
It will take the same amount of time to make 1 full bar of steel through this method as would the 56 real-life hour strait-up "hardcore" method.
We know that 1 full bar of steel requires 56 real-life hours. We also know that 100 coins = 1 bar of metal. So, each steel coin can be said to be worth 1/100th the time of 56 hours, or 33.6 minutes. (56*60*60 = gives 201600 seconds/60 = 33.6 minutes * 100 (which is how many coins there are in 1 bar) = 3360 minutes *60 = 56 hours, which proves my maths didn't fail!

So, how much is 1 steel coin worth, in time? 33 minutes, 36 seconds (33.6 minutes) ... Which for my suggestion is exactly how much time it will take to turn 1 wrought iron coin into 1 steel coin, as the math states. Each coin will, obviously, increases the time needed to turn all the coins inside into steel ones. 2 coins would be 67.2 minutes, 3 coins would be 100.8 minutes, etc. Until you reach 20 coins -- turning these many coins into steel ones takes nearly the entire real-life 12 hour time limit for a fully fueled steel crucible, assuming no one refueled it. (more geek emotes go here)
The downside? 5 times the charcoal use, and using a lone steel crucible five times. Oh, and of course, losing 1 wrought iron bar for every 100 steel produced because 100 coins -> 1 bar -> 99 coins. (and for that 100 steel produced-by-coins, 400 charcoal was "wasted" due to charcoal not having fractional values like metal does)
tl;dr
My intent with this entire suggestion, is to give players a way to make steel without spending 56 hours in a serial, grindy and nerve-wracking progression. The player can spread out his steel making in a non-linear fasion (over many more days then the 2.4 days normally, if he wants to) similar to the "fire-and-forget" dough cooking in the oven. Except instead of dough, it's 20 wrought iron coins, 1 peice of charcoal, and a fully fueled and lit steel crucible that will produce 20 steel coins after 12 hours, the time it takes for a fully fueled steel crucible to burn itself out without refueling.
And of course, give thieves a better chance to steal precious, precious steel... coins.

... Yes, if you want to, you can also convert 100 wrought iron coins (99, and a single) and follow the standard 56-hour way, and then turn those coins into whole steel.
Huge post done. You can murder me for using gratuitous amounts of math, now.