Claims:We Claim:

1. Converter cum electric arc (CONARC) furnace adapted for enhanced number of heats workability with 100% hot metal free of complexities of roof jam, elbow jam and top lance delta jam comprising:
lower shell of furnace with refractory lining of reduced thickness replacing combination of magnesia carbon brick lining and ramming mass thickness with only ramming mass to accommodate more workability with 100% hot metal;

gantry means for electrode support;

height adjustable lancing facility ;

slag door ; and

eccentric bottom tap (EBT) hole.

2. Converter cum electric arc (CONARC) furnace as claimed in claim1 wherein said ramming mass based refractory lining of bottom shell comprise of thickness between 670 mm to 675 mm preferably about 672 mm.
3. Converter cum electric arc (CONARC) furnace as claimed in anyone of claims 1 or 2 wherein said height adjustable lancing facility comprise means to adjust lance height to maintain slag layer just below slag door.
4. Converter cum electric arc (CONARC) furnace as claimed in claim 3 wherein said means to adjust lance height comprises maintaining lance height of about 1.2 m from the bath level during blowing start to generate sufficient volume of slag and subsequently maintaining lance height of around 1 m to maintain slag layer just below slag door.
5. Converter cum electric arc (CONARC) furnace as claimed in anyone of claims 1 to4 wherein said ramming mass comprises (by weight percent):

MgO: 75 to 80; preferably about 78 wt.%,
CaO: 17 to 19; preferably about 18 wt.%,
Al2O3: 0.2 to 0.4; preferably about 0.3 wt.%,
SiO2: 0.4 to 0.8; preferably about 0.6 to 1 wt.%,
Fe2O3: 0.6 to 1; preferably about 0.8 wt.%,
Maximum service temperature of ramming mass is 17000C, and
Bulk density is 2.35 gm/cc.
6. Converter cum electric arc (CONARC) furnace as claimed in anyone of claims 1 to 5 which comprises of 100 % hot metal heat working with liquid steel quantity of 190 to 200 tonne suitable for single pouring facility.
7. Converter cum electric arc (CONARC) furnace as claimed in anyone of claims 1 to 6 wherein said CONARC furnace is twin shell furnace equipped with a gantry which comprises one set of 3 phase electrode and dedicated water cooled top lance for oxygen blowing for each furnace shell.
8. Converter cum electric arc (CONARC) furnace as claimed in anyone of claims 1 to 7 wherein said twin shell CONARC furnace having Eccentric Bottom Tapping (EBT) and side lancing facility.
9. A method for carrying out primary steel making with enhanced number of heats workability with 100% hot metal free of complexities of roof jam, elbow jam and top lance delta jam in Converter cum electric arc (CONARC) furnace as claimed in anyone of claims 1 to 8 comprising:
involving only ramming mass based reduced thickness bottom shell based furnace to accommodate more working volume with 100% hot metal;
carrying out controlled blowing sequence for generating sufficient slag; and
adjusting the lance during processing for desired enhanced number of heats workability with 100% hot metal free of complexities of roof jam, elbow jam and top lance delta jam.

10. A method as claimed in claim 9 comprising
checking for refractory erosion in pillar/cojet/elbow area, water leakage in elbow, WC panel, hot heel quantity and taking corrective measures based thereon on;
cleaning and filling of EBT and gunning at pillar and slag door;
adding 1 tonne DRI, 1tonne dolomite and required amount of lime (according to percentage Si in hot metal) in furnace;
adding/pouring 165 to 175tonne preferably 170 tonne of hot metal from ladle inside furnace through Launder;
carrying out selective oxygen blowing and addition pattern of flux and DRI as coolant so as to process 100% hot metal heat with reduced flux addition and reduced cycle time;
adding ferroalloys, flux and aluminium bar depending on required steel chemistry after blowing and transferring primary processed steel to secondary refining ladle.

11. A method as claimed in anyone of claims9 or 10 wherein said pouring of hot metal is carried out with Furnace angle at 0 degree and said hot metal ladle maintained 2 feet above the launder during pouring.

12. A method as claimed in anyone of claims 9 to 11 wherein said selective blowing and addition pattern comprising

starting oxygen bowing after hot metal pouring through lance disposed at greater than 1meter height from hot metal bath level with flow rate 250 Nm3/minute;
removing slag by keeping the furnace at -2 to -3 degree at a point after around 3500 Nm3 oxygen blowing and when main decarburization starts;
carrying out lime addition along with the oxygen blowing at the feed rate of 357 Kg/minute till 3500 Nm3 of total oxygen blowing, after which feed rate of lime addition is reduced to 150 Kg/minute till 8250 Nm3 of total oxygen blowing and then again feed rate of lime addition is increased to 460 Kg/minute till 9250 Nm3 of total oxygen blowing and stopping lime addition after that and draining out last slag;
carrying out DRI addition at the feed rate of 1800 Kg/minute after 1000 Nm3 of oxygen blowing which is continued upto 3500 Nm3 of oxygen blowing and reducing thereafter DRI feed rate to 1340 Kg/minute till 9250 Nm3 of total oxygen blowing.
13. A method as claimed in anyone of claims 9 to 12 wherein blow end carbon is maintained between 0.05 to 0.08 wt%.
14. A method as claimed in anyone of claims 9 to 13 wherein, bath basicity is maintained between 1.5 to 2.5 during the whole process.
15. A method as claimed in anyone of claims 9 to 14 wherein a first temperature checking is done at 5000 Nm3 of total oxygen blowing and a second checking of temperature is done just after the final DRI addition.
16. A method as claimed in anyone of claims 9 to 15 wherein bath sampling is done at 3500 Nm3 (after first slag drain out) and 9500 Nm3 (last slag drain out) of total oxygen blowing and dissolved oxygen content of liquid steel bath is measured at 9500 Nm3 of total oxygen blowing.
17. A method as claimed in anyone of claims 9 to 16 wherein said hot metal for processing having composition comprising
Chemical composition of hot metal, wt%
C Si Mn S P Ti Ni Cr Fe
4.1-4.7 0.61-1.20 0.23-0.69 0.04-0.08 0.10-0.15 0.01-0.04 0.015-0.023 0.002-0.007 Rest of the amount

18. A method as claimed in anyone of claims 9 to 17 wherein said DRI for addition as coolant having composition comprising
Chemical composition of DRI, wt%
C S Total Fe Metallic Fe % Metallization
1.8-2.2 0.005-0.006 83-85 77-79 91-93

19. A method as claimed in anyone of claims 9 to 18 wherein soft blowing is maintained during the starting of the blowing operation to generate sufficient volume of slag and after that, operators adjust the lance height to maintain the slag layer just below the slag door. Lance height of around 1.2 m from bath level is maintained during soft blowing. Soft blowing refers to less penetration of oxygen jet inside the bath and wider impact area with the top bath surface to enhance the chemical reaction of oxygen with iron to produce sufficient volume of slag.

20. A method as claimed in anyone of claims 9 to 19 enabling processing all the heats as 100% hot metal heat during the lifetime of a CONARC furnace, including the lead heat, with reduced heat cycle time of 64 to 70 minutes, preferably 66 minutes.
Dated this the 3rd day of March, 2020
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199

, Description:FIELD OF THE INVENTION:
Present invention relates to Converter cum electric arc (CONARC) furnace adapted for enhanced number of heats workability with 100% hot metal and an improved primary steel making practice to process 100% hot metal heat in such CONARC furnace.The total heat size of a 100% hot metal heat comprisesaround 190tonne amount of liquid steel quantity tapped from CONARC furnace to ladle refining furnace from a charge of hot metalalong with DRI as chiller.In this invention, killing of hot heel is done by DRI,eliminating need of addition of coke and associated logistic involvement to kill hot heel.Present invention is adapted to tackling the problems of fewer number of heats with 100% hot metal by proper lance adjustment during processing, thereby providing opportunity to process more number of 100% hot metal heats, during the lifetime of a CONARC furnace.In this present invention, the lower shell of the CONARC furnace is modified wherebytotal thickness of bottom lining of the furnace has been decreased further via replacing carbon-magnesia brick by dry hearth ramming mass. In this present invention, slag basicity value is maintained in the range of 1.5 – 2.5 as the standard procedure throughout the process.This process modification has decreased overall flux consumption per heat.The capacity of Gas Cleaning Plant (GCP) has been increased to allow increasing the blowing rate from 200 Nm3/min to 250 Nm3/min. Further, new operating procedure involving selective addition and blowing pattern has been introduced now to process 100% hot metal heat. These multiple number of modifications lead to more number of 100% hot metal heats with reduced heat cycle time from 83 minute to 66 minute.

BACKGROUND OF THE INVENTION:
Steel is basically an alloy of iron (predominately), carbon and other elements like silicon (Si), manganese (Mn), phosphorus (P), sulphur (S) etc. It is produced from iron ore mainly by a two stage process. In the first stage, iron is produced by reducing iron ore briquette (sinter, pellet etc.). In the second stage (steel making stage), product iron is refined where mainly carbon content and other impurity elements like Si, Mn, P, S etc. are lowered by oxidation process. At the same time, different alloying elements are also added to liquid steel in this stage depending on the application of steel. Nowaday, the demand for increasing the productivity of steel plant has been increased significantly to reduce the overall cost of production.However, at the same time, the requirement for producing steel having stringent quality is an essential condition now. The necessity to accomplish these two important aspects simultaneously, steel makers divided the steel making process into two stages: (a) Primary steel making and (b) Secondary steel making. In case of primary steel making process, basically carbon rich iron is converted to steel. Secondary steel making process includes any post primary steel making process prior to casting. Secondary steel making process consists of variety of different processes through which final composition of steel is adjusted.

Primary and secondary steel making at JSW Steel Limited Dolvi Works are done in Converter Cum Electric Arc Furnace (CONARC) and Ladle Furnace (LF) respectively.

The objective of developing CONARC Furnace based steel making process was to utilize the profits of both the converter steel making (BOF) and electric arc furnace (EAF) steel making. The price and availability of different type of raw materials required for steel making changes frequently. In this regard, CONARC process is an ideal choice. This process is highly flexible to operate with different percentage of hot metal quantity.

JSW Steel Limited Dolvi Works is well-known for a number of “first in the country” initiatives. This is India's first steel plant started to use CONARC technology. The Dolvi plant is the first in India to accept the challenge for adopting combination of CONARC technology for steel making and compact strip production (CSP) for producing hot rolled coils (thin slab casting). In CONARC process Dolvi has created benchmark by successfully using 100% hot metal in CONARC furnace route and hold one patent (Indian patent no 204622) on “A Process of making steel with 60-100% hot metal in Twin Shell Electric Arc Furnace”.

In case of 100% hot metal heat in a CONARC furnace, it does not mean that the total heat size accommodates only liquid hot metal. Direct Reduced Iron (DRI) is also required as chiller material inside the furnace. The total heat size of a 100% hot metal heat comprises: 80-85% charge material(hot metal) and 15-20% chiller materials (DRI or combination of DRI and scrap).

In case of earlier steel making process with 100% hot metal in CONARC furnace, plant operators used to kill hot heel (left over steel quantity inside the CONARC furnace after tapping of liquid steel from CONARC furnace to ladle furnace) with injection of coke through consumable door lance or side lance using compressed air as carrier gas. This type of killing process requires 400-500 Kg coke per heat and it increases logistic involvement. Direct Reduced Iron (DRI) was generally used only as coolant material to handle the excess amount of heat generated during oxygen blowing.

Earlier, in case of steel making process with 100% hot metal in a CONARC furnace, operators used to pour the total amount of hot metal (170-180 tonne) from hot metal ladle to furnace by double pouring process (1st pouring amount: 120-140 tonne, 2nd pouring amount: rest amount of hot metal). The capacity of old hot metal ladle was 150 tonne. Single pouring process was possible only when the quantity of hot metal is less than 150 tonne. In case of double pouring process, first oxygen blowing is done just after the first hot metal pouring step. Then, after a certain amount of time, oxygen blowing is stopped and second hot metal pouring is done. The reaction between hot metal carbon and lance oxygen (2C + O2 = 2CO? (gas)) gets discontinued after end of first oxygen blowing step which creates low pressure region inside the furnace. Therefore, air comes through the slag door and fill up the furnace before second hot metal pouring. After that, when blowing starts, nitrogen from air can dissolve into the metal bath. It is not desirable to get extra nitrogen into the metal bath.

The lower shell of the CONARC furnace was modified once to accommodate increased amount of hot metal quantity.

In case of earlier steel making process with 100% hot metal in a CONARC furnace, it was very difficult to process more than 10%number of heats (life of a newly lined CONARC furnace: 500 heats)in a CONARC furnace due to roof jam, elbow jam and top lance delta jam. In case of earlier steel making process with 100% hot metal in a CONARC furnace, operators used to maintain slag basicity in the range of 2.5 – 3.

In general, steel making process in CONARC furnace consists of several sequential processes. At first, furnace is prepared properly before taking hot metal charge. This step includes tap hole filling, repairing of furnace etc. After that, hot heel killing inside the furnace is carried out by addition of aluminium/ferro alloys/coke. Then, hot metal charging is done through launder or from top of the furnace. Sometimes, single charging of hot metal is not enough to fill up the furnace volume. In that case, double charging of hot metal is carried out. In this situation, little bit of oxygen blowing (25-28 Nm3 of O2/tonne of hot metal) is done between the two charging steps. After charging of hot metal inside the furnace, oxygen blowing is done through top lance to reduce carbon, manganese, silicon and phosphorous content. This step is called converter process. In this stage, excessive heat generated due to the blowing operation can damage the upper shell steel panel and refractory of the furnace. Therefore, coolants (steel scrap or DRI) are added to control and utilize this excessive heat. After the converter process, generated slag inside the furnace is removed through the slag door. The next step is called arcing process. In this stage, balance amount of DRI or scrap is fed inside the furnace to achieve the desired tapping weight of the heat. Electric energy is used for melting of the solid charge and for superheating of the molten bath to tapping temperature. In this stage, the hot metal is called liquid steel and total amount of liquid steel inside the furnace is termed as “heat size”. Then, checking of liquid steel chemistry and temperature are done to ensure that the liquid steel bath has achieved the desired tapping condition. After that, the heat is deslagged and liquid steel is tapped into a teeming ladle.

JSW Steel Ltd. Dolvi Works globally known for it’s technological excellence, located at west coast of India, is one of the India’s leading integrated steel plant having 5 MTPA capacity currently. This steel plant is planning to increase the capacity to 10 MTPA by the end of 2020.In the earlier days, mainly DRI and scrap materials were treated to produce liquid steel. In the year of 2000, this plant saw the erection and commissioning of blast furnace.That time, this plant started to use hot metal for steel making. JSW Steel Limited Dolvi Works has two twin shell CONARC furnaces in their steel melting shop. Each pair of furnace is equipped with a gantry which comprises one set of 3 phase electrode and dedicated water cooled top lance for oxygen blowing for each furnace shell.This twin shell CONARC furnace have Eccentric Bottom Tapping (EBT) and side lancing facility also.Average heat size is around 190tonne (amount of liquid steel quantity tapped from CONARC furnace to ladle refining furnace). Considering around 30 tonne hot heel, the overall capacity of each furnace is 230 tonne of liquid steel. Considering the benefits regarding the flexibility of raw materials (solid feed and hot metal) and flexibility with respect to energy sources (electricity, coal, fuel and gas), CONARC is an ideal choice. The possibility of continuous slag off from CONARC furnace through the slag door allows faster dephosphorization and faster desiliconization. Presence of eccentric bottom tap (EBT) hole in this CONARC furnace permits almost slag free tapping from CONARC furnace to ladle furnace.

However, besides these advantages, JSW Steel Limited Dolvi Works used to face multiple number of disadvantages during primary steel making process in CONARC furnace. In the earlier days of primary steel making process in CONARC furnace, there was a limitation in terms of hot metal usage in the furnace. Hot metal can be used up to maximum 60% only (130-135 tonne). Furnace volume was the limitation as the depth of the furnace was shallow. Higher oxygen blowing rate to reduce blowing time was not possible due to the shallow depth. Increasing oxygen flow rate can increase churning below lance area leading to high erosion of bottom refractory which is significant in case of CONARC furnace because of lower bath height. Also, there was always possibility of loss of liquid metal through slag door during deslagging.Shallow depth of the CONARC furnace increases the possibility of roof and elbow jam because of material splashing during higher oxygen blowing rate. In addition to that, higher oxygen blowing rate was not possible due to the less capacity of GCP also (200 Nm3/minute). As in the earlier days, CONARC furnace was unable to take 100% hot metal as charge material, and only 60% of hot metal could be accommodated, balance 40% charge wasaccommodated by DRI and steel scrap material. Therefore, electrical energy was required to melt the solid charge materials (DRI and steel scrap). Electrical energy is a costly energy source, thereby increasing liquid steel production cost. Further, electrode gantry breakdown used to create production delays that time. Additionally, use of electrical energy involves higher maintenance cost of electrode gantry and water cooled cables. In the earlier days, operators used to kill hot heel using costlier materials like aluminium or ferro-aluminium or ferro-silicon or ferro-manganese, which were fed from top of the furnace.

There has been thus a need to avoid the limitation and disadvantages in existing CONARC process and to develop an improved CONARC Furnace process for primary steel making with 100% hot metal heats in higher number throughout the furnace service life, consuming less flux, less power input and reduced cycle time of operation.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to Converter cum electric arc (CONARC) furnace adapted for enhanced number of heats workability with 100% hot metal free of complexities of roof jam, elbow jam and top lance delta jam and an improved primary steel making practice to process 100% hot metal heat in suchCONARC furnace.

A further object of the present invention is directed to an improved primary steel making process with 100% hot metal heat in a CONARC furnace route wherein furnace bottom is modified to hold higher volume of hot metal for processing.

A still further object of the present invention is directed to an improved primary steel making process with 100% hot metal heat in a CONARC furnace route wherein double pouring practice has been replaced by single pouring practicewith new hot metal ladle capacity of 180 tonne, wherebypossibility of air coming through the slag door and nitrogen from air being dissolve into the metal bath can be avoided.

A still further object of the present invention is directed to an improved primary steel making process with 100% hot metal heat in a CONARC furnace route whereinslag basicity value is maintained in the range of 1.5 – 2.5 as the standard procedure throughout the process so thatoverall flux consumption per heat is decreased.

A still further object of the present invention is directed to an improved primary steel making process with 100% hot metal heat in a CONARC furnace route wherein increasing the capacity of Gas Cleaning Plant(GCP) is increased to allow increased oxygen blowing rate.

A still further object of the present invention is directed to an improved primary steel making process with 100% hot metal heat in a CONARC furnace route wherein various modifications implemented in operating procedure for processing 100% hot metal heat leading to reduce the heat cycle time from 83 minute to 66 minute.

A still further object of the present invention is directed to an improved primary steel making process with 100% hot metal heat in a CONARC furnace route wherein killing of hot heel is done by addition of DRIso that killing process eliminates the conventional practice of addition of coke and associated logistic involvement to kill hot heel.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to Converter cum electric arc (CONARC) furnace adapted for enhanced number of heats workability with 100% hot metal free of complexities of roof jam, elbow jam and top lance delta jam comprising:
lower shell of furnace with refractory lining of reduced thickness replacing combination of magnesia carbon brick lining and ramming mass thickness with only ramming mass to accommodate more workability with 100% hot metal;

gantry means for electrode support;

height adjustable lancing facility;

slag door; and

eccentric bottom tap (EBT) hole.

A further aspect of the present invention is directed to said Converter cum electric arc (CONARC) furnace wherein said ramming mass based refractory lining of bottom shell comprise of thickness between 670 mm to 675 mm preferably about 672 mm.

A still further aspect of the present invention is directed to said Converter cum electric arc (CONARC) furnace wherein said height adjustable lancing facility comprise means to adjust lance height to maintain slag layer just below slag door.

A still further aspect of the present invention is directed to said Converter cum electric arc (CONARC) furnace wherein said means to adjust lance height comprises maintaining lance height of about 1.2 m from the bath level during blowing start to generate sufficient volume of slag and subsequently maintaining lance height of around 1 m to maintain slag layer just below slag door.

A still further aspect of the present invention is directed to said Converter cum electric arc (CONARC) furnace wherein said ramming mass comprises (in weight percent):

MgO: 75 to 80; preferably about 78 wt.%,
CaO: 17 to 19; preferably about 18 wt.%,
Al2O3: 0.2 to 0.4; preferably about 0.3 wt.%,
SiO2: 0.4 to 0.8; preferably about 0.6 to 1 wt.%,
Fe2O3: 0.6 to 1; preferably about 0.8 wt.%,
Maximum service temperature of ramming mass is 17000C, and
Bulk density is 2.35 gm/cc.

A still further aspect of the present invention is directed to said Converter cum electric arc (CONARC) furnace which comprises of 100 % heat working with liquid steel quantity of 190 to 200 tonne suitable for single pouring facility.

Another aspect of the present invention is directed to said Converter cum electric arc (CONARC) furnace wherein said CONARC furnace is twin shell furnace equipped with a gantry which comprises one set of 3 phase electrode and dedicated water cooled top lance for oxygen blowing for each furnace shell.

Yet another aspect of the present invention is directed to said Converter cum electric arc (CONARC) furnace wherein said twin shell CONARC furnace having Eccentric Bottom Tapping (EBT) and side lancing facility.

A further aspect of the present invention is directed to a method for carrying out primary steel making with enhanced number of heats workability with 100% hot metal free of complexities of roof jam, elbow jam and top lance delta jam in Converter cum electric arc (CONARC) furnace as described above comprising:
involving only ramming mass based reduced thickness bottom shell based furnace to accommodate more working volume with 100% hot metal;carrying out controlled blowing sequence for generating sufficient slag; and adjusting the lance during processing for desired enhanced number of heats workability with 100% hot metal free of complexities of roof jam, elbow jam and top lance delta jam.

A still further aspect of the present invention is directed to said method comprising
checking for refractory erosion in pillar/cojet/elbow area, water leakage in elbow, WC panel, hot heel quantity and taking corrective measures based thereon on;
cleaning and filling of EBT and gunning at pillar and slag door;adding 1 tonne DRI, 1tonne dolomite and required amount of lime (according to percentage Si in hot metal) in furnace;adding/pouring 165 to 175 tonne preferably 170 tonne of hot metal from ladle inside furnace through Launder;carrying out selective oxygen blowing and addition pattern of flux and DRI as coolant so as to process 100% hot metal heat with reduced flux addition and reduced cycle time; adding ferroalloys, flux and aluminium bar depending on required steel chemistry after blowing and transferring primary processed steel to secondary refining ladle.

A still further aspect of the present invention is directed to said method wherein said pouring of hot metal is carried out with Furnace angle at 0 degree and said hot metal ladle maintained 2 feet above the launder during pouring.

A still further aspect of the present invention is directed to said method wherein said selective blowing and addition pattern comprising
starting oxygen bowing after hot metal pouring through lance disposed at greater than 1meter height from hot metal bath level with flow rate 250 Nm3/minute;
removing slag by keeping the furnace at -2 to -3 degree at a point after around 3500 Nm3 oxygen blowing and when main decarburization starts;
carrying out lime addition along with the oxygen blowing at the feed rate of 357 Kg/minute till 3500 Nm3 of total oxygen blowing, after which feed rate of lime addition is reduced to 150 Kg/minute till 8250 Nm3 of total oxygen blowing and then again feed rate of lime addition is increased to 460 Kg/minute till 9250 Nm3 of total oxygen blowing and stopping lime addition after that and draining out last slag;
carrying out DRI addition at the feed rate of 1800 Kg/minute after 1000 Nm3 of oxygen blowing which is continued upto 3500 Nm3 of oxygen blowing and reducing thereafter DRI feed rate to 1340 Kg/minute till 9250 Nm3 of total oxygen blowing.

A still further aspect of the present invention is directed to said method wherein blow end carbon is maintained between 0.05 to 0.08 wt%.

Another aspect of the present invention is directed to said method wherein, bath basicity is maintained between 1.5 to 2.5 during the whole process.

Yet another aspect of the present invention is directed to said method wherein a first temperature checking is done at 5000 Nm3 of total oxygen blowing and a second checking of temperature is done just after the final DRI addition.

A further aspect of the present invention is directed to said method wherein bath sampling is done at 3500 Nm3 (after first slag drain out) and 9500 Nm3 (last slag drain out) of total oxygen blowing and dissolved oxygen content of liquid steel bath is measured at 9500 Nm3 of total oxygen blowing.

A still further aspect of the present invention is directed to said method wherein said hot metal for processing having composition comprising
Chemical composition of hot metal, wt%
C Si Mn S P Ti Ni Cr Fe
4.1-4.7 0.61-1.20 0.23-0.69 0.04-0.08 0.10-0.15 0.01-0.04 0.015-0.023 0.002-0.007 Rest of the amount

A still further aspect of the present invention is directed to said method wherein said DRI for addition as coolant having composition comprising
Chemical composition of DRI, wt%
C S Total Fe Metallic Fe % Metallization
1.8-2.2 0.005-0.006 83-85 77-79 91-93

A still further aspect of the present invention is directed to said method wherein soft blowing is maintained during the starting of the blowing operation to generate sufficient volume of slag and after that, operators adjust the lance height to maintain the slag layer just below the slag door.Lance height of around 1.2 m from bath level is maintained during soft blowing. Soft blowing refers to less penetration of oxygen jet inside the bath and wider impact area with the top bath surface to enhance the chemical reaction of oxygen with iron to produce sufficient volume of slag.

A still further aspect of the present invention is directed to said method enabling processing all the heats as 100% hot metal heat during the lifetime of a CONARC furnace, including the lead heat, with reduced heat cycle time of 64 to 70 min preferably 66min.

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

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1: Shows the Current process flow diagram at JSW Steel Limited Dolvi Works.
Fig. 2:Shows earlier condition of bottom shell refractory lining of CONARC furnace.
Fig.3: Shows Current condition of bottom shell refractory lining of CONARC furnace according to present invention.
Fig.4: Shows the old hot metal ladle.
Fig. 5: Shows the modified new hot metal ladle used in the process according to according to present invention.
Fig. 6: shows the schematic sectional front view and plan view of the CONARC furnace according to present invention along with modified bottom shell refractory lining, top lance position, electrode gantry system and electrode delta.
Fig. 7: Shows the schematic process flow diagram for old 100% hot metal heat processing.
Fig. 8: Shows the schematic process flow diagram for current 100% hot metal heat processing according to present invention.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPNAYING DRAWINGS

Present invention relates to Converter cum electric arc (CONARC) furnace adapted for enhanced number of heats workability with 100% hot metal and an improved primary steel making practice to process 100% hot metal heat in such CONARC furnace. Present invention enable CONARC Furnace process for primary steel making with 100% hot metal heats involving single pouring in higher number of cycles throughout the furnace service life, consuming less flux, less power input and reduced cycle time of operation free of complexities of roof jam, elbow jam and top lance delta jam.

As already stated to overcome the deficiencies of the existing CONARC process with 100% hot metal, JSW Steel Limited Dolvi Woks did some modifications in processing steps of CONARC furnace and it’s refractory layer thickness. Based on this modifications, the present applicant filed a patent (Indian patent no 204622) on the topic named “A Process of making steel with 60-100% hot metal in Twin Shell Electric Arc Furnace”. JSW Steel Limited Dolvi Works started to make liquid steel using 60% to 100% hot metal according to this patent. In case of 100% hot metal heat, operator used to add DRI only as a chiller material into liquid hot metal. In case of 100% hot metal heat, there is no requirement of electrical energy, thereby reducing liquid hot metal processing cost and time. Conferring to this patent, in case of steel making process with 100% hot metal in a CONARC furnace, operators started to kill hot heel with injection of coke through consumable door lance or side lance using compressed air as carrier gas instead of using aluminium or ferro-aluminium or ferro-silicon or ferro-manganese from the top of the furnace. This was started to avoid costly aluminium and other ferro alloys. However, processing of each heat requires around 400-500 Kg coke for hot heel killing and this process increases logistics involvement also as it needs separate bunker for coke. All these factors increase the cost of the final product.

Accompanying Fig. 1shows the schematic view of current process flow diagram at JSW Steel Limited, Dolvi Works.

The composition of hot metal and DRI used in steel making are as follows:

Table 1. Chemistry of hot metal at JSW Steel Limited Dolvi Works
Chemical composition of hot metal, wt%
C Si Mn S P Ti Ni Cr Fe
4.1-4.7 0.61-1.20 0.23-0.69 0.04-0.08 0.10-0.15 0.01-0.04 0.015-0.023 0.002-0.007 Rest of the amount

Table 2. Chemistry of DRIat JSW Steel Limited Dolvi Works
Chemical composition of DRI, wt%
C S Total Fe Metallic Fe % Metallization
1.8-2.2 0.005-0.006 83-85 77-79 91-93

In case of present invention, killing of hot heel is done by DRI. This way of killing process eliminates the need of addition of coke and associated logistic involvement to kill hot heel. DRI contains around 1.8-2.2% carbon. Further, DRI is already required as chiller material and charge material during primary steel making in CONARC furnace. Therefore, present invention can avoid the requirement of coke and associated logistics involvement. In the earlier days of primary steel making, processing of 100% hot metal heat was not possible in CONARC furnace due to the shallow depth of the furnace. In case of the above stated patent, the bottom shell of the CONARC furnace was modified (modified magnesia-carbon brick thickness: 300 mm and modified ramming mass thickness: 449 mm) to accommodate increased amount of hot metal quantity and retain slag inside the furnace. Accompanying Fig 2 shows the earlier condition of bottom shell refractory lining of CONARC furnace.

However, in that case also, it was not possible to accommodate 100% hot metal from the first heat (or lead heat) in a particular CONARC furnace. Only after processing of fifty number of heats in a particular CONARC furnace, processing of 100% hot metal heat was possible due to the increased volume of the furnace. Processing of fifty number of heats essentially erodes the inside refractory layer of the CONARC furnace, thereby increasing it’s internal available volume. In case of the present invention, the refractory layer thickness of the bottom shell of the CONARC furnace has been modified(current magnesia-carbon brick thickness: 0 mm and current ramming mass thickness: 672 mm) further. Therefore, in case of the present invention, processing of 100% hot metal heat (170 tonne hot metal) in CONARC furnace is possible from the lead heat also.Accompanying Fig. 3 shows the further modified current condition of bottom shell refractory lining of CONARC furnace for processing 100% hot metal heat.

In case of earlier steel making process with 100% hot metal in a CONARC furnace, it was very difficult to process more than 10% number of heats (life of a newly lined CONARC furnace: 500 heats) in a CONARC furnace due to roof jam, elbow jam and top lance delta jam. Earlier, it was very difficult to maintain slag inside the furnace (below the slag door). Generated slag used to drain out of the furnace from the slag door due to the lower working volume of CONARC furnace. Further, operators used to perform hard blowing practice during that time. Hard blowing practice generates less volume of iron oxide (FeO), thereby decreasing the total slag volume. Slag layer above liquid metal acts as a shield and reduce metal splashing during blowing operation. Therefore, in case of earlier practice, slag layer above the liquid metal layer was unable to provide proper shielding to reduce metal splashing. Henceforth, roof jam, elbow jam and top lance delta jam were very common. In case of the present invention, the refractory layer thickness of bottom shell of the CONARC furnace has been modified to increase the working volume of the CONARC furnace. Subsequently, new blowing pattern has also been introduced. Currently, soft blowing is maintained during the starting of the blowing operation to generate sufficient volume of slag. After that, operators used to adjust the lance height to maintain the slag layer just below the slag door. In this way, present invention is tackling these problems by proper lance adjustment during processing, thereby providing opportunity to process more number of 100% hot metal heats. It is now possible to process all the heats as 100% hot metal heat during the lifetime of a CONARC furnace. In case of earlier steel making process with 100% hot metal in a CONARC furnace, operators used to pour the total amount of hot metal (170-180 tonne) from hot metal ladle to furnace by double pouring process (1st pouring amount: 120-140 tonne, 2nd pouring amount: rest amount of hot metal). The capacity of old hot metal ladle was 150 tonne. Accompanying Figure 4 shows the structural configuration details of old hot metal ladle. In case of double pouring process, first oxygen blowing is done just after the first hot metal pouring step. Then, after a certain amount of time, oxygen blowing is stopped and second hot metal pouring is done. Initial oxygen blowing produces FeO rich slag above the liquid hot metal layer. Then, during second hot metal pouring, carbon of poured hot metal reacted with FeO vigorously. Thus, boiling occurs which is aunsafe practice. Also,the reaction between hot metal carbon and lance oxygen (2C + O2 = 2CO? (gas)) gets discontinued after end of first oxygen blowing step which creates low pressure region inside the furnace.Therefore, air comes through the slag door and fill up the furnace before second hot metal pouring. After that, when second oxygen blowing starts, nitrogen from air can dissolve into the liquid metal bath. It is not desirable to get extra nitrogen into the metal bath.

In this present invention, the double pouring practice has been replaced by single pouring practice. The height of the old hot metal ladle has been increased by 250 mm. The capacity of new hot metal ladle is 180 tonne. Accompanying Figure 5 shows the view of new modified hot metal ladle used for single pouring. The benefits of introducing 180 tonne of hot metal ladle includes shifting from double pouring to single pouring operation, saving in electrical energy, improved crane logistics and improved man power utilization.

Accompanying Fig. 6 shows the schematic sectional front view and plan view of the CONARC furnace according to present invention wherein all its essential components part like modified bottom shell refractory lining, eccentric bottom tap hole, slag door, top lance position, electrode gantry system and electrode delta have been illustrated.

In case of earlier steel making process with 100% hot metal in a CONARC furnace, operators used to maintain slag basicity in the range of 2.5 – 3. However, in this present invention, maintaining basicity value in the range of 1.5 – 2.5 has been made the standard procedure throughout the process. In case of earlier 100% hot metal heat processing, total flux consumption was around 14900Kg. Accompanying Figure 7 illustrates the process flow diagram for old 100% hot metal heat processing.

In this present invention, total flux consumption is 11600 Kg. Considering average heat size of around 190 tonne, flux consumption hasbeen decreased from 78.42 Kg/per tonne of liquid steel to 61.05 Kg/per tonne of liquid steel. Therefore, present invention has decreased overall flux consumption per heat.Comparison of flux consumption during primary steel making process in CONARC process is shown in following Table 3.

Table 3:

Flux type Flux consumption during old 100% hot metal heat processing, Kg Flux consumption during current 100% hot metal heat processing, Kg
Lime 10900 10600
Dolomite 4000 1000

Total flux consumption 14900 11600

In case of present invention, the capacity of Gas Cleaning Plant (GCP) has been increased. This modification in GCP allows to increase the blowing rate from 200 Nm3/min. to 250 Nm3/min. All these modifications lead to reduce the heat cycle time for primary steel making process from 83 minute to 66 minute.Comparison of cycle time for primary steel making process in CONARC furnace is shown in following Table 4.

Table 4:

Process steps Time required for different process steps during old 100% hot metal heat processing Time required for different process steps during current 100% hot metal heat processing
Set up time including 1st hot metal pouring 16 16
1st oxygen blowing 34 40
2nd hot meta pouring 4 0
2nd oxygen blowing 19 0
Oxygen blowing end to tap start 6 6
Tapping 4 4

Total time consumption 83 66

Considering these above stated multiple number of modifications, new operating procedure has been introduced to process 100% hot metal heat. Accompanying Figure 8 illustrate the process flow diagram for current 100% hot metal heat processing according to present invention.

The process steps are as follows:

1. Furnace inspection check points: In this stage, checking is done for the following points- refractory erosion in pillar/cojet/elbow area, water leakage in elbow, WC panel, hot heel quantity etc. and based on these points corrective actions are taken.

2. Set up operation: In this stage, cleaning and filling of EBT and gunning at pillar and slag door are done. After that, 1 tonne DRI, 1tonne dolomite and required amount of lime (according to percentage Si in hot metal) are added.

3. Hot metal pouring: Around 170 tonne of hot metal is added inside furnace through Launder. Furnace angle should be 0 degree during pouring. Hot metal ladle should be maintained 2 feet above the launder.

4. Blowing operation: Oxygen bowing should be started at greater than 1meter height from hot metal bath level with flow rate 250 Nm3/minute. After around 3500 Nm3 oxygen blowing, main decarburization starts. At this point,slag should be removed by keeping the furnace at -2 to -3 degree. Blow end carbon should be maintained between 0.05 to 0.08 wt%. Lime addition should be done along with the oxygen blowing at the feed rate of 357 Kg/minute till 3500 Nm3 of total oxygen blowing. After that, feed rate of lime addition should be reduced to 150 Kg/minute till 8250 Nm3 of total oxygen blowing. Then again feed rate of lime addition should be increased to 460 Kg/minute till 9250 Nm3 of total oxygen blowing. After that, stop lime addition. During this whole process, basicity should be maintained between 1.5 to 2.5. DRI addition should be done at the feed rate of 1800 Kg/minute after 1000 Nm3 of oxygen blowing. It should be continued upto 3500 Nm3 of oxygen blowing. After that, DRI feed rate should be reduced to 1340 Kg/minute till 9250 Nm3 of total oxygen blowing. First temperature checking should be done at 5000 Nm3 of total oxygen blowing. Again, checking of temperature should done just after the final DRI addition. Bath sampling should be done at 3500 Nm3 (after first slag drain out) and 9500 Nm3 (last slag drain out) of total oxygen blowing. Dissolved oxygen content of liquid steel bath should be measured at 9500 Nm3 of total oxygen blowing. Soft blowing is maintained during the starting of the blowing operation to generate sufficient volume of slag and after that, operators adjust the lance height to maintain the slag layer just below the slag door. Lance height of around 1.2 m from bath level is maintained during soft blowing. Soft blowing refers to less penetration of oxygen jet inside the bath and wider impact area with the top bath surface to enhance the chemical reaction of oxygen with iron to produce sufficient volume of slag.

5. Tapping process: In this stage, required amount of ferro alloys, flux and aluminium bar should be added to the liquid bath depending on the required steel chemistry. After that, liquid steel should be transferred to secondary refining ladle.

It is thus possible by way of the present invention to carry out primary steel making process in CONARC furnace, specially configured to allow processing of 100% hot metal heat by single pouring along with modified blowing and addition pattern. In this way, present invention is tackling the problems of roof jam, elbow jam and top lance delta jam, by proper lance adjustment during processing, thereby providing opportunity to process more number of 100% hot metal heats. It is now possible to process all the heats as 100% hot metal heat during the lifetime of a CONARC furnace. The present CONARC furnace process of primary steel making allow increased oxygen blowing rate and leads to reduce the heat cycle time from 83 minute(as in double pouring process followed earlier) to 66 minute.