The steels covered by this SAE Recommended Practice have enhanced mechanical properties obtained by quench and temper treatment. Grade Q550 is a carbon-manganese steel, while grades Q550B, Q620B, and Q690B are carbon-manganese boron steels. Other grades (designated by suffix A) represent steels containing one or more additional alloying elements as required to achieve higher strengths and to accommodate greater thicknesses. These steels are produced fully deoxidized and to fine grain practice. Since these steels are characterized by their mechanical properties, care must be exercised in the selection of grade, especially where fabrication by welding or forming is required. Special procedures may be applicable to varying compositions and section sizes, as produced by a given supplier; therefore, the purchaser should consult with the producer in order to be aware of these variables. This SAE Recommended Practice covers three strength levels of quenched and tempered carbon-manganese, carbon-manganese-boron, and alloy steel plates for structural use.