Strength Dual Phase Steel Research Articles

Microstructure-based modelling of formability for advanced high strength dual-phase steels

The macroscopic formability of advanced high strength dual-phase steels (AHSDPSs) relies highly on the microstructure. Therefore, building the relationship between the microstructure and the formability is vital to enhancing the formability by microstructure optimisation. In this study, a formability model based on microstructure for AHSDPSs was proposed. The tensile simulation of representative volume element (RVE) was implemented to acquire the stress-strain curve (SSC). The forming limit curve (FLC) was constructed based on the method developed in our previous research. The simulation of hole-expansion with the pre-damage of the central-hole induced by the blanking process was conducted to obtain the hole-expansion ratio (HER), in which the vital failure model is developed based on the simulated FLC. For a typical advanced high strength dual-phase steel of DP1180 steel, the prediction errors of SSC, FLC and HER are 5.83 %, 6.85 % and 10.71 %, respectively. The results also depicted that the damage of the prefabricated hole has a noticeable influence on the HER, which could not be ignored for the hole-expansion simulation.

Correlation between individual phase constitutive properties and plastic heterogeneities in advanced-high strength dual-phase steels

The present study comprehensively investigates the individual phase constitutive properties and plastic heterogeneities in advanced high-strength steels (AHSS), particularly DP590 and DP780 dual-phase (DP) steels. A machine learning-based model is implemented to identify the ferrite and martensite phases in the microstructures of DP590 and DP780. Then, the constitutive properties of ferrite and martensite phases are successfully obtained through a hybrid approach of in-situ neutron diffraction coupled with the crystal plasticity finite element method (CPFEM). The distinct microstructures between DP590 and DP780 result in different macroscopic and microscopic properties among the two materials. Owing to different martensite volume fractions (Vm) in DP590 (Vm = 8.3 %) and DP780 (Vm = 35.4 %), a noticeable dependency of plastic heterogeneities during deformation on martensite fraction and its spatial distribution is revealed. Compared to DP590, the deformed microstructure of DP780 exhibits a more heterogeneous distribution of stress and strain fields, along with significant formation of plastic strain localization leading to a remarkable increase in strain partitioning index. It shows that a lower fraction of martensite with its discrete distribution decreases martensite ability to hinder the ferrite deformation, thus strain localization is primarily concentrated within the ferrite phase as the predominant failure mode in DP590. In contrast, a higher martensite fraction in DP780 causes more pronounced strain localization which occurs in the ferrite and at the ferrite/martensite interface. In addition, interconnect distribution between martensite islands enhances the inhibition of martensite to ferrite deformation, thereby high strain gradient at their interface leads to prevalence of ferrite/martensite interface decohesion in DP780.

Characterization of corrosion products formed in high-strength dual-phase steels under an accelerated corrosion test

There are some researches in the literature on the mechanical characteristics of dual-phase (DP) steels used in the automotive industry, but there is no comprehensive research on the corrosion behavior of these steels. In this work, the corrosion behavior of DP steels (DP440, DP590, DP980) exposed to two cycles of accelerated corrosion testing in accordance with Ford CETP 00.00-L-467 was observed. Raman and X-ray diffraction (XRD) techniques were used to classify the corrosion products, and the morphology of the samples was studied using a scanning electron microscope (SEM). Goethite and haematite were the primary chemical compounds determined. In high-mechanical strength DP steels, akaganeite was also identified in corroded specimens. The compounds formed due to corrosion were revealed by SEM images. In this work, according to the results of Raman spectroscopy, which was employed for the first time to reveal corrosion products in high-strength dual-phase steels, it was discovered that corrosion products increased with increasing mechanical strength due to an increasing martensite phase volume percentage. Polarization tests were carried out to support the electrochemical data reported by the Raman analysis. Similarly, an increase in the amount of martensite phase in the microstructure led to a decrease in the material’s corrosion resistance. Polarization experiments were carried out to support the electrochemical data interpreted by Raman analysis. In addition, an increase in the amount of martensite phase in the microstructure led to a decrease in the corrosion resistance of the material. In addition, information regarding the material’s electrochemical performance was obtained through Raman analysis. As shown by Raman, XRD, and polarization tests, the increase in corrosion products formed due to the increase in the amount of martensite led to a decrease in corrosion resistance.

Multiscale analysis of microstructure-based bending characteristics of advanced high strength dual-phase steel

Multiscale analysis of microstructure-based bending characteristics of advanced high strength dual-phase steel

Research on strain aging behavior of ultra-high strength dual-phase steel

The bake hardening value is one of the vital strength indexes of dual-phase steel, representing the strengthening ability of materials after pre-strain and baking, playing an important role in vehicle safety and lightweight design. Studying and improving the strain aging mechanism of dual-phase steel helps one to understand the material characteristics and enhances its utilization value. However, the ultra-high strength dual-phase steel is often prone to fracture outside the gauge length of a tensile specimen of the bake hardening value test. No suitable theory explains the fundamental law of dislocation pinning during the saturation stage at present. This paper used FEA, DIC, SEM, TEM, internal friction, and metallographic methods to study the strain aging behavior of dual-phase steels under different pre-strain, bake time, and bake temperature conditions. The results show that the fracture outside the gauge length is related to factors such as the uneven distribution of pre-strain and the ultra-high upper yield strength. The rolling pin shape tensile specimen testing has successfully solved this testing problem. The measured results at the saturation stage of dislocation pinning are in good agreement with the fitting results of the dislocation pinning strengthen mechanism based on the probability event quantization assumption.

Towards ultra-high strength dual-phase steel with excellent damage tolerance: The effect of martensite volume fraction

Ultra-high strength (> 1 GPa) dual-phase (DP) steels have been extensively investigated in literature. Yet, the damage tolerance of these DP steels remains mostly unexplored, whereas this parameter is crucial for the sheet formability and anti-crushing performance. In this work, a medium-Mn DP steel was proposed in order to develop strong but damage-resistant DP steel by refining the microstructure. Ultra-fine grained (< 1 µm) DP steels were obtained by a simple cold rolling and intercritical annealing process. The developed DP steel can achieve an outstanding ultimate tensile strength (UTS) of over 1470 MPa with appreciable elongation to failure (EtF) of over 9 % when the martensite volume fraction (Vm) is 55 %. Higher Vm results in higher strength. However, the fracture strain, which is connected with damage tolerance, decreases significantly with increasing Vm, and it is even much lower than that of a full-martensite steel. Furthermore, the necking behaviour transfers from diffuse plus localized necking to diffuse necking with increasing Vm. The sufficient ferrite content in the proposed DP1470 steel enables void growth after nucleation, consequently leading to the occurrence of localized necking and higher fracture strain. Thus, a moderate Vm ranging from 45 % to 65 % is suggested for the development of a 1470 MPa grade DP steel with excellent damage tolerance.

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.

Tuning strength-ductility combination of AISI 1008 steel through large strain single-roll drive rolling and intercritical annealing

In this study, the influence of large-strain single-roll drive rolling and intercritical annealing on the microstructure evolution and mechanical behavior of AISI 1008 steel was investigated. The microstructures of all fabricated dual-phase (DP) steels exhibited equiaxed and homogeneously distributed fine ferrite grains and martensite islands. Compared to the as-received sample, the hardness of the produced dual-phase steels increased by up to 100%. As the temperature of intercritical annealing treatment increased, the hardness of the dual-phase samples increased from 233.2 HV to 258.4 HV. All fabricated DP steels showed much higher strength and toughness than the as-received sample. Overall, with increasing the temperature of intercritical annealing, the strength, ductility, and toughness of DP steels were improved due to increasing the martensite fraction. Also, by increasing the temperature of intercritical annealing from 770 to 860°C, the work hardening rate of the DP sample was enhanced. The hardness and strength of dual-phase steels had a good linear correlation to the fraction of martensite. The failure mechanism of the 80% rolled sample was mainly brittle while all DP samples exhibited a perfect ductile fracture characterized by many homogeneous deep dimples without any cleavage facets.

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.

The effect of vanadium on microstrain partitioning and localized damage during deformation of unnotched and notched DP1300 steels

The use of vanadium-microalloying in ultrahigh strength dual phase (DP) steels has been shown to yield a fine dispersion of nano-scale vanadium carbonitrides (V(C,N)) in ferrite along with a pronounced grain refinement, leading to enhanced micromechanical compatibility and increased local ductility. Here we present data on microstrain partitioning and the evolution of damage in vanadium-free (V-free) Fine-Grained (FG) and vanadium-added (V-added) Ultra Fine-Grained (UFG) DP steels, each with a UTS of about 1300 MPa (DP1300), using quasi in-situ tensile tests coupled with scanning electron microscopy, followed by microscopic Digital Image Correlation (µDIC). Quantitative analysis shows that the homogenization of microstrain between ferrite and martensite is locally enhanced and the strain gradients at the ferrite/martensite (F/M) interfaces reduced in the V-added steel. This trend was also evident in the V-added steel exhibiting different states of stress obtained with unique notched microtensile specimen designs. Three different µDIC-based computational techniques were used to quantify the extent of microstrain partitioning, in order to determine the mechanistic basis for the increase in true strain to fracture with vanadium-microalloying. This work was supplemented with damage evolution studies in both V-free and V-added materials using high resolution, field emission scanning electron microscope (FESEM) imaging, and X-ray computed microtomography (µXCT). These corroborate the microscopic analyses and confirm that both vanadium-microalloying and stress-state impacts the local strain gradient at ferrite/martensite (F/M) interfaces, and thereby changes the way damage is initiated and grows within the material.

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

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.

A novel approach to producing architectured ultra-high strength dual phase steels

An ultrafine-grained dual phase (DP) microstructure has been produced in a steel that contains a combination of composition and deformation gradients. A carbon gradient was created by controlled decarburization of the steel. The compositionally graded steel was cold rolled to produce a large number of shear bands prior to recrystallization annealing at 650 °C for 5 h followed by intercritical annealing at 740 °C for 10 min. Microstructural characterization reveals a very fine heterogeneous dual phase microstructure with submicron grains appearing along the traces of the prior shear bands that had developed during rolling. Martensite volume fraction changes from the surface to the center and also within these shear band traces. Tensile tests of the new dual phase structure reveal that the work hardening rate changes spatially with martensite volume fraction. Detailed fractography using ex-situ X-ray computed tomography (XCT) reveals a unique form of multiple cup and cone fracture features that can be related to the evolution of damage along the shear band traces with a gradient of void density from the surface to the center. This strategy for fabricating dual phase steels enables the development of architectured ultra-high strength DP steels with different properties that can be tailored by controlling the composition and deformation gradients and subsequent annealing treatments.

Investigation of the critical factors controlling sheared edge stretching of ultra-high strength dual-phase steels

The present study aimed to explore dual-phase (DP) steels with a good combination of high strength and reasonable global ductility (i.e., total elongation and general stretch formability) and local ductility (i.e., sheared edge ductility or hole expansion ratio). Therefore, a series of ultra-high strength dual-phase steels were designed, melted, rolled, annealed and formed. These steels contained various aluminum additions and vanadium contents and were processed with different coiling temperatures and continuous galvanizing line (CGL) thermal path simulations conducted using a Gleeble 3800 system. The microstructures, tensile properties and hole expansion behaviors of all candidate DP steels were determined and compared. The microstructural and damage evolutions in the process of both hole punching and hole expansion were examined. The results indicated that hole expansion ratios of DP steels could be correlated well with (i) the burnished-to-fracture zone ratios in shear surfaces after hole punching, (ii) the values of reduction in area of tensile specimens after fracturing, and (iii) nanohardness difference between soft ferrite and hard constituents. The micro-voids and micro-cracks introduced by hole punching acted as crack initiation sites, which severely affected the subsequent hole expansion process. Therefore, better sheared edge ductility may benefit from microstructures that retard the crack propagation or void growth and coalescence during hole expansion.

Study on Bending Beam Delayed Cracking of Ultra High-strength DP Steel for Automotive Use

Automobile lightweight project can effectively reduce energy consumption and exhaust emission, but it will reduce automobile safety. Therefore, using a large number of high-strength or ultra-high-strength steels in automobiles is an effective way to give consideration to both lightweight and safety. Delayed cracking is a symptom-free cracking phenomenon, which is extremely destructive. It is necessary to evaluate the delayed cracking performance of ultra-high strength steel before it is used. In this paper, the delayed cracking properties of three ultra-high strength dual-phase steels DP780, DP980 and DP1180 for automobile are studied by bending beam delayed cracking test. The experimental results show that the delayed cracking resistance of DP780, DP980 and DP1180 steel becomes worse in turn, and DP1180 steel has the worst delayed cracking resistance. In addition, the content and morphology of martensite will affect the delayed cracking performance. The morphology of martensite in the tested steel is lathy, and the higher the martensite content, the worse the delayed cracking resistance of the steel.