Shigefusa Kasumi Yanagiba

Does anyone know the type of steel this uses? How reactive is it?

There is lots of info on this around, a quick search should give you plenty to read.

It's his "own" Swedish carbon steel. The core steel is fairly simple carbon steel without alloys to give it wear resistance.

It is quite reactive, although some more than others, reportedly. It really depends where you live and the conditions (humidity, etc). Once you have developed a patina it will behave better but it will always need a watchful eye, I would never "store it" with out oil. And I wouldn't use it to cut onions... Although it does a great job at it.

I cut pounds of onions with my shig. the first couple were unsightly, buy since then I've had zero problems with reactivity. I don't think people put the knife properly through it's paces before griping about reactivity.

Yup, home use here.