What is Kawagane in Japanese Katana terminology?

"皮鉄" (Kawagane) refers to the relatively high-carbon, hard steel that wraps the exterior of a sword blade. Kawagane is used because its high carbon content and hardness allow the sword to cut well. On the other hand, "心鉄" (Shingane) is often made of softer steel to prevent the sword from breaking or bending. This combination of different hardness levels in the construction of a Japanese sword is a unique technique that resolves the contradiction of needing both hardness for cutting and softness for resilience.