Uh, that just doesn't sound right. (I also see that Jeffrey edited it). Roasting is a very intense process and using the air-roasting process, you need to get it over 400 deg F.
However if you really want to take the time and effort to do it. Here's a
Link to do it right.
I think I'll just stick to buying roasted at the store. (where would you even find green beans?)