Hot Rolled vs Cold Rolled Steel
Steel comes in many grades, specifications, shapes, and finishes—the World Steel Association lists over 3,500 different grades of steel, each with unique properties. The various types mean that steel can by widely used in infrastructure, appliances, vehicles, wind turbines, and many more applications.
Optimizing steel’s properties for each application goes beyond changing the chemical composition, however. The manufacturing processing of steel can also have a significant impact on steel products—even when the grades and specifications are the same. One key distinction among pre-fabricated steel products is the difference between hot rolled and cold rolled steel.
Hot Rolled Steel
Hot rolling is a mill process which involves rolling the steel at a high temperature (typically at a temperature over 1700° F), which is above the steel’s re-crystallization temperature. When steel is above the re-crystallization temperature, it can be shaped and formed easily, and the steel can be made in much larger sizes.
Hot rolled steel is typically cheaper than cold rolled steel due to the fact that it is often manufactured without any delays in the process, and therefore the reheating of the steel is not required (as it is with cold rolled). When the steel cools off it will shrink slightly thus giving less control on the size and shape of the finished product when compared to cold rolled.
Hot rolled steel can often be identified by the following characteristics:
- A scaled surface—a remnant of cooling from extreme temperatures
- Slightly rounded edges and corners for bar and plate products (due to shrinkage and less precise finishing)
- Slight distortions, where cooling may result in slightly trapezoidal forms, as opposed to perfectly squared angles
The mechanical properties of steel often depend on their grade, or chemical makeup. For that reason, it is never safe to make assumptions about the mechanical properties of hot rolled vs cold rolled unless they are the same grade. 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.
Hot Rolled Cold Rolled Tensile Strength 67,000 psi 85,000 psi Yield Strength 45,000 psi 70,000 psi Reduction of Area 58 55 Elongation in 2″ 36 28 Brinell Hardness 137 167
Uses: Hot rolled products like hot rolled steel bars are used in the welding and construction trades to make railroad tracks and I-beams, for example. Hot rolled steel is used in situations where precise shapes and tolerances are not required.
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.
Cold rolled steel can often be identified by the following characteristics:
- Better, more finished surfaces with closer tolerances
- Smooth surfaces that are often oily to the touch
- Bars are true and square, and often have well-defined edges and corners
- Tubes have better concentric uniformity and straightness
Uses: Any project where tolerances, surface condition and straightness are the major factors.
The following are some of the key differences in hot rolled vs cold rolled steel:
- Hot rolled steel is rolled or shaped at a high temperature while cold rolled steel is rolled at room temperature.
- Cold rolled has a smooth and shiny finish while hot rolled has a grey and scaly finish.
- Cold rolled has sharper corners and more precise dimensions than hot rolled.
- In general, cold rolled has better mechanical properties than hot rolled.
- Cold rolled is more expensive than hot rolled.