It's not specifically the import of steel that threatens national security, but rather the loss of domestic steel production capacity. If the U.S. (or any country, for that matter) is relying on most of its steel being imported (especially if it's from less-than-friendly countries,) then your ability to produce things necessary to fight a war can be dramatically curtailed if those steel imports suddenly stop.
Steel is needed to make very nearly everything. Perhaps not quite as much as several decades ago, but it's still pretty high on the list. You can't make cars without it. You can't make tanks without it. You can't make most weapons without it. The list goes on.
This risk is not merely theoretical. Prior to 1940, the U.S. was Japan's primary supplier for oil, steel, iron, and other such high-importance commodities. However, in 1940, as Japan continued its invasion of China, the U.S. began to slow the shipment of those materials to Japan. By mid-1941, once Japan had officially allied itself with Germany and Italy and expanded its invasions of Southeast Asia, the U.S. implemented a full embargo on exports to Japan. The resulting steel and oil (and rubber, etc.) shortages were a MAJOR problem for the Empire of Japan and ultimately contributed heavily to its eventual loss of the war. It was unable to build new ships and aircraft - or to repair or upgrade the ones it had - at anywhere near the U.S. production capacity. For that matter, the same was true for pretty much all of its war material needs. Eventually, it became unable to even fuel what Navy it had left and the Imperial Japanese Navy was rendered more or less useless for the remainder of the war. Once that point was reached, Japan's ultimate surrender was only a matter of time.
Germany tried to force a similar fate on the U.K. during WWII by attempting to disrupt its supply lines from North America. Thankfully, that ultimately didn't work out, as the British and American navies were able to keep the supply lines mostly open.
