Method of manufacturing liquid steel

Abstract

The invention relates to a method of manufacturing liquid steel having a reduced amount of residual elements, in particular Mo, for casting into rolling stock.

Description

[0001] METHOD OF MANUFACTURING LIQUID STEEL

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a method of manufacturing liquid steel having a reduced amount of residual elements, in particular molybdenum, for casting into rolling stock.

[0004] BACKGROUND TO THE INVENTION

[0005] Steelmaking requires the use of iron-containing material, such as steel scrap, direct reduced iron or pig iron. In order to reduce the carbon footprint of the steel industry, the use of steel scraps is seen as key. However, steel scrap contains residual elements, notably Cu, Cr, Mo, Ni, Sn and / or Zn. The use of steel scrap is therefore not widely spread for all steel grades as those residual elements can have a detrimental effect on the steel properties. During steel production by means of direct reduced iron and / or pig iron, small amounts of residual elements are inevitably left behind in the liquid steel. When using steel scrap, the amount of residual elements is much greater compared to pig iron coming from a blast furnace or direct reduced iron. Too high an amount of residual elements may adversely affect subsequent production steps such as hot rolling, annealing and coating processes. It may also adversely affect engineering properties of the resultant steel product such as for example cold forming properties and machinability. Hence there is a need to avoid the introduction of too high an amount of residual elements into the steelmaking process. In part this can be achieved by a good scrap sorting management thereby avoiding the use of ferrous scrap parts having too high an amount of alloying elements. Where this is not feasible or not fully successful due to technical or economical reasons, the amount of these residual elements may increase over time. At present, for example molybdenum cannot be removed from steel once it is dissolved in the molten metal. As molybdenum is more noble than iron the standard refining technique of oxidation in steelmaking will not work. The better way to lower the amount of Mo in steel is to dilute the steel with virgin material, viz. steel made from ore for example originating from a blast furnace operation. But this approach lowers also the circularity. Consequently, a removal method for molybdenum from steel will become necessary on the long run.

[0006] DESCRIPTION OF THE INVENTION

[0007] It is an object of the invention to provide a method of manufacturing liquid steel having a reduced amount of residual elements, in particular a reduced amount of molybdenum. This and other objects and further advantages are met or exceeded by the present invention according to claim 1 and with preferred embodiments set out in the dependent claims and this description.

[0008] In order to achieve these objects, the present invention proposes, in a first aspect, a method of manufacturing liquid steel in a production line comprising at least a ferrous scrap melting unit, preferably it is an electrical ferrous scrap melting unit, a refining unit, a secondary metallurgy station and a casting machine, the method comprising the sequential steps of: providing and charging ferrous scrap, and optionally ferrous scrap in combination with a direct reduced iron (DRI) product, into the ferrous scrap melting unit; melting the ferrous scrap, and optionally the ferrous scrap in combination with the DRI product, to produce molten iron with a carbon content of less than 5.0 wt.%, and preferably of less than 4.8 wt.%; transferring said molten iron to the refining station; subjecting said molten iron having a temperature of less than 1600°C to a molybdenum removal treatment by adding a sulphur containing material into said molten iron thereby forming molten iron having a reduced Mo content and a layer of Mo-containing slag on top of the molten iron; removing the layer of Mo-containing slag from the molten iron; transferring the molten iron after said molybdenum removal treatment to the secondary metallurgy station for further melt treatment; and casting the molten iron following the secondary melt treatment in the casting station into rolling stock.

[0009] The method according to the invention provides a significant reduction of the molybdenum (Mo) content in molten iron originating from the processing of substantial amounts of ferrous scrap as input or source material in an ferrous scrap melting unit or ferrous scrap melting furnace. This enables the production of low-carbon steel rolling stock or forging stock having a reduced amount of molybdenum as residual element. The method according to the invention allows for a reduction of the molybdenum content in the treated molten iron of 25% or more, and in best practices of 40% or more.

[0010] The ferrous scrap melting unit or ferrous scrap melting furnace can be any melting unit capable of holding and melting large quantities of ferrous scrap material. In an embodiment it is an induction furnace or an electrical melting unit or electrical melting furnace.

[0011] An electrical ferrous scrap melting unit uses electric energy provided by several electrodes to melt the ferrous scrap, and optional in combination with a DRI product, and produce molten iron having a carbon-content of less than 5.0 wt.%, and preferably less than 4.7 wt.%. The electrical ferrous scrap melting unit is composed of a vessel able to contain hot metal. The vessel may have a circular or a rectangular shape. The vessel is closed by a roof provided with some apertures to receive electrodes to be inserted into the vessel and with other apertures to allow charging of the raw materials into the vessel and the removal of off gas. The electrodes provide the required electric energy to melt the charged raw materials and to form molten iron. During the melting of the raw materials, two layers are formed, a molten iron layer which is the densest and is thus located at the bottom of the vessel and a slag layer located on top of the molten iron. The slag layer can be partially covered by piles of raw materials waiting to be melted. Preferably, the electrical scrap melting unit or electrical melting furnace is an electric arc furnace (EAF).

[0012] The molten iron is subsequently transferred to a molten iron ladle, e.g. through at least one tap hole provided with at least one runner. Such tap holes are located in the lower part of the vessel. These may be located in the lateral walls of the vessel or in its bottom wall. In particular in the embodiment of an EAF, there are usually as many auxiliary runners as tap holes, said runners then intersecting to form a main runner to lead the extracted molten iron to the molten iron ladle. This molten iron ladle may be a simple ladle but could also be a torpedo ladle.

[0013] The molten iron tapped from the ferrous scrap melting unit is subsequently transferred to a refining station to perform a demolybdenisation step or molybdenum removal treatment in accordance with the invention. This demolybdenisation may be performed in a dedicated vessel or preferably directly in the molten iron ladle to avoid further molten metal transfer and associated heat losses.

[0014] In an embodiment the sulphur containing material added during the molybdenum removal treatment is in a solid form. In an embodiment the sulphur in solid form is selected from the group of FeS, FeS2(pyrite) and solid sulphur. Pyrite is being preferred. Pyrite is very effective in this application and is readily available against low costs. The sulphur containing material in solid form has preferably a largest diameter of less than about 1000 micron, and preferably of less than about 500 micron. In an embodiment the sulphur containing material in solid form has an average diameter in a range of about 40 to 300 micron, and preferably in a range of about 80 to 300 micron. A smaller particles size provides a larger reaction surface and thereby facilitates the demolybdenisation step. The particles should not be too small as in dispensers, silos or bunkers too small a particles may form a solid block under pressure, completely blocking the dispenser, soli or bunker.

[0015] In an embodiment the sulphur containing material in a solid form is added or injected for the purpose of the molybdenum removal treatment via powder injection from a lance or by shooting a particle-filled wire containing the sulphur containing solid material into the molten iron. On a less preferred basis the sulphur can be injected in a gas form, and which may be preheated, into the molten iron.

[0016] During the molybdenum removal treatment the molten iron is preferably agitated to increase the kinetics of the demolybdenisation. The creation of sufficient turbulence can be achieved for example by mechanical stirring such as using a rotor, blowing a gas (e.g. nitrogen or argon) into the molten iron via a lance or by gas injection via bottom stirring in the ladle of the refining station using bottom tuyeres or porous plugs.

[0017] During the molybdenum removal treatment the molybdenum concentration in the molten iron is reduced as the sulphur reacts with the molybdenum to form MoS2. This MoS2phase moves to the slag layer floating on top of the molten iron. It is preferred to remove this Mo-enriched slag layer from the molten iron prior to any further processing of the molten iron. The slag removal operation may be carried out by any well known method such as a method using slag skimmer, a method using a slag suction device, or a method of inclining the hot metal vessel to discharge slag in the vessel, and the method may be selected as suitable for facilities each steel plant has.

[0018] The concentration of molybdenum contained in the molten iron after demolybdenisation according to the present invention may be set in accordance with the purpose of the resultant steel, however, it is preferably set out to be about 0.05 wt.% or less, more preferably to 0.03 wt.% or less, and most preferably to 0.01 wt.% or less.

[0019] With the demolybdenisation treatment in accordance with the invention, the concentration of sulphur is inevitably increased in the molten iron. Therefore, in a preferred embodiment after the demolybdenisation treatment, a subsequent desulphurisation treatment is carried out. The desulphurisation treatment may be carried out by any well-known method and may be performed in a dedicated separate vessel or preferably directly in the same ladle, vessel or unit used for the demolybdenisation treatment to avoid molten metal transfer and associated heat losses.

[0020] The desulphurisation is carried out through letting the desulphurisation agent react with the hot metal. Specifically, the desulphurisation may be carried out by any well-known method of a method using refining equipment with mechanical stirrer, or a method of injecting powder (of desulphurisation agent) from a lance. For example, after the molybdenum removal treatment in the refining station, any generated slag may be removed and then, desulphurisation may be carried out in the same refining equipment with an injection lance or mechanical stirrer. An overview of suitable desulphurisation treatments is given in the publication by Schrama et aL, “Sulphur removal in ironmaking and oxygen steelmaking”, in Ironmaking & Steelmaking, Volume 44-5, pp. 333-343 (2017). As the desulphurisation agent, a desulphurisation agent having CaO as the main component, a desulphurisation agent having calcium carbide as the main component, a desulphurisation agent having lime, soda ash as the main component, metal Mg or the like may be used. The desulphurised molten iron has preferably a concentration of sulphur of less than about 0.05 wt.%, and preferably lower than about 0.02 wt.%, and more preferably less than 0.01 wt.%. Desulphurisation of the hot metal is followed by removal of the sulphide containing slag layer as is known in the art.

[0021] The molten iron thus formed is then transferred to a secondary metallurgy station having one or more secondary metallurgy units, such as a ladle furnace, RH (Ruhrstahl-Heraeus) vacuum vessel, vacuum tank degasser, alloying and stirring stations, for further melt treatment to reach the required steel temperature and composition according to the steel grades to be produced. In the secondary metallurgy station amongst others the nitrogen content in the molten steel is reduced to a level of less than 200 ppm, and preferably to less than 100 ppm, and more preferably to less than 50 ppm. It has been found that the removal of nitrogen from the molten steel should be performed after any desulphurisation treatment as a high sulphur concentration has an adverse effect on the nitrogen removal yield. Liquid steel with the required composition and temperature can then be transferred to a casting facility or casting station such as a continuous caster where it can be cast into solid products forming rolling stock, such as rolling slabs, billets, or strips, suitable for further processing by hot and / or cold rolling, heat treatment and other downstream processes to manufacture high steel grade products having a low concentration of Mo as residual element.

[0022] In an embodiment the ferrous scrap melting unit is charged with at least 90% of ferrous scrap, and preferably with 100% of ferrous scrap.

[0023] In an embodiment the ferrous scrap melting unit is charged with a combination of ferrous scrap and directly reduced iron (DRI) product and subsequently melting of the DRI product and ferrous scrap to produce molten iron. The ferrous scrap melting unit can be charged with a combination of DRI product and ferrous scrap, the combination having 90 wt.% to 10 wt.% ferrous scrap and 10 wt.% to 90 wt.% of DRI product, based on the combined amount of DRI and ferrous scrap charged.

[0024] In an embodiment the ferrous scrap melting unit is charged with at least 40 wt.% DRI product, based on the combined amount of DRI product and ferrous scrap charged. In a preferred embodiment with at least 50 wt.%, and more preferably with at least about 60 wt.% DRI product. In a preferred embodiment the ferrous scrap used for charging the ferrous scrap melting unit, and optionally also for charging a converter, has been shredded, and preferably having a size smaller than 100 mm, and preferably smaller than 80 mm. In a preferred embodiment the charged ferrous scrap is E40 specification scrap according to EU-27 steel scrap specification, last update of May 2007. Alternatively, or in addition to the E40 scrap, the charged ferrous scrap are E5H or E5M turnings.

[0025] In an embodiment of the method according to this invention, the molten iron at the beginning of the molybdenum removal treatment has a carbon content of less than 1.5 wt.%, preferably less than 1.0 wt.%, and more preferably less than 0.5 wt.%. The carbon content of the molten iron can already be adjusted in the ferrous scrap melting unit to a target composition to produce molten steel by blowing oxygen gas into the molten iron, for example using one or more injection devices such as lances positioned for example in the slag door, tap hole, side walls or the roof of the ferrous scrap melting unit. In this embodiment the molybdenum removal treatment in the refining station is performed at a temperature of less than 1600°C, and preferably at a temperature of at least about 1450°C.

[0026] To enhance the demolybdenisation treatment it is advantageous to increase the carbon content of the molten iron. To that effect the carbon content of the molten iron for the molybdenum removal treatment is preferably in a range of 3.0 to 5.0 wt.% to achieve maximum reactivity of the molybdenum with the sulphur. The higher carbon concentration lowers the melting temperature of the molten iron and thus reduces the amount of energy required. And it leads to a lower sulphur capacity of the melt, causing a higher reactivity of sulphur. In an embodiment the molten iron for the molybdenum removal treatment has a saturated carbon content. To that effect it is at least 4.0 wt.%, and more preferably at least about 4.1 wt.%, and most preferably at least about 4.3 wt.%, to increase the sulphur activity which enhances the sulphidation reaction. In an embodiment the carbon content is not more than about 4.8 wt.%, and preferably not more than about 4.7 wt.%.

[0027] Thus it may be necessary to add a carbon-rich containing material to the molten iron to increase the concentration of the carbon. This carbon addition can be done while the molten iron is still in the ferrous scrap melting unit for example through an injection device such as lances positioned in the slag door, tap hole, side walls or the roof of said melting unit or via one or more bottom tuyeres. This addition can be done also in the runner while the molten iron is transferred from the ferrous scrap melting unit to the refining station for the molybdenum removal treatment and / or it can be injected through an injection device directly in the molten iron when in said refining station. The carbon-rich containing material may come from different sources. It may be chosen, for example, among coke, coal, graphite, anthracite, silicon carbide, charcoal or a mixture of any of those sources, but can also advantageously come from renewable sources like biomass for part or all the carbon loads. In particular, biochar can be used.

[0028] The carbon-rich containing material to be injected through an injection device preferably has a particle size below about 3 mm. In a preferred embodiment said material has a particle size less than or equal to about 2 mm.

[0029] In another embodiment, the carbon containing material may also be made of composite briquettes of an iron source mixed with one or several of the previously mentioned carbon-rich material sources.

[0030] In the embodiment of performing the demolybdenisation treatment while the molten iron has a carbon content of more than 3.0 wt.%, it is advantageous that said molybdenum removal treatment in the refining station is performed at a temperature of less than about 1250°C, and preferably less than about 1200°C. At temperatures above about 1250°C the formation of the detrimental FeS phase may prevail as it leads to loss of Fe. In a preferred embodiment the temperature is less than about 1190°C. In an embodiment the temperature is at least about 1110°C to achieve maximum reactivity of molybdenum with sulphur.

[0031] To arrive at a temperature of less than 1250°C, and preferably of less than 1200°C, of the molten iron for the demolybdenisation treatment, it is very advantageous to add all or at least the majority of required carbon-rich material to the molten iron while the molten iron is still in the ferrous scrap melting unit or ferrous scrap melting furnace, for example an induction furnace or EAF. To arrive at a given batch of molten iron, in a first step part a substantial part, e.g. about 35-60% of the required amount, of the solid ferrous scrap is melted and saturated with carbon by adding carbon-rich material to the molten iron having a temperature of about 1600°C. In a second step, the remaining solid ferrous scrap is added to the molten iron and simultaneous adding the carbon-rich material. In this approach, the addition of the second batch of ferrous scrap cools the molten iron from about 1600°C to a target temperature below about 1250°C while it is being saturated with carbon favouring the subsequent demolybdenisation treatment in the refining station. This approach provides significant energy savings. It avoids that firstly the full amount of ferrous scrap is melted and being at a temperature of about 1600°C followed by adding the carbon-rich material to arrive at a carbon content of more than 3.0 wt.%, and preferably at a saturated carbon level, lowering the melting point of the iron enabling a liquid state at temperatures below 1600°C. As the batch of molten iron remains at 1600°C it has to cool down to the desired demolybdenisation treatment temperature either in the melting unit or the refining unit. This leads to significant loss of energy and production time.

[0032] This skilled person in the art will understand that other variations to this embodiment can be effected for achieving the same effect, for example adding the ferrous scrap to the melting unit in more than two steps.

[0033] In the embodiment of performing the demolybdenisation treatment while the molten iron has a carbon content in a range of 3.0 to 5.0 wt.%, the demolybdenised molten iron, and preferably subsequently desulphurized molten iron, is then transferred into a converter. The converter basically turns the molten iron into liquid steel by blowing oxygen through molten metal to decarburize it to a carbon content below 1.5 wt.%, and preferably to below 1 wt.%, and more preferably to below 0.5 wt.%. It is commonly a named Basic Oxygen Furnace (BOF). The high carbon content at the beginning of the BOF processing is required to enable a sufficient control of the exothermic process in the BOF plant. The vigorous flushing of the bath also efficiently removes nitrogen gas in the molten metal and nitrogen inclusions, commonly present in EAF based steels are hereby prevented. Ferrous scrap coming from recycling of steel may also be charged into the converter to take benefit of the heat released by the exothermic reactions resulting from the oxygen injection into molten iron.

[0034] Following the converter process the molten steel is transferred to a secondary metallurgy station for further melt treatment and next the molten steel is transferred to a casting station for casting into rolling stock, as herein described and claimed.

[0035] In an embodiment wherein the carbon content of the molten iron for the molybdenum removal treatment is in a range of 3.0 to 5.0 wt.% to achieve maximum reactivity of the molybdenum with the sulphur, the molten iron after the molybdenum removal treatment, and preferably subsequently desulphurized molten iron, is transferred to a submerged arc furnace and is charged as input material into said submerged arc furnace in combination with DRI product, and optionally in combination with DRI product and ferrous scrap. In a submerged arc furnace (SAF), in the art also known as a reducing electrical furnace (REF) or open slag bath furnace (OSBF), the electrodes are immersed into the slag layer. The SAF is operated preferably with a reducing atmosphere. A SAF is ideal for processing large amounts of DRI product into molten iron.

[0036] The SAF is charged with at least 40 wt.% DRI product. In a preferred embodiment with at least 50 wt.%, and more preferably with at least about 60 wt.% DRI product, the remaining being ferrous scrap. Now the SAF is also charged with the molten iron having been demolybdenised in accordance with the invention, and preferably subsequently desulphurized, to replace all of the ferrous scrap or part of the ferrous scrap charged together with the DRI product. DRI product has typically a melting point of about 1120-1150°C and ferrous scrap has a melting point of about 1500°C. When an SAF is charged with a combination of DRI product and ferrous scrap it requires an operational temperature for the molten iron of around 1600°C. Now the use of molten iron as feed material for the SAF to replace in part or in whole the solid ferrous scrap has the advantage that it can operate at lower temperatures, e.g. in a range of about 1300-1450°C, and thereby provides energy savings and significantly increases the operational efficiency. As the molten iron used has also a high carbon consent, there is also a reduced need for adding carbon-rich material to the molten iron in the SAF prior to transferring it to a converter. Since the molten iron used has a relatively low Mo content and where the DRI product originates from iron ore having no molybdenum as impurity, the molten iron from the SAF operation does not require any further molybdenum removal treatment.

[0037] A further important advantage is that the composition, e.g. the amount of impurities, of the molten iron can be determined very accurately. Having this exact composition enables an operator or operating system to adjust accordingly the amount of charged DRI product and / or the amount and type of charged ferrous scrap into the SAF. For example, if the demolybdenised and desulphurized molten iron is very clean, the SAF can be charged with cheaper ferrous scrap having a higher amount of impurities or more ferrous scarp can be added in combination with less DRI product. The charged product mix of molten iron, ferrous scrap and DRI product can be further optimized with respect to costs and impurity level. And, mutatis mutandis, if the demolybdenised and desulphurized molten iron still contains a high amount of impurities, the SAF can be charged with costly ferrous scrap having a low amount of impurities or less ferrous scarp can be added in combination with more DRI product. Thus a favourable extra control parameter is obtained to arrive more precisely at the required low level of impurities in the molten iron leaving the SAF / REF / OSBF.

[0038] The molten iron transferred from the SAF has a carbon content in a range of 3.0 to 5.0 wt.%, with preferred ranges as herein described, is transferred to a converter turning the molten metal into liquid steel by blowing oxygen through molten metal to decarburize it to a carbon content below 1.5 wt.%, and preferably to below 1 wt.%, and more preferably to below 0.5 wt.%, as herein described and claimed. Ferrous scrap coming from recycling of steel may also be charged into the converter to take benefit of the heat released by the exothermic reactions resulting from the oxygen injection into molten iron. Following the converter process the molten steel is transferred to a secondary metallurgy station for further melt treatment and next the molten steel is transferred to a casting station for casting into rolling stock, as herein described and claimed. In another embodiment wherein the carbon content of the molten iron for the molybdenum removal treatment is in a range of 3.0 to 5.0 wt.%, with preferred ranges as herein described, to achieve maximum reactivity of the molybdenum with the sulphur, the molten iron after the molybdenum removal treatment is transferred to an electric arc furnace (EAF) and is charged as input material into said EAF in combination with ferrous scrap, and optionally in combination with DRI product and ferrous scrap. In a preferred embodiment, the molten iron after the molybdenum removal treatment is firstly desulphurized as herein described and claimed prior to its transfer to the EAF.

[0039] The advantage of charging the EAF with molten iron having been demolybdenised, and preferably desulphurized, in accordance with the invention is that it may replace substantial amounts of solid ferrous scrap charged to the EAF. This achieves the effect of energy savings at the EAF.

[0040] The carbon content of the molten iron in said EAF can be adjusted to a target composition to produce molten steel having a carbon content of less than 1 .0 wt.%, and preferably of less than 0.5 wt.%, by blowing oxygen gas into the molten iron, for example using one or more injection lances as herein described. Following this the molten steel having the low carbon content is transferred to a secondary metallurgy station for further melt treatment. In the secondary metallurgy station amongst others the nitrogen content in the molten steel is reduced to a level of less than 200 ppm, and preferably less than 100 ppm, and more preferably less than 50 ppm. Following this the molten steel is transferred to a casting station for casting into rolling stock as is known in the art.

[0041] In another aspect, the invention relates to a production line for manufacturing liquid steel and reducing the molybdenum content as residual element in the liquid steel, the production line comprising at least a ferrous scrap melting unit, a refining unit or station, and a secondary metallurgy station or unit, wherein the refining unit comprises a metallurgical vessel allowing to hold molten iron and is provided with injection means, i.e. powder injection from a lance or allowing to shoot a particle-filled wire containing a sulphur containing material, allowing to inject a sulphur containing material directly in said molten iron in the metallurgical vessel, the sulphur in said sulphur containing material reacting with molybdenum in said molten iron to form MoS2.

[0042] The production line may further comprise a continuous steel caster. In an embodiment of the production line it further comprises a converter.

[0043] In a further aspect, the invention relates to the use of a sulphur containing material in solid form for the reduction of the molybdenum content in molten iron in a method according to this invention as herein described and claimed. Preferably the sulphur containing material in solid form is selected from the group of FeS, FeS2(pyrite) and solid sulphur. Pyrite is being preferred. The sulphur containing material in solid form has preferably a largest diameter of less than about 1000 micron, and preferably of less than about 500 micron. In an embodiment the sulphur containing material in solid form has an average diameter in a range of about 20 to 300 micron, and preferably in a range of 100 to 300 micron.

[0044] EXAMPLE

[0045] The invention will now be illustrated with reference to following, non-limiting example.

[0046] A molten model iron alloy consisting of 90.7 wt.% of iron, 5.0 wt.% of molybdenum and 4.3 wt.% of carbon was provided to a refractory crucible (the refining station). The crucible was closed with a lid. The molten iron alloy was shielded from the surrounding atmosphere by the lid and by means of a constant flow of oxygen free argon gas between the lid and the molten iron alloy to prevent oxidation of the alloy.

[0047] To this alloy in the crucible the sulphur source was added in the form of FeS2(pyrite) in a ratio of 1 :20 (in weight) of pyrite:iron alloy. The iron alloy with the added FeS2was then held at 1150°C for 120 minutes (the “molybdenum removal treatment”). The molybdenum forms MoS2with sulphur from the FeS2and becomes part of the slag layer floating on top of the - now depleted in Mo - molten iron alloy.

[0048] After solidification of the thusly treated iron alloy the molybdenum and sulphur content in the solidified iron alloy and the slag phase was determined. SEM-EDS analyses were conducted on a JEOL7001 F Field Emission Gun (FEG)-SEM with two Oxford Ultim Max (170mm2) EDS detectors. Beam currents were between 1-3nA. The acceleration voltage was set to 20kV instead of the normal 15kV in order to be able to separate the signal coming from S and Mo. The Ka line of S and the La line of Mo have a similar characteristic X-ray energy (2.307 keV and 2.293 keV, respectively) and the resulting peaks in the energy spectrum are difficult to separate. However, Mo has a peak at higher X-ray energy for the Ka line (17.441 keV) with no interference of S on that peak. Counts were used to determine the ratio between sulphur and molybdenum.

[0049] The analysis revealed that in the solidified treated iron alloy the molybdenum content was reduced. The slag phase on the other hand has a significant molybdenum content in the form of MoS2. The composition is given in table 1.

[0050] Table 1 The distribution coefficient for Mo is therefore:

[0051] Mo in Sulphide rich phase 3.2 D(Mo) = - - - - - = - = 0.15

[0052] Mo in Sulphide poor phase 21.3

[0053] The slag phase comprising the MoS2is to become the slag layer floating on top of the molten iron as described herein above and below and the molten iron below this slag layer has a lower molybdenum content as a result.

[0054] DESCRIPTION OF THE FIGURES

[0055] The invention will also be illustrated with reference to non-limiting Figs. 1 to 4, each figure illustrating an embodiment of the method according to the invention.

[0056] In the embodiment of Fig. 1 ferrous scrap 15 is charged into a ferrous scrap melting unit 20, such as an EAF or an induction furnace, to produce molten iron having a carbon content of less than 5.0 wt.%. When the ferrous scrap 15 predominantly originates from low-carbon steel scrap the molten iron has a typical carbon content of less than 1.5 wt.%, preferably less than 1.0 wt.%, and more typically of less than 0.5 wt.%. The carbon content of the molten iron in the ferrous scrap melting unit 20 can be adjusted to a target composition to produce molten steel by blowing oxygen gas into the molten iron, for example using one or more injection lances. The molten iron having the low carbon content is transferred to a refining station 30 where it is subjected to demolybdenisation by injecting a sulphur containing agent, for example solid FeS2, into the molten metal. During the metal treatment molten iron is formed having a reduced Mo content and a layer of Mo-containing slag on top of said molten iron. The slag layer is removed after the melt treatment. Next the molten iron having a reduced Mo-content is preferably subjected to a desulphurization treatment (not shown) removing sulphur contained in the molten iron prior to transferring said molten iron to a secondary metallurgy station 50 for further melt treatment. In the secondary metallurgy station 50 amongst others the nitrogen content in the molten metal is reduced to a level of less than 200 ppm, and preferably less than 100 ppm, and more preferably less than 50 ppm. Next the molten metal is transferred to a casting station 60 for casting into rolling stock as is known in the art.

[0057] In an embodiment the ferrous scrap 15 is charged in combination with DRI product 16, the combination having 90 wt.% to 10 wt.% ferrous scrap and 10 wt.% to 90 wt.% of DRI product, based on the combined amount of DRI and ferrous scrap charged. The DRI product 16 is produced in a direct reduction plant 10 from the direct reduction of iron ore conglomerates (mainly hematite, Fe2O3) in the form of lumps, pellets, or fines into iron by a reducing gas, e.g. natural gas, hydrogen or a combination thereof. Direct reduction refers to a solid-state process which reduce iron oxides to metallic iron at temperatures below the melting point of iron. There are several processes for producing DRI 16 known to the person skilled in the art. Known examples of direct-reduction processes include the MIDREX process, Tenova’s HYL process, Tenova’s HYL-I, the HYL-II and the HYL-III process, Posco’s HyREX process, and the HYBRIT process. A known process relates to a direct reduction plant or DRI reactor comprising a direct reduction shaft furnace having a reduction zone and a lower discharge zone from which DRI in solid form is discharged at a regulated rate by means of a suitable discharge mechanism.

[0058] In view of the low carbon content in the molten iron, in this embodiment the molybdenum removal treatment in the refining station 30 is performed at a temperature of less than about 1600°C, and preferably at a temperature of at least about 1450°C.

[0059] In the embodiment of Fig. 2, which is in part based on Fig. 1 , the further process step of subjecting the molten iron following the demolybdenisation by injecting a sulphur containing agent in refining station 30 to a converter process 40 is shown. For an efficient converter process it is required that the carbon content in the molten iron is sufficiently high at the beginning of the process of blowing oxygen. At the beginning of the converter process the carbon content should be in a range of 3.0 to 5.0 wt.%, and preferably the molten iron has a saturated carbon content. Preferably the carbon content does not exceed 4.8 wt.%, and more preferably does not exceed 4.7 wt.%. The minimum carbon content is preferably at least 4.0 wt.%. To that exceed it is required that the carbon content of the molten iron during the demolybdenisation treatment is in the same range. As the input material for the ferrous scrap melting unit 20, viz. the ferrous scrap 15 and optionally in combination with DRI product 16, has a relatively low carbon content, it is thus necessary to add a carbon-rich containing material 17 to the molten iron to increase the concentration of the carbon. This addition can be done while the molten iron is still in the ferrous scrap melting unit 20 for example through an injection device such as lances positioned in the slag door, tap hole, side walls or the roof of the furnace or via one or more bottom tuyeres. This addition can be done in the runner while the molten iron is transferred from the ferrous scrap melting unit 20 to the refining station 30 for the molybdenum removal treatment and / or it can be injected through an injection device directly in the molten iron in said refining station 30 as schematically shown in Fig.2. The demolybdenised molten iron, and preferably subsequently desulphurized molten iron (not shown), is then transferred into a converter 40. The converter basically turns the molten iron into liquid steel by blowing oxygen through the molten metal to decarburize it to a carbon content below 1.5 wt.%, and preferably to below 1 .0 wt.%, and more preferably below 0.5 wt.%. It is commonly named a Basic Oxygen Furnace (BOF). Ferrous scrap 15 coming from recycling of steel may also be charged into the converter 40 to take benefit of the heat released by the exothermic reactions resulting from the oxygen injection into the molten iron. Next the liquid steel is transferred to a secondary metallurgy station 50 for further melt treatment. In the secondary metallurgy station 50 amongst others the nitrogen content in the liquid steel is reduced to a level of less than 200 ppm, and preferably less than 100 ppm, and more preferably less than 50 ppm. Next the liquid steel is transferred to a casting station 60 for casting into rolling stock as is known in the art.

[0060] The increased carbon content in a range of about 3.0 to 5.0 wt.%, and preferred ranges as herein described, has the effect that it reduces the melting temperature of the molten iron and increases the sulphur activity which enhances the sulphidation reaction lowering the Mo-content. Too high a carbon content leads to a lower sulphur capacity of the hot iron.

[0061] In a preferred embodiment when having the defined high carbon content in the molten iron, the molybdenum removal treatment in the refining station 30 is performed at a temperature of the molten iron of less than about 1250°C, and preferably less than about 1200°C. At temperatures above 1250°C the formation of detrimental the FeS phase may prevail. In a preferred embodiment the temperature is less than about 1190°C. In an embodiment the temperature is at least about 1110°C to achieve maximum reactivity of molybdenum with sulphur.

[0062] In the embodiment of Fig. 3, which is in part based on Fig. 2, the ferrous scrap melting unit 20 is charged with ferrous scrap 15. Similar as in the embodiment of Fig.2, the carbon content of the molten iron in the refining unit 30 is in a range of about 3.0 to 5.0 wt.%, and with preferred ranges as herein described. The carbon content can be increased by the addition of carbon-rich material while the molten iron is still in the ferrous scrap melting unit 20, or in the runner while the molten iron is transferred from the ferrous scrap melting unit 20 to the refining station 30 for the molybdenum removal treatment and / or it can be injected through an injection device directly in the molten iron in said refining station 30 as herein described. The increased carbon content has the effect that it reduces the melting temperature of the molten iron and increases the sulphur activity which enhances the sulphidation reaction lowering the Mo-content.

[0063] Also in this embodiment, the molybdenum removal treatment in refining station 30 is performed preferably at a temperature of the molten iron of less than 1250°C, and preferably less than about 1200°C. At temperatures above 1250°C the formation of the detrimental FeS phase may prevail. In a more preferred embodiment the temperature is less than about 1190°C. In an embodiment the temperature is at least about 1110°C to achieve maximum reactivity of molybdenum with sulphur. The demolybdenised molten iron is preferably desulphurized (not shown) and this may be done in the refining unit 30 or a separate vessel or ladle. Following this the molten iron is transferred to a submerged arc furnace 70 (SAF, REF, OSBF) and charged as input material into said submerged arc furnace in combination with DRI product 16, and optionally in combination with DRI product 16 and ferrous scrap 15. As set out above, an important advantage is that the composition of the molten iron can be determined very accurately. Having this exact composition enables an operator or operating system to adjust accordingly the amount of charged DRI product and / or the amount and type of charged ferrous scrap into the SAF. The charged product mix of molten iron, ferrous scrap and DRI product can be further optimized with respect to costs and impurity level. An extra control parameter is provided to arrive more precisely at the required low level of impurities in the molten iron leaving the SAF 70.

[0064] The molten iron output material of the submerged arc furnace 70 has a carbon content in a range of 3.0 to 5.0 wt.% and is transferred, analogue to the embodiment of Fig. 2, to a converter 40 turning the molten iron into liquid steel, a secondary metallurgy station 50, and casting station 60 for casting into rolling stock.

[0065] In the embodiment of Fig. 4, which is in part based on Fig. 3, the ferrous scrap melting unit 20 is charged with ferrous scrap 15. Similar as in the embodiment of Fig. 3, the carbon content of the molten iron in the refining unit 30 is in a range of about 3.0 to 5.0 wt.%, and with preferred ranges as herein described. The carbon content can be increased by the addition of carbon-rich material while the molten iron is still in the ferrous scrap melting unit 20, or in the runner while the molten iron is transferred from the ferrous scrap melting unit 20 to the refining station 30 for the molybdenum removal treatment and / or it can be injected through an injection device directly in the molten iron in said refining station 30 as herein described. The increased carbon content has the effect that it reduces the melting temperature of the molten iron and increases the sulphur activity which enhances the sulphidation reaction lowering the Mo-content.

[0066] Also in this embodiment, the molybdenum removal treatment in refining station 30 is performed preferably at a temperature of the molten iron of less than about 1250°C, and preferably less than about 1200°C. At temperatures above 1250°C the formation of detrimental the FeS phase may prevail. In a more preferred embodiment the temperature is less than about 1190°C. In an embodiment the temperature is at least about 1110°C to achieve maximum reactivity of molybdenum with sulphur.

[0067] The demolybdenised molten iron is preferably desulphurized (not shown) and may be done in the refining unit 30 or a separate vessel or ladle. Next the molten iron is transferred to an electric arc furnace 80 (EAF) and charged as input material into said electric arc furnace 80 in combination with DRI product 16 and ferrous scrap 15. The carbon content of the molten iron in the EAF 80 can be adjusted to a target composition to produce molten steel by blowing oxygen gas into the molten iron, for example using one or more injection lances.

[0068] Next the molten steel having the low carbon content is transferred to a secondary metallurgy station 50 for further melt treatment. In the secondary metallurgy station 50 amongst others the nitrogen content in the molten steel is reduced to a level of less than 200 ppm, and preferably less than 100 ppm, and more preferably less than 50 ppm. Following this the molten iron or molten steel is transferred to a casting station 60 for casting into rolling stock as is known in the art.

[0069] The above-discussion is intended to be merely illustrative of the present system and should not be construed as limiting the appended claims to any particular embodiment or group of embodiments. Accordingly, the specification and drawings are to be regarded in an illustrative manner and are not intended to limit the scope of the appended claims. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

[0070] Any reference signs in the claims should not be construed as limiting the scope of the appended claims.

[0071] List of reference numbers:

[0072] (10) direct reduction plant;

[0073] (15) ferrous scrap;

[0074] (16) DRI product;

[0075] (17) carbon-rich containing material;

[0076] (20) ferrous scrap melting unit or ferrous scrap melting furnace;

[0077] (30) refining unit or refining station;

[0078] (40) converter;

[0079] (50) secondary metallurgy unit(s);

[0080] (60) continuous steel caster;

[0081] (70) submerged arc furnace (SAF, REF, OSBF);

[0082] (80) electric arc furnace (EAF);

Claims

CLAIMS1. Method of manufacturing liquid steel in a production line comprising at least a ferrous scrap melting unit (20), preferably an electrical melting unit, a refining unit (30), a secondary metallurgy station (50), and a casting machine (60), the method comprising the steps of: charging ferrous scrap (15), and optionally ferrous scrap (15) in combination with a direct reduced iron product (16), into the ferrous scrap melting unit (20); melting the ferrous scrap (15), and optionally the ferrous scrap (15) in combination with the DRI product (16), to produce molten iron with a carbon content of less than 5.0 wt.%; transferring said molten iron to the refining station (30); subjecting said molten iron having a temperature of less than 1600°C to a molybdenum removal treatment by adding a sulphur containing material into said molten iron thereby forming molten iron having a reduced molybdenum content and a layer of Mo-containing slag on top of the molten iron; removing the layer of Mo-containing slag from the molten iron; transferring the molten iron after said molybdenum removal treatment to the secondary metallurgy station (50) for further melt treatment; and casting the molten iron following the secondary melt treatment in the casting station (60) into rolling stock.

2. Method according to claim 1 , wherein the molten iron having a reduced molybdenum content is subjected to a desulphurization treatment removing sulphur contained in the molten iron prior to transferring said molten iron to a secondary metallurgy station (50) for further melt treatment.

3. Method according to claim 1 or 2, wherein the sulphur containing material added during the molybdenum removal treatment step is in a solid form, and preferably selected from the group of FeS, FeS2, and solid sulphur.

4. Method according to any one of claims 1 to 3, wherein the sulphur containing material is added in a solid form during the molybdenum removal treatment step via powder injection or by a particle-filled wire.

5. Method according to any one of claims 1 to 4, wherein the molten iron after the molybdenum removal treatment comprises less than 0.05 wt.% of Mo, and preferably less than 0.03 wt.% of Mo.

6. Method according to any one of claims 1 to 5, wherein the ferrous scrap melting unit (20) is an electric arc furnace (EAF).

7. Method according to any one of claims 1 to 6, wherein the ferrous scrap melting unit (20) is charged with at least 90% of ferrous scrap (15), and preferably with 100% of ferrous scrap (15).

8. Method according to any one of claims 1 to 7, wherein the ferrous scrap melting unit (20) is charged with a combination of ferrous scrap (15) and DRI product (16), the combination having 90-10 wt.% of ferrous scrap and 10-90% of DRI product, based on the combined amount of ferrous scrap and DRI charged.

9. Method according to any one of claims 1 to 8, wherein the molten iron at the beginning of the molybdenum removal treatment has a carbon content of less than 1.5 wt.%, preferably less than 1.0 wt.%, and more preferably less than 0.5 wt.%.

10. Method according to any one of claims 1 to 8, wherein the molten iron at the beginning of the molybdenum removal treatment has a carbon content in a range of 3.0 to 5.0 wt.%, and preferably in a range of 4.0 to 4.8 wt.%.

11. Method according to claim 10, wherein the molybdenum removal treatment is performed while the molten iron is at a temperature of less than 1250°C, and preferably of less than 1200°C.

12. Method according to claim 10 or 11 , wherein the carbon content of the molten iron transferred from the ferrous scrap melting unit (20) to the refining station (30) is increased by adding a carbon-rich material (17) to the molten iron.

13. Method according to any one of claims 10 to 12, wherein the molten iron after the molybdenum removal treatment is transferred to a converter (40) where the carbon content of said molten ironis lowered to a value below 1.5 wt.% by oxygen blowing, preferably below 1.0 wt.%, and more preferably below 0.5 wt.%, so as to obtain liquid steel.

14. Method according to any one of claims 10 to 12, wherein the molten iron after the molybdenum removal treatment is transferred to a submerged arc furnace (70) and charged as input material into said submerged arc furnace in combination with DRI product (16), and optionally in combination with DRI product (16) and ferrous scrap (15), and preferably the molten iron discharged from the submerged arc furnace (70) is transferred to a converter (40) where the carbon content of said molten iron is lowered to a value below 1.5 wt.% by oxygen blowing, preferably below 1.0 wt.%, and more preferably below 0.5 wt.%, so as to obtain liquid steel.

15. Method according to any one claims 10 to 12, wherein the molten iron after the molybdenum removal treatment is transferred to an electric arc furnace (80) and charged as input material into said electric arc in combination with ferrous scrap (15), and optionally in combination with DRI product (16) and ferrous scrap (15).

16. Use of FeS, FeS2 or solid S for the reduction of molybdenum in molten iron in a method according to any one of claims 1 to 15.

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