Deformation-induced Ferrite Transformation Research Articles

Grain refinement, strain hardening and fracture in thermomechanically processed ultra-strong microalloyed steel

The current research in microalloyed steel has witnessed a continuous improvement in strength-ductility combination. The intention is to reduce the gauge thickness of automotive body sheets without compromising formability, thereby enhancing fuel efficiency. In this light, the present work has devised a novel thermomechanical controlled processing (TMCP) for a low-carbon automotive-grade (Nb+V)-stabilized microalloyed steel. The microstructural modification by TMCP has resulted in an excellent combination of yield strength (782 MPa), ultimate tensile strength (1059 MPa) and ductility (21%). The critical factor translating the investigated alloy towards advanced high strength steel is the ferrite grain refinement by dynamic recrystallization (DRX) and deformation-induced ferrite transformation (DIFT). The DRX predominated during TMCP at the intercritical temperature domain of 800–700 °C, imparting the average recrystallized grain size of 2 ± 0.8 µm. In contrast, the DIFT starts well above the upper intercritical temperature regime (i.e., around 810 °C), but it contributes less to the ferrite grain refinement. The thermodynamic reason has been analyzed keeping in view the changes in grain boundary energy and dislocation energy. The plastic deformation additionally leads to precipitation of cubic (Nb, V)-rich carbides in ferrite. The incoherency, coarse precipitate size (70–80 nm), and a poor phase fraction (⁓0.5%) are the key factors that the strengthening mechanism is governed by ferrite grain refinement majorly, followed by dislocation and solid solution strengthening. The segregation of Mn, Nb and S leads to MnS-type inclusion formation, which acts as potential sites for crack initiation due to plasticity mismatch with ferrite matrix during the tensile deformation.

Work hardening behavior and substructure evolution of a low-density steel during compressive deformation

In the present work, the room temperature work hardening behavior and substructure evolution of Fe–17.5Mn–8.3Al–0.74C-0.14Si lightweight steel were investigated during compressive deformation. A series of compression tests were conducted under strain rates of 0.001s−1, 0.01s−1, and 0.1s−1. In addition, the compression tests were interrupted at strain levels of 0.25, 0.75, and 1.25 under the strain rate of 0.001s−1 to study the evolution of the substructure under compression. The starting and deformed microstructures were characterized through high-resolution electron back scatter diffraction (EBSD) technique. The obtained flow curves indicate the high capability of the experimented material for strain hardening. According to the microstructural observation, the outstanding mechanical properties are attributed to the capability of the alloy in cell structure formation and its progressive evolution to subgrains. Such extended dynamic recovery was followed by continuous dynamic recrystallization (CDRX) and deformation-induced ferrite transformation (DIFT) to accommodate the high amount of compressive strain (∼2.5). These two mechanisms result in a final refined microstructure with ultrafine equiaxed grains which provides a great combination of strength and formability.

Microstructure evolution and parameters optimization of follow-up hammering-assisted hybrid wire arc additive manufacturing

Coarse columnar grains are the typical microstructure of wire arc additive manufacturing, resulting in the poor mechanical properties. A follow-up hammering-assisted (FH) hybrid wire arc additive manufacturing process is proposed to improve the microstructure of deposited weld bead. The effects of different hammering temperatures (higher than Ae3 line) and different deformation amount on the microstructure are systematically studied, and the effect on mechanical properties is investigated correspondingly by fabricating a deposited block. The Electron Back Scatter Diffraction (EBSD), Scanning Electron Microscope (SEM), and Optical Microscopy (OM) are used for characterization. The results show that the FH process can significantly refine the grain size of deposited weld bead, and meanwhile weaken the texture strength. The grain refinement mechanism for hammering higher than the austenite recrystallization temperature is austenite dynamic recrystallization. Instead, for hammering lower than the austenite recrystallization temperature, the more significant grain refinement is attributed to the high-temperature deformation increasing the ferrite phase transformation nucleation site, nucleation rate and the deformation-induced ferrite phase transformation. Hammering temperature 850 °C and hammering deformation 40% are determined to be the optimal process parameters to refine the grains. Using the optimal process parameters, the strength and hardness of the deposited layers are greatly promoted, and the anisotropy is also improved, while the ductility is reduced to some degree, but improved obviously compared to the low-temperature hammering.

Effect of different strains on acicular ferrite and martensite-austenite islands in oxide metallurgy steel

The microstructure transformation of oxide metallurgy steel was studied by deforming to strains of 0.1–1 at strain rates of 1 s−1 at 900 °C. The variation trends of AF laths and M-A islands size with increasing strain were investigated. The kernel average misorientation(KAM) data obtained by EBSD are used to calculate the geometrically necessary dislocation(GND) density of AF laths (ρAFGND) induced by inclusions and matrix (ρAveGND). The results showed that the large number of dispersed spherical inclusions in experimental steel can effectively induce the nucleation of AF under different strains, which is composed of Al–Ti–Mg–Zr–O and MnS. With the increase of strain, the deformation-induced ferrite transformation(DIFT) behavior dominates the microstructure evolution, the nucleation and growth of more ferrite led to the gradual increase of HAGBs and decrease of ρAveGND, and the microstructure transformed from granular bainite(GB) to granular structure(GS). The size and aspect ratio of AF laths and M-A islands gradually decreased, and the M-A islands gradually replaced by more carbides distributed inside the grains and on the grain boundaries. The growth of AF laths induced by inclusions is hindered by the influence of pre-nucleated proeutectoid ferrite, resulting the increasing of ρAFGND. Meanwhile, the AF laths nucleated on inclusions satisfies the K–S orientation relationship with parent austenite.

Micro-laminated and ultrafine-grained dual-phase steel plates generated via intercritical rolling followed by water quenching

In this study, a kind of heterogeneous structured dual-phase (DP) steels with micro-laminated and ultrafine-grained microstructures are obtained via intercritical rolling followed by water quenching. The microstructures are tailored by changing rolling temperature, i.e., reheating temperature. We find that with the decrease in reheating temperature from 810 °C to 750 °C, martensite aspect ratio increases from 4.5 ± 3.5 to 6.1 ± 2.9, and martensite long axis elongates from 1.72 ± 0.09 μm to 2.86 ± 0.98 μm. Meanwhile, the size of ferrite grain surrounded by high-angle grain boundaries refines from 1.98 ± 2.32 μm to 1.47 ± 2.14 μm. The yield strength improves 78 MPa from 432 ± 3 MPa to 510 ± 6 MPa, and the tensile strength increases 114 MPa from 745 ± 5 MPa to 859 ± 1 MPa. An important find is that the strain hardening rate increases abnormally with the refinement in ferrite grain due to enhanced hetero-deformation induced (HDI) hardening. So, the downtrend of tensile ductility varies with the improvement in strength is nearly stopped when reheating temperature reduces from 780 °C to 750 °C. Therefore, better comprehensive mechanical properties of DP steels are obtained compared with the approximately equiaxed and coarse-grained DP steel plate. The microstructure evolution mechanism is discussed from the perspective of deformation-induced ferrite transformation and ferrite dynamic recovery. The change of HDI hardening is related to the microstructure feature of the tested DP steels.

Microstructure Evolution during Accelerated Cooling Followed by Coiling of an Nb-Ti/HSLA Steel

An Nb-Ti low carbon HSLA steel with the application of Coiled Tubing steel in petroleum industry, was subjected to hot rolling followed by the subsequent cooling procedures including cooling and coiling. The effect of cooling rate and coiling temperature on final microstructure and texture was investigated via optical microscopy, SEM, and EBSD analyses. It was found that rolling at 800 °C results in the appearance of elongated ferrites due to the activation of the deformation-induced ferrite transformation mechanism. Cooling from the finish rolling with different rates of 2.4 to 15 °C/s leads to the formation of polygonal ferrite and retained austenite. Higher cooling rate increases the retained austenite, decreases the ferrite grain size and improves the grain size homogeneity. Coiling at 600 °C results in coarser ferrite with the texture of γ fiber which is due to the recrystallization of ferrite. On the other hand, coiling at 500 °C intensifies the {332} $$\left\langle {113} \right\rangle$$ and {113} $$\left\langle {110} \right\rangle$$ components indicating deformed austenite transformed to ferrite.

Microstructure Evolution during Controlled Rolling of an Nb–Ti Microalloyed Steel

The microstructural evolution of an Nb–Ti microalloyed steel with the application of coiled tubing is studied in detail during hot rolling followed by accelerated cooling and simulated coiling by means of optical microscopy complemented with electron backscattered diffraction and transmission electron microscopy. It is found that the main microstructural characteristics, i.e., grain size, grain morphology, and texture, vary during different thermomechanical steps, largely in finish rolling. Finish rolling at 850 °C, in the nonrecrystallization region, activates the deformation‐induced ferrite transformation (DIFT) mechanism and leads to the appearance of γ‐fiber, and {332}<113> and {113}<110> components. Decreasing the finish rolling temperature to 750 °C promotes this mechanism and results in finer final microstructure consisting of low angle grain boundaries. However, it is found that to obtain a homogenous microstructure with γ‐fiber texture, the final rolling temperature should be high enough for the sufficient occurrence of static recrystallization in ferrite just after finish rolling. More deformation in the nonrecrystallization region, especially for the last rolling steps, strongly affects the DIFT mechanism and leads to fine‐grained microstructure. Refining of ferrite to the grain size of 3.5 ± 0.5 μm results in 65% enhancement of yield strength and reduction of strain hardening by 25%.

Effect of Annealing on the Microstructure, Texture and Mechanical Properties of a Dual-Phase Ultrahigh-strength TWIP Steel

The present study investigates the effect of annealing time and temperature on the microstructural restoration (by recovery and recrystallization), texture evolution and tensile properties of a dual-phase TWIP steel. The samples, which were initially hot rolled-air cooled followed by being in solution treated condition, subsequent 50 pct cold rolled condition and different annealing conditions (temperatures ranging from 500 °C to 1000 °C for 5 minutes to 2 hours), were subjected to microstructural characterization using optical and transmission electron microscopy, electron backscatter diffraction analysis, tensile testing and fractographic study. The deformation-induced ferrite (DIF) transformation due to cold deformation, the evolution of the grain structure and texture in both austenite and ferrite, and the change in the DIF fraction (accompanying the recrystallization annealing treatment) are critically analyzed. The optimum combination of strength and ductility is achieved in the partially recrystallized samples annealed at 700 °C for 30 minutes (UTS: 859 MPa, total elongation: 37 pct) and 900 °C for 30 minutes (UTS: 708 MPa, total elongation: 63 pct).The effects of DIF on the evolution of microstructure and texture during annealing and the final properties are discussed.

Fracture toughness characteristics of ultrafine grained Nb–Ti stabilized microalloyed and interstitial free steels processed by advanced multiphase control rolling

Aim of the current study is to analyze the fracture toughness values along with other mechanical properties and correlating the microstructures of ultrafine grained (UFG) microalloyed and interstitial free (IF) steels produced through advanced 3-steps control multiphase rolling. The analysis of fracture toughness was carried out through computing KQ (conditional fracture toughness), J-integral (crack initiation energy) and Kee (equivalent energy fracture toughness) values from 3-point bend test data of rolled specimens. Microstructural analysis was performed through transmission electron microscopy (TEM) along with selected area electron diffraction (SAED) and Electron backscatter diffraction (EBSD). The quantitative measurement of low and high angle grain boundaries and their distribution in the deformed state were determined through EBSD analysis. The good combinations of fracture toughness, yield strength (YS) and percent elongation (%El.) (i.e. ductility) were achieved through innovative 3-phase control rolling (microalloyed steel: Kee = 68.9MPa√m, J = 81.4 kJ/m2, YS = 923MPa, %El. = 13.6; IF steel: Kee = 72MPa√m, J = 87.7 kJ/m2, YS = 623Mpa and %El. = 19). This is ascribed to the development of homogeneously distributed submicron size (0.69μm) ferritic + martensitic structure in the microalloyed steel and submicron size (0.83μm) ferritic grains along with high density dislocation substructure in the IF steel. These dislocation cells and substructures could effectively block the crack initiation and propagation. The development of UFG microstructure has been analyzed in the light of deformation induced ferrite transformation (DIFT) and dynamic recrystallization (DRX) mechanisms. Superior fracture toughness of the UFG steels along with better combination of mechanical properties is very demanding for high strength structural applications.

Effect of Pre-deformation on Grain Ultrafining by Intercritical Deformation in Low-Carbon Microalloyed Steels

In this study, the effect of pre-deformation at recrystallization and non-recrystallization zone on the grain ultrafining by the subsequent intercritical deformation (ID) was investigated on low-carbon microalloyed steel. The results showed that ultrafine grain microstructure with an average size of ~ 1.0 μm was fabricated through pre-deformation in the recrystallization zone followed by ID. When pre-deformed at the non-recrystallization zone prior to ID, the grain size increased to 1.6 μm with a heterogeneous distribution along with the well-developed dynamic recovery of ferrite. The grain ultrafining mechanism was attributed to the combined action of the deformation-induced ferrite transformation and the continuous dynamic recrystallization. In particular, the continuous dynamic recrystallization process during ID occurred on the pro-eutectoid ferrite as a result of the subgrain rotation mechanism and the absorbing dislocations mechanism.

Fabricating ultrafine grain by advanced thermomechanical processing on low-carbon microalloyed steel

The ultrafine grain with the effective grain size of 1.10 μm was fabricated by intercritical deformation on low-carbon microalloyed steel, which was finer than the deformation induced ferrite transformation (DIFT) at 1.55 μm. During intercritical deformation, the DIFT process was enhanced, showing a colony distribution of the refined grains. Meanwhile, the continuous dynamic recrystallization of pro-eutectoid ferrite was developed by both the subgrains progressively trapping dislocations and the serrated boundary pinching off the elongated grains. These processes were directly observed through the grain average misorientation maps of electron back-scattered diffraction and confirmed by transmission electron microscopy micrographs.

The effect of thermomechanical controlled processing on recrystallisation and subsequent deformation-induced ferrite transformation textures in microalloyed steels

The evolution of texture components for two experimental 0.06 wt% C steels, one containing 0.03 wt% Nb (Nb steel) and the second containing both 0.03 wt% Nb and 0.02 wt% Ti (Nb–Ti steel), was investigated following a new thermomechanical controlled process route, comprising first deformation, rapid reheat to 1200 °C and final deformation to various strains. Typical deformation textures were observed after first deformation for both steels. Following subsequent reheating to 1200 °C for various times, the recrystallisation textures consisted primarily of the α- leftlangle {011} rightrangle //RD texture fibre with a weak γ-{111}//ND texture fibre, similar to deformation textures, indicative of the dominance of a strain-induced boundary migration mechanism. The texture components after finish deformation were different from the rough deformation textures, with a strong α- leftlangle {011} rightrangle //RD texture fibre at the beginning, and then the strong peaks move to (111) leftlangle {1bar{2}1} rightrangle and (111) leftlangle {bar{1}bar{1}2} rightrangle textures due to the deformation-induced ferrite (DIF) transformation. The effect of Ti on the recrystallisation textures and deformation textures has also been analysed in this study. The results illustrate that Ti significantly influences the γ-{111}//ND texture fibre. Finally, the textures after deformation and recrystallisation in the austenite were calculated based on the K–S orientation relationship between the austenite and ferrite. This allowed the understanding of the mechanism of recrystallisation between first and final deformation and the DIF textures during phase transformation.

The effect of deformation-induced-ferrite-transformation on nanometre-sized carbides in Ti–Mo ferritic steel

ABSTRACTIn this study, the effect of deformation-induced-ferrite-transformation (DIFT) rolling on precipitation in Ti–Mo ferrite matrix micro-alloyed steel was investigated by comparing to rolling in austenite non-recrystallisation region. The precipitate volume fraction and precipitation-starting time-temperature (PTT) curves under two kinds of process were calculated by thermodynamic and kinetic calculation, and the effects of non-recrystallisation rolling and DIFT rolling on the contribution of strengthening mechanisms were quantitatively analysed. The results showed that comparing with rolling in the austenite non-recrystallisation region, carbides in the steel by DIFT rolling were finer and more uniform. Moreover, DIFT rolling could enhance fine grain strengthening and precipitation strengthening simultaneously, and the increments of fine grain and precipitation strengthening were 34 and 63.7 MPa, respectively.

Grain refinement in surface layers through deformation-induced ferrite transformation in microalloyed steel plate

A novel thermo-mechanical process was adopted to obtain deformation-induced ultrafine-grained ferrite in the surface layers of a microalloyed steel plate. The new process referred as rolling coupling water-cooling control process applies water-cooling in the midst of rolling. Deformation-induced ferrite transformation led to ultrafine ferrite grains (1–3μm) in the surface layer of the plate, with ultrafine-grained layer accounting to ~13.5% of the total thickness of the plate. In the mid-thickness, relatively coarse ferrite grains (~7.8μm) and pearlite microstructure was obtained. The ductile-brittle transition temperature corresponding to the ultrafine-grained structure was −118°C and excellent toughness was obtained in the surface layer such that the average impact toughness of the complete plate was high. The mechanism of formation of ultrafine-grained structure at the surface is elucidated.

Thermomechanical processing route to achieve ultrafine grains in low carbon microalloyed steels

A new thermomechanical processing route is described for a microalloyed steel, with roughing deformation below the recrystallisation-stop temperature (T5%), followed by a rapid reheat to 1200 °C for 10s, and then finish deformation at the same temperature as the rough deformation. The new route focused on optimising the kinetics of strain-induced precipitation (SIP) and the formation of deformation-induced ferrite transformation (DITF). For comparative purposes, two experimental 0.06 wt% C steels were studied: one with 0.03 wt% Nb (Nb steel), and a second with both 0.03 wt% Nb and 0.02 wt% Ti (NbTi steel). Two processing routes were studied. The first was a conventional route, which consisted of a simulated rough deformation schedule with the final roughing pass taking place at 850 °C, which produced fully unrecrystallised austenite grains during deformation with no strain-induced ferrite formation. The second, new, thermomechanical processing route used the same roughing step, after which the steels were reheated at 10 °C/s to a temperature of 1200 °C, isothermally held for 10s allowing for precipitate dissolution, prior to air cooling to a finishing deformation temperature of 850 °C. This route resulted in DIFT primarily on the prior-austenite grain boundaries. The precipitate solution during the reheat treatment increased the supersaturation of Nb and Ti in the austenite matrix on subsequent cooling, which therefore increased the undercooling due to the increased Ae3. The observation of nanoscale cementite in the DIFT supports the view that it formed through a massive transformation mechanism. The volume fraction of SIP after finish deformation was influenced by the supersaturation of microalloy elements in solution during heat treatment. The new process route led to a significant refinement of the final ferrite grain size.

Ferrite grain refinement in low carbon Cu–P–Cr–Ni–Mo weathering steel at various temperatures in the (α + γ) region

Self-designed Cu–P–Cr–Ni–Mo weathering steel was subjected to compression test to determine the mechanism of ferrite grain refinement from 750°C to 925°C. Optical microscopic images showed that ferrite grain size declined, whereas the ferrite volume fraction increased with increasing compression temperature. Electron backscatter diffraction patterns revealed that several low-angle boundaries shifted to high-angle boundaries, thereby generating fine ferrite grains surrounded by high-angle boundaries. Numerous low-angle boundaries were observed within ferrite grains at 750°C, which indicated the existence of pre-eutectoid ferrite. Results showed that ferrite grain refinement could be due to continuous dynamic recrystallization at 750°C and 775°C, and deformation-induced ferrite transformation could be the main mechanism at 800°C and 850°C. Fine equiaxed ferrite grains with size ranging from 1.77μm to 2.69μm were produced in the (α+γ) dual-phase region.

Ultra-fine grained structure formation through deformation-induced ferrite formation in duplex low-density steel

In the present study, the applicability of compressive straining for grain refinement in a duplex low-density steel was evaluated. The microstructure of the steel deformed at 1000°C under strain rate of 1s−1 was characterized by the presence of micro-shear bands formed as a result of flow localization along the prior austenite grain boundaries. Electron backscattered diffraction analysis indicated that the shear bands were the preferred sites to trigger the dynamic recrystallization of austenite and the deformation-induced ferrite transformation. As a result, a trimodal microstructure consisted of coarse austenite grains (~10μm), refined austenite grains (~2μm) and a network of ultra-fine ferrite grains (~0.3μm) was achieved. Consequently, a superior combination of strength and ductility was obtained as a result of considerable grain refinement.

Different Applications of the Gleeble® Thermal–Mechanical Simulator in Material Testing, Technology Optimization, and Process Modeling

Abstract In this study, our aim is to present a compilation about different applications of the Gleeble® thermal–mechanical system concerning material testing, characterization of advanced thermomechanically controlled processes and modeling the multiaxial forging technique. The first topic aims to provide an accurate input dataset for developing an energy-based fatigue model for 15Ch2MFA reactor steel. The process characterization intends to quantify the effect of deformation-stored energy on the start temperature and kinetic of the austenite–ferrite transformation. The modeling topic purposes to predict the evolution of the flow stress and the hardening behavior during processing a mild steel by multiaxial forging. All of these experiments were carried out on a Gleeble® 3800 simulator using different mobile conversion units. The fatigue tests were conducted under uniaxial tension–compression loading with in-phase thermal cycles. To develop a new energy-based low-cycle fatigue (LCF) model, the energy balance of the plastic deformation was also investigated. Moreover, the evolution of the microstructure was observed by transmission electron microscopy (TEM) studies at the different stages of the nominal failure lifetime. The deformation-induced ferrite transformation (DIFT) effect was characterized by a high-resolution dilatometer under uniaxial hot compression tests. An advanced simulation program and experimental setup have been developed for the calculation of reliable flow curve from the data, sampled during the multiaxial forging simulation. Concerning the thermomechanical fatigue tests, the experimental results correspond to the Coffin–Manson curve. Furthermore, temperature evolution measurements were performed to provide input data to the calculation of the plastic strain energy partition converted to heat. Finally, the effect of the deformation stored energy on the start and finish temperature of austenite transformation was obtained and quantified.

An ultrahigh strength steel with ultrafine-grained microstructure produced through intercritical deformation and partitioning process

Abstract In this work, a novel design scheme in which deformation-induced ferrite transformation (DIFT) was applied to produce fine-grained steel and the quenching is controlled by quenching and partitioning (Q&P) process has led to the development of a new kind of steel. This steel possesses excellent mechanical properties and the ductility can be further improved without compromising strength because the refined microstructure contains martensite, retained austenite and deformation-induced ferrite. The highest elongation of 15% allied with strength of 1700 MPa is obtained through hot deformation followed by Q&P treatment at 300 °C. The microstructure evolutions are discussed in terms of the current knowledge of the Q&P process and the experimental observations. The results show that the designed multiphase steels are a promising candidate for the development of the third generation of advanced high strength steels.

Investigation of the Effect of Deformation Stored Energy on the Allotropic Transformation Temperature

The aim of our investigation was to study the effect of deformation on the start temperature of the austenite-ferrite transformation. The deformation was carried out during cooling before the beginning of the transformation. The transformation start temperature was determined by dilatometric measurement with a Gleeble 3800 thermomechanical simulator. A novel method was developed to study the deformation induced ferrite transformation (DIFT) effect and successful experiments were carried out on S460MC grade steel specimens to determine the effect of the deformation stored energy on the transformation temperature of the austenite. Evaluating the dilatograms a strong relationship was observed between the temperature of the deformation and the austenite transformation start and finish temperatures. Lower deformation temperature resulted finer microstructure after austenite transformation.