As I recall, the national security interest was the industrial base of steal steel production, not that Canada was a threat. Theory was less steal steel from foreign sources, yes even Canada, would result in more production in the US, bolstering a vital industrial base. Did it work? Dunno. Haven't bothered to look for the original source data.
I don't get to be the spelling police very often as I'm a fellow spell check cripple. ?
It was a mixed bag. I buy millions of dollars of steel each year in various forms of strand, mild reinforcement, rebar, embeds, lifting devices, and plate material. It didn't immediately allow US mills to ramp up production as production is still at the mercy of supply and demand, but it did allow them to control the floor of the market. I.E. they immediately raised their sell price to the new tariffed price on foreign steel, thus expanding profit margins for the US mills. Fast forward to today, and there has been an increase in demand...a big one. However, that's not tied to the tariffs, that's tied to the ridiculously low interest rates that have facilitated a pretty nice boost in commercial & industrial construction starts (use of steel is much more prevalent in commercial and industrial construction than residential). Lead times on steel is becoming extended, perhaps overly so, because manufacturers may want to ramp up production, but can't find labor for a variety of reasons whether it be COVID, lack of labor availability, lack of work ethic by labor, and let's face it: everyone thinks they should go to college, hold out for a management position, and start at $100K per year. Anyway, I digress....mixed bag, but mostly let mill owners pocket more cash.