For the first time, 16 steel balls are divided into 3 groups, 7 and 2, which are called 2 groups 7. If the quality is equal, the one you are looking for is in the remaining two (so that you can find it again), otherwise it is in the light seven.

If seven is divided into 3,3, 1 for the second time, it is called two groups of 3. If equal, the remaining one, if not equal, is in light three.

Finally, weigh any two of the three. If they are equal, the rest are; If they are not, then the light one is.

So you can find it at least three times.