Title of Invention

A PROCESS FOR CONTINUOUS CASTING OF ALUMINIUM KILLED STEEL THROUGH BILLET CASTER

Abstract

The invention relates to a process for continuous casting of aluminium killed steel without causing nozzle clogging in 100 mm X 100 mm billet caster. More particularly the present invention relates to a process of continuous casting of billet with significantly high level of sulfur vis-a-vis aluminium content in steel. The invention is further related to a process of identifying the narrow window of calcium treatment practice in order to avoid probable precipitation of calcium sulphide at comparatively high sulphur level vis- a-vis aluminium content in steel, aluminium rich calcium aluminate or pure alumina - the causes for nozzle clogging in billet caster.The invention further relates to a process of formation of liquid calcium aluminate at steel making temperature by identifying suitable calcium treatment practice in aluminium killed steel at comparatively high sulphur level to avoid nozzle clogging in billet caster.The invention further relates to adoption of suitable slag engineering practice under the shop logistics in order to effectively use calcium and aluminium and produce cleaner steel without nozzle dogging during billet casting.

Full Text

A PROCESS FOR CONTINUOUS CASTING OF ALUMINIUM KILLED STEEL THROUGH BILLET CASTER FIELD OF THE INVENTION The invention relates to a process for continuous casting of aluminium killed steel without causing nozzle clogging in 1(K) mm X100 mm billet caster. More particularly the present invention relates to a process of continuous casting of billet with significantly high level of sulfur vis-a-vis aluminium content in steel. The invention is further related to a process of identifying the narrow window of calcium treatment practice in order to avoid probable precipitation of calcium sulphide at comparatively high sulphur level vis- a-vis aluminium content in steel, aluninium rich calcium aluminate or pure alumina - the causes for nozzle clogging in billet csster. The invention further relates to a process of formation of liquid calcium aluminate at steel making temperature by identifying suitable calcium treatment practice in aluminium killed steel at comparatively high sulphur level to avoid nozzle clogging in billet caster. The invention further relates to adoption of suitable slag engineering practice under the shop logistics in order to effectively use calcium and aluminium and produce cleaner steel without nozzle clogging during billet casting. BACKGROUND OF THE INVENTION Continuous casting in small size billet section (100 mm X 100 mm) as existing in DSP (Durgapur) requires extremely low content of aluminium ( alumina build up in metering nozzle and subsequent nozzle clogging/choking. Such steels killed by weak deoxidisers (Si-Mn) are characterized by product defects like pin holes, blow holes, high inclusions ard total oxygen. Resulting slag has low basicity and high (FeO+MnO) leading to high dissolved oxygen in liquid steel, billet macro with pin holes/blow holes and high total oxygen and inclusion in finished products. In order to ensure better quality of the product and to cater to the demand of wider market it is imperative to use strong deoxidiser (aluminium) but at the same time to ensure smooth castability, as aluninium content more than 0.007% lead to nozzle choking in open billet caster (i.e. without submerged entry nozzle). Calcium injection, which is practiced to bind calcium with alumina arising out of aluminium deoxidation forming low melting calcium aluminate to avoid nozzle choking, may result in calcium sulphide (CaS) formation at relatively high sulphur level (requirement of S is less than 0.015% for Al level of > 0.01%) which is again a source of nozzle choking. Further, stringency of S level increases with increase in Al content Sulphur level at DSP is normally >0.03%. Hence, criticality of Al killing ir volves designing of process chart for steel making and casting so that generation of nozzle clogging compound is minimal and whatever is generated gets removed from liquid steel to avoid abortion of casting due to nozzle clogging. PRIOR ART AND DRAWBACKS Steel produced through BOF is tapped into ladle. During tapping silico manganese, ferro manganese along with ferro silicon is added through bunker/chute into the ladle to meet the specification of manganese and silicon. Part of these ferroalloys are consumed for deoxidation, i.e., removal of dissolved oxygen in the metal bath. Rests go for alloying. Carry over slag from the furnace along with the slag generated through deoxidation float over the metal bath. This slag being low in basicity (0.8 - 1.6) and high in FeO + MnO (17-30%) content result in high dissolved oxygen content in metal bath due to slag/metal equilibrium phenomenon. No slag engineering is carried out through suitable addition of fluxes resulting in his type of low basicity, high (FeO +MnO) and consequent high dissolved oxygen in liquid steel, leading to high total oxygen and inclusion in finished product. Further, wrthin the specification desired Mn/ Si ratio is not maintained, which leads to entrapment cf solid deoxidation product termed as inclusion In addition to above additives, other minor alloying is also made during tapping to meet the steel grade specifications. Then the ladle is transported to ladle treatment station (LTS), where argon/nitrogen is purged from bottom of the ladle for homogenization of bath and temperature adjustment and, if required trim additions are made for composition correction. At LTS, dissolved oxygen in bath varies from 40-75 ppm depending on carbon content mainly. After treatment in LTS, the ladle is taken to continuous casting shop and kept over a turret Casting starts with opening of the ladle nozzle. Metal stream passes to the six strand molds through acid board tundish fitted with six metering nozzles at the bottom. Tundish sample and temperature are taken time to time. Though arrangements for use of ladle to tundish shroud to protect the stream from reoxidation is there in the shop but normally not used. Formation of AI2O3 during Al-killing or reoxidation of Al to AI2O3 due to reaction of Al with Silica board, oxidizing slag (i.e. high FeO + MnO in slag) and surrounding oxygen around open stream during casting lead to nozzle choking in the metering nozzle. This is why Al-killed steel is not made in open billet caster. In view of the above, extensive, experimentation has been carried out at Durgapur Steel Plant, SAIL in an open billet caster to develop suitable steel making and casting practice for casting of aluminum-killed steel needed for high value products with stringent quality requirements.. OBJECTS OF THE INVENTION Thus, according to the present invention, there is provided a process for continuous casting of aluminium-killed steel in billet size 100 mm X 100 mm without nozzle choking. The process comprises of the following steps: (i) After tapping steel from Baric Oxygen Furnace into ladle Si-Mn deoxidation is carried out maintaining proper Mn/Si ratio and more than critical value for generation of liquid deoxidation product. (ii) This is followed by slag deoxidation through appropriate slag engineering. Slag engineering involves addition of fluxes like, lime, calcined bauxite/AI dross/calcium aluminate and Al cubes over slag in order to change the top slag composition to enhance deoxidation. (iii) In the ladle treatment station, calcium treatment of steel is earned out at existing S level of about 0.03% and with very low level of Al ( (iv) Further deoxidation of steel with Al-wire feeding to achieve up to 0.015% Al in steel, (v) Prevention of reoxidaticn and formation of Al2O3 by using ladle-tundish shroud and basic tundist lining during continuous casting of billets (100 mm X 100 mm size). The steps (i) and (ii) are camed out in ladle during tapping of steel from BOF while steps (iii) and (iv) take place n ladle al LTS and step (v) during billet casting in continuous casting machine shop, further, steps (iii), (iv) and (v) are essential steps while steps (i) and (ii) are preferable. According to further aspects of the present invention, there is provided a process for successfully continuous casting of steel with 0.015% Al in LTS and 0.0008-0.011% Al during casting and S level as high as 0.03%. According to another aspect of the present invention the deposits like CaS, Al2O3 , alumina rich calcium aluminate in nozzle, causes for choking during casting, are avoided. According to another aspect of the present invention improved product quality in respect of inclusion content, total oxygen and pinholes/blow holes is ensured. DETAILED DESCRIPTION OF THE INVENTION In steel making through BOF LTS-billet caster route weekly deoxidized steel is made due to nozzle choking problem n billet caster (100 X 100 mm) in highly deoxidized Al-killed steel. In the present invention, a process has been developed wherein solid deposits of AI2O3 in nozzle causing nozzle choking has been successfully avoided. Even the generation of CaS or Al2O3 rich cilcium aluminate responsible for nozzle choking in Ca treated Al-killed steel could be suppressed leading to smooth casting. The process may be effected in existing BOF-LTS-billet caster route. The various process steps of the inventions are discussed below in greater details. • Mn/Si ratio Within the specification of the grade Mn/Si ratio of the steel tapped from BOF had been so maintained that the deoxidation product (reaction product of Mn, Si and residual Al in ferroalloys with bath oxygen) is liquid spessartite. Being liquid at steel making temperature, it floats up to slag layer easily making the steel cleaner. • Slag Engineering Slag chemistry was so designed that the deoxidation product (MnO- AI2O3- SiO2 type) generated after Si-Mn deoxidation is converted to CaO- Al2O3-SiO2 type. This slag in equilibrum with steel bath has a capacity to reduce bath oxygen by 10-20 ppm. Under the constraints of shop logistics and temperature management problem this was effected by addition of lime (500- 800 kg/ht), calcined bauxita/AI drossffused calcium aluminate (150-200 kg/ht) and Al cube (75-100 kg/ht) over slag towards the end of tapping in ladle. Basicity (CaO/ SiO2) was rare than 2.0 and FeO + MnO varied between 8- 12%. • Ca-Si treatment followed by Al-wire injection at LTS In conventional practice of Ca treatment in Al-killed steel, Ca-Si/Ca-Fe is injected into the bath at a very low S level (preferably 0.015% Al) after Al deoxidation. This practice helps suppress formation of solid CaS, AI2O3 rich calcium alumnate or AI2O3 by forming liquid calcium aluminate as deoxidation product and hence, nozzle choking is averted in continuous casting. With increase in Al content, stringency in S increase in order to avoid nozzle choking. Again with increase in Ca injection beyond a certain proportion, CaS formation is enhanced. On the other hand, with low proportion of Ca injection AI2O3 rich calcium aluminate is formed. Further, in billet caster (100 X 100 mm) nozzle choking problem aggravates due to its metering nozzle. Since, shop facilities do not permit desulphurisation to achieve from existing level of about 0.03%, the present technology of Ca-treatment was invented to cast 0.0(18-0.015% Al containing steel successfully under existing condition of the plant. In the present invention CaSi (250-400m) was injected into the bath at relatively low Al level ( prevented formation of Al2O3 rich calcium aluminate and also CaS. This was followed by Al wire injection (60-150rn) for achieving Al at LTS 0.008-0.015%. With the increase in aim AI content amount of CaSi injection was increased. This process helped formation of liquid calcium aluminate and in turn led to smooth casting. Further, d ssolved oxygen at LTS could be brought down to 18-35 ppm compared to 43-75 ppm in conventional heats which resulted in improvement in quality of billets in respect to inclusions and total oxygen. • Control of reoxidation trough use of ladle to tundish shroud and basic tundish Any Al containing steel gets reoxidised through reaction with oxygen of atmosphere in open stream casting and silica in acid tundish. This reoxidised Al2O3 again causes nozzle coking. In order to reduce reoxidation phenomenon during casting, ladle to tundish metal stream was shrouded through fixing of refractory shrouds and basic board tundish was used thereby protecting the meal from reoxidation, particularly in high Al heats (about 0.012-0.015% Al at LTS). For heats with aim Al of about 0.01%, successful casting could be achieved even with acid board tundish and without shroud. DETAILED DESCRIPTION IN RELATION TO EXAMPLRY ILLUSTRATIONS Example-1 Heat no. 1-9839/01 have been cited as example no. 1 for showing details of one heat made through conventional practice at DSP Durgapur. Aluminium content in steel in this heat is only 0.004%. It may be noted that ratio of Mn and Si (i.e. Mn/Si) is 2.9 which is less than critical value as per literature. The critical value of Mn/Si ratio, above which formation of liquid inclusion is favored is dependent on percentage of Silicon content of steel. Further, neither slag engineering was carried out by addition of any suitable flux nor aluminium and calcium wire were injected. As a result slag basicity was only 1.62, (FeO + MnO) as high as 23.27% and dissolved oxygen at LTS was 68.6 ppm. In conventional practice, aluminium content in steel is kept as low as possible ( avoid abortion of casting due to nozzle choking. Such heats do not meet the quality requirements of Al-killed grades of steel. Example-2 One heat was performed as example-2 as per present invention for 0.01% content Al in steel during casting. Heat no. 1-9731/01 was aimed for 0.01% Al. Details of the heat making, castability and billet quality are given below and for comparison with conventional heat 1-9839/01 heat cited as example-2. Example-3 The heat performed as example-3 was aimed for 0.015% Al at LTS. Heat details, castability and product quality of heat no. 1-1817/82 have been given below as example- 3, which may be compared with conventional heat cited in Example-1. 11. Nozzle deposit analysis No trace of deposits like , Cas, rich Ca Aluminate responsible for nozzle choking. The effect of invented process (Slag Engineering + CaSi treatment + Al wire injection) over conventional process on various quality parameters is given below. For more than 0.01% aim Al, basic board and shroud were used. Heat No. 1-9731/01 and 1-1817/02 are cited as example 2 and 3 respectively for showing efficacy of the present invention in comparison to example-1. It is seen that maintaining of Mn/Si ratio above the critical value, implementation of slag engineering through addition of suitable fluxes, aluminium and calcium injection and use of shroud (ladle to tundish) during casting were practiced as per process design in example 2 and 3. As a result, heats with aluminium 0 31% (example-2) and 0.015% (example-3) at LTS stage were fully cast without nozzle choking. Basicity of slag was 2.01 and 2.64, (FeO + MnO) 12.01% and 9.4% in example-2 and 3 respectively. Dissolved oxygen at LTS was 29 ppm in example-2 and (FeO +MnO of 9.4% in slag in example-3 reflects lower value of dissolved oxygen even compared to example -2 (dissolved oxygen in this heat could not be measured). Figure 1 shows comparison of dissolved oxygen level at LTS. It may be mentioned that the product quality improves with lower dissolved oxygen level and the trial heats contain lower dissolved oxygen compared to conventional. Figure 2 shows the comparative frequency distribution of percentage inclusion volume fraction and figure 3 shows comparative frequency distribution of total oxygen in billets. It may be noted that in conventional heats 100% heats have percentage inclusion volume fraction more than 0.5 whereas in present invention 100% of heats have less than 0.5 and 50% of heats have less than 0.35. With the increase in Al and improved slag characteristics percentage inclusion volume fraction decrease. From figure 3 it is seen that total oxygen is more than 153 ppm in 67% heats and >100 ppm in 100% heats in conventional heats whereas in invented process 57% of heats have less than 100 ppm and 100% heats have less than 150 ppm. Figure 4 represents the flow chart of the entire process of present invention for successful casting of Al-killed steel. WE CLAIM: A process for continuous casting of aluminium killed steel in billet size of 100 mm X 100 mm or more /without nozzle choking, the process comprising the following steps: (i) tapping the steel from Basic Oxygen Furnace into ladle where Si-Mn deoxidation is caried out, the Mn/Si ratio is maintained at >3.0 for Si level of 0.2 to 0.25% by the addition of silico-managanese and ferro alloys like ferro manganese and ferro silicon so that part of the ferro alloys are consumed for deoxidation i.e., removal of dissolved oxygen in the metal bath while the rest is consumed in alloying generating liquid deoxidation product; (ii) the slag engineering step is carried out on the above product by the addition of lime, calcined bauxite/Aluminium, dross/fused calcium aluminate and aluminium cube over the slag towards the end of tapping in order o change the top slag composition to enhance deoxidation; (iii) the Ca-Si injection at LTS is carried out by injecting Calcium silicide wire into the ladle at LTS at relatively low Al level just after slag deoxidation; (iv) where after, the Aluminium injection was also carried out at LTS by Aluminium wire injection which resulted in liquid Calcium Aluminate which lead to smooth casting without nozzle clogging; (v) and the earlier step (iv) being carried out by shrouding ladle to tundish stream by refractory shrouding to prevent reoxidation of the steel through contact with oxygen of the atmosphere, while using a basic board tundish during casting. 2. A process for continuous casting of aluminium killed steel in billet size of 100 mm X 100 mm as claimed in claim 1 wherein, steps (i) -(ii) are carried out in ladle during tapping from EOF while steps (iii)- (iv) take place in ladle at LTS and step (v) during casting 3. A process for continuous casting of aluminium killed steel in billet size of 100 mm X 100 mm as claimed in claim 1 and 2 wherein, 'S' in steel is more than 0.015% for billet casting and wherein CaSi is injected prior to Aluminium injection. 4. A process for continuous casting of aluminium killed steel in billet size of 100 mm X 100 mm as claimed in claim 1 to 3 wherein, Aluminium is raised to 0.008-0.015% for billet casing of 100 mm X 100 mm size or more. 5. A process for continuous casting of aluminium killed steel in billet size of 100 mm X 100 mm as claimei in claim 1to 4 wherein, ladle to tundish shroud along with basic board is employed for casting of more than 0 01% Aluminium containing steel in billet caster. 6. A process for continuous casting of aluminium killed steel in billet size of 100 mm X 100 mm as claimed in claim 1 wherein, calcined bauxite/Aluminium, dross/fused calcium alumirate (1:3-1.8 kg/t) is added in ladle in BOF-LTS-CC route, 1kg aluminium cube shots per ton of steel is added in ladle over slag in BOF-LTS-CC route and 2-4 m CaSi per ton of steel is injected for billet casting of 0.008-0 015% Al containing steel. 7. A process for continuous casting of aluminium killed steel in billet size of 100 mm X 100 mm as claimed in claim 1 to 6 wherein, lime of 4-7 kg/t of steel is added in ladle in BOF-LTS-CC route. 8. A process for continuous casting of aluminium killed steel in billet size of 100 mm X 100 mm or more without nozzle choking. Dated this the 10th day of March. 2003 The invention relates to a process for continuous casting of aluminium killed steel without causing nozzle clogging in 100 mm X 100 mm billet caster. More particularly the present invention relates to a process of continuous casting of billet with significantly high level of sulfur vis-a-vis aluminium content in steel. The invention is further related to a process of identifying the narrow window of calcium treatment practice in order to avoid probable precipitation of calcium sulphide at comparatively high sulphur level vis- a-vis aluminium content in steel, aluminium rich calcium aluminate or pure alumina - the causes for nozzle clogging in billet caster.The invention further relates to a process of formation of liquid calcium aluminate at steel making temperature by identifying suitable calcium treatment practice in aluminium killed steel at comparatively high sulphur level to avoid nozzle clogging in billet caster.The invention further relates to adoption of suitable slag engineering practice under the shop logistics in order to effectively use calcium and aluminium and produce cleaner steel without nozzle dogging during billet casting.

Documents:

150-kol-2003-granted-abstract.pdf

150-kol-2003-granted-claims.pdf

150-kol-2003-granted-correspondence.pdf

150-kol-2003-granted-description (complete).pdf

150-kol-2003-granted-drawings.pdf

150-kol-2003-granted-examination report.pdf

150-kol-2003-granted-form 1.pdf

150-kol-2003-granted-form 13.pdf

150-kol-2003-granted-form 18.pdf

150-kol-2003-granted-form 2.pdf

150-kol-2003-granted-form 26.pdf

150-kol-2003-granted-form 3.pdf

150-kol-2003-granted-pa.pdf

150-kol-2003-granted-reply to examination report.pdf

150-kol-2003-granted-specification.pdf