High Strength Dual Phase Research Articles

Ferrite channel effect on ductility and strain hardenability of ultra high strength dual phase steel

This study describes an effect of controlled austenite decomposition on microstructure evolution in dual phase steel. Steel sheets austenitized at various annealing temperatures were rapidly cooled to the inter-critical annealing temperature of 800°C for the isothermal decomposition of austenite and then ultra fast cooled to room temperature. The scanning electron microscope analysis of evolving microstructure revealed ferrite nucleation and growth along prior austenite grain boundaries leading to ferrite network/channel formation around martensite. The extent of ferrite channel formation showed a strong dependence on the degree of undercooling in the inter-critical annealing temperature regime. Uniaxial tensile deformation of processed steel sheets showed extensive local inter-lath martensite damage activity. Extension/propagation of these local micro cracks to neighboring martensite grains was found to be arrested by ferrite channels. This assisted in delaying the onset of global damage which could lead to necking and fracture. The results demonstrated an alternate possible way of inducing ductility and strain hardenability in ultra high strength dual phase steels.

Influence of Microalloying Elements Ti and Nb on Recrystallization during Annealing of Advanced High-Strength Steels

Microalloying elements Ti and Nb are commonly added to high-strength Dual Phase steels as they can provide efficient means for additional strengthening due to grain refinement and precipitation strengthening mechanisms. In the form of solute elements or as fine carbonitride precipitates, Ti and Nb are also expected to have a significant effect on the microstructural changes during annealing and especially on recrystallization kinetics. The present work investigates the influence of microalloying elements Ti and Nb on recrystallization in various cold-rolled Dual Phase steel grades with the same initial microstructure but different microalloying contents. Using complementary experimental and modeling approaches makes it possible to give some clarifications regarding both the nature of this effect and the comparative efficiency of Ti and Nb on delaying recrystallization. It is shown that niobium is the most efficient micro-alloying element to impede recrystallization and that the predominant effect is solute drag.

Mechanism based mean-field modeling of the work-hardening behavior of dual-phase steels

Low-alloyed dual-phase (DP) steels exhibit good mechanical properties due to their composite-like microstructure of strong martensitic inclusions embedded in a ductile ferritic-matrix. This work presents an efficient two-scale approach to model the work-hardening of DP steels based on physically-motivated work-hardening contributions from geometrically necessary dislocations. The resulting mean-field model of Hashin-Shtrikman type comprises a minimal amount of free parameters and incorporates two main physical aspects motivated from presented experimental data: First, the constitutive equations for ferrite incorporate the averaged microstructural morphology (obtained by electronic backscatter diffraction) in terms of grain size and martensite coverage. Second, a direct interaction between ferrite and martensite phases is achieved via kinematic-hardening which models the long-range stresses experimentally observed in tensile tests of the bulk material. The presented approach is able to both reproduce the measured DP600 tensile curve and predict the distinctly different work-hardening rates observed for high-strength DP steels.

Strain localization and ductile fracture in advanced high-strength steel sheets

Abstract An experimental-numerical approach is applied to determine the strain localization and ductile fracture of high-strength dual-phase and martensitic steel sheet materials. To this end, four different quasi-static material tests were performed for each material, introducing stress states ranging from simple shear to equi-biaxial tension. The tests were analysed numerically with the nonlinear finite element method to estimate the failure strain as a function of stress state. The effect of spatial discretization on the estimated failure strain was investigated. While the global response is hardly affected by the spatial discretization, the effect on the failure strain is large for tests experiencing necking instability. The result is that the estimated failure strain in the different tests scales differently with spatial discretization. Localization analysis was performed using the imperfection band approach, and applied to estimate onset of failure of the two steel sheet materials under tensile loading. The results indicate that a conservative failure criterion for ductile materials may be established from localization analysis, provided strain localization occurs prior to ductile fracture.

Effects of Mn additions on microstructure and properties of Fe–TiB2 based high modulus steels

We studied the effects of Mn additions from 0 to 30wt.% on microstructure, mechanical and physical properties of liquid metallurgy synthesised high modulus steels in hypo- and hyper-eutectic TiB2 concentrations. While Mn has little effect on density, both Young's modulus and mechanical properties were strongly affected by the achieved wide spectrum of matrix microstructures, ranging from ferrite to martensite, reverted austenite, ε-martensite and austenite. Mn additions of 20 and 30wt.% did not translate into enhanced mechanical performance despite the higher inherent ductility of the predominantly austenitic matrix, and instead eliminate the intended weight saving potential by significantly reducing the Young's modulus. Martensitic matrices of Mn concentrations of 10wt.%, on the other hand, are favourable for improved matrix/particle co-deformation without sacrificing too much of the composites' stiffness. In hypo-eutectic Fe – TiB2 based steels, mechanical properties on the level of high strength dual phase steels could be achieved (~900MPa UTS and 20% tensile elongation) but with an enhanced Young's modulus of 217GPa and reduced density of 7460kgm−3. These significantly improved physical and mechanical properties render Mn alloyed high modulus steels promising candidate materials for next generation lightweight structural applications.

Effect of Fine Grained Dual Phase Steel on Bake Hardening Properties

High strength dual phase (DP) steels have been increasingly used in car body structure for reducing weight and increasing safety performance. Bake hardening process is widely applied for improving final mechanical properties of formed components by means of carbide precipitation and occurred Cottrell cloud. In this work, effects of bake hardening on mechanical behavior of coarse and fine grain DP steels are investigated. Initially, fine grain ferritic–pearlitic structure is produced from a low carbon steel by constrained groove pressing (CGP) based on severe plastic deformation. Ferrite grains with a submicro size of 470 nm can be hereby achieved. DP microstructures are generated afterward by intercritical annealing. Finally, bake hardening at the temperature of 160 °C for 20 min is performed under consideration of various pre-strains between 0 and 10%. It is found that bake hardenability of the DP steels can be significantly increased due to the fine grain structure induced by the CGP, which provide enhanced heterogeneous nucleation sites for precipitates and accelerated carbon diffusion.

Influence of the Welding Process on the Martensitic and Dual Phase High Strength Steels

The subject of the study are martensitic 22MnB5 steel and dual phase steel with the ferrite-martensitic structure, which are used in the automotive industry. The main purpose of the performed analyses is a study of strength differences in heat affected zones of the spot welding. For the needs of the strength decrease assessment, the critical layer of the heat affected area was experimentally simulated by different thermal influence procedures. The aim of the work is to determine the most suitable methodology for evaluating the local changes of the elastic-plastic material response. The yield strength and the deformation hardening are required constructions of safety carbody parts.

Experimental investigation and numerical simulation of resistance spot welding for residual stress evaluation of DP1000 steel

Resistance spot welding of advanced high strength dual phase steel, with tensile strength of 980 MPa at elongation of 10 %, was experimentally investigated and numerically simulated. The FEM simulation was performed using SYSWELD in order to calculate the thermal and residual stress fields for research of effect of RSW fracture behavior. Thermo-mechanical properties of the material were modified from DP600 data to experimentally determined DP1000 from hot tensile tests made with a Gleeble system. The predefined G-electrode shape was changed to the F16-electrode used in practice. Effect of welding current on nugget size and residual stresses is examined for lower, medium, and upper currents inside welding range. Simulation nugget and heat-affected zone of the 2D model show a correlation if compared to experimental results from metallography of RSW cross sections. Residual stresses are measured using the drill hole method, but they differed from simulated stresses. Possible reasons are evaluated and discussed.

Constitutive modeling of evolving plasticity in high strength steel sheets

In this paper, biaxial tensile tests with cruciform specimens were conducted to obtain the experimental plastic work contours of DP590, DP780 and DP980 sheets. It was found that for high strength dual phase steel sheets, the outlines of the experimental work contours in the first quadrant evolve evidently with the increase of plastic dissipation. In order to calibrate the evolving isotropic and anisotropic plastic behaviors simultaneously, the Yld2000-2d yield criterion with variable exponent and coefficients (Yld2000-2d-Var) was introduced. In this model, the exponent m and coefficients αi of the Yld2000-2d was assumed as the function of the equivalent plastic strain. The identification procedures of the introduced parameters were then proposed based on minimizing the difference between the experimental and the model predicted yield points, using Levenberg–Marquardt optimization method. Afterwards, the proposed model was implemented into ABAQUS as user subroutine VUMAT with semi-implicit back-Euler integration algorithm. The numerical simulations of hydro-bulging and cylindrical deep drawing tests were then carried out to verify the validity of the proposed model. The results showed that the proposed model could enhance the predicting accuracy in both tests due to its flexibility.

Examination and modeling of void growth kinetics in modern high strength dual phase steels during uniaxial tensile deformation

Ductile fracture mechanisms during uniaxial tensile testing of two different modern high strength dual phase steels, i.e. DP780 and DP980, were studied. Detailed microstructural characterization of the strained and sectioned samples was performed by scanning electron microscopy as well as EBSD examination. The results revealed that interface decohesion, especially at martensite particles located at ferrite grain boundaries, was the most probable mechanism for void nucleation. It was also revealed that the creation of cellular substructure can reduce stored strain energy and thereby, higher true fracture strain was obtained in DP980 than DP780 steel. Prediction of void growth behavior based on some previously proposed models showed unreliable results. Therefore, a modified model based on Rice-Tracey family models was proposed which showed a very lower prediction error compared with other models.

Formability Investigations of High-Strength Dual-Phase Steels

Car manufacturing is always regarded as the key industry behind sheet metal forming, and thus, the requirements of and developments in car manufacturing play a decisive role in the development of sheet metal forming. The automotive industry is faced with contradictory demands and requirements: better performance with lower consumption and less harmful emissions, more safety and comfort; these are extremely difficult to supply simultaneously with conventional materials and conventional manufacturing processes. The fulfillment of these often contradictory requirements is one of the main driving forces in the automotive industry and thus in the material and process developments in sheet metal forming, as well. In recent years, significant developments can be observed in the application of high-strength steels. In this respect, the application of various dual-phase steels is one of the best examples. However, the application of these high-strength steels often leads to formability and manufacturing problems. One formability problem is the springback occurring after sheet metal forming. In the current research, we have dealt mainly with advanced high-strength steels, primarily with dual-phase steels. When applying them, the springback phenomenon is one of the most critical issues. To reduce the tremendous amount of experimental work needed, we also applied numerical simulation using isotropic–kinematic hardening rules. The isotropic–kinematic hardening behavior of a given material in the applied AutoForm numerical package may be characterized with three independent material parameters γ, χ and K (a detailed explanation of their meaning will be given in the main part of this paper). However, we found that the material data included in simulation packages for these new high-strength steels are not fully adequate. For the determination of more reliable material parameters and to achieve better simulation results, a new testing device was developed. Numerical simulations were performed using the material parameters determined by the new device to show the sensitivity of springback behavior to these material parameters.

Microstructural and Micromechanical Effects of Cold Roll-forming on High Strength Dual Phase Steels

In this work correlation between the 1000 MPa dual phase (DP) steel microstructure and the strain gained after roll-forming process have been studied by both microstructural and micromechanical analysis. The scanning electron microscope (SEM) inspection in the bent area reveals changes in the ferrite-martensitic microstructure. The plastic deformation of DP steels originates defects at the edges of bent sheet make them partly responsible for the damage caused. In addition, electron backscatter diffraction (EBSD) measurements have been carried out for an in-depth characterization after roll-forming. A high density of misorientation of the crystal lattice within the ferrite strained grains is observed, mainly concentrated in the ferrite/martensite grain boundaries. Furthermore, the ultramicrohardness tests exhibit little dependence between mechanical parameters and the material properties.

Void coalescence and fracture behavior of notched and un-notched tensile tested specimens in fine grain dual phase steel

Due to growing global concern about the environmental issues, steel developers have been forced by automobile makers to produce more efficient steel grades with high strength to weight ratios along with high crashworthiness performance. In order to find deficiencies of the existing steels and develop superior steel products, detailed understanding of deformation and damage behavior in the existing steels is needed. In the present research, deformation and damage evolution during room temperature uniaxial tensile test of a modern high strength Dual Phase Steel, i.e. DP780, were studied. Detailed scanning electron microscopy (SEM) examination of the microstructures of notched and un-notched tensile fractured specimens revealed that in notched specimen, plastic deformation was concentrated more within the notched region. Therefore, much higher reduction in thickness with a high reduction gradient occurred in this region, In the un-notched specimen, however, plastic deformation was more uniformly distributed in larger parts of the gauge length, and therefore, thickness reduction happened with a lower gradient. Although geometric notch on the specimen did not change the void nucleation and growth mechanisms, the kinetics of these phenomena was influenced. On the other hand, voids linkage mechanism tended to change from void coalescence in the un-notched specimen to void sheeting in the notched specimen. Moreover, three different models developed by Brown & Embury (BM), Thomason and Pardoen were employed to predict the final fracture strain. It was revealed that, BM model showed much more accurate predictions for the studied DP steel in comparison with those of Thomason and Pardoens’ models.

Work-Hardening Behavior of Cold Rolled Interstitial-Free Steel Sheet and Dual Phase High Strength Steel Sheets Subjected to Two-Stage, Coaxial and Non-Coaxial Tension/Compression

In-plane tension/compression tests of a cold rolled interstitial-free (IF) steel and sheet a 980MPa dual phase high strength steel sheet (980DP) were carried out to investigate the work-hardening behavior under two-stage loading paths. The two-stage loading paths consist of the uniaxial tension/compression for the rolling direction (RD) followed by unloading and subsequent uniaxial tension/compression in the 0°, 45° and 90° directions from the first loading direction (0°-, 45°- and 90°-loading). The work hardening behavior in the second loading was different between the 980DP and the IF steel. It was found that the work hardening behaviors were significantly affected by the inner product of the strain rate mode tensors for the first and second loading and that the effect of the deformation mode (tension/compression) was small.

A random effect autologistic regression model with application to the characterization of multiple microstructure samples

ABSTRACTThe microstructure of a material can strongly influence its properties such as strength, hardness, wear resistance, etc., which in turn play an important role in the quality of products produced from these materials. Existing studies on a material's microstructure have mainly focused on the characteristics of a single microstructure sample and the variation between different microstructure samples is ignored. In this article, we propose a novel random effect autologistic regression model that can be used to characterize the variation in microstructures between different samples for two-phase materials that consist of two distinct parts with different chemical structures. The proposed model differs from the classic autologistic regression model in that we consider the unit-to-unit variability among the microstructure samples, which is characterized by the random effect parameters. To estimate the model parameters given a set of microstructure samples, we first derive a likelihood function, based on which a maximum likelihood estimation method is developed. However, maximizing the likelihood function of the proposed model is generally difficult as it has a complex form. To overcome this challenge, we further develop a stochastic approximation expectation maximization algorithm to estimate the model parameters. A simulation study is conducted to verify the proposed methodology. A real-world example of a dual-phase high strength steel is used to illustrate the developed methods.

Microstructures, Mechanical Properties, and Strain Hardening Behavior of an Ultrahigh Strength Dual Phase Steel Developed by Intercritical Annealing of Cold-Rolled Ferrite/Martensite

A dual phase (DP) steel was produced by a new process utilizing an uncommon cold-rolling and subsequent intercritical annealing of a martensite–ferrite duplex starting structure. Ultrafine grained DP steels with an average grain size of about 2 μm and chain-networked martensite islands were achieved by short intercritical annealing of the 80 pct cold-rolled duplex microstructure. The strength of the low carbon steel with the new DP microstructure was reached about 1300 MPa (140 pct higher than that of the as-received state, e.g., 540 MPa), without loss of ductility. Tensile testing revealed good strength–elongation balance for the new DP steels (UTS × UE ≈ 11,000 to 15,000 MPa pct) in comparison with the previous works and commercially used high strength DP steels. Two strain hardening stages with comparable exponents were observed in the Holloman analysis of all DP steels. The variations of hardness, strength, elongation, and strain hardening behavior of the specimens with thermomechanical parameters were correlated to microstructural features.

Realistic microstructural RVE-based simulations of stress–strain behavior of a dual-phase steel having high martensite volume fraction

ABSTRACT Micro-mechanical behavior of a low carbon, high-strength (980 MPa grade) dual-phase (DP) steel of current technological interest that contains high volume fraction of martensite is simulated and uniaxial stress–strain curve is computed. Polygon-based vectorized microstructural images obtained via a new image analysis procedure are used for simulations of the micro-mechanical behavior. Simulations are performed using physics-based constitutive relationships for martensite and ferrite constituents and generalized plane-strain mesh elements. It is shown that there are significant variations in the computed stress–strain curve from one microstructural field of view to another for microstructural windows of 110 µm×110 µm size or less. A random statistical sample of 10 microstructural windows of 110 µm×110 µm size or larger constitutes a RVE of this class of microstructures such that the simulated stress–strain curves of such RVEs are in close agreement with one another, and therefore, represent the global stress–strain response of the simulated material. For the constitutive equations, boundary conditions, and type of mesh elements used in the present work, such average simulated stress–strain curves are also in close agreement with the experimentally measured uniaxial stress–strain curve of the DP980 steel. It is also shown that in this dual-phase microstructure, martensite carries most of the stress whereas most of the strain is contained in ferrite. Consequently, the strain localization and failure is predicted to go preferentially through ferrite compared to martensite, which is in agreement with experimental quantitative fractographic observations reported elsewhere.

Damage mechanism and modeling of void nucleation process in a ferrite–martensite dual phase steel

Damage initiation and growth behavior in high strength dual phase sheet steel, i.e. DP780, was investigated using smooth and notched tensile specimens. By SEM microstructural analyses of pulled and sectioned tensile specimens, void initiation and growth behavior were quantified. Finally, a simple model was proposed to predict the void nucleation kinetics for both kinds of specimens. The predicted values were in good agreement with the experimental data using the model.

Study on the strength and failure modes of laser welded galvanized DP980 steel lap joints

The mechanical properties of different sections of the laser welded galvanized high strength dual phase (DP) 980 steel lap joint such as the hardened zone (fusion zone and the grain coarsened heat affected zone (HAZ)), the softened zone (subcritical HAZ), and the base material were determined through mini-tensile tests. The numerically-predicted load–displacement curve and the sample rotation angle were verified by experimental measurements. The FE model considering the non-homogeneous mechanical properties was built based on the weld cross section geometries that were obtained under various laser welding conditions. The numerically-predicted von Mises equivalent strain concentrations and failure modes of the galvanized DP980 lap joints with respect to different laser welding conditions exhibit reasonable agreement with the experimental results.

수소주입시킨 DP박강판의 SP시험과 수소취성 관계 해석

고강도 DP강의 수소취성 거동을 소형펀치시험을 통해 평가하였다. 이를 위해 첨가원소가 각기 다른 3종의 DP강 시험편에 전기화학적 방법으로 수소를 강제 주입시켰다. 수소주입 후, 수소주입량을 측정하였다. 수소주입량은 마르텐사이트 부피분율에 크게 의존하는 것으로 조사되었다. 전류밀도 150, <TEX>$200mA/cm^2$</TEX> 조건에서 25시간이 포화상태에 도달하는 수소주입조건으로 나타났다. SP시험 후 SP에너지와 SP bulb 형상을 비교한 결과, 수소주입량의 증가에 따라 SP에너지와 SP bulb 높이가 감소하는 것으로 나타났다. 또한 SP bulb 파단면에서는 뚜렷한 facet와 층상형태의 벽개 파단면이 관찰되어 수소취성화를 관찰할 수 있었다. Small punch(SP) tests were performed on high strength Dual Phase(DP) steels in order to evaluate the behavior of hydrogen embrittlement. For this purpose, three different kinds of DP steel specimens were charged with hydrogen by electochemical hydrogen charging experiment. After charging with hydrogen, the amount of charged hydrogen was measured. The measurement results showed that amounts of charged hydrogen were largely dependent on the martensite volume fraction of DP steel. The hydrogen charging time of 25 hrs with current densities of 150 and <TEX>$200mA/cm^2$</TEX> was investigated as saturation condition with hydrogen. The analysis results on the SP energy and height of SP bulbs after SP tests showed that those were decreased as the amount of charged hydrogen increased. Fractographs of SP bulbs were observed a brittle fracture mixed with quasi-cleavage fractures, layered structures and clear facets.