JPH06200322A - Production of spheroidal graphite cast iron member - Google Patents

Abstract

PURPOSE:To remove work hardening on the surface part in accordance with the removal of an oxidized film formed in the case of heat treatment without causing an oxidized film and large thermal strain deformation at the time of improving the toughness of spheroidal graphite cast iron stock by relatively simple heat treatment and producing a high strength and low-cost spheroidal graphite cast iron member for automotive under carriage parts, and therefore to satisfactorily express the toughness of the spheroidal graphite cast iron member. CONSTITUTION:By heat treatment in a primary stage, a mixed structure of ferrite and pearlite is formed on the inside of spheroidal graphite cast iron stock as cast, and a ferrite decarburized layer with a prescribed depth is formed on the surface part. An oxidized film formed on the surface in accordance with the same heat treatment is removed away by shot blasting in a secondary stage. After that, work hardening on the surface part generated by the shot blasting is removed away by annealing at a low temp. in a third stage.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing 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 automobiles, cast iron castings, steel forgings and the like are generally used as underbody parts such as steering knuckles and spindle supports. However, the former has a problem in strength, and the latter has a problem in that the cost is high although the strength is high. Therefore, there is a great need for members for underbody parts that have relatively high strength and can be obtained at low cost.

On the other hand, among the cast irons, attention is paid to the toughened spheroidal graphite cast irons to improve the characteristics thereof.
For example, the one disclosed in JP-A-60-187621 is known. This is austenite treatment after austenite treatment of spheroidal graphite cast iron material, which makes a part or all of the retained austenite structure a martensite structure, thereby improving the toughness and wear resistance as well as machinability. It is designed to be improved. However, the disadvantage is that it involves a high temperature liquid treatment such as austempering treatment, so that the cost is still high. Also, after machining finish, heat treatment at high temperature (3
50 to 600 ° C.), large thermal strain deformation is likely to occur.

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

[0005]

By the way, when an electric heat treatment furnace or a gas heat treatment furnace is used to heat treat the surface of a spheroidal graphite cast iron material to form a ferrite layer on the surface portion, an oxide film is simultaneously formed on the surface. Occurs. Generally, shot blasting is performed to remove such an oxide film.

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

[0007] The removal of the work hardening itself is easily performed by annealing, but if the annealing temperature is high, an oxide film is again generated due to the high temperature and a large thermal strain deformation occurs. If this is disliked and annealed at a low temperature, the oxide film and large thermal strain deformation can be avoided, but this time work hardening cannot be sufficiently removed.

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

[0009]

In order to achieve the above object, according to the invention of claim 1, the oxide film is removed by shot blasting, and then the work hardening of the ferrite layer is annealed at a low temperature. I tried to remove it.

Specifically, in the present invention, first, a ferrite layer having a predetermined depth is formed by heat treatment on the surface portion of a spheroidal graphite cast iron material having a mixed structure of ferrite and pearlite. Next, shot blasting is performed on the surface. After that, the surface portion is annealed at a low temperature.

According to a second aspect of the present invention, in the first aspect of the present invention, the mixed structure is formed by heat treatment in a coexistence temperature range of ferrite and austenite, and the heat treatment simultaneously forms a ferrite layer having a predetermined depth. Form. As a heating means used for this heat treatment, a general heat treatment furnace, a salt furnace capable of rapid heating, high-frequency heating or the like can be mentioned, but heat treatment in a two-phase temperature range (coexistence temperature range of ferrite and austenite) is possible. Anything will do.

[0012] In the invention of claim 3, the above-mentioned claim 1 or 2
In the invention, the ferrite layer is formed deeper than the depth of the surface portion that undergoes work hardening by the shot blasting treatment. Generally, the depth dimension of work hardening by shot blasting is about 0.1 mm in the depth direction from the surface, and in that case, to remove work hardening at low temperature, the ferrite layer should be removed. It must be formed to a depth of 0.1 mm or more.

[0013]

With the above structure, in the invention of claim 1, when the surface portion 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 portion, the surface is At the same time, an oxide film is formed, but this oxide film is removed by shot blasting. However, the surface portion, that is, the ferrite layer is work-hardened by the shot blast treatment. Then, by annealing the surface portion, the work hardening of the ferrite layer is removed and the ferrite layer is softened again. At this time, since the above-mentioned 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 occurs due to this annealing.

In the invention of claim 2, since the formation of the mixed structure and the formation of the ferrite layer are simultaneously performed in one heat treatment, the spheroidal graphite cast iron member of the present invention can be obtained from an as-cast spheroidal graphite cast iron material. It is possible to reduce the number of steps up to and further reduce the cost.

According to the third aspect of the present invention, the ferrite layer is formed deeper than the depth of the surface portion that undergoes work hardening by the shot blasting treatment, so that work hardening in the pearlite material of the two-phase structure is prevented. Therefore, it is possible to avoid the risk of insufficient removal of work hardening due to low temperature annealing.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. In this example, as the spheroidal graphite cast iron material, 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 is as follows: C component 3.67%, Si component 2.34%, Mn
Component is 0.31%, P component is 0.048%, S component is 0.009%, Mg component is 0.037%, and the balance is Fe.
Is. Then, as shown in FIG. 1, the heat treatment of the previous step,
A spheroidal graphite cast iron member is manufactured according to the first step heat treatment, the second step shot blast 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., soaked at that temperature for 2 hours, and then continuously heated for 3 hours to 7 ° C.
Soaking is maintained at 30 ° C. and furnace cooling is performed.

The first step is to simultaneously perform the generation of ferrite and pearlite and the formation of the ferrite layer on the surface portion in one heat treatment, and in this step, as shown in FIG.
As shown in, heating to 770 to 850 ° C., which is a two-phase temperature range (coexistence temperature range of ferrite and austenite),
After soaking at the heated temperature for 1 hour, the mixture is air-cooled.

The second step is to remove the oxide film formed on the surface by the heat treatment of the first step, and in this step, the surface is subjected to shot blasting.

The third step is for removing the work hardening received on the surface portion by the shot blasting treatment of the second step, and in this step, as shown in FIG.
The mixture is heated to 0 ° C., soaked at the heated temperature for 1 hour, and then air-cooled.

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

According to this embodiment, the matrix structure of the spheroidal graphite cast iron material is made into all-ferrite by the previous step.
Also, since high concentration Si around graphite is dispersed, S
i is uniformized, and the chill portion generated during casting is decomposed.

By the first step, a mixed structure of ferrite and pearlite is generated. Further, upon receiving this heat treatment, an oxidation reaction proceeds from the surface, whereby the surface portion is decarburized and a ferrite layer is formed on the surface portion. 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 required to obtain the spheroidal graphite cast iron member of the present invention from the as-cast spheroidal graphite cast iron material is reduced. Therefore, the cost can be further reduced.

Further, an oxide film is simultaneously formed on the surface due to the heat treatment in the first step, but this oxide film is removed by the shot blasting process in the second step. However, the surface portion, that is, the ferrite layer is work-hardened by the shot blast treatment.

Then, 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., it is possible to avoid a situation in which an oxide film is again formed on the surface or a large thermal strain deformation occurs due to the annealing.

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 in the pearlite base of the phase structure is prevented, it is possible to avoid the risk of insufficient work hardening removal due to low temperature annealing in the third step. Generally, the depth dimension of work hardening by shot blasting is about 0.1 mm in the depth direction from the surface, and in that case, to remove work hardening at low temperature, the ferrite layer should be removed. It must be formed to a depth of 0.1 mm or more.

Therefore, the spheroidal graphite cast iron member produced in this manner has a mixed structure of soft ferrite and relatively hard pearlite inside, and a ferrite layer is formed on the surface thereof to have high strength, Moreover, since it does not involve the high temperature liquid treatment unlike the conventional example, it can be obtained at a relatively low cost.

Finally, with respect to an example in which a spheroidal graphite cast iron member is specifically manufactured by the manufacturing method of this embodiment, the spheroidal graphite cast iron material used in this embodiment is directly subjected to the surface without undergoing the previous step and the first step. It will be described in comparison with a comparative example in which the second step of shot blasting is performed.

First, with respect to the example of the present invention after the second step and the comparative example, the Vickers hardness (HV) at each depth from the surface was examined with the test load F being F = 200 gf. The result is shown in FIG. 7. From this result, it can be seen that the surface portions of both the inventive example and the comparative example are hard. The value of the Vickers hardness on the surface portion of the present invention example is smaller than that of the comparative example because the ferrite layer is formed on the surface portion of the present invention example.

Next, the low temperature annealing of the third step was added to the above-mentioned present invention example and comparative example to obtain the respective Vickers hardness (HV).
Was examined under the same test load F as above. The result is shown in FIG. From this result, work hardening is removed in the example of the present invention,
It can be seen that the surface is softer than the inside. On the other hand, the processing hardness is 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 inventive example and the comparative example after the third step, a bar-shaped member having a diameter of 30 mm and a length of 400 mm was prepared for each, and the span of each bar-shaped member was prepared. A load was applied to the central portion in a state of being horizontally mounted on a 300 mm two-point support base, and a test for bending breaking load and bending amount was performed. The result is shown in FIG. From these results, the bending amount is about 30 mm in the comparative example for approximately the same bending breaking load, but the bending amount is about 55 mm in the example of the present invention, which is 1.8 times that of the comparative example. The toughness of the spheroidal graphite cast iron member is markedly improved.

In the above embodiment, the ferrite layer is formed by decarburization, but it may be formed by heating only the surface portion above the austenitizing temperature and then gradually cooling. In that case, for example, one side of the spheroidal graphite cast iron material is controlled to a temperature above the austenitizing temperature by using a high-frequency coil, while the other side is gradually cooled by blowing a cooling gas to provide a temperature gradient. Then, if the positions of heating and slow cooling are sequentially changed over the entire surface, the ferrite layer can be formed on the surface portion with an arbitrary depth relatively easily.

[0033]

As described above, according to the invention of claim 1, after forming the ferrite layer on the surface portion of the spheroidal graphite cast iron material having a mixed structure of ferrite and pearlite, a by-product at the time of forming the ferrite layer. The work hardening of the surface part that occurs with the removal of the oxide film can be removed without causing the oxide film and large thermal strain deformation by annealing at a low temperature using the ferrite layer. It is possible to actually manufacture a spheroidal graphite cast iron member having excellent toughness by heat treatment at a relatively low cost that does not involve high temperature liquid treatment as in the conventional example. As a result, for example, in the field of automobiles, a member for underbody parts having high strength and low cost can be obtained.

According to the second aspect of the present invention, since the formation of the mixed structure and the formation of the ferrite layer are performed by one heat treatment, the spheroidal graphite cast iron member of the present invention can be obtained from an as-cast spheroidal graphite cast iron. It is possible to reduce the number of steps to
Further cost reduction can be achieved.

According to the third aspect of the present invention, since the ferrite layer is formed deeper than the depth of the surface portion subjected to work hardening by the shot blast treatment, the pearlite ground of the internal structure is work hardened by the shot blast treatment. Since it can be prevented, the 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 expressed.

[Brief description of drawings]

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

FIG. 2 is a heat treatment diagram of a previous 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 micrograph showing a metal structure in a surface portion of a spheroidal graphite cast iron member at 100 times magnification.

FIG. 6 shows the metallographic structure inside the spheroidal graphite cast iron member.
It is a microscope picture shown by enlarging it to 00 times.

FIG. 7 is a diagram showing the Vickers hardness of the surface portion after the shot blasting of the inventive example and the comparative example.

FIG. 8 is a diagram showing the Vickers hardness of the surface portion of the present invention example and the comparative example after annealing at low temperature.

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

[Table 1]

Front page continued (72) Inventor Masahiko Shibahara 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda Motor Corporation

Claims (3)

[Claims] 1. A ferrite layer having a predetermined depth is formed on a surface portion of a spheroidal graphite cast iron material having a mixed structure of ferrite and pearlite by heat treatment, and then the material surface is subjected to shot blasting, and then the surface is formed. A method for manufacturing a spheroidal graphite cast iron member, characterized in that the part is annealed at a low temperature. 2. The method for manufacturing a spheroidal graphite cast iron member according to claim 1, wherein the mixed structure is generated by heat treatment in a coexisting temperature range of ferrite and austenite, and the heat treatment simultaneously produces a predetermined depth. A method for manufacturing a spheroidal graphite cast iron member, which comprises forming a ferrite layer. 3. The spheroidal graphite cast iron member manufacturing method according to claim 1, wherein the ferrite layer is formed deeper than the depth of the surface portion subjected to work hardening by shot blasting. Method for manufacturing cast iron member.