JP4773118B2 - Crankshaft with excellent bending fatigue strength - Google Patents

Description

  The present invention relates to a crankshaft having a hardened layer formed by induction hardening on the surfaces of a crankpin portion and a journal portion, and particularly intends to advantageously improve the bending fatigue strength thereof.

A crankshaft is used as a part for changing the reciprocating motion of a piston in an internal combustion engine such as an automobile engine to a rotational motion.
As shown in FIG. 1, the crankshaft 1 is provided with a journal part 2 for a cylinder, a crankpin part 3 which is a bearing part of a connecting rod for a piston, a crank web part 4 and a counterweight part 5, and particularly a journal. The part 2 and the crankpin part 3 are induction hardened to improve their fatigue strength.

  However, with the recent increase in output of internal combustion engines, further enhancement of the crankshaft, particularly improvement in bending fatigue strength, has been demanded. Here, in order to improve the bending fatigue strength of the crankshaft, the above-described journal portion 2 and crankpin portion 3, particularly the boundary between the journal portion 2 and the counterweight portion 5 and the crank web portion 4 of the crankpin portion 3 are used. It is effective to increase the fatigue strength at the fillet r portion 7 which is a boundary point.

  This is because this part is a part where stress is concentrated in shape and tends to be a starting point of fatigue damage of the crankshaft.

  In order to improve the fatigue strength, it is effective to improve the grain boundary strength. From this viewpoint, for example, Patent Document 1 proposes a technique for refining the prior austenite grain size by dispersing TiC.

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 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 Patent Document 1 described above, the demand for the bending fatigue strength of the crankshaft could not be sufficiently met.

  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-hardened, and the average Vickers hardness Hc of the shaft center after induction hardening is expressed as C There has been proposed a shaft member for a machine structure with improved torsional fatigue strength by controlling the amount within a predetermined range according to the amount.

  However, in this part, no consideration is given to the prior austenite grain size over the entire thickness of the hardened hardened layer. For this reason, the bending fatigue strength as good as expected in the present invention could not be obtained.

Furthermore, Patent Document 3 discloses that rolling fatigue strength is improved by making the surface γ grain size 10 or more in accordance with JIS G0551 grain size No. and finely dispersing carbides by two-step induction hardening after molding. Technology has been proposed.
However, this technique cannot obtain a bending fatigue strength as good as expected in the present invention, because the quench hardened layer depth is shallow.
JP 2000-154819 A (claim, paragraph [0008]) JP-A-8-53714 (Claims) JP-A-7-118791 (Claims)

  The present invention has been developed in view of the above-mentioned present situation, and an object thereof is to propose a crankshaft that is superior in bending fatigue strength than the conventional one.

The inventors have intensively studied to effectively improve the bending fatigue strength of the crankshaft.
As a result, as described below, the crankshaft having excellent bending fatigue strength can be obtained by optimizing the chemical composition of the crankshaft, the structure and the prior austenite grain size over the entire thickness of the hardened layer after quenching. The following findings (i) to (iii) were obtained.

(I) Bending fatigue strength is remarkably improved by quenching the necessary part of the crankshaft adjusted to an appropriate chemical composition and making the prior austenite grain size over the entire thickness of the quenched and hardened layer 10 μm or less, preferably 5 μm or less. To do. 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 hardened hardened layer is effectively refined, and as a result, the bending fatigue strength is remarkably improved.

(Ii) When the structure of the base material of the crankshaft, that is, the structure before quenching, is a structure containing a bainite structure and / or a martensite structure at a specific fraction, the bainite structure or the martensite structure is a ferrite-pearlite structure. Compared with, the carbide is finely dispersed in the structure, so that 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 the bending fatigue strength.

(Iii) Further, when a necessary portion of the quenching portion in the crankshaft was examined, stress is concentrated on the pin portion and the fillet r portion of the journal portion under the usage environment of the crankshaft. It was found that there is a need to secure enough. And when the quenching depth (hardened layer thickness) of this part is shallower than 2.0mm, it turns out that early fatigue failure starts from the internal quenching boundary (between the hardened layer and the non-hardened layer). did. Accordingly, it has been found that it is particularly effective in improving the bending fatigue strength to suppress the early fracture of the internal origin by setting the thickness of the hardened layer of the fillet r portion to 2.0 mm or more.

The present invention is based on the above findings.
That is, the gist configuration of the present invention is as follows.
(1) A crankshaft having a hardened hardening layer on the surfaces of the crankpin portion and the journal portion,
C: 0.40 to 0.51 mass%,
Si: 0.30 ~ 0.80mass%,
Mn: 0.50-1.50 mass%,
Al: 0.040 mass% or less (including zero),
Ti: 0.005-0.06mass%,
Mo: 0.15-0.50mass%,
B: 0.0005 to 0.0030 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and
Cr: contains 0.2 mass% or less , and
Te: 0.2 mass% or less,
Se: 0.2 mass% or less,
Ca: 0.02 mass% or less,
REM: 0.03 mass% or less,
Zr: 0.2 mass% or less,
Sn: 0.3 mass% or less,
Sb: 0.2 mass% or less,
Mg: 0.02 mass% or less and
Hf: 0.1 mass% or less
1 or 2 or more selected from among them, the balance is composed of Fe and inevitable impurities, the base material structure has a bainite structure and / or a martensite structure, and the bainite structure The total structure fraction of the martensite structure is 10% or more, and the old austenite grain size of the hardened surface layer after induction hardening is 10 μm or less over the entire thickness of the hardened layer. Crankshaft with excellent bending fatigue strength, characterized by having a bending fatigue limit strength of 800 MPa or more.

(2) In the above (1), the component composition of the steel material is further
Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0mass% or less,
A crankshaft excellent in bending fatigue strength, characterized by having a composition containing one or more selected from Nb: 0.1 mass% or less and V: 0.5 mass% or less.

  Thus, according to the present invention, it is possible to stably obtain a crankshaft having a bending fatigue strength that is remarkably superior to that of the conventional one, and as a result, it is effective for the demand for weight reduction of automotive parts. .

Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of the crankshaft is limited to the above range in the present invention will be described.
C: 0.40 ~ 0.51mass%
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 0.40 mass%, in order to ensure the required fatigue strength, the quench hardening depth must be drastically increased, and the occurrence of quenching cracks is remarkable at that time. In addition, since it becomes difficult to form a bainite structure, 0.40 mass% or more is added. On the other hand, if the content exceeds 0.51 mass%, the grain boundary strength decreases, and accordingly, the bending fatigue strength decreases, and the machinability, cold forgeability and fire cracking resistance also decrease. For this reason, C was limited to the range of 0.40 to 0.51 mass%. In addition, 0.45 mass% or more is a more preferable range.

Si: 0.30 ~ 0.80mass%
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 generation of a bainite structure, and these improve the bending 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 by setting the Si amount to 0.30 mass% or more, it is possible to obtain fine particles having a particle diameter of 10 μm or less over the entire thickness of the cured layer. However, when the Si content exceeds 0.80 mass%, the hardness increases due to 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 0.80 mass%. A more preferable Si amount is 0.40 mass% or more.

Mn: 0.50 to 1.50 mass%
Mn is an indispensable component for improving the hardenability and securing the hardening depth during quenching, so it is actively added, but if the content is less than 0.50 mass%, the effect of addition is poor, so 0.50 More than mass%. On the other hand, if the amount of Mn exceeds 1.50 mass%, the base material is hardened, which is disadvantageous for machinability. Therefore, Mn is set to 1.50 mass% or less. A more preferable amount of Mn is 0.70 mass% or less.

Al: 0.040 mass% or less (including zero)
Al is an element effective for deoxidation. Moreover, it is an element useful also in refinement | miniaturizing the particle size of a hardening hardening layer by suppressing the austenite grain growth at the time of quenching heating, and can be contained preferably in 0.005 mass% or more as needed. However, even if the content exceeds 0.040 mass%, the effect is saturated. Rather, the increase in oxides may cause a decrease in fatigue strength, so Al is limited to a range of 0.040 mass% or less.

Ti: 0.005-0.06mass%
Ti binds to N mixed as an unavoidable impurity, thereby preventing B from becoming BN and eliminating the effect of improving the hardenability of B, and having the effect of sufficiently exerting the effect of improving the hardenability of B. .
In order to obtain this effect, it is necessary to contain at least 0.005 mass%. However, if it exceeds 0.06 mass%, TiN coarsens and oxides are generated, which causes fatigue fracture. Ti is limited to the range of 0.005 to 0.06 mass% because it causes a significant decrease in bending fatigue strength. A more preferable range is 0.030 mass% or less.

Mo: 0.15-0.50mass%
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. 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.
As described above, Mo is a very important element in the present invention, and if the content is less than 0.15 mass%, the particle size of the hardened layer over the entire thickness is adjusted no matter how the manufacturing conditions and quenching conditions are adjusted. It cannot be made as fine particles of 10 μm or less. However, if the content exceeds 0.50 mass%, the hardness of the rolled material is remarkably increased, and the workability is reduced. Therefore, Mo was limited to the range of 0.15 to 0.50 mass%. A more preferable range is 0.30 to 0.50 mass%.

B: 0.0005-0.0030mass%
B has an effect of promoting the formation of a bainite structure or a martensite structure. Further, B has an effect of improving hardenability by adding a small amount and improving bending fatigue strength by increasing the quenching depth during quenching. Furthermore, B preferentially segregates at the grain boundaries, reduces the concentration of P segregating at the grain boundaries, improves the grain boundary strength, and thus has the effect of improving the bending fatigue strength.
For this reason, in the present invention, B is positively added. However, if the content is less than 0.0005 mass%, the addition effect is poor. On the other hand, if the content exceeds 0.0030 mass%, the effect is saturated, and the boron compound Since the fatigue strength is reduced due to the increase of B, B is limited to the range of 0.0005 to 0.0030 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.

P: 0.02 mass% or less P segregates at the austenite grain boundaries and lowers the grain boundary strength, thereby lowering the bending 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 0.02 mass%.

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

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 for improving the hardenability, and is dissolved in ferrite, and this solid solution strengthening improves the bending fatigue strength. Moreover, by suppressing the formation of carbides, the decrease in grain boundary strength due to carbides is suppressed, and the bending 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.

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 a bending fatigue strength. However, Ni is an extremely expensive element, and adding more than 3.5 mass% increases the cost of the steel material, so the addition is made 3.5 mass% or less. 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.

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 bending 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 is 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.

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 bending 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, since addition effect for precipitation strengthening and tempering softening resistance is small when added less than 0.005%, it is desirable to add 0.005 mass% or more.

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 bending 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 bending 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%.

Te: 0.2 mass% or less
Se: 0.2 mass% or less
Te and Se combine with Mn to form MnTe and MnSe, respectively, which improve machinability by acting as a chip breaker. However, if the content exceeds 0.2 mass%, the effect is saturated and the component cost is increased, so that the content is 0.2 mass% or less. In addition, a suitable range is 0.1 mass% or less. In order to improve machinability, it is preferable to contain 0.003 mass% or more in the case of Te and 0.003 mass% or more in the case of Se.

Ca: 0.02 mass% or less
REM: 0.03 mass% or less
Zr: 0.2 mass% or less
Ca, REM and Zr each form a sulfide together with MnS, which improves the machinability by acting as a chip breaker. However, even if Ca, REM, and Zr are contained in amounts exceeding 0.02 mass%, 0.03 mass%, and 0.2 mass%, respectively, the effect is saturated and the component cost is increased. It was supposed to be. More preferably, the ranges are Ca: 0.01 mass% or less, REM: 0.015 mass% or less, and Zr: 0.1 mass% or less. In order to improve the machinability, it is preferable to contain 0.0001 mass% or more of Ca, 0.0001 mass% or more of REM, and 0.003 mass% or more of Zr.

Sn: 0.3 mass% or less
Sb: 0.2 mass% or less
Both Sn and Sb are elements that improve machinability by the embrittlement effect. However, even if Sn: 0.3 mass% and Sb: more than 0.2 mass% are added, the effect is saturated and the cost rises and becomes economically disadvantageous. . More preferably, it is the range of Sn: 0.15 mass% or less and Sb: 0.1 mass% or less. In order to improve the machinability, it is preferable to contain Sn of 0.005 mass% or more and Sb of 0.0005 mass% or more.

Mg: 0.02 mass% or less
Hf: 0.1 mass% or less
Both Mg and Hf are not only deoxidizing elements, but also have the effect of improving the machinability as a stress concentration source, and can be added as necessary. However, if added excessively, the effect is saturated and the component cost is increased. More preferably, the ranges are Mg: 0.01 mass% or less and Hf: 0.05 mass% or less. In order to improve the machinability, it is preferable to contain Mg at 0.0001 mass% or more and Hf at 0.005 mass% or more.

As mentioned above, although the range of the suitable component composition was demonstrated, in this invention, it is insufficient only to limit a component composition to said range, and adjustment of a steel structure is also important.
That is, in the present invention, the base material structure of the crankshaft, that is, the structure before quenching (corresponding to a structure other than the hardened layer after induction hardening) has a bainite structure and / or a martensite structure, and these bainite structures The total tissue fraction of the martensite structure needs to 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 during quenching heating, increases. This is because the generated austenite is refined, and thus contributes effectively to refine the grain size of the quenched and hardened layer. And grain boundary intensity | strength rises and bending 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 remaining structure other than the bainite structure or martensite structure may be ferrite, pearlite, or the like, and is not particularly defined.
Also, 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 point of view, the bainite structure is better than the martensite structure. Since the addition amount of the alloy element is small and it can be produced at a low cooling rate, it is advantageous in production.

  In the crankshaft of the present invention, it is also important to adjust the prior austenite grain 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 10 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 10 μm, sufficient grain boundary strength cannot be obtained, and as a result, satisfactory improvement in bending fatigue strength cannot be expected. In addition, Preferably it is 5 micrometers or less, More preferably, it is 2 micrometers 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 crankshaft of the present invention after induction hardening, the outermost 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.

And about this hardened layer, the average prior austenite particle size in each position of 1/5 position, 1/2 position, and 4/5 position of the hardened layer thickness from the surface was measured, and any average prior austenite particle diameter was 10 μm or less. In this case, it is assumed that the prior austenite grain size over the entire thickness of the quenched and hardened layer is 10 μm or less.
The average prior austenite grain size is measured by optical microscope from 400 times (one visual field area: 0.25 mm x 0.225 mm) to 1000 times (one visual field area: 0.10 mm x 0.09 mm) at each position. This is done by observing 5 fields of view and measuring the average particle size with an image analyzer.

  Further, in the crankshaft of the present invention, it is necessary that the bending fatigue limit strength is 800 MPa or more particularly in the fillet r portion of the crankpin portion and the journal portion. This is because the engine can be made compact or lightweight.

  In order to set the bending fatigue limit strength of the fillet r part to 800 MPa or more, the thickness of the hardened layer of the fillet r part needs to be 2.0 mm or more. FIG. 2 is a diagram schematically showing a cross section of the pin portion or the journal portion. The thickness of the hardened layer of the fillet r portion is a distance from the fillet r portion 7 to the nearest burning boundary as shown in FIG. Say L. Under the operating environment of the crankshaft, stress concentrates on the pin portion and the fillet r portion of the journal portion, so it is necessary to ensure a sufficient quenching depth in this portion. If the thickness of the hardened layer in the fillet r portion is less than 2.0 mm, it is difficult to ensure a bending fatigue limit strength of 800 MPa or more.

  Moreover, it is necessary to ensure 2.0 mm or more also about the hardened layer thickness M of the axial smooth part 8 of a pin part and a journal part. This is because if the thickness of the hardened layer of the shaft smoothing portion 8 is not sufficiently ensured, internal fracture will occur from the burned-in surface of the shaft smoothing portion 8. In addition, the hardened layer thickness of an axial smooth part means the distance from an axial smooth part surface to the nearest burning boundary.

Next, the manufacturing conditions of the present invention will be described.
In the present invention, a steel material adjusted to a predetermined component composition is formed into a round bar by hot rolling, then cut to a predetermined length, then formed into a crankshaft by hot forging, and then normalized as necessary. After applying cutting, it is subjected to cutting, and then subjected to induction hardening and tempering, and then finished or shot peened as necessary to obtain a product.

  By making hot forging and subsequent cooling appropriate conditions, the above-mentioned base material structure, that is, the structure before quenching is made into a bainite structure and / or a martensite structure, and the fraction of these bainite structure and martensite structure Is 10% or more. The hot forging temperature may be 800 ° C. or higher, and cooling may be performed at a cooling rate of 0.2 ° C./s or higher after hot forging.

  Further, in order to make the prior austenite grain size of the hardened layer 10 μm or less, it is necessary that the maximum temperature achieved during induction hardening is 800 to 1050 ° C. and the heating rate is 200 ° C./s or more. When the maximum temperature is less than 800 ° C., a hardened layer cannot be formed by quenching. On the other hand, when the maximum temperature reached is higher than 1050 ° C. or the heating rate is lower than 200 ° C./s, the prior austenite grain size of the hardened layer exceeds 10 μm, and the bending fatigue limit strength cannot be ensured. The maximum temperature achieved during induction hardening is more preferably 950 ° C. or less, and if it is 950 ° C. or less, an ultrafine hardened layer having a prior austenite grain size of 5 μm or less can be obtained.

Steel materials having the composition shown in Table 1 were melted by a converter and cast into continuous slabs. The slab size was 300 × 400 mm. This slab was rolled into a 90 mmφ steel bar by hot rolling. Next, after cutting this steel bar to a predetermined length and heating to 1200 ° C, each hot forging from bending to finishing is performed, further deburring and forming into a crankshaft shape, about 0.7 ° C / s It was cooled with. For the steel symbol C, for comparison, a steel with cooling after forming at a condition of 0.08 ° C./s was also produced. Furthermore, the crankshaft was finished by oil drilling and finishing. The finishing temperature for hot forging was over 900 ° C. from the viewpoint of bainite or martensite structure formation.
Next, as shown in a cross-sectional view of the crankshaft in FIG. 3, induction hardening (heating rate: 250 ° C./s, maximum heating temperature: 890 ° C. (No. 1 to 33, 39 to 42) or 1030 ° C. (No. 34 to 38 in Table 2 without holding time) to form the cured layer 6, and then using a heating furnace at 170 ° C. for 30 minutes Tempering and finishing were performed to obtain a product. The hardened layer depth was adjusted by adjusting the high frequency output conditions.

Table 2 shows the results of examining the bending fatigue strength of the crankshaft thus obtained. Here, the bending fatigue strength of the crankshaft was evaluated as follows.
That is, as shown in FIG. 4, with the connecting rod attached to the crankpin portion of the crankshaft and the end portion of the crankshaft being fixed, an endurance test was performed in which a constant repeated load was applied to each connecting rod. The number of repetitions until the pin portion or journal portion was damaged was determined, and the upper limit stress that did not break even 10 ⁷ times or more was defined as the bending fatigue limit strength.

  In addition, as bending strength, the load stress of the fillet r part in said load state was evaluated by the value calculated | required by the finite element method analysis.

In addition, for the same crankshaft, the average hardened layer particle size (old austenite particle size) obtained over the base material structure, the hardened layer thickness of the shaft smoothed part and the fillet r part after quenching, and the total thickness of the hardened layer (former austenite particle diameter) And measured.
These results are also shown in Table 2.
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%.
Furthermore, as for 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 in 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, sodium dodecylbenzenesulfonate: 11 g, and first chloride. The addition of iron (1 g) and oxalic acid (1.5 g) was allowed to act as a corrosive solution to reveal the prior austenite grain boundaries.

  As is apparent from Table 2, all crankshafts satisfying the component composition range and the structure defined in the present invention satisfy the prior austenite grain size of the hardened layer of 5 μm or less over the entire thickness. As a result, the fillet r An excellent bending fatigue strength with a bending fatigue limit strength of 800 MPa or more was obtained with partial stress.

On the other hand, No. 9 is a comparative example in which the cooling rate after hot forging was reduced to 0.08 ° C./second, but the total structure fraction of bainite and martensite was less than 10%. Coarse and low bending fatigue strength.
No. No. 18 has a fine hardened layer particle size, but its C content is higher than the range of the present invention, which leads to a decrease in grain boundary strength, and therefore its bending fatigue strength is inferior.
In Nos. 19.20 and 21, 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 bending fatigue strength is inferior.
No. 22 has a B content. No. 23 had Mn content, No. 24 had S and P contents, Since No. 25 has an Al content and No. 26 has a Cr content exceeding the appropriate range of the present invention, both of them cause a decrease in grain boundary strength and are inferior in bending fatigue strength.
No. 27 is inferior in bending fatigue strength because the Ti content exceeds the appropriate range of the present invention. Since 28 has a low Ti content, the hardened layer particle size is coarse and the bending fatigue strength is inferior.
In Nos. 39 and 41, the cured layer thickness of the fillet r part is not sufficient, and in Nos. 40 and 42, the cured layer thickness of the shaft smooth part is insufficient, and the bending fatigue strength is inferior.

It is a schematic diagram of a crankshaft. It is a schematic diagram explaining the hardened layer thickness of a fillet r part and an axial smooth part. It is the figure which showed the position of the induction hardening of a crankshaft. It is the figure which showed the outline | summary of the endurance test according to this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Crankshaft 2 Journal part 3 Crankpin part 4 Crank web part 5 Counterweight part 6 Hardened layer 7 Fillet r part 8 Axis smooth part

Claims (2)

A crankshaft having a hardened hardening layer on the surfaces of the crankpin portion and the journal portion,
C: 0.40 to 0.51 mass%,
Si: 0.30 ~ 0.80mass%,
Mn: 0.50-1.50 mass%,
Al: 0.040 mass% or less (including zero),
Ti: 0.005-0.06mass%,
Mo: 0.15-0.50mass%,
B: 0.0005 to 0.0030 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and
Cr: contains 0.2 mass% or less , and
Te: 0.2 mass% or less,
Se: 0.2 mass% or less,
Ca: 0.02 mass% or less,
REM: 0.03 mass% or less,
Zr: 0.2 mass% or less,
Sn: 0.3 mass% or less,
Sb: 0.2 mass% or less,
Mg: 0.02 mass% or less and
Hf: 0.1 mass% or less
1 or 2 or more selected from among them, the balance is composed of Fe and inevitable impurities, the base material structure has a bainite structure and / or a martensite structure, and the bainite structure The total structure fraction of the martensite structure is 10% or more, and the old austenite grain size of the hardened surface layer after induction hardening is 10 μm or less over the entire thickness of the hardened layer. Crankshaft with excellent bending fatigue strength, characterized by having a bending fatigue limit strength of 800 MPa or more. In Claim 1, the component composition of steel materials is further
Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0mass% or less,
A crankshaft excellent in bending fatigue strength, characterized by having a composition containing one or more selected from Nb: 0.1 mass% or less and V: 0.5 mass% or less.

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