reply by Udo Schlengermann firstname.lastname@example.org
The parameters you gave are valid for an unfocused flat probe. Assuming a (1/4) inch probe of 10 MHz in water (6,35 mm dia) this probe has its natural focus at 68 mm distance (nearfield length) and a beam width of 1,6 mm at a -6dB-threshold (measured with a point like reflector).
The 9 mm steel path is equivalent to 36 mm water path, so you have to use a water delay of 32 mm (68 mm -36 mm) to position the focus at the far surface of the steel plate.
This will not influence the beam width, if the beam is perpendicular to the plate.