In recent years considerable researches have been carried out on the development of high strength steels for applications in automotive industry. The main issues are to reduce weight and increase safety of vehicles by use of high strength and high ductility steels. Among the candidates, so-called TRIP(Transformation Induced Plasticity)-aided multi-phase steels have been regarded as one of the most promising materials due to their excellent combinations of strength and ductility. The high formability of these transforms to martensite by plastic deformation. To lessen economic and environmental burdens, development of TRIP steels have been focused on the investigation of less expensive grades with simple constituents, such as C-Mn-Si steels. In low-alloyed C-Mn-Si TRIP steels, the austenite is stabilized by C and partly by Mn, and cementite formation during isothermal bainitic transformation is suppressed by Si. In order to further improve mechanical and galvanizing properties, or substituted to Si in the conventional, C-Mn-Si TRIP steels.
Cu as austenite stabilizer is not soluble in cementite like Si, and it has been known that Cu showed beneficial roles in strengthening steels through various mechanism such as, solid solution hardening, refinement of ferrite grain size and precipitation hardening. Therefore, Cu is expected to be an effective alloying element in TRIP-aided steels.
In the present study, effects of copper addition to the C-Si-Mn TRIP-aided multi-phase steels on the mechanical properties, amount of retained austenite and its stability subjected to plastic deformation have been investigated. Three kinds of steels whose basic composition is 0.15C-1.5Mn-1.5Si, ECO-A(no Cu addition), ECO-B(0.5wt.%Cu) and ECO-C(1.0wt.%Cu), respectively, were fabricated according to the conventional two stage treatment at slightly above Ms temperature. Microstructure observation and mechanical tests were carried out, and variations of fraction of retained austenite with straining were determined by X-ray diffraction measurement.
In the three type of steels employed in this study, typical microstructures of TRIP-aided multi-phase steels, which were composed of ferrite, bainite and retained austenite were well developed, and amount of retained austenite were increased with increasing Cu content. Tensile strength was increased with an addition of Cu, however, ductility did not show monotonic increase with Cu contents. In the 1.0wt.% Cu-added steel, amount of retained austenite was increased but its stability with straining is decreased, which caused to high strength with low ductility. However, in the 0.5wt.% Cu-added steel, stability of retained austenite did not show severe variation against plastic deformation, which was effective to utilize TRIP effect and leaded to good strength and ductility balance. It is concluded that strength of the TRIP steel is closely related to amount of retained austenite stabilized at room temperature. In addition, martensite formed after cooling from heat treatment might be affect strength of the TRIP steels, which need further study. And it is shown that the stability if the retained austenite is the most important factor governing ductility level of the TRIP steels rather than its volume fraction.