Hot Rolled vs Cold Rolled Steel
Rolling is a process where metal is passed through a pair of rollers, to change its shape and make it suitable for certain purposes. Metal rolling has a long history, which takes back to 17th century. Earlier than this, there were slitting mills where flat bars of metal were passed through rollers to form a plate of metal. Then they were passed through slitters, to produce metal rods. Early rolling mills were for iron. But later mills for lead, copper and brass were also developed. Modern rolling was introduced by Henry Cort in 1783. Rolling can be classified into two based on the temperature of the metal being rolled. These are hot rolling and cold rolling.
Steel is an alloy, which mostly consists of iron. It has a low percentage of carbon and other elements mixed to enhance certain characteristics. For example, it is made with increased hardness, rust-resistance, etc.
Hot Rolled Steel
This is a metalworking process where it happens in high temperatures. Usually the temperature is above the recrystallization temperature of the steel. First large steel pieces are directly sent to the rolling mills which are at the proper temperature. During the hot rolling process, the temperature should be maintained above the recrystallization temperature. At any occasion if the temperature drops, then the steel should be re-heated. When steel is pushed through rollers, they squeeze the metal and give it a shape. Hot rolled steel is rough, and it has a blue-grey tone to it. This is because the red hot metal goes through rolling for a longer period. Therefore, metal surface has a longer period to oxidize and produce a thick metal oxide layer which also has this grey, blue color. Hot rolled steel has many numbers of shapes. This is because heated steel can be easily molded into any shape. When this is cooled back, the given shape will remain in the steel.
Cold Rolled Steel
This is the process where final rolling is happening below the recrystallization temperature of the metal. Since cold steels are strong, they cannot be changed into so many different shapes. So there are only few shapes as flat, round, etc. Cold rolled steel has a smooth and grey finish. Since the final step is happening in the room temperature, they do not get oxidized. Therefore, they show the actual grey color of steel.
What is the difference between Hot Rolled and Cold Rolled Steel?
• Hot rolled steel has a rough, blue-grey finish, whereas cold rolled steel has a smooth grey finish.
• In hot rolled steel, final rolling is done when the steel is hot. In cold rolled steel, final rolling is done when the steel is cooled to the room temperature.
• Finished product in hot rolled steel has oxidized steel layer, but the finished product of cold rolled steel is unoxidized.
• Hot rolled steel has so many shapes, but cold rolled steel has few shapes.
• Cold rolling cannot reduce the thickness as hot rolling does. Therefore, a sheet of steel processed by a single pass through a roller in cold rolling is thicker than that in hot rolling.