When you say steam, do you mean "steam room" steam? Because that is more of a hot fog. With the dangers being dehydration and overheating.
True steam (water heated to it's boiling point and turned into water vapor) has a temperature of 212 degrees Fahrenheit - or more -, and it will kill you, or at least give you terrible burns.
Its not the breathing part that'll kill you... its just water vapor... which can open nasal passages and your bronchi, which makes it easier to breather. But the heat is the threat, causing dehydration and overheating, like bimjob said...
If it's hot steam then most probably yeah. For sauna steam rooms in the gym, they recommend that you only stay for 10-15 minutes tops and bring some water to drink or your body will be dehydrated out.
I work with steam in industrial settings that will cut a human in half lol. also interesting is superheated steam is completely invisible, not like your typical steam full of water droplets. but in general no it wont kill you just breathing it in, the heat is the only danger of steam, steam is just the gas form of water, only achieved with heat. Heat as you know can kill you.
what I mean is can breathing allot in kill you
If it really hot than maybe. Hot hot hot!!
If exposed to it for a prolonged period of time then yes