I just had a discussion with a fire investigator on this topic, so I thought I should advise the general community. Steel does not melt at any temperature associated with fires in normal combustibles and normal oxidizers, even arsonist-set fires. So if you encounter molten steel (make sure it's not aluminum or some other metal), then you need to get very curious.

Basically, to cause ultra-high temperatures needed for this to happen, you need either some sort of energy attack, or some unusual, highly specialized chemistry. The latter can happen in the case of some narrow categories of industries, but certainly not out in the forest or in an office building, or in a residential house, or at a vehicle fire out on the highway. If you are not in one of these appropriate industrial facilities (often, they will be places dealing with powdered metals), then you need to really delve into things in detail.

I would urge that any folks encountering such situations document them here or in some other appropriate forum. Years ago, it used to be common for fire investigators to publish short articles along the lines of "Look at what oddity I found in this fire" in the Fire & Arson Investigator magazine. But this has not been happening for many years. I don't know whether it's because the editorial folks don't seek out this kind of contribution, or what other reason. However, investigators should publicize unusual findings, since this one way the field can advance.

At the very least, if you don't feel like publishing, you can tell me about what happened.