President Trump has directed the Secretary of Commerce to use authorities in the Trade Expansion Act of 1962 to determine if domestic steel production capacity is sufficient for national defense requirements. And:

Pursuant to section 232(b) of the Act (19 U.S.C. 1862(b)), if the Secretary finds that steel is being imported into the United States in such quantities or under such circumstances as to threaten to impair the national security, the Secretary shall, in the report submitted under subsection (a) of this section, recommend actions and steps that should be taken to adjust steel imports so that they will not threaten to impair the national security.

Adjusting steel imports suggests imposing tariffs (or quotas). Scott Summer provides an excellent breakdown of the inherent problems with imposing steel tariffs for national security reasons.

Though steel, along with many other commodities, is essential for national security, tariffs will hurt other manufacturers by driving up the cost of an important raw material.

Overall economic prosperity contributes more to national security than any particular industry.