If the source is China, in a hostile period they might stop sending it. But according reportings I have read, the US DoD only uses about 0.5% of steel in the US, and only ~30% of US steel is imported.

Their argument is that you need strong domestic steel industry to build tanks, ships, etc, in case of war. Not saying I'm agreeing with it just pointing out the stated rationale behind the tarrifs.

The concern was the depression in prices was cratering local high-quality smelting capacity.

However, the DOD issued a memo (penned by Mattis iirc) indicating that there was no supply related concerns. The lack of aluminum used to justify the tariffs was in fact just the result of the LME's anti-market-tampering rules creating an incentive for metals traders to stockpile the material in private stores rather than in LME warehouses.

There was a nice article about this yesterday, but I don't have the link.

It doesn't. They claimed that so they could enact the tariffs, otherwise it would be a WTO illegal tariff. I think Canada/others are arguing that it is not a security risk and therefore is indeed an illegal tariff. This is what I remember from some articles. Please correct/elaborate.

Maybe because it undermines fundamental US business interests, and therefore wellbeing? If so it still seems like a stretch to me. Doubtful we'd have no where else to source steel if war or disaster struck.