The ornamental design for a steeltongue drum, as shown and described.
FIG. 1 is a first perspective view of a steel tongue drum showing my new design;
FIG. 2 is a second perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a back view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The broken lines shown in the drawings depict portions of the steel tongue drum in which the design is embodied that form no part of the claimed design.