Mechanical Properties Of Dual Phase Steel Research Articles

Effect of martensite hardness on mechanical properties and stress/strain-partitioning behavior in ferrite + martensite dual-phase steels

Low-carbon dual-phase (DP) steels composed of ferrite and martensite are widely used in automotive industries due to their good combination of strength and ductility and remarkable strain-hardening ability. Such exceptional mechanical properties of DP steels arise from microstructural deformation heterogeneity due to the different deformation roles of soft ferrite and hard martensite. The present study investigated mechanical properties and local deformation behavior of DP steels having different martensite hardness (strength) using digital image correlation (DIC) technique and in-situ X-ray diffraction (XRD) experiment during tensile deformation. The martensite hardness was reduced by tempering treatment, and its mechanical properties were compared with the as-quenched DP specimen by tensile testing. The tempered DP specimen exhibited reduced strength but enhanced ductility, especially in post-uniform elongation, compared to the as-quenched DP specimen. DIC-strain analysis indicated that the tempering effectively inhibited strain localization, allowing a greater contribution of martensite to plastic deformation. On the other hand, the in-situ XRD analysis revealed that the tempering significantly restricted the stress-increasing potential of martensite. Our results suggest that martensite, acting as a hard domain, plays a significant role in enhancing strain-hardening ability and achieving large uniform elongation due to its high stress-increasing potential. However, the presence of hard martensite also imposes restrictions on post-uniform elongation due to the strong strain localization induced in microstructures.

DEFECT SENSITIVITY OF DUAL-PHASE STEELS: A STATISTICAL MICROMECHANICAL INVESTIGATION OF THE DUCTILITY LOSS DUE TO PREEXISTING DEFECTS

As a result of their heterogeneous two-phase microstructure, dual-phase (DP) steels reveal various damage mechanisms leading to the nucleation of voids, microcracks, and other defects at all stages of deformation. Defects may also preexist in the microstructure due to thermomechanical processing of the material. The literature has ample evidence that DP steels, while offering a good compromise between ductility and strength, are sensitive to these types of preexisting defects. However, the quantitative dependency of mechanical properties of DP steels on such preexisting defects is still to be explored. In this paper, a systematic statistical analysis of this sensitivity is carried out using an idealized microstructural model of randomly generated two-phase volume elements with embedded preexisting defects. The proposed model also enables a methodological study probing the influence of mechanical phase contrast (i.e., the hardness difference between the constituent phases) and volume fractions. It is observed that high phase contrast microstructures are less sensitive to initial defects since the inherent extreme heterogeneity of the microstructure leads to the nucleation of new damage incidents irrespective of the presence of preexisting defects. At constant contrast, the volume fraction of the hard phase has less influence. These conclusions are insensitive to the precise type of defect considered.

Development of Desirable Fine Ferrite Grain Size and Random Second Phase Dual-Phase Steel Microstructures Using Composition and/or Processing Modifications

Microstructural morphology is known to have a significant impact on the mechanical properties of dual-phase steels. A fine ferrite grain size and random distribution of small second phase islands are desirable to provide superior isotropic properties compared to the banded second phase distribution that is typical for this type of steel. A rapid alloy prototyping (RAP) facility has been used to investigate three different DP 800 variants by systematically varying the compositions and/or process parameters compared to the ‘standard’ DP800 composition and processing that gives a banded microstructure. For Variant 1, the heating rate during the annealing cycle after cold rolling varied between 0.65 and 30 °C/s for the 45%, 60% and 75% cold reduction samples. It was found that a cold reduction of 75% and heating rate of 15 °C/s resulted in the microstructure that can give the best combination of strength and ductility because of the fine grain size and high martensite volume fraction. For Variant 2, the effect of changing the hot rolled (HR) microstructure (ferrite–pearlite, ferrite–bainite or martensite) on the final microstructure was investigated. Both the ferrite–50% bainite and fully martensite/bainite HR materials for all cold reductions resulted in annealed microstructures with necklace martensite morphology and finer ferrite grains compared to the ferrite–pearlite HR material, which gave a typical banded ferrite–martensite microstructure with a coarser ferrite grain size. For Variant 3, the Mn content was reduced, and increased Nb was used to achieve higher pancaking during the hot rolling stage, which refined ferrite grains in the HR condition with the same hardness. After annealing with the standard parameters only the 45% cold-reduced material produced a finer ferrite grain size than the standard material, whereas the 60% and 75% cold-reduced samples required a higher heating rate to achieve finer ferrite grain sizes due to rapid recrystallisation and growth kinetics.

Effect of intercritical annealing on the mechanical properties of dual-phase steel

ABSTRACT A dual-phase steel chemistry has been processed with varying intercritical annealing temperatures and the properties investigated. It was found that increasing the temperature in the intercritical region led to an increase in the volume fraction (52%–65%) and grain size (1.8–3.3 µm) of martensite whilst the volume fraction and grain size of ferrite decreased. The highest tensile strengths were found at low intercritical temperatures, achieving 935 MPa whilst the highest elongation values of 21% were achieved at the highest intercritical temperatures.

Microstructural Characteristics and Strengthening Mechanisms of Ferritic–Martensitic Dual-Phase Steels: A Review

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.

Micro-laminated and ultrafine-grained dual-phase steel plates generated via intercritical rolling followed by water quenching

In this study, a kind of heterogeneous structured dual-phase (DP) steels with micro-laminated and ultrafine-grained microstructures are obtained via intercritical rolling followed by water quenching. The microstructures are tailored by changing rolling temperature, i.e., reheating temperature. We find that with the decrease in reheating temperature from 810 °C to 750 °C, martensite aspect ratio increases from 4.5 ± 3.5 to 6.1 ± 2.9, and martensite long axis elongates from 1.72 ± 0.09 μm to 2.86 ± 0.98 μm. Meanwhile, the size of ferrite grain surrounded by high-angle grain boundaries refines from 1.98 ± 2.32 μm to 1.47 ± 2.14 μm. The yield strength improves 78 MPa from 432 ± 3 MPa to 510 ± 6 MPa, and the tensile strength increases 114 MPa from 745 ± 5 MPa to 859 ± 1 MPa. An important find is that the strain hardening rate increases abnormally with the refinement in ferrite grain due to enhanced hetero-deformation induced (HDI) hardening. So, the downtrend of tensile ductility varies with the improvement in strength is nearly stopped when reheating temperature reduces from 780 °C to 750 °C. Therefore, better comprehensive mechanical properties of DP steels are obtained compared with the approximately equiaxed and coarse-grained DP steel plate. The microstructure evolution mechanism is discussed from the perspective of deformation-induced ferrite transformation and ferrite dynamic recovery. The change of HDI hardening is related to the microstructure feature of the tested DP steels.

Characterization of the Trace Phosphorus Segregation and Mechanical Properties of Dual-Phase Steels

Characterization of the Trace Phosphorus Segregation and Mechanical Properties of Dual-Phase Steels

Numerical simulation of dual-phase steel based on real and virtual three-dimensional microstructures

Dual-phase steel shows a strong connection between its microstructure and its mechanical properties. This structure–property correlation is caused by the composition of the microstructure of a soft ferritic matrix with embedded hard martensite areas, leading to a simultaneous increase in strength and ductility. As a result, dual-phase steels are widely used especially for strength-relevant and energy-absorbing sheet metal structures. However, their use as heavy plate steel is also desirable. Therefore, a better understanding of the structure–property correlation is of great interest. Microstructure-based simulation is essential for a realistic simulation of the mechanical properties of dual-phase steel. This paper describes the entire process route of such a simulation, from the extraction of the microstructure by 3D tomography and the determination of the properties of the individual phases by nanoindentation, to the implementation of a simulation model and its validation by experiments. In addition to simulations based on real microstructures, simulations based on virtual microstructures are also of great importance. Thus, a model for the generation of virtual microstructures is presented, allowing for the same statistical properties as real microstructures. With the help of these structures and the aforementioned simulation model, it is then possible to predict the mechanical properties of a dual-phase steel, whose three-dimensional (3D) microstructure is not yet known with high accuracy. This will enable future investigations of new dual-phase steel microstructures within a virtual laboratory even before their production.

The synergistic effects of ultrafine grains and nano-size Cu-rich precipitates on the mechanical properties of DP steels

Simultaneous improvement in strength and ductility has always been a major concern for ferrite/martensite dual-phase (DP) steels. To this end, the synergistic effects of ultrafine grains and nano-size Cu-rich precipitates on the mechanical properties of DP steels were studied using an experimental method combined with first-principles calculations and crystal plasticity (CP) simulations. Ultrafine-grained DP steels consisting of soft ferrite, with an approximately 10% volume fraction of martensite and dispersed nano-sized Cu-rich precipitates, were obtained through warm rolling and subsequent aging heat treatment. Compared to warm deformation with a 50% reduction, that with a 75% reduction leads to finer grains and a higher tensile strength of 976 ± 15 MPa, accompanied by a small loss in elongation due to the aligned connection of the martensite phase, preferred grain orientation distribution, and relatively higher dislocation density. The optimal mechanical properties are a yield strength of 876 ± 13 MPa, tensile strength of 976 ± 15 MPa and total elongation of 15.2%. The CP simulation results indicate that the heterogeneous distribution of martensite leads to strong strain and stress partitioning, thus facilitating early damage nucleation. Additionally, the strengthening effect of precipitation was evaluated based on shearing mechanisms, and the results suggest that the precipitation of nano Cu-rich precipitates contributes to a strengthening of approximately 300 MPa in the steels.

Competitive mechanism of phosphorus capturing between MC-carbide (M = Ti, Mo) and ferrite/martensite interface in dual-phase steel

Segregation behavior and capturing mechanism of phosphorus (P) in dual-phase steel were systematically investigated by Atom probe tomography (APT). The dual-phase steel is composed of lath martensite, irregular ferrite and MC-carbide (M = Ti, Mo). The P is mainly dissolved in ferrite and martensite grains, and a small amount of P segregates around MC-carbide. APT analysis confirms that the carbide is a type of (Ti, Mo) C. The MC-carbide has a competitive advantage over the ferrite/martensite interface in capturing P atoms. Therefore, it can be concluded that P segregation around the MC-carbide weakens P enrichment at the ferrite/martensite interface. In the end, it is expected that the MC-carbide significantly reduces the P damage to the mechanical properties of dual-phase steel, especially the impact properties.

Improved properties of dual-phase steel via pre-intercritical annealing treatment and thermal cycling

The synergistic effects of pre-intercritical annealing treatments and multiple heating to the two-phase region (followed by quenching) on the microstructure and mechanical properties of dual-phase (DP) steel were studied. Intercritical annealing of ferritic-pearlitic, martensitic, and cold-rolled martensitic microstructures was investigated. The effects of tempering, austenitisation, grain growth, and recrystallisation were unravelled and an optimum holding time was characterised for each initial microstructure. Grain refinement by recrystallisation of cold-rolled martensitic microstructure resulted in the enhancement of tensile properties. Thermal cycling through multi-step intercritical annealing resulted in grain refinement and enhanced mechanical response. It was revealed that thermal cycling combined with the carefully controlled pre-intercritical microstructure could be simply used for the improvement of mechanical properties of DP steel.

Experimental and Numerical Investigation on the Effect of the Tempcore Process Parameters on Microstructural Evolution and Mechanical Properties of Dual-Phase Steel Reinforcing Rebars

One of the most efficient and cheapest ways to improve steel reinforcement bars is the Tempcore process. In this study, the effects of water pressure, water temperature, bar diameter, and its initial temperature, as Tempcore process parameters on microstructural and mechanical properties of reinforcing rebars were numerical and experimental investigated by using the Taguchi experimental design method. Results showed that these Tempcore process parameters influence the cooling intensity, which causes the formation of martensite at the surface layer and fine-grained ferrite–pearlite at the core and improvement of tensile strength. All of the considered parameters strongly affect the volume fraction of ferrite and pearlite in the rebar center and thickness of the martensite layer, but among them, water pressure with a 59% impact has a more substantial effect. Ultimately, to figure out the mechanical characteristics of hot rolled and optimal sample’ rebar, tensile tests were conducted. Outcomes displayed that the Tempcore process increases the yield and ultimate tensile strengths as much as 1.5 and 1.3 times, respectively, but slightly decreases the elongation.

Stress–strain partitioning behavior and mechanical properties of dual-phase steel using finite element analysis

Various industrial applications require materials with both high strength and good ductility. However, strategies for enhancing material performance are usually trapped in a strength–ductility trade-off. In this study, the effect of martensite morphologies (including geometry, connectivity, and distribution direction) on the stress–strain partitioning behavior and mechanical properties of a ferrite-martensite dual phase steel was studied using the secant method and finite element analysis. The results demonstrated that the combination of rhombus and horizontal geometries provided a good balance of strength and ductility. Thus, a combination of 45° and 0° regarding the angle between the directions of martensite distribution and deformation was further determined to be beneficial to the strength–ductility balance. These results are expected to provide a general understanding of strengthening and toughening mechanisms and will promote the development of high-performance steels.

Response surface modelling and optimization of temperature and holding time on dual phase steel

Low carbon steel of composition 0.98 wt% quenched in water, was subjected to heat treatment at different holding times, to improve on the mechanical properties of Dual Phase (DP) steel. This study investigates the influence of operating parameters on the mechanical properties of DP steels, using response surface methodology (RSM) to develop a prediction model. To reduce the number of experiments and eliminate trial by error approach, design of experiment (DOE) approach was adopted. In developing the model, temperature and holding time were considered as the model variables. The optimum operating parameters were predicted using the user-defined design (UDD) under RSM and the result was validated through experiments. Improved mechanical properties with increasing temperature and holding time were observed. At a holding time of 30 min and 650 °C, DP steel quenched in water compared with a holding time of 60 min at a temperature of 800 °C, shows a Hardness value of 143.26 HV as against 168.82 HV, Ultimate tensile strength of 500.641 MPa as against 598.317 MPa, and Yield strength of 257.333 MPa as against 303.246 MPa. These findings show that at a longer holding time and higher temperature, low carbon steel can be exploited for the production of Dual Phases steel.

Effect of post-weld heat treatment on microstructure and mechanical properties of DP800 and DP1200 high-strength steel butt-welded joints using diode laser beam welding

Among the available high-strength steels, there is growing demand for dual phase (DP) steels for wide application in the automotive industry owing to their good combination of high strength, ductility and formability. Also, the use of innovative welding technologies like laser beam welding (LBW) has growing importance in the field of high-strength steel because of its excellence in providing high-quality welds, high welding speed, high power density, low heat input, a narrow heat-affected zone and low heat distortions as compared to the conventional gas metal arc welding process. However, the hardening and softening in the heat-affected zone is a major issue when welding high-strength steel, i.e. DP steel grades, greatly affecting the strength, formability and plasticity of the whole-welded joint and thus affecting service performance and reliability. Based on preliminary experiments, the optimal welding condition was a nominal laser power of 1.0 kW and a welding speed of 8 mm/s. The aim of this work is to analyse and compare the weld and heat-affected zone characteristics, microstructure and mechanical properties of DP steels with 1-mm thick butt joints of DP800 and DP1200 high-strength steel (HSS) by diode laser beam welding. The effects of post-weld heat treatment (PWHT) on the strengthening of the laser-welded joints were evaluated by microstructural examinations under optical microscope and scanning electron microscope, and mechanical properties were examined by microhardness test, three-point bending tests and tensile tests.

Microstructure tailoring for property improvements of DP steel via cyclic intercritical annealing

Multi-step intercritical annealing treatment was applied on the quenched and cold rolled martesnitic microstructures to unravel the effects of thermal cycling on dual phase (DP) steel. During intercritical annealing of the quenched microstructure, hardness decreased with increasing holding time and reached a plateau. However, during intercritical annealing of the cold rolled microstructure, there was a rapid fall of hardness due to recrystallization. Afterwards, hardness increased as a result of austenitization, and then reached a plateau. A rapid grain growth was observed during repetition of intercritical annealing cycle for the DP steel originated from the quenched microstructure with adverse effects on the tensile properties. Conversely, decreased size of martensite islands and formation of chain like morphology of martensite around ferrite grains was observed during thermal cycling of the DP steel originated from the cold rolled microstructure with the consequent enhanced work hardening behavior. Accordingly, the combination of cold rolling and cyclic intercritical annealing was recommended in this work to enhance the work hardening behavior and mechanical properties of DP steels.

The bake hardening mechanism of dual-phase silicon steels under high pre-strain

In this paper, the effect of silicon content in the range of 0.34–2.26 wt% was investigated on the mechanism of bake hardening of dual-phase steels. It was observed that with increasing silicon content before the bake hardening process, the yield and ultimate tensile strength decreased. However, uniform and total elongation increased. Bake hardening value increased initially with the silicon content. However, it decreased with further increasing silicon content, which this was discussed based on the microstructural evolutions during bake hardening. It was seen that the martensite phase during the bake hardening process was tempered significantly by increasing in the silicon content. The changes in the yield and tensile strength due to increase in silicon content were proportional to the bake hardening behavior. The total elongation after bake hardening improved with increasing silicon content to 1.51%. However, it was reduced with further increasing silicon content. Silicon partitioning in the ferrite in the bake-hardened samples was higher than unbaked samples and thus the ferrite solid solution strengthening increased during bake hardening. By comparing mechanical properties of dual-phase steels before and after the bake hardening process, it was found that the bake hardening process has a positive effect on the yield strength, while its effect on the tensile strength was negative in the sample with the high silicon content. The ductility of all samples increased significantly after bake hardening, indicating a useful effect of bake hardening on ductility. Fractography also displayed that the fracture of all bake-hardened samples is ductile type.

Modeling and optimization of operating parameters using RSM for mechanical behaviour of dual phase steels

In this study, the influence of operating parameters on the mechanical properties of Dual Phase (DP) steels was investigated, using response surface methodology (RSM) to develop a prediction model. In developing the model, temperature and holding time were considered as the model variables. To establish this fact, an experiment based on statistical four-level two factorial design method was carried out. Based on the statistical analysis, surface hardness (SH) shows a correlation coefficient of R2 = 0.9607 while Ultimate Tensile strength (UTS) gave R2 = 0.9297, this suggests that the proposed quadratic model is suitable for use. The optimum operating parameters were predicted using the user-defined design (UDD) under RSM and the result was confirmed through experiments, which predict at a temperature (T) of 800 °C and holding time (HT) of 60 min, hardness value was given as 157.798 HV and UTS as 553.648 Mpa. This result indicates that RSM is a suitable method to optimize the process variables for mechanical properties of DP steels.

Effect of Heating Rate during Continuous Annealing on Microstructure and Mechanical Properties of High-Strength Dual-Phase Steel

The study concerns the influence of heating rate during continuous annealing on microstructure and mechanical properties of high-strength ferrite–martensite dual-phase (DP) steel. The study suggests that the superheat (ΔT), carbon diffusion and distribution were strongly influenced by increasing the heating rate, leading to different microstructure and mechanical properties of DP steel. The morphology of martensite was observed to shift from network along ferrite grain boundaries to banded structure to uniformly dispersed, and the average ferrite grain size was refined from 5.3 to 2.0 µm. When the heating rate was 300 °C/s, the steel exhibited excellent mechanical properties with tensile strength (UTS) of 982 MPa, total elongation (TEL) of 17.5%, strength–elongation balance (UTS × TEL) of 17.2 GPa%, because of finer microstructure, uniform martensite distribution and low carbon content of martensite. Moreover, the precipitation strengthening effect of NbC is an important reason for the high-strength DP steels. In addition, the variation in strain-hardening behavior and fracture mechanism of steel subjected to different heating rates is discussed in relation to the microstructure.

Optimization of the Continuous Galvanizing Heat Treatment Process in Ultra-High Strength Dual Phase Steels Using a Multivariate Model

The main process variables to produce galvanized dual phase (DP) steel sheets in continuous galvanizing lines are time and temperature of intercritical austenitizing (tIA and TIA), cooling rate (CR1) after intercritical austenitizing, holding time at the galvanizing temperature (tG) and finally the cooling rate (CR2) to room temperature. In this research work, the effects of CR1, tG and CR2 on the ultimate tensile strength (UTS), yield strength (YS), and elongation (EL) of cold rolled low carbon steel were investigated by applying an experimental central composite design and a multivariate regression model. A multi-objective optimization and the Pareto Front were used for the optimization of the continuous galvanizing heat treatments. Typical thermal cycles applied for the production of continuous galvanized AHSS-DP strips were simulated in a quenching dilatometer using miniature tensile specimens. The experimental results of UTS, YS and EL were used to fit the multivariate regression model for the prediction of these mechanical properties from the processing parameters (CR1, tG and CR2). In general, the results show that the proposed multivariate model correctly predicts the mechanical properties of UTS, YS and %EL for DP steels processed under continuous galvanizing conditions. Furthermore, it is demonstrated that the phase transformations that take place during the optimized tG (galvanizing time) play a dominant role in determining the values of the mechanical properties of the DP steel. The production of hot-dip galvanized DP steels with a minimum tensile strength of 1100 MPa is possible by applying the proposed methodology. The results provide important scientific and technological knowledge about the annealing/galvanizing thermal cycle effects on the microstructure and mechanical properties of DP steels.