Secondary Steel Making

Presented by :-

1. Abhishek Kumar (12MT10003)

2. Jagdip Singh (12MT10015)

IIT Kharagpur

(Metallurgical and Materials Engineering)

Secondary steelmaking is a critical step in the steel production process between the Primary processes (Basic Oxygen Furnace or Electric Arc Furnace) and Continuous Casting.

Some elements are added and some have to be removed during secondary steelmaking in order to fine-tune the composition of the steel to meet the specification and the customer’s requirements.

The temperature, internal quality and the inclusion content of the steel also have to be carefully controlled during secondary steelmaking.

Stirring and Homogenization

Ladle stirring is an essential operation during secondary steelmaking in order to:• Homogenize bath composition• Homogenize bath temperature• Facilitate the slag metal interaction essential for the process such as de-sulphurisation• Accelerate the removal of inclusion in the steel

In practice, stirring is achieved by:

1. Argon Bubbling - through the liquid steel, either via a submerged lance, or by porous plugs in the bottom of the ladle

2. Electromagnetic Stirring (EMS) - uses the electromagnetic induction principle to give efficient mixing of the entire molten steel while maintaining an unbroken protective slag layer in the ladle.

Advantages-• Simple• good slag-metal contact, therefore good sulfur and phosphorus removal• safe ladle lining• lower capital and running costs than EMS

Disadvantages –• turbulent surface• nitrogen and hydrogen pickup• alloy oxidation and loss• reduced alloy cleanness• stirring only at stir station• slightly lower ferrostatic pressure effect than bottom plug injection• significant basal 'dead zone' (dead-zones are the regions of the steel

bath where there is little or no circulation)

Advantages –• minimized basal 'dead zone' (dead-zones are the regions of the steel bath

where there is little or no circulation)• uniform dispersed stirring action• maximized ferrostatic pressure effect• strong off-center circulation• excellent slag-metal contact, therefore good sulfur and phosphorus removal• moderate nitrogen and hydrogen pickup• cleaner steel• ability to stir ladle continously anywhere• lower capital and running costs than EMS

Disadvantages –• heavy localized refractory wear• more rigorous bricking regime• danger of 'breakout'

Adavantages –• minimal splash, meaning less exposure of steel to atmosphere and

reduced 'freeboard' required (i.e. the height of the ladle wall above the bath surface)

• smooth uniform flow• cleanest steel• lowest nitrogen and hydrogen pickup• reversible flow - useful when making alloy additions• maximum safety and ease of bricking• reduced refractory wear• ability to stir ladle continously using EMS ladle car• low alloy loss and oxidation

Disadvantages –• high capital and running costs• poor slag-metal contact, so lower sulfur and phosphorus removal than

argon bubbling

Homogenization of bath temperature and composition by argon bubbling is primarily caused by the dissipation of the buoyant energy of the injected gas.

The following equations are used to calculate the stirring power and the mixing time (time to achieve 95% homogenization)

Mixing Time Stirring Power

P0 = gas pressure at the bath surface, atm

H = depth of gas injection, atm

T = bath temperature, K

The ladle furnace is used to heat the steel . Argon bubbling is applied to homogenize the steel composition and temperature.

Heating of up to 3°C per minute is achieved by a set of graphite electrodes, which are lowered into the slag layer, just above the molten steel surface.

Another purpose of the ladle furnace is to act as a holding furnace between the BOF and the continuous casting machine.

Ladle Furnace

LADLE FURNACE

Electrode –Heating of up to 3°C per minute is achieved by a set ofgraphite electrodes, which are lowered into the slag layer, just above the molten steel surface.

Additional Chute -It is used to add alloying elements and/or slag components.

Water cooled roof –Water cooled parts of the roof provide a quick and efficient cooling.

Fume offtake -Fumes formed during the operation in the LF are extracted through the cover.

Degassing

The tank degasser is used to remove gaseous elements and sulfur from the steel.

The removal of sulfur is achieved through slag-metal reactions, which arepromoted by strong argon 'flushing' (bubbling) within the vacuum envelope.

Fe methods of degassing are as follow –Ladle degassing

VD,VAD,VOD

Stream degassing

Circulation degassing

RH (Ruhrstahl Heraus)DHRH-OB

Circulation degassing

DH Process

RH Process

Recirculation Degasser (RH-OB Process)

The recirculation (RH) degasser is used for the removal of carbon and other impurity elements.

It comprises a pair of 'snorkels' which are lowered into the liquid steel. The pressure in the vessel is reduced to about 1-3 torr(1 torr = 1 mmHg).

Argon is injected through tuyeres in one of the snorkels,forcing the steel up into the unit and out again through

the other snorkel.

In some units, oxygen is injected through a lance in order to assist decarburization.

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Stream Degasser

In stream degassing, molten steel is poured into another vessel which is under vacuum. Sudden exposure of molten stream in vacuum leads to very rapid degassing due to increased surface area created by breakup of stream into droplets.

This process helps the hydrogen dissolved in steel, to be evacuated by the vacuum pump.

The major amount of degassing occurs during the fall of molten stream. Height of the pouring stream is an important design parameter.

SN Unit Purpose

1 VAD Heating, Degassing

2 VD Degassing

3 VOD,

AOD

Chemical heating, decarburization

4 LF Heating, Homogenization,

desulphurization, deoxidation,

alloying

5 IM Deoxidation, de-sulphurization,

inclusion modification etc.

Ladle Degassing

Methods involved in this are as follow -

VOD process VAS Process

CAS-OB (Composition Adjustment by Sealed Argon Bubbling – Oxygen Bubbling)

The CAS-OB (Composition Adjustment by Sealed Argon Bubbling - Oxygen Blowing) allows alloy additions to be made under an inert argon environment.

The unit is lowered onto the liquid steel over an 'eye' in the slag formed by argon bubbling.

In particular, it allows the simultaneous addition of Al and O2 gas blown through a top lance. These react to form Al2O3 plus considerable exothermic heat energy - the steel temperature can be raised by up to 10°C per minute.

The CAS-OB is therefore used for CHEMICAL REHEATING. Note however that the Al2O3 must subsequently be removed.

Deoxidation

Deoxidation is the process to eliminate oxygen, which may be free dissolved in steel or may have reacted to form various oxides.

Aluminum is a very powerful deoxidizing agent and controls the oxygen activity in the liquid steel [1] .

Using the equilibrium constant K to determine the composition of the Al-O system at equilibrium, the Al-O equilibrium curves are plotted. As can be seen in the graph below, deoxidation with aluminum is more efficient at lower temperatures.

De-oxidation hierarchy: Ca > Al > Si > Mn

Larger particles floats up much faster than smaller particles

Gas agitation helps in inclusion removal

Liquid product of de-oxidation is helpful

Amount of Oxygen present can be determined with the help of Cellox Probe.

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Decarburization

The removal of dissolved carbon from the steel during vacuum degassing arises from the following reaction: [C] + [O]→CO (g)

Using the equilibrium constant K to determine the composition of the C-O system at equilibrium, the C-O equilibrium curves are plotted.

For RH degassers the rate constant for the carburization is given by the relationship [1] while the time needed to decarburize is given by relationship [2]:

Desulpurisation

Desulfurization in the ladle is achieved by:

Adding a synthetic CaO based desulfurizing slag at vessel tapping;

Aluminum deoxidizing the steel to very low oxygen activity (otherwise the Al will react preferentially with O);

Vigorously stirring the steel in the tank degasser in order to thoroughly mix the metal and slag.

The chemical reaction of the desulphurization process in the ladle is: - 3(CaO)+2[Al]+3[S] ---- 3(CaS)+(Al2O3)

Injection Metallurgy

It’s a technique evolved for adding reactive and volatile material.

Al2O3 helps in oxygen removal but it cause nozzle clogging, owing to its dendritic structure. As to add Ca , we use injection metallurgy.

Mainly calcium, its alloy and compounds are added Ca-Si,CaO+Al2O3+CaF2, CaC2

Purpose:

Deep Deoxidation

Desulphurization

inclusion modification

1. Fine inclusions which could not be eliminated are modified

2. Liquid product (C12A7, C3A) avoids nozzle clogging

3. Spherical deformable product (C12A7 with sulphide ring of Mn and Ca) improves mechanical properties

Inclusion modification

Techniques involved for injection are –

1. Powder Injection

2. Wire Addition

The aim of powder Injection in ladle furnace is to reduce sulphur content in molten steel.

A desulphurizing agent in the form of a fine powder, is introduced in the steel bath that is stirred with argon. As previously said, argon bubbles promote desulphurization, along with thermal and chemical homogenization of the bath.

Powder Injection

Need to desulpurise steel

Effective desulphurization allows to achieve high quality steel with ultra low concentrations of sulphur and decrease sulphide inclusions, thus improving the steel toughness properties.

WIRE FEEDING

A number of ferroalloys, aluminum and calcium additions can be made in the form of cored-wires - i.e. where the additive is encased in a steel sheath.

This addition technique was primarily developed for calcium additions, since the boiling point of Ca (1491°C) is below the bath temperature.

Component of ladle furnace

Wire feeding is also useful for additions that:

• are less dense than molten steel and might otherwise float to the surface;• have limited solubility;• have a high vapour pressure;• have a high affinity for oxygen;• are very expensive and/or added in very small quantities;• are toxic;

Aluminium is often added by wire feeding to improve recovery rate, control of Al content and improve steel cleanness.

1. Speed of wire additionThere exists an optimum speed

2. Bath superheatHigher the bath superheat lower is the melting time of the wire

3. Grade of steelCored wire penetrates more in high carbon heats

Factors affecting the core wire addition -

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