A Perspective on Processing and Development of Tough Ductile High Strength Steels through TMR-DQP ProcessTuesday (27.09.2016) 16:45 - 17:00 Part of:
In recent years, quenching and partitioning (Q&P) processing has shown enormous potential in providing interesting combinations of strength, ductility, toughness and strain hardening capacity in advanced high strength steels. The University of Oulu in collaboration with its industrial partners have embarked on a program to develop tough ductile ultra-high strength steels based on a recently developed novel process comprising thermomechanically controlled rolling combined with direct quenching and partitioning (TMR-DQP). The approach used was to design suitable chemical compositions based on high Si and/or Al content, with or without addition of Nb, Ni or Mo, to determine appropriate DQP processing conditions with the aid of physical simulation on a Gleeble simulator and finally test DQP material processed on a laboratory rolling mill including slow cooling in a furnace simulating the coiling in industrial processing. The dilatation measurements conducted on Gleeble samples also enabled understanding the possible microstructural mechanisms operating during Q&P and thereby get closer to the thorough understanding needed to design optimized compositions and processing routes. Besides carbon partitioning, formation of isothermal martensite, lower bainite and in some cases, twinned, high carbon martensite during final cooling were clearly evident from the dilatation curves. Recent studies on some high-aluminium steels showed small volume contractions at the start of partitioning that have been tentatively attributed to the interface migration from austenite to martensite.
Preliminary simulations and promising results suggest that TMR-DQP processing route should be applicable to at least industrial hot strip production. Metallographic studies clearly revealed presence of extended pools and laths of retained austenite between martensite laths. Slow cooling even at relatively low quenching temperatures enabled effective partitioning, thus stabilizing 4-7% austenite at RT. High values of yield and tensile strengths and hardness combined with improved uniform and total elongations and remarkable low temperature impact properties suggest that there is good potential for the TMR-DQP route not only for tough ductile structural steels, but also for hard abrasion-resistant steels. The results also indicate that it is not the amount of austenite, but presumably its size and distribution that decide its influence on uniform and total elongation and low-temperature impact toughness.