Japanese swords at their best involved folding soft iron over up to twelve times usually, never more than 20 times. This gave a flexible backbone for the sword. Onto this was welded a hard steel edge, a single bar. The height of this art was maybe 250 to 150 years ago.
Now, about 2,500 years before the Japanese were doing that, the various Celtic peoples of Europe had a blade-smithing technique known as pattern-welding. This involved first twisting together several thin rods of iron to produce a backbone pretty much exactly the same as that used much later in the katana, then welding a hard steel edge to each side. There is one pattern-welded blade in a family collection in Europe which saw use in war over three centuries, remaining in service to one family.