PROCESSING AND PROPERTIES OF COLD-ROLLED TRIP STEELS

The influence of the annealing cycle typical of continuous annealing and hot dip galvanizing on the microstructure and mechanical properties of TRIP grades was investigated on large scale produced cold-rolled materials in the laboratory by means of an annealing simulator. In particular, the impact of the annealing temperature in the intercritical range, the start temperature for rapid cooling, the rapid cooling rate and the overaging temperature were investigated for continuous annealing conditions. Concerning the production via a hot-dip galvanizing line annealing experiments with different intercritical temperatures, cooling rates, overaging temperatures and overaging times were conducted for three line speeds. Based on these results optimized annealing cycles were selected and a TRIP 600 grade was industrially produced via continuous annealing and via hot dip galvanizing. The microstructure and the mechanical properties of the materials were investigated in detail. The welding behavior and the formability were described, too. The mechanical properties of the continuously annealed and hot-dip galvanized materials are almost the same. In comparison to a Dual Phase grade with the same tensile strength level, the TRIP grade distinguishes itself by its excellent high uniform and total elongation and high n-values at high strain levels. These outstanding mechanical properties of the TRIP grade result in a significantly better formability and higher energy absorption during crash tests. The welding behavior of the investigated TRIP grade is comparable to other high-strength steel grades.