JP4135665B2 - Crankshaft cast steel - Google Patents

Description

  The present invention relates to a cast steel for a crankshaft, and more particularly to a cast steel for a hollow crankshaft excellent in machinability.

  For example, a crankshaft of a positive displacement engine of an automobile is a member that is pivotally supported by a main bearing in a crankcase and converts a downward movement of a piston in a combustion stroke transmitted by a connecting rod into a rotational movement. Since this crankshaft requires strength and rigidity to rotate at a high speed while receiving a large load, high-carbon steel, chrome molybdenum steel, nickel chrome steel, etc. that have been die-forged have been widely used so far. However, recently, cast irons such as mehanite cast iron and spheroidal graphite cast iron have been used in order to give more rigidity by increasing the speed of the engine.

  In recent years, in order to further reduce fuel consumption of automobiles, it is strongly desired to reduce the weight of crankshafts. For this reason, hollow crankshafts with hollow shafts have been actively developed. .

  If such a hollow crankshaft is made of cast iron, for example, the required rigidity cannot be satisfied. On the other hand, it is difficult to make the shaft portion hollow even when trying to manufacture by forging.

  Patent Document 1 does not apply a hollow crankshaft, but C: 0.5 to 1.5% (in this specification, “%” means “% by mass” unless otherwise specified) ), Si: 1.5 to 3.5%, Mn: 0.7% or less, P: 0.05% or less, S: 0.1% or less, and a crank made of cast steel having the balance Fe. An invention for manufacturing a shaft is described. By using cast steel, it is possible to reduce the weight of cast iron.

JP-A-61-284550

When a hollow crankshaft is manufactured using the cast steel described in Patent Document 1, the shaft portion is hollowed by performing machining such as cutting after casting. However, as described above, the invention disclosed in Patent Document 1 is intended for a solid crankshaft and not a hollow crankshaft. For this reason, this invention does not consider the machinability at all.

  According to the examination results of the present inventors, the cast steel disclosed in Patent Document 1 does not have a high-speed drilling workability equivalent to that of cast iron. Therefore, a hollow crank using the cast steel described in Patent Document 1 is used. A shaft cannot be manufactured.

  The present invention provides a crankshaft cast steel that is suitably used for a hollow crankshaft because it has the same level of strength as forged steel and the same level of machinability as cast iron.

  In the present invention, C: 0.3-0.6%, Si: 0.1-2.0%, Mn: 0.1-2.0%, Cr: 0.05-2.0%, P: 0.03% or less, S: 0.05 to 0.5%, N: 0.003 to 0.02%, Al: 0.001 to 0.2%, balance Fe and inevitable impurities are included. Crankshaft cast steel.

  In the crankshaft cast steel according to the present invention, (i) B: 0.0003 to 0.0030% and / or Ti: 0.003 to 0.2%, (ii) Nb: 0.01 to 0 It is desirable to contain 10% and / or V: 0.01 to 0.20%, or (iii) Ca: 0.0005 to 0.02%.

  In these cast steels for crankshafts according to the present invention, “cast steel” refers to a product that is cast without being forged after melting and further machined as necessary. Fe-C based alloy.

  According to the present invention, there is provided a crankshaft cast steel that is suitably used for a member having a complicated shape, for example, for a hollow crankshaft, because it has the same level of strength as forged steel and the same level of machinability as cast iron. did it.

Hereinafter, the best mode for carrying out the present invention will be described in detail.
First, the reason why the composition of the crankshaft cast steel used in the present embodiment is limited will be described.

C: 0.3% or more and 0.6% or less When the C content is less than 0.3%, the surface hardness after induction hardening is lowered, and a desired strength cannot be ensured. On the other hand, if it exceeds 0.6%, the machinability is lowered although the strength and hardness are increased. Therefore, in the present invention, the C content is limited to 0.3% to 0.6%. From the same viewpoint, the lower limit of the C content is preferably 0.32%, and the upper limit is preferably 0.50%.

Si: 0.1% or more and 2.0% or less By containing 0.1% or more, it acts as a deoxidizer during melting, hardens the soft ferrite phase, and is effective in improving fatigue strength. Also improves. However, if it exceeds 2.0%, the toughness decreases. Therefore, in the present invention, the Si content is limited to 0.1% to 2.0%. From the same viewpoint, the lower limit of the Si content is desirably 0.3%, and the upper limit is desirably 1.5%.

Mn: 0.1% or more and 2.0% or less 0.1% or more increases strength and toughness, improves hardenability, raises the eutectoid concentration of C, and suppresses precipitation of proeutectoid ferrite. However, if the content exceeds 2.0%, the formation of a bainite structure is caused, and the wear resistance and machinability are adversely affected. Particularly, when the Mn content exceeds 2.0%, the wear resistance and machinability are affected. The reduction of the becomes remarkable. Therefore, in the present invention, the Mn content is limited to 0.1% to 2.0%. From the same viewpoint, the lower limit of the Mn content is desirably 0.5%, and the upper limit is desirably 1.6%.

Cr: 0.05% or more and 2.0% or less Cr is contained in an amount of 0.05% or more, thereby reducing the ferrite ratio and narrowing the interval between transformation points of Ac ₁ and Ac ₃ to bainite to the martensite grain boundary. Uniform martensite can be obtained by eliminating the precipitation and remaining of the undissolved ferrite. However, if the Cr content exceeds 2.0%, the formation of bainite is caused, which adversely affects the wear resistance and machinability. In particular, if the content exceeds 2.0%, the wear resistance and coverage are deteriorated. The machinability is significantly reduced. Therefore, in the present invention, the Cr content is limited to 0.05% to 2.0%. From the same viewpoint, the lower limit of the Cr content is desirably 0.05%, and the upper limit is desirably 1.2%.

S: 0.05% or more and 0.5% or less S contains 0.05% or more, so that it precipitates as a sulfide and improves machinability. However, if the S content exceeds 0.5%, the castability deteriorates. Therefore, in the present invention, the S content is limited to 0.05% to 0.5%. From the same viewpoint, the lower limit of the S content is desirably 0.05%, and the upper limit is desirably 0.2%.

N: 0.003% or more and 0.02% or less N is contained in an amount of 0.003% or more, thereby precipitating a carbonitride such as AlN to refine the austenite crystal grains during high-frequency heating. However, if the N content exceeds 0.02%, a large amount of solid solution N is present at the grain boundary, and further BN is formed, resulting in a decrease in solid solution B. Conversely, the grain boundary strength is lowered and delayed fracture occurs. Is likely to occur. Therefore, in the present invention, the N content is limited to 0.003% or more and 0.02% or less. From the same viewpoint, the lower limit of the N content is preferably 0.005%, and the upper limit is preferably 0.01%.

Al: 0.001% or more and 0.2% or less Al is added as a deoxidizing element and a crystal grain refining element by containing 0.001% or more. Moreover, although it couple | bonds with N in steel and turns into AlN, solid solution N is fixed completely and added for BN precipitation prevention. If the Al content is less than 0.001%, such an effect is insufficient. On the other hand, if it exceeds 0.2%, the effect is saturated and the toughness is rather lowered. Therefore, in the present invention, the Al content is limited to 0.001% or more and 0.2% or less. From the same viewpoint, the lower limit of the Al content is desirably 0.002%, and the upper limit is desirably 0.1%.

The optional additive elements will be described below.
B: 0.0003% or more and 0.0030% or less, and / or Ti: 0.003% or more and 0.2% or less B is contained by 0.0003% or more, thereby increasing the strength of the surface by induction hardening or the like. This is effective in improving delayed fracture resistance, which is a concern when B is easily segregated at the austenite grain boundary in a solid solution state, and embrittles elements such as P and Cu are excluded from the grain boundary to increase the grain boundary strength. However, if it exceeds 0.0030%, the grain boundary strength is conversely reduced. Therefore, when B is added, its content is desirably limited to 0.0003% or more and 0.0030% or less.

  On the other hand, Ti has an effect of improving fatigue strength in steel. B binds to N to precipitate carbonitride, completely fixes the solid solution N, and is effective in preventing BN precipitation. From such a viewpoint, Ti is preferably added in combination with B, and the content at that time is preferably 0.003% to 0.2%.

Nb: 0.01% or more and 0.10% or less, and / or V: 0.01-0.20%
Nb and V both contribute to improvement of fatigue strength in steel and contribute to improvement of base material strength. Therefore, when Nb is added, its content is preferably limited to 0.01% or more and 0.10% or less, and when V is added, its content is 0.01% or more and 0.20%. It is desirable to limit to the following.

Ca: 0.0005% or more and 0.02% or less When the Ca content is 0.0005% or more, the machinability is improved. However, when the Ca content exceeds 0.02%, such an effect is saturated and only the cost increases. Therefore, when Ca is added, its content is desirably limited to 0.0005% or more and 0.02% or less.

  The balance other than the above is Fe and inevitable impurities. As an inevitable impurity, P: 0.03% or less can be illustrated. If the P content exceeds 0.03%, grain boundary segregation occurs at the austenite grain boundaries, the grain boundary strength is lowered, and delayed fracture tends to occur. Therefore, the P content is desirably 0.03% or less.

  The crankshaft cast steel according to the present embodiment having the above composition not only can cope with a complex shape similar to cast iron, but also has higher fatigue strength or Young's modulus than cast iron.

  Further, regarding machinability, since it is not necessary to suppress MnS, which has been a cause of cracking in hot forging, machinability can be ensured by adding a large amount of S. Further, in the case of forged steel, MnS is elongated and tends to be a starting point for occurrence of grinding cracks, but in the cast product, grinding cracks are hardly generated.

  For this reason, since the cast steel for crankshaft of this embodiment has the same level of strength as forged steel and has the same level of machinability as cast iron, for example, for a member having a complicated shape such as for a hollow crankshaft. It can be used suitably.

  For this reason, even if the crankshaft made of cast steel for crankshaft of the present embodiment is hollow, it can maintain high strength and can exhibit excellent machinability during machining for hollowing. . For this reason, the crankshaft cast steel of the present embodiment can greatly contribute to reducing the weight and fuel consumption of the engine.

Furthermore, the present invention will be described more specifically with reference to examples.
A steel material having the chemical composition shown in Table 1 was melted in a 150 kg vacuum induction heating furnace, cast into a 210 mm diameter ingot in the case of forged steel, and cast into an ingot with a diameter of 65 mm in the case of cast steel and cast iron.

  About forged steel, after heating said ingot to 1250 degreeC, it hot-forged and forged into the 65-mm round bar. The forging finishing temperature was 1075 ° C., and after hot forging, the forging was allowed to cool to room temperature. Moreover, cast iron and cast steel were normalized at 850 ° C. for 1 hour, allowed to cool, and used as test steels.

  Then, fatigue strength, machinability test (carbide drill, high-speed drill), and grinding test were performed by the following test methods. The results are summarized in Table 1.

[Fatigue strength]
The Ono type rotating bending fatigue test (smooth) was performed to measure the fatigue strength. More than 300 MPa was accepted.

[Machinability test (Carbide drill)]
A 6 mm diameter carbide drill with a cutting speed of 100 m / min, a feed rate of 0.15 mm / rev, and a depth of 100 mm, and stably drilling to a depth of 200 mm using a synthetic ester lubricant (5 cc / Hr). It was determined whether or not it was possible.

[Machinability test (high-speed drill)]
After drilling to a depth of 100 mm using a high-speed drill with a diameter of 6 mm at a cutting speed of 18.5 m / min, a feed rate of 0.15 mm / rev and a depth of 50 mm, using a lubricant (20 L / min) made of water-soluble coolant. Tool wear was measured. And the amount of wear was less than 30 μm as acceptable.

[Grinding test]
The above-mentioned 65 mm diameter round bar is turned by a normal method to produce a ring with an outer diameter of 60 mm, an inner diameter of 24 mm and a height of 15 mm, and induction hardening so that the depth of Hv ≧ 450 is 3 mm. Grinding was performed under the following conditions to check for the presence of grinding cracks.

(Grinding conditions)
Whetstone (80A 80N 8V 201, dimensions: outer diameter 405mm, width 25mm, hole diameter 152.4mm)
Wet plunge grinding method, grinding wheel peripheral speed 60m / s (workpiece 0.2m / s), feed 2.0mm / s, cutting speed 0.5mm / min, margin diameter 0.2mm / 1 cut

  No. in Table 1 A to J are examples of the present invention manufactured by cast steel satisfying the composition specified in the present invention, all of which have the targets of fatigue strength of more than 300 MPa, no grinding crack, less than 30 μm of high-speed wear, and carbide life of 200 mm. All are satisfied, and it can be seen that even when hollowed, high strength can be maintained and excellent machinability is exhibited during machining for hollowing.

In contrast, No. 1 in Table 1 was obtained. Since No. 1 was cast iron, fatigue strength was insufficient.
No. No. 2 is forged steel, and since the S content is below the lower limit of the present invention, the carbide life, the amount of high-speed wear, and the grinding cracks all became unsatisfactory.

  No. 3 is forged steel, and since the C content exceeds the upper limit of the range of the present invention and the S content is lower than the lower limit of the present invention, the carbide life, the high-speed wear amount and the grinding crack are all. The value was unsatisfactory.

No. Since No. 4 is forged steel, grinding cracks were unsatisfactory.
No. Since the S content was below the lower limit of the range of the present invention, both the cemented carbide life and the high speed wear amount were unsatisfactory.

No. No. 6, since the C content exceeded the upper limit of the range of the present invention, the cemented carbide life, the high-speed wear amount, and the grinding crack were all unsatisfactory.
No. In No. 7, the fatigue strength decreased because the C content was below the lower limit.

No. In No. 8, since the N content exceeded the upper limit of the range of the present invention, the amount of high-speed wear and grinding cracks were unsatisfactory.
Furthermore, no. No. 9 caused grinding cracks because the P content exceeded the upper limit of the range of the present invention.

Claims (4)

  In mass%, C: 0.3-0.6%, Si: 0.1-2.0%, Mn: 0.1-2.0%, Cr: 0.05-2.0%, P: 0.03% or less, S: 0.05 to 0.5%, N: 0.003 to 0.02%, Al: 0.001 to 0.2%, balance Fe and inevitable impurities are included. Cast steel for crankshaft.   Furthermore, the cast steel for crankshafts described in Claim 1 which contains B: 0.0003-0.0030% and / or Ti: 0.003-0.2% by mass%.   Furthermore, the cast steel for crankshafts of Claim 1 or Claim 2 which contains Nb: 0.01-0.10% and / or V: 0.01-0.20% by the mass%.   Furthermore, the cast steel for crankshafts described in any one of Claim 1- Claim 3 which contains Ca: 0.0005-0.02% by mass%.