Title of Invention

A PROCESS FOR FACILITATING DEPHOSPHORISATION FOR STEEL MANUFACTURE IN LD CONVERTER

Abstract

A process for improved dephosphorization of steel in BOF/LD Converter by pellet coolant directed to achieve a lower average bath phosphorous content below the existing level of 0.015% and preferably around 0.011%. Importantly the process involves use a combined iron ore and pellet addition pattern wherein after the addition of iron-ore, selectively up to 3-4 tons of pellets maximum for optimum benefits, is used. The improved dephosphorization efficiency of the oxidizing slag results from addition of iron-ore pelletized with lime and dolomite, that causes effective slag-metal interface reaction with a selective control of flow rate of oxygen-lancing .The invention achieves a lower limit of phosphorous up to around 0.011% in resulting steel, even without bottom purging and higher yield by about 1%, due to increased FeO in converter on pellet addition.

Full Text

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 PROCESS FOR DEPHOSPHORIZATION OF STEEL IN LD CONVERTER BY PELLET ADDITION 2 APPLICANT (S) Name : JSW Steel Limited, Nationality : An Indian Company Address : Jindal Mansion, 5-A, Dr. G. Deshmukh Marg ,Mumbai - 400 026, State of Maharastra, India 3 PREAMBLE TO THE DESCRIPTION COMPLETE The following specification particularly descibes the invention and the manner in which it is to be performed. FIELD OF THE INVENTION: The present invention is related to producing low phosphorous steel and, in particular, to a process for producing low phosphorous steel involving iron-ore pellets as a coolant source without impairing the process for dephosphorization during making of steel in LD converters. Advantageously, the process involving the use of pellets to selective extent favour optimum benefits with high dephosphorisation efficiency of slag wherein it is possible to bring down the average bath phosphorous content from existing level of 0.015% with only iron ore as coolant to about 0.011% under optimum slag condition with such selective iron-ore pellet replacement of iron-ore even in the absence of bottom purging. BACKGROUND AND PRIOR ART: Production of steel with extremely low phosphorous content has been a challenge to the steel makers. The process conditions sensitively decide the quantum of phosphorous pick up by slag from the liquid metal. The principal conditions essential for dephosphorisation are: a relatively low temperature, quick formation of ferruginous-limy slag, formation of an oxidizing furnace atmosphere and oxidizing slag with a high activity of iron oxides (FeO) in it. These favorable conditions for removal of phosphorous occur at the end of the blowing when the carbon content decreases and the oxidation degree of the bath increases respectively, and the slag thus formed is rich in iron oxides. In normal addition practice, this is achieved by the use of iron ore as coolant. This extensive slag-metal interfacial reaction is usually achieved by bottom purging and stirring with inert gas at the end of the oxygen blowing, because the control on temperature is very difficult so long the top lance blowing continues. According to the existing art of controlling the composition in the basic oxygen converters that are operated at higher life, say exceeding 7000 heats, in between successive relining with basic refractory on the inside wall of converter the availability of the bottom purging is minimized due to build-up in the bottom. This results in lower specific volume of the converter and hence reduced chances of foamy slag formation favorable for dephosphorisation. As a result of this constraint, phosphorous removal becomes extremely difficult. This leads to the heats being finished with the output of steel having higher phosphorous than the specified limits of a particular grade of steel. Such situation can only be brought under control with a short re-blow thus prolonging the blowing time and 2 increasing the time in between successive tapings. As a result of this short re-blows the metallic yield of each heat is decreased and the overall productivity of the shop also declines. OBJECTS OF THE INVENTION: It is therefore the basic objective of the present invention to provide for effective dephosphorisation of steel in high life BOF converter which would avoid the afore discussed limitations and complexities of dephosphorisation of steel especially when converters are operated at higher life (> 7000 heats) with problems of bottom purging. Another object of the present invention is directed to process of dephosphorisation of steel which would favour effective dephosphorisation of slag such as in high life converters and even without the need for any bottom purging. A further object of the present invention is directed to steel making involving a new coolant system which would enable dephosphorisation in high life converter in the absence of bottom purging. Yet another object of the present invention is directed to steel making involving a new coolant system which would favour achieving desired essential conditions for dephosphorisation at a relatively low temperature ,quick formation of ferruginous -limy slag and providing for an oxidizing furnace atmosphere for effective dephosphorisation and generation of good quality steel. A further object of the present invention is directed to steel making involving a new coolant system which would reduce the oxygen consumption as compared to the conventional processes , and can advantageously also be effective even in absence of bottom purging. A further object of the present invention is directed to steel making involving a new coolant system which would favour reducing the average bath phosphorous content further from the level of about 0.015%, as presently usually achieved in existing practice. 3 Yet another objective of the invention is directed to improving the dephosphorisation of steel and in the process reduce the consumption of and dependence on exclusive use of iron ore in steel melting operation in BOF furnace. According to yet another objective of the invention the same is directed to favour the steel manufacture by providing for favourable reduction in the number of re-blows in steel manufacture by involving a new coolant system and in the process restrict the phosphorous content to low values in the resulting steel composition. A further objective of the present invention is to increase the converter yield by selectively utilizing the characteristics of the invented process. SUMMARY OF THE INVENTION: Thus according to the basic aspect of the present invention there is provided a process for slag dephosphorisation in LD converter comprising: selectively carrying out the step of oxidizing slag for its dephosphorisation using a coolant system comprising iron ore pellets thus conditioning /shaping -up of the BOF slag with minimum flux consumption and accelerated dephosphorisation. The above process of the invention thus provides for advantageous use of a selective coolant system including iron-ore pellets instead of simply the conventionally used iron-ore coolant whereby the dephosphorisation efficiency of the slag could be increased. Importantly, it is found by way of the present invention that the selective use of iron-ore pellets as a coolant favours in conditioning or shaping -up of the BOF slag. In particular, considering that the CaO and MgO percentage in pellets are higher because of the lime and dolomite addition during the pelletization process , such selective use of iron-ore pellets favour faster slag formation with increased FeO in the converter on such pellet addition as a coolant. In accordance with a preferred aspect of the present invention there is provided a process for slag dephosphorisation in LD converter comprising: 4 selectively carrying out the step of oxidizing slag for its dephosphorisation using a coolant system comprising a combination of iron ore and iron ore pellets which would avoid any bottom purging thus favour simple and easy conditioning /shaping -up of the BOF slag with minimum flux consumption and accelerated dephosphorisation. Importantly, as clearly apparent from the above process of the invention, the process is favoured to achieve the dephosphorisation without the need for any bottom purging which is a clear advantage and benefit simple and cost-effective high quality steel manufacture with low phosphorous content. Importantly, the process as above which do not require any bottom purging is specifically advantageous when converters are being operated at higher life (>7000 heats) where the availability of the bottom purging decreases due to build-up in the bottom. Preferably, in the coolant system the use of the iron-ore pellets is selectively made such to favour required foaming of the slag for dephosphorisation.For such purpose preferably the said addition of the iron-ore pellets is made at the later part of the blow of oxygen after initial addition of the iron-ore. Also, the oxygen flow rate is selectively optimized based on the addition of the iron-ore and the iron-ore pellets such as to increase the dephopsphorisation efficiency of the slag towards the end of the blow. The above process of the invention involving the afore discussed iron-ore pellets as a coolant thus favours reducing the phosphorous content wherein average bath phosphorous is reduced less than 0.015% preferably to about 0.011%.Also, advantageously following the above process of the invention, the number of re-blows required due to the presence of phosphourous is reduced to less than about 20% preferably to about 10%. In accordance with yet further preferred aspect of the invention the coolant system includes the said iron-ore pellets comprising about l/4th of the coolants added rest being iron-ore. Advantageously, the iron -ore pellets used in the above process of the invention comprises iron ore along with CaO and MgO preferably CaO percentage in the pellets is in the range of 1.5 to 2.0 and MgO percentage in the pellets in the range of 0.3 to 0.6 Thus such percentage of the CaO and MgO in pellets are higher because of the lime and dolomite addition during pelletization and advantageously favour the dephosphorisation process. 5 The present invention is thus directed to achieving dephosphorization efficiency in steel making in BOF converter by way of the aforediscussed effective addition of iron-ore pellets as coolant as a replacement of iron ore to reduce the phosphorous content below 0.015% in the resulting steel at the end of the process. Preferably, such addition of palletized iron-ore towards the end of the top lance blowing at selectively controlled rate in the range of 0.5 to 2 t/min in the BOF furnace, and after the usual iron-ore addition can be followed. The quantity of pellets is selectively substituted for the iron-ore addition up to a maximum limit of 3-4 tons in each heat/bath for desired optimum benefits of dephosphorization. Following the above process the average bath phosphorous percentage can be brought down to a level of 0.011% in the resulting steel at the end of the process. The use of pellets as above is found to minimize the flux consumption and accelerates dephosphorization. Also, the number of re-blows, required for dephosphorization specially for the heats that ends up with higher phosphorous content than the specified limits after the normal lance blowing can be effectively reduced from existing level of 20% for operation with only iron-ore, to a much lower level of 10% when pellets (iron ore pelletized with lime and dolomite) are used in combination with iron-ores in partial replacement of it. Importantly, following the above process also the converter yield has increased by 1% with increased FeO in converter due to pellet addition. According to a further aspect of the present invention the advantageous use of iron-ore pellets in the process eliminates the problems of wetting of iron-ores in rainy seasons. Further the use of the pellets avoid the cementation of iron-ore with fine limes in the bunker and thereby also favour discharge from the storage bunker easier. The details of the invention, its objects and advantages are explained hereunder in greater detail in relation to the following non-limiting exemplary illustrations: 6 BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES: Figure 1 is the illustrative graphical presentation of an existing addition pattern of charge and iron-ore in the converter in terms of the oxygen-lancing/blowing rates on a time scale. Figure 2 illustrates graphically the new addition pattern according to the process of the invention involving the iron ore pellets in addition to the usual charge and iron ore in terms of the oxygen-lancing/blowing at effectively controlled rates on a time scale. THE DETAILED DESCRIPTION OF THE INVENTION IN RELATION TO THE ACCOMPANYING FIGURES: As is discussed hereinbefore usually, the production of steel with extremely low phosphorous content is always a challenge to the steel industry. The process conditions sensitively decide the quantum of phosphorous pick up by slag from the liquid metal. The principal conditions essential for dephosphorisation include quick formation of ferruginous-limy slag, formation of an oxidizing furnace atmosphere and oxidizing slag with a high activity of iron oxides (FeO) in it. Usually, these favorable conditions for removal of phosphorous occur at the end of the blowing when the carbon content decreases and the oxidation degree of the bath increases respectively, and the slag thus formed is rich in iron oxides. Conventionally such conditions are achieved by the use of iron ore only as coolant. Reference is first invited to accompanying figure 1 which illustrates such an existing method of iron-ore addition with the molten metal charge at about an intermediate stage of blowing as apparent from the graphical representation on a time-scale. Importantly, as clearly represented in the figure, the oxygen flow rate since the initiation of the blowing was controlled to favour the dephosphorisation requirements. However, as further discussed hereinbefore according to such existing art of controlling the composition in the basic oxygen converters that are operated at higher life, say exceeding 7000 heats, in between successive relining with basic refractory on the inside wall of converter, the availability of the bottom purging is minimized due to build-up in the bottom. This lead to lower specific volume of the converter and hence reduced chances of foamy slag formation favorable for dephosphorisation. As a result of this constraint, phosphorous removal following such conventional process was extremely difficult and consequentially the heats finished with the output of steel having higher phosphorous than the specified limits of a particular grade of 7 steel. Such situation could be brought under control with a short re-blow thus prolonging the blowing time and increasing the time in between successive tapings. As a result of this short re-blows the metallic yield of each heat decreased and the overall productivity also got affected. Reference is now invited to accompanying figure 2 that illustrates the present invention directed to avoid the afore discussed limitations of the conventional dephophorisation involving only the iron ore as the coolant. As apparent from the figure 2, in accordance with the present invention, the process for slag dephosphorisation in LD converter involves selectively carrying out the step of oxidizing slag for its dephosphorisation using a coolant system comprising a selective combination of iron ore and iron ore pellets which would not require any bottom purging thus favour simple and easy conditioning /shaping-up of the BOF slag with minimum flux consumption and accelerated dephosphorisation. The above process for dephosphorisation of steel in BOF converter of the invention thus includes pellet addition in combination with and after the addition of iron-ore, as an effective atleast partial replacement of the iron ore.The accompanying figure 2 thus shows the sequential addition of the iron ore followed by the addition of the iron-ore pellets as per the present process. Importantly, as clearly apparent from the figure , the oxygen flow rate under the present process was also selectively controlled to favour the effective dephosphorisation of the slag in the steel making process. Preferably, towards the later part of lancing and after the iron-ore addition, the pellets are charged in the converter. The said addition of the iron-ore pellets constituting 20 to 25% of the coolant system is made at the later part of the blow of oxygen after initial addition of the iron-ore constituting 25 to 30 % of the coolant system. During the entire period of Oxygen blowing, the flow rate is selectively controlled to achieve the optimum result in controlling the phosphorous percentage in resulting steel and maintaining a favorable slag character that interact in a best way with the liquid metal at the interface, even in absence of bottom purging/stirring with inert gases. The flow rate of Oxygen is preferably controlled at stages depending upon the additions and the coolant system charging preferably as detailed hereunder : 8 According to an aspect of the present invention the oxygen flow rate is controlled based on the sequential charging of the additions and coolants in selective amounts of iron- ore and the iron- ore pellets preferably following the level of oxygen flow in the range of 350 to 400 Nm3/min for a period of 0 to 4 mins during incorporation of conventional additions including 380 to 420 l\lm3/min for a period of 5 to 8 mins during the initial iron-ore coolant addition and 400 to 440 Nm3/min for a period of 9 to 11 mins during the final pellet coolant addition. The selective control on flow rate of oxygen lancing in combination with pellet addition, enriched with Cao and MgO during pelletization ( by addition of lime and dolomite), result in faster slag formation, reduce flux consumption, increased availability of FeO in converter due to pellet addition and accelerates dephosphorisation. The new process thus makes advantageous use of selective combination of oxygen flow control and optimum level of pellet addition replacing iron-ore up to a maximum of 3-4 tons for best results and an optimum level of about 0.011% phosphorous in resulting steel was achieved. Importantly, as clearly apparent from the above process of the invention, the process is favoured to achieve the dephosphorisation without the need for any bottom purging which is a clear advantage and benefit simple and cost-effective high quality steel manufacture with low phosphorous content. Importantly, the process as above which do not require any bottom purging is specifically advantageous when converters are being operated at higher life (>7000 heats) where the availability of the bottom purging decreases due to build-up in the bottom. 9 WE CLAIM: 1. A process for slag dephosphorisation in LD converter comprising: selectively carrying out the step of oxidizing slag for its dephosphorisation using a coolant system comprising iron ore pellets thus conditioning /shaping -up of the BOF slag with minimum flux consumption and accelerated dephosphorisation. 2. A process for slag dephosphorisation in LD converter comprising: selectively carrying out the step of oxidizing slag for its dephosphorisation using a coolant system comprising a combination of iron ore and iron ore pellets which would avoid any bottom purging thus favour simple and easy conditioning /shaping -up of the BOF slag with minimum flux consumption and accelerated dephosphorisation. 3. A process for slag dephosphorisation in LD converter as claimed in anyone of claims 1 or 2 wherein in the coolant system the use of the iron-ore pellets is made such to favour required foaming of the slag for dephosphorisation. 4. A process for slag dephosphorisation in LD converter as claimed in anyone of claims 1 to 3 wherein said addition of the iron-ore pellets constituting 20 to 25% of the coolant system is made at the later part of the blow of oxygen after initial addition of the iron-ore constituting 25 to 30 % of the coolant system. 5. A process for slag dephosphorisation in LD converter as claimed in anyone of claims 1 to 4 wherein the oxygen flow rate is selectively optimized based on the addition of the iron-ore and the iron-ore pellets such as to increase the dephopsphorisation efficiency of the slag towards the end of the blow. 6. A process for slag dephosphorisation in LD converter as claimed in anyone of claims 1 to 5 wherein average bath phosphorous is reduced to less than 0.015% and preferably about 0.011%. 7. A process for slag dephosphorisation in LD converter as claimed in anyone of claims 1 to 6 wherein the oxygen flow rate is controlled based on the sequential charging of 10 the additions and coolants in selective amounts of iron- ore and the iron- ore pellets preferably following the level of oxygen flow in the range of 350 to 400 Nm3/min for a period of 0 to 4 mins during incorporation of conventional additions including 380 to 420 Nm3/min for a period of 5 to 8 mins during the initial iron-ore coolant addition and 400 to 440 Nm3/min for a period of 9 to 11 mins during the final pellet coolant addition. 8. A process for slag dephosphorisation in LD converter as claimed in anyone of claims 1 to 7 wherein the iron-ore pellets comprises about l/4th of the coolants added rest being iron-ore. 9. A process for slag dephosphorisation in LD converter as claimed in anyone of claims 1 to 8 wherein the iron-ore pellets comprises iron ore alongwith CaO and MgO preferably CaO percentage in the pellets in the range of 1.5 to 2.0 % and MgO percentage in the pellets in the range of 0.3 to 0.6 %. 10. A process for slag dephosphorisation in LD converter substantially as hereindescribed and illustrated with refernce to the accompanying figure 2. Anjan Sen Anjan Sen & Associates Applicant's Agent Dated : 31st August. 2006 ABSTRACT A PROCESS FOR DEPHOSPHORIZATION OF STEEL IN LD CONVERTER BY PELLET ADDITION A process for improved dephosphorization of steel in BOF/LD Converter by pellet coolant directed to achieve a lower average bath phosphorous content below the existing level of 0.015% and preferably around 0.011%. Importantly the process involves use a combined iron ore and pellet addition pattern wherein after the addition of iron-ore, selectively up to 3-4 tons of pellets maximum for optimum benefits, is used. The improved dephosphorization efficiency of the oxidizing slag results from addition of iron-ore pelletized with lime and dolomite, that causes effective slag-metal interface reaction with a selective control of flow rate of oxygen-lancing .The invention achieves a lower limit of phosphorous up to around 0.011% in resulting steel, even without bottom purging and higher yield by about 1%, due to increased FeO in converter on pellet addition.

Documents:

1412-MUM-2006-ABSTRACT(31-7-2013).pdf

1412-mum-2006-abstract.doc

1412-MUM-2006-CLAIMS(AMENDED)-(31-7-2013).pdf

1412-MUM-2006-CLAIMS(AMENDED)-(4-10-2011).pdf

1412-MUM-2006-CLAIMS(MARKED COPY)-(31-7-2013).pdf

1412-mum-2006-claims.doc

1412-MUM-2006-CORRESPONDENCE(1-3-2012).pdf

1412-mum-2006-correspondence(10-3-2008).pdf

1412-MUM-2006-CORRESPONDENCE(13-4-2012).pdf

1412-MUM-2006-CORRESPONDENCE(18-7-2013).pdf

1412-MUM-2006-CORRESPONDENCE(22-3-2012).pdf

1412-MUM-2006-CORRESPONDENCE(22-7-2013).pdf

1412-MUM-2006-CORRESPONDENCE(3-6-2013).pdf

1412-mum-2006-form 1(12-12-2006).pdf

1412-mum-2006-form 18(10-3-2008).pdf

1412-MUM-2006-FORM 2(TITLE PAGE)-(31-7-2013).pdf

1412-mum-2006-form 2(title page)-(4-9-2006).pdf

1412-mum-2006-form-2.doc

1412-mum-2006-general power of attorney(4-9-2006).pdf

1412-MUM-2006-REPLY TO EXAMINATION REPORT(4-10-2011).pdf

1412-MUM-2006-REPLY TO HEARING(31-7-2013).pdf

1412-MUM-2006-SPECIFICATION(AMENDED)-(31-7-2013).pdf

1412-mum-abstract.pdf

1412-mum-claims.pdf

1412-mum-correspondance-received..pdf

1412-mum-description (complete).pdf

1412-mum-drawings.pdf

1412-mum-form-1.pdf

1412-mum-form-2.pdf

1412-mum-form-26.pdf

1412-mum-form-3.pdf

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