Instead of replacing coal with hydrogen, the direct route promises to be much more efficient. You would only need enough mirrors to capture sufficient sunlight, which has hardly been practical up to now. A new material is now set to change this.https://www.notebookcheck.net/1-050-C-from-sunlight-better-blast-furnaces-for-steel-production.843472.0.html
Even if we can achieve temperature, thats only half the problem. Coal / Coke isnt just the fuel source, its also part of the chemical reaction.
If you go back to your GCSE chemistry, you'll remember that in a blast furnace, Carbon (C) in the form of coke is partially combusted to produce carbon monoxide (CO) which then reacts with the iron ore (Fe2O3) to strip off the oxygen and leave iron (Fe) and carbon dioxide (CO2).
What's used to strip the Oxygen off when using a solar furnace (or electric arc furnace for that matter)?
First comment was correct it takes more than energy to cook iron also the process to make the mirrors and equipment will take plenty of energy and produce plenty of waste, stick to solar panels and windmills!!