We cut up to 3" thick plate with a large handheld crcular saw. It is a big air powered industrial unit.

For sheet / plate up to 1/2" we use a milwaukee panel saw http://www.milwaukeetool.com/tools/s...el-saw/6480-20

there are quite a few companies that make metal specific circular saws and blades. A standard circular saw will work as well for light duty use.