High-strength Dual-phase Steel Research Articles

Dependence of mechanical properties on the phase composition of intercritically annealed medium-Mn steel as the main competitor of high-strength DP steels

Depending on the alloy composition, intercritical annealing may provide different phases in the microstructure. For low-alloyed dual-phase (DP) steels it is usually ferrite and martensite, while for medium-Mn steels retained austenite is also formed. In a present study, a wide intercritical temperature range was applied to a 5% Mn steel to investigate possible microstructure combinations: ranging from fully ferritic, through ferritic-austenitic, multiphase, to fully martensitic, which were next investigated in terms of mechanical properties to clarify the behavior of this type of material. The obtained results together with technological issues and economic indicators were next compared to mechanical properties of typical DP steels in order to assess the possibility of replacing this material in car production. The mechanical properties were evaluated using static tensile and hardness tests. The phase composition was determined qualitatively and quantitatively using dilatometry, X-ray diffraction measurements, and electron backscatter diffraction analysis. The results suggest that both initial austenite and martensite fractions have a decisive influence on the yielding and elongation of steel; however, the tensile strength depends mainly on the sum of martensite initially present in the microstructure and the strain-induced martensite formed from the plastically deformed austenite regardless of the initial retained austenite—martensite ratio. The results indicate superior total elongation of medium-Mn steels reaching 30% compared to DP steels with a similar strength level in the range between 900 and 1400 MPa. However, medium-Mn steels could be a significant competitor to dual phase steels only if some technological problems like discontinuous yielding and serrations are significantly reduced.

Semi in-situ investigations on deformation-induced micro-damage in high-strength dual-phase steels

This work investigates deformation-induced micro-damage modes in ferrite-martensite dual-phase (DP) steels utilizing a state-of-the-art semi in-situ deformation apparatus integrated with scanning electron microscope. By integrating secondary electron imaging, electron backscatter diffraction, and electron channeling contrast, we provide comprehensive statistical results on micro-damages and strain localization events of their formation in DP780 and DP980 steels. The current observations indicate that interface decohesion plays a critical role in DP780, particularly under plastic instability. Based on maps of Kernel averaged misorientation, interface decohesion arises from the accumulation of geometrically necessary dislocations, which serve to compensate for the deformation incompatibility between ferrite and martensite. In contrast, the dominant damage mode in DP980 under plastic instability involves ferrite cracking within martensite-surrounded ferritic grains. This type of micro-damage is a result of strain localization by shear bands that accommodate the global deformation while the grain is constrained by martensite islands. Martensite cracking does not show as predominate sites under the condition of plastic instability in both steels. In summary, our findings highlight the importance of deformation incompatibility and constraint on occurrence of micro-damages through distinct strain localization events, which are determined by both microstructure of damage site and its environmental microstructure. In this work, we shed light on the role of microstructural engineering as a critical strategy for enhancing the resistance of DP steels or hetero-phase steels against deformation-induced damage.

Axial load behavior of reinforced concrete columns with high-strength steel coiled strips as confinement

This paper experimentally investigates the axial load behavior of square reinforced concrete columns confined using dual-phase high-strength (100 ksi [690 MPa] yield strength) steel coiled strips. Two phases of testing were conducted on reduced-scale specimens (8 × 8 in. and 10 × 10 in. [203 × 203 mm and 254 × 254 mm]). Varied parameters include con¬finement type (strip versus reinforcing bar), confine¬ment reinforcement ratio, confinement layout (hoops and ties, single spiral, two spirals), and strip anchor¬age. Although the reinforcing-bar-confined columns demonstrated better postpeak residual strength and ductility, important findings include the following: strip-confined columns had peak strengths exceeding the nominal axial strength predicted by code, strip-con¬fined columns were able to achieve similar normalized peak strengths and prepeak stiffness as columns with reinforcing bar hoop confinement, two strip spirals are necessary to achieve the desired postpeak residual strength and ductility for an eight-bar layout, and strip spirals and hoops may provide better restraint against buckling of corner bars compared with reinforcing bar hoops.

Research on Forming Limit Stress Diagram of Advanced High Strength Dual-Phase Steel Sheets.

The Forming Limit Stress Diagram (FLSD) can accurately describe the forming process of high-strength steel. However, obtaining FLSD is relatively difficult. In order to predict fracture in advanced high-strength dual-phase (DP) steels, limit maximum and limit minimum principal strains of sheet were obtained through multiple sets of test and simulation. Two material parameters, strength coefficient K and hardening exponent n are introduced into the FLSD function which is established by the strain-stress transformation function. The function shows that the k-value determines the value of the maximum principal stress, while the n-value affects the curvature of the curve. Verification of correctness by testing and simulation to within 10% accuracy. This paper explores a new approach to FLSD research based on material properties, which can expand the application scope of FLSD.

Microvoid Formation of Ferrite-martensite Dual-phase Steel via Tensile Deformation after Severe Plastic Shear-deformation

One of the objectives for the development of high-strength dual-phase (DP) steel is improving the stretch-flangeability. Large-strained sheared edges are deformed and frequently cracked during stretch-flange formation. Considering shearing as the first deformation, the stretch-flange deformation may be regarded as a secondary deformation. To improve the stretch-flangeability of the DP steels, many researchers have analyzed the microvoid formation. However, in these analyses, the shearing process was not considered. With this background, ex-situ mini-bending tests combined with scanning electron microscopy (SEM) monitoring of microvoid formation were conducted during the secondary deformation. Prior to the secondary deformation, several microvoids were observed on the sheared surface and fine subgrains formed in the ferrite. During secondary deformation, the preliminary microvoids present at the ferrite-martensite interface propagated into the ferrite phase. In contrast, this behavior was not observed for the reamed surface deformation, which was formed without preliminary deformation. Furthermore, microvoids were initiated on ferrite grains that were not present at the ferrite-martensite interface, and martensite islands were not cracked during secondary deformation. This result is noteworthy because martensite cracking was the main factor involved in microvoid initiation, in the absence of shearing. Electron backscattering diffraction analysis revealed that the work hardening of ferrite, prior to the secondary deformation, caused a deviation in the strain concentration sites from those found in the reamed surface deformation. Therefore, this study elucidated microvoid formation on preliminary deformed surfaces via shearing and provided insights for material development considering deformations on the sheared surfaces of materials.

Ductile fracture of dual-phase steel at elevated temperatures

Structural steels can be subjected to a range of stress and temperature conditions, and accurate characterization of their limit states is a requirement for structural design. Yet data on ductile fracture at elevated temperatures remains relatively scarce especially for high-strength steel grades. This paper describes an experimental and numerical investigation of the ductile fracture of a high-strength dual-phase steel under multiaxial stress states at temperatures up to 700 °C. Experiments were carried out on five geometries of specimens and at four temperatures to identify the temperature-dependent fracture locus across a range of stress triaxialities. Finite element analyses of the experiments provided the equivalent strain at fracture initiation. It was found that the fracture initiation strain varies markedly with stress triaxiality and increases with temperature. The effect of strain rate become significant at 500 °C. A temperature-dependent plasticity-damage model was calibrated to predict ductile fracture initiation in the dual-phase steel. The proposed model can be used in finite element analyses to support performance-based design of steel structures at elevated temperatures resulting from accidental events such as fire.

Peculiar roles of nickel diffusion in intermetallic compound formation at the dissimilar metal interface of magnesium to steel spot welds

There is an increased need to join magnesium alloys to high-strength steels to create multi-material lightweight body structures for fuel-efficient vehicles. Direct joining of these two metals is difficult, primarily because pure Mg and pure Fe are immiscible. Although a variety of interlayer materials have been used to facilitate the dissimilar joining in the literature, it is not well understood how the interlayer material affects the joint properties. In this study, a newly developed process, ultrasonic interlayered resistance spot welding (Ulti-RSW), was used to join an Mg alloy to a dual phase high-strength steel with two different interlayer materials: austenitic stainless steel and pure nickel. The IMCs formed with the two interlayers were drastically different, and computational thermodynamics and diffusion simulations revealed that such different IMCs resulted from the peculiar difference in nickel dissolution governed by local chemical potential gradients and diffusion kinetics. The joint with austenitic stainless steel interlayer had an under one micron thick Al-Fe IMC layer and a joint efficiency superior than that with the nickel interlayer as well as other joining processes in the literature.

Corrosion mechanism of advanced high strength dual-phase steels by electrochemical noise analysis in chloride solutions

The corrosion behavior of ferrite-martensite and ferrite-bainite dual-phase steels in NaCl, CaCl2, and MgCl2 solutions was studied by cyclic potentiodynamic polarization (CPP) and electrochemical noise (EN) techniques. The results showed a trend for mixed corrosion due to no passivation by a protective oxide layer. For the ferrite-martensite dual-phase steels, the ferrite phase was preferentially corroded compared with the martensite phase in test solutions. Bainite-microstructure in ferrite-bainite dual-phase steels was preferentially corroded compared with ferrite-phase in CaCl2 and MgCl2 solutions. The corrosion behavior in test solutions in dual-phase steels could be divided into three steps: dissolution in anodic-phase/microstructure, non-uniform corrosion, and localized corrosion. The results indicated that factors such as galvanic coupling between ferrite/martensite, ferrite/bainite, metallic-matrix/inclusion-phases, metallic-matrix/secondary phases, and self-corrosion of each phase and microstructure contributed to the non-uniform and localized corrosion in dual-phase steels.

EFFECT OF RETAINED AUSTENITE ON PLASTIC CHARACTERISTICS OF DUAL PHASE STEELS

In terms of the current trend of research and development of new materials and optimization of current materials in the automotive industry, the greatest attention is paid to progressive high-strength dual-phase (DP) steels with increased stampability, which are designed for cold stamping for specific internal car body components of the current market. New grades of DP steels provide a combination of high strength and good formability and contribute to the weight savings of vehicle parts by 10 to 20 %, compared to current DP grades. Thanks to their top properties, DP steels with increased formability can absorb more crash energy using less steel. As a result, high-strength DP780GI and DP780GI-HF materials of first generation (hereinfater DP780GI-HF) were analyzed. The stampability improvement of DP steels was demonstrated by the experimentally determined Forming Limit Curves for both steels.

In-situ analysis of the elastic-plastic characteristics of high strength dual-phase steel

Modeling the elastic behavior of dual-phase steels is complex due to the strain dependency of Young's modulus and high elastic nonlinearity. Since it is assumed that reasons for this are to be found in microstructural behavior, microscopic in-situ analysis are necessary, but due to the overlap of the martensite and ferrite peaks, the evaluation of diffraction profiles is highly complex. Within this work, CR590Y980T (DP1000) is investigated in a continuous cyclic tensile and tension-compression test under synchrotron radiation at High Energy Material Science beamline P07 in Petra III, DESY. On basis of additional EBSD measurements, an evaluation approach is shown to analyze the dual-phase diffraction profiles in such a way that martensite and ferrite can be separated for three lattice planes. The origin of the specific elastic-plastic behavior of dual-phase steels in terms of onset of yielding, anelasticity or early re-yielding is analyzed on the basis of lattice strains and interphase stresses. For this, the time-synchronously measured micro data is correlated with the macro stress-strain relationship and thermoelastic effect. The results help to better understand strain-dependent elastic-plastic behavior of DP steels on a micro level and provide great potential to improve characterization and modeling in terms of springback prediction.

Microstructure-Based Modelling of Elastoplastic Properties and Deformation Characteristics of Advanced High Strength Dual-Phase Steel

Microstructure-Based Modelling of Elastoplastic Properties and Deformation Characteristics of Advanced High Strength Dual-Phase Steel

Optimization of mechanical properties of high strength dual phase steel through thermo-mechanical simulation

Optimization of mechanical properties of high strength dual phase steel through thermo-mechanical simulation

Study on Ductility Failure of Advanced High Strength Dual Phase Steel DP590 during Warm Forming Based on Extended GTN Model

Under warm forming, the damage parameters of extended GTN model are easily affected by temperature, and the failure of materials under warm forming is less studied by using this model. In this paper, based on the extended GTN model, the damage parameters of DP590 at different temperatures are determined by experiment and numerical simulation, studying the trend of damage parameters changing with temperature. Through the analysis of shear specimens at different angles and temperatures, we studied the changes in shear damage and void damage under different conditions, discussing the influence of shear damage and void damage on competitive fracture failure under warm forming. We modify the damage by using a function based on stress triaxiality, and present a competitive failure equation considering temperature and stress triaxiality. It is found that the extended GTN model can be applied to the failure study of DP590 steel under warm forming.

Investigating the Effect of Electrode Pressure on Nugget Size, Microstructure and Tensile Shear Strength of Resistance Spot Welded Advanced High Strength Dual Phase Steel Joints

Investigating the Effect of Electrode Pressure on Nugget Size, Microstructure and Tensile Shear Strength of Resistance Spot Welded Advanced High Strength Dual Phase Steel Joints

Influence of the forming process on springback

The use of advanced high-strength steels (AHSS) is associated with increased springback and represents a great challenge for today’s press shops with regard to the dimensional accuracy of cold-formed sheet metal parts. Springback, its appearance and characteristics are mainly dependent on the stress state introduced by the forming process and part geometry before opening the tools. With the aim of determining the influence of the forming process on springback, sheet metal forming simulation is used in this contribution to analyze a hat profile from a high-strength dual-phase steel (DH1000) by three different forming processes: crash forming, bending and drawing. For this purpose, the stresses generated by these processes and resulting springback are examined considering the respective forming history. The results show the strong influence of the material flow controlled by the process and tool geometry on the stress state before springback, which leads to significant differences in the springback of the hat profile among the forming processes: springback increases from crash forming to bending and drawing.

Resistance spot welding of high-strength DP steel and nano/ultrafine-grained IF steel sheets

In the current study, microstructural characteristics, mechanical behavior, and failure analysis of resistance spot welded joints for high-strength dual-phase (DP) steel and nano/ultrafine-grained interstitial free (IF) steel was investigated. The DP steel side indicated three different heat-affected zones including intercritical, fine-grained, and coarse-grained. The volume fraction of martensite in the coarse-grained heat-affected zone was higher than in other regions due to more growth of γ grains. The IF steel side exhibited three different subregions in the heat-affected zone, namely, ultrafine-grained, fine-grained, and coarse-grained heat-affected zone. In the coarse-grained heat-affected zone, the iron oxide was effective on the microstructure by suppressing the grain growth. The uniformity of microhardness in the fusion zone of 10 kA welding current was more than other samples due to faster homogenization of melt. All welded samples indicate hardening at fine-grained heat-affected zone (on both sides) and ultrafine-grained heat-affected zone (on the interstitial free steel side). With increasing the current of welding from 6 kA to 10 kA, the peak load increased from 7.38 kN to 11.23 kN and the energy absorption improved from 6.01 kJ to 11.72 kJ due to increasing the diameter of the fusion zone. All samples failed through partial thickness-partial pullout mode. Cleavage facets and shear dimples were observed in all samples, which indicated a mixture of brittle and ductile fractures.

Analysis of the Dual-phase Steel DP500 Stress-strain Characteristics During the Plane Shear Test

Nowadays, the tendency to survey and accelerate technological processes in order to increase produc-tion efficiency on the one hand and to reduce the ecological impact of production on the environ-ment on the other hand is increasingly prevailing. These aspects encourage both the applilcation of mathematical modelling of production processes and utilization of new material types ensuring suffi-cient strength and ductility while reducing the overall weight of the final asembly. This paper focuses on investigation and evaluation of the mechanical properties of the material under plane shear stress conditions. Specifically, it concerns the testing of a dual-phase high-strength steel DP500 (designa-tion according to EN 10346) having structure based on a ferritic-martensitic matrix. The test material is loaded by means of a testing machine with a continuous static load and condition of the plane shear stress is achieved due to the geometry of the test specimen (ASTM B831). In the experimental part of this paper, plane shear tests are performed to determine the basic mechanical properties and stress-strain characteristics of the material with respect to the given loading conditions. These deter-mined parameters and material characteristics can be further used as important input data to improve results of the numerical simulations regarding the aspect of metal forming technologies – basically stamping process.

Investigation on microstructural features and tensile shear fracture properties of resistance spot welded advanced high strength dual phase steel sheets in lap joint configuration for automotive frame applications

Abstract The main objective of this research paper is to study the microstructural features and mechanical properties of resistance spot welded advanced high-strength steel of dual phase grade in lap joint configuration which is mainly employed in sheet form for fabrication of the automotive structure. Resistance spot welding (RSW) being a solid-state welding (SSW) process is used to overcome the problems in fusion welding of AHSS-DP steel such as heat affected zone (HAZ) softening, solidification cracking and distortion which significantly deteriorates the mechanical properties of AHSS-DP800 steel joints. The straight lap (SL-TSFL) and cross lap tensile shear fracture load (CL-TSFL) of spot joints were evaluated. Optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques were used to characterize the weld nugget. The X-ray diffraction (XRD) results are also presented for phase identification in the weld nugget. The fracture surface of failed TSFL specimens was analyzed using SEM. The lap joints made using RSW disclosed superior SL-TSFL, CL-TSFL, and WNZH of 21.7 kN, 17.65 kN, and 589 HV0.5 The superior joint strength and hardness of the weld nugget zone are correlated to the evolution of lath martensite in the nugget zone.

Corrosion Mechanism of Advanced High Strength Dual-Phase Steels by Electrochemical Noise Analysis in Chloride Solutions

Corrosion Mechanism of Advanced High Strength Dual-Phase Steels by Electrochemical Noise Analysis in Chloride Solutions

Hydrogen detection in high strength dual phase steel using scanning Kelvin probe technique and XPS analyses

Hydrogen permeation through high strength DP1180 steel was studied by Scanning Kelvin Probe (SKP) and X-ray photoelectron spectroscopy (XPS). The XPS analyses showed that hydrogen desorption from the steel increased the ratio Fe(II)/Fe(III) related to oxide film reduction. In parallel, a drop of the electrochemical potential in the oxide film was measured by SKP. Analyses of the composition and potential of the surface were correlated based on Nernst red-ox thermodynamic equilibrium. From this approach, it was shown that the SKP potential can be a measure of hydrogen affecting the surface oxide, but additional contributions should be considered.