JPH01100218A - Manufacture of spheroidal graphite cast-iron product - Google Patents

Abstract

PURPOSE:To improve workability and also to prevent deterioration in physical properties, such as toughness and fatigue strength, by austenitizing a spheroidal graphite cast-iron stock, quenching this stock to carry out isothermal transformation treatment, cooling the above, and then heating the above to the prescribed temp. to apply tempering. CONSTITUTION:After austenitizing treatment is applied to a spheroidal graphite cast-iron stock, is subjected to austempering treatment consisting of isothermal transformation treatment in which quenching is performed by immersion in a cooling medium and also of air cooling. By the above treatment, the metallic structure of the stock is transformed to a mixed structure of bainite and residual austenite. Then, heating is applied to the above at 250-400 deg.C to carry out tempering, by which martensite contained in residual austenite is transformed to another structure. This stock is machined so as to be formed into a spheroidal graphite cast-iron product.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to an improvement in the manufacturing method of spheroidal graphite cast iron products.
In particular, it relates to the modification of its metallographic structure.

(Prior Art) In general, spheroidal graphite cast iron has been frequently used as a material for mechanical parts because it has excellent tensile strength and elongation and toughness compared to other cast irons.

By the way, when crankshafts and their bearings for engines for automobiles and other vehicles are manufactured from spheroidal graphite cast iron, the crankshafts are parts that require strength, so the toughness, fatigue strength, etc. of the spheroidal graphite cast iron, etc. It is desirable to further improve the physical properties of.

Therefore, as disclosed in, for example, JP-A No. 50-127823 and JP-A No. 60-187821, spheroidal graphite cast iron material is subjected to austenitization treatment and then subjected to austempering treatment, which is a constant temperature transformation treatment. This modifies the metal structure to a mixed structure of bainite and retained austenite to improve physical properties such as toughness and fatigue strength, and the presence of retained austenite slightly reduces hardness to facilitate machining. , then
A method has been proposed for increasing hardness and improving wear resistance by tempering at a predetermined temperature after machining.

(Problem to be solved by the invention) However, although the methods described in both of the above-mentioned publications are said to facilitate machining after austempering, they do not actually achieve the desired effect. The current situation was that it was not possible.

As a result of investigating the cause of this impediment to workability, we found that some of the residual austenite (unstable blocky It was found that highly hard martensite was formed in the retained austenite, and that the presence of this martensite inhibited workability.

The present invention has been made in view of the above, and its purpose is to remove martensite, which inhibits machining workability, by taking appropriate measures after austempering treatment of spheroidal graphite cast iron materials. By decomposing only the unstable lumpy retained austenite containing
This moderately reduces the hardness of the spheroidal graphite cast iron material after the lithium oxide treatment, improving its machinability, while improving toughness and fatigue strength due to the presence of stable residual austenite that does not contain martensite. The aim is to prevent the deterioration of physical properties such as

(Means for Solving the Problems) In order to achieve the above object, the solving means of the present invention first subjects a spheroidal graphite cast iron material to austenitization treatment. Next, it is rapidly cooled to a predetermined temperature, subjected to isothermal transformation treatment, and then cooled. Thereafter, it is heated and tempered at a temperature of 250 to 400°C for 20 minutes or more. After that, the method is to obtain a spheroidal graphite cast iron product by machining.

(Function) With the above configuration, in the method of the present invention, the metal structure of the spheroidal graphite cast iron material is modified into a mixed structure of bainite and residual austenite by austempering treatment, which is a constant temperature transformation treatment after austenitization treatment. Among the retained austenite, only the unstable massive retained austenite containing martensite that inhibits machining workability is decomposed by subsequent tempering, and the martensite contained in the massive retained austenite is transformed into other structures. It will be done. From this, the hardness of the spheroidal graphite cast iron material after austempering is moderately reduced and machining workability is improved, while the presence of stable retained austenite that does not contain martensite improves toughness and A spheroidal graphite cast iron product is obtained in which deterioration of physical properties such as fatigue strength is prevented.

(Example) Hereinafter, a method for manufacturing a spheroidal graphite cast iron product according to an example of the present invention will be described.

First, as a spheroidal graphite cast iron material, C2.5 to 4.1% by weight, Si 1.5 to 3.5% by weight, MnO, 1 to 1% by weight.
0% by weight, Po, 15% by weight or less, 30.03% by weight
For the following and Mg 0°025-0.060% by weight, NiO, 1-2.0% by weight, Mo0.03-0.5
0% by weight and 3 to 1.5 ffim% of CuO are added as necessary, and the remainder is prepared. In the above composition, Ni, Mo and Cu are added as necessary to improve the hardenability of the spheroidal graphite cast iron material,
This is to prevent hardening defects due to differences in wall thickness. Then, the Ni content in the spheroidal graphite cast iron material was reduced to 0.
．． The reason why it is set to 1 to 2.0% by weight is that if it exceeds 2.0% by weight, brittle fracture is likely to occur. The reason why the Mo content is set to 0.03 to 0650% by weight is that if it exceeds 0.50% by weight, the elongation after tempering will be inhibited due to segregation. Furthermore, the Cu content was reduced to 0.
The reason why it is set at 3 to 1.5% by weight is that if it exceeds 1.5% by weight, the graphite spheroidization rate decreases.

Then, the spheroidal graphite cast iron material is treated under austenitizing conditions such as an austenitizing temperature of 800 to 950°C and an austenitizing time of 5 minutes or more, and then 350 to 950°C.
2 in a cooling medium (salt bath, oil bath, fluidized bed, etc.) at 420℃
After a constant temperature transformation treatment in which the material is immersed for 0 minutes to 4 hours and rapidly cooled, an austempering treatment is performed in which the material is air cooled. As a result, the metal structure of the spheroidal graphite cast iron material is modified to a mixed structure of bainite and retained austenite. In addition, in a part of the above-mentioned retained austenite, that is, in the unstable lumpy retained austenite, martensite with high hardness that inhibits machining workability is generated.

The reason why the austenitizing temperature was set at 800 to 950°C is that below 800°C, the alloying elements are not sufficiently dissolved in solid solution, making it impossible to achieve uniform austenitizing, whereas when it exceeds 950°C, the austenite becomes carbon solid. This is because the amount of melt becomes excessive and the rate of transformation into bainite decreases. Further, the reason why the austenitizing time was set to 5 minutes or more is that this amount of time is enough to fully austenitize.

In addition, the above isothermal transformation treatment temperature was set at 350 to 420°C because if it is lower than 350°C, the hardness increases due to a decrease in the amount of retained austenite, making machining difficult.
This is because if the temperature exceeds 20°C, the mechanical properties will deteriorate due to the precipitation of troostite. In addition, the constant temperature transformation treatment time is 2
The reason for setting the range from 0 minutes to 4 hours is that if the time is less than 20 minutes, bainite formation will be insufficient and martensite will precipitate due to the transformation of retained austenite, making machining difficult.
This is because if the time is exceeded, fine carbides will precipitate due to the decomposition of retained austenite, making machining difficult as described above.

Next, by heating and tempering the spheroidal graphite cast iron material that has been austempered as described above at a temperature of 250 to 400° C. for 20 minutes or more, the remaining austenite,
Only the unstable massive retained austenite containing martensite that inhibits machining workability is decomposed, and the martensite contained in the massive retained austenite is transformed into other structures.

The reason why the above tempering temperature was set at 250 to 400°C was 2.
At temperatures below 50°C, the decomposition of unstable massive retained austenite and martensite, which is the purpose of tempering, does not occur, or even if it does decompose, the carbides generated due to low-temperature tempering become fine and distributed, resulting in hardening. This is because it is no longer possible to expect a decrease in the quality. On the other hand, if the temperature exceeds 400°C, all residual austenite in the base will decompose, resulting in a decrease in toughness and coefficient of thermal expansion.

In addition, the reason why the tempering time was set to 20 minutes or more is that it is necessary to completely decompose the unstable lumpy residual austenite and martensite by completely raising the temperature to the specified temperature to the center of the spheroidal graphite cast iron material. It is the body,
The time may be determined depending on the thickness of the spheroidal graphite cast iron material. Note that if the tempering time exceeds 4 hours, no effect can be expected over time and it is uneconomical, so the upper limit is 4 hours.
It is preferable to set the time.

Thereafter, a spheroidal graphite cast iron product is obtained by performing predetermined machining.

As described above, in this example, among the retained austenite of the spheroidal graphite cast iron material that has been modified to a mixed structure of bainite and retained austenite by austempering treatment, only unstable massive retained austenite containing high hardness martensite is used. is broken down by subsequent tempering and then machined.

Therefore, in the spheroidal graphite cast iron material to be subjected to machining, the high hardness martensite described above is decomposed into other structures along with the decomposition of the massive retained austenite, and thus does not exist in the spheroidal graphite cast iron material. Spheroidal graphite can moderately reduce the hardness of the material and improve machining workability, while the presence of stable residual austenite that does not contain martensite prevents deterioration of physical properties such as toughness and fatigue strength. You can get cast iron products.

Next, a specific example will be shown to demonstrate this.

First, test pieces made of spheroidal graphite cast iron (40×35
0x25mm) is melted and cast. The process is as follows: First, spheroidal graphite cast iron is melted in a high-frequency furnace, then the melted spheroidal graphite cast iron is placed in a ladle, Fe-5L-4%Mg is added thereto, and spheroidized. %Fe-5t06
Add 4% and perform primary inoculation. Thereafter, in Specific Examples I and H, test pieces were obtained by casting, while in other Specific Examples ■ to ■, Fe-3i-A,
After 0.07% of 17-Ca was added and inoculated by pouring, a test piece was obtained by casting. In addition, in Table 1, Fe
The content has been omitted.

Furthermore, the units of numerical values in each element column are all % by weight.

After that, each of the above test pieces was austempered under the austempering conditions shown in Table 2 below, tempered at varying tempering temperatures, and then drilled with a drill under the following processing conditions. I went there.

Drill Diameter 5mm Stub drill HSSCO Cutting speed 15m/min Feed 0. 15mm/rev number of holes processed
120 holes depth 15mm Equipment used Machining center, MC65 cutting oil
The tool wear amount at that time is shown in Table 3 below and FIG. 1, and the amount of retained austenite is shown in FIG. 2.

Table 2 Table 3 As is clear from this test data, when judging the total amount of tool wear from each specific example, it can be seen that the amount of tool wear is the lowest when the tempering temperature is 400°C. This can be seen in Figure 3, which shows a micrograph showing the metal structure of spheroidal graphite cast iron magnified 400 times, and Figure 4, which shows the metal structure of spheroidal graphite cast iron at 1.
This is also clear from the micrograph shown at 1,000 times magnification.

In addition, in Figures 3 and 4, the photograph with symbol (a) shows the case without tempering, the photograph with symbol (b) shows the case with tempering at 300°C, and the photograph with symbol (c) shows the case without tempering. The attached photograph shows the case of tempering at 400°C, and the photograph with symbol (d) shows the case of tempering at 500°C. Further, in FIGS. 3(a) and 4(a), the needle-shaped black parts are bainite, the white parts are retained austenite, and the spherical black parts are retained austenite. In addition, the white part in the center of Figure 3(a) is unstable massive retained austenite, and as shown in enlarged detail in Figure 4(a), the unstable massive retained austenite has a gray color. The part is made of high hardness martensite. Then, when tempered at 300°C, as shown in Figures 3(b) and 4(b), only the unstable massive retained austenite and martensite are decomposed together and the amount is not tempered. (a) and Fig. 4 (a), and furthermore, when tempered at 400°C, Fig. 3 (c)
As shown in FIG. 4(c), it can be seen that the unstable massive retained austenite and martensite have been completely decomposed and transformed into other structures and no longer exist. However, when tempering at 500°C, even the stable residual austenite that does not contain martensite is decomposed and transformed into other structures, as shown in Figures 3(d) and 4(d). As shown in Figure 2, it can be seen that the amount of retained austenite has become zero. Therefore, the tempering temperature is
250 to 400 in order to achieve both improvement in machining workability and improvement in physical properties such as toughness and fatigue strength due to the presence of stable retained austenite that does not contain martensite.
It can be said that it is preferable to set it in the range of ℃.

(Effects of the Invention) As explained above, according to the method of the present invention, a spheroidal graphite cast iron material is austempered by austenitizing treatment and isothermal transformation treatment, and then heated at a temperature of 250 to 400°C for 20 minutes or more. Since the spheroidal graphite cast iron product is obtained by tempering and then machining, the metal structure of the spheroidal graphite cast iron material can be modified to a mixed structure of bainite and retained austenite through the austempering process.

Furthermore, among the retained austenite, only unstable lumpy retained austenite containing martensite, which inhibits machining workability, can be decomposed by subsequent tempering, thereby improving the hardness of the spheroidal graphite cast iron material after austempering. It is possible to improve the workability of machining by reducing the amount appropriately. Furthermore, due to the presence of stable retained austenite that does not contain martensite, it is possible to obtain a spheroidal graphite cast iron product that prevents deterioration of physical properties such as toughness and fatigue strength.

[Brief Description of the Drawings] Figures 1 and 2 are test data showing the amount of tool wear and amount of retained austenite, respectively, with respect to changes in tempering temperature in a specific example of spheroidal graphite cast iron products manufactured by the method of the present invention, and Figure 3 and FIG. 4 are micrographs showing the metal structure of spheroidal graphite cast iron magnified 400 times and 1000 times, respectively. Patent applicant Mazda Motor Corporation representative
In front of people 1) Hiro Diagram 1 Bladder return temperature (°C) Diagram 2 Reinstatement temperature 35 degrees (°C) 3rd (Q) Figure 4 (Q) Procedural amendment (method) % formula % 1. Incident 1986 Patent Application No. 257852 2.1 @ Ming Name Spheroidal Graphite Cast Iron Product % 3ti Method 3, Relationship with the Amendment Person Case Patent Applicant Address 3 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture 1 Name (313) Mazda Motor Corporation Representative Furu 1)
Masao Toku Agent 550 fffi06 (445) 21
28 Address: 1-4-8 Utsubohonmachi, 4-ku, Osaka City Taihei Pillto, Name: Patent Attorney <7793) 1)
Hiromu 72. S. Date 7 of the amendment order, contents of the amendment (1) "Second is" on page 17, line 11 of the specification is changed to "
Figure 2 has been corrected to J. (a. “Test data” in the 17th member, line 14 of the Memorandum
is corrected to "Test data diagram". that's all

Claims (1)

[Claims] (1) A spheroidal graphite cast iron material is austenitized, then rapidly cooled to a predetermined temperature, subjected to isothermal transformation treatment, cooled, then heated at a temperature of 250 to 400°C for 20 minutes or more to temper, and then, A method for producing a spheroidal graphite cast iron product, characterized by obtaining a spheroidal graphite cast iron product by machining.