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The influence of the friction stir process (FSP) on the hole expansion formability of dual-phase 600 steel was examined by experimental and finite element analyses. Due to the microstructure of refining ferrite, martensite, and bainite, the yield strength and tensile strength increased after FSP by 95% and 63%, respectively. FSP led to the formation of (101) || ND and (111) || RD texture components in the processed zone. The observation of (111) || RD texture components in the FSPed steel resulted in an increment in the plastic strain ratio from 0.86 to 0.98. FSP provided a 20% improvement in the hole expansion ratio. The microstructural refinement and crystallographic texture played an important role in this increase. The FSP contributed to the increase in the hole expansion formability capability by delaying the crack formation during hole expansion and by causing the hole to fail at a higher load. It has been observed by experimental and finite element (FE) simulation results that the highest circumferential stress occurs at the hole edge where the main crack is formed. The FE simulation predictions agree with less than 2% error with the experimental observations. [ABSTRACT FROM AUTHOR]

This research presents the modification of the microstructure on the surface of a dual phase steel (DP) using three different treatments: surface mechanical attrition treatment (SMAT), surface mechanical carburizing treatment (SMCT), and surface mechanical composite coating treatment (SMCCT). We used SMAT to refine the grain size under high strain and strain rate conditions and successfully induced a phase transformation to martensite. The introduction of carbon during SMAT resulted in surface mechano-chemical carburizing, which enhanced the martensite transformation. Furthermore, the addition of silicon during the process led to the formation of SiC and Fe3C particles dispersed within the ferrite and martensite matrix, creating a surface nano-composite. Analysis using SEM and XRD revealed that the carbon and silicon-treated steel exhibited 41.4% martensite, 4.4% bainite, and 54.1% ferrite, indicating increased transformation by these elements. As a result, the alterations in the surface microstructure led to changes in the mechanical properties of the DP steel. Initially, the DP steel had Y.S. of 250 MPa and U.T.S. of 350 MPa. After SMAT treatment, its Y.S. and U.T.S. increased to 280 MPa and 380 MPa, respectively. The addition of carbon and silicon further enhanced the Y.S. and U.T.S. to 350 MPa and 450 MPa. A significant change in microhardness is achieved from 150HV0.05 to 255HV0.05 while after SMCT and SMCCT is 275HV0.05. Despite an increase in surface roughness contributing to heighten friction, there was a significant reduction in wear. This phenomenon could be attributed to grain refinement, dislocation structures, and martensite phase transformations associated with surface hardening. [ABSTRACT FROM AUTHOR]

Understanding the effects of martensite volume fractions (V m) in dual phase (DP) steel resulting from heat treatment is crucial for designing structures for mechanical impact resistance and optimizing manufacturing processes. DP steel's material behaviour depends heavily on its microstructure properties. While stress-strain curves for individual phases in DP steels are often determined using empirical models, extensive experimental data is required to establish empirical model constants. This research aims to achieve two main objectives: firstly, to calibrate stress-strain curves for pure ferrite and pure martensite using limited experimental data using micromechanical adaptive iteration algorithm (MAIA). This calibration involves using stress-strain data from DP steels with varying V m during the calibration stage and additional data for verification. Secondly, to conduct a comprehensive sensitivity analysis of MAIA to assess its capabilities and limitations. Microstructure-based finite element (FE) models, simulated with ABAQUS/Standard, are employed to predict stress-strain curves under uniaxial tensile test conditions. The MAIA approach successfully calculated ferrite and martensite stress-strain curves that could predict plastic behaviour of DP steel with different V m, which agreed with experimental work. Key advantages of this approach include avoiding complex 3D microstructure geometries and requiring only two experimentally obtained stress-strain curves with different V m for material constant calibration, along with another curve for validation. However, the experimental data selected for calibration must have a V m difference between 20%–50% and one of the DP steels must have a low martensite volume fraction. The FE micromechanical model could capture the effect of softening of martensite phase and strengthening of ferrite phase as compared to its bulk properties for DP steel. The effect of V m on strain hardening rate was also successfully captured. This technique comes with obvious shortcomings, such as excluding crystal plasticity behaviour, and change in chemical composition within the individual phase with varying martensite volume fraction. [ABSTRACT FROM AUTHOR]

In this work, the deformation behavior of fine grained dual phase steel with intragranular martensite distribution as thin boundaries is studied via in-situ EBSD investigation and full field fast Fourier transform based crystal plasticity simulation. The effect of grain size and intragranular martensite fraction on the crystal plasticity of grains is examined by analyzing the geometrically necessary dislocation (GND) density evolution and reorientation magnitude. The size of the grains/subgrains is found to profoundly govern the rotation magnitude and development of GNDs by controlling the effect of intragranular and grain boundary martensite on the stress/strain state of grains during deformation. It is observed that for comparable grain sizes and similar initial orientation, higher intragranular martensite fraction leads to higher initial GND density, lesser lattice rotation, and higher development of GND density under deformation. Alternatively, among grains of similar initial orientation and martensite content, coarse grain evolves with higher GND density and rotation magnitude under deformation than the finer ones. However, the initial GND density is higher in fine grains. The stress-strain development obtained from crystal plasticity simulation corroborates the above observations and demonstrates a strong dependence on the intragranular martensite. On the whole, the combined investigation of experimental and simulation results suggests that intragranular martensite fraction and ferrite grain size are the two major, interdependent parameters that govern the crystal plasticity behavior of dual phase steel studied with martensite morphology as thin boundaries. [ABSTRACT FROM AUTHOR]

The present investigation is aimed at revealing the effect of martensite content and impingement angle on the erosive wear of dual phase (DP) steels developed via intercritical annealing (ICA) heat treatment by conducting erosion tests using air jet erosion tester at different angles of impact at a constant velocity of 90 m/s, using alumina (Al2O3) as the erodent. DP steels having four different martensite volume fractions (MVF) 0.39, 0.50, 0.65 and 0.79 were developed by holding normalized steel for different intercritical holding times, and their erosion behavior was examined at 15, 45, 70, and 90° angles of impingement. Normalized (N) steel was used as reference material for comparative purpose. The results indicate that steady state erosion rate decreases with increasing angle of impact for N steel as well as DP steels. However, DP steels showed a significantly reduced erosion rate in comparison with N steel reflecting the role of microstructure and consequent increase in hardness. Steady state erosion rate also decreased with increasing martensite content from 39 to 79%, and DP4 steel containing 79% martensite showed the best performance in terms of erosion resistance. The mechanism of material removal changed from micro-ploughing to crater formation with the increase in the angle of impingement. Micro-ploughing, lip formation, and embedded chips were identified as the operative erosion mechanisms for N steel, whereas ploughing, craters and cracks formation were the operative erosion mechanisms for DP steels. [ABSTRACT FROM AUTHOR]

The initial corrosion process of dual phase (DP) steel produced by the continuous annealing process (CAP) in NaCl solution was systematically studied in this paper. The microstructure, nobility of the phases, and micro-galvanic corrosion behavior were characterized by scanning electron microscope (SEM), electron backscattered diffraction (EBSD) technique, scanning Kelvin probe force (SKPFM), and electrochemical impedance spectroscopy (EIS). Results indicate that there are two types of martensite, namely fresh martensite (FM) and tempered martensite (TM), embedded in the ferrite matrix. The two types of martensite are surrounded by dislocations and connect to each other as in a chain-like network at the ferrite grain boundary. Both FM and TM can act as a micro-cathode to promote the anodic dissolution of adjacent ferrite matrix since FM is up to 15 mV noble to the ferrite matrix and TM is to 30 mV noble. The initial corrosion process of DP steel can be divided into two stages. The first stage involves an increasing corrosion rate of the steel due to the successive emergence of cathodic TM and FM, while the corrosion rate decreases in the second stage because of the separation of martensite.

[Display omitted] • Specific spatial microstructure features for studying mechanical properties of DP steel. • Machine learning trained by RVE simulations included plastic and damage behavior for forward prediction. • MCMC inverse analyses solved non-uniqueness of structure–property relationships of steel. • Approach fairly dealt with multi-target optimization and high dimensional problem. This work aims to investigate complex relationship between microstructure characteristics and mechanical properties of dual phase (DP) steel through an inverse analysis based on Markov chain Monte Carlo (MCMC) method combined with meso -scale material modelling. In this framework, a machine learning approach as surrogate model was developed, in which support vector regression (SVR) and artificial neural network (ANN) were trained using results from representative volume element (RVE) simulations coupled with damage model, which were previously calibrated with experimental data of commercial DP steel grades. Moreover, specific microstructure descriptors including Moran's index, martensite band index and martensite orientation were proposed for representing effects of spatial distributions of martensitic phase. As a result, inverse predictions of microstructure characteristics of DP steels for achieving defined yield strength, tensile strength, uniform elongation and toughness were presented. The inverse analysis could solve the non-uniqueness of structure–property relationships of steel, whereby significances of dispersed structures and aligned martensite bands were highlighted in details. The approach fairly dealt with multi-target optimization and high dimensional problem, which can be further applied as a guideline for designing DP microstructures with enhanced mechanical properties. [ABSTRACT FROM AUTHOR]

This study uses direct quenching (DQ) heat treatment at an intercritical annealing temperature (IAT) of 800 °C to form a martensite-ferrite dual phase microstructure of X70 steel. The effects of tempering temperatures ranging from 200 to 500 °C on tensile properties in a dual-phase X70 steel are investigated. Carbon diffusion and redistribution in the microstructure are influenced by tempering. It was discovered that the amount of carbides increases with the tempered temperature, resulting in depleted carbon in martensite. Conversely, increasing the tempered temperature causes a decrease in ultimate tensile strength and yield strength while increasing elongation. [ABSTRACT FROM AUTHOR]

[Display omitted] This study investigates the potential of generative adversarial networks (GANs) and finite element method (FEM) for assessing the tensile properties of dual-phase (DP) steels, which are essential for lightweight and crash-resistant automotive applications. DP steels are renowned for their high strength and ductility, and their microstructure plays a crucial role in determining these tensile properties. GANs are a promising approach to learning and extracting the complex morphological features of real microstructure and precisely generating realistic images with diversity. This study employs GANs to produce synthetic microstructural images of DP steels, allowing the rapid generation of a diverse dataset. These GAN-generated microstructural images accurately reflect the metallurgical and morphological features of real DP steels. FEM simulations are then utilized to determine the corresponding tensile properties. The simulation results reveal that the martensite fraction is highly correlated with tensile strength, and uniform elongation depends on the microstructural morphology as well as the martensite fraction. Generally, GAN-generated microstructures exhibit properties similar to those of real microstructures, with some generated microstructures surpassing their real counterparts in strength–ductility products. This study emphasizes the critical role of microstructural morphology in determining tensile properties and demonstrates the potential of GANs and FEM for the efficient evaluation of the properties of DP steel and optimization of microstructures to improve performance. These insights offer valuable contributions to materials research in the automotive industry. [ABSTRACT FROM AUTHOR]

Dual phase steels are materials whose microstructure is composed of a ferrite matrix with martensite islands. Ferrite provides excellent ductility, while martensite increases the strength of steel, this provides a special appeal in the automotive industry. The main objective of this research is to obtain dual phase steels from AISI 8620 steel with a high Mn content, performing heat treatments in the intercritical range to obtain martensite percentages of 27, 33, 41, and 48%, respectively. Microstructural characterization is performed using optical microscopy and scanning electron microscopy techniques, the mechanical characterization is carried out using hardness, tension and charpy impact tests. The highest mechanical resistance is achieved in steel with 41% martensite phase, while the highest ductility is given for the material with 27% martensite, a fractographic analysis of all materials allowed to determine that the type of fracture presented is ductile. When the martensite fraction increases, the impact energy exhibits a decreasing behavior, while the hardness behaves in an increasing way.

Dual phase steels are materials whose microstructure is composed of a ferrite matrix with martensite islands. Ferrite provides excellent ductility, while martensite increases the strength of steel, which provides a special appeal in the automotive industry. The main objective of this research is to obtain dual phase steels from AISI 8620 steel with a high Mn content, using heat treatments in the intercritical range to obtain approximate percentages of martensite of 27, 33, 41, and 48%. Microstructural characterization is performed using optical microscopy and scanning electron microscopy techniques. The mechanical characterization is carried out using hardness, tension and charpy impact tests. The highest mechanical resistance is achieved in steel with 41% martensite phase, while the highest ductility was obtained by the material with 27% martensite. A fractographic analysis of all materials allowed determining that the type of fracture presented is ductile. When the martensite fraction increases, the impact energy exhibits a decreasing behavior, while the hardness behaves in an increasing way. [ABSTRACT FROM AUTHOR]

In the present investigation, some electrochemical properties of dual phase X70 steels with different martensite morphologies which have undergone boriding were studied. To obtain a variety of martensite morphologies, Direct Quenching (DQ), Intermediate Quenching (IQ) and Step Quenching (SQ) heat treatments were applied at an intercritical annealing temperature (IAT) of 760°C. The treatment (DQ) allowed the formation of fine martensite evenly distributed in the ferrite matrix. (IQ) treatment showed the formation of martensite along the ferrite / ferrite grain boundaries. In contrast, treatment (SQ) induced the formation of a banded morphology of martensite and ferrite. The realization of borides on X70 (DP) steel was carried out in a powder mixture containing 5% of B4C as source of boron, 5% of NaBF4 as activator and 90% of SiC as diluent at 950°C for a period of time from 4 h. The corrosion behavior of X70 (DP) steel has been explored by the Tafel extrapolation method in a 5 wt. % H2SO4 solution. The corrosion resistance of steel which has undergone boriding (BDP) is higher than that of steel which has not undergone it (DP). [ABSTRACT FROM AUTHOR]

The main aim of the research work is to examine the fracture toughness of the dual phase steels prepared using the intermediate quenching method. The ferrite-martensite dual phase (DP) steel is produced using low carbon micro alloyed steel by the heat treatment and intercritical quenching technique using various intercritical temperatures such as 740,760,780,800 and 820oC. The samples of the produced dual phase steel are analyzed for their microstructure using optical microscope. The fracture toughness investigations for the dual phase steel have been carried out using ASTM standard testing procedure. From the results it is observed that the fine distribution and equal volume fraction of ferrite and martensite phases at 780oC. The effect of which, the A780 steel has demonstrated excellent fracture toughness which is the result of intermediate quenching technique. The fractography analysis it is clear that the ductile initiated brittle fracture is occurred which is due to the increased hard martensite phase and the increment in the stress accumulation at the ferrite which also leads to the higher elongation.

The main aim of the research work is to examine the fracture toughness of the dual phase steels prepared using the intermediate quenching method. The ferrite-martensite dual phase (DP) steel is produced using low carbon micro alloyed steel by the heat treatment and intercritical quenching technique using various intercritical temperatures such as 740,760,780,800 and 820oC. The samples of the produced dual phase steel are analyzed for their microstructure using optical microscope. The fracture toughness investigations for the dual phase steel have been carried out using ASTM standard testing procedure. From the results it is observed that the fine distribution and equal volume fraction of ferrite and martensite phases at 780oC. The effect of which, the A780 steel has demonstrated excellent fracture toughness which is the result of intermediate quenching technique. The fractography analysis it is clear that the ductile initiated brittle fracture is occurred which is due to the increased hard martensite phase and the increment in the stress accumulation at the ferrite which also leads to the higher elongation. [ABSTRACT FROM AUTHOR]

In this paper, microhardness and microstructural characteristics of dual phase DP590 steel resistance spot weld joint under two different welding conditions (i. e. single pulse and multi pulse) were studied. It was observed that applying multi pulse currents resulted in quasi‐equiaxed grains of tempered martensite in fusion zone (FZ) of the weldment. The refinement of microstructure in fusion zone using multi pulse current treatment resulted in reduced hardness of the weld joint. [ABSTRACT FROM AUTHOR]

The quantitative measure of irregular arrangement, configuration and geometric pattern of microstructural features is often termed as fractal dimension (D f). The measurement of D f of a complex mixture of multiphase microstructure, comprised of crystals consisting of martensite, bainite, Fe 3 C and martensite/austenite (M/A) constituents' chaos networks in a typical dual phase steel (matrix ferrite) is carried out and interpreted with its respective phase fractions and resulting mechanical responses as a function of intercritical annealing schedule. Image–texture characteristics have also been analyzed to comprehend the s t o r e d – e n e r g y and c o n f i g u r a t i o n a l – s t a b i l i t y of such complex phase-mixture with annealing temperature. The invasive-type fractal characteristics, complexities and chaos of such micro layered hierarchical multiphases' networks in the multiphase steel has been convincingly revealed. • Multiphase DP steel is chosen at different intercritical annealed states. • Box-counting method is employed to estimate fractal dimension of second phase mixture. • Image–texture analysis is performed to understand the relative 'energy generation' of multiphases. • Fractality-image texture-microhardness correlated at different microstructural states. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • The dual phase steel initiating more damage sites exhibits longer uniform elongation, as the damage evolution is suppressed. • The suppression of damage growth primarily benefits from higher strain hardening capacity of ferrite in this steel grade. • The improved strain hardening of ferrite originates from alloy elements Cr and Ti. • Strain hardening capacity of ferrite is key to tailor the tradeoff of strength and ductility in dual phase steels. Dual phase (DP) steels are amongst the most widely used structural steels for automotive applications. It is essential to understand the damage initiation and damage growth in these high strength steels and further shed light on improving mechanical properties. In this work, two DP800 dual phase grades are investigated, which exhibit identical ultimate tensile stress but significantly different elongation in the uniaxial tensile test. To explain the difference in ductility, particularly described by uniform elongation, we investigate the damage initiation and growth mechanisms by analyzing microstructural changes upon deformation, such as voids, dislocation structures and the grain morphology. Furthermore, ferrite micropillars in pre-strained samples are tested in situ to capture the strain hardening capability of ferrite. We found that the DP steel with harder martensite and softer ferrite exhibits more damage initiation sites after deforming to an identical strain. However, void growth is much slower compared to the DP steel grade with fewer initiation sites. We explain the suppressed void growth by significant strain-hardening of ferrite surrounding the voids, which is observed in the micropillar compression experiments. The improved strain hardening of ferrite originates primarily from the difference in chromium content considering the negligible influence of dispersed particles. [ABSTRACT FROM AUTHOR]

The aim of this research was to investigate ferrite and martensite micromechanical behavior in dual phase (DP) steels. For this purpose, a low carbon steel with ferrite-pearlite initial structure was cold-rolled up to 80% and annealed at 600 ºC for 20 min and subsequently water quenched. The final processing step involved heating to the intercritical annealing region and holding for 10 min at 760, 780, 800 and 820 ºC followed by water quenching. DP steels consisting different volume fractions of ferrite and martensite and different ferrite grain size were produced. Scanning electron microscopy was supplemented by energy dispersive spectroscopy (EDS) and nanoindentation test to follow microstructural changes and their correlations to the variation in phase’s hardness. Nanoindentation study of ferrite and martensite hardening response in the DP microstructures showed that the average ferrite and martensite nanohardness has significantly increased from about 181 to 281 HV10mN and decreased from about 644 to 337 HV10mN with increasing intercritical annealing temperatures from 760 to 820 ºC, respectively. Higher intercritical annealing temperatures resulted in finer and harder ferrite grains in DP microstructures. Martensite nanohardness variation with intercritical annealing temperatures is related to change in its carbon content. By applying the rule of mixtures, the calculated hardness values meet well with the experimental values. [ABSTRACT FROM AUTHOR]

Abstract Laser heat treatment of galvanised steels with a martensite content superior to 80% were performed on a 1 cm wide area over an extensive temperature range [620 K - 1350 K]. The material softening induced is studied through uniaxial tensile testing and SEM microstructural observation. A treatment temperature close to Ac3 yields a massive increase in the ductility of the specimens while reducing the mechanical strength. This change in mechanical properties is associated with the nucleation of new austenite islands and the vanishing of the initial martensite laths. The results presented in this paper pave the way to localised variations of the strength-ductility trade-off, which could be useful for several industrial applications, particularly for enabling plastic forming or stamping of the martensitic steel sheets at low temperature. [ABSTRACT FROM AUTHOR]

Abstract A 0.1C5Mn3Al steel with laminated ferrite/martensite and layered ferrite/martensite microstructures was processed by hot-rolling, cold forging and subsequent annealing. Tensile tests revealed that hot rolled sample (laminated) and cold forged + 650 °C annealed sample (layered) showed similar tensile strengths but different elongations. It is confirmed that laminated structure is good for retaining elongation. Furthermore, nanoindentation tests were performed at locations at or near interfaces and in the ferrite or martensite matrix, revealing that the nanohardness difference across the interface is important in affecting the elongation of the material. The underlying mechanisms were discussed with regarding to the dislocation-interface interactions. Graphical abstract fx1 [ABSTRACT FROM AUTHOR]

Lath martensite is important in industry because it is the key strengthening component in many advanced high strength steels. The study of crystallography and chemistry of lath martensite is extensive in the literature, however, mostly based on fully martensitic steels. In this work, lath martensite in dual phase steels is investigated with a focus on the substructure identification of the martensite islands and microstructural bands using electron backscattered diffraction, and on the influence of the accompanied tempering process during industrial coating process on the distribution of alloying elements using atom probe tomography. Unlike findings for the fully martensitic steels, no martensite islands with all 24 Kurdjumov-Sachs variants have been observed. Almost all martensite islands contain only one main packet with all six variants and minor variants from the remaining three packets of the same prior austenite grain. Similarly, the martensite bands are typically composed of connected domains originating from prior austenite grains, each containing one main packets (mostly with all variants) and few separate variants. The effect of tempering at ~450 °C (due to the industrial zinc coating process) has also been investigated. The results show a strong carbon partitioning to lath boundaries and Cottrell atmospheres at dislocation core regions due to the thermal process of coating. In contrast, auto-tempering contributes to the carbon redistribution only in a limited manner. The substitutional elements are all homogenously distributed. The phase transformation process has two effects on the material: mechanically, the earlier-formed laths are larger and softer and therefore more ductile (as revealed by nanoindentation); chemically, due to the higher dislocation density inside the later-formed laths, which are generally smaller, carbon Cottrell atmospheres are predominantly observed. [ABSTRACT FROM AUTHOR]

The paper of authors deals with the quasi static nanoindentation technique of the DP 600 steel. It is dual phase steel consists of martensite and ferite. The phases have different mechanical properties where their optimal combination gives benefit final mechanical properties of the steel. Nanoindentation testing is a method that consists essentially of touching the material of interest whose local mechanical properties as are hardness and elastic modulus are unknown. Nanoindentation testing is simply an indentation test in which the length scale of the penetration is measured in nanometres (10-9 m) rather than microns (10-6 m) or millimetres (10-3 m), the latter being common in conventional hardness tests. Both load and depth of penetration are recorded at each load increment (ultimately providing a measure of modulus and hardness as a function of depth beneath the surface). The presented paper also quantifies the difference in hardness and elastic modulus of both present phases. [ABSTRACT FROM AUTHOR]

Requirements for weight reduction and increased formability in the automotive industry have largely contributed to the development of dual phase (DP) steels, which is a widespreadly kind of high strength steels. Due to the microstructural of dual phase (DP) steels consisting of hard martensite dispersed in a soft ferrite matrix and with different mechanical behaviors of the two phases, the inhomogeneous deformation of the microstructure are generated during the plastic deformation process. In this study, the strain distribution and plastic strain gradient of microstructure is obtained by Point Interpolation Method (PIM). The PIM interpolates a cluster of feature data points to construct its shape function to attain the displacement increments, which shows great convenience and certain precision in computing the strain and gradient distribution of individual phases. A nonlinear function is developed to describe the relationship of individual phases strain and overall material strain. The overall strain of material is greater than the strain of martensite and less than ferrite’s. Considering the different stain value of the two phases during the deformation process, the plastic strain gradient is generated to connect the inhomogeneous deformation in the two phases to ensure the deformation compatibility. The average plastic strain gradient is the second derivative of the deformation field of microstructure. The average plastic strain gradient of both phases is built with a power function of overall strain. The results show that greater strain gradient value in martensite phase than ferrite. The effects of grain size on the plastic strain and strain gradient have been studied in this paper. The both of plastic strain and strain gradient of ferrite increase with decreasing of grain size. The grain size has greater impact on strain gradient than plastic strain. The rules of martensite are similar to ferrite and the effects of grain size on martensite are less than ferrite. The grain size plays a critical role in plastic strain and strain gradient of dual phase steel. [ABSTRACT FROM AUTHOR]

The effect of microstructural factors on ferrite/martensite (F/M) interface decohesion in DP980 steel under uniaxial tension was investigated using ex-situ tensile testing and the crystal plasticity finite element method (CPFEM). Ex-situ tensile testing of a miniaturized specimen with a 100 μm-thick grooved gauge region revealed that the main ductile failure mechanism in DP980 steel was F/M interface decohesion. First, the effect that the crystallographic orientation of ferrite exerted on the ductile fracture by F/M interface decohesion was investigated using the representative volume elements (RVEs) of single-crystalline ferrite matrix containing a hard martensite particle. Then, the effects of martensite morphology and grain boundary alignment on the heterogeneity of strain-stress partitioning, in-grain orientation gradient for the ferrite matrix, and the corresponding void formation by F/M interface decohesion were investigated using three types of RVEs with a bi-crystalline ferrite matrix that contained one of the followings: (i) a circular martensite particle, (ii) an elliptical martensite particle, and (iii) two adjacent elliptical martensite particles at the F/F grain boundary. To capture the void formation by F/M interface decohesion, a cohesive zone model based on traction-separation law was introduced along the interfaces between the ferrite and martensite phases. The simulation revealed that the crystallographic orientation for ferrite, the martensite morphology, and the grain boundary alignment all significantly affected the heterogeneity of strain-stress partitioning, the in-grain orientation gradient for the ferrite matrix, and the void formation by F/M interface decohesion in DP980 steel under uniaxial tension. [ABSTRACT FROM AUTHOR]

Friction stir lap welding of dual phase 590 steel has been successfully performed within the A to A temperature range upon adjusting the process parameters. The microstructures and the texture have been characterized using scanning electron microscopy and electron back scattered diffraction analysis. The martensite volume fraction increases with an increasing travel speed from 100 to 300 mm/min for a constant tool rotational speed of 300 rpm. Under severe deformation and high strain rate during friction stir welding, the grain orientation 〈101〉 shifts toward the 〈111〉 and 〈001〉 directions compared to the base metal. The base metal shows γ fiber components, whereas the friction stir welded specimen shows strong brass with weak cube fibers. [ABSTRACT FROM AUTHOR]

This paper reports on the microstructural evolution of resistance spot welded 1000 MPa dual phase steel under two different welding conditions, and their relation to the mechanical performance and failure mechanisms. It is shown that a double pulse weld scheme leads to an enhancement in cross-tension strength compared to single pulse welding. The second pulse subdivides the initial fusion zone of the first pulse into two zones. The inner part is solidified with a columnar structure after the second pulse, whereas with the second pulse the outer layer becomes recrystallized (named as Rex-zone) leading to the formation of an equiaxed structure of prior austenite grains. Characteristics of martensite formed in the Rex-zone and coarse-grained heat affected zone, where the crack initiated and propagated, were investigated using orientation imaging microscopy. It was found that a change in welding scheme from single to double pulse effectively alters the characteristics of martensitic microstructure of weld zones. The results obtained demonstrate that the Rex-zone has a lower fraction of high-angle grain boundaries and coarser structure of Bain groups as opposed to the coarse-grained heat affected zone with large fraction of high-angle grain boundaries and finer Bain groups. Besides, double pulse welding creates softer sub-critical heat affected zone which reduces stress concentration at the nugget edge during cross-tension test. The better mechanical performance of double pulse weld is attributed to the significant softening at sub-critical heat affected zone, formation of thick Rex-zone with lower residual strain and high fraction of high-angle grain boundaries and finer Bain groups in the coarse grained heat affected zone. [ABSTRACT FROM AUTHOR]

Microscopic strain localizations were explored with correlative characterization and J2 plasticity modeling. The former involved electron backscattered diffraction (EBSD), nano-indentation mapping and microscopic digital image correlation. These 'mapped' the evolution of strain localizations during controlled tensile deformation of dual phase (DP) steel microstructures. Though the initial multi-phase microstructures were similar, they had different hardness differentials (ΔH) between ferrite and martensite. It was shown both experimentally and from the numerical model that ΔH controlled the strain partitioning between the phases, which triggered the microscopic strain localizations and determined the macro-tensile behavior. For example, highest ΔH provided most severe strain localizations in the ferrite phase, ultimately leading to micro-cracking in the martensite. Further, the experimental microstructures were used in our plasticity modeling, and the state of stress and strain were simulated. In the regions of interest in the present study, both interface decohesion and cracking inside martensite was observed, although the martensite interior experienced negative stress triaxiality. This was possibly triggered by severe strain localizations at the interface. This study thus provided a niche combination of correlative characterization with plasticity modeling, which were adopted to explore the mechanical behavior of complex multi-phase microstructure(s). • Strain localization (SL) behavior of DP microstructures investigated. • Correlative experimental characterizations and numerical modeling used. • Higher ΔH specimens show more severe micro-SL at ferrite/martensite interfaces. • Similar trends observed in evolution of geometrically necessary dislocation density. • Failure micro-mechanisms of DP microstructures correlated to SL. [ABSTRACT FROM AUTHOR]

In this study, microstructure development with respect to mechanical properties in ultrafine grained dual phase (martensite/austenite) by chemical composition of Fe-24Ni-0.1C (wt%) steel processed by 1-cycle and 6-cycle of accumulative roll bonding (ARB) process was investigated. Microstructure characterization by electron backscatter diffraction (EBSD) system showed that significant grain refinement occurred after 6 cycles ARB process and an average grain size smaller than 200 nm was obtained. Moreover, phase analysis map obtained from EBSD system showed that the microstructure of starting material is mostly constituted of martensite phase (more than 70%) and transforms to retained austenite by 6 cycles of ARB process. This phenomenon has proved that retained austenite is stabilized during the ARB process through grain refinement. Uniaxial tensile tests exhibited that yield strength did not significantly improve by 1-cycle reverse transformation compared to the starting material. In contrast, further continuation of the ARB process caused gradual increase in yield and ultimate tensile strengths. The significant improvement in yield strength should be originated from significant grain regiment introduced by 6 cycles of the ARB process. Surprisingly, the uniform tensile elongation for the starting material and the 6 cycles ARB processed specimen are almost the same, though yield and ultimate tensile strengths are considerably improved by employing 6 cycles of ARB process. [ABSTRACT FROM AUTHOR]

The austenite formation behavior of 80% cold rolled low carbon steel with ferrite-martensite microstructure during intercritical annealing for different times was studied. Increasing annealing time led to the formation of austenite at the edge of fine recrystallized ferrite grain boundaries. The value of Avrami exponent for the austenite formation was 2 with an activation energy of 90.67 kJ/mol. The variation of hardness, strength and strain-hardening exponent of annealed samples were studied in terms of the microstructural features. The dual phase steel generated by annealing of deformed ferrite-martensite microstructure at 800 °C for 120 s showed a superior strength-elongation balance. [ABSTRACT FROM AUTHOR]

A new and simple approach was introduced to produce dual phase (DP) steels with excellent ductility. This technique included intercritical annealing of a cold-rolled ferrite-pearlite structure followed by water quenching to produce a ferrite-martensite duplex structure, and a subsequent short intercritical annealing at a lower temperature followed by water quenching to produce the final DP steel. It was found that the microstructure of the newly developed DP steel consisted of chain-like networked martensite in the ferrite matrix with both ultrafine and coarse grain structure. Despite the same value of ultimate tensile strength (∼736 MPa), the uniform elongation and total elongation of the newly developed DP steel were 52% and 54% higher than those for a DP steel with the same composition and martensite volume fraction produced by conventional intercritical annealing. This new DP steel also exhibited a superior strength-elongation balance (UTS×TE >30,000 MPa%) in comparison with commercial and thermomechanically processed DP steels. The newly developed DP steel showed two stages of work hardening based on the Kocks-Mecking approach corresponding to an initial rapid decrease of work hardening rate ( θ = d σ /d ε ) with stress followed by stage III hardening. This steel also exhibited larger values of work hardening exponent in the Hollomon and Ludwik equations compared with the intercritically annealed DP steel. [ABSTRACT FROM AUTHOR]

Strong and ductile dual-phase (DP) steels are an important class of advanced high strength steels that are commonly used by the automotive industry. Micromechanical computational modeling is imperative for guiding the material design of DP steels with simultaneous enhancements in strength and ductility. In this study, the effect of the morphology of the martensite hard phase on the overall stress-strain response of DP steels is studied. Through generating realistic virtual representative volume elements (RVEs) and adapting a microstructural-based approach using a finite deformation elastic-viscoplastic constitutive model, it was possible to conduct various parametric studies of the effects of martensite phase on the strength and ductility of DP steel. Morphological parameters of the martensite phase that are studied include (1) effect of aspect ratio (equiaxed versus elongated), (2) effects of the combination of various aspect ratio within an RVE, and (3) effect of interconnectivity between martensite particles and its thickness. It is shown that elongated martensite particles lead to a better overall performance than that of equiaxed martensite particles. It is found that by having the martensite phase interconnected, both strength and ductility increase. Furthermore, it is shown that the increase in the volume fraction of equiaxed martensite particles within microstructures of elongated particles reduces the strength and ductility simultaneously. Overall, it is concluded that the morphology of the martensite phase in DP steels impacts more the ductility than strength. [ABSTRACT FROM AUTHOR]

The mechanical properties of dual Phase steels (DP steels) are strictly related to the spatial distribution and the geometry of the two phases composing the steel, ferrite and martensite. Due to the high costs to obtain images of sections of steel samples, one important industrial problem is the reduction of the number of 2D sections needed to build and simulate a geometric model which may reproduce in a realistic way the 3D geometry of the material. In this context, the availability of suitable techniques of parameter estimation or identification is fundamental to solve the problem. In this work we present a germ-grain model which approximates the main geometric characteristics of the martensite, taking into account the inhomogeneities of the material. The parameters of the model are estimated on the basis of the morphological characteristics of the images of about 150 tomographic sections of a real sample, quantified by the Minkowski functionals. Here we replace the Mahalanobis distance, introduced in previous literature, with the $\mathcal{N}$ -distance, which provides computational advantages. In order to test if the estimated model is reproducing the distribution of the Minkowski functionals of the real material, both confidence bands from the simulated model are computed and compared with the real data and techniques for the detection of functional outliers are applied to quantify the accuracy of fit of the estimated model. [ABSTRACT FROM AUTHOR]

The effect of martensite spatial distribution and its interface morphology on the bake hardening characteristics of a dual phase steel was investigated. In one case, typical industrial continuous annealing line parameters were employed to anneal a 67% cold rolled steel to obtain a dual phase microstructure. In the other case, a modified annealing process with changed initial heating rates and peak annealing temperature was employed. The processed specimens were further tensile pre-strained within 1–5% strain range followed by a bake hardening treatment at 170 °C for 20 min. It was observed that industrial continuous annealing line processed specimen showed a peak of about 70 MPa in bake-hardening index at 2% pre-strain level. At higher pre-strain values a gradual drop in bake-hardening index was observed. On the contrary, modified annealing process showed near uniform bake-hardening response at all pre-strain levels and a decrease could be noted only above 4% pre-strain. The evolving microstructure at each stage of annealing process and after bake-hardening treatment was studied using field emission scanning electron microscope. The microstructure analysis distinctly revealed differences in martensite spatial distribution and interface morphologies between each annealing processes employed. The modified process showed predominant formation of martensite within the ferrite grains with serrated lath martensite interfaces. This nature of the martensite was considered responsible for the observed improvement in the bake-hardening response. Furthermore, along with improved bake-hardening response negligible loss in tensile ductility was also noted. This behaviour was correlated with delayed micro-crack initiation at martensite interface due to serrated nature. [ABSTRACT FROM AUTHOR]

The morphology and distribution of the dispersed martensite islands in the ferrite matrix plays a key role in the formation of shear bands in dual phase steels. In this study, we investigate the relationship between the martensite dispersion and the strain localization regions due to the formation of shear bands in fine-grained DP 780 steel, employing experimental observations as well as numerical simulations. SEM studies of the deformed microstructure showed that voids nucleated at ferrite-martensite interface within larger ferrite grains and regions with low local martensite fraction. The experimental results were precisely analyzed by finite element simulations based on the theory of crystal plasticity. A parametric study was then performed to obtain a deeper insight in to the effect of martensite dispersion on the strain localization of the neighboring ferrite. Crystal plasticity simulation results revealed that in a more regular structure compared to a random structure, a greater region of the ferrite phase contributes to accommodate plasticity. In addition, these regions limit the formation of main shear bands by creating barriers against stress concentration regions, results in lower growth and interaction of stress concentration regions with each others. [ABSTRACT FROM AUTHOR]

Segregation of solutes on grain boundaries is usually quantified by the peak value of the concentration profile measured across the grain boundary. This procedure is inadequate both from the theoretical and from the practical point of view. Instead, we put forward the concept of interfacial excess of solute, as having a thermodynamic background and being of operative efficiency. Our analyses reveal carbon and boron co-segregation at a martensite lath boundary of a Dual Phase steel. [ABSTRACT FROM AUTHOR]

Among generations of advanced high-strength steel alloys, dual-phase steels exhibit a unique combination of strength and formability making them excellent candidates for use in the automotive industry. In this study, we seek to establish a relation between mechanical properties and microstructure of DP980. Electron backscatter diffraction (EBSD)and nanoindentation are used to identify and characterize martensite and ferrite phases. Spatial distributions of martensite and ferrite phases of subjected to various annealing treatments are found using a 2-point correlation function. Micro- and macro-mechanical properties are measured with nanoindentation, Vickers hardness and tensile tests and the results are used to determine the relation between martensite and ferrite phases and the strength of the metal. During the annealing/recovery process, the strength of the martensite phase decreases, the dislocation structure relaxes in the phase boundary region of the ferrite, and the martensite alignment along the rolling direction decreases resulting in the observed metal strength reduction. It is also shown that the higher the annealing temperature, the more homogeneous and equiaxed the distribution of martensite. [ABSTRACT FROM AUTHOR]

In this study, finite element simulations synchronized with in-situ tensile tests have identified microstructural damage models in a 600 MPa grade dual-phase steel with 10% martensite volume fraction. Firstly, in-situ tensile tests have been carried out on a double-notched, flat-plate, tensile specimen. These experiments have shown that the damage in this steel is dominated, up to moderate tensile strains, by fracture of the martensite islands and decohesion at the martensite/ferrite interfaces. Secondly, FE simulations have been conducted, by using 2.5D meshes that were adjusted to the real shape of the martensite islands, as revealed by the micrographs. The ductile fracture of the martensite was described by a three-parameter fracture envelope defined by Bao and Wierzbicki, where the equivalent strain to fracture is expressed as function of the stress triaxiality. The stiffness of the ferrite/martensite interfaces has been described by a four-parameter cohesive model of Xu-Needleman type. The comparison of the experimentally determined sequence of damage events with the FE simulations has allowed adjusting the damage parameters of the model and validating them up to equivalent plastic strains exceeding locally 0.60. After determination of the damage parameters, the event sequence of martensite fracture and ferrite/martensite decohesion, as well as the pattern of the strain localization in the ferrite phase, were quite satisfactorily predicted by the FE simulations. Furthermore, the results obtained have shown that the martensite fracture and the interface decohesion have a significant influence on the stress and strain partitioning between the two phases, and hence also on the pattern and intensity of the strain localization and on the void growth and coalescence during the subsequent deformation stages. [ABSTRACT FROM AUTHOR]

The low cycle fatigue (LCF) behaviour of a dual phase (DP) steel with different martensite morphologies has been investigated in the present work. DP steels with coarse martensite morphologies show inferior LCF life in comparison with fine martensite morphologies for all martensite volume fractions examined. It is suggested that this is be due to the development of larger local plastic strain concentrations in the ferrite with a coarser microstructure, compared to the finer microstructural morphology. Fatigue cracks were observed to initiate inside ferrite grains, and to preferentially propagate through the softer ferrite phase. The average sub-cell size was finer in samples with higher martensite volume fractions, but the sub-cell size was almost unaffected by the martensite morphology. [ABSTRACT FROM AUTHOR]

The effect of volume fraction and hardness of martensite on the Bauschinger effect in Dual Phase (DP) steel was investigated for strain levels close to those observed in automotive stamping. Five different grades of DP steel were produced by controlled heat treatment allowing the examination of the Bauschinger effect for three different volume fractions of martensite and three levels of martensite hardness. Compression–tension and shear reversal tests were performed to examine the Bauschinger effect at high levels of forming strain. Good correlation between the shear reversal and the compression–tension test was observed suggesting that for DP steel, shear stress strain data, converted to equivalent stress–strain, may be applied directly to characterize kinematic hardening behavior for numerical simulations. Permanent softening was observed following strain reversal and increased with martensite volume fraction and pre-strain level. While the Bauschinger ratio saturates at 3% pre-strain, the Bauschinger strain increases linearly with forming strain without showing saturation. This suggests that to model material behavior accurately in forming processes involving complex loading paths and high levels of strain, test data generated at high strain is required. [ABSTRACT FROM AUTHOR]

Dual phase (DP) steels consisting different volume fractions of ferrite and martensite and different ferrite grain size were produced by a new route utilizing cold-rolling and subsequent intercritical annealing of ferrite/martensite duplex starting structure at 770 °C for different times. Scanning electron microscopy has been supplemented by nanoindentation and tensile test to follow microstructural changes and their correlations to the variation in phase's hardness and mechanical properties. The results showed that longer holding times resulted in coarser and softer ferrite grains in DP microstructures. Martensite nanohardness variation with holding time is related to change in its carbon content. Mechanical properties such as strength, elongation and toughness are well correlated with the martensite/ferrite hardness ratio. [ABSTRACT FROM AUTHOR]

The low cycle fatigue (LCF) behaviour of a dual phase (DP) steel with different martensite volume fractions has been investigated, with particular focus on fatigue life, cyclic hardening/softening behaviour and microstructural evolution. DP steels with martensite volume fractions between 13% and 88% were produced and their monotonic and cyclic deformation behaviours evaluated. The LCF life has been examined in depth and compared with published literature. It has been concluded that, once normalised for plastic strain amplitude, the fatigue life was found to be significantly reduced by an increase in the martensite volume fraction. All alloys were observed to show some initial cyclic hardening followed by cyclic softening. Clear sub-cell formation occurred in ferrite grains irrespective of the martensite volume fraction, and it is suggested that this cell formation and martensite softening are responsible for the cyclic softening behaviour. [ABSTRACT FROM AUTHOR]

Micro-mechanical behavior of a low carbon, high-strength (980 MPa grade) dual-phase (DP) steel of current technological interest that contains high volume fraction of martensite is simulated and uniaxial stress–strain curve is computed. Polygon-based vectorized microstructural images obtained via a new image analysis procedure are used for simulations of the micro-mechanical behavior. Simulations are performed using physics-based constitutive relationships for martensite and ferrite constituents and generalized plane-strain mesh elements. It is shown that there are significant variations in the computed stress–strain curve from one microstructural field of view to another for microstructural windows of 110 µm×110 µm size or less. A random statistical sample of 10 microstructural windows of 110 µm×110 µm size or larger constitutes a RVE of this class of microstructures such that the simulated stress–strain curves of such RVEs are in close agreement with one another, and therefore, represent the global stress–strain response of the simulated material. For the constitutive equations, boundary conditions, and type of mesh elements used in the present work, such average simulated stress–strain curves are also in close agreement with the experimentally measured uniaxial stress–strain curve of the DP980 steel. It is also shown that in this dual-phase microstructure, martensite carries most of the stress whereas most of the strain is contained in ferrite. Consequently, the strain localization and failure is predicted to go preferentially through ferrite compared to martensite, which is in agreement with experimental quantitative fractographic observations reported elsewhere. [ABSTRACT FROM AUTHOR]

Two different morphologies of martensite in dual phase (DP) steel were obtained using two different processing routes. In one case, intermediate quenching (IQ) was adapted, where DP steel was water-quenched to obtain martensite phase, followed by inter-critical annealing. In the second case, the steel was cold rolled, followed by inter-critical annealing (CR-IA). For IQ and CR-IA steels, the inter-critical temperatures varied from 750 °C to 850 °C to obtain different volume fractions of martensite. An understanding of structure–property was obtained using a combination of scanning electron microscope (SEM), transmission electron microscope (TEM), and tensile tests. It was observed that fibrous martensite presented in IQ samples, gradually transformed to blocky martensite with increase in inter-critical temperature, resembling the CR-IA steels. The fibrous martensite encouraged martensite cracking, however, the martensite cracking was dramatically decreased in the IQ samples with increase in martensite fraction. The strain hardening behavior studied using the differential C – J model indicated multistage depending on the fraction of martensite. The low volume fraction of martensite in the DP steel provided high ductility–toughness combination and improved strain hardening ability due to the presence of soft ferrite phase in DP steel. Fibrous martensite in DP steel resulted in less strain hardening than blocky martensite, prior to exceeding a threshold volume fraction. The threshold value was significantly smaller for DP steel with blocky martensite. [ABSTRACT FROM AUTHOR]