Plasma cutters and oxy-acetylene torches are the preferred way to cut sheets of metal. But if you're not geared up with those tools and still need to cut some steel, you have a few other options.
Abrasive cutoff wheels are used for cutting metal with an angle grinder, but they aren't the best option for cutting flat sheets of metal. A circular saw will fit an 18cm cutoff wheel. Find an old and junky circular saw, as it won't last long due to the debris.
If you're really desperate, the gentleman in the video above claims to use a 10-inch (25cm) cutoff wheel with his table saw to make even more precise cuts. Again, he recommends using a cheaper saw for something like this and warns of sparks flying absolutely everywhere since the blade will be spinning up. Make sure your shop is extremely clean before trying this.
Lastly a reciprocating saw with a short cutoff blade can be positioned vertically over the metal sheet and used like a jigsaw.
Always wear safety goggles and gloves when using a cutoff wheel.