Oxygen SteelmakingIntroduction

MATERIALS 3F03M A RC H 2 3 , 2 0 1 5

Hot Metal Chemistry

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Hot Metal is saturated in C, due to hearth conditions Hot metal in coke bed

Typical hot metal chemistry: 4.5 - 5.0 % C 0.3-1.0 % Si 0.1 – 0.7 % Mn 0.05-0.10 % S 0.01-0.08 % P

External desulphurization after BF is typical in industry

Carbon content of hot metal needs to be substantially lowered to create steel

Oxygen SteelmakingRefers to augeneous process for converting hot metal into steel: Top blown

LD (Linz-Donowitz)

BOF (Basic Oxygen Furnace) or BOS

Bottom Blown OBM, Q-BOP

Combined Blowing KOBM, LBE

4% C to less than 0.1 % C in ~16 minutes (~30 minutes total)

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Process Sequence

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BOF BlowUsually 16-25 minutes

Pure oxygen blown in a supersonic rates generates slag/metal emulsion for high reaction rate

~100% oxygen utilization

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Process ReactionsThere are three major stages in the BOF process:

1) Slag Formation 2) Constant Decarburization Rate 3) Carbon mass transfer control

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Slag FormationSoft blowing to start to make a SiO2-FeO rich slag (Fayalitic-type)

Once the slag is formed, harder blowing creates slag-metal emulsion

Oxidation at the end

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Mass and Energy BalanceMore heat generated from

C Oxidation Si Oxidation

Than required for:◦ Heating metal◦ Heating and melting slag

Coolants added: Scrap (70/30 hot metal ratio common in NA) Iron ore

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Mass and Energy Balance

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Bottom BlowingMost BOF vessels have some form of bottom stirring to improve mixing: C & O closer to equilibrium Better dephosphorization Quicker slag formation Less iron oxide in slag for better iron and

alloy yield

Looking at mixing times, a small amount of bottom gas is almost like total bottom flow LH is lance height QB and QT are bottom and top flow rates

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Bottom BlowingLower iron yield loss (as FeO in the steelmaking) associated with bottom blowing C & O closer to equilibrium More decarburization before entering

carbon mass transport control regime

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OS ReactionsOxygen is the driver for most reactions

Controlled by oxygen potential Involve oxygen directly

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OS ReactionsOxygen is the driver for most reactions

Controlled by oxygen potential Involve oxygen directly

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Oxidation of SiliconRate Controlled by mass transfer of silicon in metal: [Si] + 2(FeO) = (SiO2) + 2[Fe]

Shows first order behavior until Si content <0.05% Si

Silicon oxidation largely completely in early stages of the blow

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Oxidation of ManganeseBy direct oxidation at hot spot, and:

[Mn] + [O] = (MnO)

[Mn] + (FeO) = (MnO) + Fe

Second reaction predominant later in blow

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Oxidation of PhosphorousP2O5 is acidic, so basic slags are required

Requires oxidizing conditions

Bottom blown processes closer to slag-metal equilibrium Bottom lime injection with O2

Initial slag has high FeO content

Mid-blow: FeO content decreases, more reducing conditions in slag Possibility for P reversion back to steel

End blow: More oxidizing conditions, opportunity for further phosphorous oxidation

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Sulphur RemovalGenerally poor because of oxidizing conditions

S partition is worse with acidic slags

Better to maximize desulphurization in the BF, use external desulphurization facility

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Critical Carbon ContentOnce carbon mass transfer control regime commences: Supply of C to reaction sites is not

sufficient to consume O Oxygen dissolution in steel substantially

increases Oxidation of Fe increases, higher FeO

content in slag

Carbon content where constant decarburization regime ends is called Critical Carbon Content

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Critical Carbon ContentCarbon content where constant decarburization regime ends is called Critical Carbon Content

1 – Slag Formation regime

2- Constant Decarburization rate regime

3- Carbon Mass transport control

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Critical Carbon ContentCarbon content where constant decarburization regime ends is called Critical Carbon Content

Options to reduce critical carbon content: Slower oxygen blowing (productivity

impact)

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Critical Carbon ContentTo reduce carbon content lower than the critical carbon content means that higher yield loss of Fe to slag must be accepted Increased oxygen dissolution into

steel

Other options include vacuum processes for ultra-low carbon grades

Reminder: Bottom blowing practice means lower oxidation of metal for a given carbon content

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References 1 Bramha Deo and Rob Boom, Fundamentals of Steelmaking Metallurgy, Prentice Hall, 1993, Chapters 5.1-5.2 and 6.1-6.6

2 Geerdes et Al, Blast Furnace Ironmaking: An introduction, 2009

Much of the content is taken directly from or adapted from Materials 4C03 Oxygen Steelmaking slides prepared Dr. Gord Irons.