JP2007238965A - Crankshaft - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a crankshaft which is manufactured with an induction hardening method and is superior in seizure resistance. <P>SOLUTION: The crankshaft comprises, by mass%, 0.3-0.6% C, 0.15% or less Si, more than 1.0% but 2.0% or less Mn, 0.020% or less P, 0.005-0.30% S, 0.05-0.85% Cr, 0.050% or less Al, 0.020% or less N, and the balance Fe with impurities; has a heat conductivity of 40 W/mK or higher; and such a surface hardness Hv after having been induction-hardened as to satisfy the expression of Hv>2.7×κ+420, wherein κ represents heat conductivity (W/mK). The crankshaft has preferably an induction-hardened layer having a ferrite fraction of 1 to 8%. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a crankshaft for an engine manufactured by forging or casting.

  As the engine becomes smaller and lighter, the sliding condition between the crankshaft (crankshaft) and the metal bearing becomes more severe, and the crankshaft steel is required to have higher seizure resistance. Currently, improvement in seizure resistance has been achieved by engine design, metal-side surface modification, and engine oil improvements, but efforts to improve seizure resistance on the crankshaft steel side are strongly demanded. ing.

  In the case of a crankshaft, the dominant factor for seizure is the temperature rise between the shaft and the bearing. An increase in temperature at the sliding interface between the shaft and the bearing causes breakage of the lubricating oil film and reduces seizure resistance.

  The most effective way to prevent the increase in the interface temperature is to reduce the friction between the shaft and the bearing. As a method for reducing the friction, there are measures such as reducing the friction coefficient by reducing the surface roughness, reducing the amount of wear by increasing the hardness of the shaft, and avoiding an increase in the friction coefficient due to wear powder. is there. Furthermore, methods such as forming an oil film on the shaft surface to improve lubricity have been studied.

  For example, in the invention disclosed in Patent Document 1, soft nitriding treatment is performed on steel, and an average compound layer of 12 μm or more is uniformly formed on the surface, thereby securing surface hardness and obtaining seizure resistance. .

  Moreover, in invention of patent document 2, while surface-hardening is ensured by giving chromium plating to the carburized steel surface, surface unevenness | corrugation is provided by carrying out shot peening to a chromium plating layer, and the oil film in a sliding surface By securing the above, excellent seizure resistance is obtained.

  However, the above technique has the following problems.

  Steel surface hardening processes such as nitrocarburizing or carburizing are expensive. Therefore, it is desirable to achieve surface hardening by induction hardening without going through these processes. On the other hand, if an oil reservoir is formed on the surface by shot peening or the like, it is effective to secure an oil film, but this method is also expensive, and cost reduction is indispensable for use as a general-purpose technology.

  On the other hand, the following proposals have been made for the material of the crankshaft. For example, Patent Document 3 proposes a steel material of Si: 0.02 to 0.15% (hereinafter, “%” regarding the component content is mass%) for induction hardening parts. However, the Mn content of this steel material is 0.5 to 1%, and the required hardness may not be ensured depending on the quenching conditions.

  Patent Document 4 proposes a steel material having a low Si content of Si: 0.05 to 0.8%. Since the nitride material such as Ti is added to the steel material, it is possible to ensure the required strength. However, evaluation regarding seizure resistance, which is an important performance required for a crankshaft, has not been made.

Japanese Unexamined Patent Publication No. 7-18379 JP 2004-59935 A JP 2003-253395 A Japanese Patent Laid-Open No. 9-78183

  An object of the present invention is to provide a crankshaft to which an induction hardening method which is an inexpensive processing method is applied, which has a high thermal conductivity and excellent seizure resistance.

  The present invention has been made on the basis of the following knowledge obtained by research conducted with the intention of developing a crankshaft on the assumption that surface hardening is performed by induction hardening.

  (1) As described above, an increase in temperature at the sliding interface causes the lubricating oil film to break and reduces seizure resistance. In order to prevent this temperature rise at the sliding interface, it is effective to increase the thermal conductivity of the steel material that is the material of the crankshaft.

  (2) It is known that the thermal conductivity of a steel material depends greatly on the amount of Si added. For example, although the thermal conductivity of pure iron is 68.7 W / mK, it decreases to about 1/3 when Si is added by several percent. Therefore, in order to ensure the seizure resistance of the steel material, it is important to ensure a high thermal conductivity by reducing the Si content of the steel material in addition to the elements such as hardness and surface irregularities described above. Thus, if the thermal conductivity of the steel material is improved by reducing the Si content of the steel material, the temperature of the sliding interface between the crankshaft and the bearing is lowered, and the occurrence of seizure is suppressed.

  (3) It is necessary to optimize the amount of elements added to improve the hardenability of steel. In particular, by optimizing the content of Mn, which is widely used as an element for improving hardenability, the structure after induction hardening can be made uniform and the hardness of the shaft surface can be secured stably.

  (4) Further, it is desirable to contain several percent of ferrite in the hardened layer by induction hardening. If the abrasive grain size is equivalent to the crystal grain size of the steel material, this ferrite grain will wear preferentially by grinding the grinding wheel, so that when used as a crankshaft material, the surface will be uneven. become. This unevenness becomes an oil reservoir during sliding, and an oil film on the sliding surface is secured.

  The gist of the present invention based on the above knowledge is the following crankshaft.

  In mass%, C: 0.3 to 0.6%, Si: 0.15% or less, Mn: more than 1.0% to 2.0%, P: 0.020% or less, S: 0.005 to 0.30%, Cr: 0.05 to 0.85%, Al: It contains 0.050% or less, N: 0.020% or less, the balance consists of Fe and impurities, the thermal conductivity is 40 W / mK or more, and the surface hardness Hv after induction hardening is expressed by the following formula (1) Crankshaft characterized by satisfaction.

Hv> 2.7 × κ + 420 (1)
Where κ is the thermal conductivity (W / mk).

  The crankshaft preferably has a ferrite fraction of 1 to 8% in the induction hardened layer.

  According to the present invention, a crankshaft having excellent seizure resistance that has not been obtained so far can be obtained by increasing the thermal conductivity of the steel material.

  First, the principle of the present invention will be outlined.

(1) Si content and thermal conductivity As already mentioned, the interface temperature between the crankshaft and the bearing has a great influence on the fracture of the lubricating oil film. Therefore, the interface temperature is an important factor governing seizure resistance. Since the heat generated between the crankshaft and the bearing is dissipated inside the crankshaft material, increasing the thermal conductivity of the shaft material is extremely effective for lowering the interface temperature. As described above, Si is known to reduce the thermal conductivity of steel materials. Therefore, it is possible to increase the thermal conductivity of the steel material by reducing the Si content as much as possible.

(2) About homogenization of surface structure by addition of hardenability element When ferrite is present in the induction hardened layer, the hardness itself decreases, and as a result, it tends to adversely affect the seizure resistance. Therefore, by optimizing the content of Mn, a hardenability-enhancing element, and even with a two-phase structure containing soft ferrite, the martensite structure is made uniform to ensure that the martensite phase has a certain hardness or higher. Can be secured. Of course, even when ferrite is not included, it is very useful for seizure resistance that the martensite structure is uniform and a certain level of hardness is ensured.

(3) Providing surface irregularities to ferrite-martensite duplex steel By adding ferrite, the entire structure becomes soft. However, microscopically, the steel material hardness (Vickers hardness, Hv) is uneven. For example, in the case of steel with C of 0.4% generally used as a material of a crankshaft, the ferrite portion is about Hv150. Martensite is about Hv600.

  Grinding stones are applied to the steel used for the crankshaft. When the abrasive grain size is several μm in diameter, the steel material is detached in units of several μm 3. When the ferrite area ratio is large, the adjacent martensite is also detached together with the ferrite, so the amount of wear increases and the seizure resistance deteriorates. However, when the ferrite ratio is small (less than 8%), the ferrite Only crystal grains are preferentially detached and martensite grains remain. Accordingly, the martensite grains are convex and the part where the ferrite grains are present is concave, and fine irregularities are formed on the steel material surface.

  Normally, the crankshaft and the bearing operate in a fluid lubrication state. When the surface of the shaft is uneven, the lubricating oil stored in the recess is supplied to the interface. The lubricating oil in the concave portion can compensate for the shortage of lubricating oil at the convex portion at the sliding interface, and as a result, the lubricity is greatly improved.

  As a result, it was possible to suppress the increase in the interface temperature and improve the seizure resistance. In the present invention, the Si content is suppressed in order to lower the interface temperature, and the Mn content is optimized in order to ensure hardenability and ensure the surface layer hardness after induction quenching. We have a crankshaft that shows a good fit. Furthermore, if necessary, the ratio of the ferrite phase in the surface layer portion is adjusted according to the induction hardening conditions, and the surface is given unevenness by the preferential wear, thereby further improving the lubricity.

  Hereinafter, the effect of the component of the crankshaft of this invention and the reason for limitation of content are demonstrated.

C: 0.3-0.6%
C is an element necessary for obtaining martensite, but if its content is less than 0.3%, martensite having sufficient hardness cannot be obtained. On the other hand, if it exceeds 0.6%, the steel is excessively hardened and the machinability is lowered. Therefore, the content of C is set to 0.3 to 0.6%.

Si: 0.15% or less In the present invention, the Si content is specified to be 0.15% or less. The reason for setting it to 0.15% or less is to secure seizure resistance by setting the thermal conductivity of the steel material to 40 W / mK or more. The smaller the Si content, the better the thermal conductivity.

Mn: Over 1.0% to 2.0%
Since Mn has the effect of increasing the hardenability of steel, it is indispensable for the formation of martensite. Furthermore, there exists an effect | action which raises an intensity | strength and toughness. These effects are obtained when the Mn content exceeds 1.0%. On the other hand, excessive addition of Mn has an adverse effect on the formation of ferrite. In particular, when the Mn content exceeds 2.0%, austenite is generated during quenching, which may adversely affect the mechanical strength. Therefore, the Mn content is set to more than 1.0% and up to 2.0%.

P: 0.020% or less P is an impurity. P segregates at the austenite grain boundaries and lowers the fatigue strength by lowering the grain boundary strength. Therefore, it is desirable to reduce as much as possible in steel materials for crankshafts. 0.020% is the allowable upper limit.

S: 0.005-0.30%
S precipitates as sulfides and has the effect of improving the machinability of steel. To obtain this effect, a content of 0.005% or more is required. However, if it exceeds 0.30%, the hot workability of the steel decreases. Therefore, the content of S is set to 0.005 to 0.30%.

Cr: 0.05-0.85%
Cr has the effect of improving the strength and toughness of steel. Furthermore, since it also has the effect of increasing hardenability, it is indispensable for the formation of martensite. These effects are obtained when the Cr content is 0.05% or more.

An appropriate amount of Cr narrows the interval between the Ac ₁ transformation point and the Ac ₃ transformation point. As a result, it is possible to obtain uniform martensite by eliminating precipitation of bainite at the martensite grain boundaries and residual undissolved ferrite. On the other hand, if the Cr content exceeds 0.85%, the formation of a bainite structure is caused, which adversely affects the machinability and may cause a problem in processing as a crankshaft. Therefore, the appropriate Cr content is 0.05 to 0.85%.

Al: 0.050% or less
Al is a deoxidizing element. If the Al content exceeds 0.050%, not only the deoxidation effect is saturated, but also the machinability of the steel may be reduced. Therefore, the Al content is set to 0.050% or less. Since the crankshaft of the present invention contains Mn, which is a deoxidizing element, Al need not necessarily be added. However, when sufficient deoxidation is desired, Al and Mn may be used in combination.

N: 0.020% or less The smaller the N content, the better. When N is contained excessively, toughness is lowered. Therefore, the N content is preferably suppressed to 0.020% or less.

  The thermal conductivity κ is 40 W / mK or more. This is for ensuring good thermal conductivity of the crankshaft and obtaining excellent seizure resistance. In addition, although it is desirable that the thermal conductivity is large, it is difficult to obtain a thermal conductivity of about 68 W / mK or more with an alloy containing iron as a main constituent element.

  Next, the equation (1) will be described.

  It is also one of the features of the present invention that the relationship between the surface hardness Hv of the steel material after induction hardening and the thermal conductivity κ (W / mK) satisfies the following expression (1).

Hv> 2.7 × κ + 420 (1)
In the case of a steel material having a high thermal conductivity, even if the friction coefficient increases due to the generation of wear powder, an increase in temperature at the sliding interface can be avoided and seizure can be prevented. On the other hand, in the case of a steel material having a low thermal conductivity, the temperature of the sliding interface rises and the lubrication is hindered, so it is necessary to avoid the generation of wear powder as much as possible. Therefore, the surface of the crankshaft needs to have a certain hardness or more.

  Considering these circumstances, the relationship between the surface hardness of the crankshaft that can prevent seizure and the thermal conductivity was confirmed by experiments. As a result, it became clear that seizure can be prevented if the surface hardness satisfies the equation (1).

  FIG. 1 is a diagram showing a region where seizure does not occur due to the relationship between thermal conductivity and surface hardness Hv. The thermal conductivity is 40 W / mK or more within the scope of the present invention. The straight line A is 2.7 × κ + 420. As shown, this straight line A indicates that the surface hardness Hv may be low when the thermal conductivity κ of the material steel is small.

  Finally, the reason why it is desirable to make the fraction of ferrite contained in the induction hardened layer 8% or less will be described.

  When the ferrite fraction exceeds 8%, as described above, the amount of detachment (wear) at the time of sliding increases, and it is practically difficult to use it for a crankshaft or the like. On the other hand, if the steel material has the same thermal conductivity, better seizure resistance is exhibited when it contains 1% or more of ferrite.

  The amount of ferrite can be controlled by adjusting the conditions for induction hardening. For example, the ferrite fraction in the martensite can be adjusted by changing the high frequency output and / or frequency during the induction hardening process and heating the steel material to change the surface temperature.

  Steel having the chemical composition shown in Table 1 was melted in a 150 kg vacuum induction heating furnace into an ingot having a diameter of 210 mm.

  Each of the above ingots was heated to 1250 ° C. and then hot forged into a round bar having a diameter of 65 mm. The forging finishing temperature was 1000 ° C., and after hot forging, the forging was allowed to cool to room temperature in the air.

The above round bar with a diameter of 65 mm was turned to produce a round bar with an outer diameter of 42.3 mm, which was subjected to induction hardening. During the induction hardening process, the output and frequency of the high frequency were changed, and the surface temperature of the round bar was heated so as to be above and below the Ac ₃ point, thereby adjusting the amount of undissolved ferrite in the martensite. The conditions for induction hardening in the case of a round bar having an outer diameter of 42.3 mm are as follows.

(1) Conditions for a ferrite ratio of 0% Voltage (Vol): 550
Voltage EP (kV): 8.2
Current IP (A): 11.6
Lattice Ig (A): 1.75
Heating temperature (° C.): 957 (Ac is a temperature exceeding ₃ points)
Refrigerant: water
(2) Conditions for a ferrite ratio of 4.9% Voltage (Vol): 450
Voltage EP (kV): 6.6
Current IP (A): 9.2
Lattice Ig (A): 1.15
Heating temperature (° C.): 806 (It is an intermediate temperature between Ac ₁ point and Ac ₃ point.)
Refrigerant: Water Each round bar having a diameter of 42 mm thus obtained was lapped to finish a round bar having a diameter of 40 mm. As schematically shown in FIG. 2, the lapping process is performed by pressing the round bar 1 that is turned against the lapping grindstone 2 with a constant load and rotating the round bar 1 in one direction that is the same as the turning direction. went. As the grinding wheel, one with F800 count described in JIS R6001 was used.

  While measuring the Hv hardness of the steel material surface after grinding, the ferrite rate was calculated from the microstructure photograph. The thermal conductivity was determined by a laser flash method. Table 1 also shows the test results.

  Moreover, the level difference of the surface unevenness | corrugation by cross-sectional SEM observation was measured. FIG. 3A is a photomicrograph of the surface of a martensite single phase (left figure) and the surface of a two-phase structure composed of martensite and about 40% ferrite (right figure). It is shown that the latter has a step (unevenness) due to the mixture of ferrite and martensite. FIG. 3B is a cross-sectional structure photograph of the surface of the two-phase structure. From this figure, it can be seen that a recess having a depth of about 0.5 μm is formed near the ferrite side of the boundary between ferrite and martensite.

  Since the difference in height varies depending on the cut surface, it is difficult to determine, but the F800 count grindstone uses an average of 11 μm abrasive grains, so it is actually 11 μm or less for any steel material containing ferrite. It is estimated that irregularities having a height difference of about several μm are formed.

  The seizure test was performed under the following conditions using a high surface pressure tester.

Rotation speed: 1300rpm
Surface pressure: 10 MPa / 45 min steps (until seizure occurs)
Oil temperature: 130 ° C
Oil type: 5W-20 (American Petroleum Institute standard)
Bearing: SA162 (Daiho Kogyo Co., Ltd .: Al alloy bearing, Pb-free material)
The seizure test was performed as described above, and as shown by the arrow in FIG. 4, it was determined that seizure occurred at the stage where the frictional force generated between the shaft and the bearing suddenly increased, that is, the stage where the unsteady state was exhibited. The surface pressure was taken as the seizure pressure. Table 1 shows the seizure pressure (P) of each specimen.

  As is clear from Table 1, the thermal conductivity decreases as the Si content of the material steel increases. The seizure pressure varies from 70MPa to 160MPa depending on the steel type, but the seizure pressure of 100MPa or higher was considered acceptable for use in automobile crankshafts.

  In the test materials from No. 1 to No. 4-B in Table 1, the thermal conductivity κ is all 40 W / mK or more and satisfies the above equation (1), so the seizure pressure is high and the seizure resistance is high. Is excellent. On the other hand, No. 5 and No. 6 have poor seizure resistance because the relationship between hardness and thermal conductivity does not satisfy the formula (1). In No. 7 and No. 8, the Si content is too high, so the thermal conductivity is smaller than 40 W / mK and the seizure resistance is inferior.

  FIG. 4 shows the results of a seizure test using samples No. 4-A and No. 4-B in Table 1. As is clear from this figure, even with steel materials having the same composition, the test material (No. 4-B) with a 4.9% ferrite ratio is 0% compared to the test material (No. 4-A) with a ferrite ratio of 0%. As the surface pressure increases, seizure is less likely to occur despite the friction torque once increasing. This is an influence of fine irregularities generated on the surface of the test material.

  The crankshaft of the present invention has a significantly improved seizure resistance by avoiding a temperature rise at the sliding interface by suppressing the Si content of the material steel to a low level and increasing the thermal conductivity. This crankshaft steel has high hardenability by controlling the Mn content. Thereby, the surface hardness after induction hardening can be ensured. In addition, by generating an appropriate amount of ferrite in the hardened hardened layer, providing surface irregularities, making it a lubricating oil reservoir and ensuring lubricity, the friction coefficient is further reduced, and the temperature at the sliding interface is increased. It is possible to avoid it.

  Since the crankshaft of the present invention does not require a high-cost process such as carburizing or shot peening in its manufacture, it can be manufactured at a low cost.

It is a figure which shows the range of the heat conductivity of the steel materials which show favorable seizure resistance, and the surface hardness after induction hardening. It is a figure which shows typically the aspect of grindstone grinding | polishing (lapping process). It is the microscope picture of the surface of the martensite single phase and the surface of the two-phase structure containing ferrite, and the section structure picture showing the crevice of the boundary part of ferrite and martensite. It is a figure which shows the seizing test data and seizure occurrence (arrow).

Explanation of symbols

1: Test material (round bar), 2: Wrapping wheel, 3: Abrasive grain

Claims (2)

In mass%, C: 0.3 to 0.6%, Si: 0.15% or less, Mn: more than 1.0% to 2.0%, P: 0.020% or less, S: 0.005 to 0.30%, Cr: 0.05 to 0.85%, Al: It contains 0.050% or less, N: 0.020% or less, the balance is Fe and impurities, the thermal conductivity is 40 W / mK or more, and the surface hardness Hv after induction hardening is expressed by the following formula (1) Crankshaft characterized by satisfaction.
Hv> 2.7 × κ + 420 (1)
Where κ is the thermal conductivity (W / mK). The crankshaft according to claim 1, wherein the induction-hardened layer has a ferrite fraction of 1 to 8%.

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