Oxygen Steelmaking Introduction MATERIALS 3F03 MARCH 23, 2015

Hot Metal Chemistry 2 Figure Source: 2  Hot Metal is saturated in C, due to hearth conditions  Hot metal in coke bed  Typical hot metal chemistry:  % C  % Si  0.1 – 0.7 % Mn  % S  % P  External desulphurization after BF is typical in industry  Carbon content of hot metal needs to be substantially lowered to create steel

Oxygen Steelmaking  Refers 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) 3 Figure Source: 1

Process Sequence 4 Figure Source: 1

BOF Blow  Usually minutes  Pure oxygen blown in a supersonic rates generates slag/metal emulsion for high reaction rate  ~100% oxygen utilization 5 Figure Source: 1

Process Reactions  There are three major stages in the BOF process:  1) Slag Formation  2) Constant Decarburization Rate  3) Carbon mass transfer control 6 Figure Source: 1

Slag Formation  Soft 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 7 Figure Source: 1

Mass and Energy Balance  More 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 8 Figure Source: 1

Mass and Energy Balance 9 Figure Source: 1

Bottom Blowing  Most 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 10 Figure Source: 1

Bottom Blowing  Lower 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 11 Figure Source: 1

OS Reactions  Oxygen is the driver for most reactions  Controlled by oxygen potential  Involve oxygen directly 12 Figure Source: 1

OS Reactions  Oxygen is the driver for most reactions  Controlled by oxygen potential  Involve oxygen directly 13 Figure Source: 1

Oxidation of Silicon  Rate 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 14 Figure Source: 1

Oxidation of Manganese  By direct oxidation at hot spot, and:  [Mn] + [O] = (MnO)  [Mn] + (FeO) = (MnO) + Fe  Second reaction predominant later in blow 15 Figure Source: 1

Oxidation of Phosphorous  P2O5 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 16 Figure Source: 1

Sulphur Removal  Generally poor because of oxidizing conditions  S partition is worse with acidic slag s  Better to maximize desulphurization in the BF, use external desulphurization facility 17 Figure Source: 1

Critical Carbon Content  Once 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 18 Figure Source: 1

Critical Carbon Content  Carbon 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 19 Figure Source: 1

Critical Carbon Content  Carbon content where constant decarburization regime ends is called Critical Carbon Content  Options to reduce critical carbon content:  Slower oxygen blowing (productivity impact) 20 Figure Source: 1

Critical Carbon Content  To 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 21 Figure Source: 1

References 1 Bramha Deo and Rob Boom, Fundamentals of Steelmaking Metallurgy, Prentice Hall, 1993, Chapters and 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.