step in vs replace

step in and replace both are verbs.

Word	Noun	Adjective	Verb	Adverb
step in	No	No	Yes	No
replace	No	No	Yes	No

As verbs, replace is a hypernym of step in; that is, replace is a word with a broader meaning than step in:

Other hypernyms of step in include supersede, supervene upon, supplant.

step in (verb)	replace (verb)
get involved, so as to alter or hinder an action, or through force or threat of force	substitute a person or thing for (another that is broken or inefficient or lost or no longer working or yielding what is expected)
act as a substitute	put something back where it belongs
	put in the place of another; switch seemingly equivalent items
	take the place or move into the position of