You need >700C to decarburise iron you don't need much for that but coal and a fan, i.e. a bloomery. Pretty much defines the beginning of the iron age. Getting very low-carbon iron is quite easy, you just need enough fuel and air, the trick is then adding the right amount of carbon back. Also, getting rid of impurities, slag inclusions, etc, long story short: Lots of hammering and folding different carbon grades together, though some work can be saved by building very large bloomeries and processing very large batches producing quite slag-free low-carbon iron which then can be case-hardened.

The way out of that is crucible steel, melting your ingredients in an air-tight crucible, but that requires advanced furnace technology that somehow noone came up with before industrialisation, with the exception of India. The banding btw is due to alloy not production method (though you need to follow specific steps to bring out the banding).