Steel plants sometimes have deals to operate at certain times for load shedding anyway, but how about they use the times when we have an excess of solar/wind and its not well suited to steel production (maybe due to shift patterns) to generate onsite Hydrogen using that excess power? Then they have readily available onsite green hydrogen.

I think things like this will become an absolute necessity soon, using green power at its peaks to store resources for use later. (Obviously batteries will also be essential, although I would rather see gravity storage with big reservoirs like they have in Wales)

Largely the negative prices are a side-effect of feed-in tariffs, where (some) renewable projects get paid a fixed price/kWh regardless of what the actual spot market price is. So when there's an oversupply of these subsidized renewables it pushes the spot price to negative.

I think newer renewable support schemes have been designed to avoid this problem, either by cutting off the feed-in tariff when the spot price goes to zero, or then by restructuring the subsidies as some kind of investment support (state guaranteed very low interest loans or such) rather than paying for the generated electricity.

That of course doesn't change the fundamental issue, in that in a grid with increasing amounts of variable production renewables and the price determined by marginal cost of the most expensive producer, the price swings will be larger than what we previously were used to.

I recently heard that, even in Germany where there is lots of solar, there were only 9 days in the last year where spot prices were negative. Sadly my source is just "someone said".

As for intermittent steel production, from what I know about aluminum it is essentially impossible to turn the process off, because the molten stuff solidifies and is impossible to melt again.