Decarburization and Desulphurization in Steel Making

Carbon is the single most important alloying element for steel. Below is a table of typical carbon levels, properties and applications.

The 'hot metal' that arrives from the blast furnace still contains about 4.7 %C - i.e. far in excess of the levels given above. Basic Oxygen Steelmaking involves the reduction of most of the carbon, silicon, manganese and phosphorus in the hot metal by blasting a supersonic jet of oxygen at the charge. The steel at tap will contain between 0.03-0.07 %C.

For many grades/applications therefore, carbon must be added to the steel again during secondary steelmaking.

However for many demanding grades, a further reduction in carbon is required. In this section you will learn how this is achieved.

Decarburization Chemistry

Carbon removal from the steel is achieved through reaction with dissolved oxgygen to form gaseous carbon monoxide bubbles, i.e.

[C] + [O] → CO (Equ 1)

Applying the mass action law:

KCO = pCO / aC aO (Equ 2)

where KCO is the equilibrium constant, pCO is the CO pressure, and aC and aO are the activities (or concentrations) of C and O respectively.

Ultra Low Carbon steel

Low Carbon steel Medium Carbon steel

High Carbon steel

Carbon <0.01 %C <0.25 %C 0.25-0.7 %C 0.7-1.3 %C Properties • High strength

• High toughness • Good formability

• Reasonable strength• High ductility • Excellent fabrication

• High strength • High toughness

• Very hard • Low toughness

Treatment Quenched Annealed, normalized Quenched and tempered

Quenched and tempered

Applications Sheet materials for automobiles, pipe-lines, etc.

Structural steels, sheet materials for automobiles, white goods, etc.

Shafts, gears, connecting rods, rails, etc.

Springs, dies, cutting tools, etc.

The equilibrium constant itself is given by:

log KC-O = −(1168/T) + 2.07 (Equ 3)

where T is the absolute temperature, in K.

Equations (2) and (3) tell us that the reaction will be influenced by both pressure and temperature.

Effect of Pressure and Temperature I

By re-arranging the mass action law, we can write:

[O] = pCO / [C] KCO

Explore this relationship between the carbon and oxygen concentrations using the interactive graph.

How does varying temperature affect [C]? Would you chose a higher or lower decarburization temperature?

How does varying pressure affect [C]? Would vacuum treatment aid decarburization?

Which of the two parameters has the greatest effect on [C]?

Effect of Pressure and Temperature II

If the pressure above the steel is reduced to 0.1 atmosphere, pCO will also drop to 0.1 atmosphere and the equilibrium [C].[O] product will decrease accordingly.

A driving force for carbon monoxide evolution from the steel now exists - C and O will be be removed in the ratio 12:16, as determined by their respective atomic masses.

The injection of additional oxygen into the steel can be used to produce very low carbon levels (e.g. <0.001%).

Degassing Processes

The most usual forms of vacuum treatment are:

• tank degassing, and • recirculation degassing

The main process control parameters are pressure and Ar-stirring.

Tank Degasser

Recirculation degasser

Why Control Sulphur Content? Sulphur is present in solid steel as manganese sulphide (MnS) inclusions. Their volume fraction, size, shape and distribution depend on the sulphur content, oxygen content, solidification rate, degree of hot and cold deformation and hot working temperatures. These MnS inclusions have several effects on the processing and properties of steel. Mainly these effects are detrimental, as these inclusions are more plastic than steel and, hence, during deformation they act as crack initiation sites and zones of weakness. Sulphur is detrimental to ductility, toughness, formability, weldability and corrosion resistance. However, sulphur is beneficial to machinability. Sufficient manganese must be present in the steel to prevent the formation of iron sulphide, which is highly detrimental to hot workability and leads to severe cracking during hot rolling (hot shortness).

Sulphur Specifications

The most demanding applications of steel require the sulphur 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 sulphur levels are required.

Removal of Sulphur

The sulphur content from the BOS or EAF is typically 0.01 - 0.02%. In order to satisfy the low sulphur specifications, it is necessary to remove sulphur from the steel during secondary steelmaking. Sulphur is removed by slag-liquid metal reactions under reducing conditions in which sulphur is transferred from the steel through the slag-metal interface and into the slag.

The basic chemical reaction in desulphurization is:

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

( ) in slag and [ ] in steel.

The partitioning (Ls) between the sulphur in the slag (S) and steel [S] is given by:

where Cs is the sulphur capacity of the slag and a0 is activity of oxygen in the steel.

Slag is an ionic solution of various oxides and fluxes. The type of slag required for these reactions is quite different from the highly oxidizing slag produced in the BOS/EAF. An important part of secondary steelmaking is the creation of the required sulphur reducing slag.

Source: Steeluniversity.com