Hydraulic shock using explosives will crush steel. The cube of steel needs to be immersed in water, held firmly in position, and an explosive charge is detonated. The resulting shock wave will crush the cube into whatever shape the outer structure holding it underwater is.
Methods of Explosive Forming
Explosive Forming Operations can be divided into two groups, depending
on the position of the explosive charge relative to the workpiece.
· Standoff Method
In this method, the explosive charge is located at some predetermined
distance from the workpiece and the energy is transmitted through an
intervening medium like air, oil, or water. Peak pressure at the
workpiece may range from a few thousand psi (pounds/inch2) to several
hundred thousand psi depending on the parameters of the operation.
In this method, the explosive charge is held in direct contact with
the workpiece while the detonation is initiated. The detonation
produces interface pressures on the surface of the metal up to several
million psi (35000 MPa).
This will not smash the steel cube to smithereens, but it will crush it out of the shape of a cube.
Amit Mukund Joshi Explosive Forming an Overview found here
D E Strohecker, R J Carlson, S W Porembka, Jr., and F W Boulger, Explosive Forming of Metals (DMIC Report 203, May 8, 1964) [Defence Metals Information Center, Battele Memorial Institute, Columbus, Ohio 43201] [found here]2
D J Mynors, B. Zhang, "Applications and capabilities of explosive forming", Journal of Materials processing Technology, 125-126 (2002) pp 1-25 found here