Effects of Tramp Elements on Formability of Low-carbon TRIP-aided Multiphase Cold-rolled Steel Sheets

Four kinds of TRIP-aided cold-rolled steel sheets were fabricated with additions of tramp elements of Cu, Cr, and Ni to a basic 0.15C-1.5Mn-1.5Si-0.5Cu (hereafter all in wt%) steel, and their microstructures, mechanical properties, and formability were investigated to analyze the effects of the addition of tramp elements. These steel sheets were intercritically annealed at 780-790°C, and isothermally treated at 430°C. Tensile tests and limiting dome height (LDH) tests were conducted, and the changes of retained austenite volume fractions as a function of tensile strain were measured using X-ray diffraction. The formability of the Cu- and Ni-containing cold-rolled steel sheets was greatly improved as the strain-induced transformation of retained austenite was sustained up to the high strain region due to their high volume fraction and stability of retained austenite. On the other hand, the Cr-containing steel sheets showed a dual-phase structure of ferrite and martensite resulting from low volume fraction and stability of retained austenite and a large amount of transformed martensite. These findings indicated that when tramp elements such as Cu, Cr, and Ni were positively used, low-carbon TRIP-aided cold-rolled steel sheets having mechanical properties suitable for various application purposes and excellent formability could be successfully achieved.