When we talk about precision sheet metal fabrication, cold-rolled steel usually enters the conversation, and for good reason. A cold-rolled steel sheet is essentially hot-rolled steel that has undergone further processing, usually involving compression between rollers. Hot-rolled steel sheet comes from a heated slab that is flattened and run through rollers to achieve its final dimensions, then cooled after processing. This locks the microstructure into place by preventing diffusion, but it also creates a less controlled final shape than its cold-rolled counterpart. The process of holding hot-rolled steel at extremely high temperatures also creates a scaly surface.
Hot-rolled steel has built this country. Buildings are made of it, as is much of the heavy equipment that makes those buildings and the trains that carry the raw material. More and more of these applications are calling for greater precision, which is why more hot-rolled plates are ending up on cutting beds under a high-power laser cutting beam. The one hurdle when laser cutting hot-rolled steel is the material’s surface quality. These challenges were the impetus behind a recent study Steel Warehouse conducted to test the effects of hot-rolled steel surfaces on laser cutting.
The study revealed that throughout the thickness range of hot-rolled material, surface quality has the greatest impact on cutting performance. Get the material surface right, and dialing in the remaining laser cutting parameters becomes much easier.
Overall, lighter-gauge stock offers more flexibility of laser adjustment, while thicker stock offers a smaller process-parameter window to obtain a good cut edge. The smaller the window, the more challenging it is to dial in laser cutting parameters for optimal cutting.
Any operator who has pushed a machine to its material-thickness limits knows this all too well, and it makes sense intuitively. Machine variables abound: gas flow, laser power, focal point, kerf width setting, and gating or pulsing frequency. (Heavier materials tend to respond well to a lower gating frequency, which puts more heat into the kerf at the same laser power level.)
But these variables can be pushed only so far; at some point, the only variable that can be improved upon in a significant way is the material quality. The study basically quantified this experience, but it also showed that, at any thickness, material surface quality has an outsized effect on cut quality.
What defines “quality” material for laser cutting? It must be flat, of course, but it also must be smooth—and the study quantified this assumption. For apples-to-apples comparison, cutting parameters were kept at the machine’s factory settings; cutting speed was the only parameter that was changed between runs.
The study tested three varieties of hot-rolled material. It first cut straight-from-the-mill, hot-rolled material, scale jacket and all—commonly called hot-rolled black (HR-black). The study also tested hot-rolled pickled and oiled (HRP&O) material as well as blasted material, including hot-rolled blasted (HR-blasted) sheet and blasted plate.