Physical Simulation Based on Dynamic Transformation Under Hot Plate Rolling of a Nb-Microalloyed Steel

In this work, the presence of dynamically formed ferrite above the Ae3 temperature during the physical simulation of hot rolling was presented. This unusual metallurgical process is known as dynamic transformation (DT). The metastable ferrite phase undergoes a reverse transformation when the temperature is held above the Ae3 by means of a diffusion process. These phenomena affect the rolling load during high-temperature plate rolling. Therefore, a linepipe X70 steel was studied under plate rolling with two-pass roughing and seven-pass finishing strains of 0.4 and 0.2, respectively, applied at strain rate of 1 s−1 and interpasses of 10, 20, and 30 s. The samples were cooling down during deformation, which mimics the actual industrial hot rolling. It was observed that the alloy softens as the hot rolling progresses, as depicted by flow curves and mean flow stress plots, which are linked to the combined effects of dynamic transformation and recrystallization. The former initially occurs at lower strains, followed by the latter at higher strains. The critical strain to DT was affected by the number of passes and temperature of deformation. Shorter interpass time allows higher amounts of ferrite to form due to higher retained work hardening. Similarly, the closer the deformation temperature to the Ae3 permits a higher DT ferrite fraction. The information from this work can be used to predict the formation of phases immediately after hot rolling and optimize models applied to the accelerated cooling.

Strain-Induced Ferrite Formation During Steckel Mill Simulations with Varying Roughing Pass Schedules

It has been previously demonstrated that austenite may undergo partial dynamic transformation (DT) during the plate rolling process. Austenite dynamically transforms into unstable ferrite during hot deformation even at very high temperatures. In this work, the plate rolling simulations, with emphasis on Steckel mill operations, through torsion testing under isothermal conditions were performed on an X70 steel. Four different roughing schedules were tested followed by five finishing passes with pass strains of 0.3 applied at 900 °C. The roughing schedules had zero, one, two and three roughing passes at a temperature of 1100 °C, strain of 0.4 and strain rate of 1 s−1. The stress–strain curves as well as the mean flow stress (MFS) behaviors indicated that both dynamic transformation (DT) and dynamic recrystallization (DRX) occurred during straining. The critical strains for the onset of DT and DRX were determined by means of the double differentiation method and the critical strain values decreased with the number of roughing and finishing strains from the first going to the last pass. It was observed that the volume fraction of the dynamically formed ferrite increased sharply during the finishing stage as the number of previous roughing passes increased, which can be attributed to higher strain accumulation. The results presented here indicate that improved models are needed to control the microstructure of the material during subsequent cooling.

Physical Simulation Methods Applied to Hot Rolling of Linepipe Steels

The simulation of industrial rolling processes has been shown to be an important method to optimize rolling parameters, reduce production costs and improve product quality. Previous works have shown the value of hot rolling simulation by means of torsion tests where the mean-flow-stress (MFS) can be successfully predicted. In the present work, three rolling schedules are simulated by hot torsion tests and compared. It is important to note this methodology provides the flexibility to test different ideas without the risk of downtime or damage to plant equipment that could result from an unsuccessful industrial trial. The simulation analysis considered the production steps from reheating through the final accelerated cooling as well as the final product microstructures. The study provides important information to the production of various steel grades such as pipeline, shipbuilding, structural and other high-end products.

Hot Torsion Tests - A Reliable Rolling Simulation Method for C-Mn Steels

Torsion tests have been proven to be a successful method to simulate the hot rolling of steels. Simulation work performed at a laboratory scale together with the analysis of the resulting mean-flow-stress behavior, leads to important metallurgical information to be considered during full-scale rolling processes. In this work, two different hot deformation schedules of C-Mn steels have been performed on a Gleeble simulation system in hot torsion mode. In addition to the torsion tests, the mean-flow-stresses of industrial rolling data were analyzed. Industrial hot deformation schedules simulated using hot torsion and the mean-flow-stress values were plotted versus the inverse of absolute temperature in the same graph. All points match the same behavior showing that torsion testing is a reliable hot working simulation method.

Dynamic Transformation during Plate and Strip Rolling

Torsion simulations were carried out of both plate (long interpass times) and strip (short interpass times) rolling. Both isothermal and continuous cooling conditions were employed. The dynamic transformation of austenite to ferrite was observed under all conditions and at all temperatures within the austenite phase field. About 8 to 10 volume percent ferrite was formed in a given pass, leading to about 50 - 70 % ferrite at the end of selected simulations. During the interpass intervals, some retransformation to austenite took place, the amount of which increased with holding time and temperature and decreased with the addition of alloying elements. It is shown that the driving force for the transformation is the softening associated with the replacement of work-hardened austenite grains by the softer alpha phase. The implications with respect to rolling load (i.e. mean flow stress) are also discussed.