What is it?

Seams are the most common of defects. They are long and thin, and may be closed at the surface. However, they are not welded shut. Seams can vary in length, be seen by the naked eye, and are straight in shape. They are commonly mistaken for scratches. This defect happens as a result of bubbles being formed when the steel is cooled and solidified, and their variance in length is due to the rolling process.