The steelworks Billy is dragged into in "Stranger Things" season 3 is Brimborn Steel Works, commonly shortened to Brimborn.
Billy Hargrove reaches the abandoned industrial site after a supernatural force causes him to crash near it. When he investigates, the Mind Flayer attacks and drags him into the steelworks’ basement. This event becomes the turning point of Billy’s season 3 storyline, linking him to the Mind Flayer and its plans for Hawkins.
Brimborn Steel Works is also where the Mind Flayer’s larger operation takes shape. Rats are drawn into the building, dissolve into organic matter, and contribute to the monster’s physical form. Later, other “Flayed” residents are brought there as the creature prepares to assemble itself.
Some viewers remember the location simply as “the steel mill,” but the show’s name is Brimborn Steel Works. The location is especially important because it connects Billy’s possession, the Flayed victims, and the Mind Flayer’s attempt to destroy Eleven.