What is the quickest way to dissolve steel? What is the quickest way to dissolve a large chunk of steel? I mean a very quick reaction, assuming that the interaction occurs in a sealed or semi-sealed environment? Reaction times of the order of a few minutes versus tens of minutes, hours or days are the objective.

I'm curious as to how quickly acids like $\ce{HNO3}$ and $\ce{H2SO4}$ can react with steel containers.
The fastest way is dissolving the stainless steel electrochemically using $\ce{H2SO4}$ at elevated temperature where the steel block is attached as an anode. Higher current densities lead to higher removal rate. 

Alternatively a $\ce{FeCl3}$ with minimal content of $\ce{HCl}$ (up to 3%) heated at 45°C or higher.