Multiphase Steels Research Articles

Microscopic study on the origin of poor phosphatability in a multi-phase steel

The origin of poor phosphatability in multi-phase steel is studied by various microscopic methods. The effect of the three influential factors that are a phase including compositional difference, crystallographic orientation, and surface oxidation on the zinc phosphate (ZP) formation are investigated by the comparative study of the matrices under the ZP-formed and -missing areas. The inferior ZP formation in bainite/martensite than ferrite is clarified by the analyses of a transmission electron microscopy and an electron backscatter diffraction. The matrix orientation doesn't show apparent deference in the ZP-formed and -missing matrices. The ZP-missing area shows only the enrichment of carbon in the analyses of an electron probe microanalyzer, an Auger electron spectroscopy, and a secondary ion mass spectrometry. The alloy showed no oxide film at the ZP-missing area due to the pickling process. The poor phosphatability in the multi-phase steel is further discussed on the relatively lower matrix dissolution rate at the carbon-rich bainite/martensite area in the ZP solution.

A “new” empirical equation to describe the strain hardening behavior of steels and other metallic materials

The strain hardening behavior of five different steels of varying carbon content, ranging between 0.007 and 0.71%, such as an Interstitial free (IF), a microalloyed, and a low, medium and high carbon steels, have been critically examined using tensile loading. The test results for the IF and the microalloyed steels having only ferritic structure, exhibit a two-stage strain hardening behavior. On the other hand, the low, medium, and high carbon steels having a two-phase microstructure (ferrite-pearlite/cementite) exhibit three-stage strain hardening. A number of existing and frequently used empirical equations, such as the Hollomon, Ludwik, Ludwigson and Voce were used to explain the strain-hardening characteristics of these steels; however, none of the above could fully and satisfactorily explain the flow behavior especially at high strain region. In order to remedy this deficiency, a modification to Ludwigson equation has been suggested by the authors by introducing an extra exponential term to the existing Ludwigson equation. The modelled flow curves, on the basis of this proposed new equation, could explain the strain hardening behavior of these steels very satisfactorily over the entire strain range. In addition, the excellent matching of the flow curves, based on the proposed equation with the experimental flow curves of a number of metallic materials, as diverse as multiphase steel, TRIP-assisted steel, dual-phase steel, stainless steel (316L), Mg-alloy, Ti-alloy and a high entropy (Cantor) alloy, indicates its wider applicability.

Predictive dual-scale finite element simulation for hole expansion failure of ferrite-bainite steel

A dual-scale finite element model is proposed to investigate the failure of ferrite-bainite (FB) dual-phase steel in the hole expansion test. The first level simulation solves the elastic-plastic deformation behavior with phenomenological isotropic elastic-anisotropic plastic constitutive models, and its resulting local deformation histories are supplied to the second level simulation as boundary conditions. In the second level simulation, the local microstructure evolution is solved and provides the dislocation densities, equivalent plastic strains, and stress triaxiality that measure the local fracture in grain scale. A special formulation for calculating the dislocation density distribution in the form of dislocation pile-up at grain boundary areas is highlighted as the microscale level constitutive law. The microstructural information is provided from image analyses based on grain average image quality (GavgIQ) and grain average misorientation (GAM) values observed using electron backscatter diffraction (EBSD). The data were used to identify the constituent phases of the FB steel as the major input for the microstructure-based representative volume element (RVE). Nanoindentation tests are employed to validate the identified phase and to extract the phase-level mechanical properties. The onset of failure at the hole edge during the hole expansion test is simulated by the proposed dual-scale numerical approach. Thus, both the hole expansion ratio (HER) and the location of failure can be successfully predicted. The example clarifies that the present approach based on local deformation histories and the resultant microstructure evolution with grain-level deformation inhomogeneity can be utilized for understanding the deformation and fracture of multi-phase steels.

Ensuring the strength and ductility synergy in an austenitic stainless steel: single- or multi-phase hetero-structures design

Heterogeneous microstructures have been proposed to optimize the mechanical properties in the respect of strength and ductility synergy in many alloy systems. In this work, the multi-phase hetero-structured microstructure consisting of fine grains and coarse grains where the ferrite is embedded in the vicinity of the nano-grains was obtained by regulating the thermomechanical processing parameters. A yield strength of 1.2 GPa which is more than two times higher than the coarse grained counterpart with less uniform elongation loss is achieved in the multi-phase hetero-structured steel. The microscopic load transfer in this complex microstructure is investigated by the in situ high energy synchrotron X-ray diffraction. The persistent strain hardening is ascribed to the hetero-deformation induced stress associated with the joint activation of transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects. As a comparison, the single-phase hetero-structured steel exhibits little lower strength but higher uniform ductility.

Intercritical annealing to achieve a positive strain-rate sensitivity of mechanical properties and suppression of macroscopic plastic instabilities in multi-phase medium-Mn steels.

This study investigates the high strain-rate tensile properties of a cold-rolled medium-Mn steel (Fe-12Mn-3Al-0.05C % in mass fraction) designed to have a multi-phase microstructure and positive strain-rate sensitivity. At the intercritical annealing temperature of 585 °C, increasing the annealing time from 0.5 h to 8 h increased the phase volume fraction of ultrafine-grained (UFG) austenite from 2% to 35% by reversion. The remainder of the microstructure was composed of UFG ferrite and recovered α'-martensite (the latter resembles the cold-rolled state). Servo hydraulic tension testing and Kolsky-bar tension testing were used to measure the tensile properties from quasi-static strain rates to dynamic strain rates ( to ). The strain-rate sensitivities of the yield strength (YS) and ultimate tensile strength (UTS) were positive for both annealing times. Tensile properties and all non-contact imaging modalities (infrared imaging and digital image correlation) indicated an advantageous suppression of Lüders bands and Portevin Le Chatelier (PLC) bands (a critical challenge in multi-phase medium-Mn steel design) due to the unique combination of microstructural constituents and overall composition. Fracture surfaces of specimens annealed for 0.5 h showed some instances of localized cleavage fracture (approximately 30 μm wide areas and lath-like ridges). Specimens annealed for 8 h maintained a greater product of strength and elongation by at least 2.5 GPa % (on average for each strain rate). The relevant processing-structure-property relationships are discussed in the context of recommendations for design strategies concerning multi-phase steels such that homogeneous deformation behavior and positive strain-rate sensitivities can be achieved.

Effect of various phase fraction of bainite, intercritical ferrite, retained austenite and pearlite on the corrosion behavior of multiphase steels

The work discusses the corrosion behavior of a set of newly developed high strength and highly ductile multiphase steels consisting of various fractions of bainite, intercritical ferrite, retained austenite and pearlite from the composition 0.67 % C, 1.71 % Si, 0.86 % Mn and 96.69 % Fe (wt.%). The steels were made by various combinations of continuous cooling in the ferrite-pearlite zone followed by austempering. Multiphase interaction and rust constituents uniquely control the corrosion of the steels. At a particular isothermal temperature, initial increase in corrosion rate with increment of continuous cooling time is followed by decrease in corrosion rate once pearlite starts appearing.

Phase boundary segregation-induced strengthening and discontinuous yielding in ultrafine-grained duplex medium-Mn steels

The combination of different phase constituents to realize a mechanical composite effect for superior strength-ductility synergy has become an important strategy in microstructure design in advanced high-strength steels. Introducing multiple phases in the microstructure essentially produces a large number of phase boundaries. Such hetero-interfaces affect the materials in various aspects such as dislocation activity and damage formation. However, it remains a question whether the characteristics of phase boundaries, such as their chemical decoration states, would also have an impact on the mechanical behavior in multiphase steels. Here we reveal a phase boundary segregation-induced strengthening effect in ultrafine-grained duplex medium-Mn steels. We found that the carbon segregation at ferrite-austenite phase boundaries can be manipulated by adjusting the cooling conditions after intercritical annealing. Such phase boundary segregation in the investigated steels resulted in a yield strength enhancement by 100–120 MPa and simultaneously promoted discontinuous yielding. The sharp carbon segregation at the phase boundaries impeded interfacial dislocation emission, thus increasing the stress required to activate such dislocation nucleation process and initiate plastic deformation. This observation suggests that the enrichment of carbon at the phase boundaries can enhance the energy barrier for dislocation emission, which provides a favorable condition for plastic flow avalanches and thus discontinuous yielding. These findings extend the current understanding of the yielding behavior in medium-Mn steels, and more importantly, shed light on utilizing and manipulating phase boundary segregation to improve the mechanical performance of multiphase metallic materials.

Influence of Multiphase High Silicon Steel (Retained Austenite-RA, Ferrite-F, Bainite-B and Pearlite-P) and Carbon Content of RA-Cγ on Rolling/Sliding Wear

In this research work, heat-treatment processes have been utilized to obtain multiphase microstructure in the silicon rich steel samples, silicon in the steel helps in the development of multiphase microstructure and to keep away from carbide precipitation development through the austempering. The desired multiphase microstructure (Retained austenite-RA, Ferrite-F, Bainite-B and Pearlite-P) consisting of continuous cooling (CC) for 0, 20 and 40 s respectively after austenization followed by austempering at (300, 350 and 400 °C) to form a high wear resistance multiphase steels with microstructure varies amount of F,B, P and RA during continuous cooling. Steels with varies retained austenite up to (5±1.1 to 18±1.9%) along with excellent specific wear rate (2.038 × 10−9-1.061 × 10−8 m3/N-m) were obtained. Further, the rolling/sliding wear rate has been obtained through the disc-on-disc experimental setup. The effect of continuous cooling on retained austenite, carbon content of retained austenite on specific rolling/sliding SWR (specific wear rate) has been studied; phase fraction (P, B and F) and the materials characterization with the help of XRD, AFM and FE-SEM.

Measurement and prediction of the transformation strain that controls ductility and toughness in advanced steels

New-generation multi-phase martensitic steels derive their high strength from the body-centered cubic (BCC) phase and high toughness from transformation of the metastable face-centered cubic (FCC) austenite that transforms into martensite upon loading. In spite of its critical importance, the in-situ transformation strain (or “shape deformation” tensor), which controls ductility and toughness, has never been measured in any alloy where the BCC lath martensite forms and has never been connected to underlying material properties. Here, we measure the in-situ transformation strain in a classic Fe-Ni-Mn alloy using high-resolution digital image correlation (HR-DIC). The experimentally obtained results can only be interpreted using a recent theory of lath martensite crystallography. The predicted in-situ transformation strain agrees with the measurements, simultaneously demonstrating the method and validating the theory. Theory then predicts that increasing the FCC to BCC lattice parameter ratio substantially increases the in-situ transformation strain magnitude. This new correlation is demonstrated using data on existing steels. These results thus establish a new additional basic design principle for ductile and tough alloys: control of the lattice parameter ratio by alloying. This provides a new path for development of even tougher advanced high-strength steels.

Quantification of hydrogen trapping in multiphase steels: Part II – Effect of austenite morphology

We tackle the role of austenite in multiphase steels on hydrogen diffusion systematically for the first time, considering a range of factors such as morphology, interface kinetics and the additional effect of point traps using both experiments and modelling. This follows the findings from part I where we showed that austenite cannot be parametrised and modelled as point traps under the assumption of local equilibrium, unlike grain boundaries and dislocations. To solve this, we introduce a 2D hydrogen diffusion model accounting for the difference in diffusivities and solubilities between the phases. We first revisit the as-quenched martensite permeation results from part I and show that the extremely low H diffusivity there can be partly explained with the new description of austenite but is partly likely due to quench vacancies. We then also look at the H absorption and desorption rates in a duplex steel as a case study using a combination of simulations and experiments. The rates are shown to depend heavily on austenite morphology and the kinetics of H transition from ferrite to austenite and that an energy barrier is likely associated to this transition. We show that H diffusion through the ferrite matrix and austenite islands proceeds at similar rates and the assumption of negligible concentration gradients in ferrite occasionally applied in the literature is a poor approximation. This approach is also applicable to other austenite-containing steels as well as other multiphase alloys.

The impact of grain-scale strain localization on strain hardening of a high-Mn steel: Real-time tracking of the transition from the γ → ε → α’ transformation to twinning

Strain partitioning and localization were investigated in a high-Mn steel (17.1 wt.% Mn) during tensile testing by a correlative probing approach including in-situ synchrotron X-ray diffraction, micro- digital image correlation (μ-DIC) and electron microscopy. By combining Warren's theory with the μ-DIC analysis, we monitored the formation of planar faults (stacking faults and mechanical twins) and correlated them with the local strain partitioning behavior within the microstructure. Starting with an initial microstructure of austenite (γ) and athermally formed ε- and α’-martensite, strain accumulates preferentially near the γ/ε interfaces during tensile straining. The local microscopic von Mises strain (εvM) maps obtained from μ-DIC probing show that these local strain gradients produce local strain peaks approximately twice as high as the imposed macroscopic engineering strain (ε), thus locally triggering formation of ε-martensite already at early yielding. The interior of the remaining austenite, without such interfacial strain peaks, remained nearly devoid of planar faults. The local strain-driven growth of the ε-domains occurs concomitantly with the α’-martensite formation. At intermediate macroscopic applied strains, austenite grain size is considerably reduced to a few nanometers and the associated γ/ε interfacial microscopic strain peaks increase in magnitude. This scenario favors twinning to emerge as a competing strain hardening mechanism at engineering strain levels from ε = 0.075 onwards. At large tensile strains, the γ → ε → α’ transformation rates tend to cease making both twinning and SFs formation to operate as the main strain hardening mechanisms. The findings shed light on the transformation micro-mechanisms in multiphase Mn-TRIP steels by revealing how strain localization among the constituents can directly influence the kinetics of the competing strain hardening mechanisms.

Fatigue behaviour of a multiphase medium carbon steel: Comparison between ferrite/pearlite and tempered microstructures

AbstractFatigue crack growth is the essential process in the total life of components and is unavoidable in most structures and machines. In this study, the effects of cementite particles on the fatigue properties of medium carbon steel were investigated. A quenching and tempering treatment was developed to obtain uniformly distributed cementite particles with dozens of nanometres. To evaluate the fatigue properties, fatigue strength, fatigue crack growth rate and dynamic mechanical behaviour were examined. It was found that the fatigue strength of the tempered steel is much higher than that of air cooling ferrite/pearlite steel because the cementite particles act as barriers and hinder crack propagation during cyclic loading. More importantly, the high sensitivity of the damping peak to prior fatigue may play an important role on the fatigue history of multiphase medium carbon steel.

Effect of Deformation Temperature on the Mechanical Behavior and Stability of Retained Austenite in TRIP-Assisted Medium-C Multiphase Steel.

The temperature-dependent microstructural evolution and corresponding mechanical stability of retained austenite in medium-C TRIP-assisted 0.43C-1.45Mn-0.98Si-1Al-0.033Nb-0.01Ti steel obtained by thermomechanical processing was investigated using static tensile tests and microstructural studies. The light microscopy, image analysis, XRD diffraction and the Jaoul–Crussard analysis were applied to reveal relationships between microstructure and mechanical properties. Specimens were deformed in the static tensile tests in a temperature range of −20–140 °C. It was found that an increase in deformation temperature resulted in the reduced intensity of the TRIP effect due to the higher stability of retained austenite. An increase in the retained austenite stability along with a smaller grain size and a change from its blocky morphology to thin layers was also indicated. The impact of strengthening mechanisms at different temperatures was analyzed. The best combination of strength and ductility was obtained in the samples deformed at 20 and 60 °C, which is associated with the moderate work hardening in this temperature range. The Jaoul–Crussard analysis showed much less strengthening during the second phase of deformation at 100 and 140 °C due to the high stability of retained austenite. The higher C content in the investigated TRIP steel resulted in substantial volume fractions of retained austenite stable after completing deformation.

Quantification of hydrogen trapping in multiphase steels: Part I – Point traps in martensite

We quantified systematically the H trap density in martensite resulting from the presence of dislocations, grain boundaries and retained austenite through a combination of detailed microstructural characterisation, H permeation, thermal desorption and diffusion modelling. This thorough analysis allowed for the first time to deconvolve key microstructural constituents affecting H diffusion in multi-trap martensite. Three microstructures were investigated – as-quenched, tempered at 300 °C and tempered at 450 °C. The first two microstructures had identical dislocation densities and grain size, while the as-quenched one also contained 3.5 vol.% of retained austenite. The two tempered microstructures showed a large difference in dislocation density with few other microstructural differences. The as-quenched microstructure exhibited over an order of magnitude lower H diffusivity and increased H trapping due to retained austenite, while the tempered samples exhibited very similar diffusivities, indicating that dislocations have a limited effect on H trapping. Trap density scaling laws were successfully identified and validated through diffusion simulations and experiments. It was also shown that in martensite and heavily deformed ferrite, where the average grain size is small, grain boundaries are more effective trapping sites than dislocations. Our results also show that retained austenite cannot be effectively modelled as a point trap under the local equilibrium assumption, which is frequently used to model its effect on H diffusion, and that bulk trapping must be considered at least in two dimensions, which is addressed in part II of this series.

Influence of surface preparation method on retained austenite quantification

Phase identification and quantification in multiphase steels has been a major challenge due to the preparation effects and the specificities of the analysis technique. This work aimed to identify and quantify the retained austenite fraction in 15XX class steel, in addition to verifying the best metallographic preparation methodology. X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) methods were used to identify and quantify retained austenite. It was possible to observe the difference between the methods of analysis adopted and to conclude that the XRD quantification results are less affected by surface preparation. Under the conditions reported in this study, electrolytic polishing was found to be the best methodology of surface preparation for EBSD.

The Selection of Materials for the Auto-Body Deformation Zones

Nowadays, it is important to select the proper material for parts of the car-body’s deformation zones in terms of passenger safety. Due to wide range of high-strength steels on the market, the article presents the way of selecting the material by determining the deformation work and the stiffness constant, both measured by the stretch-bending test. The procedure was verified on a high-strength micro-alloyed steel HSLA, high-strength multi-phase steels DP 600 and TRIP, and an austenitic stainless steel AISI 304. The results indicate the austenitic steel is better to use in the deformation zones for the frontal impact, while multi-phase steels are more suitable for deformation zones at the lateral impact. The regression model describing the dependence of the deformation work measured at the stretch-bending test and the deformation work measured at uniaxial tensile test was defined as well. The model allows predicting the deformation work at stretch-bending test based on the mechanical properties measured by the tensile test.

Strain-Ageing of Low-Alloyed Multiphase High-Strength Steels

The strain-aging of low alloyed, multiphase high-strength steels with strain-induced austenite to martensite transformation was studied. The influence of prestrain, aging time, and temperature dependence of the static strain aging was carried out. Ageing temperatures between 60 and 220 ∘ C and aging times from 20 to 10,000 min were investigated. The choice of steel composition allowed studying the influence of alloying elements, such as Si and Al, on the static strain aging behavior. Samples after aging were studied using light-optical microscopy, X-ray diffraction, and in-depth transmission electron microscopy (TEM). The Harper model was used to describe the precipitation mechanisms occurring during aging. The study of thin foils after aging using TEM showed the precipitation of low temperature transition carbides in the microstructure, which was observed between 60 and 5000 min. By using X-ray diffraction, it was revealed that aging at 170 ∘ C for a long time caused a slight decrease of the retained austenite volume fraction, but the C content remained constant.

Abnormal mechanical response in a silicon bearing medium carbon low alloy steel following quenching and bainitic holding versus quenching and partitioning treatment

Multiphase advanced high strength low alloy steels having a refined microstructure of hard bainite and/or martensite laths, interspersed with finely divided retained austenite films, are the potential candidate materials for futuristic industrial applications, such as automotive industry, structural parts, crankshafts and shafts, and powertrain components. In this study, a silicon-bearing, medium-carbon low alloy steel grade of DIN 1.5025 was subjected to both quenching and bainitic holding (Q&B), as well as quenching and partitioning (Q&P) isothermal heat treatments at temperatures closely above and below the martensite start temperature (Ms), respectively. The mechanical response of heat treated samples was evaluated by conducting hardness and tensile tests, corroborated by XRD analysis and metallography using both scanning electron microscopy combined with electron backscatter diffraction as well as transmission electron microscopy. The experimental results showed that a number of Q&B heat treated samples displayed a superior combination of high strength levels with good ductility and work hardening capacity in comparison to that of Q&P ones, akin to the requirements for third generation high strength multiphase steels. It was also found that the superior mechanical response of Q&B heat treated samples was rationalized in respect of the formation of tough strong multiphase microstructures involving a fine mixture of hard bainite and/or martensite laths along with thin films of mostly interlath retained austenite that imparted superior combination of ductility and tensile strength through TRIP phenomenon. Moreover, limited tensile test results conducted in this study, supported by XRD and electron microscopy analyses, suggested that the TRIP response was quite effective in the Q&B multiphase samples containing more than 18 vol.% of retained austenite.

Effect of Microstructure on Stress Dependence of Transformation Induced Plasticity in TRIP800 Low-Alloy Multiphase Steels

Effect of Microstructure on Stress Dependence of Transformation Induced Plasticity in TRIP800 Low-Alloy Multiphase Steels

Influence of carbon concentration and magnetic transition on the austenite lattice parameter of 30Mn-C steel

In this study, the effects of carbon concentration and magnetic transition on the austenite lattice parameter of Fe-30Mn-xC alloys were assessed. A linear relationship was obtained between the carbon content and the lattice parameter determined by neutron diffraction and neutron Bragg-edge transmission imaging for austenitic steels bearing 0.007–1.20 mass% C. The results obtained through both methods showed good agreement. In addition, carbon partitioning was evaluated in artificially piled-up discs with different carbon contents in order to develop a new technique for advanced multi-phase steels. The analysis of asymmetry and broadening of the diffraction line profile revealed that the carbon partitioning could be determined. In addition, the Néel temperature was determined by in situ neutron diffraction upon heating and cooling with magnetic diffraction peaks and the change in the lattice parameter with temperature being monitored. The temperature dependence of the austenite lattice parameter was found to become smaller below the Néel temperature. In conclusion, the results presented herein expand the current knowledge on the properties of high Mn austenite.