SL – Steel Story 1

What is Steel?

·       Not one material – there are thousands of different steels.

·       General name for a family of alloys of iron with carbon and a variety of different elements. (see fig1 page 165) Read green box on page 165 to remind yourself what an alloy is.

·       Composition of steel is determined by its use as even small differences in composition can have dramatic affect on properties.

·       This is especially true of carbon content;

o      With 4% C steel is brittle and of limited use.

o      With 0.1% C steel easily drawn into wires for staples or paper clips.

o      With 1% C steel is strong but not brittle and can be used to make cables to support huge suspension bridges.

·       Other elements also added to modify properties – cables of Humber bridge are made of steel containing manganese, chromium and silicon (see table 1 page 165 for other elements added.

·       Some elements must not be present – phosphorous, sulphur or dissolved gases such as oxygen, nitrogen or hydrogen. Presence of these will lead to brittle, poor quality steel.

·       The final properties of steel can also be modified by heat treatment (subjecting it to varying degrees of heating and cooling) or by work treatment (rolling or hammering) These processes modify the metal structure and therefore its properties.

·       Because composition and structure of steel can easily be modified to suit various applications it is a very versatile material.

·       We are going to study composition and its effect in this unit.

·       Every batch of steel is made to a different specification to suit the needs of a particular customer. It will have a specific composition and be treated in ways which will produce properties suitable for its eventual use.

  SL SS2 – How is steel made.

• Impure iron from blast furnace is starting point for nearly all new steel. This iron already has other elements dissolved in it including C, Si, Mn, P, and S. (see table 2 page 166)
• This iron is very brittle. To make steel from it must lower C content, remove most of P and S and add other elements before the material is allowed to solidify.
• This is achieved in the Basic Oxygen Steelmaking (BOS) process.
• Using this process batches of 300 tonnes of high quality steel are produced in about 40 mins. It is a spectacular process!!

Now have a go at assignment 2

Removing Sulphur.

• 300 tonnes of molten iron from blast furnace poured into a huge container called a ladle.
• Of the elements to be removed S is the first. This is done by reduction with magnesium.
• Several hundred Kg’s of powdered magnesium are injected through a vertical tube called a lance.
• Sulphur is reduced to magnesium sulphide in a violent exothermic reaction, the MgS floats and can be raked off.

   Mg    +     S     ®     MgS

Now have a go at assignment 3

Removing other elements.

• The molten, desulphurised iron goes into the converter – which already contains some scrap steel. (see Fig 5 page 167)
• The converter turns to a vertical position and a water cooled lance is lowered to the surface of the iron.
• A supersonic blast of oxygen under pressure creates a seething foam of molten metal and gas – Very violent!!
• Over 20 mins most of the impurities of C, Si, Mn and P (as well as  some Fe) are oxidised. (see Fig 8 page 168)

C        +        0.5  O₂   ®     CO

Si       +          O₂      ®     SiO₂

Mn      +          0.5 O₂         ®       MnO

4P          +        5O₂        ®        P₄O₁₀

Also unavoidably;

Fe          +       0.5 O₂      ®        FeO

• CO escapes as a gas and is collected by a hood over the vessel.
• Oxides of P and Si are acidic, so a mixture of CaO and MgO are added which are basic (the B in BOS). These react with the acidic oxides and form a molten slag which floats on the molten metal and can be scraped off.
• The oxides of Mn and Fe are also collected in the slag.

Now do assignment 4.

Keeping Track.

• Process is closely monitored from control room.
• Computers model conditions in the converter and two mins before the predicted end of the oxygen blow a sublance is lowered into the mixture to measure temp and carbon content and remove a sample.
• Computer uses new info to predict how much more O₂ is needed and sample is analysed quickly to determine % of the elements in the steel.
• Analysis involves emission spectroscopy – read green box on page 168 and revise CI6.1
• The following graph shows the sequence of oxidation of various elements in the converter.

Controlling the temperature.

• At the end of the oxygen blow the temp must be around 1700-1740°C – this is the target tapping temperature.
• Too high a temp can waste energy and damage converter linings.
• Too low a temp means the metal may solidify – a disaster!!
• The oxidation reactions are exothermic and provide the necessary energy to raise converter temp – no external heating is required.
• The scrap steel added to the converter at the start absorbs some of the heat as it melts maintaining a  heat balance – hence this scrap acts as both a coolant and a source of recycled steel.

Now have ago at assignment 5.

At the end of the blow.

• Converter is rotated to pour of molten steel then tilted in opposite direction to remove slag. See fig. 11, 12 and 13 page 169/170.
• Al metal is added to the molten steel to react with excess O₂ and form aluminium oxide which floats to the surface.

Meeting the specification.

• Unwanted elements have now been removed.
• Elements which are wanted now have to be added (or put back!).
• Computer models predict the exact quantity of each substance to add to meet the specification required.

Now do assignment 6

• C, Mn, Si, Cr, and Al are commonly added (some of these have just been removed!!)
• Exotic elements like niobium, molybdenum and tungsten are also sometimes added.
• A lance blows argon through the liquid steel to stir the mixture and ensure the composition and temperature are uniform.
• The steel may be refined whilst still in the ladle e.g. the ladle arc furnace allows steel in the ladles to be reheated electrically so the temp can be precisely controlled. This is especially important when making high C/ low P steels for steel rods.
• During the addition samples are frequently taken and the computer model updated.
• A final trimming adjusts the concⁿ to the required values then the steel is ready to be cast, either continuously into strands (see fig 14), or batch wise into moulds to produce ingots.

Can Steelmaking be Improved

• World production of steel is rising, especially high quality specialist steels.
• New ideas are being introduced into the BOS process all the time – e.g. direct rolling of hot steel from the caster – to save energy.
• Larger, more efficient electric arc furnaces now produce over 30% of steel worldwide (read green box on page 171)
• The mini mill route allows smaller batches of specialist steels to be made (as few as 75 tonnes) so again saving energy.

SL SS3 – Rusting.

A return to nature.

·        Many metals, including Fe, occur in Earth’s crust as oxides – because their oxides are more stable than the metal – i.e. the change from the metal to its oxide is an energetically favourable process.

·        In fact it takes energy to extract the metal from its oxide.

·        No wonder then that, given the opportunity, iron reforms its oxide – it rusts.

·        Rusting is the common name for the corrosion of iron. (see fig.16 page 172)

·        Rusting occurs when iron or steel react with oxygen and water in the atmosphere.

·        Hydrated iron(III) oxide of variable composition is produced – (Fe₂O₃.xH₂O)

·        The oxide is permeable to air and water so does not protect surface – so metal continues to corrode further underneath the layers of rust.

·        Rate of rusting influenced by other factors;

·        Impurities in iron.

·        Presence of acid or other electrolytes in solution.

·        Availability of dissolved oxygen.

What happens during rusting?

·        Rusting is an electrochemical process.

·        Electrochemical cells are set up on metal surface where different areas act as sites of oxidation and reduction. Half reactions are;

Fe²⁺ (aq) + 2e^-  ®   Fe (s)                              E^q = -0.44V

¹/₂ O₂ (g)  +  H₂O (l) + 2e^-   ®   2OH^- (aq)                E^q = +0.40V

·        Reduction of O₂ occurs at more +ve potential so electrons flow to this half cell from iron half cell where Fe is oxidised to Fe²⁺ ions.

·        Concⁿ of O₂ dissolved in water drop determines which areas are sites of oxidation or reduction.

·        At edges of drop concⁿ of O₂ is higher so oxygen is reduced to hydroxide ions.

·        At centre of drop concⁿ of O₂ is lower so Fe²⁺ ions pass into solution and Fe is oxidised.

·        Electrons released here flow in the metal surface to the edges of the drop to reduce the oxygen.

·        Hence iron rusts at centre of drop (or under a layer of paint!) where O₂ is limited. Pits form here where iron has dissolved away.

·        Rust then forms as a series of secondary processes within the solution;

Fe²⁺ (aq)   +   2OH^- (aq)   ®    Fe(OH)₂ (s)

                   Fe(OH)₂ (s)     ^{O  (aq)}          (Fe₂O₃.xH₂O)

·        Some ionic impurites e.g. NaCl from sea spray promote rusting by ­ conductivity of H₂O.

·        Other ionic compounds interfere with the electrochemical reactions and inhibit rusting, e.g. if positive ions form an insoluble hydroxide with the OH^- ions, or negative ions form an insoluble Fe(II) compound.

·        pH  is also important – rusting is accelerated by acidic conditions, but inhibited under alkaline conditions.

Activity SS3.4 investigates the chemistry of rusting.

Keeping Nature at Bay.

·        Simplest way to avoid rusting is to place a barrier between the iron and the atmosphere e.g. paint, grease or oil.

·        Increasingly the barrier is an organic polymer – a plastic film. E.g.’s are refrigerators, dishwasher shelves or bicycle baskets.

·        Iron can be covered with a thin layer of another metal e.g. steel galvanised with zinc is used for cars – The zinc layer is protected by a coating of zinc oxide, and if the zinc layer get scratched the zinc corrodes in preference to the iron – this is sacrificial protection.

·        Fig 20 page 173 shows how cars can be treated to prevent rusting for up to 10 years – however you can never eliminate the problem completely – only postpone it!

·        Sacrificial protection was used as early as 1824 to protect the metal sheathing on sailing ships – nowadays zinc blocks are attached to the steel supports of oil rigs for the same reason – the blocks are more easily replaced than the supports!! See figs 21 and 22 page 174.

·        Zinc is used because its E^q is more negative than that of iron – any metal with a more negative E^q could be used.

·        Tin cannot be used as a sacrificial metal but it is used to coat iron to make tin cans to preserve food and has been used this way since 1812.

Now have a go at assignment 7

A Prize Winning Invention.

·        Napoleon had a problem supplying food to his widespread army – he offered a prize of 12 000 Francs to anyone who could solve this problem.

·        Nicolas Apert was awarded the prize for preserving cooked food by sealing it in an airtight glass jar whilst it was still hot.

·        Peter Durand – an Englishman – adapted this method by using a tin plated iron canister instead of a glass bottle – hence the tin can was born.

Read the interesting story of Captain Edward Parry’s 100 year old veal and gravy!! Then have a go at assignment 8.

Stainless steel – the perfect solution?

·        Stainless steel was developed in 1913 by a Sheffield chemist – Harry Brearley.

·        He was adding chromium to steel to see if it would prolong lifetime of rifle barrels.

·        Analysis of steel involves dissolving it in acid – but Brearley found his high chromium steel would not dissolve, and also that it stayed shiny when left lying around for a while.

·        He realised the potential of this steel for use in cutlery – it would not need to be dried immediately after washing or require frequent polishing. (There was some prejudice to the idea of using steel for this purpose!!)

·        Stainless steel does not corrode because it forms a surface layer of chromium(III) oxide  -  Cr₂O₃.

·        This oxide is not hydrated and adheres closely to the metal surface.

·        The oxide layer is only nanometres thick so is invisible to the naked eye, so 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!

·        The one big drawback of stainless steel is the cost – it is very expensive – and cost must be taken into account when deciding the best steel for a particular job.

SL SS4 – Recycling Steel.

Why Recycle?

·         Approximately 40% of world steel production is from recycled steel.

·         Over 200 million tonnes of iron recovered per year.

·         Saving the equivalent energy to 160 million tonnes of coal or 100 million tonnes of oil – about 40% of UK’s annual energy consumption!!

·         Recycled scrap is an integral part of BOS process (about 18% of every ‘cast’ of new metal.

·         In the electric arc furnace only scrap is used.

·         Most of scrap comes from steelworks itself – waste batches or miscasts – or from industries which make steel products.

·         The composition of this type of scrap is well known.

·         Scrap from other sources e.g. scrap cars or 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.

·         Some elements improve quality of steel e.g. low concentrations of transition metals like Ni and Cr.

·         Other elements e.g. Sn and Cu can cause problems.

Recycling old Tin Cans

·         The tin must be removed before tin cans can be recycled.

·         In past this was only done using waste metal from tin plating works.

·         Only since 1980s have attempts been made to recycle tin cans from household waste on a large scale.

·         Cans are shredded and unwanted food and residual paper are removed.

·         Burning would be one way to clean cans – but unfortunately this would cause the tin to diffuse into the steel.

·         Mechanical shredding devices now shred and clean the cans and the steel fragments are picked out magnetically – this removes about 98% of unwanted material.

·         Cleaned, shredded cans are treated with a hot solution of NaOH in the presence of 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

·         Steel left behind is pressed into bales and sent to steel plant.

·         Tin is recovered by electrolysis.

Now have a look at assignment 9.

SL SS5 – A Closer Look at the Elements in Steel.

·         There are many elements in steel as well as iron;

·         A few non metals like C and Si

·         Other metals – mainly from the d-block of the periodic table. See fig 27 page 177.

·         To understand how steel behaves when exposed to weathering, or how to prevent corrosion, or how fruit juices may affect the inside of a tin can – we need to consider the chemistry of these d-block elements. (CI 11.5)

·         The d-block elements are also called the transition metals because they show a transition in properties between the reactive s-block metals, and the less reactive metals on the left of the p-block.

·         The chemistry of the d-block is very characteristic and is a direct result of their electronic structure, typical properties being;

·         Variable oxidation states.

·         Catalytic activity (of elements and their compounds)

·         Strong tendency to form complexes.

·         Form coloured compounds.

·         Metals like Mg or Na have only one oxidation state – transition metals may have a range, with some oxidation states having very characteristic colours.

Catalysis

·         E.g. iron is a catalyst in the production of ammonia in the Haber process.

·         Ni is used in hydrogenation of vegetable oils to make margarine.

·         Pt/Rh in catalytic converters.

·         Transition metal catalysts also play a vital role in biological systems – e.g. Co, Cu, Mn and Mo (all ultra trace elements), they allow complex reactions to occur quickly in dilute aqueous solutions at moderate temp. and pH.

Have a go at assignment 10.

Complex Chemistry.

·         Typically, a central metal ion is surrounded by 6 electron-donating ligands.

·         Complexes with 2 and 4 ligands are also common.

·         Fig 29 page 178 shows the structure of the complex haemoglobin formed by iron, which is responsible for transporting oxygen round the bloodstream.

·         Part (b) shows oxygen bound to the haemoglobin to form oxyhaemoglobin.

·         The oxygen molecule is only loosely attached to the complex and can easily be released when needed.

·         The complexing of metal ions in solution affects electrode potentials of metal-metal ion half cells, therefore may affect the way metals corrode – See fig 30 page 179.

Coloured compounds.

·         The colour of a transition metal complex depends on;

v      The oxidation state of the metal ion.

v      The nature of the ligand.

v      The spatial arrangement of the ligands.

·         Many colours are observed. Table 3 page 179 shows the metal ions responsible for the colours of some gemstones.

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.

·         E.g. stainless steel contains a minimum of 12% Cr (typically 18%) and also Ni (typically 8%). 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;

v      Its abundance in Earth’s crust.

v      Cost of mining.

v      Ease of extraction.

v      Demand.

v      Transport costs.

v      Political factors depending on where the ore is found.

·         Some elements are strategically critical – because they are only found in one or two countries, e.g’s include;

v      70% of worlds chromium is in the republic of South Africa.

v      Zimbabwe have over half of remainder!

v      Most of world reserves of molybdenum are found in US and Canada.