The Production of Steel

 

Most steel manufactured and used is called carbon steel due to its chemical composition.  The addition of different alloys can result in the creation of alloy steel or stainless steel, depending on the type and amount of the alloying materials used. 

 

Traditionally, steel has been made by companies that combine iron ore, limestone, coke and scrap in a blast furnace to produce pig iron.  This is then refined in a basic oxygen furnace to produce liquid steel.  Such steel manufacturers are known as integrated steelmakers. Their facilities are very capital sensitive and typically have the capacity to produce 2 to 4 million tons of steel annually.

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However, steel can also be made using scrap and direct reduced iron that is melted in an electric arc furnace to produce liquid steel.  Such steel manufacturers are referred to as minimill steelmakers.

 

In the past the liquid steel was poured into a mould to form ingots.  Today this process is too expensive, yields inferior quality steel and is technology obsolete.  The most efficient way of making steel is through continuous casting where liquid steel is poured continuously into the caster to make slabs, blooms or billets.  These are semi-finished steel products, which must undergo further processing to make finished steel products.

 

Until about the 1960’s, integrated steelmakers manufactured all steel products.  This included long products (ie. rod, bars, wire, structurals, rails, pipe and tube) and flat products (ie. plate, hot rolled products, cold rolled products, tin plate).  In modern industrial economies, the long product portion of the market accounts for about 30% of total steel demand, while the flat product portion accounts for about 70%.  However, with the emergence of minimills, integrated steelmakers began to loose markets in the long products sector of the business because the minimills had lower costs and could charge lower prices.  Consequently today, most bar, rod, wire, structural and rail products are manufactured in minimills.  The flat products business though has largely remained the preserve of integrated steelmakers.  The emergence of thin slab minimills capable of producing hot rolled plate and sheet products, such as Nucor, mean that integrated steelmakers will now be challenged in this portion of the market.

 

Finished steel products are produced either as a result of hot rolling or cold rolling.  Hot rolled product is heated and rolled at high temperatures to impart the shape, chemical and mechanical properties required in the finishing item.  Cold products are rolled at low or room temperatures.  For example, hot rolling on a continuous strip mill usually begins at about 2200 degrees F.  and is completed above 1300 degrees F.  In cold rolling, the product is not heated immediately prior to rolling although the temperature of the steel will rise due to the frictional effects of rolling so the finished product may have a temperature of 250 to 450 degrees F.

 

There are 4 principal hot rolled finished flat products: bars, plate, hot rolled strip and hot rolled sheet.  Dimensions, particularly thickness and width, are the principle bases of classification.  There are also 4 principal cold finished flat rolled products: flat bars, cold rolled strip, cold rolled sheet and black plate.  The latter is the product, which is the starting material to manufacture tin plate that is used for food packaging and beverage containers.  Galvanised products can be manufactured from either hot or cold rolled strip and sheet.

 

Most steel products are the result of hot rolling.  The difference between hot rolled steel and cold rolled steel is the mechanical and chemical properties imparted to the steel.  This in turn depends on the customer’s application for the steel and what properties they want in the product they are buying.

 

Rails, structurals, concrete-reinforcing bars, wire rod, plates, and sheet and strip are all examples of hot rolled products.  The plates, sheet ans strip products could be used to manufacture pipes and tubes, construction materials and body panels and stampings for the appliance and automotive industries.  Wire, sheet and strip are examples of products that can be cold rolled.  Wire is often used to manufacture cold formed or cold headed fasteners (eg. screws, bolts) while cold rolled sheet and strip are increasingly being demanded by the appliance and automotive industry for exterior applications due to their superior surface finish particularly as galvanised outer body panels on automobiles.