JP3301801B2 - Method of manufacturing spheroidal graphite cast iron member - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a spheroidal graphite cast iron member, and more particularly to a measure for improving toughness at low cost.

[0002]

2. Description of the Related Art For example, in an automobile, cast iron castings and steel forgings are generally used as undercarriage parts such as steering knuckles and spindle supports. However, the former has a drawback in strength, and the latter has a drawback in that although the strength is high, the cost is high. Therefore, there is a great need for members for underbody parts which are relatively high in strength and can be obtained at low cost.

On the other hand, among the cast irons, attention has been paid to tough spheroidal graphite cast iron, and its characteristics have been improved.
For example, one disclosed in JP-A-60-187621 is known. This is an austenite treatment of a spheroidal graphite cast iron material, followed by austempering, to make a part or all of the retained austenite structure a martensite structure, thereby improving the toughness and wear resistance and machinability. Improvements have been made. However, the disadvantage is that high temperature liquid processing such as austempering is involved, so that the cost is also high. After the machining finish, heat treatment at high temperature (3.
(50 to 600 [deg.] C.), large thermal strain deformation is likely to occur.

Therefore, the present inventors focused on a spheroidal graphite cast iron material having a mixed structure of soft ferrite and relatively hard pearlite, and formed a high-strength, low-cost spheroidal graphite by forming a ferrite layer on the surface thereof. The method of obtaining graphite cast iron members was studied. This utilizes the phenomenon that cracks are less likely to occur when the surface is soft, and therefore, it is difficult to break when bent.

[0005]

When a surface of a spheroidal graphite cast iron material is subjected to a heat treatment using an electric heat treatment furnace or a gas heat treatment furnace to form a ferrite layer on the surface, an oxide film is simultaneously formed on the surface. Occurs. Generally, shot blasting is performed to remove such an oxide film.

However, when the above shot blasting is performed, the surface portion is work-hardened, which causes a problem that toughness is impaired.

[0007] The removal of the work hardening itself is easily performed by annealing. However, if the annealing temperature is high, an oxide film is formed again due to the high temperature, and large thermal strain deformation occurs. If the temperature is lowered to avoid this, although the oxide film and the large thermal deformation can be avoided, work hardening cannot be sufficiently removed.

[0008] The present invention has been made in view of the above points, and an object of the present invention is to make it possible to remove work hardening caused by removal of an oxide film without causing an oxide film or a large thermal distortion deformation. Accordingly, it is an object of the present invention to provide a high-strength and low-cost spheroidal graphite cast iron member.

[0009]

Means for Solving the Problems To achieve the above object,
According to the first aspect of the present invention, first, the mixed structure is formed by heat treatment in a temperature range where ferrite and austenite coexist, and the heat treatment forms a ferrite layer having a predetermined depth at the same time. Examples of the heating means used for this heat treatment include a general heat treatment furnace, a salt furnace capable of rapid heating, and high-frequency heating, but heat treatment in a two-phase temperature range (a coexisting temperature range of ferrite and austenite) is possible. Anything should do.

Next, a shot blast treatment is performed on the surface. Thereafter, the above-mentioned surface portion is annealed at a low temperature.
That is, the oxide film was removed by shot blasting.
After that, work hardening in ferrite layer is annealed at low temperature
To remove it.

According to a second aspect of the present invention, in the first aspect of the present invention, the ferrite layer is formed to be deeper than a depth of a surface portion subjected to work hardening by shot blasting.
In general, the depth dimension of work hardening by shot blasting is about 0.1 mm from the surface in the depth direction. In this case, in order to remove work hardening at a low temperature, the ferrite layer must be removed. It must be formed to a depth of 0.1 mm or more.

[0012]

According to the above construction, in the first aspect of the present invention, the surface of the spheroidal graphite cast iron material having a mixed structure of ferrite and pearlite is heat-treated to form a ferrite layer on the surface. At this time, since the formation of the mixed structure and the formation of the ferrite layer are simultaneously performed by one heat treatment, the number of steps from the as-cast spheroidal graphite cast iron material to the spheroidal graphite cast iron member of the present invention is reduced. And cost reduction can be achieved.

When the ferrite layer is formed, an oxide film is simultaneously formed on the surface, and the oxide film is removed by a shot blast process. However, the surface portion, that is, the ferrite layer is work-hardened by the shot blasting. Then, when the surface portion is annealed, the ferrite layer is softened again after the work hardening is removed. At this time, since the annealing is performed at a low temperature, it is possible to avoid a situation in which an oxide film or a large thermal strain deformation is caused by the annealing.

According to the second aspect of the present invention, since the ferrite layer is formed deeper than the depth of the surface portion subjected to the work hardening by the shot blasting, the work hardening in the pearlite ground in the two-phase structure is prevented. Therefore, it is possible to avoid a possibility that the work hardening is insufficiently removed due to the low temperature annealing.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. In this embodiment, the spheroidal graphite cast iron material is F
An as-cast product of CD 45 (JIS G 5502) is used. As shown in Table 1, the chemical composition of this as-cast product was 3.67% for the C component, 2.34% for the Si component, and Mn.
The component is 0.31%, the P component is 0.048%, the S component is 0.009%, the Mg component is 0.037%, and the balance is Fe.
It is. Then, as shown in FIG.
A spheroidal graphite cast iron member is manufactured according to the first step heat treatment, the second step shot blasting treatment, and the third step heat treatment.

First, as a pre-process, as shown in FIG.
The as-cast product was heated to 920 ° C., kept at that temperature for 2 hours, and then kept at 7 ° C. for 3 hours.
The temperature is maintained at 30 ° C. and the furnace is cooled.

In the first step, the formation of ferrite and pearlite and the formation of the ferrite layer on the surface portion are simultaneously performed by one heat treatment.
As shown in the above, heating to 770-850 ° C. which is a two-phase temperature range (coexistence temperature range of ferrite and austenite),
After maintaining the temperature at the heated temperature for 1 hour, the air is cooled.

The second step is for removing an oxide film formed on the surface by the heat treatment in the first step. In this step, the surface is subjected to shot blasting.

The third step is for removing the work hardening applied to the surface by the shot blasting in the second step. In this step, as shown in FIG.
After heating to 0 ° C. and maintaining the temperature at the heated temperature for 1 hour, air-cool.

The microstructures of the spheroidal graphite cast iron members obtained by the above steps are shown in the micrographs of FIGS.
In addition, this spheroidal graphite cast iron member is 820 ° C. in the first step.
In the third step, heat treatment was performed at 300 ° C., respectively. As shown in FIG. 5, a ferrite layer (white band extending in the left-right direction in FIG. 5) is provided on the surface of the spheroidal graphite cast iron member, and ferrite (white in FIG. 6) is provided in the inside thereof. It can be seen that a mixed structure of (a part) and pearlite (a relatively black part in the figure) is formed. In FIGS. 5 and 6, a substantially circular black portion is graphite. Also, no oxide film was found on the surface of the spheroidal graphite cast iron member.

According to this embodiment, the base structure of the spheroidal graphite cast iron material is all-ferritized in the previous step.
Also, a high concentration of Si around the graphite is dispersed and S
i is made uniform, and the chill portion generated during casting is also disassembled.

In the first step, a mixed structure of ferrite and pearlite is formed. In addition, an oxidation reaction proceeds from the surface in response to this heat treatment, whereby the surface is decarburized and a ferrite layer is formed on the surface. In this step, since the formation of the mixed structure and the formation of the ferrite layer are simultaneously performed in one heat treatment, the number of steps from the as-cast spheroidal graphite cast iron material to the spheroidal graphite cast iron member of the present invention is reduced. And further cost reduction can be achieved.

In addition, an oxide film is simultaneously formed on the surface due to the heat treatment in the first step, and the oxide film is removed by the shot blasting in the second step. However, the surface portion, that is, the ferrite layer is work-hardened by the shot blasting.

Therefore, in the third step, the surface portion is annealed, so that the work hardening of the ferrite layer is removed and the ferrite layer is softened again. At this time, since the annealing is performed at a low temperature of 200 to 300 ° C., a situation in which an oxide film is formed again on the surface or a large thermal distortion is generated due to the annealing is avoided.

Further, in the first step, the ferrite layer is formed deeper than the depth of the surface portion which is subjected to work hardening in the shot blasting process of the second step.
Since work hardening at the pearlite ground in the phase structure is prevented, it is possible to avoid the possibility that work hardening is insufficiently removed by low-temperature annealing in the third step. In general, the depth dimension of work hardening by shot blasting is about 0.1 mm from the surface in the depth direction. In this case, in order to remove work hardening at a low temperature, the ferrite layer must be removed. It must be formed to a depth of 0.1 mm or more.

Accordingly, the spheroidal graphite cast iron member thus manufactured has a mixed structure of soft ferrite and relatively hard pearlite inside, and has a high strength with a ferrite layer formed on the surface thereof, In addition, since it does not involve high-temperature liquid treatment as in the conventional example, it can be obtained at a relatively low cost.

Lastly, regarding an example in which a spheroidal graphite cast iron member was specifically manufactured by the manufacturing method of the present embodiment, the surface of the spheroidal graphite cast iron material used in the present embodiment was directly applied without passing through the previous step and the first step. This will be described in comparison with a comparative example in which the shot blast processing in the second step is performed.

First, the Vickers hardness (HV) at each depth from the surface of each of the inventive examples after the second step and the comparative examples were examined with a test load F of 200 gf. FIG. 7 shows the result. From this result, it can be seen that both the present invention example and the comparative example have hardened surface portions. The reason why the value of the Vickers hardness at the surface of the present invention is smaller than that of the comparative example is that the ferrite layer is formed on the surface in the present invention.

Next, the low-temperature annealing in the third step was added to the above-mentioned inventive examples and comparative examples, and the respective Vickers hardnesses (HV) were obtained.
Was tested at the same test load F as above. FIG. 8 shows the result. From this result, work hardening is removed in the present invention example,
It can be seen that the surface is softer than the inside. In contrast, work hardening was not removed in the comparative example,
It can be seen that the surface is still harder than the inside.

Further, in order to evaluate the toughness of each of the present invention example and the comparative example after the third step, a rod member having a diameter of 30 mm and a length of 400 mm was prepared, and each rod member was spanned. A load was applied to the central portion in a state of being horizontally laid on a two-point support base of 300 mm, and a test was performed on the bending rupture load and the amount of bending. The result is shown in FIG. From this result, the bending amount is about 30 mm in the comparative example for substantially the same bending rupture load, but the bending amount in the present invention example is about 55 mm, which is 1.8 times larger than that of the comparative example. The spheroidal graphite cast iron member has a remarkable improvement in toughness.

In the above embodiment, the ferrite layer is formed by decarburization. However, the ferrite layer may be formed by heating only the surface portion to a temperature higher than the austenitizing temperature and gradually cooling. In this case, for example, a state in which one side of the spheroidal graphite cast iron material is controlled to a temperature equal to or higher than the austenitizing temperature using a high-frequency coil, while the other side is gradually cooled by spraying a cooling gas to provide a temperature gradient. If the heating and cooling positions are sequentially changed over the entire surface, the ferrite layer can be formed relatively easily on the surface at an arbitrary depth.

[0032]

As described above, according to the first aspect of the present invention, after a ferrite layer is formed on the surface of a spheroidal graphite cast iron material having a mixed structure of ferrite and pearlite, a by-product during the formation of the ferrite layer is formed. The work hardening of the surface caused by the removal of the oxide film can be removed without causing an oxide film or large thermal deformation by annealing at a low temperature using the ferrite layer. A spheroidal graphite cast iron member excellent in toughness can be actually manufactured by a relatively low-cost heat treatment that does not involve a high-temperature liquid treatment as in the conventional example. In addition, the generation of the mixed tissue and
Since the formation of the ferrite layer is performed by one heat treatment,
The spheroidal graphite cast iron member of the present invention is formed from as-cast graphite cast iron.
The number of steps until it is obtained can be reduced,
Cost reduction can be achieved. These results, for example in the field of automobile, it is possible to obtain a low-cost underbody component members with high strength.

According to the second aspect of the present invention, since the ferrite layer is formed deeper than the depth of the surface portion subjected to the work hardening by the shot blasting process, the pearlite of the internal structure is work hardened by the shot blasting process. Therefore, work hardening of the surface portion can be sufficiently removed, and the toughness of the spheroidal graphite cast iron member of the present invention can be sufficiently exhibited.

[Brief description of the drawings]

FIG. 1 is a process chart showing a method for manufacturing a spheroidal graphite cast iron member according to an embodiment of the present invention.

FIG. 2 is a heat treatment diagram of a pre-process.

FIG. 3 is a heat treatment diagram of a first step.

FIG. 4 is a heat treatment diagram of a third step.

FIG. 5 is a photomicrograph showing the metal structure on the surface of the spheroidal graphite cast iron member at a magnification of 100 times.

FIG. 6 shows the metal structure inside the spheroidal graphite cast iron member as 4
It is a microscope picture which expands and shows 00 times.

FIG. 7 is a view showing Vickers hardness of a surface portion after a shot blasting process of an example of the present invention and a comparative example.

FIG. 8 is a view showing Vickers hardness of a surface portion after annealing at a low temperature in the inventive examples and comparative examples.

FIG. 9 is a diagram showing the relationship between the bending rupture load and the amount of bending in the present invention example and the comparative example.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Masahiko Shibahara 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture Inside Mazda Co., Ltd. (56) References JP-A-52-62116 (JP, A) JP-A-2 -270933 (JP, A) JP-A-62-280344 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 5/00 C21D 8/00

Claims (2)

(57) [Claims] 1. A spheroidal graphite cast iron material having a temperature of 770 to 850 ° C., which is a temperature range in which ferrite and austenite coexist.
Subjected to heat treatment for heating every time, off the spherical graphite cast iron material
At the same time as producing a mixed structure of ferrite and perlite
After forming a ferrite layer of a predetermined depth on the surface of the spheroidal graphite cast iron material , then performing a shot blast treatment on the surface of the material , annealing the surface at a low temperature of 200 to 300 ° C. A method for producing a spheroidal graphite cast iron member. 2. A process according to claim 1 Symbol placement of spheroidal graphite cast iron member, spheroidal graphite cast iron in ferrite layer, characterized in that deeper than the depth of the surface portion for receiving a work hardening by shotblasting Manufacturing method of the member.