Commercial Dual-phase Steel Research Articles

Effects of spatial microstructure characteristics on mechanical properties of dual phase steel by inverse analysis and machine learning approach

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

Development of a High Ductility DP Steel Using a Segregation Neutralization Approach: Benchmarked Against a Commercial Dual Phase Steel

Commercial dual-phase steels are typically synonymous with a banded distribution of martensite in their microstructures, which can degrade ductility and increase the anisotropy of mechanical properties. The concept of neutralizing the effect of Mn segregation is employed to change the distribution of martensite to a non-banded distribution. To this end, the ratio of austenite and ferrite stabilizer elements has been changed in the composition of dual-phase steel. Microstructural analysis has been carried out on both hot-rolled (ferrite + pearlite) and heat-treated (ferrite + martensite) microstructures by optical microscope and EBSD, respectively. The microstructural examinations have confirmed the non-banded distribution of second phase and more equiaxed ferrite grains in the segregated neutralized grade microstructures compared to a commercially benchmarked dual-phase steel. Tensile properties of two grades have also been assessed in hot-rolled and heat-treated conditions in RD, TD, and 45 deg tensile directions. In the case of heat-treated condition, total elongation in RD direction has been improved from 20.9 pct in benchmark dual-phase steel to 25.4 pct in segregated neutralized dual-phase steel. Tensile anisotropy results showed a significant difference in tensile strength by tensile direction in benchmark dual-phase steel in both hot-rolled (~ 85 MPa) and heat-treated conditions (~ 48 MPa), while the corresponding differences for the segregated neutralized grades were 14 and 15 MPa, respectively.

Long-time strain aging responses of commercial dual-phase steels at room temperature

ABSTRACT In this study, the effects of room temperature static strain aging period on the tensile properties of DP600 dual-phase steel were investigated. 6% and 12% of pre-strained DP600 steels were carried out static strain aging treatment at room temperature for up to 4 months. It was observed that with increasing both pre-strain rate and room temperature static aging period, the tensile strength values increased and ductility values decreased significantly. Also, dual-phase steels maintain the continuous yield behaviour which is general characteristic. The fracture surfaces exhibited a lower depth dimple type mode with increasing room temperature static strain aging period.

Segregation Neutralised Steels: Microstructural Banding Elimination from Dual-Phase Steel Through Alloy Design Accounting for Inherent Segregation

Banding in commercial dual-phase steels, such as banded ferrite and pearlite or ferrite and martensite microstructures, is inherited from segregation during solidification in continuously cast material, predominantly from Mn segregation, and subsequent rolling. The banded microstructures lead to anisotropic mechanical properties which is generally undesirable. This paper presents an alloy design approach (termed “segregation neutralised” steels) to remove banding of the second phase by utilising co-segregation of both austenite and ferrite stabilisers to reduce local variability in second phase stability. The new composition proposed also considers achieving the same strength levels through maintaining the same second phase fraction, grain size and solid solution strengthening increments. Phase field modelling has been used to predict the segregation and phase transformation behaviours for a commercial composition dual-phase steel and the new composition segregation neutralised steel. A 5 kg laboratory alloy production route (casting, hot rolling and coiling simulation, cold rolling and annealing) has shown that the banded structure seen in commercial dual-phase steels is accurately reproduced and that banding has been reduced dramatically in both the hot rolled condition as well as after cold rolling and annealing in the new segregation neutralised steel. Chemical analysis has shown that in the segregation neutralised alloy the second phase distribution shows no correlation to the segregation bands, due to the achieved balance in austenite and ferrite stabilisers.

Microstructural Analysis of Strain Rate Dependence for Commercial Dual Phase Steel DP590 Undergoing Finite Strain Deformation

This paper analyses the correlation of strain rate dependency of the material with sheet metals manufactured orientation that would influenced the microstructural changes in terms of martensite-ferrite and voids characteristics. The investigation on the deformation behaviour of Advanced High Strength Steel (AHSS) DP 590 was conducted using a uniaxial tensile test along the two principal directions that is through longitudinal (0°) and transverse (90°) through the strain rates of 1x10-1s-1to 1x10-4s-1. This allows for a strain rate dependency analysis conducted via stress-strain curve development. Scanning Electron Microscope (SEM) is used to examine the deformed microstructure for micro structural behaviour and for the void analysis on the number of voids and the average void sizes. Referring to the stress-strain curve, it is shown that the mechanical properties of the specimen behaved differently at those preferred orientations and are sensitive to the changes in the loading conditions. This could be observed as the DP590 at 0⁰ of rolling direction have higher stiffness value as compared to the 90° of transverse direction, the variant of the response of the material could be observed to be mildly an isotropically inclined instead of behaves as an isotropically behaved material. The response is also observable within the deformed microstructure. For instance, the higher strain rate indicates a higher number of voids, with a larger average void sizes, from this response it is observed the changes of the strain rate of the material would affect the material behaviour. As for the ferrite-martensite analysis, it could be observed in both different material orientations that as the lower strain rate was applied to the specimen, the strength of the material tends to become weaker since the martensite island becomes smaller at smaller strain rate levels.

Consideration of Magnetic Measurements for Characterisation of Ferrite–Martensite Commercial Dual-Phase (DP) Steel and Basis for Optimisation of the Operating Magnetic Field for Open Loop Deployable Sensors

The magnetic properties of commercial dual-phase (DP) steels (DP600, DP800 and DP1000 grades) were evaluated using initial permeability, incremental permeability and coercivity and correlated with the key microstructural differences between the grades. The ferrite grain sizes and ferrite fractions have been compared with the magnetic parameters obtained from minor and major magnetisation loops within each DP grade. It has been revealed that the incremental permeability increases with the applied magnetic field amplitude to reach a peak and then drops at a higher magnetic field, with the values being different for the three DP grades at a lower field and converging to a similar permeability value at the high field. The effects of ferrite grain size and phase fraction on the incremental permeability are considered, and it has been shown that the influence of ferrite grain boundaries on magnetic permeability is more dominant than the effect of ferrite fraction in commercial DP steel samples. An analysis of the correlation between coercivity and initial permeability with tensile strength shows that the initial permeability provides a slightly better prediction of strength for the steels examined, which is believed to be due to the fact that a combination of reversible and irreversible domain components affect the coercivity value, while the initial permeability is predominantly affected by reversible domain movements. Based on the trend between incremental permeability and applied magnetic field and the commercial EM sensor (EMspec) operating parameters, the effect of lift-off and hence magnetic field strength on the sensitivity to DP steel properties can be assessed.

Segregation Neutralised Steels: Microstructural Banding Elimination from Dual Phase Steel Through Alloy Design Accounting for Inherent Segregation

Banding in commercial dual phase steels, such as banded ferrite and pearlite or ferrite and martensite microstructures, is inherited from segregation during solidification in continuously cast material, predominantly from Mn segregation, and subsequent rolling. The banded microstructures lead to anisotropic mechanical properties which is generally undesirable. This paper presents an alloy design approach (termed “segregation neutralised” steels) to remove banding of the second phase by utilising co-segregation of both austenite and ferrite stabilisers to reduce local variability in second phase stability. The new composition proposed also considers achieving the same strength levels through maintaining the same second phase fraction, grain size and solid solution strengthening increments. Phase field modelling has been used to predict the segregation and phase transformation behaviours for commercial dual phase steels and the new composition. A 5 kg laboratory alloy production route (casting, hot rolling and coiling simulation, cold rolling and annealing) has shown that the banded structure seen in commercial dual phase steels is accurately reproduced and that banding has been reduced dramatically in both the hot rolled condition as well as after cold rolling and annealing in the new composition steel. Chemical analysis has shown that in the modified alloy the second phase distribution shows no correlation to the segregation bands, due to the achieved balance in austenite and ferrite stabilisers.

A comparative study on mechanical behavior and damage scenario of DP600 and DP980 steels

In this study, the mechanical behavior of two cold rolled commercial dual phase steels (DP) were analyzed on micro and macro scales. First, the anisotropic behavior of these steels were investigated by standard tensile tests in three directions. The results showed that the anisotropy behavior of the fracture strain of DP980 is more pronounced than of DP600. Then, in order to assess the influence on the stress state on the mechanical behavior, four specimens with different stress states were analyzed by ARAMIS approach which is based on digital image correlation (DIC). The ARAMIS results were compared with 3D numerical simulations using the Abaqus/Explicit solver. In this part, the effect of stress state on flow curve and strain distribution in the specimens (i.e. tensile, notched-tensile, shear and bulge specimens) were investigated. To predict the fracture behavior of DP600 and DP980 steels under various loading conditions, the Modified Mohr-Coulomb (MMC) damage model was utilized. A VUMAT subroutine was developed to include a MMC damage model in the 3D models, and good agreement between the numerical and experimental results was observed. Finally, the microstructure failure mechanisms at three different stages (i.e. strain localization, micro-crack initiation and micro-crack coalescence) during uniaxial tensile loading were investigated inside the microstructure of DP600 and DP980 steels using an interrupted in-situ setup. In the results three different damage localizations were observed in both materials, and strain localization in the center of large ferrite phases and at the boundary of ferrite and martensite phases were dominant in DP600 and DP980, respectively. Two different micro-crack initiation mechanisms were observed that were similar in both materials.

On the mechanical heterogeneity in dual phase steel grades: Activation of slip systems and deformation of martensite in DP800

We used micropillar compression experiments to study the plasticity of ferrite and martensite of two commercial dual phase steel grades (DP800). The activation of all three slip plane families, namely {110}, {112}, {123}, was observed in single crystalline ferrite pillars. They exhibit a comparable mean critical resolved shear stress (CRSS) of 147 ± 6, 143 ± 9, 146 ± 4 MPa for 3 µm pillars and are predominantly following Schmid´s law. A distinct size effect occurs when comparing the CRSS of 2 µm and 3 µm pillars. Martensite islands show uniform deformation and exhibit high compressive yield strength up to nearly 3 GPa. In most cases martensite pillars deform in an isotropic fashion without distinct slip traces. Despite the identical ultimate tensile stress of two steel grades their ferrite CRSS and martensite strength are largely different. It is found that the softer ferrite results in a lower macroscopic yield strength and a higher elongation to failure during macroscopic tensile testing. The results suggest that an increased local strain hardening capability suppresses global damage. The data provided here can serve as input parameter for crystal plasticity modeling.

Non-destructive measurement of microstructure and tensile strength in varying thickness commercial DP steel strip using an EM sensor

It is desirable to be able to non-destructively evaluate the microstructural features and/or mechanical properties of steel strip. An electromagnetic sensor – sample FE model, developed in COMSOL multi-physics software, has been developed to determine the relationship between the low magnetic field relative permeability and microstructure (phase balance and grain size), and hence tensile strength, for commercial dual phase (DP) steel sheets of any thickness. The model for the effect of thickness on the sensor signal has been validated using DP steel sheets of 1 to 4 mm. It was found that the relative permeability is strongly dependent on the phase balance (fraction of ferrite and martensite) in the DP steels with an increasing relative permeability being seen for a higher fraction of ferrite. In addition, changes in the ferrite grain size also affect the relative permeability (increase in grain size increases relative permeability) and hence EM sensor signal, this being more significant for the higher ferrite content DP grades. Both these microstructural features affect the tensile strength (which decreases with an increase in ferrite fraction and increase in grain size), which results in a good correlation between the relative permeability and tensile strength. The results from the relative permeability – microstructure – tensile strength investigation and FE model to account for strip thickness have been combined to allow the prediction of tensile strength from EM sensor measurements on as-received commercial DP steel sheets of varying thickness and the approach has been tested.

Effect of holding time at an intercritical temperature on the microstructure and tensile properties of a ferrite-martensite dual phase steel

To analyze the influence of the initial microstructure in the production of a dual phase (DP) steel on the mechanical properties, a commercial DP steel was subjected to two heat treatments (HT): step quenching (SQ) and intermediate quenching (IQ). SQ samples were austenitized at 930 °C for 20 min, then annealed at 830 °C for 5, 15, 30, 60 and 120 min and finally water quenched. Shorter annealing times show allotriomorphic ferrite and martensite while at longer time polygonal ferrite is formed. Tensile tests showed that yield and tensile strength are higher than those of the commercial DP steel and besides, the uniform elongation is smaller. IQ samples were water quenched after austenitization and then annealed at 830 °C for the same holding times and water quenched again. The microstructure consists of martensite dispersed in a ferrite matrix. The remarkable results are that these samples had similar tensile properties to that of the as-received DP steel. Tensile properties were analyzed as a function of the characteristic microstructure and the assessment of the alloying elements partition during the different HTs. Evaluation of the Kolmogorov-Johnson-Mehl-Avrami theory indicates that the transformation in SQ and in IQ samples corresponds to a diffusional process.

Influence of microstructure and pre-straining on the bake hardening response for ferrite-martensite dual-phase steels of different grades

This work aims to investigate the effects of pre-straining and microstructure on bake hardening (BH) response of dual phase (DP) steels. Four kinds of different grades of commercial DP steels, DP340/600, DP420/780, DP500/780 and DP550/980, were pre-strained in tension to 0.5%, 1%, 2%, 4%, 6% and 8%, then baked at 170°C for 20min followed by restraining, or just baked at 170, 190, 210, 230, 250°C for 20min and at 250°C for 40min followed by straining without pre-straining. Electron backscattered diffraction (EBSD) measurements and temperature-dependent internal friction measurements were conducted to characterize geometrically necessary dislocations (GNDs) and analyze solute carbon content and interactions between point defects and dislocations in DP steels. The results show that for all grades of DP steels investigated, BH values increase to peak values at pre-straining ranging from 0.5% to 2% and then decline with further pre-straining. At BH condition of 170°C/20min, peak BH values with pre-straining are 33MPa for DP340/600, 34MPa for DP420/780, 78MPa for DP500/780 and 90MPa for DP550/980 respectively. Pre-straining can cause increase in tensile strength and decrease in total elongation after baking, especially for DP500/780 and DP550/980. DP steels without pre-straining can reach very high BH0 values by applying either higher temperature or longer holding time. Solute carbon content in ferrite controls the speed of BH response in DP steels. Microstructures of higher volume fraction of martensite, smaller martensite islands and smaller ferrite grains can produce higher BH values.

Analysis and design of dual-phase steel microstructure for enhanced ductile fracture resistance

The goal of this paper is to predict how the properties of the constituent phases and microstructure of dual phase steels (consisting of ferrite and martensite) influence their fracture resistance. We focus on two commercial low-carbon dual-phase (DP) steels with different ferrite/martensite phase volume fractions and properties. These steels exhibit similar flow behavior and tensile strength but different ductility. Our experimental observations show that the mechanism of ductile fracture in these two DP steels involves nucleation, growth and coalescence of micron scale voids. We thus employ microstructure-based finite element simulations to analyze the ductile fracture of these dual-phase steels. In the microstructure-based simulations, the individual phases of the DP steels are discretely modeled using elastic-viscoplastic constitutive relations for progressively cavitating solids. The flow behavior of the individual phases in both the steels are determined by homogenizing the microscale calibrated crystal plasticity constitutive relations from a previous study (Chen et al. in Acta Mater 65:133–149, 2014) while the damage parameters are determined by void cell model calculations. We then determine microstructural effects on ductile fracture of these steels by analyzing a series of representative volume elements with varying volume fractions, flow and damage behaviors of the constituent phases. Our simulations predict qualitative features of the ductile fracture process in good agreement with experimental observations for both DP steels. A ‘virtual’ DP microstructure, constructed by varying the microstructural parameters in the commercial steels, is predicted to have strength and ductile fracture resistance that is superior to the two commercial DP steels. Our simulations provide guidelines for improving the ductile fracture resistance of DP steels.

Predicting grid-size-dependent fracture strains of DP980 with a microstructure-based post-necking model

Ductile fracture is a local phenomenon, and it is well established that fracture strain levels depend on both stress triaxiality and the resolution (grid size) of strain measurements. Two-dimensional plane strain post-necking models with different model sizes are used to predict the grid-size-dependent fracture strain of a commercial dual-phase steel, DP980. The models are generated from the actual microstructures, and the individual phase flow properties and literature-based individual phase damage parameters for the Johnson–Cook model are used for ferrite and martensite. A monotonic relationship is predicted: the smaller the model size, the higher the fracture strain. Thus, a general framework is developed to quantify the grid-size-dependent fracture strains for multiphase materials. In addition to the grid-size dependency, the influences of intrinsic microstructure features, i.e., the flow curve and fracture strains of the two constituent phases, on the predicted fracture strains also are examined. Application of the derived fracture strain versus model size relationship is demonstrated with large clearance trimming simulations with different element sizes.

Failure assessment in sheet metal forming using a phenomenological damage model and fracture criterion: experiments, parameter identification and validation

Abstract In this contribution microstructural and macroscopic experimental findings are used to calibrate the phenomenological damage model GISSMO (Generalized Incremental Stress State dependent Model) and the fracture criterion FFL/SFFL (Fracture Forming Limit line/ Shear Fracture Forming Limit line) to assess failure in sheet metal forming simulation. It is shown that macroscopic failure in a commercial dual-phase steel is initiated through a combination of ductile damage mechanisms and local shear banding at a characteristic, stress-state dependent optical or tactile measureable fracture strain. The parameter identification for both models is based on ductile fracture experiments representing characteristic stress states. GISSMO is calibrated inversely using experimental stress-strain curves, optical measured fracture strains and simulation data. The FFL and SFFL are constructed in a direct manner with the optically and tactilely measured fracture strains. Both models are validated comparatively on a cross-die cup showing ductile fracture with slight necking. The use of the fracture criterion in combination with a direct method of determining the fracture lines based on tactile strain measurements leads to an overestimation of the instant of fracture initiation. With the inversely identified parameters of the phenomenological damage model the onset of fracture initiation can be accurately predicted.

A multiscale approach to modeling formability of dual-phase steels

A multiscale modeling approach is used to predict how the formability of dual-phase (DP) steels depend on the properties of their constituent phases and microstructure. First, the flow behavior of the steels is predicted using microstructure-based finite element simulations of their 3D representative volume elements, wherein the two phases (ferrite and martensite) are discretely modeled using crystal plasticity constitutive models. These results are then used to calibrate homogenized constitutive models which are then used in large-scale finite element simulations to compute the forming limit diagrams (FLDs). The multiscale approach is validated by predicting the FLDs of two commercial DP steels and comparing the predictions with experimental measurements. Subsequently, the approach is used to compute flow behavior and FLDs of a series of ‘virtual’ DP steels, constructed by varying the microstructural parameters in the commercial DP steels. The results of these computations suggest that combining the ferrite from one of the two commercial steels with the martensite of the other and optimizing the phase volume fractions can yield ‘virtual’ steels with substantially improved properties. These include a material with an FLD0 (plane strain) that exceeds those of the commercial steels by 75% without a degradation in strength; and a material with a flow strength (0.2% offset) that exceeds those of the commercial steels by ~30% without degradation of formability.

Self-Heating Measurements for a Dual-Phase Steel under Ultrasonic Fatigue Loading for stress amplitudes below the conventional fatigue limit

The aim of the present research was to study the self-heating behavior of a dual-phase steel under ultrasonic fatigue loading for stress amplitudes lower than the conventional fatigue limit. The steel studied in this research was DP600 commercial dual phase steel. Fatigue tests were conducted for different values of stress amplitudes up to 107 cycles using an ultrasonic fatigue machine at a testing frequency of 20 kHz with flat specimens. An infrared camera was used to measure the mean temperature evolution during the tests. A specific form of heat diffusion equation was adopted in this work to calculate the intrinsic dissipation from temperature measurements. The variation of the dissipated energy versus stress amplitude under cyclic loading was also studied.

Effects of Microstructure on the Variation of the Unloading Behavior of DP780 Steels

The nonlinear unloading behavior of three different commercial dual-phase steels (DP780 grade equivalent) was examined. These steels exhibited small variations in chemical composition (0.07 to 0.10 mass percent carbon) and martensite volume fraction (0.23 to 0.28), and they demonstrated similar hardening behavior. Uniaxial loading-unloading-loading tests were conducted at room temperature and quasi-static strain rates between engineering strains of 0.5 and 8%. Steel microstructures were examined using electron backscatter diffraction and nanoindentation techniques. The microplastic component of the unloading strain exhibited no dependence on the martensite volume fraction or the ferrite grain size within the small range encountered in this investigations. Instead, the magnitude of the microplastic component of the unloading strain increased as the strength ratio between the martensite and ferrite phases increased. Correspondingly, the apparent unloading modulus, or chord modulus, exhibited a greater reduction for equivalent increments of strain hardening as the strength ratio increased. These results suggest that springback can be reduced in structures containing two ductile phases if the strength ratio between the harder and softer phases is reduced.

Temperature and strain rate effects on the mechanical behavior of dual phase steel

The effect of temperature and strain rate on the mechanical behavior of a commercial dual phase steel (DP 800) has been investigated experimentally by uniaxial tensile tests in this study, covering temperatures (−60°C to 100°C) and strain rates (1×10–4 to 1×102s−1) encompassing conditions experienced in automotive crash situations. Yield and ultimate tensile strength, ductility, temperature effects and strain rate sensitivity have been determined and discussed. It was found that the Voce equation [σ=σs−(σs−σ0)exp(−εθ0/σs))] can be satisfactorily applied to describe the tensile flow curves by means of a modified Kocks–Mecking model. In this model the parameter θ0 is fixed, whereas both σ0 and σs consist of athermal and thermal stress components. The athermal component is only weakly dependent on temperature through the elastic shear modulus μ. The thermal stress component is governed by temperature and strain rate. Statistical analysis based on the experimental data has allowed all parameters in the Voce equation to be quantified.

The influence of temperatures and strain rates on the mechanical behavior of dual phase steel in different conditions

This study deals with the mechanical behavior of DP steel. A commercial dual phase steel (DP 800) was strained to 3.5% followed by annealing at 180°C for 30min to simulate the pressing of the plates and the paint-bake cycle involved in the manufacturing process of automobile body structures. The effect of temperature and strain rate on the mechanical behavior of this material was investigated by uniaxial tensile tests, covering applicable temperatures (−60°C to 100°C) and strain rates (1×10−4 to 1×102s−1) experienced in automotive crash situations. Yield and ultimate tensile strength, ductility, temperature effects and strain rate sensitivity as well as strain hardening rate have been determined and discussed.