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  1. 1. SS1 What is steel? • Steel is a general name given to alloys of iron with carbon plus a variety of other elements – Alloys are mixtures of a metal with one or more other elements – They are mixed while molten and allowed to cool – The presence of other elements disrupts the arrangement of the atoms and so changes its properties • Small differences in composition can have a big effect on its properties • This is particularly true for carbon… – 4% carbon – very brittle – 1% carbon – stronger but not too brittle (e.g. cables for bridges) – 0.1% carbon – easy to bend and pull into wires (e.g. paper clips) • Elements added to improve a steel’s properties are frequently transition elements such as manganese, chromium and vanadium
  2. 2. • Some elements must not be present as they lead to brittle, poor quality steel and so need to be removed: – phosphorous, sulphur and gases such as oxygen and nitrogen • We can also modify steel’s structures in other ways; – heat treatment (heating and cooling) – work treatment (rolling or hammering) • Because composition and properties can easily be modified steel is a very versatile material – SS1.1 – SS1.2 – CI Appendix 1 – CI 6.7 – CI 3.1 + CI 9.1 (revision) – CI1.1 – CI1.5 (revision)
  3. 3. SS2 From Iron to Steel • Nearly all steel is made from impure molten iron from the blast furnace A. Coke (C), iron ore (Fe2O3) and limestone (CaCO3) added B. Hot air blast C. Coke burns at 1500ºC C + O2 CO2 CO2 + C CO D. Reactions occur… Fe2O3 + 3CO Fe + 3CO2 CaCO3 CaO + CO2 CaO + SiO2 CaSiO3 (“slag”) E. CO2 escapes F. Molten iron sinks to bottom G. Slag forms from waste rock and limestone. It floats on the molten iron A G B E F D C
  4. 4. • Typical blast furnace iron has the following composition: – Ass 1 • This metal will be very brittle • To turn it into steel we must; – Lower the carbon content – Remove the P, S and Si – Add wanted elements – All while molten • This is achieved in the Basic Oxygen Steelmaking (BOS) process • This is a batch process • It can make 300 tonnes of high quality steel in just 40 minutes – SS2 – CI5.1 (revision) Element Fe C Si Mn P S % by mass 94.0 4.42 0.66 0.41 0.045 0.027
  5. 5. • Stage 1 - removing sulphur – 300 tonnes of molten blast furnace iron put into a ladle – Powdered Mg injected through a lance Mg + S MgS – Very exothermic – Raked off • Ass 2 • Stage 2 – the oxygen blow – Iron transferred to a convertor which contains some scrap steel – A water-cooled lance is lowered to the surface of the iron – A supersonic blast of oxygen (the O in BOS) creates a foam of molten metal and gas
  6. 6. • This very violent reaction removes most of the impurities over the next 20 minutes… C + ½O2  CO Si + O2  SiO2 Mn + ½O2  MnO 4P + 5O2  P4O10 • Also unavoidably… Fe + ½O2  FeO • CO escapes as a gas and is collected by a hood over the vessel • The other oxides need to be removed from the molten metal
  7. 7. • Stage 3 - removing other elements – Oxides of P and Si are acidic – A mixture of basic oxides (CaO and MgO) are added (the B in BOS). – These react with the acidic oxides and form a molten slag – This slag floats on the molten metal – MnO and FeO are also collected in the slag • Keeping track – Temp and carbon content are monitored and a sample is removed – Computers use this data to predict how much more O2 is needed – The sample is analysed to find % of the elements in the steel. – Analysis involves emission spectroscopy • CI6.1 (revision) • Ass 3 + 4
  8. 8. Controlling the temperature • At the end of the oxygen blow the temp must be around 1700-1740C • Too high: – energy is wasted – convertor linings get damaged • Too low: – the metal may solidify • The oxidation reactions are exothermic and provide all the necessary heat – no external heating is needed • The scrap steel added absorbs some of the heat as it melts and therefore acts as a coolant (and is recycling steel)
  9. 9. • The converter is rotated to pour off molten steel into a ladle • It is then tilted in opposite direction to remove slag. • Al metal is added to the steel in the ladle to react with excess O2 and form Al2O3 which floats to the surface • Other elements are now added (some of which have just been removed) • These include C, Mn, Si, Cr, and Al plus things like Nb, Mo and W • A lance then blows argon through the mixture to stir it • The steel is then cast, either into strands or into moulds to produce ingots.
  10. 10. Steel (of desired specification) Blast furnace iron (molten) + lance ( )( mins) (very exothermic) CONVERTOR Powdered Mg Sulphur raked off Magnesium Sulphide scrap steel lance 20 Oxygen Carbon monoxide Stage 1 Manganese (II) oxide Iron (II) oxide Stage 2 MgO and CaO (basic oxides) Magnesium and Calcium Phosphates Aluminium oxide Magnesium and Calcium Silicates Phosphorus (V) oxide and Silicon dioxide Excess oxygen Required elements Aluminium Stirs mixtureargon Stage 3 Stage 4
  11. 11. The Electric Arc Furnace • This uses scrap steel • A spark produced between carbon electrodes melts the steel • Lime (CaO) reacts with impurities to form slag • Other elements are then added • If the scrap steel is carefully selected, relatively small batches of steel can be made (75 tonnes!)
  12. 12. SS3 Steel for a purpose A return to nature • Extracting iron from its oxide requires a lot of energy • This means: – iron oxide must more stable than iron itself – Changing iron into iron oxide is energetically favourable – iron will readily reform its oxide – it will rust • Rusting is the common name for the corrosion of iron • Rusting occurs when iron or steel react with oxygen and water in the atmosphere • Rust is a hydrated form of iron(III) oxide (Fe2O3.xH2O) • It is permeable to air and water • This means the metal will continue to corrode further underneath the rust
  13. 13. • Iron and steel will rust whenever they are in contact with water and oxygen • Rusting is a redox process or an electrochemical process • It is affected by: – Impurities – Presence of acids – Amount of dissolved oxygen – CI9.2 (Redox and standard electrode potentials) – CI9.3 (Predicting reactions using standard electrode potentials) – SS3.1 – SS3.2
  14. 14. What happens during rusting? • The two half reactions involved are: Fe2+ (aq) + 2e- Fe (s) Eo= -0.44V ½ O2 (aq) + H2O(l) + 2e- 2OH- (aq) Eo= +0.40V • The oxygen half reaction has a more +ve electrode potential • It will therefore attract electrons from the less positive iron half reaction • Oxygen is reduced, iron is oxidised… Fe (s) Fe2+ (aq) + 2e- ½ O2 (aq) + H2O(l) + 2e- 2OH- (aq) • These occur at different areas on the iron…
  15. 15. • At edge of the water drop, O2 concentration will be higher so reduction reaction is most likely • Electrons needed for this are taken from middle where O2 concentration is lowest • ‘Pits’ form where the iron has dissolved • Rust forms through a series of further reactions… Fe2+ (aq) + 2OH- (aq) Fe(OH)2 (s) Fe(OH)2 (s) + O2 (aq) Fe2O3 .xH2O(s) • It is permeable to air and water • This means it will continue to rust underneath the surface • The presence of ions generally speeds up rusting as they increase the conductivity of the water (e.g. beside the sea) • It is accelerated by acids but inhibited by alkalis – SS3.3
  16. 16. Keeping nature at bay • Simplest is to put a barrier between the steel and the atmosphere – Paint, – Oil, – Grease, – Plastic – Metal – tin cans • Better is to use something that will corrode in place of the iron… • …a metal with a more –ve Eo • Ass 6
  17. 17. • Car bodies are often coated in a polymer • They are also are made from galvanised steel… • …steel coated in zinc – The zinc forms a layer of zinc oxide – However, unlike rust this is impermeable and so protects the zinc and the iron – Better still, if the zinc is scratched, it corrodes in preference to the iron – We say it is a sacrificial metal – Nowadays some cars are guaranteed for as long as 12 years against corrosion – Nevertheless, even then rusting is only postponed
  18. 18. • This same idea is used to protect ships and oil rigs • However, rather than cover the whole body in zinc, blocks are attached • Zn, Mg and Al are commonly used • These have more negative standard electrode potentials than Fe/Fe2+ • As a result they will donate electrons more readily than the iron and so corrode in preference • Corroded blocks can then be removed and replaced with new ones • SS3.4
  19. 19. Stainless steel – the perfect solution? • Stainless steel was developed in 1913 by Sheffield chemist Harry Brearley • Brearley found that steel high in chromium kept its shine • He realised this would be very useful for cutlery • No longer would they need to be dried or polished immediately after being washed • Not everyone liked the idea; • One of the foremost cutlers in Sheffield described this as “contrary to nature” • Stainless steel forms a surface layer of chromium(III) oxide (Cr2O3) • This layer is only nanometres thick so is invisible to the naked eye • This means it lets the natural shine of the metal show through. • It is also impervious to air and water so protects the metal surface. • Best of all, if you scratch the oxide layer – it simply reforms!
  20. 20. SS4 Recycling Steel • 45% of world’s steel production is from recycled steel • 200 million tonnes of iron are recovered each year • An energy-saving equivalent to 160 million tonnes of coal • Using scrap reduces the cost of mining, transport and production • There is, however, the added cost of collecting and sorting the scrap • Scrap makes up about 18% of BOS steel • The electric arc furnace uses only scrap • Most scrap comes from the steelworks or from industries which make steel products • Scrap from other sources (cars, washing machines etc…) must be carefully graded and selected • Steel makers must be aware of composition of scrap to avoid adding unwanted elements to steel
  21. 21. Recycling used ‘tin’ cans • The tin must be removed before cans can be recycled • This has been done in industry for a long time • Only tried with household waste since 1980s • Mechanical shredders shred and clean the cans • The steel fragments are then picked out magnetically • This removes about 98% of unwanted material • The cans are then treated with hot NaOH(aq) and an oxidising agent • The tin dissolves as a compound of tin(IV) Sn (s) + 6OH- (aq)  [Sn(OH)6] 2- (aq) + 4e- Stannate (IV) ion • The steel left behind is sent to steel plant • The tin is recovered by electrolysis and reused
  22. 22. SS5 Partners in Steel • Many of the elements in steels(including iron) are d-block elements • To understand and explain how steels behave, we need to understand these elements • CI 3.1 (Chemical formulae) (revise) • CI 2.4 (electron sub shells and orbitals) • Typical properties of transition elements include; – Form compounds in a variety of oxidation states – Form coloured compounds – Have a strong tendency to form complexes – Frequently show catalytic activity (as elements or compounds) • CI 11.5 • CI 11.6 • SS 5.1 • SS 5.2
  23. 23. Complex Chemistry • A complex is made up of a central metal ion surrounded by electron-donating ligands • The commonest coordination number is 6 • Haemoglobin is an example of a biological complex • It is formed by a porphyrin ring with an Fe2+ at the centre • The globin protein forms a 5th bond to the Fe2+ • O2 then acts as a ligand and bonds to the remaining site to form oxyhaemoglobin • The O2 molecule is only loosely attached and can easily be released when needed • Carbon monoxide can also act as a ligand • However it bonds more strongly, preventing O2 bonding and therefore ‘suffocating’ you
  24. 24. Tinned peaches • A small amount of the tin coating dissolves: Sn (s)  Sn2+ (aq) + 2e- • The fruit juice contains carboxylic acids (-COOH) • These partially dissociate to form anions (-COO-)… • …which form complexes with any Sn2+ ions… • …and encourages the tin to dissolve (Le Chatelier’s) • These tin complexes are non-toxic (and even add to the tangy taste!) • However, the tin is being sacrificially oxidised and gradually stripped from the steel • Once the steel is exposed, the acids will react with the iron in preference. • Hence the shelf-life of tinned fruit is less than other tinned foods
  25. 25. d-Block Metals can be Expensive • Iron is the most abundant d-block metal in the Earth’s crust and is relatively cheap. • Other d-block metals are expensive and the cost of steel rises if they are used… – Stainless steel contains typically 18% Cr and 8% Ni – This can be 5 – 6 times more expensive than ordinary steel – So it is only used selectively! • The price of a metal depends on several factors: - Abundance - Political factors - Cost of mining - Transport costs - Ease of extraction - Demand • Some elements are only found in one or two countries. They are described as strategically critical – 70% of world’s chromium is in South Africa – Zimbabwe have over half of remainder! – Most of world’s molybdenum is found in USA and Canada