JPS5867844A - Spherical graphite cast iron excellent in tenacity and preparation thereof - Google Patents

Abstract

PURPOSE:To obtain spherical graphite cast iron high in tenacity and tensile strength provided with easy machinability such as cutting after heat treatment, by converting the base structure of spherical graphite cast iron to a two-phase mixed structure consisting of bainite and ferrite. CONSTITUTION:Spherical graphite cast iron has a composition consisting of, on the weight basis, 1.9-2.7% C, 1.8-3.0% Si, below 0.5% Mn, 0.01-0.04% Mg and the remainder Fe and having an Si/C ratio exceeding 0.9 and comprises a two-phase mixed structure consisting of ferrite and bainite in which graphite is uniformly dispersed. This spheroidal graphite cast iron is prepared as described hereinafter. That is, a molten metal is cast according to a usual method and the obtained base material having a ledeburite structure constituted by uniformly dispersing fine graphite particles in a cast structure is used to be heated at about 900-1,000 deg.C for about 0.5-5hr in first stage heat treatment. In the next step, the heat treated base material is heated at a temp. directly below an A1 transformation point for about 0.5-5hr and, after this second stage heat treatment, heated to the coexisting temp. of the aforementioned two- phase and subjected to austempering treatment wherein the final heat treated base material is rapidly cooled to about 220-420 deg.C and held at this temp. for about 0.1-3hr to obtain the product.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in the properties of spheroidal graphite cast iron and a method for producing the same.

Conventionally, various spheroidal graphite cast irons have excellent properties, but are inferior in toughness against high impact loads compared to ferritic terrestrial graphite cast irons whose matrix structure is ferrite. On the contrary, although ferritic terrestrial graphite cast iron has excellent toughness against high impact loads as described above, it has properties that are contradictory to bainitic terrestrial graphite cast iron, such as a significant decrease in tensile strength.

Furthermore, products using bainitic terrestrial graphite cast iron require machining before heat treatment in order to ensure rigidity, resulting in the problem of variations in dimensional accuracy due to thermal deformation.

The present invention is basically a spheroidal graphite cast iron that combines the excellent properties of bainitic terrestrial graphite cast iron and ferritic terrestrial graphite cast iron in a well-balanced manner. It is a spheroidal graphite cast iron,
Moreover, it is an object of the present invention to provide spheroidal graphite cast iron that can be easily machined such as cutting after heat treatment, and to provide a method for manufacturing spheroidal graphite cast iron having such characteristics.

The spheroidal graphite cast iron according to the present invention is suitable for products that require excellent impact resistance and sufficient tensile strength, such as steering knuckles that support wheels.

The spheroidal graphite cast iron according to the present invention has C1 as a chemical component.
, 9 to 2.7% by weight (all percentages below indicate weight%),
Sl 1.8-3.0%, Mn (Q, 5%, Mg0.
01-0.04%, including the remainder of re, Si/C is 0.9
It has the above composition and has a two-phase mixed matrix structure of ferrite and bainite in which graphite is uniformly dispersed.

The above chemical component ratio and structure are mainly castable and black.

The reasons for the limitations are listed below.

C: When the content is 1.9% or less, castability is significantly reduced and a casting method similar to that of cast steel is required.

When it is 2.7% or more, there is a strong tendency to graphitize during solidification,
In particular, a stable ledebrite structure cannot be obtained in the center of the casting. Further, even if a sufficient cooling rate is obtained, the amount of carbides becomes extremely large and the first stage graphitization heat treatment time becomes long, which is not preferable.

8i; The smaller the 8i content, the lower the hardness of the ferrite, improving the workability, and the toughness. However, if it is less than 2%, the first and second stage graphitization heat treatment times become longer, and even after annealing. The risk of residual carbide increases. Still, lowering the carbon equivalent results in poor castability.

If it exceeds 3%, the toughness of the ferrite itself decreases, thereby impairing the properties aimed at by the present invention.

Mn; Q, if it exceeds 5%, graphitization will be significantly impaired.

Mg: An essential element for making annealed graphite spheroidal, but if it is less than 0.01%, the effect is weak, and if it is more than 0.04%, the effect is saturated and inclusions such as M'g oxide are formed. This is not desirable because it increases

8i/C; As mentioned above, the basic characteristics are determined by limiting the component ratio of C and Si, but especially Si/C
When is less than 0.9, ledebrite grows significantly and graphitization becomes extremely difficult.

Next, a method for producing spheroidal graphitized cast iron according to the present invention will be explained.

The method of the present invention involves casting a molten metal having the above composition in a conventional manner,
A casting material having a ledebrite structure in which fine graphite grains are uniformly dispersed is used, and the temperature is 900 to 1000 r.
The first stage heat treatment (first stage graphitization treatment) is carried out by heating for 0.5 to 5 hours at a temperature of 0.5 to 5 hours, and then at a temperature just below the A1 transformation point.
Second stage heat treatment (second stage graphitization treatment) by heating for 5 to 5 hours
After that, the graphite is heated to a temperature at which ferrite and austenite coexist, rapidly cooled to a temperature of 220 to 420C, and held at this temperature for 0.1 to 3 hours to perform an austempering treatment (two-phase treatment). It is characterized by a two-phase mixed structure of ferrite and bainite in which ferrite and bainite are uniformly dispersed.

First stage graphitization treatment> The first stage graphitization treatment is to decompose cementite and make graphite spheroidal. At temperatures below 900C, graphitization becomes difficult, and at temperatures above 1000C, ferrite crystals form. As the grains become coarser, they lose their spherical shape.

Further, if the treatment time is less than 0.5 hours, graphitization will be insufficient, and if the treatment time is more than 5 hours, it is wasteful.

Second-stage graphitization treatment> The second-stage graphitization treatment is for completely converting the matrix structure into ferrite.

The temperature just below the transformation point is chosen because if it is lower than that, undecomposed pearlite remains, and if it is higher than that, complete ferrite cannot be obtained. Further, if the treatment time is 0.5 hours or less, undecomposed pearlite will remain, and if the treatment time is 5 hours or more, it is not necessary.

<Two-phase treatment (austempering treatment)> - This two-phase treatment converts the completely ferrite structure through the heat treatment described above into two layers: ferrite and bainite, in which graphite is evenly dispersed.
It is for forming a phase mixed structure, and the treatment temperature is 220
Below C, martensite is generated and the toughness is impaired.
At 20C or higher, upper bainite precipitates and tensile strength and toughness decrease. Furthermore, if the treatment time is 0.1 hour or less, untransformed austenite becomes martensite, resulting in a decrease in toughness. No need for more than 3 hours.

Next, examples of the present invention will be shown.

Table 1 shows spheroidal graphite cast iron produced according to the present invention together with comparative examples.

Each sample above was heated to 1630 to 1650 in a 150 kp high frequency furnace.
After being heated and melted in C and spheroidized with Fe-8i-Mg alloy, CO2O2 mold mouth block 4501Z'
25 ran X cast at a temperature of ~1470tZ'
This is a graphitized test piece of 40Wrrn×200W.

The first stage graphitization treatment is heated at a temperature of 950C for 1.5 hours, and the second stage graphitization treatment is performed at 10C below the A1 transformation point of each component.
Heated at low temperature for 8 hours.

In addition, S i /C is 0. except for 1.8.6.8 in the sample.
It is 9 or less.

These samples according to the present invention had a stable ledebrite structure in which fine graphite was uniformly dispersed.

Next, Table 2 shows the test results of the various properties of the above sample 1.2.6.7 after the two-phase treatment, and the comparison results of the mechanical properties with conventionally known spheroidal graphite cast iron. is shown in Figure 1.

In addition, in Figure 1, the chemical composition of ordinary Guiteil cast iron is as follows:
All C8, 46%, 8i 2.74%, Mn Q.

32%, 80.10%, Po, 025%, Mg0.04
3%, the balance being Fe.

Further, FIG. 2 shows the relationship between the heating temperature and the bainitic rate during the two-phase treatment (austempering treatment) for Sample 7.

For the sample l shown in Table 2, Fig. 3 shows (X10
FIG. 4 shows a micrograph of (X400) which was subjected to 5% nital corrosion treatment.

As is clear from the above examples, the spheroidal graphite cast iron according to the present invention has a good balance of both toughness and tensile strength, and can be easily machined after heat treatment.

[Brief explanation of drawings]

Fig. 1 is a graph showing the elongation and tensile strength of spheroidal graphite cast iron according to the present invention and conventional spheroidal graphite cast iron, Fig. 2 is a graph showing the relationship between heating temperature and bainitic rate during austempering treatment in the method of the present invention, and Fig. 3 The figure is for sample 1 (xtoo)
Fig. 4 is a micrograph (X400) of sample 1 after 5% nital corrosion treatment, in which the black part is graphite, the white part is ferrite, and the colored part (gray part) is bainite. .

Claims (2)

[Claims] (1) C1.9 to 2.7% by weight (all percentages below indicate weight%), Si 1.8 to 3.0%, Mn (Q, 5
%, MgO1O1~0. ．． 04 X, Fe balance, 5r
Spheroidal graphite cast iron with excellent toughness, having a composition of 7 co, 9 <, and a two-phase mixed structure of ferrite and bainite in which graphite is uniformly dispersed. (2) C1.9-2.7% by weight (all percentages below indicate weight%), Si 1.8-10%, Mn (0.5%,
Mg0.01-0.08%, Fe balance, Si/CQ, 9
A cast iron material with a composition of
．． The first stage heat treatment is performed by heating for 5 to 5 hours, then the second stage heat treatment is performed by heating at a temperature just below the transformation point for 0.5 to 5 hours, and then the material is heated to a temperature where ferrite and austenite coexist. Then, spheroidal graphite cast iron with a two-phase mixed structure of ferrite and bainite in which graphite is uniformly dispersed is produced by rapidly cooling it to a temperature of 220 to 420 tr and performing an austempering treatment by holding it at this temperature for 0.1 to 3 hours. Method.