replace vs step in

replace and step in both are verbs.

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

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

Other hyponyms of replace include deputise, deputize, substitute, displace, preempt, usurp, oust.

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