Rare technical term; in modern Japanese, ステンレス鋼 or simply ステンレス is overwhelmingly more common.
See also: ステンレス鋼
In old documents it is written as 不銹鋼, but nowadays it is normally called ステンレス鋼.
The kanji 不 (not), 銹 (rust), and 鋼 (steel) literally mean 'rustless steel'. The reading ふしゅうこう is a Sino-Japanese compound. The term is now largely replaced by the loanword ステンレス鋼.