Static Recrystallization Characteristics Research Articles

Microstructure evolution and static recrystallization kinetics in hot-deformed austenite of coarse-grained Mo-free and Mo containing low-carbon CrNiMnB ultrahigh-strength steels

The static recrystallization characteristics and microstructure evolution in hot-deformed austenite were evaluated for a newly developed low-carbon CrNiMnB ultrahigh-strength steel with and without molybdenum addition. The time for 50% static recrystallization (t 50% ) over a wide range of strains and hot-deformation temperatures were obtained using the stress-relaxation technique on Gleeble thermomechanical simulator. Moreover, effect of deformation parameters on the size distribution and average size of prior austenite grains are investigated. A novel semi-automatic stress relaxation test reading tool with a graphical user interface was created and used successfully for the current study. The obtained results of strain´s power and the apparent activation energy are within the range stated in literature for C-Mn and microalloyed steels. Addition of molybdenum increase the power of strain and the apparent activation energy from -1.9 to -2.6 and 206 to 212 kJ/mol, respectively. The retardation effect of molybdenum addition was shown by a new regression equation devised for calculating t 50% . The developed equations show a good agreement with the experimental data and can be used in the designing of roughing during thermomechanical processing. The deformation parameters i.e., temperature, strain and holding time have a significant effect on the size distribution and average size of prior austenite grains. • The static recrystallization characteristics and microstructure evolution in hot-deformed austenite were evaluated for Mo-free and Mo-containing ultrahigh-strength steels. • A novel semi-automatic stress relaxation test reading tool with a graphical user interface is created to overcome the difficulties in determination of the onset of static recrystallization manually from stress relaxation curves. • SRX kinetic equations are derived for explaining static recrystallization in hot-deformed austenite of Mo-free and Mo-containing ultrahigh-strength steels. • Quantitative analysis of microstructural feature during SRX is performed.

Flow Stress Behaviour and Static Recrystallization Characteristics of Hot Deformed Austenite in Microalloyed Medium-Carbon Bainitic Steels

In the past decade, efforts have been focused on developing very fine, medium-carbon bainitic steels via the low-temperature (typically 300–400 °C) ausforming process, which not only enables shorter isothermal holding times for bainitic transformation at low temperatures, but also offers significantly improved strength. This paper describes static recrystallization (SRX) characteristics of austenite in four medium-carbon 2%Mn-1.3%Si-0.7%Cr steels with and without microalloying intended for the development of these steels. The stress-relaxation method on a Gleeble simulator resulted in recrystallization times over a wide range of temperatures, strains and strain rates. Also, the occurrence of precipitation was revealed. Powers of strain (−1.7 to −2.7) and strain rate (−0.21 to −0.28) as well as the apparent activation energies (225–269 kJ/mol) were in the ranges reported in the literature for C-Mn and microalloyed steels with lower Mn and Si contents. The new regression equations established for estimating times for 50% SRX revealed the retardation effects of microalloying and Mo addition showing reasonable fits with the experimental data, whereas the previous model suggested for ordinary microalloyed steels tended to predict clearly shorter times on average than the experimental values for the present coarse-grained steels. The Boratto equation to estimate the non-recrystallization temperature was successfully modified to include the effect of Mo alloying and high silicon concentrations.

Static recrystallization characteristics and kinetics of high-silicon steels for direct quenching and partitioning

Abstract In the direct quenching and partitioning (DQ&P) process, tough ultra-high-strength steel is made by combining thermomechanical processing with quenching and partitioning to obtain martensite toughened by thin films of retained austenite. The hot rolling stage with deformation and recrystallization between the rolling passes affects the state of the austenite before quenching and partitioning. This paper describes the static recrystallization kinetics of two steels with compositions suited to DQ&P processing, viz. (in wt.%) 0.3C-1Si-2Mn-1Cr and 0.25C-1.5Si-3Mn. The stress relaxation technique on a Gleeble thermomechanical simulator provided recrystallization times over a wide range of temperature, strain, strain rate and initial grain size. The higher levels of Si and Mn made the recrystallization kinetics less sensitive to strain, strain rate and temperature. The equations derived to describe the recrystallization kinetics can be used in the design of the rough rolling part of thermomechanical processing.

Effect of As-Rolled Microstructure on Static Recrystallization Characteristics and Texture Evolution during Annealing

Magnesium AZ31 alloy sheets were rolled at 100 °C at a high rolling speed of 1000 m/min. After 30% reduction, the microstructure was heavily twinned and shear banded, while a partially dynamically recrystallized and twinned microstructure was seen at the reduction of 49%. The as-rolled specimens were then annealed at 500 °C for increasing times. Microstructure and texture were characterized by optical microscopy, electron backscattered diffraction (EBSD) and X-ray diffraction (XRD). Texture weakening was found during annealing of the specimens at both reductions. However, the texture weakening was more effective in the fully twinned and shear banded specimen than the partially DRXed and twinned specimen. Effects of as-rolled microstructure on static recrystallization characteristics and texture evolution during annealing were studied.

Microstructure Evolution Characterization of Multi-Pass Hot Rolling Simulation of Nb-Ti Microalloyed Steel

In this study, a improved mathematical model was developed for Nb-Ti microalloyed steel during hot rolling simulation. Using the compression test, the dynamic and static recrystallization characteristics of Nb-Ti microalloyed steel were studied. Though multi-pass hot rolling simulation, it is found that the recrystallization during hot rolling can play an important role, it can make the mean flow stress lower and refine the grains. And respective comparison between calculated and measured data of microstructure showed some of the validation of the built model. Meanwhile, the evolution characteristic of average austenite grain size during hot rolling can be achieved by theoretical model and experiment.

Effect of Niobium on Static Recrystallization Characteristics of TWIP Steels

In this study, the constitutive flow behavior and static recrystallization characteristics of a Nb-microalloyed TWIP (Fe-20Mn-1.5Al-0.1Nb) steel under hot deformation conditions have been determined and results compared with those of Fe-25Mn-Al TWIP steels. Investigations using compression testing in a Gleeble simulator, including the double-hit technique, enabled the acquisition of flow stress and recrystallization data. These were analyzed to determine the powers of strain and strain rate as well as the activation energies of deformation and recrystallization (Qdef and Qrex). For given deformation and grain size parameters, the time for 50% recrystallization (t50) of the 0.1% Nb TWIP steel was significantly longer than for the Nb-free TWIP steel: it was comparable to that of Nb-microalloyed carbon steel. Qrex was higher than that of Type 304 stainless steel that has nevertheless much longer t50 times.

Modelling of Microstructure Evolution during Recrystallization Controlled Rolling.

The static recrystallization characteristics (grain size, kinetics) have been established for Ti-V-(Nb) austenites and used as the basis for a theoretical evaluation of microstructural evolution during hot rolling of plate. In this context, particular attention has been focussed on so-called recrystallization controlled rolling, whereby a fine as-rolled ferrite garin size is obtained via transformation from an austenite which has been substantially grain refined via static recrystallization. The model is shown to forecast a behaviour which is in acceptable accord with practical plate-rolling experience. Furthermore, the rolling model has been used in a systematic theoretical investigation of the effect of principal rolling variables on the degree of microstructural refinement during processing of Ti-V-(Nb) steels via recrystallization controlled rolling.

Static recrystallization after hot deformation of α iron

The dependence of static recrystallization characteristics on deformation and annealing conditions has been examined for vacuum melted and zone-refined iron deformed in torsion at high temperatures in the α range. The kinetics of recrystallization are strongly accelerated by increasing deformation at low strains, but are virtually independent of strain in the “steady-state” region of the high temperature flow curve. In the latter case the kinetics of recrystallization are greatly influenced by the dynamic restoration process operative during hot deformation. In particular, for the same material, static recrystallization is faster in dynamically recovered than in dynamically recrystallized structures due to a decrease in growth rate with tie in the latter. This and other differences in the static recrystallization behavior of these two structures are discussed in terms of the differences in their microstructural features.