Also know that when you slow the drill down with a voltage type control, the drill motor will get hot faster. When the motor turns slower the cooling fan won't move as much air.
JimB has the right answer if your dulling the bits fast. The drill motors speed is correct for the diameter bits it is made for, even when cutting steel. I spent my life as a metal worker, machinist, fabricator and if your going to work with metal buy tools (drill motors, saw blades and drill bits) that are made for the job. It's money well spent if that's what you want to do.