Why Control Why Control Sulfur Sulfur Content?Content?

SulfurSulfur is present in solid steel as is present in solid steel as manganesemanganese sulfide (MnS)sulfide (MnS) inclusions inclusionsMnSMnS inclusions affect the processing and inclusions affect the processing and properties of steelproperties of steelEffects are Effects are detrimentaldetrimental, as these inclusions , as these inclusions areare more plastic than steel and, hence, more plastic than steel and, hence, during deformation they act as crack during deformation they act as crack initiation sites and zones of weaknessinitiation sites and zones of weakness

Their volume fraction, size, shape and Their volume fraction, size, shape and distribution depend on the sulfur content, distribution depend on the sulfur content, oxygen content, solidification rate, degree of oxygen content, solidification rate, degree of hot and cold deformation and hot working hot and cold deformation and hot working temperaturestemperatures

Sulfur is Sulfur is detrimentaldetrimental to to ductilityductility, , toughnesstoughness, , formabilityformability, , weldabilityweldability and and corrosion corrosion resistanceresistancesulfur is sulfur is beneficialbeneficial to to machinabilitymachinability

Sufficient manganese must be present in the Sufficient manganese must be present in the steel to prevent the formation of iron sulfide, steel to prevent the formation of iron sulfide, which is highly detrimental to hot workability which is highly detrimental to hot workability and leads to severe cracking during hot and leads to severe cracking during hot rolling (hot shortness).rolling (hot shortness).

Elongated MnS inclusions

Sulfur Specifications

The most demanding applications of steel require the sulfur levels to reduce to very low levels (<0.001% S) in order to achieve the required combination of strength, ductility, formability and weldability(For applications which are extensively machined during manufacturing higher sulfur levels are required)

%S

0.300 Hydraulic connectors0.070 Crankshaft0.030 Gear0.015 Spanner0.0050 High Strength Car Wheel0.0015 Sub-frame assembly0.0010 Linepipe for Sour service0.0010 Aircraft undercarriage

Removal of Sulfur Removal of Sulfur

The sulfur content from the BOS or EAF is The sulfur content from the BOS or EAF is typically 0.01 - 0.02% typically 0.01 - 0.02% To satisfy the low sulfur specifications is To satisfy the low sulfur specifications is necessary to remove sulfur from the steel necessary to remove sulfur from the steel during secondary steelmakingduring secondary steelmaking

Sulfur is removed by slag-liquid metal Sulfur is removed by slag-liquid metal reactions under reducing conditions in which reactions under reducing conditions in which sulfur is transferred from the steel through sulfur is transferred from the steel through the slag-metal interface and into the slagthe slag-metal interface and into the slag

The basic chemical reaction in desulfurization The basic chemical reaction in desulfurization is:is:

3(CaO) + 2[Al] + 3[S] 3(CaS) + (Al2O3)3(CaO) + 2[Al] + 3[S] 3(CaS) + (Al2O3)

( ) in slag and [ ] in steel.( ) in slag and [ ] in steel.

CCss : The sulfur capacity of the slag : The sulfur capacity of the slag aa00 : Activity of oxygen in the steel : Activity of oxygen in the steel

The partitioning (LThe partitioning (Lss) between the sulfur in the ) between the sulfur in the slag (S) and steel [S] is given by:slag (S) and steel [S] is given by:

Slags for Desulfurization Slags for Desulfurization Slag : An ionic solution of various oxides and Slag : An ionic solution of various oxides and

fluxesfluxesSeveral functions :Several functions :

Protects liquid steel from reactions with Protects liquid steel from reactions with the atmospherethe atmosphere

reducing pick-up of nitrogen and hydrogenreducing pick-up of nitrogen and hydrogen thermal insulationthermal insulation It It reacts with refractories causing reacts with refractories causing

refractory wearrefractory wear reservoir for inclusions (refining medium reservoir for inclusions (refining medium

for desulfurization, deoxidation and for desulfurization, deoxidation and dephosphorization )dephosphorization )

The composition of the slag during The composition of the slag during secondary steelmaking has to be controlled secondary steelmaking has to be controlled very carefully to achieve the required very carefully to achieve the required inclusion content and steel compositioninclusion content and steel composition

Slag important factors controlling the Slag important factors controlling the reactions with steel and refractorie :reactions with steel and refractorie :

Density, volume (depth), melting point, Density, volume (depth), melting point, viscosity and interfacial energy of the slagviscosity and interfacial energy of the slag

Desulfurization Slag Desulfurization Slag

Slag from the primary steelmaking furnace Slag from the primary steelmaking furnace (BOS/EAF) :(BOS/EAF) : 45% CaO - 23% FeO - 15% SiO45% CaO - 23% FeO - 15% SiO22 - 8% MnO - 7% - 8% MnO - 7% MgO 2% PMgO 2% P22OO55

Basic slag contains free oxygen ions Basic slag contains free oxygen ions encouraged by the presence of CaO and MgOencouraged by the presence of CaO and MgO

Acid slags deficient in OAcid slags deficient in O22 and contain SiO and contain SiO22 and and PP22OO55

The basicity (B) of slag is defined as: The basicity (B) of slag is defined as:

A A high slag basicityhigh slag basicity is required for is required for desulfurizationdesulfurization

Reference to the CaO-AlReference to the CaO-Al22OO33-SiO-SiO22 at 5% MgO at 5% MgO ternary diagrams at 1600 °C and 0.03% Al, ternary diagrams at 1600 °C and 0.03% Al, with a superposition of the sulfur partition with a superposition of the sulfur partition ratio (Lratio (Lss) values reveals the slag ) values reveals the slag compositions suitable for high degrees of compositions suitable for high degrees of desulfurization.desulfurization.

Kinetics of Desulfurization I Kinetics of Desulfurization I

Reaction has to occur quickly to be Reaction has to occur quickly to be economically viable economically viable

These factors encourage fast desulfurization :These factors encourage fast desulfurization : High sulfide capacity of slagHigh sulfide capacity of slag High temperatureHigh temperature Stirring of liquid metalStirring of liquid metal VacuumVacuum Low viscosity Low viscosity ( The most important factor )( The most important factor )

Kinetics of Desulfurization II Kinetics of Desulfurization II

The degree of desulfurization is given by:The degree of desulfurization is given by:

wherewhere

kkss ~ 0.0013 ε0.25 for ε  < 60 W tonne−1 ~ 0.0013 ε0.25 for ε  < 60 W tonne−1 kkss ~ 8.1 × 10−6 ε2.1 for ε > 60 W tonne−1 ~ 8.1 × 10−6 ε2.1 for ε > 60 W tonne−1

SStt Sulfur at time t in steel Sulfur at time t in steelSS00 Sulfur at time 0 in steel Sulfur at time 0 in steelKKss rate constant rate constant t timet timeLLss Sulfur coefficient (%S)/[%S] Sulfur coefficient (%S)/[%S]WWmm Mass of steel Mass of steelWWs s Mass of slag Mass of slagε stirring powerε stirring power

Stirring energy / W Stirring energy / W tonne-1tonne-1

Other Factors : Refractories Other Factors : Refractories

Which type of ladle gives best Which type of ladle gives best desulfurization?desulfurization?Magnesite……….Yes ,but expensive Magnesite……….Yes ,but expensive Fireclay…………..No ( Fireclay…………..No ( Fireclay is Silica rich )Fireclay is Silica rich )Dolomite………….Yes Dolomite………….Yes ( ( Dolomite is lime rich )Dolomite is lime rich )

High slag basicity is needed for desulfurization → basic refractory is High slag basicity is needed for desulfurization → basic refractory is neededneeded

Ladle Stirring Ladle Stirring

There are several techniques for stirring the There are several techniques for stirring the liquid metal in the ladle.liquid metal in the ladle.

Desulfurization: Self Assessment Exercise Desulfurization: Self Assessment Exercise

Addition should be made for Addition should be made for desulfurization :desulfurization :

CaO – Al – AlCaO – Al – Al22OO3 3 – CaF– CaF2 2 - Ar- Ar

Calcium Injection I Calcium Injection I

Calcium is a powerful desulfurizer and Calcium is a powerful desulfurizer and deoxidizer deoxidizer Added to the ladle as a cored wire containing Added to the ladle as a cored wire containing calcium metal - calcium metal - CaSiCaSi - - CaSi powderCaSi powder

( Calcium is rarely used as a deoxidizer in its own right as it is ( Calcium is rarely used as a deoxidizer in its own right as it is more more expensiveexpensive than other deoxidizers ) than other deoxidizers )

Calcium boiling point :1491 °C Calcium boiling point :1491 °C Is lower that the melting point of steel Is lower that the melting point of steel → → Is a vapor Is a vapor →→ Introduce calcium or its compounds as Introduce calcium or its compounds as deep into the ladle as possible when added deep into the ladle as possible when added to steel to steel →→ Ferrostatic pressure prevents it from Ferrostatic pressure prevents it from vaporizingvaporizing

Calcium InjectionCalcium Injection

Calcium Injection IICalcium Injection II

Calcium effects :Calcium effects : Modifies the shape of sulfide inclusions at Modifies the shape of sulfide inclusions at low sulfur levels low sulfur levels Enhances machinabilityEnhances machinability Makes them more globularMakes them more globular Reduce the detrimental effect of sulfur on Reduce the detrimental effect of sulfur on ductility and toughnessductility and toughness modifies alumina inclusions to a molten calcium modifies alumina inclusions to a molten calcium aluminate phase which prevents nozzle blockage aluminate phase which prevents nozzle blockage by aluminaby alumina