OU-31 is a high-performance micro-fine steel developed by Nippon Koshuha Steel Co., Ltd. in Japan. It is most famous for being the signature steel used by the legendary master bladesmith Kiku Matsuda.
It is designed to outperform traditional tool steels like D2 by providing a superior combination of extreme toughness and edge retention.
Key Characteristics
Exceptional Toughness: OU-31 is significantly tougher than D2. It is highly resistant to chipping and breaking, making it an ideal choice for large, heavy-duty tactical and outdoor knives.
High Hardness and Wear Resistance: It is typically heat-treated to 60–62 HRC. Despite its toughness, it contains high levels of Molybdenum and Vanadium, allowing it to hold a sharp edge much longer than most conventional tool steels.
Fine Grain Structure: The steel is produced using a specialized process that results in a very fine, uniform grain. This allows the blade to take a "scary sharp" edge that remains stable even under heavy stress.
Non-Stainless (Semi-Stainless): With approximately 8.5% Chromium, it is not a stainless steel. While it resists rust much better than 1095 or O1, it will develop a patina and can rust if not kept dry and oiled.
Exclusive Status: Because it is difficult to source outside of Japan and requires a very specific heat treatment, it is rarely seen in mass-produced knives and is mostly found in high-end custom or mid-tech Japanese tactical blades.
Chemical Composition (Approximate)
The exact recipe is proprietary, but it is known for its high alloy content:
Carbon (C): ~0.90% (For high hardness)
Chromium (Cr): ~8.50% (For wear resistance and moderate rust protection)
Molybdenum (Mo): ~2.00% (Increases toughness and strength)
Vanadium (V): ~0.50% (Refines grain for a sharper edge)
Common Applications
Kiku Matsuda Knives: This is the primary steel for almost all of Kiku's world-renowned tactical and hunting knives.
Heavy-Duty Tactical Fixed Blades: Used for tools that need to be both extremely sharp and physically indestructible.
Custom Japanese Field Knives: Favored by makers who want a performance edge over standard D2 or A2.
Comparison: OU-31 vs. D2
OU-31 is much tougher, easier to sharpen to a fine edge, and holds that edge longer.
D2 is more corrosion-resistant and significantly cheaper, but much more prone to chipping.
Would you like to compare OU-31 with CPM-3V, its closest American rival in the "unbreakable tool steel" category?