Scrap steel is a known hazardous cargo. The issue is a type of scrap known as ‘turnings’ which are generally shavings of metal from industrial lathes discarded during the manufacturing process.
Steel oxidizes (rusts) very rapidly when it is in a raw state. Turnings have huge surface area to volume ratios so there is a lot of oxidizing happening. The oxidizing process creates heat. When large amounts of turnings are heaped up in piles the center of the pile can get very hot. Turnings are soaked in oil during the manufacturing process in order to keep the tooling sharp and keep the metal temperature reasonable. But in large piles of hot turnings, the oil will combust. Thus the fire. Such fires are difficult to fight as the source is deep within the pile. Pouring water on will often make things worse as the water turns to steam, breaks down to component parts (H2O), the hydrogen burns producing more heat and the oxygen attacks more steel to make more ferrous oxide.
In international shipping, scrap metal is classed as a hazardous cargo liable to spontaneous combustion. Before shipping a certificate must be produced certifying that there are no turnings in the cargo. This is not the case for domestic shipping and steel mills like turnings as it is pure steel.
On a ship, the fire would not be fought with water. Instead the hatches would be sealed tight and a blanket of CO2 gas introduced into the hold to limit the amount of oxygen available for the fire. Cooling would be effected with water sprayed around the outside of the cargo hold. (Boundary Cooling)
On an open barge, it is much more difficult. The quickest way is to use a crane with a clamshell bucket to dig out the pile and spread it out over a large dirt area where it can then be hosed off. Failing that the only option is massive amounts of water, hoping to eventually cool enough material to extinguish the fire. This is usually fruitless and it will continue to burn for a long time. By the time the fire is out though, all the turnings and oil will have been consumed.
If I were investigating this incident this would be my first area of focus.