Generally they contain less than 0.25% carbon. SAE Grade 2 (Metric class 4.6,4.8 & 5.8) fasteners are made with AISI grades 1006, 1010, 1016, 1018, 1022 & 1035 being low carbon steels.
They have a carbon content between 0.25% to 0.60%. SAE Grade 5 (Metric class 8.8) fasteners are made out of medium carbon steels with AISI grades 1038, 1541, 5132 & 5135 and also ASTM A194 Grade 2, 2H, & 2HM, ASTM A325, ASTM A449.