WO2006068487A1

WO2006068487A1 - Modifying agents for cast iron

Info

Publication number: WO2006068487A1
Application number: PCT/NO2005/000022
Authority: WO
Inventors: Torbjørn SKALAND
Original assignee: Elkem Asa
Priority date: 2004-12-23
Filing date: 2005-01-19
Publication date: 2006-06-29
Also published as: NO20045611D0

Abstract

A method of controlling the microstructure of a cast iron which comprises: forming an iron melt; removing a portion of the melt; adding on inoculant to the removed portion; and then casting the removed portion. The inoculant comprises a graphite phase modifying compound and a matrix structure modifying component. The matrix structure modifying component can be a pearlite promoter, such as Sn or Sb or As, or a ferrite promoter, such as B.

Description

Title

Modifying agents for cast iron Field of Invention

This invention relates to modifying agents for cast iron (grey, ductile and compacted) and more particularly inoculants that make it possible to control not only the graphite structure in cast iron, but also the matrix microstructure of the cast iron.

Background Art

Inoculants are traditionally designed to manipulate graphite nucleation in cast iron, and promote desirable structures such as type A graphite in grey irons and high nodule counts in ductile irons. A wide range of inoculant alloys is known that cover different needs for graphite structure control. However, graphite typically accounts for only about 10% of the cast iron microstructure, and the remaining 90% being constituents such as ferrite, pearlite or carbides. Today there are no specific inoculant materials available in the market that make it possible to control deliberately the ferrite and/or pearlite contents in the cast iron microstructure. Some inoculants are however recognised as being useful in directing microstructures towards ferritic or pearlitic grades, but no special attempt has been made in order to enhance this situation deliberately.

Elements that are recognised as ferrite promoters in cast iron are Si, Ni and Al, but generally concentrations need to be high in order to ensure fully ferritic conditions and there are other downsides to this as well. The most practical way to produce as- cast ferritic microstructures in ductile iron today is simply to ensure that the concentration of pearlite promoters is kept low, and this often implies the use of high pig iron contents in the charge. Therefore it would be useful to introduce an inoculant that could promote ferritic microstructures out of a more common ferritic/pearlite base iron, without having to restrict the content of pearlite promoters in the iron.

Some pearlite promoters that are used today in making high strength pearlitic grades include Mn, Cu, Sn, Sb and Cr. However, the introduction of such elements into the charge for producing cast iron would restrict the usefulness of the charge for other grades of iron than pearlitic grades.

The practice today is therefore to add either pearlite or ferrite conditioners to cast iron before any nodularising treatment and before inoculation.

It is an object of the invention to provide a system which would enable the production of cast irons with different microstructures from the same melt.

Description of Invention

According to one aspect of the invention, there is provided a method of controlling the microstructure of a cast iron which comprises: forming an iron melt; removing a portion of the melt; adding a modifying agent to the removed portion; and then casting the removed portion; the modifying agent comprising a graphite phase modifying compound and a matrix structure modifying component.

The cast iron melt may be formed from one or more of scrap iron, steel scrap, cast iron returns and pig iron, optionally together with one or more of ferrosilicon, ferromanganese, manganese and carbon to adjust the final cast iron composition.

Preferably, the said portion is removed to a mould and the inoculant is added to the removed portion in the mould or as it enters the mould. Conventionally, the amount of inoculant added represents from 0.1 to 1.5 wt % of the removed portion.

The matrix structure modifying component in the additive may be a pearlite promoter, such as tin (Sn) or antimony (Sb) or arsenic (As), or a ferrite promoter, such as boron (B). In this way, the desired microstructure of the matrix can be achieved.

Thus, the inoculant may include 5 to 40 wt % Sn and/or 3 to 15 wt % Sb₃ and/or 0.3 to 7 wt % As, or 0.3 to 6 wt % B, based on total weight of the inoculant. Preferably the content would be 6 to 15 wt % Sn and/or 3.3 to 10 wt % Sb, and/or 0.5 to 5 wt % As₃ or 0.3 to 6 wt % B. Combinations of these are also contemplated. Preferably, the inoculant is ferrosilicon-based and the graphite phase modifying component comprises from 40 to 80 wt % Si and from 0.5 to 10 wt % Ca and/or Sr and/or Ba, the balance being predominantly Fe. The graphite phase modifying component may also include up to 10 wt % Ce and/or La, up to 5 wt % Mg, up to 5 wt % Al, up to 10 wt % Mn and/or Ti and/or Zr (preferably 0.5 to 5 wt %), and may also include from 0.5 to 10 wt % oxygen in the form of one or more metal oxides and from 0.1 to 10 wt % sulphur in the fonn of one or more metal sulphides.

The inoculant may be in the form of a solid mixture of a ferrosilicon based alloy, the metal oxide and the metal sulphide. Conveniently, the metal oxide is selected from the group consisting of FeO, Fe₂O₃, Fe₃O₄, SiO₂, MnO, MgO, CaO, Al₂O₃, TiO₂ and CaSiO₃, CeO₂, ZrO₂ and the metal sulphide is selected from the group consisting of FeS, FeS₂, MnS, MgS, CaS and CuS. Preferably, the oxygen content is between 1 and 6% by weight and the sulphur content is between 0.1 and 3% by weight.

Conventionally, the inoculant is added in an amount in the range 0.2 to 1.5 wt % based on the total weight of the melt to which it is added.

In a modified method for the production of nodular or compacted cast iron, a portion of the melt is removed and transferred to a treatment ladle, a nodulariser is added to the removed portion of the melt in the ladle, and the inoculant is then added to at least a part of the removed portion. The nodulariser can be placed in the treatment ladle before the melt is poured in or can be added to the melt in the treatment ladle. The nodularised melt may then be poured into a pouring ladle to be poured into one or a plurality of moulds. The inoculant can then be added to the stream during transfer to the pouring ladle or added to the stream as the melt enters each mould.

Any suitable nodulariser may be used. Suitable nodularisers include a ferrosilicon- based nodulariser comprising: 40 to 60 wt % Si; 3 to 20 wt% Mg; and optionally up to 5 wt % Ca; up to 10 wt % rare earth metals, Ce and/or La; and up to 5 wt % Al; based on the total weight of the nodulariser; the balance being Fe.

The nodulariser may contain tin and/or antimony and/or arsenic or boron as matrix structure modifying components. The contents, based on the total weight of the nodulariser, are preferably 1 to 15 wt % Sn, and/or 0.4 to 10 wt % Sb and/or 0.1 to 5 wt % As or 0.2 to 3 wt % B, more preferably 2.5 to 10 wt % Sn, and/or 0.5 to 5 wt % Sb and/or 0.2-3% As or 0.25 to 2.5 wt % B.

The invention extends to a cast iron modifying agent, such as an inoculant or a nodulariser, comprising a graphite phase modifying component and a matrix structure modifying component, the matrix structure modifying component comprising Sn and/or Sb, and/or As or B. Preferably, the graphite phase modifying component comprises: Si, Ca and/or Sr and/or Ba, and Al, and may comprise rare earth metals, Ti, Zr and Mn.

The matrix structure modifying component may be a pearlite promoter comprising 5 to 40 wt % Sn and/or 3 to 15 wt % Sb and/or 0.3 to 7 wt % As, based on the total weight of the modifying agent, preferably 6 to 15 wt % Sn and/or 3.3 to 10 % Sb and/or 0.5 to 5 wt % As.

Alternatively, the matrix structure modifying component is a ferrite promoter comprising 0.3 to 6 wt % B, based on the total weight of the modifying agent, preferably 0.5 to 3.5 wt % B.

Preferably, the graphite phase modifying component is as described above in relation to the method of the invention. Where the modifying agent is an inoculant, it may also include the various other components and have the various properties mentioned above in relation to the inoculant. When it is a nodulariser, it may include the various components and have the properties mentioned above in relation to the nodulariser. The invention also extends to a method of treating molten cast iron with a modifying agent as described above. More specifically, the invention extends to forming an iron melt, removing a portion of the melt, treating the removed portion with the inoculant and casting the treated portion. Preferably, the portion is removed to a mould and the inoculant is added to the removed portion in the mould or as it enters the mould. Preferably, the amount of inoculant added represents from 0.2 to 1.5 wt % of the removed portion.

Prior to inoculation, the melt may be treated with a nodulariser, preferably as described above.

According to a further aspect, the invention provides a method of producing cast irons with differing microstructures from the same melt, which comprises: forming an iron melt; removing a first portion of the melt; adding a first inoculant including a first matrix structure modifying component to the first portion; removing a second portion of the melt; adding a second inoculant including a second matrix structure modifying component to the second portion; and casting the two portions; the two matrix structure modifying components being a pearlite promoter and a ferrite promoter. Thus, one of the two will be a pearlite promoter and the other a ferrite promoter.

The preferred and optional features of the previously described aspects of the invention are also applicable. Thus, the pearlite promoter is preferably Sn and/or Sb and/or As, and the ferrite promoter is preferably B. Both inoculants are preferably ferrosilicon-based, and both preferably include a graphite phase modifying component, as set out above.

Preferably, each inoculant is added to its respective melt portion in an amount in the range 0.2 to 1.5 wt % based on the total weight of the melt. Prior to inoculation, optionally one or both portions of the melt may be treated with a nodulariser. The nodulariser is preferably added as set out above, and is preferably similar to the nodulariser described above in its composition. Thus, it will be appreciated that the invention enables cast irons of differing properties and differing microstructures, both the graphite and the matrix, to be produced from the same base melt, without the need to modify the entire melt itself. This in turn means that a foundry will be able to produce, sequentially or even simultaneously, different grades of cast iron.

Short Description of Drawings

The invention may be carried into practice in various ways and some embodiments and Examples will now be described. In the accompanying drawings;

Figures Ia and 2b are sequential, schematic representations of a process possibility in according with the invention;

Figures 2a to 2c are sequential representations of another process possibility in accordance with the invention; and

Figures 3, 4, 5, 6 and 7 are photomicrographs showing the structure of various test alloys;

Detailed Description of Invention

Figures Ia and Ib show a melting furnace 11 (though this could equally well be a holding furnace), a pouring ladle 12 and a series of two-part moulds, two of which 13,14 are shown. The furnace 11 contains an iron melt 15 which has not been treated with any inoculant.

A portion 16 of the melt 15 is poured into the pouring ladle 12 from the furnace 11 as shown in Figure Ia. The portion of melt 16 in the pouring ladle 12 is then poured sequentially into the two-part moulds 13,14 as shown in Figure Ib. Any slag on the surface of the melt 16 is held back by a weir 17 in the pouring ladle 12. As the melt 16 is poured, an inoculant 18 is added to the stream 19 of molten iron as enters the mould 13,14. The iron is then allowed to cool and solidify in the moulds. The inoculant 18 includes not only a graphite phase modifying component, as is conventional, but also a matrix structure modifying component which determines the nature of the matrix. When a pearlitic structure is required, an inoculant 18 is used which has tin (Sn) and/or antimony (Sb) and/or as arsen (As) the matrix structure modifying component. When a ferritic structure is required, an inoculant 18 is used which has boron (B) as the matrix structure modifying component.

The process shown in Figure 2a to 2c is similar to that in Figures Ia and Ib and includes a furnace 21 holding an iron melt 22. However, in this case, as shown in Figure 2a, a portion 24 of the melt 22 is transferred to a treatment ladle 23. The treatment ladle 23 includes a nodulariser 25 which dissolves in the melt portion 24 in the treatment ladle 23.

The nodulariser-treated molten iron is then poured from the treatment ladle 23 into a pouring ladle 26, as shown in Figure 2b. Again, a weir 27 in the treatment ladle 23 prevents slag entering the pouring ladle 26.

The nodularised iron melt 28 in the pouring ladle 26 is poured sequentially into two part moulds 31, 32 as shown in Figure 2c. Any slag on the surface of the melt 28 is held back by a weir 29 in the pouring ladle 26. As the melt 28 is poured, an inoculant 33 is added to the stream 34 of molten iron as it enters the mould 13,14.

The iron is then allowed to cool and solidify in the moulds. The inoculant 33 and its use are similar to the inoculant 18 and its use in the embodiment of Figures Ia and Ib.

In both embodiments, the inoculant comprises a graphite phase modifying component and a matrix structure modifying component. The matrix structure modifying component can be a pearlite promoter such as Sn, Sb or As, or a ferrite promoter such as B.

The invention will now be further illustrated in the following Examples. EXAMPLE 1

Antimony bearing inoculant in ductile iron

Heats of liquid, iron were made up from steel scrap, pig iron, ferrosilicon, ferromanganese, and graphite, and melted in an induction furnace. Nodularising treatment was conducted in a conventional tundish ladle process by adding 1.6 wt% of a 6%Mg-bearing ferrosilicon alloy followed by addition of 0.5 wt% inoculant at transfer to the pouring ladle. Final iron composition was 3.6%C, 2.6%Si, 0.50%Mn, 0.030%Mg, and 0.007%S.

The compositions of ferrosilicon based inoculants are given in Table 1, while Table 2 shows the resulting nodule count and pearlite content in 5, 20 and 40 mm section size sand moulded ductile iron plates.

Table 2: Nodule count and pearlite content in experimental ductile iron castings.

Inoculants A and C are conventional reference materials to the experimental inoculant B containing 3.7% Sb in addition to Ca, Al and Ce. The objective with inoculant B was to promote a predominantly pearlitic microstructure in all section sizes of the experimental ductile iron castings. Table 2 shows that the reference inoculants A and C gave pearlite contents ranging from about 60% in the 5 mm section to about 8% in the 40 mm section plates. The experimental antimony bearing inoculant B gave pearlite contents around 80% for all casting plate thicknesses. This means that inoculant B is useful in achieving higher and more uniform pearlite contents in complex castings of different section thicknesses.

Particularly for the thicker and slower cooling 20 and 40 mm plates, the antimony bearing inoculant B is found to be useful in promoting significantly more pearlite in the microstructure. Such inoculants may therefore be used as means to make higher strength pearlitic microstructures in ductile iron of a relatively low alloying content that would otherwise solidify as predominantly ferritic microstructures.

The graphite nodule structures for the experimental inoculant B are also found to be good and fully acceptable for the various sections of ductile iron castings. EXAMPLE 2

Boron bearing inoculants in ductile iron

Heats of liquid iron were made up from steel scrap, pig iron, ferrosilicon, ferramanganese, copper, and graphite, and melted in an induction furnace. Nodularising treatment was conducted in a conventional tundish ladle process by adding 1.6 wt% of a 6%Mg-bearing ferrosilicon alloy followed by addition of 0.3 wt% inoculant at transfer to the pouring ladle. Final iron composition was 3.7%C, 2.6%Si, 0.6%Mn, 0.62%Cu, 0.03%Mg, and 0.01%S.

The compositions of experimental ferrosilicon-based inoculants are given in Table 3, while Table 4 shows the resulting nodule count and pearlite content in 20 mm section size sand moulded ductile iron plates.

Table 3: Composition of experimental ferrosilicon-based inoculants.

Table 2: Nodule count and pearlite content in 20 mm ductile iron plates.

Inoculants A and B are conventional reference materials to the experimental inoculants C and D containing 1.1% B in addition to Ca, Al, Ce, S, and O. The objective with inoculants C and D was to promote more ferrite in the microstructure of the experimental ductile iron castings. Table 4 shows that the reference inoculants A and B gave pearlite contents of about 70% in the 20 mm section plates, while the experimental boron bearing inoculants C and D gave pearlite contents of only around 20 to 30% for the same conditions. This means that inoculants C and D are useful in achieving higher ferrite contents in castings where the base alloying conditions (Mn and Cu) are promoting mostly pearlitic microstructures.

The boron pick-up to the final iron was found to be about 10 to 15 ppm for the experimental irons No. 3 and 4. Reference irons No. 1 and 2 have no detectable boron content in the final treated metals.

Boron bearing inoculants are found to be useful in promoting significantly more ferrite and less pearlite in the microstructure of 0.6% Mn and 0.6%Cu alloyed ductile iron. Such inoculants may therefore be used as means to make higher ductility grades of ductile iron out of pearlitic base iron conditions that would otherwise solidify having predominantly pearlitic microstructures.

EXAMPLE 3

Tin and antimony bearing inoculants in grey iron

Heats of liquid grey iron were made up from steel scrap, pig iron, ferrosilicon, ferromanganese, sulphur, and graphite, and melted in an induction furnace. Inoculation was conducted by the addition of 0.25 wt% inoculant at transfer to the pouring ladle. Final iron composition was 3.3%C, 2.0%Si, 0.45%Mn, 0.23%Cr, and 0.06%S.

The compositions of experimental ferrosilicon-based inoculants are given in Table 5, while Table 6 shows the resulting chill wedge level, graphite structure (type A graphite), and pearlite content in 20 mm section size sand moulded grey iron plates. 1

Table 5: Composition of experimental ferrosilicon-based inoculants.

Inoculant A is a conventional Sr-bearing reference material for the experimental inoculants B and C containing 3.9% Sb and 10% Sn, respectively in addition to Sr. Inoculant D is another experimental material containing 3.7% Sb in addition to Ca, Al, and Ce. The objective with the experimental inoculants B, C and D was to promote more pearlite in the microstructure of grey iron castings. Table 6 shows that the reference inoculant A gave pearlite contents of about 88% in the 20 mm section plates, while the experimental Sb and Sn bearing inoculants B, C and D gave pearlite contents of 97 to 100% for the same conditions. This means that inoculants B, C and D were useful in achieving fully pearlitic conditions in castings that would otherwise solidify with a certain ferrite content. Table 6 shows that the experimental inoculants gave a moderately increased chilling tendency in standard cast chill wedges from 7 mm chill for the reference inoculant to about 10 mm chill for the experimental inoculants. This is expected since Sb and Sn alloying tend to give pearlite and chill promotion effects simultaneously. Chill wedge levels were however fully acceptable, and could be largely offset by slightly increasing the inoculant alloy addition rate.

Base iron conditions were chosen to results in a certain formation of undercooled graphite structures. Table 6 shows that the reference irons No. 1 and 4 contain about 60 to 65% type A graphite, the balance being mostly undercooled type D graphite. The experimental inoculant treated irons No. 2, 3, 5, and 6 show type A graphite ranging from about 60 to 70% for similar conditions.

Consequently, antimony and tin bearing inoculants are found to be useful in ensuring fully pearlitic microstructures in somewhat undercooled and low alloyed grey iron conditions. Such inoculants may be used as means to promote higher strength pearlitic grades of grey iron of a relatively low alloying content that would otherwise risk to contain undercooled type D graphite and associated ferrite formation with the accompanied risk of inferior tensile strength properties.

EXAMPLE 4

Antimony bearing inoculants in ductile iron

Heats of liquid iron were made up from steel scrap, pig iron, ferrosilicon, ferromanganese, and graphite, and melted in an induction furnace. Nodularising treatment was conducted in a conventional tundish ladle process by adding 1.6 wt% of a 6%Mg-bearing ferrosilicon alloy followed by the addition of 0.4 wt% inoculant at transfer to the pouring ladle. Final iron composition was 3.7%C, 2.6%Si, 0.50%Mn, 0.033%Mg, and 0.006%S. - Composition of experimental ferrosilicon-based inoculants are given in Table 7, while Table 8 shows the resulting nodule count, nodularity, pearlite content and antimony content in 20 mm section size sand moulded ductile iron plates.

Table 7: Composition of experimental ferrosilicon-based inoculants.

Inoculant A is a conventional reference material to the experimental inoculants B through E containing from 2 to 8 % Sb in addition to Ca, Al, Zr, Mn and Ba. The objective with inoculants B through E was to investigate the pearlite promoting effect as a function of increasing Sb-content in the inoculant. Table 8 shows that the reference inoculant A gave a pearlite content of 37% in the present iron for the 20 mm section plate. The experimental antimony bearing inoculants B through E gave pearlite contents ranging from 62% for the 2% Sb-containing inoculant B to 99% for the 8% Sb-containing inoculant E. This means that increasing Sb-contents in conventional ferrosilicon based inoculants is useful to increase the pearlite content from initially less than 50% and towards 100% pearlite.

Figures 3a, 3b and 3c show the microstructure in experimental 20 mm plate castings No. 1, 3, and 5 respectively in Table 8. The residual Sb-content in these castings is increasing from <0.003% in the reference iron to 0.03% Sb in iron No. 5 treated with the 8%Sb-containing inoculant E.

Sb-containing ferrosilicon inoculants may therefore be useful as a means to make higher strength pearlitic grades of ductile iron having a relatively low initial alloying content that would otherwise solidify with predominantly ferritic microstructures. The content of Sb in the inoculant may be adjusted between say 3 and 15% in order to achieve the required level of pearlite and consequently the required strength level of the ductile iron casting.

According to Table 8, nodularity and nodule count for the experimental inoculant test series also show improvements with increasing Sb-content in the inoculant.

EXAMPLE 5

Boron bearing inoculants in ductile iron

Heats of liquid iron were made up from steel scrap, pig iron, ferrosilicon, ferromanganese, graphite, and copper, and melted in an induction furnace. Nodularising treatment was conducted in a conventional tundish ladle process by adding 1.6 wt% of a 6%Mg-bearmg ferrosilicon alloy followed by addition of 0.4 wt% inoculant at transfer to the pouring ladle. Final iron composition was 3.75%C, 2.60%Si, 0.73%Mn, 0.040%Mg, 0.006%S, and 0.72%Cu.

The compositions of experimental ferrosilicon-based inoculants are given in Table 9, while Table 10 shows the resulting nodule count, nodularity, pearlite content and boron content in 20 mm section size sand moulded ductile iron plates. Table 9: Composition of experimental ferrosilicon-based inoculants.

Table 10: Nodule count, nodularity, pearlite and boron-contents in ductile irons.

Inoculant A is a conventional reference material to the experimental inoculants B through E containing from 0.7 to 2.8 % B in addition to Ca and Al. The objective with inoculants B through E was to investigate the ferrite promoting effect as a function of increasing B-content in the inoculant. Table 10 shows that the reference inoculant A resulted in a pearlite content of 96% in the present 0.7%Mn and 0.7%Cu containing iron for the 20 mm section plate. The experimental boron bearing inoculants B through E gave pearlite contents ranging from 70% for the 0.7% B-containing inoculant B to 51% for the 2.8% B-containing inoculant E. This means that increasing B-contents in conventional ferrosilicon based inoculants is useful to reduce the pearlite content from initially close to 100% and towards 50% pearlite. Figures 4a, 4b and 4c show examples of the microstructure in experimental 20 mm castings No. 1, 3, and 5 respectively from Table 9. The residual B-content in these castings is increasing from <5 ppm in the reference iron to 70 ppm B in iron No. 5 treated with the 2.8% B-containing inoculant E.

B-containing ferrosilicon inoculants may therefore be useful as a means to soften higher strength pearlitic grades of ductile iron that would otherwise solidify with predominantly pearlitic microstructures. The content of B in the inoculant may be adjusted between say 0.3 and 6% in order to achieve the required level of ferrite and pearlite and consequently the required strength and ductility level of the ductile iron casting.

According to Table 10, nodularity and nodule count for the experimental inoculant test series is relatively unaffected by the increasing B-content in the inoculant.

EXAMPLE 6

Antimony bearing nodulariser in ductile iron

Heats of liquid iron were made up from steel scrap, pig iron, ferrosilicon, ferromanganese, graphite, and copper, and melted in an induction furnace.

Nodularising treatment was conducted in a conventional tundish ladle process by adding 1.6 wt% of experimental 6%Mg-bearing ferrosilicon alloys followed by addition of 0.4 wt% inoculant at transfer to the pouring ladle. Final iron composition was 3.6%C, 2.9%Si, 0.18%Mn, 0.038%Mg, and 0.01%S for tests 1 through 5 and. 3.75%C, 2.55%Si, 0.6%Mn, 0.035%Mg, 0.006%S, and 0.23%Cu for tests 6 through

8.

The compositions of experimental ferrosilicon-based nodularisers and inoculants are given in Tables 11 and 12, while Table 13 shows the resulting nodule count, nodularity, pearlite content and antimony content in 20 mm section size sand moulded ductile iron plates. Table 11: Composition of experimental ferrosilicon-based nodularisers.

Si Mg Ca Al RE Sb

No. % % % % % %

A 46.7 6.1 1.1 0.6 1.0

B 46.4 6.0 1.2 0.7 1.1 0.8

lαδ/e 12: Composition of experimental ferrosilicon-based inoculants.

Jαέfe i3: Nodule count, nodularity, pearlite and antimony-contents in ductile irons.

MgFeSi nodulariser A is a conventional reference material containing about 6%Mg and l%Ca and RE, while the experimental nodulariser B also contains 0.8% Sb. Inoculants A, B, and D are conventional reference materials to the experimental inoculants C and E containing 3 and 4% Sb, respectively in addition to Ca, Al, Zr, and Mn. The objective of nodulariser B and inoculants C and E was to investigate the pearlite promoting effects for Sb-bearing nodulariser and inoculant alloys.

Two different base iron conditions were tested, a ferritic iron and a mixed ferrite/pearlite containing iron. Table 13 shows that the ferritic reference irons No. 1 and 2 contain only 4% pearlite for the low Mn-content of 0.18% and in the 20 mm section plate. The experimental 0.8% antimony bearing nodulariser B resulted in an increase in pearlite content to 29 and 34% for tests No. 3 and 4, respectively. When combining the Sb-containing nodulariser B and the Sb-containing inoculant C in test No. 5, the resulting pearlite content was increased to 59%.

The ferritic/pearlitic reference iron No. 6 contained 29% pearlite for the 0.6%Mn and 0.2%Cu additions in the 20 mm section plate. The experimental 0.8% antimony bearing nodulariser B resulted in an increase in pearlite content to 93% for test No. 7. Similarly, the Sb-containing inoculant E resulted in an increase in pearlite content to 97% for test No. 8.

This means that Sb-containing ferrosilicon based nodulariser and inoculant alloys are useful to increase the pearlite content in ferritic irons of initially only 4% pearlite, towards 30 to 60% pearlite when the alloys are used alone or in combinations. Similarly, Sb-containing nodularisers and inoculants may be useful to increase the pearlite content in ferritic/pearlitic irons having initially about 30% pearlite towards a fully pearlitic condition of 90 to 100% pearlite.

Figures 5 a, 5b and 5c show examples of the microstructure in experimental 20 mm plate castings No. 1, 3, and 5 respectively in Table 11. The residual Sb-content in these castings is increasing from <0.003% in the reference iron to 0.014% Sb in iron No. 3 and 0.025% Sb in iron No. 5 (ref. Table 13).

Sb-containing ferrosilicon-based nodularisers and inoculants may therefore be useful as means to make higher strength pearlitic grades of ductile iron having a relatively low initial alloying content that would otherwise solidify with predominantly ferritic or ferritic/pearlitic microstructures. The content of Sb in the nodulariser and inoculant alloys may be adjusted in order to achieve the required level of pearlite and consequently the required strength level of the ductile iron casting. Sb-bearing nodularisers and inoculants may also be used either alone or in combination to obtain the desired effects.

EXAMPLE 7

Boron hearing nodulariser in ductile iron Heats of liquid iron were made up from steel scrap, pig iron, ferrosilicon, ferromanganese, graphite, and copper, and melted in an induction furnace. Nodularising treatment was conducted in a conventional tundish ladle process by adding 1.6 wt% of experimental 6%Mg-bearing ferrosilicon alloys followed by addition of 0.4 wt% inoculant at transfer to the pouring ladle. Final iron composition was 3.75%C, 2.5%Si, 0.73%Mn, 0.040%Mg, 0.006%S, and 0.73%Cu.

Composition of experimental ferrosilicon-based nodularisers and inoculants are given in Tables 14 and 15, while Table 16 shows the resulting nodule count, nodularity, pearlite content and boron content in 20 mm section size sand moulded ductile iron plates.

Table 14: Composition of experimental ferrosilicon-based nodularisers.

1

Table 15: Composition of experimental ferrosilicon-based inoculants.

7αδ/e id: Nodule count, nodularity, pearlite and boron-contents in ductile irons.

MgFeSi nodulariser A is a conventional reference material containing about 6%Mg and l%Ca and RE, while the experimental nodulariser B also contains 0.3% B. Inoculant A is a conventional reference material containing Ca and Al, while the experimental inoculant B is also containing 1% B. The objective of nodulariser B and inoculant B is to find investigate ferrite promoting effects of boron-bearing nodulariser and inoculant alloys.

Table 16 shows that the pearlitic reference iron No. 1 contains 96% pearlite for the 0.7%Mn and 0.7%Cu additions in the 20 mm section plate. The experimental 1% boron containing inoculant B resulted in a reduction in pearlite content to 70% for test No. 2, while the experimental 0.3% boron bearing nodulariser B resulted in a reduction in pearlite content to 66% for test No. 3.

This means that B-containing ferrosilicon based nodulariser and inoculant alloys are useful to significantly increase the ferrite content in otherwise pearlitic iron conditions. Figures 6a, 6b, and 6c shows examples of the microstructure in experimental castings No. 1, 2, and 3 respectively in Table 16. The residual B-content in these castings is <5 ppm for the reference iron No. 1, 15 ppm for iron No. 2, and 20 ppm for iron No. 3 (ref. Table 16).

Boron-containing ferrosilicon-based nodularisers and inoculants may therefore be useful as means to soften higher strength pearlitic grades of ductile iron that would otherwise solidify with predominantly pearlitic microstructures. The content of boron in the nodulariser and inoculant alloys may be adjusted in order to achieve the required level of ferrite and pearlite and consequently the required strength and ductility levels in the final ductile iron castings.

EXAMPLE 8 Comparison of Inoculants Figure 7a to 7C show an example of a standardised base iron that is being transformed into different microstructure conditions through late inoculant additions. The ferrosilicon based Reseed^® inoculant (73%Si, l%Ca, 1%A1 and 1.75%Ce) is used as the reference material.

When ductile iron castings are poured with a simultaneous addition of a conventional Reseed inoculant, the resulting microstructure is found to contain about 29% pearlite (and thus 71% ferrite), as shown in Figure 7a.

When the inoculant is changed from conventional Reseed into a 1% boron containing Reseed inoculant, while keeping the liquid base iron composition constant, the resulting microstructure is found to contain only 11% pearlite (and thus 89% ferrite) as shown in Figure 7b.

Alternatively, when the inoculant is changed to a 4% antimony bearing Reseed inoculant, while still keeping the base iron composition constant, the microstructure is found to contain as much as 73% pearlite (and only 27% ferrite), as shown in Figure

7c. Consequentially, using the very same liquid base iron composition through melting, holding, transfer, treatment and pouring, it is now possible to manipulate the ferrite/pearlite ratio, and thus the grade of iron being produced, from high ferritic to high pearlitic microstructures simply by changing the type of inoculant material being added to the metal immediately before pouring into the moulds.

This technique of alloying and microstructure control through the nodularising and inoculating additions offer some unique opportunities to simplify foundry operations, raw material handling and liquid metal processing. Further, the use of cheaper and less clean raw material sources (e.g. steel scrap) may be permissible, since the late alloying will be able to adjust ferrite and pearlite contents in order to compensate for incorrect or incomplete base iron alloying conditions. ■

Nodularisers and inoculants are traditionally used to control the nucleation and growth of the graphite phase in a cast iron microstructure. The modifying agents according to the invention will also enable control of the general matrix microstructure of cast iron materials in order to adjust material properties such as strength, ductility, hardness, machinability and other important properties at a latest possible stage in the foundry processing.

Claims

1. A method of controlling the microstructure of a cast iron which comprises: forming an iron melt; removing a portion of the melt; adding a modifying agent to the removed portion; and then casting the removed portion; the modifying agent comprising a graphite phase modifying compound and a matrix structure modifying component.

2. A method as claimed in Claim 1, in which the modifying agent is an inoculant, the removed portion is poured into a mould and the inoculant is added to the removed portion or as it is poured into the mould.

3. A method as claimed in Claim 2, in which the matrix structure modifying component in the inoculant is a pearlite promoter, such as Sn and/or Sb and/or As.

4. A method as claimed in any of Claims 1 to 3, in which the matrix structure modifying component in the inoculant is a ferrite promoter, such as B.

5. A method as claimed in any preceding Claim, in which the graphite phase modifying component comprises from 40 to 80 wt % Si and from 0.5 to 10 wt % Ca and/or Sr and/or Ba, the balance predominantly being Fe.

6. A method as claimed in Claim 5, in which the graphite phase modifying component further comprises up to 10 wt % Ce and/or La, up to 5 wt % Al and up to 10 wt % Mn and/or Ti and/or Zr.

7. A method as claimed in any preceding Claim, in which the removed portion is transferred to a treatment ladle, a nodulariser is added to the removed portion of the melt in the treatment ladle, whereafter an inoculant is added to at least a part of the removed portion, prior to casting.

8. A method as claimed in Claim 7, in which the nodularised melt is transferred to a pouring ladle to be poured into one or more moulds and the inoculant is added to the melt stream during transfer to the pouring ladle or is added to the melt stream as it enters each mould.

9. A method as claimed in any of Claims 7 to 8, in which the nodulariser comprises: 40 to 60 wt % Si; 3 to 20 wt% Mg; and optionally up to 5 wt % Ca; up to 10 wt % rare earth metals, Ce and/or La; and up to 5 wt % Al; based on the total weight of the nodulariser, the balance being Fe.

10. A method as claimed in any of Claims 7 to 9, in which the nodulariser contains Sn and/or Sb or As and/or B as matrix structure modifying components.

11. A cast iron modifying agent for the manufacture of cast iron, comprising a graphite phase modifying component and a matrix structure modifying component, the matrix structure modifying component comprising Sn and/or Sb, and/or As, or B.

12. A modifying agent as claimed in Claim 11, in which the matrix structure modifying component is a pearlite promoter comprising 5 to 40 wt % Sn and/or 3 to 15 wt % Sb and/or 0.3 to 7% As, based on the total weight of the modifying agent.

13. A modifying agent as claimed in Claim 12, in which the matrix structure modifying component is a ferrite promoter comprising 0.3 to 6 wt % B, based on the total weight of the modifying agent.

14. A modifying agent as claimed in any of Claims 11 to 13, in which the graphite phase modifying component comprises from 40 to 80 wt % Si and from 0.5 to 10 wt % Ca and/or Sr and/or Ba, the balance preferably predominantly being Fe.

15. A modifying agent as claimed in Claim 14, in which the modifying agent further comprises up to 10 % Ce and/or La, up to 5 % Al and up to 10 wt % Mn and/or Ti and/or Zr.

16. A method of producing cast irons with differing matrix microstructures from the same melt, which comprises: forming an iron melt; removing a first portion of the melt; adding a first inoculant including a first matrix structure modifying component to the first portion; removing a second portion of the melt; adding a second inoculant including a second matrix structure modifying component to the second portion; and casting the two portions; the two matrix structure modifying components being a pearlite promoter and a feπite promoter.

17. A method as claimed in Claim 16, in which the pearlite promoter is Sn and/or Sb and/or As, and the respective inoculant includes 5 to 40 wt % Sn and/or 3 to 15 wt % Sb and/or 0.3 to 7 wt % As, based on the total weight of the inoculant.

18. A method as claimed in Claim 16 or Claim 17, in which the ferrite promoter is B and the respective inoculant includes 0.3 to 6 wt % B, based on the total weight of the inoculant.

19. A method as claimed in any of Claims 16 to 18, in which, prior to adding the inoculants, at least one of the two portions of the melt is treated with a nodulariser.

20. A method as claimed in Claim 19, in which the nodulariser contains Sn and/or Sb and/or As, or B.