TPT March 2022

Reducing scale-related problems during the process of making steel for tube production By Daniel Vila-Flor, Dipl.-Ing, DMK Oy

Yield loss is not the only problem associated with scale. Other major problems that affect quality are rolled in scale defects. Scale by its nature is harder than the hot piece of steel being formed. If it is left on the surface of the steel when the billet/ shell/slab/etc is rolled, the scale is rolled into the surface causing pits, scratch, cracks, or other-shaped defects, in the surface. As the billet/shell/slab/etc is then elongated, the size of these defects are multiplied by the elongation and can stretch up to the entire length of the final product. The result is significantly poor surface quality with potentially the whole piece needing to be recycled. In addition to the surface of the product being damaged by scale, the scale may also stick to and damage the surface of the rolls being used to shape the billet/shell/slab/etc. When this happens, the rolls stamp markings onto the billet/shell/ slab/etc causing repetitive surface damage. Depending on the process, this damage may worsen the surface quality and can also cause the end product to be recycled. The surface of the rolls are also damaged by the scale, something that degrades the tool life of the rolls and increases tooling costs. The industry standard solution is descaling. Descaling is the process where a hot billet/shell/slab/etc comes out of the furnace and the surfaces are blasted with high pressure water to remove the scale. This solution works but is not without its limitations. The first problem is efficiency. It has been reported that descaling by water is not effective with the more specialty steels. As a result higher pressure more expensive descalers have been installed in an attempt to overcome this obstacle. The second, perhaps more significant problem, is heat loss. Because the surface of the steel is blasted with water, this cools the surface down; with a lower temperature it becomes harder to roll multiple pass with the billet/shell/slab/etc. Because of this billet/shell/slab/etc are typically descaled only once, because otherwise the temperature drops so low that the steel becomes extremely difficult to roll. This is problematic because during each rolling pass, the surface is elongated, allowing for additional new scale to form. While not all steel-forming processes require it, some processes sandblast or pickle the end piece during finishing to remove scale and other surface defects. This process, depending on the level of surface quality, entails an additional cost, which, in some cases, can be significant. Depending on the pickling technology being used, the cost can vary between €12-15/ ton. With a mill producing 1.3million tons annually, the annual pickling cost may vary between €15.6 to €19.5mn. The solution to all of these problems is ScaleFree technology. ScaleFree technology consists of two components: equipment and a coating product.

The formation of scale begins when the surface of a steel billet/shell/slab/bar/etc, is heated to high temperatures and exposed to oxygen. High temperature is required to enable the reaction of oxygen to the steel, thus forming scale (or oxides). Depending on the process and material conditions, scale formation, and resulting yield losses, have been typically reported to be between 2-4 per cent but, in some cases, actual losses may be much greater than that. The cost to the producer of steel can be easily calculated: The problems that scale causes occur immediately after formation and may compound during the following rolling and forming processes. Although there have been signifi- cant studies conducted by various steel mills, and at the EU level, no proper technology by these parties has been developed that would solve the problems associated with scale. ScaleFree technology, as the name suggests, is a technology developed, tested and proven to work to reduce scale-related problems extending from casting to finishing. Within the process of making steel, regardless of shape, size or material, the process creates scale. Scale (also known as oxide) forms when the surface of the steel is heated to temperatures exceeding 600 o C while being exposed to oxygen.

1,3 million tons/year * 3%

*

500eu/ton = €19,500,000

Annual production

scale formation

cost of

cost of

losses

steel

scale loss

As demonstrated above, the cost of yield losses due to the formation of scale is significant. However, as the formation of scale is inherent to the process of making steel, steel producers have grown accustomed to accepting these needless losses. ScaleFree technology has been proven to reduce scale formation from up to around 45 per cent and, in some cases, up to 90 per cent.

Effect of ScaleFree technology

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TUBE PRODUCTS INTERNATIONAL March 2022

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