I looked this up before. The US produces 85% of the steel it consumes. Of the 15% they import, China was a 2% bit player. I looked Canada's stats and only 10% of our total steel imports come from China. ie: a drop in the bucket compared to the US market. Therefore, there is literally next to nothing for Chinese steel that could possibly be funneled through Canada to the US.
The US may be getting fucked in some trade practices, but steel is a piss poor example of it.