Flow-Stress Model of 300M Steel for Multi-Pass Compression

In this work, multi-pass compressions were performed at various strain rates (0.01 s−1, 0.1 s−1, 1 s−1, 10 s−1), temperatures (950 °C, 1050 °C, 1150 °C), inter-pass holding time (1 s, 10 s, 30 s, 120 s, 600 s), interrupt strains (0.3, 0.4, 0.5, 0.6), and total pass numbers (1, 2, 3, 4). The intriguing finding was that the recrystallized fraction, average dislocation density, and plastic cumulative strain were partly eliminated during inter-pass holding, resulting in the early occurrence of recrystallization in subsequent compression. Therefore, a parameter (Θ) to evaluate the overall softening fraction due to recrystallization was proposed, and it was then used to iteratively rectify the average dislocation density and plastic cumulative strain in flow-stress modeling. The flow-stress model parameters of 300M steel for multi-pass compression were identified using an optimization technique based on non-derivative method integrated in MATLAB software. The average deviation of calculated and experimental flow-stress was 0.88 MPa (1.35%), showing good accuracy of the flow-stress model. The microstructure evolution of 300M steel was analyzed by the change of softening fraction during multi-pass compression, which provided a useful reference for the research of stress–microstructure relationships of high-strength steels.

Critical Strain for Complete Austenite Recrystallisation during Rough Rolling of C-Mn Steel and Nb-Ti-V Microalloyed Steel

The influence of roughing strain on the extent of austenite recrystallisation in plain carbon steel and Nb-Ti-V microalloyed steel was investigated. Reheating and roughing simulations were conducted on a deformation dilatometer using industrial heating, soaking times, strain and strain rates. The roughing schedules comprised of varying the pass strain magnitude within a typical roughing temperature range. The double stroke method was used to determine the austenite softening fraction. The austenite grain size, prior to and after rolling, was measured on quenched specimens. Grain refinement was achieved in both steels after all rolling schedules. An applied pass strain of 0.15 was sufficient to completely soften the austenite after the first pass and produced the finest recrystallised grain size. This was attributed to sufficient nucleation sites and driving force for recrystallisation compared to lower strains. Partial recrystallisation occurred after the first pass due to the relatively coarse initial grain size. The steel chemistry played an insignificant role in controlling the recrystallisation kinetics at high roughing temperatures. The through-thickness strain distribution calculated from FEM simulations showed that, for a given applied strain, a similar magnitude of deformation is achieved at the centre of a hot-rolled plate.

Study of Recrystallization in High Manganese Steels by Means of the EBSD Technique

The softening processes that take place after hot deformation in two high Mn steels, one of them microalloyed with 0.1%Nb, have been investigated. Double hit torsion tests were carried out in order to determine the mechanical softening at temperatures ranging from 900 to 1100°C. In addition, the applicability of different parameters obtained by means of the Electron Back Scattering Diffraction (EBSD) technique to estimate the recrystallized fraction was investigated. It has been found that the Grain Orientation Spread (GOS) is the parameter that best allows quantifying the recrystallized fraction under the conditions investigated. The correlation between the mechanical softening and the recrystallized fraction measured by EBSD depends on the material and deformation conditions. A good correlation between both values is observed for the base steel at all the temperatures. However, for the Nb microalloyed steel, although at high temperatures a good correlation is also observed, at low temperatures the mechanical softening fraction tends to be larger than the recrystallized fraction denoting that recovery has an important contribution.

Effect of Initial Austenite Microstructure on the Softening – Precipitation Interaction in a Low Nb Microalloyed Steel

The influence of initial grain size on the softening-precipitation interaction in a low niobium microalloyed steel has been investigated. The study has revealed that for the largest initial grain size (1000 μm), the recrystallised fraction remains lower than the softening fraction until relatively long times are reached. In contrast, for the smallest initial grain size (166 μm) both magnitudes are similar. As a result, precipitation interacts with recrystallisation in the case of the finest austenite grain size, whereas for the coarsest one, since recrystallisation is significantly retarded, interaction with recovery process is observed. Apparently, the initial austenite grain size does not affect precipitation kinetics.