Fluid steel was invented by that great British engineer, Sir Joseph Whitworth, and he first used it for his guns (which included artillery pieces) in the early 1880s.
It was called “fluid” because it was compressed into shape while in an extremely hot, pasty state.
Fluid steel was also made by Krupp’s of Essen, but by a slightly different process to Whitworth’s.
Was it stronger than Damascus steel? Initially, probably not.
In 1880 the Birmingham Proof House tested a number of shotgun barrel materials to ever-increasing pressures until they started to fail, and laminated steel (a form of Damascus steel) proved strongest, with fluid steel second.
However, the metallurgy of fluid steel improved rapidly in the following few years, and similar Proof House tests in 1895 put fluid ahead of Damascus steel.
Even Purdeys were eventually to use Whitworth’s steel.
But do be careful.
There is a belief in some quarters that any barrel marked “Fluid Steel” is safe with modern ammunition.
This is not so – it could well have only black powder proof.
I also note that in the USA there is one chap who is active on websites who believes that all Damascus steel barrels are so dangerous they should be destroyed.
But then you are likely to get piffle like this from a nation that does not have gun barrel proof legislation.