JPS61264156A - Spheroidal graphite cast iron having bainite matrix and its manufacture - Google Patents

Abstract

PURPOSE:To obtain spheroidal graphite cast iron having superior impact tensile strength by carrying out bainite forming heat treatment at a specified temp. so as to regulate the amount of retained austenite. CONSTITUTION:Austenitized cast iron is subjected to bainite forming heat treatment by isothermal holding in a salt bath at 375-400 deg.C and furnace cooling to obtain spheroidal graphite cast iron having a bainite matrix contg. >=30% retained austenite. By the treatment, the impact tensile strength is especially improved, so a new use for the spheroidal graphite cast iron can be found.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a bainitic-based terrestrial graphite cast iron having excellent impact tensile strength and a method for producing the same.

(Prior art) Recently, increasing the strength of spheroidal graphite cast iron has been studied in order to make castings lighter or thinner, or as a partial substitute for high-grade materials. It is known that bainitic-based terrestrial graphite cast iron obtained by tempering has high strength and toughness, but how to obtain bainitic-based terrestrial graphite cast iron with particularly excellent impact tensile strength? There was no clear explanation as to what should be done.

(Problems to be Solved by the Invention) This invention has been made to solve the above-mentioned problems, and specifically aims to obtain bainitic-based terrestrial graphite cast iron with excellent impact tensile strength. be.

(Means for Solving the Problems) In order to achieve the above-mentioned object, in this invention, the retained austenite barrier of the bainite-based terrestrial graphite cast iron is reduced to 30%.
In order to achieve this, the bainitic temperature is set at 375 to 400°C.

(Function) As described above, the present invention allows the bainitic temperature to be set to 375 to 375.
By setting the temperature to 400℃, the amount of retained austenite can be reduced to 3
When the content is 0% or more, bainitic-based geomorphic graphite cast iron particularly excellent in impact tensile strength can be obtained.

(Example) Next, this invention will be explained in detail.

Table 1 Chemical composition % Sample materials with the chemical composition shown in Table 1 were melted and 9
After holding at 00℃ for 1 hour to austenite, 22
5. 250. 275. 300. 325. 35
After being kept at a constant temperature for 1 hour in a salt bath at 0°375°C and 400°C, it was cooled in air and heat-treated to form bainitization.

Instrumented Charpy impact tests were performed on U-notch test pieces made of these samples, and the Charpy impact values (
+cg-m/cze) and the bainitic temperature ("C)", the characteristic diagram shown in FIG. 1 was obtained.

In addition, as a result of investigating the relationship between the Charpy impact tensile strength (kg-m/C) and the bainitic temperature (°C) using test pieces with the shape and dimensions (unit: ms) shown in Figure 2, it was found that
The characteristic diagram shown in the figure was obtained.

Furthermore, the mechanical properties of bainitic base geomorphic graphite cast iron are
Since it is greatly influenced by the amount of retained austenite, the relationship between the bainitization temperature and the amount of retained austenite was investigated and the result was as shown in FIG. 4.

That is, as shown in Figure 1, the Charpy impact value is high in the bainitic temperature range of 300 to 400°C and shows a flat value on a plateau, but the Charpy impact tensile strength is As shown in Figure 2, as the bainitization temperature increases, it increases almost linearly up to 375℃, and
It reaches a peak between 5°C and 400°C, and rapidly decreases above 400°C.

The Charpy impact tensile strength at a bainitic temperature of 375°C to 400°C is equivalent to the impact tensile strength of 555C, SCMn3, and 80M3, which means that it is a very high value for spheroidal graphite cast iron. Recognize.

Therefore, by setting the bainitic temperature to 375 to 400°C, it is possible to obtain a bainitic base terrestrial graphite cast iron having excellent impact tensile strength.

Furthermore, as shown in Fig. 4, the amount of retained austenite at a baenization temperature of 375°C to 400°C is 30% or more, so bainitic-based terrestrial graphite cast iron with a retained austenite amount of 30% or more is The tensile strength is very good.

(Effects of the Invention) As described above, according to the present invention, by setting the bainitic temperature to 315 to 400°C, a bainitic base structure having an extremely excellent impact tensile strength with a retained austenite content of 30% or more can be obtained. Since graphite cast iron can be obtained, this invention has the excellent effect of being able to develop new uses for bainite-based terrestrial graphite cast iron.

[Brief explanation of the drawing]

Figure 1 is a characteristic diagram showing the relationship between bainitic temperature and impact value, Figure 2 (a) is a side view of the impact tensile test piece, Figure (b) is its A-8 sectional view, Figure 3 is FIG. 4 is a characteristic diagram showing the relationship between bainitization temperature and impact tensile strength. FIG. 4 is a diagram showing the relationship between bainitization temperature and residual austenite amount. Patent Applicant: Asahi Malleable Iron Co., Ltd. Fig. 1 Temperature degree ℃ Fig. 2 Fig. 3 Fig. 3 Beginner temperature 6 degrees 0C Fig. 4 Possible - Knight α Hirohiro

Claims (1)

[Claims] 1. Bainitic-based terrestrial graphite cast iron with excellent impact tensile strength, characterized by having a retained austenite content of 30% or more. 2. A method for producing bainite-based terrestrial graphite cast iron with excellent impact tensile strength, characterized by setting the bainitic temperature to 375 to 400°C.