Did you see if it's not written on the standard's text itself? Check into the ASTM book.
Giovanni S. Crisi
Sao Paulo - Brazil

Minimum specified tensile strength for ASTM A709 Gr. 50 = 65,000 psi.

Approx. hardness for tensile strength of 65,000 psi = 74 Rockwell "B".
per ASTM A370 Table 2B.

So the answer would be an approx. minimum hardness of 74 RB.

The material would need to be upwards of 110,000 psi in tensile strength to be in the Rockwell "C" range.

Regards,
Mike