Claims:WE CLAIM:

1. A method of designing Ladle Top Slag for Secondary Refining comprising the steps of:
• Assessment of carry over slag chemistry based on hot metal quality

• Calculating the slag mass balance keeping in view the grade mix

• Change in De-oxidation and existing flux practice; and

• Adjustment of MgO and Alumina level by addition of Calcined dolomite along with calcined bauxite in ladle during tapping to maintain the fluidity of ladle top slag with increased MgO thereby reducing slag zone erosion.

2. A method as claimed in claim 1, wherein the Slag Viscosity reduced from 1.27 to 1.03 Pa. s at 16000C and melting point reduced from 15130C to 15070C.

3. A method as claimed in claim 1, wherein the flux practice for vacuum degassed heats was redesigned by changing the practice of addition of calcined dolomite in BOF during blowing to maintain the saturation limit of MgO in ladle top slag in VAD heats.

4. A method as claimed in claim 1, wherein the slag is less corrosive, fluid and saturated for specific de-oxidation practice.

Dated: this 05th day of March, 2016.

(N. K. Gupta) Patent Agent, Of NICHE,
For SAIL

To,
The Controller of Patents, The Patent Office, Kolkata.
, Description:FORM 2

THE PATENTS ACT, 1970

(39 of 1970) COMPLETE SPECIFICATION (Section 10 and rule 13)

TITLE

METHOD OF DESIGNING LADLE TOP SLAG FOR SECONDARY REFINING

APPLICANT

STEEL AUTHORITY OF INDIA LIMITED, A GOVT. OF INDIA ENTERPRISE,
RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, DORANDA, RANCHI-834002, STATE OF JHARKHAND

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

METHOD OF DESIGNING LADLE TOP SLAG FOR SECONDARY REFINING

FIELD OF INVENTION

The present invention relates to the designing of ladle top slag in secondary refining of steel. More specifically, the present invention relates to the optimization of slag composition, depending on hot metal quality, type of de- oxidation practice, ladle lining pattern, porous plug configuration, targeted steel chemistry and process route to make the slag soft and fluid in nature for smooth operation of secondary refining units.

BACKGROUND OF INVENTION

The steel production utilizes blast furnace hot metal and a scrap charge in a Basic Oxygen Furnace ("BOF") or scrap melting in an electric arc furnace to produce slabs / blooms / billets / beam blank / rounds / ingots of cast steel for reheating and rolling into manufacturing stock. Increasingly demanding applications have led to the development of more stringent physical and chemical specifications for the final steel products. The ladle refining stations (ladle furnace, Vacuum arc degassing and refining etc.) are additional steel refining step that has become a widely used tool to ensure consistent conformance to the rigid steelmaking requirements set by customers to achieve desired end quality requirements. This additional refining step lowers the level of elements such as sulphur and phosphorous and decreases the content of non- metallic inclusions such as alumina and various sulphide and oxide species. Ladle top slag chemistry is function of many process parameters and decides the techno-economic indices and quality of steel. Poorly designed ladle top slag leads to high treatment time, low ladle life, high level of inclusions, higher cost of production and lower productivity.

Ladle refining is probably the most important part of steel making since it is where the liquid steel from primary vessel is changed into a scalable high quality steel product. Ladle refining implies the following;

• Deoxidation and alloying of the steel

• Temperature and composition homogenization

• Desulfurization or re-sulphurisation of the steel

• Improved steel cleanliness; inclusion flotation and sulfide and oxide shape control
• Designing of soft and fluid slag which serves above mentioned purposes

OBJECT OF THE INVENTION

The goal of ladle refining is to deliver a ladle of homogeneous liquid steel to the caster (or ingot station) on time, at the right temperature and right quality, and meeting total chemical and metallurgical specifications.

The cost of developing this invention and the well known seemingly economic disadvantages of the basic lined ladle have also materially contributed, along with the several operational factors already outlined. In the ladle refining of steel, a surface slag covering is required to provide specific chemical and physical functions. The slag composition is designed for the different grades of steel being produced with the majority requiring desulfurization. Regardless of the chemical refining requirements of the slag, it is advantageous for the ladle slag to become as fluid as possible immediately upon tapping the metal from the furnace into the ladle.

In order to improve delivery times of the desired quality of steel to the steel caster refining slag must rapidly achieve fluidity upon tapping the steel from the melting furnace into the refining transfer ladle to expedite chemical processing of molten steel in the ladle. Rapid and efficient chemical refining of steel requires large quantities of lime in solution within a top slag layer to provide the high basicity needed for maximum sulfur capacity to promote sulfur transfer between the steel and the slag. Basicity has been traditionally defined as the slag % CaO/% SiO2 ratio (the "V-ratio") and it is well known that a highly basic fluid slag is beneficial in the removal of sulfur from a ladle of molten steel. With respect to the V-ratio, a number less than 1 is acidic and a number more than 1 is basic. The reaction at the slag/metal interface between calcium oxide in the slag and

dissolved sulfur in molten steel produces calcium sulfide that remains stable within the top slag layer as long as a reducing chemistry is maintained. By increasing the refining slag fluidity, the effective slag interfacial area available to contact the molten steel surface is likewise increased.

SUMMARY OF THE INVENTION

Therefore such as herein described there is disclosed a method of designing Ladle Top Slag for Secondary Refining comprising the steps of: calculating the slag mass balance keeping in view the grade mix, de-oxidation practice and existing flux practice; and adding Calcined dolomite in ladle during tapping along with calcined bauxite to maintain the fluidity of ladle top slag with increased MgO thereby reducing slag zone erosion.

The flux practice for vacuum degassed heats was redesigned by changing the practice of addition of calcined dolomite in BOF during blowing to maintain the saturation limit of MgO in ladle top slag in VAD heats as per CaO-SiO2-MgO ternary diagram.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

Fig. 1A: illustrates a view of ladle bottom build up as prior art;

Fig. 1B: illustrates the ring formation in top and bottom portion of ladle as prior art;
Fig. 2: illustrates the view of ladle bottom after slag dumping (after innovation) in accordance with the present invention.

DETAILED DESCRIPTION

Ladle top slag plays a very important role in steel making process. Ladle top slag also plays a very important role in refining of liquid steel in vacuum refining and ladle furnaces. Designing of grade specific slag chemistry is very important to achieve soft fluid slag with desired slag basicity for smooth functioning, low noise level during arcing, effective de-sulphurisation, inclusion absorption and

avoidance of ladle bottom build up & slag roof formation in ladles. Poor designing / improper slag chemistry leads to either viscous slag or watery slag. Viscous slag in secondary refining units leads to many operational issues like overflow tendency during deep degassing for removal of gases (H2& N2) in vacuum reefing units, high noise level during arcing in ladle furnace, poor slag - metal interaction hence poor desulphurisation, slag ring formation, ladle bottom build up etc. Contrary to that, a watery slag rich in silica (SiO2) is very corrosive in nature and increases the erosion rate of ladle lining specially in slag zone area, does not leave a coating on ladle lining during teeming / casting operation and results in low ladle lining life. Designing a ladle top slag chemistry depends on type of ladle lining, type of de-oxidation practice (Si-killed, Al-killed, Si-Al killed), targeted steel grade (low S or Sulphur bearing) and end quality requirement by customer.

The study of existing flux and de-oxidation practice for steel production at DSP, Durgapur and ISP Burnpur revealed that optimization of slag composition, depending on process route (BOF-LF-CC, BOF-VAD-BP, BOF- LF-VAD-CC) is necessary to make the slag soft and fluid in nature for smooth operation of secondary refining units.

INDUSTRIAL TRIAL:

Based on theoretical calculation, slag analysis and knowledge base, flux addition practice was modified to achieve soft, saturated and fluid ladle top slag to meet the end quality requirements. Plant trial carried out mainly for two different process routes i.e. BOF-VAD-BP/CC & BOF-LF-CC.

BOF-VAD-BP/CC:

One of the chronic problems in vacuum degassed heats through VAD was slag overflow during deep degassing leading to loss of time, temperature, high flux consumption and in worst case breakdown in VAD. The analysis of existing flux practice revealed very high level of MgO in ladle top slag of VAD heats (Table-
1). Very high level of MgO in ladle top slag beyond saturation limit keeps MgO in solid form in slag and was making slag viscous in nature and problem during deep degassing.

Table-I: Analysis of ladle top slag having overflow tendency during deep degassing

The flux practice for vacuum degassed heats is redesigned by changing the practice of addition of calcined dolomite in BOF during blowing to maintain the saturation limit of MgO in ladle top slag in VAD heats as per CaO-SiO2-MgO ternary diagram. The detail ladle top slag analysis for modified flux practice has been given in Table-II.

Table-II: Analysis of modified ladle top slag analysis

BOF-LF-CC:

Heats route through ladle furnace route requires special type of ladle top slag to reduce slag zone erosion, formation of slag zone ring in top & bottom and ladle bottom build up of ladle. The basic requirement for ladle top slag is same i.e. it should be less corrosive, fluid and saturated for specific de-oxidation practice. ISP, Burnpur was facing a chronic problem of ladle bottom build up (Fig. 1A) and ring formation in top & bottom area (Fig. 1B). Keeping in view the grade mix, de- oxidation practice and existing flux practice, the slag mass balance was calculated. The output MgO of mass balance based on addition of fluxes (Table- III) was not matching with actual slag analysis at LF out stage (Table-IV) clearly revealed that MgO is coming from ladle lining refractory to attain the MgO saturation limit of ladle top slag.

Keeping in view the grade mix, de-oxidation practice and existing flux practice, the slag mass balance was calculated. The output MgO of mass balance based on addition of fluxes (Table-III) was not matching with actual slag analysis at LF out stage (Table-IV) clearly revealed that MgO is coming from ladle lining refractory to attain the MgO saturation limit of ladle top slag.

Table-III: Slag mass balance for ISP, Burnpur

Table-IV: Ladle top slag chemistry at LF before innovation

Change in flux practice by addition of Calcined dolomite in ladle during tapping along with calcined bauxite to maintain the fluidity of ladle top slag with increased MgO helped to reduce slag zone erosion. The ladle condition after cast over was also monitored and found that slag dumping improved and ladle bottom was comparatively clean (Fig. 2).

Regular use of calcined dolomite coupled with calcined bauxite helped to maintain a fluid, non-corrosive top slag eliminating the problem of ladle bottom build up, thick slag roof formation and damage to slag zone lining; and lead to gradual improvement in ladle lining life (Table-V & VI).

Table-V: Ladle top slag analysis after change in flux practice

Table-VI: Comparison of melting point and viscosity of ladle top slags

Results:

? Problem of overflow tendency in Vacuum Degassing eliminated.

? Slag fluidity improved in all the trial heats at ISP, Burnpur. Slag Viscosity reduced from 1.27 to 1.03 Pa. s at 16000C and Melting Point reduced from 15130C to 15070C.
? Problem of ladle bottom build up and slag roof formation in ladle has

been drastically reduced

? Ladle lining life improved by ~10 heats.

Although the foregoing description of the present invention has been shown and described with reference to particular embodiments and applications thereof, it has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the particular embodiments and applications disclosed. It will be apparent to those having ordinary skill in the art that a number of changes, modifications, variations, or alterations to the invention as described herein may be made, none of which depart from the spirit or scope of the present invention. The particular embodiments and applications were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such changes, modifications, variations, and alterations should therefore be seen as being within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.