KR100883716B1 - Composition for Machine Structure, Method of Producing the Same and Material for Induction Hardening - Google Patents

Abstract

In the mechanical structural part which has a hardened layer by high frequency quenching in at least one part, by making the hardened layer into Hv 750 or more and the average particle diameter of old austenite particle being 7 micrometers or less over the whole thickness of a hardened layer, The present invention provides a mechanical structural component having further improved fatigue strength.

Mechanical structure, high frequency quenching, hardened layer, fatigue strength, austenite

Description

Composition for Machine Structure, Method of Producing the Same and Material for Induction Hardening

The present invention relates to a machine structural part having a hardened layer at least partially by high frequency quenching. Here, the mechanical structural components include a drive shaft, an input shaft, an output shaft, a crankshaft, an inner ring, an outer ring, and a hub for automobiles. And gears.

Conventionally, for example, mechanical structural parts such as driving shafts and constant velocity joints for automobiles are subjected to hot forging, further cutting, cold forging, etc., to be processed into a predetermined shape, and then subjected to high frequency quenching-tempering. It is common to secure fatigue strength such as torsional fatigue strength, bending fatigue strength, rolling fatigue strength and sliding rolling fatigue strength, which are important characteristics of mechanical structural parts.

On the other hand, in recent years, there is a strong demand for weight reduction of automotive parts from environmental problems, and from this point of view, further improvement of fatigue strength in automotive parts has been demanded.

As a means of improving the fatigue strength as described above, various methods have been proposed until now.

For example, in order to improve the torsion fatigue strength, it is conceivable to increase the quenching depth by high frequency quenching. However, fatigue strength is saturated at any depth even if the quenching depth is increased.

Moreover, the improvement of grain boundary strength is also effective for the improvement of a torsional fatigue strength, In this viewpoint, the technique of refine | purifying a spherical austenite grain size by dispersing TiC is proposed (for example, refer patent document 1).

In the technique described in Patent Document 1, since fine TiC is dispersed in a large amount at the time of high frequency quenching heating, it is necessary to miniaturize the former austenite particle diameter. Therefore, it is necessary to solidify TiC before quenching. In the hot rolling step, a step of heating to 1100 ° C. or more is adopted. Therefore, it is necessary to raise heating temperature at the time of hot rolling, and there existed a problem that productivity fell.

Moreover, the technique disclosed in the said patent document 1 also left a problem that it cannot fully satisfy the demand for recent fatigue strength.

In addition, Patent Literature 2 limits the ratio (CD / R) of the cured layer depth CD to the radius R of the high frequency quenched shaft component to 0.3 to 0.7, and then 1 mm from the surface after the CD / R and the high frequency quenched. The value A defined by the austenite particle diameter γf up to, the average Vickers hardness Hf up to (CD / R) = 0.1 in the high frequency quenching zone, and the average Vickers hardness Hc in the center of the shaft after high frequency quenching, is determined according to the amount of C A mechanical structural axis part which improves torsional fatigue strength by controlling to the range is proposed.

However, even if the CD / R is controlled, there is a limit to the improvement of the fatigue characteristics, and it has not been sufficiently able to meet the recent demand for torsional fatigue strength.

Patent Document 1: Japanese Patent Laid-Open No. 2000-154819 (claims, paragraph [0008])

Patent Document 2: Japanese Patent Laid-Open No. 8-53714 (Scope of Claim)

Disclosure of the Invention

Problems to be Solved by the Invention

It is an object of the present invention to provide a mechanical structural part and a method of manufacturing the same, and furthermore, a material for high frequency quenching, which can further improve fatigue strength after conventional high frequency quenching.

Means to solve the problem

The inventors made intensive studies to effectively improve the fatigue strength of the steel by high frequency quenching. In particular, attention has been paid to torsional fatigue strength as a representative example of the fatigue strength, and detailed examination has been conducted, and the following knowledge has been obtained.

(i) The fatigue strength is improved by increasing the intragranular strength, that is, the hardness of the cured layer by high-frequency quenching, but when the hardness is increased to Vickers hardness Hv 750 or more, fracture is broken from intragranular fracture to old austenite grain In order to move to fracture in), the fatigue strength does not improve even if the hardness is further increased.

(ii) By miniaturizing the old austenite grain size of the hardened layer by high-frequency quenching, the strength of the old austenite grain boundary can be improved, and the average old austenite grain size being 7 μm or less, even if the hardness is Hv 750 or more. The increase in fatigue strength can also be achieved.

(iii) In order to increase the hardness of the hardened layer to Hv 750 or higher, it is effective to increase the content of one or two or more of C, Si, and P in the raw material, and the former austenite particle diameter of the hardened layer by high frequency quenching. In order to refine the microstructure, Mo, B, Ti is contained in the material, and the high-frequency quenching pre-tissue is made into fine bainite or martensite into which the processing deformation by cold working is introduced. It is effective to use rapid heating to lower the heating temperature and to shorten the residence time at 800 ° C or higher.

(iv) Moreover, although another tempering is usually performed after high frequency quenching, it is also possible to increase the intraparticle strength by omitting this.

This invention is based on the said knowledge.

That is, the summary structure of this invention is as follows.

1. At least a portion of the hardened layer by high-frequency quenching, the hardened layer has a hardness of at least Hv 750, and the average particle diameter of the old austenite particles is 7㎛ or less over the entire thickness of the hardened layer .

2.C: 0.3-1.5 mass%,

Si: 0.05-3.O mass%,

Mn: 0.2-2.20 mass%,

A1: 0.25 mass% or less,

Ti: 0.005-0.1 mass%,

Mo: 0.05-0.6 mass%,

B: 0.0003-0.006 mass%

S: 0.1 mass% or less, and

P: 0.10 mass% or less

And any one of the following formulas (1) to (3) satisfies at least one formula, and the balance has a component composition of Fe and unavoidable impurities. .

Below

C> 0.7 mass%. (One)

Si> 1.1 mass%. (2)

P> 0.02 mass%. (3)

3. The content of A1 in the above composition

Al: 0.005-0.25 mass%

Machine part for machine structure as described in said 2 characterized by that.

4. As said ingredient composition,

Cr: 2.5 mass% or less,

Cu: 1.O mass% or less,

Ni: 3.5 mass% or less,

Co: 1.O mass% or less,

Nb: 0.1 mass% or less,

V: 0.5 mass% or less,

Ta: 0.5 mass% or less,

Hf: 0.5 mass% or less, and

Sb: 0.015 mass% or less

The machine structural part as described in said 2 or 3 which further contains 1 type (s) or 2 or more types selected from among.

5. As the ingredient composition,

W: 1.0 mass% or less,

Ca: 0.005 mass% or less,

Mg: 0.005 mass% or less,

Te: 0.1 mass% or less,

Se: 0.1 mass% or less,

Bi: 0.5 mass% or less,

Pb: 0.5 mass% or less,

Zr: 0.01 mass% or less, and

REM: 0.1 mass% or less

The machine structural part in any one of said 2-4 characterized by further containing 1 type (s) or 2 or more types selected from among.

6.C: 0.3-1.5 mass%,

Si: 0.05-3.O mass%,

Mn: 0.2-2.20 mass%,

A1: 0.25 mass% or less

Ti: 0.005-0.1 mass%,

Mo: 0.05-0.6 mass%,

B: 0.0003-0.006 mass%

S: 0.1 mass% or less, and

P: 0.10 mass% or less

Wherein the remainder is composed of a component composition of Fe and unavoidable impurities, and the cured layer is not tempered.

7. The content of Al in the composition of the composition,

Al: 0.005-0.25 mass%

The mechanical structural part as described in said 6 characterized by the above-mentioned.

8. As the ingredient composition,

Cr: 2.5 mass% or less,

Cu: 1.O mass% or less,

Ni: 3.5 mass% or less,

Co: 1.O mass% or less,

Nb: 0.1 mass% or less,

V: 0.5 mass% or less,

Ta: 0.5 mass% or less,

Hf: 0.5 mass% or less, and

Sb: 0.015 mass% or less

The machine structural part as described in said 6 or 7 which further contains 1 type (s) or 2 or more types selected from among.

9. As the ingredient composition,

W: 1.0 mass% or less,

Ca: 0.005 mass% or less,

Mg: 0.005 mass% or less,

Te: 0.1 mass% or less,

Se: 0.1 mass% or less,

Bi: 0.5 mass% or less,

Pb: 0.5 mass% or less,

Zr: 0.01 mass% or less, and

REM: 0.1 mass% or less

The machine structural part in any one of said 6-8 which further contains the 1 type (s) or 2 or more types selected from among.

10. The mechanical structural part according to any one of 2 to 9 above, wherein the Mo-based precipitates are dispersed at 500 or more per 1 μm ^{3 and} the average particle diameter of the Mo-based precipitates is 20 nm or less.

11.C: 0.3-1.5 mass%,

Si: 0.05-3.O mass%,

Mn: 0.2-2.20 mass%,

Al: 0.25 mass% or less,

Ti: 0.005-0.1 mass%,

Mo: 0.05-0.6 mass%,

B: 0.0003-0.006 mass%

S: 0.1 mass% or less, and

P: 0.10 mass% or less

At least one of the following formulas (1) to (3), wherein the remainder satisfies at least one formula, and the balance is at least one time of high frequency quenching on at least a part of the material which is a component composition of Fe and unavoidable impurities. In the manufacture of mechanical structural components,

The sum of either or both of the bainite structure and martensite structure in the stressed yarn before the high frequency quenching of the material is adjusted to 10 volume% or more,

A method for producing a mechanical structural part, characterized in that the attainment temperature of the high frequency quenching is set to 1000 ° C or less.

Below

C> 0.7 mass%. (One)

Si> 1.1 mass%. (2)

P> 0.02 mass%. (3)

12. The content of Al in the composition of the composition,

Al: 0.005-0.25 mass%

The manufacturing method of the machine structural part of 11 characterized by the above-mentioned.

13. As the ingredient composition,

Cr: 2.5 mass% or less,

Cu: 1.0 mass% or less,

Ni: 3.5 mass% or less,

Co: 1.O mass% or less,

Nb: 0.1 mass% or less,

V: 0.5 mass% or less,

Ta: 0.5 mass% or less,

Hf: 0.5 mass% or less, and

Sb: 0.015 mass% or less

The manufacturing method of the machine structural part of 11 or 12 characterized by further containing 1 type (s) or 2 or more types selected from among.

14. As said ingredient composition,

W: 1.0 mass% or less,

Ca: 0.005 mass% or less,

Mg: 0.005 mass% or less,

Te: 0.1 mass% or less,

Se: 0.1 mass% or less,

Bi: 0.5 mass% or less,

Pb: 0.5 mass% or less,

Zr: 0.01 mass% or less, and

REM: 0.1 mass% or less

The manufacturing method of the machine structural part in any one of said 11-13 characterized by further containing 1 type (s) or 2 or more types selected from among.

15. A material for high frequency quenching for making a structural steel material having a hardened layer having an average sphere austenite grain size of 7 μm or less due to high frequency quenching on at least part of the surface thereof,

C: 0.3-1.5 mass%,

Si: 0.05-3.O mass%,

Mn: 0.2-2.20 mass%,

Al: 0.25 mass% or less

Ti: 0.005-0.1 mass%,

Mo: 0.05-0.6 mass%,

B: 0.0003-0.006 mass%

S: 0.1 mass% or less, and

P: 0.10 mass% or less

And any one of the following formulas (1) to (3) satisfies at least one formula, and the balance has a component composition of Fe and unavoidable impurities, and further, any one of bainite structure and martensite structure. A material for high frequency quenching, characterized in that it has a stressed weave in which the sum of one or both sides is 10% by volume or more.

Below

C> 0.7 mass%. (One)

Si> 1.1 mass%. (2)

P> 0.02 mass%. (3)

16. The content of Al in the composition of the composition,

A1: 0.005-0.25 mass%

The high frequency quenching material as described in said 15 characterized by the above-mentioned.

17. As said ingredient composition,

Cr: 2.5 mass% or less,

Cu: 1.O mass% or less,

Ni: 3.5 mass% or less,

Co: 1.O mass% or less,

Nb: 0.1 mass% or less,

V: 0.5 mass% or less,

Ta: 0.5 mass% or less,

Hf: 0.5 mass% or less, and

Sb: 0.015 mass% or less

The high frequency quenching material as described in said 15 or 16 which further contains 1 type (s) or 2 or more types selected from among.

18. As said ingredient composition,

W: 1.O mass% or less,

Ca: 0.005 mass% or less,

Mg: 0.005 mass% or less,

Te: 0.1 mass% or less,

Se: 0.1 mass% or less,

Bi: 0.5 mass% or less,

Pb: 0.5 mass% or less,

Zr: 0.01 mass% or less, and

REM: 0.1 mass% or less

The high frequency quenching material according to any one of 15 to 17, further comprising one or two or more selected from among them.

19. The high frequency quenching material according to any one of 15 to 18 above, wherein the Mo-based precipitates are dispersed at 500 or more per 1 μm ^{3 and} the average particle diameter of the Mo-based precipitates is 20 nm or less.

Effects of the Invention

According to the present invention, it is possible to stably obtain a mechanical structural component having excellent fatigue characteristics, which is typical of torsional fatigue characteristics and electric fatigue characteristics, and as a result, exhibits a great effect on the demand for weight reduction of automotive components and the like. do.

1 is a graph showing the effect of the heating temperature during high-frequency quenching on the former austenite grain size of the hardened layer for Mo-added steel and Mo-free steel.

2 is a transmission electron micrograph of a fine precipitate (Mo-based precipitate) effective for ultrafine γ particles.

3 is a graph showing the relationship between the average spherical austenite grain size and the torsional fatigue strength for Mo-added steel and Mo-free steel.

4 is a graph showing the relationship between the average sphere austenite particle diameter and the torsional fatigue strength with or without tempering.

5 is a partial cross-sectional view of a constant velocity joint.

6 is a cross-sectional view showing a quenched tissue layer in a constant velocity joint inner ring.

Implement the invention  Best form for

Hereinafter, the present invention will be described in detail.

The mechanical structural parts of the present invention are composed of various shapes and structures for each component, such as driving shafts, input shafts, output shafts, crankshafts, constant velocity joints, inner and outer rings, hubs, and gears for automobiles. It is important to have a hardened layer quenched in part or all of which strength is required, and this hardened layer is Hv 750 or more and it is important that the average particle diameter of old austenite particle is 7 micrometers or less over the whole thickness of a hardened layer.

Below, the result of the research which led to this knowledge is demonstrated.

[Old Austenitic Particle Size of Hardened Layer]

If the average spherical austenite grain size of the hardened layer by high frequency quenching exceeds 7 µm, even if the hardness of the hardened layer is increased to Hv 750 or more to improve the intragranular strength, as described later, fatigue fracture will occur. It occurs from the grain boundary. Therefore, the old austenite particle diameter of the hardened layer is 7 µm or less, preferably 6 µm or less, more preferably 5 µm or less, and more preferably 3 µm or less. This is because the grain boundary strength becomes remarkably strong as the grain size is made smaller. Conventionally, even if the intensity | strength in a particle | grain is raised, the grain boundary strength does not rise, but it becomes a grain boundary intensity rate, and further high strength could not be desired. However, by making the grain size finer, the grain boundary strength rises dramatically, so that a new high strength can be expected.

Here, the average old austenite particle diameter of the high frequency quenched portion was measured as follows.

The outermost layer of the hardened layer after high frequency quenching has a martensite structure of 100% in area ratio. And as it goes from the surface of the hardened layer to the inside, the area of the 100% martensite structure continues up to a certain thickness, but thereafter, the area ratio of the martensite structure suddenly decreases. In this invention, the area | region until the area ratio of martensite structure is reduced to 98% from the surface of a high frequency quenching part was made into the hardened layer, and the average depth was made into the hardened layer thickness from the surface.

And about the hardened layer, the average old austenite particle diameter in the 1/5 position, 1/2 position, and 4/5 position of the whole thickness is measured from the surface, and the average old austenite in either position is measured. When the nit particle diameter was 7 µm, the average particle diameter of the old austenite particles was 7 µm or less over the entire thickness of the cured layer.

On the other hand, the average particle diameter of the old austenite particles was obtained by dissolving 50 g of piclinic acid in 500 g of water after curing the cross section of the hardened layer, followed by corrosion with a corrosion solution to which 11 g of dodecylbenzenesulfonate, 1 g of ferrous chloride, and 1.5 g of oxalic acid were added. By using an optical microscope, 5 fields were observed for each position at a magnification of 400 times (area of 1 field: 0.25mm × 0.225mm) to 1000 times (area of 1 field: 0.10mm × 0.09mm). Was measured.

Incidentally, in the case of relying only on the structure near the pole surface layer, such as electric fatigue, even if the thickness of the cured layer is about 1 mm, its effect can be obtained, but in the case of torsion fatigue strength, the thickness of the cured layer Is preferably at least 2 mm. More preferably at least 2.5 mm, still more preferably at least 3 mm.

[Vickers Hardness of Cured Layer]

If the Vickers hardness Hv of the cured layer is less than 750, the intragranular strength of the cured layer is weak. Therefore, even if the old austenite particles are made fine, the improvement of the fatigue strength suitable for it cannot be expected. In other words, even if the austenite particles are made finer to increase grain boundary strength as described above, if the strength in the particles is not increased, the rate of breakage rate in the particles is increased and the increase in static strength and fatigue strength cannot be expected. Therefore, in the present invention, the Vickers hardness (corresponding to the strength in the particles) Hv of the cured layer needs to be 750 or more. On the other hand, the upper limit of the Vickers hardness Hv of the cured layer is not particularly defined, but since the amount of added elements also increases in excess of 900, the machinability, cold forging property, and quenching cracking property of the base material are lowered. It is preferable that it is the following.

Here, the Vickers hardness is an average of five RBIs at 98 N (10 kgf) at a position 1/5 on the surface of the cured layer thickness.

[Composition]

Next, the preferable component composition is demonstrated in order to raise along the intraparticle strength of the hardened layer which has the above-mentioned former austenite particle diameter and Vickers hardness.

C: 0.3-1.5 mass%

C is the element which has the greatest influence on the high frequency hardenability, and increases the intra-particle strength of the hardened layer, and also thickens the high frequency hardened part to contribute to the improvement of the fatigue strength. However, if the amount is less than 0.3% by mass, in order to secure the required torsional fatigue strength, the hardened layer must be significantly thickened. As a result, hardening cracks are remarkable or the bainite structure described later is formed. It is difficult to obtain. On the other hand, when it exceeds 1.5 mass%, it becomes disadvantageous about ensuring cutting property, cold forging property, and hardenability cracking resistance. Therefore, it is preferable that the amount of C is 0.3-1.5 mass%.

Si: 0.05-3.O mass%

Si raises the intraparticle strength of a hardened layer and contributes to the improvement of fatigue strength. Moreover, it is an element useful also in obtaining the bainite structure demonstrated later, From this meaning, it is preferable to contain 0.05 mass% or more. However, when it exceeds 3 mass%, it becomes difficult to solidify the ferrite to secure cutting property and cold forging, so it is preferable to set it to 3 mass% or less.

Mn: 0.2-2.20 mass%

Mn is an element which is indispensable for improving the high frequency hardenability and securing the thickness of the cured layer. However, when the amount is less than 0.2 mass%, the effect is lacking. Therefore, it is preferable to make Mn amount 0.2 mass% or more, and also 0.3 mass% or more. On the other hand, when it exceeds 2.0 mass%, residual austenite will increase after hardening, and the hardness of a surface layer part will fall easily. Therefore, it is preferable to make Mn into 2.0 mass% or less. On the other hand, when there is too much Mn amount, there exists a tendency for machinability to become disadvantageous, It is more preferable to set it as 1.2 mass% or less, and it is more preferable to set it as 1.0 mass% or less.

Al: 0.005-0.25 mass%

Al is an effective element for deoxidation of steel. Moreover, it is an element effective also in suppressing grain growth of austenite at the time of heating of high frequency quenching, and refining a high frequency quenching part. On the other hand, when it exceeds 0.25 mass%, the effect will be saturated, but it will raise the component cost rather. Therefore, it is preferable to make Al amount into 0.25 mass% or less. On the other hand, since the effect of Al is not expressed when the quantity is less than 0.001 mass%, it is more preferable to set it as 0.001 mass% or more. It is more preferable to set it as 0.005 mass% or more.

Ti: 0.005-0.1 mass%

Ti binds to N mixed with steel as an unavoidable impurity, and B described later becomes BN, and has an effect of preventing the loss of high frequency hardenability. Therefore, it is preferable to contain the quantity 0.005 mass% or more. On the other hand, when it exceeds 0.1 mass%, TiN will be formed in a large quantity, and this will become a starting point of fatigue destruction, and it will tend to reduce fatigue strength, It is preferable to make Ti amount into 0.005 to 0.1 mass%. More preferably, it is 0.01-0.07 mass%. In addition, in order to reliably precipitate solid solution N as TiN and to effectively exhibit the hardenability of B, it is preferable to control the amount of Ti and N so that Ti (mass%) / N (mass%) ≧ 3.42.

Mo: 0.05-0.6 mass%

Mo has a function of facilitating formation of bainite structure after hot working, thereby miniaturizing austenite during heating of high frequency quenching, and atomizing the hardened layer. In addition, it has the effect of suppressing grain growth of austenite during high frequency quenching heating and making the cured layer fine. In particular, when the heating temperature of the high-frequency quenching is set to 800 to 1000 ° C, preferably 800 to 950 ° C, the growth of austenite particles can be significantly suppressed. Moreover, since it is an element effective for hardening improvement, it is used also for adjustment of hardenability. In addition, it has the effect of inhibiting the formation of carbides and preventing the decrease in grain boundary strength.

Thus, Mo is a very useful element in order to obtain the effect of the present invention, but if the amount is 0.05% by mass or more, the average spherical austenite particle diameter of the cured layer can be easily 7 μm or less, so it is preferably 0.05% by mass or more. Do. On the other hand, when Mo amount exceeds 0.6 mass%, the hardness of the steel material at the time of hot working for forming into a part shape will rise markedly, and workability will fall. Therefore, the Mo amount is preferably 0.05 to 0.6 mass%. More preferably, it is 0.1-0.6 mass%, More preferably, it is 0.3-0.4 mass%.

On the other hand, according to a review by the inventors, as a possibility of the miniaturization effect of the old austenite particles by Mo, the attracting effect (Solute Drug Effect: Solute Drug Effect), pinning effect, etc. by solid solution atoms are considered. . It is not always clear at this point how much effect both or other effects will have, but it has been confirmed that at least a pinning effect may occur. Details will be described later.

B: 0.0003-0.006 mass%

B contains a bainite structure or martensite structure as described below for the entire high frequency quenching structure, and is further useful for miniaturizing the former austenite grain size of the hardened layer. In addition, by adding a small amount, high-frequency hardenability is improved, and the cured layer is thickened, thereby improving the fatigue strength. It also has the effect of preferentially segregating at grain boundaries, reducing the concentration of P segregating at grain boundaries, increasing grain strength, and improving fatigue strength. However, if the amount is less than 0.0003 mass%, the effect is lacking. On the other hand, when it exceeds 0.006 mass%, the effect is saturated, but rather raises a component cost. Therefore, the amount of B is preferably 0.0003 to 0.006 mass%. More preferably, it is 0.0005-0.004 mass%, More preferably, it is 0.0015-0.003 mass%.

S: 0.1 mass% or less

S is an element which forms MnS and improves the machinability of steel, but when it exceeds 0.1 mass%, it segregates at grain boundaries and lowers grain boundary strength. Therefore, it is preferable to make S amount into 0.1 mass% or less. More preferably, it is 0.06 mass% or less.

P: 0.10 mass% or less

P raises the intraparticle strength of a hardened layer, and contributes to the improvement of fatigue strength. However, when it exceeds 0.10 mass%, it will segregate in a grain boundary and will reduce grain strength. Therefore, it is preferable to make P amount 0.1 mass% or less.

The balance other than the above elements is good for Fe and unavoidable impurities. However, it is particularly preferable to adjust the component composition so as to satisfy at least one of the following formulas (1) to (3).

Below

C> 0.7 mass%. (One)

Si> 1.1 mass%. (2)

P> 0.02 mass%. (3)

By satisfying any one of the above formulas (1) to (3), the Vickers hardness Hv of the cured layer can be increased to 750 or more, thereby increasing the intraparticle strength, and the fatigue caused by miniaturizing the average sphere austenite grain size to 7 µm or less. The strength improving effect is remarkably expressed.

In addition, in this invention, the tempering process normally performed after high frequency quenching can also be abbreviate | omitted. In this case, tempering softening does not occur. Therefore, even if none of the above formulas (1), (2) and (3) is satisfied, Hv 750 or more can be satisfied within the composition range. Therefore, when omission is omitted, it is not necessary to satisfy at least one of the above (1), (2) and (3).

In addition to the above component composition, it is effective to improve the new fatigue strength by containing one or two or more selected from the elements shown below.

Cr: 2.5 mass% or less

Cr is effective for improving hardenability and may be added since it is an element useful for securing a curing depth. However, when excessively contained, the carbides are stabilized to encourage the formation of residual carbides, and the grain boundary strengths are lowered to deteriorate the fatigue strength. Therefore, it is preferable to reduce Cr content as much as possible, but it can tolerate up to 2.5 mass%. Preferably it is 1.5 mass% or less. On the other hand, in order to express the improvement effect of hardenability, it is preferable to contain 0.03 mass% or more.

Cu: 1.O mass% or less

Cu is effective for improving hardenability, and is dissolved in ferrite, thereby enhancing fatigue strength. In addition, by suppressing the formation of carbides, the reduction of grain boundary strength due to carbides is suppressed, and the fatigue strength is improved. However, if the content exceeds 1.0% by mass, cracking occurs during hot working. Therefore, the content is 1.0% by mass or less. More preferably, it is 0.5 mass% or less. On the other hand, when the addition is less than 0.03% by mass, the effect of improving the hardenability and the effect of suppressing the decrease in grain boundary strength is small. Preferably it is 0.1-1.0 mass%.

Ni: 3.5 mass% or less

Since Ni is an element which improves hardenability, it is used when adjusting hardenability. It is also an element that suppresses the formation of carbides, suppresses the decrease in grain boundary strength due to carbides, and improves the fatigue strength. However, since Ni is an extremely expensive element and the steel material cost rises when it exceeds 3.5 mass%, it is set as 3.5 mass% or less. On the other hand, when the addition is less than 0.05% by mass, the effect of improving the hardenability and the effect of suppressing the decrease in grain boundary strength is small. Preferably it is 0.1-1.10 mass%.

Co: 1.O mass% or less

Co is an element that suppresses the formation of carbides, suppresses the decrease in grain boundary strength due to carbides, and improves the fatigue strength. However, Co is an extremely expensive element, and if it is added in excess of 1.O mass%, the steel material cost rises, so the addition of 1.O mass% or less. On the other hand, when the addition is less than 0.01% by mass, since the effect of suppressing the decrease in grain boundary strength is small, it is preferable to add 0.01% by mass or more. Preferably it is 0.02-0.5 mass%.

Nb: 0.1 mass% or less

Nb not only has an effect of improving hardenability, but also combines with C and N in steel to act as a precipitation strengthening element. Moreover, it is an element which improves temper softening resistance, and a fatigue strength is improved by these effects. However, even if it contains exceeding 0.1 mass%, since an effect is saturated, 0.1 mass% shall be an upper limit. On the other hand, when the addition is less than 0.005% by mass, it is preferable to add 0.005% by mass or more because the effect of enhancing the precipitation strengthening effect and the temper softening resistance is small. Preferably it is 0.01-0.05 mass%.

V: 0.5 mass% or less

V combines with C and N in steel and acts as a precipitation strengthening element. Moreover, it is an element which improves temper softening resistance, and a fatigue strength is improved by these effects. However, since the effect is saturated even if it contains exceeding 0.5 mass%, you may be 0.5 mass% or less. On the other hand, since the effect of improving fatigue strength is small in addition of less than 0.01 mass%, it is preferable to add 0.01 mass% or more. Preferably it is 0.03-0.3 mass%.

Ta: 0.5 mass% or less

Ta may be added because it is effective in delaying microstructure change and in preventing fatigue strength, in particular, deterioration of electric fatigue. However, even if the content increases more than 0.5 mass%, since it does not contribute to further strength improvement, it is made into 0.5 mass% or less. On the other hand, in order to express the improvement effect of fatigue strength, it is preferable to set it as 0.02 mass% or more.

Hf: 0.5 mass% or less

Hf may be added because it is effective against the delay of the microstructure change and has the effect of preventing the deterioration of fatigue strength, in particular, rolling fatigue. However, even if the content exceeds 0.5% by mass, the content is not increased further. Therefore, the content is made 0.5% by mass or less. On the other hand, in order to express the improvement effect of fatigue strength, it is preferable to set it as 0.02 mass% or more.

Sb: 0.015 mass% or less

Sb is effective against the delay of the microstructure change, and may be added because it is effective in preventing the fatigue strength, in particular, the deterioration of the electric fatigue. However, since the toughness deteriorates when its content exceeds 0.015 mass%, since its toughness deteriorates, it is 0.015 mass% or less, Preferably it is 0.010 mass% or less. On the other hand, in order to express the improvement effect of fatigue strength, it is preferable to set it as 0.005 mass% or more.

Moreover, in order to improve the machinability of steel, it is preferable to contain the element shown below.

W: 1.O mass% or less

W is an element which improves machinability by embrittlement. However, even if it adds in excess of 1.0 mass%, since an effect not only saturates but a cost rises and it becomes economically disadvantageous, it is preferable to make it contain 1.0 mass% or less. On the other hand, in order to improve machinability, it is preferable to contain W 0.005 mass% or more.

Ca: 0.005 mass% or less

Ca forms a sulfide together with MnS, which acts as a chip breaker, thereby improving the machinability and can be added as necessary. However, even if it contains exceeding 0.005 mass%, since the effect is not only saturated but also raises a component cost, it was made into 0.005 mass% or less. On the other hand, even if it contains less than 0.0001 mass%, since the machinability improvement effect is small, it is preferable to contain 0.0001 mass% or more.

Mg: 0.005 mass% or less

Mg is not only a deoxidation element but also becomes a stress concentration source, and has an effect of improving machinability and can be added as necessary. However, when added excessively, not only the effect was saturated but also the component cost increased, so it was made into 0.005 mass% or less. On the other hand, even if it contains less than 0.0001 mass%, since the machinability improvement effect is small, it is preferable to contain 0.0001 mass% or more.

Te: 0.1 mass% or less

Se: 0.1 mass% or less

Se and Te combine with Mn to form MnSe and MnTe, respectively, which improves machinability by acting as chip breakers. However, when the content exceeds 0.1% by mass, not only the effect is saturated but also an increase in the component cost. Therefore, the content is set to 0.1% by mass or less. Moreover, in order to improve machinability, it is preferable to contain in 0.003 mass% or more in the case of Se, and 0.003 mass% or more in case of Te.

Bi: 0.5 mass% or less

Bi improves machinability by melting, lubrication and embrittlement at the time of cutting, and therefore Bi can be added for this purpose. However, even if it adds exceeding 0.5 mass%, since the effect is not only saturated but a component cost rises, it was made into 0.5 mass% or less. On the other hand, even if it contains less than 0.01 mass%, since the machinability improvement effect is small, it is preferable to contain 0.01 mass% or more.

Pb: 0.5 mass% or less

Pb improves machinability by melting, lubrication and embrittlement at the time of cutting, and can therefore be added for this purpose. However, even if it adds exceeding 0.5 mass%, since the effect is not only saturated but a component cost rises, it was made into 0.5 mass% or less. On the other hand, even if it contains less than 0.01 mass%, since the machinability improvement effect is small, it is preferable to contain 0.01 mass% or more.

Zr: 0.01 mass% or less

Zr forms sulfides with MnS, which improves machinability by acting as chip breakers. However, even if it contains exceeding 0.01 mass%, since the effect is not only saturated but also raises a component cost, it was made into 0.01 mass% or less. On the other hand, even if it contains less than 0.003 mass%, since the machinability improvement effect is small, it is preferable to contain 0.003 mass% or more.

REM: 0.1 mass% or less

REM forms sulfides with MnS, which improves machinability by acting as chip breakers. However, even if it contains more than 0.1 mass% of REM, since not only the effect is saturated but also raises a component cost, it was made to contain in the said range, respectively. On the other hand, in order to improve machinability, it is preferable to contain REM 0.0001 mass% or more.

As mentioned above, although the preferable component composition range was demonstrated, the average particle diameter of 7 micrometers or less old austenite particle mentioned above is obtained by restrict | limiting a component composition to the said range and making into the structure demonstrated below the high frequency quenching before high frequency quenching. Can be.

That is, the structure of the base material, that is, the structure before quenching (corresponding to a structure other than the hardened layer after high frequency quenching) has bainite structure and / or martensite structure, and any one of bainite structure and martensite structure It is preferable to make the sum total of one or both sides into 10 volume% or more. This is because the bainite or martensite structure is a finely dispersed carbide structure compared to the ferrite-perlite structure, so that the area of the ferrite / carbide interface, which is the nucleation site of austenite during quenching heating This is because the increased austenite is made finer, which contributes effectively to miniaturizing the particle size of the hardened hardened layer. As grain size of the hardened hardened layer is refined, the grain boundary strength rises and the fatigue strength improves.

It is more preferable that the sum of either or both of the bainite structure and martensite structure is 20% by volume or more.

Moreover, it is preferable that the upper limit of the structure fraction of the sum total of either or both of bainite structure and martensite structure is about 90 volume%. That is, when the total tissue fraction exceeds 90% by volume, the micronizing effect of the former austenite particles of the hardened layer due to quenching is saturated, and the machinability deteriorates rapidly.

On the other hand, in terms of the grain size of the hardened layer after quenching, the martensite structure has the same effect as that of the bainite structure, but from an industrial point of view, the amount of alloy element added is smaller in the bainite structure than in the martensite structure. It is also advantageous in terms of machinability, and is advantageous in manufacturing because it can be produced at a low cooling rate.

In addition, the ratio of the volume fraction of bainite and martensite is preferably about bainite: martensite = 100: 0 to 40:60. The martensite structure is preferable for the microstructure of the former austenite grain size of the martensite of the hardened layer after the high frequency quenching. However, since martensite is hard, when a large amount is contained in a mother phase, machinability falls. Therefore, the fraction ratio of bainite and martensite is preferably bainite: martensite = 100: 0 to 40:60.

Next, the manufacturing method of the mechanical structural component of this invention is demonstrated.

In the machine structural part of the present invention, the steel material having the above-described composition is subjected to hot working such as bar rolling or hot forging to form a part, and at least a part of the part is subjected to high frequency quenching under a heating temperature of 800 to 1000 ° C. It can manufacture by doing. At least part of this is to be a site where fatigue strength is required.

In order to make the average old austenite particle diameter of a high frequency quenching part into 7 micrometers or less, there is the following method.

The total processing rate at 800-1000 degreeC at the time of hot working is made into 80% or more, and the temperature range of 700-500 degreeC is cooled at the speed of 0.2 degree-C / s or more after that. According to this condition, the structure before quenching can be made into uniform fine bainite and / or martensite structure (tissue fraction 10 volume% or more). That is, since bainite and martensite are structures in which carbides are finely dispersed compared to ferrite-perlite structure, the area of the interface of ferrite / carbide, which is the nucleation site of austenite, is increased during high-frequency quenching heating. One austenite is advantageous for miniaturization. For this purpose, the tissue fraction of the sum of one or both of bainite and martensite is required to be 10 vol% or more. And when the cooling rate in the temperature range of 700-500 degreeC is less than 0.2 degree-C / s, the structure fraction of the sum of one or both of bainite and martensite cannot be 10 volume% or more. More preferably, the cooling rate is 0.5 ° C / s or more. On the other hand, the ratio of the volume fraction of bainite and martensite is preferably about bainite: martensite = 100: 0 to 40:60 as described above.

Further, by performing 20% or more processing (hereinafter referred to as a second processing step) in a temperature range of less than 800 ° C. before the high frequency quenching, the bainite and / or martensite structure before the high frequency quenching can be made finer, Since new refinement | miniaturization of the old austenite particle after high frequency quenching can be achieved, it is preferable to add a 2nd processing process. Machining in the temperature range below 800 ° C is a hot working step and may be performed before the cooling rate (temperature range of 700 to 800 ° C) of the cooling rate, or separately cold working after cooling, or below the A ₁ transformation point. You may reheat to temperature and perform warm processing. It is preferable to make processing below 800 degreeC into 30% or more.

On the other hand, as a processing method, cold forging, cold drawing, rolling process, shot etc. are mentioned, for example. By processing below 800 ° C, the bainite or martensite structure before the high frequency quenching becomes fine, and as a result, the average particle diameter of the old austenite particles in the hardened layer obtained after the high frequency quenching becomes finer, resulting in further fatigue strength. Improve.

In accordance with the adjustment of the whole quenching structure by processing and cooling described above, and according to the following high frequency quenching conditions, spherical austenite particles having an average particle diameter of 7 µm or less are obtained for the first time.

First, when heating temperature is less than 800 degreeC, formation of austenite structure becomes inadequate and a hardened layer cannot be obtained. On the other hand, when the heating temperature exceeds 1000 ° C. and the heating rate of 600 to 800 ° C. is less than 300 ° C./s, the growth of austenite particles is accelerated and the variation in particle size is increased, resulting in a decrease in fatigue strength. Cause. That is, the final austenite grain size of the cured layer finally obtained becomes important to how the grain growth is prevented in the austenite region during quenching heating. By placing the pre-tissue as a structure having fine bainite or martensite as described above, the number of nucleation sites of reverse transformation to austenite is large, so that a large number of generated austenite particles do not grow. If cooling is initiated during this period, the average sphere austenite particle diameter of the quenched tissue can be refined. As the growth of the austenite particles proceeds as the temperature is higher and the retention time in the austenite region is longer, the growth of the austenite particles is prevented during the heating, and finally, in order to obtain the old austenite particles having an average particle diameter of 7 µm or less, The temperature reached is 1000 ° C or lower, and the temperature increase rate of 600 to 800 ° C is set to 300 ° C / s or higher.

On the other hand, the temperature attained during heating is preferably 800 to 950 ° C, and the temperature increase rate of 600 to 800 ° C is preferably 700 ° C / s or more. More preferably, it is 1000 ° C / s or more.

In addition, when the residence time of 800 ° C. or more becomes longer during high-frequency heating, the austenite grains grow and the final austenite grain size tends to increase to more than 7 μm, and the residence time of 800 ° C. or more is 5 seconds or less. It is desirable to. More preferred heating time is 3 seconds or less.

On the other hand, the effect is more remarkable in the steel containing Mo in the scope of the present invention. That is, FIG. 1 shows the results obtained by investigating the relationship between the heating temperature at the time of high frequency quenching and the former austenite grain size of the hardened layer of Mo-added steel and Mo-free steel.

Here, the result shown in FIG. 1 is obtained as follows.

In other words, the steel material of the composition shown in the following steels a, b, c, d, and e is melted in a 150 kg vacuum melting furnace, and hot forged at 150 mm angle, and then dummy billet. After preparing a dummy billet and performing 80% hot working at 850 degreeC, the temperature range of 700 degreeC-500 degreeC was cooled to 0.7 degreeC / s, and the bar rolled material was produced. In addition, some of the steel bars were subjected to 20% cold working after the cooling as a second processing step.

(a steel) C: 0.8 mass%, Si: 0.1 mass%, Mn: 0.78 mass%, P: 0.011 mass%, S: 0.019 mass%, Al: 0.024 mass%, Ti: 0.017 mass%, B: 0.0013 mass %, N: 0.0043 mass%, O: 0.0015 mass%, balance Fe and unavoidable impurities.

(b steel) C: 0.53 mass%, Si: 0.1 mass%, Mn: 0.74 mass%, P: 0.011 mass%, S: 0.019 mass%, Al: 0.024 mass%, N: 0.0039 mass%, Mo: 0.37 mass %, Ti: 0.018 mass%, B: 0.0013 mass%, balance Fe and an unavoidable impurity.

(c steel) C: 0.9 mass%, Si: 0.1 mass%, Mn: 0.78 mass%, P: 0.011 mass%, S: 0.019 mass%, Al: 0.024 mass%, Mo: 0.37 mass%, Ti: 0.017 mass %, B: 0.0013 mass%, N: 0.0043 mass%, balance Fe and unavoidable impurities.

(d steel) C: 0.42 mass%, Si: 1.5 mass%, Mn: 0.78 mass%, P: 0.011 mass%, S: 0.019 mass%, Al: 0.024 mass%, Mo: 0.37 mass%, Ti: 0.017 mass %, B: 0.0013 mass%, N: 0.0043 mass%, balance Fe and unavoidable impurities.

(e steel) C: 0.42 mass%, Si: 0.2 mass%, Mn: 0.78 mass%, P: 0.05 mass%, S: 0.019 mass%, Al: 0.024 mass%, Mo: 0.37 mass%, Ti: 0.017 mass %, B: 0.0013 mass%, N: 0.0043 mass%, balance Fe and unavoidable impurities.

A torsion fatigue test piece was taken from the obtained steel bar, and high frequency quenching was performed at a frequency of 10 to 200 kHz and a heating temperature of 870 to 1050 ° C, and further tempered under a condition of 170 ° C for 30 minutes using a heating furnace. (V) was set. In the high-frequency quenching conditions, the temperature increase rate was adjusted so that the residence time at 300 ° C / s or more and 800 ° C or more was 1 second or less.

For the test specimen thus obtained, the torsion fatigue test determined the stress to break 10 ^{5 times} by a stepped torsional test piece having a diameter of 18 mm. In addition, the average sphere austenite particle diameter of the hardened layer by high frequency quenching was measured by the method mentioned above. In addition, the Vickers hardness at the position of 1/5 of the total thickness on the surface of the hardened layer was measured. The Vickers hardness scored five points at 98 N (10 kgf), and adopted the average value.

As shown in Fig. 1, in either of Mo-added steel and Mo-free steel, the austenite grain size of the hardened layer can be reduced by lowering the heating temperature during high-frequency quenching, but in Mo-added steel, the temperature attained during heating By making 1000 degrees C or less, Preferably it is 950 degrees C or less, remarkably refinement | miniaturization of a hardened layer particle diameter is achieved.

The reason for this phenomenon is not clear, but it can be estimated as follows from the relationship with carbonitride containing Mo and Ti mentioned above. That is, in the Mo-added steel, it is considered that the Mo-based fine carbonitride is precipitated and refines the austenite particles by a strong pinning force, so that the Mo-added steel becomes finer. However, even if it is a short time high frequency quenching, when heating temperature greatly exceeds 1000 degreeC, fine (Mo, Ti) ₂ (C, N) will melt | dissolve and it is thought that the effect of pinning will become small.

On the other hand, from FIG. 1, it can be seen that in the Mo-added steel, the former austenite grain size can be further refined when the second processing step (cold processing) is added.

In addition, the inventors have found that, in the steel containing Mo, it is possible to further refine the average sphere austenite particles of the hardened layer by high-frequency quenching to improve fatigue strength, so that fine Mo-based precipitates are dispersed at a high density. It was estimated that the pinning effect would increase.

Therefore, after a steel was melted and rolled, forging was performed at 80% at 850 ° C and 25% at 750 ° C to perform air cooling (cooling rate at air cooling at 0.8 ° C / s). Then, a sample for observing the transmission electron microscope was taken from the material before high frequency quenching, and the situation of fine precipitates was observed. The sample for observation of the transmission electron microscope was prepared by taking a flat sample from the center of the material and thinning it by electropolishing using an electrochloric acid-methanol-based electrolyte solution. If the observation area is too thin, the dropping frequency of the precipitated particles becomes high, and if the thickness is too thick, it becomes difficult to recognize the precipitated particles, so that the thickness of the observation area is in the range of 50 to 100 nm. In addition, the sample thickness was computed from the electron energy loss spectrum.

An example of the transmission electron microscope image actually obtained in FIG. 2 is shown. Considering the sample thickness of about 0.1 µm in this field of view, it has been found that fine precipitates having a diameter of about 5 to 10 nm are dispersed at about 3000 densities per 1 µm ³ .

In high frequency quenching, austenite nucleates to grain boundaries of bainite or martensite, packet boundaries, carbides and the like to grow particles. The fine precipitate suppresses the movement of the grain boundary surface when the austenite grain boundary surface reaches the precipitate and moves toward the direction, as in the case where the balloon is pressed with a finger (precipitate). This inhibition of interfacial movement is called pinning. Pinning is stronger when the total amount of precipitation is constant, the larger the smaller the precipitate, and when the diameter of the precipitate is constant, the larger the amount of precipitate.

In the high frequency heating in the present invention, pinning occurs due to the fine precipitate as shown in Fig. 2, and it is estimated that the miniaturization of the average sphere austenite particle diameter is further promoted. In addition, it is confirmed that the fine precipitate illustrated in FIG. 2 is also present in the material after the high-frequency quenching treatment of 1000 ° C. or less, and it is difficult to dissolve in high temperature and short time heat treatment effectively acts to suppress the austenite grain growth during high-frequency quenching. I think that.

Next, the inventors performed a model calculation in which the precipitation volume ratio of Mo was varied in order to see the influence of the precipitation dispersion state on the average particle diameter of the old austenite particles during the high frequency heating treatment. That is, assuming that as employed by different precipitation phase (析出相) of a small amount of Mo, Mo system precipitate volume percentage of fine precipitates: f and the average grain size: when d is determined, 1㎛ ³ per Mo in the case of uniformly distributed precipitate The system fine precipitate number (precipitation density) is checked. If the average austenite grain diameter is governed by the pinning of fine precipitation, the size is inversely proportional to the precipitation density. Therefore, in consideration of the particle size of the precipitate in FIG. 2 and its density achieving 2 µm of the old austenite particles, the particle size and the precipitate density expressing the pinning effect were examined. As a result, the number of precipitates per 1 μm ³ , which is directly effective for controlling the average austenite grain size, fluctuates depending on the volume fraction of the precipitate, but, for example, sufficient pinning when the volume fraction is about 0.2 to 0.4%. It turned out that the preferable range which can express an effect and implement | achieve refinement | miniaturization of old austenite particle is as follows.

That is, in order to achieve better refinement of the old austenite particles, it is preferable to secure 500 or more fine precipitated particles having a diameter of 20 nm or less. In addition, it is preferable to secure 1000 or more fine precipitated particles having a diameter of 15 nm or less, and more preferably to secure 2000 or more precipitated particles having a diameter of 12 nm or less.

Next, this precipitate was extracted from the base material and the residue was identified by X-ray diffraction, whereby it was presumed to be mainly (Mo, Ti) ₂ (C, N) in the form of hcp. In addition, the result of EDX analysis attached to the transmission electron microscope showed that the atomic ratio of Mo and Ti was about 8: 2, and Mo was a main component. On the other hand, the precipitate here includes the thing removed from the stoichiometric composition of complete (Mo, Ti) ₂ (C, N). In any case, it is considered to be a composite carbonitride including Mo and Ti.

By the way, it is known that (Mo, Ti) ₂ (C, N) precipitates are relatively hard, unlike precipitates such as Cu, and are considered to have high ability to block passage of grain boundaries. In addition, considering that the component composition ratio is that Mo is predominantly large relative to Ti and that Mo is difficult to diffuse, such (Mo, Ti) ₂ (C, N) is (Mo, Ti) ₂ (C, N) Even if it keeps for a short time in the temperature range of about 600-700 degreeC which is precipitation temperature, it does not think that it becomes large rapidly. Therefore, in order to increase the amount of deposition of (Mo, Ti) ₂ (C, N) and increase the distribution density, within a range where the structure fractions of bainite and martensite described later can be obtained,等溫) held by, already deposited, and that _{(Mo, Ti) 2 (C} , N) of the expected precipitation of a new _{(Mo, Ti) 2 (C} , N) , while suppressing to a minimum roughness (粗大化) Can be.

3 shows the relationship between the former austenite grain size and the torsional fatigue characteristics of the cured layer. From the same drawings, it can be seen that in the Mo-added steel, the particle size was reduced and the fatigue characteristics were improved even in the region of the old austenite particle diameter of 7 µm or less. On the other hand, in the Mo-free steel, when the particle diameter becomes 7 micrometers or less, it turns out that fatigue strength does not improve even if it is made smaller than that. This is because in the Mo-free steel, the hardness of the hardened layer is lower than that of the Mo-added steel. Therefore, when the old austenite grain size becomes finer to a certain extent, fatigue fracture becomes intragranular and does not affect the old austenite grain size. I think.

In addition, when the content of either of C, Si, or P is increased in the Mo-added steel (c steel, d steel, e steel), the torsion fatigue strength of the austenite grain size is 7 µm or less. It can be seen that the improvement effect is large. This is considered to be because the intraparticle strength of the hardened layer is increased by increasing the amount of C, Si or P. Therefore, the Vickers hardness of the cured layer was investigated. Hv700 in steel a, Hv740 in steel b, Hv902 in steel c, Hv775 in steel d, Hv760 in steel e and Hv760 in the hardened layer. It could be supported that the fatigue strength improvement effect by the miniaturization of nit particles becomes very large.

Next, the inventors raised the content of either C, Si, or P based on the knowledge that by increasing the hardness of the above cured layer, the fatigue strength increase value due to the refinement of the old austenite grain size can be increased. Even if the strength is not increased, the particle strength can be increased by omitting the tempering treatment after high-frequency quenching. Therefore, it was conceived that the fatigue strength may be increased by omitting the tempering treatment.

Therefore, the torsion fatigue strength was similarly investigated for the a steel and the b steel as described above for the omission of the tempering treatment in the above-described torsional fatigue test piece production step. The Vickers hardness at the time of omitting tempering of steel a and steel b was Hv740 and Hv780, respectively.

In FIG. 4, the relationship between the former austenite particle diameter and the torsional fatigue characteristic of the hardened layer is shown by comparing the case with tempering and the case without tempering. It can be seen from FIG. 4 that fatigue strength can be improved by omitting tempering.

As described above, in the present invention, a method of not performing a tempering process can also be actively adopted. In general high strength steel, when tempering is not performed, a part may crack. For this reason, tempering after high frequency quenching is a process normally performed. This crack is usually grain boundary fracture and is due to lack of grain boundary strength. However, in the present invention, since the grain boundary strength is high due to the miniaturization of the old austenite particles, cracking is unlikely to occur even if the tempering treatment is omitted. Omission of tempering treatment is effective in suppressing softening by tempering and reducing tempering cost.

Example 1

100 kg of the steel shown in Table 1 was solvent-processed, it was heated at 1200 degreeC, and it processed into the torsion fatigue test piece by the hot working conditions and cold working conditions shown in Table 2. The sample after processing was quenched first at 1050 ° C., and then quenched under the conditions shown in Table 2. On the other hand, test No. 10 did not perform high frequency quenching at 1050 degreeC. In addition, except test No. 29 and 30, tempering of 160 degreeC x 1 h. Was performed after high frequency quenching. The old austenite particle diameter and hardness of the high frequency quenched portion were measured in the same manner as described above. In the torsion fatigue test, the stress of breaking 10 ^{5 times} by a stepped torsional test piece of 18 mmφ was determined. In addition, the entire high frequency quenched tissue was observed using an optical microscope to identify the tissues and to determine the tissue fraction (volume%) of bainite and martensite alone or in total.

As shown in Table 2 together with the above measurement results, Nos. 7 and 25 are comparative examples in which all of C, Si, and P are low, and the inventive examples further show that the torsion fatigue strength is further improved. Can be. In addition, as in Nos. 26, 27, and 28, when Mo, B, or Ti is insufficient, the former austenite grain size becomes coarse, and the torsion fatigue strength is lowered. Further, in Nos. 26, 27, and 28, in particular, since the base metal structure became ferrite-pearlite, the former austenite grain size was coarsened, and the fatigue strength was lowered. In addition, as in Nos. 29 and 30, fatigue strength is further improved compared with No. 1 steel and No. 7 steel, unless tempering treatment after high frequency quenching is performed, respectively. In addition, since No.31 steel has a small total workability of 800 to 1000 ° C. during hot working, the old austenite grain size is large and the fatigue strength is low.

Example 2

As the mechanical structural part of this invention, the constant velocity joint 12 which interposes in order to transmit power to the hub 11 of a wheel from the drive shaft 10 shown in FIG. 5 was manufactured.

This constant velocity joint 12 consists of a combination of the outer ring 13 and the inner ring 14. That is, the inner ring 14 is pivotably fixed to the inner side of the mouse portion 13a via the ball 15 fitted into the ball track groove formed on the inner surface of the mouth portion 13a of the outer ring 13, and the inner ring The drive shaft 10 is connected to the shaft 14, while the stem portion 13b of the outer ring 13 is splined to the hub 11, for example, so that power from the drive shaft 10 is transferred to the hub 11 of the wheel. To convey.

The steel material which becomes the component composition shown in Table 3 was melted with the converter, and it was set as the casting piece by continuous casting. Casting piece size was 300 × 400 mm. The cast piece was rolled into a 150 mm square billet through a breakdown step, and then rolled into a steel bar having a diameter of 55 mm.

Next, after cutting the steel bar to a predetermined length, the inner ring of the constant velocity joint (outer diameter: 45 mm and inner diameter: 20 mm) is formed by hot forging, and thereafter, for spline coupling to the fitting surface by cutting or rolling. A groove was formed. Moreover, the rolling surface of the ball was formed by cutting or cold forging. Cooling after hot forging was made as the conditions shown in Table 4. Here, the total processing rate in hot forging and cold forging was performed by adjusting the cross-sectional reduction rate of the cross section orthogonal to the axial direction of the raceway.

As shown in FIG. 6, the high frequency quenching apparatus is quenched at 1050 degreeC using the high frequency quenching apparatus of frequency: 15Hz to the raceway 14a of the ball interposed between the constant velocity joint outer ring, and the conditions shown in Table 4 are continued After high frequency quenching to form the quenched tissue layer 16, tempering was carried out under a condition of 180 ° C. × 2 h using a heating furnace. On the other hand, tempering was omitted for some constant velocity joints. The constant velocity joint inner ring thus obtained is fitted with a drive shaft to the fitting engagement surface, and is mounted through a ball (steel ball) on the mouth of the outer ring of the constant velocity joint, and a hub is fitted to the stem of the constant velocity joint outer ring. The unit was used (see FIG. 5). On the other hand, the specifications of the ball, outer ring, drive shaft and hub are as follows.

Below

Ball: Quenched Tempered Steel of High Carbon Chrome Bearing Steel SUJ2

Outer ring: high frequency quenching tempered steel of mechanical steel

Hub: high frequency quenching tempered steel of mechanical steel

Drive shaft: high frequency quenching tempered steel of mechanical steel

Next, using this constant velocity joint, in a power transmission system which transmits the rotational motion of the drive shaft to the hub via the inner and outer rings of the constant velocity joint, an endurance test regarding the electric fatigue strength of the raceway of the ball was performed.

The electric fatigue test is carried out under the condition of torque: 900Nm, operating angle (angle between inner ring axis and drive shaft axis): 20 ° and rotation speed: 300rpm, and until peeling occurs on the raceway of the inner ring of constant velocity joint. The time of was evaluated as the electric fatigue strength. In addition, the dimension and shape of a drive shaft, a constant velocity joint outer ring, etc. were set here so that a ring without a constant velocity joint might become the weakest part at the endurance test.

Moreover, about the inner ring of the constant velocity joint manufactured on the same conditions, the former austenite particle average diameter and hardness of the hardened layer were calculated | required by the method similar to the above-mentioned method.

Table 4 also lists these results.

From Table 4, No. 7 and 25 are the comparative examples in which all of C, Si, and P are low, and it turns out that a rolling fatigue life improves with respect to these comparative examples. Further, as in Nos. 26, 27, and 28, when Mo, B, or Ti is insufficient, the former austenite grain size becomes coarse, and the rolling fatigue life decreases. In addition, in Nos. 26, 27, and 28, in particular, since the base metal structure became ferrite-pearlite, the former austenite grain size was coarsened, and the fatigue strength decreased.

In addition, as in Nos. 29 and 30, when the tempering treatment after high-frequency quenching is not performed, the rolling fatigue life is further improved as compared with No. 1 steel and No. 7 steel, respectively. In addition, since No.31 steel has a small total workability of 800 to 1000 ° C at the time of hot working, the old austenite grain size is large and the rolling fatigue life is low.

Claims (33)

delete delete delete delete delete C: 0.3-1.5 mass%, Si: 0.05-3.O mass%, Mn: 0.2-2.20 mass%, A1: 0.25 mass% or less Ti: 0.005-0.1 mass%, Mo: 0.05-0.6 mass%, B: 0.0003-0.006 mass% S: 0.1 mass% or less, and P: 0.10 mass% or less , The balance has the composition of Fe and inevitable impurities, At least a part has a hardened layer by high frequency quenching, the hardened layer has a hardness of at least Hv 750 and an average particle diameter of the old austenite particles of 7 μm or less over the entire thickness of the hardened layer, And the hardened layer remains quenched. The method of claim 6, The content of Al in the composition of the composition, Al: 0.005-0.25 mass% Machine structural component, characterized in that the. The method according to claim 6 or 7, As the ingredient composition, Cr: 2.5 mass% or less, Cu: 1.O mass% or less, Ni: 3.5 mass% or less, Co: 1.O mass% or less, Nb: 0.1 mass% or less, V: 0.5 mass% or less, Ta: 0.5 mass% or less, Hf: 0.5 mass% or less, and Sb: 0.015 mass% or less Machine structural parts, characterized in that it further contains one or two or more selected from. The method according to claim 6 or 7, As the ingredient composition, W: 1.0 mass% or less, Ca: 0.005 mass% or less, Mg: 0.005 mass% or less, Te: 0.1 mass% or less, Se: 0.1 mass% or less, Bi: 0.5 mass% or less, Pb: 0.5 mass% or less, Zr: 0.01 mass% or less, and REM: 0.1 mass% or less Machine structural parts, characterized in that it further contains one or two or more selected from. The method according to claim 6 or 7, A mechanical structural part having Mo-based precipitates dispersed at 500 or more per 1 μm ³ , and the average particle diameter of the Mo-based precipitates is 20 nm or less. C: 0.3-1.5 mass%, Si: 0.05-3.O mass%, Mn: 0.2-2.20 mass%, Al: 0.25 mass% or less, Ti: 0.005-0.1 mass%, Mo: 0.05-0.6 mass%, B: 0.0003-0.006 mass% S: 0.1 mass% or less, and P: 0.10 mass% or less At least one of the following formulas (1) to (3) satisfies at least one of the formulas, the balance being at least one time of high frequency quenching on at least a part of the material which is composed of Fe and inevitable impurities: In the manufacture of mechanical structural components, The total processing rate at 800 to 1000 DEG C during hot working is 80% or more so that the sum of either or both of the bainite structure and martensite structure in the stressed yarn before the high frequency quenching of the material is 10% by volume or more. After that, the temperature range of 700 to 500 ° C. is adjusted by cooling at a rate of 0.2 ° C./s or more, A method for producing a mechanical structural part, characterized in that the attainment temperature of the high frequency quenching is set to 1000 ° C or less. Below C> 0.7 mass%. (One) Si> 1.1 mass%. (2) P> 0.02 mass%. (3) The method of claim 11, The content of Al in the composition of the composition, Al: 0.005-0.25 mass% Method for producing a mechanical structural component, characterized in that. The method according to claim 11 or 12, wherein As the ingredient composition, Cr: 2.5 mass% or less, Cu: 1.0 mass% or less, Ni: 3.5 mass% or less, Co: 1.O mass% or less, Nb: 0.1 mass% or less, V: 0.5 mass% or less, Ta: 0.5 mass% or less, Hf: 0.5 mass% or less, and Sb: 0.015 mass% or less Method for producing a mechanical structural part, characterized in that it further comprises one or two or more selected from. The method according to claim 11 or 12, wherein As the ingredient composition, W: 1.0 mass% or less, Ca: 0.005 mass% or less, Mg: 0.005 mass% or less, Te: 0.1 mass% or less, Se: 0.1 mass% or less, Bi: 0.5 mass% or less, Pb: 0.5 mass% or less, Zr: 0.01 mass% or less, and REM: 0.1 mass% or less Method for producing a mechanical structural part characterized in that it further contains one or two or more selected from. A material for high frequency quenching for making a structural steel material having a hardened layer having an average sphere austenite grain size of 7 μm or less due to high frequency quenching on at least part of the surface, C: 0.3-1.5 mass%, Si: 0.05-3.O mass%, Mn: 0.2-2.20 mass%, Al: 0.25 mass% or less Ti: 0.005-0.1 mass%, Mo: 0.05-0.6 mass%, B: 0.0003-0.006 mass% S: 0.1 mass% or less, and P: 0.10 mass% or less And any one of the following formulas (1) to (3) satisfies at least one formula, and the balance has a component composition of Fe and inevitable impurities, During the hot working, the total processing rate at 800 to 1000 ° C. is 80% or more, and then the temperature range of 700 to 500 ° C. is cooled at a rate of 0.2 ° C./s or more, thereby allowing any of bainite structure and martensite structure. A material for high frequency quenching, characterized in that the sum of one or both sides is 10% by volume or more. Below C> 0.7 mass%. (One) Si> 1.1 mass%. (2) P> 0.02 mass%. (3) The method of claim 15, The content of Al in the composition of the composition, A1: 0.005-0.25 mass% The material for high frequency quenching which is characterized by the above-mentioned. The method according to claim 15 or 16, As the ingredient composition, Cr: 2.5 mass% or less, Cu: 1.O mass% or less, Ni: 3.5 mass% or less, Co: 1.O mass% or less, Nb: 0.1 mass% or less, V: 0.5 mass% or less, Ta: 0.5 mass% or less, Hf: 0.5 mass% or less, and Sb: 0.015 mass% or less High frequency quenching material, characterized in that it further contains one or two or more selected from. The method according to claim 15 or 16, As the ingredient composition, W: 1.O mass% or less, Ca: 0.005 mass% or less, Mg: 0.005 mass% or less, Te: 0.1 mass% or less, Se: 0.1 mass% or less, Bi: 0.5 mass% or less, Pb: 0.5 mass% or less, Zr: 0.01 mass% or less, and REM: 0.1 mass% or less High frequency quenching material, characterized in that it further contains one or two or more selected from. The method according to claim 15 or 16, A high frequency quenching material, which has Mo-based precipitates dispersed at 500 or more per 1 μm ^{3 and} the average particle diameter of the Mo-based precipitates is 20 nm or less. delete The method of claim 8, As the ingredient composition, W: 1.0 mass% or less, Ca: 0.005 mass% or less, Mg: 0.005 mass% or less, Te: 0.1 mass% or less, Se: 0.1 mass% or less, Bi: 0.5 mass% or less, Pb: 0.5 mass% or less, Zr: 0.01 mass% or less, and REM: 0.1 mass% or less Machine structural parts, characterized in that it further contains one or two or more selected from. delete delete delete The method of claim 8, A mechanical structural part having Mo-based precipitates dispersed at 500 or more per 1 μm ³ , and the average particle diameter of the Mo-based precipitates is 20 nm or less. The method of claim 9, A mechanical structural part having Mo-based precipitates dispersed at 500 or more per 1 μm ³ , and the average particle diameter of the Mo-based precipitates is 20 nm or less. delete The method of claim 21, A mechanical structural part having Mo-based precipitates dispersed at 500 or more per 1 μm ³ , and the average particle diameter of the Mo-based precipitates is 20 nm or less. The method of claim 13, As the ingredient composition, W: 1.0 mass% or less, Ca: 0.005 mass% or less, Mg: 0.005 mass% or less, Te: 0.1 mass% or less, Se: 0.1 mass% or less, Bi: 0.5 mass% or less, Pb: 0.5 mass% or less, Zr: 0.01 mass% or less, and REM: 0.1 mass% or less Method for producing a mechanical structural part characterized in that it further contains one or two or more selected from. The method of claim 17, As the ingredient composition, W: 1.O mass% or less, Ca: 0.005 mass% or less, Mg: 0.005 mass% or less, Te: 0.1 mass% or less, Se: 0.1 mass% or less, Bi: 0.5 mass% or less, Pb: 0.5 mass% or less, Zr: 0.01 mass% or less, and REM: 0.1 mass% or less High frequency quenching material, characterized in that it further contains one or two or more selected from. The method of claim 17, A high frequency quenching material, which has Mo-based precipitates dispersed at 500 or more per 1 μm ^{3 and} the average particle diameter of the Mo-based precipitates is 20 nm or less. The method of claim 18, A high frequency quenching material, which has Mo-based precipitates dispersed at 500 or more per 1 μm ^{3 and} the average particle diameter of the Mo-based precipitates is 20 nm or less. The method of claim 30, A high frequency quenching material, which has Mo-based precipitates dispersed at 500 or more per 1 μm ^{3 and} the average particle diameter of the Mo-based precipitates is 20 nm or less.

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