There was an episode on planet money about precisely this with sugar tarrifs and how it had unintended consequences on the candy industy which had to move overseas. Very complicated topic.

His stated goal seems to be related to “bringing back American jobs” but it’s really all marketing for himself as usual since while this action will cause more American job loses than it creates, that doesn’t actually matter because it’s just about optics.

Also the authority he invokes is specifically for national security so increasing jobs or any other non national security reason shouldn’t even be a factor. His comments that it’s a negotiating tactic also prove there’s no actual national security rationale.

“...the Trump administration promised its voters more big action on trade. Further WTO challenges could take years, so to make policy more quickly, Trump invoked section 232 of the Trade Expansion Act of 1962, which grants the US the ability to impose trade restrictions in the name of national security.”

https://www.google.com/amp/s/qz.com/1221853/donald-trumps-ta...

There is a general consensus that the world produces too much steel, with a large part of that being Chinese overproduction. I don't have the actual statistics in front of me, but the amount of new steel production that were brought online in China in the past 5 years is on the order of the entire world surplus of steel production.

Which is why it's so confusing that Trump didn't just slap anti-dumping duties on China, which quite frankly everyone agrees with. The best guess is that Trump thought that he could use the steel tariffs as an easy way to wring concessions out of trade partners... and declaring the tariffs a matter of national security is the one way he can do that without having to get Congress's approval (as even many Republicans in Congress are horrified by this).