JP4983098B2 - Steel material with excellent fatigue characteristics and method for producing the same - Google Patents

Description

  The present invention relates to a steel material excellent in fatigue characteristics after quenching, which is suitable for application to automobile drive shafts, constant velocity joints, etc. having a hardened layer by induction quenching on the surface, and a method for producing the same.

Conventionally, machine structural parts such as automotive drive shafts and constant velocity joints are processed into a predetermined shape by hot forging, further cutting, cold forging, etc. on hot rolled steel bars, and then induction hardening- By tempering, it is common to ensure torsional fatigue strength, which is an important characteristic as a machine structural component.
On the other hand, in recent years, there is a strong demand for weight reduction of automobile members due to environmental problems, and from this viewpoint, it is required to further improve the fatigue strength of automobile parts.

As means for improving the fatigue strength as described above, various methods have been proposed so far.
For example, in order to improve the torsional fatigue strength, it is conceivable to increase the quenching depth by induction quenching. However, even if the quenching depth is increased, the fatigue strength is saturated at a certain depth.
Further, improvement of the grain boundary strength is also effective for improving the torsional fatigue strength. From this viewpoint, a technique for refining the prior austenite grain size by dispersing TiC has been proposed (see, for example, Patent Document 1). )

In the technique described in Patent Document 1 described above, since fine TiC is dispersed in a large amount during induction quenching heating, the prior austenite grain size is refined, so that TiC is solutionized before quenching. It is necessary to adopt a process of heating to 1100 ° C or higher in the hot rolling process. Therefore, it is necessary to increase the heating temperature during hot rolling, and there is a problem that productivity is inferior.
Furthermore, even with the technique disclosed in Patent Document 1 described above, there is still a problem in that it cannot sufficiently meet the recent demand for torsional fatigue strength.

In Patent Document 2, the ratio (CD / R) between the hardened layer depth CD and the radius R of the induction-hardened shaft component is limited to 0.3 to 0.7, and the CD / R and the surface after induction hardening are used. The value A defined by the austenite grain size γf up to 1 mm, the average Vickers hardness Hf up to (CD / R) = 0.1 as induction-quenched, and the average Vickers hardness Hc at the center of the shaft after induction hardening is expressed as C There has been proposed a shaft object part for a machine structure in which torsional fatigue strength is improved by controlling it within a predetermined range according to the amount.
However, in this component, since the prior austenite grain size over the entire thickness of the hardened hardened layer is not taken into consideration, it is still impossible to sufficiently meet the recent demand for torsional fatigue strength.

Furthermore, Patent Document 3 discloses that the ratio (CD / R) of the hardened layer depth CD and the radius R of the induction-hardened shaft component is limited to 0.3 to 0.7, and then the surface of the component is subjected to shot peening. A shaft component having improved torsional fatigue strength by setting the compressive residual stress to 700 N / mm ² or more has been proposed.
However, in this component, since the prior austenite grain size over the entire thickness of the hardened hardened layer is not taken into consideration, it is still impossible to sufficiently meet the recent demand for torsional fatigue strength.
Also, since compressive residual stress is mechanically applied by shot peening, it is difficult to apply compressive residual stress evenly to the surface, and the shot peening treatment conditions are strictly controlled to obtain the desired characteristics. It was necessary and it was hard to say that it was a practical method. In addition, since the process is added, there is a problem that the cost increases.

JP 2000-154819 A (Claims, paragraph [0008]) JP-A-8-53714 (Claims) JP-A-7-90379 (Claims)

  The present invention has been developed in view of the above-described present situation, and an object thereof is to propose an advantageous method of manufacturing a steel material in which fatigue strength is greatly improved by induction hardening.

The inventors have intensively studied to effectively improve the fatigue strength as described above. In particular, a detailed study was conducted focusing on torsional fatigue strength as a representative example of such fatigue strength.
As a result, excellent torsional fatigue strength can be obtained by optimizing the austenite grain size and surface residual stress over the entire thickness of the hardened layer after quenching, as described below. And gained knowledge.

(1) The torsional fatigue strength is remarkably improved by quenching steel adjusted to an appropriate chemical composition and setting the prior austenite grain size over the entire thickness of the hardened hardened layer to 12 μm or less. Specifically, regarding the chemical composition, especially by adding Si and Mo in an appropriate range, the number of nucleation sites of austenite during induction hardening increases, and the growth of austenite grains is suppressed. The grain size of the quenched and hardened layer is effectively refined, and as a result, the torsional fatigue strength is significantly improved. In particular, by adding 0.30 mass% or more of Si, a cured layer having a particle size of 12 μm or less can be obtained over the entire thickness of the cured layer after induction hardening.

(2) When the structure of the base material, that is, the structure before quenching, is a structure in which the bainite structure and / or the martensite structure is contained in a specific fraction, the bainite structure or the martensite structure is compared with the ferrite-pearlite structure. Since the carbide is a finely dispersed structure, the area of the ferrite / carbide interface, which is the nucleation site of austenite, increases during quenching heating, and the generated austenite becomes finer. As a result, the grain size of the quenched and hardened layer becomes fine, which improves the grain boundary strength and increases the torsional fatigue strength.

(3) As described above, using hardened steel with a chemical composition and structure, and appropriately controlling induction hardening conditions (heating temperature, time, number of times of quenching), the hardened layer particle size is remarkably reduced, Grain boundary strength is improved. Specifically, by setting the heating temperature to 800 to 1000 ° C., more preferably 800 to 950 ° C., and the heating time to 5 seconds or less, fine particles having a particle size of 12 μm or less can be stabilized over the entire thickness of the cured layer. Can be obtained. In particular, by subjecting the Mo-added steel to induction quenching by controlling the heating temperature: 800 to 1000 ° C., more preferably 800 to 950 ° C., a finer hardened layer particle size can be obtained. Furthermore, by repeating the quenching process under the above conditions twice or more, a finer hardened layer particle size can be obtained as compared with one quenching.

(4) As mentioned above, by using steel materials with adjusted chemical composition and structure, and by appropriately controlling the cooling rate during tempering after induction hardening, the compressive residual stress is significantly increased and the fatigue strength is improved. To do. Specifically, the cooling rate during tempering is 30 ° C / s or higher, preferably 50 ° C / s or higher, more preferably 100 ° C / s or higher, so that the surface residual stress is -700 MPa or less. Stress can be obtained stably. The heating temperature and holding time during tempering are not particularly limited as long as general tempering by furnace heating or high-frequency heating is performed.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. C: 0.35-0.7 mass%
Si: 0.30 to 1.1 mass%,
Mn: 0.2-2.0 mass%
Al: 0.005 to 0.25 mass%,
Ti: 0.005 to 0.1 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and
Cr: 0.2 mass% or less is contained under Ti (mass%) / N (mass%) ≧ 3.42, the balance is composed of Fe and inevitable impurities, and the base material structure is bainite structure and / or martensite. Having a structure, and the total structure fraction of these bainite structure and martensite structure is 10 vol % or more, and the old austenite grain size of the hardened layer after induction hardening is 12 μm or less over the entire thickness of the hardened layer, A steel material with excellent fatigue characteristics characterized by a surface residual stress of -700 MPa or less after induction hardening and tempering.

2. In the above 1, the steel material excellent in fatigue characteristics in which the thickness of the hardened layer after induction hardening is 2 mm or more.

3. In 1 or 2, the steel material is further
Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0 mass% or less,
Nb: steel material excellent in fatigue characteristics having a composition containing one or more selected from 0.1 mass% or less and V: 0.5 mass% or less.

4). C: 0.35-0.7 mass%
Si: 0.30 to 1.1 mass%,
Mn: 0.2-2.0 mass%
Al: 0.005 to 0.25 mass%,
Ti: 0.005 to 0.1 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and
Cr: 0.2 mass% or less is contained under Ti (mass%) / N (mass%) ≧ 3.42, and the remainder is hot-worked with a steel material having a composition of Fe and inevitable impurities, and then 0.2 ° C / After cooling at a rate of at least s, induction heating is performed at a temperature of induction quenching of 800 to 1000 ° C, and tempering is performed at a cooling rate of at least 30 ° C / s. A method for producing steel with excellent fatigue properties.

5. In 4, the steel material is further
Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0 mass% or less,
A method for producing a steel material having excellent fatigue characteristics, containing one or more selected from Nb: 0.1 mass% or less and V: 0.5 mass% or less.

6). 4. The method for producing a steel material having excellent fatigue characteristics according to 4 or 5, wherein after the cooling, induction hardening is repeated a plurality of times, and a heating temperature at the final induction hardening is 800 to 1000 ° C.

7). 6. The method for producing a steel material having excellent fatigue characteristics, wherein the heating temperature during induction hardening is 800 to 1000 ° C. for all of the plurality of induction hardenings.

8). 4. The method for producing a steel material having excellent fatigue characteristics according to any one of 4 to 7, wherein a heating time in the heating temperature range is 5 seconds or less per induction hardening.

9. 4. The method for producing a steel material excellent in fatigue characteristics according to any one of 4 to 8, wherein the thickness of the hardened layer on the steel material surface by induction hardening is 2 mm or more.

  According to the present invention, it is possible to stably obtain a steel material excellent in all fatigue characteristics such as bending fatigue characteristics, rolling fatigue characteristics, and sliding rolling fatigue characteristics as well as torsional fatigue characteristics. This is a great achievement for the demand for lighter parts.

Hereinafter, the present invention will be specifically described.
First, the reason why the steel material and the component composition of the steel material are limited to the above range in the present invention will be described.
C: 0.35-0.7 mass%
C is an element having the greatest influence on the hardenability, and contributes to the improvement of fatigue strength by increasing the hardness and depth of the hardened hardened layer. However, if the content is less than 0.35 mass%, the quench hardening depth must be drastically increased in order to ensure the required fatigue strength. Therefore, 0.35 mass% or more is added. On the other hand, when the content exceeds 0.7 mass%, the grain boundary strength is lowered, and accordingly, the fatigue strength is lowered, and the machinability, cold forgeability and fire cracking resistance are also lowered. For this reason, C was limited to the range of 0.35 to 0.7 mass%. Preferably it is the range of 0.4-0.6 mass%.

Si: 0.30 to 1.1 mass%
Si has the effect of increasing the number of nucleation sites of austenite during quenching heating, suppressing the grain growth of austenite, and reducing the grain size of the quenched hardened layer. Moreover, carbide | carbonized_material production | generation is suppressed and the fall of the grain boundary strength by carbide | carbonized_material is suppressed. Furthermore, it is an element useful for the formation of a bainite structure, and these improve the fatigue strength.
Thus, Si is a very important element in the present invention, and it is essential to contain 0.30 mass% or more. This is because if the Si content is less than 0.30 mass%, it is impossible to obtain fine grains with a prior austenite grain size of 12 μm or less over the entire thickness of the hardened layer, no matter how the manufacturing conditions and quenching conditions are adjusted. is there. However, if the Si content exceeds 1.1 mass%, the hardness increases due to the solid solution hardening of ferrite, leading to a decrease in machinability and cold forgeability. Therefore, Si was limited to the range of 0.30 to 1.1 mass%. Preferably it is the range of 0.40-1.0 mass%.

Mn: 0.2 to 2.0 mass%
Mn is an essential component for improving the hardenability and ensuring the hardening depth during quenching, so it is actively added, but if the content is less than 0.2 mass%, the effect of addition is poor, so 0.2% More than mass%. Preferably it is 0.3 mass% or more. On the other hand, when the amount of Mn exceeds 2.0 mass%, the retained austenite after quenching increases, and on the contrary, the surface hardness decreases, and consequently the fatigue strength decreases. Therefore, Mn is set to 2.0 mass% or less. It should be noted that if the content of Mn is large, the base material is hardened, which may be disadvantageous in machinability. Therefore, it is preferable to set the content to 1.2 mass% or less. More preferably, it is 1.0 mass% or less.

Al: 0.005 to 0.25 mass%
Al is an element effective for deoxidation. Moreover, it is an element useful also in refine | miniaturizing the particle size of a hardening hardening layer by suppressing the austenite grain growth at the time of quenching heating. However, if the content is less than 0.005 mass%, the effect of addition is poor. On the other hand, even if the content exceeds 0.25 mass%, the effect is saturated, and rather, a disadvantage that causes an increase in the component cost occurs. Limited to a range of ~ 0.25 mass%. Preferably it is the range of 0.05-0.10 mass%.

Ti: 0.005 to 0.1 mass%
Ti combines with N mixed as an unavoidable impurity to prevent B from becoming BN and the effect of improving the hardenability of B to disappear, and has the effect of sufficiently exerting the effect of improving the hardenability of B. . In order to obtain this effect, the content of at least 0.005 mass% is required. However, if the content exceeds 0.1 mass%, a large amount of TiN is formed. Ti is limited to the range of 0.005 to 0.1 mass% because it causes a significant decrease. Preferably it is the range of 0.01-0.07 mass%. Furthermore, it is preferable to satisfy Ti (mass%) / N (mass%) ≧ 3.42 from the viewpoint of securing N securely and improving the hardenability by B to obtain a bainite and martensite structure.

Mo: 0.05-0.6 mass%
Mo promotes the formation of a bainite structure, thereby minimizing the austenite grain size during quenching and heating and reducing the grain size of the quenched hardened layer. Moreover, it has the effect | action which refines | miniaturizes the particle size of a hardening hardening layer by suppressing the grain growth of austenite at the time of quenching heating. In particular, this effect becomes more prominent by setting the heating temperature during induction hardening to 800 to 1000 ° C, more preferably 800 to 950 ° C. Furthermore, since it is an element useful for improving hardenability, it is used for adjusting hardenability. In addition, Mo is an element that suppresses the formation of carbides and effectively prevents a decrease in grain boundary strength due to carbides.
Thus, Mo is a very important element in the present invention, and if the content is less than 0.05 mass%, the prior austenite grains over the entire thickness of the hardened layer no matter how the manufacturing conditions and quenching conditions are adjusted. Fine particles having a diameter of 12 μm or less cannot be obtained. However, if the content exceeds 0.6 mass%, the hardness of the rolled material is remarkably increased, and the workability is reduced. Therefore, Mo is limited to the range of 0.05 to 0.6 mass%. Preferably it is the range of 0.1-0.6 mass%. More preferably, it is in the range of 0.3 to 0.4 mass%.

B: 0.0003 to 0.006 mass%
B has an effect of promoting the formation of a bainite structure or a martensite structure. B also has the effect of improving the torsional strength by improving the hardenability by adding a small amount and increasing the quenching depth during quenching. Further, B preferentially segregates at the grain boundary, reduces the concentration of P segregating at the grain boundary, improves the grain boundary strength, and thus has the effect of improving fatigue strength.
For this reason, in the present invention, B is positively added, but if the content is less than 0.0003 mass%, the effect of addition is poor. On the other hand, if the content exceeds 0.006 mass%, the effect is saturated, rather the component In order to raise the cost, B is limited to the range of 0.0003 to 0.006 mass%. Preferably it is the range of 0.0005-0.004 mass%. More preferably, it is the range of 0.0015-0.003 mass%.

S: 0.06 mass% or less S is a useful element that forms MnS in steel and improves the machinability. However, when it exceeds 0.06 mass%, it segregates at the grain boundary and lowers the grain boundary strength. , S was limited to 0.06 mass% or less. Preferably it is 0.04 mass% or less.

P: 0.020 mass% or less P segregates at the austenite grain boundaries and lowers the grain boundary strength, thereby reducing the fatigue strength. In addition, there is a negative effect of promoting burn cracking. Therefore, it is desirable to reduce the P content as much as possible, but it is allowed up to 0.020 mass%.

Cr: 0.2 mass% or less
Cr stabilizes carbides and promotes the formation of residual carbides, lowers the grain boundary strength, and degrades fatigue strength. Therefore, it is desirable to reduce the Cr content as much as possible, but up to 0.2 mass% is acceptable. Preferably it is 0.05 mass% or less.

The basic components have been described above. However, in the present invention, other elements described below can be appropriately contained.
Cu: 1.0 mass% or less
Cu is effective in improving the hardenability, and also dissolves in ferrite, and this solid solution strengthening improves fatigue strength. Furthermore, by suppressing the formation of carbides, a decrease in grain boundary strength due to carbides is suppressed, and fatigue strength is improved. However, if the content exceeds 1.0 mass%, cracks occur during hot working, so 1.0 mass% or less is added. In addition, Preferably it is 0.5 mass% or less.

Ni: 3.5 mass% or less
Since Ni is an element that improves hardenability, Ni is used when adjusting hardenability. Moreover, it is an element which suppresses the production | generation of a carbide | carbonized_material and suppresses the fall of the grain boundary strength by a carbide | carbonized_material, and improves fatigue strength. However, Ni is an extremely expensive element, and adding more than 3.5 mass% increases the cost of the steel material. In addition, since the effect of improving hardenability and the effect of suppressing the decrease in grain boundary strength are small when added at less than 0.05 mass%, it is desirable to add 0.05 mass% or more. Preferably it is 0.1-1.0 mass%.

Co: 1.0 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 strength and fatigue strength. However, Co is an extremely expensive element, and if added in excess of 1.0 mass%, the cost of the steel material increases, so 1.0 mass% or less should be added. In addition, since addition of less than 0.01 mass% has a small effect of suppressing the decrease in grain boundary strength, it is desirable to add 0.01 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 and acts as a precipitation strengthening element. It is also an element that improves temper softening resistance, and these effects improve fatigue strength. However, since the effect is saturated even if it contains exceeding 0.1 mass%, 0.1 mass% is made an upper limit. It should be noted that addition of 0.005% by mass or more is desirable since addition of less than 0.005% has little effect of improving precipitation strengthening and temper softening resistance. 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. It is also an element that improves temper softening resistance, and these effects improve fatigue strength. However, since the effect is saturated even if it contains exceeding 0.5 mass%, it shall be 0.5 mass% or less. In addition, since the improvement effect of fatigue strength is small when added less than 0.01 mass%, it is desirable to add 0.01 mass% or more. Preferably it is 0.03-0.3 mass%.

The preferred component composition range has been described above, but in the present invention, it is not sufficient to limit the component composition to the above range, and adjustment of the matrix structure is also important.
That is, in the present invention, the structure of the base material, that is, the structure before quenching (corresponding to the structure other than the hardened layer after induction hardening) has a bainite structure and / or a martensite structure, and these bainite structure and martensite structure. The total tissue fraction of the site organization must be 10% or higher in volume fraction (vol%). The reason for this is that the bainite structure or martensite structure is a structure in which carbides are finely dispersed compared to the ferrite-pearlite structure, so the area of the ferrite / carbide interface, which is an austenite nucleation site, is increased during quenching heating. This is because the generated austenite is refined, and thus contributes effectively to refine the grain size of the quenched and hardened layer. And the grain boundary intensity | strength rises and fatigue strength improves by refinement | miniaturization of the particle size of a hardening hardening layer.
Here, the total structure fraction of the bainite structure and the martensite structure is more preferably 20 vol% or more.
The upper limit of the total structure fraction of the bainite structure and the martensite structure is preferably about 90 vol%. This is because when the total structural fraction exceeds 90 vol%, not only the refinement effect of the prior austenite grains of the hardened layer by quenching is saturated, but also machinability deteriorates rapidly.

  In addition, regarding the refinement of the grain size of the hardened layer after quenching, the martensite structure has the same effect as the bainite structure, but from an industrial viewpoint, the bainite structure is more preferable than the martensite structure. The addition amount of the alloy element is small, it is advantageous in terms of machinability, and it can be produced at a lower cooling rate, which is advantageous in production.

Here, in FIG. 1, the result of having investigated about the influence which the bainite structure fraction and martensite structure fraction in steel exert on machinability and high strength is shown.
As shown in the figure, the bainite structure and the martensite structure were almost the same in terms of increasing the strength by miniaturization, but the bainite structure was superior in terms of machinability (hardness). In particular, when the bainite structure fraction was in the range of 25 to 85%, both high strength and machinability could be obtained with a good balance. A particularly preferred bainite structure fraction is in the range of 30 to 70%.

  The remaining structure other than the bainite structure or martensite structure may be ferrite, pearlite, or the like, and is not particularly defined.

  Furthermore, in the present invention, it is also important to adjust the prior austenite particle size of the hardened layer after induction hardening. That is, regarding the hardened layer after induction hardening, the prior austenite grain size needs to be 12 μm or less over the entire thickness. This is because if the grain size over the entire thickness of the quenched and hardened layer exceeds 12 μm, sufficient grain boundary strength cannot be obtained, and satisfactory improvement in fatigue strength cannot be expected. The thickness is preferably 10 μm or less, more preferably 5 μm or less.

Here, the measurement of the prior austenite grain size over the entire thickness of the hardened hardened layer is performed as follows.
In the steel material of the present invention after induction hardening, the outermost steel layer of the induction-hardened portion has a martensite structure of 100% in area ratio. And as it goes from the surface to the inside, the area of 100% martensite structure continues at a certain depth, but the area ratio of the martensite structure decreases rapidly from a certain depth.
In the present invention, for the induction-quenched portion, the depth region from the steel material surface until the area ratio of the martensite structure is reduced to 98% is defined as a hardened layer.
For this hardened layer, the average prior austenite grain size was measured from the surface at each of the 1/5 position, 1/2 position and 4/5 position of the hardened layer thickness. In this case, it is assumed that the prior austenite grain size over the entire thickness of the quenched and hardened layer is 12 μm or less.
The average prior austenite grain size is measured 400 times (1 field area: 0.25 mm x 0.225 mm) to 1000 times (1 field area: 0.10 mm x 0.09 mm) at each position using an optical microscope. This is done by observing 5 fields of view and measuring the average particle size with an image analyzer.

  Furthermore, in the present invention, the residual stress after induction hardening and tempering needs to be −700 MPa or less. This is because the residual stress greatly affects the fatigue strength, and when a tensile residual stress, that is, a positive (+) stress is generated, the fatigue strength deteriorates. When compressive residual stress, that is, negative (-) stress is generated, the fatigue strength is improved. In the case of compressive residual stress, a reasonable effect can be obtained even if it exceeds −700 MPa, but when trying to obtain a very high fatigue strength such as 740 MPa or more as in the present invention, a high compressive residual stress of −700 MPa or less is required. is required. Note that -700 MPa or less means an absolute value greater than 700.

  Furthermore, in the present invention, the thickness of the hardened layer by induction hardening is preferably 2 mm or more. This is because when the desired properties depend only on the structure near the extreme surface layer such as the rolling fatigue life, even if the thickness of the hardened layer is about 1 mm, a certain effect can be obtained. If strength is a problem, the thicker the hardened layer, the more advantageous. Therefore, a more preferable hardened layer thickness is 2.5 mm or more, and further preferably 3 mm or more.

The production conditions of the present invention will be described below.
The steel material adjusted to a predetermined component composition is subjected to induction hardening and then subjected to induction hardening after being subjected to cold rolling, cold forging or cutting as necessary after steel bar rolling or hot forging.
In the present invention, the base material structure has the above-described bainite structure and / or martensite structure, and the total structure fraction of these bainite structure and martensite structure is 10 vol% or more. The material steel before quenching needs to be cooled at a rate of 0.2 ° C / s or higher after being processed into a predetermined shape by hot working such as rolling and forging. This is because when the cooling rate is less than 0.2 ° C./s, it becomes difficult to obtain a bainite or martensite structure, and the total structure fraction of these structures may not reach 10 vol%. The preferable range of the cooling rate after hot working is 0.3 to 30 ° C./s.
In addition, it is preferable to perform hot working in a temperature range of over 900 ° C. to 1150 ° C. If the temperature is 900 ° C. or lower, the necessary bainite structure and / or martensite structure cannot be obtained. On the other hand, if it exceeds 1150 ° C., the heating cost increases, which is economically disadvantageous.

  Next, in the present invention, induction hardening is performed in order to obtain the above-described cured layer, and the heating temperature range during this induction hardening needs to be 800 to 1000 ° C. This is because when the heating temperature is less than 800 ° C., the austenite structure is not sufficiently generated, and as a result of insufficient generation of the hardened layer structure described above, sufficient fatigue strength cannot be ensured. When the heating temperature exceeds 1000 ° C., the growth of austenite grains is promoted and becomes coarse, and the grain size of the hardened layer becomes coarse, so that the fatigue strength is also lowered. A more preferable heating temperature range is 800 to 950 ° C.

In addition, said effect expresses more notably in steel which contained Mo in the range of this invention.
Here, FIG. 2 shows the results of examining the relationship between the heating temperature during induction hardening and the prior austenite grain size of the hardened layer for Mo-added steel (Mo: 0.05 to 0.6 mass%) and Mo-free steel.
As shown in the figure, in both Mo-added steel and Mo-free steel, the prior austenite grain size of the hardened layer can be reduced by lowering the heating temperature during induction hardening. By setting the temperature to 1000 ° C. or lower, preferably 950 ° C. or lower, the hardened layer particle size can be remarkably reduced.

When the above-described induction hardening is repeated a plurality of times, at least the final induction hardening may be performed at a heating temperature of 800 to 1000 ° C. Furthermore, when the induction hardening is repeated a plurality of times, the heating temperature is most preferably set to 800 to 1000 ° C. for all induction hardening. Further, by performing repeated quenching twice or more, a finer cured layer particle size can be obtained as compared with the single quenching.
When induction hardening is repeated a plurality of times, it is preferable that at least the final quenching depth by induction hardening be equal to or greater than the previous quenching depth by induction hardening. This is because the crystal grain size of the hardened layer is most strongly affected by the last induction hardening, so if the hardening depth by the last induction hardening is smaller than the previous hardening depth by the induction hardening, This is because the average grain size over the entire thickness is rather increased and the fatigue strength tends to decrease.

Furthermore, in the induction hardening, the heating time in the above heating temperature range is preferably 5 seconds or less. This is because when the heating time is set to 5 seconds or less, the austenite grain growth can be further suppressed as compared with the case where the heating time exceeds 5 seconds, and a very fine hardened layer particle size can be obtained. . A more preferable heating time is 3 seconds or less.
Furthermore, the Most small, cooling rate after holding at a heating rate and the heating time at the time of induction hardening, the grain growth of the austenite is likely to occur, the heating rate and cooling rate after heating and holding at the time of induction hardening 200 ° C. / s or more is preferable. More preferably, it is 500 ° C./s or more.

  Tempering is performed following induction hardening, but in the present invention, in order to obtain the above-described surface residual stress, the cooling rate during tempering needs to be 30 ° C./s or more. This is because when the cooling rate is less than 30 ° C./s, the generation of thermal stress during cooling becomes insufficient, and as a result of insufficient generation of the surface residual stress described above, sufficient fatigue strength can be secured. It is not possible. A more preferable cooling rate is 50 ° C./s or more, and a further preferable cooling rate is 100 ° C./s or more.

  Steel materials having the composition shown in Table 1 were melted by a converter and cast into continuous slabs. The slab size was 300 × 400mm. The slab was rolled into a 150 mm square billet through a breakdown process, and then rolled into a steel bar having a diameter of 24 to 60 mm. The finishing temperature of rolling was over 900 ° C. as a suitable temperature from the viewpoint of bainite or martensite structure formation. Cooling after rolling was performed under the conditions shown in Table 2.

  Next, a torsional test piece having a notch with a parallel part: 20 mmφ and a stress concentration factor α = 1.5 was produced from this bar, and the heating rate was 800 ° C. using an induction hardening apparatus with a frequency of 15 kHz. / s, the temperature drop rate after heating and holding is 1000 ° C./s, and after quenching at the heating temperature and holding time shown in Table 2, using a heating furnace or high-frequency heating device, tempering under the conditions shown in Table 2 And then a torsional fatigue test was conducted.

The residual stress was measured using an X-ray residual stress measuring device at the parallel portion (position adjacent to the notch) of the torsion test piece.
The torsional fatigue test is performed using a torsional fatigue testing machine with a maximum torque of 4900 N · m (= 500 kgf · m), changing the stress conditions with both swings, and fatigue the stress that will result in a life of 1 × 10 ⁵ times. The strength was evaluated.
The obtained results are also shown in Table 2.

In addition, for the torsional test piece prepared under the same conditions, the average hardened layer particle size (old austenite particle size) obtained over the base material structure of the steel material, the hardened layer thickness after quenching, and the total thickness of the hardened layer was measured using an optical microscope. Measured.
Table 2 also shows these results.

  Here, as described above, the thickness of the hardened layer was from the steel surface to the depth at which the area ratio of the martensite structure was reduced to 98%. Moreover, about what performed induction hardening several times, the hardening layer thickness after each hardening was measured. Further, regarding the hardened layer particle size, the average prior austenite particle size at each of the 1/5 position, 1/2 position and 4/5 position of the hardened layer thickness from the surface was measured, and the maximum value was shown. The particle size of the hardened layer was measured with respect to a cross section cut in the thickness direction of the hardened layer, in a picric acid aqueous solution in which 50 g of picric acid was dissolved in 500 g of water, 11 g of sodium dodecylbenzenesulfonate, and chloride. What added 1 g of ferrous iron and 1.5 g of oxalic acid was made to act as a corrosive liquid, and the prior austenite grain boundary was made to appear. Moreover, about what performed induction hardening several times, the average prior-austenite particle size after final hardening was measured.

As apparent from Table 2, all the steel materials that satisfy the component composition range defined in the present invention and satisfy the induction hardening condition and the tempering condition of the present invention have a total austenite grain size of the hardened layer. As a result, it was possible to obtain a high torsional fatigue strength of 700 MPa or more.
When No. 1 and 2 in Table 2 are compared, it can be seen that by increasing the number of times of quenching from one to two, the grain size of the hardened layer becomes finer and the torsional fatigue strength further increases.

When comparing No.7, No.35 and No.36, when the number of quenching is increased from 1 to 2 and the second quenching depth is shallower (No.35) The torsional fatigue strength is rather lower than the case where it was applied only once, whereas the torsional fatigue strength was increased when the second quenching depth was increased (No. 36) compared to the case where it was applied only once. The strength was greatly improved. In No. 36, in the thickness direction of the hardened layer, the oldest austenite grain size was 3.5 μm at the 4/5 position from the surface to the hardened layer thickness. ), The prior austenite grain size is 2.6μm, and the refinement of the surface grain size is thought to have contributed to the improvement of fatigue strength.
Nos. 37 to 46 are those in which the Al amount is in a more preferable range (0.05 to 0.1 mass%), but all of these have finer particle sizes and improved fatigue strength.

In contrast, No. 10 has a low cooling rate after processing, so the total structural fraction of bainite and martensite is less than 10%. As a result, the hardened layer particle size becomes coarse, and the torsional fatigue strength Is low.
In No. 23, although the hardened layer particle size is fine, the C content is higher than the range of the present invention, so that the grain boundary strength is lowered, and therefore the torsional fatigue strength is inferior.
In Nos. 24, 25 and 26, the contents of C, Si and Mo are lower than the proper range of the present invention, respectively, so that the hardened layer particle size is coarse and the torsional fatigue strength is inferior.
No. 27 has a low B content, No. 28 has an Mn content, No. 29 has an S and P content, and No. 30 has a Cr content exceeding the proper range of the present invention. For this reason, any of these results in a decrease in grain boundary strength and inferior torsional fatigue strength.
No.31 is inferior in torsional fatigue strength because the Ti content exceeds the proper range of the present invention, and conversely, No.34 has a low Ti content, resulting in a coarse hardened layer particle size and torsion. Fatigue strength is inferior.
In No. 32, the heating temperature during induction hardening is too high, and the particle size of the hardened layer becomes coarse. On the other hand, in No. 33, the heating temperature during induction hardening is too low, and no hardened layer is formed. It is inferior in fatigue strength.
Nos. 51 to 52 are cases of steel not containing Mo, but as is clear from comparison with Nos. 5 and 3, respectively, the heating effect is 1000 ° C. or less, and the effect of refining by Mo becomes remarkable. I understand that.
No. 4 and No. 8 are examples in which the cooling rate during tempering was reduced compared to No. 3 and No. 7, respectively, but the residual stress was over -700 MPa, and the torsional fatigue strength decreased. is doing.

  In the above-described embodiments, the torsional fatigue characteristics are mainly exemplified as the fatigue characteristics. However, according to the present invention, other fatigue characteristics, that is, bending fatigue characteristics, rolling fatigue characteristics, and sliding rolling fatigue are described. Needless to say, the same excellent effects can be obtained with respect to fatigue properties such as fracture and crack advancement in the prior austenite grain boundaries.

It is the graph which showed the influence which the bainite structure fraction and martensite structure fraction in steel have on machinability and high strength. It is the graph which showed the influence which the heating temperature at the time of induction hardening has on the prior austenite grain size of a hardening layer about Mo addition steel (Mo: 0.05-0.6 mass%) and Mo non-addition steel.

Claims (9)

C: 0.35-0.7 mass%
Si: 0.30 to 1.1 mass%,
Mn: 0.2-2.0 mass%
Al: 0.005 to 0.25 mass%,
Ti: 0.005 to 0.1 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and
Cr: 0.2 mass% or less is contained under Ti (mass%) / N (mass%) ≧ 3.42, the balance is composed of Fe and inevitable impurities, and the base material structure is bainite structure and / or martensite. Having a structure, and the total structure fraction of these bainite structure and martensite structure is 10 vol % or more, and the old austenite grain size of the hardened layer after induction hardening is 12 μm or less over the entire thickness of the hardened layer, A steel material with excellent fatigue characteristics characterized by a surface residual stress of -700 MPa or less after induction hardening and tempering.   The steel material excellent in fatigue characteristics according to claim 1, wherein the thickness of the hardened layer after induction hardening is 2 mm or more. The steel material according to claim 1 or 2, further comprising:
Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0 mass% or less,
Nb: steel material excellent in fatigue characteristics having a composition containing one or more selected from 0.1 mass% or less and V: 0.5 mass% or less. C: 0.35-0.7 mass%
Si: 0.30 to 1.1 mass%,
Mn: 0.2-2.0 mass%
Al: 0.005 to 0.25 mass%,
Ti: 0.005 to 0.1 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and
Cr: 0.2 mass% or less is contained under Ti (mass%) / N (mass%) ≧ 3.42, and the remainder is hot-worked with a steel material having a composition of Fe and inevitable impurities, and then 0.2 ° C / After cooling at a rate of at least s, induction heating is performed at a temperature of induction quenching of 800 to 1000 ° C, and tempering is performed at a cooling rate of at least 30 ° C / s. A method for producing steel with excellent fatigue properties. 5. The steel material according to claim 4, further comprising:
Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0 mass% or less,
A method for producing a steel material having excellent fatigue characteristics, containing one or more selected from Nb: 0.1 mass% or less and V: 0.5 mass% or less.   6. The method for manufacturing a steel material having excellent fatigue characteristics according to claim 4 or 5, wherein after the cooling, induction hardening is repeated a plurality of times, and a heating temperature at the final induction hardening is set to 800 to 1000 ° C.   The method for manufacturing a steel material having excellent fatigue characteristics according to claim 6, wherein the heating temperature during induction hardening is set to 800 to 1000 ° C for all of the plurality of induction hardenings.   The method for producing a steel material having excellent fatigue characteristics according to any one of claims 4 to 7, wherein the heating time in the heating temperature range is 5 seconds or less per induction hardening.   9. The method for manufacturing a steel material excellent in fatigue characteristics according to claim 4, wherein the thickness of the hardened layer on the surface of the steel material by induction hardening is 2 mm or more.

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