Your buttplate might benefit from annealing, before you attempt to bend it.
Most of the steel castings, I have used, though they be few, appear to have benefited by annealing by laying said part on a couple of layers of charcoal briquettes, with several more layers built up over the part. The charcoal pile is lit and allowed to burn down on its own. After four, or so, hours, the cast parts seem to be softer and more ductile.