This was already posted, but I'm adding my own research.

This steel is produced by taking cast iron and submitting it to a process whereby heating it using charcoal and some chemical processes which I do not entirely understand, excess carbon contents of the cast iron are sucked out of the material, thus lending additional malleability to the material. This is advantageous over the cast iron of a kettle or pan in that it is more resistance to shattering, while also maintaining it's strength.

I am including a 知恵袋 page because it was really well explained there (for people who don't know anything about the process). This process was developed in China originally. The similar process that was used in Japan involved heating the iron to a lower temperature and then introducing it to wooden material in order to suck out the carbon contents and was called 踏鞴製鉄（たたら製鉄）, as a bellows was used to heat the material.