Due to its excellent properties, mild steel has become an in-demand material in various industries. It has unparalleled weldability and machinability, which has led to an exponential increase in its usage.
In this article, we will discuss the importance of mild steel, its uses and how it’s made.
What Is Mild Steel?
Mild steel is a type of low carbon steel. Carbon steels are metals that contain a small percentage of carbon (max 2.1%) which enhances the properties of pure iron. The carbon content varies depending on the requirements for the steel. Low carbon steels contain carbon in the range of 0.05 to 0.25 percent.
There are different grades of mild steel. But they all have carbon content within the above-mentioned limits. Other elements are added to improve useful properties like corrosion resistance, wear resistance and tensile strength.
How Is Low Carbon Steel Made?
Carbon content is uniformly increased by heat treating steel. As carbon content increases, steel develops hardness but loses ductility. This means that the metal becomes brittle and may fracture instead of bending when applying an excess load.
The manufacturing processes for mild steel are similar to other carbon steels. Higher carbon steels just contain more carbon, resulting in different properties like high strength and hardness values compared to mild steel.
These processes have developed over time and are now much more cost-effective than before. In modern manufacturing, three steps are involved in manufacturing mild steel out of pure iron.
In this leg of the steelmaking process, the iron ore is mixed with coal and lime and heated in a blast furnace having a 100 to 400-ton capacity. Lime acts as a flux and forms a protective layer on top of this hot molten metal.
Modern primary steelmaking uses Basic Oxygen Furnace (BOS) or Electric Arc Furnace (EAF) processes . These are both based on the original Bessemer process for steelmaking.
In the BOS process, oxygen is blown through the molten iron while scrap steel is added in the converter. This reduces the carbon content to a maximum of 1.5%.
In developed countries, the Electric arc furnace is used to feed scrap steel through extremely high powered electric arcs. This results in impressive quality steel.
The second step in the process of making mild steel includes further reducing its carbon contents and adding alloying elements.
Manipulating the furnace conditions (temperature, cooling rate, etc.) also aids this process. Ultimately, the type of steel desired will decide the secondary steelmaking process. One may choose from the following processes:
- Ladle furnace
- Ladle injection
Casting and Primary Forming
Once the steel has the specified carbon content and other elements that enhance its performance, the molten steel is poured into a mould. This is the casting process. Here, the steel is shaped and allowed to solidify. Afterwards, cutting the metal into desired shapes like slabs, blooms and billets takes place.
The product obtained still needs to be imparted a good surface quality free from casting defects. The primary forming process uses hot rolling quite often for this purpose. Hot rolled steel products are usually categorised into flat products, custom products, long products, and seamless tubes.
Although we have the completed mild steel product by the end of the above process, it is usually formed one more time to improve its mechanical properties in line with application requirements.
Secondary forming gives it the final shape and properties. The forming methods include:
- Shaping through cold rolling
- Surface treatment, etc.
Chemical Properties of Mild Steel
As mentioned above, mild steel has lower carbon content than medium and high carbon steels. The carbon content is up to 0.25% in mild steel but some schools of thought consider a carbon steel as mild steel up to a carbon content of 0.45%.
The low carbon content makes this steel a highly machinable metal. It can be cut, machined, formed into intricate shapes without adding proportional stresses to the workpiece. It also facilitates better weldability.
A host of alloying elements an improve the chemical properties. These elements will affect the physical/chemical properties favourably and make the final product suitable for the application. The elements that may be added include chromium (Cr), cobalt (Co), phosphorus (P), sulphur (S), manganese (Mn), among others.
For instance, chromium imparts the property of corrosion resistance and increases mild steel’s hardness. In its pure form, mild steel will rust easily due to oxidation. Unlike iron oxide, chromium metal on exposure to atmosphere forms a dense layer of chromium oxide which does not fall off, and ultimately protects the metal underneath from further corrosion attacks.
Copper in limited quantities also works like chromium oxide. Mild steel pipes may be galvanised for better protection from the atmosphere.
Other elements may be added to improve wear resistance, tensile strength, and heat resistance.
Physical Properties of Mild Steel
Its impressive properties are responsible for a growing use in a variety of industries. Some mild steel’s physical properties are as follows:
- High tensile strength
- High impact strength
- Good ductility and weldability
- A magnetic metal due to its ferrite content
- Good malleability with cold-forming possibilities
- Not suitable for heat treatment to improve properties
Applications and Use-Cases
It would not be an exaggeration to say that if you looked out your window, you would see something made from mild steel. It is the most common type of metal used around us.
From buildings to massive sea-going vessels, there is mild steel involved. Some of the applications are as follows.
In construction, there is a need for materials that can be joined easily and able to withstand changing loads. Mild steel, therefore, is the perfect material for extensive use. Some grades of mild steel are actually called structural steels for that reason.
It also meets strict seismic and wind requirements, cannot be damaged by insects and is resistant to rot or fire.
The fact that it is relatively cheaper compared to other steels doesn’t hurt either. Mild steel is therefore used in structures big or small, from bridges to buildings.
Mild steel is widely used in machinery and automobile manufacturing. It is cheap, suitable for different cutting and coating methods and has good weldability while providing good enough physical properties. These attributes make it useful for producing frames, panels, etc.
Pipelines and Poles
Excellent ductility of mild steel has made it the perfect contender for the manufacturing of pipelines and poles that need to weather extreme atmospheres.
While mild steel pipes can be easily welded to each other, they also retain a certain degree of flexibility. As the pipelines may shrink in cold weather or expand on hot days, this is a necessary feature for the metal.
Cutlery and Cookware
While previously the exclusive domain of stainless steel, modern-day chefs are coming to love mild steel cookware because of its advantages.
The cutting equipment stays sharp for a longer time, has a higher temperature limit and can be modified to become a non-sticking material.
Seasoning the metal helps to overcome the factor of corrosive wear.
Mild steel can be shaped into incredibly specific shapes which makes it perfect to be used in fencing. It also has an amazing visual appeal, so it looks beautiful too.
Mild steel may also be galvanised or coated with the right paint to make it long-lasting and rustproof.
The following are some of the common steel grades. A common denominator for all of them and other similar metals is their universal nature. Their properties make them a popular choice for a wide range of applications.
Equivalent grades: AISI 1008; C10; DC01
EN 1.0301 carbon steel contains 0.1% carbon, 0.4% manganese and 0.4 percent silicon. It also contains small amounts of copper (Cu), nickel (Ni), chromium (Cr), aluminium (Al), and molybdenum (Mb).
This grade has excellent weldability and is commonly used for extruded, forged, cold headed, and cold-pressed parts and forms. It is primarily used in automotive equipment, furniture, and appliances.
Equivalent grades: AISI 1010
EN 1.1121 carbon steel contains carbon in the range of 0.08% to 0.13%. Manganese is present in the range of 0.3% to 0.6%. It is used in the manufacturing of cold headed fasteners and bolts.
This grade also had good formability and ductility and can be formed using traditional methods. It also supports joining by all welding techniques. Its strength may be improved by heat treatment, quenching, and tempering but the cost of carrying out these processes is high.
A general purpose steel largely present in structures and the automotive industry.
Equivalent grades: AISI 1020; C22
This grade of steel has excellent weldability. EN 1.0402 is especially suitable for carburised parts. It also has a good balance between ductility, strength and toughness.
It has a carbon content from 0.18% to 0.23% with a manganese content range of 0.3% to 0.6%. Compared to EN 1.1121, it is not used as much but it does have great machinability in its as-forged condition.
This grade finds use in machinery in the form of hydraulic parts and camshafts.