New metallurgy for microalloyed TRIP steels

This project investigated the possibility of applying precipitation hardening techniques to TRIP metallurgy. The objective was to obtain a TRIP grade in hot and cold rolled formats with a tensile strength of 1000MPa based on: (1) existing 800MPa silicon TRIP (0.2C-1.5Mn-1.5Si) compositions microalloyed with titanium or vanadium with a high nitrogen content; (2) a new low silicon microalloyed TRIP grade suitable for continuous galvanising. Concerning silicon TRIP hot strips, good results were obtained in laboratory studies with additions of 0.2% wt. V and 0.1 % wt. Ti using two step cooling in the intercritical region to promote interphase precipitation. Strength levels of 936MPa with a total elongation of 25% were achieved for 0.2V steels coiled at 400°C. For the Ti steel coiled at 350°C, a good compromise in tensile strength-total elongation (913MPa-26%) was attributed to the presence of martensite and a high retained austenite percentage (10%) rather than to precipitation strengthening. For high silicon cold strips excellent mechanical properties were obtained in the laboratory with additions of 0.15%wt. V. After intercritical annealing at 775°C and overageing at 400°C strength levels up to 999MPa with total elongations >25% were achieved. A new low Si TRIP composition (0.2C-0.4Si-1.8Mn-0.08P-0.2V-0.01N) was determined on the basis of laboratory simulations. A first industrial trial produced galvanised cold strips with a dual phase microstructure that was attributed to the lack of an overageing section on the galvanising line. A large dispersion in the tensile strength (879MPa < UTS < 1072MPa) and total elongation (10% < A tot < 19.2%) along the length and width of the coil was found. This was due to the high coiling temperature (∼630°C) which caused a heterogeneous hot strip precipitation state due to thermal gradients in the coil. A second industrial trial (same composition) using a lower coiling temperature (550°C) and a galvanising line with an overageing section produced a final microstructure with 15% retained austenite in a ferrite/bainite matrix with some martensite. The mechanical properties were improved (YP = 425MPa, UTS = 922MPa, Ago = 17.5%) however a true TRIP effect could not be observed.