Differences between Hot rolled steel vs cold rolled steel, advantages, disadvantages, characteristics, and usage.
HOT ROLLED STEEL
1. Hot rolled steel is easier to make, to shape and form. It has its source in a mill process involving rolling the steel at high temperature.
2. It starts from a piece of still billet which is heated up 1700 degrees Fahrenheit (926° Celsius) and then the steel is rolled through the mill into the particular shape.
3. The whole process is done at high temperature and at the end is being cooled down. The cooling down may cause the steel to shrink and therefore there is less control over the final size and shape.
4. Hot rolled steel is commonly used when precise shapes and tolerances are not essential.
5. Characteristics - 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
6. Advantages
– Easier to make: heat it up, push through, cool down and that’s it!
– Cheaper than cold rolled
– Hot rolled steel is allowed to cool at room temperature and it’s free from internal stresses that can arise from quenching or work-hardening processes
– Most popular shapes are hot-rolled (UC, UB, SHS, RHS, PFC, flats etc.)
7. Disadvantages
– dimensional imperfections caused by heating (expanding) and cooling down (shrink, warpage)
– rough texture on a surface, need to be removed and buffed before painting
– Slight distortions
8. 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
1. While hot rolled steel is heated then cooled, cold rolled steel is heated and cooled at the room temperature and then rolled after again.
2. The steel is processed further in cold reduction mills, where the material is cooled (at the room temperature) followed by forming the material by either press-braking or cold roll forming to achieve the desired shape.
3. The term “rolled” is often used just to describe a range of finishing processes such as turning, grinding, and polishing, each of which modifies existing hot rolled stock into a more refined product.
4. Technically, “cold rolled” applies only to sheets that undergo compression between rollers. But forms like bars or tubes are “drawn” not rolled.
5. Hot rolled bars and tubes once cooled, are processed into what we call “cold finished” tubes and bars.
6. Characteristics - 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
7. Advantages
– accurate shape (consistent and straight)
– a wider range of surface finishes
– a smooth and shinier surface
– bars are true and square and have well-defined edges and corners
– Tubes have better concentric uniformity and straightness.
8. Disadvantages
– more expensive
– fewer shapes available cold-rolled (sheets, box section shapes: CHS, SHS, RHS)
– additional treatments can create internal stress within the material; this can cause unpredictable warping if the steel is not stress relieved prior to cutting, grinding, or welding.
9. 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.
10. All cold products provide a superior surface finish, and are superior in tolerance, concentricity, and straightness when compared to hot rolled.
11. Uses: Any project where tolerances, surface condition, concentricity, and straightness are the major factors.
CARBON CONTENT
1. 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.
Source:
https://www.reliance-foundry.com/blog/hot-vs-cold-rolled-steel
https://www.metalsupermarkets.com/difference-between-hot-rolled-steel-and-cold-rolled-steel/
