Papers by Keyword: DP Steel

Abstract: Effect of microstructure size and type on the hardness for the duplex steel were disclosed by using of optical microscope (OM), scanning electron microscope (SEM) and nanoindenter for the samples hot compressed under different temperature with reduction of 10%, 30%, 50% and 70%. OM and SEM were used to measure the average martensite lamellar width, space and indenter morphology. nanoindenter test characterized the microstructure hardness for the samples under different process. Experiment results show that martensite hardness for the sample hot compressed at 950°C has larger diversity than that of sample hot compressed at 1200°C. The martensite hardness fluctuation range for the sample compressed at 950°C is almost from about 7GPa to 12GPa, while, for the sample compressed at 1200°C, the fluctuation range is basically from about 9GPa to 12GPa. However, the average hardness for the samples hot compressed at 950°C is comparably smaller, which is related with lower quench temperature. The larger martensite hardness fluctuation is mainly related with induced ferrite formation and finer martensite lamellar width. For the ferrite phase, the hardness fluctuation range is lower.

Abstract: Dual precipitates of carbide and copper particles formed within Cu-Ti microalloyed DP (Dual Phase) steel in the present study. The precipitation behaviors of tiny precipitates, especially in the ferrite matrix, were checked precisely by several methods, such as optical microscopy, high resolution transmission electron microscopy, and hardness testing. It was found that copper particles nucleated only on the interphase precipitated TiC and were not dispersed randomly within the ferrite matrix. Therefore, the formation of dual precipitates within the ferrite grains should be considered as separated phase transformation; initially, only titanium carbides form during the austenite decomposition reaction, after which copper particles heterogeneously nucleate on these carbides. Furthermore, as compared to Cu microalloyed DP steel, the tempering behavior of martensite in the Cu-Ti microalloyed DP steel showed a tempering hardening characteristic.

Abstract: Typical microstructures of dual-phase (DP) steels consist of hard martensite particles dispersed within a ductile ferritic matrix. These microstructures possess a complex network of grain and interphase boundaries, which, together with the mechanical contrast of their phase composition, control micro-damage initiation mechanisms, induced by deformation. Accordingly, in this study we analyze the influence of individual microstructural features and interfaces on damage nucleation and progression in DP steels with respect to applied tensile strain. Prominent micro-damage mechanisms include cracking of martensite and damage initiation at interphase boundaries. Influence of martensite morphology is discussed based on a statistical analysis of the damage features as observed by electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) maps. Prior austenite grain boundaries (PAGbs) in martensite show a brittle behavior and are highly susceptible to crack propagation.

Abstract: Hot-rolling direct quenching and dynamical partitioning (HDQ&DP) processes were applied to a low-carbon steel containing silicon and manganese based on thermo-mechanical control process (TMCP) technology and ultra-fast cooling (UFC) technology. The microstructures and phase compositions were characterized and analyzed using SEM, EBSD, TEM and XRD. The mechanical properties and tensile deformation behaviors were investigated by means of uniaxial tensile test. The microstructures and tensile deformation behaviors of both HDQ&DP steel with and without proeutectoid ferrite were comprehensively expounded by comparing with each other. Results show that the amount of retained austenite in the HDQ&DP steel with proeutectoid ferrite can reach up to 17.3%, which is higher than that in the HDQ&DP steel without proeutectoid ferrite (15.7%). The HDQ&DP steel without proeutectoid ferrite possesses extremely high ultimate tensile strength (UTS) up to 1700 MPa with yield ratio about 0.73 and elongation about 11.5%. The introduction of proeutectoid ferrite can result in a moderate decrease of UTS to 1240-1400 MPa, a drastic decrease of yield ratio to 0.51-0.69 and a certain increase of elongation to 13.0-13.7%. The existence of the proeutectoid ferrite can partly enhance the work hardening ability of the steel and may improve its formability. It is concluded that the HDQ&DP steel with relatively large amount of proeutectoid ferrite and certain amount of bainite has extensive application prospects.

Abstract: Recently, the weight reduction of automotive body and crash safety become much more important factors. In addition, the corrosion resistance must be ensured for any material used in a structural part of automotive components. In an effort to satisfy these requirements, high strength galvannealed DP steels with 590 MPa in tensile strength have been developed in Shougang. Steel chemistry with low Si was designed to reduce the iron oxidation with chemical composition (Si, Mn etc.) and to improve the wettability by liquid zinc. And the alloying elements (Cr, Mo etc.) were added to improve hardenability of sheet to obtain DP microstructure. Newly developed 590MPa grade hot dip galvannealed DP steels have good mechanical properties and hole expansibility. The results revealed the Cr addition effectively suppresses the formation of ferrite during the continuous annealing to improve the hole expansibility of steels. The galvannealing temperatures are increased to improve the hole expansibility of steels by generating the appropriate amount pearlite.

Abstract: A new cold rolled hot-dip galvanizing DP450 steel with low carbon and high chromium was designed and effect of the galvanizing processes on the microstructure and mechanical properties was also investigated. At last, it was compared with the referenced DP450 steel which was already produced in industry. The results show that microstructure of the experimental steel consists of regular polygonal ferrite, almost 1% MA island and little bainite. The industrial experimental cold-rolled galvanizing DP steel has excellent combined mechanical properties as follow: yield strength is 306 MPa, tensile strength is 467 MPa and elongation (A80) is 33%. Compared to the referenced steel, the experimental DP450 steel has lower yield strength, better elongation and better forming performance, accordingly.

Abstract: With the requirement of vehicle performance and fuel economy, dual-phase (DP) steels as one of the advanced high stress steels (AHSS) are increasingly used in the automotive industry due to the excellent combination of the tensile strength and ductility. On a microscale the ductile fracture is governed by the void nucleation, growth and coalescence mechanism. In the dual-phase steels this damage mechanism exhibits a rather complex situation: voids are generated by the debonding of the hard phase from the matrix and the inner cracking of the hard phase besides by inclusions. On a macroscale fracture of these materials is observed in the automotive industry with the absence of strain localization or minimal post-necking deformation. Consequently the failure during the forming process is caused by a competitive or combined mechanism of internal damage evolution and metal instability. In this study, the target is to develop a simple and generalized model for metal forming processes accounting for instability, damage and ductile fracture. Theoretical predictions of metal instability by the Hill–Swift necking criterion and the modified maximum force criterion are considered. The damage model is developed by the combination of the prediction of metal instability and ductile fracture of sheet metals. The model is developed in 3D triaxial stress state and the accumulation of damage is stress state dependent. Furthermore, the influence of the hardening curve effected by damage on the forming limit curve is investigated.

Abstract: The work experimentally analyses the effect of various factors on hardness measured values on thin steel sheets using Ultrasonic Contact Impedance (UCI) technique. The conditions experimentally used are compared with that according the ASTM A 1038-08 standard. UCI is an experimental technique for indirect hardness measurement. The equipment uses a Vickers indenter and the hardness measurement is based on the change of the resonance frequency during indenter ́s penetration [.The UCI hardness may depend on some factors, therefore optimal measurement conditions must be determined. The effect of distance between indents, zinc coating, sample weight, sample mounting and adhesive material for sample fixation were determined.

Abstract: Anisotropic plastic behavior of advanced high strength steel sheets of grade DP780 and DP980 were investigated using three different yield functions, namely, the von Mises, Hills 48 and Barlat2000 (Yld2000-2d) criteria. Uniaxial tensile and balanced biaxial (hydraulic bulge) tests were conducted for the examined steels in order to characterize flow behavior and plastic anisotropy for different stress states. Additionally, disk compression and In-plane biaxial tension tests were performed for obtaining balanced r-value of DP780 and DP980, respectively. All these data were used to determine the anisotropic coefficients. According to the different yield criteria, yield stresses and r-values for different directions were calculated corresponding to these yield criteria. The results were compared with experimental data. It was found that the Yld2000-2d model precisely predict well with experimental data than the other models.

Abstract: Recently, due to the harsh demands for automobile lightweight and safety, more and more attention is focused on the warm forming process of various high strength steel sheet. In the present work, aiming to the formed parts safety problems caused by elevating temperature, take B340/590DP steel as the research object, the dent resistance of the warm-forming parts with free-form surface is studied. Firstly, combing the warm tensile tests under various conditions with the secondary room temperature tensile tests, a secondary yield constitutive model is established for the researched material by the regression analysis method, which reveals the influences of temperature, strain rate and pre-deformation on the secondary yield behaviors. Secondly, based on the plastic deformation theory and the free-form curves and surfaces theory, a dent resistance evaluation system is proposed for the warm-forming high-strength steel parts with free-form surface. Finally, design a dent resistance model experiment, validate the dent resistance of the warm-forming B340/590DP steel specimens, and determine the relevant coefficient value in the proposed dent resistance evaluation system by means of the obtained experiment data. The research results can be used directly to select the reasonable warm-forming process conditions, control and improve the warm-forming parts quality and performances.