1. General

1.1 Evolution of the microalloy

Until the 1950s, steels were divided into ordinary and special steels.

Ordinary steels and unalloyed special steels were alloyed only with carbon, sometimes with manganese or silicon as quenching elements, and possibly with a little aluminum.

Special alloy steels generally had alloy contents above 0.2%.

The first exception, linked to the presence of an element in very small quantities in solid solution, concerned aluminum when it was considered active in the metallic state in thin or strong NAI-forming sheets.

For simplicity's sake, aluminum will not be considered as a microalloying element in this article, despite its solution content of around 0.03 to 0.06%,...