Hardened and Tempered Steel Strips are produced by two heat treatment stages with cooling in between. The flatness is significantly affected by several factors, such as line speed, pass-line, temperatures and air-cooling angles. The picture below shows (from the side) how a uniform strip changes shape with different parameter changes.
The main conclusion is that several parameters affect the flatness to a high extent and by changing them; the flatness can be changed and improved. Since precision strip lines as well as the chemical material composition are different from producer to producer, recipes and control strategies must be worked out case-by-case. However, without a systematic and reliable automatic measurement this will be impossible.
Shapeline has a long reference list of special steel producers, starting with the very first Shapeline flatness gauge from 1997. The gauges are used both for quality assurance and process parameter tuning.