Because bearing steel not only can obtain higher hardness, but also has better toughness and plasticity, which is suitable for forging. Although tungsten steel is hard, it is brittle, impact-resistant, easy to crack or break, and cannot be sharpened very sharply. Kurogane should be manganese steel, which can resist impact and has high toughness, and can also be used for punching holes. But the hardness after quenching is not as high as that of bearing steel. In non-impact occasions such as daggers, hardness should be given priority, so bearing steel is the most suitable.