Production of Iron and Steels

Transcription

1 MME 131: Lecture 24 Production of Iron and Steels Prof. A.K.M.B. Rashid Department of MME BUET, Dhaka Topics to discuss 1. Importance of iron and steels 2. The smelting of iron in Blast Furnace 3. The B/F iron and its refinement to produce steel Lec 24, Page 1/14

2 Ferrous alloy: an important engineering material What is ferrous metallurgy? Historically, ferrous metallurgy is the foundation for virtually everything we know about the structure and properties of metals. Process Metallurgy Extraction and refinement Physical Metallurgy Thermodynamics and kinetics of microstructural transformations Mechanical Metallurgy Deformation behavior and failure mechanisms Lec 24, Page 2/14

3 Overview of iron and steel making to manufacture of different steel products Blast furnace plant overview Lec 24, Page 3/14

4 The blast furnace proper Top (charging section) Stack Bosh Hearth Common iron-bearing ores Oxide Ores Stoicheometry Iron Content (wt%) Magnetite Fe 3 O % Hematite Fe 2 O % Ilmenite FeTiO % Limonite FeO(OH) / HFeO % sinter pellets Ore Beneficiation Processes Crushing Screening Blending Grinding Concentrating Classifying Agglomeration Physical form of iron ore nodules briquettes Lec 24, Page 4/14

5 Blast furnace operation Dust Removal Exhaust Gas Furnace Charge (ore, coke, limestone) Preheated Charge Heat Exchanger Fe2O3 CO Fe CO2 Fe2O3 H2 Fe H2O Fe3O4 CO Fe CO2 Fe3O4 H2 Fe H2O FeO & Fe Secondary reduction (~950 0 C) Hot Air Blast CO H 2 FeO+C Fe+CO C+H 2 O CO+H 2 C+ 1 --O 2 CO 2 Primary reduction (~ C) Blast furnace mass balance Raw Materials Ore = 1935 kg Coke = 965 Stone = 480 Preheated Air 7240 m 3, or 3965 kg 200 C 400 C 700 C 900 C 1300 C Blast Furnace Gas 9360 m 3, or 5680 kg used as energy source to preheat cold air, power the blowing engine, and electrification of the plant Dust 90 kg Slag 575 kg aggregate for concrete work making of Portland cement heat insulating materials asphalt roofing Pig iron 1000 kg making of steels and cast irons Lec 24, Page 5/14

6 Blast furnace iron Element Typical Pig Iron (wt%) C Mn Si P < 0.2 S Typical Steel (wt%) < 0.50 < 0.05 < 0.05 Impurity levels must be lowered by oxidation!! Goal of steelmaking process The Steelmaking Processes Overall refinement scheme of B/F iron Reduction of iron and other metallics from ore Resulting pig iron containing high carbon and other impurities Oxidation of C, Si, Mn, P by exposing to oxygen & flux Resulting steel contains high oxygen levels S removed in slag by fluxing Linked to deoxidation Oxygen and other dissolved gases must be removed Vacuum gassing and other methods Ladle additions for fine chemistry control Controls residual O, N, H, P, S and adds alloys (Mn, Nb, V, etc.) Lec 24, Page 6/14

7 Common refinement processes of B/F iron Bessemer Converter Open Hearth Furnace Steelmaking Processes Basic Oxygen Furnace Electric Arc Furnace Two principal steelmaking processes Acid processes Use costly pig iron having low S and P Cheap production process Basic processes Use cheap pig iron having high S and P Costly production process Lec 24, Page 7/14

8 The open-hearth process Large vessel >200 tons Slower process (~6 hr) better chemical control Can handle large scrap The Bessemer process Small furnaces (~20 tons) Liquid pig iron charged Air blown from bottom No fuel required; heat evolved from exothermic oxidation reaction acts as fuel. Very quick operation (~20 min) chemical control difficult Lec 24, Page 8/14

9 The basic oxygen process Top-blown with pure oxygen More efficient heating Very flexible with respect to charge Much larger capacity than Bessemer Much faster than open-hearth The electric-arc process Large scrap Mediumsized scrap Molten iron Oxygen Lance Small scrap Lec 24, Page 9/14

10 Ladle metallurgy Presence of high levels of oxygen (0.04 to 0.1 wt%) results oxide inclusions Lime particle injection - Ladle deoxidation - Purification - Total residuals < 50 ppm Alloying additions also can be made here - Mn, Cr, Mo, V, Nb Ingot casting Freezing times may be very long Lec 24, Page 10/14

11 Killed steel deoxidized with ferrosilicon, pig iron, silico-manganese rimming action chemically suppressed very homogeneous ingot used when structural soundness is critical generally required 0.3 wt% or more carbon large shrinkage cavity or pipe formed semi-killed steel does not exhibit large pipe Capped physical suppression of the rimming action less macrosegregation thinner (if any) rimmed zone blowholes and seams typically used for 0.15 to 0.3 wt% carbon Rimmed high oxygen content (no deoxidation practice) excellent surface quality significant chemical inhomogeneity typically used for < 0.15 wt% carbon Continuous casting Lec 24, Page 11/14

12 Steelmaking in Bangladesh Plant size in this country is so small that they can be categorised possibly as micro steel mills, rather than mini steel mills Steelmaking practice in Bangladesh: very peculiar and unique To be viewed in the context of our proximity to India Chittagong Steel Mills Ltd. Open Hearth Furnace Very small steelmaking units started to grow in the private sector Initially, small electric arc furnace (EAF), mainly imported from India; Induction furnace (IF), also imported mainly from India, replaced them At present almost all steelmaking plants in Bangladesh use IF Bangladesh is probably the only country in the world where all rebars are produced by induction furnace route!! Bar/rod manufacture in Bangladesh 1. Scrap IF LRF Con Cast Billets Rolling Rod/ Bar 2. Scrap IF Con Cast Billets Rolling Rod/ Bar 3. Scrap IF Pencil Ingots Rolling Rod/ Bar 4. Scrap IF Channel Ingots Rolling Rod/ Bar 5. Scrap Rolling Rod/ Bar Lec 24, Page 12/14

13 Induction furnace as a melting unit Garbage in, Garbage out Very little, if any, refining takes place in IF Steel quality is directly related to quality of scrap Inconsistency in input is directly reflected in output Nature of scrap Ship scrap Imported bundle scrap Local scrap Unknown, uncertain composition Variable chemical mix Presence of dirt, grease, oil, tramp elements Lec 24, Page 13/14

14 Solidification / casting of liquid steel Channel Ingot /Pencil Ingots Most of the local producers solidify their molten steel in channel or pencil ingots Such solidification practice is liable to defects and is not allowed in any standard specification for rebar Continuous Casting Acceptable casting processes for billets include continuous casting which is practiced by only a handful of local producers Next Class MME 131: Lecture 25 Classification and Properties of Iron and Steels Lec 24, Page 14/14