Microstructural evolution in an ultralow-C and high-Nb bearing steel during continuous cooling

Evolution of microstructure and precipitation state during thermomechanical processing of a X80 microalloyed steel

“…Several authors have previously found that samples that suffered hot deformation can present acicular ferrite microstructures after very rapid cooling [8,24] and acicular ferrite can be formed in X70-80 steels even at cooling rates near 75 • C/s [20]. If prior austenite grain size is very fine, as is the case of present study, hardenability is lowered and quasi-polygonal or polygonal ferrite can also be found after deformation followed by fast cooling [10]. Fig.…”

“…On the other hand, lower values of generate more acicular structures after cooling. Several authors have observed that the higher density of substructure and dislocations in deformed austenite displace the transformation products from bainitic microstruc- tures to acicular ferrite firstly [18,20] or even to polygonal ferrite and pearlite for slower cooling rates or low hardenabilities [8,10,16,24,25,27,[57][58][59]. It has been reported that prior deformation of the austenite below T nr and before final cooling could enhance acicular ferrite formation (at the expense of bainite) even more than the addition of molybdenum [26].…”

“…These steels include a strict combination of alloying and microalloying elements to obtain good weldability, adequate strength in corrosive environment and a precise balance of microconstituents in order to achieve excellent mechanical properties at a reduced cost [4][5][6][7]. To obtain an adequate fine-grained final microstructure, the strict control of thermomechanical processing (reheating temperature, percentage reduction, interpass times, roughing and finishing temperatures) and accelerated cooling (delay time between finish rolling and the onset of accelerated cooling, cooling start temperature, cooling rate, cooling stop temperature) is also crucial for the strength and toughness properties [8][9][10][11][12].…”

“…In pipeline steels, an increase in the amount of hot-deformation in the austenite non-recrystallization región during thermomechanical processing increases the volume fraction of acicular ferrite after cooling at the expense of bainitic ferrite. The latter is a consequence of the higher density of substructure and dislocations within the austenite grains, which increases the nucleation rate of acicular ferrite [8,10,18,20,[24][25][26][27]. This also impedes the growth of /˛interfaces and accelerates the diffusion of carbon to the /˛interface, which can give rise to carbonenriched austenite due to the partitioning of carbon [20].…”

Development of Ti and Mo micro-alloyed hot-rolled high strength sheet steel by controlling thermomechanical controlled processing schedule

“…As a result, the austenite grains became somewhat large (2 À30 mm), as shown in Fig. 3b, and this finally resulted in somewhat large ferrite grains ( $ 7 mm) ( Table 1); ferrite grain size is closely related to austenite grain size because the austenite grain boundaries serve as the nucleation sites for g-a transformation during the coiling process [20][21][22][23]. On the other hand, for the rolling condition of the FRT 880 1C which corresponds to the rolling at g non-recrystallization region, the deformation applied during the rolling process cannot be accommodated completely unlike the case of FRT 1050 1C in which the deformation applied can be accommodated via the recrystallization of austenite grains.…”