EP0691410A1

EP0691410A1 - Additive for treating molten metal, in particular for treating molten iron or steel

Info

Publication number: EP0691410A1
Application number: EP95304716A
Authority: EP
Inventors: Keiji Miyauchi
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-07-05
Filing date: 1995-07-05
Publication date: 1996-01-10
Also published as: JPH0820811A; JP3286469B2

Abstract

According to the present invention there is provided a molten metal additive easy to manufacture and handle and capable of affording a high quality product in a small amount thereof used and in a simple and stable manner. The molten metal additive is obtained by covering an additive necessary for the treatment of molten iron with a solid metal plate whose main component is iron, nickel or aluminum, and then making the solid metal plate with the additive contained therein into a granular or massive shape.

Description

[Background of the Invention]

The present invention relates to a molten metal additive necessary for the production of molten iron in cupola, cast iron, cast steel, aluminum alloy, castings, etc. More particularly, the present invention is concerned with a molten metal additive useful in spheroidizing of graphite, vermicularization of graphite, inoculation, desulphurization, decarburization, deoxidation and degassing which are necessary for the production of those metals.
Reference will now be made to the spheroidizing of graphite as a typical example of using a molten metal additive. It is about half a century since spheroidal graphite cast iron was developed. The use of spheroidal graphite cast iron has been expanding year after year and is now becoming a main product in casting. Since spheroidal graphite cast iron is superior in mechanical properties to cast iron, it is possible to attain the reduction in weight of castings. Further, in comparison with cast steel, it is superior in dimensional accuracy, casting yield rate and machinability and is lower in casting cost. Consequently, the field of cast iron and cast steel has been being substituted by spheroidal graphite cast iron. In the spheroidizing of graphite there usually is employed a magnesium alloy. There has been used a magnesium alloy prepared by combining magnesium with fifteen or more kinds of metals. At present, a ferrosilicon-magnesium alloy incorporating calcium and rare earth elements therein is mainly in use.
The boiling point of magnesium is about 1,370 K, and the temperature of molten iron to be treated is about 1,770 K. Therefore, the instant that magnesium comes into contact with molten iron, it vaporizes and the resulting vapor pressure is as high as 12 atm. or so, thus inducing an explosive reaction.
Besides, since the solubility of magnesium in molten iron is extremely low, it is necessary that the contact time thereof with molten iron be taken as long as possible.
In view of the above-mentioned points, in the case of a conventional ferrosilicon-magnesium spheroidizing additive, there has been adopted means wherein the magnesium content is set at a value (usually 4% to 5%) of not larger than about 10% to mitigate an excessive reaction and thereby prolong the contact time with molten iron, and the spheroidizing agent is placed at the bottom of molten iron and covered with a covering material to prevent floating.
The following are typical spheroidizing additive handling methods which have heretofore been adopted:

(a) Deposit Additive Process
A spheroidizing additive is placed in a pocket portion formed at the bottom of a molten iron receiving ladle, then is covered with steel plate scraps from banking or ferrosilicon, followed by pouring of molten iron. This method is a simple method and is therefore most popular at present.
(b) Tundish Process
With a cover applied to the top of the ladle used in the deposit additive process, molten iron is poured into the ladle.
(c) Pressure Adding Process
A spheroidizing additive of high purity magnesium is fed under pressure into molten iron by means of a high-pressure cylinder or is reacted in a high-pressure vessel.
(d) Converter Process
A ladle with a spheroidizing additive set therein is inverted, allowing reaction to take place.
(e) Inmold Process
A spheroidizing additive is reacted with molten iron within a mold.

In the most popular, deposit additive process, however, white smoke is generated markedly during treatment, and the yield rate metal magnesium after treatment is as low as 50% to 60%. This is because the density of the ferrosilicon-magnesium alloy, which is about 4.3 g/cm³, is fairly lower than that of molten iron which is about 7.2 g/cm³, thus causing floating of the spheroidizing additive in an early stage.
Consequently, before the vaporized magnesium is absorbed by the molten iron, it reacts with oxygen in the air, so that it becomes magnesium oxide and generates MgO mist, thus leading to decrease in the amount of magnesium captured in the molten iron.
As methods for producing a spheroidizing additive which does not float in the deposit additive process, there have been proposed such a method as disclosed in Japanese Patent Publication No. 32337/1970 (International Nickel Limited, U.S.A.) wherein a powder mixture of magnesium, carbonyl nickel, iron and copper is compressed and sintered to afford briquettes ranging in porosity from 20% to 50% and having a surface area/volume ratio of larger than 8, and such a method as disclosed in JP56-5436B wherein powders of magnesium, calcium and iron are compressed into briquettes exceeding 4.5 g/cm³ in density. At present, however, it is required that a covering material (e.g. steel plate scraps from blanking or ferrosilicon) be applied to the ferrosilicon-magnesium spheroidizing additive.
It is also required to repair the pocket portion of the ladle frequently because the temperature thereof is apt to drop, thus permitting easy deposition of metals thereon.
The foregoing process (b), (c) and (d) require special equipment, and in the foregoing process (e), the casting yield rate lowers and the cost becomes high.
Further, the conventional ferrosilicon-magnesium spheroidizing additive contains about 45% of silicon, so in the case of increasing or decreasing the amount of the spheroidizing agent, it becomes difficult to make chemical arrangement of product, or when the ferrosilicon-magnesium spheroidizing agent is left standing in the air over a long period, its surface is oxidized, resulting in deterioration of its performance.
Although problems of the prior art have been mentioned above with respect to the spheroidizing of graphite as an example, such problems are also common to other treatments using a molten metal additive.
It is the object of the present invention to solve the above mentioned problems of the prior art and particularly provide a molten metal additive which permits stable processing possible, can afford product of a high quality in a small amount thereof and is superior in both working efficiency and economy.

[Summary of the Invention]

The present invention resides in one embodiment in a molten metal additive for use in a deposit additive process wherein the additive is placed beforehand in a molten metal treating vessel, then molten metal is poured over the additive and is thereby treated, the said additive being in the form of small pieces each covered with a solid metal wall containing iron, nickel, or aluminum, as a main component.

[Brief Description of Drawing]

Fig. 1 shows an example of a molten metal additive with a metal wall formed from a metallic pipe.
Fig. 2 shows another example of a molten metal additive with a metal wall formed from a metallic plate.
Fig. 3 is a schematic diagram of a ladle for use in a deposit additive process; and
Fig. 4 is a graph showing thermal behaviors of a molten metal additive during treatment.
In the Figures, 1 denotes covering metal wall, 2 additive, 3 degassing hole, 4 welded portion, 5 ladle, 6 molten metal additive, 7 molten metal, 8 convection of molten metal, 9 molten metal and 10 molten metal additive.

[Detailed Description of the Preferred Embodiments of the Invention]

As the additive required for the treatment of molten metal in the present invention, there is used a suitable known additive according to each treatment. Preferred examples are as follows:

(a) Molten metal additives each covered with a solid metal plate containing iron as a main component which additives are necessary for desulphurization, vermicularization, graphite spheroidizing, inoculation to help graphitization and to strengthen the matrix, of molten metals (e.g. molten iron from cupola, cast iron and cast steel) containing iron as a main component, and for deoxidation or decarburization of cast steel. In this case, it is preferred that the apparent density of each molten metal alone be in the range of 4.5 to 7.6 g/cm³.
(b) Molten metal additives each covered with a solid metal plate containing nickel as a main component which additives are necessary for the treatment of molten metals containing iron as a main component. In this case, it is preferred that the apparent density of each molten metal additive alone be in the range of 4.5 to 8.2 g/cm³.
(c) Molten metal additives preferable each in the form of a small piece covered with a metallic wall containing aluminum as a main component and which are necessary for the treatment of molten metals containing aluminum as a main component. In this case, the apparent density of each molten metal additive alone is preferably in the range of 1.5 to 2.5 g/cm³.

As the molten metal additive used in the present invention, as mentioned above, any of known additives (components) necessary for various treatments is covered with a metallic wall (plate-like) which does not exert a bad influence on the treatment concerned and which can enhance the specific gravity (apparent density) without substantial floating during treatment, and the so-covered small piece is used in the deposit additive process. The shape of such small piece is not specially limited; for example, it may be in the shape of a cylinder, disk or box. Its size is usually in the range of 0.1 to 100 cm³, preferably 1 to 30 cm³. The thickness of the metallic plate or wall is usually in the range of 0.5 to 10 mm, preferably 1 to 5 mm.
For example, the small piece can be obtained by charging the molten metal additive into a metallic pipe, e.g. steel pipe, and then sealing both ends of the pipe with a press or the like or by forming a metallic plate such as steel plate into the shape of the small piece by suitable means such as, for example, machining, pressing (forging), welding, bonding with an adhesive, the use of retainer means, or spark bonding.
According to another preferred method, the molten metal additive is charged in a powdered or melted state into a metallic pipe such as steel pipe or nickel alloy pipe by suction under reduced pressure and then the pipe is cut into small pieces. In this case, as the metallic pipe, a round pipe having an inside diameter of 1 to 5 mm, preferably 3 to 10 mm, is suitable, but a square pipe having a sectional area corresponding to that of the round pipe is also preferred.
In covering the molten metal additive with the metallic wall, it is optional whether the covering should be throughout the whole of the additive or whether it should be only over a portion, preferably a principal portion. In the case where the inside additive is powdered, it is desirable that substantially the whole of the additive be covered in a sealed state to prevent leakage thereof to the exterior from the small piece during storage for example. On the other hand, in the case where the additive is in an integrally solidified state from molten condition and is difficult to undergo a change in quality during storage, its covered portion may be relatively small. For example, the additive may be in the foregoing cut state into a small piece after charging into a metallic pipe, that is, the cut portion may be left open. In the case of sealing the whole, it is desirable to provide a degassing portion.
As noted previously, as the molten metal additive to be used in the process of the present invention, there may be used any of known molten metal additives which have been used in treating metals in molten condition, for example in such treatments as graphitization, spheroidizing of graphite, inoculation and desulphurization. Thus, the kind thereof is not specially limited. As examples are mentioned such known additives as magnesium alloy, ferrosilicon alloy, fluorite, soda ash, and carbide.
Thus, the molten metal additives according to the present invention are employable in various molten metal treatments, some of which will be explained below each individually.
Reference will first be made to the spheroidizing of graphite. By using the molten metal additive of the present invention in the deposit additive process which is a typical example of the said treatment, it is possible to substantially prevent the floating of the additive during the treatment, prevent the generation of MgO mist during the treatment and afford a high quality product in a small amount of the additive used. Besides, the pocket portion of the ladle and the covering material are not required, whereby the working efficiency is improved.
As a typical example of the metallic plate for covering the molten metal additive, mention is made of steel plate. For example, magnesium and rare earth elements are charged into the interior of a steel pipe defined by JIS G 3461 and having an outside diameter of 16 to 32 mm and a wall thickness of 1 to 5 mm, followed by pressing and cutting into the shape shown in Fig.1, or a like steel plate is formed into the shape shown in Fig.2.
The steel has a characteristic of easy deformation on heating, while magnesium as a component of the additive to be covered melts at 923 K. Under these characteristics, when the steel pipe is heated to a temperature of about 770 to 970 K, the deformability thereof becomes three times or more in comparison with that at normal temperatures. In this state, the steel pipe, with magnesium incorporated therein, can be formed into a desired shape with a very weak force. Also at normal temperatures it is possible to forge steel pipe or plate in the cold and seal the additive therein automatically.
For some particular applications, nickel alloys or aluminum alloys, other than steel, are also employable as metallic materials for covering.
There was conducted an experiment wherein in accordance with the method illustrated in Fig.3 the molten metal additive for use in spheroidizing, prepared above, was placed at the bottom of each of 200 kg and 500 kg ladles and molten iron was poured into the ladle. Then, floating and thermal behavior of the additive, as well as the magnesium yield, metallogical structure and mechanical properties of the resulting products, were checked. The results obtained are as outlined below.

(a) Thermocouples were placed in 200 kg of the molten metal additive to check the thermal behavior of the additive from the surface layer to the interior. After the ladle had been filled with molten iron, the additive did not float, but melted gradually from the surface layer side, and both spheroidizing additive and inoculant were diffused gradually into the molten iron. Besides, it turned out that at the beginning of the diffusion the molten metal additive was in a more easily absorbed state into the molten iron because a large amount of molten iron and a high molten iron pressure are present in the upper portion.
(b) When the density of the molten metal additive was set in the range of 5.6 to 7.6 g/cm³, there was obtained a satisfactory result particularly in the prevention of floating. The density permitting the most stable prevention of floating was not lower than 6.2 g/cm³. Another reason for promoting the prevention of floating is that the melting temperature of the steel plate for covering is about 1,700 K, which is lower than the molten iron pouring temperature of about 1,770 K, and that therefore the surface of the steel plate melts and the melting heat is taken off, resulting in drop of the temperature and the small additive pieces being fused together strongly in a half-melted state.
(c) 20.7 kg of the molten metal additive for spheroidizing were placed at the bottom of the 500 kg ladle and then 500 kg of molten iron were poured onto the additive at 1,770 K. The pouring was effected without floatig of the additive. Just after completion of the pouring there occurred bubbling and convection of the molten iron, which continued for about 58 seconds. There was little MgO mist generated and the reaction was mild.
(d) The yield rate of magnesium in the spheroidal graphite cast iron produced by using the above molten metal additive was about 80%, and in comparison with that of 54% in the conventional process, the amount of the additive used could be decreased by about 40%. Also in point of metallogical structure and the number of graphite spheroids, the results obtained were about twice that in the conventional process. Mechanical properties were also found to be good.
(e) The molten metal additive of the present invention can be formed in desired shape and size in accordance with the amount of molten iron to be treated. Since the thus-formed product is covered with a strong steel plate, the handling thereof is easy.
(f) The molten metal additive of the present invention can be produced less expensively than ferrosilicon type additives.

From the above results it is seen that not only the molten metal additive using a covering material which contain iron as a main component can be used as an additive for vermicular graphite cast iron [CV (compacted vermicular) cast iron] or as an inoculant for gray cast iron, but also the features thereof can be utilized effectively as a molten metal additive for the desulphurization of pig iron in cupora or the like.
If there is used a covering material containing nickel as a main component in place of iron, the molten metal additive can be used as an additive for an alloy cast iron (e.g. Ni-resist cast iron) having a high nickel content, and is also applicable as a decarburizing additive for stainless steel cast steel.
Further, if there is used a covering material containing aluminum as a main component, the additive can be utilized as a dehydrogenating additive for molten aluminum.
The above advantages are attained by the feature of the molten metal additive of the present invention such that the additive is diffused uniformly in molten metal by virtue of bubbling and convection of molten metal, without floating during treatment, and thus functions efficiently to meet the expected object.
Reference will now be made to a molten metal additive for vermicular graphite cast iron (CV cast iron) according to the present invention. Vermicular graphite cast iron has a vermicular shape of graphite and exhibits an intermediate form between gray cast iron and spheroidal graphite cast iron. Since vermicular graphite cast iron is by no means inferior to spheroidal graphite cast iron in point of strength and is superior in castability, it is evaluated as a material having commonness well balanced industrially and its application to automobile parts and ingot cases is being expanded.
Vermicular cast iron is a new material and as methods for producing the same there have heretofore been adopted (a) a method wherein the amount of magnesium to be added is adjusted, (b) a method wherein both antispheroidizing agent (mainly titanium) and magnesium are used and (c) a method wherein calcium or selenium is used.
However, since vermicular graphite cast iron is a new material and is required to have a metallogical structure which is vermicular and unstable, the above method (a) requires controlling the additive in a very narrow range, that is, the process control is difficult, while in the above methods (b) and (c) it is easy to control the additive because the amount thereof can be controlled, but the quality of the resulting product is apt to be influenced by the amount of the additive used.
A molten metal additive according to the present invention was produced by covering magnesium with steel plate in the manner mentioned above and it was then added 2.0% to molten iron in accordance with the deposit additive process. Separately, as a conventional method, commercially available additive (magnesium content 5%) for vermicular graphite cast iron and covering material were used 1% and 3%, respectively. Then, the resulting products were evaluated for metallogical structure and mechanical properties. There were obtained equal results and thus it can be said that the molten metal additive of the present invention is also applicable to vermicular graphite cast iron as effectively as in the case of spheroidal graphite cast iron.
Description is now directed to a molten metal additive for inoculation according to the present invention. Inoculation for molten cast iron is usually performed by adding 0.2-0.3% of a granular additive to accelerate the graphitization of cast iron and improve the graphite shape and the number of graphite spheroids, whereby it is intended to improve the material quality and mechanical properties.
The inoculant loses its efficacy in a short time after application thereof, so for efficient use of the inoculant it is important that the inoculant be dispersed uniformly in molten metal and be applied at a timing as close to pouring as possible. To this end, heretofore, there has been adopted, for example, (a) a ladle inoculation process wherein at the time of pouring into a ladle the inoculant is set at the bottom of the ladle or is added along the flow of molten metal, (b) a stream process or wire process wherein the inoculant is added to molten metal at the time of pouring from a ladle into a mold, or (c) an inmold process wherein the inoculant is set within a mold.
However, according to the above ladle inoculation process (a), the inoculant is not dispersed to a sufficiently uniform extent, so its amount increases and fading is apt to occur. In the above process (b) it is necessary to provide equipment for feeding a constant amount of inoculant, and the above process (c) also involves the problem that the product yield rate becomes lower.
A commercially available ferrosilicon, calcium and carbon based inoculant was covered with steel plate to obtain a molten metal additive for inoculation having such a shape as shown in Fig.1, which additive was then set at the bottom of a ladle. As a result, equal inoculation effects were obtained in an amount of 0.18% of the inoculant used and in an amount of 0.3% in the conventional process. This is presumed to be because the convection of molten metal due to bubbling was continued for about 30 seconds and the diffusion of the inoculant was thereby effected to a satisfactory extent.
The following description is now provided about a molten metal additive for the desulphurization of pig iron according to the present invention.
In the case where the sulphur content of molten iron is high, magnesium is consumed for sulphur and hence the spheroidizing of graphite is impeded. Usually, it is considered desirable that the sulphur content of molten iron to be treated be not higher than 0.02%. If the amount of the spheroidizing additive is increased in the case of a high sulphur content of molten iron, not only increase of the cost but also the occurrence of dross or inclusion defect will result. Therefore, in the case of producing spheroidal graphite cast iron using pig iron from cupola which contains about 0.1% of sulfur, it is necessary to conduct desulfurization first.
In connection with additives for desulphurization, Japanese Patent Publication No. 4242/1985 (magnesium Electron Co., Great Britain) proposes a briquette, as a desulphurizing additive for converter molten iron, produced by hardening magnesium, magnesium oxide and calcium fluoride with a binder, and JP60-4242A proposes a briquette, as a desulphurizing additive for cupola converter molten iron, produced by hardening magnesium and calcium, or alloys thereof, with a binder. But the following desulphurizing processes using carbide or soda ash have mainly been adopted so far.

(a) Porous Plug Process
This process involves introducing granular carbide into a ladle and blowing nitrogen gas into the ladle through a porous, refractory plug provided at the bottom of the ladle to stir molten iron. This process is most popular.
(b) Intra-ladle Stirring Process
Carbide and molten iron both introduced into a ladle are stirred with rotating blades.
(c) Injection Process
Powdered carbide or soda ash is blown into molten iron by the use of nitrogen gas.

In the above processes (b) and (c), however, it is necessary to provide special equipment, the amount of the additive used is large, and a large amount of slag is formed.
An experiment was conducted in the following manner. Carbide, soda ash and fluorite were each covered with steel plate to produce molten metal additives for desulphurization. Then, the additives were each set in a ladle and molten iron from cupola was poured into the ladle. When the additives were added 1.4 - 2.7% into original molten metals before treatment ranging in sulphur content from 0.085% to 0.091%, there were obtained high values, 78 - 83%, of percent desulphurization.
Description will be directed below to a molten metal additive for a metal having a high nickel content.
Ni-resist spheroidal graphite cast iron is one of high alloy cast irons, and heretofore nickel magnesium -ferrosilicon alloy has mainly been used for the treatment of such Ni-resist spheroidal graphite cast iron. The reason is that Ni-resist cast iron contains not less than about 15% of nickel and that therefore by incorporating nickel in the additive it is made possible to increase the density of the additive without exerting any influence on the molten metal components and thereby possible to enhance the treatment efficiency.
In the case of stainless steel, it is necessary that the carbon content be kept not higher than 0.03% to maintain the resistance to corrosion. Therefore, it is necessary to remove carbon which is incorporated at the time of production. Usually, such carbon is removed by introducing oxygen in the course of melting, but the supply of excess oxygen will result in increase in the content of oxygen of molten metal, thus causing gas defect or inclusion defect. In this connection, by using a molten metal additive according to the present invention capable of being diffused uniformly in molten metal and reacting efficiently, it becomes possible to effect efficient decarburization.
When magnesium was covered with stainless steel pipe defined by JIS G-3459 Schedule 40, then formed into a desired shape, and Ni-resist spheroidal graphite cast iron was produced by using the additive thus produced, the reaction was mild, with no floating of the additive, and there were attained the same mechanical properties as in the prior art.
When iron oxide (Fe₂O₃) was covered with stainless steel pipe defined by JIS G-3459 Schedule 40, then formed into a desired shape, to afford a decarburizing additive, and molten stainless steel (SCS13) was treated using the decarburizing additive thus produced, it was possible to reduce the carbon content of the original molten metal before treatment from 0.05% to 0.02%. The density of the molten metal additive was 8.1 g/cm³, which was almost equal to that of the original molten metal, about 8.0 g/cm³, so that the floating of the additive was not observed. In this case, not only calcium but also various metal oxides are employable as decarburizing additives.
Further, if a reducing agent is used as the additive to be covered, it can be utilized for deoxidation.
The following description is now provided about a molten metal additive for aluminum alloy according to the present invention.
As one of the casting defects which pose problems in aluminum casting, mention is made of gas defect. The gas defect is caused by hydrogen gas. During the melting of metal, hydrogen gas is easily absorbed into molten metal from air, and in the case of a crucible furnace using heavy oil, the solubility of hydrogen reaches a maximum of 0.35 cc/100 g. Since the solubility of hydrogen at the time of solidification is as low as 0.036 cc/100 g, excess hydrogen is discharged during soldification and forms air bubbles as gas defect. For the removal of hydrogen, an inert gas or chlorine gas is introduced in the melting stage, or a degassing additive (flux) is used. It is generally said that gas defect occurs when the solubility of hydrogen in molten metal reaches 0.2 cc/100 g. Since flux comprises a chloride of potassium or sodium and a fluorine compound and is highly hygroscopic to the extent of becoming a hydrogen source, so it is necessary to dry it before use.
When flux was covered with aluminum pipe of 3 mm in thickness defined by JIS A-1050, then formed into a desired shape, to produce a molten metal additive, and 20 kg of molten aluminum was treated with the molten metal additive and poured into a mold. The resulting product was a sound product free of gas defect. Since this molten metal additive is covered with aluminum, it is superior in bench light, which is also a characteristic feature.
As explained above in an illustrative manner, the molten metal additives according to the present invention are simple in structure and can be produced easily and yet exhibit excellent effect. The basic idea of the present invention resides in making the density of the additive as close as possible to the density of molten metal by covering it with a solid metal plate, preventing the floating of the additive by virtue of fusion between small pieces of the covering material, causing the additive to be melted gradually from the surface layer and allowing it to be diffused uniformly by the convention of molten metal. Optimum density differs depending on the purpose of treatment, but in the present invention it is possible to make adjustment to an optimum density by adjusting the thickness, size and shape of the metallic plate for covering. As to an overall density range, adjustment may be made preferably in the range of 1.5 to 8.2 g/cm³ in terms of apparent density of the additive alone.
In order to make the density of the additive as close as possible to the density of molten metal, it is preferable that limitation be made to only the constituent elements required for each treatment and that the proportion of a component low in density such as ferrosilicon (density: about 5 g/cm³) be made as small as possible.
Since the molten metal additive of the present invention has a solid metal plate as a covering layer, its bench life is ensured and the damage thereof during conveyance can be prevented; besides, it is easy to effect machining such as formation of a small hole to adjust the internal pressure.
The following examples are given to illustrate the present invention in more detail.

Example 1

A commercially available steel pipe having an outside diameter of 15.9 mm and a wall thickness of 2.9 mm, which is defined by JIS G-3461, was pressedflatwise so as to leave an inside space of 3 mm. Then, one end of the pipe was closed by pressing and a granular magnesium alloy ("Mg alloy" hereinafter) and ferrosilicon ("FeSi" hereinafter) for inoculation were charged into the inside space of the pipe thus pressed flatwise, followed by sealing, to afford a molten metal additive for spheroidizing. The weight and volume of this additive alone were 30 g and 4.3 cm³, respectively, and an apparent density threof was 7.04 g/cm³.
When the steel pipe was heated to about 1,000 K and both granular Mg alloy and FeSi were charged into the heated pipe, forming was easy and the apparent density of the molten metal additive increased. This is because of marked deterioration in strength and great increase in elongation of the covering steel pipe in a hot condition and also because Mg melts at about 920 K and hence the spacing between adjacent FeSi granules became narrower under the forming pressure.
8.3 kg of the molten metal additive was set in such a 200 kg ladle as shown in Fig.3, then molten metal of 1,773 K was poured into the ladle, and the floating of the molten metal additive and thermal behaviors thereof during treatment were checked. The marks a, b and c in Fig.4 represent positions of thermocouples, which are located at distances of 10, 30 and 60 mm, respectively, from the molten metal contact surface to observe thermal behaviors in the interior. In Fig.4, the numeral 9 represents 200 kg of the original molten metal before treatment, and numeral 10 represents the molten metal additive.
From the start of pouring of the molten metal until the lapse of 20 seconds when the pouring was completed, there was no floating of the molten metal additive. Thereafter, such a convection of molten metal as indicated at 8 in Fig.3 continued for about 25 seconds. The reaction was mild, with little MgO mist, and uniform diffusion of the additive was observed.
On the basis of the above results, floating of the molten metal additive, continuation time of the convection of molten metal, environment, Mg yield rate, and metallogical structure, were checked while varying the density of the additive. Table 1 shows the results of having treated 500 kg of molten metal.
As can be seen from the above table, in the case of using the molten metal additives according to the present invention, there is no floating if the density is not lower than 5.6, the reaction is mild, with little MgO mist, the Mg yield rate is as about 80%, and the number of graphite spheroids is about twice or more as large as that in the prior art.

Example 2

20.7 kg of a molten metal additive produced in the same way as in Example 1 and having a density of 7.0 g/cm³, an Mg content of 1.21% and an FeSi content of 4.7% was set in a 500 kg ladle, as shown in Fig.3, and molten metal of 1,773 K was poured into the ladle. Separately, as a conventional process, 7.6 kg of a commercially available ferrosilicon-magnesium ("Fe-Si-Mg" hereinafter) spheroidizing additive having an Mg content of 5% and 500 kg of an Fe-Si type inoculant were set in a 500 kg ladle and then covered with about 24 kg of a covering material, followed by pouring of molten metal. Table 2

C% Si% Mn% P% S%

3.72 1.89 0.16 0.042 0.015
According to the process of the present invention there was no floating of the molten metal additive during the pouring of molten metal, the reaction was mild, and the convection of molten metal continued for 58 seconds. Table 3 shows chemical components of products after treatment. In the process of the present invention, the Mg yield rate was as high as about 80%, mechanical properties were good, and the number of graphite spheroids in microstructure was not less than twice as large as that in the conventional process, thus indicating a dense structure.
From the above results it is seen that by using the molten metal additive for the spheroidizing of graphite according to the present invention, the amount of the additive used can be greatly decreased and the working environment is improved. Further, since the molten metal additive for graphite inoculation according to the present invention is diffused uniformly in the original molten metal before treatment, the graphite is rendered fine and the number of graphite spheroids increases to a great extent, resulting in that the variations (chilling and fading) of the metallogical structure caused by changes in the product wall thickness are avoided and hence mechanical properties are improved.

Example 3

Using a commercially available steel pipe defined by JIS G-3461 and having an outside diameter of 15.9 mm and a wall thickness of 2.9 mm, a molten metal additive for compacted vermicular graphite cast iron ("CV graphite cast iron" hereinafter) having the shape shown in Fig.1 was produced in the same way as in Example 1. 4.0 kg of this molten metal additive (Mg content: 1.3%, density: 7.1 g/cm³), as well as 2 kg of a commercially available pheroidizing additive (Fe-Si, Ca, Mg, Ti) for CV and 6 kg of a covering material as a conventional process, were each added into 200 kg of molten metal in accordance with the deposit additive process.
By using the molten metal additive produced according to the present invention, the prevention of floating of the additive and the diffusion thereof in the molten metal were effected to a satisfactory extent. As a result, both chemical components and mechanical properties of the treated product were equal to those in the conventional process, the Mg yield rate was about 80%, and even in a small amount of the additive added to such an unstable material as CV graphite cast iron there could be obtained stable metallogical structure and mechanical properties.

Example 4

Using commerically available Fe-Si, Ca-Si and Si-C inoculants and in the same way as in Example 1, molten metal additives for inoculation were produced so as to be 7.0, 6.6 and 6.4 g/cm³, respectively, in density. Then, the molten metal additives were each set in a ladle and 500 kg of molten metal was poured into the ladle at 1,773 K. The compositions of the molten metal additives for inoculation and the amounts thereof added to 500 kg of molten metal are as set forth in Table 5.
In the case of Ca-Si and Si-C inoculants, it is necessary to increase the proportion of the covering material because they are lower in density than the Fe-Si inoculant. However, in a chill test for checking the effect of inoculation, the use of each additive according to the present invention, in an amount of about 60% of that in a conventional process, could afford an equal chill depth to that in the conventional process. It turns out that the inoculants according to the present invention were dissolved little by little into the molten metal and were diffused uniformly by virtue of convection of the molten and that a small amount thereof attained durable inoculation.

Example 5

Molten metal additives for the desulphurization of molten iron were produced using commercially available soda ash, carbide and fluorite and in accordance with the method described in Example 1, then were each set in a 200 kg ladle, followed by pouring of molten iron. The density of each molten metal additive was adjusted to 6.5 or 6.6; as a result, the desulphurizer contents of the additives varied in the range of 12.5 to 16.5%. This is because of different densities of the desulphurizers.
The molten metal additives for desulphurization were each added into molten iron from cupola in an amount capable of desulphurizing the whole of sulphur contained in the molten iron, the molten iron containing 3.6% C, 2.45% Si, 0.5% Mn, 0.55% P and 0.085-0.91% S, and the effect of desulfurization was checked. Separately, as a conventional process, the same amount of carbide as in the process of the present invention was used and nitrogen was introduced through a porous plug at the ladle bottom, allowing desulphurization to proceed with stirring. The results obtained are as set forth in Table 6.
The molten metal additives for desulfurization according to the present invention did not float, permitted continuous stirring of the molten metal, generated only a small or trace amount of dust, and each afforded a high desulfurization efficiency of about 80%. When 200 kg of molten iron was treated at 1,753 K with the additive containing carbide, there was not observed any drop of the molten iron temperature by reason of exothermic reaction with the molten iron.

Example 6

In the same way as in Example 1, Mg and FeSi for inoculation were covered with stainless steel pipe defined by JIS G-3463, followed by forming into the shape shown in Fig.1. The thus-covered additive uses stainless steel in place of iron as a covering material for the spheroidizing of Ni-resist spheroidal graphite cast iron of a high nickel (Ni) content. Since the density of Ni is high, it becomes possible for the additive to be high in density. The composition of the molten metal additive used in this Examples is as shown in Table 7, from which it is seen that the density of the additive is as high as 7.4 g/cm³. 3,040g of the additive each piece of which is 29.5 g (Mg content: 1.15%) was added into 50 kg of the original molten metal before treatment and reaction was allowed to proceed at 1.773 K. Floating of the additive did not occur and stirring of the molten metal was maintained. The reaction was mild.
Table 8 shows chemical components and mechanical properties of the product obtained.
The Ni % of the original molten metal before treatment was set low by 0.5% in advance; as a result, such appropriate chemical components and mechanical properties as shown in Fig.8 were attained and there was obtained a metallogical structure of Ni-resist spheroidal graphite cast iron.

Example 7

The stainless steel covering material used in Example 6 can also be utilized in the decarburization of stainless cast steel. In the same manner as in Example 1, iron oxide (Fe₂O₃) was covered with stainless steel pipe defined by JIS G-3459 Schedule 40 to afford a molten metal additive for the decarburization of stainless cast steel. Then, at 1,853 K the molten metal additive thus produced was added into 50 kg of molten stainless steel and reacted. The relation between density and floating of this molten metal additive, as well as the effect thereon in decarburization, are shown in Table 9.
It turns out that in the case of using this molten metal additive, floating is prevented if the density thereof is not lower than 6.4 g/cm³ and that the higher the density of the molten metal additive, the higher the decarburization efficiency.

1.015 g of a molten metal additive covered with stainless steel plate and having a density of 8.1 g/cm³ was added into and reacted with 50 kg of molten stainless steel at 1,583 K. Then, decarburization efficiency was checked. Tables 10 and 11 show properties of this molten metal additive and chemical components of product.

Table 10

Composition of Molten Metal Additive			Amount of Additive added to 50 kg Molten Metal
Density g/cm³	Fe₂O₃ %	Covering Material %	Fe₂O₃	Covering Material	Total
8.1	8.4	91.6	85g	930g	1015g

Table 11

	C %	Si%	Mn%	P %	S %	Ni%	Cr%
Before treatment	0.05	0.76	1.12	0.03	0.008	8.39	18.32
After treatment	0.02	0.64	1.05	0.04	0.009	8.54	18.67

Example 8

In the same way as in Example 1 a commercially available aluminum (A1) degassing flux was covered with A1 pipe defined by JIS A-1050 and having a wall thickness of 3 mm to afford a molten A1 additive for degassing. Then, using this additive and 20 kg of molten A1, the relation between density and floating of the additive was checked.
The floating of this additive is prevented if the density thereof is not lower than 1.5 g/cm³. The results obtained are as set out in Table 12. Table 12

Composition of Molten Metal Additive Floating

Density g/cm³ Flux % Covering Material %

1 1.2 92 8 yes

2 1.5 86 14 slight

3 2.1 38 62 no

4 2.3 19 81 no

5 2.5 9 91 no

In view of the results of the floating test, 610g of the molten metal additive having a density of 2.3 g/cm³ and a flux content of 9.8% was added into and reacted with 20 kg of molten A1 to check floating and the quality of product. A potato had been added beforehand into the molten A1 before treatment to maximize the hydrogen content of the molten metal. Floating of this molten metal additive was not observed. In the case of product obtained by pouring the molten metal before treatment into a mold, there occured gas defect, while a product obtained by pouring the molten metal after treatment with the said molten metal additive into a mold, was a sound product. The results obtained are as set out in Table 13.

Table 13

Composition of Molten Metal Additive			Treatment of 20 kg Molten Metal
Density g/cm³	Flux %	Covering Material %	Original Molten Metal	Amount of Additive used	Floating	Quality of Product
2.3	9.8	90.2	with a potato added	60 g	no	free of gas defect

As set forth hereinabove, the molten metal additive according to the present invention possesses the following features.

(1) The density of the additive which is granular or massive can be made very close to the density of molten metal to be treated, whereby the floating of the additive during treatment can be prevented surely.
The density of the conventional spheroidizing additive for spheroidal graphite cast iron is 4.5 g/cm³ or so, and even if the additive, which is powdery, is compressed and sintered with a view to increasing density, it is difficult to attain a density of 6 g/cm³ or higher, so in molten iron having a density of 7.2 g/cm³ or so, there will occur floating of the additive. To avoid this, it is necessary to use a conventional covering material or special equipment.
(2) In the case of the molten metal additive according to the present invention, the solid metal plate as a covering material takes off the melting heat to lower the ambient temperature during treatment, whereby the covering metallic pieces are fused together to assist the prevention of floating. For this reason, the molten metal additive is difficult to float even if its density is low.
(3) After treatment, the molten metal additive according to the present invention melts gradually from the upper surface and induces convection of molten metal, so that the inoculation time of the additive is long and the diffusion thereof is uniform. Consequently, the resulting product has a very fine metallogical structure and hence the material characteristics are improved.
(4) In the molten metal additive according to the present invention there can be incorporated a minimum amount of additive required for each treatment, so the proportion of impurities is low and the occurrence of defects, e.g. inclusion defect, decreases.
(5) The molten metal additive according to the present invention is applicable not only to the spheroidizing of graphite but also to other treatments whose improvement has so far been difficult such as desulphurization, inoculation, decarburization, and Al degassing.
(6) Since the molten metal additive according to the present invention is covered and protected with a strong metallic plate, it is possible to prevent damage of the additive during conveyance, and its bench life against oxidation, moisture absorption, etc. can be ensured.
(7) By using the molten metal additive according to the present invention it is possible to improve the working environment involving the generation of MgO mist and dust, and it becomes easy to effect cleaning and storage of a ladle.
(8) Since the molten metal additive according to the present invention is easily portable, it is possible to select an arbitrary position of a ladle for treatment.
(9) Since the molten metal additive according to the present invention can be produced in a simple manner, the manufacturing cost is low.

Claims

A molten metal additive comprising small pieces each being obtained by covering an additive with a solid metal wall containing, as a main component iron, nickel or aluminum.
A molten metal additive for use in the treatment of molten metal in accordance with a deposit additive process involving placing the additive beforehand at the bottom of a ladle and subsequent pouring of molten metal for casting into the ladle, characterized in that said molten metal additive is in the form of small pieces, said small pieces being each obtained by covering an additive necessary for treating the molten metal for casting with a solid metal wall containing iron, nickel or aluminum as a main component.
A molten metal additive as set forth in Claim 1 or 2, having an apparent density in the range of 1.5 to 8.2 g/cm³.
A molten metal additive as set forth in any one of Claims 1 to 3, comprising an additive covered with a solid metal plate whose main component is iron and having an apparent density in the range of 4.5 to 7.6 g/cm³, said additive being necessary for any of such treatments as desulphurization of molten metal containing iron as a main component, vermicularization, graphite spheroidizing, inoculation for the promotion of graphitization and strengthening of matrix, and deoxidation or decarburization of cast steel.
A molten metal additive as set forth in any one of Claims 1 to 3, comprising an additive necessary for the treatments of molten metal containing iron as a main component and containing nickel, said additive being covered with a solid metal plate containing nickel as a main component, and said additive having an apparent density in the range of 4.5 to 8.2 g/cm³.
A molten metal additive as set forth in any one of Claims 1 to 3, comprising an additive necessary for the treatment of molten metal containing aluminum as a main component, said additive being covered with a solid metal plate containing aluminum as a main component, and said additive having an apparent density in the range of 1.5 to 2.5 g/cm³.
A molten metal additive as set forth in any one of Claims 1 to 6, wherein the covering layer is formed by a metal integrally or by a metallic pipe or plate in the shape of an enclosure having an additive in the interior thereof, said enclosure being formed by machining, forging, welding, using an adhesive or a retainer means, or spark bonding.
A process for producing a molten metal additive, which process comprises charging an additive for use in the treatment of molten metal for casting into a hollow metallic pipe whose main component is iron, nickel or aluminum, by suction under reduced pressure, then cutting the pipe thus charged with the additive into small pieces, and if desired, sealing the cut portions.
The process of Claim 8, wherein the additive to be charged into the hollow metallic pipe is in the form of powder.
The process of Claim 8, wherein the additive to be charged into the hollow metallic tube is in a melted state.