GB1564275A - Method of producing high tensile spheroidal graphite cast iron - Google Patents

Description

(54) METHOD OF PRODUCING HIGH TENSILE SPHEROIDAL GRAPHITE CAST IRON (71) We, NISSAN MOTOR COMPANY LIMITED, a corporation organized under the laws of Japan, of No. 2, Takaramachi, Kanagawa-ku, Yokohama City, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a method of producing a high tensile low carbon equivalent spheroidal graphite cast iron.

Generally, spheroidal graphite cast iron (also called nodular cast iron) is a product of relatively high carbon and high silicon content in which most of the carbon has been crystallized into a spheroidal form as proeutectic graphite during processing in the liquid state. This type of cast iron has an improved strength compared with ordinary gray cast iron, in which most of the carbon is present in the form of graphite flakes and exhibits a so-called notch effect as the origin of an insufficient strength of the product, and usually is used without effecting any particular heat treatment. Accordingly, molten iron for producing spheroidal graphite cast iron needs to contain carbon in an amount sufficient to allow crystallization of graphite during a casting process together with sufficient amount of silicon to promote the graphitization and gain a good castability. In addition it is necessary to add a graphitizing agent comprising Mg as its essential component to the molten iron. The carbon and silicon content of cast iron is commonly expressed by carbon equivalent (CE) which is defined as: CE(%) = C(%) + t (Si + P) (%). For spheroidal graphite cast iron, it is a common practice to make carbon equivalent fall within the range of about 4.05.0%.

Conventional spheroidal graphite cast iron is produced through sand casting and accordingly tends to suffer internal defects such as shrinkage cavities caused by movement of relatively weak casting walls.

As another inconvenience of spheroidal graphite cast iron, heat treatment of castings is liable to cause swell or crack on the surface particularly when the castings have internal defects. Spheroidal graphite cast iron, therefore, is put to use in the state as cast or with a light annealing merely for the sake of stress relieving.

There is another type of high duty cast iron called malleable cast iron. This material is of relatively low carbon and low silicon content: the carbon equivalent of this cast iron in practice is in the range of about 2.53.5%. As a fundamental difference from spheroidal graphite cast iron, a solidification process (sand mold casting) in the production of malleable cast iron is performed so as to give white cast iron structure without causing crystallization of graphite during solidification. The production of malleable cast iron is completed by subjecting the white cast iron to a long time heat treatment to cause precipitation of a so-called compact graphite or aggregate, graphite which affords toughness to the product. A disadvantage of malleable cast iron is the need of an immense amount of heating time to convert the white cast iron into the graphitized cast iron. Generally it takes 20 to 50 hours to complete this heat treatment and sometimes up to about 100 hours for large-sized castings.

It is an object of the present invention to provide a method of producing spheroidal graphite cast iron which is superior to conventional high duty cast irons in toughness.

It is another object of the invention to provide a method of producing high tensile spheroidal graphite cast iron, which method is far advantageous over a conventional method of producing malleable cast iron in productivity.

A method according to the invention is characterized in that a molten iron of relatively low carbon and low silicon content with the addition of a graphitizingspheroidizing agent which comprises magnesium is quickly solidified by a metalmold casting process to give a casting of white cast iron structure and that the casting is subjected to a short time heat treatment to cause precipitation of fine and spheroidal graphite.

More definitely, the carbon equivalent of molten iron in a method of the invention is made to range from 2.0 to 4.3% and the graphitizing-spheroidizing agent is used in such a quantity that the amount of retained Mg in the molten iron is 0.01 to 0.2 Wit%.

The graphitizing-spheroidizing agent is selected from conventional graphitizing agents for cast irons. A metal-mold casting process in the present invention implies every kind of known permanent-mold casting process insofar as a metal mold is used and includes gravity metal-mold casting, low pressure die casting, pressure injection die casting and forge casting.

However, the employment of a pressure casting process, which may be a pressure injection die casting process, is highly preferred. The heat treatment for graphitization is performed for about 0.2-10 hours at temperatures between 750 and 1200"C but usually can be completed in less than about 5 hours.

A spheroidal graphite cast iron produced by a method of the invention is remarkably superior to that produced by conventional methods both in tensile strength and abrasion resistance.

Fig. 1 is a photomicrograph showing the metallurgical structure in the unetched state of an intermediate cast iron before heat treatment for graphitization in a method of the invention; Figs. 2A, 2B, 2C and 2D are photomicrographs showing the metallurgical structure in the unetched state of a spheroidal graphite cast iron repectively for four different specimens obtained from the specimen of Fig. 1 by performing the heat treatment for different amounts of time; Fig. 3 is a photomicrograph showing the structure in an etched state of the specimen of Fig. 2A; Fig. 4 is a graph showing the relationship between the carbon equivalent of molten iron used in a method of the invention and the amount of heating time necessary for completing graphitization; Fig. 5 is a graph showing the relationship between the heating temperature in a graphitizing process according to the invention and the amount of time necessary for completing graphitization; Fig. 6 is a photomicrograph showing the metallurgical structure in an etched state of a spheroidal graphite cast iron produced by the use of a pressure die casting process as the metal-mold casting in a method of the invention; and Fig. 7 is a graph showing the abrasion resistance of a spheroidal graphite cast iron produced by a method of the invention in comparison with three types of conventional cast irons.

The composition of molten metal for use in a method of the invention is of course one of important factors in the success of this method. The properties of cast iron are determined fundamentally by the amounts of C and Si contained therein though various other ingredients including property improving elements such as Ni, Cr, Mo, Mn, etc. cannot be neglected. In general an increase in the total carbon of cast iron results in the presence of an increased amount of graphite in castings as cast, and Si has a similar effect in addition to its ability of affording a good castability to molten iron. It is a common practice, therefore, to indicate a fundamental composition of cast iron by carbon equivalent the definition of which is given hereinbefore. Cast iron can contain an increasing amount of graphite with increase in its carbon equivalent.

In this invention, molten iron as the starting material is limited to one whose carbon equivalent is within the range of 2.04.3% from the following reasons.

When- carbon equivalent is less than 2.0%, such iron is hardly called cast iron but is rather regarded as steel. Such a low carbon equivalent iron exhibits a poor castability particularly for permanent-mold casting and, besides, requires a considerably high casting temperature which is undesirable from the viewpoint of service life of metal molds. The upper limit to carbon equivalent is set at 4.3% because of appropriateness for avoiding the crystallization of graphite in a coarse form at the solidification step and for obtaining an intermediate cast iron which has fundamentally a white iron structure. In iron-carbon phase diagram a composition in which carbon equivalent is above 4.3% is in a region above a eutectic point, so that crystallization of proeutictic graphite is probable from molten iron of such a composition. Furthermore, a carbon equivalent in excess of 4.3% tends to hinder a method according to the invention from giving cast iron of improved toughness.

Of course molten iron for use in this invention may comprise known modifying or strength improving elements such as Cr, Ni, Mo, Al, B, Nb, Ti and/or V.

Table 1 shows a general composition of spheroidal graphite cast iron produced by a method of the invention in comparison with typical compositions of three types of conventional cast irons.

TABLE 1 (WtSo)

C Si Mn P S Cast iron according to the invention 2.0-4.0 0.9-3.0 0.15-0.9 50.2 < 0.1 Conventional S 1.3 malleable cast iron 2.2-3.0 0.9-11.6 (usually) < 0.1 < 0.15 Conventional ductile cast iron 3.5-4.2 2.0-3.5 0.2-1.0 < 0.1 < 0.025 Conventional ordinary cast iron 3.0-3.8 1.5-2.5 0.5-0.9 S0.2 < 0.15 The solidification of molten iron in a method of the invention is accomplished by a metal-mold casting process with a principal aim of quickly completing the solidification. This solidification method possesses manifold advantages over a traditional solidification method by sand casting. First, a quicker solidification in metal molds is quite effective for suppressing crystallization of graphite during solidification of liquid cast iron.

Secondly, a quick solidification causes the resultant casting to have a fine grain structure. Thirdly, internal strain in the sense of crystal lattice theory is forced to occur in the casting and serves for the generation of nuclei of precipitate at a great many points in the succeeding heat treatment for precipitation of spheroidal graphite from a white iron structure.

Besides, this process gives a high grade intermediate casting having little internal casting defects such as shrinkage cavities since a volumetric expansion of the cast iron attributable to crystallization of graphite during solidification is suppressed and there occurs no movement of casting walls during solidification.

There is a clear difference between a casting (as cast) obtained from molten iron in which carbon equivalent is 4.3% or somewhat below by a conventional sand casting process and the product of a metalmold casting for molten iron of the same composition. In the product of a sand casting process a certain portion of its carbon will be present in the form of crystallized graphite. However, the product of a metal-mold casting process has the structure of white cast iron due to a quicker completion of solidification. The employment of metal-mold pressure casting process such as pressure injection die casting or molten metal forging is particularly effective for realizing quick solidification because of a good contact of the poured liquid iron with molds, so that a white cast iron structure can readily be given by such a casting process even for molten iron having a carbon equivalent of 4.3%. The solidification of molten iron in a method of the invention, therefore, is preferably accomplished by a metal-mold pressure casting process.

A method of the invention has some resemblance to a known method of producing malleable cast iron in initially producing a white cast iron structure. A distinctive point of the invention is the addition of a spheroidizing agent (which is a graphitizing agent by nature) into molten iron to be solidified by metal-mold casting.

In this respect (apart from the casting process) a method of the invention resembles a conventional method of producing spheroidal graphite cast iron.

The shape of the precipitated graphite has a great influence on the tensile strength of cast iron as mentioned hereinbefore. In this regard compact, aggregate or dendritic graphite is desirable than flaky graphite but spheroidal graphite is still more desirable. A method of the invention is devised so as to realize precipitation of spheroidal and very fine graphite particles. As in the production of conventional spheroidal graphite cast iron, Mg is the essential component of a graphitizing and spheroidizing agent used in the present invention. It is possible to use Mg alone but it is more convenient and preferable to introduce Mg into molten iron in the form of alloy or mixture with other elements. Typical examples of useful and available Mg-containing graphitizingspheroidizing agents are alloys such as Mg Fe-Si, F-Ni-Mg, Mg-Fe and Mg-Ni.

Furthermore, alloys and mixtures of Mg with either rare-earth element(s) or alkaline-earth metal(s) are also of use. The quantity of a graphitizing-spheroidizing agent added to molten iron in a method of the invention is made such that the amount of retained Mg in the casting ranges from 0.01 to 0.2% by weight of the casting. As in conventional methods, an inoculant such as Fe-Si or Ca-Si may be added to the molten iron together with a Mg-containing graphitizing-spheroidizing agent.

The presence of a Mg-containing spheroidizing agent in the molten iron poured into a metal mold produces a supercooling effect which, coupled with a quick solidification effect of the metal mold, brings about a very fine grain structure of the casting. Besides, the solidification in the presence of Mg causes the resultant white cast iron structure to occlude nuclei of spheroidal graphite (or a foundational condition for generation of such nuclei in the sense of atomic theory), so that the decomposition of cementite and the diffusion and coagulation of the separated graphite at the subsequent graphitizing step (heat treatment) proceed through the aid of the nuclei or foundational condition.

It is also an important feature of a method of the invention that the heat treatment for precipitation of graphite from a white cast iron structure can be completed in a short time. This is derived from a very fine structure of the white cast iron obtained by the preceding metal-mold casting. In the production of conventional malleable cast iron, it is necessary to take an immense amount of time, some tens of hours, to work out a heat treatment for graphitization such that the decomposition of a white cast iron structure, i.e. a mixed phase of cementite and austenite (or cementite), gives a matrix of pearlite or ferrite, or a mixed matrix of pearlite and ferrite, with compact or aggregate graphite dispersed therein. In contrast, a white cast iron structure obtained in a method of the invention decomposes readily with precipitation of graphite in a fine and spheroidal form by a heat treatment of far shorter time and gives a high tensile cast iron. This is presumably attributable from a physical metallurgical consideration to the easiness of the decomposition of cementite and the migration or diffusion of carbon during the heat treatment of the fine structure white cast iron. The heat treatment for graphitization is conducted for about 0.2 to 10 hr at temperatures between 750 and 1200"C as mentioned hereinbefore, and it is perferable to employ temperatures between 800 and 950"C.

The completion of the heat treatment for graphitization in a method of the invention in a remarkably short time compared with a similar treatment is conventional methods for the production of malleable cast iron of course brings about a superior productivity.

As an additional advantage of a method of the invention, the fine and spheroidal graphite appeared in the product does not exhibit appreciable change in its form and particle size even if the heat treatment is continued further. Accordingly the heat treatment has a freedom in the amount of time: there is little need of paying a strict attention to the termination of the heat treatment.

The heat treatment is preferably performed in a non-oxidizing atmosphere or a reducing atmosphere from a point of view of maintaining a good casting surface obtained as a merit of the preceding metalmold casting. It will be understood that the graphitizing step may be followed by known quench hardening and tempering processes or a known refining process such as austempering or martempering.

The invention will be illustrated further by the following examples.

EXAMPLE 1.

Pig iron and scrap iron were melted in an electric furnace with the addition of a Fe-Si alloy and a conventional deoxidizer to give a molten iron whose carbon equivalent was 3.5%. The molten iron was transferred into a ladle and added with 5 Wt% of a commercial Mg-containing graphitizing and spheroidizing agent. When the molten metal was poured into a metal mold which was made of a die steel and had die sinks to give test pieces (each in the form of a 100 x 50 x 10 mm strip). The mold was preheated to about 150"C and a releasing agent was sprayed to the engraved faces of the mold prior to the pouring. After completion of the solidification of the poured iron a several number of test pieces were cut out from the obtained casting.

Fig. l is a photomicrograph (100 magnification) showing the structure of the casting for one of these test pieces (as cast) which-had been polished and left unetched.

This structure was of white cast iron. The remaining test pieces were subjected to a heat treatment for graphitization, which was carried out in a nitrogen atmosphere at a constant temperature of 900"C for various amounts of time, and then polished (but not etched) to give photomicrographs (all 100 magnification) of Figs. 2A to 2D. Fig. 2A shows the structure resulting from the heat treatment for 0.5 hr, and Figs. 2B, 2C and 2D show the results of 1 hr, 2 hr and 5 hr heating, respectively. As can be seen, graphite had precipitated in a fine and spheroidal form in any of these four cases.

Common to these spheroidal graphite cast iron structures, the matrix was of pearlite.

Figs. 2A and 2D demonstrate that a certain extent of prolongation of heat treatment time in a method of the invention does not cause an appreciable transformation or enlargement of the precipitated graphite.

Etching of the test piece subjected to the heat treatment of 0.5 hr (as a representative) gave the photomicrograph (also 100 magnification) of Fig. 3. It is apparent from Figs. 2A and Fig. 3 that the white cast iron structure shown in Fig. 1 turned completely into a spheroidal graphite cast iron structure through the short time heat treatment.

EXAMPLE 2.

The preparation of molten iron was similar to that in Example 1 (carbon equivalent was 3.5%), but the Mgcontaining graphitizing-spheroidizing agent of Example was added to this molten iron in variously different amounts so as to vary the amount of Mg retained in the castings as shown in Table 2. The permanent-mold casting was performed in accordance with Example I and the graphitization was effected by heating the cast iron specimens at 9000C for I hr. Table 2 presents analytical values for the retained Mg, the shape of the precipitated graphite and the type of matrix for spheroidal graphite cast iron specimens produced in this example.

TABLE 2

Retained Mg Shape of Run No. (Wt graphite Matrix 1 0.009 quasi- cementite and spheroidal pearlite 2 0.021 spheroidal pearlite 3 0.052 spheroidal pearlite 4 0.103 spheroidal pearlite but partly ferrite 5 0.188 spheroidal pearlite but partly ferrite .6 0.226 quasi- pearlite spheroidal and cementite The data presented in Table 2 show the preferableness of using a graphitizingspheroidizing agent in such a quantity that 0.01--0.2 Wt% Mg is retained in the solidified iron from the viewpoint of the shape of the precipitated graphite. It is difficult to obtain a good spheroidal graphite cast iron if the retained Mg is less than 0.01 Wt%. The retention of more than 0.2 Wt% Mg in the cast iron is also unfavorable because of a deformation tendency of the graphite and consumption of an unnecessarily large amount of graphitizing-spheroidizing agent.

EXAMPLE 3.

Several batches of molten iron were prepared from pig iron, scrap iron, a conventional recarburizer and a Fe-Si alloy with various values for the carbon equivalent as shown in Table 3. The measurement of carbon equivalent was made by means of a commercial rapid analysis instrument. The graphitizingspheroidizing agent used in Example I was added to each of these batches aiming at Q.05 Wt% of retained Mg in the product.

The metal-mold casting according to Example I was performed for each batch of molten iron. The thus obtained castings had a white cast iron structure irrespective of the carbon equivalent values. These castings were subjected to heat treatment for graphitization at a constant temperature of 900"C to examine the relationship between the carbon equivalent and the amount of heating time necessary for the completion of graphitization. The result of this experiment is presented in Fig. 4. This experiment was carried out also on castings obtained through generally the above described procedures but without introducing Mg into molten iron and the result is presented in Fig. 4 as the curve drawn with broken line.

A tensile strength test was made by means of an Amsler universal testing machine on graphitized specimens obtained through 3 hr heat treatment at 9000 C, using five tensile test pieces for each batch of molten iron, that is, for each carbon equivalent.

The test result is presented in Table 3 together with counterparts for conventional cast irons and hardness data (Rockwell hardness, C-scale of these specimens.

TABLE 3

Tensile -Specimen Carbon Retained Mg Hardness strength No. equivalent (Wt RC) (kg/mm3 1 2.25 0.048 29--31 91--95 2 2.55 0.052 26-30 85-91 2 2.55 0.052 26-30 85-91 3 3.08 0.046 28-33 91-96 4 3.51 0.055 25-29 89-93 5 4.12 0.051 25-29 85-90 Ref. I (conventional spheroidal graphite cast iron) 40-60 Ref. 2 (conventional malleable cast iron) 35-70 The data in Table 3 demonstrate a superior tensile strength of a spheroidal graphite cast iron produced by a method of the invention.

Example 3 was repeated except that a pressure die casting process (by means of a horizontal cold-chamber die-casting machine) was employed in place of the gravity casting process in Example 3.

Tensile and hardness tests on the specimens obtained through the pressure die casting process gave substantially the same results as those presented in Table 3.

EXAMPLE 4.

The preparation of molten iron and the metal-mold casting process in Example 1 were repeated. The graphitization of the resulting white cast iron was performed in a nitrogen atmosphere at three different temperatures, 800, 850 and 900"C, to examine the amount of heating time necessary for causing a sufficient precipitation of fine and spheroidal graphite at each temperature. The experimental result is presented graphically in Fig. 5, showing that a desirable graphitization was accomplished by continuing the heat treatment only for 0.5-1 hr when the heating temperature was 900"C and about 3 hr even when the temperature was as low as 800"C as a demonstration of the rapidness of graphitization in a method of the invention.

EXAMPLE 5.

Molten iron was prepared in an electric furnace to have a carbon equivalent of 4.2% and about 2 kg of this molten iron was transferred into a ladle. The introduction of Mg into this molten iron was performed according to Example 1. Then the molten iron was casted into a rectangular plate (100 x 50 x 10 mm) by a pressure injection die casting process under the following condition, using a horizontal cold-chamber die-casting machine and tungsten alloy dies.

Injection pressure : 200 kg/cm2 Injection rate : 0.8 m/sec Die temperature : 250"C Pouring temperature : 13500C The casting obtained by this process had a white cast iron structure which was confirmed to be cementite of a finer grain structure than the as-cast structures of the specimens 2-5 in Example 3. The finer structure in this example was attributable to the employment of a pressure die casting process wherein a forced contact of the molten iron with die faces caused a further quicker solidification. Graphitization of this casting was performed by a 1 hr heat treatment in a nitrogen atmosphere at 900"C. Fig. 6 is a photomicrograph (100 magnification) showing the etched structure of the graphitized casting.

Abrasion resistance of the spheroidal graphite cast iron obtained in this example was examined by means of an Ogoshi abrasion test machine under a dry abrasion condition of 100 mm abrasion distance and 3.3 kg load against a mild steel surface. For comparison, conventional acicular cast iron, gray cast iron (JIS FC25) and spheroidal graphite cast iron (JIS FCD70) were tested under the same condition. The results, presented in Fig. 7, demonstrate that a cast iron produced according to the invention is far superior in abrasion resistance to conventional gray cast iron and spheroidal graphite cast iron and is comparable to acicular cast iron which is known as a highly abrasion resistant cast iron.

EXAMPLE 6.

The spheroidal graphite cast iron produced in Example 5 was quench hardened by immersion into oil at a temperature of 8500C and then tempered at 600"C. The observation of the metallurgical structure of the thus treated cast iron revealed that the cementite matrix had turned into tempered martensite accompanied with no appreciable change in the shape and size of the spheroidal graphite. The tensile strength of this cast iron was 9W100 kg/mm2 and the hardness (MAC) was 28-33.

As illustrated by Examples 46, a spheroidal graphite cast iron produced by a method of the invention has a remarkably high tensile strength together with an excellent abrasion resistance compared with conventional tough or high grade cast irons.

Thus the invention has a great industrial significance in that the use of cast iron can be greatly widened.

WHAT WE CLAIM IS: 1. A method of producing a high tensile spheroidal graphite cast iron, comprising the steps of: preparing a molten iron in which carbon equivalent is in the range from 2.0 to 4.3%; adding a graphitizing-spheroidizing agent comprising as its essential component magnesium to said molten iron in such a quantity that the amount of retained magnesium in the resultant molten iron is 0.01 to 0.2 Wt%; solidifying the magnesium-containing molten iron by a metal-mold casting process quickly into a casting of a white cast iron structure; and heating said casting for about 0.2 to about 10 hours at temperatures between 750 and 1200"C to cause precipitation of fine and spheroidal graphite.

2. A method as claimed in Claim 1, wherein the heating step is performed at temperatures between 800 and 950"C.

3. A method as claimed in Claim 2, wherein the heating step is performed for about 0.5 to about 5 hours.

4. A method as claimed in Claim 1, wherein the heating step is performed in a non-oxidizing atmosphere.

5. A method as claimed in Claim 1, wherein said metal-mold casting process is a pressure casting proces

Claims (13)

**WARNING** start of CLMS field may overlap end of DESC **. structure in this example was attributable to the employment of a pressure die casting process wherein a forced contact of the molten iron with die faces caused a further quicker solidification. Graphitization of this casting was performed by a 1 hr heat treatment in a nitrogen atmosphere at 900"C. Fig. 6 is a photomicrograph (100 magnification) showing the etched structure of the graphitized casting. Abrasion resistance of the spheroidal graphite cast iron obtained in this example was examined by means of an Ogoshi abrasion test machine under a dry abrasion condition of 100 mm abrasion distance and 3.3 kg load against a mild steel surface. For comparison, conventional acicular cast iron, gray cast iron (JIS FC25) and spheroidal graphite cast iron (JIS FCD70) were tested under the same condition. The results, presented in Fig. 7, demonstrate that a cast iron produced according to the invention is far superior in abrasion resistance to conventional gray cast iron and spheroidal graphite cast iron and is comparable to acicular cast iron which is known as a highly abrasion resistant cast iron. EXAMPLE 6. The spheroidal graphite cast iron produced in Example 5 was quench hardened by immersion into oil at a temperature of 8500C and then tempered at 600"C. The observation of the metallurgical structure of the thus treated cast iron revealed that the cementite matrix had turned into tempered martensite accompanied with no appreciable change in the shape and size of the spheroidal graphite. The tensile strength of this cast iron was 9W100 kg/mm2 and the hardness (MAC) was 28-33. As illustrated by Examples 46, a spheroidal graphite cast iron produced by a method of the invention has a remarkably high tensile strength together with an excellent abrasion resistance compared with conventional tough or high grade cast irons. Thus the invention has a great industrial significance in that the use of cast iron can be greatly widened. WHAT WE CLAIM IS:

1. A method of producing a high tensile spheroidal graphite cast iron, comprising the steps of: preparing a molten iron in which carbon equivalent is in the range from 2.0 to 4.3%; adding a graphitizing-spheroidizing agent comprising as its essential component magnesium to said molten iron in such a quantity that the amount of retained magnesium in the resultant molten iron is 0.01 to 0.2 Wt%; solidifying the magnesium-containing molten iron by a metal-mold casting process quickly into a casting of a white cast iron structure; and heating said casting for about 0.2 to about 10 hours at temperatures between 750 and 1200"C to cause precipitation of fine and spheroidal graphite.

2. A method as claimed in Claim 1, wherein the heating step is performed at temperatures between 800 and 950"C.

3. A method as claimed in Claim 2, wherein the heating step is performed for about 0.5 to about 5 hours.

4. A method as claimed in Claim 1, wherein the heating step is performed in a non-oxidizing atmosphere.

5. A method as claimed in Claim 1, wherein said metal-mold casting process is a pressure casting process.

6. A method as claimed in Claim 5, wherein said pressure casting process is a pressure injection die casting process.

7. A method as claimed in Claim 5, wherein said pressure casting process is a forge casting process.

8. A method as claimed in Claim 1, wherein said molten iron contains about 2.04.0 Wt% carbon and about 0.9-3.0 Wt% silicon.

9. A method as claimed in Claim 1, wherein said graphitizing-spheroidizing agent comprises metallic magnesium.

10. A method as claimed in Claim 1, wherein said graphitizing-spheroidizing agent is a magnesium alloy seTected from Ni-Mg, Fe-Mg, Fe-Si-Mg and Fe-Ni-Mg alloys, alloys of magnesium with at least one rare-earth metal and alloys of magnesium with at least one alkaline-earth metal.

11. A method as claimed in Claim 1, further comprising the step of quench hardening the product of the heating step.

12. A spheroidal graphite cast iron produced by a method according to Claim 1.

13. A method according to Claim 1; substantially as described hereinbefore in any of Examples.