Would you buy a spring steel katana?

It isn't specifically any problem with Japanese iron ores, but a general problem with bloomery furnaces (the tatara is a bloomery furnace). The reduction of the ore to metallic iron takes place with the iron in the solid state - it doesn't melt. So there's lots of slag trapped in the bloom (the spongy iron-and-slag). Heat it and hammer it and fold it, to get rid of most of the slag. So folded steel comes with the bloomery. It's folded, but not really pattern welding, since it's a single steel. You do get a grain pattern. This technique is as old as the bloomery furnace, so dates back to 1000BC or so. Iron-making appears to turn up in Japan only in the 6th century AD, so they were about 1500 years behind.

You can run the furnace at a higher temperature, and keep it at that temperature for a long time, and you can get steel, by diffusion of the carbon into the iron in the bloom. You don't end up with a uniform carbon content, so the pieces of the bloom are sorted by carbon content. Then you can do iron body, steel edge, etc. That's the Japanese technique.

Or you can hammer the iron into thin strips, and carburise in a forge, and then weld the strips together (piled construction). This goes back to very early steel technology (500BC or so, in Europe), and you could call it "pattern welding" (but many people wouldn't).

Proper pattern welding, as opposed to folded steels, or piled or welded edge construction, dates to about 2nd century AD in Europe.

Rather than carburising strips or making bloomery steel, other steel-making technologies will give a purer product (with less slag), and no/less need for folding. The earliest such technology was crucible steel, which appears in India a few centuries BC. Next, decarburisation of cast iron, China, about 1st century BC.

This last process is essentially the Bessemer process, our modern steelmaking method. Yes, modern steels are better in that they're purer. More importantly, we know what we're doing, theoretically, so we can consistently produce steel of a particular type, with particular properties.



5'4" is about average adult male Japanese height from late prehistory through to Medieval. I expect that Edo period heights were much the same. So I'd expect quite a few samurai taller than 5'6". But yes, larger modern people should use longer katana (and often do). The length of a katana is limited by the length of blade that can be drawn (readily) with the sword worn at the waist. A longsword, worn differently, can be drawn as easily despite a much longer blade. So "modern sized" katana would be bigger than old katana, but not as big as longswords. Note that tachi, worn rather like many longswords, were, on average, longer than katana.

The katana is also essentially a single-handed sabre with a long handle. If it was to be exclusively (or predominantly) two-handed, like a longsword, then it could be bigger and longsword weight. But it maintained its one-handed function. There were longer swords in Japan intended for specialised two-handed use, but they weren't worn as everyday sidearms like katana. It isn't that Japanese people were too small to use larger swords, but that larger swords are less convenient sidearms.