JP3733967B2 - Steel material with excellent fatigue characteristics and method for producing the same - Google Patents

Description

The present invention is provided with a hardened layer by induction hardening on the surface, and applied to a motor vehicle drive shaft and a constant velocity joint suitable relates excellent steel and methods for their preparation in fatigue characteristics.

Conventionally, mechanical structural members such as automobile drive shafts and constant velocity joints are processed by hot forging, hot cutting, further cutting, cold forging, etc. into a predetermined shape and induction hardening. By tempering, it is common to ensure fatigue strength such as torsional fatigue strength, bending fatigue strength, rolling fatigue strength, and sliding rolling fatigue strength, which are important characteristics as a machine structural member. .
On the other hand, in recent years, there is a strong demand for weight reduction of automobile members due to environmental problems, and further improvement in fatigue strength of automobile members is required from this viewpoint.

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, there is a problem that it is necessary to increase the heating temperature during hot rolling, resulting in poor productivity.
Further, even with the technique disclosed in the above-mentioned Patent Document 1, there is still a problem in that it cannot sufficiently meet the recent demand for torsional fatigue strength.

Further, 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.

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

The present invention has been developed in view of the above situation, the than conventional fatigue strength is above further improvement steel, and an object thereof is to propose with its advantageous production method.

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, as described below, the knowledge that excellent torsional fatigue strength can be obtained by optimizing the chemical composition, structure, quenching conditions of steel, and the prior austenite grain size over the entire thickness of the hardened layer after quenching. Obtained.

(1) The torsional fatigue strength is remarkably improved by quenching the 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, the addition of Si and Mo in an appropriate range increases the number of austenite nucleation sites during induction hardening and suppresses the growth of austenite grains. 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 hardened layer having a particle size of 12 μm or less can be obtained over the entire thickness of the hardened 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, thereby improving the grain boundary strength and increasing 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 heating temperature: 800-1000 ° C., more preferably 800-950 ° C. and heating time: 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. Obtainable. 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.
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: 0.25 mass% or less,
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 martense. It has a site structure, the total structure fraction of these bainite structure and martensite structure is 10% or more, and the prior 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.

2. In the above 1, wherein the hardened layer thickness after induction hardening is 2mm or more, excellent steel fatigue properties.

3. In the above 1 or 2 , the steel material further includes
Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0 mass% or less,
A steel material excellent in fatigue characteristics, characterized by having a composition containing one or more selected from Nb: 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: 0.25 mass% or less,
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. A method for producing a steel material with excellent fatigue characteristics, characterized by performing induction hardening under conditions of a heating temperature of 800 to 1000 ° C. after cooling at a rate of at least / s.

5. In the above 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, characterized in that the composition contains one or more selected from Nb: 0.1 mass% or less and V: 0.5 mass% or less.

6. 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 set to 800 to 1000 ° C.

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

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

9. The method for producing a steel material excellent in fatigue characteristics according to any one of the above 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.

The present invention will be specifically described below.
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, in order to secure the required fatigue strength when the content is less than 0.35 mass%, the quench hardening depth must be dramatically increased, and the occurrence of quenching cracks becomes remarkable at that time, and the bainite structure 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 Mn content is large, the base material is hardened, which may be disadvantageous in machinability. Therefore, the content is preferably 1.2 mass% or less. More preferably, it is 1.0 mass% or less.

Al: 0.25 mass% or less
Al is an element effective for deoxidation. Moreover, it is an element useful also for refinement | miniaturizing the particle size of a hardening hardening layer by suppressing the austenite grain growth at the time of quenching heating , Preferably it adds at 0.005 mass% or more. On the other hand, even if it contains more than 0.25 mass%, the effect is saturated, and disadvantages that cause an increase in the component cost occur. Therefore, Al is limited to the range of 0.25 mass% or less . Preferably it is the range of 0.10 mass% or less .

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 necessary to satisfy Ti (mass%) / N (mass%) ≧ 3.42 from the viewpoint of securing N securely and improving 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 harmful effect that promotes burning cracks. 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 tempering and 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. In addition, if less than 0.005% is added, the effect of improving precipitation strengthening and tempering softening resistance is small, so it is desirable to add 0.005 mass% or more. 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 tempering and 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 more in terms of 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 generated at a low cooling rate, which is advantageous in production.

FIG. 1 shows the results of examining the influence of the bainite and martensite structure fractions in steel on the machinability and the strengthening.
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 the martensite structure may be ferrite, pearlite, or the like, and is not particularly defined.

  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 with an area ratio of 100%. 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 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. This is because when the strength is a problem, the hardened layer thickness is preferably as thick as possible. Therefore, a more preferable cured layer thickness is 2.5 mm or more, and more preferably 3 mm or more.

Next, the manufacturing conditions of the present invention will be described.
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.
The hot working is preferably performed in a temperature range of more than 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 hardened 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. 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.

Further, in the present invention, the induction hardening is preferably performed with the heating time in the above heating temperature range being 5 seconds or less. This is because when the heating time is 5 seconds or less, it is possible to further suppress the grain growth of austenite and to obtain a very fine hardened layer particle size as compared with the case of exceeding 5 seconds. . A more preferable heating time is 3 seconds or less.
Furthermore, since the austenite grain growth is likely to occur when the heating rate during induction hardening and the temperature decreasing rate after holding for the above heating time are large, the heating rate during induction hardening and the temperature decreasing rate after heating holding are 200 ° C / It is preferable to set it as s or more. More preferably, it is 500 ° 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 of 20 mmφ and a stress concentration factor α = 1.5 was prepared from this bar, and a heating rate of 800 kHz was used for this torsional test piece using an induction hardening apparatus with a frequency of 15 kHz. ℃ / s, the cooling rate after heating and holding is 1000 ℃ / s. After quenching at the heating temperature and holding time shown in Table 2, tempering is performed in a heating furnace at 170 ℃ for 30 minutes. Thereafter, a torsional fatigue test was conducted.
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 produced 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. In addition, the measurement of the particle size of the hardened layer was carried out with respect to a cross section cut in the thickness direction of the hardened layer. 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 is clear from Table 2, all the steel materials manufactured under the conditions satisfying the component composition range defined in the present invention and satisfying the induction hardening conditions of the present invention have a prior austenite grain size of the hardened layer of 12 μm over the entire thickness. As a result, high torsional fatigue strength of 700 MPa or higher was obtained.
In addition, comparing No. 1 and 2 or No. 4 and 5 in Table 2, increasing the number of times of quenching from 1 to 2 refines the grain size of the hardened layer and further increases the torsional fatigue strength. I understand that.
In addition, comparing No.8, No.36, and No.37, when the number of quenching is increased from 1 to 2 and the second quenching depth is shallower (No.36), The torsional fatigue strength decreases rather than the case where it is only applied once, whereas the torsional fatigue is increased when the second quenching depth is increased (No. 37) compared to the case where it is only applied once. The strength was greatly improved. In No. 37, in the direction of the thickness 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. 38 to 47 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. 11 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. 24, although the hardened layer particle size is fine, since the C content is higher than the range of the present invention, the grain boundary strength is lowered, and therefore the torsional fatigue strength is inferior.
In Nos. 25, 26, and 27, the contents of C, Si, and Mo are lower than the appropriate ranges of the present invention, respectively, so that the hardened layer particle size is coarse and the torsional fatigue strength is inferior.
No. 28 has a low B content, No. 29 has an Mn content, No. 30 has an S and P content, and No. 31 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.32 is inferior in torsional fatigue strength because the Ti content exceeds the appropriate range of the present invention, and conversely No.35 has a low Ti content, resulting in a coarse hardened layer particle size and Fatigue strength is inferior.
In No.33, 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.34, the heating temperature during induction hardening is too low to form a hardened layer. It is inferior in fatigue strength.
Nos. 52-54 are for steels that do not contain Mo, but as is clear from comparisons with Nos. 6, 4, and 3, respectively, the heating effect is 1000 ° C or less, and the effect of grain refinement by Mo is remarkable. I understand that

  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 effect 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: 0.25 mass% or less,
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 metal structure is a bainite structure and / or martensite. It has a site structure, the total structure fraction of these bainite structure and martensite structure is 10% or more, and the prior 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. The steel material having excellent 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,
A steel material excellent in fatigue characteristics, characterized by having a composition containing one or more selected from Nb: 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: 0.25 mass% or less,
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. A method for producing a steel material with excellent fatigue characteristics, characterized by performing induction hardening under conditions of a heating temperature of 800 to 1000 ° C. after cooling at a rate of at least / s. 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, characterized in that the composition contains one or more selected from Nb: 0.1 mass% or less and V: 0.5 mass% or less.   6. The method for producing a steel material with excellent fatigue characteristics according to claim 4, 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 producing a steel material with 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.   The method for producing a steel material having excellent fatigue characteristics according to any one of claims 4 to 8, 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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