JPS6196054A - Spheroidal graphite cast iron and manufacture thereof - Google Patents

Abstract

PURPOSE:To obtain a material suitable for manufacturing thick-wall machine parts which is to sustain a high repeated stress, by providing specific amounts of C, Si, Mn, Mo, and 1 kind among Cu, Ni, and Co, making a matrix form into bainite and reducing graphite grains into fine particles to be dispersed uniformly. CONSTITUTION:The spheroidal graphite molten cast pig consists of, by weight, 3-4% C, 1.8-3% Si, <0.7% Mn, 0.1-1% Mo, 0.1-3% of 1 kind among Cu, Ni and Co, and the balance Fe. This molten pig is subjected to metal mold casting and rapid cooling, and the heat-treated at 800-1,000 deg.C to be austenized which is subjected to austempering at a constant temp. of 200-450 deg.C to make the matrix form into bainite. In this way, the graphite grains are reduced to fine particles of 15-20mu to be dispersed uniformly.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention is applicable to transmission equipment, hydraulic equipment, etc. subjected to repeated stress.
This article relates to spheroidal graphite cast iron that is applied to thick-walled mechanical parts such as vehicle equipment parts, and its manufacturing method.

(Prior art and its problems) Recently, there has been a shift from cast and forged steel products to spheroidal graphite cast iron products with the aim of reducing the manufacturing cost of machine parts. is not used.

The reason for this is that the graphite grains in spheroidal graphite cast iron are large and have extremely low strength compared to the base, so they become a type of P'3 defect against fatigue and become a source of stress concentration. This is because fatigue strength is reduced. To explain this point in more detail, one of the conventional methods for producing spheroidal graphite cast iron is to
~3.0% by weight, Mn 0.5% by weight or less, Mg0.
A molten spheroidal graphite cast iron consisting of 02 to 0.08% by weight, the balance Fe, and other inevitable impurities was cast into a sand mold, and then q9
Heat treated at 00C to austenitize, then 300C
There is a method in which the base is subjected to austempering treatment at around 0 to turn the base into bainite, and then cooled.

By the way, in such a manufacturing method of spheroidal graphite cast iron, the molten spheroidal graphite cast iron is cast into a sand mold, so the molten metal is gradually cooled, and the spheroidal graphite in the molten metal is low and the spheroidal graphite is large. Particularly when casting thick machine parts, the inside becomes cold.
Because of this, the spheroidizing group of graphite becomes lower, spheroidal graphite decreases, and lumpy graphite increases. This lumpy graphite does not change in any way during the subsequent heat treatment, and its fatigue strength improves by turning it into bainite, and its strength is significantly lower than that of nine bases. Therefore, when high repeated stress is applied, stress will be applied to the lumpy graphite part. There was a problem in that the fatigue limit of spheroidal graphite cast iron was lowered.

(Object of the invention) The present invention has been made to solve the above problems,
The object of the present invention is to provide spheroidal graphite cast iron and its manufacturing method that can be applied to the manufacture of thick-walled machine parts that can be subjected to high repeated stress by making graphite grains finer and dispersed to increase the fatigue limit. It is.

(Structure of the Invention) The spheroidal graphite cast iron of the present invention contains C3, O to 4.0% by weight, S
i1.8-3.0% by weight, Mn 0.7% by weight or less, M
o O, 1-1.0% by weight, Cu, Ni, Co
Each of these consists of one type to, 1 to 3.0% by weight, the balance is Fe, the base is bainitic, and the graphite grains are 15 to 20μ
It is characterized by being finely divided and uniformly dispersed.

Further, the method for producing spheroidal graphite cast iron of the present invention includes C3, 0 to 4
．． 0% by weight, Si1.8-3.0% by weight, Mn0.
7% by weight or less, Mo0.1-1.0% by weight, Cu +
Ni+C.

Any one of 1 at-0.1 to 3.0% by weight, balance Fe
Molten spheroidal graphite cast iron is cast in a mold, rapidly cooled, and then heat treated at 80O-1000C to austenite.
The base is then subjected to austempering treatment at a constant temperature of 200 to 450 tl to turn the base into bainite.

The reason why the composition of the spheroidal graphite cast iron of the present invention is limited as described above is that C and Si work together to produce a good casting during casting and cooling of the spheroidal graphite cast iron, and the optimal combination of these two elements is The content of t is generally 0% Si-Si%
=4.2 to 4.4chi. Therefore, the C content is 3.0 to 4.0% by weight, and the St content r'' is 1.8 to 3.0% by weight.
C is set at 0% by weight, and outside these ranges it becomes difficult to obtain good quality castings.

Calm is an alloying element that affects the heat treatment of the spheroidal graphite cast iron of the present invention, and is 0.7% by weight? If the content exceeds 017N, there is a risk of carbide formation and the time required for bainite formation becomes longer, so it is preferable that the 017N content be less than 100%.

MO is an alloying element for converting spheroidal graphite cast iron into bainite. By adding it in combination with any one of Cu, Ni, and Co, MO is a type of quenching treatment that can austemper treatment up to the isothermal transformation temperature. , without forming a structure such as pearlite, it is possible to add cold supplements without forming a structure such as pearlite.
If it is less than -0-1 Raitochi, it has no effect and is 1.0% by weight.
If it exceeds 0.1 to 1.0 weight, it not only forms carbides and inhibits the spheroidization of graphite, but also impairs toughness.

Any one of C, u, Ni, and Co has the function of inhibiting the grain boundary carbide formation of Mo, and can contribute to improving toughness. If it is less than 0.1% by weight, it has no effect. Even if it exceeds 3.0% by weight, the effect does not change much, but rather increases the cost, so o,i to 3.0% by weight is preferable.

However, in the method for producing spheroidal graphite cast iron of the present invention, the reason why the molten spheroidal graphite cast iron is cast in a die and rapidly cooled is to refine the graphite particles and disperse them uniformly, thereby improving fatigue strength. The reason for heat treatment at 800 to 100Oc to austenite cast iron is 5ooc.
If it is less than 1000Ct, the austenitization is insufficient and ferrite and pearlite remain, and if it exceeds 1000Ct, the amount of solid solute carbon in the austenite increases and the toughness is impaired.
．． It also costs 200-450 to turn the base into Bainite.
The reason for heat treatment at a constant temperature of C is that below 200 C, martensite is produced and toughness is lost, and above +501:', troostite is produced and strength and toughness are impaired.

(Example) Examples of the spheroidal graphite cast iron of the present invention and its manufacturing method will be explained together with comparative examples. A Y-shaped block was made by mold casting, and a Y-shaped block was made by sand casting molten spheroidal graphite cast iron having the compositions of Comparative Examples 1 to 6 shown in Table 1, and each was subjected to the heat treatment shown in Table 1. A test piece was made.

However, the base structure of each test piece of Examples 1 to 6 and Comparative Examples 1 to 6? Inspect and graphite particle size.

When tensile strength, elongation, impact value, and fatigue strength were measured, the results shown in Table 2 below were obtained.

As is clear from Table 2 above, the test pieces of Examples 1 to 6 were rapidly cooled by die casting, and the graphite grains were refined to 15 to 23 μm. is as large as 35 to 45/1, but it can be seen that the fatigue strength after heat treatment is significantly improved under the same conditions. Furthermore, it can be seen that the tensile strength, elongation, impact value, etc. were greatly improved under the same conditions, and the toughness was significantly improved.

The average cooling rate of the molten spheroidal graphite cast iron produced by mold casting at 1300C to 100OC is 1.2 C/sec, while that of the molten spheroidal graphite cast iron produced by sand casting is 1.2 C/sec.
C to 1000C! The average cooling rate at is 0.3 r:/
Seconds. By cooling the spheroidal graphite cast iron molten metal using different cooling means, the structure after austempering was that of Example 1, and as seen in the metal structure photograph in Figure 1, the graphite grains were 15 to 20μ. While the fatigue strength threshold is improved due to finely and uniformly dispersed particles, in comparison example 1, it can be seen in the metal structure photograph in Figure 2: Uni graphite grains 7 > 130 to 45μ, which is large. And'':) Because it is dispersed in one piece, the fatigue strength is low.

(Effects of the Invention) As can be seen from the above explanation, the spheroidal graphite cast iron of the present invention has a bainite base, graphite grains are finely divided to 15 to 20μ and are uniformly dispersed, and have excellent toughness and fatigue. Since it has extremely high strength, it has the advantage that it can be applied to the production of thick-walled mechanical parts that are subject to high repeated stress.

The uniform method of the present invention involves casting a spheroidal graphite cast iron bath into a mold, rapidly cooling it, then heat-treating it at 800-1000C to austenite, and then subjecting it to austempering treatment at a constant temperature of 200-450t:'. The spheroidal graphite cast iron has high fatigue strength because the graphite grains are finely and uniformly dispersed in a size of 15 to 20 microns, and the base is bainitic and has excellent toughness. It has the effect of being easily obtainable.

[Brief explanation of the drawing]

FIG. 1 is a photomicrograph showing the metallographic structure of an example of the spheroidal graphite cast iron of the present invention, and FIG. 2 is a photomicrograph showing the metallographic structure of the spheroidal graphite cast iron of the comparative example.

Claims (1)

[Claims] 1) C 3.0 to 4.0% by weight, Si 1.8 to 3.0% by weight, Mn 0.7% by weight or less, Mo 0.1 to 1.0% by weight
, 0.1 to 3.0% by weight of any one of Cu, Ni, and Co, and the balance is Fe, the base is bainitic, and the graphite grains are refined to 15 to 20μ and uniformly dispersed. Spheroidal graphite cast iron is characterized by: 2) C3.0-4.0% by weight, Si 1.8-3.0% by weight, Mn 0.7% by weight or less, Mo 0.1-1.0% by weight
A molten spheroidal graphite cast iron consisting of 0.1 to 3.0% by weight of any one of Cu, Ni, and Co, and the balance Fe is cast in a mold, rapidly cooled, and then heat treated at 800 to 1000°C to form austenite. 1. A method for producing spheroidal graphite cast iron, which comprises: austempering treatment at a constant temperature of 200 to 450°C to turn the base into bainite.