Optical strip flatness and shape measurement in Hot Strip Mills

SSAB is a Swedish company in the steel sector and in many ways a pioneer in the production of world-leading steel grades such as Hardox© for wear plates, Strenx© for performance steel, Docol© for automotive steel. SSAB is also a world-leader in the ambition to make steel production and use sustainable. One example is SSAB´s target to use hydrogen to produce fossil free steel in 2035. Another example is to provide material and services for making lighter and more environmentally friendly constructions. A base for this is Advanced High-Strength Steels (AHSS) and Quenched & Tempered Steels (Q&T). Since year 2012, SSAB produces Hardox directly in the hot strip mill, a great challenge in terms of direct quench capacity and maintaining material properties and shape over strip width and length. The benefit is both economical (one processing step is omitted) and environmental (less energy and capacity is required). It has turned out that flatness is the most important and difficult property to master in a direct quench line.

Shapeline is a Swedish company focused on in-line optical measurement of flatness and other dimensions of flat metal. Shapeline has supplied flatness measurement systems since 1997 and now has a large number of systems in the world’s steel and metal industries. Shapeline´s ambition is to develop new technical solutions which deliver performance, reliability and information required by a developing steel and metal industry.

In year 2014, Shapeline and SSAB commenced a cooperation to develop a concept for flatness measurement which could be used to analyze how flatness affects laminar cooling and how incoming flatness transforms into outgoing flatness after the accelerated laminar cooling. The project, Opticool, has had financing support from Sweden´s innovation agency, Vinnova.

The first step in this was to develop means for reliable and high performance flatness measurement after the mill and the second step to measure both before and after laminar cooling to understand the mechanisms behind the flatness transformations.

The first part of the project to realize the flatness gauge for hot environments was reported by Kierkegaard and Hedberg in an earlier paper [5]. This paper focuses on how the two gauges have been used to understand how the laminar cooling affects flatness.

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