Steel

 Steel

https://www.millfieldvelo.com.au/columbus-steel-tubing-timeline/

http://italiancyclingjournal.blogspot.com/2007/11/columbus-steel-tubing-for-frame.html  

https://forums.roadbikereview.com/merckx/merckx-columbus-tubing-tsx-vs-strada-os-189342.html