DP600 Steel Sheets Research Articles

High-strength dual-phase steel produced through fast-heating annealing method

Fast-heating annealing method provides a new possibility to produce high-performance dual-phase (DP) steel by controlling austenite decomposition upon cooling. A quenching dilatometer with programmable heating-holding-cooling cycles was employed to stimulate the fast-heating annealing process of a cold-rolled dual-phase steel in the present work. Electron probe microanalysis and X-ray diffraction were conducted to reveal the microstructural evolution and phase transformation behavior in combination with dilatometric measurements. Experimental results showed that in case the cold-rolled DP980 steel sheet was rapidly austenitized just above the full austenitization temperature at a heating rate of 300 ​K/s, ultrafine and discontinuous martensite surrounded by interconnected network ferrite could be obtained after holding at 1073 ​K for 100 ​s. A phase transformation model was established for the cold-rolled dual-phase steel subjected to fast-heating annealing treatment. The produced fast-heating DP980 steel sheet possessed an ultimate tensile strength of about 1200 ​MPa and a fracture elongation up to 16.7%.

Springback comparison between DP600 and DP800 steel grades

A comparative study of the springback effect, otherwise known as elastic return, on Dual Phase DP600 and DP800 steel sheets is reported herein, which have been widely used in the automotive industry due to having good mechanical properties, such as high strength and ductility if compared to conventional steels. For such a purpose, it is proposed to analyze whether anisotropy and varying forming parameters interfere with the springback effect or not. The parameters selected to compare DP600 with DP800 dual phase steels were descending speed of 4 mm min−1 along the vertical axis of sheets until reaching internal bending angle of 30 degrees during bending tests at two rolling angles (0 and 90 degrees), thus forming a U-shaped steel sheet. In addition to bending tests, tensile tensting and Vickers microhardness tests have been performed at three rolling angles (0, 45 and 90 degrees). It was concluded that punching rate, internal bending angle, observation time and rolling angle exert an influence on the springback effect. Thereby, there is an important contribution to areas that require quality in formability, such as vehicle structure, which must have high impact strength and energy absorption. Steel sheets with increasingly smaller thicknesses are able to reduce density, product cost and greenhouse gases emissions from automobiles.

Influence of a Hydrostatic Pressure Shift on the Flow Stress in Sheet Metal

The stack compression test (SCT) and the strain-rate controlled hydraulic bulge test (HBT) enable to determine the large strain flow curve of sheet metal under an identical deformation mode and balanced biaxial tension. This equivalence should lead to identical flow behavior if plastic yielding is independent of the hydrostatic stress. However, a discrepancy is observed in the flow curves of DP600 steel sheet determined by the SCT and the HBT. In order to avoid uncertainty with respect to dissimilar test conditions, the average strain-rate in both material tests is carefully controlled. Additionally, evidence is provided that friction can be sufficiently minimized yielding a homogeneous SCT up to a true plastic strain of 0.3. Assuming reliable experimental data, the hypothesis that the hydrostatic pressure shift between the stress states in the SCT and the HBT causes the observed difference in flow behavior is scrutinized. Theoretical considerations regarding the effect of a superimposed hydrostatic pressure (i.e. putting a material under a pressure environment for a certain stress state) on the flow stress, as suggested by Spitzig et al. [1] and Spitzig and Richmond [2], are used to understand the effect of a pressure shift between two stress states on the flow stress. Finally, the theoretical considerations are experimentally validated using the discrepancy in flow behavior of DP600 measured by the SCT and the HBT.

A Comparison of Tensile Properties of Single-Sided and Double-Sided Laser Welded DP600 Steel Sheets

Dual Phase (DP) steels are the most commonly used steels in the automotive industry to reduce vehicle weight and improve car safety. DP600 steel is one of the most used steels for the automotive industry because this steel has high strength and good elongation properties. During the manufacture of automotive parts, welding is the most commonly used joining process, and especially the laser welding is getting more and more importance. This study was executed to evaluate how the welding type (single-sided and double-sided) affects the tensile properties of Nd:YAG (Neodymium-Doped Yttrium Aluminum Garnet; Nd:Y3Al5O12) laser welded DP600 steel sheets using different pulse frequencies. The laser welded samples were investigated by the methods of tensile test and fractography. Experimental results indicated that the tensile properties varied significantly depending on the welding type and pulse frequency. The tensile properties of the double-sided laser welded joints were significantly higher than those of the single-sided laser welded joints. Tensile strength and elongation of the single-sided and the double-sided laser welded joints increased almost linearly with increasing pulse frequency. The higher pulse frequency resulted in larger fully bonded section size, which led to higher tensile properties. The maximum tensile strength and elongation (611 MPa and 10.06 %) were obtained with the double-sided laser welded joint in the pulse frequency of 8.5 Hz. The tensile strength of this joint was almost equal to that of the base metal, but its elongation value was lower that of the base metal.

Laser beam welding of DP980 dual phase steel at high temperatures

Dual phase steels have been used in safety components for the automotive industry and represents a standard in terms of energy absorbing steel alloys. After welding, these components present a martensitic transformation in the fusion and heat-affected zones with an intrinsic brittleness. The current contribution presents a procedure where the DP980 steel sheet is kept at a given temperature during and after the laser weld in order to generate bainite instead of martensite. The results using an isothermal treatment at 527 °C shown a microstructure composed by grain boundary and bainitic ferrite and retained austenite with a constant hardness in the base material and heat-affected and fusion zones of 280 HV. The room temperature welds present hardness values between 320 and 500 HV. The high-temperature welds also shown a decrease in the maximum residual stress at the weld centerline by 1/3, with a consequential reduction in the warp of the joined component.

Forming limit diagram of DP600 steel sheets during electrohydraulic forming

Electrohydraulic forming is a typical high-speed forming which can significantly improve the formability of sheet metal. The objectives of this paper are to explore the influence of preset wire length and lifting height on the equi-biaxial stretching forming limit of DP600 steel sheets during electrohydraulic forming. The results showed that the maximum equi-biaxial stretching forming limit was obtained at the optimal wire length of 45 mm and lifting height of 37 mm, then the entire FLC was generated to evaluate the formability of DP600 steel sheets with these optimal parameters: a relative increase of 17.5–70% in major strain obtained by EHF was observed in all three regions of the FLC (tension-tension, plane strain, and tension-compression) relative to that obtained by quasi-static forming. Numerical simulation using ANSYS/LS-DYNA was conducted to reproduce the dynamic deforming behavior of a DP600 blank bulged by EHF, with an error of about 8%. A narrower radiation zone, a wider shear lip zone, and a mass of deeper and larger dimples on the fracture morphologies during EHF were observed compared with quasi-static conditions.

Mechanisms of die wear and wear-induced damage at the trimmed edge of high strength steel sheets

Die wear during trimming of advanced high strength steel (AHSS) sheets deteriorates the edge quality of trimmed sheets. In this work, the mechanisms of AISI D2 steel trim die wear and their effects on plastic deformation and fracture behaviour of the sheared edge of DP980 steel sheet were examined. A mechanical press equipped with D2 inserts was used to trim DP980 sheets with a clearance of 0.14 mm (10%). Abrasion and microchipping were identified as the wear mechanisms operating at the upper die, with microchipping becoming more dominant after 60,000 trimming cycles. The burr height and the length of the burnish zone of sheared DP980 sheets increased linearly with the chipped die edge percentage. The shear strains in the shear effected zone (SAZ) were estimated using martensite displacements as metallographic markers in the ferrite matrix as a function of the depth beneath the sheared edge. The depth of SAZ, and the plastic strains at a given depth within the SAZ increased with the number of trimming cycles. Correlation of the local stress and strain values generated in the SAZ showed that a saturation flow stress was reached near the sheared edge. The damage in SAZ occurred in the form of crack formation at the martensite/ferrite interfaces and fracture of martensite. Die wear reduced the tensile ductility of the DP980 sheets, and the fracture mode in tension changed from ductile with localized neck to more brittle sheared edge fracture initiating from surface cracks after 60,000 cycles.

Edge Effect Investigation of DP980 Steel Sheet in Multiple Laser Scanning Process

Due to free spring back, laser forming is an effective non-contact forming technique for hard forming materials. A desired shape is obtained by residual plastic strain which is induced by thermal stresses depending on multiple laser scanning. In this study, the combination of experimental and numerical analysis is employed to investigate the temperature distribution and bending angle of the dual phase (DP980) high strength steel sheet. The edge effect of six cases of laser scanning process involving 1, 2, 3, 4, 5 and 10-time laser scanning are investigated by analyzing their bending angle relative variations and transverse plastic strains. It is found that the edge effect at the middle part of laser scanning line reduces with the increasing number of laser scanning cycles. It is attributed to the fact that the relative variation of the residual plastic strain reduces with the increase of laser scanning cycles. To efficiently reduce the edge effect of DP980 steel sheet, a locally repeated scanning strategy is proposed. The numerical simulation results indicate that the edge effect reduces significantly in the 10-time laser scanning process and the average bending angle simultaneously increases around 20% under the new laser scanning strategy.

Lap Joint of 6061 Aluminum Alloy Sheet and DP590 Steel Sheet by Magnetic Pulse Welding and Characterization of Its Interfacial Microstructure

Lap Joint of 6061 Aluminum Alloy Sheet and DP590 Steel Sheet by Magnetic Pulse Welding and Characterization of Its Interfacial Microstructure

Evaluation of wear-induced plastic deformation at the trimmed edge of DP980 steel sheets

In order to increase the use of advanced high strength steel (AHSS) for automotive light-weighting, the significant trimming-induced wear damage that AHSS sheets cause to the trim dies should be reduced so as to decrease die maintenance costs and deterioration of the quality of sheared edge. In this research, the wear characteristics in the upper AISI D2 die inserts used for trimming DP980-type AHSS sheets and the wear-induced plastic deformation at the sheared edges of DP980 were studied. Observation of wear profiles at the edge of the upper trim die revealed that material abrasion was the main damage feature. The trimmed edge quality of DP980 deteriorated with the number of strokes, as indicated by an increase in burr width. Plastic strains near the surface of the sheared edge estimated using the displacements of martensite plates, increased from 2.9 at 2.5 μm below the trimmed edge after 40,000th cycles, to 55 after 80,000th cycles. Micro-hardness tests performed to estimate the local flow stresses in the shear affected zone (SAZ) indicated that the flow stress at a distance of 4.5 μm below the trimmed edge increased with the number of trimming cycles to 1.73 GPa in the 80,000th trimmed part.

Micromechanical Modeling of Dual-Phase DP600 Steel Sheet Plastic Behavior Based on a Representative Volume Element Defined from the Real Microstructure

Dual-phase steels offer very attractive combinations of strength and ductility owing to the coexistence of different microstructures components and their interactions. These steels are suitable to the automotive industry due to their improved impact resistance increasing the passenger safety along with the vehicle weight reduction. The properties of the dual-phase steels are attributed to the chemical composition, type, size, amount and spatial distribution of different phases that can be obtained during thermomechanical treatments, namely, ferrite and martensite. In this work, the microstructure of as-received DP600 cold rolled steel sheet with 1.2 mm nominal thickness was firstly characterized by means of scanning electron microscopy technique. Then, a representative volume element was obtained from the DP600 microstructure and a micromechanical finite element model is proposed considering the steel chemical composition, average ferrite grain size, martensite volume fraction and mechanical properties of both ferrite and martensite phases. The uniaxial tension loading was simulated by assuming either plane-stress and plane-strain conditions. The numerical predictions corresponding to the plane-strain model are in good agreement with the experimental true stress-strain curve determined along the sheet rolling direction. The proposed finite element micromechanical approach based on the real microstructure proved to be an important tool to evaluate both local and overall behaviors of DP600 steel grade.

Hosford-Coulomb ductile failure model for shell elements: Experimental identification and validation for DP980 steel and aluminum 6016-T4

When using shell elements, the Domain of Shell-to-Solid Equivalence (DSSE) needs to be defined in addition to the Hosford-Coulomb (HC) fracture initiation model to guarantee the convergence of the initiation of ductile fracture with respect to the mesh size. A comprehensive plasticity and fracture testing program is performed on 0.8 mm thick DP980 steel sheets. It includes uniaxial tension experiments along different material directions to characterize the plastic response, as well as simple shear, V-bending, and punch experiments to characterize the HC model parameters. Nakazima, tension, and bending experiments are performed on flat specimens with different notch radii to validate the predictions obtained with the combined DSSE-HC model. Aside from calibrating and validating the model for DP980 steel sheets, the model parameters are also identified from experiments on 1 mm thick aluminum 6016-T4 sheets and validated through numerical simulations of the deep drawing of a triangularly-shaped cup. The good agreement of the shell element simulations with all experiments demonstrates that the DSSE-HC model is capable of providing reasonable predictions of the onset of ductile failure for both membrane- and bending-dominated loading conditions.

Suggestion of an Indicator to Evaluate Material Deposition in Resistance Spot Welding: Weld–Surface Interaction Index

Resistance spot welding (RSW) is still leading joining process in the automotive industry due to the simplicity and speed of the process. On the other hand, galvanized steel sheets provide improvement in corrosion resistance of the auto body. Material deposition has an influence on the corrosion resistance and weld strength; however, in the literature very little effort has been made to identify any weld–surface interaction numerically. In this study, a numerical approach called “Weld–Surface Interaction Index” (IWSI) has been introduced to investigate the effect of material deposition on the corrosion sensitivity of the welding zone and its periphery. A systematic investigation was carried out to better understand the effects of an electrode cap–galvanized steel sheet interface on the corrosion resistance of spot-welded steel. The chemical composition of DP600 steel sheet and Z-Trode electrode cap was analyzed by using a spectrometer. The specimens were then joined by using same caps up to 1200 welds. SEM–EDS analysis was also performed on the chosen specimens to determine the weight percentages (wt.%) of Fe, Zn and Cu. EDS analysis results and coefficient of variation wt.% of Fe, Zn and Cu were used to compose IWSI formula. In the proposed formula, wt.% of Zn represented resistivity of the cover to corrosion, while wt.% of Fe represents the vulnerability.

Microstructure and property assessment of dissimilar joints of 6061-T6 Al/dual-phase steel fabricated by friction stir spot welding

Sound dissimilar joints of 6061-T6 Al (top member)/DP590 steel sheets (1.6 and 1.2 μm thickness, respectively) were achieved using friction stir spot welding. A joint microstructure showed fine aluminum grains, dominant ultrafine ferrite grains, and a minor martensite phase at ferrite grain boundaries which showed no marked effect on tensile-shear load. Instead, the thickness of the intermetallic compound layer (IMC) and the hook shape at the joint interface were found to be the dominant and minor factors on tensile-shear load, respectively. The IMC layer showed linear kinetics growth, which indicates a reaction-controlled mechanism. A well-shaped hook was formed by the strategy of selecting a relatively high pin length, low plunge rate, and high dwell time. A maximum tensile-shear load of ~ 2950 N was achieved for the joint with an IMC layer of ∽ 2 μm thickness and a well-shaped hook; this is higher than the ultimate shear load of the weaker member (6061-T6 Al). Joints with thinner IMC layers showed weak Al/IMC interfaces, but those with thicker ones exhibited brittle IMC layers since a second IMC layer with lower Al content is formed; their lower joint strengths were attributed to the formation of Kirkendall voids at the interfaces of the Al/IMC layer and initial/second IMC layers, respectively.

Correlation of the maximum shear stress with micro-mechanisms of ductile fracture for metals with high strength-to-weight ratio

Mechanisms of ductile fracture are investigated experimentally in a wide range of loading conditions from compressive upsetting to the balanced biaxial tension for two metals with high strength-to-density ratio of DP980 (t1.2) steel sheets and a bulk aluminum alloy of AA7075. Specimens are carefully designed to achieve various loading conditions from shear at low stress triaxiality to the balanced biaxial tension at high stress triaxiality for DP980, while both tensile and compressive tests are conducted for AA7075. Fractured specimen surfaces are analyzed macroscopically focusing on their relations with the maximum shear stress. It is observed that all the specimens tend to fail along the direction of the maximum shear stress in various loading states of plane strain compression, uniaxial compression, shear, uniaxial tension, plane strain tension and the balanced biaxial tension. Scanning electron microscope analyses of fracture surfaces are also conducted to explore the underlying mechanism of void coalescence since coalescence of voids is viewed as the last step of ductile fracture after nucleation and growth of voids. It is noted that fractured voids elongate along the direction of the maximum shear stress for all specimens with the stress triaxiality ranging from about −0.57 in compression to 0.67 in the balanced biaxial tension. The experiments of DP980 and AA7075 reveal that ductile fracture takes place along the direction of the maximum shear stress in the wide loading conditions of compressive upsetting, shear, uniaxial tension, plane strain tension and the balanced biaxial tension with stress triaxiality below 0.67. Thus, ductile fracture is expected to be governed by the maximum shear stress in these wide loading conditions of compression, shear and tension. It is suggested that effect of the maximum shear stress must be correctly coupled in modeling of ductile fracture in these loading conditions with uncoupled and coupled ductile fracture criteria.

Lap joint of 6061 aluminum alloy sheet and DP590 steel sheet by magnetic pulse welding and characterization of its interfacial microstructure

Lap joint of 6061 aluminum alloy sheet and DP590 steel sheet by magnetic pulse welding and characterization of its interfacial microstructure

Measurement of the strength differential effect of DP980 steel sheet and experimental validation using pure bending test

The strength differential effect (SDE), i.e., the difference between the flow stress of uniaxial tension and compression, of a dual-phase steel sheet with a tensile strength of 980 MPa (DP980) was measured using an in-plane uniaxial tension–compression test apparatus for sheet metals. The SDE data were obtained for a logarithmic total strain range of 0≤|ε|≤0.086 in both the rolling (RD) and transverse (TD) directions of the test material. It was observed that the magnitude of the compressive flow stress,σC, was consistently higher than that of the tensile flow stress, σT, for a strain range of |ε|>0.009 in both the RD and TD; the amount of SDE, defined as σC−σT/σT×100 (%), at |ε|=0.086 was 5.2% in the RD and 6.2% in the TD. Moreover, in order to validate the SDE, a novel test apparatus for obtaining the pure bending moment M versus curvature κ (i.e., M−κ) diagram was designed and manufactured. The obtained M−κ curves were consistent with those based on the elastoplastic numerical analysis considering the SDE. Thus, the SDE of the test material is experimentally validated using the pure bending test.

Anisotropic fracture forming limit diagram considering non-directionality of the equi-biaxial fracture strain

This paper is concerned with modeling of anisotropic fracture forming limit diagram considering non-directionality of the equi-biaxial fracture strain. A new anisotropic ductile fracture criterion is developed based on the Lou–Huh ductile fracture criterion (Lou et al., 2012). In an attempt to predict the forming severity of advanced high-strength steel (AHSS) sheets, the proposed fracture criterion is converted into a Fracture Forming Limit Diagram (FFLD) and anisotropic fracture locus considering the sheet metal orientation. Tensile tests of the DP980 steel sheet with the thickness of 1.2 mm are conducted using various specimen geometries including pure shear, dog-bone, and flat grooved specimens. With Digital Image Correlation (DIC) method, equivalent plastic strain distribution on the specimen surface is computed until surface crack initiates. The fracture predictability of the proposed fracture criterion is evaluated with the experimental results which cover a wide range of stress states in various loading directions. By comparing fracture strains obtained from the experiments with the ones predicted from the proposed fracture criterion, it is clearly confirmed that the fracture criterion proposed is capable of predicting the equivalent plastic strain at the onset of fracture with great accuracy over a wide range of stress states while keeping non-directionality of the equi-biaxial fracture strain.

Bendability and failure mechanisms of dual phase steel under air-bending at elevated temperatures

In this study, process parameters such as the bending angle and bending temperature of the metal sheets for pure bending of advanced high strength steel sheet DP980 were investigated using the air-bending method. This investigation focused on bendability and failure mechanisms. Experimental studies were carried out with various bending temperatures and the bending angle parameters to identify effect on springback angle, initial crack, and the evolution of the microstructure. Prior to testing, the DP steel sheets were heattreated at different temperatures (room temperature, 200 °C, 400 °C, and 600 °C) with a heating rate of 2 °C/S and 5 minutes of holding time in an electric furnace. As a result of the experiments performed at different temperature values and bending angle values, it was determined that an increase in the bending angle reduces the size of the springback angle. It was also determined that an increase in air-bending-temperature reduces the amount of springback. Air-bending-temperature also affected the microstructure evolution and slowed the cracking of the bent surface.

The Relationship between Tensile Strain and Residual Stress of High Strength Dual Phase Steel Sheet

The relation between residual stress and tensile strain is an important factor for evaluating plastic formation grade of steel sheet. The degree of plastic deformation (Δl) and elastic recovery (δ) were obtained by measuring the length of DP600 steel sheet sample under different tensile test conditions, i.e. five tensile strains (ε). Furthermore, the average residual stress value in the surface middle (the diameter of 10 mm) region of above tensile samples was analyzed by x-ray diffraction (XRD) in the crystal plane of (211). By processing the diffraction peak angle (2θ) with half width high method (FWHM), the relationship between sin2(ψ) and diffraction angle is attained by least squares method. On this basis, a mathematical model was established to correlate the tensile strain with the residual stress in the present study. The results show that the residual stress decreases and the elastic recovery increases with the increase of tensile strain (ε≤0.205). The relation between residual stress and tensile strain can be described with an exponential function . Finally, a function of tensile strain, elastic recovery and surface residual stress is established, by which a reasonable forming condition, viz. ε=0.205, δ=2.65 mm is determined for achieving the smallest σψ.