Steel is made from iron ore, a compound of iron, oxygen and other minerals that occurs in nature. The raw materials for steelmaking are mined and then transformed into steel using two different processes: the blast furnace/basic oxygen furnace route, and the electric arc furnace route.
Both processes are being continually improved to meet the challenge of low-emission steelmaking.
What Is Steel Made Of?
Iron is made by removing oxygen and other impurities from iron ore. When iron is combined with carbon, recycled steel, and small amounts of other elements it becomes steel.
Steel is an alloy of iron and carbon containing less than 2% carbon and 1% manganese and small amounts of silicon, phosphorus, Sulphur and oxygen.
Steel is the world’s most important engineering and construction material. It is used in every aspect of our lives; in cars and construction products, refrigerators and washing machines, cargo ships, and surgical scalpels.
How Steel Is Made
Steel is primarily produced using one of two methods: Blast Furnace or Electric Arc Furnace.
The blast furnace is the first step in producing steel from iron oxides. The first blast furnaces appeared in the 14th century and produced one ton per day.
Even though equipment is improved and higher production rates can be achieved, the processes inside the blast furnace remain the same. The blast furnace uses coke, iron ore and limestone to produce pig iron.
Coal traditionally has been a key part of the coke-making process. The coal is crushed and ground into a powder and then charged into an oven where it is heated to approximately 1800°F in the absence of oxygen.
As the oven is heated, the coal begins to melt so most of the volatile matter such as oil, tar, hydrogen, nitrogen and sulfur are removed. The cooked coal, called coke, is removed from the oven after 18 to 24 hours of reaction time.
The coke is cooled and screened into pieces ranging from one inch to four inches. The coke is a porous, hard black rock of concentrated carbon (contains 90 to 93 percent carbon), which has some ash and sulfur but compared to raw coal is very strong.
The strong pieces of coke with a high energy value provide permeability, heat and gases which are required to reduce and melt the iron ore, pellets and sinter. Today, natural gas is increasingly being added in place of coke to the same degree in the blast furnace to reduce carbon emissions.
The first electric arc furnaces (EAFs) appeared in the late 19th Century. The use of EAFs has expanded and now accounts for over 70 percent of steel production in the United States.
The EAF is different from the blast furnace as it produces steel by using an electrical current to melt scrap steel, direct reduced iron, and/or pig iron, to produce molten steel.
6 Steps of the Steel Manufacturing Process
The steel manufacturing process can be divided into six steps: Making the iron, primary steelmaking, secondary steelmaking, casting, primary forming, and secondary forming.
#1. Making the Iron.
Steel is a metal alloy made of iron and carbon. Thus, the steel manufacturing process starts by making iron. To do this, limestone, coke, and iron ore are combined and put into a blast furnace. The elements are melted together to create a hot metal known as molten iron.
#2. Primary Steelmaking.
The second step of the steel manufacturing process can be completed with two different pieces of equipment: a basic oxygen furnace and an electric arc furnace. With a basic oxygen furnace, the molten metal produced in step 1 is infused with scrap steel.
Then, oxygen is forced through the furnace to remove the impurities in the molten iron. With an electric arc furnace, as the name suggests, electricity is forced through the furnace to purify the iron. The completion of step 2 results in raw steel.
#3. Secondary Steelmaking.
Just like there are different grades and families of stainless steel, there are also different types of regular steel. The different grades are determined by the elements that remain in the metal at the completion of the manufacturing process.
Secondary steelmaking refines the composition of the steel to create the desired grade. This is done with different techniques such as stirring and ladle injections.
#4. Casting.
During the fourth step of steel manufacturing, molten iron is cast into molds for cooling. This process starts to set the shape of the steel and causes a thin, hard shell to form.
The strands of the shell are malleable and can be worked into the desired shape of flat sheets, beams, wires, or thin strips.
#5. Primary Forming.
Primary forming continues the shaping process. A hot roller is used to fine-tune the casting. The steel is molded into the desired shape and surface finish. Some examples include bloom, billet, and slab.
#6. Secondary Forming.
The final step of the steel manufacturing process creates the final shape and properties of the steel.
Secondary forming is accomplished with different methods such as shaping (cold rolling methods), machining (drilling), joining (welding), coating, heat treatment, and surface treatment.
After step 6, the steel is fully shaped, formed, and ready for use and processing in various applications.
Steel Production: A Story Of Recycling
One of the best features of steel (and other metals) is that scrap can become completely new, high-quality metal. The secondary steelmaking process creates alloys as good as any that come from pig iron. Metal items can degrade from use, but the elemental chemistry of metal means that melting and alloying creates a completely new product.
Growth in steelmaking output therefore doesn’t require matching growth in smelting of new ore (although pig iron production remains a vital part of the steel supply chain.) Reclaiming and processing scrap steel mean that yesterday’s car panel can be tomorrow’s I-beam.
With 98% of steel reclaimable, the metal is one of the world’s most recyclable products. Still, it is not without environmental challenges. Coke, a form of coal, is usually used as the carbon input to steelmaking.
Additionally, the high energy required to melt or smelt and oxidization and other processes of production do create chemicals and carbon dioxide. Fortunately, there is a lot of research being done in the steelmaking sector to mitigate issues with production.
Some include recycling carbon dioxide back into the steel itself, as the source of carbon, lowering need for other sources like coke.
With these technologies refined and implemented, steelmaking will continue to be one of the major industries of the future. It undergirds, drives, and builds our economy.