R2 steel vs SG2 — are they the same?

Yes, R2 and SG2 are the same steel — R2 is the name used by Kobelco Steel, and SG2 (Super Gold 2) is the name used by Takefu Special Steel for the identical powdered metallurgy formula. SG2 / R2 is a powder (PM) steel from Takefu Special Steel, Japan, with a composition of 1.25% carbon, 14.0% chromium, 1.8% vanadium, 2.3% molybdenum, and a hardness range of 62–64 HRC. So if you see a knife labeled R2 or SG2, you are getting the exact same material.