Cast iron inoculant and method for production of cast iron inoculant

Abstract

An inoculant for the manufacture of cast iron with lamellar, compacted or spheroidal graphite is disclosed. The inoculant has a particulate ferrosilicon alloy having about 40 to 80 wt % silicon, about 0.1 to 10 wt % calcium, 0 and 10% by weight of rare earths, for example cerium and/or lanthanum, and up to 5 wt % aluminium the balance being iron and incidental impurities in the ordinary amount, wherein the inoculant additionally has 0.1 to 10 wt %, based on the total weight of inoculant, antimony oxide where said antimony oxide is in particulate form and is mixed or blended with the ferrosilicon alloy particles, or is simultaneously added to cast iron together with the particulate ferrosilicon alloy particles.

Claims

1. An inoculant for the manufacture of cast iron with lamellar, compacted or spheroidal graphite wherein said inoculant comprises a particulate ferrosilicon alloy comprising between about 40 to 80 wt % silicon, between about 0.1 to 10 wt % calcium, between 0 and 10% by weight of rare earths, and up to 5 wt % aluminium the balance being iron and incidental impurities in the ordinary amount, wherein said inoculant additionally comprises 0.1 to 10 wt % Sb.sub.2O.sub.3, based on the total weight of inoculant, where said Sb.sub.2O.sub.3 is in particulate form.

2. Inoculant according to claim 1, wherein the ferrosilicon alloy comprises between 45 and 60% by weight of silicon.

3. Inoculant according to claim 1, wherein the ferrosilicon alloy comprises between 60 and 80% by weight of silicon.

4. Inoculant according to claim 1, wherein the ferrosilicon alloy comprises between 0.5 and 5% by weight of calcium.

5. Inoculant according to claim 1, wherein the ferrosilicon alloy comprises between 0.5 and 5% by weight aluminum.

6. Inoculant according to claim 1, wherein the ferrosilicon alloy comprises up to 6% by weight of rare earths.

7. Inoculant according to claim 1, wherein the inoculant comprises 0.2 to 5% by weight of particulate Sb.sub.2O.sub.3.

8. Inoculant according to claim 1, wherein the rare earths are cerium and/or lanthanum.

9. Inoculant according to claim 1, wherein the inoculant is in the form of a mixture or blend of the ferrosilicon alloy particles and the Sb.sub.2O.sub.3 particles.

10. Inoculant according to claim 1, wherein the inoculant is in the form of agglomerates made from a mixture of the particulate ferrosilicon alloy particles and the Sb.sub.2O.sub.3 particles.

11. Inoculant according to claim 1, wherein the inoculant is in the form of briquettes made from a mixture of the particulate ferrosilicon alloy particles and the Sb.sub.2O.sub.3 particles.

12. A method for producing an inoculant for the manufacture of cast iron with lamellar, compacted or spheroidal graphite, comprising: providing a particulate ferrosilicon alloy comprising 40 to 80 wt % silicon, between about 0.1 to 10 wt % calcium, between 0 and 10% by weight of rare earths, and up to 5 wt % aluminium the balance being iron and incidental impurities in the ordinary amount, mixing with said particulate base alloy 0.1 to 10 wt %, based on the total weight of inoculant, Sb.sub.2O.sub.3 particles to produce said inoculant.

13. The inoculant according to claim 1, wherein the rare earths are cerium and/or lanthanum.

14. The method according to claim 12, wherein the rare earths are cerium and/or lanthanum.

Description

DETAILED DESCRIPTION OF THE INVENTION

Description of Drawings

(1) FIG. 1 shows a test bar of iron casting,

(2) FIG. 2 is a diagram showing nodule number density in cast iron samples.

(3) FIG. 3a-b show SEM photos of an inoculant according to the present invention; FeSi coated with Sb.sub.2O.sub.3 powder.

(4) In the manufacturing process for producing cast iron with spheroidal graphite the cast iron melt is normally treated with a nodularizing agent, conventionally using an MgFeSi alloy, prior to the inoculation treatment. The nodularization treatment has the objective to change the form of the graphite from flake to nodule when it is precipitating and subsequently growing. The way this is done is by changing the interface energy of the interface graphite/melt. It is known that Mg and Ce are elements that change the interface energy, Mg being more effective than Ce. When Mg is added to a base iron melt, it will first react with oxygen and sulphur. It is only the “free magnesium” that will have a nodularizing effect. The nodularization reaction results in agitation, is violent and generates slag floating on the surface. The violence of the reaction will result in most of the nucleation sites for graphite that were already in the melt (introduced by the raw materials) and other inclusions being part of the slag on the top are removed. However some MgO and MgS inclusions produced during the nodularization treatment will still be in the melt. These inclusions are not good nucleation sites as such.

(5) The primary function of inoculation is to prevent carbide formation by introducing nucleation sites for graphite. In addition to introducing nucleation sites, the inoculation also transforms the MgO and MgS inclusions formed during the nodularization treatment into nucleation sites by adding a layer (with Ca, Ba or Sr) on the inclusions.

(6) In accordance with the present invention, the particulate FeSi base alloys should comprise from 40 to 80% by weight Si. The FeSi base alloy may be a high silicon alloy containing 60 to 80 wt %, e.g. 70 to 80 wt %, silicon or a low silicon alloy containing 45 to 60 wt %, e.g. 45-55 wt % silicon. The FeSi base alloy should have a particle size lying within the conventional range for inoculants, e.g. between 0.2 to 6 mm, e.g. 0.2 to 3 mm.

(7) In accordance with the invention, the particulate FeSi based alloy comprises between 0.5 and 10% by weight of Ca. Using a higher amount of Ca may reduce the performance of the inoculant, increase slag formation and increase the cost. Good inoculating performance is achieved also when the amount of Ca in the FeSi base alloy is about 0.5-6% by weight. Preferably the amount of Ca in the FeSi base alloy is about 0.5-5% by weight.

(8) The FeSi base alloy comprises up to 10% by weight of rare earths (RE). The RE may for example be Ce and/or La. In some embodiments the amount of RE should be up to 6% by weight. The amount of RE should preferably be at least 0.1% by weight. Preferably the RE is Ce and/or La.

(9) The Sb.sub.2O.sub.3 particles should have a small particle size, i.e. micron size, e.g. 10-150 μm, resulting in very quick melting and/or dissolution of the Sb.sub.2O.sub.3 particles when introduced in the cast iron melt. Advantageously, the Sb.sub.2O.sub.3 particles are mixed with the particulate FeSi base alloy prior to adding the inoculant into the cast iron melt. The FeSi particles are completely covered by the Sb.sub.2O.sub.3 particles, see FIG. 3. Mixing the Sb.sub.2O.sub.3 particles with the FeSi base alloy particles results in a stable, homogenous inoculant. It should however be noted that mixing and/or blending the Sb.sub.2O.sub.3 particles with the particulate FeSi base alloy is not mandatory for achieving the inoculating effect. The particulate FeSi base alloy and Sb.sub.2O.sub.3 particles may be added separately but simultaneously to the liquid cast iron.

(10) The addition of Sb.sub.2O.sub.3 particles together with FeSi base alloy particles, instead of alloying Sb with the FeSi alloy, provides several advantages. Both the antimony and oxygen of the Sb.sub.2O.sub.3 compound is essential for the performance of the inoculant. Another advantage is the good reproducibility of the inoculant composition since the amount and the homogeneity of particulate Sb.sub.2O.sub.3 in the inoculant is easily controlled. The importance of controlling the amount of inoculants and having a homogenous composition of the inoculant is evident given the fact that antimony is normally added at a ppm level. Adding an inhomogeneous inoculant may result in wrong amounts of inoculating elements in the cast iron. Still another advantage is the more cost effective production of the inoculant compared to methods involving alloying antimony in a FeSi based alloy.

EXAMPLES

(11) Four inoculation trials were performed out of one ladle of 600 kg molten cast iron treated with magnesium by addition of 1.3 wt % MgFeSi nodularizing alloy. The MgFeSi nodularizing alloy had the following composition by weight: 5.8 wt % Mg, 1 wt % Ca, 1 wt % RE, 0.7 wt % Al, 46 wt % Si, the balance being iron.

(12) The four trials were divided into two repetitions using two different inoculants.

(13) The two inoculants consisted of a ferrosilicon alloy, Inoculant A, containing 71.8 wt % Si, 1.07 wt % Al, 0.97 wt % Ca, 1.63 wt % Ce, the remaining being iron. To one part of Inoculant A it was added 1.2 wt % Sb.sub.2O.sub.3 in particulate form, and mechanically mixed to provide the inoculant of the present invention. To another part of Inoculant A it was added 1 wt % FeS and 2 wt % Fe.sub.2O.sub.3, and mechanically mixed. This is the inoculant according to WO 99/29911 produced by Elkem AS under trademark Ultraseed®.

(14) The four trials were divided into two repetitions of the two different inoculants. Two trials with added FeS and Fe.sub.2O.sub.3 powder to make Ultraseed® inoculant, and two trials with added Sb.sub.2O.sub.3 powder to make the inoculant of the present invention.

(15) Table 1 shows an overview of the inoculants used. The amounts of antimony oxide, iron oxide and iron sulphide are based on the total weight of the inoculants.

(16) TABLE-US-00001 TABLE 1 Addition rates (wt %) # Base inoculant FeS Fe.sub.2O.sub.3 Sb.sub.2O.sub.3 Reference Ladle 1 Inoculant A 1% 2% — Ultraseed (Prior art) Ladle 2 Inoculant A — — 1.2% Sb.sub.2O.sub.3 (Invention) Ladle 3 Inoculant A — — 1.2% Sb.sub.2O.sub.3 (Invention) Ladle 4 Inoculant A 1% 2% — Ultraseed (Prior art)

(17) The inoculants were added to cast iron melts in an amount of 0.2 wt %. The inoculated cast irons were cast into 28 mm diameter cylindrical test bars. Microstructures were examined in one test bar from each trial. The test bars were cut, prepared and evaluated by image analysis in position 2 shown in FIG. 1. The nodule number (number of nodules/mm.sup.2) was determined. The results are shown in FIG. 2.

(18) As can be seen from FIG. 2 the results show a very significant trend in that the cast irons treated with Sb.sub.2O.sub.3 containing inoculants have higher nodule number density compared to same cast iron melts treated with the prior art Ultraseed® inoculant.

(19) Having described preferred embodiments of the invention it will be apparent to those skilled in the art that other embodiments incorporating the concepts may be used. These and other examples of the invention illustrated above and in the accompanying drawings are intended by way of example only and the actual scope of the invention is to be determined from the following claims.