September 27, 2023
What is the difference between CR and HR steel?
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. 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. 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.
Hot rolled steel is commonly used when precise shapes and tolerances are not essential.
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.)
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
While hot rolled steel is heated then cooled, cold rolled steel is heated and cooled at the room temperature and then rolled after again. 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.
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. Technically, “cold rolled” applies only to sheets that undergo compression between rollers. But forms like bars or tubes are “drawn” not rolled. Hot rolled bars and tubes once cooled, are processed into what we call “cold finished” tubes and bars.
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.
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.
