, Description:TECHNICAL FIELD
The present disclosure relates to the field of steel making, in particular, interstitial-free (IF) steel making. It relates to a method for producing low phosphorous IF steel using secondary emission dust, a method for dephosphorization of IF steel using secondary emission dust, and uses of secondary emission dust for producing low phosphorus IF steel. Particularly, the present disclosure relates to all the aspects to produce low phosphorus IF steel.

BACKGROUND OF THE DISCLOSURE
Interstitial-free (IF) steel is used in exterior and interior body applications in automobiles. Removal of phosphorus is an important requirement in steel manufacturing for automotive sectors due to its detrimental effect on ductility, fracture toughness and cold shortness of steel. The demand for lower phosphorus (P) values in steel combined with the availability of high phosphorus iron ores makes dephosphorization of steel very challenging. In view of the deep drawability requirement in the automotive sector application, the P value in the IF steels is required to be less than 0.015%.

To maintain lower phosphorus in steels before casting, control of phosphorus during and after blow is necessary. In the basic oxygen furnace (BOF) process, molten iron from blast furnace is refined under an oxidizing and basic environment. Due to the criticality of P in IF grade steels, after the main blow, some amount of re-blow is also done along with extra additions of fluxing agents (e.g., iron ore, lime). This provides further reduction in initial phosphorus content measured in the steel after blow. However, this process increases the cycle time of the BOF process as extra time is required to perform these corrections. Also, increasing the blow in the process can also lead to refractory erosion of the furnace and reduces its life, leading to early change in the refractory lining of the vessel.

After the BOF process, the molten steel is tapped into a ladle. Phosphorus can re-enter steel during tapping if there is a slag carryover from BOF, as the slag contains P2O5 ranging from 3-5%. To mitigate this, use of slag stopping devices such as dart and infrared based slag detection systems is helpful. However, there is always some amount of slag which goes into the ladle during tapping leading to re-entry of at least some phosphorus from the slag into the steel.
Use of fluorspar (CaF2) based slags is known to help in dephosphorization of steel during and after blow. However, CaF2 based slags are known to erode the refractory linings of the furnace vessel and ladles and create an environmental hazard, limiting its use.

In IF grade steelmaking, after BOF process and before tapping, sample and temperature are taken for chemistry and downstream purposes. Based on the sample and temperature readings, if necessary, correction is done to ensure lower phosphorus levels and optimum temperature, and the steel is thereafter tapped in an unkilled state (i.e., oxidized state). During tapping, lime is added into the steel for slag-making at vacuum process downstream. After tapping, steel is taken for vacuum degassing process for further decarburization and dehydrogenation, and thereafter suitable alloying is done after deoxidation of steel, followed by sending the steel for casting (solidification) process.

There is a need in the art to develop simple, efficient and cost-effective methods for dephosphorization of steel that is free of limitations such as requirement of addition of separate/costly materials, erosion of the linings of the steel-making equipments etc. The present disclosure attempts to address said need.

STATEMENT OF THE DISCLOSURE
The present disclosure relates to a method for producing low phosphorous IF steel comprising contacting molten steel with secondary emission dust to produce the low phosphorous IF steel.

The present disclosure also relates to a method for dephosphorization of IF steel comprising contacting molten steel with secondary emission dust to dephosphorize the steel.

The present disclosure further provides use of secondary emission dust as a dephosphorization agent for production of low phosphorous IF steel.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1A shows an exemplary system employed for collection/recovery of secondary emission dust and gaseous byproducts produced during the basic oxygen furnace process.

Figure 1B shows relevant parts of the system showed in Figure 1A that are employed for collection/recovery of secondary emission dust generated during the basic oxygen furnace process.

Figure 2 shows an exemplary embodiment of the step of contacting molten steel with the secondary emission dust.

DETAILED DESCRIPTION OF THE DISCLOSURE
With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity. The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results. Throughout this specification, the word “comprise”, or variations such as “comprises” or “comprising” or “containing” or “has” or “having”, or “including but not limited to” wherever used, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment may be included in at least one embodiment of the present disclosure. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification may not necessarily all refer to the same embodiment. It is appreciated that certain features of the disclosure, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the disclosure, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

As used herein, the term “interstitial-free” steel refers to the type of steel that does not contain interstitial solute atoms to strain the solid iron lattice.

As used herein, the term “dephosphorization” refers to partial or complete removal of phosphorus from steel.

The present disclosure provides a method for producing low phosphorous interstitial-free (IF) steel comprising contacting molten steel with secondary emission dust to produce the low phosphorous IF steel. In some embodiments, the molten steel is produced in the basic oxygen furnace. Accordingly, in some embodiments, a method for producing low phosphorous IF steel comprises contacting molten steel obtained from the basic oxygen furnace with the secondary emission dust to produce the low phosphorous IF steel. In some embodiments, the molten steel is produced in any steel making furnace, including an electric arc furnace. While methods to dephosphorize steel before or during the basic oxygen furnace (BOF) process are known, the methods of the present disclosure comprise dephosphorization of steel after the BOF process. In exemplary embodiments, the methods of the present disclosure comprise addition of the secondary emission dust after the BOF process is completed, to produce a low phosphorous IF steel.

The secondary emission dust employed in the present disclosure is a by-product of the steel-making process and comprises components such as metal oxides and volatile components.

In some embodiments, the secondary emission dust comprises components selected from the group consisting of: iron oxides, calcium oxide, silicon dioxide, magnesium oxide, aluminium oxide, manganese oxide, titanium oxide, chromium oxide, phosphorous pentoxide, and a combination thereof.

In some embodiments, iron oxides are present in an amount of about 30-40%, 32-38%, 33-38%, 34-38%, or 35-40% including values and ranges therefrom, by weight of the secondary emission dust. In some embodiments, calcium oxide is present in an amount of about 15-20%, 16-20%, 15-19%, or 17-20%, including values and ranges therefrom, by weight of the secondary emission dust. In some embodiments, silicon dioxide is present in an amount of about 0-5%, 0.1-5%, 0.5-5%, 1-5%, 2-5%, or 3-5%, including values and ranges therefrom, by weight of the secondary emission dust. In some embodiments, magnesium oxide is present in an amount of about 0-5%, 0.1-5%, 0.5-5%, 1-5%, 2-5%, or 3-5%, including values and ranges therefrom, by weight of the secondary emission dust. In some embodiments, aluminium oxide is present in an amount of about 0-2%, 0.1-2%, 0.5-2%, or 1-2%, including values and ranges therefrom, by weight of the secondary emission dust. In some embodiments, manganese oxide is present in an amount of about 0-2%, 0.1-2%, 0.5-2%, or 1-2%, including values and ranges therefrom, by weight of the secondary emission dust. In some embodiments, titanium oxide is present in an amount of about 0-0.5%, 0.1-0.2%, 0.1-0.3%, or 0.1-0.5%, including values and ranges therefrom, by weight of the secondary emission dust. In some embodiments, chromium oxide is present in an amount of about 0-0.1%, 0.02-0.1%, 0.02-0.08%, or 0.04-0.1%, including values and ranges therefrom, by weight of the secondary emission dust. In some embodiments, phosphorous pentoxide is present in an amount of about 0-0.5%, 0.1-0.3%, or 0.1-0.5%, including values and ranges therefrom, by weight of the secondary emission dust. In some embodiments, volatile components, expressed as loss on ignition (LOI), are present in an amount of about 32-45%, 32-42%, 32-40%, 35-45%, or 35-40%, including values and ranges therefrom, by weight of the secondary emission dust.

The secondary emission dust comprises one or more components described herein in any of the amounts described herein or any combinations thereof.

In some embodiments, the secondary emission dust comprises iron oxides in an amount of about 30-40%, calcium oxide in an amount of about 15-20%, silicon dioxide in an amount of about 0-5%, magnesium oxide in an amount of about 0-5%, aluminium oxide in an amount of about 0-2%, manganese oxide in an amount of about 0-2%, titanium oxide in an amount of about 0-0.5%, chromium oxide in an amount of about 0-0.1%, and phosphorous pentoxide in an amount of about 0-0.5%, including values and ranges therefrom, by weight of the secondary emission dust.

In some embodiments, the secondary emission dust comprises iron oxides in an amount of about 30-40%, calcium oxide in an amount of about 15-20%, silicon dioxide in an amount of about 0-5%, magnesium oxide in an amount of about 0-5%, aluminium oxide in an amount of about 0-2%, manganese oxide in an amount of about 0-2%, titanium oxide in an amount of about 0-0.5%, chromium oxide in an amount of about 0-0.1%, phosphorous pentoxide in an amount of about 0-0.5%, and LOI (volatile components) in an amount of about 32-45%, including values and ranges therefrom, by weight of the secondary emission dust.

An exemplary composition of the secondary emission dust is shown in Table 1 below:
Table 1
Fe(T) CaO SiO2 MgO MnO Al2O3 TiO2 Cr2O3 LOI P2O5
36.56% 17.75% 3.53% 3.05% 0.209% 1.08% 0.117% 0.043% 37.48% 0.18%

The secondary emission dust employed in the present invention is a by-product of the BOF process. In some embodiments, the secondary emission dust is collected/recovered in the BOF process as shown in Figures 1A and 1B. In brief, suction is created near the skirt area (see part A in Figure 1B) of the BOF vessel to suck the emission dust and gaseous by-products. Sucked dust and gases are passed through evaporation cooler and gas cooler and sent towards an ESP (Electrostatic Precipitator). The dust is segregated in ESP (see part B in Figure 1B) through various electrodes mounted in it. Dust collected with electrodes is discharged in the ESP chamber bottom (see part C in Figure 1B) at a defined frequency automatically. From ESP chamber bottom, dust is conveyed through chain conveyor to nearby collection silo, also called day bin. From the collection silo, the dust is discharged in a tanker with the help of a pneumatic valve. Finally, dust is collected in gunny bags, each containing about 20 kilogram of dust from tanker discharge chute.

In some embodiments, the method for producing low phosphorous IF steel comprises contacting molten steel with the secondary emission dust in an amount of about 0.10-0.30%, including values and ranges therefrom, by weight of the molten steel. In some embodiments, the secondary emission dust is added to the molten steel in an amount of about 0.10-0.30%, 0.12-0.30%, 0.15-0.30%, 0.17-0.30%, or 0.20-0.30%, including values and ranges therefrom, by weight of the molten steel. In some embodiments, the secondary emission dust is added to the molten steel in an amount of about 0.20-0.30%, 0.20-0.28%, 0.20-0.25%, 0.22-0.28%, 0.22-0.26%, 0.23-0.28%, or 0.24-0.28%, including values and ranges therefrom, by weight of the molten steel. In some embodiments, the secondary emission dust is added to the molten steel in an amount of about 0.20%, 0.21%, 0.22%, 0.23%, 0.24%, 0.25%, 0.26%, 0.27%, 0.28%, 0.29% or 0.30%.

In an exemplary embodiment, about 350-400 kilogram of secondary emission dust is added to about 160-165 tons of molten steel.

The inventors have observed that very high or very low amounts of secondary emission dust addition may not be desirable. For instance, if secondary emission dust is added at very high amounts by weight of the molten steel, it could lead to an increase in the temperature drop of the molten steel due to addition of more dust. In such case, to compensate for the higher temperature drop, the temperature of molten steel at tapping is required to be higher. However, consistently running the basic oxygen furnace at very high temperatures can have a detrimental effect on the refractory lining of the BOF vessel. On the other hand, for instance, if the secondary emission dust is added at very low amounts by weight of the molten steel, the amount of phosphorus removed from the steel may be very less or negligible. Hence, maintaining optimal amounts of secondary emission dust and optimal temperature of molten steel are necessary.

In the methods of the present disclosure, molten steel is contacted with the secondary emission dust at tap. The term “tap” or “tapping” is used in the steelmaking art to refer to pouring or transfer of molten steel from the steel-making furnace, e.g., basic oxygen furnace, into a ladle.
Accordingly, in some embodiments of the present disclosure, contacting molten steel with the secondary emission dust comprises adding the secondary emission dust in a ladle and tapping the molten steel in the ladle, wherein the molten steel falling in the ladle comes in contact with the secondary emission dust (Figure 2).
In the steel-making process, molten steel from the steel-making furnace is tapped into the ladle and mixed with fluxing agents for further refining.

In some embodiments, the present methods comprise optionally adding fluxing agents to the molten steel. That is, in some embodiments, the present methods do not include addition of fluxing agents. In some other embodiments, the present methods comprise addition of fluxing agents. In the embodiments where the fluxing agents are added, they are added to the ladle at the same time as the molten steel (i.e., simultaneously at the time of tapping) or they are added to the ladle before tapping of the molten steel. In some embodiments, the fluxing agent is lime. In some embodiments, the fluxing agent is a lime-based material. Exemplary lime-based materials include, but are not limited to, secondary emission dust collected from BOF, ladle furnace emission dust, BOF sludge, or a combination thereof. Exemplary lime-based materials employed in the present methods comprise calcium oxide (e.g., CaO), iron oxides, aluminium oxide (e.g., Al2O3), silicon oxide (e.g., SiO2), and volatile components expressed as LOI. In one embodiment, a lime-based material comprises CaO in an amount of about 15 to about 25% and iron oxides in an amount of about 30 to about 45% by weight, including values and ranges thereof. In some embodiments, a lime-based material comprises CaO in an amount of about 15-25%, 15-20%, 15-22%, 16-24%, 16-22%, 15-19%, 18-25%, 18-22%, 17-20%, or 20-25%, including values and ranges therefrom, by weight of the lime-based material. In some embodiments, a lime-based material comprises iron oxides in an amount of about 30-45%, 30-40%, 35-45%, 32-42%, 32-40%, 32-38%, 33-40%, 33-38%, 34-38%, or 35-40%, including values and ranges therefrom, by weight of the lime-based material. In some embodiments, the fluxing agent is a combination comprising lime and lime-based material. The addition of lime and/or lime-based materials provide basicity to the ladle slag that is necessary for efficient dephosphorization reaction.

As the secondary emission dust comprises calcium oxide (lime), the methods of the present disclosure require lower amounts of separately-added lime or lime-based material (fluxing agent) compared to conventional methods. For example, in conventional methods, lime is added to the molten steel in an amount of about 0.24-0.25% by weight of the molten steel. In contrast, in some embodiments, the methods of the present disclosure comprise adding lime (i.e., separate addition of lime excluding the amount present in the secondary emission dust) in an amount of about 0.15-0.20%, including values and ranges therebetween, by weight of the molten steel. In some embodiments, the methods of the present disclosure comprise adding lime in an amount of about 0.16-0.20%, 0.17-0.20%, 0.18-0.20%, or 0.18-0.19%, including values and ranges therebetween, by weight of the molten steel. In an exemplary embodiment, the present methods comprise addition of about 300 kilogram of separately-added lime to 160-165 tons of molten steel compared to a conventional method that comprises addition of about 400 kilogram of lime to 160-165 tons of molten steel. In this embodiment of the present method, the remaining 100 kilogram of lime is provided by the secondary emission dust. Thus, the present methods possess an additional advantage of reducing the amount of lime/lime-based material separately-added to the molten steel by employing the secondary emission dust.

In the methods of the present disclosure, molten steel, fluxing agent(s), and secondary emission dust are combined in the ladle. To facilitate effective dephosphorization reaction, intermixing of slag and steel phases is important. Therefore, after contacting molten steel with the secondary emission dust, the contents in the ladle including the secondary emission dust, the molten steel, and the fluxing agent(s), are purged. The step of purging promotes proper mixing of steel and slag phases and facilitates effective dephosphorization reaction. An exemplary dephosphorization reaction can be described as:
[P]+5/2 [O]+3/2 (O^(2-) )=(?PO?_4^(3-)).
In this reaction, [O] is provided by iron oxides and (O2-) is provided by lime.

In some embodiments, the steel and slag phases in the ladle are purged for about 1 to 2 minutes. In some embodiments, top purging is done after tapping is complete to agitate the bath so that there is proper mixing between the steel and slag phases for better dephosphorization. In some embodiments, purging is done by employing inert gases as a purging agent. In some embodiments, inert gases that can be employed for purging include argon, carbon dioxide, nitrogen, helium, neon, krypton, xenon and radon. In some embodiments, materials such as wooden blocks/chips which evolve gases on combustion are employed as a purging agent. In this embodiment, wooden blocks and/or wooden chips are placed at the bottom of the ladle before tapping, where combustion of wood takes place upon contact with steel, leading to evolution of carbon dioxide, that provides purging of steel and slag phases.

For dephosphorization of steel, the following conditions are required:
1) The oxygen activity of the molten steel should be sufficiently high. High oxygen activity can be achieved by tapping the steel in unkilled form, i.e., in an oxidized state. The molten steel can be tapped in unkilled state, for example, by not adding deoxidizing agents at tap and by introducing new sources of oxygen, e.g., oxides of iron.
2) Basicity should be maintained in the ladle slag to combine it with phosphorus from the steel during tap. In the steelmaking process, basicity is maintained by addition of fluxing agents such as lime or lime-based material into the steel. Lime (CaO) helps in the removal of phosphorus from steel and stabilizes P2O5 in the slag form.
3) Slag should be fluid enough to increase the dissolution of fluxing agents into the slag and to improve dephosphorization kinetics. Addition of fluidizers such as CaF2 or Al2O3 helps in breaking the fluxing agents and maintaining the fluidity of the slag to improve the activity of fluxing agents and kinetics of the dephosphorization reaction.

In the present methods, before tapping molten steel into the ladle, temperature and sample measurements are taken. Exemplary sample measurements include measuring levels of phosphorous (P), carbon (C), sulphur (S), silicon (Si), manganese (Mn), chromium (Cr), nickel (Ni), and nitrogen (N). Oxygen levels are measured if required. Based on these measurements, re-blow of oxygen is done in the basic oxygen furnace to maintain oxygen activity in the molten steel. In some embodiments, oxygen is re-blown at least once before tapping molten steel into the ladle. In some embodiments, oxygen is re-blown one, two or three times before tapping molten steel into the ladle.

Further, the inventors have observed that the oxides present in the secondary emission dust, in particular iron oxides, also contribute to providing oxygen activity. Additionally, to maintain oxygen activity, in the present methods, molten steel is tapped in unkilled state, i.e., deoxidizing agents are not added to the molten steel at tapping. In exemplary embodiments, tapping of steel is done without addition of any deoxidizing agent and/or alloying material.

Basicity of ladle slag also affects the kinetics of dephosphorization reaction. As described above, separately-added lime and calcium oxide from the secondary emission dust contribute to maintaining basicity of the ladle slag.

In the present methods, the temperature of molten steel is monitored. The inventors observed that tapping of molten steel, addition of the secondary emission dust and other fluxing agents, and purging of steel and slag phases in the ladle result in a drop in the temperature of molten steel. For example, it is observed that there is a temperature loss of about 30? due to tapping into the ladle, a loss of about 25? due to addition of the secondary emission dust and fluxing agents, and a loss of about 10-15? due to purging. This loss of temperature in molten steel decreases the fluidity and oxygen activity of molten steel. Therefore, to maintain fluidity of molten steel (which thereby maintains fluidity of ladle slag), in some embodiments, the present methods comprise conducting the basic oxygen furnace process at a temperature of at least 1650?. In some embodiments, the basic oxygen furnace process is carried out at a temperature of about 1650? to about 1720?, about 1650? to about 1700?, about 1655? to about 1700?, about 1660? to about 1700?, about 1665? to about 1700?, about 1670? to about 1700?, including values and ranges therebetween. In some embodiments, the basic oxygen furnace process is carried out at a temperature of at least 1670?. In some embodiments, the basic oxygen furnace process is carried out at a temperature of about 1670? to about 1680?, including values and ranges therebetween. For example, in some embodiments, the basic oxygen furnace process is carried out at a temperature of about 1670?, 1671?, 1672?, 1673?, 1674?, 1675?, 1676?, 1677?, 1678?, 1679?, or 1680?. In preferred embodiments, the basic oxygen furnace process is carried out at a temperature of about 1670? to about 1680?; however, effective removal of phosphorous according to the present methods is obtained over the BOF temperature range of about 1650?-1710?.

In some embodiments, the method for producing low phosphorous IF steel comprises:
i) contacting molten steel with secondary emission dust, and
ii) purging the molten steel to promote mixing between steel phase and slag phase to produce low phosphorous steel.

In some embodiments, the method for producing low phosphorous IF steel comprises:
i) contacting molten steel with secondary emission dust;
ii) adding a fluxing agent (e.g., lime or lime-based material) to the molten steel; and
iii) purging the molten steel comprising the secondary emission dust and the fluxing agent to promote mixing between steel phase and slag phase to produce low phosphorous steel. The fluxing agent is added to the ladle simultaneously with the molten steel or prior to addition of the molten steel.

In some embodiments, the method for producing low phosphorous IF steel comprises:
i) carrying out the basic oxygen furnace process at a temperature of at least 1650?, preferably at a temperature of at least 1670?;
ii) contacting molten steel produced after the basic oxygen furnace process with secondary emission dust;
iii) adding a fluxing agent (e.g., lime or lime-based material) to the molten steel; and
iv) purging the molten steel comprising the secondary emission dust and the fluxing agent to promote mixing between steel phase and slag phase, to produce low phosphorous steel. The fluxing agent is added to the ladle simultaneously with the molten steel or prior to addition of the molten steel.

In some embodiments, the method for producing low phosphorous IF steel comprises:
i) carrying out the basic oxygen furnace process at a temperature of at least 1650?;
ii) optionally re-blowing the oxygen during the basic oxygen furnace process, preferably if temperature of molten steel is lower than 1650?;
iii) contacting molten steel produced after the basic oxygen furnace process with a fluxing agent (e.g., lime or lime-based material) and secondary emission dust in any order; and
iv) purging the molten steel comprising the secondary emission dust and the fluxing agent to promote mixing between steel phase and slag phase, to produce low phosphorous steel.

In some embodiments, the method for producing low phosphorous IF steel comprises:
i) carrying out the basic oxygen furnace process at a temperature of at least 1670?;
ii) optionally re-blowing the oxygen during the basic oxygen furnace process, preferably if temperature of molten steel is lower than 1670?;
iii) contacting molten steel produced after the basic oxygen furnace process with a fluxing agent (e.g., lime or lime-based material) and secondary emission dust in any order; and
iv) purging the molten steel comprising the secondary emission dust and the fluxing agent to promote mixing between steel phase and slag phase, to produce low phosphorous steel.

In some embodiments, the basic oxygen furnace process is conducted at a temperature of about 1670?-1680?. In some embodiments, the basic oxygen furnace process is conducted at a temperature of about 1670?. In some embodiments, the basic oxygen furnace process is conducted at a temperature of about 1680?.

In some embodiments, molten steel is contacted with the secondary emission dust in an amount of about 0.10-0.30% by weight of the molten steel. In some embodiments, molten steel is contacted with the secondary emission dust in an amount of about 0.20-0.30% by weight of the molten steel.

In some embodiments, contacting molten steel produced after the basic oxygen furnace process with secondary emission dust comprises:
a) adding the secondary emission dust in a ladle; and
b) tapping the molten steel produced by the basic oxygen furnace process in the ladle wherein said molten steel falling in the ladle comes in contact with the secondary emission dust.

In some embodiments, the molten steel is tapped in unkilled state, i.e., without the addition of deoxidizing agents.

The present disclosure also provides a method for dephosphorization of interstitial-free (IF) steel comprising contacting molten steel with secondary emission dust to dephosphorize the steel. The steps for contacting molten steel with secondary emission dust and steps prior to and after contacting of molten steel with secondary emission dust are described throughout this disclosure and are encompassed by the method for dephosphorization of IF steel. For example, in some embodiments, the method for dephosphorization of IF steel comprises: i) contacting molten steel with secondary emission dust, and ii) purging the molten steel to promote mixing between steel phase and slag phase to dephosphorize the steel.

In some embodiments, the method for dephosphorization of IF steel comprises: i) contacting molten steel with secondary emission dust and fluxing agent (e.g., lime or lime-based material) in any order; and ii) purging the molten steel comprising the secondary emission dust and the fluxing agent to promote mixing between steel phase and slag phase to dephosphorize the steel.

In some embodiments, the method for dephosphorization of IF steel comprises: i) carrying out the basic oxygen furnace process at a temperature of at least 1650?, preferably at a temperature of at least 1670?; ii) contacting molten steel produced after the basic oxygen furnace process with secondary emission dust; iii) adding a fluxing agent (e.g., lime or lime-based material) to the molten steel; and iv) purging the molten steel comprising the secondary emission dust and the fluxing agent to promote mixing between steel phase and slag phase, to dephosphorize the steel.

In some embodiments, the method for dephosphorization of IF steel comprises: i) carrying out the basic oxygen furnace process at a temperature of at least 1650?; ii) optionally re-blowing the oxygen during the basic oxygen furnace process, preferably if temperature of molten steel is lower than 1650?; iii) contacting molten steel produced after the basic oxygen furnace process with secondary emission dust; iv) adding a fluxing agent (e.g., lime or lime-based material) to the molten steel; and v) purging the molten steel comprising the secondary emission dust and the fluxing agent to promote mixing between steel phase and slag phase, to dephosphorize the steel.

In some embodiments, the method for dephosphorization of IF steel comprises: i) carrying out the basic oxygen furnace process at a temperature of at least 1670?; ii) optionally re-blowing the oxygen during the basic oxygen furnace process, preferably if temperature of molten steel is lower than 1670?; iii) contacting molten steel produced after the basic oxygen furnace process with secondary emission dust; iv) adding a fluxing agent (e.g., lime or lime-based material) to the molten steel; and v) purging the molten steel comprising the secondary emission dust and the fluxing agent to promote mixing between steel phase and slag phase, to dephosphorize the steel.

In some embodiments of the method for dephosphorization of IF steel, the basic oxygen furnace process is conducted at a temperature of about 1670?-1680?. In some embodiments, the basic oxygen furnace process is conducted at a temperature of about 1670?. In some embodiments, the basic oxygen furnace process is conducted at a temperature of about 1680?. In some embodiments, molten steel is contacted with the secondary emission dust in an amount of about 0.10-0.30% by weight of the molten steel. In some embodiments, molten steel is contacted with the secondary emission dust in an amount of about 0.20-0.30% by weight of the molten steel.

In some embodiments of the method for dephosphorization of IF steel, contacting molten steel produced after the basic oxygen furnace process with secondary emission dust comprises: a) adding the secondary emission dust in a ladle; and b) tapping the molten steel produced by the basic oxygen furnace process in the ladle wherein said molten steel falling in the ladle comes in contact with the secondary emission dust. In some embodiments, the molten steel is tapped in unkilled state, i.e., without the addition of deoxidizing agents.

In some embodiments, methods of the present disclosure lower the phosphorous content of the IF steel by at least 50%, 45%, 40%, 35%, 30%, 25%, 20%, 15%, or 10%. In one embodiment, methods of the present disclosure lower the phosphorous content of the IF steel by at least 50%.

In some embodiments, methods of the present disclosure provide IF steel having a phosphorus content of 0.02% or lower by weight of the steel, preferably 0.015% or lower by weight of the steel. In some embodiments, methods of the present disclosure provide IF steel having a phosphorus content of 0.02-0.005%, 0.015-0.005%, 0.015-0.006%, 0.015-0.007%, 0.015-0.008%, 0.015-0.009%, 0.015-0.010%, 0.015-0.011%, or 0.015-0.012%, by weight of the steel.

The present disclosure also provides use of secondary emission dust as a dephosphorization agent for production of low phosphorous interstitial-free (IF) steel. The secondary emission dust is a by-product of the basic oxygen furnace process. An exemplary method to collect/recover the secondary emission dust and the composition of secondary emission dust are described above.

In some embodiments, use of the secondary emission dust as a dephosphorization agent lowers the phosphorous content of IF steel by at least 50%, 45%, 40%, 35%, 30%, 25%, 20%, 15%, or 10%. In one embodiment, use of the secondary emission dust as a dephosphorization agent lowers the phosphorous content of the IF steel by at least 50%.

In some embodiments, use of the secondary emission dust as a dephosphorization agent provides IF steel having a phosphorus content of 0.02% or lower by weight of the steel, preferably 0.015% or lower by weight of the steel. In some embodiments, use of the secondary emission dust as a dephosphorization agent provides IF steel having a phosphorus content of 0.02-0.005%, 0.015-0.005%, 0.015-0.006%, 0.015-0.007%, 0.015-0.008%, 0.015-0.009%, 0.015-0.010%, 0.015-0.011%, or 0.015-0.012%, by weight of the steel.

The methods and uses provided by the present disclosure produce IF steel with desired phosphorus levels (e.g., = 0.02%, or preferably = 0.015%) without requirement of further processing of steel in the BOF vessel, and without the use of environmentally hazardous materials like CaF2. The methods and uses of the present disclosure are performed after the molten steel is produced in the BOF process, unlike conventional methods which are primarily aimed at reducing phosphorous content before or during the production of molten steel in BOF process. The methods and uses of the present disclosure are environment-friendly and economical as they re-use the secondary emission dust generated during the BOF process and thereby decrease the need for disposal of the dust. The methods and uses of the present disclosure also decrease the amount of separately-added lime or lime-based material thereby reducing the costs and improving overall efficiency of manufacturing low phosphorous IF grade steel.

The methods and uses of the present disclosure also decrease the cycle time of steelmaking process by cutting down the time in corrective actions and re-blows required to meet the low phosphorus requirements in conventional methods. It also reduces the amount of fluxes (e.g., iron ore, lime) consumed in correction process, and reduces the detrimental impact on refractory lining of the vessels due to additional blowing used in correction process.

The present disclosure described for the first time a method for dephosphorization of steel in the ladle after the BOF process. The methods and uses of the present disclosure do not require any additional set-up, avoid/minimize the necessity of slag stopping devices and at the same time provide a dephosphorization efficiency of at least 10-50%.

It is to be understood that the foregoing descriptive matter is illustrative of the disclosure and not a limitation. While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. Those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein. Similarly, additional embodiments and features of the present disclosure will be apparent to one of ordinary skill in art based upon description provided herein.

Descriptions of well-known/conventional methods/steps and techniques are omitted so as to not unnecessarily obscure the embodiments herein. Further, the disclosure herein provides for examples illustrating the above described embodiments, and in order to illustrate the embodiments of the present disclosure certain aspects have been employed. The examples used herein for such illustration are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the following examples should not be construed as limiting the scope of the embodiments herein.

INCORPORATION BY REFERENCE
All references, articles, publications, patents, patent publications, and patent applications (if any) cited herein are incorporated by reference in their entireties for all purposes. However, mention of any reference, article, publication, patent, patent publication, and patent application cited herein is not, and should not be taken as, an acknowledgment or any form of suggestion that they constitute valid prior art or form part of the common general knowledge in any country in the world.

EXAMPLES

Example 1: Production of IF steel
IF steel samples were prepared by employing methods of the present disclosure broadly comprising the following steps: i) molten pig iron from a blast furnace was subjected to the basic oxygen furnace process performed at a temperature ranging from 1650?-1720?; ii) optionally, oxygen was re-blown during the basic oxygen furnace process; iii) secondary emission dust at 0.10-0.30% by weight of the molten steel was added to an empty ladle; iv) molten steel produced after the basic oxygen furnace process was tapped into the ladle containing the secondary emission dust (temperature of molten steel was maintained between 1650?-1720?); and v) the contents of the ladle were purged by employing argon. After ladle processing, steel was taken for vacuum degassing process for further decarburization and dehydrogenation, suitable alloying was done after deoxidation of steel, and then casting (solidification) of the steel was done.

Table 2 shows the phosphorus (P) levels (%) of the above IF steel samples measured at various stages after BOF process and according to the methods of the present disclosure.
Table 2
Sample No. Temperature of molten steel after BOF process P in molten steel after BOF process Secondary emission dust added at tap
(Kg) P after On-line purging (OLP) P in steel at caster Final P drop
1686 0.022 400 0.014 0.014 0.008
1685 0.023 450 0.016 0.015 0.008
1653 0.013 450 0.011 0.011 0.002
1656 0.017 300 0.011 0.013 0.004
1668 0.016 300 0.012 0.012 0.004
1667 0.018 300 0.014 0.014 0.004
1665 0.026 400 0.014 0.014 0.012
1651 0.014 350 0.009 0.009 0.005
1655 0.016 300 0.010 0.011 0.005
1706 0.016 400 0.015 0.014 0.002
1700 0.014 400 0.011 0.011 0.003
1670 0.019 400 0.016 0.015 0.004
1673 0.024 400 0.014 0.022 0.002
1668 0.024 400 0.014 0.014 0.010
1653 0.027 400 0.012 0.011 0.016
1681 0.028 225 0.024 0.02 0.008
1659 0.026 300 0.019 0.018 0.008
1655 0.015 400 0.01 0.009 0.006
1708 0.023 400 0.019 0.018 0.005
1718 0.04 225 0.022 0.02 0.02
1696 0.024 400 0.016 0.015 0.009

As seen from the above results of Table 2, employing secondary emission dust according to the methods of the present disclosure successfully lowers the phosphorous content in IF steel leading to low phosphorous IF steel products.

Example 2: Collection/recovery of secondary emission dust
To collect secondary emission dust from the BOF vessel, suction is created near the skirt area (see part A in Figure 1B) of the BOF vessel; dust is sucked and sent towards an ESP (Electrostatic Precipitator). The dust is segregated in ESP (see part B in Figure 1B) through various electrodes mounted in it. Dust collected with electrodes is discharged in the ESP chamber bottom (see part C in Figure 1B) at a defined frequency automatically. From ESP chamber bottom, dust is conveyed through chain conveyor to nearby collection silo, also called day bin. From the collection silo, the dust is discharged in a tanker with the help of a pneumatic valve. Finally, dust is collected in gunny bags, each containing about 20 kilogram of dust from tanker discharge chute.

Claims:1. A method for producing low phosphorous interstitial-free (IF) steel comprising contacting molten steel with secondary emission dust to produce the low phosphorous IF steel.
2. The method as claimed in claim 1, wherein the molten steel is produced after basic oxygen furnace process.
3. The method as claimed in claim 1, wherein the secondary emission dust comprises components selected from the group consisting of: iron oxides, calcium oxide, silicon dioxide, magnesium oxide, aluminium oxide, manganese oxide, titanium oxide, chromium oxide, phosphorous pentoxide, and a combination thereof.
4. The method as claimed in claim 3, wherein iron oxides are present in an amount of about 30-40%, calcium oxide is present in an amount of about 15-20%, silicon dioxide is present in an amount of about 0-5%, magnesium oxide is present in an amount of about 0-5%, aluminium oxide is present in an amount of about 0-2%, manganese oxide is present in an amount of about 0-2%, titanium oxide is present in an amount of about 0-0.5%, chromium oxide is present in an amount of about 0-0.1%, phosphorous pentoxide is present in an amount of about 0-0.5%, and volatile components are present in an amount of about 32-45%, by weight of the secondary emission dust.
5. The method as claimed in any one of the preceding claims, wherein the secondary emission dust is added in an amount of about 0.10-0.30% by weight of the molten steel.
6. The method as claimed in any one of claims 2-5, wherein the basic oxygen furnace process is carried out at a temperature of at least 1650?, preferably at least 1670?.
7. The method as claimed in claim 6, wherein the basic oxygen furnace process is carried out at a temperature of 1670? to 1680?.
8. The method as claimed in any one of claims 2-7, wherein oxygen is re-blown during the basic oxygen furnace process.
9. The method as claimed in any one of the preceding claims, wherein the method further comprises adding a fluxing agent.
10. The method as claimed in claim 9, wherein the fluxing agent is lime or a lime-based material; and wherein the fluxing agent is added in an amount of about 0.15-0.20% by weight of the molten steel.
11. The method as claimed in any one of the preceding claims, wherein the method comprises purging the molten steel after contacting with the secondary emission dust, wherein said purging promotes mixing between steel phase and slag phase.
12. The method as claimed in claim 11, wherein the purging is carried out by employing inert gas or a material that evolves gases on combustion.
13. The method as claimed in claim 1, wherein said contacting comprises:
i) adding the secondary emission dust in a ladle; and
ii) tapping the molten steel in the ladle wherein said molten steel falling in the ladle comes in contact with the secondary emission dust.
14. The method as claimed in claim 2, wherein said method comprises:
i) carrying out the basic oxygen furnace process at a temperature of at least 1650?;
ii) said contacting molten steel with secondary emission dust comprising: a) adding the secondary emission dust in a ladle; and b) tapping the molten steel produced by the basic oxygen furnace process in the ladle wherein said molten steel falling in the ladle comes in contact with the secondary emission dust;
iii) optionally re-blowing oxygen during the basic oxygen furnace process; and
iv) purging the molten steel to promote mixing between steel phase and slag phase, to produce low phosphorous steel.
15. A method for dephosphorization of interstitial-free (IF) steel comprising contacting molten steel with secondary emission dust to dephosphorize the steel.
16. The method as claimed in claim 15, wherein the molten steel is produced after basic oxygen furnace process.
17. The method as claimed in claim 15 or 16, wherein the secondary emission dust is added in an amount of about 0.10-0.30% by weight of the molten steel.
18. Use of secondary emission dust as a dephosphorization agent for production of low phosphorous interstitial-free (IF) steel.
19. The method as claimed in any one of claims 15-17 or the use as claimed in claim 18, wherein the secondary emission dust comprises components selected from the group consisting of: iron oxides, calcium oxide, silicon dioxide, magnesium oxide, aluminium oxide, manganese oxide, titanium oxide, chromium oxide, phosphorous pentoxide, and a combination thereof.
20. The method or use as claimed in claim 19, wherein iron oxides are present in an amount of about 30-40%, calcium oxide is present in an amount of about 15-20%, silicon dioxide is present in an amount of about 0-5%, magnesium oxide is present in an amount of about 0-5%, aluminium oxide is present in an amount of about 0-2%, manganese oxide is present in an amount of about 0-2%, titanium oxide is present in an amount of about 0-0.5%, chromium oxide is present in an amount of about 0-0.1%, phosphorous pentoxide is present in an amount of about 0-0.5%, and volatile components are present in an amount of about 32-45%, by weight of the secondary emission dust.
21. The method or use as claimed in any one of the preceding claims, wherein said method or said use lowers the phosphorous content of the IF steel by at least 10%.
22. A low phosphorous interstitial-free (IF) steel obtained by the method as claimed in any one of claims 1-17.