Steel has always been the main component that makes up a car. However a lot of research has been done recently to see id a magnesium alloy could replace the steel. Magnesium is the lightest metal on earth. (33% lighter than Aluminum) It does not dent easily, it is non toxic, and there is a large amount of it on the earth. Magnesium is already starting to replace some of parts of 
the car that do not have to do with the frame to make the car lighter and require less gasoline. Currently magnesium can be found in the 2011 Ford Explorer, Chevy Corvette Z06, and a few Jeep models. Magnesium is twice as expensive as steel by weight. However, magnesium is 5 times as light at steel so it is actually cheaper than steel but it is not as strong as steel. Right now magnesium is not ready to replace steel because it is not as strong as steel but Mg could be used for seat frames or for the roofs of cars.
http://www.chemical-engineering.co/2011/12/22/magnesium-alloys/