Effect of Composition and Deformation on Coarse-Grained Austenite Transformation in Nb-Mo Microalloyed Steels

Abstract

Thermomechanical processing of microalloyed steels containing niobium can be performed to obtain deformed austenite prior to transformation. Accelerated cooling can be employed to refine the final microstructure and, consequently, to improve both strength and toughness. This general rule is fulfilled if the transformation occurs on a quite homogeneous austenite microstructure. Nevertheless, the presence of coarse austenite grains before transformation in different industrial processes is a usual source of concern, and regarding toughness, the coarsest high-angle boundary units would determine its final value. Sets of deformation dilatometry tests were carried out using three 0.06 pct Nb microalloyed steels to evaluate the effect of Mo alloying additions (0, 0.16, and 0.31 pct Mo) on final transformation from both recrystallized and unrecrystallized coarse-grained austenite. Continuous cooling transformation (CCT) diagrams were created, and detailed microstructural characterization was achieved through the use of optical microscopy (OM), field emission gun scanning electron microscopy (FEGSEM), and electron backscattered diffraction (EBSD). The resultant microstructures ranged from polygonal ferrite (PF) and pearlite (P) at slow cooling ranges to bainitic ferrite (BF) accompanied by martensite (M) for fast cooling rates. Plastic deformation of the parent austenite accelerated both ferrite and bainite transformation, moving the CCT curves to higher temperatures and shorter times. However, an increase in the final heterogeneity was observed when BF packets were formed, creating coarse high-angle grain boundary units.

Modeling of CCT Diagrams and Ferrite Grain Size Prediction in Low Carbon Nb–Mo Microalloyed Steels

“…This cooling rate range covers the gradual shift from QF + GF microstructures to GF + BF constituents for the displacive transformations. If the dilatometry curves are analyzed in detail 2,3) a clear differentiation of the start-finish temperatures for the individual phases is not noticeable. Therefore T bi and T bf refer to the initial and final transformation temperatures of all bainitic like transformation products without distinguishing between individual constituents.…”

“…Nerea ISASTI, 1) Pedro Manuel GARCÍA-RIESCO, 1) Denis JORGE-BADIOLA, 1) Mitra TAHERI, 2) Beatriz LÓPEZ 1) and Pello URANGA 1) * microstructure after a selected thermomechanical cycle. In this context, in the last few years special effort has been made to develop models that are capable of reproducing continuous cooling transformation diagrams, incorporating the effects of chemical composition, prior austenite grain size, accumulated strain and cooling rate.…”

“…The starting point of the present study are the continuous cooling transformation (CCT) diagrams derived in previous papers by Isasti et al 2,3) and the PhD thesis by García-Riesco 4) for a series of low carbon Nb and Nb-Mo microalloyed steels tested at different prior austenite conditions. These diagrams were built with the help of the information provided by dilatometry tests (transformation start and finish temperatures), microstructural characterization, and Vickers hardness measurements.…”

“…[2][3][4] The current regression analysis takes into account 38 different CCT diagrams. It must be pointed out that the predictive equations are only valid within the mass concentration ranges and test conditions presented in Table 1.…”

Effect of Austenite Deformation on the Microstructure Evolution and Grain Refinement Under Accelerated Cooling Conditions

“…[52] This difference can be noted in another investigation. [53] In that study, a wide cooling rate range (0.1 to 100 K/s) was used, and under low to medium cooling rates (0.1 to 20 K/s), the austenite deformation has a beneficial effect on the effective grain size reduction for ferrite-pearlite microstructures and AF-like microstructures, which can be attributed to the increased intragranular nucleation sites. However, in the high cooling rate range (20 to 100 K/s), the beneficial microstructure refining effect of austenite deformation completely disappears, and the increase of effective grain size occurs when BF becomes the dominant phase.…”