No abstract

The effects of alloying elements and coiling temperature on recrystallization behavior and bainitic transformation were investigated based on 0.07C-Mn-Cr-Nb steel with a low carbon equivalent. Based on the ferrite recrystallization behavior, the proper intercritical annealing temperature of all studied steels was suggested to produce TRIP steel with good strength and elongation balance. All steels coiled at 550°C showed much faster ferrite recrystallization behavior than steels coiled at 700°C. In addition to the coiling temperature, the effect of increasing carbon content on the ferrite recrystallization was minor at a coiling temperature of 550°C, but much more prominent at a coiling temperature of 700°C. The highest Mo added steel showed the best strength and elongation balance, and the highest carbon and Mo added steel showed the highest tensile strength at a coiling temperature of 550°C. The steel containing a higher amount of elemental Al (0.7 wt.% Al) exhibited much better elongation than the lower Al added steel (0.04 wt.% Al) in TS 780 MPa grade, about 24 % and 19 %, respectively.

The effect of microstructural changes on magnetic Barkhausen noise (BN) and magnetomechanical acoustic emission (MAE) in Mn–Mo–Ni pressure-vessel steel with various microstructures was investigated. The BN and MAE signals were strongly influenced by the microstructural features which were characterized by transmission electron micrograph observation. BN energy and the measured area under the rectified signal envelope varied inversely with hardness, and changed with the microstructure. It indicates that they are closely related to the dislocation density and residual stress which act as a barrier to irreversible domain wall motion.

The yielding of a high Mn twinning-induced plasticity steel was examined in three fine-grained specimens recrystallized at 700°C for 5 min with different cooling conditions. While the stress-strain curves of furnace-cooled and air-cooled specimens exhibit a stress drop at yielding, the drop was not observed in the water-quenched specimen. A simple analysis of the displacement data indicates the occurrence of localized deformation at the beginning of the plastic deformation in the three tensile specimens with different cooling conditions. The localized deformation of all three specimens was confirmed as Lüders strain by digital image correlation (DIC) analysis. Based on this observation, the role of yielding behavior on the strain hardening rate evolution at an early stage of the tensile deformation was discussed.

A new modified Jominy holder and specimen were developed to study the effect of chemical composition, intercritical annealing temperature, and cooling rates on the transformation behavior of a series of dual-phase sheet steels. Correlations between temperature, time, and the distance from the water quenched end were developed using a laboratory view data acquisition program. In addition, the hardness and microstructure along the length of the sheet steel sample were determined as a function of cooling rate from the intercritical annealing temperature. Special chemical regents were used to reveal new ferrite, bainite, and other microstructural components, which were also evaluated by optical, scanning, and transmission electron microscopy. Critical cooling rate equations, based on steel composition, volume fraction of austenite and intercritical annealing temperatures, were developed to avoid or minimize the formation of new ferrite and bainite. The results of this work show that the modified Jominy test is an effective and efficient way to evaluate the hardenability of sheet steels.