Source: Antique objects and classical literature
Nanban tetsu (南蠻鉃) literally means "Southern barbarian steel". The word referred to foreigners that typically came sailing to Japan from the south, and basically just means "foreign".
Various sword and spear blades are known with inscriptions of them being made of nanban tetsu. Swordsmiths like Hizen Tadayoshi, Kunikiyo, and Yasutsugu and were known to use this steel for their swords and inscribe it on the tangs. See for example a Yasutsugu auctioned at Christie's.
The exact origins of the steel are unknown, and there may have been various sources. The first generation Yasutsugu may have worked with materials from the Dutch ship "de Liefde" which stranded in 1600, its contents many iron objects including 18 cannon that came into the possession of Ieyasu Tokugawa (1543-1616), the first shōgun of Japan, by whom Yasutsugu was directly employed.1
Other sources may have been scrap metal ballast from Dutch trading ships, and there are also references to the Japanese importing steel from China, Thailand and even wootz steel from India.2
Swords inscribed as being made using nanban tetsu are predominantly made in the early 17th century, up to around the Kanbun era of 1661-1667. Sword guards that state they are specifically made of nanban steel are very rare and probably date from around the same period.
"For example, the cuirass of Japanese armours, which had bow to be bullet proof, were called nanban-dō (南蛮胴) and paintings with European sujets were called nanban-ga (南蛮画).
This means that the term nanban stood for the new, the exotic flair and for fascinating items which had never been seen before, even if some things like steel did not come from that far." 3
-Markus Sesko, 2014
A Japanese sword guard or tsuba inscribed as being made of nanban tetsu.
1. See: The nanban tetsu project.
2. Markus Sesko; Encyclopedia of Japanese Swords, Lulu Enterprises, Inc., 2014. Page 319-320.