EP1283913B1 - Method for producing spheroidal graphite cast iron - Google Patents

Abstract

A method for producing high-purity spheroidal graphite cast iron. The method comprises at least the steps of pretreating the melt in a closed converter and adding an inocculant when casting it into the mold. The solubility of the substances formed during the pretreatment and during the addition of the inocculant an be adjusted. The nucleation in the melt and the solidification in the mold can be brought into conformity by coordinating the pretreatment and the addition of inocculant.

Description

The invention relates to a method for producing spheroidal graphite cast iron comprising at least the method steps pretreatment with pure magnesium in a converter and Impfmittelzugabe when poured into the mold.

From US 5098651 a method and an apparatus for the production of nodular cast iron are known. In a converter, a treatment vessel for the molten cast iron, pure magnesium is introduced into a vessel compartment which is accessible from the outside and which is in contact via openings in the dividing wall between the vessel compartment and the remaining treatment vessel. When the converter is placed in a vertical position from a horizontal position taken to introduce the magnesium, the magnesium is wet by the melt and dissolved to generate heat of reaction. The reaction with the pure magnesium causes the following: From the non-metallic constituents of the melt arise mineral compounds consisting of oxides, sulfates, silicates and other compounds with magnesium. Due to the vapor pressure of magnesium, the melt is put into intensive motion. The mineral compounds coagulate, agglomerate and float due to the lower specific gravity at the bath surface. From the bath surface, the slags thus formed can be removed by skimming. This method is characterized, inter alia, by a bath composition having a high purity of, for example, less than 10 ppm of dissolved oxygen and a low nucleating agent concentration of, for example, less than 100 ppm of non-metallic particles.

In WO 99/45156 A1, a method is given in which prior to the actual treatment with magnesium, the molten cast iron in successive steps with calcium carbide to remove oxygen and Sulfur is treated with mill scale for supplying oxygen and with alumina or with calcium oxide to form calcium aluminates or calcium aluminum silicates. This ensures that after the subsequent treatment with magnesium, the cast iron has non-metallic inclusions, which are plastic in the machining and thus serve as a lubricant. In the subsequent process steps too large amounts of inoculant are often added. When poured into the mold then there is a risk that the quality is impaired because the reaction products lead to uncontrollable errors, such as slags, oxides and unresolved Impfmittelpartikeln. There are uncontrollable conditions with respect to the nucleation state. Because the nucleation state is not well known, the dosage of inoculants can not be determined accurately. In ignorance of the nucleation state is often overdosed. This is expensive and not advantageous for the quality of the cast iron.

Based on this prior art, it is an object of the invention to provide a method for producing nodular cast iron, which comprises at least the steps pre-treatment with pure magnesium in a converter and Impfmittelzugabe when poured into the mold, with as little unnecessary oxides, slags or Inoculant residues are introduced into the melt and into the casting.

This object is achieved by a method for producing spheroidal graphite cast iron comprising at least the steps of pretreatment with pure magnesium in a converter and Impfmittelzugabe when poured into the mold, wherein the solubility of the substances formed in the pretreatment and in the vaccine is adjustable, and the Pretreatment with a mixture or compound of pure metallic magnesium is carried out with only one or more metals from groups IIa and IIIa of the periodic table and with sulfur.

US-A-2,837,422 discloses an additive for the treatment of cast iron smelter for the production of nodular cast iron. The additive contains 30-50% Mg, 2.8-10% Ca, at least 35% Si and up to 30% Fe.

Preferred developments of the invention will become apparent from the dependent claims.

It is advantageous that as far as possible no impurities are introduced into the cast iron for the production of nodular cast iron. This is achieved by carrying out the pretreatment with a mixture or compound of pure metallic magnesium with one or more metals from groups IIa and IIIa of the periodic table and with sulfur. This is also accomplished by carrying out the pretreatment with a mixture or compound of pure metallic magnesium with sulfur and with Ba, Ca, La or rare earths, the mixture or compound forming oxysulfides in the molten metal.

It is also advantageous that for the production of nodular cast iron, the total amount of added substances is kept as low as possible. This is accomplished by making the amount of an FeSi or CaSi based inoculant added to the mold upon pouring substantially less, preferably four times smaller than the amount of oxysulfide-forming compound or mixture used in the pretreatment with magnesium.

The method proposed here starts from the known pretreatment with pure metallic magnesium in the closed converter. The process leads to an improvement in the quality of the nodular cast iron produced in the magnesium converter, because casting defects by oxides and slags and other defects caused by inoculants are avoided. In existing foundries with the magnesium converter, the process can be applied easily and without the intervention of additional process steps. By adding the oxysulfide-forming compound or mixture with the pure metallic magnesium, both the purification of the melt and the preconditioning of the nucleation state in a single process step are achieved in the pretreatment. The chemical oxygen and sulfur activity in the melt is adjusted as accurately as possible, while as little foreign bodies per unit weight of cast iron must be added. Because the formed Oxysulfide a relatively poor solubility in the Having cast iron melt, the nuclei formed after the addition of the oxysulfide-forming compound or mixture by the reaction with the melt have a sufficiently long life in the melt. It can be said of a high nucleation capacity of Oxysulfiden. The formation of an oxysulfide in the pretreatment has shown that this least adversely affects the final purity of the cast iron alloy while positively affecting the solubility of the nucleating substances.

By adding the oxysulfide-forming compound or mixture in the pretreatment, the melt is preconditioned, i. optimally prepared for solidification in the mold. This optimum preparation substantially reduces the need for inoculant during pouring. The amount and type of compound or mixture introduced in the pretreatment in the magnesium converter can be chosen such that even the process step of adding imbibants during casting in the mold can be dispensed with. Due to the small addition of foreign substances, the purity of the molten cast iron is influenced as little as possible negatively. The use of oxysulphides also has the following advantages: The specific weight differs only slightly from the specific weight of the melt, so that a homogeneous mixing can be achieved easily. The grain size and the grain shape are selectable so that the germs are retained over a longer time. As an addition to the magnesium in the pretreatment, it is advantageous to use elements from groups IIa and IIIa of the periodic table, for example Mg, Ca, Ba, La or rare earths.

During the subsequent pouring from the casting furnace into the mold, only a small addition of inoculating agent is required for the final adjustment of the nucleation state. Because the oxysulfides formed in the pretreatment have a higher nucleation capacity than the heretofore known inoculants, a small amount, ie at most a quarter, is sufficient from the amount added in the pretreatment to control nucleation in the mold. With the inoculant, which is added when poured into the mold, the speed, crystal form and crystal distribution during solidification can be controlled and finally adjusted. For example, an FeSi or CaSi-based alloy or compound with Ca, Ba, Mg, Au, Ge, La or rare earths is used as the inoculant during casting.

The pretreatment with oxysulfide-forming compounds or mixtures combined with the magnesium treatment in the converter on the one hand and the reduced addition of inoculant during pouring on the other hand has the following further advantages: Because few foreign bodies are introduced, considerably less slags are formed than in the conventional method, thus less iron is used with the slag lost. Because less gum is supplied to the molten cast iron altogether, less foreign matter is trapped between the crystals during solidification. This reduces the tendency for casting defects. Less inclusions of magnesium, silicon or oxygen compounds have a favorable influence on the microporosity of the cast iron and promote a uniform solidification in the mold. A lower microporosity is also advantageous for the interaction of the melt, with the molding material mixture or with the sand from which the mold is formed. As a result, the surface of the cast iron product becomes cleaner and more uniform, d. H. There are fewer surface defects. This makes the workability of the surface of the cast iron product easier.

Figures 1 and 2 show a micrograph of a nodular graphite particle having therein an undissolved core of an inoculant. Figure 2 shows an enlargement from the central region of the particle of Figure 1. The nodular graphite particle was obtained from a melt which was seeded with a compound of iron, silicon and lanthanum. The analysis of the core of the ductile iron particle showed the following composition: 41.7% La, 47.2% S, 9.1% Mg and 1.7% Si. It is shown that the poor solubility of the low added compounds or mixtures forming oxysulfides has a favorable influence on the formation of nodular graphite particles and on the casting properties.

The spheroidal graphite cast iron products produced by this process are characterized by high purity at relatively low production costs. Fewer slags need to be disposed of or recycled, and the dosage of inoculants can be made significantly smaller. The effort for measurement and control of the casting process, in particular the apparatus required for the analysis of the melt, because the process is more stable and better reproducible, can be significantly reduced. The amount ratio of the added foreign bodies can be adjusted between pretreatment and inoculant. By mutual adjustment between pretreatment and Impfmittelzugabe both the nucleation in the melt and the solidification in the mold can be matched. By the right choice of the type and the quantities of foreign bodies, it may be possible to dispense with an addition during casting into the mold. Ballistic graphite cast iron (GJS) is characterized by mechanical properties that are very close to the mechanical properties of cast steel.

Claims (11)

1. A process for the production of spheroidal graphite cast iron comprising at least the process steps of pretreatment with pure magnesium in a converter and addition of an inoculant during mould filling, wherein the solubility of the substances formed during pretreatment and addition of inoculant is adjustable, characterised in that pretreatment is performed with a mixture or a compound of pure metallic magnesium only with one or more metals from groups IIa and IIIa of the periodic system of elements and with sulfur.
2. A process for the production of spheroidal graphite cast iron according to claim 1, characterised in that the quantity of an inoculant based on FeSi or CaSi which is added during mould filling is substantially smaller, preferably four times smaller than the quantity of oxysulfide-forming compounds or mixtures used in the pretreatment with magnesium.
3. A process for the production of spheroidal graphite cast iron according to claim 1 or claim 2, characterised in that the pretreatment is performed with a mixture or a compound of pure metallic magnesium with sulfur and with Ca, Ba, La or rare earths, wherein the mixture or the compound forms oxysulfides by reaction with the metal melt.
4. A process for the production of spheroidal graphite cast iron according to any one of claims 1 to 3, characterised in that, in the subsequent addition of inoculant, the inoculant based on FeSi or CaSi contains Mg, Ca, Ba, Au, Ge, Bi, La or rare earths.
5. A process for the production of spheroidal graphite cast iron according to any one of claims 1 to 4, characterised in that the oxysulfides formed have a grain size of 0.001 to 0.03 mm.
6. A process for the production of spheroidal graphite cast iron according to any one of claims 1 to 5, characterised in that the oxysulfides formed have a specific weight of 4000 to 6000 kg/m³.
7. A process for the production of spheroidal graphite cast iron according to any one of claims 1 to 6, characterised in that the oxysulfides formed have a melting point of 1250 to 1550°C.
8. A process for the production of spheroidal graphite cast iron according to any one of claims 1 to 7, characterised in that the specific number of nuclei of the oxysulfides formed is between 60 and 1200 mm^-2.
9. A process for the production of spheroidal graphite cast iron according to any one of claims 1 to 8, characterised in that the compounds or mixtures used for pretreatment are introduced into the converter separately from the pure magnesium, as a compound or alloy with the magnesium or as wire, as pellets or in the form of a sandwich embedded in the magnesium.
10. A process for the production of spheroidal graphite cast iron according to any one of claims 1 to 9,
    characterised in that the quantity and nature of the inoculants introduced during mould filling are adjusted to the quantity and nature of the oxysulfide-forming compounds or mixtures with magnesium and sulfur introduced into the converter during pretreatment.
11. A process for the production of spheroidal graphite cast iron according to any one of claims 1 to 10, characterised in that the quantity and the nature of the compounds or mixtures introduced into the converter during pretreatment are selected in such a manner that it is possible to dispense with any addition of inoculant during mould filling.

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