Claims:We Claim:

1. A flux composition to remove nitrogen from molten steel to form denitrogenated molten steel including a ferro-titanate comprising: 48-60 weight % of TiO2, 25-42 weight % of FeO and 2-6 weight % SiO2.

2. The flux composition to remove nitrogen from molten steel to form denitrogenated molten steel as claimed in claim 1, wherein the particle size of the flux composition is in the range of 65-850 µm and morepreferably particle size is in the range of 75 - 850 µm.

3. The flux composition to remove nitrogen from molten steel to form denitrogenated molten steel as claimed in anyone of the claims 1 or 2, wherein the crystalline phases of the flux composition comprising:
65-78 weight % Ilmenite,
15-22 weight % Rutile, and
5-9 weight % Diiron Silicate.

4. A method to remove nitrogen frommolten steel to form denitrogenatedmolten steelinvolving the flux composition as claimed in anyone of claims 1 to 3,comprising the steps of:

adding said flux composition into the bottom of a steel ladle,wherein the flux composition including a ferro-titanate comprising: 48-60 weight % of TiO2, 25-42 weight % of FeO and 2-6 weight % SiO2;

pouring/tapping the molten steelover the flux compositioninthe steel ladlefor desired denitriding treatment to form denitrogentaed molten steel; and

refining of the denitrogenatedmolten steel.

5. The method to remove nitrogen from molten steel to form denitrogenatedmoltensteel as claimed in claim 4, wherein the molten steel is made in a Basic Oxygen Furnace (BOF) by using pig iron as the main iron source, the molten steel is poured/tapped into the steel ladle, the nitrogen removal treatment is performed in the steel ladle and the denitrogenated molten steel is refined in a ladle heating furnace (LHF) and degassing unit.

6. The method to remove nitrogen from molten steel to form denitrogenatedmolten steel as claimed in anyone of the claim 4 to 5, wherein theflux composition and molten steelis subjected to a temperature 1610-1670 °Cfor a time 15-40 minutes for said denitriding treatment so that the molten steel comes in contact with the flux composition suchthat nitrogen in the molten steel migrates toward the flux composition to form a denitrogenatedmolten steel and recovering the denitrogenatedmolten steel.

7. The method to remove nitrogen from molten steel to form denitrogenatedmolten steel as claimed in anyone of the claim 4 to 6, wherein flux composition added is in a range of 1.0 kg to 2.5 kg per ton of liquid steel (TLS) and more preferably is in the range of 1.7-2.3 kg per ton of liquid steel (TLS).

8. The method to remove nitrogen from molten steel to form denitrogenated molten steel as claimed in anyone of the claim 4 to 7, wherein the nitrogen content in the denitrogenated molten steel at ‘LHF entry’ is less than 35 ppm and more preferably less than 30 ppm and Ti content in denitrogenated molten steel is in the range of 0.006 weight % to 0.013 weight %.

9. The method to remove nitrogen from molten steel to form denitrogenated molten steel as claimed in anyone of the claim 4 to 8, wherein the denitrogenated molten steel is suitable for line pipe applications and has minimum Ti content of 0.01 weight %.

10. The method to remove nitrogen from molten steel to form denitrogenated molten steel as claimed in anyone of the claim 4 to 8, wherein due to the pickup of titanium from the flux composition in the denitrogenated molten steel, there is reduced FeTi alloy addition by 40-100% in secondary steel making depending on the steel grade.

Dated this the 20th day of October, 2020
Anjan Sen
Of Anjan Sen & Associates
(Applicants’ Agent)
IN/PA-199

, Description:FORM 2
THE PATENT ACT 1970
(39 OF 1970)
&
The Patent Rules, 2003
COMPLETE SPECIFICATION
(See Section 10 and Rule 13)

1 TITLE OF THE INVENTION :
A DENITRIDING FLUX COMPOSITION AND A METHOD TO REMOVE NITORGEN FROM MOLTEN STEEL TO FORM DENITROGENATED MOLTEN STEEL.



2 APPLICANT (S)

Name : JSW STEEL LIMITED.

Nationality : An Indian Company incorporated under the Companies Act, 1956.

Address : JSW CENTRE,
BANDRA KURLA COMPLEX,
BANDRA(EAST),
MUMBAI-400051,
MAHARASHTRA,INDIA.



3 PREAMBLE TO THE DESCRIPTION

COMPLETE

The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION

The present invention relates to the making of steel involving a denitriding flux composition, and in particular, it relates to the making of steel with low nitrogen content. Still more particularly, it relates to a denitriding flux composition and method to remove nitrogen for a steel containing titaniumduring pouring/tapping of molten steel to steel ladle prior to the secondary refinement stage. The steel produced by such process is intended for use in line pipe applications in which titanium content in the steel is important.

BACKGROUND OF THE INVENTION

Nitrogen pick-up in steel occurs from numerous sources during various stages of processing of steel such as melting, ladle processing and casting operations. Sources of nitrogen in primary steelmaking process by BOF include hot metal, scrap, impurity in oxygen and the stirring gas. Further, during tapping of liquid steel into a steel ladle, air bubbles get entrained into the steel and act as a source of nitrogen. Any leakage during vacuum degassing process may also increase nitrogen content in the steel. Nitrogen pick-up can also happen through ladle slag, coke, and various ferroalloys.

Currently, there is a demand for Line Pipe grade steels having less nitrogen content. The maximum solubility limit of nitrogen in liquid steel is approximately 450 ppm whereas it is less than 10 ppm at ambient temperature. Upon solidification, the nitrogen in steel may form blowholes, precipitate as nitride compounds and/or solidify in interstitial solid solution. This in turn affect various chemical, mechanical and physical properties of the steel. For example, nitrogen in steel affects significantly the drawability, toughness, welding properties, hardness, abrasion-resistance and corrosion performance of the steel. Thus, nitrogen is considered as a detrimental element in many steels and its content, therefore, should be reduced to a minimum. For example, nitrogen in steel causes poor deep drawability of thin sheets and low toughness of thick plates. Nitrogen content also affects the magnetic, electrical and welding properties of sheet steel. The hardness, abrasion-resistance and corrosion performance is also affected by the presence of nitrogen. Nitrogen may also be one of the causes of porosity because of gas evolution during solidification. Hence, the control of nitrogen has become extremely important in view of its influence on chemical, mechanical and physical properties of steel.

In principle, nitrogen is removed from molten steel by vacuum degassing and flux treatment. Vacuum degassing was first tested to remove nitrogen from molten steel, since hydrogen in molten steel can be effectively removed by vacuum degassing to about 1 ppm. Though vacuum degassing is widely used to remove nitrogen during processing of steel, unfortunately, it can remove only 10-30% of nitrogen in molten steel, thus making the method ineffective for nitrogen critical grades. This lower efficiency of nitrogen removal can be attributed to its smaller diffusion coefficient in molten steel. Furthermore, the different stages of steelmaking process have a chance of nitrogen pick-up in steel during processing.

Therefore, research has focused on exploring various alternative methods to remove nitrogen from molten steel effectively.

Other method of removing nitrogen includes flux treatment. Many flux composition containing CaO, BaO, MgO, Al2O3, SiO2, B2O3, CeO2, ZrO2 and TiO2 and many other oxides have been discovered to effectively remove nitrogen from molten steel.

Patent No. US7901482B2 by Tada et al, dated 6th Feb 2007, provides a method of nitrogen removal from interstitial free (IF) steel with ultra-low carbon by addition of 3-20 kg of metallic Al containing material per ton of molten steel, 1-50 kg of CaO per ton of molten steel and supply of an oxygen containing gas to the bath of molten steel tapped from an electric arc furnace (EAF) wherein the nitrogen content in the steel achieved was less than equal to 40 ppm. The process involves blowing of oxygen into the molten steel which has a chance of oxidising of the steel by forming FeO, increased oxygen content in the bath and increased requirement of killing agents to remove the oxygen from the steel bath.This method requires lot of modification to the existing steel making process.

European Patent No. EP2045338A1 by Ban-Yaet al, dated 8th Apr 2009, discloses about reduction in nitrogen content in steel for automobile applications by addition of a fluoride free flux consisting of 30-57 weight % CaO, 35-64 weight % Al2O3 and 5-17 weight % MgO, and having a composition in which the activity of alumina is 10-2 or less. They were able to reduce the nitrogen content to below 80 ppm which could further be reduced to 40 ppm or less by combination with vacuum degassing and to 20 ppm or less under preferable operating conditions. However, this flux reduces the sulphur and oxygen content as well in the steel.

Chinese Patent No CN105648138A by Lu et al, dated 26th Jan 2018, describes a method to control the tapping nitrogen content in low-carbon steel by converter smelting process so that the nitrogen content in the final molten steel is maintained below 15 ppm. The method comprises the steps that a, the molten iron ratio of converter steelmaking is controlled to be higher than 0.9; b, in the later stage of blowing, low-lance-height operation is adopted, and the oxygen lance height is lowered till steel tapping is conducted when the oxygen blowing quantity reaches 70%-80% of the total oxygen quantity with the oxygen blowing quantity of the converter as the standard; C, when steel tapping is conducted by two thirds, dolomite is added, lime is added at the same time, a layer of top slag is formed on the molten steel surface, and the thickness of the slag layer is smaller than 60 mm; d, after steel tapping is completed, a top slag modifier is added; and e, a steel outlet is maintained, so that the effect that a steel outlet flow is not diffused is guaranteed. This method requires lot of modification to the existing steel making process.

Despite progress made by previous studies as shown in above prior arts, further efforts are required to find more effective fluxcompositions.

OBJECTS OF THE INVENTION

Accordingly, the primary object of the present invention is to provide a denitridingflux composition which can be incorporated into the steelmaking processes with little or no modification than the existing steel making processes.

Yet a further object is to provide a method to remove nitrogen from molten steel to form denitrogenated molten steel.

Yet a further object is to provide a method of making denitrogenated steel in which titanium is a desired element.

Another object of the present invention is directed to said method to remove/reduce the nitrogen content in steel at an early stage of steelmaking process, particularly during pouring/tapping of molten steel to steel ladle prior to the secondary refinement stage.

Another object of the present invention is directed to provide denitrogenated steel for line pipe applications.

A further object of the present invention is directed to provide a TiO2 based denitriding flux composition.

Yet a further object of the present invention is directed to minimise the cost of production of denitrogenated steel by reducing the consumption of expensive ferroalloys.

Other objects and advantages will become apparent from the following description of the present invention.

SUMMARY OF THE INVENTION

In accordance with the basic aspect of the present invention, a flux composition to remove nitrogen from molten steel to form denitrogenated molten steel is provided. The flux composition includes a ferro-titanate comprising 48-60 weight % of TiO2, 25-42 weight % of FeO and 2-6 weight % SiO2.

There is further disclosed in the present invention, wherein the particle size of the flux composition is in the range of 65-850 µm and more preferably particle size is in the range of 75 - 850 µm.

There is disclosed that the crystalline phases of the flux composition comprising: 65-78 weight % Ilmenite, 15-22 weight % Rutile, and 5-9 weight % Diiron Silicate.

There is further provided herein a method to remove nitrogen from molten steel to form denitrogenated molten steel. The method comprising the following steps. Firstly, adding flux composition into the bottom of a steel ladle, wherein the flux composition including a ferro-titanate comprising: 48-60 weight % of TiO2, 25-42 weight % of FeO and 2-6 weight % SiO2. Secondly, pouring/tapping the molten steel over the flux composition on the steel ladle to form denitrogenated molten steel. Lastly, refining of the denitrogenated molten steel.

In addition, the present invention provides said method wherein the molten steel is made in a Basic Oxygen Furnace (BOF) by using pig iron as the main iron source. The molten steel is tapped/poured into the steel ladle. The nitrogen removal treatment is performed in the steel ladle. The denitrogenated molten steel is then refined in a ladle heating furnace (LHF) and degassing unit.

A further aspect of the present invention is directed to said method wherein the flux composition and molten steel is subjected to a temperature 1610-1670 °C for a time 15-40 minutes so that the molten steel comes in contact with the flux composition such that nitrogen in the molten metal migrates toward the flux composition to form a denitrogenated molten steel and recovering the denitrogenated molten steel.

A still further aspect of the present invention is directed to said method wherein the flux composition added is in a range of 1.0 kg to 2.5 kg per ton of liquid steel (TLS) and more preferably is in the range of 1.7-2.3 kg per ton of liquid steel (TLS).

A further aspect of the present invention is directed to said method wherein the nitrogen content in the denitrogenated molten steel at ‘LHF entry’ is less than 35 ppm and more preferably less than 30 ppm and Ti content in denitrogenated molten steel is in the range of 0.006 weight % to 0.013 weight %.

A still further aspect of the present invention is directed to the denitrogenated molten steel that is suitable for line pipe applications and has minimum Ti content of 0.01 weight %.

A still further aspect of the present invention is directed tosaid method enabling pickup of titanium from the flux composition in the denitrogenated molten steel. Due to this there is reduced or no FeTi alloy addition in secondary steelmaking.

The above and other objects and advantages of the present invention are described hereunder in greater details with reference to following accompanying non limiting illustrative drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING

FIG. 1 illustrates the flow chart of a method to remove nitrogen from molten steel to form denitrogenated molten steel;

FIG. 2 illustrates the particle size distribution of the flux composition;

FIG. 3 illustrates the phase analysis of the flux composition;

FIG. 4 illustrates the distribution of nitrogen content in denitrogenated steel at ‘LHF entry’;

Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity to help to improve understanding of embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS

The accompanying figure together with the detailed description below forms part of the specification and serves to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention.

The present invention is now discussed in more detail referring to the drawings that accompany the present application. In the accompanying drawings, like and/or corresponding elements are referred to by like reference numbers.

Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or the like parts.

Before describing in detail embodiments that are in accordance with the invention, it should be observed that the embodiments reside primarily to a denitriding flux composition to remove nitrogen from molten steel to form denitrogenated molten steel and a method using the flux composition to remove nitrogen from molten steel to form denitrogenated molten steel. The denitrogenated steel is intended to be used for line pipe applications.

In the present invention the nitrogen is removed from the molten steel involving ferro-titanate based denotriding flux in the early stage of steelmaking process that is prior to the secondary steelmaking stageso that the nitrogen content in the final product is achieved within the specified limit.Further, the quantity of consumption of expensive Ferro-Titanium (FeTi) alloy during secondary steel making is reduced as the required titanium (Ti) in steel is compensated from the flux composition. It is thus possible by way of the present invention to process steel grades with lower production cost.

The present invention relates to a flux composition to remove nitrogen from molten steel to form denitrogenated molten steel is disclosed. The flux composition includes a ferro-titanate comprising 48-60 weight % of TiO2, 25-42 weight % of FeO and 2-6 weight % SiO2. The particle size of the flux composition is in the range of 65-850 µm and more preferably particle size is in the range of 75 - 850 µm. FIG. 2 illustrates the particle size distribution of the flux composition. The crystalline phases of the flux composition comprising: 65-78 weight % Ilmenite, 15-22 weight % Rutile, and 5-9 weight % Diiron Silicate. The Ilmenite phase helps in nitrogen removal from the molten steel, the Rutile phase primarily takes part in the reduction reaction to become a source of Ti in the molten steel.FIG. 3 illustrates the phase analysis of the flux composition.

The nitride capacity of an oxide is as follows:
TiOx> CeO2> ZrO2> SiO2> Al2O3> B2O3>MgO>CaO>BaO

The flux composition containing TiOx (x = 1.5 to 2) phase has been observed to be the most effective constituent for removing nitrogen. Titanium ions exist in three valence states viz. Ti4+, Ti3+ and Ti2+ in the molten slag and may have different effects on nitride capacity. According to reported studies, when slag contains TiO2 as the initial composition, a part of TiO2 gets reduced to Ti2O3 when it reaches slag-metal or slag-gas equilibrium. As a result, both TiO2 and Ti2O3 coexist in the system.

Further, it has been observed that the ability to remove nitrogen from the molten steel is relatively higher for Ti2+ or Ti3+ containing slag than Ti4+ containing slag. However, the ferro-titanate flux composition used in the present invention, is a naturally occurring mineral which shows good fluidization behaviour and is easily available at a lower cost as it does not involve any processing cost other than size reduction. In addition, presence of both Ilmenite and Rutile phases in the material not only helped in removal of nitrogen from steel, but also acted as a source of Ti that ultimately reduced the consumption of expensive ferroalloy. Hence the flux composition was preferred for the present invention.

For steels deoxidized with aluminium (Al) and slags containing TiO2, the TiO2 gets reduced by Al at the slag-metal interface and an equilibrium is established for titanium (Ti) as per the following reaction:
2[Al] + 3(TiO2) = (Al2O3) + 3[Ti] …Eq(1)

In addition, FeTiO3 can also get reduced by Al as per the following equation:

2[Al] + (FeTiO3) = [Fe] + [Ti] + (Al2O3) …Eq(2)

In the present invention, titanium being a strong nitride former, forms nitride particles at the slag-metal interface that are captured by the slag. Furthermore, the Ti thus formed at the slag-metal interface, reacts with the nitrogen of air ingresses through the slag layer, and helps in minimizing nitrogen pick-up by the steel. A part of dissolved nitrogen may also get removed by getting floated to the slag layer through heterogeneous nucleation sites. The temperature during tapping and time between tapping start to ‘LHF entry’ facilitates the migration of nitrogen in the liquid steel toward the slag-metal interface to form a denitrogenated steel.

There is further disclosed herein a method 100 to remove nitrogen from molten steel to form denitrogenated molten steel as shown in FIG. 1. The method 100 comprising the following steps. In step 102, the flux composition is added into the bottom of a steel ladle. The flux composition including a ferro-titanate comprising: 48-60 weight % of TiO2, 25-42 weight % of FeO and 2-6 weight % SiO2. The molten steel which is made in a Basic Oxygen Furnace (BOF) by using pig iron as the main iron source. In step 104, the molten steel over the flux composition is poured/tapped on the steel ladle to form denitrogenated molten steel. The molten steel is poured/tapped into the steel ladle. The nitrogen removal treatment is performed in the steel ladle.The flux composition and molten steel is subjected to a temperature 1610-1670 °C for a time 15-40 minutes so that the molten steel comes in contact with the flux composition such that nitrogen in the molten metal migrates toward the flux composition to form a denitrogenated molten steel and recovering the denitrogenated molten steel. In step 106, the denitrogenated molten steel is refined. The denitrogenated molten steel is refined in a ladle heating furnace (LHF) and degassing unit.

The flux composition added is in a range of 1.0 kg to 2.5 kg per ton of liquid steel (TLS) and more preferably is in the range of 1.7-2.3 kg per ton of liquid steel (TLS). Addition of flux composition below 1.0 kg per TLS was less effective at reducing the nitrogen content in steel to less than 35 ppm at ‘LHF entry’ whereas addition of flux above 2.5 kg per TLS increased the Ti content above the specified upper limit of the steel grades at ‘LHF entry’.

The nitrogen content in the denitrogenated molten steel is less than 35 ppm and more preferably less than 30 ppm and Ti content in denitrogenated molten steel is in the range of 0.006 weight % to 0.013 weight %. The nitrogen and titanium content is measured at ‘LHF entry’.

The denitrogenated molten steel is suitable for line pipe applications and has minimum Ti content of 0.01 weight %. Recovery of Ti from the flux composition compensates the Ti content in the molten steel and helped to achieve a Ti content in the range of 0.006-0.013 weight % at ‘LHF entry’ and remaining addition of the ferroalloy is done as per the grade requirement. The method further includes the addition of the alloy after slag killing is complete and obtaining the steel sample chemistry confirmation.

Line pipes are welded or seamless pipes used for construction of pipelines primarily to convey gas, oil, water, chemical products, sewage and even solid material like coal and ores over long distances. They are available with the ends plain, bevelled, grooved, cold expanded, flanged or threaded. The pipelines require to exhibit high level of safety and integrity throughout their designed life as they traverse through diverse locations which are difficult to approach during regular operations. Furthermore, increased transportation efficiency and use in sub-critical atmosphere require the pipelines to exhibit high yield and tensile strengths, high low temperature impact toughness, susceptibility to hydrogen induced cracking and corrosion resistance.

Some titanium is picked up of from the flux composition in the denitrogenated molten steel. During secondary steel making there is reduced or no FeTi alloy addition required. The quantity of FeTi alloy addition is adjusted as per steel grade requirement. The denitrogenated steel is further cast in continuous casting unit. Hence, the present invention helps in reducing the production cost of these steel grades by decreasing the consumption of expensive FeTi alloy added during secondary steelmaking process at LHF and RH degassing units. As Ti could be recovered from the flux partially, the reduction in total FeTi alloy consumption achieved is in the range of 40-100% depending on the steel grade.
In this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article or composition that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article or composition. An element proceeded by "comprises...a" does not, without more constraints, preclude the existence of additional identical elements in the process, method, article or composition that comprises the element.

In the present specification, when a part “includes” a certain component, it means that the component may further include other components, not to exclude other components, unless specifically stated otherwise.

Although not defined differently, all terms including technical terms and scientific terms used herein have the same meaning as those generally understood by those skilled in the art to which the present invention pertains. Commonly used dictionary-defined terms are additionally interpreted as having meanings consistent with related technical documents and currently disclosed contents, and are not interpreted as ideal or very formal meanings unless defined.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art to which the present invention pertains can easily practice. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein.

In addition, unless otherwise specified, % means weight%.

Advantages and features of the present invention, and methods for achieving them will be clarified with reference to embodiments described below in detail together with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various different forms, and only the embodiments allow the disclosure of the present invention to be complete, and are conventional in the art to which the present invention pertains. It is provided to fully inform the knowledgeable person of the scope of the invention, and the invention is only defined by the scope of the claims. The same reference numerals refer to the same components throughout the specification.

Thus, in some embodiments, well-known techniques are not specifically described to avoid obscuring the present invention. Unless otherwise defined, all terms (including technical and scientific terms) used in the present specification may be used as meanings commonly understood by those skilled in the art to which the present invention pertains.

The present invention is described further hereinafter by reference to accompanying example.

Experiments that were actually performed are now described by way of following examples.

Example 1:Effectiveness of denitriding flux composition during early stage of steelmaking and amount of Ti picked up in the denitrogenated steel was evaluated:

In this example, the effectiveness of denitriding flux composition is experimentally shown by measuring the nitrogen content.

1.8 kg per TLS flux composition was added to the bottom of the steel ladle followed by tapping of molten steel from BOF and subsequent refinement in LHF and degassing unit. The nitrogen and Ti contents in steel at ‘LHF entry’ are 30 ppm and 0.012 weight % respectively.

2.24 kg per TLS flux composition is added to the bottom of the steel ladle followed by tapping of steel from BOF and subsequent refinement in LHF and degassing unit. The nitrogen and Ti contents in steel at ‘LHF entry’ are 20 ppm and 0.01 weight % respectively.

1.02 kg per TLS flux composition is added to the bottom of the steel ladle followed by tapping of steel from BOF and subsequent refinement in LHF and degassing unit. The nitrogen and Ti contents in steel at ‘LHF entry’ are 23 ppm and 0.007 weight % respectively.

In the above example, the flux composition and molten steel is subjected to a temperature 1610-1670 °C for a time 15-40 minutes in steel ladle so that the molten steel comes in contact with the flux composition to complete denitriding treatment.

The trial as per the present invention was conducted in 64 heats and FIG. 4 shows the distribution of nitrogen content in the denitrogenated molten steel at ‘LHF entry’ as per the present invention where the content varied between 16 to 35 ppm. The result further indicates that in 75% trial heats, the nitrogen content in the denitrogenated molten steel at ‘LHF entry’ is less than 30 ppm. Thus the result confirms that the flux is effectively helping in obtaining a denitrogenated molten steel at the early stage of steel making prior to secondary refining process.