Why do we need to convert most iron into steel alloys?

Why do we need to convert most iron into steel alloys?

Most metals in everyday use are alloys.

Iron is a good conductor of heat and can be bent or hammered into shape (malleable), quite strong physically – made stronger when alloyed with other materials.

This makes iron useful as structural material and for making things that must allow heat to pass through easily and useful construction materials.

Steel is an alloy because it is a mixture of a metal (iron) with other elements (carbon and perhaps other metals too).

Iron from the blast furnace contains about ~96% iron with ~4% of impurities including carbon, silica and phosphorus.

In this state the cast iron is too hard and too brittle for most purposes.

Cast iron is hard and can be used directly for some purposes eg manhole covers, ornamental railings because of its strength in compression and is very hard wearing.

However, if all the impurities are removed, the resulting very pure iron is too soft for any useful purpose.

Therefore, strong useful steel is made by controlling the amount of carbon and selected metals to produce an alloy mixture with the right physical properties fit for a particular application e.g. steel for car bodies, chrome stainless steel, extremely hard and tough tungsten–iron steel alloys etc.

The real importance of alloys is that they can be designed to have properties for specific uses in terms of eg compression/tensile strength or corrosion reduction ie less susceptible to rusting.

Low–carbon steels (0.1% carbon) are easily shaped for car bodies

High–carbon steels (1.5% carbon, often with other metals too) are hard wearing and inflexible and can be used for cutting tools, bridge construction.

Stainless steels have chromium(and maybe nickel) added and are much resistant to corrosion (from oxygen/water) than iron or plain steel which readily rust. Objects made of iron or plain steel, particularly those exposed to the weather, regularly have to be painted or coated with some other protective layer from the effects of water and oxygen.

The properties of iron can be altered by adding small quantities of other metals or carbon to make steel, one of the useful metal alloys in widespread use today.

Steels are alloys since they are mixtures of iron with other metals or with non–metals like carbon or silicon.

Making Steel:

(1) Molten iron from the blast furnace is mixed with recycled scrap iron

(2) Then pure oxygen is passed into the mixture and the non–metal impurities such as silicon or phosphorus are then converted into acidic oxides (the BOS oxidation process) ..

e.g. Si + O2 ==> SiO2, or 4P + 5O2 ==> P4O10

(3) Calcium carbonate (a base) is then added to remove the acidic oxide impurities (in an acid–base reaction). The salts produced by this reaction form a slag which can be tapped off separately.

e.g. CaCO3 + SiO2 ==> CaSiO3 + CO2 (calcium silicate slag)

Reactions (1)–(3) produce pure iron.

Calculated quantities of carbon and/or other metallic elements such as titanium, manganese or chromium are then added to make a wide range of steels with particular properties.

Because of the high temperatures the mixture is stirred by bubbling in unreactive argon gas!

Economics of recycling scrap steel or ion: Most steel consists of >25% recycled iron/steel and you do have the 'scrap' collection costs and problems with varying steel composition* BUT you save enormously because there is no mining cost or overseas transport costs AND less junk lying around! (NOTE: * some companies send their own scrap to be mixed with the next batch of 'specialised' steel they order, this saves both companies money!). More on this below.