Method for producing spheroidal graphite cast iron

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

[0001] The invention relates to a process for producing nodular cast iron, at least comprising the process steps of pretreatment with pure magnesium in a converter and an addition of inoculant during casting into the mold.

[0002] U.S. Pat. No. 5,098,651 has disclosed a process and a device for producing nodular cast iron. In a converter, a treatment vessel for the molten cast iron, pure magnesium is introduced in a vessel compartment which is accessible from the outside and is in contact with the remainder of the treatment vessel via openings in the partition between the vessel compartment and the remaining treatment vessel. When the converter is moved from a horizontal position, which has been adopted in order to introduce the magnesium, into a vertical position, the magnesium is wetted by the molten material and dissolved, generating reaction heat. The reaction with the pure magnesium acts in the following way: inorganic compounds, comprising oxides, sulfates, silicates and other compounds with magnesium, are formed from the nonmetallic constituents of the melt. The melt is set in intensive motion by the vapor pressure of the magnesium. The mineral compounds coagulate, agglomerate and float, on account of the lower relative density, on the bath surface. The slags which are formed in this way can be removed from the bath surface by skimming. This process is distinguished, inter alia, by a bath composition with a high purity of, for example, less than 10 ppm of dissolved oxygen and a low concentration of nucleating agent of, for example, less than 100 ppm of nonmetallic particles.

[0003] WO 99/45156 A1 describes a process in which, before the actual treatment with magnesium, the molten cast iron is treated in successive steps with calcium carbide for removal of oxygen and sulfur, with rolling scale to supply oxygen and with aluminum oxide or with calcium oxide to form calcium aluminates or calcium aluminum silicates. The result of this is that following the subsequent treatment with magnesium, the cast iron includes nonmetallic inclusions which are in a plastic condition during the material-removing machining and therefore serve as lubricants. Excessive quantities of inoculant are often added in the subsequent process steps. Then, during casting into the mold, there is a risk of the quality being adversely affected because the reaction products lead to uncontrollable defects, such as slags, oxides and undissolved inoculant particles. Uncontrollable conditions with regard to the nucleation state arise. Because the nucleation state is not accurately known, it is impossible to accurately determine the metering of the inoculants. Lack of certainty about the nucleation state often leads to excessive quantities being metered in. This is expensive and is not advantageous for the quality of the cast iron.

[0004] Working on the basis of this prior art, it is an object of the invention to provide a process for producing nodular cast iron which at least comprises the process steps of pretreatment with pure magnesium in a converter and addition of inoculant during casting into the mold, with the minimum possible quantity of unnecessary oxides, slags or inoculant residues being introduced into the melt and into the casting.

[0005] This object is achieved by a process for producing nodular cast iron, at least comprising the process steps of pretreatment with pure magnesium in a converter and an addition of inoculant during casting into the mold, in which process the solubility of the substances formed in the pretreatment and inoculation is adjustable.

[0006] Preferred refinements of the invention will emerge from the dependent claims.

[0007] It is advantageous for the minimum possible amount of impurities to be introduced in the cast iron for production of nodular cast iron. This is achieved by the fact that the pretreatment is carried out using a mixture or a compound of pure metallic magnesium with one or more metals from groups IIa and IIIa of the periodic system and with sulfur. This is also achieved by the fact that the pretreatment is carried out using a mixture or a compound of pure metallic magnesium with sulfur and with Ca, Ba, La or rare earths, the mixture or compound forming oxysulfides in the molten metal.

[0008] It is also advantageous for the total quantity of substances added to be kept as low as possible for the production of nodular cast iron. This is achieved by the fact that the quantity of an inoculant based on FeSi or CaSi which is added during casting into the mold is significantly lower, preferably four times lower, than the amount of the compounds or mixtures which form oxysulfides which is added in the pretreatment with magnesium.

[0009] The process proposed here is based on 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 which is produced in the magnesium converter, since casting defects caused by oxides and slags and further defects caused by inoculants are avoided. In existing foundries with a magnesium converter, the process can be implemented easily and without the need to add additional process steps. The addition of the compound or mixture which forms oxysulfides with the pure metallic magnesium results, in the pretreatment, both in purification of the melt and in preconditioning of the nucleation state in a single process step. The chemical oxygen and sulfur activity in the melt can be set as accurately as possible, while the smallest possible number of foreign bodies per unit weight of cast iron needs to be added. Since the oxysulfides which are formed have a relatively poor solubility in the cast iron melt, the nuclei which are formed by reaction with the melt after the addition of the compound or mixture which forms oxysulfides have a sufficiently long life in the melt. It is possible to talk of a high nucleation capacity of the oxysulfides. The formation of an oxysulfide in the pretreatment has shown that this has the least possible adverse effect on the final purity of the cast iron alloy and, at the same time, has a positive influence on the solubility of the nucleating substances.

[0010] The addition of the compound or mixture which forms oxysulfides in the pretreatment leads to preconditioning of the melt, i.e. optimum preparation of the melt for solidification in the mold. This optimum preparation significantly reduces the demand for inoculant during casting. The amount and nature of the compound or mixture which is introduced during the pretreatment in the magnesium converter can be selected in such a manner that it may even be possible to dispense with the process step of adding inoculants during casting in the mold. The low level of foreign substances added has the minimum possible adverse effect on the purity of the cast iron melt. Furthermore, the use of oxysulfides has the following advantages: the relative density deviates only slightly from the relative density of the melt, so that it is easy to achieve a homogenous mixture. The grain size and grain shape can be selected in such a manner that the nuclei are retained for a longer time. Elements from groups IIa and IIIa of the periodic system, for example Mg, Ca, Ba, La or rare earths, are advantageously selected as addition to the magnesium in the pretreatment.

[0011] The subsequent casting from the casting furnace into the mold only requires a small amount of inoculant to be added in order to definitively set the state of nucleation. Since the oxysulfides which have been formed in the pretreatment have a higher nucleation capacity than the inoculants which have been known hitherto, a small quantity, i.e. at most a quarter of the quantity which was added in the pretreatment, is sufficient to control the nucleation in the mold. The inoculant which is added during casting into the mold can be used to control and definitively set the rate, crystal shape and crystal distribution during solidification. The inoculant used during casting is, for example, an alloy or compound which is based on FeSi or CaSi with Ca, Ba, Mg, Au, Ge, La or rare earths.

[0012] The pretreatment with compounds or mixtures which form oxysulfides in combination with the magnesium treatment in the converter, on the one hand, and the reduced addition of inoculant during casting, on the other hand, has the following further advantages: since only small amounts of foreign bodies have been introduced, considerably less slag is formed than in the conventional process, and consequently less iron is lost with the slag. Since overall less inoculant is fed to the cast iron melt, fewer foreign bodies are included between the crystals during solidification. As a result, the likelihood of casting defects being formed is reduced. The fact that there are fewer inclusions of magnesium, silicon or oxygen compounds has a beneficial influence on the microporosity of the cast iron and promotes uniform solidification in the mold. A lower microporosity is also advantageous for the interaction of the molten material 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 is cleaner and more uniform, i.e. there are fewer surface defects. This makes the surface of the cast-iron product easier to machine.

[0013] FIGS. 1 and 2 show a microscopic image of a nodular graphite particle with an undissolved core of an inoculant inside it. FIG. 2 shows an enlarged view of the central area of the particle from FIG. 1. The nodular graphite particle was obtained from a melt which has been inoculated with a compound of iron, silicon and lanthanum. The analysis of the core of the nodular graphite particle showed the following composition: 41.7% of La, 47.2% of S, 9.1% of Mg and 1.7% of Si. This shows that the poor solubility of the compounds or mixtures which are supplied in small quantities and form oxysulfides has a beneficial influence on the formation of nodular graphite particles and on the casting properties.

[0014] The nodular cast iron products which are produced using this process are distinguished by a high level of purity combined with relatively low production costs. Less slag has to be disposed of or recycled, and the amount of inoculants metered in can be considerably reduced. The outlay on measuring and controlling the casting process, in particular the outlay on equipment for analysis of the melt, can be reduced considerably because the process is more stable and more reproducible. The quantitative ratio of the foreign bodies added can be modified between pretreatment and inoculant. The matching between pretreatment and addition of inoculant allows both the nucleation in the melt and the solidification in the mold to be adapted to one another. An appropriate selection of the nature and amounts of the foreign bodies means that under certain circumstances it is possible to dispense with the step of adding inoculant during casting into the mold. Nodular cast iron (SG iron) is distinguished by mechanical properties which are very close to the mechanical properties of cast steel.

Claims

1. A process for producing nodular cast iron, at least comprising the process steps of pretreatment with pure magnesium in a converter and an addition of inoculant during casting into the mold, characterized in that the solubility of the substances formed in the pretreatment and in the addition of inoculant is adjustable.

2. The process for producing nodular cast iron as claimed in claim 1, characterized in that the pretreatment is carried out using a mixture or a compound of pure metallic magnesium with one or more metals from groups IIa and IIIa of the periodic system and with sulfur.

3. The process for producing nodular cast iron as claimed in claim 1 or 2, characterized in that the quantity of an inoculant based on FeSi or CaSi which is added during casting into the mold is significantly lower, preferably four times lower, than the amount of the compounds or mixtures which form oxysulfides which is added in the pretreatment with magnesium.

4. The process for producing nodular cast iron as claimed in one of claims 1 to 3, characterized in that the pretreatment is carried out using a mixture or a compound of pure metallic magnesium with sulfur and with Ca, Ba, La or rare earths, the mixture or compound forming oxysulfides by reacting with the molten metal.

5. The process for producing nodular cast iron as claimed in one of claims 1 to 4, characterized in that during the subsequent addition of inoculant, the inoculant based on FeSi or CaSi contains Mg, Ca, Ba, Au, Ge, Bi, La or rare earths.

6. The process for producing nodular cast iron as claimed in one of claims 1 to 5, characterized in that the oxysulfides formed have a grain size of 0.001 to 0.03 mm.

7. The process for producing nodular cast iron as claimed in one of claims 1 to 6, characterized in that the oxysulfides formed have a relative density of 4000 to 6000 kg/m3.

8. The process for producing nodular cast iron as claimed in one of claims 1 to 7, characterized in that the oxysulfides formed have a melting point of 1250 to 1550° C.

9. The process for producing nodular cast iron as claimed in one of claims 1 to 8, characterized in that the specific number of nuclei of the oxysulfides formed is between 60 and 1200 mm−2.

10. The process for producing nodular cast iron as claimed in one of claims 1 to 9, characterized in that the compounds or mixtures used for the pretreatment are introduced into the molten material in the converter separately from the pure magnesium, joined or alloyed to the magnesium or embedded in the magnesium as a wire, as granules or in the form of a sandwich.

11. The process for producing nodular cast iron as claimed in one of claims 1 to 10, characterized in that the amount and type of inoculants introduced during casting into the mold are matched to the amount and type of the compounds or mixtures with magnesium and sulfur forming oxysulfides which are introduced during the pretreatment in the converter.

12. The process for producing nodular cast iron as claimed in one of claims 1 to 11, characterized in that the amount and type of the compounds or mixtures introduced during the pretreatment in the converter are selected in such a manner that there is no need to add inoculant during casting into the mold.