DP600 Dual Phase Steel Research Articles

Influence of strain path on nucleation and growth of voids in dual phase steel sheets

The objectives of this work are to describe the nucleation and growth of voids in DP600 dual phase steel sheets formed along different strain paths, and to investigate the correlation between the micro-mechanisms of damage and the forming limits. DP600 sheet specimens were stretch-formed in uniaxial, plane strain and biaxial tension using the Marciniak–Kuczynski formability test. The evolution of damage in the microstructure was correlated to the formability in each strain path by quantitatively analyzing void density, void area fraction, void aspect ratio and mean void size. Results showed that DP600 steel sheets have less formability in plane strain compared to uniaxial and biaxial tension due to the more rapid elongation and growth of voids. Moreover, the sheets safely deformed to much greater effective strains in biaxial tension due to the smaller mean void size and slower void growth. As proposed by Gurson Gurson (1977) , Tvergaard and Needleman Tvergaard and Needleman (1984) , void volume fraction is an important damage parameter. However, this work shows that void aspect ratio and mean void size also significantly influence the evolution of damage and the formability of dual phase steels.

The effect of texture in modeling deformation processes of bcc steel sheets

In textured polycrystals, combinations of averaging the direction cosines of crystals (CADCC) were found which define interdependences between the crystallographic texture and the anisotropy of the kinetic and elastic properties of a steel sheet. It was shown that in case of bcc materials, three of these parameters exist which can be calculated using the orientation distribution function (ODF).Based on the ODF, the CADCC were calculated for dual-phase steel DP600 sheets before and after annealing at 220°C and subsequent deformation by tension to 3%, 6% and 10%. Additionally, damage development was analyzed in this study.

Evaluation of bending limit curves of aluminium alloy AA6014-T4 and dual phase steel DP600 at ambient temperature

Bending/hemming operations are extensively used in automotive industries for assembling the car body panels. Besides the process mechanics, bending operation differs from biaxial sheet metal forming operations in failure mechanism also. Hence, the limit strains that material can sustain are different for both the operations. Thus, the conventional FLC proposed by Keeler and Goodwin fails to predict formability in bending/hemming operations. This necessitates the development of bending limit curves. In this work, bending limit curves are determined experimentally for AA6014-T4 and DP600. Effect of punch radius, nature and level of pre-strain on the bending limits is also studied. Thus, BLC can be used as a post-processing criterion in finite element simulations to assess formability during bending/hemming operations.

Effects of electrode tip morphology on resistance spot welding quality of DP590 dual-phase steel

Electrode pitting (EP) and electrode tip diameter enlargement (ETDE) are the two major representations of electrode wear, which lead to the change of electrode tip morphology and have significant impacts on the resistance spot welding (RSW) quality. In order to analyze the effects of electrode tip morphology on RSW quality of DP590 dual-phase steel, the RSW finite element (FE) model is developed to predict the nugget shape and size, whose reliability is verified by experimental observation. Tensile-shear tests are carried out to illustrate the failure behaviors. Three nugget shapes and two failure modes are obtained from different conditions of electrode tip morphology. The results show that EP morphologies produce incomplete fusion at the center of the workpieces’ faying surface, while ETDE morphologies reduce the nugget sizes. Both obvious EP and ETDE morphologies resulted in a decline of lap shear fracture load (LSFL).

Microstructure and fatigue crack growth behavior in tungsten inert gas welded DP780 dual-phase steel

Abstract The purpose of this study was to evaluate microstructural and mechanical change of DP780 steel after tungsten inert gas (TIG) welding and the influence of notch locations on the fatigue crack growth (FCG) behavior. The tempering of martensite in the sub-critical heat affected zone (HAZ) resulted in a lower hardness (~ 220 HV) compared to the base material (~ 270 HV), failure was found to originate in the soft HAZ during tensile test. The fusion zone (FZ) consisted of martensite and some acicular ferrite. The joint showed a superior tensile strength with a joint efficiency of 94.6%. The crack growth path of HAZ gradually deviated towards BM due to the asymmetrical plastic zone at the crack tip. The FCG rate of the crack transverse to the weld was fluctuant. The Paris model can describe the FCG rate of homogeneous material rather well, but it cannot precisely represent the FCG rate of heterogeneous material. The fatigue fracture surface showed that the stable expanding region was mainly characterized by typical fatigue striations in conjunction with secondary cracks; the rapid expanding region contained quasi-cleavage morphology and dimples. However, ductile fracture mechanism predominated with an increasing stress intensity factor range (Δ K ). The final unstable failure fractograph was subtotal dimples.

Effects of Strain Rates on Mechanical Properties and Fracture Mechanism of DP780 Dual Phase Steel

The mechanical properties of DP780 dual phase steel were measured by quasi-static and high-speed tensile tests at strain rates between 0.001 and 1000 s−1 at room temperature. The deformation and fracture mechanisms were analyzed by observation of the tensile fracture and microstructure near the fracture. Dynamic factor and feret ratio quantitative methods were applied to study the effect of strain rate on the microstructure and properties of DP780 steel. The constitutive relation was described by a modified Johnson-Cook and Zerilli-Armstrong model. The results showed that the strain rate sensitivity of yield strength is bigger than that of ultimate tensile strength; as strain rate increased, the formation of microcracks and voids at the ferrite/martensite interface can be alleviated; the strain rate effect is unevenly distributed in the plastic deformation region. Moreover, both models can effectively describe the experimental results, while the modified Zerilli-Armstrong model is more accurate because the strain-hardening rate of this model is independent of strain rate.

Phase-Field Modeling for Intercritical Annealing of a Dual-Phase Steel

A phase-field model has been developed to describe microstructure evolution during intercritical annealing of a commercial DP600 dual-phase steel. The simulations emphasize the interaction between ferrite recrystallization and austenite formation from a cold-rolled pearlite/ferrite microstructure at high heating rates. The austenite-ferrite transformations are assumed to occur under conditions where only carbon partitions between the phases by long-range diffusion. A solute drag model has been integrated with the phase-field model to describe the effect of substitutional alloying elements on the migration of the ferrite/austenite interface. Experimental results including recrystallization and transformation kinetics as well as austenite morphology have been successfully described by carefully adjusting both the austenite nucleation scenario and the interface mobilities.

Microstructures and properties of friction stir spot welded DP590 dual phase steel sheets

DP590 steel sheets were joined by friction stir spot welding using polycrystalline cubic boron nitride tool with an objective to produce bond diameters similar to conventional spot welding nuggets. A range of spindle rotation (400–2400 rev min−1) and plunge speeds (0·03–3·8 mm s−1) were exercised to attain defect free welds in 1·6 mm thick sheets. A bond diameter of 4t1/2, alike minimum nugget diameter criteria for resistance spot welds, resulted in superior mechanical properties than conventional spot welds. The heat inputs corresponding to different welding parameters influenced the weld microstructure, including grain size, phases and their morphology. The bond diameter was higher for higher heat inputs. However, low heat input welds with weld time cycles ∼4 s produced more refined microstructure and exhibited similar strengths even with reduced bond size. Plug type failure was associated with larger bond diameters (∼7·1 mm), while interfacial failure was observed with smaller welds (∼5·4 mm).

Influence of Milling and Abrasive Waterjet Cutting on the Fatigue Behaviour of DP600 Steel Sheet

Theobservation of cracks in mechanical parts shows that cracks often initiate oncutting edges. A lot of effort has been done to developing theories to predictfatigue behaviour of welds. However for the cut-edges available data is veryscarce on the fatigue behaviour. Thispaper presents theresults obtained in fatigue tests on DP600 dual-phase steel sheet specimens, underthree types of cutting edges processes: milling with two cutting parameters andabrasive waterjet cutting. The tests were carried out using smooth specimens (Kt=1)and with a fatigue constant amplitude loading with R=0.1. Surface roughness andresidual stresses induced by these different cutting conditions were measuredand analysed. It was found that the fatigue strength of the abrasive waterjetcutting specimens was smaller than the predicted fatigue strength of themilling specimens and these may be attributed to the surface roughness inducedby the cutting process. Finally failure mechanisms were studied with the scanningelectron microscope (SEM) at fracture surfaces, including the identification ofthe fatigue crack initiation region. It was also observed that fatigue crackinitiation takes place on the cut-edges.

A study of the unloading behaviour of dual phase steel

It is important to understand the strain recovery of a steel sheet in order to predict its springback behaviour. During strain recovery, the stress–strain relation is non-linear and the resulting unloading modulus is decreased. Moreover, the unloading modulus will degrade with increasing plastic pre-straining. This study aims at adding new knowledge on these phenomena and the mechanisms causing them. The unloading behaviour of the dual-phase steel DP600 is characterised experimentally and finite element (FE) simulations of a representative volume element (RVE) of the microstructure are performed. The initial stress and strain state of the micromechanical FE model is found by a simplified simulation of the annealing processes. It is observed from the experimental characterisation that the decrease of the initial stiffness of the unloading is the main reason for the degrading unloading modulus. Furthermore, the developed micromechanical FE model exhibits non-linear strain recovery due to local plasticity caused by interaction between the two phases.

Microstructural deformation pattern and mechanical behavior analyses of DP600 dual phase steel

Deformation pattern is the most important issue which can be effective on the prediction of mechanical behavior of dual phase steel in microscale. In this paper, simulation with large and small deformation theories using the real microstructure is employed to investigate the deformation pattern in microstructure of DP600 dual phase steel. Deformation pattern is mostly effective on the transition of rotation and displacement between ferrite and martensite phases and stress–strain behavior. Different deformation theories affect the predicted stress–strain behavior of dual phase steel using the finite element method. The obtained results from both numerical and experimental methods are used to investigate the effect of deformation in the microstructure. The performed experiments were interrupted in three stages to obtain the deformation pattern of components microstructure: necking initiation, localization and failure. Metallographic and SEM images were prepared to study the ferrite matrix and martensite grains deformation, and failure pattern in the microscale. Localization which causes severe deformation, rotation and displacement of the microstructure is defined as the failure criteria and the main source of void formation and rupture phenomenon. The predicted results such as stress–strain behavior and deformation pattern using small and large deformation theories are compared with the experimental findings and observable voids in the SEM images.

Research on Resistance Spot Welding of DP980 Dual Phase Steel

The process parameters and quality of resistance spot welding for DP980 dual phase steel were studied through the orthogonal experiment method, and the influence of welding current, welding time and electrode force on the strength of welding joint has been discussed. The results show that the welding current has the greatest influence on the quality of welding joint for DP980 dual phase steel, and it needs relatively lower welding current for the DP980 dual phase steel as it has high resistivity, and appropriate increasing of electrode force is a feasible way to avoid the defect of shrinkage and it improves the joint strength.

Microstructure and dynamic tensile behavior of DP600 dual phase steel joint by laser welding

Dual phase (DP) steels have been widely used in the automotive industry to reduce vehicle weight and improve car safety. In such applications welding and joining have to be involved, which would lead to a localized change of the microstructure and property, and create potential safety and reliable issues under dynamic loading. The aim of the present study is to examine the rate-dependent mechanical properties, deformation and fracture behavior of DP600 steel and its welded joint (WJ) produced by Nd:YAG laser welding over a wide range of strain rates (0.001–1133s−1). Laser welding results in not only significant microhardness increase in the fusion zone (FZ) and inner heat-affected zone (HAZ), but also the formation of a softened zone in the outer HAZ. The yield strength (YS) of the DP600 steel increases and the ultimate tensile strength (UTS) remains almost unchanged, but the ductility decreases after welding. The DP600 base metal (BM) and WJ are of positive strain rate sensitivity and show similar stress–strain response at all studied strain rates. The enhanced ductility at strain rates ranging from 1 to 100s−1 is attributed to the retardation of the propagation of plastic strain localization due to the positive strain rate sensitivity and the thermal softening caused by deformation induced adiabatic temperature rise during dynamic tensile deformation. The tensile failure occurs in the inner HAZ of the joint and the distance of failure location from the weld centerline decreases with increasing strain rate. The mechanism for the changing failure location can be related to the different strain rate dependence of the plastic deformation behavior of the microstructures in various regions across the joint. The DP600 WJ absorbs more energy over the whole measured strain rates than that of the BM due to the higher strength at the same strain when the deformation only up to 10% is considered.

Separation of nucleation and growth of voids during tensile deformation of a dual phase steel using synchrotron microtomography

The damage evolution in a DP980 dual phase steel is followed in situ by synchrotron microtomography during tensile deformation focusing on the effect that the triaxiality, induced by different sample geometries, exerts on damage formation and damage evolution. The growth of existing voids is separated from the voids nucleated between consecutive deformation steps using three-dimensional image analysis. The experimental results are correlated with those obtained by finite element analysis using a Gurson–Tvergaard–Needleman framework with a Chu and Needleman formulation to introduce the effect of nucleation of cavities. A relatively simple way to determine the nucleation parameters is proposed based on the volume of nucleated voids obtained from the tomographies. The evolution of the total volume fraction of cavities obtained from the calculations shows a good agreement with the experiments for the notched samples and reflects the effect of triaxiality on damage. Contrarily to experiments, the calculated accumulated volume fraction of nucleated voids does not reflect the effect of triaxiality suggesting the necessity to implement this parameter in the nucleation model.

Effect of Nd:YAG Laser Welding Parameters on the Hardness of lap joint: experimental and numerical approach

The observation on microstructure and microhardness has been evaluated by experimental methods and also approached to simulation work. One found out higher volume fraction of martensite inside the HAZ and the FZ that is the main cause of higher hardness insides these zones and no softening zone created by laser Nd: YAG on sheet metal of dual phase steel DP600. So the precaution during laser welding Nd: YAG at any welding speed, focal point position and power should be considered in order to prevent a crack and fracture of laser lap joint DP600 during service.

On failure mode of resistance spot welded DP980 advanced high strength steel

This paper aims at investigating the correlation between metallurgical and mechanical characteristics of DP980 dual phase steel resistance spot welds with attention focused on the transition in failure mode from interfacial to pullout mode during the tensile shear test. It was studied whether the conventional/industrial weld size criteria can produce pullout failure modes for DP980 spot welds. Based on theoretical analysis and experimental results, there is a minimum fusion zone size which beyond that the interfacial failure mode is avoided. Results showed that by increasing welding current, pullout failure mode was promoted due to increasing the fusion zone size and encouraging martensite tempering in subcritical heat affected zone (HAZ). Fusion zone size was proved to be key factor controlling mechanical properties of DP980 welds in terms of peak load, ductility and failure energy.Ce document vise à examiner la corrélation entre les caractéristiques métallurgiques et mécaniques de soudures par points par résistance de l’acier biphasé DP980, en mettant l’accent sur la transition du mode de défaillance de mode d’interface à mode par arrachement lors de l’essai de traction-cisaillement. On voulait savoir si les critères conventionnels ou industriels de taille de la soudure pouvaient produire des modes de défaillance par arrachement des soudures par points de DP980. L’analyse théorique et les résultats expérimentaux indiquent qu’il y a une taille minimale de la zone de fusion au-delà de laquelle on évite le mode de défaillance d’interface. Les résultats ont montré qu’en augmentant le courant de soudage, le mode de défaillance par arrachement était favorisé, à cause de l’augmentation de la taille de la zone de fusion et de l’encouragement du revenu de la martensite dans la HAZ sous critique. On a démontré que la taille de la zone de fusion constituait une composante clé contrôlant les propriétés mécaniques des soudures de DP980 en ce qui a trait à la charge de pointe, à la ductilité et à l’énergie de défaillance.

Tensile and fatigue properties of fiber laser welded high strength low alloy and DP980 dual-phase steel joints

The study was aimed at evaluating the microstructure and mechanical properties of high-speed fiber laser welded high strength low alloy (HSLA) and DP980 dual-phase steel joints with varying weld geometries. Fusion zone (FZ) consisted of martensitic structure, and heat-affected zone (HAZ) contained newly-formed martensite in both steels and partially tempered martensite in DP980. While HAZ-softening was present in DP980, it was absent in HSLA. A distinctive “suspension bridge”-like hardness profile with the FZ hardness as a “pylon” appeared in the fiber laser welded joints. Both HSLA and DP980 joints showed a superior tensile strength, with a joint efficiency of 94–96% and 96–97%, respectively, despite a reduced elongation in DP980 joints. Fatigue strength was higher in DP980 joints than in HSLA joints at higher stress amplitudes, but had no obvious difference at lower stress amplitudes. DP980 multiple linear welds exhibited a larger scatter and lower fatigue strength. Fatigue failure of HSLA joints occurred in the base metal at a stress amplitude above 250MPa, and at weld concavity at a lower stress amplitude below 250MPa. Fatigue crack in DP980 joints initiated predominantly from the weld concavity at both high and low levels of stress amplitudes.

Microstructure and fatigue performance of single and multiple linear fiber laser welded DP980 dual-phase steel

The aim of this study was to identify the effect of fiber laser welding (FLW) on the microstructure, hardness, tensile properties and fatigue performance of high-strength (UTS≥980MPa) DP980 dual-phase steel with single linear and multiple linear joint geometry. While FLWed joints showed tempered martensite at the outer heat-affected zone (HAZ) which caused softening in the welds, a lower extent of martensite tempering and higher hardness value were observed in the narrower HAZ of the FLWed joint than in the wider HAZ of the diode laser welded (DLWed) joint due to a higher power density, faster welding speed, lower heat input, and faster cooling rate. A characteristic “suspension bridge”-like hardness profile with the fusion zone (FZ) hardness as a “pylon” appeared in the FLW due to the formation of almost fully martensitic structure in the FZ. Despite the lower ductility after FLW, a joint efficiency of about 96–97% was achieved with the yield strength essentially unchanged. At higher stress amplitudes, fatigue life of the FLWed joints was equivalent to that of BM, but at lower stress amplitudes no direct improvement in the fatigue resistance was observed due to the presence of soft zone and weld concavity. Multiple linear welds appeared to increase the probability of premature dynamic fatigue failure at lower stress amplitudes as a result of increasing number of weld concavities and soft zones. Fatigue crack initiation occurred from the specimen surface where the weld concavity met with a soft zone, and crack propagation was mainly characterized by the typical fatigue striations along with secondary cracks.

Susceptibility to interfacial failure mode in similar and dissimilar resistance spot welds of DP600 dual phase steel and low carbon steel during cross-tension and tensile-shear loading conditions

Abstract The paper investigates the failure mode transition from interfacial to pullout in similar and dissimilar combinations of DP600 dual phase steel and low carbon steel (LCS) under tensile-shear (TS) and cross-tension (CT) loading conditions. In both CT and TS loading conditions, a transition in the failure mode from interfacial to pullout was observed with increasing fusion zone size beyond a critical value (DC). The tendency to fail in interfacial mode during the CT loading was increased in the order of LCS/LCS, DP600/LCS and DP600/DP600. It was shown that interfacial to pullout failure mode transition during the CT test is governed by the fracture toughness of the fusion zone and strength of the pullout failure location (i.e. heat affected zone). It was shown that increasing carbon equivalent of the fusion zone promoted interfacial failure mode in CT loading condition. The high carbon equivalent of DP600 steel led to formation of hard and brittle martensite, which in turn promotes crack propagation through fusion zone. In DP600/LCS combination, decreased carbon equivalent and fusion zone hardness through dilution with the LCS promotes pullout failure at smaller weld sizes. DC during the TS loading is increased in the order of DP600/LCS, LCS/LCS and DP600/DP600. No correlation between fusion zone carbon equivalent and the tendency to fail in IF mode during TS loading was found. Failure mode transition during the TS loading is controlled by hardness of fusion zone and stiffness of the joint. The lowest DC for DP600/LCS is a function of its high fusion zone hardness (in comparison to LCS/LCS combination) and its low stiffness (in comparison to DP600/DP600 combination).

A cyclic twin bridge shear test for the identification of kinematic hardening parameters

A twin bridge cyclic shear test with in-plane torsion is proposed for the identification of kinematic hardening parameters for metallic sheets. Besides its simplicity, noteworthy advantages of the test are (a) reduced loads on the experimental device as compared to a one-sided shear test, (b) identical orientation of the principal stresses with respect to the rolling direction in both of the shear bridges, e.g. which cannot be realized by the classical Miyauchi shear test, and (c) no premature termination by instability mechanisms such as buckling or necking. Two main disadvantages appear to be (a) a preclusion of the use of analytically solved initial value problem in parameter identification due to a diffusivity of the plastic region around the shear bridges, (b) smeared out anisotropic material response proportional with the width of the shear bridge. As a remedy for the former, an inverse parameter identification methodology is used to determine the hardening parameters using an objective function devising the measured moment and rotation angle. For the latter, an optimum shear bridge width is selected which also minimizes the edge effect where a shear equilibrium is not possible. A combined non-linear isotropic and kinematic hardening model respectively based on Voce and Armstrong–Frederick is selected as the material model which is implemented as a VUMAT subroutine for ABAQUS/EXPLICIT. Strain-controlled tests are conducted using three different classes of steel sheet materials, namely a mild steel DC06, a dual phase steel DP600 and a Transformation Induced Plasticity steel TRIP700. These tests with one cycle including a forward shearing and a reverse shearing phase merely focus on the Bauschinger effect. Variations with different stress and strain based loading cycles for phenomena like shakedown, ratcheting, mean stress relaxation, cyclic hardening and softening are not explored and left beyond the scope of the current study. The results, besides showing the applicability of the test to the kinematic hardening parameter identification purposes, also show that the Armstrong–Frederick model falls short to capture the cyclic response of the selected materials, especially advanced high strength steels DP600 and TRIP700.