Cold-Rolled Steel

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About Cold-Rolled Steel

Steel is produced using the hot-rolled or the cold-rolled process. Cold-rolled steel is a sub-product of hot-rolled steel. The processing of steel at the mill is the significant difference between the two types. Hot-rolled steel is rolled at high melting point temperatures, while cold-rolled steel uses hot-rolled normalised steel which is processed further in the cold reduction mills using annealing or tempering techniques. Cold-rolled steel refers to flat-rolled sheets of steel and steel coil products. Often, the term cold-rolled steel is incorrectly used for products that are not necessarily cold-rolled, but may be cold-formed, which is a completely different process of producing steel.

Manufacturing of Cold-rolled Steel

Hot-rolled steel is allowed to cool to a specific room temperature in the cold reduction mill before any type of further processing can take place on the metal. This is called “normalizing” the steel which gives it its hardness and strength.

The cooling of steel is necessary for allowing the recrystallised grains to set in place uniformly and for the carbon to be evenly distributed. Before the cold-rolling process can be initiated, the hot-rolled, normalised steel has to undergo a pickling process. The pickling process is an essential step to descale the steel and have a clean surface ready for the subsequent process of annealing or tempering.

Ductility of the steel is greatly reduced due to the various dislocations during the process of cold-rolling. This reduced ductility is improved by either the process of annealing or tempering.

Annealing The annealing process consists of 3 stages:

  • Stress relief: Steel is heated in a controlled environment between 480°C and 500°C, which allows the atoms inside the steel to move freely and reduce the internal stress
  • Recrystallisation: The temperature is further increased to 500°C to 550°C to allow for new crystal formation which assists in helping the free moving atoms, and new grains formed by recrystallisation, to bind with the cold-rolled grains
  • Grain growth: The recrystallised steel is then soaked at the same temperature till it gets heated uniformly, allowing the steel grains to grow consuming the newly formed crystals
  • The annealing process can be of two types, namely batch annealing and continuous annealing.

In the batch annealing process, protective gases are filled in the interior cover of the base unit, while the steel coils are placed on top of each other and the heating hood is placed in position. The burner heats up the surface of steel which is soaked in the temperature for some time. A cooled hood is then placed, allowing the steel to cool down slowly to room temperature.

In the continuous annealing process, the steel coil is placed near the decoiler and a set of two rolls perform the continuous movement of feeding the steel into the heat-treating section. Once the soaking of steel is completed, it is allowed to cool down to room temperature.

Tempering The tempering process is mostly done on quench-hardened steel to remove its brittleness. In the tempering stage, steel is reheated between 150°C to 400°C, allowing it to soften while gaining toughness. After uniformly soaking the steel in heat, it is allowed to cool gradually. Tempering of steel is done in three different ways using a range of temperatures which help the steel to gain the required toughness depending on its further usage. The cold-rolling of steel helps to achieve a smooth surface finish, improved strength, and uniform dimensions.

Advantages of Cold-rolled Steel

  • Cold-rolled steel products have a smoother surface finish in comparison to the steel products manufactured using the hot-rolling technique
  • The dimensions of cold-rolled steel are precise and its thickness is more uniform.
  • There is no internal stress in these products and they are not brittle
  • The cold-rolled steel has a higher tolerance unlike other steels
  • This steel is hardened and exhibits high strength with low ductility and more formability.

Uses of Cold-rolled Steel

Cold-rolled steel is produced as coils, sheet, strips, and plates in multiple grades and finishes. They are primarily used in the automotive sector in the manufacturing of the vehicle body. As cold-rolled steel is highly ductile and has excellent formability, it is suitable for manufacturing automotive parts. The other big contender for using cold-rolled steel is the construction sector. Plates and sheets cut out of cold-rolled steel are used for various types of fabrication processes.

Apart from the above mentioned applications, cold-rolled steel has a multitude of other uses due to its superior smooth surface, including manufacturing of steel tables and equipment for medical, hospitality, and general household.

Related Metal Products Supplied by SteelScout

We also supply a wide range of mild steel including Structural Sections, Tubing, Sheet, Merchant Bar, Re-Rolled Steel, Hollow Sections, Rebar, Flooring and Mesh, available in standard as well as custom dimensions for all your project needs. Simply tell us what you need, and we will provide you with a quote in 20 minutes to 3 hours. Once we receive the purchase order, we will complete your order and deliver to suit your requirements.

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