SAE Type 630 stainless steel, commonly known as 17-4 PH, or simply 17-4. For the technically inclined, it’s a martensitic precipitation hardened stainless steel containing about 15–17.5% chromium and 3–5% nickel, as well as 3–5% copper. Machinability is comparable to 304 steel, but 17-4 is magnetic and combines high levels of strength, hardness and corrosion resistance. For the less technically inclined, all one really need to know is that it’s a good material for making stainless steel razors.
Which is probably a small part of the reason why Phil of BullGoose has picked it as the material for the new stainless steel razor he hopes to have out in time for the holidays. More on the backstory and the other reasons for picking 17-4 in this thread1 over on my favourite2 shaving forum, which is also where Phil shared a photo of the plastic prototype. I rather like the clean, classic lines of this upcoming razor.
1) Phil also posted about it on BullGoose, if you prefer reading it there.
2) Disclosure; I’m on the moderator team, so off course it’s my favorite.