I don't know, but I don't think its as straightforward as blocking a merger on antitrust grounds. Maybe there is a national security angle, perhaps the national defense authorization act that gets trotted out for like everything these days. After all steal is important in wartime (but it's not like factories are being moved to Japan). But even then you'd have to give the middle finger to an important ally, tell shareholders their not going to get a big windfall from the sale, and then sit back and wait for the inevitable consolidation layoffs to roll in during the middle of an election. Not great options. Or you could try to educate the American public that the sale is actually overall a good thing, but good luck with that, especially with shameless Republicans, especially trump, howling about globalism and whatnot. Even Dems who understand this, like Sherrod Brown, are agitating against it cause the politics are rotten. If the public was literate and capable of nuance, this would be a lot easier, but that ship sailed at least 40 years ago.

Only on "day one"

It can be stopped under foreign investment regulations.

It’ll never happen. Steel is part of national security.

They only make construction grade steel for buildings.

The result of any blocked takeover would be job losses and possibly bankruptcy.

The US government has given itself far reaching powers over such matters. International investments of strategic importance must go through a government committee (CFIUS), which decides for itself what "strategic importance" means. The government can also compel individuals, e.g. it has banned US citizens outside the US from working for Chinese semiconductor firms. A lot of this is done at the executive level, with no legislative oversight and no avenue for recourse by affected parties.

The idea of the US government staying in its lane, away from private sector affairs, is pretty much dead and buried at this point.