As part of the agreement on U.S. Section 232 tariffs, Canada and the United States agreed to implement effective measures to prevent the importation of aluminum and steel that is unfairly subsidized and/or sold at dumped prices and to prevent the trans-shipment of aluminum and steel made outside of Canada or the United States to the other country.

https://www.international.gc.ca/trade-commerce/trade-agreements-accords-commerciaux/agr-acc/cusma-aceum/economic_assessment-analyse_economiques.aspx?lang=eng