All types of hot and cold rolled steel are passed through either two or four rolls. The rolls squeeze the steel and apply pressure to flatten or shape it into the desired shape, which changes the physical properties of the steel. The steel almost always goes through rolls more than once until the shape or approximate shape of the steel is achieved.
Hot Rolled Steel
The process of hot rolling involves rolling steel at a temperature above the steel’s recry-stallization temperature, which takes place over 1700 degrees Fahrenheit. Steel that is above the recry-stallization temperature can be shaped and formed much more easily than colder steel, and the steel can start in much larger sizes. It is also typically cheaper to manufacture than cold rolled steel and is often manufactured without pauses or delays in the process so the steel doesn’t need to be reheated. During the cooling process, hot rolled steel will shrink and the size and shape of the steel will be less predictable than cold rolled steel.
Hot rolled products will have a scaly grey finish and more rounded and less precise corners than cold rolled steel. This makes hot rolled steel more ideal for applications where extremely precise dimensions are not necessary, and neither is the appearance. Sometimes the scaly finish is preferred for the end product in machining or metalworking.
Using the very common 1018 as an example, the chemical properties of 1018 steel will be the same. However, we can see there are key differences between cold rolled and hot rolled.
Reduction of Area
Elongation in 2"
Cold Rolled Steel
Cold rolled steel is manufactured at temperatures below its recry-stallization temperature, typically at around room temperature. Because the steel is manufactured at a much lower temperature, there’s no need to worry about the steel shrinking or changing form or appearance.
The cold rolled steel end products like cold finished steel bar has a much smoother appearance and may have square corners more accurate in dimension and finish.
See the chart above. The yield strength and tensile strength of cold rolled will be higher than hot rolled, which makes cold rolled less likely to fracture under pressure.
In a word, some of the key differences in hot rolled vs cold rolled steel are as the followings
1. Hot rolled steel is rolled or shaped at a high temperature while cold rolled steel is rolled at room temperature.
2. Cold rolled has a smooth and shiny finish while hot rolled has a grey and scaly finish.
3. Cold rolled has sharper corners and more precise dimensions than hot rolled.
4. In general, cold rolled has better mechanical properties than hot rolled.
5. Cold rolled is more expensive than hot rolled.