JPS5945728B2 - Manufacturing method of granular graphite cast iron - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a gray cast iron casting having high elongation.

Conventional flake graphite cast iron has low strength, especially exhibiting almost no ductility.

In order to increase strength and impart ductility, it is necessary to add a spheroidizing agent such as Mg to make so-called spheroidal graphite cast iron.

However, in order to obtain spheroidal graphite cast iron, highly pure raw materials are required, high sulfur content such as in the case of melting in a cupola requires desulfurization treatment, and the whiskers are large and the method must be devised. aka fading
), easy to chill, easy to produce dross defects, easy to form gas bubbles, etc. It has many disadvantages in terms of industrial technology.

Moreover, in recent years, social problems have arisen in the disposal of slag resulting from the use of large amounts of CaC2, etc. in desulfurization treatment, and in the disposal of basic slag such as MgO generated during excessive spheroidization treatment and remelting. This has become a major problem today and a future challenge for the production of spheroidal graphite cast iron in our country.

An object of the present invention is to provide a method for producing gray cast iron castings that can obtain gray cast iron castings with high elongation without using any spheroidizing agent and without any problems even in the case of high sulfur content such as cupola molten metal. shall be.

This will be explained below. The method for manufacturing gray cast iron castings having high elongation according to this invention has iron as the main component, carbon content of 2 to 4.5%, and silicon content of 0.5 to 4%.
, molten cast iron with a manganese content of 0.2% or less is poured into a mold to form a ferrite base casting material containing fine flaky graphite with a thickness of 2 microns or less, and then heated at 750° to 1050°.
It is characterized in that it is subjected to a heat treatment in which flaky graphite is divided by holding it at least once at a temperature of °C.

To carry out this more effectively, from low temperature to 9200 ~
It is preferable to repeat a periodic heat treatment in which the temperature is raised to a high temperature of 1050° C., maintained at that temperature for 10 minutes or more, and then cooled again, twice or more.

Further, the sulfur (S) content of the cast material is preferably 0.1% or less, and the phosphorus (P) content is preferably 0.3% or less.

[Explanation of principle]

It is well known that the fine flaky graphite (D-type graphite) in cast iron is continuous flakes, so the iron part of the base is divided, and the strength is low and it has almost no toughness. be.

The present inventors discovered the following fact as a result of research on breaking up the continuity of graphite by heat treatment.

(1) The solubility of carbon in ferrite and austenite,
There is a significant difference.

(2) Carbon has a much higher diffusion rate than iron in the austenite region.

(3) When ferrite-based flaky graphite cast iron is heated to the austenite region, due to the large difference in carbon solubility, carbon near the graphite-austenite interface melts into austenite, forming voids.

The graphite piece is partially divided in a thin part where the thickness of the graphite piece is 2μ or less.

(4) Iron self-diffuses and precipitates into the void, filling the void.

This action causes the volume to decrease and the whole to shrink.

This phenomenon is illustrated in FIGS. 1 and 2.

(5) In this way, the fine flaky graphite is divided and granulated.

[Explanation of Examples]

Nippon Steel Pig No. 1 B (4,], 4%C12, 1%Si, 02
%Mn, 0.082%P, 0.03%S) as the raw material iron, the composition was adjusted with pure iron and ferrosilicon, and 3.4%C12
, 7% Si in a mold (diameter 20X) at 1350℃.
250 mm round bar: 0.3 ynm basic coating mold, acetylene suction coating mold, preheated to 150° C.), the mold was released after 90 seconds, and the bar was placed in an annealing box and subjected to the next heat treatment in an Elema furnace.

The heat treatment (graphite cutting treatment) conditions were to raise the temperature to 920° C. in about 3 hours, hold it at that temperature for 2 hours, and then cool it in air or in a furnace.

As a result, part of the fine graphite was dissolved and the strength was improved as shown in Table 1.

As cast, it has a ferrite base with fine graphite and some pearlite, and when cooled in a furnace, a complete ferrite base remains, and when cooled in air, the structure is almost the same as that as cast, with a slight pearlite remaining.

Because the graphite is fine and has a high carbon content as well as an extremely high silicon content of 27% Si, it is thought that almost no pearlite remains even after air cooling.

The microscopic structure shows almost no change in the graphite structure when viewed at 100x magnification, but when viewed at 500x magnification, the graphite is clearly fused and separated, and a rounded tip can be seen.

This is considered to be the main reason why the strength improved by 10% to 27%.

The reason why there is almost no difference between air cooling and furnace cooling is that almost no pearlite was observed even in air cooling, and the structure was almost the same as that in furnace cooling.

In this way, even in ordinary cast iron, a remarkable improvement in strength was observed when graphite was melted and annealed by holding it in the austenite region for a short time.

Although it is not clear because elongation was not measured in this experiment, it is thought that a certain amount of elongation is exhibited due to the dissolution of graphite shown in the microscopic structure.

Low S Kamaishi ductile pig (4.3%C10.6%Si, 0
．． 2%Mn, 0.06%P, 0.002%S10.00
4% Cr, 0.052% Ti, 0.002% As), the components were adjusted with pure iron and metal Si, inoculated with 0.3% ferrosilicon, and the mold was heated to 1350°C (preheating temperature It was cast at 150℃ (other conditions are the same) (chemical composition is approximately 3,
(5% C12, 5% Si) mold release time was 1 to 2 minutes.

Separately, we investigated the case where 0.5% Ti (targeted content of 0.25%) was added as a graphite refinement treatment so that it could be applied to thick-walled objects.

Furthermore, the 0.5% Ti-added molten metal was placed in a CO2 mold with the same shape as the mold.
Graphite fragmentation treatment was also applied to the cast material cast into the sand mold.

The graphite separation treatment conditions are as follows.

Warm (repeated heating 3 times) The above five types are a, b, cld, and e.

The tensile testing machine was an Instron type, and a stress-strain curve was obtained.

Representative examples (magnification: 2000x) of the microscopic structures obtained by the heat treatment in (d) above are shown in Fig. 3 (as cast) and Fig. 3d (heated twice at 1050°C).

The examples below all show an improvement in strength due to heat treatment, but this is due to the granulation of graphite as shown in this photo.

[Characteristics of mechanical strength and stress-strain curve] (I) Changes in strength (tensile strength, elongation) under heat treatment conditions a), (b), and (c) are shown in Table 2.

As is clear from this table, the tensile strength increases slightly at 750'C, but increases by 8% at 920'C and 13% at 1050°C.

Especially the elongation is 4 times even at 750℃, 920'C11050'
C showed an increase of 6.8 times.

Their microscopic structures are shown in FIGS. 4, 4a, 4b, and 4c (each magnification 13o) and FIGS. 5, 5a, 5b, and 5c (each magnification 65o).

As a result, some graphite melting was already observed by heating at 750°C, and the extent of the splitting was 920'C11, 050°C.
It becomes more significant as the holding temperature increases, and the correspondence with strength, especially elongation, is clearly recognized.

As shown in the stress-strain curve in Figure 6, it takes 4 to reach the yield point.
It is understood that the difference in the subsequent plastic deformation is that the elongation increases due to the division of graphite in the gap.

(II) Effects of heat treatment on Ti-treated molds and sand casting materials Next, the molten metal should be treated with 0.5% Ti.Target value: 0.2
5%) and was cast into the mold and CO□ sand mold.

Considering that if the mold material was too thick, the graphite would become coarse and it would be difficult to separate the graphite, so an attempt was made to make all the precipitated graphite finer by Ti treatment in advance.

The reason why a sand mold was used was because it was thought that as long as the graphite was fine even in a sand mold, the heat treatment would be effective in the present invention.

The results are shown in Table 3.

As is clear from this table, the tensile strength of the mold material increased by 15% at 1050°C compared to that at 750°C, and the elongation increased by more than twice.

The strength of the as-cast condition is not clear, but the strength of the as-cast condition is 7.
Since it is expected to be considerably worse than 50'C, T
Even with the mold material added with Ti, the graphite melting and separation effect is obvious as in the case without the addition of Ti.
As shown in Figure A (each magnification is 800), it can be seen from these figures that the progress of graphite granulation is more pronounced as the holding temperature increases.

In the sand mold material, almost no heat treatment effect is observed when held at 750°C, but it is slightly improved at 920°C, and at 1050°C, the tensile strength increases by 26% and the elongation increases by about 2.5 times.

However, the improvement in elongation is not as noticeable in other cases.

However, it is truly remarkable that even in the case of fine graphite in a sand mold, the effect was evident.

The microscopic structures are shown in Fig. 9, Fig. 9a, Fig. 9b, Fig. 9c (each magnification: 130), Fig. 10, Fig. 10a,
This is shown in Figures 10b and 10c (each magnification is 650).

From this, it can be seen that the structure of the as-cast graphite is obviously coarser than that of the mold material, and therefore the as-cast strength is considerably inferior.

When the graphite is coarse like this, the graphite melting and cutting effect of the present invention becomes somewhat small, and repeated heating at a high temperature such as 1050° C., which will be described later, becomes effective.

(e) In case of repeated decoration d) and heat treatment (e) 10
Table 4 shows the results of repeated heating at 50° C. twice and at 920° C. three times.

Tensile strength is approximately 23% at 1050°C twice, and at 920°C
It increased by about 30% after 3 times, and the elongation increased significantly by about 7 times and 9 times, showing the extremely remarkable effect of repeated heating.

Typical examples of changes in the structure are shown in Figures 4 to 4c (Figures 5 to 5c), where the graphite is neatly granulated by the heat treatments (a), (b), and (C). I know that there is.

Next, a cast material obtained using the same molten cast iron as above was subjected to a periodic heat treatment as shown in FIG.
Figure 12 shows the results of repeated heat treatment in which the temperature was raised from below 10°C to a high temperature of 950°C, held at that temperature for 2 hours, and then cooled again to 600°C or below1. As is clear from FIG. 12, it is effective to repeat such periodic heat treatment three or more times.

In this case, the temperature to be raised (high temperature) is a temperature in the ausanite range, which is practically 800° to 1000°C,
More preferably, the temperature is around 950°C.

If the temperature is too high, equipment problems will occur.

In addition, when heat treatment by repeated heating is performed, in all cases, an effect was obtained with a holding time at a high temperature of about 10 minutes, but in practical terms, a holding time of 1 to 3 hours was preferable.

Next, carbon (q, silicon (Si)
), if we talk about manganese (Mn) content, the 13th
The figure is a graph showing the results of investigating the relationship between carbon equivalent CE value (C+1/3Si), graphitization rate, and graphite thickness.From the figure, carbon content is 2 to 4.5%, silicon content is It can be seen that it is preferable to set the amount to 0.5 to 4%.

In addition, Fig. 14 is a graph showing the results of investigating the influence of Mn content on graphitization rate at various S contents.
If the n content is 0.2% or less, a desirable graphitization rate of 80% or more can be obtained.

As explained above, the method for producing a gray cast iron casting with high elongation according to the present invention does not require any special desulfurization treatment and can produce a gray iron casting with high elongation through a relatively simple process. It is extremely practical.

[Brief explanation of the drawing]

FIG. 1 is a thermal expansion change curve diagram due to fragmentation of flaky graphite, which explains the principle of the present invention, and FIG. Figures 3 and 3d are micrographs (X100O) of the structures of cast iron as it is and a typical example of the heat treatment of the present invention, Figure 4 (as cast iron), Figures 4a, 4b, and 4c. The figures are similar micrographs (X130) of three other examples, Figures 5, 5a, 5b,
Figure 5c is a photomicrograph of the same specimen at a magnification (X650). FIG. 6 is a graph showing the stress-strain curves of the cast iron as-is and three examples heat-treated according to the present invention, and FIGS. 7 and 7A, and FIGS. In the case of using a mold, these are microscopic photographs of the structure after heating (X180) and magnification (xsoo). Figures 9, 9a, 9b, and 9c as well as Figures 10, 10a, 10b, and 10c show the same magnification when using a sand mold of Ti-treated cast iron ( These are micrographs of the same (X130) and the same (X650). FIG. 11 is a schedule diagram of periodic heat treatment;
The figure is a correlation diagram between the number of repetitions and the effect. FIG. 13 is a graph showing the relationship between CE value, graphitization rate, and graphite wall thickness, and FIG. 14 is a graph showing the influence of Mn content on graphitization rate at various S amounts.

Claims (1)

[Scope of Claims] 1 Molten cast iron containing iron as a main component and having a carbon content of 2 to 4.5%, a silicon content of 0.5 to 4%, and a manganese content of 0.2% or less is used as a mold. The material is poured into a ferrite base casting material containing fine flaky graphite with a thickness of 2 microns or less, and then heat-treated by holding it at a temperature of 750° to 1050°C at least once to break up the flaky graphite. A manufacturing method for gray cast iron castings with a characteristic high elongation. 2. The method according to claim 1, in which a periodic heat treatment in which the cast material is heated from a low temperature to a high temperature of 920° to 1050°C, held at that temperature for 10 minutes or more, and then cooled again is repeated two or more times. A method for manufacturing gray iron castings with high elongation. The method for manufacturing a gray cast iron casting having high elongation according to claim 1 or 2, wherein the holding time at 3750°C or higher is 1 to 3 hours.