EP4023775A1

EP4023775A1 - Method and additive composition for preparing ductile cast iron, and ductile cast iron obtainable by said method

Info

Publication number: EP4023775A1
Application number: EP20383172.2A
Authority: EP
Inventors: Aitor LOIZAGA; Gorka Zarrabeitia; Beñat BRAVO; Susana Méndez; Ramón Suárez
Original assignee: Fundacion Azterlan
Current assignee: Fundacion Azterlan
Priority date: 2020-12-29
Filing date: 2020-12-29
Publication date: 2022-07-06

Abstract

The present invention relates to a method for preparing ductile cast iron, comprising the steps of (a) melting a base iron to obtain a molten iron; (b) treating the molten iron with a nodulazing composition; (c) introducing the nodulized molten iron into a pouring unit; (d) holding the molten iron in the pressurized pouring vessel of the pouring unit resulting in a molten iron having a sulphur content from 0.002 wt. % to 0.006 wt. %; (e) pushing the molten iron from the pressurized pouring vessel to the pouring basin of the pouring unit; (f) pouring a stream of the molten iron from the pouring basin into a mould; and (g) allowing the molten iron to solidify in said mould; wherein the molten iron is treated with an additive composition and with an inoculating composition after step d) and prior to step g); wherein the additive composition comprises a source of sulphur, a source of selenium and, optionally, a source of tellurium and/or a source of rare earth elements, and comprises a carrier; and wherein the total content of sulphur, selenium, and optionally, tellurium and/or rare earth elements, in the resulting molten iron is increased by a total amount from 0.002 to 0.012 wt.% respect to total weight of the molten iron. The present invention relates also to an additive composition for preparing ductile cast iron according to said method, and to a kit comprising said additive composition and an inoculating composition. Finally, the present invention relates to a ductile cast iron obtainable by said method and to the use of the additive composition in the production of ductile cast iron.

Description

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method for preparing ductile cast iron. Additionally, the present invention relates to an additive composition for preparing ductile cast iron. The present invention also relates to ductile cast iron obtainable by said method.

BACKGROUND OF THE INVENTION

Cast iron typically contains between 2% to 4 wt.% carbon. The carbon is intimately mixed with the iron and the form, which the carbon takes in the solidified cast iron, is very important to the characteristics and properties of the iron castings. If the carbon upon solidification takes the form of spheroidal or nodular graphite, the cast iron is referred to as ductile cast iron.
For the manufacturing of ductile cast iron, raw materials (such as pig iron, end-of-life automotive package scrap, steel scrap, foundry returns) are usually melted in cupola or induction melting furnaces. A typical chemical composition for generic ductile iron applications may comprise:

Carbon 3.0-4.0 wt. %
Silicon 1.5-4.5 wt. %
Manganese 0.1-0.8 wt. %
Sulphur 0.005-0.010 wt. %
Magnesium 0.025-0.055 wt. %

Among the companies that manufacture ductile cast iron-based components, it is very common use pressurized pouring units with a high storage capacity. For the maintenance of the metal in molten form inside the pouring unit, they may be provided with heating devices, e.g., based on induction heating circuits.
An essential treatment of the molten iron for the production of ductile cast iron is called spheroidisation, in which the prerequisites of carbon precipitation process are met in order to allow the graphite precipitating from iron to be formed into spheres. The precipitation of this spheroidal graphite is usually controlled by adding magnesium-rich and/or rare earths-rich ferro-silicon alloys. Magnesium (and/or rare earth elements) reacts with surface active elements (as sulphur or oxygen) acting as nucleation sites for graphite, which promotes the growth of spherical graphite. The nature, shape and size of these nuclei in the molten iron will determine the ability of the graphite to precipitate over them, and therefore, will determine the tendency to precipitate undesired carbides and the possibility to reduce or eliminate the formation of micro shrinkage in the last stages of solidification.
Another iron melting treatment is inoculation in which, typically, grained ferro-silicon alloyed with some specific elements is added to the melt for increasing the number of nucleation points. In particular, inoculants are a mixture of elements that offer the possibility to form stable compounds with, e.g., sulphur, oxygen, nitrogen, titanium, silicon, or combinations thereof. The resulting atomic cluster compounds provide a substrate surface with nucleation sites upon which carbon dissolved in the molten iron can start to grow as graphite flakes or nodules. This treatment may be typically done just before pouring the molten metal into the casting mould or it can be done in the casting mould itself.
Before the inoculation treatment, the time that the metal remains in the pressurized pouring unit before being cast into the moulds, may result in a degradation of the metallurgical quality of the molten iron. The holding time and high temperatures achieved in this unit may considerably reduce the stability of the nucleation sites, which tend to float to the surface and to be absorbed in the slag, instead of remaining within the molten iron that is further processed. In particular, it has been observed that cast iron melts held in a holding furnace for a long period (e.g., more than 1 hour), end up with insufficient nucleation site levels. This results in that these molten irons do not respond well to traditional inoculation methods.
As consequence, a metal degraded by permanence during long periods in the pressurized pouring unit and at high temperature, has a greater tendency to form micro porosities (micro shrinkage) in the solidified cast iron parts, when compared to cast iron parts manufactured in casting units without heating and that use shorter stay times. This is due to the lower potential of said degraded metal to form graphite nodules throughout the solidification process, even if a final inoculation is performed.
In order to recover the quality of the metal, joint addition of some elements to provide nucleation sites to molten cast irons has been proposed to improve the performance of cast iron inoculants. Fresh additions of sulphur and oxygen have been described to enhance the formation substrates for graphite precipitation throughout solidification period. In addition, it has been claimed that the controlled additions of the elements sulphur and oxygen to commercially available ferrosilicon-based inoculants can provide enhanced performance. This is an attempt to ensure that sufficient sulphur and oxygen will be available for subsequent reactions with the elements added as inoculants.
For instance, the US patent N° 6,866,696 discloses an additive for iron and steel production comprising a sulphur-containing material (such as pyrite), an oxide-containing material (such as iron oxide), and a mechanical binder (such as iron powder).
In spite of the proposed methods and additives, there is still the necessity of providing a method for recovering the quality of the molten iron treated with Mg and stored in pressurized pouring units with or without heating.

BRIEF DESCRIPTION OF THE INVENTION

The inventors of the present invention have developed a method for preparing ductile (spheroidal) cast iron. The method of the invention comprises adding an additive composition to molten iron which allows to improve the metallurgical quality of the iron. Without being bound to any theory, it is believed that the additive generates sequential nucleation sites on which graphite grows, thereby decreasing the micro porosity on the solidified parts obtained therefrom.
Thus, in a first aspect, the present invention refers to a method for preparing ductile cast iron, comprising the steps of:

a) melting a base iron to obtain a molten iron, the base iron having a carbon content from 3.00 wt. % to 4.00 wt. %, a silicon content from 1.50 wt. % to 4.50 wt. % and a sulphur content less than or equal to 0.025 wt. %, wherein the wt. % is based on the total weight of base iron;
b) treating the molten iron with a nodulazing composition comprising a source of magnesium and, optionally, a source of rare earth elements to obtain a nodulized molten iron;
c) introducing the nodulized molten iron into a pouring unit, wherein the pouring unit comprises an inlet channel, a pressurized pouring vessel, a pouring basin, a stopper/nozzle mechanism, and, optionally, a heating inductor; wherein the nodulized molten iron is introduced into the pouring unit through the inlet channel into the pressurized pouring vessel;
d) holding the molten iron in the pressurized pouring vessel resulting in a molten iron having a sulphur content from 0.002 wt. % to 0.006 wt. %;
e) pushing the molten iron from the pressurized pouring vessel to the pouring basin;
f) pouring a stream of the molten iron from the pouring basin into a mould through the stopper/nozzle mechanism; and
g) allowing the molten iron to solidify in said mould;

A second aspect of the present invention refers to an additive composition for preparing ductile iron according to a method as defined herein, the additive comprising a source of sulphur, a source of selenium and, optionally a source of tellurium and/or a source of rare earth elements, and comprising a carrier.
The additive composition of the present invention is based on a phenomenon of compensatory metallurgy targeted to restore the optimal nucleation capacity in the molten iron coming from a pressurized vessel for ductile cast iron production. The aim of the additive in the molten iron is to generate some of the disappeared nuclei, as well as other new ones, in order to stimulate the precipitation of graphite minimizing the micro shrinkage tendency of the liquid iron during solidification.
Another aspect of the present invention relates to a kit comprising an additive composition as defined herein and an inoculating composition, the inoculating composition preferably comprising elements selected from Ca, Zr, rare earth elements and Al.
A further aspect of the present invention relates to a ductile cast iron obtainable by a method as defined herein.
A final aspect of the present invention relates to the use of an additive composition as defined herein in the production of ductile cast iron.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

To better understand the invention, its objects and advantages, the following figures are attached to the specification in which the following is depicted:

Figure 1. Theta angle representation in the cooling curve of a standard thermal test sample, wherein - refers to measures (interpolated), ---- refers to measures (1^st derivative) and -·-·-·- refers to measures (2^nd derivate).
Figure 2A. Size distribution of nodular graphite in the inoculated standard cups used for the cooling registration obtained for curve INTO in Test 0.
Figure 2B. Optical micrograph of a nodular graphite in the inoculated standard cups used for the cooling registration obtained for curve INTO in Test 0.
Figure 3A. Size distribution of nodular graphite in the inoculated standard cups used for the cooling registration obtained for curve INT1 in Test 1.
Figure 3B. Optical micrograph of a nodular graphite in the inoculated standard cups used for the cooling registration obtained for curve INT1 in Test 1.
Figure 4A. Size distribution of nodular graphite in the inoculated standard cups used for the cooling registration obtained for curve INT1 in Test 2.
Figure 4B. Optical micrograph of a nodular in the inoculated standard cups used for the cooling registration obtained for curve INT1 in Test 2.
Figure 5A. Size distribution of nodular graphite in the ductile cast iron obtained for curve INTO in Test 0.
Figure 5B. Optical micrograph of a nodular graphite in the ductile cast iron obtained for curve INTO in Test 0.
Figure 6A. Size distribution of nodular graphite in the ductile cast iron obtained for curve INT1 in Test 1.
Figure 6B. Optical micrograph of a nodular graphite in the ductile cast iron obtained for curve INT1 in Test 1.
Figure 7A. Size distribution of nodular graphite in the ductile cast iron obtained for curve INT1 in Test 2.
Figure 7B. Optical micrograph of a nodular graphite in the ductile cast iron obtained for curve INT1 in Test 2.
Figure 8. SEM analysis of three graphite nuclei (A, B and C respectively) and corresponding spectrum of their chemical composition.

DETAILED DESCRIPTION OF THE INVENTION

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs.

Method for preparing ductile cast iron

The term "ductile cast iron", also known as "ductile iron", "nodular cast iron", "spheroidal graphite iron", "spheroidal graphite cast iron" and "SG iron", is a type of graphite-rich cast iron wherein graphite is in the form of nodules. The usual microstructure of ductile cast iron is a matrix of ferrite and pearlite with graphite nodules dispersed throughout the structure. The size, shape and distribution of the graphite nodules is important to the physical properties of the ductile iron. Rounded graphite nodules inhibit the creation of cracks, thus providing the enhanced ductility that gives the alloy its name. Therefore, ductile cast iron has much more impact and fatigue resistance compared to most varieties of cast iron, due to its nodular graphite inclusions.
It is an object of the present invention to improve the metallurgical quality of molten iron for ductile cast iron production, generating a sequential nucleation sites on which graphite grows, decreasing the microporosity (micro shrinkage) on the solidified parts. In particular, the method of the present invention comprises a step a) of melting a base iron to obtain a molten iron, the base iron having a carbon content from 3.00 wt. % to 4.00 wt. %, a silicon content from 1.50 wt. % to 4.5 wt. % and a sulphur content less than or equal to 0.025 wt. %, wherein the wt. % is based on the total weight of base iron.
The term "base iron" refers to raw materials for the manufacturing of ductile cast iron. The base iron can be obtained from primary raw materials or from recycled raw materials. Non-limiting examples of suitable raw materials for the manufacturing of ductile cast iron are pig iron, end-of-life automotive package scrap, foundry returns and steel scrap. These raw materials are usually melted in cupola or induction melting furnaces.
Suitable base irons for a method as described herein, may have a typical chemical composition for generic ductile iron applications.
A base iron for a method described herein has:

a carbon content: 3.00-4.00 wt. %;
a silicon content: 1.50-4,50 wt. %: and
a sulphur content: ≤ 0.025 wt. %

Iron based raw materials with a higher sulphur content may be used if subjected to a desulphurization process to provide a base iron with a sulphur content of less than or equal to 0.025 wt. %. Desulphurization is known to a skilled person and may be performed by means known in the art.
In several embodiments the base iron may be subjected to a composition adjustment, e.g., to achieve the targeted contents of carbon, silicon or sulphur, or to provide the base iron with other desired elements. Composition adjustments are known to a skilled person and may be performed by means known in the art. For instance, a composition adjustment may be performed by treating the molten base iron with, e.g., ferrosilicon and/or graphite recarburizer.
A method as described herein further comprises a step b) of treating the molten iron with a nodulazing composition comprising a source of magnesium and, optionally, a source of rare earth elements to obtain a nodulized molten iron.
The term "nodulation" or "spheroidization" refers to an essential treatment in the manufacturing of ductile cast iron, in which the prerequisites of carbon precipitation process are met in order to allow graphite precipitating from iron to be formed into spheres. This treatment is performed either as a ladle treatment or inside a mould by using a "nodulizing composition".
A "nodulizing composition" comprising a source of magnesium and, optionally, a source of rare earth elements used in a method as described herein may typically be a magnesium-rich, and optionally rare earth-rich (usually cerium). For instance, the nodulizing composition may comprise from 4.0 wt. % to 100 wt. % Mg and optionally up to 3 wt. % of rare earth elements. The nodulizing composition may preferably be a magnesium-rich, and optionally rare earth-rich, ferro-silicon alloy composition, such as magnesium ferrosilicon alloy, also referred to as FeSiMg alloy. In particular a ferro-silicon alloy composition to be used as a nodulizing composition may comprise from 4.0 wt. % to 100 wt. % Mg and optionally up to 3 wt. % of rare earth elements. Magnesium (and optionally rare earth elements) acts as nodulizing element forming compounds (with sulphur, oxygen, calcium, silicon, nitrogen and other elements), which promote the growth of spherical graphite. In some cases, alloys such as silicon carbide, pure graphite and other proprietary alloy mixtures may be added to molten ductile irons prior to magnesium (and optionally rare earth elements) treatment.
A method as described herein further comprises a step c) of introducing the nodulized molten iron into a pouring unit, wherein the pouring unit comprises an inlet channel, a pressurized pouring vessel, a pouring basin, a stopper/nozzle mechanism, and, optionally, a heating inductor; wherein the nodulized molten iron is introduced into the pouring unit through the inlet channel into the pressurized pouring vessel.
Suitable pouring units are known in the art and need not further elucidation here.
A method as described herein further comprises a step d) of holding the molten iron in the pressurized pouring vessel resulting in a molten iron having a sulphur content from 0.002 wt. % to 0.006 wt. %.
In a particular embodiment, the molten iron in step d) is hold in the pressurized vessel at least one hour.
As described above, the time that the molten iron remains in the pressurized pouring vessel before being cast into the moulds, may result in a degradation of the metallurgical quality of the molten iron, because the holding time and high temperatures achieved in this vessel tend to reduce the nucleation sites, as with time nucleation sites tend to go to the slag and are no longer available for nucleation during casting, i.e., cooling of the molten iron in the mould.
Accordingly, after holding the molten iron in the pouring vessel the molten iron has a lower sulphur content than the starting base iron, and typically has a sulphur content from 0.002 wt. % to 0.006 wt. %, with respect to the weight of molten iron.
A metal degraded by permanence of long periods in the pouring unit and at high temperature, has a greater tendency to the formation of micro porosities in the solidified parts, compared to parts manufactured in casting units without heating and shorter stay times, due to the low potential to form graphite throughout the solidification process even in spite of the final inoculation.
A method as described herein further comprises a step e) of pushing the molten iron from the pressurized pouring vessel to the pouring basin; a step f) of pouring a stream of the molten iron from the pouring basin into a mould through the stopper/nozzle mechanism; and a step g) of allowing the molten iron to solidify in said mould.
A skilled person knows how these steps are to be performed so no further elucidations are needed.
A method as described herein additionally comprises treating the molten iron with an additive composition and with an inoculating composition after step d) and prior to step g).
The additive composition comprises a source of sulphur, a source selenium and, optionally, a source of tellurium and/or a source of rare earth elements (preferably lanthanum and/or cerium), and comprises a carrier. An additive composition suitable for a method as described herein is elucidated in detail further below.
An additive composition as described herein has been found to allow for recovering the quality of molten iron treated with the source of magnesium and, optionally, the source of rare earth elements, and stored in the pouring unit, typically at high temperatures. Without being bound to any theory, an additive composition as described herein has been found to generate a sequential nucleation sites to help graphite to grow sequentially and to reduce the tendency to microporosity (also referred to as micro shrinkage) in solid castings.
The total content of sulphur, selenium, and optionally, tellurium and/or rare earth elements in the resulting molten iron is increased by a total amount from 0.002 to 0.012 wt. % respect to the weight of the molten iron.
An "inoculating composition" used in a method as described herein, typically comprises elements that offer the possibility to form stable compounds with, e.g., sulphur, oxygen, nitrogen, titanium, silicon, or combinations thereof. Such stable compounds forming elements may be selected from Al, Ca, Ba, Sr, Ce, La, Mn, Bi, S, O, Ti, Mg, and Zr. It may be preferred for the inoculating composition to comprise elements selected from Ca, Zr, rare earth elements (e.g., Sr, Ce and La) and Al.
There are many different types of inoculating compositions which are commercially available and suitable in a method as described herein. As a mode of example, an inoculation composition may be a calcium silicon, a calcium bearing ferrosilicon alloys or other ferrosilicon-based alloys that contain small percentages of said stable compounds forming elements. The aim of inoculation is to control the microstructure of cast irons (more fine-grained) as well as reduce the chilling tendency or the formation of iron carbides (or cementite). The inoculant allows to form stable compounds with sulphur, oxygen, nitrogen, titanium, silicon, or combinations thereof. These stable compounds (or atomic clusters) provide a substrate surface with nucleation sites upon which dissolved carbon in the molten iron can start to grow as graphite flakes or nodules, before sufficient undercooling occurs that favours the formation of iron carbide. The presence of iron carbide in the iron matrix is undesirable because this constituent is hard and brittle and can result in poor mechanical properties and machinability.
As indicated above, the molten iron is treated with an additive composition and with an inoculating composition after step d) and prior to step g).
For instance, the treatment may be performed during the step e) of pushing the molten iron from the pressurized pouring vessel to the pouring basin; or prior to or during the step f) of pouring the molten iron into the mould.
In a particular embodiment, the molten iron is first treated with the additive composition, and is subsequently treated with the inoculating composition. For instance, the molten iron may be treated with the additive composition during step e) and treated with the inoculating composition during step f).
In a particular embodiment, the molten iron is first treated with the inoculating composition, and is subsequently treated with the additive composition. For instance, the molten iron may be treated with the inoculating composition during step e) and treated with the additive composition during step f).
In another particular embodiment, the molten iron is simultaneously treated with the additive composition and the inoculating composition. For instance, the molten iron may be simultaneously treated with the additive composition and the inoculating composition during step e) or during step f).
In a particular embodiment, the molten iron is treated with the additive composition and/or the inoculating composition by adding the additive composition and/or the inoculating composition to the molten iron at the pouring basin.
In another particular embodiment, the molten iron is treated with the additive composition and/or the inoculating composition by adding the additive composition and/or the inoculating composition to the stream of molten iron before the molten iron gets into the mould. In a particular embodiment, the additive composition and/or the inoculating composition can be added to the stream of molten iron by blowing the additive composition and/or the inoculating composition, e.g., in powder form, onto the stream of molten iron.
In another particular embodiment, the molten iron is treated with the additive composition and/or the inoculating composition, by providing the mould with the additive composition and/or the inoculating composition, typically before the molten iron gets into the mould.
In a more particular embodiment, the molten iron is first treated with the additive composition at the pouring basin (i.e., during step e)), and is subsequently treated with the inoculating composition by adding the inoculating composition to the stream of molten iron before the molten iron gets into the mould (i.e., during step f)).
In another particular embodiment, the molten iron is simultaneously treated with the additive composition and the inoculating composition by adding the additive composition and the inoculating composition to the stream of molten iron before the molten iron gets into the mould.
In a particular embodiment, the additive composition and the inoculating composition can be added to the stream of molten iron by blowing them together or separately.
In another more particular embodiment, the molten iron is treated with the additive composition, by providing the mould with the additive composition (i.e., during step f)).
In another more particular embodiment, the molten iron is treated with the inoculating composition, by providing the mould with the inoculating composition (i.e., during step f)).
In yet another more embodiment, the molten iron is simultaneously treated with the additive composition and the inoculating composition, by providing the mould with the additive composition and the inoculating composition (i.e., during step f)).
An additive composition used in the method as defined above and suitable forms thereof, will be described in more detail in the following section of this specification.

Additive composition

A further aspect of the present invention is directed to an additive composition for preparing ductile cast iron according to a method as described herein, the additive comprising a source of sulphur, a source of selenium and, optionally a source of tellurium and/or a source of rare earth elements, and comprising a carrier.
The additive composition may also be referred to as compensative additive, given that the treatment of the molten iron with the additive composition aims to compensate for the elements lost during the holding of the molten iron in the pouring unit.
Without being bound to any theory, it has been found, that an additive composition as defined above improves the metallurgical quality of molten iron for preparing ductile cast iron generating a sequential nucleation sites in which graphite growths decreasing the microporosity (micro shrinkage) on the solidified parts.
The sources of sulphur, the sources of selenium, the sources of tellurium and the sources of rare earth elements in the additive composition can be pure sulphur, pure selenium, pure tellurium and pure rare earth elements, or compounds, alloys or salts of such elements. Such sources of sulphur, selenium and of tellurium and of rare earth elements may be added to molten iron independently or as a mixture of two or more of them.
Non-limiting examples of sources of sulphur suitable for an additive composition as defined above are pure sulphur and pyrite.
Non-limiting examples of sources of selenium suitable for an additive composition as defined above are pure selenium and ferroselenium.
Non-limiting examples of sources of tellurium suitable for an additive composition as defined above are pure tellurium and ferrotellurium.
Non-limiting examples of sources of rare earth elements suitable for an additive composition as defined above are pure elements (such as cerium, and lanthanum) added in a ferrosilicium base.
In a particular embodiment, an additive composition as described herein comprises from 2 wt. % to 60 wt. % of sulphur based on the total weight of the additive composition, more preferably from 5 wt. % to 30 wt. %.
In another particular embodiment, an additive composition as described herein comprises from 0.5 wt. % to 20 wt. % of selenium based on the total weight of the additive composition, more preferably from 1 wt. % to 10 wt. %.
In another particular embodiment, an additive as described herein comprises from 0.001 wt. % to 1 wt. % of tellurium, more preferably from 0.001 wt. % to 0.5 wt. %.
In another particular embodiment, an additive as described herein comprises from 0.5 wt. % to 10 wt. % of rare earth elements.
In a particular embodiment, an additive as described herein comprises lanthanide elements (such as Ce and La), Sc and Y as rare earth elements. La and Ce may be preferred as rare earth elements, and Ce may be particularly preferred.
In another particular embodiment, an additive composition as described herein may further comprise one or more additional elements selected from elements of group 2 such as Mg, Ca, Sr and Ba; elements of group 4 such as Ti and Zr; elements of group 13 such as Al; elements of group 15 such as Bi, such additional elements being preferably selected from Ca, Zr, and Al, which can act both as additive and inoculant simultaneously.
In another particular embodiment, an additive as described herein comprises up to 10 wt. % of the above-mentioned additional elements.
In a particular embodiment, the additive composition comprises elements which may act as inoculant. In a method as described herein the molten iron may be treated simultaneously with the additive composition and inoculant composition, using such additive compositions.
Generally, if the additive composition is added separately from the inoculating composition, the additive does not typically comprise elements that act as inoculants. However, in several particular embodiments an additive composition may comprise elements that act as inoculants and additional elements that act also as inoculants may be added separately as an inoculating composition.
The above listed elements can be present in the additive composition as pure elements or as compounds, alloys or salts of such elements. They may be added to molten iron independently or as a mixture of two or more of them. Non-limiting examples of these compounds, alloys or salts for the additive composition as defined above are oxides, sulphides, oxysulphides, selenides, tellurides, silicates, nitrides among others.
According to the scope of the invention, besides the addition of the above defined elements, other elements may be added for other purposes, without affecting the final characteristics of the ductile cast iron due to the presence of these elements.
An additive composition as described herein further comprises a carrier. Non-limiting examples of carriers suitable for the additive composition as defined herein are ferrosilicon, iron powder, iron oxide and commercial inoculants among others.
In a particular embodiment, the carrier comprises ferrosilicon and/or iron powder.
In another particular embodiment, the carrier comprises ferrosilicon having from 45 wt. % to 75 wt. % silicon based on the total weight amount of ferrosilicon, for instance, a ferrosilicon having a 75 wt.% of silicon is also known as 75% FeSi.
Non-limiting suitable forms for the additive composition of the present invention are powder, cored wire (e.g., the additive composition being encapsulated in a steel wire), cast insert or compressed powder.
In particular embodiments, in a method described herein the additive composition may be added to molten iron in the pouring basin in the form of a cored wire (e.g., wherein the wire is made of steel and the core is an additive composition as described herein).
In particular embodiments, in a method described herein the additive composition may be added to the stream of molten iron in the form of a powder (e.g., by blowing) or in the form of a cored wire (e.g., encapsulated in a steel wire and placed in the stream of molten iron).
In particular embodiments, in a method described herein the mould may be provided with the additive composition as a cast insert or as a compressed powder (also referred to in the art as a briquette), before the molten iron gets into the mould.
An additive composition as defined above can be comprised in a kit together with an inoculating composition, the inoculating composition preferably comprising elements selected from Ca, Zr, rare earth elements (e.g., Ce and La) and Al.
As discussed above for additive compositions separate from inoculating compositions, it may be preferred that the additive composition of the kit does not comprise elements that act as an inoculating agent, but if desired may also comprise such elements.
As it has been explained above, the additive composition as defined herein can be used in the production of ductile cast iron. Thus, another aspect of the present refers to the use of the additive composition as defined above in the production of ductile cast iron.

Ductile cast iron

Another aspect of the present invention, is a ductile cast iron obtainable or obtained by a method as described herein.
The metallurgical quality of said ductile cast iron is improved. In particular, without being bound to any theory, it is believed that such an improvement may be due to the fact that sequential nucleation sites are created, due to the addition of the additive composition to the molten iron, on which graphite grows decreasing the micro porosity on the solidified parts.
Due to the decreased micro porosity, ductile cast iron obtainable according to a method of the present invention presents reduced defect volume respect to ductile cast iron obtainable by the same method but without adding the additive composition.
Furthermore, again without being bound to any theory, the improvement of the metallurgical quality may also be due to the increase of the number of nodules observed in cast iron obtainable or obtained by a method as described herein, as a consequence of the increase of nucleation sites associated to the treatment with an additive composition as described herein.
The improvement of the metallurgical quality may also be due to the nodule size distribution observed for ductile cast iron obtainable or obtained by a method as described herein. In particular, ductile cast iron obtainable or obtained by a method as described herein may have a nodule size distribution with over 20% of nodules in the sizes between 5 µm and 10 µm in diameter.
The total content of sulphur, selenium, and optionally, tellurium, in the molten iron obtainable according to the method as defined above is increased by a total amount from 0.002 to 0.012 wt. % respect to total weight of the molten iron. Accordingly, a ductile cast iron obtainable or obtained by a method as described herein, wherein prior to the treatment with the additive composition and the inoculating composition has a sulphur content of 0.002 wt.% - 0.006 wt.%, may typically have from0.004 wt. % to 0.018 wt. % of total content of sulphur, selenium, and optionally, tellurium and/or rare earth elements As described, an additive composition as described herein has been found to generate sequential nucleation sites to help graphite grow sequentially as reflected by the size distribution of the nodules in the final cast iron, and to reduce the tendency to micro porosity in solid castings.
All the features described in this specification (including the claims, description and drawings) and/or all the steps of the described method can be combined in any way, with the exception of combinations of such mutually exclusive features and/or steps.
The invention will be further illustrated by means of examples, which should not be interpreted as limiting the scope of the claims.

EXAMPLES

Materials and methodology

The following tests were designed to evaluate the effectiveness of additive compositions in ductile cast iron preparation, also referred to as compensation evaluation, which was poured by means of pressurized induction heating pouring unit, using two different locations and procedures for the treatment of molten iron with the additive composition, also referred to as additive integration, (Tests 1 and 2) and one procedure to settle the standard situation without performing the treatment with the additive composition (Test 0).
The results obtained in each test were analysed by thermal analysis technique, metallographic study of the standard thermal analysis samples and of the produced ductile cast irons (both metallography studies including a complete analysis of the nodule count, shape parameters of graphite (roundness and aspect ratio) and size distribution), X-ray soundness evaluation of the poured castings, and tomography evaluation of the shrinkage level the casting poured.
The moulds for each test were produced in consecutive batches in an intensive vertical moulding machine DISAMATIC 240: 20 moulds batch, marked as "0" from the 10th to 12^th mould for the standard process evaluation in Test 0; 20 moulds batch marked as "1" from the 10^th to 12^th mould for pouring basin compensation evaluation in Test 1; and 20 moulds marked as "2" from the 10^th to 12^th for pouring stream compensation evaluation in Test 2. Marked moulds were segregated. So, each bunch have 6 cavities and 3 marked mould per test then 18 castings from each test were segregated and identified by the test number for the shrinkage and micrography evaluation.
The melting was done in a medium frequency induction furnace (250Hz, 8.000Kw) with 12 t crucible capacity.
The base iron consisted of 53 wt. % foundry returns, 45.1 wt. % automotive steel scrap, 1.78 wt. % coke recarburizer and 0.12wt. % ferrosilicon (75% in Si). In order to determine the composition of the base iron produced, a chemical analysis disk (38mm of diameter × 4mm of thickness) was poured and analysed thereafter by means of spark emission spectrometry (see Table 1). Table 1. Base iron composition by spark emission spectrometry.

Base metal chemical composition [wt. %]

C Si Mn Cu P S Ti

3.92 1.72 0.34 0.34 0.018 0.008 0.025

Iron (Fe) balance
The composition adjustment was done in the treatment ladles whose capacity was 2500kg. The adjustment additions consisted of 29 kg of 75% FeSi and 0,64 kg graphite recarburizer.
Two nodularization treatments were carried out by the tundish cover method using 1,05wt. % FeSiMg alloy (grain size 2-5mm, 6wt. % Mg, 2wt. % Ca, 0,9wt. % Al, 1wt. % rare earth elements, 45 wt. % X and Fe in balance) covered by 0,95 wt. % steel chips as the nodulizing composition. The tapping temperature was of 1490°C in the first treatment and of 1486°C for the second one. When the reaction was finished, the Mg-treated batches were transferred to the skimming area and finally tapped in the press-pour unit (heating pouring unit).
The press-pour unit vessel has a max capability of 12 t and a minimum heal level of 6 t. In the present trial, the initial level was 6.5 t and after the tapping two consecutive treated ladles, the trial started with 11.5 t of treated iron in the vessel.

The composition of the alloy in the pouring unit was analysed before pouring the trial by means of spark emission spectrometry (Table 2).

Table 2. Chemical composition at the pouring unit basin prior to the trial starting.

Chemical Composition [wt. %]
C	Si	Mn	P	S	Ni	Cu	Mg	Se	Te	Al	Ti	Sn
3.83	2.24	0.3	0.017	0.003	0.02	0.21	0.037	<0.002	<0.002	<0.010	0.034	0.016
Iron (Fe) in balance

For the compensation evaluation, the three batches of moulds were poured in the following way.
The inoculation, for all moulds, was performed in the pouring stream by blowing 0,12 wt. % of a standard inoculating composition during the pouring of the moulds.

Test 0 ― Ductile cast iron obtained according to standard method (without additive)

The pouring temperature was registered by an immersion thermocouple, recording 1406°C. Two thermal analysis cups (PT0, INTO) were collected during the pouring of the first 20 moulds. The melt for both cups were obtained directly at the same time, from the pouring base: PT0 was plane standard thermal analysis cup and INTO was standard thermal analysis cup with the addition of 0,12 wt. % of the stream inoculant.

Test 1 ― Ductile cast iron obtained introducing the additive composition in the pouring basin by immersion.

The pouring following up to the 10th mould of the second batch get the pouring spot, then line was stopped.
The additive composition of the invention (also referred to as compensative additive) was introduced to the molten iron prior to the inoculating composition, by immersion in the pouring basin close to the stopper/nozzle. After stirring the liquid metal in the immersion spot, the marked molds of the second batch were poured. The addition of this compensative additive consisted of 0,234 kg encapsuled in a steel container (i.e., a cored wire) to minimize fading in the immersion operation whose composition is shown in Table 3. In the additive, sulphur was added as pyrite, selenium as pure element, and iron and silicon as ferrosilicon. Table 3. Additive composition analysed by the combination of following techniques: Xray fluorescence for Si content, combustion for Sulphur content and ICP-MS for Selenium.

Si [wt. %] S [wt. %] Se [wt. %] Fe [wt. %]

40 25 10 Balance
At the same time of pouring, two thermal analysis cups were sampled (PT1, INT1). The melt for both cups were obtained at the same time, from the pouring basin: PT1 was plane standard thermal analysis cup, and INT1 was standard thermal analysis cup with the addition of 0,12% of the stream inoculant.
The pouring of the second batch kept on going up to finishing it.

Test 2 - Ductile cast iron obtained by introducing the additive composition simultaneously with the inoculant by dosage and blowing in the pouring stream.

The moulding line was stopped as the last mould of the Test 1 was poured. Then it was checked the proper dosage of a new compensative additive by means of a blowing dosage system at the pouring stream.
The addition of this new compensative additive consisted of 0,10 wt.% of poured melt whose composition is shown in the Table 4. Table 4. Additive composition analysed by the combination of following techniques: Xray fluorescence for Si content, combustion for Sulphur content, ICP-MS for Selenium and ICP-OS for Rare Earth.

Si [wt. %] S [wt. %] Se [wt. %] R.E. [wt. %] Fe [wt. %]

60 6 3 1.4 Balance
Then, the pouring of the moulds belonging to this second test was done up to the end of the batch. During the pouring, the melt temperature was checked recording 1398°C.
As in the previous tests, two thermal analysis cups (PT2, INT2) were poured, obtaining the melt directly from the pouring base: PT2 was plane standard thermal analysis cup, and INT2 was standard thermal analysis cup with the addition of 0,12 wt. % of the stream inoculant and 0.1 wt. % of the new compensative additive.

Results and discussion

The main parameters obtained in the cooling curves analyses in the trial were liquidus temperature (T_liquidus), minimum eutectic temperature (Tmin _euctic), recalescence (Recal.) and solidus temperature (T solidus calculated by means of the second derivative). Additionally, for a better understanding of the solidification behaviour it is included the Theta angle, referred to as A_Theta, (or simply Θ in figure 1), which is the angle of the first derivative at the last minimum before solidification ending ( Figure 1 ).

Table 5 summarizes the cooling curve parameters obtained from the test done. Unfortunately, PT1 reading failed because the thermocouple broke during the cooling.

Table 5. Cooling curves parameters for each test

Test	Curve	T _liquidus (°C)	Tmin _euctic (°C)	Recal. (°C)	T_solidus (°C)	A _Theta
T0	PT0	1139.0	1119.7	17.0	1103.4	----------
T0	INT0	1148.9	1141.9	7.0	1122.4	167.3°
T1	PT1	-----------	-----------	-----------	-----------	-----------
T1	INT1	1149.4	1144.5	5.0	1131.7	161.1°
T2	PT2	1138.7	1116.4	5.6	1103.8	-----------
T2	INT1	1148.6	1144.1	5.1	1129.6	161.7°

Metallographic analyses of the inoculated standard cups used for the cooling registration in the tree tests were performed. The nodule count distribution (Table 6 and Figures 3, 5 and 7) and size distribution (Figures 2, 4 and 6) were analyzed for each test. Table 6. Metallographic analyses result in standards cups

Test Curve Nodule count (nodules/mm²)

T0 INT0 340

T1 INT1 511

T2 INT1 410

In order to confirm the improved behaviour of the alloy treated with the compensative additive, in terms of shrinkage tendency, the casting segregated during the tests were inspected by X-ray. So, 54 castings (6 cavities per bunch and three bunches each test) were inspected. Table 7 register the number of casting showing detectable shrinkage per cavity.

Table 7. X-ray inspection result recorded by cavity and test number.

Test	Number of casting inspected	Cavities with detectable defect						% defective castings detected
Test	Number of casting inspected	#1	#2	#3	#4	#5	#6
T0	18	3	3	3	2	3	3	94
T1	18	-	1	-	-	-	1	11
T2	18	-	3	1	-	-	1	28

Analyzing by means of tomography those casting from cavity 6 with bigger defects from each test, according Xray inspection, the result was summarized in Table 8. Table 8. Size of the shrinkage defects by tomography inspection.

Test Defect volume (mm³)

Test 0 162

Test 1 36

Test 2 62
Inspection results both by X-ray and tomography, showed coherence with the metallography analyses done in the thermal analyses cups. The defects size has been reduced from 60% in Test 2 up to 80% in the casting from Test 1, taken reference in the standard process represented by Test 0.
Finally, to confirm the results obtained, the metallography study was replicated for the samples gotten from the castings inspected by tomography. Samples were extracted at the same place in the castings. Table 9. Metallographic analyses result in the castings.

Test Curve Nodule count (nodules/mm²)

T0 INT0 293

T1 INT1 360

T2 INT1 340

Table 10 compares the results of the compensative additive treatment carried out in the pouring basin (Test 1) and in the pouring stream (Test 2) with those of standard metallurgy, without using a compensative additive (Test 0).

Table 10. Results comparison of the compensative additive treatment carried out in the pouring basin (Test 1) and in the pouring stream (Test 2) with those of standard metallurgy, without using a compensative additive (Test 0).

Test	*Reduction of shrinkage volume defect compared to Test 0 (%)**	Solidus temperature (°C)	Nodule count at the casting (nodules/mm²)	Nodule sized (5-15 µm) at the thermal analyses cup (%)
Test 0	-	1122.4	293	41
Test 1	80	1131.7	360	65
Test 2	48	1129.6	340	48

*Shrinkage volume defect was 162 mm³

The reference selected for the trial present significative shrinkage tendency. All the tests were done starting with the same liquid metal carrying out further addition of the compensative additive at the pouring basin (Test 1) or at the pouring stream (Test 2). Moulds were formed in a continuous way to guaranty the same properties of the sand and mold conformation.
The nature of the graphite nuclei was analyzed by Scanning Electron Microscopy (SEM), and it was found the role of selenium in the nuclei formation, mainly, together with the rare earth. In a minor percentage it was found in compounds together with calcium. In Figure 8, selenium was observed inside the graphite nodules.

Claims

A method for preparing ductile cast iron, comprising the steps of:
a) melting a base iron to obtain a molten iron, the base iron having a carbon content from 3.00 wt. % to 4.00 wt. %, a silicon content from 1.50 wt.% to 4.5 wt. % and a sulphur content less than or equal to 0.025 wt. %, wherein the wt.% is based on the total weight of base iron;

b) treating the molten iron with a nodulazing composition comprising a source of magnesium and, optionally, a source of rare earth elements to obtain a nodulized molten iron;

c) introducing the nodulized molten iron into a pouring unit, wherein the pouring unit comprises an inlet channel, a pressurized pouring vessel, a pouring basin, a stopper/nozzle mechanism, and, optionally, a heating inductor; wherein the nodulized molten iron is introduced into the pouring unit through the inlet channel into the pressurized pouring vessel;

d) holding the molten iron in the pressurized pouring vessel resulting in a molten iron having a sulphur content from 0.002 wt. % to 0.006 wt. %;

e) pushing the molten iron from the pressurized pouring vessel to the pouring basin;

f) pouring a stream of the molten iron from the pouring basin into a mould through the stopper/nozzle mechanism; and

g) allowing the molten iron to solidify in said mould;
wherein the molten iron is treated with an additive composition and with an inoculating composition after step d) and prior to step g);
wherein the additive composition comprises a source of sulphur, a source of selenium and, optionally, a source of tellurium and/or a source of rare earth elements, and comprises a carrier; and
wherein the total content of sulphur, selenium, and optionally, tellurium and/or rare earth elements, in the resulting molten iron is increased by a total amount from 0.002 to 0.012 wt.% respect to the weight of the molten iron.
The method according to claim 1, wherein the molten iron is first treated with the additive composition, and is subsequently treated with the inoculating composition.
The method according to claim 1, wherein the molten iron is first treated with the inoculating composition, and is subsequently treated with the additive composition.
The method according to claim 1, wherein the molten iron is simultaneously treated with the additive composition and the inoculating composition.
The method according to any one of claims 1-4, wherein the molten iron is treated with the additive composition and/or the inoculating composition by adding the additive composition and/or the inoculating composition to the molten iron at the pouring basin.
The method according to claim 1-5, wherein the molten iron is treated with the additive composition and/or the inoculating composition by adding the additive composition and/or the inoculating composition to the stream of molten iron before the molten iron gets into the mould.
The method according to claim 1-6, wherein the molten iron is treated with the additive composition and/or the inoculating composition, by providing the mould with the additive composition and/or the inoculating composition before the molten iron gets into the mould.
The method according to claim 2, wherein the molten iron:
- is first treated with the additive composition at the pouring basin, and

- is subsequently treated with the inoculating composition by adding the inoculating composition to the stream of molten iron before the molten iron gets into the mould.
The method according to claim 4, wherein the molten iron is simultaneously treated with the additive composition and the inoculating composition by adding the additive composition and the inoculating composition to the stream of molten iron before the molten iron gets into the mould.
The method according to any of claims 1 to 9, wherein the additive composition is according to any one of claims 11-18.
An additive composition for preparing ductile cast iron according to the method of claim 1, the additive comprising a source of sulphur, a source of selenium and, optionally a source of tellurium and/or a source of rare earth elements, and comprising a carrier.
The additive composition according to claim 9, comprising from 2 wt. % to 60 wt. % of sulphur based on the total weight of the additive composition.
The additive composition according to any of claims 10-12, comprising from 0.5 wt. % to 20 wt. % of selenium based on the total weight of the additive composition.
The additive composition according to any of claims 10-13, from 0.001 wt. % to 1 wt. % of tellurium based on the total weight of the additive composition.
The additive composition according to any of claims 10-14, further comprising rare earth elements, in particular lanthanide elements, Sc and Y.
The additive composition according to any of claims 10-15, further comprising one or more additional elements selected from elements of group 2 such as Mg, Ca, Sr and Ba; elements of group 4 such as Ti and Zr; elements of group 13 such as Al; elements of group 15 such as Bi; such additional elements being preferably selected from Ca, Ce, Zr and Al.
The additive composition according to any of claims 10-16, wherein the carrier comprises ferrosilicon and/or iron powder.
The additive composition according to any of claims 10-17, wherein the additive is in the form of powder, cored wire, cast insert or compressed powder.
A kit comprising an additive composition according to any one of claims 10-18 and an inoculating composition, the inoculating composition preferably comprising elements selected from Ca, Zr, rare earth elements and Al.
A ductile cast iron obtainable by the method according to any of claims 1-10.
Use of the additive composition according to any of claims 11-18 in the production of ductile cast iron.