There is an upper limit to how hot a solar furnace can get. It is the temperature of the surface of the sun. Thermodynamic law says that energy cannot flow from a colder object to a hotter object (even with a lens between them). So the upper limit of a solar furnace is is 6,000 C. See the Physics Fact Book. That is still pretty hot and it is possible to melt steel at that temperature. So the real question is how much energy is coming from the sun to a particular square area at the surface of the Earth. That depends on the latitude, season, time of day, and cloud cover. It can get pretty technical. You can find the instructions for building a solar furnace at Bill Beatty's Hombrew Science page. Be careful! The Sun is hot. You could injure yourself or light your house on fire if you leave a section of a focussing collector lying around uncovered.