What is hot rolled and cold rolled?

style="text-align: left;"> What is Hot rolled steel? 

Steel mill is a place where is steel is produced and rolled.  Hot rolling is the rolling of steel in mill with high temperature. The temperature is about 1700 degree F. This temperature is above recrystaline temperature. At that temperature, crystal lattice structure of the metal become reoriented. As such metal become more workable. The recrystaline temperature of steel is between 400 - 700 deg C. Recrystaline reduces hardness of metal as well as increase ductility. Hot rolled steel is cheaper than cold rolled steel. If we cool down steel , that will shrink and shrinks as such there is less control on its shape and size. As such it is used where particular shape and size is not essential. For example railroad and I beams. 

Cold Rolled Steel

Cold rolled steel is essentially hot rolled steel that has had further processing. The steel is processed further in cold reduction mills, where the material is cooled (at room temperature) followed by annealing and/or tempers rolling. This process will produce steel with closer dimensional tolerances and a wider range of surface finishes. The term Cold Rolled is mistakenly used on all products, when actually the product name refers to the rolling of flat rolled sheet and coil products.

When referring to bar products, the term used is “cold finishing”, which usually consists of cold drawing and/or turning, grinding and polishing. This process results in higher yield points and has four main advantages:

• Cold drawing increases the yield and tensile strengths, often eliminating further costly thermal treatments.
• Turning gets rid of surface imperfections.
• Grinding narrows the original size tolerance range.
• Polishing improves surface finish.

All cold products provide a superior surface finish, and are superior in tolerance, concentricity, and straightness when compared to hot rolled.

Cold finished bars are typically harder to work with than hot rolled due to the increased carbon content. However, this cannot be said about cold rolled sheet and hot rolled sheet. With these two products, the cold rolled product has low carbon content and it is typically annealed, making it softer than hot rolled sheet.

Uses: Any project where tolerances, surface condition, concentricity, and straightness are the major factors.

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You can check out our video below to find out the difference between hot rolled steel and cold rolled steel: