Deformation Behavior Of Dual-phase Steels Research Articles

Quantitative characterization of local deformation-fracture behavior in ferrite-martensite dual-phase steels with different martensite distributions

Low-carbon dual-phase (DP) steels, composed of a soft ferrite phase and a hard martensite phase, are known as promising advanced high-strength steels (AHSSs) due to their high strength and good ductility at low fabrication cost. However, the deformation behavior of DP steels is still not fully understood because of their complex mixed-phase distribution and mechanical interactions between two phases during deformation. The present study quantitatively investigated the effect of martensite distribution on mechanical properties and local deformation-fracture behavior, using the digital image correlation (DIC) technique. Two types of DP structures were prepared: one with a chained martensite distribution (chained DP) and one with an isolated martensite distribution (isolated DP). The chained DP specimen exhibited a superior tensile property, achieving both high strength and large ductility compared to the isolated DP specimen. DIC strain analysis revealed that the chained DP structure showed relatively homogeneous deformation due to the greater contribution of martensite to plastic deformation. In contrast, the isolated DP specimen experienced significant strain localization in the soft ferrite grains. Despite high global strains, non-plastically deformed zones were observed in the central regions of large, isolated martensite particles. Notable differences in micro-void evolution were also observed between the two specimens. The chained DP specimen had a large number of randomly distributed micro-voids, ranging from 0.5 μm to 2 μm in size. In contrast, the isolated DP specimen contained fewer micro-voids, with some aligned at an angle of ∼65° to the tensile direction, potentially leading to early tensile fracture.

Three-Dimensional Micromechanical Modeling of Martensite Particle Size Effects on the Deformation Behavior of Dual-Phase Steels.

The objective of this study was to examine the influence of martensite particle size on the formation of stress and strain in microstructures of dual-phase steels. In order to achieve this objective, the 3D representative volume element (RVE) method was utilized. Particle size distributions were obtained from the microstructures of DP600 and DP1000 dual-phase steels as they actually exist. Virtual dual-phase steel microstructures were generated according to the above distribution and subsequent validation analyses were performed. In the subsequent phase, microstructures of varying martensite particle sizes (1 µm, 1.98 µm, 3 µm for DP600 and 1.15 µm, 2 µm, 3 µm for DP1000) were formed, and the effects of particle size on deformation behavior under tensile loads were determined. The findings indicated that an increase in martensite particle size resulted in a reduction in tensile strength, accompanied by an increase in deformation amount.

Microstructure and deformation behavior of a novel steel rebar: Effect of the heterogeneous microstructure of soft ferrite and Hard bainite

This study reports a detailed investigation of the deformation behavior and microstructural evolution of a novel ultra-low carbon Cr-Mo alloyed dual-phase steel rebar aimed for marine applications. The rebar matrix consists of the lamellar ferrite/bainite dual phases with the lamellar interfaces along the rolling direction. The soft ferrite phase is composed of larger grains with a low dislocation density, while the hard bainite phase is composed of finer grains with a much higher dislocation density. By comparing the strain hardening behavior from different sites at the rebar with varying ferrite and bainite volume fractions, it shows that the hetero-deformation induced (HDI) hardening is strong and dominates the overall work hardening behavior in the early stage of plastic deformation by prevailing over the conventional sum-up of contribution of each phase alone. In this stage, the plastic deformation of ferrite was constrained by the disproportionally-strained, neighboring bainite, creating the accumulation of geometrically necessary dislocations (GNDs) at the phase interface and the long-range HDI stress. The results also reinforce the understanding of deformation behavior of dual-phase steels, especially around the role of HDI stress and hardening.

Tensile deformation behavior of dual-phase steels at elevated temperatures

Tensile deformation behavior of dual-phase steels at elevated temperatures

Impact of Martensite Spatial Distribution on Quasi-Static and Dynamic Deformation Behavior of Dual-Phase Steel

The effects of microstructure parameters of dual-phase steels on tensile high strain dynamic deformation characteristic were examined in this study. Cold-rolled steel sheets were annealed using three different annealing process parameters to obtain three different dual-phase microstructures of varied ferrite and martensite phase fraction. The volume fraction of martensite obtained in two of the steels was near identical (~ 19 pct) with a subtle difference in its spatial distribution. In the first microstructure variant, martensite was mostly found to be situated at ferrite grain boundaries and in the second variant, in addition to at grain boundaries, in-grain martensite was also observed. The third microstructure was very different from the above two with respect to martensite volume fraction (~ 67 pct) and its morphology. In this case, martensite packets were surrounded by a three-dimensional ferrite network giving an appearance of core and shell type microstructure. All the three steels were tensile deformed at strain rates ranging from 2.7 × 10−4 (quasi-static) to 650 s−1 (dynamic range). Field-emission scanning electron microscope was used to characterize the starting as well as post-tensile deformed microstructures. Dual-phase steel consisting of small martensite volume fraction (~ 19 pct), irrespective of its spatial distribution, demonstrated high strain rate sensitivity and on the other hand, steel with large martensite volume fraction (~ 67 pct) displayed a very little strain rate sensitivity. Interestingly, total elongation was found to increase with increasing strain rate in the dynamic regime for steel with core–shell type of microstructure containing large martensite volume fraction. The observed enhancement in plasticity in dynamic regime was attributed to adiabatic heating of specimen. To understand the evolving damage mechanism, the fracture surface and the vicinity of fracture ends were studied in all the three dual-phase steels.

Microstructure-Based RVE Approach for Stretch-Bending of Dual-Phase Steels

Fracture behavior and micro-failure mechanism in stretch-bending of dual-phase (DP) steels are still unclear. Representative volume elements (RVE) have been proved to be an applicable approach for describing microstructural deformation in order to reveal the micro-failure mechanism. In this paper, 2D RVE models are built. The deformation behavior of DP steels under stretch-bending is investigated by means of RVE models based on the metallographic graphs with particle geometry, distribution, and morphology. Microstructural failure modes under different loading conditions in stretch-bending tests are studied, and different failure mechanisms in stretch-bending are analyzed. The computational results and stress-strain distribution analysis indicate that in the RVE models, the strain mostly occurs in ferrite phase, while martensite phase undertakes most stress without significant strain. The failure is the results of the deformation inhomogeneity between martensite phase and ferrite phase. The various appearance and growth of initial voids are different depending on the bending radius.

Prediction of 3D Microstructure and Plastic Deformation Behavior in Dual-Phase Steel Using Multi-Phase Field and Crystal Plasticity FFT Methods

The plastic deformation behavior of dual-phase (DP) steel is strongly affected by its underlying three-dimensional (3D) microstructural factors such as spatial distribution and morphology of ferrite and martensite phases. In this paper, we present a coupled simulation method by the multi-phase-field (MPF) model and the crystal plasticity fast Fourier transformation (CPFFT) model to investigate the 3D microstructure-dependent plastic deformation behavior of DP steel. The MPF model is employed to generate a 3D digital image of DP microstructure, which is utilized to create a 3D representative volume element (RVE). Furthermore, the CPFFT simulation of tensile deformation of DP steel is performed using the 3D RVE. Through the simulations, we demonstrate the stress and strain partitioning behaviors in DP steel depending on the 3D morphology of DP microstructure can be investigated consistently.

Uniaxial Tension Simulation Using Real Microstructure-based Representative Volume Elements Model of Dual Phase Steel Plate

Dual-phase steels have become a favored material for car bodies. In this study, the deformation behavior of dual-phase steels under uniaxial tension is investigated by means of 2D Representative Volume Elements (RVE) model. The real metallographic graphs including particle geometry, distribution and morphology are considered in this RVE model. Stress and strain distributions between martensite and ferrite are analyzed. The results show that martensite undertakes most stress without significant strain while ferrite shares the most strain. The tensile failure is the result of the deforming inhomogeneity between martensite phase and ferrite phase, which is the key factor triggering the plastic strain localization on specimen section during the tensile test.

Effect of material inhomogeneity on the cyclic plastic deformation behavior at the microstructural level: micromechanics-based modeling of dual-phase steel

The microstructure of dual-phase (DP) steels typically consists of a soft ferrite matrix with dispersed islands of hard martensite phase. Due to the composite effect of ferrite and martensite, DP steels exhibit a unique combination of strain hardening, strength and ductility. A microstructure-based micromechanical modeling approach is adopted in this work to capture the tensile and cyclic plastic deformation behavior of DP steel. During tensile straining, strain incompatibility between the softer ferrite matrix and the harder martensite phase arises due to a difference in the flow characteristics of these two phases. Microstructural-level inhomogeneity serves as the initial imperfection, triggering strain incompatibility, strain partitioning and finally shear band localization during tensile straining. The local deformation in the ferrite phase is constrained by adjacent martensite islands, which locally results in stress triaxiality development in the ferrite phase. As the martensite distribution varies within the microstructure, the stress triaxiality also varies in a band within the microstructure. Inhomogeneous stress and strain distribution within the softer ferrite phase arises even during small tensile straining because of material inhomogeneity. The magnitude of cyclic plastic deformation within the softer ferrite phase also varies according to the stress distribution in the first-quarter cycle tensile loading. Accumulation of tensile/compressive plastic strain with number of cycles is noted in different locations within the ferrite phase during both symmetric stress and strain controlled cycling. The basic mode of cyclic plastic deformation in an inhomogeneous material is cyclic strain accumulation, i.e. ratcheting. Microstructural inhomogeneity results in cyclic strain accumulation in the aggregate DP material even in symmetric stress cycling.

Effects of martensite morphology and tempering on dynamic deformation behavior of dual-phase steels

The effects of martensite morphology and tempering on the quasistatic and dynamic deformation behavior of dual-phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens, which had different martensite morphologies and tempering conditions, using a torsional Kolsky bar, and then the test data were compared via microstructures, tensile properties, and fracture mode. Bulky martensites were mixed with ferrites in the step-quenched (SQ) specimens, but small martensites were well distributed in the ferrite matrix in the intermediate-annealed (IA) specimens. Under a dynamic loading condition, the fracture mode of the SQ specimens was changed from cleavage to ductile fracture as the tempering temperature increased, whereas the IA specimens showed a ductile fracture mode, irrespective of tempering. These phenomena were analyzed in terms of a rule of mixtures applied to composites, microstructural variation, martensite softening and carbon diffusion due to tempering, and adiabatic shear-band formation.

Effects of martensite morphology and volume fraction on quasi-static and dynamic deformation behavior of dual-phase steels

The effects of martensite morphology and volume fraction on the quasi-static and dynamic deformation behavior of dual-phase steels were investigated in this study. Quasi-static and dynamic torsional tests were conducted using a torsional Kolsky bar for four steel specimens, which had different martensite morphology and volume fraction, and then the test data were compared via microstructures, tensile properties, and fracture mode. In the intermediate quenched (IQ) steel specimens, very fine fibrous martensites were well distributed in the ferrite matrix, but bulky martensites were mixed with ferrites in the step quenched (SQ) specimens. Quasi-static torsional properties were similar to tensile properties, and fracture occurred in a ductile mode in IQ specimens, whereas cleavage fracture was predominated in SQ specimens. Under a dynamic loading condition, the fracture mode of SQ specimens was changed from cleavage to ductile fracture, whereas IQ specimens had a ductile fracture mode, irrespective of loading rate. These phenomena were analyzed using a shear lag model, phase continuity, and the thermal softening effect of martensite.

Effects of microstructural variables on the deformation behaviour of dual-phase steel

An expression for the stress of martensite in dual-phase steel was developed, which shows the interdependence of the stress of martensite and strain hardening in the ferrite matrix and the contribution of microstructural variables (the volume fraction of martensite f m, ferrite grain size d f, and martensite particle size d m). The onset of plastic deformation of martensite in dual-phase steel was predicted to depend on its yield strength and the microstructural variables, and this was verified by the modified Crussard-Jaoul analysis. It was found that for this dual-phase steel, refining the grain size and increasing f m increase the flow stress and raise the strain hardening rate at low strains, but little affect the strain hardening rate at high strains. The effect of the ferrite grain size on the flow stress of this dual-phase steel was found to obey the Hall-Petch relation, i.e. σ = σ 0 e + K e d f − 1 2 , where the Hall-Petch intersection σ 0 e and slope K e are functions of strain, f m and d m. The effects of the plastic deformation of martensite and the microstructural variables on the strain hardening rate and the Hall-Petch behaviour were analysed in terms of the densities of statistically stored dislocations and geometrically necessary dislocations using the previously developed theoretical model.

Modeling tensile deformation of dual-phase steel

An analytical model has been developed which can describe the tensile deformation behavior of dualphase steel containing retained austenite which transforms to martensite during deformation. The model takes into account the internal back stresses created in the material as a result of the deformation. The influence of various metallurgical factors, such as the amounts of the secondary phases (martensite and retained austenite), strength ratio of the phases, work hardening coefficients, and the stability of retained austenite with respect to the strain-induced transformation, was analyzed. The strongest influence on both strength and ductility was found to result from a large work hardening coefficient of the martensite. Increasing the stability of retained austenite to strain-induced transformation improved the ductility remarkably. The model developed was used to predict the tensile deformation behavior of a commercial dual phase steel fairly accurately.

On the deformation behavior of dual-phase steels

On the deformation behavior of dual-phase steels