I suggested using peroxide not hydrogen, as the peroxide reaction is more exothermic. Either way I suspect that you'd need to burn a lot of extra hydrogen to overcome the latent heat of water and create sufficient heat to melt the ore / alloy.
A quick back of the envelope calculation would be at least 1 million litres of H2 per ton of steel. So maybe 2~3 million litres of H2 per manufactured car just for the steel alone?
[EDIT] Peroxide also has the added benefit of being a liquid at room temperature.
Yes, peroxide being liquid certainly is nice, being a light gas is certainly one major obstacle in using H2. However I am surprised that the peroxide reaction would be more exothermic, as you would have to break the hydrogen from the peroxide first.
1 million litres of H2 are just 90kg though, that isn't that much for a ton of steel.
It is only a bit more exothermic, about 20% more (again from my rusty enthalpy calculations). The downside of course is making the peroxide in the first place.
Also, peroxide is relatively safe (compared to gaseous hydrogen) to handle up to ~150C.
A quick back of the envelope calculation would be at least 1 million litres of H2 per ton of steel. So maybe 2~3 million litres of H2 per manufactured car just for the steel alone?
[EDIT] Peroxide also has the added benefit of being a liquid at room temperature.