My guess is this move is more about national security than anything. Trump probably realizes that if we were ever to go to war against China we would need to massively improve our steel and aluminum production so we can provide our own for military vehicles, aircraft and ships. This might be an effective way to make sure that we, as a country, are self-sufficient in case of a war.

Don't misunderstand, I don't agree with the decision, I am just analyzing it. I am for complete free trade and no wars! :)