Are these values definitive?

The World Steel Association collects a lot of data, which has been used to calculate the value of 0.762 t/tonne using the System Expansion methodology, which is the highest ranked method in the ISO 14040 series of standards.

There are other values in circulation, some are higher and some are lower than 0.762 t/tonne. Most are derived by less rigorous methods than System Expansion in an attempt to have a common method for all materials. They are intended to provide a fair comparison, but do not take account of steel’s unique environmental qualities.