Microstructure Control And Mechanical Properties Of Hot-rolled Microalloyed TRIP Steels

The microstructural evolution of hot-rolled TRIP steels with different microalloying condition (not added/Nb added alone/Nb+Ti or V+Ti added) during different thermo-mechanical processes were investigated by hot uniaxial compression using a Gleeble-1500 hot simulator, in combination with optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), inductively coupled plasma optilal emission spectrometry (ICP-OES), room-temperature tensile test and in situ high-energy X-ray diffraction (HEXRD), and the effects of microalloying elements and multi-phase microstructure states on the mechanical behaviors of hot-rolled microalloyed TRIP steel were analyzed. In addition, the trial manufacture of hot-rolled microalloyed TRIP steels was conducted using a reversing rolling pilot test mill. The results obtained are as follows.Because of the high C content in the used TRIP steels, the added microalloying elements could not be dissolved completely after austenitization at 1250℃ for 5min, and there was a certain amount of undissolved particles presented in the microstructures of the used TRIP steels. When the used TRIP steel was microalloyed with Nb alone, the content of Nb dissolved into austenite could be up to about 70% of the total amount of Nb added. While the used TRIP steel was microalloyed with Nb+Ti or V+Ti, the dissolved content of Ti was quite low and led to the lower dissolved content of Nb, but had no significant effect on the dissolved content of V.The deformation in austenitic nonrecrystallization region could be realized by the precipitation of solid solution Nb during thermal processing. In the case of the hot-rolling process based on controlled cooling, the higher grain boundary area per unit volume (5v) in the pancaked sustenite state, in comparison with that in the recrystallized sustenite state, promoted the transformation of austenite to ferrite and the refinement of ferrite grain. As a result, the deformation in austenite nonrecrystallization region led to the finer ferrite grains, smaller bainite packets with chaotic orientation, and more precipitation of Nb in the final multi-phase microstructures, thus resulting in an improvement of ultimate tensile strength and elongation of the used TRIP steels. In the case of the hot-rolling process based on dynamic transformation,-the influence of the prior austenite state on the microstructure evolution during subsequent thermo-mechanical process became weak, and the difference of the amount of Nb precipitation was small, thus the hot-rolling process based on dynamic transformation exhibited higher process stability. In comparison with that based on controlled cooling processing, TRIP steels obtained by the process based on dynamic transformation demonstrated the markedly improved combination of strength and ductility, due to the much refined and uniform microstructures with smaller sizes of ferrite and bainitic packets and more stable retained austenite.The stability of retained austenite in TRIP steels could be significantly improved by the addition of Nb. On the one hand, the dissolved Nb in retained austenite increased the value of stacking fault energy of austenite, thus decreased the rate of shear bands formation, retarding the strain-induced transformation of retained austenite to martensite. On the other hand, because the transformation of austenite to ferrite was retarded by the addition of Nb, most of retained austenite was layer-like between the bainitic ferrite laths with higher stability. Retained austenite with high stability was not easy to transform to martenite during deformation, which would be beneficial to improve the work-hardening ability and achieve a better combination of strength and ductility.In comparison with that in the case of the addition of Nb alone, the co-addition of Nb and Ti reduced the amount of dissolved Nb in austenite after austenitization. The Nb-Ti steel demonstrated higher ultimate tensile strength and lower elongation than Nb steel, under similar state of multi-phase microstructures. The addition of V had a markedly weaker effect on the refinement of austenite grains in comparison with the addition of Nb, and showed less effect on the transformation of austenite to ferrite and the refinement of ferrite grains, as well as the stability of retained austenite. The main effect of the addition of V was improving the yield strength of the used steels through the strong precipitation strengthening.The trial manufacture of hot-rolled microalloyed TRIP steels based on dynamic transformation was conducted using a reversing rolling pilot test mill. Under the experiment condition with relatively simple cooling control and simulation coiling, the ultimate tensile strength of Nb steel could be higher than 800MPa with the total elongation up to 40%, i.e. the product of tensile strength and total elongation was higher than 30GPa%. The ultimate tensile strength of V-Ti steel was close to 900MPa with the total elongation up to 30%, i.e. the product of tensile strength and total elongation was higher than 27GPa%. It was confirmed that the hot-rolling process based on dynamic transformation for hot-rolled microalloyed TRIP steels was feasible in industrial production, and hot-rolled microalloyed TRIP steels with well mechanical properties can be fabricated though adjusting the addition of microalloying elements and optimizing the processing parameters.