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.

Phase Transformation Law of Nb Microalloyed Steel at Different Cooling Rates

Through the Gleeble3500 thermal simulation test machine, the phase transformation law of Nb microalloyed steel was studied and tested. After the compression deformation, it was cooled to room temperature at different speeds. Obtain the dynamic continuous cooling transformation diagram and the scanning structure diagram of the test steel, and then analyze the phase composition under different cooling speeds through JMatPro material performance simulation. The results show that: at a lower cooling speed (0.1°C/s), austenite decomposition is a diffusion-type phase change that takes place in a high-temperature region, and carbon atoms can diffuse sufficiently. At a moderate cooling rate (1°C/s), the bainite phase transition is a semi-diffusion phase transition in which carbon atoms are displaced in a non-cooperative thermally activated transition mode. When the cooling rate is high (15°C/s), the martensitic transformation is a non-diffusion-type transformation carried out in the low temperature region, and the atoms are directly transferred from the austenite lattice to the martensite lattice. With the increase of the cooling rate and the decrease of the transition temperature, from low-speed cooling→medium-speed cooling→high-speed cooling, respectively, the diffusion type phase transition→semi-diffusion type phase transition→the non-diffusion type phase transition. At different cooling rates, the continuous cooling transition diagram simulated by JMatPro is basically the same as the phase transition in the dynamic continuous cooling transition diagram of the test steel, which proves that the simulation prediction of the dynamic continuous cooling transition of the test steel by the JMatPro software has high accuracy and applicability.

Determination of Non-Recrystallization Temperature for Niobium Microalloyed Steel

In the present investigation, the non-recrystallization temperature (TNR) of niobium-microalloyed steel is determined to plan rolling schedules for obtaining the desired properties of steel. The value of TNR is based on both alloying elements and deformation parameters. In the literature, TNR equations have been developed and utilized. However, each equation has certain limitations which constrain its applicability. This study was completed using laboratory-grade low-carbon Nb-microalloyed steels designed to meet the API X-70 specification. Nb- microalloyed steel is processed by the melting and casting process, and the composition is found by optical emission spectroscopy (OES). Multiple-hit deformation tests were carried out on a Gleeble® 3500 system in the standard pocket-jaw configuration to determine TNR. Cuboidal specimens (10 (L) × 20 (W) × 20 (T) mm3) were taken for compression test (multiple-hit deformation tests) in gleeble. Microstructure evolutions were carried out by using OM (optical microscopy) and SEM (scanning electron microscopy). The value of TNR determined for 0.1 wt.% niobium bearing microalloyed steel is ~ 951 °C. Nb- microalloyed steel rolled at TNR produce partially recrystallized grain with ferrite nucleation. Hence, to verify the TNR value, a rolling process is applied with the finishing rolling temperature near TNR (~951 °C). The microstructure is also revealed in the pancake shape, which confirms TNR.