Cast iron inoculant and method for production of cast iron inoculant

Abstract

An inoculant for manufacturing cast iron with lamellar, compacted or spheroidal graphite is disclosed. The inoculant has a particulate ferrosilicon alloy having 40 and 80% by weight of silicon, 0.5-5 wt % of calcium and/or strontium and/or barium, 0-10 wt % of rare earths, 0-5 wt % of magnesium, less than 5% by weight of aluminium, 0-10 wt % of manganese and/or zirconium, and the balance being iron, wherein the inoculant additionally contains 0.1-10 wt % of particulate bismuth oxide particles and optionally 0.1-10 wt % of one or more particulate metal sulphides and/or one or more particulate iron oxides, where the particulate bismuth oxide is mixed or blended with the ferrosilicon particles, or is simultaneously added to cast iron together with the particulate ferrosilicon particles.

Claims

1. An inoculant for the manufacture of cast iron with lamellar, compacted or spheroidal graphite consisting of: a particulate ferrosilicon alloy consisting of: 40-80 wt % of silicon, 0.5-5 wt % of calcium, 0.1-10 wt % of rare earths, 0.5-5 wt % of aluminum, the balance being iron and incidental impurities in the ordinary amount, and 0.1-2.2 wt % of particulate bismuth oxide, based on the total weight of inoculant, wherein said ferrosilicon alloy particles are coated with the particulate bismuth oxide.

2. The inoculant according to claim 1, wherein the silicon in the particulate ferrosilicon alloy is between 45 and 60 wt %.

3. The inoculant according to claim 1, wherein the silicon in the particulate ferrosilicon alloy is between 60 and 80 wt %.

4. The inoculant according to claim 1, wherein the calcium in the particulate ferrosilicon alloy is between 0.5 and 3 wt %.

5. The inoculant according to claim 1, wherein the rare earths in the particulate ferrosilicon alloy is 0.1-6 wt %.

6. The inoculant according to claim 1, wherein the particulate bismuth oxide in the inoculant is 0.2 to 2.2 wt %.

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

8. The inoculant according to claim 1, wherein the inoculant is in the form of a blend or mixture of the particulate ferrosilicon alloy and the particulate bismuth oxide.

9. The inoculant according to claim 1, wherein the inoculant is in the form of agglomerates made from a mixture of the particulate ferrosilicon alloy and the particulate bismuth oxide.

10. The inoculant according to claim 1, wherein the inoculant is in the form of briquettes made from a mixture of the particulate ferrosilicon alloy and the particulate bismuth oxide.

11. The inoculant according to claim 1, wherein the rare earths are cerium, lanthanum, or a combination thereof.

12. An inoculant for the manufacture of cast iron with lamellar, compacted or spheroidal graphite, said inoculant consisting of a particulate ferrosilicon alloy consisting of: 40-80 wt % of silicon, 0.5-5 wt % of calcium, 0.1-10 wt % of rare earths, 0.5-5 wt % of aluminum, the balance being iron and incidental impurities in the ordinary amount, and 0.1-2.2 wt % of particulate bismuth oxide, based on the total weight of inoculant, wherein the particulate ferrosilicon alloy inoculant and the particulate bismuth oxide are added separately but simultaneously to liquid cast iron.

13. A method for producing the inoculant of claim 1 for the manufacture of cast iron with lamellar, compacted or spheroidal graphite, comprising: providing the particulate ferrosilicon alloy consisting of 40-80 wt % of silicon, 0.5-5 wt % of calcium, 0.1-10 wt % rare earths, 0.5-5 wt % of aluminium, the balance being iron and incidental impurities in the ordinary amount, and mixing to or blending with said particulate ferrosilicon alloy 0.1-2.2 wt % of particulate bismuth oxide based on the total weight of inoculant to produce said inoculant.

14. The method according to claim 13, wherein the rare earths are cerium, lanthanum, or a combination thereof.

15. The inoculant according to claim 13, wherein said particulate bismuth oxide is simultaneously added to liquid cast iron with the particulate ferrosilicon particles, instead of mixing or blending with the ferrosilicon particles.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows a test bar for cast irons.

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

(3) FIG. 3a-b show SEM photos of the inoculant according to the present invention; FeSi coated with Bi.sub.2O.sub.3 powder. Bi.sub.2O.sub.3 is visible as white particles.

DETAILED DESCRIPTION OF THE INVENTION

(4) In the manufacturing process for producing cast iron with spheroidal graphite the cast iron melt is normally nodularisation treated, conventionally using an Mg—FeSi alloy, prior to the inoculation treatment. The nodularisation 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 nodularising effect. The nodularisation 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 and removed. However some MgO and MgS inlcusions produced during the nodularisation 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 transform the MgO and MgS inclusions formed during the nodularisation 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% silicon or a low silicon alloy containing 45 to 60% 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.

(7) In accordance with the invention, the particulate FeSi based alloy comprises between 0.5 and 5% by weight of Ca and/or Sr and/or Ba. Using a higher amount of Ca, Ba and/or Sr may reduce the performance of the inoculant, increase slag formation and increase the cost. The amount of Ca and/or Sr and/or Ba in the FeSi base alloy may e.g. be 0.5-3% by weight.

(8) The FeSi base alloy comprises up to 10% by weight of rare earths (RE). RE may for example be Ce and/or La. Good inoculating performance is also achieved when the amount of RE is 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 presence of small amounts of elements like Bi in the melt (also called subversive elements) will prevent magnesium having the desired nodularising effect. This negative effect can be neutralized by using Ce. Introducing Bi.sub.2O.sub.3 together with the inoculant is adding a reactant to an already existing system with Mg inclusions floating around in the melt and “free” Mg. This is not a violent reaction and the Bi yield (Bi/Bi.sub.2O.sub.3 remaining in the melt) is expected to be high. Good inoculating effect is also observed when the inoculant contains 0.2 to 5% by weight, based on the total weight of inoculant, of particulate Bi.sub.2O.sub.3. The amount of particulate Bi.sub.2O.sub.3 may in some embodiments e.g. be from about 0.5 to about 3.5% by weight, based on the total weight of inoculant

(10) The Bi.sub.2O.sub.3 particles should have a small particle size, i.e. micron size (e.g. 1-10 μm), resulting in very quick melting or dissolution of the Bi.sub.2O.sub.3 particles when introduced in the cast iron melt. Advantageously, the Bi.sub.2O.sub.3 particles are mixed with the particulate FeSi base alloy prior to adding the inoculant into the cast iron melt. FIG. 3 shows an inoculant according to the present invention where Bi.sub.2O.sub.3 particles are mixed with FeSi alloy particles. The Bi.sub.2O.sub.3 particles are visible as white particles. Mixing the Bi.sub.2O.sub.3 particles with the FeSi base alloy particles results in a stable, homogenous inoculant. The following Examples show that the addition of Bi.sub.2O.sub.3 particles together with FeSi base alloy particles results in an increased nodule number density when the inoculant is added to cast iron, thus reducing the amount of inoculant necessary to achieve the desired inoculating effect.

EXAMPLES

(11) Two cast iron melts P and Q were treated with 1.05 wt % MgFeSi nodularizing alloy based on the weight of the cast irons in a tundish cover ladle. The MgFeSi nodularizing alloy had the following composition by weight: 5.8% Mg, 1% Ca, 1% RE, 0.7% Al, 46% Si, balance being iron.

(12) The Mg treated cast iron melts P and Q were inoculated with a ferrosilicon Inoculant A containing 71.8 wt % Si, 1.07 wt % Al, 0.97 wt % Ca, 1.63 wt % Ce, the remaining being iron and incidental impurities in the ordinary amount. Different amounts of bismuth oxide in particulate form, iron sulphide in particulate form and iron oxide in particulate form were added to Inoculant A and mechanically mixed using to obtain homogenous mixtures of the different inoculants.

(13) For comparison purposes the same cast iron melts were inoculated with Inoculant A to which were added only iron oxide and/or iron sulphides (prior art).

(14) The chemical composition of the final cast irons were 3.5-3.7 wt % C, 2.3-2.5 wt % Si, 0.29-0.30 wt % Mn, 0.009-0.011 wt % S, 0.040-0.050 wt % Mg.

(15) The added amounts of bismuth oxide, iron oxide and iron sulphide to the FeSi base alloy are shown in Table 1. The amounts of bismuth oxide, iron oxide and iron sulphide are based on the total weight of the inoculants.

(16) TABLE-US-00001 TABLE 1 Inoculant mixtures based on Inoculant A and various additions by weight % of Bi.sub.2O.sub.3, FeS and Fe.sub.2O.sub.3, Additions Base inoculant FeS Fe.sub.2O.sub.3 Bi.sub.2O.sub.3 Reference Melt P 1 Inoculant A 1% 2% — P1 (prior art) 2 Inoculant A — — 1.1% P2 (Invention) 3 Inoculant A 1% 2% 1.1% P3 (Invention) 4 Inoculant A 1% 2% — P4 (prior art) Melt Q 1 Inoculant A — — 2.2% Q1 (Invention) 2 Inoculant A 1% — 1.1% Q2 (Invention) 3 Inoculant A 1% 2% — Q3 (prior art) 4 Inoculant A 1% 2% — Q4 (prior art) 5 Inoculant A 1% — 2.2% Q5 (Invention) 6 Inoculant A 1% 2% 1.1% Q6 (Invention)

(17) The different inoculants were added to cast iron melts P and Q in an amount of 0.2 wt %. The inoculated cast irons were cast into 28 mm diameter cylindrical test bar samples. 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. Nodule number density (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 Bi.sub.2O.sub.3 containing inoculants, P2, P3, Q1, Q2, Q5 and Q6, according to the invention, show higher nodule number density compared to cast iron melts treated with the prior art inoculants, P1, P4, Q3, Q4.

(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.