Ladle Metallurgy and Practices at Lf for Uninterrupted Casting

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LADLE METALLURGY AND PRACTICES AT LF FOR UNINTERRUPTED CASTING By Shubham Indoria, Shashi Bhushan Prasad, Sachin Srivastva

Submitted in partial fulfilment of the requirements for the degree

B.tech.(hons.): metallurgy and material science engineering

Nation Institute of Technology, Jamshedpur Project Supervisor/Mentor BALAM SINGH SR. MANAGER, CCS (O)

June 2013

CERTIFICATE TO WHOM IT MAY CONCERN This is to certify that Shubham Indoria, Sachin Srivastva, Shashi Bhushan Prasad, VACATIONAL TRAINEES at SMS-2, B.S.P. Have completed their technical project successfully at SMS-II. Their performance was good and the project assigned to them was completed within the specified time period. They were given the project “LADLE METALLURGY AND PRACTICES AT LF FOR UNINTERRUPTED CASTING”. I wish them all the success and believe that this project training will stand them in good stead in their future.

Mr. BALAM SINGH Sr. Manager CCS (O) S.M.S -2

AKNOWLEDGEMENT The work contained within this report was performed at SAIL, BHILAI. This work carries with it the kind support, inspiration and guidance by various people at various levels, to whom I am grateful and sincerely indebted. I would like to express my sincere gratitude and appreciation to the following persons and/or institutions for their assistance and contributions in completion of this project: •Mr. Balam Singh, sr. mgr. Ladle furnace for his support, guidance and continued encouragement during the course of my studies. His enthusiastic and diligent approach to life, as well as his dedication and loyalty towards his students will always be appreciated. •Mr. Shobhit Sharad Gottlieb, for his continuous support and interest in the progress of our work. •Mr. Rajesh Devangan, sr. mgr. training at SMS-II, & Mr. Sudhir Kumar, AGM (secondary steel making) for their guidance, continued encouragement throughout this project. •Mr. P Subba Rao, AGM (Contracts), & Mr. C b Rao sir, for all the help and resources that were made available to me. •A. V. Fuley, VT co-ordinator BSP, for his constant support and guidance. •SAIL, BHILAI for allowing us to visit their laboratory facilities and instrumentation, and the helpful of the staff at times when I was in need of advice. • Our families and friends who always encouraged me during my studies.

Shashi Bhushan Indoria Srivastava

Shubham Sachin

Vacational TRAINEEs SMS-2 BHILAI STEEL PLANT; SAIL

Table of Contents LADLE METALLURGY AND PRACTICES AT LF FOR UNINTERRUPTED CASTING...........1 CERTIFICATE...................................................................................................... 2 AKNOWLEDGEMENT........................................................................................... 3 Table of Contents............................................................................................... 4 Table of Figures................................................................................................. 7 1.

INTRODUCTION TO BHILAI STEEL PLANT........................................................ 8 1.1. HISTORICAL BACKGROUND............................................................................................... 9 1.2. PRODUCTS OF BSP..................................................................................................10 1.3. ACHIEVEMENTS....................................................................................................... 11 1.4. PROCESS DESCRIPTION IN BHILAI STEEL PLANT..................................................................11 1.4.1 Plant facilities...................................................................................................... 12 1.5. PURPOSE OF VISITING BSP............................................................................................. 14 1.6. PURPOSE OF SELECTING THIS TOPIC.................................................................................14

2.

Introduction to Steel Melting Shop II and Its Units....................................... 15 2.1 INTRODUCTION TO SMS-II............................................................................................. 16 2.2 CONVERTOR SHOP....................................................................................................... 16 2.2.1 Process Flow in Convertor Shop...........................................................................18 2.2.2 Mixer Section....................................................................................................... 20 2.2.3 Scrap Yard........................................................................................................... 20 2.2.4 Ladle Preparation Section....................................................................................20 2.2.5 Slag Pot Handling Section....................................................................................20 2.2.6 Bulk Material Handling......................................................................................... 21 2.2.7 Gas Cleaning Section........................................................................................... 21 2.3 CONTINUOUS CASTING SHOP..........................................................................................22 2.3.1 Argon Rinsing Bay............................................................................................... 23 2.3.2 Secondary Steel Making Processes:.....................................................................23 2.3.2.1 2.3.2.2 2.3.2.3

VAD (Vacuum Arc Degassing Unit)............................................................................. 23 Ladle Furnace............................................................................................................. 26 RH Degasser.............................................................................................................. 28

2.3.3 Tundish preparation Bay......................................................................................29 2.3.4 Casting Bay......................................................................................................... 30 2.3.5 Slab & Bloom Storage Yard (SBS).........................................................................31 2.3.6 Mould Repair Shop............................................................................................... 31 2.3.7 Casting Powder Plant (SFMPP).............................................................................31 2.3.8 Brief Description Of Continuous Casting Shop.....................................................32 2.4 COORDINATION BETWEEN CONVERTOR AND CONTINUOUS CASTING MACHINES.........................33 2.4.1 Temperature........................................................................................................ 33 2.4.2 Chemistry............................................................................................................ 33 2.4.3 Time.................................................................................................................... 34 3.

Literature Survey: Secondary Steel Making................................................. 35 3.1 3.2

HISTORY OF SECONDARY STEEL MAKING...........................................................................36 PURPOSE OF SECONDARY STEEL MAKING..........................................................................37

3.3 GASES IN STEEL.......................................................................................................... 38 3.3.1 Sievert’s Law....................................................................................................... 38 3.3.2 Oxygen in Steel................................................................................................... 38 3.3.3 Nitrogen In Steel.................................................................................................. 39 3.3.4 Hydrogen In Steel................................................................................................ 40 3.3.4.1 3.3.4.2 3.3.4.3

4.

Hair line cracks (flakes) & Hydrogen embrittlement...................................................40 Hydrogen blistering.................................................................................................... 41 Loss of tensile ductility............................................................................................... 42

Ladle Metallurgy........................................................................................ 43 4.1 SLAG MAKING/TREATMENT............................................................................................. 44 4.1.1 Function of slag in Ladle furnace:........................................................................44 4.1.2 Philosophy of charging of lime & deoxidisers for making slag.............................44 4.1.3 Problem of carryover of slag................................................................................45 4.1.4 Use of Synthetic Slag........................................................................................... 45 4.2 DESULPHURIZATION...................................................................................................... 46 4.3 TEMPERATURE CONTROL................................................................................................47 4.4 ALLOYING ADDITIONS.................................................................................................... 48 4.4.1 Alloying Addition Calculation...............................................................................49 4.4.2 Cooling Effect of Ferro-alloys...............................................................................49 4.4.3 Mode of Alloying Additions...................................................................................49 4.5 HOMOGENIZATION OF TEMPERATURE AND CHEMISTRY OF THE BATH........................................50 4.6 METALLURGICAL ASPECTS OF LF FOR UNINTERRUPTED CASTING............................................50

5.

Practices at Ladle Furnace.......................................................................... 52 5.1 ARCING...................................................................................................................... 53 5.1.1 Basic Principles.................................................................................................... 53 5.1.2 Arcing Electrodes................................................................................................. 53 5.1.3 Electrode Slipping................................................................................................ 53 5.1.4 Temperature rise in Ladle Furnace.......................................................................54 5.1.1.1 5.1.1.2 5.1.1.3 5.1.1.4 5.1.1.5 5.1.1.6 5.1.1.7

Tap Changer/Current & Voltage.................................................................................. 54 Chilling due to alloying elements............................................................................... 54 Purging Condition....................................................................................................... 55 Ladle Condition.......................................................................................................... 55 Heat Size................................................................................................................... 55 Lime Addition............................................................................................................. 55 Electrode Regulation System..................................................................................... 55

5.2 INERT GAS PURGING..................................................................................................... 56 5.2.1 Purging Philosophy.............................................................................................. 56 5.2.2 Porous Plug – Design and Specifications..............................................................56 5.2.3 Emergency Lancing............................................................................................. 57 5.3 SAMPLING & TEMPERATURE MEASUREMENT.......................................................................58 5.3.1 Sample Probe...................................................................................................... 58 5.3.2 Temperature Probe.............................................................................................. 58 5.3.3 Pneumatic Probe.................................................................................................. 58 5.4 ALLOYING ADDITIONS.................................................................................................... 59 5.4.1 Ferro Alloys Addition System (Using Hoppers).....................................................59 5.4.2 Wire Feeder......................................................................................................... 60 5.5 FUME EXTRACTION SYSTEM............................................................................................. 61

5.5.1 Equipments of Fume Extraction System..............................................................61 1. 2. 3. 4. 5. 6. 7. 8. 9.

Water cooled fume duct..................................................................................................... 61 Damper............................................................................................................................. 61 Spark arrestor:................................................................................................................... 61 Bag Filter:.......................................................................................................................... 61 Rotary Air Lock:................................................................................................................. 61 Screw Conveyor:................................................................................................................ 62 I.D. Fan:............................................................................................................................. 62 Chimney:........................................................................................................................... 62 Dust Collector:................................................................................................................... 62

5.5.2 Description of system and main equipment........................................................62 5.6 UTILITIES AND SERVICES............................................................................................... 63 5.6.1 Industrial Water................................................................................................... 63 5.6.2 Soft Water............................................................................................................ 63 5.7 LINING PRACTICES AND PERFORMANCE OF STEEL LADLE......................................................64 5.8 HYDRIS PROBE............................................................................................................ 64 5.8.1 Configuration....................................................................................................... 64 5.8.2 Hydris Components............................................................................................. 65 5.8.2.1 5.8.2.2

The Hydris probe........................................................................................................ 65 The Pneumatic Unit.................................................................................................... 66

5.8.3 Measurement Principle........................................................................................67 5.8.4 Measurement procedure......................................................................................69 6.

Optimisation of Casting Process.................................................................. 70 6.1 SOFT REDUCTION......................................................................................................... 71 6.2 CLEAN STEEL FOR CONTINUOUS CASTING.........................................................................72 6.3 PREVENTION OF CONTAMINATION.....................................................................................73 6.3.1 Ladle to Tundish................................................................................................... 73 6.3.2 In the Tundish...................................................................................................... 73 6.3.3 Tundish to Mould.................................................................................................. 74 6.4 THE FIRST STAGE OF SOLIDIFICATION – IN THE MOULD........................................................74 6.5 EFFECT OF STEEL COMPOSITION ON SHELL FORMATION.......................................................75 6.6 THE INFLUENCE OF STEEL COMPOSITION ON AS CAST QUALITY.............................................76

7)

Data Analysis............................................................................................. 77

8)

Conclusions............................................................................................... 78

Table of Figures Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure

1 Process flow at BSP..................................................................................... 13 2 Schematic LD Vessel...................................................................................17 3 Process flow diagram of SMS-II...................................................................19 4 Schematic: Vacuum Degasser.....................................................................25 5 Schematic Ladle Furnace............................................................................26 6 Schematic R.H. Degasser............................................................................29 7 Binary system of CaO - Al2O3......................................................................34 8 Hydrogen induced cracking........................................................................41 9 Hydrogen Blistering and Embrittlement......................................................41 10 Solubility of Gas Decreases with Decrease In Partial Pressure of Gas.......42 12 CaO-Al2O3 Binary system..........................................................................50 13 Schematic Porous Plug.............................................................................. 56 14 Alloy feeding Mechanism..........................................................................60 15 Multi-Lab Hydris, pneumatic unit, lance and Hydris sensors.....................65 16 Hydris insert before final assembly in a cardboard tube...........................66 17 Hydris pneumatic unit............................................................................... 67 18 Hydris measurement principle..................................................................68 19 A typical measurement in the calculation only mode...............................69 20 Graphical representation of the soft reduction zone.................................71

1.INTRODUCTION TO BHILAI STEEL PLANT 1) Historical Background 2) Process Description in BSP 3) Achievements 4) Products of Bhilai Steel Plant 5) Purpose of visiting BSP 6) Purpose of selecting this topic

1.1.

Historical Background

Bhilai Steel Plant, a symbol of Indo-Soviet techno-economic collaboration, is one of the first three integrated steel plants set up by Government of India to build up a sound base for the industrial growth of the country. The agreement for setting up the plant of 1 MT capacity Ingot steel was signed between the Government of erstwhile USSR and India on 2nd February, 1955. And only after a short period of 4 years, India entered the main stream of the steel producers with the commissioning of its first Blast Furnace on 4th February, 1959 by the then President of India, Dr Rajendra Prasad. Commissioning of all the units of 1 MT stage was completed in 1961. BSP expanded its production capacity in two phases - first to 2.5 MT which was completed on Sept. 1, 1967 and then on to 4 MT which was completed in the year 1988. The plant now consists of ten coke oven batteries. Six of them are 4.4 meters tall. The 7 meter tall fully automated Batteries No 9 & 10 are among the most modern in India. Of BSP's seven blast furnaces, three are of 1033 cu. meter capacity each, three of 1719 cu. meter and one is 2000 cu. meter capacity. Most of them have been modernised incorporating state-of-theart technology. All the units of the plant have been laid out in sequential formation according to technological inter-relationship so as to ensure uninterrupted flow of in-process materials like Coke, Sinter, Molten Iron, Hot Ingots, as well as disposal of metallurgical wastages and slag etc., minimizing the length of various inter-plant communications, utilities and services. BSP meets its raw material requirements by its own captive mines spread over 10929.80 acres. Iron ore comes from Rajhara group of mines, 85 kms south-west of Bhilai. Limestone requirements are met by Nandini mines, 20 kms north of Bhilai and dolomite comes from Hirri in Bilaspur district, 135 kms east of the plant. To meet the future requirement of iron ore, another mining site Rowghat , situated about 100 km south of Rajhara, is being developed. Steel Authority of India limited is the largest steel producer in India with a turnover of Rs. 47, 041 crores in the financial year 2011-2012. Steel and its products are undoubtedly the pillar and anchor of material developments through the ages. It is a substantial part of material science and a key material in product development in modern technological advancement. It is the base material for over 2500 different grades of products. The world production of crude steel as reported by world steel association is to a great extent more than any other metal product; this also proves its wide versatility in material Consumption. Its world productions in million metric tons are 1327, 1219, 1413 and 1490 in the year 2008, 2009, 2010 and 2011 respectively. India is on 4th in the production of steel and the

productions in million metric tons are 58, 63, 69 and 72.2 in the year 2008, 2009, 2010 and 2011 respectively.

1.2.

PRODUCTS OF BSP

BSP is the sole manufacturer of rails and producer of the widest and heaviest plates in India. BSP specializes in the high strength UTS 90 rails, high tensile and boiler quality plates, TMT bars, and electrode quality wire rods. It is a major exporter of steel products with over 70% of total exports from the Steel Authority of India Limited being from Bhilai. The products of Bhilai Steel Plant are:-

Product Type

Product

End use / consumers

Rails in 13m, 26m, 65/78 m length and welded Rail & FINISHED panels of 130m/260m length Indian Railways, Export Structural PRODUCTS Heavy Structural Construction Crane Rails Cranes Mill Crossing sleepers Broad gauge sleepers Merchant Light structural engineering and construction, Mill medium rounds (plain & TMT), heavy rounds (plain) Wire Rod Wire rods - plain construction wire rods - TMT EQ wire Mill rods electrodes Plates boilers, defence, railways, shipbuilding, LPG Plate Mill cylinders, export Billets SEMIS (from Re-rollers BBM) Blooms (from BBM) Narrow width slabs CC blooms Killed slabs IRON Pig iron Foundry Coal chemicals, ammonium sulphate (fertilizer), tar By CHEMICALS and tar products, (pitch, naphthalene, creosote oil, products road tar, Anthracene

1.3.

ACHIEVEMENTS

A leader in terms of profitability, productivity and energy conservation, BSP has maintained growth despite recent difficult market conditions. Bhilai is the only steel plant to have been awarded the Prime Minister's Trophy for the best integrated steel plant in the country nine times. The distinction of being the first integrated steel plant with all major production units and marketable products covered under ISO 9002 Quality Certification belongs to BSP. This includes manufacture of blast furnace coke and coal chemicals, production of hot metal and pig iron, steel making through Twin Hearth and Basic Oxygen processes, manufacture of steel slabs and blooms by Continuous Casting, and production of hot rolled steel blooms, billets and rails, structural plates, steel sections and wire rods. The plant's Quality Assurance System has subsequently been awarded ISO 9001:2000. Bhilai Steel Plant also has ISO 14001 certification for its Environment Management System and its Dalli Mines. It also has introduced various environment-friendly technologies like Coal Dust Injection System in the Blast Furnaces, de-dusting units and electrostatic precipitators in other units together with a vigorous afforestation program.

1.4.

Process Description in Bhilai Steel Plant

Steel is made through Twin Hearth furnaces in Steel Melting Shop I as well as through LD Convertor - Continuous Casting route in Steel Melting Shop II. Production of cleaner steel is ensured by flame enrichment and oxygen blowing in SMS I while secondary steel making processes such as Vacuum Arc Degassing unit, RH (Ruhshati Heraus) Degassing Unit, and ladle furnace refining in ensures homogenous steel chemistry in SMS II. Rh degasser mainly used to remove hydrogen from rail steel and Ladle Furnace is used for minor alloying addition and temperature control for casting shop. The Rolling Mill complex consists of the Blooming & Billet Mill, Rail & Structural Mill, Merchant Mill, Wire Rod Mill and also a most modern Plate Mill. While input to the BBM and subsequently to Merchant Mill and Wire Rod Mill comes from the Twin Hearth Furnaces, the Rail & Structural Mill and Plate mill roll long and flat products respectively from continuously cast blooms and slabs only. Also there are Ore Handling Plant, three Sintering Plants, two 110 MW generating captive Power Plants, two Oxygen Plants, Engineering Shops, Machine Shops and a host of other supporting agencies giving Bhilai a lot of self-sufficiency in fulfilling the rigorous demands of an integrated steel plant.

1.4.1 Plant facilities Blast Furnaces:  3 of 1033 Cu m capacity each  3 of 1719 Cu m capacity each  1 of 2355 Cu m capacity Hot Metal Capacity : 4.70 MT / year Steel Melting Shop: Steel-making through BOF, VAD/Ladle Furnace/RH-Degasser and Continuous casting route  3 converters of 110/130 T  VAD unit, 2 RH degasser,2 Ladle furnace  4 Slab Casters, 1 bloom caster, 1 Combi caster Annual Capacity: 1.425 MT Cast steel Converter Shop : 3 BOF 110/130 T Convertors Secondary Refining facilities : 1 VAD unit, 2 RH degassers, 2 Ladle furnaces, 1 Desulphurisation Unit Continuous Casting Shop: 4 Slab Casters, 1bloom caster, 1Combi caster Steel-making through Twin Hearth Furnace (THF) route : 4 THFs of 250 T capcity each Annual capacity 2.5 MT ingot steel Blooming & Billet Mill 14 pairs of recuperative soaking pits Capacity to produce 2.14 MT/year of blooms Capacity to produce 1.50 MT/year of billets Rail & Structural Mill Capacity - 7,50,000 T Merchant Mill Capacity - 5,00,000 T Wire Rod Mill Capacity - 4,20,000 T Plate Mill Capacity Plates thickness Width Length

-

9,50,000 T 8-120 mm 1500-3270 mm 5-12.5 M

Figure 1 Process flow at BSP

1.5.

Purpose of visiting BSP

Bhilai Steel Plant (BSP) is one of the giant leaders in steel industry in terms of profitability, productivity and energy conservation and it has also maintained growth despite recent difficult market condition. Steel grades confirming to various national and international specifications are produced here. Production of cleaner steel is ensured by flame enrichment and oxygen blowing in SMS I while secondary refining in Vacuum Arc Degassing ensures homogenous steel chemistry in SMS II. Also in SMS II there are two 130 T capacity RH (Ruhshati Heraus)Degassing Units , installed mainly to remove hydrogen for rail steel and Ladle Furnaces to meet present and future requirements of quality steel. BSP is capable of providing the cleanest and finest grade of steel. It consists of the Blooming & Billet Mill, Rail & Structural Mill, Merchant Mill and also a most modern Plate Mill. The total length of rails rolled at here so far would circumvent the globe more than 4.5 times.

1.6.

Purpose of selecting this topic

Production of Rail Steel & various defense steels require stringent control over the chemistry of steel (H2 < 1.5 ppm for rail steel and specific amount of various alloying elements for greater hardness and toughness in defense alloys) and higher production rates to meet demands. We undertook this project to grasp the underlying concepts, the Metallurgy and practices used at Ladle furnace and Rh degasser to make these fine adjustments and to assist in increasing production rates by adjusting temperature and chemistry for uninterrupted casting.

2.Introduction to Steel Melting Shop II and Its Units 1) Introduction to SMS-II 2) Convertor shop 3) Continuous Casting Shop  Secondary Steel Making Units  Casting Bay  Other Units 4) Co-ordination Between Convertor and Continuous Casting Machines

2.1 Introduction to SMS-II Steel Melting Shop-II (SMS-II) is designed to produce 1.5 MT of Cast Steel in the form of Slabs and Blooms. The shop was commissioned on 29th of July'1984. With every passing day, the shop has improved its systems and processes. Many in-house modifications along with acquisition of most modern steel making equipment’s have helped in fulfilment of ever increasing customer demands in terms of quality and quantity. The revised production capacity is estimated to be about 1.8 MT. SMS-II is equipped with secondary steel making units like VAD, Ladle Furnace and RH Degasser to produce low hydrogen, low sulphur and micro-alloy steels.

SMS II comprises of 2 units A) Converter Shop B) Continuous Casting Shop

2.2 Convertor Shop The Converter Shop has 3 converters of 130 T capacities each. The working lining of the Converter is of 690 mm Magnesia Carbon bricks. The lining life has increased from 100 Heats with TBDB in the beginning to a level of 6252 Heats in SEP'2006 with Magnesia carbon bricks.

Figure 2 Schematic LD Vessel

Tap to tap time of the Converters is around 60 minutes, with an average heat weight of nearly 120 Tons. There are two hot metal charging cranes of 180 + 50 T capacity and a semi-portal crane of 40 + 40 T capacity. Lance handling cranes of 30 / 5 T capacity have also been provided for fixing and changing of oxygen lances, converter relining and other related maintenance activities. The operation cycle of the converter is as follows : a)

Charging

-

5 Min

b)

Oxygen blowing

-

20 min

c)

Sampling & temperature measurement

-

8 min

d)

Corrections before tapping

-

5 min

e)

Tapping

-

5 min

f)

Nitrogen splashing, coating & deslagging

-

7 min

TOTAL

-

50 min

2.2.1

Process Flow in Convertor Shop

Before charging, the converter is inspected for lining condition, after which scrap and hot metal are charged. For getting the prediction of O 2 and bulk material, hot metal weight analyses, temperature and scrap weight is fed to the computer before start of blow. As per perdition O2 is blown and the bulk materials are charged through a computerised addition system during the oxygen blowing process. Oxygen blowing is done for about 18 min according to computer model prediction. The oxygen is blown through a 5-hole nozzle, water cooled lance, at a pressure of 16 Kg/Cm 2 and a flow rate of 400 - 450 NM 3 /min. After the blow, convertor is tilted for sample & temp. Reconditioning of bath is done with O2 and lime addition if bath analysis deviate from desired analysis i.e. reblowing is done. The analysis of the steel sample is checked in the site laboratory. Tapping of steel is done after necessary adjustment in temperature and chemistry. The steel is tapped into a 130 T steel ladle, placed on steel transfer car moving below the converter. Ferro-Alloy additions are made in the ladle as per the requirement of the grade of steel. The steel tapped from here has various possible routes: i) Through Argon Rinsing Unit to the Con-cast machines (in case of normal slab grade heats). ii) Through VAD unit (in case of special steel heats like API, Boiler Quality and Rail Steels). iii)

Through Ladle Furnace & RH Degasser route (in case of Rail Steels).

The figure below represents the process flow at SMS-II

Figure 3 Process flow diagram of SMS-II

2.2.2

Mixer Section

The main function of Mixer is to supply Hot Metal (Pig Iron) to the converters at a fairly uniform temperature and composition, in required quantity whenever demanded. The mixer acts as a buffer between the Blast Furnaces and the Converters. There are two mixers of 1300 T capacity each. At a time only one mixer is kept in operation. The pig iron reaches mixer in ladles of 100 T capacities. The metal from these ladles is poured into the mixer with the help of two 125 + 30 T cranes. Burners are provided in the mixer to maintain the temperature of hot metal. Metal from the mixer is supplied to the converters by hot metal ladles, kept on self- propelled hot metal transfer cars. Weigh bridges are provided to weigh the quantity of hot metal supplied.

2.2.3

Scrap Yard

Scrap yard receives the scraps generated from all over the plant in wagons and trucks. Two 30 T magnet cranes do the unloading / loading of scrap. Scrap is supplied to the converters in 11 M3 scrap boxes kept on two scrap transfer cars. Two weigh scales below the scrap transfer car track check quantity of scrap supplied.

2.2.4

Ladle Preparation Section

As the name suggests, the steel casting ladles, required for tapping of steel from converters are prepared here. There are 28 ladles of 130 T capacities each. All the ladles have provision for bottom purging through porous plugs and fitted with slide gate FLOCON 6300 system for pouring of steel for casting. Two 75+15 T (100 + 20 T MODIFIED) cranes and 5 T cantilever cranes are provided here. The ladles are lined with magnesia carbon bricks to withstand the heat and erosion during treatment in the VAD, LF and RH units. Repair of ladle lining, pre-heating, fixing of slide gate system, purging system is the major activity of this section.

2.2.5

Slag Pot Handling Section

The slag produced during steel making is poured out of the converter into 16 M3 slag pots (also called thimbles). The slag cups are loaded on to the tilting type slag cars with the help of two 100/20 T slag pot cranes. The tilting slag cars are taken by locomotives to the slag yard where the slag cups are tilted and emptied. They are then coated with lime and received back in slag pot handling section to be used again.There are 26 Thimbles and 20 Slag tilting cars to handle Convertor shop slag.

2.2.6

Bulk Material Handling

The following Bulk Materials are handled in this section : 1) Lime. 2) Calcined Dolomite. 3) Lime stone. 4) Coke. 5) Raw Dolomite. 6) Iron Ore Lime and calcined dolomite are used as fluxes; Iron Ore & Limestone are used as coolants. Coke is used for pre-heating the converter and coating. Lime and calcined dolomite are received from RMP-II & RMP-I. Limestone, Iron ore and dolomite come from OHP. Coke is received from Coke Ovens Department. All these materials come in the bunkers at a location called junction-34, from where it is supplied by conveyors to the Converter Shop via junction-35, and distributed to their respective bunkers by means of reversible shuttle conveyor. Each converter has 9 bunkers, 3 for lime, 2 for calcined dolomite, 1 for raw dolomite, 1 for Iron ore, 1 for limestone and 1 for coke. Required quantities of these materials are charged into the converter by means of weigh scales of 5 T and 1 T capacity. The 5 T weigh scale is for Lime and 1 T scale for other materials. Very sophisticated microprocessor based system, of weighing and then charging of bulk materials into converter, is installed in the main control pulpit.

2.2.7

Gas Cleaning Section

This is a modern and sophisticated plant, which cools, cleans, and collects the gases emerging from the mouth of the converter during oxygen blowing. The system is based on suppressed combustion principle (air factor 0.1) i.e. the gases rich in CO are not allowed to come in contact with the atmospheric oxygen and are stored for use as a fuel. The gases at converter mouth are at a temperature of 1850 ° - 1900 °C and contain above 230 Gms of dust per NM3. They pass through water cooled hood and stack. The gas cools to 1019 °C at the end of the stack. The gas then enters the quencher, where water is sprayed on by nozzles, cooling the gas to 72 °C, and removing the dust which goes out in the form of slurry. The cooled and partially cleaned gas enters the kinpactor venturi where it is cleaned further. The kinpactor venturi has a variable throat, opening of which is adjusted to maintain a pressure of ± 1 mm w.c at converter mouth by regulating the flow of gas to avoid any infiltration of air into the system or any possibility of gases escaping to atmosphere. The gases then pass through a mist eliminator to remove any water droplets. After this gases go to ID Fan, and then to the changeover valve which directs the gases to the recovery side if the carbon monoxide content is more than 45 %, otherwise, the gases are burnt and let off into the atmosphere through a flare stack.

The gases to be recovered pass through a booster fan and a hydraulic non-return valve before going to the gasholder of 40,000 M3 capacities where they are collected over water. Before gasholder, the dust content of the gases is 100 Mg/NM3. The gases pass through an electrostatic precipitator after the gasholder to reduce the dust content to 10 mg/ NM3. This gas is called LD Gas and is used as a fuel. The calorific value of the converter gas is 2000 k.cal/Cu.m and the average composition is : CO

=

65 - 70 %

CO2 = 15 - 20 %

O2

N2

=

=

0.10 %

15 - 20 %

About 85 M3 gas is recovered per ton of steel.

2.3 Continuous Casting Shop Continuous Casting Shop comprises the following sections : 1) 2) 3) 4) 5) 6) 7) 8) 9) 10)

Argon Rinsing Bay Tundish preparation Bay Casting Bay VAD (Vacuum Arc Degassing Unit) Ladle Furnace RH Degasser Discharge Bay Slab & Bloom Storage Yard (SBS) Mould Repair Shop Casting Powder Plant (SFMPP)

This shop has three single strand radial slab casters, one 4-strand radial bloom caster and a combi caster - which can be converted to a slab caster or a 3-strand bloom caster as per requirement. All sections of the Continuous Casting Shop except the Mould Repair Shop & Casting Powder Plant are parallel to Converter Shop bays. Converter Shop has 3 LD Converters of capacity 130 Tons each. Liquid steel is taken in 130 T steel casting ladle transfer car. There are 3 such cars, each running on separate rail track - stretching from below each converter to casting bay via on-line Argon Rinsing Station. An additional ladle car is provided for returning the empty ladles.

2.3.1

Argon Rinsing Bay

At Argon Rinsing Bay, the ladle tapped (with liquid steel) is received on the steel car. Argon / Nitrogen gas is bubbled into the liquid metal from top through a refractory lined lance called the argon rinsing lance. The rinsing operation does homogenisation of temperature & composition of liquid metal, as well as deoxidation is carried out by Al addition. Temperature is measured after rinsing and steel sample is sent to the lab for analysis. Trimming additions are done as per requirement, for achieving targeted chemistry

of Carbon, Manganese and Aluminium content in steel. If the temperature of the liquid metal is high, it is brought down by further rinsing, after which heats are sent to CCS machines at required temperature or sent to secondary steel making units like VAD, LF or RHD.

2.3.2

Secondary Steel Making Processes:

LD process is most fitting for tonnage steel production but it has its limitations on the quality front due to dissolved gases such as H 2 and N2. So for the production of high quality steel, secondary refining units are used all over the world. SMS-II secondary refining units comprise three units: 1)

Vacuum Arc Degassing Unit (VAD)

2)

Ladle Furnace (LF)

3)

RH Degasser (RHD)

2.3.2.1 VAD (Vacuum Arc Degassing Unit) The VAD unit was commissioned on 17th January'1991. The unit consists of a vacuum chamber where ladle tapped with liquid steel from converter is placed for treatment. There is a provision of inert gas (Argon) purging from the bottom. The vacuum chamber is made air tight with a metallic cover with the help of special type of rubber sealing. and is connected to vacuum pumps comprising a series of ejector system and condensers. High pressure super-heated steam is used for creating vacuum as low as 1 millibar or less. The unit is also provided with heating facility and ferro alloy addition system for temperature and chemistry adjustments. Heating is done by submerged arcing with the help of three columns of graphite electrodes using electrical energy. A dedicated transformer is provided for the purpose. Partial vacuum is maintained in the vacuum chamber to suck out any fume or dust generated during arc heating. Continuous purging of bath from the bottom helps in temperature and composition homogenisation. A separate pump house along with cooling towers and settling tanks is provided to cater to the water requirement of VAD unit. Continuous chemical dozing of water current is ensured to avoid any deposition and also to ensure auto corrosive coating inside the narrow pipe lines. A CaO / Alumina rich slag is formed by addition of lime and prefused synthetic slag for facilitating desulphurisation if required. A 50 T / 15 T EOT crane, 3 T telpher and 1 T jib crane are provided for handling and maintenance purposes.

The VAD unit thus serves following functions : 1) 2) 3) 4) 5)

Temperature adjustment Chemistry adjustment Desulphurisation Removal of dissolved gasses ( H2, N2, O2 ) De-oxidation as per requirement.

TECHNICAL DATA OF VAD Heat size nominal

:

110 - 115 T

Free Board necessary

:

600 - 800 mm

Transformer rating

:

24 MVA

Roof lifting

:

Electrode lifting Type of electrode

Hydraulic, 2 cylinder :

:

Hydraulic

Graphite low density

No of electrodes in a column

:

4

Diameter of electrode

:

457 mm

Electrical power HT

:

11 KV

Electrical power LT

:

415 V

Heating rate

:

3 - 4 °C / minute of arcing

Industrial Water: Pressure

:

3.5 Kg / Cm2 minimum

Inlet temperature

:

32 °C Max

Outlet temperature

:

41 °C approx.

Maximum flow rate

:

650 M3 / Hr

Flow rate during heating :

50 M3 / Hr

Soft Water: Pressure

:

5.0 Kg / Cm2

Inlet temperature

:

32 °C

Flow Rate

:

80+20m3/Hr (for heat shield) primary, & 100 m3/Hr Secondary.

Compressed Air: Pressure

:

3 Kg / Cm2

Consumption / treatment

:

2 - 3 NM3

Argon: Pressure at manifold Flow rate for stirring

: :

4 - 10 Kg / Cm2 70 NL/ min (Avg.) & 200 NL/ min

Max Nitrogen pressure: For releasing electrode clamps Type of Vacuum Pump pump with

: :

25 Kg / Cm2 6 stage steam jet vacuum

1 starting ejector and 1 heating ejector. Steam: Pressure at manifold

:

13 Kg / Cm2

Temperature at take over point Soft water for de-superheating Temperature at manifold : Flow rate during degassing (ejector 1 to 6) : Flow rate during heating :

: 350 - 375 °C : 1.6 M 3 / Hr Approx. 220 °C 10.5 T / Hr 0.800 T / Hr

Figure 4 Schematic: Vacuum Degasser

2.3.2.2 Ladle Furnace Ladle Furnace and RH Degasser units are recent additions to SMS-II. The equipment and technology of 130 T Ladle Furnace was given by M/s GA Danieli. The unit was started on 23rd December'1999. Ladle Furnace is a heating unit where liquid steel tapped in ladle from converter can be heated using the similar principle that of VAD unit. Continuous inert gas (Argon) bottom purging is done for temperature and chemistry homogenisation. The lid of Ladle Furnace is water cooled and is provided with three holes for three columns of electrodes, one hole for ferro alloy addition and one for aluminium / Calcium silicide wire injection. A 2 T jib crane is provided for electrode column preparation and replacement of same in Ladle Furnace. A dedicated fume extraction system with bag filters for dust separation is

provided to suck out the fumes and dust generated during arcing. Ladle Furnace performs all the functions of VAD excepting removal of dissolved gasses. Figure 5 Schematic Ladle Furnace

TECHNICAL DATA OF LADLE FURNACE Heat size nominal

:

130 T

Transformer rating (continuous)

:

Roof lifting

Hydraulic Cylinder

:

28 MVA

Electrode lifting

:

Hydraulic Cylinder

Electrode de-clamping

:

Hydraulic Cylinder

Type of electrode

:

Graphite (High density)

No of electrodes in 1 column:

3

Diameter of electrode

:

457 mm

Heating rate

:

3 - 5 °C / min of arcing

Electrical Power HT

:

11 KV

Electrical Power LT Industrial water: Pressure Inlet temperature Circulation rate Make up rate Emergency requirement Soft Water: Pressure Inlet temperature Circulation rate Make up rate

:

415 V

: :

4 Kg / Cm2 35 °C : 260 M3 / Hr 15 M3 / Hr : 100 M3

:

: : :

4 Kg / Cm2 35 °C : 60 M3 / Hr 3.5 M3 / Hr

Compressed Air: Pressure Requirement

: :

3 - 4 Kg / Cm2 235 M3 / Hr

Argon: Pressure Requirement

: :

4 ~ 16 Kg / Cm2 48 NM3 / Hr

Nitrogen: Pressure : Requirement (Normal) Requirement (Intermittent) :

4 ~ 8 Kg / Cm2 : 35 NM3 / Hr 48 NM3 / Hr

2.3.2.3 RH Degasser The state of the art RH Degasser unit was supplied, erected and commissioned by M/s Technometal, Germany and M/s Voest Alpine India on a turnkey basis. The unit was started on 30th March'2000. RH Degasser is basically a degassing unit. The principle of creating vacuum is similar to that of VAD unit but there is basic difference in the working principle of the two. VAD is a tank degasser while RH Degasser belongs to circulating degassing system. There is a vessel with inlet and outlet snorkels both lined with refractory. The vessel is immersed into the liquid steel. Inside the inlet snorkel two layers of inert gas supply lines are installed. The vessel is subjected to low vacuum. The metal level rises in both the snorkels due to barometric pressure. Inert gas stirring via the lift gas nozzles in the inlet snorkel causes a partial quantity of melt to be lifted into the RH vessel which subsequently comes back to ladle through down leg snorkel. Consequently a high turbulent flow from the inlet to the down leg snorkel takes place. Once into the ladle, the steel flows quite slowly to the bottom of the ladle and turns back upwards to the uplid snorkel, when velocity is increased again. Thus the recirculation of molten steel is started and complete heat thus passes several times through the

RH vessel. The metal inside the RH vessel is exposed to vacuum level of 1 millibar or less and so dissolved gasses (H2, N2, O2) in the steel is reduced. The circulation rate of molten steel is as high as 130 T / minute. The RH process is thus faster and effective than VAD process. The process is most suitable for making Rail Steel which requires H2 concentration level less than 1.6 ppm. Also the obtained purity of Al-Si killed grade steel is very high due to effective separation and removal of non-metallic inclusions during the process. The unit is provided with ferro-alloy addition system for any trimming addition required during degassing. A separate pump house along with cooling towers and settling tanks are provided to meet the water requirements of RH Degasser unit. A unique swivel joint system is provided for lifting and lowering of RH vessel along with its suction pipes. An off line pre-heating burner is used for heating the vessel along with snorkels after fresh lining. An extremely sophisticated burner is provided for heating the vessel at the treatment place whenever required.

Figure 6 Schematic R.H. Degasser

2.3.3

Tundish preparation Bay

Tundish preparation bay has been provided with the facilities for relining and preparation of tundishes. The bay is serviced by two 50 T / 10 T EOT cranes. Tundish capacity is of 10 Tons and 20 Tons, for slab and bloom caster respectively.

2.3.4

Casting Bay

In the casting bay all the continuous casting machines are located. This bay is divided into two blocks. In one block there are three slab casters and in the other block one combi caster & one bloom caster are located. The casting bay is serviced by three 180 T / 50 T /15 T / EOT cranes and two 50 T semi-portal cranes. The working platform of the bay is at + 13.15 Metres elevation. The rated capacity of Converter Shop & Continuous Casting Shop is raised to 1.8 MT of cast steel production per annum. GENERAL FEATURES OF SLAB CASTERS: 1. 2. 3. 4. 5. 6. 7.

Radius of machine & mould : 12000 mm Length of copper mould : 1000 mm Maximum casting speed : 1.2 M / min Metallurgical length of machine : 23 metres Dummy bay insertion speed : 4 M / min (Max) Range of frequency of mould oscillation : 6 to 13 mm Range of frequency of mould oscillation : 15 - 100 cycles / min (Increasing with casting speed) Grades of steel cast : Killed, structural & Alloy steel Cross section of slabs produced : 200 x 1300, 200 x 1500,250

8. 9. x 1500 Total tonnage of cast from all the slab casters

:

1.18

OPERATIONAL DATA FOR SLAB CASTER: 1. 2. 3. 4. 5.

Rated casting speed Casting time for 120 T heat Water consumption mould Total water consumption Secondary cooling zone Length of slab cut at gas cutting machine

: : : :

0.5 - 1.0 M / min 55 - 70 min 350 - 500 Cu M / Hr 90 - 130 Cu.M / Hr

:

6 - 10.5 M

GENERAL FEATURES OF BLOOM CASTER: 1. 2.

Radius of machine and mould Length of copper moulds

:

: 12000 mm 1000 mm

3. 4. 5. 6. 7. 8. 9.

Maximum casting speed of strands Metallurgical length Dummy bar insertion speed Range of amplitude of mould oscillation Range of frequency of mould oscillation Grade of steel cast Cross section of cast blooms

2.3.5

: : :

: :

6 M / min 24 M 4 M / min : 6 - 13 mm : 15 - 100 cycles / min Rail and structural steels. 300 x 340

Slab & Bloom Storage Yard (SBS)

The slab & Bloom storage yard is 374 M long and 108 M wide arranged in 3 bays, each 36 M wide. All the cast products of CCS are received here. Slabs for plate mill are cut into desired size, inspected for visible surface defects and conditioned by scarfing. Accepted slabs are sent to plate mill for rolling as per rolling plan. Slabs for despatch are inspected, conditioned and despatched to designated customers through wagons placed on 4 different tracks laid perpendicular to bays. Blooms are despatched through wagons to Rail & Structural Mill for rolling. The yard consists of following facilities for handling, inspection, conditioning, storage & despatch of cast products. 1) 2) 3) 4) 5)

Gas cutting Machine - 3 Nos for Slab, 1 No for Bloom 5 EOT cranes of 46 T & 64 T hook capacity. Crane Nos 2106, 2107, 2108, 209 & 2110 3 finger cranes 16 T each. Crane Nos F1, F2 & F3 3 gripper cranes 18 T each. Crane Nos 1, 2, 4 with GCM's 6 semi-portal cranes 32 T each. Crane Nos 1 to 6

2.3.6

Mould Repair Shop

In the Mould repair shop, old (used) moulds for casting are repaired and new moulds are also assembled.

2.3.7

Casting Powder Plant (SFMPP)

In the Powder Plant, also called SFMPP (slag formation mixture preparation plant) casting powder for slab and bloom casters is prepared by mechanical mixing of ingredients.

2.3.8

Brief Description Of Continuous Casting Shop

After tapping of liquid steel into steel teeming ladle, the ladle is moved to the Argon Rinsing Bay. Here the steel is rinsed for proper homogenisation and achieving desired temperature of liquid steel for casting. After this the ladle is transferred to casting bay by steel transfer car or sent to secondary steel units for further processing. The steel ladle received from Secondary steel units or Argon Bay is placed on the Lift & Turn Stand of Casting machines by means of casting cranes of 180 T capacities. Tundish is brought to the casting position and the sub-entry nozzle is aligned with the centre of mould. The ladle is turned to the casting position and placed above the tundish. After this command from the casting In-charge is received for opening the ladle slide gate. After the slide gate is opened metal from the ladle is taken into the tundish through a refractory lined pipe called the shroud. When the metal level in tundish is attained, metal is taken into the mould of machine through a sub-entry nozzle. When the mould is filled to the working level, mould oscillation is started and the casting is started at 0.2 m / min speed, and speed is increased gradually to a constant working speed. When the dummy bay comes out of the roll zone the holding device hook holds it. When the dummy bar head reaches the withdrawal roll stand, it is separated from the stand (slab or bloom) The strand is cut at the gas cutting machine into the desired lengths and cut slabs / blooms are transported to SBS Yard. 10 minutes before the metal in the ladle finishes, another ladle is placed on the reserve arm of L & T stand & kept ready. As soon as the metal in the first ladle finishes, the L & T is rotated by 180 º and the sequence ladle comes to casting position and the ladle is opened for casting. Thus sequence of casting is maintained. Sequence of casting is maintained depending on the grade of steel being cast. At the end of casting, tail end of strand is quickly removed form the machine by increasing the speed and the machine is checked & prepared for next casting. MAJOR IMPROVEMENTS & ACHIEVEMENTS (SINCE INCEPTION OF SMS-II) : 1. 2. 3. 4.

INCREASE IN LINING LIFE OF CONVERTER TO 6252 HEATS. PRODUCTION OF RAIL STEEL WITH LESS THAN 2 PPM HYDROGEN AND LOW SULPHUR STEELS LIKE API AND BS GRADES. IN-HOUSE IMPROVEMENT IN CASTING MACHINES TO IMPROVE QUALITY OF STEEL AND YIELD. 5. REDUCTION IN CONSUMPTION OF INPUTS LIKE HOT METAL, LIME AND FERRO ALLOYS. 6. HIGHEST EVER PRODUCTION OF 84 HEATS ON 29-03-2005.

2.4 Coordination between Convertor and Continuous Casting Machines Secondary Steel Making Units are designed for maintaining proper coordination between Convertor & CCM for an uninterrupted casting operation. Among other parameters, the parameters Temperature, Chemistry and Time majorly influence the casting operations. Heat, Therefore, before going to continuous casting shop, is taken through various secondary steel making units ((VAD)/ (LF-Rh)) for maintaining proper temperature and chemistry. The parameters influence the Casting operation in the following manner.

2.4.1

Temperature

When Heat is transferred from converter to ladle furnace via ladle transfer cars and cranes, there is temperature drop which can later on prove detrimental to casting. Heat is subjected to temperature rise at ladle furnace by arcing. A temperature rise of almost 4°C/min is observed. Proper attention should be paid to temperature rise and it should not rise beyond 1635°C at any cost otherwise it can lead to solidification problems at CCS. Generally the heat is taken out of LF at 1620 C and sent to R-H degasser where there is temperature drop of almost 2.5 C/min. Degassing takes place for 15 minutes. The temperature at Lf is so adjusted that even after a temperature drop at R-H degasser; it should have a Superheat of 30-35C When it reaches CCS. High as well as low Temperature poses serious problems in CCS. Problems in casting due to low temperature: - If the Heat is sent to CCS at or near the liquidus temperature, it may lead to freezing of metal in submerged entry nozzle (SEN) or freezing of metal in mould. Problems in casting due to High temperature: - If heat is sent to CCS at much higher superheat, we have to reduce metal through put to avoid problems in solidification which later on leads to clogging on SEN due to accumulation of Non-Metallic Inclusions.

2.4.2

Chemistry

Chemistry of the Heat plays an important role in uninterrupted casting. Aluminium wire is added to the heat to produce killed steel. Alumina inclusions occur as deoxidation products in the aluminium based deoxidation of steel. Pure alumina has a melting point above 2000°C,i.e., these alumina inclusions are present in a solid state in liquid steel. The addition of calcium to steel which contains such inclusions changes the composition of these inclusions from pure alumina to CaO-containing calcium aluminates, as

it can be observed from the CaO-Al2O3 binary system, the melting point of the calcium aluminates will decrease as the CaO content increases, until liquid oxide phases occur at about 22% of CaO, i.e., when the CaO.2Al2O3 compound is first exceeded at 1600 C. The liquid phase content continues to increase as CaO content rises further and is 100% at 35% of CaO. The minimum melting temperature for the liquid calcium aluminates is around 1400°C, i.e., when C12A7 forms. Such liquid calcium aluminates may be present in liquid form until, or even after, the steel solidifies .If the sulphur content of the steel is high, calcium will react with sulphur forming solid CaS, which could clog up the continuous casting nozzle.so the sulphur content should be low to avoid such a Possibility.

That is why in BSP there are strict rules for addition of Ca-Si wire to the melt. They are as follows:1. 2. 3.

When sulphur 0.025%, no Ca-Si to be added

Figure 7 Binary system of CaO - Al2O3

2.4.3

Time

Time also plays an important role in proper coordination between Ladle furnace and continuous casting. Heat should reach in proper time at CCS from LF otherwise there would

be significant temperature drop from LF to CCS which would pose serious problems in casting. Moreover, delay in transfer of heat from LF to CCS may lead to discontinuity in continuous casting operation.

3.Literature Survey: Secondary Steel Making 1) History of Secondary Steel Making 2) Purpose of Secondary Steel Making 3) Gases in steel a. Sievert’s Law b. Nitrogen In Steel c. Hydrogen In Steel

3.1 History of Secondary Steel Making Prior to 1950 or so, after steel was made in furnaces such as open hearths, converters, and electric furnaces, its treatment in a ladle was limited in scope and consisted of deoxidation, carburization by addition of coke or Ferro coke as required, and some minor alloying. However, more stringent demands on steel quality and consistency in its properties require controls that are beyond the capability of these steelmaking furnaces. This is especially true for superior-quality steel products in sophisticated applications. This requirement has led to the development of various kinds of treatments of liquid steel in ladles, besides deoxidation. These have witnessed massive growth and, as a result, have come to be variously known as secondary steelmaking, ladle metallurgy, secondary processing of liquid steel, or secondary refining of liquid steel. However, the name secondary steelmaking has more or less received widest acceptance and hence has been adopted. Secondary steelmaking has become an integral feature of modern steel plants. The advent of the continuous casting process, which requires more stringent quality control, is an added reason for the growth of secondary steelmaking. Steelmaking in furnaces, also designated now as primary steelmaking, is therefore increasingly employed only for speedy scrap melting and gross refining, leaving further refining and control to secondary steelmaking. There are processes, such as vacuum arc refining (VAR) and electro slag remelting (ESR), that also perform some secondary refining. However, they start with solidified steel and remelt it. Hence, by convention, these are not included in secondary steelmaking. Harmful impurities in steel are sulphur, phosphorus, oxygen, hydrogen, and nitrogen. They occupy interstitial sites in an iron lattice and hence are known as interstitials. The principal effects of these impurities in steel are loss of ductility, impact strength, and corrosion resistance. When it comes to detailed consideration, each element has its own characteristic influence on steel properties. Oxygen and sulphur are also constituents of nonmetallic particles in steel, known as inclusions. These particles are also harmful to properties of steel and should be removed as much as possible. Carbon is also present as interstitial in iron lattice. However, unlike the other interstitials, it is generally not considered to be harmful impurity and should be present in steel as per specification. But, today, there are grades of steel in which carbon also should be as low as possible. Historically, the Perrin process, invented in 1933, is the forerunner of modern secondary steelmaking. Treatment of molten steel with synthetic slag was the approach. Vacuum degassing (VD) processes came in the decade of 1950–1960. The initial objective was to lower the hydrogen content of liquid steel to prevent cracks in large forging-quality ingots. Later on, its objective also included lowering of nitrogen and oxygen contents. Purging with inert gas (Argon) in a ladle using porous bricks or tuyeres (IGP) came later. Its primary objective was stirring, with consequent homogenization of temperature and composition of melt. It offered the additional advantage of faster floating out of non-metallic particles. It was also found possible to lower carbon to a very low value in stainless steel by treatment of the melt with oxygen under vacuum or along with an argon stream. This led to development of vacuum-oxygen decarburization (VOD) and argon-oxygen decarburization (AOD). Synthetic slag treatment and powder injection processes of molten steel in a ladle

were started in late 1960s and early 1970s with the objective of lowering the sulphur content of steel to the very low level demanded by many applications. This led to the development of what is known as injection metallurgy (IM). Injection of powders of calcium bearing reagents, typically calcium silicide, was also found to prevent nozzle clogging by Al2O3 and lead to inclusion modification, which are of crucial importance in continuous casting as well as for improved properties. The growth of secondary steelmaking is intimately associated with that of continuous casting of steel. Up to the decade of the 1960s, ingot casting was dominant. Now, most of world‟s steel is cast via the continuous casting route. The tolerance levels of interstitial impurities and inclusions are lower in continuous casting than in ingot casting, and this has made secondary refining more important. For good quality finished steel, proper macrostructure of the casting is also important, in addition to the impurity level. This requires close control of the temperature of molten steel prior to teeming into the continuous casting mold. In traditional pit side practice, without ladle metallurgical operations, the temperature drop of molten steel from furnace to mold is around 20–40°C. An additional temperature drop of about 30–50°C occurs during secondary steelmaking. Continuous casting uses pouring through a tundish, causing some further drop of 10–15°C. Therefore, provisions for heating and temperature adjustment during secondary steelmaking are very desirable. This has led to the development of special furnaces such as the vacuum arc degasser (VAD), ladle furnace (LF). These are very versatile units, capable of performing various operations.

3.2 Purpose of Secondary Steel Making Secondary steel making processes are adopted primarily to achieve various objectives. The fulfilment of these objectives results in a steel which meets the desired stringent requirements. These objectives also called the functions and goals of secondary steel making include: 1. Control of gases: degassing (decreasing the concentration of hydrogen and oxygen in steel 2. Low sulphur contents (normally less than 0.010 % and to as low as 0.002 % ) 3. Micro cleanliness (removal of undesirable non metallic, primarily oxides and sulphides) 4. Inclusion morphology ( since the steelmakers cannot remove undesirable oxides completely, this step allows steelmakers to change the composition or the shape of the undesired matter left in the steel to make it compatible with the mechanical properties of finished steel) 5. Mechanical properties 6. Homogenization of liquid steel composition and temperature 7. Achievement of correct temperature of liquid steel for subsequent casting 8. Achievement of correct chemical composition by means of trimming addition

3.3 Gases In Steel Impurities like C, Si, Mn, S, P etc are eliminated but gases like oxygen, nitrogen and hydrogen may still remain in solution as deleterious impurities. On solidification, the excess dissolved gases are liberated which may form skin or pin holes, blow holes etc. these cavities are in general detrimental to the mechanical properties of steel. The amount of dissolved gases depends on 

Quality of raw materials used.



Steel making process.



Composition and temperature.

3.3.1

Sievert’s Law

The equilibrium solubility of diatomic gasses like O 2, N2, H2 in steel is given by SIEVERT’S LAW which states that for any diatomic gas dissolved in liquid steel we have the relationship %Gas dissolved=k.

√P

Where P is the partial pressure of the gas in ambient atmosphere k is constant k depends on interaction of the gas atom with iron &other constituents present in steel.

3.3.2

Oxygen in Steel

Oxygen is supplied for refining iron and hence a certain fraction is inevitably left over as dissolved oxygen in liquid steel. Excess oxygen causes defects like blow holes and non metallic inclusions. Oxygen is lowered by deoxidizers like Mn, Si, and Al etc. Through RH degasser, oxygen is removed as CO. Oxygen, as a principle refining agent, plays an important role in determining the final composition and properties of steel. Oxygen dissolved in steel greatly influences the consumption of the deoxidizers and thus affect the quality of steel. The control of oxygen in liquid steel is a prime objective in steelmaking because it enables the desired final chemical composition and solidification structures to be achieved easily. The cleanliness of the steel is improved by lowering its oxygen content. If the oxygen content in the molten steel is sufficiently high during vacuum degassing, the oxygen will react with some of the carbon in the steel to produce carbon monoxide (CO). The evolved carbon monoxide escapes and is removed from the system by the vacuum pump along with the other gases. When used for this purpose, the vacuum degassing process is often referred to as vacuum carbon Deoxidation. It’s difficult and time consuming to produce the steel with the carbon content below 0.03% by conventional steelmaking procedures. However, if for example, the

non-deoxidized molten steel at about 0.04 % carbon is exposed to a vacuum, carbon is readily removed to a level of about 0.01 % by reaction with oxygen in steel.

In non-deoxidized molten steel, the carbon and oxygen contents will approach the equilibrium at a given temperature and pressure according to the following reaction C + O = CO The equilibrium constant is K= pCO/ [%C] [%O] For carbon content below about 0.5 % and at steelmaking temperatures, the product of (%C) (%O) is about 0.002 for one atmospheric pressure of carbon monoxide. If the steel is subjected to lower and lower pressures, the equilibrium between carbon and oxygen will change and they will react in effort to establish a new equilibrium. Carbon monoxide produced by this reaction escapes from the system as a gas, and thus most of the oxygen is no longer available to form non metallic inclusions with other substances that may be later added to the steel. Strong deoxidizer such as aluminium, titanium and silicon, when added to the molten steel are effective in reducing the oxygen content so that carbon can no longer react with oxygen when the steel is vacuum degassed. However the strong deoxidizers form non metallic inclusions as a product of their reactions with oxygen, and these inclusions may become trapped in the steel during solidifications and impair its cleanliness and mechanical properties.

3.3.3

Nitrogen In Steel

Nitrogen may have an undesirable effect on the properties of steel depending on its composition, subsequent treatment and intended use. Nitrogen contents should be as low as possible in low carbon steels intended for deep drawing applications. Low nitrogen contents (0.004% maximum) are particularly important, especially with new sheet steels developed for processing in continuous annealing lines. Nitrogen like hydrogen, also obeys SIEVERT‟S LAW

% [N2] = K

√ pN 2

One would expect to remove nitrogen by reducing the partial pressure of nitrogen above the liquid bath. Unfortunately this does not readily occur. Although some nitrogen is removed from molten steel by inert gas flushing or vacuum degassing, the amount is extremely slight. Instead, nitrogen tends to form stable nitrides that cannot be removed effectively by commercial methods of vacuum treatment. As a result low nitrogen contents are attained mainly by the control of primary steel making practices. Nitrogen in steel comes from atmospheric air, raw material charges, process adopted and purity of oxygen used. RH degasser is adopted to reduce some nitrogen contents as well. Reaction is given below:

2[N] = N2 Where N stands for nitrogen

To get very low nitrogen, vacuum must be very low. Compared to Hydrogen, nitrogen removal rate is low due to low diffusibility.

3.3.4

Hydrogen In Steel

H2 is formed when water vapour comes in contact with steel &slag .The amount of H2 dissolved in steel varies with the partial pressure of H 2, composition of steel & its temperature. The temperature of liquid steel is bounded to drop during vacuum treatment. The more is the surface area of steel & the more the prolonged is the treatment, the more will be the heat loss. The tapping temperature of the steel is generally kept 50~70 ⁰C higher than the steel melting temperature. The degree of vacuum employed depends upon the degree of the degassing required & the cost of generation of vacuum. The degree of degassing increases with the degree of vacuum. Hydrogen is a particularly troublesome gas. It is the cause of bleeding ingots, embrittlement, low ductility and the presence of blow holes. In solid steel it causes internal ruptures called thremal flakes. Until recently effective boiling periods in the steelmaking vessel and the drying of addition agents were necessary precautions taken during steelmaking to limit the amount of hydrogen in the liquid steel. Even with these precautions, after solidification the steel had to be subjected to lengthy and complicated heating and cooling cycles to promote the diffusion of hydrogen that steel might have absorbed. If the hydrogen content of liquid steel exceeds the solubility limit of hydrogen in solid iron, it leads to pinhole formation and porosity in steel. Just a few parts per million of hydrogen in dissolved steel can cause-

3.3.4.1 Hair line cracks (flakes) & Hydrogen embrittlement Hydrogen embrittlement is the process by which various metals, most importantly high-strength steel, become brittle and fracture following exposure to hydrogen. Hydrogen embrittlement is often the result of unintentional introduction of hydrogen into susceptible metals during forming or finishing operations. The mechanism starts with lone hydrogen atoms diffusing through the metal. At high temperatures, the elevated solubility of hydrogen allows hydrogen to diffuse into the metal (or the hydrogen can diffuse in at a low temperature, assisted by a concentration gradient). When these hydrogen atoms re-combine in minuscule voids of the metal matrix to form hydrogen molecules, they create pressure from inside the cavity they are in. This pressure can increase to levels where the metal has reduced ductility and tensile strength up to the point where it cracks open (hydrogen induced cracking, or HIC).

Figure 8 Hydrogen induced cracking

3.3.4.2 Hydrogen blistering

Figure 9 Hydrogen Blistering and Embrittlement

3.3.4.3 Loss of tensile ductility Hair line cracks are formed all through the section and are revealed only after deep etching. Hydrogen is desorbed very slowly even after cooling the steel for days or even weeks. Sufficient time is allowed for hydrogen to diffuse out.

Figure 10 Solubility of Gas Decreases with Decrease In Partial Pressure of Gas

4.Ladle Metallurgy 1) Slag Making/Treatment a) Functions of Slag in Ladle Furnace b) Philosophy of charging lime and deoxidisers for making slag c) Problem of carryover of slag 2) Desulphurization 3) Temperature Control 4) Alloying Additions a) Alloy Addition Calculation b) Cooling Effect of Ferro-Alloys c) Mode of Alloying Additions 5) Homogenization of Temperature and Chemistry of the bath 6) Metallurgical Aspects of Lf for Uninterrupted Casting

4.1 Slag Making/Treatment As the famous saying goes “Take care of slag & steel will take care of itself” I.e. The better will be the slag; the better will be the quality of steel. In Bhilai Steel Plant, we use Oxidising slag in convertor as we seek to remove impurities like carbon, silicon, phosphorous... etc., Whereas after tapping we use Reducing slag as we seek for alloying addition and also sometimes for Desulphurization. In ladle furnace slag plays an important role in Arcing and refining of steel.

4.1.1

Function of slag in Ladle furnace:

a) Slag provides necessary resistance in the bath thus giving voltage drop and increasing temperature of the bath. b) Slag stabilises the arc. c) Reduces noise level during arcing d) A good reducing slag is necessary for good recovery of alloying elements. e) Slag provides necessary atmosphere for Desulphurisation. f)

Prevent from reoxidation of bath.

g) Absorbs Inclusions. h) Prevent form nitrogen pickup.

4.1.2 Philosophy of charging of lime & deoxidisers for making slag After tapping of heat in ladle at convertor, we add lime and deoxidisers in the ladle to make basic reducing slag to ensure full or excellent recovery of alloying elements. When heat comes to Lf operator ensures that a thick slag is present in the heat, if he finds thin slag or week slag then a precautionary measure is taken. Operator adds lime to the heat while purging, and if necessary then deoxidisers are also added. Deoxidisers are added in order to reduce the slag thus alloying additions such as Mn, C, Mo, Ti do not get oxidised and enters directly in the steel. Reducing the oxidation potential of bath also helps in desulphurisation. The use of deoxidisers is generally not necessary and not recommended but, However; use of deoxidisers becomes inevitable if we have some carry over slag from previous heat. This is called the Problem of carry-over of slag.

4.1.3

Problem of carryover of slag

In spite of all care a certain amount if oxidising slag from primary steel making furnace does get entrained and carried over, along with the liquid steel, into the Lf during tapping. This slag always contains some SiO2, besides some FeO and MnO. Aluminium is consumed unnecessarily in reducing the FeO & MnO thus present, during deoxidation. The consumption of Al for this purpose is considered as a waste and a drain on cost of deoxidation. If the refining slag carried over into the Lf there is no alternative but to stand this loss. While deoxidising with Al, the SiO 2 present in the bath whether in slag or in any other source, tends to get reduced as 3(SiO2) + 4Al  2(Al2O3) + 3Si Because of the shift in equilibrium of Si/O in the direction of more silicon dissolved in the bath. This creates problem when silicon specification in the bath is very low. The residual Al in the steel therefore needs to be controlled to minimize reversion in this way.

4.1.4

Use of Synthetic Slag

Synthetic Slag can be used to serve two purposes 1. Deoxidation of Bath 2. For Desulphurization Slags having low content of (FeO+MnO) can be used for deoxidation purpose. The effect of (FeO+MnO) content on O2ppm is shown in fig. below

In Ladle Furnace the Deoxidation is based on principles of diffusion deoxidation combined with intense stirring slag and metal. In practice the area of contact between metal and slag is increased by pouring molten metal from considerable height and at great speed into metal containing synthetic slag. The passage of FeO from metal to slag, [FeO] (FeO), which is slow in normal conditions of diffusion deoxidation is accelerated by intense mixing of molten metal and slag. The basic slag with low content of (Feo+MnO) obtained in Ladle Furnace can be utilized for deoxidation. Their composition may be as folows: CaO 65%, SiO2 13%, Al2O3 2%, CaF2 15%, MgO 4%, FeO