You will not find Factorio in a Humble Bundle or Fanatical pack, which are the primary sources for legitimate "spare" keys shared in the community.
: It serves as a "benchmark" to see if your brain enjoys the "factory must grow" loop before spending money. Why It’s Rarely on Sale Reviewers from highlight that the developers, Wube Software , believe the game's value is constant. Fixed Pricing
If you are looking for a free Steam key , you should know that they almost certainly don't exist in any legitimate form