Strain-induced Ferrite Transformation Research Articles

Investigation on Microstructure Evolution and Strengthening Mechanism in Boron-Bearing Advanced High Strength Steel

Investigation on the casting, hot-rolling and heat treatment process of a high boron-bearing advanced high strength steel was conducted. The scanning electron microscopy (SEM), electron-probe micro-analyzer (EPMA), electron backscattered diffraction (EBSD) and X-ray diffraction (XRD) were employed to analyze the evolution of microstructure which includes the phase constitution and boride morphology during hot processing. Through the control of as-casted, heat treated microstructure and using interrupted tensile tests, the strengthening mechanism was revealed. Results showed that, the strain-induced ferrite transformation of austenite and grain size were the decisive factors which control the strength and ductility, while the defects and internal stress etc. were the secondary factors.

Multi-Phase-Field Analysis of Stress–Strain Curve and Ferrite Grain Formation during Dynamic Strain-Induced Ferrite Transformation

A numerical model of dynamic strain-induced ferrite transformation (DSFT) was developed by combining the multi-phase field model with the Kocks–Mecking model. Using the developed model, a three-dimensional simulation of the DSFT in a Fe-C alloy was performed to study the correlation between the variation in flow stress and the microstructure evolution during the DSFT. The simulation results indicated that the developed model successfully simulated the characteristic DSFT behavior, i.e., both the stress–strain curve with a single peak and the formation of an ultrafine-grained ferrite microstructure. The variation in the flow stress during the DSFT was characterized by the volume fraction of the ferrite phase.

The effect of strain induced ferrite transformation on the microstructural evolutions and mechanical properties of a TRIP-assisted steel

The capability of strain-induced ferrite transformation in refining the microstructure of a promising transformation-induced plasticity (TRIP) steel has been studied in the present investigation. This was performed employing the hot compression testing technique to deform the experimental TRIP steel at temperatures below the austenite-to-ferrite transformation temperature (i.e., Ac3). The strain-induced ferrite transformation (DIFT) has been identified as the most effective mechanism contributing to the grain refinement at temperatures (860 and 880°C) just below the corresponding Ac3. The influence of this phenomenon on the mechanical properties of the material during and after deformation has been fully addressed. The results show that the DIFT would lead to a work softening behavior during deformation. However, the grain refinement through DIFT has significantly improved the mechanical properties of the deformed specimens characterized by the shear punch testing method.

Low-temperature strain-induced ferrite transformation in twinning-induced plasticity steel

The occurrence of deformation-induced ferrite transformation (DIFT) has been identified in dual-phase twinning-induced plasticity steel during compression testing. The results indicate that the intergranular and intragranular nucleation contribute to dynamic transformation of ferrite from parent austenite . In addition, the massive transformation mechanism is believed to control the reported DIFT in the cold deformation regime. The characteristics of induced ferrite phase and its subsequent effect on the related mechanical behavior have been fully addressed.

Microstructure Characterization of Friction Stir Spot Welded TRIP Steel

Transformation Induced Plasticity (TRIP) steels have not yet been successfully joined by any welding technique. It is desirable to search for a suitable welding technique that opens up for full usability of TRIP steels. In this study, the potential of joining TRIP steel with Friction Stir Spot Welding (FSSW) is investigated. The aim of the study is to investigate whether acceptable welds can be produced, and additionally, to obtain an understanding of the microstructural changes during welding. The microstructure was investigated with a combination of microscopical techniques with the aim of identifying the transformations occurring during welding. Reflected light microscopy, scanning electron microscopy, and electron backscatter diffraction were among the methods applied for detailed investigations. The microstructure adjacent to the welds can generally be subdivided in two thermo-mechanically affected zones (TMAZ), and two heat-affected zones (HAZ). The dual behavior of the microstructure in the zones is related to the two transition temperatures in steel: A1 and A3. In parts of the TMAZ the microstructure contains ultra fine-grained ferrite. This finding parallels the observation in thermo-mechanically processed steels, where severe deformation at elevated temperatures is used to produce ultra fine-grained microstructures. Several possible transformation mechanisms could in principle explain the development of ultra fine-grained ferrite, e.g. dynamic recrystallization, strain-induced ferrite transformation and dynamic recovery.

Dynamic Strain-Induced Ferrite Transformation during Hot Compression of Low Carbon Si-Mn Steels

The dynamic strain-induced transformation (DSIT) of austenite to ferrite was investigated under different undercooling conditions using three low carbon Si-Mn steels. The undercooling of austenite (ΔT) was controlled by varying the cooling rate between austenitization and deformation temperatures. Uniform DSIT ferrite grains (∼2.3 μm) were produced at a relatively high deformation temperature above 840°C using a low carbon high Si steel (0.077C-0.97Mn-1.35Si, mass%) in connection with a larger ΔT. The critical conditions for DSIT were determined based on the flow stress-strain curves measured during hot compression tests. Influence of deformation temperature on DSIT of low carbon Si-added steel was also discussed.

Austenite Recrystallization and Controlled Rolling of Low Carbon Steels

The dynamic recrystallization and static recrystallization in a low carbon steel were investigated through single-pass and double-pass experiments. The results indicate that as the deformation temperature increases and the strain rate decreases, the shape of the stress-strain curve is changed from dynamic recovery shape to dynamic recrystallization shape. The austenite could not recrystallize within a few seconds after deformation at temperature below 900 °C. According to the change in microstructure during deformation, the controlled rolling of low carbon steel can be divided into four stages: dynamic recrystallization, dynamic recovery, strain-induced ferrite transformation, and rolling in two-phase region. According to the microstructure after deformation, the controlled rolling of low carbon steel can be divided into five regions: non-recrystallized austenite, partly-recrystallized austenite, fully-recrystallized austenite, austenite to ferrite transformation, and dual phase.

The Nature and Consequences of Coherent Transformations in Steel.

Advanced research on structural steels has recently focused on the improvement of properties through the control of grain size. Grain refinement increases strength via the Hall-Petch relation, lowers the ductile-brittle transition by increasing resistance to transgranular cleavage, and reduces hydrogen embrittlement by minimizing interfacial fracture along grain or lath boundaries. However, given their different mechanisms, these properties require slightly different measures of the effective grain size. When the grains are smooth and random, all measures of the effective grain size are roughly equivalent. However, transformations in steel are often crystallographically coherent, producing a martensitic, bainitic or ferritic product that has either a Kurdumov-Sachs (KS) or a Nishiyama-Wasserman (NW) relation to the parent austenite. The 24 KS variants and 12 NW variants divide into three sets of eight, corresponding to the three Bain variants of the fcc→bcc transformation. Grain, packet or block boundaries that separate different Bain variants have signifi-cant misorientations of the {100} cleavage planes, but may have only slight misorientations of the {110} slip planes. It follows that grain refinement through coherent transformation is very effective in improving resistance to cleavage fracture and, if the boundary facets are small, to hydrogen embrittlement, but is often relatively ineffective in increasing strength. For this reason, grain refinement for increased strength is best done with incoherent transformations (such as the strain-induced ferrite transformation) while grain refinement for low-temperature toughness or hydrogen resistance is best done with coherent transformations that refine the effective grain size without overstrengthening to unacceptably low ductility.