Heatran is a legendary PokeMon meant to represent the Earth's molten core. It's Fire-typing comes from, obviously, the core's "melted-ness". Steel is because the core is mostly made up of molten iron ore and copper ore. It's body is made of molten iron (as seen in its PokeDex entries) which explains both of Heatran's typings. However, due to Heatran's spider-like battle movement in Generation 5, and its said ability to climb walls, this can be just a misconception.