High-strength Dual-phase Steel Research Articles

Effects of tempering on the mechanical properties of high strength dual-phase steels

Abstract Dual-phase (DP) steels are generally characterized by the combination of high strength with good ductility, but some formability characteristics of these steels are low compared to other steel classes. In particular, stretched edge ductility and bendability are poor by comparison with other less ductile strip steel classes such as complex-phase (CP) steels. This paper describes the effect of short time tempering of the order of few seconds to few minutes on the mechanical properties of two cold rolled and hot-dip galvanised dual-phase (DP) steels. The uni-axial tensile characteristics, hole-expansivity, Erichsen cupping height and bendability were assessed after tempering at temperatures between 200 and 450 °C for relatively short time scales up to 10 min. The short tempering treatments significantly changed the character of the DP steel leading to a decrease in tensile elongation and an increase in hole-expansitivity and bendability. The properties observed in tempered DP steels where found to be closely comparable to those expected for CP steels. Microstructural analyses using the SEM showed carbide precipitation within the martensite and ferrite that increased with tempering.

スプリングバックの影響を考慮したスポット溶接箱型断面部材の疲労寿命評価

Most automobile body members are fabricated from formed steel sheets joined by spot welding, and these welded joints typically tend to cause fatigue failure owing to local stress concentrations. In this study, fatigue tests were conducted under cyclic torsion loads using spot-welded rectangular section members made of 590 MPa grade dual-phase high-strength steel to investigate the springback effect on fatigue properties. These members were assembled using the rectangular sections that were formed under different blank holder forces, as this would change the amount of springback. Furthermore, the prediction of the fatigue life of these members was attempted through springback analysis, an elastic analysis together with a method of fatigue life prediction for spot welds using nominal structural stress. In this prediction, residual stress caused by springback was regarded as the mean stress. Concerning the experimental results, every fatigue failure was initiated close to the spot weld edge located at the center of the tested members, and fatigue life decreased with springback amount increasing. In addition, the predicted results were generally in agreement with the experimental results in terms of the effect of springback.

Desing of Dual Phase Steel Sheets for Auto Body

This paper outlines the considerations of strength and energy absorbing car body by applying mechanical properties of steel sheets with higher strength properties. Good combination of strength and ductile properties of dual-phase steel helps reduce weight and improve crash safety body. High-strength dual-phase steels have better deep-drawing capacity and stretching capacity compared to microalloyed at the same strength. Dual phase steel sheet have comparable streching capacity but poorer deep drawing capacity than DC04. Measured values LDR and FLD0 have to serve engineers who haven't experience with application of high strength steel sheets for car body.

The influence of robotic MAG process welding parameters

In the modern manufacturing sector, new materials and current fabrication technologies force a critical re-examination of all conventional knowledge. Increasing production and reducing waste remain fundamental objectives for every business: the trend is moving towards increasingly reduced labour, which is gradually being replaced by automation. On the other hand, especially in such a fundamental sector as the automotive industry, there is a prime requirement: reduced vehicle weight, due to the incessant demand for energy and cost savings, in addition to meeting commitments relating to harmful emissions and environmental protection. The metallurgical development of high-strength steels has allowed the use of new materials with increasingly enhanced mechanical characteristics, benefiting mankind with increased safety and the ability to absorb ever greater quantities of energy without cracking. To analyse the problems associated with the types of welding used with increasing frequency in the automobile sector, research has been focussed on joints made between conventional materials such as S355 steel and high-strength Dual Phase steels. This study has attempted to highlight the use of a welding robot in a mass production setting, and to study the best welding parameters when not working with components with the same metallurgical characteristics. Furthermore, the ability to obtain good quality welds when there are geometrical problems, such as misalignments between the pieces to be welded, has been evaluated.

Henry Granjon Prize Competition 2010 Winner Category a: “joining and Fabrication Technology” Study On Friction Stir Spot Welding Of Dual-Phase High-Strength Steel Sheets

Friction stir spot welding (FSSW) was applied to interstitial free (IF) steel sheet and dual-phase high-strength steels for automobiles with an uncoated Si3N4 tool. The tool with TiN-coating and Ar shielding gas were also attempted. Defect-free joints were successfully obtained. Microstructure distribution of the joint and its evolution during FSSW were systematically examined as well as joint strength. The joint exhibited the heterogeneous microstructure distribution, which could be mainly explained by phase transformation from the different peak temperatures during FSSW. However, contaminations of oxygen, silicon and nitrogen were detected in an elliptical region observed near the pin hole, and strongly affected the microstructural evolution. Additionally, they caused the increase in hardness in the elliptical zone. On the other hand, by using coated tool and shielding gas, contaminations and hardness increase in elliptical region were suppressed and joint strength was improved.

A finite element modeling and prediction of stamping formability of a dual-phase steel in cup drawing

A numerical technique is presented in order to assess formability conditions for tearing type sheet metal failures in automotive stamping applications. The proposed technique is based on the plastic instability and uses Swift’s diffuse necking and Hill’s localized necking concepts. Both necking models are transformed into a set of differential equations that may be applied both for proportional and non-proportional loadings and expressed an incremental form suitable for finite element (FE) analysis. Next, the numerical models are implemented into Ls-Dyna FE program and applied to predict the forming limit curve (FLC) of a high-strength dual-phase steel using in-plane proportional loadings of a single shell element. Then cup drawing processes of the same steel grade are simulated and failure heights for three square blanks are predicted. Allowable maximum punch strokes predicted with the necking models are compared with the results from square cup drawing tests under the same blankholder force. Model predictions were in accord with the experimental data and determined to be conservative for this steel grade. An investigation of cup failure heights determined with conventional technique revealed that the FE post-processing using the experimental FLC resulted in erroneous failure predictions and not conservative.

Effect of Si content on wettability of dual phase high strength steels by liquid Zn-0.23 wt.%Al

The wettability of dual phase steels by liquid Zn-0.23 wt.%Al at 450 °C (723 K) was investigated using the dispensed sessile drop method. Dual phase steels of six different Si contents (0 wt.%, 0.12 wt.%, 0.18 wt.%, 0.3 wt.%, 0.5 wt.%, and 1.0 wt.%) annealed in a 15 %H2-N2 gas atmosphere at a dew point of −60 °C (213 K) were examined. As the Si content increased from 0 wt.% to 0.5 wt.%, the wettability worsened. However, as the Si content increased to 1.0 wt.%, the wettability improved to be better than that of the 0.3 wt.%Si steel. Based on the surface oxide analysis results using SEM, FT-IR, and TEM analyses, the effect of the surface oxide type on the wettability is discussed. Although the morphology of the nanometer-sized surface oxides did not change, the surface oxides changed from MnO to Mn2SiO4 to MnSiO3 and then to SiO2 as the Si content increased. After comparison of the present results with the wettability of pure oxides, it was concluded that the coating property of the dual phase high strength steel was more closely related to the type of surface oxide.

Effect of Process Parameters on Microstructural Evolution and Grain Refinement in a Low Carbon High Strength Microalloyed Dual Phase Steel

The effects of process parameters on microstructural evolution and grain refinement are determined in a Nb-Ti microalloyed high strength dual phase steel. With the increase of cooling rate, final microstructures change from a mixture of acicular ferrite (AF)+martensite/retained austenite (M/A) to conventional bainite (CB)+M/A. Accordingly, the morphology of M/A constituent changes from an equiaxed island in AF to an elongated interlath in CB. The length and width of bainite packets become smaller with the increase of cooling rate and the decrease of deformation temperature. The length of individual bainitic ferrite plates within the packets become smaller with the increase of cooling rate and the decrease of deformation temperature, whereas the thickness of them varies slightly with them. The optimized relaxing time on grain refinement is 60 s. The reheating temperature, reduction ratio and interrupt temperature has no obvious effect on the formation of dual phase of acicular ferrite or bainite and M/A.

Quantitative Fractographic Analysis of the Variability in Tensile Ductility of a High Strength Dual Phase Steel

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Modelling of flow stress of dual phase steel under warm tensile deformation

In order to evaluate the flow stress of dual phase high strength steel in warm forming processes, the flow stress behaviours of dual phase steel were investigated at different temperatures and strain rates. A mathematical model is proposed to predict the stress–strain curves of dual phase steel during warm tension by employing the hyperbolic sine function of Zener–Hollomon parameter, which can describe the relationship among temperature, strain rate and flow stress under warm tensile deformation correctly. Material constants in the equation are expressed in polynomial form of strain. Parameters in the polynomials were obtained by least square method. The predicted stress–strain curves are in good agreement with experimental results, which confirms that the proposed model is accurate.

A micro-level strain analysis of a high-strength dual-phase steel

Abstract The deformation behaviour of a dual-phase steel with a tensile strength of about 1 000 MPa is investigated at the micro level. In-plane strains are determined by means of in-situ tensile tests in the scanning electron microscope and automatic local deformation analysis (based on digital image correlation). Maps of strain at high and low magnification reveal the network-like formation of deformation bands. In detailed high resolution analyses the sites of highest strain (“hot spots”) are identified. Observations in metallographic cross-sections indicate that these hot spots are frequently the origin of damage initiation.

Prediction of Continuous Cooling Transformation Diagrams for Dual-Phase Steels from the Intercritical Region

The purpose of the present work is the implementation and validation of a model able to predict the microstructure changes and the mechanical properties in the modern high-strength dual-phase steels after the continuous annealing process line (CAPL) and galvanizing (Galv) process. Experimental continuous cooling transformation (CCT) diagrams for 13 differently alloying dual-phase steels were measured by dilatometry from the intercritical range and were used to tune the parameters of the microstructural prediction module of the model. Mechanical properties and microstructural features were measured for more than 400 dual-phase steels simulating the CAPL and Galv industrial process, and the results were used to construct the mechanical model that predicts mechanical properties from microstructural features, chemistry, and process parameters. The model was validated and proved its efficiency in reproducing the transformation kinetic and mechanical properties of dual-phase steels produced by typical industrial process. Although it is limited to the dual-phase grades and chemical compositions explored, this model will constitute a useful tool for the steel industry.

The limit drawing ratio and formability prediction of advanced high strength dual-phase steels

Dual phase (DP) steels, being among advanced high-strength steels (AHSS), have been successfully used in the sheet metal stamping of automotive components for its great benefit in reducing vehicle weight while improving car safety. In their practical application, one of the major challenges is related to formability prediction of onset crack. This paper first conducts limiting drawing ratio (LDR) experiments to identify the maximum blank diameter of SPFC340, DP600, DP800 and DP1000 with onset crack. The fracture modes of these four types of blanks are then compared and classified into two categories: necking crack and shear crack. Further for DP1000, appropriate hardening formula is determined to fit the flow curve derived by the tensile test. Three yielding models (Hill-48, Batlat-89 and Banabic-2005) are compared with each other in the numerical simulation of DP1000 LDR onset crack. The investigation shows that a Swift and Hockett–Sherby combined formula is in good agreement with the flow curve of the tensile test and Batlat-89 yield model successfully predicts the onset shear crack of DP AHSS.

Improvement of the Galvanized Coating Quality of High Strength Dual Phase Steels by Pre‐Electroplating Nickel Layer

AbstractGalvanized dual phase steel sheets are used extensively in the industrial applications because of their excellent mechanical properties and superior corrosion resistance, but the segregation of alloying elements and the formation of oxides on the steel surface often have a deleterious effect on coating adhesion during the galvanizing process. In order to improve the coating quality, a nickel layer was pre‐electroplated on the steel substrate before galvanizing and it's found that there is an improvement in the coating quality. The coating microstructures were investigated by scanning electron microscopy together with energy dispersive X‐ray spectroscope, glow discharge optical emission spectroscope and X‐ray diffractions. The experimental results show that the compact Ni3Zn22 intermetallic layer formed at the zinc/nickel interface during the galvanizing process, prohibiting the nucleation and the growth of the ζ‐Zn phase layer and resulting in the improvement of the zinc coating adhesion.

A numerical performance comparison of a dual-phase steel and aluminium alloy bumper bar system

Bumper bar systems are important automotive structures to help protect passengers during frontal and rear collisions. This paper presents the outcomes of a numerical modelling study of a simple rectangular cross section bumper bar system using the explicit finite element code LS Dyna for a low velocity (16 km/h) centre pole frontal impact. The study utilises and compares a dual-phase steel (DP600) and two aluminium alloys (A16061T6 and A17108T6). A simple material model using isotropic elastic plastic material behaviour is utilised together with a failure criterion. When the bumper system weight is held constant by reducing the wall thickness of the steel bumper system, both aluminium alloy bumpers outperform the high-strength dual-phase steel.

Local plastic strain evolution in a high strength dual-phase steel

The evolution of local plastic deformation in a dual-phase (DP) steel has been studied using Digital Image Correlation (DIC) and in-situ tensile testing inside a scanning electron microscope. Tests were performed using specially designed samples to study the initiation and evolution of damage in DP1000 steel by measuring the strains at the scale of the microstructure. Micrographs have been analysed using DIC at different stages throughout a tensile test to measure local strain distributions within the ferrite–martensite microstructure. The results show progressive localisation of deformation into bands orientated at 45° with respect to the loading direction. Strain magnitudes are higher in the ferrite phase with local values reaching up to 120%. Several mechanisms for damage initiation are identified and related to the local strains in this steel. The procedure used and the results obtained in this work may help the development of models aimed at predicting the properties of new generation automotive steels.

Microstructures in a resistance spot welded high strength dual phase steel

The microstructure of a high strength dual phase steel resistance spot welded with tempering-pulse technology is characterized in this paper. In the fusion zone, there is a needle-like microstructure identified as acicular or side plate ferrite that has a cube-on-cube orientation relationship with respect to the surrounding martensite. In contrast to the microstructures produced by the lower cooling rate arc or laser welding techniques, the nucleation of this fine intragranular ferrite takes place independent of inclusions. Further, a leaf-like microstructure within the martensitic matrix is found to contain primitive orthorhombic Cr 3C 2 and face-centered cubic CrC chromium carbides, rather than Cr 23C 6 or Cr 7C 3 as is commonly observed in steel alloys. The formation histories of both the ferrite phase and the chromium carbides are analyzed.

Effect of Dwell Time on Friction Stir Spot Welded Dual Phase Steel

Friction stir spot welding (FSSW) is a relatively new welding technique which has already been successfully commercially applied on automotive aluminum alloys which, in turn, lead to significant energy savings and infrastructure cost-reductions. Due to the success of FSSW on Al alloys FSSW has generated tremendous interest in the automotive industry as an alternative joining process to resistance sport welding for steel body panels. Currently there is limited data on the mechanical properties and microstructure of friction stir spot welded steel thus the current aim was to investigate how these parameters chamged as a function of some critical process variables. In this instance spot welding of 2 mm thick high strength dual phase steel (DP800) was undertaken using a 3.5mm pin length tungsten rhenium tool with a tool rotation speed of 1000 rpm and plunge rate of 100 mm/min. Different dwell times of 0, 0.5, 1, 2 and 4 seconds were then applied prior to tool retraction to study the effect of dwell time on the microstructure and mechanical properties of the spot welds. Investigation of the weld spot joints revealed three distinct microstructural zones; a) thermo-mechanically affected zone (TMAZ), b) heat affected zone (HAZ), and c) parent material zone. The TMAZ shows an acicular martensite microstructure while the HAZ shows a ferrite/martensite structure in a very fine scale when compared to parent material zone. The Parent material is a typical dual phase structure which is ferrite matrix with a dispersion of martensite. Therefore, the TMAZ was harder while HAZ was softer when compared with parent material zone. The tensile strength obtained from tensile tests and the bonding ligament which has been reported to be measures of strength were also found to increase with dwell time.

Modeling mixed microstructures using a multi-level cellular automata finite element framework

Abstract Thermo-mechanical processing (TMP) of steels are designed to yield mixed microstructures with a potential of superior mechanical properties. Prominently, creation of martensite or bainite islands in ferrite resulting in high strength dual phase steels. With the increasing demands of such steels from the automotive sector, the need to understand and predict the deformation behaviour and load requirements during the manufacture of such steels has become important. Finite elements are best suited to analyse the deformation behaviour as a function of stress, strain and temperature fields. However, representing all the facets of these mixed microstructures computationally within the finite element framework is a challenge. This is because constituents of the mixed microstructure evolve at different length scales e.g. sub-microns to microns while the finite element technique works best at the millimeter scale. The paper describes a multi-level cellular automata framework amenable to capture micro-features across the sub-micron and microns length scales. The framework is integrated with a commercially available numerical code, ABAQUS™ and tested for a plane strain compression simulation. The paper concludes with the potential of using the framework for creating designer microstructures.

Formability Analysis of Diode-Laser-Welded Tailored Blanks of Advanced High-Strength Steel Sheets

Currently, advances due to tailored blanking can be enhanced by the development of new grades of advanced high-strength steels (HSSs), for the further weight reduction and structural improvement of automotive components. In the present work, diode laser welds of three different grades of advanced high-strength dual-phase (DP) steel sheets (with tensile strengths of 980, 800, and 450 MPa) to high-strength low-alloy (HSLA) material were fabricated by applying the proper welding parameters. Formability in terms of Hecker’s limiting dome height (LDH), the strain distribution on the hemispherical dome surface, the weld line movement during deformation, and the load-bearing capacity during the stretch forming of these different laser-welded blanks were compared. Finite element (FE) analysis of the LDH tests of both the parent metals and laser-welded blanks was done using the commercially available software package LS-DYNA (Livermore Software Technology Corporation, Livermore, CA); the results compared well with the experimental data. It was also found that the LDH was not affected by the soft zone or weld zone properties; it decreased, however, with an increase in a nondimensional parameter, the “strength ratio” (SR). The weld line movement during stretch forming is an indication of nonuniform deformation resulting in a decrease in the LDH. In all the dissimilar weldments, fracture took place on the HSLA side, but the fracture location shifted to near the weld line (at the pole) in tailor-welded blanks (TWBs) of a higher strength ratio.