JPH0613727B2 - Manufacturing method of spheroidal graphite cast iron casting - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a spheroidal graphite cast iron cast product, and more specifically, for a spheroidal graphite cast iron cast product that undergoes austempering, is machined before austempering treatment. The present invention relates to a method for manufacturing a spheroidal graphite cast iron casting when performing.

[Prior Art] In a spheroidal graphite cast iron casting, the casting is subjected to an austenitizing treatment and then to a constant temperature transformation treatment, that is, a so-called austempering treatment is performed, and the base has a mixed structure of bainite and retained austenite, It is well known in the art that a spheroidal graphite cast iron casting having higher strength and higher toughness can be obtained as compared with a ferrite type or ferrite-pearlite type.

However, in this case, agglomerated retained austenite is precipitated by the austempering treatment, and the retained austenite undergoes work-induced transformation during processing to become martensite, which causes a problem that machinability is significantly deteriorated.

In order to avoid this problem of machinability, machining is performed before austempering, but even in this case the material hardness in the as-cast state is high, so it also serves as a primary heat treatment and strain relief. Softening and annealing, followed by machining.

By the way, when producing a spheroidal graphite cast iron casting, in general,
Inoculation is carried out for the purpose of preventing chilling and promoting graphitization. Conventionally, this inoculation is generally performed by the ladle inoculation method in which the inoculant is added in the ladle before pouring the molten metal from the ladle into the mold. Since it takes a long time to solidify, the inoculation effect is reduced, and there is a risk that chills, which have a significant adverse effect on machinability and toughness, may precipitate. Further, since the number of graphite particles is generally reduced when the chill is precipitated, segregation of alloy elements,
Precipitation of carbides having high hardness occurs, and machinability is also hindered in this respect.

[Problems to be Solved by the Invention] As a measure for deterioration of machinability due to this chill precipitation, after casting,
In the primary heat treatment that is performed before machining, the casting is heated and held at a predetermined temperature of the A ₁ transformation point or higher for a predetermined time prior to the annealing treatment to perform chill decomposition, and then is heated at a predetermined temperature of the A ₁ transformation point or lower for a predetermined time. It has been proposed to carry out so-called two-step annealing, in which the annealing is performed while holding. However, this method has problems that the heat treatment cost is high and the time is long.

By the way, an inoculant is added at the time of casting, for example, by a pouring-flow inoculation method in which a molten metal is poured into a mold, or a chamber is provided at an intermediate part of a runner in the mold and the inoculant is filled in the chamber. Compared with the conventional ladle inoculation method, if it is done by a method such as the in-mold inoculation method that is added by
Since the time from inoculation to solidification is shortened, the inoculation effect is improved, the number of graphite particles is increased, and chill precipitation can be expected to be suppressed.

Then, by increasing the number of graphite particles in this way, chill precipitation is reduced and the matrix structure is promoted to be ferritic, and improvement in machinability can be expected.

[Object of the Invention] The present invention has been made in view of the above problems, and in the method for producing a spheroidal graphite cast iron casting, by increasing the number of graphite particles to a certain value or more, machinability in an as-cast state is improved. A basic object of the present invention is to provide a manufacturing method which can improve the heat treatment cost even when performing the primary heat treatment.

[Means for Solving Problems] Therefore, according to the present invention, a spheroidal graphite cast iron casting containing graphite having a graphite particle number of 250 / mm ² or more is formed, and then A ₁
It is heated and held at a temperature below the transformation point for 0.5 to 8 hours, then machined, then heated and austenitized, and then rapidly cooled to be subjected to a constant temperature transformation.

Was the number of graphite grains of the spheroidal graphite cast iron castings is limited to 250 / mm ² or more, by the number of graphite grains to 250 / mm ² or more, to promote ferritization without causing precipitation of the chill It is possible to improve machinability and prevent deterioration of toughness, and the number of graphite particles is 250.
This is because if the number is less than the number of pieces / mm ² , chill may be precipitated to deteriorate machinability and reduce toughness.

When the primary heat treatment is applied after casting and before machining, the heat treatment temperature is A ₁ transformation point or lower and the heating holding time is 0.5.
The reason for limiting to ~ 8 hours is as follows.

The temperature was less than the A ₁ transformation point, by ensuring the number of graphite grains 250 / mm ² or more, the chill of precipitation is prevented, chill decomposition treatment is unnecessary, the above A ₁ transformation point This is because it is not necessary to heat, and the Young's modulus is lowered when the heat treatment at the A ₁ transformation point or higher is repeated. The temperature below the A ₁ transformation point is preferably in the temperature range of 700 to 750 ° C.

The reason why the heating and holding time is set to 0.5 to 8 hours is that the ferrite formation is not sufficiently performed in less than 0.5 hours, and the effect is saturated and the cost is increased only in 8 hours or more.

As the conditions for the austempering treatment, it is preferable that the austenitizing treatment is performed by heating and holding at a temperature of 800 to 950 ° C. for 4 hours or less, and the isothermal transformation treatment is 250 to 42.
It is preferable to hold at a temperature of 0 ° C. for 15 minutes or more.

[Effects of the Invention] According to the present invention, by increasing the number of graphite particles to 250 / mm ² or more, it is possible to suppress the precipitation of chill and promote the ferritic transformation of the matrix structure. And deterioration of toughness can be prevented. Further, segregation of alloying elements can be prevented, so that deterioration of toughness after austempering can be prevented at the same time.

Furthermore, since the precipitation of chill can be suppressed as described above, even when performing the primary heat treatment before machining,
Even when performing the primary heat treatment above the A ₁ transformation point, it is not necessary to perform chill decomposition process of heating held above the A ₁ transformation point,
Since only low temperature annealing in which the material is heated and held at a temperature not higher than the A ₁ transformation point for 0.5 to 8 hours is required, the heat treatment temperature can be lowered and the heat treatment time can be shortened, and energy consumption can be reduced and manufacturing cost can be reduced. it can. Further, by repeating the heat treatment at the A ₁ transformation point or higher, it is possible to prevent the Young's modulus from lowering due to voids around the graphite.

[Examples] In the present invention, the number of graphite particles is increased to 250 particles / mm ² or more by adding an inoculant having a high inoculation effect, for example, a pouring flow inoculation method or an in-mold inoculation method. Examples will be described in comparison with Comparative Examples using the conventional ladle inoculation method.

Even if an inoculant is added by the ladle inoculation method, the number of graphite particles can be 250 pieces / mm ² or more if the cooling rate is sufficiently fast, but the whole material is more reliably 250 pieces. / Mm
^In order to achieve ² or more, it is desirable to use a pouring method or an in-mold method that has a high inoculation effect as the inoculation method. By adopting such an inoculation method, the number of graphite particles in the entire material is surely 250 / It becomes possible to make it to be ² mm ² or more, and it is possible to promote ferritic conversion without causing the precipitation of chill.

(1) Inoculation method Four test materials as examples of the present invention and two as comparative examples
The test material of the book was cast.

As shown in Table 1, the inoculants were added by the pouring flow inoculation method for Examples 1 and 2 of the present invention, and by the in-mold inoculation method for Examples 3 and 4 of the present invention. The comparative example is based on the ladle inoculation method. As the inoculant, Fe-Si (50% by weight) system, Fe-Si (60 to 75% by weight) system to which Ca, Al, Zr, Bi, a rare earth element or the like is added can be used. The one used in the experiment was Fe-S according to the present invention.
An i (60 to 75% by weight) -Ca-Al inoculant was added in an amount of 0.1 to 0.2% by weight, while a comparative example used the same inoculant in an amount of 0.3 to 0.6% by weight.

(2) Chemical composition The chemical compositions (% by weight) of the test materials of Examples 1, 2, 3, 4 of the present invention and Comparative Examples 1 and 2 were as shown in Table 1.

(3) Base structure and number of graphite grains For each as-cast sample material, the volume percentage of ferrite and pearlite in the base composition and unit area (1 mm ² )
The number of graphite particles per unit was measured.

The measurement results are as shown in Table 2. In each of the examples of the present invention using the pouring flow inoculation method or the in-mold inoculation method, the number of graphite particles was 250 or more / mm ² , and the comparison was made. It was confirmed that the ferritic formation was remarkably promoted as compared with the example.

In the examples and comparative examples of the present invention, the casting material in an as-cast state is heated and held at a temperature of 890 ° C. for 1.5 hours to perform an austenitizing treatment, and then heated and held at a temperature of 395 ° C. for 2.0 hours. It has been transformed.

(4) Machinability test For each of the inventive examples 1, 2, 3, 4 and comparative examples 1 and 2, a round bar test piece having an outer diameter of 45 mm and a length of 360 mm was prepared, and the outer diameter portion was cut. It machined and measured the tool life. The tools used and the cutting conditions were as follows.

Tool used: AC10G (aluminum coating) Cutting conditions Cutting speed: 120 m / min Feed: 0.4 mm / rotation Depth of cut: 1.5 mm Cutting oil: Oil-free (dry) About the tool life for each test piece obtained in the above test, Based on the tool life for Comparative Example 1 (1.0),
The test results expressed as coefficients are shown in Table 2.

As can be seen from Table 2, according to the examples of the present invention, it was confirmed that the tool life was remarkably extended and the machinability was improved as compared with the comparative examples.

As described above, according to the embodiment of the present invention, by adding an inoculant at the time of casting by, for example, a pouring flow inoculation method or an in-mold inoculation method, as compared to a ladle inoculation method which is generally performed conventionally, The time from the inoculation to the solidification of the molten metal is shortened, so that the inoculation effect is improved and the number of graphite particles can be increased to 250 / mm ² or more.

As a result, since the precipitation of chill can be suppressed and the ferritic transformation of the matrix structure can be promoted, the machinability in the as-cast state can be improved.

Claims (1)

[Claims] 1. A spheroidal graphite cast iron casting containing graphite having a number of graphite particles of 250 / mm ² or more is formed, then heated and held at a temperature of A ₁ transformation point or less for 0.5 to 8 hours, and then machined. The method for producing a spheroidal graphite cast iron casting is characterized in that, after that, heating is performed to perform austenitizing treatment, and then quenching is performed to perform isothermal transformation treatment.