What biomass grows the fastest without being waterlogged - I imagine bamboo or sugarcane or something
Grow that, and burn it to make carbon neutral steel; bonus points if you do it in a highrise/underground farm but frankly some medium term reversible environmental damage is preferable to killing off way more with climate change
Coal has a bunch of impurities compared to charcoal I thought?
And if the repeated melting is done by burning biomass/charcoal or with clean(er) energy then it’s not a huge issue