That steel is only cheap short term. Once China has a monopoly they can charge whatever they want. The only reason Chinese steel is cheaper is because it's lower quality and their government subsidizes it with long term strategic intent
However, this policy is being sold as a way to help US manufacturers and that's not quite the same thing. The benefits could easily go to other imports with weaker tariffs. This coupled with the problem of hurting down stream US manufacturers means that the only benefit is hedging against a Chinese monopoly and not the direct benefits that are usually talked about.