Mild steel is the most popularly used steel which plays a vital role in different businesses. It is a type of alloy in which carbon plays the role of a primary alloying element. An alloy is a mixed product of metals and non-metals. In cases when pure metals do not fulfil all the manufacturing properties of a product, manufacturers include additives to obtain the necessary properties like hardness.
In the case of mild steel, the level of carbon is one of the most important factors which control its mechanical properties. Along with the carbon, the other elements like iron, zinc, etc. are mixed to improve the ductility, tensile strength and hardness of the mild steel.
What is the composition of Mild Steel?
Mild steel composition is as follows –
- Carbon between 0.16 – 0.18 % (max allowable 0.25%)
- Manganese between 0.70 – 0.90 %
- Silicon up to 0.40% (maximum)
- Sulphur 0.04% (maximum)
- Phosphorous 0.04% (maximum)
Properties of Mild steel
Mild steel is widely used in various industries. Hence, people dealing with manufacturing and production business must have sufficient knowledge of the essential properties of mild steel.
- The modulus elasticity of mild steel is 210,000 Mpa which is similar to industry grade steel with a density of 7860 kg/m3.
- As mild steel conducts electricity, hence it is readily used for the welding process.
- Mild steel is malleable, and due to this property, it is used as constructing materials like pipelines.
- Mild steel is ductile and hard.
- Mild steel contains ferromagnetic property. Hence it can be magnetised easily.
- Mild steel is ideal as a structural steel.
- The hardness of the Mild steel is usually increased by applying carbon. easily
- Mild steel contains iron. Hence it easily forms rust.
- Subject to oxidation if not adequately coated
- Mild steel contains a negligible amount of alloying elements which also helps it to be affordable than other steels. Hence, the affordability, machinability, and weldability have made it a popular choice among the steel consumers.
How to make Mild steel?
Mild steel is manufacturing is a three-stage process:
- Primary making of steel
- Secondary making of steel
Different finishing processes follow the stages mentioned above which directly affect the characteristics of the final product.
Steps of Primary steelmaking
Primary steel can be made in two ways –
- From 100% recycled scrap steel
- By combining virgin steel and recycled material.
Process 1: Using 100% recycled material
Using Electric Arc Furnace metal steel can be made by reprocessing recycled scrap steel. In this process, high power electric arcs melt the metal at a high temperature up to three thousand degrees Fahrenheit. Once the scrap steel is melted, manufacturers add more scraps in the furnace until it capacity permits. After a flat bath of molten steel is created, manufacturers blow oxygen through it to oxidize excess carbon.
Process 2: Combination of virgin steel and recycled material
This process follows the below steps:
Blast furnace application
This is the first stage of primary steelmaking where a manufacturer produces virgin steel from coke, iron core and lime putting them in a blast furnace with a temperature of 3000 degrees Fahrenheit. In this process, the carbon is released from the mixed of melting iron ore and burning coke. The lime absorbs the impurities.
Basic Oxygen Furnace (BOF) application
The product produced at the first stage contains almost 4% carbon with a certain amount of impurities present in it. To remove it Basic Oxygen Furnace (BOF) is used where the virgin steel is delivered in a molten state. Once it is transferred a blow of oxygen which is 100% pure is applied to the oxidisation of the extra carbon. This forms a finished steel which contains carbon density between 0% -1.5%.
In both cases, for further processing, manufacturers tap molten steel into steel baths and ladles out of the furnace. Also, the surface slag impurities are removed.
Steps of Secondary steelmaking
Secondary steelmaking is an on-demand process when there is a need for products with high-quality steel along with consistent properties. This is made by altering the temperature of the individual components. Below processes are followed to make secondary steelmaking.
- Stirring: In this process to separate non-metallic parts of the steel, the ladle is induced with electromagnetic fields. The turbulent currents isolate the non-metallic portions from the surface. As a result, it generates the steel with a homogeneous composition.
- Ladle furnace: A ladle controls temperature of the furnace, and it performs the function of a secondary electrode. Besides that, it measures alloy components injection.
- Ladle injection: In this process, using an inert gas a stirring effect is introduced. Once the manufacturers inject the gas into a steel bath, it raises the temperature of the molten steel causing a stirring effect.
- Degassing: Gases like hydrogen, nitrogen, and oxygen often stay inside molten gas. To degas various techniques like vacuums, temperature variation or inert gas injection is used
- Sealed Argon bubbling using Oxygen Blowing for Composition Adjustment (CAS-OB): Oxygen is applied to the bath along with aluminium by a lance to increase the temperature level and to accurate steel composition.
Deoxidation is a critical process as oxygen present in molten steel can react with carbon and releases carbon monoxide. Now based on the level of Deoxidation the properties of the finished steel product is determined and so as the Mild steel properties.
Casting method follows the below steps:
- In traditional casting method, the ladle is lifted by crane to fill it up with molds that are placed on the rail cars. For Ingot molds, they are removed after solidification. After that Ingots are placed into soaking pits for reheating.
- Through casting process, molten steels are also brought into shapes which enable more suitable downstream processing.
- Once the manufacturers lift the ladles to elevated position molten steel is discharged into an open container, commonly known as tundish.
- From this open container, the molten material is fed to the casting machine.
- From the casting machine, the manufacturers feed the steel in a molten state into a rotating bottom plate which contains mold that is water-cooled.
- As the steel starts to solidify, the movable plate is gradually lowered to allow extra molten steel into the mold.
- Steel slabs are formed using a casting machine.
- Rollers pull the solidified steel before they take a straight form and cut it at the edge of the machine. Additionally, this process continues for weeks or days without interruption.
Finishing procedures for Mild steel
This stage is required to cast the solid ingots into useful sizes and shapes. In this stage –
- Rotating rolls compress the steel and pull it as well. Once the rolls start to revolve at a higher speed than the steel, it starts to enter the machine. As a result, the steel experience the forward thrust by the rolls and applies compression between them. The process can be of two categories hot forming and cold forming:
- In case of hot forming, a temperature above recrystallization temperature is applied to the steel, and it breaks up the microstructure. Within the steel, this results in an enhanced uniform size and an evenly distributed carbon.
- In cold forming, a less temperature than the recrystallisation temperature is applied to the steel. This process enhances the strength of the steel almost 20% by strain hardening. Besides that, it improves the finish tolerances.
- The rolling phase generates the semi-finished steel with various forms like slabs, billets or blooms based on its required final dimensions.
- Further processing is performed in a rolling mill on the semi-finished products to generate intermediate products. This makes it market ready for the downstream companies.
- Once the steel is out from the rolling mill and ready for the downstream companies, various secondary processing methods are applied on it to prevent the corrosion. This, in turn, improves the characteristics of the mild steel.
What are the basic differences between steel and mild steel?
- Mild steel is more malleable and softer than Steel.
- Steel is more brittle than Mild steel
- Mild steel is less resistant to corrosion than Steel.
- Steel contains very little carbon and it is usually alloyed with metals like nickel, chromium, molybdenum etc to improve its chemical and mechanical properties.
What are the standard applications of mild steel?
There are several common areas in which mild steel might be used:
- Structural steel
- Furniture and decorations
- Different automobile manufacturers also use mild steel for making the body and parts of the vehicle.
As we have discussed on mild steel and its several aspects how to create it and mould it to maintain a good structure, it is obvious to keep the quality in the overall process. However, with an experienced and quality manufacturer of mild steel, we can easily maintain all the parameters.