JPH07113125B2 - Manufacturing method of high-toughness spheroidal graphite cast iron casting - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a high-toughness spheroidal graphite cast iron casting having high deformability.

[Problems to be Solved by Prior Art and Invention]

Since its invention, spheroidal graphite cast iron has undergone improvements in material quality and production technology.According to JIS G5502, it is now common to have mechanical properties such as tensile strength of 80 kgf / mm ² in terms of strength and elongation of 17% in terms of toughness. It is well known that it has been widely manufactured and has been widely adopted for important safety parts which are suspension device parts for automobiles, such as steering knuckles, transverse links, torque rods, trailing arms and the like.

By the way, there are tension, compression, bending, twisting, and the like as the types of loads applied to such parts, and the most frequent one of them is the bending load, which has a great influence on the stress level. In the case of breakage due to bending, as the theory of material mechanics shows, the maximum stress (or maximum strain) occurs on the surface, so the strength of the part depends on the characteristics of the surface, such as mechanical properties, hardness, residual It is known to be strongly affected by stress and surface roughness.

Therefore, it is well known that, especially for fatigue fracture due to bending load, a work-hardened layer is generated in the surface layer by shot peening, cold rolling, etc., and residual stress of compression is generated to positively improve the strength. Is a fact of.

On the other hand, for so-called toughness such as static bending or impact bending, that is, for fracture in which the elongation property is the dominant factor among mechanical properties, almost all the consideration is given to the above-mentioned characteristics of the surface layer and its influence. The reality is not. But,
Many of the parts are required to have toughness against such a load, that is, cracks must not be generated or broken up to a predetermined bending amount, and improvement thereof is desired.

For cast parts, not only the processed surface but also the strength and toughness of the surface to be cast are required, but since the surface properties of the processed surface are stable, it is easy to obtain the desired characteristics. The surface properties of the portions to be subjected to the dispersion vary greatly, and it is difficult to obtain desired characteristics. Therefore, normally, due to various restrictions in the manufacturing method, no consideration is given to the strength as an inevitable thing.

In general, cast iron castings are generally cleaned by blasting with an iron-based shot material (hereinafter referred to as steel shot blasting) in order to remove sand and scale attached after casting and / or heat treatment. . The steel shot blasting may have a clear intention of work hardening of the surface layer portion by shot like the above-mentioned shot peening treatment, but in most cases, it is usually performed simply for the purpose of cleaning. Considering the merits and demerits of this steel shot blasting,
Work hardening occurs by shots regardless of the intention of work hardening of the surface layer, which increases fatigue strength as described above, which is convenient for parts where fatigue strength is the highest priority. Is.

However, on the other hand, the toughness of the work-hardened layer, that is, the amount of deformation until crack initiation or fracture is lower than that of the inside, so for parts that require toughness against static bending and shock bending, it is initially set. Since the toughness of the above material is much lower than expected, it causes a big problem.

This phenomenon does not cause much problem when the hardness of the base material itself is high because the hardening rate of the surface layer due to steel shot blasting is small, but in the case of a material with a low base material hardness, for example, JIS FCD40 or less Indicates that the surface layer has a large curing rate, and its effect is remarkable.

In this way, although steel shot blasting, which is usually used to clean the casting surface, has both merits and demerits, only the good aspect of improving fatigue strength is emphasized in the past, while the toughness is reduced. At present, this cleaning method is used without paying attention to the negative aspects.

Especially for parts that require toughness against static bending and shock bending rather than bending fatigue limit of the casting surface, sufficient toughness cannot be ensured as-cast, and heat treatment to decompose primary cementite after casting and / Or a spheroidal graphite cast iron casting having a toughness improved by heat treatment for decomposing pearlite tends to be used. In this case, since the steel shot blasting is usually performed to remove the adhered scale after the heat treatment, the toughness improved by the bending heat treatment causes a problem that the surface layer portion is significantly hardened as a result of work hardening. .

An object of the present invention is to provide a method for producing a high-toughness spheroidal graphite cast iron cast product which does not have a work-hardened layer on the surface layer of the spheroidal graphite cast iron cast product and has excellent deformability.

[Means for solving problems]

The method for producing a high-toughness spheroidal graphite cast iron cast product of the present invention is, after performing a heat treatment for decomposing primary cementite and / or a heat treatment for decomposing pearlite, performing blasting cleaning with an iron-based blast material, and then 550 to 670 ° C. It is characterized in that the work-hardened layer by blasting is decomposed and removed by holding the temperature at 1 to 3 hours.

Hereinafter, embodiments of the present invention will be described in more detail with reference to the drawings.

[Example 1] A static bending test was performed using a test piece in order to clarify the influence of work hardening of the surface layer on the bending toughness. The test conditions are as follows.

(1) Test piece shape Rectangle length of 350 mm with cross section 31 mm × 31 mm 350 mm (2) Material of test piece JIS G5502 FCD37 equivalent HB 143 to 163 Chemical composition is shown in Table 1.

(3) Manufacturing method of test piece (A) After casting, perform steel shot blasting cleaning, and then raise the temperature to 850 ° C in 5 hours, and then perform 3 at this temperature.
After holding for 2 hours, cool down to 660 ℃ in 2 hours and let cool.
The scale that has adhered again is removed by steel shot blasting.

(This step is a normal manufacturing step of heat-treated casting) (B) 630 ° C in 3 hours after carrying out the step (A)
The work-hardened layer is decomposed and removed by steel shot blasting by raising the temperature to 2, holding at this temperature for 2 hours, and then allowing to cool. (This is a process of the present invention.) (4) Test method As shown in Fig. 1, the test piece 1 was freely supported with a span of 300 mm, a load P was applied to the center, and the amount of deformation until crack initiation was determined.

The test results are shown in Table 2. A typical example of the load-deformation amount curve is shown in FIG. The hardness distribution from the surface layer of both test pieces is shown in FIG.

As is clear from Table 2, the deformation amount of the test piece of (B) is about twice the deformation amount of the test piece of (A). The cause of such a large difference in deformation amount is clear from FIG.

That is, in the test piece of (A), a work-hardened layer of 0.3 to 0.4 mm is formed in the surface layer portion, so the toughness of this portion is lowered, whereas in the test piece of (B), work hardening is caused by heat treatment. This is because the layer has been decomposed and removed, and the surface layer portion also has toughness close to that of the base material. Therefore, in order to improve the bending toughness of the casting surface, it is found that it is extremely important not to leave the work-hardened layer on the surface layer portion.

[Example 2] Next, in order to clarify the influence of the heat treatment temperature for decomposing and removing the work-hardened layer, a static bending test was conducted using the same test piece and the same test method as in Example 1.

(1) Material of test piece Equivalent to FCD37 as in Example 1. HB143 to 163 (2) Manufacturing method of test piece After performing step (A) of Example. 1, holding at each heat treatment temperature for 2 hours. Then, the work-hardened layer was removed.

(A) 700 ℃ (b) 670 ℃ (c) 650 ℃ (d) 600 ℃ (e) 550 ℃ (f) 500 ℃ Table 3 shows the test results.

It can be seen from Table 3 that the recovery of the deformation amount is small at 500 ° C and the release method for the work hardened layer is insufficient, but the recovery of the deformation amount is sufficient at 550 ° C or higher.

On the other hand, the thickness of the oxide scale layer associated with the heat treatment increases as the temperature increases. Here, one point of the present invention is to set a heat treatment temperature at which the oxide scale layer can be suppressed within a range that does not hinder the coating. In other words, if the oxide scale becomes thick and it causes trouble in coating, it is necessary to perform some blast cleaning again, and this causes a work-hardened layer to occur again.

As a result of conducting a coating experiment with this test product, it could be confirmed that at a temperature up to 670 ° C, there is no problem even if coating is performed with the oxide scale layer attached. Therefore, the heat treatment temperature at which a sufficient amount of deformation can be recovered and the oxide scale can be suppressed within a range that does not hinder coating is 550 to 670 ° C.

Example 3 An example of improving the bending toughness of the torque rod, which is one of the suspension parts for the truck shown in FIG. 4, is shown. This part connects the axles of the rear two axes, and is mainly subjected to tensile and compression in the front-back direction, but is also subjected to bending due to vertical force and lateral force. In particular, there is a demand that cracks should not occur up to a predetermined bending amount when it is mounted on a stone or the like on the road, and it is a component that requires high bending toughness.

The flexural toughness was evaluated by measuring the amount of displacement at the load point until a crack was generated by applying a load to the center with both ends freely supported as shown in FIG. The material is JIS.FCD37 equivalent and the manufacturing method is as follows.

(A) After casting, the casting was held at a temperature of 850 ° C. for 3 hours, allowed to cool, and then scaled off by steel shot blasting, and the hardness was HB146 to 152.

(B) After the above step (A), the work hardened layer was decomposed and removed by holding it at a temperature of 630 ° C. for 2 hours and has a hardness of HB143.
It was ~ 149.

The test results are shown in Table 4. From this result, the deformation amount of the manufacturing method (B) is about 2.5 times the deformation amount of the manufacturing method (A).

[Embodiment 4] An example of improving the bending toughness of the passenger car control arm (also known as transverse link, lower arm, etc.) shown in FIG. 6 is shown.

This part is one of the main suspension parts for front-wheel drive passenger cars, and is an important safety part that receives longitudinal force and lateral force from tires.
The required strength is, of course, fatigue strength, but in the unlikely event of an accident, cracks must never occur up to a predetermined bending amount, which is also a part requiring high bending toughness.

The bending toughness was evaluated by fixing two points as shown in FIG. 7, applying a load assuming the longitudinal force from the tire, and measuring the amount of deformation until crack initiation. The material is JIS.FCD37 equivalent and the manufacturing method is as follows.

(A) After casting, the product was held at a temperature of 850 ° C. for 3 hours, allowed to cool, and scaled off by steel shot blasting, and the hardness was HB142 to 146.

(B) After the step (A), the work-hardened layer was decomposed and removed by holding it at a temperature of 650 ° C. for 2 hours and has a hardness of HB142 to 149.
Met.

The test results are shown in Table 5. From this result, the deformation amount of the production method (B) was about 1.8 times the deformation amount of the production method (A).

[Advantages of the Invention] As is clear from the above description, conventionally, only the good aspect of improving fatigue strength has been emphasized, that is, the negative aspect of steel shot blasting cleaning, that is, a significant decrease in bending toughness against static or impact load. It was clarified that the fact is clear, and a method for producing a high-toughness spheroidal graphite cast iron casting was developed by a heat treatment that eliminates the work-hardened layer in a temperature range where there is little oxide scale adhesion.

And the high toughness spheroidal graphite cast iron casting of the present invention is a suspension component for an automobile, for example, a torque rod, a control arm, a steering knuckle, a rear axle housing, a suspension arm as shown in FIGS. 4, 6, 8, 9, 10, and 11.
For important safety parts such as links, which require high bending toughness, they have industrially significant hardening such as improved safety and longer service life.

[Brief description of drawings]

Figures 1,5,7 are front views showing the test method, Figure 2 is the load-deformation curve, Figure 3 is the hardness distribution chart, and Figures 4,6,8,9,10,11 are suspensions for automobiles. It is a bird's-eye view of a device component.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shigeki Yokoyama 5200 Mikkajiri, Kumagaya-shi, Saitama, Institute of Magnetic Materials, Hitachi Metals Co., Ltd. (56) References JP-A-60-106944 (JP, A)

Claims (1)

[Claims] 1. In producing a high-toughness spheroidal graphite cast iron casting, after heat treatment for decomposing primary cementite and / or heat treatment for decomposing pearlite, blasting cleaning is carried out with an iron-based blast material, followed by 550- A method for producing a high-toughness spheroidal graphite cast iron casting, characterized by decomposing and removing a work-hardened layer by blasting for 1 to 3 hours at a temperature of 670 ° C.