Papers by Keyword: Static Recrystallization

Abstract: Excellent mechanical properties (high strength and toughness) of microalloyed steels are mainly caused by induced precipitation during thermomechanical treatment (TMT) and grain refinement. It has been recently found that TMT of Nb-microalloyed steels can give rise to two different kinds of precipitates, manifested by the double plateau in the statically recrystallised fraction (Xa) against time curves. This work presents an electron diffraction study performed in a transmission electron microscope, equipped with an EDS analytical system. Lattice parameters of a great deal of particles, smaller than 200 nm and with face cubic centred structure, have been measured. Frequency distribution of the values of lattice parameters shows that these are grouped in two sets whose mean values are close. Comparison of these values with those found in the literature for carbides, nitrides and carbonitrides usually present in microalloyed steels demonstrates that they are Nb carbonitrides with slight stoichiometric differences (NbCxNy).

Abstract: We have investigated the static and metadynamic behaviour of the interstitial free steels and in particular the effects of the steeling elements (phosphorous and boron) on kinetics of recrystallisation. The results showed that the strain for the initiation of strain independent softening (often referred to as metadynamic recrystallisation) varies with the Zener-Hollomon parameter and steel composition. Strain rate had a strong influence on kinetics of metadynamic recrystallisation. An increase in temperature from 930oC to 1100oC led to a decrease in time for 50% softening (about one order of magnitude) in the SRX region. However, for the same temperature range, the time for 50% MDRX did not change significantly.

Abstract: The stress relaxation method has been applied to the nickel-based alloy 80A to predict meta-dynamic (MDRX) and static recrystallization (SRX) kinetics. Compression tests were performed on a Gleeble 3800 system atr different temperatures (950-1200°C). The strain rate was varied in the case of MDRX and the pre-strain in the case of SRX. To investigate MDRX, the prestrain was set to twice the peak strain in order to reach steady state before holding. To focus on the interaction of MDRX and SRX, the pre-strain was set to the peak strain, where dynamic recrystallization (DRX) starts but does not yet reach steady state. Avrami type equations for the prediction of both the MDRX and SRX were adapted to feed a semi-empirical grain structure model.

Abstract: This work has studied the influence of different Ti and N compositions on hot deformation strength by determining the peak stress of flow curves and the activation energy (dynamic recrystallisation). It has also assessed their influence on static recrystallisation by means of the statically recrystallised fraction versus time and the activation energy. A precipitate study performed by SEM and TEM has yielded a better understanding of the influence of the Ti/N ratio and precipitation state in hot deformation (dynamic and static recrystallisation). A correlation was found between for the finer distribution of precipitates, Ti/N ratio close to 1.5, smaller austenite grain, maximum activation energy for hot deformation (dynamic recrystallisation) and maximum activation energy for static recrystallisation.

Abstract: A linear regression model consisting of the weighted sums of certain alloying elements has recently been developed to predict the activation energy (Qrex) and kinetics of static recrystallisation (SRX) for hot-deformed austenite based on stress relaxation test results for over 40 different carbon steels. The validity of the model has been further assessed here by determining the Qrex and the kinetics of SRX of certain high-Nb bearing steels, extra-low and low carbon Nb-Mo bainitic and high-Si dual phase and TRIP steels, and Nb-Ti grades with the varying N content. The validity of the model is shown to be fairly good for the Nb-Ti, Nb-Mo and Cr-Mo grades. The approach of maximum effective concentration of Nb and Si and the weight factor for Cr enable reasonable fit for DP, TRIP and Nb-Cr steels, as well. Possible influences of C and N on Qrex and the kinetics of SRX were checked, but none was observed in microalloyed steels.

Abstract: It has been demonstrated recently that when a reversion of the strain is applied during the hot working of a Nb-microalloyed steel, the subsequent static recrystallization kinetics is significantly affected. However, depending on the magnitude of the reversion, the static softening kinetics can be accelerated or delayed. This relates to the substructure dissolution taking place by the effect of the reversal. In the present work, new microstructural results obtained by EBSD on an austenitic stainless steel hot deformed by torsion is used to explain better the observed effect of the reversion of the strain.

Abstract: The present work examines the effect of strain and Zener Hollomon parameter, Z, on deformation and recrystallization of Ti-IF steel deformed in the warm temperature region. Torsion tests were performed at temperatures of 765oC and 850oC and strain rates of 0.003s-1 and 1s-1. For some conditions, an annealing treatment at the temperature of deformation was carried out using a fluidized bed furnace. Electron Back Scatter Diffraction (EBSD) maps were implemented to study the microstructure evolution. At a low value of Z abnormally large grains formed during annealing. At the higher values of Z, a more classical recrystallization reaction occurred during annealing. The latter is more desirable in most commercial applications.