Steel Shell is a Equip Spell Card.
Spell Cards are usually activated from the hand on your turn unless their type gives another timing window.
On the current TCG and OCG Forbidden & Limited lists tracked here, Steel Shell is Unlimited.
YGOPRODeck lists Steel Shell for these formats: GOAT, OCG GOAT, Duel Links, Edison, TCG, OCG, Master Duel.
A listed print of Steel Shell appears in Tournament Pack: 1st Season (TP1-007) at Rare rarity.
Among markets shown on this page, the lowest listed snapshot for Steel Shell is about $1.57 on Cardmarket (prices change frequently).
