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Carbon Steel: Categories and Finishing

Steel is produced in large numbers each year for the purpose of making a diverse range of products- from structural beams for skyscrapers to sewing needles. Carbon steel is one of the most widely used alloys which makes up approximately 85% of all production processes. It has a carbon content of 0-2% which affects the steel’s microstructure, lending it toughness and strength. It also consists of silicon, copper, and manganese in small amounts.
Carbon steel categories
Based on the chemical composition and characteristics, carbon steel can be categorized into many different types. Mild steel also has a similar carbon content and is a type of low-carbon steel. There are 4 categories of plain carbon steel which is free from alloys:

1. Low carbon steel or mild steel
Being the most common carbon steel grade, it comprises carbon content in the range of 0.04-0.3%. Mild steel has a carbon content of 0.05-0.25% and is highly formable, ductile, and can be used for wire products, plates, and automobile body parts.

2. Medium carbon steel
It has a 0.6-1.65% of manganese and carbon content in the range of 0.31-0.06%. For further adjustment of its mechanical properties and microstructure, this steel can be heated and quenched accordingly. Its popular application includes axles, rails, shafts, gears, and railway wheels.

3. High carbon steel
The carbon content range of high carbon steel is 0.6-1% while the manganese content is 0.3-0.9%. It has properties that make it ideal to be used in high-strength wires and springs. Its welding process is a bit complex. It is mostly used for springs, high-strength wires, and edged tools.

4. Ultra-high carbon steels
Comprising a carbon content of 1.24-2%, it is also known as experimental alloy. It has a great hardness level and is ideal for applications such as axles, knives, and punches.

Carbon steel manufacturing
The manufacturing of carbon steel takes place in three stages:
1. Primary steelmaking
2. Secondary steelmaking
3. Casting

Carbon steel finishing
Post the completion of the carbon steel manufacturing process, it is finished using heat treatment, rolling, surface treatment, or downstream secondary processing.

Product rolling
Under the process, solid cast ingots are rolled into useful sizes and shapes which is done via compression and pulling. The rolling is conducted at a fast pace which pushes the steel further and compresses it.

Hot forming
Under this, steel is heated above its recrystallization temperature for the purpose of breaking up the as-cast microstructure. This lends it a uniform grain size and an even carbon distribution within the steel.

Cold forming
Under this, the steel is heated below its recrystallization temperature which leads to an increased strength through strain hardening by up to 20%. The resultant cold-rolled carbon steel has an improved finish and allows tighter tolerances. The steel, post the rolling process is in the form of semi-finished products either as billets, blooms, or slabs which depends on the final dimensions. While bloom is a thick rectangular slab, billet has a narrow width and similar thickness. A slab is thinner. These semi-finished products are further processed into intermediate products. This makes them ready to be manufactured and finally processed by downstream cold-rolled steel suppliers.