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Ferritic–martensitic dual-phase (DP) steels are prominent and advanced high-strength steels (AHSS) broadly employed in automotive industries. Hence, extensive study is conducted regarding the relationship between the microstructure and mechanical properties of DP steels due to the high importance of DP steels in these industries. In this respect, this paper was aimed at reviewing the microstructural characteristics and strengthening mechanisms of DP steels. This review article represents that the main microstructural characteristics of DP steels include the ferrite grain size (FGS), martensite volume fraction (MVF), and martensite morphology (MM), which play a key role in the strengthening mechanisms and mechanical properties. In other words, these can act as strengthening factors, which were separately considered in this paper. Thus, the properties of DP steels are intensely governed by focusing on these characteristics (i.e., FGS, MVF, and MM). This review article addressed the improvement techniques of strengthening mechanisms and the effects of hardening factors on mechanical properties. The relevant techniques were also made up of several processing routes, e.g., thermal cycling, cold rolling, hot rolling, etc., that could make a great strength–ductility balance. Lastly, this review paper could provide substantial assistance to researchers and automotive engineers for DP steel manufacturing with excellent properties. Hence, researchers and automotive engineers are also able to design automobiles using DP steels that possess the lowest fuel consumption and prevent accidents that result from premature mechanical failures.

A good combination of strength and fracture resistance is highly desired for the development of high-strength ferrite–martensite dual-phase (DP) steels for automotive application. But the increase in strength is usually compromised by a reduction in fracture resistance, and the guideline for microstructure optimization remains to be established. This study is dedicated to the DP steels with tensile strength above 1 GPa, and the influences of the equiaxed and fibrous morphologies on the mechanical properties were investigated by both the uniaxial tensile tests and the essential work of fracture (EWF) method. The fibrous morphology is efficient in increasing strength due to the ferrite grain refinement effect. Under uniaxial tension, the fibrous DP morphology does not lead to higher fracture strain. But when evaluating with the EWF method, the fibrous DP steels present a superior fracture resistance, which is attributed to the larger crack tip necking. The interpretation of the fracture resistance measurements was substantiated by the detailed damage observations. Therefore, the fibrous DP concept could provide an efficient pathway to improve the combination of strength and fracture resistance.

In this study, investigations on stretch flangeability were carried out on as-received DP 600 steel which had 22 volume % of martensite. Stretch flangeability is normally characterized by the hole expansion test and refers to the ability to avoid cracking during stretch flanging or hole expansion. Two intercritical annealing treatments (soaking temperatures of 760 °C and 800 °C followed by water quenching) were selected in order to obtain lower and higher volume fractions of martensite respectively. The influences of four different hole preparation techniques, electrical discharge machining, drilling, drilling + reaming, and punching, on the hole expansion ratios were investigated. Higher surface roughness at the surface of the hole showed inverse correlation with HER values. The highest hole expansion ratio of 156% was obtained for the as-received DP 600 steel with the hole prepared by electrical discharge machining. The HER values observed for various hole preparation conditions can be normalized with respect to the electrical discharge condition so that the HER values can be estimated when one of the HER values is known.

The martensitic transformation mechanism in the heat-affected zone of DP1180 steel plays a decisive role in the strength of welded joints. In this work, the nucleation and growth kinetics of martensite laths in the coarse grain heat-affected zone (CGHAZ) are analyzed by a high-temperature laser scanning confocal microscope (LSCM). The grain distribution and stress distribution of the samples after in situ observation are analyzed by electron backscatter diffraction (EBSD). The results reveal that austenite grain growth is realized by continuous grain boundary annexation and grain boundary migration of small grains by large grains during the heating process. Seven growth modes of CGHAZ martensitic laths under laser welding conditions are proposed. Additionally, the end growth of martensitic laths is mostly attributed to the collision with grain boundaries or other laths to form a complex interlocking structure. The results of this study could provide important data support for the development of dual-phase steel materials and improvement of welding quality.

This study investigated the manufacturability of ultra-strength steel-Al plate composite, the punch-cutting performance of these composites and base materials, and the cutting surface properties. Therefore, experimental studies included explosive welding, punching process, and materials characterisations (microstructure and sheared surface image analyses via SEM, hardness and tensile test). In the punching process, five different types of punches were used. Additionally, the load cell was mounted on the punch and connected to the computer by an amplifier and a data acquisition card; the force values corresponding to the processing times were directly monitored, and the data were obtained. Shear process analyses were performed using finite elements. Microstructure and mechanical test results show that the DP1200 steel-Al 1100 aluminium laminated bimetal composite was produced by explosive welding of acceptable quality. The flat punch showed the highest shearing force, and other punches (angled and concave ends) reduced the shearing forces P R2, P V16, P R1, and P 16, respectively. Punches with different cutting-edge forms reduced the shearing force by up to 75%. The sheared surfaces were investigated in detail. Shearing marks were observed on both sides of the bimetal similarly. The punching process force vs time results showed good agreement between modelling and experimental punching values.

This work systematically investigates the high-cycle fatigue (HCF) properties and fatigue crack growth (FCG) behavior of hot-rolled dual-phase (DP) steels with comparable tensile strength but distinctly different yield strength (458 MPa for the FG sample and 355 MPa for the CG sample), grain sizes and morphologies. Contrary to the conventional Hall–Petch relationship, the coarse-grained (CG) sample demonstrates superior fatigue performance. This enhancement is reflected in its higher fatigue strength, combined with an elevated FCG threshold and a reduced FCG rate in the Paris regime of FCG behavior. Fracture morphologies and FCG path analyses reveal that this enhanced fatigue resistance attributes to pronounced crack path tortuosity in the CG microstructure. The tortuous crack path enhances roughness-induced crack closure effects in the near-threshold regime while promoting more frequent crack deflection during stable propagation, collectively reducing the effective driving force for crack growth. The experimental evidence confirms that properly designed CG microstructures with appropriate phase distribution can provide superior fatigue resistance in hot-rolled DP steels.

With the rapid development of the automotive industry, the requirements for bodywork materials are not only focused on high strength but also on improved forming properties. To develop a new generation of automotive steels with higher strength–plasticity matching, a high elongation 1200 MPa grade V-Nb microalloyed cold-rolled reinforced formable dual-phase steel was developed in this experiment through rational compositional design and precise process machining. The properties of the test steel are improved by varying the over-aging temperature as well as the annealing temperature to achieve a good strength–plasticity balance. The results show that as the aging temperature increases, the tensile strength and yield strength of the test steel decrease, while the elongation continues to increase. At an aging temperature of 310 °C, the steel exhibits not only high strength but also better ductility. As the annealing temperature increases, the tensile strength and yield strength of the test steel initially increase and then decrease, while the elongation continues to increase. When the heat treatment process involves an annealing temperature of 860 °C and an over-aging temperature of 310 °C, the test steel achieves the best strength–plasticity balance.

The conventional melting methods were used to obtain in situ TiC particle-reinforced dual-phase steel, followed by hot rolling and heat treatment processes. The aim was to investigate the effect of TiC particles on the fracture behavior of dual-phase steel at different annealing temperatures, by analyzing the microstructure and tensile behavior of the multiscale TiC particle-reinforced dual-phase steel. The results showed that TiC particles precipitated in the as-cast microstructure of dual-phase steel were distributed along the grain boundaries. During hot rolling, the grain boundary-like morphology of the micron-sized TiC particles was disrupted, and the particles became more refined and evenly distributed in the matrix. The tensile tests revealed that the strength of the TiC particle-reinforced dual-phase steel increased with increasing martensite content, while the elongation decreased. These results were similar to those of conventional steel. The addition of 1 vol.% multiscale TiC particles improved the strength of the dual-phase steel but did not affect elongation of the steel. Cracks and holes were primarily concentrated around the TiC particles rather than at the interface of martensite and ferrite. The main causes of crack sprouting were TiC particle interface cracking and TiC particle internal fragmentation. Overall, the study demonstrated the potential of multiscale TiC particle-reinforced dual-phase steel as a strong and tough material. The refined distribution of TiC particles in the matrix improved the strength of the material without compromising its elongation. The results also highlighted the importance of careful selection of reinforcement particles to avoid detrimental effects on the fracture behavior of the material.

In a deep-learning-based algorithm, generative adversarial networks can generate images similar to an input. Using this algorithm, an artificial three-dimensional (3D) microstructure can be reproduced from two-dimensional images. Although the generated 3D microstructure has a similar appearance, its reproducibility should be examined for practical applications. This study used an automated serial sectioning technique to compare the 3D microstructures of two dual-phase steels generated from three orthogonal surface images with their corresponding observed 3D microstructures. The mechanical behaviors were examined using the finite element analysis method for the representative volume element, in which finite element models of microstructures were directly constructed from the 3D voxel data using a voxel coarsening approach. The macroscopic material responses of the generated microstructures captured the anisotropy caused by the microscopic morphology. However, these responses did not quantitatively align with those of the observed microstructures owing to inaccuracies in reproducing the volume fraction of the ferrite/martensite phase. Additionally, the generation algorithm struggled to replicate the microscopic morphology, particularly in cases with a low volume fraction of the martensite phase where the martensite connectivity was not discernible from the input images. The results demonstrate the limitations of the generation algorithm and the necessity for 3D observations.

In this research, the effect of 2D and 3D Representative Volume Element (RVE) on the ductile damage behavior in single-phase (only ferrite) and dual-phase (ferrite and martensite) steels is analyzed. Physical and fitting parameters of the constitutive model for bcc-ferrite and bcc-martensite phases are adapted from the already published work. Crystal plasticity (CP) based numerical simulations without damage consideration are run and, later, ductile damage criteria for the ferrite phase is defined for all cases. The results of the non-damage (-nD-) and damage (-D-) simulations are compared to analyze the global and local differences of evolving stresses and strains. It is observed that for the same model parameters defined in all cases, damage initiation occurs at the overall higher global strain in the case of 3D compared to 2D. Based on statistical data analysis, a systematic comparison of local results is carried out to conclude that the 3D RVEs provide better quantitative and qualitative results and should be considered for such full phase simulations. Whereas 2D RVEs are simple to analyze and provide appropriate qualitative information about the damage initiation sites.