JP2000345279A - Spheroidal graphite cast iron article and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a spheroidal graphite cast iron article having a high heardness part excellent in wear resistance and a tough part enough in strength and toughness in an integrated casting and to provide a method for producing it. SOLUTION: This article is composed of spheroidal graphite cast iron having difference in hardness as cast after casting and is provided with a high hardness part 1h having >=255 HB Brinell hardness and a tough part 1a. In a wheel hub 1, a wheel rim 3 of a wheel is fixed to the edge face of a screw part 1a. To the outer diameter part 1h of a spline part 1g at the outside of the spline part, a spline 1e slid and engaged with a brake disk 4 is worked. It is produced by inoculating a molten metal with a graphite nuclei generating element by 0.002 to 0.05 wt.% (expressed in terms of Bi), executing casting, thereafter rapidly cooling the outer diameter part 1h of the spline part and slowly cooling the screw part 1a. By selectively executing the rapid cooling to the outer diameter part 1h of the spline part when cooling after the casting, high hardness can be obtd. even in the case of a structure as cast.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spheroidal graphite cast iron product having a high hardness portion having excellent wear resistance and a tough portion having high strength and toughness in an integrated casting.
In particular, the present invention relates to a spheroidal graphite cast iron product exhibiting sufficient mechanical properties such as hardness and strength without heat treatment after casting and a method for producing the same.

[0002]

2. Description of the Related Art The prior art will be described by taking a wheel hub for a vehicle as an example. The wheel hub is a part for attaching a tire of a car. As a material and manufacturing method of the wheel hub, there is one disclosed in Japanese Patent Application Laid-Open No. Hei 5-287438. According to the method, a cast product is made of spheroidal graphite cast iron having a large amount of ferrite ground, and the outer peripheral portion is subjected to high-frequency induction heating for a short time, followed by forced air cooling. Thus, a spheroidal graphite cast iron wheel hub in which the outer periphery of the wheel hub is hardened to HB285 to 341 to a depth of about 15 mm. Others
FCD450 equivalents are also quenched and tempered as a whole.

[0003]

According to the above method, it is considered that a molded material for a wheel hub having satisfactory performance can be obtained. However, in recent years, at a manufacturing site where further cost reduction is required, there is a demand for manufacturing a wheel hub having desired properties without any heat treatment. Heat treatment itself is costly. Further, in the case of quenching and tempering a large-sized spheroidal graphite cast iron product having a complicated shape, it is necessary to pay attention to the cooling process in order to prevent cracking due to transformation stress. At present, the heat treatment process requires 18 hours or more. In addition, since the parts are deformed during the heat treatment, the machining cost must be set higher in anticipation of the deformation, which increases the cost of the machining.

[0004] On the other hand, in the method in which only the spline portion is subjected to induction heating and then forced cooling, the surface (black scale) becomes too hard, and the tool is easily damaged during cutting. Further, there is a case where the hardness is not reached (defective) due to a variation in hardness. Therefore, if a wheel hub having desired performance can be obtained without heat treatment (with a cast structure), it is extremely advantageous in terms of cost reduction.

The present invention has been made in view of such problems, and a spheroidal graphite cast iron having a high hardness portion having excellent wear resistance and a tough portion having high strength and toughness in an integrated casting. An object of the present invention is to provide a product and a method for manufacturing the same. In particular, it is an object of the present invention to provide a spheroidal graphite cast iron product exhibiting mechanical properties such as sufficient hardness and strength without heat treatment after casting, and a method for producing the same.

[0006]

In order to solve the above-mentioned problems, a spheroidal graphite cast iron product of the present invention is made of spheroidal graphite cast iron having a metal structure (as-cast structure) simply cooled after casting. It is characterized by including a high hardness portion 1h having a hardness of HB 255 or more and a tough portion 1a.

In the method for producing a spheroidal graphite cast iron product of the present invention, the nucleation element of graphite is set to 0.002 to 0.
05% by weight (in terms of Bi)
Is rapidly cooled, and the tough part 1a is gradually cooled.

[0008] By selectively quenching the high hardness portion 1h during cooling after casting, high hardness can be obtained even in an as-cast structure. Therefore, the heat treatment can be omitted, and the cost for the heat treatment step can be reduced. In addition, it is not necessary to provide an extra cutting allowance for absorbing the heat treatment deformation, which leads to a reduction in cutting cost.

[0009]

In the present invention, the spheroidal graphite cast iron has a TC of 3.0 to 4.0 and a Si of 2.0 to 2.0% by weight.
3.0, Mg: 0.03 to 0.06, Mn: 0.2 to
0.8, Cu: 0.5-2.0, Sn: 0.03-0.0.
10 (however, Mn, Cu, and Sn can be replaced with equivalent pearlite-stable elements) and Bi: 0.002-0.
05 (but can be replaced by equivalent graphite nucleation elements). The above TC means total carbon, which is precipitated as graphite, Fe
C contained in the solid solution and C contained in the intermetallic compound.

Mn, Cu, S as pearlite stable elements
Those that can be substituted for n are Cr, Mo, W, V, and the like.
For example, when replacing Mn with Cr, 0.05% by weight
About 0.2 can be contained. When replacing Cu with Mo, it can be contained in an amount of about 0.05 to 0.2 by weight%. Elements that can be substituted for Bi as the graphite nucleation element are Re, Ca, Al, Si and the like. For example, Ca
In the case of substituting with, the content can be about 0.002 to 0.05 by weight%. Alternatively, when substituting with Al, it can be contained in an amount of about 0.03 to 0.1 by weight%.

A more preferred range of the components is as follows (the figures are% by weight). Tc: 3.55-3.65, Si: 2.25-2.4
0, Mn: 0.30 to 0.50, Cu: 0.90 to 1.
10, Sn: 0.035 to 0.05, Mg: 0.04 to
0.05, Bi: about 0.003

In the spheroidal graphite cast iron product of the present invention, it is preferable that the high hardness portion 1h has a metal structure different from a chill structure. Here, the chill structure refers to a structure in which redebrite (a mixed structure of austenite and cementite) is crystallized by quenching the molten metal. If this chill structure appears, it becomes too hard, so that cutting becomes difficult, and the Charpy impact value decreases, which is not preferable.
A preferred specific structure is, for example, a dense pearlite structure.

In the spheroidal graphite cast iron product of the present invention, the number of graphite particles in the high hardness portion 1h is preferably 740 / mm ² or more. If the number of graphite particles is large, carbon dissolved in the matrix is likely to precipitate as graphite, and the compound cementite (Fe ₃ C) is hardly crystallized. As a method of increasing the number of graphite particles, there is a method of adding (inoculating) a small amount of the above-described graphite nucleation element such as Bi into the molten metal at the time of casting.

The tough part 1a of the spheroidal graphite cast iron product of the present invention comprises:
The tensile strength is preferably 450 N / mm ² or more, and the Charpy impact value (without notch) is preferably ² kgf · m / mm ² or more. The hardness of the high hardness portion 1h is Brinell hardness HB28.
It can be 5 or more.

In the method for producing a spheroidal graphite cast iron product of the present invention, the high-hardness portion 1h is heated at a temperature of 20 ° C./sec or more from the time when the temperature of the high-hardness portion reaches a temperature below the liquidus temperature and above the A ₁ transformation point. Cooling at a cooling rate is preferred. for that reason,
By applying a chill to the high-hardness portion 1h and / or removing the mold of the high-hardness portion 1h (frame release), the high-hardness portion 1h can be rapidly cooled.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a wheel hub for a construction machine vehicle will be described below. FIG. 1 is a side cross-sectional view for explaining the shape of the wheel hub and the names of each part. The wheel hub 1 has a hollow cylindrical shape as a whole. The left side of the figure is the outside of the vehicle, and the right side of the figure is the inside of the vehicle. A relatively thick ring-shaped portion at the left end in the drawing is called a thread portion 1a, and a wheel rim 3 of a wheel is fixed to this end surface by a bolt 2.

Inside the screw portion 1a (right side in the figure) is a body portion 1b having a relatively thin and large diameter. The inside of the body 1b is connected to a disk-shaped flange 1c. A small diameter body portion 1d is connected to the inside of the inner diameter portion of the flange portion 1c. A relatively thick ring-shaped spline portion 1g is connected to the inside of the small-diameter body portion 1d. Spline part 1g
The spline 1e with which the brake disk 4 is slidably engaged is machined to the outer diameter of. The inside of the spline portion 1g is referred to as a spline portion inner diameter portion 1f, and the outside thereof is referred to as a spline portion outer diameter portion 1h.

The wheel hub 1 of this embodiment has an outer diameter of 530 mm in a cast state. The wall thickness of each part is 50
mm, large diameter body 1b is 15mm, small diameter body 1d is 40m
m, 1 g of spline portion is 57 mm.

The following table shows the standard values of the mechanical properties of each part of the wheel hub of this embodiment. The spline portion is required to have hardness and strength in terms of wear resistance. Other parts require toughness.

[Table 1]

FIG. 2 is a schematic sectional view showing the structure of a mold for casting the wheel hub of FIG. (A) shows a state before casting, and (B) shows a state in which the lower mold is removed and the spline portion is rapidly cooled after casting. The mold 11 is an upper mold 1
3, a middle mold 21, a core 23, a cooling material 31, and a lower mold 33. Of these, the cooling material 31 is made of a graphite material, and the others are those obtained by solidifying molding sand.

The upper mold 13 is located above the cavity 25 and includes a feeder 15 and a gate 17. The middle mold 21 is located on the outer periphery of the upper / central part of the cavity 25. Core 23
Is located in the cavity 25. The lower mold 33 is located below and on the outer periphery of the spline cavity 25b. A cooling material 31 is arranged on the outer periphery of the spline portion cavity 25b.

In this embodiment, the cooling material 31 is made of graphite. Specifically, homogeneous graphite SMG manufactured by Showa Denko KK
Was used. The cooling material 31 had an inner diameter of 300 mm, a thickness of 30 mm, and a height of 80 mm. This graphitic cooling material has a higher cooling effect than a casting chill, and has the effect of stably increasing the hardness of the spline portion to a depth of about 30 mm.

The molten metal was poured from the gate 17 in the state shown in FIG. The melting raw material of the molten metal is 40% of steel plate scrap,
0% and 30% ductile pig iron were used. The inoculant is 0.3% (primary), 0.1% (secondary) of Fe-75Si in a ladle.
added. Further, 0.1% of Fe-75Si-2Bi was added in order to refine graphite and increase the number of graphite particles at the time of pouring.

The components in the molten metal were measured by a Cantback (emission spectrometer), and as a% by weight, Tc: 3.6% and Si:
2.4, Mn: 0.4, Cu: 1.0, Sn: 0.04
Degree. Mg was added by a sandwich method (place-pour method) so as to be about 0.04 by weight% with respect to the molten metal. The amount of the inoculated Bi was 0.1% by weight based on the molten metal.
003. Since Bi has a low boiling point, it is gasified in the molten metal and a part thereof is lost, but it is considered that the residual amount in the solidified cast iron does not significantly decrease.

The temperature of the molten metal was about 1350 ° C. In addition,
It is estimated that the solidus temperature of this molten metal is about 1150 ° C., and the A ₁ transformation point of the solidified cast iron is about 730 ° C.

At the time of this experiment, a thermometer (platinum rhodium wire thermocouple type, the position of which will be described later) was placed in the spline cavity 25b, and when the temperature reached 850 ° C., FIG. As shown in (B), the lower mold 33 and the cooling material 31 were removed (unframed). Thereby, the spline portion 1g of the cast iron product (wheel hub 1) was opened to the outside air and cooled. At this time, in this embodiment, no air was blown on the spline portion 1g, but a sufficient cooling rate was obtained as described later. In addition, in the case of actual product production, the timing of unraveling is managed by the time after casting (about 17 minutes).

FIG. 3 shows a cooling state of the cast iron spline after casting. (A) is a cooling line for 17 minutes after casting, (B) is an enlarged view of a part of the curve of (A),
FIG. 3C is a cross-sectional view illustrating an arrangement position of the temperature sensor. In this example (experimental example), as shown in FIG. 3C, temperature sensors were set at two positions D and E on the outer periphery of the spline portion 1g. D is 1mm spline 1g from the inner surface of the cooling material 31
And E is the position where it entered 1 g of the 10 mm spline portion.

Since point D is close to the cooling material 31, FIG.
As shown in the figure, cooling is faster than point E. Both point D and point E cool rapidly up to about 900 to 1,000 ° C., but the cooling rate thereafter decreases. As shown in FIG. 3B, the initial cooling rate at the point D is 1,398 ° C. → 1,0 ° C.
22.9 ° C / sec between 55 ° C, 1,398 ° C → 997
C. is 13.4 ° C./sec, 1,398 ° C. → 940 ° C. is 7.6 ° C./sec, and 1,500 ° C. → 910 ° C. is 4.1 ° C./sec.

Since the experiment using the data shown in FIG. 3 was merely for examining the effect of the cooling material after casting, the lower mold was not unframed. In this case, as can be seen from FIG. 3A, the cooling rate at 900 ° C. or lower is slow, and it is difficult to obtain the desired mechanical properties. Therefore, the temperature at point D is actually 850.
2C, the lower die 33 (including the cooling material 31) around the spline portion 1g of the cast iron product is removed, and the spline portion 1g is exposed to the outside air, as shown in FIG. I do. The cooling rate at this time is preferably about 1.5 ° C./sec from 850 ° C. to 600 ° C. from the viewpoint of obtaining sufficient hardness.

FIG. 4 is a graph showing the mechanical properties of each part of the wheel hub cast iron product. (A) shows the hardness of each part of the cast iron product, (B) shows the tensile strength, (C) shows the elongation, and (D) shows the Charpy impact value. In the figure, a range surrounded by a solid line as “850 ° C. air-cooled product” indicates a range of mechanical properties of the above-described lower mold opening. The slowly cooled product does not have the lower mold unframed, but exhibits a range of mechanical properties. The part names of the cast iron products shown on the horizontal axis of the graph are the same as the names of the respective parts in FIG. In addition, the test piece of a mechanical property was extract | collected from the part enclosed with the dashed-dotted line in each part of the cast iron product of FIG.

With respect to the hardness, the air-cooled product at 850 ° C. obtained about 300 HB at the outer diameter and about 290 HB at the inner diameter. Note that the thread is away from the spline, so 8
There is not much change in hardness of the air cooled product at 50 ° C. or the slowly cooled product.

The tensile strength of the 850 ° C. air-cooled product was higher, and was 800 N / mm ² or more at the spline portion. The thread part also had a tensile strength of 600 N / mm ² or more, which was equivalent to FCD600.

The elongation of the 850 ° C. air-cooled product was also higher. The Charpy impact value is also 850 ° C.
The air-cooled product was higher, achieving the target value of ² kgf · m / mm ² . The conditions for the Charpy impact value test were as follows: no notch, 10.780 kg, 0.515 m hammer, and room temperature. The fact that the 850 ° C air-cooled product also improved the elongation and the Charpy impact value was because the cooling rate was high and the number of graphite particles increased,
It is considered that the pearlite was densified as a result of the addition of Mn and Cu.

Next, prevention of generation of a chill structure on the surface of a cast iron product will be described. FIG. 5 shows the cooling rate (horizontal axis) and the number of chill critical graphite particles (vertical axis) of spheroidal graphite cast iron (equivalent to FCD450).
4 is a graph showing the relationship between the presence and absence of cementite. The generation of cementite can be prevented at the upper left side of the critical line shown by the solid line in the figure, and the generation of cementite at the lower right side of the curve. That is, it is considered that the generation of cementite is strongly affected by the number of graphite particles. In the wheel hub cast iron product of the present embodiment, as shown in FIG. 3B, the maximum cooling rate of the surface in contact with the cooling material during solidification is considered to be about 23 ° C./sec. Reading the number of chill critical graphite particles at the intersection S with the line at the speed of 23 ° C./sec, N = 740 /
a mm ^2. That is, by setting the number of graphite particles N to 740 / mm ² or more, generation of a chill structure can be prevented. In this example, in order to increase the number of graphite particles, a small amount of Bi is inoculated as a pouring stream as described above.

FIG. 6 is an optical micrograph showing a metal structure of a cast iron product having a high hardness portion (about 5 mm from the surface). (A) is 100 times that without corrosion, and (B) is 400 times with corrosion.
It is twice. The corrosion was performed with 3% nital. As can be seen from these photographs, the graphite particles have a very high spheroidization rate (about 95%) and a fairly small graphite particle size (about 10 μm on average). The number of graphite particles was 1,140 /
mm ² , sufficiently clearing the above-mentioned 740 critical graphite particles / mm ² , and there was a margin.

The base structure is bainite and a dense pearlite structure as can be seen from FIG. 6 (B). The structure of the cast iron product that was heat treated after casting was fine pearlite +
Ferrite (10%).

While the embodiment of the present invention has been described above, there are other methods such as water cooling, forced air cooling, oil cooling, etc., instead of the frame air cooling.

[0038]

As is apparent from the above description, according to the present invention, a spheroidal graphite cast iron having a high hardness portion having excellent wear resistance and a tough portion having high strength and toughness in an integrated casting. Goods can be obtained. In particular, a spheroidal graphite cast iron product exhibiting mechanical properties such as sufficient hardness and strength can be obtained without heat treatment after casting. Therefore, the heat treatment can be omitted, so that the cost for the heat treatment step can be reduced and energy can be saved. In addition, it is not necessary to provide an extra cutting allowance for absorbing the heat treatment deformation, which leads to a reduction in cutting cost.

[Brief description of the drawings]

FIG. 1 is a side sectional view for explaining the shape and names of parts of a wheel hub of the present invention.

FIG. 2 is a schematic sectional view showing a configuration of a mold for casting the wheel hub of FIG. (A) shows a state before casting, and (B) shows a state in which the lower mold is removed and the spline portion is rapidly cooled after casting.

FIG. 3 shows a cooling state after casting of a spline portion of a cast iron product. (A) is a cooling line for 17 minutes after casting, (B) is an enlarged view of a part of the curve of (A), and (C) is a cross-sectional view showing an arrangement position of a temperature sensor.

FIG. 4 is a graph showing mechanical properties of various parts of a wheel hub cast iron product. (A) shows the hardness of each part of the cast iron product,
(B) shows tensile strength, (C) shows elongation, and (D) shows Charpy impact value.

FIG. 5 is a graph showing the relationship between the cooling rate of spheroidal graphite cast iron (horizontal axis) and the number of chill critical graphite grains (vertical axis), and the presence or absence of generation of cementite.

FIG. 6 is an optical micrograph showing a metal structure of a cast iron product in a high hardness part. (A) is 100 times that of no corrosion and (B)
Is 400 times greater than with corrosion.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Wheel hub 1a Screw part 1b Large-diameter body part 1c Flange part 1d Small-diameter body part 1e Spline 1f Spline inner diameter part 1g Spline part 1h Spline outer diameter part 2 Wheel mounting bolt 3 Wheel rim 4 Brake disk 11 Mold 13 Upper mold 15 Cooling water 17 Gate 21 Middle mold 23 Core 25 Cavity 25a Screw cavity 25b Spline cavity 31 Cooling material 33 Lower mold

Continued on the front page (72) Koji Ito 18-34 Shakacho, Hitachinaka-shi, Ibaraki Prefecture Inside Ito Foundry Ironworks Co., Ltd. (72) Inventor Haruo Tanabe 26 Matsuyama-cho, Moka-shi, Tochigi Pref. (72) Inventor Hikaru Ishizaki 26 Matsuyama-cho, Moka-shi, Tochigi Pref. Inside the Purchasing Department, Komatsu Seisakusho Moka Factory (72) Inventor, Kiyomito Takase 26, Matsuyama-cho, Mooka-shi, Tochigi Pref.

Claims (11)

[Claims] 1. A spheroidal graphite cast iron product comprising a spheroidal graphite cast iron having a part having a hardness difference as cast after casting and having a high hardness part 1h having a Brinell hardness of HB255 or more and a tough part 1a. . 2. The spheroidal graphite cast iron has a TC of 3% by weight.
0 to 4.0, Si: 2.0 to 3.0, Mg: 0.03 to
0.06, Mn: 0.2-0.8, Cu: 0.5-2.
0, Sn: 0.03 to 0.1 (however, Mn, Cu, Sn
And Bi: 0.002 to 0.05 (however, it can be replaced by an equivalent graphite nucleation element).
The described spheroidal graphite cast iron product. 3. The spheroidal graphite cast iron product according to claim 1, wherein the high hardness portion 1h has a metal structure different from a chill structure. 4. The high-hardness portion 1h has a graphite particle count of 740 / mm ² or more.
The described spheroidal graphite cast iron product. 5. The tensile strength of the tough part 1a is 450 N / mm.
² or more, 2kg Charpy impact value (no notch)
The spheroidal graphite cast iron product according to any one of claims 1 to 4, wherein the value is not less than f · m / mm ² . 6. A method for producing a spheroidal graphite cast iron product having a high hardness part 1h and a tough part 1a; inoculating 0.002-0.05% by weight (in terms of Bi) of a graphite nucleating element into a molten metal; A method for producing a spheroidal graphite cast iron product, wherein the high hardness portion 1h is rapidly cooled after casting, and the tough portion 1a is gradually cooled. 7. The high hardness portion 1h has a Brinell hardness HB2.
55 or more, and the tensile strength of the tough part 1a is 450 N / mm ² or more,
The method for producing a spheroidal graphite cast iron product according to claim 6, wherein the Charpy impact value (without notch) is 2 kgf · m / mm ² or more. 8. The spheroidal graphite cast iron has a TC of 3% by weight.
0 to 4.0, Si: 2.0 to 3.0, Mg: 0.03 to
0.06, Mn: 0.2-0.8, Cu: 0.5-2.
0, Sn: 0.03 to 0.10 (however, Mn, Cu, S
n can be replaced by an equivalent graphite nucleating element) and B
8. The method for producing a spheroidal graphite cast iron product according to claim 6, wherein i: 0.002 to 0.05 (however, it can be replaced with an equivalent spheroidizing inhibitory element). 9. The spherical shape according to claim 6, wherein the high hardness portion 1h is cooled from about 1130 ° C. to about 920 ° C. at a cooling rate of 20 ° C./sec or more. Manufacturing method of graphite cast iron products. 10. The graphite nucleating element to be inoculated is B
The method for producing a spheroidal graphite cast iron product according to any one of claims 6 to 9, further comprising i, Ce, and other Re. 11. The high-hardness portion 1h is rapidly cooled by applying a chill to the high-hardness portion 1h and / or removing (unframe) the mold of the high-hardness portion 1h. 11. The method for producing a spheroidal graphite cast iron product according to any one of items 10 to 10.