Evaluation of the Austenite Recrystallization by Multideformation and Double Deformation Tests

Abstract

A high amount of deformation below the non‐recrystallization temperature (Tnr) is a common industrial practice to achieve a good combination of toughness and strength in microalloyed steels. To combine the industrially relevant optimum combination of high productivity and product quality, an accurate knowledge of Tnr and the recrystallization kinetics is required. Although a lot of literature data is available on the recrystallization behaviour of microalloyed steels, correlations are often difficult to be made due to the effect of different experimental set‐ups and test schedules used to obtain this data. Although it would significantly improve the knowledge about these steels, so far, no systematic comparison has been presented in literature to correlate the different techniques one to another. In this study, different hot rolling simulation techniques and testing schedules were compared, within the experimental constraints of the used equipment, to determine the Tnr temperature of two microalloyed steels. Good agreement was found between the results from different test equipment. Furthermore, the results from the multideformation tests under continuous cooling conditions could be correlated with the results from isothermal double deformation tests.

Recrystallization–precipitation interaction during austenite hot deformation of a Nb microalloyed steel

“…In order to quantify the amount of softening between these extremes, the 2% offset method was used. With that method the softening fraction is linearly related to the statically recrystallized volume fraction [23,24]. All deformations were performed in uni-axial compression and under vacuum in a Bähr ® -dilatometer equipped with an induction coil and a deformation unit.…”

“…1 after reheating for 5 min at 1250 • C and subsequent cooling at a rate of 30 • C/s to a chosen deformation temperature, a first deformation pass with a strain of 0.2 and a strain rate of 0.5/s was given. After different interpass times, a second deformation pass (again with ε = 0.2, dε/dt = 0.5/s) was applied [23,24]. The recrystallized fraction corresponding to these different interpass times was determined with the help of the second stress-strain curve.…”

“…It can thus be argued that all precipitates were formed during deformation on nucleation sites formed by dislocations. The evolution of the dislocation density during deformation is expressed by the Mecking-Kocks [24][25][26] theory that relates the evolution of the dislocation density as a function of strain ε to the production and annihilation of dislocations:…”

Effect of Finishing Hot‐Working Temperature on Microstructure of Thermomechanically Processed Mn–Al Multiphase Steels

“…It is well known that adding Nb to the steel increases the no‐recrystallization temperature ( T nr ) below which no complete static recrystallization occurs between successive rolling passes. Below T nr the strain is accumulated with increasing passes leading to austenite pancaking …”

“…Physical simulation is usually integrated with mathematical and numerical models describing the microstructure evolution during hot-working and cooling. [17][18][19][20][21] Thermomechanical processing models of C-Mn and HSLA steels are well developed and documented in literature. Presently, the focus is placed on modeling of thermomechanical treatment of different AHSS grades (DP, TRIP, CP, QP -Quenching and Partitioning), in which a final multiphase microstructure is substantially dependent on decomposition of austenite during run-out table cooling and coiling.…”