The Ultra Steels project also wants to develop a superstrong steel with a tensile strength of 1,500 Mpa or more. Currently 1,000 Mpa is considered the limit because higher-Mpa steel suffers delayed fractures. Delayed fractures are caused by hydrogen. In such steels as spring steel, internal fractures become significant when the tensile strength exceeds 1,200 Mpa and the number of repetitive movements surpasses 107. Researchers want to develop a new process to microrefine large grains that can cause fractures, Sato says.
Grain boundaries are the surfaces between individual grains in a metal.
Hypereutectoid piano wire with a high strength of 5.7 gigapascals already is available. But its use is limited because the small amount of hydrogen in the steel causes delayed fractures. The project is trying to create a new metallic structure similar to those found in high-strength steels that are resistant to fatigue, Sato says. They're developing a new heat-treatment process to achieve a martensitic organization in which no carbide exists in the grain boundaries. They'll also continue to probe the fundamental nature of deformations and fractures.