tread vs step on

tread is a noun but step on is not a noun.

Word	Noun	Adjective	Verb	Adverb
tread	Yes	No	Yes	No
step on	No	No	Yes	No

As verbs, step on is a hyponym of tread; that is, step on is a word with a more specific, narrower meaning than tread:

Other hyponyms of tread include tread on.

tread (noun)	step on (noun)
a step in walking or running
structural member consisting of the horizontal part of a stair or step
the part (as of a wheel or shoe) that makes contact with the ground
the grooved surface of a pneumatic tire

tread (verb)	step on (verb)
crush as if by treading on	place or press the foot on
brace (an archer's bow) by pressing the foot against the center
mate with (used of male birds)
tread or stomp heavily or roughly
put down or press the foot, place the foot
apply (the tread) to a tire