Formability of Ultrafine-Grained Steel: Forming Microscrews by Cold Heading and Rolling

While uniform elongation is a measure of ductility of the material, the reduction in area in tensile tests is also an important measure of ductility. Ultrafine-grained steels with different carbon contents from ultralow carbon to high carbon were produced by warm caliber rolling. It was found that the reduction in area- tensile strength balance is far better than the conventional ferrite+pearlite steels and even superior to martensite steels for all materials. Formability of ultrafine-grained steel is examined by applying to form an M 1.7 micro screw using these ultrafine-grained steels. The screws are formed by the process of cold heading and rolling. The relationship between cold heading, rolling, uniform elongation and reduction in area are investigated to clarify the formability of ultrafine-grained steels. Low-carbon ultrafine-grained steel has excellent cold headability and favorable rolling properties, i.e., excellent formability. The reduction in area is a measure to determine formability on cold heading.