Effect of deformation schedule on the microstructure and mechanical properties of a thermomechanically processed C-Mn-Si transformation-induced plasticity steel

Thermomechanical processing simulations were performed using a hot-torsion machine, in order to develop a comprehensive understanding of the effect of severe deformation in the recrystallized and nonrecrystallized austenite regions on the microstructural evolution and mechanical properties of the 0.2 wt pct C-1.55 wt pct Mn-1.5 wt pct Si transformation-induced plasticity (TRIP) steel. The deformation schedule affected all constituents (polygonal ferrite, bainite in different morphologies, retained austenite, and martensite) of the multiphased TRIP steel microstructure. The complex relationships between the volume fraction of the retained austenite, the morphology and distribution of all phases present in the microstructure, and the mechanical properties of TRIP steel were revealed. The bainite morphology had a more pronounced effect on the mechanical behavior than the refinement of the microstructure. The improvement of the mechanical properties of TRIP steel was achieved by variation of the volume fraction of the retained austenite rather than the overall refinement of the microstructure. <br /

Effect of Austenite Deformation on the Microstructure Evolution and Grain Refinement Under Accelerated Cooling Conditions

Although there has been much research regarding the effect of austenite deformation on accelerated cooled microstructures in microalloyed steels, there is still a lack of accurate data on boundary densities and effective grain sizes. Previous results observed from optical micrographs are not accurate enough, because, for displacive transformation products, a substantial part of the boundaries have disorientation angles below 15 deg. Therefore, in this research, a niobium microalloyed steel was used and electron backscattering diffraction mappings were performed on all of the transformed microstructures to obtain accurate results on boundary densities and grain refinement. It was found that with strain rising from 0 to 0.5, a transition from bainitic ferrite to acicular ferrite occurs and the effective grain size reduces from 5.7 to 3.1 μm. When further increasing strain from 0.5 to 0.7, dynamic recrystallization was triggered and postdynamic softening occurred during the accelerated cooling, leading to an inhomogeneous and coarse transformed microstructure. In the entire strain range, the density changes of boundaries with different disorientation angles are distinct, due to different boundary formation mechanisms. Finally, the controversial influence of austenite deformation on effective grain size of low-temperature transformation products was argued to be related to the differences in transformation conditions and final microstructures

Crystallographic analysis of martensite in 0.2C-2.0Mn-1.5Si-0.6Cr steel by EBSD

The crystallography of martensite formed in 0.2C-2.0Mn-1.5Si-0.6Cr steel was studied using the EBSD technique. The results showed that the observed orientation relationship was closer to the Nishiyama- Wassermann (N-W) than to the Kurdjumov-Sachs (K-S) orientation relationship (OR). The microstructure of martensite consisted of parallel laths forming morphological packet-like structures. Typically, there were three different lath orientations in a morphological packet consisting of three specific N-W OR variants sharing the same {111} austenite plane. A packet of martensite laths with common {111} austenite plane was termed as a crystallographic packet. Generally, the crystallographic packet size corresponded to the morphological packet size, but occasionally the morphological packet was found to consist of two or more crystallographic packets. Therefore, the crystallographic packet size appeared to be finer than the morphological packet size. The relative orientation between the variants in crystallographic packets was found to be near 60°/. This appears to explain the strong peak observed near 60° in the grain boundary misorientation distribution. Martensite also contained a high fraction of boundaries with their misorientation in the range 2.5-8°. Typically these boundaries were found to be located inside the martensite laths forming lath-like sub-grains, whose long axes were parallel with the long axis of the martensite laths

Crystallographic Analysis of Isothermally Transformed Bainite in 0.2C-2.0Mn-1.5Si-0.6Cr Steel Using EBSD

The crystallography of bainite, transformed isothermally at 450 °C in 0.2C-2.0Mn-1.5Si-0.6Cr steel, was investigated by electron backscatter diffraction (EBSD) analysis. The orientation relationship (OR) was found to be closer to Nishiyama-Wassermann (N-W) than Kurdjumov-Sachs orientation relationship. Bainite microstructure consisted of parallel laths forming a morphological packet structure. Typically, there were three different lath orientations in a morphological packet. These orientations were dictated by a three specific N-W OR variants sharing the same {111} austenite plane. A packet of bainite laths with common {111} austenite plane was termed as crystallographic packet. Generally, the crystallographic packet size corresponded to the morphological packet size. Locally, crystallographic packets with only two dominant orientations were observed. This indicates strong local variant selection during isothermal bainite transformation. The relative orientation between the variants in crystallographic packets was found to be near 60°/. This appears to explain the strong peak observed in the grain boundary misorientation distribution near 60°. Bainite also contained pronounced fraction of boundaries with their misorientation in the range of 2.5°-8° with quite widely dispersed rotation angles. Spatially these boundaries were found to locate inside the bainite laths, forming lath-like sub-grains. © 2013