High strength steel nuts are now available. Most options are covered; standard or flat, different pitches standard (coarse) or fine, one bevel or two sided bevel, and standard or small head (refer to the drawing for sizing).
In the Japanese industrial standard (JIS B 1181), S45C nuts are typically manufactured to "Strength Class 6" or "8" (equivalent to the 10.9T bolts)
S45C: This is a Medium Carbon Steel (JIS G4051). The "45" indicates a carbon content of approximately 0.45%.
-H: The "H" typically stands for Hardenability Guaranteed (often referred to as "H-band" steel).