We claim the ornamental design for a steam iron, as shown and described.
FIG. 1 is a front perspective view of a steam iron of the present invention;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a first side view thereof;
FIG. 4 is a second side view thereof;
FIG. 5 is a top view thereof;
FIG. 6 is a bottom view thereof;
FIG. 7 is a front view thereof; and,
FIG. 8 is a rear view thereof.
The broken lines shown represent unclaimed subject matter and form no part of the claimed design. Additionally, the unevenly broken lines in the drawings define the limits of the claimed design.