One of the most common questions in forming thick steel plate by plate rolling is whether the plate needs to be heated before bending. This article looks at preheating in plate rolling for thick sections, when it becomes necessary, and the limits of cold forming.
How Thick Can Cold Rolling Go?
Most plate rolling machines operate at room temperature, that is, cold. As long as the roll diameter, the distance between rolls and the drive power of the machine are sufficient, most carbon steel plate can be brought to the target diameter cold. As plate thickness increases, the bending moment required to reach the same diameter also increases, so the limit of cold rolling capacity cannot be assessed separately from the machine's mechanical power and roll dimensions. When the maximum moment the machine can deliver is no longer enough for a given thickness and diameter combination, the plate cannot be brought to the required curvature cold.
Where Preheating Comes Into Play
Mechanical Capacity Limit
On thick plate, especially when a small diameter-to-thickness ratio is required, the force needed for cold rolling can exceed the machine's capacity. In this case, heating the plate to a certain temperature lowers the material's yield strength, allowing the same machine to form it with less force. Here, preheating in plate rolling is not a quality preference but a mechanical necessity.
Material Ductility and Cracking Risk
Some high-strength or low-ductility steel grades can show a tendency to crack where tensile stress concentrates on the outer surface during cold rolling. This risk becomes more pronounced as thickness increases and the inner bend radius decreases. The elongation and impact values stated on the material certificate should be used as a reference when assessing whether cold rolling is safe. In borderline cases, preheating is chosen as a measure to reduce cracking risk.
Residual Stress and Downstream Processing
On thick plate rolled cold, residual stresses build up inside the material after bending. If the part will later be welded or machined, the level of these stresses becomes significant. On some projects, preheating before rolling or stress-relief annealing after rolling is chosen to reduce residual stress; which path is followed depends on the part's intended use and the project specification.
Setting the Preheat Temperature
Where preheating is found necessary, temperature is set according to material grade and thickness; since excessive temperature can adversely affect the material's microstructure, heating is generally kept within the range given by the material supplier. Heating must be uniform across the plate's cross-section to avoid local hot spots, since uneven heating can cause unexpected differences in curvature during rolling. The scale layer that can form on the plate surface after heating should also be checked before rolling, as surface roughness there can affect roll marks.
Precautions When Cold Rolling Is Preferred
If thick plate will be rolled without preheating, keeping the rolling speed low and advancing the bending passes gradually limits sudden stress buildup. Setting the distance between rolls to suit the thickness, and feeding the plate symmetrically between the rolls, prevents uneven deformation through the thickness. On critical projects, carrying out a trial bend on a sample from the same plate batch before production shows whether cold rolling is safe before committing the full run.
Interaction With Springback and the Flat End Problem
Whether or not preheating is applied to thick plate also affects the amount of springback and the length of the flat end zone. Since heated material generally shows lower yield strength, the same target diameter can be reached with a smaller compensation allowance; in cold rolling, the compensation allowance needs to be calculated more carefully. For more detail on the flat end problem, pre-bending and springback control in plate rolling, see our article on the plate rolling process.
The Situation on Stainless and Special Alloy Plate
On materials such as stainless steel, the preheating decision is not limited to mechanical capacity alone; additional factors such as surface integrity and roll contamination also come into play. If heating is applied to these materials, surface oxidation must be kept under control. For more detail on surface protection and roll contamination in stainless steel plate rolling, see this article.
Comparison With Other Cutting Processes
The concept of preheating is a decisive parameter not only in plate rolling but also in other fabrication steps such as oxy-fuel cutting of thick plate. Looking at the relationship between preheating and cutting speed for thick plate in oxy-fuel cutting is useful for seeing how preheating serves different purposes across processes; see our article on preheating and cutting speed for thick plate.
A Practical Approach to the Decision
When deciding whether preheating in plate rolling is needed for a given project, plate thickness, target diameter, material grade, the machine's mechanical capacity and the part's intended downstream use should be evaluated together. Deciding on a single parameter alone can create both unnecessary cost and quality risk. A trial bend and a review of the material certificate confirm this decision before production begins.
DMK Makina's Approach to Thick Plate Rolling
Based in Lüleburgaz and serving customers across Thrace and Marmara as well as industrial projects throughout Turkey and Europe, DMK Makina assesses the preheating decision for thick plate rolling based on material grade, thickness and target diameter. Where necessary, controlled preheating is applied; otherwise, cold rolling passes are advanced in a gradual and controlled manner to preserve the part's geometric accuracy and material integrity.
Conclusion
Preheating in plate rolling for thick plate is not a step that must be applied automatically on every project; it is a decision that depends on the machine's mechanical capacity, material ductility and the part's intended use. Preheating comes into play when the thickness and diameter ratio pushes the machine's limits or the material carries cracking risk; otherwise, controlled cold rolling can be sufficient. Making the right call here is decisive for both production efficiency and part quality.
