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).