Development of Highly Formable Giga Strength Steels by Q&P Process at POSCOTuesday (27.09.2016) 16:30 - 16:45 Part of:
Recently, Q&P (Quenching and Partitioning) is widely investigated as a potential annealing process to develop GPa tensile strength grades and the third generation advanced high strength sheet steels[1~4]. The attractive points of Q&P are that it makes possible leaner alloying elements and higher elongation than conventional AHSS. Even though MA constituent or fresh martensite increases tensile strength in conventional TRIP steels, it deteriorates hole expansion ratio(HER). Since hard martensite is formed from the metastable austenite by insifficient partitioning of carbon during isothermal bainite transformation treatment. However, a certain amount of martensite is transformed from austenite to martensite during quenching and then bainite transformation during partitioning in 2 step Q&P process. Untransformed austenite can be stabilized with the carbon available in the martensite and bainite formed during quenching and partitioning respectively. Thus, final phase constituents obtained are tempered or fresh martensite, bainite, retained austenite and ferrite. Thus, POSCO adopted Q&P for lean alloying TRIP steels with good hole expansion ratio. Elongation≥21% and HER≥20% were developed for tensile strength 980MPa grade, elongation≥14% and HER≥30% were developed for tensile strength 1180MPa grade by the optimization of process variables with carbon below 0.2% containing steels. Excellent HER type of tensile strength 980MPa grade which has 17% of elongation and 55% also developed. HER increases as the volume fraction of MA constituent or fresh martensite decreases and ferrite phases are isolated. From the calculation of constrained para-equilibrium model, carbon partitioning between martensite and austenite is not dominant in carbon containing below 0.2% steel. However, the volume fraction of martensite increases at quenching step, bainte transformation during partitioning step is promoted. This transformation behavior considered to improves HER and elongation.