JP2001347357A - Spheroidal graphite cast iron member and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a spheroidal graphite cast iron member formed into a double structure of that essentially consisting of ferrite low in notch sensitivity and large in the number of graphite grains on the surface layer and that high in strength on the inside and to provide its production method. SOLUTION: This method for producing a spheroidal graphite cast iron member includes a step in which at least one or more kinds of pearlite transforming elements selected from the groups consisting of, by weight, 0.1 to 1.0% Cu, 0.1 to 0.5% Mn and 0.02 to 0.1% Cr and 2.0 to 2.8% silicon are added to molten metal and a step in which the molten metal is cooled to the eutectic point of the molten metal in such a manner that the rate in the surface part 13 is controlled to 140 to 400 deg.C/min which is also higher than that in the inside 17 of the molten metal by >=10% and a step in which the molten metal is cooled to the eutectic point of the molten metal at 15 to 50 deg.C/min and also in such a manner that the cooling rate in the surface part 13 is made higher than that in the inside 17.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high toughness spheroidal graphite cast iron member which is applied to, for example, a vehicle suspension strength component such as a steering knuckle and a lower arm, and a method of manufacturing the same.

[0002]

2. Description of the Related Art Normally, high load and impact are applied to undercarriage strength parts of automobiles, so that high static strength, fatigue strength and impact strength are required. In addition to recent demands for improved fuel efficiency, reduction in weight is required, and high flexibility in shape is required in order to cope with complicated shapes. In order to improve the above strength, the inside of the part has a pearlite structure,
There is a method of controlling the structure of a ferrite structure having a prolonged portion near the surface. Conventionally, in order to control the structure, means by heat treatment, addition of Cr, and rapid cooling of a mold have been used.

The above heat treatment is disclosed in Japanese Patent Application Laid-Open Nos. Hei 6-200322, Hei 6-17186 and Hei 9-2.
As described in Japanese Patent No. 96215, various heat treatments are performed after casting a suspension component. However, this method requires a heat treatment and a step of removing scale accompanying the heat treatment, so that the cost is increased. Also,
The means for adding Cr is described in JP-A-5-125480, and the means for rapidly cooling the mold is described in JP-A-60-95.
As described in Japanese Patent Publication No. 72-72, all means have a high Si content of 2.7% by weight or more in order to prevent chilling. When the Si content is high, the ferrite becomes brittle ferrosilicon and the impact strength is inferior. However, in order to improve the impact strength, there is a problem that the method using all ferrite reduces the static strength.

[0004]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems and forms a double-structured structure having a structure mainly composed of ferrite having a low notch sensitivity and having a large number of graphite grains and having a high strength inside in a surface layer. It is an object of the present invention to provide a spheroidal graphite cast iron member and a method for manufacturing the same.

[0005]

According to the present invention, there is provided a method for manufacturing a spheroidal graphite cast iron member, the method comprising:
~ 1.0 wt% Cu, 0.1-0.5 wt% Mn,
At least one or more pearlitic elements selected from the group consisting of 0.02 to 0.1% by weight of Cr;
Adding 8% by weight of silicon to the molten metal, and cooling the molten metal to a eutectic point of the molten metal at a rate of 140 to 400 ° C./min and at least 10% faster than the inside of the molten metal. Cooling the molten metal to the eutectoid point of the molten metal at 15 to 50 ° C./min. According to the above manufacturing method, the number of graphite particles in the surface layer is larger than that in the inside, and the amount of ferrite and the amount of pearlite in the surface layer and the inside are optimized, and high static strength, fatigue strength, and impact strength are obtained. The high toughness spheroidal graphite cast iron can be obtained as-cast without special heat treatment.

In one aspect of the manufacturing method according to the present invention, the cooling rate of the surface portion and the inside is adjusted by adjusting a cooling metal, a hot water pool, or an uneven shape formed on the surface of the member. Performed by at least one or more means. According to the above method, the cooling rates of the surface layer and the inside can be controlled within an appropriate range. Further, in the spheroidal graphite cast iron member according to the present invention, the graphite is formed on the surface portion by 150% / mm ² or more by 20% or more than the inside by the above method. In the spheroidal graphite cast iron member, the toughness of the surface layer is improved due to the increase in the number of graphite particles, and the tendency of the surface layer to become ferritic becomes stronger with respect to the inside.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a spheroidal graphite cast iron member according to the present invention and a method for manufacturing the same will be described in detail with reference to the drawings. The present invention provides a high toughness spherical structure having a double layer structure in which the surface layer is mainly composed of notch-sensitive ferrite having a low susceptibility and a large number of graphite grains, and the inside has a high strength and a mixed structure of pearlite and ferrite. This is a method for producing graphite cast iron as-cast. In the above surface layer, the number of graphite particles is at least 150 / mm ² or more and 20%
The ferrite area ratio is 60% or more and 10% or more higher than the inside, and the thickness of the layer is 0.1% or more.
5 mm or more.

A method for manufacturing a spheroidal graphite cast iron member having the above structure will be briefly described. First, the amount of Si is set to 2.0 to 2.8.
% By weight, and Cu, M
At least one of n and Cr is added in an appropriate amount.
In this state, the cooling rate to the eutectic point of the surface layer of the molten metal is set to 1
At a rate of 40 ° C./min or more, it is 10% or more faster than the inside.
In addition, the cooling rate to the eutectoid point is controlled to 50 ° C./min or less for the surface layer and 15 ° C./min or more for the inside to control the amount of ferrite and pearlite in the surface layer and the inside to a predetermined range. .

[Amount of Si added] By appropriately controlling the amount of Si added, the amount of pearlite can be changed according to the number of graphite particles, the cooling rate, and the pearlitized element. Here, if the amount of Si added is lower than 2.0% by weight, the number of graphite particles decreases, and it becomes difficult to control the structure by the number of graphite particles. On the other hand, if the added amount of Si is higher than 2.8% by weight, it becomes difficult to form pearlite, and the ferrite becomes brittle ferrosilicon to lower the toughness. Therefore, the added amount of Si is set to 2.0 to 2.8% by weight. In order to further improve the toughness of ferrite,
2.0-2.5% by weight is preferred.

[Addition amount of pearlite element] Cu, Mn, and Cr are used as pearlite elements, and Cu: 0.1 to 1.0.
% By weight, Mn: 0.1 to 0.5% by weight, Cr: 0.02
At least one is added in the range of 0.1% by weight. Along with this, the cooling rate of the member is changed according to the product thickness and the casting method, and the addition amount is controlled according to the change in the cooling rate so that the amount of pearlite in the surface layer and the inside becomes appropriate. Here, if each component of the pearlitized element is below the lower limit, a sufficiently pearlitized structure cannot be obtained.
Conversely, when Mn exceeds 0.5% by weight and Cr exceeds 0.1% by weight, the tendency of chilling becomes strong. Further, if Cu is added in an amount of more than 1.0% by weight, the spheroidization of graphite is impaired, so it is necessary to make the amount of Cu not more than 1.0% by weight. In order to prevent an increase in the ductile brittle transition temperature, the addition amount of Cu is 0.1 to 0.5% by weight, and the addition amount of Mn is 0.1 to 0.5%.
More preferably, the amount of addition of Cr is set to 0.02 to 0.05% by weight.

[Cooling Rate (Up to Eutectic Point)] The cooling rate of a product during casting will be described with reference to FIG. In FIG. 1, a solid line is a cooling curve of the surface layer of the member, and a broken line is an internal cooling curve. Cooling rate to eutectic point in surface layer 1
Is set to 140 ° C./min or more, and 10% or more faster than the internal cooling rate 3. Thereby, the number of graphite particles in the surface layer can be increased by 20% or more than that in the inside. Due to the large number of graphite particles, the crystallization amount of graphite increases and the tendency of the surface layer to become ferrite becomes strong.Moreover, the large number of graphite particles itself has an effect of preventing propagation of cracks,
Improve toughness. Here, if the cooling rate 1 to the eutectic point in the surface layer is not 10% or more higher than the internal cooling rate 3, the number of graphite particles in the surface layer will not be sufficiently larger than that in the interior, and a double structure will be obtained. I can't. Further, when the cooling rate 1 of the surface layer is lower than 140 ° C./min, a sufficient number of graphite particles (1
50 / mm ² or more) cannot be obtained.

[Cooling rate (from eutectic point to eutectoid point)] On the other hand, the cooling rate from the eutectic point to the eutectoid point in the surface layer and inside is 15 to 50 ° C./min. This is within a range in which the amount of pearlite can be controlled by controlling the amount of the pearlite element added. Here, when the cooling rate 5 of the surface layer up to the eutectoid point exceeds 50 ° C./min, the surface layer is excessively added (40% in area ratio) without adding a pearlitizing element.
The above pearlite precipitates. Furthermore, if the internal cooling rate 7 is lower than 15 ° C./min, sufficient (10% or more) pearlite cannot be obtained inside even if the pearlitizing element is added. The cooling rate 5 of the surface layer up to the eutectoid point may be 50 ° C./min or less, but it is not substantially lower than the internal cooling rate 7 and at least 15 ° C./min.
Minutes or more. Similarly, the internal cooling rate up to the eutectoid point 7
Is at most 50 ° C./min.

[Method of Controlling Cooling Rate] As a method of controlling the cooling rate, if the speed is to be increased, a chiller is installed, and if the speed is to be reduced, warm water is turned near the product to gradually cool the product. There is a way. In addition, the cooling rate in the surface layer is higher than that of the inside as it is because heat is taken by the mold. However, when the temperature difference is insufficient, as shown in FIG. Height h is 1mm
The method of providing the above-described linearly formed protrusions 9 and FIG.
As shown in the figure, a concave portion 11 formed in a lattice shape having a width w2 of 10 mm or less and a depth d of 1 mm or more is effective. The surface area of the product is increased by the unevenness formed by the projections, and the cooling rate of the surface layer can be increased with respect to the inside.

[Structure of Spheroidal Graphite Cast Iron] By the treatment method described above, as shown in FIG. 4, the surface layer 13 is mainly composed of ferrite and the number of graphite 15 grains is large, and the inside 17 is a mixture of ferrite and pearlite. The toughened spheroidal graphite cast iron member 18 having a structure can be obtained by as-casting. Here, the ferrite area ratio of the surface layer is 60% or more, the area ratio of the inside 17 is at least 10% or more, and the number of graphite particles of the surface layer 13 is 150 / mm ² or more, and 20% or more of the inside 17. high. The hardness of the surface layer 13 is 91H.
RB or less, and the thickness is at least 0.5 mm or more.

The number of graphite particles in the surface layer 13 is 150 / mm ²
Below this, the effect of preventing crack propagation cannot be expected, and the toughness decreases. Further, when the number of particles of the graphite 15 is 20% or less with respect to the inside 17, the notch sensitivity is lowered without ferrite of the surface layer 13. When the ferrite area ratio of the surface layer 13 is less than 60% and the hardness is 91 HRB or more, the notch sensitivity of the surface layer 13 increases, and sufficient toughness cannot be obtained. The same applies when the thickness of the surface layer 13 is less than 0.5 mm. The pearlite area ratio of the inside 17 is at least 10%, at least 10% higher than the surface layer 13, and the hardness is at least 85 HRB. If the pearlite area ratio is less than 10% and the hardness is less than 85 HRB, sufficient internal strength cannot be obtained.

[0016]

Next, a description will be given of an embodiment in which the method for manufacturing a spheroidal graphite cast iron member according to the present invention is applied to a steering knuckle which is a vehicle suspension part. In this example, an example of the present invention in which the components and the cooling rate of the material were controlled, Comparative Example 1 manufactured by the conventional method, and Comparative Example 2 in which only the components were changed from the conventional method are shown.

First, Table 1 shows the component values of the materials used in the present invention and Comparative Example 1. Steering knuckle 33
In the strut arm 35 and the tie rod arm 37 (see FIG. 5), which require particularly high toughness,
Tables 2 to 4 show the measurement results of the cooling rate of the surface layer 13 and the inside 17, the number of grains of the graphite 15, the area ratio of ferrite, and the hardness. Among them, Table 2 shows the cooling rate and the structure of the present invention,
Table 3 shows the cooling rate and structure of Comparative Example 1, Table 4 shows the cooling rate and structure of Comparative Example 2, and Table 5 shows the results of each strength test.

[0018]

[Table 1]

In the example of the present invention, as shown in Table 1, 0.14 wt% of Cu was added as a pearlitizing element.
A cooling basin 39 shown in FIG. 6 was arranged on the thin tie rod arm 37 to adjust the cooling rate. In addition, 41 of FIG. 6 is a gate. On the other hand, in Comparative Example 1, FCD4 in Table 1 was used.
The spheroidal graphite cast iron consisting of 50 pieces was produced by a conventional casting method. In Comparative Example 2, Cu was added as the pearlitizing element in the same manner as in the present invention in an amount of 0.14 wt%, and was produced by a conventional casting method.

The results of these structures and hardness are shown in Tables 2 to 4.
7 to 10. 7 to 10.
2 is a structural photograph of the example of the present invention in Table 2. 7 shows the inside of the strut arm, FIG. 8 shows the vicinity of the surface of the strut arm, FIG. 9 shows the inside of the tie rod arm, and FIG. 10 shows the vicinity of the surface of the tie rod arm.

[0021]

[Table 2]

[0022]

[Table 3]

[0023]

[Table 4]

The embodiment of the present invention has a surface layer 13 having a thickness of 0.5 mm or more.
At 0 / mm ² or more, the content was 20% or more higher than that of the inside 17 and the structure was 60% or more in ferrite area ratio and 10% or more higher than the inside 17. The hardness of the inside 17 is 85
The surface hardness was not less than HRB and not more than 91 HRB.
Comparative Example 1 was made of an FCD450 material containing almost no pearlitizing element.
Is not sufficient and the hardness is low.
Further, in Comparative Example 2, as shown in Table 4, the cooling rate to the eutectoid point of the surface layer 13 was too high in the thin tie rod arm 37, the ferrite area ratio of the surface layer 13 was low, and the hardness was low. it was high.

Next, Table 5 shows the strength test results of the present invention and Comparative Examples 1 and 2.

[0026]

[Table 5]

As shown in Table 5, in the examples of the present invention, high tensile strength and bending strength as well as high bending impact strength were obtained.

The present invention is not limited to the above-described embodiments of the present invention, and various modifications and changes can be made based on the technical idea of the present invention. For example, as a part to which the present invention is applied, in addition to the steering knuckle 33, a rear knuckle, a lower arm, a trailing arm, and other suspension parts, a differential case, a differential mount bracket, and other complicated shapes requiring high toughness are required. It can also be suitably used for parts.

[0029]

According to the present invention, since the number of graphite particles in the surface layer is larger than that in the inside, the toughness of the surface layer is improved, and the ferrite structure is mainly in the surface layer with respect to the inside.

[Brief description of the drawings]

FIG. 1 is a graph showing a cooling curve of a spheroidal graphite cast iron member according to the present invention.

2A is a plan view of a spheroidal graphite cast iron member according to the present invention, and FIG. 2B is a cross-sectional view taken along line AA in FIG.

3A is a plan view of another spheroidal graphite cast iron member according to the present invention, and FIG. 3B is a cross-sectional view taken along line BB of FIG. 3A.

FIG. 4 is a schematic view showing the structure of a spheroidal graphite cast iron member according to the present invention.

FIG. 5 is a plan view of a steering knuckle to which the manufacturing method according to the present invention is applied.

6 is a plan view showing a state in which a water pool is provided on the steering knuckle of FIG. 5;

FIG. 7 is a photograph (magnification: 100 times) showing a structure inside a strut arm portion in the inventive example of Table 2;

FIG. 8 is a photograph (magnification: 100 times) showing a structure near the surface of a strut arm portion in the example of the present invention in Table 2.

FIG. 9 is a photograph (magnification: 100 times) showing a structure inside a tie rod arm portion in the example of the present invention in Table 2.

FIG. 10 is a photograph (magnification: 100 times) showing a structure near a surface of a tie rod arm portion in the example of the present invention in Table 2.

[Explanation of symbols]

 1 Cooling rate to eutectic point (surface layer) 3 Cooling rate to eutectic point (internal) 5 Cooling rate from eutectic point to eutectoid point (surface layer) 7 Cooling from eutectic point to eutectoid point Speed (internal) 9 convex portion 11 concave portion 13 surface layer 15 graphite 17 inside 18 spheroidal graphite cast iron member 33 steering knuckle 35 strut arm portion 37 tie rod arm portion 39 basin 41 gate

Claims (3)

[Claims] 1. The method according to claim 1, wherein the Cu is 0.1 to 1.0% by weight.
0.5% by weight of Mn, at least one or more perlitizing elements selected from the group of 0.02 to 0.1% by weight of Cr, and 2.0 to 2.8% by weight of silicon are added to the molten metal. Step and the surface of the molten metal is 140-400
Cooling the molten metal to the eutectic point of the molten metal at a rate of at least 10 ° C./min and at least 10% faster than the inside of the molten metal;
Cooling the molten metal to the eutectoid point of the molten metal at a rate of ° C./minute to the eutectoid point of the molten metal. 2. Adjustment of the cooling rate of the surface portion and the inside,
2. The method for producing a spheroidal graphite cast iron member according to claim 1, wherein the method is performed by at least one of a cooling metal, a hot water pool, and an uneven shape formed on the surface of the member. 3. The method according to claim 1, wherein the surface portion is formed with graphite of 150 to 600 particles / mm ² , which is 20% or more larger than that of the inside, and the number of particles in the inside is set to 70 to 300 / mm ^2. Characterized spheroidal graphite cast iron members.