This is the 17122nd most frequent Russian word.
стальных
"Стальных" means "of steel" or "steel" (adjective, genitive plural form).
Here, 'стальных' is used to describe the material that the knives are made of, indicating they are made of steel.
У него было пять стальных ножей.
He had five steel knives.
In this sentence, 'стальных' metaphorically refers to 'steel,' meaning strong and unyielding, connected to emotional resilience or fortitude.
Стальных нервов ему явно не хватало.
He clearly lacked steel nerves.
Here, 'стальных' is used to characterize the constructions as being structurally made of steel material.