Materials engineering. Iron and steel making

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1 Materials engineering Iron and steel making

2 Metals: rarely exist in pure state mostly in ores Ore: Metallic and other compounds, mostly oxides Metallic content: Iron ores: 30-70% Fe Copper ores: % Cu Molybdenum: % Mo 4 basic way to gain the metallic parts from ore: Reduction by carbon Electrolytic way Metallotermical process Dissociation costs

3 1) Reduction by carbon MeO + C Me + CO FeO + C [Fe] + {CO} molten metals gas 2) Electrolytic way Al 2 O 3 Al O 2- on the cathode: Al e - Al 3) Metallothermical process TilCl 4 + 2Mg [Ti] + 2MgCl 4) Dissociation MeX [Me] + [x] only at high energy level

4 Iron and steel

5 Iron and steel making Blast furnace Foundry Steel making plant Foundry

6 Production of molten steel

tailings: silicates, sand, other non ferrous MnO, Al 2 O 3, P 2 O 5, etc cost cost

7 Iron producing processes Purpose: Iron ore Pig Iron ore types: Fe 3 O 4 magnetite ~70% Fe Fe 2 O 3 hematite ~70% Fe FeCO 3 siderite ~50% Fe Concentration of ore + tailings: silicates, sand, other non ferrous MnO, Al 2 O 3, P 2 O 5, etc cost cost of pig iron cost of concentrating cost of blast furnace 30 ~ 70 % Fe % in ore magnetite

8 Blast Furnace Plant

10 Blast Furnace Plant Tasks 1) Reduction of the ore 2) Extraction of tailings 3) Melting separation of the molten iron from the molten tailings (spec. weight difference) Charge: Ore + Coke + Limestone Dimension of the BF: Diameter: 4-10 m Height: m Volume: m 3 For 1000 t of iron: 2000 t ore t coke t limestone + ~ 4000t hot air

11 Processes in blast furnace Charge moves down (6-8 hours) - Preheating by gas: coke burns more efficient Formation of CO CO reacts with iron ore - Coke reduces CO 2 in the gas C + CO 2 2CO - CO reduces the surface of the iron ore. Indirect reduction FeO + CO Fe + CO 2 - Slag producing by limestone. CaCO 3 CaO + CO 2 MgCO 3 MgO + CO 2 - In the bosh the coke burns C + O 2 CO 2 + Heat - The coke reduces the molten ore. Direct reduction FeO + C Fe + CO - Molten limestone + other slag components produce eutectic slag Slag floats over molten iron

12 Processes in blast furnace thermodynamics C+O 2 CO 2 Gibbs free energy Reduction of FeO from 690 C

13 Processes in blast furnace Carbon reduces the oxides: FeO + C Fe + CO MnO + C Mn + CO SiO 2 + 2C Si + 2CO P 2 O 5 + 5C 2P + 5CO SO 2 + 2C S + 2CO alloying elements impurities in molten iron gas In BF carbon can reduce S, P, Cr, Mn, Si 70-90% and Ti 10-20% Sulfur and phosphorous are harmful in pig iron, and they must be removed.

14 Processes in blast furnace Desulfurization FeS + CaO FeO + CaS in molten iron in molten slag Dephosphorization P 2 O 5 + 5FeO + 5C + 4 CaO CaO 4 P 2 O 5 + 5Fe + 5CO in molten iron in molten slag in molten iron gas Result: pig iron

Product of blast furnace At the bottom of the BF: Slag on the top Molten iron on the bottom with~4% C Near to eutectic composition Taping at different heights: different composition different purpose

16 Product of blast furnace At the bottom of the BF: Slag on the top Molten iron on the bottom with~4% C Near to eutectic composition Taping at different heights: different composition different purpose C% Mn% Si% S% P% for casting 3-4 < 1 < 4 < 0.1 < 0.1 for steel with Bessemer method ~ 3 < 0.1 < 0.1 for steel with Thomas method ~ 2 < 0.1 < 0.1 for steel with Siemens-Martin method ~ 1 < 0.1 < 0.1

17 Product of blast furnace Taping:

18 Product of blast furnace Metallurgy

19 Steel making Purpose Pig iron steel by fire refining treatments that decrease the C content and impurities. Main steps 1) Charging 2) Oxidation decreasing C content 3) Increasing temp. with decreasing C% the T melt increases! 4) Deoxidation decrease FeO and O in molten steel 5) Alloying 6) Casting, solidification casting of ingots or continuous casting of bars and billets

20 Steel making Processes Siemens Martin (open hearth furnace) Bessemer converter process Thomas converter process Oxygen converter process (Linz-Donawitz process - LD) Electric arc steel furnace

21 Siemens Martin process (1864) Charging pig iron+scrap pig iron + ore Capacity t 6-12 h Too expensive carbon 0,3 %/hour burns out

22 Siemens Martin process (1864)

23 Bessemer process (1856) Charging molten iron ºC ~3% Si Capacity 5-60 t min First converter method No external heat Acidic lining (slag react.) Si + O2 SiO 2 from the air

24 Bessemer process (1856) During the blow C, Si, Mn % decreases. % ºC 1700ºC 1250ºC 4% 3% C Si Mn O 1% N 15 min. blowing time

Thomas converter process (1878) Charging molten iron 1210-1250ºC ~2% P No external heat

25 Thomas converter process (1878) Charging molten iron ºC ~2% P No external heat Similar to Bessemer, but basic lining for slag reaction. 4 P +5 O 2 2 P 2 O 5 from the air

26 Oxygen-converter process (LD) Charging molten iron ~3% C ~0.5% Mn ~1% Si ~0.1% P, S Capacity t No external heat To avoid overheating when blowing iron ore or scrap are changed. Limestone is changed for desulfurization & dephosphorization.

27 Variants of Oxygen-converter process OLP process oxygen limestone powder oxygen & CaO powder is blown through the lance AOD process argon oxygen decarbonizing oxygen & argon is blown through the lance Mixed gas system decreased partial pressure of oxygen C% decreases up to 0.002% C e.g. for stainless steels

28 Electric Arc Steel Furnace Charging scrap + solid pig iron ~3% C ~0.5% Mn ~1% Si ~0.1% P, S Capacity t For high grade steels T > 2500ºC intensive reaction, N2 dissociation

29 Electric Arc Steel Furnace Charging scrap

30 Electric Arc Steel Furnace during work

31 Electric Arc Steel Furnace Electrode in the furnace

32 Steel making - oxidization Purpose decrease C% and oxidize the impurities (S, P) In open hearth and electric arc furnace C + FeO Fe + CO from scrap or iron ore turbulence in the charge In air or oxygene blowing converters 2C + O 2 2CO from blowing air The dissolved oxygen content increases

33 Hamilton s law At the given temperature [C][O]=constant O% p=1bar Stainless steels oxidization requires vacuum C < 0.02% O < 0.01% p=1mbar C%

34 The law of distribution and mass action The law of distribution At a given temperature the ratio of the amount of a given compound In the molten iron and in the molten slag is constant. L(T) = FeS in the slag FeS in the iron The law of mass action Determines the direction of the reaction ma + nb v 1 v 2 pab v 1 =k 1 (C AB ) p V 2 =k 2 (C A ) m (C B ) n At equilibrium v 1 =v 2

35 Effect of nonmetallic elements S, P, O, N Effect of sulfur S does not dissolve, forms FeS eutectic with iron. FeS at grain boundaries Crystallization at the grain boundaries. Cold and hot brittleness T grain 0% ~80% 100% FeS % To reduce the effect: desulfurization 1) Alloy with Mn 2) Increase S content FeS + Mn MnS + Fe generally S < 0.035% MnS is formable at high temperature

36 Effect of nonmetallic elements S, P, O, N Effect of sulfur - desulfurization L = FeO in the slag FeO in the iron increases with temperature K = CaS in the slag FeO in the iron FeS in the iron CaO in the slag = CaS in the slag FeO in the slag FeS in the iron CaO in the slag L FeS in the iron = CaS in the slag FeO in the slag K CaO in the slag L To achieve low S% increase L : increase the temperature. increase CaO content in the slag increase CaS content in the slag increase FeO content in the slag The slag must be changed

37 Effect of nonmetallic elements S, P, O, N Effect of nitrogen nitride compounds precipitation and/or the solidification of nitrogen in interstitial solid solution. Increases strength decreases toughness. Ageing

38 Effect of nonmetallic elements S, P, O, N Effect of phosphorous Keep P content under 0.035% T (0.001%) γ α % 15% Impact energy R m R p02 Z R m R p02 TTKV P% TTKV T Z P [%]

39 Effect of nonmetallic elements S, P, O, N Effect of phosphorous - dephosphorization 2 P + 5 FeO + 4 CaO (CaO 4 P 2 O 5 ) + Fe in molten iron in molten slag Dissolves only in slag in molten iron K = [CaO 4 P 2 O 5 ] [Fe] 5 P 2 FeO 5 [CaO]4 [P] = [CaO 4 P 2 O 5 ] [Fe] 5 K FeO 5 [CaO]4 To achieve low P% decrease the temperature. increase CaO content in the slag increase phosphate content in the slag increase FeO content in the slag The slag must be changed

40 Effect of nonmetallic elements S, P, O, N Effect of oxygen In form of O or FeO. stress after long service period after forming Ageing initial state strain Work done till fracture Impact energy To reduce the effect: deoxidation T TTKV TTKV O% Brittel-to-ductile transition temperature Methods: Settling Diffusional deox. Synthetic slag Ladle metallurgy

41 Effect of nonmetallic elements S, P, O, N Effect of oxigene R m Z R m Z O [%]

42 Deoxidation: settling Deoxidizing elements are loaded into the molten steel. General reaction: FeO + Me MeO + Fe The amount of deoxidizing elements are limited by their disadvantageous effect on the properties: Mn < 1% causes grain coarsening & brittleness Si < 0.5 % it decreases the toughness. V Ti Al < 0.1 % they decreases the toughness.

43 Deoxidation: settling Effect of deoxidizing element on the dissolved oxygen O [%] 0.1 Mn Si 0.01 V C Zr Al Ti Me [%]

44 Deoxidation: settling Rimmed steel Deoxidizing with Mn only susceptible on ageing Semi-killed steel Deoxidizing with Mn + Al for continuous casting Killed steel Deoxidizing with Mn + Si lower TTKV than rimmed steel Dead killed steel Deoxidizing with Mn + Si + Al/V/Ti/Zr best quality from the point of brittleness

45 Deoxidation: diffusional and synthetic slag meth. Diffusional method Deoxidizing element loaded on the top of the molten slag. Diffusion of O to slag. Diffusional synthetic slag method The molten steel is poured on the top of prepared FeO-free slag. molten steel FeO free slag

46 Deoxidation: ladle metallurgy Ladle metallurgy Powder injection deoxidizing, desulfurizing, and dephosphorising powder with Ar gas are blown into the molten steel. - Inclusions are lifting to the slag. - Almost isotropic molten steel This technology with the converter method is the most up-to-date steel making process

47 Vacuum handling A process for deoxidation and degasification The effect of vacuum on steel 1) Decreasing the partial pressure of the gas above the molten steel 2) Decrease the content of oxygen. 3) Increase the vaporization rate of low melting point metals (Zn, Pn, Sn, As) 4) Separates the compounds by dissociation. ~10-6 bar ~10-9 bar bar Fe 4 N CrN FeO AlN SiO 2 Al 2 O 3 Practically impossible TiN

48 Vacuum Vacuum handling Two type of process Molten steel Vacuum chamber Steel stream ladle Degasificated steel Vacuum ladle degassing Vacuum stream degassing

49 Effect of dissolved gases on steel CO - in the rimmed steel produces gas bubbles O 2 - produces gas inclusions and oxide and silicate inclusions N 2 - increase the ability for aging and nitride inclusions H 2 - flocking H 2 bubbles cracking

50 Effect of dissolved gases on steel flocking H 2 bubbles reason T melting A 4 [H] [H] A 3 Temp A 3 A 4 T melting

51 Effect of dissolved gases on steel H 2 solution let the gas atoms depart by diffusion slow cooling after casting (several days) forging by very soft deformation to make cohesion between the surfaces of the cracks + for N make stable nitrides my mircoalloying elements Al, V, Ti AlN, VN

52 Alloying, casting Alloying Can only take place after a perfect deoxidation, otherwise alloying elements would burn. Casting Two types: casting of ingots continuous casting

53 Casting of ingots simple High productivity More homogeneous slow

54 Casting of ingots Solidification process for ingots - Shrinking effect - Crystallisation, grain-arrangement, mircostructure - Segregation

55 Casting of ingots Shrinking effect the top 12-15% of the total weight of killed steel ingot must be cut off (rimmed steel only 3-5%)

56 Casting of ingots Crystallisation, grain-arrangement, mircostructure R rate of crystall growing R N N number of crystal nuclei ΔT Supercooling under the equilibrium

57 Casting of ingots Segregation normal segregation During the solidification the liquid phase becomes enriched with alloying elements and impurities. The difference can be 300% for S P% % for P S% T R rate of crystall growing C% cross section of the ingot B [%] Concentration of the liquid phase

58 Casting of ingots Segregation inverse segregation dendrite Because shrinkage the alloying elements and impurities can move inwards between dendrites. liquid phase The impuritiy concentration is higher between the dendrites arm. Concentration %

59 Casting of ingots Microstricture and segragation in ingot

60 Casting of ingots

61 Continuous casting

62 Continuous casting 72gc6I-_E

63 Steel refining methods All of these methods have a remelting and solidification period to: - Decrease the dissolved gas content and the amount of inclusions - Produce a homogeneous fine grained crystal structure - Produce a homogeneous distribution of alloying elements Used for - tool steels - high alloy steels

Steel refining methods Vacuum arc remelting process Removal of dissolved gases, such as hydrogen, nitrogen and CO; Reduction of undesired trace elements with high vapor

64 Steel refining methods Vacuum arc remelting process Removal of dissolved gases, such as hydrogen, nitrogen and CO; Reduction of undesired trace elements with high vapor pressure; Improvement of oxide cleanliness; Achievement of directional solidification of the ingot from bottom to top, thus avoiding macro-segregation and reducing micro-segregation.

65 Steel refining methods Vacuum induction remelting process Removal of undesired trace elements with high vapor pressures Removal of dissolved gases (hydrogen and nitrogen)

66 Steel refining methods Electroslag remelting process Similar technology: electron beam remelting process

Production of Iron and Steels

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