Behavior Of Dual-phase Steels Research Articles

The effect of microstructure on the local wear behavior of dual phase steels

For experimental purposes we used a micro-scale ball-cratering tribometer. In case this test, the process parameters (load, angular speed) were constant, the solution of the wear-kinetic differential equation could be expressed in a simple, closed form. It was tested four types of samples according to the next microstructure, low martensitic steel sample (30% martensite and 70% ferrite), medium martensitic steel sample (50% martensite and 50% ferrite), high martensitic steel sample (70% martensite and 30% ferrite), full martensitic steel sample (100% martensite).

On the Development and Computational Implementation of Complex Constitutive Models and Parameters’ Identification Procedures

Nowadays, the automotive industry has focused its attention to weight reduction of the vehicles to overcome environmental restrictions. For this purpose, new materials, namely, advanced high strength steels and aluminum alloys have emerged. These materials combine good formability and ductility, with a high tensile strength due to a multi-phase structure (for the steel alloys) and reduced weight (for the aluminum alloys). As a consequence of their advanced performances, complex constitutive models are required in order to describe the various mechanical features involved. In this work, the anisotropic plastic behavior of dual-phase steels and high strength aluminum alloys is described by the non-quadratic Yld2004-18p yield criterion, combined with a mixed isotropic-nonlinear kinematic hardening law. This phenomenological model allows for an accurate description of complex anisotropy and Bauschinger effects of the materials, which are essential for a reliable prediction of deep drawing and springback results using numerical simulations. To this end, an efficient computational implementation is needed, altogether with an inverse methodology to properly identify the constitutive parameters to be used as numerical simulation input. The constitutive model is implemented in the commercial finite element code ABAQUS as a user-defined material subroutine (UMAT). A multi-stage return mapping procedure, which utilizes the control of the potential residual, is implemented to integrate the constitutive equations at any instant of time (pseudo-time), during a deformation process. Additionally, an inverse methodology is developed to identify the constitutive model parameters of the studied alloys. The identification framework is based on an interface program that links an optimization software and the commercial finite element code. This methodology compares experimental data with the respective results numerically obtained. The implemented optimization process aims to minimize an objective function, which defines the difference between experimental and numerical results using the Levenberg-Marquardt gradient-based optimization method. The proposed integrated approach is validated in a number of benchmarks in sheet metal forming, including monotonic and cyclic loading, with the goal to infer about the modelling of anisotropic effects.

Effect of martensite volume fraction on stress triaxiality and deformation behavior of dual phase steel

Strain incompatibility among softer ferrite matrix and harder martensite phase arises during tensile straining due to difference in the flow characteristics of two phases. Strain incompatibility between ferrite and martensite phases during tensile straining results strain partitioning, inhomogenous deformation and finally deformation localization. The local deformation in ferrite phase is constrained by adjacent martensite islands which results in local stress triaxiality development. As martensite distribution varies within the microstructure, the stress triaxiality also varies in a range within the microstructure. Effect of martensite volume fraction on stress triaxiality and tensile deformation behavior on dual phase steels are examined in the current investigation by conducting rigorous finite element study on representative volume elements. The investigation reveals that with increasing martensite volume fraction the band of stress triaxiality distribution increases (i.e. locally stress triaxiality approaches towards plane strain condition) and as a consequence uniform elongation reduces.

Effect of Nonlinear Kinematic Hardening Model on Draw-Bend Springback Behavior of Dual Phase Steel

The aim of this study is to evaluate the effects of two nonlinear kinematic hardening models on springback prediction of dual phase steel. Young’s modulus degradation with plastic strain was also considered. Material parameters of both hardening models were identified by fitting the experimental tension-compression-tension stress-strain curves using optimization software LS-OPT. The results showed that the nonlinear kinematic hardening models gave good prediction compared with measured data. And a 2-D draw-bend springback benchmark was applied to assess the material models with identified parameters. The accuracy of springback simulation improved when both kinematic hardening and Young’s modulus degradation were considered. Yoshida model gave the best overall springback prediction in the draw-bend springback of dual phase steel.

Microstructure based prediction of strain hardening behavior of dual phase steels

In the automotive industries, dual phase (DP) steels have increasingly used for various car body parts due to their excellent combination of high strength and good formability. The microstructure of DP steel basically consists of a matrix of ferrite embedded by martensitic islands. For the mechanical and fracture behaviors of the DP steel, effects of martensite phase fraction, morphology, and phase distribution play an important role. In this work, dual phase steel sheets with different martensite contents were produced by intercritical annealing process. Subsequently, a Finite Element (FE) based modeling using Representative Volume Elements (RVEs) approach was proposed for predicting overall stress–strain behavior of the investigated DP steels. Two dimensional RVE models were generated from micrographs of the DP steels on the microstructure level. For the individual single phases flow curves based on dislocation theory and local chemical composition were applied. Additionally, Geometrically Necessary Dislocations (GNDs), which accumulates at the phase boundaries due to the austenite–martensite transformation during quenching process, was taken into account. A local hardening effect due to these phase boundary dislocations was applied at the interface layer between ferrite and martensite. The calculated stress–strain responses of the DP steels were verified with experimental results determined from tensile tests. The micromechanics model could be then used to describe the local stress and strain evolution of the individual phases in the DP microstructures.

Dynamic Tensile Characteristics of DP600 Steel Sheets for Automotive Applications

To reduce fuel consumption and greenhouse gas emission, dual phase (DP) steels have been considered for automotive applications due to their higher tensile strength, better initial work hardening along with larger elongation compared to conventional grade of steels. In such applications, which would create potential safety and reliability issues under dynamic loading, the mechanical behavior of DP steel considering the strain rate must be examined. In the present study, the dynamic tensile behavior of DP600 steel sheets was investigated using a high-speed tensile testing machine at various strain rates. And the quasi-static tensile testing was also conducted on the steel to understand the effect of the strain rate on the tensile property. The fracture mechanisms of the steel were also analyzed. The results show that the mechanical properties of DP600 steel are noticeably influenced by the strain rates. As the strain rate increases, the strength of the steel increases and the obvious yield phenomenon can be observed when the strain rate is above 0.01 s-1. The fracture elongation of DP600 steels decreases with increasing strain rate from 0.001 to 1 s-1, then increases up to the strain rate of 100 s-1 and reaches the lowest value at the strain rate of 1000 s-1. DP600 steel sheet exhibit typical ductile fracture characteristics with dimples morphology of the facture surface when tensile deformed at various strain rates.

The Assessment of Work Hardening Behaviour of Dual Phase Steels with Coarse Martensite Islands

In this paper, the tensile properties of dual phase (DP) steels with different martensite contents have been studied. Two steels with 0.09 and 0.15% carbon were used for the production of DP steel samples via intercritical annealing treatment, and the deformation behaviour of DP steel samples were examined during tensile loading. Although DP steels usually show two-stage work hardening behaviour, DP steels in this study with relatively high martensite content and coarse islands size exhibit a three-stage work hardening behaviour. This unusual behaviour was attributed, with the help of SEM micrographs, to decohesion of martensite-ferrite interface and martensite cracking.

Thermo-Mechanical Behaviour of Dual-Phase Steels in Various Structural Morphologies: Experiments and Modelling

The thermo-mechanical behaviour of the hot rolled dual-phase steel 10MnSi7 has been determined as a function of temperature and time within the two-phase field α+γ as well as at lower deformation temperatures down to Ms-temperature. The corresponding microstructures, phase hardness, state of recrystallisation have been determined using quantitative metallography. Using this information, a finite element model of a representative volume element (RVE) has been set up. Each element represents a subdomain of a grain that transforms according to the kinetics found in the measurements. The RVE has been subjected to the same loading history as in the accompanying experiments and the overall stress-strain response is monitored during ongoing transformation. The effect of a preferred orientation of the two phase structure has been studied taking into account the effects of a significant plastic deformation. The model is validated by comparison with the experimental evidence.

Work Hardening Behavior of Fe-0.1 C Dual Phase Steel

The strain hardening exponent (n) in the stress strain relationship of metals and alloys is an indicator of their stretchability during press forming operations. The larger the n value, the more the material can deform before instability, and the material can be stretched further before necking starts. This paper aims to investigate work hardening behavior of dual phase steels. A series of dual-phase (DP) steels containing ferrite and martensite with different volume fractions of martensite (Vm) were produced by intercritical heat treatment. Work hardening behavior was analyzed in terms of Holloman analysis. Results showed that in DP steels with less than %50 Vm, the work hardening took place in one stage and the work hardening exponent increased with increasing Vm. By increasing the volume fraction of martensite (Vm>%50) more than one stage will be observed in the Holloman analysis.

Affect of the tempering temperature on the microstructure and mechanical properties of dual phase steels

Abstract In this study, affect of tempering temperature on the microstructure and mechanical properties of dual phase steels was studied. The C–Mn steel specimens were put under intercritical annealing treatment (ICT) at 760 °C and quenched in water to obtain 31% martensite and then tempered within the range of 100–600 °C. The tempering changes the mechanical properties of the dual phase steels through influencing both martensite and dislocation density of ferrite. Low tempering temperature results in formation of fine carbides in the martensite phase. Density of carbides increases and then carbon concentration of martensite decreases as tempering temperature increases. The continuous yield behavior of dual phase steel remains until tempering up to 300 °C. At tempering temperatures above 300 °C the discontinuous yielding and upper–lower yield points return which means degradation in mechanical properties. Yield strength increases by tempering up to 200 °C slightly, but it decreases at higher tempering temperatures. Ultimate tensile strength does not change at tempering temperature up to 200 °C considerably, but it decreases at higher tempering temperatures. Uniform elongation and total elongation increased slightly with increasing tempering time. Hardness of martensite decreases after tempering but this is not the case with ferrite resulting in a decreased hardness difference between martensite and ferrite. According to obtained results it seems that the tempering temperatures lower than 300 °C (1 h) is suitable for attaining continuous yielding and optimum strength and ductility.

Influence of Microstructure on Ductile Damage Behavior of Dual Phase Steel

High tensile strength steel is used to save weight of the automotive body for reduction of CO2 emissions. Because of its high tensile strength, the deformation of high tensile strength steel is difficult. Many kinds of high strength steel are developed to achieve a good balance between high strength and formability. Dual phase steel is one of these steels. It has ferrite and martensite phase as a soft phase and hard phase respectively. In Dual Phase steel, ferrite phase has formability and martensite phase has strength part. However, influences of their volume fraction, chemical composition and heat treatment conditions on material properties are not clear. Therefore, in this study, mechanical properties and micro void behavior on ductile fracture of dual phase steel are investigated. The specimens with difference chemical composition and heat treatment condition are provided. Basic material test is carried out to obtain these mechanical properties. Additionally, the change of void volume fraction during forming is measured and void nucleation site is observed by using laser microscope and image analysis. In the result, dual phase steel containing large volume fraction of ferrite phase shows high uniform elongation and that of martensite phase shows high local elongation. Furthermore, the gradient of void volume fraction of the steel which shows high equivalent fracture strain is lower than that of the steel which shows low equivalent fracture strain.

Experimental Research of Prestrain Effects on Mechanical Behavior of Dual Phase Steel

Dual phase (DP) steel is increasingly utilized in automotive industry to match the requirement of both performance and lightweight of autobody for economical and ecological considerations. Most of autobody structures are manufactured by stamping process which imports prestrain effects into original material and alters its static and cyclic mechanical behaviors. In this study, mechanical behavior of prestrained DP steel was researched. Firstly, microstructure of commercial DP steel with different prestrain levels was examined with optical microscope of 500X magnification. Then, static tensile experiments and strain-controlled fatigue experiments were carried out at room temperature. Test results of differenet prestrain levels were compared. Static stress-strain relationship and local strain-life model were determined using linear regression method. Furthermore, fatigue performance and notch fatigue resistance capability were compared with prestrain level based on Neuber factor. Finally, explanation of the change with prestrain was presented with crack closure theory.

Finite element analysis of the influence of the restraining force in the draw bend test

Draw restraining force is used in sheet metal forming processes to regulate material flow and control springback in the parts. In this paper, the influence of draw restraining force on springback behaviour of dual phase steels is investigated, through numerical simulations of the draw bend test. Three grades of dual phase steels are submitted to different normalized back forces in the range from 0.5 to 1.1 of the initial yield stress. All simulations were performed with the DD3IMP in-house code. The study highlights the importance of an accurate modelling of the isotropic work hardening of dual phase steels for an accurate springback prediction.

Effect of chemical composition, martensite volume fraction and tempering on tensile behaviour of dual phase steels

In this work an investigation was conducted into the tempering effects on mechanical properties of a carbon steel and a microalloyed steel, both with dual phase microstructure. The UTS and YS decreased and elongations increased with an increase of tempering temperature. However, the loss in UTS and YS at tempering temperatures of 100, 200, 300, 400, 500 and 600 °C was a minimum for microalloyed dual phase steel. This is thought to be due to tempering of the martensite and precipitation in the ferrite.

Influence of draw restraining force on the springback in advanced high strength steels

Draw beads are used in sheet metal forming processes to regulate material flow and achieve higher quality in the parts. The draw beads impose draw restraining force on the blank which eliminates defects such as wrinkling, tearing and reduces springback. In a draw-bend test, the draw restraining force is imitated by the back force applied through a hydraulic ram. As the back force increases, large strain is induced which reduces springback tendency. This study evaluates the influence of back force on the springback behavior in advanced high strength steel (AHSS). Three grades of dual-phase steel, namely DP600, DP800, DP980, are used with normalized back forces ranging from 0.5 to 1.1. The springback decreases as the back force increases. Anticlastic curvature as well as curl radius has significant influence on the springback behavior of dual phase steels.

Effect of aging temperature and martensite by volume on strain aging behaviour of dual phase steel

A study has been made of the cold deformation aging susceptibility of dual phase steel with 14 and 22% martensite by volume. It was found that yield strength (YS) and ultimate tensile strength (UTS) increased with increasing testing temperature up to 200°C and then decreased at 250°C which indicated that static strain aging is dominant up to 200°C. This aging is associated with free dislocations in the ferrite as a result of the austenite–martensite transformation, which become preferential sites for solute atom diffusion. At higher temperature (250°C) a softening effect is dominant due to tempering of martensite. It was also observed that at a given aging temperature YS, UTS and ΔY (increase in stress due to strain aging) increased with by volume of martensite.

Static strain ageing behaviour of dual phase steels

In this work an investigation was conducted into the cold deformation ageing susceptibility of a carbon steel and a microalloyed steel, both with dual phase micro-structure. Ageing experiments after different prestrains were carried out at temperatures ranging from 25 to 250 °C. It was found that yield strength (YS) and tensile strength (UTS) of the steels with different dual phase micro-structures exhibit maximum values at ageing temperature of 100 °C after different prestrains. It is assumed that the first rise is based on the formation of solute atom atmospheres around dislocations and the further strengthening in the second step is caused by the low-temperature carbide precipitation in ferrite. When the ageing temperature increased to 150, 200 or 250 °C, YS decreased due to tempering effect in martensite. It was also found that the ageing of the microalloyed steel occurred more slowly than that of the carbon steel. The slow occurrence of ageing was clearly observed at temperatures of 100, 150, 200 and 250 °C and was attributed to the chemical composition of the steels.

Effect of microstructure on the dynamic deformation behavior of dual phase steel

The high-speed deformation behavior of a commercially available dual phase steel composition was studied by means of split Hopkinson bar testing in shear punch mode. The as-received cold-rolled material was subjected to a variety of heat treatment conditions to produce several different microstructures, namely ferrite plus pearlite, ferrite plus bainite and/or acicular ferrite, ferrite plus bainite and martensite, and ferrite plus different fractions of martensite. A strong effect of microstructure on both static and dynamic properties and on the dynamic factor (ratio of dynamic to static strength) was observed. According to the variation of dynamic factor with static strength, three groups of microstructures with three distinct behaviors were identified. Under the same static strength level, the classic dual phase microstructure was found to have the highest value of dynamic factor and therefore can absorb more dynamic energy than other microstructures.

Phase Transformations in Two C–Mn–Si–Cr Dual Phase Steels

The continuous cooling transformation behaviour of dual phase steels were studied with emphasis on the effects of addition of chromium, cooling rate and intercritical annealing temperature on the volume fractions of different transformed products of austenite. The results show that addition of 0.21% chromium promotes the formation of acicular products in the final microstructure and does not have much effect on the volume fractions of the co-existing phases at intercritical annealing temperatures. A mathematical model based on Avrami's equation and Scheil's additivity principle was developed to predict the volume fractions of polygonal ferrite as a function of cooling rate for one of the continuous cooled steels. The predicted results from the model show good agreement with experimental observations.

Effect of martensite content, its dispersion, and epitaxial ferrite content on Bauschinger behaviour of dual phase steel

Dual phase microstructures were produced in a low carbon steel, in which the martensite volume fraction was kept constant at two levels, of 18 and 25%, while the epitaxial ferrite content was varied independently. The microstructures were produced with two dispersions of martensite, a relatively coarse dispersion by intercritical annealing of a ferrite/pearlite starting microstructure and a finer dispersion from an initial martensitic microstructure. Bauschinger tests were conducted using prestrains in both tension and compression of 0.4, 1, and 2.2%. Prestrain direction had no measurable effect on plastic flow behaviour after strain reversal. Mean back stresses increased with increasing strain and martensite content, and were higher for the finer martensite dispersion. They decreased significantly with increasing epitaxial ferrite content in the case of the finer dispersion, but less significantly in the coarser dispersion. These effects of microstructure are discussed in terms of stress transfer to martensite, work hardening, and tensile properties. It is concluded that about half of the mean back stress developed during early plastic deformation was generated by stress transfer to the martensite, the remainder arising from strain hardening of the matrix. A small permanent softening in the Bauschinger test resulted from a reduction of effective stress in the ferrite matrix when the strain path was reversed.