces is low, the propagation speed of cracks is slow, and as a result, (iii) low cycle bending fatigue strength is improved.
However, if the surface hardness is less than HV550, the wear resistance is impaired, so the surface hardness is set to HV550 to HV800 (see “← →” in the figure). HV600 to HV750 is preferable, and HV620 to HV720 is more preferable.
When the surface hardness exceeds HV800, the toughness of the surface is remarkably lowered, the propagation speed of cracks is increased, and the low cycle bending fatigue strength is lowered.
Since the surface hardness is the hardness of the carburized structure forming the carburized layer, the surface hardness can be adjusted by adjusting the carbon potential during carburizing and the tempering temperature after carburizing and quenching.
For example, a steel part is carburized and quenched at a carbon potential of 0.8, then tempered at 150 ° C., and then a low cycle bending fatigue test is performed. If the low cycle bending fatigue strength is lower than the required value, The carbon potential is lowered to 0.7, or the tempering temperature is raised to 180 ° C. to reduce the surface hardness, thereby improving the low cycle bending fatigue strength.
Core hardness: HV400 to HV500
The inventors investigated the influence of the core hardness (HV) on the 500 cycle bending fatigue strength (kN) in the range of the core hardness HV270 to HV650. The result is shown in FIG.
From FIG. 5, it can be seen that the core hardness is in the range of HV400 to HV500, and the higher the core hardness is, the more the low cycle bending fatigue strength is improved.
As a result of verifying the fracture surface of the damaged product, if the core hardness is low, the core (hardened structure) directly under the carburized layer yields, and the carburized layer cannot receive stress more than the stress when yielded. It has been found that the stress on the surface of the steel part increases.
In order to make the low cycle bending fatigue strength significantly higher than the low cycle bending fatigue strength of conventional JIS SCr420, SCM420, etc., the core hardness is required to be HV400 or more, so the core hardness is HV400 to HV500. (See “← →” in the figure). HV430 to HV500 is preferable, and HV450 to HV500 is more preferable.
If the core hardness exceeds HV500, the toughness of the core portion is significantly reduced, the crack propagation speed of the core portion is increased, and the low cycle bending fatigue strength is reduced.
A core part is a location where C which penetrates from the surface of a steel part reaches by carburizing treatment. For example, it is a portion from a location where the material C increases by 10% (0.22% when the material C is 0.20%) to the material C. The core can be identified by EPMA-C line analysis or the like.
The carburizing method does not need to use a special method, and the effects of the present invention are exhibited even when a gas carburizing method, a vacuum carburizing method, a gas carbonitriding method, or the like, which is a general carburizing method, is used.
After carburizing, when heating to the austenite region (around 850 ° C.) and quenching (secondary quenching), the crystal grains become finer and the low cycle bending fatigue strength is further improved.
In the present invention, the carburized structure is responsible for the surface hardness, and the hardened structure is responsible for the core hardness, so the component composition is adjusted to give the steel the required carburizability and hardenability. The core hardness can be adjusted separately. This point is also a feature of the present invention.

Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited.
The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.
(Example)
Steel materials having the composition shown in Tables 1 and 2 were forged and then subjected to soaking and normalization to produce a roughing test piece for a low cycle bending fatigue test and a roughing test piece for a wear test. .

Test No. 1 to 21 (invention example), test no. 23 to 25 (comparative examples) and test no. The rough-processed test pieces 28 to 44 (invention examples) were subjected to carburizing treatment at 930 ° C. for 5 hours in a modified gas carburizing furnace, and then subjected to oil quenching at 130 ° C.
Test No. About the rough processing test piece of 22 (invention example), carburizing treatment at 930 ° C. for 5 hours was performed in a modified gas carburizing furnace, followed by oil quenching at 130 ° C. Then, oil quenching at 130 ° C. was performed.
Specimen No. About the rough processing test piece of 26 (comparative example), carburizing treatment of 930 ° C. × 5 hours was performed in a modified gas carburizing furnace, and then oil quenching at 220 ° C. was performed.
Test No. About the rough-processed test piece of 27 (Comparative Example), it was subjected to carburizing treatment at 930 ° C. for 5 hours in a modified gas carburizing furnace, followed by oil quenching at 20 ° C., followed by tempering for 1.5 hours. gave.
The carbon potential during the carburizing treatment was adjusted in the range of 0.5 to 0.8, and the tempering temperature was adjusted in the range of 150 to 300 ° C. to adjust the surface hardness and core hardness.
After the heat treatment, only the carburized layer on the side surface was removed by machining for the rough machining test piece for the low cycle bending fatigue test, and the 13 mm square notched test piece 1 (low cycle bending fatigue test piece shown in FIG. ) Was produced.
About the rough-processed test piece for an abrasion test, only the grip part was removed by machining to produce a test piece (wear test piece) having a cylindrical part with a diameter of 26 mm and a width of 28 mm.
The surface hardness (HV) and core hardness (HV) of the low cycle bending fatigue test piece were measured. The results are shown in Table 3. The surface hardness of the wear test piece was equivalent to the surface hardness of the low cycle bending fatigue test piece.
As shown in FIG. 1, the low cycle bending fatigue test is a four-point bending that applies a load 2 with a stress ratio of 0.1 to a 13 mm square low cycle bending fatigue test piece 1 having a notch X with a sine wave at a frequency of 1 Hz. A fatigue test was performed.
The frequency of 1 Hz (about 0.01 s ⁻¹ in terms of strain rate) is smaller than the strain rate actually applied to the automobile gear. In general, the repetition rate affects the fatigue test value when the strain rate is 10 s ⁻¹ or more. This is a region, and 10 s ⁻¹ is much larger than the strain rate actually applied to the automobile gear, so there is no problem in the evaluation with the frequency of 1 Hz.
In addition, it was confirmed by actually measuring the temperature of the test piece that the test piece did not generate heat during the test at a frequency of 1 Hz.
The stress ratio of an actual automobile gear is 0, but the reason for setting the stress ratio to 0.1 in this test is to prevent the test piece from slipping during unloading during the test.
This test was carried out in 10 ² to 10 ⁴ cycles with a load at which the test piece broke, and the 500 cycle bending fatigue strength (kN) obtained by interpolating the test results was taken as the low cycle bending fatigue strength. The low cycle bending fatigue strength is also shown in Table 3.

In the wear test, a roller made of bearing steel (SUJ2) having a diameter of 130 mm, a width of 18 mm, and a crowning of R = 150 mm on the outer periphery was pressed against the wear test piece with a surface pressure at a Hertzian stress of 1500 MPa. Rotate with the same peripheral speed direction of both rollers at -100% (slip rate is -100% (the peripheral speed of the contact part is 100% higher than the wear test piece)) After reaching, the wear depth of the wear specimen was measured. The wear depth is also shown in Table 3.
As shown in Table 3, the test No. of the invention example. In 1 to 22 and 28 to 44, the low cycle bending fatigue strength is excellent at 20 kN or more, and the wear depth is also excellent at 20 μm or less.
In contrast, Test No. of the comparative example. In No. 23, the low cycle bending fatigue strength is low. This is because the core hardness increased due to the C of the steel material exceeding 0.6%.
Test No. of the comparative example. In 24, the wear depth is large. This is because the carburizability is hindered and the surface hardness is lowered due to the fact that Si in the steel material exceeds 1.5%.
Test No. of the comparative example. In 25, the low cycle bending fatigue strength is low. This is because P is segregated at the grain boundaries and grain boundary fracture has occurred due to the fact that P of the steel material exceeds 0.02%.
Test No. of the comparative example. In No. 26, the low cycle bending fatigue strength is low. This is because the steel component composition is within the range of the present invention, but the core hardness is lower than HV400.
The reason why the core hardness is lower than HV400 is that the temperature of the quenching oil is as high as 220 ° C. and the quenching is insufficient.
Test No. of the comparative example. In No. 27, the low cycle bending fatigue strength is low. This is because the component composition of the steel is within the scope of the present invention, but the core hardness exceeds HV550.
The reason why the core hardness exceeds HV550 is that the amount of C is relatively high at 0.6% and the temperature of the quenching oil is as low as 20 ° C.

As described above, by using the carburized steel part having excellent low cycle bending fatigue strength according to the present invention, gears such as a differential gear for automobiles can be significantly reduced in size and weight. It becomes possible to improve fuel consumption and reduce CO ₂ emission. Therefore, the effect of the present invention is extremely remarkable, and the present invention has great industrial applicability.

1 Test piece 2 Load X Notch

Claims

% By mass
C: 0.1 to 0.6%
Si: 0.01 to 1.5%,
Mn: 0.3 to 2.0%,
P: 0.02% or less,
S: 0.001 to 0.15%,
N: 0.001 to 0.03%,
Al: 0.001 to 0.06%, and
O: contains 0.005% or less,
A steel part in which the balance is substantially iron and inevitable impurities, carburized and then tempered,
A carburized steel part excellent in low cycle bending fatigue strength, characterized by having a surface hardness of HV550 to HV800 and a core hardness of HV400 to HV500.
The carburized steel part having excellent low cycle bending fatigue strength according to claim 1, wherein the low cycle bending fatigue strength is 20 kN or more.
The carburized steel part having excellent low cycle bending fatigue strength according to claim 1 or 2, wherein the steel material further contains B: 0.0002 to 0.005% by mass%.
The carburization excellent in low cycle bending fatigue strength according to any one of claims 1 to 3, wherein the steel further contains Cr: 1.20 to 3.0% by mass. Steel parts.
The carburization excellent in low cycle bending fatigue strength according to any one of claims 1 to 4, wherein the steel material further contains Ti: 0.01 to 0.2% by mass%. Steel parts.
The steel material further contains, as an inevitable component, one or more of Mo: less than 0.1%, Cu: less than 0.1%, and Ni: less than 0.1% by mass% The carburized steel part excellent in low cycle bending fatigue strength according to any one of claims 1 to 5.
The steel material is further one or two kinds by mass% of Mo: 0.1 to 1.5%, Cu: 0.1 to 2.0%, and Ni: 0.1 to 5.0%. The carburized steel part excellent in low cycle bending fatigue strength according to any one of claims 1 to 5, characterized by containing the above.
The steel material further contains one or two of Nb: 0.01 to 0.2% and V: 0.03 to 0.2% by mass%. The carburized steel part excellent in low cycle bending fatigue strength of any one of ~ 7.
Further, the steel material is one or two of mass%, Ca: 0.0002 to 0.005%, Zr: 0.0003 to 0.005%, and Mg: 0.0003 to 0.005%. The carburized steel part excellent in low cycle bending fatigue strength according to any one of claims 1 to 8, characterized by containing the above.
The carburized steel part excellent in low cycle bending fatigue strength according to any one of claims 1 to 9, wherein the carburized steel part is a differential gear or a transmission gear.