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About 15-20 years ago I read a short story (I believe as part of a collection) which as its main plot line featured a metal bar that appeared one day. A guy picked it up and threw it in the recycling. The bar itself was indestructable and unworkable, and any metal that it touched was "infected" to have the same properties after some amount of time. So it damaged the machine that tried to process it for recycling, and turned the metal recycling machine into the same type of metal. And it spread from there.

IIRC this didn't go very well for civilization. I think one of the characters calculated how this might spread rapidly along railways, etc. and committed suicide at the end...maybe?

This is probably A. E. van Vogt's "Juggernaut" (1944), which was previously asked about and answered here. Though that answer was not accepted, it is almost certainly correct.

The story concerns the mysterious appearance of a bar of what looks like plain steel, which is collected in a wartime scrap metal drive. Metal from the batch including that bar can be shaped as normal steel, but, once fully "set" (which takes about two weeks), it cannot be remelted and has supernatural hardness (i.e. can't be bent at all):

"As you all know, obtaining information from a metallurgist"—he paused
and grinned inoffensively at Nadderly, whom he had invited down—"is
like obtaining blood from a turnip. Mr. Nadderly embodies in his
character and his science all the caution of a Scotchman who realizes
that it’s time he set up the drinks for everybody, but who is waiting
for some of the gang to depart.

"I might as well warn you, gentlemen,
that he is fully aware that any statement he has made on this metal
might be used against him. One of his objections is that thirty days
is a very brief period in the life of an alloy. There is an aluminum
alloy, for instance, that requires forty days to age-harden.

"Mr. Nadderly wishes that stressed because the original hard alloy, which
seems to have been a bar of about two inches square by a foot long,
has in fifteen days imparted its hardness to the rest of the bar, of
which it is a part.

"Gentlemen"—he looked earnestly over the
faces—"the hardness of this metal cannot be stated or estimated. It is
not just so many times harder than chromium or molybdenum steel. It is
hard beyond all calculation.

"Once hardened, it cannot be machined,
not even by tools made of itself. It won’t grind. Diamonds do not even
scratch it. Cannon shells neither dent it nor scratch it. Chemicals
have no effect. No heat we have been able to inflict on it has any
softening effect.

"Two pieces welded together—other metal attaches to
it readily—impart the hardness to the welding. Apparently, any metal,
once hardened by contact with the hard metal, will impart the hardness
to any metal with which it in turn comes into contact.

"The process is
cumulative and endless, though, as I have said, it seems to require
fifteen days. It is during this fortnight that the metal can be
worked.

The cause of the effect is discovered too late and the "infected" metal is too widely distributed to stop its spread. The government intentionally distributes the infected metal into Europe to stop the German war machine.

The story was originally published in the August 1944 edition of Astounding Science Fiction, and can be read online in its entirety courtesy of archive.org.